Elsevier

Chemical Engineering Journal

Volumes 215–216, 15 January 2013, Pages 721-730
Chemical Engineering Journal

Efficient degradation of RhB over GdVO4/g-C3N4 composites under visible-light irradiation

https://doi.org/10.1016/j.cej.2012.11.074Get rights and content

Abstract

GdVO4/g-C3N4 composite photocatalysts were synthesized by a milling and heating method. The samples were characterized by X-ray diffraction (XRD), thermogravimetry and differential thermal analysis (TG–DTA), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–vis diffuse reflection spectroscopy (DRS). The photocatalytic activities were evaluated in the degradation of rhodamine B aqueous solution. The result shows that g-C3N4 and GdVO4 present suitable band potentials, which induces a charge transfer at the heterojunction interfaces of the two semiconductors. The recombination of photogenerated electron–hole pairs is thus retarded and, consequently, the photocatalytic activity of GdVO4/g-C3N4 composites is enhanced. The 10 wt.% GdVO4/g-C3N4 composite presents the highest photocatalytic activity of with a degradation rate of 0.0434 min−1, which exceeded that of pure g-C3N4 by more than 3.1 times. The proposed mechanism for the enhanced visible-light photocatalytic activity of GdVO4/g-C3N4 composites was further proven by photoluminescence (PL) spectroscopy, photocurrent–time measurement, and hydroxyl radicals trapping measurement.

Highlights

• Novel GdVO4/g-C3N4 composite photocatalyst is developed by milling and heating method. • The g-C3N4/GdVO4 composite degraded RhB 3× faster than g-C3N4 under visible-light. • The synergetic effect of GdVO4 and g-C3N4 is proposed. • Holes and O2- are the main active species for the RhB photodegradation.

Introduction

Since 1972, Fujishima and Honda first found that water can be decomposed into H2 and O2 by TiO2 under light irradiation, photocatalysis using semiconductor has been undoubtedly considered as the most promising technique in removal of environmental pollutant and production of hydrogen [1]. In terms of the high activity and chemical stability, the semiconductor TiO2 has been known as the most excellent photocatalyst [2], [3]. Unfortunately, due to its large band gap of 3.2 eV, TiO2 can only absorb UV light which occupies no more than 4% of the solar spectrum. In order to utilize the solar light in the visible region (λ > 420 nm) which covers the largest proportion of the solar spectrum, a visible-light active photocatalyst is desired.

To date, great efforts have been devoted to develop visible-light-driven (VLD) photocatalyst, and many novel VLD photocatalysts were reported, such as Ag3PO4, CaBi2O4, and Ag/AgBr [4], [5], [6]. Most of the reported VLD photocatalysts were developed based on the idea of energy band engineering. Three approaches have been widely used to narrow the bandgap of semiconductors: (I) modification of the valence band (VB) (such as N–TiO2 and CaBi6O10) [7], [8], (II) adjustment of the conduction band (CB) (such as Ag3PO4 and AgMO2 (M = Al, Ga, In)) [4], [9], [10], and (III) continuous modulation of the VB and/or CB (such as (ZnO)x(GaN)1−x and AgAl1−xGaxO2) [11], [12]. However, some problems, such as the choice of doping element, the discrete doping levels, and the low mobility of carriers in the doping levels, often agonizes the researchers. The energy band configuration of a semiconductor can not be handled easily. Hence, some researchers pay much attention on another route: coupling of two kinds of semiconductor particles with different redox energy levels of their corresponding CB and VB. Since this method was first suggested by Serpone et al. in 1982, a large variety of coupled semiconductor systems have been reported [13], [14], [15], [16], [17], [18], [19], [20], [21]. The most typical example is CdS/TiO2 [13]. Due to the suitable band potential, the photo-excited electrons can migrate from the CB of CdS to that of TiO2. In this configuration, two advantages can be obtained: (1) an improvement of visible-light absorption; (2) an increase in the lifetime of the charge carrier. A high activity under visible-light is thus acquired. Now, coupling two different semiconductors with suitable band potential has become an important way to synthesize the VLD photocatalyst.

