Influence of exposed facets,morphology and hetero-interfaces of BiVO4 on photocatalytic water oxidation: A review

The photocatalytic water splitting has a stupendous role to generate renewable hydrogen. However, the overall water splitting reaction is limited by the sluggish oxidation step. Bismuth vanadate (BiVO₄) has been identified as one of the most promising n-type semiconductors for photocatalytic water oxidation due to its small energy gap and favorable band alignment. Thus, it is necessary to summarize the recent progress done on BiVO₄ for the guidance of future research. In this review, we have discussed recent strategies that have been adopted to boost its photoconversion efficiency at three different levels: (1) facet control, (2) morphological control and, (3) interfacial control. The roles of high indexed facets, anisotropic morphologies, and mediator-based Z schemes are comprehensively discussed and the emphasis is given to find the suitable structural, morphological, and interfacial combination of BiVO₄ for efficient water oxidation.     

光化学合成太阳能燃料的研究进展

光电转换、光热转换和光化学转换是太阳能利用的三种主要途径。近年来,太阳能燃料的研究已引起了人们的广泛关注。本文针对光化学合成太阳能燃料,简要综述了光解水制H₂及CO₂光化学还原为CO等燃料的研究进展,并展望了基于利用太阳能制取的H₂和CO进一步光费托合成碳氢燃料的前景。     

Recent advances in the design of semiconductor hollow microspheres for enhanced photocatalyticv water splitting

Photocatalytic water splitting is a promising method to produce clean and renewable energy, which provides an alternative solution to solve environmental and resource problems. New catalysts based on semiconductor nanoparticles have received increasing attention since they facilitate all the reactions needed for “artificial photosynthesis”. In recent decades, hollow microspheres have provided an ideal platform for efficient utilization. Scientists are working to understand the basic principles, band structures, and modification strategies of hollow microspheres to enhance photocatalytic performance. In this paper, the research progress of hollow microsphere photocatalysts in the field of water splitting is reviewed. In particular, the photocatalytic principles of hollow microspheres and the methods to improve the performance of semiconductor photocatalysts are discussed in depth. The structural advantages and defects of hollow microspheres and modification methods of semiconductor band structure are introduced. Finally, the remaining challenges are summarized, and some insights into new trends and improvement directions for hollow materials are provided. This review will provide new insights for understanding hollow microspheres and help researchers in related fields to have a deeper and more comprehensive understanding of hollow microspheres in photocatalytic water splitting.     

A first-principles study of electronic structure and photocatalytic performance of two-dimensional van der Waals MTe2–As (M = Mo,W) heterostructures

To efficiently produce green energy and to overcome energy crises and environmental issues, photocatalytic water splitting has become the core heart of recent research. Fabricating heterostructures with type-II band alignment can enhance the photocatalytic activity. By first-principles computations, we study Mo(W)Te₂–As van der Waals (vdW) heterostructures as promising photocatalysts for overall water splitting. The bandgap, band edge position and optical properties can be modified by biaxial strain. With appropriate compressive strain of 2% and 3%, the WTe₂–As heterostructures show transition from type-I to type-II band alignment, which could slow down electron-hole pair recombination. Compared with Mo(W)Te₂ and As monolayers, the band edge of Mo(W)Te₂–As heterostructures is on favorable positions for straddling the water redox potentials. Moreover, Mo(W)Te₂–As heterostructures fascinatingly show strong absorption peaks in both visible and near ultra-violet region, making them promising candidates for overall water splitting photocatalysts.     

Photocatalytic membrane reactors for hydrogen production from water

Hydrogen is considered today a promising environmental friendly energy carrier for the next future, since it produces no air pollutants or greenhouse gases when it burns in air, and it possesses high energy capacity. In the last decades great attention has been devoted to hydrogen production from water splitting by photocatalysis. This technology appears very attractive thanks to the possibility to work under mild conditions producing no harmful by-products with the possibility to use renewable solar energy. Besides, it can be combined with the technology of membrane separations making the so-called photocatalytic membrane reactors (PMRs) where the chemical reaction, the recovery of the photocatalyst and the separation of products and/or intermediates simultaneously occur. In this work the basic principles of photocatalytic hydrogen generation from water splitting are reported, giving particular attention on the use of modified photocatalysts able to work under visible light irradiation. Several devices to achieve the photocatalytic hydrogen generation are presented focusing on the possibility to obtain pure hydrogen employing membrane systems and visible light irradiation. Although many efforts are still necessary to improve the performance of the process, membrane photoreactors seem to be promising for hydrogen production by overall water splitting in a cost-effective and environmentally sustainable way.     

