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Article

Modulation of Photocatalytic CO2 Reduction by np Codoping Engineering of Single-Atom Catalysts

1
School of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo 255000, China
2
Hunan Key Laboratory of Micro-Nano Energy Materials and Devices, Xiangtan University, Xiangtan 411105, China
*
Authors to whom correspondence should be addressed.
Nanomaterials 2024, 14(14), 1183; https://doi.org/10.3390/nano14141183
Submission received: 6 June 2024 / Revised: 4 July 2024 / Accepted: 9 July 2024 / Published: 11 July 2024
(This article belongs to the Special Issue Advances in Stimuli-Responsive Nanomaterials: 2nd Edition)

Abstract

:
Transition metal (TM) single-atom catalysts (SACs) have been widely applied in photocatalytic CO2 reduction. In this work, np codoping engineering is introduced to account for the modulation of photocatalytic CO2 reduction on a two-dimensional (2D) bismuth-oxyhalide-based cathode by using first-principles calculation. np codoping is established via the Coulomb interactions between the negatively charged TM SACs and the positively charged Cl vacancy (VCl) in the dopant–defect pairs. Based on the formation energy of charged defects, neutral dopant–defect pairs for the Fe, Co, and Ni SACs (PTM0) and the −1e charge state of the Cu SAC-based pair (PCu−1) are stable. The electrostatic attraction of the np codoping strengthens the stability and solubility of TM SACs by neutralizing the oppositely charged VCl defect and TM dopant. The np codoping stabilizes the electron accumulation around the TM SACs. Accumulated electrons modify the d-orbital alignment and shift the d-band center toward the Fermi level, enhancing the reducing capacity of TM SACs based on the d-band theory. Besides the electrostatic attraction of the np codoping, the PCu−1 also accumulates additional electrons surrounding Cu SACs and forms a half-occupied dx2y2 state, which further upshifts the d-band center and improves photocatalytic CO2 reduction. The metastability of Cl multivacancies limits the concentration of the np pairs with Cl multivacancies (PTM@nCl (n > 1)). Positively charged centers around the PTM@nCl (n > 1) hinders the CO2 reduction by shielding the charge transfer to the CO2 molecule.

1. Introduction

Artificial photocatalytic CO2 reduction is an intriguing research area aiming to reduce fossil consumption and mitigate the greenhouse effect [1,2]. Photocatalysts are the crucial materials for readily converting CO2 into environmentally friendly fuels via photolysis using abundant solar energy [3,4,5]. With maximal atom-utilization efficiency, transition metal (TM) single-atom photocatalysts (SACs) exhibit excellent catalytic performance comparable to precious metals [6,7,8,9,10,11]. Due to its sufficient optical absorption and adequate activation centers, two-dimensional (2D) bismuth oxyhalide BiOX (X = F, Cl, Br, I) is an outstanding host to anchor the TM SACs photocatalyst and accelerate the CO2 reduction [12,13,14,15,16]. For TM-doped bismuth oxyhalide, the dopant of TM SACs can form impurity levels in the forbidden band, promoting the generation and separation of photogenerated carriers [17,18]. However, the most outstanding advantage is that doped TM SACs regulate the surface state, which enhances the CO2 adsorption by promoting the charge transfer between the activation centers and absorbed CO2 molecules [17,18,19]. For example, isolated Cu SACs on BiOBr (Cu@BiOBr) establish a strong built-in electric field which serves as an electron trap to facilitate charge transfer and stabilize charge carriers. As a result, 0.5% Cu@BiOBr has a higher CO2 absorption uptake (2.7 cm3 g−1) than BiOBr (2.3 cm3 g−1) [20]. In Co-SAC-doped Bi3O4Br, the isolation of Co2+ by replacing Bi3+ enables Co@Bi3O4Br layers to be more negatively charged, improving the CO2 adsorption and stabilizing the *COOH intermediate on the surface [17]. Charge localization induced by Fe SAC doping strengthens CO2 bonding strengths and improves the CO2 absorptive capacity in both porous Bi5O7I Micro-flower and Bi4O5I2 [21,22].
Some issues remain to be addressed in understanding the CO2 photolysis by the TM SACs on BiOX. The defect configurations around the TM SACs are ambiguous, especially whether halogen atoms are present or not near the TM impurity sites [17,18,19,20,21]. Both experimental and theoretical works confirm that the [Bi2O2]2+ layer is sandwiched by two X layers in BiOX [23,24,25,26,27,28,29]. However, the unlocked bismuth (Bi) surface without a halogen covering is usually used to simulate photocatalytic CO2 reduction around the TM SACs [17,18,19,20]. This divergence highlights the necessity of assessing the effects of halogen vacancy (Vhalogen) on the microstructures and reduction performance around the TM SACs. Vhalogen usually acts as an n-type defect, whereas TM SACs exhibit p-type characteristics and form negatively charged centers to promote the reduction reaction [23,24].
The np codoping concept is established based on the electrostatic attraction between the n- and p-type dopants (or defects) with opposite charge states, which affects the ionization, solubility, and charge transfer of the doped semiconductors [30,31,32,33]. For example, the electrostatic attraction within the np pair enhances both thermodynamic and kinetic solubilities, creating tunable intermediate bands to effectively narrow the band gap and enhance the visible-light photoactivity of TiO2 [32]. The np pairs limit the applications of Ga2O3 by affecting the dopant ionization [33]. On BiOX, np codoping can be established via the electrostatic interactions in the combination of p-type TM dopants and n-type Vhalogen defects. The effects of np codoping on the stability and reducing capacity of TM SACs must be determined. For TM SACs, the d-band theory is universally recognized as a means of evaluating the reducing capacity [34,35]. Consequently, the dependence of the orbital alignment and d-band center of TM_d on np codoping deserves systematic study regarding the CO2 reduction on TM SACs on BiOX.
In this paper, we investigate the effects of np codoping on the stability and reducing capacity of TM SACs for photocatalytic CO2 reduction reaction (CO2RR) on BiOCl by using first-principles calculation. Previous experimental works found that Fe, Co, Ni, and Cu are all effective SACs for accelerating CO2RR on BiOX [17,18,19,20]. Consequently, we constructed a dopant–defect combination by pairing these TM SACs and their surrounding Cl vacancies. Thermodynamically, the balance between n- and p-defects is determined by the defect equilibrium, which can be solved by calculating the formation energies of charged defects (ΔHf) [36,37,38]. Based on the ΔHf, we find that the Cl monovacancy (VCl) is more stable than the Cl multivacancies. np codoping is established by the coulomb attraction between the negatively charged p-type TM SACs and positively charged n-type VCl. The electrostatic attraction of np codoping enhances the stability of TM SACs. Neutral pairs are formed for the Fe, Co, and Ni SACs (PTM0), while the Cu SAC-based pair is dominated by the q = −1 charge state (PCu−1). In the np pairs, electrostatic interaction settles the electron accumulation around TM SACs. Accumulated electron occupies localized TM_d orbital and upshifts the d-band center toward the Fermi level, enhancing the reducing capacity of TM SACs based on the d-band theory. As a result, the CO2 absorption is improved along with the enhancement of charge transfer and the decrease in Gibbs free energies. The PCu−1 also locates additional electrons surrounding the Cu SACs and forms a half-occupied dx2−y2 state, further unshifting the d-band center and improving the reducing capacity of TM SACs. The metastability of Cl multivacancies limits the concentration of the np pairs with Cl multivacancies (PTM@nCl (n > 1)). Positively charged centers around PTM@nCl (n > 1) shield the charge transfer between the PTM@nCl (n > 1) and the CO2 molecule, hindering the CO2 reduction.

