MoS2 nanocomposites anchored on reduced graphene oxide

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AgS/MoS nanocomposites anchored on reduced graphene oxide: Fast interfacial charge transfer for hydrogen evolution reaction Getachew Yirga Solomon, Raffaello Mazzaro, Shujie You, Marta Maria Natile, Vittorio Morandi, Isabella Concina, and Alberto Vomiero ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05086 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Ag2S/MoS2 nanocomposites anchored on reduced graphene oxide: Fast interfacial charge transfer for hydrogen evolution reaction Getachew Solomon,a Raffaello Mazzaro,a,b Shujie You,a Marta Maria Natile,c Vittorio Morandi,b Isabella Concina,a Alberto Vomiero,*a,d

a. Division of Materials Science, Department of Engineering Science and Mathematics, Luleå University of Technology, SE-971 98 Luleå, Sweden b. CNR- Institute of microelectronics and microsystem (IMM), 40129 Via Piero Gobetti 101, Bologna, Italy c. 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 d. Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Via Torino 155, 30172 Venezia Mestre, Italy

ACS Paragon Plus Environment

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KEYWORDS: electrocatalyst, hydrogen evolution, silver sulfide, molybdenum sulfide, reduced graphene oxide.

Corresponding author: *[email protected]


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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 one-pot synthesis. The RGO support assists the growth of 2D MoS2 nanosheets partially covered by silver sulfides as revealed by HR-TEM. As compared to the bare MoS2 and MoS2/RGO, the Ag2S/MoS2 anchored on RGO surface (the ternary system Ag2S/MoS2/RGO) demonstrated a high catalytic activity towards hydrogen evolution reaction (HER). Its superior electrochemical activity towards HER is evidenced by the positively shifted (190 mV vs RHE) over potential at a current density of -10 mA/cm2, and a small Tafel slope (56 mV/dec) compared to 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 (EIS) was applied to understand the charge transfer resistance, the ternary sample shows a very small charge transfer resistance (98 Ohm) 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



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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 a great promise for the development of a clean energy landscape.


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INTRODUCTION Hydrogen is a promising alternative to fossil fuels. Its superior properties such as lightweight, high energy density, and sustainability make it the ideal candidate for our future energy resources. Most hydrogen production processes produce undesirable by-products such as CO2.1 Electrocatalytic and photocatalytic methods, instead, represent a sustainable way for hydrogen evolution, which is currently attracting huge attention from the 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 decreasing it. Platinum and platinum-based catalysts are generally considered as the most efficient catalyst for HER, but their high-cost and scarcity limits its widespread use. For this reason, highly efficient, stable, 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 di-chalcogenides (TMDs),3 layered metal oxides4 and carbides5. 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.



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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 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 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 (RGO),8 TiO2,6 graphitic carbon nitrides9 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 system, 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 to their binary counterpart. Combining MoS2 with highly


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conductive graphene substrate can facilitate electron transfer rate from the MoS2 to the substrate. Ag2S is another metal sulfide that has been used as a catalyst for HER13. In this study, we report a new ternary system composed of Ag2S, MoS2, and RGO. The asprepared Ag2S/MoS2/RGO ternary composites synthesized via hydrothermal method have a significant and enhanced electrocatalytic activity compared to 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 to minimize the need of an expensive catalyst for hydrogen evolution, holding a great promise for clean energy.

RESULTS AND DISCUSSION The morphological and structural characterization of as prepared MoS2, MoS2/RGO, and Ag2SMoS2/RGO composites are reported in Fig. 1 and Fig. 1, electronic supporting information (ESI†). FE-SEM characterization (Fig. 1, ESI†) shows only a small variation of the microscale morphology of the binary and the bare MoS2 samples, while an agglomerated Ag2S nanoparticle is observed in the ternary samples. The MoS2 sheet 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 (Fig. 1 a ESI†) suggested the elemental distribution of Mo, and S elements, consistent with the successful preparation of the MoS2



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nanosheets. HR-TEM characterization of the MoS2 sample (Error! Reference source not found. a) displays wrinkled micro-sheets, with the typical morphology and thickness associated with MoS2. The wrinkles and folded edges are characterized by the 0.61 nm spacing, as expected for crystalline Molybdenite.14 The crystal domains are very small, as confirmed by the Selected Area Electron Diffraction (Fig. 1 (b)) performed on the aggregates, which shows an extremely broad ring, consistent with a low crystallinity sample. The HR-TEM images of MoS2/RGO (Fig. 1 (c, d)) are characterized by wrinkled and folded micro-sheets. Several areas of the sample display the typical graphene (0,0,2) pattern, as well as

