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Vol. 9. Issue 1.
Pages 610-621 (January - February 2020)
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Vol. 9. Issue 1.
Pages 610-621 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.11.001
Open Access
Performance of Ag/BiOBr/GO composite photocatalyst for visible-light-driven dye pollutants degradation
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Chenyang Lia,b, Boqiang Wangb,c, Fengjun Zhangb, Ningning Songb, Gang Liud, Cong Wangc, Shuang Zhonga,b,
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zhongshuang13@hotmail.com

Corresponding author.
a Key Laboratory of Songliao Aquatic Environment, Ministry of Education, Jilin Jianzhu University, Changchun 130118, PR China
b Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130021, PR China
c China Northeast Municipal Engineering Design and Research Institute Co., Ltd, Changchun 130000, PR China
d Jilin Province Shi Ze Environmental Protection Technology Co., Ltd, Changchun 130000, PR China
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Abstract

Based on BiOBr, an Ag/BiOBr/GO composite catalyst was constructed by doping the noble-metal Ag and the non-metal GO. A Xenon lamp was used to simulate a source of visible light, and Rhodamine B was used as target pollutant. The catalytic activity of the novel catalyst was investigated via degradation experiment. The catalysts were characterized and analyzed by XRD, SEM, TEM, XPS, FT-IR, UV vis-DRS, and EIS. Compared with BiOBr, Ag/BiOBr, and GO/BiOBr, the prepared Ag/BiOBr/GO catalyst (doped with 2.0wt.% GO and 1.5wt.% Ag) exhibited the highest photocatalytic activity, with a RhB degradation rate of up to 98% within 120min. Adding the metal Ag and GO significantly improved the photocatalytic activity of the BiOBr catalyst. This is a result of the Schottky barrier, surface plasmon resonance, and good electrical conductivity of the metal Ag as well as the large specific surface area and excellent electron transport efficiency of the non-metal GO. Their combination promoted the transfer and separation of photo-generated carriers of catalyst, and effectively limited the recombination rate of the photogenerated electron–hole. In a subsequent repeatability experiment, the catalysts still showed good photocatalytic activity and stability after five times of reuse, indicating that the composite catalyst has good stability and reusability value.

Keywords:
BiOBr
Ag
GO
Photocatalyst
Water treatment
Full Text
1Introduction

Dye wastewater (usually containing aromatic groups such as Rhodamine B, methyl orange, and methyl blue) generally has high chemical stability and complex biological toxicity [1,2]. Dye wastewater contains a large amount of organic and inorganic pollutants, and once discharged into the water without treatment, will reduce the transparency of the water and consume a large amount of dissolved oxygen. The ecological environment in the water is consequently disrupted and the biological chain in the water is eventually destroyed [3,4] At present, the common processing methods for dye wastewater include physical methods, biological methods, advanced oxidation methods, and photocatalysis methods [5–8]. Among these, semiconductor photocatalysts have attracted much attention because they can convert solar energy and have great potential for the decomposition of organic pollutants [9].

Among many semiconductor photocatalysts, bismuth-based photocatalysts are important due to their unique electronic structure [10] and good visible light response [11]. Common bismuth-based semiconductor photocatalysts mainly include bismuth tungstate [12,13] and halogenated oxygen bismuth [14–16]. Among these catalysts, bismuth oxyhalide has attracted notable research attention due to its good visible light response ability, suitable band gap, and good chemical stability [17–19]. Bismuth oxyhalide belongs to tetragonal semiconductor materials and has a unique heterogeneous layered structure, which greatly reduces the recombination rate of free electron and hole [20]. At the same time, BiOX with a layered structure has anisotropy. Such a loose structure supports polarization of adjacent atoms and orbit [17], and also facilitates the generation and separation of hole-electrons, which results in improved photocatalytic activity [21,22]. However, BiOX still has disadvantages such as low utilization of sunlight and low light quantum efficiency. Combined with the details mentioned above, researchers have developed ways to improve the migration efficiency of photogenerated electrons, further restrict electron–hole recombination, and broaden the absorption range of visible light [23,24]. Common modification methods mainly include noble-metal surface deposition and non-metal doping [25]. The Schottky barrier between the semiconductor and the noble-metal is formed by noble-metal surface deposition, where a small amount of simple substance noble-metal is deposited on the surface of the semiconductor [26], thus improving photocatalytic activity. The more commonly deposited noble-metals mainly include Pd, Au, Ag [27,28], and Pt [29]. Via modification of other anions such as carbon, nitrogen, and graphene (GO) [30–32], non-metal doping can effectively increase the surface area of materials and promote the transport and separation of charge. The adsorption effect on the pollutant is increased and the stability of the photocatalyst is improved [33].

