Understanding and manipulating synthetic progress for precisely controlling the components and defects of nanomaterials is an important and challenging task in materials synthesis and nanocatalysis. Metal phosphides (MPs) have been explored as cheap advanced materials in various catalytic fields. MP materials are usually synthesized through gas-solid phosphorization reaction in a trial-to-error manner, but their formation mechanism and the origin of controlled synthesis remain unclear. Here, we combine in situ thermogravimetric analysis-mass spectrometry (TG-MS) and quasi-in situ X-ray powder diffraction (XRD) analysis to probe the transformation mechanism from metal oxides (MOs) to MPs during the phosphorization process mediated by hypophosphite. Temperature, time, and the amount of hypophosphite are revealed as the driven forces while oxophilicity and crystallinity as the impeded forces, simultaneously control the component and defect level of a series of MP (M = Ni, Co, W, Mo, and Nb). The as-obtained WO_2.9/WP is proved to be an efficient Z-scheme photocatalyst for oxidative coupling of methane with the total C₂₊ production and C₂H₄ selectivity in C₂₊ products reaching 10.75 μmol·g⁻¹ and 98.25%. Our work provides a fundamental understanding of the phosphorization treatment and thereby guides a viable synthetic route to the controlled synthesis of MO_x−δ, MP, MO_x−δ/MP, and MP/M heterostructured materials.

The development of facile strategies to tune the oxygen vacancy (OV) content in transition metal oxides (TMOs) is paramount to obtain low-cost and stable electrocatalysts, but still highly challenging. Taking NiCo₂O₄ as a model system, we have experimentally established a facile calcination and electrochemical activation (EA) methodology to dramatically increase the concentration of OVs and provide theoretical insight into how the concentration of OVs affects the performance of spinel TMOs towards the electrochemical hydrogen evolution reaction (HER). A self-supported cathode of OV-rich NiCo₂O₄ nanowire arrays was found to exhibit higher HER activity and better stability in alkaline media than its counterparts with fewer OVs. The electrocatalytic HER activity was in good agreement with the increasing concentration of OVs in the studied samples. A large current density of 360 mA·cm^–2 was reached with an overpotential of only 317 mV. Additionally, such a facile strategy was able to efficiently generate OVs in other TMOs (e.g., CoFe₂O₄ and NiFe₂O₄) for enhanced HER performance. In addition, our theoretical results suggest that the increasing OV concentration reduces the adsorption energy of water molecules and their dissociation energy barrier on the surface of the catalyst, thus leading to performance improvement of spinel TMOs toward the electrochemical HER. This work may open a new avenue to increase the concentration of OVs in TMOs in a controlled manner for promising applications in a variety of fields.