Improving Selectivity and Activity of CO2 Reduction Photocatalysts with Oxygen

Mechanistic Insights Into H2O Dissociation in Overall Photo-/Electro-Catalytic CO2 Reduction

Photo-/electro-catalytic CO2 reduction with H2O to produce fuels and chemicals offers a dual solution to address both environmental and energy challenges. For a long time, catalyst design in this reaction system has primarily focused on optimizing reduction sites to improve the efficiency or guide the reaction pathway of the CO2 reduction half-reaction. However, less attention has been paid to designing activation sites for H2O to modulate the H2O dissociation half-reaction. Impressively, the rate-determining step in overall CO2 reduction is the latter, and it influences the evolution direction and formation energy of carbon-containing intermediates through the proton-coupled electron transfer process. Herein, we summarize the mechanism of the H2O dissociation half-reaction in modulating CO2 reduction performance based on cutting-edge research. These analyses aim to uncover the potential regulatory mechanisms by which H2O activation influences CO2 reduction pathways and conversion efficiency, and to establish a mechanism-structure-performance relationship that can guide the design and development of high-efficiency catalytic materials. A summary of advanced characterization techniques for investigating the dissociation mechanism of H2O is presented. We also discuss the challenges and offer perspectives on the future design of activation sites to improve the performance of photo-/electro-catalytic CO2 reduction.

Electrochemical CO2 reduction to liquid fuels: Mechanistic pathways and surface/interface engineering of catalysts and electrolytes

The high energy density of green synthetic liquid chemicals and fuels makes them ideal for sustainable energy storage and transportation applications. Electroreduction of carbon dioxide (CO2) directly into such high value-added chemicals can help us achieve a renewable C cycle. Such electrochemical reduction typically suffers from low faradaic efficiencies (FEs) and generates a mixture of products due to the complexity of controlling the reaction selectivity. This perspective summarizes recent advances in the mechanistic understanding of CO2 reduction reaction pathways toward liquid products and the state-of-the-art catalytic materials for conversion of CO2 to liquid C1 (e.g., formic acid, methanol) and C2+ products (e.g., acetic acid, ethanol, n-propanol). Many liquid fuels are being produced with FEs between 80% and 100%. We discuss the use of structure-binding energy relationships, computational screening, and machine learning to identify promising candidates for experimental validation. Finally, we classify strategies for controlling catalyst selectivity and summarize breakthroughs, prospects, and challenges in electrocatalytic CO2 reduction to guide future developments.

Proposing Oxalic Acid as Chemical Storage of Carbon Dioxide to Achieve Carbon Neutrality

Increasing emissions of carbon dioxide into the atmosphere due to the use of fossil fuels and ongoing deforestation are affecting the global climate. To reach the Paris climate agreement, in the coming decades low emission technologies must be developed, which allow for carbon removal on a Gt per year-scale. In this regard, we propose the electrochemical conversion of carbon dioxide to oxalic acid as a potentially viable pathway for large scale CO2 utilization and storage. Combined with water oxidation, in principle this transformation does not need stoichiometric amounts of co-reagents and minimize the necessary electrons for the reduction of carbon dioxide.

Floatable artificial leaf to couple oxygen-tolerant CO2 conversion with water purification

To enable open environment application of artificial photosynthesis, the direct utilization of environmental CO2 via an oxygen-tolerant reductive procedure is necessary. Herein, we introduce an in situ growth strategy for fabricating two-dimensional heterojunctions between indium porphyrin metal-organic framework (In-MOF) and single-layer graphene oxide (GO). Upon illumination, the In-MOF/GO heterostructure facilitates a tandem CO2 capture and photocatalytic reduction on its hydroxylated In-node, prioritizing the reduction of dilute CO2 even in the presence of air-level O2. The In-MOF/GO heterostructure photocatalyst is integrated with a porous polytetrafluoroethylene (PTFE) membrane to construct a floatable artificial leaf. Through a triphase photocatalytic reaction, the floatable artificial leaf can remove aqueous contaminants from real water while efficiently reducing CO2 at low concentrations (10%, approximately the CO2 concentration in combustion flue gases) upon air-level O2. This study provides a scalable approach for the construction of photocatalytic devices for CO2 conversion in open environments.

