Effects of Catalyst Phase on the Hydrogen Evolution Reaction of Water Splitting: Preparation of Phase-Pure Films of FeP, Fe 2 P, and Fe 3 P and Their Relative Catalytic Activities
The comparative catalytic activities of iron phosphides, Fe x P (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 Fe 2 P, 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 H 2 SO 4 . HER activity follows the trend Fe 3 P > Fe 2 P > FeP, with Fe 3 P 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 Fe 3 P and Fe 2 P 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 Fe 2 P were grown using Fe(CO) 4 PH 3 (1), while the films of FeP were prepared using either Fe(CO) 4 P t BuH 2 (2) or the new molecule {Fe(CO)4P(H)tBu}2 (3) on quartz and FTO. Compound 3 was prepared from the reaction of PCl 2 t Bu with a mixture of Na[HFe(CO) 4 ] and Na 2 [Fe(CO) 4 ] and characterized by single-crystal X-ray diffraction, ESI-MS, elemental analysis, and 31 P/ 1 H NMR spectroscopies. Films of Fe 3 P were prepared as previously described from H 2 Fe 3 (CO) 9 P t Bu (4).