Pt nanoparticles supported on TiO₂ is found to be active for solar heterogeneous photocatalysis. However, the tendency of Pt nanoparticles to grow/aggregate into larger secondary particles in the photochemical synthesis is an obstruction for good performance. Temperature can modulate the size and electronic properties of Pt particles from synthesis in solution. Unlike high temperature, photochemical solution-phase reaction at low temperature leads to a sluggish diffusion-controlled nucleation and growth kinetics, which holds great promise to suppress aggregation. Herein, we report a zero-degree photochemical synthesis to obtain a highly dispersed Pt/TiO₂ catalyst at an impressive hydrogen generation rate of 643.29 mmol g⁻¹ h⁻¹ and good stability upon solar light illumination, superior to the counterparts at room temperature (25 °C) or even higher solution-phase temperature (60 °C). Comparative characterizations evidence that zero-degree photochemical synthesis prefers small stable Pt nanoparticles to promote photoelectron transfer and collection, which provides an experimental guideline for constructing advanced photocatalysts.