Density Functional Theory Study of B, N, and Si Doped Penta-Graphene as the Potential Gas Sensors for NH₃ Detection

Abstract

Designing a high-performance gas sensor to efficiently detect the hazardous NH₃ molecule is beneficial to air monitoring and pollution control. In this work, the first-principles calculations were employed to investigate the adsorption structures, electronic characteristics, and gas sensing properties of the pristine and B-, N-, P-, Al-, and Si-doped penta-graphene (PG) toward the NH₃, H₂S, and SO₂ molecules. The results indicate that the pristine PG is insensitive to those toxic gases due to the weak adsorption strength and long adsorption distance. Nevertheless, the doping of B, N, Al, and Si (B and Al) results in the transition of NH₃ (H₂S and SO₂) adsorption from physisorption to chemisorption, which is primarily ascribed to the large charge transfer and strong orbital hybridizations between gas molecules and doping atoms. In addition, NH₃ adsorption leads to the remarkable variation of electrical conductivity for the B-, N-, and Si-doped PG, and the adsorption strength of NH₃ on the B-, N-, and Si-doped PG is larger than that of H₂S and SO₂. Moreover, the chemically adsorbed NH₃ molecule on the N-, B-, and Si-doped PG can be effectively desorbed by injecting electrons into the systems. Those results shed light on the potential application of PG-based nanosheets as reusable gas sensors for NH₃ detection.

1. Introduction

In recent years, the excessive emission of poisonous ammonia (NH₃) has brought a serious threat to the ecological environment and human health [1]. Long-term exposure to NH₃ can cause bronchitis, lung swelling, and even death [2]. In this regard, the highly efficient detection of trace NH₃ gas is regarded as an effective method to restrain its negative effects. Nowadays, many researchers have focused on the fields of seeking the high-performance sensing materials for NH₃ detection [3,4,5]. Among them, the two-dimensional (2D) nanomaterials have great potential application in gas senors due to the good stability, large surface-to-volume ratio, and massive active sites [6,7,8,9]. For instance, C-vacancy defected Ti₂CO₂ monolayer was predicted by the first-principles method to be a reusable gas sensor for NH₃ detection [10]. In addition, NH₃ adsorption based on borophene [11], C₂N monolayer [12], phosphorus carbide monolayer [13], and Ni-doped InN monolayer [14] has been theoretically studied to evaluate the possibility of those materials as an NH₃ sensor. However, some intrinsic shortcomings, including the insufficient sensitivity, excessively large adsorption strength, and long recovery time, restrict their industrial utilization. Thus, it is necessary to explore the high-performance sensor material for NH₃ detection.

Penta-graphene (PG), a new 2D allotrope of graphene, was recently proposed and predicted to have excellent dynamical and mechanical stability up to 1000K [15]. Moreover, many theoretical investigations showed that PG has a great potential application in gas capture and sensing [16,17,18], hydrogen storage [19], and lithium-ion batteries [20] due to its good mechanical, electrical, and optical properties [21,22]. Cheng et al. [23] investigated the electronic and transport properties of NH₃, NO, and NO₂ adsorbed on PG monolayers by using the first-principles and non-equilibrium Green’s function methods, and revealed that PG was highly sensitive and selective to NO and NO₂ molecules, while it showed poor NH₃ sensing performance because of the weak physisorption. As known, the heteroatom dopant method has been proven to significantly improve the interactions between gas molecules and substrate [24,25]. For example, the adsorption strength, charge transfer, and sensing properties of CO and CO₂ on Fe-doped PG were higher than those on the pristine PG [26]. By using the density functional theory (DFT) calculations, Chen et al. [17] found that the doping of B and N into PG could obviously enhance the adsorption strength of SF₆ decomposed products and demonstrated the B-doped PG could be a good sensing material for H₂S molecules. In addition, the N-doped PG exhibited high CO₂ selectivity from the mixtures of H₂, N₂, and CH₄ in an external electric field [16]. In addition, the Pt-doped PG was reported to have good detection ability of H₂ molecules [27]. Nevertheless, the investigations into the sensing properties of PG with different dopants toward the poisonous NH₃ are still lacking.

Generally, the gaseous pollutants of NH₃, H₂S, and SO₂ are simultaneously released from the combustion of fuel and industrial production. Therefore, the first-principles calculations based DFT method were performed to study the adsorption behaviors of these toxic gases on the pristine and B-, N-, P-, Al-, and Si-doped PG monolayers, and the corresponding sensing characteristics were discussed in this work. Meanwhile, the adsorption structure and electronic properties (e.g., energy band, density of states, difference charge density, and charge transfer) were also systematically analyzed. Our results indicate that the B-, N-, and Si-doped PG nanosheets could be promising NH₃ sensing materials with high selectivity and sensitivity, which provide the fundamental basis for the development of novel PG-based gas sensor.

