Si-Doped Nitrogenated Holey Graphene (C₂N) as a Promising Gas Sensor for O-Containing Volatile Organic Compounds (VOCs) and Ammonia

Abstract

Two-dimensional (2D) crystalline materials have been regarded as promising sensor materials due to their large specific surface area, high sensitivity, and low cost. In the present work, based on the density functional theory (DFT) method, the sensor performance of novel silicon (Si)-doped nitrogenated holey graphene (SiC₂N) toward five typical VOCs (HCHO, CH₃OH, C₃H₆O, C₆H₆, and C₂HCl₃) and ammonia were systematically investigated. The results demonstrated that Si doping could effectively decrease the band gap of C₂N and simultaneously provide active sites for gas adsorption. Through comprehensive analyses of adsorption energies and electronic properties, the SiC₂N was found to exhibit high selectivity for O-containing VOCs (HCHO, CH₃OH, and C₃H₆O) and NH₃ via a covalent bond. Moreover, after the HCHO, CH₃OH, C₃H₆O, and NH₃ adsorption, the band gap of SiC₂N greatly decreases from 1.07 eV to 0.29, 0.13, 0.25, and 0.12 eV, respectively, which indicated the enhancement the conductivity and enabled the SiC₂N to be a highly sensitive resistive-type sensor. In addition, the SiC₂N possesses a short recovery time. For instance, the recovery time of HCHO desorbed from SiC₂N is 29.2 s at room temperature. Our work anticipates a wide range of potential applications of Si-doped C₂N for the detection of toxic VOCs and ammonia, and supplies a valuable reference for the development of C₂N-based gas sensors.

1. Introduction

Along with the improvement of residential air tightness, the issues related to indoor air pollution have aroused increasing critical attention. Volatile organic compounds (VOCs) and ammonia are identified as the main indoor contaminants that are mainly emitted from building and furnishing materials [1]. Long-term exposure to these harmful gases can cause severe health problems such as headaches, respiratory tract irritation, nervous system injury, and even cancer such as leukemia, etc. [2,3]. Given the increasing health hazard, the environmental monitoring of exposed VOCs is desired.

Over time, researchers have devoted continuous efforts to develop novel and high-performance capture agents for gaseous contaminants detection [4,5,6]. Two-dimensional (2D) and nonmetal materials, such as graphene, graphdiyne, phosphorene, carbon nitrides, etc., have been widely used in interdisciplinary regions, such as gas sensors [7,8,9,10,11,12,13,14,15], spintronics [16,17], photo- or electro- catalysis [18,19], and so on, due to their large specific surface area, unique electronic properties, and low cost. Additionally, such 2D materials that act as sensor materials exhibited high sensitivity in minute concentrations. For instance, the graphene/polyaniline nanocomposite proposed by Wu et al. [9] exhibited high sensitivity for ammonia molecules at ppm level. In addition, Ahmad’s group [12] reported that the detection limit of black phosphorus toward NO₂ was as low as 5 ppb. Very recently, a novel 2D nitrogenated holey graphene (C₂N) was synthesized by Mahmood’s group [20] through a bottom-up wet-chemical reaction. The newly synthesized C₂N possesses a high specific surface area and uniformly distributed pores and is deemed a potential candidate for gas detection. Previous computational works manifested that C₂N-based materials can be used in the detection of some toxic gases such as NI₃ [21], H₂S [22], and so on [23,24]. However, the pristine C₂N exhibited finite adsorption ability, low selectivity, and low sensitivity toward VOCs and ammonia (NH₃) [25,26,27]. Can the sensor performance of C₂N toward VOCs and ammonia be regulated through appropriate functionalization strategies?

Previous studies have demonstrated that heteroatom doping can not only offer active sites for gas adsorption but also promote the charge transfer between gases and substrate, thereby effectively enhancing the sensor performance of materials. For instance, metal oxides doped with Sc [28] and Ag [29] atoms exhibited enhanced sensor performance toward VOCs in contrast with their pure phase. Additionally, it has been reported that B [30], N [31], and Li [32] atoms doping could effectively enhance the selective recognition of carbon-based materials toward VOCs and ammonia. In addition, previous theoretical studies have reported that metal (e.g., Al and Ti)-doped C₂N were potential sensing materials for certain VOCs [27,33]. At present, it remains curious if nonmetal-modified C₂N is an efficient sensor material for VOCs. Silicon (Si), as one of the most earth-abundant elements, is non-toxic and environmentally friendly. Ren and coworkers [34] have prepared Si-doped metastable ε-phase WO₃ as a gas sensor for acetone (C₃H₆O). It was found that Si element doping could not only provide abundant target–receptor sites for target gases but also enhance the charge transfer performance of WO₃. The synthesized Si-doped ε-WO₃-based detector exhibited high sensitivity (a detection limit of 10 ppb) and outstanding selectivity for C₃H₆O. In addition, Guntner and coworkers found that the sensor response of MoO₃ toward NH₃ increased twice via Si doping when compared to its pristine phase [35]. Very recently, the DFT method has been utilized by Singsen et al. [36] to investigate the chemical detection performance of Si-doped green phosphorene (Si-GreenP) for biotoxin volatile organic compounds. The results demonstrated that the Si-GreenP presented enhanced adsorption ability (almost ~five times greater than pure GreenP) and selectivity toward formaldehyde (HCHO) and C₃H₆O. Additionally, considerable changes have been found in the band gap and work function of Si-GreenP after gas adsorption, suggesting its excellent sensitivity as a gas sensor. Given these advances, Si-doped C₂N may be a potential gas sensor for VOCs and NH₃. However, at present, the understanding of electronic characteristics and potential sensor performance of Si-doped C₂N remain in its infancy and needs further exploration.

