Tuning of structural and optical properties with enhanced catalytic activity in chemically synthesized Co-doped MoS₂ nanosheets

Abstract

Molybdenum disulfide (MoS₂) nanosheets, due to having a highly active nature, being low cost and having unique physical and chemical properties, have shown their efficacy in the catalytic reduction of nitroarenes. Doping of transition metal ions in molybdenum disulfide (MoS₂) nanosheets is a well-known strategy to enhance their catalytic efficiency for the reduction of nitroarenes, however, finding the optimum dopant amount is still a subject of ongoing research. Herein, we have synthesized few-layered cobalt (Co) doped MoS₂ nanosheets with different cobalt content (2%, 4%, 6% and 8%) through the solvothermal approach, taking sodium molybdate dihydrate (Na₂MoO₄·2H₂O), thiourea (CH₄N₂S) and cobalt acetate tetrahydrate [Co(CH₃COO)₂·4H₂O] as precursors and their catalytic performance has been affirmed by monitoring the reduction of p-nitrophenol by NaBH₄ in real time using UV-visible absorption spectroscopy. The 6% Co doped MoS₂ nanosheets have exhibited superior catalytic activity with a pseudo-first order rate constant of 3.03 × 10⁻³ s⁻¹ attributed to the abundant defects in the active edge sites having a dominant metallic 1T phase with Co ion activated defective basal planes, sulphur (S) edges, synergistic structural and electronic modulation between MoS₂ and Co ions and enhanced electron transfer assisted through redox cycling in the active sites. An attempt has also been made to study the manipulation of structural and optical properties with cobalt doping in MoS₂ nanosheets to establish a correlation between the catalytic efficiency and dopant content. This study demonstrates that proper tuning of Co doping in MoS₂ nanosheets paves the way in searching for a potential alternative of a noble metal catalyst for the catalytic reduction of nitroarenes.

The optimal cobalt (6% Co) doped MoS₂ catalyst has shown the highest catalytic activity due to the presence of abundant defects in the active edge sites, having dominant metallic 1T phase with Co ion activated defective basal planes.

1. Introduction

Graphene-like two-dimensional (2D) transition metal dichalcogenides¹ (TMDs) have garnered a lot of interest because of their fascinating structural and electronic properties which make them suitable as catalysts in various technologically important applications such as electrochemical generation of hydrogen from water^2–4 and hydro-desulfurization of petroleum.^5,6 One of the most widely studied TMD systems includes layered MoS₂. It is commonly known to exist in two crystallographic forms: (i) the thermodynamically stable trigonal prismatic (2H) phase (space group P6₃/mmc)⁷ and (ii) the metastable octahedral (1T) phase (space group P3m1).⁸ Among these 1T MoS₂ is reported to show metallic behaviour⁹ whereas the 2H MoS₂ is an indirect semiconductor having a band gap of ∼1.2 eV.¹⁰ The electronic properties of 2H MoS₂ can be tailored by changing the layer thickness,¹⁰ doping of TM metal ions¹¹ and introducing tensile lattice strain.¹² Theoretical and experimental studies indicate that the catalytic activity of thermodynamically stable 2H polymorph of MoS₂ is associated with its metallic edges, whereas its semiconducting basal plane is proved to be catalytically inert.^2,4

p-Nitrophenol (p-NP) is one of the most common and stubborn harmful organic pollutants found in industrial wastewaters. However, its reductive derivative, p-aminophenol (p-AP) is widely used in manufacturing of drugs such as paracetamol and clofibrate and also used as a developing agent in photography and as a petroleum additives.^13–15 Therefore, the catalytic transformation of p-NP into p-AP has been extensively studied as a model reaction to verify the activity of a catalyst during the past few years.^16–18 Most of these studies were focused on the activity of noble-metal nanoparticles (NPs) such as Ag, Au, Pd and Pt. Although they possess high catalytic potential, however, their usage may lead to depletion of rare resources, environmental contamination and health issues. In addition, the agglomeration of colloidal noble metal NPs during the catalytic reaction is often inevitable.

In this regard, MoS₂, a 2D layered material, acts as an efficient candidate in catalytic reduction of p-NP as its basal plane and catalytic edges with increased exposed active sites facilitate the electron transfer process.¹⁹ Recently, various nanostructures of MoS₂, amorphous^20–22/crystalline^23–26 and vertically aligned structures^27,28 have been explored to maximize the number of active edge sites. Hybridization of MoS₂ with conducting/semiconducting/magnetic materials (graphene^28–32/CoSe₂ (ref. 33)/CoS^34–37/CdS^38,39/Fe₃O₄ (ref. 40)) is able to enhance the catalytic activity through synergistic 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. The 1T phase facilitates electrode kinetics by increasing the electrical conductivity and proliferation of the catalytic active sites.^41–43 Noble metal (Au, Ag, Pd, Pt)-MoS₂ nanocomposite has proven to be as efficient catalyst, although such type of synthesis procedure includes controlled reaction condition, stabilizing agent and high cost.⁴⁴ 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.^23,45,46 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)⁴⁷ or increase the conductivity by stabilizing the 1T MoS₂ polytype.^32,41,42

Introduction of cobalt ion in 2H MoS₂ may be a fascinating strategy in doping as it can increase the types and numbers of active sites in the basal planes in addition to edges^48,49 along with introduction of 1T phase in 2H phase. It also exhibits ready redox Co²⁺ ↔ Co³⁺ switching and can alter the band positions favourably. Intrinsic defects in the form of Mo vacancies and sulphur vacancies as well as extrinsic donor and acceptor defects resulting from doping are used to improve the semiconducting properties generally through band gap reduction by lowering the conduction band, raising the valence band, and/or introducing midgap states. These defects can act as electron and hole traps, which can increase the recombination times and alter the recombination rates resulting enhancement in catalytic efficiency than the pristine one.

