Pt Single Atom Co-Catalysts on Thin, Defined Anatase Layers: Critical Factors for Photocatalytic Hydrogen Generation

Single-atom catalysts (SACs) have emerged as promising co-catalysts in photocatalytic hydrogen generation. However, key challenges remain in this field, particularly in reliably controlling their surface density, dispersion, and integration into semiconductor systems. This study systematically investigates platinum single atoms deposited on anatase TiO₂ thin films, grown on fluorine-doped tin oxide with defined thickness and crystallographic structure, by direct-current sputter-deposition of titania. Comprehensive parameter screening shows that variations in Pt precursor concentration, pre-annealing temperature, and TiO₂ layer thickness significantly influence the photocatalytic performance. Crucially, a Langmuir-type adsorption behavior for Pt single atoms is demonstrated and extract an optimal surface density of ≈4 × 10⁵ SAs µm^-2 (around 0.26 at.% Pt) - higher loading does not further enhance photocatalytic activity. For optimized co-catalyst loading, variations in TiO₂ thickness and structure remain the primary factors influencing photocatalytic performance through charge transport, and light absorption as key parameters. This optimal loading is confined to the surface of the TiO₂ film and is sufficient to fully utilize the photogenerated electron flux under ultravoilet excitation. This work introduces the concept of a critical co-catalyst density that separates the transition from co-catalyst-limited to absorber-limited behavior in SAC-based photocatalysis, thereby contributing to a further rational design of photocatalytic systems.