Functional Regulation of ZnAl-LDHs and Mechanism of Photocatalytic Reduction of CO2: A DFT Study

Defect engineering and heteroatom doping can significantly enhance the activity of zinc-aluminum layered double hydroxides (ZnAl-LDHs) in photocatalytic CO2 reduction to fuel. However, the in-depth understanding of the associated intrinsic mechanisms is limited. Herein, we systematically investigated Zn vacancies (VZn), oxygen vacancies (VO), and Cu doping on the geometry and electronic structure of ZnAl-LDH using density functional theory (DFT). We also revealed the related reaction mechanism. The results reveal the concerted roles of VO, VZn, and doped-Cu facilitate the formation of the unsaturated metal complexes (Znδ+-VO and Cuδ+-VO). They can localize the charge density distribution, function as new active centers, and form the intermediate band. Simultaneously, the intermediate band of functionalized ZnAl-LDHs narrows the band gap and lowers the band edge location. Therefore, it can broaden the absorption range of light and improve the selectivity of CO. Additionally, the unsaturated metal complex lowers the Gibbs free energy barrier for effective CO2 activation by bringing the d-band center level closer to the Fermi level. The work provided guidance for developing LDH photocatalysts with high activity and selectivity.


Introduction
CO 2 gas is widely used as a common chemical in people's lives, and with mankind's dependence on fossil fuels, the huge emissions of CO 2 have led to many climate problems such as the "greenhouse effect" [1][2][3][4]. Recently, CO 2 gas capture and conversion technologies have become a hot topic of research, and solar-light-driven CO 2 conversion is a potential strategy for the production of sustainable fuels such as CO, HCOOH, and CH 4 . However, the efficiency is severely limited by the high inertness of the CO 2 molecule and the inherent limitations of semiconductor photocatalysts, i.e., fast electron-hole recombination and no reaction sites [4,5]. Layered double hydroxides (LDHs) [6,7] have received increasing attention as a potential two-dimensional (2D) photocatalyst because of their mutable metal cation composition and interlaminar anions, which allows for both electronic structure tuning and bandgap manipulation [8][9][10][11][12][13]. The general formula of LDHs is [M 2+ 1−x M 3+ x (OH) 2 ] x+ (A n− x/n )·zH 2 O, where M 2+ , M 3+ , and A n− represent divalent, trivalent cations, and charge-balancing anions, respectively. By adjusting the M 2+ and M 3+ cations, or the LDHs' size, the energy band structure [14][15][16] can be altered, which can improve the absorption of visible light, and generally does not introduce additional carrier recombination centers [17][18][19]. However, the photocatalytic efficacy of bulk LDHs is inhibited due to their limited exposed surface, poor light absorption, inefficient adsorption and separation of gas, and sluggish photo-induced charge transferability [4,13,15]. Various

Structure Construction of the Functionalized ZnAl-LDHs
The rationality and reliability of the structure of pristine ZnAl-LDH and the computation method are vital since they are the basis for the further calculation of the modified ZnAl-LDHs. The pristine ZnAl-LDH model is developed based on previous reports, and the optimized structure and lattice parameters are shown in Figure 1 [31][32][33][34]. A 3 × 3 × 1 supercell is adopted for ZnAl-LDH according to the position of the characteristic diffraction peaks (110) and (003) of LDHs powder X-ray diffraction (XRD) experimentally, [31] and the obtained unit lattice parameters are (a = b= 3.08 Å, c= 7.75 Å). The Zn:Al ratio of the supercell is 2:1, six Zn atoms surround one Al atom to form an octahedral structure, and the intercalated anions are nitrate anions. A 15 Å vacuum layer is constructed along the z-direction for eliminating the interactions between duplicating plates [35][36][37][38].
change versus the variation of the lattice parameters is plotted as shown in Figure S1a, suggesting the lowest energy point is the one with the experimental lattice parameter [34]. As shown in Figure 2c, the calculated cohesive and surface energies are −1.35 eV/atom and −1.34 eV/Å 2 , respectively, further confirming the surface stability of ZnAl-LDH. The AIMD results as plotted in Figure S1b show that the ZnAl-LDH can maintain its original structure at 600 K with the energy change within 0.001 eV, again indicating its excellent structural stability and validity of the model and calculation method.  [31] in blue.