One of the standard reactions to test the catalytic action is the reduction of p-NP by NaBH₄. Nitrophenol, with aromatic rings associated with H-bonding –NO₂ and –OH groups, enables the reduction to be carried out in water making it an environment-friendly reaction. In this work we have investigated in detail the tuning with Co doping on structural, optical properties and catalytic activity in chemically synthesized MoS₂ nanosheets having the mixture of both 1T and 2H phases. Attempt has also been made to establish a correlation between its observed optical properties with the catalytic efficiency as the doping concentration of Co is increased in MoS₂ nanosheets. This study demonstrates that catalytic efficiency of MoS₂ nanosheets can be enhanced with increased amount of cobalt doping percentages (2 at. wt%, 4 at. wt%, 6 at. wt%). As Co-doping percentage is increased in MoS₂ nanosheets, the band gap is reduced along with increased active sites which facilitate electron transfer easily resulting exceptional catalytic activity in the reduction of nitrophenol to aminophenol. The observed turnover frequency (TOF) in Co–MoS₂ (6 at. wt%) is far superior in comparison with other MoS₂ architectures and noble-metal-based catalysts, graphene based nanocomposites etc. reported so far.

2. Experimental details

2.1. Chemicals used

High purity (99.9% purity) chemicals, like sodium molybdate dihydrate (Na₂MoO₄·2H₂O), thiourea (CH₄N₂S), sodium borohydride (NaBH₄), acetic acid (CH₃COOH), p-nitrophenol (C₆H₅NO₃), cobalt acetate tetrahydrate (CO(CH₃COO)₂·4H₂O), ethyl alcohol (CH₃CH₂OH) were procured from Sigma-Aldrich. All the reagents are analytical reagent grade and used without further purification.

2.2. Synthesis of ultra-thin MoS₂ nanosheets

Pure and Co doped MoS₂ nanoparticles/nanosheets were synthesized in aqueous solution using sodium molybdate dihydrate (Na₂MoO₄·2H₂O), thiourea (CH₄N₂S) via simple hydrothermal technique in the starting solution mixture. In a typical synthesis, 2 mM of Na₂MoO₄·2H₂O and 5 mM of CH₄N₂S were dissolved in 40 mL of deionized water and were stirred for 30 minutes to obtain the clear starting solution mixture. The pH of the solution mixture was maintained at 5.0 by adding 0.02 mol L⁻¹ of acetic acid (CH₃COOH) solution under stirring. This solution mixture was then transferred to a 50 mL Teflon lined stainless steel autoclave and was subjected to hydrothermal treatment at 200 °C for 24 hours under autogenous pressure. The autoclave was then allowed to cool naturally and the obtained black precipitate was separated through centrifugation for 30 minutes. Then the precipitate was washed thoroughly with ethanol and water several times for removal of surface adsorbed ions and impurities. The final product was dried in oven at 80 °C overnight and used for further characterization.

2.3. Synthesis of cobalt (2 at. wt%, 4 at. wt%, 6 at. wt% and 8 at. wt%) doped MoS₂ NSs

For the synthesis of 2 at. wt%, 4 at. wt%, 6 at. wt% and 8 at. wt% Co-doped MoS₂ samples, appropriate amount of cobalt acetate tetrahydrate [Co(CH₃COO)₂·4H₂O] aqueous solution was added to the former starting solution mixture and stirred very well prior to hydrothermal treatment at 200 °C for 24 hours. The rest of the procedure of separation, purification and collection of the dried as-synthesized samples was repeated as before (Fig. 1).

Fig. 1. Diagram of experimental procedure.

2.4. Characterization of nanostructures

The crystalline phase of respective as-synthesized undoped and Co doped MoS₂ samples were ascertained through powder X-ray diffraction (XRD) studies. It was carried out employing a Bruker AXS diffractometer D8, Germany using CuK_α radiation (λ = 1.5418 Å) at an applied voltage of 40 kV and at a scan rate of 3° min⁻¹ in the 2θ range of 10°–70°. The presence of surface functional groups in the synthesized MoS₂ samples were established through Fourier-transformed infrared (FTIR) spectroscopy carried out between 4000 to 400 cm⁻¹ using a Perkin–Elmer Spectrum RX-II Model 73713, USA. The microstructural features of the synthesized MoS₂ samples were analyzed by transmission electron microscopy (TEM, FEI-TECNAI G2 20S-TWIN, USA) imaging and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100, Japan) imaging at an acceleration voltage of 200 kV. Zeta potential measurement was undertaken to determine the surface charge of the samples using a Malvern Nano ZS instrument. The optical and catalytic activity of the samples was investigated through a Shimadzu-2450 UV-vis-NIR spectrophotometer, Japan. The band gap energies of the samples were evaluated from UV-vis absorption spectra using the Tauc plot. 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). The Raman spectra of the samples were recorded on HORIBA Jobin-YvonLabRAM HR800 at 532 nm excitation wavelength.

