Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting

Carbonized polymer dots modified ZnIn2S4 microspheres for visible-light-driven hydrogen evolution promotion performance

To effectively separate electron-hole pairs produced by light, a heterojunction arrangement can be employed, thereby improving photocatalytic efficiency. In this study, a simple hydrothermal process is used to manufacture carbonized polymer dots/ZnIn2S4 (CPDs/ZIS) heterostructure, which enhances the light absorption and charge carrier lifetime in comparison to bare ZnIn2S4 (ZIS). Upon irradiation with visible light, the 3-CPDs/ZIS composite generates hydrogen at a rate of 133 μmol g-1 h-1, which is 8.9 times faster than that of pure ZIS. The addition of CPDs can increase the range of light that can be absorbed, extend the service life of the optical charge, increase the specific surface area, and promote charge separation and transmission, which could effectively accelerate the photocatalytic reduction reaction. The presence of CPDs results in the introduction of multiple transition energy states and a decrease in the H* adsorption free energy, which enhances the hydrogen evolution activity according to the theoretical calculation findings of density functional theory (DFT) and Gibbs free energy of the hydrogen evolution process. Combining theoretical calculations and experimental results, a direct Z-type heterojunction mechanism is proposed for the hydrogen evolution promotion effectiveness of CPDs/ZIS under visible light.

Nanoparticle Exsolution on Perovskite Oxides: Insights into Mechanism, Characteristics and Novel Strategies

Supported nanoparticles have attracted considerable attention as a promising catalyst for achieving unique properties in numerous applications, including fuel cells, chemical conversion, and batteries. Nanocatalysts demonstrate high activity by expanding the number of active sites, but they also intensify deactivation issues, such as agglomeration and poisoning, simultaneously. Exsolution for bottom-up synthesis of supported nanoparticles has emerged as a breakthrough technique to overcome limitations associated with conventional nanomaterials. Nanoparticles are uniformly exsolved from perovskite oxide supports and socketed into the oxide support by a one-step reduction process. Their uniformity and stability, resulting from the socketed structure, play a crucial role in the development of novel nanocatalysts. Recently, tremendous research efforts have been dedicated to further controlling exsolution particles. To effectively address exsolution at a more precise level, understanding the underlying mechanism is essential. This review presents a comprehensive overview of the exsolution mechanism, with a focus on its driving force, processes, properties, and synergetic strategies, as well as new pathways for optimizing nanocatalysts in diverse applications.

Key Strategies for Enhancing H2 Production in Transition Metal Oxide Based Photocatalysts

Transition metal oxides (TMOs) were one of the first photocatalysts used to produce hydrogen from water using solar energy. Despite the emergence of many other genres of photocatalysts over the years, TMO photocatalysts remain dominant due to their easy synthesis and unique physicochemical properties. Various strategies have been developed to enhance the photocatalytic activity of TMOs, but the solar-to-hydrogen (STH) conversion efficiency of TMO photocatalysts is still very low (<2 %), which is far below the targeted STH of 10 % for commercial viability. This article provides a comprehensive analysis of several widely used strategies, including oxygen defects control, doping, establishing interfacial junctions, and phase-facet-morphology engineering, that have been adopted to improve TMO photocatalysts. By critically evaluating these strategies and providing a roadmap for future research directions, this article serves as a valuable resource for researchers, students, and professionals seeking to develop efficient energy materials for green energy solutions.

Ti3C2-MXene/NiO Nanocomposites-Decorated CsPbI3 Perovskite Active Materials under UV-Light Irradiation for the Enhancement of Crystal-Violet Dye Photodegradation

