MoS₂ quantum dot decorated g-C₃N₄ composite photocatalyst with enhanced hydrogen evolution performance

Abstract

A novel MoS₂ quantum dot (QD) decorated g-C₃N₄ (MoS₂-QDs/CN) heterostructured photocatalyst has been synthesized via a simple wetness impregnation method. The as-prepared MoS₂-QDs/CN composite exhibited strong absorption in the visible light region. Its photocatalytic activity on hydrogen evolution was evaluated by splitting water under visible light irradiation (λ > 420 nm). The highest H₂ yield of 19.66 μmol h⁻¹ was achieved on 50 mg 1 wt% MoS₂-QDs/CN photocatalyst (loaded with 1 wt% Pt), which was 6.6 and 1.84 times those on pure g-C₃N₄ and 1 wt% Pt loaded g-C₃N₄, respectively.

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1 Introduction

The giant gap between the rapid development of industrialization and the efficiency to harness fossil fuel has led to the imminent energy crisis.¹ Furthermore, the continuous consumption of fossil fuel has caused ever-increasing environmental pollution. Therefore, the search for clean energy has become the global research emphasis. Hydrogen, with the fuel value (∼143 kJ g⁻¹) three times higher than gasoline and the characteristics of environment friendliness and transportation convenience, is a possible candidate to replace gasoline.^2,3 In 1972, Fujishima and Honda discovered a novel photocatalytic hydrogen evolution method.⁴ Since then, photocatalysis has been considered as one of the most promising strategies to achieve green and low-cost energy.

Most of the studies on photocatalysis have been focused on the preparation of efficient photocatalysts. TiO₂ has been once considered as an option for efficient photocatalyst over the past decades. But the over-wide band-gap of TiO₂ (3.2 eV) limits its absorption range in the ultra violet region. Then in 2009, Antonietti et al. first reported the visible light photocatalytic activity of graphitic carbon nitride (g-C₃N₄),⁵ making it a promising candidate for efficient photocatalysis.

The discovery of g-C₃N₄ can be dated back to 1834, when g-C₃N₄ was synthesized by Berzelius in 1834 and named as “melon” by Liebig.⁶ In 1922, Franklin did further research on melon and firstly introduced the concept of carbon nitride.⁷ g-C₃N₄ is the most stable form of carbon nitride, in which carbon and nitrogen atoms are sp² bonded, leading to the π-conjugated, 2-dimensional graphite-like structure.^8–10 Compared with other inorganic semiconductor photocatalysts, g-C₃N₄ is more stable with respect to thermal and chemical attack. Also, low health and environment risks make it safer to put into application. Beyond these, g-C₃N₄ has an appealing electronic structure (band gap = ∼2.7 eV, conduction band at ∼−0.8 eV and valence band at ∼1.9 eV), which enormously widens its light adsorption range to visible region.

However, g-C₃N₄ still suffers problems such as low quantum efficiency and high rate of carrier recombination, which have enormously affected the photocatalytic performance of pure g-C₃N₄. Series of methods have been tried to improve its photocatalytic performance, including metal doping,^11,12 graphene¹³ and semiconductor^14,15 coupling, copolymer constructing^16,17 etc.

Molybdenum disulfide (MoS₂) has recently become an attractive material in photodetector,¹⁸ photovoltaic cells,¹⁹ electrocatalyst²⁰ and photocatalyst,¹⁵ due to its high mobility at room temperature (∼410 cm² V⁻¹ s⁻¹) with on/off ratio of 10⁸ and excellent optical absorption property. The S–Mo–S coordination in the crystal lattice generates unsaturated Mo and S atoms at the edge (Fig. S1†), leading to the unique “edge activity” of MoS₂.^21,22 MoS₂ achieved large enhancement in electrocatalytic H₂ evolution reaction (HER) when it was hybridized with Au,²³ and graphene.^24,25 MoS₂ has been fabricated into different forms to achieve this goal, such as nanoparticles,²⁶ nanosheets,²⁷ and nanoflowers.²⁸ Above them, the higher quantum confinement and small-size effect of MoS₂ quantum dots (MoS₂-QDs) leads to unsaturated bonds both on surfaces and edges, providing more opportunity to connect with other atoms and obtain higher activity toward HER.²⁰ Also, MoS₂-QDs can gather the photo-induced electron around the edges and efficiently prevent charge recombination. Therefore, MoS₂-QD is a promising candidate as cocatalyst in photocatalytic hydrogen evolution.

