Adsorption of methylisocyanate and ethylisocyanate on a Co-decorated TiS₂ monolayer: understanding chemical interactions using DFT and COHP

Abstract

Designing efficient nanosensors is highly desirable for the detection of toxic substances. Herein, the adsorption behavior of the two specified VOCs, methylisocyanate (MIC) and ethylisocyanate (EIC), was investigated using density functional theory (DFT) and COHP calculations on pristine and cobalt-decorated titanium disulfide (Co–TiS₂) monolayer systems. The decoration of Co on the TiS₂ monolayer induces a semiconductor-to-metal transition in it. The stability of the Co–TiS₂ monolayer system was confirmed by the calculated adsorption energy (E_ads) of −3.18 eV. DFT calculations revealed that the pristine TiS₂ monolayer systems are chemically inert towards the above-specified VOCs. However, functionalizing the TiS₂ monolayer with a Co atom substantially improved the adsorption of MIC and EIC molecules. The estimated E_ads values upon MIC and EIC adsorption on Co–TiS₂ monolayer systems were −0.81 eV and −0.95 eV, respectively. The adsorption of the investigated VOCs changed the electronic and magnetic properties of Co–TiS₂ monolayer systems, which were examined through spin-polarized density of states calculations. Bader charge analysis revealed that VOCs are charge acceptors and the Co–TiS₂ monolayer is a charge donor. A considerable increase in work function was observed upon adsorption, correlating with sensitivity enhancements of up to 10% for MIC and 8% for EIC. COHP analysis revealed that both molecules interact with the adsorbent by forming Co–N ionic bonding caused by the Co(4s)-N(2s) orbital overlapping. The Ti–S, Co–S, and Co–N bonding pairs possess an ICOBI value of 0.32, 0.31, and 0.15, respectively, showing weak ionic bonding. Our findings propose that Co–TiS₂ could be a better choice for the detection of MIC and EIC.

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1 Introduction

The uncontrolled release of toxic gases and volatile organic compounds (VOCs) from industrial activities, automobile exhaust, and the routine usage of organic solvents in paints, adhesives, and other products into the environment is deteriorating living standards and food safety.^1,2 VOCs belonging to the family of low-molecular-weight compounds, consist of carbon and hydrogen atoms, and vaporize easily at ambient conditions.³ The dermal contact or inhalation of VOCs has both acute and chronic impacts on human health.⁴ Short-term exposure to VOCs can lead to eye and throat irritation, headaches, and dizziness.^5,6 In contrast, long-term exposure has been associated with skin allergies, hormonal imbalances, neurotoxicity, cancer, kidney damage, and effects on reproductive and metabolic functions.^7–9

Methyl isocyanate (MIC), a member of the VOCs family, with the chemical formula C₂H₃NO, is a colorless, highly flammable liquid having a sharp, strong smell.¹⁰ It rapidly evaporates when exposed to air. MIC is extensively used in the synthesis of polyurethane foams, plastics, rubbers, and adhesives, as well as in the production of carbamate pesticides.¹¹ Even though it is useful, but has a reactive and highly toxic nature, creating several health hazards, including reproductive and developmental toxicity, respiratory irritation, cancer, and even death.^12,13 The deaths of about 8000 people within 48 hours were reported due to the leakage of MIC in the Bhopal disaster, leaving more than 200 000 victims who were experiencing long-term health consequences.¹⁴ Ethyl isocyanate (EIC), a derivative of the isocyanate family, is a colorless, transparent, flammable liquid with a pungent smell. The electrophilic nature of the isocyanate unit in its structure makes it suitable for the synthesis of various organic compounds. Although it is used as an intermediate for the synthesis of agrochemicals, pharmaceuticals, and polyurethane-based materials, it has some adverse health effects.¹⁵ Its acute exposure can induce irritation in the eyes and cause skin allergies, while chronic exposure may lead to shortness of breath, wheezing, cough, and chest infections.¹⁶ Considering the challenges of health and safety in the laboratory and industry, the detection of VOCs, such as MIC and EIC, is inevitable.

Metal oxide-based nanosensors have mainly been used for the detection of VOCs because of their excellent sensing potential and ease of operation. Still, high operating temperature and poor sensitivity have limited their feasibility for sensing purposes.^17,18 Sensors based on conducting polymers are also not suitable for detecting VOCs due to certain drawbacks, such as degradation and humidity effects.¹⁹ Two-dimensional (2D) materials have attracted considerable attention in sensor technology owing to their large surface-to-volume ratio and high density of active sites.²⁰ Among 2D materials, thanks to their tunable band gap and sensitive surfaces, transition metal dichalcogenides (TMDs) have received considerable attention for sensor applications.²¹ Within the family of TMDs, the titanium disulfide (TiS₂) remains unexplored for VOCs sensing applications. TiS₂ is a layered TMD similar to MoS₂ and WS₂; however, its crystallographic structure differs significantly, crystallizing in a CdI₂-type hexagonal close-packed (hcp) phase.²² A monolayer TiS₂ nanosheet is composed of a layer of Ti atoms positioned between two S atomic layers. TiS₂ can crystallize in different polymorphs, most notably the 1T and 2H phases. In the 1T phase, each Ti atom is coordinated by six S atoms in an octahedral coordination environment. Whereas, in the 2H phase, Ti atoms are surrounded by six S atoms arranged in a trigonal prismatic geometry.²³ TiS₂ favors the 1T phase over the 2H phase, being 142 meV per atom lower in energy.²⁴ Henceforth, the analysis pertains to the 1T phase. The weak interlayer bonding in TiS₂ enables the isolation of individual monolayers. Several techniques, such as atomic layer deposition, chemical vapor deposition, and exfoliation techniques, have been employed for the synthesis of 2D TiS₂.^25,26

