銅基光催化劑二氧化碳轉化研究進展

關鍵詞：光催化；CO2 還原；銅基催化劑；改性策略；梯形異質結

1 Introduction

In recent decades， excessive fossil fuel consumption hasresulted in both an energy crisis and significant CO2 emissions 1–5.These emissions have led to serious environmental issues likeglobal warming and ocean acidification 6. Thus， it is crucial tominimize CO2 emissions while simultaneously advancingsustainable CO2 conversion technologies. Various methods havebeen developed to convert CO2， including electrocatalytic 7，thermocatalytic 8， and photocatalytic methods 9. Among them，photocatalytic CO2 conversion to solar fuel is regarded as aviable solution for mitigating CO2 emissions and addressingenergy shortages due to the availability of inexhaustible solarenergy 10–13.

Considerable effort has been put into designing effectivephotocatalysts to facilitate CO2 conversion under mildconditions. Of the numerous photocatalysts explored， noblemetals， including Au， Pt， Ag， and Pd have been thoroughlyinvestigated and shown to enhance photocatalytic performanceowing to their remarkable catalytic abilities 14–16. In particular，the incorporation of Au and Ag can lead to localized surfaceplasmon resonance （LSPR）， which significantly enhances thephotoresponse and aids in separating photogenerated chargecarriers in semiconductor photocatalysts 17. However， thewidespread application of these materials is restricted due totheir high cost and limited availability in the Earth’s crust.Therefore， the design of cost-effective， non-noble metal catalystsis essential to achieve large-scale photocatalytic applications 18.

In recent years， copper-based materials have become a focusof interest for photocatalytic CO2 reduction because they offerexcellent photoactivity， are readily available， and are non-toxic 19.Developing effective copper-based systems for photocatalyticCO2 reduction reactions （CO2RR） requires a comprehensivereview of the advances in copper nanocatalysts. In this paper， wefirst introduce the general photocatalytic principles for CO2RR.Next， we focus on discussing representative copper-basednanocatalysts， namely， copper nanoparticles， cuprous oxide， andcopper oxide， and the different means of modulating their CO2photoreduction performance. Specifically， we highlight thevarious techniques such as morphology design， size design，semiconductor compounding and heterojunction constructionthat can be utilized to enhance their effectiveness for CO2RR.Finally， we discuss the key issues and potential benefits of usingcopper-based catalysts for CO2 photoreduction. We anticipatethat this review will serve as a practical reference for producingcertain products and a useful resource for researchers developingcopper-based photocatalysts.

2 General mechanisms of CO2 reduction

The process of photocatalytically reducing CO2 is intricate，involving multiple stages and the transfer of electrons 20. Fig. 1shows the three fundamental stages of a typical CO2photoreduction process. Initially， the photocatalyst is excited bylight， resulting in the formation of electron-hole pairs （e?/h+）.Photons with energy greater than the semiconductor’s band gapcause electrons to jump from the valence band （VB） to theconduction band （CB）， creating e?/h+， where electrons （e–） are inthe CB and holes （h+） are in the VB 21. The subsequent stageinvolves spatially separating the photoinduced e?/h+. The e? andh+ generated by photoexcitation are separated from the bulkphase of the photocatalyst and migrate to the surface， wheresome of them will be recombined and lost inside the catalyst，while the others will successfully move to the catalyst surface toparticipate in the subsequent reaction 22. Finally， the electronholepairs undergo redox reactions. The electrons that reach thephotocatalyst’s surface participate in the reduction reaction withadsorbed CO2 to yield fuels and chemicals， while the holes areconsumed in an oxidation reaction with water or sacrificialagents 23.

For the CO2RR to be effective， the energy levels of the CBand VB of the semiconductor photocatalyst must meet thethermodynamic requirements of the CO2 reduction reaction 24，25.Specifically， the CB position of the semiconductor catalyst mustbe more negative than the standard CO2 reduction potential，whereas the VB position must be more positive than the potentialfor H2O oxidation. Table 1 presents the necessary potentials forreducing CO2 to different compounds， including methane （CH4），formic acid （HCOOH）， formaldehyde （HCHO）， acetic acid（CH3COOH）， and ethylene （C2H4）， listed relative to a standardhydrogen electrode （vs. NHE） at pH 7. For instance， to convertCO2 into CH4， the CB position of the photocatalyst must be morenegative than ?0.24 V.

The photocatalytic process for reducing CO2 involvesmultiple steps and involves the transfer of numerous electronsand protons， ultimately leading to the synthesis of valuablechemicals like formaldehyde， methanol， methane， and glyoxal.Several factors influence the product selectivity， and the reactionmechanism is dependent on the number of electrons producedand their transfer to the CO2 molecule. The process involvesC=O bond breaking， formation of intermediate species，oxidation of H2O to produce protons， coupling of intermediatespecies with protons， and the formation of new bonds 26. Fig. 2depicts the generation pathways for formaldehyde， methanol，methane， and glyoxal. In the photoreduction of CO2 toformaldehyde， CO2 is activated by combining O atoms withactive sites of the catalyst. An electron is transferred to the CO2molecule to generate a free radical that then combines with aproton to produce a *COOH free radical intermediate.Subsequently， a proton and an electron are added to the *COOHintermediate to generate formic acid. Lastly， formic acid acceptstwo protons to produce formaldehyde and water. Methanol andmethane can also be produced through these pathways insubsequent steps， depending on how the intermediate speciescombine with electrons and protons 27. Researchers havetheoretically and experimentally analyzed these pathways.However， due to the intricate nature of the reaction， withnumerous steps， intermediates， and byproducts， achievingproduct selectivity is challenging. For instance， C2 productselectivity necessitates stabilization of the *CH3 radical 28， butthis selectivity is generally low due to rapid hydrogenation ofintermediates to produce the C1 compounds and the repulsiveinteraction between intermediates with the same charge，hindering C―C bond formation 29.

Here， we divide the photocatalytic CO2 reduction productsinto two categories， one is the product of the two-electronreaction CO （without HCOOH）， and the other is the product ofmulti-electron reactions （CH4， CH3OH， CH3COOH， etc.）.

3 Characterizations

Numerous material characterization techniques are applicableto copper-based catalysts for unveiling their composition，morphology， and other physicochemical properties. This sectionprovides a concise overview of the most pivotal and widelyadopted techniques. X-ray diffraction （XRD） serves as a crucialtool for elucidating crystal structures and crystallinity. Thismethod is extensively utilized for determining lattice parametersof copper-based catalysts. On the other hand， X-rayPhotoelectron Spectroscopy （XPS） is primarily utilized forsurface analysis， examining the elemental composition andchemical states of surface atoms. It can detect copper oxides ornanoparticles （NPs）， determine the Cu oxidation state， andevaluate their adhesion properties. Scanning electronmicroscopy （SEM） and transmission electron microscopy（TEM） are pivotal for determining the morphology and detailedstructure of many nanomaterials， including copper-basedcatalysts. The advancement of electron microscopy has spurredadditional techniques， including such as energy dispersive X-rayspectroscopy （EDS）， high-resolution TEM （HRTEM）， highangleannular dark-field scanning TEM （HAADF-STEM），scanning tunneling microscopy （STM）， and elemental mapping.These means are commonly employed to analyze nanocatalysts，exploring texture， composition， adhesion to support materials，and other aspects.

4 CO2RR by copper-based photocatalysts

4.1 Light-driven reduction of CO2 to CO

The conversion of CO2 to CO through photocatalysis entailsthe transfer of two electrons. It is a common pathway for theconsumption of CO2 with fewer kinetic barriers than theprocesses that generate more complex organic compounds.

4.1.1 Copper oxides

Recent investigations have proven that modifying copperoxide （CuO） is a viable strategy for regulating the redoxpotential and minimizing the recombination of photogeneratede? and h+ 30. Utilizing these effects can enhance the process ofconverting CO2 into CO. To this end， researchers have employedseveral typical strategies， including semiconductor compounds，heterojunction designs， and morphological designs. Theseapproaches aim to optimize the catalytic properties of CuObasedmaterials， which is essential for achieving high efficiencyin CO2 reduction. By adopting these strategies， CuO-basedphotocatalysts have demonstrated significant potential for futureapplications in carbon capture and utilization.