Influence of Copper Valence in CuOx/TiO2 Catalysts on the Selectivity of Carbon Dioxide Photocatalytic Reduction Products

The Cu cocatalyst supported on the surface of TiO2 photocatalysts has demonstrated unique activity and selectivity in photocatalytic CO2 reduction. The valence state of copper significantly influences the catalytic process; however, due to the inherent instability of copper's valence states, the precise role of different valence states in CO2 reduction remains inadequately understood. In this study, CuOx/TiO2 catalysts were synthesized using an in situ growth reduction method, and we investigated the impact of various valence copper species on CO2 photocatalytic reduction. Our results indicate that Cu+ and Cu0 serve as primary active sites, with the selectivity for CH4 and CO products during CO2 photoreduction being closely related to their respective ratios on the catalyst surface. The adsorption and activation mechanisms of CO on both Cu+ and Cu0 surfaces are identified as critical factors determining product selectivity in photocatalytic processes. Furthermore, it is confirmed that Cu+ primarily facilitates CH4 production while Cu0 is responsible for generating CO. This study provides valuable insights into developing highly selective photocatalysts.

Structurally and surficially activated TiO2 nanomaterials for photochemical reactions

Renewable fuels and environmental remediation are of paramount importance in today's world due to escalating concerns about climate change, pollution, and the finite nature of fossil fuels. Transitioning to sustainable energy sources and addressing environmental pollution has become an urgent necessity. Photocatalysis, particularly harnessing solar energy to drive chemical reactions for environmental remediation and clean fuel production, holds significant promise among emerging technologies. As a benchmark semiconductor in photocatalysis, TiO2 photocatalyst offers an excellent solution for environmental remediation and serves as a key tool in energy conversion and chemical synthesis. Despite its status as the default photocatalyst, TiO2 suffers from drawbacks such as a high recombination rate of charge carriers, low electrical conductivity, and limited absorption in the visible light spectrum. This review provides an in-depth exploration of the fundamental principles of photocatalytic reactions and presents recent advancements in the development of TiO2 photocatalysts. It specifically focuses on strategic approaches aimed at enhancing the performance of TiO2 photocatalysts, including improving visible light absorption for efficient solar energy harvesting, enhancing charge separation and transportation efficiency, and ensuring stability for robust photocatalysis. Additionally, the review delves into the application of photodegradation and photocatalysis, particularly in critical processes such as water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide generation, and alcohol oxidation. It also highlights the novel use of TiO2 in plastic polymerization and degradation, showcasing its potential for converting plastic waste into valuable chemicals and fuels, thereby offering sustainable waste management solutions. By addressing these essential areas, the review offers valuable insights into the potential of TiO2 photocatalysis for addressing pressing environmental and energy challenges. Furthermore, the review encompasses the application of TiO2 photochromic systems, expanding its scope to include other innovative research and applications. Finally, it addresses the underlying challenges and provides perspectives on the future development of TiO2 photocatalysts. Through addressing these issues and implementing innovative strategies, TiO2 photocatalysis can continue to evolve and play a pivotal role in sustainable energy and environmental applications.

Surface-Integrating Oxygen Vacancy and CuxO Nanodots Enabling Synergistic Electric Field and Dual Catalytic Sites Boosting CO2 Photoreduction

