Effects of Catalyst Phase on the Hydrogen Evolution Reaction of Water Splitting: Preparation of Phase-Pure Films of FeP, Fe₂P, and Fe₃P and Their Relative Catalytic Activities

The comparative catalytic activities of iron phosphides, Fe_xP (x = 1–3), have been established with phase-pure material grown by chemical vapor deposition (CVD) from single-source organometallic precursors. This is the first report of the preparation of phase-pure thin films of FeP and Fe₂P, and their identity was established with scanning-electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. All materials were deposited on fluorine-doped tin oxide (FTO) for evaluation of their activities toward the hydrogen evolution reaction (HER) of water splitting in 0.5 M H₂SO₄. HER activity follows the trend Fe₃P > Fe₂P > FeP, with Fe₃P having the lowest overpotential of 49 mV at a current density of 10 mA cm^–2. Density functional theory (DFT) calculations are congruent with the observed activity trend with hydrogen binding favoring the iron-rich terminating surfaces of Fe₃P and Fe₂P over the iron-poor terminating surfaces of FeP. The results present a clear trend of activity with iron-rich phosphide phases outperforming phosphorus rich phases for hydrogen evolution. The films of Fe₂P were grown using Fe­(CO)₄PH₃ (1), while the films of FeP were prepared using either Fe­(CO)₄P^tBuH₂ (2) or the new molecule {Fe­(CO)₄P­(H)^tBu}₂ (3) on quartz and FTO. Compound 3 was prepared from the reaction of PCl₂^tBu with a mixture of Na­[HFe­(CO)₄] and Na₂[Fe­(CO)₄] and characterized by single-crystal X-ray diffraction, ESI-MS, elemental analysis, and ³¹P/¹H NMR spectroscopies. Films of Fe₃P were prepared as previously described from H₂Fe₃(CO)₉P^tBu (4).