Ti3C2-MXene material, known for its strong electronic conductivity and optical properties, has emerged as a promising alternative to noble metals as a cocatalyst for the development of efficient photocatalysts used in environmental cleanup. In this study, we investigated the photodegradation of crystal-violet (CV) dye when exposed to UV light using a newly developed photocatalyst known as Ti3C2-MXene/NiO nanocomposite-decorated CsPbI3 perovskite, which was synthesized through a hydrothermal method. Our research investigation into the structural, morphological, and optical characteristics of the Ti3C2-MXene/NiO/CsPbI3 composite using techniques such as FTIR, XRD, TEM, SEM-EDS mapping, XPS, UV-Vis, and PL spectroscopy. The photocatalytic efficacy of the Ti3C2-MXene/NiO/CsPbI3 composite was assessed by evaluating its ability to degrade CV dye in an aqueous solution under UV-light irradiation. Remarkably, the Ti3C2-MXene/NiO/CsPbI3 composite displayed a significant improvement in both the degradation rate and stability of CV dye when compared to the Ti3C2-MXene/NiO nanocomposite and CsPbI3 perovskite materials. Furthermore, the UV-visible absorption spectrum of the Ti3C2-MXene/NiO/CsPbI3 composite demonstrated a reduced band gap of 2.41 eV, which is lower than that of Ti3C2-MXene/NiO (3.10 eV) and Ti3C2-MXene (1.60 eV). In practical terms, the Ti3C2-MXene/NiO/CsPbI3 composite achieved an impressive 92.8% degradation of CV dye within 90 min of UV light exposure. We also confirmed the significant role of photogenerated holes and radicals in the CV dye removal process through radical scavenger trapping experiments. Based on our findings, we proposed a plausible photocatalytic mechanism for the Ti3C2-MXene/NiO/CsPbI3 composite. This research may open up new avenues for the development of cost-effective and high-performance MXene-based perovskite photocatalysts, utilizing abundant and sustainable materials for environmental remediation.

Self-assembly synthesis of hollow phosphorus-doped graphitic carbon nitride microboxes for the photodegradation of organic pollutants

The rational design of photocatalysts with efficiency and stability is highly desirable but remains challenging. Here, we report a supramolecular self-assembly strategy to construct hollow phosphorus-doped g-C3N4 microboxes (PCNMs). Considering the effects of multiple parameters on the structure and activity of samples, the orthogonal design is innovatively introduced to optimize technology parameters for screening high-performance g-C3N4. Under visible light irradiation (λ ≥ 420 nm), rhodamine B (RhB, 4 mg L-1) is completely degraded in just 80 seconds in the presence of the optimal PCNM. The kinetic rate constant of RhB degradation with the PCNM is 3.4633 min-1, demonstrating unprecedented activity that is about 112 times higher than that of bulk g-C3N4 (0.0309 min-1) synthesized by direct polycondensation of melamine. Additionally, the optimal PCNM also shows enhanced degradation efficiency for tetracycline. The outstanding properties are primarily attributed to the hollow architecture, high specific surface area, and phosphorus doping. This work advances the design of photocatalysts correlating various factors, opening an avenue for optimizing photocatalytic synthesis and activity.

Enhanced Light Trapping in GaAs/TiO2-Based Photocathodes for Hydrogen Production

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising materials due to their tunable band gaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuring of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light, and their improved charge separation and collection abilities and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO2 shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin-film counterpart. Morphological and electronic parameters, such as the aspect ratio of the nanowires and their doping pattern, were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co nanoparticles used as a hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities comparable with previous noble-metal-based systems. This study demonstrates the potential of a GaAs nanostructured semiconductor and its reliable use for photodriven hydrogen production.

Surface-Degenerate Semiconductor Photocatalysis for Efficient Water Splitting without Sacrificial Agents via a Reticular Chemistry Approach

The production of green hydrogen through photocatalytic water splitting is crucial for a sustainable hydrogen economy and chemical manufacturing. However, current approaches suffer from slow hydrogen production (<70 μmol ⋅ gcat -1  ⋅ h-1 ) due to the sluggish four-electrons oxygen evolution reaction (OER) and limited catalyst activity. Herein, we achieve efficient photocatalytic water splitting by exploiting a multifunctional interface between a nano-photocatalyst and metal-organic framework (MOF) layer. The functional interface plays two critical roles: (1) enriching electron density directly on photocatalyst surface to promote catalytic activity, and (2) delocalizing photogenerated holes into MOF to enhance OER. Our photocatalytic ensemble boosts hydrogen evolution by ≈100-fold over pristine photocatalyst and concurrently produces oxygen at ideal stoichiometric ratio, even without using sacrificial agents. Notably, this unique design attains superior hydrogen production (519 μmol ⋅ gcat -1  ⋅ h-1 ) and apparent quantum efficiency up to 13-fold and 8-fold better than emerging photocatalytic designs utilizing hole scavengers. Comprehensive investigations underscore the vital role of the interfacial design in generating high-energy photoelectrons on surface-degenerate photocatalyst to thermodynamically drive hydrogen evolution, while leveraging the nanoporous MOF scaffold as an effective photohole sink to enhance OER. Our interfacial approach creates vast opportunities for designing next-generation, multifunctional photocatalytic ensembles using reticular chemistry with diverse energy and environmental applications.

