Mechanism on Carbon Vacancies in Polymeric Carbon Nitride for CO₂ Photoreduction

1. INTRODUCTION

In recent years, environmental problems have become more and more serious, gradually attracting the attention and becoming a research hotspot mainly due to the continuous accumulation of CO₂ and other greenhouse gases produced by the burning of fossil fuels, which has caused a series of huge impacts such as global climate change^{[1, 2]}. Therefore, reducing CO₂ emissions and improving CO₂ conversion efficiency are key methods to alleviate global warming^{[3, 4]}. However, it is well known that CO₂ is a very stable inactive molecule, which has the characteristics of difficult adsorption and low reactivity, while traditional emission reduction methods (including adsorption, ultrafiltration and coagulation) rely on too much high energy input. It will also have a great burden on the environment^[3-6].

As an emerging technology, photocatalysis has gained people's favor due to its mild reaction conditions and low energy input. Recently, the use of TiO₂, ZnO, WO₃, In₂O₃, SrTiO₃, CeO₂, BiPO₄, Fe₂O₃ and other semiconductors for photocatalytic reduction of CO₂ has been more in-depth research progress^[7-14]. In addition, because the polymer photocatalysts like PCN are cheap, metal-free and have unique properties including semiconductivity, alkaline, high hardness, chemical robustness, and high chemical and mechanical stability, causing a wide range of attention^[15-17]. However, the photocatalytic performance of PCN is limited due to its low specific surface area and fast recombination of photogenerated electron-hole pairs caused by the intrinsically p-conjugated electronic system^{[18, 19]}. Many attempts have been made to improve the catalytic activity, such as doping metals and non-metals, forming vacancies, creating heterojunctions and building nanostructures^[20-22]. Recently, our collaborative research has shown that the ability of formed carbon vacancies melon-based PCN (named as CV) to capture CO₂ has been enhanced because the unsaturated nitrogen atoms give the CV Lewis basis function, thereby prolonging the lifetime of the photo-generated charges. Moreover, previous experiment has detected that CV displayed a 45-fold improvement in CO₂ to CO activity over the melon-based PCN (the most in-depth study and the most widely reported PCN), which indicates that CV can be functionalized as an active site for activating CO₂, thereby effectively promoting CO₂ emission reduction^{[23, 24]}. In general, the experiment provides an effective new means of CO₂ conversion, but its microscopic mechanism is still unclear and worth to explore.

In this work, we use density functional theory (DFT) to study the reaction mechanism of CV and CV_+H (hydrogenated CV) to reduce CO₂ to CO, and combine the electronic structure to analyze the reason of difference for photocatalytic CO₂ reduction activity.

2. CALCULATION METHODS

The DFT calculations in this work were carried out via the Vienna ab initio simulation package (VASP)^{[25, 26]}. Projector augmented wave (PAW) potentials were used to describe the electron-ion interactions and Perdew, Burke, and Ernzerh functional of the generalized gradient approximation (GGA-PBE) were used to describe the exchange and correlation interactions^[27-29]. DFT-D2 method was included for the van del Waals corrections^[30]. In the first-principle DFT calculations, we set cutoff energy to 520 eV, which was used to expand the wave function. The Brillouin zone of unit cells (1×1×1) was sampled with a 5×5×1 Monkhorst-Pack mesh of k points. At the same time, we also consider the spin polarization of electrons. All configurations were fully relaxed until the conventional energy was smaller than 10^-4 eV and the Hellmann-Feynman forces acting on the atoms were smaller than 0.01 eV/Å. As such, we add a 10 Å vacuum layer along the z-direction to avert the interaction between adjacent layers.

