Delineating the role of crystallinity on the electrocatalytic activity of colloidally synthesized MoP nanocrystals

The synthesis of non-noble metal nanocrystals as cheaper alternatives to Pt for use as hydrogen evolution reaction (HER) electrocatalysts is highly desired for the future of renewable energy. In this work, amorphous molybdenum phosphide (MoP) nanocrystals were synthesized via the colloidal synthesis by heating MoCl5 (Mo source), 1-octadecene (solvent and reducing agent) and trioctylphosphine (P source) at 340 °C for 6 h. Subsequently, the amorphous nanocrystals were converted into crystalline nanocrystals through annealing. Broad XRD peaks in conjunction with diffuse rings on SAED pattern confirmed the amorphous nature of as synthesized nanocrystals. Alternatively, sharp XRD peaks and spotty diffraction rings in the SAED pattern confirmed the crystalline nature of the annealed nanocrystals. The amorphous nanocrystals were small, highly dispersed and had a semi-spherical morphology. The annealed nanocrystals retained their dispersity and semi-spherical morphology but increased significantly in size due to Ostwald ripening. XPS and EDS analysis demonstrated that the amorphous and crystalline nanocrystals had a composition corresponding to that of MoP. Both catalysts exhibited electrocatalytic activity and stability which compares favourably to other non-noble metal HER catalysts. Benefiting from a high density of unsaturated active sites and low charge transfer resistance (Rct), the amorphous MoP exhibited superior activity compared the crystalline MoP nanocrystals. To achieve current density of 10 mA cm-2, amorphous and crystalline MoP required an overpotential of 235 and 317 mV in acidic media, respectively. The negligible decrease in the catalytic activity of the MoP nanocrystals for up to 1000 cycles indicated their potential as catalysts in hydrogen generation. These findings demonstrate that colloidal synthesis can be used to prepare amorphous MoP nanocrystals with well-defined morphology which exhibit electrocatalytic activity that is superior than that of its crystalline counterpart.