Oil-in-Water Self-Assembled Synthesis of Ag@AgCl Nano-Particles on Flower-like Bi2O2CO3 with Enhanced Visible-Light-Driven Photocatalytic Activity

In this work, a series of novel flower-like Ag@AgCl/Bi2O2CO3 were prepared by simple and feasible oil-in-water self-assembly processes. The phase structures of as-prepared samples were examined by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), X-ray fluorescence spectrometer (XRF), etc. The characterization results indicated that the presence of Ag@AgCl did not affect the crystal structure, but exerted a great influence on the photocatalytic activity of Bi2O2CO3 and enhanced the absorption band of pure Bi2O2CO3. The photocatalytic activities of the Ag@AgCl/Bi2O2CO3 samples were determined by photocatalytic degradation of methylene blue (MB) under visible light irradiation. The Ag@AgCl (10 wt %)/Bi2O2CO3 composite showed the highest photocatalytic activity, degrading 97.9% MB after irradiation for 20 min, which is over 1.64 and 3.66 times faster than that of pure Ag@AgCl (calculated based on the equivalent Ag@AgCl content in Ag@AgCl (10 wt %)/Bi2O2CO3) and pure Bi2O2CO3, respectively. Bisphenol A (BPA) was also degraded to further prove the degradation ability of Ag@AgCl/Bi2O2CO3. Photocurrent studies indicated that the recombination of photo-generated electron–hole pairs was decreased effectively due to the formation of heterojunctions between flower-like Bi2O2CO3 and Ag@AgCl nanoparticles. Trapping experiments indicated that O2−, h+ and Cl° acted as the main reactive species for MB degradation in the present photocatalytic system. Furthermore, the cycling experiments revealed the good stability of Ag@AgCl/Bi2O2CO3 composites. Based on the above, a photocatalytic mechanism for the degradation of organic compounds over Ag@AgCl/Bi2O2CO3 was proposed.

Up until now, various morphologies of Bi 2 O 2 CO 3 have been obtained. For instance, Zheng and co-workers have synthesized flower-like, sponge-like, porous spheres and plate-like Bi 2 O 2 CO 3 samples, and have investigated their photocatalytic properties with regards to the degradation of organic pollutants under solar light [14]. They found that the flower-like Bi 2 O 2 CO 3 assembled by nanoflakes showed the best photocatalytic performance in which over 95% of the RhB solution (10-5 M) was degraded after approximately 50 min. The Bi 2 O 2 CO 3 microflowers synthesized by Cheng et al. could degrade 83% methyl orange (MO) within 150 min, which was much higher than that of commercial Bi 2 O 2 CO 3 [15].
However, Bi 2 O 2 CO 3 with the wide band gap can only absorb UV light (less than 5% of the solar light spectrum), restricting its practical application under solar light [16,17]. On the other hand, the photocatalytic efficiency of a photocatalyst is also usually limited by its ability to separate and/or combine photo-generated electron-hole pairs. Therefore, it is necessary to develop an effective strategy to improve charge separation efficiency and to enhance the visible light responsive activity of photocatalysts.
To address this issue, an enormous amount of research effort has been focused on combining Bi 2 O 2 CO 3 with lower band semiconductors such as MoS 2 [18], g-C 3 N 4 [19], Fe 2 O 3 [20], etc. to extend the absorption range. Previous reports revealed that silver@silver halide [21] (Ag@AgX (X = Cl, Br and I)) photocatalysts possess excellent visible light driven photocatalytic degradation efficiencies for organic pollutants and disinfection capabilities due to the localized surface plasmon resonance (SPR) effect exhibited by Ag nanoparticles (NPs). The surface plasmon resonance (SPR) of Ag nanoparticles has been used for photocatalysts due to its excellent absorption in the visible light region. Ag/AgX (X = Cl, Br and I) have become the focus of the photocatalysts due to their extraordinary plasma effect for specific applications, such as visible light harvesting for bacteria destruction and the degradation of organic pollutants in water [22,23]. Importantly, recent works have led to the development of Ag@AgX-based composite photocatalysts with the advantages of efficient visible light absorption, low cost, and relative stability. For example, it has been reported that Ag@AgCl/WO 3 , Ag@AgCl/MCM-41, Ag@AgI/Al 2 O 3 , Ag@AgCl/GO, and AgI/TiO 2 showed enhanced visible light photocatalytic degradation and disinfection.
To the best of our knowledge, there is no work focused on the preparation of and the photocatalytic activity of Ag@AgCl modified Bi 2 O 2 CO 3 photocatalysts. Based on previous studies, Bi 2 O 2 CO 3 modified with Ag@AgCl to form Ag@AgCl/Bi 2 O 2 CO 3 heterojunctions may improve visible light activated photocatalytic activity. On one hand, 3D hierarchical architectures could make pure Bi 2 O 2 CO 3 visible light-active through surface scattering and reflecting (SSR) effects [24]. On the other hand, the surface plasmon resonance effects (SPR effects) of Ag@AgCl decorated on the Bi 2 O 2 CO 3 surface could further strengthen the visible light absorption of 3D Bi 2 O 2 CO 3 hierarchical microspheres [25]. In this article, Ag@AgCl-decorated Bi 2 O 2 CO 3 (hereafter designated as NPs Ag@AgCl/Bi 2 O 2 CO 3 ) was successfully synthesized via a multistep route, in which a hydrothermal precipitation was employed to synthesize flower-like Bi 2 O 2 CO 3 . An oil-in-water self-assembly synthesis was used to deposit NPs Ag@AgCl on Bi 2 WO 6 . The characterization and a possible photocatalytic mechanism for the Ag@AgCl/Bi 2 O 2 CO 3 composites were discussed in detail.

