Single-atom Cu anchored catalysts for photocatalytic renewable H2 production with a quantum efficiency of 56

Natural AIEgens as Ultraviolet Sunscreens and Photosynergists for Solar Fuel Production

Bio-nano hybrids (BNH), combining semiconductors and microorganisms, have shown great promise for effective solar-to-fuel energy conversion. However, the high-energy ultraviolet (UV) photons in the solar spectrum can cause severe photocorrosion of semiconductors and irreversible photodamage to microorganisms within BNH. Here, we developed an encapsulation strategy using natural luminogens with aggregation-induced emission characteristics (AIEgens) to construct a protective layer for BNH, effectively shielding them against high-energy UV photons. We incorporated natural berberine (BBR) into the BNH composed of Methanosarcina barkeri and polymeric carbon nitrides (CNx). The self-assembled BNH-BBR system displayed a 2.75-fold higher CH4 yield than BNH under simulated solar irradiation. Mechanism analysis revealed that BBR acted as a UV sunscreen for BNH by converting high-energy short wavelengths into low-energy long wavelengths, thereby reducing the accumulation of reactive oxygen species and alleviating the photocorrosion of CNx. Furthermore, BBR functioned as a photosynergist for BNH by regulating photoelectron production and utilization, enhancing the intracellular energy formation in M. barkeri for growth and metabolism. This work provides important insights into the effective and scalable conversion of CO2 into valuable biofuels with BNH under light illumination containing high-energy photons.

Oriented Crystal Polarization Tuning Bulk Charge and Single-Site Chemical State for Exceptional Hydrogen Photo-Production

Rapid bulk charge recombination and mediocre surface catalytic sites harshly restrict the photocatalytic activities. Herein, the aforementioned concerns are well addressed by coupling macroscopic spontaneous polarization and atomic-site engineering of CdS single-crystal nanorods for superb H2 photo-production. The oriented growth of CdS nanorods along the polar axis, vectorially superimposing substantial polar units with orderly arrangement, renders a strong polarization electric field (20.1 times enhancement), which boosts bulk charge separation with an efficiency up to 72.4% (80.4-fold). Remarkably, polarization electric field alters the chemical state of Pt single sites by orderly reducing the binding energy of Pt atom with stepwise polarization enhancement of CdS substrate, which increases the onsite electron density of Pt from 10.232 to 10.261e- and *H key intermediates, providing preponderant Volmer-Tafel/Volmer-Heyrovsky reaction pathways with significantly decreased energy barriers for H2 production. Thus, highly polarized CdS nanorods with atomically dispersed Pt sites perform an outstanding H2 space-time yield of 118.5 mmol g-1 h-1 and apparent quantum efficiency of 57.7% at λ = 420 nm, and a record-high H2 turnover frequency of 57798.4 h-1, being one of the best catalysts for photocatalytic H2 evolution. This work highlights the function of polarization in manipulating charge separation and catalytic reaction.

Vacancy-Induced Symmetry Breaking in Titanium Dioxide Boosts the Photocatalytic Hydrogen Production from Methanol Aqueous Solution

The key to optimizing photocatalysts lies in the efficient separation and oriented migration of the photogenerated carriers. Herein, we report that breaking continuous TiO6 tetragonal (D4h) symmetry in titanium dioxide material by oxygen vacancy engineering could induce a dipole field within the bulk phase and thus facilitate the separation and transfer of photogenerated electron-hole pairs. After further loading of Cu single-atom co-catalysts, the obtained catalyst attained a hydrogen (H2) yield rate of 15.84 mmol g-1 h-1 and a remarkable apparent quantum yield of 12.67% at 385 nm from methanol aqueous solution. This catalyst also demonstrated impressive stability for at least 24 h during the photocatalytic tests. The innovative concept of producing dipole fields in semiconductors by breaking the crystal symmetry offers a new perspective for designing photocatalysts.

Photocatalytic Methanol Dehydrogenation Promoted Synergistically by Atomically Dispersed Pd and Clustered Pd

Supported metal in the form of single atoms, clusters, and particles can individually or jointly affect the activity of supported heterogeneous catalysts. While the individual contribution of the supported metal to the overall activity of supported photocatalysts has been identified, the joint activity of mixed metal species is overlooked because of their different photoelectric properties. Here, atomically dispersed Pd (Pd1) and Pd clusters are loaded onto CdS, serving as oxidation and reduction sites for methanol dehydrogenation. The Pd1 substitutes Cd2+, forming hole-trapping states for methanol oxidation and assisting the dispersion of photodeposited Pd clusters. Therefore, methanol dehydrogenation on CdS with supported Pd1 and Pd clusters exhibits the highest turnover frequency of 1.14 s-1 based on the Pd content and affords H2 and HCHO with a similar apparent quantum yield of 87 ± 1% at 452 nm under optimized reaction conditions. This work highlights the synergistic catalysis of supported metal for improved photocatalytic activity.

Dipole field as charge-transfer bridge between Cu atomic clusters/PtCu alloy nanocubes and nitrogen-rich C3N5 for superior photocatalytic hydrogen evolution

Utilizing spontaneous polarization field to harness charge transfer kinetics is a promising strategy to boost photocatalytic performance. Herein, a novel Cu atom clusters/PtCu alloy nanocubes coloaded on nitrogen-rich triazole-based C3N5 (PtCu-C3N5) with dipole field was constructed through facile photo-deposition and impregnation method. The dipole field-drive spontaneous polarization in C3N5 acts as a charge-transfer bridge to promote directional electron migration from C3N5 to Cu atom clusters/PtCu alloy. Through the synergistic effects between Cu atom clusters, PtCu alloy and dipole field in C3N5, the optimized Pt2Cu3-C3N5 achieved a record-high performance with H2 formation rate of 4090.4 μmol g-1 h-1 under visible light, about 154.4-fold increase compared with pristine C3N5 (26.5 μmol g-1 h-1). Moreover, the apparent quantum efficiency was up to 25.33 % at 320 nm, which is greatly superior than most previous related-works. The directional charge transfer mechanism was analyzed in detail through various characterizations and DFT calculations. This work offers a novel pathway to construct high-efficiency multi-metal photocatalysts for solar energy conversion.

Single-Atom based Metal-Organic Framework Photocatalysts for Solar-Fuel Generation

The growing demand for fossil fuels and subsequent CO2 emissions prompted a search for alternate sources of energy and a reduction in CO2. Photocatalysis driven by solar light has been found as a potential research area to tackle both these problems. In this direction, SAC@MOF (Single-atom loaded MOFs) photocatalysis is an emerging field and a promising technology. The unique properties of single-atom catalysts (SACs), such as high catalytic activity and selectivity, are leveraged in these systems. Photocatalysis, focusing on the utilization of Metal-Organic Frameworks (MOFs) as platforms for creating single-atom catalysts (SACs) characterized by metal single-atoms (SAs) as their active sites, are noted for their unparalleled atomic efficiency, precisely defined active sites, and superior photocatalytic performance. The synergy between MOFs and SAs in photocatalytic systems is meticulously examined, highlighting how they collectively enhance photocatalytic efficiency. This review examines SAC@MOF development and applications in environmental and energy sectors, focusing on synthesis and stabilization methods for SACs on MOFs and also characterization techniques vital for understanding these catalysts. The potential of SAC@MOF in CO2 Photoreduction and Photocatalytic H2 evolution is highlighted, emphasizing its role in green energy technologies and advances in materials science and Photocatalysis.