2. Computational Details

Density functional theory (DFT) [39] calculations were performed in the Vienna Ab initio Simulation Package (VASP) [40]. The interaction of ions with electrons was determined by projector augmented wave potentials (PAWs) [41]. The electron exchange–correlation energy was treated based on local-density approximation (LDA) [42] because the lattice constants and band structure of BiOCl calculated by LDA are in good agreement with experimental values [43]. The results were also checked based on the Perdew–Burke–Ernzerhof (PBE) functional [44]. The kinetic energy cutoff energy was 500 eV, and the total energy convergence criterion was set to 10−5 eV. A 4 × 4 × 1 supercell with over 15 Å of lattice constant was chosen to avoid the interactions of adjacent point defects. The K-mesh was optimized based on the minimum energy principle, and 3 × 3 × 1 Monkhorst–Pack grids were found to be sufficient for sampling the Brillouin zone for the supercell. A vacuum space of over 15 Å was applied to avoid the interactions of neighbor images along the z direction.
To solve the defect equilibrium between TM dopants and the compensating VCl, the formation energy of charged defects was calculated with the following formula [45,46]:
Δ H f = E t o t ( q , α ) E t o t ( h o s t ) + i n i ( E i + μ i ) + q ( ε V B M + E f )
where Etot(q,α) and Etot(host) are the total energy of the defect system and pristine BiOCl, respectively; n is the number of Cr, Bi, and TM dopants; q is the number of electrons transferred from supercell to reservoirs in forming the defect; and μi is the chemical formula of constituent i with energy Ei. Ef is the Fermi energy with respect to the valence band maximum (VBM) and ranges from the VBM to the conduction band minimum (CBM). The chemical potentials of Bi (μBi) and TM (μTM) were derived from the energies of their corresponding metals. The chemical potentials of O and Cl were limited to avoid the formation of elementary substances and maintain a stable BiOCl compound. μCl and μO are defined as:
Δ H f ( B i O C l ) = 2 μ B i + 2 μ C l + 2 μ O ,
μ C l 0 ,
where ΔHBiOCl is the formation energy of BiOCl. Under Cl-poor conditions, the chemical potential of the Cl atom is defined as the energy in the species Cl+ (−5.36 eV). The total energies of the charged systems should be corrected for the interaction of the charged defect with the compensating background and its periodic images. We used Makov–Payne (M-P) corrections, formulated as q2 α/2Ɛ L, where L is the linear dimension of the supercell, Ɛ is the static dielectric constant, and α is the Madelung constant.
To evaluate the reaction coordinate of CO2 reduction, Nørskov’s method [47] was used to calculate the Gibbs free energy difference:
Δ G = Δ E + Δ E Z P E + T Δ S
E, ∆EZPE, and ∆S denote the absorbed energy difference, the zero-point energy difference, and the entropy difference between the adsorbed state and the corresponding free state for density functional theory calculations, respectively. T is the temperature of the system, 298.15 K.
The reactions of CO2 adsorption and the *CO2 plus H formation *COOH process are defined as follows [48]:
* + C O 2 * C O 2
* C O 2 + H + + e * C O O H
where * represents adsorption sites, and *CO2 and *COOH represent adsorption intermediate states. Therefore, the CO2RR is calculated as:
Δ G C O 2 * = G C O 2 * G * G C O 2
Δ G C O O H * = G C O O H * 1 2 G H 2 G C O 2 *
Charge density redistribution is determined by the charge density difference between the pairs and corresponding isolated systems [49]. The charge density difference is calculated as follows:
Δ ρ = ρ A B ρ A ρ B
where ρA and ρB are the charge densities of isolated systems A and B, respectively, and ρAB is the charge density of the pairs.