Herein, we synthesized MoS₂-QDs decorated g-C₃N₄ (MoS₂-QDs/CN) via a facile impregnation route. MoS₂-QDs were obtained via a simple sonication method from commercialized MoS₂ powder,²⁹ which is environment-friendly and cost-effective. The MoS₂-QDs/CN composite showed much enhanced photocatalytic activity for hydrogen evolution by water-splitting under visible light irradiation.

2 Experimental

2.1 Synthesis of MoS₂-QDs/CN photocatalyst

All reagents, including (NH₂)₂CO (AR, Sinopharm Chemical Reagent Corp, P. R. China), MoS₂ powder (99%, <2 μm, Sigma Aldrich) and N-methyl-2-pyrrolidinone (98%, Sigma-Aldrich) were used directly without any further purification.

The metal-free g-C₃N₄ powder was prepared by calcining 20 g of urea in an alumina crucible at 550 °C for 4 h with a ramp of 5 °C min⁻¹. The product was collected and grinded into powder.

MoS₂-QDs synthesis involves bath sonication and probe sonication of MoS₂ powder.²⁹ 200 mg of MoS₂ was added into 20 mL of N-methyl-2-pyrrolidinone (NMP) and sonicated continuously in ultrasonic bath for 3.5 h. Then, the dispersion was further sonicated with a sonic probe for 3.5 h. The as-prepared dispersion was kept undisturbed overnight to settle down bulk MoS₂. The top two-thirds of the dispersion was sucked out and centrifuged at 5500 rpm for 90 min. Finally, the quantum dots were extracted and washed with distilled water and ethanol, and then re-dispersed in a certain amount of distilled water to obtain a 5 mg mL⁻¹ MoS₂-QDs suspension.

MoS₂-QDs/CN composite photocatalysts were prepared as follows: 0.3 g bulk g-C₃N₄ was dispersed in 300 mL deionized water and sonicated for 30 min. Certain amount of MoS₂-QDs suspension was added drop-wisely into the as-dispersed g-C₃N₄ and stirred for 24 h. After that, the product was collected by centrifugation, washed with ethanol, and dried at 70 °C for 24 h. Finally, the sample was calcined at 300 °C for 1 h and named as x% MoS₂-QDs/CN, where x = 0.5, 1, 3 and 7 wt, representing the mass percentage of MoS₂-QDs in the composite.

2.2 Characterizations

The crystal structure of MoS₂-QDs/CN composite photocatalysts was investigated by X-ray diffraction (XRD; Rigaku D/Max 2200PC) with Cu Kα radiation operated at 40 kV of acceleration voltage and 40 mA of applied current. The scanning rate was maintained at 4° min⁻¹ in the 2θ range of 10–80°. Infrared spectra were obtained on KBr pellets on a Thermofischer Nicolet IS50 spectrometer. The morphology of the samples was recorded by transmission electron microscopy (TEM; FEI Tecnai G²F20) operated at 200 kV. UV-Vis diffuse reflection spectroscopy was examined on a Shimadzu UV-3600 spectrometer, using BaSO₄ as reference. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific Escalab 250 spectrometer with Al Kα radiation line source and the binding energies for the high resolution spectra were calibrated by setting C 1s to 285 eV. The photoluminescence (PL) spectra of the samples were obtained by Edinburgh Instruments FLSP-920 fluorescence spectrophotometer with an excitation wavelength of 400 nm at room temperature.

2.3 Photocatalytic activity

The photocatalytic activity of MoS₂-QDs/CN photocatalyst for water-splitting hydrogen evolution was conducted in a glass reactor. The reactor was connected with gas chromatography (Techcomp GC7900, high purity nitrogen as carrier gas), and 300 W Xe lamp (λ > 420 nm) was used as light source. 50 mg of photocatalyst was dispersed in 90 mL of deionized water, along with 10 mL of triethanolamine. As for loading 1 wt% Pt on the photocatalysts, 1 mL of 0.5 g L⁻¹ H₂PtCl₆ was added into the dispersion. Then, the reactor was swept by nitrogen flow for 15 min to remove the dissolved oxygen. The dispersion was stirred and irradiated under 300 W Xe lamp for 1 h to reduce Pt species. Next, the reactor was swept again by nitrogen flow for 15 min to remove the additional hydrogen. During the photocatalytic experiment, 1.5 mL gas sample in the reactor was extracted and analyzed by gas chromatography every hour.

