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.

Atomically Contacted Cs3Bi2Br9 QDs@UiO-66 Composite for Photocatalytic CO2 Reduction.

Metal halide perovskite quantum dots (QDs) are widely studied in the field of photocatalytic CO2 due to their strong light absorption and long carrier migration length. However, it can not exhibit high catalytic performance because of the radiative recombination and the lack of effective catalytic sites. Metal organic frameworks (MOFs) encapsulated QDs can not only solve the aforementioned problems, but also maintain their own unique characteristics with ultra-high specific surfaces area and abundant metal sites. In this work, lead-free bismuth-based halide perovskite QDs are encapsulated into Zr-based MOF (UiO-66), which combines the advantages with high power conversion efficiency of QDs and the high surface area and porosity of UiO-66. In addition, benefiting from the close contact between the Cs3Bi2Br9 QDs and the UiO-66 enables the photogenerated electrons in the QDs to be rapidly transferred to the MOF. As a result, the Cs3Bi2Br9@UiO-66 composite exhibits a higher yield for photocatalytic CO2 reduction than that of the prepared large-sized composite of Cs3Bi2Br9 and UiO-66.

Advances and challenges in the modification of photoelectrode materials for photoelectrocatalytic water splitting.

With the increasing consumption of fossil fuels, the development of clean and renewable alternative fuels has become a top priority. Hydrogen (H2) is an ideal primary clean energy source for its extremely high gravimetric energy density, carbon-free combustion, and abundant natural resources. Photoelectrocatalytic (PEC) water splitting is among the most promising approaches for converting sunlight and water into H2. However, the cost-effectiveness and the overall solar to hydrogen conversion efficiency of PEC water splitting are still big challenges. In the past few decades, several studies have been devoted to this technology, and it is essential to develop economical photoelectrocatalysts with high conversion efficiency. Therefore, there is an urgent need for a comprehensive and updated review of recent advances in the design, manufacture, and modification of efficient PEC water splitting systems. This review first starts with the basic mechanism of photoelectrochemical water splitting. Then the problems in PEC water splitting are discussed, and the methods of photoelectrode modulation such as nanostructure fabrication, doping engineering, surface modification, and heterojunction construction are introduced. Finally, the critical challenges and future trends/perspectives in the PEC water splitting are discussed.

In-Plane Palladium and Interplanar Copper Dual Single-Atom Catalyst in Bulk-Like Carbon Nitride for Cascade CO2 Photoreduction.

Dual single-atom catalysts (DSACs) are promising for breaking the scaling relationships and ensuring synergistic effects compared with conventional single-atom catalysts (SACs). Nevertheless, precise synthesis and optimization of DSACs with specific locations and functions remain challenging. Herein, dual single-atoms are specifically incorporated into the layer-stacked bulk-like carbon nitride, featuring in-plane three-coordinated Pd and interplanar four-coordinated Cu (Pd1-Cu1/b-CN) atomic sites, from both experimental results and DFT simulations. Using femtosecond time-resolved transient absorption (fs-TA) spectroscopy, it is found that the in-plane Pd features a charge decay lifetime of 95.6 ps which is much longer than that of the interplanar Cu (3.07 ps). This finding indicates that the in-plane Pd can provide electrons for the reaction as the catalytically active site in both structurally and dynamically favorable manners. Such a well-defined bi-functional cascade system ensures a 3.47-fold increase in CO yield compared to that of bulk-like CN (b-CN), while also exceeding the effects of single Pd1/b-CN and Cu1/b-CN sites. Furthermore, DFT calculations reveal that the inherent transformation from s-p coupling to d-p hybridization between the Pd site and CO2 molecule occurs during the initial CO2 adsorption and hydrogenation processes and stimulates the preferred CO2-to-CO reaction pathway.

Single-Atom Heterogeneous Catalysts: Human- and AI-Driven Platform for Augmented Designs, Analytics and Reality-Enabled Manufacturing.

