Bottom-up Electrosynthesis of Highly Active Tungsten Sulfide (WS3âx

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Bottom-up Electrosynthesis of Highly Active Tungsten Sulfide (WS3−x) Films for Hydrogen Evolution Shu Min Tan and Martin Pumera* School of Physical and Mathematical Sciences, Division of Chemistry and Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 S Supporting Information *

ABSTRACT: Transition metal dichalcogenides have been extensively studied as promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER). However, despite the intention to achieve sustainable energy generation, conventional syntheses typically use environmentally damaging reagents and energy-demanding preparation conditions. Hence, we present electrochemical synthesis as a green and versatile alternative to traditional methods. In this fundamental study, we demonstrated the bottom-up synthesis of a mixed WS2/WS3 film-like material via cyclic voltammetry (CV). The film-like material can be directly electrosynthesized on any conductive substrates and renders the catalyst immobilization step redundant. Through stepwise analysis of deposition voltammograms facilitated by straightforward modification of CV conditions, and characterization using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), a two-step mechanism involving the initial WS3 deposition and subsequent partial reduction to WS2 was proposed. The WS2/WS3 material was determined to possess composition of WS2.64. Compared to non-electrosynthesized WSx materials, its predominantly basal orientation limited the heterogeneous electron transfer rate toward surface-sensitive redox couples. However, WS2.64 demonstrated excellent HER activity, with the lowest Tafel slope of 43.7 mV dec−1 to date; this was attributed to different metal-chalcogen binding strengths within WS2.64. Fundamental understanding of the electrosynthesis process is crucial for green syntheses of inexpensive and highly electrocatalytically active materials for sustainable energy production. Albeit, the process may be different for a myriad of nanomaterials, this study can be exploited for its analyses from which the conclusions were made, to empower electrochemical synthesis as the prime fabrication approach for HER electrocatalyst development. KEYWORDS: tungsten sulfide, hydrogen evolution, electrodeposition, electrocatalysis, transition metal dichalcogenides

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1T-WS2 nanosheets,8 WS2 particles supported on different substrates (e.g., graphene),9 doped WS2,10,11 and WS3 on Si nanowires.12 With the availability of numerous synthesis routes toward WSx materials, it is unsurprising that large variations in overpotentials at −10 mA cm−2 (142−750 mV) and Tafel slopes (55−164 mV dec−1) have been revealed.8,10,12−14 Due to the wide range of Tafel slopes reported, their mechanism toward the electrocatalysis of HER has yet to be elucidated. Generally, the conventional fabrication methods of TMDs include: (1) top-down approach where the bulk material undergoes chemical exfoliation using organolithium intercalants or liquid exfoliation in solvents of similar surface energy to that of the layers,15,16 and (2) bottom-up techniques such as chemical vapor deposition (CVD),17,18 solvothermal/hydrothermal synthesis,2,9 wetness impregnation followed by processing with respective chalcogen gases,11,19 and treatment of transition metal films with corresponding chalcogen gases.20

INTRODUCTION In view of the looming energy crunch, moving away from dependence on nonrenewable resources is imperative. With this in mind, hydrogen has been proposed to be a viable source of energy, with high energy density, harmless byproducts, and renewable production methods.1 Electrochemical hydrogen evolution reaction (HER) offers the possibility of sustainable hydrogen production. However, it currently suffers from high costs of the archetypal platinum-group electrocatalysts, which limit their utilization on the industrial scale. As such, many have turned to inexpensive, earth-abundant electrocatalysts such as transition metal dichalcogenides (TMDs) as attractive alternatives. Research on MoS2 as HER electrocatalyst has taken center stage in such investigations.2−4 However, recently, burgeoning interest in other Group VI TMDs (e.g., MoSe2 and WS2) has led to an array of nanoassembled HER electrocatalysts.5,6 In contrast to MoS2, whose S edges are the active sites, both the metal and chalcogen edges of MoSe2 and WS2 are active toward HER; the respective basal planes are electrocatalytically inert.7 Prior studies have fabricated different forms of WSx, including © XXXX American Chemical Society

Received: November 17, 2015 Accepted: January 7, 2016

A

DOI: 10.1021/acsami.5b11109 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

