Synthesis and characterization of doped/undoped/co-doped ZnO nanoparticles prepared by

different techniques - Gas sensor/ Photocatalytic applications:

ZnO nanoparticles has been prepared by different techniques such as sol gel spin and dip

coating, thermal evaporation method, doctor blade technique, simple heat treatment method,

simple combustion method and rf magnetron sputtering technique. Herein synthesis of

nanoparticles has been done by varying the different parameters such as annealing

temperature, aging time, doping elements (Al, In, Sn, Sb, Ni, Er, Ce ), stabilizer, solvents, pH

value, codoping (In-Sn) and rare earth doping (Er, Ce, Eu, Nd) elements by varying atomic

weight % etc. and their characteristics has been studied (FTIR, FESEM, XRD, UV VIS,

DTA, LCR, RAMAN, Photoluminscence). On the basis of good optoelectronic properties the

prepared nanoparticles has been studied for gas sensor and photodegradation applications.

Sol gel spin coating technique:

Zinc nitrate hexahydrate with different molar concentrations

(0.20M,0.25M,0.10M,0.15M,0.05M,0.02M) as solute, dissolve this in different solvents as

methanol, 2 methoxy ethanol, ethanol and isopropyl alcohol at room temperature. Di

ethanolamine (DEA)/ Mono ethanolamine (MEA) used as stabilizer. The molar ratio of

stabilizer to zinc acetate was kept at 1.0. The resulting solution was stirred at 60-800c for 0.5/

1 hr. At last transparent solution (ZnO) has been observed. Different atomic weight

percentage of various types of doping can be added.

Zinc oxide films have been prepared by spin coating method on glass substrate, which

was cleaned thoroughly and dried. The spin coating time is 30 sec in the beginning 10sec,

spin speed was 1000rpm and in later 20 sec spin speed was 2000 rpm. After each coating

samples were dried at 80- 1000c for 10 min and pre annealed it at 300-400

0c for 10-15 min.

The procedure from spin coating to pre annealing was repeated for several times to make

desired thickness. At last all the samples post annealed at 5000c for an hour.

Simple heat treatment method:

ZnO thin films and powder has been prepared using thermal evaporation technique (deposit

on glass substrate) and simple Combustion method. Fig. shows the systematic scheme of

ZnO nanoparticles prepared by simple heat treatment and thermal evaporation method.

0.17M of Zn acetate dihydrate dissolve in 10 ml triple deionised water and then stirrer it at

700C for 30 min. After stirring the mixture, solution was filtered with whatman filter paper

(No. 1) and transferred into the crucible, placed in the muffle furnance at 4000C for 10-20

min. During heating, spongy like material is obtained. Then anneal it again for an hour at

5000C for an hour. Finally white ZnO powder is obtained. And Al as doping element is used

by varying different annealing temperature (125-2250C). and their different characteristics

has been studied.

Thermal evaporation Technique:

ZnO thin films and powder has been prepared using thermal evaporation technique (deposit

on glass substrate) and simple Combustion method. Fig. 1 (a). shows the systematic scheme

of ZnO nanoparticles prepared by simple heat treatment and thermal evaporation method.

0.17M of Zn acetate dihydrate dissolve in 10 ml triple deionised water and then stirrer it at

700C for 30 min. After stirring the mixture, solution was filtered with whatman filter paper

(No. 1) and transferred into the crucible, placed in the muffle furnance at 4000C for 10-20

min. During heating, spongy like material is obtained. Then anneal it again for an hour at

5000C for an hour. Finally white ZnO powder is obtained. Fig. 1(b). shows the experimental

set up of ZnO films prepared by thermal evaporation technique in two zone split furnance.

For this, prepared ZnO nanoparticles from simple heat treatment are taken as source and glass

slide as substrate by maintain source temperature at 8000C and substrate at 400

0C for an hour.

Fig. Systematic scheme of ZnO nanoparticles prepared by simple combustion and thermal

evaporation method.

