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Magnetic Co-Doped MoS2 Nanosheets for Efficient Catalysis of Nitroarene Reduction

Co-doped MoS2 nanosheets have been synthesized through the hydrothermal reaction of ammonium tetrathiomolybdate and hydrazine in the presence of cobalt acetate. These nanosheets exhibit a dominant metallic 1T phase with cobalt ion-activated defective basal planes and S-edges. In addition, the nanosheets are dispersible in polar solvents like water and methanol. With increased active sites, Co-doped MoS2 nanosheets exhibit exceptional catalytic activity in the reduction of nitroarenes by NaBH4 with impressive turnover frequencies of 8.4, 3.2, and 20.2 min–1 for 4-nitrophenol, 4-nitroaniline, and nitrobenzene, respectively. The catalyst is magnetic, enabling its easy separation from the reaction mixture, thus making its recycling and reusability simple and efficient. The enhanced catalytic activity of the Co-doped 1T MoS2 nanosheets in comparison to that of undoped 1T MoS2 nanosheets suggests that incorporation of cobalt ions in the MoS2 lattice is the major reason for the efficiency of the catalyst. The dopant, Co, plays a dual role. In addition to providing active sites where electron transfer is assisted through redox cycling, it renders the nanosheets magnetic, enabling their easy removal from the reaction mixture.


INTRODUCTION
Two-dimensional (2D) MoS 2 nanosheets 1 have garnered interest as a potential noble-metal-free catalyst for the electrochemical generation of hydrogen from water 2−4 and hydrodesulfurization of petroleum. 5,6 Theoretical and experimental studies indicate that the catalytic activity of the thermodynamically stable 2H polymorph of MoS 2 is associated with its metallic edges, whereas its semiconducting basal plane is catalytically inert. 2,4 In this context, nanostructures of MoS 2 , amorphous 7−9 / crystalline, 10−13 and vertically aligned structures 14,15 have been explored to maximize the number of active edge sites. MoS 2 is also hybridized with conducting/semiconducting/magnetic materials (graphene 15 4 27 ) to enhance the catalytic activity through synergetic coupling effects. Metastable, intrinsically metallic, octahedral 1T MoS 2 obtained through exfoliation of trigonal prismatic 2H MoS 2 has proven to be an excellent catalyst for H 2 evolution reactions as the 1T phase facilitates electrode kinetics by increasing the electric conductivity and proliferation of the catalytic active sites. 28−30 Introducing transition metal ions (Co, Ni, Fe) into the MoS 2 matrix has been the classic route to maximize the catalytic activity of MoS 2 , as the doped ions alter the electronic properties at the coordinatively unsaturated catalytic S-edges. 10,31,32 These strategies have been designed, largely, to either optimize the density of active edge sites by reducing the dimensions along the z direction or xy direction (nanostructures) 33 or increase the conductivity by stabilizing the 1T MoS 2 polytype. 19,28,29 The question is, would it be possible to tune both the structural features and electronic properties simultaneously to increase the catalytic active sites? Doping 2H MoS 2 with Co has been shown to increase its catalytic efficiency through increased active sites in the basal planes in addition to edges. 34 It would be of interest to prepare Co-doped 1T MoS 2 because in addition to all of the above effects, there would be increased conductivity.
One of the standard reactions to test the electron transfer catalytic action is the reduction of nitroarenes by NaBH 4 . Nitroarenes, with aromatic rings associated with H-bonding −NH 2 and −OH groups, enable the reduction to be carried out in water, making it a green reaction. This study demonstrates a single-step robust strategy to synthesizing 1T Co-doped MoS 2 nanosheets. With increased active sites, Co-doped MoS 2 nanosheets exhibit exceptional catalytic activity in the reduction of nitroarenes. The observed turnover frequency (TOF) is far superior in comparison to that of other MoS 2 architectures and noble-metal-based catalysts, reported so far.

RESULTS AND DISCUSSION
The XRD pattern of as-prepared Co-doped MoS 2 nanosheets ( Figure 1A,a) exhibits a broad 002 reflection at 11.0 Å, indicating the presence of guest species in the interlayer. 35,36 The guest entity could possibly be NH 3 /NH 4 + ions released as byproducts of hydrazine used as a reductant in the hydrothermal reaction.
