(Invited) Particulate Photocatalysts with Perovskite-Type Structures and Their Application in Water Splitting
Authors: Hisatomi Takashi 
Published: July 2021

Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen as a storable and transportable energy carrier. Both the efficiency and scalability of water- splitting systems are essential factors for practical application of renewable solar hydrogen because of the low areal density of solar energy [1]. It is desirable to develop particulate photocatalysts and their reaction systems that efficiently split water, because such systems can be spread over wide areas by inexpensive processes potentially. The author’s group has studied various oxide and oxynitride semiconductors as photocatalysts for water splitting. In my talk, recent progress in photocatalytic materials and reactors will be presented with focus on perovskite-type materials.

SrTiO3 loaded with proper cocatalysts has been known to be active in overall water splitting under ultraviolet light irradiation since 1980s. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved drastically [2]. The author's group has found that doping Al3+ into the titanium site of SrTiO3 boosts the water splitting activity by two orders of magnitude [3]. By refining the preparation conditions of the Al-doped SrTiO3 (SrTiO3:Al) photocatalyst and the loading conditions of cocatalysts working as hydrogen and oxygen evolution sites, the AQY of overall water splitting has reached more than 90% at 365 nm [2], equivalent to an internal quantum efficiency of almost unity. This quantum efficiency is the highest yet reported and indicate that a particulate photocatalyst can drive the greatly endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained from photon-to-chemical or photon-to-current conversion in photosynthesis or photovoltaic systems, respectively.

Photocatalyst sheets based on SrTiO3:Al contained in a panel-type reactor split water into hydrogen and oxygen and release gas bubbles at a rate corresponding to a solar-to-hydrogen energy conversion efficiency of 10% under intense ultrviolet illumination even when the water depth is merely 1 mm [4]. Moreover, the photocatalyst can maintain 80% of its initial activity during 1300 h of constant simulated sunlight irradiation at ambient pressure with appropriate surface modifications [5]. A prototype 1-m2-sized panel reactor containing SrTiO3:Al photocatalyst sheets splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. A solar hydrogen production system with a greater size (100 m2) was recently built, and its performance and system characteristics are under investigation.

SrTiO3:Al harvests only the ultraviolet light of the sunlight. It is essential to develop photocatalysts active under visible light irradiation. BaTaO2N is a visible light that can evolve hydrogen and oxygen from aqueous solutions containing sacrificial electron donors and acceptors, respectively. Recently, the AQYs of BaTaO2N in these sacrificial reactions have been improved significantly. Highly-crystalline BaTaO2N can be synthesized by directly nitriding a mixture of BaCO3 in the presence of RbCl flux [6]. The resultant BaTaO2N shows decent hydrogen evolution activity when a platinum cocatalyst is loaded by impregnation. By optimizing the cocatalyst loading conditions, the electron transfer from the BaTaO2N photocatalyst to the platinum cocatalyst, and thus the hydrogen evolution activity, was enhanced. The AQY of the resultant Pt-loaded BaTaO2N reached 6.8% at 420 nm in the sacrificial hydrogen evolution reaction. This is exceptionally high among nitride-type photocatalysts with visible light absorption of up to 650 nm.

In conventional synthesis, BaTaO2N is nitrided from a mixture of BaCO3, Ta2O5, Ba- and Ta-containing complex oxides, often in the presence of flux. However, these starting materials have different crystal structures from the product BaTaO2N, and so the crystal structures have to be change during the nitridation. This is thought to induce defects in the product material. Therefore, a perovskite-type oxide with a Ba/Ta molar ratio of unity was designed, namely a complex oxide nominally denoted as Na1/4Ba3/4Zn1/4Ta3/4O3 [7]. This oxide can be nitrided into BaTaO2N because Na and Zn components are volatile at high temperature. The resultant BaTaO2N exhibited an AQY of 11.9% at 420 nm in the sacrificial oxygen evolution reaction after loading of a cobalt oxide cocatalyst.

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