Bi2O2CO3 Composite Photocatalyst with

Aug 14, 2014 - School of Physics and Information Technology, Shaanxi Normal University, Xi'an 710062, People's Republic of China. ‡ Materials and St...
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A Plasmonic Ag−AgBr/Bi2O2CO3 Composite Photocatalyst with Enhanced Visible-Light Photocatalytic Activity Lei Jin,† Gangqiang Zhu,*,† Mirabbos Hojamberdiev,‡ Xiancong Luo,† Congwei Tan,† Jianhong Peng,§ Xiumei Wei,† Jinping Li,† and Peng Liu† †

School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, People’s Republic of China Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan § College of Physics and Electronic Information Engineering, Qinghai University of Nationalities, Xining 810007, People’s Republic of China ‡

ABSTRACT: A plasmonic Ag−AgBr/Bi2O2CO3 composite photocatalyst was prepared by a two-step synthesis method. The asprepared Ag−AgBr/Bi2O2CO3 was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray, X-ray photoelectron, ultraviolet−visible diffuse reflection, and photoluminescence spectroscopy. As probe pollutants, rhodamine B (RhB) and methylene blue (MB) were adopted to investigate the photocatalytic activity of the Ag−AgBr/Bi2O2CO3 composite photocatalyst under visible light irradiation. The as-prepared photocatalyst showed an enhanced photocatalytic activity for the degradation of RhB and MB under visible light, which was attributed to the heterostructured Ag−AgBr/Bi2O2CO3 and surface plasmon resonance (SPR) exhibited by Ag nanoparticles. Simultaneously, high stability of the sample was also investigated by five successive photodegradations of RhB under visible light. The relationship between photocatalytic activity and the structure of Ag−AgBr/Bi2O2CO3 is discussed, and possible reaction mechanisms are also proposed.

1. INTRODUCTION Semiconductor-based photocatalysis, completely eliminating harmful organic pollutants in water, shows a tempting prospect and attracts considerable attention.1,2 In recent years, bismuth subcarbonate (Bi2O2CO3) has been one of the most appealing candidates in the treatment of dye-containing wastewaters.3,4 The application of Bi2O2CO3 as a photocatalyst for degradation of methyl orange under UV light was first reported by Cheng et al.4 The Bi2O2CO3 demonstrated high photocatalytic activity because a number of Aurivillius-related oxide families, including Bi2O2CO3, with a structure characterized by an intergrowth of [Bi2O2]2+ layers and CO32− layers orthogonally to each other, are in favor of forming platelike nanostructures and accelerating the separation of electron−hole pairs.4,5 Zheng et al.3 also studied the photocatalytic activity of Bi2O2CO3 nanostructures and found that Bi2O2CO3 platelike nanostructures with exposed {001} facets showed the best photocatalytic activity. A unique electronic structure and the ability to degrade organic pollutants efficiently make Bi2O2CO3 a good candidate for photocatalyst.6 However, Bi2O2CO3 has a wide band gap (3.2− 3.4 eV) and can only be excited by UV light.7 In order to enhance absorption of visible light, a number of strategies have so far been developed, including metal doping and fabrication of heterostructured semiconductor composites.8,9 However, use of surface plasmon resonance on noble metal to ameliorate photocatalytic activity of Bi2O2CO3 has not been reported yet. Plasmonic photocatalysts consisting of silver nanoparticles were demonstrated to be promising and suitable visible-lightactive photocatalysts.10,11 It is well-known that, under visible light irradiation, the surface of Ag nanoparticles can induce collective oscillations of the conduction electrons with a resonant frequency, named the surface plasmon resonance © 2014 American Chemical Society