On treating Co-doped MoS 2 nanosheets with 1 N HCl solution, the 002 reflection ( Figure 1A,b) disappears, indicating deintercalation of the guest species. However, the low intensity of the 002 reflection or its absence ( Figure 1A,a,b) suggests that Co-doped MoS 2 nanosheets are poorly ordered along the stacking direction and comprise largely exfoliated layers. The asymmetric 2D reflections at 2θ = 33 and 57°reveal the presence of stacking faults 37,38 within the few-layered Co-doped MoS 2 . The undoped MoS 2 is also poorly ordered and exhibits increased basal spacing due to NH 3 /NH 4 + intercalation ( Figure  1A,c).
The Raman spectrum of Co-doped MoS 2 ( Figure 1B) exhibits the in-plane E 2g (380 cm −1 ) and out-of-plane A 1g (406 cm −1 ) Mo−S vibration modes, characteristic of the MoS 2 layered structure. An additional peak at 220 cm −1 in Figure  S1 (Supporting Information, SI) indicates the presence of 1T polytype. Increased full width at half-maximum and the shift in the A 1g and E 2g modes of Co-doped MoS 2 in comparison to those of bulk MoS 2 clearly indicate softening of A 1g and E 2g modes and phonon confinement that is expected for mono-to few-layer MS 2 , thus indicating that the Co-doped MoS 2 comprises mono to few layers. 39,40 The chemical composition of the Co-doped MoS 2 was further probed by X-ray photoelectron spectroscopy (XPS). Mo 3d and S 2p spectra (Figure 2a Table 1) correspond to Mo 4+ and S 2− of the 1T polytype of MoS 2 . A small proportion of the 2H polytype coexists with the 1T phase. 41 The N 1s spectrum ( Figure 2c; Table 1) indicates the presence of NH 3 and NH 4 + ions, which are accommodated in the interlayer of MoS 2 nanosheets, as suggested by the XRD pattern ( Figure  1a). 35 The core-level Co 2p spectra (Figure 2d; Table 1) confirm the presence of Co 2+ species. The XRD pattern ( Figure  1a)  To understand the nature of the chemical environment of Co 2+ , the Co-doped MoS 2 was treated with 1 N HCl when the intercalated or undoped Co 2+ species, if any, was expected to be leached out. Cobalt estimation of the leachate showed that only about 30% of the cobalt could be leached out by acid. This was further confirmed by the atomic percentages (Co-3.36, Mo-27.11, S-69.54) in the acid-leached Co-doped MoS 2 arrived at from XPS data. The composition of the acid-leached sample is Co 0.1 Mo 0.78 S 2 . The XPS spectra of acid-treated Co-doped MoS 2 (SI, Figure S2) indicate that whereas the nitrogen-containing species, NH 3 and NH 4 + ions, are absent, Co 2+ and 1T phase of MoS 2 nanosheets are retained. These further suggest that Co 2+ is present in the MoS 2 lattice. Hydrothermally synthesized MoS 2 has been shown to have a defective basal plane as well as unsaturated S-edges. 42 Recent studies by Liu et al. 34 demonstrate that Co 2+ is doped at S vacancies in basal planes as well as at the unsaturated S-edges.
The magnetic hysteresis loop measured on the powder sample indicates a weak ferromagnetic behavior ( Figure 3). The saturation magnetization (M S ) at 300 K of Co-doped MoS 2 nanosheets is 0.0029 emu g −1 , which is comparable to that of exfoliated 1T MoS 2 reported in the literature. 43 Because our control 1T MoS 2 is nonmagnetic, it is fair to assume that the magnetism in Co-doped MoS 2 arises as a consequence of doping. The magnetism in monolayer MoS 2 and its doped analogues depends on the nature of edges, type of edge defects, lattice strain, and the dopant concentration. Theoretical  calculations by Wang et al. 44 reveal that low concentrations of 4 and 6% of Co 2+ doping in the Mo vacant sites of the basal planes result in stable magnetic moments at room temperature. Yun et al. 45 and Saab et al. 46 also reported tuning of electronic and magnetic properties due to doping of metal ions in the MoS 2 lattice. The very low M S observed for Co-doped MoS 2 suggests that the weak ferromagnetism here originates from the strain in the layer rather than from ordering of Co 2+ ions. Codoped MoS 2 ( Figure 3) as well as the acid-leached product is weakly magnetic, suggesting that Co 2+ ions are doped in the MoS 2 layers. In addition, the presence of Co 2+ in the MoS 2 lattice could be the reason for the retention of metastable 1T structure even after deintercalation of the intercalants (SI, Figure S2). All of these results indicate that Co 2+ Figure 4d clearly shows that the layers are crystalline, exhibiting (100) lattice planes. Except for the circled regions representing the 2H phase, the layers largely exist as the 1T polytype. 43 Figure 5 schematically depicts the catalytic reduction of nitroarenes. Catalytic performance of Co-doped MoS 2 in the reduction of 4-nitrophenol (4-NP) in water is summarized in Figure 6a,d−f. UV−visible absorption spectra (Figure 6a) of the reaction mixture indicate that 4-nitrophenol converts to 4aminophenol within 7 min. The absorption peak at 400 nm is due to the nitro phenolate ion, and the intensity of this peak decreases with time and disappears completely at 7 min. Peaks at 235 and 308 nm emerge due to the formation of amino  Table 2.