Insight into mechanism for remarkable photocatalytic hydrogen evolution of Cu/Pr dual atom co-modified TiO2

The development of high-activity photocatalysts is crucial for the current large-scale development of photocatalytic hydrogen applications. Herein, we have developed a strategy to significantly enhance the hydrogen photocatalytic activity of Cu/Pr di-atom co-modified TiO2 architectures by selectively anchoring Cu single atoms on the oxygen vacancies of the TiO2 surface and replacing a trace of Ti atoms in the bulk with rare earth Pr atoms. Calculation results demonstrated that the synergistic effect between Cu single atoms and Pr atoms regulates the electronic structure of Cu/Pr-TiO2, thus promoting the separation of photogenerated carriers and their directional migration to Cu single atoms for the photocatalytic reaction. Furthermore, the d-band center of Cu/Pr-TiO2, which is located at -4.70 eV, optimizes the adsorption and desorption behavior of H*. Compared to TiO2, Pr-TiO2, and Cu/TiO2, Cu/Pr-TiO2 displays the best H* adsorption Gibbs free energy (-0.047 eV). Furthermore, experimental results confirmed that the photogenerated carrier lifetime of Cu/Pr-TiO2 is not only the longest (2.45 ns), but its hydrogen production rate (34.90 mmol g-1 h-1) also significantly surpasses those of Cu/TiO2 (13.39 mmol g-1 h-1) and Pr-TiO2 (0.89 mmol g-1 h-1). These findings open up a novel atomic perspective for the development of optimal hydrogen activity in dual-atom-modified TiO2 photocatalysts.

Shaping and Doping Metal-Organic Framework-Derived TiO2 to Steer the Selectivity of Photocatalytic CO2 Reduction toward CH4

Steering selectivity in photocatalytic conversion of CO2, especially toward deep reduction products, is vital to energy and environmental goals yet remains a great challenge. In this work, we demonstrate a facet-dependent photocatalytic selective reduction of CO2 to CH4 in Cu-doped TiO2 catalysts exposed with different facets synthesized by a topological transformation from MIL-125 (Ti) precursors. The optimized round cake-like Cu/TiO2 photocatalyst mainly exposed with the (001) facet exhibited a high photocatalytic CO2 reduction performance with a CH4 yield of 40.36 μmol g-1 h-1 with a selectivity of 94.1%, which are significantly higher than those of TiO2 (001) (4.70 μmol g-1 h-1 and 52.6%, respectively), Cu/TiO2 (001 + 101) (18.95 μmol g-1 h-1 and 69.6%, respectively), and Cu/TiO2 (101) (14.73 μmol g-1 h-1 and 78.9%, respectively). The results of experimental and theoretical calculations demonstrate that the Cu doping dominating the promoted separation and migration efficiencies of photogenerated charges and the preferential adsorption on (001) facets synergistically contribute to the selective reduction of CO2 to CH4. This work highlights the significance of synergy between facet engineering and ion doping in the design of high-performance photocatalysts with respect to selective reduction of CO2 to multielectron products.

Atomistic understanding of enhanced selectivity in photocatalytic oxidation of benzyl alcohol to benzaldehyde using graphitic carbon nitride loaded with single copper atoms

The loading of graphitic carbon nitride (gCN) with transition metals has received significant attention for efficient light-driven catalysis. However, the contribution of the loaded metals to enhanced performance remains unclear. In this study, Cu is loaded onto gCN to understand how photocatalytic activity is regulated by the loaded metals. Loading gCN with 3 wt% of Cu increases the electron population by 8.1 and 4.6 times under UV (λ < 370 nm) and visible light (390 < λ < 740 nm), respectively. This sample shows nearly 100% selectivity for oxidizing benzyl alcohol to benzaldehyde and a high yield-to-power ratio, reaching 0.35 mmol g-1 h-1 W-1. The loaded Cu species exist as single atoms with a +1-oxidation state. Each Cu+ cation is coordinated to two (at 3 wt% Cu) or four (at 6 wt% Cu) N atoms within the cavity of the gCN framework. Doubling the Cu loading results in a smaller electron population and coordinatively more saturated Cu+ cations, making it catalytically less reactive. Ab initio molecular dynamics simulations show that Cu+ cations produce filled mid-gap states above the valence band, which function as hole traps and hence oxidation centers. The Cu+ cation and the neighboring N atoms are electron-depletion and electron-accumulation sites due to Cu → N electron transfer, making it highly reactive for oxidative transformations via the hole transfer pathway. The role of Cu as a hole-transfer site updates the received understanding that surface-loaded Cu serves as an electron-accumulation site. A strong correlation is observed between the electron population at steady-state and the product yield, indicating that it could serve as a promising performance indicator for the design of future photocatalysts.

Defect Engineering Simultaneously Regulating Exciton Dissociation in Carbon Nitride and Local Electron Density in Pt Single Atoms Toward Highly Efficient Photocatalytic Hydrogen Production

The high exciton binding energy (Eb) and sluggish surface reaction kinetics have severely limited the photocatalytic hydrogen production activity of carbon nitride (CN). Herein, a hybrid system consisting of nitrogen defects and Pt single atoms is constructed through a facile self-assembly and photodeposition strategy. Due to the acceleration of exciton dissociation and regulation of local electron density of Pt single atoms along with the introduction of nitrogen defects, the optimized Pt-MCT-3 exhibits a hydrogen production rate of 172.0 µmol h-1 (λ ≥ 420 nm), ≈41 times higher than pristine CN. The apparent quantum yield for the hydrogen production is determined to be 27.1% at 420 nm. The experimental characterizations and theoretical calculations demonstrate that the nitrogen defects act as the electron traps for the exciton dissociation, resulting in a decrease of Eb from 86.92 to 43.20 meV. Simultaneously, the stronger interaction between neighboring nitrogen defects and Pt single atoms directionally drives free electrons to aggregate around Pt single atoms, and tailors the d-band electrons of Pt, forming a moderate binding strength between Pt atoms and H* intermediates.

Toward Functionality and Deactivation of Metal-Single-Atom in Heterogeneous Photocatalysts

Single-atom heterogeneous catalysts (SAHCs) provide an enticing platform for understanding catalyst structure-property-performance relationships. The 100% atom utilization and adjustable local coordination configurations make it easy to probe reaction mechanisms at the atomic level. However, the progressive deactivation of metal-single-atom (MSA) with high surface energy leads to frequent limitations on their commercial viability. This review focuses on the atomistic-sensitive reactivity and atomistic-progressive deactivation of MSA to provide a unifying framework for specific functionality and potential deactivation drivers of MSA, thereby bridging function, purpose-modification structure-performance insights with the atomistic-progressive deactivation for sustainable structure-property-performance accessibility. The dominant functionalization of atomically precise MSA acting on properties and reactivity encompassing precise photocatalytic reactions is first systematically explored. Afterward, a detailed analysis of various deactivation modes of MSA and strategies to enhance their durability is presented, providing valuable insights into the design of SAHCs with deactivation-resistant stability. Finally, the remaining challenges and future perspectives of SAHCs toward industrialization, anticipating shedding some light on the next stage of atom-economic chemical/energy transformations are presented.

