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Article

The Adsorption Mechanisms of SF6-Decomposed Species on Tc- and Ru-Embedded Phthalocyanine Surfaces: A Density Functional Theory Study

by
Rou Xue
1,
Wen Jiang
1,*,
Xing He
1,
Huihui Xiong
2,
Gang Xie
3 and
Zhifeng Nie
1,*
1
Yunnan Key Laboratory of Metal-Organic Molecular Materials and Device, School of Chemistry and Chemical Engineering, Kunming University, Kunming 650214, China
2
School of Metallurgy Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
3
Kunming Metallurgical Research Institute Co., Ltd., Kunming 650031, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(20), 7137; https://doi.org/10.3390/molecules28207137
Submission received: 11 August 2023 / Revised: 18 September 2023 / Accepted: 2 October 2023 / Published: 17 October 2023
(This article belongs to the Topic Computational Chemistry in Metallurgy, Materials and Energy)
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Versions Notes

Abstract

:
Designing high-performance materials for the detection or removal of toxic decomposition gases of sulfur hexafluoride is crucial for both environmental monitoring and human health preservation. Based on first-principles calculations, the adsorption performance and gas-sensing properties of unsubstituted phthalocyanine (H2Pc) and H2Pc doped with 4d transition metal atoms (TM = Tc and Ru) towards five characteristic decomposition components (HF, H2S, SO2, SOF2, and SO2F2) were simulated. The findings indicate that both the TcPc and RuPc monolayers are thermodynamically and dynamically stable. The analysis of the adsorption energy indicates that H2S, SO2, SOF2, and SO2F2 underwent chemisorption on the TcPc monolayer. Conversely, the HF molecules were physisorbed through interactions with H atoms. The chemical adsorption of H2S, SO2, and SOF2 occurred on the RuPc monolayer, while the physical adsorption of HF and SO2F2 molecules was observed. Moreover, the microcosmic mechanism of the gas–adsorbent interaction was elucidated by analyzing the charge density differences, electron density distributions, Hirshfeld charges, and density of states. The TcPc and RuPc monolayers exhibited excellent sensitivity towards H2S, SO2, and SOF2, as evidenced by the substantial alterations in the band gaps and work functions of the TcPc and RuPc nanosheets. Our calculations hold significant value for exploring the potential chemical sensing applications of TcPc and RuPc monolayers in gas sensing, with a specific focus on detecting sulfur hexafluoride.

Graphical Abstract

1. Introduction

Sulfur hexafluoride (SF6) is a non-flammable gas with outstanding insulating medium properties and quenching arc performance [1]. Due to its robust dielectric properties, relatively low toxicity, and exceptional chemical inertness, SF6 is frequently employed in diverse industrial applications and processes [2,3,4]. During prolonged equipment operation, insulation defects are inevitable and partial discharges take place, leading to the decomposition of SF6 and the production of harmful fluoride sulfides. When trace amounts of water and oxygen are present, significant reactions occur with these by-products, resulting in the formation of gaseous substances, including hydrogen fluoride (HF), hydrogen sulfide (H2S), sulfur dioxide (SO2), thionyl fluoride (SOF2), and sulfuryl fluoride (SO2F2) [5,6,7]. Collectively, these substances are referred to as the characteristic components of SF6. These characteristic components not only affect the insulation performance of electrical equipment and corrode solid insulation materials [8,9], but also pose a risk to human health, such as causing eye/skin irritations, allergies, and even cancers [10,11]. Therefore, the online detection and scavenging of SF6-decomposed gases has become an urgent task for evaluating the safety of gas-insulated substation (GIS) operation and safeguarding human well-being [12,13,14].
More recently, two dimensional (2D) phthalocyanine (Pc) nanomaterials, which were successfully synthesized using an experimental method, have received widespread attention due to their unique properties [15,16,17]. Interestingly, phthalocyanine displays a variety of morphologies and possesses exceptional properties such as a high specific surface area, unique electronic characteristics, and attractive optical performance [18,19]. The exceptional properties exhibited by phthalocyanine make it a highly valuable material for advanced technical applications, including photovoltaics [20,21], optoelectronics [22,23], electrocatalysis [24,25], and spintronics [26,27]. Furthermore, a subset of phthalocyanine materials, which operate at ambient temperatures, is both safe and non-hazardous, demonstrating significant potential for integration into wearable technology [28,29]. With the remarkable advancements in preparation methods, including organic vapor deposition (OVPD) [30], chemical synthesis [31], and epitaxial growth [32], the production of phthalocyanine materials with high yields and large surface areas has significantly improved, which has led to phthalocyanine-based nanomaterials being extensively used in resistance, optical, and micro-weight precision sensors [33,34,35]. However, their limited sensitivity and selectivity, as well as their slow sensing processes, impede their practical application [36].
Currently, the decoration of transition metal (TM) is considered an effective method for enhancing the sensing performance of 2D materials. Similarly, the semiconductor properties and sensing abilities of H2Pc can be altered by introducing a TM into the center of the H2Pc monolayer [37,38,39]. For instance, incorporating Cr into pristine phthalocyanine has been shown to improve its selectivity and sensitivity towards H2CO [40]. With the aid of DFT computations, Zou et al. discovered that MnPc exhibited excellent sensing abilities towards CO, NO, O2, and NO2 by examining the adsorption behaviors and sensing characteristics of six different gas molecules on an MnPc monolayer [41]. Furthermore, various TM-atom-modified phthalocyanine monolayers, such as CoPc [42,43,44], FePc [45], CuPc [42], and NiPc [42,46], have demonstrated exceptional gas-sensing properties. The outstanding performance achieved with 3d TM atoms doped into an H2Pc monolayer raises the following question: can 4d TM atoms, when similarly doped into an H2Pc monolayer, serve as exceptional gas sensors or adsorbents for detecting or scavenging SF6 decomposition species?
In this work, we propose TMPc (TM = Tc and Ru) nanomaterials as a possible gas-sensing material for the detection of SF6 decomposition products based on the first-principles calculations. First of all, the adsorption performance of five SF6 decomposition gases (HF, H2S, SO2, SOF2, and SO2F2) on TcPc and RuPc monolayers are systematically studied. Then, the electronic characteristics of the different adsorption systems are computed and analyzed, including the charge transfer, charge density differences (CDDs), electron density distributions (EDDs), density of states (DOSs), and partial density of states (PDOSs). Subsequently, we discuss the gas-sensing mechanisms of TcPc and RuPc towards these toxic gases by examining changes in band structures and work functions before and after gas adsorption. Finally, the feasibility of employing TcPc and RuPc monolayers as promising materials for detecting SF6 characteristic components is assessed based on their adsorption strengths and sensing performances.