Heterogeneous catalysts with targeted functionality can be designed with atomic precision, but it is challenging to retain the structure and performance upon the scaled-up manufacturing. Particularly challenging is to ensure the "atomic economy", where every catalytic site is most gainfully utilized. Given the emerging synergistic integration of human- and artificial intelligence (AI)-driven augmented designs (AD), augmented analytics (AA), and augmented reality manufacturing (AM) platforms, this minireview focuses on single-atom heterogeneous catalysts (SAHCs) and examines the current status, challenges, and future perspectives of translating atomic-level structural precision and data-driven discovery to next-generation industrial manufacturing. We critically examine the atomistic insights into structure-driven SAHCs functionality and discuss the opportunities and challenges on the way towards the synergistic human-AI collaborative data-driven platform capable of monitoring, analyzing, manufacturing, and retaining the atomic-scale structure and functions. Enhanced by the atomic-level AD, AA, and AM, evolving from the current high-throughput capabilities and digital materials manufacturing acceleration, this synergistic human-AI platform is promising to enable atom-efficient and atomically precise heterogeneous catalyst production.

Understanding the unique S-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction.

Understanding charge transfer dynamics and carrier separation pathway is challenging due to the lack of appropriate characterization strategies. In this work, a crystalline triazine/heptazine carbon nitride homojunction is selected as a model system to demonstrate the interfacial electron-transfer mechanism. Surface bimetallic cocatalysts are used as sensitive probes during in situ photoemission for tracing the S-scheme transfer of interfacial photogenerated electrons from triazine phase to the heptazine phase. Variation of the sample surface potential under light on/off confirms dynamic S-scheme charge transfer. Further theoretical calculations demonstrate an interesting reversal of interfacial electron-transfer path under light/dark conditions, which also supports the experimental evidence of S-scheme transport. Benefiting from the unique merit of S-scheme electron transfer, homojunction shows significantly enhanced activity for CO2 photoreduction. Our work thus provides a strategy to probe dynamic electron transfer mechanisms and to design delicate material structures towards efficient CO2 photoreduction.

Effect of Functional Group Modifications on the Photocatalytic Performance of g-C3 N4.

In recent years, photocatalysis has received increasing attention in alleviating energy scarcity and environmental treatment, and graphite carbon nitride (g-C3 N4 ) is used as an ideal photocatalyst. However, it still remains numerous challenges to obtain the desirable photocatalytic performance of intrinsic g-C3 N4 . Functional group functionalization, formed by introducing functional groups into the bulk structure, is one of the common modification techniques to modulate the carrier dynamics and increases the number of active sites, offering new opportunities to break the limits for structure-to-performance relationship of g-C3 N4 . Nevertheless, the general overview of the advance of functional group modification of g-C3 N4 is less reported yet. In order to better understand the structure-to-performance relationship at the molecular level, a review of the latest development of functional group modification is urgently needed. In this review, the functional group modification of g-C3 N4 in terms of structures, properties, and photocatalytic activity is mainly focused, as well as their mechanism of reaction from the molecular level insights is explained. Second, the recent progress of the application of introducing functional groups in g-C3 N4 is introduced and examples are given. Finally, the difficulties and challenges are presented, and based on this, an outlook on the future research development direction is shown.

Functionalized MOF-Based Photocatalysts for CO2 Reduction.

Metal-organic frameworks (MOFs) materials have become a research forefront in the field of photocatalytic CO2 reduction attributed to their ultra-high specific surface area, adjustable structure, and abundant catalytic active sites. Particularly, MOFs can be facilely tuned to match CO2 photoreduction by utilizing post-modification of metal nodes, functionalization of organic linkers, and combination with other active materials. Herein, the recent advances in the construction strategy of MOF-based photocatalysts materials for CO2 reduction are highlighted. Some systematic modification strategies on MOF-based photocatalysts are also discussed, such as modification of metal sites and organic ligands, construction of heterojunction, introduction of single/dual-atom, and strain engineering. Finally, the future development directions of MOF-based photocatalysts in the field of CO2 reduction are presented.

