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Université Catholique de Louvain
Institut de la Matière Condensée
et des Nanosciences
Pole Bio & Soft Matter
Université de Yaoundé I
Faculté des Sciences
Departement de Chimie
Inorganique
Nickel-zinc mixed metal oxide:
Coprecipitation synthesis and application
as gas sensors
Roussin LONTIO FOMEKONG
Thèse présentée en vue de l’obtention du grade de Docteur en Sciences
de l’Ingénieur de l’UCL et Docteur/PhD en Chimie de l’UYI
Promoteurs: Pr. John LAMBI NGOLUI (UYI/ENS, Cameroun)
Pr. Arnaud DELCORTE (UCL/IMCN/BSMA, Belgique)
Membres du jury:
Pr. Daniel PEETERS (Président), UCL/IMCN/MOST, Belgique
Pr. Sophie HERMANS (Secrétaire), UCL/IMCN/MOST, Belgique
Dr Sami YUNUS, UCL/IMCN/BSMA, Belgique
Dr. Driss LAHEM, Materia Nova, Belgique
Pr. Daniel NJOPWOUO, UYI/FS/CI, Cameroun
Pr. Isabel VAN DRIESSCHE, Ugent/FS, Belgique
Avril 2016
ii
iii
Remerciements
Avant toute chose, je rends grâce à Dieu qui pour ma part a permis
que tout ce travail soit achevé et je remercie toutes les personnes qui
ont contribué de près ou de loin à la réalisation de cette thèse. Par
ailleurs je voudrai mettre un accent sur certaines personnes.
Je souhaite de prime à bord remercier mes promoteurs, les Professeurs
John LAMBI NGOLUI et Arnaud DELCORTE. Plus qu’un
promoteur le Pr. LAMBI, est un père qui m’a accueilli dans son
laboratoire depuis 2007 et m’a initié à la recherche. Ses conseils, sa
rigueur, sa personnalité et son aide multiforme tant sur le plan de la
recherche que sur d’autres plans de la vie m’ont permis de réaliser ce
travail. En ce qui concerne le Pr. DELCORTE, il m’a fait confiance
dès les premiers contacts et m’a accepté dans sa grande équipe depuis
2013, me permettant ainsi de réaliser ce travail et je tiens
particulièrement à le remercier pour la grande liberté d’action dont j’ai
bénéficié durant toute ma thèse. Son ouverture d’esprit, son sens de
l’écoute au-delà même de la recherche, ses conseils et sa grande
disponibilité m’ont permis de mener à bien cette thèse.
Je tiens à remercier aussi particulièrement les membres de mon comité
d’accompagnement, les Pr. Sophie HERMANS et Dr. Sami YUNUS,
pour m’avoir accompagné effectivement et efficacement durant ce
travail. Merci au Pr. Sophie HERMANS de m’avoir ouvert les portes
de son laboratoire pour réaliser non seulement certaines synthèses et
décompositions thermiques mais aussi des analyses tel que la
thermogravimétrie. Merci au Dr Sami YUNUS pour son ouverture
d’esprit et précisément pour avoir initié la collaboration avec Materia
Nova où j’ai réalisé une bonne partie de cette thèse.
Je remercie les autres membres du jury, le Dr Driss LAHEM, le Pr.
Daniel NJOPWOUO, le Pr. Isabel VAN DRIESSCHE et le Pr. Daniel
iv
PEETERS pour avoir pris de leur précieux temps pour lire ce travail
ainsi que pour leurs remarques et commentaires qui ont permis
d’améliorer la qualité de ce manuscrit. Je remercie particulièrement le
Dr. Driss LAHEM, qui m’a ouvert les portes de Materia Nova et qui
m’a suivi tout au long des expériences réalisées là-bas.
Merci à tous ceux qui m’ont apporté leur aide sur le plan expérimental
et pour les analyses réalisées durant ma thèse. Je pense ainsi :
- Au Pr. Eric GAIGNEAUX et son équipe, précisément, Chiara
PEZZOTTA, François DEVRED, josefine SCHNEE pour les
analyses DRX et BET.
- Au Dr. Marc DEBLIQUY dont l’aide a été très précieuse pour
la partie expérimentale liée aux senseurs à gaz ;
- A Claude POLEUNIS et Pierre ELOY pour les analyses ToF-
SIMS et XPS respectivement;
- A Anne ISERENTAND pour les analyses ICP-AES ;
- A Jean-Francois STATSYNS pour les analyses
thermogravimétriques;
- Au Dr. Delphine MAGNIN pour les analyses SEM;
Je désire remercier tous les enseignants du département de Chimie
Inorganique de l’Université de Yaoundé I et ceux de l’Ecole Normale
Supérieure de Yaoundé pour les enseignements reçus depuis le début
de mon cursus universitaire et qui ont constitué une bonne base
scientifique pour mener à bien ce travail.
Je tiens à remercier mes collègues de laboratoire pour l’atmosphère
cordiale, les discussions autour de mon sujet et leur soutien
multiforme dans les épreuves difficiles. Je pense ainsi au Dr. Patrice
KENFACK TSOBNANG, à Hypolite Mathias TEDJIEUKENG
KAMTA, Cedrik NGNINTEDEM YONTI, Gilbert FEUYIT,
Mohammad ZARSHENAS, Eva POSPISILOVA, Vanina
v
CRISTAUDO, Supriya SURANA, Henok MESFIN, Jean-Louis
STANUS, aux Dr. Victor LEBEC et Dr. Bartlomiej CZERWINSKI
(les anciens membres) et à tous les étudiants de DIPES II et Master.
Merci à tous les membres du pole BSMA et précisément aux
secrétaires Françoise BOUVY, Rose-anne JACOB et Aurore
BECQUEVORT pour leur aide sur le plan administratif.
Je remercie l’Université Catholique de Louvain pour le financement
de 24 mois octroyé via le programme «Coopération au
Développement» et qui m’a permis de réaliser cette thèse en cotutelle
sans trop de difficultés.
Mes remerciements vont naturellement à ma famille et tous mes amis
(Saustin DONGMO, Floriant DOUNGMENE, Antoine TIYA, Duclair
KUETE, Yvan NGANDJUI, Verlaine FOSSOG, Armand NANHA,
Stephane NGAKO, Alvine DJOMATCHIE, Paule CHOUMI, Trésor
MELACHIO) qui m’ont soutenu et encouragé depuis le début,
particulièrement à mes parents (Julienne WANA et feu Mr. LONTIO),
mes frères (Maxim LONTIO KAHABI et Herman LONTIO
TCHIFFO) et surtout mon épouse Fridiane MANIDZE TIWA pour sa
disponibilité et sa patience.
vi
vii
TABLE OF CONTENTS
Remerciements iii
Foreword xi
Abstract xiii
List of abbreviations xv
GENERAL INTRODUCTION ………………………………...…. 2
I. Context of the research …………………………….……………... 3
II. Overview on mixed metal oxide systems……………….………. 10
II.1. General information ………………………………..…. 10
II.2. Synthesis methods ………………………………..…… 10
II.3. Applications ………………………………………....... 18
II.4. Review on nickel oxide (NiO) and zinc oxide ( ZnO) ....19
III. Characterization techniques …...…………..……………..……. 26
III.1. ICP-AES analysis ………………………………..…... 26
III.2. FTIR analysis ………………………...………………. 27
III.3. TGA ………………………………………...………... 28
III.4. XRD analysis ……………………...…………………. 29
III.5. SEM and TEM analyses ……………………......……. 31
III.6. XPS analysis ………………………………………..... 33
III.7. ToF-SIMS analysis ……………………………...…… 34
IV. Gas sensors …………………………………………...….…….. 37
IV.1. Introduction …………...………………………..……. 37
IV.2. Semiconductor metal oxide gas sensors ……………... 38
V. Aim and objectives of the work ………………...……..........….. 45
VI. Outline of the thesis …………………………………..……….. 46
VII. References …………………………………………..………… 48
EXPERIMENTAL RESULTS ………………………………...… 64
PART I: SYNTHESIS AND CHARACTERIZATION OF
NICKEL ZINC MALONATE AND ITS DECOMPOSITION
PRODUCTS (NICKEL ZINC MIXED OXIDES) ……………... 66
viii
CHAPTER I
Coprecipitation of nickel zinc malonate: a facile and reproducible
synthesis route for Ni1-xZnxO nanoparticles and Ni1-xZnxO/ZnO
nanocomposites via pyrolysis ………………....…………………. 68
I.1. Abstract ………………………………...…..………….. 68
I.2. Introduction ……………………………………...…….. 69
I.3. Experimental section ………………...………………… 71
I.4. Results and discussions ……………………………..…. 75
I.5. Conclusion …………………….……………………….. 90
I.6. References ……………………………………………... 91
CHAPTER II
Effective reduction in the nanoparticle sizes of NiO obtained via
the pyrolysis of nickel malonate precursor modified using
oleylamine surfactant …………………………………………….. 96
II.1. Abstract ……..………………………………………… 96
II.2. Introduction ……………...…………………………..... 97
II.3. Experimental section …………………...…….……….. 99
II.4. Results and discussions ………………...…...…..…… 101
II.5. Conclusion …………………...……………………… 112
II.6. References …………………………….…………..…. 113
PART II: GAS SENSING APPLICATIONS ………………..… 116
CHAPTER III
Coprecipitation synthesis by malonate route, structural
characterization and gas sensing properties of Zn-doped Ni … 118
III.1. Abstract ………..……………………….…………… 118
III.2. Introduction ………………...………………………. 119
III.3. Experimental section …………………...…...…........ 120
III.4. Results and discussions …………….………...…..… 123
III.5. Conclusion …………………………….……….…… 127
III.6. References …………………………….…...…..…… 129
ix
CHAPTER IV
Ni0.9Zn0.1O/ZnO nanocomposite prepared by malonate
coprecipitation route for gas sensing .………………..………… 130
IV.1. Abstract …………...…………………...………..….. 130
IV.2. Introduction ………………………………………… 131
IV.3. Experimental section ……………………......……… 133
IV.4. Results and discussions …......……………………… 136
IV.5. Conclusion ………………………….………….…… 152
IV.6. References ……………………………...………..…. 153
GENERAL CONCLUSION ………………………….………… 160
I. Main results …………………………………………………….. 161
II. Perspectives …………………………………...……...……..… 163
Annex …………….……………………………………...……….. 166
x
xi
Foreword
This dissertation is divided in three main parts:
1) The first part is an overview, including a general introduction
with literature review.
2) The second part is a detailed presentation of main results,
consisting in four chapters in the form of published article and
submitted manuscripts, as follows:
Chapter I: Coprecipitation of nickel zinc malonate: a facile and
reproducible synthesis route for Ni1-xZnxO nanoparticles and Ni1-
xZnxO/ZnO nanocomposites via pyrolysis. Roussin Lontio Fomekong,
Patrice Kenfack Tsobnang Delphine Magnin, Sophie Hermans,
Arnaud Delcorte, John Lambi Ngolui, Journal of Solid State
Chemistry, 2015, 230, 381-389;
Chapter II: Effective reduction in the nanoparticle sizes of NiO
obtained via the pyrolysis of nickel malonate precursor modified using
oleylamine surfactant. Roussin Lontio Fomekong, Patrice Kenfack
Tsobnang, Hypolyte Mathias Tedjieukeng Kamta, Ebede Samuel,
Arnaud Delcorte, John Lambi Ngolui. (Submitted);
Chapter III: Coprecipitation synthesis by malonate route, structural
characterization and gas sensing properties of Zn-doped NiO. Roussin
Lontio Fomekong, Driss Lahem, Marc Debliquy, Vedi Dupont, John
Lambi Ngolui, Arnaud Delcorte., Material Today: proceeding, 2016,
3, 586-591;
Chapter IV: Ni0.9Zn0.1O/ZnO nanocomposite prepared by malonate
coprecipitation route for gas sensing. Roussin Lontio Fomekong, Driss
Lahem, Marc Debliquy, Sami Yunus, John Lambi Ngolui, Arnaud
Delcorte. (under revision to Sensor & Actuators B: Chemistry);
3) The last part is a general conclusion with perspectives
xii
xiii
ABSTRACT
As the atmospheric pollution has considerably increased in the
recent years, the detection of harmful and flammable gases is a subject
of growing importance in both domestic and industrial environments.
Gas sensors using metal oxide semiconductors have been the subject
of extensive investigations because they have several advantageous,
features such as simplicity in device structure, low cost for fabrication,
robustness in practical applications and adaptability to a wide variety
of reducing or oxidizing gases. One of the main challenges in the
development of metal-oxide gas sensors is the enhancement of
selectivity to a particular gas. For this purpose, the sensor optimal
temperature, the use of doping elements, the formation of composite
materials are investigated. To achieve better selectivity in the case of
composites, the synthesis method has a great influence on the sensor
performance. This thesis reports the synthesis, characterization and
gas sensor investigation of nickel-zinc mixed metal oxide prepared by
thermal decomposition of the corresponding precursors (nickel-zinc
malonate), presynthesized by coprecipitation.
The precursors, nickel zinc malonate blends, with various
Ni/Zn ratio, were synthesized by coprecipitation in an aqueous
solution by finding the optimal condition (pH =7, T=90 °C and t=3H)
and characterized by ICP-AES, FTIR and TG. The obtained results
showed that the precursor is a homogeneous mixture of nickel
malonate and zinc malonate. TG indicates that the decomposition take
place at around 360 °C. Alternatively the nickel malonate precursor
was modified by a surfactant (oleylamine) and the characterization of
this modified precursor showed clearly the presence of the surfactant
in the precursor.
The thermal decomposition products were characterized by
FTIR, XRD, SEM, TEM, XPS and ToF-SIMS. XRD indicates the
xiv
formation of one phase (Ni1-xZnxO), identified as the cubic NiO
structure when the Zn percentage is lower than 20 %, and two phases
(Ni1-xOZnxO/ZnO) when the Zn percentage is equal or greater than 20
%. In the case of Zn doped NiO the variation of the NiO lattice
parameter indicates that the zinc has substituted nickel in its crystal
structure. ToF-SIMS confirms the presence of the Zn and Ni in the
same structure while XPS indicates that they are both under Zn2+
and
Ni2+
form. The morphology of the particles was determined by SEM
and TEM and the results indicate that the pure NiO and ZnO are
spherical while the composite material has undefined morphology. For
all samples, the particle size was less than 80 nm. It was found that the
oleylamine reduces considerably the particle size of NiO (56 % of
reduction).
The sensing performance of all the samples was investigated.
The sensitivity of films of mixed nickel zinc oxide to CO, H2, NO2
was tested for different concentrations and temperatures. For the
doped material, it is found that the conductivity of NiO increases with
increasing the amount of Zn up to 4 %. The optimal sensitivity and the
selectivity to CO of NiO increase with 2 % of Zn doping. For the
composite material, it is found that the Ni1-xZnxO/ZnO (1:1)
composite material is more sensitive than the pure Ni1-xZnxO and ZnO
at 300 °C to CO. A good selectivity to NO2 at 225 °C was also
observed for the composite material. The performance of the
composite material was explained by the formation of a p-n
heterojunction.
xv
List of abbreviations
Abbreviations Descriptions
PCB polychlorinated biphenyl
PBB polybrominated biphenyl
CFC chlorofluorocarbon
COP 21 conference of parties 21
WHO world health organisation
UV ultra-violet
MOCVD metal-organic chemical vapor deposition
RF radio frequency
ALD atomic layer deposition
PVD physical vapor deposition
DC direct current
FSP flame spray pyrolysis
YAG yttrium aluminium garnet
YIG yttrium iron garnet
AACVD aerosol-assisted chemical vapor
deposition
ODH oxidation dehydrogenation
XRD x-ray diffraction
xvi
Abbreviations Descriptions
MO methyl -orange
MB methyl –blue
ICP-AES inductively coupled plasma atomic
emission spectroscopy
FTIR fourier transform infrared spectroscopy
TGA thermogravemetric analysis
SEM scanning electron microscopy
TEM transmission electron microscopy
XPS x-ray photoelectrons spectroscopy
ToF-SIMS Time-of-Flight secondary ions mass
spectrometry
SDTA simultaneous differential thermal
analysis
NiM nickel malonate
NiMO nickel malonate modified by oleylamine
Ek (J) kinetic energy
Eb (J) binding energy
UHV ultra-high vacuum
S sensitivity
xvii
Abbreviations Descriptions
Rair resistance in air
Rgas resistance in gas
ppm part per million
Tcrit critical temperature
E(f) Fermi energy
OAm oleylamine
HAD hexadecylamine
ODA octadecylamine
FWHM: full width at half maximum
1
2
GENERAL
INTRODUCTION
3
I. Context of the research
Two of the greatest problems that the world is facing today are
those of environmental pollution and global warming. As far as
environmental pollution is concerned it is one of the various human
impacts on the biosphere. In other words even though mankind has
impacted upon the biosphere in several ways including damage to the
habit, alteration of the vegetation and soils, over-exploitation of
natural resources and disturbing the equilibrium of important
environmental systems, environmental pollution is one of the most
important factor to be taken into consideration [1]. On the other hand,
global warming which contributes enormously to the disruption of the
equilibrium of the environment is also an important factor deserving
concern and control, and its link with atmosphere pollution is now
well established.
With regards to environmental pollution, there are three
aspects involved: geospheric (soil) pollution, hydrospheric (water or
aquatic) pollution and atmospheric (air) pollution. Soil pollution, in
particular, also called soil contamination, as shown in figure 1a,
results from acid rain, polluted water and chemicals such as fertilizers,
detergents, hydrocarbons, trace elements and heavy metals,
herbicides, pesticides and chlorinated hydrocarbons being released by
spill or underground storage tank leakage into the soil. All of this may
lead for instance to poor crop yields and health issues for the animal
and human populations [2]. Water pollution is the degradation of the
quality of water by the introduction into lake, streams, rivers and
oceans of waste materials of chemical, physical or biological origin
(figure 1b). Waste materials of chemical origin, in addition to those
cited above, include, petroleum wastes; oil spills; chemical
carcinogens like PCB’s and PBB’s; radionuclides like radium,
plutonium and uranium; poorly or non-biodegradable trace organic
4
pollutants like aromatics and organo-halides (CFC’s and plastics) and
gases like oxygen and carbon dioxide. Physical materials can be
present in water in the form of dissolved, suspended and volatiles
solids or simply as sediments. Biological materials will include those
that contribute to the lowering the amount of oxygen like algue and
other microorganisms through eutrophication [1]. All of this has effect
on the fish population and drinkable water resources.
Figure 1: example of soil pollution (a) [3] and water pollution (b) [4].
Atmospheric (air) pollution is by far the most harmful form of
pollution in our environment because the intake of air by human
beings is inevitable and pollution released in the atmosphere cannot be
contained. The atmosphere provides the life-support system on which
human beings depend and that is why there is no life on other planets
which lack an oxygen atmosphere. Air pollution is caused by the
emission of particulates (unburnt hydrocarbons produce by incomplete
combustion of fossil fuels) and gaseous species like oxides of sulphur
(SO2 and SO3), carbon (CO2 and CO) and nitrogen (NO, NO2, N2O5,
etc.); O3; H2S; Cl2; H2 and unburnt hydrocarbons into the atmosphere
(figure 2) [5]. The sources or agents of these atmospheric pollutants
(a) (b)
5
include emissions from car bus, truck and train exhausts, factories,
human and animal respiration, forest fires and volcanoes. Evidence of
increasing air pollution is seen in lung cancer, asthma, allergies, and
various breathing problems along with severe and irreparable damage
to flora and fauna and global warming (resulting from greenhouse
gases: water vapor, carbon dioxide, methane, nitrous oxide and ozone
in the troposphere and the destruction of ozone layer in the
stratosphere by chlorofluorocarbons (CFCs)). Increased agriculture,
deforestation, landfills, industrial production, and mining also
contribute a significant share to gas emissions. The sources of
production of CFC, which contribute to the destruction of the ozone
layer and eventually lead to global warming are refrigerators, air-
conditioners, deodorants and insect repellents [1]. The importance of
the effect of global warming as a consequence of air pollution is
witnessed by the fact that, about 200 nations decided to meet recently
(December 2015) in Paris for the 21st World Conference of Parties
(COP21), to evaluate and find a lasting solution to this global
phenomenon. This work therefore, part of which is in the detection of
some air pollutants like CO, NO2 and H2 (gases) is our modest
contribution to air pollution control by not only detecting but also
evaluating their quantities in air.
6
Figure 2: Example of air pollution [6].
In fact, the World Health Organisation (WHO) has fixed a
limit to the concentration of each gas in the environment [1]. To be
able to follow these recommendations, we need to be able to detect
and quantify these gases in the atmosphere. This is the reason for
which gas sensors have been developed. The different types of gas
sensor are: electrochemical, infrared, ultrasonic, holographic and
semiconductor. Among them, the semiconductor sensors have
attracted considerable attention from the scientific community because
of their numerous advantages some of which include their simplicity
of use, high sensitivity to small concentrations of gases, low
fabrication cost and long term stability [7]. The working principle of
this type of sensors is simply based on the change in the electrical
resistance of the material in the presence of toxic gases. However,
their main drawbacks are the poor selectivity and the high working
temperatures required which, therefore, limit their practical
applications [8]. To overcome this problem, a lot of work is being
done on the semiconductor metal oxide sensing layer of the sensors.
7
For example, the selectivity of this type sensor can be enhanced by the
use of composite or doped materials [9]. The size and morphology of
the materials also have a direct influence on their sensing
performance. This implies that, the enhancement of the performance
of the semiconductor gas sensor is directly related to the employed
material.
Nowadays, material science, which is the study of materials, is
a keys to technological and industrial development [10]. It studies
many categories of materials such as polymers, ceramics and alloys
[11–13] which are widely used in industry. Among these materials
ceramics in general, and metal oxides in particular, attract more
scientific attention because of their diversity and very interesting
properties (electrical, optical and magnetic) as well as their overall
beneficial characteristics of hardness, thermal stability and chemical
resistance [14,15]. To improve on the existing properties of metal
oxides or to generate new ones, the mixed metal oxide systems have
been intensively studied in the last decade [16–19]. In fact, these types
of materials possess superior and unique properties since they
combine the most desirable physicochemical properties of their
constituents while suppressing the least desirable ones.
Some mixed metal oxide systems include: nanocomposite-
based mixed metal oxides[18,20] and solid solutions including doped
metal oxides [21–23]. As far as nanocomposite-based mixed metal
oxides are concerned, their superior properties could result, for
example, from the formation of p-n junctions in the case where the
composite consists of p-type and n-type semiconductors. A p-n
junction improves the photocatalytic property by avoiding the
recombination of electron-hole pairs generated under UV radiation
[24–26]. In gas sensors, p-n junctions can enhance the sensitivity and
selectivity of materials by modifying the electrical states of the surface
8
materials at the p-n junction points [9,16,27]. In the case of doped
metal oxides, in particular, the impurities introduced by the dopant
could result in new material properties. In other words, the presence of
new atoms in the crystal structure of the pure metal oxide
semiconductor can modify the position of the Fermi level and, hence,
change the electrical properties of the resulting materials [28–32].