4.1.1.1 Semiconductor compound

Owing to its narrow band gap （1.34-1.70 eV）， CuO oftenexperiences fast recombination of e? and h+， which can hinder itsperformance in photocatalytic applications. To solve thisproblem， the transfer of photogenerated carriers can beaccelerated by introducing highly conductive materials. Qianand his coworkers synthesized CuxO-modified La2Sn2O7（CuxO/La2Sn2O7） through a two-step hydrothermal procedurefor the CO2 photoreduction without sacrificial reagents 31. Theoptimized 2.4% CuxO/La2Sn2O7 catalyst demonstratedremarkable CO2 reduction activity， yielding 109.4 μmol?g?1 ofCO after 3 h of light exposure. The total number ofphotoelectrons utilized was 650.8 μmol?g?1， which surpassedthat of the reference La2Sn2O7 and Au/La2Sn2O7 byapproximately 6 and 3 times， respectively. The presence ofCuO/Cu2O/Cu clusters is responsible for the increasedphotocatalytic activity （Fig. 3a，b）， as they improve thephotoresponse， facilitate the transfer of electrons， and provideactive sites for CO2 molecule adsorption and subsequentreactions. Consequently， these clusters contribute significantlyto the improved photocatalytic performance.

4.1.1.2 Heterojunction construction

Constructing heterojunction structures can effectivelyseparate photogenerated carriers and improve both the efficiencyand stability of photocatalysis. S-scheme heterojunctions arebeing increasingly recognized for their role in photocatalyticCO2 reduction due to their excellent carrier separation efficiencyand the redox ability of different active components 32–35. Zhanget al. demonstrated the preparation of a high-performance Sschemeheterojunction of CCB/CuO for CO2 photoreductionreaction 36. This was achieved by incorporating lead-freeCs2CuBr4 PQDs into mesoporous CuO skeletons. Experimentaland theoretical investigations confirmed the S-scheme chargetransfer mechanism in the CCB/CuO heterojunction. （Fig. 3c）.Notably， the rate of electron depletion （Relectron） in the CCB/CuOsystem was found to be 2.8 times faster compared to that ofpristine Cs2CuBr4. The improved charge separation and greaterCO2 adsorption capacity are crucial factors contributing to thesuperior photocatalytic efficiency of the CCB/CuO sample.These results underscore the advantages of the S-schemeheterojunction configuration. CuO/Cu2O hybrid heteronanosheets（HNSs） containing Cu （II）-O-Cu （I） interfacialbridges were successfully synthesized by transforming bare CuOnanospheres using an in situ topotactic strategy 37. TheCuO/Cu2O-HNSs exhibited notable CO2 reduction photoactivityunder visible-light illumination， achieving a high CO productselectivity of 94.4%. Mechanistic studies revealed that the Cu（II）-oxygen-Cu （I） bridge significantly contributes to theeffective separation of photoinduced e?/h+. This bridgefunctioned as a rapid channel （S-scheme） for directing thetransfer of these carriers （Fig. 3d）. Furthermore， the existence ofthe Cu （I） center on the Cu （II）-oxygen-Cu （I） bridge functionedas an active site for CO2 reduction， effectively reducing theenergy barrier for CO2RR-CO conversion. Additionally， the Cu（I） center’s location lowered the d-band center， facilitating thedesorption of *CO intermediates during the CO2RR process.Using an ultrafast spray-calcination method， Shi et al.synthesized a series of CuBi2O4/CuO photocatalysts on glasssubstrates 38. The optimized CBO/CuO composites exhibitedsignificantly improved photocatalytic CO2 reductionperformance compared to pure CuBi2O4 and CuO， with a COyield of 1599.1 μmol?m?2 after visible light illumination for 9 h.Based on the photocatalytic activity and in situ XPS analysis， thecharge transfer mechanism of the S-scheme as shown in Fig. 3eis proposed. The results show that the enhanced photocatalyticactivity is due to the effective separation of photoinducedcharges brought about by the CuBi2O4/CuO S-schemeheterojunction with well-aligned staggered energy bandstructure. Using a new interfacial engineering method thatmerges optical and catalytic activity sites， Liu et al. designed andsynthesized CuO@In2O3 S-scheme heterojunction composites 39.The CuO@In2O3 photocatalyst achieved a CO2 to CO evolutionrate of 500.46 μmol?g?1?h?1， representing a 10.3-fold and 7.9-foldincrease over pure CuO and In2O3， respectively. As illustrated inFig. 3f， a built-in electric field （IEF） is established at the In2O3-CuO interface， directed towards CuO and causing the energybands to bend. This interfacial electric field， along with theenergy band bending， facilitates the transfer of e– from the CBof CuO to the VB of In2O3. As a result， this migrationsignificantly improves the charge separation efficiency.

4.1.1.3 Morphology design

The photocatalytic performance depends on photocatalyst toa great extent 40，41. The morphological design of catalysts hasgarnered increasing interest in recent years. Fang et al. preparedthe CuO-TiO2 hollow microsphere catalyst by a one-pottemplate-free approach （Fig. 4a） 42. The CuO-TiO2 sampleshows outstanding activity for CO2 photoreduction， with a COproduction rate of 14.5 μmol?g?1?h?1， which is over four timesthat of P25 TiO2 and TiO2 hollow microsphere catalysts. Severalkey factors account for the superior performance. Primarily， thesubstantial specific surface area of CuO-TiO2 hollowmicrospheres ensures a high density of catalytic active sites forreactant interaction. Additionally， the catalyst?s hierarchicalnanostructure promotes efficient mass transfer of reactants andproducts through its porous framework. Furthermore， the hollowmacroporous core and mesoporous shell nanostructure increaseslight scattering and reflection， improving the capture ofexcitation light and thereby enhancing the photocatalyticefficiency of the CuO-TiO2 hollow microspheres. Throughseveral pulsed atomic layer deposition cycles， numerous CuOnanowires （108 cm?2） were surface-modified alongside denselypacked ZnO islands 43. The SEM images of representative CuOand ZnO-CuO nanowires are displayed in Fig. 4b. Thesenanowires undergo CO2 photoreduction based on UV-Visradiation under saturated humidity （CO2 + H2O mixture）conditions. The highest CO conversion rate （1.98mmol?gcat?1?h?1） and quantum efficiency （0.0035%） occur whenZnO islands collide at a diameter of 1.4 nm （8 ALD cycles）.These findings suggest that optimizing the morphology of CuOis advantageous for enhancing the photocatalytic reaction.

4.1.2 Cuprous oxide

Cuprous oxide （Cu2O） is a preferred semiconductor for CO?photoreduction owing to its advantageous narrow band gap （Eg =1.9-2.2 eV）， natural abundance and potential to activate CO2.Nonetheless， the material is susceptible to photocorrosion， whichleads to the performance degradation over extended periods ofoperation. A number of means have been used by researchers toboost the activity of Cu2O in CO2 photocatalytic reduction toCO， including semiconductor compounding， heterojunctionconstruction and morphological regulation.

4.1.2.1 Semiconductor compound

Integrating Cu?O with LDHs has been established as asuccessful method for boosting photocatalytic efficiency. Jianget al. prepared a series of Cu2O-modified Zn-Cr LDHs（xCu2O@Zn2?2xCr LDH） by in situ reduction using Cu-Zn-Crternary layered hydroxides as raw materials 44. Among thevarious loaded LDHs， 0.1Cu2O@Zn1.8Cr LDH achieved thehighest activity for CO? reduction to CO in pure water，exceeding the performance of Cu-Zn-Cr ternary LDHs and pureZn2Cr LDHs. Isotope analysis （Fig. 5b） has revealed that uponultraviolet light exposure， 0.1 Cu2O @ Zn1.8Cr LDH immersedin H218O under 13CO2 atmosphere significantly increases themass peaks of 18O2 （m/z = 36） and 13CO （m/z = 29）. Thisobservation confirms that photoinduced e– are responsible forreducing CO2 to CO， whereas holes facilitate the oxidation ofwater to O2. The photocatalytic efficiency of Cu2O NPs issignificantly improved by the surface modification of LDH.Cu2O’s ability to function as an electron trap， as shown in Fig.5a， reduces recombination of e?/h+， leading to improvedphotocatalytic performance.