In the current work, the dispersion corrected density functional theory was implemented to investigate the electronic properties of Si-doped nitrogenated holey graphene (SiC₂N) as well as its sensor ability for typical VOCs (HCHO, methanol (CH₃OH), C₃H₆O, benzene (C₆H₆), and trichloroethylene (C₂HCl₃)) and NH₃. Firstly, the detailed doping modes of SiC₂N and the inherent regulation of the Si atom on the electronic structures of C₂N were exhaustively discussed. Next, we systematically analyzed the adsorption performance and selectivity of pure C₂N and SiC₂N toward various VOCs and NH₃. It was found that after Si doping, the capture ability and selectivity of C₂N toward O-containing VOCs and NH₃ were dramatically improved. In addition, the SiC₂N possessed high sensitivity and appropriate recovery time as a resistive-type sensor. These explorations aim to contribute a theoretical basis to existing knowledge and provide new insights for the design and development of high-performance C₂N-based sensing materials used for VOCs and ammonia detection.

2. Materials and Methods

The 2 × 2 supercell of C₂N was used as the basic computational model in the present work. A vacuum space of 20 Å in the z-direction was employed to avoid the interlayer interaction. All the spin-polarized density functional theory calculations were executed by using the Vienna ab initio simulation package (VASP) code [37]. The Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was adopted to treat the exchange–correlation energies and potentials [38]. The interactions between core ions and valence electrons were depicted with the projector augmented wave (PAW) pseudopotential [39] with the cutoff energy set to 500 eV. Additionally, the D3 vdw correction proposed by S. Grimme was performed for the consideration of van der Waals interactions [40,41]. The convergence criteria for the total energy and the Hellmann–Feynman force were 10⁻⁵ eV and 0.01 eV/Å, respectively. The 3 × 3 × 1 and 7 × 7 × 1 Monkhorst–Pack grids were utilized for the geometric optimizations and electronic characteristics, respectively. In addition, the electronic structure calculations with HSE06 functional [42] were also carried out for partial systems with the Monkhorst–Pack grids of 7 × 7 × 1 to obtain accurate band structure and band-gap values. The formation energies for the Si atom doping process were calculated by the following equation:

E_form = E(SiC₂N) − E(C₂N) − μ(Si)

(1)

where the E(SiC₂N) and E(C₂N) were the total electronic energies of SiC₂N and pristine C₂N, respectively. The μ(Si) represented the potential energy of Si atoms, which was adopted as the cohesive energy of each Si atom in the Si-bulk (−4.63 eV).

The adsorption energies (E_ad) of SiC₂N toward gases were obtained by Formula (2):

E_ad = E(SiC₂N_gas) − E(SiC₂N) − E(gas)

(2)

where the E(SiC₂N_gas) and E(SiC₂N) donated the total electronic energies of SiC₂N_gas and SiC₂N, respectively. Additionally, the E(gas) were the energies of isolated VOCs and NH₃. In addition, the conductivity and adsorption of Gibbs free energies were found within the VASPKIT code [43]. Moreover, the Atomistix ToolKit (ATK) package was performed to investigate the transport properties of the SiC₂N-based systems [44].

3. Results and Discussion

3.1. Geometric Configurations and Electronic Properties of Si-Doped C₂N

Firstly, the incorporation configurations and formation energies of Si-doped C₂N (SiC₂N) were comprehensively investigated. As shown in Figure 1a, there are five distinct sites, including the C atom, N atom, and interstitial space (I1, I2, and I3) for Si doping; the optimized configurations are shown in Figure S1. The computational results demonstrate that the Si atom is more favorable to anchor on the I1 site with the formation energy of −1.51 eV. This configuration was filtered out and displayed in Figure 1b. The negative formation energy indicates that the doping process is exothermic, which provides significant feasibility in terms of experimental synthesis of Si-doped C₂N. Herein, the doping density of the Si atom is at a relatively low level of 1.37 at%. Then, we only focus on this configuration. The stability of SiC₂N was further validated by the ab initio molecular dynamics simulations (AIMD) [45] at 300 and 500 K for 10 ps, respectively. As shown in Figure S2, the geometric structure of SiC₂N is well preserved at room temperature and even at 500 K, indicative of the high thermodynamic stability of SiC₂N.