3. Results and discussion

3.1. Structural study (XRD data analysis)

The room temperature HRXRD (Fig. 2) patterns of undoped and the Co-doped MoS₂ nanostructures of different doping concentration were recorded over 2θ range of 10°–70° to verify the formation of crystalline phases of MoS₂. The HRXRD data reveals that 2H MoS₂ phase is formed having diffraction peaks at 14.2°, 33.4°, 39.7°, 49.7°, 58.8° (PDF 65-1951). It is clearly seen that the crystalline quality is improved with doping of maximum percentages of cobalt. The broadening of peaks has been observed with increase of Co doping concentration due to appearance of lattice strain. Since the ionic radius of Co²⁺ (0.074 nm) is slightly larger than that of Mo⁴⁺ (0.065 nm), it is expected to develop a lattice strain. The crystallite sizes of the MoS₂ nanostructures were estimated by Scherrer formula.D_hkl = kλ/(β_hkl cos θ)Here, k is a coefficient (k = 0.9), β is the full width at half maximum of the (002) peak, and λ is the incident X-ray wavelength (λ = 0.154 nm). The crystallite sizes D of the undoped and the 8% Co doped MoS₂ samples are estimated to be 8.11 nm and 7.44 nm, respectively. The microstrain (ε) and dislocation density (δ) appearing in the lattice due to some substitution of Mo atoms by Co atoms can be calculated from the following relations:ε = β cos θ/4, δ = 1/D²

Fig. 2. XRD pattern of undoped and Co doped MoS₂.

The values of crystallite size, microstrain and dislocation density are calculated and summarized in Table 1.

The microstructural parameters of the undoped and Co doped MoS₂ nanostructures.

Co doping percentages	Crystallite size (nm)	Microstrain ε	Dislocation density δ (lines per m2)
0%	8.11	3.98 × 10−3	1.52 × 1016
8%	7.44	5.17 × 10−3	1.81 × 1016

3.2. Raman spectroscopy study

The recorded Raman spectra of MoS₂ and Co doped MoS₂ are shown in Fig. 3(a). The characteristic peaks at 282, 380, 406 and 458 cm⁻¹ correspond to the E_1g, E¹_2g, A_1g, and A_2u phonon modes of 2H-MoS₂. The peak arising due to 1 T-phase of MoS₂ is identified at 345 cm⁻¹ (ref. 50) suggesting that the as-synthesized flower-like MoS₂ consists of mixed 2H and 1T phases. 18% incorporation of 1T phase in undoped sample and 73% incorporation of 1T phase in 6% Co doped MoS₂ samples were obtained. In addition to these peaks, two new peaks emerge at 359 cm⁻¹ and 480 cm⁻¹ for Co²⁺ doped MoS₂ nanostructures. Furthermore, the peak intensity is generally higher with higher doping concentration.⁵¹ This suggests that these peaks may belong to the local vibrational modes associated with the Co²⁺ dopants.

Fig. 3. (a) Raman spectra for undoped and Co doped MoS₂ (b) zoomed view.

It is clearly observed that with increase in cobalt doping, E¹_2g vibration mode is decreased, while the A_1g vibration mode is increased, which are related to the tensile strain⁵² appearing in the lattice due to doping of cobalt ion in Mo site and number of layer in MoS₂.⁵³ When the layer number increases, the interlayer van der Waals force in MoS₂ suppresses atomic vibration, resulting in higher force constants resulting in stiffening of A_1g mode (blue-shift). On the contrary, E¹_2g peak shows a red-shift, instead of a blue-shift, suggesting that the increased interlayer van der Waals force plays a minor role while stacking-induced structure changes or long-range coulombic interlayer interactions in multilayer MoS₂ may dominate the change of atomic vibration.⁵⁴ Additionally, the intensity ratio of A_1g to E¹_2g mode is 2.17 which concludes that it favours the vibration of A_1g mode and thus the best edge-terminated structure.⁵⁵ According to the dependence of A_1g full width at half maximum (FWHM) on the MoS₂ thickness, this decrease in FWHM in 6% Co doped MoS₂ is an indication of thinning down the effective MoS₂ thickness.⁵⁰ The positions of the characteristic Raman E¹_2g and A_1g peaks and their separations with varying cobalt doping concentrations are summarized in Table 2.

Comparison of characteristic Raman peak position with varying doping concentration of cobalt ions in MoS₂.

Co doping percentages	E12g (cm−1)	A1g (cm−1)	Δk (cm−1)
0%	381.45	405.77	24.32
2%	379.49	406.59	27.10
4%	378.63	407.09	28.46
6%	378.23	407.23	29.00

3.3. Morphology and microstructure

SEM image analysis

The observed surface morphology of the as-prepared samples was analyzed based on recorded SEM and TEM images. As shown in Fig. 4(a), the pristine MoS₂ exhibits dispersed nanosheet like morphology. Fig. (b)–(d) show the nanostructures of 2%, 4% and 6% Co doped MoS₂ samples. With the incorporation of dopant ion, the dispersed flakes are interlaced by rolling up of surface edges. They form nano-flowers by summing up of nano-flakes (thickness within 10 nm). From EDX analysis of 4% cobalt doped MoS₂ it can be seen that S : Mo atomic ratio is 1.4 : 1. It indicates that some oxygen vacancies are created along with doping. It also shows accurate detection of the amount of cobalt doped in the sample.

