Graphene composite and its photocatalytic performance in phenol degradation under visible light

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 44, Issue 8, August 2016 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2016, 44(8), 937942

RESEARCH PAPER

Preparation of Au/BiOBr/Graphene composite and its photocatalytic performance in phenol degradation under visible light YU Xue, WANG Liang*, FENG Li-juan, LI Chun-hu* College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China

Abstract: BiOBr, BiOBr/Graphene and Au/BiOBr/Graphene composites were prepared by hydrothermal synthesis and dopamine in-situ reduction method; their morphology, composition, phase structure and optical absorption properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflection spectroscopy (DRS) and photoluminescence (PL) emission spectroscopy. The photocatalytic performance of Au/BiOBr/Graphene in phenol degradation under visible light was investigated. The results indicate that the Au/BiOBr/Graphene composite exhibits enhanced absorption in the visible light region as well as superior photocatalytic activity in the degradation of aqueous phenol, in comparison with BiOBr and BiOBr/Graphene, owing to the enhanced quantum efficiency, narrowed band gap (2.25 eV) and surface plasmon resonance of Au nano particles. Over Au/BiOBr/Graphene composite, the degradation rate of phenol reaches 64% in 180 min under visible light irradiation. Key words:

BiOBr; graphene; gold; phenol; degradation; photocatalysis

The semiconductor photocatalysis has been considered as a “green chemical method” for the environmental remediation and solar energy conversion to solve the problems of environmental pollution and energy crisis. Traditional semiconductor photocatalysts such as TiO2[1] and ZnO[2] have been applied in the degradation of contaminants in water because of their excellent performance in oxidation and photoinduction. However, these photocatalysts can only utilize the UV light, which covers only 4% of the solar spectrum, due to their wide band gaps (for example, the band gap of TiO2 is about 3.2 eV). Therefore, great effort has been devoted to the development of new photocatalysts [3,4]. Recently, a series of ternary bismuth oxyhalides (BiOX, X = Cl, Br, or I) has been widely investigated, owing to their high photocatalytic activity under visible light[5], as the layered structure of BiOX can promote the atomic polarization and improve the efficiency of charge separation. Among these BiOX photocatalysts, BiOBr with the PbFCl layer crystal structure has drawn much attention due to its high stability, suitable band gap and excellent photocatalytic activity. In comparison with the single-component semiconductors, the binary-component[6] and multi-component[7,8] counterparts

exhibited enhanced photocatalytic activity, as the synergy of multi-components may overcome the shortages of single-component such as wide band gap and insufficient charge separation ability. Therefore, the photocatalytic activity of BiOBr, with an indirect-transition band gap of about 2.75 eV, may be effectively improved by integrating with other materials. Meanwhile, graphene, with peculiar properties such as low density, high surface area and high conductivity, can be used as a promising supporting platform for anchoring guest nanoparticles, as well as an excellent electron transfer mediator and acceptor to facilitate charge separation [9,10]. Theoretically, under visible light irradiation, the noble metal like Au can be induced to release electrons by surface plasmon resonance effect (SPR)[11,12]. The electrons induced by SPR can be injected to the conductive band of the semiconductor, whose conductive band energy is more positive than –1.3 eV (vs. NHE, the normal hydrogen electrode). The enhanced local electric field generated by the SPR of Au nano particles (NPs) near the semiconductor surface allows for the formation of the charges in the near surface region of the composite.

Received: 12-Apr-2016; Revised: 22-Jun-2016. *Corresponding author. Tel: 0532-66782502, E-mail: [email protected], [email protected]. Copyright  2016, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

YU Xue et al / Journal of Fuel Chemistry and Technology, 2016, 44(8): 937942

Fig. 1

XRD patterns of BiOBr, BiOBr/Graphene and

Fig. 2

EDS pattern of Au/BiOBr/Graphene

Au/BiOBr/Graphene

The photons produced by SPR of Au NPs can be scattered to extend the optical path to increase the quantities of the effective electrons and holes[13,14]. In this work, the Au/BiOBr/Graphene composite photocatalyst was prepared by hydrothermal method using Bi(NO3)3·5H2O and KBr without any surfactant and in-situ reduction method using dopamine (DA) as reducing agent to reduce the Au(III) salt. The photocatalytic performance of Au/BiOBr/Graphene nanocomposite in the degradation of phenol was then investigated.

