Preparation of MoS2-based polydopamine-modified core–shell nanocomposites with elevated adsorption performances

New molybdenum disulfide (MoS2)-based core–shell nanocomposite materials were successfully prepared through the self-assembly of mussel-inspired chemistry. Characterization by Fourier transform infrared, thermogravimetric analysis, scanning electron microscope and transmission electron microscopy revealed that the surface of the flaked MoS2 was homogeneously coated with a thin layer of polydopamine (PDA). Dye adsorption performances of the synthesized MoS2–PDA nanocomposites were investigated at different pH values and reaction times. Compared with pure MoS2 nanosheets, the obtained core–shell nanocomposites showed elevated adsorption performances and high stability, indicating their potential applications in wastewater treatment and composite materials.


Introduction
Over the past several decades, graphene has been attracting a great deal of attention due to the unusual properties associated with its ultrathin structure. 1-7 Among other twodimensional layer materials, [8][9][10][11] molybdenum disulde (MoS 2 ) is one of the most attractive and has a similar structure to graphite. [12][13][14][15][16] MoS 2 has a sandwich structure consisting of two layers of sulfur atoms and a middle layer of molybdenum atoms, which are bonded by van der Waals forces. 17-20 MoS 2 nanosheets have many excellent physical and chemical properties, and demonstrate important applications in sensors, [21][22][23][24] optoelectronic devices, 25,26 catalysis 27,28 and other elds. 29 In addition, MoS 2 nanocomposites have attracted wide study in the eld of polymer nanocomposites. [30][31][32][33] For example, Li et al. investigated polyethylene glycol (PEG)-modied MoS 2 surfaces, 34 and the obtained products showed good removal efficiency toward some dyes.
In recent years, mussel-inspired chemistry has become a hot research topic in materials science, chemistry and other elds. [35][36][37] In the marine environment, mussels can secrete proteins through their feet, which have excellent adhesion and good biocompatibility. Dopamine, as an imitation musselprotein material, has strong adhesion properties through a complex self-assembly process to form a polydopamine (PDA) coating with various functions. 38 The formed PDA layer can be used to modify the surface of inorganic and organic materials, so it demonstrates wide prospects for application in the elds of separation membranes, adsorbent materials, biomedical materials, biological binders and so on. [39][40][41][42] Now core-shell materials have shown great potential applications in biology, electricity, catalysis and so on. [43][44][45][46] For example, Zhao et al. synthesized core-shell diamond-based nanocomposites, exhibiting high activity and high catalytic performance. 47 Although the catalytic performance of prepared composite materials may seem ideal, there are still some deciencies, such as high cost and harsh operating conditions. Thus, musselinspired chemistry has the advantages of mild reaction conditions, a wide range of applications and diverse functions, etc. On the other hand, core-shell nanocomposites have unique structural characteristics, integrating the properties of both internal and external materials.
In this work, we used PDA to modify the surface of MoS 2 nanosheets to synthesize core-shell nanocomposites. The asprepared composites were characterized by a series of morphological and spectral characterization techniques. The results showed that we had successfully prepared MoS 2 -PDA polymeric materials. The obtained MoS 2 -PDA composites were used as adsorbents for the removal of methylene blue and Safranine T, and they showed enhanced adsorption ability.

Fabrication of the MoS 2 -PDA nanocomposites
The synthesis of core-shell MoS 2 -PDA nanocomposite was undertaken according to the method given in previous literature. 48 Briey, 300 mg of MoS 2 nanosheets were rst dispersed in 100 mL of Tris buffer (10 mM, pH ¼ 8.5) with sonication for 10 minutes, and then 200 mg of dopamine was added to the above solution with stirring. Aer stirring at room temperature in the dark for 12 h, 24 h, and 48 h, the obtained composite products were washed several times with water and ethanol, respectively, then centrifuged at 6000 rpm for 10 min. Finally, the products were freeze-dried for 2-3 days for the next experiments.

