Efficient photocatalytic hydrogen production over eosin Y-sensitized MoS₂

Abstract

MoS₂ is considered as a promising alternative to noble metals for H₂ evolution. In this work, a MoS₂ catalyst was synthesized via a simple and feasible solvothermal process and characterized by XRD, SEM, TEM and XPS techniques. Though the thus-obtained MoS₂ was stabilizing agent-free and composed of relatively large agglomerated particles, it shows superior hydrogen production performance under sensitization of eosin Y (EY), significantly different form earlier reports. Under optimized conditions, the hydrogen production rate was high, up to 35 mmol h⁻¹ g⁻¹, much higher than those of previous reports. We attribute this to its relatively high surface roughness and the small thin petal-like nanosheets constituting the MoS₂, which result in more active sites for hydrogen evolution. Meanwhile, the obtained MoS₂ shows excellent photostability with the H₂-evolving activity remaining nearly unchanged after 4 cycles.

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1 Introduction

Hydrogen has been considered as an ideal candidate to replace fossil fuels owing to its high calorific value and non-polluting characteristics.^1,2 Among various methods, solar splitting of water to hydrogen has gained tremendous interest for decades.^3,4 It is well-known that in general, a catalyst is necessary in order to split water efficiently and an ideal catalyst must be efficient, durable, and cost-effective. However, so far the majority of photocatalytic systems have employed expensive precious metals, such as Pt, Pd, Rh, as catalysts. Their scarcity and high-cost limit their application on an industrial scale.^5,6 Therefore, considerable efforts have been invested in developing new hydrogen evolution reaction (HER) catalysts that are highly active, stable, earth-abundant and environmentally friendly.^7–10 Up to now, in addition to biomimetic catalysts, such as hydrogenases and their mimics,^11,12 cobaloximes^13,14 and nickel thiolates,^15,16 relatively stable inorganic HER catalysts, such as NiS¹⁷ and MoS₂ ¹⁸ have gained great interest due to their low cost and better photostability than the above-mentioned biomimetic catalysts.

Molybdenum disulfide (MoS₂), acting as a high-performance commercial dehydrosulfurization catalyst,¹⁹ has also been exploited in electrochemical and photochemical hydrogen production processes and much progress has been made in recent years.^20–26 Although MoS₂ itself does not possess photocatalytic H₂ generation activity, it has been proven to be an efficient HER catalyst when combined with a semiconductor or photosensitizer. Heretofore, much research has been focused on hydrogen evolution of MoS₂ loaded on different supports, such as CdS, TiO₂, ZnS and graphene (G).^27–32 For example, eosin Y (EY)-sensitized MoS₂/RGO (RGO = reduced graphene oxide) showed a hydrogen evolution rate of 88.5 μmol h⁻¹ in 80 mL of 15% (v/v) TEOA aqueous solution (pH = 9.0) under visible light irradiation (λ ≥ 420 nm) with 0.4 mM EY and 20 mg catalyst.³³ Furthermore, as far as we know, for the above MoS₂-based photocatalytic hydrogen production systems, the highest H₂ evolution activity was 9.0 mmol h⁻¹ g⁻¹ over 2.0 wt% MoS₂/G-CdS composite.³⁰

In contrast, investigations on employing unsupported MoS₂ as H₂-evolving catalyst in photochemical systems are rare. In 2009, Zong et al. discussed the hydrogen evolution activity of poly(vinyl pyrrolidone) (PVP)-stabilized colloidal MoS₂ nanoparticles (NPs). In a 2 : 1 acetonitrile–methanol solution (150 mL), about 900 μmol of H₂ was generated within 6 h under visible light irradiation when 12.5 μmol MoS₂ and 20 μmol [Ru(bpy)₃]²⁺ were used as catalyst and sensitizer, respectively.³⁴ In addition, in 2014, Yuan et al. also investigated the hydrogen production activity of PVP-stabilized colloidal MoS₂ NPs in a 1 : 1 methanol–water solution (100 mL) when a series of cyclometalated Ir(III) complexes were used as sensitizers. About 1923 μmol H₂ was evolved after 20 h with 100 μM MoS₂ and 100 μM sensitizer.³⁵ However, for these two homogeneous systems,^34,35 several important points are worth noting. First, similar to previous results,^22,23 only “soluble” colloidal MoS₂ NPs demonstrated unique catalytic activities, bulk MoS₂ did not exhibit any photocatalytic activity. Second, the MoS₂ used in the above two systems were both stabilized by PVP and the uncapped large MoS₂ clusters (8–10 nm) had no H₂-evolving activity at all. Third, not only the above two hydrogen production reactions were conducted in organic solvents, but also the sensitizers were both noble metal complexes.

