Effect of pH on the morphology and gas sensing properties of ZnO nanostructures

Sensors and Actuators B 166–167 (2012) 438–443

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Effect of pH on the morphology and gas sensing properties of ZnO nanostructures Onkar Singh a , Manmeet Pal Singh b , Nipin Kohli a , Ravi Chand Singh a,∗ a b

Department of Physics, Guru Nanak Dev University, Amritsar 143005, India Department of Applied Sciences, Khalsa College of Engineering and Technology, Amritsar 143001, India

a r t i c l e

i n f o

Article history: Received 23 August 2011 Received in revised form 24 February 2012 Accepted 28 February 2012 Available online 5 March 2012 Keywords: Nanorods Nanoparticles Sensors Precipitation

a b s t r a c t Morphology dependent gas sensing behaviour of zinc oxide has been reported in this paper. Nanostructures of zinc oxide have been synthesized by following a precipitation route at various pH values of the precursor solution. Structural and morphological analyses were carried out by using XRD and FESEM techniques. The XRD pattern confirmed wurtzite hexagonal structure of ZnO. The FESEM study revealed that ZnO synthesized at pH 8 developed nanorod like structure, rods got fused together when synthesized at pH 9 and 10, whereas synthesis at pH 11 resulted in transformation of rods into nanoparticles. The thick films of synthesized samples were deposited on alumina substrate and their sensing response to methanol, ethanol and propanol was investigated at different operating temperatures. It was observed that all the sensors exhibited optimum sensing response at 400 ◦ C. It has also been observed that sample prepared at pH 11, constituting nanoparticles, exhibited high sensing response than an assembly of nanorods prepared at pH 8–10. Sensing response of all the samples tested was significantly higher towards propanol vapour than towards that of methanol and ethanol. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In the past few decades, metal oxides like ZnO, SnO2 , WO3 have been considered to be the most prominent materials for gas sensing application due to the sensitivity of their electrical conductivity to the ambient gas composition, which arises from charge transfer interactions with reactive gas like CO, O2 , hydrocarbons and volatile organic compounds. It is well known that the gas sensing mechanism is based on the reaction between the test gas molecules and adsorbed oxygen species on the surface of metal oxide. The amount of adsorbed oxygen is strongly dependent on morphology and structure, surface area and grain size of the sensing material. Thus many research groups are trying to develop novel methods for the synthesis of zinc oxide having different morphologies for potential application in gas sensing [1–5]. Moreover, nanostructures have become the focus for many researchers because of their wide range of applications. Zinc oxide possessing direct wide band gap (3.37 eV) and a large exciton binding energy (60 meV), is one of the most versatile metal oxide semiconductors having applications which are quite diverse and magnificent. In the literature, one can encounter many applications of this utilitarian metal oxide semiconductor. Nanostructured ZnO may be employed for solar cell applications

∗ Corresponding author. Tel.: +91 9914129939. E-mail address: [email protected] (R.C. Singh). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.085

[6], LEDs [7], optoelectronic devices [8], biosensors [9], sensors [10], etc. [11–13]. As the morphology and structure of a material depend upon synthesis conditions and parameters, therefore, many techniques have been reported to synthesize material bearing various geometries. Different approaches prevalent for synthesizing nanosized ZnO are r.f. sputtering [1], sol–gel [14], chemical vapour deposition [15], pulsed laser deposition [16], ultrasonic spray pyrolysis [17], wet chemical routes [2], hydrothermal synthesis [18], etc. In wet chemical routes crystallization of compound takes place out of supersaturated solution. The structure and properties of precipitate and the rate of its deposition are dictated by many factors, such as cooling rate, pH, ionic strength of a solution, and impurity concentration [19]. Although pH of solution is one of the most easily measurable and controllable parameters during crystallization of an ionic compound yet its influence on morphology and structure has not been studied extensively. From the available literature, we envisage that pH variation of precursor solution affects the morphology of the nanostructures significantly [20]. Wahab et al. [20] found variation from plate-like to flower-like morphology on increasing pH from 6 to 12; they concluded that lower pH was suitable for obtaining 2D structure where as higher pH values can result in rod like structure. 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 morphologies through a low-temperature hydrothermal process by adjusting the initial and final pH values of the reaction mixture.