Direct Z-scheme GaSe/ZrS2 heterojunction for overall water splitting

In this paper, a novel GaSe/ZrS₂ heterojunction is designed and systematically studied. The monolayers in this heterojunction can be easily fabricated and the heterojunction has good stability. More importantly, electronic structures, band alignment, differential charge density and the built-in electric field show that it is a direct Z-scheme photocatalyst that can not only effectively separate photogenerated carriers but also improve the redox ability of carriers. The mobility of the electrons in this heterostructure is as large as 1450.38 cm²V⁻¹s⁻¹ (same magnitude order of black phosphorene) along the zigzag direction, which is beneficial to photocatalytic performance. The band alignment shows that GaSe/ZrS₂ is suitable for overall water splitting and the splitting can happen spontaneously. Moreover, GaSe/ZrS₂ heterojunction has excellent absorption coefficients (higher than 10⁵cm⁻¹) in both visible and ultraviolet ranges which is very beneficial to photocatalytic overall water splitting. The study of electric field and biaxial strain effects on GaSe/ZrS₂ heterojunction show that the light absorption coefficients of GaSe/ZrS₂ heterojunction can be obviously further improved by applying biaxial strain. Our work shows that GaSe/ZrS₂ heterojunction is a promising photocatalysts for overall water splitting.     

Recent progress in photocatalysts for overall water splitting

Photocatalytic splitting of water with solar energy is considered as the most promising approach for the production of hydrogen fuel. However, its solar to hydrogen conversion efficiency is much below the industrial requirement (10%). This situation has stimulated intensive efforts to improve photocatalytic overall water splitting (namely, simultaneously providing unassisted oxidation and reduction of water), leading to the invention of novel catalysts in the recent years. The evaluation of these recent progresses constitutes this review article, with emphasis on the strategies employed for the development of catalysts. The catalysts were deeply reviewed and were classified into four types: (a) perovskite compounds, (b) metal oxides (sulfides and nitrides), (c) Bi‐ and In‐based materials, and (d) multicomponent catalysts. Furthermore, the challenges that remain with the process and catalysts and the potential advances were discussed as an outlook for future research.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H₂ evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H₂ production have been discussed. In the mainstream, recent developments in TiO₂ and g-C₃N₄ based photo-catalysts and their potential for H₂ production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H₂ production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H₂ production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.     

A review on bismuth-based composite oxides for photocatalytic hydrogen generation

Bismuth-based composite oxides are always considered the best visible-light photocatalysts for oxygen production. However, they are failed to photocatalytic reduce the hydrogen from water, due to their lower conduction band made up by Bi 6p and O 2p. Thus, it is significant to modulate their levels of the conduction and valence bands satisfying the redox potential for both H⁺/H₂ and O₂/H₂O, which will directly lead to discovering new visible-light materials for photocatalytic hydrogen generation. Recent years, some modified bismuth-based composite oxides have been reported to achieve photocatalytic hydrogen production. In this paper, a review of photocatalytic hydrogen generation by bismuth-based composite oxides is presented, mainly including energy band engineering, Z-scheme overall water splitting, and strategies for photocatalytic activity improvement.     