Besides the above problems, for the synthesis of MoS₂, though various methods including ultrahigh-vacuum processing, high-temperature treatment,²² sulfidization using H₂S gas,³⁶ electrodeposition³⁷ and photodeposition³⁸ have been developed, it is still a challenge to achieve highly efficient MoS₂ catalyst through a relatively facile, environmentally friendly and scalable method. Then in recent years, much research has been focused on the synthesis of MoS₂ via simple and feasible hydro(solvo)thermal method.^{28,30,31,33–35,39} However, more research is necessary because it is well-known that the property of a material is related to its structure, and likewise, the structure is related to the synthetic procedure and parameters, such as the kind and molar ratio of raw materials, the reaction temperature and time, and the solvents, etc.

Bear all above points in mind, in this paper, a MoS₂ catalyst was successfully prepared by the solvothermal method and its hydrogen production performance was evaluated in a noble-metal-free aqueous solution using readily accessible and cheap organic dye, EY as sensitizer. The key factors affecting its hydrogen production activity and the photostability of the obtained MoS₂ were investigated in detail. Surprisingly, the results demonstrated that the thus-prepared MoS₂ exhibits superior H₂-evolving activity and excellent photostability, though it is uncapped and composed of agglomerates up to 1 μm in diameter.

2 Experimental

2.1 Preparation of catalyst MoS₂

All reagents were of analytical grade and used without further purification. The water used throughout all experiments was deionized water purified through a Millipore system.

The MoS₂ catalyst was synthesized by a solvothermal reaction of Na₂MoO₄·2H₂O and KSCN in a 1 : 1 (v/v) DMF/H₂O. For a typical synthesis, 1.0 mmol Na₂MoO₄·2H₂O and 3.0 mmol KSCN were dissolved in 60 mL DMF/H₂O under continuous stirring, and the pH value of the reaction system was adjusted to 1–2 by 1 : 1 HCl. Then the reaction mixture was stirred at room temperature for 2 h before transferred into a 100 mL Teflon-lined autoclave. After heated in an oven at 220 °C for 20 h, a black product was obtained through centrifugation, washing thoroughly with water and anhydrous ethanol, and drying overnight in a vacuum oven at 60 °C.

2.2 Material characterization

X-ray diffraction (XRD) measurements were conducted on a Rigaku MiniFlex X-ray diffractometer. UV-vis diffuse reflectance spectra were recorded on a Meipuda UV-1800PC UV-visible spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Escalab 250XI X-ray photoelectron spectrometer with a monochromatic X-ray source. Transmission electron microscopy (TEM) of the catalyst was determined using an FEI Tecnai G2 F20 microscope operated at 200 kV. Field-emission scanning electron microscopy (FESEM) images were obtained on a FEI Quanta 200F microscope operated at 5 kV equipped with an energy-dispersive spectrometer (EDS). Photoluminescence (PL) spectra were measured on a Hitachi F-4500 fluorescence spectrometer. The surface area of MoS₂ was obtained by the Brunauer–Emmett–Teller (BET) nitrogen adsorption method on a Micromeritics ASAP 2420 accelerated surface area and porosimetry system.

2.3 Photocatalytic activity evaluation

The photocatalytic reactions were carried out in a sealed quartz flask (230 mL) with a flat window and a silicone rubber septum for sampling. A 300 W xenon arc lamp equipped with a 420 nm cutoff filter (10 cm away from the reactor) was used as a visible light source. In a typical experiment, 25 mg (0.156 mmol) MoS₂ and 40 μmol (0.4 mM in the reaction mixture) EY were dispersed in 100 mL aqueous solution of 15% (v/v) TEOA by ultrasonication for 25 min. Then the pH value of the TEOA solution was adjusted to the required value by 1 : 1 HCl. Prior to irradiation, the suspension was thoroughly degassed by bubbling nitrogen gas for 20 min to remove the dissolved oxygen. During the whole reaction, a continuous magnetic stirring was used to maintain the homogeneity of the reaction mixture and a water circulation system was used to keep the temperature at about 25 °C. The H₂ evolved was measured by a gas chromatograph (GC, Agilent 4890D) equipped with a thermal conductivity detector (TCD) and a porapak Q stainless column. Nitrogen was served as the carrier gas at a flow rate of 20 mL min⁻¹. All of the H₂ evolution experiments were repeated three times for reliability of the results (±standard deviation).

