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Ag2S/Ag Heterostructure: a Promising Electrocatalyst for Hydrogen Evolution Reaction Mrinmoyee Basu, Roshan Nazir, Chavi Mahala, Pragati Fageria, Sumita Chaudhary, Subhashis Gangopadhyay, and Surojit Pande Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00029 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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Ag2S/Ag Heterostructure: a Promising Electrocatalyst for Hydrogen Evolution Reaction Mrinmoyee Basu,1,* Roshan Nazir, 1 Chavi Mahala, 1Pragati Fageria, 1 Sumita Chaudhary,2Subhashis Gangopadhyay,2 Surojit Pande 1,* 1

Department of Chemistry, BITS-Pilani, Pilani, Rajasthan-333031, India 2

Department of Physics, BITS-Pilani, Pilani, Rajasthan-333031, India

Different metal chalcogenides, being a potential candidate for hydrogen evolution catalysts, have attracted enormous attention in the field of water splitting. In present study, Ag2S/Ag is revealed as an efficient catalyst for hydrogen evolution. Using a sacrificial template of CuS nanostructure, Ag2S/Ag heterostructures are synthesized following simple wet-chemical technique. Two different routes such as wet-chemical as well as hydrothermal are followed to modulate the morphology of the CuS templates from flower ball to wire like structures, which subsequently results in the formation of Ag2S nanostructure. Finally, Ag layer is deposited on Ag2S with the help of photo reduction technique. The unique heterostructure of Ag2S/Ag shows efficient catalytic activity in H2 evolution reaction. Ag2S/Ag wire can successfully generate 10 mA/cm2 current density at -0.199V potential. Ag2S/Ag contains the micro-nanostructure; where nanoplates of Ag2S/Ag assemble to give rise microstructure like flower ball and wire.

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KEYWORDS: Hydrogen evolution, Metal chalcogenide, Microwire, Heterostructure, Flower ball.

Introduction Increasing demand of renewable and green energy sources becomes a very urgent issue which needs to be addressed as there will be a huge energy crisis in the near future. Fossil fuel present in the earth curst is limited whereas there is increasing energy demand day by day and also there is increased rate of pollution globally. Hence, an alternative source of green energy such as H2 can be considered as one of the most promising candidates.1,2 Photoelectrochemical (PEC) splitting3,4 of water is developed as an efficient way to produce hydrogen in large scale. However, for the successful implementation in PEC water splitting, development of suitable catalysts for oxygen and hydrogen evolution is of high practical importance. In this present scenario exploration of highly efficient and earth abundant catalysts for hydrogen evolution is of main focus.5,6 Pt is known as a renowned and efficient catalyst for H2 evolution, which can work almost having the onset potential of ‘0’ V.7,8 On the other hand, high cost and low availability of Pt also lead to search for new catalysts of high performance with long durability. Although, extensive research work has already been carried out to find the Pt-free catalysts for H2 evolution, further development of low cost, stable and efficient catalysts for hydrogen evolution is still of high practical needs. In this aspect metal sulfides are trying to establish themselves as efficient alternative of Pt where Mo, Co, W, Ni, Fe sulfides are found to be most efficient.9-14 Ekspong et al., reported that MoS2+x anchored on N-doped carbon nanotube exhibits an onset potential of -135 mV for HER in 0.5 m H2SO4 solution having Tafel slope of 36 mV dec-1 with high stability.15 Lin et al., discovered that ‘S’ depletion in MoS2 increases the catalytic activity which shows overpotential


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of about 60-75 mV and the Tafel slope is 29 mV/decade.16 An enhanced electrocatalytic activity of exfoliated WS2 sheets for H2 evolution reaction is also reported by Voiry et al.17 Sun et al., observed an enhanced HER activity for N-doped WS2 sheets with a low onset potential of 86 mV.18 Metal decorated metal chalcogenides have also established themselves as efficient catalyst for hydrogen evolution reaction. MoS2@graphen decorated with Pt NPs demonstrates superior electrocatalytic performance with very low Tafel slope of 39.4 mV/dec.19 Similarly Shen et al., reported that trace amount of Pt loaded MoS2 shows efficient activity toward hydrogen evolution reaction with Tafel slope 49 mV/decade.20 Huang et al., studied that MoS2 and WS2 decorated with Au nanoparticle shows significantly enhanced electrocatalytic activity toward HER reaction.21 Recently, CuS is also checked as a catalyst for hydrogen evolution reaction.22 There is limited report on the electrocatalytic activity of Ag2S or Ag2S/Ag. Ag2S/Ag is mainly studied for its antibacterial property.23,24 Recently Ren et al., reported HER activity of porous Ag2S/CuS catalyst25 where ~0.2 V is required to generate 10mA/cm2 and the value of Tafel slope is 75 mV/decade. It has been also found that Ag2S was used as an assistant material to improve the activity of CuS and MoS2 catalyst.26 MoS2 decorated with Ag2S shows enhanced electrocatalytic activity with Tafel slope of 42 mV/decade, which is due to the improved electrical conductivity results from decorated Ag2S.27 Being inspired by the previous report, we were motivated to synthesize Ag2S and to study the HER activity. In this present work, a photo-induced wet chemical approach is introduced for the synthesis of Ag2S/Ag, using CuS as a sacrificial template. Different morphologies (wire and flower ball) of Ag2S/Ag are synthesized following two different growth routes with a subsequent deposition of Ag on Ag2S to form the heterostructure. Here we explore the electrocatalytic activity of Ag2S/Ag towards the HER reaction where wire like nanostructure of Ag2S/Ag is


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found to be most effective which can generate current density of 10 mA/cm2 upon application of -0.199 V potential with excellent stability where as in case of Ag2S/Ag flower ball -0.257 V is required to generate 10 mA/cm2. Therefore, the novelty of this work is manifold-(1) design a new route for the synthesis of Ag2S/Ag wire and flower ball, (2) shape dependent study of the electrocatalytic property of Ag2S/Ag, (3) establish Ag2S/Ag wire as an efficient, stable electrocatalyst for HER reaction.

