Hydrothermal synthesis of In2O3 for detecting H2S in air

Sensors and Actuators B 115 (2006) 642–646

Hydrothermal synthesis of In2O3 for detecting H2S in air Jiaqiang Xu a,b,∗ , Xiaohua Wang a , Jianian Shen b a

College of Materials and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, PR China b Institute of Materials, Shanghai University, Shanghai 200072, PR China Received 19 April 2005; received in revised form 21 October 2005; accepted 24 October 2005 Available online 29 November 2005

Abstract Nanocrystalline In2 O3 gas sensing material was prepared by sintering a precursor In(OH)3 at 600 ◦ C which was hydrothermally synthesized at 250 ◦ C for 24 h by using InCl3 ·4H2 O as a starting material. The nanopowder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry–differential scanning calorimetry (TG–DSC) and X-ray photoelectron spectrometer (XPS). The results indicated that the precursor of indium oxide was cubic indium hydroxide with range size of 50–80 nm, and indium oxide was composed of In and O. Gas sensing properties of the sensors were tested by mixing a gas in air at static state, the tested results showed that the sensor based on In2 O3 nanocrystals had satisfying H2 S gas sensing properties at rather low temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrothermal reaction; Indium oxide; Nanomaterial; Gas sensor

1. Introduction In2 O3 semiconductive materials have been extensively studied as chemical sensors for a long time due to their advantageous features such as a wide band-gap around 3 eV and a low resistance and good catalysis [1,2]. In2 O3 nanometer materials, which possess ultra-high surface-to-volume ratios, are expected to be superior gas sensor candidates and alternatives of thin-film sensors that have been found to have many crucial limitations, especially for their cost. In2 O3 -based materials had been studied as various gas-sensors. Chung [3] reported that an In2 O3 film as a H2 gas sensor had fine sensitivity and selectivity against CO. Gurlo et al. [4] and Epifani et al. [5] studied an In2 O3 film as an O3 gas sensor which had very high sensitivity even if the concentration of O3 was 5–500 ppb. Gurlo et al. [4] explained the sensing mechanism of In2 O3 to oxidizing gases in terms of chemical adsorption and electron transfer. Jiao et al. [6] found that an In2 O3 film had high sensitivity to low concentration of NO2 but low sensitivity to deoxidizing gases. The sensitivity of different kinds of oxides to 1000 ppm CO at different temperatures was compared by Belysheva et al. [7]. The results

∗

Corresponding author. Tel.: +86 371 63556078; fax: +86 371 63556078. E-mail address: [email protected] (J. Xu).

0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.10.038

showed that In2 O3 had the maximum sensitivity, and 1% Audoped In2 O3 was better than 1% Pd-doped In2 O3 as a CO gas sensor. Zhan et al. [8] studied the sensitivity of In2 O3 doped with Au and some oxides such as SiO2 and Fe2 O3 as a liquefied petroleum gas sensor with low power consumption (<100 mW). Traditional synthesis of In2 O3 includes physical methods such as sputtering [9,10], and gas phase deposition [11], and soft chemical methods such as sol–gel [6,12], homogeneous precipitation [13], templating [14], microemulsion [15]. In recent years, nano- and micrometer particles with uniform size and shape have been produced by hydrothermal/solvothermal process. It is a new technique for inorganic materials synthesis, and the reaction system is aquiferous solvent or organic solvent sealed in a Teflon-lined stainless autoclave. Chemical reactions occur among chemicals in the solution at appropriate temperature and pressure. Uniform of crystal growth, ideal and perfect crystallization with few or no disfigurement, can be gained by hydrothermal/solvothermal synthesis. However, there are few reports about hydrothermal/solvothermal synthesis of In2 O3 [16,17] so far. In the present work, nanocrystalline In2 O3 was prepared by sintering the precursor hydrothermal synthesized at 250 ◦ C for 24 h, and used as a H2 S gas sensor. Its gas sensing properties were superior to In2 O3 prepared by chemical precipitation, and the sensor showed satisfying behavior at rather low temperature,

