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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 4: 575 - 585

DOI: https://doi.org/10.7536/PC230817

Review

Application of Carbon Dots and Its Composites in the Field of Photocatalytic CO2 Reduction

  • Dang Yongqiang , 1, * ,
  • Huang Rui 1 ,
  • Feng Xiangyu 1 ,
  • Liu Guoyang 1 ,
  • Zhu Youyu 1 ,
  • Zhang Yating , 1, 2, *
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  • 1 College of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, China
  • 2 Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Natural Resources, Xi'an 710021, China
*e-mail: (Yating Zhang);
(Yongqiang Dang)

Received date: 2023-08-17

  Revised date: 2024-01-15

  Online published: 2024-03-15

Supported by

Natural Science Foundation of Shaanxi Province(2022JQ-132)

Innovation Capability Support Program of Shaanxi(2019-TD-021)

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Abstract

The massive consumption of fossil fuels has caused the continuous increase of carbon dioxide concentration in the atmosphere,resulting in serious climate and environmental problems such as greenhouse effect and sea level rise.The use of solar photocatalysis to reduce CO2to hydrocarbon fuel with added value is regarded as one of the most promising potential solutions.Researchers have developed a variety of photocatalysts,among which carbon dots are a new type of carbon nanomaterials with a size of less than 10 nm.They have unique up-conversion luminescence properties and can promote electron transfer.The synthesis method is friendly and safe.They are widely used in the field of photocatalytic reduction of CO2.In this paper,starting from the mechanism of photocatalytic reduction of CO2,the action mechanism and performance evaluation of carbon dots and carbon dot composite materials in photocatalytic reduction of CO2are reviewed in detail in terms of light absorption efficiency,carrier separation efficiency,CO2adsorption capacity and multiple interactions.The advantages of carbon dots in the field of photocatalytic reduction of CO2are summarized.The existing challenges and possible ways to address the challenges in the future are analyzed.And the future development is prospected.It provides a new idea for promoting the development of carbon dot-based photocatalysts 。

Contents

1 Introduction

2 Introduction of carbon dots

3 Mechanism of photocatalytic reduction of CO2by semiconductors

4 Application of carbon dots in photocatalytic reduction of CO2

4.1 Application of individual carbon dots in photocatalytic reduction of CO2

4.2 Application of Carbon Dots-Based Composite Materials in the Field of CO2Reduction

5 Conclusion and prospect

Key words： carbon dots; carbon dot composites; photocatalysis; carbon dioxide reduction; mechanism of photocatalytic CO2 reduction

Cite this article

Dang Yongqiang , Huang Rui , Feng Xiangyu , Liu Guoyang , Zhu Youyu , Zhang Yating . Application of Carbon Dots and Its Composites in the Field of Photocatalytic CO2 Reduction[J]. Progress in Chemistry, 2024 , 36(4) : 575 -585 . DOI: 10.7536/PC230817

1 Introduction

In recent years,a large number of fossil fuels such as coal and oil have been consumed,resulting in the continuous increase of CO2in the air,which has caused serious environmental problems such as greenhouse effect,global warming and sea level rise[1]。 How to reduce the concentration of CO2is one of the urgent problems to be solved.Converting CO2into hydrocarbon fuels can not only reduce the concentration of CO2,but also produce hydrocarbons with high added value,which is helpful to realize the carbon cycle and is an attractive way[2]。
At present,the main methods of conversion of CO2are electrocatalysis,photocatalysis,photoelectrocatalysis,thermocatalysis and photothermal catalysis,among which photocatalytic technology uses inexhaustible solar energy and has the advantages of green,clean and pollution-free[3][4,5]。 Photocatalytic technology was first used by Professor Fujishima in 1972 for the photoelectrocatalytic decomposition of H2O to produce O2[6]。 In 1978,Somorjai et al.Used SrTiO3as a catalyst for the photocatalytic reduction of CO2,and in 1979,Inoue and Fujishima et al.Proposed the reaction mechanism of photocatalytic reduction of CO2.More and more researchers have carried out the research on photocatalytic reduction CO2,and are committed to more efficient photocatalytic reduction CO2[7][8]。
Photocatalyst is the core in the process of photocatalytic reduction of CO2.At present,a large number of photocatalysts have been reported,but there are still some deficiencies in photocatalytic activity,product selectivity and light stability.Photocatalytic materials should have the following advantages:(1)large absorption coefficient for sunlight,which can adapt to different spectra[9]; (2)that material has stable state,light corrosion resistance and long service life;And(3)that cost is low,and the control and manufacture are easy.Carbon dots(CDs)have been widely used in the field of photocatalytic reduction of CO2due to their excellent electron transfer properties and upconversion luminescence properties of nanomaterials.In 2011,Sun et al.Published an article on the photocatalytic reduction of CO2by carbon dots,and found that CO2was successfully reduced to formic acid,which laid the research foundation for the photocatalytic reduction of CO2by carbon dots[10]。 Then they deposited gold or platinum on the surface of carbon dots and used photocatalyst to reduce CO2.Formic acid was detected in the product.The local plasma resonance effect of the metal can enhance the use of light by the catalyst.The metal transfers and concentrates the photogenerated electrons,weakens the recombination,and is conducive to the reduction reaction[11⇓~13]。 Since then,more and more applications of carbon dots in photocatalytic reduction of CO2have been studied 。