Doctor blade Technique

In order to develop nanoscale gas sensing device, doctor blade technique is used. These

nanoparticles were dispersed in deionised water to prepare the slurry for the fabrication of

nanodevice. The prepared paste was then coated on substrate (glass, silica, sapphire) by using

glass rod. For sensing device assembly pair of Au electrodes on the top of its surface and

microheater on its bottom surface is arranged. Synthesized nanoparticles were assembled on

the substrate by connecting Cu wires using silver paste to Au electrodes. To measure the gas

sensing properties static gas sensing set up is used.

Rf Magnetron Sputtering

ZnO based thin film has been prepared by rf magnetron sputtering technique by using

different substrates. Prior to the deposition, substrate was cleaned with distilled water,

followed by isopropyl alcohol and methanol in ultrasonic cleaner for 10 min, dried in

nitrogen gas. Sputtering system (IISC Bangalore) has ZnO target (99.99%) pure for 2 inch

diameter was sputtered in argon atmosphere for about 200 second to remove the surface layer

of target before deposition of film. Magnetron sputtering rf power of frequency 13.5 MHz,

the base pressure in the chamber prior to the deposition was maintained at 5x10-6

Torr,

controlling by pirani gauge, distance between target and substrate is 7.5 cm. The rf power

during the growth was kept constant at 100 Watt. This deposition was done under argon gas

flow of 200 sccm. After the deposition films were annealed at 6000C for an hour. Different

thickness of the films has been deposited by varying the deposition time.

Structural Properties:

XRD spectra corresponds to different molar concentration of doped/undoped by varying

annealing temperature is carried out and was found the preferential orientation of all the sol

gel spin coated films were along (002) plane but aluminium doped ZnO films for 0.15 M by

heat treatment shows decrease in intensity of (002) plane as compared to spin coated ZnO

films Fig. In case of spin coated films, the mixture first stirred for 1 hr at 70°C and then

cooled and this cycle was repeated two times and it was observed that intensity of plane (002)

was also found to decrease when the temperature was brought down by keeping the mixture

for I hr at 10°C and then again magnetically stirred at about same temperature (70°C) for 1hr

as shown in Fig shows the prominent peaks when the film was not quenched to lower

temperature before coating, but annealed at 4500C after coating.

The presence of prominent peaks showed the polycrystalline nature of films. This may be due

to the fact that when the mixture was suddenly quenched to lower temperature and then

heated again, the crystallanity of the films might get deteriorated due to formation of stress

due to varied temperature conditions.

XRD reveals that the most prominent peak is 101 and the crystal size lies in the range of 20-

75 nm. Which can be determined from scherrer formula

XRD spectra corresponding to annealing temperature from 400-600˚C were carried out and

was found that preferential orientation of all films were along (101) crystal plane at 2Ɵ =

36.48° at different annealing temperature. This shows that these films prepared by simple

combustion method are polycrystalline in nature and show a good c-axis orientation.

20 30 40 50 60 70 80

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Structural property of Er- ZnO nanoparticles

To examine crystallinity, the prepared Er- ZnO nanoparticles were characterized by X Ray

Diffraction (XRD). Fig. 2 shows the XRD pattern of prepared Er doped nanoparticles. All the

observed pattern shows the diffraction planes 2θ = 31.80˚, 34.44˚, 36.22˚, 47.59˚, 56.67˚,

62.91˚, 66.43˚, 68.09˚, 69.15˚and 77.04˚corresponding to hexagonal wurtzite ZnO planes of

(100), (002), (101), (102), (110), (103) (200), (112), (201) and (202) respectively. All the

prepared samples shows most prominent diffraction peak at 36.22˚. A weak reflection at

28.35˚ attributed to (222) plane for Er2O3 crystalline structure [9]. These observed diffraction

planes well matched with standard card number (JCPDS 36 -1451). The lattice spacing was

calculated from Bragg’s formula

2d sinθ = nλ

Where d is lattice spacing and θ is angle of incidence, λ is wavelength and n is diffraction

order.

Crystal size can be found from Scherrer’s formula

Where D is crystalline size, K is constant (0.92), λ = 0.154 nm, mean wavelength of CuKα1

radiation, β is full width half maxima and θ is Bragg’s angle in radians. Fig. 2 shows the

intensity of (101) peak increases, indicating preferential orientation along c axis.