High carrier separation efficiency and rapid surface catalytic reaction are crucial for enhancing catalytic CO2 photoreduction reaction. Herein, integrated surface decoration strategy with oxygen vacancies (Ov) and anchoring CuxO (1 < x < 2) nanodots below 10 nm is realized on Bi2MoO6 for promoting CO2 photoreduction performance. The charge interaction between Ov and anchored CuxO enables the formation of enhanced internal electric field, which provides a strong driving force for accelerating the separation of photocharge carriers on the surface of Bi2MoO6 (ηsurf ≈71%). They can also cooperatively reduce the surface work function of Bi2MoO6, facilitating the migration of carrier to the surface. Meanwhile, surface-integrated Ov and CuxO nanodots allowing dual catalytic sites strengthens the adsorption and activation CO2 into *CO2 over Bi2MoO6, considerably boosting the progression of CO2 conversion process. In the absence of co-catalyst or sacrificial agent, Bi2MoO6 with Ov and CuxO nanodots achieves a photocatalytic CO generation rate of 12.75 µmol g-1 h-1, a remarkable increase of over ≈15 times that of the original counterpart. This work provides a new idea for governing charge movement behaviors and catalytic reaction thermodynamics on the basis of synergistic improvement of electric field and active sites by coupling of the internal defects and external species.

Contact-electro-catalytic CO2 reduction from ambient air

Traditional catalytic techniques often encounter obstacles in the search for sustainable solutions for converting CO2 into value-added products because of their high energy consumption and expensive catalysts. Here, we introduce a contact-electro-catalysis approach for CO2 reduction reaction, achieving a CO Faradaic efficiency of 96.24%. The contact-electro-catalysis is driven by a triboelectric nanogenerator consisting of electrospun polyvinylidene fluoride loaded with single Cu atoms-anchored polymeric carbon nitride (Cu-PCN) catalysts and quaternized cellulose nanofibers (CNF). Mechanistic investigation reveals that the single Cu atoms on Cu-PCN can effectively enrich electrons during contact electrification, facilitating electron transfer upon their contact with CO2 adsorbed on quaternized CNF. Furthermore, the strong adsorption of CO2 on quaternized CNF allows efficient CO2 capture at low concentrations, thus enabling the CO2 reduction reaction in the ambient air. Compared to the state-of-the-art air-based CO2 reduction technologies, contact-electro-catalysis achieves a superior CO yield of 33 μmol g-1 h-1. This technique provides a solution for reducing airborne CO2 emissions while advancing chemical sustainability strategy.

Surface Coordination Environment Engineering on PtxCu1-x Alloy Catalysts for the Efficient Photocatalytic Reduction of CO2 to CH4

Alloy catalysts have been reported to be robust in catalyzing various heterogeneous reactions due to the synergistic effect between different metal atoms. In this work, aimed at understanding the effect of the coordination environment of surface atoms on the catalytic performance of alloy catalysts, a series of PtxCu1-x alloy model catalysts supported on anatase-phase TiO2 (PtxCu1-x/Ti, x = 0.4, 0.5, 0.6, 0.8) were developed and applied in the classic photocatalytic CO2 reduction reaction. According to the results of catalytic performance evaluation, it was found that the photocatalytic CO2 reduction activity on PtxCu1-x/Ti showed a volcanic change as a function of the Pt/Cu ratio, the highest CO2 conversion was achieved on Pt0.5Cu0.5/Ti, with CH4 as the main product. Further systematic characterizations and theoretical calculations revealed that the equimolar amounts of Pt and Cu in Pt0.5Cu0.5/Ti facilitated the generation of more Cu-Pt-paired sites (i.e., the higher coordination number of Pt-Cu), which would favor a bridge adsorption configuration of CO2 and facilitate the electron transfer, thus resulting in the highest photocatalytic CO2 reduction efficiency on Pt0.5Cu0.5/Ti. This work provided new insights into the design of excellent CO2 reduction photocatalysts with high CH4 selectivity from the perspective of surface coordination environment engineering on alloy catalysts.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Nanoscale Janus Z-Scheme Heterojunction for Boosting Artificial Photosynthesis

