Investigation of ethanol vapor sensing properties of ZnO flower-like nanostructures

Measurement 73 (2015) 588–595

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Investigation of ethanol vapor sensing properties of ZnO flower-like nanostructures S. Safa a, R. Azimirad a,⇑, Kh. Mohammadi a, R. Hejazi b, A. Khayatian c a

Malek-Ashtar University of Technology, Tehran, Iran Department of Chemistry, Iran University of Science and Technology, Tehran, Iran c Department of Physics, University of Kashan, Kashan, Iran b

a r t i c l e

i n f o

Article history: Received 23 November 2014 Received in revised form 30 May 2015 Accepted 4 June 2015 Available online 26 June 2015 Keywords: Zinc oxide Nanoflowers Hydrothermal Gas sensor Sensitivity

a b s t r a c t ZnO flower-like nanostructures were synthesized using zinc nitrate, zinc acetate and zinc sulfate precursors by hydrothermal method. The samples were characterized by X-ray diffraction, scanning electron microscopy and Fourier transform infrared analyses. All of the ZnO samples synthesized by the different precursors were crystallized in a same wurtzite hexagonal structure with different properties such as various morphologies, effective surface areas, concentrations of OH surface groups, and gas sensing performances. It was observed that flower-like ZnO nanostructures grown by zinc nitrate and zinc acetate included nanopetals with thicknesses of 30 and 50 nm, respectively. For the ZnO sample based on zinc sulfate, microspheres were constructed from nanoplatelets with thickness of 100 nm. The gas sensing properties of the samples toward ethanol vapor were examined and compared at various working temperatures. It was observed that ZnO[nitrate] nanoflower sample was the optimal choice for practical applications (with considerable sensitivity of 70% beside fast response time of 25–27 s at low working temperature of 130 °C). The suitable ethanol sensing characteristics of ZnO[nitrate] sample was explained by its highly porous structure as the mean surface area of the samples calculated from BET method followed from ZnO[nitrate] > ZnO[acetate] > ZnO[sulfate] sequence. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction As a functional semiconducting material, zinc oxide (ZnO) with wide direct band-gap energy of 3.37 eV, high chemical stability in operating conditions [1] and easy-to-use for microelectronic processing has been widely employed in photonic crystals [2], light emitting diodes [3], room temperature UV lasers [4] and chemical gas sensors [5–7]. In the recent years, the development of gas sensors to detect harmful and toxic gases has persuaded many researchers to work on this field due to the concerns for the environmental protection, domestic gas ⇑ Corresponding author. E-mail address: [email protected] (R. Azimirad). http://dx.doi.org/10.1016/j.measurement.2015.06.001 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.

alarms and safety requirements for human health [6]. Among the various semiconducting gas sensors, ZnO has been considered as the most promising material because of its high electrochemical stability, non-toxicity, suitability to doping, low cost and considerable sensitivity at even low temperatures [7]. Since the sensing mechanism of semiconductor gas sensors (like ZnO) is based on the direct interaction between gas molecules and surface of the sensor, therefore improving the sensing characteristics through different strategies like using hierarchical morphologies with the highest surface area [5–8], manipulation of gas–solid contact points by decoration with highly active metal elements [9] and doping the sensor material [10,11] have been studied by various groups. Concerning the importance of the

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morphology, it is known that the gas sensing characteristics are influenced greatly by the dimension and porosity of nanostructures, and therefore their surface areas [8]. Generally, when the nanoparticles are accumulated into the sensing materials, the aggregation between the nanoparticles due to the van der Waals attraction becomes very strong [12], and as a direct result, the inner parts of the accumulated particles remain inactive. On the other hands, lowering the particle size would have devastating side-effects on gas sensing performance [13]. In order to obviate this problem, new morphologies with less agglomerated configuration like 1D nanowires [5], mesoporous structures [13,14] and hierarchical morphologies like nanoflowers [8,15] have been developed. For example, Yao et al. [16] compared the gas sensing properties of the nanoparticles and nanoflowers ZnO toward benzene vapor. They found that by changing the morphology from regular nanoparticles to nanoflowers, the gas sensitivity increased about three fold. Recently, it has been demonstrated that gas sensing properties of ZnO nanostructures significantly depends on the size and morphology of them [17]. Therefore, it is rational to investigate the influences of shape, size and surface defects on the gas sensing performances of the ZnO nanostructures with ultra-high surface area which is the objective of this study. It is known that among the various synthesis routes for nanostructures, hydrothermal method is efficient, cost-effective, easy to control, feasible and template free as compared to other techniques [18]. The hydrothermal method is generally defined as any heterogeneous chemical reaction in the presence of an aqueous or non-aqueous solvent at relatively low temperature under pressure greater than or equal to 1 atmosphere. In the present study, a simple hydrothermal method was employed to prepare ZnO flower-like microspheres using different zinc salt precursors (zinc nitrate, zinc acetate and zinc sulfate). The structural and morphological characteristics of the samples were investigated. Finally, the gas sensing properties of the synthesized ZnO nanostructures were examined and compared.

