Composition-Graded MoWSx Hybrids with Tailored Catalytic Activity

Nov 27, 2017 - Composition-Graded MoWSx Hybrids with Tailored Catalytic Activity by Bipolar Electrochemistry. Shu Min Tan and Martin Pumera. School of...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 41955−41964

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Composition-Graded MoWSx Hybrids with Tailored Catalytic Activity by Bipolar Electrochemistry 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, Singapore S Supporting Information *

ABSTRACT: Among transition metal dichalcogenide (TMD)-based composites, TMD/graphene-related material and bichalcogen TMD composites have been widely studied for application toward energy production via the hydrogen evolution reaction (HER). However, scarcely any literature explored the possibility of bimetallic TMD hybrids as HER electrocatalysts. The use of harmful chemicals and harsh preparation conditions in conventional syntheses also detracts from the objective of sustainable energy production. Herein, we present the conservational alternative synthesis of MoWSx via one-step bipolar electrochemical deposition. Through bipolar electrochemistry, the simultaneous fabrication of composition-graded MoWSx hybrids, i.e., sulfur-deficient MoxW(1−x)S2 and MoxW(1−x)S3 (MoWSx/BPEcathodic and MoWSx/BPEanodic, respectively) under cathodic and anodic overpotentials, was achieved. The best-performing MoWSx/BPEcathodic and MoWSx/BPEanodic materials exhibited Tafel slopes of 45.7 and 50.5 mV dec−1, together with corresponding HER overpotentials of 315 and 278 mV at −10 mA cm−2. The remarkable HER activities of the composite materials were attributed to their small particle sizes, as well as the near-unity value of their surface Mo/W ratios, which resulted in increased exposed HER-active sites and differing active sites for the concurrent adsorption of protons and desorption of hydrogen gas. The excellent electrocatalytic performances achieved via the novel methodology adopted here encourage the empowerment of electrochemical deposition as the foremost fabrication approach toward functional electrocatalysts for sustainable energy generation. KEYWORDS: transition metal dichalcogenides, hydrogen evolution reaction, bipolar electrochemistry, electrodeposition, composition gradient, electrocatalysis, molybdenum disulfide, tungsten disulfide

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INTRODUCTION Increasing trepidation over diminishing energy reserves and deteriorating climatic conditions has led to escalating exigency for sustainable energy generation. The high energy density of molecular hydrogen and harmless byproducts from hydrogen production make it a desirable energy carrier for the ever growing energy demand.1,2 Among the current methods of hydrogen production, the electrochemical hydrogen evolution reaction (HER) offers the most efficient strategy toward renewable energy production. However, the laggard kinetics of HER gives rise to the need for highly catalytic electrocatalysts to improve the HER overpotential. Platinum (Pt), the current archetypal HER electrocatalyst, exhibits greatly enhanced HER kinetics but suffers from high costs due to its scarcity, limiting its application for industrial-scale hydrogen production. Toward the goal of surpassing the benchmark Pt electrocatalytic performance, transition metal dichalcogenides (TMDs) have been widely studied and various structures of TMDs have been synthesized. Of particular note are the group VI TMDs (MX2, M = Mo, W and X = S, Se), which present an assortment of nanoarchitectured HER electrocatalysts.3−7 © 2017 American Chemical Society

Composite TMD materials comprising two or more materials have generated much interest due to the synergistic effects within the hybrids.8−10 Among the composite materials, TMD/ graphene-related material hybrids are the most extensively scrutinized for their electrocatalytic properties;11−16 considering pure TMD hybrids, the available literature reporting on the HER performances of bichalcogen TMD composites (e.g., MoS2(1−x)Se2x) is relatively widespread as well.17−21 Comparatively, scarcely any investigation into the HER electrocatalytic performances of bimetallic TMD hybrids is reported. The available studies on such composite materials are centered on MoS2/WS2 hybrids, which were reported to exhibit high elasticity,22 outstanding optical23 and thermoelectric24 properties, and tunable bandgap.25,26 Most of these MoxW1−xS2 composites were synthesized via bottom-up methods that employed a highly regulated high-temperature environment and/or long fabrication periods (>24 h).10,27−29 In a prior Received: June 30, 2017 Accepted: November 8, 2017 Published: November 27, 2017 41955

DOI: 10.1021/acsami.7b09435 ACS Appl. Mater. Interfaces 2017, 9, 41955−41964

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Figure 1. (A) Illustration of the bipolar electrochemistry apparatus setup. An optical image of the setup is presented in Figure S1A of the Supporting Information. (B, C) Scanning electron microscopy (SEM) images of MoWSx/BPEcathodic/anodic across the indium tin oxide (ITO) electrode at (left to right) decreasing ηcat and ηan, as indicated.

