Zhang Lizhong

(School of Materials Science and Engineering, North University of China, Taiyuan 030051)

Abstract: Energy crisis and environmental pollution have become vitally severe challenges for the current society. During the circumstances of pursuing carbon peak and carbon neutrality, photocatalytic CO2 reduction will be an essential, necessary and even inevitable development requirement and trend. However, there are some bottlenecks that need to be addressed immediately, such as low quantum efficiency and narrow light absorption range for single photocatalysts. Therefore,efficient and stable visible light driving materials are the core challenge in the field of photocatalytic CO2 conversion. This paper reviewed the photocatalysts used for CO2 reduction classified as metal photocatalysts, non-metal photocatalysts, and MOFs photocatalysts, and the CO2 reduction capacity and mechanism of different photocatalysts were described. In the end,the problems to be solved and the future development of photocatalytic CO2 reduction were summarized.

Key words: photocatalysis; CO2 reduction; metal catalyst; non-metal catalyst

1 Introduction

Excessive anthropogenic CO2emissions created by overused fossil fuel have caused an energy crisis and environmental issues like global warming, greenhouse effect, and rising sea level, which have immeasurable negative influences on our society and human lives directly. It has been reported that the level of carbon dioxide in the atmosphere represented a milestone with the challenge of achieving a 400 μL/L target in March 2014. The average temperature was 16.48 °C in August 2015, which was 0.88 °C higher than the average of 15.6 °C in the 20th century and broke the record from 1880 to 2015[1]. The CO2concentration increased to 407 μL/L in the atmosphere in 2017[2]and was extremely frightful with an increase above the concentration of 413 μL/L in 2020. Thus, it is imminent to develop a novel CO2emission reduction or utilization technology to reduce CO2concentration in the atmosphere.

Currently, usual routes for decreasing the CO2content include carbon capture and storage (CCS) and carbon capture and utilization (CCU). CCS refers to pumping the collected CO2into the ground or the deep sea for storage, which is aimed to reduce CO2concentration.However, the problems of storage space and cost are more difficult. Furthermore, it would lead to other issues,such as excessive energy consumption, gas seepage, and destruction of fragile ecosystems as well. Consequently,CCU is extensively considered as a robust route to mitigate environmental issues caused by the combustion of fossil fuels and environmental pollution[3]. CCU means that CO2is transformed into clean, low-cost,and renewable fuel, which is a doable and promising treatment to meet the growing requirement for energy and serious situation simultaneously.

Up to date, the methods of CCU mainly involve electrocatalysis, photocatalysis and thermochemistry.Electrocatalysis receives different products with strong selectivity, specificity, and high reduction efficiency by adjusting the solvent system and electrode materials,but its shortcomings including high cost, high energy consumption and easy decomposition of solvent seriously hinder its development and application. The chemical conversion method is the so-called Sabatier reaction that is a favorable exothermic reaction, the CH4formation rate is high in the full temperature range from 200 °C to 550 °C, but the reaction rate is restricted due to the high motion barrier during the electron reduction process[4].Photocatalytic reduction of CO2can create abundant products with different carbon oxidation states ranging from CO, CH4to higher hydrocarbons in the gas phase,as well as various oxygenates in the liquid phase such as CH3OH and HCOOH[5-6]. However, the major limiting factors of catalysts include low energy conversion efficiency, uncontrollable selectivity and instability,and competitive hydrogen evolution in the aquatic environment[6]. Hence, it is still a serious challenge in the aspect of designing and fabricating photocatalysts with good selectivity and outstanding conversion efficiency.

In this review, the author attempts to focus on classifying and discussing various photocatalysts, which consist of metal catalysts (noble metal and non-noble metal), nonmetal catalysts and MOFs for CO2reduction. Noble metal catalysts contain Ag, Pd, Au, Pt and Ru based compounds, with the research progress for CO2reduction such as AgX/Ag, Ag NPs/ACF, TiO2-Au-CuO, Au/TiO2,CaTiO3@Ni-Pt and Pd/TiO2being discussed. Non-noble metal catalysts can be divided into three categories,i.e.: non-noble metal oxides, sulfides, and nitrides. The research progress of TiO2, WO3, Sr3Ti2O7, AG/CdS,G-C3N4/SnS2and Ta3N5/Bi in the field of photocatalytic reduction of CO2was summarized. Non-metal catalysts contain g-C3N4and other non-metal catalysts, with the research progress for CO2reduction such as g-CNQD/PMOF, LaPO4/ g-C3N4,Cu-CNT/pCN, Au/CN, CuInZnS/g-C3N4,Cu/GO, CQDs/Cu2O and SiC being discussed.The MOFs include MILs, UiOs and ZIFs and PCNs,with the research progress for CO2reduction such as 3J-2DT, CsPbBr3@ZIF, Cu2O@Cu@UiO-66-NH2,UIO-66-NH2/GR and PCN-601 being discussed. In addition,the catalytic activity of various catalysts was described,and the possible reaction mechanism in the photocatalytic process was analyzed, while the obvious advantages of each type of catalyst in the catalytic process were summarized, and the development trend of the catalysts in the future was prospected.