Tuning Electrons Migration of Dual S Defects Mediated MoS2-x/ZnIn2S4-x Toward Highly Efficient Photocatalytic Hydrogen Production

Photocatalytic hydrogen production is a prevalent method for hydrogen synthesis. However, high recombination rate of photogenerated carriers and high activation energy barrier of H remain persistent challenge. Here, the two-step hydrothermal method is utilized to prepare dual S-defect mediated catalyst molybdenum sulfide/zinc indium sulfide (MSv/ZISv), which has high hydrogen production rate of 8.83 mmol g-1h-1 under simulated sunlight. The achieved rate is 21.91 times higher than pure ZnIn2S4 substrate. Defects in ZIS within MSv/ZISv modify the primitive electronic structure by creating defect state that retaining good reducing power, leading to the rapid separation of electron-hole pairs and the generation of additional photogenerated carriers. The internal electric field further enhances the migration toward to cocatalyst. Simultaneously, the defects introduced on the MoS2 cause electron rearrangement, leading to electron clustering on both S vacancies and edge S. Thereby MSv/ZISv exhibits the lowest activation energy barrier and |ΔGH*|. This work explores the division of synergies between different types of S defects, providing new insights into the coupling of defect engineering.

Constructing Nano-Heterostructure with Dual-Site to Boost H2O2 Activation and Regulate the Transformation of Free Radicals

A major issue with Fenton-like reaction is the excessive consumption of H2O2 caused by the sluggish regeneration rate of low-valent metal, and how to improve the activation efficiency of H2O2 has become a key in current research. Herein, a nano-heterostructure catalyst (1.0-MnCu/C) based on nano-interface engineering is constructed by supporting Cu and MnO on carbon skeleton, and its kinetic rate for the degradation of tetracycline hydrochloride is 0.0436 min-1, which is 2.9 times higher than that of Cu/C system (0.0151 min-1). The enhancement of removal rate results from the introduced Mn species can aggregate and transfer electrons to Cu sites through the electron bridge Mn-N/O-Cu, thus preventing Cu2+ from oxidizing H2O2 to form O2 •-, and facilitating the reduction of Cu2+ and generating more reactive oxygen species (1O2 and ·OH) with stronger oxidation ability, resulting in H2O2 utilization efficiency is 1.9 times as much as that of Cu/C. Additionally, the good and stable practical application capacity in different bodies demonstrates that it has great potential for practical environmental remediation.

Ultra-fast green synthesis of a defective TiO2 photocatalyst towards hydrogen production

An ultra-fast green synthesis of defective titanium dioxide (TiO2) photocatalysts was conducted by the microwave-assisted method using l-ascorbic acid (l-As) as a reducing agent. Effect of l-As concentrations on the chemical-, optical- and photoelectrochemical properties as well as the photocatalytic performance towards the hydrogen (H2) production was explored. The obtained TiO2 nanoparticles (NPs) illustrated the brown fine powders with different brownness levels depending on the concentrations of l-As. A high l-As concentration provided a high brownness of TiO2 NPs with a high generation of Ti3+ defects and oxygen vacancies (Ov), which can extend the light absorption towards the visible and near-infrared regions, suppress the recombination rate of electron-hole pairs, promote the photocurrent response and minimize the interface charge transfer resistance. An appropriate quantity of generated defects and good porous properties played a crucial role in photocatalytic H2 production. Under fluorescence illumination, the sample synthesized with a TiO2 and l-As weight ratio of 1 : 0.25 (PAs0.25) exhibited the highest H2 production rate (∼162 μmol g-1 h-1 in the presence of 1 wt% Au co-catalyst) with a slight drop (∼8.2%) after the 5th use (15 h). The synthesis method proposed in this work provides a new insight to an ultra-fast synthesis of defective TiO2 NPs using an eco-friendly chemical precursor under non-severe conditions.

General negative pressure annealing approach for creating ultra-high-loading single atom catalyst libraries

Catalyst systems populated by high-density single atoms are crucial for improving catalytic activity and selectivity, which can potentially maximize the industrial prospects of heterogeneous single-atom catalysts (SACs). However, achieving high-loading SACs with metal contents above 10 wt% remains challenging. Here we describe a general negative pressure annealing strategy to fabricate ultrahigh-loading SACs with metal contents up to 27.3-44.8 wt% for 13 different metals on a typical carbon nitride matrix. Furthermore, our approach enables the synthesis of high-entropy single-atom catalysts (HESACs) that exhibit the coexistence of multiple metal single atoms with high metal contents. In-situ aberration-corrected HAADF-STEM (AC-STEM) combined with ex-situ X-ray absorption fine structure (XAFS) demonstrate that the negative pressure annealing treatment accelerates the removal of anionic ligand in metal precursors and boosts the bonding of metal species with N defective sites, enabling the formation of dense N-coordinated metal sites. Increasing metal loading on a platinum (Pt) SAC to 41.8 wt% significantly enhances the activity of propane oxidation towards liquid products, including acetone, methanol, and acetic acid et al. This work presents a straightforward and universal approach for achieving many low-cost and high-density SACs for efficient catalytic transformations.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Changeable Active Sites by Pr Doping CuSA-TiO2 Photocatalyst for Excellent Hydrogen Production

Photocatalytic water splitting for clean hydrogen production has been a very attractive research field for decades. However, the insightful understanding of the actual active sites and their impact on catalytic performance is still ambiguous. Herein, a Pr-doped TiO2-supported Cu single atom (SA) photocatalyst is successfully synthesized (noted as Cu/Pr-TiO2). It is found that Pr dopants passivate the formation of oxygen vacancies, promoting the density of photogenerated electrons on the CuSAs, and optimizing the electronic structure and H* adsorption behavior on the CuSA active sites. The photocatalytic hydrogen evolution rate of the obtained Cu/Pr-TiO2 catalyst reaches 32.88 mmol g-1 h-1, 2.3 times higher than the Cu/TiO2. Innovatively, the excellent catalytic activity and performance is attributed to the active sites change from O atoms to CuSAs after Pr doping is found. This work provides new insight for understanding the accurate roles of single atoms in photocatalytic water splitting.

Defect Regulation Strategy of Porous Persistent Phosphors for Multiple and Dynamic Information Encryption

Optical encryption technologies based on persistent luminescence material have currently drawn increasing attention due to the distinctive and long-lived optical properties, which enable multi-dimensional and dynamic optical information encryption to improve the security level. However, the controlled synthesis of persistent phosphors remains largely unexplored and it is still a great challenge to regulate the structure for optical properties optimization, which inevitably sets significant limitations on the practical application of persistent luminescent materials. Herein, a controlled synthesis method is proposed based on defect structure regulation and a series of porous persistent phosphors is obtained with different luminous intensities, lifetime, and wavelengths. By simply using diverse templates during the sol-gel process, the oxygen vacancy defects structures are successfully regulated to improve the optical properties. Additionally, the obtained series of porous Al2O3 are utilized for multi-color and dynamic optical information encryption to increase the security level. Overall, the proposed defect regulation strategy in this work is expected to provide a general and facile method for optimizing the optical properties of persistent luminescent materials, paving new ways for broadening their applications in multi-dimensional and dynamic information encryption.

Nickel-Doped Decatungstate as a Robust Photocatalyst for Violet Light-Triggered Redox Coupling Conversion of Alcohol and Water to Aldehyde/Ketone and Hydrogen

The effective coupling of photoinduced alcohol oxidation and water reduction may economically produce hydrogen (H2) from water, which is of great significance in solving the current energy crisis. This study discloses that decatungstate (DT) and especially Ni2+ions-doped DTs are active for the photoreaction of benzyl alcohol with H2O, and under 48 h of violet light illumination, the best 1%Ni-DT yields ca. 86.1% benzoic acid and a 4.65 h-1 H2 generation efficiency (turnover frequency, TOF). Also, 1%Ni-DT is efficient for the photoredox coupling reaction of aliphatic and especially aromatic primary/secondary alcohols with water. A series of characterizations support that the doubled-reduced H2DT produced from the photoreaction plays a key role in water reduction to H2, which is accelerated by the doped Ni2+. In particular, it and the derived Ni3+ may construct a Z-type catalyst for water overall splitting, thereby hoisting the acid yield and H2 amount in the later stage of the photoreaction.

