Visible-Light-Driven Photocatalytic Z-Scheme Overall Water Splitting in La5 Ti2 AgS5 O7 -based Powder-Suspension System

Aliovalent metal cation doping of La5Ti2AgO7S5 particles for improved photocatalytic and photoelectrochemical water splitting

The doping of photocatalytic La5Ti2AgO7S5 particles with aliovalent metal cations was investigated. The incorporation of lower-valence Al3+ cations at Ti4+ sites improved the photocatalytic hydrogen evolution activity. In addition, the anodic photocurrent during oxygen evolution was increased upon adding higher-valence Ta5+ ions. The effect of doping on the carrier density in the photocatalytic particles was also examined in this work.

Interfacial Design of Particulate Photocatalyst Materials for Green Hydrogen Production

Green hydrogen production using particulate photocatalyst materials has attracted much attention in recent years because this process could potentially lead to inexpensive and scalable solar-to-chemical energy conversion systems. Although the development of efficient particulate photocatalysts enabling one-step overall water splitting (OWS) with solar-to-hydrogen efficiencies in excess of 10 % remains challenging, promising photocatalyst candidates exhibiting OWS activity have been demonstrated. This review provides a comprehensive introduction to the solar-to-hydrogen energy conversion process of semiconductor photocatalyst materials and highlights recent advances in photocatalytic OWS via both one-step and two-step photoexcitation processes. The review also covers recent developments in the photocatalytic OWS of SrTiO3, including the establishment of large-scale photocatalytic systems, interfacial design using cocatalysts to enhance water splitting activity, and its photoelectrochemical (PEC) properties at the electrified solid/liquid interface. In addition, there is a special focus on visible-light-absorbing oxynitride and oxysulfide particulate photocatalysts with absorption edges near 600 nm. Methods for photocatalyst preparation and surface modification, as well as PEC properties, are also discussed. The semiconductor properties of particulate photocatalysts obtained from photoelectroanalytical evaluations using particulate photoelectrodes are evaluated. This review is intended to provide guidelines for the future development of particulate photocatalysts capable of efficient and stable OWS.

Recent Advances in Redox-Based Z-Scheme Overall Water Splitting under Visible Light Irradiation

Photocatalytic overall water splitting (OWS) using suspended particulate photocatalysts to produce green hydrogen has inspired continuous interest due to its low cost for easy large-scale application. The two-step photoexcitation system (Z-scheme) mimicking natural photosynthesis was proposed to efficiently use visible light for realization of efficient conversion of solar irradiation. In this Perspective, we will introduce recent advances in redox-based Z-scheme OWS systems, including iodine-based, iron-based, metal complex-based, and other special ion redox couples. The advantages and challenges of each couple and the factors affecting the Z-scheme OWS efficiency are discussed in detail. Finally, the challenges and feasible solutions for the achievement of highly efficient Z-scheme OWS are then outlined. This Perspective provides guidance on how to construct a Z-scheme OWS system and enhance photocatalytic performance.

Recent developments, advances and strategies in heterogeneous photocatalysts for water splitting

Photocatalytic water splitting (PWS) is an up-and-coming technology for generating sustainable fuel using light energy. Significant progress has been made in the developing of PWS innovations over recent years. In addition to various water-splitting (WS) systems, the focus has primarily been on one- and two-steps-excitation WS systems. These systems utilize singular or composite photocatalysts for WS, which is a simple, feasible, and cost-effective method for efficiently converting prevalent green energy into sustainable H2 energy on a large commercial scale. The proposed principle of charge confinement and transformation should be implemented dynamically by conjugating and stimulating the photocatalytic process while ensuring no unintentional connection at the interface. This study focuses on overall water splitting (OWS) using one/two-steps excitation and various techniques. It also discusses the current advancements in the development of new light-absorbing materials and provides perspectives and approaches for isolating photoinduced charges. This article explores multiple aspects of advancement, encompassing both chemical and physical changes, environmental factors, different photocatalyst types, and distinct parameters affecting PWS. Significant factors for achieving an efficient photocatalytic process under detrimental conditions, (e.g., strong light absorption, and synthesis of structures with a nanometer scale. Future research will focus on developing novel materials, investigating potential synthesis techniques, and improving existing high-energy raw materials. The endeavors aim is to enhance the efficiency of energy conversion, the absorption of radiation, and the coherence of physiochemical processes.

