- Email: [email protected]
Competition of Hg2+ adsorption and surface oxidation on MoS2 surface as affected by sulfur vacancy defects
Applied Surface Science 483 (2019) 521–528
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Competition of Hg2+ adsorption and surface oxidation on MoS2 surface as affected by sulfur vacancy defects ⁎
Hao Yia,b,1, Xian Zhangb,1, Feifei Jiaa,b, , Zhenlun Weib, Yunliang Zhaoa,b, Shaoxian Songa,b, a b
T
⁎
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenum disulfide Hg2+ adsorption Surface oxidation DFT calculations AFM imaging
Two dimensional molybdenum disulfide (2D-MoS2) has been recently developed to be used as superb adsorbents for removing heavy metals from water due to its huge sulfur-rich surface area. In this work, single adsorption and co-adsorption performances of H2O, Hg2+ and O2 on MoS2 surface with and without S-vacancy defects have been theoretically studied to explore the Hg2+ adsorption and surface oxidation through density function theory (DFT) calculations. Moreover, the Hg2+ adsorption and surface oxidation have experimentally studied through atomic force microscopy (AFM) to verify the theoretical calculation results. It has been found that S-vacancy defects make MoS2 surface more reactive, leading to the much stronger adsorption energy of H2O, Hg2+ and O2 on defective MoS2 surface. O2 can only be physically adsorbed on the perfect MoS2 surface, while it bonds to the unsaturated Mo atoms at the vacancy site on defective MoS2 surface. Besides, co-adsorption results illustrate that Hg2+ have priority to react with MoS2 surface than O2 due to the much stronger binding affinity, and surface oxidation occurs only when there are enough reaction sites for the Hg2+ adsorption and surface oxidation simultaneously. These theoretical co-adsorption results are consistent with the experimental Hg2+ adsorption and surface oxidation results obtained by AFM. These findings in this study are of great significance for the development and utilization of MoS2-based nanomaterials as a heavy metal adsorbent.
1. Introduction Hg2+ is the primary form of mercury (Hg) in water normally found in the global environment [1]. As a highly toxic heavy metal, the existence of Hg2+ in the environment often causes long-term contamination problems, which has a serious impact on the public health and environmental protection. Excessive accumulation of Hg2+ in the human body leads to many neurological disorders such as memory loss, insomnia, behavioral issues, and cerebral palsy [2]. Therefore, the latent huge danger of Hg2+ discharged into environment makes it particularly urgent to treat with Hg2+ wastewater. Adsorption is regarded as the most promising approach for Hg2+ wastewater treatment compared with other techniques, such as ion exchange, coagulation and chemical precipitation, in terms of its low cost, simple operation, and regeneration capability. Thanks to the strong soft-soft interactions between Hg2+ and sulfur [3], some sulfurcontaining materials have recently been reported to demonstrate substantial improvements in Hg2+ removal performance [4–7].
Molybdenum disulfide (MoS2), as a typical layered transition-metal dichalcogenide (TMD), is composed of a hexagonal plane of Mo atoms sandwiched by two hexagonal planes of S atoms [8–10]. Due to the abundance of intrinsic sulfur atoms (potential binding sites) on the formed S-Mo-S layered MoS2 surface, it could be applied as natural, efficient and perfect adsorbents to remove Hg2+ from polluted water. Recently, study on the adsorption of Hg2+ on natural MoS2 has been performed by atomic force microscope (AFM), and the AFM imaging clearly demonstrated the strongly adsorption of Hg2+ on MoS2 (001) surface [11]. In order to improve specific surface area and adsorption capacity, a novel porous MoS2-based aerogels has been successfully designed to remove Hg2+ in aqueous solution and exhibited a very high adsorption capacity [12]. Considering that the gallery height of 0.298 nm between two neighboring layers is too narrow to allow hydrated Hg2+ ions into the interior spaces where the vast majority of potential binding sites are located. MoS2 with widened interlayer spacing have been prepared and exhibited a tremendous capturing capacity, fast adsorption kinetics, extraordinary affinity and excellent
⁎
Corresponding authors at: Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected] (F. Jia), [email protected] (S. Song). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2019.03.350 Received 25 September 2018; Received in revised form 26 March 2019; Accepted 31 March 2019 Available online 01 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
selectivity for Hg2+ in water [3]. Because of the weak out-of-plane van der Waals interactions between individual S-Mo-S layers, exfoliating MoS2 into two-dimensional (2D) materials may be another effective method to improve the ability of Hg2+ removal for bring more surfaces with sulfur exposed outside. And a previous study proved that 2D-MoS2 could be used as a superb adsorbent for removing Hg2+ from water [13]. Above all, MoS2 has been turned out to be a highly efficient adsorbent for Hg2+ removal. However, some problems are remained to be studied. The structural defects on MoS2 layer have been observed and extensively investigated recently [14–18]. Even though an ideal surface of MoS2 would be free from dangling bonds or other types of defects, several imperfections can appear during the exfoliation or growth processes [19]. MoS2 layers have intrinsic defects like vacancies generated during mechanical or chemical exfoliation, and a single vacancy with one S missing defect (S-MoS2) is most frequently observed for the lowest defect formation energy [20,21]. It is believed that defect-rich features on MoS2 surface will have a tremendous influence on the adsorption performance [3,16,22,23]. However, the details that how Svacancy defect affects the Hg2+ adsorption behaviors need to be further studied. Besides, due to the existence of defect, the MoS2 surface is easy to be oxidized in humid or aqueous environment. Study has shown that a perfect single-layer sheet of MoS2 stays intact when exposed in O2 due to the weak physical adsorption, while monolayers of MoS2 with Svacancy defects adsorb O2 through chemical adsorption [24]. Besides, the oxidation process of MoS2 sheet in water was in situ observed by AFM, it was found that the oxidation led to the partial etching of the surface layer and needlelike protrusions on the MoS2 surfaces [25]. Consequently, the unavoidable occurrence of S-vacancy defects on monolayer MoS2 has a huge influence both on the Hg2+ adsorption and surface oxidation. However, it is not clear how the S-vacancy defects influence both the Hg2+ adsorption and surface oxidation, and the competition of Hg2+ adsorption and surface oxidation on MoS2 surface have not been investigated yet. Therefore, an attempt was made to explore the competition of the coexisting surface oxidation and Hg2+ adsorption behaviors under the influence of S-missing defects on MoS2 surface. AFM has been recently established as a powerful technique for direct nanoscale observation of the surface morphology of adsorbent and the adsorption behaviors of ions or molecules on an adsorbent, and in situ observation of the surface or interface reaction process can be also achieved [25–27]. Another powerful tool for studying microscopic phenomena and mechanisms is molecular simulations [28–30]. Density function theory (DFT) performs very well in the simulation of configuration and electronic structure of mineral surface, which can gain insight into the fundamental aspect of adsorption and oxidation at the atomic level [31–34]. Hence, with the purpose of better understanding of the competition of Hg2+ adsorption and surface oxidation on MoS2 surface as well as providing reasonable explanation, Hg2+ adsorption and oxidation on MoS2 surface were explored by using DFT calculations combined with AFM imaging. Studies in this work provide a clear understanding about the surface oxidation phenomenon of the MoS2 surface in the process of adsorbing heavy metal in water, which are of great significance for the development and utilization of molybdenum disulfide as a heavy metal adsorbent.
Fig. 1. MoS2 (001) surface models: (a) perfect MoS2 surface, (b) S-MoS2 surface with one S-missing defect, (c) S-MoS2 surface with two S-missing defects.
from the Sinopharm Chemical Reagent Co., Ltd. (China) were used to adjust pH, and the pH of Hg2+ solution was 5 throughout the experiment. The sample for AFM studying was of cuboid form with dimensions of 5 × 5 × 1 mm. 2.2. Computational method The models of MoS2 (001) surface with a dimension 16.45 Å by 15.83 Å were built by Materials Studio package. Based on this MoS2 surface model, perfect MoS2 surface model, MoS2 surface model with one S-missing defect (S-MoS2) and two S-missing defect (SS-MoS2) have been built to investigate the Hg2+ adsorption, surface oxidation and their competitive behaviors. The central site on the perfect MoS2 surface or the defect site on S-MoS2 surface were chosen as the initial adsorption sites. These MoS2 surface models and adsorption sites ①–③ were shown in Fig. 1. All the computations were performed using all-electron DFT with a double numerical basis set plus dynamic polarization function (DNP), as implemented in the Dmol3 module [35]. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional including van der Waals corrections was applied. Self-consistent field (SCF) computations were performed with a convergence criterion of 10−6 au for the total energy. After the geometrical optimization, the adsorption energy, electron density maps and density of states have been calculated. The adsorption energies (Eads) for H2O, Hg2+ or O2 on MoS2 and S-MoS2 surface were calculated defined by Eq. (1).
Eads =EM − MoS2 − (EM + E MoS2)
2. Experiments
(1)
where EM−MoS2 represents the total energy of the adsorbed molecules (H2O, Hg2+ or O2) and MoS2 substrate. EM and EMoS2 are the energies of the adsorbed molecules and MoS2 substrate, respectively.