Synergistic effect of atomically dispersed Cu species and Ti-defects for boosting photocatalytic CO2 reduction over hierarchical TiO2

The photocatalytic water-mediated CO2 reduction reaction, which holds great promise for the conversion of CO2 into valuable chemicals, is often hindered by inefficient separation of photogenerated charges and a lack of suitable catalytic sites. Herein, we have developed a glycerol coordination assembly approach to precisely control the distribution of atomically dispersed Cu species by occupying Ti-defects and adjusting the ratio between Cu species and Ti-defects in a hierarchical TiO2. The optimal sample demonstrates a ∼4-fold improvement in CO2-to-CO conversion compared to normal TiO2 nanoparticles. The high activity could be attributed to the Ti defects, which enhance the photogenerated charge separation and simultaneously facilitate the adsorption of water molecules, thereby promoting the water oxidation reaction. Moreover, by means of in situ EPR and FTIR spectra, we have demonstrated that Cu species can effectively capture photogenerated electrons and facilitate the adsorption of CO2, so as to catalyze the reduction of CO2. This work provides a strategy for the construction of atomic-level synergistic catalytic sites and the utilization of in situ techniques to reveal the underlying mechanism.

Plasmonic coupling-boosted photothermal composite photocatalyst for achieving near-infrared photocatalytic hydrogen production

The development of photocatalysts that effectively utilize low-energy photons for efficient photocatalysis still faces a number of challenges. Herein, an efficient NIR-driven system based on WO3-x/ZnIn2S4 (WO3-x/ZIS) prepared by a simple low-temperature water-bath method, and the optimal WO3-x/ZIS-3 composites can reach a hydrogen-production efficiency of 14.05 μmol g-1h-1 under NIR light irradiation. The localized surface plasmon (LSPR) resonance effect in WO3-x quantum dots (QDs) not only broadens the ZIS photo-response range, but also the photothermal effect of WO3-x can increase the local reaction temperature of WO3-x/ZIS composite system, thus enhancing the photothermal-assisted photocatalytic activity. In addition, density functional theory (DFT) calculations show that the difference in work function between WO3-x and ZIS can lead to the formation of interfacial electric field (IEF), which not only promotes the separation and migration efficiency of photogenerated carriers, but also facilitates the photocatalytic water splitting for hydrogen production. This study provides possible directions for the construction of NIR-driven photothermal-assisted photocatalytic hydrogen production system.

Noble-Metal-Free Single- and Dual-Atom Catalysts for Artificial Photosynthesis

Artificial photosynthesis enables direct solar-to-chemical energy conversion aimed at mitigating environmental pollution and producing solar fuels and chemicals in a green and sustainable approach, and efficient, robust, and low-cost photocatalysts are the heart of artificial photosynthesis systems. As an emerging new class of cocatalytic materials, single-atom catalysts (SACs) and dual-atom catalysts (DACs) have received a great deal of current attention due to their maximal atom utilization and unique photocatalytic properties, whereas noble-metal-free ones impart abundance, availability, and cost-effectiveness allowing for scalable implementation. This review outlines the fundamental principles and synthetic methods of SACs and DACs and summarizes the most recent advances in SACs (Co, Fe, Cu, Ni, Bi, Al, Sn, Er, La, Ba, etc.) and DACs (CuNi, FeCo, InCu, KNa, CoCo, CuCu, etc.) based on non-noble metals, confined on an arsenal of organic or inorganic substrates (polymeric carbon nitride, metal oxides, metal sulfides, metal-organic frameworks, carbon, etc.) acting as versatile scaffolds in solar-light-driven photocatalytic reactions, including hydrogen evolution, carbon dioxide reduction, methane conversion, organic synthesis, nitrogen fixation, hydrogen peroxide production, and environmental remediation. The review concludes with the challenges, opportunities, and future prospects of noble-metal-free SACs and DACs for artificial photosynthesis.

Advances in Defect Engineering of Metal Oxides for Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction technology, capable of converting low-density solar energy into high-density chemical energy, stands as a promising approach to alleviate the energy crisis and achieve carbon neutrality. Semiconductor metal oxides, characterized by their abundant reserves, good stability, and easily tunable structures, have found extensive applications in the field of photocatalysis. However, the wide bandgap inherent in metal oxides contributes to their poor efficiency in photocatalytic CO2 reduction. Defect engineering presents an effective strategy to address these challenges. This paper reviews the research progress in defect engineering to enhance the photocatalytic CO2 reduction performance of metal oxides, summarizing defect classifications, preparation methods, and characterization techniques. The focus is on defect engineering, represented by vacancies and doping, for improving the performance of metal oxide photocatalysts. This includes advancements in expanding the photoresponse range, enhancing photogenerated charge separation, and promoting CO2 molecule activation. Finally, the paper provides a summary of the current issues and challenges faced by defect engineering, along with a prospective outlook on the future development of photocatalytic CO2 reduction technology.

Self-Organized Gradually Single-Atom-Layer of Metal Osmium for an Unprecedented Hydrogen Production from Seawater

Atomic thick two-dimensional (2D) materials with exciting physical, chemical, and electronic properties are gaining increasing attention in next-generation science and technology, showing great promise in catalysis and energy science. However, the precise design and synthesis of efficient catalytic systems based on such materials still face many difficulties, especially in how to control the preparation of structurally determined, highly active, atomic-scale distribution of material systems. Here, we report that a highly active zerovalent osmium single-atom-layer with a thickness of single atom size has been successfully and controllably self-organized on the surface of 2D graphdiyne (GDY) material. Detailed characterizations showed that the incomplete charge transfer effect between the Os atoms and GDY not only stabilized the catalytic system but also improved the intrinsic activity, making the Gibbs free energy reach the best and resulting in remarkable performance with a small overpotential of 49 mV at 500 mA cm-2, large specific j0 of 18.6 mA cm-2, and turnover frequency of 3.89 H2 s-1 at 50 mV. In addition, the formation of sp-C-Os bonds guarantees the high long-term stability of 800 h at a large current density of 500 mA cm-2 in alkaline simulated seawater.

Ultrahigh Bifunctional Photocatalytic CO2 Reduction and H2 Evolution by Synergistic Interaction of Heteroatomic Pt-Ru Dimerization Sites

Diatomic-site catalysts (DASCs) inherit the excellent performance of single-atom catalysts (SACs) by utilizing two adjacent atomic metal species to achieve functional complementarity and synergistic effects that improve the carbon dioxide reduction reaction (CO2RR) and H2 evolution reaction (HER) kinetics. Herein, we report a method to further improve the catalytic efficiency of Pt by using Pt and Ru single atoms randomly anchored on a g-C3N4 surface, yielding partial Pt-Ru dimers. The synthesized catalyst exhibits extraordinary photocatalytic activity and stability in both the CO2RR and HER processes. In-depth experimentation, the pH-dependent chemical exchange saturation transfer (CEST) imaging nuclear magnetic resonance (NMR) method, and theoretical analyses reveal that the excellent performance is attributed to orbital coupling between the Pt atoms and the neighboring Ru atoms (mainly dxy and dxz), which decreases the orbital energy levels and weakens the bond strength with intermediates, resulting in improved CO2RR and HER performance. This study successfully applies the pH-dependent CEST imaging NMR method to catalytic reactions, and CO2 adsorption is directly observed using CEST 2D imaging maps. This work presents significant potential for a variety of catalytic reaction applications by systematically designing bimetallic dimers with higher activity and stability.