2. Experimental details 2.1. Synthesis procedure Zinc nitrate hexahydrate (Zn(NO3)26H2O, Merck, 99.0%), zinc acetate dihydrate (Zn(CH3COO)22H2O, Merck, 99.5%) and zinc sulfate hepta hydrate (ZnSO47H2O, Merck, 99.0%) were used as the starting materials to synthesize ZnO nanostructures by the hydrothermal method. Urea (CO(NH2)2, Merck, 99.99%) was employed as the precipitation agent. All precursors were in analytical grade and used as-purchased without further purification. At first, 20 mL of urea aqueous solution (1.0 M) was dripped into 25 mL of zinc nitrate aqueous solution (0.1 M) under vigorous stirring until all reagents were completely dissolved and a transparent solution was obtained. The starting pH value was set at about 5.5. The mixed solution was then poured into a teflon-lined stainless steel autoclave with 100 mL capacity

and was heated at a constant temperature of 120 °C for 6 h. Then the autoclave was cooled to room temperature in air, while the final pH value was 9. The obtained precipitate was centrifuged and washed with absolute ethanol and distilled water for three times. Finally, the obtained white product was dried in an oven at 60 °C and annealed in air at 400 °C for 2 h. To investigate the effect of anions of zinc salts on the structure and morphology of the obtained ZnO powders, similar procedures were repeated again by zinc acetate and zinc sulfate aqueous solutions as the initial precursors. The different synthesized samples were denoted as ZnO[nitrate], ZnO[acetate] and ZnO[sulfate]. 2.2. Characterizations The crystalline structure of the as-prepared ZnO nanomaterials was characterized using X-ray diffraction (XRD) technique (Inel. EQUNOX 3000) with Cu ka radiation source at k = 1.54056 Å. Scanning electron microscopy (SEM, VEGA TESCAN II) was employed to study the size and morphology of the prepared ZnO powders. The SEM samples had been coated by a gold thin film by a desktop sputtering system (Nanostructured coating Co., Iran). The Fourier transform infrared (FT-IR) spectra of the samples were obtained using a Shimadzu-8400S spectrometer in the range of 400–4000 cm1 with KBr pellets. The active surface area of the samples were measured and calculated through N2 gas physical adsorption at liquid nitrogen temperature (77 K) which is commonly known as BET method using a Belsorp mini II (BelJapan) apparatus. 2.3. Gas sensing measurements For preparation of a sensor, at first, platinum comb-like electrodes were deposited on a glass substrate by sputtering technique and copper wires were contacted on the both coated electrodes by silver paste. Afterwards, 0.05 g of each of the ZnO powders was suspended in 2.5 mL ethanol by ultrasonication for 10 min and then dripped between the fabricated electrodes. The samples were heated at 120 °C for 20 min for final dry out. The static measuring method was used for the gas sensing tests at temperatures between 130 and 230 °C to different ethanol concentrations (50–1000 ppm). Prior to each test, the gas sensor was heated at 230 °C for 30 min in air to burn out the organic residues from the sensing layers to achieve a stabilized point. The signal current (I) with ±0.1 lA accuracy was collected by a computer connected multimeter at a bias voltage of 5.00 ± 0.01 V. The sensor response to ethanol (reductive gas) is defined as:

Sensitivity ð%Þ ¼

Igas  100 Iair

ð1Þ

where Iair and Igas are the currents in air and the testing gas, respectively. The reported accuracy of sensitivities is based on statistical errors. The ethanol sensing properties of the synthesized ZnO nanostructures were also compared with a commercial ZnO nanopowder (TECNAN, 99.98%) denoted as ZnO[com].

