“…Thus, our results showed that the interaction between Cu and the hexaniobate surface is highly beneficial for photocatalysis and in our case, it was achieved at mild temperatures (90 °C) and without any chemical reducing agent. Indeed, recent studies with TiO 2 and Cu showed that heat treatments at around 100–150 °C can improve TiO 2 and Cu interaction without inducing Cu diffusion into the oxide, and consequently, it enhances the photocatalytic H 2 production [30] . In contrast, when no heat treatment was applied, the Cu−TiO 2 linkage strength was found to be weak, since more than 60 % of the Cu content leached from TiO 2 after washing, or at high temperatures, ion diffusion into the semiconductor lattice hinders the proton reduction.…”