2 Photocatalysts for Photocatalytic Conversion of CO2

2.1 Metal-based catalysts

2.1.1 Noble metal-based catalysts

Metallic nanoparticles characteristics (Au, Pd, Pt etc.)of the localized surface plasmon resonance have been applied to study the photo-thermal effect, which absorbs visible light to drive the chemical reactions, such as water decomposition, CO2reduction, organic matter degradation, etc[7].

2.1.1.1 Ag-based catalysts

Ag particles express superior potential due to the localized surface plasmon effects (LSPR) and stable catalytic performance. An, et al.[8]contracted cubic quadruped AgCl/Ag nanoparticles and AgBr/Ag nanoplates via a facile glycerol mediated method. The as-prepared AgX/Ag catalysts possess superior absorption capability under visible light irradation and promising properties to reduce CO2to methanol, and the methanol release rate achieved by AgCl/Ag and AgBr/Ag can reach 188.68 μmol/g and 108.696 μmol/g, respectively. In addition, the activity of photocatalysts is still satisfactory in the cyclic reaction.Accordingly, the efficient activity and stability of the as-prepared AgX/Ag nano photocatalysts denote their promising potential for organic fuel production. Wang,et al.[9]pickled carbon fibers (CFs) with concentrated nitric acid firstly and fabricated novel photocatalyst Ag NPs/ACF decorated with plasmonic Ag nanoparticles employing solution dipping combined with the ultrasonic method. The results show that the conversion rate of CO2to methanol is 13.9 μmol/(g·h) without sacrificial agents under visible light irritation, which is 2.5 times higher than pure CFs with a methanol yield of 5.5 μmol/(g·h).The photocatalytic enhancement is attributed mainly to exceptional carriers separation activity caused by the strong interaction between Ag NPs and CFs. This work plays a positive role in promoting the development of potential applications of plasma photocatalysts.

2.1.1.2 Au-based catalysts

The potential of Au nanoparticles in photocatalysis has attracted wide attention. It is reported that the absorption of visible light caused by Au surface plasma and the electron storage capacity of Au improves photocatalytic performance. Li, et al.[10]fabricated an Au-mediated Cu2O-based Z-scheme heterostructure system by electrodepositing a Cu2O layer on the surface of Au particle-coated TiO2nanorods. For TiO2-Au-Cu2O, the embedded Au particles act as a charge transfer medium to enhance electron transport from the conduction band of TiO2to the valence band of Cu2O. Wang, et al.[11]successfully construct 0D/2D Au/TiO2plasmon Ohmicjunction composites by the hydrothermal method, which possess superior photocatalytic performance for CO2reduction to CH4and CO. The main reduction products of the optimized 0D/2D Au/TiO2composite are CH4and CO, with the reduction rate equating to 70.34 μmol/(g·h)and 19.75 μmol/(g·h), respectively, coupled with a CH4selectivity of 80%. The results suggest that the improved photocatalytic CO2reduction performance and high CH4selectivity are ascribed to the synergistic effect between plasmonic Au NPs and Ohmic-junction of Au and TiO2.In addition, the Au/TiO2plasma ohm-junction can be used as a highly efficient photocatalyst for CO2reduction for future energy conversion applications.

Cui, et al.[12]grafted metal cations (Fe2+, Co2+, Ni2+and Cu2+) to gold nanoclusters (Au NCs) using L-cysteine as the bridging ligand. The result demonstrates that the photocatalyst reduces CO2to CO at a rate of 3.54 μmol/(g·h) and a selectivity of 65.2%. The metal-S bonding bridge facilitates the electron transfer from Au NCs to metal cations so that the grafted metal cations can receive photo-induced electrons and work as catalytic sites for CO2reduction.