2.4 Quantum efficiency measurement

A 300 W xenon lamp loaded with a bandpass filter (460 nm) was used for quantum efficiency measurements. The quantum yield (QY) was calculated by following equation: QY = N_e/N_p = 2N_H2/N_p, where N_e, N_p and N_H2 are the numbers of reacted electrons, incident photons and evolved H₂ molecules, respectively. N_p = t × S × Q, where t is the irradiation time, S is the effective light irradiation area, and Q is the photon flux of the incident light. An optical meter (Newport, 840-C) was used for measuring the light intensity, from which the number of incident photons was calculated.

3 Results and discussions

3.1 Synthesis of MoS₂

Herein, a solvothermal method was adopted for the synthesis of MoS₂ with Na₂MoO₄ and KSCN as the starting materials. During the reaction process, MoO₄²⁻ was reduced to MoS₂ by KSCN, which plays the roles of reducing agent and sulfur donor by releasing H₂S.⁴⁰ On the basis of the common knowledge, the reaction routes could be expressed as follows:

KSCN + 2H₂O + HCl = NH₃ + H₂S + CO₂ + KCl (1)

4Na₂MoO₄ + 9H₂S = 4MoS₂ + Na₂SO₄ + 6NaOH + 6H₂O (2)

3.2 MoS₂ characterizations

The powder X-ray diffraction pattern of the as-prepared MoS₂ is shown in Fig. 1. The low and broad diffraction peaks show that the MoS₂ as-prepared is poorly crystalline.⁴¹ In addition, it can be seen that the (002) diffraction peak at about 14.1° is absent, while the (100) and (110) diffraction peaks located at about 33.6 and 58.8° are still retained, indicating the successful formation of MoS₂ and a low stacking height along the c axis for the synthesized MoS₂.⁴² Furthermore, according to literature,⁴² the thus-prepared MoS₂ can be indexed as hexagonal 2H-MoS₂ (JCPDS 37-1492).

Fig. 1 XRD patterns of the MoS₂ as-prepared and recovered after four runs of H₂-evolving reaction.

To reveal the surface morphology of the prepared MoS₂, SEM measurements were performed at different magnifications. A general view (Fig. 2a) shows that the MoS₂ sample displays a nonuniform morphology, which is composed of agglomerated particles with the size ranging from about 100 nm to 1 μm. Higher magnified SEM images (Fig. 2b and c) reveal that these agglomerates are constituted by small NPs of about 10–30 nm in diameter with nonuniform shape and size and their surfaces are quite rough. Simultaneously, we tried our best to characterize the microstructure and morphology of the MoS₂ sample by TEM technique. From the TEM and HRTEM images (Fig. 2d–f), on one hand, it can be seen that almost similar to the SEM results, the as-prepared MoS₂ is composed of irregularly shaped NPs (around 10–50 nm in diameter). However, the small NPs agglomerate together to form larger clusters. In other words, serious aggregation occurs, which results in a relatively bad dispersion of the sample in the TEM determination. On the other hand, from the HRTEM image (Fig. 2f), lots of short and disordered lattice fringes with a spacing of around 0.66 nm corresponding to (002) plane of hexagonal-phase MoS₂ can be observed, which indicate that the obtained MoS₂ is layered structure. Then combining TEM results with the above SEM ones, we can infer that the obtained MoS₂ is made up of small NPs, which stack each other to form large aggregated particles. In addition, every small NP is composed of small thin and curving nanosheets, looking like petals in morphology. Besides this, the fuzzy feature of lattice fringes shows the low crystallinity of MoS₂ agreeing well with the XRD results. However, it is a pity that no more high-quality TEM and HRTEM images were obtained due to the very serious aggregations. Furthermore, the composition of as-prepared MoS₂ was preliminarily determined by EDS (see Fig. S2a†), which shows that it is mainly composed of Mo, S and a quantity of C, N and Cl, and the calculated atomic ratio of S to Mo is about 2.01, further verifying the product is MoS₂. The element Cl should originate from HCl used to adjust the pH value of the reaction mixture, and the C and N come from the reaction materials of solvothermal process.

