Structure and Photocatalytic Properties of In(OH)<sub>3</sub>/InOOH Natural Heterojunction Nanocrystals Prepared by Hydrothermal Synthesis

In(OH)3/InOOH nanocrystals are synthesized using InCl3∙4H2O as the indium source in the presence of ethylenediamine by a one-step hydrothermal process, and the effect of synthesis temperature on the morphology, crystallization, structure, and photocatalytic properties of the photocatalysts is investigated. The results show that all the synthesized In(OH)3/InOOH samples have granular nanoparticles with a size of 20–30 nm. When the synthesis temperature is lower than 150°C, only the In(OH)3 phase exists. With increasing synthesis temperature, In(OH)3 gradually dehydrates to form InOOH to achieve a phase equilibrium between InOOH and In(OH)3. The concentration of InOOH is the maximum at 180°C and decreases at 210°C. The photocatalytic activity is evaluated from the degradation of a Rhodamine B solution under ultraviolet light irradiation, which follows the first-order reaction kinetics. The increase in synthesis temperature can significantly improve the photocatalytic performance of In(OH)3/InOOH because of the formation of a large number of natural heterojunctions in the nanocrystals, which can enhance ultraviolet light absorption and facilitate charge transfer and separation. The first-order kinetic constant of the nanocrystals synthesized at 180°C is almost 2.5 times that of the nanocrystals synthesized at 150°C. The active species capture experiments demonstrate that holes and superoxide radicals are the main active species in the photocatalytic systems.