Polymeric graphitic carbon nitride (g-C3N4) is a novel metal-free VLD semiconductor with narrow bandgap energy of 2.7 eV. Recently, Wang and Yan et al. reported that g-C3N4 has the photocatalytic performance for hydrogen or oxygen production from water splitting and photodegradation of organic pollutant under visible-light irradiation [22], [23]. Graphitic C3N4 is inexpensive, and can be easily prepared via numerous facile methods. Furthermore, g-C3N4 has the properties of high thermal and chemical stability. It indicates that the metal-free g-C3N4 has promising potential in the photocatalysis fields. Therefore, several studies were followed to improve the visible-light photocatalytic performance of g-C3N4. Ge et al. synthesized Ag doped g-C3N4 and used it as a photocatalyst to decompose water under visible-light irradiation [24]. Liu et al. prepared sulfur doped g-C3N4 [25]. The prepared g-C3N4−xSx displays much higher activity than g-C3N4 for water splitting. Besides the metal or non-metal doped photocatalysts, the g-C3N4 based composite is another important part of the reported photocatalysts. Yan et al. prepared g-C3N4/TaON composite which presented stronger activity than either single phase of g-C3N4 or TaON in rhodamine B (RhB) photodegradation [26]. Very recently, the novel Bi2WO6/g-C3N4 N–In2TiO5/g-C3N4, graphene/g-C3N4 composite photocatalysts were prepared and used for the photodegradation of RhB as well as water splitting [27], [28], [29]. The vanadate (such as BiVO4, m-LaVO4) present higher photoabsorption ability than Bi2WO6 or TaON, and might be the good choice of the coupling semiconductor. To the best of our knowledge, however, the study of vanadate coupled g-C3N4 photocatalyst has not been reported before. In our previous work, GdVO4 was reported to display good photocatalytic activity in acetone degradation [30]. It is a pale yellow semiconductor with a band gap of ca. 2.40 eV. The conduction band and valence band level of GdVO4 was 0.00 eV and 2.40 eV, respectively, both of which are lower than that of g-C3N4. The suitably matching bands indicate that GdVO4 and g-C3N4 can constitute an appropriate system to improve the photocatalytic activity.

In the present study, the composite photocatalyst GdVO4/g-C3N4 was prepared by mixing and heating methods. The synthesized photocatalysts were characterized by BET, XRD, FT-IR, SEM, TEM, XPS, DRS, and PL technologies. RhB was used as mode compound to investigate the activities of GdVO4/g-C3N4 composite under visible-light irradiation (λ > 420 nm). The results demonstrated that compared with pure GdVO4 and g-C3N4, the GdVO4/g-C3N4 composite photocatalyst had a remarkably enhanced RhB photodegradation activity under visible-light irradiation.

Section snippets

Preparation of catalysts

All chemicals were reagent grade and used without further purification. Pure g-C3N4 powders were prepared by directly heating melamine. In a typical synthesis run, 6 g of melamine was placed in a semi-closed alumina crucible with a cover. The crucible was then heated to 520 °C for 4 h to obtain g-C3N4 powder.

Pure GdVO4 was prepared by precipitation method: Solutions of NH4VO3 and Gd(NO3)3 with a V to Gd mole ratio of 1:1 were mixed to give a deposit. The pH value of the solution was adjusted to 7

TG–DTA and BET analysis

The TG–DTA experiment was carried out to investigate the thermal stability of pure g-C3N4 in air. The result is shown in Fig. 1a. Only a sharp weight loss with one endothermal peak at 743 °C, which can be attributed to the sublimation of g-C3N4, is observed. The beginning temperature of the weight loss is located at approximately 600 °C. It indicates that g-C3N4 is stable below 600 °C, and can be used to construct the composite photocatalyst at a moderate temperature. However, most of the

Conclusion

Novel GdVO4/g-C3N4 composite photocatalysts were prepared via a simple mixing and heating method. In comparison of pure g-C3N4, the prepared composite exhibits enhanced photocatalytic activity under visible-light irradiation, which can be attributed to the synergetic effect of GdVO4 and g-C3N4 in separation of electron–hole pairs. The sample with the weight concentration of GdVO4 of 10 wt.% shows the highest photocatalytic activity for RhB degradation. It might be attributed to the good disperse

Acknowledgements

We acknowledge Doctor Xiaodong Yi in Xiamen University for the help of XPS analysis. This work was supported by the National Natural Science Foundation of China (21003109, 51108424) and the program for Zhejiang Leading Team of Science and Technology Innovation (2009R50020).

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