Achieving highly efficient electrocatalytic hydrogen evolution with Co-doped MoS₂ nanosheets

Abstract

MoS₂ is a promising hydrogen evolution reaction (HER) catalyst because of the Pt-like activity at the side edges, but the whole activity is restricted by the inert basal plane. Herein, Co-doped 1T-MoS₂ nanosheets are grown on carbon cloth (CC) through hydrothermal synthesis and exhibit superior HER activity with an overpotential of 69 mV@10 mA cm⁻² and a Tafel slope of 81.84 mV dec⁻¹ as well as durability for over 100 h at 100 mA cm⁻² in an alkaline medium. The detailed structural tests demonstrate that the improved HER activity is attributed to Co doping and the high 1T phase content. Co doping induces transformation from the 2H to the 1T phase (67%), and further TMA⁺ addition increases the doping amount and the 1T phase content (79%). The excellent durability is due to the strong interface binding between MoS₂ nanosheets and CC associated with the heterogeneous nucleation and growth and the high growth temperature (230 °C). This provides an inspiration for developing efficient and stable MoS₂ catalysts by element doping.

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New concepts

Enhancing the intrinsic conductivity of MoS₂ and rationally tailoring its catalytic sites are highly desirable and challengeable for high electrocatalytic activity for the hydrogen evolution reaction (HER). Doping is an effective strategy to optimize the electronic structure of MoS₂ and improve its catalytic performance. In this study, we propose a new concept for heterogeneous nucleation and growth of Co-doped 1T-MoS₂ of high HER electrocatalytic activity in alkaline media. Our experiment was conducted under the conditions of low supersaturation in an extremely low-concentration solution, which ensures heterogeneous nucleation at active sites rather than homogeneous nucleation randomly. This low concentration requires the reaction to occur necessarily at a high temperature (230 °C), which leads to the strong bonding between the MoS₂ nanosheets and the substrate and then the excellent electrocatalytic stability of MoS₂ under high current densities (100 mA cm⁻²). This is distinctly different from the previous reports using high concentrations and lower temperatures (about 200 °C). In addition, the introduction of TMA⁺ into the solution generated a large number of defects which facilitated Co doping and significantly enhanced HER activity.

Introduction

Water electrolysis for hydrogen production is crucial for resolving the problems associated with intermittent renewable energy from wind and solar power.¹ Developing hydrogen evolution electrocatalysts is challenging to achieve efficient, stable, and cost-effective water electrolysis.^2,3 MoS₂ has been considered a highly promising candidate catalyst for the HER due to its hydrogen adsorption energy close to Pt group metals (ΔG_H* ≈ 0) at the edge sites.^4–6 Moreover, MoS₂ exhibits excellent anticorrosion resistance to most chemical reagents including acids and bases. However, MoS₂ generally exists in the most readily available 2H phase, which shows poor HER activity because of the semiconductor property and the large inert basal plane.^7–11

Compared to the semiconductor 2H phase, the 1T phase MoS₂ has metal-like conductivity and more electrochemically active sites along the basal plane, predicting an active HER catalyst.^12–15 Various top-down and bottom-up methods have been developed to produce 1T-MoS₂.^4,16–20 For the top-down method, the bulk 2H phase is exfoliated to the sheet-like 1T phase by guest species intercalating into the van der Waals gap between the 2H layers. The intercalation enlarges the interlayer spacing and weakens the interlayer van der Waals forces, leading to interlayer distortion or exfoliation, producing few- or single-layer nanosheets. The reported guest species for intercalation include alkali metal ions (such as Li⁺),^21–25 H₂,²⁶ NH₃,²⁷ NH₄⁺,^28–30 TMA⁺,^31–33etc. For the bottom-up method, the 1T phase is directly synthesized using hydrothermal methods.³⁴ Solution concentration, additives, pH, temperature, etc. can be smartly regulated to modify the interlayer spacing, electronic structure of catalytically active sites, and the 1T phase content to enhance the catalytic activity. For instance, defect-rich 1T-MoS₂ was obtained by adding organic or inorganic acids, achieving an alkaline HER overpotential of 135 mV at 10 mA cm⁻².^32,35 Furthermore, various routes, including activating inert atoms in the basal plane to generate sulfur vacancies by argon (Ar) plasma exposure,^36,37 H₂ annealing,⁶ and electrochemical⁵ or chemical etching desulfurization, have been explored to improve the HER activity. The optimal alkaline HER overpotential of 131 mV@10 mA cm⁻² was achieved by the H₂O₂ etching strategy.³⁸ Despite these efforts, the activity is still far below most reported HER catalysts. In this context, heteroatom doping emerges as a promising avenue to bridge this performance gap and achieve electrocatalytic activity comparable to other highly efficient HER catalysts.³⁹ Foreign atoms, such as Pt,^40,41 Co,^42–46 Ni,^47,48 and Ru,⁴⁹ could alter the local electronic structure and activate sulfur atoms within the basal plane and have been shown to enhance the HER activity, and the optimal alkaline overpotential is 59 mV@10 mA cm⁻².²⁶ During these synthesis processes, auxiliary measures such as injection of hydrogen are needed to promote the doping effect. However, the synthesized samples are usually in powder form and have to be bound to electrodes with binders, which shields active sites, degrades conductivity, and compromises stability especially under a large current density.⁵⁰ In comparison, catalysts grown directly on the electrodes have strong adhesion and improved activity.⁵¹