Artificial photosynthesis for CO2 reduction coupled with water oxidation currently suffers from low efficiency due to inadequate interfacial charge separation of conventional Z-scheme heterojunctions. Herein, an unprecedented nanoscale Janus Z-scheme heterojunction of CsPbBr3 /TiOx is constructed for photocatalytic CO2 reduction. Benefitting from the short carrier transport distance and direct contact interface, CsPbBr3 /TiOx exhibits significantly accelerated interfacial charge transfer between CsPbBr3 and TiOx (8.90 × 108 s-1 ) compared with CsPbBr3 :TiOx counterpart (4.87 × 107 s-1 ) prepared by traditional electrostatic self-assembling. The electron consumption rate of cobalt doped CsPbBr3 /TiOx can reach as high as 405.2 ± 5.6 µmol g-1 h-1 for photocatalytic CO2 reduction to CO coupled with H2 O oxidation to O2 under AM1.5 sunlight (100 mW cm-2 ), over 11-fold higher than that of CsPbBr3 :TiOx , and surpassing the reported halide-perovskite-based photocatalysts under similar conditions. This work provides a novel strategy to boost charge transfer of photocatalysts for enhancing the performance of artificial photosynthesis.

Recent Advances and Perspectives of Core-Shell Nanostructured Materials for Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into solar fuels is a promising technology to alleviate CO₂ emissions and energy crises. The development of core-shell structured photocatalysts brings many benefits to the photocatalytic CO₂ reduction process, such as high conversion efficiency, sufficient product selectivity, and endurable catalyst stability. Core-shell nanostructured materials with excellent physicochemical features take an irreplaceable position in the field of photocatalytic CO₂ reduction. In this review, the recent development of core-shell materials applied for photocatalytic reduction of CO_{2 is introduced} . First, the basic principle of photocatalytic CO₂ reduction is introduced. In detail, the classification and synthesis techniques of core-shell catalysts are discussed. Furthermore, it is also emphasized that the excellent properties of the core-shell structure can greatly improve the activity, selectivity, and stability in the process of photocatalytic CO₂ reduction. Hopefully, this paper can provide a favorable reference for the preparation of efficient photocatalysts for CO₂ reduction.

Facilitated Photocatalytic CO2 Reduction in Aerobic Environment on a Copper-Porphyrin Metal-Organic Framework

Herein, we fabricated a π-π stacking hybrid photocatalyst by combining two two-dimensional (2D) materials: g-C₃ N₄ and a Cu-porphyrin metal-organic framework (MOF). After an aerobic photocatalytic pretreatment, this hybrid catalyst exhibited an unprecedented ability to photocatalytically reduce CO₂ to CO and CH₄ under the typical level (20 %) of O₂ in the air. Intriguingly, the presence of O₂ did not suppress CO₂ reduction; instead, a fivefold increase compared with that in the absence of O₂ was observed. Structural analysis indicated that during aerobic pretreatment, the Cu node in the 2D-MOF moiety was hydroxylated by the hydroxyl generated from the reduction of O₂ . Then the formed hydroxylated Cu node maintained its structure during aerobic CO₂ reduction, whereas it underwent structural alteration and was reductively devitalized in the absence of O₂ . Theoretical calculations further demonstrated that CO₂ reduction, instead of O₂ reduction, occurred preferentially on the hydroxylated Cu node.

Co-embedding oxygen vacancy and copper particles into titanium-based oxides (TiO2, BaTiO3, and SrTiO3) nanoassembly for enhanced CO2 photoreduction through surface/interface synergy

