effect of ph on the morphology and gas sensing properties of zno nanostructures
TRANSCRIPT
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Sensors and Actuators B 166– 167 (2012) 438– 443
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical
j o ur nal homep a ge: www.elsev ier .com/ locate /snb
ffect of pH on the morphology and gas sensing properties of ZnO nanostructures
nkar Singha, Manmeet Pal Singhb, Nipin Kohli a, Ravi Chand Singha,∗
Department of Physics, Guru Nanak Dev University, Amritsar 143005, IndiaDepartment of Applied Sciences, Khalsa College of Engineering and Technology, Amritsar 143001, India
r t i c l e i n f o
rticle history:eceived 23 August 2011eceived in revised form 24 February 2012ccepted 28 February 2012vailable online 5 March 2012
eywords:
a b s t r a c t
Morphology dependent gas sensing behaviour of zinc oxide has been reported in this paper. Nanostruc-tures of zinc oxide have been synthesized by following a precipitation route at various pH values of theprecursor solution. Structural and morphological analyses were carried out by using XRD and FESEMtechniques. The XRD pattern confirmed wurtzite hexagonal structure of ZnO. The FESEM study revealedthat ZnO synthesized at pH 8 developed nanorod like structure, rods got fused together when synthesizedat pH 9 and 10, whereas synthesis at pH 11 resulted in transformation of rods into nanoparticles. The
anorodsanoparticlesensorsrecipitation
thick films of synthesized samples were deposited on alumina substrate and their sensing response tomethanol, ethanol and propanol was investigated at different operating temperatures. It was observedthat all the sensors exhibited optimum sensing response at 400 ◦C. It has also been observed that sam-ple prepared at pH 11, constituting nanoparticles, exhibited high sensing response than an assemblyof nanorods prepared at pH 8–10. Sensing response of all the samples tested was significantly highertowards propanol vapour than towards that of methanol and ethanol.
. Introduction
In the past few decades, metal oxides like ZnO, SnO2, WO3 haveeen considered to be the most prominent materials for gas sensingpplication due to the sensitivity of their electrical conductivity tohe ambient gas composition, which arises from charge transfernteractions with reactive gas like CO, O2, hydrocarbons and volatilerganic compounds.
It is well known that the gas sensing mechanism is based onhe reaction between the test gas molecules and adsorbed oxygenpecies on the surface of metal oxide. The amount of adsorbed oxy-en is strongly dependent on morphology and structure, surfacerea and grain size of the sensing material. Thus many researchroups are trying to develop novel methods for the synthesis ofinc oxide having different morphologies for potential applicationn gas sensing [1–5]. Moreover, nanostructures have become theocus for many researchers because of their wide range of appli-ations. Zinc oxide possessing direct wide band gap (3.37 eV) and
large exciton binding energy (60 meV), is one of the most ver-atile metal oxide semiconductors having applications which areuite diverse and magnificent. In the literature, one can encounter
any applications of this utilitarian metal oxide semiconductor.anostructured ZnO may be employed for solar cell applications∗ Corresponding author. Tel.: +91 9914129939.E-mail address: [email protected] (R.C. Singh).
925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2012.02.085
© 2012 Elsevier B.V. All rights reserved.
[6], LEDs [7], optoelectronic devices [8], biosensors [9], sensors [10],etc. [11–13].
As the morphology and structure of a material depend uponsynthesis conditions and parameters, therefore, many techniqueshave been reported to synthesize material bearing various geome-tries. Different approaches prevalent for synthesizing nanosizedZnO are r.f. sputtering [1], sol–gel [14], chemical vapour deposi-tion [15], pulsed laser deposition [16], ultrasonic spray pyrolysis[17], wet chemical routes [2], hydrothermal synthesis [18], etc. Inwet chemical routes crystallization of compound takes place outof supersaturated solution. The structure and properties of precip-itate and the rate of its deposition are dictated by many factors,such as cooling rate, pH, ionic strength of a solution, and impurityconcentration [19].
