Synergistic Pd Single Atoms, Clusters, and Oxygen Vacancies on TiO2 for Photocatalytic Hydrogen Evolution Coupled with Selective Organic Oxidation

Local weak hydrogen bonds induced dipole-dipole interactions in polymer for enhancing photocatalytic oxidation

Functionalizing organic polymers is an effective strategy for enhancing their photocatalytic performance. However, this approach is currently limited by specific motifs, complex preparation methods, and an unclear electron transfer mechanism. Here, we present a meticulously designed structure of perylene diimide connected with poly (barbituric acid trimer) through self-assembled hydrogen bonding. In particular, the local chemical environment of the two components is adjusted by hydrogen bond-induced dipole-dipole interactions, leading to the emergence of a significant inherent electric field. Additionally, the formation of hydrogen bonds provides electronic pathways that facilitate charge transfer from perylene to adjacent units. Moreover, the distinctive electronic structure enhances polarity transfer and improves activation and adsorption capabilities for reactive molecules. Ultimately, B-PDI exhibits outstanding oxidation rates for benzylamine to N-benzylidene-benzylamine (10.03 mmol g-1h-1) and selectivity (>99.99 %). Our work offers a widely popular approach for enhancing the photocatalytic activity of organic semiconductor materials by constructing hydrogen bonds in heterogeneous molecules.

Simultaneously activating molecular oxygen and surface lattice oxygen on Pt/TiO2 for low-temperature CO oxidation

Developing high-performance Pt-based catalysts with low Pt loading is crucial but challenging for CO oxidation at temperatures below 100 °C. Herein, we report a Pt-based catalyst with only a 0.15 wt% Pt loading, which consists of Pt-Ti intermetallic single-atom alloy (ISAA) and Pt nanoparticles (NP) co-supported on a defective TiO2 support, achieving a record high turnover frequency of 11.59 s-1 at 80 °C and complete conversion of CO at 120 °C. This is because the coexistence of Pt-Ti ISAA and Pt NP significantly alleviates the competitive adsorption of CO and O2, enhancing the activation of O2. Furthermore, Pt single atom sites are stabilized by Pt-Ti ISAA, resulting in distortion of the TiO2 lattice within Pt-Ti ISAA. This distortion activates the neighboring surface lattice oxygen, allowing for the simultaneous occurrence of the Mars-van Krevelen and Langmuir-Hinshelwood reaction paths at low temperatures.

Photoredox coupling of carbon dioxide reduction with tetracycline oxidation using excited-state bismuth and cobalt dual sites over cobalt-tailored bismuth oxychloride

Photocatalytic carbon dioxide (CO2) conversion and simultaneous pollutant oxidation in a single system are promising approaches to mitigate energy and environmental challenges. However, the limited availability of active photocatalyst sites led to slow reaction kinetics and poor selectivity. Current research has predominantly focused on ground-state reactive sites of semiconductors, with less emphasis on active sites in their excited states. Therefore, gaining insights into the active sites in the excited state of semiconductors could provide a significant breakthrough in understanding the photocatalytic reaction mechanism. In this study, cobalt-doped bismuth oxychloride nanosheets containing abundant oxygen vacancies (OVs) were used as a model to investigate the active sites in excited states. These nanosheets were used to integrate CO2 reduction with tetracycline (TC) oxidation. Combining theoretical calculations with in situ characterizations revealed that under excited-state conditions photogenerated electrons transfer from cobalt (Co) dopants to OVs and subsequently to bismuth (Bi) atoms, forming Bi(3-x)+ sites enriched with excited electrons. These excited-electron-rich Bi(3-x)+ sites and electron-deficient Co sites contribute to CO2 reduction and TC oxidation, respectively. This study provides a comprehensive understanding of active sites in the excited state in doped semiconductors at the atomic level, reinforcing their potential for synergistic CO2 reduction and pollutant degradation.

Cu@CuOx/WO3 with photo-regulated singlet oxygen and oxygen adatoms generation for selective photocatalytic aromatic amines to imines

Photocatalysts can absorb light and activate molecular O2 under mild conditions, but the generation of unsuitable reactive oxygen species often limits their use in synthesizing fine chemicals. To address this issue, we disperse 1 wt% copper on tungsten trioxide (WO3) support to create an efficient catalyst for selective oxidative coupling of aromatic amines to imines under sunlight irradiation at room temperature. Copper consists of a metallic copper core and an oxide shell. Experimental and density functional theory calculations have confirmed that Cu2O is the primary activation site. Under λ < 475 nm, the light excites electrons of the valence bands in Cu2O and WO3, which activate O2 to superoxide radical •O2-. Then rapidly transforms into oxygen adatoms (•O) and oxygen anion radicals (•O-) species on the surface of Cu2O. Simultaneously, it is captured by holes in the WO3 valence band to generate singlet oxygen (1O2). •O bind to 1O2 promoting the coupling reaction of amines. When λ > 475 nm, intense light absorption due to the localized surface plasmon resonance excites numerous electrons in Cu to promote the oxidative coupling with the adsorbed O2. This study presents a promising approach towards the design of high-performance photocatalysts for solar energy conversion and environmentally-friendly oxidative organic synthesis.