Experimental Section: Materials: Cu(II)-sulfate pentahydrate, Thioacetamide, Cetyltrimethylammonium bromide was purchased from SRL. Cu(II)-chloride, Thiourea, Ethanol and H2SO4 were purchased from Merck. AgNO3 and Nafion were purchased from sigma-aldrich. Pt/C (5%) was purchased from spectrochem. Isopropanol was from Alfa Aesar. All the chemicals were used without any further purification. Synthesis: Initially CuS was synthesized which further used as the solid sacrificial template for the synthesis of Ag2S. Two different methodologies were followed for the CuS nanostructure synthesis to finally achieve different morphologies of the Ag2S (Scheme 1). Synthesis of CuS flower ball nanostructure: CuS flower balls were synthesized following a simple wet-chemical route where Cu(II)-sulfate and thioacetamide were used as the precursor of Cu2+ and S2-. In the following procedure Cetyltrimethylammonium bromide(CTAB) was used as the growth directing agent. Initially, 0.25 g of CuSO4 was dissolved in 20 mL water and then 0.36 g of CTAB was added in stirring condition and stirring was continued for 30 min to have a clear solution and named as solution A. On the other hand, solution B was prepared by dissolving 0.15g of Thioacetamide (TAA) in 30 mL DI water. Solution A was kept on water bath (WB) maintaining the temperature ~80 °C and


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solution B was added drop wise to solution A under stirring condition. Finally, the whole mixture was kept on water bath for 1h. The bluish green precipitate was collected and washed several time using DI water and ethanol. Synthesis of CuS wire nanostructure: For the synthesis of CuS wires, hydrothermal route was followed. In this procedure initially Cu-Thiourea complex (Cu-Tu) was synthesized following a previously reported method.28 20 mL of 0.01M of aqueous solution of Cu(II)-chloride was taken in a 9 cm diameter petridish and then to it 20 mL 0.02M Thiourea (Tu) was slowly added and stirred for 5 min. After that the mixture was kept in undisturbed condition for 24h at room temperature. After 24h a white spongy precipitate was collected and washed with ethanol. In this way, [Cu(Tu)]Cl. 1/2H2O was collected in three different batches and taken in a Teflonlined autoclave. To this 0.046 g of extra Tu was added and the final volume was kept 80 mL. After that hydrothermal reaction was carried out for 5h at 150 °C. Hydrothermal set-up was allowed to cool down naturally and the greenish blue ppt was collected, washed with ethanol and DI water. Synthesis of Ag2S/Ag heterostructure: The as-synthesized CuS nanostructures (both flower ball and wires) are separately used as template for the synthesis of Ag2S/Ag heterostructure. First, 30 mg of CuS is dispersed in 30 mL of DI water through 10 min ultrasonication. Dispersed CuS solution was kept under irradiation of visible light and 500 µL of 1M AgNO3 was added drop by drop and stirred the solution for 1h. After immediate addition of AgNO3, it was observed that the dispersion becomes aggregated and flocculated at the bottom and the solution is colorless. After 1h stirring, solution shows light blue color of Cu(II) which indicates about the exchange of Cu(II) ion in CuS by Ag(I). Unless


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otherwise indicated Ag2S/Ag follower ball and Ag2S/Ag wire samples are named as Ag2S/Ag-1 and Ag2S/Ag-2. Mechanism for the synthesis of Ag2S/Ag: For the synthesis of Ag2S/Ag heterostructure a green pathway is introduced and CuS is being used as a sacrificial template. Upon addition of AgNO3 in the suspension of CuS, there is ready exchange between Cu(II) and Ag (I), ion which results in the formation of Ag2S nanostructure. The whole process was carried out under irradiation of visible light which further helps in the deposition of Ag (0) on Ag2S. Following this method Ag2S/Ag heterostructure can be successfully synthesized. Preparation of working electrode: Ink of Ag2S/Ag-1 and Ag2S/Ag-2 were prepared by dispersing 5 mg of the sample in 300 µL of Isopropanol. Then 30 µL of nafion was added as binder and sonicated for 30 min for uniform dispersion. After that 5 µL of the dispersion was drop casted carefully on GC electrode having diameter 3 mm and it leads to catalyst loading ~1.06 mg/cm2. Electrochemical measurement: Electrochemical measurements were carried out in a three-electrode system. In the cell 10 mL of 0.5M aqueous solution of H2SO4 was used as electrolyte. In the present condition Ag/AgCl used as reference electrode, Pt-wire as the counter electrode and catalyst modified on glassy carbon electrode was used as working electrode. All the electrochemical data was recorded in CH Instrument (CHI604E) at 25 °C. The linear-sweep voltammogram of Ag2S/Ag-1 and Ag2S/Ag-2 was obtained from the potential range of 0.2V to -0.8V vs. Ag/AgCl with a scan rate of 20 mV/S. Electrochemical impedance spectroscopy:


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Electrochemical impedance measurement was also performed in a three electrode system. Onset potentials of different materials were chosen as the performing bias for this measurement with the sweeping of frequency from 50 KHz to 1 Hz with a 10 mV AC dither. Characterization of Materials: Rigaku Mini Flex II diffractometer with Cu-Kα radiation was utilized to monitor powder X-ray diffraction pattern, with a scanning rate 2° per min. Raman analysis was carried out using Airix (STR 500) instrument. Morphology of CuS, Ag2S/Ag samples were investigated with the help Nova NanoSem 450 FESEM. Bruker XFlash 6130, attached with FESEM instrument was used for EDS analysis. Morphology of CuS, Ag2S/Ag sample was determined using Transmission electron microscopy (Bruker microscope operated). X-ray photoelectron spectroscopy (XPS) analysis was carried out using a commercial Omicron EA 125 source with Al-Kα radiation (1486.7eV). For all measurements, emission current of the X-ray source was fixed at 20 mA for an anode voltage of 15 kV. High resolution XPS spectra were collected using a pass energy of 20 eV, with a step size of 0.02 eV. The UHV chamber base pressure was maintained < 10-9 mbar throughout the measurements. To compensate any kind of charging effect, C 1s binding energy peak at 284.5 eV has been used as a reference. Calculation Method: Details of the calculations of mass activity and specific activity are shown below.29,30 Mass activity value (A/g) was calculated from the catalyst loading and the observed current density (mA/cmgeo2) at potential -0.25 V. Specific activity is calculated by normalizing the current at fixed potential (-0.25 V) by the electrochemically active surface area. Mass activity = observed current density at a fixed potential / catalyst loading Specific Activity = observed current / electrochemically active surface area


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Results and Discussion: To investigate the phase purity and crystallinity of the as synthesized Ag2S/Ag-1 and 2, X-ray diffraction analysis was performed as depicted in Figure 1. PXRD pattern shows the presence of both Ag2S and Ag crystals. All the diffraction peaks nicely match with the lattice planes of the monoclinic Ag2S structure (JCPDS no, 14-0072)31 along with the cubic Ag crystals (JCPDS No, 04-0783)32 as marked within the figure 1. In both the spectra, it has also been observed that the peak intensity of the Ag appears slightly weaker than that of Ag2S. However, XRD patterns do not exhibit any peak originated from CuS, the sacrificial template for the synthesis of Ag2S. This finding can be explained in terms of complete exchange of the Cu(II) ions by the Ag(I) ions, leading to the formation of Ag2S/Ag heterostructure. No other impurity peak is detected through XRD which also suggest a high material purity. From all the XRD analysis it is quite clear that pure phase Ag2S/Ag heterostructure is successfully grown following both the growth methods. XRD pattern of the CuS wire-like structures, synthesized from the Cu-Thiourea complex (CuTu) is also shown in Figure S1. Crystallinity of the as-synthesized Ag2S/Ag-2 was checked with the help of Raman spectroscopy and shown in Figure S2. It can be seen that a broad band centered at 243 cm-1 and a strong peak at 480 cm-1 which are the characteristic of Ag2S.33 Scanning electron microscopy (SEM) has been used to determine the surface morphology and possible growth methodology of CuS, and Ag2S/Ag. Different morphologies of CuS were synthesized following two different growth routes. SEM images with different magnifications of CuS nanostructures, prepared using method 1 are shown in Figure S1 and Figure 2a, Low magnification SEM image shows the presence of balls of CuS flowers with high density. High magnification SEM images reveal that the CuS flowers are composed of very small plates. Plates


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of CuS flower balls have smooth edges with the edge length of ~200 nm (Figure S1c and the inset of Figure S1c). Using these CuS flower balls as templates, Ag2S/Ag flower ball were synthesized (Figure 2b). Figure S2 shows the SEM images of Ag2S/Ag flower balls in different magnifications. Low magnification image shows that CuS and Ag2S/Ag bears nearly similar morphology of flower ball which are also composed of small petals. Whereas, high magnification image shows that the edges of the petals are not smooth like CuS rather become rough after CuS to Ag2S/Ag conversion. Figure 3a shows the SEM image of CuS wire synthesized following method 2. Initially wires of Cu-Tu complex were synthesized. Using these Cu-Tu complexes as Cu precursor, CuS was synthesized. Low magnification images show that the wire like morphology was retained in case of CuS (Figure S3). Size of CuS wires are not very uniform but lengths in micrometer region where as diameter is ~600 nm. High magnification SEM image shows that surface of CuS wires are composed of very small hexagonal plates of CuS with smooth surfaces. Edge lengths of CuS plates are ~100 nm. EDS spectra of CuS wire shows the presence of Cu and S and EDS mapping clearly show the uniform distribution of Cu and S (Figure in S4). Figure S5 and Figure 3b shows the SEM image of Ag2S/Ag in different magnifications. Low magnification SEM images shows that Ag2S/Ag synthesized from CuS wires, maintained its wire like morphology. Ag2S/Ag wires have the length in micrometer region where as diameter is ~600 nm which is in accordance with CuS wires. High magnification image of Ag2S/Ag wires shows that wires are composed of small plates. But the edges of Ag2S/Ag plates are not smooth like CuS. In both the cases, Ag2S/Ag flower ball and wires, same trend of conversion was observed from SEM analysis: Ag2S/Ag can retain the similar morphology like CuS but the well-defined edges of the very small plates are lost. EDS spectra shows the presence of Ag and S and EDS mapping clearly shows the uniform