2.1.1.3 Pt-based catalysts

Yadav, et al.[13]synthesized Si-containing platinum nanoparticles (Pt NPs) on mesoporous TiXSi1-XO2carrier.The experimental results show that the catalyst with a silicon content of 24 mol% has higher selectivity for methanol than the catalyst with a silicon content of 18 mol% and 28 mol% (molar mass fraction). The DFT calculation shows that the strong interaction between Pt NPs and the support makes CO2easy to be activated.Lee, et al.[14]successfully synthesize CaTiO3@Ni-Pt via the sol-gel method and the impregnation method. The compound of CaTiO3@Ni-Pt exibited a carbon dioxide conversion of 46.48% and a methane selectivity of 99.46% at 180 °C. The results of photoluminescence and photocurrent showed that the photocharge recombination of nickel and platinum was inhibited and the catalytic activity was improved.

2.1.1.4 Pd-based catalysts

Among various metals used for doping, Pd is widely perceived as a potential candidate that Pd can extend the range of spectral response to 400-800 nm[15]. Pd/TiO2exhibits a higher reduction performance compared to pure TiO2to produce hydrocarbon[16]. Nishimura, et al.[17]synthesized the Pd/TiO2thin film photocatalyst by applying high-pressure pulsed plasma method to load palladium nanoparticles on TiO2and evaluate the performance of CO2reduction with H2and H2O under the illumination condition of Xe lamp or UV light. When the blending ratio of CO2/H2/H2O is set at 1: 0.5: 0.5, the CO2reduction over Pd/TiO2has the best performance under visible light irritation, with the maximum molar quantities of CO and CH4produced per unit weight of photocatalyst equating to 30.3 µmol/g and 22.1 µmol/g, respectively.Su, et al.[18]fabricated various Pd-loaded TiO2nanowire(Pd/TiO2-NW) catalysts by the hydrothermal method and investigated the CO2reduction performance to CH4and CO. The Pd loaded TiO2NWs can boost the yield of CH4and CO. The 0.5% Pd/TiO2-NWs illustrate an ideal CO and CH4yield of 50.4 μmol/g and 26.7 μmol/g,respectively. As the active centers of CO2photoreduction,Pd nanoparticles improved the photocatalytic activity.Suitable palladium loading inhibited the recombination of photoexcited electron hole pairs and enhanced the photocatalytic activity.

2.1.1.5 Ru-based catalysts

Wei, et al.[19]successfully prepared the novel photocatalyst of bimetallic Pt-Ru alloy nanoparticles (NPs) selectively deposited on the facet of TiO2nanocrystals via the photon-assisted gas bubbling-membrane reduction(P-GBMR) method. The synergistic effect of Pt and Ru is identified in PtRu/TiO2catalyst, and the preferred catalyst shows the best catalytic performance for the reduction of CO2to CH4under simulated solar radiation (The CH4formation rate (38.7 μmol/(g·h)) was about 29 times that of commercial P25, and the selectivity of CH4products was 93.7%). The design of PtRu/TiO2would open up an exploration for the development of highly efficient photocatalysts.

2.1.2 Non-noble metal catalysts

2.1.2.1 Non-noble metal oxygen-based catalysts

Various semiconductors have been widely available for CO2reduction such as TiO2, WO3[20], and ZnO[21]in recent years. In particular, TiO2is one of the widely availabe materials in the photocatalytic application field due to its cheap, efficient, stable, and non-toxic performance.Nevertheless, TiO2with a 3.2 eV bandgap can absorbs merely UV light at a wavelength of 387.5 nm accounting for less than 5% of the solar spectrum[22]. In addition,TiO2also has ultra-high electron hole recombination rate,therefore, modification of TiO2has become the focus of current research. Nematollahi, et al.[23]prepared a series of Ni and Bi doped TiO2catalysts with different Ni and Bi contents by the sol-gel method and explored the photocatalytic activity for CO2reduction under visible light irradiation. Compared with pure TiO2, the doubledoped TiO2samples have narrow bandgap energy and therefore have relatively high light absorption in the visible region.

The Bi-Ni metals doped TiO2exhibited a methane yield of 21.13 μmol/g by up to 6.5 times that obtained over the pure TiO2. Zhao, et al.[24]prepared ZnPc/TiO2by the gelsol method. After 10 hours of visible light irradiation,the highest yield of formic acid reached 978.6 mmol/g, and the conversion rate was 0.37%, which was higher than that of pure TiO2and ZnPc catalysts (with formic acid yield reaching 321.0 mmol/g and 756.2 mmol/g,respectively).

