Density functional theory study of the adsorption of elemental mercury on a 1T-MoS2 monolayer

Elemental mercury has become a global concern because of its significant impact on human health and the ecosystem. A lot of effort has been put towards the removal of elemental mercury from the 2H-MoS2 (prismatic structure of MoS2). However, the mechanism of 1T-MoS2 (polytype structure of MoS2) in Hg0 capture remains unexplored. In this study, density functional theory (DFT) was adopted to investigate the adsorption mechanism of Hg on a 1T-MoS2 monolayer. The different possible adsorption positions on the 1T-MoS2 were examined. For different adsorption configurations, the changes in electronic property were also studied to understand the adsorption process. The results elucidated that chemisorption dominates the adsorption between Hg0 atoms and the 1T-MoS2. It was found that the TMo (on top of the Mo atom) position is the strongest adsorption configuration among all the possible adsorption positions. The adsorption of Hg0 atoms on the 1T-MoS2 monolayer is influenced by adjacent S and Mo atoms. The adsorbate Hg0 atom is found being oxidized on the TMo position of the 1T-MoS2 with an adsorption energy of −1.091 eV. From the partial density of states (PDOS) analysis of the atoms, the strong interaction between Hg0 and the 1T-MoS2 surface is caused by the significant overlap among the d orbitals of the mercury atom and the s orbital of the S atom and p and d orbitals of the Mo atom.


Introduction
Elemental mercury (Hg 0 ) is the most toxic form of mercury in flue gas released from industrial activity (Galbreath and Zygarlicke, 1996;Presto and Granite, 2006). Because it is insoluble in water, highly volatile, and chemically inert, it can be trans-ported in air over long distances and periods of time (Gao et al., 2013;Lim and Wilcox, 2013). Once it returns to the biosphere, mercury can bio-accumulate in the ecosystem and become a major threat to human health (UNEP, 2013a;2013b;Liang et al., 2015). As a consequence, since 1990 a series of policies and legislation on mercury control has been published to prevent the further invasion of mercury, such as Clean Air Act Amendments (1990) (Johansen 2003), Mercury and Air Toxics Standards (MATS) (2011) (EPA, 2011), the Minamata Convention (2013) (UNEP, 2013c). Driven by a series of policies, mercury emissions in Canada, for example, were reduced from about 33 t to 6 t per annum between 1990 and 2000 (UNEP, 2002). Nowadays, more stringent emission legislation has been introduced by many regions 2 worldwide (Jones, 1999;Praveen, 2003). There is therefore an urgent need for the development of new materials for the control of airborne mercury.
Our previous research showed that MoS2 is an elemental Hg trapping agent with a trigonal prismatic (2H) structure (Zhao et al., 2016), wherein the layer of Mo atoms is sandwiched between two layers of S atoms, such that each Mo is coordinated to six S atoms in a trigonal prismatic geometry (Yin et al., 2011;Chhowalla et al., 2013). Experimental and theoretical studies have demonstrated that MoS2, a type of graphene-like 2D material, is an excellent Hg adsorbent (Zhao et al., 2016). In our previous research, the 2H-MoS2, the most stable phase of a MoS2 monolayer, has been examined extensively for the adsorption of mercury on the defect-free to defective surface. However, not much work has been carried out on the adsorption of mercury on the other geometric structure of MoS2, the polytype (1T) with a tetragonal symmetry, which has one MoS2 layer per repeat unit in the octahedral phase.
The 1T-MoS2 monolayer is a metastable phase of MoS2, and this has attracted attention because of its unique properties since it was fabricated for the first time (Wypych and Schollhorn, 1992) and a series of techniques has been developed to characterize the structure of 1T-MoS2 (Wypych et al., 1998). Compared with 2H-MoS2, the metastable phase has a metallic character, and as such, may provide significantly improved catalytic activity compared with its 2H counterpart. Furthermore, one pronounced property of mercury is its affinity to other metals, such as gold, silver, and copper (Aboud et al., 2008). It was reported that Hg 0 atoms can easily bind to sulfurized adsorbents (Asasian and Kaghazchi, 2015). It was also found that the 1T phase could replace noble metals, such as Pt in hydrodesulfurization reactions and hydrogen evolution reaction (HER) processes (Putungan and Kuo, 2014). Therefore, to better understand the potential of MoS2 in mercury capture, in addition to the understanding of mercury adsorption on 2H-MoS2, it is essential to examine the adsorption of mercury on the 1T-MoS2.
Because of the metallic structure of the 1T-MoS2, the 1T phase can transform to the 2H phase at a temperature in the range of around 100-300 °C (Goki et al., 2012). Although there is still no clear consensus on the optimum structural phase transition temperature, there is still a lot of work being carried out on the stabilization of the metallic structure (Gao et al., 2015). However, very little has been done on the adsorption properties of the metallic 1T phase of MoS2, especially the 1T phase of MoS2 material for the adsorption of elemental Hg. In this study, the possible configurations for the adsorption of Hg atoms on the 1T-MoS2 were examined by density-functional theory (DFT) modelling. The partial density of states (PDOS) and charge transfer for the most stable configuration were studied in detail to show the mechanism of mercury adsorption on the 1T-MoS2 surface.
In the calculation, although ab initio methods such as DFT have been proposed to point out weak interactions, the main difficulty is that weak interactions are long-range. In addition, commonly used approximations such as the local-density approximation (LDA) are suitable for short range and are not appropriate for these long-range interactions. In this simulation work, the overlap of electronic densities around each atom is the basis of all calculation and when this overlap is estimated to be too small, it is not possible to get an accurate result for the system. Therefore, to overcome this problem, a semi-empirical r −6 term such as density functional theory-dispersion correction (DFT-D) is adopted (Sato and Nakai, 2009;Hasnip et al., 2014). Because of the great precision and efficiency of simulations of localized d and 2p orbital with plane-waves basis sets, ultrasoft pseudo-potentials (USPPs) are employed in first-row and transition-metal systems (Vanderbilt, 1990).

