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Unlike planar photoelectrodes, bendable and malleable photoelectrodes extend their application to mechanical flexibility beyond conventional rigid structures, which have garnered new attention in the field of photoelectrochemical water splitting. A bendable metal (Hastelloy), which has both bendability and compatibility with various oxide layers, allows high-temperature processes for crystallization; therefore it is far superior as a substrate than a conventional flexible polymer. In this study, we fabricate bendable BiVO4 crystalline thin films on the metal substrates by employing template layers (SrRuO3/SrTiO3) to reduce the structural misfits between BiVO4 and the substrate. The crystallinities were verified through X-ray diffraction and transmission electron microscopy, and photocatalytic performances were examined. The crystallinity of BiVO4 was significantly improved by utilizing similar lattice constants and affinities between BiVO4 and the oxide template layers. We also formed a type II heterojunction by adding a WO3 layer which complements the charge separation and charge transfer as a photoanode. The photocurrent densities of tensile-bent BiVO4/WO3 thin films with a bending radius of 10 mm are comparable to those of pristine BiVO4/WO3 thin film in various aqueous electrolytes. Moreover, photostability tests showed that the tensile-bent crystalline photoanodes retained 90% of their initial photocurrent density after 24 h, which proved their exceptional durability. Our work demonstrates that the bendable photoelectrodes with crystallinity hold great potential in terms of device structure for solar-driven water splitting.

Copper nitride (Cu3N) has attracted wide attention for solar energy conversion applications owing to suitable Eg of 1.6–1.9 eV, non-toxicity, and possibility to fabricate homojunction solar cells. The Cu3N has a body-centered site of anti-ReO3 structure, which can be easily occupied by transition metals. Hence, many studies on the band gap tuning of Cu3N have been conducted with doping of various elements. However, N-doped Cu3N film has not been reported experimentally yet although it was theoretically predicted to have very stable doping and new partially filled narrow band gap. Herein, via systematically controlling the nitrogen partial pressure (R = N2/N2 + Ar), we successfully fabricated N-doped Cu3N film using reactive RF sputtering. The N-doped Cu3N film (only specific R = 0.5) exhibits significantly reduced optical bandgap (1.1 eV) and improved photocurrent density (1.66 mA/cm2 at 10 V) compared with that of pristine Cu3N film.

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-...

Superlattice comprising of ferroelectric (FE) and non-ferroelectric (non-FE) material has been developed to induce a ferroelectric response in the non-ferroelectric layer. In this study, a superlattice thin film comprising of BiFeO3 (BFO) as FE oxide and La0.85Ce0.15MnO3 (LCMO) as semiconducting non-FE oxide is fabricated and investigated the FE and piezoelectric responses in LCMO. We observed the piezoelectric response of individual layers using time-resolved X-ray microdiffraction experiment under an external electric field. Piezoelectric response of individual layers was resolved by comparing it with the values obtained from kinematic diffraction calculation. Piezoelectric coefficient (d33) of LCMO monolayer is found to be approximately 27 pm/V, which is a similar value as that of the BFO layer.

Although there is considerable interest in BiFeO3 owing to its versatile physical properties, which make it suitable for a wide range of applications, its high leakage current is a significant limi...

While controlling the cation contents in perovskite rare-earth nickelate thin films, a metal-to-insulator phase transition is reported. Systematic control of cation stoichiometry has been achieved by manipulating the irradiation of excimer laser in pulsed laser deposition. Two rare-earth nickelate bilayer thin-film heterostructures with the controlled cation stoichiometry (i.e. stoichiometric and Ni-excessive) have been fabricated. It is found that the Ni-excessive nickelate film is structurally less dense than the stoichiometric film, albeit both of them are epitaxial and coherent with respect to the underlying substrate. More interestingly, as a temperature decreases, a metal-to-insulator transition is only observed in the Ni-excessive nickelate films, which can be associated with the enhanced disproportionation of the Ni charge valence. Based on our theoretical results, possible origins (e.g. anti-site defects) of the low-temperature insulating state are discussed with the need of future work for deeper understanding. Our work can be utilized to realize unusual physical phenomena (e.g. metal-to-insulator phase transitions) in complex oxide films by manipulating the chemical stoichiometry in pulsed laser deposition.

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/Bi4V2O11 type-II heterojunction thin-film photoanode fabricated by using pulsed laser deposition on the basis of only one BiVO4 ceramic target using the transition between BiVO4 and Bi4V2O11 crystalline 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/Bi4V2O11 heterojunction thin film is achieved by controlling only the oxygen partial pressure based on ba...

In photoelectrochemical (PEC) water splitting, BiVO4 is considered the most promising photoanode material among metal oxide semiconductors because of its relatively narrow optical bandgap and suitable band structure for water oxidation. Nevertheless, until now, the solar-to-hydrogen conversion efficiency of BiVO4 has shown significant limitations for commercialization because of its poor charge transport. Various strategies, including the formation of a heterojunction and doping of electron donors, have been implemented to enhance the charge transport efficiency; however, fundamental approaches are required for further enhancement. In this regard, we report the fundamental approach for BiVO4 thin film photoanodes by fabricating epitaxial oxide thin films with different crystallographic orientations for PEC water splitting. The crystalline anisotropy generally reveals distinct physical phenomena along different crystallographic orientations. In the same vein, in terms of the anisotropic properties of BiVO4...