Photocatalyst Synthesis
All chemical reagents purchased were of analytical grade and used without further purification. The flower-like Bi 2 O 2 CO 3 precursor was synthesized by a hydrothermal method. In a typical procedure to prepare flower-like Bi 2 O 2 CO 3 precursor, 3 mmol of Bi(NO 3 ) 3¨5 H 2 O were dissolved in 20 mL of 1 M HNO 3 , and then 2 mmol of citric acid were introduced to the solution. After being magnetically stirred for 10 min, the pH of the solution was adjusted to 4-4.2 with the addition of NaOH solution under vigorous stirring. The white precursor that formed was transferred to a Teflon-lined stainless steel autoclave and maintained at 160˝C for 24 h. After cooling the hydrothermal system to room temperature, the flower-like Bi 2 O 2 CO 3 precursor was separated by a centrifugation process, washed several times with distilled water and ethanol, and dried under vacuum at 80˝C for 8 h.
The Ag@AgCl/Bi 2 O 2 CO 3 composite was prepared by a novel oil-in-water self-assembly method under dark conditions (As shown in Figure 1). Typically, 0.5g Bi 2 O 2 CO 3 powder and AgNO 3 were dissolved into 20 mL deionized water and a certain amount (n AgNO3 :n CTAC = 1:1.5) of Cetyl trimethyl ammonium chloride (CTAC)/carbon tetrachloride solution (40 mL) was added dropwise at room temperature under vigorous magnetic stirring over 20 min. After the CTAC addition, the reaction mixture was magnetically stirred for another 20 min. The resulting suspension was then filtered and washed with deionized water and ethanol, respectively. The obtained AgCl/Bi 2 O 2 CO 3 powder was dispersed in 30 mL distilled water, and irradiated with a 250 W metal halide lamp (Philips), equipped with wavelength cutoff filters for λ ě 420 nm. The resulting sample (Ag@AgCl/Bi 2 O 2 CO 3 ) was washed with distilled water and anhydrous ethanol to remove the surfactant, and the final products (hereafter designated Ag@AgCl/Bi 2 O 2 CO 3 ) were dried at 80˝C for 8 h in the dark. Ag@AgCl/Bi 2 O 2 CO 3 composites with different molar ratios of Ag@AgCl to Bi 2 O 2 CO 3 were prepared according to the typical procedure described above.
Materials 2016, 9,486 3 of 18 transferred to a Teflon-lined stainless steel autoclave and maintained at 160 °C for 24 h. After cooling the hydrothermal system to room temperature, the flower-like Bi2O2CO3 precursor was separated by a centrifugation process, washed several times with distilled water and ethanol, and dried under vacuum at 80 °C for 8 h. The Ag@AgCl/Bi2O2CO3 composite was prepared by a novel oil-in-water self-assembly method under dark conditions (As shown in Figure 1). Typically, 0.5g Bi2O2CO3 powder and AgNO3 were dissolved into 20 mL deionized water and a certain amount (nAgNO3:nCTAC = 1:1.5) of Cetyl trimethyl ammonium chloride (CTAC)/carbon tetrachloride solution (40 mL) was added dropwise at room temperature under vigorous magnetic stirring over 20 min. After the CTAC addition, the reaction mixture was magnetically stirred for another 20 min. The resulting suspension was then filtered and washed with deionized water and ethanol, respectively. The obtained AgCl/Bi2O2CO3 powder was dispersed in 30 mL distilled water, and irradiated with a 250 W metal halide lamp (Philips), equipped with wavelength cutoff filters for λ ≥ 420 nm. The resulting sample (Ag@AgCl/Bi2O2CO3) was washed with distilled water and anhydrous ethanol to remove the surfactant, and the final products (hereafter designated Ag@AgCl/Bi2O2CO3) were dried at 80 °C for 8 h in the dark. Ag@AgCl/Bi2O2CO3 composites with different molar ratios of Ag@AgCl to Bi2O2CO3 were prepared according to the typical procedure described above. For comparison, an Ag@AgCl/Bi2O2CO3 sample was prepared via a precipitation method with Bi2O2CO3 powder, NaOH, AgNO3 and CTAC. The as-prepared catalyst was denoted as P-Ag@AgCl/Bi2O2CO3.
Pure Ag@AgCl was synthesized similarly using an oil-in-water self-assembly method with AgNO3, and CTAC.

Photocatalyst Characterization
The crystal structures and phase data for the prepared samples were determined by X-ray diffractometry (XRD) using a Rigaku D/MAX2500 PC diffractometer (D/MAX2500 PC, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation, with an operating voltage of 40 kV and an operating current of 100 mA. The morphologies of the samples were investigated with a scanning For comparison, an Ag@AgCl/Bi 2 O 2 CO 3 sample was prepared via a precipitation method with Bi 2 O 2 CO 3 powder, NaOH, AgNO 3 and CTAC. The as-prepared catalyst was denoted as P-Ag@AgCl/Bi 2 O 2 CO 3 .
Pure Ag@AgCl was synthesized similarly using an oil-in-water self-assembly method with AgNO 3 , and CTAC.

Photocatalyst Characterization
The crystal structures and phase data for the prepared samples were determined by X-ray diffractometry (XRD) using a Rigaku D/MAX2500 PC diffractometer (D/MAX2500 PC, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation, with an operating voltage of 40 kV and an operating current of 100 mA. The morphologies of the samples were investigated with a scanning electron microscope (SEM) (Hitachi, Chiyoda, Japan, s-4800) and energy dispersive X-ray spectroscopy (EDX), as well as by transmission electron microscopy (TEM) (JEOL Ltd., Akishima, Japan, JEM-2010). UV-visible light (UV-vis) diffuse reflectance spectra were recorded on a UV-vis spectrometer (UV-1901, Puxi, Beijing, China). The chemical compositions of the samples were tested by an X-ray fluorescence spectrometer (XRF, Rigaku, Tokyo, Japan, ZSX Promusll). Surface areas of the samples were determined by the Brunauer-Emmett-Teller (BET) method based on the adsorption and desorption isotherms of N 2 collected on a Quantachrome Nova 4200e automatic analyzer (Monorosb, Quantachrome, Boynton Beach, FL, USA). The photoluminescence of the powdered samples was measured by a spectrofluorometer (Hitachi, f7000). Electrochemical and photoelectrochemical measurements were performed in a constructed three electrode quartz cell system. A Pt sheet was used as a counter electrode and Hg/Hg 2 Cl 2 /sat. KCl was used as a reference electrode, while the thin film on indium-tin oxide (ITO) was used as the working electrode for investigation. The photoelectrochemical experimental results were recorded with a CHI 660B electrochemical system.