Tungsten oxide (WO3) with a bandgap energy of 2.4-2.8 eV has been widely certified to be an outstanding visible light photocatalyst[25]. More importantly, the valence band potential of WO3is high enough to span the reaction potential of H2O oxidation and promote the reduction of CO2. Thus, WO3is generally selected to be combined with some other semiconductors to oxidize H2O[26]. Zhao,et al.[27]firstly designed and prepared PI/WO3aerogel photocatalysts by chemical amide reaction, coupled with an ethanol supercritical drying technique. PI/WO3aerogel with strong CO2adsorption capacity and a suitable band structure to reduce CO2and oxidize H2O simultaneously exhibits the highest yield of CO (5.72 μmol/(g·h)), which is 11-fold higher than that of the pristine PI aerogel. This work might provide a new route to developing highly efficient and stable PI-based aerogel photocatalysts for CO2reduction.

Perovskite metal oxide Sr3Ti2O7has attracted extensive attention owing to its layered structure leading to faster charge carriers transport, and the interlayer space is used as the active sites of redox reaction to promote the separation of charge carriers. Jeyalakshmi, et al.[28]prepared layered perovskite type Sr3Ti2O7catalysts doped with N, S and Fe by the modified polymer complex method and also characterized and evaluated the performance for photoreduction of CO2in the aqueous alkaline medium under UV-visible light irradiation. The results illustrate that the dopants can form additional energy levels within the bandgap, increasing visible light absorption and retarding recombination of the charge carriers. Sr3Ti2O7co-doped with N, S and Fe together displays a maximum apparent quantum yield of CO2reduction products. The work proves that Sr3Ti2O7is a kind of promising candidate for CO2reduction.

2.1.2.2 Non-noble metal sulfide-based catalysts

Sulfides are referred to among the most promising materials for CO2reduction with relatively excellent conductivity, solar performance, and abundant advantages by comparison with conventional metallic oxides.

With a suitable bandgap of 2.4 eV, cadmium sulfide (CdS)has attracted enormous interest for its relatively high visible light activity[29]. Generally, photocatalytic activity for CO2reduction improves evidently via modification methods like atomic doping[30], heterojunctions either with metals[31]and defect engineering[32]. Therefore, the optimized CdS based photocatalysts usually can achieve higher CO2reduction efficiency and selectivity. Cho, et al.[33]fabricated the amine-functionalized graphene/CdS composite (AG/CdS) in two steps. It exhibited a methane formation rate of 2.84 μmol/(g·h) under visible light irradiation and a CO2pressure of 1 bar, corresponding to 3.5 times the yield observed for rGO/CdS.

The high toxicity of Cd evidently limits the practical application of CdS. Alternatively, SnS2has attracted significant attention as a visible-light photocatalyst for its low cost, abundant resource, and friendly benign elemental components and narrow bandgap (2.0-2.4 eV)[34]. Di, et al.[35]constructed a novel type of direct Z-scheme g-C3N4/SnS2heterojunction by depositing SnS2quantum dots onto the g-C3N4surface in situ via a simple one-step hydrothermal method. The g-C3N4/SnS2hybrid exibited robust photocatalytic CO2reduction compared with individual g-C3N4and SnS2, which should be attributed to the IEF-induced direct Z-scheme and improved CO2adsorption capacity.

In addition, extensive research about MoS2has been done due to its high specific surface area, corrosion resistance,easy recovery, controllable energy band and excellent catalytic capacity. Lee, et al.[36]successfully prepared a novel heterostructure composed of Ru and Cu co-doped ZnS nano-powder (RCZS) and MoS2graphene mixture(MSG) by microwave-assisted solvothermal method.RCZS nanopowder is fabricated on the surface of MSG,which produces a nanoscale interface between RCZS and MSG. Optimized operating conditions for CO2reduction have been obtained, such as the concentration of NaOH solution, the amount of RCZS/MSG photocatalyst,and the content of co-doped ruthenium. The optimal conditions covering a NaOH concentration of 2 mol/L and a 0.5RCZS/MSG dosage of 0.05 g/L have achieved a maximum methane gas yield of 58.6 μmol/(g·h).

2.1.2.3 Non-noble metal nitride-based catalysts

Recent studies have proved that under ultraviolet and visible light irradiation, the surface of non-polar metal nitrides can directly generate H2from water, and the strong reactivity of nitrides is very attractive for CO2reduction.