Switch Volmer-Heyrovsky to Volmer-Tafel Pathway for Efficient Acidic Electrocatalytic Hydrogen Evolution by Correlating Pt Single Atoms with Clusters

As cost-effective catalysts, platinum (Pt) single-atom catalysts (SACs) have attracted substantial attention. However, most studies indicate that Pt SACs in acidic hydrogen evolution reaction (HER) follow the slow Volmer-Heyrovsky (VH) mechanism instead of the fast kinetic Volmer-Tafel (VT) pathway. Here, this work propose that the VH mechanism in Pt SACs can be switched to the faster VT pathway for efficient HER by correlating Pt single atoms (SAs) with Pt clusters (Cs). Our calculations reveal that the correlation between Pt SAs and Cs significantly impacts the electronic structure of exposed Pt atoms, lowering the adsorption barrier for atomic hydrogen and enabling a faster VT mechanism. To validate these findings, this work purposely synthesize three catalysts: l-Pt@MoS2, m-Pt@MoS2 and h-Pt@MoS2 with low, moderate, and high Pt-loading, having different distributions of Pt SAs and Cs. The m-Pt@MoS2 catalyst with properly correlating Pt SAs and Cs exhibits outstanding performance with an overpotential of 47 mV and Tafel slope of 32 mV dec-1. Further analysis of the Tafel values confirms that the m-Pt@MoS2 sample indeed follows the VT reaction mechanism, aligning with the theoretical findings. This study offers a deep understanding of the synergistic mechanism, paving a way for designing novel-advanced catalysts.

Surface Engineered Single-atom Systems for Energy Conversion

Single-atom catalysts (SACs) are demonstrated to show exceptional reactivity and selectivity in catalytic reactions by effectively utilizing metal species, making them a favorable choice among the different active materials for energy conversion. However, SACs are still in the early stages of energy conversion, and problems like agglomeration and low energy conversion efficiency are hampering their practical applications. Substantial research focus on support modifications, which are vital for SAC reactivity and stability due to the intimate relationship between metal atoms and support. In this review, a category of supports and a variety of surface engineering strategies employed in SA systems are summarized, including surface site engineering (heteroatom doping, vacancy introducing, surface groups grafting, and coordination tunning) and surface structure engineering (size/morphology control, cocatalyst deposition, facet engineering, and crystallinity control). Also, the merits of support surface engineering in single-atom systems are systematically introduced. Highlights are the comprehensive summary and discussions on the utilization of surface-engineered SACs in diversified energy conversion applications including photocatalysis, electrocatalysis, thermocatalysis, and energy conversion devices. At the end of this review, the potential and obstacles of using surface-engineered SACs in the field of energy conversion are discussed. This review aims to guide the rational design and manipulation of SACs for target-specific applications by capitalizing on the characteristic benefits of support surface engineering.

Hollow structured CdS@ZnIn2S4 Z-scheme heterojunction for bifunctional photocatalytic hydrogen evolution and selective benzylamine oxidation

Photocatalytic hydrogen evolution (PHE) is frequently constrained by inadequate light utilization and the rapid combination rate of the photogenerated electron-hole pairs. Additionally, conventional PHE processes are often facilitated by the addition of sacrificial reagents to consume photo-induced holes, which makes this approach economically unfavorable. Herein, we designed a spatially separated bifunctional cocatalyst decorated Z-scheme heterojunction of hollow structured CdS (HCdS) @ZnIn2S4 (ZIS), which was prepared by a sacrificial hard template method followed by photo-deposition. Consequently, PdOx@HCdS@ZIS@Pt exhibited efficient PHE (86.38 mmol·g-1·h-1) and benzylamine (BA) oxidation coupling (164.75 mmol·g-1·h-1) with high selectivity (97.34 %). The unique hollow core-shelled morphology and bifunctional cocatalyst loading in this work hold great potential for the design and synthesis of bifunctional Z-scheme photocatalysts.

Multisite Cocatalysis: Single atomic Pt2+/Pt0 active sites synergistically improve the simulated sunlight driven H2O-to-H2 conversion performance of Sb2S3 nanorods

Single atomic metal (SAM) cocatalysis is a potential strategy to improve the performance of photocatalytic materials. However, the cocatalytic mechanism of SAM sites in different valence states is rarely reported. Herein, single atomic Pt2+/Pt0 active sites were anchored on Sb2S3 nanorods to synergistically improve the photoactivity for hydrogen production under simulated sunlight. Experimental results and density functional theory calculations indicated that the coexistence of single atomic Pt2+/Pt0 sites synergistically improves the broadband light harvesting and promotes the Sb2S3-to-Pt electron transfer following inhibited photoexciton recombination kinetics and enhanced H proton adsorption capacity, resulting higher and more durable photoactivity for hydrogen production. Therefore, the optimal Sb2S3-Pt0.9‰ composite catalyst achieved remarkably enhanced hydrogen evolution rate of 1.37 mmol∙g-1∙h-1 (about 105-fold greater of that of Sb2S3 NRs) under faintly alkaline condition, and about 5.41 % of apparent quantum yield (AQY700 nm) was achieved, which shows obvious superiority in hydrogen production by contrasting with the reported Sb2S3-based photocatalysts and conventional semiconductor photocatalytic materials modified with noble metals. This study elucidate a well-defined mechanism of multisite cocatalysis for photoactivity improvement.

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.