Earlier research has shown that the exceptional catalyticefficiency of these materials stems from their electron transfermechanisms. However， the specific impact of electron transferon catalytic activity remains unclear. Zhu et al. conducted astudy to investigate the link between electronic structure andcatalytic activity in Cu NPs combined with Cu2O （Cu/Cu2O-1，-2， -3）， employing the acid disproportionation approach withdifferent processing durations 45. Among the Cu/Cu2O samplestested， Cu/Cu2O-2 exhibited superior activity in CO2 reduction，yielding CO at a rate of 10.43 μmol?g?1?h?1. This rate was overfourfold higher than that of pure Cu2O and surpassed the resultsof both Cu/Cu2O-1 and Cu/Cu2O-3. This result emphasizes thepivotal importance of electron transfer in influencing thecatalytic performance of these materials. Interestingly， thecatalyst with the highest electron transfer （Cu/Cu2O-1） does notshow the best photoreduction performance， whereas the samplewith moderate electron transfer （Cu/Cu2O-2） demonstrates thehighest photoreduction efficiency. To verify these results， theGibbs free energy of the CO2 reduction and the adsorptionenergy of reaction intermediates *CO2， *COOH， and *CO ondifferent models （Fig. 5c） were calculated. The findings suggestthat Cu/Cu2O has the lowest rate-determining step for COproduction， as indicated by a CO desorption energy barrier of0.48 eV， which is much lower than the 0.69 eV barrier for Cuand the 3.01 eV barrier for Cu2O. This result aligns well with theexperimental results. According to Fig. 5d， the adsorptionenergies of Cu/Cu2O with CO2， COOH， and CO were measuredto be ?1.0， ?2.5， and ?1.0 eV， respectively， which weresignificantly higher than the counterparts of Cu2O （?2.4， ?4.6，and ?3.5 eV） and lower than the counterparts of Cu （?0.02， ?1.6，and ?1.1 eV）， showing a moderate adsorption capacity. It hasbeen found that the changes in Bader charge of active centers arelinearly related to the adsorption energy of intermediates in theCO2 reduction. This finding indicates that a moderate chargetransfer is associated with an optimal adsorption energy level，which significantly enhances the photoreduction activity of CO2.It is important to note that the highest level of electron transferdoes not necessarily equate to the best catalytic performance.Instead， moderate adsorption is advantageous in reducing thereaction energy barrier to its lowest level， thereby resulting insuperior CO2 reduction reaction performance.

4.1.2.2 Heterojunction construction

MOFs have been extensively researched as both adsorbentsfor CO2 and multiphase photoreduction materials for CO2，thanks to their large specific surface area， customizable poresize， rich array of catalytic centers， and effective gas adsorptioncapability. Dong et al. prepared a variety of Co-MOF/Cu2O（XCMC） composites using a stepwise self-assembly method 46.The 33 wt% CMC hybrids show maximum photocatalyticefficiency for CO2 reduction to CO， operating withoutphotosensitizers or sacrificial reagents. After 4 h of lightexposure， the CO product yield reaches 15.3 μmol?g?1?h?1 withalmost 100% selectivity. Fig. 6a illustrates the possible pathwayof CO2 reduction by Co-MOF/Cu2O photocatalysts. Initially， theFermi energy levels of n-type Co-MOF and p-type Cu2O arelocated near their respective valence and conduction bands.Upon contact between Cu2O and Co-MOF， a p–n junction isformed， resulting in an IEF that aligns the semiconductors’Fermi energy levels. Upon exposure to visible light， the excitede? in the CB of Cu2O are efficiently transferred to the CB of Co-MOF， facilitated by the p-n junction’s electric field.Subsequently， the e? within the CB of the Co-oxo cluster in Co-MOF interact with CO2 molecules adsorbed on the surface toform CO， whereas the h+ generated in Co-MOF’s VB migratetoward Cu2O. This binary heterostructure design ensureseffective separation of photoexcited electrons， enabling rapidtransfer to the Co-oxo cluster of the catalyst， therebysignificantly enhancing the photocatalytic CO2 reductionactivity.

The p-n heterojunction discussed above is a common type IIheterojunction that results in the reduction of the photogeneratedelectron and hole redox abilities 47，48. To address this problem，researchers in recent years have designed and constructed Sschemeheterojunctions for photocatalytic CO2 reduction. Bydesigning an effective g-C3N4/Cu2O@Cu plasmonic S-schemeheterojunction， the authors significantly improved the CO2reduction efficiency of Cu2O 49. Under visible light illumination，the optimized photocatalyst produces CO at a rate of 10.8μmol?g?1?h?1， which is 13.5 times greater than that of pristineCu2O. The key parameters enhancing the photocatalytic activityare outlined below. （1） the construction of S-schemeheterojunctions effectively separates and transfers charges; （2）LSPR of Cu NPs efficiently separates interfacial charges andenhances light absorption. Fig. 6b illustrates the proposedmechanism for CO2RR using the g-C3N4/Cu2O@Cu plasmonicheterojunction. A 3D ReS2@Cu2O/Cu heterojunctionphotocatalyst was rationally constructed using a thermaloxidation method by Zhang et al 50. The optimizedReS2@Cu2O/Cu photocatalyst exhibits superior performance inconverting CO2 to CO than ReS2@Cu2O/Cu-100 andReS2@Cu2O/Cu-300， with the ReS2@Cu2O/Cu-180photocatalyst having the highest CO yield （14.3 μmol?g?1）.Enhanced visible light absorption is observed in theReS2@Cu2O/Cu photocatalyst， and the S-scheme charge transferpathway between ReS2 and Cu2O facilitates more efficientcharge separation （Fig. 6c）， leading to improved photocatalyticperformance. Ling et al. synthesized g-C3N4/Cu2Oheterojunctions and dispersed palladium nanoparticles on theirssurface as a co-catalyst 51. Under visible light， the g-C3N4/Cu2OPdS-scheme heterojunction catalyst outperforms Cu2O， g-C3N4，and g-C3N4/Cu2O in CO2 reduction， reaching a CO yield of 14.6μmol?mg?1 after 2 h of irradiation and maintaining highphotocatalytic activity through the fourth cycle. The increasedphotocatalytic performance results from the effective chargeseparation achieved through the S-scheme heterojunction in theg-C3N4/Cu2O-Pd composites （Fig. 6d）. The authors constructedp-n heterojunctions by incorporating CsPbBr3 NCs （n-type） intomesoporous Cu2O microspheres （p-type） 52. Under visible lightexposure， the CPB/Cu2O heterojunction functions as an Sschemephotocatalyst， demonstrating markedly improvedactivity and stability for CO2 reduction to CO. Fig. 6edemonstrates the charge transfer process in the CPB/Cu2O Sschemeheterojunction. Since Cu2O is a p-type semiconductorand CsPbBr3 is an n-type semiconductor， they form a closelycontacted p-n heterojunction where electrons spontaneouslymigrate from CsPbBr3 to Cu2O until their Fermi levels align.Consequently， CsPbBr3 forms an electron depletion layer，acquiring a positive charge at the interface， while Cu2O developsan electron accumulation layer， acquiring a negative charge. Theestablishment of this space charge layer induces band bendingand an IEF directed from CsPbBr3 to Cu2O. Under the influenceof band bending， IEF， and Coulombic attraction between e– andh+， e? in the CB of Cu2O traverse the interface and recombinewith h+ in the VB of CsPbBr3， thereby forming an effective Sschemeelectron transfer pathway that significantly improves thecharge separation efficiency.

4.1.2.3 Morphological regulation

Material structural engineering is a common approach used toimprove electron transfer in photocatalysts. By employing a 3Dporous structure， multiple advantages are achieved， includingsignificantly enhancing visible light absorption through multiplescattering and increasing CO2 adsorption. Additionally， such astructure improves the efficiency of interfacial reactions and gastransfer. Cui et al. demonstrated this by preparing 3D porousCu2O with a dendritic structure via electrodeposition followedby thermal oxidation at 220 °C 53. The transformation of the 3Dporous copper structure into Cu2O results in a highelectrochemical specific surface area and outstandingphotocatalytic CO2 reduction performance. The 3D porous Cu2Oexhibits a 2.5-fold increase in photoinduced carrierconcentration （4.3 × 1020 cm?3） and a 24-fold improvement inCO2 to CO conversion （13.4 nmol?cm?2?h?1） compared tononporous Cu2O. Fig. 7a，b illustrate the SEM image and theproposed mechanism of photocatalytic CO2 reduction occurringon 3D porous Cu2O. The notable improvement in photocatalyticperformance results from the 3D porous structure?s ability toenhance CO2 gas mass transfer efficiency and the nanodendriticstructure?s capacity to increase light trapping and electrontransport efficiency. In addition， the 3D porous Cu2O is highlyresistant to photocorrosion， owing to the rapid separation andhigh reactivity of its photogenerated e? and h+. 3D g-C3N4 foamswith micron-scale porous structures were prepared by a noveland simple method by Sun et al. 54. In addition， Cu2O quantumdots （QDs） were then deposited onto the 3D g-C3N4 foamthrough a straightforward photodeposition approach. Theresulting g-C3N4 foam/Cu2O QDs composite exhibitsconsiderably better performance than pristine g-C3N4 foam andg-C3N4 powder， with enhancements by factors of 3.9 and 11，respectively. Fig. 7c illustrates the pathway of CO2RR in thiscomposite. The exceptional photocatalytic performance isassigned to the porous structure and the synergistic interactionbetween g-C3N4 foam and Cu2O QDs， which facilitate efficientcharge carrier transfer and promote the accumulation of e? on theCu2O QDs. Chang et al. combined submicron-sized Cu2Ocrystals with various morphologies with graphite nitride （gCN）to assess their CO2 photoreduction activity under visible lightillumination 55. The findings indicate that the morphology ofCu2O crucially influences its band structure， optical properties，and the effectiveness of charge transfer. Fig. 7d illustrates theband structures of xCu2O and gCN.