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distribution of Ag and S throughout the wires (Figure in S6). EDS analysis of Ag2S/Ag wire does not show the presence of Cu; it can be concluded that there is complete exchange of Cu(II) by Ag(I). With the help of transmission electron microscopy, morphology of both the Ag2S/Ag samples was checked. Figure 4a shows the TEM image of flower balls of CuS. Low magnification TEM image shows the presence of flower balls of CuS. Inset of Figure 4a shows the high magnification image which clearly dictates that the flower balls are composed of very small plates of CuS. Diameters of the flower balls are in micrometer region but the lengths of the petals are in nm region which confirms the micro-nanostructure of CuS. HRTEM analysis was carried out to determine the crystallinity. Figure S7 shows the HRTEM image of CuS flower ball. The measured lattice spacing of 0.301 nm is in agreement with the ‘d’ (102) spacing of CuS.34 Area mapping and also line mapping in EDS analysis shows the presence of Cu and S only (Figure S8). TEM image of Ag2S/Ag flower ball is shown in Figure 4b. Low magnification image shows the presence of ball structure of Ag2S/Ag having diameter in micrometer region. Nature of CuS flower ball and Ag2S/Ag are quite different. Ag2S/Ag does not contain sharp length petals rather petals of Ag2S/Ag are bit blunt (inset of Figure 4b). EDS Mapping shows the uniform distribution of Ag and S throughout the flower ball structure. Line scan across the flower ball also shows the relative intensities of Ag and S (Figure S9) and the area mapping shows the uniform distribution of Ag and S throughout the flower ball. Figure 5a shows the TEM image of CuS synthesized in method 2. Low magnification TEM shows that CuS has wire like structure with spikes on its surface throughout uniformly. High magnification TEM image clearly shows that spikes are very small plates (inset of Figure 5a). It means CuS wires are composed of very small plates of CuS having smooth and defined edges.


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CuS wires have length in micrometer region where as diameter is ~600 nm which is well matched with the SEM image. Inset of Figure 5a shows that the edge length of CuS is ~100 nm. TEM image of Ag2S/Ag synthesized using CuS of method 2 also shows that Ag2S/Ag retained the wire structure of CuS. Figure 5b shows the TEM image of Ag2S/Ag. High magnification TEM shows that although the length of Ag2S/Ag is in micrometer region but the diameter is ~600 nm which is also in accordance with CuS. Previously, from SEM images it was observed that after synthesis of Ag2S/Ag from CuS, although the wire like structure remain same but the well-defined edges of CuS plates are lost in Ag2S/Ag (inset of Figure 5b). TEM also delivers the same conclusion like SEM. EDS mapping on a single Ag2S/Ag wire shows the uniform distribution of Ag and S, which confirms that the wire is made of only Ag and S and the line scan across the wire shows that the relative intensities of Ag and S (Figure S10). To investigate the surface chemical properties and relative material compositions of the metal sulfide heterostructures as well as the oxidation states of the metal (Ag) and sulfur (S), XPS technique was employed as a characterizing tool. Successful transformation of CuS nanostructures into Ag2S nanostructures as well as formation of Ag2S/Ag heterostructures and their quantitative analysis are depicted in Figure 6. A wide scan survey spectrum for Ag2S/Ag heterostructure with strong existence of Ag 3d (364 eV, 374 eV) and S 2p (161 eV, 162.2 eV) doublets along with C 1s (285 eV), O 1s (532 eV), and other Ag and S related core level binding energy and Auger peaks are shown in Figure 6a. The clear existence of Ag 3d and S 2p doublets indicates the successful formation of metal sulfide nanostructures. High resolution scans of Ag 3d and S 2p core level spectra along with their various deconvolution components are shown in Figure 6b and 6c, respectively, for Ag2S/Ag heterostructure surface. The deconvoluted Ag 3d binding energy spectrum is fitted with two


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silver doublets. The Ag+ BE peaks centered at 373.95 eV (3d3/2) and 367.96 eV(3d5/2) are contributed to the silver sulfide formation, whereas BE peaks at 373.6 eV (3d3/2) and 367.65 eV (3d5/2) are attributed to the Ag0, the metallic state of silver of the sulfide heterostructures (Figure 6b). This finding is also complementary with our XRD results where formation of Ag clusters has also noticed. A relative BE shift of about 0.35 eV for Ag0 oxidation state towards the higher energy as compared to Ag+ state has also been observed. Moreover, the ratio of Ag+ BE peak intensities for 3d1/2 to 3d