gradients,37,38 which facilitates simple and efficient analysis of the optimal composition for the best HER performance. Besides simultaneous electrodeposition of composite electroactive materials, bipolar electrochemistry was also employed for electrocatalyst screening, namely, through anodic dissolution of metal films coupled with the cathodic reaction of interest, either oxygen reduction reaction or HER.37,39 However, these screening platforms proceeded with qualitative screening by the naked eye. To overcome this challenge, electrochemiluminescence (ECL) was exploited for more quantitative analysis; cathodic potentials were referenced against the ECL images instead of the typical Ag/AgCl reference to give apparent potentials.40 Nonetheless, this method does not allow the usual examination for HER parameters, e.g., Tafel slope and overpotentials at fixed current density, which prohibits comparisons between past literature and present studies. For the first time, we synthesized MoWSx hybrid materials on an indium tin oxide (ITO)-coated glass slide BPE through bipolar electrochemistry, which generates composition gradients through the application of an external electric field across the ITO BPE. The composites were comprehensively characterized through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), examined at the graduated regions of cathodic and anodic overpotentials (ηcat and ηan, respectively) of the ITO BPE. By cutting the ITO BPE into smaller dimensions, each portion can be used directly as an electrode for quantitative HER measurement, which is pioneering for materials synthesized via bipolar electrochemistry. The lowest Tafel slopes of 45.7 and 50.5 mV dec−1, as well as corresponding HER overpotentials of 315 and 278 mV at −10 mA cm−2, were obtained for the respective cathodic and anodic composites. These excellent performance parameters were attributed to the comparable molybdenum and

report, using a hydrothermal method, Zhu et al. synthesized Mo1−xWxS2 nanopetals, the best of which displayed the lowest Tafel slope of 89 mV dec−1.30 Further computational calculations revealed lower Gibbs free energy of hydrogen adsorption (ΔGH), which improved the HER activity of the composite materials. Separately, Lu et al. reported a monotonic trend of decreasing HER overpotential with increasing fraction of W in the MoxW1−xS2 composites.10 The hydrothermal synthesis suffers from the usage of environmentally damaging chemicals and requirement of severe preparation conditions of ultrahigh vacuum and high temperatures.31,32 Conversely, electrochemical synthesis imparts a myriad of advantages, including versatility in the employed substrate and electrochemical method, fast synthesis, straightforward operation, scalability, and autonomy over thickness of electrodeposited material.33,34 The electrocatalyst immobilization step is also unnecessary with direct coating of the conducting substrates with the material of interest.35 Bipolar electrochemistry occurs through the exposure of a conducting substrate to an adequately strong electric field to generate a polarization that is directly dependent on the applied electric field as well as the dimension of the object aligned to the field. This results in oxidation and reduction reactions transpiring at either ends of the conducting object, eliminating the need for direct contact between the object and the power supply, as in conventional electrodeposition.36 Beyond the advantages bestowed by conventional electrodeposition, bipolar electrochemistry presents a highly viable means for the electrodeposition of composite materials because of its unique gradient overpotentials (η), which decrease from the extreme edges of a bipolar electrode (BPE) toward the zero overpotential point (η0) (Figure 1A). This enables a one-step deposition of composite materials exhibiting composition 41956

DOI: 10.1021/acsami.7b09435 ACS Appl. Mater. Interfaces 2017, 9, 41955−41964

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Figure 2. (A) Cyclic voltammograms of electrodeposition of MoWSx/CV, MoSx/CV, and WSx/CV conducted at −1.3 to +0.6 V vs Ag/AgCl. Conditions: 10 mM (NH4)2WS4 and/or 2 mM (NH4)2MoS4 in 0.1 M KCl as the supporting electrolyte, scan rate of 50 mV s−1, 20 cycles, purged with N2. Polarization curves of MoWSx/CV electrodeposited (B) over 20, 30, 40, and 50 cycles and (C) over 20 cycles, in electrolytes containing 1:5, 1:10, and 1:20 (NH4)2MoS4 and (NH4)2WS4. (Insets) Enlarged views of (B) as indicated by the shaded areas. Polarization curves of (D) MoWSx/− 1.3 and (E) MoWSx/+0.6 electrodeposited over 60, 300, 600, 1200, and 1800 s. (F) Polarization curves of best-performing MoWSx/CV, MoWSx/− 1.3 and MoWSx/+0.6 composite materials, benchmarked against Pt/C and bare GC.

catalytic performances of MoWSx/BPEcathodic and MoWSx/ BPEanodic were ascribed to the approximately equivalent surface Mo and W contents, as well as the small sizes of the deposited nanoparticles, which represented high surface area for HER catalysis. Typically, electrochemistry is performed via manipulating the potential of a conducting object, which is directly appended to a power supply. The electron transfer event is generally propelled by a disparity in potential between the electrode and the electrolyte; hence, by varying the potential of the electrolyte, successful control over the electrochemical reaction can be achieved. Bipolar electrochemistry allows control over the electrolyte potential through application of an external voltage exceeding a threshold value across the electrolyte. The solution resistance leads to a linearly decreasing potential gradient from one end of the BPE to the other, and the potential difference between the two ends (ΔEBPE) is evaluated

tungsten contents obtained through surface elemental analysis using XPS, as well as the evenly distributed nanoparticles with a high surface area-to-volume ratio. The novel method presented here proffers the possibility for rapid development and screening of composite materials as electrocatalysts for potential sustainable energy production in the future.