In this paper, noble-metal deposition and non-metal doping were performed to improve the photocatalytic activity of BiOX. The composite photocatalyst was synthesized via two steps. Firstly, Ag was deposited on BiOBr to obtain the Ag/BiOBr catalyst. GO was further added to the Ag/BiOBr catalyst to obtain the final Ag/BiOBr/GO composite photocatalyst. Through the fabrication of ternary composite photocatalyst and utilization of the advantages of good electrical conductivity, surface plasmon resonance, the specific Schottky barrier of the metal, the large specific surface area, and excellent photoelectric performance of the non-metal, doping and deposition were performed on the material. The low quantum efficiency and the weak visible light absorption of BiOBr were improved. Furthermore, Rhodamine B (50mg/L) was used as target pollutant, and the photocatalytic performance was analyzed via photocatalytic degradation experiment.

2Materials and methods2.1Preparation of the Ag/BiOI/GO photocatalyst

In this paper, BiOBr was synthesized via micro-emulsion assisted solvent-thermal method. Firstly, 4.0mmol of Bi(NO3)3·5H2O was dissolved in 80.00mL of ethylene glycol monomethylether to obtain solution A. Then, 6.0mmol of (C16Mim) Br (1-hexadecyl-3-methylimidazolium bromide) was dissolved in 80.00mL of ethylene glycol monmethylether to obtain solution B. After the obtained solutions A and B were respectively stirred for 1h, solution B was slowly poured into solution A. After uniformly mixing and stirring for 1h, the obtained mixed solution was transferred to a 100.00mL high-pressure reaction kettle with polytetrafluoroethylene tank. The solution volume in the reactor did not exceed 80% of the total volume. The high-pressure reaction kettle was then heated in an electric heating constant-temperature oven at 160°C for 2h, after which, it was cooled in room temperature. The precipitate in the reactor was collected and washed three times with absolute alcohol and ultrapure water. Finally, the obtained solid was placed in a vacuum drying oven at 60°C for 2 days.

The Ag/BiOBr was prepared via the deposition method in which the Ag was deposited in the BiOBr. Firstly, 1.00g BiOBr photocatalyst was added to 100.00mL ultrapure water and stirred on a magnetic stirrer for 30min. Then, it was sonicated in an ultrasonic cleaner for 10min and stirred on a magnetic stirrer for 30min. Different amounts of AgNO3 were separately added to 50.00mL ultrapure water and briefly stirred before being added to the solution of the above photocatalyst. After stirring for 10min via magnetic stirrer, a total of 75.00mL NaBH4 was added by dropwise addition, and after brief stirring, it was washed four times with ultrapure water. Finally, the obtained solid was placed in a vacuum drying oven at 60°C for 1–2 days.

Graphene oxide (GO) was prepared from graphite powder by a modified Hummers method [34]. And the Ag/BiOBr/GO photocatalyst was synthesized by a facile chemical method. 1.00g of BiOBr photocatalyst was added to 100mL ultrapure water. Then, 0.100g AgNO3 was added to 50.00mL ultrapure water, and after brief stirring, it was added to the solution of the BiOBr photocatalyst. After magnetic stirring for 10min, the corresponding 7.50mL NaBH4 (0.010052mol/L) was added by dropwise addition. After stirring for 1h, different amounts of GO were added. After adding GO into the solution, it was stirred for 1h and then washed four times with ultrapure water. Finally, the obtained solid was placed in a vacuum drying oven at 60°C for 1–2 days.

