We used a liquid phase reaction to synthesize a nickel (Ni) and iron (Fe)-containing layered double hydroxide (NiFe-LDH), having a lateral size less than 10 nm. A chelating agent introduced into the media was thought to increase the concentration of metal hydroxide nuclei and suppress excessive growth of the LDH crystal, resulting in the synthesis of nanometer-sized LDH. The NiFe-LDH catalyzed the oxygen evolution reaction (OER) at an overpotential of 247 mV and current of 10 mA cm–2, which is superior to the performance of conventional iridium oxide (IrO x ) catalysts. Notably, a membrane electrode assembly (MEA) for anion exchange membrane water electrolysis using NiFe-LDH as an anode catalyst exhibited an energy conversion efficiency of 74.7% for flowing 1.0 A cm–2 in 1 M KOH at 80 °C. This efficiency is the highest among MEAs implemented with non-noble metal-based catalysts reported to date and offers a viable replacement for IrO x anode catalysts.

We report on structural and optical properties for the (0001)-oriented α-(CrxFe1-x)2O3 (0 ≤ x ≤ 1) epitaxial films prepared on c-sapphire substrates by using pulsed-laser deposition. Pure corundum phase with atomically flat surface was obtained in the entire composition range. Optical absorption spectra for the films with 0.2 < x < 0.9 showed a nearly constant band-gap (1.7 eV), which is narrower than those of α-Fe2O3 (2.1 eV) and α-Cr2O3 (3.0 eV). The result suggests that the band-gap narrowing arises from a type-II band alignment of these oxides and the fundamental band-gap lies between the Cr t2g and O 2p occupied states and the Fe t2g* empty state.

Epitaxial α-Fe2O3 thin films on conducting Ta-doped SnO2 bottom layers are prepared by pulsed-laser deposition in order to elucidate variation of the photoelectrochemical (PEC) properties for the water oxidation reaction on the α-Fe2O3 photoanodes with different orientation and thickness. Cyclic voltammetry under illumination shows that current density (photocurrent onset potential) of the m-axis oriented samples is considerably larger (lower) than that of the c-axis oriented ones. Using electrochemical impedance spectroscopy under illumination, we ascribe these behaviors to difference in surface states between c- and m-planes besides anisotropic conductivity along the individual axes. Our study demonstrates the power of utilizing well-defined epitaxial thin films to investigate the anisotropic PEC properties of visible-light-driven photoelectrodes.

We have studied the atomic ordering of B-site transition metals and magnetic properties in the pulsed-laser deposited films of La 2 CrFeO 6 (LCFO) and La 2 VMnO 6 (LVMO), whose bulk materials are known to be single perovskites with random distribution of the B-site cations.Despite similar ionic characters of constituent transition metals in each compound, the maximum B-site order attained was surprisingly high, ~90% for LCFO and ~80% for LVMO, suggesting a significant role of epitaxial stabilization in the spontaneous ordering process. Magnetization and valence state characterizations revealed that the magnetic ground state of both compounds was coincidently ferrimagnetic with saturation magnetization of ~2μ B per formula unit, unlike those predicted theoretically. In addition, they were found to be insulating with optical band gaps of 1.6 eV and 0.9 eV for LCFO and LVMO, respectively. Our results present a wide opportunity to explore novel magnetic properties of binary transition-metal perovskites upon epitaxial stabilization of the ordered phase.

We report the photoelectrode properties of an n-type -Ga 2 O 3 single crystal in aqueous solutions. The conduction and valence band-edge potentials were found to be 1.1 V higher and 2.5 V lower than the H þ /H 2 and O 2 /H 2 O redox potentials, respectively. Photocurrent drastically increased as photogenerated carriers were excited at a photon energy higher than the fundamental absorption edge of 4.7 eV. The incident photon-to-current conversion efficiency was 36% at 5.2 eV. Gaseous oxygen and hydrogen evolved from the photoelectrode and Pt counter electrode, respectively. The stoichiometric water splitting was demonstrated by applying an external bias of 1 V.