The Role of Ni-Based Cocatalyst in Inhomogeneous RVO<sub>4</sub> Photocatalyst Systems (R = Y, Gd)

We extend here our inspection, via first-principles dynamical simulations supported by experiments, to the inhomogeneous photocatalytic RVO<sub>4</sub> systems. At variance with our former paper, we limit the present analysis to the most promising photocatalysts (R = Y, Gd) for which the addition of a Ni-based cocatalyst provides the best activities. The specific role of Ni-based cocatalysts is elucidated, showing that a surface Ni doping, as opposed to a bulk one, is the most efficient way to enhance the catalytic activities of these compounds. This is a key step in the improvement of the photocatalysts and a roadmap to optimize the amount of Ni dopant needed for both hydrogen and oxygen generation.