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) MoS2 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 MoS2 is associated with its metallic edges, whereas its semiconducting basal plane is catalytically inert.[2,4]

In this context, nanostructures of MoS2, amorphous[7−9]/crystalline,[10−13] and vertically aligned structures[14,15] have been explored to maximize the number of active edge sites. MoS2 is also hybridized with conducting/semiconducting/magnetic materials (graphene[15−19]/CoSe2[20]/CoS[21−24]/CdS[25,26]/Fe3O4[27]) to enhance the catalytic activity through synergetic coupling effects. Metastable, intrinsically metallic, octahedral 1T MoS2 obtained through exfoliation of trigonal prismatic 2H MoS2 has proven to be an excellent catalyst for H2 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 MoS2 matrix has been the classic route to maximize the catalytic activity of MoS2, 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 MoS2 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 MoS2 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 MoS2 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 NaBH4. Nitroarenes, with aromatic rings associated with H-bonding −NH2 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 MoS2 nanosheets. With increased active sites, Co-doped MoS2 nanosheets exhibit exceptional catalytic activity in the reduction of nitroarenes. The observed turnover frequency (TOF) is far superior in comparison to that of other MoS2 architectures and noble-metal-based catalysts, reported so far.

Results and Discussion

The XRD pattern of as-prepared Co-doped MoS2 nanosheets (Figure A,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 NH3/NH4+ ions released as byproducts of hydrazine used as a reductant in the hydrothermal reaction.

Figure 1

(A) XRD patterns of Co-doped MoS2 nanosheets (a) as-prepared and (b) treated with 1 N HCl and of (c) MoS2 prepared in the absence of cobalt. (B) Raman spectrum of Co-doped MoS2 nanosheets in comparison with bulk MoS2.

On treating Co-doped MoS2 nanosheets with 1 N HCl solution, the 002 reflection (Figure A,b) disappears, indicating deintercalation of the guest species. However, the low intensity of the 002 reflection or its absence (Figure A,a,b) suggests that Co-doped MoS2 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 MoS2. The undoped MoS2 is also poorly ordered and exhibits increased basal spacing due to NH3/NH4+ intercalation (Figure A,c).

The Raman spectrum of Co-doped MoS2 (Figure B) exhibits the in-plane E2g (380 cm–1) and out-of-plane A1g (406 cm–1) Mo–S vibration modes, characteristic of the MoS2 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 A1g and E2g modes of Co-doped MoS2 in comparison to those of bulk MoS2 clearly indicate softening of A1g and E2g modes and phonon confinement that is expected for mono- to few-layer MS2, thus indicating that the Co-doped MoS2 comprises mono to few layers.[39,40]

The chemical composition of the Co-doped MoS2 was further probed by X-ray photoelectron spectroscopy (XPS). Mo 3d and S 2p spectra (Figure a,b; Table ) correspond to Mo4+ and S2– of the 1T polytype of MoS2. A small proportion of the 2H polytype coexists with the 1T phase.[41] The N 1s spectrum (Figure c; Table ) indicates the presence of NH3 and NH4+ ions, which are accommodated in the interlayer of MoS2 nanosheets, as suggested by the XRD pattern (Figure a).[35] The core-level Co 2p spectra (Figure d; Table ) confirm the presence of Co2+ species. The XRD pattern (Figure a) and the XPS Co 2p spectra confirm the absence of CoS2 and CoMo2S4. The binding energy of 779.2 eV is close to what has been observed for CoMo2S4, suggesting that Co2+ substitutes Mo atoms along the {002} or the S-edge planes of MoS2. The atomic percentages of Co, Mo, S, and N are 4.68, 24.66, 59.38, and 11.27, respectively, leading to a chemical composition of Co0.16Mo0.83S2(NH3)0.38.

Figure 2

XPS spectra showing Mo 3d (a), S 2p (b), N 1s (c), and Co 2p (d) core-level peak regions of Co-doped MoS2.

Table 1

Summary of the Binding Energies of Mo, S, Co and N in Co-doped MoS2

binding energy (eV)
	Mo 3d	phase	S 2p	Co 2p	N 1s
Mo0.83Co0.16S2(NH3)0.38	228.6 & 231.8	1T	161.49 & 162.80	779.2 & 794.0	397.6 – NH3
	229.0 & 232.5	2H	163.97 & 164.77		400.4 – NH4+

