Reaction systems for solar hydrogen production via water splitting with particulate semiconductor photocatalysts

Efficient photocatalytic overall water vapor splitting over amorphous Ni(OH)2/Ni2B heterojunctions

Developing efficient photocatalysts for solar-driven overall water vapor splitting is crucial for sustainable hydrogen production. However, current photocatalytic efficiencies are limited by inadequate hygroscopicity, sluggish proton transport, and rapid recombination of photogenerated electron-hole pairs. Here, we report the successful synthesis of an amorphous Ni(OH)2/Ni2B heterojunction material with a core-shell structure by regulating the reducing environment during the formation of nickel boride. This material exhibits highly efficient overall water vapor splitting performance without any cocatalysts. Under simulated solar irradiation, the optimized sample achieves a hydrogen production rate of 976 μmol·g-1·h-1, with near-stoichiometric evolution of hydrogen and oxygen, an apparent quantum yield of 5.4 %, and a solar-to-hydrogen conversion efficiency of 3.8 %. The enhanced performance is attributed to the unique amorphous/amorphous heterojunction structure that promotes effective charge separation, the abundant surface hydroxyl groups that improve proton transport and hygroscopicity, and the formation of photo-induced frustrated Lewis pairs (FLPs). Our findings shed light on the critical role of amorphous structures and surface chemistry in boosting photocatalytic activity, paving the way for the rational design of advanced photocatalysts for overall water vapor splitting.

Isolated asymmetric CoN4 sites on nitrogen-doped hollow carbons as electrocatalysts for hydrogen evolution reactions in dual pH electrolytes

Atomic transition metal-nitrogen (M-Nx) active sites dispersed on carbon matrix have emerged as highly promising electrocatalysts for energy conversion systems, owing to their maximized metal center utilization and excellent conductivity of the carbon substrate. However, conventional symmetric M-N4 configurations often exhibit limited hydrogen evolution reaction (HER) activity in alkaline due to the inferior water dissociation capacity. In this work, isolated Co single atoms anchored on N-doped hollow carbon substrates (CoSA/NHCs) are successfully fabricated by the pyrolysis of Co2+-containing polypyrrole (Co-PPY) precursor, and the subsequent removal of embedded Co particles. Thanks to the high density of atomically distributed asymmetric Co-N4 moieties, which facilitate efficient water activation andoptimal adsorption of the active hydrogen (H*), the synthesized CoSA/NHCs electrocatalysts show remarkable HER activity and stability in alkaline electrolyte. Besides, they can effectively electrolyze HER in acidic medium, undergoing the Volmer-Heyrovsky pathway. Remarkably, CoSA/NHC-800, pyrolyzed at 800 °C, requires small overpotentials of 230 and 228 mV to achieve the current density of -10 mA cm-2 in 1.0 M KOH and 0.5 M H2SO4 solutions, respectively. Moreover, it displays small Tafel slopes, high Faradic efficiencies (∼100 %) and superior long-term stability. This study paves a strategic approach for designing high-performance single-atom electrocatalysts through asymmetric structural engineering.

D1-D2-A ternary conjugated microporous polymers synthesized via direct CH arylation for enhancing photocatalytic hydrogen evolution

Conjugated microporous polymers (CMPs), featured by broad tunability in molecule design, structure and properties, have been widely used as photocatalysts for water splitting to produce hydrogen. However, the conventional donor-acceptor (D-A) binary CMPs have not achieved satisfactory performance so far. In this contribution, a series of D1-D2-A ternary CMPs are synthesized by the atom-economical direct CH arylation polymerization (DArP), wherein the dibenzo[b,d]thiophene-S,S-dioxide (BTDO), tetraphenylethylene (TPE) and 3,4-ethylenedioxythiophene (EDOT) units serve as the acceptor (A), donor D1 and donor D2, respectively. The structure-property correlations of the CMPs are systematically investigated by optical, electrochemical, water contact angle, and hydrogen production performance tests, revealing that the ternary D1-D2-A CMPs can maximize hydrophilicity and charge separation through the synergistic effect of BTDO, EDOT, and TPE building blocks. As a result, the ternary CMP-3 with an optimal D/A ratio achieves the highest photocatalytic hydrogen evolution rate up to 81.4 mmol g-1 h-1 without the aid of Pt co-catalyst, which has a 26-fold and 101-fold improvement compared to the pristine D1-A and D1-D2 binary CMPs, respectively. Meanwhile, a high apparent quantum yield of 11.1 % at 500 nm is successfully achieved. Density functional theory calculation discloses that D1-D2-A ternary CMPs possess the desirable molecular geometry and superior charge separation. This work provides a new design and synthetic strategy for the high-performance CMP-based photocatalysts.

Chemistry of Materials Underpinning Photoelectrochemical Solar Fuel Production

Since its inception, photoelectrochemistry has sought to power the generation of fuels, particularly hydrogen, using energy from sunlight. Efficient and durable photoelectrodes, however, remain elusive. Here we review the current state of the art, focusing our discussion on advances in photoelectrodes made in the past decade. We open by briefly discussing fundamental photoelectrochemical concepts and implications for photoelectrode function. We next review a broad range of semiconductor photoelectrodes broken down by material class (oxides, nitrides, chalcogenides, and mature photovoltaic semiconductors), identifying intrinsic properties and discussing their influence on performance. We then identify innovative in situ and operando techniques to directly probe the photoelectrode|electrolyte interface, enabling direct assessment of structure-property relationships for catalytic surfaces in active reaction environments. We close by considering more complex photoelectrochemical fuel-forming reactions (carbon dioxide and nitrogen reduction, as well as alternative oxidation reactions), where product selectivity imposes additional criteria on electrochemical driving force and photoelectrode architecture. By contextualizing recent literature within a fundamental framework, we seek to provide direction for continued progress toward achieving efficient and stable fuel-forming photoelectrodes.

Hybrid Perovskite Solar Cells: A Disruptive Technology for Hydrogen Production through Photocatalytic Water Splitting

Perovskite solar cells (PSCs) have recently emerged as a viable technology for photovoltaic applications, offering high efficiency and cost-effective manufacturing. Beyond generating electricity, PSCs can also facilitate hydrogen production through water splitting. This article provides a comprehensive review of current research on PSCs for hydrogen production, highlighting their potential as a transformative technology in this field. The challenges and opportunities associated with using PSCs for hydrogen production are discussed, including their stability and efficiency under various operating conditions. The impact of device design, system integration, and materials engineering on PSC performance for hydrogen production is also examined. Furthermore, an overview of hydrogen demand is provided and how PSCs can be integrated with other renewable energy sources to contribute to a sustainable energy future through green hydrogen production is explored. The analysis suggests that hydrogen production using PSCs has the potential to become a groundbreaking technology, significantly impacting the energy sector and the transition to low-carbon energy.

Highest Solar-to-Hydrogen Conversion Efficiency in Cu2ZnSnS4 Photocathodes and Its Directly Unbiased Solar Seawater Splitting

Despite being an excellent candidate for a photocathode, Cu2ZnSnS4 (CZTS) performance is limited by suboptimal bulk and interfacial charge carrier dynamics. In this work, we introduce a facile and versatile CZTS precursor seed layer engineering technique, which significantly enhances crystal growth and mitigates detrimental defects in the post-sulfurized CZTS light-absorbing films. This effective optimization of defects and charge carrier dynamics results in a highly efficient CZTS/CdS/TiO2/Pt thin-film photocathode, achieving a record half-cell solar-to-hydrogen (HC-STH) conversion efficiency of 9.91%. Additionally, the photocathode exhibits a highest photocurrent density (Jph) of 29.44 mA cm-2 (at 0 VRHE) and favorable onset potential (Von) of 0.73 VRHE. Furthermore, our CTZS photocathode demonstrates a remarkable Jph of 16.54 mA cm-2 and HC-STH efficiency of 2.56% in natural seawater, followed by an impressive unbiased STH efficiency of 2.20% in a CZTS-BiVO4 tandem cell. The scalability of this approach is underscored by the successful fabrication of a 4 × 4 cm2 module, highlighting its significant potential for practical, unbiased in situ solar seawater splitting applications.

