Co–N2 Bond Precisely Connects the Conduction Band and Valence Band of g‐C3N4/CoCo‐LDH to Enhance Photocatalytic CO2 Activity by High‐Efficiency S‐Scheme

Precise regulation of photogenic electron transfer path plays an important role in improving photocatalytic carbon dioxide reduction efficiency and product selectivity. Herein, under the guidance of density functional theory calculation, the interface chemical bond (CoN2 bond) at the atomic level is designed, and g‐C3N4/CoCo‐layered double hydroxide (LDH) heterostructure is manufactured. CoCo‐LDH with water oxidation ability and g‐C3N4 were combined to construct S‐scheme heterojunction with redox ability. The valence band and conduction band of g‐C3N4 and CoCo‐LDH are precisely connected by the interfacial CoN2 bond, which realizes the high‐speed transfer of electron transport. Despite the absence of cocatalyst, the heterojunction exhibits high water oxidation and carbon reduction capacity due to the precise regulation of CoN2 bonds. Theoretical calculations and experimental results show that the addition of CoCo‐LDH: reduces the oxidation overpotential of water to provide more H protons; regulates the delocalization charge of g‐C3N4; and reduces the energy barrier of the CO2 intermediate (*COOH) in the reduction half‐reaction. The results show that the selectivity of carbon‐based substances in the products was 100%, and the optimal CO yield was 71.39 μmol g−1 h−1, which is among the highest values of g‐C3N4‐based photocatalysts.

Precise regulation of photogenic electron transfer path plays an important role in improving photocatalytic carbon dioxide reduction efficiency and product selectivity.Herein, under the guidance of density functional theory calculation, the interface chemical bond (Co─N 2 bond) at the atomic level is designed, and g-C 3 N 4 /CoCo-layered double hydroxide (LDH) heterostructure is manufactured.CoCo-LDH with water oxidation ability and g-C 3 N 4 were combined to construct S-scheme heterojunction with redox ability.The valence band and conduction band of g-C 3 N 4 and CoCo-LDH are precisely connected by the interfacial Co─N 2 bond, which realizes the high-speed transfer of electron transport.Despite the absence of cocatalyst, the heterojunction exhibits high water oxidation and carbon reduction capacity due to the precise regulation of Co─N 2 bonds.Theoretical calculations and experimental results show that the addition of CoCo-LDH: reduces the oxidation overpotential of water to provide more H protons; regulates the delocalization charge of g-C 3 N 4 ; and reduces the energy barrier of the CO 2 intermediate (*COOH) in the reduction half-reaction.The results show that the selectivity of carbon-based substances in the products was 100%, and the optimal CO yield was 71.39 μmol g À1 h À1 , which is among the highest values of g-C 3 N 4 -based photocatalysts.
enhancing the water oxidation half-reaction can provide abundant H protons and improve the activity of PCR. [11]However, it is still difficult to achieve the goal of selectively generating carbon-based products because precious metals are reducing cocatalysts, which is also the cocatalyst to promote HER. [12][15] Therefore, it is worth exploring to introduce an oxidation-assisted catalyst in PCR to enhance the half-reaction of water oxidation.In recent years, the layered double hydroxide (LDH) has shown low overpotential in oxygen evolution reaction (OER) reaction as electrocatalyst, which is a potential water oxidizing material. [16]Among them, 2D ultrathin Co-based LDHs has been widely studied because of its abundant active sites, and is the most promising electrocatalyst for water oxidation. [17,18]owever, the final carbon products of PCR are carried out on the reductive half-reaction.How to introduce oxidation cocatalyst to achieve efficient photocatalytic reduction half-reaction has not been reported.
21] 2D carbon nitride (g-C 3 N 4 ) has a negative conduction potential, which can meet the thermodynamic conditions of CO 2 reduction, and it has negligible HER capacity. [5,22]Combining reducing g-C 3 N 4 with oxidizing LDH is a good scheme to achieve simultaneous regulation of carbon reduction and water oxidation reactions in PCR.However, electron transfer in heterojunctions is usually carried out in a type-II manner, resulting in reduced redox capacity in heterojunctions, [23] making it difficult to achieve complex PCR reactions.Recent studies have shown that the strong redox capacity of the system can be preserved by constructing step scheme (S-scheme) heterojunction. [24,25]he S-scheme heterojunction was first proposed by Prof. Yu group. [25]The system includes reducing photocatalyst (RP) and oxidizing photocatalyst (OP) which have different Fermi energy levels (E f ).As they come into contact with each other, the free electrons in the RP migrate to the OP until their Fermi levels flatline.Due to the transfer of electrons, a built-in electric field (BEF) is formed at the interface between the two. [26]When the heterojunction is illuminated by light, the photoelectrons in the conduction band of the OP are transferred to the valence band of the RP by an internal electric field.Because of these unique advantages, S-scheme heterojunctions have been used in many photocatalytic reactions. [17,27]For instance, Yu et al. explored the synthesis of TiO 2 /Bi 2 O 3 , TiO 2 @ZnIn 2 S 4 and other systems, showing excellent photocatalytic performance in H 2 O 2 production and PCR and other fields. [28,29]Wang et al. also synthesized C 3 N 4 /polydopamine S-scheme heterojunction which showed excellent photocatalytic performance for H 2 O 2 production. [30]lthough the construction of S-scheme heterojunctions can improve the performance of photocatalysis, the current S-scheme heterojunctions are mostly connected by van der Waals forces, resulting in a weak binding force between the two semiconductors and a lack of direct electron transfer channels, which restricts the development of efficient S-scheme photocatalysts. [26]ecently, by forming chemical bonds at the interface, electron transport channels can be established to achieve directional electron transport at the interface. [31]For example, Xia et al. constructed Bi 2 Sn 2 O 7 /BiOBr S-scheme heterojunction by enhancing the internal electric field by forming new Bi─O bonds from Bi and O atoms exposed by defects, which improved the photocatalytic N 2 reduction activity. [32]The establishment of atomic electron channels facilitates electron transfer.However, the construction of these chemical bonds does not directly connect the conduction band of the reducing material with the valence band of the oxidizing material, which results in that the chemical bond cannot effectively act as the electronic channel and cannot make the electrons transfer efficiently through the S-scheme.
In this study, we use density functional theory (DFT) to investigate the band structure composition of g-C 3 N 4 and CoCo-LDHs.On the basis of theoretical research, we construct N─Co bond in the heterojunction to precisely connect the valence and conduction bands of g-C 3 N 4 and CoCo-LDH, forming efficient S-scheme heterojunction, achieving ultrahigh CO 2 reduction activity, and 100% carbon-based products.In situ Fourier transform infrared spectroscopy (FTIR) shows that adding CoCo-LDH improved the adsorption and oxidation capacity of water, ensure the release of proton H species, and promoted the protonation of intermediates.DFT and experimental results show that the introduction of CoCo-LDH not only reduces the overpotential of water oxidation, but also selectively reduces the energy barrier of *COOH in the PCR process by regulating the microscopic charge structure.The BEF constructed by Fermi level flattening allows for electron transfer by means of the S-scheme, and ensure the maximum redox potential of g-C 3 N 4 /CoCo-LDH is retained.Without adding a sacrificial agent, 100% carbon-based products can be selectively generated in the photocatalytic CO 2 process, and the CO yield can reach 71.39 μmol g À1 h À1 .

