Metal-Decorated Phthalocyanine Monolayer as a Potential Gas Sensing Material for Phosgene: A First-Principles Study.

Research into a gas sensing material with excellent performance to detect or remove toxic phosgene (COCl2) is of great significance to environmental and biological protection. In the present work, the adsorption performance of COCl2 on pristine phthalocyanine (Pc) and metal-decorated Pc (MePc, Me = Cu, Ga, and Ru) monolayers was studied by first-principles calculations. The results show that the absorption process of COCl2 on pristine Pc and CuPc both belong to physisorption, indicating that they are not suitable gas sensing materials for COCl2. When Pc sheets are decorated by Ga and Ru atoms, the adsorption of COCl2 is changed into chemisorption, and the corresponding adsorption energies are -0.57 and -0.50 eV for GaPc and RuPc, respectively. The microcosmic mechanism between COCl2 and adsorbents (GaPc, RuPc) was clarified by the analysis of the density of states, the charge density difference, and the Hirshfeld charge. In addition, the COCl2 adsorption results in a significant conductivity variation of the RuPc monolayer, demonstrating it exhibits a high sensitivity to the COCl2 molecule. Meanwhile, quick desorption processes were noticed at various temperatures for the COCl2/RuPc system. Consequently, the RuPc monolayer can be considered as a potential candidate for phosgene sensors because of the moderate adsorption strength, high sensitivity, and fast desorption speed.

Introduction

Phosgene (COCl2) is a colorless, highly toxic gas that has been used in chemical warfare as well as in industrial processes including the making of dyestuffs and polyurethane resins.[1] Once exposed to phosgene, the human respiratory system is severely damaged, leading to many diseases such as noncardiogenic pulmonary edema, emphysema, and even death.[2,3] Because of the potential threat of phosgene, monitoring and controlling the concentration of phosgene in the environment is urgent. Currently, traditional metal oxide materials have been widely employed in detecting and capturing toxic gas.[4−7] However, the industrial applications of these materials are limited because of harsh working conditions, weak adsorption strength, poor sensitivity, and selectivity. To overcome these shortcomings, it is urgent to develop a kind of advanced gas sensing material with excellent performance for phosgene.

In recent years, massive researchers have conducted various investigations to improve the performance of gas sensing materials.,[8−12] Compared with the traditional sensing materials, two-dimension (2D) sensing materials have unique advantages in the design of new gas sensors because of the high surface area, excellent mechanical properties, suitable adsorption capacity, and numerous active sites.[13,14] Metal decorated phthalocyanine (MePc), which has a planar π aromatic structure, is a significant metal–organic molecular 2D material and exhibits a surprising variety of function.[15,16] Very recently, several metal-embedded phthalocyanine monolayers have been successfully synthesized in experiments, where these MePc exhibit high stability and good selectivity by decorating different metal atoms. For instance, Maggioni et al.[17] discovered that CuPc is a good gas sensor for oxynitride. Li et al.[18] studied the adsorption characteristics of H2S on CuPc thin-film transistors by experiments and found that CuPc-based OTFT devices attached with a 195 nm insulator layer are potential gas sensors for hydrogen sulfide detection.

These aforementioned studies have confirmed the feasibility of metal phthalocyanines for the gas sensing, however, the microcosmic mechanism of gas-adsorbent interaction is difficult to be explained experimentally.[19,20] Thus, many researchers employed the density function theory (DFT) calculations to develop efficient and inexpensive gas sensing materials, and analyzed the microscopic interaction mechanism at the atomic or even electronic level. Generally, to detect and capture phosgene, two primary gas sensing materials have been widely studied by the method of DFT: (1) the graphene family, such as metal-atom doped graphene,[21,22] white graphene,[23] and graphdiyne nanoflake;[24] and (2) 2D chalcogenides, such as MoS2.[25] However, the microcosmic mechanism of phosgene gas adsorbed on metal-decorated phthalocyanine is lacking investigation. In this regard, we use the DFT method to simulate the adsorption behaviors of phosgene on phthalocyanine with and without metal atoms (Cu, Ga, and Ru) doping. The adsorption energy, density of states (DOS), total charge density (TCD), charge difference density (CDD), and band structure are systematically investigated. Meanwhile, the sensing mechanism based on the conductivity change and desorption time is also evaluated, which provides theoretical support to explore the MePc monolayer as a potential phosgene gas sensor/adsorbent.

