Advanced Powder Technology 21 (2010) 609–613

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Advanced Powder Technology

journal homepage: www.elsevier .com/locate /apt

Original Research Paper

Synthesis, characterization, electrical and sensing properties of ZnO nanoparticles

AK Singh *

Defence Institute of Advanced Technology, Girinagar, Pune 411025, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 December 2009Accepted 2 February 2010

Keywords:ZnONanofluidsPhotoluminescenceUV–VISImpedance spectroscopyThermal conductivity

0921-8831/$ - see front matter � 2010 The Society ofdoi:10.1016/j.apt.2010.02.002

* Tel.: +91 20 24304173; fax: +91 20 24389411.E-mail addresses: draksingh@hotmail.com, aksingh

The present study investigates the electrical and sensing properties of mechanically compacted pellets ofnanosized zinc oxide powders synthesized by chemical method at room temperature in alcohol baseusing Triethanolamine (TEA) as capping agent. Synthesized ZnO particles has been characterized for itsoptical, structural, morphological properties using UV–VIS spectrophotometer, X-ray diffraction (XRD)and Scanning Electron Microscopy (SEM). The ZnO particles have hexagonal wurtzite structure and theparticles are of 20–30 nm in size. The electrical properties of the prepared material have been investi-gated with Impedance Spectroscopy at different temperatures and frequencies and other laboratorysetup. Resistivity, I–V curves, AC impedance of ZnO nanoparticles pellets with temperature was investi-gated and response was compared with commercial ZnO. Piezoelectric and oxygen sensing property ofZnO were also examined. Dynamic hysteresis of sintered ZnO pellet using axis ACCT TF analyzer2000HS did not show polarization retention by sample. Oxygen sensing of ZnO pellet has been investi-gated for different concentrations of oxygen for the temperature range of 200–350 �C. The decrease ofthe current flow through the ZnO pellet with increasing oxygen concentration indicates the applicationof ZnO in oxygen sensing. The prepared ZnO particles were also used for preparing nanofluids of differentconcentrations and were characterized by measuring thermal conductivity using hot wire method whichshows sigmoidal behavior over a temperature range of 10–50 �C.� 2010 The Society of Powder Technology Japan.. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

In the past few years, intense research activities have been de-voted to the synthesis, structural characterization and investiga-tion of physical and other properties of nanostructured materials.These nano materials has also been used in preparing Nanofluids(colloidal suspensions of nanoparticles) for efficient heat transferapplications. Zinc oxide NPs exhibits remarkable physical proper-ties that make metal oxide nanoparticles strong candidates innanofluids and other applications. ZnO, a key technological mate-rial, has received wide attention due to its specific chemical, elec-trical, surface and microstructural properties. ZnO being directband gap semiconductor has high exciton binding energy(60 meV) would allow excitonic transitions at room temperature,meaning high radiative recombination efficiency for spontaneousemission as well as lower threshold voltage for emission. ZnO isconsidered as a substitute to GaN and ITO, as transparent conduct-ing oxide. It has high electrochemical stability and control overresistance (10�3 to 10�5 O). The microstructural and physical prop-erties of ZnO can be modified by introducing changes into the pro-

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cedure of its chemical synthesis. The studies for nanosized ZnOpowders have been conducted due to their size-dependent elec-tronic and optical properties, which offer possibilities for micro-electronic devices [1,2]. As one of the major materials for solidstate gas sensor, bulk and thin films of ZnO have been proposedfor its sensing applications. ZnO sensor elements have been fabri-cated in various forms including single crystal [3,4] sinter pellet[5–7] thin film [8,9] and thick film [10].

The preparation methods of nanosized metal oxide particleshave been extensively studied for the precise control of the mor-phology at the nanometric scale. In this study, ZnO nanoparticlesare synthesized by wet chemical method, under ambient atmo-sphere at room temperature which is a promising option for largescale production. Also, properties of ZnO can be tailored other thanthe advantage of range of precursors, temperature, time, pH. etc. Torestrict the growth of the particles, TEA has been used as cappingagent. Synthesized ZnO powder has been used to prepare a pellet.Mechanically compacted pellet has been studied for electrical, pie-zoelectric and oxygen gas sensing properties. Effect of annealing inair atmosphere was also studied on the electrical properties of ZnOnanomaterial. Prepared ZnO response has been studied for threedifferent concentrations (100 ml, 150 ml and 200 ml) of oxygenin the temperature range of 200–350 �C. Synthesized ZnO has alsobeen used for preparing nanofluids and have been characterized

ed by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Fig. 1. UV–Vis absorption spectra of ZnO nanoparticles.