hexacyanoferrate(II) trihydrate, hexaammineruthenium(III) chloride and sulfuric acid were purchased from Sigma-Aldrich. Glassy carbon electrodes, Ag/AgCl reference electrode and platinum counter electrode were obtained from CH Instruments. Reticulated vitreous carbon foam was acquired from ERG Aerospace Corporation, USA. Milli-Q water with a resistivity of 18.2 MΩ cm was utilized throughout for the preparation of electrolytes. Apparatus and Methods. XPS was carried out by employing a conventional nonmonochromated X-ray source using the Mg Kα line (SPECS XR50, hν = 1253 eV, 200 W) and a multichannel energy analyzer (SPECS Phoibos 100 MCD-5). The electrodeposition of materials was performed on screen printed electrodes (SPEs) which were then attached on sample holders for measurements. Powder samples were secured onto sample holders via carbon tape. Wide-scan and high resolution W 4f and S 2p core-level spectra were obtained. Calibration was achieved using the adventitious C 1s peak, referenced at 284.5 eV. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were carried out with a JEOL 7600F fieldemission SEM (JEOL, Japan), at an acceleration voltage of 5 and 20 kV, respectively. The GC foam samples were coated through electrodeposition via CV, by varying the potential ranges. For the powder samples, the solids were affixed onto a sample stub with carbon tape. Electrochemical depositions and measurements using chronoamperometry, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques were executed with a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) operated using the NOVA 1.8 (Eco Chemie) software. These three electrochemical techniques were conducted in a 5 mL electrochemical cell at room temperature using a three-electrode configuration, with the bare GC, Ag/AgCl and Pt electrodes as the working, reference, and counter electrodes, respectively. Electrodeposition Procedure. First, 10 mM of (NH4)2WS4 was prepared in 0.1 M KCl solution. Bare GC electrode was polished to a mirror finish using 0.05 μm alumina particles on a polishing pad, before it is employed for electrodeposition via chronoamperometry or CV. Chronoamperometry was first performed at +1.2 V for 900 s, followed by treatment at −1.3 V for 60 s. A thin coating of film was observed on the GC electrode surface, whereby full coverage was not achieved. Electrodeposition via CV was investigated based on varying potential ranges, scan rates and number of cycles. In particular, the CVs obtained from varying potential ranges and scan rates are plotted in Figure 1 and Figure S4, respectively. On the basis of our study, we selected three specific potentials for further characterizations: −1.5 to 0 V, 0 to +1.5 V, and −1.3 to +1.1 V. These represent reductive,

These approaches typically require energy-demanding preparation conditions (e.g., high temperatures and ultrahigh vacuum) and relatively large quantities of reagents that may be detrimental to the environment.21 Furthermore, they usually result in materials exhibiting varying degrees of exfoliation. Methods such as hydrothermal synthesis are also sensitive to temperature.9 On the other hand, electrochemical synthesis is a simple green bottom-up synthesis process that utilizes aqueous electrolytes at room temperature with minimal set up required.22 The electrochemical synthesis presents multiple advantages over the above-mentioned techniques, which includes versatility in the choice of substrate and electrochemical technique, rapid, ease of operation, scalability, and control over thickness of deposited material.22,23 It also renders the catalyst immobilization step redundant,24 dispelling doubts of porosity and roughness resulting in the observed catalytic activity.25,26 In light of sustainable energy generation, electrodeposition is currently an established sample preparation method in the field of solar cells,27−29 and toward lab-on-chip applications, it has also been experimented for microelectronic manufacturing.30−32 For the electrochemical synthesis of transition metal chalcogenides, chronoamperometry, chronopotentiometry, and cyclic voltammetry (CV) are generally techniques of choice. In particular, the electrodeposition of TMDs was first explored by Belanger et al. to form amorphous MoS3 films.33 Using CV, an irreversible oxidative peak was observed at +0.3 V (vs SCE) at scan rate of 10 mV s−1. Subsequent characterizations by the same group using X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and elemental analysis, revealed the presence of additional sulfur on the electrodeposited material.34 As for WS2, chronopotentiometry and CV were employed for its electrodeposition onto conducting tin oxide substrate and graphene respectively, demonstrating the adaptability of electrodeposition as a synthetic pathway.24,35,36 The latter study reported good HER electrocatalytic activity by the WS2/graphene composite material, exhibiting a Tafel slope of 67 mV dec−1. The proliferation of electrosynthesis as a mainstream TMD fabrication method is still in its nascent stage; much investigations into the mechanisms of different electrosynthesis systems, as well as comparisons of electrocatalytic performances between electrosynthesized and non-electrosynthesized materials, are required to pave the way for its establishment. In this paper, we report the electrochemical synthesis of a film-like material consisting of a mixture of WS2 and WS3 (WS3−x, x = proportion of WS2) via CV, together with characterization by XPS, scanning electron microscopy coupled to energydispersive X-ray spectroscopy (SEM-EDS). On the basis of these characterization outcomes, we proposed the electrosynthesis mechanism and structure of WS3−x. Additionally, the electron transfer capabilities as well as HER electrocatalytic activity of WS3−x are explored and compared with those of nonelectrosynthesized materials as well. Fundamental understanding of the electrosynthesis of WS3−x will assist in the widespread use of electrosynthesis as a fabrication pathway, and in the development of low cost and highly active electrocatalysts for sustainable energy production.

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Figure 1. Cyclic voltammograms of electrodeposition synthesis of tungsten sulfide materials performed at (black) −1.3 to +1.1 V, (red) 0 to +1.5 V and (blue) −1.5 to 0 V. Conditions: 10 mM (NH4)2WS4 in 0.1 KCl as supporting electrolyte, scan rate of 50 mV s−1, 50 cycles, purged with N2. All scans start from 0 V with initial scan in the cathodic direction (with the exception of 0 to +1.5 V). (Inset) Corresponding polarization curves of the electrodeposited tungsten sulfide materials. Conditions: 0.5 M H2SO4 as supporting electrolyte, scan rate of 2 mV s−1, purged with N2.

