SEMICONDUCTING oxides with many novel properties

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1 4700 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015 Study on Effect of [OH-] Linkages on Physical, Electrical, and Gas Sensing Properties of ZnO Nanoparticles Shweta Jagtap and Kaustubh Priolkar Abstract This paper reports the growth mechanism of sol gel derived nanocrystalline ZnO powder. The influence of ph value on the particle size and its properties are studied using X-ray diffraction, UV, Fourier transform infrared spectroscopy, photoluminescence (PL), and impedance spectroscopy. UV analysis reveals an increase in bandgap due to decrease in particle size. The strong UV emission along with some defect sites is also observed from PL spectra. The presence of Zn-OH linkages are observed in the case of ZnO with lower ph values (ph 7, 8, and 9) which are responsible for lower sensitivity toward volatile organic compounds. However, higher ph values (ph 10 and 11) have absence of Zn-OH linkage, which improves the gas sensing performance. Index Terms ZnO, nanoparticles, ph, sensor response, VOCs. I. INTRODUCTION SEMICONDUCTING oxides with many novel properties from electrical, chemical, optical, magnetic are fundamental to the development of smart and functional materials, devices and systems. ZnO is one of the n type semiconductor materials having a wide band gap of 3.37eV and large exciton binding energy of 60 mev at room temperature. In ZnO, zinc coordination is an indicator of covalent bonding, however the ZnO bond also possess a very strong ionic character therefore, ZnO is useful in many applications such as solar cell, gas sensor, piezoelectric devices, and acoustic [1] [3]. Soft chemistry routes in particular sol gel procedure have many advantages such as ability to achieve superior purity material, compositional homogeneity of the products at moderate temperatures. Although aqueous sol-gel approaches were highly successful in the synthesis of bulk metal oxides, some major limitations are brought forward when it comes to the preparation of their nanoscale counterparts. The complexity arises mainly due to the high reactivity of the metal oxide precursor and double role of water as ligand and solvent [4] [6]. Non aqueous sol gel processes in organic solvents are able to overcome some of the limitations of aqueous systems. Organic compounds are not only the Manuscript received March 12, 2015; revised April 8, 2015; accepted April 8, Date of publication April 27, 2015; date of current version June 23, This work was supported by the University Grants Commission D. S. Kothari Post-Doctoral Fellowship. The associate editor coordinating the review of this paper and approving it for publication was Prof. Michael J. Schöning. The authors are with the Department of Physics, Goa University, Panaji , India ( shweta.jagtap@gmail.com; krp@unigoa.ac.in). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JSEN oxygen supplying agent for metal oxides but also strongly influence the particle size, shape, surface and assembly properties. The slow reaction rates that cause moderate reactivity, in combination with the stabilizing effect of the organic species lead to the formation of highly crystalline products that are often characterized by uniform particle morphologies and particle size in the range of just few nanometers. The oxygen, for nanoparticle formation is provided by the solvents (ethers, alcohols, aldehydes etc) or by organic constituents of precursors. Although ph of a solution is one of the most easily measurable and controllable parameters, 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 [7]. Rizwan et al. [8] found variation from plate-like to flower-like morphology on increasing ph from 6 to12; they concluded that lower ph was suitable for obtaining 2D structure where as higher ph values can result in rod like structure. Meng et al. [9], [10] has also reported SnO 2 -graphene nanocomposite for ppb level benzene detection and ZnO flower like hierarchical structures for VOCs. The theoretical investigations on the growth of nanocrystals tells that the growth mechanism could be either controlled by diffusion of the particles, i.e., Ostwald ripening, or controlled by the reaction at the surface or by both parameters, even in the absence of capping agents. Several investigations in the literature report the growth of ZnO nanocrystals carried out in the presence of capping agents. In these cases, the growth would be a product of several complex processes, interactions with the capping agents, which is necessary to control the size and size distribution of the ZnO nanocrystals. Further ZnO has been traditionally employed as a gas sensor by making use of the change in resistivity on the exposure to the relevant gases or vapours. 