Fig. 2 SEM (a–c), TEM (d and e) and HRTEM (f) images of the MoS₂ sample as-prepared.

In order to investigate the chemical states of Mo and S in the obtained MoS₂ sample, high-resolution XPS spectra of Mo 3d and S 2p were also determined, as given in Fig. 3. For the Mo 3d spectrum (Fig. 3a), peaks located at 229.3 and 232.5 eV can be ascribed to Mo 3d_5/2 and Mo 3d_3/2, respectively, and the peak at 226.4 eV corresponds to S 2s. The S 2p spectrum (Fig. 3b) consists of a doublet with binding energies of 162.1 (S 2p_3/2) and 163.6 eV (S 2p_1/2). All this indicates the presence of Mo⁴⁺ and S²⁻ in the MoS₂ sample.^37,40 The surface element contents derived from XPS quantification are 44.3% for S and 22.8% for Mo, respectively, which is close to a Mo to S atomic ratio of 1 : 2, consistent with the above EDS result. In addition, UV-vis diffuse reflectance spectrum of the MoS₂ prepared was determined and a wide absorption in the region of from 400 to 800 nm was observed (Fig. S4†), consistent with literature result.⁴³

Fig. 3 High-resolution XPS spectra of Mo 3d (a) and S 2p (b) of the MoS₂ as-prepared.

3.3 Photocatalytic hydrogen production

It is well known that the hydrogen-evolving conditions, such as the pH value of the reaction system, the concentrations of sensitizer, sacrificial reagent and the catalyst play crucial roles on the hydrogen-evolving activity, so various experimental parameters were studied in detail at first.

Fig. 4 depicts the time courses of H₂ generation with different pH values. It can be seen that the highest rate of 876 μmol h⁻¹ was obtained at pH 7. Either a lower or higher pH value leads to a significant decrease in the H₂ evolution efficiency with the maximum H₂ evolution rate of 68 μmol h⁻¹ at pH 5 and 380 μmol h⁻¹ at pH 9, respectively, consistent with previous results.^33,44 This phenomenon can be explained as follows. At a lower pH value, the protonation of TEOA will reduce its ability to act as an effective electron donor and TEOA⁺ decomposition will become less facile.⁴⁵ On the other hand, a higher pH value will make a lower proton concentration in solution, and then the H₂ evolution reaction becomes more thermodynamically unfavorable.⁴⁶

Fig. 4 Time courses of photocatalytic H₂ evolution as a function of the pH value (MoS₂ = 25 mg, EY = 0.4 mM, TEOA = 15 vol%).

The effects of EY and TEOA concentrations were also investigated. As shown in Fig. 5a, the optimal EY concentration is 0.4 mM. A further increasing or decreasing both leads to a decrease in the hydrogen production activity. This phenomenon is probably because that at lower EY concentrations, the EY molecules adsorbed on MoS₂ surface increase with EY concentration increasing, which benefits for improving the light absorption efficiency,^47,48 however, at relative high EY concentrations, shielding effect of free EY molecules in solution as well as concentration quenching would occur,⁴⁹ which will result in a decrease in the electron injection efficiency and then in the hydrogen production efficiency.^50–53

Fig. 5 Time courses of photocatalytic H₂ evolution as a function of (a) EY²⁻ (MoS₂ = 25 mg, TEOA = 15 vol%, pH = 7) and (b) TEOA (MoS₂ = 25 mg, EY = 0.4 mM, pH = 7) concentrations.

Similarly, Fig. 5b indicates that the H₂ evolved gradually increases with increasing the TEOA concentration from 10% to 15% and the highest rate of 876 μmol h⁻¹ was achieved at 15%, whereas it decreases to 582 μmol h⁻¹ at 20%, consistent with literature results.^45,48,54 According to literatures,^55,56 this is probably because that in the present system, TEOA can be oxidized by photo-generated holes (h⁺) on EY molecules to form TEOA⁺. As a result, the increase in TEOA concentration makes more electrons transferred from TEOA to EY, and then faster reductive quenching of the excited EY molecules.

