Pt Single Atoms on TiO 2 Polymorphs—Minimum Loading with a Maximized Photocatalytic Efficiency

For more than 20 years, Pt/TiO$_2$ represents the benchmark photocatalyst/co-catalyst platform for photocatalytic hydrogen (H$_2$) generation. Here, single atom (SA) Pt is decorated on different polymorphs of TiO$_2$ (anatase, rutile, and the mixed phase of P25) using a simple immersion anchoring approach. On P25 and anatase, Pt SAs act as highly effective co-catalyst for pure water splitting with a photocatalytic H$_2$ evolution activity (4600 µmol h$^{−1}$ g$^{−1}$)—on both polymorphs, SA deposition yields a significantly more active photocatalyst than those decorated with classic Pt nanoparticles or conventional SA deposition approaches. On rutile, Pt SAs provide hardly any co-catalytic effect. Most remarkable, for P25, the loading of Pt SAs from precursor solution with a very low concentration (<1 ppm Pt) leads already to a maximized co-catalytic effect. This optimized efficiency is obtained at 5.3 × 10$^{5}$ atoms µm$^{−2}$ (at macroscopic loading of 0.06 at%)—for a higher concentration of Pt (a higher density of SAs), the co-catalytic efficiency is significantly reduced due to H$_2$/O$_2$ recombination. The interactions of the SA Pt with the different polymorphs that lead to this high co-catalytic activity of SA Pt at such low concentrations are further discussed.