Identification of the Active Sites on Metallic MoO2-x Nano-Sea-Urchin for Atmospheric CO2 Photoreduction Under UV, Visible, and Near-Infrared Light Illumination.

We report an oxygen vacancy (Vo )-rich metallic MoO2-x nano-sea-urchin with partially occupied band, which exhibits super CO2 (even directly from the air) photoreduction performance under UV, visible and near-infrared (NIR) light illumination. The Vo -rich MoO2-x nano-sea-urchin displays a CH4 evolution rate of 12.2 and 5.8 µmol gcatalyst -1 h-1 under full spectrum and NIR light illumination in concentrated CO2 , which is ca. 7- and 10-fold higher than the Vo -poor MoO2-x , respectively. More interestingly, the as-developed Vo -rich MoO2-x nano-sea-urchin can even reduce CO2 directly from the air with a CO evolution rate of 6.5 µmol gcatalyst -1 h-1 under NIR light illumination. Experiments together with theoretical calculations demonstrate that the oxygen vacancy in MoO2-x can facilitate CO2 adsorption/activation to generate *COOH as well as the subsequent protonation of *CO towards the formation of CH4 because of the formation of a highly stable Mo-C-O-Mo intermediate.

Alkali metal-modified crystalline carbon nitride for photocatalytic nitrogen fixation.

Alkali metal chlorides have been used as molten salts for further preparing crystalline carbon nitride, but the effect of alkali metal types on the properties of crystalline carbon nitride has not been systematically studied. Therefore, in this paper, a series of crystalline carbon nitride samples doped with different alkali metals were successfully prepared using LiCl-KCl, KCl-NaCl, LiCl-KCl-NaCl and LiCl-NaCl as molten salts: LK-HTCN (Li-K co-doping), KN-HCN (K-Na co-doping), LKN-HTCN (Li-K-Na co-doping) and LN-HTCN (Li-Na co-doping). The experimental results show that KN-HCN contains only the heptazine unit structure, while the other samples contain heptazine and triazine unit structures. Meanwhile, the concentration of the cyano group in the KN-HCN sample structure is significantly higher than that of other samples, which may be related to the fact that there is only a heptazine unit in the structure of the KN-HCN sample. In addition, the loading mode of K and Na ions in the KN-HCN structure is favorable for the migration and separation of photogenerated charges. Thanks to these characteristics, KN-HCN showed excellent photocatalytic ammonia production activity. This study can provide theoretical insight for the development of crystallized carbon nitride.

Highly Strained Bi-MOF on Bismuth Oxyhalide Support with Tailored Intermediate Adsorption/Desorption Capability for Robust CO2 Photoreduction.

Herein, using as-designed surface-mounted Bismuth-based metal-organic framework (Bi-MOF) on two-dimensional BiOBr support, as an operable platform for site-specific strain engineering to tailor the intermediate adsorption/desorption capability in CO2 photocatalytic conversion is proposed. Giant compressive strain up to 7.85 % is successfully induced on the surface-mounted Bi-MOF revealed by HRTEM images and geometric phase analysis as well as in situ Raman characterization, which largely downshifts the p band center of Bi nodes and intensifies their unsaturated state. In-depth explorations are put onto p-p (Bi 6p and CO2 /CO 2p) orbital hybridization. Taking the adsorption process as an example, the 1π and 7σ frontier molecule orbitals of CO2 2p for both the strain-free and strained models shift downwards the Fermi level, indicative of fast adsorption of CO2 . Meanwhile, strain engineering further induces new non-degenerate orbital overlapping near 1π and intensified overlapping of 7σ orbitals, stimulating the fast activation of absorbed CO2 molecules.

Site-Specific Electron-Driving Observations of CO2 -to-CH4 Photoreduction on Co-Doped CeO2 /Crystalline Carbon Nitride S-Scheme Heterojunctions.

