Density functional theory study of B- and Si-doped carbons and their adsorption interactions with sulfur compounds

Abstract

Understanding the adsorption interactions between carbon materials and sulfur compounds has far-reaching impacts, in addition to their well-known important role in energy storage and conversion, such as lithium-ion batteries. In this paper, properties of intrinsic B or Si single-atom doped, and B–Si codoped graphene (GR) and graphdiyne (GDY) were investigated by using density functional theory-based calculations, in which the optimal doping configurations were explored for potential applications in adsorbing sulfur compounds. Results showed that both B or Si single-atom doping and B–Si codoping could substantially enhance the electron transport properties of GR and GDY, improving their surface activity. Notably, B and Si atoms displayed synergistic effects for the codoped configurations, where B–Si codoped GR/GDY exhibited much better performance in the adsorption of sulfur-containing chemicals than single-atom doped systems. In addition, results demonstrated that, after B–Si codoping, the adsorption energy and charge transfer amounts of GDY with sulfur compounds were much larger than those of GR, indicating that B–Si codoped GDY might be a favorable material for more effectively interacting with sulfur reagents.

INTRODUCTION

Carbon materials play a very important role in energy storage and conversion, such as being commonly used as the electrode materials of lithium-ion batteries and supercapacitors.^1,2 Recent investigations have illustrated that doping materials with various heteroatoms can manifest their electrochemical properties, resulting in greatly enhanced performance in energy storage and electrocatalysis.^3,4 For the further improvement of the performance of batteries and supercapacitors, understanding the interactions between carbon electrodes and electrolytes, as well as other intermediate moieties, has become increasingly critical. Indeed, knowing the adsorption interaction between carbon and sulfur compounds not only is critical for developing advanced lithium-ion batteries but also has far-reaching impacts in other areas as well, such as pollution control and remedy.^5,6

Regarding environmental pollution of sulfur compounds, it mainly refers to the damage of sulfates, thiocyanates, and hydrogen sulfide. At present, sulfur-containing reagents in natural water in China primarily originate from untreated or partially treated industrial wastewater.⁷ There is also pollution of agricultural water bodies caused by the extensive use of pesticides and fertilizers, as well as domestic water pollution caused by improper disposal of waste and washing water.⁸ Sulfur elements in natural water bodies exist mostly in the form of sulfate (SO₄²⁻), sulfur hydrogen (HS⁻), and thiocyanate (SCN⁻), which are highly soluble and easily enriched in water. Sulfides are also highly toxic and their enrichment in water can disrupt the ecological balance of water, lowering the pH value of natural water and thereby destroying the cytochromes of plants and animals living in the water and rendering the cellular tissues of plants and animals hypoxic. In severe cases, these living species might be endangered. Therefore, effective measures must be taken to detect and remove sulfides from water. Due to the high solubility of sulfide ions in water, it is difficult to remove them using sedimentation methods. Recently, a series of technologies such as the biological method,⁹ ion exchange method,¹⁰ and chemical oxidation method^11,12 have been developed to remove sulfides from water. However, these methods are difficult to apply to a wide range of water due to the need for large reactors, complicated operations, and a risk of secondary pollution. As a new method to treat pollutants in water, the adsorption method is attracting increasing interest in removing sulfides in water because of its advantages of simple operation, low cost, and no secondary pollution.^13–15 Currently, the commonly used adsorbents include polymer adsorbents,¹⁶ metal oxide adsorbents,^17,18 molecular sieve-based adsorbents,¹⁹ and carbon nanomaterial adsorbents.^20,21