2.2Characterization

The crystal structures and phase analysis of samples were determined by X-ray diffraction (XRD) in the range of 10–80° (2θ) using a Rigaku D/max 2500 diffractometer (Rigaku Corp., Tokyo, Japan) with Cu-K. Morphologies and microstructures were characterized with a field emission scanning electron microscope (FESEM; SU8000, Hitachi High-Technologies Corp., Tokyo, Japan) and a transmission electron microscope (TEM; Tecnai F20, FEI Co., Hillsboro, OR, USA), respectively. X-ray photoelectron spectroscopy (XPS) data were obtained on an ESCALab-250i-XL device (Thermo Fisher Scientific Inc., Waltham, MA, USA). Specific surface areas were measured through a nitrogen adsorption BET method (BET/BHJ Surface Area, 3H-2000PS1; Baishide Co., Beijing, China). Raman spectra of prepared samples were recorded on a microscopic confocal Raman spectrometer (Horiba HR800, Horiba Jobin Yvon Ltd., Kyoto, Japan) with excitation by 514.5nm laser light. UV–vis diffuse reflectance spectra (DRS) of the samples were measured by using a UV-Vis spectrophotometer (TU-1901). Photocurrent and electrochemical impedance spectroscopies (EIS) were measured by an electrochemical analyzer (CHI 660D CH Instruments, Inc., Austin, TX, USA). The photoluminescence (PL) spectra, obtained at room temperature at an excitation wavelength of 280nm, were recorded on a Hitachi F-4600 fluorescence spectrophotometer.

2.3Photocatalytic activity experiment

A 300W Xenon lamp with a UV cutoff filters (λ>400nm) was used as light source and 50mg/L of Rhodamine B was used as target pollutant. The catalyst was added at a solid-liquid ratio of 1.00g/L. Firstly, the adsorption performance of the catalyst in the experiment was examined. The mixed solution was stirred in the dark before the visible light reaction. Samples were taken every 10min, and the amount of each sample was 3.00mL. The extracted sample was centrifuged, and the concentration of Rhodamine B in the supernatant was detected via ultraviolet-visible spectrophotometer. The adsorption efficiency of the catalyst was calculated. The adsorption equilibrium time of the photocatalytic reaction was further determined. Then, the visible light source was turned on for the photocatalytic reaction experiment, and the sample container was placed 15cm away from the Xenon lamp light source. Furthermore, it was equipped with a magnetic stirring device for the photocatalytic degradation experiment, and the continuous irradiation time was 120min. Samples were extracted every 10min, and the amount of each sample was 3.00mL. The extracted sample was centrifuged, and the concentration of Rhodamine B in the supernatant was determined via ultraviolet-visible spectrophotometer. Furthermore, the degradation efficiency of the catalyst on the pollutant was calculated.

2.4Catalyst stability experiment

To evaluate the stability performance of the photocatalyst, a cycle experiment was performed on the catalyst. The reaction vessel was magnetically stirred for 0.5h in the dark prior to visible light irradiation. After visible light irradiation for 6h, the syringe was extracted with 0.5mL of the solution and filtered for the determination of Rhodamine B. The supernatant was discarded via centrifugation, and the material was washed three times with ultrapure water, and dried in a vacuum drying oven at 60°C to collect the solid. An equal amount of Rhodamine B was added for the next photocatalytic degradation reaction. This experiment was repeated five times.

3Results and discussion3.1Structure and morphology

The chemical phase and composition of different catalysts were analyzed via their X-ray diffraction pattern. Fig. 1 shows the XRD pattern analysis of different materials.

Fig. 1.

XRD patterns of different catalysts.

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Fig. 1 shows that similar diffraction peaks are observed for BiOBr, Ag/BiOBr, BiOBr/GO, and Ag/BiOBr/GO photocatalysts. Observing pure BiOBr showed that the diffraction angles (2θ) of the material exhibited strong diffraction peaks at 25.26°, 32.31°, 39.4°, 46.3°, 57.3°, 67.6°, and 76.7°, respectively. Comparison with the standard card of pure BiOBr in the standard database (JCPDS 73-2061, space group peak P4/nm) showed that the positions of diffraction peaks in the material are consistent with those of the peaks in the BiOBr standard card. These diffraction peaks correspond to the crystal diffraction planes of (011), (110), (112), (020), (212), (220), and (212) in BiOBr, respectively.

Comparison showed that the diffraction peak of the material at 2θ of 32.31° has a larger increasing tendency, indicating that the material may preferentially grow toward the (110) crystal plane. Comparison of Ag/BiOBr/GO, BiOBr/GO, Ag/BiOBr, and BiOBr showed that the characteristic peak positions of all materials remain unchanged. Remarkable enhancement occurs only at 2θ of 32.31°, 46.3°, and 57.3°. This may be due to the interaction after Ag deposition and GO doping on the BiOBr. Further comparative analysis indicated that on the XRD pattern, the peaks of Ag at 3θ, 34.2°, 44.2°, or 64.4° are not observed. Furthermore, no obvious characteristic peaks of GO are found. This may be because the amount of Ag in the sample is small and in a highly dispersed state and GO is evenly distributed on the surface of the material and covered by other diffraction peaks. Therefore, it was not found in the XRD pattern. These results indicate that the prepared samples have high crystallinity and no other impurities.