According to the previous articles and experiments, the possible reaction mechanism of the CO₂ reduction for the CV₁/CV₂ and CV_1+H/CV_2+H were calculated as follows^{[23, 31]}:

The Gibbs free energy changes (ΔG) for each species were calculated at T = 298.15 K with the following equation^[32]:

where ΔE_ele and ΔE_ZPE are the change of the electronic and zero-point energies and ΔS is the entropy change. We can get the zero point energy by calculating the frequency of the optimized configuration, and obtain S from NIST database query. ΔG_pH is the free energy associated with pH, which is defined as ΔG_pH = k_BT × ln 10 × pH, where k_B is the Boltzmann constant, and pH = 0 is considered in this work. ΔG_U = –neU, where n and U are the number of electrons transferred and the electrode potential applied, and the value of U was set to be zero as well. We calculate the ΔG of each reaction step by the difference between the free energy of the product and the reactant or only relative to the initial state of the gaseous CO₂ and CV₁/CV₂ surface.

3. RESULTS AND DISCUSSION

3.1 Melon-based PCN and its CV

It is widely accepted that the CO₂ photoreduction reaction is mainly composed of three steps: (a) light absorption, (b) photo-generated carrier separation and transport and (c) surface redox reaction^[31]. However, for the CO₂ reduction reaction, melon-based PCN still has some shortcomings, such as the light absorption range limited to the ultraviolet region, low light conversion efficiency, and low product selectivity^[33]. Fortunately, defect engineering has been recently proved to be a powerful method, which can improve the surface electronic structure to increase the visible light absorption range, enhance the lifetime of widely generated charges, and accelerate the reaction rate^[34-36]. In this case, Yang et al. constructed CV by water vapor etching and used positron annihilation spectrometer (PAS) and X-ray photoelectron spectrometer (XPS) confirmed the existence and location of CV in the melon-based PCN skeleton^[23]. On the basis of the experiment, we removed carbon atoms (C1, C2, C3, C4, C5, and C6 in Fig. 1a) at different positions on melon-based PCN to form the corresponding carbon defects CV_n (i.e., CV₁, CV₂, CV₃, CV₄, CV₅, and CV₆), as shown in Fig. S1.

Figure 1

Figure 1.  (a) Top and side views of melon-based PCN and the possible doping sites (C1, C2, C3, C4, C5, and C6); (b) Geometric structure of CV₁; and (c) geometric structure of CV₂. The brown, blue, and pink balls represent C, N, and H atoms, respectively

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As shown in Fig. 1a, although these six C atoms in the heptazine (-C₆N₇) unit of the melon-based PCN are all three-fold coordinated, the chemical environments of them are slightly different. We first calculated the formation energy (E_form) of each configuration to characterize the thermodynamic stability of each vacancy. The E_form is defined as^[37]:

where E(CV) is the total energy of the CV system, E(PCN) is the total energy of melon-based PCN, and μ(C) is the energy of each C atom in the graphite phase. The optimized CV is divided into two main types: one is a substrate with a closed conjugated ring structure similar to the heptazine ring comprising two 6-membered and one 5-membered rings, such as the CV₁, CV₃ and CV₅ in Fig. S1a, S1c, and S1e, respectively, and the other is the open-type conjugated PCN lacking a 6-membered ring, like CV₂, CV₄ and CV₆ in Fig. S1b, S1d, and S1f, respectively. Among them, CV₁ and CV₂ have the lowest formation energy (4.45 and 4.85 eV) for each type of the defects. Hence, we decided to use CV₁ and CV₂ as the representatives to discuss the microscopic reaction mechanism of CO₂ to CO convergence. Interestingly, our research also confirmed that the corrugated structure is formed after defects in the C1 position, while the structure with defect in C2 position still maintains flat structure. This result inspired that the corrugated structure is more stable than the planar structure, which is consistent with the study of Azofra et al^[38].