Fig. 4. SEM images of (a) undoped (b) 2% Co, (c) 4% Co and (d) 6% Co doped MoS₂ nanostructures (e) EDS elemental mapping of Mo, S and Co components for 4% Co doped MoS₂ (f) Mo, (g) S, (h) Co EDAX mapping for 4% Co doped MoS₂.

HRTEM image analysis

Fig. 5 shows detailed microstructure of the MoS₂ nanosheet samples recorded by using HRTEM imaging. Fig. 5(a) clearly shows the magnified image of interlaced flakes of undoped MoS₂ where interlayer distance is found to be 0.62 nm forming a 14 layer system. Inset of Fig. 5(b) shows the interplanar distance between (100) planes and it is found to be 0.27 nm, which matches very well with SAED ring pattern corresponding to (100) plane as shown in Fig. 5(c). The undoped MoS₂ reveals polycrystalline nature which is clear from SAED pattern and four distinct rings appears corresponding to planes (002), (100), (103) and (110) which are also seen in XRD pattern. Similar results can also be obtained from Fig. 5(f) of 6% Co doped MoS₂. Lattice spacing for (002), (110), (103) planes are 0.63 nm, 0.15 nm and 0.23 nm, respectively. It exactly matches with results obtained from SAED pattern in Fig. 5(e) (inset). Again (004) plane is prominent in 6% Co doped MoS₂ sample which is a layered material as confirmed from our SEM and TEM results. By comparing Fig. 5(b) and (e) of microstructures corresponding to undoped and 6% Co doped MoS₂ we can ascertain that the multi-layered nanoflakes in undoped MoS₂ are exfoliated and thinned down to few layers in 6% Co doped MoS₂ due to cobalt ion intercalation in the basal plane of the layered structure. This is also consistent with AFM image analysis (ESI, Fig. S1†).

Fig. 5. HRTEM images (a) showing layer numbers and interlayer separation between (002) planes (b) interlaced flake size having lateral dimension ∼40 nm and (100) interplanar spacing (inset) 0.27 nm (c) SAED pattern for different orientation of planes for undoped MoS₂; (d) interlaced nanoflakes with lateral dimension ∼20 nm, (e) layer distribution and SAED pattern (inset) and (f) lattice fringes and interlayer separation between (002) planes for 6% Co²⁺ doped MoS₂ samples.

3.4. FTIR data analysis

FTIR measurements were carried out in order to obtain the bending and stretching vibrations of functional groups present in the undoped and Co doped MoS₂ samples. Fig. 6 depicts the FTIR spectra of all the samples under investigations. The weak peak observed at about 452 cm⁻¹ is assigned to Mo–S vibration in undoped MoS₂. The peak corresponding to 1035 cm⁻¹ is probably due to S–O (sulfoxide) asymmetric stretching or C–O bondstretching (alcohol group). The peaks around 1200 and 1628 cm⁻¹ in the FTIR spectra of undoped MoS₂ and Co doped MoS₂ confirm the existence of C–S and C–N bonds, respectively. Moreover, a broad absorption band at about 3134 cm⁻¹ appears in the FTIR spectra is assigned to stretching vibration of the O–H bond corresponding to intercalated water. The peak around 1400 cm⁻¹ is probably due to C–H bending mode. In addition, the new peaks arising at 561 cm⁻¹, 567 cm⁻¹ and 573 cm⁻¹ for 2%, 4% and 6% Co doped MoS₂, respectively corresponding to Co–O stretching mode in tetrahedral site and octahedral environment. The peak corresponding to Mo–S vibration at 452 cm⁻¹ for undoped MoS₂ shifts to 455 cm⁻¹ for 2% Co, 461 cm⁻¹ for 4% Co and 463 cm⁻¹ for 6% Co doped MoS₂, respectively as shown in Fig. 6(b). Here the red shift of wave number indicates Co substitution in Mo site causing decrease in effective mass of the whole system, oxygen vacancies and structural defects.^56,57

Fig. 6. (a) FTIR spectra for undoped and Co doped MoS₂ (b) zoomed view of shifting of Mo–S peak.

3.5. Zeta potential measurement

The surface charges of undoped and Co doped MoS₂ were determined from zeta potential measurement using the aqueous dispersion in neutral condition (pH-7). High value of the zeta potential (either + or −) is indicative of the stability of particles against aggregation or flocculation in the dispersion medium. All four samples showed negative surface charge, may be due to presence of carboxyl group (–COOH) which has been added during synthesis. The zeta potential values for undoped, 2%, 4%, and 6% Co doped MoS₂ showed −48.3 mV, −41.2 mV, −37.5 mV and −30.8 mV, respectively.

