In this paper, we explore the drastic differences in transport properties and catalytic activities for two structural polymorphs of NiP 2 : cubic (Pa3; No. 205) and monoclinic NiP 2 (C2/c; No. 15). The former one has been long assumed to be the high-pressure metastable phase, as it had been originally synthesized through high-pressure methods. Synthetic and in situ synchrotron X-ray diffraction studies unambiguously show that the cubic polymorph can be synthesized at ambient pressure but irreversibly transforms into the monoclinic structure above 876 K. Band structure calculations and transport measurements show that cubic NiP 2 is a semimetal, while the monoclinic polymorph is an ntype direct band gap semiconductor. Both compounds exhibit low thermal conductivities, with cubic NiP 2 exhibiting a value of 1.7 W•m −1 K −1 at 300 K. The bulk structure of the phosphides may affect the surface-related properties. Unlike the monoclinic polymorph, cubic NiP 2 excels in both half-cell HER and OER measurements. In alkaline half-cell OER, cubic NiP 2 outperforms the RuO 2 standard. More importantly, HER tests in a PEM electrolysis single cell showed high promise for cubic NiP 2 , which requires only 13% higher overpotentials when compared to state-of-the-art Pt/IrRuO x -based assemblies, far surpassing any reported properties of metal pnictide or chalcogenide full cells.

AlFe2B2 pre-catalyst produces a nanocrystalline Fe3O4 shell in alkaline media. The system acts as a remarkably stable water oxidation electrocatalyst.

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 CuInSe2 nanoparticles 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 P63 mc), 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 CuInSe2 nanoparticles. 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.

The effect of the crystal structure of ordered transition-metal phosphide catalysts on the hydrogen-evolution reaction is investigated using single crystals of iron-phosphide (FeP) and monoclinic nickel-diphosphide (m-NiP2).