g-C3N4 heterojunction photocatalysts with enhanced hydrogen evolution activity

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ScienceDirect www.elsevier.com/locate/matchar

Synthesis of novel MoS2/g-C3N4 heterojunction photocatalysts with enhanced hydrogen evolution activity Tian Yuminga , Ge Leib , Wang Kaiyuea,⁎, Chai Yueshenga a

Taiyuan University of Science and Technology, Taiyuan 030024, China Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum Beijing, Beijing 102249, China

b

AR TIC LE D ATA

ABSTR ACT

Article history:

Novel MoS2/g-C3N4 heterojunction photocatalysts were synthesized via a simple

Received 12 July 2013

impregnation and heating methods. The products were characterized by X-ray diffraction,

Received in revised form

transmission electron microscopy and UV–vis diffuse reflectance spectra. The photocata-

18 October 2013

lytic activities of MoS2/g-C3N4 samples were evaluated based on the hydrogen evolution

Accepted 19 October 2013

experiments under visible light irradiation (λ > 400 nm). The UV–vis diffuse reflectance spectra revealed that the MoS2/g-C3N4 photocatalysts had strong absorption in the visible

Keywords:

light region. The photocatalytic results indicated that the highest H2 evolution rate of

Photocatalysis

23.10 μmol·h−1 was achieved on the 0.5 wt.% MoS2–g-C3N4 sample, which was enhanced by

Nanoparticles

11.3 times compared to pure g-C3N4. This study may provide an approach to the

Functional

development of novel heterojunction photocatalysts for hydrogen production under visible

Semiconductors

light irradiation. © 2013 Elsevier Inc. All rights reserved.

1.

Introduction

Photocatalytic hydrogen evolution from water splitting using semiconductor photocatalysts offers a promising strategy for resolving the global energy security and environmental protection issues [1]. The exploration of visible light driven photocatalysts is of marvelous significance for the practical purpose of photocatalytic system [2]. In various types of compounds, such as oxides, sulfides, and oxynitrides semiconductor photocatalysts have been surveyed and proved to be able to produce hydrogen under visible illumination [3]. Lately, polymeric g-C3N4 has allured considerable attentions as a promising photocatalyst for its outstanding light absorption in the visible region [4]. With a narrow band gap (2.7 eV), the polymer compound g-C3N4 material has been

⁎ Corresponding author. Tel./fax: + 86 3516998145. E-mail address: [email protected] (K. Wang). 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.10.020

investigated as an attractive ingredient for the photocatalytic water splitting and degradation of organic pollutants [5]. Furthermore, the g-C3N4 is less expensive and easy to prepare, which makes the g-C3N4 based material a more suitable choice for fundamental understanding. However, pure g-C3N4 demonstrated low photocatalytic H2 evolution activity under visible light irradiation due to low efficiency of charge carriers [6]. Coupling g-C3N4 with another semiconductor (such as: TiO2 and Ni(OH)2) with matching band potentials to form a heterojunction is an effective approach to separate photogenerated electron–hole pairs [7–9]. As a promising electrocatalyst for H2 production, molybdenum disulfide (MoS2) with layered structure has been reported to demonstrate enhanced photocatalytic performance when coupled with CdS and TiO2 [10,11]. According to our

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knowledge, no previous work regarding the application of a MoS2/g-C3N4 heterojunction photocatalysts has been reported to date. Herein, we report the synthesis of MoS2/g-C3N4 hybrid photocatalysts via meek and mild impregnation method for photocatalytic H2 evolution under visible light irradiation. The consequences indicate that the novel MoS2/g-C3N4 heterojunction material is a promising photocatalyst for use in H2 evolution. The work may provide more insight into synthesizing novel heterojunction photocatalysts for applications in solar energy conversion and utilization.

2.

Experimental

2.1.