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

Mechanism-Guided Design of Photocatalysts for CO2 Reduction toward Multicarbon Products

Photocatalytic reduction of CO2 to high value-added multicarbon (C2+) products is an important way to achieve sustainable production of green energy but limited by the low efficiency of catalysts. One fundamental issue lies in the high complexity of catalyst structure and reaction process, making the rational catalyst design and targeted performance optimization a grand challenge. Herein, we performed a mechanism-guided design of photocatalysts for CO2 reduction by using the experimentally reported Cu doped TiO2 (Cu-TiO2) with high C3H8 selectivity and well-defined structure as the prototype. Our mechanistic study highlights three key factors for C3H8 formation, i.e., formation of double O vacancies (Vdi-O) for selectivity, C-C coupling for activity, and Vdi-O recovery for durability. More importantly, Vdi-O formation/recovery and C-C coupling are negatively correlated, indicating that ideal candidates should achieve a balance between oxygen vacancy (VO) formation and C-C coupling. On this basis, TiO2 with the doping of two adjacent Cu atoms (Cu-Cu-TiO2) was designed with enhanced performance for CO2 photoreduction toward C3H8. Furthermore, a simple descriptor (Nμ, "effective d electron number") based on inherent atomic properties was constructed to uncover the underlying causes of the performance variation of different systems. These results provide new insights into the "structure-performance" relation of metal oxide-based photocatalysts, thus offering useful strategies for the rational design of excellent catalysts for CO2 photoreduction.

Rapid Charge Transfer Endowed by Interfacial Ni-O Bonding in S-scheme Heterojunction for Efficient Photocatalytic H2 and Imine Production

Cooperative coupling of H2 evolution with oxidative organic synthesis is promising in avoiding the use of sacrificial agents and producing hydrogen energy with value-added chemicals simultaneously. Nonetheless, the photocatalytic activity is obstructed by sluggish electron-hole separation and limited redox potentials. Herein, Ni-doped Zn0.2 Cd0.8 S quantum dots are chosen after screening by DFT simulation to couple with TiO2 microspheres, forming a step-scheme heterojunction. The Ni-doped configuration tunes the highly active S site for augmented H2 evolution, and the interfacial Ni-O bonds provide fast channels at the atomic level to lower the energy barrier for charge transfer. Also, DFT calculations reveal an enhanced built-in electric field in the heterojunction for superior charge migration and separation. Kinetic analysis by femtosecond transient absorption spectra demonstrates that expedited charge migration with electrons first transfer to Ni2+ and then to S sites. Therefore, the designed catalyst delivers drastically elevated H2 yield (4.55 mmol g-1  h-1 ) and N-benzylidenebenzylamine production rate (3.35 mmol g-1  h-1 ). This work provides atomic-scale insights into the coordinated modulation of active sites and built-in electric fields in step-scheme heterojunction for ameliorative photocatalytic performance.

Anchoring Bi2S3 quantum dots on flower-like TiO2 nanostructures to boost photoredox coupling of H2 evolution and oxidative organic transformation

The rational integration of semiconductor quantum dots (QDs) with anatase TiO2 nanostructures is a promising strategy to develop efficient photocatalysts. Herein, Bi2S3QD/TiO2 photocatalyst was constructed by controllably depositing Bi2S3 QDs on flower-like TiO2 nanostructures and used for the photocatalytic redox-coupling reaction of H2 evolution and oxidative transformation of benzyl alcohol. The abundant amino groups in TiO2 nanostructures served as the anchoring sites for uniform growth of Bi2S3 QDs. The anchoring of Bi2S3 QDs onto TiO2 nanostructures not only enhanced the photoabsorption ability and the photogenerated charge separation efficiency but also afforded powerful photogenerated charge carriers and abundant active sites for the photocatalytic reaction. As a result, the Bi2S3QD/TiO2 photocatalyst exhibited a favorable performance in the redox-coupling reaction, providing the high production rates of H2 up to 4.75 mmol·gcat-1·h-1 and benzaldehyde up to 6.12 mmol·gcat-1·h-1, respectively, as well as an excellent stability in the long-term photocatalytic reaction. Meanwhile, a trace amount of water in the reaction system could act as a promoter to accelerate the photocatalytic redox-coupling reaction. The photocatalytic mechanism following S-scheme heterojunction was proposed according to the systematic characterizations and experimental results. This work offers some insight into the rational construction of efficient and cost-effective photocatalysts for the conversion of solar to chemical energy.

Construction of novel photocatalysts for efficient hydrogen evolution: The key role of natural halloysite nanotubes

Hydrogen (H2) evolution by photocatalytic water splitting is a potential strategy to solve worldwide energy shortage. Sulfide nanocatalysts showed great potential for H2 evolution, but suffered from low charge separation efficiency and easy agglomeration. In this work, ZnIn2S4 (ZIS) nanoflowers were anchored onto the surface of halloysite nanotubes (HNTs) modified by ethylenediaminetetraacetic acid (EDTA). Photocatalyst 3ZnIn2S4-HNTs/EDTA3 (3ZIS-HNTs/E3) displayed the optimum H2 evolution rate of 10.4 mmol·g-1·h-1, being 3.4 times as that of the original ZIS. Moreover, 3ZIS-HNTs/E3 presented satisfied property in the photocatalytic hydrogenation reaction of 4-nitrophenol to produce 4-aminophenol. HNTs as substrates not only hindered the growth and agglomeration of ZIS, but also induced more S vacancies in ZIS. The production of Schottky junctions between ZIS and Pt, the high utilization of light energy in tubular HNTs, and the trapping effect of EDTA for photogenerated h+ were all favorable for enhancing the catalytic property. The density functional theory (DFT) calculations showed that 3ZIS-HNTs/E3 with more S vacancies had the lowest adsorption energy and the most appropriate ΔGH* for H* to enhance the H2 evolution efficiency, which was consistent with the experimental catalytic results. This study contributes a novel thought for synthesizing composites on the basis of natural minerals for taking part in and enhancing the catalytic performance.

