Optoelectronic devices based on hybrid halide perovskites have shown remarkable progress to high performance. However, despite their apparent success, there remain many open questions about their intrinsic properties. Single crystals are often seen as the ideal platform for understanding the limits of crystalline materials, and recent reports of rapid, high-temperature crystallization of single crystals should enable a variety of studies. Here we explore the mechanism of this crystallization and find that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization. We use this knowledge to demonstrate a broader range of processing parameters and show that these can lead to improved crystal quality. Our findings are therefore of central importance to enable the continued advancement of perovskite optoelectronics and to the improved reproducibility through a better understanding of factors influencing and controlling crystallization.

Lead halide perovskites are materials with excellent optoelectronic and photovoltaic properties. However, some hurdles remain prior to commercialization of these materials, such as chemical stability, phase stability, sensitivity to moisture, and potential issues due to the toxicity of lead. Here, we report a new type of lead-free perovskite related compound, CsPdBr. This compound is solution processable, exhibits long-lived photoluminescence, and an optical band gap of 1.6 eV. Density functional theory calculations indicate that this compound has dispersive electronic bands, with electron and hole effective masses of 0.53 and 0.85 m, respectively. In addition, CsPdBr is resistant to water, in contrast to lead-halide perovskites, indicating excellent prospects for long-term stability. These combined properties demonstrate that CsPdBr is a promising novel compound for optoelectronic applications.

Chemical vapor deposition (CVD) is used to grow thin films of 2D MoS 2 with nanostructure for catalytic applications in the hydrogen evolution reaction (HER). Tailoring of the CVD parameters results in an optimized MoS 2 structure for the HER that consists of large MoS 2 platelets with smaller layered MoS 2 sheets growing off it in a perpendicular direction, which increases the total number of edge sites within a given geometric area. A surface area to geometric area ratio of up to ∼340 is achieved, benefiting from the edge-exposed high-porosity network structure. The optimized thickness of the MoS 2 film is determined for maximum performance, revealing that increasing thickness leads to increased impedance of the MoS 2 film and reduced current density. The current density of the optimum sample reaches as high as 60 mA/cm 2 geo (normalized by geometric area) at an overpotential of 0.64 V vs RHE (in 0.5 M H 2 SO 4 ), with a corresponding Tafel slope of ∼90 mV/dec and exchange current density of 23 μA/cm 2 geo . The lowered Tafel slope and large exchange current density demonstrate that the high-porosity edge-exposed MoS 2 network structure is promising as a HER catalyst.

The sluggish kinetics of oxygen reduction to water remains a significant limitation in the viability of proton-exchange-membrane fuel cells, yet details of the four-electron oxygen reduction reaction remain elusive. Herein, we apply in situ infrared spectroscopy to probe the surface chemistry of a commercial carbon-supported Pt nanoparticle catalyst during oxygen reduction. The IR spectra show potential-dependent appearance of adsorbed superoxide and hydroperoxide intermediates on Pt. This strongly supports an associative pathway for oxygen reduction. Analysis of the adsorbates alongside the catalytic current suggests that another pathway must also be in operation, consistent with a parallel dissociative pathway.

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