Enhanced photoactivity towards bismuth vanadate water splitting through tantalum doping: An experimental and density functional theory study

The activation of hole trap states in bismuth vanadate (BiVO4) is considered an effective strategy to enhance the photoelectrochemical (PEC) water-splitting activity. Herein, we propose a theoretical and experimental study of tantalum (Ta) doping to BiVO4 leading to the introduction of hole trap states for the enhanced PEC activity. The doping of Ta is found to alter the structural and chemical surroundings via displacement of vanadium (V) atoms that cause distortions in the lattice via the formation of hole trap states. A significant enhancement of photocurrent to ∼4.2 mA cm-2 was recorded attributing to the effective charge separation of efficiency of ∼96.7 %. Furthermore, the doping of Ta in the BiVO4 lattice offers improved charge transport in bulk and decreased charge transfer resistance at the electrolyte interface. The Ta-doped BiVO4 displays the effective production of hydrogen (H2) and oxygen (O2) under AM 1.5 G illumination with a faradaic efficiency of 90 %. Moreover, the density functional theory (DFT) study confirms the decrease in optical band gap and the activation of hole trap states below the conduction band (CB) with a contribution of Ta towards both valence and CB that increases the charge separation and majority charge carrier density, respectively. The findings of this work propose that the displacement of V sites with Ta atoms in BiVO4 photoanodes is an efficient approach for enhanced PEC activity.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Progress on Single-Atom Photocatalysts for H2 Generation: Material Design, Catalytic Mechanism, and Perspectives

Solar energy utilization is of great significance to current challenges of the energy crisis and environmental pollution, which benefit the development of the global community to achieve carbon neutrality goals. Hydrogen energy is also treated as a good candidate for future energy supply since its combustion not only supplies high-density energy but also shows no pollution gas. In particular, photocatalytic water splitting has attracted increasing research as a promising method for H2 production. Recently, single-atom (SA) photocatalysts have been proposed as a potential solution to improve catalytic efficiency and lower the costs of photocatalytic water splitting for H2 generation. Owing to the maximized atom utilization rate, abundant surface active sites, and tunable coordination environment, SA photocatalysts have achieved significant progress. This review reviews developments of advanced SA photocatalysts for H2 generation regarding the different support materials. The recent progress of titanium dioxide, metal-organic frameworks, two-dimensional carbon materials, and red phosphorus supported SA photocatalysts are carefully discussed. In particular, the material designs, reaction mechanisms, modulation strategies, and perspectives are highlighted for realizing improved solar-to-energy efficiency and H2 generation rate. This work will supply significant references for future design and synthesis of advanced SA photocatalysts.

Photothermal Effect- and Interfacial Chemical Bond-Modulated NiOx/Ta3N5 Heterojunction for Efficient CO2 Photoreduction

Photothermal catalysis, which combines light promotion and thermal activation, is a promising approach for converting CO2 into fuels. However, the development of photothermal catalysts with effective light-to-heat conversion, strong charge transfer ability, and suitable active sites remains a challenge. Herein, the photothermal effect- and interfacial N-Ni/Ta-O bond-modulated heterostructure composed of oxygen vacancy-rich NiOx and Ta3N5 was rationally fabricated for efficient photothermal catalytic CO2 reduction. Beyond the charge separation capability conferred by the NiOx/Ta3N5 heterojunction, we observed that the N-Ni and Ta-O bonds linking NiOx and Ta3N5 form a spatial charge transfer channel, which enhances the interfacial electron transfer. Additionally, the presence of surface oxygen vacancies in NiOx induced nonradiative relaxation, resulting in a pronounced photothermal effect that locally heated the catalyst and accelerated the reaction kinetically. Leveraging these favorable factors, the NiOx/Ta3N5 hybrids exhibit remarkably elevated activity (≈32.3 μmol·g-1·h-1) in the conversion of CO2 to CH4 with near-unity selectivity, surpassing the performance of bare Ta3N5 by over 14 times. This study unveils the synergistic effect of photothermal and interfacial chemical bonds in the photothermal-photocatalytic heterojunction system, offering a novel approach to enhance the reaction kinetics of various catalysts.