Photochemical tuning of dynamic defects for high-performance atomically dispersed catalysts

Developing active and stable atomically dispersed catalysts is challenging because of weak non-specific interactions between catalytically active metal atoms and supports. Here we demonstrate a general method for synthesizing atomically dispersed catalysts via photochemical defect tuning for controlling oxygen-vacancy dynamics, which can induce specific metal-support interactions. The developed synthesis method offers metal-dynamically stabilized atomic catalysts, and it can be applied to reducible metal oxides, including TiO2, ZnO and CeO2, containing various catalytically active transition metals, including Pt, Ir and Cu. The optimized Pt-DSA/TiO2 shows unprecedentedly high photocatalytic hydrogen evolution activity, producing 164 mmol g-1 h-1 with a turnover frequency of 1.27 s-1. Furthermore, it generates 42.2 mmol gsub-1 of hydrogen via a non-recyclable-plastic-photoreforming process, achieving a total conversion of 98%; this offers a promising solution for mitigating plastic waste and simultaneously producing valuable energy sources.

Exceptional piezocatalytic H2 production of nitrogen-doped TiO2@carbon nanosheets induced by engineered piezoelectricity

Piezocatalytic hydrogen evolution is a promising strategy to generate sustainable energy. In this report, nitrogen-doped (N-doped) TiO2@ carbon nanosheets (N-TiO2@C NSs) was successfully synthesized using C3N4 as a multifunctional template. During the synthesis, the two-dimensional (2D) architecture of C3N4 nanosheets directed the synthesis of TiO2 nanosheets. In addition, nitrogens of C3N4 were doped into the TiO2 lattice. Simultaneously, C3N4 was transformed into N-doped carbon nanosheets. N doping broke the crystal symmetry of TiO2, which endowed TiO2 with promising piezoelectric properties. The N-doped carbon nanosheets derived from C3N4 improved charge carrier separation efficiency and served as a flexible support to inhibit structural damage under sonication. Therefore, the N-TiO2@C NSs exhibited highly efficient activity for piezocatalytic H2 production (6.4 mmol·g-1·h-1) in the presence of methanol, much higher than those of the previously reported piezocatalysts. Our method is hoped to provide a new strategy for designing highly efficient piezocatalysts.

Catalyzing Artificial Photosynthesis with TiO2 Heterostructures and Hybrids: Emerging Trends in a Classical yet Contemporary Photocatalyst

Titanium dioxide (TiO2) stands out as a versatile transition-metal oxide with applications ranging from energy conversion/storage and environmental remediation to sensors and optoelectronics. While extensively researched for these emerging applications, TiO2 has also achieved commercial success in various fields including paints, inks, pharmaceuticals, food additives, and advanced medicine. Thanks to the tunability of their structural, morphological, optical, and electronic characteristics, TiO2 nanomaterials are among the most researched engineering materials. Besides these inherent advantages, the low cost, low toxicity, and biocompatibility of TiO2 nanomaterials position them as a sustainable choice of functional materials for energy conversion. Although TiO2 is a classical photocatalyst well-known for its structural stability and high surface activity, TiO2-based photocatalysis is still an active area of research particularly in the context of catalyzing artificial photosynthesis. This review provides a comprehensive overview of the latest developments and emerging trends in TiO2 heterostructures and hybrids for artificial photosynthesis. It begins by discussing the common synthesis methods for TiO2 nanomaterials, including hydrothermal synthesis and sol-gel synthesis. It then delves into TiO2 nanomaterials and their photocatalytic mechanisms, highlighting the key advancements that have been made in recent years. The strategies to enhance the photocatalytic efficiency of TiO2, including surface modification, doping modulation, heterojunction construction, and synergy of composite materials, with a specific emphasis on their applications in artificial photosynthesis, are discussed. TiO2-based heterostructures and hybrids present exciting opportunities for catalyzing solar fuel production, organic degradation, and CO2 reduction via artificial photosynthesis. This review offers an overview of the latest trends and advancements, while also highlighting the ongoing challenges and prospects for future developments in this classical yet rapidly evolving field.

Photocatalytic H2 Generation: Controlled and Optimized Dispersion of Single Atom Co-Catalysts Based on Pt-TCPP Planar Adsorption on TiO2

When using single atoms (SAs) as a co-catalyst in photocatalytic H2 generation, achieving a well-dispersed, evenly distributed and adjustable SA surface density on a semiconductor surface is a challenging task. In the present work we use the planar adsorption of tetrakis-(4-carboxyphenyl)-porphyrin (TCPP) and its platinum coordinated analogue, Pt-TCPP, onto anatase TiO2 surfaces to establish a spatially controlled decoration of SAs. We show that the surface Pt SA density can be very well controlled by co-adsorption of Pt-TCPP and TCPP in the planar monolayer regime, and by adjusting the Pt-TCPP to TCPP ratio a desired well dispersed surface density of SAs up to 2.6×105  atoms μm-2 can be established (which is the most effective Pt SA loading for photocatalysis). This distribution and the SA state are maintained after a thermal treatment in air, and an optimized SA density as well as a most active form of Pt for photocatalytic H2 evolution can be established and maintained.

P-N Heterojunction Embedded CuS/TiO2 Bifunctional Photocatalyst for Synchronous Hydrogen Production and Benzylamine Conversion

The coupling of photocatalytic hydrogen production and selective oxidation of benzylamine is a topic of significant research interest. However, enhancing the bifunctional photocatalytic activity in this context is still a major challenge. The construction of Z-scheme heterojunctions is an effective strategy to enhance the activity of bifunctional photocatalysts. Herein, a p-n type direct Z-scheme heterojunction CuS/TiO2 is constructed using metal-organic framework (MOF)-derived TiO2 as a substrate. The carrier density is measured by Mott-Schottky under photoexcitation, which confirms that the Z-scheme electron transfer mode of CuS/TiO2 is driven by the diffusion effect caused by the carrier concentration difference. Benefiting from efficient charge separation and transfer, photogenerated electrons, and holes are directedly transferred to active oxidation and reduction sites. CuS/TiO2 also exhibits excellent bifunctional photocatalytic activity without noble metal cocatalysts. Among them, the H2 evolution activity of the CuS/TiO2 is found to be 17.1 and 29.5 times higher than that of TiO2 and CuS, respectively. Additionally, the yields of N-Benzylidenebenzylamine (NBB) are 14.3 and 47.4 times higher than those of TiO2 and CuS, respectively.