Then, the band structures and partial density of states (PDOS) plots were investigated with the HSE06 functional in Figure 1c–f to obtain accurate electronic characteristics of intrinsic C₂N and SiC₂N. Firstly, as shown in Figure 1c,d, the pristine C₂N exhibits a semiconductor feature with a direct band gap of ~2.41 eV, which is quite consistent with previous studies [46]. As shown in Figure 1e, after Si doping, the band gap of SiC₂N considerably decreases to 1.07 eV, which is conducive to electron transmission. Additionally, the Si doping yields a new state on the VBM, which is mainly localized around the Si atom. More notably, from the PDOS plot of SiC₂N, we can observe that the VBM mainly consists of active p_z electrons, which ensure high activity of SiC₂N for gas adsorption. In addition, strong hybridization can be observed between Si and N atoms, signing strong covalent characters. Herein, the band structures of pure C₂N and SiC₂N were also calculated with the PBE functional, as displayed in Figure S3a,b. Additionally, the obtained band gap values for C₂N and SiC₂N with the PBE functional are 1.65 and 0.60 eV, respectively. To sum up, Si doping can effectively regulate the electronic structure of C₂N, which will be beneficial for electron transmission and gas adsorption.

The electron localization function (ELF) map of SiC₂N is presented in Figure 2a. The ELF map has been deemed a useful tool to directly ascertain the bonding nature and the degree of electron localization [47,48]. In general, the region with an ELF value > 0.5 signs covalent interaction and high electron localization [49]. Firstly, topographical analysis of ELF manifests that the formed Si-N exhibits a covalent feather. In addition, the highly localized electrons are located on the Si atom, which is susceptible to interaction with the incoming gas molecules. In addition, the charge density difference (CDD) plot of SiC₂N between the Si atom and C₂N was also analyzed in Figure 2b, from which we can see remarkable charge transfer between the Si atom and the substrate. Additionally, the Bader charge result demonstrates that the Si atom donates 1.28 e to the C₂N.

3.2. Adsorption of Gases on Pristine C₂N

In this section, the adsorption behavior of gaseous pollutants on pristine C₂N was investigated. Four typical and toxic VOCs (HCHO, CH₃OH, C₃H₆O, C₆H₆, and C₂HCl₃) and NH₃ were selected as the target adsorbates. Various initial adsorption configurations were constructed to determine the most stable structures of C₂N_gas systems. According to previous studies, the gases preferred to adsorb on the large hole site of C₂N (see Figure S3c) due to its copious π-electrons; thus, it was chosen as the adsorption site for VOCs and NH₃ [24,50,51]. For volatile organic compounds, the adsorption models were established by laying the pollutants in parallel to vertical positions on the hole site. In the vertical scenarios, the interaction sites of VOCs were also considered. For NH₃, only the interaction sites of NH₃ (N and H) were taken into account. The initial binding configurations, the optimized structures, and the adsorption energies of gases on pristine C₂N were recorded in Figures S4–S7.

The optimal binding configurations were filtered out and depicted in Figure 3, and the corresponding adsorption energies were listed in

Table 1. The adsorption energies (E_ad) and Bader charge transfer (Δq) in C₂N_gas and SiC₂N_gas systems. The minus sign in Δq represents the electron reduction of gases.

	Ead (eV)	Δq (Gas)		Ead (eV)	Δq (Gas)
C2N_HCHO	−0.47	0.017	SiC2N_HCHO	−1.04	0.388
C2N_CH3OH	−0.53	0.019	SiC2N_CH3OH	−0.90	0.057
C2N_C3H6O	−0.44	0.005	SiC2N_C3H6O	−0.60	0.134
C2N_C6H6	−0.38	0.003	SiC2N_C6H6	−0.36	−0.016
C2N_C2HCl3	−0.48	0.001	SiC2N_C2HCl3	−0.41	0.017
C2N_NH3	−0.38	−0.004	SiC2N_NH3	−1.21	−0.046

. As displayed in Figure 3, the HCHO, C₆H₆, and C₂HCl₃ molecules tend to adsorb on the C₂N sheet in a vertical orientation, in which their C-H bonds point toward the large hole of C₂N. The vertical distances (named as D_v) between the HCHO, C₆H₆, and C₂HCl₃ molecules and the substrate are 1.03, 1.46, and 1.66 Å, respectively, while the CH₃OH and C₃H₆O molecules prefer to lie flat on the C₂N surface with vertical distances of 1.41 and 1.85 Å, respectively. When it comes to the NH₃ molecule, the NH₃ is adsorbed on the surface of C₂N with its two H atoms downwards; the D_v between NH₃ and C₂N is 1.29 Å. Notably, scrutinizing the binding configurations of C₂N_gas systems, there are no obvious chemical bonds between them, which portends weak interaction. Moreover, the adsorption energy of −0.50 eV is always used to distinguish physical and chemical adsorption [52]. As summarized in Table 1, the E_ad of C₂N for HCHO, CH₃OH, C₃H₆O, C₆H₆, C₂HCl₃, and NH₃ molecules are −0.47, −0.53, −0.44, −0.38, −0.48, and −0.38 eV, respectively, demonstrating physisorption or weak chemisorption. Their relatively weak interaction ascribes to the van der Waals affinity of π-electrons in C₂N toward VOCs (or NH₃). Additionally, it can be seen that the adsorption energies of pure C₂N toward these gases are at a comparable level, indicating poor selectivity.