Molecule level precise construction of donor-acceptor polymeric carbon nitride for photocatalytic hydrogen evolution

Constructing donor-acceptor structures in polymeric carbon nitride (CN) provides an attractive pathway for facilitating charge carrier separation in photocatalytic reactions. However, achieving the implantation of donor or acceptor moieties at molecule level precision remains challenging. Here we develop a three-dimensional (3D) porous thiophene implanted carbon nitride (TCN) with donor-acceptor structure via a supramolecular assembly strategy. The specific-designed triazine derivatives with similar hydrogen bonding sites allow for the uniform introduction of thiophene groups at molecule level precision during the supramolecular assemble stage. The electron-donating thiophene groups in TCN can continuously tune electronic band structure, expand visible light absorption range, and promote charge carriers' separation. The optimized properties enable TCN-3 an outstanding H2 evolution rate of 5620 μmol h-1 g-1, greatly exceeding bulk CN (95 μmol h-1 g-1). Briefly, our work may offer opportunities to prepare highly active photocatalysts with molecule level precise donor-acceptor structure.

Efficient photocatalytic hydrogen production over ZnIn2S4 by producing sulfur vacancies and coupling with nickel-based polyoxometalate

A composite catalytic system using sulfur-vacancy-containing ZnIn2S4-Sv as a light-harvesting material and nickel-based polyoxometalate Na6K4[Ni4(H2O)2(PW9O34)2] (Ni4POM) as a co-catalyst was developed. The Ni4POM/ZnIn2S4-Sv composite gave a good hydrogen production rate of 337.5 μmol h-1, a value 11.8 times higher than that of ZnIn2S4-Sv. The direction of electron transfer, from ZnIn2S4-Sv to Ni4POM, was verified using surface photovoltage spectra.

Regulation of d-Band Centers in Localized CdS Homojunctions through Facet Control for Improved Photocatalytic Water Splitting

The accelerated kinetic behaviour of charge carrier transfer and its unhindered surface reaction dynamic process involving oxygenated-intermediate activation and conversion are urgently required in photocatalytic water (H2 O) overall splitting, which has not been nevertheless resolved yet. Herein, localized CdS homojunctions with optimal collocation of high and low index facets to regulate d-band center for chemically adsorbing and activating key intermediates (*-OH and *-O) have been achieved in H2 O overall splitting into hydrogen. Density functional theory, hall effect, and in situ diffuse reflectance infrared Fourier transform spectroscopy confirm that, electrons and holes are kinetically transferred to reductive high index facet (002) and oxidative low index facet (110) of the localized CdS homojunction induced by facet Fermi level difference to dehydrogenate *-OH and couple *-O for hydrogen and oxygen evolution, respectively, along with a solar conversion into hydrogen (STH) of 2.20 % by Air Mass 1.5 Global filter irradiation. These findings contribute to solving the kinetic bottleneck issues of photocatalytic H2 O splitting, which will further enhance STH.

A Data-Driven Search of Two-Dimensional Covalent Organic Frameworks for Visible-Light-Driven Overall Water Splitting

Two-dimensional (2D) covalent organic frameworks (COFs) with versatile structural and optoelectronic properties that can be tuned with building blocks and topological structures have received widespread attention for photocatalytic water splitting in recent years. However, few of these have been reported for overall water splitting under visible light. Here, we present a data-driven search of 2D COFs capable of visible-light-driven overall water splitting by combining high-throughput first-principles computations and experimental validations. Seven 2D COFs were identified to be capable of overall water splitting from the CoRE COF database, and their photocatalytic activities were further verified and optimized by our preliminary experiments. The production rates of H2 and O2 reached 80 and 32 μmol g-1 h-1, respectively, without using sacrificial agents. This work represents an attempt to explore 2D COFs for visible-light-driven overall water splitting with a data-driven approach that could accelerate the discovery and design of COFs toward photocatalytic overall water splitting.

Three-Dimensional Fully Conjugated Covalent Organic Frameworks for Efficient Photocatalytic Water Splitting

Covalent organic frameworks (COFs) are promising photocatalysts for water splitting, but their efficiency lags behind that of inorganic counterparts partly due to the limited charge transport and optical absorption properties. To overcome this limitation, we proposed to employ three-dimensional (3D) fully conjugated (FC) COFs with a topological assembly of cyclooctatetraene derivatives for photocatalytic water splitting. On the basis of first-principles calculations, we demonstrated that these 3D FC-COFs are semiconductors with exceptional charge transport and optical absorption properties. The carrier mobilities are comparable to those of inorganic semiconductors and superior to the record mobility observed in two-dimensional COFs. Additionally, the 3D FC-COFs exhibit broad visible light absorption with direct band gaps and high optical absorption coefficients. Among them, two 3D FC-COFs are identified for overall water splitting, while three others can facilitate the hydrogen evolution half-reaction. This study pioneers the design of 3D FC-COF photocatalysts, potentially advancing their applications in photocatalysis and optoelectronics.

