Recently, germanium selenide (GeSe) has emerged as a promising van der Waals semiconductor for photovoltaics, solar light harvesting, and water photoelectrolysis cells. Contrary to previous reports claiming perfect ambient stability based on experiments with techniques without surface sensitivity, here, by means of surface-science investigations and density functional theory, it is demonstrated that actually both: i) the surface of bulk crystals; and ii) atomically thin flakes of GeSe are prone to oxidation, with the formation of self-assembled germanium-oxide skin with sub-nanometric thickness. Surface oxidation leads to the decrease of the bandgap of stoichiometric GeSe and GeSe 1−x , while bandgap energy increases upon surface oxidation of Ge 1−x Se. Remarkably, the formation of a surface oxide skin on GeSe crystals plays a key role in the physicochemical mechanisms ruling photoelectrocatalysis: the underlying van der Waals semiconductor provides electron-hole pairs, while the germanium-oxide skin formed upon oxidation affords the active sites for catalytic reactions. The self-assembled germanium-oxide/germanium-selenide heterostructure with different bandgaps enables the activation of photocatalytic processes by absorption of light of different wavelengths, with inherently superior activity. Finally, it is discovered that, depending on the specific solvent-GeSe interaction, the liquid phase exfoliation of bulk crystals can induce the formation of Se nanowires.

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Reduction of Cu2O by hydrogen is a common preparation step for heterogeneous catalysts; however, a detailed understanding of the atomic reaction pathways is still lacking. Here, we investigate the interaction of atomic hydrogen with the Cu2O­(100):(3,0;1,1) and Cu2O­(111):(√3 × √3)R30° surfaces using scanning tunneling microscopy (STM), low-energy electron diffraction, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The experimental results are compared to density functional theory simulations. At 300 K, we identify the most favorable adsorption site on the Cu2O­(100) surface: hydrogen atoms bind to an oxygen site located at the base of the atomic rows intrinsic to the (3,0;1,1) surface. The resulting hydroxyl group subsequently migrates to a nearby Cu trimer site. TPD analysis identifies H2 as the principal desorption product. These observations imply that H2 is formed through a disproportionation reaction of surface hydroxyl groups. The interaction of H with the (111) surface is more complex, including coordination to both Cu+ and OCUS sites. STM and XPS analyses reveal the formation of metallic copper clusters on the Cu2O surfaces after cycles of hydrogen exposure and annealing. The interaction of the Cu clusters with the substrate is notably different for the two surface terminations studied: after annealing, the Cu clusters coalesce on the (100) termination, and the (3,0;1,1) reconstruction is partially recovered. Clusters formed on the (111) surface are less prone to coalescence, and the (√3 × √3)R30° reconstruction was not recovered by heat treatment, indicating a weaker Cu cluster to support interaction on the (100) surface.

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