The value of lattice parameters a and c for Er doped ZnO nanoparticles were

calculated using the following equation

(1)

Where h,k and l are the Miller Indices, d is interplanar distance and a, c are lattice

parameters.

D is the density and it was calculated by using formula

D =

Table 1 shows the density of Er doped ZnO is double than ZnO (JCPDS 36-1451). Thus Er

doping has been demonstrated by the modification of lattice parameters in comparison with

pure ZnO. XRD peaks reveal a small shift towards higher angle which shows the variation in

lattice parameters. After a certain limit of doping concentration non uniform distribution of

dopant ions has been found. This shows the importance of lattice space distribution of Er in

the ZnO:Er.

Strutural properties of In- Sn co-doped ZnO nanoparticles by varying Aging time

The crystallinity of In-Sn co-doped ZnO nanoparticles (A1-A3) was analyzed by the

X- Ray Diffraction (XRD) method (Xpert Pro with CuKα, nickel metal is used asbeta filter).

Figure 2 shows the typical XRD pattern of prepared In-Sn codoped ZnO nanoparticles for

A1-A3. All the observed pattern shows the diffraction planes 2θ = 31.60˚, 34.54˚, 36.32˚,

47.49˚, 56.37˚, 62.71˚, 66.53˚, 68.18˚, 69.17˚, 77.04˚ corresponding to hexagonal wurtzite

ZnO planes of (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202)

respectively. All the prepared samples shows most prominent diffraction peak at 36.32˚.

These observed diffraction peaks are well matched with standard card number (JCPDS 36-

1451). However, compared to their result, crystallization nature in this study is much better

due to several peaks observed in XRD spectra.

FIG.2 XRD spectra of codoped ZnO nanoparticles (a) 0 hrs (b) 24 hrs and (c) 36 hrs.

The lattice spacing are calculated from Bragg’s formula

2d sinθ = nλ

Where d is lattice spacing and θ is angle of incidence, λ is wavelength and n is the diffraction

order.

Aging Time (hrs) Crystal Size (nm) Along diffraction plane

(100) (002) (101)

0 32.00 21.70 27.25

24 30.75 21.11 26.15

36 29.95 19.35 25.85

Table I. Crystal size of ZnO nanoparticles for (A1-A3).

Crystal size can be found from scherrer’s formula

Aging Time (hrs) Crystal Size (nm) Along diffraction plane

(100) (002) (101)

0 2.817 2.602 2.476

24 2.815 2.603 2.477

36 2.814 2.601 2.479

Table 2. Lattice spacing of ZnO nanoparticles for A1-A3.

Morphological Study:

Fig. shows the micrographs of ZnO crystallites prepared under different conditions and

techniques. Fig. shows the spin coated films on glass substrates. Here in this case before

spin coating the mixture were first stirred for 1 hr at 70°C and then cooled for I hr at 10°C

and then again magnetically stirred at about same temperature (70°C) for 1hr, the observed

crystallite width size was of order of 60-70 nm and also observed that few crystallites have

oriented more along length wise having size of order of 200-250nm whereas the crystallites

in which this process is not carried out the observed crystallites were of order of 110-130 nm

and more regular crystallites densely packed were observed. shows the crystallites of order of

40-50 nm for 0.15M concentration by heat treatment with the content of 1.8% Al as dopant

followed by annealing and it is observed that the size is more as compared to undoped. The

results obtained are in well agreement with XRD results. It is also observed that more ordered

and dense crystallites are observed in case of Aluminium doped under annealing.

FESEM images reveals that the grain size is of the order of nanometer. Different

morphology is observed which is due to the variation in annealing temperature. The size of

grain increases with increase in temperature.

It is evident from the FESEM images that the prepared materials possessed

nanosphere shape having granular nature. It can be clearly seen from Fig. shows that samples

consist of ZnO nanostructures. The average width and length of ZnO nanoparticles are found

to be 18.7 nm and 20.1 nm for isopropyl alcohol and 19.8 nm and 21.7 nm for triple

deionized water. The typical size of nanoparticles is in the range of 40-50 nm.

