Visible-light-induced hydrogen evolution reaction with WS _x Se_2−x

Abstract

WS₂ is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic properties of ternary sulphoselenides of tungsten (WS _x Se _2−x ) by the dye-sensitized hydrogen evolution. WS _x Se _2−x solid solutions are found to exhibit high activity reaching 2339 μmol h⁻¹ g⁻¹ for WSSe, which is three times higher than that of WS₂ alone (866 μmol h⁻¹ g⁻¹ ). The turnover frequency is also high (0.7 h⁻¹ ). Such synergistic effect of selenium substitution in WS₂ is noteworthy.

1 Introduction

Transition metal dichalcogenides (TMDs) have emerged as an important class of materials with unique properties [1–4]. They have layered sandwiched X–M–X units (M = metal and X = chalcogenides) with strong in-plane bonding and weak out-of-plane van der Waals interaction between layers. Among the layered TMDs, MoS₂ and MoSe₂ have attracted the significant attention in the last few years due to their properties related to transistors [5,6], catalysis [8–10]and gas sensors [10]. They are also useful for hydrogen evolution catalysis. Theoretical and experimental studies indicate that the (0001) planes are catalytically inert, while (10–10) planes are catalytically active due to the low Gibbs free energy for hydrogen evolution reaction (HER) [11,12]. Various strategies have been employed to improve the HER either by creating much active edge sites [13–17]or by chemical modification with Fe, Co, Ni and Re [17,18]. The metastable 1T-phase exhibits improved catalytic activity for HER. Ternary composites of TMDs MoS_2(1−x)Se_2x [19,20] and MoS _x Cl _y [21] are reported to show improved performance compared with binary compounds in electrochemical HER. We have synthesized ternary WS _x Se_2−x (x = 0.0, 0.50, 1.0, 1.5, 2.0) and examined their photocatalytic HER activity. Interestingly, we found synergistic effect wherein the ternary sulphoselenides are superior catalysts compared with the binary compounds.

2 Experimental

Tungsten sulphoselenides, WS _x Se_2−x (x = 0, 0.5, 1, 1.5, 2.0), were synthesized by the solid-state reaction of a stoichiometric mixture of tungsten (W) (99.99 + % Sigma Aldrich) with sulphur (S) (99.99% Sigma Aldrich) and selenium (Se) (Sigma Aldrich) of 99.99% purity. The mixture was loaded in a high-quality quartz tube (12 mm OD, 10 mm ID, 22 cm length) and sealed under vacuum at 10⁻⁶ Torr. The sealed tube was introduced into a furnace and slowly heated to 850°C for over 15 h. The quartz tube was kept at the same temperature for 48 h and cooled slowly to room temperature. The products were also characterized by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer (Cu K _α ; λ = 1.54 Å). Raman spectra were recorded with 514 nm Argon laser using a Jobin-Yvon Labram HR spectrometer. The scanning electron microscope (SEM) images were taken using a NOVA nano-FESEM. The BET surface area measurement was calculated by N₂ adsorption using a Quantachrome Autosorb instrument at 77 K.

WS _x Se_2−x samples (x = 0.0, 0.50, 1.0, 1.5, 2.0) were dispersed in a solution of triethanolamine (TEAO, 15% v/v) in water. To this dispersion 0.014 mM of Eosin Y dye was added and the system purged with N₂. Hydrogen evolution studies were carried out with a 100 W halogen lamp under constant stirring. The evolved gas was analysed over a period using a Perkin Elmer 580 C Clarus GC-TCD.

Turnover frequency (TOF) was calculated using the equation

$$ \mathrm{TOF } \left( \mathrm{h}^{-1}\right)\,{=}\,\,\frac{\text{Activity of the catalyst}}{\text{Moles of the catalyst used}} $$

(1)

3 Results and discussion

XRD patterns confirm the hexagonal 2H-structure for all the WS _x Se_2−x compositions, with a prominent peak due to the (002) reflection (figure 1a). There is a shift in the reflection to lower angles with increasing substitution of sulphur by selenium (figure 1b). Typical SEM images shown in figure 2a and b show the flake-like morphology of the samples. The lateral size of the flakes is ∼10 μm, while the thickness is < 1 μm. The composition of the samples was ascertained by energy-dispersive spectra (EDS).

Figure 1

(a) X-ray diffraction (XRD) and (b) shift in the (002) peak in XRD of bulk WS _x Se_2−x (x = 0, 0.5, 1.0, 1.5, 2.0).

Figure 2

Typical scanning electron microscope image of the (a) horizontal and (b) cross-sectional view of the particles of the compound. The size and thickness of the particles are on an average ∼10 and ∼1 μm, respectively.