Although pH of solution is one of the most easily measurableand controllable parameters during crystallization of an ionic com-pound yet its influence on morphology and structure has not beenstudied extensively. From the available literature, we envisage thatpH variation of precursor solution affects the morphology of thenanostructures significantly [20]. Wahab et al. [20] found varia-tion from plate-like to flower-like morphology on increasing pHfrom 6 to 12; they concluded that lower pH was suitable for obtain-ing 2D structure where as higher pH values can result in rod likestructure. Alias et al. [21] reported that increase in the pH of sol
led to the decrement of the particle size of ZnO nanostructures.Pal et al. [22] prepared ZnO nanostructures with different mor-phologies through a low-temperature hydrothermal process byadjusting the initial and final pH values of the reaction mixture.
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he variation of pH brings about the rate of nucleation and growthf nanostructures under control. Bai et al. [23] reported the synthe-is of rose like zinc oxide nanostructures from ZnCl2 and ammonia25%) through a hydrothermal decomposition method on a copperlate substrate. They too observed that pH and concentration sig-ificantly affected the morphology, orientation and density of thes-grown ZnO nanostructures. In one of their works, Kukushkin andemna [19] have reported the effect of solution pH on crystalliza-
ion kinetics of some classes of ionic inorganic compounds at thetage of nucleation and pH dependences of supersaturation, criticalize, and nucleus flux. They demonstrated that the pH of systemsnder consideration is the driving force for phase transition.
In this paper, we are reporting a change in the morphology ofhe ZnO nanostructure brought about by the variation of pH ofeaction mixture. In the papers discussed above, authors have syn-hesized nanostructures by using high temperatures, sophisticatedechniques or complex reactions. In this work, we have adopted
relatively simpler technique namely crystallization in the liq-id phase, where the increase in pH of solution has transformedanorods into nanoparticles. Furthermore the effect of morphologyn gas sensing behavior of ZnO towards different organic vapouras been investigated. The novelty in the present study is that were reporting complete transfer of nanorods to nanoparticles byarying pH of the solution and morphological effects on gas sensingehaviour.
. Experimental details
.1. Synthesis of nanostructured ZnO by crystallization in liquidhase at different pH
For obtaining precipitate, we initiated with the 0.2 M solutionf ZnCl2 (AR grade) in distilled water to which ammonia solution25%) was added drop wise to get a desired pH of the solution whileontinuous stirring for half an hour. Following similar procedure,e prepared four different reaction mixtures and maintained theirH at 8, 9, 10 and 11, respectively. The resulting precipitate in eachase was separated from solutions, washed and dried at 120 ◦C, andamples thus collected were sintered at 500 ◦C for 3 h to completehe process of zinc oxidation.
.2. Material characterization
For crystal structure analysis, the prepared samples were char-cterized by powder X-ray diffraction (XRD) using Cu K� radiationith Shimadzu 7000 Diffractometer. Morphology of the samplesas analyzed by the field emission scanning electron microscope
FESEM) with FEI Quanta 200F.
.3. Fabrication of thick film sensor and sensor testing set up
Following procedure was followed to fabricate thick film sen-ors. A pinch (2–3 mg) of ZnO powder was mixed properly withwo drops of distilled water to make paste. The paste was paintedith a fine brush onto an alumina substrate (12 mm × 5 mm size)aving pre-deposited gold electrical contacts to obtain a thick filmf thickness around 25–27 �m (thickness of the film is controlledy masking of cellulose tape of known thickness). For depositionf gold contacts, liquid bright gold (manufactured by Hobby Col-robbia Bright Gold) was painted with brush on alumina substrateseaving a 2 mm gap in the middle which followed heat treatmento convert the paint into metallic gold. To obtain sensors of identi-
al geometry, alumina substrates were appropriately masked usingommercially available cellulose tape and after painting them withensing material, extra wet material was removed. The sensoresign is shown in Fig. 1.12 mm
Fig. 1. Schematic of gas sensor.