All-Optical Electrochemiluminescence

Electrochemiluminescence (ECL) is widely employed for medical diagnosis and imaging. Despite its remarkable analytical performances, the technique remains intrinsically limited by the essential need for an external power supply and electrical wires for electrode connections. Here, we report an electrically autonomous solution leading to a paradigm change by designing a fully integrated all-optical wireless monolithic photoelectrochemical device based on a nanostructured Si photovoltaic junction modified with catalytic coatings. Under illumination with light ranging from visible to near-infrared, photogenerated holes induce the oxidation of the ECL reagents and thus the emission of visible ECL photons. The blue ECL emission is easily viewed with naked eyes and recorded with a smartphone. A new light emission scheme is thus introduced where the ECL emission energy (2.82 eV) is higher than the excitation energy (1.18 eV) via an intermediate electrochemical process. In addition, the mapping of the photoelectrochemical activity by optical microscopy reveals the minority carrier interfacial transfer mechanism at the nanoscale. This breakthrough provides an all-optical strategy for generalizing ECL without the need for electrochemical setups, electrodes, wiring constraints, and specific electrochemical knowledge. This simplest ECL configuration reported so far opens new opportunities to develop imaging and wireless bioanalytical systems such as portable point-of-care sensing devices.

Selective Exposure of Robust Perovskite Layer of Aurivillius-Type Compounds for Stable Photocatalytic Overall Water Splitting

Aurivillius-type compounds ((Bi2 O2 )2+ (An -1 Bn O3 n +1 )2- ) with alternately stacked layers of bismuth oxide (Bi2 O2 )2+ and perovskite (An -1 Bn O3 n +1 )2- are promising photocatalysts for overall water splitting due to their suitable band structures and adjustable layered characteristics. However, the self-reduction of Bi3+ at the top (Bi2 O2 )2+ layers induced by photogenerated electrons during photocatalytic processes causes inactivation of the compounds as photocatalysts. Here, using Bi3 TiNbO9 as a model photocatalyst, its surface termination is modulated by acid etching, which well suppresses the self-corrosion phenomenon. A combination of comprehensive experimental investigations together with theoretical calculations reveals the transition of the material surface from the self-reduction-sensitive (Bi2 O2 )2+ layer to the robust (BiTiNbO7 )2- perovskite layer, enabling effective electron transfer through surface trapping and effective hole transfer through surface electric field, and also efficient transfer of the electrons to the cocatalyst for greatly enhanced photocatalytic overall water splitting. Moreover, this facile modification strategy can be readily extended to other Aurivillius compounds (e.g., SrBi2 Nb2 O9 , Bi4 Ti3 O12 , and SrBi4 Ti4 O15 ) and therefore justify its usefulness in rationally tailoring surface structures of layered photocatalysts for high photocatalytic overall water-splitting activity and stability.

Metallic Powder Promotes Nitridation Kinetics for Facile Synthesis of (Oxy)Nitride Photocatalysts

Nitrogen-containing semiconductors (including metal nitrides, metal oxynitrides, and nitrogen-doped metal oxides) have been widely researched for their application in energy conversion and environmental purification because of their unique characteristics; however, their synthesis generally encounters significant challenges owing to sluggish nitridation kinetics. Herein, a metallic-powder-assisted nitridation method is developed that effectively promotes the kinetics of nitrogen insertion into oxide precursors and exhibits good generality. By employing metallic powders with low work functions as electronic modulators, a series of oxynitrides (i.e., LnTaON2 (Ln = La, Pr, Nd, Sm, and Gd), Zr2 ON2 , and LaTiO2 N) can be prepared at lower nitridation temperatures and shorter nitridation periods to obtain comparable or even lower defect concentrations compared to those of the conventional thermal nitridation method, leading to superior photocatalytic performance. Moreover, some novel nitrogen-doped oxides (i.e., SrTiO3- x Ny and Y2 Zr2 O7- x Ny ) with visible-light responses can be exploited. As revealed by density functional theory (DFT) calculations, the nitridation kinetics are enhanced via the effective electron transfer from the metallic powder to the oxide precursors, reducing the activation energy of nitrogen insertion. The modified nitridation route developed in this work is an alternative method for preparing (oxy)nitride-based materials for energy/environment-related heterogeneous catalysis.