2.1. Materials Natural Molybdenite with high purity were collected from the Wuzhou mine (Guangxi, China). Standard Hg2+ solution (1000 ppm, 3% HNO3) was purchased from Shanghai Zhanyun Chemical Co., Ltd. (China), which was diluted volumetrically with the Milli-Q ultrapure water (resistivity 18.2 MΩ·cm) to prepare 100 ppb and 10 ppm Hg2+ solution. Sodium hydroxide (NaOH) and nitric acid (HNO3) purchased
2.3. AFM measurement In situ observations of Hg2+ adsorption and surface oxidation behaviors were performed using a MultiMode 8 AFM (Bruker, Billerica, MA) with PeakForce Tapping mode. ScanAsyst-Air and ScanAsyst-fluid 522
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
−0.275 eV, which demonstrates a stronger binding affinity between water molecules and defective MoS2 surface. This theoretical study indicates that defect-free MoS2 surface is not very favorable for water adsorption due to the very weak interaction, in agreement with previous study [37,38]. Besides, the more negative adsorption energy of H2O molecule on S-MoS2 surface demonstrates the much stronger reactivity of defective surfaces due to the existence of S-vacancy defect. Fig. 3 shows the interface adsorption configurations as well as the electron density maps of Hg2+ on MoS2 and S-MoS2 surfaces. The distance between Hg2+ and surfaces decrease from 2.30 Å on MoS2 to 1.47 Å on S-MoS2. The adsorption energies of Hg2+ on MoS2 and SMoS2 surface are −1.196 eV and −1.473 eV, respectively, which indicates a strong chemical adsorption. Furthermore, the electron density maps in Fig. 3 show no overlap of electron cloud, thus the interaction between Hg2+ and S atom on MoS2 or S-MoS2 surface is more likely to be an ionic bond. Due to this high binding affinity between Hg2+ ions and the sulfur sites on the surface of MoS2 sheets, Hg2+ ions can be strongly bound to the MoS2 surface even without S-missing defects. Some experimental studies have also demonstrated this strong interaction between Hg2+ and sulfur on MoS2 surface [3,11,13,39]. In this study, this strong interaction between the MoS2 and Hg2+ has been verified from the perspective of theoretical calculations, and the hollow sites at the center of a hexagonal ring as well as the defect sites are the potential adsorption sites for Hg2+. Besides, the existence of S-vacancy defect makes the MoS2 a much more excellent adsorbent due to higher binding affinity between Hg2+ and sulfur atoms. Fig. 4 shows the interface adsorption configurations as well as the electron density maps of O2 on MoS2 and S-MoS2 surface. When on the defect-free MoS2 surface, two O atoms are just absorbed above the S atom, 3.57 Å and 3.37 Å distant from the S-plane surface, respectively. The very small adsorption energy of −0.007 eV indicates a very weak physical adsorption. Such long distances and weak van der Waals interactions clearly shows that perfect MoS2 monolayers cannot be oxidized. By comparison, the adsorbed O2 molecule formed covalent bonds with three surrounding Mo atoms at the vacancy site. The adsorption energy of −0.661 eV between the O2 and the S-MoS2 surface is significantly stronger than that on MoS2 surface and is in the range of typical covalent bond energy [40], which also demonstrates that O2 can be adsorbed onto defective MoS2 monolayer chemically. Electron density maps of O2 and Mo atom on defect-free (Fig. 4a) and S-vacancy (Fig. 4b) MoS2 surface also shows the influence of S-missing vacancy, the overlap of electron cloud shown in Fig. 4b indicates the covalent interaction between the O2 and Mo atom on S-MoS2 surface. As a consequence, S-vacancy defect on MoS2 surface has a huge influence on the surface oxidation behaviors. O2 molecule stays a little far from the defect-free MoS2 surface while it bonds to the unsaturated Mo atoms at the vacancy site on defective MoS2 surface, similar conclusion has been obtained by previous study [24,41–43]. Thus, it can be concluded that Hg2+ adsorption and surface oxidation can both take place on MoS2 surface only in the presence of S-missing defect. In conclusion, it is obvious that MoS2 surface with a S-vacancy is more reactive, in agreement with previous study [44]. To further investigate how S-vacancy defect influences the surface property, the partial density of states (PDOS) of the S and Mo atom near the S-vacancy site were determined. Fig. 5 clearly shows the differences between PDOS of S and Mo atoms on MoS2 and S-MoS2 monolayer. Svacancy mainly introduces a shallow state into the band gap (Fig. 5a), leading to the more reactive S atom around the defect site. As for PDOS of Mo on MoS2 and S-MoS2 surface, shown in Fig. 5b, S-vacancy defect mainly causes two different effects in the PDOS. One effect is a defect state close to the conduction band minimum (CBM), arising from the dangling bonds of the Mo 4d orbitals due to their unsaturated charges. The other one is a shallow state change close to the Fermi level, as arrowed in Fig. 5b. As electrons are very active at around the Fermi level [45,46], both Mo and S near the S-vacancy on S-MoS2 surface become more active than that on MoS2 surface. Hence, it is easy to
Fig. 2. Adsorption configurations of H2O on MoS2 surface (a) and S-MoS2 surface (b).
+ Si3N4 probes (tip radius r = 2 nm) were used, which were fixed on a V-shaped cantilever with the spring constants k of 0.4 and 0.7 N/m, respectively. All the sample of MoS2 sheets were freshly cleaved with adhesive tape just prior to use. When in-situ observing the surface oxidation or the Hg2+ adsorption, the freshly cleaved MoS2 surface was mounted in the AFM liquid cell followed by injecting 2 mL deionized water or 2 mL Hg2+ solution into the cell, and then to capture the AFM images. All the observations were performed with automatically optimized scan parameters including setpoint, feedback response, and scan rate for 5 h at 27 °C. All images were 512 pixels and analyzed by NanoScope Analysis 1.5 software. 3. Results and discussion 3.1. DFT calculations 3.1.1. Isolated adsorption of H2O, Hg2+ or O2 on MoS2 or S-MoS2 surface The adsorption behaviors of isolated Hg2+, O2, H2O on the MoS2 surface with and without S-vacancy defects were theoretically studied though DFT calculations to explore the interface interaction. Fig. 2 summarizes the interface adsorption configurations of H2O on MoS2 and S-MoS2 surfaces. And Table 1 shows the adsorption energy between adsorbate (H2O, Hg2+ or O2) and MoS2 or S-MoS2 surface. For MoS2, the H2O is 3.59 Å distant from the top S-plane on MoS2 surface. While on the S-MoS2 surface, the H2O molecule is 1.32 Å distant from the SMoS2 surface, and just above the S-vacancy defect site. The adsorption energy of H2O on MoS2 is 0.218 eV, which means an endothermic reaction [36]. This theoretical calculation result is in good agreement with real situation that molybdenite is of strong hydrophobicity. While on S-MoS2 surface, the adsorption energy for S-MoS2/H2O is Table 1 Adsorption energy (Eads) of H2O, Hg2+ and O2 on the MoS2 or S-MoS2 surface. Structure
Eads/eV
MoS2/H2O S-MoS2/H2O MoS2/O2 S-MoS2/O2 MoS2/Hg2+ S-MoS2/Hg2+
0.218 −0.275 −0.007 −0.661 −1.196 −1.473
523
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
Fig. 3. Adsorption configurations and electron density maps of Hg2+ on perfect MoS2 surface (a) and S- MoS2 surface (b).
Fig. 4. Adsorption configurations and electron density maps of O2 on perfect MoS2 surface (a) and S-MoS2 surface(b).
Density of states (electrons/eV)
2.0
(a)
S on MoS2 surface
1.5
s p d
1.0 0.5 0.0 2.0
S on S-MoS2 surface
1.5
s p d
1.0 0.5 0.0 -6
Density of states (electrons/eV)
3
-4
(b)
-2
Energy (eV)
0
2
Mo on MoS2 surface
s p d
2 1 0
Fig. 6. Co-adsorption configurations with H2O (a), Hg2+ (b) and O2 (c) right above the S-vacancy site on S-MoS2 surface, respectively.
3
Mo on S-MoS2 surface
s p d
2
understand that Mo atom close to S-vacancy on S-MoS2 surface tends to be oxidized while defect-free MoS2 surface just physically adsorb O2 molecule. Besides, the local density of states at the Fermi level increases due to the S-vacancy, which accounts for the larger adsorption energy of adsorbates on defective S-MoS2 surface reasonably.
1
0
-6
-4
-2
Energy (eV)
0
2
Fig. 5. PDOS of S atom (a) and Mo atom (b) near the S-vacancy on MoS2 and SMoS2 surface.
3.1.2. Co-adsorption configurations of H2O, Hg2+ and O2 on S-MoS2 surface After finding out that competition of Hg2+ adsorption and surface 524
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
In conclusion, HgeO complex would occur no matter Hg2+ or O2 was placed on the defect site. HgeO bond has also been determined through X-ray photoelectron spectroscopy (XPS) measurement in previous experimental study [13], which demonstrates the rationality of our theoretical calculation results. Therefore, the solution environment and dissolved oxygen have not shown to have adverse effect on the removal of Hg2+ from water even in the presence of S-vacancy on MoS2 surface, because they can bond to Hg2+ and then being adsorbed onto MoS2 surface together. A MoS2 monolayer model with both O2 and Hg2+ occupy two Svacancy sites has also been built to further investigate the competition of Hg2+ adsorption and surface oxidation. As shown in Fig. 7, Hg2+ bonds to O atom of H2O molecule and forms a HgeO complex, which was adsorbed close to one defect site. As for the O2 above another defect site, it reacts chemically with the Mo atom near the S-vacancy which suggests the occurrence of the surface oxidation. That is to say, the coexisting of Hg2+ adsorption and surface oxidation can take place at the same time only in a case where there are enough reaction sites for Hg2+ and O2 simultaneously. To sum up, Hg2+ will be adsorbed onto the MoS2 surface in two ways, which depends on the location of H2O, Hg2+ and O2 around the S-missing vacancy defect. One way is to form HgeO complex with O2 or H2O and then be absorbed onto MoS2 surface, and another is being directly adsorbed and form SeHg ionic bond with S atom on MoS2 surface. In addition, MoS2 surface has the preference adsorption for Hg2+ than O2 due to the much stronger binding affinity, and surface oxidation takes place only when there are enough reaction sites for the Hg2+ adsorption and surface oxidation simultaneously.
Fig. 7. Co-adsorption configurations with both Hg2+ and O2 above the S-vacancy sites on SS-MoS2 surface.
oxidation can only occur on the defective S-MoS2 surface, sequential coadsorption of H2O, Hg2+, and O2 on S-MoS2 surface have been investigated. Three adsorption modes have been considered for the further understanding of competition behaviors of the Hg2+ adsorption and oxidation on MoS2 surface. The adsorption behaviors have been simulated by placing H2O, Hg2+ and O2 right above the S-vacancy site on S-MoS2 surface, respectively. Fig. 6 shows the co-adsorption configurations with H2O (Fig. 6a), Hg2+ (Fig. 6b) and O2 (Fig. 6c) right above the S-vacancy site on SMoS2 monolayer, respectively. It was easy to find that no matter which one of the H2O, Hg2+, and O2 was placed on the defect site, the Hg2+ would be adsorbed onto the MoS2 surface firstly, indicating the strongest binding affinity between Hg2+ and S-MoS2 surface. Significantly, Hg2+ would be directly adsorbed onto the MoS2 surface without bonding to H2O or O2 molecule if the H2O was put above the S-missing vacancy defect. When placing the Hg2+ above the S-vacancy site initially, Hg2+ would combine with the O atom of H2O molecule to form a HgeO complex and then being adsorbed onto the MoS2 surface together. In comparison, when placing the O2 above the S-vacancy site, Hg2+ would connect to the O atom of O2 molecule to form a HgeO complex, after which this complex was adsorbed onto the MoS2 surface.
