Synergistic Regulation of S-Vacancy of MoS2-Based Materials for Highly Efficient Electrocatalytic Hydrogen Evolution

Low or excessively high concentration of S-vacancy (C_S-vacancy) is disadvantageous for the hydrogen evolution reaction (HER) activity of MoS₂-based materials. Additionally, alkaline water electrolysis is most likely to be utilized in the industry. Consequently, it is of great importance for fine-tuning C_S-vacancy to significantly improve alkaline hydrogen evolution. Herein, we have developed a one-step Ru doping coupled to compositing with CoS₂ strategy to precisely regulate C_S-vacancy of MoS₂-based materials for highly efficient HER. In our strategy, Ru doping favors the heterogeneous nucleation and growth of CoS₂, which leads to a high crystallinity of Ru-doped CoS₂ (Ru-CoS₂) and rich heterogeneous interfaces between Ru-CoS₂ and Ru-doped MoS_2-x (Ru-MoS_2-x). This facilitates the electron transfer from Ru-CoS₂ to Ru-MoS_2-x, thereby increasing C_S-vacancy of MoS₂-based materials. Additionally, the electron injection effect increases gradually with an increase in the mass of Co precursor (m_Co), which implies more S^2- leaching from MoS₂ at higher m_Co. Subsequently, C_S-vacancy of the as-synthesized samples is precisely regulated by the synergistic engineering of Ru doping and compositing with CoS₂. At C_S-vacancy = 17.1%, a balance between the intrinsic activity and the number of exposed Mo atoms (EMAs) to boost highly active EMAs should be realized. Therefore, the typical samples demonstrate excellent alkaline HER activity, such as a low overpotential of 170 mV at 100 mA cm⁻² and a TOF of 4.29 s⁻¹ at -0.2 V. Our results show promise for important applications in the fields of electrocatalysis or energy conversion.

GRAPHICAL ABSTRACT

GRAPHICAL ABSTRACT

Introduction

The rapid development of the economy has made the fast consumption of fossil energy, resulting in the energy crisis and severe environmental pollution (Xie et al., 2013b; Xu et al., 2017; Jiang et al., 2022). Hydrogen energy, as one of the clean and renewable energies, has received extensive attention around the world (Huang et al., 2019; Venkateshwaran and Senthil Kumar, 2019; Zhu et al., 2019; Wang F. et al., 2020). Electrocatalytic water splitting (Lin et al., 2017; Lin et al., 2019; Djara et al., 2020; Liu L. et al., 2022) is regarded as an eco-friendly technology for hydrogen (H₂) production. So far, platinum-based materials are still the best electrocatalysts for acidic water electrolysis (Chen et al., 2020). Nevertheless, the high cost and low abundance seriously restrict the wide applications of such precious metals (Li et al., 2011; Qi et al., 2019; Jing et al., 2020). On the other hand, alkaline water electrolysis is most likely to be utilized in the industry, owing to the unrestricted reactant availability, desirable safety, and satisfactory output (Yin et al., 2015). However, the sluggish dynamics of hydrogen evolution reaction (HER) in alkaline environments results in excessive energy consumption (Yin et al., 2015). Therefore, it is of great significance for the development of highly efficient and low-cost electrocatalysts to overcome energy barriers for accelerating kinetics and to decrease overpotential during the hydrogen evolution reaction (HER) process.

It is well known that molybdenum disulfide (MoS₂) can be designed as an alternative to Pt due to the excellent H* adsorption–desorption properties, special layered structures, good stability, and low cost (Hinnemann et al., 2005; Deng et al., 2017; Wang Y. et al., 2019; Gao et al., 2020). However, the pristine 2H-MoS₂ tends to aggregate under van der Waals forces, resulting in poor edge active sites (Kibsgaard et al., 2012; Wang et al., 2017; Wang et al., 2021b). More importantly, there remain a lot of active sites from the vast basal planes of such MoS₂ to be developed (Kibsgaard et al., 2012; Wang et al., 2017; Wang et al., 2021b). To address these drawbacks, defect-rich, double-gyroid, and amorphous structures have been introduced into such MoS₂ nanosheets to increase the unsaturated sulfur atoms as active sites for HER (Kibsgaard et al., 2012; Xie et al., 2013a; Liu et al., 2018). Doping transition metals (TMs) into MoS₂ is another important method for the advancement of electrocatalysis. This is because doping TMs not only increases the number of unsaturated sulfur atoms but also regulates the adsorption free energy of hydrogen atoms (ΔG_H) of active sites to favor HER (Wang et al., 2015; Wang D. et al., 2019; Wang et al., 2021b). Among these TMs, ruthenium (Ru) belongs to the Pt group, but its price is as low as about 5% of Pt (Zhang J. et al., 2019). A quintessential example demonstrates that ΔG_H at the Ru-doped in-plane sulfur sites decreases to approximately 0.19 eV (Zhang X. et al., 2019). This indicates that doping Ru can efficiently modulate the electronic features of the adjacent sulfur atoms, thereby leading to the optimal H atom binding energy (Yan et al., 2018). On the other hand, after doping Ru into 2H-MoS₂ (Li J. et al., 2021) or 2H-WS₂ (Li J. et al., 2022), these Ru sites can significantly accelerate the water dissociation in alkaline environments. For example, the energy barrier of the H−OH cleavage (E) is as high as 2.42 eV before doping Ru. Remarkably, E of such Ru sites decreases to 2.02 eV, which is advantageous for water dissociation to OH and H intermediates.

Modulating S-vacancy into MoS₂-based materials has been developed as an efficient strategy (Tsai et al., 2017; Park et al., 2018; Li Y. et al., 2021) to activate inert basal planes because the exposed Mo atoms can be tailored into newborn active sites (Wang X. et al., 2020). Following this opinion, much effort has so far been devoted to introducing S-vacancy into the basal planes of mono-layered or multi-layered 2H-MoS₂ nanosheets (Li et al., 2016b; Tsai et al., 2017; Park et al., 2018; Li et al., 2019; Wang X. et al., 2020). Among these strategies, chemical vapor deposition coupled to plasma has been developed to modulate S-vacancy into MoS₂ (Li et al., 2016b). In this case, S atoms escape from the MoS₂ lattice more easily than Mo atoms due to the lower formation energy of S-vacancy compared to that of the Mo interstitial (Li et al., 2016b). Subsequently, the controllable electrochemical preparation of S-vacancy for multi-layered MoS₂ has been proposed by simply adjusting desulfurization parameters, such as desulfurization potential and time (Tsai et al., 2017). More recently, the single Ru atom doping technology promotes the phase transition of 2H-MoS₂ and the formation of S-vacancy, which greatly enhances its HER activity (Zhang J. et al., 2019). Nevertheless, a low or excessively high concentration of S-vacancy (C_S-vacancy) is disadvantageous for the hydrogen evolution reaction (HER) activity of MoS₂-based materials. Consequently, it is of great importance for the development of a novel approach to fine-tuning C_S-vacancy to significantly improve alkaline hydrogen evolution. In addition, the aforementioned electrocatalysts based on S-vacancy only consist of a single component (that is, MoS₂) rather than hybrid catalysts. As one of the traditional semiconductor materials, MoS₂ suffers from another drawback of unsatisfactory charge-transfer resistance (R_CT), leading to low HER activity (Li Y. et al., 2021; Wang et al., 2021b; Wang et al., 2021c). Hence, compositing with the metallic phase will favor fast electrode kinetics, realizing synergistically regulating C_S-vacancy and R_CT of MoS₂-based electrocatalysts for highly efficient HER.