In our previous study, MoS₂ nanosheets containing the 1T phase (88%) were grown directly on CC through a supercritical hydrothermal growth process and showed improved HER activity and durability.³² In this work, Co-doped 1T-MoS₂ nanosheets were grown directly on CC through this process, where the high-concentration of sulfuric acid ensures Co doping into MoS₂ but not forming sulfide. The critical growth requires a higher temperature (230 °C), which enhances the adhesion between MoS₂ and the substrate and consequently the mechanical stability for durable H₂ generation. The as-grown Co-doped 1T MoS₂ (Co-1T-MoS₂) nanosheets contained a high content of 1T phase (79%) and showed excellent HER performance with an overpotential of only 69 mV@10 mA cm⁻² and good stability for over 100 hours at 100 mA cm⁻² (1 M KOH), surpassing most reported MoS₂-based catalysts.

Experimental section

Synthesis of MoS₂ catalysts

0.083 mmol of Na₂MoO₄, 0.5 mmol of CS(NH₂)₂, and 3 mL of concentrated sulfuric acid (95–98%) were dissolved in 60 mL of deionized water. Additionally, 0.125 mmol of CoSO₄ and 0.096 mol of tetramethylammonium sulphate (C₈H₂N₂SO₄, TMA⁺) were added to control the composition and structure of MoS₂ nanosheets to synthesize Co-doped 1T-MoS₂ (Co-1T-MoS₂). The mixing solution was transferred into a 100 ml autoclave, and then a carbon cloth (CC), pre-treated in a mixture of 98% sulfuric acid and 65% nitric acid at a ratio of 1 : 3 for 12 hours, was placed upright in the autoclave. The autoclave was maintained at 230 °C in an oven for 24 hours. After cooling down to room temperature, the samples were retrieved and ultrasonically cleaned with deionized water for 5 minutes. The sample prepared without CoSO₄ and TMA⁺ is marked as MoS₂-blank, those with CoSO₄ and TMA⁺, CoSO₄, and TMA⁺ as Co-1T-MoS₂, Co-MoS₂, and 1T-MoS₂, respectively. The Co-1T-MoS₂ sample is repeatedly grown using two (Co-1T-MoS₂-2) and three growing cycles (Co-1T-MoS₂-3) to increase the nanosheet density.

HER activity measurements

All electrochemical measurements were conducted on a DH7000C electrochemical workstation. A three-electrode system was utilized, with Hg/HgO (1M KOH) as the reference electrode, a carbon block (2 × 3 cm²) as the counter electrode, and the synthesized samples as the working electrode. The testing solution was 1 M KOH. Linear sweep voltammetry (LSV) measurements were conducted within a voltage range from −0.9 V to −1.25 V at a scan rate of 5 mV s⁻¹ with 90% iR compensation. The solution resistance was determined using electrochemical impedance spectroscopy (EIS). The recorded potentials were converted to the reversible hydrogen electrode (RHE) scale using the equation E_RHE = E_Hg/HgO + 0.0591 V × pH + 0.098 V. From the LSV plot, the logarithm of current density was taken as the abscisic coordinate with the value of overpotential as the ordinate to obtain the relevant Tafel curve. ECSA was evaluated by cyclic voltammetry (CV) from −0.1 V to −0.5 V versus Hg/HgO at sweep rates of 20–100 mV s⁻¹. The electrochemical impedance spectroscopy (EIS) measurements were carried out at −1.2 V control potential with the frequency ranging from 10 000 to 0.1 Hz.