Results and Discussion
2.1.Synthesis and Structural of g-C 3 N 4 /CoCo-LDH First, DFT calculation was carried out to understand the valence band and conduction band composition of carbon nitride and CoCo-LDH, and to explore the model of accurately constructing interfacial chemical bonds.The calculation results are shown in Figure 1. Figure 1a shows the optimized structures of g-C 3 N 4 , CoCo-LDH, and g-C 3 N 4 /CoCo-LDH from left to right.Density of states (DOS) calculated according to the structure is shown in Figure 1b-d.It can be seen from the figure that the valence band of carbon nitride is mainly contributed by element N, and the conduction band of CoCo-LDH is mainly contributed by element Co. [33] If N─Co bond is constructed at the interface, the valence band and conduction band of the two can be accurately connected.This will provide precise electron transfer channels for the S-scheme.More importantly, after the formation of heterojunction, the N─Co hybrid band becomes gold when it crosses the Fermi level (E f ), as shown in Figure 1d, indicating that a hybrid band of N and Co atoms is formed near E f , [34] which means that the formation of heterostructure is conducive to electron transfer at the interface.
The g-C 3 N 4 /CoCo-LDH heterojunction was prepared using g-C 3 N 4 loaded in situ with ZIF-67 and etched by Co 2þ .Figure 1e shows the detailed synthesis process of the sample.The surface of the g-C 3 N 4 was negatively charged (À50.3 mV) (Figure 2a) and rough (Figure S1a,b, Supporting Information)-which allows Co 2þ to be easily fixed to nitrogen (Tri-S-triazine). [35]The Zeta value of g-C 3 N 4 /ZIF-67 became positive (44.6 mV), which indicates that ZIF-67 grows on g-C 3 N 4 .X-ray diffraction (XRD) tests show that ZIF-67 cubes grew on the g-C 3 N 4 (Figure S2, Supporting Information).Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) tests show that ZIF-67 cubes grew on the g-C 3 N 4 nanosheet (Figure S1c,d, Supporting Information).
After Co 2þ etching, the diffraction peak of ZIF-67 in the sample disappeared (Figure 2b), but no new diffraction peaks appear, which may be due to a low CoCo-LDH content and low crystallinity. [36]In the enlarged red region in Figure 2c, it can be seen that the peak value of g-C 3 N 4 shifts after the formation of CoCo-LDH, which not only indicates that there is some material growing on the surface of g-C 3 N 4 , but also proves that there is a strong interaction between the two.After Co 2þ etching, the cubic ZIF-67 disappeared and became an ultrathin nanosheet, which was closely combined with the g-C 3 N 4 nanosheet (Figure 2d,e and S1e,f, Supporting Information).
The distribution of elements in Figure 2g shows that the elements C, N, Co, and O are uniformly distributed, indicating that they have a good contact area.The detailed process of Co 2þ etching was explored through time-resolved experiments (Figure S3-S5, Supporting Information).Unlike ZIF-67, which evolved into hollow CoCo-LDH alone, ZIF-67 transformed into ultrathin nanosheets with folds on g-C 3 N 4 , which may be due to the strong interaction between g-C 3 N 4 and CoCo-LDH, and the two were strongly connected through electrostatic interaction.The above research shows that through the process of electrostatic adsorption and in situ growth and transformation, the constructed g-C 3 N 4 and LDH have a strong interaction-formation of chemical bonds, which is conducive to the electron migration at the interface.Moreover, after the heterojunction formed, the higher specific surface area could have resulted in more active sites (Figure S6, Supporting Information).