Herein, we develop a one-step Ru doping coupled to compositing with the CoS₂ strategy to synergistically regulate C_S-vacancy of MoS₂-based materials for highly efficient alkaline HER. Ru doping is advantageous for the formation of S-vacancy in the basal planes of MoS₂. On the other hand, Ru doping favors heterogeneous nucleation and growth of CoS₂, which leads to rich heterogeneous interfaces between Ru-doped CoS₂ (Ru-CoS₂) and Ru-doped MoS_2-x (Ru-MoS_2-x). This facilitates the electron transfer from Ru-CoS₂ to Ru-MoS_2-x, thereby increasing C_S-vacancy of MoS₂-based materials. At fixed Ru dopant, the electron injection effect increases gradually with an increase in the mass of Co precursor, which means more S^2- escaping from Ru-MoS₂ nanosheets. Therefore, synergistically regulating C_S-vacancy of the as-synthesized samples, from 2.1 to 27.5%, is realized by a new one-step Ru doping coupled to compositing with the CoS₂ strategy. On regulating C_S-vacancy to 17.1%, a balance between the intrinsic activity and the number of exposed Mo atoms (EMAs) to boost highly active EMAs should be realized. As a consequence, the typical samples demonstrate the optimal alkaline HER activity among all samples, such as a low overpotential of 170 mV at 100 mA cm⁻², a large specific alkaline HER current density of 77.6 μA cm⁻², and a turnover frequency of 4.29 s⁻¹ at -0.2 V as well as excellent long-term stability. Our results pave a novel approach to unlocking the potential of inert basal planes in MoS₂-based materials for highly efficient HER and promise important applications in the field of electrocatalytic hydrogen evolution.

Experimental Section

Fabrication of the Typical Samples (Ru-MoS_2-X-CoS₂/CC)

The typical samples were synthesized by a one-pot hydrothermal strategy using the following precursors such as Na₂MoO₄.2H₂O, CH₄N₂S, and Co(NO₃)₂·6H₂O. In the typical experiments, 160 mg Co(NO₃)₂·6H₂O, 160 mg Na₂MoO₄.2H₂O, and 600 mg CH₄N₂S were dissolved in 46.0 ml deionized water under magnetic stirring. Subsequently, 4.0 ml RuCl₃ solution (5 mmol L⁻¹) was introduced into the above cobalt salt solution under magnetic stirring for 0.5 h. Carbon cloth (CC, 4 cm²) was pretreated according to the related literature (Yu et al., 2015). Then, the above solution and the pretreated CC were transferred into a 100.0 ml Teflon-lined stainless-steel autoclave and heated to 200°C for 20 h. After the hydrothermal reaction, the mixture was cooled to room temperature. The typical samples were harvested after being washed with water thoroughly and vacuum-dried at 60 °C for 12.0 h and abbreviated as Ru-MoS_2-x-CoS₂/CC.

Fabrications of Other Ru-MoS_2-X-CoS₂/CC Samples

Other Ru-MoS_2-x-CoS₂/CC samples were synthesized at various volumes of RuCl₃ solution (5 mmol L⁻¹) of 1.0, 7.0, 10.0, and 30.0 ml under otherwise similar conditions as the typical experiments. These Ru-MoS_2-x-CoS₂/CC samples are denoted as Ru-MoS_2-x-CoS₂/CC-1.0, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0.

In addition, Ru-MoS_2-x-CoS₂/CC-80, Ru-MoS_2-x-CoS₂/CC-240, Ru-MoS_2-x-CoS₂/CC-280, and Ru-MoS_2-x-CoS₂/CC-320 were synthesized at various masses of Co(NO₃)₂·6H₂O of 80, 240, 280, and 320 mg under otherwise similar conditions of the typical experiments, respectively.

Figure 1 displays a schematic representation of the fabrication of all Ru-MoS_2-x-CoS₂/CC samples.

FIGURE 1

FIGURE 1. Schematic representation of the fabrication of Ru-MoS₂-CoS₂/CC samples.

Fabrications of Ru-MoS₂/CC, MoS₂/CC, MoS₂-CoS₂/CC, and Ru-CoS₂/CC

Ru-doped MoS₂ nanosheets assembled on CC are abbreviated as Ru-MoS₂/CC. Fabrication of Ru-MoS₂/CC is almost the same as the typical samples except for the absence of Co(NO₃)₂.

MoS₂ nanosheets assembled on CC are abbreviated as MoS₂/CC. Fabrication of MoS₂/CC is almost the same as Ru-MoS₂/CC except for the absence of RuCl₃ solution.

MoS₂ nanosheets coated with CoS₂ assembled on CC are abbreviated as MoS₂-CoS₂/CC. Fabrication of MoS₂-CoS₂/CC is almost the same as the typical samples except for the absence of RuCl₃ solution.

Ru-doped CoS₂ assembled on CC are abbreviated as Ru-CoS₂/CC. Fabrication of Ru-CoS₂/CC is almost similar to the typical samples except for the absence of Na₂MoO₄.2H₂O.

Other experimental details about materials, characterization, and performance measurements are supplied in Supplementary Material S1.

Results and Discussion

Characterization of the Typical Samples

Scanning electron microscopic (SEM) images display the morphology of the typical samples in Supplementary Figures SJ–L. From these figures, it can be observed that lots of nanosheets are grown on the smooth surface of CC (Supplementary Figure S2). Powder X-ray diffraction (XRD) pattern of Ru-MoS_2-x-CoS₂/CC (Figure 2A) shows the diffraction peaks located at 2θ = 14.1°, 32.7°, and 58.8°, matching well with the (002), (100), and (110) planes of 2H-MoS₂ (PDF#75-1539) (Nguyen et al., 2021), respectively. Other sharp diffraction peaks of Ru-MoS_2-x-CoS₂/CC, such as 32.4° and 55.1°, are attributed to the (200) and 311) planes of CoS₂ (PDF#70-2865) (Yao et al., 2019), respectively, suggesting the presence of highly crystallized CoS₂ besides 2H-MoS₂. From Figure 2A, we also observe the diffraction peak at 26.2° originating from CC. The Raman spectrum of Ru-MoS_2-x-CoS₂/CC (Supplementary Figure S3) displays the typical E¹_2g and A_1g vibration models of the Mo-S bonds, further verifying the phase structure of 2H-MoS₂ in Ru-MoS_2-x-CoS₂/CC. As shown in Figure 2B, the Ru 3p high-resolution X-ray photoelectron spectroscopy (XPS) spectrum of Ru-MoS_2-x-CoS₂/CC is divided into two characteristic peaks at the binding energies of 462.4 and 484.7 eV, corresponding to Ru 3p_3/2 and Ru 3p_1/2 (Ge et al., 2022). Nevertheless, no peak of Ru is detected in MoS₂ nanosheets coated with CoS₂ assembled on CC (MoS₂-CoS₂/CC) in Figure 2B. Furthermore, Figures 2C, D exhibit Co 2p and Mo 3 day XPS spectra of Ru-MoS_2-x-CoS₂/CC, respectively. It is seen that Co 2p and Mo 3 day peaks positively shift by about 0.27 and 0.15 eV compared to those of MoS₂-CoS₂/CC (Hao et al., 2017), respectively. Additionally, the characteristic peaks about Ru₂S₃ or RuCl₃ are not observed in the XRD pattern of Ru-MoS_2-x-CoS₂/CC (Figure 2A). Therefore, these data confirm the successful doping of Ru into the following two phases of Ru-MoS_2-x-CoS₂/CC, 2H-MoS₂, and CoS₂.