Other important examples of doping in materials science are in steel
where doping iron with 4 % carbon enhances tremendously the
hardness and strength of the material and in the ruby laser (the first
laser material invented by Théodore Maiman [33]) where doping
Al2O3 with Cr (0.05 %) enhances the optical properties. These
interesting properties of mixed metal oxide systems have galvanized
scientists to develop different synthetic routes for this category of
materials [15,34]. It is well established that the synthetic route has a
tremendous influence on the size, shape and subsequently the
properties of the materials [35]. In fact, some specific properties are
even linked to the nanometer size of the particles and some unusual
optical and electrical properties have been attributed to the
phenomenon known as quantum confinement [36]. The large
surface/volume ratio is also important for some applications such as
catalysis [37]. Various techniques are being employed in the synthesis
of mixed metal oxides both in thin film and bulk forms. For thin films
there is: Metal-Organic Chemical Vapor Deposition (MOCVD) [38],
Radio Frequency (RF) sputtering [39], Molecular beam [40], aerosol
spray [41], pulsed laser deposition [42] and Atomic Layer Deposition
(ALD) [43] while for bulk materials there is high temperature solid
state synthesis [44,45], electro-spinning [46], sol-gel [47],
hydrothermal [48], solvothermal [49], plasma-assisted [50],
combustion [51] and coprecipitation [52].
Among all the mixed metal oxide systems studied, that
constituted by NiO and ZnO is interesting because it combines the
9
individual high potentials of the n-type ZnO and the p-type NiO. In
fact, ZnO is a transparent n-type semiconductor with a band gap of 3.3
eV and is widely used in piezoelectric transducers, varistors,
phosphors, sensors, solar cells, and transparent conducting films as a
result of its good electrical, catalytic, optical and gas sensor properties
as well as its a chemically stability and non-toxicity [53]. NiO is a p-
type semiconductor with a wide band gap between 3.6 and 4.0 eV. It
possesses many properties such as magnetic, electronic, catalytic and
other properties and as a result it is used as a solar thermal absorber,
catalyst, anode in fuel cells and gas sensor [54]. Many techniques
have been used to synthesize mixed metal oxides containing NiO and
ZnO, some of which include: sol-gel [55], high temperature solid state
method [56], ion beam sputtering [57], electrospinning [58], pulsed
laser deposition [59]. In general, all these techniques need high
vacuum, high temperature (≥ 900 °C) technology and very expensive
apparatii. Coprecipitation is however quite simple and does not
require high working temperatures. In the coprecipitation method, the
precipitating agent or ligand has been known to have an influence on
the morphology and size of the particles [34]. These parameters would
have a direct influence on the properties of the resultant materials. For
example, Abdul Hameed et al. have shown in their work on NiO/ZnO
composite that , by using oxalate, hydroxide and carbonate as different
precipitating agents, the size of the particles changes as well as the
catalytic property [25] while Qingshui Xie et al. have synthesized the
same system using citrate resulting in a totally different morphology
[60].
This thesis investigates the synthesis of the nickel zinc mixed
metal oxide system by pyrolysis of the corresponding malonate
precursor obtained by coprecipitation in aqueous medium and also
explores the gas sensor properties of material obtained. We also intend
to study the optimum conditions (time, temperature, composition etc.)
10
for obtaining the appropriate material for the most suitable gas sensor
applications.
II. Overview on mixed metal oxide systems
II.1. General information
Mixed metal oxide systems can be classified into three main
groups. The first group includes chemical compounds resulting from
the chemical interaction between various oxides. Examples include
the ferrites (MFe2O4, with M= divalent cations) resulting from
interaction between Fe2O3 and MO [61], the cobaltites (MCo2O4, M=
divalent cations) obtained by interaction between Co2O3 and MO [62],
stannates (ZnSnO3 and Zn2SnO4) compounds formed in the ZnO-SnO2
system [63], cuprates (MCu2O4, M = divalent cations) [44] and the
crystal CdIn2O4 which is an effective sensor for CO, produced by the
interaction of CdO with In2O3 [64]. The phase α-SnWO4, sensitive to
small amounts of CO and NO gas, was observed in the system SnO2-
WO3 [65]. The second group includes mixed oxides, which form
partial and total solid solutions. An example of such a system is the
mixture of TiO2 and SnO2, forming total solid solutions over the entire
range of compositions even though only above a certain critical
temperature [66]. NiO and ZnO form partial (with 30 % of ZnO) solid
solution at higher temperature (at least 900 °C) [67]. The third group
consists of systems that give neither individual compounds nor solid
solutions. But rather mixtures of metal oxide nanocrystals interacting
with each other. Examples include, CuO-ZnO [50] and Fe2O3-ZnO
[68]. From these three groups, only the third is considered as mixed
metal oxides composite materials.
II.2. Synthesis methods
Various techniques have been used for the synthesis of mixed
metal oxide systems in thin film and bulk form such as sol-gel, aerosol
11
spraying, reactive metal sputtering, evaporation technique, spin
coating, solvo/hydrothermal technique, combustion method,
microwave, flame pyrolysis and coprecipitation.
II.2.1. Solvo/hydrothermal technique
Hydrothermal synthesis can be defined as the technique of
crystallizing substances from high-temperature aqueous solutions at
high vapor pressures and depends on the solubility of minerals under
these conditions. The interfacial tension is usually greatly reduced and
mass transfer between the different phases is enhanced. When the
solvent is water, it is commonly referred to as “hydrothermal”. It is
performed in an apparatus consisting of a steel pressure vessel called
an autoclave (figure 3), in which a nutrient is supplied along with
water [34]. The synthesis conditions (temperature, pressure, time, ratio
of reactants …) have a great influence on the product material [69].
The advantages of the hydrothermal method over other types of
crystal growth technique include the ability to create crystalline
phases which are not stable at the melting point, the growth of
materials which have a high vapor pressure near their melting point
and the growth of large good-quality crystals while ensuring good
control over their composition. The disadvantages of the method
include the need of expensive autoclaves and the difficulty in
observing the crystal as it grows. Many important mixed metal oxides
for different applications have been synthesized using this method and
some of these oxides include CuO-SnO2 and CuO-SnO2-Fe2O3 [70],
Co3−xFexO4, ZnxCo3−xO4 [71], SnO2/Fe2O3 [72], ZnO-MgO, ZnO-NiO
[49] and α-Fe2O3–ZnO–Au [73].
12
Figure 3: Schematic of a stainless autoclave for solvothermal
synthesis [74].
II.2.2. Sol-Gel technique
The sol-gel process is a method for producing solid materials
which involves the conversion of monomers into a colloidal solution
(sol) that acts as the precursor for an integrated network (or gel) of
either discrete particles or network polymers as illustrated in figure 4.
The method is based on the hydrolysis of the precursor (generally
metalorganic compound) followed by condensation. The advantages
of this method include, the ease shaping materials into complex
geometries in a gel state, the synthesis of high purity products, good
composition control and relatively low working temperatures (200-
600°C). In spite of its numerous advantages, the sol-gel technique has
some limitations, such as, the process takes long, the difficulty of
controlling some parameters of the material [34], etc. Some of the
mixed metal oxides prepared by the sol-gel technique include Fe2O3-
ZnO [68], TiO2-MgO [75], CoAl2O4, CoCr2O4, ZnFe2O4 and CuCr2O4
[76], Cu doped cobalt oxide [77] and ZnAl2O4/SiO2 [78].
13
Figure 4: Illustration of Sol-Gel technique [79].
II.2.3. Sputter deposition
Sputter deposition or sputtering is a physical vapor deposition
(PVD) technique often used for the synthesis of thin films. It involves
ejecting a material (usually the sample material) from a "target" (or
source) onto a "substrate" such as a silicon wafer. In other words,
atoms at the surface of the target material (sample) are bombarded by
high energy particles or ions resulting in their removal and subsequent
physical transfer, in vapor form, after acceleration by an electric field,
and condensation at the surface of substrate as illustrated in figure 5.
When the desired material is a mixed metal oxide system, a
multisource from two “targets” is employed. The main types of
sputtering techniques used are DC, RF, magnetron, ion-assisted and
reactive sputtering. The sputter deposition method has a lot of
advantages which include film thickness control, good film uniformity
14
and the most important advantage is that, even materials with very
high melting points are easily sputtered whereas evaporation of these
materials in a resistance evaporator or Knudsen cell is usually either
problematic or impossible. It also presents some disadvantages like
high equipment cost, low rate of deposition for some materials (SiO2
for example) and the difficulty to combine it with a lift-off for
structuring the film [80]. This technique has been used for the
synthesis of various mixed metal oxides and some examples include:
SnWO4 for gas sensing [65], ZrO2–Al2O3, HfO2–Al2O3, ZrO2–Y2O3,
and ZrO2–TiO2 [81], hafnium–aluminum–oxide and zirconium–
aluminum–oxide [82].
Figure 5: Schema of working principle of Sputter deposition [83].
15
II.2.4. Flame spray pyrolysis
Spray pyrolysis is a powder material synthesis method in
which the solution is atomized in a hot wall reactor where the aerosol
droplets undergo evaporation, drying, thermolysis of the precipitate
particles concentrated within them at higher temperature to form a
microporous particle and, eventually, sintering. When the heat source
is the flame the method is called flame spray pyrolysis (FSP). The
working principle is shown in figure 6. The advantages of FSP include
the ability to dissolve the precursor directly in the fuel, simplicity of
introduction of the precursor into the hot reaction zone (e.g. a flame),
and flexibility in using the high-velocity spray jet for rapid quenching
of aerosol formation. The main disadvantages include, low production
rates and hazardous gaseous reactants and byproducts. Another major
disadvantage is the formation of hard agglomerates in the gas phase
leading to difficulties in producing high quality bulk materials [84].
This method has been successfully applied for the synthesis of mixed
metal oxides such as β-SrMnO3 and NiMn2O4 [85], LiCoO2 [86],
BaTiO3 [87] and MgAl2O4 [88]
Figure 6: Schema of working principle of flame spray pyrolysis [89].
16
II.2.5. Mechanosynthesis
Mechanosynthesis is a term for hypothetical chemical
synthesis in which the reaction outcomes are determined by the use of
mechanical constraints to direct reactive molecules to specific
molecular sites. The mechanical energy (e.g. in ball milling) is used
here to activate chemical reactions and structural changes in the
powder mixture. Although no heat is applied, high temperatures may
be generated locally as the mechanical energy is transformed into heat
energy and this acts to speed up interdiffusion rates and the formation
of a product phase on a very fine scale. Even though this method is
time consuming (at least 7 hours) and there can be contamination from
the milling media, its main advantage is the possibility of reducing
side reactions leading to higher yields and/or better conversions [90].
BiFeO3 [91], Y3Fe5O12 [92], LaMnO3 [93] and Iron–cobalt spinel
oxide [94], have been successfully synthesized by this method.
II.2.6. Coprecipitation synthesis
It is a wet chemical method used for the synthesis of nano and
ultra-dispersed inorganic powders from solution. It involves
precipitating simultaneously two or more metals salts in solution by
using a precipitating agent (or ligand) and subsequently thermally
decomposing the precipitate. This method emerged in contrast to
conventional solid-state synthesis methods of compounds and
materials widely used also in ceramics manufacturing. The main
differences between coprecipitation products and those obtained by
solid-phase synthesis are much smaller grains (crystallites) and,
usually, lower temperatures (<600°C) and shorter duration of phase
formation. The precipitating agent, the decomposition temperature and
the time are the main parameters which can influence the morphology,
the size and, consequently, the properties of the target materials. It is a
simple, low cost, controllable, versatile method used to synthesize
17
mixed metal oxide systems at relatively low temperatures (< 600°C)
and at atmospheric pressure. One of the most interesting advantages of
this method is the possibility to control the stoichiometry of the
desired materials. However one of the major difficulties in using this
method is finding a good common solvent in which the desired metal
ions can easily coprecipitate while avoiding agglomeration during the
thermal decomposition process [95]. Various mixed metal oxides have
been synthesized via this method and some include: CuO-ZnO [96],
mixed calcium zinc metal oxide [97], Cu-doped ZnO [98], Fe2O3-
Al2O3 [52] and Ni1-xZnxFe2O4 [99].
II.2.6.1. Influence of the precipitating agent
In coprecipitation synthesis, a precipitating agent (or ligand) is
used to coprecipitate the target metal ions by forming a metallo-
organic compound which is decomposed afterward to give the mixed
metal oxide. The precipitating agent has a great influence on the
decomposition temperature, the morphology and the size of particles
of the coprecipitated material. The ligand can also play an important
role in determining the limit of solid solution formation between the
two metal oxide compounds [100]. It is also reported in the literature
by M. Zeng et al. that the temperature to synthesize yttrium aluminum
garnet (YAG) and yttrium iron garnet (YIG) by coprecipitation
depends directly on the ligand used [101]. This assertion is aptly
demonstrated in this article in which using ammonium hydrogen
carbonate as precipitating agent, pure YIG and YAG are obtained in
2h whereas using ammonium carbonate under the same conditions,
YIG is obtained alongside Fe2O3 and Y2O3 as impurities while YAG
is obtained with Y2O3 as impurity. Another example is the work
published by Abdul Hameed et al. [25] in which they obtained
NiO/ZnO by coprecipitation with different particle sizes by varying
the precipitating agent (oxalate, hydroxide, and carbonate) while, at
18
the same time affecting the photocatalytic properties of the material.
To buttress this point they obtained particle sizes of 22, 25 and 51 nm
by using, respectively, oxalate, carbonate and hydroxide as
precipitating agents after 6h of thermal decomposition of precursors at
500 °C in air.
II.2.6.2. Influence of the temperature
The decomposition temperature of the precursor is also a very
important parameter in the coprecipitation method especially on the
nature of the final product. For example, Stefano Diodati et al.
obtained one pure phase of MnFeO3 after thermal decomposition of
Mn-Fe oxalate at 700°C [102]. In the same logic, thermal
decomposition of Fe-Cu malonate gives a mixture of Fe2O3 and CuO
at 275°C while one pure phase of CuFe2O4 is obtained at 350°C
[103]. This temperature is very low relative to that at which this same
single phase compound is obtained via a solid state reaction (~
800°C).
II.3. Applications
Mixed metal oxide are widely used in industry because of their
electrical, magnetic, optic, mechanic and opto-electric properties [12]
which are both interesting and varied. As for its magnetic properties,
mixed metal oxides are used in data storage (audio and video tapes,
hard and floppy discs, etc.), as permanent magnets (loudspeakers,
small generators, small motors and sensors) and for power
management (transformers). With regards to its optical properties, it is
used in photocatalysis (photodegradation of pollutants) and as
photonic crystals. Electrical properties include their uses as electrical
insulating materials for materials with dielectric properties (zirconium
barium titanate, barium zirconate, lead magnesium niobate, etc.) and
as gas sensors (ZnO-SnO2, Fe2O3-NiO, ZnO-CuO …) for those with
19
semiconductor properties [104,105]. They are also used in solid oxide
fuel cells as electrolytes (Yttria stabilized zirconia, Ce0.9Gd0.1O2−δ,
La0.85Sr0.15Ga0.9Mg0.1O3−δ ) [106,107] which offers a good oxide ion
transport, while blocking electronic transport and as anode (NiO/ZnO,
Cu–Zirconia, Cu–Ceria, nickel–zirconia cermet) [108]. Mixed metal
oxides also find wide uses in catalysis: the catalytic combustion of
methane (LaBO3 with B = transition element, SrFeO3−δ , Mn, Co, Cr,
and Fe doped ZrO2) and the oxidative dehydrogenation of alkanes
(Cr0.28Al0.68Nb0.04Ox, Ni0.62Ta0.10Nb0.28Ox, Mg2V2O7, etc.) [17].
II.4. Review on nickel oxide (NiO) and zinc oxide (ZnO)
II.4.1. Nickel (II) oxide (NiO)
Nickel (II) oxide is a chemical compound with the formula
NiO, which adopts the [NaCl] structure, with octahedral Ni(II) and
O2−
sites (figure 7). This conceptually simple structure is commonly
known as the rocksalt structure. Like many other binary metal oxides,
NiO is often non-stoichiometric, meaning that the Ni:O ratio deviates
from 1:1. In nickel oxide this non-stoichiometry is accompanied by a
color change, with the stoichiometrically correct NiO being green and
the non-stoichiometric NiO being black. The density, refractive index,
and melting point of NiO are respectively 6.72 g/cm3, 2.1818 and
1955 °C. It is soluble in ammonia, hydroxide and KCN. NiO which is
a chemically and thermally stable compound is a p-type semi-
conductor with a band gap between 3.6-4 eV [109].
20
Figure 7: Nickel (II) oxide crystal structure [110].
NiO has been intensively synthesized in the bulk (powder) and thin
film forms using various techniques bearing in mind that the synthesis
route and conditions adopted could influence the size, morphology
and, hence the properties of the materials. Several authors have
reported the synthesis of different morphologies of NiO powders
using various techniques:
- Spherical rods: By varying the synthesis conditions, spherical
NiO with different sizes have been synthesized by the
solvothermal (2-10 nm) [111], micro-emulsion (47-86 nm)
[112] and Sol-Gel (~ 3 nm) [113] techniques.
- Nanowires: Nanowires of NiO have been prepared by many
techniques such as hydrothermal [114], precipitation [115] and
Sol-Gel [116].
- Nanoplates: Microwave [117] and hydrothermal [118]
techniques have been used to synthesize NiO nanoplates.
NiO thin films have also been deposited via a wide range of methods
such as reactive sputtering [119], spin coating [120], aérosol-assisted
chemical vapor deposition (AACVD) [121] and metal organic
chemical vapor deposition (MOCVD) [122].
NiO finds many applications in different domains owing to its many
interesting properties. It is used as a catalyst for the oxidation
21
dehydrogenation (ODH) of ethane [123] and for CO oxidation at low
temperatures [124]. It is also used in electrochromic devices [125].
NiO, which is able to absorb visible sunlight and not infrared sunlight,
is used as a solar thermal absorber in industry [126]. Owing to its p-
type semi-conductivity, NiO is widely used as sensor for the detection
of such gases as hydrogen (H2), carbon monoxide (CO) and
formaldehyde (HCHO) [127–129].
II.4.2. Zinc oxide (ZnO)
Zinc oxide is an inorganic compound with formula ZnO and
crystalizes into two main structures: wurtzite (hexagonal) and zinc
blende (cubic). The hexagonal structure (figure 8), which is more
stable at ambient conditions, has as point group 6mm (Hermann-
Mauguin notation), space group P63mc and lattice constants a = 3.25
Å and c = 5.2 Å. The density, refractive index, and melting point of
this white powder are, respectively, 5.606 g/cm3, 2.008, and 1975 °C
[109]. Crystalline zinc oxide is thermochromic, changing from white
to yellow when heated in air and reverting to white on cooling. This
color change is due to a small loss of oxygen to the environment at
high temperatures to form the non-stoichiometric Zn1+xO [53].
Figure 8: Zinc oxide crystal structure [130].
22
ZnO with different morphologies and size particles synthesized using
various techniques, has been reported in the literature [115-124].
- Spherical rods: ZnO with spherical morphology and different
particles size have been prepared by various techniques such
as precipitation (10-85 nm) [131], hydrothermal (2-3 μm)
[132] and sonochemical (0.53-1.1 μm) [133].
- Nanowires: ZnO nanowires have been successfully
synthesized by hydrothermal [134], solution route and
electrochemical method [135].
- Nanoplates: hydrothermal [136], solution route [137] and
microwave [138] techniques have been used to prepare ZnO
nanoplates.
- Nanocubes: ZnO nanocubes were synthesized by ultrasonic
assisted hydrolysis [139] and hydrothermal processes [140].
Some of the numerous applications of zinc oxide are summarized
here. Zinc oxide has a high refractive index, high thermal
conductivity, binding, antibacterial and UV-protection properties;
therefore, it is added into materials and products including plastics,
ceramics, glass, cements, rubbers, lubricants, paints, ointments,
adhesives, sealants, concrete manufacturing, pigments, foods,
batteries, ferrites or fire retardants. ZnO has a wide direct band gap
(3.37 eV or 375 nm at room temperature) and for that its most
common potential applications are in laser and light emitting diodes
[141]. ZnO exhibit piezoelectric property and therefore it is used for
“self-powered nanosystems” [142]. When ZnO is doped with 1-10 %
of a magnetic element (Mn, Fe, Co…), it also finds application in
spintronic [143]. ZnO is widely used in photocatalysis for the
degradation of dye like rhodamine B and methylene orange [144,145].
It is also used in solar cell [146] and in gas sensor [147] where the
sensor performance depends on the morphology of the particles.
23
II.4.3. Nickel zinc mixed metal oxide systems
Mixed metal oxide system is of particular interest because it
combines the more useful properties of the constituent oxides.
Furthermore, interactions between the two components can generate
new properties. Nickel zinc mixed metal oxide is a typical example of
this category of systems whose study can be grouped into two
headings: the solid solution and composite material.
II.4.3.1. Solid solution of ZnO and NiO
A solid solution is a homogeneous mixture of two or more
substances in which both the solute (s) and the solvent are in the solid
state (or phase). In such a case, the crystal structure of the solvent
remains unchanged by addition of the solute (s) since the latter simply
incorporates itself into the solvent crystal lattice. Two types of solid
solutions exist: “substitutional solid solution” whereby a solute atom
replaces a solvent atom in the lattice, and “interstitial solid solution”,
whereby the solute atom fits itself into the spaces between the solvent
atoms in the lattice. Both of these types of solid solutions affect the
properties of the material by distorting the crystal lattice and
disrupting the physical and electrical homogeneity of the solvent
material. Some mixtures could result in complete or partial solid
solutions over a range of concentrations, while others will not form
solid solutions at all. The propensity for any two substances to form a
solid solution is a complicated matter involving the chemical,
crystallographic, and quantum properties of the substances in
question. While in interstitial solid solutions the size of the
incorporating atom should be small (0.4-0.8 Å), substitutional solid
solutions may be formed between a solute and solvent if the following
Hume-Rothery rules are obeyed, that is if the solute and solvent have:
24
- Similar atomic radii ( differences between the radii should be
15% or less);
- Same crystal structure;
- Similar electronegativities (or electropositivities);
- Similar valency (similar chemistry).
