RGO heterostructural aerogel with enhanced and selective photocatalytic properties under visible light

Accepted Manuscript Title: A Three-Dimensional BiOBr/RGO Heterostructural Aerogel with Enhanced and Selective Photocatalytic Properties under Visible Light Author: Xue Yu Junjie Shi Lijuan Feng Chunhu Li Liang Wang PII: DOI: Reference:

S0169-4332(16)32677-0 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.219 APSUSC 34524

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APSUSC

Received date: Revised date: Accepted date:

13-9-2016 26-11-2016 28-11-2016

Please cite this article as: Xue Yu, Junjie Shi, Lijuan Feng, Chunhu Li, Liang Wang, A Three-Dimensional BiOBr/RGO Heterostructural Aerogel with Enhanced and Selective Photocatalytic Properties under Visible Light, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.219 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Three-Dimensional BiOBr/RGO Heterostructural Aerogel with Enhanced and Selective Photocatalytic Properties under Visible Light 



Xue Yu1, Junjie Shi2 , Lijuan Feng1, Chunhu Li1, Liang Wang1

1. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qing dao 266100,China

2. Institute of Applied and Physical Chemistry and Center for Environmental Research and Sustainable Technology, University Bremen, Leobener Str. UFT, 28359 Bremen, Germany



Corresponding author.Tel: +86 532 66782502. E-mail addresses: [email protected], [email protected]

Scheme 1. Illustration for the photocatalytic degradation of MO over BiOBr/RGO aerogels.

Highlights 1. A BiOBr/RGO aerogel photocatalyst was synthesized using dopamine as reducing agent. 2. BiOBr/RGO aerogel can be easily controlled morphology by a simple two-step method. 3. BiOBr/RGO aerogel photocatalyst exhibited superior performance in MO decoloration.

Abstract: A series of BiOBr/reduced graphene oxide (RGO) aerogel was fabricated using a two steps hydrothermal method. Various methods such as SEM, TEM, DRS and Raman spectroscopy were employed to fully characterize the as-obtained BiOBr/ RGO. Their photocatalytic degradation of methyl orange (MO) were studied under visible light irradiation. The combination of BiOBr and RGO result in an improved activity. The sample with 10 wt% RGO abbreviated as BiOBr-G10 shows the highest activity. Moreover, this sample exhibits a selective visible-light photocatalytic behavior as the degradation rate over MO (80%) is much higher than that over Rhodamin B (50%) and phenol (35%) in 60 mins. The XRD and photoluminescence emission spectroscopy characterization of the BiOBr-G10 samples indicates an increased crystallization of BiOBr and efficient quenching of photo-excited electrons and holes contributes to the improved photocatalytic activities.

Keywords: RGO aerogel; BiOBr; Photocatalysis pollutant remediation; Visible Light

1. Introduction Reduced graphene oxide (RGO) aerogel is a three-dimensional (3D) porous solid material

[1-3]

. This monolithic material combines the advantages from both graphene

and aerogel[4-6], for example, high specific surface areas, high mechanical strength, good electrical and thermal conductivity and easy availability from graphite

[7-9]

. It

has proved to be a promising and versatile building block for the design of new catalyst, sorption materials [10,11]. Recently, the loading of semiconductor and metal nanoparticles onto RGO aerogels has spurred great interest due to their potential use in solar energy conversion and remediation of environmental pollutants[12-15]. Among the many RGO aerogels-based materials that have been reported, those involving BiOBr nanocrystals are of particular interest. As reported such a hybrid system shows promising applications in

the degradation of organic contaminants in the liquid phase through a photocatalysis process. Since the BiOBr is immobilized on the RGO aerogel surface, which could greatly simplify the separation and recycling process during practical applications. Most of the previous investigations have focused on TiO2 based catalysts because of the extensive use of such catalysts for photocatalysis

[16]

. Traditional TiO2- based

catalysts are indeed very active, stable, low cost and non-toxicity

[17]