(SPR), which can significantly amplify absorption of visible light.12,13 Particularly, the Ag−AgX system has been proposed to be an important photosensitive semiconductor, and several methods have been developed to fabricate the silver halide system. Wang et al.14,15 successfully synthesized efficient and stable plasmonic photocatalysts AgX/Ag (X = Cl, Br) and found that the heterostructured photocatalysts could promote the separation of photoexcited electron−hole pairs through various carrier-transfer pathways. Huang et al.16 prepared Ag/ AgBr/WO3·3H2O composite photocatalyst by an ion-exchange method, and this photocatalyst not only could kill Escherichia coli under visible light but also could degrade methylene blue in aqueous solution. Several studies have demonstrated that the Ag−AgX (X = Cl, Br, I) system can enhance photocatalytic performance of a photocatalyst under visible light by SPR of Ag nanoparticles. Furthermore, among the AgX group, the band gap of AgBr (2.6 eV)17 is narrower than that of AgCl (3.3 eV)18 and AgI (2.8 eV)19 and is conducive to be used as a highly efficient visible-light-responsive photocatalyst. Nevertheless, the micrometer-sized particles of Ag−AgBr may cause a decrease in surface area and high recombination rate during the photocatalytic process.20 The Bi2O2CO3-based composites can overcome this issue due to the unique electronic and crystal structure of Bi2O2CO3. Moreover, the Ag−AgBr/Bi2O2CO3 system allows photoexcited electron transfer between Ag−AgBr and Bi2O2CO3, suppressing the recombination of electrons and holes during the photocatalytic process. Received: Revised: Accepted: Published: 13718

May 25, 2014 August 11, 2014 August 14, 2014 August 14, 2014 dx.doi.org/10.1021/ie502133x | Ind. Eng. Chem. Res. 2014, 53, 13718−13727

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resolution transmission electron microscopic (HRTEM) images were taken with a JEM-2100F (JEOL) at an acceleration voltage of 200 kV. The chemical states of elements in the final product were analyzed on an Escalab MKII X-ray photoelectron spectrometer (XPS, VG Scienta) with Mg Kα radiation. UV− vis diffuse reflectance spectra were recorded on a Lambda 950 UV−vis−NIR spectrophotometer (PerkinElmer) in the wavelength range 200−800 nm. The photoluminescence (PL) spectra of the samples were measured with a Horiba Fluoromax-4 spectrophotometer (Horiba Jobin Yvon) with an excitation wavelength of 320 nm at room temperature. The specific surface area (SBET) values were obtained from N2 adsorption−desorption isotherms at 77 K (ASAP 2010, Micromeritics), on samples preheated at 120 °C for 12 h in vacuum. The SBET values were calculated by the Brunauer− Emmett−Teller (BET) method. 2.3. Photodegradation Experiments. Photocatalytic activity of the prepared samples was evaluated toward the photodegradation of rhodamine B (RhB) and methylene blue (MB) in aqueous solutions at ambient temperature. In detail, 0.05 g of the prepared photocatalyst sample was dispersed in 50 mL of RhB/MB (10 mg/L) aqueous solution in a glass reactor. Prior to irradiation, the suspension was kept in the dark for 30 min to ensure adsorption−desorption equilibrium. The light source was a 300 W xenon lamp, and two cutoff filters were used to occlude light below 420 nm and above 760 nm to ensure the photoreaction occurs under visible-light irradiation. During visible light irradiation, 2 mL of suspension was taken out at certain time intervals for subsequent analysis of RhB/MB concentration. The RhB/MB concentration was analyzed by use of a U-3010 UV−vis spectrophotometer (Hitachi). Photodegradation experiments with mixed RhB and MB aqueous solutions (CMB = 10 mg/L and CRhB = 10 mg/L) were also performed under identical experimental conditions.