FIGURE 2

FIGURE 2. (A) XRD patterns of Ru-MoS₂/CC, MoS₂-CoS₂/CC, Ru-CoS₂/CC, and the typical samples; (B) Ru 3p, (C) Co 2p, and (D) Mo3d high-resolution XPS spectra of the typical samples and MoS₂-CoS₂/CC; (E) TEM image and (F,G) HR-TEM images of the typical samples; (H) EPR spectra, and (I) molar ratio of Mo³⁺ to Mo⁴⁺, and the atom ratio of S to Mo of MoS₂/CC, MoS₂-CoS₂/CC, and the typical samples. (J) EDX mapping profiles of Mo, Co, Ru, and S over the typical samples. The inset of panel (E) is the SAED pattern of the typical samples. Scale bars of panels (E), (F), (G), and (J) are 100 nm, 2 nm, 2 nm, and 2 μm, respectively.

Nanosheets of Ru-MoS_2-x-CoS₂/CC are also clearly observed from transmission electron microscopic (TEM) image (Figure 2E), which is consistent with the results of SEM images. According to the high-resolution TEM (HR-TEM) image (Figure 2F), the lattice spacings of 0.625 and 0.245 nm that are seen in Ru-MoS_2-x-CoS₂/CC correspond to the (002) plane of MoS₂ and the (210) plane of CoS₂ (He et al., 2020; Liu X. et al., 2022), respectively. These also conform to the results of XRD. As we know, MoS₂ is regarded as one of the traditional semiconductor materials, and CoS₂ is a metallic phase due to its high Fermi level. Therefore, there are lots of Schottky heterojunctions among Ru-MoS_2-x-CoS₂/CC. In addition, the inset of Figure 2E (selected area electron diffraction, SAED) pattern indicates that Ru-doped MoS₂ (Ru-MoS₂) and Ru-doped CoS₂ (Ru-CoS₂) of Ru-MoS_2-x-CoS₂/CC are polycrystalline (Huang et al., 2015). We can clearly observe that many defects exist in such Ru-MoS₂ from Figure 2G due to lattice distortion by Ru doping (Zhang J. et al., 2019) and the electron injection effect (Gan et al., 2018; Zhang J. et al., 2019). Energy-dispersive X-ray (EDX) spectroscopy mapping (Figure 2J) profiles further confirm the uniform distribution of Mo, Co, Ru, and S elements throughout Ru-MoS_2-x-CoS₂/CC. Considering that Ru-MoS₂ assembled on CC (Ru-MoS₂/CC) and Ru-CoS₂ assembled on CC (Ru-CoS₂/CC) tend to form nanosheets (Supplementary Figures S1B, C) and nanoparticles (Supplementary Figures SH, I), respectively, and that Ru-CoS₂ nucleates and grows prior to Ru-MoS₂ (Figures 4A–F), we can reasonably deduce as follows: during the hydrothermal process, Ru-CoS₂ nanoparticles are firstly assembled on CC; then, Ru-CoS₂/CC are densely coated by Ru-MoS₂ nanosheets to construct the typical samples, Ru-MoS_2-x-CoS₂/CC.

According to XPS data in Figure 2D and Supplementary Figure S4, we characterize molar ratios of Mo³⁺ to Mo⁴⁺ of MoS₂/CC, MoS₂-CoS₂/CC, and Ru-MoS_2-x-CoS₂/CC in Figure 2I. The presence of Mo³⁺ can induce S-vacancy of the basal planes in Ru-MoS₂ (Ma et al., 2020) of Ru-MoS_2-x-CoS₂/CC. From Figure 2I, Ru-MoS_2-x-CoS₂/CC exhibits a higher value of about the molar ratio of Mo³⁺ to Mo⁴⁺ than MoS₂/CC or MoS₂-CoS₂/CC, reaching 1.35. To confirm such defective structures, electron paramagnetic resonance (EPR) (Liu et al., 2017; Gong et al., 2020; Wang J. et al., 2021) is further employed to estimate the S-vacancy of all samples. As expected, Ru-MoS_2-x-CoS₂/CC demonstrates the highest EPR signal at g = ∼ 2.002 among all samples (Figure 2H). Moreover, the EPR signal of MoS₂-CoS₂/CC is higher than that of MoS₂/CC. These data straightforwardly indicate that the formation of S-vacancy might be closely related to both Ru doping into MoS₂ and compositing with CoS₂.

Doping Ru Coupled to Compositing With CoS₂ to Regulate Microstructures of the As-synthesized Samples

First, a series of Ru-MoS₂/CC samples were synthesized at the various volumes of RuCl₃ solution (V) of 1.0, 4.0, 7.0, 10.0, and 30.0 ml under otherwise similar conditions of the typical experiments except for the absence of Co(NO₃)₂. These Ru-MoS₂/CC samples are denoted as Ru-MoS₂/CC-1.0, Ru-MoS₂/CC-7.0, Ru-MoS₂/CC-10.0, and Ru-MoS₂/CC-30.0. Similar Ru 3p XPS spectra to the typical samples are observed in Supplementary Figure S5A, which is responsible for successfully doping Ru into all Ru-MoS₂/CC samples. In addition, the atom ratios of Ru to Mo (A) of all Ru-MoS₂/CC samples are characterized by XPS in Supplementary Figure S5B. From this figure, A increases with increasing V, indicating that more Ru will be doped into the Ru-MoS₂/CC samples at higher V. In this work, the atomic ratio of S to Mo (S: Mo) of MoS₂/CC is firstly measured (Supplementary Table S1) and is normalized to 2.00. Then, it is employed as a reference to confirm the normalized S: Mo of MoS_2-x in all Ru-MoS₂/CC samples in terms of XPS data (Xu et al., 2016; Wang J. et al., 2021). Here, the measured S: Mo for all Ru-MoS₂/CC samples are the atom ratios of S minus double Ru to Mo, which is abbreviated as [(S–2Ru)/Mo]. Therefore, the normalized S: Mo of MoS_2-x in all samples are listed in Supplementary Table S2. From Figure 3E, C_S-vacancy of Ru-MoS₂/CC samples increases with the increase in Ru dopants. For example, C_S-vacancy of Ru-MoS₂/CC-30.0 increases to 10.5% at V = 30.0 ml. That is to say, doping Ru can regulate C_S-vacancy of Ru-MoS₂/CC samples varying from 2.1 to 10.5%.

FIGURE 3

FIGURE 3. (A) Atom ratio of Ru to Mo based on ICP-OES, (B) XRD patterns, (C) RC of Ru-MoS_2-x and Ru-CoS₂, (D) Mo3d high-resolution XPS spectra, and (E) C_S-vacancy of the related samples. Meanwhile, 1, 2, 3, 4, 5 and 6 are denoted as MoS₂-CoS₂/CC, Ru-MoS_2-x-CoS₂/CC-1.0, the typical samples, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0, respectively. Fabrications of {2}, {3}, {4}, {5}, and {6} are the same as those of Ru-MoS_2-x-CoS₂/CC-1.0, the typical samples, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0 except for the absence of Co(NO₃)₂, to be denoted as Ru-MoS₂/CC-1.0, Ru-MoS₂/CC, Ru-MoS₂/CC-7.0, Ru-MoS₂/CC-10.0, and Ru-MoS₂/CC-30.0, respectively. The inset of panel (A) is the atom ratio of Co to (Co + Mo) of the related samples based on ICP-OES.