3.6. XPS study

X-ray photoelectron spectroscopic (XPS) measurements were carried out to investigate the chemical composition of undoped and 6% Co doped MoS₂. As shown in the survey spectrum (wide scan) in Fig. 7(c) and (f), the elements C, Mo, S, and O can be clearly identified in undoped and C, Mo, S, O and Co in 6% Co doped MoS₂ nanosheets. The typical Mo 3d_5/2 and Mo 3d_3/2 peaks of 2H phase appeared at 229, 231.5 eV in undoped and 229, 232 eV binding energies in 6% Co doped MoS₂ along with two additional peaks shifted by about 1 eV which were assigned to Mo 3d_5/2 and 3d_3/2 of the 1T phase characteristic of the trigonal prismatic environment in 2H phase⁵⁸ as shown in Fig. 7(a) and (d). Similarly, the S 2p_3/2 and S 2p_1/2 peaks of 2H phase appeared at 162.96, 163.37 eV in undoped and 162.07, 163.57 eV in 6% Co doped MoS₂ binding energies, respectively as shown in Fig. 7(b) and (e). Two additional peaks shifted by about 1 eV (161.78, 162.03 eV (undoped), 161.48, 162.75 eV (6% Co doped)), from the S 2p_3/2 and S 2p_1/2 in 2H phase, were exhibited in both samples as expected for 1T sites. The peaks of Mo 3d_5/2 and Mo 3d_3/2 are shifted to higher binding energy with gradually increasing Co doping as observed in 6% Co doped MoS₂. The ratios of Mo⁵⁺ to Mo⁴⁺ for undoped, 2%, 4% and 6% Co doped MoS₂ are 0.28 : 1, 0.34 : 1, 0.63 : 1 and 0.71 : 1, respectively indicating the increased proportion of Mo⁵⁺ with increase of cobalt ion dopant. The separation of binding energy between Mo 3d_3/2 and 3d_5/2 is 3.13 eV (1T phase) and 3 eV (2H phase) in 6% Co doped MoS₂, can be attributed to the dominant presence of Mo⁵⁺ ions in the cobalt doped samples⁵⁹ by redox reaction of Mo⁴⁺ ↔ Mo⁵⁺ supporting the earlier result. There are four main peaks corresponding to Co 2p_3/2 at 779.25 eV (Co³⁺), 781.25 eV (Co²⁺) and Co 2p_1/2 at 794.75 eV (Co³⁺), 797.75 eV (Co²⁺) in 6% Co doped MoS₂ as shown in Fig. 7(g). These peaks are very close to the binding energies of the Co species in CoS₂.^60–63 Also, another four peaks arising at 784.77, 787.75, 800.25, 802.62 eV signifies the presence of satellite shake up peaks. The binding energy of 779.25 eV is closed to what has been observed for CoMo₂S₄, suggesting that Co²⁺ substitutes Mo atoms along the {002} or the S-edge planes of MoS₂ indicating the uniform doping of cobalt ions in the MoS₂ substrate.⁴⁸ As cobalt doping percentages is increased, so a greater number of cobalt ions are substituting the Mo lattice site. All the binding energies, peak positions corresponding to Mo, S, Co and O in case of undoped and 6% Co doped MoS₂ are summarized in Table 3. The Mo 3d, S 2p and Co 2p spectrum with de-convoluted peaks of 2% and 4% Co doped MoS₂ nanosheets are shown in Fig. S3(a), (d), (b), (e), (c) and (f) respectively.† The presence of higher valence state of cations Mo⁵⁺ and oxygen existing in the doped sample suggests the probability of having partially oxidized Mo₂O₅ species in the doped sample which could result in the generation of oxygen vacancies in the sample. The binding energy of O 1s peaks found in both the species of MoS₂ centred at 530.78 eV (O (i)) belongs to the O²⁻ ions in the hexagonal structure of undoped MoS₂. The peaks centred at 530.88 eV (O (ii)) belong to the oxygen vacancy defects in 6% Co and they can also serves as active sites for catalysis. In addition to redox couple present in the MoS₂ matrix, a third charge compensation mechanism is possible for matrix and dopant ions through intervalence charge transfer (IVCT).Mo⁴⁺ + Co³⁺ ↔ Mo⁵⁺ + Co²⁺

Fig. 7. XPS narrow scan spectra of Mo 3d (a) and (d) and S 2p (b) and (e), for undoped, 6% Co doped MoS₂, narrow scan spectra of Co 2p (g) of 6% Co doped MoS₂. Survey spectra of Mo (3d, 3p, 3s), S 2p, C 1s, Co 2p (c), (f) for undoped and 6% Co doped MoS₂.

Variation of oxidation peaks of Mo, S and Co in MoS₂ with cobalt doping.

Doping%	1T		2H		1T		2H		Co3+		Co2+		O 1s (eV)
	Mo 3d5/2 (eV)	Mo 3d3/2 (eV)	Mo 3d5/2 (eV)	Mo 3d3/2 (eV)	S 2p3/2 (eV)	S 2p1/2 (eV)	S 2p3/2 (eV)	S 2p1/2 (eV)	Co 2p3/2 (eV)	Co 2p1/2 (eV)	Co 2p3/2 (eV)	Co 2p1/2 (eV)
0%	228.25	231.12	229	231.5	161.78	162.03	162.96	163.37	—	—	—	—	530.78
6%	228.62	231.75	229	232	161.48	162.75	162.07	163.57	779.25	794.75	781.25	797.75	530.88

The XPS data presented here subsequently confirm that the precursor valence of Mo⁴⁺ and Co³⁺ partially alter to Mo⁵⁺ and Co²⁺, which supports this mechanism.