2. Calculation Method and Details

All the gas sensing simulations on pristine penta-graphene (PG), doped-PG, and charged PG were performed using the Dmol³ module [28] of Materials studio software (Accelrys, California, USA). The generalized gradient approximation in the form of the Perdew–Burke–Ernzerhof [29] and the double numerical plus polarization (DNP) basis set [30] were chosen for all the spin calculations. The Grimme approach [31] and DFT semi-core pseudopotential (DSSP) method [32] were used to describe the van der Waals interactions and core treatment, respectively. The 3 × 3 ×1 supercell of PG (54 atoms) with 20 Å along the z direction was adopted during the simulations, and the Monkhorst−Pack meshes of 4 × 4 ×1 (10 × 10 ×1) were employed to conduct the geometry optimizations (electronic properties calculations), and the Hirshfeld charge was calculated to analyze the charge transfer between gas molecules and substrates. In addition, the global orbital cutoff radius was set as 4.6 Å, and the convergence tolerances were 1.0 × 10⁻⁵ Ha, 0.002 Ha/Å, and 0.005 Å for total energy, maximum force, and maximum displacement, respectively. In doped PG systems, the formation energy (E_for) of B, N, P, Al, or Si atoms could be determined by the following equation [17]:

$${\mathrm{E}}_{\mathrm{for}}{=\mathrm{E}}_{\mathrm{doped}\text{-}\mathrm{PG} }{- \mathrm{E}}_{\mathrm{PG}}{+\mathrm{E}}_{\mathrm{C}} {- \mathrm{E}}_{\mathrm{doped}\text{-}\mathrm{atom}}$$

(1)

where ${\mathrm{E}}_{\mathrm{PG}}$ and ${\mathrm{E}}_{\mathrm{doped}\text{-}\mathrm{PG}}$ are the total energies of pristine and doped PG, and ${\mathrm{E}}_{\mathrm{C}}$ and ${\mathrm{E}}_{\mathrm{doped}\text{-}\mathrm{atom}}$ are the chemical potential of C and doped atoms, respectively. The chemical potentials of C (B, P, Al, and, Si) were obtained from the average energy for every C (B, P, Al, and Si) atom in bulk diamond (B, P, Al, and Si), while the chemical potential of the N atom was calculated by the N₂ molecule.

The adsorption characteristics of NH₃, H₂S, and SO₂ over pristine and doped PG were studied in this work, and the adsorption energy (E_ads) could be obtained by [33,34]

$${ \mathrm{E}}_{\mathrm{ads}}{=\mathrm{E}}_{\mathrm{gas}+\mathrm{substrate} }{- \mathrm{E}}_{\mathrm{substrate}} {- \mathrm{E}}_{\mathrm{gas}}$$

(2)

where ${\mathrm{E}}_{\mathrm{gas}+\mathrm{substrate}}$ and ${\mathrm{E}}_{\mathrm{substrate}}$ are the total energies of pristine PG (doped PG) with and without gas adsorptions, and ${\mathrm{E}}_{\mathrm{gas}}$ is the energy of a single gas molecule. As known, a more negative E_ads means a stronger gas–substrate interaction.

3. Results and Discussion

3.1. Electronic Properties and Stability of Doped PG

The optimized lattice constant of PG with P-421M symmetry is 3.63 Å, and the calculated band gap is 2.43 eV (Figure 1c), both of which match well with previous reported results [35]. There are two types of carbon atoms, i.e., the sp² hybridized carbon atom (C1 site) and sp³ hybridized carbon atom (C2 site), as shown in Figure 1a. Thus, the C1 and C2 sites substituted by B, N, P, Al, or Si atoms are both taken into account, and the obtained formation energy of different doped PG is listed

Table 1. The formation energy of penta-graphene with different dopants.

Structures	Efor/eV
	sp2 Hybridized Carbon Atom (C1 Site)	sp3 Hybridized Carbon Atom (C2 Site)
B-doped PG	+0.92 (+0.67) [17]	+1.00 (+0.74) [17]
N-doped PG	−0.10 (−0.01) [17]	+1.80 (+1.90) [17]
P-doped PG	−0.14	+0.95
Si-doped PG	+0.87	+0.96
Al-doped PG	+1.51	+3.23

. The difference between our results and other studies may be ascribed to the different supercell size and dopant concentration. One can see that N and P substitutions at the C1 site are the exothermic process, while the other doping cases are the endothermic process. Additionally, all the dopant atoms prefer to substitute the C1 atom due to the smaller formation energy, which is consistent with previous calculated results [17,21]. Therefore, the cases of B, N, P, Al, and Si substituting the C1 site are discussed in the following sections.