Photocatalytic Activity
The photocatalytic activities of Ag@AgCl/Bi 2 O 2 CO 3 composites were evaluated by the degradation of methylene blue (MB) under irradiation of visible light. For the first series of tests, a 250 W halide lamp (Royal Philips, Amsterdam, The Netherlands) with a 420 nm cutoff filter was used at a distance of 10 cm from the top of an unsealed beaker. In a second series of tests, a glass reactor with circulating water flowing outside to control the temperature to 25˘2˝C was employed. For each test, 0.4 g catalyst powder was added to 100 mL 10 mg/L MB solution. Prior to irradiation, the test solution was stirred in the dark for 30 min. During irradiation, a 3 mL aliquot of the reaction suspension was withdrawn every 5 min and centrifuged at 10,000 rpm for 6 min to remove the particles. The collected supernatant solutions were then analyzed by a UV-vis spectrophotometer.
The degradation efficiency (%) was calculated as follows: where C 0 is the initial concentration of MB; and C is the concentration of MB at time t. Photocatalytic activities while degrading MB in the dark in the presence of the photocatalyst and under visible-light irradiation in the absence of the photocatalyst were also evaluated.
Additionally, the degradation procedure of Bisphenol A (BPA) was the same as that of MB.

Catalyst Characterization
The XRD study was carried out to confirm the crystalline structure of the as-prepared Ag@AgCl/Bi 2 O 2 CO 3 composites. Figure 2 shows the XRD patterns of Ag@AgCl, Bi 2 O 2 CO 3 and Ag@AgCl/Bi 2 O 2 CO 3 heterojunctions with different Ag@AgCl contents. The Ag@AgCl was found to be mainly composed of the chlorargyrite AgCl phase. The XRD peaks of AgCl were in good agreement with the cubic crystalline phase of AgCl (JCPDS 31-1238), revealing three distinct diffraction peaks at 27  The morphology of the Ag@AgCl/Bi2O2CO3 composite was studied by SEM, TEM and HRTEM. It can be observed from Figure 3a that the diameter of a flower-like Bi2O2CO3 microsphere is 1-2 μm, built from two-dimensional nanoplates with a smooth surface. The as-synthesized Ag@AgCl nanoparticles have a nearly spherical shape with a diameter of 80-100 nm (Figure 3b). Figure 3c shows that some Ag@AgCl particles of the size 0.3-0.5 μm are randomly attached to the surface of Bi2O2CO3 microspheres via a precipation method. However, the sample containing Ag@AgCl deposited onto Bi2O2CO3 prepared by a self-assembly method is shown in Figure 3d. The shape of Bi2O2CO3 did not change upon NPs Ag@AgCl-loading, which indicates that the NPs Ag@AgCl deposition process does not damage its flower-like structure. In addition, Ag@AgCl-loading could very well promote the scattering of Ag@AgCl, and reduce its grain size. From Figure 3d, some spherical particles with a size range of 5 to 20 nm were observed to be uniformly deposited on the surface of Bi2O2CO3, indicating the increased surface roughness of the flower-like structure [26]. The BET results revealed that the specific surface area of the Ag@AgCl (25 wt %)/Bi2O2CO3 (24.23 m 2 ·g −1 ) nano-composite was larger than that of pure Bi2O2CO3 (13.61 m 2 ·g −1 ) (as shown in Table 1). The light absorption was considered to be enhanced due to the increased light scattering. Figure 3e shows the TEM image of flower-like microstructures. According to the TEM image in Figure 3f, Ag@AgCl nanoparticles with a size of about 10 nm were attached to the flower-like nanostructures of Bi2O2CO3. It can be seen from the HRTEM images (Figure 3g), that the clear lattice fringe indicates the high-crystallinity of the composite. The lattice fringes of Ag, AgCl and Bi2O2CO3 coexist in the Ag@AgCl/Bi2O2CO3 composite. The measured interplanar spacings of 0.24 nm, 0.28 nm and 0.27 nm correspond to the (111) plane of Ag, the (200) plane of AgCl and the (110) plane of Bi2O2CO3. This revealed that the nanoparticles formed on Bi2O2CO3 were Ag@AgCl. The SAED pattern collected from Ag@AgCl/Bi2O2CO3 is shown in Figure 3h, where polycrystalline diffraction rings can be observed. The characteristic peak of Ag@AgCl/Bi2O2CO3 can be obviously observed in EDX ( Figure 3i). This further proves the formation of Ag@AgCl on Bi2O2CO3. In addition, the elemental compositions of the composite nanoparticles were further analyzed by XRF. The results are shown in Table 2, and the atomic ratio of Ag to Cl was found to be 1.49:1, indicating the presence of Ag in the Ag@AgCl nanojunction system. Based on the XRF results, the atomic ratio of Bi to C to O was found to be 1.9:1:5.5, which was less than the theoretical ratio of 2:1:5 expected from Bi2O2CO3. Since the percentages of O and C were higher in the experimental sample than in the theoretical ratio for Bi2O2CO3, H2O and/or C were thought to possibly be present in the prepared Ag@AgCl/Bi2O2CO3 composite. The morphology of the Ag@AgCl/Bi 2 O 2 CO 3 composite was studied by SEM, TEM and HRTEM. It can be observed from Figure 3a that the diameter of a flower-like Bi 2 O 2 CO 3 microsphere is 1-2 µm, built from two-dimensional nanoplates with a smooth surface. The as-synthesized Ag@AgCl nanoparticles have a nearly spherical shape with a diameter of 80-100 nm ( Figure 3b). Figure 3c shows that some Ag@AgCl particles of the size 0.3-0.5 µm are randomly attached to the surface of Bi 2 O 2 CO 3 microspheres via a precipation method. However, the sample containing Ag@AgCl deposited onto Bi 2 O 2 CO 3 prepared by a