2.4 Electrochemical characterizations

The electrochemical performance of MoS₂-QDs/CN was tested on a CHI 660A electrochemical workstation (Chenhua Instrument, Inc.) in a standard three-electrode system consisting of Pt wire as counter electrode, Ag/AgCl (in saturated KCl) as reference electrode and sample-coated indium tin oxides (ITO) glass as working electrode. Na₂SO₄ (0.2 M) aqueous solution was used as the electrolyte. The working electrode was prepared as follows: 20 mg of the grinded sample was mixed with 40 mg iodine in a testing tube, followed by adding acetone to full and sonicating the slurry for 30 min. Then, the dispersion was transferred into a 20 mL beaker and two pieces of 2 cm × 0.5 cm ITO glass were sunk into the dispersion with 10 V applied voltage for 5 min. Finally, the ITO glass with sample film was calcined at 200 °C for 2 h. EIS curves were obtained at −0.4 V vs. Ag/AgCl. Irradiation used in the photocurrent tests was from a Xe lamp with a UV filter (λ > 420 nm) and obtained at 0.4 V vs. Ag/AgCl. Mott–Schottky plots were collected at a frequency of 962 Hz in dark.

3 Results and discussion

3.1 Characterizations of MoS₂-QDs/CN

The FT-IR spectra in Fig. 1 show that both the MoS₂-QDs/CN composite and pure g-C₃N₄ have peaks at 810 cm⁻¹, 1200–1690 cm⁻¹ and 3000–3300 cm⁻¹, which can be assigned to the graphite-like structure and stretching modes of C–N heterocylces.^13,30 For MoS₂-QDs/CN composite and MoS₂-QDs, there is a peak at 669 cm⁻¹ corresponding to MoS₂, indicating the successful incorporation of MoS₂ in the composite. The XRD patterns (Fig. 1c) also support this result. There are two diffraction peaks in the XRD pattern of pure g-C₃N₄. The strong peak at 27.6° represents the interlayer stacking of the conjugated aromatic compounds, indexing to the graphite-like material with the d-spacing of 0.323 nm. The weak one at 12.7° corresponds to the (100) plane of the melon structure with an interplanar separation of 0.696 nm. For MoS₂-QDs/CN composite, one strong peak at (002) plane and six other small peaks that fit well with MoS₂ (JCPDS 77-1716), can be observed. It can be concluded that the loading of MoS₂ has not destroyed the graphite-like structure of g-C₃N₄. However, in the previous study of Gopalakrishnan et al.,²⁹ only one peak of (002) plane was observed in the XRD pattern of MoS₂-QDs. The possible reason is that in our composite, some MoS₂-QDs form nanoclusters on the surface of g-C₃N₄ as observed by TEM (Fig. 2a). And yet considering the relatively stronger intensity of the (002) plane compared with the other six peaks, most MoS₂-QDs are still well dispersed on the surface of g-C₃N₄.

Fig. 1 (a and b) The FT-IR spectra of g-C₃N₄, MoS₂-QDs/CN and MoS₂-QDs; (c) XRD patterns of g-C₃N₄ and MoS₂-QDs/CN composite.

Fig. 2 (a and b) HRTEM images of 1 wt% MoS₂-QDs/CN composite.

The TEM image in Fig. 2a exhibits the micrograph of MoS₂-QDs/CN composite, where the sheet-like g-C₃N₄ is decorated with crystallized MoS₂-QDs. The diameter of MoS₂-QDs prepared by sonication has been well controlled at ∼10 nm and the crystallinity of MoS₂ has been well preserved, as revealed by the striation observed in the partially amplified image inserted in Fig. 2b. However, the increasing introduction of MoS₂-QDs has led to the agglomeration of MoS₂-QDs as observed in Fig. S2.†

The chemical composition and elemental states in 1 wt% MoS₂-QDs/CN composite and g-C₃N₄ was investigated by XPS (Fig. 3). The XPS survey spectrum of the composite is shown in Fig. 3a. Except for three peaks at 529.2 eV, 399.3 eV and 291.9 eV corresponding to O, N, and C, respectively, which can also be observed in survey spectrum of g-C₃N₄, two more peaks at the binding energies of 234.4 eV and 161.5 eV assigned to Mo and S can be observed.