4.1.3 Copper

Copper metal， known for its excellent conductivity， does notinherently possess photocatalytic activity. However， it caneffectively collaborate with other photocatalysts to boost theirperformance. In this context， copper metal plays various crucialroles， such as serving as an electron trap， adsorption site， orheterojunction. These functions contribute to enhancing theperformance of the CO2 photoreduction process. It is worthmentioning that copper-based catalysts can function asphotocatalysts in the photocatalytic process， directly engaging inlight absorption and charge separation. Additionally， they canserve as co-catalysts in certain cases， strengthening theefficiency of the entire photocatalytic system by improvingsurface charge transfer processes.

4.1.3.1 Semiconductor compound

Cu-Ti3C2Tx nanosheets were prepared by Xiao et al. throughthe spontaneous reduction of Cu ions on the surface of Ti3C2Tx，without the need for an applied reducing agent. CuTi3C2Tx/g-C3N4 photocatalysts were formed by coupling Cu-Ti3C2Tx withg-C3N4， using an electrostatic self-assembly strategy （Fig. 8a） 56.The close contact between Cu-Ti3C2Tx and g-C3N4 is essentialfor enhancing the separation of e?/h+. In addition， Cu andTi3C2Tx function as efficient channels for electron transport，while the interface between Cu and Ti3C2Tx serves as an activesite for CO2 adsorption and activation. These combined effectslead to the exceptional activity of the CuTi3C2Tx/g-C3N4catalyst， which achieves a CO yield of 49.02 μmol?g?1，representing a 9.0-fold increase over that of the pristine g-C3N4.In another study， Zhu and coworkers developed Cu/TiO2composites with different levels of integration using a two-stepprocess that included hydrothermal treatment followed by photodeposition57. The various electronic structures of the Cu/TiO2composites were modulated by adjusting the degree of bonding.When compared to pure TiO2， the Cu/TiO2 compositedemonstrates markedly superior performance， with the Cu/TiO2-3 sample exhibiting exceptional photocatalytic activity when thecritical level of composite Cu is reached. This sample achievesa CO evolution rate of 15.27 μmol?g?1?h?1， with a remarkableselectivity of 95.9% （Fig. 8b，c）. The superior performance stemsfrom the robust interaction between Cu and TiO2， which inducesstructural changes in Cu/TiO2 that impact the electronicproperties of TiO2. Specifically， the characteristic electronicstructure of TiO2 is occupied by low-energy orbitals in theCu/TiO2-3 structure. Furthermore， alterations in the Ti-Ovibration mode and the surrounding environment of Cu/TiO2 inthis structure enhance carrier transport efficiency. However， theperformance of Cu/TiO2-4 and Cu/TiO2-5 declines due to theagglomeration of Cu particles resulting from increased Cucontent. This agglomeration reduces the interaction between Cuand TiO2， leading to decreased electron transfer between the twocomponents. To pinpoint the origin of the produced CO， isotopelabeling experiments were carried out， followed by productanalysis using gas chromatography-mass spectrometry （GCMS）（Fig. 8d）. In the GC analysis， a peak with a retention timeof 10.8 min corresponds to CO， which is further confirmed in theMS analysis as 13CO （m/z = 29）. These GC-MS results provideevidence that CO is produced through CO2 photocatalysis.Zhang et al. synthesized a copper-based boron imidazolate cage（BIF-29） with six mononuclear copper centers exposed for CO2photoreduction reaction 58. The cage with unsaturatedcoordinated Cu sites （BIF-29） shows greater efficiency intransforming CO2 to CO compared to the coordination-saturatedcage （BIF-33）， with a CO formation rate of 3334 μmol?g?1?h?1and a selectivity of 82.6% （Fig. 8e）. The high photoactivity ofBIF-29 is ascribed to the existence of unsaturated coordinationcopper sites. DFT calculations （Fig. 8f） demonstrate that theCOOH intermediates in BIF-29 have a formation energy of 1.26eV at the Cu site. In contrast， BIF-33 has a formation energy of1.63 eV for COOH intermediates at the copper site， reflectingthe impact of its saturated Cu coordination. This finding showsthat BIF-29 promotes the generation of *COOH and introducesa new state in the CB near the Fermi energy level （Fig. 8g），enhancing its reactivity compared to BIF-33. In summary， theunsaturated coordination copper in BIF-29 facilitates the CO2adsorption and improves the CO2 reduction efficiency. Hu et al.introduced an innovative method for transforming biomass intoeffective photocatalysts for artificial CO2 reduction 59. Theirapproach utilizes hydrothermal processes to convertcarbohydrates from biomass into hydrothermal carbon （HTCC），which features abundant sp2 hybridized structures that efficientlyabsorb sunlight for CO2RR. The incorporation of a coppercocatalyst further boosts the CO2 reduction performance.Notably， Cu-HTCC exhibits a CO2 reduction activity that is 32times greater than that of commercial TiO2 and 1.7-fold highercompared to HTCC， as demonstrated in Fig. 8h. The theoreticalcalculations shown in Fig. 8i highlight the substantial role of theCu cocatalyst in enhancing CO2 reduction efficiency.Specifically， the generation of COOH* intermediates and COoccurs preferentially with the Cu cocatalyst， resulting in a morenegative generation energy compared to pure HTCC. Thesefindings align with the observed higher CO yield of Cu-HTCC.

photocatalysts significantly boosts their ability to absorb visiblelight， enhancing the efficiency of photogenerated carrierseparation and transfer. This alteration affects thesemiconductor’s core electronic properties and band gap，resulting in enhanced photoactivity. Cao et al. employed a cationexchange approach to create CuCdS-x catalysts， incorporatingatomically dispersed copper centers and sulfur vacancies on theCdS surface for superior photocatalytic CO2 reduction 60. Theresulting CuCdS-5 catalyst achieves remarkable photocatalyticactivity， producing CO at a rate of 8.5 μmol?g?1?h?1 and a 92%selectivity， over three times the efficiency of original CdS， asillustrated in Fig. 9a. Through the combination of experimentaldata and DFT calculations （Fig. 9b，c）， it is apparent thatatomically dispersed copper sites and sulfur vacancies are vitalfor creating more CO2 adsorption centers. They further assist incharge redistribution and decrease the dissociation adsorptionenergy of CO2， ultimately elevating the photocatalytic efficiencyof the system. Wang and colleagues fabricated a series ofCu/CeO2?x?0.1 samples using a simple hydrothermal approach 61.The Cu/CeO2?x?0.1 sample demonstrated the highestphotocatalytic performance after 5 h of irradiation， producing8.25 μmol?g?1 of CO， as depicted in Fig. 9d. This outstandingresult， 26 times greater than CeO2?x， results from the integrationof Cu， which enhances the catalyst's optical absorption across theUV-visible spectrum. This improvement assists in the effectiveseparation and transport of e– and h+， thereby extending thelifespan of the carriers， as evident in the time-resolvedfluorescence spectra （Fig. 9e）. In addition， Bao et al. prepared aphotocatalyst using a molten salt method with highly dispersedCu nanoclusters （Cu NCs） loaded on the TiO2 surface containingoxygen vacancies， to boost the CO2 photoreduction process 62.With a CO evolution rate of 40.23 μmol?g?1?h?1， the optimizedcatalyst COCT-3 significantly outperforms pure TiO2 and themajority of previously reported TiO2-based photocatalysts. Theremarkable catalytic activity is primarily due to the synergisticinteraction between Cu NCs and OVs. The function of OVs inboosting CO2 adsorption and activation， coupled with thesupport of Cu NCs in moderating H2O dissociation， isdemonstrated through experimental results and DFT calculations（Fig. 9g，h）. Specifically， the interaction between OVs and CuNCs fine-tunes the thermodynamics and kinetics of CO2 to COreduction， creating a pathway with a lower energy barrier.Additionally， Fig. 9i offers a detailed view of the reactionpathway for the CO2-to-CO reduction process using COCT-3.