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 [7], [11]. In this paper we have demonstrated a simple nanoparticle synthesis route without using any special capping agent. The effect of changing ph of the solution on X 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 JAGTAP AND PRIOLKAR: STUDY ON EFFECT OF [OH-] LINKAGES ON PHYSICAL, ELECTRICAL, AND GAS SENSING PROPERTIES 4701 particle size is studied. It is observed that this variation is due to the presence/absence of Zn-OH linkage. Further the effect of OH linkages is studied for the sensitivity towards gas sensing. To our information, as on today, no research group has reported this OH linkages effect in case of VOCs. II. EXPERIMENTAL The ZnO sols were prepared by dissolving Zinc Acetate dihydrate (CH 3 COO) 2 Zn.2H 2 O, (SD Fine-chem. Ltd) in methanol at room temperature. A clear solution of 0.2 M was obtained by continuous stirring of the prepared solution at room temperature for 60 min. The solution was found to be stable and transparent with no precipitate. ZnO powders were prepared by varying the ph value of the solution from 7 to 11. The ph value of the solution was adjusted to the desired value using sodium hydroxide (0.1N NaOH, Qualigens) solution. The solution was kept for 48 hrs, to complete the gelation and hydrolysis process. During this period of time, white ZnO precipitates slowly crystallize and settle down at the bottom of the flask. The white precipitate was filtered and washed with excess methanol to remove the starting material and dried at 120 C [2]. The pellets of identical dimensions (0.95 cm diameter and 0.15 cm thickness) were made by using Hydraulic press and then were used for measurements. Prepared ZnO powders were characterized by X-ray diffraction (XRD) collected using RIGAKU diffractometer with Cu Kα radiations in the range 2 =20 to 80. Fourier transform infrared spectroscopy (FTIR) spectra were recorded with a SHIMADZU FTIR-8900 spectrometer in the range of cm 1. The spectral absorption was determined using UV visible spectrometer (Model: 2401PC, Make: Shimadzu) in the wavelength ranging from 200 nm to 800 nm. For transmission electron microscopy (TEM) studies, JEOL Transmission Electron Microscope-1200EX (120KV) was employed and images were recorded. The photoluminescence (PL) spectra were measured by using Agilent Cary eclipse spectrofluorometer. Impedance measurements were made using Wayne Kerr Precision component analyzer (6440B). Measurements were carried out at room temperature to extract the electrical properties of the prepared ZnO pellets in the frequency range of 20 Hz to 3 MHz. Fig. 1. XRD of ZnO powders. TABLE I DATA OF PARTICLE SIZE AND LATTICE PARAMETERS III. RESULTS AND DISCUSSION A. XRD Analysis The XRD pattern of prepared powder samples with variable ph values are shown in figure 1. The XRD analysis of all the powder samples depicts the wurtzite hexagonal structure (JCPDS data no ). In case of ph 7, 8 and 9 powder samples, peak broadening can be seen as compared to the samples with higher ph. Additionally, for ph 7, 8 and 9 powder samples, the XRD pattern shows the presence of Zn(OH) 2 phases (JCPDS data no ). It is also noted that the degree of crystallinity increases with increasing the ph of the solution. The particle size for all powders was calculated from the FWHM in XRD pattern using Scherrer s formula. Lattice parameters for all samples are also calculated and given in table 1. Fig. 2. UV-visible absorption spectra for ZnO nanoparticles with variable ph. B. UV-Visible Absorption The optical absorption measurement is an initial step to observe the nanoparticle behavior. Figure 2 gives the room temperature absorption spectra of ZnO nanoparticles prepared using different ph values. The excitonic peak for all ZnO nanoparticles prepared with different ph are found to be blue shifted with respect to their bulk. This can be due to the confinement effects. The absorption exhibits a blue shift from 3.37 to 3.47 ev as the size of ZnO nanocrystalline decreases from31to14nm.