Characterization of Interfacial Chemical Bonds
To further verify the chemical bond composition of the heterogeneous interface, X-ray photoelectron spectroscopy (XPS) analysis was performed.N 1s spectra were divided into 398.92,399.36, and 401.28 eV, which corresponds with N(C─N═C), N(N─(C) 3 ) groups, and N─H bonds.peak at 399.14 eV, which corresponds to the Co─N x bond in the g-C 3 N 4 /CoCo-LDH composite (Figure 3a). [37]Figure 3b shows peaks at 781.11 and 796.90 eV that belong to Co 3þ .The peaks at 783.11 and 799.31 eV belong to Co 2þ .Two peaks also appear at 781.41 and 789.21 eV, which belong to the peak of the Co─N x bond, which indicates the formation of a chemical bond between the two substances. [38]o investigate the interfacial chemical structure of g-C 3 N 4 / CoCo-LDH in more detail, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) tests were performed, as shown in Figure 3c-g 3c).The shift of the peak position of the absorption side band indicates that the chemical environment of Co species has changed. [39,40]The frontier peak position of CoCo-LDH and g-C To thoroughly determine the local structure of Co species, the Fourier transform EXAFS spectra were obtained through data transformation (Figure 3d-f and S7, S8, Supporting Information).In g-C 3 N 4 /CoCo-LDH, the first shell and the second shell are basically next to each other, and similar results can be seen from wavelet transform (WT) (Figure 3g), which proves that Co─O and Co─N bonds are formed.Since the Co─O was longer than that of Co─N in most cases: the first shell was a Co─N bond with a bond length of 2.06 Å and a coordination number of 2; the second shell was a Co─O bond with a length of 2.47 Å and a coordination number of 6.Compared to CoCo-LDH alone, the bond length (2 Å) of Co─O in g-C 3 N 4 /CoCo-LDH was longer.This stretching of the bond length may be due to an interface effect.Finally, as shown in Figure 3g and Table S1, Supporting Information, it can be seen from the WT that in addition to the Co─O bond, the heterojunction also produces the Co─N bond, which further proves the existence of the Co─N 2 bond.The formation of chemical bonds at the interface is very important for heterojunction: on the one hand, it can regulate the electronic structure and reduce the energy barrier of PCR; on the other hand, it can accelerate the transmission rate of the electronic interface like a highway.
Moreover, DFT was used to calculate the interface with and without the Co─N bond.The Δρ distribution contour diagram shows a large redistribution of the charge in the presence of Co─N bonds between the interfaces (Figure 4a,c).The result of <Δρ(z)> shows that when the Co─N 2 bond is constructed at the interface between g-C 3 N 4 and CoCo-LDH, the local charge is redistributed (Figure 4b,d).It can be seen from the calculation results that after the formation of Co-N 2 bond, the electron density at the interface is significantly enhanced, which can promote the adsorption and reaction of CO 2 at the interface. [41]he electronic structure of the material was further studied by solid-state electron spin resonance (ESR) (Figure 4e).After the introduction of LDH, the signal peak of ESR was significantly enhanced, and the increase of ESR signal confirmed the enhancement of delocalized electrons.This indicates that the construction of an interfacial chemical bond can regulate the delocalized charge of g-C 3 N 4 . [42]The study showed that the extended delocalization was beneficial to the separation of excitons in g-C 3 N 4 . [43]