Above procedure was followed to fabricate three sensors foreach pH value, which followed curing at 350 ◦C for 30 min. The mea-surements of gas sensor response were carried out with a homebuilt apparatus consisting of a simple potentiometer arrangement,a 40 L test chamber in which a sample holder, a small tempera-ture controlled oven and a circulating fan were installed. We havetaken care of other ambient vapour, and we have placed a humid-ity meter inside the testing chamber to operate all the samplesat same conditions. We have checked all samples at about 20%humidity. Fabricated sensor was placed in the test chamber ovenat a desired temperature and a known quantity of alcohol species(e.g. 25 �L of ethanol gives 250 ppm in 40 L chamber) was injectedinto test chamber using Hamilton syringe. Due to small quantity ofVOC, it instantaneously vapourizes and mixes with air in the cham-ber. After keeping it in the chamber for about 40 s, the mixture isallowed to exhaust out of lab through chamber door. The chamberdoor is kept open for 30 min to replenish the lost oxygen on thesensor surface and recovery of sensor resistance to the base value.Variation of real time voltage signal across the resistance connectedin series with the sensor, was recorded with an experimental setup consisting of Keithley Data Acquisition Module KUSB-3100 and acomputer. The sensor response magnitude was determined as Ra/Rg
ratio, where Ra and Rg are the resistances of sensor in air ambienceand air–gas mixture, respectively. All the sensors were tested threetimes by following same procedure by varying temperature from250 to 450 ◦C with 50 ◦C intervals.
3. Results and discussion
3.1. Material characterization
Fig. 2 represents the X-ray diffraction pattern of materials syn-thesized at various pH values (from 8 to 11). The peaks visible inthe graphs are in well agreement with standard available data, andthese depict the wurtzite hexagonal structure of nanosized zincoxide.
Fig. 3(a)–(d) represents the FESEM images of the nanostruc-tured ZnO powders at pH values of 8, 9, 10 and 11, respectively.From these images it is clear that at pH 8, well chiseled rod likestructures of ZnO are formed. At pH 9 and 10 fusion and agglom-eration of these rods have been observed; moreover nanorods areno longer morphologically distinct. With further increase in pH to11, a riveting situation has emerged where material did not growas rods but morphed into particles instead. Lu and Yeh [24] haveobtained results otherwise; during hydrothermal processing withthe increase in pH of starting solution from 9 to 12, morphologyof obtained ZnO powder has changed from ellipsoidal-shape to rodlike shape.
In the following text we have tried to discuss the effect of pH
on zinc oxide nanostructures. In the basic solution (above pH 7),concentration of hydroxyl (OH−) ion is high and these ions arestrongly attracted by the positively charged, Zn-terminated, sur-faces [20,25]. At pH 8 we acquired fine nanorods of ZnO, at this stage
440 O. Singh et al. / Sensors and Actuato
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Fig. 3. FESEM images of the powders synthesized at
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the solution is basic with hydroxyl ions dominating the reaction.Li et al. [25] found that growth velocities along certain crystallinedirection depend upon the pH, and preferred growth direction forbasic and neutral solution is along (0 0 0 1), thus ultimately result-ing in synthesis of fine nanorods at pH 8. According to another pointof view, anisotropic crystal growth is required for the formationof nanowires or nanorods, i.e. the crystal grows along a certainorientation faster than other directions. The driving force for thesynthesis of nanorods and nanowires by spontaneous growth isthe decrease in Gibbs free energy, which arises from either recrys-tallization or a decrease in supersaturation [26].