DFT calculations

All first-principles DFT calculations were performed using the VASP software package. The exchange and correlation effects of electrons were described using the Perdew–Burke–Ernzerhof functional of generalized gradient approximation (GGA-PBE). During structure optimization, the cutoff energy was set to 400 eV and the k-point was set to 2 × 2 × 1. In addition, the force and energy convergence criteria were set to 0.03 and 1 × 10⁻⁵ eV, respectively. In the subsequent calculation of charge distribution and density of states (analysis of electronic properties), the k-point is set to 6 × 6 × 1 to ensure accuracy. We constructed correlative theoretical models to simulate MoS₂ with S-vacancies and Co doping. Based on defect formation energy calculations reported in the literature,^52,53 sulfur vacancies near the Co site are found to have the lowest defect formation energy, and so sulfur vacancy was selected as the adsorption site to ensure a reproducible theoretical experiment.

Material characterization

X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Rigaku SmartLab 9 kW) with Cu Kα radiation at 40 kV and 150 mA. Raman spectroscopy analysis of the catalysts was performed using a Raman spectrometer (LabRAM Odyssey, Horiba). The morphology was observed by field emission scanning electron microscopy (FESEM, Gemini 300, Zeiss) at an accelerating voltage of 2 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB Xi +, Thermo) was used to obtain the photoelectron spectra using Al Kα radiation. Transmission electron microscopy (TEM, JEOL JEM2100F) was employed to observe the microstructure at an accelerating voltage of 200 kV. The quantitative analysis of Co doping in the sample is performed using an inductively coupled plasma optical emission spectrometer (ICP-MS-Agilent 7850). The electron paramagnetic resonance (EPR) spectra were collected at room temperature using a Bruker E300 spectrometer.

Results and discussion

Analysis and characterization of catalysts

All MoS₂ nanosheets were grown on CC via a supercritical hydrothermal process, as schemed in Fig. 1a. The SEM images in Fig. 1b show the morphological feature of the as-prepared Co-doped 1T MoS₂ (Co-1T-MoS₂) nanosheets. For comparison, the morphologies of samples with and without TMA⁺ or CoSO₄, including MoS₂-blank, Co-MoS₂, and 1T-MoS₂, are all presented. It can be seen that the Co-1T-MoS₂ nanosheets are directly grown on the carbon fibres (Fig. 1b). The nanosheets exhibit an edge-curled structure with individual nanosheet sizes around 100–200 nm, seeming extremely thin and even translucent. The distribution density of nanosheets is relatively low on the surface of carbon fibre after the first growing cycle, and after the second (Co-1T-MoS₂-2) and third (Co-1T-MoS₂-3) growing cycles, the nanosheet density increases significantly but the thickness does not increase notably (Fig. S1, ESI†). In the 1T-MoS₂ sample (Fig. 1c) merely with TMA⁺ but no Co addition, the nanosheets are curled more seriously while the thickness is similar to that of the Co-1T-MoS₂ sample. In contrast, for the MoS₂-blank and Co-MoS₂ samples without TMA⁺ addition, nanosheets extend fully and grow vertically on the carbon fibres and are much thicker (Fig. 1d and e). The results suggest that the introduction of large cations (TMA⁺) during the growth prevents the layer stacking and then the increase in thickness. It is believed that the reduced nanosheet thickness increases the internal stress in the layer and causes curling.⁵⁴ Despite the difference in morphology and thickness for the samples, the nanosheet density remains similar, which is related to the critical growth that supports the heterogeneous nucleation preferentially on the active sites of the fibres. The heterogeneous nucleation is confirmed by the observation on the inner wall of the reaction autoclave under varied conditions (Fig. S2, ESI†). In the absence of TMA⁺, homogeneous nucleation occurs, and a noticeable black precipitate is formed on both the inner wall and the bottom. In contrast, in the presence of TMA⁺, less black residue is observed after the first growing circle, and no residue is produced after the second and third growing cycles. It can be concluded that TMA⁺ adsorption on the surface of MoS₂ nanosheets inhibits homogeneous nucleation and favours heterogeneous nucleation and growth on the pretreated carbon cloth. The heterogeneous nucleation and growth are ascribed to the extremely low reactant concentration and TMA⁺ adsorption. This is further confirmed by the reduced residue during the second and third growing cycles compared to the first growing one since the formed nanosheets will consume the reactants in the growth solution, which lowers supersaturation that is more favourable to the heterogeneous nucleation and growth.