Coupling Fe-Co atomic pair to promote the selective reduction of nitroaromatics under mild conditions

Selectively reducing nitroaromatics into aromatic amines will not only remove nitroaromatic pollutants in waste effluents to reduce environmental risks, but also yield important feedstocks for chemical industrial manufactures. In this study, a FeCo-co-embedded N-doped Carbon (FeCo-N-C) catalyst with Fe-Co atomic pair has been identified with favorable activity, superior selectivity, excellent reusability, as well as outstanding performance in the treatment of real water. The combined results from theoretical study and experimental tests indicate that the improved catalytic performance of FeCo-N-C is owing to the narrowed band gap and electron delocalization caused by the Fe-Co atomic pair which can improve electron transport in its catalytic reaction. The results of isotope experiments and H* quenching experiments confirm that H2O is the source of hydrogen in catalytic reduction of PNP. FeCo-N-C is identified as a superior catalyst to replace multitudinous currently used noble-metal catalysts for the selective catalytic reduction of nitroaromatics in wastewater treatment.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Improved Charge Separation and CO2 Affinity of In2O3 by K Doping with Accompanying Oxygen Vacancies for Boosted CO2 Photoreduction

The CO2 photocatalytic conversion efficiency of the semiconductor photocatalyst is always inhibited by the sluggish charge transfer and undesirable CO2 affinity. In this work, we prepare a series of K-doped In2O3 catalysts with concomitant oxygen vacancies (OV) via a hydrothermal method, followed by a low-temperature sintering treatment. Owing to the synergistic effect of K doping and OV, the charge separation and CO2 affinity of In2O3 are synchronously promoted. Particularly, when P/P0 = 0.010, at room temperature, the CO2 adsorption capacity of the optimal K-doped In2O3 (KIO-3) is 2336 cm3·g-1, reaching about 6000 times higher than that of In2O3 (0.39 cm3·g-1). As a result, in the absence of a cocatalyst or sacrificial agent, KIO-3 exhibits a CO evolution rate of 3.97 μmol·g-1·h-1 in a gas-solid reaction system, which is 7.6 times that of pristine In2O3 (0.52 μmol·g-1·h-1). This study provides a novel approach to the design and development of efficient photocatalysts for CO2 conversion by element doping.

Construction of Monoatomic-Modified Defective Ti4+αTi3+1-αO2-δ Nanofibers for Photocatalytic Oxidation of HMF to Valuable Chemicals

Efficiently upgrading 5-hydroxymethylfurfural (HMF) into high-value-added products, such as 2,5-diformylfuran (DFF) and 2,5-furan dicarboxylic acid (FDCA), through a photocatalytic process by using solar energy has been incessantly pursued worldwide. Herein, a series of transition-metal (TM = Ni, Fe, Co, Cu) single atoms were supported on Ti4+αTi3+1-αO2-δ nanofibers (NFs) with certain defects (Ov), denoted as TM SAC-Ti4+αTi3+1-αO2-δ NFs (TM = Ni, Fe, Co, Cu), aiming to enhance the photocatalytic conversion of HMF. A super HMF conversion rate of 57% and a total yield of 1718.66 μmol g-1 h-1 (DFF and FDCA) surpassing that of the Ti4+αTi3+1-αO2-δ NFs by 1.6 and 2.1 times, respectively, are realized when TM is Co (Co SAC-Ti4+αTi3+1-αO2-δ NFs). Experiments combined with density functional theory calculation (DFT) demonstrate that the TM single atoms occupy the Ti site of Ti4+αTi3+1-αO2-δ NFs, which plays a dominant role in the photo-oxidation of HMF. Raman, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) characterizations confirm the strong electron local exchange interaction in TM SAC-Ti4+αTi3+1-αO2-δ NFs and demonstrate the substitution of Ti by the TM SACs. The projected density of states and charge density difference reveal that the strong interaction between metal-3d and O-2p orbitals forms Ti-O-TM bonds. The bonds are identified as the adsorption site, where TM single atoms on the surface of Ti4+αTi3+1-αO2-δ NFs reduce HMF molecule adsorption energy (Eads). Furthermore, the TM single atom modulates the electronic structure of TM SAC-Ti4+αTi3+1-αO2-δ NFs through electron transfer, leading to narrow band gaps of the photocatalysts and enhancing their photocatalytic performance. This study has uncovered a newer strategy for enhancing the photocatalytic attributes of semiconducting materials.

A Full-Spectrum ZnS Photocatalyst with Gradient Distribution of Atomic Copper Dopants and Concomitant Sulfur Vacancies for Highly Efficient Hydrogen Evolution

A rarely discussed phenomenon in the realm of photocatalytic materials involves the presence of gradient distributed dopants and defects from the interior to the surface. This intriguing characteristic has been successfully achieved in the case of ZnS through the incorporation of atomic monovalent copper ions (Cu+) and concurrent sulfur vacancies (Vs), resulting in a photocatalyst denoted as G-CZS1-x. Through the cooperative action of these atomic Cu dopants and Vs, G-CZS1-x significantly extends its photoabsorption range to encompass the full spectrum (200-2100 nm), which improves the solar utilization ability. This alteration enhances the efficiency of charge separation and optimizes Δ(H*) (free energy of hydrogen adsorption) to approach 0 eV for the hydrogen evolution reaction (HER). It is noteworthy that both surface-exposed atomic Cu and Vs act as active sites for photocatalysis. G-CZS1-x exhibits a significant H2 evolution rate of 1.01 mmol h-1 in the absence of a cocatalyst. This performance exceeds the majority of previously reported photocatalysts, exhibiting approximately 25-fold as ZnS, and 5-fold as H-CZS1-x with homogeneous distribution of equal content Cu dopants and Vs. In contrast to G-CZS1-x, the H adsorption on Cu sites for H-CZS1-x (ΔG(H*) = -1.22 eV) is excessively strong to inhibit the H2 release, and the charge separation efficiency for H-CZS1-x is relatively sluggish, revealing the positive role of a gradient distribution model of dopants and defects on activity enhancement. This work highlights the synergy of atomic dopants and defects in advancing photoactivity, as well as the significant benefit of the controllable distribution model of dopants and defects for photocatalysis.

P-doped Mn0.5Cd0.5S coupled with cobalt porphyrin as co-catalyst for the photocatalytic water splitting without using sacrificial agents

Photocatalytic water splitting over semiconductors is an important approach to solve the energy demand of human beings. Most photocatalytic H2 generation reactions are conducted in the presence of sacrificial agent. However, the use of sacrificial reagents increases the cost of hydrogen generation. Realizing photocatalytic water splitting for hydrogen production without the addition of sacrificial agents is a major challenge for photocatalysts. The porphyrin MTCPPOMe and P doped MnxCd1-xS make a significant contribution in facilitating the MnxCd1-xS photocatalytic pure water splitting to H2 reaction. Herein, a novel MTCPPOMe/P-MnxCd1-xS (M = 2H, Fe, Co, Ni) composite catalyst which can efficiently split pure water without using sacrificial agents is developed. As a result, the H2 generation rate of CoTCPPOMe/P-Mn0.5Cd0.5S is as high as 2.10 μmol h-1, which is 9.1 and 4.2 times higher than that of Mn0.5Cd0.5S (MCS) and P-Mn0.5Cd0.5S (P-MCS), respectively. P doped MnxCd1-xS inhibits the recombination of photogenerated carriers, and introduction of MTCPPOMe as co-catalyst enhances the reduction capacity. In summary, an efficient and economical photocatalystis prepared for pure water splitting to prepare hydrogen.

Copper Single-Atom Catalysts-A Rising Star for Energy Conversion and Environmental Purification: Synthesis, Modification, and Advanced Applications

Future renewable energy supply and green, sustainable environmental development rely on various types of catalytic reactions. Copper single-atom catalysts (Cu SACs) are attractive due to their distinctive electronic structure (3d orbitals are not filled with valence electrons), high atomic utilization, and excellent catalytic performance and selectivity. Despite numerous optimization studies are conducted on Cu SACs in terms of energy conversion and environmental purification, the coupling among Cu atoms-support interactions, active sites, and catalytic performance remains unclear, and a systematic review of Cu SACs is lacking. To this end, this work summarizes the recent advances of Cu SACs. The synthesis strategies of Cu SACs, metal-support interactions between Cu single atoms and different supports, modification methods including modification for carriers, coordination environment regulating, site distance effect utilizing, and dual metal active center catalysts constructing, as well as their applications in energy conversion and environmental purification are emphatically introduced. Finally, the opportunities and challenges for the future Cu SACs development are discussed. This review aims to provide insight into Cu SACs and a reference for their optimal design and wide application.

