MoS2 Nanocomposites Anchored on Reduced Graphene Oxide

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22380−22389

www.acsami.org

Ag2S/MoS2 Nanocomposites Anchored on Reduced Graphene Oxide: Fast Interfacial Charge Transfer for Hydrogen Evolution Reaction Getachew Solomon,† Raffaello Mazzaro,†,‡ Shujie You,† Marta Maria Natile,§ Vittorio Morandi,‡ Isabella Concina,† and Alberto Vomiero*,†,∥

Downloaded via BUFFALO STATE on July 20, 2019 at 02:41:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Division of Materials Science, Department of Engineering Science and Mathematics, Luleå University of Technology, SE-971 98 Luleå, Sweden ‡ CNR-Institute of Microelectronics and Microsystem (IMM), Via Piero Gobetti 101, Bologna 40129, Italy § CNR-Institute of Condensed Matter Chemistry and Technologies for Energy (ICMATE), Department of Chemical Sciences, University of Padova, Via Francesco Marzolo, 1, 35131 Padova PD, Italy ∥ Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Via Torino 155, 30172 Venezia Mestre, Italy S Supporting Information *

ABSTRACT: Hydrogen evolution reaction through electrolysis holds great potential as a clean, renewable, and sustainable energy source. Platinum-based catalysts are the most efficient to catalyze and convert water into molecular hydrogen; however, their large-scale application is prevented by scarcity and cost of Pt. In this work, we propose a new ternary composite of Ag2S, MoS2, and reduced graphene oxide (RGO) flakes via a one-pot synthesis. The RGO support assists the growth of two-dimensional MoS2 nanosheets partially covered by silver sulfides as revealed by high-resolution transmission electron microscopy. Compared with the bare MoS2 and MoS2/RGO, the Ag2S/MoS2 anchored on the RGO surface (the ternary system Ag2S/MoS2/RGO) demonstrated a high catalytic activity toward hydrogen evolution reaction (HER). Its superior electrochemical activity toward HER is evidenced by the positively shifted (−190 mV vs reversible hydrogen electrode (RHE)) overpotential at a current density of −10 mA/cm2 and a small Tafel slope (56 mV/dec) compared with a bare and binary system. The Ag2S/MoS2/RGO ternary catalyst at an overpotential of −200 mV demonstrated a turnover frequency equal to 0.38 s−1. Electrochemical impedance spectroscopy was applied to understand the charge-transfer resistance; the ternary sample shows a very small chargetransfer resistance (98 Ω) at −155 mV vs RHE. Such a large improvement can be attributed to the synergistic effect resulting from the enhanced active site density of both sulfides and to the improved electrical conductivity at the interfaces between MoS2 and Ag2S. This ternary catalyst opens up further optimization strategies to design a stable and cheap catalyst for hydrogen evolution reaction, which holds great promise for the development of a clean energy landscape. KEYWORDS: electrocatalyst, hydrogen evolution, silver sulfide, molybdenum sulfide, reduced graphene oxide

■

INTRODUCTION Hydrogen is a promising alternative to fossil fuels. Its superior properties such as light weight, high energy density, and sustainability make it the ideal candidate for our future energy resources. Most hydrogen production processes produce undesirable byproducts such as CO2.1 Electrocatalytic and photocatalytic methods, instead, represent a sustainable way for hydrogen evolution, which is currently attracting huge attention from researchers. Hydrogen evolution reaction (HER) is the cathodic half-reaction of water splitting (WS) and plays a critical role in the efficiency of the overall WS process. Unfortunately, because of the slow kinetics at the electrode interfaces, HER suffers from large overpotential, hence requiring efficient catalysts2 to decrease it. Platinum and platinum-based catalysts are generally considered the most efficient catalysts for HER, but their high cost and scarcity limit their widespread use. For this reason, highly efficient, stable, © 2019 American Chemical Society

environmentally friendly, and cheap catalysts are required to sustainably produce hydrogen from water. Recently, several layered materials analogous to graphene have gained enormous attention such as transition-metal dichalcogenides (TMDs),3 layered metal oxides,4 and carbides.5 TMDs such as MoS2, WS2, MoSe2, and WSe2 with multiple layers stacked and held together by weak van der Waals interactions are becoming an active area of research due to their unique structures. Among the TMDs, MoS2 is one of the most promising catalysts for HER. A number of studies have revealed that the catalytic activities of MoS2 arise from its edge site, leaving the basal plane inert.6 Successful activation of the inert basal plane sites, while maintaining the edge activity, would make MoS2 Received: March 21, 2019 Accepted: May 30, 2019 Published: May 30, 2019 22380

DOI: 10.1021/acsami.9b05086 ACS Appl. Mater. Interfaces 2019, 11, 22380−22389

Research Article

ACS Applied Materials & Interfaces

Figure 1. HR-TEM micrographs and selected area electron diffraction pattern of (a, b) pristine MoS2; (c, d) HR-TEM micrographs of the MoS2/ RGO sample and relative fast Fourier transform (FFT) (in the inset) exhibiting the typical hexagonal RGO pattern. (e, g) HR-TEM micrographs of the Ag2S/MoS2/RGO sample displaying crystalline nanoparticles surrounded by MoS2 nanosheets. (f) EDS elemental mapping Ag2S/MoS2/RGO sample, exhibiting the core−shell structure. (h) Top left, FFT of the HR-TEM image displayed in (g), and relative spatial distribution of the crystal phases on the original micrograph, extracted from the reflections highlighted in different colors on the FFT (dark J plugin, ImageJ software). Specifically: red and yellow, Ag2S acanthite [0,0,1] and [1,1,0] zone axis; green, MoS2 [0,0,2] zone axis. Scale bar is 10 nm.

■

RESULTS AND DISCUSSION The morphological and structural characterizations of asprepared MoS2, MoS2/RGO, and Ag2SMoS2/RGO composites are reported in Figures 1 and S1, Supporting Information. Field emission scanning electron microscopy characterization (Figure S1, Supporting Information) shows only a small variation of the microscale morphology of the binary and the bare MoS2 samples, whereas an agglomerated Ag2S nanoparticle is observed in the ternary samples. The MoS2 sheet is partially covered by Ag2S, mainly because of the complex formation of L-cysteine with both metals (Ag and Mo). The energy dispersion spectroscopy (EDS) spectrum taken on MoS2 nanosheets (Figure S2a, Supporting Information) suggested the elemental distribution of Mo and S elements, consistent with the successful preparation of MoS2 nanosheets. High-resolution transmission electron microscopy (HR-TEM) characterization of the MoS2 sample (Figure 1a) displays wrinkled microsheets, with the typical morphology and thickness associated with MoS2. The wrinkles and folded edges are characterized by 0.61 nm spacing, as expected for crystalline molybdenite.14 The crystal domains are very small, as confirmed by the selected area electron diffraction (Figure 1b) performed on the aggregates, which shows an extremely broad ring, consistent with a low crystallinity sample. The HR-TEM images of MoS2/RGO (Figure 1c,d) are characterized by wrinkled and folded microsheets. Several areas of the sample display the typical graphene (0,0,2) pattern, as well as wrinkles with 0.34 nm d-spacing, consistent with graphene folded (0,0,2) planes. Some larger wrinkles can also be observed (0.61 nm) corresponding to MoS2 grown on the surface of RGO. Some darker nanoparticles can be recognized on the surface of RGO (Figure 1d), indicating the presence of MoS2 nucleation sites. In the Ag2S/MoS2/RGO composite (Figure 1e−h), the HRTEM images display some large nanoparticles (10−50 nm range) surrounded by 2D nanosheets, whose d-spacing is compatible with MoS2. The wrinkles and some crystalline features of MoS2 are clearly visible (Figure 1e,g), with the