In addition to these conditions, specific experimental synthesis
conditions are usually necessary (for instance, high temperature and
pressure and reaction time) to obtain the solid solution [148]. A search
in the literature [22], it is possible to form substitutional solids
solution between NiO and ZnO under specific experimental
conditions. Literature results show that the solid solution between
ZnO and NiO can be formed at high temperatures (between 800 °C
and 1200 °C) and long reaction times, achieving a limit of 30 % ZnO
and 70 % NiO (that is, 30 % solubility of ZnO in NiO) by using high
temperature solid state synthesis [149]. However, some other authors
have reported the formation of a solid solution between NiO and ZnO
at 450 °C for 8 h attaining a 5 % of solubility limit, by using
electrochemical synthesis [150]. From these results, we can say that
the method employed plays a key role in the formation of a solid
solution. It is worth emphasizing here that it is a solid solution of ZnO
in NiO and not vice versa. In other words, that Zn atoms substitute
those of Ni in its lattice and not the reverse. This assertion is
confirmed by the fact that phase identification of this solid solution by
XRD analysis shows only the NiO phase. The insertion of zinc in the
nickel oxide structure induces an increase in the lattice parameter of
nickel oxide since the radius of Zn is slightly larger than that of Ni. It
is worth noting that the formation of solid solution follows very
closely Vegard’s law [67]. Literature studies on the effect of Zn
insertion in the NiO structure on the gas sensor properties of NiO are
virtually non-existent. It has been found that the presence of 5 % of Zn
into the NiO structure, reduces considerably the response time and
25
enhances the selectivity of the NiO sensor to ammonia (NH3) [150]
and also its electrochromic performance [151].
II.4.3.2. Composite material: ZnO/NiO
A composite material can be defined generally as one in which
one of the constituents (the dispersed or discontinuous phase) is
dispersed in a continuous phase (the matrix). A mixed metal oxide
composite system is, therefore, a simple mixture between nanocrystals
interacting with each other by simple physical contact in such a
manner that one is dispersed in the other. NiO and ZnO which are,
respectively, p-type and n-type semiconductors can form composite
materials with a p-n junction accounts for the new properties of the
resulting material. In fact by creating intimate electrical contact at the
interface between two dissimilar semiconducting materials (NiO and
ZnO for example), the Fermi levels across the interface can equilibrate
to the same energy, usually resulting in charge transfer and the
formation of a charge depletion layer. This is the basis for unique
effects that can lead to increased sensor performance. The contact
between a p-type semiconductor and its n-type counterpart will induce
an internal electric field that can extend the probability of electron–
hole separation, thus, enhancing the photocatalytic activity of the
composite. In the case of the NiO/ZnO composite, in particular, which
is also the object of our study, the literature reports several works
carried out on it. On the photocatalytic properties of this system, in
particular, it has been shown that the p-n junction really enhances the
photocatalytic degradation of some dyes like methyl orange (MO),
methylene blue (MB) and rhodamine B [58,152]. For gas sensor
applications, this composite has been used for the detection of many
gases such as acetone and H2S, and the effect of the p-n junction has
been well established to be responsible of the enhancement of gas
detection [152,153]. NiO/ZnO has also been used as supercapacitor
26
electrode material [154] and as anode material for lithium ion batteries
[155]. The solid state reaction method [108], hydrothermal [155] and
coelectrospinning [153] techniques have been used for the synthesis of
the NiO/ZnO composite material.
III. Characterization techniques
For the characterization of precursor and decomposition
products in order to furnish in-depth knowledge with respect to their
composition, structure, morphology and various properties a number
of methods and techniques have been developed. These include,
inductively coupled plasma atomic emission spectroscopy (ICP-AES),
Fourier transform infrared spectroscopy (FTIR), thermogravimetric
analysis (TGA), X-ray diffraction (XRD), scanning electron
microscopy (SEM), transmission electron microscopy (TEM), X-ray
photoelectrons spectroscopy (XPS), time-of-flight secondary ions
mass spectrometry (ToF-SIMS). These highlighted techniques are just
those that were used in the present work. A brief discussion on each of
them with regards to their use and information obtainable from is
given below.
III.1. Inductively coupled plasma atomic emission
spectroscopy (ICP-AES)
Inductively coupled plasma atomic emission spectroscopy,
also referred to as inductively coupled plasma optical emission
spectrometry (ICP-OES), is a solution analytical technique used for
the quantitative analysis of metals (even in trace amounts) in
materials. It makes use of inductively coupled plasma to produce
excited atoms and ions that emit electromagnetic radiation at
wavelengths characteristic of the constitutive elements. The intensity
of this emission is indicative of the concentration of the element
within the sample. In actual fact, when the sample solution is
27
introduced into the spectrometer, it becomes atomized into a mist-like
cloud. This mist is carried into the argon plasma by a stream of argon
gas. The plasma (ionized argon) produces temperatures close to 7000
°C, which thermally excite the outer-shell electrons of the elements in
the sample. The relaxation of the excited electrons as they return to
the ground state is accompanied by the emission of photons with an
energy characteristic of the element. Because the sample contains a
mixture of elements, a spectrum of light wavelengths are emitted
simultaneously and the spectrometer uses a grating to disperse the
light, separating the particular element emissions and directing each of
them to a dedicated photomultiplier tube detector. The more intense
this light is, the more concentrated the element. A computer converts
the electronic signal from the photomultiplier tube into concentrations.
The sample preparation is the critical step of this analysis because the
sample has to be totally in solution. To achieve this, many methods
are used including acid digestion, pressure digestion and microwave
digestion. Among these methods, acid digestion is most widely used
and consists of completely transfering the analytes into solution by
using mineral acids (HCl, HNO3, HF, H2SO4, etc.). The advantages of
ICP-AES are: excellent limit of detection and linear dynamic range,
multi-element capability, low chemical interference and a stable and
reproducible signal. The disadvantages are mainly spectral
interferences (many emission lines), cost and operating expense, and
the fact that the samples must typically be in solution [156].
III.2. Fourrier Transform Infrared Spectroscopy (FTIR)
Fourier transform infrared spectroscopy is a technique used to
obtain an infrared spectrum of the absorption of a solid, liquid or gas
sample. The term Fourier transform infrared spectroscopy originates
from the fact that a Fourier transform (a mathematical process) is
required to convert the raw data into the actual spectrum. In infrared
28
(IR) spectroscopy, infrared radiation passes through a sample and
while some of it is absorbed by the sample, the rest is transmitted. The
resulting spectrum represents the molecular absorption and
transmission, creating a molecular fingerprint of the sample. This
makes infrared spectroscopy useful for several types of analysis. FTIR
can provide much information on the material including, the
identification of unknown materials, the determination of the quality
or consistency of a sample and the determination of the amount of
each component in the mixture. An infrared spectrum represents a
fingerprint of the sample with absorption peaks corresponding to the
frequencies of vibrations between the bonds of the atoms making up
the material. Therefore, infrared spectroscopy can result in a positive
identification (qualitative analysis) of various organic and inorganic
materials. In addition, the size of the peaks in the spectrum can be an
indication of the quantity of a specific compound in the sample. With
modern software algorithms, infrared is a good tool for quantitative
analysis. The advantages of FTIR include, the high speed of
measurement (few seconds), the high sensitivity (high signal-to-noise
ratio for a given scan-time), the mechanical simplicity and the
instrument calibration which is automatic. The disadvantages are: the
fact that FTIR cannot use advanced electronic filtering techniques
[156], the instrument has a single beam whereas dispersive
instruments usually have a double beam, making the highly sensitive
work and experiment which take a long time difficult and less
accurate by FTIR.
III.3. Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) is a method of thermal
analysis in which changes in physical and chemical properties of
materials are measured as a function of increasing temperature (at a
constant heating rate) or as a function of time (at constant temperature
29
and/or constant mass loss). The TGA can provide information about
physical phenomena such as second-order phase transitions, including
vaporization, sublimation, absorption, adsorption, desorption and
about chemical phenomena including chemisorption, desolvation
(especially dehydration), decomposition and solid-gas reactions (e.g.
oxidation or reduction). TGA is commonly used to determine selected
characteristics of materials that exhibit either mass loss or gain due to
decomposition, or loss of volatiles (such as moisture). Common
applications of TGA include materials characterization through
analysis of characteristic decomposition patterns, studies of
degradation mechanisms and reaction kinetics, determination of
organic content in a sample and determination of inorganic (e.g. ash)
content in a sample, which may be useful for corroborating predicted
material structures or for chemical analysis. A TGA instrument
continuously weighs a sample as it is heated to high temperatures. As
the temperature increases, various components of the sample are
decomposed and the weight percentage of each resulting mass change
measured. Results are plotted with temperature on the X-axis and
mass loss on the Y-axis. The TGA is generally coupled with
Simultaneous Differential Thermal Analysis (SDTA) which indicates
if the phenomena that take place are the endo or exothermic [157].
III.4. X-Ray Diffraction analysis (XRD)
X-ray diffraction is a material analysis method used for
identifying the atomic and molecular structure of a crystal based on
the fact that the lattice atoms cause a beam of incident X-rays to
diffract into many specific directions. By measuring the angles and
intensities of these diffracted beams, it is possible to produce a three-
dimensional picture of the density of electrons within the crystal, from
which the mean positions of the atoms in the crystal can be
determined, as well as their chemical bonds, their disorder and various
30
other informations. Since many materials can form crystals such as
salts, metals, minerals, semiconductors, as well as various inorganic,
organic and biological molecules, X-ray crystallography has been
fundamental in the development of many scientific fields. For ceramic
materials, two techniques are often employed: powder X-ray
diffraction (PXRD) and single-crystal X-ray diffraction depending on
whether the material is a powder or a monocrystal. The information
obtainable from the powder diffraction patterns includes phase
identification, crystallite size, unit cell symmetry, stress, strain and
growth orientation. The working phenomenon behind the generation
of diffraction patterns can be described as follows: When an X-ray
beam of wave length λ strikes the solid crystal at an angle Ɵ, the
resulting scattered radiation can be determined by virtue of Bragg’s
law: nλ = 2dsinƟ, where n is called the diffraction order and d denotes
the distance between the diffracting planes (figure 9). It is noteworthy
that the set of d-planes is unique for each and every material and
thereby provide information on the crystal structure. It is important to
point out that, by manipulating different parameters like the geometry
of incident rays and the orientation of the detector and crystal, one can
obtain all the possible diffraction directions of the lattice [156].
31
Figure 9: Illustration of X-ray diffraction analysis [158].
III.5. Scanning and transmission electron microscopy
(SEM and TEM) analyses
Scanning electron microscopy is one of the most important
techniques for the study of the overall appearance of the material. It
helps us to analyze different parameters like, shape, density, diameter,
thickness, length and orientation of the material. SEM belongs to the
family of microscopies, but it uses a beam of electrons in place of
light in order to make an image. The beam of electrons passes through
the electromagnetic lenses and strikes the surface of the sample. The
detector (a scintillator) collects the secondary/backscattered electrons
ejected from the sample and passes them onto a photomultiplier which
converted them into a signal and directs them towards a displaying
screen. In SEM, the sample is scanned with a focused beam of
electrons. The electron acceleration voltages are normally in the range
of 5-20 kV for the SEM. Depending on the sample and experimental
condition, the bombardment of electrons does not cause any damage
to the samples. SEM has the ability to capture the images from the
32
visible range to a few nanometers, while the magnification range is
around 20X- 30000X along with a spatial resolution of 50-100 nm
[157].
Transmission electron microscopy, like SEM, belongs to the
family of electron microscopies. In order to get structural information
at atomic level TEM is one of the most commonly used techniques.
The working principle of TEM is just like for the optical microscopy
except that it makes use of a relatively high energy electron source of
around 200 keV. It is a microscopy technique in which the beam of
electrons is transmitted through an ultra-thin specimen, interacting
with it as passes through. TEM is a very valuable tool for the
material’s analysis, since it can furnish a variety of information
including chemical composition, crystal/surface structure information
and image resolutions of up to a few angstroms just by switching the
operational mode. However, some of its disadvantages, which must
taken into consideration [157], include the fact that it,
- is a destructive technique;
- is time consuming (with respect to sample preparation);
- only provides local information;
- is relatively difficult to operate.
The comparison between the SEM and TEM analysis is illustrated in
table 1 and figure 10.
Table 1: Comparison between SEM and TEM.
SEM TEM
Based on backscattered electrons Based on transmitted electrons
Information: morphology and
composition (via X-Ray
Fluorescence)
Information: morphology,
composition, crystallization,
stress
Resolution: less than 1 nm Resolution: around 10 nm
Image: three- dimensional Image: two-dimensional
33
Figure 10: Illustration of SEM and TEM techniques [159].
III.6. X-ray Photoelectrons Spectroscopy (XPS)
X-ray photoelectron spectroscopy (also known as Electron
Spectroscopy for Chemical Analysis-ESCA) is a surface-sensitive
quantitative spectroscopic technique that determines the elemental
composition at the part per thousand ranges, empirical formulae and
the chemical and electronic state of the elements that exist within a
material. It is based on the photoemission process. In other words, a
beam of X-rays irradiates a surface and transfers its photon energy to
the surface electrons, some of which after receiving sufficient energy,
escape from that surface with a specific kinetic energy (called as
photoelectron) and can, thus, be detected (figure 11). The relation
between kinetic energy, Ek, and x-ray photon energy, hν, can be
expressed as Ek = hν - Eb, where, Eb is the binding energy. The XPS
analyzer determines the number and the intensity of the energy of
emitted photoelectrons. Since each element has a characteristic
binding energy, XPS, therefore, provides quantitative information on
34
the elemental composition of the outermost 100 Å of sample surface.
The internal or inner photoelectrons lose energy by collision and so do
not have sufficient energy to be emitted by the atom. Electron binding
energy is strongly influenced by the chemical group in which the
considered element is involved, and so XPS can be used to determine
the oxidation state of the element. The main advantages of XPS
include its ability to provide vital chemical information with
sensitivity of 0.1 at %, simple quantitative information (at the first 10
nm), low sample damage and the fact that it can be used to study
insulating materials. The main disadvantages of XPS are its relatively
poor lateral resolution and the need for ultra-high vacuum (UHV)
[157].
Figure 11: Basic principle of XPS [160].
III.7. Time-of-Flight secondary ion mass spectrometry
(ToF-SIMS)
Time-of-flight secondary ion mass spectrometry is a surface-
sensitive analytical method that uses a pulsed ion beam (Cs, Ar, or
microfocused Ga) to remove molecules from the very outermost
35
surface of the sample (~ 1 nm). In fact, the incident ion penetrates the
surface and transfers its energy to the sample atoms via a collision
cascade process. The particles which are removed from atomic
monolayers on the surface (positive or negative secondary ions) are
then accelerated into a "flight tube" and their mass-to-charge ratio is
determined by measuring the exact time at which they reach the
detector (i.e. time-of-flight). The illustration is shown in figure 12.
This technique is highly surface sensitive because the molecular
secondary ions only come from the first monolayers. Depending on
the current of the beam of primary ions, one can distinguish static
SIMS (with low primary ion current) and dynamic SIMS (with higher
primary ion current). Three operational modes are available with ToF-
SIMS: surface spectrometry, surface imaging and depth profiling.
ToF-SIMS is widely used in material science disciplines for studies of
materials such as polymers, pharmaceuticals and semiconductors. The
three main modes of data acquisition include: elemental or molecular
survey, elemental or molecular mapping and depth profiles [161]. The
strengths of ToF-SIMS include:
- Survey of all ion masses (positive or negative) including
atomic ions individual isotopes and molecular compounds;
- Elemental and chemical mapping on a sub-micron scale;
- High mass resolution, to distinguish species of similar nominal
mass;
- High sensitivity for trace elements or compounds, in the order
of ppm to ppb for most species;
- Surface analysis of insulating and conducting samples;
- Molecular depth profiling and 3D imaging of organic materials
and
- Retrospective analysis, for post-data acquisition analysis and
interpretation of stored images and spectra.
The main limitations are:
36
- It generally does not produce quantitative analyses (semi-
quantitative, at best);
- Charging may be a problem in some samples, although charge
compensation routines are generally sufficient to overcome
these problems;
- There is commonly an image shift when changing from
positive to negative ion data collection mode; this makes it
difficult to collect positive and negative ion data on exactly the
same spot and
- Too much data which can take days or weeks to fully analyse.
Consequently, it is extremely important to have a very clear
purpose in collecting ToF-SIMS data, and focus on analyzing
and interpreting the data that are specifically related to the
question at hand.
Figure 12: Illustration of ToF-SIMS technique [162].
37
IV. Gas sensors
IV.1. Introduction
A gas sensor can be defined as a device, which upon exposure
to gaseous species or molecules, alters one or more of its physical
properties, such as mass, electrical conductivity, or dielectric
properties, in a way that is possible to measure and quantify [163].
These changes deliver an electrical signal, with a magnitude that is
proportional to the concentration of the gas under test. These changes,
in some cases, may be spontaneous or they could be a slow response
taking several minutes or more. The device should also show a reverse
property after the gas has been removed. This reversal may take
several minutes to hours depending on the nature of material and the
gas involved. However, in certain cases, this reversal may not take
place. According to the working principle, there is different type of
gas sensors and each type has certain advantages and disadvantages as
well.
- Electrochemical: It works by allowing gases to diffuse through a
porous membrane to an electrode where it is either chemically
oxidized or reduced. The amount of current produced is determined by
how much of the gas is oxidized at the electrode, indicating the
concentration of the gas. The main disadvantages of electrochemical
sensors are: They have a narrow temperature range, a short shelf life,
they are subject to several interfering gases such as hydrogen and their
lifetime will be shortened in very dry and very hot areas [164].
- Infrared sensor: It is based on the principle that most gases have a
characteristic absorption band in the infrared region of the spectrum
and this can be used to detect each of them. The disadvantages of this
type of sensors are: they cannot monitor all gases (only non-linear
molecule), they can be affected by humidity and water, they can be
38
expensive and dust and dirt can coat the optics and impair response
[164].
- Catalytic sensor: This sensor operates by burning the gas in air at the
surface of the bead and measuring the resultant resistance change,
proportional to concentration. Their main drawbacks are that they are
only sensitive to combustible gases and the have a poor sensitivity
[164].
- ChemFET sensor: It is a chemical potentiometric sensor based on
Field-Effect Transistors where the charge on the gate electrode is
applied by a chemical process. Depending on what is detected, there
could be ISFET (for the detection of ion in electrolyte) or ENFET (for
the detection of specific biomolecules using enzymes).
- Semiconductor: It is based on the change in electrical resistance of
the semiconductor material in the presence of gases owing to the
chemical reaction (oxidation or reduction) between sensing layer and
target gas [165].
Gas sensors have widespread and common application in the
following areas: industrial production (e.g., methane detection in
mines) [166]; automotive industry (e.g., detection of polluting gases
from vehicles) [167]; medical applications (e.g., medical diagnostic)
[168]; indoor air quality control (e.g., detection of carbon monoxide,
formaldehyde) [169] and environmental studies (e.g., greenhouse gas
monitoring) [170].
IV.2. Semiconductor metal oxide gas sensors
The atmospheric air we live in contains numerous kinds of
chemical species, natural and artificial, some of which are vital to our
life while many others are more or less harmful. A vital gas like O2
and humidity should be kept at adequate levels while hazardous gases
39
should be controlled to designated levels. Among the different kinds
of monitoring methods, semiconductor-based sensors are being used
for many applications owing to their low cost, robustness and simple
measurement electronics. The semiconductor-based sensor is a
conductometric (or resistive) sensor based on the change in the
resistance of the material in the presence of a gas. It is also a chemical
sensor which detects gases by a chemical reaction that takes place
when the gas comes in direct contact with it. The chemical reaction
between the gas and semiconductor converts it into an electrical
signal. In general, a gas sensor must possess at least two functions: to
recognize a particular gas and to transduce the gas recognition into a
measurable sensing signal. The gas recognition is carried out through
surface chemical processes involving gas solid interactions. These
interactions may be in the form of adsorption, or chemical reactions.
The transducer function of a gas sensor is dependent on the sensor
material itself. For semiconductor metal oxides, the transduction
modes employed rely on the change in the electrical properties.
During the detection process, if the gas molecules are chemisorbed
onto the semiconductor surface, there is electron transfer between the
adsorbed gas and the semiconductor. This induces a change of
conductivity of the semiconductor which is associated to the
concentration and chemical form of the detected gas [171].
Semiconductor gas sensors consist of an insulating substrate
(generally alumina or silicon oxide) fitted with interdigitated gold
electrodes and a platinum heater (figure 13). The electrodes ensure the
electrical conductivity measurement of the sensitive material, while
the platinum heater is used for the control of the temperature of the
sensors. The sensitive layer, an organic or inorganic (mostly metal
oxide) semiconductor is deposited on the substrate. Usually for metal
oxides, the sensitive layer is maintained at high temperature (from 150
40
to 500 °C) because the adsorption process is activated and controlled
by the temperature.
Figure 13: Principle scheme of the sensors.
Various semi conducting metal oxides (SnO2, ZnO, WO3, In2O3),
catalytic oxides (V2O5, MoO3, CuO, NiO) and mixed metal oxides
(LaFeO3, ZnFe2O4, BaTiO3 and Cd2Sb2O6.8) have been studied for gas
sensing properties and more and more new oxides are currently being
explored [8,9]. There are two types of semiconducting metal oxide
materials: p-type, where the charge carriers are the holes and n-type,
where the charge carriers are electrons. There could be an increase or
decrease of resistance depending on the nature of the sensor material
(n-type or p-type) and the gas (reducing or oxidizing). The response of
a sensor is characterized by following five parameters: the sensitivity,
selectivity, stability, response time and recovery time. These
parameters are discussed below.
41
IV.2.1. Sensitivity
This is the device characteristic that define the variation in
physical and /or chemical property of the sensing material under gas
exposure. The sensitivity (S) is generally defined, for n-type
semiconductors, by S = (Rgas/Rair-1) x 100 for oxidizing gases or
S = (Rair/Rgas – 1) x 100 for reducing gases, and for p-type
semiconductors, by S = (Rgas/Rair-1) x100 for reducing gases or
S = (Rair/Rgas – 1) x 100 for oxidizing gases (where Rair is the
resistance of the material in air and Rgaz is the resistance of the
material in the presence of the test gas). A sensor is very sensitive
when a small variation of gas concentration leads to an appreciable
change of resistance. High sensitivity is an interesting advantage of
semiconductor metal oxide gas sensors, because the resistance
changes with a small variation (few ppm) of the test gas concentration.
The sensitivity is highly dependent on film porosity, film thickness,
operating temperature, humidity, presence of additives, crystallite size
[171].
IV.2.1.1. Influence of Operating Temperature on the
Sensitivity
The gas sensors based on a metal oxide semiconductor usually
works at high temperatures (≥ 150 °C) and their response strongly
depends on the temperature. In fact, the detection of a specific gas
depends partially on adsorption phenomena and precisely on the
thermodynamics of gas adsorption on the surface of semiconductor
material. For a given sensor, the highest response for a specific gas is
obtained at a specific temperature. The optimization of the working
temperature of the sensor to a gas is usually used to enhance its
selectivity. In general, the response and recovery time of a sensor
depend also on the temperature because the kinetics of the reactions
between semiconductors and gases are temperature dependent. Thus,
42
at low temperatures sensors have long responses and recovery times
because the kinetics of the reaction is slow [171].