Charge Separation Characteristic
Photoelectrochemical tests show that the formation of interfacial chemical bonds greatly improves the separation efficiency of photogenerated carriers (Figure 4f and S9-S11, Supporting Information).The average electron lifetime of the obtained g-C 3 N 4 /CoCo-LDH is shorter (Figure 4f ), indicating that the interfacial electrons can be rapidly transferred at the interface. [43]To clarify the nature of the enhancement of g-C 3 N 4 properties when adding CoCo-LDH, the change in the material's photoelectric properties was studied.To further study the effect of CoCo-LDH on the photogenerated electrons, transient fluorescence testing was done to study the lifetime of the photogenerated electrons.According to the electrochemical impedance of the material (Figure S9, Supporting Information), it can be seen that adding CoCo-LDH can reduce both the impedance radius of the material and the resistance of the photogenerated electron transmission.The photocurrent density of the material was studied.As can be seen from Figure S10, Supporting Information, compared with single g-C 3 N 4 , the addition of CoCo-LDH can significantly increase the photogenerated current density of the material, which indicates that CoCo-LDH can significantly increase the number of effective electrons.The fluorescence spectrum of the materials was tested, see Figure S11, Supporting Information.In order to explore the role of interfacial chemical bonds in electron transfer, photoluminescence tests were performed on mechanically mixed samples as reference samples, and it was found that the presence of N─Co bonds significantly reduced the fluorescence intensity, making the material to have excellent carrier separation efficiency.

Characterization of Band Structure and Electron Transfer Path
The electron transfer mechanism was studied next.The band gap width of CoCo-LDH and g-C 3 N 4 is 2.21 and 2.64 eV, respectively (Figure S12a-c, Supporting Information).The flat band potential of g-C 3 N 4 and CoCo-LDH were À0.89 and À0.10 V (Figure S12d, e, Supporting Information).The detailed energy band structure is shown in Figure S12f, Supporting Information.The reduction potential of CO 2 cannot be satisfied by the mode of electron transfer through type II heterojunction (Figure S13, Supporting Information).Fermi energy levels were obtained by means of the kelvin probe experiment, see Figure 5a.The surface work functions of g-C 3 N 4 , CoCo-LDH, and g-C 3 N 4 /CoCo-LDH were measured using a Kelvin probe, in order to study the photogenerated charge transfer pathway of the samples after heterojunction formation.As shown in Figure 5a, the contact potential differences (CPDs) between the Au probe and g-C 3 N 4 , CoCo-LDH, and g-CN/ LDH are À1390, À400, and À980 mV, respectively.The work functions of g-C 3 N 4 , CoCo-LDH, and g-CN/LDH can be calculated using the formula The value of φAu in the formula was 5.1 eV.Therefore, the Fermi energy levels (E f ) values of the g-C 3 N 4 , CoCo-LDH, and g-CN/CoCo-LDH samples are 3.71, 4.70, and 4.12 eV, respectively. [44]ccording to the test results, g-C 3 N 4 had a more negative E f than CoCo-LDH.Therefore, when the two form a heterojunction and make contact, the free electrons in the g-C 3 N 4 will be transferred to the CoCo-LDH until the equilibrium state is reached. [45]hus, when the two form a heterojunction and come into contact, the free electrons in g-C 3 N 4 flow into CoCo-LDH until equilibrium is reached. [45]A BEF is formed at the interface between the two, and the photogenerated electrons move through the S-scheme, driven by the BEF and the chemical bond at the interface (Figure 5b-d).The presence of internal electric field and chemical bonds at the interface can make the photogenerated electrons to move rapidly through the S-scheme at the interface, which greatly improves the separation efficiency of photogenerated electron holes.
Moreover, the S-scheme retains the maximum reduction and oxidation capacity of the system, and provides a guarantee for the efficient reaction of the two half-reactions in the PCR process.In the case of no reduction cocatalyst, the maximum retention of the redox capacity of the system is the premise to promote the efficient occurrence of complex redox reactions in CO 2 conversion reactions.Moreover, XPS test was used to study the variations in the position of the binding energy peak of different elements, which is consistent with the kelvin probe results (Figure S14, Supporting Information).The number of free radicals in different samples was obtained by an electron paramagnetic resonance (EPR) testing.As shown in Figure 5e,f, the superoxide free radicals and superoxide free radicals produced by g-CN/LDH were significantly enhanced.The potential for producing superoxide and hydroxyl radicals was À0.33 [46] and 1.99 V, [47] respectively.When the holes produced by CoCo-LDH are transferred to the valence band of g-C 3 N 4 and the electrons of g-C 3 N 4 are transferred to the conduction band of CoCo-LDH, the number of free radicals does not increase.Therefore, only when the material is transferred by way of the S-scheme, leaving the valence band of CoCo-LDH and the conduction band of g-C 3 N 4 can be beneficial to increase the freedom and quantity.The above studies show that the addition of CoCo-LDH can enhance the oxidation capacity of H 2 O and the reduction capacity of g-C 3 N 4 (Figure 5g), so as to achieve the regulation of reducing capacity without adding reducing cocatalyst.