3.2. AFM observation of Hg2+ adsorption and oxidation on MoS2 surface AFM observations of Hg2+ adsorption and surface oxidation have been performed to verify our theoretical calculation results. Firstly, the behavior of MoS2 surface in pure water in the presence of dissolved oxygen was studied by AFM observation. Fig. 8 shows the AFM images
(b)
(a)
(c)
(d)
Fig. 8. AFM images of fresh MoS2 surface before and after exposure to pure water: (a) Freshly cleaved basal plane of MoS2, and after immersion in pure water for 1 h (b), 3 h (c), 5 h (d), respectively. 525
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
(b)
(a)
Fig. 9. AFM images of MoS2 surface after immersion in Hg2+ solution at concentration of 10 ppm for 1 h and 5 h.
(a)
(b)
(c)
(d)
Fig. 10. AFM images of fresh MoS2 surface before and after exposure to Hg2+ solution at concentration of 100 ppb. Freshly cleaved basal plane of MoS2 (a), and after immersion in Hg2+ solution for 1 h (b), 3 h (c), 5 h (d), respectively.
morphologies of protrusions on the surface of MoS2 were observed after immersion in Hg2+ solution for 1 h (Fig. 9a). The density of bright spots increased with increasing adsorption time. When the adsorb time extended to 5 h, the morphology of protrusions was still bright spot and nearly no short chainlike structures formed (Fig. 9b), indicating no surface oxidation occurred during the process of Hg2+ adsorption on the MoS2 nanosheet. Besides, the formation of more and more bright spot protrusions indicates the adsorption of Hg2+ on MoS2 surface, in agreement with previous study [11]. Therefore, it could be concluded that only Hg2+ adsorption occurred on MoS2 surface when it was exposed to Hg2+ solution at high concentration of 10 ppm. These results are in agreement with our theoretical calculation results that MoS2 surface has the preference adsorption for Hg2+ than O2 when there are limited reaction sites due to the much stronger binding affinity. Finally, the behavior of MoS2 surface in Hg2+ solution at low concentration was also studied by AFM observation. Fig. 10 shows the AFM images of freshly cleaved MoS2 sheets before and after immersion in Hg2+ solution at concentration of 100 ppb. The smooth surface of freshly cleaved MoS2 was captured in air (Fig. 10a). After immersion in
of freshly cleaved MoS2 sheets (Fig. 8a) and the surface of MoS2 after immersion in pure water for 1 h (Fig. 8b), 3 h (Fig. 8c), 5 h (Fig. 8d), respectively. Before exposure to pure water, the surface of MoS2 captured in air is very smooth. After the sample had been immersed in water for 1 h, the needlelike protrusions are observed on the surface of MoS2 (Fig. 8b). Some needlelike protrusions linked together to form short chainlike structures in 3 h (Fig. 8c), and increasing chainlike structures occurred after immersion in pure water for 5 h (Fig. 8d). Previous study has also observed this phenomenon and figured out that chainlike structures is caused by surface oxidation, and the oxidative product is MoO3·H2O crystals [25,47]. In our calculation results, the chemical bond between Mo atom and O atom has also been observed, in agreement with this AFM results. Thus, it can be concluded that the formation of chainlike structures protrusions stands for the occurrence of surface oxidation on MoS2 nanosheet. Then, the behavior of MoS2 surface in Hg2+ solution at high concentration was studied by AFM observation. Fig. 9 shows the AFM images of MoS2 surface after immersion in Hg2+ solution at concentration of 10 ppm for 1 h and 5 h. A number of bright spots 526
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
Hg2+ solution for 1 h, the morphology of protrusions on MoS2 sheet was mainly bright spots and some needlelike protrusions (Fig. 10b). When immersed 3 h or 5 h, two changes have taken place. One is the appearance of an increasing number of bright spots, and the other is the formation of short chainlike structures linked together from needlelike protrusions (Fig. 10c–d). These phenomena demonstrate that both Hg2+ adsorption and oxidation has taken place on the MoS2 surface. According to our theoretical calculation result shown in Fig. 6 and Fig. 7, MoS2 surface has the preference for Hg2+ adsorption when there are limited reaction sites due to the much stronger binding affinity, and surface oxidation takes place only when there are redundant reaction sites for Hg2+ adsorption and surface oxidation simultaneously. Hg2+ solution with low concentration means excess surface reaction site, thus there are redundant reaction sites provided for the oxidation. These AFM images have verified the correctness of the DFT calculations results. In conclusion, high concentration of Hg2+ solution means insufficient reaction sites on MoS2 surface, the only observed Hg2+ adsorption can be interpreted by the stronger affinity between Hg2+ and MoS2 based on our calculation results. While in Hg2+ solution with low concentration, there are surplus surface reaction sites, thus surface oxidation can also take place in addition to Hg2+ adsorption, which can be proved both by our calculation results and AFM images in this work.
[7] H.Y. Jeong, K. Sun, K.F. Hayes, Microscopic and spectroscopic characterization of Hg(II) immobilization by mackinawite (FeS), Environ. Sci. Technol. 44 (2010) 7476–7483, https://doi.org/10.1021/es100808y. [8] M. Fojtů, W.Z. Teo, M. Pumera, Environmental impact and potential health risks of 2D nanomaterials, Environ. Sci. Nano. 4 (2017) 1617–1633, https://doi.org/10. 1039/C7EN00401J. [9] Z. Wang, B. Mi, Environmental applications of 2D molybdenum disulfide (MoS2) Nanosheets, Environ. Sci. Technol. 51 (2017) 8229–8244, https://doi.org/10.1021/ acs.est.7b01466. [10] Q. Wang, L. Peng, Y. Gong, F. Jia, S. Song, Y. Li, Mussel-inspired Fe3O4 @ Polydopamine(PDA)-MoS2 core–shell nanosphere as a promising adsorbent for removal of Pb2+ from water, J. Mol. Liq. 282 (2019) 598–605, https://doi.org/10. 1016/j.molliq.2019.03.052. [11] F. Jia, X. Zhang, S. Song, AFM study on the adsorption of Hg2+ on natural molybdenum disulfide in aqueous solutions, Phys. Chem. Chem. Phys. 19 (2017) 3837–3844, https://doi.org/10.1039/C6CP07302F. [12] L. Zhi, W. Zuo, F. Chen, B. Wang, 3D MoS2 composition aerogels as chemosensors and adsorbents for colorimetric detection and high-capacity adsorption of Hg2+, ACS Sustain. Chem. Eng. 4 (2016) 3398–3408, https://doi.org/10.1021/ acssuschemeng.6b00409. [13] F. Jia, Q. Wang, J. Wu, Y. Li, S. Song, Two-dimensional molybdenum disulfide as a superb adsorbent for removing Hg2+ from water, ACS Sustain. Chem. Eng. 5 (2017) 7410–7419, https://doi.org/10.1021/acssuschemeng.7b01880. [14] R. Addou, L. Colombo, R.M. Wallace, Surface defects on natural MoS2, ACS Appl. Mater. Interfaces 7 (2015) 11921–11929, https://doi.org/10.1021/acsami. 5b01778. [15] W. Zhou, X. Zou, S. Najmaei, Z. Liu, Y. Shi, J. Kong, J. Lou, P.M. Ajayan, B.I. Yakobson, J.C. Idrobo, Intrinsic structural defects in monolayer molybdenum disulfide, Nano Lett. 13 (2013) 2615–2622, https://doi.org/10.1021/nl4007479. [16] J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, Controllable disorder engineering in oxygen-incorporated MoS 2ultrathin nanosheets for efficient hydrogen evolution, J. Am. Chem. Soc. 135 (2013) 17881–17888, https://doi.org/10.1021/ja408329q. [17] K. Zhang, H.J. Kim, J.T. Lee, G.W. Chang, X. Shi, W. Kim, M. Ma, K.J. Kong, J.M. Choi, M.S. Song, J.H. Park, Unconventional pore and defect generation in molybdenum disulfide: application in high-rate lithium-ion batteries and the hydrogen evolution reaction, ChemSusChem. 7 (2014) 2489–2495, https://doi.org/ 10.1002/cssc.201402372. [18] F. Jia, C. Liu, B. Yang, S. Song, Microscale control of edge defect and oxidation on molybdenum disulfide through thermal treatment in air and nitrogen atmospheres, Appl. Surf. Sci. 462 (2018) 471–479, https://doi.org/10.1016/j.apsusc.2018.08. 166. [19] S. Kc, R.C. Longo, R. Addou, R.M. Wallace, K. Cho, Impact of intrinsic atomic defects on the electronic structure of MoS2 monolayers, Nanotechnology. 25 (2014), https://doi.org/10.1088/0957-4484/25/37/375703. [20] J.F. Paul, E. Payen, Vacancy formation on MoS2 hydrodesulfurization catalyst: DFT study of the mechanism, J. Phys. Chem. B 107 (2003) 4057–4064, https://doi.org/ 10.1021/jp027668f. [21] D. Liu, Y. Guo, L. Fang, J. Robertson, Sulfur vacancies in monolayer MoS2 and its electrical contacts, Appl. Phys. Lett. 103 (2013) 183113, , https://doi.org/10.1063/ 1.4824893. [22] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X.W.D. Lou, Y. Xie, Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution, Adv. Mater. 25 (2013) 5807–5813, https://doi.org/10.1002/adma.201302685. [23] F. Jia, K. Sun, B. Yang, X. Zhang, Q. Wang, S. Song, Defect-rich molybdenum disulfide as electrode for enhanced capacitive deionization from water, Desalination. 446 (2018) 21–30, https://doi.org/10.1016/j.desal.2018.08.024. [24] H. Liu, N. Han, J. Zhao, Atomistic insight into the oxidation of monolayer transition metal dichalcogenides: from structures to electronic properties, RSC Adv. 5 (2015) 17572–17581, https://doi.org/10.1039/C4RA17320A. [25] X. Zhang, F. Jia, B. Yang, S. Song, Oxidation of molybdenum disulfide sheet in water under in situ atomic force microscopy observation, J. Phys. Chem. C 121 (2017) 9938–9943, https://doi.org/10.1021/acs.jpcc.7b01863. [26] Z. Xian, Y. Hao, Y. Zhao, S. Song, Quantitative determination of isomorphous substitutions on clay mineral surfaces through AFM imaging: a case of mica, Colloids Surf. A Physicochem. Eng. Asp. 533 (2017) 55–60, https://doi.org/10. 1016/j.colsurfa.2017.08.024. [27] H. Yi, X. Zhang, Y. Zhao, Y. Liu, S. Song, A novel method for the quantitative determination of defects on graphene surfaces, J. Colloid Interface Sci. 499 (2017) 62–66, https://doi.org/10.1016/j.jcis.2017.03.094. [28] H. Yi, X. Zhang, Y. Zhao, L. Liu, S. Song, Molecular dynamics simulations of hydration shell on montmorillonite (001) in water, Surf. Interface Anal. 48 (2016) 976–980, https://doi.org/10.1002/sia.6000. [29] X. Zhang, H. Yi, Y. Zhao, F. Min, S. Song, Study on the differences of Na- and Camontmorillonites in crystalline swelling regime through molecular dynamics simulation, Adv. Powder Technol. 27 (2016) 779–785, https://doi.org/10.1016/j. apt.2016.03.005. [30] H. Yi, F. Jia, Y. Zhao, W. Wang, S. Song, H. Li, C. Liu, Surface wettability of montmorillonite (001) surface as affected by surface charge and exchangeable cations: a molecular dynamic study, Appl. Surf. Sci. 459 (2018) 148–154, https://doi. org/10.1016/j.apsusc.2018.07.216. [31] Z. Liu, W. Peng, Z. Xu, K. Shih, J. Wang, Z. Wang, X. Lv, J. Chen, X. Li, Molybdenum disulfide-coated lithium vanadium fluorophosphate anode: experiments and firstprinciples calculations, ChemSusChem. 9 (2016) 2122–2128, https://doi.org/10. 1002/cssc.201600370.