Tantalum nitride (Ta3N5) has received extensive attention because it exhibited good photocatalytic activity in water splitting and degradation of organic pollutants[37].Moreover, the CB and VB energy level for Ta3N5are about -0.29 V and 1.81 V vs. reversible hydrogen electrode (RHE), which meets the thermodynamic requirements for photocatalytic reduction of CO2by H2O into CH4[38]. In particular, modifying Ta3N5by metal particles is a facile route to improve the separation efficiency of carriers[39]. Wang, et al.[40]prepared an ohmic junction photocatalyst of Ta3N5/Bi by using metal Bi with low work function to modify n-type Ta3N5. It receives an approximately 5 times the CH4yield compared with that achieved byTa3N5.

The conduction band minimum (CBM) of GaN is more negative than those of most metal oxides, and hence GaN is kinetically more probable for the reduction of highly stable CO2molecules[41]. AlOtaibi, et al.[42]demonstrated firstly high-conversion-rate photochemical reduction of carbon dioxide (CO2) on galliuam nitride (GaN) nanowire arrays into methane (CH4) and carbon monoxide (CO).The results illustrate that the reduction of CO2to CO dominates on the as-prepared GaN nanowires under ultraviolet light irradiation. However, the production of CH4is increased considerably via using the Rh/Cr2O3core/shell cocatalyst, with an average yield of 3.5 μmol/(g·h)in 24 h. The rate of photoreduction of CO2is 14.8 μmol/(g·h) after loading Pt nanoparticles on the lateral m-plane surfaces of GaN nanowires, which is nearly by an order of magnitude higher than that measured on the as-grown GaN nanowire arrays. This work reveals the immense potential of engineered core/shell cocatalysts for the excellent selectivity toward hydrocarbon compounds.

2.2 Non-metal catalysts

2.2.1 g-C3N4-based catalysts

Among multitudinous nonmetallic semiconductors, the graphite carbonitride (g-C3N4) has become a significantly promising material due to its low cost, simple preparation process, relatively good stability, suitable electronic structure and excellent photocatalytic performance.Nevertheless, the activity of bulk g-C3N4is suppressed by the defects of small surface area and rapid charge recombination rate[43]. Hence, a lot of researches has been done to the design and building of highly efficient g-C3N4based catalysts by loading with metals and coupling with other semiconductors. Furthermore, the structure of catalysts plays also a vitally key role in CO2reduction progress besides the carrier separation and transport.

Zeng, et al.[44]fabricated a novel hybrid catalyst of g-CNQDs/PMOF by integrating the zero-dimensional(0D) carbon nitride quantum dots (g-CNQDs) with twodimensional (2D) ultrathin porphyrin MOF (PMOF). The prepared hybrid catalyst exhibits a 2.34-fold enhancement in the CO generation rate (16.10 μmol/(g·h)) and a 6.02-fold enhancement in the CH4evolution rate (6.86 μmol/(g·h)) as compared to the bare PMOF

Li, et al.[45]synthesized a series of LaPO4/g-C3N4coreshell nanowires via an in-situ hydrothermal growth of LaPO4nanorods in tubular g-C3N4and investigated their photocatalytic activity in CO2reduction. A maximum CO yield of 0.433 µmol has been obtained from CO2reduction within 1 h of irradiation on 30 mg of nanocomposite photocatalyst in the absence of any noble metal. Finally, a possible mechanism, which is featured with LaPO4activation due to significantly promoted separation of photo-generated charge carriers, is proposed.The encouraging performance in CO2photoreduction demonstrates that this novel nanocomposite will be a prospective material in environmental protection and energy conversion.

Tahir, et al.[46]reported the well-designed 1D/2D composite, consisting of carbon nanotubes modified protonated carbon nitrides (CNTs/pCN) coupled with Cu-NPs by a facile ultrasound assisted chemical method.Loading 3% of Cu and 0.5% of CNTs into pCN gives the highest activity for the production of CO, CH4, and CH3OH under visible light illumination. The highest CO yield as the main product over Cu-CNTs/pCN composite is 560 μmol/(g·h), which is 1.21, 1.36, 3.78 and 9.03 folds higher than using Cu/pCN, CNTs/pCN, pCN, and g-C3N4samples, respectively. The findings of this work would be attractive for the development of CO2conversion systems with renewable fuels production under solar energy.