Employing Dilute Bimetallic Dispersion on Nano-Photocatalysts for Enhanced Green Hydrogen Production Under Visible Light

Photocatalytic water splitting offers a sustainable pathway for producing clean H2 fuel. However, conventional heterojunction photocatalysts face severe challenges, including diminished redox potential due to complex band alignments, interfacial defects accelerating charge recombination, and long charge-carrier paths reducing photocarrier and material utilization. Here, we achieve efficient, visible-light-driven H2 generation by employing a dilute bimetallic dispersion on a metal chalcogenide nano-photocatalyst. Using CdS nanorod as a model photocatalyst, we strategically position Cu in lattice sites and Co in interstitial locations to preserve CdS's strong optical properties and redox potential. In this design, Cu species serve as electron sinks to drive H2 evolution, while the Co2+/Co3+ couple functions as a redox shuttle to efficiently channel photogenerated holes to the reactant. This optimized photocatalyst demonstrates a high H2 production rate of ≈52 mmol g-1 h-1, surpassing bare CdS and other emerging photocatalytic designs by >100-fold and >65-fold, respectively. Mechanistic studies highlight the roles of Cu and Co as electron and hole sinks and active redox sites, thereby facilitating directed photocarrier migration and enhanced light-to-chemical conversion. By establishing spatially distinct redox sites, this work provides a foundational framework for designing next-generation photocatalytic platforms, paving the way for sustainable energy and chemical applications using light.

Self-anchored nickel cocatalyst on nitrogen-doped carbon dots for enhanced photocatalytic hydrogen evolution

Self-anchored nickel on nitrogen-doped carbon dots as an efficient co-catalyst achieved an impressive HER activity and durability. Mechanistic studies revealed that the NiC co-catalyst optimized photoabsorption, charge transport dynamics, and surface reactions and reduced the HER overpotential.

Protons Accumulate at the Graphene-Water Interface

Water's ability to autoionize into hydroxide and hydronium ions profoundly influences surface properties, rendering interfaces either basic or acidic. While it is well-established that protons show an affinity to the air-water interface, a critical knowledge gap exists in technologically relevant surfaces like the graphene-water interface. Here we use machine learning-based simulations with first-principles accuracy to unravel the behavior of hydroxide and hydronium ions at the graphene-water interface. Our findings reveal that protons accumulate at the graphene-water interface, with the hydronium ion predominantly residing in the first contact layer of water. In contrast, the hydroxide ion exhibits a bimodal distribution, found both near the surface and further away from it. Analysis of the underlying electronic structure reveals local polarization effects, resulting in counterintuitive charge rearrangement. Proton propensity to the graphene-water interface challenges the interpretation of surface experiments and is expected to have far-reaching consequences for ion conductivity, interfacial reactivity, and proton-mediated processes.

Enhanced Photocatalytic O2 Evolution over Layered Perovskite Oxyiodide Ba2Bi3Nb2O11I through Flux Synthesis and Surface Modifications

Sillén-Aurivillius oxyiodides, particularly Ba2Bi3Nb2O11I with double-perovskite layers, are promising photocatalysts for visible-light-driven water splitting due to their excellent light absorption and carrier transport properties. However, efforts to enhance their photocatalytic performance through advancements in synthesis methods or surface modifications remain limited. Here, we report the flux synthesis of Ba2Bi3Nb2O11I and the optimization of cocatalyst loading. Single-phase Ba2Bi3Nb2O11I was successfully synthesized using molten alkali iodide salts under appropriate reaction conditions. The heating rate during the synthesis significantly influenced crystallinity and carrier lifetime, as shown by time-resolved microwave conductivity measurements. By optimizing the reaction conditions to enhance crystallinity (prolong carrier lifetime), the flux-synthesized sample exhibited a higher sacrificial O2 evolution rate than that prepared via the conventional solid-state reaction. Furthermore, precise control over the loading conditions of the iron-ruthenium oxide cocatalyst ((Fe,Ru)Ox) significantly enhanced nonsacrificial O2 evolution from an aqueous Fe3+ solution. Electrochemical analysis revealed that the tuned loading conditions enhanced the catalytic activity of the (Fe,Ru)Ox cocatalyst for both water oxidation and Fe3+ reduction. Finally, Z-scheme water splitting using the optimized (Fe,Ru)Ox-loaded Ba2Bi3Nb2O11I photocatalyst showed superior efficiency than that using the previously reported unoptimized sample. This study provides valuable insights into enhancing the O2 evolution activity of oxyiodide photocatalysts for water-splitting applications.

Recent developments in atomically precise metal nanocluster-based photocatalysts for hydrogen production

Photocatalytic hydrogen production offers a sustainable approach for utilising light energy, providing a promising solution to global energy challenges. The efficiency of this process relies on developing photocatalysts with broad light responsiveness and effective charge carrier separation capabilities. Atomically precise metal nanoclusters (NCs) have emerged as a highly favourable class of materials for this role due to their unique atomic arrangements, ultrasmall size, quantum confinement effects, and plenty of surface-active sites. These exceptional properties endow NCs with semiconductor-like behaviour, allowing for the generation of electrons and holes under light excitation, thus driving the hydrogen production reaction. Moreover, their robust light-absorption properties across the UV to near-IR spectrum, coupled with tuneable optical properties controlled by their composition and structure, promise NCs as next-generation photocatalysts. This review explores recent developments in the application of NCs for photocatalytic hydrogen production, emphasising strategies to enhance charge carrier separation and transfer efficiency, as well as photostability. The discussion also highlights the challenges and future opportunities in using NCs for efficient hydrogen production.

Type-II heterostructure of semiconducting CdS nanoparticle-ZnO nanoflake arrays for visible light dependent enhanced photocatalytic activity

Type-II heterostructure semiconductors are very attractive for optoelectronics, environmental and energy-related applications. In this report, the heterostructure (Hs) semiconductor nanocrystalline CdS-ZnO was grown by a cost-effective chemical precipitation method and study of photocatalytic performance from the view of type-II semiconductor heterostructure. The array of nano flake (NF)-particle (NP) morphology of the Hs was observed from FESEM images. Different optical parameters like refractive index, optical conductivity, energy functions, and others were studied from UV-Vis spectroscopy. Tuning of the excitonic peaks (355 nm to 464 nm), band gap energy (3.78 eV to 2.8 eV), and Urbach energy (1 eV to 2.35 eV) have been observed from optical spectroscopy. The crystal phase matching in the Hs has been verified from the observed hexagonal wurtzite structure of both CdS NPs and ZnO NPs. The greater ultrafast life time of the CdS NP-ZnO NF Hs (59 nS) was found compare with pure CdS NPs (17 nS) and ZnO NPs (4.41nS). The observed room temperature current in the heterostrcture enhanced heavily compare with pure ZnO NPs at any voltage (-10 V to + 10 V). A photocatalytic degradation test showed that the highest efficiency (≈ 95%) degradation of MB within 28 min was obtained using type II CdS NP-ZnO NF heterostructure (Hs) semiconductors compare with pure ZnO NPs (75%) and pure CdS NPs (83.5%) under visible light irradiation. This highly efficient activity of the HS was induced by enhanced charge separation and interfacial charge transfer in nanocrystal heterostructure semiconductors.