The sensing efficiency of a pristine TMDs monolayer sensor towards adsorbates is relatively weak, but can be enhanced through various approaches, like heterojunction engineering, doping, exposure to high-energy beams, morphology modification, and decoration with metals.^27–31 H. Liu et al. reported using first-principles calculations that the Ir-decorated MoS₂ exhibits good potential for detecting C₂H₂ and C₂H₄ gas molecules.³² H. Wu et al. showed that Pd decoration improves the H₂ sensing behavior of MoS₂.³³ J. Shi et al. reported that doping TaS₂ with metal atoms not only tunes the Fermi level but also introduces positive charge centers, which contribute to superior gas-sensing performance.³⁴ F. Altalbawy et al. performed the DFT simulation to examine the Pd and Pt transition metals decorated WS₂ nanosheets for the efficient detection of SO₂ and SO₃ molecules.³⁵ Toularoud et al. studied the electronic and magnetic properties of monolayer TiS₂ doped with 3d transition-metal atoms and found that the Cr-doped TiS₂ monolayer system showed the most significant magnetic moment.³⁶ Ghani et al. emphasized that the presence of defects in TiS₂ monolayers enhances their adsorption of CO molecules.³⁷ T. Tang et al. reported that titanium oxysulfide (TiS_xO_2−x) significantly improves the sensitivity and stability of NO₂ detection at room temperature.³⁸ S Zhang et al. have made a COHP analysis of metal-doped WSe₂ to analyze the bonding and antibonding interactions between the adsorption sites and gas molecules.³⁹ To the best of our knowledge, transition-metal-decorated TiS₂ has not yet been investigated for the efficient detection of VOCs like MIC and EIC. Therefore, it is of great interest to analyze the adsorption behavior and sensing mechanisms governing the interaction of targeted VOCs on the surface of 2D TiS₂. Hence, analyzing how VOCs adsorb and interact with TiS₂ monolayer systems is essential for understanding their sensing performance.

This study examines the structural, electronic, and magnetic properties of pristine TiS₂ (p-TiS₂) and cobalt-decorated TiS₂ (Co–TiS₂) monolayers for the adsorption of MIC and EIC using DFT calculations. To characterize the adsorption mechanism, we consider adsorption energy, adsorption height, charge transfer, and the modifications in electronic and magnetic properties. Additionally, electron localization function (ELF) and crystal orbital Hamiltonian population (COHP) analysis was employed to illustrate the bonding interactions between Co–TiS₂ monolayer systems and the adsorbed VOCs molecules. Thus, this study offers a theoretical foundation for developing efficient VOCs sensors.

2 Computational method

In the present study, first-principles calculations were performed with the Vienna Ab initio Simulation Package (VASP) code based on density functional theory (DFT).⁴⁰ The projector-augmented wave (PAW) potentials were considered to describe the interactions between the electron and ions.⁴¹ For treating electron interactions, the exchange-correlation energy was described using the generalized gradient approximation (GGA) within the Perdew–Burke–Ernzerhof (PBE) functional.⁴² To consider the long-range interactions, van der Waals (vdW) effects were incorporated using the Grimme correction (i.e., DFT-D3) with a vdW correction parameter (IVDW) of 12.⁴³ The on-site coulomb interactions between partially filled d-orbitals of Ti and Co atoms were corrected using the DFT + U_eff approach, where U_eff for Co and Ti are 3.42 eV and 2.58 eV, respectively.⁴⁴ A cutoff kinetic energy of 500 eV was employed for the plane-wave expansion, and the size of the k-point mesh was set to 25 × 25 × 1, which was validated through convergence tests and presented in Fig. S11. For initial calculations, a unit cell of the pristine T-TiS₂ monolayer was obtained from the TiS₂ bulk structure, and a 20 Å vacuum along the z-axis was used to avoid interactions between periodic images. The convergence criterion of 10⁻⁵ eV and smearing width of 0.0 eV was used for full relaxation of the 1 × 1-TiS₂ unit cell. To investigate the effect of Co-decoration, a supercell of 5 × 5 × 1 was constructed from a 1 × 1 TiS₂ unit cell, and the k-point mesh of 4 × 4 × 1 was employed for its geometry optimization. To analyze the stability of the Co-decorated TiS₂ monolayer systems and the adsorbed molecules, the following equation was used.⁴⁵

E_ads = E_Co–TiS2 − E_TiS2 − E_Co (1)

where E_Co–TiS2 denotes the total energy of the Co–TiS₂ monolayer systems E_TiS2 refers to the total energy of TiS₂ monolayer systems, and E_Co corresponds to that of the isolated Co-atom.