Design of cocatalyst loading position for photocatalytic water splitting into hydrogen in electrolyte solutions

Photocatalytic water splitting into hydrogen is a very attractive and desirable technology to realize sustainable and renewable green energy conversion. Up to now, many research results have confirmed that cocatalyst such as Pt is essential for a high efficiency photocatalytic H₂ evolution system. In a traditional view, the cocatalyst should be closely combined with the photocatalyst to achieve a high H₂ or O₂ photo-productive rate. In this work, an unusual point has been put forward that the suitable loading position of cocatalyst Pt for film-type TiO₂ catalyst is on the bare Fluorine-doped tin oxide (FTO) substrate instead of on the surface of TiO₂ in the electrolyte solutions. Especially, in acidic electrolyte, the hydrogen production rate of this new designed catalyst with Pt loaded on FTO (TiO₂-Pt/FTO) reaches 2.4 times that of the common catalyst with Pt loaded on TiO₂ (Pt/TiO₂-FTO). According to the experiment results, it is supposed that this loading way of cocatalyst on the substrate can construct a self-bias photocatalytic electrochemical cell system, drive electrolyte ions' movement directionally, and obtain high photocatalytic H₂ production efficiency. The universality of this innovation has been verified by CdS and CdS@TiO₂ film-type catalysts. This study provides a new guide in exploring high-efficiency film-type photocatalytic system for water splitting into hydrogen in the electrolyte solution.     

Analysis of a photochemical water splitting reactor with supramolecular catalysts and a proton exchange membrane

In this paper, a new type of photochemical water splitting system is proposed and analyzed. The system comprises two reactors in which photocatalytic half-reactions of water reduction and water oxidation are conducted. The two reactors are divided by a proton conducting membrane. Complex molecular devices based on ruthenium-(bipyridine)₃²⁺ photosensitizers are dissolved in both reactors, which generate electrons or holes when exposed to high energy photonic radiation, and act as catalysts for water splitting. The selected molecular devices for water reduction have a unique property to enhance the existence time of photoelectrons, such that the likelihood of generated electron pairs to produce a molecule of hydrogen is increased. In this study, the physical-chemical processes are analyzed and the main design parameters of the reactor are determined. The engineering design consists of a photochemical reactor, a mass flow controller to supply the fresh water, and two fans that extract the products of water splitting.     

Plasmonic hot electrons: Potential candidates for improved photocatalytic hydrogen production

The photocatalytic water splitting is an emerging way of solar to hydrogen conversion due to its cheap and clean nature. The plasmon excitations in metallic sub-wavelength nanostructures are improving the semi-conductor industry through broadening of absorption range, enhancing charge separation and surging optical density of states in coupled semi-conductor. Besides these traditional enhancement mechanisms, the semi-conductor can be externally charged by additional carriers generated from plasmon-decay in ultrafast time scales. The mechanisms behind the generation of these metallic hot electrons and their transfer to semiconductor for enhanced photocatalytic hydrogen production have been reviewed. Furthermore, the road barriers limiting the efficiency of photocatalytic water splitting and its possible solutions through coupling with plasmonic resonators are presented. The possible problems and the expected solutions are proposed with the help of reasonable literature providing perspective for future research. This research finds its importance not only in green energy but opto-electronic industry as well.     

Solar photocatalytic reactor performance for hydrogen production from incident ultraviolet radiation

Solar based hydrogen production is a promising alternative to methods based on fossil fuels, such as steam methane reforming (SMR) and coal gasification. A more economically viable way of producing hydrogen from water is under active investigation by many researchers, to convert solar energy to chemical energy with higher efficiency. In this paper, supramolecular complexes developed by Brewer (2006) for photocatalytic hydrogen production are examined, particularly for larger scale engineering reactors that can use visible light to dissolve the photocatalysts in water, causing the splitting of water molecules into hydrogen and hydroxyl ions. This paper analyzes and optimizes the system parameters associated with this system. A predictive model for the reactor is developed for a batch type photocatalytic reactor. Results are presented and discussed to evaluate how the system parameters affect the hydrogen production rate, and solar to hydrogen efficiency, using a monochromatic LED array and Rhodium based photocatalysts.     

Borate particulate photocatalysts for photocatalytic applications: A review

Borate is a kind of wide-bandgap semiconducting material with rich structure and variety, which is usually used in optical properties research. Its unique crystal structure has essential research value for expanding the applications of photocatalysis. This review summarizes the recent research progress of borate photocatalysis, including novel borate photocatalyst, borate-based composite, and borate glass photocatalyst. Furthermore, the energy and environmental photocatalysis applications of borate photocatalysts are discussed, such as overall water splitting reaction, water decomposition to generate hydrogen or oxygen, degradation of pollutants, and others. The strategy of the combination of theoretical calculation and experiment to explore new borate photocatalysts was also introduced. Finally, some developing directions and challenges in borate photocatalytic materials are summarized.     

