Dynamic regulation of ferroelectric polarization using external stimuli for efficient water splitting and beyond

Establishing and regulating the ferroelectric polarization in ferroelectric nano-scale catalysts has been recognized as an emerging strategy to advance water splitting reactions, with the merits of improved surface charge density, high charge transfer rate, increased electronic conductivity, the creation of real active sites, and optimizing the chemisorption energy. As a result, engineering and tailoring the ferroelectric polarization induced internal electric field provides significant opportunities to improve the surface and electronic characteristics of catalysts, thereby enhancing the water splitting reaction kinetics. In this review, an interdisciplinary and comprehensive summary of recent advancements in the construction, characterization, engineering and regulation of the polarization in ferroelectric-based catalysts for water splitting is provided, by exploiting a variety of external stimuli. This review begins with a detailed overview of the classification, benefits, and identification methodologies of the ferroelectric polarization induced internal electric field; this offers significant insights for an in-depth analysis of ferroelectric-based catalysts. Subsequently, we explore the underlying structure-activity relationships for regulating the ferroelectric polarization using a range of external stimuli which include mechanical, magnetic, and thermal fields to achieve efficient water splitting, along with a combination of two or more fields. The review then highlights emerging strategies for multi-scale design and theoretical prediction of the relevant factors to develop highly promising ferroelectric catalysts for efficient water splitting. Finally, we present the challenges and perspectives on the potential research avenues in this fascinating and new field. This review therefore delivers an in-depth examination of the strategies to engineer the ferroelectric polarization for the next-generation of water electrolysis devices, systems and beyond.

Fabrication of zirconium-based metal-organic frameworks@tungsten trioxide (UiO-66-NH2@WO3) heterostructure on carbon cloth for efficient photocatalytic removal of tetracycline antibiotic under visible light

Designing recyclable photocatalysts with high activity and stability has drawn considerable attention in the fields of sewage treatment. Herein, a series of heterojunctions constructed by zirconium-based metal-organic frameworks (UiO-66-NH₂) and tungsten trioxide (WO₃) is immobilized on carbon cloth via a facile solvothermal method, resulting in highly recyclable photocatalysts. Multiple characterization techniques, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, verify the successful synthesis of UiO-66-NH₂ nanospheres on the surface of needlelike WO₃ modified carbon cloth. Results show that the optimal heterojunction photocatalyst exhibits excellent photocatalytic degradation efficiency for the removal of tetracycline (TC) from water, for which nearly 100% of TC is degraded within 60 min under visible light. Trapping experiments and electron spin resonance (ESR) spectra analyses demonstrate that the superoxide radicals O₂^- and photogenerated hole h⁺ play a dominant role in the degradation process. Excellent photocatalytic activity is dominantly attributed to the effective separation of photoinduced carriers in this type-Ⅱ heterostructure system. Moreover, the possible photocatalytic oxidation degradation pathway is confirmed by analyzing intermediates using liquid chromatography mass spectrometry (LC-MS). This study offers a highly efficient strategy to design recyclable heterojunction photocatalysts for the degradation of refractory antibiotics in sewage.

Interfacial internal electric field and oxygen vacancies synergistically enhance photocatalytic performance of bismuth oxychloride

Solar-to-chemical energy conversion is valuable and sustainable strategy for energy and environmental crisis through photocatalysis. The amorphous SnO_x modified BiOCl (Sn-BiOCl) full-spectrum-responsive catalysts were designed and synthesized through solvothermal method. The introduced Sn regulates the growth of BiOCl to form ultrathin nanosheets with surface oxygen vacancies. And the surface modification of SnO_x induces interfacial internal electric field via charge redistribution on the interface of BiOCl and SnO_x to accelerate the photogenerated charge separation. The modification of SnO_x decreased work function of Sn-BiOCl and thus elevated its conduction band and valence band simultaneously, leading enhanced photocatalytic reducibility with the improved generation rate of ·O₂^-. The surface SnO_x and oxygen vacancies of Sn-BiOCl broadened light absorption range and enhanced photocatalytic performance synergistically, resulting in 14-fold increased photodegradation rate of phenol compared with pure BiOCl under full spectrum. This method is also able to expand to other metal ions (such as Fe³⁺, In³⁺ and Sb³⁺). This work provides a valuable concept in structure regulating for enhanced photocatalytic performance in the removal of organic pollutants by interfacial internal electric field and surface oxygen vacancies.