4. Conclusions (1) S-vacancy defect on MoS2 surface makes MoS2 more reactive due to introducing the local density of states around the Fermi level. (2) Perfect and defective MoS2 surface both has very weak adsorption energy of H2O but has very strong binding affinity to Hg2+, while surface oxidation can only take place in the presence of S-vacancy defect. (3) Hg2+ can be absorbed onto the MoS2 surface in two ways which depends on the distribution of Hg2+, O2 and H2O around the defect sites. One is to form HgeS complex with S atom on MoS2 surface, and another is to form HgeO complex with H2O or O2 and then being absorbed onto the MoS2 surface. (4) MoS2 surface has the preference for Hg2+ adsorption when there are limited reaction sites due to the much stronger binding affinity, and surface oxidation takes place only when there are redundant reaction sites for Hg2+ adsorption and surface oxidation simultaneously. Acknowledgements The financial supports to this work from the National Natural Science Foundation of China under the projects No. 51674183 and 51704220 were gratefully acknowledged. References [1] P. Miretzky, A.F. Cirelli, Hg(II) removal from water by chitosan and chitosan derivatives: a review, J. Hazard. Mater. 167 (2009) 10–23, https://doi.org/10.1016/j. jhazmat.2009.01.060. [2] L. Trasande, P.J. Landrigan, C. Schechter, Public health and economic consequences of methyl mercury toxicity to the developing brain, Environ. Health Perspect. 113 (2005) 590–596, https://doi.org/10.1289/ehp.7743. [3] K. Ai, C. Ruan, M. Shen, L. Lu, MoS2 nanosheets with widened interlayer spacing for high-efficiency removal of mercury in aquatic systems, Adv. Funct. Mater. 26 (2016) 5542–5549, https://doi.org/10.1002/adfm.201601338. [4] M.M. Hyland, G.E. Jean, G.M. Bancroft, XPS and AES studies of hg(II) sorption and desorption reactions on sulphide minerals, Geochim. Cosmochim. Acta 54 (1990) 1957–1967, https://doi.org/10.1016/0016-7037(90)90264-L. [5] D.M. Manohar, K.A. Krishnan, T.S. Anirudhan, Removal of mercury (II) from aqueous solutions and chlor-alkali industry wastewater using, Water Res. 36 (2002) 1609–1619, https://doi.org/10.1016/S0043-1354(01)00362-1. [6] K. Anoop Krishnan, T.S. Anirudhan, Removal of mercury(II) from aqueous solutions and chlor-alkali industry effluent by steam activated and sulphurised activated carbons prepared from bagasse pith: kinetics and equilibrium studies, J. Hazard. Mater. 92 (2002) 161–183, https://doi.org/10.1016/S0304-3894(02)00014-6.
527
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
https://doi.org/10.1007/s12274-014-0658-x. [40] Y.Y. Chen, M. Dong, Z. Qin, X.D. Wen, W. Fan, J. Wang, A DFT study on the adsorption and dissociation of methanol over MoS2 surface, J. Mol. Catal. A Chem. 338 (2011) 44–50, https://doi.org/10.1016/j.molcata.2011.01.024. [41] S. Kc, R.C. Longo, R.M. Wallace, K. Cho, Surface oxidation energetics and kinetics on MoS2 monolayer, J. Appl. Phys. 117 (2015), https://doi.org/10.1063/1. 4916536. [42] A.V. Krivosheeva, V.L. Shaposhnikov, V.E. Borisenko, J.L. Lazzari, C. Waileong, J. Gusakova, B.K. Tay, Theoretical study of defect impact on two-dimensional MoS2, J. Semicond. 36 (2015) 1–6, https://doi.org/10.1088/1674-4926/36/12/122002. [43] T. Liang, W.G. Sawyer, S.S. Perry, S.B. Sinnott, S.R. Phillpot, Energetics of oxidation in MoS2 nanoparticles by density functional theory, J. Phys. Chem. C 115 (2011) 10606–10616, https://doi.org/10.1021/jp110562n. [44] R. Kronberg, M. Hakala, N. Holmberg, K. Laasonen, Hydrogen adsorption on MoS2 -surfaces: a DFT study on preferential sites and the effect of sulfur and hydrogen coverage, Phys. Chem. Chem. Phys. 19 (2017) 16231–16241, https://doi.org/10. 1039/C7CP03068A. [45] C. Ataca, S. Ciraci, Dissociation of H2O at the vacancies of single-layer MoS2, Phys. Rev. B Condens. Matter Mater. Phys. 85 (2012) 1–6, https://doi.org/10.1103/ PhysRevB.85.195410. [46] M. Ghorbani-Asl, A.N. Enyashin, A. Kuc, G. Seifert, T. Heine, Defect-induced conductivity anisotropy in MoS2 monolayers, Phys. Rev. B 88 (2013) 7, , https://doi. org/10.1103/PhysRevB.88.245440. [47] Y. Ryu, W. Kim, S. Koo, H. Kang, K. Watanabe, T. Taniguchi, S. Ryu, Interfaceconfined doubly anisotropic oxidation of two-dimensional MoS2, Nano Lett. 17 (2017) 7267–7273, https://doi.org/10.1021/acs.nanolett.7b02621.
[32] W. Song, T. Yang, X. Wang, Y. Sun, Y. Ai, G. Sheng, T. Hayat, X. Wang, Experimental and theoretical evidence for competitive interactions of tetracycline and sulfamethazine with reduced graphene oxides, Environ. Sci. Nano. 3 (2016) 1318–1326, https://doi.org/10.1039/c6en00306k. [33] T. Hüffer, H. Sun, J.D. Kubicki, T. Hofmann, M. Kah, Interactions between aromatic hydrocarbons and functionalized C60 fullerenes – insights from experimental data and molecular modelling, Environ. Sci. Nano. 4 (2017) 1045–1053, https://doi. org/10.1039/C7EN00139H. [34] N. Ding, X. Chen, C.-M.L. Wu, Interactions between polybrominated diphenyl ethers and graphene surface: a DFT and MD investigation, Environ. Sci. Nano. 1 (2014) 55–63, https://doi.org/10.1039/C3EN00037K. [35] N. Yu, L. Wang, M. Li, X. Sun, T. Hou, Y. Li, Molybdenum disulfide as a highly efficient adsorbent for non-polar gases, Phys. Chem. Chem. Phys. 17 (2015) 11700–11704, https://doi.org/10.1039/C5CP00161G. [36] C.H. Zhao, J.H. Chen, B.Z. Wu, X.H. Long, Density functional theory study on natural hydrophobicity of sulfide surfaces, Trans. Nonferrous Met. Soc. China (English Ed. 24 (2014) 491–498. doi:https://doi.org/10.1016/S1003-6326(14) 63087-9. [37] K.K. Ghuman, S. Yadav, C.V. Singh, Adsorption and dissociation of H2O on monolayered MoS2 edges: energetics and mechanism from ab initio simulations, J. Phys. Chem. C 119 (2015) 6518–6529, https://doi.org/10.1021/jp510899m. [38] S. Tongay, J. Zhou, C. Ataca, J. Liu, J.S. Kang, T.S. Matthews, L. You, J. Li, J.C. Grossman, J. Wu, Broad-range modulation of light emission in two-dimensional semiconductors by molecular physisorption gating, Nano Lett. 13 (2013) 2831–2836, https://doi.org/10.1021/nl4011172. [39] S. Jiang, R. Cheng, R. Ng, Y. Huang, X. Duan, Highly sensitive detection of mercury (II) ions with few-layer molybdenum disulfide, Nano Res. 8 (2015) 257–262,
528
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Competition of Hg2+ adsorption and surface oxidation on MoS2 surface as affected by sulfur vacancy defects ⁎
Hao Yia,b,1, Xian Zhangb,1, Feifei Jiaa,b, , Zhenlun Weib, Yunliang Zhaoa,b, Shaoxian Songa,b, a b
T
⁎
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenum disulfide Hg2+ adsorption Surface oxidation DFT calculations AFM imaging
Two dimensional molybdenum disulfide (2D-MoS2) has been recently developed to be used as superb adsorbents for removing heavy metals from water due to its huge sulfur-rich surface area. In this work, single adsorption and co-adsorption performances of H2O, Hg2+ and O2 on MoS2 surface with and without S-vacancy defects have been theoretically studied to explore the Hg2+ adsorption and surface oxidation through density function theory (DFT) calculations. Moreover, the Hg2+ adsorption and surface oxidation have experimentally studied through atomic force microscopy (AFM) to verify the theoretical calculation results. It has been found that S-vacancy defects make MoS2 surface more reactive, leading to the much stronger adsorption energy of H2O, Hg2+ and O2 on defective MoS2 surface. O2 can only be physically adsorbed on the perfect MoS2 surface, while it bonds to the unsaturated Mo atoms at the vacancy site on defective MoS2 surface. Besides, co-adsorption results illustrate that Hg2+ have priority to react with MoS2 surface than O2 due to the much stronger binding affinity, and surface oxidation occurs only when there are enough reaction sites for the Hg2+ adsorption and surface oxidation simultaneously. These theoretical co-adsorption results are consistent with the experimental Hg2+ adsorption and surface oxidation results obtained by AFM. These findings in this study are of great significance for the development and utilization of MoS2-based nanomaterials as a heavy metal adsorbent.