Fig. 1 HR-TEM micrographs and SAED pattern of a, b) pristine MoS2; c,d) HR-TEM micrographs of ) MoS2/RGO sample and relative

FFT (in the inset) exhibiting the typical

hexagonal RGO pattern. e,g) HR-TEM micrographs of Ag2S/MoS2/RGO sample displaying crystalline nanoparticles surrounded by MoS2 nanosheets. f) EDS elemental mapping Ag2S/MoS2/RGO sample, exhibitingACS theParagon core-shell structure. h) top left, FFT of the HR-TEM Plus Environment image displayed in g), and relative spatial distribution of the crystal phases on the original

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wrinkles with 0.34 nm d-spacing, consistent with graphene folded (0,0,2) planes. Some larger wrinkles can be also observed (0.61nm) corresponding to MoS2 grown on the surface of RGO. Some darker nanoparticles can be recognized on the surface of RGO (Fig. 1d), pointing out the presence of MoS2 nucleation sites. In the Ag2S/MoS2/RGO composite (Fig. 1 (e-h)), the HR-TEM 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 (Fig. 1(e, g)), with the expected Molybdenite phase fringes. EDS spectrum (Fig. 1 b, ESI†) and EDS mapping on the nanoparticles (Fig. 1 f) highlights a homogeneous concentration of Ag and S within the nanoparticles, while a complimentary shell of Mo suggests a core-shell structure where Ag2S nanoparticles are embedded in MoS2 nano-sheets. The nanoparticles are crystalline and the FFT pattern (h) is compatible with Acanthite Ag2S phase. Again, these nanoparticles are surrounded by a layer of semi-crystalline MoS2, suggesting a strong interaction between the two sulfides. These crystalline nanoparticles were related to the formation of Ag2S nanocrystals as confirmed by XRD (JCPDS card no. 96-901-1415). The set of lattice fringes observed in silver sulfide nanoparticle corresponds to monoclinic nanocrystalline Acanthite phases (Fig. 1Error! Reference source not found. g). The FFT pattern (Fig. 1 h, top left) obtained from the Fast Fourier Transform



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(FFT) of Ag2S/MoS2/RGO nanocomposite displayed in Fig. 1 g highlights the presence of Ag2S Acanthite phase (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 (Fig. 13, ESI†.). 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 panel). The lower yield in the sample MoS2/RGO, compared to 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 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


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Fig. 2 (a-c). MoS2 and MoS2/RGO exhibits very similar XRD pattern. 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 P 63/m mc (according to JCPDS card no. 96-101-1287). The two peaks 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 (Fig. 2b), which can be attributed to RGO (JCPDS card no. 96-120-0018), confirming the presence of reduced graphene oxide in the binary system15. The ternary composite sample Ag2S/MoS2/RGO (Fig. 2 c) exhibits a diffraction peak typical of Ag2S nanoparticle, well matched with JCPDS card no. 96-901-1415.



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In Ag2S/MoS2/RGO sample a monoclinic crystal structure, with lattice parameters a=4.23Å, b=6.93Å, c=8.2Å, and space group P121/c1, associated to the acanthine phase of Ag2S was obtained, which is in agreement with HRTEM analysis and with literature results.16 In addition, a very small amount of cubic silver (Ag) is found (JCPDS card no. 96-901-1608). This silver might come from the reduction of silver by some organics before it forms Ag2S. This is also in agreement with some literature result on hydrothermally synthesized Ag2S nanoparticles.17 In the ternary system, Ag2S is the dominant detectable phase, compared to RGO and MoS2, suggesting that

Fig. 2 XRD patterns (a, b, c) and Raman spectra (d, e, f) of a, d) MoS2, b, e) MoS2/RGO c, f) Ag2SMoS2/RGO. crystallization of Ag2S is more effective. Furthermore, the characteristic peaks of MoS2 and RGO


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are not clearly distinguishable in the XRD pattern of the composite. This could be because of the peaks from Ag2S nanoparticles are more intense. In general, the two peaks in MoS2 and MoS2/RGO are broad and weak, which indicates poor crystallization, caused by the hydrothermal synthesis method. Semi-crystalline and amorphous MoS2 based catalyst shows a higher density of active sites and exhibits comparable HER activity with respect to crystalline MoS218. Raman spectroscopy can be used to investigate phase composition, defects, and vacancies in the crystalline structure of composites,19 benefitting of 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.20 The 2H-MoS2 phase exhibit, peaks at 286 cm-1 (E1g), 383 cm-1 (E12g), and 408 cm-1 (A1g). Among them, the peaks at 383 cm-1 and 408 cm-1 with some blue and redshifts are the most intense ones.14 Also in this work, as shown in