Density functional theory (DFT) calculation reveals that the Ru-S bonding energy (about 0.92 eV) decreases by 0.87 eV compared to the Mo-S bonding energy (about 1.79 eV) (Luo et al., 2020), which is advantageous for the formation of S-vacancy in the basal planes of MoS₂. In this work, Ru atoms substituting for Mo atoms in MoS₂ are abbreviated as Ru_(Mo). According to the previously reported literature (Zhang J. et al., 2019; Zhang X. et al., 2019), Ru doping results in an increase in C_S-vacancy due to the strong attractive interaction between Ru_(Mo) and the adjacent S-vacancy. Furthermore, S-vacancy is exclusively stable around Ru_(Mo) (Kang, 2021).

To investigate the synergistic effect of doping Ru coupled to compositing with CoS₂ on the microstructure, other Ru-MoS_2-x-CoS₂/CC samples are further synthesized at various V of 1.0, 7.0, 10.0, and 30.0 ml under otherwise similar conditions of the typical experiments to be denoted as Ru-MoS_2-x-CoS₂/CC-1.0, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0, respectively. According to Supplementary Figure S6, these Ru-MoS_2-x-CoS₂/CC samples almost display the same Ru 3p XPS spectra as the typical samples, indicating a successful doping of Ru into them as well. The values of A of all samples are characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES). In terms of Figure 3A, the values of A of these samples increase with the increase in V. Moreover, other Ru-MoS_2-x-CoS₂/CC samples also exhibit the same phase structure as the typical samples, since the XRD pattern of each sample is similar to that of the typical samples (Figure 3B). Figure 3C further presents the relative crystallinity (RC) of Ru-MoS_2-x and Ru-CoS₂ for all Ru-MoS_2-x-CoS₂/CC samples based on the corresponding XRD data such as the (002) plane for molybdenum disulfide and (200) (210), (211) (220), and 311) planes for cobalt disulfide. From this figure, the RC of Ru-MoS_2-x decreases with the increase in V, which implies that more defective structures or S-vacancy would be modulated into the basal planes of Ru-MoS_2-x at higher V. However, the influence of V on RC of Ru-CoS₂ is not coincident with the trend of the former. Initially, the RC of Ru-CoS₂ gradually increases with increasing V. At V = 4.0 ml, the RC of Ru-CoS₂ of the typical samples exhibits the highest value among all samples. However, further increasing V leads to a decrease in the RC of Ru-CoS₂.

In addition, Mo³⁺ can be observed in Mo 3 day XPS spectra of all Ru-MoS_2-x-CoS₂/CC samples (Figure 3D), implying the formation of S-vacancy of the basal planes in Ru-MoS_2-x. Taking into account that MoS₂-CoS₂/CC and other Ru-MoS_2-x-CoS₂/CC samples (Supplementary Figures S1E, F; Supplementary Figure S7) are almost similar to the morphology of the typical samples (Supplementary Figures S1K, L), such core–shell structure, Ru-MoS_2-x nanosheets densely coated Ru-CoS₂ composites, facilitates to correctly reflect the S: Mo of Ru-MoS_2-x in all samples by XPS analysis. As mentioned above, the S: Mo of MoS₂/CC is also employed as a reference to confirm the normalized S: Mo of Ru-MoS_2-x in all Ru-MoS_2-x-CoS₂/CC samples (Li et al., 2016a; Xu et al., 2016) based on XPS data (Supplementary Figure S8). Here, the measured S: Mo for all Ru-MoS_2-x-CoS₂/CC samples is the atom ratio of S minus double (Co + Ru) to Mo, which is abbreviated as [(S–2Co–2Ru)/Mo]. Subsequently, we further calculate the related C_S-vacancy, as shown in Figure 3E; Supplementary Table S1. From Figure 3E, C_S-vacancy of Ru-MoS_2-x-CoS₂/CC increases to 7.9% compared to that of MoS₂-CoS₂/CC (about 6.0%). Out of expectation, C_S-vacancy of the typical samples rather than Ru-MoS_2-x-CoS₂/CC-30.0 reaches the highest value among all samples, about 17.1% from Figure 3E. At the same time, Ru-MoS_2-x-CoS₂/CC-30.0 exhibits a lower EPR signal than the typical samples (Supplementary Figure S9), further confirming fewer S-vacancy for Ru-MoS_2-x-CoS₂/CC-30.0 in comparison with the typical samples. In other words, at a fixed mass of Co precursor, C_S-vacancy of Ru-MoS_2-x can be precisely regulated from 7.9 to 17.1% by synergistic Ru doping and compositing with CoS₂ engineering. As a consequence, we can infer that compositing with CoS₂ should be another key factor to regulate C_S-vacancy of Ru-MoS_2-x nanosheets in Ru-MoS_2-x-CoS₂/CC samples. This is because Ru doping also affects the microstructure of CoS₂, which may have a significant influence on C_S-vacancy of Ru-MoS_2-x nanosheets in Ru-MoS_2-x-CoS₂/CC samples in turn.

Following this viewpoint, we deeply investigate the influence of doping Ru on the microstructure of cobalt disulfide. As mentioned above, CoS₂ nucleates and grows prior to MoS₂. To eliminate the effect of MoS₂ as much as possible during the characterization of microstructures of CoS₂, the related samples such as [MoS₂-CoS₂/CC], [1] [Ru-MoS_2-x-CoS₂/CC], [2], [3], and [4] are synthesized at t = 6 h (Figures 4A–F) rather than 20 h. Fabrications of [MoS₂-CoS₂/CC], [1] [Ru-MoS_2-x-CoS₂/CC], [2], [3], and [4] are the same as those of MoS₂-CoS₂/CC, Ru-MoS_2-x-CoS₂/CC-1.0, the typical samples, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0 except for t, respectively. Their SEM images and XRD patterns are displayed in Figures 4A–G, respectively. Besides the diffraction peak at 26.2° of CC, other peaks are indexed to CoS₂ without the signal of MoS₂ for all samples (Figure 4G), further confirming the heterogeneous nucleation and growth of CoS₂ prior to MoS₂. As shown in Figure 4A, a small number of spherical CoS₂ nanoparticles are grown on CC for [MoS₂-CoS₂/CC]. Moreover, the similar phenomena are observed for Ru-doped samples from Figures 4B–F. Interestingly, with an increase in V, the size and number of Ru-CoS₂ nanoparticles gradually increase (Figure 4B). At V = 4.0 ml, both the size and number of Ru-CoS₂ nanoparticles for [Ru-MoS_2-x-CoS₂/CC] reach up to the maximum value (Figure 4C). However, further increasing V leads to a decrease in both the size and number of Ru-CoS₂ for other [Ru-MoS_2-x-CoS₂/CC] (Figures 4D–F). Next, we characterize the RC of Ru-CoS₂ for all samples synthesized at 6 h according to XRD data such as (200) (210), (211) (220), and (311) planes of cobalt disulfide, as shown in Figure 4H. Impressively, their trend in the variation is almost consistent with that of the RC of Ru-CoS₂ of Ru-MoS_2-x-CoS₂/CC samples synthesized at 20 h (Figure 3C). This confirms that doping Ru can improve the crystallinity of CoS₂ under certain conditions. To find out the reason for this issue, we investigate the Fourier-transforming infrared (FT-IR) spectrum of MoS₂-CoS₂/CC in Figure 4I. The peak located at 1020 cm⁻¹ straightforwardly indicates the formation of CoS₂ (Chakraborty et al., 2006) at V = 0.0 ml. At V increasing to 1.0 ml, the peak of Ru-MoS_2-x-CoS₂/CC-1.0 located at 1024 cm⁻¹ shows a blue-shift of about 4 cm⁻¹ compared to that of MoS₂-CoS₂/CC. At V = 4.0 ml, the related peak blue-shifts to about 1036 cm⁻¹, as shown in Figure 4I. This indicates that doping Ru into CoS₂ can decrease the Co−S bond length and enhance the bonding energy (Feng et al., 2018), which favors the formation of Ru-CoS₂. However, a decrease in both the size and number of CoS₂ with further increasing V (Figures 4D–F) can be explained as follows: introducing excessive Ru into the lattice of CoS₂ may induce an increase in the oxidation state of neighboring Co ions to maintain the charge neutrality. The similar result has been observed for other transition metal compounds, such as NiO (Smyth, 2000; Ge et al., 2022). This implies that excessive dopant will hinder the formation of CoS₂ in turn.