the most promising catalyst for HER. A number of studies have been conducted to increase the catalytic performances of MoS2: the formation of structural defects including point defects, sulfur vacancies, cracks,7 and the creation of composites with other two-dimensional (2D) materials like graphene or carbon nitrides8 are some of the directions pursued up to now. Growing MoS2 on the surfaces of other nanomaterials, like 2D materials such as reduced graphene oxide (RGO),8 TiO2,6 and graphitic carbon nitrides,9 could induce a synergetic effect because of their intrinsic high charge conduction properties. Doping with various metals (Co, Ni, Fe, and Zn) has also been proved to improve the HER activity in MoS2.10,11 Composite catalysts, including binary and ternary systems, are also an effective method for improving the electrocatalytic activity. The growth of metal/nonmetal nanoparticles on the surface of 2D materials such as MoS2 and RGO is another strategy to create an effective catalyst for HER. So far, many ternary hydrogen evolution electrocatalysts, including MoS2/ RGO,8 Mn/MoS2/RGO,11 CoS2@MoS2/RGO,12 and other composites, have been reported, with higher electrocatalytic activity, compared with their binary counterpart. Combining MoS2 with highly conductive graphene substrates can facilitate the electron transfer rate from the MoS2 to the substrate. Ag2S is another metal sulfide that has been used as a catalyst for HER.13 In this study, we report a new ternary system composed of Ag2S, MoS2, and RGO. The as-prepared Ag2S/MoS2/RGO ternary composites synthesized via the hydrothermal method have a significant and enhanced electrocatalytic activity compared with the bare MoS2/RGO and MoS2. The reason for that is the synergetic effect of both sulfides, which also offers a promising pathway to the design of a new catalyst for water splitting and help minimize the need of an expensive catalyst for hydrogen evolution, holding a great promise for clean energy. 22381


Research Article


Figure 2. XRD patterns (a, b, c) and Raman spectra (d, e, f) of (a, d) MoS2, (b, e) MoS2/RGO, and (c, f) Ag2SMoS2/RGO.

are broad and weak, indicating the poor crystallization of MoS2. The MoS2/RGO sample presents an additional peak with very low intensity at around 25.5° (Figure 2b), which can be attributed to RGO (JCPDS card no. 96-120-0018), confirming the presence of reduced graphene oxide in the binary system.15 The ternary composite sample Ag2S/MoS2/ RGO (Figure 2c) exhibits a diffraction peak typical of Ag2S nanoparticles, in agreement with JCPDS card no. 96-901-1415. In the Ag2S/MoS2/RGO sample, a monoclinic crystal structure, with lattice parameters a = 4.23 Å, b = 6.93 Å, c = 8.2 Å and space group P121/c1, associated with the acanthine phase of Ag2S was obtained, which is in agreement with HRTEM analysis and literature results.16 In addition, a very small amount of cubic silver (Ag) is found (JCPDS card no. 96-9011608). This silver might come from the reduction of silver by some organics before it forms Ag2S. This is also in agreement with some literature results on hydrothermally synthesized Ag2S nanoparticles.17 In the ternary system, Ag2S is the dominant detectable phase, compared with RGO and MoS2, suggesting that crystallization of Ag2S is more effective. Furthermore, the characteristic peaks of MoS2 and RGO are not clearly distinguishable in the XRD pattern of the composite. This could be because the peaks from Ag2S nanoparticles are more intense. In general, the two peaks in MoS2 and MoS2/RGO are broad and weak, which indicate poor crystallization, caused by the hydrothermal synthesis method. The semicrystalline and amorphous MoS2-based catalyst shows a higher density of active sites and exhibits comparable HER activity with respect to crystalline MoS2.18 Raman spectroscopy can be used to investigate phase composition, defects, and vacancies in the crystalline structure of composites,19 benefitting from its extreme sensitivity to the type of bonding and coordination symmetry, which makes it possible to identify different phases of MoS2 from their characteristic Raman peaks.14,20 The 2H-MoS2 phase exhibits peaks at 286 cm−1 (Eg1), 383 cm−1 (E2g1), and 408 cm−1 (A1g). Among them, the peaks at 383 and 408 cm−1 with some blue and red shifts are the most intense ones.14 Moreover, in this work, as shown in Figure 2d, the E2g1 (379 cm−1) and A1g (405 cm−1) peaks are intense in the MoS2 sample and are ascribed

expected molybdenite phase fringes. The EDS spectrum (Figure S2b, Supporting Information) and EDS mapping on the nanoparticles (Figure 1f) highlight homogeneous concentrations of Ag and S within the nanoparticles, whereas a complementary shell of Mo suggests a core−shell structure where Ag2S nanoparticles are embedded in MoS2 nanosheets. The nanoparticles are crystalline, and the FFT pattern (h) is compatible with the acanthite Ag2S phase. Again, these nanoparticles are surrounded by a layer of semicrystalline MoS2, suggesting a strong interaction between the two sulfides. These crystalline nanoparticles were related to the formation of Ag2S nanocrystals as confirmed by X-ray diffraction (XRD) (JCPDS card no. 96-901-1415). The set of lattice fringes observed in silver sulfide nanoparticles corresponds to monoclinic nanocrystalline acanthite phases (Figure 1g). The FFT pattern (Figure 1h, top left) obtained from the fast Fourier transform (FFT) of the Ag2S/MoS2/RGO nanocomposite displayed in Figure 1g highlights the presence of the Ag2S acanthite (red, zone axis (0,0,1); and yellow, zone axis (1,1,0)) and MoS2 molybdenite (green, 0,0,2 reflections) phases, confirming the core−shell structure of the composite. Rutherford backscattering (RBS) investigated the composition of the as-synthesized powder (Figure S3, Supporting Information). The RBS spectra of the three different samples indicate the presence of Mo and S in the sample MoS2 (see surface edges for S and Mo panels). The lower yield in the sample MoS2/RGO, compared with that in the MoS2 sample, is due to the presence of C. In the sample Ag2S/MoS2/RGO, the presence of Ag is clearly seen in the shift of the high-energy surface edge around channel 570 (panel a). RUMP code simulations enable one to quantify Mo/S/Ag atomic ratio in the various samples. The Ag/Mo/S atomic ratio is about 0.8:1.0:2 in the composite, in agreement with EDS analysis. The powder X-ray diffraction (XRD) patterns of the catalysts are presented in Figure 2a−c. MoS2 and MoS2/ RGO exhibit very similar XRD patterns. The two diffraction peaks at around 32.6 and 57.4° correspond to the (100) and (110) planes of hexagonal MoS2 with cell parameters of a = 3.14 Å, b = 3.14 Å, c = 12.5 Å and space group of P63/mmc (according to JCPDS card no. 96-101-1287). The two peaks 22382