The microstructure and structural characteristics of the prepared Ag/BiOBr/GO composite catalyst were analyzed via SEM and TEM, and energy dispersive spectroscopy (EDS) was conducted. The results are shown in Fig. 2.

Fig. 2.

(a) SEM image of Ag/BiOBr/GO composite catalyst. (b) Magnified SEM image. (c) TEM image. (d) High-resolution TEM image. (e–j) EDS mapping distribution.

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Fig. 2(a) shows the SEM image of the Ag/BiOBr/GO composite catalyst. The composite catalyst consists of a large number of microsphere structures with a diameter of about 1–2μm. Fig. 2(b) shows the SEM image of the sample at high magnification. Each nano/micro-sphere of BiOBr is composed of a large number of loose nanosheets, which cross each other. Each nano-micro-sphere has a higher pore volume and pore size. This further increases the specific surface area of the material. The transfer rate of the target pollutant is increased and the photocatalytic performance is enhanced. Moreover, the multi-beam visible light can be diffracted and reflected in the material, thus increasing the photon capture rate and reducing the recombination of photogenerated electron and hole.

Fig. 2(c) and (d) shows low and high magnification TEM images of the Ag/BiOBr/GO composite catalyst, respectively. Fig. 2(c) shows that the uniform distribution of GO sheet in the material is more obvious, and at the same time, the Ag particles and the BiOBr with microsphere structure are uniformly distributed on the GO. Compared with Fig. 2(d), the adjacent lattice fringe spacing of 0.277nm can clearly be observed in the high magnification TEM image. Calculation of the Bragg equation and XRD pattern analysis indicates that this corresponds to the (110) crystal plane of BiOBr [35]. Similarly, the lattice fringe spacing of another region is 0.238nm, which corresponds to the (111) crystal plane of Ag [28]. This indicates that the material loading was successful. Therefore, through EDS mapping of the Ag/BiOBr/GO composite catalyst in Fig. 2(e)–(j), five elements of Bi, Br, C, O, and Ag were detected on the surface of the catalyst, which further proves that the composite catalyst consists of the above elements.

The morphology of each element, which was presented in the Ag/BiOBr/GO composite catalyst, was further characterized by XPS, as shown in Fig. 3.

Fig. 3.

(a) Full spectrum of the Ag/BiOBr/GO composite catalyst; (b) Bi 4f; (c) Br 3d; (d) Ag 3d; (e) C 1s; (f) O 1s.

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Fig. 3 shows the XPS spectrum of the Ag/BiOBr/GO composite catalyst. Fig. 3(a) shows the full spectrum of the sample. The Bi, Br, Ag, C, and O are all present in the sample, which corresponds to the previous EDS mapping. Fig. 3(b) shows the XPS spectrum of Bi 4f. The binding energies are 164.2eV and 158.6eV, which represents Bi 4f5/2 and Bi 4f7/2, indicating that the Bi element in the material exists in the form of Bi3+. Fig. 3(c) shows the XPS spectrum of Br 3d. The binding energy at the position where the absorption peak appears in Br 3d is 68.8eV, which corresponds to Br 33/2. Further observation of the XPS spectrum of Ag 3d in Fig. 3(d) shows that the binding energy of two characteristic absorption peaks are 367.8eV and 374.4eV, respectively, which corresponds to the morphology of Ag 3d3/2 and Ag 3d5/2, and indicates that Ag exists in metal form [20,36,37].

In Fig. 3(e), the C 1s has three characteristic peaks. The binding energies were 288.5eV, 285.9eV, and 284.6eV, respectively, which corresponds to hybrid CO bonds, COC, and CC bonds in the sp2 orbital [38,39], indicating the presence of GO in the material. Fig. 3(f) shows the XPS spectrum of O 1s. Similarly, the O 1s has also three characteristic peaks. The binding energy of 531.6eV corresponds to the characteristic peak of CO bonds, the binding energy of 529.6eV corresponds to the characteristic peak of OH bonds, and the binding energy of 523.8eV corresponds to the characteristic peak of HOCO bonds [40]. XPS spectrum analysis shows a composite catalyst with higher stability.