FIGURE 4

FIGURE 4. SEM images of (A) [MoS₂-CoS₂/CC] ([1]), (B) [Ru-MoS_2-x-CoS₂/CC-1.0] ([2]), (C) [the typical samples] ([3]), (D) [Ru-MoS_2-x-CoS₂/CC-7.0] ([4]), (E) [Ru-MoS_2-x-CoS₂/CC-10.0] ([5]), and (F) [Ru-MoS_2-x-CoS₂/CC-30.0] ([6]); (G) XRD patterns and (H) RC of Ru-CoS₂ of the corresponding samples; and (I) FT-IR spectra of the related samples. Meanwhile, 1, 2, 3, 4, 5, and 6 are MoS₂-CoS₂/CC, Ru-MoS_2-x-CoS₂/CC-1.0, the typical samples, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0, respectively. Additionally, [1], [2], [3], [4], [5], and [6] are the corresponding samples synthesized at t = 6 h rather than 20 h. The insets of panels (A–F) are the low-magnified SEM images of the corresponding samples. The insets in red and blue solid lines of panel (I) are the high-magnified images of panel (I) indicated in red and blue solid frames, respectively. Scale bars of panels (A–F) and their insets are 2 and 5 μm.

Possible Formation Mechanism of S-Vacancy in Ru-MoS_2-X-CoS₂/CC Samples

Based on these results and discussion, a possible formation mechanism of S-vacancy of the basal planes in Ru-MoS_2-x of Ru-MoS_2-x-CoS₂/CC samples can be proposed as follows: on the one side, S-vacancy is modulated into the basal planes of MoS₂ due to the strong attractive interaction between Ru_(Mo) and the adjacent S-vacancy. Furthermore, S-vacancy is exclusively stable around Ru_(Mo). On the other hand, by compositing with CoS₂, Schottky heterojunctions provide a feasible opportunity for the electron transfer from Ru-CoS₂ to Ru-MoS_2-x (Li et al., 2018) due to a high Fermi level of CoS₂. This effect can promote S^2- escaping from Ru-MoS_2-x nanosheets to remain charge neutrality (Liu et al., 2017; Gan et al., 2018), indicating the formation of the positively charged defects, S-vacancy. Subsequently, we further characterize the atom ratios of Co to (Co + Mo) of the related samples by ICP-OES in the inset of Figure 3A. From this inset, the atom ratio of Co to (Co + Mo) of the typical samples is as high as about 16.1% at V increasing to 4.0 ml, thereby resulting in more Ru-CoS₂, and rich heterogeneous interfaces between Ru-CoS₂ and Ru-MoS_2-x as well. More heterogeneous interfaces favor the electron injection from Ru-CoS₂ to Ru-MoS_2-x, which leads to an increase in C_S-vacancy (Figure 3E). However, further increasing V results in a decrease in atom ratios of Co to (Co + Mo) (inset of Figure 3A) and poor heterojunctions. This could indicate a weaker electron injection effect of Ru-CoS₂ and lower C_S-vacancy at excessively high V. The schematic illustration of the possible formation mechanism of S-vacancy of the basal planes in Ru-MoS_2-x of Ru-MoS_2-x-CoS₂/CC samples is presented in Figure 5.

FIGURE 5

FIGURE 5. Schematic illustration of the possible formation mechanism of the S-vacancy of the basal planes in Ru-MoS_2-x of Ru-MoS_2-x-CoS₂/CC samples.

To confirm this viewpoint, we systematically investigate the effect of compositing with CoS₂ on C_S-vacancy of the as-synthesized samples at fixed V or Ru dopants in Figure 6. Figure 6A exhibits XRD patterns of Ru-MoS_2-x-CoS₂/CC-80, Ru-MoS_2-x-CoS₂/CC -240, Ru-MoS_2-x-CoS₂/CC-280, and Ru-MoS_2-x-CoS₂/CC-320. With an increase in the mass of Co precursor (m_Co), the intensity of the diffraction peak of the (002) plane at 14.1° for Ru-MoS_2-x greatly decreases, while intensities of peaks of the (200) and 311) planes for Ru-CoS₂ gradually increase (Figure 6A), indicating a decrease in the RC of Ru-MoS_2-x and an increase in the RC of Ru-CoS₂ (Supplementary Figure S10). At m_Co increasing to 240 mg, Ru-CoS₂ can be densely coated by Ru-MoS_2-x nanosheets (Supplementary Figure S11B). Nevertheless, further increasing m_Co to 280 or 320 mg leads to incompact Ru-MoS_2-x shells (Supplementary Figures S11C, D) due to the formation of more Ru-CoS₂ or a decrease in the content of Ru-MoS_2-x. These data are in good agreement with the results of Figure 6D, which displays atom ratios of Co to (Co + Mo) of the as-synthesized samples by XPS analysis. These atom ratios also increase with increasing m_Co. For example, the atom ratio of Co to (Co + Mo) of R Ru-MoS_2-x-CoS₂/CC-320 increases up to 10.1%. To further confirm the effect of electron injection from Ru-CoS₂ to Ru-MoS_2-x, we characterize Mo3d high-resolution XPS spectra of Ru-MoS_2-x-CoS₂/CC-80, Ru-MoS_2-x-CoS₂/CC-240, Ru-MoS_2-x-CoS₂/CC-280, and Ru-MoS_2-x-CoS₂/CC-320 in Figure 6B. Mo 3 days in XPS data move to low binding energy (Huang et al., 2019), implying an increase in the electron density of Ru-MoS_2-x of the as-synthesized samples. For example, Mo 3 day peaks of the typical samples negatively shift by about 0.06 eV compared to those of Ru-MoS_2-x-CoS₂/CC-80 in terms of Figure 2D and Figure 6B. After careful investigations, similar results are observed in Mo3d XPS spectra of other Ru-MoS_2-x-CoS₂/CC samples (Figure 6B), yielding the related data about negative shifts of 0.12 eV for Ru-MoS_2-x-CoS₂/CC-240, 0.45 eV for Ru-MoS_2-x-CoS₂/CC-280, and 0.50 eV for Ru-MoS_2-x-CoS₂/CC-320. It is seen that such an electron injection effect increases gradually with the increase in m_Co, which means more S^2- escaping from MoS_2-x nanosheets (Liu et al., 2017; Gan et al., 2018) at higher m_Co. Therefore, the atom ratios of S to Mo for all Ru-MoS_2-x-CoS₂/CC samples decrease (Supplementary Table S3), and C_S-vacancy increases (Figure 6E) with increasing m_Co. For instance, Ru-MoS_2-x-CoS₂/CC-320 demonstrate the highest C_S-vacancy (about 27.5%) among all samples.