Low-Temperature Photothermal Therapy Platform Based on Pd Nanozyme-Modified Hydrogenated TiO2

In photothermal treatments (PTTs), normal tissues around cancerous tumors get injured by excessive heat, whereas damaged cancer cells are easily restored by stress-induced heat shock proteins (HSPs) at low temperatures. Therefore, to achieve a unique tumor microenvironment (TME), it is imperative to increase PTT efficiency and reduce normal tissue injury by adopting appropriate reactive oxygen species (ROS) and lipid peroxides (LPO) cross-linked with HSPs. In the present research, a potential strategy for mild photothermal treatments (mPTTs) was proposed by initiating localized catalytic chemical reactions in TME based on Pd nanozyme-modified hydrogenated TiO2 (H-TiO2@Pd). In vitro and in vivo evaluations demonstrated that H-TiO2@Pd had good peroxidase-like activities (POD), glutathione oxidase-like activities (GSHOx), and photodynamic properties and also satisfactory biocompatibility for 4T1 cells. Localized catalytic chemical reactions in H-TiO2@Pd significantly depleted GSH to downregulate the protein expression of GPX4 and promoted the accumulation of LPO and ROS, which consumed HSP70 or inhibited its function in 4T1 cells. Hence, the as-constructed low-temperature photothermal therapeutic platform based on Pd nanozyme-modified H-TiO2 can be a promising candidate to develop a safe and effective mPTT for cancer treatments.

DNAzyme-Mediated Biodeposition Coupling Adjustable Cascade Electric Fields for Photoelectrochemical Telomerase Activity Monitoring

Telomerase, as a specialized reverse transcriptase, plays a vital role in early cancer diagnostics and prognosis; thus, developing efficient sensing technologies is of vital importance. Herein, an innovative "signal-on-off" photoelectrochemical (PEC) sensing platform was developed for ultrasensitive evaluation of telomerase activity based on an electron-transfer tunneling distance regulation strategy and DNAzyme-triggerable biocatalytic precipitation. Concretely, cascade internal electric fields between CuInS2 quantum dots (QDs), graphitic carbon nitride nanosheets (g-C3N4 NSs), and TiO2 nanorod arrays (NRAs) were developed to realize cascade electron extraction and hole transfer. Enabled by such a design, an effective "signal-on" state to gain a progressively enhanced PEC output was designed by suppressing the photogenerated electron-hole pair recombination. With the introduction of hairpin probe H2 and the subsequent extension of the primer sequence driven by the target telomerase, the CuInS2 QDs labeled with hairpin probe H1 were programmatically unfolded, resulting in CuInS2 QDs' close proximity to the working electrode away from the cascade interface, accompanied by the formation of G-quadruplex/hemin complexes. The gradual undermining of tunneling distance and implantation of DNAzyme-initiating biocatalytic precipitation tremendously induced the sluggish migration kinetics of the photoinduced charge, accompanied by the photocurrent intensity decrement, leading to the "signal-off" state. Under optimized conditions, the as-prepared PEC biosensor realizes ultrasensitive detection of telomerase activity from 10 to 105 cell·mL-1 with a detection limitation of 3 cells·mL-1. As a proof of concept, this well-designed method provides new insights into signal amplification for telomerase activity evaluation and also presents promising potential for further development in drug screening, healthcare diagnostics, and biological assays.

Improved Sensitivity and Selectivity of Glutathione Detection through Target-Driven Electron Donor Generation in Photoelectrochemical Electrodes

Glutathione (GSH) plays a vital role in many physiological processes, and its abnormal levels have been found to be associated with several diseases. In contrast to traditional methods using electron donor-containing electrolytes for photoelectrochemical (PEC) sensing, in this study, a target-driven electron donor generation in a PEC electrode was developed to detect GSH. Using well-aligned TiO2 nanotube arrays (TNTs) as the PEC substrate, mesoporous MIL-125(Ti) was grown in the TNTs through an in situ solvothermal method and subsequent two-step annealing treatment. The accommodation capacity of mesoporous MIL-125(Ti) allows a well loading of cystine and Pt nanoclusters (NCs). Taking advantage of the specific cleavage ability of disulfide bonds by GSH, cystine was converted to cysteine, which served as the electron donor for the PEC process. Benefiting from the confinement effect of mesoporous MIL-125(Ti), cysteine was effectively oxidized to cysteine sulfinic acid by the photogenerated holes. Importantly, the highly active Pt NCs decorated in the mesopores not only improved the charge transfer but also accelerated the above oxidation reaction. The synergistic effect of these factors enabled the efficient separation of the photogenerated electron-hole pairs, which induced a significant photocurrent increase and in turn led to the high-sensitivity detection of GSH. Consequently, the proposed PEC biosensor exhibited excellent performance in the detection of GSH in serum specimens. The target-driven electron donor generation designed in this study might open a new route for developing sensitive and selective PEC biosensors with application in complex biological environments.

Zinc Vacancy Promotes Photo-Reforming Lignin Model to H2 Evolution and Value-Added Chemicals Production

Lignin, rich in β-O-4 bonds and aromatic structure, is a renewable and potential resource for value-added chemicals and promoting H2 evolution. However, direct photo-reforming lignin remains a huge challenge due to its recalcitrant structure. Herein, a collaborative strategy is proposed by dispersing Pt on zinc-vacancy-riched ZnIn2 S4 (Pt/VZn -ZIS) for revealing the effect of lignin structure during photo-reforming process with lignin models. And a series of theoretical calculations and experimental results show that lignin model substances with more nucleophilic group structures will have a stronger tendency to occur the photo-reforming reactions. In addition, benefiting of Pt-S electronic channel is formed by occupying Pt atom onto zinc vacancies in ZnIn2 S4 , which can effectively reduce the energy barrier of H2 evolution and accompany the selective oxidation of lignin model from Cα-OH to Cα = O under simulated sunlight. The natural lignin is used to further demonstrate this selective oxidation mechanism. The presented work demonstrates the photo-reforming lignin model mechanism and the influence of lignin-structure during the process of photo-reforming.