3.7. Optical property study

UV-vis absorption spectroscopy and Tauc's plot

Fig. 8 shows the UV-visible absorption spectra of the undoped and different percentages of cobalt doped MoS₂ dispersed in DMF (dimethyl formamide). As expected, three typical characteristic absorption peaks of undoped and Co doped MoS₂ are clearly observed. A prominent peak centred in the region of 430–470 nm (C exciton), and two additional peaks with relatively lower intensity are observed around ∼615 nm (B exciton) and ∼670 nm (A exciton) which correspond to excitonic transitions between the split valence band and the minima of the conduction band at the K point of Brillouin zone for MoS₂ nanosheets.⁶⁴ The band gaps E_g (in eV) for undoped and cobalt doped MoS₂ nanostructures were calculated using the Tauc equation, (αhν)^1/n = B(hν − E_g)where α is the absorption coefficient (in cm⁻¹), hν is the incident photon energy (in eV), B is a constant relative to the materials, and n = ½ (for direct allowed transition) and n = 2 (for indirect transition). Direct excitonic transition corresponding to A exciton (∼670 nm) and B exciton (∼610 nm) could be detected from UV-vis absorption spectra. Moreover, significant amount of indirect band gap contribution was observed for all the nanostructures from the Tauc plot (Fig. 10) having smaller in magnitude in comparison with direct band gap.⁶⁵ The average band gap (E_g) values were estimated from the intercept of the linear portion of the (αhν)² and (αhν)^1/2versus hν plots on the hν axis (i.e., (αhν)² = 0 and (αhν)^1/2 = 0) as shown in Fig. 10. The most interesting results, observed from individual Tauc's plot of undoped MoS₂ and 2%, 4%, 6% Co²⁺ doped MoS₂, is that the E_gap, calculated from the absorption spectrum, first decreased and then increased with increasing Co²⁺ doping.

Fig. 8. UV-vis absorption spectroscopy for undoped and Co doped MoS₂.

Fig. 10. Individual Tauc's plot for direct band gap (a), (c), (e), (g), (i) and indirect band gap (b), (d), (f), (h), (j) of undoped, 2%, 4%, 6% and 8% Co doped MoS₂.

The observations indicate that the incorporation of TM ions lifts the valence band edge causing decrease in band gap which is consistent with theoretical result obtained from DFT calculation.⁶⁶ In case of 8% Co doped MoS₂ sample, due to incorporation of heavy amount of dopant ion, defect states appear in conduction band. Consequently, the absorption edge of the conduction band is pushed to the higher energies resulting in widening of band gap to 2.25 eV (blue shift of the excitation peaks) attributing to Moss–Burstein effect⁶⁷ (Fig. 9(a)). The variation of band gap (Fig. 9(b)), exciton energies and spin–orbit splitting with various (2%, 4%, 6%, 8%) amount of cobalt doping percentages (A, B excitons) are listed in Table 4.

Fig. 9. Schematic view of the transitions corresponding to excitons A (K₄ → K₅) and B (K₁ → K₅) in (a) in 2H MoS₂ and (b) Burstein–Moss (BM) band-filling effect in heavy cobalt ion doping. Labels are according to ref. 64. (c) Variation of direct and indirect band gap with doping concentration.

Variation of band gap, exciton energies and spin–orbit splitting with doping (A, B excitons).

Doping concentration	Direct B. G (eV)	Indirect B. G (eV)	Exciton A (eV)	Exciton B (eV)	Spin–orbit splitting (eV)
0%	2.30	1.60	1.85	2.01	0.16
2%	2.24	1.54	1.83	1.98	0.15
4%	2.11	1.53	1.82	1.96	0.14
6%	2.09	1.52	1.83	1.96	0.13
8%	2.25	1.53	1.84	2.02	0.18

3.8. Catalytic activity

The catalytic activity of undoped and Co doped MoS₂ nanostructures have been investigated through the reduction of p-nitrophenol (NP) to p-aminophenol (AP) in an excess amount of NaBH₄. For the experiment, aqueous stocks of MoS₂ samples, p-NP, and NaBH₄ were prepared at concentrations of 2 g L⁻¹, 10⁻² (M), and 0.1 (M), respectively. The stock of NaBH₄ was maintained at 4 °C to restrain the evolution of H₂ gas. Aliquots of 3.0 mL of 0.1 (M) NaBH₄, 20 μL of 10⁻² (M) p-NP, and 5 μL of MoS₂ samples were taken from the respective stocks and mixed together and their absorption spectra were monitored between 450 nm and 600 nm at different time intervals. For control, the study was also carried out in the absence of MoS₂. All the graphs have been plotted based on the average values obtained from three repeat sets of the experiments.