1 1.1

Experimental Reagents and catalyst preparation

All solvents and reagents, including Bi(NO3)2·5H2O (AR, Sinopharm Chemical Reagent Co., Ltd.), KBr (AR, Sinopharm Chemical Reagent Co., Ltd.), dopamine (AR, Shanghai Aladdin biochemical Polytron Technology Co., Ltd.), HAuCl4·4H2O (AR, Shanghai Aladdin biochemical Polytron Technology Co., Ltd.), and Graphene (Beijing Deco Daojin Technology Co., Ltd.), were obtained from commercial suppliers and used without further purification. Typically, 0.595 g of Bi(NO 3)3·5H2O was dissolved into 10 mL of 2 mol/L HNO3 solution under magnetic stirring (Solution A), and 0.2975 g of KBr was dissolved in 10 mL of deionized water under magnetic stirring (Solution B). Solution B was then added dropwise into Solution A and stirred magnetically for 1 h at room temperature. After that, a given amount of graphene was added into the mixture with stirring for 1 h. The mixture was transferred into a Teflon-lined stainless steel autoclave and maintained at different temperatures for 24 h. Subsequently, the autoclave was cooled to room temperature naturally. The obtained samples were taken out and washed with ethanol three times and then dried in air.

A certain concentration of dopamine (DA) solution was added to the obtained BiOBr/Graphene suspension with vigorous stirring, following which a given amount of HAuCl4·4H2O aqueous solution was added with stirring for 20 min. Finally, the Au NPs/BiOBr/Graphene nanocomposites were obtained by filtering and drying under room temperature. 1.2

Characterization

The phase composition of the as-prepared catalyst samples was characterized on an X-ray diffractometer (XRD, Bruker D8 Advance with Cu K radiation at 40 kV and 30 mA). The sample size and morphology were determined by scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEM-2100). UV-vis diffusion reflectance spectra were obtained on a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan). The photoluminescence (PL) spectra were measured with a Varian Cary-Eclipse500 fluorescence spectrophotometer at room temperature. The solution was irradiated for degradation from the top of the reactor by a 300-W Xenon lamp (CEL-HXF300/CEL-HXUV300) with a cut-off ( > 400 nm) to filter out UV light; the concentration of phenol was analyzed under its characteristic absorption wavelength of 510 nm on a UV-vis spectrophotometer (TU-1800). 1.3

Photo-degradation test

The photocatalytic degradation of phenol was performed in a 100 mL magnetically stirred cylindrical reactor at room temperature (25°C); irradiation was provided by a 300W xenon lamp with a cut-off filter ( > 400 nm). In a typical run, 100 mL of 10 mg/L phenol aqueous solution was added to the reactor; the pH value of the phenol solution was kept at the initial state.

YU Xue et al / Journal of Fuel Chemistry and Technology, 2016, 44(8): 937942

Fig. 3

SEM images of BiOBr (a), BiOBr/Graphene (b) and Au/BiOBr/Graphene (c); TEM image of Au/BiOBr/Graphene (d)

After 100 mg of photocatalyst was added to the reactor, the system was first stirred in the dark for 30 min to achieve adsorption equilibrium. The samples were centrifuged to sediment the photocatalyst; the phenol concentration in the supernatants was monitored by measuring the solution absorbance at 510 nm wavelength with a UV-1800 spectrophotometer, using the 4-APP spectrophotometry. The photocatalytic activity can be evaluated by the following equation: X = (C0 – C)/C0 × 100% (1) where C0 (mg/L) and C (mg/L) are the concentrations of phenol before visible light irradiation and after visible light irradiation, respectively.

