In2O3 Nanotower Hydrogen Gas Sensors Based on Both Schottky Junction and Thermoelectronic Emission
© Zheng et al. 2015
Received: 16 March 2015
Accepted: 5 July 2015
Published: 15 July 2015
Indium oxide (In2O3) tower-shaped nanostructure gas sensors have been fabricated on Cr comb-shaped interdigitating electrodes with relatively narrower interspace of 1.5 μm using thermal evaporation of the mixed powders of In2O3 and active carbon. The Schottky contact between the In2O3 nanotower and the Cr comb-shaped interdigitating electrode forms the Cr/In2O3 nanotower Schottky diode, and the corresponding temperature-dependent I-V characteristics have been measured. The diode exhibits a low Schottky barrier height of 0.45 eV and ideality factor of 2.93 at room temperature. The In2O3 nanotower gas sensors have excellent gas-sensing characteristics to hydrogen concentration ranging from 2 to 1000 ppm at operating temperature of 120–275 °C, such as high response (83 % at 240 °C to 1000 ppm H2), good selectivity (response to H2, CH4, C2H2, and C3H8), and small deviation from the ideal value of power exponent β (0.48578 at 240 °C). The sensors show fine long-term stability during exposure to 1000 ppm H2 under operating temperature of 240 °C in 30 days. Lots of oxygen vacancies and chemisorbed oxygen ions existing in the In2O3 nanotowers according to the x-ray photoelectron spectroscopy (XPS) results, the change of Schottky barrier height in the Cr/In2O3 Schottky junction, and the thermoelectronic emission due to the contact between two In2O3 nanotowers mainly contribute for the H2 sensing mechanism. The growth mechanism of the In2O3 nanotowers can be described to be the Vapor-Solid (VS) process.
KeywordsNanotowers Hydrogen Gas sensor Schottky junction Thermoelectronic emission
Chemical sensors based on semiconductor oxide materials have been extensively researched due to their advantageous features, such as high sensitivity, low cost, and simplicity in fabrication . Among them, indium oxide (In2O3) semiconductive materials have been extensively studied as chemical sensors for a long time due to their advantageous features such as a wide bandgap around 3 eV, a low resistance, and good catalysis .
1-D In2O3 nanostructures show high gas-sensing response due to their high electric conductance, high transparency to visible light, strong interaction with the gas molecules, high surface-to-volume ratio, and large surface activities . However, up to now, few In2O3 nanostructures exists good sensing properties to hydrogen. Hydrogen is the most attractive and sustainable energy for the future generations due to its high efficiency and renewable properties.
In this contribution, we report that new In2O3 tower-shaped nanostructures with high surface-to-volume ratio have been fabricated using thermal evaporation of the mixed powders of In2O3 and active carbon. The synthesized In2O3 nanotowers distributing on Cr comb-shaped interdigitating electrodes with relatively narrower interspace of 1.5 μm have excellent gas-sensing characteristics to hydrogen concentration ranging from 2 to 1000 ppm at operating temperature of 120–275 °C.
The fabrication of In2O3 nanotower hydrogen sensors includes two parts. One is the preparation of comb-shaped interdigitating electrodes with relatively narrower interspace of 1.5 μm on the silicon substrate, and the corresponding processes are basically the same with our previous works [4, 5].
Another part is the synthesis of In2O3 nanotowers on the silicon substrate by thermal evaporation of active carbon and In2O3 powders. The active carbon and In2O3 powders are mixed in a 1:1 weight ratio as the reaction source and put near the silicon substrate, which are placed inside the little quartz tube that is pulled into a large quartz tube in a horizontal tube electric furnace. After the whole system is evacuated by a vacuum pump for 20 min, the argon gas is guided into the system at 250 sccm and the pressure is kept at 350 Torr. Then, the system is rapidly heated up to 1050 °C at 40 °C/min from the room temperature and kept this temperature for 1 h. Finally, the system is cooled down to the room temperature for several hours by natural cooling. The In2O3 nanotowers are synthesized. Field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) are used to identify the morphology and structure of the products.
The heater of the gas sensor and the corresponding schematic diagram are shown in Fig. 1b, c. The heater steel silks on the thermal insulation material are connected with the metal pins “1” and “4.” Each gas sensor chip is put on the heater steel silks and bonded to another two metal pins so as to constitute a sensor element. The sensor element and the corresponding schematic diagram are shown in Fig. 1d, e. When the heating voltage is applied to metal pins “1” and “4,” the temperature of the heater steel silks will rise up according to Joule’s law so as to make the temperature of gas sensor chip rise up. When the bias voltage is applied to metal pins “3” and “6,” the electrical signal measurement for the gas sensor can be carried out. The sensor temperature at different heating voltages is measured by contacting a thermocouple to the upper side of the gas sensor chip.