Synthesis of the Photocatalyst

The metal-free g-C3N4 powders were prepared by heating 2.0 g of cyanamide in an alumina combustion boat under nitrogen gas flow (10 ml/min) to 550 °C at a heating rate of 10 °C min−1 followed by 4 h at that temperature prior to cooling. The product was collected and ground into a powder. Layered MoS2 were synthesized by hydrothermal method: 1.236 g ammonium molybdate [(NH4)6Mo7O24·4H2O] and 2.665 g thiourea were dissolved in 30 ml distilled water, the solution was transferred into a 23 ml Teflon-lined autoclave and treated at 220 °C for 24 h. The products were collected after centrifugation to remove the impurities, and then dried in an oven at 85 °C for 24 h. The synthesis of MoS2/g-C3N4 composite photocatalysts was described as follows: 1.0 g g-C3N4 was ground and dispersed into 20 ml distilled water. Then, 1 ml MoS2 solution (1 mg/ml) was added dropwise into the g-C3N4 solution under magnetic stirring to form a mixture. After stirring for 1 h and ultrasonic dispersion for 2 h, the mixture was dried in an oven at 85 °C for 24 h to evaporate the solvents. The obtained sample was ground and heated at 300 °C for 2 h under nitrogen gas flow (10 ml/min).

2.2.

Photocatalytic Activity

The photocatalytic H2 evolution experiments were performed in a 300 ml quartz reactor at ambient temperature. The reactor is connected to a closed circulating system. PLS-SXE 300UV Xe arc lamp with a UV-cutoff filter (> 400 nm) was used as the light source. 0.1 g of photocatalyst powder was suspended in 120 ml of aqueous solution containing 25% methanol by volume. The loading of 1.0 wt.% Pt co-catalyst was conducted by directly dissolving H2PtCl6 into the mentioned suspension. Next, the suspensions were stirred and irradiated (300 W Xe arc lamp) for 30 min at room temperature to reduce the Pt species. Before photocatalytic experiments, the reaction vessel was evacuated for 30 min to remove the dissolved oxygen and to ensure the anaerobic conditions. The products were analyzed by gas chromatography (Shimadzu GC-8A, high purity Argon as a carrier gas). The quantum efficiency for the H2 evolution was measured using a similar experimental setup, only a band pass filter of 420 nm.

3.

71

Results and Discussion

3.1. Characterization of MoS2/g-C3N4 Heterojunction Photocatalysts The XRD patterns of pure MoS2, g-C3N4 and MoS2/g-C3N4 heterojunction samples are illustrated in Fig. 1. All the diffraction peaks observed correspond to the hexagonal phase of MoS2 (JCPDS 87-2416, Molybdenum disulfide 2H). The result indicates that the layered hexagonal MoS2 has been successfully synthesized via hydrothermal method. The pure g-C3N4 has two distinct diffraction peaks at 27.5° and 13.2°, which can be indexed for graphitic materials as the (002) and (100) peaks in JCPDS 87-1526. These two diffraction peaks are in superb constant with the g-C3N4 reported in the literatures [4,12]. Nevertheless, no diffraction peaks corresponding to MoS2 co-catalysts can be recognized in Fig. 1. This may be because of the small amount of MoS2 contents and high dispersion in polymeric g-C3N4 photocatalysts. The existence of MoS2 in MoS2/g-C3N4 heterojunction photocatalysts can be confirmed by HRTEM as discussed later. Fig. 2 shows the HRTEM pictures of the 0.5 wt.% MoS2/ g-C3N4 photocatalyst. The layered MoS2 co-catalysts are well dispersed and decorated on the surface of g-C3N4, thus a close neighborhood of MoS2 and g-C3N4 components achieved in the heterojunction photocatalyst. The lattice fringes of individual MoS2 particles with a d spacing of 0.615 nm can be assigned to the (002) plane of hexagonal MoS2 (JCPDS 87-2416) [11]. The HRTEM consequences indicate that the presence of MoS2 forms the intimate interfaces in the heterojunction samples, which could favor the electron–hole pair separation and enhance the photocatalytic efficiency. The UV–vis diffuse reflectance spectra of the as-prepared MoS2/g-C3N4 heterojunction photocatalysts were investigated and presented in Fig. 3. The natural g-C3N4 sample displays absorption from the UV through the visible range up to 460 nm, corresponding to the band gap of 2.7 eV [4,13]. After drawing in MoS2, the MoS2/g-C3N4 heterojunction samples presented similar absorption edge and enhanced light absorption in the visible region as compared to g-C3N4, The absorption intensity of the as-prepared samples strengthen with increasing MoS2 contents, which agrees with the color of the prepared samples that vary from light yellow to grey. The UV–vis results suggest that the composite samples may be able to absorb more visible light to produce electron–hole pairs and improve catalytic activity.

3.2.