3. Results and Discussion

3.1. Structures and Stability

In stable BiOCl, the [Bi2O2]2+ layer is sandwiched by two Cl layers (Figure 1), and the lattice constant is calculated to be 3.854 Å, agreeing with previous works [13,50,51]. The TM SACs are doped on the BiOX by substituting a bismuth atom based on experiments [16,17,18,19,20]. The TM SAC dopants are denoted as the BiTM on the intrinsic BiOX surface as shown in Figure 1b. The dopant–defect pair (PTM@nCl) comprises the BiTM and its surrounding Cl vacancy as shown in Figure 1. The PTM@3Cl is the combination of a TM SAC and Cl trivacancy. In the Cl trivacancy, the proportion of unlocked Bi atoms is up to 19%, which is large enough to comprise the unlocked Bi region. The structures of the BiTM and PTM@1Cl (PTM for short) are given in Figure 1b,c, and the PTM@nCl (n > 1) is shown in Figure S1. To assess the stability of the pairs, the formation energy (ΔHf) of charged defects is investigated as a function of Ef based on Equation (1). The cathode where the CO2RR takes place is an electron reservoir and corresponds to the n-type semiconductor. Therefore, we focus on the electron-rich (e-rich) condition where the Ef is close to the CBM. The ΔHf calculated following Equation (1) also depends on μCl. We focus on the Cl-rich limit (μCl = 0 eV), considering that the electrolyte solution usually accelerates the surface Cl ion exchange in photocatalytic experiments.
Based on previous experiments, surface halogen atoms significantly influence the charge transfer between activation sites and absorbed molecules [52,53]. The stability of Cl vacancies is evaluated prior to the dopant–defect pairs. Figure S2a illustrates the formation energy of the Cl monovacancy VCl, Cl di-vacancy (V2Cl), and tri-vacancy VCl (V3Cl) with varied charge states as a function of Ef. The Ef ranges from the VBM and CBM. One can see that the VCl is energy favorable in these three kinds of Cl vacancies if Ef is above the mid-gap (i.e., for the e-rich condition). In the V3Cl, 3/16 of Bi atoms are uncovered by the surface Cl atoms so that the V3Cl is large enough with regard to the unlocked Bi region. Consequently, the VCl is more stable than the unlocked Bi region in the BiOCl cathode. The VCl is capable of forming a donor defect with q = +1 charge state (VCl+1, with the superscript referring to the charge state) when the Ef is in most of the gap and becomes a neutral defect only when the Ef is closer to the CBM. The charge transition level Ɛ(+1/−1) is only 0.07 eV above the CBM, indicating that it is a donor defect. The Fermi level of the VCl0 is located in the conduction band in the spin–polarized density of states, which also indicates the donor defect characteristic.
Considering the stability of the VCl, we first investigate the PTM, which pairs the BiTM and surrounding single VCl, as shown in Figure 1c. The formation energy ΔHf is calculated to investigate the stability and ionization of the PTM and BiTM. If Ef is close to the CBM, as shown in Figure 2, the BiTM−1 dominates the BiTM for all the Fe, Co, Ni, and Cu SACs, indicating the p-type impurities characteristic of these TM SACs on pristine BiOCl in the e-rich condition. When the positively charged VCl+1 and negatively charged BiTM−1 move close to each other, as shown in Figure 1c, np codoping is established due to the coulomb attraction and forms the PTM pairs. Due to the electrostatic attraction of the np codoping, the ΔHf of the PTM is lower than the relative BiTM as shown in Figure 2. Consequently, the VCl facilitates the stability of the TM SACs on the BiOCl. When Ef is close to the CBM, neutral pairs (PTM0) are energetically favorable for the Fe, Co, and Ni SACs, while the negatively charged PCu−1 with charge state q = −1 is most stable for the Cu SACs. The PCu−1 possesses the lowest Ef (as low as −1.56 eV) in all the defects, suggesting that the PCu−1 can exist in great quantities on the BiOX cathode. The q = −1 charge state indicates that the PCu−1 further traps electrons from the surrounding lattice besides the inner electrostatic interaction in the pairs. The Cl-poor condition, where ΔHf is elevated by 5.36 eV depending on the μCl, as shown in Figure S3, hinders the TM SACs from anchoring on the BiOCl.
The electrostatic interaction is the key factor for establishing np codoping. We investigate the electrostatic interaction in the pairs by calculating the charge density redistribution. The charge density redistribution is determined as the charge density difference of the BiOCl system after and before forming PTM pairs. In Figure 3, the yellow and blue isosurfaces refer to the electron accumulation and dissipation regions, respectively. Before combining into PTM pairs, isolated TM SACs are all negatively charged, and VCl is positively charged based on the ΔHf in Figure 2 and Figure S3a. The negative charge density localizes around the TM SACs in the BiTM, while the positive charge localizes around the vacancy site in the VCl as shown in Figure 3e,f. In all the PTM pairs, negative and positive charges also separated at the adjacent TM SACs and VCl site as shown from Figure 3a–d. An obvious coulomb interaction is elicited between the closer positive and negative charges, forming more stable np codoping.