Research Article


Figure 3. XPS peaks for (a) Mo 3d XPS peaks of MoS2, MoS2/RGO, and Ag2S/MoS2/RGO; (b) S 2p XPS peaks of MoS2, MoS2/RGO, and Ag2S/ MoS2/RGO; (c) O 1s XPS peaks of MoS2, MoS2/RGO, and Ag2S/MoS2/RGO; and (d) 3d XPS peaks of Ag2S/MoS2/RGO. XPS spectra are normalized to their maximum intensity. In all of the panels, red, MoS2; green, MoS2/RGO; and blue, Ag2S/MoS2/RGO.

agrees with Mo(IV) in 2H-MoS2.24 Although the doublet relative to MoS2 (at lower binding energies) is always the more important moving to MoS2/RGO and, in particular to the ternary system, Ag2S/MoS2/RGO, an increase of the Mo 3d doublets relative to molybdenum in MoOx and MoO3 is evident. In bare MoS2, the amount of Mo as MoS2 is 65% with respect to the total amount of Mo, but they are reduced to 54 and 32% in MoS2/RGO and Ag2S/MoS2/RGO, respectively. In Ag2S/MoS2/RGO, a slight shift of the contribution at lower BEs is observed, suggesting a quite strong interaction with Ag2S. The peak positions observed for the S 2p peak (Figure 3b) in bare MoS2 and MoS2/RGO, respectively, are in agreement with the values for sulfur in MoS2 (162.0 eV; the BE value is the one corresponding to the 2p3/2 component).25 A slight shift of the S 2p peak toward lower BEs in Ag2S/MoS2/ RGO could be explained by the presence of Ag2S: S 2p3/2 in Ag2S falls at lower BEs, indeed. The high full width at halfmaximum of the O 1s peak (Figure 3c) suggests the presence of different species. The BEs of O 1s in molybdenum oxides are expected to be around 530.5−531.3 eV.26 This last contribution is particularly evident in the Ag2S/MoS2/RGO sample, where the amount of molybdenum oxides becomes more important. The contribution at higher BEs (around 532.0 eV) is characteristic of surface hydroxyl groups. The Ag 3d peak shape and positions (368.8 and 374.8 eV for Ag 3d5/2 and 3d3/2, respectively) (Figure 3d) are characteristic of Ag(I) in Ag2S.27 The XPS quantitative analysis gives a Ag/Mo atomic ratio equal to 0.5:1.0. The discrepancy compared with RBS (Ag/Mo = 0.8:1.0) can be understood in terms of the different penetration depths of the two techniques (a few nm for XPS, several hundred nm for RBS) and the structure of the samples, in which the Ag-containing particles are covered by a MoS2 sheet (see TEM results). In this respect, RBS gives a value closer to the true one, whereas XPS tends to underestimate the Ag concentration. The electrocatalytic performance of the as-prepared catalysts toward the HER activity was tested in the Ar-saturated 0.5 M H2SO4 electrolyte solution in a three-electrode system with a scan rate of 5 mV/s. The polarization curve (Figure 4a) shows the potential needed to produce current densities of −50 and −10 mA/cm2 (the so-called η50 and η10 parameters). The ternary composites (Ag2S/MoS2/RGO) can efficiently reduce H+ (proton), as evidenced by a small overpotential of −300 mV (−190 mV) to produce a current density of −50 (−10) mA/cm2. MoS2 and its binary composites (MoS2/RGO) exhibit an overpotential of −460 and −400 mV at a current density of −50 mA/cm2, respectively, versus the reversible

to the in-plane and out-of-plane vibration modes of the two sulfur atoms.19,20 For MoS2/RGO and Ag2S/MoS2/RGO samples, two additional broad peaks at 1345 and 1586 cm−1 were observed (Figure 2e,f). These contributions are characteristic of the D and G bands of RGO, respectively.21 The D and G bands of RGO in MoS2/RGO and Ag2SMoS2/RGO samples are clearly observed at almost the same position. The G band is due to the in-plane vibration of sp2 bonded carbon atoms, while the D band arises from defects and vacancies related vibration.22 The peak intensities of MoS2 and Ag2S in the composites are very weak and almost impossible to be detected by applying 0.2 mW laser power. We applied such low laser power to avoid the laser-induced transformation of MoS2 into MoO3 (Figure S4, Supporting Information). Ag2S is a triatomic and nonlinear molecule; its expected bands are related to the symmetric stretching mode, which is the only Raman-active one. The most intense band is in the range of 90−280 cm−1 with laser power of 1.48 mW.23 However, with reduced laser power (0.2 mW), a high-resolution Raman spectrum was not obtained, and we recorded, instead, a low-intensity and highly noisy baseline (Figure 2e,f). A much more detailed indication of the surface composition was obtained from the XPS data (Figure 3, and Table 1). It is Table 1. XPS Peak Positions (Binding Energies, eV) of the Electrocatalyst sample MoS2

MoS2/RGO

Ag2S/MoS2/RGO

Mo 3d 229.1 230.4 232.8 229.1 230.6 233.0 228.7 230.4 232.7

MoS2 MoOx MoO3 MoS2 MoOx MoO3 MoS2 MoOx MoO3

Ag 3d

S 2p 162.0

162.0

368.2 374.4

161.6

noteworthy that moving from bare MoS2 to ternary systems, a clear change of the Mo 3d peak (Figure 3a) is evident. The Mo 3d of bare MoS2 can be fitted with three doublets 229.1, 230.4, and 232.8 eV (the position refers to the 3d5/2 component of the doublet) corresponding to MoS2, MoOx, and MoO3 (Table 1). The oxide is mainly due to surface oxidation. As shown in Figure 3a, the additional peak at 226.2 eV corresponds to the S 2s peak. In particular, the doublet at lower binding energies 22383


Research Article


Figure 4. Electrochemical characterization of as-synthesized catalysts toward HER. (a) Polarization curves of HER for various catalysts up to −50 mA/cm2. (b) Tafel plots of the corresponding electrocatalyst, and (c) chronoamperometry for the stability test of Ag2S/MoS2/RGO. The inset in (c) compares the polarization curves before (solid line) and after (dotted line) the stability test. (d) Cyclic voltammetry of Ag2S/MoS2/RGO conducted in a non-Faradic region at different scanning rates.