Two-dimensional carbon nanomaterials, such as graphene (GR) and graphdiyne (GDY), have large specific surface areas and their three-dimensional assembly may yield high porosity, which can provide abundant adsorption sites for adsorbate molecules. Therefore, GR and GDY have tremendous development prospects as adsorbents.^22–26 However, the stability of the C–C covalent bond limits the reactivity of GR and GDY greatly, making them undergo chemisorption with only very few molecules or atoms.²⁷ Therefore, the modification of GR and GDY is necessary. As an important and effective modification method, doping is often used to improve the electron transport properties of carbon nanomaterials while also increasing their surface reactivity.^28–31 There has been considerable progress in the application of doped GR and GDY in the field of adsorption. For example, Zhang et al.³² found that Ti doping could greatly enhance the adsorption energy of GR with various gases, such as CO, NO, SO₂, and HCHO. Their density functional theory (DFT) calculations illustrate that there is a strong electron transfer between the doped Ti atom and gas molecules. In addition, it can be revealed that such Ti-doped GR has high selectivity to the abovementioned gas molecules. Tabandeh et al.³³ found that only weak interactions exist between intrinsic GDY and commonly used drugs, such as hydroxyurea (HU) and 5-fluorouracil (FU), by DFT calculations. In contrast, for B,N-codoped GDY, there is a strong interaction with O atoms in HU and FU molecules. Therefore, B,N-codoped GDY appears to be more suitable as transport carriers for drugs, such as HU and FU.

Many studies have shown that polyatom doping could improve the adsorption properties of GR and GDY to a greater extent.^34–37 Thakur et al.³⁸ conducted theoretical calculations to examine the effect of Al, Al–S, Al–N, and Al–P doping on the electronic structure of GR. Their results show that the order of the energy gap for each doping configuration is Al-doping > Al–S codoping > Al–N codoping > Al–P codoping, indicating that codoping changes the frontier orbitals of GR more strongly. For GDY, its polyatom-doped material was successfully prepared experimentally for the first time in 2014.³⁹ This breakthrough brings the study of polyatomic doping of GDY from a theoretical level to a reality. It also confirms that multiatom doping can increase the reactivity of GDY to a greater extent while maintaining the stability of doped GDY.

Currently, nonmetallic elements are frequently used in studying doping modification of carbon nanomaterials.^40–43 Due to the different electronegativity between C and the dopant elements, the doping of these nonmetallic elements changes the band structure and improves the electrical conductivity of carbon nanomaterials. On the other hand, the nonmetallic dopant will form polar covalent bonds with the C atoms in the carbon nanomaterials, and the newly created polar covalent bonds may significantly enhance the adsorption of polar compounds on the carbon nanomaterials.⁴⁴ Theoretical studies have confirmed that when elements with electronegativity less than that of C atoms are doped into carbon nanomaterials, the doping element can act as an active site in the carbon nanomaterial and as an electron donor during chemical reactions. Such effects on the electronic structure of carbon nanomaterials consequently enhance their adsorption properties for certain adsorbates.^45,46 Nonmetallic elements B and Si are known to be less electronegative than C and, therefore, are expected to act as active sites on the surface of carbon nanomaterials after doping. On the other hand, based on the weak metallic nature of B and Si, they are less likely to break the chemical bonds of the doped carbon nanomaterials. Therefore, in this study, we investigated the properties of GR and GDY doped using B and Si atoms. Currently, the first-principles calculations based on DFT have been proven to be a powerful tool for the precise study of nanoparticles and their electronic structure.^47,48 These theoretical calculations allow predicting and probing the feasibility of improving the surface properties of matter, which is difficult to achieve in experiments. Up to now, there have been very few studies addressing the electrochemical properties of GR and GDY and their adsorption properties for sulfur compounds, even though the first-principles calculations have played a key role in studying the application of adsorbents. Therefore, we examined the effect of single-atom doping and codoping of B and Si on the electronic structure and electrical conductivity of GR and GDY by using DFT calculations. Based on the outstanding adsorption properties of GR and GDY, the adsorption performance of the most stable doping configuration for sulfur compounds (SO₄²⁻, HS⁻, SCN⁻) was subsequently explored in this study.

CALCULATION METHOD AND THE INITIAL ADSORPTION CONFIGURATION Calculation method

The optimization of all models was achieved by solving the Kohn–Sham equation.⁴⁹ The electron density function ρ(r) describing the state of the system was found, from which the adsorption energy, charge transfer, and density of states of the adsorption system were calculated.