At pH 8 the formation of well-defined rods is due to the factthat system tends to minimize the total energy attributed by spon-taneous polarization. The spontaneous polarization results fromthe non-centrosymmetric ZnO crystal structure. In (0 0 0 1) facet-dominated single crystal nanorods, positive and negative ioniccharges are spontaneously established on the zinc and oxygen-terminated (0 0 0 1) surfaces, respectively [25,26].
At the elevated pH 9 and 10, agglomeration of rods has beenobserved. The reason for this may be attributed to the fact that
supersaturation of the solution increases with increase in pH.Therefore, to minimize enhanced overall energy of the system, rodstend to agglomerate and the material loses its well defined rod-like(a) pH 8, (b) 9, (c) 10 and (d) 11, respectively.
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hape. That is what has probably happened at pH 9 and 10 and thebtained entities in these cases are somewhat like bundle of rodselded together.
At pH 11 we obtained particles instead of rods, the change inorphology of the ZnO may be explained by the fact that at such a
igh pH the hydroxyl ions have dominated the reaction. The poly-eric chain at high pH is larger than the one at low pH. The high
oncentration of OH ions has resulted in the cyclization because athis high pH intermolecular reaction has dominated intramoleculareaction [27]. Plausibly rods might have taken the form of particlesue to cyclization. Another argument that may justify the resultsbtained is given by Alias et al. [21] and Wei et al. [28] and suggestedhat at higher pH 10 and 11, concentration of Zn(NH3)4
2+ reduces,nd the dissolution effect becomes more dominant. Wei et al. [28]ave found that the ends of rods became flat due to dissolution,ut we have observed that rods flattened enough and transformed
nto particles. Cao [26] has suggested that high supersaturationesults in homogenous nucleation, at high pH of 11, supersaturationncreased and thus we obtained ZnO particles instead of rods.
.2. Sensing performance
Sensors fabricated from powder synthesized at different pH val-es were exposed to 250 ppm of methanol, ethanol and propanolt different temperatures and results are shown in Fig. 4, whererror bars represent standard deviation. Study revealed that opti-um operating temperature of all the sensors for all the test species
emained invariant at 400 ◦C.The comparative response of VOCs for all the sensors based on
amples prepared at different pH values is shown in Fig. 5, whererror bars represent standard deviation. It is evident from figurehat sensing response is exceptionally higher for the sample corre-ponding to pH 11 as compared to pH 8, 9 and 10. The sensorsabricated from powder synthesized at pH 11 were exposed to50 ppm methanol, ethanol and propanol at 400 ◦C and their resis-ance variation with time is shown in Fig. 6. From this figure it islear that the response and recovery time of the fabricated sensorsre very fast.
Under similar conditions, the sensing response magnitude ofamples synthesized at various pH values varies as the follow-ng order: pH 11 > pH 10 > pH 9 > pH 8. The reason for increase inhe sensing response towards higher pH values may be ascribedo the material morphology. From FESEM images it is clear that
orphology changes from rods to particles with increase in pH.vidently, at pH 8 rods are formed, at pH 9 rods and limited numberf particles coexist whereas at pH 10 number of particles increasesignificantly. The presence of nanoparticles along with nanorodsnhance the surface area and hence the sensing response. At pH 11here are only nanoparticles and have maximum sensing responseecause of greater surface area as compared to rods obtained atther pH values. It is well known fact that smaller the size largerill be the surface area, in other words particle size has inverse
elationship with the surface area [27]. The sensing is a com-lex phenomenon which occurs on the surface of the metal oxideemiconductor. Rothschild and Komem [3] have simulated vari-us sensing parameters such as grain size, surface state density,nd carrier concentration. with the sensing response. However, theurface reactivity of particles is known to rapidly increase with thencrease in surface-to-bulk ratio because the strong curvature ofhe particle surface generates a large density of defects, which arehe most reactive surface sites. Another result obvious from theurves in Fig. 4 is that nanoparticles show better sensing response
han nanorods for all tested VOCs. In one of our works we havelready reported that ZnO nanoparticles are better candidates forensing applications than nanorods [29]. In this work we againave arrived to the same conclusion irrespective of technique usedFig. 4. Sensing response towards 250 ppm of (a) methanol, (b) ethanol and (c)propanol for the samples prepared at different pH at different operating temper-atures.