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is found to be 0.65 which is having very close proximity of the

theoretical value of 0.67. All these findings are very much in line with earlier reported values of silver sulfide and clearly indicate our assigned Ag+ state belongs to Ag2S.35,36,24 Similar to Ag 3d peak, high resolution S 2p binding energy spectrum of silver sulfide surface is presented in Figure 6c. Here, the S 2p BE spectrum is deconvoluted for spin orbit splitting of metal sulfide S2+, centered around 161 eV (2p3/2) and 162.2 eV (2p1/2). A spin orbit splitting of 1.2 eV with an intensity ratio of 0.52 (expected theoretical value is 0.5) for S 2p nicely match with earlier reported values and also suggest the formation of Ag2S.35,24 However, a small percentage of S0 state centered at 163 eV has also been observed, which might be originated as a contaminant from the used sulfur source.37 Apart from the comparison of relative positions and intensities of individual peaks, a quantitative analysis of the Ag+ and S2- absolute intensities has also been performed where the atomic sensitivity factors of 0.54 and 5.2 have been used for the S 2p(-2) and Ag 3d (+1) BE peaks, respectively. Our finding shows an atomic ratio of about 1.93 for Ag to S, which further confirms the formation of Ag2S of the Ag+ and S2- oxidation states. For a better understanding and clarity, every detail of all the deconvoluted spectra is summarized in table 1. Electrocatalytic Activity:


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Electrocatalytic performance of the synthesized material was evaluated by using linear sweep voltamograme (LSV) technique. All the electrochemical measurements for hydrogen evolution were carried out in 0.5 M H2SO4 solution with a scan rate of 20 mV/s. Potentials are measured with respect to Ag/AgCl electrode and reported against reversible hydrogen electrode. Electrocatalytic activity of bare GC, Ag2S, Ag2S/Ag-1, Ag2S/Ag-2 and Pt/C (5%) are checked and presented in Figure 7 which shows that Ag2S/Ag-2 has enhanced electrocatalytic activity compared to Ag2S/Ag-1 and Ag2S. Ag2S/Ag-2 shows increased current density with anodic shift in the onset potential. Current density of Ag2S/Ag-2 rapidly increases with further cathodic potential scans, hence, overpotential of -0.127 V and -0.199V are required to produce current densities of 2 and 10 mA/cm2, respectively. By contrast Ag2S/Ag-1 requires -0.176V and -0.257 V to generate current densities 2 and 10 mA/cm2 (Figure 7b) and pure Ag2S needs -0.342V and 0.421 V to generate 2 and 10 mA/cm2 current density. Pt/C (5%) could generate 10 mA/cm2 current density upon application of only 92 mV cathodic potential. Ag2S/Ag-1 can generate current density ~68 mA/cm2 upon applied potential of -0.4 V where as Ag2S/Ag-2 can generate 113.3 mA/cm2 under same condition. SEM images of Ag2S/Ag-1 and Ag2S/Ag-2 samples represent that both of them are having micro-nano structures, where micro structures look quite different but with very similar nanostructures. All these findings clearly indicate that nano plates of Ag2S/Ag act as a building block to give up both the microstructures such as flower balls as well as wires. Superior activity of Ag2S/Ag-2 may be attributed to the difference in their microstructure. Ag2S/Ag-2 has the micro wire structure, due to having 1D morphology, more no of H+ ions may be adsorbed on its surface. As a result of which it can generate more current density at comparatively low applied potential.


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As a support of this result, electrochemically active surface area (ECSA) was determined for Ag2S/Ag-1 and 2 samples using literature reported technique.38-41 Figure S13a, b shows the CV curve of Ag2S/Ag-1 and Ag2S/Ag-2 recorded in the potential range of 0.1157 to 0.2157 V vs. RHE at different scan rate using 0.5 M H2SO4 as an electrolyte. Double layer charging currents are measured from the CV curves at a potential of 0.1657V and are plotted with scan rates and shown in Figure S13c. Double layer capacitance (Cdl) is calculated from the slop and the values are 0.233 mF and 0.675 mF for Ag2S/Ag-1 and Ag2S/Ag-2, respectively. ECSA is calculated from Cdl and roughness factor (Rf) from ECSA. Therefore, the calculated ECSA and Rf values for Ag2S/Ag-1 and Ag2S/Ag-2 are 3.88 cm2, 54 and 11.25 cm2, 158, respectively. Higher values of ECSA and Rf of Ag2S/Ag-2 helps to provide more exposed surface area and electrochemically active sites towards HER reaction in compare to Ag2S/Ag-1. Pure Ag2S is catalytically less active compared to Ag2S/Ag. Ag being an electron dearth center, helps to enhance the charge transportation by snatching charges from Ag2S to electrolyte which further results in higher electrocatalytic activity of Ag2S/Ag. At -0.25V mass activity of Pt/C, Ag2S/Ag flower, wire and bare Ag2S are calculated and the corresponding values are 140.6, 8.2, 26.4 and 0.54 A/g, respectively which further establish Ag2S/Ag wire as an efficient electrocatalyst compared to the flower ball. Specific activity of Ag2S/Ag-1 and 2 are calculated from the ECSA and the values are 0.16 and 0.18 mA/cm2, respectively which further supports higher electrocatalytic activity of Ag2S/Ag-2. All the values of mass activity, specific activity are shown in Table 2. To validate the performance of a catalyst, Tafel slope is an important tool which can determine the mechanism pathway of the HER reaction. To determine the Tafel slope the linear portion of the Tafel plots were fitted to the Tafel equation (ߟ = ܾ݈‫ ܬ݃݋‬+ ܽ, where ߟ is overpotential, ‫ ܬ‬is the current density, ܽ is exchange current density and ܾ is the Tafel slope).42 The value of Tafel