Each sensor element is packaged with a stainless steel mesh cap as shown in Fig. 1f. After that, the six metal pins of each sensor element are inserted in the corresponding holes of one measurement unit in the test gas chamber as shown in Fig. 1g. There are 10 measurement units in the test gas chamber, so 10 sensor elements can be tested at the same time. Then, the test gas chamber is sealed. By controlling gas-sensing characterization system (Gyjf Technology Co. Ltd., People’s Republic of China/JFO2E) consisting of gas mixer instrument, measurement instrument, and computer, certain concentration gas or air is passed into the test gas chamber based on a flow-through technique in different circumstances , and the corresponding schematic diagram is shown in Fig. 1h. In our case, the concentration of hydrogen gas is ranging from 2 to 1000 ppm. At the same time, bias voltage of 8.9 V is as working voltage, and certain heating voltage ranging from 3 to 5.1 V is as heating voltage which is supplied to test the gas chamber so as to apply to each tested sensor element. The resistance change of the gas sensor can be caught by the measurement instrument and displayed on the computer.
Results and Discussion
where, φ B is the effective barrier height, A is the junction area, A* is the Richardson constant, n is the ideality factor, R is the series resistance, and β = q/kT. In such a case, the generalized Norde method can be used to evaluate the effective barrier height and diode ideality factor from I-V measurements [13, 14]. The effective barrier height and ideality factor measured at different temperatures for Cr/In2O3 nanotower Schottky diode are shown in Fig. 4b. The barrier height can be seen to increase linearly with temperature. On the other hand, the ideality factor for the Schottky diode decreases from 2.93 to 0.82 with an increase in temperature. Deviation of the ideality factor from unity may be due to the existence of high series resistance .
The long-term stability of the sensor is also displayed in Fig. 8b. We measure the response of the sensor to 1000 ppm H2 at an operating temperature of 240 °C every 3 days. Clearly, the sensor shows relatively stable response in 30 days.
Hydrogen-sensing response of the In2O3 nanotowers sensor operated at 240 °C in comparison to those of the reported sensors using In2O3-based nano- or micro-structured materials
Operated temperature (°C)
Response% (ΔR/R o*100 %) toward H2 under the following concentrations (ppm)
In2O3 nanowires 
In2O3 nanoneedles 
ZnO/In2O3 nanorods 
In2O3 thin films 
In2O3 nanowires 
Au-loaded In2O3 nanofibers 
In2O3 hollow nanospheres 
In2O3 nanowires 
In2O3/ZnO nanowires 
In2O3 nanowires 
In2O3 flower-like microspheres 
In2O3 urchin-like microspheres 
In2O3 nanofibers 
In2O3 nanowires 
The unique hydrogen-sensing properties based on the In2O3 nanotower can be explained as follows. In the open air, the chemisorbed oxygen ions (O2 − or O−) form on the surface of the In2O3 nanotowers leading to a depletion region in surface layer owing to the electron shift from In2O3 to oxygen. When the hydrogen is injected, H2 reacts with chemisorbed oxygen ions, which releases electrons back to the surface of the In2O3 nanotowers and alter its electrical conductivity of the structures. According to the XPS spectrum of O1s shown in Fig. 3c, it appears that a high concentration of chemisorbed oxygen ions will favor the forward reaction . For example, it is found that the ethanol-sensing responses of In2O3 increase with the increasing intensity of chemisorbed oxygen . In addition, because of the response of the metal oxide sensor to methanol depending on the basicity of the metal oxide  and the basicity controlled by the electronic density which is generated from the Vo , the hydrogen-sensing properties of the In2O3 nanotowers are excellent due to lots of Vo being introduced in the In2O3 nanotowers in this paper.
The hydrogen-sensing mechanism of the sensors is attributed to not only the hydrogen-sensing properties based on the In2O3 nanotower discussed above but also the structure characteristics of the sensors as schematically shown in Fig. 9b–e. Besides the change of the Schottky barrier height in the Cr/In2O3 Schottky junction as shown in Fig. 9b, c, the thermoelectronic emission due to the contact between two In2O3 nanotowers as shown in Fig. 9d, e will contribute for the hydrogen-sensing mechanism too.
