Development of an alcohol sensor based on ZnO nanorods synthesized using a scalable solvothermal method

Sensors and Actuators B 185 (2013) 735–742

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

Development of an alcohol sensor based on ZnO nanorods synthesized using a scalable solvothermal method Mingli Yin a,b,∗ , Mengdi Liu a , Shengzhong Liu a,c,∗ a b c

Key Laboratory of Applied Surface and Colloid Chemistry, MOE, Shaanxi Normal University, Xi’an, Shaanxi 710062, PR China School of Science, Xi’an Technological University, Xi’an, Shaanxi 710062, PR China Dalian Institute of Chemical Physics, National Laboratory for Clean Energy, Dalian, Liaoning 116023, PR China

a r t i c l e

i n f o

Article history: Received 27 December 2012 Received in revised form 3 May 2013 Accepted 14 May 2013 Available online 25 May 2013 Keywords: ZnO Nanorod Gas sensor Alcohol

a b s t r a c t An one-step solvothermal synthesis technique has been developed to prepare uniform ZnO nanorods for gas sensor applications. Material characterization has included X-ray diffraction (XRD), scanning electron microcopy (SEM) and photoluminescence (PL). It has been found that ZnO nanorods prepared with ethanol solvent not only exhibit cleaner and smoother surfaces, larger crystallite size, reduced strain, smaller diameter and more donor-related surface defects, but also gave better gas sensing performance comparing to the ZnO nanorods prepared with pure water solvent. It was found that at alcohol level of 500 ppm, such sensor showed the response of 142, among the highest reported values achieved for ZnO nanorod sensors. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor oxide gas sensors have been widely used to detect toxic, inflammable and foul-smelling gases, due to their advantages in high response, low cost, portability and good reversibility, etc. Likewise, they also hold great potential to replace traditional chemical analyses in detecting volatile organic compounds [1,2]. In recent years, a great deal of research has been devoted to semiconductor oxides such as SnO2 [3], Fe2 O3 [4], CuO [5], TiO2 [6], Ga2 O3 [7] and ZnO [8]. Among them, ZnO has been considered as one of the most promising gas sensing materials because of its high electrochemical stability, non-toxicity, suitability to doping, and low cost [8–11]. The combination of direct wide band gap (3.35 eV), large exciton binding energy (60 meV), piezoelectric properties, excellent chemical and thermal stability, has made ZnO a distinguished candidate for electronic and photonic applications. In the past decades, ZnO has been widely used in various forms of ultraviolet (UV) and blue light-emitting devices, solar cells, piezoelectric devices, acousto-optic devices, varistors, transparent thin film transistors and chemical sensors, etc. [12,13]. It is known that the characteristic of nanostructured gas sensing materials greatly depends on their shapes and sizes [14]. One

∗ Corresponding authors at: Key Laboratory of Applied Surface and Colloid Chemistry, MOE, Shaanxi Normal University, Xi’an, Shaanxi 710062, PR China. Tel.: +86 029 8153 0785. E-mail addresses: [email protected] (M. Yin), [email protected] (S. Liu). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.055

dimensional (1D) nanostructured ZnO, due to its low electron/hole recombination rate, together with its high surface-to-volume ratio, has attracted much attention [15]. Hitherto, various 1D ZnO nanostructures, such as nanorods [16], nanotubes [17], nanowires [18], nanobelts [19,20], and nanofibers [21], have been prepared by different groups. For example, using a low-temperature hydrothermal process, Wang et al. fabricated ZnO nanorods with diameter 90–200 nm and attained sensor response ∼26 and ∼34 for 100 and 200 ppm ethanol, respectively [10]. Singh et al. synthesized In and Pd doped ZnO nanorods with smaller diameter of 30–70 nm, fabricated pulse-like gas sensors and achieved response of ∼35 for ethanol at working temperature 550 ◦ C [22]. Using a solvothermal process, Chen et al. harvested ZnO nanorods with narrow size distribution at diameter as low as 15 nm. Utilizing the small size nanorods, they demonstrated a response as high as ∼47 for 200 ppm ethanol at temperature 400 ◦ C [23]. Likewise, Wan et al. fabricated ∼25 nm diameter ZnO nanowires using microelectromechanical system (MEMS) technology, and succeeded a response ∼47 toward 200 ppm ethanol at 300 ◦ C [24]. On the other hand, Xue et al. synthesized ZnSnO3 nanowires by thermal evaporation of ZnO, SnO, and graphite powder and demonstrated a high gas response ∼44 toward 500 ppm ethanol at temperature of 300 ◦ C [25]. Hydrothermal reaction is a very common wet chemical method to create ZnO nanostructures [26], nevertheless, this method often requires high temperature, elevated pressure, complicated preparation process and extended reaction time [15,27]. In this article, we report our development of a simple one-step atmospheric