2. Results and Discussion

2.1. Structure and Stability of H2Pc, TcPc, RuPc, and SF6

In the TMPc (TM = Tc and Ru) monolayer, as displayed in Figure 1b,c, the Tc/Ru atom is surrounded by four nearest-neighbor N atoms (N4) in the center vacancy, and the suspended bonds of C atoms are passivated with the H atoms. Moreover, all atoms in the three optimized geometric structures are coplanar. According to our previous investigation [47,48], the hollow site of the H2Pc and TMPc monolayer is the preferential adsorption site for the “porphyrin-like” configurations. When the SF6 decomposition components approach the H2Pc/TMPc monolayers, various adsorption styles of SF6 decomposition gases are considered, as shown in Figure 1d. For instance, two different adsorption configurations of HF on the H2Pc/TMPc monolayers are considered: HF adsorption on the substrate with the H atom orients towards the surface (defined as the H-end) and HF adsorption on the substrate with the F atom orients towards the surface (defined as the F-end).
Figure 2 illustrates that the binding energy (Ebin) of RuPc is smaller than the cohesive energy (Ecoh) of bulk Ru, indicating robust thermal stability in the RuPc monolayer [47]. Despite TcPc having a larger Ebin than the Ecoh of bulk Tc, the negative value of the binding energy is sufficient to maintain the structural stability of the TcPc monolayer [48]. The high stability of both the RuPc and TcPc monolayers is further supported by an ab initio molecular dynamics (AIMD) simulation, as depicted in Figure 3. Here, the temperature and potential energy of TcPc and RuPc exhibit only slight fluctuations around the equilibrium state after heating at 300 K for 20 ps with a time step of 1 fs. Therefore, both the TcPc and RuPc monolayers exhibit sufficient stability for application as sensing materials.
Figure 4 depicts the CDD and DOSs of the TcPc and RuPc monolayers. In Figure 4a, the Tc atom donates 0.213 e, acting as an electron donor, while its nearest-neighbor N4 atoms accept 0.068 e, functioning as electron acceptors. Additionally, charge accumulation primarily occurs on the C and N atoms, while charge depletion surrounds the central Tc atom, highlighting the strong affinity between the Tc atom and N4 atoms. As shown in Figure 4b, the robust interactions between Tc and the Pc monolayer are predominantly due to the strong hybridizations of Tc-d and N-p orbitals in the range of −5.0 to 5.0 eV. Consequently, the pronounced affinity between Tc and Pc contributes to the robust stability of the TcPc monolayer. From Figure 4c–d, it is evident that the CDD and DOS plots of the RuPc nanosheet share similarities with those of the TcPc monolayer. The central Ru atom also acts as an electron donor, interacting with the N4 atoms across a wide energy range of −7.5 to 7.5 eV. Notably, at the Fermi level, there is an overlap between the Ru-sp orbital and the p orbital of the N4 atom, resulting in the formation of a strong Ru-N4 bond.

2.2. Adsorption Characteristics of SF6 Decomposition Gases on H2Pc Monolayer

In this section, the adsorption of bare H2Pc for five characteristic decomposed species of SF6 was first analyzed. The corresponding calculated data, including adsorption energy (Eads), adsorption distance (D), electron transfer (Qt) and bandgap (Bg), are listed in Table 1. Table 1 shows that the SF6-decomposed gas molecules, except for HF, prefer to adsorb onto the H2Pc substrate via the S-end. For different adsorption systems, the adsorption energy follows the order: H2S < HF < SO2F2< SOF2 < SO2. Meanwhile, all of the absolute values of adsorption energy are below the critical value of 0.8 eV [49]; thus, the adsorption process can be classified as physisorption, which is mainly contributed by the van der Waals interaction [50]. In addition, the five characteristic species have little effect on the energy gap of pristine H2Pc. Therefore, the pure H2Pc monolayer is not suitable to detect and remove SF6 decomposition species due to the poor sensitivity and weak adsorption strength.
Figure 5 shows the lowest-energy CDD and EDD of different adsorption systems. One can see that the SF6 decomposition component has a negligible impact on the structure of the H2Pc monolayer, with the coplanar structures of H2Pc remaining intact after the adsorption of these gases. Furthermore, as observed in the CDD plots of Figure 5b2–e2, only a minimal amount of electron transfer occurs in the HF@H2Pc and H2S@H2Pc adsorption systems. This suggests that the interactions between HF, H2S, and the H2Pc substrate are relatively weaker compared to the other three adsorption systems. Simultaneously, as demonstrated in the EDD plots of Figure 5b3–e3, the electron densities of all SF6 decomposition products do not overlap with those of the H2Pc substrate, further indicating that their interactions are not particularly strong. Those results are in good agreement with the calculations listed in Table 1. Consequently, the pristine H2Pc nanosheet cannot become a potential sensing material for SF6-decomposed species in terms of adsorption strength and electron transfer.