Fig. 1 (a) Schematic synthesis process of MoS₂ nanosheets on CC. SEM images of samples (b) Co-1T-MoS₂, (c) 1T-MoS₂, (d) Co-MoS₂, and (e) MoS₂-blank. HRTEM image of (f) Co-1T-MoS₂, (g) and (h) magnified 1T phase and 2H phase region, and (i) the corresponding TEM-EDX elemental mappings of Co-1T-MoS₂.

Fig. 1f–i presents the HRTEM images and the corresponding EDX elemental mapping of nanosheets scraped from the Co-1T-MoS₂ sample. The vertical two-, three-, and six-layer stacked MoS₂ nanosheets as well as the horizontally lying single sheets suggest the extremely thin nanosheet thickness (Fig. 1f). Distinct distortions and defective atomic arrangement are observed in regions marked by yellow dashed circles. The enlarged image in Fig. 1g reveals that the nanosheet, comprising six MoS₂ monolayers, exhibits an interlayer spacing of 0.712 nm, obviously larger than that of the standard 2H phase (002) plane spacing (0.624 nm). This enlarged interlayer spacing may facilitate a phase transformation from 2H to 1T. The magnified region marked by the red rectangle in Fig. 1h displays two distinct crystal phase regions. The lattice spacing on the left side is 0.27 nm, with an interplane angle of 60°, which aligns with the lattice structure characteristic of the 2H-MoS₂ (100) plane. The lattice structure on the right clearly displays the trigonal lattice structure of the 1T phase with octahedral coordination.⁵⁵ The observation suggests the coexistence of 1T and 2H phases within these nanosheets. Additionally, extensive areas devoid of discernible lattice features are attributed to lattice mismatch between the 1T and 2H phases and play a role in alleviating interfacial stress between different crystal phases and stabilizing the 1T phase.³⁸ Energy-dispersive X-ray spectroscopy (TEM-EDS) shows that Co, Mo, and S elements are uniformly distributed on the nanosheets, and the calculated atomic ratio of Mo to S is 0.88 : 1, significantly deviating from the stoichiometry (1 : 2) due to S vacancies (Fig. S3, ESI†). Actually, S vacancies were proved by the calculated g-value of 1.991 according to the electron paramagnetic resonance (EPR) spectroscopy results (Fig. S4, ESI†).

Fig. 2a and b presents the XRD patterns of the samples and the standard MoS₂ powder card (PDF#75-1539). The XRD patterns of all samples align well with those of the standard 2H phase MoS₂, showing characteristic diffraction peaks at the (002), (110), and (101) crystal planes.^56,57 It can be found that Co doping alone does not change the peak intensity obviously compared to TMA⁺. However, with the assistance of TMA⁺, the diffraction intensity is greatly weakened by Co doping. The narrow and strong peaks at 14.1° for MoS₂-blank correspond to the standard interlayer spacing (6.15 Å) of the 2H phase. Although the 1T phase is clearly observed in TEM, no diffraction peak of 1T-MoS₂ appears in the XRD patterns of all samples grown once. It is believed that 1T phase MoS₂ is mostly in few-layer or monolayer structures, and no diffraction peak of the 1T phase can be detected, especially because most of the sheets are vertically grown on the substrate. It should be noted that with repeated growing, the increased nanosheet density (Fig. S1, ESI†) suppresses and weakens the carbon peaks in the XRD patterns gradually (Fig. 2b).

Fig. 2 XRD patterns of samples. (a) Grown with Co doping or TMA⁺ addition and (b) grown with Co doping and TMA⁺ addition and grown once, twice, and thrice. (c) XPS survey spectra, (d) high-resolution Mo 3d–S 2s and (e) Co 2p of Co-MoS₂ and Co-1T-MoS₂, and (f) Raman spectra of MoS₂-blank and Co-1T-MoS₂.

