All-inorganic metal halide perovskite-related phases are semiconducting materials that are of significant interest for a wide range of applications. Nanoparticles of these materials are particularly useful because they permit solution processing while offering unique and tunable properties. Of the many metal halide systems that have been studied extensively, cesium cadmium chlorides remain underexplored, and synthetic routes to access them as nanoscale materials have not been established. Here we demonstrate that a simple solution-phase reaction involving the injection of a cesium oleate solution into a cadmium chloride solution produces three distinct cesium cadmium chlorides: hexagonal CsCdCl3 and the Ruddlesden–Popper layered perovskites Cs2CdCl4 and Cs3Cd2Cl7. The phase-selective synthesis emerges from differences in reagent concentrations, temperature, and injection rates. A key variable is the rate at which the cesium oleate solution is injected into the cadmium chloride solution, which is believed to influence the local Cs:Cd concentration during precipitation, leading to control over the phase that forms. Band structure calculations indicate that hexagonal CsCdCl3 is a direct band gap semiconductor while Cs2CdCl4 and Cs3Cd2Cl7 have indirect band gaps. The experimentally determined band gap values for CsCdCl3, Cs2CdCl4, and Cs3Cd2Cl7 are 5.13, 4.91, and 4.70 eV, respectively, which places them in a rare category of ultrawide-band-gap semiconductors.

The production of hydrogen fuels, via water splitting, is of practical relevance for meeting global energy needs and mitigating the environmental consequences of fossil-fuel-based transportation. Water photoelectrolysis has been proposed...

Copper selenide nanoparticles are important materials with desirable properties for a broad scope of applications. Copper selenides are additionally known to adopt several different crystal structures and compositions. Here, we report the direct solution-phase synthesis of colloidal Cu2–x Se nanoparticles that adopt a structure distinct from other known copper selenide phases. This Cu2–x Se phase was determined to be structurally related to weissite Cu2–x Te, which is a layered compound containing alternating Cu-rich and Cu-deficient layers sandwiching distorted hexagonal layers of chalcogen atoms. When the weissite-like Cu2–x Se nanoparticles were annealed in solution, they converted to the more stable cubic berzelianite phase, indicating that they are metastable. Optical characterization of the weissite-like Cu2–x Se nanoparticles showed a broad plasmon absorption band centered around 1550 nm, and the position of this absorption band shifted only slightly when subjected to reductive and oxidative conditions.

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