Photocatalytic Performance of PCR
PCR was carried out on the prepared catalysts using water as the proton donor.With the single g-C 3 N 4 , there was a lower CO generation rate of 2.67 μmol g À1 h À1 (Figure 6a,b).After adding CoCo-LDH, the photocatalytic yield reached 71.39 μmol g À1 h À1 , that was 26.7 times that of g-C 3 N 4 alone, which was one of the highest reported on g-C 3 N 4 to date.For the oxidation products, a certain amount of oxygen is detected, as shown in Figure S15, Supporting Information, and it can be found that the oxygen production is improved after the formation of heterojunction.Moreover, the activity of g-CN/LDH-3 is 11.3 times higher than that of mechanically mixed g-CN/LDH-M (6.31 μmol g À1 h À1 ), as shown in Figure S16a, Supporting Information.The main product of reduction reaction was CO, and no H 2 was detected (Figure S16b, Supporting Information).
In order to explore the reasons for the high selectivity of carbon-based products, the reduction and oxidation capacity of materials to water was investigated by electrochemical method.As shown in Figure S17a, Supporting Information, the oxidation capacity of photocatalyst for water decomposition can be explained by the test results of OER. [48]Compared with g-C 3 N 4 , heterojunction exhibits a lower overpotential and a higher current density, which favorably proves that the presence of LDH can promote the oxidation of water, thus providing more H þ .Subsequently, the reducing capacity of the material to water was studied using HER. [49]As can be seen from Figure S17b, Supporting Information, the curves of g-C 3 N 4 , and heterojunction basically coincide, indicating that the addition of LDH cannot promote the occurrence of HER, so it is difficult to produce H 2 , so that H þ released by water oxidation can effectively participate in the photocatalytic CO 2 reduction process. [50]More importantly, the selectivity of CO in carbon products is nearly 100% (99.01%) (Figure S18, Supporting Information), which proves that the activation of water: generates 100% carbon products; achieves extremely high selectivity for CO in carbon products.The combination of different LDH with g-C 3 N 4 resulted in the formation of only CO in the product, see Figure S19, Supporting Information.This indicates that promoting the water half-reaction efficiency is an important way to improve the activity and selectivity of PCR reaction.
To prove the source of CO 2 , a number of control tests were performed (Figure 6c).The results showed that, in the absence of light, a catalyst or CO 2 gas, the CO produced was very low.This provides indirect confirmation that the CO product came from CO 2 gas, and the catalyst and light source are the necessary conditions.The stability of photocatalysts was then tested further, see Figure 6d.After 40 h cycle, the PCR of g-C 3 N 4 /CoCo-LDH did not decrease significantly, which proved that g-C 3 N 4 /CoCo-LDH was very stable in this system.In order to prove the stability of the material, XRD, SEM, and TEM tests were carried out.As shown in Figure S20, Supporting Information, compared with the XRD of the previous material, the peak pattern basically did not change.Through SEM and TEM, it can be seen that there is no agglomeration of materials compared with before.These characterizations further indicate that the material has excellent stability.
The first step of PCR is adsorption of CO 2 on the surface of the photocatalyst.Therefore, in order to investigate the adsorption capacity of the catalyst for CO 2 , CO 2 -temperature programmed desorption (TPD) testing was done on the catalyst.The results are shown in Figure 6e.For pure g-C 3 N 4 , there was an adsorption peak in the low-temperature region, which indicates the physical adsorption of CO 2 .After the CoCo-LDH composite, the peak moves to the high-temperature region, which indicates that the adsorption capacity of the material for CO 2 was enhanced.There was also a peak in the high-temperature region, which indicates the chemical adsorption between the material and CO 2 .The peak indicates chemical adsorption of CO 2 on the material surface, which played a role in the activation of CO 2 . [51]The results of CO 2 -TPD further prove that the addition of CoCo-LDH can regulate the electronic structure of carbon nitride, change the adsorption mode of CO 2 , and improve the adsorption strength of CO 2 on the material surface.
On the other hand, the formation of intermediate products in the PCR process involves the core of H proton. [52] Therefore, the interaction of catalyst and H 2 O molecules greatly affects the PCR activity.The contact angle experiment shows that the contact angle between the material and water decreases after the addition of CoCo-LDH, indicating that the hydrophilicity of g-C 3 N 4 /CoCo-LDH is better than g-C 3 N 4 , which is conducive to the reaction of the material with H 2 O to generate the required H proton (Figure S21, Supporting Information) Moreover, in order to obtain the desired H protons and hydroxyl radical, the H 2 O molecules need to be oxidized on the surface of the material. [53,54]he number of free radicals in different samples was obtained by EPR testing. [55]As shown in Figure 6f, the signal strength of hydroxyl radical generated by g-CN/LDH-3 is greatly enhanced compared with that of g-C 3 N 4 , indicating that the addition of different CoCo-LDH can promote the dissociation of water and thus provide more H protons.