To further analyze the phase composition and content of the as-synthesized MoS₂ samples, Fig. 2c–e presents the detailed XPS analysis results. The XPS survey spectra of all samples indicate the presence of Mo 3d and S 2p. In the XPS spectrum of MoS₂-blank, the doublet is deconvoluted into two types of doublets: the Mo 3d_5/2 and Mo 3d_3/2 orbitals of 2H-MoS₂ at 228.57 eV and 232.44 eV, and the Mo 3d_5/2 and Mo 3d_3/2 orbitals of 1T-MoS₂ at 228.95 eV and 231.77 eV, the binding energy of which is lower than that of 2H-MoS₂.⁵⁸ The peak area analysis reveals that the MoS₂-blank sample contains 42% of the 1T phase, indicating that Co doping induces a partial phase transformation from 2H to 1T. For the sample with only CoSO₄ addition (Co-MoS₂), the concentration of the 1T phase is about 67% according to XPS analysis, which is significantly higher compared to the MoS₂-blank sample. Moreover, in the XPS survey spectrum (Fig. 2e), characteristic peaks of Co 2p_3/2 and 2p_1/2 are observed. The deconvoluted high-resolution Co 2p spectrum shows peaks at 778.5 eV and 793.6 eV, attributed to Co 2p_3/2 and Co 2p_1/2 of Co–Mo–S formed after Co is doped into the MoS₂ lattice.⁵⁹ It is worth noting that no peaks corresponded to Co²⁺,⁴⁵ indicating that Co exists purely as a dopant but not in any sulfide form. Additionally, the doping concentration is calculated to be 3.84% based on the area integration of the deconvoluted Co 2p and Mo 3d peaks. For the sample with only TMA⁺ addition (1T-MoS₂), the 1T phase is also detected in the deconvoluted spectrum. The calculated 1T phase content is 68%, confirming that the large cations promote the formation of the 1T phase during MoS₂ growth. Moreover, with Co doping into the sample with TMA⁺ (Co-1T-MoS₂), the 1T phase content further increased to 79% and the calculated Co doping concentration is 4.33%. These findings demonstrate that during the hydrothermal synthesis Co can be effectively doped into the MoS₂ lattice, and the doping concentration is increased with the assistance of TMA⁺. In particular, there is no Mo⁶⁺ peak in all of the deconvoluted high-resolution Mo 3d spectra (Fig. 2d), although it is commonly observed in the reported hydrothermal results.^26,45 This is associated with our hydrothermal process. The extremely low reactant concentration leads to a slow reaction rate and then heterogeneous nucleation and growth, which avoids rapid grain accumulation from the homogenous nucleation. Furthermore, the high hydrothermal temperature provides enough driving kinetics for reducing Mo⁶⁺ to MoS₂, which becomes the prerequisite for the good HER performance of MoS₂ nanosheets.

Considering the low intensity of the Co characteristic peak in the XPS spectrum might lead to a large deviation in quantifying the Co doping level, and we further performed inductively coupled plasma (ICP) analysis for the precise determination of the Co doping content (Table S2, ESI†). By comparing the Co/Mo molar ratios for Co-MoS₂ (1.35%) and Co-1T-MoS₂ (7.8%), it is deduced that Co can be effectively doped into the MoS₂ lattice during the present hydrothermal synthesis, and TMA⁺ addition promotes Co doping.

Furthermore, from the Raman spectroscopy results of Co-1T-MoS₂ and MoS₂-blank samples (Fig. 2f), it is found that the MoS₂-blank sample exhibits only two characteristic peaks of the 2H phase located at 380.8 cm⁻¹ (E¹_2g) and 409.2 cm⁻¹ (A_1g)^60,61 while the Co-1T-MoS₂ sample displays signals not only for the 2H phase (368.6 cm⁻¹ (E¹_2g) and 402.5 cm⁻¹ (A_1g)) but also for the 1T phase with peaks corresponding to the J₁, J₂, and J₃ vibrational modes at 144.6, 193.1, and 333.9 cm⁻¹, respectively.^62–64 Compared to MoS₂-blank, the redshift in the E¹_2g and A_1g modes in the Raman spectrum of Co-1T-MoS₂ confirms the successful incorporation of Co into the MoS₂ two-dimensional (2D) plane, which weakens the Mo–S related modes, thereby lowering their vibrational frequencies.

Evaluation of electrochemical HER activity

The HER performance of all samples was evaluated in an alkaline electrolyte. The tests were conducted using a three-electrode system in 1 M KOH solution with 90% IR compensation. The LSV polarization curves in Fig. 3a show that the MoS₂-blank sample of the pure 2H phase exhibits an overpotential of 201 mV at 10 mA cm⁻². The introduction of TMA⁺ (1T-MoS₂) or doping with Co alone (Co-MoS₂) improves the catalytic activity with the overpotential decreasing to 184 mV and 174 mV at 10 mA cm⁻², respectively. When TMA⁺ and CoSO₄ are added together (Co-1T-MoS₂), the HER activity is significantly enhanced. The overpotential decreases to 95 mV, surpassing most of the previously reported transition metal sulfides (Table S1, ESI†). The reaction kinetics of the samples were studied using EIS (Fig. 3b). The Nyquist plots show that with TMA⁺ and Co added sequentially, the charge transfer resistance (R_ct) decreases progressively from the initial 17.68 Ω to 7.86 Ω, 1.47 Ω, and 0.98 Ω in the sequence of MoS₂-blank, Co-MoS₂, 1T-MoS₂, and Co-1T-MoS₂, corresponding to guest intercalation, heteroatom doping, and the transformation from the 2H to the 1T phase, respectively. This indicates that the introduction of Co and TMA⁺ enhances the charge transfer efficiency. The R_ct of the Co-1T-MoS₂ sample is significantly smaller than the minimum results reported in the literature for alkaline solutions (3.3 Ω) (Table S1, ESI†).

