While searching for an efficient, non-noble, earth-abundant catalyst for the hydrogen evolution reaction (HER), we synthesized hexagonal dinickel phosphide with different nanostructures using solvothermal phosphidation. Coupled atop p-type Si, this catalyst performed as a p-n heterojunction photocathode assembly and the performance varied when under different electrolyte media. Apart from changing the surface morphology, Ni2P was crystallized with an increase in the Ni delta+/Ni2+ ratio as the phosphidation temperature gradually increased. A systematic evaluation of the water splitting reaction shows that a very small amount of catalyst (>85% transmittance for the catalyst layer) exhibits a photocurrent of -10 mA cm(-2 )with a positive applied potential of 0.05 V versus reversible hydrogen electrode under simulated solar irradiation of AM 1.5G. We discuss the substantial charge transfer process at the depletion layer of the electrode/catalyst and the catalyst/electrolyte interface. Mott-Shottky analysis showed a shift in the flat band potential for Ni2P, which reveals the underlying mechanism for the role of the p-n junction for enhanced photoelectrochemical cell performance. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.