3 4702 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015 Fig. 3. Variation of band gap with particle size of ZnO powder. Fig. 5. PL spectra of ZnO nanoparticles with different ph values. Fig. 4. FTIR of the ZnO powders with variable ph. The variation in the band gap of nanoparticles can be also evaluated from the effective mass model expression as, E g = E bulk g + h2 π 2 ( 1 2er ) 1.8e2 (1) m e m h 4ππε 0 r Where E bulk g is the bulk energy gap, r is the crystallite radius, m e is the effective mass of the electrons, m h is the effective mass of the holes, ε is the relative permittivity, ε 0 is the permittivity of free space, h is the plank s constant, e is the charge of the electron. For ZnO, E bulk g = 3.35eV, m e = 0.24m 0,m h = 0.45m 0,wherem 0 is the free electron mass. Figure 3 shows a plot of the band gap variation with variation in the size of the ZnO nanoparticles. The solid curve shows the theoretical results of Eq. (1) for theoretical calculations; the values of r have been taken from the XRD data. It is noticed that the band gap measured from absorption spectra closely coincides with the theoretical results. C. Fourier Transform Infrared Spectroscopy Figure 4 shows FTIR analysis of the ZnO powders in the range of 350 to 4000 cm 1 at room temperature. It is seen from the figure that presence of strong stretching mode of Zn-O at 450 cm 1 was also noted for all samples. Along with stretching mode, for lower ph values i.e. ph 7, ph8 and ph 9, two bands at 1560 and 1393 cm 1 corresponding to C=O and C-H stretching vibrations are observed. These bands are the characteristic of acetate group complexes to zinc species [12]. This impurity is due to the unreacted acetate at lower ph. Also a small peak appearing at 1335 cm 1 could be due to stretching of CO bond of alcohols [13]. In addition to that spectra showed a strong band at 3400 cm 1, which is attributed to OH stretching mode [14]. Even though all the samples were prepared in identical conditions, samples with lower ph showed presence of OH ions. It is possible that presence of OH ions has its roots in preparation process. As these samples were prepared using Zn acetate and NaOH as starting chemicals, some of the OH ions present in starting materials could be linked to the Zn ions in the surface layers on ZnO particle. It can also be seen that the stretching mode progressively disappears with increase in ph and is nearly absent for powders prepared using solution of ph 10 and 11. Concomitantly, the ZnO stretching mode at 450 cm 1 sharpens lending weight to hypothesis that ph ions are indeed bonded to Zn ions in the surface layers of ZnO nanoparticles. It may be mentioned here that all IR spectra were recorded under identical conditions. D. Photoluminescence The room temperature PL spectra for ZnO powders with different ph values are shown in figure 5. The excitation wavelength used here is 255nm. The strong UV emission for all samples are observed at wavelength of nm. The other defect bands are also observed for all the samples. It is known that different types of defects are responsible for green, yellow, and orange-red emissions [15]. It is well known that, the UV emission was attributed to the near band edge emission of the wide band gap ZnO, which is responsible for the excitonic recombination. It has been already demonstrated that, the green band emission corresponds to

4 JAGTAP AND PRIOLKAR: STUDY ON EFFECT OF [OH-] LINKAGES ON PHYSICAL, ELECTRICAL, AND GAS SENSING PROPERTIES 4703 the crystal defects such as vacancies, interstitial sites in ZnO. These green emissions originated from the recombination of the holes with the electrons occupying the singly ionized oxygen vacancies. The mechanism of the green emission has been suggested to be mainly due to the concentration of free electrons and the existence of various point defects. The deeplevel emission, i.e. green PL, of ZnO is usually ascribed to structural defects and impurities [15]. The emission band found at 420 nm is attributed to the interstitial oxygen [14]. Blue emissions at 465 nm and 488 nm is due to the intrinsic defects i.e. O and Zn vacancies or interstitials and their complexes [17]. In addition to this, samples with ph 7, 8 and 9 shows considerable emission at 590 nm. This emission is