Fig. 3 FESEM images of In-Sn co-doped ZnO nanoparticles.

Figure 3 shows the FESEM (FESEM- JSM6100 (Jeol)) images of pure and In – Sn co-doped

ZnO nanoparticles (A1-A3). Figure 3(a) shows morphology of pure ZnO. It is observed that

ZnO nanoparticles become more uneven with aging time. As for A1(0hrs) sample surface is

relatively rough and non uniform grain size is observed. However, surface roughness

decreases with increase in aging time. It is clear from figure 3 (b,c) that codoped ZnO

nanoparticles produce almost uniformly distributed rectangular shaped crystallites. Non

uniform circular rod shapes nanoparticles are observed in figure 3 (a,d). These different

Morphologies is greatly influenced by the time rate of photocatalytic activity. In the

photpcatalytic study rectangular shaped nanoparticles shows best photocatalytic efficiency

which is due to the large surface area to volume ratio of rectangular shaped ZnO

nanoparticles. This indicates that 24 hrs aging time is the ideal for preparation of ZnO based

nanoparticles. Aggregated rod shapes additive ZnO nanoparticles are observed with average

diameter 40- 90 nm. Large size is observed in case of pure ZnO nanoparticles. The average

width and length of codoped ZnO nanoparticles are found to be of the order of 19 nm and 21

nm for A1 (0hrs), 18 nm and 20 nm for A2 (24 hrs), 22 nm and 25 nm for A3 (36hrs) (Table

3). These results show that the synthesized nanoparticles have relatively rough surface and

non uniform grains. With the increase in aging time sol get more homogenous and stable.

Accordingly, the quality of prepared nanoparticles gets improved.

FTIR Characterization:

To detect the presence of functional group and phase transformation, IR spectra of samples

were taken in the transmittance mode. It gives information about the way in which molecules

are bonded.

3100—3450 Cm-1

OH mode

2400-3100 Cm-1

O-C-O

1020-1070 Cm-1

C=O

400-500 Cm-1

ZnO

FTIR study of Er-doped ZnO nanoparticles

UV VIS Spectra (Transmittance, Absorbance, Optical Band gap)

According to Beer Lambert’s Law

A= log (I0/I)= e.c.l,

Where, A is absorbance, I0 is the intensity of incident light at given wavelength, I is

transmitted intensity, L is path length, c is concentration of the solution and e is molar

absorbtivity. molar absorbtivity.

The absorption peaks in Fig. showed the strong absorption peak in the wavelength range

350-390 nm. Due to electron transitions from valance band to conduction band this can be

assigned to intrinsic band gap absorption of ZnO. From absorbance spectra it is clear that

absorbance is proportional to the concentration of the solution as described by Beer

Lambert’s law. According to Beer Lambert’s law, high absorbance means high concentration

of ZnO nanoparticles. Beer Lambert’s law states that

A= log (I0/I)= e.c.L

Here A is the absorbance. I0 is the intensity of incident light at given wavelength, I is

transmitted intensity, L is path length, c is concentration of solution and e is molar

absorbtivity.

The optical absorbance spectrum in Fig. showed that there is increase in absorbance with

annealing temperature. This above spectrum shows that ZnO crystals have a low absorbance

in the visible region, which is a characteristic of ZnO. The absorbance coefficient for direct

transition semiconductor is related to the optical band gap (Eg)

αhν = A(hν-Eg)1/2

where hν is photon energy, Eg is optical band gap, A is constant. Fig. Shows (αhν)2 plotted as

function of photon energy (eV) for ZnO. The extrapolation of straight line to αhν = 0 gives

the value of direct optical band gap. It is observed that optical direct transition lies in the

range of 3.00-3.27eV. analyzing the optical band gap data it is clear that band gap decreases

with increase in molar concentration of the solution. This decrease in band gap is due to the

stress relaxation. The decrease in stress with increase in molar concentration is attributed to

the densification and an increase in oxygen contents which reduces stress and relax optical

band gap.