In this work, the Ag−AgBr/Bi2O2CO3 nanojunction system consisting of Ag, AgBr, and bismuth subcarbonate was fabricated as an efficient visible-light-responsive photocatalyst because Bi2O2CO3 provides large specific surface area, Ag nanoparticles give a surface plasmon resonance (SPR) effect, and AgBr is visible-light-responsive, and the visible-light-active Ag−AgBr and Bi2O2CO3 demonstrate synergism in the composite photocatalyst. Therefore, the ternary Ag−AgBr/ Bi2O2CO3 system is expected to show much higher photocatalytic activity compared to the binary systems (Ag−AgBr, Ag/Bi2O2CO3, and AgBr/Bi2O2CO3) and single-component (Bi2O2CO3) photocatalyst. Therefore, the photocatalytic activity of Ag−AgBr/Bi2O2CO3 composite photocatalyst was investigated by degradation of organic pollutants in contrast to Bi2O2CO3, Ag/Bi2O2CO3, Ag−AgBr, and AgBr/Bi2O2CO3. Possible photodegradation mechanisms of rhodamine B and methylene blue over the prepared Ag−AgBr/Bi 2 O 2CO3 composite photocatalyst was discussed.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ag−AgBr/Bi2O2CO3 Composite Photocatalyst. 2.1.1. Preparation of Bi2O2CO3 Porous Microspheres. All chemicals were of analytical grade and used without further purification. The Bi2O2CO3 porous microspheres were prepared by a typical synthesis procedure: first, Bi(NO3)3·5H2O was dissolved in 40 mL of dilute HNO3 solution (1 M) under stirring for 10 min until a clear solution was formed. After that, 3 mmol of citric acid was introduced into the solution. The pH of the solution was adjusted to 4.2 with the dropwise addition of NaOH aqueous solution under magnetic stirring at room temperature. The well-homogenized suspension was then transferred into a 100 mL Teflon-lined stainless steel autoclave with a filling capacity of about 80% and maintained at 180 °C for 24 h. After the hydrothermal reaction, the milky-white-colored precipitates were collected, washed with deionized water and ethanol several times, and dried at 75 °C for 8 h. 2.1.2. Preparation of Ag−AgBr/Bi2O2CO3 Composite Photocatalyst. The Ag−AgBr/Bi2O2CO3 composite photocatalyst was prepared by deposition−precipitation and photoreduction methods. First, the Bi2O2CO3 powders (0.5 g) were dispersed in 30 mL of deionized water under vigorous stirring for 30 min, and then 0.087 g of cetyltrimethylammonium bromide (CTAB) was introduced into the suspension. Afterward, a moderate amount of AgNO3 in 2 mL of NH4OH was added to the above suspension and it was magnetically stirred for 3 h. The resulting suspension was then dispersed in deionized water under light irradiation (300 W xenon lamp) for 30 min to obtain Ag nanoparticles. The product was collected, washed with deionized water and ethanol several times, and dried at 75 °C for 8 h. Finally, a graycolored Ag−AgBr/Bi2O2CO3 composite photocatalyst was obtained. 2.2. Characterization. Crystalline phases of the prepared samples were identified by X-ray powder diffraction (XRD) on a D/Max2550 X-ray diffractometer (Rigaku) with Cu Kα radiation (λ = 1.5406 Å). The powder samples were scanned at a scanning rate of 8°·min−1 in the 2θ range 10−70° at 40 kV and 50 mA, respectively. The morphology, particle size, and chemical composition of the prepared samples were examined by use of a Quanta 200 scanning electron microscope (SEM; FEI) equipped with an energy-dispersive X-ray spectrometer (EDS). Transmission electron microscopic (TEM) and high-

3. RESULTS AND DISCUSSION 3.1. Characterization of Prepared Composite Photocatalyst. Figure 1 shows XRD patterns of the prepared Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/ Bi2O2CO3 composite photocatalysts. As can be seen in Figure 1a, the diffraction peaks of the XRD pattern can be indexed to tetragonal phase Bi2O2CO3 (ICDD-PDF 41-1488). Figure

Figure 1. XRD patterns of prepared (a) Bi2O2CO3, (b) Ag/Bi2O2CO3, (c) AgBr/Bi2O2CO3, and (d) Ag−AgBr/Bi2O2CO3. 13719

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Figure 2. (a) XPS survey spectra of Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3. (b−d) High-resolution XPS spectra of (b) Bi 4f, (c) C 1s, and (d) Ag 3d.

assigned to Bi 4f7/2 and Bi 4f5/2, respectively. In Figure 2c, the peaks at binding energy 284.6 eV are attributed to carbon contaminant,21 whereas the peaks at binding energy 288.7 eV are related to the carbonate ion in the Bi2O2CO3 sample. As shown in Figure 2d, for the Ag−AgBr/Bi2O2CO3 sample, the peaks at 368 and 375 eV correspond to Ag 3d5/2 and Ag 3d3/2, respectively. It is noted that the Ag 3d5/2 peak is further divided into two separate peaks at 367.6 and 368.3 eV, while the Ag 3d3/2 peak is also divided into two separate peaks at 373.6 and 374.3 eV. The peaks at 367.6 and 373.6 eV are attributed to Ag+ of AgBr, and the peaks at 368.3 and 374.3 eV are assigned to metallic Ag.22 The results of XPS analysis confirm the presence of Ag and AgBr in the prepared composite samples. The morphology and microstructure of the prepared samples were investigated by SEM. As shown in Figure 3, Bi2O2CO3, Ag/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples consist of spherical particles with an average diameter of about 6−8 μm. The magnified SEM images of those samples depicted in Figure 3 show that porous spherical structures were formed by selfassembly of nanosheets with thicknesses ranging from 20 to 40 nm. This kind of porous spherical structure is expected to have a large specific surface area (SBET) that is beneficial for the adsorption of various contaminants from aqueous solution. According to the SEM images of Ag−AgBr/Bi2O2CO3 sample shown in Figure 3e,f, there are some Ag and AgBr particles attached to the nanosheets of Bi2O2CO3. TEM and HRTEM images of Bi2O2CO3 and Ag−AgBr/ Bi2O2CO3 samples are shown in Figure 4. From the TEM images shown in Figure 4a,c, it can be said that the porous spherical structures are composed of nanosheets, which is