3.2. Adsorption of NH₃, H₂S and SO₂ on Pristine PG

In order to evaluate the detection ability of PG, the adsorption behaviors of NH₃, H₂S, and SO₂ on the pristine PG were investigated in this section. Considering the structure symmetry of PG, six possible adsorption sites, including two top sites (labeled as A and B), three bridge sites (labeled as C, D, and E), and one hollow site (labeled as F) were taken into account, as displayed in Figure 1a. For convenience, one gas molecule adsorption on the PG is defined as “gas@PG”, for instance, NH₃@PG means the NH₃ adsorption on PG. The calculated adsorption energy, adsorption distance, charge transfer, and band gap of three toxic gases adsorbed on PG are presented in

Table 2. Adsorption energy (E_ads), the Hirshfeld charge transferred from PG to gases (Q_T), adsorption distance (d), and band gap (E_g) of different gases adsorbed on pristine PG.

Gas Molecules	Eads/eV	QT/e	d/Å	Eg/eV
NH3	−0.382	0.058	2.970 (N-C)	2.35
H2S	−0.298	0.040	3.169 (S-C)	2.22
SO2	−0.328	−0.072	2.661 (S-C)	1.90

. It is found that the adsorption energies of the NH₃, H₂S, and SO₂ molecules are −0.298 eV∼−0.382 eV, and the adsorption distances are in the range of 2.661 Å to 3.169 Å (Figure 2). This indicates that all the toxic gases belong to the representative physisorption, which can be also demonstrated by the small Hirshfeld charge transferred from PG to gases. Moreover, compared with the pristine PG, the band gap of PG after gas molecule adsorptions shows little change. Therefore, the pristine PG exhibits poor detection ability toward the NH₃, H₂S, and SO₂ molecules due to weak interactions and negligible resistance variation.

To further comprehend the micro-interaction mechanism between gas molecules and PG, the density of states (DOSs) and charge density difference (CDD) of various adsorption systems were calculated, as shown in Figure 3 and Figure 4. For the NH₃@PG, H₂S@PG, and SO₂@PG, the gas–substrate interactions are mainly contributed by the weak hybridizations between PG and p orbitals of gas molecules. By contrast, NH₃-s, H₂S-s, and SO₂-s orbitals show the weaker interactions with the penta-graphene due to the small peak value of DOSs (see Figure 3). For the NH₃@PG and H₂S@PG, the partial electrons of NH₃ or H₂S are transferred to PG (Figure 4a,b), indicating the NH₃ and H₂S molecules act as electron donors, which agrees well with the results of the Hirshfeld charge (Table 1). In addition, compared with the case of H₂S, NH₃ loses more electrons and thus has the lower adsorption energy. For the SO₂@PG, a part of the charge of PG is transferred to the SO₂ molecule, as shown in Fig. 4c, which suggests the SO₂ molecule acts as the electron acceptor. However, the small transfer charge between gas molecules and PG still results in weak interactions, and thus the pristine PG shows poor detection ability toward these toxic gases.

3.3. Adsorption of NH₃, H₂S, and SO₂ on Doped PG

The adsorption behaviors of NH₃, H₂S, and SO₂ on M-doped penta-graphene (M-PG, M=B, N, Al, and Si) were also investigated, and the obtained calculation results and corresponding lowest energy configurations are displayed in Table 2 and Figure 5. Compared with the NH₃ on pristine PG, doping B, N, Al, or Si atoms into PG nanosheets can obviously enhance the NH₃ adsorption strength with the E_ads of −1.06 eV∼−2.46 eV, resulting in the transition of NH₃ adsorption from physisorption to chemisorption. In addition, the adsorption distances are significantly shortened (Figure 5a–d), and the charge number transferred from NH₃ to the doped PG is remarkably increased in comparison with the case of NH₃ adsorption on the pristine PG. By contrast, only the doping of Al and B atoms can strengthen the adsorption of PG toward H₂S and SO₂, the rest of the dopants show a slight change (see Table 2). In particular, the H₂S is dissociation-chemisorbed on the Al-PG surface with one H bonding to the nearest neighbor C atom, as shown in Figure 5f. Therefore, the doping of B, N, P, Al, and Si atoms may be able to improve the detection ability of the NH₃, H₂S, and SO₂ molecules.

