Evaluating H₂ Production by Ultraviolet-Induced Water Splitting over (Cu or Ni)-TiO₂ Nanoparticle Photocatalysts

This study explores the surface modification of TiO₂ P25 with nanosized copper (Cu) or nickel (Ni) for photocatalytic hydrogen production via water splitting. Photocatalysts with 1 or 5 wt % metal loadings were synthesized using a cost-effective impregnation method and were tested as-prepared (XM/P25) and after thermal reduction under hydrogen flow (XM/P25-r). Pre-calcination was omitted to simplify the synthesis. The characterization of the photocataysts revealed key physicochemical, optical, and electronic properties, including the surface distribution and oxidation states of metal species and their interaction with TiO₂. TEM and EDS showed well-distributed metal species forming nanometric domains on the P25 surface. XPS and Auger spectroscopy allowed the identification of different metal oxidation states and species. After reduction, Cu⁰ and Ni⁰ phases coexisted with oxidized species, enhancing light absorption and reducing the electron-hole recombination, confirmed by UV-vis DRS and PL, respectively. Cu species seemed to exhibit stronger interaction with TiO₂ than Ni ones. Photochemical results showed that only the thermally reduced photocatalysts led to immediate hydrogen production, highlighting the need for zero-valent metal species for an efficient hydrogen generation. 5Ni/P25-r and 5Cu/P25-r outperformed P25, achieving 331 and 284 μmol H₂·g_cat ^-1·h^-1, respectively, compared to 261 μmol H₂·g_cat ^-1·h^-1 for P25. Although in terms of hydrogen production 5Ni/P25-r slightly outperformed 5Cu/P25-r, the latter demonstrated greater stability, possibly due to the oxidation of Ni during the tests. As-prepared photocatalysts can be activated after extended UV irradiation; for instance, 5Cu/P25 starts producing hydrogen after 10 h of irradiation as a result of the in situ metal reduction, confirmed by the color change in the photocatalyst.