1. Introduction Hg2+ is the primary form of mercury (Hg) in water normally found in the global environment [1]. As a highly toxic heavy metal, the existence of Hg2+ in the environment often causes long-term contamination problems, which has a serious impact on the public health and environmental protection. Excessive accumulation of Hg2+ in the human body leads to many neurological disorders such as memory loss, insomnia, behavioral issues, and cerebral palsy [2]. Therefore, the latent huge danger of Hg2+ discharged into environment makes it particularly urgent to treat with Hg2+ wastewater. Adsorption is regarded as the most promising approach for Hg2+ wastewater treatment compared with other techniques, such as ion exchange, coagulation and chemical precipitation, in terms of its low cost, simple operation, and regeneration capability. Thanks to the strong soft-soft interactions between Hg2+ and sulfur [3], some sulfurcontaining materials have recently been reported to demonstrate substantial improvements in Hg2+ removal performance [4–7].
Molybdenum disulfide (MoS2), as a typical layered transition-metal dichalcogenide (TMD), is composed of a hexagonal plane of Mo atoms sandwiched by two hexagonal planes of S atoms [8–10]. Due to the abundance of intrinsic sulfur atoms (potential binding sites) on the formed S-Mo-S layered MoS2 surface, it could be applied as natural, efficient and perfect adsorbents to remove Hg2+ from polluted water. Recently, study on the adsorption of Hg2+ on natural MoS2 has been performed by atomic force microscope (AFM), and the AFM imaging clearly demonstrated the strongly adsorption of Hg2+ on MoS2 (001) surface [11]. In order to improve specific surface area and adsorption capacity, a novel porous MoS2-based aerogels has been successfully designed to remove Hg2+ in aqueous solution and exhibited a very high adsorption capacity [12]. Considering that the gallery height of 0.298 nm between two neighboring layers is too narrow to allow hydrated Hg2+ ions into the interior spaces where the vast majority of potential binding sites are located. MoS2 with widened interlayer spacing have been prepared and exhibited a tremendous capturing capacity, fast adsorption kinetics, extraordinary affinity and excellent
⁎
Corresponding authors at: Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei, 430070, China. E-mail addresses: [email protected] (F. Jia), [email protected] (S. Song). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2019.03.350 Received 25 September 2018; Received in revised form 26 March 2019; Accepted 31 March 2019 Available online 01 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
selectivity for Hg2+ in water [3]. Because of the weak out-of-plane van der Waals interactions between individual S-Mo-S layers, exfoliating MoS2 into two-dimensional (2D) materials may be another effective method to improve the ability of Hg2+ removal for bring more surfaces with sulfur exposed outside. And a previous study proved that 2D-MoS2 could be used as a superb adsorbent for removing Hg2+ from water [13]. Above all, MoS2 has been turned out to be a highly efficient adsorbent for Hg2+ removal. However, some problems are remained to be studied. The structural defects on MoS2 layer have been observed and extensively investigated recently [14–18]. Even though an ideal surface of MoS2 would be free from dangling bonds or other types of defects, several imperfections can appear during the exfoliation or growth processes [19]. MoS2 layers have intrinsic defects like vacancies generated during mechanical or chemical exfoliation, and a single vacancy with one S missing defect (S-MoS2) is most frequently observed for the lowest defect formation energy [20,21]. It is believed that defect-rich features on MoS2 surface will have a tremendous influence on the adsorption performance [3,16,22,23]. However, the details that how Svacancy defect affects the Hg2+ adsorption behaviors need to be further studied. Besides, due to the existence of defect, the MoS2 surface is easy to be oxidized in humid or aqueous environment. Study has shown that a perfect single-layer sheet of MoS2 stays intact when exposed in O2 due to the weak physical adsorption, while monolayers of MoS2 with Svacancy defects adsorb O2 through chemical adsorption [24]. Besides, the oxidation process of MoS2 sheet in water was in situ observed by AFM, it was found that the oxidation led to the partial etching of the surface layer and needlelike protrusions on the MoS2 surfaces [25]. Consequently, the unavoidable occurrence of S-vacancy defects on monolayer MoS2 has a huge influence both on the Hg2+ adsorption and surface oxidation. However, it is not clear how the S-vacancy defects influence both the Hg2+ adsorption and surface oxidation, and the competition of Hg2+ adsorption and surface oxidation on MoS2 surface have not been investigated yet. Therefore, an attempt was made to explore the competition of the coexisting surface oxidation and Hg2+ adsorption behaviors under the influence of S-missing defects on MoS2 surface. AFM has been recently established as a powerful technique for direct nanoscale observation of the surface morphology of adsorbent and the adsorption behaviors of ions or molecules on an adsorbent, and in situ observation of the surface or interface reaction process can be also achieved [25–27]. Another powerful tool for studying microscopic phenomena and mechanisms is molecular simulations [28–30]. Density function theory (DFT) performs very well in the simulation of configuration and electronic structure of mineral surface, which can gain insight into the fundamental aspect of adsorption and oxidation at the atomic level [31–34]. Hence, with the purpose of better understanding of the competition of Hg2+ adsorption and surface oxidation on MoS2 surface as well as providing reasonable explanation, Hg2+ adsorption and oxidation on MoS2 surface were explored by using DFT calculations combined with AFM imaging. Studies in this work provide a clear understanding about the surface oxidation phenomenon of the MoS2 surface in the process of adsorbing heavy metal in water, which are of great significance for the development and utilization of molybdenum disulfide as a heavy metal adsorbent.
Fig. 1. MoS2 (001) surface models: (a) perfect MoS2 surface, (b) S-MoS2 surface with one S-missing defect, (c) S-MoS2 surface with two S-missing defects.
from the Sinopharm Chemical Reagent Co., Ltd. (China) were used to adjust pH, and the pH of Hg2+ solution was 5 throughout the experiment. The sample for AFM studying was of cuboid form with dimensions of 5 × 5 × 1 mm. 2.2. Computational method The models of MoS2 (001) surface with a dimension 16.45 Å by 15.83 Å were built by Materials Studio package. Based on this MoS2 surface model, perfect MoS2 surface model, MoS2 surface model with one S-missing defect (S-MoS2) and two S-missing defect (SS-MoS2) have been built to investigate the Hg2+ adsorption, surface oxidation and their competitive behaviors. The central site on the perfect MoS2 surface or the defect site on S-MoS2 surface were chosen as the initial adsorption sites. These MoS2 surface models and adsorption sites ①–③ were shown in Fig. 1. All the computations were performed using all-electron DFT with a double numerical basis set plus dynamic polarization function (DNP), as implemented in the Dmol3 module [35]. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional including van der Waals corrections was applied. Self-consistent field (SCF) computations were performed with a convergence criterion of 10−6 au for the total energy. After the geometrical optimization, the adsorption energy, electron density maps and density of states have been calculated. The adsorption energies (Eads) for H2O, Hg2+ or O2 on MoS2 and S-MoS2 surface were calculated defined by Eq. (1).
Eads =EM − MoS2 − (EM + E MoS2)
2. Experiments
(1)
where EM−MoS2 represents the total energy of the adsorbed molecules (H2O, Hg2+ or O2) and MoS2 substrate. EM and EMoS2 are the energies of the adsorbed molecules and MoS2 substrate, respectively.