1panels b, c, and d show XRD patterns of the prepared Ag/ Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 composite photocatalysts, respectively. However, the diffraction peaks belonging to cubic Ag phase are not observable in the XRD patterns, making it difficult to confirm the presence of Ag phase in the composite photocatalyst. Nevertheless, the color of both composite photocatalysts (Ag/Bi2O2CO3 and Ag−AgBr/ Bi2O2CO3) became gray after irradiation with a 300 W xenon lamp for 30 min. Therefore, it can be deduced that Ag nanoparticles were formed on the surface of the samples. The diffraction peaks corresponding to cubic AgBr phase (ICDDPDF 06-0438) can be seen in the XRD patterns of AgBr/ Bi2O2CO3 and Ag−AgBr/Bi2O2CO3 composite photocatalysts (Figure 1c,d). Hence, it can be inferred that AgBr was precipitated by the reaction between Ag+ and Br− in aqueous solution. In order to further determine the chemical state of elements present in the synthesized samples, Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples were analyzed by X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 2. Figure 2a shows wide-scan XPS spectra of Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples. As shown, the presence of bismuth (Bi 4f, Bi 4d, and Bi 5d), oxygen (O 1s), and carbon (C 1s) in the four samples is confirmed. For Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples, the XPS spectra represent a peak at approximately 370 eV corresponding to Ag 3d. Furthermore, high-resolution XPS spectra of Bi, C, and Ag of the synthesized samples are separately shown in Figure 2b−d. In Figure 2b, the peaks at 158.8 and 164.3 eV are 13720

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with spacing d = 0.288 nm match with the (200) crystallographic plane of AgBr. It is clear from TEM and HRTEM results that AgBr nanoparticles were deposited on flat surfaces of Bi2O2CO3 nanosheets with exposed {001} facets. Additionally, the elemental distribution of Ag−AgBr/ Bi2O2CO3 composite photocatalyst was analyzed by energydispersive X-ray spectrometry (EDS). Figure 5a shows a SEM

Figure 3. SEM images of (a, b) Bi2O2CO3, (c, d) Ag/Bi2O2CO3, and (e, f) Ag−AgBr/Bi2O2CO3 samples.

Figure 5. (a) SEM image, (b−e) elemental maps, and (f) EDS spectrum of spherical structure of Ag−AgBr/Bi2O2CO3 composite photocatalyst.

image of a single spherical structure of Ag−AgBr/Bi2O2CO3 composite photocatalyst. The elemental distribution of the Ag− AgBr/Bi2O2CO3 composite photocatalyst is shown in Figure 5b−e. As can be seen, the elements (O, Br, Bi, and Ag) are uniformly distributed in the spherical structure of Ag−AgBr/ Bi2O2CO3 composite photocatalyst. Particularly, the results of elemental mapping in Figure 5 evidence that Bi and O are inside the porous spherical structure, whereas Br and Ag are scattered on the outer surface of the porous spherical structure of Ag−AgBr/Bi2O2CO3 composite photocatalyst. The elemental composition of the Ag−AgBr/Bi2O2CO3 composite photocatalyst was approximately estimated by EDS analysis. The EDS spectrum shown in Figure 5f also reveals that the Ag−AgBr/ Bi2O2CO3 composite photocatalyst contains only Bi, C, O, Ag, and Br. The UV−vis diffuse reflectance spectra of Bi2O2CO3, Ag/ Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples are shown in Figure 6a. From the UV−vis diffuse reflectance spectra (Figure 6a), it is evident that all the samples have absorption edges in the UV region and no obvious shift is observed. For the Ag/Bi2O2CO3 sample, the absorption in the range 400−700 nm is ascribed to surface plasmon resonance (SPR) of Ag nanoparticles on the surface of the Ag/Bi2O2CO3 sample. For the AgBr/Bi2O2CO3 sample, absorption in the

