In this paper, we present a solvothermal synthesis of iron phosphide electrocatalysts using a triphenylphosphine (TPP) precursor. The synthetic protocol generates Fe2P phase at 300 degrees C and FeP phase at 350 degrees C. To enhance the catalytic activities of obtained iron phosphide particles heat-treatments were carried out at elevated temperatures. Annealing at 500 degrees C under reductive atmosphere induced structural changes in the samples: (i) Fe2P provided a pure Fe3P phase (Fe3P-500 degrees C) and (ii) FeP transformed into a mixture of iron phosphide phases (Fe2P/FeP-500 degrees C). Pure Fe2P films was prepared under argon atmosphere at 450 degrees C (Fe2P-450 degrees C). The electrocatalytic activities of heat-treated Fe2P-450 degrees C, Fe3P-500 degrees C, and Fe2P/FeP-500 degrees C catalysts were studied for hydrogen evolution reaction (HER) in 0.5 M H2SO4. The HER activities of the iron phosphide catalyst were found to be phase dependent. The lowest electrode potential of 110 mV vs. a reversible hydrogen electrode (RHE) at 10 mA cm(-2) was achieved with Fe2P/FeP-500 degrees C catalyst. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.