Fig. 3 XPS survey spectra of (a) 1 wt% MoS₂-QDs/CN composite and (b) g-C₃N₄; high resolution XPS spectra of (c) C 1s, (d) N 1s of g-C₃N_​4; and the high resolution XPS spectra of (e) C 1s, (f) N 1s, (g) Mo 3d and (h) S 2p of 1 wt% MoS₂-QDs/CN composite.

The high-resolution C 1s and N 1s XPS spectra of the 1 wt% MoS₂-QDs/CN composite and g-C₃N₄ are shown in Fig. 3c–f. The C 1s peak in Fig. 3c and e can be deconvoluted into two peaks with binding energies of ∼288.0 eV and ∼284.6 eV, which can be assigned to carbon sp² bonded to the three nitrogen atoms in triazine rings³⁸ and the surface graphitic or amorphous carbons^39,40 respectively. Both N 1s spectra in Fig. 3d and f exhibit an asymmetrical peak, indicating the existence of nitrogen atoms in multiple chemical-environment. The N 1s peak of pure g-C₃N₄ can be deconvoluted into three fitting Gaussian peaks at 401.0 eV, 399.7 eV and 398.5 eV. The former two peaks can be attributed to the conjugated nitrogen atom⁴¹ and amino groups.⁴² The peak at 398.5 eV corresponds with the nitrogen atom sp² bonded to two carbon atoms, further confirming the sp²-bonded graphite-like structure of g-C₃N₄.¹⁰ After MoS₂-QDs is coupled with g-C₃N₄, the two peaks of the conjugated nitrogen atom and the nitrogen atom of amino groups shift to 400 and 400.9 eV, respectively, revealing the formation of substantial interface and the intimate interaction between MoS₂ and g-C₃N₄. The peak at 398.5 eV is identically same as pure g-C₃N₄, suggesting that the triazine structure of g-C₃N₄ has been well-preserved after the introduction of MoS₂-QDs.

In Mo 3d spectrum of 1 wt% MoS₂-QDs/CN composite (Fig. 3g), three peaks at 235.3 eV, 232.0 eV and 228.6 eV can be observed. The Mo 3d_3/2 peak at 235.3 eV is attributed to Mo⁶⁺ in MoO₃.⁴³ The latter Mo 3d_5/2 peaks at 232.0 eV and 228.6 eV correspond to the Mo⁶⁺ in MoO₃ ⁴⁶ and Mo⁴⁺ in MoS₂,⁴⁴ respectively. The S 2p spectrum in Fig. 3h shows a doublet located at 161.5 and 162.5 eV corresponding to the S 2p_3/2 strongly adsorbed to MoS₂ (110)⁴⁵ and the S 2p_3/2 in MoS₂,⁴⁴ respectively. Both the high resolution spectra of Mo 3d and S 2p confirm the existence of MoS₂-QDs in the composite.

The optical absorption of g-C₃N₄, MoS₂-QDs and MoS₂-QDs/CN composite was investigated by UV-Vis absorption spectra (Fig. 4). The absorption edge of g-C₃N₄ is at ∼450 nm, corresponding to the band gap of 2.65 eV. And the absorption intensity of MoS₂-QDs triumphs over all other samples. The composite samples' absorption edges slightly red-shift. The excitonic peak at 660 nm arising from the K point of the Brillouin zone of MoS₂ can be clearly observed both in the composite samples and in MoS₂-QDs, indicating MoS₂-QDs has been successfully decorated on the surface of g-C₃N₄.²⁹ The absorption of the composite samples intensifies with the increasing content of MoS₂-QDs, suggesting the close connection between the introduction of MoS₂-QDs and the improved light absorption of the MoS₂-QDs/CN composite. And the band-gap results shown in Table S1† exhibit the decreased band-gap of the composite with the increasing content of MoS₂-QDs. Previous studies exhibit that nanostructured semiconductor usually has larger band-gap than its bulk counterpart because of quantum confinement effect and consequently makes the adjusting of its absolute CB and VB potentials.^31–34 According to TEM observation, higher content of MoS₂-QDs is more likely to form nanoclusters, leading to narrower band-gap of the quantum dots and thus shortening the band-gap of the composite probably by shrinking the potential difference between the CB edge of MoS₂-QDs and the VB edge of g-C₃N₄.

