Template Engineering of Metal-to-Insulator Transitions in Epitaxial Bilayer Nickelate Thin Films

Understanding metal-to-insulator phase transitions in solids has been a keystone not only for discovering novel physical phenomena in condensed matter physics but also for achieving scientific breakthroughs in materials science. In this work, we demonstrate that the transport properties (i.e., resistivity and transition temperature) in the metal-to-insulator transitions of perovskite nickelates are tunable via the epitaxial heterojunctions of LaNiO₃ and NdNiO₃ thin films. A mismatch in the oxygen coordination environment and interfacial octahedral coupling at the oxide heterointerface allows us to realize an exotic phase that is unattainable in the parent compound. With oxygen vacancy formation for strain accommodation, the topmost LaNiO₃ layer in LaNiO₃/NdNiO₃ bilayer thin films is structurally engineered and it electrically undergoes a metal-to-insulator transition that does not appear in metallic LaNiO₃. Modification of the NdNiO₃ template layer thickness provides an additional knob for tailoring the tilting angles of corner-connected NiO₆ octahedra and the linked transport characteristics further. Our approaches can be harnessed to tune physical properties in complex oxides and to realize exotic physical phenomena through oxide thin-film heterostructuring.

Bendable BiVO4-Based Photoanodes on a Metal Substrate Realized through Template Engineering for Photoelectrochemical Water Splitting

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.

Template Engineering of CuBi2 O4 Single-Crystal Thin Film Photocathodes

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 V_RHE 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.

Using Soft Polymer Template Engineering of Mesoporous TiO2 Scaffolds to Increase Perovskite Grain Size and Solar Cell Efficiency

The mesoporous (meso)-TiO₂ layer is a key component of high-efficiency perovskite solar cells (PSCs). Herein, pore size controllable meso-TiO₂ layers are prepared using spin coating of commercial TiO₂ nanoparticle (NP) paste with added soft polymer templates (SPT) followed by removal of the SPT at 500 °C. The SPTs consist of swollen crosslinked polymer colloids (microgels, MGs) or a commercial linear polymer (denoted as LIN). The MGs and LIN were comprised of the same polymer, which was poly(N-isopropylacrylamide) (PNIPAm). Large (L-MG) and small (S-MG) MG SPTs were employed to study the effect of the template size. The SPT approach enabled pore size engineering in one deposition step. The SPT/TiO₂ nanoparticle films had pore sizes > 100 nm, whereas the average pore size was 37 nm for the control meso-TiO₂ scaffold. The largest pore sizes were obtained using L-MG. SPT engineering increased the perovskite grain size in the same order as the SPT sizes: LIN < S-MG < L-MG and these grain sizes were larger than those obtained using the control. The power conversion efficiencies (PCEs) of the SPT/TiO₂ devices were ∼20% higher than that for the control meso-TiO₂ device and the PCE of the champion S-MG device was 18.8%. The PCE improvement is due to the increased grain size and more effective light harvesting of the SPT devices. The increased grain size was also responsible for the improved stability of the SPT/TiO₂ devices. The SPT method used here is simple, scalable, and versatile and should also apply to other PSCs.