Table 1

Table 1.  Formation Energy of CVs

DownLoad: CSV

	CV1	CV2	CV3	CV4	CV5	CV6
Eform (eV)	4.45	4.85	4.50	5.05	6.22	5.21

3.2 Hydrogenated CV (CV_+H)

Previous experiments have shown that hydrogen atoms can easily fill into the unsaturated sites of CV. In this case, we thoroughly studied the position and number of hydrogen atoms adsorbed on CVs, where the adsorption energy formula is as follows^[39]:

where E_A–S, E_S, and E_A are the total energies of the complex of adsorbate-substrate (A–S), the substrate (S), and the absorbate (A), respectively. In general, the hydrogen adsorption energy of CV₁ and CV₂ follows the same trend (Fig. 2 and structures as shown in Fig. S2). When the coordination of N atoms on both sides of the vacancy is not saturated, the hydrogen is adsorbed more easily with negative adsorption energy. For example, the adsorption energy is –2.98 eV from CV₁ to CV_1+H, as shown in Fig. 2a. When the coordination of N atoms is saturated, the adsorption energy becomes positive, which means that it becomes very difficult to adsorb hydrogen atom. Analogously, the CO₂ adsorption process also faces the same trend. When in an unsaturated state, the N atom is easier to adsorb CO₂ and form a covalent bond with the C atom. When the coordination of the N atom is gradually saturated, the adsorption capacity decreases, which is similar to the reports of Wu et al. that CO₂ is easily adsorbed on exposed N atoms^[40]. Therefore, this also can be seen as a competitive process between the hydrogen and carbon dioxide adsorption stages. In addition, the presence of hydrogen atom may lead to hydrogen bond and it would benefit the fixation of CO₂. Therefore, in the following research, CV₁/CV₂ and CV_1+H/CV_2+H are chosen as models to explore the mechanism of CO₂ reduction reaction.

Figure 2

Figure 2.  Adsorption energies of hydrogen adsorbed on (a) CV₁ and (b) CV₂; Top and side views of geometric structure of (c) CV_1+H and (d) CV_2+H; The brown, blue, and pink balls represent C, N, and H atoms, respectively

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3.2 Catalytic performance of the CO₂ reduction on CV

Azofra et al. proposed that the CO₂ reduction reaction pathway catalyzed by PCN is as follows: CO₂→*COO→*COOH→CO→HCO*→HCHO→CH₃O*→CH₃OH^[38]. Since the release of a large amount of CO observed in the experiment, we designed two possible reaction pathways (Scheme 1) that use CV as a catalyst to convert CO₂ to CO:

Scheme 1

Scheme 1.  Schematic diagram of the CO₂ reduction reaction process, where * is CV and *′ is CV_+H (A hydrogen atom is adsorbed on the CV)

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where * is CV₁/CV₂ (substrate) and *΄ is CV_1+H/CV_2+H (A hydrogen atom is adsorbed on the substrate). We considered each possible structure in these two paths until the most stable structure is found. In terms of CV₁, the CO₂ is chemically adsorbed with the adsorption energy as high as –1.30 eV and the C–N bond length (the distance between the C atom of CO₂ and the N atom of CV) is 1.42 Å. The CV₂ has a much weak adsorption capability, with CO₂ adsorption energy as low as –0.34 eV and the C–N bond length as 2.91 Å (Fig. S3, Fig. S4). The result shows that the corrugated structure formed by CV₁ leads to better exposure of N atoms, while the planar CV₂ structure has the influence of steric hindrance of CO₂ adsorption.