FIGURE 6

FIGURE 6. (A) XRD patterns and (B) Mo3d high-resolution XPS spectra of Ru-MoS_2-x-CoS₂/CC-80 (Ⅰ), Ru-MoS_2-x-CoS₂/CC-240 (Ⅱ), Ru-MoS_2-x-CoS₂/CC-280 (Ⅲ), and Ru-MoS_2-x-CoS₂/CC-320 (Ⅳ); (C) XRD patterns of MoS₂-CoS₂/CC-80 ({Ⅰ}), MoS₂-CoS₂/CC-240 ({Ⅱ}), MoS₂-CoS₂/CC-280 ({Ⅲ}), and MoS₂-CoS₂/CC-320 ({Ⅳ}); (D) atom ratios of Co to (Co + Mo) by XPS data, (E) C_S-vacancy of the related samples. Meanwhile, fabrications of MoS₂-CoS₂/CC-80, MoS₂-CoS₂/CC-240, MoS₂-CoS₂/CC-280, and MoS₂-CoS₂/CC-320 are almost the same as those of Ru-MoS_2-x-CoS₂/CC-80, Ru-MoS_2-x-CoS₂/CC-240, Ru-MoS_2-x-CoS₂/CC-280, and Ru-MoS_2-x-CoS₂/CC-320 except for the absence of RuCl₃ solution.

Additionally, Figure 6 also demonstrates the influence of compositing with CoS₂ on C_S-vacancy of the as-synthesized samples without Ru dopants. Figure 6C shows XRD patterns of MoS₂-CoS₂/CC-80, MoS₂-CoS₂/CC-240, MoS₂-CoS₂/CC-280, and MoS₂-CoS₂/CC-320. The influences of m_Co on intensities of the related peaks and RC of MoS₂ and CoS₂ are shown in Figure 6C and Supplementary Figure S12, suggesting a decline in the crystallinity of MoS₂ and an increase in crystallinity of CoS₂ as well. The trend in the variation of atom ratios of Co to (Co + Mo) of the as-synthesized samples without Ru dopants is the same as that of Ru-doped samples in Figure 6D, which implies the formation of more CoS₂ and richer heterojunctions at higher m_Co. Similarly, the more heterojunctions are, the stronger electron injection effect from CoS₂ to MoS_2-x is obtained. For instance, Mo 3 day peaks of MoS₂-CoS₂/CC-240 display a negative shift of 0.15 eV in comparison with those of MoS₂-CoS₂/CC-80 from Supplementary Figure S13. At m_Co increasing to 320 mg, Mo 3 day peaks of MoS₂-CoS₂/CC-320 present a negative shift of about 0.36 eV compared to those of MoS₂-CoS₂/CC-80 (Supplementary Figure S13). Therefore, the atom ratios of S to Mo for these samples without Ru dopants also decrease with increasing m_Co (Supplementary Table S3), leading to an increase in C_S-vacancy (Figure 6E) as well. Therefore, C_S-vacancy of the basal planes of Ru-MoS_2-x can be efficiently regulated by simply changing m_Co based on the effect of electron injection from Ru-CoS₂ to Ru-MoS_2-x.

Together, either doping Ru or compositing with CoS₂ only regulates C_S-vacancy of the as-synthesized samples within a narrow range, such as between 2.1 and 10.5% for doping Ru (Figure 3E) and 4.5 and 11.5% for compositing with CoS₂ (Figure 6E). Interestingly, synergistically regulating C_S-vacancy of the as-synthesized samples, from 2.1 to 27.5% (Figure 6E), is realized by a new one-step doping-assisted compositing strategy.

HER Activities of Ru-MoS_2-X-CoS₂/CC Samples Synthesized at Various V

To find out the relationship between the microstructure and HER activity, we carefully investigate a series of electrochemical performances of Ru-MoS_2-x-CoS₂/CC samples synthesized at various V under the fixed m_Co (160 mg) in Figures 7, 8. According to Figures 7A–D, the typical samples exhibit the highest HER activity among these samples, such as an overpotential of about 170 mV at a current density of 100 mA cm⁻² and a Tafel plot of 71 mV dec⁻¹ in alkaline environments (1 M KOH), implying that low or excessive Ru dopant is disadvantageous for the HER activity of Ru-MoS_2-x-CoS₂/CC samples.

FIGURE 7

FIGURE 7. (A) Polarization curves, (B) Tafel plots, (C) C_dl at different scan rates, and (D) Nyquist plots of Ru-MoS_2-x-CoS₂/CC-1.0 (2), Ru-MoS_2-x-CoS₂/CC-7.0 (4), Ru-MoS_2-x-CoS₂/CC-10.0 (5), and Ru-MoS_2-x-CoS₂/CC-30.0 (6). The inset of panel (C) is C_USAs of the related samples. The electrolyte is N₂-saturated 1 M KOH.

FIGURE 8

FIGURE 8. (A) Polarization curves, (B) Tafel plots, (C) C_dl at different scan rates, (D) Nyquist plots, (E) R_f and SCD_HER, and (F) TOF at 0.2 V of Ru-MoS₂/CC, MoS₂-CoS₂/CC, Ru-CoS₂/CC, and the typical samles. The electrolyte is N₂-saturated 1 M KOH.

Figure 7C displays double-layer capacitance (C_dl) to evaluate electrochemical active surface area (ECSA), which is calculated by cyclic voltammetry (CV) curves in Supplementary S14, 15. From Figure 7C and Figure 8C, the typical samples also demonstrate the highest C_dl among all Ru-MoS_2-x-CoS₂/CC samples, reaching up to ca. 131 mF cm⁻². For 2H-MoS₂, its active sites are mainly from unsaturated sulfur atoms (Kibsgaard et al., 2012; Xie et al., 2013a; Liu et al., 2018). This is because the adsorption free energy of hydrogen atom (ΔG_H) of unsaturated sulfur atoms approaches zero (about -0.06 eV), which indicates their excellent H* adsorption–desorption property (Hinnemann et al., 2005). More recently, modulating sulfur vacancy into MoS₂-based materials has been developed as an efficient strategy (Tsai et al., 2017; Park et al., 2018; Li Y. et al., 2021) to activate inert basal planes because EMAs can be tailored into newborn active sites (Wang X. et al., 2020). Therefore, the active sites of these Ru-MoS_2-x-CoS₂/CC samples should include unsaturated sulfur atoms and EMAs.