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3. Results and discussion 3.1. Material characterizations The effects of zinc salt precursors on the size and morphology of the synthesized powders were studied by SEM (Fig. 1a–i). Fig. 1a–f shows flower-like ZnO nanostructures with scaled flakes grown under the effects of nitrate (Fig. 1a–c) and acetate (Fig. 1d–f) anions in the hydrothermal bath. The nanoflowers are consisted of many petals. Fig. 1a–f suggest that both nitrate and acetate anions in the present of urea surfactant may cause the spontaneous growth of planes with different directions and consequently formation of nanoflakes bonded to each-other to form a flower-like micro/nanostructure [19,20]. The thickness and lateral dimension of the flakes of ZnO[nitrate] nanoflowers were 30 nm and 2 lm, while these values for ZnO[acetate] nanoflowers were 50 nm and 2 lm, respectively. Also, as can be seen, the thinner petals of ZnO[nitrate] are involved regular leaf twists in the outskirts. Fig. 1g–i shows that zinc sulfate bath resulted in formation of the surface exfoliated microspheres. The microspheres are constituted by nanoplatelet structures with thicknesses of 100 nm. It is suggested that the negatively charged sulfate anions adsorb over the positively charged surface of ZnO planes leading to the formation of layered structure of microspheres.

a

Recently, Abbasi et al. [21] found a sever dependence between morphology of ZnO and anionic group of the precursors. However, in the present work, it is observed that urea precipitation agent in the pH of 5.5 does not let a major morphological evolution by changing the precursors. It is suggested that the decomposition of urea can produce OH and CO2 3 anions (see the Eqs. (2)–(5)), which would rapidly capture the Zn2+ cations and form Zn5(CO3)2(OH)6 nanoflowers as well as ban the other preferential growth. The X-ray diffraction patterns of the hydrothermally synthesized ZnO nanostructures as well as the commercial nanopowder are shown in Fig. 2. All of the identified peaks of samples match with Bragg reflections for ZnO with hexagonal wurtzite structure (JCPDS card no. 36-1451), and no diffraction peaks arising from any impurity can be detected. Thus, the results clearly show that the products are pure ZnO. With comparing the sharpness and intensity of the peaks, it can be concluded that the nanoflowers (ZnO[nitrate] and ZnO[acetate]) have higher crystallinity than the microspheres (ZnO[sulfate]). Applying the well-known Scherer equation, the average crystallite sizes of ZnO nanostructures synthesized using zinc nitrate, zinc acetate and zinc sulfate were calculated to be 40, 55 and 25 nm, respectively. This is due to the sulfate anion effect which inhibits the growth of crystallites [21]. Meanwhile, the observed peak positions show that the inter-planar

c

b

5 μm

1 μm

e

d

10 μm

g

f

250 nm

1 μm

i

h

10 μm

250 nm

1 μm

250 nm

Fig. 1. SEM lower and higher magnification images of the ZnO nanostructures grown in hydrothermal bath of (a–c) zinc nitrate, (d–f) zinc acetate, and (g–i) zinc sulfate precursors.

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Fig. 2. XRD patterns of ZnO nanostructures grown in various precursors compared with the commercial ZnO nanopowder.

spacing of samples follows from ZnO[sulfate] > ZnO [nitrate] > ZnO[acetate]. Fig. 3a shows the FT-IR spectra of the synthesized ZnO[nitrate] before calcination. As can be seen, the hydrothermally synthesized sample has various surface functional groups. The broad band at 3363 cm1 was assigned to the OAH stretching vibrations [22]. The sharp bands at 1503 and 1393 cm1 were attributed to the C@O asymmetrical and symmetrical stretching vibrations, respectively. In addition, the bands at 1110 and 1038 cm1 corresponded to the CAO asymmetrical and symmetrical stretching vibrations, respectively [23]. The bands at 952, 837 and 707 cm1 were assigned to the OAC@O bending vibrations ascribed to the CO2 anions. 3 Therefore, it is reasonable to conclude that the unstable Zn5(CO3)2(OH)6 complex was then crystallized by ion supply from the decomposition of urea [24,25]. To more explain, urea as a homogeneous precipitation agent would be decomposed in the hydrothermal bath as below:

COðNH2 Þ2 þ H2 O $ CO2 þ 2NH3

ð2Þ

Subsequently in the alkaline bath, the simultaneous chain reactions result in the formation of unstable Zn5(CO3)2(OH)6 complex by following reactions [26,27]:

NH3 þ H2 O $ NHþ4 þ OH

ð3Þ

CO2 þ 2OH $ CO2 3 þ H2 O

ð4Þ

 5Zn2þ þ 2CO2 3 þ 6OH $ Zn5 ðCO3 Þ2 ðOHÞ6 ðSÞ

ð5Þ

Finally, Zn5(CO3)2(OH)6 can be pyrolysized into pure hexagonal ZnO phase with annealing at 400 °C. Moreover, the FT-IR spectra of the synthesized ZnO nanostructures and commercial nanopowder are compared in Fig. 3b. As can be seen, all spectra contain main characteristic ZnO band at 467 cm1. The weak and broad bands around 3473 and 1534 cm1 were attributed to the OAH stretching of absorbed water and surface OH groups on the surface of nanopowders [22,23].