Computational Methodology

The density functional theory (DFT) calculations have been completed by the Dmol package in Materials Studio in this study.[26] The Perdew–Burke–Ernzerhof (PBE) of generalized gradient approximation (GGA) was used to treat the exchange-correlation effect of electrons.[27,28] The weak long-range interaction was corrected by adopting the empirical dispersion correction of Grimme (DFT-D).[29−31] The double numerical with polarization (DNP) basis and DFT semicore pseudopotential (DSSP) method was employed in the simulation process.[32,33] Moreover, the k points sampled by the Monkhorst–Pack scheme in the geometry optimization (electronic properties) calculations were set to 6 × 6 × 1 (12 × 12 × 1) and the global cutoff radius was selected as 5.2 Å.[34] Considering the effect of periodic image interaction, the vacuum layer along the Z direction was set to 20 Å.[35] In geometric optimization, energy tolerance accuracy, maximum force tolerance, and maximum displacement were selected as 1.0 × 10–5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively.[36]

The binding energy (Ebin) for metal decorated phthalocyanine monolayer is defined asWhere EPc, and EMe+Pc represent the total energies of pristine and metal-doped Pc, respectively. EMe is the energy of the corresponding volume of an isolated metal atom (EMe(bulk)). In addition, the aggregation possibility of metal atoms in the phthalocyanine monolayer was explored by calculating the cohesive energy (Ecoh). Ecoh = (EMe(bulk) – Eiso-Me)/n, where Eiso-M is the energy of a single metal atom and n is the volume number of metal atoms.

To assess the sensing ability, the adsorption energy (Eads) of phosgene molecule on the pristine Pc and MePc monolayers are defined asWhere Egas+MePc is the total energy of phosgene adsorbed on metal doped phthalocyanine, EMePc and Egas are the total energies of MePc monolayer and phosgene molecule, respectively.

In general, when Eads is negative, the adsorption should be spontaneous, and a more negative value means a better stability.[36,37] If the absolute value of negative adsorption energy Eads > 0.5 eV, the adsorption process can be judged as chemical adsorption, otherwise, it is physical adsorption.[10,38] In this work, the adsorption behavior of phosgene molecules on phthalocyanine and metal phthalocyanine monolayers are calculated at room temperature (298 K). To reveal the interaction between substrate and gas molecules, Hirshfeld charge is calculated to discuss the corresponding charge transfer (Qt) according to the following equation:Qadsorbed molecule and Qisolated molecule represent the charge numbers of target molecules before and after adsorption, respectively. When the charge transfer Qt > 0, it indicates that the electrons are transferred from gas molecule to phthalocyanine surface, otherwise, the electrons are transferred from phthalocyanine surface to gas molecule.

Results and Discussion

Structure and Stability of Pristine Pc, MePc, and COCl2

Figure shows the optimized geometries of pristine Pc, metal doped phthalocyanine (MePc, Me= Cu, Ga, and Ru) monolayers and gas molecule COCl2. As shown in Figure a, the full-relaxed pristine Pc monolayer contains 20 C atoms, 8 N atoms, and 4 H atoms. All the atoms in each molecule are coplanar, indicating the whole molecule is fully delocalized and conjugated. When the Pc monolayer is decorated with Cu, Ga, and Ru atoms (Figure b–d), the CuPc, GaPc, and RuPc monolayer remain in the stable “graphene-like” plane configuration. The lattice constants of both phthalocyanine and metal phthalocyanine are about 10.69 Å, which is in good agreement with the published experimental and theoretical results.[39−41]Figure e gives the optimized geometrical structure of COCl2 gas molecule. The bond lengths of C–O bond and C–Cl bond in the COCl2 molecule are 1.190 and 1.758 Å, respectively, and their bond angles are 124.136°, which matches well with the other calculated results.[42]

Figure 1

Geometric optimized model: (a) pristine Pc monolayer, (b) CuPc monolayer, (c) GaPc monolayer, (d) RuPc monolayer, and (e) gas molecule COCl2. The C, H, N, Cu, Ga, Ru, O, and Cl atoms are indicated by gray, white, blue, orange, gray purple, blackish green, red, and green balls, respectively.

The binding energy (Ebin) between the central metal and Pc monolayer is plotted in Figure , one can see that the Ebin of MePc (Me= Cu, Ga, and Ru) monolayer are −7.63 to −10.11 eV, and all of the values are below the cohesive energy of corresponding metal bulk, demonstrating they are stable. According to previous study, the center site, namely, the vacancy in the center of Pc or just above the doped metal atom in MePc, is selected as the preferential adsorption site.[43]

Figure 2

Cohesive energy of metal bulk (Ecoh) and binding energy between central metal and Pc monolayer (Ebin) as a function of the different decorated metals.

