In situ X-ray diffraction guided synthesis of Ni$_2$P nanoparticles for the oxygen evolution reaction

Nickel phosphides are considered excellent candidates as catalysts for the oxygen evolution reaction (OER) under alkaline conditions. This study first investigates the nucleation and growth of Ni2P during hydrothermal synthesis by in situ powder X-ray diffraction. It is found that Ni2P nanoparticles are formed throughout the temperature range of 150 °C to 300 °C, but above 225 °C a transition of Ni2P to Ni12P5 is observed. Data recorded at multiple temperatures allow activation energies for the nucleation of Ni2P, growth of Ni2P, and the Ni2P to Ni12P5 phase transitions to be estimated as 91.0(5) kJ mol−1, 62.3(9) kJ mol−1, and 115.5(7) kJ mol−1, respectively. The in situ data further reveals that the Ni2P crystallite sizes can be controlled by varying the reaction time and temperature, and correspondingly ex situ autoclave syntheses were performed to obtain phase-pure Ni2P nanoparticles with sizes of ∼20 nm, ∼25 nm, and ∼30 nm. Furthermore, with short reaction times partly amorphous ∼20 nm and ∼25 nm Ni2P nanoparticles are obtained. Using fully crystalline standards, the crystallinity of the nanoparticles is determined to infer amorphous impurities, and the effects of both crystallinity and crystallite size for the nanoparticle Ni2P catalyst towards OER under alkaline conditions are established. The crystalline Ni2P nanoparticle samples show an almost constant overpotential ranging from 413 to 417 mV and a minor increase in the Tafel slope from 60.5 to 71.7 mV dec−1 with increasing crystallite size. In contrast, the Ni2P nanoparticles with excess amorphous phosphorus exhibit significantly higher Tafel slopes of 95.8 (∼20 nm) and 89.6 mV dec−1 (∼25 nm). Absolute crystallinity is very rarely quantified in studies of nanoparticle catalysts, but the present results highlight that crystallinity determination can be used to suggest the presence of amorphous impurities, which in this case have a larger impact than crystallite size when optimizing electrode characteristics for electrocatalytic water splitting.