. Nevertheless,

TiO2 have a low quantum yield with the fast charge carriers (e−/h+) recombination, therefore can only be excited by UV light, which occupies only 3-4 % of the solar irradiation. Different from TiO2, the BiOBr is characterized by a layered structure in which Br atoms are situated between [Bi2O2] layers [18,19]. This special structure could contribute to the inducing of an efficient separation of photogenerated electron–hole pairs, thus will lead to an enhanced photocatalytic activity of the catalysts under visible-light[20-23]. For instance, Wang’s group reported that BiOBr had a higher photocatalytic activity than P25 (TiO2 nanoparticles) under UV irradiation[24]. Recently our group has developed a hydrothermal method and prepared BiOBr/semi-coke composites as an emerging photo-catalyst for nitrogen monoxide oxidation under visible light

[25]

. Some recent studies point that the combining of

BiOBr in RGO arogel coould result in some new properties, especially in photocatalysis. For example, Tu et al. reported the good electronic conductivity of graphene could effectively inhibit the recombination of the electron–hole pairs in the BiOBr–RGO, which leads to the improved the efficiency of photocatalysts [26]. Several methods, including the one-pot hydrothermal method and a in-situ solvothermal method

[27]

, have been used to form inorganic BiOBr nanocrystals on

RGO aerogels. However, one of the challenges related to the integration is how to ensure the nucleation and growth of BiOBr nanocrystals selectively on the graphene via a free growth in the solution. Because it is difficult to adapt their compatibilities and interactions in addition to regulating the reduction of graphene oxide (GO)

[28]

.

The other challenge is how to control the structure, uniform dispersion, and the loading of the nanocrystals on the RGO. In this work, we report a facile two-steps hydrothermal self-assembly strategy for the

synthesis of a three-Dimensional BiOBr/RGO heterostructural aerogels. The nano BiOBr is first synthesized through a hydrothermal method, aiming to control micro-structure

and

increasing

hydrothermal-synthesized

BiOBr

its

crystallinity.

flakes

exhibit

photo-degradation of methyl orange under visible light

Jiang

et

al.

noticeable [29]

reported the activity

for

. Following that, a second

hydrothermal step is applied to anchor the micro-structured BiOBr on the surface of GO, meanwhile, dopamine (DA) worked as a reducing agent and cross-linker was added. After the hydrothermal self-assembly and a super-drying process the BiOBr/RGO hybrid aerogels were formed, the as-synthesized BiOBr/RGO composite aerogels with promoted photocatalytic performance, the controllable micro-structure morphology, specific surface area and spectral properties were investigated. The photocatalytic performance of BiOBr/RGO composite aerogel was systematically studied by the methyl orange (MO) degradation under visible-light irradiation. Moreover, the degradation activity for Rhodamine B and phenol were compared. In terms of the floating and recycling ability, we expected that the BiOBr/RGO composite aerogels could become a promising photocatalyst for broadening applications.

2. Experimental 2.1. Materials All chemical regents were of analytical grade reagents and used without further purification. Graphite powder (325 mesh, 99.8% purity) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Dopamine was provided by Shanghai Aladdin biochemical Polytron Technology Co., Ltd. Sulphuric acid (H2SO4, 98%), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30 wt %) were purchased from Tianjin Bo Di Ltd. Deionized water was used throughout this study.

2.2 Fabrication of BiOBr functionalized GO photocatalyst 2.2.1 Synthesis of BiOBr Pure BiOBr was first synthesized via a facile hydrothermal method. In a typical

procedure, 1 mmol of Bi(NO3)3·5H2O( AR, Sinopharm Chemical Reagent Co., Ltd.) was dissolved into 10 mL HNO3 solution (2 M) under magnetic stirring for 0.5 h and was indicated as suspension A: 1.2 mmol of KBr(AR, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in 10 mL deionized water under magnetic stirring for 10 min and was indicated as suspension B. Afterwards, suspension B was added dropwise into the suspension A, the mixed solution was allowed to proceed for 1h at room-temperature under ultrasonication. The resulting precursor solution was transferred into a 50 ml Teflon-lined stainless autoclave. The hydrothermal treatment was carried out at 160℃ for 12 h. After cooling, the obtained precipitates were collected and washed with deionized water for three times, and dried at 60℃ for 12 h.