Based upon the absorbance spectra optical band gap of the prepared ZnO nanoparticles can

be calculated from the equation.

Eg

=

Here λ is the maximum wavelength of well defined absorbance peaks.

The absorbance spectra of these films (for different aging time) were measured in the range

of 200-750 nm. “Figs” showed the strongest absorbance peak in the range of 350-380 nm.

The transmittance spectra can be found from absorbance spectra by using relation

Absorbance = 2 – log10 T (1)

The transmittance of the sample can be defined as the ratio of photon that passes through the

sample over the incident number of photons. The UV absorption peak and Peak shifts from

361 nm to 358 nm were observed due to size difference.

Table 1. Effect on optical band gap with different molar concentration

Molar concentration Technique Used Optical band gap (eV)

0.10M Sol Gel 3.07

0.10M Simple Heat Treatment 3.21

0.15M Sol Gel 3.02

0.15M Simple Heat Treatment 3.02

0.15M Simple Heat Treatment a) 3.02, b) 3.16, c) 3.27

Electrical Properties:

To study the semiconductor in the field of electronics various methods are proposed by

different researchers. Fig. showed the variation in capacitance and applied frequency

(0.2MHz- 3.6MHz). Low value of capacitance is observed at high frequency. This is due to

the fact that more value of capacitance resulting from interface states in equilibrium with

ZnO nanoparticles can follow the ac signal. It has also observed that the more value of

capacitance has been observed in case of thermal evaporation technique in comparison with

simple heat treatment method.

Variation of capacitance with frequency of ZnO nanostructure.

Many semiconductors has been investigated to study their utilization in the field of

electronics, by using different solvents the study of capacitance and dielectric constant as

function of frequency is one of the convenient method.

The value of dielectric constant can be found from the formula

C = (€0 €r A)/d .

Where C is capacitance, €0 is dielectric constant, A is the area of circular pellet and d is its

thickness. It was observed from the Fig that the dielectric constant decreases with frequency.

This variation with frequency is due to charge transport relaxation time.

Thermal properties:

Thermal analysis starts from room temperature up to 10000C at the heating rate of 5

0 C/ min

in the presence of nitrogen gas. The associated peak near 1500C is associated with OH group.

An exothermic peak near 3500C is related to crystallization of ZnO. These results confirm

that the prepared ZnO films have good thermal stability, which can be used for the electronic

applications such as for the fabrication of solar cell and gas sensor.

Thermal analysis of the ZnO has been carried out from DTA. This analyses start from

room temperature up to 10000C at the heating rate 100C/ min in the presence of nitrogen

gas. Fig. showed exothermic and endothermic peaks. The exothermic peak appearing in

the range 90-1100C is attributed to the loss of absorbed water. Exothermic peak near around

300- 3400C is due to decomposition and crystallization of the precursor. An exothermic peak

in the DTA curve up to 6500C which is due to Wurtzite phase and possibly related to the

crystallization of ZnO. These results confirmed that ZnO composite have good thermal

Stability. This gives the information that ZnO nanoprticles are helpful for the fabrication of

solar cell based devices.

Gas sensing property:

Fig. shows the response of doped/ undoped ZnO films to different ethanol vapour gas

concentrations. It is observed from the figure that the response of both the samples was

enhanced with gas concentration and more response is observed in case of Al doped ZnO. At

various concentrations of ethanol gas very less change in response and recovery time. The

calculated value of response and recovery time is 15s and 20s. The rapid response and

recovery time of the sensor reveals it has potential applications.

Photocatalytic applications

Fig. represents the absorbance spectra recorded from 200-650 nm wavelength for DR-

23 dye solutions irradiated at different time interval in the presence of UV radiations and

ZnO photocatalyst. A continuous decrease in the absorbance intensity clearly confirms the

fact that as synthesized ZnO nanoparticles are acting as photocatalyst for the degradation of

the dye under UV irradiations. A/Ao value almost approaches to zero after 60 min of UV

irradiation of the aqueous suspensions of DR-23 dye and ZnO photocatalyst.