Reactive Deposition Versus Strong Electrostatic Adsorption (SEA): A Key to Highly Active Single Atom Co-Catalysts in Photocatalytic H2 Generation

In recent years, the use of single atoms (SAs) has become of a rapidly increasing significance in photocatalytic H2 generation; here SA noble metals (mainly Pt SAs) can act as highly effective co-catalysts. The classic strategy to decorate oxide semiconductor surfaces with maximally dispersed SAs relies on "strong electrostatic adsorption" (SEA) of suitable noble metal complexes. In the case of TiO2 - the classic benchmark photocatalyst - SEA calls for adsorption of cationic Pt complexes such as [(NH3 )4 Pt]2+ which then are thermally reacted to surface-bound SAs. While SEA is widely used in literature, in the present work it is shown by a direct comparison that reactive attachment based on the reductive anchoring of SAs, e.g., from hexachloroplatinic(IV) acid (H2 PtCl6 ) leads directly to SAs in a configuration with a significantly higher specific activity than SAs deposited with SEA - and this at a significantly lower Pt loading and without any thermal post-deposition treatments. Overall, the work demonstrates that the reactive deposition strategy is superior to the classic SEA concept as it provides a direct electronically well-connected SA-anchoring and thus leads to highly active single-atom sites in photocatalysis.

Progress in photocatalytic CO2 reduction based on single-atom catalysts

Reduced CO₂ emissions, conversion, and reuse are critical steps toward carbon peaking and carbon neutrality. Converting CO₂ into high-value carbon-containing compounds or fuels may effectively address the energy shortage and environmental issues, which is consistent with the notion of sustainable development. Photocatalytic CO₂ reduction processes have become one of the research focuses, where single-atom catalysts have demonstrated significant benefits owing to their excellent percentage of atom utilization. However, among the crucial challenges confronting contemporary research is the production of efficient, low-cost, and durable photocatalysts. In this paper, we offer a comprehensive overview of the study growth on single-atom catalysts for photocatalytic CO₂ reduction reactions, describe several techniques for preparing single-atom catalysts, and discuss the advantages and disadvantages of single-atom catalysts and present the study findings of three single-atom photocatalysts with TiO₂, g-C₃N₄ and MOFs materials as carriers based on the interaction between single atoms and carriers, and finally provide an outlook on the innovation of photocatalytic CO₂ reduction reactions.

In Situ Monitoring of the Spatial Distribution of Oxygen Vacancies and Enhanced Photocatalytic Performance at the Single-Particle Level

Oxygen vacancies (OVs) on specific sites/facets can strengthen the interaction between reactants and oxide surfaces, facilitating interfacial charge transfer. However, precise monitoring of the spatial distribution of OVs remains a grand challenge. We report here that a single-particle spectroscopy technique addresses this challenge by establishing a positive correlation relationship between defects and bound exciton luminescence across different facets. Taking monoclinic BiVO₄ as an example, on the basis of theoretical guidance, by in situ tracking the PL lifetimes and PL spectra of different facets on single particles before and after hydrogen treatment, we provide evidence that the PL emission originates from the OV state and determine that OVs is more inclined to be generated at the {010} facets. This anisotropic defect engineering significantly prolongs the lifetime of carriers and accelerates the activation of molecular oxygen. These findings not only verify preference rules of OVs in metal oxides but also provide a time-space-resolved monitoring method.

Enhancement of Visible-Light-Driven Hydrogen Evolution Activity of 2D π-Conjugated Bipyridine-Based Covalent Organic Frameworks via Post-Protonation

Photocatalytic hydrogen (H₂ ) evolution represents a promising and sustainable technology. Covalent organic frameworks (COFs)-based photocatalysts have received growing attention. A 2D fully conjugated ethylene-linked COF (BTT-BPy-COF) was fabricated with a dedicated designed active site. The introduced bipyridine sites enable a facile post-protonation strategy to fine-tune the actives sites, which results in a largely improved charge-separation efficiency and increased hydrophilicity in the pore channels synergically. After modulating the degree of protonation, the optimal BTT-BPy-PCOF exhibits a remarkable H₂ evolution rate of 15.8 mmol g^-1  h^-1 under visible light, which surpasses the biphenyl-based COF 6 times. By using different types of acids, the post-protonation is proved to be a potential universal strategy for promoting photocatalytic H₂ evolution. This strategy would provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

Light-Induced Agglomeration of Single-Atom Platinum in Photocatalysis

With recent advances in the field of single-atoms (SAs) used in photocatalysis, an unprecedented performance of atomically dispersed co-catalysts has been achieved. However, the stability and agglomeration of SA co-catalysts on the semiconductor surface may represent a critical issue in potential applications. Here, the photoinduced destabilization of Pt SAs on the benchmark photocatalyst, TiO₂ , is described. In aqueous solutions within illumination timescales ranging from few minutes to several hours, light-induced agglomeration of Pt SAs to ensembles (dimers, multimers) and finally nanoparticles takes place. The kinetics critically depends on the presence of sacrificial hole scavengers and the used light intensity. Density-functional theory calculations attribute the light induced destabilization of the SA Pt species to binding of surface-coordinated Pt with solution-hydrogen (adsorbed H atoms), which consequently weakens the Pt SA bonding to the TiO₂ surface. Despite the gradual aggregation of Pt SAs into surface clusters and their overall reduction to metallic state, which involves >90% of Pt SAs, the overall photocatalytic H₂ evolution remains virtually unaffected.

