Atomic-scale direct probing of active sites and subsequent elucidation of the structure-activity relationship are important issues involving oxide-based electrocatalysts to achieve better electrochemical conversion efficiency. By generating Ruddlesden-Popper (RP) two-dimensional homologous faults via simple control of the cation nonstoichiometry in LaNiO thin films, we demonstrate that strong tetragonal distortion of [NiO] octahedra is induced by more than 20% elongation of Ni-O bonds in the faults. In addition to direct visualization of the elongation by scanning transmission electron microscopy, we identify that the distorted [NiO] octahedra in the faults show considerably higher electrocatalytic activities than other surface sites during the electrochemical oxygen evolution reaction. This unequivocal evidence of the octahedral distortion and its impact on electrocatalysis in LaNiO suggests that the formation of RP-type faults can provide an efficient way to control the octahedral geometry and thereby remarkably enhance the oxygen catalytic performance of perovskite oxides.

We demonstrated template-free inclusion of the third dimension into the graphitic frameworks while retaining π-conjugation and conductivity, which was verified by their activity as metal-free electrocatalysts for the hydrogen evolution reaction.

Crystalline Si and III-V compound semiconductors with appropriate band edge positions for the reduction of water have been widely utilized in photoelectrochemical (PEC) cells for the hydrogen evolution reaction (HER). However, the high cost of manufacturing those PEC cell photoabsorbers makes it difficult to achieve cost-effective hydrogen production. To overcome this issue, a new approach to fabricate a photoabsorber with low cost yet high performance for the HER is highly necessary. Here, we present a controlled fracture method, the so-called spalling process, to fabricate a cost-effective thin semiconductor applicable to the PEC HER. Using this method, a wafer-scale thin Si, whose thickness can be controlled from a few micrometers to sub-50 μm, was fabricated from a thick Si mother substrate without material loss. Pt nanoparticle-decorated 16 μm thick spalled Si with an np rear junction exhibited an HER onset potential of 332 mV (vs reversible hydrogen electrode (RHE)) and a photocurrent density of 20.1 mA cm at 0 V (vs RHE), which are the best performances among previously reported planar-type thin Si-based photocathodes. Finally, we demonstrated that 20 μm thick GaAs could also be successfully fabricated by the spalling process, while exhibiting a PEC HER performance comparable to 350 μm thick bulk GaAs.