Table 2. Summary of Tafel Slope, Mass Loading, Charge-Transfer Resistance, and Double-Layer Capacitance at a Specific Overpotential catalyst

η10 (mV)

Tafel slope (mV/dec) (voltammetry method)

mass loading (mg/cm2)

Rct (Ω) at −155 mV

Cdl (mF/cm2)

Ag2S/MoS2/RGO MoS2/RGO MoS2

−190 −279 −315

56 71 90

0.295 0.295 0.295

96 259 495

23.7 16 8.9

η = α + b log j0

hydrogen electrode (RHE). In all cases, a fast increase in the cathodic current density was observed with increasing negative potential. On further increasing the potential, the evolution of gas bubbles significantly decreased the catalytic activity. In Ag2S/MoS2/RGO samples, these bubbles are started at a relatively lower potential, as compared with the other binary and bare counterparts, which is a clear indication of the presence of the higher density of active sites and increased electrocatalytic activity. The comparison of the electrochemical performance of the various electrocatalysts based on overpotential is tabulated in Table 2. In addition, we compared the new catalyst to a platinum foil (area A = 0.42 cm2). The current density of the Pt foil shows a rapid increase in the cathodic potential; overpotentials of −100 and −6 mV vs RHE are required to produce current densities of −50 and −10 mA/cm2, respectively. The Tafel slope is another parameter to evaluate the kinetics of HER catalytic activity. If the measured Tafel parameters show a Tafel behavior, then the HER on the catalyst is a purely facial and kinetically controlled reaction, which can be described by the Tafel equation28−30

(1)

where η is the applied overpotential, j0 is the resulting current density, b is the Tafel slope, and α is the intercept related to symmetry. Depending on the electrochemical reaction condition and the corresponding Tafel slope, it is possible to understand the main mechanisms involved in the formation of molecular hydrogen. Accordingly, there are three possible HER reaction mechanisms in the acidic electrolytic condition10 H 2O+ + e̅ → Hads + H 2O → Volmer step 2.34RT ≅ 120 mV/dec b= αF

(2)

followed by a combination of adsorbed species to form H2 gas Hads + Hads → H 2 → Tafel step b=

2.34RT ≅ 30 mV/dec (1 + α)F

(3)

or the desorption step, which is characterized by 22384


Research Article


Figure 5. (a) Current density at −0.18 V versus scan rate and linear fit for estimation of double-layer capacitance. (b) Nyquist plots (solid lines), fitting data (dot), and equivalent circuit (inset) of the as-synthesized catalyst at 100% mass loading.

Hads + H+ + e− → H 2 → Heyrovsky step b=

2.34RT ≅ 40 mV/dec 2F

produce H2. As a result, the values of the slope may change from literature to literature. To further assess the long-term stability of the as-prepared catalyst, chronoamperometry experiments for the ternary catalyst were performed (Figure 4c). The stability tests were carried out at overpotential of −224 mV for 17 h. The catalyst is stable and demonstrated a current density of ∼−17 mA/cm2, with no significant change in the polarization curve after 17 h of the stability test (inset in Figure 4c). Another parameter used to compare different electrocatalysts is the electrochemical active surface area (ECSA). The rates of the HER electrocatalyst activity are directly related to ECSA. However, measuring ECSA for binary and ternary catalysts is very complicated because of the inaccurate measurement of the specific capacitance of the composites. Therefore, comparing the electrocatalysts with their electrochemical double-layer capacitance (Cdl), which have a direct relationship with ECSA, is an alternative way to have insight on the catalytic active surface area.33−35 Herein, the double-layer capacitance was obtained using cyclic voltammetry in the non-Faradic region within a very narrow potential range (0.06−0.22 V vs RHE) (Figures 4d and S5, Supporting Information). The Cdl values were estimated by plotting (Ja − Jc)/2 at −0.18 V vs RHE against the scan rate (v). The calculated slope is the value of the double-layer capacitance (Figure 5a): the values of Cdl were calculated as 23.7, 16, and 8.9 mF/cm2 for Ag2S/MoS2/RGO, MoS2/RGO, and MoS2 catalysts, respectively, which is an indication of the increased active surface sites for the ternary system. A higher Cdl of catalysts can provide a high density of reactive sites for adsorption and facilitate efficient charge transfer, leading to high catalytic activity. To understand the effect of MoS2 on the performances of the ternary catalyst, linear sweep voltammetry (LSV), cyclic voltammetry, and Cdl analysis of Ag2S/RGO were performed. As depicted in Figure S5a, Ag2S/RGO binary composites exhibit overpotentials of −340 and −460 mV at current densities of −10 and −50 mA/cm2, respectively, versus the reversible hydrogen electrode (RHE). The double-layer capacitance values were estimated by plotting (Ja − Jc)/2 at −0.16 V vs RHE against the scan rate (v). The double-layer capacitance (Figure S5b), Cdl, was calculated as 7.7 mF/cm2 for the Ag2S/RGO sample. Taking into account the values of

(4)

From the above three heterogeneous reactions, which occur on the electrode surface, the reaction mechanism has a characteristic slope, which can help compare the activity of the electrocatalysts. Therefore, a combination of possible reaction steps 1 and 2 or 1 and 3 can lead to the generation of the H2 molecule. Under a restricted set of conditions, if the Volmer step is the rate-determining step (RDS), a slope of 120 mV/ dec can be obtained, whereas if the RDS is the Heyrovsky or Tafel step, slopes between 30 and 40 mV/dec are observed.31,32 As shown in Figure 4b, the Tafel slope values obtained from the polarization curves (after removing the Ohmic contribution to the measured potential; see Supporting Information) are 90, 71, 56, and 38 mV/dec for pristine MoS2, MoS2/RGO, Ag2S/MoS2/RGO, and platinum foil, respectively. For the ternary Ag2S/MoS2/RGO composite, a lower Tafel slope was observed compared with the other nonprecious catalysts. This lower Tafel slope correlates with the lower overpotential to attain a threshold cathodic current density, which is beneficial for efficient proton reduction. The Tafel slope of 56 mV/dec for the Ag2S/MoS2/RGO electrocatalyst suggests the occurrence of a very fast Volmer step, followed by a slow electrochemical desorption Heyrovsky step. For pristine MoS2 and MoS2/RGO, the RDS was the Volmer step, which is evidenced by the large Tafel slopes of 90 and 70 mV/dec, respectively. The small value of the Tafel slope (38 mv/dec) for Pt indicates that the RDS is the Tafel step, namely, the combination of adsorbed species to form H2 gas. Therefore, in Ag2S/MoS2/RGO, the RDS is the desorption mechanism (Hads + H+ + e− → H2), indicating that the presence of both metal sulfides promotes the catalytic activities, dramatically contributing to HER. The above-suggested pathway may not be the only mechanism to obtain molecular hydrogen but can be considered as a guideline for determining the HER mechanism step. The value can vary for different experimental conditions, the different surface coverages of the adsorbed hydronium ion, and also different mechanisms may also occur in parallel, significantly influencing the overpotential needed to 22385


