Photoelectrochemical (PEC) water splitting is a promising method for conversing solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α-Fe2O3) is one of the most attractive materials for highly efficient charge carrier generation and collection due to its large specific surface area and shortening minority carrier diffusion length required to reach the surface. In the present work, PEC water splitting performance of α-Fe2O3 prepared by anodization of thin iron layers on an FTO glass and subsequent annealing in low O2-Ar ambient with only 0.03% O2 was investigated. The key finding is that annealing the anodic nanostructures with low oxygen concentration provides a strongly enhanced PEC performance compared with classic air annealing. The photocurrent of the former at 1.5 V vs. RHE results in 1.1 mA/cm 2 , being 11 times higher than that of the latter. The enhancement of the PEC performance for α-Fe2O3 annealed in low oxygen atmosphere can be attributed to controlled morphology, Sn doping, and introduction of oxygen vacancies, which contribute to the enhancement of the hole flux from the photogenerated site to the reactive surface and additionally lead to an enhanced hole transfer at the interface between the α-Fe2O3 and the electrolyte. From the obtained results, it is evident that low oxygen annealing is a surprisingly effective method of defect engineering and optimizing α-Fe2O3 electrodes for a maximized PEC water splitting performance.

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α‐Fe2O3) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α‐Fe2O3 is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O2 in Ar). It was found that low oxygen annealing can activate a significant PEC response of α‐Fe2O3 even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α‐Fe2O3 annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm−2, being 20 times higher than that of annealing in air. The obtained results show that the α‐Fe2O3 annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α‐Fe2O3. Additionally, we demonstrate the photocurrent of α‐Fe2O3 annealed in low oxygen ambient can be further enhanced by Zn‐Co LDH, which is a co‐catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α‐Fe2O3 photoanodes.

Synopsis :The influence of the Mn content of Si-added steel sheets on the Fe-Zn galvannealing reaction was investigated. Three steel sheets, 1.5 mass%-Si-1.4, 1.9 and 2.7 mass%Mn, were annealed in a 10vol%H 2 -90vol%N 2 atmosphere. Si and Mn oxides were analyzed by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. SiO 2 and Mn 2 SiO 4 formed as selective oxides at the steel surface after recrystallization annealing. The ratio of the oxide species changed depending on the Mn content in the steel. When the Mn content was lower, formation of SiO 2 was promoted and that of Mn 2 SiO 4 was suppressed. In the selective oxide layer which formed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets, Mn 2 SiO 4 formed at the outer side, and SiO 2 formed at the inner side. This can be explained by consideration of the thermodynamic oxygen potential gradient. Furthermore, areas where SiO 2 mainly formed and those where Mn 2 SiO 4 mainly formed were distributed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets. In this case, the Fe-Zn intermetallic compound (IMC) formed preferentially on the Mn 2 SiO 4 between the zinc coating and the substrate steel after galvanizing, and the Fe-Zn galvannealing reaction was suppressed on the SiO 2 layer. It is considered that a dense and continuous protective SiO 2 layer acted as a barrier to the Fe-Zn galvannealing reaction.

The influences of external and internal Si oxides on the galvannealing reaction of 0-1.8%Si-added steel were investigated. Internal and external Si oxides formed by recrystallization annealing at the dew points of 223-263 K. The distribution of Si oxides in the subsurface region of the substrate steel was analyzed using glow discharge optical emission spectroscopy, reflection electron microscopy, and transmission electron microscopy. External Si oxides suppressed the galvannealing reaction. Then the ζ phase formed locally at the interface between the substrate steel and the zinc coating after galvanizing, which suggests that the external Si oxides at the steel surface acted as a barrier to the Fe-Zn reaction. When Si oxidized internally, a depleted zone of solute Si was observed in the subsurface region of the substrate steel. In this case, the galvannealing reaction was accelerated in comparison with that when Si did not oxidize. Furthermore, decrease of the solute Si content led to acceleration of the galvannealing reaction, nucleation of the ζ phase, and growth of the Γ phase. From these results, it was concluded that depletion of solute Si attributable to internal oxidation of Si accelerated the galvannealing reaction.