Intermediate Active Species in PCR
In order to gain insight into the PCR process, in situ FTIR spectra were collected on g-C 3 N 4 and g-CN/LDH-3.CO 2 and H 2 O were preadsorbed on both samples and then subjected to irradiation for a period of time.The adsorption peak intensity of H 2 O (1670 cm À1 ) in g-CN/LDH-3 is significantly stronger than that in g-C 3 N 4 , that is conducive to the participation of water in the reaction.As depicted in Figure 7a-d, bidentate carbonate (b-CO 3 2À , 1650 cm À1 ) and monodentate carbonate (m-CO 3 2À , 1510, 1540, and 1560 cm À1 ) were detected on the g-CN/LDH-3 and g-C 3 N 4 after exposure to the humidified CO 2 atmosphere, which indicates chemisorption of CO 2 . [56]The vibrational peaks at 1340 and 1720 cm À1 were detected, which indicates the formation of *COOH on these samples, [57,58]

DFT Calculations of PCR
To understand the potential photocatalytic enhancement mechanism during PCR process, the intermediate transition barrier was studied using DFT calculations, see Figure 7e-g.The energy curve in Figure 7e shows the calculated energy barrier for the conversion of the four steps in the PCR process, which changed significantly after CoCo-LDH was added.The adsorption process from original CO 2 to *CO 2 is exothermic on the above photocatalyst.It can be seen that the heterojunction is more likely to adsorb CO 2 to form *CO 2 .Figure 7f,g show the atomic structures of g-C 3 N 4 and g-CN/LDH-3 for CO 2 adsorption and transformation, respectively.Subsequently, *CO 2 renders into *COOH, which dissociates into *CO.The formation energy barriers of *COOH on g-C 3 N 4 / CoCo-LDH was À0.218 eV (Figure 7c), lower than that of g-C 3 N 4 (À0.904eV).Therefore, in g-C 3 N 4 /CoCo-LDH, the *COOH intermediate can be easily desorbed on the surface of the material to form *CO. [59] *CO is an important intermediate for reducing CO 2 to CO, and its energy barrier on the g-C 3 N 4 /CoCo-LDH is much smaller than that of g-C 3 N 4 .A lower energy barrier indicates that it is easy to dissociate from the surface of g-C 3 N 4 / CoCo-LDH and accelerate the reaction.DFT and experimental results show that the addition of CoCo-LDH has many effects: on the one hand, it reduces the energy barrier of water decomposition and promotes the water reaction; on the other hand, it selectively regulates the reaction energy barrier of the reducing half-reaction, selectively promotes the reduction of carbon and avoids the generation of H 2 .