2 Introduction to carbon dots

carbon dots were discovered by Scrivens et al.of the University of South Carolina in 2004 during the preparation of single-walled carbon nanotubes by arc discharge method.They are carbon nanoparticles with a size of less than 10 nm and fluorescent effect[14]。 Carbon Dots are composed of Graphene Quantum Dots Carbon and oxygen/nitrogen groups or Polymer aggregates,which can be divided into three types according to their structures:Graphene Quantum Dots(GQDs),Carbon Nanodots(CNDs)and Carbonized Polymer Dots(CPDs)[15,16]。 GQDs have a carbon core composed of one or several layers of graphene and edge-connected chemical groups,which are anisotropic and have lateral dimensions greater than The height.CNDs are always spherical,and they are divided into carbon nanoparticles without a lattice and carbon quantum dots(CQDs)with a distinct lattice;CPDs are aggregated or cross-linked polymers prepared from linear polymers or monomers.the electronic structure of carbon dots is usually described by molecular orbital theory,and some literatures point out that carbon dots have valence bands and conduction bands,and give explicit measurement methods[17][18,19]。
the synthesis methods of carbon dots mainly include"top-down"and"bottom-up".The"top-down"method is to peel off The carbon source with large size by physical or chemical methods to produce carbon dots with very small size[20]。 Uch as arc discharge method,laser ablation method,electrochemical synthesis method and ultrasonic method[21][22][23]。 The"bottom-up"method is to synthesize carbon dots from small carbon materials in molecular or ionic States,such as microwave method,hydrothermal/solvent method,pyrolysis method and template method,and often uses small organic molecules or oligomers as carbon sources,such as citric acid,glucose and polyethylene glycol[24][25][26][27]。 the synthesis method of carbon dots is relatively simple,and the carbon source and synthesis method have a certain impact on the properties of carbon dots,which can be changed by adjusting the synthesis method and carbon source of carbon dots。
carbon dots have many advantages,such as unique fluorescence mechanism,excellent optical properties,good water solubility,low toxicity,low cost and good biocompatibility,which make Carbon dots have a very good development in the fields of bioimaging,fluorescent probes and catalysis[28,29]。 the upconversion luminescence property of carbon dots can emit short-wavelength light under the excitation of long-wavelength light,which can improve the utilization of visible light for catalysts with weak visible light absorption and expand the visible light absorption range of wide band gap semiconductors[30,31]。 As a good electron/hole carrier,carbon dots can effectively reduce the recombination rate of electrons and holes in the photocatalytic process.When carbon dots are hole-accepting carriers,they can effectively prevent the holes from oxidizing the catalyst,improve the stability of the catalyst and the selectivity of the product,and are one of the potential photocatalysts for photocatalytic reduction of CO2[32]。

3 Mechanism of Photocatalytic Reduction of CO2 by Semiconductor

the internal band structure of semiconductor photocatalyst essentially determines the thermodynamic characteristics of photogenerated carriers and photocatalytic activity of semiconductor materials.The energy band structure is composed of valence band(VB),conduction band(CB)and forbidden band.The valence band is The highest energy band occupied by electrons at absolute zero of solid materials,which is composed of atomic orbitals filled with electrons,and the conduction band is composed of empty orbitals not filled with electrons[33,34]。 The energy gap between the valence band and the conduction band is the forbidden band width,also known as the band gap(Eg).The mechanism of photocatalysis is that when the semiconductor photocatalyst is excited by light greater than or equal to the band gap energy,the electron（e-）will jump from VB to CB,leaving a hole（h＋）on VB.The generated electron and hole have the ability to reduce and oxidize respectively(Figure 1A).With the help of the oxidation and reduction co-catalyst,the redox reaction occurs on the surface of the photocatalyst.For the photocatalytic process,the selection of photocatalyst is very important.Different photocatalysts have different energy band structures(Figure 1b).Selecting a photocatalyst with a suitable energy band structure is the key to the smooth progress of the photocatalytic process 。
图1 (a) 由合适的氧化还原助催化剂介导的用于太阳能燃料生产的半导体光催化剂上光催化 CO2 转化的可能机理示意图[35];(b) 一些半导体光催化剂的能带位置和水溶液中 pH = 7 时 CO2 还原的氧化还原电位[36]