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RESULTS AND DISCUSSION In this fundamental study, cyclic voltammetry (CV) and chronoamperometry were first explored to assess the feasibility and conditions required for MoWSx deposition, and the resulting materials were labeled as MoWSx/CV and MoWSx/ −1.3/+0.6, respectively. Because of the low overpotentials of oxidation and reduction of MS42− (M = Mo or W) precursors, bipolar electrochemical deposition resulted in two different materials, MoWSx/BPEcathodic and MoWSx/BPEanodic. Through thorough characterization across the MoSx/WSx composition gradients on the ITO BPE, the outstanding HER electro41957

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Figure 3. XPS (A) W 4f, (B) Mo 3d, and (C) S 2p core-level spectra of MoWSx/CV, MoWSx/−1.3, and MoWSx/+0.6. (D) SEM images of MoWSx/CV, MoWSx/−1.3, and MoWSx/+0.6. (Inset) SEM image of MoWSx/+0.6 indicating poor coverage of the screen-printed electrode (SPE) substrate.

voltammograms are shown in Figure 2A. It is apparent that the current profile of MoWSx/CV exhibits much higher values compared to that of the direct addition of the profiles of MoSx/ CV and WSx/CV; the anodic peak at ca. 450 mV (vs Ag/AgCl) achieved 434 μA, which is 4 times the current of MoSx/CV and 3 times that of WSx/CV, and the cathodic peak garnered an even greater enhancement. This suggests that the coelectrodeposition of MoSx and WSx allowed for more favorable interactions with the underlying substrate and between the layers of material, leading to the augmented deposition of the composite material. The MoWSx/CV material was then optimized for HER electrocatalytic activity by tuning the number of CV cycles and the ratio of Mo and W precursors employed. As shown in Figure 2B (bottom inset) and C, 20 cycles in an electrolyte of Mo/W, ratio 1:5, produced the composite with the best HER performance, whereby the lowest overpotential at a current density of −10 mA cm−2 was attained among the different materials. A lower overpotential indicates that less input energy is required to achieve the same current density. A special feature of the HER polarization curve of MoWSx/CV, which is not found for MoWSx deposited through other techniques, is the inherent reduction prepeak at ca. −0.1 V (Figure 2B, top inset). We hypothesized this peak to represent the reduction of MoWSx to MoWS2. On the basis of a prior study by Merki et al., a similar inherent peak was observed and attributed to the reduction of S22− to S2−.31 Indeed, from the XPS S 2p spectrum of MoWSx/CV (Figure S2), the S22− peaks disappeared after HER, with the emergence of a sulfate peak. It is intriguing that the trend obtained from Figure 2B follows that of the inherent peak current in which a larger peak current corresponded to lower overpotentials at −10 mA cm−2. This is conceivably due

as the proportion of the decline in total applied potential (ΔEapplied) across the length of the BPE (LBPE): ΔE BPE =

L BPE × ΔEapplied Lcell

By increasing ΔEapplied to beyond the threshold voltage, sufficient ΔEBPE can be achieved and electrochemical redox reactions will occur concurrently at both ends of the BPE. The schematic diagram and actual setup of the bipolar electrochemistry deposition are shown in Figures 1A and S1A, respectively. An ITO BPE was placed between two platinum electrodes, which were used as driving electrodes for deposition. Because of the need for charge balance, the currents occurring on both ends of the BPE have to be equal. As the onset potentials of both cathodic reactions, i.e., HER and reduction of MS42−, are in close proximity, it is likely that both reactions occur at the cathodic end, resulting in a higher current. Thus, the zero overpotential point (η0) was displaced from the midpoint of the ITO electrode toward the cathodic edge. Using this setup, the electrodeposition of MoWSx via bipolar electrochemistry was performed, and the SEM images in Figure 1B,C verified the successful deposition of the material, which displayed distinctly different morphology from that of the bare ITO electrode (Figure S1B). From the edges of the ITO electrode toward η0, decreasing sizes of the deposited MoWSx material were observed as lower η represents lower rates of electrodeposition. To better appreciate the process of bipolar electrochemical deposition, initial insights were gained from CV and chronoamperometry electrodepositions. MoWSx/CV, MoSx/ CV, and WSx/CV were first deposited via CV from −1.3 to +0.6 V (vs Ag/AgCl) over 20 cycles, and the resulting cyclic 41958

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thus resulting in the overlapping HER curves. It is likely that the presence of both MX2 and MX3 on the composite surface increased its activity toward HER electrocatalysis, possibly through concurrent adsorption of protons and desorption of molecular hydrogen on separate active sites.41 The modest HER performance of MoWSx/+0.6 can be attributed to the poor coverage of the underlying substrate, as seen in Figure 3D, inset, as a result of the short deposition duration of 60 s. Before embarking on bipolar electrochemical deposition of MoWSx, it is imperative that the possible reactions that may occur are identified. In this study, the open configuration for bipolar electrochemistry was adopted; hence, both ends of the BPE were exposed to the same electrolyte composition. Some of the possible reactions include water splitting reactions as well as the oxidation and reduction reactions of the precursors MS42− to MS3 and MS2, respectively. The respective competing chemical equations and their onset potentials are displayed in Table 1.