Pristine Cu NPs， powered by LSPR， demonstrate improvedphotocatalytic activity compared to other metals. Interestingly，the photocatalytic activity varies significantly with the numberof phenothiazine units attached to the benzene rings and linkedto Cu nanoparticles. Zeng et al. employed a straightforward insitu photoreduction method to combine phenothiazine benzene（XPB） with adjustable bonding units with plasmonic Cunanoparticles 63. Among a series of prepared samples， Cunanoparticles encapsulated in triphenothiazine benzene （TPB）have better photocatalytic properties， with a formation rate of1308.8 μmol?g?1?h?1 for CO without cocatalyst （Fig. 10a）. Thisresults from the unique concave structure and efficientconjugation system， which create an extensive contact area withCu NPs， facilitating the efficient migration of hot electrons of CuNPs （Fig. 10b）. There has been a notable rise in interest inexploring photocatalysts that feature 3D hierarchical structuresto boost photocatalytic efficiency. Sun et al. successfullyprepared g-C3N4 foam with a 3D micron-scale pores by using acombination of template and microwave methods， and skillfullyloaded with Cu NPs to produce the Cu-NPs/g-C3N4 foamcomposite 64. The material， with its exceptional CO2 adsorptionand diffusion properties， achieves a CO production rate of10.247 μmol?g?1?h?1， surpassing g-C3N4 foam and g-C3N4powder by factors of 2.56 and 6.34， respectively （Fig. 10c）. Thissignificant enhancement in photocatalytic efficiency is mainlyattributed to the marked increase in CO2 adsorption andimproved charge separation （Fig. 10d）. Furthermore， Shi et al.prepared multiple g-C3N4 nanosheets with various Cunanoparticle loadings by means of secondary roasting andmicrowave hydrothermal methods， with g-C3N4 as a substratematerial 65. The engineered catalyst demonstrates exceptionalphotocatalytic properties for converting CO2 to CO， with themost effective samples producing CO at a rate threefold highercompared to pristine g-C3N4 under visible light （Fig. 10e）.Introducing Cu NPs leads to the improvement of carrier transferand separation efficiency of the catalytic system， theincreasement of surface-active centers， resulting in thephotocatalytic activity is greatly enhanced （Fig. 10f）.

4.1.3.2 Morphological design

Lai et al. reported a facile synthetic process to prepare daggerlikeCu@Co core-shell bimetallic catalysts 66. The synthesisdiagram is shown in Fig. 11a. This Cu@Co bimetallic catalystshows outstanding activity in terms of CO production rate withsuperhigh selectivity. In addition， the Cu@Co sample exhibitsstability with no deactivation even during a long time （48 h） ofCO reduction. Fig. 11b shows the electron transfer mechanismfor the CO2 photoreduction in Cu@Co samples. The superiorcatalytic efficiency of Cu@Co is ascribed to the combination ofthe advantages of Cu and Co， which allows for rapid CO2capture， and the electronic coupling of Co and Cu leading to fastinterfacial charge transfer kinetics. Tang et al. synthesizedhollow copper spheres with linear defects by a controlled moltensalt method （Fig. 11c） 67. The erosion force of the molten saltinduces line defects， and the spatial confinement along with theliquid environment ensures the development of a homogeneoushollow structure. The distinctive design of this structure providesa multitude of active sites and greatly optimizes the migration ofcharge carriers. The wire-defective hollow copper sphere （CCu）demonstrates exceptional photocatalytic activity for pure water（1028.57 μmol?g?1）， even at extremely low CO2 levels，surpassing the catalytic efficiency of the majority ofsemiconductor-based catalysts.

4.1.3.3 Heterojunction construction

He et al. developed 3D honeycomb-like nitrogen-dopedgraphitic carbons （N-GCs） featuring embedded core-shellCu@Cu2O nanoparticles using a straightforward polymerthermal treatment approach 68. The catalyst Cu@Cu2O/N-GC-600 achieves the highest photocatalytic efficiency， generatingCO at 27.78 μmol?g?1 under visible light， and retains stableperformance through 6 cycles， surpassing the Cu@Cu2O/N-GC-500 and Cu@Cu2O/N-GC-700 in both activity and stability. Asillustrated in Fig. 12a， the Cu2O and N-GC semiconductors，which have interleaved energy band configurations， are capableof forming S-scheme heterojunctions. N-GC’s CB and VB aresituated at higher energy levels compared to those of Cu2O andCu. Upon visible light exposure， both N-GC and Cu2O producephotogenerated e– and h+. In this system， the excited e– from theCB of Cu2O are transferred to the Fermi level of N-GC， wherethey recombine with the photoinduced h+ in N-GC?s VB. Thistransfer leads to a prolonged lifetime of the photoexcited e– inthe CB of N-GC， thereby promoting the CO2 reduction reactionbetween these electrons and CO2 to generate CO. Furthermore，the Cu@Cu2O/N-GC heterojunction allows e– from the CB ofCu2O to be utilized by the h+ in the VB of N-GC， which extendsthe lifetime of the photoexcited h+ in Cu2O’s VB beyond thatobserved in pristine Cu2O. This improved electron-hole pairtransfer and separation within the heterojunction effectivelyimpedes their recombination， thereby leading to a notableenhancement in the photocatalytic performance. To boost thephotocatalytic efficiency of nano-heterostructures， potentialstrategies such as S-scheme configurations and co-catalysts areconsidered. In a study conducted by the authors， Cu NPscombined with a 2D/2D van der Waals heterojunction （g-C3N4/MoS2） were developed for the photoreduction of CO2 69.The g-C3N4/MoS2/Cu sample shows remarkable efficiency inreducing CO2 to CO， achieving a formation rate of 146.7μmol?g?1?h?1 and a selectivity of 100%. The authors employedvarious means to demonstrate that the charge transfer pathwayadheres to an S-scheme mechanism （Fig. 12b）. In addition， the2D/2D morphology results in a tightly constricted interface， andthe Cu-NPs function as a synergistic co-catalyst to enhance thecatalytic properties， facilitate the extraction of electrons， andmodulate the product selectivity. The authors successfullysynthesized dendritic CdxZn1–xSe nanostructures by a cationexchangemethod and further coupled them with Cu2O@Cu toform an S-scheme heterojunction 70. Characterization studiesindicate that CdxZn1?xSe effectively captures visible light，benefiting from its large specific surface area and rich activesites for CO2 adsorption and catalysis. Furthermore， the Sschemeheterojunction promotes the separation and transport ofe? and h+ （Fig. 12c）. Therefore， the optimized CZS/CC5composite photocatalysts exhibited high activity， high stabilityand reusability.

4.2 Light-driven reduction of CO2 to organicchemical

4.2.1 Copper oxides

excellent capacity to absorb and utilize visible light.Nevertheless， a narrow bandgap often leads to the fastrecombination of e– and h+， thereby posing a significantchallenge for its practical application. In light of theselimitations， researchers have investigated numerous approachesto improve the photocatalytic properties of CuO-based materialsfor CO2 reduction， including morphological modulation， sizemodulation， and heterojunction design， etc.

4.2.1.1 Morphological modulation

Nanostructured photocatalysts with stable structures and highspecific surface areas are anticipated to be extensively utilizedfor reducing CO2 through photocatalysis. Schaak et al. pioneereda method involving the use of uniform Cu3N nanocubes as bothstructural and compositional templates. Their approach involvedcoating these templates with a thin layer of TiO2， then oxidizingthe Cu3N core to generate hollow CuO nanocubes.Subsequently， they doped the released nitrogen into the TiO2framework at a moderate temperature of 450 °C， resulting in thecreation of crystalline TiO2?xNx （Fig. 13a） 71. The methane yieldof the CuO-TiO2?xNx hollow nanocubes reaches 41.3ppm?g?1?h?1， marking a 2.5-fold enhancement over Degussa P25TiO2 under identical conditions （Fig. 13b）. Duan et al. employeda straightforward approach that combined thermal oxidationwith the continuous ion layer adsorption and reaction （SILAR）technique to produce columnar cactus-like CdxCu1?xS/CuO/CMcomposites 72. The TEM images of CdxCu1?xS/CuO/CM aredepicted in Fig. 13c. In comparison to pure CuO nanowires，CdxCu1?xS/CuO/CM demonstrates improved photoactivity in theCO2 reduction to methanol. Following a 4-h irradiation period，the CdxCu1?xS/CuO/CM catalyst achieved a methanol yield of3.60 μmol?gcat?1， representing a 2.81-fold increase over that ofpure CuO/CM （Fig. 13d）. The synergy between CdxCu1?xSnanosheets and CuO nanowires boosts the specific surface area，enhances light absorption， and improves charge carrierseparation， thus elevating the photocatalytic efficiency.