Research Article


Figure 6. Nyquist plot showing the EIS response of Ag2S/MoS2/RGO at (a) 75% mass loading and (b) 50% mass loading, as a function of the overpotential, from −215 mV (smallest Rct) to −70 mV (largest Rct) at steps of 20 mV. (c) Semilogarithmic plot of the inverse of charge-transfer resistance (Rct) as a function of the overpotential (η) with a Tafel slope of 56 mV/dec for 50% mass loading and a Tafel slope of 46.5 mV/dec for 75% mass loading.

first time constant (CPE1-Rct) relates to the kinetics of the HER.30 The second time constant (CPE2-Rc) is related to the porosity of an electrode surface and does not change with overpotential.30 The values of Rs obtained from the Nyquist plot at high frequency for all catalysts vary from 10 to 20 Ω and are attributed to resistance arising from the solution. We calculated Rct at −155 mV because, in this potential region, the charge transfer is only purely due to the Faradic current. The charge-transfer resistances at overpotential of −155 mV vs RHE (Figure 5b) were calculated as 96, 259, and 495 Ω for Ag2S/MoS2/RGO, MoS2/RGO, and MoS2, respectively, with 100% mass loading (0.291 mg/cm2). The Ag2S/MoS2/RGO catalyst exhibited lower charge-transfer resistance compared with the other catalysts, which is a clear indication of the fast electron transfer at the solution electrode interfaces, leading to the higher current density. As previously discussed, the Tafel slope calculated from the polarization curves is lower for the Ag2S/MoS2/RGO catalyst, indicating that the superior catalytic activity of the ternary samples can be ascribed to favorable kinetics toward the formation of molecular hydrogen. Furthermore, at mass loading of 0.291 mg/cm2 (100%), the potential independent semicircles at the high-frequency end are not observed, implying that there is no layer formed across the glassy carbon electrode and the as-synthesized electrocatalyst. Tafel parameters can also be extracted from the dependency of impedance on the overpotential, which minimizes the contribution of the resistance from the solution and the wiring circuit.31,37 For the efficient Ag2S/MoS2/RGO catalyst, the Tafel parameters are extracted from the EIS measurement by varying the overpotential, with an increment of −20 mV (Figure 6a,b). We also change the mass loading of the catalyst on the GC electrode to get an insight on how the catalytic activity changes as a function of the mass loading (Figure 6a,b, the impedance measured at 50 and 75% mass loading, respectively) and compared with 100% mass loading in Figure 5b. The results show that the charge-transfer resistance (diameter of semicircles in the Nyquist plot) decreases with increasing overpotential. The charge-transfer resistance (Rct) was obtained by fitting the EIS data with the same circuit model used for the 100% mass loading. The overpotential vs the inverse of Rct on a semilogarithmic scale was plotted to get

Cdl for the other systems, this result indicates that MoS2 has a vital effect on increasing the performance of the ternary system. To check the effect of the platinum counter electrode on the catalyst performances via dissociation, the stability test for 17 h using a graphite carbon electrode as a counter electrode was conducted. As shown in Figure S7a,b, the stability test using both the counter electrodes (graphite and platinum) produces similar results. As shown in Figure S7a, the LSV before and after the stability test results in similar performances. This indicates that the platinum counter electrode does not have any positive effect on the catalyst performances within the potential window of our measurement. Recently, silver leakage from the reference electrode Ag/ AgCl has been reported,36 which affects the measured performance of the catalyst. To check this problem, the sample MoS2/RGO (which does not contain silver) was used and analyzed using EDS analysis. The stability test was performed for 17 h at two different overpotentials (−0.36 and −0.57 V vs RHE), which is around −0.58 and 0.79 V vs the Ag/AgCl reference electrodes (Figure S8). The EDS analysis before and after 17 h of stability test is shown in Figures S9 and S10, respectively. The EDS mapping analysis was carried out directly on the sample deposited on the glassy carbon electrode. The result provides a piece of evidence for having MoS2 and RGO nanoparticles inside the binary system. Also, after bulk electrolysis (Figures S9b and S10), EDS indicates that there is no platinum and silver deposition on the assynthesized catalyst. Therefore, based on our investigations, we do not have any impurities from the silver reference electrode or the platinum counter electrode. The catalytic performance is purely from the as-deposited samples. EIS measurements were also conducted to study the chargetransfer kinetics between the electrode and solution interfaces. The Nyquist plot of the synthesized catalyst was measured at overpotential −155 mV (Figure 5b), and data were fitted using a two-series constant phase element equivalent circuit (inset of Figures 5b and S6, Supporting Information). The ease of charge transport reflects on the charge-transfer resistance obtained by fitting the EIS data. The equivalent circuit consists of series resistance, Rs, in series with two parallel model time constants (CPE1-Rct) and (CPE2-Rc). In this model, only the 22386


Research Article


Table 3. Charge-Transfer Resistances, Turnover Frequency, and Exchange Current Densities of the Prepared Electrocatalyst catalyst

charge-transfer resistances (Rct), at η−155 mV and 100% mass loading

active sites n (mol) at η−200 mV

TOF (s−1) at η−200 mV

exchange current density (jo) (mA/cm2)

Ag2S/MoS2/RGO MoS2/RGO MoS2

96 259 495

3.144 × 10−8 1.68 × 10−8 7.59 × 10−9

0.38 0.25 0.1

1.88 × 10−3 6.98 × 10−4 3.65 × 10−4

Table 4. Comparison of the MoS2-Based Catalyst That is Related to the Present Work catalyst Mn−MoS2/RGO MoS2/C3N4 Ag2S/MoS2 MoS2/RGO C/MoS2/GO g-C3N4/MoS2/RGO CoS2/MoS2/RGO CoS2@MoS2/RGO Mo2N−MoS2 CoS2−C@MoS2 Ag2S/MoS2/RGO

medium (electrolyte)

η (mV)a

Tafel slope (mV/dec)