The catalytic efficiency of MoS₂ and Co doped MoS₂ nanostructures has been evaluated by studying the model reaction of reduction of p-NP by excess NaBH₄ using UV-visible spectroscopy.⁴⁴ The reduction of p-NP to p-AP by NaBH₄ is not a kinetically favourable process due to large difference in the redox potential of the donor and acceptor couple (i.e., E{(H₃BO₃)(aq.)/(BH₄⁻)(aq.)} = −1.33 V and E{(p-NP)(aq.)/(p-AP)(aq.)} = 0.76 V versus Normal Hydrogen Electrode (NHE), respectively.⁶⁸ In the presence of a catalyst, the reaction rate is accelerated by overcoming the kinetic restrictions⁶⁹ on the electron transfer from donor BH₄⁻ to acceptor p-NP. In the present reduction process, the aqueous solution of p-NP (λ = 317 nm) having light yellow colour was observed to change to deep yellow along with a spectral shift of the absorbance peak to 400 nm. The alkalinity of the solution is increased with the addition of excess of freshly prepared ice-cold aqueous solution of NaBH₄ by the formation of the p-nitrophenolate ion in solution. The reduction reaction was manifested by a decrease in absorbance at λ = 400 nm with fading of the yellow colour of the solution and concurrent appearance of a gradually intensifying new peak at λ = 295 nm due to the formation of the reduced product, p-AP [Fig. 11(a)–(e)]. The reduction process of p-NP to p-AP in presence of Co doped MoS₂ catalyst and NaBH₄ is depicted step by step in Fig. 13. The progress of the reaction with time or the kinetics of the reaction was observed in terms of the concentrations of p-nitrophenolate ions in the reaction mixture by monitoring the changes in absorbance at λ = 400 nm at different time intervals. The reaction between p-NP and excess NaBH₄ has been taken as the control, since the absorbance at λ = 400 nm was found to remain unaltered with time in the absence of undoped and Co doped MoS₂ samples in the reaction mixture. It is noteworthy to mention that the presence of Co doped MoS₂ nanostructures did not affect the absorbance spectra of either the p-nitrophenolate ions or p-NP because of their very low concentration⁶⁹ (∼3 ppm) in the system. For the studied reduction reaction, the concentration of the reducing agent, NaBH₄, has been used in large quantities (∼50 times) with respect to p-NP and catalysts. High concentration, increased pH, and ice-cold condition of the freshly prepared aqueous solution of NaBH₄ ensured the stability of the borohydride anion in the reaction medium by dampening its decomposition/protonolysis to liberate hydrogen gas. Small amount of hydrogen gas was released assisted in circumventing the aerial oxidation of p-AP⁶⁹ by providing a reducing atmosphere in situ. The bubbles of hydrogen gas released during the reaction also helped in stirring the reaction mixture.⁶⁸ A large excess of NaBH₄ compared with the concentration of p-NP also ensured that the concentration of BH₄(−) ions in the reaction medium remained invariant during the reaction. In addition, occurrence of only one isosbestic point in the UV-visible absorption spectra of the reduction of p-NP (as is evident from Fig. 11) signified that the reaction mixture involved only a pair of species, p-NP and p-AP⁷⁰ that varied in concentration and activity of nanocatalysts. From these observations, it may be inferred that the reduction of p-NP followed pseudo-first-order kinetics.⁶⁹ Thus, by taking the absorbance of the principal species (i.e., p-nitrophenolate ions) to be proportional to their respective concentrations, the rate constant of the reaction, in the presence of undoped, 2%, 4%, 6%, 8% Co doped MoS₂ was determined using the equation−ln(C_t/C₀) = −ln(A_t/A₀) = ktwhere A_t and A₀ are the absorbance (at λ = 400 nm) at times t and 0, respectively, while C_t and C₀ are the equivalent concentrations of p-nitrophenolate ions at times t and 0, respectively. The apparent rate constants (k) in the presence of each of the undoped and Co doped MoS₂ nanostructured samples were determined from the slopes of the respective linear plots of [−ln(A_t/A₀)] versus time, as shown in Fig. 12. The plots shown in Fig. 12 depict the linearity of [−ln(A_t/A₀)] with time; exponential decay profiles of the absorbance with time support pseudo-first order kinetics with respect to p-nitrophenolate ion. From Fig. 11(d), the 6% Co doped MoS₂ was found to be the most superior catalyst compared to the other. The turnover frequencies (TOF) of the different MoS₂ nanostructures with various cobalt dopant concentrations have been calculated using the expressionwhere 1 × 10⁻² M of p-NP has been taken as the substrate and 2g L⁻¹ of the nanostructured MoS₂ as the catalyst. The variation of rate constant and turn over frequency (TOF) with cobalt doping in MoS₂ substrate are listed in Table 5. Table 6 presents the comparison of TOF values of different nanocomposites of MoS₂ reported earlier and our obtained results. The catalytic efficiency of MoS₂ nanostructure is enhanced with the increase of cobalt doping percentages. In Co-doped MoS₂, Co²⁺ takes residence at the coordinative unsaturated sulphur vacancies on the basal plane, edge sites^23,41 substituting Mo atoms. This leads to a conversion of a fraction of Mo⁴⁺ to Mo³⁺, thus stabilizing the 1T polytype.²⁸ Doped Co²⁺ distorts the close-packed sulphur layer of MoS₂ and induces lattice strain.^71,72 These sites would lower the reaction free energy.⁴¹ Nitrophenol is adsorbed at the strained active sites of the MoS₂ surface.^71–73 At these sites, the electron transfer to the MoS₂ surface 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.⁷¹ Also, metallic Co²⁺ ions may act as traps for the photo-generated electrons, due to the Schottky barrier created at the metal–semiconductor interface. As a consequence of this trapping phenomenon, the electrons generated in the photocatalytic process and trapped in the metal ions can be transferred to the adsorbed p-NPhate ion on the surface of MoS₂. The electron transfer from the semiconductor to the nitro group of the p-nitrophenolate ion favours the reaction to the formation of the p-AP product. Co substituted sites not only raise faster electron transfer for nitrophenol 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. In our study the optimal dopant concentration obtained is 6 at. wt%. since higher concentration (8 at. wt%) of cobalt ion loading possibly leads to partial blockage of catalytic active sites of MoS₂ support surface resulting in enhanced frequency of charge carrier recombination.