4.2.1.2 Size modulation

Through the phase transfer method， Xiang et al. prepared 15nm CuO NPs， which were uniformly dispersed on NaTaO3nanocubes via hydrothermal treatment 73. The principle behindthis method is that smaller catalyst particles lead to greaterinternal surface utilization， which enhances the macroscopicreaction rate. Consequently， the photocatalytic properties ofCuO can be enhanced by adjusting its size or by creating CuOQDs. Using isopropanol， the CNTO catalyst can selectivelyreduce CO2 to methanol， with the optimal activity occurring at 5wt% CNTO and yielding methanol at 1302.22 μmol?g?1?h?1.Compared to 5 wt% CNTO-L， prepared through conventionalliquid-phase reduction， this yield is 2.6 times greater. Asdepicted in Fig. 14b， the higher methanol yield is attributed tothe prepared surface CuO particles with small， uniformdistribution and tight contact， which can increase the lightcapture and carrier separation and improve the CO2 utilization.Quantum dots， as a special type of semiconductor material，possess a remarkable light trapping ability and play a crucial rolein promoting photogenerated electron transport by impedingcharge recombination. In their study， Li et al.， developed amethod to encapsulate CuO quantum dots in the pore channelsof the MIL-125（Ti） MOF， utilizing a straightforwardcomplexation oxidation technique 74. These quantum dots werethen integrated with g-C3N4 to create a composite known as g-C3N4/CuO@MIL-125（Ti）. The synthesis process of the g-C3N4/CuO@MIL-125（Ti） composite is illustrated in Fig. 14c，revealing that the size of the CuO quantum dots produced isapproximately 2 nm， as depicted in Fig. 14d. The studydemonstrates that the optimal g-C3N4/CuO@MIL-125（Ti）sample exhibits a remarkable increase in photocatalytic activityfor solar fuels， including methanol， acetaldehyde， and ethanol.This improvement is due to the close interaction between theCuO quantum dots and titanium's active sites within the MIL-125（Ti） matrix， facilitating the smooth migration of e? fromMIL-125（Ti） and g-C3N4 to the CuO quantum dots.

4.2.1.3 Heterojunction construction

By constructing heterojunction structures， photogeneratedcarriers can be effectively separated， leading to improvedphotocatalytic activity and stability. This separation allowsphotogenerated electrons and holes to exist on two differentmaterials， reducing the chance of carrier recombination due tothe built-in electric field. Nogueira et al. conducted a studywhere they prepared a Nb2O5/CuO heterojunction using asolvothermal method and examined the impact of various copperoxide doping ratios on the selectivity of resulting products 75.Their findings revealed that CuO modification enhances theactivity of Nb2O5; however， the CuO content alters thedistribution pattern of by-products. The proposed charge transfermechanisms of photocatalytic CO2 reduction on Nb2O5/CuO areillustrated in Fig. 15a. The authors suggest that the CuOmodification promotes the formation of C ― H bonds，influencing the dimerization of the ― CH3 and ― COOHgroups， ultimately resulting in the production of CH3COOH.Zhang et al. successfully synthesized WO3-110 nanowires andWO3-001 nanosheets combined with CuO NPs using astraightforward approach 76. It was found that the CO2photoreduction ability of WO3 nanostructures is greatlyinfluenced by their exposed crystalline surfaces. The higher CO2photoreduction activity of WO3-110 nanowires compared toWO3-001 nanosheets is primarily attributed to their negative CBposition and rapid e?/h+ separation efficiency. The fabrication ofa p-n heterojunction in the CuO-WO3 nanocomposite throughcoupling with CuO NPs leads to better separation of e–/h+ andmore efficient charge transfer， resulting in improvedphotocatalytic conversion of CO2 to CH4. The charge transferpathway between p-type CuO and n-type WO3 is illustrated inFig. 15b. With its reduced e?/h+ recombination rate and efficientcarrier transport， the CuO-WO3-110 nanowire compositedemonstrates higher CO2 photoreduction performance than theCuO-WO3-001 nanosheet composites. Song et al. synthesizedCuO/BiOCl heterostructures by a hydrothermal method withenhanced interfacial electron-induced interactions to promotethe CO2 photoreduction efficiency 77. In the CO2 photoreductionprocess， the optimized BC2 photocatalyst （containing 2% CuOby mass） achieved yielding rates of 114.1 μmol?g?1?h?1 for CH4and 36.2 μmol?g?1?h?1 for CH3OH， surpassing the performanceof both pure CuO and BiOCl. In this structure （Fig. 15c）， BiOClserves as a bridge for electron transfer， facilitating enhancedcontact between the two components and boosting the separationand migration efficiency of e? and h+. In another study，CuO/ZnO p-n heterojunctions with a layered structure weredeveloped by combining CuO and ZnO nanospheres using asimple hydrothermal synthesis technique 78. The photocatalystsperformed efficiently and stably in the photoreduction of CO2 tomethanol under visible light， with dimethylformamide andtriethylamine as electron donors in the aqueous medium. Amongthe various photocatalysts synthesized， ZC3 exhibited thehighest methanol generation activity. The methanol yields of thephotocatalysts were measured to be 3855.36 μmol?gcat?1. Asillustrated in Fig. 15d， the superior performance results from theheterogeneous structure， which enhances e– and h+ separation onthe catalyst surface and reduces recombination. TheAg/CuO@ZnIn2S4 photocatalyst with a plasma S-schemeheterojunction was successfully prepared via the combination ofhydrothermal and photodeposition processes 79. Under lightirradiation， the optimized 15-ACZ composite achieves the bestphotocatalytic activity， yielding 54.4 μmol?g?1?h?1 of CH4 and aselectivity of 92.8%. The high conversion rate observed in thisstudy can be explained by several factors. Primarily， the IEFcreated within the S-scheme heterostructure as well as theformation of a Cu-S covalent bond at the junction of the twocomponents significantly enhance the directional separation ofcharge carriers. Additionally， Ag serves as a co-catalyst forelectron trapping in the CB of ZnIn2S4. Moreover， Ag utilizesthe LSPR effect to promote light harvesting， thus providing morehot e? to drive the reaction effectively. Furthermore， rGOBi2S3/CuO S-scheme heterojunction photocatalysts weresuccessfully prepared by hydrothermal treatment 80. The lightabsorption capacity of Bi2S3 nano hollow flowers in the visiblerange is extended through the combination of rGO and CuO.This is achieved by the formation of rGO-Bi2S3/CuO S-schemeheterojunction photocatalysts， demonstrating the capability toconvert CO2 to methanol at approximately 423.52μmol?gcat?1?h?1， with a methanol selectivity of 98.6%. Theimproved efficiency of the photocatalysts is a result of theirfavorable band gap of the rGO-Bi2S3/CuO heterojunctioncatalysts， which results in significantly lower charge transferresistance and enhanced conductivity. These factors collectivelylead to enhanced charge separation efficiency. Unlike the aboveproposed structures， Kim and colleagues developed a unique pn-p heterojunction catalyst （Cu2O/S-TiO2/CuO） with synthesissteps detailed in Fig. 16e 81. This catalyst shows excellentphotocatalytic performance in CO2 conversion， reaching a CH4evolution rate of 2.31 μmol?m2?h?1 due to its superior lightabsorption and efficient charge separation.

4.2.2 Cuprous oxide

To boost the efficiency of CO2 photoreduction to organiccompounds， various strategies can be employed on Cu2O，including surface crystallographic tuning， morphologicaloptimization， elemental doping， and the creation ofheterostructures.

4.2.2.1 Crystalline surface regulation

By employing correlated scanning fluorescence X-raymicroscopy and ambient transmission electron microscopy atatmospheric pressure， Wu et al. were able to directly detect thephotoactivity of the （110） crystal face of a single Cu2O particlein the reduction of CO2 to CH3OH 82. In contrast， they found thatthe （100） crystal face of the same particle is inert. Theconfigurations of the two crystalline surfaces of Cu2O （100） and（110） are depicted in Fig. 16a，b. Deng et al. confirmed that the（111） face of Cu2O is a selective source of CH4 production bycomparing the photocatalytic CO2 reduction behavior ondifferent active faces of Cu2O catalysts 83. The results indicatethat the methane yield of Cu2O-100 is negligible， while themethane yield of Cu2O-111 is 12.24 μmol?g?1?h?1 and aselectivity of 91.4%. Theoretical calculations （Fig. 16c，d） showthat Cu2O-111 （Eads = ?2.120 eV） has a strong CO adsorptioncapacity， making it suitable for efficient hydrogenation toproduce valuable products.