In addition, the influence of the catalyst concentration on hydrogen evolution performance was also discussed. As demonstrated in Fig. 6, increasing the catalyst concentration results in an increase in the hydrogen evolution rate, while the amount of H₂ evolved does not scale-up linearly with the catalyst amount. Specifically, increasing the MoS₂ amount from 12.5 to 25 mg makes an increase of the hydrogen evolution rate from 429 to 876 μmol h⁻¹, while when 50 and 75 mg MoS₂ were used, the hydrogen evolution rate was only 1202 and 1380 μmol h⁻¹, respectively. Moreover, the system lifetime was prolonged to some extent when a higher catalyst concentration was used, possibly due to the stability increase of sensitizer EY.⁵⁷ Except for the above experiments, a series of control experiments were also conducted and the results revealed that no H₂ was detected either in the absence of EY, MoS₂, TEOA, or when the reaction was performed in the dark, suggesting this reaction is a photocatalytic process.

Fig. 6 Time courses of H₂ evolution as a function of MoS₂ amount (EY = 0.4 mM, TEOA = 15 vol%, pH = 7).

Furthermore, in view of photosensitizer (PS) also plays a crucial role in the dye-sensitized H₂-generating systems,⁵⁸ other four xanthene dyes, eosin B (EB), rose bengal (RB), erythrosin B disodium salt (EB²⁻) and rhodamine B (RhB) with different substitutions on the xanthene ring have also been employed to investigate the effect of PS species on the hydrogen-evolving activity of MoS₂. As demonstrated in Fig. 7, the PS species significantly affect the H₂ generation activity and the highest H₂ evolution rate of 876 μmol h⁻¹ was obtained for EY. Concretely, the activity order is EY > EB²⁻ > RB > EB > RhB, similar to the results reported in literatures.^46,48,59 In addition, under the optimal condition of 25 mg MoS₂, 0.4 mM EY, 15% TEOA and pH 7.0, the quantum yield at 460 nm achieved to 22.8%.^33,60

Fig. 7 (a) Time courses of hydrogen production and (b) the amounts of hydrogen evolved in 2.5 h with different sensitizer species (MoS₂ = 25 mg, sensitizer = 0.4 mM, TEOA = 15 vol%).

From the above results, we also found that for the EY–MoS₂–TEOA system, hydrogen generation ceased after about 5 h. To elucidate the deactivation reasons, more experiments were performed and the results are displayed in Fig. 8. Firstly, the photocatalytic stability of MoS₂ was determined by a recycling test (Fig. 8a). It can be seen that after four consecutive cycles, there is no significant decrease in the H₂-evolving activity and more than 90% of the activity is still retained with respect to the first cycle, indicating that the MoS₂ catalyst is stable during the photocatalytic reaction.

Fig. 8 (a) Recycling H₂ evolution test for MoS₂ catalyst (MoS₂ = 25 mg, EY = 0.4 mM, TEOA = 15 vol%, pH = 7. After each cycle, fresh EY and TEOA were added to the recovered MoS₂ sample). (b) UV-vis absorption spectra of the reaction solutions before and after 5.5 h irradiation (the MoS₂ catalyst in reaction mixture was removed by sedimentation before analysis).

To further clarify this, the MoS₂ sample recovered after four cycles was also characterized by SEM, XPS and EDS, and the results were compared with those of freshly prepared MoS₂. It can be found that no significant changes occurred in the size and morphology of the MoS₂ catalyst after the photocatalytic reaction, though a little more aggregations happened, as demonstrated in Fig. S1a–c.† In addition, the EDS and XPS results showed that the atomic ratio of S to Mo was also unchanged (see Fig. S2 and S3†). Concretely, for the recovered MoS₂ sample, the elemental contents determined by XPS are 41.6% for S and 21.2% for Mo respectively, namely, the atomic ratio of Mo to S is still 1 : 2. Furthermore, no obvious changes were observed for the UV-vis spectrum of the MoS₂ recovered after reaction (see Fig. S4†). So the deactivation of the present photocatalytic system could be mainly attributed to the loss of EY under the irradiation of visible light.⁵⁸ This assumption was further supported by the changes on the absorption spectra of the solutions before and after reaction. As indicated in Fig. 8b, the absorption intensity of EY decreases drastically together with an obvious blue shift of the characteristic absorption peak after reaction, which is associated with the loss of Br atoms from EY molecule,³³ namely, the photodegradation of EY molecules happened. On the other hand, the color change from red to green-yellow of the reaction solution during the photocatalysis process further verified the above assumption.