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Fig. 1. (a) An illustration of gas sensor structure and (b) a scheme of measuring electric circuit with photograph of a sensor.

pressure technique to produce relatively large quantity of uniform ZnO nanorod material. Both material properties and gas sensing performance of ZnO nanorods prepared with ethanol and pure water solvent were analyzed. Even though stability is known to be a key quality indicator for commercial gas sensors, it has been largely neglected in literature devoted to conductometric gas sensors [28]. We present significantly improved gas sensing stability and performance using ethanol solvent. The solvent effect is analyzed and discussed in detail. 2. Experimental 2.1. Synthesis All chemicals used for the present work are AR grade from Beijing Chemicals Co. Ltd., and used as received without further purification. The synthesis method for ZnO nanorods was an improved one upon what was used in ref. [29]. More specifically, the reference technique used non-organic solvent. We found that by utilizing ethanol solvent, the ZnO nanorod quality has been significantly improved. In a typical preparation, nutrient solution was prepared by mixing 200 ml of 20 mM Zn(NO3 )2 and 200 ml of 20 mM hexamethylene tetramine (HMTA) in a 600 ml beaker. Solvent was either doubly distilled water or a mixture of doubly distilled water and anhydrous ethanol in 4:1 volume ratio. Precursors were prepared by dissolving them in their respective solvents. For each synthesis, after mixing Zn(NO3 )2 and HMTA precursor solutions, the container was covered with a layer of plastic wrap, and then was put into a blast oven at 90 ◦ C for 100 min. Upon cooling down to room temperature, discard liquid in upper portion of the container and collect only the white sludge deposited on the bottom of the container. Clean the sludge using a combination of centrifuge and water rinsing process for 5 cycles. At last, wash the sludge with pure ethanol for a few times. The sludge was then dried at 60 ◦ C for 12 h to form powder sample, it was then annealed at 500 ◦ C for 1 h in air atmosphere. The as-fabricated, annealed sample prepared with ethanol, and the annealed sample prepared using aqueous solution was labeled as a1, a2 and a3, respectively. 2.2. Characterization Crystal structure of the annealed samples was characterized by a DX-2700 X-ray diffractometer (XRD) using Cu K˛ radiation ( = 0.15418 nm) with 2 range of 20–80◦ . The dimension, morphology and surface details of the samples were characterized on a FEI Quanta 200 scanning electron microscopy (SEM) operated at 20 kV. The photoluminescence (PL) spectrum was recorded on a PE LS55

spectrophotometer by a 325 nm excitation from Xe lamp at room temperature.