Then, the electronic properties of C₂N_gas systems, including PDOS and Bader charge, were investigated to further comprehend the interaction behavior between C₂N and contaminants. The density of states of gases before and after adsorption, the partial density of states of gases, and C₂N in the adsorption systems were displayed in Figure S8. On the one hand, there are tiny state variations in the DOS of gas molecules after the adsorption process. On the other hand, the gas molecules hybridize with C₂N at a low degree. Additionally, as recorded in Table 1, only thimbleful charge transfers can be observed in C₂N_gas systems. The density of states and Bader charge results further prove the relatively weak interaction between C₂N and VOCs (or NH₃).

3.3. Adsorption of Gases on SiC₂N

3.3.1. Binding Configurations and Adsorption Energies

The adsorption behavior of VOCs and NH₃ on Si-doped C₂N was probed in this section. Herein, the Si and hole sites were selected as the adsorption sites for gas molecules (see Figure S3d). Similarly, various binding configurations of SiC₂N_gas systems were examined in Figures S9–S20 to search for the optimal adsorption structures. The most energetically favorable binding configurations of SiC₂N_gas systems are depicted in Figure 4, and the corresponding adsorption energies are displayed in Table 1. Combining the binding configurations and adsorption energies, we can see that Si doping can effectively promote the adsorption of HCHO, CH₃OH, C₃H₆O, and NH₃. Firstly, as shown in Figure 4a, the HCHO molecule prefers to interact with SiC₂N in a parallel pattern, in which the Si atom and the N atom of the C₂N skeleton act as dual adsorption sites for HCHO. Stable Si-O (1.67 Å) and N-C (1.52 Å) bonds are formed between HCHO and SiC₂N, which ultimately leads to appreciable adsorption energy of −1.04 eV. In the CH₃OH, C₃H₆O, and NH₃ adsorption systems, the O atoms in hydroxyl and ketone groups, as well as the N atom of NH₃, are chemically active to interact with the dopant Si atom. The newly formed Si-O (or Si-N) bond lengths in SiC₂N_CH₃OH, SiC₂N_C₃H₆O, and SiC₂N_NH₃ systems are 1.82, 1.85, and 1.91 Å, respectively, which are quite close to the sum covalent radii of Si and O (N) atoms (1.84 for Si-O and 1.86 for Si-N). These results indicate the formation of covalent bonds between Si and O (or N) atoms. Additionally, the obtained adsorption energies of SiC₂N for CH₃OH, C₃H₆O, and NH₃ are −0.90, −0.60, and −1.21 eV, respectively, which are much higher than that of pristine C₂N, whereas the C₆H₆ and C₂HCl₃ still tend to physically adsorb on the hole site of SiC₂N, accompanied by small adsorption energies of −0.36 and −0.41 eV, respectively. There is no noticeable enhancement in the C₆H₆ and C₂HCl₃ adsorption after Si doping. Overall, from both structural and energetical perspectives, doping the Si atom can remarkably boost the selective adsorption performance of SiC₂N for target O-containing VOCs (HCHO, CH₃OH, and C₃H₆O) and NH₃. Additionally, the outstanding capture capacity and selectivity may enable the SiC₂N to be a potential semiconductor material for the environmental monitoring of gaseous pollutants.

It should be noted that as a sensor material, the SiC₂N should be insensitive to main ambient gases (N₂ and O₂). Thus, the adsorption of these two ambient gases on the Si site of SiC₂N was also evaluated in Figure S21. The computational results reveal that the adsorption of N₂ on the SiC₂N can be summarized as a weak van der Waals interaction (E_ad = −0.12 eV), indicating that the nitrogen almost does not interfere with VOC and NH₃ detection. However, the O₂ has a stronger interaction with Si-doped C₂N with the adsorption energy of −2.05 eV, which will cause the Si site to be occupied and thereby decrease the sensor performance. Therefore, it is advisable to utilize appropriate O₂ filters or control the detection environment to minimize the interference of O₂ in actual application [35].