Fig. 11. Time-dependent UV-visible absorption spectra of the reduction of p-NP by excess NaBH₄ in the presence of: (a) undoped MoS₂; (b) 2% Co doped MoS₂; (c) 4% Co doped MoS₂; (d) 6% Co doped MoS₂; and (e) 8% Co doped MoS₂. Condition: [p-NP] = 10⁻² M, [NaBH₄] = 0.1 M, and [Catalyst] = 2 g L⁻¹.

Fig. 13. Diagram of the reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) by NaBH₄ catalysed by cobalt doped MoS₂ NP.

Fig. 12. Plots of −ln(A_t/A₀) versus time for the reduction of p-NP by NaBH₄ in the presence of undoped MoS₂, 2% Co, 4% Co, 6% and 8% Co doped MoS₂ nanostructures. Conditions: 3.0 mL of 0.1 M NaBH₄, 20 μL of 10⁻² M 4-NP, and 5 μL of 2 g L⁻¹ nanocatalysts.

Enhancement of rate constant and TOF with increasing cobalt doping in MoS₂.

Doping percentages	Rate constant (k; s−1)	TOF (molecules per g per s)
0%	2.55 × 10−5	1.02 × 1017
2%	5.16 × 10−4	1.06 × 1018
4%	1.95 × 10−3	2.70 × 1018
6%	3.03 × 10−3	4.18 × 1018
8%	2.45 × 10−3	3.13 × 1018

Comparison of the TOF values for the catalytic reduction of p-NP.

Catalyst sample	TOF (min−1)	Reference
2H-chemically exfoliated MoS2	1.5 × 10−2	(Guardia et al. 2014)74
MoS2–rGO	1.50 × 10−1	(Anto Jeffery et al. 2017)75
MoS2–Pd	3.20 × 10−3	(Qiao et al. 2017)76
MoS2–Fe3O4	4.00 × 10−2	(Lin et al. 2015)77
MoS2–Fe3O4/Pt	6.00 × 10−4	(Cheng et al. 2015)78
MoS2	5.11 × 10−3	(Qiao et al. 2018)79
MoS2	2.50 × 10−3	This work
2% Co doped MoS2	3.10 × 10−2	This work
4% Co doped MoS2	1.17 × 10−1	This work
6%Co doped MoS2	1.82 × 10−1	This work
8% Co doped MoS2	1.47 × 10−1	This work
Ni–Co alloy	4.63 × 10−3	(Wu et al. 2011)80
Au–Fe3O4 NPs	2.80 × 10−3	(Lin et al. 2011)81
Pd–Fe3O4@SiO2	1.95 × 10−2	(Wang et al. 2014)82
AgNPs	2.09 × 10−3	(Liang et al. 2013)83

4. Conclusion

Undoped and various (2%, 4%, 6% and 8%) atomic weight percentages of cobalt doped MoS₂ nanosheets with the cobalt ion activated on defective basal planes, S-edges, Mo sites are synthesized in a single-step hydrothermal reaction process. The effectiveness of the prepared MoS₂ nanostructures as catalysts for the reduction of 4-nitrophenol to 4-aminophenol in excess NaBH₄ has been assessed. All prepared morphologies of MoS₂ nanostructures were found to be effective and stable catalysts and followed pseudo-first-order kinetics. The catalytic activity property was clearly observed to enhance with cobalt doping percentages and also provides far more improved result compared to noble metal and rGO-based MoS₂ nanocomposites reported earlier. Readily dispersible in polar solvents like water or methanol, the 6% Co-doped MoS₂ nanosheets exhibit exceptional catalytic activity toward reduction of harmful nitrophenol from waste waters at ambient temperature exhibiting high turnover frequency (4.18 × 10¹⁸ molecules per g per s). Apart from that the observed high negative zeta potential indicates that our reported samples are stable against aggregation in dispersion medium. The superior catalytic activity of the highest 6% Co doped MoS₂ layers is possibly due to a combination of (a) stabilization of the metallic 1T phase with increasing electrical conductivity (decreasing band gap and activation energy) and (b) better electron capture from the hydride and electron supply to the nitrophenol substrates through reversible redox reactions at the Co sites. In a nutshell, our study clearly exhibits that proper tuning of Co doping in MoS₂ nanosheets is another potential alternative of noble metal catalyst for the catalytic reduction of nitroarene.

Conflicts of interest

There are no conflicts to declare.