J. Xu et al. / Sensors and Actuators B 115 (2006) 642–646

643

such as high sensitivity, selectivity as well as fast recovery responses. 2. Experimental 2.1. Hydrothermal preparation of In2 O3 The precursor In(OH)3 was prepared by a simple hydrothermal reaction. InCl3 ·4H2 O (In% = 39.0–40.0%, Guoyao Group Chemical Reagent Co. Ltd., Shanghai) was used as a raw material without further purification. In a typical synthesis, 0.305 g InCl3 ·4H2 O was dissolved in 25 ml distilled water and added into a Teflon-lined stainless autoclave of 40 ml capacity with proper quantity of sodium dodecyl benzene sulfonate as a surfactant. The autoclave was sealed and heated at 250 ◦ C for 24 h, then allowed to cool to room temperature. The precipitate was poured out into a tube for centrifugal separating, washing, drying at 105 ◦ C for 3 h. In2 O3 nanocrystals can be gained by sintering the dried precursor at 600 ◦ C for 1 h. 2.2. Characteristic of In2 O3 The crystal structure of samples, including the precursor of indium oxide and indium oxide itself, was characterized by XRD ˚ incident radiation with monochromatized Cu K␣ (λ = 1.5418 A) (X’Pert Pro. Holand). The morphology of the synthesized samples was examined by TEM (JEM-100CX II type, Japanese). TG–DSC (NETZSCH STA 409PC, Germany) was used to analyze the variation of heat and weight while In2 O3 precursor was being annealed. The components of In2 O3 were confirmed by XPS (AXIS ULTRA type, England). 2.3. Fabrication of In2 O3 gas sensor The structure of the gas sensor belongs to side-heated type. The basic fabrication process is as follows. The final powders were mixed and ground with an adhesive in an agate mortar to form gas-sensing paste. The paste was coated on an alumina tube on which a pair of Au electrodes were previously printed, then dried under IR radiation for several minutes in air, and then sintered at 600 ◦ C for 1 h. Finally, the electrodes were jointed on a base and a Ni–Cr heating wire was inserted. The gas sensors prepared were aged at 300 ◦ C for 240 h in order to improve their stability.

Fig. 1. The measuring electric circuit of gas sensor.

was defined as follows: Rair Response = Rgas Rair is the resistance of a sensor in air, and Rgas is that in a detecting gas. 3. Results and discussion 3.1. Crystal structure and morphology of the powder The XRD pattern confirmed that the precursor prepared by hydrothermal reaction was cubic In(OH)3 ; all the diffraction peaks could be indexed to the body-centered In(OH)3 (JCPDS card No.76-1464, a = 0.7974 nm) with perfect crystallization. XRD analysis also showed that the sintered sample was cubic In2 O3 ; all the diffraction peaks completely agreed with those of JCPDS card No.71-2194, a = 1.011 nm. Average grain size determined by full width half maximum (FWHM) of XRD peaks of the precursor and final powder were 50.6 and 29.5 nm, respectively. According to the above results, In2 O3 nanocrystals may be formed as follows: In3+ + OH− → In(OH)3 ; 2In(OH)3 → In2 O3 + 3H2 O.

2.4. Test of In2 O3 gas sensing properties A stationary state gas distribution method was used for the test of gas sensing properties. In the measuring electric circuit for gas sensors (Fig. 1), a load resistor was connected in series with a gas sensor. The circuit voltage was 10 V, and outputs Vout were the terminal voltage of the load resistor. The working temperature of a sensor was adjusted through varying the heating voltage. The resistance of a sensor in air or test gas was measured by monitoring Vout . The test was operated in a measuring system of HW-30A (Hanwei Electronics Co. Ltd., Henan, PR China). The concentration of gas was 50 ppm except liquefied petroleum gas (LPG) of 500 ppm. In this paper, gas response of a sensor

Fig. 2. TEM of precursor of indium oxide.