IV.2.1.2. Influence of the Humidity on the Sensitivity
Water vapor, which acts as a reducing gas, can strongly affect
the baseline and the sensitivity of the sensor. The variation of the
water vapor quantity on the surface of semiconductor can affect the
sensor performances during its use. For metal oxide sensors, the
reliability is guaranteed at relative humidities superior to 10 % at an
operating temperature of 20 °C. The baseline and the sensitivity can
be affected by humidity below this value. For example, the response
of SnO2 to carbon monoxide (CO) gas is strongly increased in the
presence of water vapor (ref). To explain the influence of water vapor,
there are two considerations. First, in the presence of humidity, the
adsorption of water molecules reduces the number of available
adsorption sites which, in turn, reduces the number of the adsorbed
oxygen on the semiconductor. The second point is that the water vapor
can also act also as an electron donor to the semiconductor, thus,
changing its baseline resistance [7]. The presence of water molecules
can also create OH sites, on the surface at a specific operating
temperature which can then influence the response. Considering the
CO gas as an example, the proposed mechanism could be:
CO +O- CO2 + e-
CO+2OH- CO2+H2O+2e-
IV.2.2. Selectivity
This characteristic is related to the discrimination capacity of a
sensor towards a mixture of gases. Selectivity plays a major role in
gas identification. The selectivity of the sensor to a specific gas (gas
A) relative to another interfering gas (gas B) can be defined by the
43
ratio SA/SB, where SA is the sensitivity of the sensor due to the
presence of the gas A and SB is the sensitivity of the sensor due to the
presence of the interfering gas B. Limited selectivity is the main
drawback of gas sensors based on semiconductor metal oxides. In fact,
different gases often produce very similar sensor sensitivity (except
when comparing reducing to oxidizing gases) [172]. For example,
gases such as ethanol, carbon monoxide, and methane have
appreciable cross-sensitivity that hinders the development of a
domestic gas sensor that can distinguish these species. Common
techniques of improving the selectivity of gas sensors include the
control of the sensor operating temperature [172], the orientation of
specific adsorption, the use of selective gas filters and additives, by
doping with impurities or catalyst [172].
Different operating temperatures allow the control of the
sensitivity toward a particular gas when there is a unique Tcrit (critical
temperature / optimum temperature with maximum sensor response)
for each gas; thus allowing the sensor to produce distinguishable
signals for two gases at a selected temperature [173]. The deposition
of chemical compounds on the surface of the metal oxide, which react
as adsorbent or react specifically with certain gases can enhance the
selectivity. For example, sulfanilic acid, which has affinity with NO2,
can enhance specific adsorption of this gas and increase the selectivity
to NO2. The use of a selective filter, which will act as a barrier and
block certain interfering gases will allow to eliminate those gases
before their reaction with the sensitive material [173]. One example of
filter consists of a thin and compact layer of SiO2 on the sensitive
semiconductor layer for hydrogen detection. Indeed the SiO2 layer is
relatively impermeable and only hydrogen can pass through it to
interact with the sensitive layer [173]. The introduction of impurities
in the metal oxide structure can modify the position of the Fermi level
(Ef), which determines the kinetics of the reaction for the partial
44
pressures of diffusing gas and its coverage rate. Then the equilibrium
and the kinetics of adsorption can be modified and this will promote
some specific reactions which will increase the selectivity toward
certain gases [150]. When the sensor is loaded by a catalyst, the
selectivity can be also enhanced [174]. For example an increase of the
selectivity toward CO was observed when the sensor layer is loaded
by Pd [29].
IV.2.3. Stability
It is a characteristic that takes into account the reproducibility
of device measurements after a long use. The success of the sensor
will be limited if the sensor performance is not demonstrated as
reproducible and stable over a long-term testing. Problems of stability,
as outlined by Park and Akbar [175] may be attributed to three factors.
The first is that a sensing layer can suffer from surface contamination.
Secondly, changes in the sensor characteristics (such as intergranular
connectivity) can occur due to thermal expansion coefficient
mismatch and / or interfacial reactions at the metal electrode/ ceramic
interface. Lastly, the film morphology may change over time due to
the relatively high operating temperatures of the sensor, which may
also cause migration of additives. To avoid the effects of non-
reproducibility after long term use, the sensor materials are submitted
to a thermal pre- treatment, which decreases posterior material
instabilities. In general, during this treatment, the samples are
submitted to high calcinations temperatures (from 400 to 1000 °C in
the range of 1 to 8 hours). Most often the sensor element gets covered
with decomposition products like carbon, CO2, and H2O causing a
gradual decrease in the sensitivity at the operating temperature. A
periodic change in sensor temperature removes all the adsorbed
species and unburnt organic contaminants from the surface [164].
45
IV.2.4. Response Time
The response time is the time interval over which resistance
attains a fixed percentage (usually 90%) of the final value when the
sensor is exposed to a full scale concentration of the gas. Time
response is especially dependent on the operating temperature and on
the sensor characteristics such as crystallite size, additives, electrode
geometry, electrode position and diffusion rates. In general at high
temperatures, the sensor has a short response time (few second). The
smaller the response time the better the sensor.
IV.2.5. Recovery Time
This is the time interval over which, after taking the sensor out
of the target gas, the resistance returns to 90% of the resistance
variation. The recovery time is also influenced by the temperature. At
high temperatures the recovery time is short. A good sensor should
have a small recovery time (few second) so that it can be used over
and over again.
V. Aim and Objectives of the Work
The aim of the present work is to synthesize mixed nickel zinc
oxide systems via thermal decomposition of nickel-zinc malonates
precursor presynthesized by co-precipitation. The optimum synthesis
conditions to yield Zn doped NiO and the nanocomposite, ZnO-NiO,
formed by the two metal oxides will be investigated. We will also
investigate the gas sensor properties of all the materials synthesized
with the aim of finding the best Ni/Zn ratio for the desired gas sensor
properties. The main objectives of the work set out in this thesis are 4-
fold, namely:
- to synthesize mixed nickel zinc malonates with different Ni/Zn
ratios by co-precipitation;
46
- to characterize all the nickel zinc malonate precursors
synthesized;
- to decompose the precursors and then characterize the
decomposition products and
- to investigate the gas sensor properties of all the
decomposition products prepared as thick film.
VI. Outline of the Thesis
The following section gives the layout of the work as presented
in the thesis in the form of chapters. Apart from general introduction
which contains the state of art of the research and the main objective
of the work, subsequent chapters present the work carried out in the
form of articles either published or submitted for publication. Thus,
Chapter I is dedicated to the synthesis and characterization of the
nickel zinc malonate precursors, as well as the Zn doped NiO (Ni1-
xZnxO) and Ni1-xZnxO/ZnO nancomposites. Here it has been shown
that the nickel-zinc malonate precursor consists of a homogeneous
mixture of nickel malonate and zinc malonate. After thermal
decomposition of the precursors, various instrumental techniques such
as XRD, ToF-SIMS and XPS prove the insertion of Zn into the
structure of NiO.
Chapter II focuses on the influence of the surfactant (oleylamine
precisely) on the nanoparticle size of NiO obtained via the pyrolysis
of the simple and the modified nickel malonate. FTIR, TG and ToF-
SIMS were used to prove the modification of the simple nickel
malonate precursor by oleylamine while XRD, SEM and BET
analyses combined, clearly demonstrate the particle size reduction of
the decomposition product from the modified nickel malonate
precursor by oleylamine.
47
The results of the gas sensor properties of the Zn doped NiO are
detailed in Chapter III. Various gases are also tested and the influence
of doping on the sensor properties of NiO is investigated.
The results of the gas sensor properties of the nanocomposite
materials investigated in this work are presented in Chapter IV.
Various reducing and oxidizing gases and different operating
temperatures were tested for several composite materials in order to
determine the best composition and temperature for the detection of a
specific gas.
48
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EXPERIMENTAL RESULTS
65
66
PART I
SYNTHESIS AND CHARACTERIZATION
OF NICKEL ZINC MALONATE AND ITS
DECOMPOSITION PRODUCTS (NICKEL
ZINC MIXED OXIDES).
The results obtained in this work can be grouped into two
parts: the synthesis and characterization of the materials and their
sensor properties. In the first part, for the synthesis of the precursors,
two ligands have been tested (malonate and acetylacetonate) and
many parameters (solvent, temperature, pH and time) investigated. To
the best of our knowledge, these two ligands with similar structure
have never been used for the coprecipitation of nickel and zinc ions.
They were chosen because both are stable in air, inexpensive and can
be easily and safely handled. Also due to their nature (chelating
ligand), they usually lead to nanoparticle decomposition products. By
varying all the parameters with acetylacetonate, we didn’t manage to
coprecipitate the zinc and nickel ions in solution while we have
succeeded to do so with malonate and the detailed results are present
in chapter I. The particle size is a very important parameter which
influences the material properties. In the chapter II we have
investigated the effect of a specific surfactant (oleylamine) on the
particle size of the final product.
67
68
CHAPTER I
Coprecipitation of nickel zinc malonate: A facile and
reproducible synthesis route for Ni1-xZnxO
nanoparticles and Ni1-xZnxO/ZnO nanocomposites
via pyrolysis
(Journal of Solid State Chemistry 2015, 230, 381-389)
I.1. Abstract
Nanoparticles of Ni1-xZnxO and Ni1-xZnxO/ZnO, which can be
good candidates for selective gas sensors, were successfully obtained
via a two-step synthetic route, in which the nickel zinc malonate
precursor was first synthesized by co-precipitation from an aqueous
solution, followed by pyrolysis in air at a relatively low temperature
(~500 °C). The precursor was characterized by ICP-AES, FTIR and
TG and the results indicate the molecular structure of the precursor to
be compatible with Ni1-xZnx(OOCCH2COO).2H2O. The
decomposition product, characterized using various techniques (FTIR,
XRD, ToF-SIMS, SEM, TEM and XPS), was established to be a
doped nickel oxide (Ni1-xZnxO for 0.01≤ x ≤0.1) and a composite
material (Ni1-xZnxO/ZnO for 0.2≤ x≤0.5). To elucidate the form in
which the Zn is present in the NiO structure, three analytical
techniques were employed: ToF-SIMS, XRD and XPS. While ToF
SIMS provided a direct evidence of the presence of Zn in the NiO
crystal structure, XRD showed that Zn actually substitutes Ni in the
structure and XPS is a bit more specific by indicating that the Zn is
present in the form of Zn2+
ions.
69
I.2. Introduction
Recently, there has been an increased interest in the synthesis
of mixed metal oxides due certainly to their great potential in
advanced applications, such as electronics, optoelectronics,
photocatalysis and gas sensors [1-4]. In fact, this type of material
possesses superior and unique properties since it combines the most
desirable physicochemical properties of its constituents while
suppressing their least desirable properties such as poor selectivity in
the case of gas sensor application for example [5,6]. Mixed metal
oxide systems can be grouped into two main categories:
nanocomposite based mixed metal oxides and doped metal oxides.
The new properties of nanocomposite materials are mainly ascribed to
the build-up of an inner electric field at the p/n junction interface of
the two oxides since the composite material consists of n and p type
semiconductor oxides mixed at the nanoscale [7]. In the case of doped
metal oxides, the new property can result from the dopant in the metal
oxide [8]. The presence of new atoms in the crystal structure of pure
metal oxide semiconductors can modify the position of the Fermi level
and hence change the electrical properties of the resulting materials.
For the synthesis of mixed metal oxide systems, several methods such
as electrospinning [9, 10], sol–gel [11, 12, 13], hydrothermal [14,15],
plasma assisted route [16], high-temperature solid-state reaction [17],
coprecipitation [18-22] have been employed. Among these methods,
co-precipitation offers several advantages over the other methods:
good dispersion of the constituent oxides, low operating temperatures,
low cost and good control of stoichiometry. The method employed for
the synthesis of the composite materials certainly has an influence on
their properties. For instance, Abdul Hameed et al.[23] have
demonstrated in the synthesis of ZnO/NiO composites from
hydroxides, carbonates and oxalates that the size of the particles and,
70
evidently, the properties change depending on precipitating agent
used.
Nickel oxide (NiO) is a transparent p-type semiconductor with a wide
band gap in the range of 3.6–3.8 eV [24, 25]. It has very interesting
characteristics such as good chemical stability and good electrical
properties. This material has been used as a solar thermal absorber,
catalyst in fuel cells, electrochromic material in display devices and as
a gas sensor material [26-30].
Zinc oxide (ZnO) is a transparent n-type semiconductor with a
band gap of 3.2 eV and ZnO nanoparticles, in particular, have
received great attention because of their unique catalytic, electrical,
gas sensing, and optical properties [31-33]. In general, their non-
toxicity, good electrical, optical, and piezoelectric behavior, low cost
and extensive applications in diverse areas such as solar cells,
photocatalysis, ultraviolet lasers, transparent conductive oxides,
spintronics and gas sensors[34-39] are some of the reasons for the
sustained interest in this material.
Most of the methods used so far in the synthesis of mixed
metal oxide systems [9-17] are generally costly, requiring
sophisticated apparatus such as high vacuum and high temperature
systems. In this paper, we report, for the first time, to the best of our
knowledge, a controlled synthesis of nanoparticles of Ni1-xZnxO and
Ni1-xZnxO/ZnO by a relatively low temperature method which
involves the pyrolysis of a precursor obtained by a coprecipitation
route in aqueous solution using the malonate ligand as the
precipitating agent. The structures of the precursors were partially
elucidated by ICP-AES, FTIR, TGA. The formation of mixed-phase
nanoparticles after thermal decomposition of the precursor was
confirmed by a combination of the following characterization
techniques: FTIR, XRD, ToF-SIMS, XPS and SEM.
71
I.3. Experimental section
I.3.1. Synthesis procedure
Zinc chloride (ZnCl2), nickel chloride (NiCl2.H2O), lithium
hydroxide (LiOH.H2O), malonic acid (HOOCCH2COOH) obtained
from Aldrich and isopropanol, acetone obtained from MERK were
used without further purification.
The nanocomposites and doped materials were both obtained in the
same manner via two experimental steps: the synthesis of the
precursor by co-precipitation in aqueous solution containing nickel
and zinc ions using the malonate ligand as precipitating agent and its
subsequent decomposition. Using zinc nickel malonate precursor as
example, lithium malonate was first prepared in aqueous solution by
adding lithium hydroxide (10 mmol) to malonic acid (5 mmol) and the
pH adjusted to 7. After 5 mins, an aqueous solution containing zinc
and nickel chloride in the desired proportion (the number of moles of
zinc in the solution varying between 0.01-0.04 and 0.1-0.5,
corresponding respectively to molar percentages of 1-4 % and 10-50
%) was poured drop by drop into the previously prepared solution of
lithium malonate with continuous stirring. The resulting green
solution was then stirred for 3H at 90 °C under reflux. It should be
mentioned here that attaining those specific synthesis conditions
(pH=7, T=90 °C, t=3H) was a crucial step in the synthesis. Variations
of these conditions could affect the final product. The precipitate so
obtained was filtered, washed successively with distilled water,
isopropanol and acetone to ensure the total removal of impurities and
dried in an oven at 70-80 °C to obtain a light green powder. The as
prepared precursor powder was calcined in ceramic combustion boat
holder at 500 °C in a muffle oven (10 °C/min) for 1 hour under air
flow. The scheme of the synthetic route is shown in figure 1
72
Figure 1: Flow diagram for the synthesis route of the materials.
I.3.2. Characterization methods
The synthetized precursors were characterized by ICP-AES,
FTIR and TGA and decomposition products by XRD, SEM, TEM,
XPS and ToF-SIMS.
Precursor Decomposition products
73
The elemental analysis of the precursors was performed via
Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-
AES) using the Thermo Jarrell Ash Iris Advantage apparatus. A few
milligrams (~20 mg) of sample were digested in 4 mL of mix acid (3
mL of concentrated nitric acid+1 mL of concentrated chlorhydric
acid) and the obtained solution was diluted with 1000 mL of water.
Measurements were carried out on this final solution. The functional
groups of the malonate ligand were determined using Fourier
Transform Infra-Red (FTIR) spectroscopy on the QuickLock Single
Reflection Horizontal ATR Accessory from Bruker. Thermal behavior
of precursor was studied by Thermogravimetry Analysis (TGA) on a
METTLER TOLEDO Thermal Analyzer in air at a flow rate of 100
mL. min-1
, a heating rate of 10 °C.min-1
and between 25-900 °C. The
powder X-ray diffraction (PXRD) data for the decomposition residues
were collected at room temperature with D5000 Siemens Kristalloflex
θ-2θ powder diffractometer. This diffractometer which has a Bragg-
Brentano geometry, is equipped with graphite-monochromated Cu-Kα
(λ = 1.54056Å) radiation, a standard scintillation counter detector and
has an automatic sample changer which can accommodate 30 samples.
For the experiment, decomposition residue was spread out on a flat
silicon plate in such a manner to avoid preferred orientations. The
patterns were recorded in the range of 5–90° with a scan step of 0.02 °
(2θ) and a 2 s step–1
acquisition interval. These patterns were
compared to NiO and ZnO patterns of the ICCD using HighScore Plus
software for phase identification. The recorded patterns were also
indexed and the unit cell refined by using the STOE WinXPOW
software package (STOE WinXPOW Version 1.10; STOE& Cie
GmbH, Darmstadt, Germany, 2002) [40]. The morphology of powder
was determined by Scanning Electron Microscopy (SEM) using a high
resolution FEG JEOL7600F with an accelerating voltage of 1kV. The
images were obtained with the semi-inlens secondary electron
74
detector. The semi-quantitative elemental analysis of the inorganic
residue was performed by Energy dispersive X-ray (EDX). For that,
the specimens were mounted on stubs and analyses were performed
without any specimens preparation using a JEOL FEG SEM 7600F
equipped with an EDX system (Jeol JSM2300 with a resolution < 129
eV) operating at 15 keV with a working distance of 8 mm. The
acquisition time for the chemical spectra lasted 300 s with a probe
current of 1 nA. The quantitative analysis of the atomic elements was
achieved with the integrated Analysis Station software. A two-step
analysis procedure was applied in order to obtain quantitatively
reliable results for the elements profiles over the entire cross sections:
(i) the subtraction of the bremsstrahlung done with the classical "Top
Hat Filter” method [41, 42] and (ii) the quantification of the area
under each atomic peak determined by the φ (ρz) model [43-45]. The
morphologies of the samples were confirmed by transition electron
microscopy (TEM, Leo922 Model from Zeiss) with an accelerating
voltage of 120 kV. TEM samples were prepared by dropping a
sonicated water dispersion suspension of the powder samples on a
carbon-coated copper grid. The presence of Zn in the structure of NiO
was demonstrated using Time of Flight Secondary Ions Mass
Spectrometry (ToF-SIMS) on an IONTOF V spectrometer (IONTOF
GmbH, Munster, Germany). The samples were initially bombarded
with Bi5+
ions (30keV, 45°) and the resulting secondary ions
accelerated at 2 kV before entering the analyzer and post-accelerated
at 10 kV before detection. The surface area of analysis was 200 x 200
μm2, the data acquisition time was 2 min and the primary ion dose was
4.12x107 ions cm
-2. The composition, the chemical state of the various
constituents and the stoichiometry of each inorganic residue were
determined by X-ray Photoelectron Spectroscopy (XPS) using Kratos
Axis Ultra spectrometer (Kratos Analytical, Mancherster, UK)
75
equipped with a monochromatized aluminum X-ray source (powered
at 10 mA and 15 kV) and eight-channeltron detector.
I.4. Results and Discussions
The metal contents (weight percentage) of the precursors
obtained via ICP-AES are presented in table 1. They are compatible
with an homogeneous mixture of Ni(OOCCH2COO).2H2O and
Zn(OOCCH2COO).2H2O in the desired proportion. We can represent
this mixture by the following formula: Ni1-xZnx(OOCCH2COO).2H2O
with x = 0-0.04 and 0.1-0.5 (0-4 % and 10 – 50 % of molar percentage
zinc respectively) corresponding, respectively, to samples S1-S5 and
S6-S10. These results suggest the presence of water molecules in the
precursor.
Table 1: Metal contents (weight percentage) of the precursors
obtained via ICP-AES
The FTIR spectra of three samples (S1, S6 and S10) chosen as
being more representative of the ten samples are shown in figure 2.
The FTIR absorption band indicates that all the principal functional
groups expected for this mixture of nickel malonate and zinc malonate
are present. The band observed at 3453 cm-1
and 3185 cm-1
can be
assigned to lattice water present in the precursor in accordance with
the elemental analysis. The weak bands which appear at 3025 cm-1
and 2913 cm-1
correspond, respectively, to the asymmetric, νas(CH),
and symmetric, νs(CH), stretching vibrations of the methylene group
Sample Elements S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
Found (%) Ni 30.3 29.7 29.5 29.3 28.1 26.2 22.5 20.0 17.4 14.8
Zn 0.0 0.5 0.9 1.5 1.8 4.5 7.6 10.6 13.7 17.3
Calculated (%) Ni 29.8 29.5 29.2 28.9 28.6 26.7 23.7 20.6 17.6 14.6
Zn 0.0 0.3 0.7 1.0 1.3 3.3 6.6 9.9 13.1 16.3
76
(-CH2) while those at 1450 cm-1
and 1284 cm-1
can be ascribed,
respectively, to the bending deformation, δ(CH), and the wagging
mode, ρw (CH). The medium band at 1664 cm-1
and the strong band
at 1554 cm-1
can be assigned to OCO asymmetric stretching vibration
of carboxylate group while that at 1374 cm-1
results from the
symmetric stretching vibration. The absence of free carbonyl, ν(C=O),
vibration expected at 1735-1705 cm-1
is an indication that the entire
carboxylate group is probably engaged in the formation of the
coordination compound. Finally, the band at 1176 cm-1
is being
assigned to the asymmetric vibration of C-C bond. These FTIR results
are in good agreement with those reported by Mathew et al. [46], in
their article on cobalt malonate.
Figure 2: FTIR spectra of samples S1, S6 and S10.
4500 4000 3500 3000 2500 2000 1500 1000 500 0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
1,2
1,3
1,4
1,5
1,6
1,7
1,8
1,9
2,0
ab
so
rptio
n
wavelength(cm-1)
---S1
---S6
---S10
77
For the TGA, sample S10 was selected for the thermal analysis
of the precursor since it is very representative of the ten samples
produced in this work. Figure 3 shows the TG curves for the samples
S10 (a) and comparative curves for S1, S6 and S10 (b) in a dynamic
air atmosphere (100 mL/min). All the TG curves in figure 3 present
two main weight loss steps. For sample S10, precisely (figure 3a), step
1 occurs between 140 and 219 °C (17.11% weight loss) and can be
assigned to the dehydration of the precursor (cf. 17.99% theoretical
weight loss). The simultaneous differential thermal analysis (SDTA)
curve shows that the dehydration step is endothermic. Step 2
occurring between 272 and 360 °C (42.8% mass loss) can be
attributed to the loss of ethenone (CH2=C=CO) and carbon dioxide
(CO2) corresponding to a theoretical weight loss of 43 %. The SDTA
curve shows two exothermic peaks side by side which are an
indication of the decomposition of two products, zinc malonate and
nickel malonate, almost at the same temperature. The total weight loss
after decomposition is 59.91 % (cf. 60.99 % expected). The residue
after decomposition is 40.09 % which corresponds to nickel zinc
oxide (cf. 39.01 % expected). Overall, the TG curve indicates that the
precursor decomposes at relatively low temperature (~ 360 °C) in air
to give nickel zinc oxide.