The solution process of the Kohn–Sham equation is a self-consistent operation that sets the convergence criterion for single-atom energy in the self-consistent field to 1 × 10⁻⁵ eV. When choosing the exchange association generic function, we chose the commonly used generalized gradient approximation function.^50,51 The Perdew–Burke–Ernzerhof exchange-correlation function was used to calculate the exchange-correlation energy.⁵² The atomic orbitals were calculated using the double numeric with polarization basis set.⁵³ The Brillouin zone⁵⁴ was set up as a 3 × 3 × 1 k-point. The maximum energy convergence criterion of the system in the self-consistent field was set as 1 × 10⁻⁵ Ha (1 Ha = 27.2114 eV). The convergence criterion of the interaction force between atoms was 0.02 Ha nm⁻¹, and the convergence criterion of the maximum displacement between atoms was 0.0005 nm. When the above convergence criteria were satisfied, the structure optimization was completed.

Initial adsorption configuration

SO₄²⁻ has a tetrahedral structure formed by combining sp³ hybrid sulfur atoms and oxygen atoms. Only one adsorption model was considered for the initial adsorption configurations for SO₄²⁻. Both HS⁻ and SCN⁻ have a linear shape and the difference in their initial adsorption positions could affect the final adsorption patterns. Therefore, for HS⁻, three initial adsorption configurations were considered in this study: parallel adsorption (//), perpendicular adsorption at the end of the atom S (S), and perpendicular adsorption at the end of the H atom (H). Three similar initial adsorption configurations were considered for SCN⁻: parallel adsorption (//), perpendicular adsorption at the end of S (S), and perpendicular adsorption at the end of N (N). Using the adsorption system of intrinsic GR as an example, the initial adsorption configurations of the three ions are shown in Figure 1. The initial adsorption distance was set with reference to the van der Waals radii of each atom,⁵⁵ as shown in Table S1.

Figure 1. The initial adsorption configurations, where gray, red, yellow, white, and blue colors represent C, O, S, H, and N atoms, respectively: (A) GR–SO42−; (B) GR–HS−(//); (C) GR–HS−(S); (D) GR–HS−(H); (E) GR–SCN−(//); (F) GR–SCN−(S); (G) GR–SCN−(N).

RESULTS AND DISCUSSION Doped GR and GDY

When the spacing between adjacent GR cells is greater than 20 Å (1 nm = 10 Å), the interaction between them can be ignored.⁵⁶ To eliminate the influence of interactions between adjacent cells on the calculation results, a 12.3 Å × 12.3 Å × 20 Å supercell of GR is considered in this research. Based on the hexagonal symmetry of the benzene ring, the single-atom doping of GR considers only one model, while the codoping model needs to consider three situations. As for GDY, based on the presence of –C≡C– and the hexagonal symmetry of the central aromatic ring of GDY, three types of single-atom doped GDY models and nine types of codoped GDY models are constructed in this study. As shown in Figure 2, configurations of single-atom doped GR are named B–GR and Si–GR, respectively. The configurations of codoped GR are named B–Si–GR1 → B–Si–GR3. Each single-atom doped GDY is named B–GDY1 → B-GDY3 and Si–GDY1 → Si–GDY3, and the codoped configurations of GDY are named B–Si–GDY1 → B–Si–GDY9, respectively.

Figure 2. Models for doped GR and GDY, where pink and purple colors, respectively, represent B and Si atoms: (A) B–GR; (B) Si–GR; (C) B–Si–GR1; (D) B–Si–GR2; (E) B–Si–GR3; (F) B–GDY1; (G) B–GDY2; (H) B–GDY3; (I) Si–GDY1; (J) Si–GDY2; (K) Si–GDY3; (L) B–Si–GDY1; (M) B–Si–GDY2; (N) B–Si–GDY3; (O) B–Si–GDY4; (P) B–Si–GDY5; (Q) B–Si–GDY6; (R) B–Si–GDY7; (S) B–Si–GDY8; (T) B–Si–GDY9.