Photoexcited dynamic modulation, maximizing the effective utilization of photoinduced electron-hole pairs, dominates the multiple electrons-involving reduction pathways for terminal CH4 evolution during CO2 photoreduction. Yet, the site-specific regulation of directional charge transfer by modification of an S-scheme heterojunction has seldom been discussed. Herein, an atomic-level tailoring strategy by anchoring single-atomic Co into CeO2 co-catalyst rather than carbon nitride supports, which can selectively favor CO2 -to-CH4 photoreduction, is reported. Through in situ dynamic tracking investigations, this study identifies that surface Co-embedded bimetallic CeCo conjunction is the key feature driving a strong interconnection of dynamical charge states through S-scheme heterojunctions. The Co-embedded modification into CeO2 co-catalysts is demonstrated to have a critical effect on directional charge control, accelerating the driving of electrons from the carbon nitride donations to site-specific Co hubs, which thereby promotes electronic transferability for electrons-involving CH4 formation. As a result, an unprecedented CH4 yield (181.7 µmol g-1 ) is obtained with a high turnover number (411.4) through a fully gas-solid reaction, demonstrating its potential toward targeted CH4 formation without adding any sacrificial agent.

Highly Efficient Photocatalytic Reduction of CO2 to CO by In Situ Formation of a Hybrid Catalytic System Based on Molecular Iron Quaterpyridine Covalently Linked to Carbon Nitride.

Efficient and selective photocatalytic CO2 reduction was obtained within a hybrid system that is formed in situ via a Schiff base condensation between a molecular iron quaterpyridine complex bearing an aldehyde function and carbon nitride. Irradiation (blue LED) of an CH3 CN solution containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), triethylamine (TEA), Feqpy-BA (qpy-BA=4-([2,2':6',2'':6'',2'''-quaterpyridin]-4-yl)benzaldehyde) and C3 N4 resulted in CO evolution with a turnover number of 2554 and 95 % selectivity. This hybrid catalytic system unlocks covalent linkage of molecular catalysts with semiconductor photosensitizers via Schiff base reaction for high-efficiency photocatalytic reduction of CO2 , opening a pathway for diverse photocatalysis.

Construction 0D/2D heterojunction by highly dispersed Ag2S quantum dots (QDs) loaded on the g-C3N4 nanosheets for photocatalytic hydrogen evolution.

In recent years, the use of quantum dots (QDs) cocatalysts to improve the hydrogen evolution activity from the water splitting of photocatalysts has become a popular research topic. Herein, we successfully prepared a novel 0 dimension/2 dimension (0D/2D) heterojunction nanocomposite (denoted Ag2S quantum dots (QDs)/g-C3N4) with excellent photocatalytic performance by anchoring the Ag2S QDs cocatalyst on the surface of g-C3N4 through a self-assembly strategy. Ag2S QDs with an average particle size of approximately 5.8 nm were uniformly and tightly modified on g-C3N4. The Ag2S QDs/g-C3N4 composite with 0.5 wt% Ag2S QDs loading achieved the highest hydrogen evolution rate of 471.1 µmol·g-1·h-1 with an apparent quantum efficiency (AQE) of 1.48% at 405 nm. Such remarkable hydrogen evolution activity far exceeded that of undoped g-C3N4 and Ag2S nanoparticles (NPs)/g-C3N4. Moreover, it was 2.04 times the activity of Pt/g-C3N4 with Pt as the cocatalyst. The enhanced photocatalytic performance was attributed to the energy band broadening of Ag2S QDs caused by the quantum size effect and the convenient and effective charge transfer between g-C3N4 and Ag2S QDs cocatalysts. The mechanism underlying the enhanced photocatalytic H2 evolution activity was further proposed. This study demonstrates that semiconductor-based quantum dots are strong candidates for excellent cocatalysts in photocatalysis.

Dual-Single-Atom Tailoring with Bifunctional Integration for High-Performance CO2 Photoreduction.