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The variation of pH brings about the rate of nucleation and growth of nanostructures under control. Bai et al. [23] reported the synthesis of rose like zinc oxide nanostructures from ZnCl2 and ammonia (25%) through a hydrothermal decomposition method on a copper plate substrate. They too observed that pH and concentration significantly affected the morphology, orientation and density of the as-grown ZnO nanostructures. In one of their works, Kukushkin and Nemna [19] have reported the effect of solution pH on crystallization kinetics of some classes of ionic inorganic compounds at the stage of nucleation and pH dependences of supersaturation, critical size, and nucleus flux. They demonstrated that the pH of systems under consideration is the driving force for phase transition. In this paper, we are reporting a change in the morphology of the ZnO nanostructure brought about by the variation of pH of reaction mixture. In the papers discussed above, authors have synthesized nanostructures by using high temperatures, sophisticated techniques or complex reactions. In this work, we have adopted a relatively simpler technique namely crystallization in the liquid phase, where the increase in pH of solution has transformed nanorods into nanoparticles. Furthermore the effect of morphology on gas sensing behavior of ZnO towards different organic vapour has been investigated. The novelty in the present study is that we are reporting complete transfer of nanorods to nanoparticles by varying pH of the solution and morphological effects on gas sensing behaviour. 2. Experimental details 2.1. Synthesis of nanostructured ZnO by crystallization in liquid phase at different pH For obtaining precipitate, we initiated with the 0.2 M solution of ZnCl2 (AR grade) in distilled water to which ammonia solution (25%) was added drop wise to get a desired pH of the solution while continuous stirring for half an hour. Following similar procedure, we prepared four different reaction mixtures and maintained their pH at 8, 9, 10 and 11, respectively. The resulting precipitate in each case was separated from solutions, washed and dried at 120 ◦ C, and samples thus collected were sintered at 500 ◦ C for 3 h to complete the process of zinc oxidation.

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Gold contacts

Alumina substrate

5mm

Thick film of zinc oxide

12 mm Fig. 1. Schematic of gas sensor.

Above procedure was followed to fabricate three sensors for each pH value, which followed curing at 350 ◦ C for 30 min. The measurements of gas sensor response were carried out with a home built apparatus consisting of a simple potentiometer arrangement, a 40 L test chamber in which a sample holder, a small temperature controlled oven and a circulating fan were installed. We have taken care of other ambient vapour, and we have placed a humidity meter inside the testing chamber to operate all the samples at same conditions. We have checked all samples at about 20% humidity. Fabricated sensor was placed in the test chamber oven at a desired temperature and a known quantity of alcohol species (e.g. 25 ␮L of ethanol gives 250 ppm in 40 L chamber) was injected into test chamber using Hamilton syringe. Due to small quantity of VOC, it instantaneously vapourizes and mixes with air in the chamber. After keeping it in the chamber for about 40 s, the mixture is allowed to exhaust out of lab through chamber door. The chamber door is kept open for 30 min to replenish the lost oxygen on the sensor surface and recovery of sensor resistance to the base value. Variation of real time voltage signal across the resistance connected in series with the sensor, was recorded with an experimental set up consisting of Keithley Data Acquisition Module KUSB-3100 and a computer. The sensor response magnitude was determined as Ra /Rg ratio, where Ra and Rg are the resistances of sensor in air ambience and air–gas mixture, respectively. All the sensors were tested three times by following same procedure by varying temperature from 250 to 450 ◦ C with 50 ◦ C intervals. 3. Results and discussion

2.2. Material characterization 3.1. Material characterization For crystal structure analysis, the prepared samples were characterized by powder X-ray diffraction (XRD) using Cu K␣ radiation with Shimadzu 7000 Diffractometer. Morphology of the samples was analyzed by the field emission scanning electron microscope (FESEM) with FEI Quanta 200F. 2.3. Fabrication of thick film sensor and sensor testing set up Following procedure was followed to fabricate thick film sensors. A pinch (2–3 mg) of ZnO powder was mixed properly with two drops of distilled water to make paste. The paste was painted with a fine brush onto an alumina substrate (12 mm × 5 mm size) having pre-deposited gold electrical contacts to obtain a thick film of thickness around 25–27 ␮m (thickness of the film is controlled by masking of cellulose tape of known thickness). For deposition of gold contacts, liquid bright gold (manufactured by Hobby Colorobbia Bright Gold) was painted with brush on alumina substrates leaving a 2 mm gap in the middle which followed heat treatment to convert the paint into metallic gold. To obtain sensors of identical geometry, alumina substrates were appropriately masked using commercially available cellulose tape and after painting them with sensing material, extra wet material was removed. The sensor design is shown in Fig. 1.