2.2.2 Synthesis of GO Graphene oxide (GO) was prepared from purified natural graphite powder using an improved Hummer’s method reported by Marcano et al[30].

2.2.3 Synthesis of BiOBr/GO The BiOBr/RGO composite aerogels were synthesized via a hydrothermal method using BiOBr and GO as the precursor, dopamine (DA) as a reducing agent, as illustrated by Fig. 1. In a typical synthesis, 60 mg dopamine and 0.3 g of BiOBr were added into 30ml graphene oxide (GO) dispersion (1mg /mL−1) under magnetic stirring. In order to ensure the preparation of fully reduced graphene, the weight ratio of GO to DA was kept consistent at 1:2. After ultrasonic treatment at room temperature, the solution was transferred into a 50 mL Teflon-sealed autoclave and reacted at 100 ℃ for 12 h. The as-obtained hydrogel was collected, washed with deionized water for several times, and treated by freeze-drying to obtain BiOBr/RGO aerogel. The obtained BiOBr/RGO aerogel with RGO weight ratios of 5 wt%, 10 wt% and 20 wt%, were denoted as BiOBr-G5, BiOBr-G10, and BiOBr-G20. The pure BiOBr was through the same steps with BiOBr/RGO aerogels just without addition of GO.

Fig. 1. Schematic illustration of the fabrication process of BiOBr/RGO aerogel

2.3 Characterization The sizes and morphologies of as-prepared samples were determined by scanning electron-microscopy (SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEM-2100). The specific surface area was measured by a Micromeritics ASAP2020 Surface Area and Porosity Analyzer and calculated by using the BET equation. The phase composition of the as-prepared samples was characterized by means of X-ray diffractometer (XRD, Bruker D8 Advance with CuKa1 radiation at 40 kV and 30 mA). Raman spectrum was acquired with a Dilor XY microspectrometer using a 532 nm excitation wavelength. FTIR spectra were recorded in transmittance mode with a resolution of 4 cm−1 using a Nicolet Nexus 670 FTIR spectrometer. The photoluminescence (PL) spectrum was measured using a Hitachi F-7000 fluorescence spectrophotometer at room temperature. UV–vis diffusion reflectance spectrum of the samples were obtained on a UV-Vis spectrophotometer (UV-3600, Shimadzu, Japan)

2.4 Photocatalytic Activity. The photocatalytic activity of the as-prepared samples was determined by degradation of methyl orange (MO) under visible light irradiation. A 300W Xe lamp with a 360nm cut off filter was used as a light source. The experiments were carried out at 25℃ as follows: 0.1 g of sample was added into 100 mL of MO solution (10 mg/L).

The distance between the bottom of the lamp and the top of the solution was 20 cm. Before irradiation, the suspension was stirred for 30 min in dark to establish of adsorption–desorption equilibrium. At given time intervals, a 5 mL solution was taken and centrifuged to remove the catalyst in it. The concentration of MO left in the solution was determined by the UV–vis spectroscopy at its characteristic wavelength of 464 nm. The degradation of Rhodamine B (RhB, 10 mg/L) following a similar procedure, the concentration of RhB left in the solution was also determined by the UV–vis spectroscopy, however, at the wavelength of 554 nm. For the degradation of phenol, the concentration of phenol was determined by a 4-aminoantipyine colorimetry method, with the characteristic absorption peak at 510 nm.