Fig. 1 (a) Schematic illustration of probable mechanism of photocatalytic CO2 conversion over a semiconducting photocatalyst for solar fuels production mediated by suitable redox cocatalysts[35]; (b) Band positions of some semiconductor photocatalysts and the redox potentials of CO2 reduction at pH = 7 in aqueous solution[36]

The overall photocatalytic efficiency is determined by the efficiency of four tandem steps,namely:light harvesting efficiency（ηabs）,carrier separation efficiency（ηsep）,carrier transport and migration efficiency（ηcmt）,and surface carrier utilization efficiency（ηcu）.The type and selectivity of the photocatalytic products are related to the reaction mechanism.Habisreutinger et al.Summarized the formation of C1（CO by photocatalytic reduction of CO2with TiO2.Possible mechanism of HCOOH,CH2O,CH3OH,CH4）,and C2products,Due to the different connection modes between CO2and catalyst surface(oxygen coordination,carbon coordination,side/mixed coordination),it can be summarized as formaldehyde pathway,carbene pathway and glyoxal pathway[37]。 Recently,Tang et al.Further summarized the reduction pathway of CO2according to whether the reduction reaction occurs through electron transfer,proton transfer(sometimes hydroxyl transfer),or concerted electron-proton transfer(Fig.2):when the reduction produces the C1product,CO and HCOOH are produced by two-electron processes,HCHO and C are produced by four-electron processes,CHO,CHOH and CH2OH are formed by some electron and proton transfer processes,formaldehyde is subsequently produced,CH3OH is produced by six-electron processes and CH4is produced by eight-electron processes[38]; When reduction produces C2products,the two-electron process produces(COOH)2,and the eight-electron process produces CH3COOH.The ten-electron process produces CH3CHO,the twelve-electron process produces CH3CH2OH and C2H4,and the fourteen-electron process produces C2H6.They believe that if the intermediate molecules in one reaction pathway are rapidly desorbed from the surface of the catalyst before further reactions occur,the reaction in another pathway will occur,and the final products will be different.The selection of CO2reduction products can be achieved by regulating the structure of the catalyst.Some progress has been made in the study of the mechanism of photocatalytic reduction of CO2,but the mechanism is complex and not yet fully understood,and needs to be further explored 。
图2 CO2还原为常见C1和C2产品的机理途径[38]

Fig. 2 Mechanistic Pathways of CO2 Reduction to Common C1 and C2 Products[38]

The strategies to improve the photocatalytic efficiency can be summarized into five aspects:(1)enhance the photoexcitation.Carbon dots,as a substance with upconversion fluorescence properties,can effectively expand the light absorption properties of wide band gap semiconductors.Zhang et al.Used CQDs to modify TiO2nanotubes,so that the photocatalyst can absorb more sunlight and use more solar energy for photocatalytic reduction of CO2;（2）to promote carrier transfer and separation[39]。 The construction of heterojunction is one of the effective methods to promote carrier transfer and separation.As a nano-carbon material,carbon dot is one of the commonly used photocatalysts for semiconductor/nano-material heterojunction.Li et al encapsulated CDs in NH2-UiO-66(Zr),and the electron transfer from the conduction band of NH2-UiO-66(Zr))to CDs effectively inhibited carrier recombination,increased the yield of CO from 3.5µmol·g-1·h-1to 16.6µmol·g-1·h-1;（3）,and promoted CO2adsorption and activation[40,41][42]。 The CO2adsorption capacity of the CQDs/TiO2composite prepared by Gao et al.Is 1.4 times higher than that of the pure TiO2[43]; (4)to accelerate the charge transfer between the catalyst and the adsorbed CO2on its surface.Li et al.Studied the charge dynamics mechanism of the hybrid g-C3N4/[Co(bpy)3]2+(bpy=2,2°-(bipyridine)photocatalytic system by using in situ ultraviolet and other techniques.The CO2combined with[Co(bpy)2]+to form a[CoⅢ(bpy)2CO2]+intermediate,and the accumulation of the intermediate indicated that the subsequent CO2reduction reaction was the main rate-limiting step[44]。 After replacing bpy with dmbpy to obtain the g-C3N4/[Co(dmbpy)3]2+(dmbpy=4,4'-dimethyl-2,2'-bipyridine)system,the electron affinity of the cobalt complex[Co(dmbpy)3]2+and the intermediate[CoⅢ(dmbpy)2CO2]+is enhanced,and the charge transfer between the catalyst and the CO2is accelerated.Charge dynamics enhanced the photocatalytic conversion of CO2to CO by 5.4-fold with a selectivity of over 85%and an apparent quantum yield of 1.96%at 400 nm;(5)suppress undesired surface reactions,such as the formation reaction of H2and the reoxidation reaction of CO2reduction products,to improve product selectivity.Wang et al.Prepared hole-accepting carbon dots and carbon nitride composite materials,which can effectively prevent methanol from being oxidized and improve the selectivity of photocatalytic reduction of CO2to methanol[45]。

