Photoreduction of Mesoporous In₂O₃: Mechanistic Model and Utility in Gas Sensing

Abstract

A model is proposed for the drop in electronic resistance of n-type semiconducting indium oxide (In(2)O(3)) upon illumination with light (350 nm, 3.5 eV) as well as for the (light-enhanced) sensitivity of In(2)O(3) to oxidizing gases. Essential features of the model are photoreduction and a rate-limiting oxygen-diffusion step. Ordered, mesoporous In(2)O(3) with a high specific surface area serves as a versatile system for experimental studies. Analytical techniques comprise conductivity measurements under a controlled atmosphere (synthetic air, pure N(2)) and temperature-resolved in-situ Fourier transform infrared (FTIR) spectroscopy. IR measurements reveal that oxygen vacancies form a donor level 0.18 eV below the conduction band.

Temperature-dependent Photoconductance and Optical Properties of In2O3 Thin Films Prepared by Autowave Oxidation

“…The resistivity of the films reduced sharply when the UV light source was turned on. This change in resistance was approximately 25 % at room temperature, presumably because of the generation of free charge carriers and/or an increase the electron mobility, caused by desorption of oxygen ions from the grain boundaries [29]. Similar resistance behavior in In 2 O 3 films under UV irradiation was observed in another study [34].…”

“…Currently, there are a few interpretations on how light effects the electrical properties of indium oxide [29]: the generation of electron/hole pairs, desorption of oxygen adsorbates and photoreduction. The band gap of the indium oxide films, depending on the method of preparation, can vary from 3.5 eV (355 nm) to 4 eV (310 nm) [5], which is consistent with our measurements [45].…”

“…Relaxation of the normalized resistance and transmittance after the UV irradiation had been terminated, as a function of time number of surface defects and grain boundaries led to modifications of the indium oxide band structure. Hence, electron transfer is only possible when the energy of the light is smaller than the band gap energy [29,30]. An electron/hole pair can be formed as a result of this transition.…”

“…The quantitative change in the resistance in the In2O3 films irradiated with UV light was strongly dependent on film's structure and morphology [24,26,28]. This effect could be used to improve the sensitivity of gas sensors based on indium oxide thin films [25,[28][29][30]. The effect of reducing the Casimir force in ITO films under UV has been reported [31].…”

Controlling synthesis and gas-sensing properties of ordered mesoporous In2O3-reduced graphene oxide (rGO) nanocomposite

“…Various nanostructured metal oxides with high surface area have widely investigated as sensing materials [1][2][3][4][5]. Among them, ordered mesoporous metal oxides have attracted considerable attention since their accessible pores benefit not only the diffusion of gas molecules for increasing response rate, but also the reduction in aggregation and sintering for enhancing their thermal stability under high temperature during the fabrication and work process of gas sensor [6][7][8][9][10][11][12][13]. For example, Tiemann and co-workers [13] reported the improved sensitivity of mesoporous In 2 O 3 to CH 4 .…”