Photocatalytic CO₂ reduction into valuable fuel and chemical production has been regarded as a prospective strategy for tackling with the issues of the increasing of greenhouse gases and shortage of sustainable energy. A composite photocatalysis system employing a semiconductor enriched with oxygen vacancy and coupled with metallic cocatalyst can facilitate charge separation and transfer electrons. In this work, mesoporous TiO₂ and titanium-based perovskite oxides (BaTiO₃ and SrTiO₃) nanoparticle assembly incorporated with abundant oxygen vacancy and copper particles have been successfully synthesized for CO₂ photoreduction. As an example, the TiO₂ decorated with different amounts of Cu particles has an impact on photocatalytic CO₂ reduction into CH₄ and CO. Particularly, the optimal TiO₂/Cu-0.1 exhibits nearly 13.5 times higher CH₄ yield (22.27 μmol g^-1 h^-1) than that of pristine TiO₂ (1.65 μmol g^-1 h^-1). The as-obtained BaTiO₃/Cu-0.1 and SrTiO₃/Cu-0.1 also show enhanced CH₄ yields towards photocatalytic CO₂ reduction compared with pristine ones. Based on the temperature programmed desorption (TPD) and photo/electrochemical measurements, the co-embedding of Cu particles and abundant oxygen vacancy into the titanium-based oxides could promote CO₂ adsorption capacity as well as separation and transfer of photoinduced electron-hole pairs, and finally result in efficient CO₂ photoreduction upon the TiO₂/Cu, SrTiO₃/Cu, and BaTiO₃/Cu composites.

A review on TiO2-x-based materials for photocatalytic CO2 reduction

Photocatalytic CO₂ reduction technology has a broad potential for dealing with the issues of energy shortage and global warming. As a widely studied material used in the photocatalytic process, titanium dioxide (TiO₂) has been continuously modified and tailored for more desirable application. Recently, the defective/reduced titanium dioxide (TiO_2-x) catalyst has attracted broad attention due to its excellent photocatalytic performance for CO₂ reduction. In this perspective review, we comprehensively present the recent progress in TiO_2-x-based materials for photocatalytic CO₂ reduction. In detail, the review starts with the fundamentals of CO₂ photocatalytic reduction. Then, the synthesis of a defective TiO₂ structure is introduced for the regulation of its photocatalytic performance, especially its optical properties and dissociative adsorption properties. In addition, the current application of TiO_2-x-based photocatalysts for CO₂ reduction is also highlighted, such as metal-TiO_2-x, oxide-TiO_2-x and TiO_2-x-carbon-based photocatalysts. Finally, the existing challenges and possible scope of photocatalytic CO₂ reduction over TiO_2-x-based materials are discussed. We hope that this review can provide an effective reference for the development of more efficient and reasonable photocatalysts based on TiO_2-x.

Rational fabrication of cadmium-sulfide/graphitic-carbon-nitride/hematite photocatalyst with type II and Z-scheme tandem heterojunctions to promote photocatalytic carbon dioxide reduction

Artificial photosynthesis has become one of the most attractive strategies for lowering atmospheric carbon dioxide (CO₂) level and achieving the carbon balance; whereas, the fast electron-hole recombination and sluggish charge transfer in photocatalysts are themain stumbling blocks to the applications. Constructing semiconductor nano-heterostructures provides a promising strategy to accelerate the separation and transfer of photoinduced charge carriers for promoting the multielectron CO₂ reduction reaction. Herein, a CdS/g-C₃N₄/α-Fe₂O₃ three-component photocatalyst consisting of type II and Z-scheme tandem heterojunctions is skillfully fabricated via the solvothermal synthesis followed with photoinduced deposition. The CdS/g-C₃N₄/α-Fe₂O₃ tandem-heterojunction photocatalyst exhibits superior performance toward the conversion of CO₂ to fuels (CO and CH₄), compared with the single- and binary-component systems, owing to the favorable energy-level alignment, accelerated charge separation, facilitated water dissociation and sufficient reactive-hydrogen provision. The total consumed electron number of CdS/g-C₃N₄/α-Fe₂O₃ catalyst for CO₂ reduction is about 10.5 times that of pure g-C₃N₄. The photocatalytic mechanism is elucidated according to detailed characterizations and in-situ spectroscopy analyses. This work sheds light on the rational construction of heterojunction photocatalysts to promote the conversion of CO₂ to solar fuels, without using any sacrifice reagent or noble-metal cocatalysts.