4 Application of carbon dots in photocatalytic reduction of CO2;

In recent years,researchers have carried out a lot of research work on the synthesis and application of carbon dot-based photocatalysts,and have achieved exciting research results.In this paper,the research progress of carbon dots in photocatalytic reduction of CO2in recent years will be introduced in detail from two aspects of carbon dots and carbon dots composite materials 。

4.1 Application of individual carbon dots in the field of photocatalytic reduction of CO2

As a kind of zero-dimensional nanomaterials,carbon dots are mainly composed of amorphous carbon and nanocrystalline regions of sp2/sp3hybrid graphitic carbon,and there are abundant functional groups on the surface of carbon dots.By adjusting the functional groups on the surface of carbon dots,the composition structure of carbon dots can be adjusted,and then the energy band structure can be affected.By doping or surface functionalization,the photocatalytic reduction of CO2process can be enhanced[19]。
Liu et al.Prepared highly graphitized carbon dots(GCDs)for photocatalytic reduction of CO2.The nitrogen-containing functional groups can enhance the adsorption of carbon dots on the reduction intermediate CO,which is helpful for further conversion.The conjugated largeπbonds produced by graphitization can stabilize the photogenerated electrons.Therefore,the separation of photogenerated electron-hole pairs is improved,the recombination rate of electrons and holes is reduced,and the interfacial reaction is kinetically promoted,so that CO2is reduced to CH4,and the selectivity reaches 74.8%[46]。 Maimaiti et al.Prepared N and S co-doped mesoporous aminated coal-based carbon nanoparticles（NH2-CNPs）by acyl chloride chlorination and ethylenediamine passivation method,the unique mesoporous structure increased the adsorption capacity of carbon dots to CO2,and the defect structure formed by N and S co-doping could effectively separate photogenerated carriers(Fig.3 )[47]。 NH2-CNPs reduced CO2to CH3OH with a selectivity of 76.6%.Dang et al.Prepared iron-doped carbon dots by a one-step hydrothermal method.The introduction of iron regulated the energy band structure of carbon dots and reduced the carrier recombination rate,and the yield of CH3OH reached 654.28µmol·g-1·h-1[48]。 The following year,Dang et al.Reported the hydrothermal synthesis of tetraphenylporphyrin carbon dots(TPP-CDs),and its photocatalytic reduction of CO2to methanol was 2.06 times higher than that of the original carbon dots,which was attributed to the improvement of the utilization of visible light[49]。 The above studies show that the efficiency of photocatalytic reduction of CO2can be directly affected by changing the structure of carbon dots,so that CO2can be reduced to more valuable hydrocarbon fuels such as methane and methanol,and the selectivity of the products can also be improved,but the methods of controlling the structure of carbon dots and the structure-activity relationship between the structure and photocatalytic performance of carbon dots still need to be further studied.Table 1 summarizes the products,selectivity,and mechanism of the photocatalytic reduction of CO2by individual carbon dots 。
图3 在 NH2-CNPs上用水光催化CO2还原的机理示意图[47]

Fig. 3 Schematic of the mechanism of the photocatalytic CO2 reduction with water on NH2-CNPs[47]

表1 Application of individual carbon dots in photocatalytic reduction of CO2

Table 1 Application of individual carbon dots in photocatalytic reduction of CO2

Catalyst Synthesis method① Product Production rate Selectivity Mechanism Ref.
GCDs Ultrasonic-assisted CH4, CO 983, 350 μmol·g-1·h-1 74.8% CO2 adsorption 46
NH2-CNPs reflux-stirring CH3OH 61.87 μmol·g-1·h-1 76.6% CO2 adsorption site types and distribution 47
Fe-CDs Hydrothermal CH3OH 654.28 μmol·g-1·h-1 ─ High electron transfer rate 48
TPP-CDs Hydrothermal CH3OH 173.35 μmol·g-1·h-1 ─ sufficiently negative reduction potential, Light absorption 49