644

J. Xu et al. / Sensors and Actuators B 115 (2006) 642–646

Fig. 4. TG–DSC curve of the precursor of In2 O3 .

from bulk In2 O3 . We can conclude that the precursor can be completely decomposed above 250.6 ◦ C in one step to form In2 O3 . The TG–DSC analysis and XRD results confirmed the complete decomposition of the precursor In(OH)3 to In2 O3 . The observed weight loss of 15.81% was close to the calculated loss of 16.28%.

Fig. 3. TEM of indium oxide.

Fig. 2 shows a TEM micrograph of the precursor. The precursor displayed cubic form morphology, and the average particle size around 60 nm was the same as the result from the XRD peak broadening. Fig. 3 shows the morphology of indium oxide. The crystallographic form of the as-prepared product is rectangular with perfect crystallization and fine dispersion with range size of 50–80 nm. 3.2. Thermal analysis of the precursor of In2 O3 The variation of heat and weight while In2 O3 precursor been sintering was shown in Fig. 4. There is an endothermic peak at 250.6 ◦ C in the DSC curve, accompanied by a weight loss of 15.81% in TG curve at the same temperature, resulting from thermal decomposition of the precursor. When the precursor was heated above 250.6 ◦ C, there is no obviously endothermic or exothermic peak in the DSC. A slight mass change at 250.6–1000 ◦ C may be induced by desorption of oxygen

3.3. Components analysis of In2 O3 The X-ray photoelectron spectrum shown in Fig. 5(a) indicated that In2 O3 nanocrystals were composed of In and O. Elemental analysis revealed that the mass ratio of O to In was 21.97%:78.03% and the atomic ratio of O:In was 66.89:33.11, which did not agree with the stoichiometric amount of O present in In2 O3 . Excrescent O may be adsorpted oxygen. No other impurities were detected. From the spectrum of O 1s (Fig. 5(b)), it can be seen that there are four peaks separately at 529.509, 530.337, 531.546 and 532.787 eV which were attributed to four kinds of oxygen species on indium oxide. These species are assigned as molecular-type adsorbates, O2 and O2 − and a dissociative type one, O2 2− , which are discerned to desorb depending on the adsorption conditions, in addition to surface (lattice) oxygen, O2− , which desorbs in the temperature range above 350 ◦ C [18].

Fig. 5. XPS of In2 O3 (a) and spectrum of O 1s (b).

J. Xu et al. / Sensors and Actuators B 115 (2006) 642–646

645

Fig. 6. Selectivity of In2 O3 to different gases at 268.5 ◦ C.

3.4. Gas sensing properties of In2 O3 gas sensor We selected 11 kinds of gases as a detecting gas to characterize the gas sensing properties of In2 O3 synthesized by hydrothermal and chemical precipitation. Column chart in Fig. 6 shows that In2 O3 without doping any catalyst and additive has gratifying response and good selectivity to 50 ppm H2 S gas at 268.5 ◦ C. The resistance response is 124.9, the selective coefficient of H2 S against ammonia vapor is 40.27, and that against liquefied petroleum gas is 2.298. The relationship of response and working temperature from 125.6 to 307.0 ◦ C was shown in Fig. 7. Two maxima of response were appeared at 268.5 and 192.0 ◦ C that may be caused by chemisorbed oxygen O2 − and O2 2− , respectively. In Figs. 6 and 7, the gas response of the sensor based on In2 O3 synthesized by hydrothermal process was higher than that synthesized by a chemical precipitation method. The relationship of response–recovery properties and working temperature is shown in Fig. 8. It can be seen that the response is faster than the recovery; response time (less than 10 s) is quick enough to satisfy users’ requirement. When the temperature was lower than 192 ◦ C, recovery time was too long to meet the need of fast detecting, so that 268.5 ◦ C was selected as the optimal working temperature. The best resistance response of the sen-

Fig. 7. Relationship of gas response of In2 O3 and working temperature.

sor to 50 ppm H2 S was 124.9 with a response time of 2 s and a recovery time of 7 s. 3.5. Mechanism discussion of H2 S gas sensing [12,19] Stoichiometric In2 O3 would transform to non-stoichiometric In2 O3−x , and form an n-type semiconductor during calcination at high temperature, because a few crystal lattice oxygens are drawn off from the bulk In2 O3 at high temperature. It can be expressed as: ••