The variation of electric conductivity (σ) could be utilized to evaluate the sensitivity of a material before and after molecular adsorption, which is defined as σ ∝ exp(−E_g/2K_BT), where E_g, K_B, and T are the band gap, Boltzmann constant, and temperature, respectively. As mentioned above, NH₃ and H₂S adsorptions lead to a little charge in the band gap of the pristine PG, which demonstrates that PG has poor sensitivity to NH₃ and H₂S. However, the band gap of B-, N-, and Si-PG undergo great variation after NH₃ adsorption, as shown in Figure 6e,f,h, implying that NH₃ adsorption has a huge impact on the resistivity of B-, N-, and Si-doped PG, thus these doped PG exhibits high NH₃ sensitivity. Furthermore, the adsorption strength of NH₃ on the B-, N-, and Si-PG surface is much larger than that of H₂S and SO₂ on the same substrates (see

Table 3. Adsorption energy (E_ads) of various adsorption systems, Hirshfeld charge transferred from gases to doped PG (Q_T), and band gap of the system before adsorption (E_g) and after adsorption (E_g’). The positive (negative) Q_T means that gases lose (gain) charge.

Structures	NH3			H2S			SO2
	Eads	QT	Eg (Eg’)	Eads	QT	Eg (Eg’)	Eads	QT	Eg (Eg’)
B-doped PG	−2.46	0.406	1.81/1.23	−1.63	0.360	1.81/1.31	−1.13	−0.105	1.81/1.82
N-doped PG	−1.06	0.465	1.88/2.27	−0.20	0.009	1.88/1.65	−0.42	−0.133	1.88/0.00
P-doped PG	−0.57	0.147	0.00/0.00	−0.40	0.067	0.00/0.00	−0.29	−0.043	0.00/1.33
Al-doped PG	−2.17	0.312	0.54/0.76	−2.87	−0.043	0.54/0.42	−0.97	−0.203	0.54/0.46
Si-doped PG	−1.11	0.354	1.32/0.84	−0.41	0.073	1.32/1.20	−0.35	−0.168	1.32/1.10

). Those results indicate that B-, N-, and Si-PG show excellent NH₃ selectivity and sensitivity and could be a promising gas sensor for NH₃ detection. In addition, compared with the Al-PG, the B-PG also has good H₂S sensitivity due to the obvious change of band gap, as displayed in Figure 6i. Nonetheless, the band gap of Al-PG and B-PG remains unchanged in spite of SO₂ chemisorption (Figure 6k,l), which suggests that Al- and B-PG could not be the SO₂ sensing materials.

Figure 7 displays the difference charge density of NH₃, H₂S, and SO₂ adsorbed the doped PG. For the NH₃ adsorption, one sees that the charge accumulation primarily appears around the N, B, Al, and Si atoms, while the charge depletion occurs in the vicinity of the NH₃ molecule (Figure 7a–d). The Hirshfeld charge analysis indicates the NH₃ acts as an electron donor with a large charge transfer from NH₃ to N-, B-, Al-, and Si-doped PG of 0.465 e⁻, 0.406 e⁻, 0.312 e⁻, and 0.354 e⁻, respectively. Those results also demonstrate that NH₃ is chemically adsorbed on N-, B-, Al-, and Si-doped PG. The CDD plot of H₂S@B-PG shows similar results as displayed in Figure 7e. However, for the SO₂@B-PG and SO₂@Al-PG systems, the charges are mainly accumulated around the SO₂ molecule, while the charges are depleted near the B and Al dopants, thus the SO₂ acts as an electron acceptor and obtains the charge from B-PG and Al-PG by 0.105 e^- and 0.203 e⁻ (see Table 3). Considering the large adsorption strength and charge transfer, the SO₂ adsorbed on the B- and Al-doped PG also belongs to chemisorption.