Fig. 2. Room temperature PL of ZnO nanoparticles.

Fig. 3. SEM of ZnO nanoparticles.

610 AK Singh / Advanced Powder Technology 21 (2010) 609–613

for thermal behavior under low concentrations (loading) of 0.075,0.25 and 0.5%.

2. Synthesis methodology

Zincacetatedihydrate, Zn(Ac)2�2H2O dissolved in Dimethyl-sulphoxide (DMSO) and Potassiumhydroxide (KOH) dissolved inEthanol is used to synthesize ZnO and then Triethanolamine(TEA) is added as capping agent. 0.2 M Zincacetatedihydrate,Zn(Ac)2�2H2O is prepared in 20 ml Dimethylsulpoxide (DMSO)and stirred till it is completely dissolved and forms a clear solution.1.2 M solution of Potassiumhydroxide in 10 ml Ethanol is added tothe solution of Zincacetate drop wise till the solution becomesmilky white under slow stirring condition until it is uniformlywhite. 0.12 ml TEA is added and stirring is continued for propermixing of the capping agent in the milky white solution. The pre-cipitate is separated by centrifugation and then washed at leastthree times and then naturally dried to get fine white powder.The dried synthesized ZnO nanoparticles are made into pellet usinghydraulic press under 7500 psi. The pellet was silver painted usingsilver conducting paint. The dimensions of the pellet are 0.62 mmthickness and 13.11 mm diameter. All the chemicals used werepurchased from the leading suppliers without further purification.Synthesized ZnO has also been used for preparing stable nanofluidsin DI water of different concentrations. Acetylacetone has beenused for stabilization of ZnO nanoparticles in DI water.

3. Measurements and analysis

UV–VIS absorption study has been carried out using Nanodrop1000 spectrophotometer by dispersing nanoparticles in methanol.Photoluminescence (PL) measurements have been done at roomtemperature using Perkin–Elmer (LS-55) Luminescence Spectro-photometer. Analysis of crystal structure, crystal size and morphol-ogy has been carried out by XRD and SEM. Stoichiometric analysishas been done using EDAX. From Scherrer’s formula [11] usingFWHM of XRD patterns, size of particles is calculated

D ¼ 0:9kb cos h

ð1Þ

where k is wave length of X-ray source, b is full width at half max-imum in radians, h is Bragg’s diffraction angle. Impedance spectros-copy measurements were performed using Broadband DielectricSpectrometer of Novocontrol Model (Alpha ATB). Other electricaland gas sensing properties has been measured by using laboratorysetup having provision for four probe resistance measurement, con-trolled heating of pellet and flow of gas. The gas adsorbs on the ZnOsensing layer and causes a change in resistance depending on thegas concentration. Piezoelectric properties of the sample has beenanalyzed using axis ACCT TF analyzer 2000HS by dynamic hystere-sis of sintered ZnO pellet after polling. Hotwire method [12] hasbeen used to measure thermal conductivity of nanofluid.

4. Results and discussion

4.1. Optical properties

Fig. 1 shows the UV absorption spectra of ZnO capped with TEAhaving absorption peak at 360 nm which is blue shifted to, from375 nm absorption wavelength (expected for bulk ZnO having di-rect band gap of 3.3 eV) and showing sharp excitonic peaks at360 nm. Fig. 2 shows the room temperature photoluminescencespectra of ZnO excited at 322 nm for ZnO nanoparticles afterannealing for 03 h at 673 K. A strong peak has been observed at471.5 nm (blue emission attributed to intrinsic defects such as

oxygen and zinc interstials), a weak peak at 519 nm (green emis-sion attributed to singly ionized oxygen vacancies) and low inten-sity peaks at 412 nm (attributed to zinc vacancies) and 393 nm (UVemission attributed to free exciton recombination) [13].