3. Results and discussion 3.1 Morphology and microstructure characterizations

Fig. 2. (a) SEM image of pure BiOBr; (b) SEM image of BiOBr-G10 aerogel; (c) TEM and (d) HRTEM image of BiOBr-G10 aerogel; the inset of (b) is the image of the BiOBr-G10 aerogel.

The morphology and microstructure of BiOBr, BiOBr-RGO aerogel is investigated by SEM, TEM and HRTEM. The SEM image in Fig. 2a shows that the as-prepared BiOBr has flower-like microstructure. The nano-petal with thickness of about 32 nm and the dimension ranged from 0.5 to 0.7 μm. Representative scanning electron

micrographs of BiOBr-G10 aerogel are chosen to document the morphology of the formed hybrid aerogels. As shown in Fig. 2b and inset, the sample is comprised of RGO nanosheets, which are interconnected and formed a three-dimensional porous network with micrometer sized pores, suggesting that the 3D block was a good catalyst support and provided extending surface for anchoring the BiOBr nanoflakes. The density of BiOBr-G10 sample is measured to be 88 mg/cm3. The TEM image from Fig. 2c indicates a homogeneous distribution of the BiOBr nanoflakes on the RGO support, no aggregation of the composites is observed. The BiOBr nanoflakes is supposed to work as a spacer, which could inhibit the gathering of the RGO sheets. Furthermore, a HRTEM image in Fig. 2d shows the interplanar spacing of the crystal is 0.282 nm which corresponds well with the (102) plane of BiOBr.[31] These results suggest that BiOBr nanoflakes can be selectively loaded on RGO sheets without free growth in solution. And the RGO sheets can be individually separated and uniformly coated by BiOBr.

Fig.3. (a) XRD patterns of pure BiOBr, BiOBr-G5, BiOBr-G10 and BiOBr-G20 samples; (b) Nitrogen adsorption-desorption isotherms of as-synthesized BiOBr-G10 aerogel and pure BiOBr.

As shown in Fig.3a, the diffraction patterns of pure BiOBr, BiOBr-G5 and BiOBr-G10 could be indexed to a tetragonal phase of BiOBr. The samples show peaks of 2θ values at 13.3°, 24.3°, 34.3°, 35.7° and 47.3°, which correspond to the (001), (002), (012), (110) and (020) crystallographic planes of BiOBr. These patterns can be assigned to the standard cards (JCPDS Card No. 09-0393)

[31,32]

. It is worth

noting that the BiOBr-G10 displays sharper (110) and (102) peaks compared with other samples, which are assigned to preferential attachment along the (110) plane and related structural combinations [33], and which is also in accordance with the HRTEM results. The intense and clear diffraction peaks imply the good crystallinity of these samples. However, for the BiOBr-G20 sample, an obvious decrease on the relative intensity of the diffraction peaks can be found. It suggests the inorgnic BiOBr are mainly in the amorphous phase and the relative high content of RGO can hinder the crystallization of BiOBr. As inferred from Eenergy-dispersive X-ray measurement of the BiOBr-G10 sample (Fig.S1), the hybrids consist of 2 atom% Bi and Br, 79 atm% C. Fig. 3b demonstrates the N2 adsorption–desorption isotherm of the pure BiOBr and BiOBr-G10 aerogel. Compared with pure BiOBr (4.91 m2/g), the BiOBr-G10 aerogel displays a relatively high Brunauer-Emmett-Teller (BET) surface area (19.65 m2/g). Both of the bulk BiOBr and BiOBr-G10 exhibits a typical IV isotherm, indicating the two samples have a similar mesoporous structure[34]. The Barrett-Joyner-Halenda (BJH) analysis (Fig.S2-S3) demonstrated the existence of mesopores in the two samples. It should be noted that the adsorption-desorption isotherm of the BiOBr-G10 is not close at low relative pressure, which may result from the formation of poly-dopamine during the reducing reaction[35,36]. Both BiOBr and BiOBr-G10 had a relatively broad pore-size distribution varies from 2 nm to 85nm and 2 nm to 112 nm, respectively. The porous diameter was estimated to be about 2.5 nm for the pure BiOBr and 17.3 nm for the BiOBr-G10. The increased pore size is favorable for the mass transfer of both reactants and degradation products inside the mesoporous structure.