On the basis of different solvents used for the fabrication of ZnO nanoparticles, above

mentioned detailed reveals that nanoaprticles which was prepared by TDW as solvent has

better properties than IPA. So the synthesis by using deionized water used for the

photocatalytic application for DR 23 dye. Fig. (a). represents the absorbance spectra recorded

from 200-650 nm wavelength for DR-23 dye solutions irradiated at different time interval in

the presence of UV radiations and ZnO photocatalyst. A continuous decrease in the

absorbance intensity clearly confirms the fact that as synthesized ZnO nanoparticles are

acting as photocatalyst for the degradation of the dye under UV irradiations. A/Ao value

almost approaches to zero after 60 min of UV irradiation of the aqueous suspensions of DR-

23 dye and ZnO photocatalyst (Fig.(b)). The corresponding percentage photocatalytic

degradation of DR-23 dye as a function of UV irradiation time is shown in Fig. (c) Complete

degradation (100%) of the said dye was observed within 60 min of UV irradiations in the

presence of ZnO nanoparticles.

Langmuir– Hinshelwood kinetic model was used to investigate the kinetic studies of

the photodegradation process (Eq 3) is applied.

Where Co = initial concentration of the DR-23 dye and C = the concentration of DR-23 dye at

irradiation time ‘t’.

When graph between C

Cln o and irradiation time ‘t’ gives a straight line passing through

origin as shown in Fig. d. The slope of the line passing through the origin gives the rate

constant k for the photocatalytic process. Pseudo first order rate constant ‘k’ for the

photodegradation of DR-23 dye was found to be 0.0631 min-1

with correlation constant (R2)

of 0.99596.

Fig. (a) UV–Vis absorbance spectra (b) variations of A/Ao (c) percentage degradation of DR-

23 after different time intervals of UV irradiation and (d) pseudo-first order rate kinetics for

the photodegradation of DR-23dye.

Photocatalytic activity of Er-doped ZnO nanoparticles

Fig. 1 shows the systematic designed photoreactor with water circulating unit (to maintain

constant temperature) and an opening for O2 supply and another opening for withdrawing the

sample after regular interval of time. A 125 W UV lamp was used as source for UV

irradiation. This photocataytic experiment was used for evaluating the photocatalytic

applications of Er doped ZnO nanoparticles against DR 31 dye. A 100 ml aqueous solution of

ktC

Cln o

35 ppm DR 31 dye was prepared in deionised water. 0.080g of Er- ZnO nanoparticles was

suspended in to the dye solution as photocatalyst. Prior to UV irradiation, the resulting

suspension was ultrasonicated for 35 min for proper homogeneity of photocatalyst as well as

to maintain adsorption desorption equilibrium. A 5 ml of sample solution was taken out from

photoreactor after a regular interval of time followed by centrifugation at 3000 rpm for 10

min in order to remove ZnO suspension from the solution. Percentage degradation was

calculated using eq. 1

Percentage degradation =

(1)

where, Ao is initial absorbance of dye and A is absorbance of dye solution after UV light

irradiation.

3.2. Photocatalytic degradation of prepared Er-doped ZnO nanoparticles

For the photocatalytic applications, DR-31 (35 ppm) was chosen as target dye for Er doped

ZnO nanoparticles. Photocatalytic degradation was performed under UV light. Fig. 6

represents absorbance spectra recorded from 300-750 nm for DR-31 dye under regular

interval of time. Er-ZnO as photocatalyst plays a vital role for photodegradation application,

which is mainly due to adsorption of dye molecules. Decrease in absorbance intensity clearly

confirmes that Er doped ZnO are acting as photocatalyst for the degradation of dye. A strong

absorption band at 509 nm represents maximum wavelength for DR-31 dye. Fig.6 shows the

devreasing trend of absorption for each Er doped ZnO photocatalyst. Complete degradation

of dye was observed in 60 min under UV irradiation. Light excitation causes some photo

stimulated elsto optic effects [21]