The TG curves (figure 3b) for S1, S6 and S10 show that there is a
progressive decrease in the dehydration temperatures in going from S1
(pure NiO) through S6 to S10 (with 50 % of Zn molar percentage).
These decreases in temperature with increasing amount of Zn can be
explained on the basis of the strengths of ion-dipole interactions
involved here. In fact the water molecules are bonded to metal ions
via this type of interaction. This type of interaction is inversely
proportional to the distance between the cation and the water
molecule. In other word the smaller the distance, the stronger the
78
interaction. Therefore, since the ionic radius of Ni2+
is smaller (69 pm)
than for Zn2+
(74pm), its interaction with the water molecules should
be stronger than that of Zn2+
. The result is that the Ni2+
-water bond
will be more stable, making it more difficult to be broken.
Figure 3: TG curves for sample S10 (a) and comparative curves for
S1, S6 and S10 (b).
The FTIR spectra of the decomposition products of S1, S6 and
S10 (figure 4) all showed only one broad band around 370-600 cm-1
which could be assigned to the metal-oxygen bond vibration (Ni-O
and Zn-O). The absence of other vibrations is a good indication that
the decomposition of the precursor was complete.
79
Figure 4: FTIR spectra of decomposition products of sample S1, S6
and S10.
The ten decomposition products obtained by pyrolysis of the
precursors at 500 °C for 1H in air were analyzed by X-Ray powder
diffraction (PXRD). Two categories can be distinguished from the
PXRD patterns (figure 5 a,b): that which indicates just one single
phase corresponding to decomposition products of S1-S6 (figure 5a)
and the other which indicates two different phases corresponding to
decomposition products of S7-S10 (figure 5b). For the first category
4500 4000 3500 3000 2500 2000 1500 1000 500 0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
S1 residue
S6 residue
S10 residue
ab
so
rptio
n
wavelength (cm-1)
---S1
---S6
---S10
80
(Figure 5a), the single phase was identified as cubic NiO (Fm-3m, a =
4.177 Å). The absence of any phase of zinc (Zn metal and ZnO) can
be explained by the fact that during pyrolysis, the Zn was incorporated
in the NiO crystal structure by substituting Ni. The fact that the NiO
lattice parameters for all the samples show an almost linear increase
from 4.177(1) to 4.191 (1) Å with increasing amount of Zn molar
percentage from 0 to 10 % (Figure 6) confirms the hypothesis of the
incorporation of Zn into NiO since the ionic radius of Zn2+
(74 pm) is
a little bit greater than that of Ni2+
(69 pm) [47]. The increase of the
lattice parameter of NiO with increasing amount of Zn observed in
this study is in agreement with the well-known Vegard’s law [48]. For
the second category (Figure 5b), the two different phases were
identified as cubic NiO and hexagonal ZnO (P63mc, a = 3.2490(1)Å
and c = 5,2052(6)Å). As the amount of Zn molar percentage increases
from 20 to 50 %, the lattice parameter of NiO remains almost
constant, a = 4.191 Å, (figure 6). Thus, for limited amounts of Zn2+
(x
< 0.2) in Ni1-xZnx(OOCCH2COO).2H2O, the pyrolysis leads to the
formation of Ni1-xZnxO solid solution whereas when x ≥ 0.2 the
pyrolysis gives a composite material consisting of Ni1-xZnxO and ZnO.
These results contrast those reported earlier in the literature [49-52]
for the solid state reaction between NiO and ZnO at high pressures
and temperatures where they obtained solid solution for up to 30 % Zn
molar percentage and also for the solid solution obtained by an
electrolytic approach combined with subsequent temperature
oxidation (~450°C, 8H) for up to 5 % Zn molar percentage. From the
above discussion it is evident that the coprecipitation technique
combined with pyrolysis used in our study is not only simple but also
reduces considerably the time and the temperature of the synthetic
processes involved. To confirm those results, a physical mixture in the
right proportion of NiO and ZnO was realize and the resulting mixture
was heated at 500 °C for 1 H. As shown in figure 5 (c), the second
81
phase of ZnO is already present at 4 % of ZnO while at 10 % there is
no evidence phase of ZnO when the coprecipitation is used.
Figure 5: Powder XRD of S1 to S6 decomposition products (a), S6 to
S10 decomposition products (b) and physical mixture between NiO
and ZnO in different percentage (4, 6 and 10 % of Zn) (c).
(c)
82
Figure 6: Variation of lattice parameter with percentage of Zn.
The presence of Zn in the NiO crystal structure was confirmed
by ToF SIMS. For this purpose, the decomposition products of the
three samples S1, S6 and S10 were analyzed. Figure 7 shows the
resulting positive mass spectra, with additional peaks corresponding to
Zn2+ (128 and 130 u) for S6 and S10 (containing Zn) which are absent
in S1 (pure nickel oxide). Another series of peaks is also present in
the mass spectra of S6 and S10, at 122, 124 and 126 u. These
additional peaks are identified as NiZn+, with the expected isotopic
ratio. In SIMS, 10-30 Bi3-5+
primary ions eject matter from an
hemispherical crater that is smaller than 10 nm of width in inorganic
materials such as metals or their oxides (e.g. 4-5 nm for 15 keV Bi3-5
impinging on Au [53]). Since the particle sizes of our samples are
0 10 20 30 40 50
4,176
4,178
4,180
4,182
4,184
4,186
4,188
4,190
4,192
latt
ice p
ara
mete
r (Å
)
Zn content (%)
83
significantly superior to 10 nm, the presence of these additional peaks
with high intensities implies that zinc and nickel are present in the
same particle. This may be due to the substitution of Ni by Zn in the
NiO during synthesis. This confirms the XRD result reported above.
Figure 7: Partial positive ToF-SIMS spectra of S1, S6 and S10
decomposition products showing the Ni2+, NiZn
+ and Zn2
+ peak
isotopes. The peak intensities are normalized with respect to the total
spectrum intensity (total counts).
84
The variation of the normalized sum of intensities of these
additional peaks with increasing percentage of Zn was investigated
and the result is presented in figure 8. The intensity of the NiZn+ ions
increases abruptly with the Zn percentage to a maximum at 15 % and
then decreases progressively to 50 % Zn. The increase can be
explained by the fact that, in those samples, all the zinc is inserted in
the nickel oxide structure and the increase of the Zn percentage is
simply mirrored by the mixed ion intensity. When the second phase of
zinc oxide starts to form, the intensity corresponding to the NiZn+ ions
emitted from the mixed Ni-Zn nanoparticles remains constant.
However, other peaks corresponding to the zinc oxide phase start to
grow in the mass spectra, proportionally to their exposed surface area.
Consequently, the emergence of this additional phase of zinc oxide
leads to the decrease of the NiZn+ relative intensity, because of its
decreasing surface fraction.
Figure 8: Variation of the sum of the normalized ToF-SIMS
intensities of the NiZn+ peak isotopes with increasing percentage of
Zn.
0 10 20 30 40 50
0,000
0,001
0,002
0,003
0,004
0,005
0,006
0,007
pe
ak
no
rma
lize
d in
ten
sity
Zn content (%)
85
The surface morphologies of the decomposition products of
samples S1, S6 and S10 were investigated by SEM as presented,
respectively, in figure 9 (a), (b) and (c). As evident from the
micrographs, the polycrystalline nature of the S1 residue is obvious
with well-defined spherical particles with average grain sizes of 75 nm
whereas the S6 and S10 residues show less well-defined grain sizes.
The agglomeration of particles into larger sizes as the amount of Zn in
the sample increases is also evident. This observed agglomeration
could play a vital role in the properties of the target materials such as
variation in electrical conductivity due to the change in specific
surface for example.
Figure 9: SEM images of decomposition products of S1(a), S6(b),
S10(c).
The results of semi-quantitative analysis performed by EDX on
the decomposition products of S1, S6 and S10 are summarized in table
86
2. The Ni/Zn atomic ratio for S6 and S10 were respectively 5.70 (cf.
5.82 expected) and 1.04 (cf. 0.86 expected). The slight differences
observed between the expected and obtained value can be explained
not only by the presence of carbon as impurities detected by EDX but
also by the limitation of EDX analysis as a quantitative method.
Table 2: EDX results for S1, S6 and S10 decomposition residue
The TEM analysis confirms the polycrystalline nanostructure
of our particles as was shown by SEM. Figure 10 (a) indicates that the
shape of the particles are well defined and uniform for pure NiO
which is not the case for 10 % Zn doped NiO and the composite
material (figure 10, b,c).
Samples % Ni % Zn % O %C Ni/Zn Atomic ratio
(Kα, 7.471KeV) (Kα, 8.63 KeV) (Kα, 0.525 KeV) (Kα, 0.277 KeV)
S1 residue 61.80 0 27.27 11.05 0
S6 residue 43.14 7.56 40.97 8.33 5.70
S10 residue 24.64 23.58 26.88 24.91 1.04
87
Figure 10: TEM images of decomposition product of S1(a), S6(b),
S10(c).
The composition, the chemical state of the various constituents
and the stoichiometry of the decomposition products of samples S1,
S6 and S10 were investigated by XPS. XPS showed the samples to be
composed of nickel, zinc, and oxygen. As far as the chemical states of
the constituents are concerned, the high resolution XPS spectra (figure
11) showed the absence of a metallic Ni2p peak which generally
occurs at 852.5 eV, implying that there is no nickel metal cluster
formed. This is in agreement with XRD results, which confirms the
absence of a pure nickel metal phase. The Ni2p lines in the three
88
decomposition residues occur at 853.6 eV (S1, Zn absent), 854 eV
(S6, Zn present) and 853.9 eV (S10, Zn present), respectively. These
results are in agreement with those reported for NiO [54]. These
values indicate that there are slight increases of the line energies going
from S1 to S6 and S1 to S10 (0.4 eV and 0.3 eV, respectively). These
observed shifts in energy values could be ascribed to the formation of
the Ni-O-Zn bond which could slightly modify the chemical binding
state of Ni due to the small difference between the electronegativity of
Zn and Ni (~ 0.26 eV). However it’s important to note that this shift is
very small, just at the limit of the XPS capability. The energies of the
Zn2p3/2 lines in the high resolution XPS spectra were observed at
1021.8 eV and 1021.9 eV for the decomposition products of samples
S6 and S10, respectively (figure 12). These energy values are in
agreement with those reported for ZnO [55]. Comparing these values
to what is observed in the literature regarding ZnO (1022 eV [55]), the
shift is practically insignificant for the reason discussed previously.
The small shoulder observed in these spectra can be explained by the
presence of a very small amount of Zn metal or by the interaction
between Zn and Ni in the samples. All these results point to the fact
that the Zn is mainly incorporated into NiO rocksalt structure as Zn2+
ions. With regards to the stoichiometry of the decomposition product
of samples S6 and S10 the XPS results gave respectively the
following atomic ratios: Ni/Zn = 2.5 (cf. 5.82 expected) and Ni/Zn =
1.8 (cf. 0.86 expected). These observed differences in stoichiometry
could be due to the fact that XPS is a surface analysis technique and
may therefore give results different from those of the bulk material.
They could also be explained by the fact that in the doped material,
ZnO, which has a high vapor pressure [56] could produce a Zn-rich
atmosphere on the surface. In the case of the composite material, the
Ni1-xZnxO could agglomerate on the surface of ZnO thus enriching it
89
with Ni. This phenomenon has been observed earlier by M. K. Deore
et al.[57].
Figure 11: Ni2P XPS peaks for decomposition products of samples
S1, S6 and S10.
Figure 12: Zn2P XPS peaks for decomposition products of samples
S6 and S10.
90
I.5. Conclusion
We have successfully synthesized Ni1-xZnxO and Ni1-
xZnxO/ZnO nanoparticles by pyrolysis at relatively low temperature
(~500 °C) of nickel zinc malonate prepared by coprecipitation. The
obtained particles are crystalline and, ToF-SIMS, which provides
direct evidence of the presence of Zn in the NiO crystal structure,
combined with XRD and XPS, show that a Zn-doped NiO sample of
single cubic structure is obtained when the dopant concentration of Zn
ions in the NiO lattice sites is lower than 20 mol %. The composite
material (Ni1-xZnxO/ZnO) is obtained from 20 to 50 % Zn (molar
percentage). Our results demonstrate that the proposed coprecipitation
technique is not only simple, low cost, controllable, reproducible,
versatile but can also be applied to prepare a wide variety of target
compounds with a good control of purity, stoichiometry and phases.
The final materials are semiconductors and could be good candidates
for sensitive and selective gas sensors.
91
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96
CHAPTER II
Effective reduction in the nanoparticle sizes of NiO
obtained via the pyrolysis of nickel malonate
precursor modified using oleylamine surfactant.
(Submitted)
II.1. Abstract
Nickel oxide nanoparticles were synthesized via thermal
decomposition of two precursors obtained by precipitation of: a
simple nickel malonate and a nickel malonate modified by oleylamine,
a surfactant. While FTIR, TGA and ToF-SIMS were used to
characterize the two precursors and to show the presence of
oleylamine in the modified precursor, XRD, SEM and BET were used
to investigate the structure, the morphology and the specific surface
area, respectively of the decomposition products obtained after
pyrolysis. The results show that the modification of nickel malonate
by oleylamine was effective. The XRD results, which showed a cubic
structure for the NiO obtained, suggest with SEM an important
particle size reduction (at least 54 % of reduction) when oleylamine is
used to modify the nickel malonate precursor. The SEM images also
show the well-defined spherical nanoparticle morphology in both
cases, which indicates that the morphology is not affected by
oleylamine. The specific surface area of the NiO obtained by
oleylamine modified precursor was 4.7 times larger than that obtained
with the simple precursor, which confirms again the reduction of
particle size.
97
II.2. Introduction
It is well established that nanoscale materials display physical
and chemical properties that are different from those of their bulk
counterparts [1]. This difference can be attributed to surface effects
(causing smooth properties scaling as a function of the fraction of
atoms at the surface) and to quantum effects (showing discontinuous
behavior due to quantum confinement effects in materials with
delocalized electrons) [1]. In fact particle sizes influence directly
physical phenomena (electron and heat transfer, for example) which
can occur in the material. For instance, the surface area of a material
increases with decreasing particle size, and this affects directly many
properties such as catalysis and gas sensor [2, 3]. As a result, a new
range of technological applications of nanomaterials has been
developed in the last decade. For these reasons, design and
optimization of synthetic methods for the production of nanoparticles
have been intensively investigated and important progress has been
made on the synthesis of nanomaterials with tailored composition,
size, shape, and crystalline structure. However there is still a strong
need for the development of simple and inexpensive protocols for
large-scale synthesis of high quality nanoparticles.
Nickel oxide (NiO) nanoparticles have particularly attracted
the attention of scientists because of its very interesting characteristics
such as good chemical stability, good optical, magnetic and electrical
properties, and the fact that it is frequently used as a model for p-type
metal oxide materials [4-6]. The electronic properties of NiO have
specifically driven research interests in applications such as anodes for
lithium ion batteries [7], solar cells [8], electrochromic coatings [9],
composite anodes for fuel cells [10], antiferromagnetic materials [11]
and as gas sensors [12,13]. With regards to its synthesis several
methods have been reported in the literature notably,
98
solvo/hydrothermal reaction [14], sol–gel process [15], microwave
synthesis [16], solution–combustion [17] and precipitation followed
by thermal decomposition [18]. In particular, the latter is simple, low
cost, well controlled and readily yields high purity products. With this
method, it is possible to add one or many surfactants to influence the
final particle size. In fact, it has been demonstrated that the surfactant
has a great influence on the particle size of the material [19].
Oleylamine (OAm) is a long-chain primary alkylamine
surfactant which, just like octadecylamine (ODA) or hexadecylamine
(HDA), can act as an electron donor at elevated temperatures.
Interestingly, commercial OAm has a much lower cost than
commonly used pure alkylamines, though some concerns regarding
purity and reproducibility have also been raised [19]. Moreover, the
fact that OAm is liquid at room temperature, may facilitate the
washing procedures that follow the chemical synthesis of
nanoparticles. However, care should be taken in handling OAm since,
as indicated in the material safety data sheet, it can be corrosive to the
skin [19].
While a search in the literature shows that the thermal
decomposition of many metal malonates [20, 21] and nickel malonate
[22, 23], in particular, have been investigated, the previous study
addressed to the synthesis of nickel oxide nanoparticles from such a
precursor seems to be scarce [24], and much less the influence of
oleylamine as surfactant in the synthesis. We, thus, provide, for the
first time, evidence that oleylamine is an effective surfactant in the
reduction of the nanoparticle sizes of nickel oxide obtained, via the
pyrolysis of nickel malonate pre-synthesized by precipitation.
99
II.3. Experimental section
II.3.1. NiO synthesis
Hydrated nickel chloride (NiCl2.H2O), hydrated lithium
hydroxide (LiOH.H2O), malonic acid (HOOCCH2COOH) and the
surfactant, oleylamine (CH3(CH2)7CH=CH(CH2)8NH2), obtained from
Aldrich, and ethanol and acetone obtained from MERK were all used
without further purification.
NiO nanoparticles were obtained via two experimental steps:
the synthesis of the precursor and its subsequent decomposition. Two
syntheses were carried out for the precursor. In the first synthesis
(without any surfactant), the precursor, nickel malonate
(NiCH2C2O4.2H2O), code-named NiM, was prepared according to the
procedure in the literature [25] and calcined in a ceramic combustion
boat holder at 500 °C in a muffle oven (10°C/min heating rate) for 1 h
under air flow. In the second synthesis (with the surfactant), 1.2 g of
the previously synthesized nickel malonate were placed in a round
bottom flask, 10 ml of oleylamine added and the mixture heated, with
continuous stirring, in an oil bath for 1 h at 140 °C. The resulting light
green solution was cooled to room temperature and an excess of
ethanol added before filtration. The green precipitate obtained after
filtration was washed several times with ethanol and dried at 100°C
for 4 h (scheme 1). The as prepared precursor modified by the
surfactant, code-named NiMO, was calcined in a ceramic combustion
boat holder at 500 °C in a muffle oven (heating rate 10 °C/min) for 1 h
under air flow.
100
Scheme 1: Illustration of the reaction between nickel malonate and
oleylamine.
II.3.2. Characterization
The two synthetized precursors were characterized by FTIR
and TGA, and the decomposition products by XRD, SEM and BET.
The functional groups of the malonate ligand and the
oleylamine were elucidated using Fourier Transform Infra-Red (FTIR)
spectroscopy on the QuickLock Single Reflection Horizontal ATR
Accessory from Bruker. The Thermal behavior of the precursor was
studied by Thermogravimetry Analysis (TGA) on a METTLER
TOLEDO Thermal Analyzer in air and in nitrogen at a flow rate of
100 mL.min-1
,a heating rate of 10 °C.min-1
and in the temperature
range 25-900 °C. The presence of oleylamine in the NiMO precursor
was confirmed using Time of Flight Secondary Ions Mass
Spectrometry (ToF-SIMS) on an IONTOF V spectrometer (IONTOF
GmbH, Munster, Germany). The samples were initially bombarded
with Bi5+ ions (30 keV, 45°) and the resulting secondary ions
accelerated at 2 kV before entering the analyzer and post-accelerated
at 10 kV before detection. The surface area of analysis was 200 x 200
μm2, the data acquisition time was 2 min and the primary ion dose was
4.12 x 107 ions cm
-2. The Powder X-ray Diffraction (PXRD) data for
the decomposition products were collected at room temperature with a
CH2
O
O
Ni.2H2O
H2N
O
O
Oleylamine
140 °CCH2
O
O
Ni.2H2O
O
O
101
D5000 Siemens Kristalloflex θ-2θ Powder Diffractometer. This
diffractometer has a Bragg-Brentano geometry and is equipped with a
graphite-monochromated Cu-Kα (λ = 1.54056Å) radiation, a standard
scintillation counter detector and an automatic sample changer which
can accommodate 30 samples. For the experiment, the decomposition
products were spread out on a flat silicon plate in such a manner as to
avoid preferred orientations. The patterns were recorded in the range
of 5–90° with a scan step of 0.02 ° (2θ) and a 2 s step–1
acquisition
interval. These patterns were compared to the NiO pattern of the
ICCD using HighScore Plus Software for phase identification. The
recorded patterns were also indexed and the unit cell refined by using
the FullProf-suite program [26]. The morphology of the
decomposition products was determined by Scanning Electron
Microscopy (SEM) using a high resolution FEG JEOL7600F with an
accelerating voltage of 15 kV. The images were obtained with the
semi-inlens secondary electron detector. The particle sizes were also
estimated by SEM. Textural analyses were carried out on
Micromeritics Tristar 3000 equipment using N2 adsorption/desorption
at 77.150 K. Samples were degassed under vacuum at 150 °C for a
period of at least 12 h prior to surface area measurement. The specific
surface area was calculated using the Brunauer–Emmet–Teller (BET)
equation whereas the pore size distributions were derived from the
desorption branches using the Barrett–Joyner–Halenda (BJH) model.
II.4. Results and discussions
FTIR spectrometry was performed in order to verify that the
ligand (malonate) and the surfactant (oleylamine) are effectively
present in the precursor. As is evident from Figure 1, the two spectra
clearly show the absorption bands corresponding to the malonate
ligand as corroborated by literature [25]. However, there are two main
differences between the two precursors, NiM and NiMO. The first
102
difference is that, for the NiMO precursor, one additional absorption
band is observed at 1053 cm-1
which can be assigned to the bending
vibration of the C-N bond indicating the presence of the oleylamine
molecule. The second difference is that the intensities of all the
absorption bands for the NiMO precursor are higher than those of
NiM. The increase in the observed absorption intensities can be
explained on the basis of the similarity between the characteristic
functional groups of oleylamine and those of nickel malonate. In other
words, the presence of oleylamine in the nickel malonate molecule
will tend to increase the intensities of the existing absorption bands.
The absorption bands observed at 3453 cm-1
and 3185 cm-1
can be
ascribed, respectively, to lattice water present in the nickel malonate
and the symmetric and asymmetric stretching vibrations of -NH2
present in oleylamine. The bands at 2920 and 2852 cm-1
correspond
respectively to asymmetric and symmetric stretching vibrations of the
C-H bonds which are present in both the malonate ligand and
oleyamine. The bands at around 1660 cm-1
and 1560 cm-1
can be
assigned to the OCO asymmetric stretching vibrations of the
carboxylate group present in the nickel malonate and to the bending
vibrations of –C=C and -NH2 bonds, respectively, present in
oleylamine. The closeness of these values to those in the literature [19,
25], confirms that the malonate ligand and the oleylamine are
effectively present in the precursor NiMO. This implies that the nickel
malonate was successfully modified by oleylamine via our synthetic
route.