442 O. Singh et al. / Sensors and Actuators B 166– 167 (2012) 438– 443
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ig. 5. Sensing Response of the samples synthesized at different pH values to50 ppm of different VOCs at optimum operating temperature of 400 ◦C.
or material synthesis. The sensor response is quantitatively deter-ined by number of active sites on the surface of gas sensors. When
nO nanostructured sensors are exposed to air, oxygen moleculesdsorb on the surface of the materials to form O2
−, O−, O2− ions byapturing electrons from the conduction band. Thus the ZnO sen-ors show a high resistance in air [10]. All the sensors in air at roomemperature have resistances in the mega ohm range furthermorehe resistances vary between the sensors of same morphologies andetween different morphologies.
2(gas) ↔ O2(ads) (1)
2(ads) + e− ↔ O2−
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2−
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hen metal oxide based sensors are exposed to reducing agentst moderate temperature, the target gas reacts with the adsorbedxygen and as a result captured electrons go back to the conduc-ion band. This eventually increases the conductivity of metal oxideased sensors [10]. The reaction can be described as follows:
+ O−ads → RO + e− (4)
he results displayed in Fig. 4 shows the variation of sensingesponse magnitude for three alcohols in the following order:ropanol > ethanol > methanol. The number of methyl groupsttached to these alcohols is in the same order as well. Thoughas sensing is a complex phenomenon still there are some obviousarameters, which might be playing crucial role in exhibiting thisype of sensing response variation. One of the plausible explana-ions could be on the basis of complete oxidation of these alcoholsnd in the process consuming 9, 6 and 3 O−
ads by propanol, ethanolnd methanol, respectively. Gong et al. [30] have reported similarypes of results from their study.
. Conclusion
In this work we used chemical technique to synthesize nanos-ructured ZnO at different pH values of precursor solution. The pHas played a significant role in altering the morphology of the zinc
xide. At pH 8 nanorods of ZnO have been obtained due to spon-aneous polarization which results from the non-centrosymmetricnO crystal structure. With increase in pH to 9 and 10, agglom-ration due to increase in supersaturation of the system has beentemperature of 400 ◦C.
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bserved. At pH 11 it was found that structure had altered morerominently and nanorods had morphed into nanoparticles. Theriving force for this alteration is imputed to the dominance ofissolution effect and homogeneous nucleation at pH 11. The pHlayed a pivotal role in controlling the morphology of zinc oxide.ensing performance of these powders was investigated for 3lcohols and we perceived that sensing response depends uponurface morphology. We ascertained that nanoparticles have betteresponse than nanorods for all the VOCs. All the sensors demon-trated similar trend for the response towards test alcohols in theollowing manner propanol > ethanol > methanol.
cknowledgments
Authors would like to thank following: University Grants Com-ission, New Delhi, India for financial support and IIT Roorkee for
ESEM investigations.One of the authors Onkar Singh thanks CSIR, New Delhi for senior
esearch fellowship.
eferences
[1] N.H. Al-Hardan, M.J. Abdullah, A.A. Aziz, Sensing mechanism of hydrogen gassensor based on rf-sputtered ZnO thin films, Int. J. Hydrogen Energy 35 (2010)4428.
[2] T. Krishnakumar, R. Jayaprakash, N. Pinna, N. Donato, A. Bonavita, G.Micali, G. Neri, CO gas sensing of ZnO nanostructures synthesized byan assisted microwave wet chemical route, Sens. Actuators B 143 (2009)198–204.
[3] A. Rothschild, Y. Komem, The effect of grain size on the sensitivity of nanocrys-talline metal-oxide gas sensors, J. Appl. Phys. 95 (2004) 6374–6380.
