▎ 摘　　要

Solar-to-hydrogen (STH) conversion through unassisted artificial photosynthesis (APS) devices is one of the promising and environmentally friendly strategies for sustainable development. However, the practical large-scale application of the unassisted APS devices is impeded by the need for expensive noble metal-based catalysts in photovoltaics and/or electrolyzers. Herein, well-aligned 2D NixSy nanowalls (2D NixSy NWs) on a 3D nitrogen-doped graphene foam (3D NGF) are synthesized and further employed it in unassisted APS. Due to the positive synergistic effect between the highly electrocatalytic activity of NixSy NW and excellent conductivity of NGF, this low cost material of (2D/3D) NixSy NW/NGF is highly efficient as a multifunctional catalyst in various applications: a counterelectrode for dye-sensitized solar cell (DSSC) and a "bifunctional" electrocatalyst for oxygen and hydrogen evolution for electrocatalytic overall water splitting. Furthermore, three NixSy NW/NGF-based DSSCs as a tandem cell for unassisted solar-driven overall water splitting is connected, using NixSy NW/NGF itself on nickel foams as the anode and cathode. Impressively, such integrated photovoltaic-electrolyzer APS device can achieve an STH efficiency of 3.2% with an excellent stability and low cost. This work opens an avenue to advanced multifunctional materials for the low-cost and unassisted solar-driven overall water splitting.