2.3. Fabrication and test of gas sensors A proper amount of ZnO nanorod materials were ground with a few drops of water in an agate mortar to form slurry. Then the slurry was coated on an alumina tube with two Au electrodes and four Pt wires on each end. The coating was then dried at high temperature by a hair dryer for a few minutes. A Ni–Cr wire coil, set in the tube, was employed as a heater to control working temperature by tuning the heating voltage. Fig. 1(a) is an illustration of the sensor, Fig. 1(b) measures electric circuit and the inset in Fig. 1(b) is the photograph of a gas sensor. Gas sensing tests were performed using a WS-30A gas sensitivity instrument (Wei Sheng Electronics Co. Ltd., China) with a test chamber of 30 L (315 mm × 315 mm × 350 mm) in volume. The gas concentration was determined by a stationary state process: a predetermined volume of liquid alcohol was injected using a microliter syringe onto a hot stage where the alcohol evaporated upon contact into vapor and dispersed in the test chamber by two fans. The vapor concentration was calculated using its mole ratio relative to air. For present work, relative humidity was regulated at about 30%. The circuit voltage (Vc ) was set at 5 V. Out voltage (Vout ) signal was recorded at the terminal of a standard resistor (RL : 10 k), which was connected in series with the gas sensor (Rs ). The gas response (S) is defined as Ra /Rg in reducing gas atmosphere and Rg /Ra in oxidizing gas atmosphere, where Ra and Rg are resistance of the gas sensor in air and in testing gas atmosphere, respectively. The response time and recovery time are defined as the time taken by the gas sensor to achieve 90% of the entire resistance change for vapor adsorption and desorption, respectively.

3. Results and discussion 3.1. Characteristics of ZnO nanorods Fig. 2 shows XRD characteristics of the ZnO nanorod samples: a1 is an as-fabricated sample prepared with ethanol solvent, and a2 an annealed one. In comparison, we included an annealed sample a3 prepared using aqueous solution. All peaks can be indexed to hexagonal wurtzite ZnO structure (JCPDS card no. 36-1451). It is clear that all samples have strongest (1 0 1) diffraction peak, which is consistent with the standard diffraction pattern for the ZnO powder.

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Fig. 2. XRD patterns of ZnO nanorods: a1 and a2 are as fabricated, annealed samples prepared with ethanol solvent, and a3 the annealed sample prepared with pure water solvent.

For the hexagonal system, the lattice parameters can be calculated by Bragg diffraction function and the d-spacing equation [30]: 2dhkl sin  = 

  dhkl =

4 3

h2 + k2 + hk a2

(1)

  2 −1/2 l +

c

(2)

where  and  are Bragg diffraction angle and the X-ray wavelength, respectively; h, k and l the miller indices; a and c the lattice parameters. The lattice parameters for all samples are a = 3.268 A˚ and ˚ It is interesting that both lattice parameters a and c are c = 5.250 A. larger than the results of nano-particle thin film reported by Prajap˚ c = 5.21 A) ˚ [30]. They are also larger than those ati et al. (a = 3.25 A, reported in JCPDS card no. 36-1451 for randomized ZnO powder ˚ c = 5.206 A). ˚ (a = 3.249 A, Fig. 3(a) and (b) are SEM images of samples a2 and a3: the former shows uniform and well-dispersed ZnO nanorods, with diameter ranging 290–330 nm and length 3.2–3.4 ␮m; while the latter shows significantly larger diameter and shorter length, ranging 330–500 nm and 2.8–3.0 ␮m separately. The maximum aspect ratios for S2 and S3 are 11.6 and 9.1 respectively. ZnO is a polar crystal with {0001} polar facets and electricallyneutral {1010} non-polar facets. Because the polar facets have higher surface energy than the non-polar ones, naturally ZnO nuclei grow preferably along c-axis, i.e. [0001] direction to form rod-like

structure [31]. When using pure aqueous solution to grow ZnO nanorods, floccule are frequently formed on ZnO nanorod surface (Fig. 3b), leading to non-rod shape due to insufficient reaction time or lack of source material. However, the surface of a2 is clean (Fig. 2a), indicating that ethanol solvent can prevent formation of new nucleation sites for subsequent growth process and therefore promotes preferential growth of ZnO nuclei along the c-axis. HMTA was believed to act as a weak base for it can be slowly hydrolyzed in water. This process releases OH- gradually [31] and therefore helps maintain pH in mild condition, leading to preferred orientation of 1D nanostructured ZnO. The growth process can be elaborated as follows [31]: (CH2 )6 N4 + 6H2 O ↔ 4NH3 + 6HCHO