The surface functional group changes of pure BiOBr and BiOBr/GO, Ag/BiOBr and Ag/BiOBr/GO composite materials were analyzed and compared via FT-IR, as shown in Fig. 4(a). When the Ag and GO are doped, the absorption peak of the material becomes rich and the absorption peak is remarkably enhanced. The absorption peak at 525cm−1 is the stretching vibration peak of BiO, the absorption peak at 1405cm−1 is the stretching vibration peak of COO, and the absorption peak at 1610cm−1 is the stretching vibration peak of CO. This indicates that COOH functional groups exist on the surface of the material. Thus, it is identified that GO was successfully doped in the BiOBr.

Fig. 4.

FT-IR spectrum and Raman spectrum of different catalysts.

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The Raman spectra of the prepared catalysts showed that Ag/BiOBr/GO and BiOBr samples displayed two peaks at around 1334 and 1590cm−1, corresponding to the D-band and G-band, respectively (Fig. 4(b)). Neither of these peaks was present in spectra of BiOBr, thus confirming the successful combination of Ag/BiOI/GO.

In addition, according to the N2 adsorption–desorption isotherm measurements, the Langmuir Surface Area values of the BiOBr, BiOBr/GO, Ag/BiOBr, and Ag/BiOBr/GO are 12.81, 19.71, 4.2870, and 7.7796m2/g, respectively.

3.2Photocatalytic performance and stability3.2.1Effects of doped dosage on catalytic activity

In this study, the visible light activity of the catalyst was investigated by comparing the degradation ability of different catalysts to Rhodamine B (50mg/L). The results are shown in Fig. 5.

Fig. 5.

(a) Effect of Ag on the photocatalytic activity of BiOBr for the degradation of RhB under visible light. (b) Different first-order kinetics fit curves of Ag/BiOBr for catalytic degradation of RhB. (c) Different reaction rate constants k of catalytic degradation fit curves of Ag/BiOBr. (d) Effect of GO on the photocatalytic activity of Ag/BiOBr for the degradation of RhB under visible light. (e) Different first-order kinetics fitting curves of Ag/BiOBr/GOfor catalytic degradation of RhB. (f) Different reaction rate constants k of catalytic degradation fit curves of GO/Ag/BiOBr.

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Fig. 5(a) shows the experimental results of the Ag/BiOBr catalyst with different Ag doping amounts (0.5, 1, 1.5, and 2wt.%) for the degradation of Rhodamine B. Fig. 5(d) shows the experimental results of the Ag/BiOBr/GO catalyst with different GO doping amounts (0.5, 1, 2, and 3wt.%) for the degradation of Rhodamine B. The concentration of Rhodamine B used in the experiment was 50mg/L. At the initial stage of the experiment, the photocatalytic material was firstly subjected to a light-shielding adsorption experiment so that the pollutant Rhodamine B could reach the adsorption-desorption equilibrium on the surface of the photocatalytic material. Fig. 5(a) and (b) shows that the difference in the effects of doping amounts on the adsorption performance of the catalyst are small, and the adsorption rates of all materials are about 20%. At the second stage, under irradiation of a Xenon lamp (which simulated visible light) for 120min, showed that with increasing doping amounts of Ag and GO, these significantly affect the catalytic performance of the catalyst.

Fig. 5(b) and (e) shows first-order kinetic fitting curves of Ag/BiOBr and Ag/BiOBr/GO catalysts for the degradation of Rhodamine B. The equation is ln(C/C0)=kt, where k represents the first-order reaction rate constant, t represents the time abscissa, and ln(C/C0) represents the ordinate. Based on this, fitting was conducted. Fig. 5(a) and (b) shows that when the doped amount of Ag is 1.5wt.%, the photocatalytic activity of the catalyst is strongest. After continuous irradiation for 120min, the degradation rate of the catalyst for Rhodamine B reached 68.7%. At this time, the reaction rate constant of Ag/BiOBr is largest, and the corresponding k=0.0092min−1. Similarly, Fig. 5(d) and (e) shows that in the Ag/BiOBr/GO composite catalyst, the catalytic activity of Ag/BiOBr/GO composite catalyst is best when the doped amount of GO is 2wt.%. Compared with the degradation efficiency of the Ag/BiOBr composite catalyst, its degradation efficiency is increased by 22.5%. The reaction rate constant of the Ag/BiOBr/GO composite catalyst is k=0.0277min−1, which is about three times the reaction rate constant of the Ag/BiOBr composite catalyst. At the same time, when the doped amount of Ag and GO is continuously increased, the degradation efficiency of the catalyst for the pollutant does not continue to increase. However, a decreasing trend was found. This may be because when the doped particles are in excess, too many dopants are supported on the surface of the catalyst, thus reducing the distance of the capture point. The photogenerated electron–hole recombination is caused by the interaction of the Coulomb force, which increases the recombination rate of the electron–hole, thus reducing the photocatalytic activity.