Water coordinated on Cu(I)-based catalysts is the oxygen source in CO2 reduction to CO

Catalytic reduction of CO₂ over Cu-based catalysts can produce various carbon-based products such as the critical intermediate CO, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we develop a modified triple-stage quadrupole mass spectrometer to monitor the reduction of CO₂ to CO in the gas phase online. Our experimental observations reveal that the coordinated H₂O on Cu(I)-based catalysts promotes CO₂ adsorption and reduction to CO, and the resulting efficiencies are two orders of magnitude higher than those without H₂O. Isotope-labeling studies render compelling evidence that the O atom in produced CO originates from the coordinated H₂O on catalysts, rather than CO₂ itself. Combining experimental observations and computational calculations with density functional theory, we propose a detailed reaction mechanism of CO₂ reduction to CO over Cu(I)-based catalysts with coordinated H₂O. This study offers an effective method to reveal the vital roles of H₂O in promoting metal catalysts to CO₂ reduction.

Understanding the underlying mechanisms for catalytic reduction of CO₂ over Cu based catalysts remains challenging. Here, the authors develop an effective method to reveal the vital roles of H₂O in promoting metal catalysts to CO₂ reduction via a modified triple stage quadrupole mass spectrometer.

Dissection of Light-Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO₂ reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO₂ substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h^-1 , respectively. While the Fe⁰ redox state has been widely accepted as the catalytically active species, we show here that the Fe^I species is already involved in the CO₂ activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO₂ upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO₂ .

Engineering Catalytic Interfaces in Cuδ+/CeO2-TiO2 Photocatalysts for Synergistically Boosting CO2 Reduction to Ethylene

Photocatalytic CO₂ conversion into a high-value-added C₂ product is a highly challenging task because of insufficient electron deliverability and sluggish C-C coupling kinetics. Engineering catalytic interfaces in photocatalysts provides a promising approach to manipulate photoinduced charge carriers and create multiple catalytic sites for boosting the generation of C₂ product from CO₂ reduction. Herein, a Cu^δ+/CeO₂-TiO₂ photocatalyst that contains atomically dispersed Cu^δ+ sites anchored on the CeO₂-TiO₂ heterostructures consisting of highly dispersed CeO₂ nanoparticles on porous TiO₂ is designedly constructed by the pyrolytic transformation of a Cu²⁺-Ce³⁺/MIL-125-NH₂ precursor. In the designed photocatalyst, TiO₂ acts as a light-harvesting material for generating electron-hole pairs that are efficiently separated by CeO₂-TiO₂ interfaces, and the Cu-Ce dual active sites synergistically facilitate the generation and dimerization of *CO intermediates, thus lowering the energy barrier of C-C coupling. As a consequence, the Cu^δ+/CeO₂-TiO₂ photocatalyst exhibits a production rate of 4.51 μmol^-1·g_cat^-1·h^-1 and 73.9% selectivity in terms of electron utilization for CO₂ to C₂H₄ conversion under simulated sunlight, with H₂O as hydrogen source and hole scavenger. The photocatalytic mechanism is revealed by operando spectroscopic methods as well as theoretical calculations. This study displays the rational construction of heterogeneous photocatalysts for boosting CO₂ conversion and emphasizes the synergistic effect of multiple active sites in enhancing the selectivity of C₂ product.

Recent Advances in Porous Materials for Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction into solar fuels is a promising technology for addressing energy and CO₂ emission issues. Because of the superior properties in CO₂ adsorption and activation, molecular diffusion, light absorption, and charge separation and transfer, porous materials have been developed into a multifunctional platform for photocatalytic CO₂ reduction. In this Perspective, we first discuss the emerging trends of CO₂ reduction in major inorganic porous materials-based photocatalysts, such as mesoporous materials, macroporous materials, hollow materials, hierarchically porous materials, and zeolites. Prospects and challenges in the development of porous materials-based photocatalysts are then outlined. Finally, we envision feasible solutions for the deployment of porous materials to enhance photocatalytic CO₂ reduction performance.