EXPERIMENTAL SECTION

Materials. Ammonium tetrathiotungstate, potassium chloride, sodium chloride, potassium phosphate dibasic, sodium phosphate monobasic, potassium hexacyanoferrate(III), potassium B


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Figure 2. XPS spectra of tungsten-containing materials. (A) Survey, (B) W 4f core-level and (C) S 2p core-level spectra. All scans were referenced to adventitious C 1s peak at 284.5 eV. oxidative, and mixed oxidative/reductive potential ranges. Briefly, the experimental conditions that produce the best performing electrodeposited material are −1.3 to +1.1 V at 50 mV s−1 for 50 cycles. The deposited film attains full coverage of the GC electrode surface. Electrochemical Measurements. Upon the electrosynthesis step, the coated GC electrode was rinsed gently with Milli-Q water to remove excess [WS4]2− ions from its surface, and allowed to dry under a lamp. It is then used for the sensing of Ru(NH3)63+/2+, Fe(CN)63−/4−, and Eu3+/2+ redox probes and hydrogen evolution reaction (HER) measurements. A freshly electrosynthesized WS2.64 film is prepared for every measurement. For Ru(NH3)63+/2+ and Fe(CN)63−/4−, phosphate buffer solution (PBS, 50 mM, pH 7.2) was utilized as the supporting electrolyte; for Eu3+/2+ redox probes and HER measurements, potassium chloride (0.1 M) and sulfuric acid (0.5 M) were employed, respectively. All CV measurements were executed at a scan rate of 100 mV s−1, and LSV scans at 2 mV s−1. All potentials are relative to Ag/AgCl unless otherwise indicated, and all solutions were purged with nitrogen gas. For Ru(NH3)63+/2+, Fe(CN)63−/4−, and Eu3+/2+, the observed HET rate constants (k0obs) were evaluated using the Nicholson approach,37 whereby the peak-to-peak separation (ΔE) of the electron transfer process is associated with a dimensionless factor ψ and, subsequently, to k0obs. The roughness factor was not accounted for. The following diffusion coefficients of the reduced (R) and oxidized (O) species of the redox couples were used in the calculations: DR = DO = 9.1 × 10−6 cm2 s−1 (Ru(NH3)63+/2+),38 DR = DO = 7.26 × 10−6 cm2 s−1 (Fe(CN)63−/4−),39 and DR = DO = 5.7 × 10−6 cm2 s−1 (Eu3+/2+).40,41 For long-term stability, the HER measurements were obtained before and after 40 days of exposure to ambient air. For stability study in acidic medium, the HER measurements were performed before and after 100 CV cycles, conducted from +35 to −265 mV at 100 mV s−1 in 0.5 M sulfuric acid.

solution of (NH4)2WS4 in KCl supporting electrolyte. Upon optimizing various factors such as scan ranges, scan rates and number of cycles, WS3−x electrosynthesis was achieved by performing CV from −1.3 to +1.1 V at 50 mV s−1 for 50 cycles. In contrast to the tungsten sulfide powder obtained through conventional chemical exfoliation of the bulk counterpart, a uniform blue film with good adherence was obtained with full coverage of the glassy carbon (GC) substrate. Electrosynthesis via chronoamperometry was assessed as well in a preliminary screening. However, unlike CV, chronoamperometry resulted in uneven coverage of the GC electrode; subsequent HER measurements also demonstrated the superior activity of the CV film over the chronoamperometry material (Figure S1). Hence, CV was employed as the electrosynthesis technique of choice. The voltammograms obtained from the electrosynthesis of WS3−x at −1.3 to +1.1 V (Figure 1, black) presents an unusual observation. In contrast to the work by Sun et al. where the CV electrodeposition of WS2 from [WS4]2− occurred via cathodic reduction reaction,24 no reduction peak was observed on the initial cathodic scan. However, after the manifestation of an oxidative peak, a reduction peak ensued, implying reduction of products from the oxidation step. To further analyze these redox processes, the deposition step was repeated using oxidative (0 to +1.5 V) and reductive (−1.5 to 0 V) scan ranges. Here, we denote the film deposited using the mixed oxidative and reductive potential range as WS3−x, where x is the proportion of WS2 in the film. The absence of a reduction peak for the reductive range (Figure 1, blue) over 50 cycles suggests that [WS4]2− did not undergo any detectable electrochemical reduction. On the other hand, an oxidation peak was apparent for the first cycle of the oxidative range (Figure 1, red) and the electrode surface was subsequently passivated against further deposition. An analysis of their HER performances (Figure 1, inset) indicated much higher electrocatalytic activity for the mixed oxidative and reductive scan range, followed by oxidative and, subsequently, reductive ranges. This suggests that the different potential scan ranges engendered films of distinct composition. Next, we proceeded to characterize the deposited films to uncover factors responsible for the differences in HER activity. To elucidate the surface chemical compositions and species of the electrodeposited films, survey and core-level XPS were