Adsorption performance test
In order to investigate the dye adsorption performances of MoS 2 nanosheets and MoS 2 -PDA nanocomposites, we selected MB and ST as model dyes to complete the experiment. 10 mg of MoS 2 nanosheets and 10 mg of the prepared MoS 2 -PDA nanocomposites were added to 100 mL of MB solution (8 mg L À1 ) and ST solution (30 mg L À1 ), respectively. Aer different adsorption time intervals, the samples were centrifuged and the supernatant analyzed by a UV-Vis spectrophotometer (664 nm for MB; 530 nm for ST). In addition, we also studied the effect of MoS 2 and MoS 2 -PDA on MB adsorption with different initial solution pH values. The pH of the initial solution was adjusted to values of 2-11 by adding diluted aqueous HCl solution or NaOH solution. 20 All experiments were carried out at room temperature under dark conditions. The amount of adsorbed dye per unit mass of adsorbent q t (mg g À1 ) at time t (min) is calculated from the following formula: where C 0 is the initial concentration of the adsorption solution (mg L À1 ), C t is the concentration of the adsorption solution at time t (mg L À1 ), m is the total MoS 2 sample added (g), and V is the volume of the adsorption solution (L).

Characterization
The microstructures of the samples were obtained using a eldemission scanning electron microscope (SEM) (S-4800II, Hitachi, Japan) equipped with an energy dispersive X-ray spectroscope (EDS) and a transmission electron microscope (TEM) (HT7700, Hitachi High-Technologies Corporation, Japan) with an accelerating voltage of 20 kV. Thermogravimetric analysis (TGA) was performed by a NETZSCH STA 409 PC Luxx simultaneous thermal analyzer (Netzsch Instruments Manufacturing Co, Ltd, Germany). BET measurements (NOVA 4200-P, US) were taken to characterize the specic surface areas and pore diameter distribution. Fourier transform infrared (FTIR) spectra were accomplished on a Thermo Nexus 470 FT-IR spectrometer (KBr disk). X-ray photoelectron spectroscopy (XPS) was performed using a Bragg diffraction setup (SMART LAB, Rigaku, Japan) with an Al Ka X-ray source. The adsorption experiments were monitored using a Shimadzu UV2550 spectrophotometer. All experimental processes were undertaken in a beaker under dark conditions. The synthesized composite materials were completely dewatered using an FD-1C-50 freeze dryer (Beijing Boli Experimental Instruments Co., Ltd., China). All aqueous solutions were prepared with water puried in a double-stage Millipore Milli-Q Plus purication system. In the TEM image, it was obvious that the outer edge of the MoS 2 nanosheets demonstrated some ultra-thin layer structures, which indicated that the PDA layer had been successfully anchored on the surface of the MoS 2 nanosheets. Moreover, the typical XRD pattern of MoS 2 (the inset image of Fig. 2a) and the EDX pattern of MoS 2 -PDA-48 (Fig. 2f) also provided obvious characteristics and suggested the above speculations.

Characterization of nanocomposites
In addition, the thermal stability of MoS 2 and the obtained MoS 2 -PDA composites were characterized and are shown in Fig. 3. According to the TG curves, MoS 2 showed a low mass loss In order to study the microstructure characteristics of MoS 2 and MoS 2 -PDA-48, Fig. 4a shows the BET measurements using N 2 adsorption-desorption isotherms. The adsorption-desorption isotherms of both MoS 2 and MoS 2 -PDA-48 belonged to type IV isotherms. The hysteresis loop of MoS 2 has an abrupt increase in the amount of adsorption at P/P 0 ¼ 0.6-0.95, which       Table 1. The specic surface area of MoS 2 -PDA-48 was calculated to be 30.57 m 2 g À1 , which was much larger than that of MoS 2 (25.83 m 2 g À1 ). The higher specic surface area can increase the number of active sites on the surface of the MoS 2 -PDA composite and enhance the chances of dye molecules anchoring at active sites, thus making adsorbents with better adsorption properties. FT-IR spectra of MoS 2 and the MoS 2 -PDA-48 composite are demonstrated in Fig. 5. The curve of MoS 2 -PDA-48 shows a characteristic band at 3378 cm À1 , which could be ascribed to the stretch vibrations of -NH 2 , -NH-, and -OH. [49][50][51][52][53][54][55] The characteristic peak at 1607 cm À1 could be deduced as a C]C stretching vibration in the benzene ring. At the same time, the weak peaks around 1285 and 876 cm À1 could be assigned to C-OH stretching vibration from the catechol groups and C-O stretching, respectively, which indicate that the surface of MoS 2 was successfully modied by a PDA layer. [56][57][58][59] In addition, the XPS analysis (Fig. 6) demonstrates the composition of MoS 2 and MoS 2 -PDA-48. Fig. 6a demonstrates the characteristic peaks in the curve of the MoS 2 -PDA-48 composite, such as Mo (3d), O (1s), C (1s), and N (1s). Additionally, the high resolution XPS spectra are shown in Fig. 6b-f. Compared with pure MoS 2 , the relative intensity of C 1s of MoS 2 -PDA-48 decreased and the O 1s peak increased signicantly (Fig. 6b and c). In the meantime, it was clear that the relative intensity of the N 1s of MoS 2 -PDA-48 at 400 eV increased obviously in Fig. 6d. In addition, as shown in Fig. 6e and f, the relative intensity of Mo 3d and S 2p in MoS 2 -PDA-48 was signicantly reduced compared with the MoS 2 nanosheets, which also indicates that the PDA layer was successfully coated onto the surface of the MoS 2 nanosheets. On the other hand, the relative elemental analysis data based on XPS analysis is shown in Table 2. The atom percentages of Mo and S in the table fell from 13.99% and 27.71% to 3.51% and 9.11%, respectively. At the same time, the atomic percentage of N in MoS 2 -PDA-48 accounted for 7.58%, indicating successful PDA modication on the surface of the MoS 2 nanosheets.