In addition, the reaction rate constant of the Ag/BiOBr/GO composite catalyst in this study (k=0.0277min−1) is significantly larger than the similar experiment conducted by Lufeng Lu(kBiOBr=0.231min−1, kBiOBr-0.2Ag modified=0.2min−1) [37] and Yun Guo (kAg/CDots/BiOBr=0.0055min−1) [41]. In summary, by comparing different doping amounts of Ag and GO on the BiOBr and combining with the first-order kinetics fitting curves, it can be concluded that when Ag is 1.5wt.% and GO is 2wt.%, the photocatalytic activity of the Ag/BiOBr/GO composite catalyst is best.

3.2.2Activity comparison of photocatalyst

Fig. 6(a) shows that different catalysts have different efficiencies for the degradation of Rhodamine B with identical concentration. Fig. 6(a) shows that when no catalyst is added to the solution, the solution itself does not have degradation activity. Therefore, the concentration of the dye remains unchanged with time. Compared with the curves, it can be concluded that the Ag/BiOBr/GO composite catalyst exhibits the best photo-degradation ability when the relevant catalyst is added to the solution. Within 120min, the removal rate for Rhodamine B can reach about 98%. According to the first-order kinetics model, as shown in Fig. 6(b), the photocatalytic reaction rate of the Ag/BiOBr/GO composite catalyst is significantly higher than those of other materials, reaching 0.0277min−1. In summary, among the tested catalysts, the Ag/BiOBr/GO composite catalyst achieves the highest photocatalytic activity.

Fig. 6.

Different materials: (a) curve for the degradation of Rhodamine B (RhB). (b) Corresponding first-order kinetics fit curves for the degradation of RhB.

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3.2.3Stability evaluation of the catalyst

The stability of the catalyst is an important factor, which limits the wide application of the catalyst in practice. To investigate the stability of the Ag/BiOBr/GO composite catalyst, a cycle degradation experiment was performed on the catalyst, and the effect is shown in Fig. 7. Fig. 7 shows that the catalyst still exhibits good photocatalytic activity and stability after five times of reuse although the degradation rate of the pollutant is slightly reduced after each experiment. This may be related to the mass loss of the catalyst during the experiment. However, it does not affect the overall degradation of the catalyst.

Fig. 7.

Cycle experiment of the Ag/BiOBr/GO composite photocatalyst for the degradation of Rhodamine B (RhB).

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3.3Mechanism underlying the enhancement of photocatalytic activity

The optical absorption property of the Ag/BiOBr/GO composite catalyst was analyzed via UV-visible diffuse reflectance spectroscopy. Based on this, the visible light response range of the composite catalyst was investigated, and the energy gap of the sample was further estimated, as shown in Fig. 8.

Fig. 8.

BiOBr, BiOBr/GO, Ag/BiOBr, and Ag/BiOBr/GO composite catalysts: (a) UV-visible diffuse reflectance spectroscopy. (b) Fit curves of (αhv2)–(hv) for different materials. (c) Valence band energy diagram of BiOBr. (d) Schematic diagram of BiOBr, Ag/BiOBr/GO band positions.

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Fig. 8(a) shows that pure BiOBr has an absorption edge at about 430nm, indicating that the original band gap absorption mainly depends on visible light. However, the absorption range of visible light is narrow. After doping Ag and GO, the light absorption wavelength of the new catalyst has a specific red shift, indicating that the light absorption intensity of the material was significantly enhanced and the optical response range was also widened.

The calculation was performed by the Kubelka-Munk equation of αhv=K(hv−Eg)2/n, where α represents the adsorption constant, h represents the Planck constant, v represents the optical frequency, and K represents a constant that is usually 1. For BiOBr, it is n=4. The calculation result is shown in Fig. 8(b). The forbidden band energies of the composite catalyst of BiOBr, BiOBr/GO, Ag/BiOBr, and Ag/BiOBr/GO were estimated to be 2.98eV, 2.80eV, 2.81eV, and 2.76eV. Compared with pure BiOBr, the band gap of the doped material is significantly reduced. Therefore, the photocatalytic activity of the material is enhanced.