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RESULTS AND DISCUSSION In this cardinal study, we investigate the electrosynthesis of WS3−x film from its [WS4]2− precursor, and characterize the film using various elemental and structural analyses. By exploiting information garnered from the study, we propose a plausible mechanism and structure for the film. In addition, its applicability for sensing purposes and performance as an HER electrocatalyst are evaluated and compared with similar materials. The observed trends are rationalized through structural and thermodynamic considerations. Electrosynthesis and Determination of Electrodeposited WS3−x. The electrochemical synthesis of WS3−x was performed via cyclic voltammetry in a nitrogen-saturated C


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Table 1. S2−/ S22− Ratio, and Ratios of S and W Contents of Films Fabricated Through Oxidative Scan Range to Those of the Films Obtained Using Mixed Oxidative and Reductive Scan Ranges at 50 and 200 mV s−1 entry

potential rangescan rate (mV s−1)a

S2−/S22− ratio

1 2 3

Oxidative50 Ox/Red50 Ox/Red200

0.194 0.417 0.845

S

Oxidative50/SOx/Redscan rate ratiob

W

− 0.998 0.767

Oxidative50/WOx/Redscan rate ratiob − 1.000 0.850

Oxidative50 denotes potential ranging from 0 V to +1.5 V, performed at scan rate of 50 mV s−1, while Ox/Red50 and Ox/Red200 represent potentials ranging from −1.3 V to +1.1 V, performed at scan rates of 50 and 200 mV s−1 respectively. bBased on respective deconvoluted S 2p and W 4f corelevel peak areas and relative sensitivity factors. a

electrosynthesis process conducted at 200 mV s−1 (Figure S2). This peak is much smaller in the voltammogram of the synthesis at 50 mV s−1, and it decreases with increasing scan cycle. Hence, we infer, within reason, that the large disparities in W and S ratios between entries 1 and 2 can be attributed to significant redissolution of WS3 at ca. −1.05 V back to the soluble precursor [WS4]2−. Similar observations were made for MoS3 as well.21 This also suggests that the main peak at ca. −1.2 V, which arose in the mixed potential scan range, originates, to an extent, from the reduction of WS3 to WS2, giving rise to the increase in the S2−/S22− ratio. A possible mechanism and structure will be proposed later in this section. Further structural characterization and elemental analyses of various WSx materials deposited on GC foam were conducted using SEM-EDS. The corresponding SEM micrographs and EDS spectra are given in Figure 3. From the micrographs, WS3−x and WS3 materials can be clearly distinguished, whereby the latter exhibited a rough and poorly adhered film (Figure 3D, inset) whereas the former presented a smooth film with minimal defects and edges. The observed corrugations of WS3−x were attributed to natural ridges of the bare GC foam. It is imperative to note that the fabricated WS3−x material manifests itself as a film, unlike most other tungsten sulfide materials which exhibit the powder form. The electrodeposited WS3−x film demonstrated an enrichment of S content (Figure 3F), indicating the presence of S species such as S8 in the layers. After immersing in sulfuric acid, in which HER was conducted, its elemental quantification (Figure 3G) displayed much lower S content than before, indicating the removal of the excess S through HER in acidic medium. With the information gathered above, we endeavored to shed light on the electrosynthesis mechanism. The observed cyclic voltammograms for the electrosynthesis of WS3−x differs greatly from those obtained for MoS3 in a prior study by Merki and coworkers;21 instead of the current density growing to saturation point, as in the case of MoS3, WS3−x displays a very stable oxidation peak, with comparatively smaller increases in the reduction peak. This implies that the mechanism involved in the deposition of WS3−x differs from that of MoS3. Drawing on the characterization results and analyses, the following mechanism is suggested for the electrodeposition of WS3−x (the mechanism is further elaborated in the Supporting Information and illustrated by Figures S3 and S4): Oxidative adsorption step:

performed on films fabricated through the three scan ranges. The respective XPS data were compared against bulk WS2 and plotted in Figure 2. The bulk material clearly presented two pairs of deconvoluted W 4f peaks, corresponding to W6+ (ca. 36.3 and 38.5 eV) and W4+ (ca. 33.2 and 35.4 eV) species, while its S 2p spectrum displayed a pair of peaks assigned to S 2p1/2 and S 2p3/2 of S2− at approximately 162.1 and 163.2 eV, respectively.13,40 All three scan ranges produced tungstencontaining materials; while the mixed and oxidative ranges yielded films containing tungsten sulfide, the amount of S present in the film engendered through the reductive range treatment was below the detection limit (Figures 2A,C). The W 4f peak (Figure 2B) of the film obtained via reductive scan range has been fitted with a pair of peaks resulting from W6+ species at approximately 36.0 and 38.2 eV, indicating the presence of WO3. For the mixed and oxidative ranges, both W6+ and W4+ (ca. 33.2 and 35.4 eV) species were present, signifying the presence of both WO3 impurities and WS2/WS3 material. Deconvolution of the S 2p peak for these two ranges resulted in S2− and S22− species at ca. 162.3 and 163.5 eV, and 163.3 and 164.5 eV, respectively, which are assigned to the presence of WS2/WS3 and WS3 correspondingly.12 Both WS2 and WS3 contain W4+ and S2− species; however, only WS3 possess the S22− ligand,42 with a formal charge state of [W4+(S22−)(S2−)].43 By establishing that the ratio of S2− to S22− constitutes the main distinction between WS2 and WS3, we may proceed with further elucidation of a possible structure and electrosynthesis mechanism for WS3−x. To gain in-depth understanding of the film compositions to identify factors responsible for the disparity in their HER electrocatalytic activities, a closer inspection of the atomic percentages (atom %) of the S2− and S22− species is required (Table 1). WS3 has been reported to contain both S2− and S22− ligands in its structure and such oxidative procedure has been employed to produce amorphous MoS3.33 Hence, it can be concluded from the low S2−/S22− ratio (Table 1, entry 1) that the oxidative range yielded WS3 film. Further, it was revealed that the mixed film (Table 1, entry 2) engendered a higher S2−/ S22− ratio compared to the electrodeposited WS3 (Table 1, entry 1). This suggests the conversion of WS3, from the initial oxidative step, to WS2, in the subsequent reductive process, leading to increase in the S2−/S22− ratio. To identify the electrochemical reaction occurring at the main peak at ca. −1.2 V (Figure 1), we inspected the ratios of metal and chalcogen contents of the WS3 film to those of the films yielded using the mixed range, at 50 and 200 mV s−1 (Table 1, entries 2 and 3). The film obtained at 200 mV s−1 exhibits much lower W and S ratios compared to the one attained at 50 mV s−1. An additional reduction peak, whose onset was approximately 150 mV before the dominant peak, was also observed in the cyclic voltammogram of the

[WS4 ]2 − → WS3 +

1 S8 + 2e− 8

(1)

Reductive desorption step: WS3 + D

1 S8 + 2e− → [WS4 ]2 − 8

(2a)


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composition of WS3−x to be WS2.64 (calculations are detailed in the Supporting Information and substantiated by Figure S5), and its proposed structure is presented in Figure 4. The

Figure 4. Proposed structure of WS2.64.

proposed WS2.64 structure resembles chain-like assemblies of W atoms interconnected by S22− or S2− ligands with weak van der Waals interactions between W and S of adjacent chains, in good agreement with previous literature whereby the presence of bridging S22− ligands in chain-like amorphous WS3 has been reported.43 It is likely that such amorphous WS3 was initially adsorbed onto the GC electrode surface, and upon reduction, the bridging S22− ligands cleaved, leaving behind one S atom and forming the unique film-like WS2.64 material. It is important to note that this proposed structure is not definitive and permutations of the S ligand positions may vary throughout the film. By employing observations made from the cyclic voltammograms and information harnessed from the various characterization techniques, we have proposed a possible mechanism for the electrodeposition of WS3−x via CV. Calculations based on the S 2p core-level XPS spectra of both WS3 and WS3−x demonstrate the empirical formula of WS3−x to be WS2.64, allowing the its structure to be postulated. The stepwise analysis of materials deposited by varying electrochemical conditions, which facilitated the experimental elucidation of mechanism and structure, is made possible with exploitation of the versatility of electrochemical synthesis techniques. Equipped with such knowledge, we then performed subsequent studies to examine their electrochemical properties. Heterogeneous Electron Transfer Rates at Electrodeposited WS2.64. The heterogeneous electron transfer (HET) rate is frequently studied to assess the functionality of an electrode material for biological sensing and electrochemical applications.44 An inherently rapid HET rate essentially reduces the overpotential required for the electron transfer event which is desired for various applications. The HET rates of several common redox couples, Ru(NH3)63+/2+, Fe(CN)63−/4−, and Eu3+/2+, which are frequently exploited for electrochemical sensing functions, were investigated on electrosynthesized WS2.64, bulk and exfoliated WS2 as well as bulk WS3 materials using CV (Figure 5). The observed HET rate constants (k0obs) were then calculated using the peak-to-peak separation (ΔE) of the voltammograms, based on the Nicholson method.37 Ru(NH3)63+/2+ is an outer-sphere redox couple i.e. it is insensitive to surface oxides and microstructures but provides information on electronic properties such as density of states (DOS) of the electrode material.45 Figure 5A illustrates the cyclic voltammograms of Ru(NH3)63+/2+ on various WSx materials, showing relatively similar ΔE; their k0obs were also calculated to be in the same order of magnitude. This is

Figure 3. Scanning electron micrographs of (A) bare GC, (B) electrodeposited WS3−x, (C) electrodeposited WS3−x, followed by HER and (D) electrodeposited WS3, at 370× magnification. (Inset) Micrograph of electrodeposited WS3 at higher magnification of 1000×. (E−H) Corresponding energy-dispersive X-ray spectra of (A−D), as indicated. The elemental quantifications of individual materials are displayed accordingly.

