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Heavy pnictogen chalcohalide semiconductors are coming under the spotlight for energy conversion applications. Here we present the colloidal synthesis of phase pure AgBiSCl2nanocrystals. This quaternary chalcohalide compound features a quasi-two-dimensional crystal structure and a direct band gap, in contrast with the monodimensional structure and the indirect band gap peculiar to the orthorhombic, ternary Bi chalcohalides. Consistently, colloidal AgBiSCl2nanocrystals exhibit photoinduced luminescence compatible with both band edge excitons and midgap states. This is the first observation of band edge emission in chalcohalide nanomaterials at large, although exciton recombination in our AgBiSCl2nanocrystals mostly occurs via nonradiative pathways. This work further advances our knowledge on this class of mixed anion semiconductor nanomaterials and provides a contribution to establishing chalcohalides as a reliable alternative to metal chalcogenides and halides.

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

We report the colloidal synthesis of bismuth chalcohalide nanocrystals, which adopt a hexagonal phase that we describe with the Bi13S18X2(in which X = Br and I) structure. Such a model structure displays columns of Bi atoms that form dimers of subvalent Bi (formally Bi24+), which we here ascribe to Peierls-type distortions. We suggest that the Bi24+dimers are at the origin of the anomalously low band gap of this material, with the lowest energy electronic transition showing analogies with an intervalence charge transfer. Our synthetic approach and insights into the structural and electronic features of the hexagonal bismuth chalcohalides are fundamental to sustain the rapidly increasing use of this class of (nano)materials for diverse applications, such as photocatalysis and thermoelectrics.

Combining multiple species working in tandem for different hydrogen evolution reaction (HER) steps is an effective strategy to design HER electrocatalysts. Here, we engineered a hierarchical electrode for the HER composed of amorphous-TiO2/Cu nanorods (NRs) decorated with cost-effective Ru–Cu nanoheterostructures (Ru mass loading = 52 μg/cm2). Such an electrode exhibits a stable, over 250 h, low overpotential of 74 mV at −200 mA/cm2for the HER in 1 M NaOH. The high activity of the electrode is attributed, by structural analysis, operando X-ray absorption spectroscopy, and first-principles simulations, to synergistic functionalities: (1) mechanically robust, vertically aligned Cu NRs with high electrical conductivity and porosity provide fast charge and gas transfer channels; (2) the Ru electronic structure, regulated by the size of Cu clusters at the surface, facilitates the water dissociation (Volmer step); (3) the Cu clusters grown atop Ru exhibit a close-to-zero Gibbs free energy of the hydrogen adsorption, promoting fast Heyrovsky/Tafel steps. An alkaline electrolyzer (AEL) coupling the proposed cathode and a stainless-steel anode can stably operate in both continuous (1 A/cm2for over 200 h) and intermittent modes (accelerated stress tests). A techno-economic analysis predicts the minimal overall hydrogen production cost of US$2.12/kg in a 1 MW AEL plant of 30 year lifetime based on our AEL single cell, hitting the worldwide targets (US$2–2.5/kgH2).

Methylammonium lead halide perovskites have emerged as ideal materials for photovoltaics and other optoelectronic applications. Their optimal properties stem from complex mechanisms at an atomic level. In these structures, methylammonium molecules undergo continuous temperature-induced reorientations. The underlying atomistic mechanism of this phenomenon and its effect on the material properties are unexplored in many aspects. Here, we address this issue through extensive first-principles and force-field calculations. We show that the interplay between thermal energy and the energetics of molecular orientations fully explains the orthorhombic–tetragonal–cubic phase transitions of the material. The close links among methylammonium orientations, electronic structures, and optical properties are identified. Hydrogen bonding between organic and inorganic sublattices is demonstrated to be the key player in these relationships. We validate our findings against temperature-dependent one-photon and two-photon photoluminescence spectra of CH3NH3PbBr3and results from the literature. The gained insights can explain the diversity of spectral features observed in the experimental data.