2.3. Adsorption Characteristics of SF6 Decomposition Gases on TcPc Monolayer

The preferential adsorption orientation, adsorption energy, adsorption distance, electron transfer, and bandgap of the SF6 decomposition components on TcPc monolayers are summarized in Table 2. It is found that the HF species prefers to adsorb onto the TcPc nanosheet with the H-end, whereas the H2S, SO2, SOF2 and SO2F2 molecules prefer to adsorb on TcPc monolayer with the S-end. Moreover, except for the HF one, the other four decomposed species are chemically adsorbed with Eads of −1.43 eV, −1.97 eV, −1.78 eV, and −0.96 eV, respectively. Meanwhile, an electron transfer of 0.298 e, 0.099 e, 0.073 e, and 0.256 e occurs between the corresponding gas molecules and the TcPc substrate. Interestingly, there is a clear variation in the bandgap of TcPc after the adsorption of H2S, SO2, and SOF2 molecules, showing a potential sensitivity of the TcPc monolayer towards these three species.
The optimized structures, CDD, and EDD of SF6-decomposed species adsorption on TcPc monolayer are presented in Figure 6. In Figure 6a1, the HF molecule is adsorbed on the TcPc nanosheet with an adsorption energy of −0.23 eV and an adsorption distance of 2.368 Å. The adsorption energy of HF@TcPc falls below the critical threshold of 0.8 eV, suggesting that it belongs to a physical adsorption driven by van der Waals forces. In contrast, as shown in Figure 6b1–e1, the chemisorption occurs among the other four adsorption systems with Eads ranging from −0.96 to −1.97 eV. In addition, as shown in Figure 6a2–e2, there is very little electron transfer in the HF@TcPc adsorption system, indicating that the interactions between HF molecules and the TcPc nanosheet are much weaker compared to the other four adsorption systems. For the SO2@TcPc, SO2F@TcPc, and SO2F2@TcPc systems, abundant charges are accumulated around the gas molecules, whereas some charges are depleted around the corresponding TcPc monolayer. These results suggest that massive electrons from the TcPc nanosheet are transferred to the SO2, SO2F, and SO2F2. Furthermore, there is substantial overlap in the total electron density between H2S, SO2, SO2F2, SO2F2, and TcPc substrate, as shown in the EDD maps in Figure 6a3–e3. This further demonstrates that the TcPc monolayer possesses a strong trapping capability for H2S, SO2, SOF2, and SO2F2 molecules.
To gain a better understanding of the adsorption behavior of five SF6 decomposition products on the TcPc substrate, the corresponding DOSs and PDOSs of different adsorption systems before and after gas adsorption are presented in Figure 7 and Figure 8. Figure 7 illustrates the symmetric DOSs of the five adsorption systems, indicating their non-magnetic nature [51]. The data in Figure 7 make it evident that, unlike the other four systems, the HF system has a minimal impact on the DOSs near 0 eV. Additionally, there are no discernible resonance peaks between H-p and Tc-d orbits (Figure 8a). Upon adsorption of H2S, SO2, SOF2, and SO2F2 onto TcPc in the respective systems, the DOS curve shows a slight shift towards lower energy levels (Figure 7b–e). Notably, a distinct peak emerges near the Fermi level, indicating a significant transfer of electrons from the TcPc surface to the H2S, SO2, SOF2, and SO2F2 molecules. This leads to strong interactions between the gas and substrate. The notable adsorption strength is primarily attributed to the orbital hybridization between S-sp and Tc-d orbitals across the entire energy level, as illustrated in Figure 8. However, in the HF@TcPc system, there are four weak resonance peaks at approximately −6.11 eV, −0.23 eV, 2.32 eV, and 5.10 eV. This suggests that the TcPc nanosheet has a limited capacity for trapping HF. Therefore, the TcPc monolayer demonstrates significant potential as a sensing material for H2S, SO2, SOF2, and SO2F2 molecules.