It is found that the geometrical parameters of the NO3 -containing ZnAl-LDH are identical to those of the NO3 -free one, as presented in Figure 1. Additionally, since the surface is the site of the catalytic reduction occurrence, and the interlayer anions are not involved in the reaction, a single-layer NO3 -free hydrotalcite is used in this paper to reduce the calculation consumption. As shown in Figure 2, the structures of ZnAl-LDH, VZn-ZnAl-LDH and Cu-VZn-ZnAl-LDH were constructed and optimized, respectively. It should be noted that the presence of defects may cause enhanced surface polarization while the spin polarization is rectified by the spin polarization. The Cu atoms in Cu-VZn-ZnAl-LDH are thus corrected by the Hubbard correction (DFT + U) approach [41,42] for considering the interaction of highly correlated electrons of transition metals. Herein, we systematically study how Cu and VZn affect the electronic behavior, d-band center, and Gibbs free energy barrier of Cu-VZn-ZnAl-LDHs by the DFT + U method. The geometrical parameters are optimized by utilizing the PBE and PBE + vdW (DFT + D3) approaches and are compared with the available values in the experiment and theory [24,31,39,40] to screen out the most reasonable method and model. As shown in Table S1 (Supplementary Materials), the PBE method overestimates the lattice parameters since non-bonding interactions are not taken into account. With van der Waals correction included, the geometries given by PBE with the DFT + D3 method are in line with the experimental observations with a maximum difference of 0.028 Å in the Al-O bond, as shown in Table S1, and PBE + vdW is thus chosen as an appropriate method for the following calculations. To rationalize the stability of the ZnAl-LDH structure, the energy change versus the variation of the lattice parameters is plotted as shown in Figure S1a, suggesting the lowest energy point is the one with the experimental lattice parameter [34]. As shown in Figure 2c, the calculated cohesive and surface energies are −1.35 eV/atom and −1.34 eV/Å 2 , respectively, further confirming the surface stability of ZnAl-LDH. The AIMD results as plotted in Figure S1b show that the ZnAl-LDH can maintain its original structure at 600 K with the energy change within 0.001 eV, again indicating its excellent structural stability and validity of the model and calculation method. The optimized key lengths of ZnAl-LDH, VZn-ZnAl-LDH, and Cu-VZn-ZnAl-LDH are compared with the ones measured by the extended X-ray absorption fine structure spectra (EXAFS), as shown in Figure 2a [25,31]. Generally, the Zn-Zn, Zn-O, and Cu-O bond distances are very consistent with the experimental results with differences less than 0.02 Å, indicating the rationality of our structural models. The cohesion, surface, and defect formation energies of the three ZnAl-LDHs are calculated as shown in Figure 2b-d and are all increased in the order of Cu-VZn-ZnAl-LDH < VZn-ZnAl-LDH < ZnAl-LDH, demonstrating that the zinc and oxygen vacancies can stabilize ZnAl-LDH and doping Cu can further improve the stability of ZnAl-LDH. With the stepwise modification of ZnAl-LDH, the Zn-O bond is decreased and the crystal structure significantly distorts. The Zn-O bond length for VZn-ZnAl-LDH decreases to 2.06 Å compared to 2.08 Å for pristine ZnAl-LDH, the shorter bonds result in larger defect ranges and new active centers, facilitating the adsorption of CO2. The activity of photocatalytic processes is also improved by structural distortion since more active defect sites can be created on the surface. This is more obvious for Cu-VZn-ZnAl-LDH, where the Cu-O bond length on the defect site is further shortened to 1.97 Å. It could be because the 3d orbital of the Cu is active and in favor of transporting electrons, and thus, aside from defect sites in Cu-VZn-ZnAl-LDH, the largely exposed Cu atoms serve as another new active site for photocatalytic CO2 reduction. The unsaturated Zn δ+ and Cu δ+ are produced with the introduction of Zn vacancies and doping Cu, which caused structural distortions and the formation of Zn δ+ -VO/Cu δ+ -VO complexes, as shown in Figure 2a. These complexes act as the reaction's active sites to improve the catalytic activity. It is found that the geometrical parameters of the NO 3 − containing ZnAl-LDH are identical to those of the NO 3 − free one, as presented in Figure 1. Additionally, since the surface is the site of the catalytic reduction occurrence, and the interlayer anions are not involved in the reaction, a single-layer NO 3 − free hydrotalcite is used in this paper to reduce the calculation consumption. As shown in Figure 2, the structures of ZnAl-LDH, V Zn -ZnAl-LDH and Cu-V Zn -ZnAl-LDH were constructed and optimized, respectively. It should be noted that the presence of defects may cause enhanced surface polarization while the spin polarization is rectified by the spin polarization. The Cu atoms in Cu-V Zn -ZnAl-LDH are thus corrected by the Hubbard correction (DFT + U) approach [41,42] for considering the interaction of highly correlated electrons of transition metals. Herein, we systematically study how Cu and V Zn affect the electronic behavior, d-band center, and Gibbs free energy barrier of Cu-V Zn -ZnAl-LDHs by the DFT + U method.