Although the CV₁ performs well in each step in Path I (red curve in Fig. 3a), the onset potential of the CO desorption step is abnormally high (3.09 eV). Since the presence of H on the unsaturated N atom has been detected experimentally, we adopted the method of adsorbing a hydrogen atom by the intermediate before the potential-determining step and the calculation results show that the adsorption of hydrogen atom changes the substrate and the reaction process is greatly improved. The Gibbs free energy diagrams of the reaction process are shown in Fig. 3, where the trends of CV₁/CV₂ in Paths I and II are quite different. First, they have different potential-determining step. Concretely, the potential-determining step of path 1 is the CO gas desorption step (*CO → CO(g)), but the potential-determining step of path 2 changes to the *CO adsorption hydrogen atom step (*CO + H ⁺ + e^-→*'CO). More importantly, the onset potentials of CV₁ and CV₂ in path II are only 1.47 and 0.90 eV, respectively, while those in path I are 3.09 and 3.22 eV, respectively. It is recognized that the protonation and desorption of CO molecules are the key steps in determining whether to generate CH₄ or CO, which means that CV₁/CV₂ may convert CO₂ to CH₃OH in Path I, rather than directly generate CO. The comparison of the two paths shows that the adsorption of hydrogen atoms (* to *') during the reaction can not only change the potential-determining step through reducing the onset potential, but also make the reaction thermodynamically easier to proceed spontaneously.

Figure 3

Figure 3.  Gibbs free energy diagram of CO₂ reduction process from CV₁/CV₂ in (a) path I and (b) path II

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3.4 Catalytic performance for the CO₂ conversion path on CV_+H

As mentioned above, we choose CV_+H (i.e. CV_1+H and CV_2+H) as a representative, and use Gibbs free energy to characterize the effect of the compound after CV adsorbs hydrogen atoms on the reduction reaction process. Here, we design two reaction pathways based on whether hydrogen atom is adsorbed on the intermediate before the potential-determining step, as shown in Scheme 2:

Scheme 2

Scheme 2.  Schematic diagram of the CO₂ reduction reaction process, where *′ is CV_+H (A hydrogen atom is adsorbed on the CV) and *′′ is CV_+2H (The second hydrogen atom is adsorbed on the CV)

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Where *' is CV_+H (A hydrogen atom is adsorbed on the CV) and *'' is CV_+2H (The second hydrogen atom is adsorbed on the CV). Similarly, we considered the possible structure of each intermediate and selected the most stable structure for Gibbs free energy calculation. In the process of CO₂ adsorption, as far as CV_1+H/CV_2+H is concerned, the CO₂ is both between chemical adsorption and physical adsorption on CV_1+H and CV_2+H, with their adsorption energy as –0.23 and –0.13 eV, respectively. The CN bond lengths are 3.77 and 1.42 Å (Fig. S5 and S6).

As shown in Fig. 4, the free energy diagrams are different between paths I and II. The most noticeable is the change in the potential-determining step. The potential-determining step in path I is the *COOH hydrogenation to produce CO and water (*COOH + H⁺ + e^- → *CO + H₂O). The onset potentials of CV_1+H and CV_2+H are 1.45 and 1.56 eV, respectively. The potential-determining step in path II is the stage of *COOH adsorption of hydrogen atoms (*'COOH + H⁺ + e^- → *''COOH), with the onset potential of CV_1+H and CV_2+H as 0.89 and 1.21 eV, respectively.

Figure 4

Figure 4.  Gibbs free energy diagrams of CO₂ reduction process from CV_1+H/CV_2+H in (a) path I and (b) path II

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It can be concluded from the above results that the adsorption of hydrogen atoms reduces the onset potential (CV₁, CV₂ from 3.09 and 3.22 eV to 1.47 and 0.90 eV, respectively), while CV_1+H and CV_2+H decreased from 1.45 and 1.56 eV to 0.89 and 1.21 eV. This may be due to the formation of corrugated structure and the more exposed N atoms on the surface. In conclusion, CV₂ and CV_1+H have the largest reduction in the onset potential, and can effectively promote the catalytic effect of CV on the CO₂ reduction reaction, making the reaction easier to occur.

3.5 Electronic properties of CV and CV_+H

In order to understand the impact of defect engineering on the electronic properties, we have analyzed the band structures and projected density of states (PDOSs) of CV₁/CV₂ and CV_1+H/CV_2+H. PDOSs show that valance band (VB) and defect level are mainly dominated on N atoms and conduction band (CB) is contributed on C atoms (Fig. S7). After the formation of carbon vacancies, electrons mainly gather in the orbitals of N atoms. When CO₂ is adsorbed, electron transfer occurs between the N atoms around the C vacancy and the adsorbed CO₂ (Fig. 5), indicating that C vacancies are conducive to the adsorption of CO₂ and improve the reactivity of the reaction site, and promote the progress of the reaction.