2In2 O3 (s) → 2In2 O3−x (s) + xO2 (g) + 2xVO + 4xe− . This result could be seen from the slight weight loss in TG–DSC curve over 250.6 ◦ C. When the In2 O3−x semiconductor was exposed to air, oxygen would be adsorbed on its surface. At a certain temperature, oxygen in air could despoil of electrons from the In2 O3−x semiconductor to turn into chemisorbed oxygen, such as O2 − or O2 2− that was confirmed by spectrum of O 1s in Fig. 5(b). As a result, the resistance of the n-type semiconductor would increase. When In2 O3 was contacted with H2 S gas, the strong reducing gas may react with O2 x− and put back the electrons to In2 O3−x semiconductor, and thereby the resistance of In2 O3 would decrease. The H2 S sensing mechanism

Fig. 8. Relationship of response–recovery curves and working temperature.

646

J. Xu et al. / Sensors and Actuators B 115 (2006) 642–646

was presumed as follows: −

−

O2 (g) + e
O2 (ad) O2 − (ad) + e−
O2 2− (ad)

[6]

[7]

−

H2 S (g) + e
H2 S (ad) H2 S (g) + O2 x− (ad)
H2 S (ad) + O2 (g) + xe− 2H2 S (ad) + 3O2 x− (ad)
2SO2 (g) + 2H2 O (g) + 3xe−

[8]

[9]

The net reaction is: 2H2 S (g) + 3O2 x− (ad)
2H2 O (g) + 2SO2 (g) + 3xe− It appears that a high concentration of surface-adsorbed oxygen ions (O2 x− ) would favor the forward reaction. According to this mechanism, the phenomenon of this experiment is in good conformity with the presumption and XPS of O 1s. Further detailed investigation is however needed to confirm this discussion. 4. Conclusion In summary, a reproducible simple route to obtain cubic In2 O3 materials on a large scale was developed by annealing the precursor In(OH)3 under ambient pressure. The cubic form In2 O3 had nearly uniform diameter size of 50–80 nm. As a H2 S gas sensor, typically for 50 ppm of H2 S at 268.5 ◦ C, the sensor showed satisfying behavior. The response magnitude was 124.9, and the selective coefficient against ammonia vapor was 40.27, while that against liquefied petroleum gas was 2.298. The response and recovery time was 2 s and 7 s, respectively. Combining the results of TG–DSC and XPS of O 1s, a H2 S gas sensing mechanism was suggested.

[10]

[11]

[12]

[13]

[14] [15]

[16]

[17]

[18] [19]

Acknowledgments We appreciate the financial supports of NSFC (No. 20471055) and Henan Outstanding Youth Science Fund (No. 03120000800). We also show great appreciation to the Key Laboratory of Special Functional Materials of Henan University. References [1] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Self-assembled In2 O3 nanocrystal chains and nanowire networks, Adv. Mater. 16 (2004) 65–69. [2] Z.L. Zhan, J.Q. Xu, D.G. Jiang, State of In2 O3 -based gas sensor, Chin. J. Trans. Technol. 22 (2003) 1–3. [3] W.Y. Chung, Gas sensing properties of spin-coated indium oxide film on various substrates, J. Mater. Sci: Mater. Electron. 12 (2001) 591– 596. [4] A. Gurlo, N. Barsan, U. Weimar, M. Ivanovskaya, A. Taurino, P. Siciliano, Polycrystalline well-shaped blocks of indium oxide obtained by the sol–gel method and their gas sensing properties, Chem. Mater. 15 (2003) 4377–4383. [5] M. Epifani, S. Capone, R. Rella, P. Rella, P. Siciliano, L. Vasanelli, In2 O3 thin films obtained through a chemical complexation based