Fig. 3. The FT-IR spectra of (a) intermediate complex formed from sulfate salt before calcination and (b) all ZnO samples annealed at 400 °C as compared with the commercial ZnO nanopowder.

The amount of adsorbed nitrogen gas molecules onto the nanopowders shown in ads- and des-orption isotherm curves (see Fig. 4) is used as a benchmark for evaluating active surface area of the samples. The mean surface area of ZnO[sulfate], ZnO[acetate] and ZnO[nitrate] were calculated to be 30, 31 and 50 m2/g, respectively. It should be noted the surface area of ZnO[com] has been reported 45 m2/g [28,29]. The highest surface area of the ZnO[nitrate] is consistent with its finest hierarchical morphology as shown in Fig. 1.

3.2. Gas sensing results The gas sensing behavior of the synthesized ZnO nanostructures (ZnO[nitrate], ZnO[acetate] and ZnO[sulfate] as well as ZnO[com]) to ethanol vapor was measured. The response mechanism of resistive metal oxide gas sensors has been described based on the shift in the equilibrium state of the surface oxygen species in the presence of the target gas which leads to change in the resistance of the

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(1) 2

(1) ZnO[nitrate]; SA= 50.42 m /g

150

2

(2) ZnO[acetate]; SA=31.09 m /g 2

Vadsorption (cm3 /g)

(3) ZnO[sulfate]; SA=30.08 m /g

(2) (3)

100

50 ads. des.

0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 4. The N2 gas adsorption and desorption isotherms of the samples at 77 K.

sensor. When ZnO is exposed to air, the oxygen molecules are adsorbed on its surface for generation of chemisorbed 2 Od species (O , O) by capturing electrons from the 2, O conduction band. Upon exposure to reducing gases such as ethanol, gas molecules react with the oxygen ions on the surface of ZnO and reduce the concentration of surface oxygen species. Hence, trapped electrons are released back to the conduction band of ZnO giving rise to a measurable increase in the conductivity [30]. Thus, nanomaterials with desirable morphology and higher surface area may afford an optimized state for gas sensing properties. To obtain the optimum operating temperature, the sensors were maintained over a wide range of temperatures (130–230 °C) and their response to 250 ppm of ethanol vapor was measured (Fig. 5). From the Knudsen equation [31], the gas diffusion is directly proportional to the porosity and pore diameter, while it is inversely proportional to the pore tortuosity. The 3D flower like ZnO nanostructure film is suggested to allow fast diffusion of gas molecules,

Fig. 5. The sensitivity of the ZnO samples to 250 ppm of ethanol at operating temperatures up to 230 °C.

resulting in the high rates of gas adsorption and desorption and sensitivity compared with those ones of ZnO nanoparticles. This could be considered as a contributor to the highly sensitive performance of all nanoflower gas sensors. Also, the higher surface area of the severely porous ZnO[nitrate] sample provide even better condition to be swept by ethanol molecules [32,33]. On the other hand, with increasing the temperature leading to higher thermal energies for the surface reactions, the sensitivity of all of the samples was enhanced. Consequently, the optimum temperature of all sensors was at 230 °C where they showed the highest response to ethanol. When the temperature was increased above it, the activation energy barrier was overcome. The concentration of free electrons increases significantly as the ethanol gas molecules react with the oxygen species adsorbed on the surface. The sensitivity of the sensors decreases above the optimum temperature which can be attributed to change in the surface properties of the sensing materials [34]. It was reported that the stable oxygen ions on the ZnO  surface are O 2 below 100 °C, O between 100 and 300 °C, 2 and O above 300 °C [35]. Therefore, the oxygen ions adsorbed on the ZnO surface are mainly in the form of O at the operating temperature of this work (130–230 °C). The reaction between ethanol and ionic oxygen can be as following:

CH3 CH2 OHads þ 6Oads ! 2CO2 þ 3H2 O þ 6e

ð6Þ

It was also obvious the response of ZnO[com] sensor to ethanol (250 ppm) at the working temperatures below 160 °C was insignificant while the sensors fabricated using the synthesized ZnO nanostructures showed clear responses to ethanol above 130 °C. In the case of the flower-like ZnO nanostructures, gas molecule dissociation occurs at lower temperature and enhances the chemisorption reaction. Thus, the flower-like ZnO nanostructures are very promising for fabricating low power consumption sensors and for industrial applications. The results also indicated that the synthesized samples showed higher sensitivities at upper temperatures compared to the commercial sample (ZnO[com]). The sensitivity and operating temperature of sensors are mainly depends on active surface area, porosity, oxide stoichiometry, defect concentration, crystal orientation, catalytic activity, bond ionicity and work function [36]. The good ethanol sensing properties of sensors based on the flower-like ZnO nanostructures is due to higher surface area and defect density [32,37]. Among the synthesized powders, ZnO[nitrate] and ZnO[acetate] demonstrated higher response to ethanol compared to ZnO[sulfate] below 230 °C which may be attributed to the difference of the morphology where ZnO[sulfate] includes the nanoplatelet structure with higher thicknesses (see SEM results). As mentioned above, the change in resistance of the semiconductor sensors is mainly caused by the adsorption and desorption of gas species on the surface of the sensing materials. The response of these sensors generally is strongly dependent on the effective surface [38,39]. This fact evidently implies that any increase in the effective surface area of the sensing material is expected to increase the number of active surface sites for gas diffusion pathways which promotes

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Fig. 6. (a) The sensitivity of the sensors operated at 230 °C to 250 ppm ethanol and (b) the sensitivity as a function of ethanol concentration at 230 °C.

the reactions with absorbed oxygen and consequently improves the sensing properties. Hence, although the synthesized powders have the same crystalline structure, but the difference in effective surface area seems to be an affective factor causing higher sensing responses for ZnO[nitrate] and ZnO[acetate] in comparison with ZnO[sulfate]. The dynamic sensing response of sensors at 230 °C to 250 ppm ethanol is shown in Fig. 6a. The sensor response for all sensors has increased with increasing ethanol concentration (Fig. 6b). This figure also indicates that up to 500 ppm ethanol, the sensitivity of ZnO[sulfate] is lower than that for ZnO[nitrate] or ZnO[acetate] samples, while

with increasing ethanol concentration to 1000 ppm the sensitivity values of the three synthesized sensors become approximately same. The response time of the sensors was estimated at the optimum temperature to 250 ppm of ethanol. The results show that the response time of ZnO[nitrate], ZnO[acetate] and ZnO[sulfate] is about 25–27 s and it is 35 s for ZnO[com]. Firooz et al. [37] synthesized SnO2 with different morphologies as a function of concentration ratios of an ionic and cationic surfactant by hydrothermal method. They observed that sensor based on SnO2 with morphology of nanoflower-like exhibited higher response to CO than the cubic- and prism-like morphologies due to the smaller crystallite size of nanosheets. Pawar et al. [38] reported the highest gas response (95%) to acetone gas at 2000 ppm at 325 °C for ZnO nanoflower-like structure with the fast response and recovery. The high acetone gas sensitivity of ZnO nanoflowers was attributed to the surface morphology. In another study, Hemmati et al. [40] used SnO2-doped ZnO nanosheets synthesized by a precipitation method as selective ethanol sensors. They reported response times of 96–418 s for 100 ppm of ethanol at 400 °C. In present work, the sensors based ZnO nanoflowers with self-assembled nanosheets exhibited faster response at even lower temperatures which can be attributed to their unique three-dimensional structures with high pore density, enlarged pore size and enhanced surface accessibility [41]. Table 1 has compared the response time of the sensors to ethanol with similar studies. In general, the hydrothermally synthesized sheet-like ZnO nanostructures have low temperature (<250 °C) sensing properties toward various ethanol concentrations, offering the sensor potential advantages such as low power dissipation, long life time and compatibility with micromachining technology [42,7]. Also, in contrast with the commercial powder (ZnO[com]) which did not show any resistance change below 150 °C even toward ethanol concentrations as high as 1000 ppm, the synthesized nanostructure samples started to give reproducible signals to 50–1000 ppm ethanol barely above 130 °C. The relatively lower operating temperature of ZnO[acetate], ZnO[nitrate], and ZnO[sulfate] can be partly attributed to their porous structure and higher effective surface area compared with the commercial powder. Another important parameter for the evaluation of a gas sensor to be applied in practical applications is related to its stability and repeatability. The gas sensing stability of the ZnO[acetate] sample was measured four sequential times against 250 ppm of ethanol vapor at 230 °C. It was observed that the performance of the sample was decreased to 94%, 92%, and 92% of the maximum point

Table 1 Contrast of ethanol sensors based on different ZnO nanostructures. Sensing material

Temperature (°C)

Concentration (ppm)

Response time (s)

Ref.