①synthesis method of carbon dots

4.2 Application of Composite Materials Based on Carbon Dots in Reduction CO2 Field

Light absorption efficiency and carrier separation efficiency are the main factors limiting the photocatalytic efficiency.The upconversion luminescence performance of carbon dots can broaden the visible light absorption performance of the catalyst,and carbon dots can also act as electron/hole acceptors to promote the carrier transport rate.In addition,carbon dots can promote the adsorption of CO2on the catalyst,thus improving the catalytic activity.These advantages make carbon dots popular in the construction of composite photocatalysts.The role of carbon dots is described separately below 。

4.2.1 Enhanced light absorption efficiency

Because of the wide band gap of traditional photocatalysts,photocatalysis mainly uses ultraviolet light in sunlight at the beginning,and the use of visible light and infrared light is minimal.Carbon dots are a kind of upconversion luminescent nanoparticles,and the combination of carbon dots and wide band gap semiconductor photocatalyst can effectively broaden the utilization of sunlight by photocatalysis.Zhang et al.Decorated biomass-derived carbon quantum dots on TiO2nanotubes,and compared with unmodified TiO2nanotubes,the photocatalytic reduction of CO2to CO and CH4was improved by 2.4 and 2.5 times,respectively[39]。 The upconversion luminescence property of carbon quantum dots effectively improves the defect that the TiO2only responds in the ultraviolet region.Zhong synthesized TD-COF from TP and DBT,and encapsulated the metalloporphyrin-based carbon dots inside the COF.The CO selectivity of Ni-PCD@TD-COF reached 98%,and the metalloporphyrin-based carbon dots as a photosensitizer improved the light absorption performance of the catalyst[50]。
combined with the upconversion luminescence technology of carbon dots and photocatalysts with absorption in the visible range,it is expected to realize the utilization of near infrared/infrared light in the photocatalytic process.the band gap of ZnO is about 3.3 eV at room temperature.it is difficult to make full use of visible light.If ZnO and carbon dots are Combined,it will be a breakthrough in the field of photocatalysis to extend the light absorption range of ZnO to the near infrared by using the upconversion luminescence performance of carbon dots[51,52]。 Lin et al.Synthesized a composite material of carbon dots with composite hollow spheres and zinc oxide,and the yield of CO could reach 60.77µmol·g-1·h-1,which was 54.7 times that of pure ZnO and 11.5 times that of commercial TiO2.The photocatalytic reaction was also successfully realized under near-infrared illumination alone,and the yield of CO was 15.98µmol·g-1·h-1[52]。 They further synthesized ZnO/carbon dots composites containing oxygen defects(OD-ZnO/CDs)in a furnace aerosol reactor(FuAR),and it can be seen from Fig.4 that under the irradiation of UV-Vis-NIR,The yield of CO is 118.8µmol·g-1·h-1,with abundant oxygen vacancies to promote the transfer of electrons,and the synergistic effect between the intermediate band generated in OD-ZnO and the up-conversion luminescence of carbon dots to improve the light utilization efficiency,which is the reason for the improved yield[53]。 In these works,CDs extended the use of photocatalyst to the light region to the near-infrared region,and realized the photocatalytic reduction of CO2in the whole spectrum,but the main product was CO,while CH4and CH3OH had greater value.Kong et al.Used CQDs to modify ultrathin Bi2WO6nanosheets,and the yield of CH4was 9.5 times higher than that of the original Bi2WO6under visible light irradiation,so it is necessary to explore the reduction of CO2into more value-added products while utilizing the full spectrum[54]。
图4 在不同的FuAR温度下，在不同的照明波长范围内合成样品TiO2（P25）、OD ZnO/C-N2和OD ZnO/C-空气的平均CO产率（5 h）[53]

Fig. 4 Average CO yields (5 h) on TiO2 (P25), OD-ZnO/C-N2, and OD-ZnO/C-Air samples, synthesized at various FuAR temperatures, under different illumination wavelength ranges[53]