Visible Light-Active Ternary Heterojunction Photocatalyst for Efficient CO2 Reduction with Simultaneous Amine Oxidation and Sustainable H2O2 Production

The present work described a unique approach for CO₂ reduction to methanol along with the oxidation of various amines to the corresponding imines and photocatalytic H₂O₂ production from H₂O and molecular O₂ using a heterojunction photocatalyst made up of ZnIn₂S₄/Ni₁₂P₅/g-C₃N₄(NCZ) under visible light irradiation. The photocatalysts were synthesized via a high-temperature treatment of nickel and phosphorous precursors with g-C₃N₄ followed by decoration of ZnIn₂S₄. The synthesized photocatalysts were characterized using various spectroscopic and microscopic techniques. The density functional theory (DFT) studies suggested the participation of the valence band maximum (VBM) from Ni₁₂P₅ and the conduction band maximum (CBM) from ZnIn₂S₄ in the ternary NCZ heterojunction. The ternary composite exhibited superior photocatalytic activity compared to that of its individual components due to the formation of a heterojunction, thereby enhancing the transfer efficiency of electrons from the conduction band of g-C₃N₄ to that of ZnIn₂S₄ using Ni₁₂P₅ as an electron bridge. Moreover, the reduced band gap of the ternary heterojunction played a key role in its higher efficiency.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

Durable CuxO/mesoporous TiO2 photocatalyst for stable and efficient hydrogen evolution

Heterogeneous structures containing highly dispersed semiconductor nanoparticles on a photoactive support are effective for the photocatalytic hydrogen evolution reaction (HER). In this work, the interlayer ion-exchange and space confining nature of layered titanate nanosheets was used to embed copper ions in titanates, which were then transitioned to mesoporous Cu_xO/TiO₂ with highly dispersed Cu_xO nanostructures. Both experimental and density functional theory (DFT) studies demonstrated that the fine-decoration of Cu_xO nanostructures and the reducible valence of the copper species enabled stable superior photocatalytic activity. The HER efficiency was enhanced to 12.45 mmol g^-1 h^-1 for the mesoporous Cu_xO/TiO₂ composites in comparison to an efficiency of 0.38 mmol g^-1 h^-1 for the non-modified TiO₂. Steady HER performances over 10 h, cyclic HER measurement over 60 h, and testing of the composite kept under ambient conditions for over one year, demonstrated excellent stability of the composite against photochemical and wet-chemical erosion.

In situ Cu single atoms anchoring on MOF-derived porous TiO2 for the efficient separation of photon-generated carriers and photocatalytic H2 evolution

Single atom catalysts (SACs) have an extremely high atom utilization and distinctive structures and properties in the field of photocatalysis. However, the premise of conducting scientific research and applications is still the stability and catalytic activity of single atoms on suitable substrates. Metal organic frameworks (MOFs), as one of the most suitable single-atom substrates, have tunable internal structures, unsaturated coordination bonds, and high specific surface areas. In this work, Ti-based MOF, MIL-125, was adopted as the precursor to prepare mesoporous Cu-loaded TiO₂. During the synthesis of MIL-125, a Cu source was added, and Cu atoms were fixed by partly replacing Ti atoms in the Ti-O octahedron to coordinate with O atoms, resulting in a good dispersity, good stability and high loading amount. Experimental investigations demonstrated that dispersed Cu single atoms act as reaction centres, besides being able to accelerate the transfer of photoelectrons. Under simulated sunlight, the H₂ evolution rate of the optimum Cu-TiO₂ sample reaches 17.77 mmol g^-1 h^-1, nearly 101 times higher than that of the pure mesoporous TiO₂. The apparent quantum efficiency (AQE) is 20.15% under 365 nm irradiation. This research opens a new thinking to preparing high stability and high activity single atom photocatalysts.

Abatement of dichloromethane with high selectivity over defect-rich MOF-derived Ru/TiO2 catalysts

The regulation of oxygen vacancies and Ru species using metal-organic frameworks was synergically adopted in a rational design to upgrade Ru/TiO₂ catalysts, which are highly active for the catalytic oxidation of dichloromethane (DCM) with less undesired byproducts. In this work, Ru/M-TiO₂ and Ru/N-TiO₂ catalysts were synthesized by the pyrolysis of MIL-125 and NH₂-MIL-125 incorporated with Ru, the existence of Ru nanoclusters and nanoparticles was detected by XAFS, respectively, and the catalytic performance was analyzed comprehensively. Complete oxidation of DCM was obtained at ∼290 °C over Ru/M-TiO₂ and Ru/N-TiO₂ catalysts, while Ru/N-TiO₂ showed quite less monochloromethane (MCM) and higher CO₂ yields, and better dechlorination capacity in oxidation. The distinction comes down to that the easier desorption of chlorine could be achieved over Ru⁴⁺ which act as the main activated adsorption sites for DCM in Ru/N-TiO₂, compared to oxygen vacancies that serve as the main dissociation sites in Ru/M-TiO₂. Additionally, Ru/N-TiO₂ exhibited superior stability and excellent resilience in moisture. An in situ DRIFTS experiment further indicated the different DCM catalytic degradation process as well as the reaction mechanism over the as-prepared catalysts.