The optimized key lengths of ZnAl-LDH, V Zn -ZnAl-LDH, and Cu-V Zn -ZnAl-LDH are compared with the ones measured by the extended X-ray absorption fine structure spectra (EXAFS), as shown in Figure 2a [25,31]. Generally, the Zn-Zn, Zn-O, and Cu-O bond distances are very consistent with the experimental results with differences less than 0.02 Å, indicating the rationality of our structural models. The cohesion, surface, and defect formation energies of the three ZnAl-LDHs are calculated as shown in Figure 2b-d and are all increased in the order of Cu-V Zn -ZnAl-LDH < V Zn -ZnAl-LDH < ZnAl-LDH, demonstrating that the zinc and oxygen vacancies can stabilize ZnAl-LDH and doping Cu can further improve the stability of ZnAl-LDH. With the stepwise modification of ZnAl-LDH, the Zn-O bond is decreased and the crystal structure significantly distorts. The Zn-O bond length for V Zn -ZnAl-LDH decreases to 2.06 Å compared to 2.08 Å for pristine ZnAl-LDH, the shorter bonds result in larger defect ranges and new active centers, facilitating the adsorption of CO 2 . The activity of photocatalytic processes is also improved by structural distortion since more active defect sites can be created on the surface. This is more obvious for Cu-V Zn -ZnAl-LDH, where the Cu-O bond length on the defect site is further shortened to 1.97 Å. It could be because the 3d orbital of the Cu is active and in favor of transporting electrons, and thus, aside from defect sites in Cu-V Zn -ZnAl-LDH, the largely exposed Cu atoms serve as another new active site for photocatalytic CO 2 reduction. The unsaturated Zn δ+ and Cu δ+ are produced with the introduction of Zn vacancies and doping Cu, which caused structural distortions and the formation of Zn δ+ -V O /Cu δ+ -V O complexes, as shown in Figure 2a. These complexes act as the reaction's active sites to improve the catalytic activity.

Charge Properties of Functionalized ZnAl-LDHs
Due to the importance of the surface charge distribution for the reaction, the charge properties of the three modified ZnAl-LDHs are calculated. As shown in Figure 3, by inserting Zn defects and dopant Cu to ZnAl-LDH, the charge density increases dramatically and the charge on the surface is mainly localized around Zn δ+ /Cu δ+ , facilitating the electron transport, and Cu δ+ is enriched with more charge than Zn δ+ . Furthermore, the unsaturated metal position can effectively accept electrons from the reactant molecules (CO 2 or H 2 O) and activate them to form relevant intermediates, promoting the charge exchange reaction.

Charge Properties of Functionalized ZnAl-LDHs
Due to the importance of the surface charge distribution for the reaction, the charge properties of the three modified ZnAl-LDHs are calculated. As shown in Figure 3, by inserting Zn defects and dopant Cu to ZnAl-LDH, the charge density increases dramatically and the charge on the surface is mainly localized around Zn δ+ /Cu δ+ , facilitating the electron transport, and Cu δ+ is enriched with more charge than Zn δ+ . Furthermore, the unsaturated metal position can effectively accept electrons from the reactant molecules (CO2 or H2O) and activate them to form relevant intermediates, promoting the charge exchange reaction. To analyze the charge and bonding situation, the electron localization function (ELF) is calculated on the (001) surface of three ZnAl-LDHs, as shown in Figure 4a, where red, green, and blue indicate high, moderate, and low charge densities, respectively, and the greater the connection between a metal and the OH group and the deeper the color, the better the capacity of a metal to gain or lose electrons. The charge density is gradually increased with the involvement of VO, VZn, and the transition metal Cu, indicating that the unsaturated metal atoms can concentrate the surface charge around them and absorb and activate reactants more effectively. The electron density difference (EDD) and Bader charges of the three ZnAl-LDHs as presented in Figure 4b show that the Zn atom in VZn-ZnAl-LDH loses −0.17 e and the Cu atom in Cu-VZn-ZnAl-LDH loses more electrons (−0.55 e), which are more likely to accumulate on the OH group near the defect. Simultaneously, To analyze the charge and bonding situation, the electron localization function (ELF) is calculated on the (001) surface of three ZnAl-LDHs, as shown in Figure 4a, where red, green, and blue indicate high, moderate, and low charge densities, respectively, and the greater the connection between a metal and the OH group and the deeper the color, the better the capacity of a metal to gain or lose electrons. The charge density is gradually increased with the involvement of V O , V Zn , and the transition metal Cu, indicating that the unsaturated metal atoms can concentrate the surface charge around them and absorb and activate reactants more effectively. The electron density difference (EDD) and Bader charges of the three ZnAl-LDHs as presented in Figure 4b show that the Zn atom in V Zn -ZnAl-LDH loses −0.17 e and the Cu atom in Cu-V Zn -ZnAl-LDH loses more electrons (−0.55 e), which are more likely to accumulate on the OH group near the defect. Simultaneously, the charge density of oxygen around the unsaturated metal on the defects is enhanced from +1.36 e to +1.42 e, and Cu δ+ (+1.45 e) is more pronounced than Zn δ+ , suggesting the more efficient charge transfer in defected ZnAl-LDHs. Importantly, the DFT calculations show that when V Zn