To understand the nature of the chemical environment of Co2+, the Co-doped MoS2 was treated with 1 N HCl when the intercalated or undoped Co2+ 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 MoS2 arrived at from XPS data. The composition of the acid-leached sample is Co0.1Mo0.78S2. The XPS spectra of acid-treated Co-doped MoS2 (SI, Figure S2) indicate that whereas the nitrogen-containing species, NH3 and NH4+ ions, are absent, Co2+ and 1T phase of MoS2 nanosheets are retained. These further suggest that Co2+ is present in the MoS2 lattice. Hydrothermally synthesized MoS2 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 Co2+ 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 ). The saturation magnetization (MS) at 300 K of Co-doped MoS2 nanosheets is 0.0029 emu g–1, which is comparable to that of exfoliated 1T MoS2 reported in the literature.[43] Because our control 1T MoS2 is nonmagnetic, it is fair to assume that the magnetism in Co-doped MoS2 arises as a consequence of doping. The magnetism in monolayer MoS2 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 Co2+ 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 MoS2 lattice. The very low MS observed for Co-doped MoS2 suggests that the weak ferromagnetism here originates from the strain in the layer rather than from ordering of Co2+ ions. Co-doped MoS2 (Figure ) as well as the acid-leached product is weakly magnetic, suggesting that Co2+ ions are doped in the MoS2 layers. In addition, the presence of Co2+ in the MoS2 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 Co2+ is possibly doped in the basal plane and S-edge planes of MoS2 layers.

Figure 3

Hysteresis loop of the Co-doped MoS2 nanosheets at 300 K.

Clusters of layers are observed in the SEM image (Figure a) of as-synthesized Co-doped MoS2. The bright-field transmission electron microscopy (TEM) image (Figure b) indicates that the transparent layers are few-layer thick and few hundred nanometers in lateral dimensions. The HRTEM image (Figure c) shows lattice fringes with a spacing of 1.1 nm, which correlates with the basal spacing observed in the XRD pattern (Figure a), suggesting the presence of intercalants. The HRTEM image in Figure d 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 4

(a) SEM image, (b) low-magnification bright-field TEM image, and (c, d) HRTEM images of Co-doped MoS2.

Figure schematically depicts the catalytic reduction of nitroarenes. Catalytic performance of Co-doped MoS2 in the reduction of 4-nitrophenol (4-NP) in water is summarized in Figure a,d–f. UV–visible absorption spectra (Figure a) of the reaction mixture indicate that 4-nitrophenol converts to 4-aminophenol 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 phenolate, and their intensities increase with time. The log (absorbance) versus time plot (Figure e) is linear (R2 = 0.979), indicating a pseudo-first-order kinetics[47,48] with a rate constant of 1.976 × 10–3 s–1. The turnover frequency (TOF) values, defined as the number of moles of the product formed per unit time per mole of the catalyst, of the materials studied are given in Table .

Figure 5

Schematic representation of the catalytic reduction of nitroarenes using Co-doped 1T MoS2.

Figure 6

Reduction of nitroarenes was traced through UV–visible absorption spectra of the reaction mixture containing 10 mg of the Co-doped MoS2 catalyst, 400 mM NaBH4, and nitroarene. Evolution of absorption spectra with time in the case of 4-nitrophenol (a), 4-nitroaniline (b), and nitrobenzene (c). Plots of absorbance (d) and log (absorbance) (e) against time for 4-nitrophenol reduction. Efficiency of the catalyst (as TOF) in six consecutive cycles of 4-nitrophenol reduction (f).

Table 2

Catalytic Activity of the Catalysts in the Reduction of Nitroarenes

substrate	catalyst	time (min)	TOF (min–1)
4-nitrophenol (37 mM)	Co-doped MoS2 (4.7% doping)	7	8.41
	Co-doped MoS2 (∼2% doping)	13.5	4.36
	Co-doped MoS2 (∼1% doping)	18	3.27
	acid-leached Co-doped MoS2	8	7.36
	ammoniated MoS2	90	0.65
4-nitroaniline (14 mM)	Co-doped MoS2 (4.7% doping)	7	3.15
nitrobenzene (50 mM)	Co-doped MoS2 (4.7% doping)	4	20.2

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 ). The catalytic activity of Co-doped MoS2 remains nearly constant over a number of cycles (Figure f). The morphology and composition of the catalyst remain almost the same after six cycles of catalysis. Earlier attempts to make MoS2-based catalysts magnetic have been through hybridization of MoS2 nanosheets with magnetic nanoparticles such as Fe3O4.[27] One 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 MoS2 with an acid leads to deintercalation of interlayer NH3/NH4+ and removal of about 30% of Co2+, which were either intercalated or in the edge planes. Catalytic reduction of 4-NP using acid-leached Co-doped MoS2 exhibits a slight decrease in catalytic activity (Table ). In contrast, ammoniated 1T-MoS2 synthesized in the absence of a cobalt source exhibits relatively very poor catalytic activity toward 4-NP reduction (Table ). The comparison of the catalytic activities (Table ) of the catalysts used in 4-NP reduction suggests that incorporation of cobalt ions in the MoS2 lattice is crucial to maximizing the efficiency of the catalyst.