Conclusions
In summary, a g-C 3 N 4 /CoCo-LDH S-scheme catalyst with N-Co interfacial chemical bond for precise regulation of electron transfer was prepared.The XANES and EXAFS tests showed that the Co─N 2 bond is formed at the interface, which facilitates electron transfer at the interface.The BEF constructed by Fermi level flattening makes the electrons transfer by means of the S-scheme, the maximum redox capacity of the material is retained, which provides the maximum redox driving force.The selectivity of carbon-based substances in the product reached 100%.In the absence of sacrificial agent and cocatalyst, the ultrahigh-redox capacity of the catalyst itself makes the CO yield reach 71.39 μmol g À1 h À1 , which is among the highest values of g-C 3 N 4 -based photocatalysts.Theoretical calculations and experimental results showed that introducing CoCo-LDH can reduce the energy barrier of the key intermediate *COOH and the overpotential of water oxidation, but not the overpotential of H 2 .In this study, a redox heterojunction with interfacial chemical bond was established to realize the regulation of the redox capacity, which provided a new idea for avoiding the generation of an H 2 by-product in the PCR in water.
Synthesis of Ultrathin g-C 3 N 4 : First, a crucible with a cover was filled with 15.0 g of urea, and calcined at 500 °C for 2 h.The obtained faint yellow powder was placed into a crucible for a second round of calcination.The parameters for the second round of calcination were 400 °C for 2 h.After the two-step calcination process, ultrathin g-C 3 N 4 nanosheets were obtained.fluorescent meter (Hitachi F-4600, ex: 310 nm).The free radical test was analyzed by spin captured EPR (a JES-FA300) signals.Surface contact angles were measured using a contact angle meter (OCA20, DatPhysics Co. Ltd, Filderstadt, Germany).The reaction intermediates were tested by in situ FTIR spectroscopy (Bruker Invenio S).The optical properties of the catalyst were measured by UV-visible diffuse reflectance spectroscopy (DRS, Thermo 220).The Fermi level of the catalyst was measured through a Kelvin probe (KP020, KP Technologies LTD).
PCR Experiment: PCR experiments were performed in a closed reactor with a quartz glass window that provided illumination at 30 °C.The reaction catalyst was dispersed with 1 mg of ultrasonic water, evenly smeared on the quartz glass, and dried in vacuum.A xenon lamp with a power of 300 W was used as the light source.During the reaction, 3 mL of water was added to the reactor to provide protons.Before the photocatalytic reaction, the reactor was pumped into a vacuum, then filled with pure CO 2 (99.999%), then pumped into a vacuum again.This was repeated four times.The final pressure of CO 2 in the reaction vessel was 1 atmosphere.After a certain period of illumination, 1 mL of gas was extracted and the product was detected by means of GC-9560 gas chromatography (Huaai Chromatographic Analysis Technology Co. Ltd, Shanghai).

Figure 3
indicates another
. Co k-Edge XANES of standard Co foil, CoO and Co 3 O 4 , CoPc, and CoCo-LDH and g-C 3 N 4 /CoCo-LDH were referenced (Figure 3 N 4 /CoCo-LDH is between Co 3 O 4 and CoO, which indicates that the valence state and electronic structure of cobalt atoms in CoCo-LDH and g-C 3 N 4 /CoCo-LDH are between CoO and Co 3 O 4 .Furthermore, the Co species in the heterojunction shows negative movement compared to the Co species in the pure CoCo-LDH, which demonstrates that after the formation of g-C 3 N 4 /CoCo-LDH, a portion of the electrons transfer from N to Co, then the valence state of Co decreases.Similar results have been obtained in XPS.

Figure 2 .
Figure 2. a) Zeta potential of g-C 3 N 4 , g-C 3 N 4 /ZIF-67, and g-C 3 N 4 /CoCo-LDH.b) XRD patterns of g-C 3 N 4 and different proportions g-C 3 N 4 /LDH.c) A larger version of the red area in Figure S2a, Supporting Information.d) SEM images of g-C 3 N 4 /CoCo-LDH.e) TEM images of g-C 3 N 4 and g-C 3 N 4 /CoCo-LDH.f ) Enlarged TEM images of (e).g) EDX elemental mapping images of heterojunction.

