General applicability of nanocrystalline Ni₂P as a noble-metal-free cocatalyst to boost photocatalytic hydrogen generation

To replace the role of noble-metal cocatalysts (e.g. Pt) in photocatalytic hydrogen generation, low-cost alternatives with earth-abundant elements should not only possess high catalytic activities, but also have general applicability. Herein, nanocrystalline Ni₂P cocatalysts are used to modify CdS, TiO₂, and C₃N₄ host photocatalysts. It is observed that the Ni₂P cocatalyst boosts hydrogen generation over all the host photocatalysts, which demonstrates its good catalytic property and general applicability. To investigate its action mechanism, nanocrystalline Ni₂P was successfully integrated with TiO₂ nanorods (TiNR) for the first time. The optimized Ni₂P/TiNR sample exhibits an 85 times higher activity compared to single TiNR, and its apparent quantum efficiency was calculated to be 11.6% at 360 nm. Among the varied nickel-based semiconductor cocatalysts, Ni₂P is also proven to be the best cocatalyst. Photoluminescence and electrochemical results reveal that the Ni₂P cocatalyst promotes the charge transfer both in the photocatalyst and at the photocatalyst/solution interface, as well as accelerates the surface reaction. The enhanced charge transfer efficiency and improved surface reaction rate finally result in a dramatically improved performance. It is believed that the present work can provide basic principles for the development of noble-metal-free cocatalysts with high catalytic activity and general applicability.