Single-atom photocatalysis has been demonstrated as a novel strategy to promote heterogeneous reactions. There is a diversity of monoatomic metal species with specific functions; however, integrating representative merits into dual-single-atoms and regulating cooperative photocatalysis remain a pressing challenge. For dual-single-atom catalysts, enhanced photocatalytic activity would be realized through integrating bifunctional properties and tuning the synergistic effect. Herein, dual-single-atoms supported on conjugated porous carbon nitride polymer are developed for effective photocatalytic CO2 reduction, featuring the function of cobalt (Co) and ruthenium (Ru). A series of in situ characterizations and theoretical calculations are conducted for quantitative analysis of structure-performance correlation. It is concluded that the active Co sites facilitate dynamic charge transfer, while the Ru sites promote selective CO2 surface-bound interaction during CO2 photoreduction. The combination of atom-specific traits and the synergy between Co and Ru lead to the high photocatalytic CO2 conversion with corresponding apparent quantum efficiency (AQE) of 2.8% at 385 nm, along with a high turnover number (TON) of more than 200 without addition of any sacrificial agent. This work presents an example of identifying the roles of different single-atom metals and regulating the synergy, where the two metals with unique properties collaborate to further boost the photocatalytic performance.

State-of-the-art recent progress in MXene-based photocatalysts: a comprehensive review.

Recently, the emerging two-dimensional material MXene enjoys a high reputation due to its fascinating characteristics and shines in various research fields. Among them, MXene has received increasing attention and favor from the photocatalysis community due to its regular planar structure, outstanding metal conductivity, surface adjustable chemical properties, abundant derivatives, and excellent optical and thermal properties. There is no doubt that the introduction of MXene has endowed the photocatalytic system with extraordinary performance and has made an important impact in the field of advanced catalysis and chemical technology. Herein, on the one hand, we summarize the synthetic route of MXenes and MXene-derived 0D quantum dots (MQDs), and propose the corresponding improved synthetic strategies for the defects of MXenes obtained by the current synthetic methods. On the other hand, recent progress in MXene-based photocatalysts has been reviewed to provide a comprehensive dissection of their designs and applications. It is highly anticipated that these will present researchers with a one-stop point for the state-of-the-art development of MXene-based photocatalysts. Finally, the future opportunities and challenges of MXenes in the flourishing field of photocatalysis are also discussed.

Carbon-Graphitic Carbon Nitride Hybrids for Heterogeneous Photocatalysis.

Polymeric graphitic carbon nitride (g-C3 N4 ) and various carbon materials have experienced a renaissance as viable alternates in photocatalysis due to their captivating metal-free features, favorable photoelectric properties, and economic adaptabilities. Although numerous efforts have focused on the integration of both materials with optimized photocatalytic performance in recent years, the direct parameters for this emerging enhancement are not fully summarized yet. Fully understanding the synergistic effects between g-C3 N4 and carbon materials on photocatalytic action is vital to further development of metal-free semiconductors in future studies. Here, recent advances of carbon/g-C3 N4 hybrids on various photocatalytic applications are reviewed. The dominant governing factors by inducing carbon into g-C3 N4 photocatalysts with involving photocatalytic mechanism are highlighted. Five typical carbon-induced enhancement effects are mainly discussed here, i.e., local electric modification, band structure tailoring, multiple charge carrier activation, chemical group functionalization, and abundant surface-modified engineering. Photocatalytic performance of carbon-induced g-C3 N4 photocatalysts for addressing directly both the renewable energy storage and environmental remediation is also summarized. Finally, perspectives and ongoing challenges encountered in the development of metal-free carbon-induced g-C3 N4 photocatalysts are presented.

Crystalline Carbon Nitride Supported Copper Single Atoms for Photocatalytic CO2 Reduction with Nearly 100% CO Selectivity.