Apart from exhibiting a high TOF, the catalyst also has the advantage of recyclability. As the catalyst is weakly magnetic, it is easily separated from the reaction mixture using a strong magnet, enabling easy recycling ( Figure 5). The catalytic activity of Co-doped MoS 2 remains nearly constant over a number of cycles (Figure 6f). The morphology and composition of the catalyst remain almost the same after six cycles of catalysis. Earlier attempts to make MoS 2 -based catalysts magnetic have been through hybridization of MoS 2 nanosheets with magnetic nanoparticles such as Fe 3 O 4 . 27 One  Figure 3. Hysteresis loop of the Co-doped MoS 2 nanosheets at 300 K.  of the shortcomings of such approaches is the increased net weight of the catalyst because the magnetic component of the hybrid does not provide sites for catalytic action. Here, the advantage is that the dopant that improves the catalytic efficiency also makes the catalyst magnetic. Treating Co-doped MoS 2 with an acid leads to deintercalation of interlayer NH 3 /NH 4 + and removal of about 30% of Co 2+ , which were either intercalated or in the edge planes. Catalytic reduction of 4-NP using acid-leached Co-doped MoS 2 exhibits a slight decrease in catalytic activity (Table 2). In contrast, ammoniated 1T-MoS 2 synthesized in the absence of a cobalt source exhibits relatively very poor catalytic activity toward 4-NP reduction ( Table 2). The comparison of the catalytic activities (Table 2) of the catalysts used in 4-NP reduction suggests that incorporation of cobalt ions in the MoS 2 lattice is crucial to maximizing the efficiency of the catalyst. Figure 6b,c traces the reduction of nitroaniline and nitrobenzene, respectively, in the presence of as-prepared Codoped MoS 2 . The results show that the catalyst is universally effective in the reduction of nitro groups in different substrates and in at least two polar solvents. In fact, the catalyst is most efficient in the reduction of nitrobenzene, a reaction that is of importance in the removal of toxic nitrobenzene from effluents. In all of the cases, reduction of nitroarene does not occur in the absence of the catalyst.
In comparison to what has so far been reported in the literature (Table 3), the enhanced TOF and recyclability make Co-doped MoS 2 a superior catalyst. The TOF of Co-doped MoS 2 is 1 order greater than that of the best MoS 2 -based catalyst and ∼20% higher than the best value reported so far.