As shown in Figure 2, the covalent radius of the B atom (0.82 Å) is close to that of the C atom (0.77 Å). Therefore, the deformation of GR and GDY in the two-dimensional plane after B doping is sufficient to allow them to adapt to their structures. Specifically, when the GR and GDY are doped with B atoms, their geometry is only deformed within the two-dimensional plane. In contrast, the covalent radius of the Si atom (1.11 Å) is much larger than that of the C atom. Thus, when GR and GDY are doped with Si atoms, their structures are deformed out of the plane to a greater extent to reach a stable structure, in which protruding structures are formed on the surface of GR and GDY. Akbari et al.⁴⁰ investigated the effect of Al and N doping on the structure of graphyne by using DFT calculations. Their results show that when Al atoms replace $${{\rm{C}}}_{{\mathrm{sp}}^{2}}$$, the structure of doped graphyne undergoes a deformation outside its plane to accommodate Al atoms. When graphyne is doped with N atoms, the structure of the doped graphyne encounters only a small change in the plane. This is mainly due to the difference in radii of Al, N, and C atoms. The radius of N atoms is smaller and close to that of C atoms. Thus, doping N changes little on the chemical bond length, and the doped graphyne undergoes deformation only in the plane.

Table S2 lists the length of each chemical bond in the intrinsic and doped configurations. The lengths of the B–C, Si–C, and B–Si bonds are all greater than the length of the C–C bond. The difference between the lengths of the Si–C and C–C bonds is larger, so the deformation of the lattice during the doping of single Si and codoping of B–Si atoms is greater. In this case, the change in the total energy of the system is mainly due to the change in potential energy caused by the deformation of the lattice.⁵⁷ The difference in length between the B–C bond and the C–C bond is smaller. When GR and GDY are doped with a single B atom, the lattice of the doped system only undergoes a small deformation and the resulting change in energy is extremely small. In this case, the energy difference between breaking the C–C bond and forming the B–C bond is the main reason for the total energy change.

Table 1 lists the total energy (E_tot), cohesive energy (E_coh), energy gap (E_g), and Fermi level (E_f) for each system. Larger values of the total energy and E_coh indicate a more stable system.⁵⁸ The formula for calculating the cohesive energy is as follows⁴⁰: [Image Omitted. See PDF]where E_tot is the total energy of the system, eV; E_c is the energy of a free state C atom, eV; E_dop is the energy of a free state dopant atom, eV; n and m are the number of C and dopant atoms in the doped system, respectively.

Table 1 Total energy (E_tot), cohesive energy (E_coh), band gap (E_g), and Fermi level (E_f) of GR and GDY, eV.

Structure	Etot	Ecoh	Ehomo	Elumo	Eg	Ef
GR	−51,816.120	−7.713	−5.799	−2.930	2.869	−4.365
B–GR	−51,455.747	−7.681	−5.230	−2.974	2.256	−5.200
Si–GR	−58,653.547	−7.616	−3.883	−1.706	2.177	−4.950
B–Si–GR1	−58,291.799	−7.559	−4.991	−3.238	1.753	−4.987
B–Si–GR2	−58,287.442	−7.211	−5.285	−3.222	2.063	−5.245
B–Si–GR3	−58,286.477	−7.192	−5.184	−3.263	1.921	−5.160
GDY	−74,558.294	−7.050	−5.268	−4.830	0.438	−5.049
B–GDY1	−74,202.060	−7.461	−5.138	−4.805	0.333	−5.106
B–GDY2	−74,197.204	−7.049	−5.064	−4.825	0.239	−5.063
B–GDY3	−74,197.077	−7.047	−5.353	−5.072	0.281	−5.076
Si–GDY1	−81,401.019	−7.617	−5.402	−5.023	0.379	−5.226
Si–GDY2	−81,394.514	−7.010	−5.372	−5.031	0.341	−5.223
Si–GDY3	−81,394.284	−7.007	−5.391	−5.012	0.379	−5.242
B–Si–GDY1	−81,040.739	−7.205	−5.480	−5.042	0.438	−5.173
B–Si–GDY2	−81,042.361	−7.408	−5.293	−4.972	0.321	−5.244
B–Si–GDY3	−81,040.962	−7.202	−5.238	−4.928	0.310	−5.230
B–Si–GDY4	−81,037.436	−6.996	−5.513	−5.214	0.299	−5.242
B–Si–GDY5	−81,037.291	−6.994	−5.508	−5.257	0.251	−5.263
B–Si–GDY6	−81,037.569	−6.997	−5.538	−5.094	0.444	−5.201
B–Si–GDY7	−81,037.315	−6.994	−5.538	−5.061	0.477	−5.186
B–Si–GDY8	−81,037.462	−6.996	−5.565	−5.293	0.272	−5.304
B–Si–GDY9	−81,037.029	−6.990	−5.551	−5.211	0.340	−5.258