2 2.1

Results and discussion XRD results

The XRD patterns of BiOBr, BiOBr/Graphene and Au/BiOBr/Graphene are shown in Figure 1. All the peaks observed for BiOBr can be indexed to the typical tetragonal crystal structure of BiOBr crystalline (JCPDS Card No. 09-0393)[5]. Obviously, there is no diffraction peaks observed for graphene and Au in the XRD patterns of BiOBr/Graphene and Au/BiOBr/Graphene, probably due to the low content of graphene and well dispersed Au NPs[13,15]. This suggests that the hybridization with graphene and Au NPs has little influence on the crystal structure of BiOBr. In addition, all the

characteristic peaks for the samples containing graphene are weaker than those for pure BiOBr, probably implying that graphene and Au nanoparticles added have a minor influence on formation of BiOBr crystals[16]. Furthermore, as shown in Figure 2, the EDS results demonstrated that Au NPs (0.86%) are successfully loaded on the BiOBr/Graphene sheets and the element ratio of Bi to Br is close to 1. 2.2

Morphology and microstructure

As shown by the SEM images in Figure 3(a), BiOBr prepared by the hydrothermal method exhibits flower-like microstructure; the sphere diameter is about 3 μm and the average thickness of BiOBr flakes is about 20 nm. However, BiOBr/Graphene and Au/BiOBr/Graphene display no obvious flower-like microstructure, as shown in Figure 3((b) and (c)), suggesting that the microstructure of BiOBr is influenced by graphene and ultrasonication, in accordance with the result of XRD. The TEM image of Au/BiOBr/Graphene shown in Figure 3(d) illustrates that Au NPs are uniformly deposited and well-dispersed, with a particle size of less than 50 nm. 2.3

Optical properties

The activity of a photocatalyst is largely affected by the recombination of the photo induced electrons and holes, which can be verified by the PL spectra.

YU Xue et al / Journal of Fuel Chemistry and Technology, 2016, 44(8): 937942

Fig. 4

Photoluminescence spectra of BiOBr, BiOBr/Graphene and Au/BiOBr/Graphene

Fig. 5

UV-vis absorption spectra of BiOBr, BiOBr/Graphene and Au/BiOBr/Graphene

In general, the higher the PL intensity is, the less efficient the carriers participate in the photocatalytic process [17]. As shown in Figure 4, the PL spectrum intensity of pure BiOBr is much stronger than those of BiOBr/Graphene and Au/BiOBr/Graphene. The emission peak intensity is decreased upon the loading of graphene, as graphene is an excellent electron transfer mediator and acceptor. Especially, the Au/BiOBr/Graphene composite exhibits the lowest PL peak intensity, indicating that the surface plasmon resonance of Au NPs has great effect on the separation of electrons and holes. The enhanced local electric field generated by the SPR of Au NPs near BiOBr/Graphene surface allows for the formation of the charges in the near surface region of the composite; the photons produced by SPR of Au NPs can be scattered to extend the optical path to increase the quantities of effective electrons and holes[18]. To explore the optical absorption ability, UV-vis DRS of BiOBr, BiOBr/Graphene and Au/BiOBr/Graphene are shown in Figure 5. All three samples display optical absorption under UV and visible light, but are different in the absorption ranges, viz., in the order of Au/BiOBr/Graphene (2.25 heV) < BiOBr/Graphene (2.48 eV) < BiOBr (2.75 eV) according to

calculation with the equation Eg = 1240/ (eV). The band gap values of Au/BiOBr/Graphene and BiOBr/Graphene are decreased and the adsorption edges exhibit a red shift, which may be attributed to the incorporation of graphene and Au NPs. On one hand, graphene shows the excellent optical absorption property under UV and visible light; on the other hand, the SPR of Au NPs has a synergistic effect with graphene. Furthermore, the conduction band (CB) and valence band (VB) potentials of BiOBr are deduced according to the empirical equations: EVB = χ – Ee + 0.5Eg (2) ECB = EVB –Eg (3) where EVB and ECB represent the CB and VB edge potentials, respectively. χ is the electro-negativity of the semiconductor; for BiOBr, χ = 6.17. Ee is the free electron energy on the hydrogen scale (about 4.5 eV) and Eg is the band gap. The calculated EVB and ECB values for BiOBr are around 3.05 and 0.3 eV, respectively, suggesting that the electrons induced by SPR of Au NPs are injected to the conductive band of BiOBr, whose conductive band energy is more positive than –1.3 eV (vs. NHE) of Au NPs. Above results demonstrate that the SPR of Au NPs, the optical absorption property of graphene and the suitable band edge of BiOBr have great effect on the visible light absorption ability. 2.4