Fig.4. (a) FTIR spectra of pure GO, pure RGO, pure BiOBr, BiOBr-G5, BiOBr-G10 and BiOBr-G20 samples; (b) Raman spectra of pure RGO, BiOBr-G5, BiOBr-G10 and BiOBr-G20 samples. (c) Schematic illustration of the co-assembly and reducing process of graphene oxide with BiOBr.

Raman spectroscopy was conducted to analyze the crystal structures of graphite and graphene based materials. As shown in Fig. 4a, the Raman spectra of aerogels demonstrated two broad peaks centered at 1347 and 1597 cm-1, corresponding to D band and G band of RGO. [37]. To be noted, the BiOBr doped samples exhibits broader and less intense peaks compared with the pure RGO sample, which indicates a decreased crystallinity during the formation of the BiOBr/RGO aerogel. In addition, after the BiOBr doping the intensity ratio of the D band and G band (ID/IG = 1.0) shows slightly decrease compared with that of (ID/IG = 1.1) pure RGO, indicating the increased orderliness and decreased defects of RGO support on the hybrid materials. The obtained samples were further characterized by FT-IR spectra. In Fig. 4b, the vibration peak of Bi-O bond at 520 cm−1 was observed in the spectra of BiOBr and BiOBr/RGO aerogels, which was ascribed to the stretching vibration bonds in tetragonal BiOBr

[38]

. Moreover, except pure BiOBr all other FTIR spectras showed

peaks at 1090 cm-1, 1400 cm-1, 1570 cm-1 and 3460 cm-1, corresponding to C-O, C-OH, -C=C and -OH in hydroxyl and carboxyl[39,40]. Compared to pristine GO aerogel, the RGO sample shows decreased peaks intensity at C-OH (3430 cm-1, 1400 cm-1), C-O (1090 cm-1) , C=O (1720 cm-1), indicating that GO can be reduced by dopamine during the aerogel formation process. However, after combining with BiOBr, the peak intensity at 1090 cm-1 increased noticeably, which indicates the formation of new C-O bonds. As illustrated by Fig.4c, the increased C-O bonds also further confirms that the BiOBr nanoflakes have been selectively loaded on RGO sheets.

3.2 Optical characterization

Fig.5. (a) UV–vis diffusive reflectance spectra of pure BiOBr, BiOBr-G5, BiOBr-G10 and BiOBr-G20 and RGO samples; (b) Photoluminescence spectra of pure BiOBr, BiOBr-G5, BiOBr-G10 and BiOBr-G20. The inset shows an enlarged Photoluminescence spectra of

BiOBr-G5, BiOBr-G10 and BiOBr-G20 in the range of 425-525 nm.

The UV-vis diffuse reflectance spectroscopy (DRS) were used to analyze the optical properties of the as-prepared photocatalysts[41,42]. Fig. 5a shows all the samples exhibited strong adsorption in both UV and visible light regions. With the increasing of RGO content, there was a red shift of the absorbance edge and enhancement of absorption intensity in the visible light range. This can be attribute to the excellent optical absorption properties of RGO under UV and visible light. The absorbance of RGO and BiOBr-G20 had no obvious difference. The optical band energy were calculated by using the Tauc equation. The Eg value of BiOBr was about 2.67 eV. The conduction band (CB) and valence band (VB) potentials of BiOBr were calculated to be 3.01eV, 0.34eV respectively.(Supporting information Fig. S4) The photoluminescence emission spectroscopy was employed to monitors recombination of photo-excited electrons and holes of a material. The PL emission spectra for BiOBr and BiOBr/RGO photocatalysts in the range of 350–600 nm are presented in Fig. 5b. The intensity PL emissions represent the abilities of the bulk material to generate electron−hole pairs and their subsequent recombination under continuous light excitation.[43-45]. It can be observed that all the PL emission intensities of BiOBr/RGO aerogels at 466 nm are much weaker than that of the pure BiOBr. It demonstrated that the introduction of RGO can promote the separation of photo induced electron–hole pairs, which was beneficial to the enhancement of photocatalytic activity. In addition, compared with other BiOBr/RGO samples the BiOBr-G10 sample (Fig. 5b inset) shows the lowest PL emissions intensity at 466 nm, indicates the sample’s high efficiencies for the inhibit of electron−hole pairs recombination. The result is also consistant with the photocatalytic activity studies, which will be further discussed below.