due to the presence of OH ions at the surface [18]. Minor emission at 698 nm is also noted for all the samples, which is attributed due to the presence of excess of oxygen [19]. E. Transmission Electron Microscopy TEM images of the samples are shown in figure 6. It can be seen that the samples with higher ph values (ph 10, 11) are having larger particle size as compare to the lower ph (ph 7, 8, 9). The particle size varies from 9 to 15nm and the size trend of the ZnO nanoparticles from the TEM micrograph agrees with the XRD results. During the ZnO nanoparticle formation, each particle is surrounded by the CH3 COONa phase, because the volume fraction of the product phase is less than the percolation limit, the formation of separated ZnO nanoparticles are guarantee [2]. The initial decrease in particle size up to the critical value of ph i.e. ph 9 can be correlated with the amount of NaOH added. Here, the presence of NaOH provides the hydroxyl ions necessary for formation of ZnO nanocrystals as well as forms a protective layer limiting the growth of nanocrystals. This is supported by FTIR and PL results. The presence of OH ions in the surface layers is significant in case of powders prepared with solution of ph 7, 8 and 9 samples while it is negligibly small in case of samples prepared with ph 10 and 11 solution. Thus we can say that, for ph 7, 8 and 9 the presence of OH ions acts as a capping agent which prevents the growth. Further increase in particle size is observed in case of ph 10 and 11 can also be related to the absence of OH ions. F. Impedance Analysis The electrical impedance spectroscopy can yield a specific insight into the sensing mechanism, whereby the contribution of the sensor elements can be distinguished [16]. The sensing mechanism generally involves the conduction process (resistor R) and polarization behavior (capacitor C). The relationship between the real part (Z ) and the imaginary part (Z ) of the complex impedance (Nyquist diagrams) of ZnO-based sensors is shown in Figure 7. For ph 7, 8 and 9, incomplete semicircle at high frequency with low frequency spur is observed. Incomplete semicircle is mainly due to the increase in relaxation time occurred [20]. Here in this case, low frequency conduction process is may be due to the presence of OH ions [21], [22]. Fig. 6. TEM images of the ZnO powder with different ph values: (a) ph7, (b) ph8, (c) ph9, (d) ph10, and (e) ph11. In case of ph 10 and 11 samples, a single incomplete semicircle is observed which is may be due to increase in relaxation time which could be due to increase in particle size.

5 4704 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015 Fig. 8. Hypothetical microstructure model for ZnO. Fig. 9. Equivalent circuit for ZnO. behavior is observed. It is also seen from the figure that diameter of semicircle is also varied for variable ph values. Depressed semicircles indicate increase in conductivity of ZnO pellets. Here in this case, the overall conduction is mainly related to: intergranular contact, bulk conductivity and electrode contact as seen from the figure 8. The intergranular contacts are consisting of space charge layer depleted of electrons is usually more resistive than the bulk. The total conductance is then determined by the percolation path through the low resistance of the bulk grains in series with the high resistance of intergranular contact [23]. Electrons must overcome the intergranular contact barrier in order to cross from grain to another for conduction. The capacitive contribution of the electrode contact (EC) originates from the capacitance between the electrode and the conducting core, surrounded by a less conducting ZnO shell. Hence as seen from the figure 9, the equivalent circuit is consists of RB, R gb C gb and R C C C. Where, RB representing the bulk contribution, a parallel (R gb C gb ) represents the element for the intergranular contact, and a parallel (R C C C ) element denotes the electrode contact. Fig. 7. Nyquist plots (Z versus Z ) for the ZnO powder with different ph values. As no low frequency spur is observed (no OH ions presence), the process is dominated by charge transfer resistance from the nanoparticle interior and only high frequency capacitive G. Gas Sensing Properties Sensors fabricated from powder synthesized at different ph values were exposed to 200 ppm of methanol, ethanol and propanol at different temperatures. The comparative response of VOCs for all the sensors with different ph values is shown in Figure 10. Gas sensor response (Rs) is defined as the ratio of resistance of a sample (at an operating temperature) in air to its resistance in exposed test gas and is given as Rs = (R air /R gas ) [24]. Study revealed that optimum operating temperature of all the ZnO sensors is found to be at 250 C. The fabricated sensors showed maximum response at 250 C, and it was noted that further increased in temperature is