Fig. 4 UV-Vis absorption spectra of g-C₃N₄ and MoS₂-QDs/CN composite.

To further investigate the influence of MoS₂-QDs incorporation on the band-gap of the composite, Mott–Schottky tests were performed to confirm the electronic potentials of the composite and MoS₂-QDs. The measured potentials has been converted to the reversible hydrogen electrode (RHE) scale via the Nernst equation: E_RHE = E_Ag/AgCl + 0.05916pH + E⁰_Ag/AgCl, in which E_Ag/AgCl is the potential measured against the Ag/AgCl reference electrode while E⁰_Ag/AgCl stands for the standard potential of Ag/AgCl electrode at 298 K (0.1976 V). The Mott–Schottky curves of MoS₂-QDs and 1 wt% MoS₂-QDs/CN are shown in Fig. 5a and b, respectively. The calculated CB edge of pure MoS₂-QDs is at −1.20 eV. However, two obvious slopes can be observed in Fig. 5b, which can be assigned to g-C₃N₄ and MoS₂-QDs in the composite, whose calculated conduction band edges are located at −1.55 eV and −1.40 eV, respectively. The result reveals that the nanocluster formation of MoS₂-QDs causes the downshifting of its CB edge and thus the shrinkage of the band-gap of the composite, well supporting the above speculation on the relationship between the MoS₂-QDs content and the change of the composite band-gap. The possible mechanism is illustrated in Fig. S4.†

Fig. 5 Mott–Schottky plots collected on (a) MoS₂-QDs and (b) 1 wt% MoS₂-QDs/CN composite at a frequency of 962 Hz in dark.

The photoluminescence (PL) spectra of pure g-C₃N₄ and MoS₂-QDs/CN composite samples are shown in Fig. 6. All the photocatalysts exhibit broad PL peak centered at ∼450 nm under the 400 nm excitation at room temperature. This emission with the photon energy equal to the band-gap of g-C₃N₄ could result from the recombination of the photogenerated charge carriers corresponding to intrinsic HOMO–LUMO transition. Comparing all the spectra, we can find that PL intensity decreases with the increased incorporation of MoS₂-QDs. At ∼460 nm, there is a strong peak caused by charge transfer emission. In pure g-C₃N₄, the electron transfers from donor (the conjugated nitrogen) to acceptor (3-s-triazine ring),¹⁷ while in the composite, driven by the build-in electric field, the photogenerated electrons on the CB of g-C₃N₄ can immigrate to the CB of MoS₂-QDs and meanwhile the holes generated on the VB of MoS₂ can transfer to the VB of g-C₃N₄. Therefore, MoS₂-QDs incorporation can effectively prevent the recombination of photogenerated electrons and holes, which will directly contribute to the improved photocatalytic performance of the composite.

Fig. 6 Photoluminescence spectra of pure g-C₃N₄ and MoS₂-QDs/CN composite samples at an excitation wavelength of 400 nm.

3.2 Photocatalytic activity

Photocatalytic hydrogen evolution over different MoS₂-QDs/CN composite samples with 1 wt% Pt was evaluated under visible light irradiation (λ > 420 nm) using triethanolamine as sacrificial agent to consume photogenerated holes. Fig. 7a presents the hydrogen evolution of MoS₂-QDs/CN photocatalysts loaded with different amount of MoS₂ quantum dots.

Fig. 7 (a) Photocatalytic H₂ evolution over MoS₂-QDs/CN photocatalysts loaded with 1 wt% Pt under visible light irradiation (λ > 420 nm) using triethanolamine as sacrificial agent; (b) cycle runs for the photocatalytic H₂ production on 1 wt% MoS₂-QDs/CN.

The pure g-C₃N₄ sample without loading cocatalysts evolves H₂ at the rate of 2.99 μmol h⁻¹, while the highest H₂ evolution rate of 19.66 μmol h⁻¹ has been achieved on 50 mg 1 wt% MoS₂-QDs/CN, which is about 6.6 times higher than that on pure g-C₃N₄. On the other side, the further increased addition of MoS₂-QDs leads to an obvious decrease in photocatalytic hydrogen production. Possible reason can be explained as follows: the increased introduction of MoS₂-QDs leads to the agglomeration of quantum dots, namely the decrease of active sites on the edges, and consequently the decrease of H₂ evolution.

