First-Principles Calculations to Investigate the Oxidation Mechanism of Pristine MoS₂ and Ti-Doped MoS₂

Abstract

Generally, MoS₂ is easily oxidized when exposed to oxygen, and the antioxidation mechanism of MoS₂ is still a challenge. Thus, more efforts were made to greatly improve its antioxidation performance. It was reported that the Ti atom doped with MoS₂ was treated as the effective method to enhance its antioxidation performance; however, the detailed antioxidation mechanism was not well understood. Superior to experimental methods, the first-principles method could provide deep insight into the atomic information and serve as a useful tool to gain an understanding of the antioxidation mechanisms of the doped MoS₂; thus, the antioxidation behavior of the Ti-doped MoS₂ was investigated in detail using first-principles calculations. However, an opposing conclusion was obtained from the calculated results compared to the previous experimental results; that is, the incorporation of the Ti atom was not helpful for improving the antioxidation performance of MoS_2. The strange phenomenon was well probed and discussed in detail, and understanding the oxidation mechanism of the Ti-doped MoS₂ would be helpful for expanding its applications in the ambient atmosphere.

1. Introduction

A high-quality MoS₂ sheet was successfully deposited using the chemical vapor deposition system [1], which has been applied in many fields due to its excellent optical [2,3], magnetic [4], electrocatalytic and photocatalytic [5,6], and mechanical [7] properties. In particular, the ultrathin thickness of MoS₂ endowed the excellent tribological performance [8,9] due to its easily shearing. However, its tribological behaviors were closely sensitive to the environment conditions [10] because the formation of MoO₃ would result in an increasing friction coefficient when exposed to oxygen [11]. Ren et al. recently reported that MoS₂-based superlattice films exhibited superlubricity (0.006) in a low vacuum while showing a high friction coefficient (>0.04) in the air [12]. The poor antioxidation of MoS₂ limited its further applications in the air; thus, improving its antioxidation performance was imperative.

The oxidation mechanism of MoS₂ should initially be well understood [13]. It is a well-established oxidation mechanism that, owing to no dangling bonds on the MoS₂ plane that are terminated by S atoms, when it is exposed to air, it leads to the formation of defects that enhance its surface activity, further resulting in the strong interactions between defected MoS₂ and oxygen [14,15,16]. Indeed, Pu et al., using the first-principles method, confirmed that the defects in the MoS₂ was a virulent attack on its related properties and led to failure [17]. It is worth noting that the metal-doped MoS₂ was found as an efficient way to enhance antioxidation [18,19,20,21,22]. Ti-doped MoS₂ has been confirmed to slow down its failure in ambient atmosphere [23,24,25] while the real interactions mechanism between the oxygen and the Ti-doped MoS₂ were rarely reported, theoretically. Thus, one wonders if the incorporation of the Ti element really slowed down the failure of MoS₂ in the ambient atmosphere.

Thereafter, in this work, interactions between pristine MoS₂, Ti-doped MoS₂, and O atoms were systematically investigated using first-principles calculations in order to obtain the proposed antioxidation mechanism. The complex microscopic antioxidation mechanism was revealed by the related calculations. However, we obtained opposing results compared to the previous reports [23,24,25]; that is, the incorporation of the Ti element was not conducive to improving the antioxidation performance of MoS₂.

2. Computational Methods

As known, the first-principles method is treated as an effective tool to investigate the various properties of solid materials at an atomic level [26,27,28,29]. All calculations were performed using the CASTEP code [30]. The calculated model of pristine MoS₂ (1T) is shown in Figure 1. The electronic configurations of Mo, S, and Ti was 4d⁵ 5s¹, 3s² 3p⁴, and 3s² 3p⁶ 3d² 4s², respectively. The cutoff energy of 400 eV was selected for the plane-wave expansion, and the k-point of the Brillouin zone was sampled with a 5 × 5 × 10 Monkhorst-Pack grid. The convergence of energy, force, stress, and displacement was set as 2.0 × 10⁻⁶ eV/atom, 5.0 × 10⁻² eV/Å, 0.1 GPa, and 2.0 × 10⁻³ Å. To minimize interlayer interaction, MoS₂ sheets in the neighboring cells were separated by a distance of at least 20 Å. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) [31] was selected for all calculations. The ultrasoft pseudopotential was selected to manage the interaction between the ionic and valence electrons [32]. The phonon dispersion and related density of state of Ti-doped MoS₂ were calculated to determine the dynamical stability of the doped models [33,34]. The finite displacement method was used to calculate phonons.