2.4. Adsorption Characteristics of SF6 Decomposition Gases on RuPc Monolayer

Similarly, the adsorption properties of gas molecules including HF, H2S, SO2, SOF2, and SO2F2 on the RuPc monolayers were also investigated thoroughly. The adsorption energy, electron transfer, adsorption distance, and band structure are listed in Table 3. The adsorption energies of HF, H2S, SO2, SOF2, and SO2F2 molecules are −0.28 eV, −1.26 eV, −1.64 eV, −1.53 eV, and −0.33 eV, respectively. The corresponding adsorption distances between HF, H2S, SO2, SOF2, SO2F2, and RuPc surface are 2.268 Å, 2.243 Å, 2.110 Å, 2.088 Å, and 3.411 Å, respectively. In terms of adsorption energies, the RuPc sheet exhibits strong trapping capabilities for H2S, SO2, and SOF2 molecules. However, the trapping efficiency for HF and SO2F2 is relatively limited. Furthermore, approximately 0.165 e, 0.071 e, and 0.039 e are transferred from the RuPc nanosheet to the HF, SO2, and SOF2 molecules, respectively. Conversely, the RuPc sheet received approximately 0.275 e and 0.003 e from the H2S and SO2F2 molecules, respectively. This indicates that HF, SO2, and SOF2 (H2S and SO2F2) function as electron acceptors (donors). Consequently, the RuPc monolayer possesses a suitable adsorption capacity for H2S, SO2, and SOF2 molecules, indicating its potential as a gas sensor material. Additionally, the band structure of RuPc undergoes noticeable alterations upon the adsorption of H2S, SO2, and SOF2, further emphasizing its remarkable gas sensitivity towards these molecules.
Figure 9 illustrates the lowest energy structures, CDD, and EDD of the SF6 decomposition gas adsorbed on the RuPc monolayer. As displayed in Figure 9a1–d1, a clear trend can be observed in the adsorption energies (Eads), as follows: Eads (HF) > Eads (SO2F2) > Eads (H2S) > Eads (SOF2) > Eads (SO2). In general, adsorption processes with Eads greater than 0.8 eV are commonly classified as chemisorption [52], and the adsorption energies of H2S, SO2, and SOF2 molecules on the RuPc monolayer are −1.26 eV, −1.64 eV, and −1.53 eV, respectively. Obviously, the RuPc monolayer exhibits appropriate adsorption strength to chemically capture H2S, SO2, and SOF2 gas molecules. In contrast, the interaction between HF, SO2F2 molecules, and the RuPc monolayer is weak. The adsorption energies of HF and SO2F2 are −0.28 eV and −0.33 eV, with corresponding adsorption distances of 2.268 Å and 3.411 Å, respectively. This outcome indicates that HF and SO2F2 weakly adsorb onto the RuPc monolayer, primarily driven by van der Waals forces. The CDD plots in Figure 9a2–e2 demonstrate electron transfer occurring between H2S, SO2, SOF2, and the RuPc nanosheet. From Figure 9a2 and e2, the presence of large electron depletion between HF/SO2F2 and RuPc also demonstrates their weak interaction. However, abundant electrons of H2S/SO2/SOF2 are transferred to the intermediate region of the gas–substrate, resulting in a strong interaction between H2S/SO2/SOF2 and the RuPc monolayer. In addition, HF, SO2, and SOF2 acquire a few electrons from the RuPc monolayer, acting as electron acceptors. Conversely, the other gas molecules function as electron donors, releasing a portion of their electrons. This observation aligns well with the results obtained from Hirshfeld charge analysis (Table 3). As shown in Figure 9a3–e3, there exists a significant electron overlap between H2S, SO2, and SOF2 molecules and the RuPc monolayer, while electron overlap does not occur in the HF@RuPc and SO2F2@RuPc systems. Consequently, it is concluded that the RuPc monolayer has a strong capture ability in terms of H2S, SO2, and SOF2 molecules.
To further understand the microcosmic mechanism of the gas–substrate interaction, the DOSs and PDOSs of SF6 decomposition species on the RuPc monolayer are displayed in Figure 10 and Figure 11. From Figure 10a, it is evident that the DOSs of the RuPc monolayer with and without HF adsorption are nearly identical in the energy interval of −2.40 to 5.00 eV and −2.75 to 2.45 eV. When compared to the clean RuPc monolayer, the DOSs of the HF@RuPc system experience a slight increase after 5.0 eV and 2.45 eV, with peaks appearing at around −2.00 eV and 0.00 eV, and no significant shift observed. The unoccupied DOS peak of this system is primarily contributed by the Ru-d orbital and H-s orbital. Moreover, there is no state peak overlap between the two atoms near the Fermi level (Figure 11a); thus, the interaction is extremely weak. As shown in Figure 10b, the DOSs of clean RuPc and H2S@RuPc near the Fermi level are different: the DOS peak value of H2S@RuPc moves to the low-energy direction. From Figure 11b, the interaction between the S atom and Ru atom is evident due to the obvious resonance peaks between the S and Ru atoms at about 2.00 eV. Combining the Eads (−1.26 eV) and electron transfer (0.275 e) of the H2S@RuPc system, one can conclude that the adsorption of H2S on the RuPc monolayer belongs to chemical adsorption. The DOSs of the SO2 and SOF2 adsorption systems are similar to that of H2S, as shown in Figure 10c,d. Both of them have deviations in DOSs at the Fermi level, and their peaks are reduced. From Figure 11c,d, resonance peaks of S and Ru atoms near the Fermi level can be observed, indicating that strong interactions of SO2, SOF2, and the RuPc monolayer exist in the two systems. In conclusion, the RuPc monolayer can be used as a candidate sensing material for the detection of H2S, SO2, and SOF2.