self-assembly method is shown in Figure 3d. The shape of Bi 2 O 2 CO 3 did not change upon NPs Ag@AgCl-loading, which indicates that the NPs Ag@AgCl deposition process does not damage its flower-like structure. In addition, Ag@AgCl-loading could very well promote the scattering of Ag@AgCl, and reduce its grain size. From Figure 3d, some spherical particles with a size range of 5 to 20 nm were observed to be uniformly deposited on the surface of Bi 2 O 2 CO 3 , indicating the increased surface roughness of the flower-like structure [26]. The BET results revealed that the specific surface area of the Ag@AgCl (25 wt %)/Bi 2 O 2 CO 3 (24.23 m 2¨g´1 ) nano-composite was larger than that of pure Bi 2 O 2 CO 3 (13.61 m 2¨g´1 ) (as shown in Table 1). The light absorption was considered to be enhanced due to the increased light scattering. Figure 3 . This revealed that the nanoparticles formed on Bi 2 O 2 CO 3 were Ag@AgCl. The SAED pattern collected from Ag@AgCl/Bi 2 O 2 CO 3 is shown in Figure 3h, where polycrystalline diffraction rings can be observed. The characteristic peak of Ag@AgCl/Bi 2 O 2 CO 3 can be obviously observed in EDX (Figure 3i). This further proves the formation of Ag@AgCl on Bi 2 O 2 CO 3 . In addition, the elemental compositions of the composite nanoparticles were further analyzed by XRF. The results are shown in Table 2, and the atomic ratio of Ag to Cl was found to be 1.49:1, indicating the presence of Ag in the Ag@AgCl nanojunction system. Based on the XRF results, the atomic ratio of Bi to C to O was found to be 1.9:1:5.5, which was less than the theoretical ratio of 2:1:5 expected from Bi 2 O 2 CO 3 . Since the percentages of O and C were higher in the experimental sample than in the theoretical ratio for Bi 2 O 2 CO 3 , H 2 O and/or C were thought to possibly be present in the prepared Ag@AgCl/Bi 2 O 2 CO 3 composite.   3 . TEM images of (e,f) Ag@AgCl/Bi 2 O 2 CO 3 . HRTEM and SAED images of (g,h) Ag@AgCl/Bi 2 O 2 CO 3 . EDX spectrum of (i) Ag@AgCl/Bi 2 O 2 CO 3 samples. The EDX elemental mapping of the Ag@AgCl/Bi 2 O 2 CO 3 composites ( Figure 4) displays the even distribution of the elements Ag, Cl, Bi, C and O on Ag@AgCl/Bi 2 O 2 CO 3 . The elements of Ag and Cl are from Ag@AgCl. Bi, C and O are from Bi 2 O 2 CO 3 . The results further indicate the chemical uniformity within individual particles, which clearly confirms that the Ag@AgCl NPs were uniformly distributed on the surface of the Bi 2 O 2 CO 3 flower-like microspheres. All of the above structure characterizations indicate that a promising Ag@AgCl/Bi 2 O 2 CO 3 photocatalyst with well-designed architecture was constructed.  The EDX elemental mapping of the Ag@AgCl/Bi2O2CO3 composites ( Figure 4) displays the even distribution of the elements Ag, Cl, Bi, C and O on Ag@AgCl/Bi2O2CO3. The elements of Ag and Cl are from Ag@AgCl. Bi, C and O are from Bi2O2CO3. The results further indicate the chemical uniformity within individual particles, which clearly confirms that the Ag@AgCl NPs were uniformly distributed on the surface of the Bi2O2CO3 flower-like microspheres. All of the above structure characterizations indicate that a promising Ag@AgCl/Bi2O2CO3 photocatalyst with well-designed architecture was constructed.  It is well known that the UV-vis absorption edge is related to the energy band of the semiconductor catalyst, depending on its electronic structure [18]. The optical absorption property of Ag@AgCl/Bi 2 O 2 CO 3 composites was analyzed. Figure 5 shows the UV-vis diffuse reflectance spectra of Ag@AgCl, Bi 2 O 2 CO 3 and Ag@AgCl/Bi 2 O 2 CO 3 composites. Clearly, it can be seen that pure Bi 2 O 2 CO 3 boasted a weak visible light response with an absorption band edge of about 365 nm. After Ag@AgCl was introduced, the ability of light absorption for Bi 2 O 2 CO 3 was drastically enhanced due to the surface plasmonic resonance (SPR) of Ag nanoparticles on Ag@AgCl in visible light. Accordingly, with the increase in Ag@AgCl content, the differences in optical adsorption agree well with the colors, changing from white to dark-gray. Additionally, a slight redshift in the absorbance region was observed with the increase in Ag@AgCl content, which might be caused by a possible charge-transfer transition at the interface between Ag@AgCl and Bi 2 O 2 CO 3 , due to the presence of Ag@AgCl [27]. These results also indicate that Ag@AgCl exists on the surface of Bi 2 O 2 CO 3 and that the Ag@AgCl/Bi 2 O 2 CO 3 photocatalysts are more effective at degrading organic dyes under light irradiation. On the other hand, for a crystalline semiconductor, the optical absorption near band edge follows the following formula [19]: where α, h, ν, E g , and A are the absorption coefficient, Plank constant, light frequency, band gap, and a constant, respectively [28]. Among them, n is 1 for a direct transition and 4 for an indirect transition. Bi 2 O 2 CO 3 processes indirect transition band gaps [29,30], and so in this case, the value of n is equal to 4. The E g value can be estimated by extrapolating the straight portion of the (αhν) 1/2´( hν) plot to the α = 0 point, where the band gap of Bi 2 O 2 CO 3 is 3.4 eV (see inset of Figure 5). As a consequence, the Ag@AgCl/Bi 2 O 2 CO 3 sample can more easily produce electron-hole pairs under the same UV light irradiation and display higher photocatalytic activities. It is well known that the UV-vis absorption edge is related to the energy