4.2.2 Promote carrier separation

Low carrier separation efficiency is one of the main factors limiting the photocatalytic efficiency.Heterojunctions are often used to spatially separate photogenerated electrons and holes and inhibit their combination.Li et al.Used a microwave method to directly grow N,S co-doped carbon quantum dots(CQDs)on the surface of TiO2,and this method formed a tightly bonded interface between the carbon quantum dots and the TiO2,so that the electrons on the TiO2were more easily transferred to the carbon quantum dots,and the recombination of electron-hole pairs on the TiO2was more effectively suppressed[55]。 The yields of CH4and CO were 7.79 and 7.61 times higher than that of the original TiO2,respectively.Li et al.explored the effect of the position of CDs in the MOF on the photocatalytic activity,and loaded CDs on the outer surface（CD/NH2-UiO-66）and internal（CD@NH2-UiO-66）of the MOF,respectively.The reduction yield of CO2to CO by CD@NH2-UiO-66 is 16.6µmol·g-1·h-1,which is four times higher than that of CD/NH2-UiO-66,indicating that the CDs embedded in the MOF can effectively promote the internal charge transfer[42]。 Bi et al.Synthesized a multi-carbon species coupled CQDs/TiO2-C composite by pyrolysis,and experimentally found that this new electron transfer channel was from C to O to CQDs(C-O-CQDs),which again proved the function of carbon dots to promote carrier separation[56]。
The heterojunction between carbon dots and photocatalyst is also one of the important ways to enhance carrier separation.Kulandaivalu et al.Used biochar(EFB)as raw material to synthesize carbon dots by hydrothermal method.CQDs were combined on the Cu2O by physical adsorption.The yield of C2H6was 28.42µmol·g-1·h-1.The electron transfer from the Cu2O to the carbon quantum dots inhibited the recombination of photogenerated carriers[57]。 Li et al.Constructed a type II heterojunction of cuprous oxide and carbon quantum dots by a one-step ultrasonic method.The yield of CH3OH was 55.7µmol·g-1·h-1.The transfer of holes to CQDs not only reduced the recombination rate of electrons and holes,but also prevented the side reaction of Cu2O and CH3OH from being oxidized by holes.In addition,the repeated refraction of visible light between carbon dots also enhanced the visible light absorption of the material(Fig.5 )[58]。 In the photocatalytic reaction,Cu2O is easily oxidized into CuO,so they used a one-step ultrasonic method to synthesize a structure in which CQDs are embedded in the carbon layer and wrapped by cuprous oxide.The existence of the carbon layer can effectively prevent the oxidation of Cu2O,and the yield of methanol increases to 99.6µmol·g-1·h-1[59]。 The combination of CDs and Cu2O can reduce CO2to CH3OH with more added value,but there are few studies on selectivity,so the next step is to study how to improve the selectivity of CH3OH 。
图5 CQDs/Cu2O的机理图[58]

Fig. 5 Mechanism diagram of CQDs/Cu2O[58]

The structure of carbon dots has a great influence on the product selectivity of photocatalytic reduction of CO2.Wang et al.Synthesized two kinds of carbon dots and compounded them with carbon nitride(CN).From the experiment of photocatalytic reduction of CO2,it can be found thatmCD/CN mainly reduces CO2to CH3OH.The selectivity can reach 99.6%±0.2%,whilesCD/CN reduces CO2to CO[45]。 It is found by calculation that CH3OH is more easily adsorbed on CN,while H2O is more easily adsorbed on CDs.For themCD/CN system,holes will be transferred from CN to themCD to prevent the CH3OH from being oxidized and improve the selectivity of the CH3OH.In the photocatalytic mechanism ofsCD/CN,electrons are transferred from CN tosCD,and the resulting CH3OH is easily oxidized by holes(Fig.6).Carbon dots with different structures can affect the selectivity of products.They further synthesized a composite of carbon nitride-like polymer(FAT)with carbon dots,and the yield of CH3OH was 24.2µmol·g-1·h-1,which was twice that ofmCD/CN[60]。 Some N atoms in CN are replaced by O atoms in FAT,resulting in a lower density of trapped electrons and a higher intensity of trapped holes after photoexcitation.Spectroscopic studies have also shown that CDs can extract holes from FAT on a sub-microsecond time scale before deep trapping of FAT occurs,thus favoring the 6-electron reduction reaction of carbon dioxide to methanol.Obviously,carbon dots as hole acceptors are more conducive to the formation of methanol than as electron acceptors,but for CDs and CN composites,although the selectivity of methanol is higher,the yield of methanol needs to be improved。
图6 （a） CO2和CH3OH在CN上的吸附能和最稳定的构型，以及H2O在mCD上的吸附能;（b） mCD/CN和sCD/CN的光催化CO2还原示意图;（c）可见光下CN和mCD/CN上的CH3OH氧化试验[45]

Fig. 6 (a) Adsorption energies and most stable configurations of CO2 and CH3OH on CN, and H2O on mCD; (b) Schematic illustration of the photocatalytic CO2 reduction of mCD/CN and sCD/CN; (c) CH3OH oxidation assay on CN and mCD/CN under visible light[45]