The adsorption energies (E_ads) of the selected VOCs on Co–TiS₂ monolayer systems were calculated using the expression.

E_ads = E_{VOC/Co–TiS2} − E_Co–TiS2 − E_VOC (2)

where E_{VOC/Co–TiS2} denotes the total energy of the VOCs adsorbed Co–TiS₂ monolayer systems E_Co–TiS2 refers to the total energy of Co–TiS₂ monolayer systems, and E_VOC Corresponds to that of the isolated VOC molecule.

The equation used for the calculation of work function (Φ) was.⁴⁶

ϕ = E_vacuum − E_F (3)

where E_F is the Fermi energy and E_vacuum is the vacuum-level electrostatic potential energy.

The charge density difference (CDD) was calculated by using the equation.⁴⁷

Δρ = ρ_product − ρ_adsorbent − ρ_molecule (4)

where ρ_product, ρ_adsorbent and ρ_molecule are the total charge density of the VOCs adsorbed Co–TiS₂ monolayer systems, bare Co–TiS₂ surface, and isolated VOCs, respectively.

3 Results and discussions

3.1 Structural properties of p-TiS₂ and Co–TiS₂

Our discussion starts with the structural properties of p-TiS₂ and Co–TiS₂ monolayer systems. To find the ground state energy of p-TiS_2, it was subjected to full structural relaxation for both non-magnetic and magnetic configurations. The results confirm that p-TiS₂ is non-magnetic, as reported in the literature.⁴⁸ For the decoration of TiS₂ with Co, five different possible adsorption sites were considered, namely the bridge site, hollow site, lower S site, Ti top site, and upper S site, as shown in Fig. S1. The bridge site corresponds to the midpoint of the Ti–S bond, the hollow site lies above the center of the hexagonal ring, the lower S site is positioned above the S atom in the bottom layer, the Ti top site is directly above a Ti atom, and the upper S site is located above the S atom in the top plane of the TiS₂ monolayer. Each configuration was subjected to full relaxation for various magnetic and non-magnetic states. Based on the computed ground-state energies and from the estimated adsorption energy (E_ads), Co adsorption at the hollow site is found to be most energetically favorable. Xu et al. examined Sc adsorption on WS₂ and found that the hexagonal hollow site offered the most energetically favorable configuration.⁴⁹ Furthermore, the lowest energy is achieved in the ferromagnetic (FM) configuration, verifying that Co–TiS₂ favors a ferromagnetic (FM) ground state, in agreement with earlier studies.⁵⁰

The optimized geometry of p-TiS₂ is presented in Fig. 1(a). TiS₂ crystallizes in a hexagonal lattice structure, and side-view analysis reveals a tri-layer structure, with the Ti layer centrally positioned between two symmetric S layers. The ground state lattice constant of p-TiS₂ monolayer is a = b = 3.42 Å, and the Ti–S bond length is 2.42 Å, whereas in the literature, the reported values are 3.40 Å and 2.42 Å, respectively.⁵¹ The observed S–Ti–S bond angles are 90.66° and 54.80°. The vertical distance separating the upper and lower sulfur atom planes in monolayer TiS₂ is known as the buckling height or thickness of the monolayer, and its observed value is 2.84 Å. The optimized top and side views of the five distinct Co–TiS₂ configurations are illustrated in Fig. S2. This demonstrates that in each case, the Co atom undergoes a slight displacement from its initial position to achieve a more stable configuration. The ground state lattice constant of the Co–TiS₂ monolayer is observed to be a = b = 3.42 Å, identical to that of pristine TiS₂, while the Ti–S bond length slightly increases to 2.43 Å upon decoration. The change in bond length may be attributed to the interaction between the Co atom and the substrate.⁵² The adsorption energy for each configuration is calculated and is presented in Table 1. The negative E_ads indicated that the decoration of the Co atom on the surface of the TiS₂ is an exothermic process.⁵³ Moreover, to verify the thermal stability of Co–TiS₂, ab initio molecular dynamics (AIMD) simulations were performed at 300 K for 6000 fs and presented in Fig. S12. It is observed that despite some minor distortions, the structure did not dissociate into an amorphous phase. No local bond breaking is observed. The potential energy PE of the ions has some clear fluctuations at some intervals. These peaks correspond to the large displacement of the Co atom from the initial position. However, as the simulation process evolves, the PE variation becomes smaller. It shows the local geometry does not undergo large changes or distortions. The displacement of the Co atom also has some variations; however, these can be regarded as small. It shows that the given structure is assumed to be thermally stable.

Fig. 1 (a and d) Optimized structures, (b and e) band structures, and (c and f) the total density of states (TDOS) of p-TiS₂ and Co–TiS₂ monolayers, respectively.