4.2.2.2 Morphological modulation

Yu et al. successfully synthesized Cu2O mesoporous nanorodsencapsulated by carbon layers by a simple chemical oxidationand carbonization strategy 84. The carbon coating on Cu2Onanorods offers excellent electrical conductivity， enhancing thestructural benefits of one-dimensional nanostructures tosignificantly boosting electron transfer and reducing e?/h+recombination. Meanwhile， the large specific surface area andnumerous active centers afforded by the mesoporous structurefacilitate the CO2 adsorption process. Furthermore， the carbonlayer also serves to protect Cu2O， thereby improving itsphotostability. The CO2 reduction pathway on this catalyst isshown in Fig. 17a. The authors prepared different crystallineforms of cuprous oxide （cubic， octahedral and rhombicdodecahedral） and found that Cu2O/rGO with rhombicdodecahedral structure is 4.1 to 80.8 times more efficient thancubic， octahedral Cu2O/RGO and CuO/RGO 85. The smallerenergy band bending of the VB and CB in rhombic dodecahedralCu2O might be the reason for the improved CO2 photoreductionperformance （Fig. 17b）， as it lowers the potential barrier for theelectron transfer to the surface. Zeng et al. integrated Ti3C2 QDsonto Cu2O nanowires （NWs） using a self-assembly approach 86.Fig. 17c illustrates the schematic of the catalyst synthesisprocess. The study reveals that integrating Ti3C2 QDssubstantially improves the stability of Cu2O nanowires，enhances charge transfer， increases carrier density， and improveslight absorption. Moreover， this integration also reduces bandbending and minimizes charge recombination， leading to asubstantial boost in the photocatalytic activity of the catalyst forCH3OH production.

4.2.2.3 Elemental doping

As is well known， the physical properties of semiconductorscan be effectively altered through doping， as the doped elementsdiffer in radius and electronegativity from the host material. Thisprocess commonly leads to the formation of various defects，including substitutional， dislocation， and interstitial defects. Thepresence of these defects can greatly affect the semiconductor’soptical properties， energy band structure， and type ofconductivity. In their study， Yu et al. synthesized a Cl-dopedCu2O photocatalyst for visible light-driven CO2RR. This catalystachieves a notable apparent quantum yield （AQY） of 2.2% at400 nm， with CO and CH4 efficiencies of 1.13% and 1.07%，respectively， and demonstrates excellent stability 87. DFTcalculation， as shown in Fig. 17d， reveals that the Cl-doped Cu2Opromotes the CO2 reduction to *COOH， *CO， and *CH3Ointermediates， enhancing the production of CO and CH4.Moreover， the Cl-doped Cu2O exhibits a higher affinity for *COintermediates， which favor protonation and subsequentconversion to CH4， resulting in a higher selectivity for CH4compared to pure Cu2O.

4.2.2.4 Heterojunction construction

Zhang and coworkers demonstrated the successful preparationof distinctive one-dimensional Cu2O@Cu heterostructurednanorod arrays via a straightforward in situ reduction process 88.These Cu2O@Cu nanorod arrays display outstanding visiblelight absorption， significant carrier concentration， and efficientcharge transfer， promoting the effective separation of excitons.Additionally， the heterostructure features a good hydrophobicinterface and structural integration， contributing to its optimizedperformance. As a result， the Cu2O@Cu heterostructure attainsan AQY of 2.40% for CH4 and C2H4， with an impressive activityretention rate of up to 92% after four cycles. The process of CO2reduction on the Cu2O@Cu heterostructure is illustrated in Fig.18a. Tang et al. prepared reduced graphene oxide （rGO）encapsulated Ag/Cu2O octahedral nanocrystals（Ag/Cu2O@rGO） ternary catalysts using a water bath methodcombined with gas-bubbling-assisted membrane reduction 89.The Agn/Cu2O@rGO heterojunction catalysts exhibitoutstanding activity in the photoreduction of CO2 to CH4. TheAg4/Cu2O@rGO catalyst stands out with the highest methaneyielding rate of 82.6 μmol?g?1?h?1 and a selectivity of 95.4%among the tested catalysts. The photocatalytic CO2 reductionmechanism on the catalyst is depicted schematically in Fig. 18b.In another work， using a soft-template method， Bi et al.developed Cu2O hollow nanospheres and integrated them withTiO2 by in situ hydrolysis of Ti（OBu）4 under ultrasonictreatment 90. In comparison to Cu2O hollow nanospheres， theCu2O/TiO2 catalyst demonstrates superior photocatalyticefficiency for converting CO2 to methane. The enhancedperformance is attributed to the presence of p-n heterojunctionswithin the composite （Fig. 18c）， which efficiently reduce e?/h+recombination and enhance material stability， thereby improvingphotocatalytic activity under visible light. Efficient CO2photoconversion to methanol under visible light was achievedusing a ternary heterostructure of Cu2O， graphene， and TiO2nanotube arrays （TNA） in a dual-chamber reactor designed forlight-driven reactions 91. The increased photocatalytic activity inthe ternary heterostructure is due to its improved lightabsorption， reduced e?/h+ recombination， and effectivepromotion of electron transfer at the heterojunction interface（Fig.18d）. Additionally， an S-scheme BiOBr/Cu2Oheterostructure was successfully prepared using a simplehydrothermal process 92. The 20%-BiOBr/Cu2O compositedisplays the greatest CO2 photoreduction efficiency undersimulated visible light， outperforming pure BiOBr and Cu2Oindividually， achieving a CH4 yield of 22.78 μmol?g?1 in 4 h. TheS-scheme heterostructures （Fig.18e） between BiOBr and Cu2Oseparate the charges more efficiently and accelerate theinterfacial charge migration， which minimize e?/h+recombination and thus boost photocatalytic performance. Shi etal. have synthesized mixed Cu and Cu2O nanoparticles on WO3nanoflake films using a one-step electrodeposition method toconstruct an S-scheme Cu/Cu2O/WO3 heterojunction catalyst 93.The yield of CH4 reaches 2.43 mmol?gcat?1 after visible lightillumination for 24 h. It is noteworthy that the selectivity of CH4increases from nearly 0% to 96.7%， while the selectivity of COdecreases from 94.5% to 0% in the presence of modified Cu NPs.The efficient generation of CH4 by Cu/Cu2O/WO3 catalysts isattributed to the modification of Cu NPs that facilitates theelectron-proton transfer from CO to CH4. In situ XPSmeasurement further confirms the S-scheme charge transferpathway （Fig.18f）.

4.2.3 Copper

Incorporating metallic copper may inhibit the e?/h+recombination by introducing electron traps and creatingheterojunctions， as well as forming Schottky barriers， etc.

4.2.3.1 Semiconductor compound

The addition of copper can optimize the work function of thesemiconductor， leading to a better-aligned energy band structurethat accelerates charge separation and enhances CO2 reductionefficiency. Indrajit and coworkers successfully prepared avariety of Cu NPs modified GO photocatalysts by a facilemicrowave process 94， resulting in a pronounced boost inphotocatalytic activity for solar fuel production. When exposedto visible light （Fig. 19a）， the Cu/GO-2 composite containing 10wt% Cu reaches a solar fuel production rate of 6.84μmol?g?1?h?1， surpassing pure GO by 60 times and P25 by 240times. It was observed that adding Cu NPs successfully modifiesthe work function of GO， which leads to better charge separationand enhanced CO2 reduction efficiency. A facile in situconversion strategy was developed by Li et al. to prepare Cudopedbiphasic CsPbBr3-Cs4PbBr6 inorganic chalcogenidenanocomposites （Cu/CsPbBr3-Cs4PbBr6 NCS） 95. This materialdemonstrates a 4.2-fold boost in the efficiency of CO2 reductionto CH4 relative to unmodified Cu CsPbBr3 NPs. （Fig. 19b）.Doping Cu does not significantly affect the material system?sband gap. In Cu/CsPbBr3-Cs4PbBr6 NCS， the position of the VBchanges significantly as the electrons of Cu are easily transferredto CsPbBr3-Cs4PbBr6 NCS. This transfer increases the electroncloud density in the perovskites， resulting in the elevation of itsFermi energy level position. Consequently， with the band gapremaining unchanged and the CB position elevated， thephotoexcited electrons on the perovskites exhibit enhancedreduction ability. This enhancement is beneficial for the 8-electron reduction reaction required to produce CH4. Xiong et al.prepared TiO2 with simultaneous Cu NPs anchoring by a facilepolyol approach 96. The influence of different Cu loadings onphotoreduction efficiency is studied under simulated solar light.Compared with （TiO2-Cu-2.5%） and （TiO2-Cu-7.5%）， （TiO2-Cu-5%） indicates the best performance in photocatalyticreduction of CH4， producing CH4 at a rate of 25.73 μmol?g–1?h–1，which is over twice the amount of pure TiO2 （Fig. 19c）. Theenhanced performance results from effectively separatingphotogenerated charges by incorporating Cu NPs into the TiO2nanoclusters. Liu et al. effectively synthesized Cu NPsdepositedTiO2 nano-flower films through a hydrothermalapproach coupled with a microwave-assisted reduction process 97.Exposure to UV and visible light results in a CH3OH yield of 1.8μmol?cm?2?h?1 （Fig. 19d） for 0.5Cu/TiO2 films， marking a 6.0-fold increase relative to pure TiO2 films.

