Anisotropic Single-Crystalline Ti-Based Oxides for Photocatalytic Water Splitting

High Resolution Image Download MS PowerPoint Slide Conspectus Solar-driven water splitting provides a clean and essential pathway to green hydrogen for future, sustainable energy systems. Among the developed photocatalytic materials, inorganic semiconductors are particularly attractive because of their earth abundance, scalable synthesis, and potential for high-energy conversion efficiency. Prototypical Ti-based oxide single crystals, such as TiO 2 and SrTiO 3, have been extensively investigated as model photocatalysts, owing to their well-defined electronic structures and excellent chemical stability. However, two fundamental limitations remain: the inefficient separation and migration of photogenerated charge carriers, and the spatial colocalization of hydrogen and oxygen evolution sites. Together, these factors lead to severe charge recombination and promote back-reactions, thereby substantially constraining the overall efficiency of photocatalytic water splitting. Long-standing efforts in this field have consistently highlighted facet engineering as a powerful means of overcoming these critical limitations. Facet-dependent modulation imparts crystallographic anisotropy directly to the key functional attributes of the photocatalyst, manifesting as directional enhancements in charge separation efficiency and interfacial reaction kinetics. In this regard, facet-specific atomic electronic engineering induces internal electric fields that direct charge migration and isolate redox sites, therefore boosting photocatalytic water splitting. In this Account, we provide a comprehensive review of our group’s progress over the past two decades in controlling anisotropic facets in Ti-based oxides. By developing a suite of controllable synthetic strategies based on liquid-phase and solid-state routes, we demonstrate, guided by Wulff theory, that inorganic ions and organic small molecules can systematically modulate the surface energies of specific crystal facets, thereby enabling rational, theory-informed facet engineering. We have established a series of strategies for preparing single-crystalline TiO 2 and SrTiO 3 with well-defined anisotropic surface structures. These efforts have deepened our understanding of the intrinsic structure–activity relationships that connect crystallographic architecture to photocatalytic performance. We highlight how advances in characterization techniques in recent years have greatly sharpened our understanding of facet-dependent charge transport and surface reaction kinetics. We anticipate that these insights will provide a robust foundation for future developments in the field, including the rational design of facet-specific catalysts, detailed elucidation of interfacial reaction mechanisms, and the coordinated integration of multiscale theoretical and experimental approaches. A more explicit link between structure and function is expected to play a decisive role in guiding the development of high-efficiency photocatalytic systems and in accelerating the practical implementation of solar-driven hydrogen production.