sol–gel process and their application as gas sensor devices, J. Sol-Gel Sci. Technol. 26 (2003) 741–744. Z. Jiao, M.H. Wu, J.Z. Gu, X.L. Sun, The gas sensing characteristics of ITO thin film prepared by sol–gel method, Sens. Actuators B 94 (2003) 216–221. T.V. Belysheva, E.A. Kazachkov, E.E. Gutman, Gas sensing properties of In2 O3 and Au-doped films for detecting carbon monoxide in air, J. Anal. Chem. 56 (2001) 676–678. Z.L. Zhan, D.G. Jiang, W.H. Song, J. Chang, In2O3-based LPG gas sensor with low power consumption, Chin. J. Trans. Technol. 22 (2003) 31–32, 36. J.Q. Hu, F.R. Zhu, J. Zhang, H. Gong, A room temperature indium tin oxide/quartz crystal microbalance gas sensor for nitric oxide, Sens. Actuators B 93 (2003) 175–180. E.B. Ali, H.E. Maliki, J.C. Bernede, M. Sahnoun, A. Khelil, O. Saadane, In2 O3 deposited by reactive evaporation of indium in oxygen atmosphere influence of post-annealing treatment on optical and electrical properties, Mater. Chem. Phys. 73 (2002) 78–85. E. Gagaoudakis, M. Bender, E. Douloufakis, N. Katsarakis, E. Natsakou, V. Cimalla, G. Kiriakidis, The influence of deposition parameters on room temperature ozone sensing properties of InOx , Sens. Actuators B 80 (2001) 155–161. S. Shukla, S. Seal, L. Ludwig, C. Parish, Nanocrystalline indium oxidedoped tin oxide thin film as low temperature hydrogen sensor, Sens. Actuators B 97 (2004) 256–265. S.C. Tang, J.S. Yao, J. Chen, J.G. Luo, Preparation of indium tin oxide (ITO) with a single-phase structure, J. Mater. Proc. Technol. 137 (2003) 82–85. Y.B. Li, Y. Bando, D. Golberg, Single-crystalline In2 O3 nanotubes filled with In, Adv. Mater. 15 (2003) 581–585. Z.L. Zhan, W.H. Song, D.G. Jiang, Preparation of nanometer-sized In2 O3 particles by a reverse microemulsion method, J. Colloid Interface Sci. 271 (2004) 366–371. D.B. Yu, S.H. Yu, S.Y. Zhang, J. Zuo, D.B. Wang, Y.T. Qian, Metastable hexagonal In2 O3 nanofibers templated from InOOH nanofibers under amibient pressure, Adv. Funct. Mater. 13 (2003) 497–501. H.L. Zhu, K.H. Yao, Y.H. Wo, N.Y. Wang, L.N. Wang, Hydrothermal synthesis of single crystalline In(OH)3 nanorods and their characterization, Semicond. Sci. Technol. 19 (2004) 1020–1023. N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catal. Surv. Asia 7 (2003) 63–75. J.Q. Xu, D.L. Yan, G.Q. Wang, Z.Z. Tian, Q.P. Jiang, Studies on WO3 -based H2 S gas sensing materials, Chin. J. Ceram. Soc. 27 (1999) 591–596.

Biographies Jiaqiang Xu obtained his Master’s degree in inorganic chemistry from University of Science and Technology of China in 1988. He has been a professor in applied chemistry at Zhengzhou University of Light Industry from 2001. He has been a vice-director of China Special Committee of Gas and Humidity Sensor Technology. His research interests include the synthesis of nanometer materials and their application in gas sensor and other fields, as well as improving gas selectivity. Xiaohua Wang obtained her Bachelor degree in chemical engineering from Zhengzhou University in 2001. She had been a technician in Henan Billions Chem. Co. Ltd. for two years. Then, she started to pursue her Master’s degree in material science under the direction of Prof. Xu. Her study work is focus on the synthesis of nanometer materials and their application in gas sensor. Jianian Shen obtained his Master degree of corrosion and protection of metals from the Institute of Metal Research, Chinese Academy of Science in 1982. He is the professor of Shanghai University and the deputy director of the Institute of Materials, Shanghai University. His research interests include the corrosion and the surface modification of metals.