3.4 Photocatalytic H₂ evolution mechanism

As mentioned above, though MoS₂ is a promising candidate for solar hydrogen generation, it alone has no photocatalytic activity,^27,61 whereas under the existence of PS, it was found to exhibit excellent H₂-evolving performance.^33–35 For the present EY–MoS₂–TEOA system, the H₂ evolution reaction should possibly proceed as shown in Scheme 1. At first, the PS EY is excited under the illumination of visible light. Then the excited *EY molecules inject electrons to the conduction band (CB) of MoS₂ to initiate the catalytic reaction. In order to regenerate EY, sacrificial reagent TEOA was added to sustain the reaction cycle.^33,60,62

Scheme 1 Proposed photocatalytic H₂ evolution mechanism under visible light irradiation (a) and energy level (b) for the EY–MoS₂–TEOA system.

In order to verify the above hypothesis, photoluminescence (PL) spectra of EY and EY–MoS₂ in TEOA solution were determined. It can be seen that the fluorescence (FL) intensity of EY–MoS₂ is much weaker than that of EY (Fig. S5†), whereas the FL of EY can't be quenched by adding TEOA,³³ implying an efficient electron transfer from the singlet excited state ¹*EY to MoS₂, namely, an oxidative quenching occurs. However, on the other hand, according to literature,³³ the lowest-lying triplet excited state ³*EY, generated form singlet excited state ¹*EY via efficient intersystem crossing, can be reductively quenched by TEOA and produce EY⁻˙ with reductive potential of −0.8 V (vs. NHE),^33,63 which can also transfer electrons to the CB of MoS₂ (ca. −0.50 V vs. NHE).⁶⁴ So in the present EY–MoS₂–TEOA H₂-evolving system, the electron transfer involves both oxidative quenching and reductive quenching.

3.5 Primary elucidation on the H₂-evolving activity

Based on the above results, it is worth noting that first of all, a high H₂ production performance of 876 μmol h⁻¹ (35 mmol h⁻¹ g⁻¹) was obtained for the EY-sensitized MoS₂, which is much higher than those of previous reports.^33,65 For a similar EY–MoS₂ system in literature,³³ the H₂-evolving rate of was only 37.1 μmol h⁻¹ under almost similar conditions of 20 mg MoS₂ catalyst, 0.4 mM EY and 15 vol% TEOA aqueous solution. In addition, the hydrogen production activity of EY-sensitized single layer MoS₂ system was only 592.71 μL h⁻¹ (equal to about 26.46 μmol h⁻¹) under the same conditions.⁶⁵ Second, and more important, the MoS₂ catalyst herein is composed of relatively large agglomerates and stabilizing agent-free, significantly conflicting with previous reports,^33,34 which concluded that the particle size and dispersion of MoS₂ NPs played key roles in its catalytic activity, and only PVP-stabilized colloidal MoS₂ NPs exhibited excellent catalytic activity, namely, no H₂ was detected for the MoS₂ in bulk powder probably owing to the size-dependent energy shift.⁶⁶ To account for this phenomenon, BET specific surface area of the as-prepared MoS₂ was first measured. The result showed that it was only 6.66 m² g⁻¹, very close to that of bulk MoS₂ (6 m² g⁻¹),⁶⁷ implying that the surface area is not a vital factor for the superior activity of MoS₂. Therefore, we propose that the surface morphology of MoS₂ probably plays a crucial role in its catalytic activity. As far as we know, the origin of the H₂-evolving activity of MoS₂ is closely related to the amount of edge sites and correlated with the number of unsaturated S atoms on the edges, which enable them to absorb and release hydrogen, while the (002) basal planes of MoS₂ are catalytically inert.^21,22,68 So the quite rough surface and small thin petal-like nanosheets of the as-prepared MoS₂ may offer more active edge sites for hydrogen evolution.

4 Conclusion

In summary, different from literature results, uncapped and agglomerated MoS₂ exhibits superior hydrogen production performance (35 mmol h⁻¹ g⁻¹) in an aqueous system with EY as sensitizer. The excellent activity could be attributed to its relatively rough surface and lots of petal-like nanosheets, which increases the number of active edge sites. In addition, the MoS₂ exhibits good durability in the photocatalytic process and the deactivation can be mainly ascribed to the photolysis of sensitizer EY. More importantly, compared with the colloidal or nanosized MoS₂ samples, the MoS₂ as-prepared is more suitable for practical application due to its nature of easy recovery. We believe that this study will provide more information for the design of efficient MoS₂-based catalysts and hydrogen evolution systems.

Acknowledgements

This research was financially supported by the National Science Foundation of China (No. 21171147).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14710k

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