(3)

NH3 + H2 O ↔ NH4 + + OH−

(4)

Zn2+ + 2OH− ↔ Zn(OH)2

(5)



Zn(OH)2 −→ZnO + H2 O

(6)

PL is a useful tool for detecting the intrinsic defects in fluorescent crystal material. Fig. 4 shows room temperature PL spectra for samples (a) a2 and (b) a3. As the spectra are heavily convoluted, individual peaks are extracted using Gaussian distribution. As shown in Fig. 4, the numerical interpretation fits the measurement data well. As both weak ultraviolet (UV) emission (<400 nm)

Fig. 3. SEM images of (a) a2 – annealed sample prepared using ethanol solvent and (b) a3 – annealed sample prepared using pure water solvent.

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Fig. 4. Room temperature PL spectra of (a) a2 and (b) a3 as well as their deconvoluted peaks using Gaussian distribution. Inset in the upper right corner of each figure lists peak positions and contents (in peak area percentage) from the numerical deconvolution.

and strong visible emission peaks were observed for both samples, indicating abundant defects on sample surface. The peaks centered at 401.1 nm for sample a2 and at 409.2 nm for a3 are from shallow donor-related UV emission [30]. The peaks centered at 479.6 and 504.5 nm for sample a2; and 479.3 and 508.8 nm for sample a3 are believed to be from zinc vacancy (VZn ) and oxygen anti-site (OZn ), respectively [31,32]. There is a peak centered at 449.0 nm, about 16–21 nm higher than the interstitial zinc (Zni ) peak position [32–34] and ∼31 nm lower than the VZn peak position [33,34]. The peak area percentage for the 449.0 nm peak is 42.8% for sample a2 and 43.8% for a3. Taking into account that Zni is a widespread defect and often dominant, so we infer that the peak centered at 449.0 nm is ascribed to Zni or the complex defect of Zni. This may explain the results in Fig. 1 that the lattice parameters of a2 and a3 are larger than that of (JCPDS card no. 36-1451) for randomized ZnO powder. The weak peak centered at 392.6 nm is attributed to the near band edge emission [34]. Zni is an electron donor, and VZn as well as OZn are electron acceptors [35]. The total area percentage from the peaks of donor-related defects for a2 is 52.5%, while that for a3 is 48.4%. It is well known that oxygen can be chemisorbed easily on donor-related defects, leading to improved gas sensing response [36,37]. For example, Chen et al. reported [34] that tube-like ZnO had higher donor-related content and gave

better sensitivity than flower-like ZnO with lower donor-related content. Our result is consistent with the explanation. 3.2. Gas sensing performance In order to examine the stability of the gas sensors, we studied transient response versus number of exposure tests, as shown in Fig. 5. Note that, for the continence of description, in following text the sensors fabricated using the annealed sample prepared with ethanol solvent (a2) and with pure aqueous solvent (a3) are labeled as S1 and S2, respectively. For both sensors S1 and S2, the alcohol exposure doses were controlled at 100 ppm in a continuous measurement status, i.e., the heating temperature was maintained at working temperature of 370 ◦ C. It is clear that S1 showed optimum response initially and it degraded to 38 at the second exposure test and stabilized there. However, S2 showed a different stability profile. It started at 80 for the first exposure, dropped to ∼70 for the second exposure and continually degraded for each additional exposure. It stabilized at 34 after multiple repeated tests. All subsequent data were recorded at an intermittent measurement status, i.e., the gas sensors were cooled down to room temperature before starting next cycle of test. After two days of exposure in ambient, S1 retained its original response magnitude (Fig. 6),

Fig. 5. Transient response vs. number of exposure tests for (a) S1 and (b) S2 using 100 ppm alcohol vapor in a continuous measurement mode. The working temperature of sensors was 370 ◦ C.