Figure 5

Figure 5.  Electron density difference of (a) CV_1, (b) CV₂, (c) CV_1+H, and (d) CV_2+H adsorbed CO₂. The isovalue is set to 0.01 e/ų

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The calculated band gap of melon-based PCN is 2.50 eV (Fig. S8), which is slightly smaller than the experimental value of 2.70 eV^[41], but consistent with previous calculations^[42]. In addition, when carbon vacancies are formed, the band gaps change of CV₁ and CV₂ are not significant (2.52 and 2.42 eV, respectively); when hydrogen atoms are adsorbed on CV₁/CV₂, the band gaps of CV_1+H and CV_2+H change to 2.44 and 2.51 eV (Fig. S9). More importantly, the spin-up and spin-down states of the CV₁/CV₂ and CV_1+H/CV_2+H band structure are no longer similar with melon-based PCN, which indicates that the uneven electronic valence between carbon and nitrogen atoms after defect engineering transforms the melon-based PCN into a spin-polarized system. Therefore, defect level appears between the conduction band minimum (CBM) and the valence band maximum (VBM) and the construction of defect engineering has caused the Fermi level to move from the top of the valence band of melon-based PCN to the middle of the forbidden band, which prevents the recombination of photogenerated carriers, prolongs the lifetime of photogenerated electrons, and improves the efficiency of the CO₂ reduction reaction (Fig. S10). This is advantageous for CO₂ to capture electrons to overcome kinetic obstacles. Meanwhile, it could receive electrons from the valence band under light excitation, which can promote the absorption of visible light and improve the photocatalytic efficiency. To further support this view, we have drawn the VBM and CBM orbital diagrams and found that the VBM of melon-based PCN is mainly concentrated on nitrogen atoms, and the CBM is mainly distributed on nitrogen and carbon atoms (Fig. S10), which is consistent with the PDOS analysis. The N atom in -NH group is neither a component of VBM nor a component of CBM, indicating that photoexcited electrons are severely confined to each heptazine unit, resulting in high photo-generated carrier recombination rate and low photocatalytic efficiency. In the defect system, due to the existence of defect states, the orbital distribution of CBM and VBM has undergone a significant change. The VBM of CV₁ and CV_1+H is dispersed in the whole system, while that of CV₂ and CV_2+H is mostly formed on the heptazine unit at the defect position, which proves that it is good for the separation of photoexcited electron-hole pairs, suppresses the recombination of photo-generated charge carriers, increases the life of photo-generated charges, and improves the photocatalytic activity.

4. CONCLUSION

This work mainly carries out the defect engineering on C atoms of melon-based CN at different positions, and evaluates the chemical stability of carbon defects. According to the experiments that show the existence of hydrogen atoms, we have carried out a detailed study on the number and positions of the adsorbed hydrogen atoms. More importantly, we have conducted an in-depth study of the CO₂ reduction reaction process by the calculation of Gibbs free energy and analyzed the electronic properties by DOS and band structure. The results show that the CV₂ performs well in the process of CO₂ reduction reaction and the onset potential in path II reduces to 0.9 eV. However, we think it is the synergistic effect for the CO₂ reduction reaction in the CV systems, as the onset potentials of CV₁ and CV_2+H are around 1.2~1.5 eV with a strong chemisorbed CO₂. Furthermore, the adsorption of hydrogen atoms can indeed effectively increase the range of visible light absorption, reduce the onset potential during reaction, and promote the rate of CO₂ reduction reaction. Overall, this work provides the mechanism of CV catalyzed CO₂ reduction reaction and brings new prospects for designing better photocatalysts.

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