Table 1 A summary of adsorption energy (E_ads) in eV of Co-atom decorated TiS₂ monolayer at different adsorption sites, magnetic moment in µ_B, vertical distance (h) between Co-atom and upper S-atom layer in Å, Co–S bond length in Å, and nature of adsorption for each configuration

System	Adsorption configuration	Magnetic moment	Eads	h	Co–S bond length	Adsorption
Co-decorated TiS2	Bridge site	−2.52	−3.07	1.10	2.27	Chemisorption
	Hollow site	−2.65	−3.18	0.66	2.16	Chemisorption
	Lower S site	−2.65	−2.98	0.71	2.17	Chemisorption
	Ti top site	−2.52	−3.13	1.10	2.26	Chemisorption
	Upper S site	−2.80	−1.48	2.19	2.15	Chemisorption

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3.2 Electronic and magnetic properties of p-TiS₂ and Co–TiS₂

To understand how Co decoration modifies the electronic and magnetic behavior of the TiS₂ monolayer, we performed a detailed analysis of its electronic band structure and density of states (DOS), providing deeper insights into the underlying mechanisms of property modulation. The calculated electronic band structure and total density of states (TDOS) of the p-TiS₂ monolayer system are shown in Fig. 1(b and c). From Fig. 1(b), the missing electronic states at the Fermi level indicate that p-TiS₂ is a semiconductor. The valence band maximum (VBM) lies at the G point, whereas the conduction band minimum (CBM) appears at the M point. Thus, the material exhibits an indirect band gap nature. The CBM is positioned 0.11 eV above the Fermi level, while the VBM lies 0.11 eV below it, yielding a band gap of 0.22 eV, which closely matches the previously reported value of 0.18 eV.⁵⁴ The density of states (DOS) analysis, as presented in Fig. 1(c), further confirms the presence of an energy gap in the p-TiS₂ monolayer system due to the absence of states around the Fermi level. To gain deeper insights into the electronic properties, the partial density of states (PDOS) and orbital density of states (ODOS) of the p-TiS₂ monolayer were examined and are presented in Fig. 2(a–c). From the PDOS analysis, it is evident that both Ti and S atoms contribute to the electronic states around the band edges. The VBM is primarily derived from the 3p orbitals of S atoms, while the 3d orbitals of Ti atoms dominate the CBM. The spin-polarized electronic band structure and TDOS of the Co–TiS₂ monolayer system are presented in Fig. 1(e and f), respectively. The decoration of Co on the TiS₂ monolayer induces a semiconductor-to-metal transition in it, as seen by the presence of electronic states at the Fermi level in Fig. 1(e). This behavior is consistent with the findings of U. Iqrar et al., who observed a similar transition upon the decoration of Li, Na, and K on TiS₂.⁵⁵ Decoration with a foreign atom may distort the host lattice charge distribution, introducing new states near the Fermi level that increase the density of states and facilitate electron occupation, thereby enhancing conductivity.⁵⁶ The TDOS, PDOS, and ODOS spectra of Co–TiS₂ are presented in Fig. 2 (d–f). The Ti-3d orbitals contribute to the conduction band, while the Co-3d and S-3p orbitals dominate the valence band. The spin-polarized TDOS plot of Co–TiS₂ from Fig. 1(f) shows an unequal distribution of density of states between spin-up and spin-down channels, leading to a net magnetization in the Co–TiS₂ monolayer system.⁵⁷ The estimated total magnetic moment is −2.65 µ_B, where the negative sign indicates that the majority of carriers occupy the spin-down channel, also depicted in plot 1(f). The Co contributes to this magnetic moment is −1.902 µ_B, whereas Ti-atoms contribute −0.055 µ_B. The negative signs indicate that the Co atom and the Ti atoms are spin-polarized in parallel, and the Co–TiS₂ monolayer system is ferromagnetic. Upon Co decoration, the partially filled 3d orbitals of Co introduce unpaired electrons and spin polarization, thereby inducing magnetism in the system. Whereas in p-TiS₂, Ti⁴⁺ cations possess an empty 3d shell and therefore do not contribute any unpaired d electron to the system, and the system remains non-magnetic.

Fig. 2 Partial density of states (PDOS) and orbital density of states (ODOS); (a–c) for p-TiS₂ and (d–f) for Co–TiS₂.

4 Adsorption of MIC and EIC molecules

To explore the adsorption behavior of MIC and EIC on TiS₂ monolayer systems, six different possible initial orientations of the adsorbates, four parallel (h₁, h₂, h_3, and h₄) and two perpendicular (v₁ and v₂) to the surface, were considered, and the corresponding ground state energies were calculated. All the initial configurations are shown in Fig. S3 and S5. The relaxed geometries corresponding to the most stable orientations of MIC and EIC on p-TiS₂ are presented in Fig. 3(a) and 4(a), respectively. Our calculations revealed that both MIC and EIC preferentially adsorb on p-TiS₂ via the O site, forming a Co–O bond perpendicular to the plane. The key parameters characterizing the optimized adsorption configurations are given in Table 2. The calculated E_ads of MIC and EIC on p-TiS₂ monolayer are −0.15 eV and −0.17 eV, respectively. The optimized average adsorption height (h) between the MIC molecule and the TiS₂ surface is 2.98 Å, whereas for the EIC molecule, it is 2.88 Å. These values of h are greater than the sum of the covalent radii (SCR) of the S atom in the TiS₂ surface and the N atom in the adsorbates (1.76 Å). Coupled with the relatively low adsorption energies, these larger h values clearly indicate that the interaction of the adsorbates with the pristine TiS₂ monolayer is characterized by physisorption. Therefore, no chemical bond is formed between the S atoms of p-TiS₂ and the adsorbate atoms.