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Fig. 6. Transient responses at different temperatures from 160 to 400 ◦ C for (a) S1 and (b) S2 using 100 ppm alcohol vapor exposure. The inset in Fig. 6(a) shows transient output signal voltage response for S1 at different temperatures.

but S2 showed significant reduction after two days of idling. The response for S1 recorded at an intermittent status is a bit higher than the response recorded at a continuous measurement status in the same conditions. As floccule is much smaller than nanorod in size, its melting point should be significantly lower than that of nanorod material. We suspect that this is a cause of instability. As reported by Korotcenkov et al. [38,39], as structural changes may occur at low temperature (<300 ◦ C) when grain size is smaller than 3–5 nm, reduction in grain size of gas-sensing material frequently leads to decreased thermal stability of sensor. The modern approaches for gas sensor design based on reduced grain size material to attain high sensor response are not always the best methodology. Moreover, lower melting point floccule may cover surface adsorption sites of the ZnO nanorods due to sintering effect, leading to degradation of gas sensing performance. To identify the optimum working temperature, we investigated the transient response of S1 and S2 to 100 ppm alcohol vapor exposure in different temperature range from 160 ◦ C to 400 ◦ C. Transient responses at different temperatures from 160 to 400 ◦ C for S1 and S2 using 100 ppm alcohol vapor exposure are shown in Fig. 6. Both sensors started to show gas response at temperature as low as 160 ◦ C. For S1, as shown in Fig. 6(a), the response slowly increased as sensor temperature increased, it reached the maximum (42)

at 370 ◦ C and then it dropped when temperature went higher. S2 showed a slightly different profile. The response increased slowly from 7 to 9 only when temperature rose from 160 ◦ C to 300 ◦ C. However, when temperature reached above 300 ◦ C, it started to show drastic increase, from 9 at 300 ◦ C to 24 at 340 ◦ C and then to the maximum of 34 at 370 ◦ C. It then dropped when temperature went up further. The inset in Fig. 6(a) shows the transient output signal voltage response for S1 operated at different temperatures, ranging from 200 ◦ C to 400 ◦ C. Fig. 7 shows transient responses of S1 and S2 with exposure to different concentrations of alcohol vapor at the optimum working temperature. It is clear that the both sensors show reliable and reproducible responses toward various alcohol concentrations. In addition, the sensor response increases with alcohol concentration and both S1 and S2 seem to reach saturation when alcohol vapor concentration is increased to about 800 ppm. The response for S1 upon exposure to 100, 500 and 1000 ppm alcohol vapor are as high as 42, 142 and 222, respectively. However, the largest response is only 92 for S2 at 1000 ppm. The inset in Fig. 7(a) shows the transient output signal voltage response for sensor S1 and a typical test series. It is clear that as the sensor exposed to alcohol vapor, the voltage rises sharply and when alcohol vapor is removed, the voltage drops to baseline quickly. Moreover, the higher the alcohol vapor concentration, the higher the voltage response.

Fig. 7. Transient response for (a) S1 and (b) S2 to different alcohol vapor concentration at 370 ◦ C. The inset in Fig. 7(a) shows transient output signal voltage response.

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Fig. 8. Response-recovery curves for sensors (a) S1 and (b) S2 at 100 ppm alcohol vapor exposure.

Fig. 8 shows response-recovery curves for S1 and S2 upon exposure to 100 ppm alcohol vapor. Both S1 and S2 show a response time of 22 s, however their recovery time is different: 10 s for S1 and 27 s for S2. We believe that the shorter recovery time of S1 is caused by its larger crystallite size, leading to less scattering in the electron transfer process as well as rapid and effective gas diffusion toward its entire gas sensing surface. As the sensor response is measured by resistance change of the gas sensing material upon exposure to test gas atmosphere. The resistance changes to different direction when exposed to reducing vapor comparing to that of oxidizing vapor. For n-type semiconductor sensor material, resistance change differently comparing to p-type semiconductor material. According to oxygen adsorption theory, an electron core (semiconducting body)–shell (electron depleting layer) model and the potential barrier between the particles are often established. For an n-type semiconductor, it has high resistance in air due to oxygen adsorbed on its surface, which captures electrons from the conduction band, resulting in an electron depletion layer on the surface. The thickness of the electron depletion layer can be calculated by the Debye length equation:



D =

εε0 kB T q2 ND

(7)

where ε, ε0 , kB , T, q, ND are the relative permittivity, vacuum permittivity, Boltzmann constant, absolute temperature, charge, and donor density, respectively. When diameter of ZnO nanorod is in the order of its Debye length, D , the entire nanostructure becomes a conductance switch, as almost all electrons in conduction band can be depleted by surface acceptors [40]. Therefore, device sensitivity increases as nanorod diameter shrinks [41,42]. It is interesting that, in our case, the average diameter of a2 is larger than that of ZnO nanorods reported in literature [10,21–24] as summarized in Table 1, but its optimum response is higher comparing to exposure to the same alcohol concentration. We speculate that the following causes may account for the higher response. (1) Larger nanorod diameter renders better dispersion. With reduction in particle size, the aggregation between particles becomes increasingly apparent because the van der Waals attraction increases as nanorod diameter decreases; (2) the combination of smoother surface and better crystallinity enhances its gas response and stability. The smooth surface facilitates the gas diffusion on ZnO nanorod surface, and the large crystal size causes reduction in grain boundaries, leading to smaller number of trapped and scattering states during electron transfer. (3) SEM images show clearly that nanorods have bigger diameter in the middle and smaller at both ends, which will increase the resistance in the longitudinal direction. The thickness of the surface

Table 1 Comparison to literature sensors: sensitivity of nanorod ZnO sensors to ethanol vapor exposure. Optimum temperature (◦ C)

Ra /Rg

Reference

200

50 100 200

320

∼22 ∼26 ∼34

[10]

Pure ZnO nanorods In doped ZnO nanorods

30–70

400

≥550

∼17 ∼35

[21]

Pure ZnO nanorods

15

50 100 300

400

∼6 ∼8 ∼14

[22]

Pure ZnO nanowire

∼25

50 100 200

300

∼15 ∼33 ∼47

[23]

ZnSnO nanowire

20–90

50 100 500

300

∼13 ∼18 ∼44

[24]

Pure ZnO nanorods

290–330

50 100 500

370

∼17 ∼42 ∼142

Gas sensor

Diameter (nm)

Ethanol concentration (ppm)