4.2. Particle size and structural properties

Fig. 3 shows SEM images of ZnO capped with 0.12 ml of TEA.Observation of figure shows that ZnO particles are spherical in nat-

Fig. 4. X-ray diffraction pattern of ZnO nanoparticles. Fig. 6. I–V characteristics of nano ZnO pellet before and after annealing at roomtemperature.

AK Singh / Advanced Powder Technology 21 (2010) 609–613 611

ure and size of the particles is in the range of 40–50 nm. Fig. 4shows the XRD pattern of TEA capped ZnO nanoparticle. The XRDpattern shows broadening of the peaks indicating ultra fine natureof the crystallites. The peaks assigned to diffractions from variousplanes correspond to hexagonal structure of ZnO. The crystallitesize estimated for the same sample from Scherrer’s formula usingFWHM [11] from XRD patterns is of the order of 12–20 nm fromdifferent peaks, giving average size of 15.4 nm from all the peaks.The crystallite sizes obtained indirectly by XRD and directly byelectron microscopy may not always exactly match. Whereas theelectron microscopy can be used to determine almost any crystal-lite size, X-ray peak broadening methods give the most correct re-sults for crystallite sizes in the range of 10–100 nm.

4.3. Electrical properties

Fig. 5 shows the resistivity of ZnO pellet with temperature inthe range of 85–335 �C when biased with 20 V. Figure shows de-crease in resistivity with temperature which is a characteristic ofthe semiconductor. There are two distinct zones of conductioni.e. electronic conduction and ionic conduction. Electronic conduc-tion is referred to as n- or p-hopping-type depending on whetherthe principal charge carriers are electrons or holes, respectively.This is present at low temperatures and the second conduction re-gion which is prevalent at high temperatures is the ionic conduc-tion. Ionic conduction takes place when ions can hop from site to

Fig. 5. Variation of resistivity of pellet with temperature prepared using ZnOnanoparticles.

site within a crystal lattice as a result of thermal activation. Obser-vation of Fig. 6 shows that at lower temperatures the resistivity ofthe annealed (400 �C for 3 h) ZnO is much lower than that of theun-annealed ZnO. This is because the un-annealed ZnO is muchmore porous than annealed ZnO which results higher resistivity.In the annealed ZnO the boundaries of the grain get diffused there-by resulting in reduction in the size of the pores. At higher temper-atures the resistivity of the annealed ZnO becomes stable. This isbecause of lack of vacant oxygen ions in annealed ZnO, which havebeen removed in the annealing process. Fig. 6 shows the I–V char-acteristics of un-annealed and annealed sample at constant tem-perature (27 �C). It is observed that the breakdown voltage ofannealed ZnO is much lower than that of un-annealed ZnO. It is be-cause of the lower porosity of the annealed sample which results inthe better conductivity.

Fig. 7 shows the variation of impedance with temperature (30–300 �C) for different frequencies (1–18 MHz). It has been foundthat the impedance at lower frequency is higher than that at thehigher frequency with sharp decrease in impedance at 275 �C thisis due to sharp increase in capacitance at 275 �C (variation ofcapacitance with temperature not shown here). The impedanceof the commercial ZnO (particle size of 2–5 lm) is higher than thatof ZnO nanoparticles as shown in Fig. 8 for 100 kHz frequency. Thepeak for ZnO nano pellet is sharp and well formed.

Fig. 7. Variation of impedance of nano ZnO pellet with temperature and at differentfrequencies.

Fig. 8. Variation of impedance of commercial ZnO nanoparticle ZnO with temper-ature at different frequencies.

Fig. 9. Variation of permittivity of commercial ZnO and nanoparticle ZnO.

Fig. 10. Polarization–voltage hysteresis cur

612 AK Singh / Advanced Powder Technology 21 (2010) 609–613

The permittivity of a material describes an AC signal transmis-sion speed and dielectric material capacitance. The measurementare taken for both nano ZnO and Commercial ZnO and comparedat 1 kHz frequency as shown in Fig. 9. It is found that the permit-tivity of the nano size ZnO is much higher than that of commercialZnO, because of small particle size i.e. higher porosity compared tocommercial ZnO. Therefore the nano ZnO is much better dielectricmaterial than commercially available ZnO.

