Thiosulfate species promoting hydrogen evolution reaction at the heterointerface of Ir clusters-loaded WS₂ nanosheets

Abstract

We synthesize Ir cluster-loaded monolayer WS₂ nanosheets for water electrolysis and fabricate unique functional heterointerfaces with thiosulfate species formed from sulfide ions of WS₂. In situ XAFS measurements reveal that water dissociation is promoted between positively charged thiosulfates and negatively charged Ir clusters at the interfacial active sites for hydrogen evolution reaction.

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Among various renewable energy technologies, water electrolysis is a promising method for producing clean hydrogen.¹ However, the widespread adoption of this technology has been hindered by the inefficiencies of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), and highly active catalysts are required to minimize the overpotential and improve the overall energy efficiency of the system.² Recently, it was discovered that compounds containing various typical elements such as metal sulfides, phosphides and nitrides showed high activity in HER or OER.³ Although only constituent metal species have been considered to be active sites in HER or OER, it is gradually becoming clear that the adsorption of H or OH on neighboring sites, i.e., typical elements, also plays important roles in the reaction. For example, in neutral and alkaline HERs where H₂O is the substrate, it has been reported that the interaction of metal ions with O and sulfide ions with H on a metal sulfide electrocatalysts leads to the dissociation of H₂O and the improvement of HER activities.⁴ Therefore, in order to design highly efficient catalyst materials, it is essential to consider and analyze the properties of typical elements as well as metal species. Nevertheless, the properties of typical elements have not been sufficiently studied compared to metal species, and there are very few reports on in situ measurements to clarify the behaviors and functionalities of typical elements in catalytic materials.^5–7 The construction of heterointerfaces by synthesizing composite materials has attracted attention to developing highly active catalytic materials.^8–10 Significant progress has been achieved in the development of highly active electrocatalysts by exploiting the unique properties of heterostructures, which are interfaces between different components that can produce synergistic effects not achievable with individual materials. Therefore, it is highly expected that the construction of heterointerfaces containing typical elements and the analysis of their behaviors will lead to the discovery of new breakthroughs. In a recent study, the authors synthesized composite catalysts consisting of monolayer metal hydroxide nanosheets and Au clusters, and outstanding OER performances were achieved at the heterointerfaces of the catalysts.¹¹ Since both monolayer nanosheets and metal clusters have a high surface area-to-volume ratio, the combination is an effective catalyst design that maximizes the material interfaces to utilizes its functionalities. Metal sulfides such as WS₂ and MoS₂ are known as layered materials¹² and the monolayers of WS₂ and MoS₂ are preferred for construction of sulfur-containing heterointerfaces in composites of nanosheets and metal clusters and for development of new functionalities. In this study, to focus on the role of sulfur elements in the heterointerfaces, we synthesized a composite catalyst by combining WS₂ monolayer nanosheets and Ir clusters (Fig. 1(a) and Fig. S1, ESI†). Since Ir is active in both HER and OER under acid, neutral and alkaline conditions, heterointerfaces composed of Ir clusters are appropriate for investigating the effects of heterostructures on the reactions. Thiosulfate species are produced from S²⁻ of WS₂ by natural oxidation through the combination of the Ir clusters and the WS₂ nanosheets. In situ XAFS measurements found that the dissociation of H₂O molecules in the HER process is promoted by unique valence change of thiosulfate species. On the other hand, the thiosulfate species are simply oxidized and dissolved during the OER. This is the first report to demonstrate that sulfur oxoacid, thiosulfate, improves HER activity.

Fig. 1 (a) Schematic illustration of fabrication of the Ir/WS₂ from monolayer WS₂ and Ir clusters. (b) Low high magnification TEM image of the monolayer WS₂ nanosheets. HAADF-STEM images of (c) and (d) the monolayer WS₂ nanosheets, (e) Ir clusters and (f) Ir/WS₂. (g) STEM-EDX mapping of the Ir/WS₂. (W: green, S: red, Ir: blue).