Scalable Photochromic Film for Solar Heat and Daylight Management

The adaptive control of sunlight through photochromic smart windows could have a huge impact on the energy efficiency and daylight comfort in buildings. However, the fabrication of inorganic nanoparticle and polymer composite photochromic films with a high contrast ratio and high transparency/low haze remains a challenge. Here, a solution method is presented for the in situ growth of copper-doped tungsten trioxide nanoparticles in polymethyl methacrylate, which allows a low-cost preparation of photochromic films with a high luminous transparency (luminous transmittance Tlum = 91%) and scalability (30 × 350 cm2 ). High modulation of visible light (ΔTlum = 73%) and solar heat (modulation of solar transmittance ΔTsol = 73%, modulation of solar heat gain coefficient ΔSHGC = 0.5) of the film improves the indoor daylight comfort and energy efficiency. Simulation results show that low-e windows with the photochromic film applied can greatly enhance the energy efficiency and daylight comfort. This photochromic film presents an attractive strategy for achieving more energy-efficient buildings and carbon neutrality to combat global climate change.

Non-noble metal single-atoms for oxygen electrocatalysis in rechargeable zinc-air batteries: recent developments and future perspectives

Ever-growing demands for zinc-air batteries (ZABs) call for the development of advanced electrocatalysts. Single-atom catalysts (SACs), particularly those for isolating non-noble metals (NBMs), are attracting great interest due to their merits of low cost, high atom utilization efficiency, structural tunability, and extraordinary activity. Rational design of advanced NBM SACs relies heavily on an in-depth understanding of reaction mechanisms. To gain a better understanding of the reaction mechanisms of oxygen electrocatalysis in ZABs and guide the design and optimization of more efficient NBM SACs, we herein organize a comprehensive review by summarizing the fundamental concepts in the field of ZABs and the recent advances in the reported NBM SACs. Moreover, the selection of NBM elements and supports of SACs and some effective strategies for enhancing the electrochemical performance of ZABs are illustrated in detail. Finally, the challenges and future direction in this field of ZABs are also discussed.

Liquid-Metal Molecular Scissors

Molecules are the smallest units of matter that can exist independently, relatively stable, maintaining their physical and chemical activities. The key factors that dominate the structures and properties of molecules include atomic species, alignment commands, and chemical bonds. Herein, we reported a generalized effect in which liquid metals can directly cut off oxygen-containing groups in molecular matter at room temperature, allowing the remaining groups to recombine to form functional materials. Thus, we propose basic liquid-metal scissors for molecular directional clipping and functional transformations. As a proof of concept, we demonstrate the capabilities of liquid-metal scissors and reveal that the gallium on the surface of liquid metals directly extracts oxygen atoms from H2O or CH3OH molecules to form oxides. After clipping, the remaining hydrogen atoms from the H2O molecules recombine to form H2, while the remaining fragments of CH3OH produce H2, carbon materials, and carboxylates. This finding refreshes our basic understanding of chemistry and should lead to the development of straightforward molecular weaving techniques, which can help to overcome the limitations of molecular substances with single purposes. It also opens a universal route for realizing future innovations in molecular chemical engineering, life sciences, energy and environment research, and biomedicine.

Halogen-Modulated 2D Coordination Polymers for Efficient Hydrogen Peroxide Photosynthesis under Air and Pure Water Conditions

H2 O2 is a widely used eco-friendly oxidant and a potential energy carrier. Photocatalytic H2 O2 production from water and O2 is an ideal approach with the potential to address the current energy crisis and environmental issues. Three zig-zag two-dimensional coordination polymers (2D CPs), named CuX-dptz, were synthesized by a rapid and facile method at room temperature, showing preeminent H2 O2 photoproduction performance under pure water and open air without any additives. CuBr-dptz exhibits a H2 O2 production rate high up to 1874 μmol g-1  h-1 , exceeding most reported photocatalysts under this condition, even comparable to those supported by sacrificial agents and O2 . The coordination environment of Cu can be modulated by halogen atoms (X=Cl, Br, I), which in turn affects the electron transfer process and finally determines the reaction activity. This is the first time that 2D CPs have been used for photocatalytic H2 O2 production in such challenging conditions, which provides a new pathway for the development of portable in situ H2 O2 photosynthesis devices.

dz2 Band Links Frontier Orbitals and Charge Carrier Dynamics of Single-Atom Cocatalyst-Aided Photocatalytic H2 Production

The Cu single-atom catalyst (SAC) supported on TiO2 exhibits outstanding efficacy in photocatalytic hydrogen evolution. The precise operational mechanism remains a subject of ongoing debate. The focus resides with the interplay linking heightened catalytic activity, dynamic valence state alterations of Cu atoms, and their hybridization with H2O orbitals, manifested in catalyst color changes. Taking anatase TiO2 (101) as a prototypical surface, we perform ab initio quantum dynamics simulation to reveal that the high activity of the Cu-SAC is due to the quasi-planar coordination structure of the Cu atom after H2O adsorption, allowing it to trap photoexcited hot electrons and inject them into the hybridized orbital between Cu and H2O. The observed alterations in the valence state and the coloration can be attributed to the H atom released during H2O dissociation and adsorbed onto the lattice O atom neighboring the Cu-SAC. Notably, this adsorption of H atoms puts the Cu-SAC into an inert state, as opposed to an activating effect reported previously. Our work clarifies the relationship between the high photocatalytic activity and the local dynamic atomic coordination structure, providing atomistic insights into the structural changes occurring during photocatalytic reactions on SACs.

Two-Dimensional Porphyrinic Metal-Organic Framework Composites as a Photocatalytic Platform for Chemoselective Hydrogenation

Chemoselective hydrogenation with high efficiency under ambient conditions remains a great challenge. Herein, an efficient photocatalyst, the 2D porphyrin metal-organic framework composite AmPy/Pd-PPF-1(Cu), featuring AmPy (1-aminopyrene) sitting axially on a paddle-wheel unit, has been rationally fabricated. The 2D AmPy/Pd-PPF-1(Cu) composite acts as a photocatalytic platform, promoting the selective hydrogenation of quinolines to tetrahydroquinolines with a yield up to 99%, in which ammonia borane serves as the hydrogen donor. The AmPy molecules coordinated on a 2D MOF not only enhance the light absorption capacity but also adjust the layer spacing without affecting the network structure of 2D Pd-PPF-1(Cu) nanosheets. Through deuterium-labeling experiments, in situ X-ray photoelectron spectroscopy, electron paramagnetic resonance studies, and density functional theory calculations, it is disclosed that Cu paddle-wheel units in 2D AmPy/Pd-PPF-1(Cu) nanosheets behave as the active site for transfer hydrogenation, and metalloporphyrin ligand and axial aminopyrene molecules can enhance the light absorption capacity and excite photogenerated electrons to Cu paddle-wheel units, assisting in photocatalysis.