Fig 7 (a) shows a corresponding plot between relative change in absorption

intensity (A/A0) and irradiation time by varying Er concentration. From detailed experiment,

it was observed that Er doped ZnO nanoparticles with 2.5% doping element shows drastically

decrease in absorbance with increase in irradiation time. Fig 7 (b) represents variation in

percentage photodegradation with irradiation time. With increasing Er doping concentration

up to 2.5% there is significantly increase in percentage degradation. However, with increase

in further concentration the percentage (3.0%, 3.5%) degradation was decreased. The

obtained results confirmed that 2.5% of Er into ZnO is important for complete degradation

(99.1%) in 60 min of DR-31 dye. Thus EZ2 is the best composition for the photodegradation

of said dye. Table 2 represents Er doped ZnO nanoparticles exhibits better photocatalytic

performance as compared to other Er- ZnO as photocatalyst reported in literature.

3.3. Kinetic study of photocatalytic degradation using Er doped ZnO nanoparticles

The kinetic study for Er doped ZnO nanoparticles (photocatalyst) for photodegradation of

DR-31 dye was studied by using Langmuir – Hinshelwood Kinetic model [22] by eq. 3.

These all observations are under UV irradiations.

ln

(3)

Where C0 is initial concentration of DR-31 dye and C is concentration of DR- 31 dye at

irradiation time ‘t’. K is Pseudo first order rate constant.

Fig. 8 exhibits plot of ln (Co/C) vs irradiation time and it has been found that for

degradation of DR-31 dye for each sample obeyed pseudo-first order reaction kinetics. Fig. 9

represents the variation of rate constant for Er doped ZnO nanoparticles (EZ1-EZ4). It is

clearly observed from the bar diagram that pseudo-first order rate constant is maximum for

EZ2 sample as compared to other Er-doped ZnO nanomaterials. Linear regression coefficient

(R), (R)2 half life time and rate constant was summarised in Table 3. The maximum value of

K for the said dye was found to be 0.07010 (min)-1

and half life time was minimum for EZ 2.

These studies confirmed that different concentration of Er is responsible for the

photodegradation of dye.

In the photocatalytic activity, DR-31 dye molecules are physiorbed or

chemisorbed onto the surface of ZnO nanoparticles. Er doped ZnO is expected to increase

surface defects, which is responsible for the enhancement of photocatalytic efficiency. Er

ions can also act as effective electron scavenger to trap the conduction band electrons, which

reduce the probability of electron hole recombination. This means that Er ions on the surface

of ZnO nanoparticles may act as scavenger. In this process, the reactive species are O2-, HO2 ,

H2O2. In order to find responsible species for degradation of dyes, this scavenger study was

performed. For Er doped nanoparticles, the Er ions incorporated in ZnO are responsible to

absorb excited electrons from conduction band of ZnO. At higher Er concentration, the

number of free electrons decreases, this causes decreasing photdegradation.

Absorption spectra vs wavelength for photocatalytic degradation of DR doped ZnO as photocatalyst:

EZ1, EZ2, EZ3 and EZ4.

Percentage degradation of DR-31 dye.

Pseudo-first order kinetics for DR respectively

Gas sensing application for Sn doped ZnO nanoparticles

Sn doped ZnO nanoparticles were characterized in standard gas sensing set up wherein a particular

amount of host gas was injected into the chamber with air as background. Before exposure to organic

gas, oxygen atoms are adsorbed into ZnO surface; it takes electrons from surface and become O-

(release oxygen). This O- ion helps to create the depletion layer on the host surface. Addition of Sn

help to completes the ZnO structure. This allows more oxygen to be adsorbed, which can enhance the

response.

Fig. 8 clearly shows that variation in temperature increased the gas sensor response, which

increased sensitivity. The ethanol sensing mechanism of the sample is explained as follows.

Adsorption is a surface defect. It forms the ionic species (O2-

and O-) on sample surface. kinetic

reaction before and after the ethanol exposure is described in equations (3)-(5) [29].

O2 (gas) O2 (ads) (3)

O2 (ads) + e- O2

- (ads) (4)

O2- (ads) + e

- 2O

- (ads) (5)

Fig. 8 shows the sensor’s response to ethanol and acetone with variation of operating temperature at

300˚C, 400˚C and 500˚C respectively. These measurements help to indentify the operating

temperature of sensor device for exposure of different gases. This figure shows good linearity as a

function of operating temperature for ethanol and acetone gas. It has clearly seen from Fig.8 that the

2.0% (SZ3) Sn dopant yields the best sensing response at 400˚C.