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slope for Ag2S/Ag wire is 102 mV/decade, 128 mV/decade for flower ball (Figure 7c) and for pure Ag2S the value is 141 mV/decade. The Tafel slope of Pt/C (5%) is 37 mV/decade which is in accordance with the literature.43-45 Lower Tafel value of Ag2S/Ag wire confirms the superior catalytic activity towards HER reaction and pure Ag2S is catalytically less active compared to all three. As a comparative study, all the Tafel slopes are summarized in Table 2. HER reaction in acidic medium follows two kinds of path ways. Volmer step is called as discharge step and denoted here as (1) is the first step. The other two possibilities are first reaction now may follow desorption step called Heyrovsky process noted as (2) second, reaction may follow recombination step (3) called as Tafel step. If the rate determining step of the HER reaction is Volmer step, Tafel slope should be 120 mV/decade and for Heyrovsky process and Tafel process value should be 40 and 30 mV/decade respectively.46 HER reaction follows Volmer-Heyrovsky mechanism and the rate determining step is Volmer step for all the three cases.

It is clear that Ag2S/Ag can function as an efficient electrocatalyst for HER reaction. The overall electrocatalytic data has been summarized in Table 2. We have also made a detailed comparison with the existing literature where Ag2S or Ag2S/Ag was used as supportive material and some other cases where people used MS/M (M = metal) system for hydrogen evolution reaction and shown in Table 3. It can be seen from Table 3 that metal nanoparticle decorated on metal chalcogenide surface helps to improve electrocatalytic activity. Electrocatalytic activity of Ag2S/Ag is comparable with the existing literature.


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Stability of Ag2S/Ag-1 and Ag2S/Ag-2 was checked up to continuous 1000 cycles. Figure S14a, b shows the LSV curve of the initial and after 1000 cycles of Ag2S/Ag-1 and Ag2S/Ag-2, respectively. There is very negligible change in current density as well as onset potential (change in overpotential to generate 10 mA/cm2 for Ag2S/Ag-1 is only 0.001V and for Ag2S/Ag-2 it is 0.005V) was observed, which dictates about the robustness of the catalyst towards strong acidic condition. After electrocatalysis study, morphology of Ag2S/Ag-1 and Ag2S/Ag-2 was checked with the help of TEM analysis and it was observed that both Ag2S/Ag-1 and Ag2S/Ag-2 have the similar morphology as before (Figure S15). Line mapping shows the presence of Ag and S across the flower ball and wire. EDS mapping also shows the distribution of both Ag and S throughout flower ball (Figure S16) and wire (Figure S17). After electrocatalysis, Raman spectra of Ag2S/Ag wire shows peak at 243 and 480 cm-1 which are characteristic of Ag2S (Figure S2). To confirm the superior catalytic activity of Ag2S/Ag-2, electrochemical impedance measurement was carried out. Ease of electron transportation from the electrode surface to electrolyte can be clearly understood from the electrochemical impedance measurement. For both the samples Nyquist impedance was measured at their respective onset potentials and shown in Figure 7d. Evaluated data can be fitted using the equivalent circuit composed of one constant phase element (CPE) and R1 which speaks about the solution resistance and the charge transfer resistance from electrode surface to electrolyte (R2). The impedance curve has been fitted and the values of the resistance are summarized in table 4. R1 resistances for both the Ag2S/Ag heterostructure appear very similar but R2 sharply increases from 52.9 Ω (Ag2S/Ag-2) to 161.6 Ω (Ag2S/Ag-1). Successive increases in the charge transfer resistance indicates that wire structure of Ag2S/Ag is electro-catalytically more active compared to the follower ball like structure. Impedance result is in good agreement with the electro-catalytic data.


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Conclusion: In this work, we report an easy, simple synthetic route for the synthesis of various morphology of Ag2S/Ag. Here, CuS function as a sacrificial template for the synthesis of Ag2S/Ag nanostructure. Morphology of CuS is varied to generate different morphology of Ag2S/Ag structure. Flower ball of CuS is synthesized following a simple wet-chemical route whereas CuS nanowire is synthesized using hydrothermal route. Efficient electrocatalytic hydrogen evolution reaction on Ag2S/Ag is studied in 0.5M H2SO4. Ag2S/Ag exhibited high electrocatalytic behavior for hydrogen evolution. Ag2S/Ag-wire shows the superior activity compare to the flower ball structure. It can successfully generate 10 mA/cm2 current density upon application of -0.199V vs. RHE. Overall, Ag2S/Ag is very stable, it can generate almost unaltered current density even up to 1000 cycle run. This work opens up an opportunity for the easy preparation of an efficient and stable electrocatalyst for hydrogen evolution reaction. ASSOCIATED CONTENT Supporting Information. Detailed of the characterization of the material, experiments for checking stability are given. This material is available free of charge via at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors Mrinmoyee Basu ([email protected]); Surojit Pande ([email protected]) Author Contributions


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The manuscript was written through contributions of all authors. Roshan Nazir and Chavi Mahala both have equal contribution in this work. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements: MB thankfully acknowledges financial support from Department of Science and Technology (DST) Inspire (DST/INSPIRE/04/2015/000239) program, and DST Science and Engineering Research Board (SERB)(YSS/2015/000100), Govt. of India. SP gratefully acknowledges the financial support from DST SERB, Fasttrack (SERB/FT/CS-042/2012) grant. We are also thankful to BITS Pilani, the UGC special assistance and DST-FIST program. The instrumental support for TEM, Raman, FESEM, and XPS measurements from the Material Research Centre (MRC), MNIT Jaipur is highly acknowledged. We also thank to the Department of Physics, BITS Pilani for assistance with powder x-ray diffraction studies (DST-FIST sponsored).