to the presence of more HER-active species upon the pre-HER reduction, leading to a higher HER activity. Chronoamperometry was then performed to examine the quality of the deposited composite material via a fixed potential approach. Two potentials (−1.3 and +0.6 V vs Ag/AgCl) were selected on the basis of the extreme potentials of the CV method. A range of durations were inspected, and the resulting HER polarization curves are plotted in Figure 2D,E. On comparing these two materials, MoWSx/−1.3 demonstrated to be more active toward HER whereby all of the examined materials of MoWSx/−1.3 exhibited lower overpotentials than those of MoWSx/+0.6. The HER activities of MoWSx/−1.3 and MoWSx/+0.6 were assessed to be dependent on the duration of applied potential, though opposite trends were acquired. The MoWSx/−1.3 composite exhibited improving activity with longer durations; however, extended deposition time spans proved to be detrimental to MoWSx/+0.6. Integrating the findings of the above optimization steps, the HER polarization curves of the best-performing MoWSx/CV, MoWSx/−1.3, and MoWSx/+0.6 hybrids are plotted in Figure 2F, benchmarked against those of Pt/C and bare glassy carbon (GC). Pt/C displayed exceptional electrocatalytic performance (Tafel slope of 30.6 mV dec−1 and overpotential of 118 mV at −10 mA cm−2) surpassing all of the other materials, whereas the other extremity was bordered by bare GC’s poor HER activity (112.5 mV dec−1 and 861 mV, respectively). MoWSx/ CV and MoWSx/−1.3 exhibited almost overlapping HER curves (overpotential of ca. 249 mV at −10 mA cm−2), with the only distinction being the inherent reduction prepeak as aforementioned, resulting in differing Tafel slopes (51.5 and 37.7 mV dec−1 correspondingly). In contrast, the MoWSx/+0.6 composite showed poorer HER performance than that of MoWSx/CV and MoWSx/−1.3, with the Tafel slope of 65.7 mV dec−1 and overpotential of 336 mV at −10 mA cm−2. To elicit a deeper understanding of the trends observed, characterizations of MoWSx/CV, MoWSx/−1.3, and MoWSx/ +0.6 by XPS and SEM-EDS were conducted and the corresponding spectra and images are displayed in Figure 3. XPS was performed to examine the surface elemental composition and chemical bonding information of MoWSx, which were relevant to the application of HER electrocatalysis occurring on the surface of the material. From Figure 3A−C, similarities could be drawn between MoWSx/CV and MoWSx/ −1.3; both materials contained Mo(VI) moieties (ca. 235.1 and 232.0 eV) and large amounts of W(VI) moieties (ca. 38.4 and 36.3 eV), even though additional S22− ligands (ca. 164.9 and 163.7 eV) were observed on MoWSx/CV. Conversely, MoWSx/+0.6 possessed more W(IV) moieties (ca. 36.2 and 34.1 eV) and S22− ligands than the W(VI) and S2− species, though no Mo(VI) moieties were observed. Through SEM micrographs, the morphology of MoWSx/−1.3 manifested as nanoparticles that agglomerated in clumps of ca. 200 nm (Figure 3D), whereas MoWSx/+0.6 showed larger and lessdefined particles (>200 nm) with rough surface corrugations. The morphology of the MoWSx/CV composite material appeared to be intermediate between those of MoWSx/−1.3 and MoWSx/+0.6. The surface sulfur-to-metal (S/M) and molybdenum-to-tungsten (Mo/W) ratios evaluated from the XPS elemental quantifications (Table S1) also allude to the parallel values obtained for MoWSx/CV and MoWSx/−1.3. The congruence in elemental composition and morphology between MoWSx/CV and MoWSx/−1.3 indicated that similar HER-active species resided on the surface of these materials,

Table 1. Chemical Equations and Onset Potentials of Possible Competing Reactions in Bipolar Electrochemical Deposition nature of reaction oxidation reduction

chemical equation

onset potential (vs Ag/AgCl) (mV)

(i) 2H2O → 4H+ + O2 + 4e− (ii) MS42− → MS3 + 1/8S8 + 2e− (iii) 2H+ + 2e− → H2 (iv) MS42− + 2e− + 4H+ → MS2 + 2H2S

+582a ca. −190b −648a ca. −200b

a

Onset potentials of oxygen and hydrogen evolution reactions were calculated based on +1.23 V vs reversible hydrogen electrode (RHE) and 0 V vs RHE, respectively. bOnset potentials were obtained from cyclic voltammograms in Figure 2A.