Photocatalytic Z-scheme Overall Water Splitting: Insight into Different Optimization Strategies for Powder Suspension and Particulate Sheet Systems

Achieving solar light-driven photocatalytic overall water splitting is the ideal and ultimate goal for solving energy and environment issues. Photocatalytic Z-scheme overall water splitting has undergone considerable development in recent years; specific approaches include a powder suspension Z-scheme system with a redox shuttle and a particulate sheet Z-scheme system. Of these, a particulate sheet has achieved a benchmark solar-to-hydrogen efficiency exceeding 1.1 %. Nevertheless, owing to intrinsic differences in the components, structure, operating environment, and charge transfer mechanism, there are several differences between the optimization strategies for a powder suspension and particulate sheet Z-scheme. Unlike a powder suspension Z-scheme with a redox shuttle, the particulate sheet Z-scheme system is more like a miniaturized and parallel p/n photoelectrochemical cell. In this review, we summarize the optimization strategies for a powder suspension Z-scheme with a redox shuttle and particulate sheet Z-scheme. In particular, attention has been focused on choosing appropriate redox shuttle and electron mediator, facilitating the redox shuttle cycle, avoiding redox mediator-induced side reactions, and constructing a particulate sheet. Challenges and prospects in the development of efficient Z-scheme overall water splitting are also briefly discussed.

Enabling artificial photosynthesis systems with molecular recycling: A review of photo- and electrochemical methods for regenerating organic sacrificial electron donors

This review surveys advances in the literature that impact organic sacrificial electron donor recycling in artificial photosynthesis. Systems for photocatalytic carbon dioxide reduction are optimized using sacrificial electron donors. One strategy for coupling carbon dioxide reduction and water oxidation to achieve artificial photosynthesis is to use a redox mediator, or recyclable electron donor. This review highlights photo- and electrochemical methods for recycling amines and NADH analogues that can be used as electron donors in artificial photosynthesis. Important properties of sacrificial donors and recycling strategies are also discussed. Compounds from other fields, such as redox flow batteries and decoupled water splitting research, are introduced as alternative recyclable sacrificial electron donors and their oxidation potentials are compared to the redox potentials of some model photosensitizers. The aim of this review is to act as a reference for researchers developing photocatalytic systems with sacrificial electron donors, and for researchers interested in designing new redox mediator and recyclable electron donor species.

A perspective on two pathways of photocatalytic water splitting and their practical application systems

Photocatalytic water splitting has been widely studied as a means of converting solar energy into hydrogen as an ideal energy carrier in the future. Systems for photocatalytic water splitting can be divided into one-step excitation and two-step excitation processes. The former uses a single photocatalyst while the latter uses a pair of photocatalysts to separately generate hydrogen and oxygen. Significant progress has been made in each type of photocatalytic water splitting system in recent years, although improving the solar-to-hydrogen energy conversion efficiency and constructing practical technologies remain important tasks. This perspective summarizes recent advances in the field of photocatalytic overall water splitting, with a focus on the design of photocatalysts, co-catalysts and reaction systems. The associated challenges and potential approaches to practical solar hydrogen production via photocatalytic water splitting are also presented.

Understanding the Photocatalytic Activity of La5Ti2AgS5O7 and La5Ti2CuS5O7 for Green Hydrogen Production: Computational Insights

Green production of hydrogen is possible with photocatalytic water splitting, where hydrogen is produced while water is reduced by using energy derived from light. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of La₅Ti₂AgS₅O₇ and La₅Ti₂CuS₅O₇-two emerging candidate materials for water splitting. The electronic structure of both bulk materials was calculated by using hybrid DFT, which indicated the band gaps and charge carrier effective masses are suitable for photocatalytic water splitting. Notably, the unique one-dimensional octahedral TiO _x S_6-x and tetragonal MS₄ channels formed provide a structural separation for photoexcited charge carriers which should inhibit charge recombination. Band alignments of surfaces that appear on the Wulff constructions of 12 nonpolar symmetric surface slabs were calculated by using hybrid DFT for each of the materials. All surfaces of La₅Ti₂AgS₅O₇ have band edge positions suitable for hydrogen evolution; however, the small overpotentials on the largest facets likely decrease the photocatalytic activity. In La₅Ti₂CuS₅O₇, 72% of the surface area can support oxygen evolution thermodynamically and kinetically. Based on their similar electronic structures, La₅Ti₂AgS₅O₇ and La₅Ti₂CuS₅O₇ could be effectively employed in Z-scheme photocatalytic water splitting.