Conversion step (driven by H2S evolution): WS3 + 2e− + 2H+ → WS2 + H 2S(g)

(2b)

A mechanism similar to the proposed step 1 has been reported for the oxidation of [MoS4]2− to MoS3 at comparable oxidative potentials (Eox = +0.5 V vs Ag/AgCl and +0.3 V vs SCE respectively).33 Step 2a simply describes the backward reaction to step 1 where WS3 redissolves to form [WS4]2−. The conversion of WS3 to WS2 (step 2b) is a partial reduction, as supported by the presence of S22− species in S 2p core-level spectrum of the mixed scan range (Figure 2C). The proposed mechanism aids in expounding the observations made from Figure 1. Electrosynthesis via the oxidative range produced S8 (step 1) which has limited electrical conductivity, hence passivating the electrode surface against subsequent reaction; on the other hand, no peak occurred in the reductive range as the products of oxidation are the depolarizers for the electrochemical reduction processes (steps 2a and 2b). Using deconvoluted S 2p spectra of the mixed and oxidative scan ranges, we performed calculations and obtained the E


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main orientation aligned along the electrode/electrolyte interface, concurring with literature observations for MoS2.23,48 Previous studies have shown that for materials where the edges and defects are the active sites, increased exposed surface area and surface defects enhance the electrochemical sensing capability of these materials significantly.49,50 As illustrated in Figure S7, the other WSx materials exhibit either particles or sheets with edges exposed in contrast to the smooth film observed for WS2,64. Moreover, the film did not exhibit any conducting 1T phase as well (Figure S6). Such electrochemically active edges allow greater physical interactions with Fe(CN)63−/4− and Eu3+/2+, which, being innersphere redox couples, enables faster HET rates to be achieved. Hence, it can be concluded that the electrodeposited WS2.64 film performs as well as other WSx materials, which were fabricated via bulk production, toward the sensing of outersphere redox probes which do not form any surface interactions with the electrode material. However, it is evident that differences in electrical conductivity, surface defects and exposed surface area play a vital role in the electrochemical catalysis of inner-sphere redox couples, thus the electrochemical sensing capability of WS2.64 was limited by its deficiencies in 1T phase and electrochemically active sites. These results provide insights into the possible electrodeposition synthesis of sensor materials, and educate on the necessity to enhance the electrochemical activity through modifications to morphology and electronic properties before such materials can be exploited in sensing applications in the future. Hydrogen Evolution Reaction. The hydrogen evolution reaction (HER) on various catalysts, especially TMDs, have been thoroughly investigated experimentally and theoretically, in hopes of achieving an economical and highly active HER electrocatalyst. Generally, characteristics such as high surface area, high defect density and hence large availability of active sites, high electron density as well as good conductivity are desirable for the catalysis of HER. Given these physical and electronic attributes, many catalysts were created to be structurally catered toward HER electrocatalysis. Examples of such nanoarchitectured TMDs include vertically aligned MoS2 and MoSe2 with large edge-to-basal ratios,51 defect-rich MoS2 nanosheets,52 1T phase WS2 nanosheets8,13 and S-doped MoSe2 nanosheets.53 Unlike these manufactured TMDs which used elaborate synthesis methodologies, a simple electrosynthesis step via CV cycling was employed in this fundamental study for the fabrication of WS2.64; the film was utilized for analysis without further chemical or electrochemical treatment. Figure 6A illustrates the polarization curves of the electrodeposited WS2.64, bulk WS2 and WS3 as well as exfoliated WS2, with comparisons to Pt/C and bare GC substrate. Lower overpotential at −10 mA cm−2 generally portends better HER electrocatalytic activity as higher catalytic current density can be achieved at lower potentials. Pt/C shows the best HER electrocatalytic performance with the earliest overpotential of 161 mV at −10 mA cm−2. Unsurprisingly, the two bulk tungsten sulfide materials exhibit poor HER activity, with close proximity between the curves of bulk WS2 and bare GC. Despite the enhancement in HER performance from that of the bulk WS2 material, exfoliated WS2 displays relatively poor HER activity at with an overpotential of 652 mV at −10 mA cm−2; on the other hand, the electrodeposited WS2.64 exhibits excellent HER electrocatalytic activity, with the lowest measured overpotential of 494 mV. Hence, it is evident that

Figure 5. Cyclic voltammograms of 10 mM (A) Ru(NH3)63+/2+, (B) Fe(CN)63−/4−, and (C) Eu3+/2+ on WS2.64 and various WSx materials, as indicated. Conditions: (A and B) 50 mM phosphate buffer solution (pH 7.2) and (C) 0.1 M KCl as supporting electrolytes, scan rate of 50 mV s−1.

indicative that the WSx materials do not differ much in their DOS. On the other hand, both Fe(CN)63−/4− and Eu3+/2+ redox probes exhibit sensitivity to surface microstructures which is demonstrated in Figure 5B,C.46 The exfoliated WS2 displayed faster HET for both redox couples compared to bulk WS2 due to the exfoliation step and the occurrence of 2H → 1T phase transformation (Figure S6) during the intercalating procedure, which exposes more electrochemically active surface area as well as improves its conductivity, respectively.8,9 Comparing bulk WS3 and exfoliated WS2, the former shows smaller ΔE toward Fe(CN)63−/4− and larger ΔE toward Eu3+/2+. This may be due to differences in surface charge of the materials. Bulk amorphous WS3 was demonstrated to exist as cluster ions of [W3S7]4+ or the decomposed form [W3S4]4+ which confers positive surface charge to WS3,47 enabling stronger electrostatic attraction between WS3 and the negatively charged Fe(CN)63−/4−. Hence, larger k0obs value of 3.21 × 10−4 cm s−1 cf. 1.46 × 10−4 cm s−1 of exfoliated WS2 was achieved. The opposite is observed for the positively charged Eu3+/2+. Among all four WSx materials, WS2.64 fared the worst in terms of k0obs values at 1.17 × 10−6 and 1.84 × 10−4 cm s−1 for Fe(CN)63−/4− and Eu3+/2+ respectively. This may be attributed to the WS2.64 film being electrodeposited with basal plane as the F


