Enhanced photocatalytic activity of porous In₂O₃ for reduction of CO₂ with H₂O

In this paper, a series of indium oxides were prepared by calcining the In(OH)₃ precursors, which were synthesized by hydrothermal method using the mixed solution of ethylenediamine (En) and water as a solvent. The morphologies, particle sizes, pore structure, crystallinity and surface defect concentration of these photocatalysts were adjusted by varying the ratio of En to water during the preparation of In(OH)₃ precursors. The results revealed that In₂O₃ photocatalysts obtained from the precursor prepared in En containing solvent exhibited much higher photocatalytic activities for CO₂ reduction with H₂O when compared to that derived from the precursor prepared in pure water. This result can be ascribed to several reasons as following. Firstly, the En addition with a suitable amount can improve the crystallinity of In₂O₃ and decrease the surface defect concentration, which obviously depressed the recombination of photogenerated electrons and holes. Also, the addition of En during the preparation of precursor can decrease the particle size, increase specific surface area and pore structure, resulting into the increase of active sites. Finally, the band gap of In₂O₃ can be slightly narrowed after the addition of En, resulting in enhanced light absorption in the visible region. When the ratio of ethylenediamine to water is 1:1, the as-prepared In₂O₃ possessing the largest specific surface area and pore volume, enhanced light absorption ability and higher hole–electron separation efficiency, exhibited the highest photocatalytic activity. Under visible light irradiation, the H₂, CO and CH₄ production rates of 5.3, 8.3 and 27.2 µmol/g_cat/h can be achieved, respectively.