Methodology
Simulation in this study was conducted using the exchange-correlation functional GGA-PW91-OBS. The DFT-D takes into account the charge of the dispersion forces (Ortmann et al., 2006), as conducted by the CASTEP package (Clark et al., 2005). USPPs were applied to all ion-electron interactions in the system (Vanderbilt, 1990). The valence configurations of Mo, S, and Hg atoms were 4p 6 4d 5 5s 1 , 3s 2 3p 4 , 5d 10 6s 2 , respectively. To ensure convergence, convergence with respect to both energy cutoff and k-point mesh was set as fine quality (an energy cutoff of 310 eV and 2×2×1 k-points mesh). The total energy of the 1T-MoS2 (4×4) was converged to within 1×10 −5 eV/atom using the method of Monkhorst-Pack (Chadi, 1977). In addition, the self-consistent field (SCF) calculation was kept within the energy convergence criterion of 1×10 −6 eV/atom and a smearing width of 0.1 eV was used for the treatment of the metallic system. For supercell calculations, the same modeling parameters were applied.
Since all atomic layers are relaxed in the 1T-MoS2 (4×4) (Liu et al., 2010), to search for the most-stable configuration of the 1T-MoS2 monolayer and the Hg atoms, the total energy minimization method introduced by Broyden Fletcher Goldfarb Shanno (BFGS) was adopted in this study (Pfrommer et al., 1997). The total-energy difference was set to be within 10 −5 eV/atom, and the maximum force was set to be within 0.03 eV/Å, while the maximum stress was set to be within 0.05 GPa and the maximum atom displacement was set to be within 0.001 Å. For the interactions among periodical slabs, a vacuum region of 25 Å was also applied in the direction perpendicular to the 1T-MoS2 plane. The isolated Hg atom was calculated in a (10 Å) 3 supercell.
The adsorption energy equation ( Hg+1T-MoS E is the total energy of the Hg 0 -1T-MoS2 slab of the stable adsorption configuration. By definition, a more negative adsorption energy suggests a more favorable exothermic Hg adsorption on the 1T-MoS2 surface.