Photocatalytic Activity of Functionalized ZnAl-LDHs
To explore the photocatalytic activity of the functionalized ZnAl-LDHs, the energy band structure, charge density of the valance band maximum (VBM) and conduction band minimum (CBM), and density of states (DOS) of the relevant ZnAl-LDHs are calculated, as presented in Figure 5. The computed energy band gap (Eg) of ZnAl-LDH calculated by the HSE06 method is 3.20 eV, which is consistent with the experimental value of 3.18 eV. As shown in Figure 5a, the Eg dramatically drops to 2.67 and 1.69 eV for VZn-ZnAl-LDH and Cu-VZn-ZnAl-LDH compared to pristine ZnAl-LDH, suggesting the significant improvement of the efficiency of the available light usage and photoexcited electrons transported from VB to CB. This is due to the formation of the intermediate bands by the introduction of defects and transition metals. The intermediate bands split the original Eg into two parts, and the reduction of the energy band gap suppresses the recombination of photogenerated electrons/holes and thus promotes carrier separation and migration. The charge density of VBM and CBM in Figure 5b shows that the introduction of defects and Cu doping allows surface electrons to be more effectively concentrated around the unsaturated Zn δ+ /Cu δ+ after excitation, potentially promoting carrier migration and facilitating the surface catalytic reaction. The new intermediate band, known as defect energy level, is formed by Zn-4s and Cu-3d orbitals, respectively, as shown in Figure 5c. Interest-

Photocatalytic Activity of Functionalized ZnAl-LDHs
To explore the photocatalytic activity of the functionalized ZnAl-LDHs, the energy band structure, charge density of the valance band maximum (VBM) and conduction band minimum (CBM), and density of states (DOS) of the relevant ZnAl-LDHs are calculated, as presented in Figure 5. The computed energy band gap (Eg) of ZnAl-LDH calculated by the HSE06 method is 3.20 eV, which is consistent with the experimental value of 3.18 eV. As shown in Figure 5a, the Eg dramatically drops to 2.67 and 1.69 eV for V Zn -ZnAl-LDH and Cu-V Zn -ZnAl-LDH compared to pristine ZnAl-LDH, suggesting the significant improvement of the efficiency of the available light usage and photoexcited electrons transported from VB to CB. This is due to the formation of the intermediate bands by the introduction of defects and transition metals. The intermediate bands split the original Eg into two parts, and the reduction of the energy band gap suppresses the recombination of photogenerated electrons/holes and thus promotes carrier separation and migration. The charge density of VBM and CBM in Figure 5b shows that the introduction of defects and Cu doping allows surface electrons to be more effectively concentrated around the unsaturated Zn δ+ /Cu δ+ after excitation, potentially promoting carrier migration and facilitating the surface catalytic reaction. The new intermediate band, known as defect energy level, is formed by Zn-4s and Cu-3d orbitals, respectively, as shown in Figure 5c. Interestingly, it is near the Fermi level in the forbidden band of V Zn -ZnAl-LDH and Cu-V Zn -ZnAl-LDH, which favors the acceptance of the electrons excited on VB and then transits to the Zn-3d orbital on CB. The existence of the defect level favors the reduction of excitation energy and improves the efficiency of electron transfer. The work functions (WF) of the three ZnAl-LDHs are computed to explore the charge transfer. As shown in Figure 6a, the WF value gradually increases with the subsequent addition of VZn, VO, and Cu to ZnAl-LDH, and electrons would flow from ZnAl-LDH to the region containing unsaturated Zn or Cu until the Fermi energy level remains constant. The difference in work functions between ZnAl-LDH and VZn-ZnAl-LDH/Cu-VZn-ZnAl-  The work functions (WF) of the three ZnAl-LDHs are computed to explore the charge transfer. As shown in Figure 6a, the WF value gradually increases with the subsequent addition of V Zn , V O , and Cu to ZnAl-LDH, and electrons would flow from ZnAl-LDH to the region containing unsaturated Zn or Cu until the Fermi energy level remains constant. The difference in work functions between ZnAl-LDH and V Zn -ZnAl-LDH/Cu-V Zn -ZnAl-LDH results in a built-in electric field on the perfect surface defect ZnAl-LDH, which tends to enrich more electrons on the surface and thus promotes the surface catalytic reaction, as presented in Figure 6b.