Tuning the Bandgap of Semi-Conductive Metal-Organic Framework to Promote Photocatalytic Hydrogen Evolution from Water

The systematic study on the relationship between the bandgap of catalysts and their activity for water reduction under visible-light irradiation is of significant importance. Herein, the design flexibility and modifiability of Metal-Organic Frameworks (MOFs) are utilized to synthesize a series of M3O-containing MOFs (named NKU-M, M represents Fe, Co, Ni, FeNi, FeCo, Fe-NO2, and Fe-NH2) through structure engineering. Upon visible light irradiation, the H2 production rates of these MOFs exhibit a volcanic curve with bandgap changing, and NKU-Fe with a bandgap of 2.15 eV shows the highest rate of 263.5 µmol g-1 h-1. Theoretical calculation suggests that NKU-Fe possesses an optimal conduction and valence band position and a small value of work function, contributing to its excellent hydrogen production performance under visible light. Systematic studies reveal that photocatalysts with wider bandgaps go against absorbing sunlight, leading to unsatisfied hydrogen production. Catalysts with narrower bandgaps have higher light harvesting ability but may exhibit lower space potential, leading to an increased recombination rate of photo-generated electron-hole pairs, which hampers overall photocatalytic performance. This study illustrated that the activity of photocatalysts can be adjusted by tuning the bandgaps at the molecular degree, which may inspire the construction of more efficient catalysts for photocatalysis.

Catalytic Strategy for Chemical Analysis of Volatile Iodine with the Assistance of Machine Learning

A strategy of catalytic chemical detection (CCD) with the assistance of a machine learning (ML) approach was proposed and evaluated in this work. In the CCD method, the target analyte acts as the catalyst of the detection reaction rather than traditional reactants. The detection of a typical environmental contaminant-volatile iodine was selected as an example to establish the general routine in designing CCD. One major obstacle lies in the complex of manual selection of detection reaction, especially considering that more than 650,000 related reactions were exhibited in SciFinder database. Traditional workflow is time-consuming and material-consuming; therefore, the ML approach with descriptors directly related to CCD was employed. The reaction of indoles and aromatic aldehydes to bis(indolyl)methanes was screened out with the ML approach. After preliminary experiments, the screened reaction for iodine detection achieved desirable sensitivity, specificity, and recognizability simultaneously. The fabricated sensor devices were practicable for portable detection in real gas samples with a low concentration. This work provides a practical example of chemical analysis based on catalytic strategy and exemplifies the powerful application for the ML method in chemistry through the introduction of original descriptors.

Recyclable Molecular Ferroelectrics to Harvest Mechanical Energy for Sustained Hydrogen Generation

Production of hydrogen fuel from water and renewable energy offers one of the most promising pathways for carbon neutrality and sustainable development. However, existing hydrogen generation technologies struggle with durability issues, such as poisoning, coking, and fouling, so it is a crucial economic concern to find a long-term hydrogen generation catalyst or approach. Herein, we report a recyclable cyclic supersaturation strategy harnessing molecular ferroelectric (TMFM)0.26(TMCM)0.74CdCl3 (MF-1) for hydrogen generation, which enables cycles of recrystallization and dissolution of molecular ferroelectric nanocrystals in supersaturated aqueous solution systems. The molecular ferroelectric nanocrystals generate hydrogen through the piezoelectric effect and dissolve in aqueous solution, enabling complete hydrogen desorption. Additionally, their low acoustic impedance, closely matching that of water, facilitates efficient mechanical energy transmission, thereby enhancing hydrogen generation efficiency. We achieve a robust hydrogen generation rate of record-high 11.56 mmol g-1 h-1 (mechanical-to-hydrogen energy conversion efficiency of 35.6%), with outstanding durability surpassing 1500 h. This work not only provides a new strategy for efficient and sustainable hydrogen generation but also boosts the outlook for the application of water-soluble molecular ferroelectric materials.

Spontaneous Exciton Dissociation in Sc-Doped Rutile TiO2 for Photocatalytic Overall Water Splitting with an Apparent Quantum Yield of 30

Achieving high-efficiency photocatalytic overall water splitting with earth-abundant materials like TiO2 under ambient conditions is a compelling renewable energy solution. However, this remains challenging due to both the presence of rich deep-level defects and lack of strong driving force in particulate photocatalysts, limiting the separation of photogenerated charges. Here, we developed a scandium (Sc)-doped rutile TiO2 with fully passivated detrimental Ti3+ defects and very strong built-in electric field arising from engineered (101)/(110) facet junctions. The Sc3+ doping enables a much lower exciton binding energy of 8.2 meV (28.6 meV for undoping) than room-temperature thermal fluctuation energy, indicating spontaneous exciton dissociation. These features enable the photogenerated electrons and holes to selectively transfer to the (110) and (101) facets, respectively. The resulting Sc-doped TiO2 with cocatalyst delivers photocatalytic overall water splitting with an apparent quantum yield of 30.3% at 360 nm and a solar-to-hydrogen conversion efficiency of 0.34%, representing the highest values reported for TiO2-based photocatalysts under ambient conditions.

Light-Driven Low-Temperature and Near-Unity Conversion of Ester on a Perovskite Derivative Photothermal Catalyst via Photon-Bismuth Triggered Hotspot

Solar-driven photothermal chemical transformations are regarded as green processes to reduce energy consumption and are expected to utilize unique light-induced activation mechanisms to improve reaction kinetics. Halide perovskites and their derivatives, due to unique optoelectronic properties and compositional flexibility, are allowed for the precise regulation of energy band structures and surface electronic states, showing potentials as photoactivated catalysts with photo-thermal synergistic effects. However, the photothermal catalytic performance of halide perovskites is still unsatisfied with low conversion (<0.2%). Herein, Cs3BixSb2- xBr9 is designed as a novel and effective photothermal catalyst for light-driven degradation of ester under room temperature, achieving a near-unity conversion of ≈99% without external heating. Photothermal catalytic process shows the remarkable enhancementup to 796% and 200% compared with that in the single thermocatalysis or photocatalysis. The stable catalyst shows superior light-driven cyclic performance, as well. Mechanistic studies combined with in situ characterizations and theoretical calculations show that photon-bismuth hotspot with the synergy of photoinduced charge transfer process (photochemistry) significantly reduce the activation energy, light-to-heat effects (thermochemistry) elevate the local temperature, and bismuth active site promotes the C─O bond activation (surface adsorption), which together contribute to excellent solar-driven conversion efficiency on the perovskite derivative.

Intrafacet Charge Separation toward Efficient Overall Water Splitting on SrTiO3 Single Crystal Photocatalysts

Facet engineering of photocatalytic particles has been proven to be effective in enhancing charge separation efficiency, particularly via interfacet built-in electric field, thus improving photocatalytic performance. However, whether charge separation also occurs within a facet remains unclear. Here, we found that reduction cocatalysts and oxidation cocatalysts are distributed on a facet of single cubic SrTiO3 crystal via photodeposition. Rh are predominantly deposited on the central region, whereas CoOOH are preferentially deposited near the edges, indicating that photogenerated electrons are accumulated at the center and holes at the edges. This intrafacet charge separation leads to a 2.7-fold improvement in overall water splitting efficiency compared to cubic SrTiO3 with randomly distributed cocatalysts. These findings advance the understanding of facet-based charge separation and offer new perspectives for the rational design of high-performance photocatalysts beyond interfacet engineering.