Photocatalytic Activity of MoS2/g-C3N4 Samples

The photocatalytic activities of the MoS2/g-C3N4 heterojunction photocatalysts with 1.0 wt.% Pt co-catalysts were evaluated by hydrogen evolution via water splitting in aqueous methanol solutions under visible light irradiation (>400 nm). Fig. 4 presents the H2 evolution curves over MoS2/g-C3N4 photocatalysts with different MoS2 contents, together with that of pure g-C3N4 for a comparison. The pure g-C3N4 sample without MoS2 loading reveals visible light photocatalytic activity and the H2 production rate reaches 2.03 μmol·h−1, which can be attributed to the reasonable band gap and unique electronic

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Fig. 1 – X-ray diffraction patterns of pure MoS2, g-C3N4 and the MoS2/g-C3N4 heterojunction photocatalysts. structure of g-C3N4. After adding 0.1 wt.% MoS2, the activity of H2 evolution is significantly increased to 6.8 folds compared to the original sample. With expansion of the MoS2 doping amount, the photocatalytic H2 evolution on MoS2–g-C3N4 is enhanced further. The 0.5 wt.% MoS2–g-C3N4 shows the highest H2 evolution rate of 23.10 μmol·h−1 with a 2.8% apparent quantum efficiency, which is about 11.3 times higher than that of pure g-C3N4. However, a further increase of MoS2 content leads to a reduction of H2 evolution activity. This shrinkage can be related to the enlargement in the gradual murkiness of the composite samples (see Fig. 3). The advisable MoS2 content with well dispersion may have favored the transfer and separation of the charge carriers. At higher content, the black MoS2 can lead to shielding of the active sites on the photocatalyst surface and also decline the intensity of light through the depth of the reaction solution [14]. As a consequence, a suitable content of MoS2 is significant for optimizing the photocatalytic performance of MoS2–g-C3N4 composites. The enhanced photocatalytic H2 evolution activity of MoS2/ g-C3N4 heterojunction photocatalysts can be depicted to synergetic effect between Pt, MoS2 and g-C3N4. The CB and VB

edge potentials of polymeric g-C3N4 are determined at −1.13 and +1.57 eV [15]. The band gap of MoS2 nanosheets is estimated at −0.1 and +2.0 eV [9]. The CB and VB edge potentials of g-C3N4 are more negative than those of MoS2, which allow the electrons to transfer from the CB of g-C3N4 to that of MoS2 via contacting interfaces. Moreover, the photogenerated electrons in the CB of g-C3N4 can be conveyed to MoS2 nanosheets through the Pt particles, which act as a conductive electron transport pathway, lengthening the lifetime of the charge carriers, and then react with the adsorbed H ions at the surface of MoS2 to generate H2. Accordingly, the recombination process of the electron–hole pairs is effectively inhibited and brings about enhancement of H2 evolution for the MoS2/g-C3N4 heterojunction photocatalyst.

4.

Conclusions

Novel MoS2/g-C3N4 heterojunction photocatalysts were synthesized via a facile impregnation method. The layered MoS2 are decorated on the surface of g-C3N4 to form intimate

Fig. 2 – High resolution transmission electron microscopy images of 0.5 wt.% MoS2/g-C3N4 composite photocatalysts.

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No. KYJJ2012-06-20), and the doctoral initiating project of Taiyuan University of Science and Technology (File No. 200726).

REFERENCES

Fig. 3 – UV–vis diffuse reflectance spectra of pure g-C3N4 and MoS2/g-C3N4 composite samples.

Fig. 4 – Photocatalytic H2 evolution over MoS2/g-C3N4 heterojunction photocatalysts loaded with 1.0 wt.% Pt co-catalysts from 25% methanol aqueous solution under visible light irradiation (λ > 400 nm).

interfaces, and the MoS2/g-C3N4 samples exhibit stronger absorption in the visible light region. The 0.5 wt.% MoS2/ g-C3N4 exhibits the highest H2 evolution rate of 23.10 μmol·h−1, which is about 11.3 times higher than that of pure g-C3N4. This work may provide an insight to the development of efficient MoS2/g-C3N4 heterojunction photocatalyst in hydrogen evolution and solar energy application.

Acknowledgments This work was financially supported by the National Science Foundation of China (Grant Nos. 21003157 and 21273285), the Beijing Nova Program (Grant No. 2008B76), the Science Foundation of China University of Petroleum, Beijing (Grant

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