3.2. Modulation of CO2 Reduction

CO2 absorption (*CO2) is usually the rate-determining step in the CO2RR [15,18]. We investigate CO2 absorption on defective BiOCl systems to evaluate the effects of np codoping on the photocatalytic CO2RR. From Figure 4a, one can see that the CO2 is physically absorbed on the pristine BiOCl surface, and hardly any charges transfer between the BiOCl and the CO2. Only the surface charge density distribution of the BiOCl is disturbed by absorbed CO2. The charge transfer between the VCl+1 site and absorbed CO2 is enhanced, but still only physical absorption occurs with over 3.5 Å of distance as shown in Figure 4b. The Gibbs free energy difference (ΔG) is also calculated based on Equations (5) and (6) to evaluate the CO2 absorption. Negative ΔG refers to the chemisorption of CO2 on the photocatalyst. In Figure S4, one can see that the ΔG is 0.16 eV on the VCl+1, lower than that on pristine BiOCl (0.32 eV). Consequently, the VCl+1 mildly improves the CO2 absorption. For the CO2 absorption on the dopant–defect pairs, dramatic charge transfers are found in the neutral PFe0, PCo0, and the negatively charged PCu−1 as shown in Figure 4c, Figure 4d, and Figure 4e, respectively. The lengths of C–TM bonds between the absorbed CO2 and these pairs are 1.90 Å, 1.97 Å, and 1.97 Å as shown in Table 1, while the ΔG sharply declines to −0.52 eV, −0.83 eV, and −0.54 eV for the PFe0, PCo0, and PCu−1, respectively, as shown in Figure 5. As a result, the np codoping facilitates the absorption of the CO2 on the PFe0, PCo0, and PCu−1. The charge transfers in the PNi0 are slightly weaker, and the distance between the absorbed CO2 and Ni SACs (3.31 Å) is larger than that in other PTMs, but the ΔG still declines to −0.31 eV. Consequently, the CO2 absorption is also accelerated by the np codoping in the PNi0.
For comparison, we also evaluated the CO2 absorption on the isolated BiTM0 and BiTM−1. On the neutral BiTM0, the CO2 is only absorbed physically on the TM SAC sites with fewer charge transfers as seen in Figure S5. The distances between the CO2 and BiTM0 sites are all beyond 4 Å and are obviously larger than the distances between the CO2 and the dopant–defect pairs as shown in Table 1. The ΔGs of CO2 absorption on the BiTM0 are all positive as shown in Figure 5. The CO2 is therefore still physically absorbed at the neutral BiTM0 site. For the CO2 absorption on the negative BiTM−1, we find that the CO2 is only chemically adsorbed at the BiFe−1 but is still physically absorbed at the BiCo−1, BiNi−1, and BiCu−1. In detail, an intense charge transfer is formed between the CO2 and BiFe−1 site as shown in Figure S6a, while the length of the C-Fe bond is 1.90 Å as shown in Table 1. The ΔG of CO2 absorption is −0.18 eV in the BiFe−1. On the other hand, relatively few charge transfers are formed between the CO2 and BiCo−1, BiNi−1, and BiCu−1 sites as shown in Figure S6b, 6c, and 6d, respectively. The distances between CO2 and these activation sites are over 4.0 Å as shown in Table 1. The relative ΔGs of CO2 absorption are 0.15 eV, 0.23 eV, and 0.12 eV in the BiCo−1, BiNi−1, and BiCu−1 systems. Based on previous experiments, the carboxylate pathway is the most common route for the CO2RR on Fe-, CO-, and Cu-doped bismuth oxyhalide [16,17,18,19,20]. *CO2 hydrogenation (*COOH) follows close behind CO2 absorption (*CO2) in the carboxylate pathway and is reported to be also an important step influencing CO2 reduction [15,18]. We therefore also assess the ΔG of both *CO2 and *COOH in all defective BiOCl systems. In Figure 5, one can see that np codoping facilitates both the CO2 absorption and *CO2 hydrogenation with negative ΔG in the BiOCl systems by Fe, Co, and Cu SAC doping. Such an enhancement of CO2 photoreductions corresponds with previous experiments [18,19,20]. For the PNi0-doped BiOCl, the CO2 absorption is exothermic, but the *CO2 hydrogenation is endothermic, which is related to the fact that the CO2 activation prefers the carbide pathway, in which the *COOH is difficult to generate [17].