103
Figure 1: FTIR spectra of NiM and NiMO (a) and Oleylamine only
(b).
4000 3500 3000 2500 2000 1500 1000 500 00,0
0,3
0,6
0,9
1,2
1,51.5
1.2
0.9
0.6
0.3
1053
1560
1660
2852
2920
3185
3453
NiM
NiMO
abso
rptio
n
wavelength(cm-1)
(a)
104
To further confirm the presence of oleylamine in the NiMO
precursor and to study its thermal behavior, themogravimetric analysis
(TGA) in air and nitrogen was carried out on it and the results are
shown in figure 2. The TGA of NiM in air has already been published
in our previous paper [25]. Hence, we shall focus more attention here
on the NiMO precursor. As is evident from Figure 2, two main weight
losses are observed between 25°C and 900°C for each of the
precursors and the weight loss is more appreciable for NiMO than
NiM, which Could be an indication of the presence of more organic
compound (malonate and oleylamine) in the former. The first weight
loss is attributed to dehydration (loss of water) [25]. In air (Figure 2a),
the inorganic residue after thermal decomposition, is 37.60 %,
corresponding to NiO for simple nickel malonate, NiM (cf. 37.94 %
expected) and 28.66 % for modified one, NiMO (cf. 28.34 %
expected, assuming one oleylamine for four nickel malonate
molecules). The TG curves show that the precursor decomposes at
343 °C and 360 °C for the simple nickel malonate (NiM) and the
oleylamine modified nickel malonate (NiMO), respectively, which
indicates that the presence of oleylamine does not affect much the
decomposition temperature of the precursor. In nitrogen (Figure 2b),
the inorganic residue after thermal decomposition constitute 30.06 %
(cf. 29.85 % expected), corresponding to metallic nickel for simple
nickel malonate (NiM) and 22.93 % (cf. 22.27 % expected, again
assuming one oleylamine for four nickel malonate molecules) for the
modified one (NiMO). The results in nitrogen corroborate those in air.
105
Figure 2: Thermogravimetry of NiM and NiMO in air (a) and in
nitrogen (b).
The presence of Oleylamine in the NiMO precursor was
confirmed by ToF-SIMS. For this purpose, the NiM and NiMO
precursors were analyzed and the partially positive and negative mass
spectra are shown in Figure 3. In the negative spectra (Figure 3a), two
significative peaks are present at 116 and 118 u in both NiM and
NiMO precursors, corresponding to Ni(OOCCH2)- with the expected
isotopic ratio. These peaks confirm the presence of nickel malonate in
the two samples. Figure 3b shows the region of the positive mass
spectra corresponding to the mass of the intact oleylamine molecular
ions. The mass spectrum of the NiMO precursor exhibits intense ion
peaks attributed to C18H35NH3+ (268 u) and C18H37NH3
+ (270 u)
which are evident absent in NiM. These additional peaks indicate the
presence of oleylamine (C18H35NH2), which in turn confirms the
effective modification of the nickel malonate with the surfactant. The
0 150 300 450 600 750 900
0
20
40
60
80
100
we
ig
ht lo
ss
(%
)
Temperature (°c)
NiM
NiMO
0 150 300 450 600 750 900
0
20
40
60
80
100
we
ig
ht lo
ss
(%
)
Temperature (°C)
NiM
NiMO
(a) (b)
106
others peaks observed in the spectra, which are present in the two
precursors, are associated to nickel malonate cluster ions.
Figure 3: Partial negative (a) and positive (b) ToF-SIMS spectra of
NiM and NiMO precursors.
Ni(OOCCH2)-
Ni(OOCCH2)-
(a)
C18H35NH
3
+
C18H37NH
3
+
(b)
107
These results suggest that a simple Van der Waals interaction
(see scheme 1) exists between the nitrogen of the amine group of
oleylamine and the nickel ion. The evidence that this interaction is
weak comes from the absence of absorption band of Ni-N bond in the
FTIR spectrum and the fact that there was no peak associated to Ni-
oleylamine fragment in the ToF-SIMS spectrum. This observation is
in accordance with the literature [19].
The XRD patterns of the two samples are shown in Figure 4.
They indicate that the decomposition products are the same for NiM
and NiMO precursors. These decomposition products were identified
as the cubic NiO (Fm-3m) phase (ICDD ref N°: 04-007-8202) [27].
As it can be clearly seen from the results, for the precursor containing
oleylamine (NiMO), the decomposition product shows peaks with
smaller intensities (Figure 4a) and larger integral width (Figure 4b)
compared to the sample obtained with the precursor (NiM) containing
only nickel and the malonate ligand. These phenomena (smaller
intensity and larger integral width) could be attributed, respectively, to
the decrease in the crystallinity and the reduction in the particle size of
the decomposition products in presence of the surfactant during the
pyrolysis process. From the Scherrer formula (d= kλ/βcosƟ where d is
crystallite size, k is shape factor, λ is x-ray wavelength, β is the full
width at half maximum and Ɵ is the Bragg angle), the average
crystallite size was estimated to be 33 nm without oleylamine and 25
with the oleylamine. Considering the limits of this approach with
respect to the sample crystallinity, the morphology and the
instrumental effects, SEM analysis was also carried out to ascertain
the size and morphology of the particles after the decomposition
process. The results are presented in Figure 5.
108
Figure 4: Powder XRD of NiM and NiMO decomposition products
(a) and compared integral width of 2 peaks (b).
As the micrographs show, the polycrystalline nature of the
decomposition products from the two precursors, NiM and NiMO, is
obvious with well-defined spherical nanoparticles (Figure 5a and
Figure 5b respectively). This result means that the surfactant used here
10 20 30 40 50 60 70 80 90 100
(222
)(3
11)
(220
)
(200
)
(111
)
NiMO residue
NiM residue
2 theta (°)
(a)
(b)
109
does not affect the nature of the particle morphology but evidently
affects the particle sizes. The particle sizes were estimated by
measuring 50 particles in each case and the corresponding histograms
reveal that most of the particles are in the range of 70-80 nm for the
NiO obtained by the thermal decomposition of NiM precursor (Figure
5c) and, in the range of 25-35 nm for the ones obtained from NiMO
(Figure 5d). NiO nanoparticles obtained by the thermal decomposition
of the NiMO precursor are smaller in size (mean size equal to 33 ± 6
nm) than those obtained for NiM (mean size equal to 72 ± 15 nm),
giving a 54 % particle size reduction provoked by the oleylamine
surfactant. The difference between the particle sizes obtained from the
Scherrer formula and SEM can be tentatively explained by the
differences in the polycrystalline nature of the samples and their
degree of crystallinity. This phenomenon may be explained on the
basis that the presence of cumbersome molecule like the oleylamine
surfactant in the precursor reduces the interaction (through steric
hindrance) between adjacent nickel malonate molecules, thus,
limiting considerably the possibility of agglomeration of the
nanoparticles during the thermal decomposition process. This
observation concords with that reported in the literature [24].
110
Figure 5: SEM images and the particle sizes distribution of
decomposition products of NiM (a), (c) and NiMO (b), (d).
The nitrogen adsorption–desorption isotherm curves of the
NiO synthesized by thermal decomposition of NiM and NiMO are
presented on Figure 6. The adsorption isotherm of the two samples is
of type II and they differ only by the amount of adsorbed N2, which
follows the respective specific surface areas of the samples as
presented in Table 1. The NiO synthesized by the thermal
decomposition of NiMO has a larger specific surface area and adsorbs
more nitrogen than the one obtained from NiM. For the NiO obtained
from NiM, the desorption and the adsorption isotherms perfectly
overlap indicating that the NiO is non-porous. On the other hand, the
NiO obtained from NiMO shows a little capillary decondensation
revealing the presence of small mesopores. The difference in the
111
porosity structures of the two NiO’s is confirmed by the measurement
of pore volume. The specific surface area values and the pore volumes
of the synthesized NiO are summarized in Table 1. As these results
indicate, the surface area of the NiO and the pore volume increase
significantly in the presence of surfactant (4.75 and 3 times,
respectively, larger for NiMO than for NiM). These results are in good
agreement with those of SEM and XRD which also indicate a
reduction in particle size in the presence of surfactant. Besides, it is a
well established fact that for a spherical particle, a reduction in size
induces an increase in the specific surface area [28].
Figure 6: Nitrogen physisorption isotherms of the NiO obtained from
NiM and NiMO.
0,0 0,2 0,4 0,6 0,8 1,0
0
5
10
15
20
25
30
35
40
45
1.00.80.60.40.20.0
qu
an
tity
ad
so
rbed
(cm
3/g
ST
P)
Relative pressure (P/P0)
NiO from NiMO_adsorption
NiO from NiMO_desorption
NiO from NiM_adsorption
NiO from NiM_desorption
112
Table 1: Specific surface area values and the pore volumes of the
synthesized NiO
II.5. Conclusion
The reduction in the nanoparticle sizes of NiO obtained via the
pyrolysis of nickel malonate pre-synthesized by precipitation has been
successfully achieved by a simple modification of the precursor using
oleylamine as surfactant. The new precursor obtained (nickel
malonate-oleylamine) was characterized by FTIR and TGA and ToF-
SIMS analyses. The reduction in the NiO nanoparticle sizes, as
demonstrated by SEM, was found to be in agreement with XRD and
BET results. The nanoparticle size reduction is attributed to the
presence of oleylamine in the modified precursor which limits
agglomeration during the thermal decomposition of the precursor. The
particle size reduction observed increases significantly the specific
surface area of NiO as revealed by BET. The method employed is
inexpensive and reproducible with a potential for scale-up and can be
readily extended to the synthesis of other metal oxide nanoparticles
especially where size is an important factor.
Samples SBET (m2/g) Pore volume (cm
3/g)
NiO from NiM 4 0.018
NiO from NiMO 19 0.060
113
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[23] K. A. Jones, R. J. Acheson, B. R. Wheeler, A. K. Galwey,
Thermal decomposition of nickel malonate, Trans. Faraday Soc. 64
(1968) 1887-1897.
[24] Y. Bahari Molla Mahaleh, S. K. Sadrnezhaad, D. Hosseini, NiO
Nanoparticles Synthesis by Chemical Precipitation and Effect of
Applied Surfactant on Distribution of Particle Size, J. Nanomaterials
2008 (2008) 1-4.
[25] R. Lontio Fomekong, P. Kenfack Tsobnang, D. Magnin, S.
Hermans, A. Delcorte, J. Lambi Ngolui, Coprecipitation of nickel zinc
malonate: A facile and reproducible synthesis route for Ni1-xZnxO
nanoparticles and Ni1-xZnxO/ZnO nanocomposites via pyrolysis, J.
Solid State Chem. 230 (2015) 381-389.
[26] T. Roisnel, J. Rodríguez-Carvajal, WinPLOTR: A windows tool
for powder diffraction pattern analysis, Materials Science Forum
2001.
[27] J.H. Binks, J.A. Duffy, Chemical Bonding in Rock Salt
Structured Transition Metal Oxides, J. Solid State Chem. 87 (1990)
195-201.
[28] S. Majumdar, The effects of crystallite size, surface area and
morphology on the sensing properties of nanocrystalline SnO2 based
system, Cerem. Int. 41 (2015) 14350–14358.
116
PART II
GAS SENSING APPLICATIONS
The second part of this thesis concerns the application as gas
sensors of the two families of as-synthesized mixed metal oxides, i.e.
doped metal oxide and composite metal oxide. As explained in the
introduction, those two families of materials are usually good
candidate to improve the sensor performance. Among all the different
gases, three were chosen for our study: two reducing gases (H2 and
CO) and one oxidizing gas (NO2). NO2 is one of the most dangerous
air pollutants, generally produced by the oxidation of atmospheric
nitrogen in all combustion processes. It plays a major role in the
formation of ozone and acid rain. In fact, ozone is formed when NO2
and volatile organic compounds react in the presence of heat or
sunlight. Continued or frequent exposure to NO2 concentrations
higher than the air quality standard (less than 1 ppm) may cause an
increased incidence of acute respiratory illness in children. The
detection and measurement of NO2 gas are thus of great importance in
both environmental protection and human health. CO is a poisonous
gas produced by the incomplete burning of carbon based fuels. It can
cause immediate health problems, and even death, in high
concentrations, and some suspect it to cause long-term health
problems in low concentrations if a person experiences regular
exposure. As far as H2 is concerned, it is not a poisonous gas as the
previous two, but it is highly flammable and burns in air at a very
wide range of concentrations. Therefore, hydrogen detection is one of
the important issues of chemical and nuclear industry for insuring
their security and installation. By using these three reference gases,
beyond specific sensitivity issues, our goal was to address in detail the
important question of selectivity
117
In term of materials, the sensor properties of different Zn
doped NiO have been investigate and compare with pure NiO (chapter
III) while for the composite materials, one (with 60 % of Zn) has
specially retained our attention. Indeed among all the composite
materials investigated, the one with 60 % of Zn showed higher
sensitivity than the others. As shown in annex 1, for CO (a), H2 (b)
and NO2 (b) the composite with 60 % of Zn presents the best
sensitivity. For this reason we decided to study the sensing properties
of this composite more thoroughly and the results are presented in
detail in chapter IV.
118
CHAPTER III
Coprecipitation synthesis by malonate route,
structural characterization and gas sensing
properties of Zn-doped NiO
(Materials Today: proceedings 2016, 3, 586 –591)
III.1. Abstract
Zn-doped NiO powders have been prepared via pyrolysis at
500°C of zinc-nickel malonate precursors prepared by co-precipitation
using different molar ratios of Zn/Ni (0.01 - 0.04 and 0.1). Structural
characterization of the powders was performed by X-ray powder
Diffraction (XRD) and Time of Flight-Secondary Ion Mass
Spectrometry (ToF-SIMS). The results confirm that a single phase of
Ni1-xZnxO (x=0.01-0.04, 0.1) is formed. Electrical measurements
reveal that its activation energy decreases with increasing amount of
Zn. Gas-sensing measurements reveal that the optimal operating
temperature is 300 °C and that the use of 2 % Zn as a dopant
improved the gas-sensing properties.
119
III.2. Introduction
Various oxide semiconductor-based gas sensors have been
used to detect toxic gases [1–3] and the most representative ones are,
in general, n-type semiconductors (SnO2, ZnO, WO3 ...). In contrast,
p-type oxide semiconductors such as NiO, CuO, Co3O4, Cr2O3 and
Mn3O4 have, to date, received relatively less attention. In fact,
compared to n-type oxide semiconductor gas sensors, the p-types
exhibit shortcomings. Even though they show promising potentials for
practical applications, their response to gases is still low [4] and
should, therefore, be enhanced in order to detect more accurately trace
concentrations of various gases. Nevertheless, the crucial importance
of p-type oxide semiconductors as chemiresistive materials should not
be under-estimated considering that most p-type oxide semiconductors
such as NiO, CuO, Cr2O3, Co3O4 and Mn3O4 have been extensively
used as good catalysts [5, 6] to promote selective oxidation of various
volatile organic compounds (VOCs). From this perspective, p-type
oxide semiconductors are promising material platforms for developing
the new functionalities of chemiresistors even though their gas
response need to be enhanced to make them suitable for use in
practical sensor applications. Their gas response could be enhanced by
tuning the morphology of the nanostructures in the oxide
semiconductors, loading noble metals, using metal oxide catalysts to
chemically sensitize the semiconductors or doping with additives to
electronically sensitize the semiconductor. The latter method has
captured the attention of many researchers.
Nickel oxide (NiO) is a p-type semiconductor with a wide
band gap in the range of 3.6 - 4.0 eV [7]. It has very interesting
characteristics such as good chemical stability and good electrical
properties. It is for these reasons that it has been used as a solar
120
thermal absorber, catalyst in fuel cells, electrochromic material in
display devices and as a gas sensor material [8].
Many methods have been used to prepare doped metal oxides
[9] but co-precipitation presents a lot of advantages over the others:
low cost, better stoichiometric control, versatility… It is for these
reasons that, in this work, powders of Zn-doped nickel oxide for use
as a carbon monoxide sensor material have been prepared by thermal
decomposition of the precursors obtained using this method.
III.3. Experimental section
III.3.1. Synthesis of the compounds
Zinc chloride (ZnCl2), nickel chloride (NiCl2.H2O), lithium
hydroxide (LiOH.H2O) and malonic acid (HOOCCH2COOH) were
purchased from Aldrich, and isopropanol and acetone were purchased
from MERCK. All reagents were used without further purification.
The doped materials was obtained via two experimental steps: the
synthesis of the precursor by coprecipitation in aqueous solution
containing nickel and zinc ions using the malonate ligand as
precipitating agent and its subsequent decomposition. Using zinc
nickel malonate precursor as example, lithium malonate was first
prepared in aqueous solution by adding lithium hydroxide (10 mmol)
to malonic acid (5 mmol) and the pH adjusted to 7. After 5 min, an
aqueous solution containing zinc and nickel chloride in the desired
proportion (the number of moles of zinc in the solution varying
between 0.01-0.04 and 0.1) was poured drop by drop into the
previously prepared solution of lithium malonate with continuous
stirring. The resulting green solution was then stirred for 3 hours at 90
°C under reflux. The precipitate so obtained was filtered, washed
successively with distilled water, isopropanol and acetone to ensure
the total removal of impurities and dried in an oven at 70-80 °C to
121
obtain a light green powder. The as prepared precursor powder was
calcined in a ceramic (porcelain) combustion boat holder at 500 °C in
a muffle oven (10 °C/min) for 1 hour under air flow.
III.3.2. Characterization methods
The precursors were characterized by Inductively Coupled
Plasma Atomic Emission Spectroscopy (ICP-AES), Fourier
Transform Infra-Red (FTIR) and decomposition products by powder
X-ray diffraction (XRD) and Time of Flight Secondary Ions Mass
Spectrometry (ToF-SIMS). The metal elemental analysis of the
precursors was performed via ICP-AES using the Thermo Jarrell Ash
Iris Advantage apparatus. The functional groups of the malonate
ligand were determined using FTIR spectroscopy on the QuickLock
Single Reflection Horizontal ATR Accessory from Bruker. The XRD
data for the decomposition products were collected at room
temperature with D5000 Siemens Kristalloflex θ-2θ powder
diffractometer. The patterns were recorded in the range of 5–90° with
a scan step of 0.02 ° (2θ) and a 2 s step–1 acquisition interval. These
patterns were compared to NiO and ZnO patterns of the ICCD using
HighScore Plus software for phase identification. The presence of Zn
in the structure of NiO was demonstrated using ToF-SIMS on an
IONTOF V spectrometer (IONTOF GmbH, Munster, Germany). The
samples were initially bombarded with Bi5+ ions (30 keV, 45°) and
the resulting secondary ions accelerated at 2 kV before entering the
analyzer and post-accelerated at 10 kV before detection. The surface
area of analysis was 200 x 200 μm2, the data acquisition time was 2
min and the primary ion dose was 4.12 x 107 ions cm-2. Electrical
characterization of NiO and Zn doped NiO was performed by
measuring their resistance with a multimeter at different temperatures.
122
III.3.3. Sensor preparation
For gas-sensing measurements, NiO and Zn-doped NiO
powders were each deposited on an alumina substrate fitted with
interdigitated gold electrodes and platinum heating element using a
simple drop coating of the corresponding ink. The ink was prepared
by mixing the metal oxide powder with distilled water and the
resulting solution (charged with 10 % of metal oxide) was placed for
24h in a TURBULA Shaker-Mixer to separate all the agglomerate
particles. Finally, the NiO and Zn-doped NiO coatings were annealed
at 400 °C for 1h in air before their exposure to the gases in a Teflon
chamber. The system used for the gas sensing measurement is shown
in figure 1.
Figure 1: gas sensing measurement setup.
123
III.4. Results and discussions
III.4.1. Characterization of precursors
The metal contents of the precursors obtained via ICP-AES are
presented in table 1. They are compatible with an homogeneous
mixture of Ni(OOCCH2COO).2H2O and Zn(OOCCH2COO).2H2O in
the desired proportion. The mixture can be represented by the
following formula: Ni1-xZnx(OOCCH2COO).2H2O with x = 0-0.04
and 0.1 corresponding, respectively, to samples S1-S5 and S6.
Table 1: Metal contents of the precursors obtained via ICP-AES
The FTIR spectra of all samples (S1-S6) recorded show all the
principal characteristic absorption bands corresponding to the
malonate ligand and are in good agreement with those reported by
Mathew et al. [10], in their article on cobalt malonate.
III.4.2. Characterization of decomposition products
The decomposition products obtained by pyrolysis of the
precursors at 500 °C for 1h in air were analyzed by X-Ray powder
diffraction (PXRD). As is evident from figure 2(a), the diffractogram
obtained shows only one single phase identified as cubic NiO (Fm-
3m, a = 4.177 Å). The absence of any phase of zinc (Zn metal or ZnO)
can be explained by the fact that during pyrolysis, the Zn was
incorporated in the NiO crystal structure by substituting Ni. It can be
confirmed by the fact that the NiO lattice parameters for all the
samples show an almost linear increase from 4.177 (1) to 4.187 (1) Å
Sample Elements S1 S2 S3 S4 S5 S6
Found (%) Ni 30.3 29.7 29.5 29.3 28.1 26.2
Zn 0.0 0.5 0.9 1.5 1.8 4.5
Calculated (%) Ni 29.8 29.5 29.2 28.9 28.6 26.7
Zn 0.0 0.3 0.7 1.0 1.3 3.3
124
with increasing amount of Zn from 0 to 10 % since the ionic radius of
Zn2+
(74 pm) is a little bit greater than that of Ni2+
(69 pm) [11].
The presence of Zn in the NiO crystal structure was also
confirmed by ToF SIMS. Figure 2(b) shows the resulting positive
mass spectra, with additional peaks with the expected isotopic ratio
corresponding to ZnNi+ (122, 124 and 126 u) for S3 and S6
(containing Zn) which are absent in S1 (pure nickel oxide). In SIMS,
10-30 Bi3-5+ primary ions eject matter from an hemispherical crater
that is smaller than 10 nm of width in inorganic materials such as
metals or their oxides (e.g. 4-5 nm for 15 keV Bi3-5 impinging on Au
[12]). Since the particle sizes of our samples are significantly superior
to 10 nm, the presence of these additional peaks with high intensities
implies that zinc and nickel are present in the same particle
confirming then the substitution of Ni by Zn in the NiO during
synthesis.
Figure 2: Powder XRD of S1 to S6 decomposition products (a) and
Partial positive ToF-SIMS spectra of S1, S3 and S6 decomposition
products (b).
