Controllable Preparation And Photocatalytic H₂ Production Activity Of The Indium Selenides And Indium Titanates Nano/micro-structures

H2 production from water splitting driven by solar energy is an environmentally friendly technology to produce renewable energy, which is the best way to to solve the energy and enviromental problems. The hard and hot-point on solar photocatalytic hydrogen production studies is to prepare photocatalyst with wide spectral response and high photocatalytic activity.Indium selenides have been widely used in optoelectronic devices and solar cells owing to their strong visible-light absorption ability. Nevertheless, indium selenides have not been explored as excellent functional materials for photocatalytic hydrogen production because of their complicated synthesis craft, poor micro/nano architectures, and so on. Indium titanate is a wide band gap semiconductor and usually used in ultraviolet light photocatalytic reaction. Thus the solar energy utilization is very low. In allusion to these problems, we have designed a variety of micro- and nano-structured indium selenides and indium titanates, and carried out corresponding characterizations about structure, morphology and composition. Through the study of photocatalytic H2 production performance of as-synthesized materials, we systematically studied their photocatalytic H2 production activities. The major research results are as follows.(1) Fabrication of porous In2Se3 tetragonum by a facile two-step solvothermalcalcining process and its photocatalytic hydrogen production propertiesIn2Se3 was directly synthesized by a facile two-step solvothermal-calcining method using InCl3 and Se as the source materials. This route features low cost, environmentally benign, simple, high purity and narrow particle size distribution. The morphologies, compositions and optical absorption properties of the as-prepared samples have been deeply discussed based on the characterizations, including XRD, SEM, EDS, TEM, UV-Vis DRS, BET, PL and so on. The precursors obtained by solvothermal process were tetragonums and nanoparticles. After annealing in N2 atmosphere, the precursors turned into pure phase porous γ-In2Se3 tetragonums and γ-In2Se3 nanoparticles. In addition, the resultant In2Se3 materials with three different morphologies could be utilized as photocatalysts for photocatalytic hydrogen evolution in pure water and in the triethanolamine-water system under ultraviolet light and visible light irradiation, respectively. Among them, porous In2Se3 tetragonum shows the best photocatalytic hydrogen production effect. The relationship between morphology and photocatalytic hydrogen production activity was also discussed.(2) Fabrication of InSe and InSe-TiO2 nanosheets by a facile two-step solvothermalcalcining process and their visible light photocatalytic hydrogen production propertiesInSe nanosheets were directly synthesized by a facile two-step solvothermal-calcining method using InCl3 and SeO2 as the source materials. The morphologies, compositions, optical absorption property and element valence of the as-prepared sample have been deeply discussed based on the characterizations, including XRD, SEM, EDS, TEM, UV-Vis DRS, BET, XPS and so on. The precursors obtained by solvothermal process were regular nanosheets. After annealing in N2 atmosphere, the precursor turned into pure cubic phase InSe nanosheets. InSe nanosheets exhibit strong visible light absorption property with a band gap of 1.55 eV. The dependence of evolved H2 on different sacrificial agents was performed with InSe nanosheets as a photocatalyst.The result was that the photocatalytic reaction with ascorbic acid has the highest H2 production amount of 15.1 μmol in 1.5 h.The prepare procedure of InSe-TiO2 nanosheets was the same as that of above InSe nanosheets, and only P25 was added during the solvothermal step. UV-Vis DRS result showed that InSe-TiO2 nanosheets exhibit strong absorption peaks in the range of 250-800 nm. The visible light photocatalytic H2 production reaction were performed on InSe-TiO2 photocatalysts with different contents of TiO2. Pt was used as a cocatalyst and ascorbic acid was used as a sacrificial agent. Among all InSe-TiO2 heterostructures, InSe-TiO2(0.015) sample has the maximum H2 production rate of 1.1 mmol/(g catalyst?h). It was seen that the photocatalytic activity of InSe-TiO2(0.015) exceeds that of pure InSe nanosheets by a factor of 4.1. As confirmed by HRTEM, nitrogen adsorption-desorption isotherms, contact angles and photocurrent responses of the as-prepare InSe and InSe-TiO2 samples, the improved photoactivity by modification with TiO2 was due to the increased photoreaction sites and high interfacial charge sepatation and transportation efficiency. In addition, the visible light photocatalytic H2 production activity of InSe-TiO2(0.015) nanosheets was dependent on the concentrations of ascorbic acid and Pt. What’s more, the InSe-TiO2(0.015) catalyst can go through a continuous 18 h long-time photoreaction under the optimized condition, without evident change in H2 production rate.(3) Band gap engineering of In2TiO5 and its near-infrared light photocatalytic hydrogen production propertiesIndium titanate was synthesized by a multi-step sol-gel-calcination method using InCl3 and Ti(C4H9O)4 as the source materials. Oxygen vacancies and nitrogen doped indium titanates were obtained by annealing pristine indium titanate in NH3 flow. The composition, crystal structure, morphologies, optical absorption property, element valence and electronic band structure of the as-prepared indium titanate photocatalysts have been deeply discussed based on the characterizations, including XRD, elemental analyses, FTIR, EDS, TEM, XPS, EPR, UV-Vis-NIR DRS, theoretical calculation and so on. Indium titanate samples obtained by NH3-treated temperatures higher than 600 oC contain oxygen vacancies and substitutional nitrogen, while the samples obtained by NH3-treated temperatures lower than 500 oC have no oxygen vacancies and substitutional nitrogen. Compared to pristine indium titanate, the band gap of oxygen vacancies and nitrogen doped indium titanate was reduced. A newly formed oxygen vacancy state is observed between the conduction band and the valance band. Thus the oxygen vacancies and nitrogen doped indium titanate sample exhibits near-infrared light absorption property. The oxygen vacancies and nitrogen doped indium titanates were utilized as photocatalysts for near-infrared light photocatalytic hydrogen evolution. The effect of NH3-treated temperature and the sacrificial agents were studied. The result was that the indium titanate annealed at 700 oC has the highest H2 production rate of 169.8 μmol/(g catalyst·h) in methanol aqueous solution under near-infrared light irradiation.This paper firstly demonstrated that the indium selenides can be synthesized by a facile two-step solvothermal-calcining method. These nano-structured indium selenide materials can perform as photocatalysts in hydrogen production under visible light irradiation, which has not been reported before. In addition, this work represents the first case of realizing near-infrared light photocatalysis of indium titanate for hydrogen evolution.