Geometric structure
The positions of all atoms were fully relaxed in the 1T-MoS2 monolayer to ensure the accuracy of the results. As shown in Fig. 1, the pristine structure of the 1T-MoS2 monolayer is relaxed with the optimized unit cell (lattice constant a=b=3.166 Å), which is close to the result (a=3.169 Å) reported by other re-searchers (Enyashin and Seifert, 2012). There are four possible adsorption positions of an Hg 0 atom on the 1T-MoS2 monolayer: (1) on top of a Mo atom (TMo); (2) on top of the space between neighboring S and Mo atoms (BMo-S); (3) on top of an S atom of the top sulphur plane (TTS); and (4) on top of an S atom on the bottom sulphur plane (TBS), and these are shown in Fig. 2a-d respectively after the optimization of the adsorption configuration. It is shown that the Hg 0 atom is favorably adsorbed at the TMo adsorption position with the highest adsorption energy of −1.091 eV, which is the site that was also found as the best site for the adsorption of other transition metals on 2H-MoS2, such as Fe, Co, and Ni. (Huang et al., 2013). It is also observed in this study that the Hg atom is not adsorbed at the position right on top of a Mo atom but with a deviation from Mo atom along the Mo-S bond, as illustrated in Fig. 2a. This is common in the adsorption position of BMo-S and TBS. Under the influence of the Hg atoms adsorbed, the structures of the four adsorption positions on the 1T-MoS2 monolayer are deformed. It suggests that the geometric structure of pristine 1T-MoS2 is unstable and is easily affected by external interference.

Adsorption energy
The results of DFT-D and DFT methods are listed in Table 1. Because the DFT-D mainly focuses on the dispersion force between adsorbent and the adsorbate system, the adsorption behavior of a single Hg atom can be calculated more accurately. Therefore, the following discussion is mainly focused around DFT-D methods. Based on Eq. (1), the stability of the adsorption configurations is found to be of the order of TMo>BMo-S>TTS>TBS, as shown in Table  1. All the four stable adsorption positions show Fig. 1 Optimized structure of clean 1T-MoS2 monolayer (yellow atom is S atom, and aquamarine atom is Mo atom) Note: for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article 4 sufficient adsorption energy to allow the Hg atoms to be chemically adsorbed on the 1T-MoS2 monolayer (Atkins, 2001). Comparing Figs. 2a, 2b and 2c, it can be seen that for the stable adsorption positions with Hg atoms adsorbed, the Hg atom is initially adsorbed at the positions of TMo, BMo-S, and TBS, but eventually deviated along the bond of Mo and S atoms. However, the adsorption position of TMo is very close to the Mo atom with the shortest distance between the Hg and the nearest adjacent (NA) Mo atom (4.822 Å). Furthermore, the distance between the Hg atom and the NA Mo/S atom is the shortest among the four potential adsorption sites. That means the Mo atoms of the 1T-MoS2 monolayer may affect the behavior of Hg adsorption. The adsorption site of TTS shows a shorter distance between the Hg atom and the NA S atom than that of the TBS, which means that the ad-sorption of Hg atom on the 1T-MoS2 monolayer is also possibly contributed to by the NA S atoms in the top sulphur plane.