band of the semiconductor catalyst, depending on its electronic structure [18]. The optical absorption property of Ag@AgCl/Bi2O2CO3 composites was analyzed. Figure 5 shows the UV-vis diffuse reflectance spectra of Ag@AgCl, Bi2O2CO3 and Ag@AgCl/Bi2O2CO3 composites. Clearly, it can be seen that pure Bi2O2CO3 boasted a weak visible light response with an absorption band edge of about 365 nm. After Ag@AgCl was introduced, the ability of light absorption for Bi2O2CO3 was drastically enhanced due to the surface plasmonic resonance (SPR) of Ag nanoparticles on Ag@AgCl in visible light. Accordingly, with the increase in Ag@AgCl content, the differences in optical adsorption agree well with the colors, changing from white to dark-gray. Additionally, a slight redshift in the absorbance region was observed with the increase in Ag@AgCl content, which might be caused by a possible charge-transfer transition at the interface between Ag@AgCl and Bi2O2CO3, due to the presence of Ag@AgCl [27]. These results also indicate that Ag@AgCl exists on the surface of Bi2O2CO3 and that the Ag@AgCl/Bi2O2CO3 photocatalysts are more effective at degrading organic dyes under light irradiation. On the other hand, for a crystalline semiconductor, the optical absorption near band edge follows the following formula [19]: where α, h, ν, Eg, and A are the absorption coefficient, Plank constant, light frequency, band gap, and a constant, respectively [28]. Among them, n is 1 for a direct transition and 4 for an indirect transition. Bi2O2CO3 processes indirect transition band gaps [29,30], and so in this case, the value of n is equal to 4. The Eg value can be estimated by extrapolating the straight portion of the (αhν) 1/2 −(hν) plot to the α = 0 point, where the band gap of Bi2O2CO3 is 3.4 eV (see inset of Figure 5). As a consequence, the Ag@AgCl/Bi2O2CO3 sample can more easily produce electron-hole pairs under the same UV light irradiation and display higher photocatalytic activities. In order to further determine the chemical state of elements present in the synthesized samples, Ag@AgCl/Bi2O2CO3 samples were analyzed by X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 6. Figure 6a shows wide-scan XPS spectra of Ag@AgCl/Bi2O2CO3 samples. As shown, the presences of bismuth (Bi 4f, Bi 4d, Bi 4p and Bi 5d), oxygen (O 1s), carbon (C 1s), silver (Ag 3d) and chlorine (Cl 2p) in the Ag@AgCl/Bi2O2CO3 sample were confirmed. Furthermore, high-resolution XPS spectra of Ag in the synthesized sample are shown in Figure 6b. As shown in Figure 6b, for the Ag@AgCl/Bi2O2CO3 sample, the peaks at 373.13 and 366.85 eV In order to further determine the chemical state of elements present in the synthesized samples, Ag@AgCl/Bi 2 O 2 CO 3 samples were analyzed by X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 6. Figure 6a shows wide-scan XPS spectra of Ag@AgCl/Bi 2 O 2 CO 3 samples. As shown, the presences of bismuth (Bi 4f, Bi 4d, Bi 4p and Bi 5d), oxygen (O 1s), carbon (C 1s), silver (Ag 3d) and chlorine (Cl 2p) in the Ag@AgCl/Bi 2 O 2 CO 3 sample were confirmed. Furthermore, high-resolution XPS spectra of Ag in the synthesized sample are shown in Figure 6b. As shown in Figure 6b, for the Ag@AgCl/Bi 2 O 2 CO 3 sample, the peaks at 373.13 and 366.85 eV correspond to Ag 3d 3/2 and Ag 3d 5/2 respectively. It is noted that the Ag 3d 3/2 peak can be further divided into two separate peaks at 374.46 and 373.01 eV, while the Ag 3d 5/2 peak can also be divided into two separate peaks at 368.17 and 366.78 eV. The peaks at 366.78 and 373.01 eV can be attributed to Ag + of AgCl, while the peaks at 374.46 and 368.17 eV can be assigned to metallic Ag [31]. The results of XPS analysis confirm the presence of Ag and AgCl in the prepared composite sample.    Figure 7a shows the adsorption performance of Ag@AgCl, Bi2O2CO3 and Ag@AgCl (10 wt %)/Bi2O2CO3. During the dark period, Ag@AgCl, Bi2O2CO3 and Ag@AgCl (10 wt %)/Bi2O2CO3 removed MB from the solution via adsorption. The adsorption equilibrium was reached between 25 and 30 min for these materials. Upon visible light irradiation, as shown in Figure 7b, blank reaction was also given as controls. The blank experiments which were performed in the absence of a photocatalyst showed no obvious changes in the concentration of MB over 20 min of reaction under visible light irradiation. This verifies that MB is an organic dye which is chemically stable and difficult to decompose. The reaction in dark with Ag@AgCl/Bi2O2CO3 catalysts did not facilitate the degradation of MB. After visible light irradiation for 20 min, the degradation percent of MB by pure Bi2O2CO3 was only 26.77%, which indicates that pure Bi2O2CO3 shows weak activity. The degradation of MB in this situation may be induced by the sensitization effects of dye molecules [32], since Bi2O2CO3 cannot be excited by visible light due to its large band gap (3.4 eV). The Ag@AgCl/Bi2O2CO3 composite displayed the highest activity, and the removal percent of MB reached 97.9% within 20 min, which is over 1.64 and 3.66 times faster than that of pure Ag@AgCl (calculated based on the equivalent Ag@AgCl content in Ag@AgCl (10 wt %)/Bi2O2CO3) and pure Bi2O2CO3, respectively. According to these results, the plasmon resonance of Ag NPs and the charge transfer between nano Ag@AgCl and Bi2O2CO3 leads to a higher photocatalytic activity in Ag@AgCl/Bi2O2CO3 compared to the pure Ag@AgCl and Bi2O2CO3 materials.