To confirm the difference in active sites for such Ru-MoS_2-x-CoS₂/CC samples, the inset of Figure 7C further shows the concentration of unsaturated sulfur atoms (USAs) of all samples based on XPS data from Supplementary Figure S16. From this inset, there is no significant difference in the concentration of USAs (C_USAs) for these samples. This may be closely related to preferentially exposed sulfur-edge atoms of Ru-MoS_2-x nanosheet array vertically assembled on CC. At the same time, this inset implies that USAs are not the essential factor for the most abundant active sites of the typical samples. From Figure 3E, C_S-vacancy for Ru-MoS_2-x-CoS₂/CC-1.0, the typical samples, Ru-MoS_2-x-CoS₂/CC-7.0, Ru-MoS_2-x-CoS₂/CC-10.0, and Ru-MoS_2-x-CoS₂/CC-30.0 are 7.9, 17.1, 14.0, 12.5, and 12.0%, respectively. It is seen that C_S-vacancy of the typical samples is higher than that of other Ru-MoS_2-x-CoS₂/CC samples. Consequently, we can reasonably conclude that the difference in active sites of RCM/CC should be dependent on C_S-vacancy. The formation of one S-vacancy means the occurrence of three EMAs at C_S-vacancy < 18% (Li et al., 2016a) because S-vacancy uniformly distributes on the basal planes of 2H-MoS_2-x. Obviously, under this situation, the higher C_S-vacancy is obtained, the more EMAs or active sites are achieved. At m_Co = 160 mg, Ru doping firstly favors heterogeneous nucleation and growth of CoS₂ (Figures 4C, I) besides introducing a certain quantity S-vacancy into the basal planes. For instance, at V increasing to 4.0 ml, the typical samples demonstrate higher C_S-vacancy (Figure 3E) than Ru-MoS_2-x-CoS₂/CC-1.0, maybe due to more heterogeneous interfaces between Ru-CoS₂ and Ru-MoS_2-x. Nevertheless, further increasing V hinders the formation of Ru-CoS₂ (Figures 4F, I) and leads to a decline in C_S-vacancy. Lower C_S-vacancy could originate from fewer Schottky heterojunctions and a weaker electron injection effect of Ru-CoS₂ at excessively high V.

As another key factor for HER, the R_CT of all Ru-MoS_2-x-CoS₂/CC samples is investigated in Figure 7D and Figure 8D. These figures exhibit their Nyquist plots. Each semicircle represents the R_CT of the cathode reaction. The charge transfer of Ru-MoS_2-x-CoS₂/CC-1.0 or Ru-MoS_2-x-CoS₂/CC-7.0 is inferior to that of the typical samples. Moreover, further increasing V results in considerably unsatisfactory R_CT from Figure 7D. In our viewpoint, rich active sites will promote the occurrence of HER reaction in the cathode, implying efficient charge transfer for the typical samples. On the other hand, compositing with the metallic phase for fast electrode kinetics (He et al., 2020; Li et al., 2020; Zhou G. et al., 2021), such as CoS₂ (Li et al., 2020), carbon nanotube (Huang et al., 2017), and reduced graphene oxide (Wang Y. et al., 2020), is one of the important approaches to improving electrocatalytic HER activity. From Figure 7D and Figure 8D, Ru-MoS_2-x-CoS₂/CC-1.0 demonstrate better charge transfer compared to Ru-MoS₂/CC. Furthermore, the influence of V on R_CT is coincident with the trend in the variation of RC of Ru-CoS₂ (Figure 3C) and atom ratios of Co to (Co + Mo) (inset of Figure 3A) of all Ru-MoS_2-x-CoS₂/CC samples. Therefore, the higher crystallinity of cobalt disulfide or the more CoS₂ is obtained, the lower R_CT of the as-synthesized samples is achieved.

HER Activities of Ru-MoS_2-X-CoS₂/CC Samples Synthesized at Various m_Co

We further investigate the HER activity of Ru-MoS_2-x-CoS₂/CC samples synthesized at various m_Co under the fixed V (4.0 ml) in Supplementary Figure S17. From Supplementary Figures S17A, B, HER activities of Ru-MoS_2-x-CoS₂/CC-80 are an overpotential of about 228 mV at a current density of 100 mA cm⁻² and a Tafel plot of 102 mV dec⁻¹. At m_Co increasing to 160 mg, the typical samples demonstrate higher HER activity than other samples (Figure 8A; Supplementary Figure S17A). Further increasing m_Co leads to unsatisfactory overpotential and sluggish electrode kinetics, yielding the related data of 264 and 123 mV dec⁻¹ for Ru-MoS_2-x-CoS₂/CC-240, 294 and 130 mV dec⁻¹ for Ru-MoS_2-x-CoS₂/CC-280, and 301 and 140 mV dec⁻¹ for Ru-MoS_2-x-CoS₂/CC-320 at the same current density.

To illustrate the reason why excessive m_Co is disadvantageous for HER activity, C_dl and C_USAs of all samples are tested in Supplementary FIgure S17C and Supplementary Figure S18, respectively. The corresponding CV curves related to Supplementary FIgure S17C are shown in Supplementary Figure S19. From Supplementary FIgure S17C, C_dl gradually increases with an increase in m_Co. For example, Ru-MoS_2-x-CoS₂/CC-80 display low C_dl (about 118 mF cm⁻²) at m_Co = 80 mg. At m_Co increasing to 320 mg, the C_dl of Ru-MoS_2-x-CoS₂/CC-320 is as high as 199 mF cm⁻². We rationally hypothesize that this is closely related to C_S-vacancy, due to the fact that high C_S-vacancy favors abundant EMAs. As we know, the basal planes in 2H-MoS₂ are inert, owing to ΔG_H reaching 2.1 eV (Zhou W. et al., 2021). Impressively, inert basal planes can be efficiently activated by a low concentration of S-vacancy (ca 4%) modulating into them because the corresponding ΔG_H closes to zero (Wang X. et al., 2020). It is not difficult to understand that such EMAs should be highly active, but the number is considerably poor.

According to the related literature (Wang X. et al., 2020), ΔG_H negatively shifts with an increase in C_S-vacancy, implying that hydrogen atoms do not easily desorb at excessive C_S-vacancy. For instance, at excessive m_Co, C_S-vacancy of Ru-MoS_2-x-CoS₂/CC-320 reaches up to 27.5%. As expected, EMAs are abundant in this case, but the catalytic sites are of low activity, owing to the undesirable ΔG_H (Wang X. et al., 2020). Subsequently, a balance between the intrinsic activity and the number of EMAs to boost highly active EMAs could be realized by precisely regulating C_S-vacancy to 17.1% (Wang X. et al., 2020). Consequently, the typical samples demonstrate the optimal HER activity among all samples. Furthermore, C_USAs of all samples except for Ru-MoS_2-x-CoS₂/CC-320 is almost the same as that of the typical samples. C_USAs of Ru-MoS_2-x-CoS₂/CC-320 (about 33.9%) is lower than that of other samples, which is responsible for the incompact Ru-MoS_2-x shells (Supplementary Figure S11D), and the significant decrease in the content of molybdenum disulfide (Figure 6D). This indicates that sulfur-edge atoms of Ru-MoS_2-x nanosheets are not easy to be preferentially exposed for such samples synthesized at excessive m_Co.

Last but not least, Supplementary Figure S17Dand Figure 8D further exhibit R_CT of all samples. Higher m_Co leads to higher R_CT of the related samples (Supplementary Figure S17D). For example, at m_Co reaching up to 320 mg, Ru-MoS_2-x-CoS₂/CC-320 display the highest R_CT among all Ru-MoS_2-x-CoS₂/CC samples. It can be ascribed to insufficient highly active EMAs regardless of the higher RC of Ru-CoS₂ (Figure 6A; Supplementary Figure S10) for these samples synthesized at higher m_Co. Insufficient active sites could not promote redox half-reaction, thereby leading to inefficient charge transfer.