Constructing a new 2D Janus black phosphorus/SMoSe heterostructure for spontaneous wide-spectral-responsive photocatalytic overall water splitting

Janus MoSSe monolayer with built-in electric dipole, as another emerging two-dimensional (2D) material after MoS₂, is predicted to be an ideal photocatalyst for overall water splitting. However, in spite of the excellent hydrogen evolution reaction (HER) activity of Se-surface, the extremely poor oxygen evolution reaction (OER) activity of S-surface hinders the achievement of photocatalytic overall water splitting. Herein, we construct a new 2D van der Waals heterostructure consisting of high-OER-active black phosphorus (BP) and Janus MoSSe monolayer, and demonstrate a new strategy of Janus BP/SMoSe heterostructure to achieve wide-spectral-responsive photocatalytic overall water splitting. The electronic structures and optical properties of two different heterostructures, BP/SMoSe and BP/SeMoS, are systematically investigated via first principles density, exhibiting a type-II band arrangement. Unlike BP/SeMoS, the BP/SMoSe heterostructure shows excellent optical properties, such as a large dielectric constant of 8.14 and a small optical absorption boundary of 0.10 eV. Furthermore, BP/SMoSe heterostructure possesses greater light absorption intensity and a broader light absorption range. It is found that the BP/SMoSe heterostructure exhibits proper band alignment and enhanced intrinsic dipole, which is favorable to obtain high electron-hole separation efficiency. This work provides a feasible strategy of 2D Janus BP/SMoSe heterostructure for approaching almost perfect overall water-splitting photocatalysis.     

State-of-the-art progress in overall water splitting of carbon nitride based photocatalysts

Converting solar energy into hydrogen (H₂) by photocatalytic water splitting is a promising approach to simultaneously address the increasing energy demand and environmental issues. Half decade has passed since the discovery of photo-induced water splitting phenomenon on TiO₂ photoanode, while the solar to H₂ efficiency is still around 1%, far below the least industrial requirement. Therefore, developing efficient photocatalyst with a high energy conversion efficiency is still one of the main tasks to be overcome. Graphitic carbon nitride (g-C₃N₄) is just such an emerging and potential semiconductor. Therefore, in this review, the state-of-the-art advances in g-C₃N₄ based photocatalysts for overall water splitting were summarized in three sections according to the strategies used, and future challenges and new directions were discussed.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

CdS@Mg(OH)2 core/shell composite photocatalyst for efficient visible-light photocatalytic overall water splitting

Coating a protective agent or promoter on the surface of the photocatalyst is a proven good strategy to realize photocatalytic hydrogen production from pure water, but remains still a considerable challenge. Herein, a novel CdS@Mg(OH)₂ core/shell composite nanorods photocatalyst was synthesized by coating Mg(OH)₂ on CdS surface by hydrothermal and precipitation processes. The coated-Mg(OH)₂ layer did not change the structure of CdS, and the photocatalytic overall water splitting performance of the CdS@Mg(OH)₂ under visible light irradiation was improved obviously. After loading nano-Pt via the photodeposited method, the hydrogen production rate and stability of Pt/CdS@Mg(OH)₂ were 3.3 and 2.4 times that of the Pt/CdS under the visible light irradiation, respectively. The surface Mg(OH)₂ layer improved the hydrophilicity and stability of the core/shell composites and increased the amount of active sites, thus improving the photocatalytic properties. It is believed that Mg(OH)₂ can be used as a new co-catalyst to enhance the performance of photocatalytic overall water splitting.     

Light intensity effects on photocatalytic water splitting with a titania catalyst

Photocatalytic water splitting is a process which could potentially lead to commercially viable solar hydrogen production. In order to evaluate if solar concentration could be used to increase the feasibility of the process, the effect of light intensity on photocatalytic water splitting was examined.