Raman spectra, recorded using a 514 nm unpolarized Ar laser with backscattered geometry (figure 3a), show two prominent first-order modes E_2g and A_1g modes at 358 and 423 cm⁻¹, respectively. The other first-order mode due to the longitudinal acoustic mode appears as a small hump at 177 cm⁻¹. WSe₂ shows a prominent band at 253 cm⁻¹ due to the out-of-plane second-order mode of the longitudinal 2LA mode at 251 cm⁻¹, which overlaps with the E$^{1}_{2\mathrm {g}}$ mode [22–24]. With the increase in Se content of WS _x Se_2−x , a red-shift is observed in the A_1g band but the E_2g band shows no significant change (figure 3b). Disorder in the crystallites induces some broadening of the Raman peaks [25,26].

Figure 3

(a) Raman spectra and (b) shift in the W–Se A_1g, W–S E_2g and W–S A_1g bands in Raman of bulk WS _x Se_2−x (x = 0, 0.5, 1.0, 1.5, 2.0).

BET surface areas of WS _x Se_2−x composition are in the 4–6 m² g⁻¹ range with type-III isotherm (figure 4). Catalytic activities of these compounds would, therefore, be independent of the surface area. Dye-sensitized photochemical reaction was performed using TEAO as a sacrificial agent with Eosin Y as a photosensitizer. Eosin Y, on the absorption of photon, undergoes a transition from a singlet to a lowest lying triplet state (EY^3∗) through intersystem crossing. EY^3∗ takes up an electron from TEAO and gets converted to EY⁻. EY⁻ is highly reactive and transfers the electron to the hydrogen evolution site [8,9]. The activities in the first 3 h of WS₂ and WSe₂ were 691 and 1732 μmol h⁻¹ g⁻¹ of hydrogen with TOF of 0.17 and 0.59 h⁻¹, respectively. The activities of the solid solutions of WS_1.5Se_0.5, WS_1.0Se_1.0 and WS_0.5Se_1.5 compounds were 880, 1567 and 1202 μmol h⁻¹ g⁻¹ with TOF of 0.24, 0.36 and 0.38 h⁻¹, respectively (figure 5a). The activity of the catalyst increases with further addition of the dye, where activities of WS₂, WS_1.5Se_0.5, WS_1.0Se_1.0, WSe_0.5 S _1.5 and WSe₂ are 866, 955, 2339, 1491 and 1480 μmol h⁻¹ g⁻¹ with TOF of 0.21, 0.26, 0.70, 0.47 and 0.51, respectively. The activity of bulk WSe₂ (1732 μmol h⁻¹ g⁻¹) is slightly higher than that of WS_1.0Se_1.0 (1567 μmol h⁻¹ g⁻¹) in the first 3 h but decreases after 3 h. The activity of WSSe is 2339 μmol h⁻¹ g⁻¹, whereas it is 1480 μmol h⁻¹ g⁻¹ for WSe₂ (figure 5a and b). However, we observe a

Figure 4

The BET N₂ adsorption isotherm of WS₂, WSe₂ and WS_1.0Se_1.0.

Figure 5

Comparative study of hydrogen evolution with WS _x Se_2−x (x = 0, 0.5, 1.0, 1.5, 2) (a) over a period of time. The bar graphs compare (b) the activity and (c) TOF.

decrease in the activity of WSe₂ on further addition of dye after 3 h, suggesting that it will not be an optimal choice as a long-term catalyst. Figure 5b and c and table 1 show comparative activities and TOF of these compounds, where we can clearly see the enhanced hydrogen evolution (2–3 times higher than that from WS₂) of the solid solution with a maximum at WS_1.0Se_1.0, which indicates synergistic effect. The introduction of selenium into WS₂ induces a slight distortion in the structure because of the larger radius of selenium as compared with S. This could facilitate bond breaking of the molecules adsorbed on the basal plane. The basal plane with unsaturated bonds provides favourable hydrogen binding energy from the tensile regions on the surface, with active sites mainly located in the basal plane [27,28]. Previous studies of alloys of Se, S and W suggest that distortions can make the surface active and thus provide more sites for hydrogen samples compared with the binary compounds [29–31]. Hydrogen evolution was stable for 5 cycles (over a period of 15 h) as shown in figure 6a. There was no change in the sample before and after the HER (figure 6b and c).

Table 1 Comparison of hydrogen evolution with bulk WS _x Se_2−x .

Figure 6

The cyclic study of photocatalytic HER on WS_1.0Se_1.0. The SEM images (a) before and (b) after photocatalytic HER. The morphology is unchanged after the cycling studies.

4 Conclusions

In summary, we have synthesized layered binary solid solution of WS₂ and WSe₂ by a sealed tube reaction and characterized it appropriately. Visible-light dye-induced photocatalytic study indicates higher hydrogen evolution studied in WS _x Se_2−x compared with the parent binary counterparts, showing maximum yield at WSSe. The result suggests a synergistic effect of substitution of Se in the HER activity of the solid solutions.

Highlights

• 1. Bulk synthesis of ternary transition metal dichalcogenides, tungsten sulphoselenide

• 2. Composition-dependent X-ray diffraction patterns of sulphoselenide of tungsten

• 3. Composition-dependent Raman

• 4. Photochemical hydrogen evolution

• 5. Role of anion substitution in hydrogen evolution of tungsten sulphoselenides.