Comparison of HER Activities of the Typical Samples and Other Similar Electrocatalysts

HER activities of Ru-MoS₂/CC, MoS₂-CoS₂/CC, Ru-CoS₂/CC, and the typical samples are shown in Figure 8A. Ru-CoS₂/CC display the lowest HER activity among these samples, suggesting that the activity of the typical samples is mainly from Ru-MoS_2-x rather than Ru-CoS₂. In addition, Ru-MoS₂/CC exhibit faster electrode kinetics than MoS₂-CoS₂/CC according to Figure 8B. In our viewpoint, the Ru-doped sites can significantly accelerate the sluggish water dissociation in alkaline HER (Li J. et al., 2021).

From Figure 8C, C_dl of Ru-MoS₂/CC is only 103 mF cm⁻² and lower than that of the typical samples. Furthermore, C_USAs of Ru-MoS₂/CC (about 39.3%) is almost the same as that of the typical samples from Supplementary Figure S18. Therefore, the difference in active sites between Ru-MoS₂/CC and the typical samples should be closely related to C_S-vacancy. Higher C_S-vacancy indicates more highly active EMAs. As shown in Figure 3E, C_S-vacancy of Ru-MoS₂/CC is only about 3.1% by doping Ru, implying insufficient active sites. In our strategy, doping Ru coupled to compositing with CoS₂ synergistically regulates C_S-vacancy of the as-synthesized samples from 2.1 to 27.5%. For instance, C_S-vacancy of the typical samples reaches up to 17.1% (Figure 3E), which implies that rich highly active sites are successfully introduced into the typical samples. In terms of Figure 8D, R_CT of the typical samples is lower than that of Ru-MoS₂/CC. Besides rich active sites promoting redox half-reaction, high crystallized Ru-CoS₂ of the typical samples is another important factor to realize the efficient charge transfer during electrocatalysis. Next, the specific activity and turnover frequency (TOF) are further conducted to investigate the intrinsic activity of all samples (Figures 8E, F). C_dl of an ideal plane electrode is considered as 60 μF cm⁻² (Levine and Smith, 1971); the roughness factor (R_f) can be calculated by the formula: R_f = (C_dl 60^-1)×10⁺³. The specific alkaline HER current density (SCD_HER) is determined by the formula: SCD_HER = j R_f⁻¹ (Jovic et al., 2015; Jovic et al., 2016), where j is the current density at an overpotential of -0.2 V. From Figure 8E, the typical samples have not only the largest R_f (2.18 × 10⁺³) but also the highest specific activity (about 77.6 μA cm⁻²) among all samples. In addition, the TOF for these samples is obtained by the formula TOF = jS (2nF) ⁻¹. Here, S is the geometrical surface area in cm² and F is the Faraday constant. The value of n is the number of active sites (mol), which is confirmed according to the previously reported literature (Tian et al., 2014). The typical samples also exhibit the highest intrinsic activity among all samples, about 4.29 s⁻¹ in Figure 8F. Moreover, the comparison between the typical samples and other similar electrocatalysts is provided in Table 1. The typical samples exhibit higher HER activity than other similar electrocatalysts listed in Table 1, which is ascribed to two factors: abundant active sites and accelerated electrode kinetics during the HER process.

TABLE 1

TABLE 1. Comparison of the HER electrocatalytic activity of Ru-MoS_2-x-CoS₂/CC with some MoS₂-based HER electrocatalysts recently reported.

The Long-Term Durability of the Typical Samples

In addition, LSV curves of the typical samples before and after 1000 cycles are recorded to evaluate their durability, and just a 10 mV negative shift at a current density of -100 mA cm⁻² is seen in Supplementary Figure S20A, B; the chronoamperometric response (i ∼ t) of the typical samples displays negligible attenuation of the current density for 10 h. Moreover, no significant change is observed in the morphology and phase structure after 1000 cycles (Supplementary Figures S21, 22). All results suggest that the typical samples possess remarkable long-term durability for HER in alkaline media.

Conclusion

Herein, we develop a one-step Ru doping coupled to compositing with the CoS₂ strategy for the fabrication of Ru-MoS_2-x-CoS₂/CC. In our strategy, Ru doping is advantageous for the formation of S-vacancy in the basal planes of MoS₂. More importantly, Ru doping affects microstructures of CoS₂, which has a significant influence on C_S-vacancy of Ru-MoS_2-x nanosheets in Ru-MoS_2-x-CoS₂/CC samples in turn. At the fixed m_Co (160 mg), Ru doping favors the heterogeneous nucleation and growth of CoS₂ at V increasing to 4.0 ml, which leads to a high crystallinity of Ru-CoS₂ and rich heterogeneous interfaces between Ru-CoS₂ and Ru-MoS_2-x. This facilitates the electron transfer from Ru-CoS₂ to Ru-MoS_2-x, indicating an increase in C_S-vacancy, thereby increasing C_S-vacancy of the MoS₂-based materials. However, further increasing V results in a low crystallinity of Ru-CoS₂ and poor heterojunctions, implying a weaker electron injection effect of Ru-CoS₂ and low C_S-vacancy. At fixed V (4.0 ml of RuCl₃ solution), the electron injection effect increases gradually with the increase in m_Co, which means more S^2- escaping from Ru-MoS₂ nanosheets at higher m_Co. Therefore, synergistically regulating C_S-vacancy of the as-synthesized samples, from 2.1 to 27.5%, is realized by a new one-step Ru doping coupled to compositing with the CoS₂ strategy. High C_S-vacancy indicates abundant EMAs. Impressively, inert basal planes can be efficiently activated by modulating low C_S-vacancy into Ru-MoS_2-x-CoS₂/CC samples. However, the number is considerably poor. Additionally, hydrogen atoms do not easily desorb at excessive C_S-vacancy. This is because EMAs are abundant in this case, but the catalytic sites are of low activity, owing to the undesirable ΔG_H. By precisely regulating C_S-vacancy to 17.1%, a balance between the intrinsic activity and the number of EMAs to boost highly active EMAs should be realized. Consequently, the typical samples demonstrate the optimal alkaline HER activity among all samples, such as a low overpotential of 170 mV at 100 mA cm⁻², a large SCD_HER of 77.6 μA cm⁻², and a TOF of 4.29 s⁻¹ at - 0.2 V as well as excellent long-term stability. The results pave a new approach to activating inert basal planes in MoS₂ for efficient hydrogen evolution and promise important applications in the fields of electrocatalysis or energy conversion.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

Author Contributions

X-YL has made substantial contributions to the design of this work and has drafted the work. S-JZ has made substantial contributions to the acquisition, analysis, and interpretation of data for the work. YW has revised the work critically for the important intellectual content and has approved the final version to be published. TL, X-YY, and C-FY have made some contributions to the analysis and interpretation of data for the work. YL has given some important suggestions for this revised article. BS has given some suggestions on preparing this article. L-HC has provided some important ideas for the design of this work and has approved the final version to be published.

Funding

This work was also financially supported by the National Natural Science Foundation of China (No. U20A20122).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

L-HC acknowledges the Hubei Provincial Department of Education for the “Chutian Scholar” program. Y-LW acknowledges the Fundamental Research Funds for the Central Universities (WUT: 2020-IB-028). The authors deeply thank the 111 Projects (Grant No. B20002) and the International Science and Technology Cooperation Program of China (2021YFE0115800) for supporting this work.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2022.915468/full#supplementary-material

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