Metallic Copper-Containing Composite Photocatalysts: Fundamental, Materials Design, and Photoredox Applications

Semiconductor photocatalysis has long been regarded as a potential solution to tackle the energy and environmental challenges since the first discovery of water splitting by TiO₂ almost 50 years ago. The past few years have seen a tremendous flurry of research interest in the modification of semiconductors because of their shortcomings in the aspects of solar harvesting, electron-hole pairs separation, and utilization of photogenerated carriers. Among the various strategies, the introduction of metallic copper into the photocatalysis system can not only enhance the absorption of sunlight and the separation efficiency of photogenerated electrons and holes, but also increase the adsorption ability of substrate and the number of active sites, so as to realize the high solar to chemical energy conversion efficiency. This review focuses on the rational design of copper-based composites and their applications in photoredox catalysis. First, the preparation methods of metallic copper-containing composites are discussed. Then, the applications of different types of copper-based composites in the photocatalytic removal of pollutants, splitting of water to hydrogen production, reduction of carbon dioxide, and conversion of organic matter are introduced. Finally, the opportunities and challenges in the design and synthesis of copper-based composites and their applications in the photocatalysis are prospected.

Photocatalytic CO2 reduction on Cu single atoms incorporated in ordered macroporous TiO2 toward tunable products

The Cu/3DOM-TiO2 photocatalyst exhibits high performance toward CO2 to CH4 conversion in a gas–solid system while producing C2H4 in a liquid–solid system.

Copper-Based Plasmonic Catalysis: Recent Advances and Future Perspectives

With the capability of inducing intense electromagnetic field, energetic charge carriers, and photothermal effect, plasmonic metals provide a unique opportunity for efficient light utilization and chemical transformation. Earth-abundant low-cost Cu possesses intense and tunable localized surface plasmon resonance from ultraviolet-visible to near infrared region. Moreover, Cu essentially exhibits remarkable catalytic performance toward various reactions owing to its intriguing physical and chemical properties. Coupling with light-harvesting ability and catalytic function, plasmonic Cu serves as a promising platform for efficient light-driven chemical reaction. Herein, recent advancements of Cu-based plasmonic photocatalysis are systematically summarized, including designing and synthetic strategies for Cu-based catalysts, plasmonic catalytic performance, and mechanistic understanding over Cu-based plasmonic catalysts. What's more, approaches for the enhancement of light utilization efficiency and construction of active centers on Cu-based plasmonic catalysts are highlighted and discussed in detail, such as morphology and size control, regulation of electronic structure, defect and strain engineering, etc. Remaining challenges and future perspectives for further development of Cu-based plasmonic catalysis are also proposed.

Direct Z-Scheme Heterojunction of Ligand-Free FAPbBr3/α-Fe2O3 for Boosting Photocatalysis of CO2 Reduction Coupled with Water Oxidation

Up to now, the majority of the developed photocatalytic CO₂ reduction systems need to use expensive sacrificial reductants as electron source. It is still a huge challenge to drive the photocatalytic CO₂ reduction using water as an electron source. Herein, we report a facile strategy for the construction of direct Z-scheme heterojunction of LF-FAPbBr₃/α-Fe₂O₃, which is manufactured by the in situ and two-step controlled growth of ligand-free formamidinium lead bromide (LF-FAPbBr₃) nanocrystals on the surface of α-Fe₂O₃ nanorods. The matchable energy levels and direct contact between LF-FAPbBr₃ and α-Fe₂O₃ significantly accelerate the interfacial charge transfer, with a charge separation efficiency (η_separation) of 93%, much higher than that of 11% shown by the ligand-capped FAPbBr₃/α-Fe₂O₃ heterojunction. The resulting efficient separation and raised redox ability of photogenerated carriers endow the LF-FAPbBr₃/α-Fe₂O₃ heterojunction with an outstanding photocatalytic performance for CO₂ reduction (to CO and CH₄) coupled with water oxidation (to O₂), achieving a highest electron consumption rate of 175.0 μmol g^-1 h^-1 among the reported metal halide perovskite-based photocatalysts, which are 5 and 11 times higher in comparison with those of sole LF-FAPbBr₃ and ligand-capped FAPbBr₃/α-Fe₂O₃, respectively.