4.2.3 Nhanced CO2 adsorption capacity

The adsorption ability of the catalyst to the reactant affects the concentration of the reactant on the surface of the photocatalyst,which is one of the important factors affecting the catalytic effect.When Lin et al.Studied the photocatalytic reduction of CO2by OD-ZnO/CDs,they proved that the CO2adsorption ability on the catalyst surface played an important role in the CO2photoreduction reaction[53]。 Gao et al.Found that when carbon quantum dots were immobilized on TiO2nanosheets(CDs/TiO2-3）during the photocatalytic reduction of CO2using TiO2.The CO2adsorption capacity of CDs/TiO2-3 is about 1.4 times that of the original TiO2,and the yield of CO is increased by three times[43]。 When Liu et al.Measured the CO2adsorption curve and in situ Fourier transform infrared spectroscopy of the catalyst,they found that N-doped carbon quantum dots(N-CQDs)promoted the adsorption and activation of surface CO2,as well as the desorption of CO after loading N-CQDs on Bi4MoO9[61]。 Wei et al.Reported that graphene quantum dots(GQDs)were uniformly dispersed in the pores of Zn-MOF,and the specific surface area,surface defects and morphology affected the adsorption of CO2on the catalyst surface,thus affecting the formation of products[62]。 Li et al.Modified carbon quantum dots onto CN to solve the problems of insufficient CO2adsorption and activation sites,ineffective charge separation,and slow surface reaction kinetics in the presence of CN,and the yield of CH4was increased by 14 times[63]。 Jo et al.Found that nitrogen-doped carbon dots as a co-catalyst had a role in promoting CO2adsorption[64]。 Ma et al.Used BET to test the BiVO4（CD-BVO）of BiVO4（BVO）and carbon dot modification,and found that the specific surface area of CD-BVO is larger,which can provide more active sites for CO2[65]。 Therefore,when designing the photocatalyst for CO2reduction,the adsorption capacity of the catalyst for CO2is an important factor that can not be ignored,which is directly related to the performance of the catalyst.The selection of materials with suitable adsorption capacity for CO2is a prerequisite for obtaining photocatalysts with excellent catalytic performance 。

4.2.4 Carbon dots play multiple roles

In the photocatalytic process of the carbon dots composite photocatalyst,the carbon dots can promote the efficiency of photocatalytic reduction of CO2in many ways.Lee et al.Uniformly decorated N-doped carbon dots on a TiO2with a hollow sphere structure,and the selectivity for CH4reached 98%,and the carbon dots simultaneously played a role in enhancing the light absorption efficiency and promoting electron transfer(Fig.7 )[66]。 Wei et al.Uniformly dispersed graphene quantum dots(GQDs)in the pores of Zn-MOF,and the GQDs acted as a photosensitizer to improve the light absorption efficiency,and the heterojunction formed with the MOF also promoted the spatial separation of photogenerated electron-hole pairs,and both the yields of CH4and CO increased in a linear manner,with the generation rates of 20.9µmol·g-1·h-1and 3.7µmol·g-1·h-1,respectively[62]。 Gao et al.Used the pyrolysis method to immobilize carbon quantum dots on TiO2nanosheets(CDs/TiO2-3）,the yield of CO was increased by three times compared with that of TiO2nanosheets.It is attributed to the improvement of the separation efficiency of photogenerated carriers and the CO2adsorption capacity of TiO2by carbon quantum dots,and the CO2adsorption capacity of CDs/TiO2-3 is about 1.4 times that of the original TiO2[43]。 Li et al.Used a simple hydrothermal method to synthesize a composite of carbon dots and carbon nitride(CN),and the modification of carbon quantum dots(CQDs)not only increased the adsorption capacity of CO2,but also effectively promoted the transfer of electrons,resulting in a yield of CH414 times higher than that of pure CN[63]。 Jo et al.Synthesized nitrogen-doped carbon dots(NCD)and LDH/CN composite materials,and the selectivity of CH4can reach 99%.As a co-catalyst,NCD not only accelerates the charge transport process and hinders the charge recombination,but also improves the absorption efficiency of light and promotes the ability to adsorb CO2[64]。
图7 CD/TOH的产物分析（插图：CD/TOH的TEM）和机理图[66]

Fig. 7 Product analysis of CD/TOH (inset: TEM of CD/TOH) and mechanism diagram[66]

Table 2 summarizes the products and selectivity of photocatalytic reduction of CO2by carbon dot composite materials and the role of carbon dots in the composite materials.It can be seen that the absorption of visible/infrared light can be improved by using the upconversion luminescence performance of carbon dots,and the existence of carbon dots promotes the transfer of electrons/holes and reduces the recombination rate of electron-hole pairs.Some of these works have reduced CO2to more value-added CH4and CH3OH,but there are relatively few studies on the effect of carbon dots on product selectivity,and it is necessary to focus on the photocatalytic mechanism,so as to better improve the product selectivity of photocatalytic reduction of CO2 。
表2 Application of carbon dots composite in photocatalytic reduction of CO2;