Mass (u)
116 118 120 122 124 126 128 130
-3x10
0.5
1.0
1.5
2.0
2.5
3.0
P_NiO (lave)_1 Total Counts: 2.5572E+006
-3x10
0.5
1.0
1.5
2.0
2.5
3.0
Sca
led
In
ten
sity
P_98-2_3 Total Counts: 1.4591E+006
-3x10
0.5
1.0
1.5
2.0
2.5
3.0
P_90-10_2 Total Counts: 2.8629E+006
Mass (u)
116 118 120 122 124 126 128 130
-3x10
0.5
1.0
1.5
2.0
2.5
3.0
S1
S3
S6
(b)
ZnNi+ ZnNi+
0 20 40 60 80 100
x NiO
S6 residue
S5 residue
S4 residue
S3 residue
S2 residue
S1 residue
xx
x
x
x
2/°
(a)
ZnNi+
125
For electrical characterization, the resistances of the S1, S3, S5
and S6 decomposition products are measured at different temperatures
in air and the results are shown in figure 3(a). For all samples the
resistance decreases with increasing temperature following a classical
Arrhenius law. From these results, the activation energies (for the
transition of the electrons from valence bond to conduction band) of
these samples were calculated and figure 3(b) indicates that the
activation energy decreases with increasing amount of Zn up to 4 %,
meaning that the resistance decreases correspondingly.
Figure 3: Variation of resistance with temperature (a) and variation of
activation energy with Zn content (b).
III.4.3.Gas sensing properties
In order to check the sensing properties of NiO and Zn-doped
NiO to 200 ppm of CO gas, the samples were maintained at various
0,0014 0,0016 0,0018 0,0020 0,0022 0,0024
4
5
6
7
8
9
10
11
12
Ln
Ro
1/T (°K-1)
--- NiO
--- Ni0.98Zn0.02O
--- Ni0.96Zn0.04O
--- Ni0.9Zn0.1O
0 2 4 6 8 10
0,30
0,35
0,40
0,45
0,50
0,55
0,60
Activ
atio
n e
ne
rg
y E
a(e
V)
Zn content (%)
(a) (b)
126
temperatures in the range 150 to 400 °C (Figure 4(a)). It is observed
that the sensitivity of all the sensors (S = R1/R2, where R1 is the
resistance of sensor in the presence of gas and R2 the resistance
without gas) increases with operating temperature until a maximum
around 300°C is reached.
The best sensitivity is obtained for 2 % Zn-doped NiO. This
could be explained as follows: CO sensing mechanism depends on
adsorbed oxygen on the surface and since Zn has more affinity to
oxygen than NiO, it implies that the Zn doped material will adsorb
more oxygen on its surface, thus, enhancing the response. On the other
hand, the low response for 4 % and 10 % Zn-doped NiO is probably
due to the high conductivity of these materials and consequently the
variation of the resistance in the presence of gas is not visible. A
balance has to be found. Other gases (H2 and NO2) have been tested to
compare the selectivity of NiO and 2% Zn-doped NiO (NO2 decreases
the resistance and the sensitivity S is defined by R2/R1). Figure 4(b,c),
shows that 2% Zn-doped NiO is more selective to CO at 300°C than
NiO. NiO and 2% Zn-doped NiO sensors were exposed to CO (200
and 400 ppm) for repeated tests (Figure 4(d)) and it was observed that
the doped sensor showed a faster response than that of NiO, and it
shows a stable evolution and reproducibility.
127
Figure 4: Response of NiO and all Zn-doped NiO to 200ppm CO (a);
response of Ni0.98Zn0.02O to 200 ppm H2 200 ppm CO and 1ppm NO2
(b); response of NiO to 200 ppm H2, 200 ppm CO and 1ppm NO2 (c);
Resistance variation of Ni0.98Zn0.02O in contact with CO at 300°C
(200 and 400 ppm, RH=50 %) (d).
III.5. Conclusion
NiO and Zn doped NiO were successfully prepared via
pyrolysis of the corresponding precursor obtained by a simple and low
cost co-precipitation method. The decomposition products were
identified as a single phase Ni1-xZnxO and the absence of additional
phases of Zn or ZnO proves that substitution of Zn in the NiO
structure is total. Electrical properties investigations showed that Zn
150 200 250 300 350 400
1,00
1,05
1,10
1,15
1,20
1,25
1,30
1,35
1,40
1,45
1,50
1,55
1,60
1,65
1,70
1,75
1,80
1,85
1,90
S(R
1/R
2)
Temperature (°C)
--- Ni0.98Zn0.02O
--- Ni0.96Zn0.04O
--- Ni0
--- Ni0.9Zn0.1O
150 200 250 300 350 400
1,0
1,1
1,2
1,3
1,4
1,5
1,6
1,7
1,8
1,9
S(R
1/R
2)
Temperature (°C)
--- response to CO for Ni0.98Zn0.02O
--- response to H2 for Ni0.98Zn0.02O
--- response to NO2 for Ni0.98Zn0.02O
150 200 250 300 350 400
1,00
1,05
1,10
1,15
1,20
1,25
1,30
1,35
S(R
1/R
2)
Temperature (°C)
--- response to CO for NiO
--- response to NO2 for NiO
--- response to H2 for NiO
0 500 1000 1500 2000 2500 3000
100
120
140
160
180
200
220
240
260
280
---- NiO
---- Ni0.98Zn0.02O
CO off
400 ppm
CO off
200 ppm
R (
x 1
0 O
hm
)
time (s)
(a) (b)
(c) (d)
128
increases the conductivity of NiO and, consequently, decreases its
activation energy. The gas sensing properties towards some gases (H2,
CO, NO2) were also investigated as a function of Zn content and the
operating conditions. Of all the Zn doping percentages investigated, it
was found that 2 % doping gave the best sensitivity to CO at 300°C.
More research is on-going (the determination of the band gap for pure
NiO and all the Zn doped NiO by UPS) to better understand the effect
of the 2 % Zn-doping on the sensitivity of NiO.
129
III.6. References
[1] N. Yamazoe, Sens. Actuators B 108 (2005) 2–14.
[2] Y. Shimizu, M. MRS Bull. 24 (1999) 18–24.
[3] A. Kolmakov, M. Moskovits,Annu. Rev. Mater. Res. 34 (2004)
151–180.
[4] M. Hübner,C.E.Simion,A.Tomescu-Sta˘noiu, S. Pokhrel, N.
Barsan, U. Weimar, Sens.Actuators B 153 (2011) 347–353.
[5] K.Pirkanniemi, M. Sillanpää, Chemosphere 48 (2002) 1047–1060.
[6] M.M. Bettahar, G. Costentin, L. Savary, J.C. Lavalley, Appl.
Catal. A: Gen. 145 (1996) 1–48.
[7] E. R. Beach, K. Shqau, S. E. Brown, S. J. Rozeveld, P. A. Morrisa,
Mater. Chem. Phys.115 (2009) 371–377.
[8] I. Hotovy, J. Huran, P. Siciliano, S. Capone, L. Spiess, V.
Rehacek, Sens. Actuators B 103 (2004) 300–311.
[9] C. T. Dinh, T. D. Nguyen, F. Kleitz, O. D. Trong, Controlled
Nanofabrication: Advances and Applications, Pan Stanford
Publishing, Singapore, 2013, pp. 327-367.
[10] V. Mathew, J. Joseph, S. Jacob, P.A Varughese, K.E. Abraham,
Optoelectron. Adv. Mat. 2 (2008) 561-565.
[11] L. H. AHRENS, Geochim. Cosmochim.Acta, 2 (1952) 155-169.
[12] A. Delcorte, Ch. Leblanc, C. Poleunis, and K. Hamraoui, J. Phys.
Chem. C 117 (2013) 2740−2752.
130
CHAPTER IV
Ni0.9Zn0.1O/ZnO nanocomposite prepared by
malonate coprecipitation route for gas sensing.
(Accepted in Sensor & Actuator B: Chemical, DOI:
10.1016/j.snb.2016.03.099)
IV.1. Abstract
A nanocomposite material, Ni0.9Zn0.1O/ZnO, was synthesized
by controlled thermal treatment of the corresponding nickel zinc
malonate previously prepared by coprecipitation in aqueous solution.
The co-existence, structure and morphology of the Zn-doped nickel
oxide, Ni0.9Zn0.1O and zinc oxide, ZnO, nanoparticles was confirmed
by a set of instrumental techniques such as XRD, SEM and ToF-
SIMS. The gas-sensing properties of the as-synthesized
nanocomposites have been evaluated for CO, NO2, H2. The results
show that the gas sensors based on the Ni0.9Zn0.1O/ZnO
nanocomposites exhibit a good selectivity to NO2 at low temperature
(225 °C) and an increase in the response to CO gas at a high
temperature (300 °C) when compared to ZnO and Ni0.9Zn0.1O
individually. The significant enhancement of the sensing performance
of the Ni0.9Zn0.1O/ZnO nanocomposite is attributed to the formation of
p-n junctions (metal oxide heterojunctions) between Ni0.9Zn0.1O (p-
type semiconductor) and ZnO (n-type semiconductor) and to the
catalytic activity of these metal oxides.
131
IV.2. Introduction
Semiconducting metal oxide based sensors have been widely
used for the detection of toxic, hazardous and combustible gases for
the safety of human beings and environmental control [1-7]. This is
certainly due to their advantages such as simplicity of their use, high
sensitivity to small concentration of gases, low fabrication cost and
long term stability [7-9]. The gas detection principle of metal oxides
semiconductors is based on the variation of electrical conductivity in
the presence or absence of tested gas. In fact, when the metal oxide
semiconductor is heated, it usually adsorbs the oxygen on its surface
and, for an n-type semiconductor, this oxygen will extract electrons
from the conduction band and form a depletion layer. In the presence
of a reducing gas, for example, the negatively charged oxygen will
oxidize the tested gas and re-inject the electron back to the conduction
band and in the case of n-type semiconductor, the electrical
conductivity will increase [10,11]. For a p-type semiconductor, the
oxygen adsorption increases the hole concentration. The presence of
the target gas is then determined by the change of resistance resulting
from the reaction between surface adsorbed oxygen species and the
gas. The resistance change is directly related to the gas concentration,
allowing for its measurement. Although, this type of sensors presents
a good sensitivity, its selectivity remains a challenging issue as it
gives a similar response to a wide range of gases. Many researchers
have suggested various ways to overcome this problem of cross-
sensitivity, which include the use of specific filters like zeolite thin
films [12,13], the addition of a noble catalytic metal to promote the
reaction to specific gases [14,15], and the use of composite metal
oxides [16-19]. The latter is very important in that it can also be used
to enhance the sensitivity of the gas sensor [7]. In fact, the interaction
between the components of the metal oxide composite material can
lead to a change in the electronic structure of the nanocrystals and,
132
hence, affect their reactivity. For example, the contact between p and
n-type semiconductors induces the build-up of an inner electric field at
the p-n junction interface which could modify the properties of the
resulting material. It is remarkable that the selectivity of the sensor
response can be significantly improved by varying the composition
[20,21], structure [22] and operating temperature [20] of such
nanocomposite sensors. Indeed, various authors have reported
selective and sensitive gas sensing characteristics of composite oxide
materials prepared by different methods [23-26].
Zinc oxide (ZnO), a n-type semiconductor, has been
considered as a viable material for gas sensors because of its high
electrochemical stability, non-toxicity, suitability to doping and low
cost [27]. It has been extendedly used as effective sensing material for
gases such as H2, CO, NO2 and acetone [28-31]. Various
morphologies (nanorods [32], nanowires [33], nanoplates [34], etc.) of
nanostructured ZnO have been synthesized by different methods and
tested for gas sensor applications. It was found that the sensing
performance strongly depends on the morphology, which, in turn,
depends on the synthesis route. Nonetheless, ZnO has demonstrated
some drawbacks, such as high working temperature (between 400 and
500 °C) and poor selectivity.
As a p-type wide-bandgap semiconductor, nickel oxide (NiO)
has attracted increasing attention due to its excellent chemical stability
and good electrical properties. In addition, NiO is a p-type
semiconductor which is widely used for gas sensor applications [35]
for such gases as NO2 [36], H2 [37], formaldehyde [38] and NH3 [39].
Like all p-type semiconductors, the sensitivity of NiO is very low and,
therefore, needs to be improved. Thus, recent years, a lot of effort has
been geared towards the synthesis of nanostructured NiO with various
morphologies in order to improve its performances [40-42].
133
As far as NiO-ZnO composites are concerned, several works
have been reported in the literature on their use as gas sensors
synthesized by various techniques [43-45]. In most cases; the
formation of a p-n junction clearly affects the sensing properties.
However, this p-n junction directly depends on the electrical
properties of each component (NiO and ZnO for this specific case). To
the best of our knowledge, we are reporting for the first time the
investigation of the gas sensing properties of Ni0.9Zn0.1O/ZnO
nanocomposites forming intimate heterojunctions. This material was
obtained at a relatively low temperature (500 °C) by the thermal
decomposition of the corresponding nickel zinc malonate prepared by
co-precipitation. In fact, the insertion of Zn in the NiO structure
affects the NiO electrical properties thereby changing the resulting p-n
junction effect on the gas sensing property of the target composite
material.
IV.3. Experimental section
IV.3.1. Synthesis of the materials
The ZnO, Ni0.9Zn0.1O and Ni0.9Zn0.1O/ZnO nanoparticles were
synthesized by a simple co-precipitation followed by thermal
decomposition following the procedure reported elsewhere in our
previous paper [46]. For instance, Ni0.9Zn0.1O/ZnO was obtained via
the thermal decomposition under air flow at 500 °C for 1 h, of
corresponding mixed metal malonate co-precipitaed in aqueous
solution by mixing the malonate ligand with the nickel and zinc ions
in the desired proportion (Zn/Ni = 1.5). For doped material, however,
the nickel and zinc ratio was Zn/Ni = 0.11. As was clearly established
in our previous paper [46], the Zn/Ni = 0.11 ratio corresponds to the
solubility limit (~ 10 %) of Zn in NiO.
134
For gas-sensing measurements, the as prepared Ni0.9Zn0.1O,
ZnO and Ni0.9Zn0.1O/ZnO powders were deposited as thick films on
alumina substrates fitted with interdigitated gold electrodes and a
platinum heating element, using a simple drop-coating of each
corresponding ink. The mean values of the thicknesses evaluated by
the profilometer of the different films were 20.47 μm, 19.29 μm and
26.62 μm for Ni0.9Zn0.1O, ZnO and Ni0.9Zn0.1O/ZnO respectively. The
ink was prepared by mixing the metal oxide powder with distilled
water and the resulting solution (charged with 10 % of metal oxide)
was placed for 24 hours in a TURBULAR Shaker-Mixer to separate
the agglomerated particles. Finally, the thick films were annealed at
400 °C for 1h in open air. Before exposure to gases, the sensor was
left for 3 h under air flow (50 % of humidity) in the Teflon chamber at
the highest temperature of the tests (400 °C) in order to stabilize the
baseline resistance.
IV.3.2. Characterization of the materials
The powder X-ray diffraction (PXRD) data for the
decomposition products were collected at room temperature with a
D5000 Siemens Kristalloflex θ-2θ powder diffractometer which has a
Bragg-Brentano geometry. It is also equipped with a graphite-
monochromated Cu-Kα (λ = 1.54056Å) radiation, a standard
scintillation counter detector and an automatic sample changer which
can accommodate 30 samples. For the experiment, the decomposition
product was spread out on a flat silicon plate in such a manner as to
avoid preferred orientations. The patterns were recorded in the range
of 5–80° with a scan step of 0.02 ° (2θ) and a 2 s step–1
acquisition
interval. These patterns were compared to those of NiO and ZnO
patterns of the ICCD using the HighScore Plus software for phase
identification. The recorded patterns were also indexed and the unit
cell refined by using the STOE WinXPOW software package (STOE
135
WinXPOW Version 1.10; STOE & Cie GmbH, Darmstadt, Germany,
2002) [47]. The morphology of the particles deposited on the alumina
substrate was determined by Scanning Electron Microscopy (SEM)
using a high resolution FEG JEOL7600F with an accelerating voltage
of 5 kV. The images, taken from the top of the samples, were obtained
with the semi-inlens secondary electron detector. SIMS depth profiles
were made using a ToF-SIMS 5 (IONTOF GmbH, Münster,
Germany) time-of-flight secondary ion mass spectrometer. The
instrument is equipped with a Cs+ gun as sputtering source and a
liquid metal ion gun (Bi) as analytical source both mounted at 45°
with respect to the sample surface. The time-of-flight mass analyzer is
perpendicular to the sample surface. Depth profiles were carried out in
the ‘non-interlaced’ mode, consisting of period of 3.28 s of 30 keV
Bi5+ primary ions pulses for SIMS analysis followed by periods of
3.28 s of continuous Cs+ ion sputtering. Then, 25 s pauses whereas
low energy electrons are sent to the sample in order to recover the
initial surface potential. The Cs+ ion source was operated at 2 keV
with a d.c. current of 160 nA. For depth profiling, the focused Cs
beam of primary ions was rastered over an area of typically 450 x 450
μm2. A pulsed beam of 30 keV Bi5
+ (a.c. current of 0.12 pA) ions was
employed to provide mass spectra from a 150 x 150 μm2 area in the
centre of the sputter crater. Charge compensation was done by
electron flood gun (Ek = 20 eV). All data analyses were carried out
using the software supplied by the instrument manufacturer,
SurfaceLab (version 6.5).
IV.3.3. Gas-sensing tests
The measurements of the electrical resistances of the sensors in
the presence of the target gases were made in a Teflon chamber. The
sensors are connected to a tailor-made system to measure their
electrical resistance. Moist air (50% relative humidity at 20 °C) was
136
used as reference gas in the tests. After the electrical resistance of the
sensors was stable in reference air (Rair) and then waiting for another
20 min, NO2, H2 or CO in air diluted with the reference moist air to
get the desired target gas concentrations, were introduced into the
chamber. Rgas is obtained when the sensor resistance is stable in the
presence of target gas. The sensor response (S) for ZnO and
Ni0.9Zn0.1O/ZnO, which are n-type semiconductors is defined by
S = (Rgas/Rair -1) x 100 for oxidizing gases (the resistance increases
after the reduction of the target gas) and S = (Rair/Rgas - 1) x 100 for
reducing gases (the resistance decreases after the oxidation of the
target gas). For Ni0.9Zn0.1O, which is p-type semiconductor, S =
(Rgas/Rair - 1) x 100 for reducing gases (the resistance increases after
the oxidation of the target gas) and S = (Rair/Rgas - 1) x 100 for
oxidizing gases (the resistance decreases after the reduction of the
target gas). These measurements were carried out at different
operating temperatures ranging from 225 to 350 °C. Response time is
the time it takes sensor to reach 90% of its steady-state gain value
after introduction of target gas. The Recovery time is the time that it
takes the sensor to be within 10% of the value it had before exposure
to the target gas.
IV.4. Results and discussions
IV.4.1. Structure, morphology and composition
The decomposition products obtained after pyrolysis of the
precursors in air at 500 °C for 1h were analyzed by X-Ray diffraction
(XRD). As evident from figure 1, the diffractograms of (a) and (c)
show one single phase each, identified as hexagonal ZnO (P63mc, a =
3.2490(1) Å and c = 5.2052(6) Å) and cubic NiO (Fm–3m, a = 4.187
Å) respectively. The absence of any of zinc phase (Zn metal and ZnO)
in the pattern (c) can be explained by the fact that during pyrolysis, the
Zn was incorporated in the NiO crystal structure by substituting Ni.
137
Moreover, the lattice parameter of NiO obtained here (4.187 Å) is
greater than that of pure NiO (4,177 Å) [46]. The diffractogram of
sample (b) shows two different phases identified as cubic NiO and
hexagonal ZnO which indicate the formation of the composite
material. The lattice parameter of NiO is 4.190 Å which is different
from the pure NiO (4,177 Å). This difference again suggests the
partial insertion of Zn into the NiO structure since the radius of Zn2+
(74 pm) is slightly greater than that of Ni2+
(69 pm) [48]. This result
undoubtedly excludes the formation of a simple NiO/ZnO composite
material, rather favoring the formation of a Ni1-xZnx/ZnO composite (x
~ 0.1) which will be tested for the first time as gas sensor.
Figure 1: Powder XRD of pure ZnO (a), Zn/Ni = 1.5 (b) and Zn/Ni =
0.11 (c).
0 10 20 30 40 50 60 70 80 90
a:pure ZnO
b:Zn/Ni = 1.5
c:Zn/Ni = 0.11 o ZnOx NiO
oooo
o
oo
o
XX
X
X
X
a
b
c
2
138
The morphology of all the metal oxide thick films was
investigated by SEM analyses. As is evident from figure 2, the
samples, in general, show undefined morphology. However, ZnO (a)
and Ni0.9Zn0.1O/ZnO (b) are almost spherical while Ni0.9Zn0.1O (c) is
more similar to small pellets. It is also evident that the ZnO and
Ni0.9Zn0.1O/ZnO particles are smaller (~30 nm) than those of
Ni0.9Zn0.1O. It is, also, important to mention that the very large
particles observed in figure 2(a) and (b) were identified as constituting
the substrate (alumina) by the EDX mapping
Figure 2: SEM images of ZnO (a), Ni0.9Zn0.1O/ZnO (b) and
Ni0.9Zn0.1O (c).
100nm
(a) ZnO
(c) Ni0.9
Zn0.1
O
(b) Ni0.9
Zn0.1
O/ZnO
100nm
100nm 100nm
100nm 100nm
139
A deeper insight into the composition of the system was
obtained by ToF-SIMS, and a representative in-depth profile is shown
in Figure 3. A rough estimate of the depth of the crater (picture on
figure 3 (b)) based on the sputter yield of the oxides gives around 10
nm. As can be seen, the Ni0.9Zn0.1O shows an approximately
homogeneous composition in the depth. As observed from the Ni+/Zn
+
ion intensity ratio in figure 3 (a), despite the statistical variation, the
best linear regression of the data is almost a constant. However the
signal of Ni+ is higher than that of Zn
+ and this can be explained by
the difference of ionization probability of the considered elements.
This composition uniformity was also observed in the composite
material (not shown).
Figure 3: Ratio of the Ni+ intensity to the Zn
+ intensity with
increasing Cs+ ion fluence for Ni0.9Zn0.1O (a) and image before and
after the sputtering (b).
