A Single Molecular Stoichiometric P‐Source for Phase‐Selective Synthesis of Crystalline and Amorphous Iron Phosphide Nanocatalysts

Abstract The formation of iron phosphide nanoparticles (Fe_xP NPs) is a well‐studied process. It usually uses air‐sensitive phosphorus precursors such as n‐trioctylphosphine or white phosphorus. In this study, we report the synthesis and characterization of a remarkably stable tetrakis(acyl)cyclotetraphosphane, P₄(MesCO)₄. We demonstrate that this compound can be used as a stoichiometric source of P(0) species in order to synthesize FeP and Fe₂P nanoparticles at only 250 °C. This tunable process provides a route to monodisperse nanoparticles with different compositions and crystallinities. We combine X‐Ray photoelectron spectroscopy (XPS) and atomic pair distribution function (PDF) in order to study the local order and bonding in the amorphous and crystalline materials. We show that crystalline FeP forms via an intermediate amorphous phase (obtained at a lower temperature) that presents local order similar to that of the crystalline sample. From the results of this work, a better understanding of the mechanism of the formation of amorphous and crystalline Fe_xP NPs is provided which relies on the use of a stoichiometric and single P‐source. We then explore the electrocatalytic properties of Fe_xP nanoparticles for the hydrogen evolution reaction (HER) in acidic and neutral electrolytes. In both electrolytes, amorphous FeP is a more efficient catalyst than crystalline FeP, itself more efficient than crystalline Fe₂P. Our study paves the way for a more systematic investigation of amorphous metal phosphide phases in electrocatalysis. It also shows the beneficial properties of PDF on the characterization of such nanomaterials, which is highly challenging.