Phenol degradation

To broaden the practical application of this novel photocatalyst, Au/BiOBr/Graphene was used to degrade phenol in aqueous solution. Figure 6 shows its photocatalytic activity in phenol degradation under visible light irradiation. The adsorption of phenol on Au/BiOBr/Graphene in the dark achieves equilibrium in 30 min. Pure BiOBr displays very weak photocatalytic activity under visible irradiation.

Fig. 6

Photocatalytic activity of BiOBr, BiOBr/Graphene and

Au/BiOBr/Graphene in the degradation of phenol under visible light irradiation

YU Xue et al / Journal of Fuel Chemistry and Technology, 2016, 44(8): 937942

With the incorporation of Au and graphene, the composites show an evident increase in the efficiency for the degradation of phenol (64%) under visible light irradiation in 180 min. In comparison with pure BiOBr, the efficiencies of BiOBr/Graphene and Au/BiOBr/Graphene in phenol degradation are highly enhanced, probably due to the synergy between Au nanoparticles and graphene, which improves the quantum efficiency of photocatalysis. As a result, the Au/BiOBr/Graphene composite exhibits the highest photo-catalytic performance in phenol degradation

3

[7] Huang Y C, Fan W J, Long B, Li H B, Zhao F Y, Liu Z L, Tong Y X, Ji H B. Visible light Bi2S3/Bi2O3/Bi2O2CO3 photocatalyst for effective degradation of organic pollutions. Appl Catal B: Environ, 2016, 186: 68–76. [8] Jiang L X, Li K X, Yan L S, Dai Y H, Huang Z M. Preparation of Ag(Au)/Graphene-TiO2 composite photocatalysts and their catalytic performance under simulated sunlight irradiation. Chin J Catal, 2012, 33(12): 1974–1981. [9] Chen J W, Shi J W, Wang X, Cui H J, Fu M L. Recent progress in the preparation and application of semiconductor/graphene composite photocatalysts. Chin J Catal, 2013,34(4): 621–640.

Conclusions

[10] Liu W J, Cai J Y, Li Z H. Self-assembly of semiconductor nanoparticles/Reduced Graphene Oxide (RGO) composite

In summary, Au/BiOBr/Graphene composite was successfully synthesized by using a facile in-situ reduction method at ambient temperature. The Au/BiOBr/Graphene composite exhibits improved photocatalytic activity in the degradation of phenol under visible light; such an improvement is mainly attributed to the synergy between the surface plasmonic resonance of Au NPs and the high electron conductivity of graphene, which can efficiently promote the separation of the photogenerated holes and electrons. Over the Au/BiOBr/Graphene composite, the phenol degradation rate reaches 64% under visible light irradiation in 180 min, which is almost 1.67 times higher than that over pure BiOBr.

aerogels for enhanced photocatalytic performance and facile recycling in aqueous photocatalysis. Acs Sustainable Chem Eng, 2015, 3(2): 277–282. [11] Han D, Zhang A W, Gao G J, Su H Q. Progress in the photocatalysis of supported-gold catalysts. Chem Ind Eng Prog, 2012, 31(2): 435–440. [12] Sun L L, Zhao D X, Song Z M, Shan C X, Zhang Z Z, Li B H, Shen D Z. Gold nanoparticles modified ZnO nanorods with improved photocatalytic activity. J Colloid Interface Sci, 2011, 363(1): 175–181. [13] Bi J H, Zhou Z Y, Chen M Y, Liang S J, He Y H, Zhang Z Z, Wu L. Plasmonic Au/CdMoO4 photocatalyst: Influence of surface plasmon resonance for selective photocatalytic oxidation of

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