Fig. 2 represents the X-ray diffraction pattern of materials synthesized at various pH values (from 8 to 11). The peaks visible in the graphs are in well agreement with standard available data, and these depict the wurtzite hexagonal structure of nanosized zinc oxide. Fig. 3(a)–(d) represents the FESEM images of the nanostructured 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 like structures of ZnO are formed. At pH 9 and 10 fusion and agglomeration of these rods have been observed; moreover nanorods are no longer morphologically distinct. With further increase in pH to 11, a riveting situation has emerged where material did not grow as rods but morphed into particles instead. Lu and Yeh [24] have obtained results otherwise; during hydrothermal processing with the increase in pH of starting solution from 9 to 12, morphology of obtained ZnO powder has changed from ellipsoidal-shape to rod like 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 are strongly attracted by the positively charged, Zn-terminated, surfaces [20,25]. At pH 8 we acquired fine nanorods of ZnO, at this stage

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pH11

Intensity (a.u.)

pH10

20

30

40

50

(103) (200) (112) (201)

(110)

(102)

(101)

(100) pH8

(002)

pH9

60

70

80

2θ (deg.) Fig. 2. X-ray diffraction patterns of the powders synthesized at different pH values.

the solution is basic with hydroxyl ions dominating the reaction. Li et al. [25] found that growth velocities along certain crystalline direction depend upon the pH, and preferred growth direction for basic and neutral solution is along (0 0 0 1), thus ultimately resulting in synthesis of fine nanorods at pH 8. According to another point of view, anisotropic crystal growth is required for the formation of nanowires or nanorods, i.e. the crystal grows along a certain orientation faster than other directions. The driving force for the synthesis of nanorods and nanowires by spontaneous growth is the decrease in Gibbs free energy, which arises from either recrystallization or a decrease in supersaturation [26]. At pH 8 the formation of well-defined rods is due to the fact that system tends to minimize the total energy attributed by spontaneous polarization. The spontaneous polarization results from the non-centrosymmetric ZnO crystal structure. In (0 0 0 1) facetdominated single crystal nanorods, positive and negative ionic charges are spontaneously established on the zinc and oxygenterminated (0 0 0 1) surfaces, respectively [25,26]. At the elevated pH 9 and 10, agglomeration of rods has been observed. 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, rods tend to agglomerate and the material loses its well defined rod-like

Fig. 3. FESEM images of the powders synthesized at (a) pH 8, (b) 9, (c) 10 and (d) 11, respectively.

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a

6

pH 8 pH 9 pH 10 pH 11

5

Methanol Sensing Response

shape. That is what has probably happened at pH 9 and 10 and the obtained entities in these cases are somewhat like bundle of rods welded together. At pH 11 we obtained particles instead of rods, the change in morphology of the ZnO may be explained by the fact that at such a high pH the hydroxyl ions have dominated the reaction. The polymeric chain at high pH is larger than the one at low pH. The high concentration of OH ions has resulted in the cyclization because at this high pH intermolecular reaction has dominated intramolecular reaction [27]. Plausibly rods might have taken the form of particles due to cyclization. Another argument that may justify the results obtained is given by Alias et al. [21] and Wei et al. [28] and suggested that at higher pH 10 and 11, concentration of Zn(NH3 )4 2+ reduces, and the dissolution effect becomes more dominant. Wei et al. [28] have found that the ends of rods became flat due to dissolution, but we have observed that rods flattened enough and transformed into particles. Cao [26] has suggested that high supersaturation results in homogenous nucleation, at high pH of 11, supersaturation increased and thus we obtained ZnO particles instead of rods.

