Highly efficient mobility, separation and charge transfer in black SnO 2 -TiO 2 structures with co-catalysts: the key step for the photocatalytic hydrogen evolution.

Oxygen vacancies and co-catalysts enhance photocatalytic hydrogen production by improving the charge carrier separation. Herein, the black SnO 2 -TiO 2 structure (BST) was synthesized for the first time by two consecutive methods. First, the sol-gel nucleation method allowed TiO 2 to form on the SnO 2 nanoparticles, creating a strong interaction and direct contact between them. Subsequently, this structure was reduced by NaBH 4 during thermal treatment, generating (Ti 3+ /Sn 2+ ) states to form the BST. Then, 2 wt% of Co, Cu or Pd was impregnated onto BST. The results showed that the activity raised with the presence of Ti 3+ /Sn 2+ states, reaching a hydrogen generation rate of 147.50 μmol g -1 h -1 with BST in comparison with the rate of 99.50 μmol g -1 h -1 for white SnO 2 -TiO 2 . On the other hand, the interaction of the co-catalysts with the BST structure helped to increase the photocatalytic hydrogen production rates: 154.10 μmol g -1 h -1 , 384.18 μmol g -1 h -1 and 480.20 μmol g -1 h -1 for cobalt-BST, copper-BST and palladium-BST, respectively. The results can be associated with the creation of Ti 3+ /Sn 2+ at the BST interface that changes the lifetime of the charge carrier, improving the separation of photogenerated electrons and holes and the co-catalysts in the structures move the flat band position and increasing the photocurrent response to having electrons with greater reducing power.