The aggregates of MoS₂-QDs also cause the gradual shrinkage of the quantum dots' band-gap, resulting in the gradual decrease of the composite band-gap as illustrated in Fig. S4.† The close distance between CB edges of MoS₂-QDs and g-C₃N₄ would weaken the driven force and impede the transfer of charge carriers. Therefore, 1 wt% of MoS₂-QDs incorporation into g-C₃N₄ could lead to a balance between the suitable band-gap structure, the high mobility of charge transfer and the low probability of charge recombination, thus achieving the best photocatalytic performance.

Control experiments were conducted to further determine the role of MoS₂-QDs in photocatalytic H₂ evolution (Fig. S5†). Pure g-C₃N₄ showed a relatively low H₂ evolution rate (2.99 μmol h⁻¹) than MoS₂-QDs/CN (7.43 μmol h⁻¹) and the H₂ evolution rate on g-C₃N₄ loaded with Pt (10.71 μmol h⁻¹) was much less than that on MoS₂-QDs/CN loaded with Pt (19.66 μmol h⁻¹), suggesting the contribution of MoS₂-QDs to the improved H₂ evolution.

A 12 h cycle experiment with intermittence in every 4 h was performed on 1 wt% MoS₂-QDs/CN composite photocatalyst to examine its stability (Fig. 7b). Only a slight decrease of H₂ evolution rate happened in the third run (19.48 μmol h⁻¹) and the average H₂ evolution rate maintained at 19.58 μmol h⁻¹.

3.3 Electrochemical characterizations

The charge transfer in g-C₃N₄, 1 wt% MoS₂-QDs/CN and MoS₂-QDs were studied by EIS in dark. As shown in Fig. 8a, the semicircular diameter of MoS₂-QDs is the smallest among all the samples, demonstrating the excellent conductivity of the catalyst. And the electrochemical impedance of 1 wt% MoS₂-QDs/CN is smaller than that of CN, suggesting the decoration of MoS₂-QDs has improved the conductivity of g-C₃N₄ and further accelerated charge transfer. This phenomenon further proves the lowered recombination rate of photogenerated carriers in the composite and consequently the increased number of charge carriers evolved in photocatalytic reaction due to the introduction of MoS₂-QDs, which agrees well with the above PL result in Fig. 6.

Fig. 8 (a) Nyquist plots of electrochemical impedance spectroscopy of g-C₃N₄, 1 wt% MoS₂-QDs/CN and MoS₂-QDs at −0.4 V; (b) periodic photo-responses of g-C₃N₄, 1 wt% MoS₂-QDs/CN and MoS₂-QDs. All the electrodes were evaluated in 0.2 M Na₂SO₄ aqueous solution under visible light irradiation using Ag/AgCl as reference electrode. The gradual decrease of the composite's photocurrent is due to the exfoliation of the catalyst from ITO.

The photocurrent responses of g-C₃N₄, 1 wt% MoS₂-QDs/CN and MoS₂-QDs were investigated for several on–off cycles of irradiation (Fig. 8b). It is clear that the photocurrent intensity of 1 wt% MoS₂-QDs/CN is much higher than that of g-C₃N₄, which is in accordance with the improved photocatalytic H₂ evolution rate on 1 wt% MoS₂-QDs/CN. The spectra also suggest the improved photocatalytic activity of the composite can be attributed to the decoration of MoS₂-QDs, which has greatly improved the photo-response of the photocatalyst and the separation/transfer ability of charge carriers in the composite.

3.4 Discussion on photocatalytic mechanism

Based on the above characterizations, the enhanced photocatalytic performance of the MoS₂-QDs/CN composite can be attributed to the enhanced light absorption and the improved charge transfer ability resulting from the synergetic effect of MoS₂-QDs and g-C₃N₄ together with Pt. The possible photocatalytic mechanism of the MoS₂-QDs/CN composite was illustrated in Fig. 9.

Fig. 9 Schematic illustration of the generation, separation, and transfer of electron–hole pairs for photocatalytic hydrogen evolution in the MoS₂-QDs/CN composite.