In order to further understand the micro-interactions of the gas-substrate, we calculated the DOSs of the N (S) atom of NH₃ (H₂S/SO₂) and its nearest neighbor dopant atoms for various adsorption systems, as displayed in Figure 8. For the NH₃@N-PG system, the NH₃ is adsorbed on top of its nearest neighbor C atom, and the obvious orbital hybridizations between C-2p and N-2p of NH₃ can be observed in the range of −12.5 eV to −10 eV, while the relatively weak interactions between C-sp and N-2s are found at −21.05 eV as shown in Figure 8a. For the NH₃@B-PG, NH₃@Al-PG, and NH₃@Si-PG systems (Figure 8b–d), there are significant hybridizations between N-2p of NH₃ and B-, Al-, and Si-sp orbitals ranging from −10.0 eV to −5.0 eV. Moreover, there are also weak hybridizations of N-2s and B-, Al-, and Si-2s around −20.0 eV. In summary, the strong interactions between the N atom of NH₃ and its nearest neighbor atoms result in NH₃ chemisorption. As for the H₂S@B-PG and H₂S@Al-PG, the strong adsorption strength of the H₂S molecule is primarily contributed by the orbital interactions between S-2p of H₂S and B- and Al-sp in the range of −10.0 eV to 0.0 eV (Figure 8e–f). By contrast, the strong orbital hybridizations between S-2p of SO₂ and B- and Al-sp occur in the larger range of −10.0 eV to 2.5 eV for the SO₂@B-PG and SO₂@Al-PG systems, which leads to the SO₂ molecule being chemically adsorbed over the B- and Al-doped PG.

3.4. Recovery of Doped PG after Sensing Toxic Gases

The short recovery time at room temperature is also of great importance for a good sensing material. Consequently, the feasibility of the toxic gases (NH₃, H₂S, and SO₂) desorbed from the N-, B-, Si-, and Al-doped PG was explored in this section. As displayed in Table 3, the adsorption energies of NH₃, H₂S, and SO₂ on the doped PG with chemisorption are all lower than −0.97 eV. As known, the gas molecule can be released from a solid material when the corresponding E_ads is higher than −0.50 eV [36]. Accordingly, those toxic gases cannot be desorbed immediately after adsorption, and the recovery time becomes very long at room temperature. Recently, the introduction of a negative charge could effectively modulate the adsorption strength of a gas molecule on the substrate [37,38,39]. In this regard, the adsorption energies of NH₃, H₂S, and SO₂ on the doped PG with a different negative charge were calculated, and the results are shown in Figure 9. With the increase in injected negative charge, the adsorption strength of NH₃ on B-, Al-, and Si-doped PG is gradually decreased, while that of H₂S and SO₂ on B- and Al-doped PG is gradually increased. This indicates that the H₂S or SO₂ removal from the B-PG and Al-PG surface becomes more difficult after the introduction of electrons due to the stronger adsorption strength. Nevertheless, the adsorption energies of NH₃@B-PG and NH₃@Si-PG are increased from −2.46 eV to −0.38 eV and from −1.11 eV to −0.29 eV after adding 2.0 e⁻ into the systems. For the NH₃@N-PG system, the adsorption energy is also remarkably changed from −1.06 eV to −0.24 eV (−0.35 eV) after injecting 0.5 e⁻ (1.0 e⁻) into the N-PG substrate. Thus, we can conclude that the chemically adsorbed NH₃ molecule on the N-, B-, and Si-doped PG could be easily desorbed by controlling the number of injected electrons into the systems. Considering the N-, B-, and Si-doped PG also exhibit good selectivity and sensitivity toward the NH₃ molecule as discussed above, we can therefore say that the N-, B-, and Si-doped PG could be a promising sensing material for NH₃ detection.

4. Conclusions

In summary, the adsorption behaviors and gas sensing properties of the pristine and N-, B-, P-, Al-, and Si-doped PG toward three toxics gases (NH₃, H₂S, and SO₂) were studied by DFT calculations in this work. In addition, the adsorption characteristics, electronic properties, and the sensing mechanism were also discussed. The results of formation energy show that the substitution of the C1 atom by N, B, P, Al, or Si is energetically favorable. The pristine PG exhibits poor sensing ability of the NH₃, H₂S, and SO₂ molecules due to weak adsorption strength. In contrast, doping B, N, Al, or Si atoms into PG nanosheets can obviously improve the NH₃ adsorption strength, while only the doping of the B and Al atoms enhance the interactions between the H₂S and SO₂ molecules and PG, which was further verified by the analysis of DOSs, CDD, and the charge transfer. Additionally, the adsorption strength of NH₃ on B-, N-, and Si-doped PG is much larger than that of H₂S and SO₂ on the same substrates, demonstrating that the B-, N-, and Si-doped PG have good NH₃ selectivity from the mixtures of H₂S and SO₂. In addition, doped PG exhibits high NH₃ sensitivity due to its great variation of resistivity after NH₃ adsorption. More interestingly, the chemically adsorbed NH₃ molecule on the N-, B-, and Si-doped PG could be effectively desorbed by controlling the number of injected electrons into the systems. Therefore, the N-, B-, and Si-doped PG can be the potential materials to develop NH₃ sensors with high sensitivity and selectivity.