Development of Particulate Photocatalysts for Water Splitting

Sunlight-driven water splitting has been studied as a means of producing renewable solar hydrogen. Overall water splitting using particulate photocatalysts has been attracting growing interest, because such systems can be spread over wide 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.<br/><br/>The author’s group has studied various semiconductor oxides, (oxy)nitrides, and (oxy)chalcogenides as photocatalysts for water splitting [3]. SrTiO₃ is an oxide photocatalyst that has been known to be active in overall water splitting under ultraviolet irradiation since 1980. Recently, the apparent quantum yield (AQY) of this photocatalyst in overall water splitting has been improved to more than 90% at 365 nm, equivalent to an internal quantum efficiency of almost unity, by refining the preparation conditions of the photocatalyst and the loading conditions of cocatalysts [4]. This quantum efficiency is the highest yet reported and indicates that a particulate photocatalyst can drive the endergonic overall water splitting reaction at a quantum efficiency comparable to values obtained in photon-to-chemical and photon-to-current conversion processes by photosynthesis and photovoltaic systems, respectively.<br/><br/>For practical solar energy harvesting, it is essential to develop photocatalysts that are active under visible light irradiation. Recently, Ta₃N₅ [5], Y₂Ti₂O₅S₂ [6], TaON [7], and BaTaO₂N [8] were reported to be active in photocatalytic overall water splitting via one-step excitation under visible light irradiation. In these achievements, the synthesis of well-crystallized semiconductor particles and the loading of composite cocatalysts were important for improving the photocatalytic activity. Nevertheless, further improvements in the preparation of cocatalyst/photocatalyst composites are still required to achieve a sufficient STH value.<br/><br/>It is possible to combine hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatlaysts (OEPs) and decompose water into hydrogen and oxygen via two-step excitation. Such a process is recognized as Z-scheme widely. Particulate photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ as the HEP and Mo-doped BiVO₄ as the HEP immobilized onto Au and C layers split water into hydrogen and oxygen with STH values exceeding 1.0% [9,10]. Some other (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths are also applicable to Z-schematic photocatalyst sheets and hold the promise of realizing greater STH values.<br/><br/>References<br/>[1] Hisatomi <i>et al.</i>, <i>Nat. Catal.</i> 2, 387 (2019).<br/>[2] Nishiyama <i>et al.</i>, <i>Nature</i> <i>598</i>, 304 (2021).<br/>[3] Chen <i>et al.</i>, <i>Nat. Rev. Mater.</i> <i>1</i>, 17050 (2017).<br/>[4] Takata <i>et al.</i>, <i>Nature</i> <i>581</i>, 411 (2020).<br/>[5] Wang <i>et al.</i>, <i>Nat. Catal.</i> <i>1</i>, 756 (2018).<br/>[6] Wang <i>et al.</i>, <i>Nat. Mater. 18</i>, 827 (2019).<br/>[7] Xiao <i>et al.</i>, <i>Angew. Chem. Int. Ed.</i> 134, e202116573 (2022).<br/>[8] Li <i>et al.</i>, <i>ACS Catal.</i> <i>12</i>, 10179 (2022).<br/>[9] Wang <i>et al.</i>, <i>Nat. Mater. 15</i>, 611 (2016).<br/>[10] Wang <i>et al.</i>, <i>J. Am. Chem. Soc.</i> 139, 1675 (2017).