The cyclic voltammogram of the MoWSx composite material (Figure 2A) exhibited broad oxidation and reduction peaks corresponding to eqs (ii) and (iv), respectively. For both anodic and cathodic reactions, the redox of MS42− demonstrated earlier onset potentials compared to those of the water splitting reactions. Hence, it is anticipated that MoWSx hybrids will be deposited onto the ITO BPE on both ends. Because of solution resistance, external voltages higher than the minimum ΔEapplied (calculation of which is elaborated in Supporting Information) at 3, 5, and 7 V and varying deposition time spans of 5, 10, and 20 min were selected for bipolar electrochemical deposition of MoWSx. The resulting optical and SEM images are illustrated in Figures S3, S4, and 4A. At ΔEapplied of 3 V (Figure S3A), barely any material was electrodeposited onto the BPE after 5 min. Upon longer deposition durations of 10 and 20 min, a hint of brown material was observed at the edges of the electrode. This implied that the applied voltage was insufficiently high to enable electrochemical reactions to occur beyond the very edges of the BPE. At higher applied voltages of 5 and 7 V (Figure S3B,C), characteristic dark brown deposits were clearly detected on both edges of the BPE, which extended toward η0. It is noted that with a higher applied voltage, the length of deposited area increased, whereas longer durations led to thicker (darker) composites with higher homogeneity. The MoWSx material deposited at 5 V for 20 min (Figure S3B) was an exception to the latter finding because of the poor adherence of the deposited layer, which was partially removed upon rinsing. 41959

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Figure 4. (A) SEM images of MoWSx/BPE-modified ITO electrodeposited at 5 V after various durations, as indicated. The “−” and “+” signs represent the cathodic and anodic edges, respectively. (B) Scheme describing the deposition of MoWSx/BPE, as observed using SEM.

MoWSx/BPEcathodic and MoWSx/BPEanodic with decreasing ηcat and ηan (Figure 1B,C) depicted well-defined and evenly deposited nanoparticles with good coverage of the underlying ITO electrode across the length of the BPE. Figure S5A,C exhibits the size distributions of MoWSx/BPEcathodic and MoWSx/BPEanodic across the entire cathodic and anodic regions, and Figure S5B,D depicts the variations of average size distributions of MoWSx/BPEcathodic and MoWSx/BPEanodic with decreasing ηcat and ηan, respectively. There was a clear dependence of particle size on η, with more apparent size disparity observed for MoWSx/BPEcathodic (from 272 to 56 nm (difference = 216 nm) with decreasing overpotentials cf. 158− 34 nm (difference = 124 nm) of MoWSx/BPEanodic). Lower overpotentials represent lower rates of electrodeposition, leading to smaller particle sizes. Through employing bipolar electrochemistry, studies on variation of electrocatalytic performances with differing sizes may be conducted with respect to noncomposite materials. However, in this study, both particle size and chemical compositions of MoWSx/BPE were varied due to their composite nature; hence, both are important factors to be accounted for. In-depth elemental analyses were also conducted via EDS, and the typical EDS spectra of MoWSx/BPEcathodic and MoWSx/BPEanodic are displayed in Figure S6; the peaks that were assigned to Mo, W, and S presented the successful incorporation of all three elements into the composite material. For chemical bonding information along the cathodic and anodic composition gradients, XPS core-level Mo 3d, W 4f, and S 2p spectra were deconvoluted and the typical XPS spectra of MoWSx/BPEcathodic and MoWSx/BPEanodic are exhibited in Figure S7. Similar to those of MoWSx/−1.3, the deconvoluted S 2p peaks of MoWSx/BPEcathodic were attributed only to S2− ligands due to the highly negative applied potential, which led to the reduction of MS42− to MS2. The M(IV) and S ligands at characteristic binding energies similar to those previously obtained for CV- and chronoamperometry-based electrodeposition of MoWSx were indicative of the presence of Mo−S and W−S bonds and successful fabrication of the MoWSx/BPE composite material via bipolar electrochemistry. From the S/M and Mo/W ratios garnered from the EDS measurements, the chemical formulae of MoWSx/BPEcathodic and MoWSx/BPEanodic were calculated and are tabulated in

Of the three voltages that were examined, 5 V demonstrated evenly distributed nanoparticles of ∼100 nm (Figure 4A), whereas 7 V exhibited smaller nanoparticles, some of which aggregated and resulted in a wider spread of sizes because of more nucleation sites at the higher applied potential (Figure S4B); with scarcely any material deposited, 3 V displayed similar morphology to bare ITO (Figures S4A and S1B). An inspection of the SEM images of the materials deposited at 5 V for 5, 10, and 20 min (Figure 4A) revealed minimal deposition at the anode end after 5 min, whereas sporadic spots of uneven morphology were observed after 20 min, likely due to breakdown of the material over extended deposition durations under a high voltage. Cracks that were spotted at the cathode end after 20 min deposition were attributed to the decreased concentrations of the MS42− precursors with prolonged depositions, which resulted in the deposition of less ideal MoSx/WSx compositions in the top layers; differing thermal expansion coefficients of MoSx and WSx led to cracks in the layers upon cooling down after the deposition procedure.42 On the other hand, the MoWSx composite material displayed good coverage of the BPE at both the anodic and cathodic ends after 10 min of deposition treatment. With the observations made from SEM, a summary of the deposition process via bipolar electrochemistry is illustrated in Figure 4B and will be briefly described. At above the minimum ΔEBPE, initial nucleation of nanoparticles occurred. Subsequently, over a longer deposition period, full coverage of the BPE was achieved; further deposition resulted in the formation of nanoparticles stacked onto the bottom layers. Over prolonged deposition time spans, cracks appeared in the layers due to differing thermal expansion coefficients of the two materials forming the composite. On the basis of the above conclusions, MoWSx deposited at 5 V for 10 min (MoWSx/ BPE) was chosen for further examination and characterization. Thus far, cursory morphological information has been engendered from SEM, but more systematic morphological and elemental studies were required to confirm the identity of the deposited material and to elucidate elemental trends pertaining to varying ηcat and ηan via bipolar electrochemistry. To this end, SEM-EDS and XPS were performed along the length of the BPE modified with MoWSx, with each measurement approximately 2 mm apart. The SEM images of 41960

