Optimized loading of Pt single atoms from platinic acid solution and the sweet spot for activity

In recent years, platinum single atoms (SAs) have emerged as exceptionally efficient co-catalysts for photocatalytic hydrogen (H₂) evolution. In the present work, we systematically investigate the solution parameters that govern the deposition of Pt SAs from dilute H₂PtCl₆ precursors onto well-defined sputtered anatase TiO₂ thin films and evaluate the effect on photocatalytic H₂ evolution. We show that both precursor concentration and solution composition critically determine the oxidation state, dispersion, and reactivity of surface-bound Pt species. Ultra-dilute, additive-free solutions (0.001-0.005 mM) enable strong electrostatic adsorption (SEA) and aquation-assisted anchoring of isolated Pt²⁺ atoms, leading to atomically dispersed Pt-O-Ti surface motifs with maximal H₂ evolution per Pt. In contrast, increased ionic strength or unfavorable speciation caused by higher concentration or pH adjustment induces non-selective uptake, formation of Pt(IV)-rich 2D rafts, and diminished catalytic performance. Overall, we show that the placement and photocatalytic activity of Pt SAs are dictated by the pre-reaction deposition chemistry. From a practical view, this study defines a narrow yet actionable parameter window for scalable, efficient SA co-catalyst decoration and establishes the mechanistic basis for performance optimization via precursor-chemistry design.