Electronic structure
The adsorption mechanism is mainly determined by the change of electronic structure in the Hg-1T-MoS2 slab. The charge transfer of all atoms and the relevant PDOS analysis were conducted in this study. Hirshfeld charge analysis is based on the variation of density of the free atomic electron density (Delley, 1986). The atomic charges for the pristine 1T-MoS2 monolayer, the free Hg atom, and the Hg-1T-MoS2 slab were calculated by the Hirshfeld method. The charge transfer is in accordance with the difference of the electron state in each individual atom before and after adsorption (Table 2), the atomic

Position
Ead ( indices of which are labeled in Fig. 3. A positive value of charge transfer means the atom gains electrons after adsorption, while a negative value of that means the atom donates electrons after adsorption. As shown in Table 2, the order of the Hg charge transfer is recognized as TMo>BMo-S>TBS>TTs, which is in a similar order to that of the stability of the four adsorption configurations, except for the TBS and TTS positions. The Hg atom adsorbed on the TMo adsorption site has the largest charge transfer of −0.04 eV after adsorption, while the Hg atoms on the other three potential adsorption positions donate different electrons to the 1T-MoS2 monolayer. It indicates that the Hg is oxidized after it is adsorbed on the 1T-MoS2 monolayer. For the four stable adsorption configurations, the Mo atoms have negative charge transfer (−0.01 eV) in those different potential adsorption positions. It is worth noting that the TMo adsorption site has the most Mo atoms, i.e. Mo (8), Mo (9), Mo (11), Mo (12), Mo (16), that participate in the adsorption process. As for the TMo adsorption position, the S atoms in the top S plane, i.e. S(21), S(29) and S(31), show positive charge transfer (0.01 eV) in the Hg-1T-MoS2 monolayer system. In addition, the S atoms in the bottom S plane, i.e. S(2), S(4) and S(20), also show negative charge transfer (−0.01 eV) in the Hg-1T-MoS2 monolayer system. Consequently, the oxidized Hg atoms with positive charge are bonded with S atoms in the top S plane that accept electrons from neighboring Mo atoms, the Hg atoms, and the S atoms in the bottom S plane. Hence, the adsorption of Hg at the TMo position is very stable.
To further understand the electronic interactions of the adsorbate Hg atom with the 1T-MoS2 monolayer at the TMo adsorption site, the density of state (DOS) of the system was investigated by analyzing   Fig. 4. The PDOS peaks of d, s, and p orbitals of an isolated Hg 0 atom are approximately −3.1, 0 and 5.7 eV before adsorption, respectively. However, after adsorption, all the PDOS peaks of the Hg atom shift left while the state of the s and p orbitals significantly decrease in energy level. All these changes confirm that strong interactions occur between Hg 0 and the 1T-MoS2 monolayer. The orbitals of the NA Mo atom and the NA S atom were analyzed to show possible co-interaction with the Hg atom. As illustrated in Figs. 4a and 4c, the s, p and d orbitals of the NA Mo atom and the s and p orbitals of the S atom change slightly after adsorption. In particular, the p orbitals of the NA Mo atom and of the NA S atom become higher in PDOS value and overlap with the s and d orbitals of the Hg atom. It indicates the bonding of s orbital of the Hg atom and p orbitals of the S atom. The d orbitals of the Hg atom interact with the p and d orbitals of the Mo atom and the s orbital of the S atom strongly. It therefore suggests that there are strong interactions between the 1T-MoS2 monolayer and the adsorbate Hg atoms. It can also be concluded that the 1T-MoS2 monolayer can oxidize the adsorbate Hg atoms. Both the chemisorption and the oxidation ability of the 1T-MoS2 monolayer contribute to its good performance in Hg adsorption.

Conclusions
The DFT modelling of the adsorption of Hg 0 atom on the 1T-MoS2 monolayer showed that the Hg atoms are favorably adsorbed on the TMo of the 1T-MoS2 slab with an adsorption energy of 1.091eV. The most stable adsorption configuration in the Hg-1T-MoS2 slab is chemisorption. The PDOS and charge transfer analyses also show that the Hg atoms adsorbed on the TMo position are oxidized and interact with the 1T-MoS2 slab strongly with a number of Mo atoms and S atoms from both the top and bottom S planes participating in the adsorption. It is evident in this study that the 1T-MoS2 is an excellent adsorbent for Hg 0 capture.