Three-dimensional porous materials with micron pores can give suitable transmission channels, longer exposure time and good CO2capture ability. Therefore,the porous g-C3N4catalyst with micron pore structure is a potential for photocatalytic reduction of CO2in the gaseous medium[47]. Sun, et al.[48]prepared a unique heterostructure, which is developed based on a 3D photoactive semiconductor and a 0D Cu2O QDs for superb photocatalytic reduction of CO2into CO. The Cu2O QDs are loaded onto 3D g-C3N4foam through a simple photo-deposition strategy. The results demonstrate that g-C3N4foam is not only considered as an excellent carrier for Cu2O QDs, but also can evidently improve the photocatalytic performance by promoting CO2adsorption and gas transfer. Simultaneously, the built g-C3N4foam/Cu2O QDs possess outstanding enhancement in photocatalytic performance being 3.9 times and 11 times higher than that of g-C3N4foam and bulk g-C3N4powder,respectively. The excellent photocatalytic activity can be ascribed to the unique porous structure and the synergistic effect between g-C3N4foam and Cu2O QDs, which can accelerate the transfer of charge carriers and impose the cumulation of photo-generated electrons on the Cu2O QDs. The study provides a new idea for preparing 0D/3D hierarchical photocatalytic systems, which can provide reference on designing and constructing highly efficient photocatalysts to maximize the photocatalyst kinetics.

Li, et al.[49]constructed a series of Au/g-C3N4(Au/CN)nanocomposites where g-C3N4nanosheets (CNNSs)served as a substrate for the growth of different sized Au nanoparticles (Au NPs) by the constant temperature bathreduction method. Compared to other nanocomposites,the 3-Au/CN sample with reasonable distribution of density and small Au NPs possesses the most efficient photodegradation activity (92.66%) of RhB in 30 min under the visible light irradiation and the photoreduction performance of the catalyst for converting CO2to CO and CH4to attain a product yield of 77.5 mol/g and 38.5 mol/g,respectively, in 8 h under UV light irradiation.

Gan, et al.[50]prepared the CuInZnS/g-C3N4hybrids using L-cysteine molecules as sulfur source and simultaneously adjusted the molar ratio of In3+to Zn2+ions in the precursor, of which the g-C3N4substrates were refluxed in HNO3solution to obtain surface carboxyl and hydroxyl groups. Owing to the tunable energy band structure of CIZS and the light absorption capacity of CIZS/CCN hybrids, the optimized CIZS/CCN hybrids express a MO near-infrared degradation efficiency of 96.5% in 5 h and a CO production rate of 50.04 μmol/g in 8 h. This study provides a profitable way to adjust the band matching of heterostructures and promote the photocatalytic performance.

2.2.2 Other Nonmetallic catalysts

Other non-noble metal catalysts mainly consist of SiC,C60, C3N4, GO, rGO and CNTs, etc. Recently, graphene is widely used to prepare inorganic composites owing to its high thermal conductivity, unique carriers transfer,high flexibility structure, high transparency and specific surface area[51]. Wang, et al.[52]prepared TiO2/NG-HS by in-situ growth of ultra-thin N-doped graphene (NG)layers on interfacial TiO2by chemical vapor deposition.The CO2conversion over optimum TiO2/NG-HS is 18.11 μmol/(g·h), which is 4.6-fold more compared to pure TiO2/HS.

The graphene oxide (GO) is a highly attractive twodimensional nanostructured oxide of graphite. It is an atomically thin carbon sheet with various oxygenated functional groups on the basal plane and peripheries[53].The various isolated oxygenated functional groups and stoichiometric C/O ratio on the basal plane make GO a partially insulating wide-bandgap semiconductor-like material[54]. GO is expected to be a potential catalyst due to its wide bandgap. Shown, et al.[55]fabricated a series of copper nanoparticles modified GO (Cu/GO)composites by the single kettle microwave method. The Cu/GO-2 composite containing 10% of Cu exhibites the highest solar fuel formation rate of 6.84 μmol/(g·h)for photocatalytic CO2reduction under visible light irradiation. The photocatalytic CO2reduction rate achieved hereby is 60 times higher than that obtained by the pristine GO and 240 times higher than that achieved by commercial P-25 under visible light.

Reduced graphene oxide (rGO) has been widely applied for CO2reduction due to its excellent electronic conductivity, affinity with other materials, charge migration rate and high CO2adsorption capacity[56]. Cui,et al.[57]prepared a rhenium complex Re(PyBn)(CO)3Cl for CO2photo-reduction. Compared with the unsupported homogeneous catalyst Re(PyBn)(CO)3Cl, the covalently immobilized catalyst composite enhanced the turnover number by six times and significantly improved the catalyst stability. A plausible mechanism for CO2photoreduction by the TiO2-rGO-Re(PyBn)(CO)3Cl catalyst composite has been suggested. The results provide a novel idea for the future design of efficient catalyst for CO2photo-conversion.