2.5. Sensing Performance Evaluation of TcPc and RuPc

The adsorption of SF6 decomposition products induces a change in the conductivity (σ) of the substrate, which can serve as an indicator for the sensitivity of the material. The conductivity is determined by the bandgap (Bg), as defined by the following formula:
σ ∝ A exp − B g / 2 K B T
where A is a constant, while KB and T represent the Boltzmann constant (8.62 × 10−5 eV/K) and absolute temperature, respectively. Based on this formula, a greater variance of bandgap before and after gas adsorption indicates a higher sensitivity for a material. The Bg of the TcPc nanosheet with and without gas adsorption is illustrated in Figure 12. It is evident that all of the systems exhibit semiconducting properties with a bandgap ranging from 0 eV to 0.830 eV. Compared to the Bg of pristine TcPc, the adsorption of HF and SO2F2 leads to a negligible change, indicating that the TcPc monolayer is less sensitive to these two gases. However, upon adsorption of H2S, SO2, and SOF2, the Bg of TcPc significantly increases from 0.181 eV to 0.787 eV, 0.778 eV, and 0.830 eV, respectively. This demonstrates its exceptional sensitivity to these gases, positioning it as a promising resistance-type gas sensor for detecting H2S, SO2, and SOF2.
Figure 13 illustrates the band structures of various gas@RuPc adsorption systems. For the clean RuPc monolayer, it exhibits a zero bandgap with semimetal characteristics (Figure 13a). Upon the adsorption of H2S, SO2, and SOF2, the bandgap of RuPc is increased to 1.150 eV, 1.223 eV, and 1.238 eV, respectively. The significant changes in the bandgap of a RuPc monolayer suggest a transition from its semi-metallic properties to a semiconductor behavior. In other words, the RuPc monolayer demonstrates outstanding sensitivity towards H2S, SO2, and SOF2 molecules.
The work function is a crucial property for investigating adsorption performance. It signifies the strength of electron binding within a metal. A higher work function indicates a lower likelihood of electron emission from the metal. Therefore, assessing the sensitivity of a TMPc monolayer can be also achieved by analyzing the change in work function (φ) before and after gas adsorption, as defined by [53,54]:
φ = E vacuum − E fermi
where Evacuum and Efermi represent the vacuum level and Fermi level of the TMPc monolayer after gas adsorption.
As depicted in Figure 14a, significant changes in the work function of TcPc are observed after the adsorption of SF6-decomposed gases. Comparing it to the φ of pristine TcPc (5.116 eV), the φ increases to 5.987 eV, 5.932 eV, 5.905 eV, and 6.231 eV after the adsorption of HF, SO2, SOF2, and SO2F2, respectively. Notably, the work function of the TcPc monolayer drops to 4.109 eV after H2S adsorption, highlighting the excellent gas-sensing capabilities of TcPc. From Figure 14b, one can find the significant alterations in the work function of RuPc upon the adsorption of SF6-decomposed gases. Compared to clean RuPc (φ = 5.279 eV), a minor increase in φ is observed after HF, SO2, SOF2, and SO2F2 adsorption. However, the work function of RuPc decreases significantly to 4.299 eV after H2S adsorption. Consequently, it can be concluded that RuPc also serves as an excellent gas sensor.
The sensitivity of a gas-sensing material can be quantitatively assessed by measuring the resistance change of TMPc before and after gas adsorption. As is widely recognized, the conductivity of a substance is inversely proportional to its resistance. If the conductivity is recorded, the sensitivity of the sensor can be determined by the following equation [55]:
S = ( 1 σ TMPc / gas − 1 σ TMPc ) / 1 σ TMPc
where σTMPc/gas and σTMPc are the electrical conductivity of TMPc monolayer after and before adsorption, respectively. Figure 15 gives the sensitivity of five gas molecules on the TcPc and RuPc monolayers. As shown in Figure 15a, the sensitivity of the TcPc monolayer towards each gas gradually decreases as the temperature ranges from 298 K to 398 K, which is consistent with the sensitivity of the Rh-doped h-BN monolayer to SF6 decomposition gas [55]. Moreover, the TcPc monolayer exhibits the highest sensitivity to SOF2 at each temperature, followed by H2S, SO2, and SO2F2. Correspondingly, the TcPc monolayer exhibits outstanding sensitivity in detecting SO2, H2S, and SOF2 at its operating temperature. As illustrated in Figure 15b, it is evident that RuPc’s sensitivity to SF6 decomposition byproducts (H2S, SO2, and SOF2) at room temperature falls within the range of (5.36~29.7) × 109, significantly exceeding the sensitivity towards SF6 alone (approximately 103), and there is a slight decrease in sensitivity as the operating temperature rises. However, the sensitivity of H2S, SO2, and SOF2 at high temperature (398 K) is determined to be within the range of (1.93~6.95) × 107, surpassing the sensitivity of Rh-BN towards SF6-decomposing species [55]. The exceptional sensitivity of the RuPc monolayer enables it to possess the precise ability to detect SF6 decomposition gases, underlining its potential for gas-sensing applications. In summary, both TcPc and RuPc monolayers exhibit remarkable capability in detecting these SF6-decomposing species.

3. Calculation Method and Details

In this work, the spin calculations were performed using the DMol3 quantum chemistry module [56] based on the density functional theory (DFT) method, as implemented in the Material Studio software package. The exchange–correlation among electrons is computed using the Perdew–Burke–Ernzerhof (PBE) function within the framework of general gradient approximation (GGA), owing to its superior computational accuracy [57]. To enhance the comprehension of the van der Waals force and long-range interactions, we employed the DFT-D method (Grimme custom) [58]. The double numerical plus polarization (DNP) [59] atomic orbital basis set was employed to ensure computational accuracy, while the DFT semi-core pseudopotential (DSPP) [60] was utilized to account for relativistic effects. Moreover, a real-space global cutoff radius of 5.2 Å was employed, and the Monkhorst–Pack scheme [61] was utilized to select k-points of 6 × 6 × 1 (12 × 12 × 1) meshes for geometry optimization (electronic property calculations). A vacuum space with a thickness of 20 Å was selected in the z-direction to mitigate interactions between neighboring clusters and to avoid interlayer interactions due to periodic boundary conditions [62]. The energy convergence, maximum displacement, and maximum force were respectively set as 1.0 × 10−5 Ha, 5 × 10−3 Å, and 0.002 Ha/Å.
In order to evaluate the stability of the TMPc monolayers, the binding energy (Ebin) of the TM-doped Pc sheet was determined by [12,47]:
E bin = E TM + Pc − E Pc − E TM
where ETM+Pc and EPc are the total energies of TMPc and pristine Pc monolayers, and ETM is a single metal atom obtained by calculating the energy of corresponding bulk (ETM(bulk)). In addition, the cohesive energy (Ecoh) was also calculated to explore the aggregation possibility of TM atoms in the Pc monolayer. Ecoh = (ETM(bulk) − Eiso-TM)/n, where Eiso-TM represents the energy of an isolated TM atom, and n is the number of TM atoms in bulk.
To quantitatively evaluate the interaction strength between SF6 decomposition products and the pristine H2Pc and TMPc monolayers, the adsorption energy (Eads) was calculated by:
E ads = E gas + sur − E gas − E sur
where Egas+sur is the total energy of TMPc with the adsorbed SF6-decomposed gas molecules, and Esur and Egas are the total energy of the clean H2Pc/TMPc monolayer and particular decomposed gas molecule, respectively. When the Eads is negative, the adsorption process becomes spontaneous and releases heat, and a larger absolute value of Eads means stronger adsorption strength. The electron transfer (ΔQ) from the substrate to SF6 decomposition products based on the Hirshfeld charge can be determined by:
Q t = Q adsorbed − Q isolated
The Qisolated and Qadsorbed represent the charge of SF6 decomposition products before and after adsorption, respectively. A negative (positive) value of ΔQ indicates that the SF6 decomposition product’s gas molecule gains (loses) electrons. In addition, the charge density difference (CDD, Δρ) of different adsorption systems can be obtained by:
Δ ρ = ρ slab + gas − ρ slab − ρ gas
where ρslab+gas, ρslab, and ρgas represent the electron density of the TMPc monolayer with gas adsorption, a clean TMPc monolayer, and isolated SF6 decomposition products, respectively.