Sn-doped ZnO nanosheets ZnO flakes Ti-doped ZnO nanotetrapods Pd-doped ZnO nanoparticles Pd-modified ZnO nanorods ZnO nanosheets

400 400 260 400 200 230

100–500 300 100 250 1530 250

96–418 62 90 15 14 25–27

[40] [44] [45] [46] [47] This study

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of the first round at the subsequent cycles. This decrease could be explained by the fact that a small portion of the active surface of ZnO nanostructures may be blinded gradually due to humidity, temperature, and some additional external fluctuating factor, i.e., the sampling error [43]. 4. Conclusions In summary, ZnO nanostructured microspheres by various zinc salt precursors with the help of urea as a surfactant and by a facile aqueous chemical method at low temperature (120 °C) were successfully synthesized. It was found that the growth of ZnO nanostructures strongly depends on the zinc precursor. In fact, anionic group of zinc salt beside to urea surfactant plays a crucial role on ZnO surface morphology and consequently on the surface area and ethanol gas sensing properties. The results proved that the sensor based on ZnO[nitrate] nanoflower sensor had higher responsivity and fast response at temperatures between 130 and 230 °C. Also, this sensor had the threshold temperature response of 130 °C. Such behavior is attributed to the enlarged active surface area as well as the crystal structure of ZnO nanostructures. Thus, the sensor based on the ZnO[nitrate] nanoflower is a promising candidate as a practical detector for ethanol vapor at low temperatures. Acknowledgment Authors would like to thank the Iran National Science Foundation for the financial support of the work. References [1] X.J. Wang, W. Wang, Y.L. Liu, Enhanced acetone sensing performance of Au nanoparticles functionalized flower-like ZnO, Sens. Actuators, B 168 (2012) 39–45. [2] A. Janotti, C.G. Van de Walle, Fundamentals of zinc oxide as a semiconductor, Rep. Prog. Phys. 72 (2009) 126501. [3] A. Shimizu, M. Kanbara, M. Hada, M. Kasuga, ZnO green light emitting diode, Jpn. J. Appl. Phys. 17 (1978) 1435. [4] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Room-temperature ultraviolet laser emission from selfassembled ZnO microcrystallite thin films, Appl. Phys. Lett. 72 (1998) 3270–3272. [5] L. Wang, Yanfei Kang, Xianghong Liu, Shoumin Zhang, Weiping Huang, Shurong Wang, ZnO nanorod gas sensor for ethanol detection, Sens. Actuators, B 162 (2012) 237–243. [6] T. Gao, T.H. Wang, Synthesis and properties of multipod-shaped ZnO nanorods for gas-sensor applications, Appl. Phys. A 80 (2005) 1451– 1454. [7] H.J. Pandya, S. Chandra, A.L. Vyas, Integration of ZnO nanostructures with MEMS for ethanol sensor, Sens. Actuators, B 161 (2012) 923– 928. [8] S. Wei, S. Wang, Y. Zhang, M. Zhou, Different morphologies of ZnO and their ethanol sensing property, Sens. Actuators, B 192 (2014) 480–487. [9] Y. Zhang, J. Xu, P. Xu, Y. Zhu, X. Chen, W. Yu, Decoration of ZnO nanowires with Pt nanoparticles and their improved gas sensing and photocatalytic performance, Nanotechnology 21 (2010) 285501. [10] H. Gong, J.Q. Hu, J.H. Wang, C.H. Ong, F.R. Zhu, Nano-crystalline Cudoped ZnO thin film gas sensor for CO, Sens. Actuators, B 115 (2006) 247–251. [11] A. Khayatian, M. Almasi Kashi, R. Azimirad, S. Safa, Enhanced gassensing properties of ZnO nanorods encapsulated in an Fe-doped ZnO shell, J. Phys. D Appl. Phys. 47 (2014) 075003. [12] G. Korotchenkov, Gas response control through structural and chemical modification of metal oxide films: state of the art and approaches, Sens. Actuators, B 107 (2005) 209–232.

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