3.3.2. Interaction Mechanism

Then, the electronic characteristics, including PDOS, ELF, and CDD, were probed within the PBE method to give a deep mechanistic understanding of the interaction between SiC₂N and O-containing VOCs (or NH₃). The density of states of gases before and after adsorption, the partial density of states of working Si (or N) atoms in SiC₂N, and the O (or N) in gases for SiC₂N_HCHO, SiC₂N_CH₃OH, SiC₂N_C₃H₆O, and SiC₂N_NH₃ systems were shown in Figure 5. In the cases of HCHO adsorption, first, it can be seen that the localized electronic states of isolated HCHO split into small continuous energy states, which can be regarded as the electron redistribution caused by the strong interaction. Additionally, in the SiC₂N_HCHO system, the dopant Si atom and the N atom of C₂N act as dual adsorption sites for HCHO. The O atom of HCHO highly hybridizes with the Si at −10.69~−6.60, −5.14~−4.45, and −1.35~−0.73 eV, respectively. In addition, obvious orbital overlaps between the N atom of SiC2N and the C atom of HCHO can be observed at −14.60~−5.79 eV. These results confirm the formation of intense Si-O and N-C covalent bonds. Furthermore, as depicted in Figure 5b–d, in the SiC₂N_CH₃OH, SiC₂N_C₃H₆O, and SiC₂N_NH₃ systems, the orbitals of CH₃OH, C₃H₆O, and NH₃ molecules are also divided into several small states. Additionally, there are obvious electron hybridizations between the Si atom and the O (or N) of gaseous molecules, likewise indicating their exquisite covalent interaction. Herein, one can see that the hybrid degree between Si and the O of C₃H₆O is weaker compared with that in the other three systems, which explains the relatively weak adsorption capacity of SiC₂N toward C₃H₆O. In comparison, the electronic state deformations of the C₆H₆ and C₂HCl₃ in the adsorption process are negligible. Additionally, there are a few orbital overlaps between C₆H₆ (or C₂HCl₃) and SiC₂N in SiC₂N_C₆H₆ and SiC₂N_C₂HCl₃ systems (see Figure S22). The DOS results further demonstrate the high selective adsorption performance of SiC₂N toward O-containing VOCs and NH₃.

As discussed above, the bonding nature can be intuitively determined by ELF diagrams. The ELF plots of SiC₂N_HCHO, SiC₂N_CH₃OH, SiC₂N_C₃H₆O, and SiC₂N_NH₃ systems are given in Figure S23. It can be seen that the Si-O (or Si-N) bonds exhibit covalent bonding characteristics. Additionally, it can be seen that the covalent degree of Si-O bond in SiC₂N_C₃H₆O is lower than the Si-O (or Si-N) in other adsorption systems, which is corroborated mutually in the previous adsorption energies and DOS results. The ELF diagrams of SiC₂N_C₆H₆ and SiC₂N_C₂HCl₃ were not analyzed since there is no obvious chemical bonding between SiC₂N and C₆H₆ (or C₂HCl₃). Moreover, the CDD diagrams of the adsorption systems with regard to SiC₂N and gas molecules are presented in Figure S24. Significant charge transfer can be observed between the SiC₂N substrate and O-containing VOCs (or NH₃). The amounts of charge transfer in various adsorption systems were recorded in Table 1. It can be seen that the charge transfer from the SiC₂N monolayer to HCHO, CH₃OH, and C₃H₆O is calculated to be 0.388, 0.057, and 0.134 e, respectively. Additionally, in the SiC₂N_NH₃ system, the NH₃ molecule donates 0.046 e to the SiC₂N sheet. The remarkable charge transfer demonstrates the intense covalent bonding between O-containing VOCs (or NH₃) and SiC₂N, while the charges of C₆H₆ and C₂HCl₃ are almost unchanged after being adsorbed on SiC₂N.

3.4. Sensor Explanation of Si-Doped C₂N Sheet

3.4.1. Sensor Performance of SiC₂N

The potential of SiC₂N as resistive or work function (Φ)-type sensors was assessed in this section. Firstly, the operation of sensor materials is mainly based on the conductance variations after capturing target analytes. Additionally, the conductivity (σ) of semiconductor materials is quite related to its band gap, and the relationship between the σ and E_g is expressed as follows [53,54]:

$$\mathsf{\sigma }\propto \exp (-\frac{E_g}{2\mathrm{k}T})$$

(3)

where the E_g, k, and T donate the band gap, Boltzmann constant (8.62 × 10⁻⁵ eV/K), and temperature, respectively. The band structures of SiC₂N_gas systems with the HSE06 functional were depicted in Figure 6. One can see that the adsorption of O-containing VOCs and NH₃ inspires dramatic declines in the band gap of SiC₂N. The band gaps of SiC₂N_HCHO, SiC₂N_CH₃OH, SiC₂N_C₃H₆O, and SiC₂N_NH₃ systems considerably decrease to 0.29, 0.13, 0.25, and 0.12 eV, respectively, which will result in an evident increase in conductivity, while the band gap of SiC₂N is almost unchanged after the adsorption of C₆H₆ and C₂HCl₃. The band structures of SiC₂N_gas systems calculated by the PBE functional were displayed in Figure S25. In general, the PBE method underestimates the band-gap values; qualitatively, the PBE results are quite consistent with the HSE06 results. Herein we further calculated the electrical conductivity (σ) of SiC₂N and the adsorption systems at 300 K in Figure 7 with the PBE functional. It can be observed that the adsorption of O-containing VOCs and NH₃ generates remarkable changes in conductivity; their peaks of conductivity occur at lower values of the chemical potential. In addition, one can see that the conductivity of the carrier near or at the chemical potential is 0 V, which is consistent with the PBE band structure results (see Figure S25) and further confirms the narrower band gaps of the adsorption systems, while the adsorption of C₆H₆ and C₂HCl₃ has a negligible influence on the conductivity of SiC₂N. These results indicate that the Si-doped C₂N possesses both high sensitivity and selectivity for O-containing VOC and NH₃ detection. Furthermore, the current–voltage of the SiC₂N_NH₃ system was found as an example to ensure the effect of gas adsorption on the conductivity of SiC₂N. The I–V curves for pure SiC₂N and SiC₂N_NH₃ systems are shown in Figure 7g, in which the applied voltage ranges from 0 to 1.0 with the increment of 0.2 V. For pure SiC₂N, there is no apparent current flow at 0 and 0.2 V owing to the band gap of SiC₂N. The current appears when the applied voltage reaches 0.4 V, and then the current gradually increases along with the applied voltage. Additionally, the adsorption of NH₃ remarkably enhances the electrical conductivity of the SiC₂N sheet. Firstly, due to the quite small band gap of the SiC₂N_NH₃ system, there is an obvious current in the SiC₂N_NH₃ system even at a small applied voltage of 0.2 V. In addition, it can be observed that the current flows in the SiC₂N_NH₃ system are evidently stronger in contrast with the pure SiC₂N when using the applied voltage of 0~1 V. Additionally, the difference in the conductivity is most significant at the applied voltage of 0.4 V. One can see that at the applied voltage of 0.4 V, the current flowing through SiC₂N_NH₃ system is 18.76 μA, which is almost six times higher than pure SiC₂N (3.27 μA), which predicts that the SiC₂N sheet possesses highest sensitivity toward NH₃ at 0.4 V. Overall, the current–voltage curve further proves the high sensitivity toward ammonia.