4. Conclusions

In this work, first-principles calculations were utilized to investigate the adsorption behaviors of SF6-decomposed species (HF, SO2, H2S, SOF2, and SO2F2) on the intrinsic and Tc/Ru-doped H2Pc monolayers, aiming to find a potential Pc-based gas-sensing material for the detection or scavenging of the above five gas molecules. The main conclusions are summarized as follows:
(1)
TcPc and RuPc monolayers exhibit a semi-metallic property, and the strong hybridizations between the Tc/Ru-d orbital and N4 of Pc further demonstrate their high structural stability.
(2)
The TcPc monolayer exhibits a strong affinity towards H2S, SO2, SOF2, and SO2F2 due to the robust orbital hybridization between the Tc-d orbitals and S-sp orbitals of these gases.
(3)
The RuPc nanosheet exhibits a remarkable ability to capture H2S, SO2, and SOF2 molecules, primarily owing to the robust orbital hybridizations between the Ru-d orbitals and the S-sp orbitals of these gases. Therefore, the RuPc nanosheet holds significant promise as a scavenger for H2S, SO2, and SOF2 molecules.
(4)
The adsorption of H2S, SO2, and SOF2 induces significant changes in the bandgap and work function of the TcPc and RuPc monolayers, highlighting the strong sensitivity of these monolayers to H2S, SO2, and SOF2 molecules.
Overall, the bare H2Pc monolayer is not a potential gas-sensing material for SF6-decomposed species. However, the TMPc (TM = Tc and Ru) monolayer shows promise as a potential material for the gas detection or scavenging of SF6-decomposed species, owing to its enhanced gas capture ability and heightened sensitivity. These theoretical results can provide certain guidance for subsequent related experimental research.

Author Contributions

Formal analysis, investigation, and writing—original draft, R.X.; methodology and data curation, W.J.; visualization, X.H.; software and validation, H.X.; supervision, G.X.; conceptualization, resources, writing—review and editing, project administration, and funding acquisition, Z.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (51904137, 12264020), the Special Basic Cooperative Research Programs of Yunnan Provincial Undergraduate Universities’ Association (202101BA070001-032, 202001BA070001-153), the Basic Research Project of Yunnan Province (202201AT070017), and the Scientific Research Funds of Yunnan Education Department (2023Y0895 and 2023Y0901). The authors also thank the High-Level Talent Plans for Young Top-Notch Talents of Yunnan Province (Grant No. YNWR-QNBJ-2020-017) and High-Level Talent Special Support Plans for Young Talents of Kunming City (Grant No. C201905002).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used in this study are contained within the article.