Figure 4. TEM and HRTEM images of (a, b) Bi2O2CO3 and (c, d) Ag−AgBr/Bi2O2CO3 samples.

consistent with the SEM results shown in Figure 3. Figure 4b,d shows HRTEM images of Bi2O2CO3 and Ag−AgBr/Bi2O2CO3 samples. In Figure 4b, lattice fringes with spacing d = 0.273 and 0.272 nm are separately ascribed to the (110) and (11̅0) crystallographic planes of single-crystalline Bi2O2CO3 nanosheet, implying that the Bi2O2CO3 single crystals have exposed {001} facets. As shown in Figure 4d, lattice fringes with spacing d = 0.273 nm correspond to the (110) crystallographic plane of single-crystalline Bi2O2CO3 nanosheet, whereas lattice fringes 13721

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Figure 6. (a) UV/vis diffuse reflectance spectra and (b) PL spectra of Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples.

range 400−700 nm is enhanced, in comparison with Bi2O2CO3, due to the presence of AgBr in the composite. In comparison with Ag/Bi2O2CO3 and AgBr/Bi2O2CO3 samples, the Ag− AgBr/Bi2O2CO3 composite photocatalyst shows stronger absorption in the range 400−700 nm. As a crystalline semiconductor, optical absorption near the band edge follows the formula αhν = A(hν − Eg)n/2,7,8 where α, ν, Eg, and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively. Among them, n depends on the characteristics of the transition in a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). According to the given equation, the value of n was estimated to be 4, indicating an indirect transition of Bi2O2CO3. The band gap (Eg) values of the samples are estimated from a plot of (αhν)1/2 versus photon energy (hν), and the result is shown in the inset of Figure 6a. As is known, the separation rate of photoexcited electron−hole pairs is also an important factor that affects the photocatalytic activity of a photocatalyst. Also, efficiency of photoinduced charge carrier trapping, transfer, and separation on the semiconductor surface or near-surface was examined by photoluminescence spectrometer,23,24 and the results are shown in Figure 6b. As shown, the intensity of the emission peak at 364 nm is comparatively higher for Bi2O2CO3 sample compared to that of Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag− AgBr/Bi2O2CO3 samples. With the incorporation of Ag nanoparticles in Bi2O2CO3, the Schottky barrier is formed at the Ag/Bi2O2CO3 interface. Therefore, Ag nanoparticles can easily capture the photoexcited electrons from Bi2O2CO3 and then transfer to the conduction band, resulting in lower recombination rate of e−/h+ pairs. As Br− generated on the AgBr can be oxidized to Br0 by holes, consuming the photoexcited h+ and leading to the separation of electron− hole pairs,25 Ag−Br/Bi2O2CO3 shows a lower recombination rate of e−/h+ pairs than Bi2O2CO3. The intensity of emission peak at 364 nm for Ag−AgBr/Bi2O2CO3 is lower than that of Ag/Bi2O2CO3 and AgBr/Bi2O2CO3 samples due to electron transfer between Ag/AgBr and Bi2O2CO3. Hence, the Ag− AgBr/Bi2O2CO3 composite photocatalyst can exhibit lower recombination rate of e−/h+ pairs compared with Bi2O2CO3, Ag/Bi2O2CO3, and AgBr/Bi2O2CO3 samples, leading to higher photocatalytic activity. Nitrogen adsorption−desorption isotherms of the prepared samples are shown in Figure 7. As shown, the samples display typical type II isotherms indicating the presence of mesopores,

Figure 7. Nitrogen adsorption/desorption isotherms of Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples.

which is comparable to the SEM and TEM results. The BET surface areas of Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples are 33.61, 38.84, 41.38, and 40.05 m2/g, respectively. 3.2. Photocatalytic Activity and Photostability of Samples. Rhodamine B (RhB) and methylene blue (MB) in aqueous solutions were used as probe dye pollutants to investigate the photocatalytic activity of the prepared samples under visible light irradiation, and the results are plotted in Figure 8. Figure 8a shows MB adsorption curves of the asprepared samples in the dark, indicating that the adsorption− desorption equilibrium was reached after 15 min. After the establishment of adsorption−desorption equilibrium, the samples were irradiated by visible light. From Figure 8b, it can be seen that the characteristic absorption peak of MB at 664 nm decreases successively with increasing visible light irradiation time and disappears within 12 min. Figure 8c shows the variation of MB concentration (C/C0) against photodegradation time over the prepared photocatalyst samples. It is obvious that the MB concentration had almost no change with increasing visible light irradiation time without photocatalyst samples. As shown in Figure 8c, the Ag−AgBr/Bi2O2CO3 composite photocatalyst exhibits the highest photocatalytic activity compared with Bi2O2CO3, Ag/Bi2O2CO3, and AgBr/ Bi2O2CO3 samples under visible light irradiation. After 12 min 13722