[4] S. Dixit, A. Srivastava, A. Srivastava, R.K. Shukla, Effect of toxic gases on humid-ity sensing property of nanocrystalline ZnO film, J. Appl. Phys. 102 (2007)113114–113118.
[5] R. Martins, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, M. Bender, N. Kat-sarakis, V. Cimalla, G. Kiriakidis, Zinc oxide as an ozone sensor, J. Appl. Phys. 96(2004) 1398–1408.
[6] S. Rani, P. Suri, P.K. Shishodia, R.M. Mehra, Synthesis of nanocrystalline ZnOpowder via sol–gel route for dye-sensitized solar cells, Sol. Energy Mater. Sol.Cells 92 (2008) 1639–1645.
[7] D.C. Kim, W.S. Han, B.H. Kong, H.K. Cho, C.H. Hong, Fabrication of the hybridZnO LED structure grown on p-type GaN by metal organic chemical vapordeposition, Physica B 401 (402) (2007) 386–390.
[8] W.S. Lee, G.W. Choi, Y.J. Seo, Surface planarization of ZnO thin film for opto-electronic applications, Microelectron. J. 40 (2009) 299–302.
[9] A. Umar, M.M. Rahman, A. Al-Hajry, Y.B. Hahn, Highly-sensitive choles-terol biosensor based on well-crystallized flower-shaped ZnO nanostructures,Talanta 78 (2009) 284–289.
10] B. Shouli, C. Liangyuan, L. Dianqing, Y. Wensheng, Y. Pengcheng, L. Zhiyong,C. Aifan, C.C. Liu, Different morphologies of ZnO nanorods and their sensingproperty, Sens. Actuators B 146 (2010) 129–137.
11] X.J. Liu, C. Song, F. Zeng, F. Pan, Donor defects enhanced ferromagnetism inCo:ZnO films, Thin Solid Films 516 (2008) 8757–8761.
12] M.S. Samuel, J. Koshy, A. Chandran, K.C. George, Electrical charge trans-port and dielectric response in ZnO nanotubes, Curr. Appl. Phys. 11 (2011)1094–1099.
13] Y.J. Zhang, J.B. Wang, X.L. Zhong, Y.C. Zhoua, X.L. Yuanc, T. Sekiguchi, Influence
of Li-dopants on the luminescent and ferroelectric properties of ZnO thin films,Solid State Commun. 148 (2008) 448–451.14] J.S. Bhat, A.S. Patil, N. Swami, B.G. Mulimani, B.R. Gayathri, N.G. Deshpande, G.H.Kim, M.S. Seo, Y.P. Lee, Electron irradiation effects on electrical and optical prop-erties of sol–gel prepared ZnO films, J. Appl. Phys. 108 (2010) 043513–043520.
s B 166– 167 (2012) 438– 443 443
15] T. Terasako, M. Yagi, M. Ishizaki, Y. Senda, H. Matsuura, S. Shirakata, Growthof zinc oxide films and nanowires by atmospheric-pressure chemical vapordeposition using zinc powder and water as source materials, Surf. Coat. Technol.201 (2007) 8924–8930.
16] K. Matsubara, P. Fons, K. Iwata, A. Yamada, K. Sakurai, H. Tampo, S. Niki, ZnOtransparent conducting films deposited by pulsed laser deposition for solar cellapplications, Thin Solid Films 431–432 (2003) 369–372.
17] V.R. Shinde, T.P. Gujar, C.D. Lokhande, LPG sensing properties of ZnO films pre-pared by spray pyrolysis method: effect of molarity of precursor solution, Sens.Actuators B 120 (2007) 551–559.
18] S. Ohara, T. Mousavand, M. Umetsu, S. Takami, T. Adschiri, Y. Kuroki, M. Takata,Hydrothermal synthesis of fine zinc oxide particles under supercritical condi-tions, Solid State Ionics 172 (2004) 261–264.