Monolayer WS₂ nanosheets were synthesized using the one-pot method as reported previously.¹³ In the XRD patterns, the peak of the (002) reflection around 14° derived from the layer stacking, was not be observed for monolayer WS₂ nanosheets while the commercial WS₂ showed the strong and sharp peak (Fig. S2, ESI†).¹⁴ The SEM-EDS results showed that W and S were uniformly distributed in the synthesized WS₂ (Fig. S3, ESI†), and the composition was W : S = 1 : 1.94, which is almost the ideal ratio. Transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and atomic force microscopy (AFM) characterizations were conducted to investigate the morphology and structural characteristics of the synthesized WS₂ (Fig. 1(b), (c) and Fig. S4, ESI†). The TEM, HAADF-STEM and AFM images reveal that the nanosheets exhibit a uniform, two-dimensional morphology, with lateral sizes of several nanometers. The lattice fringes of the WS₂ nanosheets were clearly visible, with an interlayer spacing of approximately 0.28 nm, which corresponds to the (100) plane of hexagonal WS₂ (Fig. 1(d)).¹⁴ The results clearly show the successful synthesis of monolayer WS₂ nanosheets. Colloidal Ir clusters were synthesized in a mixed solvent of ethylene glycol and water.¹⁵ The STEM image (Fig. 1(e) and Fig. S5, ESI†) clearly showed that Ir clusters were uniformly dispersed without agglomeration and had an average size of approximately 1.4 nm. The STEM analysis also revealed that the Ir/WS₂ catalyst consisted of the monolayer WS₂ nanosheets and Ir clusters with uniform dispersion (Fig. 1(f), (g) and Fig. S6, ESI†). The lateral sizes of the WS₂ nanosheets in the Ir/WS₂ were smaller than those of the pristine WS₂ nanosheets, suggesting that WS₂ nanosheets were partially fragmented during the combination process due to the physically fragile nature of monolayer nanosheets. For comparison, the Ir cluster-loaded graphene (Ir/G) catalyst was also prepared, which showed a similar uniform dispersion of Ir clusters with an average particle size of 1.4 nm (Fig. S7, ESI†). The Ir/WS₂ and Ir/G showed similar electrochemical surface area (Fig. S8, ESI†). The SEM-EDX analysis indicated that the amount of Ir clusters loading was 73 wt% for the Ir/WS₂ and 63 wt% for the Ir/G, confirming that composite catalysts with similar Ir loading were successfully synthesized by using the combination method.

XAFS and XPS measurements were conducted to investigate the oxidation states of component elements for the pristine WS₂, Ir/WS₂ and Ir/G (Fig. 2 and Fig. S9, S10, ESI†). The pristine WS₂ showed the absorption edge attributable to W⁴⁺ (10210.9 eV) in the W L₃-edge XANES spectra, which was slightly higher than that of the laminated bulk WS₂ (10210.6 eV) (Fig. S10, ESI†). The absorption edge of the Ir/WS₂ (10211.2 eV) slightly shifted to the higher energy side than that of the pristine WS₂ (Fig. 2(a)), indicating that the W species in the Ir/WS₂ have more oxidative states compared to those of the pristine WS₂. The WS₂ nanosheets were fragmented during the combination process with Ir clusters, resulting in partial oxidation of nanosheets. In the Ir L₃-edge XANES spectra of the Ir/WS₂ and Ir/G (Fig. 2(b)), the absorption edges of the Ir/WS₂ and Ir/G were observed at 11221.8 eV and 11221.5 eV, respectively, which are between the Ir (Ir⁰) and IrO₂ (Ir⁴⁺). Since the average cluster size of 1.4 nm was very small, the spectra reflected the inner metallic states and the surface oxidative states of the Ir clusters. The absorption edge of the Ir/WS₂ catalyst was slightly higher than that of the Ir/G, indicating more oxidative states of the Ir clusters on the WS₂ than those on the graphene. In the S K-edge XANES spectrum of the pristine WS₂, the peak at 2471.6 eV was observed, which is attributable to S²⁻ species of typical sulfides (Fig. 2(c)).⁶ On the other hand, the Ir/WS₂ showed two peaks at 2471 eV and 2482 eV in the spectrum, which are attributable to S²⁻ and S⁶⁺ species, respectively. The results suggest the formation of thiosulfate.¹⁶ This is due to the oxidation of the S²⁻ species during the combination process, leading to the formation of thiosulfate species with cationic S⁶⁺ species on the entire surface of WS₂ (Fig. S11, ESI†).¹⁷ Based on the measurements, electronic states of W and S in the Ir/WS₂ were partially changed to more oxidative states than those of the pristine WS₂ during the combination of Ir clusters and thiosulfate species formed in the Ir/WS₂. In addition, EXAFS analysis suggested formation of heterointerfaces of Ir clusters and WS₂ (Fig. S12, ESI†).

Fig. 2 (a) W L₃-edge, (b) Ir L₃-edge and (c) S K-edge XANES spectra of the pristine WS₂, Ir/WS₂, Ir/G and reference compounds.