Phosphating CoMoO4-Modified Hematite-Based Photoanode Enhances Surface Charge Transfer and Reaction Activity for Efficient Photoelectrochemical Water Oxidation

The photoelectrochemical properties of hematite-based photoanodes are hindered by severe carrier recombination and poor reaction activity, which is a major challenge. Herein, we coupled zirconium-doped α-Fe2O3 (Zr:Fe2O3) and phosphating cobalt molybdate electrocatalyst (P-CoMoO4) to ameliorate the above difficulties. The conductivity and carrier density of hematite significantly increase by Zr doping. Synergistically, the incorporation of P-CoMoO4 constructs type II heterojunction with α-Fe2O3, realizing the photogenerated electron-hole directional separation. What is more, phosphating treated CoMoO4 loading creates an intermediate surface state on the photoanode, which plays an indispensable role in trapping and transport of photogenerated holes. Meanwhile, P-CoMoO4 as the cocatalyst not only offers rich active site of Co2+ but also reduces the barrier and expedites the kinetics for the water oxidation reaction. As a consequence, the resulting P-CoMoO4/Zr:Fe2O3 composite photoanode exhibits an impressive photocurrent density of 2.27 mA cm-2 at 1.23 V vs RHE and onset potential as low as 0.68 V vs RHE. This work presents a simple yet feasible strategy of collaborative modification and surface reconstruction to achieve efficient charge separation and transfer of photoanode materials.

An Overview of Dynamic Descriptions for Nanoscale Materials in Particulate Photocatalytic Systems from Spatiotemporal Perspectives

Particulate photocatalytic systems using nanoscale photocatalysts have been developed as an attractive promising route for solar energy utilization to achieve resource sustainability and environmental harmony. Dynamic obstacles are considered as the dominant inhibition for attaining satisfactory energy-conversion efficiency. The complexity in light absorption and carrier transfer behaviors has remained to be further clearly illuminated. It is challenging to trace the fast evolution of charge carriers involved in transfer migration and interfacial reactions within a micro-nano-single-particle photocatalyst, which requires spatiotemporal high resolution. In this review, comprehensive dynamic descriptions including irradiation field, carrier separation and transfer, and interfacial reaction processes have been elucidated and discussed. The corresponding mechanisms for revealing dynamic behaviors have been explained. In addition, numerical simulation and modeling methods have been illustrated for the description of the irradiation field. Experimental measurements and spatiotemporal characterizations have been clarified for the reflection of carrier behavior and probing detection of interfacial reactions. The representative applications have been introduced according to the reported advanced research works, and the relationships between mechanistic conclusions from variable spatiotemporal measurements and photocatalytic performance results in the specific photocatalytic reactions have been concluded. This review provides a collective perspective for the full understanding and thorough evaluation of the primary dynamic processes, which would be inspired for the improvement in designing solar-driven energy-conversion systems based on nanoscale particulate photocatalysts.

Divalent Cation Doping into SrTiO3 for Enhancing the Photocatalytic Performance of Water Splitting

Perovskite SrTiO3 (STO) is a widely used semiconductor photocatalyst whose photocatalytic activity is significantly influenced by cation doping. In this work, we explore effective divalent dopants to improve the photocatalytic performance of water splitting through both theoretical and experimental approaches. First-principles calculations suggest that divalent Mg2+ and Zn2+ are promising dopants replacing Ti4+ sites of STO to help mitigate charge recombination processes associated with defect levels caused by oxygen vacancies. Experimental analysis of synthesized STO confirms the photocatalytic performance, consistent with the theoretical predictions.

Strain Engineering the Optoelectronic and HER Behavior of MoS2/ZnO Heterojunction: A DFT Investigation

The rational design of heterojunctions by coupling two or more two-dimensional (2D) materials is regarded as a feasible strategy to efficiently enhance photocatalytic-hydrogen performance by capturing solar energy to address the increasing global energy crisis. In this work, a functional MoS2/ZnO heterojunction is proposed based on first-principles simulation. Our results reveal that the photogenerated electrons and holes in the MoS2/ZnO heterojunction follow a specific Z-scheme pathway, highly facilitating redox reactions and optimizing optical properties in the visible-light region. Under external strain, the MoS2/ZnO heterojunction demonstrates improved HER performance and remarkable optical-harvesting capabilities. Interestingly, the HER free energy for the heterojunction is only -0.04 eV under -5% compressive strain, highlighting its promising potential for photocatalytic hydrogen production. We observe that the absorption edge of the spectrum shifts gradually to the infrared region with increasing tensile biaxial strains, whereas compressive biaxial strains result in a blue-shift absorption spectrum. Additionally, all heterojunctions achieve excellent solar-to-hydrogen (STH) efficiencies exceeding 10%, demonstrating their capability to store sufficient solar energy. Our work offers a novel strategy for exploring highly efficient photocatalysts in the field of hydrogen energy with the ability to modulate their activity through external strain.

Impact of Reaction Environment on Photogenerated Charge Transfer Demonstrated by Sequential Imaging

Most photocatalysis research focuses on understanding the photogenerated charge transfer processes within the solid catalysts themselves. However, these studies often overlook the impact of the reaction environment on photogenerated charge separation and reactions. To address this gap, our study employed a sequential imaging methodology that integrates surface photovoltage microscopy (SPVM), in situ atomic force microscopy (AFM), and scanning electrochemical microscopy (SECM) to track the transfer of photogenerated charges from the space charge region to the reactants at the nanoscale on individual BiVO4 particles. It identifies the key role that surface charges at the photocatalyst-electrolyte interface play in photogenerated charge transfer. Specifically, we demonstrated that the surface charge generates an additional driving force, which adjusts the interface electric field and reverses the photovoltage of {010} facet from 90 to -25 mV in a neutral electrolyte. This competitive or even larger driving force compels the photogenerated electrons, which are confined within the bulk, to migrate to the surface, ultimately leading to the redistribution of photogenerated charges. Furthermore, our findings uncovered that the difference between the solution pH and the isoelectric point of the facet serves as the origin of the interfacial electric field. Overall, our sequential imaging research fills an important gap in understanding the driving and influencing factors of charge transfer across the solid-liquid interface for photocatalytic reactions in solution. It provides significant insights into clarifying the bottleneck issue of charge separation in photocatalytic reactions.

Design Refinement of Catalytic System for Scale-Up Mild Nitrogen Photo-Fixation

Ammonia and nitric acid, versatile industrial feedstocks, and burgeoning clean energy vectors hold immense promise for sustainable development. However, Haber-Bosch and Ostwald processes, which generates carbon dioxide as massive by-product, contribute to greenhouse effects and pose environmental challenges. Thus, the pursuit of nitrogen fixation through carbon-neutral pathways under benign conditions is a frontier of scientific topics, with the harnessing of solar energy emerging as an enticing and viable option. This review delves into the refinement strategies for scale-up mild photocatalytic nitrogen fixation, fields ripe with potential for innovation. The narrative is centered on enhancing the intrinsic capabilities of catalysts to surmount current efficiency barriers. Key focus areas include the in-depth exploration of fundamental mechanisms underpinning photocatalytic procedures, rational element selection, and functional planning, state-of-the-art experimental protocols for understanding photo-fixation processes, valid photocatalytic activity evaluation, and the rational design of catalysts. Furthermore, the review offers a suite of forward-looking recommendations aimed at propelling the advancement of mild nitrogen photo-fixation. It scrutinizes the existing challenges and prospects within this burgeoning domain, aspiring to equip researchers with insightful perspectives that can catalyze the evolution of cutting-edge nitrogen fixation methodologies and steer the development of next-generation photocatalytic systems.