3.3. Orbital Alignments and d-Band Center of the TM_3d

The TM_3d orbital is usually crucial to the reducing capacity of TM SACs [54,55]. To account for the improvement in the CO2 absorption by the np codoping, we investigate the d-band center of the TM_3d orbital states on the BiOCl systems. Based on the d-band theory, the d-band center close to the Fermi level usually facilitates the reducing capacity of TM SACs [30,31]. For comparison, the d-band centers of the BiTM0 and BiTM−1 are also calculated as shown in Table 1. One can see that the d-band centers of all BiTM0 dopants are located far away from the Fermi level. For instance, the d-band centers of the BiFe0, BiNi0, and BiCu0 are as low as −2.54 eV, −3.85 eV, and −2.81 eV, respectively. Low d-band centers correspond to the low reducing capacity of the BiTM0 and result in physical absorption of the CO2. For the BiTM−1, the d-band center of BiFe−1 is significantly shifted from −2.54 eV to −1.26 eV compared with BiFe0, resulting in the enhancement from physical absorption to chemical absorption. The d-band center in the BiCo−1 decreases from −1.81 eV to −2.45 eV, so the CO2 remains physically absorbed on the BiCo−1 site. Although the d-band centers of the BiNi−1 and BiCu−1 are shifted toward the Fermi level compared with the BiNi0 and BiCu0, they are still lower than the Fermi level by −2.09 eV and −2.31 eV. Consequently, the CO2 is also physically absorbed on these systems. As shown in Table 1, the d-band centers are upshifted to −1.27, −0.83, −1.46, and −1.45 eV for the PFe0, PCo0, PNi0, and PCu−1, respectively, and are comparable to those (−1.56 eV) of the Pt SACs anchored at the edge of graphene, which are excellent photocatalysts for H2 reduction [56]. Consequently, the modulation of CO2 absorption on the dopant–defect pairs derives from the enhancement of the reducing capacity of TM SACs with the upshift in the d-band center by the np codoping.
The d-band center is determined by the orbital alignment and occupation of the TM-3d. To account for the effects of np codoping on the d-band centers, we further calculated the spin-resolved states of density (PDOSs) of the BiOCl systems before and after np codoping to illustrate the orbital alignment and occupation. Based on the crystal field theory, the degeneracy of the TM-3d orbital states is broken in a process dependent on the molecular symmetry. As shown in Figure 1b,c, the TM ion is located at the center of a square plane formed by four nearest neighboring oxygen atoms. In this local symmetry, the TM_3d orbitals split into four groups dxz + dyz, dxy, dz2, and dx2−y2. In the neutral BiTM0, the oxidation states of the TM dopant are all +3. As shown in Figure 6a, the d electron configuration obeys Hund’s rule in BiFe0. Five spin-up d orbitals are fully occupied, while five spin-down d orbitals are entirely empty. In the BiCo0, BiNi0, and BiCu0, the d electron configurations break Hund’s rule as shown in Figure 6a. In detail, four spin-up d orbitals and only the spin-down dxz + dyz orbital are occupied in the Co_3d, when the electron configuration of Co is changed from 3d74s2 to 3d64s0 after losing three valence electrons in the BiCo0. In the BiNi0, four spin-up d orbitals and three spin-down d orbitals (dxz + dyz orbitals and dxy orbital) are occupied to form the 3d74s0 configuration. In the BiCu0, the Cu_3d orbitals are spin-degenerated, and spin-degenerated dxz + dyz, dxy and dz2 orbitals are occupied to change the 3d104s1 to a 3d84s0 configuration.
In the dopant–defect pairs PTM0, negatively charged TM dopants and positively charged VCl+1 are stabilized by the electrostatic interaction of the np codoping. The electron accumulation changes the oxidation state of the TM dopant from +3 to +2. Compared with the electron configuration in the BiTm0, electron accumulation is located around the TM while charge depletion forms around the VCl+1 and durable electrostatic attraction is established based on the Coulomb’s law as shown in Figure 4. Accumulated electrons occupy the localized empty TM_3d orbital as shown in Figure 6b. In detail, spin-down dz2 is further occupied in the PFe0 to form the 3d64s0 configuration. In the PCo0, the electron from the VCl+1 further occupies the spin-down dxy to form the 3d74s0 configuration. Spin-down Ni_dz2 is further occupied, leading to spin-degenerated Ni_3d states in the PNi0, while the spin degeneration of the Cu_3d states is broken along with the spin-up dx2−y2 being occupied. Above all, newly occupied TM_3d orbitals are localized and remain at the Fermi level in the PTM0, gathering both the spin-up and spin-down TM_3d orbitals together and bringing them to the Fermi level. As a result, the d-band centers are shifted to the Fermi level compared with those in the BiTM0. For the PCu−1 as shown in Figure 6b, the added electrons also accumulate around the Cu SACs and fill into Cu_dx2−y2, inducing spin-degenerated Cu_3d orbital states and localized half-occupied dx2−y2. A stronger localized characteristic of Cu_3d is formed around the Fermi level, further shifting the d-band center to the Fermi level.
The electron accumulation around the TM SACs plays an important role in changing the d-orbital alignment and shifting the d-band center in the PTM. In contrast to the BiFe−1, the reducing capacity in the BiTM−1 is weaker than that in the PTM. To account for the reduced capacity in the BiTM−1, we investigate its d orbital alignment and electron configuration in detail. The electron accumulation is also formed around the BiFe−1 as shown in Figure S7a. The spin–polarized PDOS of TM_3d in the BiTM−1 is shown in Figure S8a. The BiFe−1 keeps in line with the PFe0 on the d-orbital alignment, the 3d64s0 configuration, and even Hund’s rule, resulting in a similar d-band center and ΔG for the CO2 absorption. Although electron accumulation occurs around the BiCo−1 as shown in Figure S7b, the 3d74s0 configuration stays the same as that in the PCo0, but the spin–polarized d-orbital alignments exhibit a dramatic change compared with those in the PCo0. Five spin-up orbitals are occupied in the BiCo−1 as shown in Figure S8a, while only four spin-up orbitals are occupied in the PCo0 (the same as in the BiCo0) as shown in Figure 6. Such variation changes the high spin state (3μB) to a low spin state (1μB) as well as induces greater broadening of Co_3d orbital states and then a more negative d-band center in the BiCo−1. The electron localization is weakest in the BiNi−1 as shown in Figure S7c. As a result, although the electron configuration in the Ni_3d is same as the cases in the PCo0, the energy level and broadening of Ni_3d, which also influence the d-orbital alignment, are extended to a low energy level in the BiNi−1, resulting in a much lower d-band center than that in the PCo0. The BiCu−1 possesses the same orbital alignment, orbital occupation, and d-band center as the PCu0 as shown in Figure S8d. Therefore, it possess a similar ΔG for CO2 absorption. The stronger reducing capacity of the PCu−1 derives from the accumulation of additional electrons around the Cu SACs. On the other hand, compared with the BiTM0, BiFe−1, BiNi−1, and BiCu−1 upshift the d-band center because additional electrons accumulate around the TM SACs and occupy more 3d orbitals. In contrast to the BiCo0, the BiCo−1 moves the d-band center downward with negative energy following the transition of spin states. Consequently, the electron accumulation plays a key role in the d-orbital alignment and the d-band center. The transition of spin states also influences the d-band center.