Fig. 3 (a and d) Optimized structures, (b and e) band structures, and (c and f) TDOS of methylisocyanate (MIC) adsorbed on p-TiS₂ and Co–TiS₂ surfaces, respectively.

Fig. 4 (a and d) Optimized structures, (b and e) band structures, and (c and f) TDOS of ethylisocyanate (EIC) adsorbed on p-TiS₂ and Co–TiS₂ surfaces, respectively.

Table 2 Adsorption energy (E_ads) in eV for different orientations of adsorbates (VOCs molecules), vertical distance (h) between molecule and surface in Å, distance (d) between molecule and surface in Å, and total magnetic moment (M) per supercell in µ_B

System	Adsorbed molecule	Orientation of adsorbed molecule	Eads	h	d	M
p-TiS2	MIC	h1	−0.14	3.14	3.21	—
		h2	−0.10	2.98	3.17	—
		h3	−0.15	3.42	3.51	—
		h4	−0.13	2.79	2.86	—
		v1	−0.01	2.65	2.73	—
		v2	−0.03	2.91	3.23	—
Co–TiS2	MIC	h1	−0.81	2.06	2.06	−1.0036
		h2	−0.73	2.10	2.85	1.0141
		h3	−0.75	2.08	2.11	1.0135
		h4	−0.51	2.14	2.28	1.0181
		v1	−0.10	1.98	1.98	−1.0002
		v2	−0.60	2.00	2.10	2.5445
p-TiS2	EIC	h1	−0.15	3.10	3.22	—
		h2	−0.16	3.02	3.18	—
		h3	−0.17	3.29	3.35	—
		h4	−0.16	2.58	2.97	—
		v1	−0.02	2.49	2.65	—
		v2	−0.03	2.83	3.27	—
Co–TiS2	EIC	h1	−0.95	2.06	2.07	−2.5542
		h2	−0.48	2.37	2.46	−1.0105
		h3	−0.89	1.99	2.08	−2.5341
		h4	−0.27	2.01	2.01	1.0051
		v1	−0.59	1.92	1.96	2.6054
		v2	−0.17	2.06	2.10	−1.0010

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Decorating the TiS₂ monolayer with Co atoms can enhance its sensitivity toward adsorbates. Different initial configurations of MIC and EIC on Co–TiS₂ (Co–TiS₂@MIC and Co–TiS₂@EIC) are presented in Fig. S7 and S9 in the supplementary information. The optimized adsorption configurations of MIC and EIC on Co–TiS₂ in their most stable orientations are presented in Fig. 3(d) and 4(d) respectively. E_ads corresponding to each configuration were calculated and presented in Table 2. E_ads analysis indicates that MIC and EIC preferentially anchor to Co–TiS₂ through the N site, resulting in the formation of a Co–N bond, with E_ads of −0.81 eV and −0.95 eV, respectively. M. Rouhani studied the adsorption of MIC on B(OH)-doped graphene and calculated the adsorption energy to be −18.15 kcal mol⁻¹, which corresponds to −0.79 eV.⁵⁸ To date, no prior reports are available in the literature regarding the adsorption energy of EIC for comparison. The optimal adsorption height corresponding to the maximum adsorption energy is found to be 2.06 Å for both the investigated molecules on Co–TiS₂. The calculated adsorption distances between the surface and MIC(EIC) molecules are 2.06 Å (2.07 Å), respectively. Before adsorption, the average N–C bond length in the MIC and EIC molecules is 1.31 Å and 1.32 Å, which increases to 1.35 Å and 1.36 Å upon adsorption on the Co–TiS₂ surface. The change in bond length indicates the interaction between the VOC molecules and the Co–TiS₂ monolayer.⁵⁹