Reactive co-sputter deposition of Ta-doped tungsten oxide thin films for water splitting application

This study aimed to investigate the structural, optical, and electronic properties of WO3 thin films modified by Ta-doping, considering their potential application in photoelectrochemical (PEC) water splitting. Due to its unique physical and chemical properties, WO3 films have been commonly suggested as a promising photoanode for hydrogen production. However, the wide bandgap and unsuitable band edge positions of WO3 limit its PEC efficiency. Doping have been extensively applied as an effective strategy for bandgap engineering. Here, post-annealed WO3 films with different concentrations of Ta dopant were synthesized via reactive magnetron co-sputtering, while DC and RF sputtering powers were varied with the aim of achieving the desired properties. EDX analysis showed that Ta atoms were doped into WO3 in the range of 0-3.93 at%. As evident from SEM and AFM images, the surface morphology was significantly affected by increasing Ta doping, the formation of a granular structure with well-defined boundaries and increasing surface roughness (1.79-47.94 nm). XRD patterns confirmed that the incorporation of Ta atoms into a monoclinic WO3 improved the crystallinity, especially in the (002) direction. Most importantly, a decrease in the average transparency (92.82-74.27%), an increase in visible absorption, a red shift of the fundamental absorption edge corresponding to a favorable drop in the optical bandgap energy (3.07-2.61 eV) were found with increasing Ta concentration. Notably, the substitution of W6+ ions with Ta dopant (0-3.93 at%) led to an upward shift in the valence band maximum (3.62-3.31 eV) and a downward shift in the conduction band minimum (0.55-0.70 eV). The WO3 photoanode doped with 3.93 at% Ta exhibited the maximum photocurrent density of 0.65 mA/cm2 (at 1 V vs. Ag/AgCl) under simulated sunlight. Furthermore, WO3 photoanode doped with 3.93 at% Ta showed excellent photoresponsivity and slow electron-hole recombination. The obtained results predict the potential of Ta-doping coupled with post-annealing to optimize the structural and optoelectronic properties of sputtered WO3 thin films as photoanode for use in efficient PEC water splitting.

Template-free synthesis of single-crystal SrTiO3 nanocages for photocatalytic overall water splitting

In this study, we present a novel approach to achieve the template-free fabrication of nanocage-shaped SrTiO3 (N-STO) single crystals via molten salt flux treatment. Systematic characterizations demonstrate the high crystallinity and low defect density of N-STO. The N-STO single crystals enable overall water splitting (OWS) with hydrogen and oxygen evolution rates of 100.86 μmol h-1 g-1 and 44.2 μmol h-1 g-1, respectively, which is 5.7-fold higher than the porous SrTiO3 (P-STO) control.

Nearly Barrierless Four-Hole Water Oxidation Catalysis on Semiconductor Photoanodes with High Density of Accumulated Surface Holes

The sluggish water oxidation reaction (WOR) is considered the kinetic bottleneck of artificial photosynthesis due to the complicated four-electron and four-proton transfer process. Herein, we find that the WOR can be kinetically nearly barrierless on four representative photoanodes (i.e., α-Fe2O3, TiO2, WO3, and BiVO4) under concentrated light irradiation, wherein the rate-limiting O-O bond formation step is driven by accumulated surface photogenerated holes that exhibit a superior fourth-order kinetics. The activation energy is about 0.03 eV for the fourth-order kinetic pathway, which is quantitatively estimated by combining the Population model and Butler-Volmer model with the Eyring-like equation and is further confirmed by density functional theory calculations. The WOR rate under this condition shows more than 1 order of magnitude enhancement compared with that of first-, second-, or third-order kinetics. Focusing on α-Fe2O3, the accumulated high-density surface holes form adjacent FeV═O intermediates that effectively activate surface-adsorbed H2O molecules via the hydrogen-bonding effect, as revealed by operando Raman measurements and ab initio molecular dynamics simulations. This work discloses a systematic understanding of the internal relations between activation energy and reaction orders of surface holes for future WOR study.

Colloidal Design and Preparation of an Internal Electric Modulated Z-Scheme BiOI-CdS Heteronanostructure with Oxygen-Rich Vacancies

Photoelectrochemical (PEC) water splitting offers an ideal strategy for the development of clean and renewable energy. However, its practical implementation is often inhibited by the high recombination rate of photogenerated charge carriers and the instability of photoanodes. Introducing defect engineering (such as oxygen vacancies) and constructing internal electric field-modulated Z-scheme heteronanostructures (HNs) can be considered as effective approaches to overcome these obstacles. Herein, a flexible method is developed for synthesizing Z-scheme BiOI-CdS HNs with oxygen vacancies, which induce an internal electric field between ultrathin BiOI nanosheets and a CdS semiconductor. This Z-scheme mechanism significantly promotes the separation of photogenerated electron-hole pairs, thereby enhancing the PEC performance. The BiOI-CdS photoanode achieves a photocurrent density of 4.22 mA cm-2 at 1.6 V vs RHE under AM 1.5G illumination (100 mW cm-2), outperforming bare BiOI and CdS. Moreover, the photoanode exhibits exceptional stability with only a slight decrease of approximately in its initial photocurrent after a rigorous 4 h test, surpassing other counterparts in terms of durability. This work affords a better understanding of oxygen vacancies and the construction of highly efficient and stable Z-scheme photoanodes for feasible PEC application.

Multi-Bioinspired Fog Harvesting Structure with Asymmetric Surface for Hydrogen Revolution

The urgent need for sustainable energy storage drives the fast development of diverse hydrogen production based on clean water resources. Herein, a unique type of multi-bioinspired functional device (MFD) is reported with asymmetric wettability that combines the curvature gradient of cactus spines, the wetting gradient of lotus, and the slippery surface of Nepenthes alata for efficient fog harvesting. The precisely printed MFDs with microscale features, spanning dimensions, and a thin wall are endowed with asymmetric wettability to enable the Janus effects on their surfaces. Fog condenses on the superhydrophobic surface of the MFDs in the form of microdroplets and unidirectionally penetrates its interior due to the Janus effects, and drops onto the designated area with a better fog harvesting rate of 10.64 g cm-2 h-1. Most significantly, the collected clean water can be used for hydrogen production with excellent stability and durability. The findings demonstrate that safe, large-scale, high-performance water splitting and gas separation and collection with fog collection based on MFDs are possible.

Noble-Metal-Free Cocatalysts Reinforcing Hole Consumption for Photocatalytic Hydrogen Evolution with Ultrahigh Apparent Quantum Efficiency

Achieving efficient and sustainable hydrogen production through photocatalysis is highly promising yet remains a significant challenge, especially when replacing costly noble metals with more abundant alternatives. Conversion efficiency with noble-metal-free alternatives is frequently limited by high charge recombination rates, mainly due to the sluggish transfer and inefficient consumption of photo-generated holes. To address these challenges, a rational design of noble-metal-free cocatalysts as oxidative sites is reported to facilitate hole consumption, leading to markedly increased H2 yield rates without relying on expensive noble metals. By integrating femtosecond transient absorption spectroscopy with in situ characterizations and theoretical calculations, the rapid hole consumption is compellingly confirmed, which in turn promotes the effective separation and migration of photo-generated carriers. The optimized catalyst delivers an impressive photocatalytic H2 yield rate of 57.84 mmol gcat -1 h-1, coupled with an ultrahigh apparent quantum efficiency reaching up to 65.8%. Additionally, a flow-type quartz microreactor is assembled using the optimal catalyst thin film, which achieves a notable H2 yield efficiency of 0.102 mL min-1 and maintains high stability over 1260 min of continuous operation. The strategy of reinforcing hole consumption through noble-metal-free cocatalysts establishes a promising pathway for scalable and economically viable solar H2 production.