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ACS Applied Materials & Interfaces Recombination step:

Hads + Hads → H 2 (Tafel: b ≈ 30mV dec−1)

The Tafel slope value hence provides information on the mechanism undertaken during HER measurements. Generally, WS2 related materials perform less efficiently than their MoS2 counterparts which reported Tafel slopes as low as ca. 40 mV dec−1 in chemically exfoliated 1T nanosheets.17,55 Prior to this study, the electrochemical deposition of MoS3 has been reported to exhibit a Tafel slope of 40 mV dec−1.21 Thus, far, the lowest Tafel slope obtained for WS2 related materials was 55 mV dec−1 obtained by Chhowalla et al. in 1T phase WS2 nanosheets.13 Here, we performed the Tafel slope analysis on WS2.64 and various non-electrodeposited WSx materials, to evaluate and compare their HER activities. In contrast to the most electrocatalytic performance by Pt/C which demonstrated a Tafel slope value of 32.6 mV dec−1 corresponding to the Tafel mechanism, the bare GC electrode exhibited the poorest activity with a Tafel slope of 157 mV dec−1. The overall trend observed for the bulk-synthesized WSx materials indicated the Volmer mechanism as the rate-limiting step with their Tafel slopes closest to the theoretical value of 120 mV dec−1. On the other hand, the electrodeposited WS2.64 film performed the best among the WSx materials, exhibiting a Tafel slope of 43.7 mV dec−1. The HER activity of WS2.64 exceeds those of previously mentioned nanoassembled WSx, providing support for the employment of electrochemical synthesis for electrocatalyst fabrication; high catalytic activity was bestowed through this simple and green approach, the advantages of which far outweigh those of more complex synthetic routes. A variety of factors may have resulted in the variable activities across the different WSx materials, including the proportion of 1T phase, the presence of W6+ oxides, the extent of exfoliation affecting the exposed surface area and the presence of different W and S species on the electrode materials. As anticipated, the bulk WS2 and WS3 performed poorly, mainly due to their large stacked particles, with little exposed edges for HER to occur (Figure S7). Between these two materials, WS3 showed marginally superior HER performance. Comparing the exfoliated WS2 and its bulk counterpart, the higher electrocatalytic activity of the former can be attributed to its exfoliated sheets (Figure S7) as well as larger proportion of 1T phase (Figure S6) which resulted in substantially larger active sites and metallic properties.13 Despite the lack of metallic 1T phase in the electrodeposited WS2.64 film, it exhibited excellent HER activity resulting from the presence of both WS2 and WS3 which culminated in a noncrystalline structure (Figure 4). Further in-depth understanding of this enhancement can be elucidated by considering the binding strengths of W−S and H−S bonds. A prior study by Nørskov et al. uncovered an inverse relationship between the adsorption energy of chalcogen to the edge (ΔEx) and the hydrogen adsorption free energy (ΔGH), indicating that a stronger binding between the chalcogen and the edge leads to a weaker binding of protons to the chalcogen active site.7 ΔGH is a vital indicator for the extent to which a site of interest is active toward HER and the thermoneutral value where ΔGH ≈ 0 was assessed to be the most favorable value.19 The presence of bridging S in S22− ligand produced a reduction in the binding strength between the ligand and W, engendering a lower ΔES for S22− and more attractive ΔGH compared to S2−. The difference in affinity toward H adsorption may have resulted in two different sites for adsorption and desorption of H; S22−

Figure 6. (A) Polarization curves of WSx materials, as indicated. The performances of Pt/C and bare GC electrode were shown for comparison. Conditions: 0.5 M H2SO4 as supporting electrolyte, scan rate of 2 mV s−1, purged with N2. (B) Corresponding Tafel slopes of the polarization curves in panel A. (C) Bar graph of Tafel slope values of various WSx materials, together with those of Pt/C and bare GC electrode. The dotted lines at 30, 40, and 120 mV dec−1 represent the Tafel, Heyrovsky, and Volmer mechanisms, respectively. Error bars correspond to standard deviations of triplicate measurements.

the WS2.64 film demonstrates the highest HER activity among the various WSx materials. Subsequently, Tafel slope analysis was conducted based on the polarization curves obtained in Figure 6A to obtain the mechanism by which HER proceeded. The corresponding Tafel slopes and values are plotted in Figure 6B,C. Overall, HER may be described as the half-equation of an electrochemical watersplitting reaction, whereby two protons are electrochemically reduced, from which a hydrogen molecule evolves. However, the actual mechanism of electrochemical HER involves a twostep process, and may be limited by any of the three ratedetermining steps below:2,54 Electrochemical discharge step: H3O+ + e− → Hads + H 2O (Volmer: b ≈ 120mV dec−1)