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Figure 8. (a) Absorption properties, (b) UV−vis spectra evolution at given time intervals, (c) photodegradation efficiency, and (d) photocatalysis kinetics for MB in aqueous solution by prepared Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples.

and Ag−AgBr/Bi2O2CO3 samples, calculated from the data shown in Figure 9d, are 0.06584, 0.11552, 0.1993, and 0.3631 min−1, respectively. By considering the results from photodegradation experiments, shown in Figures 8 and 9, it can be concluded that the prepared Bi2O2CO3, Ag/Bi2O2CO3, AgBr/ Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples show better photocatalytic activity for degradation of RhB than MB under visible light irradiation. In general, wastewater from the textile industry contains a mixture of various kinds of organic dyes. Therefore, it is necessary to test photocatalytic activity of the prepared Ag− AgBr/Bi2O2CO3 composite photocatalyst for photodegradation of mixed dye solutions. Figure 10 shows photocatalytic activity of the Ag−AgBr/Bi2O2CO3 composite photocatalyst for degradation of mixed dyes (RhB and MB) in aqueous solution. It can be noted in Figure 10a that there are two characteristic absorption peaks at 554 and 664 nm for RhB and MB, respectively. The two characteristic absorption peaks of RhB and MB at 554 and 664 nm nearly disappear after 18 min of visible light irradiation. These results indicate that the Ag− AgBr/Bi2O2CO3 composite photocatalyst also exhibits high photocatalytic activity for degradation of mixed dye pollutants. The photodegradation efficiency for RhB and MB of the Ag− AgBr/Bi2O2CO3 composite photocatalyst is shown in Figure 10b. As shown, there is no significant difference between the photodegradation efficiency of RhB and MB. That is to say that the prepared Ag−AgBr/Bi2O2CO3 composite photocatalyst can simultaneously degrade two different dyes under one photocatalytic process, suggesting that the prepared Ag−AgBr/ Bi2O2CO3 composite photocatalyst has high photocatalytic activity for degradation of organic dye pollutants in wastewater.

of visible light irradiation, the MB photodegradation efficiency was 30%, 53%, 66%, 80%, and 100% for Bi2O2CO3, Ag/ Bi 2 O 2 CO 3 , Ag/AgBr, AgBr/Bi 2 O 2 CO 3 , and Ag−AgBr/ Bi2O2CO3 samples, respectively. In order to understand the reaction kinetics of MB photodegradation in our experiments, a pseudo-first-order model was used, as expressed by ln(C0/C) = k1t, where C0 and C are the concentrations of MB in solution at time 0 (the time to obtain adsorption−desorption equilibrium) and time t, respectively, and k1 is the pseudo-first-order reaction rate constant. The pseudo-first-order reaction rate constants k1 for the Bi2O2CO3, Ag/Bi2O2CO3, Ag/AgBr, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples, calculated from the data shown in Figure 8d, are 0.01166, 0.04046, 0.06542, 0.10098, and 0.33311 min−1, respectively. Figure 9 shows RhB adsorption curves, time-dependent UV− vis spectra, photodegradation efficiency, and photodegradation kinetics of RhB in aqueous solution over prepared samples versus time. The RhB adsorption curves of prepared samples in the dark are shown in Figure 9a, indicating that adsorption− desorption equilibrium was reached after 15 min. Figure 9b shows time-dependent UV−vis spectra of RhB in aqueous solution after the photodegradation reaction with Ag−AgBr/ Bi2O2CO3 sample at different times. Within 12 min of visible light irradiation, the absorption peak of RhB at 554 nm decreases dramatically as the irradiation time increases and completely disappears at 12 min. As shown in Figure 9c, the RhB photodegradation efficiency was 70%, 82%, 93%, and 100% for Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag− AgBr/Bi2O2CO3 samples, respectively, after 12 min of visible light irradiation. The pseudo-first-order reaction rate constants k1 for prepared Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, 13723

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Figure 9. (a) Absorption properties, (b) UV−vis spectra evolution at given time intervals, (c) photodegradation efficiency, and (d) photocatalysis kinetics for RhB in aqueous solution by prepared Bi2O2CO3, Ag/Bi2O2CO3, AgBr/Bi2O2CO3, and Ag−AgBr/Bi2O2CO3 samples.