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Figure 5. Mo/W and S/M ratios of (A, B) MoWSx/BPEcathodic and (C, D) MoWSx/BPEanodic, respectively, obtained from bulk and surface elemental measurements.

To enable quantitative monitoring of HER activities of MoWSx/BPE deposited via bipolar electrochemistry, the ITO BPE was cut into 1.25 cm × 0.2 cm strips, and each strip was wrapped with PARAFILM to expose an electrodeposited area of 0.5 cm × 0.2 cm for use as a working electrode. Through this, the variations in HER performances of MoWSx/BPEcathodic and MoWSx/BPEanodic were tracked across the BPE and important HER parameters, e.g., overpotential at −10 mA cm−2 and Tafel slope, were acquired and compared across the literature. The HER polarization curves of MoWSx/BPEcathodic and MoWSx/BPEanodic are plotted in Figure 6A, whereas Figure 6B−E presents trends in the HER parameters with decreasing ηcat and ηan. Overall, the anodic materials exhibited a wider range of HER performances (Figure 6A) and greater variations in both HER parameters (Figure 6D,E) compared to those of the cathodic materials. Across the BPE, from highest to lowest ηcat, decreasing Tafel slopes were obtained, whereas the overpotential at −10 mA cm−2 of MoWSx/BPEcathodic remained relatively stable. Prior density functional theory (DFT) studies demonstrated that only the Mo edge was HER-active for MoS2, whereas for WS2, both W and S edges were electrocatalytic toward HER. The overpotential at −10 mA cm−2 was thus expected to increase with increased MoS2 proportion on the surface (Figure 5A), but it remained relatively constant; this was ascribed to the smaller nanoparticles of MoWSx/BPEcathodic engendered at lower ηcat (Figure 1B), which led to higher exposure of HERactive sites, countering the anticipated rise in overpotential at − 10 mA cm−2. It is noted that the cathodic composite material that achieved the best Tafel slope of 45.7 mV dec−1, with the corresponding HER overpotential of 315 mV at −10 mA cm−2, possessed the smallest particle size, and the optimal surface Mo/W ratio was found to be close to unity. Previously, Zhu et al. analyzed the charge transfer between the WS2 and MoS2 layers and determined the directional electron transfer to flow from WS2 to MoS2, which enabled the ΔGH of Mo edge to approach the thermoneutral value (i.e., ΔGH ≈ 0).30 Hence, it

Table S1, together with those of MoWSx/CV, MoWSx/−1.3, and MoWSx/+0.6. Both MoWSx/BPEanodic and MoWSx/+0.6 engendered similar molecular formulae of MS3 (Mo0.7W0.3S3), whereas MoWSx/BPEcathodic and MoWSx/−1.3 presented a general formula closer to MS2 (Mo0.4W0.6S1.4 and Mo0.7W0.3S2, respectively). With the MoWSx/BPE hybrids thoroughly characterized, their HER electrocatalytic performances were subsequently analyzed. The MoWSx/BPE hybrids were expected to display chemical composition gradients with dependence on the η across the BPE due to the different applied potentials for the oxidation and reduction of each MS42− precursor. To assess that, Mo/W and S/M ratios (Figure 5) were evaluated on the basis of EDS and XPS data to illuminate potential trends across the cathodic and anodic composition gradients. Comparing the bulk (EDS) and surface (XPS) ratios, the surface of the MoWSx/BPE composites tended to be enriched with sulfur in comparison with the underlying layers; this was especially so for MoWSx/ BPEanodic because of the presence of S8 resulting from the oxidative deposition. The surface and bulk ratios were then considered separately. The surface Mo/W and S/M ratios of MoWSx/BPEcathodic (Figure 5A,B) presented increasing trends with decreasing ηcat, from 0.2 to 0.8 and 1.9 to 2.3, respectively. Conversely, for MoWSx/BPEanodic (Figure 5C,D), a declining trend in the surface Mo/W ratio, from 2.0 to 1.0, was obtained with decreasing ηan, whereas no trend was observed for the surface S/M ratio, which remained at an average of 4.1. On the other hand, on the basis of the bulk S/M and Mo/W ratios of both cathodic and anodic materials, there was no apparent trend with decreasing ηcat and ηan, apart from the Mo/W ratio of MoWSx/BPEcathodic (Figure 5A), which showed an increasing trend to approximately 1 at the lowest ηcat. Instead, relatively stable values for the bulk S/M ratios of MoWSx/BPEanodic and MoWSx/BPEcathodic (Figure 5D,B respectively) were obtained, at 3.1 and less than 2, indicating successful syntheses of MoWS3 (MoWSx/BPEanodic) and sulfur-deficient MoWS2 (MoWSx/ BPEcathodic), based on the bulk elemental analysis. 41961