Abbreviations: GDY, graphdiyne; GR, graphene.

Compared to the intrinsic GR and GDY, the value of the cohesive energy of the doped system is still negative, indicating that energy is released when the atoms in the doped GR and GDY are combined to form the compound. Thus, the doped system is stable. The increase in the value of the Fermi level of the doped systems indicates that the electron transport properties of the doped systems are improved and the metallic properties of the doped GR and GDY are enhanced. The analysis of the cohesive energy and Fermi level shows that doping with B and Si enhances the activity of GR and GDY, while the doped compounds remain thermodynamically stable.

Upon calculating the total energy, the most stable doping configuration can be selected accordingly. The total energy of the B–Si–GR1 configuration is respectively 4.357 and 5.322 eV higher than those of the B–Si–GR2 and B–Si–GR3 configurations. Therefore, the B–Si–GR1 configuration is the most stable codoped GR configuration. The cohesive energy of the B–Si–GR1 configuration is also higher than that of the other codoped configurations, which further confirms that the B–Si–GR1 configuration is the most stable B–Si codoped GR configuration. Similarly, B–GDY1, Si–GDY1, and B–Si–GDY2 are the most stable single-doped and codoped GDY configurations. Therefore, the B–GR, Si–GR, B–Si–GR1, B–GDY1, Si–GDY1, and B–Si–GDY2 configurations are selected for the following adsorption calculations of sulfur compounds in this study, and the optimal material for the adsorption of sulfur compounds is then selected from them.

Figure 3 shows the electronic structure of the intrinsic and the doped configurations. In Figure 3A,E, the individual C atoms in the intrinsic GR are neutral and the C atoms in the intrinsic GDY are charged, which is the same as the findings of Xi et al.⁵⁹ After doping GR and GDY with B or Si atoms, the doped atoms have a positive charge and the C atoms around the doping site have a negative charge. It is shown that the dopant atoms (B and Si atoms) lose electrons and act as positive charge centers in GR and GDY. Yang et al.⁴⁵ prepared B-doped carbon nanotubes using chemical deposition, and their analysis of the electrical properties of the material shows that the doped B atoms have a positive charge due to the electronegativity of B being less than that of C. The B atoms act as electron donors in the doped molecules and thus enhance the surface activity of the carbon nanotubes.⁶⁰ Niu et al.⁶¹ prepared Si-doped GR nanosheets by anneal of 400°C. After doping, the Si atom has strong valence electron transfer with adjacent C atoms, and the Si atom acts as an active site in the GR nanosheets. The doping site has strong charge interaction with the surrounding adsorbates to achieve a sensing capability for the doped GR nanosheets. It is similar to the change in electronic structure found in this research. Thus, B and Si doping can enhance the reactivity of GR and GDY with certain adsorbates.

Figure 3. Electronic structure: (A) GR; (B) B–GR; (C) Si–GR; (D) B–Si–GR1; (E) GDY; (F) B–GDY1; (G) Si–GDY1; (H) B–Si–GDY2.

Figures 4 and 5 show the band structure and the partial density of states of each configuration, respectively. As seen in Figure 4A, the intrinsic GR forms a Dirac cone at the Fermi level with a band gap of 0 eV between the conduction band and valence band, which is the same as reported in references,^62,63 confirming that the calculation parameters set in this study are reliable.