Fig. 3 HER performance of MoS₂-blank, Co-MoS₂, 1T-MoS₂, Co-1T-MoS₂, and Co-1T-MoS₂-2. (a) Polarization curves in the range of −0.9 V to −1.25 V versus Hg/HgO at a scan rate of 5 mV s⁻¹ with 90% iR compensation. (b) Nyquist plots of electrochemical impedance spectra. (c) Corresponding Tafel charts. (d) Linear regression of current densities at different scan rates for the calculation of C_dl. (e) Chronopotentiometry durability curves of Co-1T-MoS₂-2 and 1T-MoS₂ at 100 mA cm⁻² for 100 h. (f) Polarization curves of Co-1T-MoS₂-2 before and after stability tests. (g) Polarization curves of 1T-MoS₂ before and after stability tests. (h) Comparison for overpotentials at 10 mA cm⁻² of various samples.

The catalytic kinetics were analyzed according to the Tafel slope in Fig. 3c. The Tafel slopes reflect the difference in the reaction rate-determining steps (RDS) on the surface of catalysts. In an alkaline medium, the Tafel slope for the Volmer reaction (H₂O + e⁻ → H_ads + OH⁻), Heyrovsky reaction, and Tafel reaction is 120 mV dec⁻¹, 40 mV dec⁻¹, and 30 mV dec⁻¹, respectively.⁶⁵ The Tafel slope for Co-1T-MoS₂, Co-MoS₂, 1T-MoS₂, and MoS₂-blank is 81.84 mV dec⁻¹, 98.63 mV dec⁻¹, 105.75 mV dec⁻¹, and 108.21 mV dec⁻¹, respectively. All Tafel slopes are between 40 and 120 mV dec⁻¹, indicating that the HER rate is determined by both the Volmer reaction and the Heyrovsky reaction. Also, the electrochemical surface areas (ECSA) were calculated by measuring the capacity of the double layer (C_dl) to represent the area of the catalytically active sites in contact with the electrolyte.⁴⁶ ECSA is proportional to C_dl which is derived from the CV curves at different scan rates (20–100 mV s⁻¹) (Fig. S5, ESI†). As shown in Fig. 3d, with the sequential introduction of TMA⁺ and Co into the synthesis solution, the ECSA increases from 21.3 mF cm⁻² (MoS₂-blank) to 23.3 mF cm⁻² (Co-MoS₂), 43.29 mF cm⁻² (1T-MoS₂), and 44.38 mF cm⁻² (Co-1T-MoS₂). Since the samples have similar specific surface areas due to their similarity in nanosheet size and distribution density, the increased ECSA is thought to be mainly attributed to the improved active site density on the basal planes of MoS₂ nanosheets with the formation of the 1T phase and Co doping by adding TMA⁺ and Co species into the hydrothermal solution during the growth.

The heterogeneous nucleation results in selective nucleation, and thus MoS₂ nanosheets grow sparsely on the CC surface. To increase the nanosheet density, multiple-cycle growth was performed to improve the electrochemical surface area and the HER activity (Fig. S6, ESI†). Comparing the LSV polarization curves of samples grown for one to three cycles (Co-1T-MoS₂, Co-1T-MoS₂-2, and Co-1T-MoS₂-3), it is clear that the HER overpotential decreases progressively from 95 mV to 69 mV and finally to 59 mV. Although the overpotential of the sample after one growing cycle is slightly larger than the previously reported value (Table S1, ESI†), it is greatly reduced and even lower than the reported minimum value for Co-doped samples after two and three growth cycles. We attribute the improved activity to the increased ECSA with increasing the growing cycles (Fig. S6c, ESI†) but not related to the electrocatalytic kinetics. Thus, all samples grown for one to three cycles have the same Tafel plot (Fig. S6d, ESI†). However, the nucleation and growth on the previously grown MoS₂ nanosheets, undergoing multiple growing cycles, inevitably lead to the formation of interfaces between two growing cycles and produce the interfacial capacitance. This is proved by the emerging semicircles in the EIS spectra for the samples grown for two and three cycles (Fig. S6b, ESI†). It should be noted that the last semicircle in each EIS spectrum represents the charge transfer rate (R_ct). The same R_ct value for samples grown for different cycles suggests that the R_ct depicts the intrinsic activity of Co-1T-MoS₂ and has no relationship to the ECSA.