Exploiting carbon family-based materials, especially graphene and fullerene (C60) would offer greater opportunity for improving the photocatalytic efficiency to satisfy practical requirements[58]. Yadav, et al.[59]firstly prepared the C60polymer film as visible light active photocatalyst for excellent and selective reduction of CO2via covalent coupling of C60monomer units consisting of tetrasubstituted C60-pyrene conjugates through spacer groups. The formation rate of HCOOH is 5.501 mol/(g·h)with visible light irradiation.

Carbon quantum dots (CQDS) are primarily generated by sp2hybridization of carbon atoms with a size of less than 10 nm[60]. Numerous varying researches have produced some interesting findings at CQDs-based catalysts due to their unique characteristics, such as robust visible light absorption performance, charge carriers transfer ability and tunable photoluminescence[61]. Kulandaivalu,et al.[62]synthesized blue carbon quantum dots (CQDs)via the top-down hydrothermal method with biochar.Then the synthesized CQDs are coupled with commercial copper oxide (Cu2O) nanoparticles to form CQDS/Cu2O nanocomposites. The results illustrate that CQDS/Cu2O photocatalyst has excellent optical, physical and chemical properties, and can reduce efficiently CO2to C2H6under visible light irradiation. Compared with pure Cu2O, the photocatalytic activity of CQDS/Cu2O photocatalyst reaches up to 54%. The significant development of the photoactivity of nanocomposites reveals the importance of CQDSin increasing the photoreduction of CO2with high electronic requirements to C2H6.

Silicon carbide (SiC), as one of the appealing nonmetallic oxide semiconductors, has been applied universally from metallurgy to aerospace due to its high thermal conductivity and high strength[63]. SiC also exhibits a moderate wide bandgap, considerable negative conduction band potential, high chemical stability, and environmental friendliness. These properties make SiC profitable as a photocatalyst, especially for the high reduction potential reactions such as water reduction and CO2reduction in efficiently harvesting solar energy[64].Wang, et al.[65]successfully fabricated a novel hollow spherical 3D hollow-sphere structure of β-SiC with an open mouth. The optimal 2.0% of Pt loading led to a stable CH4evolution as high as 16.8 µmol/(g·h) with the simulated solar light irradiation, which is more than twice that of many reported metal oxides under similar experimental conditions. The major factor in the improvement of photocatalytic reduction is that the high specific surface area of SiC is profitable to CO2adsorption and mass transfer for CO2reduction. In addition, Pt particles are beneficial to promoting the separation of photo-induced electrons of SiC and CO2absorption.

2.2.3 MOFs-based photocatalysts

MOFs exhibit the superiority in specific surface area and CO2adsorption capacity which make them fantastic potential applications in CO2adsorption and reduction.

Kong, et al.[66]successfully synthetized the CsPbBr3@ZIF hybrids, including CsPbBr3@ZIF-8 and CsPbBr3@ZIF-67, with a facile in-situ synthetic method by directly growing a zinc/cobalt-based zeolitic imidazolate framework (ZIF) coating on the surface of CsPbBr3quantum dots. The nanocomposites possess increased CO2reduction activity with an electron consumption rate of 15.498 μmol/(g·h) and 29.630 μmol/(g·h) for CsPbBr3@ZIF-8 and CsPbBr3@ZIF-67, respectively, which is 1.39-and 2.66 times more than that achieved by pure CsPbBr3.The CsPbBr3@ZIF hybrids have elevated photo-induced carriers separation efficiency, CO2capture and stability that are basic factors to propel CO2reduction. This work would provide a new view for designing better perovskite/MOF-based catalysts.

UIO-66-NH2as the common MOFs is widely studied owing to its excellent thermal stability[67], appropriately high specific surface area, and excessive CO2absorption capacity[68]. Wang, et al.[69]successfully constructed ternary cubic nanocomposites Cu2O@Cu@UIO-66-NH2under facile solvothermal conditions. The ternary cubic nanocomposites can effectively reduce CO2to CO and CH4. The precipitation rates of CO and CH4are 20.9 μmol/(g·h) and 8.3 μmol/(g·h), respectively. This study can provide a reference for the design and creation of nanocomposites for the photoreduction of CO2.