The electrical response comes from the variation of the Schottky barrier height and barrier width as a result of adsorption of gaseous species at the Schottky contact. The response due to adsorption can be explained from the band diagram at the metal/nanostructure contact. After the exposure to H2, Cr adsorbs H2 by catalytic chemical adsorption, which reduces the Schottky barrier height . The catalytic effect of chromium can be understood in two ways . The first one is related to the work function value of the chromium which leads to an excess of free bonds near the surface region, then producing an important oxidation which favors the subsequent reactions with the H2. The second one is related to the formation of a Cr2O3 phase close to the surface region, which increases the catalytic activity of this region due to the fact that the surface chromium atoms can be easily oxidized and reach an effective valence of four. So, they can adsorb a complete monolayer of active oxygen.
In an individual In2O3 nanotower, free carriers (electrons) can transport along the conduction channel. But when transporting between two contacting In2O3 nanotowers, the electrons have to pass through a potential barrier at the junctions and a thermoelectronic emission mechanism can be used to describe the electron transportation in the junctions . The change in resistance during the adsorption or desorption process of H2 species is likely to be caused by the alteration both in the width of the surface depletion layer of each In2O3 nanotower and in the height of potential barriers built at the contacted junctions between two In2O3 nanotowers. These two-fold effects may facilitate less response time and higher response to certain chemical species .
In summary, we report that new In2O3 nanotowers synthesized via thermal evaporation and distributed on Cr comb-shaped interdigitating electrodes with relatively narrower interspace of 1.5 μm have high response, fine long-term stability, and small deviation from ideal value of power exponent β, in the hydrogen concentration ranging from 2 to 1000 ppm and at the operating temperature of 120–275 °C. The Schottky contact between the In2O3 nanotower and the Cr comb-shaped interdigitating electrode forms the Cr/In2O3 nanotower Schottky diode, and the corresponding temperature-dependent I-V characteristics have been measured. The diode exhibits a low Schottky barrier height of 0.45 eV and ideality factor of 2.93 at room temperature. The growth mechanism of the In2O3 nanotowers has been discussed in detail, which is account for the high surface-to-volume ratio of the morphology. Lots of oxygen vacancies and chemisorbed oxygen ions exist in the In2O3 nanotowers according to the XPS results. The change of Schottky barrier height in the Cr/In2O3 Schottky junction and the thermoelectronic emission due to the contact between two In2O3 nanotowers mainly contribute for the H2 sensing mechanism. The In2O3 nanotowers hydrogen sensors are promising for further practical applications.
This work was supported by National Natural Science Foundation of China (50902097), Basic Research Project of Shenzhen (JCYJ20140418193546110), Guangdong Natural Science Foundation of China (9451806001002303), Project of the Department of Education of Guangdong Province (2013KJCX0165), Outstanding Young Teacher Training Project in the institutions of higher learning of Guangdong Province (Yq2013145), and Open Project of Shenzhen Key Laboratory of Micro-nano Photonic Information Technology (MN201405).
- Zeng ZM, Wang K, Zhang ZX, Chen JJ, Zhou WL. The detection of H2S at room temperature by using individual indium oxide nanowire transistors. Nanotechnology. 2009;20:045503.View ArticleGoogle Scholar
- Xu JQ, Wang XH, Shen JN. Hydrothermal synthesis of In2O3 for detecting H2S in air. Sens Actu B. 2006;115:642.View ArticleGoogle Scholar