Table 2 shows that for all the doping concentrations, highest sensitivity was observed at an

operating temperature of 400˚C. Sensitivity response has almost same value for variations in working

temperature. This phenomenon indicates that same response may be expected for other organic gases.

Fig. 9 shows the sensitivity of Sn doped ZnO based sensor towards ethanol and acetone gas at

different operating temperatures (300˚C, 400˚C, 500˚C). The sensitivity at 5% volume of ethanol and

acetone gas concentration was 86.8% and 84.4% respectively. The response and recovery time was

found to be 22s and 31s for SZ3 (ethanol). This figure reveals that at exposure of gases, sensitivity of

sensor increased upto 400˚C and thereafter it decreased which is due to the fact that at low

temperature, a low response of sensor is observed because the gas molecules do not have different

thermal energy to react with absorbed oxygen species. Comparison of gas sensing properties is

observed in Table 3. The increasing sensitivity has positive correlation with Sn dopant up to SZ3 (2%

Sn) and after that it decreases. This result has positive correlation with XRD, FESEM and UV Vis

spectroscopy results as described above. Therefore optimum amount of SZ3 (2 at.wt%) Sn yields to

maximum sensitivity.

Fig. 8. Sensor response to 5% volume of ethanol (E1-E3) at 300-5000C and acetone (A1-A3)

at 300-500˚C for SZ1-SZ4 respectively.

0 500 1000 1500 2000 2500 3000

0

10

20

30

40

50

60

70

80

90

Temp 4000C

Re

sp

on

se

(%

)

Time (s)

SZ 1

SZ 2

SZ 3

SZ 4

AcetoneA2

0 500 1000 1500 2000 2500 3000

0

10

20

30

40

50

60

70

80

Temp 3000C

Re

sp

on

se

(%

)

Time (s)

SZ 1

SZ 2

SZ 3

SZ 4

Ethanol

E1

0 500 1000 1500 2000 2500 3000-10

0

10

20

30

40

50

60

70

80

90

Temp 4000C

Re

sp

on

se

(%

)

Time (sec)

SZ 1

SZ 2

SZ 3

SZ 4

EthanolE2

0 500 1000 1500 2000 2500 3000-10

0

10

20

30

40

50

60

70

80

Temp 5000C

Re

sp

on

se

(%

)

Time (s)

SZ 1

SZ 2

SZ 3

SZ 4

EthanolE3

0 500 1000 1500 2000 2500 3000

0

10

20

30

40

50

60

70

80

Temp 3000C

Re

sp

on

se

(%

)

Time (S)

SZ 1

SZ 2

SZ 3

SZ 4

AcetoneA1

0 500 1000 1500 2000 2500 3000

0

10

20

30

40

50

60

70

80

90

Temp 5000C

Re

sp

on

se

(%

)

Time (s)

SZ 1

SZ 2

SZ 3

SZ 4

Acetone

A3

Fig. 9. Sensitivity of Sn doped ZnO based sensor at different temperature for SZ1-SZ4 towards

ethanol and acetone gases.

Temp (0C) Response (%)

Ethanol Acetone

SZ 1 SZ 2 SZ 3 SZ 4 SZ 1 SZ 2 SZ 3 SZ 4

300 60 67 72 45 42 64 74 51

400 64 71 86 53 61 69 83 50

500 55 63 72 42 57 64 45 43

Table 2 Response of sensors towards Ethanol and Acetone.

300 350 400 450 50040

50

60

70

80

90

S

en

sit

ivit

y (

%)

Temperature (0C)

SZ 1

SZ 2

SZ 3

SZ 4

Ethanol

300 350 400 450 50020

30

40

50

60

70

80

90

Se

ns

itiv

ity

(%

)

Temperature (0C)

SZ 1

SZ 2

SZ 3

SZ 4

Acetone