Mechanism of Functionalized ZnAl-LDHs Photocatalytic Reduction of CO2
To further understand the photocatalytic CO2 reduction reaction and its competitive water-splitting reaction, the adsorption energies of CO2, H2O, and the product CO on the three ZnAl-LDHs are calculated with the corresponding structures, as shown in Figure 7. The adsorption energies for Cu-VZn-ZnAl-LDH absorbing CO2 (−1.81 eV) and H2O (−1.21 eV) are the highest, but the lowest for absorbing CO (−0.14 eV), followed by VZn-ZnAl-LDH. The oxygen vacancies and zinc vacancies on the ZnAl-LDH surface can effectively capture CO2 and H2O molecules, which may be due to the electron-rich effect of the 4s orbital of Zn. In addition, the doping of Cu on top of oxygen vacancies and zinc vacancies forms unsaturated Cu δ+ , which can capture H2O and CO2 molecules more efficiently due to the richer electrons in the 3d orbital of Cu. This suggests that defects and Cu doping can significantly increase the activities of adsorbing reactants and quick desorbing products from the surface, thus facilitating the reduction reaction.

Mechanism of Functionalized ZnAl-LDHs Photocatalytic Reduction of CO 2
To further understand the photocatalytic CO 2 reduction reaction and its competitive water-splitting reaction, the adsorption energies of CO 2 , H 2 O, and the product CO on the three ZnAl-LDHs are calculated with the corresponding structures, as shown in Figure 7. The adsorption energies for Cu-V Zn -ZnAl-LDH absorbing CO 2 (−1.81 eV) and H 2 O (−1.21 eV) are the highest, but the lowest for absorbing CO (−0.14 eV), followed by V Zn -ZnAl-LDH. The oxygen vacancies and zinc vacancies on the ZnAl-LDH surface can effectively capture CO 2 and H 2 O molecules, which may be due to the electron-rich effect of the 4s orbital of Zn. In addition, the doping of Cu on top of oxygen vacancies and zinc vacancies forms unsaturated Cu δ+ , which can capture H 2 O and CO 2 molecules more efficiently due to the richer electrons in the 3d orbital of Cu. This suggests that defects and Cu doping can significantly increase the activities of adsorbing reactants and quick desorbing products from the surface, thus facilitating the reduction reaction. It is well known that photocatalytic reduction of CO2 and water splitting are a group of competitive reactions. To screen out ZnAl-LDHs photocatalysts with high selectivity to CO2PR, the Gibbs free energy diagrams of photocatalytic CO2 and water splitting on ZnAl-LDH, VZn-ZnAl-LDH, and Cu-VZn-Zn-Al-LDH as well as the associated thermodynamic reaction pathways are calculated, as shown in Figure 8a-c. The *CO and *H2 formation barriers with values of 1.25 and 1.07 eV, respectively, are the lowest and highest for Cu-VZn-ZnAl-LDH among the three ZnAl-LDHs, suggesting its high selectivity for CO2 reduction to CO and inhibition of H2 evolution. For VZn-ZnAl-LDH, the barrier for CO2 conversion is also very low (ΔG = 1.41 eV); thus, CO is the primary reaction product for both defective ZnAl-LDHs. The results conclusively show that Cu δ+ or Zn δ+ in ZnAl-LDHs can govern the strengths of CO2 and H2O adsorption as well as those intermediates generated by their photoreduction, and hence determine the selectivity of photocatalytic CO2 reduction. It is well known that photocatalytic reduction of CO 2 and water splitting are a group of competitive reactions. To screen out ZnAl-LDHs photocatalysts with high selectivity to CO 2 PR, the Gibbs free energy diagrams of photocatalytic CO 2 and water splitting on ZnAl-LDH, V Zn -ZnAl-LDH, and Cu-V Zn -Zn-Al-LDH as well as the associated thermodynamic reaction pathways are calculated, as shown in Figure 8a-c. The *CO and *H 2 formation barriers with values of 1.25 and 1.07 eV, respectively, are the lowest and highest for Cu-V Zn -ZnAl-LDH among the three ZnAl-LDHs, suggesting its high selectivity for CO 2 reduction to CO and inhibition of H 2 evolution. For V Zn -ZnAl-LDH, the barrier for CO 2 conversion is also very low (∆G = 1.41 eV); thus, CO is the primary reaction product for both defective ZnAl-LDHs. The results conclusively show that Cu δ+ or Zn δ+ in ZnAl-LDHs can govern the strengths of CO 2 and H 2 O adsorption as well as those intermediates generated by their photoreduction, and hence determine the selectivity of photocatalytic CO 2 reduction. The d-band center (εd) theory [34][35][36][37][38][39] can be employed to clarify the reasons why Cu δ+ and Zn δ+ cations can strongly control the adsorption of CO2 and H2O and their photocatalytic conversion. When εd moves closer to the Fermi level, the influences on the adsorbate become stronger, facilitating electron injection from the surface to adsorbed gases. As illustrated in Figure 9a, by the introduction of VZn, VO, and Cu, respectively, the εd gradually approaches the Fermi level from −4.40 to −2.31 eV, indicating that the electrondonating capacity is dramatically enhanced. Therefore, the reaction energy barriers are The d-band center (ε d ) theory [34][35][36][37][38][39] can