Figure 4 .
Figure 4. a,c) The distribution of charge density difference Δρ, b,d) average charge density difference <Δρ(z)> without and with interfacial chemical bonding; the differential charge density Δρ shown by the isosurface bounding region of a) 0.0023 e Å À3 and c) 0.0607 e Å À3 .e) ESR of g-C 3 N 4 and g-CN/ LDH-3.f ) Transient fluorescence spectrum of g-C 3 N 4 and g-CN/LDH-3.

Figure 5 .
Figure 5. a) Kelvin probe test of g-C 3 N 4 , CoCo-LDH, and g-C 3 N 4 /LDH-3 composite in the dark; b-d) illustration of S-scheme charge transfer process; e,f ) EPR spectra of g-C 3 N 4 , CoCo-LDH, and g-CN/LDH-3 tested for a) •O 2À and b) •OH.g) Schematic diagram of g-C 3 N 4 and g-C 3 N 4 /CoCo-LDH photocatalytic CO 2 reduction.

Figure 6 .
Figure 6.a) Time curves of the CO yield with the prepared photocatalysts.b) CO production rate on different photocatalysts.c) Photocatalytic CO production rate of g-C 3 N 4 /CoCo-LDH under various conditions.d) Cycling measurements for g-C 3 N 4 /CoCo-LDH.e) CO 2 -TPD of g-C 3 N 4 and g-CN/ LDH-3 catalysts.f ) EPR spectra of g-C 3 N 4 and g-CN/LDH-3 tested for OH.
with *COOH being a likely reaction intermediate in PCR to CO.More importantly, compared with g-C 3 N 4 (Figure 7c,d), the *COOH peak in g-CN/LDH-3 is significantly enhanced, indicating that more proton hydrogen reacts with the intermediate in the previous step to generate *COOH (Figure 7a,b).The above results indicate that the addition of CoCo-LDH promotes the occurrence of water half-reaction and produces abundant H protons, thus providing a material basis for the formation of key intermediate *COOH.
Synthesis of g-C 3 N 4 /CoCo-LDH: First, 200 mg of ultrathin g-C 3 N 4 was dissolved in 50 mL deionized water.Different masses(10, 30, 50, and 60 mg) of Co(NO 3 ) 2 •6H 2 O were added to the above solution and stirred for 0.5 h. 10 mL CTAB (0.5 mg mL À1 ) was added to the above solution and stirred continuously for 3 min.The aqueous solutions containing 2-MIM of different masses(15, 45, 75, and 90 mg) were successively poured into the stirred solutions and stirring continued for 20 min.This was followed by washing with anhydrous methanol several times and drying at 65 °C.Next, 10 mg of above samples were dissolved in 20 mL mixed solution (water: ethanol = 2:1).This was subjected to ultrasound treatment.Different masses (10, 30, 50, and 60 mg) of Co(NO 3 ) 2 •6H 2 O were added to the above solution.The solution was transferred to the reactor for reaction at 120 °C for 2 h.The obtained sample is centrifuged, washed, and dried at 65 °C (In order to improve the readability of the text, the names of ultrathin g-C 3 N 4 /CoCo-LDH can be abbreviated as g-CN/LDH-1, g-CN/LDH-2, g-CN/LDH-3, and g-CN/LDH-4, respectively.).The prepared g-C 3 N 4 nanosheets and CoCo-LDH were put into 20 mL ethanol and stirred.The ethanol volatilized naturally, and the mixed sample was labeled as g-CN/LDH-M.Material Characterization: The crystal structure of the material was tested by XRD (Rigaku D/MAX 2500, 50 kV) with Cu Kα radiation.The microstructure of the catalyst was characterized by SEM (Hitachi S-4800) and TEM (JEOL JEM-2100).Material composition and elemental chemical states were investigated using XPS (PHI 1600 ESCA).The X-ray absorption spectra (XAS) including XANES and EXAFS of materials were collected the Singapore Synchrotron Light Source center with a Si (111) double crystal.Co foils, CoPc, and Co 3 O 4 are reference samples.The photoluminescence characteristics of the material were recorded by

Figure 7 .
Figure 7.In situ FTIR spectra showing adsorption of CO 2 on a,b) g-C 3 N 4 /CoCo-LDH and c,d) g-C 3 N 4 .e) The calculated intermediate energy (eV) in g-C 3 N 4 and g-C 3 N 4 /CoCo-LDH systems.Corresponding to the CO 2 conversion structure diagram in Figure 6c: f ) g-C 3 N 4 , and g) g-C 3 N 4 /CoCo-LDH.