Preparation of nanometer-sized In2O3 particles by a reverse

* Corresponding author.
E-mail address: zhanzili@ (Z.L. Zhan). 0021-9797/$ – see front matter 2003 Published by Elsevier Inc. doi:10.1016/j.jcis.2003.11.048
Zili Zhan,∗ Wenhui Song, and Denggao Jiang
School of Chemical Engineering, Zhengzhou University, Zhengzhou 450002, China Received 30 April 2003; accepted 24 November 2003
Abstract Nanometer-sized indium oxide (In2 O3 ) particles have been prepared by chemical reaction of inorganic indium compounds and ammonia gas in a reverse microemulsion system consisting of water, Triton X-100 (surfactant), n-heptanol (co-surfactant), and n-octane (oil). Precursor hydroxides precipitated in the droplets of water-in-oil (W/O) microemulsion were calcined at different temperatures to form indium oxide powder. The factors affecting the particle size have been discussed; the calcination temperature is considered to be the important factor for controlling the size. In2 O3 calcined at 400 ◦ C had a spherical form and a narrow size distribution. Calcination at 800 ◦ C led to the formation of particles not only of irregular shape, but also of a wide size distribution. With the increase in calcination temperature from 400 to 800 ◦ C, the average size of the particles grew from 7 to about 40 nm. The species of reactants used in the aqueous phase had a significant effect on the size of the particles. The average diameter of In2 O3 particles derived from reactant InCl3 was 7 nm; that of particles derived from In(NO3 )3 was 15 nm. The In2 O3 nanoparticles were characterized by transmission electron microscopy and X-ray diffraction. The phase behavior of the microemulsions is discussed. 2003 Published by Elsevier Inc.
because of the high degree of agglomeration and irregular particle morphology in the In2 O3 powder. It is well known that the sensitivity of gas sensors can be improved by decreasing the particle size of the gas-sensing material in order to increase the number of oxygen sites on its surfaces. Thus, for the application of In2 O3 as a sensing layer material of chemical gas sensors, nanosized particles are desirable. Reverse microemulsions are colloidal nanodispersions of water in oil stabilized by a surfactant film. These thermodynamically stable dispersions can be considered as true nanoreactors which can be used to carry out chemical reactions and, in particular, to synthesize nanomaterials. The synthesis of nanoparticles in microemulsions and the evolution of the formation mechanisms and growth control in recent years have been reviewed by López-Quintela [9,10]. The particle size is affected by the microemulsion droplets, the flexibility of the surfactant film, the reactant concentration, and the adsorption of surfactant on the particles as listed below: (1) particle size increases with increasing reactant concentration, (2) particle size decreases with increasing excess of one of the reactants, (3) particle size increases with increasing surfactant film flexibility, (4) particle size
Keywords: Indium oxide; Microemulsion; Nanoparticles
1. Introduction Indium oxide (In2O3 ) is an n-type semiconductor that shows the unusual combination of high transparency in the visible region and high electrical conductivity. These properties make In2 O3 an interesting material for a number of applications: solar cells, optoelectronic devices, and liquid crystal displays [1]. Indium oxide is also an important material for semiconductor gas sensors. Recently, In2 O3 has been used by many researchers as a gas-sensing material [2–4]. In2 O3 -based gas sensors exhibite some excellent gas-sensing properties, such as high sensitivity to O3 [5] and NO2 [6] and high sensitivity and selectivity to CO without being disturbed by the coexistence of H2 [7]. The In2 O3 used for gas sensors is usually prepared from precursor hydroxides precipitated by addition of NH4 OH to aqueous InCl3 solutions [7,8]. However, it is not easy to obtain In2 O3 nanoparticles from this precipitation method,
Journal of Colloid and Interface Science 271 (2004) 366–371 /locate/jcis
Preparation of nanometer-sized In2O3 particles by a reverse micrห้องสมุดไป่ตู้emulsion method
Z. Zhan et al. / Journal of Colloid and Interface Science 271 (2004) 366–371
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increases with increasing droplet size, and (5) the growth of the particles stops after adsorption of surfactants onto the particles. For the formation of ceramics via microemulsion techniques, the lack of methods for inhibiting the aggregation and sintering of the produced nanoparticles when they are fired to crystallize is a serious drawback. It is difficult to control the particle size of ceramics. Water-in-oil microemulsions have been used to synthesize many metal oxide particles, such as titania [11], indium– tin oxide [12], and tin oxide [13]. But the synthesis of In2 O3 nanoparticles through reverse microemulsions is unseen in the related reports. In order to obtain highly sensitive In2 O3 gas-sensing material and study the correlation between the particle size and the sensitivity of the gas sensor, the phase behavior of waterin-oil microemulsions has been observed by a laser device, and nanoparticles of indium oxide have been prepared in Triton X-100/n-octane/n-heptanol/water microemulsions. In2 O3 particle size distribution and physical properties were also studied.