High-performance ZnIn2S4/Ni(dmgH)2 for photocatalytic hydrogen evolution: Ion exchange construction, photocorrosion mitigation, and efficiency enhancement by photochromic effect

In this work, a novel photocatalyst of ZnIn₂S₄/Ni(dmgH)₂ was designed by a simple chemical precipitation method and used to enhance hydrogen evolution under visible light irradiation. Along with vigorous discharges of hydrogen bubbles, an optimal rate of 36.3 mmol/g/h was reached under UV-Vis light for hydrogen evolution, nearly 4.9 times of the one from pure ZnIn₂S₄. The heterojunction exhibits steady hydrogen evolution capability and owns a high apparent quantum efficiency (AQE) of 20.45% under the monochromatic light at 420 nm. By coupling ZnIn₂S₄ with Ni(dmgH)₂, an extraordinary photochromic phenomenon was detected and attributed to the active Ni-S component in situ formed between the nickel and sulfur composites under light irradiation. The emerging sulfide benefits light absorption of the system and separation of photogenerated electron and hole pairs. Besides providing a promising photocatalyst for visible light hydrogen production, the present work is hoped to inspire new trends of catalytic medium designs and investigations.

Bifunctional Hot Water Vapor Template-Mediated Synthesis of Nanostructured Polymeric Carbon Nitride for Efficient Hydrogen Evolution

Regulating bulk polymeric carbon nitride (PCN) into nanostructured PCN has long been proven effective in enhancing its photocatalytic activity. However, simplifying the synthesis of nanostructured PCN remains a considerable challenge and has drawn widespread attention. This work reported the one-step green and sustainable synthesis of nanostructured PCN in the direct thermal polymerization of the guanidine thiocyanate precursor via the judicious introduction of hot water vapor's dual function as gas-bubble templates along with a green etching reagent. By optimizing the temperature of the water vapor and polymerization reaction time, the as-prepared nanostructured PCN exhibited a highly boosted visible-light-driven photocatalytic hydrogen evolution activity. The highest H₂ evolution rate achieved was 4.81mmol∙g^-1∙h^-1, which is over four times larger than that of the bulk PCN (1.19 mmol∙g^-1∙h^-1) prepared only by thermal polymerization of the guanidine thiocyanate precursor without the assistance of bifunctional hot water vapor. The enhanced photocatalytic activity might be attributed to the enlarged BET specific surface area, increased active site quantity, and highly accelerated photo-excited charge-carrier transfer and separation. Moreover, the sustainability of this environmentally friendly hot water vapor dual-function mediated method was also shown to be versatile in preparing other nanostructured PCN photocatalysts derived from other precursors such as dicyandiamide and melamine. This work is expected to provide a novel pathway for exploring the rational design of nanostructured PCN for highly efficient solar energy conversion.

Tandem cells for unbiased photoelectrochemical water splitting

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

Integrated Coupling Utilization of the Solar Full Spectrum for Promoting Water Splitting Activity over a CIZS Semiconductor

Most of the existing photocatalysts can only use ultraviolet light and part of visible light, so broadening the spectrum response range and realizing the full spectrum coverage are key measures to improve the solar-to-hydrogen (STH) efficiency of photocatalytic water splitting. A spatially separated photothermal coupled photocatalytic (PTC) reaction system was designed using carbonized melamine foam (C-MF) as a substrate to absorb visible and infrared light and Cu_0.04In_0.25ZnS_y@Ru (CIZS@Ru) as a photocatalyst to absorb UV-visible light (UV-vis). By comparing the three modes of bottom, liquid level, and self-floating, it is found that the surface temperature of the system has a significant effect on the hydrogen evolution activity. The monochromatic light and activation energy experiments verify that the enhancement of photocatalytic activity comes from the strengthened photothermal effect of the substrate. Combined with theoretical calculations, it is further confirmed that the introduction of photothermal materials provides additional kinetic energy for carrier transmission and promotes directional carrier transmission efficiency. Based on the photoenergy-thermal integrated catalytic strategy, the hydrogen production rate reaches 603 mmol h^-1 m^-2. The structural design of photocatalysis has potential application in the field of photoenergy-fuel conversion.