Pure ZnO nanorods 90

This work

M. Yin et al. / Sensors and Actuators B 185 (2013) 735–742

conductive region with finite electron concentration is calculated to be on the order of tens of nanometers [43]. (4) PL results show that ZnO nanorods synthesized using present method have higher content of the donor-related defects on surface, leading to improved response. 4. Conclusion In summary, a simple and rapid one-step solvothermal method has been developed to prepare large quantity of uniform ZnO nanorod material. The nanorod material prepared with both organic and aqueous solvents were characterized, assembled into gas sensors and evaluated. It was found that solvent plays an important role in ZnO nanorod growth. The ZnO nanorods fabricated in ethanol solvent show clean surface without any floccule, while those prepared using pure water solvent are covered with floccules. Furthermore, using ethanol solvent renders final ZnO nanorod product smaller diameter, better crystallinity and more donor-related surface defects, leading to better sensor performance in both stability and sensitivity. The modern approaches for gas sensor design based on reducing grain size of the sensor material do not always guarantee both maximum sensor response and improved stability. Our approach provides a balanced approach between high sensitivity and improved stability. Acknowledgements We acknowledge financial support from Chinese National University Research Fund (GK261001009), Shaanxi Normal University, Xi’an, China. References [1] J.Q. Xu, J.J. Han, Y. Zhang, Y.A. Sun, B. Xie, Studies on alcohol sensing mechanism of ZnO based gas sensors, Sensors and Actuators B 132 (2008) 334–339. [2] X.F. Chu, X.H. Zhu, Y.P.X. Dong, T. Ge, S.Q. Zhang, W.Q. Sun, Acetone sensors based on La3+ doped ZnO nano-rods prepared by solvothermal method, Journal of Materials Science & Technology 28 (3) (2012) 200–204. [3] Z.D. Lin, W.L. Song, H.M. Yang, Highly sensitive gas sensor based on coral-like SnO2 prepared with hydrothermal treatment, Sensors and Actuators B 173 (2012) 22–27. [4] P. Sun, W.N. Wang, Y.P. Liu, Y.F. Sun, J. Ma, G.Y. Lu, Hydrothermal synthesis of 3D urchin-like ␣-Fe2 O3 nanostructure for gas sensor, Sensors and Actuators B 173 (2012) 52–57. [5] H. Kim, C. Jin, S. Park, S. Kim, C. Lee, H2 S gas sensing properties of bare and Pd-functionalized CuO nanorods, Sensors and Actuators B 161 (2012) 594–599. [6] P.M. Perillo, D.F. Rodríguez, The gas sensing properties at room temperature of TiO2 nanotubes by anodization, Sensors and Actuators B 171–172 (2012) 639–643. [7] S. Nakagomi, T. Sai, Y. Kokubun, Hydrogen gas sensor with self temperature compensation based on ␤-Ga2O3 thin film, Sensors and Actuators B: Chemistry (2013), http://dx.doi.org/10.1016/j.snb.2013.01.020. [8] J.J. Hassan, M.A. Mahdi, C.W. Chin, H. Abu-Hassan, Z. Hassan, A high-sensitivity room-temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays, Sensors and Actuators B 176 (2013) 360–367. [9] J.X. Wang, X.W. Sun, Y. Yang, C.M.L. Wu, N–P transition sensing behaviors of ZnO nanotubes exposed to NO2 gas, Nanotechnology 20 (2009) 465501/1–465501/465501. [10] L.W. Wang, Y.F. Kang, X.H. Liu, S.M. Zhang, W.P. Huang, S.R. Wang, ZnO nanorod gas sensor for ethanol detection, Sensors and Actuators B 162 (2012) 237–243. [11] O. Lupan, L. Chow, G. Chaid, A single ZnO tetrapod-based sensor, Sensors and Actuators B 141 (2009) 511–517. [12] S.O. Kucheyev, J.E. Bradley, J.S. Williams, C. Jageadish, M.V. Swain, Mechanical deformation of single-crystal ZnO, Applied Physics Letters 80 (2002) 956–958. [13] O.D. Jayakumar, N. Manoj, V. Sudarsan, C.G.S. Pillai, A.K. Tyagi, A rare defect free 3D ZnO rod structure with strong UV emission, CrystEngComm 13 (2011) 2187–2190. [14] N. Yamazoe, K. Shimanoe, Roles of shape and size of component crystals in semiconductor gas sensors I. Response to oxygen, Journal of Electrochemical Society 155 (2008) 85–92.

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Biographies Mingli Yin received his Ph.D. degree from Wuhan University in 2010. His research is focused on synthesis and gas sensing properties of low dimensional metal oxide semiconductors. Mengdi Liu is currently a senior undergraduate for Materials Science and Engineering major. Now, she is interested in synthesis of low dimensional metal oxide semiconductors.

Shengzhong (Frank) Liu received his Ph.D. degree from Northwestern University (Evanston, Illinois, USA) in 1992. Upon his postdoctoral research at Argonne National Laboratory (Argonne, Illinois, USA), he joined high-tech companies in US for research including nanoscale materials, thin film solar cells, laser processing, diamond thin films, etc. While working in United Solar Ovonic, USA, he developed an electrochemical ZnO process and scaled it up for solar cell production. His invention at BP Solar in semi-transparent photovoltaic module won R&D 100 award in 2002. In 2011, he was selected into China’s top talent recruitment program and now he is a Professor at Shaanxi Normal University and Dalian Institute of Chemical Physics, Chinese Academy of Sciences.