Table 2 Application of carbon dot composites in photocatalytic reduction of CO2

Catalyst Synthesis methoda Product Production rate Selectivity The role of CDs Ref.
BBQ Thermal annealing HCOOH ─ ─ Charge separation 11
Au/CDs Reflux HCOOH, CH3COOH ─ ─ Separate electron-hole pairs 12
Au-CDs Irradiation HCOOH 4 μmol·g-1·h-1 ─ Separate electron-hole pairs 13
CPD/Bi4O5Br2 Hydrothermal CO and CH4 132.42 μmol·g-1·h-1 ─ Light absorption, Electron transfer 19
CQD-modified TiO2 NTs Hydrothermal CO and CH4 2.71, 0.71 μmol·g-1·h-1 ─ Light absorption, Electron transfer 39
CD@NH2-UiO-66 Hydrothermal CO 16.6 μmol·g-1·h-1 ─ Charge transfer and separation 42
C/TiO2 Calcination CO 0.86 μmol·g-1·h-1 ─ Electron transfer 43
mCD/CN Microwave CH3OH 13.9 ± 1.7 μmol·g-1·h-1 99.6%±0.2% Hole acceptor, Prevents the surface adsorption of methanol 45
Ni-PCD@TD-COF Pyrolysis CO 396.5 μmol·g-1·h-1 98% Light absorption 50
ZnO1-x /C FuAR CO 60.77 μmol·g-1·h-1 ─ Light absorption 52
OD-ZnO/C Calcination CO 118.8 μmol·g-1·h-1 ─ Light absorption 53
CQDs/UBW Hydrothermal CH4 0.899 μmol·g-1·h-1 ─ Light absorption, Electron transfer 54
NSCQDs/TiO2 Ultrasound CO and CH4 0.769, 1.153 μmol·g-1·h-1 ─ Light absorption, Electron transfer 55
CQDs/TiO2 Calcination CO 46.21 μmol·g-1·h-1 100% Electron transfer 56
CQDs/Cu2O Stirring C2H6 28.42μmol·g-1·h-1 ─ Electron transfer 57
CQDs/Cu2O Calcination CH3OH 55.7 μmol·g-1·h-1 ─ Hole transfer 58
CL@CQDs/Cu2O Hydrothermal CH3OH 99.6 μmol·g-1·h-1 ─ Light absorption, Electron transfer 59
CD/FAT Suspend, Annealing CH3OH 24.2 μmol·g-1·h-1 100% Hole acceptor 60
N-CQDs/Bi4MoO9 Ultrasound CO 3.24 μmol·g-1·h-1 ─ Electron transfer, CO2 adsorption 61
Zn-Bim-His-1
@GQDs		 Ultrasound, Stirring CH4 and CO 20.9, 3.7 μmol·g-1·h-1 85% Spatially separate electron-hole pairs, Light absorption 62
CQD/OCN-x Hydrothermal CH4 1.73 μmol·g-1·h-1 ─ Charge separation and transfer, CO2 adsorption 63
NCD/LDH/CN Hydrothermal CH4 25.69 μmol·g-1·h-1 99% Light absorption, Electron transfer, CO2 adsorption 64
CD-BVO/rGH one-step in situ
growth method		 CO and CH4 61.54, 21.47 μmol·g-1·h-1 ─ Migration and separation of photoexcited carriers, Light absorption 65
CD/TOH Ultrasound CH4 26.8 μmol·g-1·h-1 98% Light absorption, Electron transfer 66
CDs@CoOx Calcination CO 8.1 μmol·g-1·h-1 89.3% Electron transfer 67

asynthesis method of composite materials

5 Conclusion and prospect

As a metal-free zero-dimensional carbon-based nanomaterial,carbon dots have good photoinduced electron transfer ability and unique upconversion fluorescence properties,which can be used as photocatalysts independently.Can also be compounded with other photocatalysts to play a role in broadening the light absorption range and promoting charge transfer,improve the photocatalytic efficiency,and has great potential in the field of photocatalytic reduction of CO2.However,there are still some challenges to be overcome,and it is necessary to continue to study the structure-activity relationship between the structure and photocatalytic efficiency of carbon dots and carbon dots composite photocatalysts 。
(1)regulate and controlling that structure of the carbon dot by means of atom doping,surface modification,carbon source regulation and the like,and further enhancing the light absorption capacity,the photogenerated charge transfer property and the surface catalytic performance of the carbon dot;
(2)In-depth study of the photocatalytic mechanism of carbon dots and explore ways to improve product selectivity;
(3)obtain high added value product of C2+（(ethane,ethylene,acetic acid,propane,propylene,etc. )。

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