The sensitivity of SiC₂N toward O-containing VOCs and ammonia was further investigated. The sensitivity was calculated by the following equation [55]:

$$S=({\sigma }_2-{\sigma }_1)/{\sigma }_1$$

(4)

where the ${\sigma }_2$ and ${\sigma }_1$ donate the electrical conductance of SiC₂N_gas and SiC₂N systems, respectively. Additionally, according to the relationship between the σ and E_g in Equation (3), the S can be written as follows:

$$S=\exp [-(E_{g2}-E_{g1})/2\mathrm{k}T]=\exp (-\Delta E_g/2\mathrm{k}T)$$

(5)

where the $E_{g2}$ and $E_{g1}$ are the band gap of SiC₂N_gas and SiC₂N systems, respectively. The sensitivity of SiC₂N toward O-containing VOCs and ammonia at 300, 350, and 400 K were summarized in Figure 8. It can be seen that the sensitivity of SiC₂N toward O-containing VOCs and NH₃ is high (8.17× > 10⁴) within the normal working temperature range (300–400 K). In addition, the sensitivity of some other sensor materials reported in previous studies is summarized in Table S1 [36,54,55,56,57,58,59,60]. The sensitivity of SiC₂N toward O-containing VOCs and NH₃ is comparable to or higher than some well-established gas sensors such as Si-GreenP toward acetone (~10⁸ at room temperature) [36], vacancy-doped black phosphorus toward CO₂ (1254%, 298 K) [56], Rh-BN toward SO₂ (4.79 × 10⁷, 298 K) [54], and so on, showing superior sensor ability.

We also assess the performance of SiC₂N as a Φ-type sensor in Figure S26 [61,62]. However, the relatively small Φ changes (−6.63%~−8.28%) after O-containing VOC and NH₃ adsorption may cause the SiC₂N to suffer from low sensitivity in the realistic application. The detailed descriptions were displayed in SI. Therefore, by comparison, the Si-doped C₂N is more suitable as a resistive-type sensor for O-containing VOC and NH₃ detection.

The recovery time is another vital factor for gas-sensor materials in practical application, and it can be improved by increasing the working temperature or applying ultraviolet (UV) light. The recovery time was determined by the following [63]:

$${\mathsf{\tau }=\mathsf{\omega }}^{-1}\exp (-\frac{E_{\mathrm{ad}}}{\mathrm{k}T})$$

(6)

where the ω, E_ad, k, and T were the attempt frequency (10¹²/s), adsorption energies for target gases, and Boltzmann constant as well as temperature, respectively. The calculated recovery time of SiC₂N_gas systems at the temperature of 300, 400, and 450 K was summarized in Figure 9a. It can be found that the recovery time of SiC₂N_C₃H₆O is only 1.19 × 10⁻², while the SiC₂N_HCHO, SiC₂N_CH₃OH, and SiC₂N_NH₃ systems require a relatively long time (1.30 × 10³~2.09 × 10⁸ s) at room temperature. Nevertheless, when using a working temperature of 400 K, the recovery time of SiC₂N_HCHO and SiC₂N_CH₃OH systems can significantly decrease to 12.6 and 2.17 × 10⁻¹ s, respectively. As the temperature continues to rise to 450 K, the SiC₂N_NH₃ system likewise presents a short recovery time of 3.53 × 10¹ s. It can be seen that only the detection of NH₃ needs a relatively high recovery temperature of 450 K; nevertheless, it is acceptable. The calculated recovery time without UV of O-containing VOCs is shorter than some well-established sensor materials. For example, the recovery time for detecting HCHO of SiC₂N is 12.6 s at 400 K, which is lower than Au-modified indium–gallium–zinc oxide (13 s at 523 K) [64], A-site cation deficiency in LaFeO₃ (13 s at 478 K) [65], In₂O₃ (40 s at 438 K) [66], SnO₂ (7 s at 488 K) [67], and ZnO/SnO₂ (9 s at 498 K) [68]. Moreover, we also estimated the adsorption Gibbs free energies of gases on SiC₂N at the temperature of 300~400 K, as shown in Figure 9b. The adsorption Gibbs free energies were calculated by the following:

∆G_ad = G(SiC₂N_gas) − G(SiC₂N) − G(gas)

(7)

where the G(SiC₂N_gas) and G(SiC₂N) donated the Gibbs free energies of SiC₂N_gas and SiC₂N, respectively. Additionally, the G(gas) was the Gibbs free energies of isolated VOCs and NH₃. It can be seen that only the NH₃ exhibits a relatively large ∆G_ad of −0.43~−0.61 eV at 300~400 K. The ∆G_ad for O-containing VOCs on SiC₂N is quite small, indicating that the entropy effect involving the temperature is conducive to the desorption of gases. In addition, according to previous reports, the recovery time can be further shortened by exposure to UV light (ω = 10¹⁶/s) due to the elimination of the desorption barrier [69,70]. Additionally, UV light has been widely used to reduce recovery time. Aasi and coworkers have reported that the desorption of indole from Pd-decorated MoS₂ needs a recovery time of 1.28 × 10⁵ s, whereas, under UV light, the recovery time can greatly decrease to 12.8 s. Herein, the recovery time of SiC₂N_gas systems under UV light at 300, 350, and 400 K is shown in Figure 9c. It can be seen that, under exposure to UV light, the SiC₂N_HCHO, SiC₂N_CH₃OH, and SiC₂N_C₃H₆O possess short recovery times of 29.2, 0.13, and 1.19 × 10⁻⁶ s, respectively, at room temperature. Even for the SiC₂N_NH₃ system, when using UV light, the recovery time can be effectively shortened to 26.2 s at 350 K. In a word, the apparent changes in conductivity and acceptable recovery time offer significant advantages for SiC₂N as a resistive-type sensor in the detection of O-containing VOCs and NH₃.

Moreover, we also investigated the co-adsorption of gases on the sensor performance of SiC₂N, and the results were recorded in Figures S27–S30 and Table S2. It was found that the coverage and co-adsorption of different gases have little effect on the sensitivity but slightly increase the recovery time. Therefore, in the real application, the environment with low gas concentration is more favorable for the utilization of SiC₂N. A detailed description is displayed in the Supporting Information. Furthermore, we also investigated the sensor performance of the other two nonmetal B- [71] and P-doped C₂N [72] for VOCs and ammonia, the results were recorded in Figures S31–S34 and Table S3. The weak affinity of P-doped C₂N towards VOCs and NH₃ manifests its poor sensor performance. And although BC₂N also presents high sensivitity for O-containing VOCs and ammonia, the BC₂N_gas systems need a long recovery time or high operation temperature. Therefore, comparing B- and P-doped C₂N, the SiC₂N is more suitable as a sensor for VOCs and ammonia.

3.4.2. The Effect of Si Doping Density

In this section, we further increase the doping density of the Si atom to evaluate the effect of doping density on the sensing performance. Two other Si doping densities were taken into consideration, including moderate and high Si doping density. In the case of moderate Si doping density, two Si atoms were incorporated into a 2 × 2 supercell of C₂N with a Si doping density of 2.70 at%. As shown in Figure S35a,b, there are two possible doping modes for Si₂C₂N. In one case, the two Si atoms are scattered in two different interstitial spaces (labeled as Si₂C₂N_a), and in another one, the Si atoms are gathered in one interstitial space (labeled as Si₂C₂N_b). Additionally, the former configuration is proved to be more energetically favorable (E_form = −2.98 eV), which is chosen as the computational model for Si₂C₂N. In addition, the Si doping density was further increased to 5.26 at%, and the optimized configuration of Si₄C₂N was shown in Figure S35c. Herein, we first analyze the effect of Si density on the electronic characteristics of Si-doped C₂N in Figure S35d–g. It can be seen that the band gaps of Si-doped C₂N decrease with the Si density. Moreover, similar to SiC₂N, the VBM of Si₂C₂N and Si₄C₂N is composed of active p_z electrons, which also guarantee the high activity of Si₂C₂N and Si₄C₂N.

Then, we further assess the doping density of Si atoms on the sensing performance of Si-doped C₂N toward O-containing VOCs and ammonia. Firstly, the adsorption of HCHO, CH₃OH, C₃H₆O, and NH₃ on Si₂C₂N was investigated. Since the two dopant Si atoms can both serve as the adsorption sites for gases, the adsorptions of single and two gaseous pollutants on Si₂C₂N were both taken into consideration. The optimized adsorption configurations are displayed in Figure S36. It can be seen that the adsorption energies of Si₂C₂N and Si₄C₂N toward O-containing VOCs and ammonia are slightly larger than that of SiC₂N, and the adsorption energies gradually increase with the Si doping density. In addition, the band gaps of Si-doped C₂N after the adsorption of O-containing VOCs and ammonia are summarized in

Table 2. The band gap and recovery time of Si_nC₂N_gas systems based on the PBE functional.