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

−230 −500 −220 −200 −240 −397 −160 −98 −190 −173 −190

76 82 42 50 46 83 56 37 59 61 56

M M M M M M M M M M M

H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4

turnover frequency 0.05 at η = −200 mV b

0.05 at η = −200 mV b b b

0.36 at η = −200 mV b b b

0.38 at η = −200 mV

loading (mg/cm2)

Cdl (mF/cm2)

0.25 0.70 0.57 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

17.37 5.4 b

4.2 b

6.9 11.9 23 0.46 3.44 23

substrate

reference

GC GC GC GC GC GC GC GC GC GC GC

11 38 39 40 41 42 43 12 44 45 present work

a

Overpotential at a current density of 10 mA/cm2. bNo information is given about the measurement.

the contribution of the mass transport effect at higher current density. The formation of bubbles is observed at a relatively low overpotential compared with the other catalysts. This process increases the needed overpotential, and the mass transfer of the hydronium ion toward the electrode will decrease. As a result, the charge-transfer process requires more overpotential to override this process. Therefore, the Tafel slope starts to increase, and the process of adsorption and desorption steps of the molecular hydrogen will decrease. However, this problem does not occur during the EIS measurements, and the Tafel values are purely due to electron transfers. The Cdl values for the ternary sample at 75 and 50% mass loadings were also obtained through EIS and are presented in Figure S8, Supporting Information. The Ag2S/ MoS2/RGO sample with 75% mass loading exhibits a higher Cdl, which testifies its higher electrochemical active surface area. To provide further insights on the intrinsic activities of the catalyst, the number of active sites (n), turnover frequencies (TOFs), and the equilibrium current density were measured. The number of active sites was determined using cyclic voltammetry collected from −0.4 to +0.4 V vs RHE in a phosphate buffer solution (PBS, PH = 7) with a scan rate of 50 mV/s. The net voltammetry charges (Q) of the catalysts were determined by subtracting charges resulting from the glassy carbon electrode as indicated in Figure S9, Supporting Information. All of the values are calculated and tabulated in Table 3 (detailed information is given in the Supporting Information). At overpotential of −200 mV (vs RHE), the TOF for Ag2S/ MoS2/RGO is 0.38 s−1, for MoS2/RGO it is 0.25 s−1, and for MoS2 it is 0.1 s−1, which confirms that the total number of H2 produced by 1 mol of active site per unit time is higher for the Ag2S/MoS2/RGO ternary catalyst. The exchange current density value obtained for Ag2S/MoS2/RGO (1.882 × 10−3 mA/cm2) is also higher than that for the other catalysts, which indicates a higher activity. The overall high electrocatalytic activity of the ternary system compared with that of the binary (MoS2/RGO) and bare MoS2 can originate from the smaller

the Tafel slope of the proton discharging step. As shown in Figure 6a, for Ag2S/MoS2/RGO at a mass loading of 75%, and at an overpotential of −215 mV (smallest circle), the chargetransfer resistance is 70 Ω. However, for the same catalyst, at the same overpotential but different mass loading (50%) (Figure 6b), the charge-transfer resistance is reduced to 240 Ω. Furthermore, the semicircles in the low-frequency region (Figure 6b) are potential dependent, whereas those the highfrequency region does not vary upon potential variation. In all cases, the low-frequency region shows a decrease of chargetransfer resistance with the increasing applied overpotential. However, when the mass loading doubles, the impedance spectrum shows an additional semicircle at high frequency, which is not dependent on the applied potential (Figure S7, Supporting Information). This semicircle might be attributed to the high surface coverage of the aggregated catalyst on the glassy carbon electrode, which inhibits the electron transfer across the GC electrode and catalyst interfaces. This trend is not observed for 100, 75, and 50% catalyst mass loadings, suggesting that the semicircle at high frequency is not related to the pseudocapacitance obtained from hydrogen adsorption. The electrochemical impedance parameters obtained under this condition were fitted for the Ag2S/MoS2/RGO catalyst, and using these values, the Tafel parameters are calculated and plotted (Figure 6c). The results are in agreement with the ones obtained by the voltammetry method (Figure 4b) with slight differences. As shown in Figure 6c, for the Ag2S/MoS2/RGO catalyst of an approximate mass loading of 75%, a Tafel slope of 46.5 mV/dec is obtained; when the mass loading decreases to 50%, the Tafel slope increases to 56 mV/dec. The Tafel parameters obtained using the EIS methods are purely due to charge-transfer kinetics of electrons at the interfaces. The Tafel slope obtained using the EIS method also indicates that the mechanism of molecular hydrogen production predominantly occurs via the Volmer−Heyrovsky step with a faster Volmer step (discharge step) and the Heyrovsky step as the rate-determining step. The very small difference of the Tafel slope values obtained using voltammetry and EIS for the Ag2S/MoS2/RGO catalyst might arise due to 22387


Research Article

ACS Applied Materials & Interfaces charge-transfer resistance, larger TOF value, and larger Cdl, which contribute significantly to its higher electrochemical performance. As shown in Table 4, a comparison of similar MoS2-based catalysts from the literature is listed. The new synthesized ternary catalyst provides a comparable and reasonable catalytic performance with a higher turnover frequency toward HER.

solidator fellowship, the European Union’s Horizon 2020 research and innovation program under grant agreement No 654002, Luleå University of Technology laboratory fund program, and Kempe foundation for partial funding. I.C. acknowledges VINNOVA under the VINNMER Marie cure incoming Grant for partial funding (project “Light Energy”, LiEN, 2015-01513).

■

■

CONCLUSIONS In this work, we present a new and efficient ternary electrocatalyst composed of Ag2S, MoS2, and RGO for catalyzing hydrogen evolution reaction. HR-TEM images of the ternary system display nanoparticles surrounded by 2D nanosheets, whose d-spacing is compatible with 2H-MoS2. Ag2S nanoparticles with the acanthite phase, homogeneously distributed and embedded by a layer of semicrystalline MoS2, suggest a strong interaction between the two sulfides. The Tafel slope of 56 mV/dec for the Ag2S/MoS2/RGO electrocatalyst suggests the occurrence of a very fast Volmer step, followed by an RDS, electrochemical desorption, or the Heyrovsky step. The presence of both metal sulfides promotes catalytic activities, dramatically contributing to HER. This superior performance is due to the uniformly distributed Ag2S nanoparticles on the surface of the MoS2 nanosheets supported by RGO, which is responsible for the ease of hydrogen adsorption and desorption. The impedance measurements also show a quite small charge-transfer resistance for the ternary Ag2S/MoS2/RGO catalyst. The large enhancement is due to the synergistic electronic and morphological effect, including the exposure of highly rich active sites. Overall, this work demonstrates that highly efficient and stable hydrogen evolution catalysts can be obtained by forming binary and ternary catalysts. These results will open a new strategy to optimize and efficiently catalyze molecular hydrogen via the synergetic effect resulting from both metal sulfides. In summary, this work provides a low-cost strategy for designing an efficient electrocatalyst for HER, which looks promising for future clean and renewable energy resources.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05086. Full experimental protocols (PDF)

