Development of a New Fabrication Route for High Quality Visible-Light-Driven Photocatalysts; Atmosphere Controlled Flux Growth for Oxynitride and Nitride Crystals
Authors: Suzuki Sayaka , Yanai Minori , Komatsu Mugi , Saito Haruka , Hisatomi Takashi , Oishi Shuji , Domen Kazunari , Teshima Katsuya 
Published: February 2020

Photocatalysts and related materials have been attracted a number of research interests due to their various application such as decomposition of organic substances, superhydrophilic surfaces, dye-sensitized solar cells, and hydrogen production by water splitting. In particular, water-splitting by photocatalysts have been investigated because of expectation to supply clean and recyclable hydrogen energy. Generally, photocalysts, as represented by TiO2, are activated by only ultraviolet light illumination due to their wide band gap. Although these UV-light-driven photocatalysts can split water in a proper condition, the solar energy conversion efficiency is rather low because UV light energy is just several percent in total energy of sun light on the earth, and visible light accounts for almost half of the solar energy.

From the viewpoint of increase the efficiency and industrial application of solar hydrogen production, visible-light-driven photocatalysts have intensively attracted research interests. In particular, oxynitride and nitride semiconductor photocatalysts are one of promising materials for construction of photocatalytic water splitting systems. Although visible-light-driven photocatalysts with sufficiently small band gap, an appropriate band gap position for water splitting, and stability during the reaction have been developed, it is still required to improve the quantum yield. Generally, lattice defects are not suitable for photocatalysts with high performance because they would become recombination centers of excited carriers.

Flux method is a liquid phase crystal growth technique which uses molten metals or molten metal salts as solvents. Flux method can afford binary or multicomponent high-quality crystals at lower temperatures below their melting points. It is interesting that flux-grown crystals usually have idiomorphic shapes with well-developed facets, which represents their high crystallinity. In addition, it provides simple, low-cost and environmentally-benign pathway compared to conventional solid-state-reaction method. Therefore, flux growth is a promising way to prepare high-quality visible-light-driven photocatalyst crystals. Herein, we present growth of oxynitride and nitride crystals for photocatalysts through atmosphere controlled flux method. LaTiO2N, Ta3N5, SrTaO2N, BaTaO2N, and BaNbO2N (and many kinds of oxynitrides and nitrides) crystals were grown under NH3 flow with using flux. Especially LaTiO2N crystals were directly grown from KCl flux and exhibited higher oxygen evolution performance compared to those prepared by conventional methods. It is attributed to elongated lifetime of excited carriers which detected by time-resolve IR spectroscopy. The flux growth is applied to crystal layer (i.e., thin and thick films) fabrication technique and then various photoelectrode consisting of the above-mentioned photocatalysts and current collector were fabricated. The details of flux growth and evaluation (e.g., photocatalytic performance) of the oxynitride and nitride crystals will be presented in 233rd ECS Meeting.

This research was supported in part by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), NEDO and JSPS Grant-in-Aid for Scientific Research (A) 25249089.