Unlocking the Key to Photocatalytic Hydrogen Production Using Electronic Mediators for Z-Scheme Water Splitting

A prevalent challenge in particulate photocatalytic water splitting lies in the fact that while numerous photocatalysts exhibit outstanding hydrogen evolution reaction (HER) activity in organic sacrificial reagents, their performance diminishes markedly in a Z-scheme water splitting system using electronic mediators. This underlying reason remains undefined, posing a long-standing issue in photocatalytic water splitting. Herein, we unveiled that the primary reason for the decreased HER activity in electronic mediators is due to the strong adsorption of shuttle ions on cocatalyst surfaces, which inhibits the initial proton reduction and results in a severe backward reaction of the oxidized shuttle ions. To address this, taking typical visible-light-responsive photocatalysts, BaTaO2N and SrTiO3:Rh, as examples, we have developed a strategy via selective surface modification of metal cocatalysts (such as Pt, Ru) with chromium oxide species (CrOx) to prevent the adsorption of shuttle ions. It is demonstrated that the photocatalytic HER activities of BaTaO2N and SrTiO3:Rh can be improved by one to two orders of magnitude in diverse shuttle ions. The introduced CrOx substantially weakens the interaction between the metal cocatalysts and shuttle ions, promotes proton adsorption for the HER reaction, and also suppresses the backward reaction between shuttle ions. Owing to the improved HER activity, the photocatalytic performance of Z-scheme water splitting is significantly enhanced, providing a feasible strategy for constructing efficient Z-scheme systems in heterogeneous photocatalysis.

A scalable solar-driven photocatalytic system for separated H2 and O2 production from water

Solar-driven photocatalytic water splitting offers a sustainable pathway to produce green hydrogen, yet its practical application encounters several challenges including inefficient photocatalysts, sluggish water oxidation, severe reverse reactions and the necessity of separating produced hydrogen and oxygen gases. Herein, we design and develop a photocatalytic system composed of two separate reaction parts: a hydrogen evolution cell containing halide perovskite photocatalysts (MoSe2-loaded CH(NH2)2PbBr3-xIx) and an oxygen evolution cell containing NiFe-layered double hydroxide modified BiVO4 photocatalysts. These components are bridged by a I3-/I- redox couple to facilitate electron transfer, realizing efficient overall water splitting with a solar-to-hydrogen conversion efficiency of 2.47 ± 0.03%. Additionally, an outdoor scaled-up setup of 692.5 cm2 achieves an average solar-to-hydrogen conversion efficiency of 1.21% during a week-long test under natural sunlight. By addressing major limitations inherent in conventional photocatalytic systems, such as the cooccurrence of hydrogen and oxygen in a single cell and the resultant severe reverse reactions from hydrogen and oxygen recombination, this work introduces an alternative concept for photocatalytic system design, which enhances both efficiency and practicality.

Solar-driven plasmon-enhanced photocatalysis: Co2+-doped ZnFe2O4 nanospheres-embedded ZnO nanosheets for effective degradation of dyes and antibiotics

To ensure sustainable management and the availability of water and sanitation for all, a UN sustainable development goal (SDG) focused on promising methods to eliminate aqueous pollutants is urgently required. In this regard, solar photocatalysis, driven by freely available sunlight using low-cost, reusable photocatalysts, is a promising approach. In this context, we present a novel full-solar-spectrum photocatalyst with promising efficiency attributed to its laddered heterojunction and Ag-based plasmon enhanced activity. Specifically, it comprised Co2+-doped zinc-ferrite nanoparticles embedded on zinc oxide sheets that were later conformally coated with a small weight fraction (2.5%) of Ag under sunlight. The photocatalyst was optimized for different synthesis methods, post-synthesis temperatures, and different compositions with orange G as a model pollutant. Unlike previous reports, without any scavengers, the photocatalyst was effective for highly polluted water with a chemical oxygen demand (COD) of ∼740 ppm, eliminating 66% of it within an hour. We have coined a new term, solar photo-oxidation efficiency (SPOE), to describe the photocatalyst's performance. SPOE was directly dependent on the pollutant concentration and was found to be 72% for 400 ppm ciprofloxacin, with an apparent quantum yield of 36%. The promising activity of our photocatalyst continued even after several reuses. The generation of hydroxyl and superoxide radicals was confirmed by respective confirmatory tests. Scavenging tests indicated the highest contribution of superoxide radicals and holes in photodegradation. Our photocatalyst is promising and holds enormous potential for use in the treatment of diverse pollutants.

Surface Oxygen Vacancies on Copper-Doped Titanium Dioxide for Photocatalytic Nitrate-to-Ammonia Reduction

Photocatalytic transformation of nitrate (NO3-) in wastewater into ammonia (NH3) is a challenge in the detoxification and recycling of limited nitrogen resources. In particular, previously reported photocatalysts cannot promote the reaction using water as an electron donor. Herein, we report that copper-doped titanium dioxide (Cu-TiO2) powders, prepared via the sol-gel method and subsequent calcination, promote NO3--to-NH3 reduction in water. The Cu2+ doping into TiO2 creates a large number of surface oxygen vacancies (OVsurf), which are stable even under aerated conditions. The Ti3+ and Cu2+ atoms adjacent to OVsurf behave as active sites for the NO3--to-NH3 reduction. Doping with an appropriate amount of Cu2+ and calcination at an appropriate temperature produce the catalysts with a large number of OVsurf, while maintaining a high conductivity, and exhibit a high photocatalytic activity.

Highly Efficient Schottky Heterojunctions for Photocatalytic Hydrogen Evolution: Facile Synthesis of Hollow Nano-ZnSe Spheres on Ti3C2-Nanosheets

Traditional photocatalysts often have limited efficiency due to the high recombination rate of photogenerated electron-hole pairs. In this work, we synthesized 3D/2D ZnSe-MXene Schottky heterojunctions by an in situ electrostatic self-assembly method. Notably, the 3 % MXene-ZnSe composite exhibited an optimized photocatalytic hydrogen production rate of 765.0 μmol g-1 h-1, about 1.6 times higher than that of pristine ZnSe. MXene's high conductivity and large surface area enhance catalytic performance by providing more active sites and efficient electron transfer pathways from ZnSe to MXene. This accelerates the separation and movement of photogenerated carriers, significantly reducing recombination. We have investigated the photocatalytic hydrogen production mechanism of the ZnSe-MXene composites using various characterization techniques. Our findings provide favourable insights into the synergistic effects within the heterojunction, offering valuable guidance for the design and development of advanced photocatalytic materials.

Bifunctional In3+ Doping toward Defect Engineering in SrTiO3 for Solar Water Splitting

Defect engineering in SrTiO3 crystals plays a pivotal role in achieving efficient overall solar water splitting, as evidenced by the influence of Al3+ ions. However, the uneven structural relaxation caused by Al3+ ions has been overlooked, significantly affecting the defect state and catalytic activity. When an Al2O3 crucible is used, optimizing this defect engineering presents a significant challenge. In this study, we introduced In3+ into the SrTiO3 crystal to achieve favorable photocatalytic performance. Notably, In3+ stabilizes at the B sites of SrTiO3, outcompeting Al3+, demonstrating a bifunctional effect by simultaneously regulating the concentration of defect charges and mitigating the negative impact of Al3+ on structural relaxation, leading to shallow-state defects. Additionally, the incorporation of In3+ ions effectively prevents the precipitation of perovskite Sr2+. Carrier behavior studies and density functional theory (DFT) calculations provide substantial evidence of the underlying modulating mechanism. Consequently, the optimized In3+-doped SrTiO3 exhibits impressive gas evolution rates of 1.40 mmol·h-1 H2 and 0.69 mmol·h-1 O2 under full-spectrum light irradiation, corresponding to a promising apparent quantum yield (AQY) of 82.36% at 365 nm and a solar-to-hydrogen (STH) efficiency of 0.54%. Such enhanced activity could be attributed to the effective incorporation of In3+ ions, which improves the structural stability of the perovskite SrTiO3 lattice.