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4

3

2

1

0 250

300

350

400

450

o

Temperature ( C)

3.2. Sensing performance

b 20 pH 8 pH 9 pH 10 pH 11

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Ethanol Sensing Response

16 14 12 10 8 6 4 2 0 250

300

350

400

450

400

450

o

Temperature ( C)

c 25 pH 8 pH 9 pH 10 pH 11

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Propanol Sensing Response

Sensors fabricated from powder synthesized at different pH values were exposed to 250 ppm of methanol, ethanol and propanol at different temperatures and results are shown in Fig. 4, where error bars represent standard deviation. Study revealed that optimum operating temperature of all the sensors for all the test species remained invariant at 400 ◦ C. The comparative response of VOCs for all the sensors based on samples prepared at different pH values is shown in Fig. 5, where error bars represent standard deviation. It is evident from figure that sensing response is exceptionally higher for the sample corresponding to pH 11 as compared to pH 8, 9 and 10. The sensors fabricated from powder synthesized at pH 11 were exposed to 250 ppm methanol, ethanol and propanol at 400 ◦ C and their resistance variation with time is shown in Fig. 6. From this figure it is clear that the response and recovery time of the fabricated sensors are very fast. Under similar conditions, the sensing response magnitude of samples synthesized at various pH values varies as the following order: pH 11 > pH 10 > pH 9 > pH 8. The reason for increase in the sensing response towards higher pH values may be ascribed to the material morphology. From FESEM images it is clear that morphology changes from rods to particles with increase in pH. Evidently, at pH 8 rods are formed, at pH 9 rods and limited number of particles coexist whereas at pH 10 number of particles increases significantly. The presence of nanoparticles along with nanorods enhance the surface area and hence the sensing response. At pH 11 there are only nanoparticles and have maximum sensing response because of greater surface area as compared to rods obtained at other pH values. It is well known fact that smaller the size larger will be the surface area, in other words particle size has inverse relationship with the surface area [27]. The sensing is a complex phenomenon which occurs on the surface of the metal oxide semiconductor. Rothschild and Komem [3] have simulated various sensing parameters such as grain size, surface state density, and carrier concentration. with the sensing response. However, the surface reactivity of particles is known to rapidly increase with the increase in surface-to-bulk ratio because the strong curvature of the particle surface generates a large density of defects, which are the most reactive surface sites. Another result obvious from the curves in Fig. 4 is that nanoparticles show better sensing response than nanorods for all tested VOCs. In one of our works we have already reported that ZnO nanoparticles are better candidates for sensing applications than nanorods [29]. In this work we again have arrived to the same conclusion irrespective of technique used

15

10

5

0 250

300

350 o

Temperature ( C) Fig. 4. Sensing response towards 250 ppm of (a) methanol, (b) ethanol and (c) propanol for the samples prepared at different pH at different operating temperatures.

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25

a

Ethanol

0.18

Methanol

Methanol 0.16

Propanol 20

Resistance (M )

Sensing Response

0.14

15

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gas in 0.12 0.10 0.08 0.06 0.04

5

gas out

0.02

8

9

10

0

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for material synthesis. The sensor response is quantitatively determined by number of active sites on the surface of gas sensors. When ZnO nanostructured sensors are exposed to air, oxygen molecules adsorb on the surface of the materials to form O2 − , O− , O2− ions by capturing electrons from the conduction band. Thus the ZnO sensors show a high resistance in air [10]. All the sensors in air at room temperature have resistances in the mega ohm range furthermore the resistances vary between the sensors of same morphologies and between different morphologies.

(ads)

−

+ e ↔ 2O

0.30

(ads)

(2)

R + O− ads → RO + e−

0.20

0.10

0.00

4. Conclusion In this work we used chemical technique to synthesize nanostructured ZnO at different pH values of precursor solution. The pH has played a significant role in altering the morphology of the zinc oxide. At pH 8 nanorods of ZnO have been obtained due to spontaneous polarization which results from the non-centrosymmetric ZnO crystal structure. With increase in pH to 9 and 10, agglomeration due to increase in supersaturation of the system has been

gas out 0

20

40

60

80

100

Time (s)

c

3.5

Propanol

3.0

(4)

The results displayed in Fig. 4 shows the variation of sensing response magnitude for three alcohols in the following order: propanol > ethanol > methanol. The number of methyl groups attached to these alcohols is in the same order as well. Though gas sensing is a complex phenomenon still there are some obvious parameters, which might be playing crucial role in exhibiting this type of sensing response variation. One of the plausible explanations could be on the basis of complete oxidation of these alcohols and in the process consuming 9, 6 and 3 O− ads by propanol, ethanol and methanol, respectively. Gong et al. [30] have reported similar types of results from their study.

gas in

0.15

(3)

When metal oxide based sensors are exposed to reducing agents at moderate temperature, the target gas reacts with the adsorbed oxygen and as a result captured electrons go back to the conduction band. This eventually increases the conductivity of metal oxide based sensors [10]. The reaction can be described as follows:

Ethanol

0.05

(1)

−

100

0.25

2.5

Resistance (M )

O2

−

80

b

Resistance (M )

Fig. 5. Sensing Response of the samples synthesized at different pH values to 250 ppm of different VOCs at optimum operating temperature of 400 ◦ C.