Stability is the prerequisite for evaluating whether a catalyst could be used as a commercial electrode. Fig. 3e shows the chronopotentiometry (CP) durability curves of 1T-MoS₂ and Co-1T-MoS₂-2 at 100 mA cm⁻² for 100 h. After 100 hours of continuous CP testing, the HER overpotential at 100 mA cm⁻² shows no essential change for both samples. Comparing the LSV polarization curves before and after the CP test, the HER catalytic activity changes slightly. For Co-1T-MoS₂-2, the overpotential at 10 mA cm⁻² is increased by 4 mV while the current density is increased at a more negative potential. As for 1T-MoS₂, the overpotential at 10 mA cm⁻² is increased by 5 mV while the current density is decreased at a more negative potential. According to previous reports, 1T-MoS₂ is a metastable phase that easily converts to the more stable 2H phase, accounting for the decrease in HER activity.⁶⁶ In contrast to the reports, the calculated 1T phase content in 1T-MoS₂ and Co-1T-MoS₂-2 after the durability test was increased to 77% and 86% (Fig. S7a and b, ESI†) compared to 68% and 79% before the durability test. This is beyond the traditional understanding of the thermodynamic metastability of the 1T phase. Firstly, the separated growth of individual nanosheets on the surface of carbon fibers effectively prevents nanosheet stacking and the transformation from the 1T phase to the 2H phase. Secondly, the shocking of gas bubbles produced during the test could expand the interlayer distance and even leads to complete separation of stacked nanosheets, which in part promotes the transformation of nanosheets from the 2H to the 1T phase. However, a high binding energy Mo⁶⁺ characteristic peak appears in the high-resolution Mo 3d spectra of both samples after the stability test (Fig. S7, ESI†) while there is no Mo⁶⁺ peak appearing in the XPS spectra of both the samples before the test (Fig. 2d). This suggests that MoS₂ is partly oxidized to MoO₃ during the test, which in a certain degree supports the activity degradation. The more negative redox potential of Mo⁶⁺ relative to H₂ evolution at 100 mA cm⁻² in the alkaline solution should contribute to the oxidization.⁶⁷ This means that MoS₂ is more appropriate for H₂ generation in the acidic solution or in the alkaline solution at much more negative potential and a larger current density.

Additionally, stability is related not only to the inherent structure of the catalyst but also to the bonding strength between the catalyst and the substrate.⁶⁸ SEM images (Fig. S8a and b, ESI†) of 1T-MoS₂ and Co-1T-MoS₂ after the durability test showed that the nanosheets maintain nearly the same morphologies after the 100-hour durability test, indicating the excellent bonding strength between MoS₂ nanosheets and the CC substrate. This is due to the heterogeneous nucleation and growth especially at extremely high temperatures (230 °C). In contrast, the controlled sample, grown at a lower temperature (210 °C) in the same solution and meanwhile reaching the same nanosheet size and density when the growth duration was extended to 48 h, shows degraded stability after a 100-hour test at 100 mA cm⁻² (Fig. S9, ESI†). That is, the nanosheets grown at the lower growth temperature have weakened bonding strength, as confirmed by the SEM image in Fig. S8c (ESI†) and the nanosheets collapse or even peel off after testing.

DFT calculations

To further understand the roles of Co doping and TMA⁺ intercalation in the HER activity of Co-1T-MoS₂, DFT calculations were performed to clarify the electronic structure of MoS₂ models, including 2H-MoS₂, 1T-MoS₂, Co-1T-MoS₂ (6.2 Å), and Co-1T-MoS₂ (7.1 Å).