be employed to clarify the reasons why Cu δ+ and Zn δ+ cations can strongly control the adsorption of CO 2 and H 2 O and their photocatalytic conversion. When ε d moves closer to the Fermi level, the influences on the adsorbate become stronger, facilitating electron injection from the surface to adsorbed gases. As illustrated in Figure 9a, by the introduction of V Zn , V O , and Cu, respectively, the εd gradually approaches the Fermi level from −4.40 to −2.31 eV, indicating that the electron-donating capacity is dramatically enhanced. Therefore, the reaction energy barriers are reduced and the CO 2 adsorption and activation ability are significantly improved due to the presence of unsaturated Cu δ+ /Zn δ+ . olecules 2023, 28, 738 11 of 17 reduced and the CO2 adsorption and activation ability are significantly improved due to the presence of unsaturated Cu δ+ /Zn δ+ . It is well known that semiconductors as CO2 reduction photocatalysts must have suitable band edge positions to match the reduction potential of CO2/hydrocarbons. The CBM of photocatalysts is more negative, the driving force behind the photocatalytic reaction is stronger and thus the capability of the photocatalyst to reduce CO2 will be stronger, as presented in Figure 9b. The standard redox potentials of CBM for ZnAl-LDHs, VZn-ZnAl-LDH, and Cu-VZn-ZnAl-LDH are −0.75, −0.86, and −0.93 V, respectively, and still more than that of CO2/CO (−0.53 V), which is sufficient to drive photocatalytic CO2PR. Therefore, all three ZnAl-LDHs are evidenced to be thermodynamically favorable for photocatalytic CO2 reduction to CO. Importantly, when VZn and Cu are introduced to ZnAl-LDH in turn, the CBM grows more and more negative from −0.75, −0.86, and −0.93 V, indicating the strengthened capacity for CO2 reduction in the order of ZnAl-LDH < VZn-ZnAl-LDH < Cu-VZn-ZnAl-LDH. The heightened performance of ZnAl-LDHs could owe to the following reasons. Firstly, the coordinative effects caused by oxygen and zinc defects in VZn-ZnAl-LDH have been verified by the fact that it has a significantly improved CBM potential compared to pristine ZnAl-LDH. The introduction of VO and VZn increases the electron aggregation and transfer efficiency of Zn atoms in defected ZnAl-LDH, and the resulting negative CBM potential dramatically enhances the reduction, and thus the photocatalytic ability. Secondly, with copper incorporation, Cu-VZn-ZnAl-LDH exhibits the smallest Eg, which is capable of significantly improving the transmission ability of It is well known that semiconductors as CO 2 reduction photocatalysts must have suitable band edge positions to match the reduction potential of CO 2 /hydrocarbons. The CBM of photocatalysts is more negative, the driving force behind the photocatalytic reaction is stronger and thus the capability of the photocatalyst to reduce CO 2 will be stronger, as presented in Figure 9b. The standard redox potentials of CBM for ZnAl-LDHs, V Zn -ZnAl-LDH, and Cu-V Zn -ZnAl-LDH are −0.75, −0.86, and −0.93 V, respectively, and still more than that of CO 2 /CO (−0.53 V), which is sufficient to drive photocatalytic CO 2 PR. Therefore, all three ZnAl-LDHs are evidenced to be thermodynamically favorable for photocatalytic CO 2 reduction to CO. Importantly, when V Zn and Cu are introduced to ZnAl-LDH in turn, the CBM grows more and more negative from −0.75, −0.86, and −0.93 V, indicating the strengthened capacity for CO 2 reduction in the order of ZnAl-LDH < V Zn -ZnAl-LDH < Cu-V Zn -ZnAl-LDH. The heightened performance of ZnAl-LDHs could owe to the following reasons. Firstly, the coordinative effects caused by oxygen and zinc defects in V Zn -ZnAl-LDH have been verified by the fact that it has a significantly improved CBM potential compared to pristine ZnAl-LDH. The introduction of V O and V Zn increases the electron aggregation and transfer efficiency of Zn atoms in defected ZnAl-LDH, and the resulting negative CBM potential dramatically enhances the reduction, and thus the photocatalytic ability. Secondly, with copper incorporation, Cu-V Zn -ZnAl-LDH exhibits the smallest Eg, which is capable of significantly improving the transmission ability of photogenerated electrons from VBM to CBM, and the energy of CO 2 adsorption brought on by the high dispersion of Cu-3d atomic electron orbits is further increased to facilitate the photocatalytic reaction. During the reaction, defects and elemental doping result in the formation of unsaturated metals Zn δ+ or Cu δ+ serving as the active center of the reaction, which facilitates the charge transfer from the ZnAl-LDH surface to CO 2 to form COOH* intermediates and ultimately remove water to form CO, as illustrated in Figure 10. Our work reveals the intrinsic mechanisms of the coordinative effects of co-doped transition metal and oxygen defects, as well as metal defects, on effective photocatalytic CO 2 reduction. This provides a new opportunity for the design of novel LDHs photocatalysts for the conversion of CO 2 .