4.4. Piezoelectric properties

Piezoelectric behavior of ZnO has been analyzed after sinteringthe sample at 800 �C. Poling was done at 90 �C, 500 V for 20 min,then 10 min at 1 KV beyond which current conduction occuredand poling could not be carried out. Fig. 10 shows the polarizationvoltage hysteresis curve of sintered ZnO pallet upto voltage appli-cation of 1100 V indicating the absence of polarization retention bysample.

4.5. Gas sensing properties

Fig. 11 shows variation of current with temperature for differ-ent concentration of gas i.e. 0.2, 0.4 and 0.8 volume percentageand is well below the explosion limit. Here a constant voltage of20 V is applied to the pellet and the current is monitored. Thegas in addition to the surface adsorbed oxygen, deplete the elec-trons from the conduction bands of ZnO and thus the currentsreducing with the increase in the concentration of the oxygen.The decrease of the current flow through the ZnO pallet withincreasing oxygen concentration indicates the application of ZnOin oxygen sensing.

4.6. Thermal conductivity of ZnO nanofluids

Fig. 12 shows the variation of thermal conductivity(K) of ZnOnanofluids for different concentrations in the temperature rangeof 10–40 �C. Thermal conductivity exhibits slow increase for0.075% and 0.25% loading nanofluids while 0.5% nanofluid showsrelatively fast increase in K for the given temperature range. Be-yond 30 �C temperature, K is found to increase abruptly for nano-

ve for nano ZnO pellet after sintering.

10 20 30 40 50

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66

0.68

K [W

/mK

]

Temperature [ 0C ]

0.075% ZnO 0.250% ZnO 0.500% ZnO

Fig. 12. Variation of thermal conductivity with temperature for different ZnOloading.

Fig. 11. Variation of current with temperature for different concentrations ofoxygen gas.

AK Singh / Advanced Powder Technology 21 (2010) 609–613 613

fluid of 0.5% loading, which can be attributed to agglomeration ofZnO nanoparticles and Brownian motion of nanoparticles as thetemperature increases. At higher temperature Acetylacetone inthe solution is not able to hold the ZnO nanoparticles in dispersedcondition and results in agglomeration. Also, with increase in tem-perature Brownion motion increases causing convection which inturn increases the effective thermal conductivity of nanofluid.

5. Conclusion

ZnO nanoparticles have been successfully synthesized usingzinc acetate, DMSO and KOH in Ethanol at room temperature usingTEA as capping agent. The semiconducting nature of ZnO is ob-

served from the measurements of resistivity with temperature.The semi conductivity in ZnO must be due to large oxygen defi-ciency in it. Then behavior of the resistivity with change in temper-ature was studied and compared with the annealed (400 �C) ZnOand found that the at lower temperature the resistivity of annealedZnO is lower than the un-annealed one because porosity is less inthe latter which results in the better conductivity. At higher tem-peratures the resistivity of annealed one is higher because of lackof singly ionized oxygen vacancies.

Using Impedance Spectroscopy (IS) the variation of impedance,permittivity and loss tangent is measured with change in temper-ature and frequencies; and then compared with the commercialZnO which is in micro sizes. We found that impedance of commer-cial ZnO is far higher than that nano ZnO. Permittivity of nano ZnOis higher than commercial one therefore latter can act as gooddielectric. The loss tangent of the nano ZnO is less than that ofcommercial ZnO, hence can be used as transmitter material. Thereis substantial increase in thermal conductivity which makes nano-fluids attractive as cooling fluids.

Acknowledgements

Authors are thankful to Vice-Chancellor, DIAT, Pune, for grant-ing permission to publish this work. Authors would like to thankProf. SK Kulkarni, Department of Physics, University of Pune, forproviding Photoluminescence facility and technical discussions;Director, DMSRDE, Kanpur, for SEM and EDAX of samples, and DrHH Kumar, ARDE, Pune for piezoelectric characterization ofsamples.

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