Recent progress in doping engineering of perovskite oxides for photocatalytic green hydrogen production via water splitting

Given the immense potential for addressing energy and environmental issues, the utilization of solar energy for hydrogen production by water splitting (WS) has garnered widespread attention in the scientific community. Perovskite oxides (POs), with their compositional flexibility and structural stability, hold significant promise in the photocatalytic hydrogen evolution reaction (HER). Rational doping of POs is the key to enhancing the rate of the photocatalytic HER. In this review, with the aim of providing guidance for the enhancement of photocatalytic efficiency, the recent advancements in ion-doping engineering of PO-based photocatalysts are summarized. The principles of photocatalytic WS, the preparation methods of ion-doped POs, the types of dopants, and the effects of doping on the photocatalytic performance of the HER are systematically reviewed and discussed. Background and advances in practical applications are further elaborated. Ultimately, prospects and challenges of the doped POs for the photocatalytic HER are proposed. This review provides a good reference for making informed decisions regarding the selection of doped ions and a valuable suggestion for establishing high-stability photocatalytic systems toward large-scale applications.

Engineering plasmonic charge kinetics and broadband photoelectrochemical spectral responses using a multi-resonant Au-TiO2 plasmonic particle grating-based optical resonator

The plasmonic integrated semiconductor has widened the operational spectral region of semiconductors for light-matter interaction-driven solar energy harvesting applications. However, a specific plasmonic resonance has moderate light absorption and is only active in a specific width of the visible spectrum. We present a tailored plasmonic particle grating-based Au-TiO2 Schottky photoelectrode-based broadband absorber that operates in the extended spectral region of 400-800 nm due to the synergistic interaction of multi-resonant photonic and plasmonic modes of the plasmonic particle grating structure. In the visible spectrum, the proposed photoelectrode increased the incoming photon to electron conversion efficiency (IPCE%) by seven and five times more than TiO2 for TM (along the grating vector) and TE (perpendicular to the grating vector) incidence, respectively. The plasmonic response of the gold nanoparticle and the grating-coupled surface plasmon polariton (SPP)-guided mode resonance (GMR) are responsible for such increments. Ultrafast pump-probe spectroscopy verifies that the plasmon-GMR interaction causes extended plasmonic charge generation and lifetime. The kinetics of plasmonic-generated charges in grating-coupled SPP and LSPR was investigated through TM and TE polarized pump and probe excitation. Such findings are consistent with the observed PEC spectral responses under their respective polarization illumination. Therefore, our research provides a simple method for integrating photonic and plasmonic materials for innovative broadband spectrum responses in photovoltaic and energy harvesting applications.

Isoreticular Squaraine-Linked Titanium-Organic Frameworks for Photocatalytic Water Splitting to Hydrogen Under Visible Light

Inspired by the excellent photocatalytic activity of TiO2, titanium metal-organic frameworks (Ti-MOFs) with broad absorption of visible light are regarded as promising photocatalysts, but carboxylate-linkers used in them are mainly limited to the large extended π-electron systems. Developing Ti-MOFs using organic linkers with a donor-acceptor-donor (D-A-D) structure is expected to improve their charge separation but is still challenging. Herein the design of two new isoreticular Ti-MOFs, Ti6-SQ1 and Ti6-SQ2 are reported, by using squaraines bearing different electron donors as organic linkers. Discrete fourier transform (DFT) calculations demonstrate that ligand-to-metal charge transfer (LMCT) from the acceptor units of squaraines to the Ti6-oxo secondary building units (SBUs) drives the photocatalytic water splitting to hydrogen reaction. Compared with Ti6-SQ2, the shorter distance between the squaraine centers and the Ti6-oxo SBUs in Ti6-SQ1 makes stronger LMCT, showing higher photocatalytic hydrogen evolution efficiency of 11.5 mmol g-1 h-1 under visible light (λ > 420 nm), which is ≈8 times that of Ti-based MOF photocatalysts reported so far. This work provides a new strategy to design Ti-MOF photocatalysts and understand their structure-property relationship.

Dual S-scheme MoS2/ZnIn2S4/Graphene quantum dots ternary heterojunctions for highly efficient photocatalytic hydrogen evolution

The layered chalcogenide ZnIn2S4 (ZIS) exhibits photo-stability and a tunable band gap but is limited in photocatalytic applications, such as hydrogen (H2) production, due to rapid carrier recombination and slow charge separation. To overcome these limitations, we have synthesized a ternary MoS2/ZIS/graphene quantum dots (GQDs) heterojunction, wherein MoS2 and GQDs are strategically attached to ZIS interlaced nanoflakes, enhancing light absorption across the 500-1500 nm range. This heterojunction benefits from dual S-scheme interfaces between MoS2-ZIS and ZIS-GQDs, establishing directed internal electric fields (IEFs). These IEFs accelerate the transfer of photoinduced electrons from the conduction bands of MoS2 and GQDs to the valence band of ZIS, promoting rapid recombination with holes and facilitating efficient catalytic reactions with plentiful photoinduced electrons stemmed from the conduction band of ZIS. As a result, the photocatalytic H2 production rate of the MoS2/ZIS/GQDs heterojunction is measured at 21.63 mmol h-1 g-1, marking an increase of 36.7 times over pure ZIS. This research provides valuable insights into designing novel heterojunctions for improved charge separation and transfer for solar energy conversion applications.

MoSe2-Sensitized Water Splitting Assisted by C60-Dendrons on the Basal Surface

To facilitate water splitting using MoSe2 as a light absorber, we fabricated water-dispersible MoSe2/C60-dendron nanohybrids via physical modification of the basal plane of MoSe2. Upon photoirradiation, the mixed-dimension MoSe2/C60 (2D/0D) heterojunction generates a charge-separated state (MoSe2⋅+/C60⋅-) through electron extraction from the exciton in MoSe2 to C60. This process is followed by the hydrogen evolution reaction (HER) from water in the presence of a sacrificial donor (1-benzyl-1,4-dihydronicotinamide) and co-catalyst (Pt-PVP). The apparent quantum yields of the HER were estimated to be 0.06 % and 0.27 % upon photoexcitation at the A- and B-exciton absorption peaks (λmax=800 and 700 nm), respectively.

Single atoms meeting 2D materials: an excellent configuration for photocatalysis

Photocatalysis has problems such as low light absorption efficiency and rapid recombination of photogenerated electron-hole pairs. Many studies have been conducted to improve these issues. This review encapsulates the progress and applications of two pioneering research fields in catalysis: single-atom and two-dimensional (2D) material catalysts. The advent of this new type of catalysts, which integrates single atoms onto 2D materials, has seen remarkable growth in recent years, offering distinctive advantages. The article delves into the array of synthesis methods employed for loading single atoms onto 2D materials, including the wet chemical approach, atomic layer deposition technique, and thermal decomposition method. A highlight of the review is the superior attributes of single-atom catalysts supported on 2D materials (SACs-2D) in photocatalysis, such as extending the light absorption wavelength range, enhancing the efficiency of photogenerated electron-hole pair separation, and accelerating redox kinetics. The review meticulously examines the diverse applications of SACs-2D photocatalysis, which encompass water splitting for hydrogen generation, carbon dioxide reduction, degradation of organic pollutants, nitrogen fixation and hydrogen peroxide synthesis. These applications demonstrate the potential of SACs-2D materials in addressing pressing environmental and energy challenges. Finally, this article evaluates the current state of this burgeoning field, discussing the opportunities and challenges ahead.

Investigation of Two Novel Heterojunction Photocatalysts with Boosted Hydrogen Evolution Performance

Among the reported photocatalysts, ZnIn2S4 has garnered significant research interest due to its advantageous layered structure and appropriate band gap. However, achieving rational design and effective interfacial regulation in heterojunctions remains challenging. In this study, we designed two novel heterojunctions: SrTiO3@ZnIn2S4 and SrCrO3@ZnIn2S4. The photocatalytic hydrogen evolution performance of prepared heterojunctions was systematically investigated under different single-wavelength light sources. Without a cocatalyst, the optimized hydrogen evolution efficiency of SrTiO3@ZnIn2S4 and SrCrO3@ZnIn2S4 reached 3.27 and 4.6 mmol g-1. The enhanced photocatalytic performance can be attributed to the formation of a type-II heterojunction, which improves light absorption capabilities and promotes the separation and transfer of photoinduced carriers. This study provides valuable insights into the strategic construction of heterojunctions for photocatalytic water splitting.