2.1. Materials Natural Molybdenite with high purity were collected from the Wuzhou mine (Guangxi, China). Standard Hg2+ solution (1000 ppm, 3% HNO3) was purchased from Shanghai Zhanyun Chemical Co., Ltd. (China), which was diluted volumetrically with the Milli-Q ultrapure water (resistivity 18.2 MΩ·cm) to prepare 100 ppb and 10 ppm Hg2+ solution. Sodium hydroxide (NaOH) and nitric acid (HNO3) purchased
2.3. AFM measurement In situ observations of Hg2+ adsorption and surface oxidation behaviors were performed using a MultiMode 8 AFM (Bruker, Billerica, MA) with PeakForce Tapping mode. ScanAsyst-Air and ScanAsyst-fluid 522
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
−0.275 eV, which demonstrates a stronger binding affinity between water molecules and defective MoS2 surface. This theoretical study indicates that defect-free MoS2 surface is not very favorable for water adsorption due to the very weak interaction, in agreement with previous study [37,38]. Besides, the more negative adsorption energy of H2O molecule on S-MoS2 surface demonstrates the much stronger reactivity of defective surfaces due to the existence of S-vacancy defect. Fig. 3 shows the interface adsorption configurations as well as the electron density maps of Hg2+ on MoS2 and S-MoS2 surfaces. The distance between Hg2+ and surfaces decrease from 2.30 Å on MoS2 to 1.47 Å on S-MoS2. The adsorption energies of Hg2+ on MoS2 and SMoS2 surface are −1.196 eV and −1.473 eV, respectively, which indicates a strong chemical adsorption. Furthermore, the electron density maps in Fig. 3 show no overlap of electron cloud, thus the interaction between Hg2+ and S atom on MoS2 or S-MoS2 surface is more likely to be an ionic bond. Due to this high binding affinity between Hg2+ ions and the sulfur sites on the surface of MoS2 sheets, Hg2+ ions can be strongly bound to the MoS2 surface even without S-missing defects. Some experimental studies have also demonstrated this strong interaction between Hg2+ and sulfur on MoS2 surface [3,11,13,39]. In this study, this strong interaction between the MoS2 and Hg2+ has been verified from the perspective of theoretical calculations, and the hollow sites at the center of a hexagonal ring as well as the defect sites are the potential adsorption sites for Hg2+. Besides, the existence of S-vacancy defect makes the MoS2 a much more excellent adsorbent due to higher binding affinity between Hg2+ and sulfur atoms. Fig. 4 shows the interface adsorption configurations as well as the electron density maps of O2 on MoS2 and S-MoS2 surface. When on the defect-free MoS2 surface, two O atoms are just absorbed above the S atom, 3.57 Å and 3.37 Å distant from the S-plane surface, respectively. The very small adsorption energy of −0.007 eV indicates a very weak physical adsorption. Such long distances and weak van der Waals interactions clearly shows that perfect MoS2 monolayers cannot be oxidized. By comparison, the adsorbed O2 molecule formed covalent bonds with three surrounding Mo atoms at the vacancy site. The adsorption energy of −0.661 eV between the O2 and the S-MoS2 surface is significantly stronger than that on MoS2 surface and is in the range of typical covalent bond energy [40], which also demonstrates that O2 can be adsorbed onto defective MoS2 monolayer chemically. Electron density maps of O2 and Mo atom on defect-free (Fig. 4a) and S-vacancy (Fig. 4b) MoS2 surface also shows the influence of S-missing vacancy, the overlap of electron cloud shown in Fig. 4b indicates the covalent interaction between the O2 and Mo atom on S-MoS2 surface. As a consequence, S-vacancy defect on MoS2 surface has a huge influence on the surface oxidation behaviors. O2 molecule stays a little far from the defect-free MoS2 surface while it bonds to the unsaturated Mo atoms at the vacancy site on defective MoS2 surface, similar conclusion has been obtained by previous study [24,41–43]. Thus, it can be concluded that Hg2+ adsorption and surface oxidation can both take place on MoS2 surface only in the presence of S-missing defect. In conclusion, it is obvious that MoS2 surface with a S-vacancy is more reactive, in agreement with previous study [44]. To further investigate how S-vacancy defect influences the surface property, the partial density of states (PDOS) of the S and Mo atom near the S-vacancy site were determined. Fig. 5 clearly shows the differences between PDOS of S and Mo atoms on MoS2 and S-MoS2 monolayer. Svacancy mainly introduces a shallow state into the band gap (Fig. 5a), leading to the more reactive S atom around the defect site. As for PDOS of Mo on MoS2 and S-MoS2 surface, shown in Fig. 5b, S-vacancy defect mainly causes two different effects in the PDOS. One effect is a defect state close to the conduction band minimum (CBM), arising from the dangling bonds of the Mo 4d orbitals due to their unsaturated charges. The other one is a shallow state change close to the Fermi level, as arrowed in Fig. 5b. As electrons are very active at around the Fermi level [45,46], both Mo and S near the S-vacancy on S-MoS2 surface become more active than that on MoS2 surface. Hence, it is easy to
Fig. 2. Adsorption configurations of H2O on MoS2 surface (a) and S-MoS2 surface (b).
+ Si3N4 probes (tip radius r = 2 nm) were used, which were fixed on a V-shaped cantilever with the spring constants k of 0.4 and 0.7 N/m, respectively. All the sample of MoS2 sheets were freshly cleaved with adhesive tape just prior to use. When in-situ observing the surface oxidation or the Hg2+ adsorption, the freshly cleaved MoS2 surface was mounted in the AFM liquid cell followed by injecting 2 mL deionized water or 2 mL Hg2+ solution into the cell, and then to capture the AFM images. All the observations were performed with automatically optimized scan parameters including setpoint, feedback response, and scan rate for 5 h at 27 °C. All images were 512 pixels and analyzed by NanoScope Analysis 1.5 software. 3. Results and discussion 3.1. DFT calculations 3.1.1. Isolated adsorption of H2O, Hg2+ or O2 on MoS2 or S-MoS2 surface The adsorption behaviors of isolated Hg2+, O2, H2O on the MoS2 surface with and without S-vacancy defects were theoretically studied though DFT calculations to explore the interface interaction. Fig. 2 summarizes the interface adsorption configurations of H2O on MoS2 and S-MoS2 surfaces. And Table 1 shows the adsorption energy between adsorbate (H2O, Hg2+ or O2) and MoS2 or S-MoS2 surface. For MoS2, the H2O is 3.59 Å distant from the top S-plane on MoS2 surface. While on the S-MoS2 surface, the H2O molecule is 1.32 Å distant from the SMoS2 surface, and just above the S-vacancy defect site. The adsorption energy of H2O on MoS2 is 0.218 eV, which means an endothermic reaction [36]. This theoretical calculation result is in good agreement with real situation that molybdenite is of strong hydrophobicity. While on S-MoS2 surface, the adsorption energy for S-MoS2/H2O is Table 1 Adsorption energy (Eads) of H2O, Hg2+ and O2 on the MoS2 or S-MoS2 surface. Structure
Eads/eV
MoS2/H2O S-MoS2/H2O MoS2/O2 S-MoS2/O2 MoS2/Hg2+ S-MoS2/Hg2+
0.218 −0.275 −0.007 −0.661 −1.196 −1.473
523
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
Fig. 3. Adsorption configurations and electron density maps of Hg2+ on perfect MoS2 surface (a) and S- MoS2 surface (b).
Fig. 4. Adsorption configurations and electron density maps of O2 on perfect MoS2 surface (a) and S-MoS2 surface(b).
Density of states (electrons/eV)
2.0
(a)
S on MoS2 surface
1.5
s p d
1.0 0.5 0.0 2.0
S on S-MoS2 surface
1.5
s p d
1.0 0.5 0.0 -6
Density of states (electrons/eV)
3
-4
(b)
-2
Energy (eV)
0
2
Mo on MoS2 surface
s p d
2 1 0
Fig. 6. Co-adsorption configurations with H2O (a), Hg2+ (b) and O2 (c) right above the S-vacancy site on S-MoS2 surface, respectively.
3
Mo on S-MoS2 surface
s p d
2
understand that Mo atom close to S-vacancy on S-MoS2 surface tends to be oxidized while defect-free MoS2 surface just physically adsorb O2 molecule. Besides, the local density of states at the Fermi level increases due to the S-vacancy, which accounts for the larger adsorption energy of adsorbates on defective S-MoS2 surface reasonably.
1
0
-6
-4
-2
Energy (eV)
0
2
Fig. 5. PDOS of S atom (a) and Mo atom (b) near the S-vacancy on MoS2 and SMoS2 surface.
3.1.2. Co-adsorption configurations of H2O, Hg2+ and O2 on S-MoS2 surface After finding out that competition of Hg2+ adsorption and surface 524
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
In conclusion, HgeO complex would occur no matter Hg2+ or O2 was placed on the defect site. HgeO bond has also been determined through X-ray photoelectron spectroscopy (XPS) measurement in previous experimental study [13], which demonstrates the rationality of our theoretical calculation results. Therefore, the solution environment and dissolved oxygen have not shown to have adverse effect on the removal of Hg2+ from water even in the presence of S-vacancy on MoS2 surface, because they can bond to Hg2+ and then being adsorbed onto MoS2 surface together. A MoS2 monolayer model with both O2 and Hg2+ occupy two Svacancy sites has also been built to further investigate the competition of Hg2+ adsorption and surface oxidation. As shown in Fig. 7, Hg2+ bonds to O atom of H2O molecule and forms a HgeO complex, which was adsorbed close to one defect site. As for the O2 above another defect site, it reacts chemically with the Mo atom near the S-vacancy which suggests the occurrence of the surface oxidation. That is to say, the coexisting of Hg2+ adsorption and surface oxidation can take place at the same time only in a case where there are enough reaction sites for Hg2+ and O2 simultaneously. To sum up, Hg2+ will be adsorbed onto the MoS2 surface in two ways, which depends on the location of H2O, Hg2+ and O2 around the S-missing vacancy defect. One way is to form HgeO complex with O2 or H2O and then be absorbed onto MoS2 surface, and another is being directly adsorbed and form SeHg ionic bond with S atom on MoS2 surface. In addition, MoS2 surface has the preference adsorption for Hg2+ than O2 due to the much stronger binding affinity, and surface oxidation takes place only when there are enough reaction sites for the Hg2+ adsorption and surface oxidation simultaneously.
Fig. 7. Co-adsorption configurations with both Hg2+ and O2 above the S-vacancy sites on SS-MoS2 surface.
oxidation can only occur on the defective S-MoS2 surface, sequential coadsorption of H2O, Hg2+, and O2 on S-MoS2 surface have been investigated. Three adsorption modes have been considered for the further understanding of competition behaviors of the Hg2+ adsorption and oxidation on MoS2 surface. The adsorption behaviors have been simulated by placing H2O, Hg2+ and O2 right above the S-vacancy site on S-MoS2 surface, respectively. Fig. 6 shows the co-adsorption configurations with H2O (Fig. 6a), Hg2+ (Fig. 6b) and O2 (Fig. 6c) right above the S-vacancy site on SMoS2 monolayer, respectively. It was easy to find that no matter which one of the H2O, Hg2+, and O2 was placed on the defect site, the Hg2+ would be adsorbed onto the MoS2 surface firstly, indicating the strongest binding affinity between Hg2+ and S-MoS2 surface. Significantly, Hg2+ would be directly adsorbed onto the MoS2 surface without bonding to H2O or O2 molecule if the H2O was put above the S-missing vacancy defect. When placing the Hg2+ above the S-vacancy site initially, Hg2+ would combine with the O atom of H2O molecule to form a HgeO complex and then being adsorbed onto the MoS2 surface together. In comparison, when placing the O2 above the S-vacancy site, Hg2+ would connect to the O atom of O2 molecule to form a HgeO complex, after which this complex was adsorbed onto the MoS2 surface.