Photocatalytic Activity
The photocatalytic degradation of organic pollutants generally follows pesudo-first-order kinetics. As shown in Figure 8 Figure 7a shows the adsorption performance of Ag@AgCl, Bi 2 O 2 CO 3 and Ag@AgCl (10 wt %)/Bi 2 O 2 CO 3 . During the dark period, Ag@AgCl, Bi 2 O 2 CO 3 and Ag@AgCl (10 wt %)/Bi 2 O 2 CO 3 removed MB from the solution via adsorption. The adsorption equilibrium was reached between 25 and 30 min for these materials. Upon visible light irradiation, as shown in Figure 7b, blank reaction was also given as controls.

Photocatalytic Activity
The blank experiments which were performed in the absence of a photocatalyst showed no obvious changes in the concentration of MB over 20 min of reaction under visible light irradiation. This verifies that MB is an organic dye which is chemically stable and difficult to decompose. The reaction in dark with Ag@AgCl/Bi 2 O 2 CO 3 catalysts did not facilitate the degradation of MB. After visible light irradiation for 20 min, the degradation percent of MB by pure Bi 2 O 2 CO 3 was only 26.77%, which indicates that pure Bi 2 O 2 CO 3 shows weak activity. The degradation of MB in this situation may be induced by the sensitization effects of dye molecules [32], since Bi 2 O 2 CO 3 cannot be excited by visible light due to its large band gap (3.4 eV). The Ag@AgCl/Bi 2 O 2 CO 3 composite displayed the highest activity, and the removal percent of MB reached 97.9% within 20 min, which is over 1.64 and 3.66 times faster than that of pure Ag@AgCl (calculated based on the equivalent Ag@AgCl content in Ag@AgCl (10 wt %)/Bi 2 O 2 CO 3 ) and pure Bi 2 O 2 CO 3 , respectively. According to these results, the plasmon resonance of Ag NPs and the charge transfer between nano Ag@AgCl and Bi 2 O 2 CO 3 leads to a higher photocatalytic activity in Ag@AgCl/Bi 2 O 2 CO 3 compared to the pure Ag@AgCl and Bi 2 O 2 CO 3 materials.
The photocatalytic degradation of organic pollutants generally follows pesudo-first-order kinetics. As shown in Figure 8 possessed the most optimal activity among all the samples, with an apparent rate constant of 0.051 min´1. The highest activity was obtained for the Ag@AgCl (10 wt %)/Bi 2 O 2 CO 3 sample, on which more than 97.9% of MB was degraded within 20 min. The photocatalytic performance significantly improved after the introduction of Ag@AgCl and increased with the increase of Ag@AgCl content from 0 to 10 wt %. However, further increase in Ag@AgCl loading resulted in a decrease in photocatalytic activity. Once the loading of Ag@AgCl increases beyond a critical value, the nanoclusters of Ag@AgCl species are thought to agglomerate and shade the active sties on the surface of Bi 2 O 2 CO 3 , resulting in lower degradation rates [33]. Simultaneously, this can facilitate the recombination of photoinduced electron-hole pairs. Therefore, suitable Ag@AgCl content was crucial for optimizing the photocatalytic performances of the Ag@AgCl/Bi 2 O 2 CO 3 composites. The above results imply that significant increases in photoactivity can be ascribed to the synergetic effects of Ag@AgCl and Bi 2 O 2 CO 3 photocatalyst. Ag@AgCl content from 0 to 10 wt %. However, further increase in Ag@AgCl loading resulted in a decrease in photocatalytic activity. Once the loading of Ag@AgCl increases beyond a critical value, the nanoclusters of Ag@AgCl species are thought to agglomerate and shade the active sties on the surface of Bi2O2CO3, resulting in lower degradation rates [33]. Simultaneously, this can facilitate the recombination of photoinduced electron-hole pairs. Therefore, suitable Ag@AgCl content was crucial for optimizing the photocatalytic performances of the Ag@AgCl/Bi2O2CO3 composites. The above results imply that significant increases in photoactivity can be ascribed to the synergetic effects of Ag@AgCl and Bi2O2CO3 photocatalyst.  To further ascertain the active species in the degradation process and to verify the supposed photocatalytic mechanism, some sacrificial agents such as tert-butyl alcohol (IPA), ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) and nitrogen (N2) were used as the hydroxyl radical (−OH) scavenger, hole (h + ) scavenger and superoxide radical (−O2 − ) scavenger, respectively [34,35]. As shown in Figure 9, the addition of 1 mM IPA produced an insignificant effect on the degradation rate, demonstrating that −OH radicals play a small role in the photocatalytic degradation of MB. In contrast, the photocatalytic degradation rates of MB decreased significantly in the presence of 1 mM EDTA-2Na and under N2-saturated conditions. Compared to the addition of EDTA-2Na, Ag@AgCl content from 0 to 10 wt %. However, further increase in Ag@AgCl loading resulted in a decrease in photocatalytic activity. Once the loading of Ag@AgCl increases beyond a critical value, the nanoclusters of Ag@AgCl species are thought to agglomerate and shade the active sties on the surface of Bi2O2CO3, resulting in lower degradation rates [33]. Simultaneously, this can facilitate the recombination of photoinduced electron-hole pairs. Therefore, suitable Ag@AgCl content was crucial for optimizing the photocatalytic performances of the Ag@AgCl/Bi2O2CO3 composites. The above results imply that significant increases in photoactivity can be ascribed to the synergetic effects of Ag@AgCl and Bi2O2CO3 photocatalyst.  