Su et al. prepared BiYO3 materials with various levels of Cudoping through a hydrothermal method 98. The Cu/BiYO3catalysts show improved photocatalytic performance in CO2RRunder visible light. The 2.0% Cu/BiYO3 catalyst， in particular，achieves the highest efficiency with a formic acid yield of 2.04μmol?mL?1， 2.2 times higher than that of pure BiYO3 （0.92μmol?mL?1） （Fig. 20a）. Zhang et al. also synthesized various Cu-TiO2 composites with different levels of Cu loading using an insitu hydrolysis method 99. These materials demonstrate notablephotocatalytic activity for CO2 photoreduction. Specifically， Cu-TiO2 shows the highest efficiency in converting CO2 to CH4 at aCu loading of 0.4 wt%， achieving a 21-fold increase in efficiencycompared to the widely used commercial photocatalyst， DegussaP25 TiO2 （Fig. 20b）. David and his collaborators studied the CO2photoreduction efficiency of g-C3N4/Cu/TiO2 100. The findingsindicate that the incorporation of metallic copper onto TiO2 andg-C3N4 created a Schottky barrier， enhancing the charge carrier’sseparation and thereby boosting the production of CH3OH andHCOOH （Fig. 20c）. Jin et al. synthesized Cu-loaded brookiteTiO2 composites （Cu-BTN） through a single-step hydrothermalprocess 101. The presence of Cu NCs on BTN surfaces wasshown to greatly increase the efficiency and selectivity of CO2photoreduction to CH4. Among the tested samples， the 1.5% Cu-BTN displayed the highest efficiency in photocatalytic CH4production， surpassing pristine BTN and Ag-BTN by 11.4 and3.3 times， respectively （Fig. 20d）. Importantly， the Cu-BTNcatalysts enhance CH4 selectivity over CO due to surface OVsand their distinctive CO2/H2O adsorption properties. Thisimprovement is assigned to Cu doping， which aids in electrontrapping and reduces the recombination of photogeneratedcarriers， thus increasing photocatalytic activity.

4.2.3.2 Heterojunction construction

Kang et al. prepared STO/TiO2 coaxial nanotube arrays byanodic oxidation combined with a subsequent hydrothermalmethod 102. Bimetallic Au-Cu nanoparticles with varying Au/Curatios were applied onto STO/TiO2 nanotube arrays using amicrowave-assisted solvothermal technique. When exposed toUV/Vis light， a CH4 evolution rate of 15.49 ppm?cm?2?h?1 （421.2μmol?g?1?h?1） is acquired on Au3Cu@SrTiO3/TiO2 nanotubearrays by using diluted CO2 （33.3% in Ar） （Fig. 21a）. During the34-h test， the CH4 gas conversion underwent a decrease from15.49 to 13.57 ppm·cm?2·h?1 after five cycles of measurements;however， this represented 87.6% of its original activity （Fig.21b）. Several critical factors contribute to the improvedphotocatalytic activity. Firstly， high-surface-area nanotubearrays featuring porous walls improve gas diffusion and enhancethe interaction between photoinduced charges and surfacespecies. Secondly， the introduction of STO/TiO2heterostructures was essential in facilitating photogeneratedcharge separation. Thirdly， including noble metal bimetallicalloy NPs as co-catalysts in the nanotube arrays significantlyimproved the efficiency of the redox process. Lastly， usinghydrous hydrazine （N2H4?H2O） as both a hydrogen source andelectron donor established a reductive environment essential forpreserving the alloying effect. Kumar et al. engineeredCs2AgBiBr6 double nanoplates and their heterostructure withCu-loaded reduced graphene oxide （Cu–RGO） viamechanochemical techniques for gas-phase photocatalytic CO2reduction， with water vapor serving as the proton source and nohole scavengers involved 103. The Cs2AgBiBr6-Cu-RGOnanocomposites demonstrate excellent photocatalyticperformance， achieving a methane yield rate of 10.7μmol?g?1?h?1 and maintaining consistent performance over threecycles （Fig. 21c）. The electron transfer process is illustrated inFig. 21d， highlighting increased adsorption of CO2 moleculesand efficient charge separation as key factors contributing totheir exceptional performance.

5 Conclusion and outlook

Copper-based catalysts have garnered significant interest fortheir potential applications in photocatalytic CO2 reduction. Thisapproach involves the conversion of CO2 into useful fuels andchemicals， driven by renewable energy sources. Despiteadvancements in the development of copper-based systems forCO2 photoreduction process， several challenges and futureperspectives still exist. One of the main challenges for copperbasedcatalysts is their low selectivity towards a specific product.Copper-based catalysts tend to produce multiple products due tothe complex mechanisms involved in the CO2 reduction process.In-depth examination is needed to uncover the mechanisms ofactive catalytic sites， the redox interactions in Cu-basedcatalysts， and the pathways that generate various compoundsduring CO2 photoconversion. Notably， Cu-based photocatalystsmainly generate C1 compounds like CO， CH4， HCOOH， andCH3OH， as opposed to the more difficult-to-synthesize C2+products. In this context， applying electric fields or localizedheating in photocatalysis shows promising potential forenhancing CO2 conversion to C2+ products. Moreover， inconventional photocatalytic reactors， coexistence of reactantsand products leads to inevitable re-oxidation of the products.Implementing a two-compartment reactor is crucial to mitigatereoxidation， thereby facilitating the production of C2+ chemicals.

Another challenge is the instability of copper-based catalystsunder harsh operating conditions. Photocorrosion is a commonissue for pure metallic Cu NPs and copper oxides duringphotocatalytic reactions， e.g.， Cu2O is easily oxidized to CuO byphotocorrosion， and Cu NPs may also be oxidized andtransformed to CuO during the reaction process. Variations inCu’s oxidation state greatly impact its photocatalytic efficiency.Thus， ensuring the stability of Cu is crucial for harnessing itsinherent properties in photocatalysis. To address the problem ofphotocorrosion， the following ways can be considered in thefuture： （1） creating alloy structures with other relatively stablemetals （including gold， platinum and palladium）; （2） creating areducing environment by using strong reducing chemicals （e.g.，N2H4?H2O）; （3） designing copper-centered core-shell structuresor carbon-based coatings encapsulating copper; and （4）combining hole-scavenging agents with copper-basedphotocatalysts.

Additionally， the efficiency of copper-based photocatalysts islimited by their narrow absorption spectra in the visible lightrange. Thus， it is crucial to develop innovative techniques toextend the absorption spectra of copper-based catalysts into thenear-infrared range， which contains a significant amount of solarenergy. In terms of future perspectives， there is a need to designcopper-based catalysts that can efficiently harvest and utilizesolar energy. This can be realized by enhancing the absorptionspectra of the photocatalyst， improving the charge transferefficiency， and increasing the surface area of the photocatalyst.Moreover， the development of new synthetic methods andoptimizations of existing ones can also enhance the properties ofcopper-based photocatalysts. These methods should allow forthe precise control of the size， shape， and surface area of thecatalyst， which can significantly affect its catalytic activity andstability. Finally， the integration of copper-based photocatalystswith advanced technologies such as electrochemical cells andmembrane reactors can enhance their performance and stability.This integration can facilitate the separation and recovery of theproducts， prevent back reactions， and boost the efficiency of theoverall CO2 reduction process. Furthermore， the LSPR effect ofCu-based photocatalysts plays a crucial role in promoting thephotocatalytic reduction of CO2 due to its ability to transferplasmonic electrons to the CB of the photocatalyst or thereactants/intermediates produced during the reaction. This canresult in an increased number of electrons in the visible or NIRrange， which is essential for achieving efficient CO2 reduction.Moreover， the LSPR effect induces thermal effects that furtherenhance CO2 reduction. The effect also creates reaction sites forC―C coupling， thereby increasing C2+ production rates. Hence，constructing plasmonic Cu photocatalysts can aid in boosting therate of the photocatalytic reaction and generating higher C2+