Enhanced Photothermal-Assisted Hydrogen Production via a Porous Carbon@MoS2/ZnIn2S4 Type II-S-Scheme Tandem Heterostructure

MoS2/ZnIn2S4 flower-like heterostructures into porous carbon (PC@MoS2/ZIS) are embedded. This ternary heterostructure demonstrates enhanced light absorption across a broad spectral range from 200 to 2500 nm. It features both Type-II and S-scheme dual heterojunction interfaces, which facilitate the generation, separation, and transfer of photoinduced carriers. The PC enveloped by MoS2/ZIS composite microspheres serves as a photothermal source, providing additional energy to the carriers. This process accelerates charge separation and migration, enhancing photothermal-assisted photocatalytic H2 evolution. The optimal H2 evolution rate for PC@MoS2/ZIS reaches an impressive 18.79 mmol g-1 h-1, with an apparent quantum efficiency of 14.1% at 400 nm. This work presents a promising approach for effectively integrating multicomponent heterostructures with photothermal effects, offering innovative strategies for efficient solar energy utilization and conversion.

Strain-induced charge delocalization achieves ultralow exciton binding energy toward efficient photocatalysis

The exciton effect is commonly observed in photocatalysts, where substantial exciton binding energy (E b) significantly hampers the efficient generation of photo-excited electron-hole pairs, thereby severely constraining photocatalysis. Herein, we propose a strategy to reduce E b through strain-induced charge delocalization. Taking Ta2O5 as a prototype, tensile strain was introduced by engineering a crystalline/amorphous interface, weakening the interaction between Ta 5d and O 2p orbitals, thus endowing a delocalized charge transport and significantly lowering E b. Consequently, the E b of strained Ta2O5 nanorods (s-Ta2O5 NRs) was reduced to 24.26 meV, below the ambient thermal energy (26 meV). The ultralow E b significantly enhanced the yield of free charges, resulting in a two-fold increase in carrier lifetime and surface potential. Remarkably, the hydrogen evolution rate of s-Ta2O5 NRs increased 51.5 times compared to that of commercial Ta2O5. This strategy of strain-induced charge delocalization to significantly reduce E b offers a promising avenue for developing advanced semiconductor photoconversion systems.

Synergistic effects of Fe and Ag doping on the structural and optical properties of a TiO2 thin film: a dual function platform for hydrogen generation and dye degradation

The global population is increasing at an alarming pace, leading to a rapid surge in industrialization and concomitant environmental degradation. In light of the present circumstances, it is crucial to find sustainable solutions to mitigate pollution and ensure a safe and clean environment for the present and future society. One promising solution is photocatalysis, which utilizes solar energy to address environmental issues while providing a renewable and sustainable energy source. Herein, a TiO2-based multi-layer thin film photocatalyst was developed, and its hydrogen generation and Rhodamine 6G dye degradation properties were analysed. As TiO2 undergoes excitation in the UV region, in order to shift the absorbance towards the visible region, bandgap engineering was performed with Fe and Ag doping. As a consequence, the band gap effectively reduces, and Fe and Ag doping result in the least gap energies measuring 2.7 eV and 2.85 eV, respectively. DFT band structure study was performed, which shows the presence of additional electronic states that lie in the conduction and valence energy bands of TiO2 owing to the doping of elements. Additionally, the effect of the number of thin film layers on photocatalysis was investigated. To confirm the presence of structural distortions and oxygen vacancies, different characterizations were performed.

Covalent Organic Framework Controls the Aggregation of Metal Porphyrins for Enhanced Photocatalytic H2 Evolution

Although different post modifications of covalent organic frameworks (COFs) have been developed for achieving hierarchical nanostructures and improved photocatalytic performance, the co-assemblies of COFs with small organic molecules were still rarely studied. Herein, COF/porphyrin composites, which were fabricated at room temperature, reveal that COFs surface can modulate the aggregation of metal porphyrins, which subsequently enhance the photocatalytic properties of COFs assemblies. Thus, the surface of COFs was decorated by porphyrins aggregations with varied thickness, dependent on the metal ions of porphyrins. Ni(II) meso-Tetra (4-carboxyphenyl) porphine (NiTCPP) formed discontinuous monolayer covering on COFs surface, while Pt(II) meso-Tetra (4-carboxyphenyl) porphine (PtTCPP) or Co(II) meso-Tetra (4-carboxyphenyl) porphine (CoTCPP) aggregated into multilayer coverage. Notably, even though NiTCPP did not show any advantages in terms of light absorption or HOMO/LUMO energy levels, COF/NiTCPP with the lowest porphyrin loading still exhibited the highest photocatalytic H2 evolution (29.71 mmol g-1 h-1), which is 2.5 times higher than that of COF/PtTCPP or COF/CoTCPP. These results open new possibilities for making highly efficient photocatalysts upon the co-assemblies of COFs with small organic molecules.

Preparing Ruthenium Complex-Contained DaTp COFs via π-π Interactions for Visible-Light-Driven Photocatalytic Hydrogen Peroxide Production

Hydrogen peroxide (H2O2) is a crucial energy carrier with growing significance in sustainable energy systems. Covalent organic frameworks (COFs) have recently emerged as promising materials for efficient H2O2 photosynthesis, while transition-metal complexes are recognized for their efficacy as molecular photocatalysts in H2O2 production. This study introduces a novel π-π interaction strategy to immobilize ruthenium complexes into COFs, using DaTp COF as a model system. This approach significantly enhances the photocatalytic activity for H2O2 production, achieving an initial rate of 3276 μmol g-1 h-1 without using scavengers under visible-light irradiation (λ > 420 nm). Notably, incorporating ruthenium complexes optimizes the oxygen reduction reaction pathways, shifting from a less efficient four-electron process to a more efficient two-electron process. Density functional theory calculations further reveal that ruthenium complexes not only broaden the light absorption spectrum of the COF but also increase water affinity, directly contributing to H2O2 generation. These findings offer a strategic framework for designing and enhancing COFs in H2O2 photosynthesis applications.

Correlation between the Molecular Properties of Semiconducting Polymers of Intrinsic Microporosity and Their Photocatalytic Hydrogen Production

Increasing the interface area between organic semiconductor photocatalysts and electrolyte by fabricating nanoparticles has proven to be an effective strategy to increase photocatalytic hydrogen production activity. However, it remains unclear if increasing the internal interface by the introduction of porosity has as clear benefits for activity. To better inform future photocatalyst design, a series of polymers of intrinsic microporosity (PIMs) with the same conjugated backbone were synthesized as a platform to independently modulate the variables of porosity and relative hydrophilicity through the use of hydrophilic alcohol moieties protected by silyl ether protecting groups. When tested in the presence of ascorbic acid and photodeposited Pt, a strong correlation between the wettable porosity and photocatalytic activity was found, with the more wettable analogue of two polymers of almost the same surface area delivering 7.3 times greater activity, while controlling for other variables. Transient absorption spectroscopic (TAS) investigation showed efficient intrinsic charge generation within 10 ps in two of the porous polymers, even without the presence of ascorbic acid or Pt. Detectable hole polarons were found to be immediately extracted by added ascorbic acid, suggesting the generation of reactive charges at regions readily accessible to electrolyte in the porous structures. This study directs organic semiconductor photocatalysts design toward more hydrophilic functionality for addressing exciton and charge recombination bottlenecks and clearly demonstrates the advantages of wettable porosity as a design principle.