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The deployment of water electrolysis as a major contributor to global hydrogen production requires the elimination of catalysts based on scarce and expensive precious metals, and amongst the most promising alternatives are first-row transition metal phosphides. This study presents the synthesis, characterisation, electrochemical testing and performance rationalisation of cobalt phosphide modified with aluminium as an improved catalyst for alkaline oxygen evolution. The electrodes were prepared by gas phase phosphorisation of Al-sputtered Co foam, and characterised by SEM, EDX, XRD, XPS, HAADF-STEM and Raman spectroscopy. Al modification enhances the oxygen evolution performance of the anodes, with a current density of 200 mA cm−2reached at an overpotential of 360 mV, representing a 50 mV improvement compared to the Al-free sample. Double layer capacitance measurements indicate that the performance enhancement results from an approximately four-fold increase in relative electrochemically active surface area (ECSA) in the Al-modified sample. In situRaman spectroscopy rationalises this ECSA increase on the grounds of an Al-induced preference for a spinel phase Co/Al oxide on the catalyst surface upon exposure to electrolyte solution, the compact crystal structure of which causes shrinkage and surface cracking. This contrasts with previous observations on Al-doped nickel phosphides, where an increase in surface area was attributed to Al dissolution. These results present a route for achieving high current density oxygen evolution without the need to alter the catalyst active species, as well as demonstrate the importance of in situtechniques for rationalising performance improvements resulting from subtle differences in surface chemistry.

Chalcogenide semiconducting nanoparticles are promising building blocks for solution-processed fabrication of optoelectronic devices. In this work, we report a new large-scale colloidal synthesis of metastable CuInSe2nanoparticles with hexagonal plate-like morphology. Powder X-ray diffraction analysis of the nanoparticles showed that the structure of the nanoparticles is not simple hexagonal wurtzite-type CuInSe2(space group P63mc), indicating the formation of an ordered superstructure. Detailed insight into this structural aspect was explored by high-resolution electron microscopy, and the results evidence an unreported chemical ordering within the synthesized CuInSe2nanoparticles. Specifically, while the Se sublattice is arranged in perfect wurtzite subcell, Cu and In are segregated over distinct framework positions, forming domains with lower symmetry. The arrangement of these domains within the hexagonal Se substructure proceeds through the formation of a number of planar defects, mainly twins and antiphase boundaries. As a semiconductor, the synthesized material exhibits a direct optical transition at 0.95 eV, which correlates well with its electronic structure assessed by density functional theory calculations. Overall, these findings may inspire the design and synthesis of other nanoparticles featuring unique chemical ordering; thus, providing an additional possibility of tuning intrinsic transport properties.