Superexchange-induced Pt-O-Ti3+ site on single photocatalyst for efficient H2 production with organics degradation in wastewater

Efficient photocatalytic H2 production from wastewater instead of pure water is a dual solution to the environmental and energy crisis, but due to the rapid recombination of photoinduced charge in the photocatalyst and inevitable electron depletion caused by organic pollutants, a significant challenge of dual-functional photocatalysis (simultaneous oxidative and reductive reactions) in single catalyst is designing spatial separation path for photogenerated charges at atomic level. Here, we designed a Pt-doped BaTiO3 single catalyst with oxygen vacancies (BTPOv) that features Pt-O-Ti3+ short charge separation site, which enables excellent H2 production performance (1519 μmol·g-1·h-1) while oxidizing moxifloxacin (k = 0.048 min-1), almost 43 and 98 times than that of pristine BaTiO3 (35 μmol·g-1·h-1 and k = 0.00049 min-1). The efficient charge separation path is demonstrated that the oxygen vacancies extract photoinduced charge from photocatalyst to catalytic surface, and the adjacent Ti3+ defects allow rapid migration of electrons to Pt atoms through the superexchange effect for H* adsorption and reduction, while the holes will be confined in Ti3+ defects for oxidation of moxifloxacin. Impressively, the BTPOv shows an exceptional atomic economy and potential for practical applications, a best H2 production TOF (370.4 h-1) among the recent reported dual-functional photocatalysts and exhibiting excellent H2 production activity in multiple types of wastewaters.

Recent Advances in Phase-Engineered Photocatalysts: Classification and Diversified Applications

Phase engineering is an emerging strategy for tuning the electronic states and catalytic functions of nanomaterials. Great interest has recently been captured by phase-engineered photocatalysts, including the unconventional phase, amorphous phase, and heterophase. Phase engineering of photocatalytic materials (including semiconductors and cocatalysts) can effectively affect the light absorption range, charge separation efficiency, or surface redox reactivity, resulting in different catalytic behavior. The applications for phase-engineered photocatalysts are widely reported, for example, hydrogen evolution, oxygen evolution, CO₂ reduction, and organic pollutant removal. This review will firstly provide a critical insight into the classification of phase engineering for photocatalysis. Then, the state-of-the-art development of phase engineering toward photocatalytic reactions will be presented, focusing on the synthesis and characterization methodologies for unique phase structure and the correlation between phase structure and photocatalytic performance. Finally, personal understanding of the current opportunities and challenges of phase engineering for photocatalysis will also be provided.

Recent Advances in Carbon Nitride-Based S-scheme Photocatalysts for Solar Energy Conversion

Energy shortages are a major challenge to the sustainable development of human society, and photocatalytic solar energy conversion is a potential way to alleviate energy problems. As a two-dimensional organic polymer semiconductor, carbon nitride is considered to be the most promising photocatalyst due to its stable properties, low cost, and suitable band structure. Unfortunately, pristine carbon nitride has low spectral utilization, easy recombination of electron holes, and insufficient hole oxidation ability. The S-scheme strategy has developed in recent years, providing a new perspective for effectively solving the above problems of carbon nitride. Therefore, this review summarizes the latest progress in enhancing the photocatalytic performance of carbon nitride via the S-scheme strategy, including the design principles, preparation methods, characterization techniques, and photocatalytic mechanisms of the carbon nitride-based S-scheme photocatalyst. In addition, the latest research progress of the S-scheme strategy based on carbon nitride in photocatalytic H₂ evolution and CO₂ reduction is also reviewed. Finally, some concluding remarks and perspectives on the challenges and opportunities for exploring advanced nitride-based S-scheme photocatalysts are presented. This review brings the research of carbon nitride-based S-scheme strategy to the forefront and is expected to guide the development of the next-generation carbon nitride-based S-scheme photocatalysts for efficient energy conversion.

Photocatalysts for a sustainable future: Innovations in large-scale environmental and energy applications

The growing environmental and energy crises have prompted researchers to seek new solutions, including large-scale photocatalytic environmental remediation and the production of solar hydrogen using photocatalytic materials. To achieve this goal, scientists have developed numerous photocatalysts with high efficiency and stability. However, the large-scale application of photocatalytic systems under real-world conditions is still limited. These limitations arise at every step, including the large-scale synthesis and deposition of photocatalyst particles on a solid support, and the development of an optimal design with high mass transfer and efficient photon absorption. The purpose of this article is to provide a detailed description of the primary challenges and potential solutions encountered in scaling up photocatalytic systems for use in large-scale water and air purification and solar hydrogen production. Additionally, based on a review of current pilot developments, we draw conclusions and make comparisons regarding the main operating parameters that affect performance, as well as propose strategies for future research.