K-N Bridge-Mediated charge separation in hollow g-C3N4 Frameworks: A bifunctional photocatalysts towards efficient H2 and H2O2 production

The development of bifunctional photocatalysts for enhancing hydrogen (H2) and hydrogen peroxide (H2O2) production from water is essential in addressing environmental and energy issues. However, the practical implementation of photocatalytic technology is still constrained by the inadequate separation of photo-generated charge carriers. Herein, potassium (K) atoms are introduced into the interlayers of graphitic carbon nitride (g-C3N4) with a hollow hexagonal structure (K-TCN) and are coordinated with N atoms in adjacent layers. The presence of K-N coordination serves as a layer bridge, facilitating the separation of charge carriers. The hollow hexagonal structure reduces the distance over which photogenerated electrons migrate to the surface, thereby enhancing the reaction kinetics. Consequently, the optimized K-TCN exhibits a dramatically improved photocatalytic H2 (941.6 μmol g-1h-1 with platinum (Pt) as the cocatalyst) and H2O2 (347.6 μmol g-1h-1) generation as compared to hollow g-C3N4 (TCN) and bulk g-C3N4 nanosheet (CN) without K-N bridge under visible light irradiation. The unique design holds promising potential for developing highly efficient bifunctional photocatalysts towards producing renewable fuels and value-added chemicals.

S-scheme Cu3P/TiO2 heterojunction for outstanding photocatalytic water splitting

TiO2 photocatalysts are of great interest in the fields of environmental purification, new energy and so on, because of their non-toxicity, high stability, high redox ability and low cost. However, the photogenerated carriers are severely recombined, which limits the application of TiO2 photocatalysts. Herein, S-scheme Cu3P/TiO2 heterojunction composites were successfully synthesized by a simple and efficient microwave hydrothermal method, and the results show that the hydrogen production rate of Cu3P/TiO2 is 5.83 mmol∙g-1∙h-1 under simulated sunlight irradiation, which is 7.3 and 83.3 times higher than that of pure TiO2 and Cu3P, respectively. This excellent performance is derived from the internal electric field (IEF) and energy band bending generated by the S-scheme heterojunction formed between Cu3P and TiO2. The density functional theory (DFT) calculation indicates that the Cu3P possess smaller work function and more negative conduction band (CB) position than that of TiO2, which is very conducive to greatly improve the H+ reduction ability and hydrogen production performance. This work provides a new idea for the reveal of electron transfer paths and active sites in S-scheme heterojunctions and deepens the mechanism understanding.

Accelerate charge separation in Cu2O/MoO2 photocathode for photoelectrocatalytic hydrogen evolution

Photoelectrocatalyzing water reduction is a potential approach to building a green and sustainable society. As a benchmark photocathode, Cu2O receives much attention but faces serious charge recombination and photocorrosion. This work prepared an excellent Cu2O/MoO2 photocathode via in situ electrodeposition. A systematical study of theory and experiment demonstrates that MoO2 not only effectively passivates the surface state of Cu2O as well as accelerates reaction kinetics as a cocatalyst, but also promotes the directional migration and separation of photogenerated charge. As expected, the constructed photocathode exhibits a highly enhanced photocurrent density and an appealing energy transformation efficacy. Importantly, MoO2 can inhibit the reduction of Cu+ in Cu2O via a formed internal electric field and shows excellent photoelectrochemical stability. These findings pave the way to designing a high-activity photocathode with high stability.

Reversed Electron Transfer in Dual Single Atom Catalyst for Boosted Photoreduction of CO2

Photogenerated charge localization on material surfaces significantly affects photocatalytic performance, especially for multi-electron CO2 reduction. Dual single atom (DSA) catalysts with flexibly designed reactive sites have received significant research attention for CO2 photoreduction. However, the charge transfer mechanism in DSA catalysts remains poorly understood. Here, for the first time, a reversed electron transfer mechanism on Au and Co DSA catalysts is reported. In situ characterizations confirm that for CdS nanoparticles (NPs) loaded with Co or Au single atoms, photogenerated electrons are localized around the single atom of Co or Au. In DSA catalysts, however, electrons are delocalized from Au and accumulate around Co atoms. Importantly, combined advanced spectroscopic findings and theoretical computation evidence that this reversed electron transfer in Au/Co DSA boosts charge redistribution and activation of CO2 molecules, leading to highly significantly increased photocatalytic CO2 reduction, for example, Au/Co DSA loaded CdS exhibits, respectively, ≈2800% and 700% greater yields for CO and CH4 compared with that for CdS alone. Reversed electron transfer in DSA can be used for practical design for charge redistribution and to boost photoreduction of CO2 . Findings will be of benefit to researchers and manufacturers in DSA-loaded catalysts for the generation of solar fuels.

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.

Highly electron-deficient ultrathin Co nanosheets supported on mesoporous Cr2O3 for catalytic hydrogen evolution from ammonia borane

The hydrolysis of ammonia borane (NH3BH3) on metal-based heterogeneous catalysts under light irradiation has been considered as an efficient technique for hydrogen (H2) generation, in which the activity of the catalyst can be improved by increasing the electron density of the active metal. However, studies focused on reducing the electron density of the active metal are rare. Here, we report an electron density manipulation strategy to prepare highly electron-deficient ultrathin Co nanosheets via transferring nanosheets to support mesoporous Cr2O3 by simple one-step in situ reduction (denoted as Co/Cr2O3). X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectra confirm the formation of electron-deficient Co nanosheets and the Co-O-Cr bond due to electron transfer from the nanosheets to mesoporous Cr2O3. Importantly, the Co-O-Cr bond can work as a bridge to accelerate the electron transfer under light irradiation and then improve the electron-deficiency degree of Co nanosheets. As a result, the optimal Co/Cr2O3 exhibits a high intrinsic catalytic performance with the turnover frequency (TOF) value of 106.8 min-1 and significantly reduces the activation energy (Ea) to 16.8 kJ mol-1 under visible light irradiation, which make it among the best ever recorded monometallic Co-based catalyst with enriched electrons. The density functional theory (DFT) calculation results suggest that the electron-deficient Co nanosheets are responsible for the greatly decreased H2O activation and dissociation energy barriers and then the acceleration of the evolution of H2. The work provides a new perspective for designing high efficiency catalysts for H2 production, which is beneficial for relative energy conversion and storage catalysis.

Slow-release synthesis of Cu single-atom catalysts with the optimized geometric structure and density of state distribution for Fenton-like catalysis

Single-atom Fenton-like catalysis has attracted significant attention, yet the quest for controllable synthesis of single-atom catalysts (SACs) with modulation of electron configuration is driven by the current disadvantages of poor activity, low selectivity, narrow pH range, and ambiguous structure-performance relationship. Herein, we devised an innovative strategy, the slow-release synthesis, to fabricate superior Cu SACs by facilitating the dynamic equilibrium between metal precursor supply and anchoring site formation. In this strategy, the dynamics of anchoring site formation, metal precursor release, and their binding reaction kinetics were regulated. Bolstered by harmoniously aligned dynamics, the selective and specific monatomic binding reactions were ensured to refine controllable SACs synthesis with well-defined structure-reactivity relationship. A copious quantity of monatomic dispersed metal became deposited on the C3N4/montmorillonite (MMT) interface and surface with accessible exposure due to the convenient mass transfer within ordered MMT. The slow-release effect facilitated the generation of targeted high-quality sites by equilibrating the supply and demand of the metal precursor and anchoring site and improved the utilization ratio of metal precursors. An excellent Fenton-like reactivity for contaminant degradation was achieved by the Cu1/C3N4/MMT with diminished toxic Cu liberation. Also, the selective ·OH-mediated reaction mechanism was elucidated. Our findings provide a strategy for regulating the intractable anchoring events and optimizing the microenvironment of the monatomic metal center to synthesize superior SACs.