19] S.A. Kukushkin, S.V. Nemna, The effect of pH on nucleation kinetics in solutions,Dokl. Phys. Chem. 377 (2001) 117–120.
20] R. Wahab, S.G. Ansari, Y.S. Kim, M. Song, H.S. Shin, The role of pH variation onthe growth of zinc oxide nanostructures, Appl. Surf. Sci. 255 (2009) 4891–4896.
21] S.S. Alias, A.B. Ismail, A.A. Mohamad, Effect of pH on ZnO nanoparticle propertiessynthesized by sol–gel centrifugation, J. Alloys Compd. 499 (2010) 231–237.
22] U. Pal, J.G. Serrano, P. Santiago, G. Xiong, K.B. Ucer, R.T. Williams, Synthesisand optical properties of ZnO nanostructures with different morphologies, Opt.Mater. 29 (2006) 65–69.
23] W. Bai, K. Yu, Q. Zhang, X. Zhu, D. Peng, Z. Zhu, Y.S.N. Dai, Large-scale synthesis ofzinc oxide rose-like structures and their optical properties, Physica E 40 (2008)822–827.
24] C.H. Lu, C.H. Yeh, Influence of hydrothermal conditions on the morphology andparticle size of zinc oxide powder, Ceram. Int. 26 (2000) 351–357.
25] J. Li, S. Srinivasan, G.N. He, J.Y. Kang, S.T. Wu, F.A. Ponce, Synthesis and lumi-nescence properties of ZnO nanostructures produced by the sol–gel method, J.Cryst. Growth 310 (2008) 599–603.
26] G. Cao, Nanostructures and Nanomaterials: Synthesis, Properties and Applica-tions, Imperial College Press, London, 2004.
27] P.K. Sharma, M.H. Jilavi, V.K. Varadan, Influence of initial pH on the particle sizeand flouorescence properties of the nano scale Eu(III) doped yttrium, J. Phys.Chem. Solids 63 (2002) 171–177.
28] A. Wei, X.W. Sun, C.X. Xu, Z.L. Dong, Y. Yang, S.T. Tan, W. Huang, Growth mech-anism of tubular ZnO formed in aqueous solution, Nanotechnology 17 (2006)1740–1744.
29] R.C. Singh, O. Singh, M.P. Singh, P.S. Chandi, Synthesis of zinc oxide nanorodsand nanoparticles by chemical route and their comparative study as ethanolsensors, Sens. Actuators B 135 (2008) 352–557.
30] H. Gong, Y.J. Wang, S.C. Teo, L. Huang, Interaction between thin-film tin oxidegas sensor and five organic vapors, Sens. Actuators B 54 (1999) 232–235.
Biographies
Onkar Singh received his M.Sc. physics degree from Guru Nanak Dev University,Amritsar, India in 2006. Presently he is pursuing for Ph.D. in the field of nano sizedmetal oxide materials and gas sensors at the same institute.
Manmeet Pal Singh received his Ph. D. in physics in 2011 and M.Sc. (Hon. School)physics degree in 2004 from Guru Nanak Dev University, Amritsar, India. Presentlyhe is working as a lecturer in the Department of Applied Sciences, Khalsa College ofEngineering and Technology, Amritsar 143001, India.
Nipin Kohli received her M.Sc. physics degree from Guru Nanak Dev University,Amritsar, India in 2005. Presently she is pursuing for Ph.D. in the field of nanostruc-tured materials and their application as gas sensors at Guru Nanak Dev University,India.
Ravi Chand Singh received his Ph.D. in physics from Guru Nanak Dev University,Amritsar, India in 1989. Since then he has had an appointment at the same insti-
Canada in 1990. He moved to Guru Nanak Dev University Amritsar in 1993, wherehe is presently working as an associate professor of physics. His recent interests arematerial research for gas sensing and development of new experiments for physicseducation.