Solid gold is usually most stable as a face-centred cubic (fcc) structure. To date, no one has synthesized a colloidal form of Au that is exclusively hexagonal close-packed (hcp) and stable under ambient conditions. Here we report the first in situ synthesis of dispersible hcp Au square sheets on graphene oxide sheets, which exhibit an edge length of 200–500 nm and a thickness of ~ 2.4 nm (~ 16 Au atomic layers). Interestingly, the Au square sheet transforms from hcp to a fcc structure on exposure to an electron beam during transmission electron microscopy analysis. In addition, as the square sheet grows thicker (from ~ 2.4 to 6 nm), fcc segments begin to appear. A detailed experimental analysis of these structures shows that for structures with ultrasmall dimensions (for example, <~ 6 nm thickness for the square sheets), the previously unobserved pure hcp structure becomes stable and isolable.

Vacancy engineering, that is, self-doping of vacancy in semiconductors, has become a commonly used strategy to tune the photocatalytic performances. However, there still lacks fundamental understanding of the role of the vacancies in semiconductor materials. Herein, the g-C₃N₄ nanosheets with tunable nitrogen vacancies are prepared as the photocatalysts for H₂ evolution and CO₂ reduction to CO. On the basis of both experimental investigation and DFT calculations, nitrogen vacancies in g-C₃N₄ induce the formation of midgap states under the conduction band edge. The position of midgap states becomes deeper with the increasing of nitrogen vacancies. The g-C₃N₄ nanosheets with the optimized density of nitrogen vacancies display about 18 times and 4 times enhancement for H₂ evolution and of CO₂ reduction to CO, respectively, as compared to the bulk g-C₃N₄. This is attributed to the synergistic effects of several factors including (1) nitrogen vacancies cause the excitation of electrons to midgap states below the conduction band edge, which results in extension of the visible light absorption to photons of longer wavelengths (up to 598 nm); (2) the suitable midgap states could trap photogenerated electrons to minimize the recombination loss of photogenerated electron–hole pairs; and (3) nitrogen vacancies lead to uniformly anchored small Pt nanoparticles (1–2 nm) on g-C₃N₄, and facilitate the electron transfer to Pt. However, the overintroduction of nitrogen vacancies generates deeper midgap states as the recombination centers, which results in deterioration of photocatalytic activities. Our work is expected to provide new insights for fabrication of nanomaterials with suitable vacancies for solar fuel generation.

Porous CuO hollow architectures with perfect octahedral morphology are synthesized simply by annealing Cu-based metal–organic framework (MOF) templates. When tested as anode materials for lithium-ion batteries, these hollow octahedra exhibit greatly enhanced performance of lithium storage with excellent cycling stability and good rate capability.