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As the BO6octahedral structure in perovskite oxide is strongly linked with electronic behavior, it is actively studied for various fields such as metal–insulator transition, superconductivity, and so on. However, the research about the relationship between water-splitting activity and BO6structure is largely lacking. Here, we report the oxygen evolution reaction (OER) of LaNiO3(LNO) by changing the NiO6structure using compositional change and strain. The 5 atom % La deficiency in LNO resulted in an increase of the Ni—O—Ni bond angle and an expansion of bandwidth, enhancing the charge transfer ability. In-plane compressive strain derives the higher dz2orbital occupancy, leading to suitable metal–oxygen bond strength for OER. Because of the synergistic effect of A-site deficiency and compressive strain, the overpotential (η) of compressively strained L0.95NO film is reduced to 130 mV at j= 30 μA/cm2compared with nonstrained LNO (η = 280 mV), indicating a significant enhancement in OER.

Nonequilibrium deposition is a remarkable method for the in situ growth of unique nanostructures and phases for the functionalization of thin films. We introduce a distinctive structure of a mixed-phase, composed of BiVO4and β-Bi2O3, for photoelectrochemical water splitting. The mixed-phase is fabricated via nonequilibrium deposition by adjusted oxygen partial pressure. According to density functional theory calculations, we find that vanadium exsolution can be facilitated by introducing oxygen vacancies, enabling the fabrication of a nanostructured mixed-phase. These unique structures enhance charge migration by increasing the interfacial area and properly aligning the band offset between two crystalline phases. Consequently, the photocurrent density of the nanostructured mixed-phase thin films is about twice that of pristine BiVO4thin films at 1.23 VRHE. Our work suggests that nonequilibrium deposition provides an innovative route for the structural engineering of photoelectrodes for the understanding of fundamental properties and improving the photocatalytic performance for solar water splitting.

Although there is considerable interest in BiFeO3owing to its versatile physical properties, which make it suitable for a wide range of applications, its high leakage current is a significant limitation. Among various methods for reducing the leakage current, substitution with transition-metal or rare-earth elements is widely recognized as the most effective approach. Herein, to enable in-depth studies of the physical properties of BiFeO3, high-quality epitaxial BiFeO3thin films with a low leakage current must be formed. However, owing to the difficulty of controlling the element doping when pulsed laser deposition is used for epitaxial thin-film growth, studies on substitutional doping based on epitaxial BiFeO3thin films have not been systematically carried out. In this regard, we establish an innovative approach for overcoming the high leakage current of BiFeO3by fabricating artificially engineered superlattice-based epitaxial BiFeO3thin films in which there is a significant reduction of the leakage current. The control of the element doping in epitaxial BiFeO3thin films is easily regulated precisely at the atomic-scale level. The results of this study strongly suggest that superlattice-based epitaxial BiFeO3thin films can be a cornerstone for exploring the reliable fundamental physical properties of substitutional doping in epitaxial BiFeO3thin films.

An important factor in the performance of photoelectrochemical water splitting is the band edge alignment of the photoelectrodes for efficient transport and transfer of photogenerated carriers. Many studies for improving charge transfer ability between the electrode and the electrolyte have been reported, while research to improve charge transfer at the interface of the photoactive semiconductor and the conducting substrate is largely lacking. Here, we demonstrate that the water-splitting performance of an oxide heterostructured photoelectrode can be increased 6-fold by inserting an atomically thin polar LaAlO3interlayer compared with that of an oxide heterostructure without an insertion to modify interfacial band offsets. The electrically lowered Schottky barrier is driven by the atomically thin layer, and the charge transfer resistance between the oxides is reduced by up to 2 orders of magnitude upon insertion of LaAlO3, a wide-gap (5.6 eV) insulator. We show that the critical thickness of the polar layer for enhancing the charge transfer is 3 unit cells. The dipole moment from the polar sheets of LaAlO3introduces an internal electric field, which modifies the effective band offsets in the device. This work serves as a proof of concept that photoelectrochemical performance can be improved by manipulating the band offsets of the heterostructure interface, suggesting a new design strategy for heterostructured water-splitting photoelectrodes.

To improve the photoelectrochemical performance of photoelectrodes, various modifications such as the doping of electron donors, morphology control, and adoption of catalysts have been widely implemented, among which the formation of type-II heterojunctions has been recognized as an effective method to significantly improve the charge transport efficiency of photoelectrodes. In this regard, we report on an in situ high-quality epitaxial BiVO4/Bi4V2O11type-II heterojunction thin-film photoanode fabricated by using pulsed laser deposition on the basis of only one BiVO4ceramic target using the transition between BiVO4and Bi4V2O11crystalline phases. Herein, for the first time, we report on the structural and chemical transition between monoclinic BiVO4(010) and orthorhombic Bi4V2O11(001) crystalline phases by simply controlling the oxygen partial pressure. Subsequently, the growth of epitaxial BiVO4/Bi4V2O11heterojunction thin film is achieved by controlling only the oxygen partial pressure based on band alignment. At 1.23 VRHE, the photocurrent density of heterojunction BiVO4/Bi4V2O11structure is also significantly higher than that of the epitaxial BiVO4thin film owing to the effective charge transfer of the Bi4V2O11thin film. This study strongly suggests that the nonequilibrium deposition of epitaxial BiVO4thin films can propose a new paradigm in the structural design of photoanodes.

We have fabricated high quality bismuth vanadate (BiVO4) polycrystalline thin films as photoanodes by pulsed laser deposition (PLD) without a postannealing process. The structure of the grown films is the photocatalytically active phase of scheelite-monoclinic BiVO4which was obtained by X-ray diffraction (XRD) analysis. The change of surface morphology for the BIVO4thin films depending on growth temperature during synthesis has been observed by scanning electron microscopy (SEM), and its influence on water splitting performance was investigated. The current density of the BiVO4film grown on a glass substrate covered with fluorine-doped tin oxide (FTO) at 230 °C was as high as 3.0 mA/cm2at 1.23 V versus the potential of the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, which is the highest value so far in previously reported BiVO4films grown by physical vapor deposition (PVD) methods. We expect that doping of transition metal or decoration of oxygen evolution catalyst (OEC) in our BiVO4film might further enhance the performance.