To gain a deeper insight into the interaction mechanism between VOCs molecules and the adsorbent, a comprehensive analysis of the electronic properties was carried out. The spin-polarized electronic band structure and TDOS spectra of Co–TiS₂@MIC and Co–TiS₂@EIC systems are presented in Fig. 3(e and f) and Fig. 4(e and f), respectively. These spectra reveal that the Co–TiS₂ monolayer system retains its metallic phase upon adsorption. The observed asymmetries in the TDOS spectra of the Co–TiS₂@MIC and Co–TiS₂@EIC systems indicate that the systems maintain their magnetic character after adsorption; however, the magnetic moments of the adsorbed systems are reduced compared to those of the unadsorbed configuration. The spin-polarized PDOS and ODOS spectra of the Co–TiS₂@MIC system are presented in Fig. 5. From Fig. 5(a), a slight increase in electron density near the Fermi level suggests higher conductivity in the system. The variations in the intensity of the TDOS spectrum across the energy range suggest that adsorption has a significant influence on the distribution of electrons within the Co–TiS₂ system.⁶⁰ The appearance of new electronic states and the suppression of some existing ones near the Fermi level upon adsorption reveal significant orbital hybridization between the adsorbent surface and the interacting molecules.⁶¹ The ODOS spectrum of Co-atom and N-atom indicates that the d-orbital of Co-atom overlaps with the p orbital of N-atom at an energy of −0.62 eV near the Fermi level, resulting in a p-d hybridization between N and Co-atom. The TDOS spectrum presented in Fig. 6 shows that the adsorption of the EIC molecule on the Co–TiS₂ surface leads to the emergence of new electronic states in the vicinity of the Fermi level. From Fig. 6(e and f), the ODOS spectrum of Co-atom and N-atom indicates that the d-orbital of Co-atom overlaps with the p orbital of N-atom at an energy of −0.43 eV near the Fermi level, resulting in a p-d hybridization between N and Co-atom. This hybridization causes the transfer of charge between the surface and molecule, which causes the bonding between the Co and Ti atoms.

Fig. 5 Total density of states TDOS, partial density of states PDOS and orbital density of states ODOS plots for MIC with Co–TiS₂; (a) TDOS, (b) PDOS, (c) PDOS for adsorbent, (d) PDOS for molecule, (e) ODOS for Co, (f) ODOS for N.

Fig. 6 Density of states DOS for Co–TiS₂ with EIC complex; (a) TDOS, (b) PDOS, (c) PDOS for adsorbent, (d) PDOS for molecule, (e) ODOS for Co, (f) ODOS for N.

To analyze the charge transfer between the Co–TiS₂ and the adsorbed VOC molecules quantitatively, Bader charge analysis was performed. This analysis helps to identify the donor and acceptor nature of the adsorbate and adsorbent. Herein, we have calculated the partial charge (ΔQ) on each atom of the Co–TiS₂@MIC and Co–TiS₂@EIC systems, which are presented in Fig. 7(a) and (b). The horizontal dotted line acts as a reference line, indicating no charge exchange. Each atom of the system is represented with a distinct color. The positive values of ΔQ indicate the charge donor character, while its negative values show charge acceptor property. From Fig. 7 (a and b), all Ti atoms possess partial positive charges, and all S atoms carry partial negative charges. In both cases, the Co atom of the substrate has a partial positive charge, and the N atom of the adsorbate has a partial negative charge. For the Co–TiS₂@MIC system, the ΔQ on the Co atom of the surface is +0.49e, while the N and O atoms of the molecule carry charges of −1.22e and −1.04e, respectively. In the case of the Co–TiS₂@EIC system, the Co atom has a charge of +0.71e, and the N and O atoms of the molecule have charges of −1.25e and −1.02e, respectively. Thus, for both cases, charges are transferred from the surface to the molecule, indicating that the surface acts as a charge donor while the molecules serve as charge acceptors. The charge acceptance of MIC and EIC might be attributed to their electrophilic nature. The sensing mechanism relies on the process of charge transfer, where a material acts as either a donor or an acceptor. This transfer of charge modifies carrier concentration or resistivity, which is converted into an electrochemical or electrical signal. Thus, the magnitude and direction of charge transfer are critical parameters for assessing sensor efficiency.⁶²

Fig. 7 Partial charge for (a) Co–TiS₂@MIC, (b) Co–TiS₂@EIC.

Furthermore, the transfer of charge between the surface and molecules alters the work function (ϕ) of the adsorbent. To examine the variations in ϕ, we have calculated the planar-average electrostatic potential (V) of p-TiS₂, Co–TiS₂, Co–TiS₂@MIC, and Co–TiS₂@EIC systems, and presented them in Fig. 8(a and b). The flat region represents the vacuum level E_vac, and the negative minima in the potential curve originate from the ionic cores and the crystal potential. The adsorption of MIC and EIC on Co–TiS₂ monolayer systems alters the vacuum level relative to the unadsorbed system. The upward shift in the vacuum level reveals the withdrawal of electrons from the adsorbent, increasing ϕ.⁶³ Upon the adsorption of MIC(EIC) on the Co–TiS₂ monolayer system, the increase in work function from 5.68 eV to 6.3(6.13) eV was observed. The change in ϕ corresponds to a change in surface dipole moment induced by the adsorption of the VOCs molecules.⁶⁴ The variations in the ϕ are correlated with sensitivity (S) values, as expressed by equation.⁶⁵

Where ϕ_i and ϕ_f are the work functions of the Co–TiS₂ system before and after adsorption, respectively. The sensitivity values for MIC and EIC are 10.9 and 7.8%, respectively.

Fig. 8 Electrostatic potential as a function of distance of (a) Co–TiS_2, Co–TiS₂@MIC, (b) Co–TiS_2, Co–TiS₂@EIC.