6 JAGTAP AND PRIOLKAR: STUDY ON EFFECT OF [OH-] LINKAGES ON PHYSICAL, ELECTRICAL, AND GAS SENSING PROPERTIES 4705 of nanoparticles plays a major role in adsorption process of gases on the surface of ZnO pellet. More the surface area available greater is the adsorption of on ZnO pellet surface hence higher the sensitivity. Since the samples prepared under lower ph conditions have smaller particle size, they naturally have higher surface to volume ratio and therefore should have exhibited higher sensitivity towards gas sensing. However this is not true for our case. For ph 7, 8 and 9 samples, even though they smaller particle size they shows less sensitivity towards VOCs. This is due to the fact that, these samples shows presence of OH linkages i.e. presence of Zn(OH) 2 phase in the diffraction patterns and presence of O-H stretching and bending modes in IR spectra. These OH ions are attached to the Zn ions present in the surface layers of nanoparticles. This Zn-OH linkages prevents adsorption of gases on the surface of nanoparticles thereby lowering the sensitivity of these samples. However, samples prepared at higher ph (ph 10 and 11) Zn(OH) 2 phase is absent. Hence more pure ZnO is available when exposed to the formed gas in the chamber. It is observed that highest sensitivity is obtained in case of samples prepared with ph 11 solution. Similarly as seen from the PL spectra, ZnO sample with ph 11 shows considerable blue emission at 465 nm as compared to other samples. This emission is attributed to oxygen vacancies. During the conduction process, the intrinsic oxygen vacancies are extracted from the conduction band and are trapped at the surface leading to an electron depleted surface region called as space charge layer. It is unlikely that a change in <1% the surface coverage causes a factor of 100 change in the total resistance. Therefore, the surface barrier at intergraular contact plays an important role in achieving high sensitivity in this case. Therefore these samples in spite of having larger particle sizes show higher sensitivity towards VOCs. H. Response and Recovery Time As the sample with ph 11 is showing highest sensitivity towards propanol, the sensing response and recovery at 200 ppm with time at the operating temperature of 250 C was measured. The response and recovery time were found to be 4 s and 10 s, respectively. Fig. 10. Sensing response of ZnO with different ph values for VOCs. responsible for decrease in Rs. It is evident from figure that sensing response was substantially higher for samples prepared at higher ph. The sensitivity order is as follows, ph7 < ph8 < ph9 < ph10 < ph11. It is also observed that the variation of sensing response magnitude for three alcohols in the following order: propanol>ethanol>methanol. The lower sensitivity for lower ph values can be correlated to the presence of OH ions. As sensitivity depends on the surface area of the nanoparticles, the surface to volume ratio I. Repeatability and Reproducibility Tests The repeatability and reproducibility test were carried out for all the samples. It is found that for cycle to cycle variation of prepared samples showed the uncertainty of ±3% in resistance and ±3% in sensor response and uncertainty of about ±5% in resistance with ±2% in sensor response in case of sample to sample variation. The sensing is a complex phenomenon which occurs on the surface of the metal oxide semiconductor. 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. 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