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those of the materials deposited through CV or chronoamperometry, as demonstrated by their lower overpotentials. Compared with the typical fabrication method, the Tafel slopes achieved by MoWSx/BPEcathodic and MoWSx/BPEanodic (45.7 and 50.5 mV dec−1) also outperformed the slope obtained by the hydrothermal MoWS2 (89 mV dec−1),30 suggesting the advantage of the electrodeposition approach over conventional bottom-up synthesis methods. Endowed with these merits, as well as short deposition durations, bipolar electrochemistry can potentially be developed into a viable industrial-scale fabrication method for HER electrocatalysts.

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EXPERIMENTAL SECTION

Materials. Ammonium tetrathiotungstate, ammonium tetrathiomolybdate, sulfuric acid, potassium chloride, and indium tin oxidecoated glass slides (ITOs) were obtained from Sigma-Aldrich. The Ag/AgCl reference electrode, glassy carbon (GC) electrodes, and platinum counter electrode were purchased from CH Instruments. Milli-Q water with 18.2 MΩ cm resistivity was used for the preparation of electrolytes. Apparatus and Methods. X-ray photoelectron spectroscopy (XPS) was conducted with a conventional nonmonochromated X-ray source, which uses the Mg Kα line (SPECS XR50, hν = 1253 eV, 200 W) and a multichannel energy analyzer (SPECS Phoibos 100 MCD5). For XPS measurements, the samples were electrodeposited on screen-printed electrodes (SPEs) or ITOs, which were then affixed on sample holders using carbon tape. Wide-scan and high-resolution C 1s, Mo 3d, W 4f, and S 2p core-level spectra were acquired. Calibration was done using the C 1s peak, referenced at 284.5 eV. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were conducted with a JEOL 7600F fieldemission SEM (JEOL, Japan), at acceleration voltages of 5 and 20 kV, respectively. The SPE (ITO) samples were coated through electrodeposition via cyclic voltammetry (CV) (bipolar electrochemistry (BPE)) before being attached onto sample stubs for measurements. Electrochemical depositions and measurements using cyclic voltammetry (CV) and linear sweep voltammetry were conducted with a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands), operated with NOVA 1.8 (Eco Chemie) software. The electrochemical techniques were conducted using a three-electrode configuration, with the bare GC, Ag/AgCl, and Pt electrodes as the working, reference, counter electrodes, correspondingly. Electrochemical depositions via bipolar electrochemistry (BPE) were conducted using DC Power Supply PS-305D (Dazheng, China). Particle size distributions were measured using Nikon NIS-Elements software and plotted across the entire cathodic and anodic regions, respectively, and with decreasing ηcat and ηan. Procedures. Synthesis of MSx/CV (M = Mo or W) and MoWSx/CV via Cyclic Voltammetry. The MoWSx hybrid was synthesized in an electrolyte containing 2 mM (NH4)2MoS4 and 10 mM (NH4)2WS4, both dissolved in 0.1 M KCl solution, which functioned as the supporting electrolyte. Different (NH4)2MoS4 to (NH4)2WS4 ratios of 1:5, 1:10, and 1:20 (Mo/W ratios) were utilized as well. The electrodeposition of MoWSx via cyclic voltammetry was conducted using the potential range of −1.3 to +0.6 V at 50 mV s−1 for 20, 30, 40, and 50 cycles to obtain MoWSx. The fabricated film attains full coverage of the electrode surface. Control materials MoSx and WSx were synthesized through the same procedure, in an electrolyte containing the corresponding precursors. Synthesis of MoWSx/−1.3 and MoWSx/+0.6 via Chronoamperometry. The MoWSx hybrid was fabricated in an electrolyte containing 2 mM (NH4)2MoS4 and 10 mM (NH4)2WS4, both dissolved in 0.1 M KCl solution, which operated as the supporting electrolyte. The electrodeposition of MoWSx via chronoamperometry was performed using an applied potential of −1.3 or +0.6 V over 60, 300, 600, 1200, and 1800 s. Synthesis of MoWSx/BPEanodic/cathodic via Bipolar Electrochemistry. The MoWSx hybrid was produced in an electrolyte comprising 2 mM

Figure 6. (A) Polarization curves of MoWSx/BPEcathodic and MoWSx/ BPEanodic. Overpotentials at −10 mA cm−2 and Tafel slopes of (B, C) MoWSx/BPEcathodic and (D, E) MoWSx/BPEanodic.