Adsorption Behavior of COCl2 on Pristine Pc and MePc

The best adsorption structure of COCl2 on the pristine Pc and MePc (Me= Cu, Ga, and Ru) monolayer is described in Figure . The adsorption energy, charge transfer, and band gap of the pristine Pc system are listed in Table . In the present work, we consider the parallel and vertical adsorption style, for instance, the structure of COCl2 parallelly adsorbed on the pristine Pc is defined as Pc-COCl2-P, as seen in Figure a. After adsorption, the COCl2 gas molecule is preferential to adsorb on the Pc substrate in parallel style, and the adsorption energy is −0.37 eV, with a charge of 0.0327 e transferred from the Pc substrate to COCl2. The interaction between the COCl2 molecule and Pc substrate can be regarded as physical adsorption, thus, the pristine Pc monolayer has poor COCl2 capture ability.[33] The calculated adsorption parameters of COCl2 on the MePc (Me = Cu, Ga, and Ru) monolayer substrate are summarized in Table . The COCl2-P on the GaPc surface is thermodynamically unstable because it is changed into the adsorption style of COCl2-V on the GaPc monolayer after relaxation. From Table , one can see that the COCl2 molecule tends to be adsorbed on MePc sheets in parallel style except for the GaPc monolayer. The introduction of Ga and Ru atoms can obviously improve the adsorption ability of Pc toward COCl2, the most stable adsorption structures are GaPc-COCl2-V and RuPc-COCl2-P with the Eads of −0.57 eV and −0.50 eV, respectively, which are concluded as chemisorption.[33] However, the decoration of Cu atom cannot promote the COCl2 capture ability because of the weak adsorption energy. From Figure c, d, the COCl2 adsorbed on GaPc and RuPc substrates exhibit little deformation, and the corresponding adsorption distance (charge transfer) is 2.143 Å (0.1236 e) and 2.100 Å (0.1654 e), respectively. Those results indicate that the Ga and Ru atom doped Pc has stronger adsorption performance to COCl2 in comparison with the pristine Pc.

Figure 3

Top and side views of the geometric optimized structure after adsorption: (a) Pc-COCl2-P, (b) CuPc-COCl2-P, (c) GaPc-COCl2-V, and (d) RuPc-COCl2-P. The C, H, N, Cu, Ga, Ru, O, and Cl atoms are indicated by gray, white, blue, orange, gray purple, blackish green, red, and green balls, respectively.

Table 1

Adsorption Energy (Eads) of Gas Molecule COCl2 on the Pc Substrate, Charge Transfer between COCl2 and Pc (Qt, Negative Value Indicates That the Gas Has Gained Electrons), and Energy Gap (Eg) for Pc Adsorption System

gas	materials	adsorption style	Eads (eV)	Qt (e)	Eg (eV)
COCl2	Pc	parallel	–0.37	0.0327	0.00
		vertical	–0.16	0.0046	0.00

Table 2

Adsorption Energy (Eads) of Gas Molecule COCl2 on CuPc, GaPc, and RuPc, Adsorption Distance (d), and Charge Transfer (Qt) for Different Adsorption Systems

gas	materials	adsorption style	Eads (eV)	d (Å)	Qt (e)
COCl2	CuPc	parallel	–0.30	3.282	–0.0504
		vertical	–0.15	2.881	–0.0369
	GaPc	vertical	–0.57	2.143	–0.1236
	RuPc	parallel	–0.50	2.100	0.1654
		vertical	–0.32	2.047	–0.1664

The DOS of the lowest-energy configuration of COCl2 on the pristine Pc is shown in Figure . One can see that the DOS of COCl2 is nonlocalized, and there is no obvious resonance peak between the COCl2 and Pc monolayer. This illustrates that the interaction between COCl2 and the pure Pc monolayer is weak, which mainly relies on the van der Waals force. Thus, the pristine Pc monolayer could not be a good gas sensing material for COCl2 because of its poor adsorption capacity.

Figure 4

DOS of COCl2, a Pc monolayer, and COCl2 adsorbed on a Pc monolayer. The Fermi level is set to zero energy and indicated by the black dashed line.