■

REFERENCES

(1) Xu, J.; Liu, T.; Li, J.; Li, B.; Liu, Y.; Zhang, B.; Xiong, D.; Amorim, I.; Li, W.; Liu, L. Boosting the Hydrogen Evolution Performance of Ruthenium Clusters through Synergistic Coupling with Cobalt Phosphide. Energy Environ. Sci. 2018, 11, 1819−1827. (2) Dubouis, N.; Yang, C.; Beer, R.; Ries, L.; Voiry, D.; Grimaud, A. Interfacial Interactions as an Electrochemical Tool to Understand Mo-Based Catalysts for the Hydrogen Evolution Reaction. ACS Catal. 2018, 8, 828−836. (3) Chia, X.; Eng, A. Y. S.; Ambrosi, A.; Tan, S. M.; Pumera, M. Electrochemistry of Nanostructured Layered Transition-Metal Dichalcogenides. Chem. Rev. 2015, 115, 11941−11966. (4) Bajdich, M.; García-Mota, M.; Vojvodic, A.; Nørskov, J. K.; Bell, A. T. Theoretical Investigation of the Activity of Cobalt Oxides for the Electrochemical Oxidation of Water. J. Am. Chem. Soc. 2013, 135, 13521−13530. (5) Naguib, M.; Mashtalir, O.; Carle, J.; Presser, V.; Lu, J.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Transition Metal Carbides. ACS Nano 2012, 6, 1322−1331. (6) Chen, B.; Meng, Y.; Sha, J.; Zhong, C.; Hu, W.; Zhao, N. Preparation of MoS2/TiO2 Based Nanocomposites for Photocatalysis and Rechargeable Batteries: Progress, Challenges, and Perspective. Nanoscale 2018, 10, 34−68. (7) Tsai, C.; Li, H.; Park, S.; Park, J.; Han, H. S.; Nørskov, J. K.; Zheng, X.; Abild-Pedersen, F. Electrochemical Generation of Sulfur Vacancies in the Basal Plane of MoS2 for Hydrogen Evolution. Nat. Commun. 2017, 8, No. 15113. (8) Lee, J. E.; Jung, J.; Ko, T. Y.; Kim, S.; Kim, S.-I.; Nah, J.; Ryu, S.; Nam, K. T.; Lee, M. H. Catalytic Synergy Effect of MoS 2 /Reduced Graphene Oxide Hybrids for a Highly Efficient Hydrogen Evolution Reaction. RSC Adv. 2017, 7, 5480−5487. (9) Wang, M.; Ju, P.; Li, J.; Zhao, Y.; Han, X.; Hao, Z. Facile Synthesis of MoS 2 /g-C 3 N 4 /GO Ternary Heterojunction with Enhanced Photocatalytic Activity for Water Splitting. ACS Sustainable Chem. Eng. 2017, 5, 7878−7886. (10) Shi, Y.; Zhou, Y.; Yang, D.-R.; Xu, W.-X.; Wang, C.; Wang, F.B.; Xu, J.-J.; Xia, X.-H.; Chen, H.-Y. Energy Level Engineering of MoS 2 by Transition-Metal Doping for Accelerating Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 15479−15485. (11) Wu, L.; Xu, X.; Zhao, Y.; Zhang, K.; Sun, Y.; Wang, T.; Wang, Y.; Zhong, W.; Du, Y. Mn Doped MoS2/Reduced Graphene Oxide Hybrid for Enhanced Hydrogen Evolution. Appl. Surf. Sci. 2017, 425, 470−477. (12) Guo, Y.; Gan, L.; Shang, C.; Wang, E.; Wang, J. A Cake-Style CoS2@MoS2/RGO Hybrid Catalyst for Efficient Hydrogen Evolution. Adv. Funct. Mater. 2017, 27, No. 1602699. (13) Basu, M.; Nazir, R.; Mahala, C.; Fageria, P.; Chaudhary, S.; Gangopadhyay, S.; Pande, S. Ag2S/Ag Heterostructure: A Promising Electrocatalyst for the Hydrogen Evolution Reaction. Langmuir 2017, 33, 3178−3186. (14) Li, M.; Wang, D.; Li, J.; Pan, Z.; Ma, H.; Jiang, Y.; Tian, Z. Facile Hydrothermal Synthesis of MoS 2 Nano-Sheets with Controllable Structures and Enhanced Catalytic Performance for Anthracene Hydrogenation. RSC Adv. 2016, 6, 71534−71542. (15) Gu, Y.; Wu, H.; Xiong, Z.; Al Abdulla, W.; Zhao, X. S. The Electrocapacitive Properties of Hierarchical Porous Reduced Graphene Oxide Templated by Hydrophobic CaCO3spheres. J. Mater. Chem. A 2014, 2, 451−459.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Getachew Solomon: 0000-0001-6039-1865 Raffaello Mazzaro: 0000-0003-4598-9556 Shujie You: 0000-0001-7475-6394 Marta Maria Natile: 0000-0001-5591-2670 Vittorio Morandi: 0000-0002-8533-1540 Alberto Vomiero: 0000-0003-2935-1165 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support from Knut & Alice Wallenberg foundation, the Swedish foundation con22388