Single metal atom photocatalysts have received widespread attention due to the rational use of metal resources and maximum atom utilization efficiency. In particular, N-rich amorphous g-C3N4 is always used as a support to anchor single metal atoms. However, the enhancement of photocatalytic activity of g-C3N4 by introducing a single atom is limited due to the bulk morphology and the excess defects of amorphous g-C3N4. Here, we report crystalline g-C3N4 nanorod supported copper single atoms by molten salts and the reflux method. The prepared single Cu atoms/crystalline g-C3N4 photocatalyst (Cu-CCN) shows highly selective and efficient photocatalytic reduction of CO2 under the absence of any cocatalyst or sacrificial agent. The introduction of single Cu atoms can be used as the CO2 adsorption site, thus increasing the adsorption capacity of Cu-CCN samples to CO2. Theoretical calculation results show that reducing CO2 to CH4 on Cu-CCN samples is an entropy-increasing process, whereas reducing CO2 to CO is an entropy-decreasing process. As a result, the Cu-CCN samples exhibited enhanced photocatalytic CO2 reduction with nearly 100% selective photocatalytic CO2 to CO conversion. The mechanism of photocatalytic CO2 reduction over Cu-CCN samples was proposed based on in situ Fourier transform infrared spectra, X-ray absorption spectroscopy, and density functional theory calculation. This work provides an in-depth understanding of the design of photocatalysts for enhancing active sites of the reactants.

Single Ni Atoms Anchored on Porous Few-Layer g-C3 N4 for Photocatalytic CO2 Reduction: The Role of Edge Confinement.

It is greatly intriguing yet remains challenging to construct single-atomic photocatalysts with stable surface free energy, favorable for well-defined atomic coordination and photocatalytic carrier mobility during the photoredox process. Herein, an unsaturated edge confinement strategy is defined by coordinating single-atomic-site Ni on the bottom-up synthesized porous few-layer g-C3 N4 (namely, Ni5 -CN) via a self-limiting method. This Ni5 -CN system with a few isolated Ni clusters distributed on the edge of g-C3 N4 is beneficial to immobilize the nonedged single-atomic-site Ni species, thus achieving a high single-atomic active site density. Remarkably, the Ni5 -CN system exhibits comparably high photocatalytic activity for CO2 reduction, giving the CO generation rate of 8.6 µmol g-1 h-1 under visible-light illumination, which is 7.8 times that of pure porous few-layer g-C3 N4 (namely, CN, 1.1 µmol g-1 h-1 ). X-ray absorption spectrometric analysis unveils that the cationic coordination environment of single-atomic-site Ni center, which is formed by Ni-N doping-intercalation the first coordination shell, motivates the superiority in synergistic N-Ni-N connection and interfacial carrier transfer. The photocatalytic mechanistic prediction confirms that the introduced unsaturated Ni-N coordination favorably binds with CO2 , and enhances the rate-determining step of intermediates for CO generation.

Amine-functionalized graphitic carbon nitride decorated with small-sized Au nanoparticles for photocatalytic CO2 reduction.

Amine-functionalized graphitic carbon nitride (g-C3N4) decorated with Au nanoparticles (CN/Au) was prepared by N2 plasma treatment of g-C3N4 powders impregnated with HAuCl4·3H2O. Well-dispersed Au nanoparticles with a small particle size were deposited on g-C3N4 nanosheets. In addition, the amino group was introduced into the CN/Au system. Without the addition of cocatalyst and sacrificial agent, CN/Au exhibited enhanced photocatalytic activity for CO2 reduction under visible-light irradiation. CO and CH4 evolution rates of CN/Au reached 28.3 and 1.3 µmol·h-1·g-1, which were 7.6 and 2.6 times higher than those of pristine g-C3N4 (CN-0), respectively. The enhanced activity can be explained by these factors. (1) The introduced amino group improved the adsorption capacity of CN/Au for CO2; (2) the hot electrons generated by Au nanoparticles activated the surrounding electrons through energy transfer and caused local temperature to rise, increasing the efficiency of the photoreduction reaction of CO2; (3) the Schottky junction between Au and g-C3N4 promoted the migration of electrons from g-C3N4 to Au nanoparticles, suppressing the recombination of the carriers. Time-of-flight secondary ion mass spectrometry confirmed the introduction of amino groups, and solid-state 13C nuclear magnetic resonance spectra provided a support for inferring the position of the amino group.