Figure shows the strong resonance peak (the light blue region) between the COCl2 molecule, the decorated Ga atom, and the Pc sheet, which implies a significant interaction in COCl2/GaPc adsorption system. By contrast, in the pristine Pc system (Figure ), there only exists a weak interaction between COCl2 and pure Pc sheet. Therefore, the doped Ga atom behaves as an electron bridge enhancing the interactions of gas-adsorbent. Additionally, there also exists the orbital hybridization between the COCl2 and Pc sheet, as indicated by the orange dashed line. The strong interactions can be further confirmed by the charge density difference (CDD) and total charge density (TCD) corresponding to the COCl2/GaPc adsorption system, as shown in Figure . The electron aggregation is mainly localized on the COCl2 molecule, which manifests the electron-withdrawing property of COCl2, this coincides with the Hirshfeld analysis (Qt = −0.1236 e). The localized electron depletion is around the Ga atom, which also demonstrates that it denotes partial electrons (0.0589 e) to the COCl2 molecule. Meanwhile, the COCl2 molecule also takes 0.0647 e from the Pc sheet. In other words, Ga bridges the charge transfer from the gas molecule to the Pc monolayer, thus facilitating electron redistribution in the adsorbed system. Besides, the TCD predicts the overlap of electron density of COCl2 gas molecules with the GaPc monolayer, as seen in Figure c, d. Consequently, the GaPc monolayer can be considered as candidate for phosgene adsorbent or gas sensor because of its strong removal ability.

Figure 5

DOS of COCl2 adsorbed on the GaPc monolayer. The Fermi level is set to zero energy and indicated by the black dashed line.

Figure 6

(a) Top and (b) side views of the charge density difference (CDD) map and (c) top and (d) side views of total charge density (TCD) map correspond to the most stable adsorption system of COCl2 on GaPc. The blue (yellow) areas are electron aggregation (depletion), and the isosurface values of CDD and TCD are ±0.02 and ±0.05 e/Å3, respectively.

To clarify the interaction mechanisms of the COCl2/RuPc adsorption system, the DOS of COCl2 adsorbed on RuPc monolayer is also calculated, as seen in Figure , indicating the strong resonance peak (the violet region) between the COCl2 molecule, the Ru-3d orbitals, and the Pc sheet, which manifests a significant interaction in COCl2/RuPc adsorption system. Thus, the decorated Ru atom acts as an electron bridge, strengthening the interactions between the adsorbed COCl2 molecule and the Pc monolayer. The strong interactions are further confirmed by the CDD diagram in panels a and b in Figure , the dense electron aggregation is located between the Ru and O atoms, indicating its strong covalent bond. Unlike the adsorption system of COCl2/GaPc (Figure ), many electrons are accumulated around the doped Ru atom, and the Ru gains 0.1108 e from the gas molecule, which is consistent with the Hirshfeld analysis (Qt = −0.1654 e), and thus the COCl2 acts as an electron donor. The total electronic charge densities (TCD) of gas molecules absorbed on the RuPc monolayer in Figure c, d predict that there is an obvious electron orbital overlap between the gas molecules and the RuPc monolayer. Thus, the RuPc monolayer can be also considered as another candidate for phosgene adsorbent or gas sensor.

Figure 7

DOS of COCl2 adsorbed on a RuPc monolayer. The Fermi level is set to zero energy and indicated by the black dashed line.

Figure 8

(a) Top and (b) side views of the charge density difference (CDD) map and (c) top and (d) side views of total charge density (TCD) map correspond to the most stable adsorption system of COCl2 on RuPc. The blue (yellow) areas are electron aggregation (depletion), and the isosurface values of CDD and TCD are ±0.02 and ±0.05 e/Å3, respectively.

Analysis of the Sensing Mechanism

The above sections exhibit the characteristics of good stability and strong adsorption capacity of GaPc and RuPc monolayers, which is suitable for the potential COCl2 gas sensing material. In this section, to better understand the microscopic sensing mechanism, we further analyzed the sensitivity and desorption time for the GaPc and RuPc monolayers. Generally, the energy gap is used as an evaluation criterion to evaluate the exciting behavior of the molecule; a small energy gap usually fits a molecule that is easy to excite, and the results also reflect the change in conductivity of the system. Figure shows the diagram of the energy gap of a Ga and Ru atom doped phthalocyanine before and after the COCl2 gas molecule was adsorbed. According to Figure a, the Ga-doped Pc monolayer shows a semimetallic feature, with the electronic energy gap of the GaPc monolayer remains the same after the COCl2 adsorption, as shown in Figure b. Thus, the conductive behavior between gas molecules and GaPc monolayer has little variation, demonstrating that it has poor COCl2 sensibility. From panels c and d in Figure , the energy gap of the RuPc monolayer enlarged from 0 to 0.605 eV after the COCl2 gas molecule adsorption, which indicated that the adsorption system changes from a semimetallic feature to a semiconductor. Because of the noticeable increase in the energy gap, the RuPc monolayers show excellent sensitivity. Consequently, compared with the GaPc monolayer, the RuPc monolayer is more suitable as a promising COCl2 sensing material in the case of sensitivity.