Research Article

ACS Applied Materials & Interfaces (16) Sadovnikov, S. I.; Gusev, A. I.; Rempel, A. A. Nonstoichiometry of Nanocrystalline Monoclinic Silver Sulfide. Phys. Chem. Chem. Phys. 2015, 17, 12466−12471. (17) Sadovnikov, S. I.; Rempel, A. A. Synthesis of Nanocrystalline Silver Sulfide. Inorg. Mater. 2015, 51, 759−766. (18) Guo, Y.; Shang, C.; Zhang, X.; Wang, E. Electrocatalytic Hydrogen Evolution Using the MS 2 @MoS 2 /RGO (M = Fe or Ni) Hybrid Catalyst. Chem. Commun. 2016, 52, 11795−11798. (19) Lee, J. U.; Kim, K.; Han, S.; Ryu, G. H.; Lee, Z.; Cheong, H. Raman Signatures of Polytypism in Molybdenum Disulfide. ACS Nano 2016, 10, 1948−1953. (20) Jagminas, A.; Niaura, G.; Ž alneravičius, R.; Trusovas, R.; Račiukaitis, G.; Jasulaitiene, V. Laser Light Induced Transformation of Molybdenum Disulphide-Based Nanoplatelet Arrays. Sci. Rep. 2016, 6, No. 37514. (21) Alam, S. N.; Sharma, N.; Kumar, L. Synthesis of Graphene Oxide (GO) by Modified Hummers Method and Its Thermal Reduction to Obtain Reduced Graphene Oxide (RGO)*. Graphene 2017, 06, 1−18. (22) Asha Jhonsi, M.; Nithya, C.; Kathiravan, A. Probing Electron Transfer Dynamics of Pyranine with Reduced Graphene Oxide. Phys. Chem. Chem. Phys. 2014, 16, 20878−20886. (23) Martina, I.; Wiesinger, R.; Schreiner, M. Micro-Raman Investigations of Early Stage Silver Corrosion Products Occurring in Sulfur Containing Atmospheres. J. Raman Spectrosc. 2013, 44, 770− 775. (24) Kondekar, N. P.; Boebinger, M. G.; Woods, E. V.; McDowell, M. T. In Situ XPS Investigation of Transformations at Crystallographically Oriented MoS 2 Interfaces. ACS Appl. Mater. Interfaces 2017, 9, 32394−32404. (25) Chowdari, B. V. R.; Mok, K. F.; Xie, J. M. R. G.. IST X-Ray Photoelectron Spectroscopy Database, 1993; Vol. 160. (26) Naumkin, A. V.; Kraut-Vass, A.; Gaarenstroom, S. W.; Powell, C. J.. NIST X-Ray Photo-Electron Spectroscopy Database, NIST Standard Reference Database 20, version 4.1, 2012; Vol. 2012. (27) Kamila, S.; Mohanty, B.; Samantara, A. K.; Guha, P.; Ghosh, A.; Jena, B.; Satyam, P. V.; Mishra, B. K.; Jena, B. K. Highly Active 2D Layered MoS 2 -RGO Hybrids for Energy Conversion and Storage Applications. Sci. Rep. 2017, 7, No. 8378. (28) Lei, Z.; Zhan, J.; Tang, L.; Zhang, Y.; Wang, Y. Recent Development of Metallic (1T) Phase of Molybdenum Disulfide for Energy Conversion and Storage. Adv. Energy Mater. 2018, 8, No. 1703482. (29) Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of Their Catalytic Activity. ACS Catal. 2012, 2, 1916−1923. (30) Damian, A.; Omanovic, S. Ni and Ni{single Bond}Mo Hydrogen Evolution Electrocatalysts Electrodeposited in a Polyaniline Matrix. J. Power Sources 2006, 158, 464−476. (31) Murthy, A. P.; Theerthagiri, J.; Madhavan, J.; Murugan, K. Highly Active MoS 2 /Carbon Electrocatalysts for the Hydrogen Evolution Reaction − Insight into the Effect of the Internal Resistance and Roughness Factor on the Tafel Slope. Phys. Chem. Chem. Phys. 2017, 19, 1988−1998. (32) Shinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K. Insight on Tafel Slopes from a Microkinetic Analysis of Aqueous Electrocatalysis for Energy Conversion. Sci. Rep. 2015, 5, No. 13801. (33) Liu, N.; Guo, Y.; Yang, X.; Lin, H.; Yang, L.; Shi, Z.; Zhong, Z.; Wang, S.; Tang, Y.; Gao, Q. Microwave-Assisted Reactant-Protecting Strategy toward Efficient MoS 2 Electrocatalysts in Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, 23741− 23749. (34) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881−17888. (35) Tang, T.; Jiang, W. J.; Niu, S.; Liu, N.; Luo, H.; Chen, Y. Y.; Jin, S. F.; Gao, F.; Wan, L. J.; Hu, J. S. Electronic and Morphological Dual

Modulation of Cobalt Carbonate Hydroxides by Mn Doping toward Highly Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. J. Am. Chem. Soc. 2017, 139, 8320−8328. (36) Roger, I.; Symes, M. D. Silver Leakage from Ag/AgCl Reference Electrodes as a Potential Cause of Interference in the Electrocatalytic Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2017, 9, 472−478. (37) Zhang, H.; Yang, B.; Wu, X.; Li, Z.; Lei, L.; Zhang, X. Polymorphic CoSe 2 with Mixed Orthorhombic and Cubic Phases for Highly Efficient Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, 1772−1779. (38) Qian, X.; Ding, J.; Zhang, J.; Zhang, Y.; Wang, Y.; Kan, E.; Wang, X.; Zhu, J. Ultrathin Molybdenum Disulfide/Carbon Nitride Nanosheets with Abundant Active Sites for Enhanced Hydrogen Evolution. Nanoscale 2018, 10, 1766−1773. (39) Wang, M.; Ju, P.; Li, W.; Zhao, Y.; Han, X. Ag2S NanoparticleDecorated MoS2 for Enhanced Electrocatalytic and Photoelectrocatalytic Activity in Water Splitting. Dalton Trans. 2017, 46, 483−490. (40) Lee, J. E.; Jung, J.; Ko, T. Y.; Kim, S.; Kim, S., Il; Nah, J.; Ryu, S.; Nam, K. T.; Lee, M. H. Catalytic Synergy Effect of MoS 2 /Reduced Graphene Oxide Hybrids for a Highly Efficient Hydrogen Evolution Reaction. RSC Adv. 2017, 7, 5480−5487. (41) Yang, C.; Li, Y.; Wang, J.; Ma, L.; Zhou, Z.; Tian, X.; Dai, C. Carbon Doped Molybdenum Disulfide Nanosheets Stabilized on Graphene for the Hydrogen Evolution Reaction with High Electrocatalytic Ability. Nanoscale 2016, 8, 1676−1683. (42) Kang, S.; Jang, J.; Ahn, S. H.; Lee, C. S. Novel Design of Hollow G-C3N4 Nanofibers Decorated with MoS2 and S, N-Doped Graphene for Ternary Heterostructures. Dalton Trans. 2019, 48, 2170−2178. (43) Liu, C.-G.; Chi, J.-Q.; Liu, Y.-Q.; Gao, W.-K.; Dong, B.; Yan, K.-L.; Shang, X.; Liu, Y.-R.; Chai, Y.-M.; Li, X. Ternary CoS 2 /MoS 2 /RGO Electrocatalyst with CoMoS Phase for Efficient Hydrogen Evolution. Appl. Surf. Sci. 2017, 412, 138−145. (44) Ojha, K.; Saha, S.; Banerjee, S.; Ganguli, A. K. Efficient Electrocatalytic Hydrogen Evolution from MoS2-Functionalized Mo2N Nanostructures. ACS Appl. Mater. Interfaces 2017, 9, 19455− 19461. (45) Zhu, Y.; Song, L.; Song, N.; Li, M.; Wang, C.; Lu, X. Bifunctional and Efficient CoS 2 −C@MoS 2 Core−Shell Nanofiber Electrocatalyst for Water Splitting. ACS Sustainable Chem. Eng. 2019, 7, 2899−2905.

22389


MoS2 Nanocomposites Anchored on Reduced Graphene Oxide

Recommend Documents