Calculated by the empirical equation of Eg=EVB−ECB, as shown in Fig. 8(d), the conduction band (ECB) positions of BiOBr and Ag/BiOBr/GO are 0.06eV and 0.28eV, respectively. The conduction band of the material moves downward, thus reducing the band gap and improving the photocatalytic activity.

The photocatalytic decomposition of organic contaminants usually happens through oxidation by OH, and O2 radicals [42]. To demonstrate the active species during the photocatalytic reaction, EPR measurement was used to scavenge the relevant active species. Fig. 9(a) shows the ESR signals of O2 trapped by DMPO in the presence of the Ag/BiOBr/GO photocatalyst. Obviously, there were no signals in the dark, but intensive characteristic signals were detected under visible light irradiation. Similarly, as exhibited in Fig. 9(b), the characteristic signals of OH can be ignored in the dark since it can also be found under visible light irradiation. The ESR results demonstrate that O2 and OH are generated during the photocatalytic degradation process, which coincides with the free radicals trapping experiments.

Fig. 9.

(a) EPR spectra for DMPO-O2 and (b) DMPO-OH under visible light irradiation with Ag/BiOBr/GO.

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Fig. 10(a) shows the PL spectra of different materials. The four photocatalytic materials have similar emission peaks, which is consistent with the positions of the absorption edge in the UV-vis DRS spectrum analysis results. Comparison of the Ag/BiOBr/GO composite catalyst with other materials shows that the luminescence peak intensity of the Ag/BiOBr/GO composite catalyst is lower than those of the other three materials. This indicates that doping of Ag and GO inhibits the recombination of the photogenerated electron–hole, thus limiting the photogenerated electron–hole recombination of the catalyst. Consequently, the catalytic activity of the catalyst is enhanced.

Fig. 10.

(a) PL spectra of different materials. (b) Schematic diagram of photocatalytic enhancement.

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Fig. 10(b) shows a schematic diagram of the photocatalytic enhancement mechanism of the Ag/BiOBr/GO composite catalyst. Ag and GO are combined with BiOBr via deposition method and doping method. Crystal structure and morphology of the Ag/BiOBr/GO composite catalyst after the combination remain unchanged. The separation efficiency of photogenerated electron and hole are significantly improved by the synergistic effect of the Schottky barrier of the noble-metal Ag and plasma surface resonance, and the large specific surface area of GO, its good electron transfer ability, and conductivity.

Furthermore, it is also possible that the Rhodamine B molecule is combined with the aromatic region on the GO sheet by π–π stacking, thus transferring the electrons on the GO to the molecules of the target pollutant, and thus enhancing reactivity. In such a case, the catalyst removes the pollutants by physical adsorption first and then via chemical reaction.

4Conclusions

In this paper, BiOBr was prepared via assisted solvothermal method using ionic liquid as template. The prepared BiOBr was observed via SEM, which showed that the material is composed of numerous microspheres. Each microsphere is about 1–2μm in diameter. Compared to the sheet and nano-belt structures, the microsphere structure has a larger specific surface area, increasing the contact area of the material and target pollutant. The photocatalytic performance is improved. The Ag/BiOBr, BiOBr/GO, and Ag/BiOBr/GO catalysts were prepared by doping the metal Ag and the non-metal GO. A Xenon lamp was used as visible light source, and Rhodamine B (50mg/L) was used as target pollutant. The added amount of catalyst was 20mg, and a photocatalytic degradation experiment was conducted. Comparison of the degradation effects of the target pollutant indicated that the Ag/BiOBr/GO composite catalyst had the best degradation effect. Within 120min, it reaches about 98% for the degradation of 50mg/L of RhB. When the doped amount of GO was 2wt.% and that of Ag was 1.5wt.%, the photocatalytic activity of the catalyst was best.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the Open Project Program of Key Laboratory of Groundwater Resources and Environment (Jilin University, Ministry of Education) and the China Postdoctoral Science Foundation [grant number 2017M621214]; also supported by the Natural Science Outstanding Young Talents Foundation of Jilin Province [grant number 20190103144JH].

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