- Qurashi A, El-Maghraby EM, Yamazaki T, Kikuta T. Catalyst supported growth of In2O3 nanostructures and their hydrogen gas sensing properties. Sens Actu B. 2010;147:48.View ArticleGoogle Scholar
- Wang B, Zhu LF, Yang YH, Xu NS, Yang GW. Fabrication of a SnO2 nanowire gas sensor and sensor performance for hydrogen. J Phys Chem C. 2008;112:6643.View ArticleGoogle Scholar
- Wang B, Zheng ZQ, Zhu LF, Wu HY, Yang YH. Self-assembled and Pd decorated Zn2SnO4/ZnO wire-sheet shape nano-heterostructures networks hydrogen gas sensors. Sens Actu B. 2014;195:549.View ArticleGoogle Scholar
- Hwang IS, Kim SJ, Choi JK, Jung JJ, Yoo DJ, Dong KY, et al. Large-scale fabrication of highly sensitive SnO2 nanowire network gas sensors by single step vapor phase growth. Sens Actu B. 2012;165:97.View ArticleGoogle Scholar
- Wang B, Zheng ZQ, Wu HY, Zhu LF. Field emission properties and growth mechanism of In2O3 nanostructures. Nano Res Lett. 2014;9:111.View ArticleGoogle Scholar
- Du JM, Huang L, Chen ZQ. A controlled method to synthesize hybrid In2O3/Ag nanochains and nanoparticles: surface-enhanced Raman scattering. J Phys Chem C. 2009;113:9998.View ArticleGoogle Scholar
- Gu FB, Zhang L, Wang ZH, Han DM, Guo GS. Fine-tuning the structure of cubic indium oxide and their ethanol-sensing properties. Sens Actu B. 2014;193:669.View ArticleGoogle Scholar
- Wang SQ, An YK, Feng DQ, Wu ZH, Liu JW. The local structure, magnetic, and transport properties of Cr-doped In2O3 films. J Appl Phys. 2013;113:153901.View ArticleGoogle Scholar
- Yan SM, Ge SH, Zuo YL, Qiao W, Zhang L. Room-temperature ferromagnetism in Er-doped ZnO thin films. Scr Mater. 2009;61:387.View ArticleGoogle Scholar
- Wu XC, Hong JM, Han ZJ, Tao YR. Fabrication and photoluminescence characteristics of single crystalline InO nanowires. Chem Phys Lett. 2003;373:28–32.View ArticleGoogle Scholar
- Das SN, Pal AK. Hydrogen sensor based on thin film nanocrystalline n-GaN/Pd Schottky diode. J Phys D App Phys. 2007;40:7291.View ArticleGoogle Scholar
- Das SN, Kar JP, Choi JH, Lee TI, Moon KJ, Myoung JM. Fabrication and characterization of ZnO single nanowire-based hydrogen sensor. J Phys Chem C. 2010;114:1689.View ArticleGoogle Scholar
- Sze SM. Physics of semiconductor devices. New York: Wiley; 1979.Google Scholar
- Zhu LF, She JC, Luo JY, Deng SZ, Chen J, Ji XW, et al. Self-heated hydrogen gas sensors based on Pt-coated W18O49 nanowire networks with high sensitivity, good selectivity and low power consumption. Sens Actu B. 2011;153:354.View ArticleGoogle Scholar
- Kim I, Rothschild A, Hyodo T, Tuller HL. Microsphere templating as means of enhancing surface activity and gas sensitivity of CaCu3Ti4O12 thin films. Nano Lett. 2006;6:193.View ArticleGoogle Scholar
- Scott RWJ, Yang SM, Chabanis G, Coombs N, Willams DE, Ozin GA. Tin dioxide opals and inverted opals: near-ideal microstructures for gas sensors. Adv Mater. 2001;13:1468.View ArticleGoogle Scholar
- Li J, Fan HQ, Jia XH. Multilayered ZnO nanosheets with 3D porous architectures: synthesis and gas sensing application. J Phys Chem C. 2010;114:14684.View ArticleGoogle Scholar
- Ahsanulhaq Q, Toshinari Y, Maghraby EME, Toshio K. Fabrication and gas sensing properties of In2O3 nanopushpins. Appl Phys Lett. 2009;95:153109.View ArticleGoogle Scholar
- Wei SH, Wang SM, Zhang Y, Zhou MH. Different morphologies of ZnO and their ethanol sensing property. Sens Actu B. 2014;192:480.View ArticleGoogle Scholar
- Huang BR, Lin JC. A facile synthesis of ZnO nanotubes and their hydrogen sensing properties. Appl Surf Sci. 2013;280:945.View ArticleGoogle Scholar
- Wang HT, Kang BS, Ren F, Tien LC, Sadik PW. Hydrogen-selective sensing at room temperature with ZnO nanorods. Appl Phys Lett. 2005;86:243503.View ArticleGoogle Scholar
- Lim W, Wright JS, Gila BP, Johnson J, Ural A, Anderson T, et al. Room temperature hydrogen detection using Pd-coated GaN nanowires. Appl Phys Lett. 2008;93:072109.View ArticleGoogle Scholar