Figure 10. (a) UV−vis spectra evolution at given time intervals and (b) photodegradation efficiency of mixed dyes (RhB and MB) in aqueous solution by prepared Ag−AgBr/Bi2O2CO3 sample.

activity of the Ag−AgBr/Bi2O2CO3 composite photocatalyst still remained high even after five cycles. After five cycles, the photocatalytic activity of the sample decreased less than 20%, probably due to the loss of photocatalyst powders during recycling or some intermediates covered on the surface of photocatalyst particles. The reused Ag−AgBr/Bi2O 2CO3 composite photocatalyst powders were collected after the fifth cycle, dried, and examined by XPS. The XPS wide-scan spectra of the Ag−AgBr/Bi2O2CO3 composite photocatalyst before the first cycle and after the fifth cycle of photodegradation reaction

Recycling as well as maintaining high photocatalytic activity are the critical issues for long-term use of photocatalysts in practical applications. It is known that photocorrosion or photodissolution may occur on the photocatalyst surface during the photocatalytic reaction. To test the photostability of Ag− AgBr/Bi2O2CO3 composite photocatalyst for degradation of RhB, the photocatalyst sample was reused five times. Each RhB photodegradation experiment with Ag−AgBr/Bi2O2CO3 composite photocatalyst was carried out under identical experimental conditions. As shown in Figure 11a, the photocatalytic 13724

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Figure 11. Recycling experiments for photodegradation of RhB by Ag−AgBr/Bi2O2CO3 composite photocatalyst under visible light for five cycles.

Figure 12. Impacts of different scavengers on (a) MB and (b) RhB photodegradation under visible light irradiation.

oxalate monohydrate to the reaction solution. That means that holes (h+) are the dominant reactive species in the photocatalytic oxidation process of MB under visible light irradiation. With the introduction of N2 into the reaction solution, photocatalytic efficiency was also decreased. The reduced photocatalytic activity of Ag−AgBr/Bi2O2CO3 composite photocatalyst in the presence of N2 indicates that O2 acts primarily as an efficient electron trap, leading to generation of • O2− and preventing the recombination of electrons and holes. Figure 12b shows the impact of different scavengers on RhB degradation under visible light irradiation. It is obvious that addition of AO and N2 shows a major effect on the photodegradation process, manifesting that h+ and •O2− played a significant role in photodegradation of RhB. Therefore, it can be deduced that h+ and •O2− have a major impact on the enhancement of photodegradation of MB and RhB dyes over Ag−AgBr/Bi2O2CO3 composite photocatalyst. 3.3. Reaction Mechanisms. As demonstrated above, Ag− AgBr/Bi2O2CO3 composite photocatalyst shows high photocatalytic activity for photodegradation of RhB and MB dyes under visible light, and the possible mechanisms for the enhancement in photocatalytic activity of Ag−AgBr/Bi2O2CO3 composite photocatalyst are schematically illustrated in Scheme 1. Mechanisms for the photodegradation of dye over Bi2O2CO3 were previously reported.27 It is a typical photosensitization degradation mechanism for dye in aqueous solution over the wide-gap Bi2O2CO3 photocatalyst under visible light irradi-

are shown in Figure 11b. As can be noticed, the XPS spectra show no significant difference between the Ag−AgBr/ Bi2O2CO3 composite photocatalyst powders before the first cycle and after the fifth cycle of photodegradation experiments. The obtained results confirm that the Ag−AgBr/Bi2O2CO3 composite photocatalyst has good photostability under visible light irradiation. In the photocatalytic oxidation process of dye molecules, a series of photoinduced reactive species, including •OH, •O2−, or h+, will be directly involved in the process after the electron− hole pairs are generated by photocatalyst under light irradiation.26 In order to discriminate the impact of photoinduced reactive species on photodegradation of MB and RhB, isopropyl alcohol (IPA), ammonium oxalate monohydrate (AO), and N2 were separately introduced into reaction solutions containing MB and RhB and the Ag−AgBr/ Bi2O2CO3 composite photocatalyst. In this study, 1.0 mmol/ L IPA and AO and N2 gas at a flow rate of 100 mL/min were separately introduced in the reaction solution to investigate the generation of •OH, h+, and •O2− under visible light irradiation, respectively. The variation of photodegradation efficiency of MB and RhB over Ag−AgBr/Bi2O2CO3 composite photocatalyst is shown in Figure 12. As shown in Figure 12a, the addition of IPA shows a minor effect on the photodegradation process, confirming that •OH played a weaker role in the photodegradation of MB. Furthermore, photocatalytic efficiency significantly decreased with addition of ammonium 13725