	Eg (eV)	$\mathit{\tau }$ (UV Light)	$\mathit{\tau }$ (UV Light)	$\mathit{\tau }$ (UV Light)
SiC2N_HCHO	0.11	$2.92 \times$ 101	$3.59 \times$ 10−2	$1.26 \times$ 10−3
SiC2N_CH3OH	~0	$1.30 \times$ 10−1	$3.94 \times$ 10−4	$2.17 \times$ 10−5
SiC2N_C3H6O	0.07	$1.19 \times$ 10−6	$2.49 \times$ 10−8	$3.61 \times$ 10−9
SiC2N_NH3	~0	$2.09 \times$ 104	$8.59 \times$ 100	$1.74 \times$ 10−1
Si2C2N_HCHO	~0	$6.68 \times$ 104	$2.26 \times$ 101	$4.15 \times$ 10−1
Si2C2N_CH3OH	0.02	$1.37 \times$ 102	$1.30 \times$ 10−1	$4.01 \times$ 10−3
Si2C2N_C3H6O	0.03	$1.79 \times$ 10−5	$2.38 \times$ 10−7	$2.75 \times$ 10−8
Si2C2N_NH3	0.02	$6.80 \times$ 105	$1.56 \times$ 102	$2.37 \times$ 100
Si4C2N_HCHO	~0	$2.21 \times$ 107	$2.84 \times$ 103	$3.22 \times$ 101
Si4C2N_CH3OH	~0	$4.46 \times$ 103	$2.37 \times$ 100	$5.45 \times$ 10−2
Si4C2N_C3H6O	~0	$5.70 \times$ 10−5	$6.26 \times$ 10−7	$6.56 \times$ 10−8
Si4C2N_NH3	~0	$4.78 \times$ 107	$5.41 \times$ 103	$5.75 \times$ 101

. For Si₂C₂N, similar to SiC₂N, a single molecule of O-containing VOCs or ammonia adsorption can cause remarkable band gap changes. It can be seen after a CH₃OH, C₃H₆O, and NH₃ molecule adsorption that the band gaps of Si₂C₂N_CH₃OH, Si₂C₂N_C₃H₆O, and Si₂C₂N_NH₃ systems considerably decrease to 0.02, 0.03, and 0.07 eV. Additionally, the band gap of Si₂C₂N_HCHO approaches 0 eV, indicating the high sensor performance of Si₂C₂N. The quantities of the adsorbed gases have little effect on sensing performance (see Table S4).

Furthermore, for Si₄C₂N, the adsorption configurations of gases on Si₄C₂N were presented in Figures S37 and S38, and the band gap, as well as band gap changes in Si₄C₂N_gas systems, are also recorded in Table 2. Since the intrinsic band gap of Si₄C₂N is quite small (~0.27 eV), after the adsorption of O-containing VOCs and ammonia, the band gap values are very close to 0 eV. The band gap changes are smaller in contrast with SiC₂N and Si₂C₂N systems, which results in low sensitivity. In addition, since Si doping density also affects adsorption energies, it can be seen that the recovery time is slightly increased along with the Si doping density (see Table 2). Therefore, taking careful consideration of sensitivity and recovery time, low or moderate Si doping densities are more favorable for Si-doped C₂N as a resistive-type sensor.

4. Conclusions

In summary, the sensor performance of Si-doped nitrogenated holey graphene (C₂N) for five kinds of representative VOCs (HCHO, CH₃OH, C₃H₆O, C₆H₆, and C₂HCl₃) and NH₃ were systematically evaluated by the first-principle calculations. Firstly, the formation energies and AIMD simulations proved the incorporation of Si atom into C₂N was thermodynamically stable. The Si doping not only induced a considerable decrease (~1.34 eV) in the band gap of C₂N, but also provided an adsorption site for gaseous pollutants. Compared with the inherent physical or weak chemical adsorption of C₂N toward gases, the SiC₂N exhibited high capture capacity and selectivity for O-containing VOCs and NH₃ through intense strong covalent bonds. Furthermore, the potential of SiC₂N as resistive- and work-function-type sensors were checked. The band gap of SiC₂N dramatically decreases after capturing O-containing VOCs and NH₃. The band gap of SiC₂N_HCHO, SiC₂N_CH₃OH, SiC₂N_C₃H₆O, and SiC₂N_NH₃ systems are 0.29, 0.13, 0.25, and 0.12 eV, respectively, which leads to great enhancement in the conductivity of the system. In comparison, the changes in work function were relatively tiny (−6.63%~−8.28%), demonstrating that the SiC₂N is more suitable to act as a resistive-type sensor. In addition, the SiC₂N was found to possess an acceptable recovery time in detecting O-containing VOCs and NH₃. When using UV light, the SiC₂N_HCHO, SiC₂N_CH₃OH, and SiC₂N_C₃H₆O possess short recovery times of 29.2, 0.13, and 1.19 × 10⁻⁶ s, respectively, at room temperature. Additionally, the recovery time of SiC₂N_NH₃ is 26.2 s at 350 K. In conclusion, our findings demonstrate that the Si-doped C₂N is a promising candidate for the environmental detection of O-containing VOCs and ammonia.