LAUSR

Platinum co‐catalysts play a critical role in promoting the photocatalytic performance of inorganic semiconductors, yet despite intensive investigation, the active platinum species responsible remains controversial. Here, the physicochemical properties of Pt nanoparticles introduced into anatase titania through three different synthetic protocols are investigated by porosimetry, XRD, X‐ray photoelectron spectroscopy (XPS), and X‐ray absorption spectroscopy (XAS) and correlated with their corresponding activity for aqueous photocatalytic hydrogen production. Conventional wet impregnation produces small but highly oxidized platinum nanoparticles owing to the classical strong metal–support interaction with titania during high‐temperature processing. Photodeposition yields predominantly metallic but large and inhomogeneous (1.5–7.5 nm) Pt nanoparticles. In contrast, a modified in situ polyol route affords metallic and highly dispersed (<2 nm) nanoparticles with minimal PtOx. Photocatalytic H2 evolution is directly proportional to the surface concentration of Pt metal, conclusively demonstrating metallic platinum as the active co‐catalyst, and offering a simple parameter to quantitatively predict the photocatalytic performance of Pt/TiO2 in H2 production. The modified in situ polyol synthesis is optimal for co‐catalyst formation, delivering rate enhancements of 25–80 % compared with the other syntheses.