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Fig. 2 d, the E12g (379cm-1) and A1g (405 cm-1) peaks are intense in the MoS2 sample and are ascribed 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 cm-1, and 1586 cm-1 were observed (


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Fig. 2 e & 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 Carbon-Carbon (sp2) bond, and the D band arises from defects and vacancies related to sp3 bonded Carbon atoms.22 The peaks intensity of MoS2 and Ag2S in the composites is very weak and is almost impossible to be detected by applying the 0.2 mW laser power. We applied such low laser power to avoid the laser induced transformation of MoS2 into MoO3 (see Fig. 1, ESI†). Ag2S is a triatomic and non-linear 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 1.48mW.23 However, with reduced laser power (0.2mW), a high-resolution Raman spectrum was not obtained, and we recorded, instead, a low intense and highly noisy baseline (



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Fig. 2(e-f)). A much more detailed indication of the surface composition was obtained from the XPS data (

Fig. 3, and Error! Reference source not found.). It is noteworthy that moving from bare MoS2 to ternary

systems

a

clear

change

of


Mo

3d

peak

(

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Fig. 3 aError! Reference source not found.) is evident. The Mo 3d of bare MoS2 can be fitted with 3 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, respectively (Table 1). The oxide is mainly due

to

surface

oxidation.

As

shown

in

Fig. 3 a, the additional peak at 226.2 eV corresponds to the S 2s peak. In particular, the doublet at lower binding energies 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 relatives to molybdenum in MoOx and MoO3 is evident. In bare MoS2 the amount of Mo as MoS2 is 65 % respect to the total amount of Mo but is reduced to 54 % and 32 % in MoS2/RGO and Ag2S/MoS2/RGO, respectively.



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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 (

Fig. 3 b) 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 S 2p peak towards lower BEs in Ag2S/MoS2/RGO could be explained with the presence of Ag2S: S 2p3/2 in Ag2S falls at lower BEs, indeed. The high FWHM of the O 1s peak (

Fig. 3 c) suggests the presence of different species. The BEs of O 1s in Molybdenum oxides is expected 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.


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The Ag 3d peak shape and positions (368.8 and 374.8 eV for Ag 3d5/2 and 3d3/2, respectively) (

Fig. 3 d) 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 to RBS (Ag:Mo=0.8:1.0) can be understood in terms of the different penetration depth 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, while XPS tends to underestimate Ag concentration. Table 1 XPS peak positions (binding energies, eV) of the electrocatalyst. Sample

Mo 3d

MoS2

229.1

MoS2

230.4

MoOx

232.8

MoO3

229.1

MoS2

230.6

MoOx

233.0

MoO3

228.7 230.4

MoS2/RGO

Ag2S/MoS2/RGO

Ag 3d

S 2p

-

162.0

-

162.0

MoS2

368.2

161.6

MoOx

374.4



232.7

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MoO3

The electrocatalytic performance of the as-prepared catalysts towards the HER activity were tested in the Ar-saturated 0.5M H2SO4 electrolyte solution in a three-electrode system with a scan rate of 5 mV/s. The polarization curve (Fig. 4 a) shows the potential needed to produce a current density of -50 and -10 mA/cm-2 (the so-called η50 and η10 parameters). The ternary composites (Ag2S/MoS2/RGO) can efficiently reduce H+ (proton) as evidenced by a small over-potential of -

Fig. 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 the panels, red: MoS2; green: MoS2/RGO; blue: Ag2S/MoS2/RGO. 300 mV (-190 mV) to produce a current density of -50 (-10) mA/cm-2. MoS2 and its binary composites (MoS2/RGO) exhibits an overpotential of -460 mV and -400 mV at a current density of -50 mA/cm-2 respectively, versus reversible hydrogen electrode (RHE). In all cases, a fast


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increase in the cathodic current density was observed with increasing negative potential. 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 to the other binary and bare counterparts, which is a clear indication of the presence of 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; an overpotential of −100 mV 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 equation: 28,29,30 𝜂 = 𝛼 + 𝑏 𝑙𝑜𝑔𝑗0