Herein, we developed a fully solution-deposited nanocomposite photoanode based on silver nanoparticle (NP)-impregnated bismuth vanadate (BiVO4) films. The synthesized Ag NPs exhibit diameters of few nanometers and uniform matrix dispersion, which were confirmed by high-resolution transmission electron microscopy. The photoanode composed of the Ag NP-incorporated nanocomposite BiVO4 showed a remarkable enhancement in both low potential and the saturated photocatalytic current densities in comparison with the pristine BiVO4 film. The observed experimental results are attributed to the improved carrier generation and enhanced charge separation by the localized surface plasmon resonance-mediated effect as suggested by electrochemical impedance spectroscopy and a numerical simulation.

UV phototransistors based on ZnO, a material considered promising owing to its wide direct bandgap and high stability in harsh environments, have been intensively investigated. However, ZnO single-layer UV phototransistors, especially solution-processed devices, still exhibit poor electrical and UV photoresponse characteristics. Herein, we report the fabrication of a low-cost, large-area, and high-performance solution-processed ZnO/SnO2 bilayer UV phototransistor with improved electrical and UV photoresponse characteristics attained by inserting a SnO2 carrier transport layer, which is the actual path of the electrons. The photogenerated electrons are readily transferred from the ZnO UV-sensitive layer to the SnO2 carrier transport layer, owing to the lower conduction band of the SnO2 carrier transport layer than the ZnO UV-sensitive layer. In addition, the efficient extraction of photogenerated electrons from the ZnO UV-sensitive layer through the SnO2 carrier transport layer with high field effect mobility contributes to the improvement in the UV photoresponse characteristics of the ZnO/SnO2 bilayer UV phototransistor. The ZnO/SnO2 bilayer UV phototransistor exhibits high responsivity and detectivity as well as fast photoresponse. These results demonstrate that the solution-processed ZnO/SnO2 bilayer UV phototransistor developed in this study provides a novel approach for improving the performance of UV phototransistors with low-cost and large-area processing.

In photoelectrochemical (PEC) water splitting, charge separation and collection by the electric field in the photoactive material are the most important factors for improved conversion efficiency. Hence, ferroelectric oxides, in which electrons are the majority carriers, are considered promising photoanode materials because their high built-in potential, provided by their spontaneous polarization, can significantly enhance the separation and drift of photogenerated carriers. In this regard, the PEC properties of BiFeO3 thin-film photoanodes with different crystallographic orientations and consequent ferroelectric domain structures are investigated. As the crystallographic orientation changes from (001)pc via (110)pc to (111)pc, the ferroelastic domains in epitaxial BiFeO3 thin films become mono-variant and the spontaneous polarization levels increase to 110 μC/cm2. Consequently, the photocurrent density at 0 V vs. Ag/AgCl increases approximately 5.3-fold and the onset potential decreases by 0.180 V in the downward polarization state. It is further demonstrated that ferroelectric switching in the (111)pc BiFeO3 thin-film photoanode leads to an approximate change of 8,000% in the photocurrent density and a 0.330 V shift in the onset potential. This study strongly suggests that domain-engineered ferroelectric materials can be used as effective charge separation and collection layers for efficient solar water-splitting photoanodes.

Self-assembled nanocomposite films containing ferroelectric and ferromagnetic phases have attracted enormous research interest because they are the most promising candidates for practical multiferroic applications. However, obtaining a genuine magnetoelectric (ME) coupling effect is still challenging in this research area. To substantially improve the ME effect, new heterostructure designs with efficient strain control between two phases are urgently needed. Herein, a novel three-dimensional (3D) nanocup architecture of a heterostructure film is developed. To establish the unique architecture, a heavily Co, Fe-doped ferroelectric Bi3.25La0.75Ti3O12 (BLT) target was used during the growth of BLT thin films via pulsed laser deposition. Consequently, 3D nanocup-structured CoFe2O4 (CFO) particles formed inside the BLT via spontaneous nucleation and agglomeration. The 3D nanocup BLT-CFO film exhibited magnetically controlled reversible dielectric switching, which is direct evidence of strong ME coupling caused by the efficient interfacial strain coupling and low leakage of the novel nanocup architecture. The obtained results strongly suggest that the 3D nanocup heterostructure film significantly improves the ME coupling effect. In addition, we propose a new paradigm in the architecture design of self-assembled nanocomposite films for diverse multifunctional devices. A technique for controlling the growth of nanostructures on thin films can improve switches that manipulate both electrical and magnetic signals. Hyunji An from the Gwangju Institute of Science and Technology in South Korea and colleagues mixed cobalt and iron atoms into a bismuth–lanthanum–titanium ceramic that responds to external electric fields. By using pulsed laser on the ceramics, the team identified that the cobalt and iron components were spontaneously assembled into teacup-shaped nanostructures on the film’s surface. Characterization of the composite revealed that this new morphology provided a large area for coupling magnetic and electrical properties, while avoiding the signal losses associated with similar switches, such as thin films infused with vertically oriented magnetic pillars. Three-dimensional nanocup Bi3.25La0.75Ti3O12–CoFe2O4 heterostructure film is obtained via a heavily Co, Fe co-doped ferroelectric Bi3.25La0.75Ti3O12 target during pulsed laser deposition. The unique 3D nanocup architecture beyond usual architectures enables reversible magnetoelectric switching of the multiferroic heterostructure film through overcoming leakage issues, as well as efficient interfacial strain coupling based on their structural benefits.