(a) (b)
Before the
sputtering
After the
sputtering
crate
r 0,00E+000 3,00E+016 6,00E+016 9,00E+0160
20
40
60
80
100
120
Ni/Z
n s
ims in
ten
sit
y/c
ou
nts.s
-1
Fluence (Cs+/cm2)
140
IV.4.2. Gas-sensing tests
The first test carried out was addressed to the study of the
response to 300 ppm of carbon monoxide (CO) gas for the as prepared
three sensors at different working temperatures. In fact, it is well-
known that the operating temperature is an important parameter for a
metal oxide semiconductor and the gas response is highly dependent
on operating temperature. The resistance of ZnO and Ni0.9Zn0.1O/ZnO,
both which are n-type semiconductors decreases in the presence of CO
but increases for Ni0.9Zn0.1O, a p-type semiconductor (figure 4 (b)). As
figure 4(a) indicates, the response of the sensor based on
Ni0.9Zn0.1O/ZnO to 300 ppm of CO increases rapidly with an increase
in the operating temperature up to 300 °C and then decreases with a
further increase in the operating temperature. This implies that this
sensor reaches its maximum response of 194 % at 300 °C (the optimal
operating temperature). A similar behavior could be observed in the
case of the sensor based on Ni0.9Zn0.1O but its maximum response at
optimal operating temperature (300 °C) is 24 % which is 8 times
lower than that of sensor based on Ni0.9Zn0.1O/ZnO. In the case of
pure ZnO, however, we observe an increase of response with
increasing operating temperature in the range of the temperature
chosen for the test. At 400 °C, the response is 85 % which is 2.2 times
less than that of the sensor based on Ni0.9Zn0.1O/ZnO at 300 °C. From
these results it can be observed that the sensor based on
Ni0.9Zn0.1O/ZnO compared to Ni0.9Zn0.1O and ZnO, shows a strong
increase of response to 300 ppm of CO and a drop in the maximum
operating temperature. This suggests an influence of the formation of
heterojunction on the gas sensing response and on the optimal
operating temperature. The existing of the heterojunction can be
proved by the dynamic response to CO at 300 °C for the three sensors
(figure 4(b)), where it can be seen that the baseline resistance in air of
the composite material (Rair = 60 MΩ) is far higher than that of the
141
single components (ZnO: Rair = 750 kΩ, Ni0.9Zn0.1O: Rair = 131 Ω).
This kind of heterojunction effect has also been reported for other
heterojunction systems like CuO-NiO [49] and Fe2O3-NiO [50].
Figure 4: Response of ZnO, Ni0.9Zn0.1O and Ni0.9Zn0.1O/ZnO to 300
ppm of CO at different operating temperatures (a) and their dynamic
response at 300 °C (b).
The same response studies were carried out on the three
different sensors with 5 ppm of NO2. As figure 5 indicates, the
response of the Ni0.9Zn0.1O/ZnO based sensor and the two others
(Ni0.9Zn0.1O and ZnO) decreases with increasing operating
220 240 260 280 300 320 340 360 380 400 420
0
20
40
60
80
100
120
140
160
180
200
CO
resp
on
se (
%)
Temperature (°C)
ZnO
Ni0.9
Zn0.1
O
Ni0.9
Zn0.1
O/ZnO
1200 1600 2000 2400 2800
130
135
140
145
150
155
160
165 CO off
CO in
Resis
tan
ce (
Oh
m)
Time (s)
Ni0.9
Zn0.1
O
0 300 600 900 1200 1500 1800
20000
30000
40000
50000
60000
CO off
CO in
Re
sis
tan
ce
(x
10
00
Oh
m)
Time (s)
Ni0.9
Zn0.1
O/ZnO
0 300 600 900 1200 1500 1800
6400
6600
6800
7000
7200
7400
7600
7800
CO off
CO in
Re
sis
tan
ce
(x
10
0 O
hm
)
Time (s)
ZnO
(a)
(b)
142
temperature. The responses are 1049 % for ZnO, 354 % for
Ni0.9Zn0.1O/ZnO and 3 % for Ni0.9Zn0.1O at 225°C, and 72 % for ZnO,
30 % for Ni0.9Zn0.1O/ZnO and 0.1 % for Ni0.9Zn0.1O at 350°C. For the
range of temperatures covered in this work, the optimum temperature
is 225 °C and at this temperature the ZnO showed higher response
than the other two.
Figure 5: Response of ZnO, Ni0.9Zn0.1O and Ni0.9Zn0.1O/ZnO
to 5 ppm of NO2 at different operating temperatures.
The response of the three sensors to 300 ppm of H2 has been
also investigated at different operating temperatures and the results are
shown in figure 6. The response of the 3 kinds of sensors follows the
same trend, increasing with temperature up to 300 °C and then
decreasing. Among the three sensors investigated at 300 °C, Zn shows
220 240 260 280 300 320 340 360
0
200
400
600
800
1000
1200
NO
2 r
esp
on
se (
%)
Temperature (°C)
Ni0.9
Zn0.1
O
ZnO
Ni0.9
Zn0.1
O/ZnO
143
a higher response (176 %) than for Ni0.9Zn0.1O/ZnO (104 %) and
Ni0.9Zn0.1O (11 %).
Figure 6: Response of ZnO, Ni0.9Zn0.1O and Ni0.9Zn0.1O/ZnO to
300 ppm of H2 at different operating temperatures.
The low gas response of sensor based on Ni0.9Zn0.1O (p-type
semiconductor) compared to the others (n-type semiconductors), can
be explained by the theory supported by Hübner et al.’s calculation
which states that the gas response of p-type oxide semiconductors is
the square root of that of n-type oxide semiconductors with the same
morphology [51]. Moreover, owing to its high conductivity (Rair =131
Ω) compared to others (Rair = 750 kΩ for ZnO and Rair = 60 MΩ for
Ni0.9Zn0.1O/ZnO), the variation of the resistance at the surface of the
particle is negligible because the Debye length is very short and the
220 240 260 280 300 320 340 3600
20
40
60
80
100
120
140
160
180
H2 r
esp
on
se (
%)
Temperature (°C)
Ni0.9
Zn0.1
O
ZnO
Ni0.9
Zn0.1
O/ZnO
144
relative charge carrier change is small when’s adsorbed gas exchanges
electrons. The high gas response of the sensor based on composite
material (Ni0.9Zn0.1O/ZnO) specifically for CO can be explained by
the formation of a heterojunction on the one hand and by the catalytic
activity of both component (ZnO and Ni0.9Zn0.1O) on the oxidation of
CO on the other hand [52-54]. This cumulative CO oxidation catalytic
activity should favor more adsorption of CO on the surface via the
active sites thus enhancing the sensor response. This will not be the
case for H2 and NO2.
Based on the fact that the heterojunction has interesting effects
on the detection of CO among the others testing gases (H2 and NO2),
further investigations on the gas sensor properties of the
nanocomposite material (Ni0.9Zn0.1/ZnO) were carried out by studying
its selectivity, dynamic response and the influence of humidity.
To this end and bearing in mind that selectivity is one of the
most problematic properties of metal oxide gas sensors, the selectivity
of Ni0.9Zn0.1O/ZnO nanocomposite with respect to CO, NO2 and H2 at
different operating temperatures was investigated. In fact, Figure 7
shows the different responses of Ni0.9Zn0.1O/ZnO to the three gases at
different temperatures. These results show that at 225°C the response
to 5 ppm of NO2 (354 %) is higher than that of 300 ppm of CO (33 %)
and 300 ppm of H2 (23 %). This implies that this sensor is at least 10.7
times and 15.4 times more sensitive to NO2 than CO and H2,
respectively, at 225°C which is indicative of its good selectivity to
NO2. This good selectivity to NO2 can be ascribed to the difference in
the optimum operating temperature for each gas. At low temperature
(225 °C) NO2 reacts more with the material than CO. In fact NO2
reacts with the sensing material in two ways: directly with free
electrons (equation 1) and adsorbed oxygen on the surface (equation
2). CO, however, reacts only with the adsorbed oxygen on the surface
145
as shown in equation 3. At low temperature, owing to the fact that the
kinetic energy of the reaction of CO with the adsorbed oxygen species
is low leading to a low oxygen desorption rate and electron release, a
decrease of the response to CO is expected while this phenomenon
does not happen for NO2.
NO2 (g) + e- → NO2
- (1)
NO2 (g) +O2-(ads)
+2e
- → NO2
− +2O
- (2)
CO + O-
(ads) CO2 + e- (3)
At 300°C and 350°C, the responses to 300 ppm of CO (194 %
and 170 %, respectively) are higher than those of 300 ppm of H2 (103
% and 65 %, respectively). This implies that at 300°C this sensor is
1.9 times more sensitive to CO than H2 while at 350°C it is 2.6 times.
Thus, even though at 300 °C the Ni0.9Zn0.1O/ZnO shows the highest
response to CO (194 %), its best selectivity to this gas relative to H2
is, however, obtained at 350°C (2.6 times).
The sensitivity of this sensor to 5 ppm of NO2 at 300 and 350
°C are 93 % and 30 %, respectively. Even though the sensitivity to
300 ppm of CO at these temperatures are 2.1 and 5.7 times higher than
the sensitivity to 5 ppm of NO2, the selectivity against NO2 is not
evident because of the difference of the test gas concentration.
However, the composite material is more sensitive to CO than ZnO
and Ni0.9Zn0.1O at this temperature as shown in figure 4. This increase
of sensitivity to CO at 300 °C is ascribed to the formation of a
heterojunction in the composite material.
From these results, we can conclude that the gas sensor material based
on Ni0.9Zn0.1O/ZnO shows a good selectivity at 225°C for NO2 and
an increased sensitivity to CO at 300 °C.
146
Figure 7: Response of Ni0.9Zn0.1O/ZnO to 300 ppm of CO, 300 ppm
of H2 and 5 ppm of NO2 at different operating temperatures.
Figure 8 shows the dynamic and reproducible response of the
Ni0.9Zn0.1O/ZnO gas sensor exposed to different concentrations of CO
at 300 °C. As can be seen, the sensor resistance reaches an equilibrium
value upon gas exposure and settles back to the original value when
the test gas is vented. This indicates a good reproducibility of the
sensor. As far the dependence of the sensor response on the
concentration of CO is concerned, it was also noticed that the response
time is limited by the reaction rate of CO with the adsorbed oxygen
species. The response of the sensor is 135, 183 and 222 for 100, 200
and 300 ppm of CO respectively. The results indicate that the gas
response increases with increasing concentration of CO from 100 to
300 ppm. Almost the same gas response was obtained after repeating
1 2 3 40
50
100
150
200
250
300
350
350 °C300 °C275 °C225 °C
Resp
on
se (
%)
CO
H2
NO2
147
the measurements one year later. The response and recovery times are
calculated to be 59 s and 370 s respectively.
Figure 8: Resistance variation of Ni0.9Zn0.1O/ZnO in the presence of
CO at 300°C (100, 200 and 300 ppm, RH = 50 %).
The effect of humidity on the performance of a sensor is very
important for practical applications. To investigate this effect, we have
measured the baseline resistance of Ni0.9Zn0.1O/ZnO at 300 °C at
different humidity levels. Figure 9 shows that in the absence of
humidity the baseline resistance has a highest value whereas with 20
% of relative humidity, this baseline resistance drops drastically to
lower values. Beyond 20 % of relative humidity, the baseline
resistance slightly decreases with the introduction of 40, 60, 80 and
100 % of relative humidity. This implies that beyond 20 % of relative
humidity the response of the sensor is no longer affected by humidity
which is interesting from the point of view of practical applications.
To further elucidate the effect of humidity on the performance of the
0 1000 2000 3000 4000 5000
10000
15000
20000
25000
30000
35000
100 ppm
300 ppm200 ppm
trec
= 370 stresp
= 59 s
Resis
tan
ce (
x1000 O
hm
)
Time (s)
148
sensor, the response of the sensor based on Ni0.9Zn0.1O/ZnO in the
absence of humidity (0 % of relative humidity) and 50 % of relative
humidity was investigated for different concentrations of CO gas at
300 °C. The results are shown in figure 10. In the former (0 % of
relative humidity) the responses are 325 %, 456 % and 702 % for 100,
200 and 300 ppm of CO, respectively. These values are, at least 2.4
times higher than those obtained in 50 % of humidity (135 %, 183 %
and 222 %) for the same gas concentrations (figure 10). This
observation can be explained as follows: firstly, in the absence of
humidity all the active sites on the surface of the sensor are occupied
by only adsorbed oxygen whereas in the presence of humidity some
active sites are occupied by water molecules. In other words, since CO
reacts with adsorbed oxygen, more oxygen adsorbed onto the surface
implies that more CO will react, thus, enhancing the sensor response.
Secondly, the drastic increase in the value of the baseline resistance in
the absence of humidity (from 32.4 MΩ in 50 % of humidity to 150
MΩ in dry air) (figure 9) implies that the decrease in the resistance in
the presence of CO gas, due to the reinjection of electrons in the
conduction band, could be more visible, thus, resulting in an increase
in the sensor response in the absence of humidity
149
Figure 9: Variation of baseline resistance of Ni0.9Zn0.1O/ZnO at
different relative humidity levels.
Figure 10: Response of Ni0.9Zn0.1O/ZnO to 300 ppm of CO at 300 °C
in the presence and in the absence of humidity.
0 20 40 60 80 100
40
60
80
100
120
140
160R
esis
tan
ce (
Mo
hm
)
Humidity (%)
Ni0.9
Zn0.1
O/ZnO
100 150 200 250 300
200
300
400
500
600
700
CO
resp
on
se (
%)
CO concentration (ppm)
0 % humidity
50 % humidity
150
IV.4.3. Gas sensing mechanism
The sensing mechanism is based on the change in resistance of
the sensor by the adsorption and desorption processes of oxygen
molecules on the surface of oxides. In general, when an n-type metal
oxide semiconductor gas sensor is exposed to air, oxygen molecules
will be adsorbed onto the surface of the sensing material and ionize
into species such as O2−, O
−, and O
2− by reducing electrons at the
valence band of the n-type semiconductor and this will form a
depletion layer on the surface. In this process, oxygen molecules act
as electron acceptors to decrease the electron concentration and, thus,
increase the resistance of the sensor. Ni0.9Zn0.1O/ZnO behaves as an n-
type semiconductor with electron carriers. Upon exposure to a CO gas
atmosphere, for example, at an appropriate temperature, these gas
molecules will react with the adsorbed oxygen ions on its surface and
release electrons, thus, decreasing the resistance of the sensor. The
reactions involved are shown in equation 3 and 4.
2CO + O2- (ads) 2CO2 + e
- (4)
The enhancement of the sensor response for CO gas in the
nanocomposite material can be ascribed to the p-n heterojunction
formation. Similar results of sensor performance improvement using
metal oxide hetero-structures have been reported by various authors
[45, 55, 56]. In fact, the creation of inner electric fields at the
semiconductor p-n interface will allow the electrons to flow from
Ni0.9Zn0.1O (with higher band gap) to ZnO while holes will flow in the
opposite direction until the Fermi levels equalizes. This creates an
electron depleted layer at the interface between ZnO and Ni0.9Zn0.1O
which bends the energy band. The formation of two depleted layers,
one at the surface of individual grains by the adsorption of oxygen
species and the other at the hetero-interface between ZnO and
Ni0.9Zn0.1O, promotes higher oxygen adsorption on the sensor surface
151
which might provide more reaction sites. Therefore, by reacting with
the CO gas, more electrons will be released back to the conduction
band, leading eventually to an enhanced response (figure 11). The
influence of the p-n junction can also be explained as follows: the
normal ambient resistance of the nanocomposite material in air (Rair)
will be higher than that of each component (Ni0.9Zn0.1O: Rair = 131 Ω,
ZnO: Rair = 750 kΩ, Ni0.9Zn0.1O/ZnO: Rair = 60 MΩ at 300 °C as
observed in figure 4(b)), due to the fact that at the contact between
both components, there is a depletion layer increasing the contact
resistance between the grains. The overall resistance of the layer is
dominated by this contact resistance which explains the drastic
increase in the resistance of the composite. When a reducing gas (CO)
is introduced, the electron releases from the CO oxidation will
decrease the holes concentration in the p-type Ni0.9Zn0.1O by
combining with them. This will result in an increase of the electron
concentration according to the law of mass action for semiconductor
(n0 · p0 = ni2). As a result, the concentration gradient of the same
charge carrier on both sides of the p-n junction is reduced. This will
leads to a decrease in the potential barrier height of the depletion layer
at the interface. The result is a large decrease in the resistance and a
corresponding increase in the sensor response compared to the
situation without a heterojunction.
152
Figure 11: Sensing mechanism
IV.5. Conclusion
A novel nanocomposite material, Ni0.9Zn0.1O/ZnO, has been
successfully obtained via the thermal decomposition of mixed
precursor compound synthesized using a facile coprecipitation route
and its gas sensing properties investigated for many gases (CO, H2 and
NO2). According to the XRD results the nanocomposite material were
an homogeneous mixture of Ni0.9Zn0.1O and ZnO, excluding the
possibility of the simple NiO/ZnO nanocomposite. Enhanced sensing
properties with respect to CO were observed by comparison with
those of pure Ni0.9Zn0.1O and ZnO structures and the optimal
operating temperature was found to be 300 °C. It was also found that
the as synthesized composite material can be used at lower operating
temperature (225°C) as selective NO2 gas sensor. The humidity has no
significant influence on the response of sensor beyond 20 %. The
heterojunction between Ni0.9Zn0.1O and ZnO is probably the principal
factor responsible for the enhanced gas sensing performance of the
composite material.
153
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GENERAL CONCLUSION
161
I. Main results
Mixed metal oxides are promising systems for sensitive and
selective metal oxide gas sensors and also for other applications. To
develop a simple and effective method for the controlled synthesis of
this kind of system is a great challenge for the scientist. In this work,
the synthesis by coprecipitation method, the characterization by a
variety of techniques and the sensing properties of Zn doped NiO (Ni1-
xZnxO) and Ni1-xZnxO/ZnO nanocomposites have been investigated.
Even though the number and the distribution of p-n junctions are not
well controlled by this synthesis method, the results obtained were
very interesting. Based on our study the following conclusions can be
drawn:
- Single batch precursors (nickel zinc malonate) with different
Ni/Zn ratio have been successfully synthesized by a simple
and versatile co-precipitation method, using malonate as a
precipitating agent. The pH, the temperature and the time of
the reaction were found to influence the desired products. The
nickel zinc malonate precursors were obtained at a pH value
equal to 7, at 90 °C in 3H.
- The characterization by FTIR, ICP-AES and TGA
demonstrates that the as-synthesized precursor is a
homogeneous mixture of nickel malonate and zinc malonate in
the expected Ni/Zn ratio. It was found that the presence of Zn
influences the dehydration of the precursor. The precursors
decompose at around 360 °C in air to give the corresponding
metal oxide.
- The decomposition product was fully characterized by FTIR,
XRD, ToF-SIMS, XPS, SEM and TEM. FTIR shows only one
absorption band corresponding to the metal-oxygen bond, and
no additional band was observed, which indicates that the
162
decomposition of the precursor was complete. The crystalline
phase was identified by XRD, and it was found that for less
than 20 % of Zn, one single phase is formed, which can be
attributed to NiO rocksalt cubic structure. No additional phase
was observed. For Zn percentages equal or greater than 20 %,
two different phases were identified, corresponding to NiO
rocksalt cubic and ZnO hexagonal structures. The increase of
lattice parameter of NiO observed by XRD, the new fragment
(NiZn+) identified in the ToF-SIMS spectra and the binding
energy of Zn2+
found by XPS indicate without doubt that the
zinc substituted nickel in the NiO cubic structure during the
synthesis. The SEM and TEM analyses showed that the pure
NiO had a polycrystalline spherical morphology, while in the
presence of zinc we observed undefined morphology.
- We demonstrated that by modifying the precursor (nickel
malonate) by a suitable surfactant (oleylamine), one can
reduce considerably the size of final oxide nanoparticles, as
observed by SEM and BET.
- The gas sensor properties of Zn doped NiO was fully
investigated and it was found that, the best sensitivity
enhancement for the CO gas was obtained at 2 % of Zinc. This
increase of sensitivity was attributed to the higher affinity of
zinc with oxygen than nickel, which changes the layer
conductivity on the one hand and leads to the adsorption of
more oxygen on the surface of the material on the other hand.
These effects influence directly the sensor properties. The
selectivity to CO was also significantly enhanced with the 2 %
Zn doping. Therefore, the doping by zinc generally improves
the sensor performance of NiO.
- The gas sensor properties of the composite materials were
investigated and one showed a better sensing performance than
163
the others (with 60 % of zinc). An exhaustive study of the gas
sensor properties of this specific composite material revealed
that at high temperature (300 °C) the sensitivity to CO is better
for the composite material than that of its components (ZnO
and Ni0.9Zn0.1O). This observation was attributed to the
presence of p-n heterojunctions in the composite material. At
lower temperature (225 °C) the composite material was
showed a good selectivity to NO2. It was also found that the
sensor performance was not affected by humidity beyond 20 %
of it, which is good for practical applications. This sensor can
be used at lower temperature to detect NO2 and at high
temperature to detect CO.
II. Perspectives
In this work, we have used malonate as precipitating agent to
synthesize a nickel zinc mixed metal oxide system by co-precipitation
follow by thermal decomposition in air. The gas sensor properties
were also investigated. Although interesting results have been
obtained, further research could be undertaken.
- To understand the effect of the precipitating agent for the
synthesis of this nickel zinc oxide system, another
precipitating agent like oxalate, acetylsalicylate, might be used
to see if we can reach the optimum conditions for this
synthesis. By changing the ligand, the decomposition
temperature could be reduced and the solubility limit could be
increased for the same decomposition time and temperature
with a possible influence on the morphology of the particles.
- The crystallization of the precursor can give us more and
precise information on the exact structure of this precursor.
Therefore finding a good solvent and adequate condition for
164
the crystallization of these precursors should constitute an
interesting future study.
- The presence of Ni and Zn in the same nanoparticle was
successfully proven by ToF-SIMS, in agreement with XRD. If
we have 3 metals like Ni, Zn, and Co in the same particle it
could be interesting to investigate by ToF-SIMS, their
presence in the same particle.
- The investigation of the effect of Zn doping on the gas sensor
properties of NiO for Volatile Organic Compounds (VOCs)
and precisely the formaldehyde (HCHO) which is a very
harmful indoor gas, could be interesting since the NiO is
known as a good sensor for formaldehyde.
- The deposition of the metal oxide thick film by other
techniques (more reproducible) like spray coating and inkjet
printing could be investigated for the suitable industrial
application.
165
166
ANNEX
Response of different composite materials to 300 ppm of CO (a), 300
ppm of H2 (b) and 5 ppm of NO2 (c) at different operating
temperatures.
150 200 250 300 350 400
1,0
1,5
2,0
2,5
3,0
CO
se
nsitiv
ity (
%)
Temperature (°c)
60/40 (Ni/Zn)
50/50 (Ni/Zn)
40/60 (Ni/Zn)
150 200 250 300 350 400
1,0
1,2
1,4
1,6
1,8
2,0
2,2
H2 s
en
sitiv
ity (
%)
Temperature (°C)
60/40 (Ni/Zn)
50/50 (Ni/Zn)
40/60 (Ni/Zn)
150 200 250 300 350 400
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
NO
2 s
en
sitiv
ity (
%)
Temperature (°C)
60/40 (Ni/Zn)
50/50 (Ni/Zn)
40/60 (Ni/Zn)
(a) (b)
(c)