3.3 Photocatalytic activity and selctivity

Fig. 6. (a) C/C0 versus time curves illustrating MO adsorption and photodegradation under visible light irradiation by pure BiOBr, BiOBr-G5, BiOBr-G10 and BiOBr-G20; (b) Cycling performance of the BiOBr-G10 aerogel for adsorption and photocatalytic degradation of MO under visible light irradiation.

The photoatalytic activities of BiOBr and BiOBr/RGO aerogels were evaluated by the MO degradation under visible light, as shown in Fig. 6a. The RGO content was found to have a significant impact on the absorption and degradation efficiency. Obviously, the BiOBr-G10 sample shows the highest activity, could degradate 94.7% MO in 2 h. For other samples, the obtained degradation rates in 2 h follow the sequence of BiOBr-G5(85.8%) > BiOBr-G20 (79.4%) > BiOBr (53.5%). The different degradation abilities of different samples is also in line with the photoluminescence emission tests, where the BiOBr-G10 sample shows the highest efficiencies for the quenching of electron−hole pairs, follows BiOBr-G5 and BiOBr-G20 exhibits the similar ability,

however are much higher than that of pure BiOBr. In addition, BiOBr-G10 aerogel had obviously higher adsorption ability, which is almost 1.33 times as high as that of BiOBr/RGO nano powder and showed the similar degradation efficiency, compared with BiOBr/RGO nano powder [46]. In a subsequent set of experiments, the kinetics of MO degradation by BiOBr/RGO aerogels was also studied (Fig. S5). In general, the photocatalytic degradation reactions could be described as a first-order reaction. The BiOBr/RGO aerogels exhibited a much higher reaction rate constant than pure BiOBr. Among them, the BiOBr-G10 sample showed the highest photocatalytic activity, with a reaction rate about 3.5 times that of pure BiOBr. To further get some indication regarding the stability of the BiOBr functionalized RGO photocatalyst during the degradation of MO, three cycles' of continues test were recorded. Fig. 6b shows the results for the BiOBr-G10 sample. Each test lasted around 150 min. After the third cycle test the photocatalytic efficiency of the BiOBr-G10 showed almost no change. The good stability achieved on the BiOBr-G10 catalyst demonstrates the RGO aerogels supported BiOBr are suitable for the recycling use in the photocatalytic degradation of pollutants in the liquid phase.

Fig.7. (a) C/C0 versus time curves illustrating MO, RhB and phenol adsorption and photodegradation under visible light irradiation by BiOBr-G10; (b) Photodegradation rate of MO, RhB and phenol in the first 60 mins under visible light irradiation by BiOBr-G10.