was likely that having a Mo/W ratio close to unity would enhance the HER performance of the MoWSx/BPEcathodic composite material. Both factors of particle size and surface Mo/W ratio, to which the best MoWSx/BPEcathodic performance was attributed, were found to be applicable to MoWSx/BPEanodic as well. The Tafel slopes and overpotentials at −10 mA cm−2 garnered decreasing trends with decreasing ηan (Figure 6D,E), in line with a declining surface Mo/W ratio to ca. 1 (Figure 5C) and diminishing particle size (Figure 1C); the MoWSx/BPEanodic composite with the best HER performance attained a Tafel slope of 50.5 mV dec−1 and a corresponding overpotential of 278 mV at −10 mA cm−2. This implied that the HER electrocatalytic process was likely to proceed via similar pathways for both sulfur-deficient MoWS 2 (MoWS x / BPEcathodic) and MoWS3 (MoWSx/BPEanodic). The S22− ligands in the MoWSx/BPEanodic structure were more likely to form bonds with Mo rather than W as the former bond is more stabilizing and forms more readily. The lower electron density of the S22− ligand compared to that of S2− further increased the disparity in charge density distribution between WS2 and MoS2, augmenting the directional flow of electrons, which engendered concurrent adsorption and desorption of protons and molecular hydrogen on different active sites. With the successful synthesis and identification of the optimal MoWSx/BPEcathodic and MoWSx/BPEanodic hybrid materials, a comparison of the HER performances of these materials against those of MoWSx/CV, MoWSx/−1.3, and MoWSx/+0.6 was rendered and is depicted in Figure S8. In addition to the advantage of simultaneous depositions of composites with varying elemental gradients, the optimal cathodic and anodic hybrids produced via bipolar electrochemistry exhibited HER activities similar to or higher than 41962

DOI: 10.1021/acsami.7b09435 ACS Appl. Mater. Interfaces 2017, 9, 41955−41964

Research Article

ACS Applied Materials & Interfaces Notes

(NH4)2MoS4 and 10 mM (NH4)2WS4, both dissolved in 0.1 M KCl solution, which acted as the supporting electrolyte. The ITOs were cut into dimensions of 1.25 cm × 3 cm, rinsed thoroughly with Milli-Q water, and dried with N2 gas prior to electrodeposition. Two Pt electrodes were dipped into the electrolyte 4.5 cm apart. With the ITO electrode placed in the middle of the electrolyte well, the direct current potential of 3, 5, or 7 V was applied for 300, 600, or 1080 s. After electrodeposition, the ITO was rinsed with Milli-Q water and dried with N2 gas before further measurements and characterization. Electrochemical Measurements. After electrodeposition, the modified GC electrode or ITO was rinsed gently with Milli-Q water to eliminate excess precursor ions and then dried under a lamp. It was then utilized for hydrogen evolution reaction (HER) measurements. A freshly electrosynthesized MoWSx hybrid material was prepared prior to each measurement. For the ITO, the glass slide was further cut into 1.25 cm × 0.2 cm strips, each of which was wrapped with PARAFILM to expose an area of 0.5 cm × 0.2 cm for use as a working electrode, with Cu tape acting as the electrical contact. Sulfuric acid (0.5 M) as the supporting electrolyte and a scan rate of 2 mV s−1 were utilized for all HER measurements. All potentials are relative to RHE, unless otherwise indicated, and all solutions, except for the BPE electrodepositions, were purged with nitrogen gas.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.P. acknowledges funding by Tier 99/13 grant by Ministry of Education, Singapore.

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CONCLUSIONS In summary, successful electrosynthesis of MoWSx composite materials was performed using the straightforward bipolar electrochemical deposition. Characterizations of the composites across the ITO BPE via SEM-EDS and XPS imparted evidence of the presence of Mo, W, and S, as well as bonding information corresponding to those obtained from CV- and chronoamperometry-based electrodepositions, indicating the effective fabrication of MoSx/WSx hybrid materials. Through subsequent analysis of their HER performances, various observations were elucidated: the HER performances of the most efficient MoWS x/BPE cathodic and MoWS x/BPE anodic composites were ascribed to the higher exposed surface area of the smaller particles, as well as their near-unity surface Mo/ W ratios. The excellent Tafel slopes and HER overpotentials obtained by the electrodeposited MoWSx composites indicated the superiority of electrodeposition as the prime synthesis approach for the development of electrocatalysts for sustainable energy production.

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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.7b09435. Optical image of the bipolar electrochemistry apparatus setup; SEM of bare ITO; core-level S 2p XPS spectra of MoWSx/CV before and after HER; elemental quantifications of MoWSx; calculation for minimum ΔEapplied; optical and SEM images of MoWSx/BPE-modified ITO; histograms illustrating particle size and average size distributions; typical EDS and core-level W 4f, Mo 3d, and S 2p XPS spectra of MoWSx/BPEcathodic and MoWSx/BPEanodic; and polarization curves of MoWSx/ CV, MoWSx/−1.3, MoWSx/+0.6, MoWSx/BPEcathodic, and MoWSx/BPEanodic (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Martin Pumera: 0000-0001-5846-2951 41963

DOI: 10.1021/acsami.7b09435 ACS Appl. Mater. Interfaces 2017, 9, 41955−41964

Research Article

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DOI: 10.1021/acsami.7b09435 ACS Appl. Mater. Interfaces 2017, 9, 41955−41964