Interface-Engineered Ni-Coated CdTe Heterojunction Photocathode for Enhanced Photoelectrochemical Hydrogen Evolution

Photoelectrochemical (PEC) water splitting for hydrogen production using the CdTe photocathode has attracted much interest due to its excellent sunlight absorption property and energy band structure. This work presents a study of engineered interfacial energetics of CdTe photocathodes by deposition of CdS, TiO₂, and Ni layers. A heterostructure CdTe/CdS/TiO₂/Ni photocathode was fabricated by depositing a 100-nm n-type CdS layer on a p-type CdTe surface, with 50 nm TiO₂ as a protective layer and a 10 nm Ni layer as a co-catalyst. The CdTe/CdS/TiO₂/Ni photocathode exhibits a high photocurrent density (J_ph) of 8.16 mA/cm² at 0 V versus reversible hydrogen electrode (V_RHE) and a positive-shifted onset potential (E_onset) of 0.70 V_RHE for PEC hydrogen evolution under 100 mW/cm² AM1.5G illumination. We further demonstrate that the CdTe/CdS p-n junction promotes the separation of photogenerated carriers, the TiO₂ layer protects the electrode from corrosion, and the Ni catalyst improves the charge transfer across the electrode/electrolyte interface. This work provides new insights for designing noble metal-free photocathodes toward solar hydrogen development.

Light-Induced Bipolar Photoresponse with Amplified Photocurrents in an Electrolyte-Assisted Bipolar p-n Junction

The p-n junction with bipolar characteristics sets the fundamental unit to build electronics while its unique rectification behavior constrains the degree of carrier tunability for expanded functionalities. Herein, a bipolar-junction photoelectrode employed with a gallium nitride (GaN) p-n homojunction nanowire array that operates in electrolyte is reported, demonstrating bipolar photoresponse controlled by different wavelengths of light. Significantly, with rational decoration of a ruthenium oxides (RuO_x ) layer on nanowires guided by theoretical modeling, the resulting RuO_x /p-n GaN photoelectrode exhibits unambiguously boosted bipolar photoresponse by an enhancement of 775% and 3000% for positive and negative photocurrents, respectively, compared to the pristine nanowires. The loading of the RuO_x layer on nanowire surface optimizes surface band bending, which facilitates charge transfer across the GaN/electrolyte interface, meanwhile promoting the efficiency of redox reaction for both hydrogen evolution reaction and oxygen evolution reaction which corresponds to the negative and positive photocurrents, respectively. Finally, a dual-channel optical communication system incorporated with such photoelectrode is constructed with using only one photoelectrode to decode dual-band signals with encrypted property. The proposed bipolar device architecture presents a viable route to manipulate the carrier dynamics for the development of a plethora of multifunctional optoelectronic devices for future sensing, communication, and imaging systems.

Molecular Characteristics of Water-Insoluble Tin-Porphyrins for Designing the One-Photon-Induced Two-Electron Oxidation of Water in Artificial Photosynthesis

Faced with the new stage of water oxidation by molecular catalysts (MCs) in artificial photosynthesis to overcome the bottle neck issue, the "Photon-flux density problem of sunlight," a two-electron oxidation process forming H₂O₂ in place of the conventional four-electron oxidation evolving O₂ has attracted much attention. The molecular characteristics of tin(IV)-tetrapyridylporphyrin (SnTPyP), as one of the most promising MCs for the two-electron water oxidation, has been studied in detail. The protolytic equilibria among nine species of SnTPyP, with eight pK_a values on the axial ligands' water molecules and peripheral pyridyl nitrogen atoms in both the ground and excited states, have been clarified through the measurements of UV-vis, fluorescence, ¹H NMR, and dynamic fluorescence decay behaviour. The oxidation potentials in the Pourbaix diagram and spin densities by DFT calculation of the one-electron oxidized form of each nine species have predicted that the fully deprotonated species ([SnTPyP(O^-)₂]^2-) and the singly deprotonated one ([SnTPyP(OH)(O^-)]^-) serve as the most favourable MCs for visible light-induced two-electron water oxidation when they are adsorbed on TiO₂ for H₂ formation or SnO₂ for Z-scheme CO₂ reduction in the molecular catalyst sensitized system of artificial photosynthesis.

Cocatalysts for Photocatalytic Overall Water Splitting: A Mini Review

Photocatalyst overall water splitting is usually restricted by low carrier separation efficiency and a slow surface reaction rate. Cocatalysts provide a satisfactory solution to significantly improve photocatalytic performance. In this review, some recent advances in cocatalysts for photocatalytic overall water splitting are gathered and divided into groups. Firstly, the loading method of the cocatalyst is introduced. Then, the role of the cocatalyst applied for the photocatalytic overall water splitting process is further discussed. Finally, the key challenges and possible research directions of photocatalytic overall water splitting are proposed. This review is expected to promote research on the design of efficient cocatalysts in photocatalytic systems for overall water splitting.