7 4706 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015 materials to form O 2,O,O 2 ions by capturing electrons from the conduction band. Thus the ZnO sensors show a high resistance in air. The surface of the ZnO material undergoes the following reactions [24], O 2 (gas) O 2 (ads) O 2 (ads) + e O 2 (ads) O 2 (ads) + e 2O (ads) O (ads) + e O 2 (ads) For ZnO, the O 2 species dominates at temperature below 150 C, and above this temperature O species dominate [24]. At grain boundaries, adsorbed oxygen forms a potential barrier, which prevents carrier from moving freely. The electrical resistance is attributed to this potential barrier. In the presence of reducing gases, reducing gases reacts with the highly reactive chemisorbed oxygen and hence the surface density of negatively charged oxygen decreases and the bound electrons are made free leading to the reduction of the barrier height in the grain boundary. The reduced barrier height decreases the sensor resistance. In presence of air, oxygen is ionosorbed on the ZnO surface and ionosorbed species acts as electron acceptor due to their relative energetic position with respect to the fermi level. Depending upon the temperature, oxygen is ionosorbed on the surface as O 2 (for lower temperature i.e. below 150 C) and O ions from 150 to 400 C. The electrons required for this process originates from the donor sites. The second reasonable explanation could be because O atoms to form O get an electronic release energy (electron affinity: 141 kj/mol), O and then get an O 2 need to absorb energy electrons (electron affinity: 780 kj/mol). Lower temperatures favor the formation of the former, high temperatures favor the formation of the latter. Furthermore, according to the molecular orbital theory, O 2 is more stable than the O 2 and O 2 2. O 2 and O 2 2 are easier to decompose at high temperatures than at low temperatures. 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: R + O (ads) RO + e The possible explanations for gas sensing is explain 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. Here the phenomenon can be studied considering ethanol for example and shown in figure 11. The target gas (ethanol) may undergo different reactions, and then can take two routes of decomposition reaction, i.e. Dehydration and dehydrogenation: C 2 H 5 OH C 2 H 4 + H 2 O (acidic oxide) 2C 2 H 5 OH 2CH 3 CHO + H 2 (basic oxide) Fig. 11. Band diagram of ZnO for ethanol sensing. These primary products thus formed are consecutively oxidized to CO, CO 2 and H 2 O. C 2 H 4 + 3O 2 2 (ads) 2CO 2 + 2H 2 O + 6e 2CH 3 CHO(ads) + 5O 2 2 (ads) 4CO 2 + 4H 2 O + 10e IV. CONCLUSION Structural and optical properties of synthesized ZnO nanoparticles with variable ph values have been studied. It is noted that, presence of OH ions are responsible for smaller particle size. The presence of Zn-OH linkages observed in case of ZnO nanoparticles with lower ph values (ph 7, 8 and 9) are responsible for lower sensitivity towards volatile organic compounds (VOCs). However higher ph values (ph 10 and 11) with absence of Zn-OH linkage improves the gas sensing performance. The repeatability and reproducibility measurements show that the prepared sensor can be used as a VOCs gas sensor. ACKNOWLEDGMENTS The authors also like to thank the Department of Physics, University of Pune for TEM characterization. REFERENCES [1] N. Singh, P. Pandey, and F. Z. Haque, Effect of heat and time-period on the growth of ZnO nanorods by sol gel technique, Opt.-Int. J. Light Electron Opt., vol. 123, no. 15, pp , [2] S. Rani, P. Suri, P. K. Shishodia, and R. M. Mehra, Synthesis of nanocrystalline ZnO powder via sol gel route for dye-sensitized solar cells, Solar Energy Mater. Solar Cells, vol. 92, no. 12, pp , [3] Z. Chen, X. X. Li, G. Du, N. Chen, and A. Y. M. Suen, A sol gel method for preparing ZnO quantum dots with strong blue emission, J. Lumin., vol. 131, no. 10, pp , [4] H.-W. Ryu et al., ZnO sol gel derived porous film for CO gas sensing, Sens. Actuators B, Chem., vol. 96, no. 3, pp , [5] G. S. Devi, V. B. Subrahmanyam, S. C. Gadkari, and S. K. Gupta, NH 3 gas sensing properties of nanocrystalline ZnO based thick films, Anal. Chim. Acta, vol. 568, nos. 1 2, pp , [6] M. Dutta, S. Mridha, and D. Basak, Effect of sol concentration on the properties of ZnO thin films prepared by sol gel technique, Appl. Surf. Sci., vol. 254, no. 9, pp , 2008.