The HER and OER activities of the synthesized catalysts were measured under acidic, neutral and alkaline conditions (Fig. 3, Fig. S13, S14 and Table S1, ESI†). The same measurements were also performed on Pt/C and IrO₂ samples for comparisons. Under alkaline and neutral conditions, the HER proceeds with H₂O molecules as the substrate, whereas protons act as the substrate under acidic conditions, and the acidic HER proceeds without dissociation of the OH bond (acidic: 2H⁺ + 2e⁻ → 2H₂, neutral, alkaline: 2H₂O + 2e⁻ → H₂ + 2OH⁻).¹⁸ In this study, all catalysts showed higher HER activities under the acidic condition than under the alkaline and neutral conditions. The Pt/C, a typical highly active catalyst for HER, exhibited high activities, while the Ir/G showed lower activities than the Pt/C under all conditions as reported previously.¹⁹ While the WS₂ alone showed very low HER activities under all conditions, the Ir/WS₂ showed good HER activities, indicating that the Ir clusters act as the main active species in the Ir/WS₂. Notably, the Ir/WS₂ showed lower HER activities than the Pt/C under the acidic condition, but showed comparable activities to the Pt/C under the alkaline and neutral conditions. Furthermore, although the Ir/WS₂ catalyst contains the same active Ir clusters as the Ir/G, the Ir/WS₂ showed similar activities to the Ir/G under the acidic condition, but showed much higher activities than the Ir/G under the alkaline and neutral conditions. This activity discrepancy clearly originated from the difference in the composite materials, i.e. WS₂ and graphene. The superior activities of Ir/WS₂ over Ir/G under the alkaline and neutral conditions suggest that the combination of Ir and WS₂ facilitates H₂O dissociation. When comparing based on the current densities per Ir mass of the catalysts, the same activity order was observed in HER (Fig. S15, ESI†), and the similar conclusion was also obtained in the impedance measurements (Fig. S16, ESI†). In the OER, the WS₂ alone showed very low activities under all conditions, indicating that Ir clusters work as the active species in the Ir/WS₂ for OER (Fig. 3(d)–(f)). The Ir/WS₂ and Ir/G showed significantly higher activities than those of the IrO₂, demonstrating that the Ir clusters can also act as a highly active OER catalyst.²⁰ The catalytic activities differed with pH, but the degree of difference in activities due to the difference in pH was smaller than the difference in activities in HER. In OER, H₂O is the substrate under acidic and neutral conditions, and OH⁻ is the substrate under alkaline conditions (acidic, neutral: 2H₂O → 4H⁺ + O₂ + 4e⁻, alkaline: 4OH⁻ → 2H₂O + O₂ + 4e⁻). Therefore, dissociation of the OH bond occurs in all conditions, leading to smaller activity differences with pH. The Ir/WS₂ showed higher OER activities than the Ir/G, but unlike HER, the difference in activities between the Ir/WS₂ and Ir/G was not large in any conditions. Therefore, it is believed that the functions of WS₂ and graphene as composite materials are similar in the OER process. When comparing the Ir/WS₂ and isolated Ir clusters without support materials, the similar results were observed (Fig. S13, ESI†).

Fig. 3 IR-corrected polarization curves of the pristine WS₂, Ir/WS₂, Ir/G, Pt/C and IrO₂ for (a)–(c) HER and (d)–(f) OER in the aqueous solution of (a) and (d) 0.5 mol dm⁻³ H₂SO₄, (b) and (e) 1 mol dm⁻³ PBS, and (c) and (f) 1 mol dm⁻³ KOH.