Acknowledgments

We thank the editor and the reviewers for their useful help in improving this paper, along with the Scientific Innovation Team of Kunming University for helpful discussions on topics related to this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 28 07137 g001
Figure 1. Top sides of the optimized geometric structures: (a) H2Pc, (b) TcPc, (c) RuPc, and (d) various adsorption styles of SF6-decomposed species. The cyan, green, blue, gray, white, yellow, red, and light blue balls are Tc, Ru, N, C, H, S, O, and F atoms, respectively.
Figure 1. Top sides of the optimized geometric structures: (a) H2Pc, (b) TcPc, (c) RuPc, and (d) various adsorption styles of SF6-decomposed species. The cyan, green, blue, gray, white, yellow, red, and light blue balls are Tc, Ru, N, C, H, S, O, and F atoms, respectively.
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Figure 2. Binding energy (Ebin) of TcPc/RuPc monolayer and cohesive energy (Ecoh) of Tc/Ru bulk.
Figure 2. Binding energy (Ebin) of TcPc/RuPc monolayer and cohesive energy (Ecoh) of Tc/Ru bulk.
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Molecules 28 07137 g003
Figure 3. Temperature and potential energy fluctuation of (a) TcPc and (b) RuPc monolayers in the AIMD simulation at 300 K. The red and green lines correspond to the temperature and potential energy fluctuations, respectively.
Figure 3. Temperature and potential energy fluctuation of (a) TcPc and (b) RuPc monolayers in the AIMD simulation at 300 K. The red and green lines correspond to the temperature and potential energy fluctuations, respectively.
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Molecules 28 07137 g004
Figure 4. CDD and DOSs of the (a,b) TcPc and (c,d) RuPc monolayer. The four nearest-neighbor N atoms of TcPc/RuPc monolayer are indicated by N4. The cyan and yellow areas correspond to the charge accumulation and consumption, respectively. The isosurface of CDD is set as ±0.01 eÅ−3, and the black dashed line is the Fermi level.
Figure 4. CDD and DOSs of the (a,b) TcPc and (c,d) RuPc monolayer. The four nearest-neighbor N atoms of TcPc/RuPc monolayer are indicated by N4. The cyan and yellow areas correspond to the charge accumulation and consumption, respectively. The isosurface of CDD is set as ±0.01 eÅ−3, and the black dashed line is the Fermi level.
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Molecules 28 07137 g005
Figure 5. Optimized geometric models, CDD, and EDD of SF6-decomposed species adsorption on H2Pc monolayer. (a1–a3) HF, (b1–b3) H2S, (c1–c3) SO2, (d1–d3) SOF2, and (e1–e3) SO2F2. The cyan and yellow areas correspond to the accumulation and consumption of charges, respectively. The isosurface values of CDD and EDD are set as ±0.003 eÅ−3 and 0.2 eÅ−3, respectively.
Figure 5. Optimized geometric models, CDD, and EDD of SF6-decomposed species adsorption on H2Pc monolayer. (a1–a3) HF, (b1–b3) H2S, (c1–c3) SO2, (d1–d3) SOF2, and (e1–e3) SO2F2. The cyan and yellow areas correspond to the accumulation and consumption of charges, respectively. The isosurface values of CDD and EDD are set as ±0.003 eÅ−3 and 0.2 eÅ−3, respectively.
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Molecules 28 07137 g006
Figure 6. Optimized geometric models, CDD, and EDD of SF6-decomposed species adsorption on TcPc monolayer. (a1–a3) HF, (b1–b3) H2S, (c1–c3) SO2, (d1–d3) SOF2, and (e1–e3) SO2F2. The cyan and yellow areas correspond to the charge accumulation and consumption, respectively. The isosurface values of CDD and EDD are set as ±0.01 eÅ−3 and 0.2 eÅ−3, respectively.
Figure 6. Optimized geometric models, CDD, and EDD of SF6-decomposed species adsorption on TcPc monolayer. (a1–a3) HF, (b1–b3) H2S, (c1–c3) SO2, (d1–d3) SOF2, and (e1–e3) SO2F2. The cyan and yellow areas correspond to the charge accumulation and consumption, respectively. The isosurface values of CDD and EDD are set as ±0.01 eÅ−3 and 0.2 eÅ−3, respectively.
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Molecules 28 07137 g007
Figure 7. DOSs of different gas@TcPc adsorption systems. (a) HF@TcPc, (b) H2S@TcPc, (c) SO2@TcPc, (d) SOF2@TcPc, and (e) SO2F2@TcPc. The black and red lines represent the DOSs of the TcPc monolayer before and after gas adsorption, respectively. The Fermi level serves as the zero-energy reference point and is represented by a vertical black dashed line.
Figure 7. DOSs of different gas@TcPc adsorption systems. (a) HF@TcPc, (b) H2S@TcPc, (c) SO2@TcPc, (d) SOF2@TcPc, and (e) SO2F2@TcPc. The black and red lines represent the DOSs of the TcPc monolayer before and after gas adsorption, respectively. The Fermi level serves as the zero-energy reference point and is represented by a vertical black dashed line.
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Figure 8. PDOSs of different gas@TcPc adsorption systems. (a) HF@TcPc, (b) H2S@TcPc, (c) SO2@TcPc, (d) SOF2@TcPc, and (e) SO2F2@TcPc. The Fermi level is indicated by the vertical dashed line at zero energy.
Figure 8. PDOSs of different gas@TcPc adsorption systems. (a) HF@TcPc, (b) H2S@TcPc, (c) SO2@TcPc, (d) SOF2@TcPc, and (e) SO2F2@TcPc. The Fermi level is indicated by the vertical dashed line at zero energy.
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Molecules 28 07137 g009
Figure 9. Optimized geometric models, CDD, and EDD of SF6-decomposed species adsorption on RuPc monolayer. (a1–a3) HF, (b1–b3) H2S, (c1–c3) SO2, (d1–d3) SOF2, and (e1–e3) SO2F2. The cyan and yellow areas correspond to the charge accumulation and consumption, respectively. The isosurface values of CDD and EDD are set as ± 0.01 eÅ−3 and 0.2 eÅ−3, respectively.
Figure 9. Optimized geometric models, CDD, and EDD of SF6-decomposed species adsorption on RuPc monolayer. (a1–a3) HF, (b1–b3) H2S, (c1–c3) SO2, (d1–d3) SOF2, and (e1–e3) SO2F2. The cyan and yellow areas correspond to the charge accumulation and consumption, respectively. The isosurface values of CDD and EDD are set as ± 0.01 eÅ−3 and 0.2 eÅ−3, respectively.
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Molecules 28 07137 g010
Figure 10. DOSs of different gas@RuPc adsorption systems. (a) HF@RuPc, (b) H2S@RuPc, (c) SO2@RuPc, (d) SOF2@RuPc, and (e) SO2F2@RuPc. The black and red lines represent the DOSs of the RuPc monolayer before and after gas adsorption, respectively. The Fermi level serves as the zero-energy reference point and is represented by a vertical black dashed line.