Hydrogen Impurities in ZnO: Shallow Donors in ZnO Semiconductors and Active Sites for Hydrogenation of Carbon Species

ZnO, as a low-cost yet significant semiconductor, has been widely used in solar energy conversion and optoelectronic devices. In addition, Cu/ZnO-based catalysts can convert syngas (H₂, CO, and CO₂) into methanol. However, the main concern about the intrinsic connection between the physical and chemical properties and the structure of ZnO still remains. In this work, efforts are made to decipher the physical and chemical information encoded into the structure. Through using NMR-IR techniques, we, for the first time, report a new ZnO model with three H⁺ cations incorporated into one Zn vacancy. ¹H magic-angle spinning NMR and IR spectra demonstrate that Ga³⁺ cations are introduced into the Zn vacancies of the ZnO lattice, which replace the H⁺ cation, and thus further confirm the feasibility of our proposed model. The exchange between the H⁺ cation in Zn vacancies and the D₂ gas phase shows that ZnO can activate H₂ because of the quantized three H⁺ cations in the defect site.

Illustration of charge transfer in graphene-coated hexagonal ZnO photocatalysts using Kelvin probe force microscopy

A graphene coated hexagonal ZnO (HZO@Gr) with enhanced activity in photocatalysis was synthesized. However, the photoinduced charge transfer behavior and the beneficial role of graphene in promoting photocatalytic reactions have not been sufficiently investigated experimentally. In this paper, the surface potentials of the ±(0001)-polar plane of HZO (Zn-polar plane and O-polar plane), graphene, graphene/Zn-polar plane and graphene/O-polar plane were measured using Kelvin probe force microscopy (KPFM). On the basis of the KPFM results, the respective Fermi levels were calculated and the internal electric field (IEF) of HZO was confirmed. Taking the IEF of HZO into consideration, the three-dimensional band diagrams of the HZO@Gr composites in methyl blue (MB) solution in the dark and under UV-visible irradiation after equilibrium were proposed. Accordingly, it is found that there could emerge different interactions between graphene and HZO at the ±(0001)-polar plane of HZO. Furthermore, the photogenerated holes and electrons tend to migrate to opposite directions. With the participation of graphene and IEF, the composites show a decrease in possibility of charge recombination. As a result, the active groups, namely ˙OH and ˙O₂⁻ radicals, could be mainly generated at/near the O-polar plane and Zn-polar plane, respectively. This work can serve as a supplemental explanation of the charge transfer during the photocatalytic process at the polar ZnO/graphene composite surface.

The Fermi levels and three-dimensional band diagrams of the synthesized HZO@Gr composites in methyl blue (MB) solution before and after equilibrium were assumed.

Sandwiched ZnO@Au@CdS nanorod arrays with enhanced visible-light-driven photocatalytical performance

The development of high-performance photocatalysts is central to efforts focused on taking advantage of solar energy to overcome environmental and energy crises. Integrating different functional materials artfully into nanostructures can deliver more efficient photocatalytic activity. Here, sandwiched ZnO@Au@CdS nanorod films were synthesized via successive ZnO nanorod electrodeposition, Au sputtering and CdS electrodeposition. The as-synthesized composites were characterized by UV-vis spectrophotometer, x-ray diffractometer, scanning and transmission electron microscopy. Their photocatalytic activity was assessed by degrading Rhodamine B solution under visible light irradiation. ZnO@Au@CdS exhibited better photocatalytic performance than ZnO@CdS throughout the visible light region, and the corresponding enhancement factor of Au nanoparticles was measured as a function of CdS loading amount, and it could reach 190% with CdS deposition for 1 min. The normalized rate constant could reach 0.387 h^-1 for ZnO@Au@CdS-1min, which was equivalent to or better than results in reference photocatalysts. The enhancement mechanism of Au nanoparticles was estimated by comparing the monochromatic photocatalytic action spectra with the absorption spectrum of ZnO@Au@CdS, and it was mainly determined by incident photon energy. With selective excitation of Au nanoparticles by incident photons, the excited hot electrons in Au NPs are transferred to the conduction band of ZnO to boost photocatalytic reaction. With selective excitation of CdS, the enhanced interband absorption of CdS and relay station effect of Au nanoparticles should be responsible for the enhanced photocatalytic performance. Our work not only opens the door to the design of efficient supported photocatalysts, but also helps to understand the enhancement mechanism of LSPR effect on the photoelectric conversion of semiconductors.

Radical macro spatial separation: a novel way to enhance the photocatalytic efficiency

Radical macro spatial separation can be achieved via a new photocatalytic system and significantly improved the photocatalytic efficiency.

Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications

Surface-bulk modification of zinc oxide for efficient photocatalysis.

Schottky or Ohmic metal-semiconductor contact: influence on photocatalytic efficiency of Ag/ZnO and Pt/ZnO model systems

The relationship between the contact type in metal-semiconductor junctions and their photocatalytic efficiencies is investigated. Two metal-semiconductor junctions, silver on zinc oxide (Ag/ZnO) and platinum on zinc oxide (Pt/ZnO) serve as model system for Ohmic and Schottky metal-semiconductor contact, respectively. Ag/ZnO, with Ohmic contact, exhibits a higher photocatalytic efficiency than Pt/ZnO, with Schottky contact. The direction of electric fields within the semiconductor is found to play a crucial role in the separation of photogenerated charges, and thus strongly influences the photocatalytic efficiency.