3.2. AFM observation of Hg2+ adsorption and oxidation on MoS2 surface AFM observations of Hg2+ adsorption and surface oxidation have been performed to verify our theoretical calculation results. Firstly, the behavior of MoS2 surface in pure water in the presence of dissolved oxygen was studied by AFM observation. Fig. 8 shows the AFM images
(b)
(a)
(c)
(d)
Fig. 8. AFM images of fresh MoS2 surface before and after exposure to pure water: (a) Freshly cleaved basal plane of MoS2, and after immersion in pure water for 1 h (b), 3 h (c), 5 h (d), respectively. 525
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
(b)
(a)
Fig. 9. AFM images of MoS2 surface after immersion in Hg2+ solution at concentration of 10 ppm for 1 h and 5 h.
(a)
(b)
(c)
(d)
Fig. 10. AFM images of fresh MoS2 surface before and after exposure to Hg2+ solution at concentration of 100 ppb. Freshly cleaved basal plane of MoS2 (a), and after immersion in Hg2+ solution for 1 h (b), 3 h (c), 5 h (d), respectively.
morphologies of protrusions on the surface of MoS2 were observed after immersion in Hg2+ solution for 1 h (Fig. 9a). The density of bright spots increased with increasing adsorption time. When the adsorb time extended to 5 h, the morphology of protrusions was still bright spot and nearly no short chainlike structures formed (Fig. 9b), indicating no surface oxidation occurred during the process of Hg2+ adsorption on the MoS2 nanosheet. Besides, the formation of more and more bright spot protrusions indicates the adsorption of Hg2+ on MoS2 surface, in agreement with previous study [11]. Therefore, it could be concluded that only Hg2+ adsorption occurred on MoS2 surface when it was exposed to Hg2+ solution at high concentration of 10 ppm. These results are in agreement with our theoretical calculation results that MoS2 surface has the preference adsorption for Hg2+ than O2 when there are limited reaction sites due to the much stronger binding affinity. Finally, the behavior of MoS2 surface in Hg2+ solution at low concentration was also studied by AFM observation. Fig. 10 shows the AFM images of freshly cleaved MoS2 sheets before and after immersion in Hg2+ solution at concentration of 100 ppb. The smooth surface of freshly cleaved MoS2 was captured in air (Fig. 10a). After immersion in
of freshly cleaved MoS2 sheets (Fig. 8a) and the surface of MoS2 after immersion in pure water for 1 h (Fig. 8b), 3 h (Fig. 8c), 5 h (Fig. 8d), respectively. Before exposure to pure water, the surface of MoS2 captured in air is very smooth. After the sample had been immersed in water for 1 h, the needlelike protrusions are observed on the surface of MoS2 (Fig. 8b). Some needlelike protrusions linked together to form short chainlike structures in 3 h (Fig. 8c), and increasing chainlike structures occurred after immersion in pure water for 5 h (Fig. 8d). Previous study has also observed this phenomenon and figured out that chainlike structures is caused by surface oxidation, and the oxidative product is MoO3·H2O crystals [25,47]. In our calculation results, the chemical bond between Mo atom and O atom has also been observed, in agreement with this AFM results. Thus, it can be concluded that the formation of chainlike structures protrusions stands for the occurrence of surface oxidation on MoS2 nanosheet. Then, the behavior of MoS2 surface in Hg2+ solution at high concentration was studied by AFM observation. Fig. 9 shows the AFM images of MoS2 surface after immersion in Hg2+ solution at concentration of 10 ppm for 1 h and 5 h. A number of bright spots 526
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
Hg2+ solution for 1 h, the morphology of protrusions on MoS2 sheet was mainly bright spots and some needlelike protrusions (Fig. 10b). When immersed 3 h or 5 h, two changes have taken place. One is the appearance of an increasing number of bright spots, and the other is the formation of short chainlike structures linked together from needlelike protrusions (Fig. 10c–d). These phenomena demonstrate that both Hg2+ adsorption and oxidation has taken place on the MoS2 surface. According to our theoretical calculation result shown in Fig. 6 and Fig. 7, MoS2 surface has the preference for Hg2+ adsorption when there are limited reaction sites due to the much stronger binding affinity, and surface oxidation takes place only when there are redundant reaction sites for Hg2+ adsorption and surface oxidation simultaneously. Hg2+ solution with low concentration means excess surface reaction site, thus there are redundant reaction sites provided for the oxidation. These AFM images have verified the correctness of the DFT calculations results. In conclusion, high concentration of Hg2+ solution means insufficient reaction sites on MoS2 surface, the only observed Hg2+ adsorption can be interpreted by the stronger affinity between Hg2+ and MoS2 based on our calculation results. While in Hg2+ solution with low concentration, there are surplus surface reaction sites, thus surface oxidation can also take place in addition to Hg2+ adsorption, which can be proved both by our calculation results and AFM images in this work.
[7] H.Y. Jeong, K. Sun, K.F. Hayes, Microscopic and spectroscopic characterization of Hg(II) immobilization by mackinawite (FeS), Environ. Sci. Technol. 44 (2010) 7476–7483, https://doi.org/10.1021/es100808y. [8] M. Fojtů, W.Z. Teo, M. Pumera, Environmental impact and potential health risks of 2D nanomaterials, Environ. Sci. Nano. 4 (2017) 1617–1633, https://doi.org/10. 1039/C7EN00401J. [9] Z. Wang, B. Mi, Environmental applications of 2D molybdenum disulfide (MoS2) Nanosheets, Environ. Sci. Technol. 51 (2017) 8229–8244, https://doi.org/10.1021/ acs.est.7b01466. [10] Q. Wang, L. Peng, Y. Gong, F. Jia, S. Song, Y. Li, Mussel-inspired Fe3O4 @ Polydopamine(PDA)-MoS2 core–shell nanosphere as a promising adsorbent for removal of Pb2+ from water, J. Mol. Liq. 282 (2019) 598–605, https://doi.org/10. 1016/j.molliq.2019.03.052. [11] F. Jia, X. Zhang, S. Song, AFM study on the adsorption of Hg2+ on natural molybdenum disulfide in aqueous solutions, Phys. Chem. Chem. Phys. 19 (2017) 3837–3844, https://doi.org/10.1039/C6CP07302F. [12] L. Zhi, W. Zuo, F. Chen, B. Wang, 3D MoS2 composition aerogels as chemosensors and adsorbents for colorimetric detection and high-capacity adsorption of Hg2+, ACS Sustain. Chem. Eng. 4 (2016) 3398–3408, https://doi.org/10.1021/ acssuschemeng.6b00409. [13] F. Jia, Q. Wang, J. Wu, Y. Li, S. Song, Two-dimensional molybdenum disulfide as a superb adsorbent for removing Hg2+ from water, ACS Sustain. Chem. Eng. 5 (2017) 7410–7419, https://doi.org/10.1021/acssuschemeng.7b01880. [14] R. Addou, L. Colombo, R.M. Wallace, Surface defects on natural MoS2, ACS Appl. Mater. Interfaces 7 (2015) 11921–11929, https://doi.org/10.1021/acsami. 5b01778. [15] W. Zhou, X. Zou, S. Najmaei, Z. Liu, Y. Shi, J. Kong, J. Lou, P.M. Ajayan, B.I. Yakobson, J.C. Idrobo, Intrinsic structural defects in monolayer molybdenum disulfide, Nano Lett. 13 (2013) 2615–2622, https://doi.org/10.1021/nl4007479. [16] J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, Controllable disorder engineering in oxygen-incorporated MoS 2ultrathin nanosheets for efficient hydrogen evolution, J. Am. Chem. Soc. 135 (2013) 17881–17888, https://doi.org/10.1021/ja408329q. [17] K. Zhang, H.J. Kim, J.T. Lee, G.W. Chang, X. Shi, W. Kim, M. Ma, K.J. Kong, J.M. Choi, M.S. Song, J.H. Park, Unconventional pore and defect generation in molybdenum disulfide: application in high-rate lithium-ion batteries and the hydrogen evolution reaction, ChemSusChem. 7 (2014) 2489–2495, https://doi.org/ 10.1002/cssc.201402372. [18] F. Jia, C. Liu, B. Yang, S. Song, Microscale control of edge defect and oxidation on molybdenum disulfide through thermal treatment in air and nitrogen atmospheres, Appl. Surf. Sci. 462 (2018) 471–479, https://doi.org/10.1016/j.apsusc.2018.08. 166. [19] S. Kc, R.C. Longo, R. Addou, R.M. Wallace, K. Cho, Impact of intrinsic atomic defects on the electronic structure of MoS2 monolayers, Nanotechnology. 25 (2014), https://doi.org/10.1088/0957-4484/25/37/375703. [20] J.F. Paul, E. Payen, Vacancy formation on MoS2 hydrodesulfurization catalyst: DFT study of the mechanism, J. Phys. Chem. B 107 (2003) 4057–4064, https://doi.org/ 10.1021/jp027668f. [21] D. Liu, Y. Guo, L. Fang, J. Robertson, Sulfur vacancies in monolayer MoS2 and its electrical contacts, Appl. Phys. Lett. 103 (2013) 183113, , https://doi.org/10.1063/ 1.4824893. [22] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X.W.D. Lou, Y. Xie, Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution, Adv. Mater. 25 (2013) 5807–5813, https://doi.org/10.1002/adma.201302685. [23] F. Jia, K. Sun, B. Yang, X. Zhang, Q. Wang, S. Song, Defect-rich molybdenum disulfide as electrode for enhanced capacitive deionization from water, Desalination. 446 (2018) 21–30, https://doi.org/10.1016/j.desal.2018.08.024. [24] H. Liu, N. Han, J. Zhao, Atomistic insight into the oxidation of monolayer transition metal dichalcogenides: from structures to electronic properties, RSC Adv. 5 (2015) 17572–17581, https://doi.org/10.1039/C4RA17320A. [25] X. Zhang, F. Jia, B. Yang, S. Song, Oxidation of molybdenum disulfide sheet in water under in situ atomic force microscopy observation, J. Phys. Chem. C 121 (2017) 9938–9943, https://doi.org/10.1021/acs.jpcc.7b01863. [26] Z. Xian, Y. Hao, Y. Zhao, S. Song, Quantitative determination of isomorphous substitutions on clay mineral surfaces through AFM imaging: a case of mica, Colloids Surf. A Physicochem. Eng. Asp. 533 (2017) 55–60, https://doi.org/10. 1016/j.colsurfa.2017.08.024. [27] H. Yi, X. Zhang, Y. Zhao, Y. Liu, S. Song, A novel method for the quantitative determination of defects on graphene surfaces, J. Colloid Interface Sci. 499 (2017) 62–66, https://doi.org/10.1016/j.jcis.2017.03.094. [28] H. Yi, X. Zhang, Y. Zhao, L. Liu, S. Song, Molecular dynamics simulations of hydration shell on montmorillonite (001) in water, Surf. Interface Anal. 48 (2016) 976–980, https://doi.org/10.1002/sia.6000. [29] X. Zhang, H. Yi, Y. Zhao, F. Min, S. Song, Study on the differences of Na- and Camontmorillonites in crystalline swelling regime through molecular dynamics simulation, Adv. Powder Technol. 27 (2016) 779–785, https://doi.org/10.1016/j. apt.2016.03.005. [30] H. Yi, F. Jia, Y. Zhao, W. Wang, S. Song, H. Li, C. Liu, Surface wettability of montmorillonite (001) surface as affected by surface charge and exchangeable cations: a molecular dynamic study, Appl. Surf. Sci. 459 (2018) 148–154, https://doi. org/10.1016/j.apsusc.2018.07.216. [31] Z. Liu, W. Peng, Z. Xu, K. Shih, J. Wang, Z. Wang, X. Lv, J. Chen, X. Li, Molybdenum disulfide-coated lithium vanadium fluorophosphate anode: experiments and firstprinciples calculations, ChemSusChem. 9 (2016) 2122–2128, https://doi.org/10. 1002/cssc.201600370.