Figure 4. Band structure: (A) GR; (B) B–GR; (C) Si–GR; (D) B–Si–GR1; (E) GDY; (F) B–GDY1; (G) Si–GDY1; (H) B–Si–GDY2.

Figure 5. Partial density of states: (A) GR; (B) B–GR; (C) Si–GR; (D) B–Si–GR1; (E) GDY; (F) B–GDY1; (G) Si–GDY1; (H) B–Si–GDY2.

As shown in Figure 4B–D, the Fermi level of doped GR drops below the top of the valence band, which indicates that the electrons of the doped atoms fill the valence band near the Fermi level of GR after B and Si doping, making the Fermi level of GR lower; thus B and Si doping improves the conductivity of GR. When B or Si atoms replace C atoms, the dopant atoms provide carriers in the form of holes, and the B or Si atoms form acceptor levels near the Fermi level, acting as a P-type semiconductor. This conclusion is also supported by the results presented in Figure 5B–D, where the s and p orbital electrons of the B atom and the s, p, and d orbital electrons of the Si atom fill the valence band below the Fermi level after doping. Mousavi-Khoshdel et al.⁶⁴ found that doping of carbon nanotubes with B resulted in an electron-deficient state at the doping site, which led to a narrowing of its valence band and a shift of the Fermi level toward the valence band. Their finding is consistent with the results of this research.

As seen in Figure 4E, the Fermi level of the intrinsic GDY lies between the conduction band and the valence band, with a band gap of 0.439 eV. The band gaps of the B–GDY1 and Si–GDY1 configurations are 0.331 and 0.367 eV, while the B–Si–GDY2 configuration has the smallest band gap of 0.323 eV. This indicates that the B–Si–GDY2 configuration has the best electron transport properties. Lu et al.⁶⁵ investigated the application of H and F codoped GDY for improving the capacity of Li-ion batteries. Their results showed that H and F codoped GDY could form a porous structure in the form of a fibrous network, and this structure made the material show good interfacial compatibility with the electrolyte, which could greatly enhance the storage capacity of Li in the energy storage element. H and F exhibit synergistic effects in the doped GDY, and energy storage elements based on H–F–GDY anode materials have an ultrahigh Li-ion storage capacity that cannot be achieved without or with only H or F single-atom doped GDY. In the present study, when GDY is codoped with B and Si, the two doping elements also show a synergistic effect in enhancing the metallic properties of GDY.

Adsorption of sulfur compounds by doped GR and GDY configurations

Parallel adsorption is used as an example, and the most stable doped configuration is selected for the adsorption calculations. The adsorption energy (E_ads) and adsorption distance (D) for each adsorption system are listed in Table 2.

Table 2 Adsorption energy and adsorption distance of each adsorption system.

Configuration	Eads (eV)	D (Å)	Configuration	Eads (eV)	D (Å)
GR–SO42−	−2.153	3.607	Si–GR–SO42−	−5.089	1.660
GR–HS−	−1.746	3.299	Si–GR–HS−	−4.703	2.202
GR–SCN−	−1.227	3.380	Si–GR–SCN−	−2.787	1.767
GDY–SO42−	−4.309	3.504	Si–GDY1–SO42−	−6.085	1.632
GDY–HS−	−4.115	3.758	Si–GDY1–HS−	−5.307	2.161
GDY–SCN−	−4.081	3.490	Si–GDY1–SCN−	−5.414	2.729
B–GR–SO42−	−3.232	3.473	B–Si–GR1–SO42−	−5.338	2.773
B–GR–HS−	−2.299	3.208	B–Si–GR1–HS−	−5.046	2.187
B–GR–SCN−	−1.581	3.305	B–Si–GR1–SCN−	−3.999	1.762
B–GDY1–SO42−	−4.864	1.493	B–Si–GDY2–SO42−	−6.117	1.581
B–GDY1–HS−	−4.710	1.516	B–Si–GDY2–HS−	−5.899	2.160
B–GDY1–SCN−	−4.383	1.978	B–Si–GDY2–SCN−	−5.737	1.733

Abbreviations: GDY, graphdiyne; GR, graphene.

