Synthesis of High Purity Layered Structure Sn3O4 and Its Photocatalytic Performance for Hydrogen Evolution Reaction Under Visible-Light Irradiation
Authors: Hashimoto Masanari , Tanabe Toyokazu , Gunji Takao , Kaneko Shingo , Abe Hideki , Matsumoto Futoshi 
Published: February 2020

Catalytic water splitting in visible light by visible-light-sensitive photocatalysts is of confocal interest because this will realize efficient conversion of the solar energy to hydrogen fuels for sustainable energy managements. Many water-splitting photocatalysts, such as metal-doped oxides and oxynitrides, can decompose water and produce hydrogen fuel when absorbing visible light [1]. However, such transition-metal oxides and oxynitrides may be not practical catalysts for large-scale solar-energy conversion, because they contain environmentally toxic heavy metals or expensive transition metals (Rh, Nb, Ta, or La), which are much less produced than abundant metals such as iron. Herein, we report that an oxide of abundantly available and environmentally non-toxic tin, Sn3O4, can efficiently catalyze Hydrogen (H2)-evolution in a methanol solution under irradiation of visible light. Sn3O4 belongs to a series of layered tin oxides such as SnO and Sn2O3, of which crystal structure consists of stacking of alternating atomic layers of tin and oxygen, but the refined structure of Sn3O4 has not been fully elucidated. Recent studies have demonstrated the photoelectric properties and photocatalytic performance of dye degradation of Sn3O4, but none has ever shed light to its potential as a photocatalyst for water splitting. We prepared high purity nanocrystals of Sn3O4 by hydrothermal synthesis using sodium citrate as a ligand [2]. Optimized hydrothermal synthesis provided high purity and highly active Sn3O4 material. SEM and TEM have shown that the Sn3O4 material consisted of thin, highly crystalline flexes (500×500×10nm3), which were predominantly surrounded by the {110} facets (the inset of Fig.1). The Sn3O4 material was orange in color and more efficiently absorbed visible light than the control SnO2. Figure 1 shows the time courses of hydrogen (H2) evolution over the SnO-, SnO2-, and Sn3O4 materials under irradiation of visible light (λ > 420 nm). The hydrogen (H2)-evolution tests have demonstrated that Sn3O4 can promote H2 evolution at a significant efficiency with and without co-catalyst (Pt), whereas neither SnO nor SnO2 is active toward the reaction, as shown in Fig.1. The activity of Sn3O4 was increased with the purity of Sn3O4 phase, where inactive SnO2 particles inhibited the H2 evolution. Theoretical calculations have elucidated that the enhanced activity of Sn3O4 is attributed to the co-existence of Sn2+ and Sn4+ in Sn3O4 leads to a desirable band gap and band-edge position for photocatalytic H2 evolution from water solution under visible light, the conduction-band minimum of Sn3O4 is higher than the reduction potential for water, and the band gap matches the photon energy of visible light. The Sn3O4 has great potential as a practical solar-energy conversion catalyst, in terms of low impacts to the environment, abundance of mineral resource, and catalytic performance for the H2 generation from water solution.
[1] M. Hara, K. Domen, et al., Stud. Surf. Sci. Catal. 2003, 145, 169−172.
[2] M. Maidhily, T. Tanabe et al., ACS Appl. Mater. Interfaces 2014, 6, 3790.

Figure 1