The Progress and Outlook of Metal Single-Atom-Site Catalysis

Single-atom-site catalysts (SASCs) featuring maximized atom utilization and isolated active sites have progressed tremendously in recent years as a highly prosperous branch of catalysis research. Varieties of SASCs have been developed that show excellent performance in many catalytic applications. The major goal of SASC research is to establish feasible synthetic strategies for the preparation of high-performance catalysts, to achieve an in-depth understanding of the active-site structures and catalytic mechanisms, and to develop practical catalysts with industrial value. This Perspective describes the up-to-date development of SASCs and related catalysts, such as dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs), analyzes the current challenges encountered by these catalysts for industrial applications, and proposes their possible future development path.

Single-Atom-Based Catalysts for Photocatalytic Water Splitting on TiO2 Nanostructures

H2 generation from photocatalytic water splitting is one of the most promising approaches to producing cost-effective and sustainable fuel. Nanostructured TiO2 is a highly stable and efficient semiconductor photocatalyst for this purpose. The main drawback of TiO2 as a photocatalyst is the sluggish charge transfer on the surface of TiO2 that can be tackled to a great extent by the use of platinum group materials (PGM) as co-catalysts. However, the scarcity and high cost of the PGMs is one of the issues that prevent the widespread use of TiO2/PGM systems for photocatalytic H2 generation. Single-atom catalysts which are currently the frontline in the catalysis field can be a favorable path to overcome the scarcity and further advance the use of noble metals. More importantly, single-atom (SA) catalysts simultaneously have the advantage of homogenous and heterogeneous catalysts. This mini-review specifically focuses on the single atom decoration of TiO2 nanostructures for photocatalytic water splitting. The latest progress in fabrication, characterization, and application of single-atoms in photocatalytic H2 generation on TiO2 is reviewed.

Elevating Photooxidation of Methane to Formaldehyde via TiO2 Crystal Phase Engineering

Photocatalytic conversion of methane to value-added products under mild conditions, which represents a long sought-after goal for industrial sustainable production, remains extremely challenging to afford high production and selectivity using cheap catalysts. Herein, we present the crystal phase engineering of commercially available anatase TiO₂ via simple thermal annealing to optimize the structure-property correlation. A biphase catalyst with anatase (90%) and rutile (10%) TiO₂ with the optimal phase interface concentration exhibits exceptional performance in the oxidation of methane to formaldehyde under the reaction conditions of water solvent, oxygen atmosphere, and full-spectrum light irradiation. An unprecedented production of 24.27 mmol g_cat^-1 with an excellent selectivity of 97.4% toward formaldehyde is acquired at room temperature after a 3 h reaction. Both experimental results and theoretical calculations disclose that the crystal phase engineering of TiO₂ lengthens the lifetime of photogenerated carriers and favors the formation of intermediate methanol species, thus maximizing the efficiency and selectivity in the aerobic oxidation of methane to formaldehyde. More importantly, the feasibility of the scale-up production of formaldehyde is demonstrated by inventing a "pause-flow" reactor. This work opens the avenue toward industrial methane transformation in a sustainable and economical way.

Considering single-atom catalysts as photocatalysts from synthesis to application

With the ever-increased greenhouse effect and energy crisis, developing novel photocatalysts to realize high-efficient solar-driven chemicals/fuel production is of great scientific and practical significance. Recently, single-atom photocatalysts (SAPs) are promising catalysts with maximized metal dispersion and tuneable coordination environments. SAPs exhibit boosted photocatalytic performance by enhancing optical response, facilitating charge carrier transfer behaviors or directly manipulating surface reaction processes. In this regard, this article systematically reviews the state-of-the-art progress in the development and application of SAPs, especially the mechanism and performance of SAPs on various reaction processes. Some future challenges and potential research directions over SAPs are outlined at the final stage.

Nanoscale Self-Wetting Driven Monatomization of Ag Nanoparticle for Excellent Photocatalytic Hydrogen Evolution

Metal nanoparticles (NPs) with <10 nm have demonstrated many novel applications including surprisingly low melting point, astonishing liquid-like pseudoelasticity, and outstanding hydrogen evolution performance. Here, a nanoscale self-wetting driven monatomization of Ag NPs with <5 nm on carbon nitride (CN) to fabricate Ag single-atom catalyst (Ag₁ /CN SAC) is demonstrated, and a thermodynamic approach to elucidate Ag NPs decomposing into single atoms is established. Dynamic dispersion process of Ag NPs into atoms on CN is recorded using in situ AC-HADDF-TEM techniques. Density functional theory calculations and molecular dynamics simulations suggest that the spontaneous dispersion origins from the nanoscale self-wetting effect in thermodynamics. In atomic scale, the driving force of self-wetting derived from the balance between cohesive energy of Ag NPs and excess energy of Ag atoms in CN vacations. The fabricated Ag₁ /CN SAC proved a higher efficiency for photocatalytic hydrogen evolution activity (3690 μmmol g^-1 h^-1 ) than Pt nanoparticles on CN (3192 μmmol g^-1 h^-1 ). This spontaneous monatomization resulting from the interaction between metal NPs and substrate provides a simple method to prepare SACs with a high active photocatalytic hydrogen evolution.