3.4. Discussion

Finally, the effect of Cl multivacancies on the np codoping is discussed. Taking the most sable PCu−1 as an example, the PCu@2Cl and PCu@3Cl are pairs with double and ternary Cl vacancies, as shown in Figure S1. Both the PCu@2Cl and PCu@3Cl exhibit lower formation energy than the PCu−1. However, a high ΔHf hinders the stability and solubility of V2Cl and V3Cl, as shown in Figure S2. The solubility of both the PCu@2Cl and PCu@3Cl is lower than that of the PCu−1. In the e-rich condition, both the PCu@2Cl and PCu@3Cl exhibit the charge state of q = 0, as shown in Figure S2b (denoted as PCu@2Cl0 and PCu@3Cl0). The ΔG profiles in Figure 7a exhibit obvious high free energy (0.61 eV and 0.43 eV) for CO2 absorption at both the PCu@2Cl0 and PCu@3Cl0 sites, which is higher than that at the PCu−1 and even higher than that at the VCl+1. Relatively high d-band centers lead to a weak reducing capacity as shown in Table 1. Obviously positive charge accumulation around the PCu@2Cl0 and PCu@3Cl0 is also observed as shown in Figure 7c,d, shielding the electron transfer to absorbed CO2 and decreasing the reducing capacity. Consequently, the PCu−1 enhances the CO2 absorption, and doping the TM on the unlocked Bi region is not an efficient strategy to facilitate the CO2RR.