A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and for exploring unusual physical phenomena. However, owing to its volatility, sulfur substitution to oxide materials is challenging, and thus the sulfurization effects on the associated properties have been limitedly studied. Here, a facile method for sulfurization to a perovskite oxide $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ is developed and demonstrated. A thiourea $(\mathrm{C}{\mathrm{H}}_{4}{\mathrm{N}}_{2}\mathrm{S})$ solution is used as a precursor for the sulfurization and its doping-level control. By manipulating the sulfur concentration ($x$), we systematically examine the physical properties of sulfur-doped $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3\ensuremath{-}x}{\mathrm{S}}_{x}$ films. An enhancement in the tetragonality and ferroelectricity by sulfurization is observed with the band-gap reduction, which is consistent with our theoretical predictions. In the sulfurized films, the ferroelectric phonon modes become softened progressively, probably due to the substitution of apical oxygens with sulfur atoms. Our work is of practical interest for designing ferroelectric photovoltaic devices with high performances.

Bismuth vanadate (BiVO4) has attracted significant attention as a promising photoanode material for hydrogen production via photoelectrochemical (PEC) water splitting because of its narrow optical band gap and suitable band edge positions for water oxidation. However, the actual photoactivity of BiVO4 is considerably limited by its poor electron transport and slow water oxidation kinetics. Although several studies have been carried out to improve its photo-efficiency via the enhancement of electron transport and water oxidation kinetics, only a few studies have reported the growth of epitaxial BiVO4 to explore the fundamental properties of BiVO4 for PEC water splitting because extremely flat epitaxial films exhibit poor photo-efficiency because of their low surface-active area. However, studies of epitaxial BiVO4 still have the potential to provide new routes for improving its photo-efficiency. In this study, the growth of epitaxial BiVO4 is investigated using a thin γ-WO3 template layer deposited on a SrTiO3(001) substrate covered by a SrRuO3 (SRO) bottom electrode using pulsed laser deposition. Consequently, at 1.23 V vs. the RHE (reversible hydrogen electrode), the photocurrent density of epitaxial BiVO4 on the γ-WO3 template layer (2.20 mA cm−2) is approximately 10 times that of bare BiVO4, related to the effective charge transfer by the γ-WO3 intermediate layers and the subsequent increase in the surface-active area of epitaxial BiVO4. These results strongly suggest that epitaxial BiVO4 grown using a template layer can be a cornerstone for the in-depth understanding of the fundamental properties of BiVO4 for PEC water splitting.

To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copper-based oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi2 O4 (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on single-crystal SrTiO3 (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate -0.4 and -0.7 mA cm-2 at even 0 VRHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H2 O2 as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.

Solar-driven water splitting technology is considered to be a promising solution for the global energy challenge as it is capable of generating clean chemical fuel from solar energy. Various strategies and catalytic materials have been explored in order to improve the efficiency of the water splitting reaction. Although significant progress has been made, there are many intriguing fundamental phenomena that need to be understood. Herein, we review recent experimental efforts to demonstrate enhancement strategies for efficient solar water splitting, especially for the light absorption, charge carrier separation, and water oxidation kinetics. We also focus on the state of the art of photoelectrochemical (PEC) device designs such as application of facet engineering and the development of a ferroelectric-coupled PEC device. Based on these experimental achievements, future challenges, and directions in solar water splitting technology will be discussed.

Tuning the bandgap in ferroelectric complex oxides is a possible route for improving the photovoltaic activity of materials. Here, we report the realization of this effect in epitaxial thin films of the ferroelectric complex oxide Bi3.25La0.75Ti3O12 (BLT) suitably doped by Fe and Co. Our study shows that Co (BLCT) doping and combined Fe, Co (BLFCT) doping lead to a reduction of the bandgap by more than 1 eV compared to undoped BLT, accompanied by a surprisingly more efficient visible light absorption. Both BLCT and BLFCT films can absorb visible light with a wavelength of up to 500 nm while still exhibiting ferroelectricity, whereas undoped BLT only absorbs UV light with a wavelength of less than 350 nm. Correlated with its bandgap reduction, the BLFCT film shows a photocurrent density enhanced by 25 times compared to that of BLT films. Density functional theory calculations indicate that the bandgap contraction is caused by the formation of new energy states below the conduction bands due to intermixed transition metal dopants (Fe, Co) in BLT. This mechanism of tuning the bandgap by simple doping can be applied to other wide-bandgap complex oxides, thereby enabling their use in solar energy conversion or optoelectronic applications.