Electrochemical desorption step: Hads + H3O+ + e− → H 2 + H 2O (Heyrovsky: b ≈ 40mV dec−1) G


Research Article


deposition of WS3 followed by partial conversion to WS2, and the chain-like structure for WS2.64 consisting of both S2− and S22− ligands have been proposed. The structural morphology of the film was observed to be predominantly basal oriented with minimal defects, which limits its electrochemical sensing capability toward various surface-sensitive redox couples. However, among other bulk-fabricated WSx materials, it demonstrated excellent electrocatalytic activity toward hydrogen evolution reaction, attaining the lowest Tafel slope of 43.7 mV dec−1 obtained for any tungsten sulfide material to date, suggesting the Volmer−Heyrovsky mechanism. The stellar electrocatalytic performance of WS2.64 was ascribed to differences in metal−chalcogen binding strengths as a result of binding to different S ligands, implying the possibility of concurrent occurrence of both adsorption and desorption steps on separate active sites of WS2.64. Further enhancements to the electrochemical and electrocatalytic activities of the electrodeposited film may be rendered through careful alterations to its morphology as well as electronic properties. Fundamental understanding of the electrodeposition process will assist in its widespread use as a versatile and green synthetic pathway, and enable the development of inexpensive and highly active electrocatalysts for sustainable energy production.

ligands act as adsorption sites for proton which is subsequently released upon electrochemical desorption from S2− sites. The low Tafel slope value of 43.7 mV dec−1 was thus attributed to this concurrent adsorption and desorption of protons and hydrogen molecules, respectively, on different S active sites. The slight disparity in electron density of the two S ligands may contribute to differences in their affinity toward the adsorption or desorption of protons as well.56 In spite of the low Tafel slope value of WS2.64, its overpotential of 494 mV at −10 mA cm−2 was high compared with several literature values ranging from ca. 142−270 mV.8,10,13 This was ascribed to the lack of active edge sites on WS2.64, as shown in Figure 3B, which limited the rate of reaction, resulting in significantly larger overpotentials. To investigate the impact of increased surface area on HER overpotentials, the electrosynthesis process and, subsequently, HER were performed on a GC foam; however, despite the much larger surface area of the GC foam (Figure S8), only a slight improvement of ca. 90 mV was obtained at a current of −0.7 mA (approximately equivalent to current density of −10 mA cm−2 in Figure 6A). From the polarization curves (Figure S8), it is evident that the rate-limiting step of HER on GC foam is altered, possibly due to the entrapment of H2 bubbles within the foam; this indicates that the HER overpotential of the film on GC foam would otherwise be much higher, implying that the observed enhancement in overpotential as a result of increased surface area is suppressed. Hence, the surface area of WS2.64 is likely to significantly affect its HER overpotential due to the considerable increase in available active sites. It is likely that the presence of large exposed active edge sites of catalysts in literature, which are definitively more HER-active compared to the predominant basal plane of WS2.64, imparted appreciably lower overpotentials. Acid and long-term stability studies were performed to investigate the susceptibility of the WS2.64 film to acid corrosion and atmospheric oxidation. The respective graphs are plotted in Figures S9 and S10. While the film exhibited a significant increase in Tafel slope (ΔTafel slope = 19.8 mV dec−1) and overpotential at −10 mA cm−2 (Δoverpotential = 46 mV) after 100 CV cycles in acidic medium, likely due to chemical changes in the WS2.64 active sites, the onset potential was observed to occur at less negative potentials. This was attributed to the removal of S impurities from the film (Figure 3C), resulting in lower overpotential required for HER. On the other hand, the Tafel slope for the long-term stability assessment over 40 days remained relatively unaltered (ΔTafel slope = 5.8 mV dec−1), implying that the kinetics of the electrocatalyst were retained and the active sites were mostly chemically unchanged. However, the onset of HER was later and the overpotential at −10 mA cm−2 demonstrated a substantial difference (Δoverpotential = 66 mV) after the 40 days; these were ascribed to the adsorption of surface impurities which hindered the active sites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11109. HER activity comparisons, electrosynthesis voltammograms, detailed proposed mechanism, detailed calculation of WS2.64XPS spectra, SEM images, acid stability and long-term stability studies. (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.P. acknowledges funding by Tier 2 grant (MOE2013-T2-1056; ARC 35/13) by Ministry of Education, Singapore.

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REFERENCES

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CONCLUSION To summarize, we have conducted an extensive examination into the bottom-up synthesis of WS2.64 and comprehensively analyzed its properties with regards to several electrochemical and electrocatalytic applications. The electrosynthesis technique has been demonstrated to be successful in the fabrication of the WS2.64 film via cyclic voltammetry. By drawing on conclusions made from various structural and elemental analyses, a two-step mechanism which involves the initial H


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