Figure 9

Band gap of GaPc (a) before COCl2 adsorption and (b) after COCl2 adsorption. The band gap of RuPc (c) before COCl2 adsorption and (d) after COCl2 adsorption. The dashed line is the Fermi energy.

It is well-known that the sensitivity can be assessed by exploring the variation of electrical conductivity (σ) of the COCl2/MePc gas adsorption system, the variation in electrical conductivity could be defined as[23]where Eg is the band gap, KB represents the Boltzmann constant (8.62 × 10–5 eV/K), and T is the working temperature of the gas sensor. According to the discussion above, the band gap of COCl2/GaPc adsorption systems remains unchanged, which implies that there are few effects on the resistivity when the phosgene adsorbed on GaPc monolayer, thus the sensitivity of GaPc monolayer to phosgene molecules is poor. Meanwhile, upon exposure to the phosgene molecule, the electronic properties of the RuPc monolayer are dramatically changed, and there is an effectively interact between RuPc and COCl2 according to the previous analysis. This illustrates that doping phthalocyanine monolayer with Ru reflects the excellent applicability as a gas sensor for the detection of COCl2.

Finally, the desorption property is also significant to assess the repeatability of a gas sensor. Moderate interactions between sheet and gas molecules imply that adsorbate can be desorbed in a short time so that the device can realize its sustainable utilization. On the basis of the transition state theory, we calculated desorption time (τ) by the following equation:[44]where υ0 is the attempt frequency, and T is the temperature. In this work, the value of attempt frequency is considered as 1 × 10–12 s–1, which was employed by the reported literature.[25,45] The desorption time for the phosgene over RuPc at various temperatures is shown in Figure , where the working temperature is from 298 to 498 K. The results indicate that the desorption time of phosgene over RuPc monolayer decreases gradually (1.144 × 10–7 to 2.841 × 10–4 s) with the increase in temperature. Therefore, the sensitivity of RuPc to phosgene is large enough, and RuPc can be used as the sensor for gas detection because of the quick desorption time.

Figure 10

Desorption time for the phosgene over RuPc at various temperatures.

A comparative analysis of different 2D materials toward COCl2 sensing with previously reported research work has been conducted, as shown in Table . First, it can be seen that the adsorption of phosgene on boromerene is physical adsorption, and the interaction is weak, so it is not suitable to be a gas sensing material for COCl2.[45] We can see that Si embedded MoS2 sheet and AlPc monolayer have strong adsorption capacity for phosgene molecules, which severely impedes gas desorption because of their long desorption time.[25,36] Therefore, they are not suitable for reusable COCl2 gas sensor materials. In contrast, the RuPc monolayer can be considered as a potential and promising candidate for the COCl2 gas sensor because of its moderate adsorption capacity and fast desorption speed.

Table 3

Comparison of Different Properties for COCl2 Adsorbed on Different 2D Materialsa

2D materials	Eads (eV)	τ (s)	refs
Si-embedded MoS2 sheet	–1.23	5.77 × 108	(25)
AlPc monolayer	–0.86	3.47 × 102	(36)
borophene	–0.31	1.74 × 10–7	(45)
RuPc monolayer	–0.50	2.84 × 10–4	this work

The desorption time τ (at 298 K) is calculated from the data in the literature.

Conclusions

In this work, the first-principles calculation was employed to study the adsorption performance of COCl2 on the pristine Pc and MePc (Me = Cu, Ga, and Ru), aiming to seek a novel MePc-based gas sensing material for detecting or removing phosgene. The results show that the pristine Pc and CuPc have less potential to be used as a gas sensing material for COCl2 because of their poor adsorption strength; However, when decorating Ga and Ru atom into a Pc sheet, the decorated Ga, Ru atom acts as an electron bridge, enhancing the interactions between the adsorbed COCl2 molecule and Pc monolayer. On the basis of the analysis of the energy gap, the RuPc monolayer exhibits a high sensitivity to the COCl2 molecule because of the variation of conductivity. In addition, a quick response on the desorption was noticed for the COCl2/RuPc substrate at various temperatures. Overall, the RuPc monolayer is one of the potential candidates for phosgene sensors, because it shows good stability, moderate adsorption strength, high sensitivity, and fast desorption speed.