The hydrogen adsorption free energy (ΔG_H*) is a critical parameter for evaluating the HER activity of catalysts. ΔG_H* close to zero indicates a lower energy barrier for hydrogen evolution, corresponding to enhanced catalytic activity. As depicted in Fig. 4a, the large absolute values of ΔG_H* for pristine 2H (2.6017 eV) and 1T (2.0704 eV) phases suggest their poor HER performance. For Co-1T-MoS₂ with a normal layer distance (6.2 Å), the ΔG_H* is 0.1942 eV. However, for the synthesized Co-1T-MoS₂ sample, the interlayer spacing is expanded to 7.1 Å due to TMA⁺ intercalation, and correspondingly ΔG_H* is reduced to 0.1285 eV. This markedly reduced ΔG_H* due to the synergistic effects of the 1T phase, Co doping, and expanded interlayer spacing is theoretically conducive to the HER catalytic activity.

Fig. 4 DFT simulation results: (a) calculated free-energy diagram of the HER on the bases of Co-1T-MoS₂ (7.1 Å), Co-1T-MoS₂ (6.2 Å), 1T-MoS₂, and 2H-MoS₂. (b) The related kinetic energy barriers of H₂O dissociation for 1T-MoS₂ and Co-1T-MoS₂ (7.1 Å), (c) charge density difference, and (d) the PDOS of Mo, S and Co.

However, the water dissociation is the RDS in the alkaline solution. The adsorption and dissociation model of water molecules on Co-1T-MoS₂ (7.1 Å) is illustrated in Fig. S10 (ESI†). In this model, Co substitution for Mo atoms introduces S vacancies and O is stabilized on the S vacancies near Mo, as shown in the constructed structural model (Fig. S10b, ESI†). Co substitution for Mo may induce lattice strain and local instability, potentially leading to the formation of S vacancies to alleviate this strain and reduce the system free energy. The calculated free energy diagram for water dissociation reveals that the energy barrier on Co-1T-MoS₂ (7.1 Å) is 0.67 eV, substantially lower than the 1.46 eV observed for the pristine 1T phase. A comparison of the charge differential density between 1T-MoS₂ and Co-MoS₂ at the doping site indicates the increased electron transfer to Co, which helps modulate the water dissociation energy (Fig. 4c).

To further elucidate the impact of Co doping and the 1T phase on the HER activity, we calculated the density of states of all models, as shown in Fig. 4d. According to the projected density of states (PDOS), the bandgap of 1T-MoS₂ is significantly narrower than that of the 2H phase, indicating that the 1T phase exhibits greater metallicity and superior intrinsic electrical conductivity. Additionally, the doped Co occupying the Mo sites increases the density of states (DOS) at the Fermi level, facilitating electron transportation. This enhanced electron transportation during the reaction process accelerates the catalytic reaction rate, in agreement with the experimental findings.

Conclusion

Co-doped 1T-phase MoS₂ nanosheets are directly grown on carbon cloth using the hydrothermal method. The heterogeneous nucleation and growth are realized in an extremely low-concentration solution at a high temperature (230 °C). The high concentration of acid in the solution ensures Co doping rather than forming a Co-based compound. TMA⁺ addition during the synthesis promotes the formation of the 1T phase and in turn increases the Co-doping content. The individual growth of each nanosheet prevents the possible stacking during the HER process and then inhibits the phase transformation from 1T to 2H. The as-synthesized Co-doped 1T-phase MoS₂ exhibits active alkaline HER performance with an overpotential of only 69 mV at 10 mA cm⁻² and a Tafel slope of 81.84 mV dec⁻¹. Moreover, the catalyst maintains excellent stability at 100 mA cm⁻² for over 100 h. DFT calculations indicate that Co doping produces S vacancies in MoS₂ nanosheets and O occupation in V_S reduces the dissociation energy of water, which is inducive to improving the HER activity of Co-doped 1T-phase MoS₂. The high hydrothermal temperature significantly enhances the binding strength of MoS₂ sheets on CC, which contributes to the HER stability. This research makes a solid stride for MoS₂ to practical application in electrocatalytic water splitting for hydrogen production.

Author contributions

Fengrui Sun: writing – review & editing, writing – original draft, methodology, investigation, formal analysis, and data curation. Kebin Yang: data curation, methodology, and validation. Xinbo Qin: data curation, formal analysis, and investigation. Weibing Wu: funding acquisition, project administration, resources, writing – review & editing, conceptualization, formal analysis, methodology, and supervision. Yizhong Lu: writing – review & editing, investigation, formal analysis, methodology, and supervision.

Data availability

All data generated or analyzed during this study are included in this published article and its ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Shandong Provincial Natural Science Foundation, China (ZR2019MEM033).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nh00111k

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