Research Progress of Zinc Stannate-Based Photocatalysts

Solar energy utilization is a significant way to solve the environmental and energy problems accompanying with the fossil fuel economy development，while artificial photosynthesis imitated semiconductor photocatalysis is one of important solar energy utilization strategies. As a sustainable tactics can be qualified for solving the issues of energy crisis and environmental pollution，semiconductor photocatalysis holds a comparably significant role among various treatment technologies. The core of semiconductor photocatalysis is inseparable from the development of efficient photocatalysts. In fact，there are a large number of binary metal oxides（TiO2，Fe2O3，ZnO，CdS，SnO2，etc.）and even more stable ternary II-IV-VI metal oxides（SrTiO3，CaTiO3，Zn2GeO4，ZnIn2S4）can work as photocatalytic materials. Among various semiconductor photocatalytic materials，zinc stannate（Zn2SnO4）with inverse spinel structure has become a research hotspot due to its good conductivity，high electron mobility，stable physicochemical properties and non-toxicity. The development of Zn2SnO4 contributes to the environmental alleviation and the energy production to some extent，of which has been considered as a rising star photocatalyst. However，the unmodified Zn2SnO4 can only respond to UV-light，in the meantime，the recombination rate of photogenerated electron-hole pairs is quite fast，the surface-active sites are relatively limited，resulting in its poor photocatalytic behavior and stability during the reaction process. Up to now，much effort has been devoted to the enhancement of photocatalytic performance over Zn2SnO4，including doping，semiconductor heterojunctions，metallic loading and morphological or defect engineering. Amongst，doping requires the introduction of alien elements into Zn2SnO4 via physical or chemical approaches，leading to the formation of new charges within the crystal，producing defects or altering the lattice types，and thus changing the electronic structure and the distribution of photo-generated carriers of Zn2SnO4. As for semiconductor heterojunctions，combining with two or more kinds of semiconductor materials in a certain way on the micro/nano scale is needed，which would be a modification of the charge of one semiconductor to the another. By combination of the advantages of two or more materials can effectively regulate the performance of a single semiconductor. Whilst for the metallic loading，a single metallic or bimetallic element loading is indispensable，and thereby expanding the light absorption range and accelerating the diffusion of photo-generated electrons to the particle surface to participate in the subsequent catalytic reaction. By comparison，morphological engineering seems to be achieved in an easy manner. Morphology，in face，directly determines the physical properties of catalytic materials such as grain size，specific surface area and pore structure. As for defect engineering，it can endow the modified Zn2SnO4 photocatalysts with the positive effects on the energy band structure，light absorption and utilization，separation and migration of photogenerated carriers，surface catalytic reactions，and photocatalytic performance. However，one problem is that there is a lack of comprehensive overview on the influence of the microstructure，crystal configuration and internal electronic structure arrangement on the catalytic performance. Herein，in this review，the synthesis methods including （microwave）hydrothermal treatment of Zn2SnO4 photocatalysts with micro/nano frameworks were firstly summarized. And then，the effects of the synthesis conditions on the crystal configuration，surface electronic structure，morphological characteristics，particle size and catalytic performance of Zn2SnO4 were systematically overviewed. In addition，the abovementioned four modification strategies for Zn2SnO4 were introduced accordingly，and the effects of different modification methods on the structure and performance of photocatalytic materials were also well expounded. On the grounds of experimental and theoretical results，it was found that the aforesaid modification tactics can effectively improve the visible light utilization and carrier separation efficiency over Zn2SnO4-based materials. At the same time，three important processes containing the light absorption，carrier dynamics and surface catalytic reaction were carefully analyzed. Importantly，the applications in photocatalysis over Zn2SnO4 were also briefly introduced，including photocatalytic reduction of CO2，photocatalytic H2 evolution by splitting H2O，removal of liquid pollutants，dye-sensitized solar cell，degradation of dyes and photocatalytic oxidation of gaseous contaminants. Although good results had been achieved in the modified Zn2SnO4 photocatalysts，there were still challenges in revealing the reaction mechanism on the catalyst surface. For example，different metals（ions/atoms/clusters）doping and co-decoration of doping and defect configurations were more complex cases，of which the corresponding mechanism in the catalytic process had not been clarified. In addition，combining theory with experiment was inevitable to have a better understanding of the reaction mechanism. Even more importantly，research interest had been directed to the obtainment of Zn2SnO4 with a greatly improved photocatalytic behavior by a facile modification method under the simulated real environmental condition，which could be expected to accomplish the industrial applications. As a result，to cover the aforementioned concerns in a feasible manner，the prospects and opportunities of the modified Zn2SnO4 as an efficient photocatalyst in various photocatalytic applications were outlooked. This review provided some new ideas and directions for the controllable synthesis and modification of Zn2SnO4-based catalytic materials with high catalytic performance. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.