To have a better understanding of the charge transfer between the surface and molecules, the charge density difference (CDD) plots of MIC and EIC molecules on the Co–TiS₂ monolayer system are presented in Fig. 9. The transfer of charge causes a redistribution of electron density, inducing depleted and accumulated charge regions that are distinguished by different colors. In Fig. 9, the yellow and cyan regions correspond to charge depletion and accumulation, respectively. The accumulation of charges occurs around the N atom of adsorbates. In contrast, depletion of charges is observed around the Co atom of Co–TiS₂ systems, indicating charge transfer from the Co atom to the N atom. This is attributed to the higher electronegativity of N compared to Co. The significant charge transfer between the N and Co atoms indicates strong adsorption of the VOC molecules on the Co–TiS₂ surface.

Fig. 9 Charge density difference (CDD) for (a) methylisocyanate (MIC), (b) ethylisocyanate (EIC) adsorbed on Co–TiS₂ monolayer. The iso-surface scale is 10⁻³ e Å⁻³. Yellow and cyan regions indicate charge accumulation and depletion, respectively.

To analyze the nature of chemical bonding between the Co–TiS₂ surface and VOC molecules, specifically whether it is covalent, metallic, or ionic, an electron localization function (ELF) analysis has been performed. Its values range from 0 to 1, where 0 represents completely delocalized and 1 denotes fully localized electrons. In ELF maps, red regions indicate strong electron localization with values close to 1, associated with covalent bonds or lone pairs, and blue regions correspond to values near 0 and represent delocalized electrons. In contrast, green regions with values around 0.5 denote intermediate localization, typically related to ionic or metallic bonding.^66,67 The ELF plots of Co–TiS₂@MIC and Co–TiS₂@EIC systems are shown in Fig. 10. The discrete red and yellow regions between the adsorbate and the surface atoms indicate electron localization and suggest that there is partial electron sharing between the Co atom and the adsorbed VOC molecules. The presence of green and cyan regions further implies moderate electron delocalization associated with charge redistribution at the interface. In contrast, the blue regions represent areas of low electron density, from where the electrons are transferred. Hence, the Bader charge analysis, CDD, and ELF analysis indicate the mixed ionic-covalent interactions.

Fig. 10 The electron localization function (ELF) for (a) methylisocyanate (MIC), (b) ethylisocyanate (EIC) adsorbed on Co–TiS₂ monolayer.

The recovery time (τ) of a sensor is a critical parameter that defines how long the sensor takes to desorb the adsorbed gas molecules, allowing it to return to its initial state and be ready for the next detection. The shorter recovery time is considered ideal for the reusability of the sensors. The equation gives the recovery time of a sensor is

where α is attempt frequency (1 × 10¹² s⁻¹), K_β is the Boltzmann constant and T is the temperature at which the sensor is set to operate, and E_ads is the adsorption energy of the system. The calculated recovery times (τ) for MIC and EIC adsorption on the Co–TiS₂ surface are 33 s and 2.6 h at 300 K, respectively, which decrease significantly to 0.46 s and 52 s at 350 K, and further reduce to 15 µs and 1 s at 400 K, indicating faster desorption at elevated temperatures. The efficient and reversible gas sensors exhibit recovery times ranging from milliseconds to a few seconds. In this context, Co–TiS₂-based nanosensors for the detection of MIC and EIC molecules demonstrate recovery times within the desirable range under high-temperature conditions.