To further ascertain the active species in the degradation process and to verify the supposed photocatalytic mechanism, some sacrificial agents such as tert-butyl alcohol (IPA), ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) and nitrogen (N2) were used as the hydroxyl radical (−OH) scavenger, hole (h + ) scavenger and superoxide radical (−O2 − ) scavenger, respectively [34,35]. As shown in Figure 9, the addition of 1 mM IPA produced an insignificant effect on the degradation rate, demonstrating that −OH radicals play a small role in the photocatalytic degradation of MB. In contrast, the photocatalytic degradation rates of MB decreased significantly in the presence of 1 mM EDTA-2Na and under N2-saturated conditions. Compared to the addition of EDTA-2Na, N2-saturated conditions have a bigger influence, indicating that h + and −O2 − radicals are the main To further ascertain the active species in the degradation process and to verify the supposed photocatalytic mechanism, some sacrificial agents such as tert-butyl alcohol (IPA), ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) and nitrogen (N 2 ) were used as the hydroxyl radical (´OH) scavenger, hole (h + ) scavenger and superoxide radical (´O 2´) scavenger, respectively [34,35].
As shown in Figure 9, the addition of 1 mM IPA produced an insignificant effect on the degradation rate, demonstrating that´OH radicals play a small role in the photocatalytic degradation of MB. In contrast, the photocatalytic degradation rates of MB decreased significantly in the presence of 1 mM EDTA-2Na and under N 2 -saturated conditions. Compared to the addition of EDTA-2Na, N 2 -saturated conditions have a bigger influence, indicating that h + and´O 2´r adicals are the main active species. However, the absolute suppression by IPA, EDTA-2Na and N 2 on photocatalytic activity was not so large, and so it is thought that another important reactive species may affect the observed activity. Considering that the accumulated holes could oxidize Cl´to Cl˝, it was thought that Cl˝atoms were the active species that participated in the degradation of MB [36,37]. Accordingly, the active species trapping experiments demonstrated that the h + and´O 2´a re the two main active species in the degradation process of MB, which was also in agreement with the supposed photocatalytic mechanism.
Materials 2016, 9,486 11 of 18 active species. However, the absolute suppression by IPA, EDTA-2Na and N2 on photocatalytic activity was not so large, and so it is thought that another important reactive species may affect the observed activity. Considering that the accumulated holes could oxidize Cl − to Cl°, it was thought that Cl° atoms were the active species that participated in the degradation of MB [36,37]. Accordingly, the active species trapping experiments demonstrated that the h + and −O2 − are the two main active species in the degradation process of MB, which was also in agreement with the supposed photocatalytic mechanism. The lifetime of the catalyst is an important parameter of the photocatalytic process. In order to examine the photocatalytic stability of the photocatalyst, the experiment was repeated with the same photocatalyst. The Ag@AgCl/Bi2O2CO3 composite was easily recycled by centrifugation and then washed with water and ethanol. After every 20 min of photocatalytic degradation, the separated photocatalysts were washed with deionized water and then with ethanol to dry them. Figure 10 shows the decrease in MB after every run. A high MB photodegradation could be maintained after five cycling runs with no obvious catalyst deactivation. The slight decrease in photoactivity may be ascribed to the minor loss of catalyst during the recycle experiment [38]. The results confirm that the photoactivity of the Ag@AgCl/Bi2O2CO3 catalyst is stable during the degradation process. The lifetime of the catalyst is an important parameter of the photocatalytic process. In order to examine the photocatalytic stability of the photocatalyst, the experiment was repeated with the same photocatalyst. The Ag@AgCl/Bi 2 O 2 CO 3 composite was easily recycled by centrifugation and then washed with water and ethanol. After every 20 min of photocatalytic degradation, the separated photocatalysts were washed with deionized water and then with ethanol to dry them. Figure 10 shows the decrease in MB after every run. A high MB photodegradation could be maintained after five cycling runs with no obvious catalyst deactivation. The slight decrease in photoactivity may be ascribed to the minor loss of catalyst during the recycle experiment [38]. The results confirm that the photoactivity of the Ag@AgCl/Bi 2 O 2 CO 3 catalyst is stable during the degradation process. The interfacial charge separation and transfer dynamics of photoelectrons were studied by monitoring their photocurrent-time responses. As shown in Figure 11, the transient photocurrent responses of Bi2O2CO3 and Ag@AgCl/Bi2O2CO3 were tested and showed three on-off cycles of irradiation. The photocurrent responses of as-prepared samples were obtained by the intermittent visible light irradiation of 30 s. The rise and fall of the photocurrent responses are evidently observed for each switch-on and switch-off event, the results demonstrating effective separation of photoinduced electron-hole pairs in Ag@AgCl/Bi2O2CO3 upon exposure to visible light [39]. The results are shown in Figure 11 (the response of the ITO has been deducted as background). It is clear that Ag@AgCl/Bi2O2CO3 shows a much higher photocurrent density than that of pure Bi2O2CO3. The photocurrent result confirms the more efficient separation of photo-generated electron-hole pairs for the Ag@AgCl/Bi2O2CO3 composite than that of pure Bi2O2CO3. This indicates that Ag/AgCl was been effectively combined with Bi2O2CO3, which leads to the efficient separation of the photo-generated electron-hole pairs in Ag@AgCl/Bi2O2CO3.  