Glycine-Functionalized CsPbBr3 Nanocrystals for Efficient Visible-Light Photocatalysis of CO2 Reduction

Capping ligands are indispensable for the preparation of metal-halide-perovskite (MHP) nanocrystals (NCs) with good stability; however, the long alkyl-chain capping ligands in conventional MHP NCs will be unfavorable for CO₂ adsorption and hinder the efficient carrier separation on the surface of MHP NCs, leading to inferior catalytic activity in artificial photosynthesis. Herein, CsPbBr₃ nanocrystals with short-chain glycine as ligand are constructed through a facile ligand-exchange strategy. Owing to the reduced hindrance of glycine and the presence of the amine group in glycine, the photogenerated carrier separation and CO₂ uptake capacity are noticeably improved without compromising the stability of the MHP NCs. The CsPbBr₃ nanocrystals with glycine ligands exhibit a significantly increased yield of 27.7 μmol g^-1  h^-1 for photocatalytic CO₂ -to-CO conversion without any organic sacrificial reagents, which is over five times higher than that of control CsPbBr₃ NCs with conventional long alkyl-chain capping ligands.

The enhancement of photocatalytic CO2 reduction by the in situ growth of TiO2 on Ti3C2 MXene

Photocatalytic CO₂ reduction is enhanced by the promoted charge transfer at the interface between TiO₂ and Ti₃C₂ after the in situ growth of TiO₂ on Ti₃C₂.

Photocatalytic Reduction of CO2 to CO over Quinacridone/BiVO4 Nanocomposites

Solar energy-driven photoreduction of CO₂ to energy-rich chemicals is of significance for sustainable development but challenging. Herein, quinacridone (QA)/nBiVO₄ (n=0.2-20, in which n stands for the mass ratio of BiVO₄ to QA) nanocomposites were developed for photoreduction of CO₂ . Characterization of the materials with Fourier-transform (FT)IR spectroscopy and X-ray photoelectron spectroscopy (XPS) pointed to QA/nBiVO₄ preparation via hydrogen-bonding-directed self-assembly of QA on BiVO₄ nanosheets. Using triethanolamine (TEOA) as a sacrifice reagent, QA/10BiVO₄ showed the best performance, affording CO with a production rate of 407 μmol g^-1  h^-1 , 24 times higher than those of pure QA. It was indicated that the Z-scheme charge-transfer mechanism of QA/nBiVO₄ could significantly improve the separation and transmission efficiency of photo-generated electrons and holes. This novel approach provides new insight for fabricating the composite photocatalytic materials of small molecule organic semiconductors and inorganic semiconductors with high efficiency for photocatalytic of reduction CO₂ .

Activity and selectivity of CO2 photoreduction on catalytic materials

Photoreduction of molecular CO₂ by solar light into added-value fuels or chemical feedstocks is an appealing strategy to simultaneously overcome environmental problems and energy challenges. However, multiple reaction steps and a large number of possible products have significantly hindered the development of highly selective catalysts capable of delivering CO₂ conversion with high efficiency. Recently, several strategies associated with different conversion mechanisms have been proposed to improve the activity and product selectivity of CO₂ photocatalysts. These are based on development of low dimensional nanomaterials, defect or facet engineering, design of tailored heterostructures, and carrier conductivity enhancement. In spite of impressive progress in the field, real-world applications are yet to be delivered. To sustain further research in this promising field, here we provide a short frontier of recent advances in activity and selectivity of CO₂ reduction photocatalysts, together with a critical discussion of further avenues of research in this field.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.

Spiers Memorial Lecture. Artificial photosynthesis: An introduction

A brief introduction into artificial photosynthesis technologies is presented. Following the basic concepts of biological photosynthesis, light energy is directly or sequentially used for the synthesis of valuable chemicals with the help of man-made catalysts. Differences between artificial, hybrid and natural photosynthesis are shown and the possible advantages and disadvantages are highlighted.