In Co-doped MoS 2 , Co 2+ takes residence at the coordinatively unsaturated sulfur vacancies on the basal plane and edge sites. 10,58 This leads to a conversion of a fraction of Mo 4+ to Mo 3+ , thus stabilizing the 1T polytype. 15 Doped Co 2+ distorts the close-packed sulfur layer of MoS 2 and induces lattice strain. 59,60 These sites would lower the reaction free energy. 28 Nitroarenes are adsorbed at the strained active sites of the MoS 2 surface. 59−61 At these sites, the electron transfer to the substrate is facilitated by Co 2+ /Co 3+ redox couple. The electrons generated by the hydrolysis of NaBH 4 are transferred  to the Co 2+ -accommodated basal and edge sites of 1T MoS 2 and are promoted into the MoS 2 conduction band. 59 The Cosubstituted sites not only instigate faster electron transfer for nitroarene reduction through reversible reduction−oxidation reactions but also serve as an electron reserve and aid in the retention of the 1T phase with enhanced electrical conductivity. The role of Co in improving the catalytic efficiency is very clear from the fact that the control MoS 2 , which has intercalated NH 3 /NH 4 + and hence has similar access to surface as that of Co-doped MoS 2 , shows poor activity (TOF is 1 order lower). As Co-doped MoS 2 is largely few-layer thick, the lattice expansion by intercalated species may not be very important and this is borne out from the almost similar activity of the acid-treated sample, which does not have intercalated species. This observation may be important when these catalysts are used in the hydrogen evolution reaction, the reaction medium of which is usually fairly acidic. To further ascertain the role of Co sites in catalysis, the catalytic activity of Co-doped MoS 2 with varying cobalt contents was studied. The increase in TOF of 4-nitrophenol reduction with an increase in Co doping ( Table 2) further confirms the importance of Co sites in the catalyst.

CONCLUSIONS
Magnetic, 1T Co-doped MoS 2 nanosheets with the cobalt ionactivated defective basal planes and S-edges are synthesized in a single-step hydrothermal reaction. Readily dispersible in polar solvents like water or methanol, Co-doped MoS 2 nanosheets exhibit exceptional catalytic activity toward reduction of nitroarenes at ambient temperature. In addition to exhibiting a high turnover frequency, the catalyst can be magnetically separated from the reaction mixture, thus enabling recyclability simple and efficient. The superior catalytic activity of the Codoped MoS 2 layers may be due to a combination of (a) stabilization of the metallic 1T phase and (b) better electron capture from the hydride and electron supply to the nitroarene substrates through reversible redox reactions at the Co sites.

Preparation of Co-Doped MoS 2 Nanosheets.
Cobalt acetate (0.214 g) was dissolved in 45 mL of water. Ammonium tetrathiomolybdate (0.442 g) was added to the pink Co 2+ solution, and the mixture was stirred for 15 min. Hydrazine hydrate (5 mL) was added to the solution, and stirring was continued for another 15 min. The black-brown solution was transferred to a teflon-lined autoclave and sealed in a stainless steel canister. The autoclave was heated in a hotair oven at 180°C for 24 h and cooled to room temperature under ambient conditions. The pH of the supernatant at the end of the reaction was ∼12. The black precipitate was washed with distilled water till the pH of the washings is ∼7, followed by washing with acetone. The product was dried in air at ambient temperature. The preparation was repeated using 0.107 and 0.054 g of cobalt acetate to vary the cobalt content in the product. In all of the cases, an excess amount of NaBH 4 was used to ensure that its concentration could be considered constant throughout the reaction and the molar ratio in each case was optimized at the lowest NaBH 4 concentration that results in the shortest reaction time.
For comparison, the nitroarene reduction reactions were carried out (a) in the absence of the catalyst, (b) using control ammoniated MoS 2 nanosheets as a catalyst, and (c) acid-treated Co-MoS 2 nanosheets. 4.5. Characterization. All of the samples were analyzed by recording powder X-ray diffraction (XRD) patterns using a PANalytical X'pert pro diffractometer (Cu Kα radiation, secondary graphite monochromator, scanning rate of 1°2θ/ min). The amount of Co 2+ was estimated by a Varian AA240 atomic absorption spectrometer using a Co hallow cathode lamp in an air−acetylene flame at a wavelength of 324.4 nm. Xray photoelectron spectroscopy (XPS) measurements were carried out with Kratos axis Ultra DLD. All spectra were calibrated to the binding energy of the C 1s peak at 284.51 eV. Scanning electron microscopy (SEM) analysis was carried out using a Zeiss, Ultra 55 field emission scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) images were acquired with Tecnai T20 operated at 200 kV. UV−visible spectra of the reaction mixtures were recorded on a PerkinElmer (LS 35) UV−visible spectrometer. The Raman spectra of the samples were recorded on HORIBA Jobin-Yvon LabRAM HR800 at 532 nm excitation wavelength. Isothermal magnetization [M vs H] was measured using a superconducting quantum interference device (SQUID) magnetometer.
■ ASSOCIATED CONTENT

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00848. Raman spectrum of Co-doped MoS 2 , and XPS data of acid leached Co-doped MoS 2 (PDF)