In the quest for mass production of hydrogen from water electrolysis, to develop highly efficient, stable and low-cost catalysts is still the central challenge.

Cuprous oxide Cu2O is a promising p-type semiconductor for photoelectrochemical (PEC) solar hydrogen generation because it has a suitable bandgap (Eg = 2.0-2.2 eV) and a band alignment adapted to water reduction. In addition, metallic Cu is earth-abundant thus making Cu2O a low-cost material. However, the reduction potential of Cu2O into metallic Cu (0.47 V versus RHE) is lower than that of water which induces a severe instability under irradiation in a PEC cell. Therefore, our recent efforts focused on the growth of a protective overlayer on top of Cu2O in order to stabilize Cu2O when used as a photocathode in an aqueous electrolyte. Among potential protective materials cuprous sulphide Cu2S is another p-type semiconductor with a 1.2 eV bandgap and an appropriate energy level alignment with Cu2O that would allow electrons flowing to the interface. We present here an original and simple method aimed at protecting a compact layer (CL) or nanowires (NWs) of Cu2O with a Cu2S coating. Our method is based on the ions exchange reaction (IER) of O(2-) into S(2-) at the surface of Cu2O itself in a solution-containing Na2S as the sulphur source. The local surface IER implies the formation of a conformal and uniform coating independently on the starting Cu2O morphology, CLs or NWs. As expected, coating Cu2O photocathodes by a conformal Cu2S layer improves their stability and PEC performances.

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Implementation of non‐planar aromatic motifs in the molecular design of π‐conjugated materials represents a paradigm shift that is interesting from a fundamental research viewpoint but remains largely underdeveloped. To further facilitate this cause, we report on the synthesis and properties of two large polycyclic aromatic compounds (38 carbon atoms). In one compound the planar π‐system pyrene is linked to bowl‐shaped hydrocarbon corannulene through a C=C bond, which is referred to as “corannulene–vinylpyrene”. Photochemical treatment of this compound results in annulation of both aromatic nuclei and formation of the other large fused structure with ten six‐membered aromatic rings, referred to as “corannulene–benzopyrene”. Photophysical and electrochemical studies reveal that the properties of these two structurally related hybrid materials arise from an extended π‐conjugation between the planar and non‐planar motifs and varies depending upon their molecular structure.

Polymorphism is a central phenomenon in material science that often results in important differences of the electronic properties of organic crystals due to slight variations of intermolecular distances and positions....