1

Where 𝜂 is the applied overpotential, 𝑗0 the resulting current density, b 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



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formation of molecular hydrogen. Accordingly, there are three possible HER reaction mechanisms in acidic electrolytic condition;10 H2O + + e → Hads + H2O b=

2.34RT αF ≅120

→ Volmer step 2

mV/dec

Followed by a combination of adsorbed species to form H2 gas: Hads + Hads →H2

→

Tafel step

2.34RT

3

b = (1 + α)F≅30 mV/dec Or the desorption step, which is characterized by: Hads + H + + e ― →H2 b=

2.34RT 2F ≅40

→

Heyrovsky step

mV/dec

4

From the above three heterogeneous reactions, which occur on the electrode surface, the reaction mechanism has a characteristic slope, which can help to 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, while if the RDS is the Heyrovsky or Tafel step, slopes between 30-40 mV/dec are observed 31,32.


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As shown in Fig. 4 b, the Tafel slope values obtained from the polarization curves (after removing the ohmic contribution to the measured potential, see ESI†) are 90, 71, 56, and 38 mV/dec for pristine MoS2, MoS2/RGO, Ag2S/MoS2/RGO, and for platinum foil, respectively. For the ternary Ag2S/MoS2/RGO composite, lower Tafel slope was observed than 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 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 slope of 90 mV/dec and 70 mV/dec, respectively. The small value of the Tafel slope (38 mv dec-1) 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 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 in determining the HER mechanisms step. The value can vary for different experimental conditions, the different surface coverage of adsorbed hydronium ion, and also different mechanisms may also occur in parallel, which significantly influences the



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overpotential needed to produce H2. As a result, the values of the slope may change from literature to literature.

Fig. 4 Electrochemical characterization of as synthesized catalysts towards 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

stability

test

of

Ag2S/MoS2/RGO. The inset in (c) compares the polarization curves before (solid line) and To further assess the long-term stability of as-prepared catalyst the Chronoamperometry after (dotted line) the stability test. d) Cyclic voltammetry of Ag2S/MoS2/RGO conducted in experiments the ternary catalyst were performed (Fig. 4 c). The stability tests were carried out non-faradicfor region at different scanning rates. at overpotential of -224 mV for 17 hr. The catalyst is stable and demonstrated a current density of ~ ― 17 mA/cm2, with no significant change in the polarization curve after 17 hr of the stability test (inset in Fig. 4 c). Another parameter used to compare different electrocatalysts is the


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electrochemical active surface area (ECSA). The rates of 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 electrocatalyst 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.33,34,35 Herein, the double layer capacitance was obtained using cyclic voltammetry in the non-faradic region within a very narrow potential range (0.06- 0.22V vs RHE) as shown in (Fig. 4(d)) and Fig. 1, ESI†). The Cdl values were estimated by plotting (Ja-Jc)/2 at -0.18V vs RHE against the scan rate (v). The calculated slope is the value of the double layer capacitance (Fig. 5 a): the values of the Cdl were calculated as 23.7 mF/cm2, 16 mF/cm2, 8.9 mF/cm2 for Ag2S/MoS2/RGO, MoS2/RGO and MoS2 catalyst, respectively, which is an indication for 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 facilitates efficient charge transfer, leading to high catalytic activity. In order to understand the effect of MoS2 on the performances of the ternary catalyst, the LSV, cyclic voltammetry, Cdl analysis of Ag2S/RGO were performed. As depicted in Error! Reference source not found.a, Ag2S/RGO binary composites exhibit an overpotential of -340 mV and -460



Page 26 of 48

mV at a current density of -10 mA/cm2, and -50 mA/cm2, respectively, versus 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 (Error! Reference source not found.b), Cdl, was calculated as 7.7 mF/cm2 for the Ag2S/RGO sample. Taking into account the values of 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 hours using a graphite carbon electrode as a counter electrode was conducted. As shown in Error! Reference source not found.a and b, the stability test using both the counter electrodes (graphite and platinum) produces similar results. As shown in Error! Reference source not found.a, 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 hours at two different overpotential (-0.36 and -0.57 V vs RHE), which is around


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-0.58 V and 0.79V Vs Ag/AgCl reference electrodes (Error! Reference source not found.). The EDS analysis before and after 17 hours of stability test is shown in Error! Reference source not found. and Error! Reference source not found., 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 (Error! Reference source not found.b and Error! Reference source not found.), EDS indicates that there is no platinum and silver deposition on the as-synthesized 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 asdeposited samples. EIS measurements were also conducted to study the charge transfer kinetics between the electrode and solution interfaces. The Nyquist plot of the synthesized catalyst was measured at overpotential -155 mV (Fig. 5 b), and data were fitted using a two series constant phase element equivalent circuit (inset of Fig. 5 b and Fig. 1, ESI†). 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 first time constant (CPE1-Rct) relates to the kinetics of the HER.


