Epitaxial Bi2FeCrO6 Multiferroic Thin-Film Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Recent Advances in Ferroelectric Materials-Based Photoelectrochemical Reaction

Inorganic perovskite ferroelectric-based nanomaterials as sustainable new energy materials, due to their intrinsic ferroelectricity and environmental compatibility, are intended to play a crucial role in photoelectrochemical field as major functional materials. Because of versatile physical properties and excellent optoelectronic properties, ferroelectric-based nanomaterials attract much attention in the field of photocatalysis, photoelectrochemical water splitting and photovoltaic. The aim of this review is to cover the recent advances by stating the different kinds of ferroelectrics separately in the photoelectrochemical field as well as discussing how ferroelectric polarization will impact functioning of photo-induced carrier separation and transportation in the interface of the compounded semiconductors. In addition, the future prospects of ferroelectric-based nanomaterials are also discussed.

Construction of novel noble-metal-free MoP/CdIn2S4 heterojunction photocatalysts: Effective carrier separation, accelerating dynamically H2 release and increased active sites for enhanced photocatalytic H2 evolution

Developing novel photocatalysts with high performance is significant for the practical application of photocatalytic H₂ production. Herein, novel and noble-metal-free heterojunction photocatalysts contained CdIn₂S₄ nanoparticles and bulk MoP were prepared. The H₂-production rate of optimal MoP/CdIn₂S₄ (MPCIS) composites achieved at 286.10 μmol g^-1h^-1, which was nearly 2.2 times that of CdIn₂S₄-1 %Pt (130.51 μmol g^-1h^-1). Electrochemical and PL results displayed that MoP cocatalyst could vastly boost the carrier separation efficiency of CdIn₂S₄. The high carrier separation efficiency maybe put down to the Fermi level rearrangement between MoP and CdIn₂S₄. The linear sweep voltammograms tests showed that, compared to CdIn₂S₄, CdIn₂S₄ with 30 %MoP (30MPCIS) has smaller Tafel slopes and lower H₂ evolution overpotentials, which contributed to facilitate H₂ release in kinetics. The 30MPCIS composites possess higher C_dl value and MoP has Pt-like electronic structure, so that MoP could offer abundant surface reaction sites of MPCIS composites for HER. Aforementioned results demonstrate that MoP may have great potential in replacing precious metal Pt for photocatalytic H₂ production. It is expected that this study can provide new insights into developing the novel sulfide-based heterojunction photocatalysts with MoP as cocatalyst for resultful photocatalytic H₂ generation.

Design of MOF-Derived NiO-Carbon Nanohybrids Photocathodes Sensitized with Quantum Dots for Solar Hydrogen Production

Nickel oxide (NiO) is a promising p-type material for a wide range of optoelectronic devices, as well as photocathode for photoelectrochemical (PEC) water splitting. However, traditional NiO photoelectrodes exhibit a wide bandgap (3.6 eV), intrinsic poor electrical conductivity, and low surface area, leading to low PEC systems performance. Herein, the authors explore a Ni-based metal-organic framework (MOF) template method to obtain hierarchical hollow spheres of carbon/NiO nanostructure by successive carbonization and oxidation treatments. After sensitization with core and core-shell quantum dots (QDs), the optimized NiO-photocathode exhibits a maximum current density of -93.6 µA cm^-2 at 0 V versus RHE (reversible hydrogen electrode) in neutral pH (6.8) and -285 µA cm^-2 at -0.4 V versus RHE. Compared to pure NiO and single-core CdSe QDs, a 2.2-fold increase in photocurrent can be obtained. The improvement in the performance of this hybrid is not only due to the high surface area for loading QDs and light scattering, but also to the presence of a highly conductive carbon matrix that promotes fast charge transfer. The proposed MOFs-based NiO/carbon photocathode sensitized with QDs can be an effective strategy to improve the efficiency of metal oxide-based PEC systems for hydrogen generation.

Controlled Porosity in Ferroelectric BaTiO3 Photoanodes

The use of ferroelectric polarization to promote electron-hole separation has emerged as a promising strategy to improve photocatalytic activity. Although ferroelectric thin films with planar geometry have been largely studied, nanostructured and porous ferroelectric thin films have not been commonly used in photo-electrocatalysis. The inclusion of porosity in ferroelectric thin films would enhance the surface area and reactivity, leading to a potential improvement of the photoelectrochemical (PEC) performance. Herein, the preparation of porous barium titanate (pBTO) thin films by a soft template-assisted sol-gel method is reported, and the control of porosity using different organic/inorganic ratios is verified by the combination of scanning electron microscopy and ellipsometry techniques. Using piezoresponse force microscopy, the switching of ferroelectric domains in pBTO thin films is observed, confirming that the ferroelectric polarization is still retained in the porous structures. In addition, the presence of porosity in pBTO thin films leads to a clear improvement of the PEC response. By electrochemical poling, we also demonstrated the tuning of the PEC performance of pBTO thin films via ferroelectric polarization. Our work offers a simple and low-cost approach to control the morphology optimization of ferroelectric thin films, which could open up the development of materials with great potential for PEC applications.

Enhancing photoelectrochemical performance of the Bi2MoO6 photoanode by ferroelectric polarization regulation

Photoelectrochemical water splitting provides a promising strategy for converting solar energy into chemical fuels and has attracted extensive interest. Herein, Bi₂MoO₆ nanopillars with large surface areas were fabricated on an ITO-coated glass substrate and their photoelectrochemical properties are enhanced through appropriate manipulation of ferroelectric polarization. The Bi₂MoO₆ photoanode with polarization orientation toward ITO shows an enhanced photocurrent density of 250 μA cm^-2 at 1.23 V vs. reversible hydrogen electrode, which is 28% higher than that of pristine Bi₂MoO₆ nanopillars without macroscopic polarization. The corresponding depolarization electric field benefits the separation of light-excited electron-hole pairs, thus minimizing the recombination of charge carriers and further enhancing the photocurrent current density. Our work offers a new strategy of Bi₂MoO₆-based photoelectrochemical devices with great potential of application in the conversion of solar energy into chemical fuels.

Epitaxial patterned Bi2FeCrO6 nanoisland arrays with room temperature multiferroic properties

Epitaxial multiferroic Bi₂FeCrO₆ nanoisland arrays with a lateral size of ∼100 nm have been successfully fabricated by patterned SiO₂ template-assisted pulsed laser deposition. The as-grown island structure exhibits promising multiferroic properties (i.e. ferroelectric and magnetic) even at nanometer dimensions at room temperature. This work demonstrates an effective strategy to fabricate high-density nonvolatile ferroelectric/multiferroic memory devices.