O2(ads) + e− ↔ O2 − (ads)

60

Time (s)

pH

O2(gas) ↔ O2(ads)

40

gas in

2.0 1.5 1.0 0.5 0.0

gas out 0

20

40

60

80

100

Time (s) Fig. 6. Sensor resistance variation with time for the samples synthesized at pH 11 for 250 ppm of (a) methanol (b) ethanol and (c) propanol at optimum operating temperature of 400 ◦ C.

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observed. At pH 11 it was found that structure had altered more prominently and nanorods had morphed into nanoparticles. The driving force for this alteration is imputed to the dominance of dissolution effect and homogeneous nucleation at pH 11. The pH played a pivotal role in controlling the morphology of zinc oxide. Sensing performance of these powders was investigated for 3 alcohols and we perceived that sensing response depends upon surface morphology. We ascertained that nanoparticles have better response than nanorods for all the VOCs. All the sensors demonstrated similar trend for the response towards test alcohols in the following manner propanol > ethanol > methanol. Acknowledgments Authors would like to thank following: University Grants Commission, New Delhi, India for financial support and IIT Roorkee for FESEM investigations. One of the authors Onkar Singh thanks CSIR, New Delhi for senior research fellowship. References [1] N.H. Al-Hardan, M.J. Abdullah, A.A. Aziz, Sensing mechanism of hydrogen gas sensor 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 by an 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 nanocrystalline 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 humidity 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. Katsarakis, 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 ZnO powder 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 hybrid ZnO LED structure grown on p-type GaN by metal organic chemical vapor deposition, Physica B 401 (402) (2007) 386–390. [8] W.S. Lee, G.W. Choi, Y.J. Seo, Surface planarization of ZnO thin film for optoelectronic applications, Microelectron. J. 40 (2009) 299–302. [9] A. Umar, M.M. Rahman, A. Al-Hajry, Y.B. Hahn, Highly-sensitive cholesterol 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 sensing property, Sens. Actuators B 146 (2010) 129–137. [11] X.J. Liu, C. Song, F. Zeng, F. Pan, Donor defects enhanced ferromagnetism in Co:ZnO films, Thin Solid Films 516 (2008) 8757–8761. [12] M.S. Samuel, J. Koshy, A. Chandran, K.C. George, Electrical charge transport 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 properties of sol–gel prepared ZnO films, J. Appl. Phys. 108 (2010) 043513–043520.

443

[15] T. Terasako, M. Yagi, M. Ishizaki, Y. Senda, H. Matsuura, S. Shirakata, Growth of zinc oxide films and nanowires by atmospheric-pressure chemical vapor deposition 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, ZnO transparent conducting films deposited by pulsed laser deposition for solar cell applications, Thin Solid Films 431–432 (2003) 369–372. [17] V.R. Shinde, T.P. Gujar, C.D. Lokhande, LPG sensing properties of ZnO films prepared 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 conditions, 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 on the 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 properties synthesized 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, Synthesis and 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 of zinc 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 and particle 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 luminescence 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 Applications, Imperial College Press, London, 2004. [27] P.K. Sharma, M.H. Jilavi, V.K. Varadan, Influence of initial pH on the particle size and 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 mechanism 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 nanorods and nanoparticles by chemical route and their comparative study as ethanol sensors, Sens. Actuators B 135 (2008) 352–557. [30] H. Gong, Y.J. Wang, S.C. Teo, L. Huang, Interaction between thin-film tin oxide gas 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 sized metal 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. Presently he is working as a lecturer in the Department of Applied Sciences, Khalsa College of Engineering 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 nanostructured 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 institute for one year, and moved to post-doctoral position at Simon Fraser University, Canada in 1990. He moved to Guru Nanak Dev University Amritsar in 1993, where he is presently working as an associate professor of physics. His recent interests are material research for gas sensing and development of new experiments for physics education.