8 JAGTAP AND PRIOLKAR: STUDY ON EFFECT OF [OH-] LINKAGES ON PHYSICAL, ELECTRICAL, AND GAS SENSING PROPERTIES 4707 [7] O. Singh, M. P. Singh, N. Kohli, and R. C. Singh, Effect of ph on the morphology and gas sensing properties of ZnO nanostructures, Sens. Actuators B, Chem., vols , pp , May [8] R. Wahab, S. G. Ansari, Y. S. Kim, M. Song, and H.-S. Shin, The role of ph variation on the growth of zinc oxide nanostructures, Appl. Surf. Sci., vol. 255, no. 9, pp , [9] F.-L. Meng et al., Parts per billion-level detection of benzene using SnO 2 /graphene nanocomposite composed of sub-6 nm SnO 2 nanoparticles, Anal. Chim. Acta, vol. 736, pp , Jul [10] F. Meng et al., Flower-like hierarchical structures consisting of porous single-crystalline ZnO nanosheets and their gas sensing properties to volatile organic compounds (VOCs), J. Alloys Compounds, vol. 626, pp , Mar [11] M. Yin, M. Liu, and S. Liu, Development of an alcohol sensor based on ZnO nanorods synthesized using a scalable solvothermal method, Sens. Actuators B, Chem., vol. 185, pp , Aug [12] N. F. Hamedani and F. Farzaneh, Synthesis of ZnO nanocrystals with hexagonal (Wurtzite) structure in water using microwave irradiation, J. Sci., Islamic Republic Iran, vol. 17, no. 3, pp , [13] G. Xiong, U. Pal, and J. G. Serrano, Correlations among size, defects, and photoluminescence in ZnO nanoparticles, J. Appl. Phys., vol. 101, no. 2, pp , [14] M. M. Ba-Abbad, A. A. H. Kadhum, A. B. Mohamad, M. S. Takriff, and K. Sopian, The effect of process parameters on the size of ZnO nanoparticles synthesized via the sol gel technique, J. Alloys Compounds, vol. 550, pp , Feb [15] A. B. Djurišić et al., Defect emissions in ZnO nanostructures, Nanotechnology, vol. 18, no. 9, 2007, Art. ID [16] Z. G. Wang, X. T. Zu, S. Z. Yang, and L. M. Wang, Blue luminescence from carbon modified ZnO nanoparticles, J. Mater. Sci., vol. 41, no. 12, pp , [17] L. Dai, X. L. Chen, W. J. Wang, T. Zhou, and B. Q. Hu, Growth and luminescence characterization of large-scale zinc oxide nanowires, J. Phys, Condens. Matter, vol. 15, no. 13, pp , [18] K. H. Tam et al., Defects in ZnO nanorods prepared by a hydrothermal method, J. Phys. Chem. B, vol. 110, no. 42, pp , [19] P.-T. Hsieh, Y.-C. Chen, C.-M. Wang, Y.-Z. Tsai, and C.-C. Hu, Structural and photoluminescence characteristics of ZnO films by room temperature sputtering and rapid thermal annealing process, Appl. Phys. A, vol. 84, no. 3, pp , [20] T. P. Hülser, H. Wiggers, F. E. Kruis, and A. Lorke, Nanostructured gas sensors and electrical characterization of deposited SnO 2 nanoparticles in ambient gas atmosphere, Sens. Actuators B, Chem., vol. 109, no. 1, pp , [21] Y.-M. Lee, C.-M. Huang, H.-W. Chen, and H.-W. Yang, Low temperature solution-processed ZnO nanorod arrays with application to liquid ethanol sensors, Sens. Actuators A, Phys., vol. 189, pp , Jan [22] E. Barsoukov and R. Macdonald, Eds., Impedance Spectroscopy: Theory, Experiment, and Applications. New York, NY, USA: Wiley, [23] J. Lee, J.-H. Hwang, J. J. Mashek, T. O. Mason, A. E. Miller, and R. W. Siegel, Impedance spectroscopy of grain boundaries in nanophase ZnO, J. Mater. Res., vol. 10, no. 9, pp , [24] T. T. Trinh et al., Improving the ethanol sensing of ZnO nano-particle thin films The correlation between the grain size and the sensing mechanism, Sens. Actuators B, Chem., vol. 152, no. 1, pp , Feb Shweta Jagtap received the M.Sc. and Ph.D. degrees in electronic science from Pune University, Pune, India, in 2006 and 2010, respectively. She is currently a Post-Doctoral Fellow with the Department of Physics, Goa University, Goa. Her research interests include the synthesis and characterization of nanomaterials and their applications in thick- and thin-film sensors. Kaustubh Priolkar received the Ph.D. degree in physics from Goa University, in He joined Goa University, in 1999, where he is currently a Professor with the Department of Physics. His current area of interest is structure and physical property correlation in complex materials using X-ray absorption spectroscopy and neutron scattering.