30

The second


Page 28 of 48

time constant (CPE2–Rc) is related to the porosity of an electrode surface and does not change


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with overpotential


30.

The values of Rs obtained from the Nyquist plot at high frequency for all



Page 30 of 48

catalysts varies from 10 to 20 Ohm 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

Fig. 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 to the faradic current. The charge transfer resistance at overpotential of-155mV vs RHE (Fig. 5 circuit (inset) of as synthesized catalyst at 100% mass loading. b) was calculated as 96, 259 and 495 Ohm for Ag2S/MoS2/RGO, MoS2/RGO, and MoS2, respectively with 100% mass loading (0.291 mg/cm2). Ag2S/MoS2/RGO catalyst exhibited lower charge transfer resistance as compared to 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 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.


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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. Table 2. Summary of Tafel slope, mass loading, charge transfer resistance, and double layer capacitance at a specific overpotential

Catalyst

𝜼10

(mV)

Ag2S/MoS2/RGO

-190

Tafel slope

Mass loading Rct

(mVdec-1)

(mg/cm2)

(Ohm)

(voltammetry

at

method)

mV

56

0.295

Cdl (mF/cm2)

-155

96

23.7 mF/cm2

MoS2/RGO

-279

71

0.295

259

16 mF/cm2

MoS2

-315

90

0.295

495

8.9 mF/cm2

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 (



Page 32 of 48

Fig. 6 (a-b)). We also change the mass loading of the catalyst on GC electrode to get an insight on how the catalytic activity changes as a function of the mass loading (

Fig. 6 a and b, the impedance measured at 50% and 75% mass loading respectively), and compared with 100% mass loading in Fig. 5 b. The results show that the charge transfer resistance (diameter of semicircles in the Nyquist plot) decreases with increasing overpotential. The charge transfer resistance (Rct) were 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 the Tafel slope of the proton discharging step. As shown in


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Fig. 6a, for Ag2S/MoS2/RGO at a mass loading of 75 %, and at an overpotential of -215mV (smallest circle), the charge transfer resistance is 70 Ω. However, for the same catalyst, at the same overpotential but different mass loading (50 %) (

Fig. 6 b), the charge transfer resistance is reduced to 240 Ω. Furthermore, the semicircles in the low-frequency region,



Page 34 of 48

Fig. 6 b are potential dependent, while the high-frequency region does not vary upon potential variation. In all cases, the low-frequency region shows a decrease of charge transfer 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 applied potential (Fig. 1, ESI†). This semicircle might be attributed to the high surface coverage of aggregated catalyst on 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 loading, 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 Ag2S/MoS2/RGO catalyst and using these values, the Tafel parameters are calculated and plotted (

Fig. 6 c). The results are in agreement with the ones obtained by the voltammetry method (see Fig. 4b) with slight differences. As shown in


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Fig. 6 c, for 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 electron at the interfaces. The Tafel slope obtained using EIS method also indicates that the mechanism of molecular hydrogen production predominantly occurs via 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 Ag2S/MoS2/RGO catalyst, might arises due to the contribution of mass transport effect at higher current density. The formation of bubbles is observed at relatively low overpotential as compared

Fig. 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) Semi-logarithmic 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 46.5mV/dec for 75% mass loading. ACS Paragon Plus Environment


Page 36 of 48

to the other catalysts. This process increases the needed overpotential, and the mass transfer of Hydronium ion towards 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 step of molecular hydrogen will decrease. However, this problem is not occurring during the EIS measurements and the Tafel values are purely due to electron transfers. The Cdl for the ternary sample at 75% and 50% mass-loading were also obtained through EIS and presented in Fig. 1, ESI†. Ag2S/MoS2/RGO sample with 75% massloading 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 V to +0.4 V Vs RHE in 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 glassy carbon electrode as indicated in Fig. 1, ESI†. All the values are calculated and tabulated in Error! Reference source not found. (detailed information are given in the ESI†). Table 3. Charge-transfer resistances, turnover frequency and exchange current densities of the prepared electrocatalyst