4. Conclusions (1) S-vacancy defect on MoS2 surface makes MoS2 more reactive due to introducing the local density of states around the Fermi level. (2) Perfect and defective MoS2 surface both has very weak adsorption energy of H2O but has very strong binding affinity to Hg2+, while surface oxidation can only take place in the presence of S-vacancy defect. (3) Hg2+ can be absorbed onto the MoS2 surface in two ways which depends on the distribution of Hg2+, O2 and H2O around the defect sites. One is to form HgeS complex with S atom on MoS2 surface, and another is to form HgeO complex with H2O or O2 and then being absorbed onto the MoS2 surface. (4) MoS2 surface has the preference for Hg2+ adsorption when there are limited reaction sites due to the much stronger binding affinity, and surface oxidation takes place only when there are redundant reaction sites for Hg2+ adsorption and surface oxidation simultaneously. Acknowledgements The financial supports to this work from the National Natural Science Foundation of China under the projects No. 51674183 and 51704220 were gratefully acknowledged. References [1] P. Miretzky, A.F. Cirelli, Hg(II) removal from water by chitosan and chitosan derivatives: a review, J. Hazard. Mater. 167 (2009) 10–23, https://doi.org/10.1016/j. jhazmat.2009.01.060. [2] L. Trasande, P.J. Landrigan, C. Schechter, Public health and economic consequences of methyl mercury toxicity to the developing brain, Environ. Health Perspect. 113 (2005) 590–596, https://doi.org/10.1289/ehp.7743. [3] K. Ai, C. Ruan, M. Shen, L. Lu, MoS2 nanosheets with widened interlayer spacing for high-efficiency removal of mercury in aquatic systems, Adv. Funct. Mater. 26 (2016) 5542–5549, https://doi.org/10.1002/adfm.201601338. [4] M.M. Hyland, G.E. Jean, G.M. Bancroft, XPS and AES studies of hg(II) sorption and desorption reactions on sulphide minerals, Geochim. Cosmochim. Acta 54 (1990) 1957–1967, https://doi.org/10.1016/0016-7037(90)90264-L. [5] D.M. Manohar, K.A. Krishnan, T.S. Anirudhan, Removal of mercury (II) from aqueous solutions and chlor-alkali industry wastewater using, Water Res. 36 (2002) 1609–1619, https://doi.org/10.1016/S0043-1354(01)00362-1. [6] K. Anoop Krishnan, T.S. Anirudhan, Removal of mercury(II) from aqueous solutions and chlor-alkali industry effluent by steam activated and sulphurised activated carbons prepared from bagasse pith: kinetics and equilibrium studies, J. Hazard. Mater. 92 (2002) 161–183, https://doi.org/10.1016/S0304-3894(02)00014-6.
527
Applied Surface Science 483 (2019) 521–528
H. Yi, et al.
https://doi.org/10.1007/s12274-014-0658-x. [40] Y.Y. Chen, M. Dong, Z. Qin, X.D. Wen, W. Fan, J. Wang, A DFT study on the adsorption and dissociation of methanol over MoS2 surface, J. Mol. Catal. A Chem. 338 (2011) 44–50, https://doi.org/10.1016/j.molcata.2011.01.024. [41] S. Kc, R.C. Longo, R.M. Wallace, K. Cho, Surface oxidation energetics and kinetics on MoS2 monolayer, J. Appl. Phys. 117 (2015), https://doi.org/10.1063/1. 4916536. [42] A.V. Krivosheeva, V.L. Shaposhnikov, V.E. Borisenko, J.L. Lazzari, C. Waileong, J. Gusakova, B.K. Tay, Theoretical study of defect impact on two-dimensional MoS2, J. Semicond. 36 (2015) 1–6, https://doi.org/10.1088/1674-4926/36/12/122002. [43] T. Liang, W.G. Sawyer, S.S. Perry, S.B. Sinnott, S.R. Phillpot, Energetics of oxidation in MoS2 nanoparticles by density functional theory, J. Phys. Chem. C 115 (2011) 10606–10616, https://doi.org/10.1021/jp110562n. [44] R. Kronberg, M. Hakala, N. Holmberg, K. Laasonen, Hydrogen adsorption on MoS2 -surfaces: a DFT study on preferential sites and the effect of sulfur and hydrogen coverage, Phys. Chem. Chem. Phys. 19 (2017) 16231–16241, https://doi.org/10. 1039/C7CP03068A. [45] C. Ataca, S. Ciraci, Dissociation of H2O at the vacancies of single-layer MoS2, Phys. Rev. B Condens. Matter Mater. Phys. 85 (2012) 1–6, https://doi.org/10.1103/ PhysRevB.85.195410. [46] M. Ghorbani-Asl, A.N. Enyashin, A. Kuc, G. Seifert, T. Heine, Defect-induced conductivity anisotropy in MoS2 monolayers, Phys. Rev. B 88 (2013) 7, , https://doi. org/10.1103/PhysRevB.88.245440. [47] Y. Ryu, W. Kim, S. Koo, H. Kang, K. Watanabe, T. Taniguchi, S. Ryu, Interfaceconfined doubly anisotropic oxidation of two-dimensional MoS2, Nano Lett. 17 (2017) 7267–7273, https://doi.org/10.1021/acs.nanolett.7b02621.
[32] W. Song, T. Yang, X. Wang, Y. Sun, Y. Ai, G. Sheng, T. Hayat, X. Wang, Experimental and theoretical evidence for competitive interactions of tetracycline and sulfamethazine with reduced graphene oxides, Environ. Sci. Nano. 3 (2016) 1318–1326, https://doi.org/10.1039/c6en00306k. [33] T. Hüffer, H. Sun, J.D. Kubicki, T. Hofmann, M. Kah, Interactions between aromatic hydrocarbons and functionalized C60 fullerenes – insights from experimental data and molecular modelling, Environ. Sci. Nano. 4 (2017) 1045–1053, https://doi. org/10.1039/C7EN00139H. [34] N. Ding, X. Chen, C.-M.L. Wu, Interactions between polybrominated diphenyl ethers and graphene surface: a DFT and MD investigation, Environ. Sci. Nano. 1 (2014) 55–63, https://doi.org/10.1039/C3EN00037K. [35] N. Yu, L. Wang, M. Li, X. Sun, T. Hou, Y. Li, Molybdenum disulfide as a highly efficient adsorbent for non-polar gases, Phys. Chem. Chem. Phys. 17 (2015) 11700–11704, https://doi.org/10.1039/C5CP00161G. [36] C.H. Zhao, J.H. Chen, B.Z. Wu, X.H. Long, Density functional theory study on natural hydrophobicity of sulfide surfaces, Trans. Nonferrous Met. Soc. China (English Ed. 24 (2014) 491–498. doi:https://doi.org/10.1016/S1003-6326(14) 63087-9. [37] K.K. Ghuman, S. Yadav, C.V. Singh, Adsorption and dissociation of H2O on monolayered MoS2 edges: energetics and mechanism from ab initio simulations, J. Phys. Chem. C 119 (2015) 6518–6529, https://doi.org/10.1021/jp510899m. [38] S. Tongay, J. Zhou, C. Ataca, J. Liu, J.S. Kang, T.S. Matthews, L. You, J. Li, J.C. Grossman, J. Wu, Broad-range modulation of light emission in two-dimensional semiconductors by molecular physisorption gating, Nano Lett. 13 (2013) 2831–2836, https://doi.org/10.1021/nl4011172. [39] S. Jiang, R. Cheng, R. Ng, Y. Huang, X. Duan, Highly sensitive detection of mercury (II) ions with few-layer molybdenum disulfide, Nano Res. 8 (2015) 257–262,
528