4. Conclusions

In summary, np codoping engineering is introduced to account for the modulation of photocatalytic CO2 reduction on a BiOCl-based cathode by using first-principles calculation. np codoping is established via the Coulomb interactions between positively charged TM SACs and the negatively charged Cl vacancy (VCl) in the dopant–defect pairs. Based on the formation energy of charged defects, we find neutral dopant–defect pairs for the Fe, Co, and Ni SACs (PTM0) and the q = −1 charge state of the Cu SAC-based pair (PCu−1). The electrostatic attraction of the np codoping in pairs strengthens the stability and solubility of TM SACs and stabilizes the electron accumulation around the TM SACs. Accumulated electrons occupy the localized TM_d orbital and change the d-orbital alignment, gathering the d-orbital states and shifting the d-band center toward the Fermi level and enhancing the reducing capacity of TM SACs based on the d-band theory. As a result, the CO2 absorption is improved with the enhancement of charge transfer and the decrease in Gibbs free energies. Besides the electrostatic attraction of the np codoping, additional electrons in the PCu−1 also accumulate, surrounding Cu SACs and forming a half-occupied dx2-y2 state, which further upshifts the d-band center and improves photocatalytic CO2 reduction. For the pairs consisting of TM SACs and Cl multivacancies PTM@nCl (n > 1), their concentration is limited due to the metastability of Cl multivacancies. The positively charged center around the TM SACs blocks the charge transfer, indicating that doping the unlocked region with TM atoms does not facilitate CO2 reduction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano14141183/s1, Figure S1: Schematic diagrams of atomic structure of the PTM@2Cl and PTM@3Cl; Figure S2: Formation energy of VnCl, PCu@nCl, PCo@nCl depending on the Ef; Figure S3: ΔHf of the BiTM (rel lines) and PTM (blue lines) at Cl-poor limit for (a) Fe, (b) Co, (c) Ni and (d) Cu, respectively; Figure S4: Gibbs free energy (ΔG) profiles for VCl+1 and BiOCl, respectively; Figure S5: Charge transfer between surface defect sites and absorbed CO2 in BiOCl with (a) BiFe0, (b) BiCo0, (c) BiNi0, (d) BiCu0, respectively.; Figure S6: Charge transfer between surface defect sites and absorbed CO2 in BiOCl with (a) BiFe−1, (b) BiCo−1, (c) BiNi−1, (d) BiCu−1, respectively; Figure S7: Polarization charge density in the (a) BiFe−1, (b) BiCo−1, (c) BiNi−1, (d) BiCu−1, respectively; Figure S8: (a) pdos of the BiTM−1 and (b) pdos of the PCu@2Cl0, PCu@3Cl0 and PCu@2Cl+1. See the supplementary material for detailed results (such as formation energy, Gibbs free energy, and charge density redistribution).

