¹ Research Initiative for Supra-Materials, Shinshu University, Japan

² Office of University Professors, The University of Tokyo, Japan

Abstract 

Sunlight-driven water splitting using particulate photocatalysts has been attracting growing interest because such systems can be spread over large areas by potentially inexpensive processes [1]. In fact, a solar hydrogen production system based on 100-m² arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built, and its performance and system characteristics including safety issues were reported recently [2]. Nevertheless, it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems. In my talk, recent progress in photocatalytic materials and reaction systems will be presented.

The author’s group has studied various semiconductor materials as photocatalysts for water splitting. Recently, the apparent quantum yield (AQY) of SrTiO₃ has been improved to more than 90% at 365 nm, equivalent to an internal quantum efficiency of almost unity, by refining the preparation of the photocatalyst and cocatalysts [3]. This observation means that particulate photocatalysts can drive the endergonic overall water-splitting reaction with almost no recombination loss. For practical solar hydrogen production, it is essential to develop photocatalysts that are active under visible light. Recently, Ta₃N₅ [4], Y₂Ti₂O₅S₂ [5], TaON [6], and BaTaO₂N [7] were found to be active in photocatalytic overall water splitting via one-step excitation under visible light. In these achievements, the synthesis of well-crystallized semiconductor particles and the loading of composite cocatalysts were important for improving photocatalytic activity. It is also possible to combine hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) and decompose water into hydrogen and oxygen via two-step excitation. Such a process is also known as Z-scheme. Particulate photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ as the HEP and Mo-doped BiVO₄ as the OEP immobilized onto Au and C layers split water into hydrogen and oxygen with STH values exceeding 1.0% [8, 9]. Some other (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths are also applicable to Z-schematic photocatalyst sheets ^[10] and hold the promise of realizing greater STH values.

References

^[1] Hisatomi et al., Nat. Catal. 2, 387 (2019).

^[2] Nishiyama et al., Nature 598, 304 (2021).

^[3] Takata et al., Nature 581, 411 (2020).

^[4] Wang et al., Nat. Catal. 1, 756 (2018).

^[5] Wang et al., Nat. Mater. 18, 827 (2019).

^[6] Xiao et al., Angew. Chem. Int. Ed. 134, e202116573 (2022).

^[7] Li et al., ACS Catal. 12, 10179 (2022).

^[8] Wang et al., Nat. Mater. 15, 611 (2016).

^[9] Wang et al., J. Am. Chem. Soc. 139, 1675 (2017).

^[10] Hisatomi et al., Catal. Sci. Technol. 8, 3918 (2018).