Prospects and challenges in designing photocatalytic particle suspension reactors for solar fuel processing

Photocatalytic approaches for the production of solar hydrogen or hydrocarbons are interesting as they provide a sustainable alternative to fossil fuels. Research has been focused on water splitting and on the synthesis of photocatalyst materials and compounds, and their characterization. The material-related challenges include the synthesis and design of photocatalysts that can absorb visible light at a high quantum efficiency, cocatalysts that are selective and can accelerate the reduction and/or oxidation reactions, and protection layers that facilitate migration of the minority carriers to the surface-active sites while reducing charge recombination and photo-corrosion. Less attention has been paid to the conceptual design of reactors, and how design and coupled transport can affect the material choice and requirements. This perspective discusses the various possible conceptual designs for particle suspension reactors and the related implications on the material requirements to achieve high energy conversion efficiencies. We establish a link between the thermodynamic limits, materials requirements, and conceptual reactor designs, quantify changes in material requirements when more realistic operation and losses are considered, and compare the theory-derived guidelines with the ongoing materials research activity.

This perspective discusses the various possible conceptual designs for particle suspension reactors and the related implications on the material and reactor requirements to achieve high STH conversion efficiencies.

Bifunctional black phosphorus: coupling with hematite for Z-scheme photocatalytic overall water splitting

Z-scheme 2D–2D BP/α-Fe₂O₃ heterostructure enables preferable photocatalytic properties for efficient overall water splitting under visible light.

Efficient photocatalytic hydrogen evolution on single-crystalline metal selenide particles with suitable cocatalysts

It is important to improve the apparent quantum yields (AQYs) of narrow bandgap photocatalysts to achieve efficient H₂ production. The present work demonstrates a particulate solid solution of zinc selenide and copper gallium selenide (denoted as ZnSe:CGSe) that evolves H₂ efficiently and is responsive to visible light up to 725 nm. This material was synthesized using a flux-assisted method and was found to comprise single-crystalline tetrahedral particles. The coloading of Ni and Rh, Pt, Pd or Ru as cocatalysts further improved the photocatalytic H₂ evolution rate over this photocatalyst. With the optimal coloading of a Ni–Ru composite cocatalyst, an AQY of 13.7% was obtained at 420 nm during a sacrificial H₂ evolution reaction, representing the highest value yet reported for a photocatalyst with an absorption edge longer than 700 nm. The present study demonstrates that the preparation of single-crystalline particles and the rational assembly of composite cocatalysts are effective strategies that allow the efficient utilization of long wavelengths by metal selenide photocatalysts for solar fuel production.

Coloading of a Ni–Ru composite cocatalyst on a 700 nm-class single-crystalline particulate selenide photocatalyst improves its hydrogen evolution activity.

Z-Scheme Photocatalytic Systems for Solar Water Splitting

As the world decides on the next giant step for the renewable energy revolution, scientists have begun to reinforce their headlong dives into the exploitation of solar energy. Hitherto, numerous attempts are made to imitate the natural photosynthesis of plants by converting solar energy into chemical fuels which resembles the "Z-scheme" process. A recreation of this system is witnessed in artificial Z-scheme photocatalytic water splitting to generate hydrogen (H2). This work outlines the recent significant implication of the Z-scheme system in photocatalytic water splitting, particularly in the role of electron mediator and the key factors that improve the photocatalytic performance. The Review begins with the fundamental rationales in Z-scheme water splitting, followed by a survey on the development roadmap of three different generations of Z-scheme system: 1) PS-A/D-PS (first generation), 2) PS-C-PS (second generation), and 3) PS-PS (third generation). Focus is also placed on the scaling up of the "leaf-to-tree" challenge of Z-scheme water splitting system, which is also known as Z-scheme photocatalyst sheet. A detailed investigation of the Z-scheme system for achieving H2 evolution from past to present accompanied with in-depth discussion on the key challenges in the area of Z-scheme photocatalytic water splitting are provided.