3. Results and Discussion

3.1. Phonon Dispersion

To investigate the stability of Ti-doped MoS₂ (the S or Mo atom is replaced with the Ti atom), called MS_Ti-S and MS_Ti-Mo, the phonon dispersion curves are calculated as shown in Figure 2. Figure 2a shows the phonon dispersion of MoS₂, which is in accord with the calculated results reported by Tornatzky et al. [35]. Generally, the occurrence of imaginary frequency corresponds to the dynamical instability [36]. Figure 2b displays the phonon dispersion of MS_Ti-S. Several imaginary frequencies are observed in MS_Ti-S, revealing the dynamical instability. Figure 2c shows the phonon dispersion of MS_Ti-Mo. No imaginary frequency indicates that MS_Ti-Mo is the dynamical stability, which is consistent with the previous work [37]. Thus, subsequently, the adsorption behaviors of O atoms are just investigated for the dynamical stable MoS₂ and MS_Ti-Mo.

3.2. Adsorption Energy

To better understand the antioxidation mechanism of Ti-doped MoS₂, we explore the stable adsorption configuration of the O atom on a MoS₂(002) slab. The stability of the O atom is determined by the adsorption energy ($E_{ad}^O$) [38], calculated by:

$$E_{ad}^O=E_{\mathrm{tot}({\mathrm{MoS}}_2(002)/O)}-E_{{\mathrm{MoS}}_2(002)}-{\mu }_{(O)}$$

(1)

where $E_{\mathrm{tot}({\mathrm{MoS}}_2(002)/O)}$, $E_{{\mathrm{MoS}}_2(002)}$, and ${\mu }_{(O)}$ are the total energy of the oxygen adsorption system, the clean relaxed MoS₂(002) surface, and the chemical potential of the O atom. Figure 3 shows the oxygen-adsorbed configuration on the MoS₂(002) surface. Four possible adsorbed sites are considered to search the most stable adsorbed sites: the Mo top site, S top site, dopant top site, and hollow site (H site). To the best of our knowledge, the stability of the adsorbed site can be reflexed from the adsorption energy [39]. The lower adsorption energy corresponds to the good stability of the adsorbed materials.

The calculated $E_{ad}^O$ values are listed in

Table 1. The calculated $E_{ad}^O$ values for O adsorbed on the MoS₂(002) and MS_Ti-Mo (002) surface.

Configurations	Adsorbed Site	${\mathit{E}}_{\mathit{a}\mathit{d}}^{\mathit{O}}$ (eV)	dS−O (Å)	dMo−O (Å)	dTi−O (Å)	dTi−Mo (Å)
MoS2	H	−5.53	1.484	-	-	-
	Mo top	−3.96	1.568	2.204	-	-
	S top	−5.52	1.483	-	-	-
Ti−MoS2	H	−1.33	1.639	2.336	2.010	-
	Mo top	−0.70	1.578	-	-	-
	S top	−1.90	1.487	-	-	-
	Ti top	−1.04	1.601	-	2.062	-

. The calculated ones of those adsorbed sites for the MoS₂(002) surface are less than zero, which indicates that O atoms easily adsorb onto the MoS₂(002) surface. The calculated $E_{ad}^O$ for the MS_Ti-Mo(002) surface is −1.33, −0.70, −1.90, and −1.04 eV for the H, Mo top, S top, and Ti top sites, respectively. Generally, a more negative $E_{ad}^O$ value reveals a stronger interaction between the adsorbent and the adsorbate [40,41]. The adsorption energies of O adsorbed on the MoS₂(002) surface are more negative than that of the MS_Ti-Mo(002) surface, indicating a weaker interaction between the O atom and atoms in the MS_Ti-Mo(002) surface. As shown in Table 1, the distance between the Mo and O atom is 2.204 and 2.336 Å for the MoS₂: Mo top site and MS_Ti-Mo: H site, respectively, which is larger than that of the calculated equilibrium bond length of the Mo−O bond (2.08 Å) [42], indicating that the adsorption of the O atom onto the Mo top site and H site (MS_Ti-Mo) results in a breaking of the Mo−S bond, leading to the damage of the MoS₂ nanosheet. In Figure 4, for the H site of the MoS₂(002) surface, the O atom moves from the hollow position to the S top site, forming the covalent bond; the nanosheet structure is maintained perfectly after the adsorption of the O atom. The distance between the Ti and O atoms is 2.010 and 2.062 Å for the MS_Ti-Mo: H site and MS_Ti-Mo: Ti top site, respectively, which is more than that of the reported equilibrium bond length of the Ti−O bond (1.933 Å) [43], indicating that no chemical bonds are formed between the Ti and O atoms. In Figure 5, for the H site of the MS_Ti-Mo(002) surface, after the O adsorption, the nanosheet structure is damaged after an attack of the O atom; the same phenomenon is found as the O atom adsorbed above the Ti top site. The nanosheet structure remains in good condition as the O atom adsorbed above the Mo and S top site. It can be concluded that the incorporation of Ti into MoS₂ only improves the oxidation resistance as O adsorbed on the Mo site.