Figure b,c traces the reduction of nitroaniline and nitrobenzene, respectively, in the presence of as-prepared Co-doped MoS2. 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 ), the enhanced TOF and recyclability make Co-doped MoS2 a superior catalyst. The TOF of Co-doped MoS2 is 1 order greater than that of the best MoS2-based catalyst and ∼20% higher than the best value reported so far.

Table 3

Comparison of TOF for the Reduction of Nitroarenes by Various Catalytic Materials Reported in the Literature

catalyst	TOF (min–1)	reference
4-nitrophenol reduction
Co-doped MoS2	8.41	present work
1T chemically exfoliated MoS2	0.74	(49)
2H chemically exfoliated MoS2	0.015
MoS2-Fe3O4	4.0 × 10–2	(27)
MoS2-Fe3O4/Pt	6.0 × 10–4	(50)
MoS2-Pd	3.2 × 10–3	(51)
MoS2-Pt
MoS2	2.5 × 10–3
MoS2-Au
MoS2-Ag
Ni0.33Co0.66	2.0 × 10–3	(52)
citrate capped Au nanoparticles	1.4	(53)
Ag dendrites	0.13	(54)
Pd supported on CNTs	6.3	(55)
4-nitroaniline reduction
Co-doped MoS2	3.15	present work
1T chemically exfoliated MoS2	1.39	(49)
Au nanowires	0.10	(56)
dodecahedral Au nanoparticles	0.10	(57)

In Co-doped MoS2, Co2+ 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 Mo4+ to Mo3+, thus stabilizing the 1T polytype.[15] Doped Co2+ distorts the close-packed sulfur layer of MoS2 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 MoS2 surface.[59−61] At these sites, the electron transfer to the substrate is facilitated by Co2+/Co3+ redox couple. The electrons generated by the hydrolysis of NaBH4 are transferred to the Co2+-accommodated basal and edge sites of 1T MoS2 and are promoted into the MoS2 conduction band.[59] The Co-substituted 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 MoS2, which has intercalated NH3/NH4+ and hence has similar access to surface as that of Co-doped MoS2, shows poor activity (TOF is 1 order lower). As Co-doped MoS2 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 MoS2 with varying cobalt contents was studied. The increase in TOF of 4-nitrophenol reduction with an increase in Co doping (Table ) further confirms the importance of Co sites in the catalyst.

Conclusions

Magnetic, 1T Co-doped MoS2 nanosheets with the cobalt ion-activated 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 MoS2 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 Co-doped MoS2 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.

Experimental Section

Preparation of Co-Doped MoS2 Nanosheets

Cobalt acetate (0.214 g) was dissolved in 45 mL of water. Ammonium tetrathiomolybdate (0.442 g) was added to the pink Co2+ 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 hot-air 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.

Acid Leaching of Co-Doped MoS2 Nanosheets

To extract the intercalated and free cobalt species, 100 mg of Co-doped MoS2 was stirred in 5 mL of 1 N HCl for 24 h. The supernatant was collected. The process was repeated thrice. The cobalt content in the supernatant was estimated. The black solid was washed with water followed by acetone and dried in air at ambient temperature.

Preparation of MoS2 Nanosheets

As a control experiment, the synthesis was repeated in the absence of cobalt acetate, which results in ammoniated MoS2 nanosheets.

Reduction of Nitroarenes Using Co-Doped MoS2 Nanosheets as a Catalyst

Reduction of 4-Nitrophenol (4-NP)

The catalyst (10 mg) was dispersed in 100 mL of water by stirring for 1 h. 4-NP (512 mg, 37 mM) was dissolved in 100 mL of the catalytic dispersion. An excess of NaBH4 (1.51 g, 400 mM) was added with constant stirring [4-NP:NaBH4 molar ratio was 1:12]. The progress of the reaction was monitored by measuring the absorbance of 4-NP at 400 nm.

Reduction of 4-nitroaniline (4-NA)

The catalyst (10 mg) was dispersed in 100 mL of water by stirring for 1 h. 4-NA (192 mg, 14 mM) was dissolved in 100 mL of the catalytic dispersion. An excess of NaBH4 (1.51 g, 400 mM) was added with constant stirring [4-NA/NaBH4 molar ratio was 1:28]. The progress of the reaction was monitored by measuring the absorbance of 4-NA at 380 nm.

Reduction of Nitrobenzene (NB)

The catalyst (10 mg) was dispersed in 100 mL of methanol by stirring for 1 h. Nitrobenzene (0.52 mL, 50 mM) was dissolved in the dispersion. An excess of NaBH4 (1.51 g, 400 mM) was added with constant stirring [NB/NaBH4 molar ratio was 1:8]. The progress of the reaction was monitored by measuring the absorbance of nitrobenzene at 259 nm.

In all of the cases, an excess amount of NaBH4 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 NaBH4 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 MoS2 nanosheets as a catalyst, and (c) acid-treated Co-MoS2 nanosheets.

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 Co2+ 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. X-ray 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.