Dual-Functional Photocatalysis for Cooperative Hydrogen Evolution and Benzylamine Oxidation Coupling over Sandwiched-Like Pd@TiO2 @ZnIn2 S4 Nanobox

Photocatalytic hydrogen evolution (PHE) over semiconductor photocatalysts is usually constrained by the limited light-harvesting and separation of photogenerated electron-hole pairs. Most of the reported systems focusing on PHE are facilitated by consuming the photoinduced holes with organic sacrificial electron donors (SEDs). The introduction of the SEDs not only causes the environmental problem, but also increases the cost of the reaction. Herein, a dual-functional photocatalyst is developed with the morphology of sandwiched-like hollowed Pd@TiO₂ @ZnIn₂ S₄ nanobox, which is synthesized by choosing microporous zeolites with sub-nanometer-sized Pd nanoparticles (Pd NPs) embedded as the sacrificial templates. The ternary Pd@TiO₂ @ZnIn₂ S₄ photocatalyst exhibits a superior PHE rate (5.35 mmol g^-1 h^-1 ) and benzylamine oxidation conversion rate (>99%) simultaneously without adding any other SEDs. The PHE performance is superior to the reported composites of TiO₂ and ZnIn₂ S₄ , which is attributed to the elevated light capture ability induced by the hollow structure, and the enhanced charge separation efficiency facilitated by the ultrasmall sized Pd NPs. The unique design presented here holds great potential for other highly efficient cooperative dual-functional photocatalytic reactions.

Emerging Single-Atom Catalysts/Nanozymes for Catalytic Biomedical Applications

Single-atom catalysts (SACs) are a type of atomically dispersed nanozymes with the highest atom utilization, which employ low-coordinated single atoms as the catalytically active sites. SACs not only inherit the merits of traditional nanozymes, but also hold high catalytic activity and superb catalytic selectivity, which ensure their tremendous application potential in environmental remediation, energy storage and conversion, chemical industry, nanomedicine, etc. Nevertheless, undesired aggregation effect of single atoms during preactivation and reaction processes is significantly enhanced owing to the high surface free energy of single atoms. In this case, appropriate substrates are requisite to prevent the aggregation event through the powerful interactions between the single atoms and the substrates, thereby stabilizing the high catalytic activity of the catalysts. In this review, the synthetic methods and characterization approaches of SACs are first described. Then the application cases of SACs in nanomedicine are summarized. Finally, the current challenges and future opportunities of the SACs in nanomedicine are outlined. It is hoped that this review may have implications for furthering the development of new SACs with improved biophysicochemical properties and broadened biomedical applications.

Single Atom Sites Catalysts based on High Specific Surface Area Supports

Catalysis is the heart of modern chemical industry. Supports with high specific surface area are crucial for the fabrication of efficient catalysts with elevated metal dispersion. Single atom sites catalysts...

Optimized Pt Single Atom Harvesting on TiO2 Nanotubes-Towards a Most Efficient Photocatalyst

In the present work the authors show that anodic TiO₂ nanotubes (NT) show excellent harvesting properties for Pt single atoms (Pt SAs) from highly dilute Pt solutions. The tube walls of anodic nanotubes, after adequate annealing to anatase, provide ample of suitable trapping sites-that is, surface Ti³⁺ -O_v (O_v : oxygen vacancy) defects that are highly effective to extract and accumulate Pt in the form of SAs. A saturated (maximized) SA density can be achieved by an overnight immersion of a TiO₂ NT layer to a H₂ PtCl₆ solution with a concentration that is as low as 0.01 mm Pt. Such TiO₂ NTs with surface trapped Pt SAs provide a maximized high activity for photocatalytic H₂ generation (reaching a turnover frequency (TOF) of 1.24 × 10⁶ h^-1 at a density of 1.4 × 10⁵ Pt atoms µm^-2 )-a higher loading with Pt nanoparticles does not further increase the photocatalytic activity. Overall, these findings show that anodic TiO₂ nanotubes provide a remarkable substrate for Pt extraction and recovery from very dilute solutions that directly results in a highly efficient photocatalyst, fabricated by a simple immersion technique.

Ultrathin tungsten-doped hydrogenated titanium dioxide nanosheets for solar-driven hydrogen evolution

Ultrathin tungsten-doped hydrogenated TiO2 (W-h-TiO2) nanosheets are highly efficient for photocatalytic hydrogen production by water splitting without a noble metal cocatalyst.

Engineering of platinum–oxygen vacancy interfacial sites in confined catalysts for enhanced hydrogenation selectivity

Confined TiO2/Pt-700-Ar with rich Pt–Ov interfacial sites exhibits superior p-ABC selectivity compared with confined TiO2/Pt and unconfined Pt/TiO2-700-Ar with poor Pt–Ov interfacial sites.

Tumor Microenvironment Responsive Single-Atom Nanozymes for Enhanced Antitumor Therapy

Single-atom nanozymes (SAzymes) with specific response to the unique tumor microenvironment (TME) feature providing 100% metal atoms utilization for high-efficient enzyme-catalyzed therapy and accurate template for the study of therapeutic mechanisms. In this review, we first introduce the various synthetic strategies of SAzymes, and the TME-responsive SAzymes activities. Next, the TME-responsive enhanced antitumor therapeutic approaches based on the enzymatic activities of SAzymes are summarized, and the corresponding therapy mechanisms are elaborated. Subsequently, a concise but concentrated summary, and the challenges and opportunities for the future design and engineering of SAzyme are outlined. As a newly-built discipline, SAzymes have vast space for development in enhanced antitumor therapy. This timely review provides guidance and constructive suggestions for the future of SAzymes.