3.3. Charge Density Difference

To further investigate the nature of the charge transfer and chemical bonds of the O-adsorbed MoS₂(002) and MS_Ti-Mo(002) surface, the charge density difference is analyzed and discussed as displayed in Figure 6. Because the electronegativity of the O atom is stronger than that of S atoms [44], the O atom can gain more valence charge from S atoms as shown in Figure 6a–c. Three different S−O bonds are related to the electronic interactions between the S and O atoms as shown in Table 1. Furthermore, in Figure 6d–g, due to the larger electronegativity of S compared to Mo [45] and Ti [46], the O atom also gains more valence charge from the S, Mo, and Ti atoms, as supported by Figure 6d–g. Four different S−O bonds also assign to the electronic interactions among the S and O atoms, listed in Table 1. Moreover, in Figure 6d,g, there is no localized hybridization between the Ti and O atoms, which further reveals no chemical bonds between the Ti and O atoms.

3.4. Density of States (DOS)

To further probe the nature of the oxidation behavior of MoS₂, the total and partial density of states of the O-adsorbed MoS₂ and MS_Ti-Mo are calculated as shown in Figure 7 and Figure 8. The vertical dotted line represents the Fermi energy level (0 eV). For the O-adsorbed MoS₂(002) surface, in Figure 7a (H site), valence bands consist of two parts; the first part (−13.42 to −11.24 eV) is composed of O-2s, S-3s, S-3p, Mo-5s, and Mo4d states, and the second part (−6.23 to 1.97 eV) mainly consists of O-2p, S-3p, and Mo-4d states. The hybridization of the O-2p and S-3p states in the valence bands is obviously observed, indicating the formation of covalence bonds between the O and S atoms. Figure 7b,c shows that the valence bands mainly consist of O-2p, S-3s, S-3p, and Mo-4d states. The hybridization of O-2p and S-3p are also detected as shown in Figure 7b,c. The covalence bonds between O and S are also formed for the Mo top and S top sites. The results are in agreement with that of the charge density difference.

Figure 8a–d shows the total density and partial density of the state of the O-adsorbed MS_Ti-Mo(002) surface. The valence bands for these four adsorption sites are mainly composed of O-2p, S-3s, S-3p, Mo-5s, and Mo-4d states. The obvious hybridization between the O-2p and S-3p states is found, which reveals the formation of the covalence bonds between the O and S atoms. However, the hybridization between the O-2p and Ti-3p is not observed, indicating that no chemical bonds are formed between the O and the Ti atoms.

According to what was previously mentioned, the Ti element in the MoS₂ does not directly bond with the O atom. The covalence bonds are formed between the S and the O atoms. The O atom inclines to attack and damage Mo−S bonds in pristine MoS₂; although the oxygen resistance of the Mo top site is improved, the O atom favors to attack and damage Ti−S bonds in the H and Ti top sites with the incorporation of Ti into MoS₂. Thus, the antioxidation of MoS₂ does not improve under the incorporation of the Ti atom. Because the sequence of electronegativity is Ti (1.54) < Mo (2.16) < S (2.58) < O (3.44), when O is adsorbed onto the MoS₂ surface, O atoms tend to move toward the S atom as supported by the charge density difference analysis, which results in the charge transferring between the O atom and the MoS₂ surface. Furthermore, the covalence bonds are easily formed between the O and S atoms leading to the broken Mo−S bonds.

4. Conclusions

In summary, the influence of the Ti atom on the antioxidation of MoS₂ were investigated using the first-principles method. The main conclusions obtained are as follows:

(1)

The oxidation mechanism of the pristine MoS₂ was well probed. Two Ti-doped models, MS_Ti-S and MS_Ti-Mo, were designed based on the structural feature of MoS₂. Due to the dynamical instability of MS_Ti-S, the antioxidation behavior of MS_Ti-Mo was investigated in detail.

(2)

Although the antioxidation property of the Mo site improved, the H site and Ti top site were easily attacked by the O atom, leading to broken Ti–S bonds. The incorporation of the Ti element into the MoS₂ was not helpful for improving its antioxidation performance.