The adsorption energy is calculated as⁵³: [Image Omitted. See PDF]where E_GR/GDY+ion is the total energy of the adsorption system, eV; E_GR/GDY is the energy of the intrinsic or doped GR/GDY configuration in its isolated state, eV; E_ion is the energy of the sulfide in its isolated state, eV.

Negative values of adsorption energy indicate that the adsorption process between the adsorbent and the sulfur compounds is exothermic, and therefore, the adsorption interaction tends to proceed spontaneously.⁶⁶ As shown in Table 2, the adsorption energy between the Si-doped or codoped configuration and the sulfur compounds is much greater than that of the intrinsic configuration for both GR and GDY, and the adsorption distance between them is also substantially reduced. This suggests that the adsorption between GR or GDY and sulfur compounds can be improved significantly when Si is one of the dopants. The increase in the adsorption energy of sulfides on the B–GR configuration is less than that on the Si–GR configuration, and the adsorption distance only changes slightly. Therefore, the adsorption between the B–GR configuration and the sulfur compounds is still unsatisfactory. The adsorption energy of the codoped configuration for sulfur compounds is generally higher than that of the single-doped configuration; that is, the codoped configuration has the best adsorption effect on these molecules. Thus, the B and Si elements synergistically improve the adsorption capacity of GR and GDY for sulfur compounds, which is consistent with the conclusions drawn from the analysis in Figure 5. Regarding the type of adsorbent, the adsorption energy of doped GDY for sulfur compounds is much higher than that of GR with the same doping type, indicating that GDY is more effective than GR for the adsorption of sulfur compounds. Therefore, GDY is more suitable as a promising material for the detection and adsorption of sulfur compounds than GR.

Using the intrinsic and codoped adsorption systems as examples, Table 3 lists the charge of each atom after sulfur compounds are adsorbed. The value of the charge can reflect the charge transfer. When the charge transfer is stronger, the interaction between the adsorbent and the sulfides is greater.²³ When the codoped configuration adsorbs sulfur compounds, the atoms in the adsorbate generally lose electrons and the change of charge is generally large, so the charge transfer between the B–Si codoped GR/GDY and the sulfur compounds is strong. When GDY is used as the adsorbent, the charge transfer of the adsorbate is generally higher than that of GR as the adsorbent. Therefore, GDY has a higher charge transfer capacity with sulfur compounds and its performance as an adsorbent is better than that of GR, which is the same as the conclusion drawn from the adsorption energy analysis in Table 2. The following is an in-depth investigation of the intrinsic and codoped GDY adsorption systems for sulfur compounds.

Table 3 Charge amount of each atom in sulfides, (e).

[Table omitted. See PDF]

Abbreviations: GDY, graphdiyne; GR, graphene.

To further investigate the differences in adsorption states between intrinsic and codoped GDY configurations and adsorbate, the charge density of each adsorption system is calculated in Figure 6. When the effective overlap in charge density between the adsorbent and the adsorbate is greater than 1.000 × 10⁻¹ e Å⁻³, it indicates that chemisorption has occurred between them.⁶⁷ As seen in Figure 6A,C,E, the effective overlap of the charge density of the adsorption system is much lower than 1.000 × 10⁻¹ e Å⁻³ when the sulfur compounds are adsorbed by the intrinsic GDY. Therefore, only physical adsorption exists in the intrinsic adsorption system, and the stability of the system is generally poor. In Figure 6B–F, however, the effective overlap of the charge density between the B–Si–GDY2 configuration and the sulfides is above 3.750 × 10⁻¹ e Å⁻³, with the maximum value of the effective overlap reaching 5.000 × 10⁻¹ e Å⁻³. Therefore, a much more stable chemisorption between the B–Si–GDY2 configuration and the sulfur compounds is formed, and the stability of such adsorption system is high.