Molecules 2023, 28, 738 12 of 17 photogenerated electrons from VBM to CBM, and the energy of CO2 adsorption brought on by the high dispersion of Cu-3d atomic electron orbits is further increased to facilitate the photocatalytic reaction. During the reaction, defects and elemental doping result in the formation of unsaturated metals Zn δ+ or Cu δ+ serving as the active center of the reaction, which facilitates the charge transfer from the ZnAl-LDH surface to CO2 to form COOH* intermediates and ultimately remove water to form CO, as illustrated in Figure 10. Our work reveals the intrinsic mechanisms of the coordinative effects of co-doped transition metal and oxygen defects, as well as metal defects, on effective photocatalytic CO2 reduction. This provides a new opportunity for the design of novel LDHs photocatalysts for the conversion of CO2. Figure 10. Illustration of the photocatalytic reduction mechanism of CO2. Atomic labels Al, Zn, Cu, C, O, and H are pink, green, blue, gray, red, and white, respectively.

Computational Method
All calculations are based on density functional theory utilizing the Vienna Ab initio Simulation Package (VASP) [43][44][45][46]. The exchange-correlation energy is evaluated utilizing Perdew-Burke-Ernzerhof (PBE) functional in the Generalized Gradient Approximation (GGA), and the core electron interaction is substituted with the Projector Augmented Wave (PAW) pseudopotential [47][48][49]. The van der Waals interaction is corrected by the DFT-D3 method [50][51][52] of Grimme and the spin polarization is considered [53,54]. To account for the strong electron correlation properties of Cu, the DFT + U methods [55] were employed with a U-value of 3.60 eV [56] for the Cu 3d state to consider the strong field Coulomb interaction of Cu local electrons [25,56,57,41,42]. The 4 × 4 × 1 Monkhorst-Pack mesh is employed to sample the k points in the Brillouin zone for structural relaxation, and the Plane Wave basis function is utilized to expand at 450 eV. To eliminate interaction between periodic units, the thickness of the vacuum layer is set to 15 Å, and a 3 × 3 × 1 periodic cell is established to avoid lateral interaction [58]. Calculations were carried out until the energy and force converged within 10 −6 eV and 0.01 eV·Å −1 , respectively [31,58]. To achieve the rapid convergence of the self-consistent field iterations, 0.1 eV Fermi smearing and Pulay mixing were utilized [39,40]. The energy band calculations of all three ZnAl-LDHs were performed using the hybrid functional HSE06 method [19]. The adsorption energies of adsorbates were determined by employing the formula [59,60]: Figure 10. Illustration of the photocatalytic reduction mechanism of CO 2 . Atomic labels Al, Zn, Cu, C, O, and H are pink, green, blue, gray, red, and white, respectively.