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22. Basu, M.; Nazir, R.; Fageria, P.; Pande, S. Construction of CuS/Au Heterostructure through a Simple Photoreduction Route for Enhanced Electrochemical Hydrogen Evolution and Photocatalysis. Sci. Rep. 2016, 6, 34738. 23. Ma, X.; Zhao, Y.; Jiang, X.; Liu, W.; Liu, S.; Tang, Z. Facile Preparation of Ag2S/Ag Semiconductor/Metal Heteronanostructures with Remarkable Antibacterial Properties ChemPhysChem 2012, 13, 2531–2535. 24. Pang, M.; Hu, J.; Zeng, H. C. Synthesis, Morphological Control, and Antibacterial Properties of Hollow/Solid Ag2S/Ag Heterodimers. J. Am. Chem. Soc. 2010, 132, 10771– 10785. 25. Ren, H.; Xu, W.; Zhu, S.; Cui, Z.; Yang, X.; Inoue, A. Synthesis and properties of nanoporous Ag2S/CuS catalyst for hydrogen evolution reaction. Electrochim. Acta 2016, 190, 221–228. 26. Xia, X.; Zhao, X.; Ye, W.; Wang, C. Highly porous Ag-Ag2S/MoS2 with additional active sites synthesized by chemical etching method for enhanced electrocatalytic hydrogen evolution. Electrochim. Acta 2014, 142,173–181. 27. Wang, M.; Ju, P.; Li, W.; Zhao, Y.; Han, X. Ag2S nanoparticle-decorated MoS2 for enhanced electrocatalytic and photoelectrocatalytic activity in water splitting. Dalton Trans., 2017, 46, 483–490. 28.

Mao, J.; Shu, Q.; Wen, Y.; Yuan, H.; Xiao, D.; Choi, M. M. F. Facile Fabrication of Porous CuS Nanotubes Using Well-Aligned [Cu(tu)]Cl·1/2H2O Nanowire Precursors as Self-Sacrificial Templates. Cryst. Growth Des. 2009, 9, 2546–2548.


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29. Anantharaj, S.; Rao Ede, S.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069−8097. 30. Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured α‑Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077−7084. 31. Wu, P-J.; Yu, J-W.; Chao, H-J.; Chang, J-Y. Silver-Based Metal Sulfide Heterostructures: Synthetic Approaches, Characterization, and Application Prospects. Chem. Mater. 2014, 26, 3485−3494. 32. Zhang, Q.; Huang, Y.; Xu, L.; Cao, J-j.; Ho, W.; Lee, S. C. Visible-Light-Active Plasmonic Ag−SrTiO3 Nanocomposites for the Degradation of NO in Air with High Selectivity. ACS Appl. Mater. Interfaces 2016, 8, 4165−4174. 33. Minceva-Sukarova, B.; Najdoski, M.; Grozdanov, I.; Chunnilall, C. J. Raman spectra of thin solid films of some metal sulfides. J. Mol. Struct. 1997, 410-411, 267-270. 34. Basu, M.; Sinha, A. K.; Pradhan, M.; Sarkar, S.; Neigishi, Y.; Govind, Pal, T. Evolution of Hierarchical Hexagonal Stacked Plates of CuS from Liquid−Liquid Interface and its Photocatalytic Application for Oxidative Degradation of Different Dyes under Indoor Lighting. Environ. Sci. Technol. 2010, 44, 6313–6318. 35. Kaushik, V. K. XPS Core Level Spectra and Auger Parameters for Some Silver Compounds. J. Electron Spectrosc. Relat. Phenom. 1991, 56, 273.


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36. Jiang, D.; Chen, L.; Xie, J.; Chen, M. Ag2S/g-C3N4 Composite Photocatalysts for Efficient Pt-free Hydrogen Production. The co-catalyst Function of Ag/Ag2S Formed by Simultaneous Photodeposition. Dalton Trans. 2014, 43, 4878-4885. 37. Fan, W.; Jewell, S.; She, Y.; Michael; Leung, K. H. In Situ Deposition of Ag–Ag2S Hybrid Nanoparticles onto TiO2 Nanotube Arrays towards Fabrication of Photoelectrodes with High Visible Light Photoelectrochemical Properties. Phys. Chem. Chem. Phys. 2014, 16, 676-680. 38. McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking

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42. Gao, M-R.; Liang, J-X.; Zheng, Y-R.; Xu, Y-F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S-H. An Efficient Molybdenum Disulfide/Cobalt Diselenide Hybrid Catalyst for Electrochemical Hydrogen Generation. Nat. Commun.2014, 6, 5982. 43. Xiao, P.; Alam Sk, M.; Thia, L.; Ge, X.; Lim, R. J.; Wang, J.-Y.; Lim, K. H.; Wang, X. Molybdenum phosphide as an efficient electrocatalyst for the hydrogen evolution reaction. Energy Environ. Sci., 2014, 7, 2624-2629. 44. Ganesan, P.; Sivanantham, A.; Shanmugam, S. Inexpensive electrochemical synthesis of nickel iron sulphides on nickel foam: super active and ultra-durable electrocatalysts for alkaline electrolyte membrane water electrolysis. J. Mater. Chem. A, 2016, 4,1639416402. 45. Anantharaj, S.; Karthik, P. E.; Subramanian, B.; Kundu, S. Pt Nanoparticle Anchored Molecular Self-Assemblies of DNA: An Extremely Stable and Efficient HER Electrocatalyst with Ultralow Pt Content. ACS Catal. 2016, 6, 4660−4672. 46. Conway, B. E.; Tilak, B. V. Interfacial Processes Involving Electrocatalytic Evolution and Oxidation of H2, and the Role of Chemisorbed H. Electrochimi. Acta 2002, 47, 35713594.


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Scheme 1: Schematic representation for the synthesis of Ag2S/Ag heterostructure with different morphologies. 300

$

$

$

$

200 $

Intensity (a.u.)

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

Page 26 of 35

$

$ $

$

$ = Peaks of Ag2S @ = Peaks of Ag

@

$

$ $ $$

@$

$

$ $ $

Ag2S/Ag wire $ $$ $$

@

100 Ag2S/Ag flower ball 0 100

20

30

40

50

60

70

80

14-0072

80 60 40 20 0 20

30

40

2θ θ

50

60

70

80

Figure 1: PXRD pattern of Ag2S/Ag heterostructure with different morphologies.


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a

b

Figure 2: FE-SEM image of (a) CuS, (b) Ag2S/Ag flower ball.

a

b

Figure 3: FE-SEM image of (a) CuS, (b) Ag2S/Ag wire.


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a

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b

Figure 4: TEM image of (a) CuS where inset shows the assembly of small plates to give rise flower ball type structure, (b) Ag2S/Ag flower ball and the inset shows the assembly of small blunt shape plates generates flower ball type structure.

a

b

Figure 5: TEM image of (a) CuS and the inset shows the sharp edge of CuS plates which serves as building block for wire, (b) Ag2S/Ag wire and the inset shows blunt edge of Ag2S/Ag plates which serves as building block for wire.


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a

b

c

Figure 6: XPS spectra of (a) wide scan survey of Ag2S/Ag surface. High resolution scans of (b) Ag 3d and (c) S 2p spectra, respectively.


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a

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b

c

d

Figure 7: (a, b) Polarization curve for blank GC, Pt/C, Ag2S, Ag2S/Ag-1 and Ag2S/Ag-2 in 0.5M H2SO4. (c) Tafel plots of Pt/C, Ag2S, Ag2S/Ag-1 and Ag2S/Ag-2 and (d) Nyquist plots of Ag2S, Ag2S/Ag-1 and Ag2S/Ag-2.


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Table 1: Binding energy peak positions and relative intensities of deconvoluted Ag 3d and S 2p spectra of Ag2S/Ag heterostructure surface.

B E peaks

Area

Position

Ag 3d3/2

85218.12

373.953

Ag 3d5/2

131707.9

367.965

S 2p1/2

4009.586

162.217

S2p3/2

7723.146

161.056

Ag(0)

Ag 3d3/2

8897.591

373.603

metallic

Ag 3d5/2

13252.7

367.652

S(0) contaminant

S 2p

646.875

163.043

Material

Ag2S


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Table 2: Comparative results of overall electrocatalytic performance of different catalyst. Overpotential required to generate 10 mA/cm2

Mass Activity (A/g) at -0.25 V

Tafel slope (mV/decade)

Specific activity

Ag2S/Ag-1

-0.257 V

8.2

128

0.16

Ag2S/Ag-2

-0.199 V

26.4

102

0.18

Ag2S

-0.421 V

0.54

141

-

Pt/C

-0.092 V

140.6

37

-

Cathode

(mA/cm2ECSA)


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Table 3: Comparison of HER activity data for different reported catalyst which are closely related to the present work.

Catalyst

Ag-Ag2S/MoS2 Ag2S/CuS MoS2/Ag2S CuS/Au WS2/Au

Loading Amount (mg/cm2) 12.06 0.57 1.06

Medium

0.2M H2SO4 0.5M H2SO4 0.5M H2SO4 0.5M H2SO4 0.5M H2SO4

MoS2/Au

0.5M H2SO4

MoS2/Pt@Graphene MoS2/Pt Ag2S/Ag

0.5M H2SO4 0.5M H2SO4 0.5M H2SO4

1.06

Overpotential to generate 10 mA/cm2 340 mV 193 mV 200 mV 179 mV 230 mV (1mA/cm2) 205 mV(1 mA/cm2) 56 mV 50 mV 199 mV

Tafel Slope

Reference

75 42 75 56.69

26 25 27 22 21

56.97

21

39.4 49 102

19 20 Current work


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Table 4: Values of Charge transfer resistance and resistance of the material for both the heterostructure.

Cathode

R1(Ω)

R2 (Ω)

Ag2S/Ag-2

17.3

52.9

Ag2S/Ag-1

13

161.6


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TOC: Schematic diagram of the use of Ag2S/Ag (both the morphology: Flower ball and Wire) in electrocatalytic hydrogen evolution reaction.


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Ag Heterostructure: A ... - ACS Publications

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