In situ XAFS measurements were performed to investigate catalytic behaviors of the Ir/WS₂ under HER and OER conditions (Fig. 4). Under the initial OCP condition, the spectra were similar to the ex situ XAFS results. When a potential of −0.05 V vs. RHE was applied, the W L₃-edge and Ir L₃-edge XANES spectra shifted to lower energy sides. Under HER conditions where negative potentials are applied, electronic states of electrodes generally change to reductive.²¹ Therefore, the W and Ir species in the Ir/WS₂ changed to reductive electronic states during HER. Under the OCP condition after the polarization, the W L₃-edge and Ir L₃-edge spectra returned to almost the same positions as the spectra under the initial OCP condition. On the other hand, the S K-edge spectra exhibited interesting behaviors. Under the initial OCP condition, peaks corresponding to S²⁻ and S⁶⁺ were observed at 2472 eV and 2482 eV, respectively, similar to the ex situ measurements (Fig. 4(e)). Under the polarization of −0.05 V vs. RHE, the intensity of the S²⁻ peak decreased and that of the S⁶⁺ peak increased, clearly indicating an oxidative change in the electronic states of the sulfur species despite the reductive condition of HER.¹⁷ Notably, after returning to the OCP condition, the S K-edge spectrum returned to its initial states, suggesting reversible change in the electronic states of the sulfur species. This unique behavior suggests that the central S⁶⁺ of the thiosulfate species interacts with the anionic O of H₂O accompanied with charge attraction from thiosulfates to H₂O at the Ir/WS₂ heterointerfaces. Since the W and Ir species were reduced under HER conditions, the cationic H of H₂O is likely to interact preferentially with them.²² The active site for HER is Ir, not W. Therefore, the dissociation of the OH bond is facilitated by the interaction between the central S⁶⁺ of the thiosulfate species and the O of H₂O, as well as the interaction between the Ir cluster and the H atom of H₂O, resulting in excellent HER activities (Fig. S17(a), ESI†). The Ir/WS₂ exhibited higher HER activities than the Ir/G in alkaline and neutral media, while thier activities were similar under acidic conditions. The HER activities of Ir/WS₂ in alkaline and neutral media were achieved by the thiosulfate-facilitated water dissociation at the heterointerface, which is the reason for the superior performances of the Ir/WS₂ those of the Ir/G. One possible explanation for the presence of S⁶⁺ in the Ir/WS₂ is sulfate species rather than thiosulfate species. However, since the S²⁻ peak decreased and the S⁶⁺ peak increased reversibly, it is highly likely that thiosulfate species containing both S²⁻ and S⁶⁺ are present within the molecule. Furthermore, an Ir/WS₂ catalyst that does not contain thiosulfate species exhibited low HER activities, suggesting that thiosulfate species play an important role in enhancing HER activities (Fig. S18, ESI†). Catalytic behaviors of the Ir/WS₂ under OER conditions were also investigated. Under the polarization of 1.50 V vs. RHE, the W L₃-edge and Ir L₃-edge XANES spectra shifted to higher energy sides, indicating an oxidative change in electronic states of the W and Ir specie (Fig. 4(f), (g), (h) and (i)). Under the OCP condition after the polarization, the Ir L₃-edge XANES spectrum returned to its initial position, while the W L₃-edge spectrum remained slightly shifted to higher energies, indicating partially irreversible oxidative alteration of WS₂. The S K-edge spectra under the OER condition showed a decrease in the S²⁻ peak intensity and an increase in the S⁶⁺ peak intensity, suggesting an oxidative change in electronic states of sulfur species (Fig. 4(j)). However, when the potential was returned to OCP, the peak intensities of both did not return to their original levels. These results indicate that thiosulfates and the original S²⁻ species were oxidized and the WS₂ nanosheets were degraded during the OER (Fig. S10(b), ESI†), as reported previously.²³ When the STEM observation and XRD measurement were performed on the Ir/WS₂ after HER and OER (Fig. S19 and S20, ESI†), the Ir/WS₂ after HER was almost the same as before the reaction, whereas the WS₂ nanosheets considerably disappeared and the Ir clusters were aggregated after OER. The Ir/WS₂ showed a similar polarization curve after 1000 cycles of CV measurement, indicating good durability of the Ir/WS₂ in HER (Fig. S21, ESI†). In situ XAFS measurements for the pristine WS₂, which does not contain thiosulfate species, revealed that reversible change in the S K-edge spectra in HER was attributed to thiosulfate species in the Ir/WS₂ (Fig. S22, ESI†). This is the first study to find that the oxoacid of sulfur, thiosulfate, can improve HER activities. In summary, we synthesized the Ir/WS₂ electrocatalyst with maximized heterointerfaces and investigated the unique catalytic behaviors of thiosulfate species. The Ir/WS₂ showed excellent activities comparable to the Pt/C in HER under the neutral and alkaline conditions. In situ XAFS analysis revealed the unique functionality of thiosulfate species at the Ir/WS₂ heterointerface which promoted water dissociation during HER and that underwent oxidative dissolution during OER. Of the thiosulfate species generated on the surface, only those close to Ir clusters would contribute to HER activities. Recently, some studies found that several oxoacids such as SeO₃²⁻ and phosphate enhanced HER activities based on the different mechanism from that of this study.^24,25 These findings in this study not only emphasize functionalities of typical elements at the heterointerfaces for electrocatalytic water splitting, but also suggest a broader application potential for oxoacid-enhanced activities in electrocatalysis fields.

Fig. 4 In situ (a), (b), (f) and (g) W L₃-edge, (c), (d), (h) and (i) Ir L₃-edge, and (e) and (j) S K-edge XANES spectra of the Ir/WS₂ in the aqueous solution of 1 mol dm⁻³ PBS. (b), (d), (g) and (i) are enlargements of (a), (c), (f) and (h), respectively.

Data availability

All data can be found in the main article or the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc06122e

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