The degradation of RhB and phenol in the presence of BiOBr-G10 under visible-light was also investigated and the results are shown in Fig. 7a. Obviously, in the first 60 min (Fig. 7b), the BiOBr-G10 catalysts show the highest efficiency for degradation of MO, with a recorded degradation rate as high as 80%, following the degradation rate for RhB at 50% and phenol at 35%. After 150 mins, the photodegradation of both MO and RhB achieving 90%, however, for phenol the degradation rate is only 50%. Wu and coworkers have reported that a high {001} facets dominated BiOBr lamellas show better the activity over RhB than over MO[19]. They attribute to the negtive charge effects of the BiOBr samples. RhB tend to form positively charged cations and MO form anions in aqueous solutions, therefore RhB could be more easily adsorbed through the ionic interactions force. In contrast, in our case, the obtained

BiOBr-reduced graphene oxide (RGO) hybrid catalysts show better the activity over MO instead of RhB. As shown in Fig. 7a, in the first 30 mins, the BiOBr-G10 products show the highest adsorption rate for MO compared to RhB and Phenol. This is also contrary to the study from Wu et al[19]. The results indicate after combining BiOBr with RGO, the adsorption characteristic was altered. Therefore, electrostatic interaction is not the only mechanism operating in this case, the interactions with the RGO support should also be considered. A quite recent study from Xiao and Song point that besides the electrostatic interaction, graphene and its derivatives usually have a strong π–π interactions with dyes through the conjugate aromatic structure[47].This idea may explain why after combing with RGO the BiOBr-G10 exhibit a high-efficiency for adsorption of anionic MO. In addition, the relatively low adsorption efficiency to RhB are probably due to the distortion of the planar structure because of the steric hindrance of neighboring groups and the rotation of single bonds. Phenol as a neutralized specie and with only one aromatic ring therefore undoubtedly shows the lowest adsorption performance. Therefore, the adsorption capabilities towards different dyes (anionic, cationic and neutral) on the BiOBr-RGO are actually a balance of the π–π interaction and electrostatic interaction, for dyes which containing more aromatic rings the π–π interaction tends to plays a main role on the RGO supported catalyst.

3.5 Photocatalytic mechanism To get some indications on the photocatalytic mechanism of this BiOBr/RGO aerogels, different oxidants radicals scavenger solvent/gas was added. For example, ammonium oxalate (0.05 g), isopropanol (0.01M) and N2 (0.01Mpa) was used to eliminate the H+, ·OH radicals and O2-, respectively[48,49]. As shown in Fig. 8, the added isopropanol almost had no influence on the MO degradation, indicating that ·OH radicals did not play a major role in the photocatalytic process. However, after adding ammonium oxalate and N2 the photocatalytic activity have been greatly suppressed, suggesting that the photo-generated holes and ∙O2- were the main

oxidative species in the degradation of MO on BiOBr/RGO photocatalyst [31].

Fig. 8. Effects of ·OH, h+ and ∙O2- scavengers on the degradation of MO in the presence of BiOBr-G10 aerogel.

Scheme 1. Illustration for the photocatalytic degradation of MO over BiOBr/RGO aerogels

4. Conclusion In summary, a novel BiOBr/RGO composite aerogel had been successfully synthesized using a two steps hydrothermal method. The resulting BiOBr/RGO composite aerogel exhibited an improved photocatalytic activity for degradation of MO under visible light. The composition of the RGO played the decisive role, resulting in an increased activity with the RGO ratio at 10 wt%. In addition, the sample (BiOBr-G10) exhibits a changed photocatalytic selectivity under visible-light irritation as compared with pure BiOBr. The degradation rate over MO (80%) is much

higher than that over Rhodamin B (50%) and phenol (35%) in the first 60 mins. The XRD and photoluminescence emission spectroscopy characterizations indicates an increased crystallization of BiOBr and improved quenching effect of photo-excited electrons and holes could contributes to the improved photocatalytic activities. A strong π–π interactions with dyes through the conjugate aromatic structure, contributes to the high efficient selective adsorption of anionic MO. Furthermore, the heterostructured BiOBr/RGO aerogel as a bulk catalyst can be easily removed from the aqueous system for recycling. This work would pave a way for the design of high-efficiency decolorization materials based on semiconductor and RGO aerogel for applications in environmental restoration.

Acknowledgments This study was supported by State Key Laboratory of Heavy Oil Processing. This is MCTL contribution NO.127.

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