- Huang JR, Gu CP, Meng FL, Li MQ, Liu JH. Detection of volatile organic compounds by using a single temperature-modulated SnO2 gas sensor and artificial neural network. Smar Mater Stru. 2007;16:701.View ArticleGoogle Scholar
- Huang BR, Lin JC. Core-shell structure of zinc oxide/indium oxide nanorod based hydrogen sensors. Sens Actu B. 2012;174:389.View ArticleGoogle Scholar
- Bari RH, Ppatil P, Patil SB, Bari A. Detection of H2S gas at lower operating temperature using sprayed nanostructured In2O3 thin films. Bull Mater Sci. 2013;36:967.View ArticleGoogle Scholar
- Chen PC, Ishikawa F, Chang HK, Ryu KM, Zhou CW. Ananoelectronic nose: a hybrid nanowire/carbon nanotube sensor array with integrated micromachined hotplates for sensitive gas discrimination. Nanotechnology. 2009;20:125503.View ArticleGoogle Scholar
- Xu XJ, Fan HT, Liu YT, Wang LJ, Zhang T. Au-loaded In2O3 nanofibers-based ethanol micro gas sensor with low power consumption. Sens Actu B. 2011;160:713.View ArticleGoogle Scholar
- Kim SJ, Hwang IS, Choi JK, Kang YC, Lee JH. Enhanced C2H5OH sensing characteristics of nano-porous In2O3 hollow spheres prepared by sucrose-mediated hydrothermal reaction. Sens Actu B. 2011;155:512.View ArticleGoogle Scholar
- Singh ND, Ponzoni A, Gupta RK, Lee PS, Comini E. Synthesis of In2O3-ZnO core-shell nanowires and their application in gas sensing. Sens Actu B. 2011;160:1346.View ArticleGoogle Scholar
- Singh ND, Ponzoni A, Comini E, Lee PS. Chemical sensing investigations on Zn–In2O3 nanowires. Sens Actu B. 2012;171:244.View ArticleGoogle Scholar
- Xu XM, Zhao PL, Wang DW, Sun P, You L, Sun YF, et al. Preparation and gas sensing properties of hierarchical flower-like In2O3 microspheres. Sens Actu B. 2013;176:405.View ArticleGoogle Scholar
- Xu XM, Mei XD, Zhao PL, Sun P, Sun YF, Hu XL, et al. One-step synthesis and gas sensing characteristics of urchin-like In2O3. Sens Actu B. 2013;186:61.View ArticleGoogle Scholar
- Kapse VD, Ghosh SA, Raghuwanshi FC, Kapse SD. Enhanced H2S sensing characteristics of La-doped In2O3: effect of Pd sensitization. Sens Actu B. 2009;137:681.View ArticleGoogle Scholar
- Zheng W, Lu XF, Wang W, Li ZY, Zhang HG, Wang Y, et al. A highly sensitive and fast-responding sensor based on electrospun In2O3 nanofibers. Sens Actu B. 2009;142:61.View ArticleGoogle Scholar
- Qurashi A, El-Maghraby EM, Yamazaki T, Shen YB, Kikuta T. A generic approach for controlled synthesis of In2O3 nanostructures for gas sensing applications. J All Comp. 2009;481:35.View ArticleGoogle Scholar
- Korotcenkov G, Cho BK, Boris I, Han SH, Lychkovsky Y, Karkotsky G. Indium oxide ceramics doped by selenium for one-electrode gas sensors. Sens Actu B. 2012;174:586.View ArticleGoogle Scholar
- Yan YG, Zhang Y, Zeng HB, Zhang LD. In2O3 nanotowers: controlled synthesis and mechanism analysis. Cryst Grow Des. 2007;7:940.View ArticleGoogle Scholar
- Ahsanulhaq Q, Maghraby EME, Toshinari Y, Toshio K. Catalyst-free shape controlled synthesis of In2O3 pyramids and octahedron: structural properties and growth mechanism. J Allo Comp. 2009;480:L9.View ArticleGoogle Scholar
- Wang B, Yang YH, Wang CX, Yang GW. Nanostructures and self-catalyzed growth of SnO2. J Appl Phys. 2005;98:073520.View ArticleGoogle Scholar
- Yan YG, Wang X, Chen HX, Zhou LX, Cao XH, Zhang J. Synthesis of ZnO nanotowers controlled by a reagent’s vapour pressure. J Phys D Appl Phys. 2013;46:155304.View ArticleGoogle Scholar
- Neri G, Bonavita A, Rizzo G, Galvagno S, Capone S, Siciliano P. Methanol gas-sensing properties of CeO2-Fe2O3 thin films. Sens Actu B. 2006;114:687.View ArticleGoogle Scholar
- Gonzalez-Vidal JL, Olvera MDL, Maldonado A, Barranca AR, Melendez-Lira M. CO sensitivity of undoped -ZnO, Cr-ZnO and Cu-ZnO thin films obtained by spray pyrolysis. Rev Mexi De Fisi S. 2006;52:6.Google Scholar
- Park YK, Kim SS. Formation of networked ZnO nanowires by vapor phase growth and their sensing properties with respect to CO. Mate Lett. 2011;65:2755.View ArticleGoogle Scholar
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.