Author Contributions

Conceptualization, C.Z. and G.Y.; methodology, Y.L.; software, Y.S.; validation, C.Z., Y.L. and Y.S.; formal analysis, C.Z. and G.Y.; investigation, G.Y.; resources, Y.S. and X.Q.; data curation, G.Y. and Y.L.; writing—original draft preparation, C.Z. and G.Y.; writing—review and editing, Y.L. and Y.S.; visualization, Y.S.; supervision, X.Q.; project administration, X.Q.; funding acquisition, Y.S. and X.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (12004321 and 11974300), Scientific Research Foundation of Education Bureau of Hunan Province (23B0157 and 19B554), Natural Science Fund for Distinguished Young Scholars of Hunan Province (2023JJ10037), and National Key R&D Program of the MOST of China (2022YFA1602602).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagrams of atomic structures for 2D BiOCl with (a) VCl, (b) BiTM, and (c) PTM. Blue circle refer to the PTM.
Figure 1. Schematic diagrams of atomic structures for 2D BiOCl with (a) VCl, (b) BiTM, and (c) PTM. Blue circle refer to the PTM.
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Figure 2. Formation energies of BiTM (red lines) and PTM (blue lines) at the Cl-rich limit for Fe, Co, Ni, and Cu. The numerical notations refer to charge states. The dotted lines refer to the formation energies of meta-stable charged defects.
Figure 2. Formation energies of BiTM (red lines) and PTM (blue lines) at the Cl-rich limit for Fe, Co, Ni, and Cu. The numerical notations refer to charge states. The dotted lines refer to the formation energies of meta-stable charged defects.
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Figure 3. Charge density redistribution of (a) PFe0, (b) PCu0, (c) PNi0, and (d) PCu0. (e,f) The charge density distribution of the isolated BiCu−1 and VCl−1. The yellow (blue) isosurface refers to the charge accumulation (dissipation) region. The isovalue is set to 0.03 e/Å.
Figure 3. Charge density redistribution of (a) PFe0, (b) PCu0, (c) PNi0, and (d) PCu0. (e,f) The charge density distribution of the isolated BiCu−1 and VCl−1. The yellow (blue) isosurface refers to the charge accumulation (dissipation) region. The isovalue is set to 0.03 e/Å.
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Figure 4. Charge transfer between absorbed CO2 and the (a) pristine BiOCl, (b) VCl+1, (c) PFe0, (d) PCo0, (e) PNi0, and (f) PCu1. The isovalue is set to 0.0003 e/Å.
Figure 4. Charge transfer between absorbed CO2 and the (a) pristine BiOCl, (b) VCl+1, (c) PFe0, (d) PCo0, (e) PNi0, and (f) PCu1. The isovalue is set to 0.0003 e/Å.
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Figure 5. Gibbs free energy (ΔG) of CO2 absorption and *CO2 hydrogenation (*COOH) at (a) Fe, (b) Co, (c) Ni, and (d) Cu SAC sites.
Figure 5. Gibbs free energy (ΔG) of CO2 absorption and *CO2 hydrogenation (*COOH) at (a) Fe, (b) Co, (c) Ni, and (d) Cu SAC sites.
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Figure 6. (a) Spin-resolved PDOS of BiTM0. The spin-resolved PDOS of the VCl0 is given for comparison. (b) Spin-resolved PDOS of the PTM0 and PCu−1. The oxidization states are Fe2+, Co2+, Ni2+, and Cu1+ in the stable PFe0, PCo0, PNi0, and PCu−1, respectively.
Figure 6. (a) Spin-resolved PDOS of BiTM0. The spin-resolved PDOS of the VCl0 is given for comparison. (b) Spin-resolved PDOS of the PTM0 and PCu−1. The oxidization states are Fe2+, Co2+, Ni2+, and Cu1+ in the stable PFe0, PCo0, PNi0, and PCu−1, respectively.
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Figure 7. (a) Gibbs free energy profiles of the PCu−1, PCu@2Cl0, and PCu@3Cl0. (bd) depict the charge density redistribution of the PCu−1, PCu@2Cl0, and PCu@3Cl0, respectively. The isovalue is set to 0.03 e/Å.
Figure 7. (a) Gibbs free energy profiles of the PCu−1, PCu@2Cl0, and PCu@3Cl0. (bd) depict the charge density redistribution of the PCu−1, PCu@2Cl0, and PCu@3Cl0, respectively. The isovalue is set to 0.03 e/Å.
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Table 1. Calculated TM–C bond length (Å) for absorbed *CO2 (d(*CO2)) and *COOH (d(*COOH)), bond angle of absorbed *CO2 molecule (θ(°)), and d-band center (eV). “/” indicates that the distance between physically absorbed CO2 and the activation site is beyond 3.50 (Å).
Table 1. Calculated TM–C bond length (Å) for absorbed *CO2 (d(*CO2)) and *COOH (d(*COOH)), bond angle of absorbed *CO2 molecule (θ(°)), and d-band center (eV). “/” indicates that the distance between physically absorbed CO2 and the activation site is beyond 3.50 (Å).
Defectd(*CO2) (Å)d(*COOH) (Å)d-Band Center (eV)Θ (°)
BiFe0/1.91−2.54179.75
BiFe−11.901.83−1.26145.77
PFe01.901.83−1.27147.15
BiCo0/1.85−1.81179.53
BiCo−1/1.80−2.45148.32
PCo01.971.81−0.83147.70
BiNi0/1.86−3.85179.66
BiNi−1/1.81−2.09179.15
PNi03.311.84−1.46177.90
BiCu0/1.94−2.81179.77
BiCu−1/1.93−2.31179.18
PCu0/1.93−2.31179.13
PCu−11.971.88−1.45153.71
PCu@2Cl0/1.88 −3.12 149.73
PCu@3Cl0/1.85−1.99179.40
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Yin, G.; Zhang, C.; Liu, Y.; Sun, Y.; Qi, X. Modulation of Photocatalytic CO2 Reduction by np Codoping Engineering of Single-Atom Catalysts. Nanomaterials 2024, 14, 1183. https://doi.org/10.3390/nano14141183

AMA Style

Yin G, Zhang C, Liu Y, Sun Y, Qi X. Modulation of Photocatalytic CO2 Reduction by np Codoping Engineering of Single-Atom Catalysts. Nanomaterials. 2024; 14(14):1183. https://doi.org/10.3390/nano14141183

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Yin, Guowei, Chunxiao Zhang, Yundan Liu, Yuping Sun, and Xiang Qi. 2024. "Modulation of Photocatalytic CO2 Reduction by np Codoping Engineering of Single-Atom Catalysts" Nanomaterials 14, no. 14: 1183. https://doi.org/10.3390/nano14141183

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Yin, G., Zhang, C., Liu, Y., Sun, Y., & Qi, X. (2024). Modulation of Photocatalytic CO2 Reduction by np Codoping Engineering of Single-Atom Catalysts. Nanomaterials, 14(14), 1183. https://doi.org/10.3390/nano14141183

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