Adsorption performances
In order to study the adsorption properties of the prepared MoS 2 -PDA nanocomposites, two model dyes (MB and ST) were     Table 3. It should be noted that PDA is a kind of hydrophilic and adhesive material. Aer PDA anchored onto the surface of the MoS 2 nanosheets, the core-shell nanostructures formed demonstrated a porous surface with a number of enhanced active sites to improve the adsorption performance of the composites. In addition, the adsorption kinetic process can be described by classical kinetic models as follows: The pseudo-rst-order model can be represented by eqn (2): The pseudo-second-order model can be represented by eqn (3): where q e is the equilibrium adsorption capacity, mg g À1 ; q t is the adsorption capacity at time t, mg g À1 ; the k 1 and k 2 values are the kinetic rate constants. Kinetic results (Table 3) could be better characterized by a pseudo-rst-order model with a correlation coefficient (R 2 > 0.9665) or a pseudo-second-order model with a correlation coefficient (R 2 > 0.8101). Comparing the above adsorption data, it could easily be observed that MoS 2 -PDA-48 showed the best adsorption capacity, which could be related to the rich functional groups on the surface of PDA. And the ability to remove MB seemed higher than that for ST. In addition, the gradual increase in the maximum adsorption indicated that the experimental method used in this study is feasible. Fig. 8 shows the effect of solution pH on the adsorption of MB to MoS 2 -PDA-48. It can be seen that as the pH of the solution increased from 2 to 11, the adsorption capacity showed an increasing trend from 36.02 mg g À1 to 55.32 mg g À1 . However, this process varied with different pH values. This difference can be presumed to be a result of the protonation of the functional groups on the nanocomposite surface as well as the p-p stacking and electrostatic interactions with the MB molecules. 60 In addition, in order to demonstrate the reuse of the obtained composites, 10 consecutive cycles were repeated using the same MoS 2 -PDA-48 nanocomposite and fresh MB solution (Fig. 9). This showed that the adsorption capacity of the MoS 2 -PDA-48 nanocomposite still retained a removal rate of 87.4% towards    Table 4 lists the MB adsorptions of the relevant materials reported in the literature. [61][62][63][64][65][66] In contrast, the present prepared MoS 2 -PDA composite materials showed a larger adsorption capacity and eco-friendly preparation process, demonstrating wide application in wastewater treatment and self-assembled core-shell composite materials.

Conclusions
In summary, we have synthesized core-shell MoS 2 -PDA nanocomposites simply by using mussel-inspired chemistry. A series of characterization techniques demonstrated that the PDA was successfully coated onto the surface of the MoS 2 nanosheets. The prepared MoS 2 -PDA composites showed effective removal capacities towards two model dyes. The adsorption process was illustrated by pseudo-rst-order and pseudo-second-order kinetic models. The present research work is expected to show potential applications in wastewater treatment and selfassembled core-shell composite materials.

Conflicts of interest
There are no conicts to declare.