Wang, et al.[70]explored a novel microwave-induced synthesis route and assembled highly dispersed UIO-66-NH2nanocrystals onto graphene (GR) and constructed resoundingly UIO-66-NH2/GR. It is found that the formic acid formation rate of UIO-66-NH2/GR is about 11 times that of the pristine UIO-66-NH2. In comparison to the same semiconductors obtained through the traditional hydrothermal method, the MIS induced catalyst possessed an outstanding activity (108%) for the photo-reduction of CO2. This work provides a reference for the design and preparation of nanocomposites aiming at effectively reducing CO2.

Among enormous various MOFs, the zirconium-based MOFs have attracted tremendous concern owing to their high thermal and chemical stability, where their intriguing performance could deal with rigmarole and grave catalytic situations[71]. Sun, et al.[72]successfully prepared and characterized a porous and visible-light-driven zirconium metalorganic framework ([Zr6O4(OH)4(L)6]·8DMF,denoted as Zr-SDCA-NH2) by employing a conjugated amine-functionalized dicarboxylic ligand (H2L =2,2′-diamino-4,4′-stilbenedi-carboxylic acid, H2SDCANH2). It demonstrated that Zr-SDCA-NH2possesses visible-light-response activity for CO2reduction with an eminent formation rate of 96.2 μmol/h on 1.0 mmol of MOF compared with that of various reported aminefunctionalized Zr-MOFs. In addition, for the Zr6oxo clusters charge transfer process (LMCT) or the catalytic role of the ligand itself, Zr-MOFs show the selective catalytic reduction of CO2. This study reveals that the combination of amino groups and highly conjugated molecules is a profitable and elementary strategy to disseminate visible light absorption of the organic ligand,which can provide ideas and methods for forming a visible light responsive MOF photocatalyst.

Ni-MOF (PCN-601) has been proved to be a value-added and durable photocatalyst for visible light responsive overall CO2reduction owing to the light collector,catalytic active sites, and high specific surface area. Fang,et al.[73]formulated and built a MOF (denoted as PCN-601), which consists of reactive Ni-oxo cluster nodes and light-harvesting metalloporphyrin ligands connected by dint of pyrazolyl groups and acts as a catalyst for gasphase CO2photoreduction at ambient temperature. PCN-601 achieves ultrafast ligand-to-node electron transport and efficient charge separation in the photocatalyst. The CH4production rate of PCN-601 with a gas yield of 10.1 μmol/(g·h), which is by 3-fold and 20-fold more than that of the analogous MOFs based on carboxylate porphyrin and the classic Pt/CdS, respectively. The study indicated that the rational attempt about MOF structures weakens the confliction between reactivity and stability and improves immensely to detract photogenerated carriers for optimization of efficiently capable catalysts.

3 Conclusions

In summary, various types of numerous nanocomposite systems have been designed and prepared. Although a wide amount of research has received significant advances recently in photocatalytic reduction of CO2, which is an effective way to overcome the status quo of environmental pollution and energy crisis. The studies of photocatalysts are at an early stage confronted with various shocks and challenges. Admittedly, nanocomposites constructed by modification with noble metal can improve the performance of photocatalysts to a large extent.Nevertheless, the high cost remains the “bottleneck”of the photocatalysts which restrict their development.In regard to the non-noble metal catalyst, TiO2is still a research priority. Sulfides have attracted multitudinous attention due to the narrow bandgap with good visible light response. There remains the problem of poor visible light utilization and rapid recombination of charge carriers. The research of nitrogen is less relative to oxides and sulfides. The g-C3N4-based nanocomposites hold high specific surface area and enriched active sites to promote the separation of photo-generated carriers and catalytic performance. Proper construction of heterostructures that could simultaneously facilitate efficient optical absorption,utilization, separation, and transportation of carriers,is a research priority for a new generation of highly efficient and outstanding photocatalysts. MOFs acting as a kind of new material exhibit tremendous potential with an ultrahigh porosity and unique porous structure in comparison with traditional catalysts. There are still shortcomings including low visible light utilization rate, poor stability, and selectivity that are inherent in MOFs through post-synthetic modifications. Therefore,more studies are needed for enhancing the efficiency of photocatalytic reduction CO2. Aimed at the problems presented above, policy should be directed at practical applications as well as aimed at efficient, low cost and environmental amicable and desirable photocatalysts to improve catalytic activity in photocatalytic reduction CO2. Only in this way, it is necessary to provide technological support to requirements for the application of industrialization and sustainable development.