4.1 Chemical interactions and COHP analysis

As discussed above, Bader charge analysis and CDD provide a quantitative analysis of the charge exchange between any two elements or reactants. To deeply examine how the two atoms interact, the crystal orbital Hamiltonian population (COHP) provides valuable information. Using COHP, the nature of chemical interaction and its strength can easily be understood. Thus, COHP has a great power to understand the chemistry of the materials. Here, we performed COHP calculations to understand the bonding among various elements, including Ti and S (Ti–S pair), Co and S (Co–S pair), and Co and N (Co–N) pair. Note that the Co–N pair involves the interaction between the adsorbent and the molecule. To easily understand this analysis, we first explain some fundamental concepts. In integrated COHP (ICOHP), the more negative value shows stronger interaction or bonding. In projected COHP (–pCOHP), the bonding states strengthen the interaction that appears in the positive interval (0, N), whereas antibonding states tend to weaken the bonding and appear in the negative interval (–N, 0). The ICOBI value acts as a descriptor for the nature of bonding, covalent, ionic, or mixed. An ICOBI value close to 1 indicates covalent bonding. An intermediate value has an ionic nature. In Fig. 11, the ICOHP value is shown for various orbital interactions. In the Ti–S bond, the strongest interaction is between Ti(4s) and S(2s) orbitals. The other s-d and p-d orbitals have a small or negligible contribution. It means the Ti–S bonding is caused by the overlapping of the 4s orbitals of the Ti atom and the 2s orbitals of the S atoms. For the Co–S bond, the dominant contribution is from the 4p_x orbital of the Co atom and the 2s orbital of the S atom. Likewise, in the Co–N pair, the 4p_z orbital of Co and the 2s orbital have a strong interaction. Using the concepts, one can understand the interactions for other cases. Fig. 12 provides a more detailed explanation of the results. In Ti–S bonding pairs, maximum energy states below the Fermi level are bonding; however, some antibonding states appear near the Fermi level. A negligible separation between bonding and antibonding states at the Fermi level indicates weak bonding, and it is also reflected by the small value of ICOBI. In Co–S pairs, the bonding states dominate over antibonding ones below −5 eV. The emerging antibonding states above this energy cause a weak interaction, which results in a small ICOBI value. Interestingly, the distribution of bonding/antibonding states in the Co–N pair is similar to that of Co–S. The only difference is in the ICOBI values. The smaller ICOBI value for the Co–N pair shows that Co–N bonding is relatively weaker than the Co–S bonding. To further understand the origin of Co–N interaction, the –pCOHP is plotted for various combinations of Co and N orbitals, shown in Fig. 13. Clearly, the Co(4s) and N(2s) orbitals have a dominant contribution in the bonding states below the Fermi level. The larger peak at −20 eV has a strong influence on the bonding states. In all other orbital interactions, the contribution of bonding states is small. All this discussion reveals that both molecules interact with the adsorbent Co–TiS₂ through the overlapping of Co(4s) and N(2s) bonding orbitals. The slight separation between bonding/antibonding states at the Fermi level and the smaller value of ICOBI confirm that bonding between Co and N is weak ionic bonding. Similarly, the Ti–S and Co–S bonding also has ionic character. Based on p-d hybridization and on the ICOBI values, we proposed that there is mixed covalent-ionic bonding between the Co-atom of the adsorbent and the N-atom of the VOC molecules.

Fig. 11 Integrated crystal orbital Hamiltonian population (ICOHP) analysis; (a) Ti–S bonding pair in Co–TiS₂ with A molecule, (b) Co–S bonding pair in Co–TiS₂ with A molecule, (c) Co–N pair in Co–TiS₂ with A molecule, (d) Ti–S bonding pair in Co–TiS₂ with B molecule, (e) Co–S bonding pair in Co–TiS₂ with B molecule, (f) Co–N pair in Co–TiS₂ with B molecule. A stands for ethylisocyanate, and B is for methylisocyanate. ICOHP values in the majority (minority) spin are shown in blue (red).

Fig. 12 Projected crystal orbital Hamiltonian population (-pCOHP) analysis; (a) Ti–S bonding pair in Co–TiS₂ with A molecule, (b) Ti–S bonding pair in Co–TiS₂ with B molecule, (c) Co–S bonding pair in Co–TiS₂ with A molecule, (d) Co–S bonding pair in Co–TiS₂ with B molecule, (e) Co–N pair in Co–TiS₂ with A molecule, (f) Co–N pair in Co–TiS₂ with B molecule. Molecules A and B are defined in Fig. 10.

Fig. 13 Projected COHP (–pCOHP) for (a) Co(3d) and N(2s) orbitals, (b) Co(4s) and N(2s), (c) Co(3d) and N(2p) orbitals, (d) Co(4s) and N(2p) orbitals.

5 Conclusion

DFT calculations were performed to explore the geometrical, electronic, and magnetic properties of pure and Co-decorated TiS₂ monolayer systems. The nature and strength of chemical bonding was examined using COHP analysis. The adsorption of two selected volatile organic compounds (VOCs), methylisocyanate (MIC) and ethylisocyanate (EIC), was also investigated on these monolayer systems. Our results showed that pristine TiS₂ systems exhibited poor adsorption towards the targeted VOC molecules. The anchoring of Co-atom on the TiS₂ surface induces a phase transformation from the semiconducting to metallic, remarkably improving its interaction with VOCs molecules. The obtained adsorption energies for MIC and EIC on the Co–TiS₂ monolayer systems are −0.81 eV and −0.95 eV, respectively. The results indicated that both MIC and EIC acted as charge acceptors, whereas Co–TiS₂ acted as an electron donor. Furthermore, the density of states, Bader charge, electrostatic potential, work function, and electron localization function analysis were performed to authenticate the adsorption of targeted VOCs on the Co–TiS₂ surface. COHP results showed that the Co atom forms ionic weak ionic bonding with the N atom of the molecule by Co(4s) and N(2s) orbitals. Although the sensitivity and selectivity of Co–TiS₂-based nanosensors are good towards MIC and EIC molecules, they demonstrate poor recovery times at room temperature, which can hinder their reusability at room temperature, the desired recovery times can be achieved at elevated temperatures. Based on our results, we propose that the Co–TiS₂ monolayer could be used as a sensor for the detection of MIC and EIC molecules efficiently.

Conflicts of interest

The authors have no conflicts of interest to declare.

Data availability

Data sets generated during the current study are available from the corresponding author (Dr M. Mushtaq) upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5ra09215a.

Acknowledgements

Dr M. Mushtaq acknowledges the support of the University of Poonch Rawalakot for the VASP code license.

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