Figure 10. DOSs of different gas@RuPc adsorption systems. (a) HF@RuPc, (b) H2S@RuPc, (c) SO2@RuPc, (d) SOF2@RuPc, and (e) SO2F2@RuPc. The black and red lines represent the DOSs of the RuPc monolayer before and after gas adsorption, respectively. The Fermi level serves as the zero-energy reference point and is represented by a vertical black dashed line.
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Molecules 28 07137 g011
Figure 11. PDOSs of different gas@RuPc adsorption systems. (a) HF@RuPc, (b) H2S@RuPc, (c) SO2@RuPc, (d) SOF2@RuPc, and (e) SO2F2@RuPc. The Fermi level is indicated by the vertical dashed line at zero energy.
Figure 11. PDOSs of different gas@RuPc adsorption systems. (a) HF@RuPc, (b) H2S@RuPc, (c) SO2@RuPc, (d) SOF2@RuPc, and (e) SO2F2@RuPc. The Fermi level is indicated by the vertical dashed line at zero energy.
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Molecules 28 07137 g012
Figure 12. Band structure of various adsorption systems. (a) TcPc, (b) HF@TcPc, (c) H2S@TcPc, (d) SO2@TcPc, (e) SOF2@TcPc, and (f) SO2F2@TcPc. The Fermi energy is set as zero, and the space between the dashed colored lines represents the bandgap.
Figure 12. Band structure of various adsorption systems. (a) TcPc, (b) HF@TcPc, (c) H2S@TcPc, (d) SO2@TcPc, (e) SOF2@TcPc, and (f) SO2F2@TcPc. The Fermi energy is set as zero, and the space between the dashed colored lines represents the bandgap.
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Molecules 28 07137 g013
Figure 13. Band structure of various adsorption systems. (a) RuPc, (b) HF@RuPc, (c) H2S@RuPc, (d) SO2@RuPc, (e) SOF2@RuPc, and (f) SO2F2@RuPc. The Fermi energy is set as zero, and the space between the dashed colored lines represents the bandgap.
Figure 13. Band structure of various adsorption systems. (a) RuPc, (b) HF@RuPc, (c) H2S@RuPc, (d) SO2@RuPc, (e) SOF2@RuPc, and (f) SO2F2@RuPc. The Fermi energy is set as zero, and the space between the dashed colored lines represents the bandgap.
Molecules 28 07137 g013
Molecules 28 07137 g014
Figure 14. Work function of (a) TcPc and (b) RuPc monolayers with and without SF6-decomposed gas adsorption. The dashed colored lines represent the work function values of pristine TcPc and RuPc monolayer, respectively.
Figure 14. Work function of (a) TcPc and (b) RuPc monolayers with and without SF6-decomposed gas adsorption. The dashed colored lines represent the work function values of pristine TcPc and RuPc monolayer, respectively.
Molecules 28 07137 g014
Molecules 28 07137 g015
Figure 15. Sensitivity of (a) TcPc and (b) RuPc monolayers toward the SF6-decomposed gases at various temperatures.
Figure 15. Sensitivity of (a) TcPc and (b) RuPc monolayers toward the SF6-decomposed gases at various temperatures.
Molecules 28 07137 g015
Table 1. Preferential adsorption orientation (Orientation), adsorption energy (Eads), adsorption distances (D), electron transfer (Qt), and bandgap (Bg) of the most stable adsorption system, where gas@H2Pc is defined as the SF6 decomposition products adsorbed on the H2Pc substrate. When H2Pc does not adsorb gas, the band gap is 1.120 eV.
Table 1. Preferential adsorption orientation (Orientation), adsorption energy (Eads), adsorption distances (D), electron transfer (Qt), and bandgap (Bg) of the most stable adsorption system, where gas@H2Pc is defined as the SF6 decomposition products adsorbed on the H2Pc substrate. When H2Pc does not adsorb gas, the band gap is 1.120 eV.
Adsorption SystemOrientationEads/eVQt/eBg/eV
HF@H2PcH-end−0.15−0.081.106
H2S@H2PcS-end−0.140.0271.125
SO2@H2PcS-end−0.33−0.0541.092
SOF2@H2PcS-end−0.27−0.3091.106
SO2F2@H2PcS-end−0.240.0431.121
Table 2. Preferential adsorption orientation (Orientation), adsorption energy (Eads), adsorption distance (D), electron transfer (Qt), and bandgap (Bg) of the most stable adsorption system, where gas@TcPc is defined as the SF6 decomposition products adsorbed on the TcPc substrate.
Table 2. Preferential adsorption orientation (Orientation), adsorption energy (Eads), adsorption distance (D), electron transfer (Qt), and bandgap (Bg) of the most stable adsorption system, where gas@TcPc is defined as the SF6 decomposition products adsorbed on the TcPc substrate.
Adsorption SystemOrientationEads/eVD/ÅQt/eBg/eV
HF@TcPcH-end−0.232.368−0.1400.000
H2S@TcPcS-end−1.432.3080.2980.787
SO2@TcPcS-end−1.972.160−0.0990.778
SOF2@TcPcS-end−1.782.132−0.0730.830
SO2F2@TcPcS-end−0.962.353−0.2560.000
Table 3. Preferential adsorption orientation (Orientation), adsorption energy (Eads), adsorption distance (D), the electron transfer (Qt), and bandgap (Bg) of the most stable adsorption system, where gas@RuPc is defined as the SF6 decomposition products adsorbed on the RuPc substrate.
Table 3. Preferential adsorption orientation (Orientation), adsorption energy (Eads), adsorption distance (D), the electron transfer (Qt), and bandgap (Bg) of the most stable adsorption system, where gas@RuPc is defined as the SF6 decomposition products adsorbed on the RuPc substrate.
Adsorption SystemOrientationEads/eVD/ÅQt/eBg/eV
HF@RuPcH-end−0.282.268−0.1650.000
H2S@RuPcS-end−1.262.2430.2751.150
SO2@RuPcS-end−1.642.110−0.0711.223
SOF2@RuPcS-end−1.532.088−0.0391.238
SO2F2@RuPcS-end−0.333.4110.0030.000
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MDPI and ACS Style

Xue, R.; Jiang, W.; He, X.; Xiong, H.; Xie, G.; Nie, Z. The Adsorption Mechanisms of SF6-Decomposed Species on Tc- and Ru-Embedded Phthalocyanine Surfaces: A Density Functional Theory Study. Molecules 2023, 28, 7137. https://doi.org/10.3390/molecules28207137

AMA Style

Xue R, Jiang W, He X, Xiong H, Xie G, Nie Z. The Adsorption Mechanisms of SF6-Decomposed Species on Tc- and Ru-Embedded Phthalocyanine Surfaces: A Density Functional Theory Study. Molecules. 2023; 28(20):7137. https://doi.org/10.3390/molecules28207137

Chicago/Turabian Style

Xue, Rou, Wen Jiang, Xing He, Huihui Xiong, Gang Xie, and Zhifeng Nie. 2023. "The Adsorption Mechanisms of SF6-Decomposed Species on Tc- and Ru-Embedded Phthalocyanine Surfaces: A Density Functional Theory Study" Molecules 28, no. 20: 7137. https://doi.org/10.3390/molecules28207137

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