Figure 6. Charge density of adsorption systems: (A) GDY–SO42−; (B) B–Si–GDY2–SO42−; (C) GDY–HS−; (D) B–Si–GDY2–HS−; (E) GDY–SCN−; (F) B–Si–GDY2–SCN−.

Figure 7 shows the band structure of the adsorption system and the density of states of the atoms in the system. Comparing the results of Figure 7A,C,E with Figure 4E, it can be known that there is no change in the size of the band gap of the intrinsic GDY after the adsorption, which means that there is no significant change in the electronic structure of the GDY near the Fermi level and also implies that the electron transfer between the sulfur compounds and the intrinsic GDY is extremely weak. Respective comparison of Figure 7A,C,E with Figure 7B,D,F shows that the density of states values for the s and p orbital electrons of the O atom in SO₄²⁻, for the p orbital electrons of the S atom in HS⁻, and for the p orbital electrons of the S, C, and N atoms in SCN⁻ drop sharply around the Fermi level after the adsorption by B–Si–GDY2. In addition, the band gap of B–Si–GDY2 is also substantially reduced in the band structure diagram, and a narrower band is formed near its Fermi level. This indicates that the orbital electrons in the adsorbate are filled near the Fermi level of B–Si–GDY2, reducing the number of holes formed due to the doping of B and Si atoms. Kim et al.⁶⁸ examined the adsorption properties of intrinsic and B/N-doped graphyne for SO₂ by using the DFT method. Their analysis of charge density differences shows that strong orbital hybridization between O atoms of SO₂ and B atoms of B-doped graphyne (GY-B1) triggers chemisorption. Charge redistribution occurs between SO₂ and GY-B1, and the valence electrons of O atoms in SO₂ are transferred to GY-B1. This supports our conclusions drawn from Figures 6 and 7 in this study.

Figure 7. Band structure and density of states of adsorption systems: (A) GDY–SO42−; (B) B–Si–GDY2–SO42−; (C) GDY–HS−; (D) B–Si–GDY2–HS−; (E) GDY–SCN−; (F) B–Si–GDY2–SCN−.

CONCLUSIONS

In this paper, the effects of B or Si single-atom doping and B–Si codoping on the geometrical and electronic properties of GR and GDY were investigated using DFT calculations. The optimal configurations of doped GR or GDY were consequently selected to explore their adsorption activity for the potential treatment of sulfur-containing chemicals in wastewater. The calculation results show that the s and p orbital electrons of the doping element B and the s, p, and d orbital electrons of Si were involved in forming the bands near the Fermi level of GR and GDY, resulting in the great enhancement of the reactivity of GR and GDY. The synergistic effects between the doping elements B and Si made the B–Si codoped carbon materials have higher reactivity than the B or Si single-atom doped configuration. The adsorption calculations show that the adsorption energy and charge transfer of the codoped carbon with several sulfur compounds are higher than those of the single-atom doped adsorption system, and therefore, the codoped materials have a stronger adsorption capacity. It can also be deduced from the adsorption energy and charge transfer that GDY is more promising than GR for adsorbing sulfur compounds when they are doped by the same heteroatoms. The orbital electrons of some of the atoms in the sulfur compounds are filled near the Fermi level of the codoped GDY after adsorption, reducing the holes formed due to the doping of B and Si atoms. Overall, B–Si codoped GDY has the strongest adsorption performance for sulfur-containing substances, and this type of codoped GDY can be a good alternative material for treating sulfur compounds in wastewater.

AUTHOR CONTRIBUTIONS

Peng Guo: Calculation; curation and validation of data; visualization; analysis; and original draft writing. Hong Zhang: Methodology; writing–reviewing and editing; project administration. Shuliang Dong: Investigation; interpretation; visualization. Libao An: Conceptualization; funding acquisition; supervision; and final approval of the version to be published.

ACKNOWLEDGMENTS

The authors would like to acknowledge the support of the National Natural Science Foundation of China (Grant No. 51472074).

CONFLICT OF INTEREST STATEMENT

The authors declare that there are no conflicts of interests.