Computational Method
All calculations are based on density functional theory utilizing the Vienna Ab initio Simulation Package (VASP) [43][44][45][46]. The exchange-correlation energy is evaluated utilizing Perdew-Burke-Ernzerhof (PBE) functional in the Generalized Gradient Approximation (GGA), and the core electron interaction is substituted with the Projector Augmented Wave (PAW) pseudopotential [47][48][49]. The van der Waals interaction is corrected by the DFT-D3 method [50][51][52] of Grimme and the spin polarization is considered [53,54]. To account for the strong electron correlation properties of Cu, the DFT + U methods [55] were employed with a U-value of 3.60 eV [56] for the Cu 3d state to consider the strong field Coulomb interaction of Cu local electrons [25,41,42,56,57]. The 4 × 4 × 1 Monkhorst-Pack mesh is employed to sample the k points in the Brillouin zone for structural relaxation, and the Plane Wave basis function is utilized to expand at 450 eV. To eliminate interaction between periodic units, the thickness of the vacuum layer is set to 15 Å, and a 3 × 3 × 1 periodic cell is established to avoid lateral interaction [58]. Calculations were carried out until the energy and force converged within 10 −6 eV and 0.01 eV · Å −1 , respectively [31,58]. To achieve the rapid convergence of the self-consistent field iterations, 0.1 eV Fermi smearing and Pulay mixing were utilized [39,40]. The energy band calculations of all three ZnAl-LDHs were performed using the hybrid functional HSE06 method [19]. The adsorption energies of adsorbates were determined by employing the formula [59,60]: where E (a−s) , E (s) , and E (a) are the energies of the total adsorbate-substrate systems, isolated adsorbate, and isolated substrate, respectively. For each step of the reaction, the Gibbs free energy (G) [61,62] is computed using the following formula: where E Total represents total electron and ion energy, C P dT represents enthalpy temperature correction, E ZPE represents zero-point vibration energy correction, TS represents entropy contribution, T represents temperature (298.15 K), and S represents entropy [63,64]. The standard hydrogen electrode (SHE) model assumes that each step of the process will experience simultaneous proton transfer, and the electron pair will depend on the applied potential. Thus, the change in free energy concerning the initial state of CO 2 gas above the empty surface can be described as follows: where * represents the appropriate adsorption state on the catalyst's surface, and "e" represents the fundamental charge. Herein, the ZnAl-LDH system is subjected to ab initio molecular dynamics (AIMD) simulations utilizing a Nose-Hoover temperature-controlled NVT regularized synthesis with a time step of 1 fs. The initial configuration for the simulations is the DFT-optimized ZnAl-LDH, and the AIMD simulation time is increased to 10 ps at a temperature of 600 K to guarantee that the loaded system reaches the equilibrium state [65].

Conclusions
This work designs defect-rich ZnAl-LDHs photocatalysts by introducing oxygen and zinc defects and the incorporation of the transition metal copper. The stability, electronic structure, energy band structure, and possible usage in catalytic CO 2 and their corresponding mechanisms have been examined by employing the DFT methods. The origins of the intriguing coordinative effects of defects and transition state metal co-doping of ZnAl-LDHs on the improvement of photocatalytic performances have been emphasized. The addition of V Zn , V O , and Cu to ZnAl-LDH significantly reduces the energy band gap and improves light absorption, and leads to the formation of the corresponding intermediate band (defective energy level/doping energy level), which can effectively suppress the recombination for creating photogenerated of electrons and holes and improve carrier mobility, facilitating electron transmission to the surface to take part in the catalytic reduction reaction. Furthermore, the CBM is lowered and the enhanced reduction capacity can drive a more efficient photocatalytic reduction of CO 2 to CO.
Upon adding V Zn and V O to ZnAl-LDH, the thermal stability is improved and the Zn δ+ -V O active site is formed since the lattice is distorted. The stability of the defect structure is further improved after introducing Cu, and the Cu δ+ -V O complex is formed, serving as the active site, which could capture the photogenerated electrons more effectively, enhancing the bonding effect and facilitating the bonding with the reactant molecules. The emergence of Zn δ+ and Cu δ+ brings the energy level of the d-band center closer to the Fermi level, improving the adsorption and activation capacity of CO 2 molecules and lowering the reaction barrier, which aids in the creation of intermediates. At the same time, the low-valent metal Zn δ+ and Cu δ+ can enhance CO selectivity by reducing CO adsorption and suppressing the HER reaction. Meanwhile, theoretical calculations show that the unsaturated complexes Zn δ+ -V O and Cu δ+ -V O form after the introduction of oxygen vacancies, metal vacancies, and metal Cu, in that order, and that their Zn-O and Cu-O bond lengths are consistent with the experimental values. DOS shows that the characteristic peaks of V Zn -ZnAl-LDH and Cu-V Zn -ZnAl-LDH in XPS correspond to Zn 4s orbitals and Cu 3d orbitals, respectively, which are electron-rich states that can be used as active sites for the reaction, effectively reducing CO 2 to CO.
Defect engineering and element doping can significantly lower ZnAl-LDH charge carrier transport resistance, leading to more active sites and increased photocatalytic activity. Compared to pure ZnAl-LDH, V Zn -ZnAl-LDH can efficiently narrow the band gap and absorb light at longer wavelengths attributed to the V O and V Zn , and the more negative CBM position can improve the selectivity for CO. Cu-V Zn -ZnAl-LDH has good thermal stability, the easiest reduction of CO 2 to reduce CO, and efficient inhibition of hydrogen release, in addition to more effective collection of more visible light and selective enhancement benefits. This work explored the mechanism of CO 2 PR reaction by functionalized ZnAl-LDHs, which provides theoretical guidance for the design of novel LDH photocatalysts.

Conflicts of Interest:
The authors state that they have no known competing financial interests or personal relationships that do not affect the work reported in this paper.
Sample Availability: Samples of the compounds are available from the authors.