Mott–Schottky plots (Fig. 5b) determine the CB potentials of g-C₃N₄ and MoS₂ at −1.55 eV and −1.40 eV, respectively. In the composite, the top part of the VB is mainly dominated by g-C₃N₄, while the bottom part of the CB is mainly contributed by MoS₂-QDs. Such band alignment results in the formation of a type-II heterojunction (Fig. S6†).^35–37 When the composite is irradiated, electrons can be excited from CBs of both g-C₃N₄ and MoS₂-QDs, which can easily transfer from the surface of g-C₃N₄ to the edges of MoS₂-QDs, inciting HER at the active edges of MoS₂-QDs. With sufficient chemical potential from the CB of MoS₂-QDs to Pt, electrons are also inclined to accumulate on Pt to prolong their life-time and finally react with protons. On the other hand, the holes generated from MoS₂-QDs can easily move to g-C₃N₄ due to the type-II band alignment, leading to the redistribution of electrons and holes so that the oxidation reaction is highly likely to take place and effectively prevent the electron–hole recombination process and resulting in the improved hydrogen evolution performance of the MoS₂-QDs/CN composite photocatalyst.

As mentioned above, no H₂ has been produced on MoS₂-QDs. But the decoration of MoS₂-QDs on g-C₃N₄ has greatly improved the H₂ evolution from 2.99 μmol h⁻¹ on pure g-C₃N₄ to 7.43 μmol h⁻¹ on MoS₂-QDs/CN in the absence of Pt, indicating the synergetic catalysis effect between MoS₂-QDs and g-C₃N₄. Specifically, MoS₂-QDs loading has enhanced the visible light absorption, and the well-matched band structure of MoS₂-QDs and g-C₃N₄ facilitates the separation and transfer of charge carriers in the composite. Moreover, the photocatalytic performance of MoS₂-QDs/CN is better than the previously reported g-C₃N₄ decorated with bulk MoS₂,¹⁵ which is more likely to be attributed to the more prominent edge activity of MoS₂-QDs. When Pt, as an electron trapper, was loaded on these catalysts, the H₂ yields have been further increased to 10.71 and 19.66 μmol h⁻¹ on Pt/CN and MoS₂-QDs/Pt/g-C₃N₄, respectively. However, no obvious synergetic effect between MoS₂-QDs and Pt has been observed. Comparing the H₂ evolution amount on 1 wt% MoS₂-QDs/CN (7.43 μmol h⁻¹) and 1 wt% Pt/CN (10.71 μmol h⁻¹), MoS₂-QDs have great potential to replace noble metal as cocatalyst for photocatalytic HER.

4 Conclusions

In summary, a new MoS₂-QDs/g-C₃N₄ heterostructured photocatalyst (MoS₂-QDs/CN) has been synthesized via a facile impregnation method. The obtained composite photocatalyst exhibits enhanced absorption in visible light range compared with pure g-C₃N₄. The introduction of MoS₂-QDs effectively prevents charge recombination and improves the photocatalytic H₂ evolution activity. The highest H₂ evolution rate of 19.66 μmol h⁻¹ has been achieved on 50 mg of 1 wt% MoS₂-QDs/CN photocatalyst, which is 6.6 times higher than that on pure g-C₃N₄. The band-structure of MoS₂-QDs/CN composite results in a type-II heterostructure, which is preferable in preventing the recombination of photo-induced carriers. The photocatalytic performance of MoS₂-QDs/CN is superior to the previously reported g-C₃N₄ decorated with bulk MoS₂,¹⁵ because of the more prominent edge activity of MoS₂-QDs. MoS₂-QDs have great potential to replace noble metal cocatalyst for photocatalytic HER. The novel MoS₂-QDs/CN composite is a promising candidate for visible light H₂ evolution, and provides inspiration on the development of other heterostructured photocatalysts for solar fuel production.

Acknowledgements

This work was financially supported by the National Key Basic Research Program of China (2013CB933200), National 863 plans projects (2012AA062703), Youth Innovation Promotion Association CAS (2012200).

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Footnote

† Electronic supplementary information (ESI) available: The illustration of MoS₂ crystal structure, HRTEM image of 7 wt% MoS₂-QDs/CN, band-gaps of all samples, explanation of the band-gap shrinkage of MoS₂-QDs/CN composite, control experiment results of photocatalytic HER and illustration of the electron–hole pair separation and transfer in the MoS₂-QDs/CN composite. See DOI: 10.1039/c6ra07060d

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