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h+ + A → CO2 + H 2O

Scheme 1. Schematic Illustration of Generation and Transformation of Electrons and Holes within Ag−AgBr/ Bi2O2CO3 Composite Photocatalyst under Visible Light Irradiation

4. CONCLUSIONS A plasmonic Ag−AgBr/Bi2O2CO3 composite photocatalyst was successfully prepared by a two-step chemical method. SEM, TEM, and HRTEM observations revealed that the Bi2O2CO3 porous spherical structures are composed of nanosheets with exposed {001} facets. The Ag−AgBr/Bi2O2CO3 composite photocatalyst exhibited higher visible-light-induced photocatalytic activity for photodegradation of RhB, MB, and a mixture of RhB and MB in aqueous solutions than the Bi2O2CO3, AgBr/Bi2O2CO3, and Ag/Bi2O2CO3 samples due to its special morphology, heterostructure, and surface plasmon resonance of Ag nanoparticles. What is more, the prepared Ag− AgBr/Bi2O2CO3 composite photocatalyst also showed good photostability under visible light and high recyclability for degradation of RhB in aqueous solution, demonstrating the possibility of its being used in industrial wastewater treatment in the future.

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ation. During visible light irradiation, organic dye (denoted as A) will be stimulated by visible light to single and triplet states (denoted as A*). The electron (e−) will transfer to the conduction band (CB) of Bi2O2CO3 (as step i) and ultimately reacts with the surface adsorbates O2 to form •O2− (as step v), inducing the degradation of organic contaminants. As a narrow band gap semiconductor, in AgBr (Eg = 2.6 eV), there will be an interband transition in electrons of the valence band (VB) and the electron−hole pairs can be segregated (as step ii) under visible light irradiation.28 The surface plasmon resonance (SPR) of Ag nanoparticles in the Ag−AgBr/Bi 2 O 2 CO 3 composite will additionally enhance visible light absorption. As the work function of AgBr (AgBr = 5.3 eV) is higher than that of Ag (Ag = 4.25 eV),29 the energetic electrons from the plasmon-excited Ag nanoparticles will inject into the conduction band of AgBr (as step iii). On the basis of relative position of conduction band, the photogenerated electrons will transfer from the CB of AgBr (0.07 eV)30 to the CB of Bi2O2CO3 (0.16 eV)8 (as step iv). This process is beneficial for separation of the photogenerated charge carriers, leading to higher photocatalytic activity with the incorporation of AgBr nanoparticles. In addition, the photogenerated holes on the surface of AgBr will react with Br− to form Br0 (as step vi). The Br0 atoms are reactive radical species that oxidize the surfaceadsorbed organic pollutants, and the Br0 atoms will be reduced to form Br− finally. The resultant Br− will react with Ag+ to form AgBr again, maintaining the stability of the photocatalyst sample under visible light irradiation. The detailed photodegradation reaction processes are as follows:

A + visible light → A*

(1)

A* + Bi 2O2 CO3 → •A+ + Bi 2O2 CO3 (e−)

(2)

Bi 2O2 CO3 (e−) + O2 → •O−2

(3)

•+

A + •O−2 → CO2 + H 2O

(4)

AgBr + visible light → e− + h+

(5)

h+ + AgBr → Br 0 + Ag +

(6)

Br 0 + A → CO2 + H 2O + Br −

(7)

Ag + + Br − → AgBr

(8)

(9)

AUTHOR INFORMATION

Corresponding Author

*Tel/fax: +86-29-81530750. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51102160, 11104175, and 51272148), the Fundamental Research Funds for the Central Universities (Grants GK201402009 and GK201401003), and College Students’ Innovative Projects of the State Ministry of Education (Programs 201310718002 and cx13035). M.H. thanks the Japan Society for the Promotion of Science (JSPS) for financial support.

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