Page 37 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Charge-transfer

active sites n (mol)

resistances (Rct), at

Catalyst

at 𝜂 ―200𝑚𝑉

Exchange current density TOF

(s-1)

(jo) (mAcm-2)

at 𝜂 ―200𝑚𝑉

𝜂 ―155𝑚𝑉 and 100% mass-loading

Ag2S/MoS2/RGO

96

3.144x10-8

0.38

1.88 x10 ―3

MoS2/RGO

259

1.68x10-8

0.25

6.98 x10 ―4

MoS2

495

7.59x10-9

0.1

3.65 x10 ―4

Table 4. Comparison of MoS2 based catalyst that is related to the present work Catalyst Mn-MoS2/RGO

Medium

𝜂

Tafel

(electrolyte

(mV)

Slope

)

a

(mV/Dec)

0.5 M H2SO4

-230

76

Turn over

loading

Cdl

Sub

Referenc

frequency

(mg/cm2

(mF/cm2

strat

e

)

)

e

0.25

17.37

GC

11

0.05 at η= -200 mV

MoS2/C3N4

0.5 M H2SO4

-500

82

-

0.70

5.4

GC

38

Ag2S/MoS2

0.5 M H2SO4

-220

42

0.05 at η=-200 mV

0.57

-

GC

39

MoS2/RGO

0.5 M H2SO4

-200

50

-

0.25

4.2

GC

40

C/MoS2/GO

0.5 M H2SO4

-240

46

-

0.25

-

GC

41

g-

0.5 M H2SO4

-397

83

-

0.25

6.9

GC

42

0.5 M H2SO4

-160

56

0.36 at η= -200

0.25

11.9

GC

43

C3N4/MoS2/RGO

CoS2/MoS2/RGO

mV 0.5 M H2SO4

-98

37

-

0.25

23

GC

12

Mo2N–MoS2

0.5 M H2SO4

-190

59

-

0.25

0.46

GC

44

CoS2–C@MoS2

0.5 M H2SO4

-173

61

-

0.25

3.44

GC

45

Ag2S/MoS2/RGO

0.5 M H2SO4

-190

56

0.38 at η= -

0.25

23

GC

Present

CoS2@MoS2/RG O

200mV a

) Overpotential at a current density of 10 mA cm−2 -)

No information is given about the measurement.


work


Page 38 of 48

At overpotential of -200 mV (vs. RHE), the TOF for Ag2S/MoS2/RGO is 0.38 s-1, for MoS2/RGO is 0.25 s-1, and for MoS2 it is 0.1s-1, which confirms that the total number of H2 produced by one mole 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 x10-3 mA/cm-2) is also higher than the other catalysts, which indicates a higher activity. The overall high electrocatalytic activity of the ternary system compared to the binary (MoS2/RGO), and bare MoS2 can originate from the smaller charge-transfer resistance, larger TOF value, and larger Cdl, which contribute significantly to its higher electrochemical performance. As shown in Table 4Table 4, a comparison of similar MoS2 based catalysts from literature is listed. The new synthesized ternary catalyst provides a comparable and reasonable catalytic performance with higher turnover frequency towards HER.

CONCLUSION 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


38

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compatible with 2H MoS2. Ag2S nanoparticles with Acanthite phase, homogeneously distributed and embedded by a layer of semi-crystalline MoS2, suggesting a strong interaction between the two sulfides. The Tafel slope of 56 mV/dec for Ag2S/MoS2/RGO electrocatalyst suggests the occurrence of a very fast Volmer step, followed by a 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 a 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 synergetic effect resulting from both metal sulfides. In summary, this work provides a low-cost strategy for designing


39


Page 40 of 48

efficient electrocatalyst for HER, which has a promise to the future clean and renewable energy resources.

ACKNOWLEDGMENT The authors acknowledge the financial support from Knut & Alice Wallenberg foundation, the Swedish foundation consolidator 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. Concina acknowledges VINNOVA under the VINNMER Marie cure incoming Grant for partial funding (project “Light Energy”, LiEN, 2015-01513).

CONFLICT OF INTEREST The authors declare no conflict of interest

ASSOCIATED CONTENT Full experimental protocols, additional electrochemical Data and details of additional experments available on supporting Information.

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(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. Dalt. 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

Sustain. Chem. Eng. 2019, 7, 2899–2905.


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Ag2S/MoS2 nanocomposites anchored on reduced graphene oxide: Fast interfacial charge transfer for hydrogen evolution reaction


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MoS2 nanocomposites anchored on reduced graphene oxide

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