The interfacial charge separation and transfer dynamics of photoelectrons were studied by monitoring their photocurrent-time responses. As shown in Figure 11, the transient photocurrent responses of Bi 2 O 2 CO 3 and Ag@AgCl/Bi 2 O 2 CO 3 were tested and showed three on-off cycles of irradiation. The photocurrent responses of as-prepared samples were obtained by the intermittent visible light irradiation of 30 s. The rise and fall of the photocurrent responses are evidently observed for each switch-on and switch-off event, the results demonstrating effective separation of photoinduced electron-hole pairs in Ag@AgCl/Bi 2 O 2 CO 3 upon exposure to visible light [39]. The results are shown in Figure 11 (the response of the ITO has been deducted as background). It is clear that Ag@AgCl/Bi 2 O 2 CO 3 shows a much higher photocurrent density than that of pure Bi 2 O 2 CO 3 . The photocurrent result confirms the more efficient separation of photo-generated electron-hole pairs for the Ag@AgCl/Bi 2 O 2 CO 3 composite than that of pure Bi 2 O 2 CO 3 . This indicates that Ag/AgCl was been effectively combined with Bi 2 O 2 CO 3 , which leads to the efficient separation of the photo-generated electron-hole pairs in Ag@AgCl/Bi 2 O 2 CO 3 . The interfacial charge separation and transfer dynamics of photoelectrons were studied by monitoring their photocurrent-time responses. As shown in Figure 11, the transient photocurrent responses of Bi2O2CO3 and Ag@AgCl/Bi2O2CO3 were tested and showed three on-off cycles of irradiation. The photocurrent responses of as-prepared samples were obtained by the intermittent visible light irradiation of 30 s. The rise and fall of the photocurrent responses are evidently observed for each switch-on and switch-off event, the results demonstrating effective separation of photoinduced electron-hole pairs in Ag@AgCl/Bi2O2CO3 upon exposure to visible light [39]. The results are shown in Figure 11 (the response of the ITO has been deducted as background). It is clear that Ag@AgCl/Bi2O2CO3 shows a much higher photocurrent density than that of pure Bi2O2CO3. The photocurrent result confirms the more efficient separation of photo-generated electron-hole pairs for the Ag@AgCl/Bi2O2CO3 composite than that of pure Bi2O2CO3. This indicates that Ag/AgCl was been effectively combined with Bi2O2CO3, which leads to the efficient separation of the photo-generated electron-hole pairs in Ag@AgCl/Bi2O2CO3.  Electrochemical impedance spectroscopy (EIS) is a very useful tool to characterize charge-carrier migration [40], and was thus used to further confirm the interfacial charge transfer effects of as-prepared composites. Generally, a higher charge mobility results in a smaller EIS arc radius. Figure 12 shows the EIS Nyquist plots of the Bi 2 O 2 CO 3 and Ag@AgCl/Bi 2 O 2 CO 3 electrodes. Obviously, Ag@AgCl/Bi 2 O 2 CO 3 nanocomposites show much smaller arc radii compared to Bi 2 O 2 CO 3 . The reduced arc radius indicates a diminished resistance of working electrodes, suggesting a decrease in the solid state interface layer resistance and the charge transfer resistance across the solid-liquid junction on the surface by forming a heterojunction between Ag@AgCl and Bi 2 O 2 CO 3 [41]. Since the radius of the arc on the EIS spectra reflects the migration rate occurring at the surface, this suggests that a more effective separation of photo-generated electron-hole pairs and a faster interfacial charge transfer occurs on the Ag@AgCl/Bi 2 O 2 CO 3 surface under these conditions [42]. Electrochemical impedance spectroscopy (EIS) is a very useful tool to characterize charge-carrier migration [40], and was thus used to further confirm the interfacial charge transfer effects of as-prepared composites. Generally, a higher charge mobility results in a smaller EIS arc radius. Figure 12 shows the EIS Nyquist plots of the Bi2O2CO3 and Ag@AgCl/Bi2O2CO3 electrodes. Obviously, Ag@AgCl/Bi2O2CO3 nanocomposites show much smaller arc radii compared to Bi2O2CO3. The reduced arc radius indicates a diminished resistance of working electrodes, suggesting a decrease in the solid state interface layer resistance and the charge transfer resistance across the solid-liquid junction on the surface by forming a heterojunction between Ag@AgCl and Bi2O2CO3 [41]. Since the radius of the arc on the EIS spectra reflects the migration rate occurring at the surface, this suggests that a more effective separation of photo-generated electron-hole pairs and a faster interfacial charge transfer occurs on the Ag@AgCl/Bi2O2CO3 surface under these conditions [42].     (Figure 14c) shown, After 20 min illumination, the BPA corresponding peaks decrease, indicating that BPA was mineralized to CO2 and H2O in the process of photocatalytic reaction.    Figure 14a shows the high performance liquid chromatography (HPLC) chromatogram during the photocatalytic degradation of BPA over Ag@AgCl (10 wt %)/Bi2O2CO3 under visible light irradiation. The BPA showed a characteristic peak at a retention time (RT) of 10.9 min. The BPA peak became weaker with increasing irradiation time, and the removal percent of BPA reached 51.4% within 20 min, indicating the excellent photocatalytic activity of Ag@AgCl (10 wt %)/Bi2O2CO3. Three-dimensional HPLC chromatography used to further demonstrate the photocatalytic activity of the Ag@AgCl (10 wt %)/Bi2O2CO3. As the degradation of organic pollutants BPA solution in the reaction of 0 min (Figure 14b) and 20 min (Figure 14c) shown, After 20 min illumination, the BPA corresponding peaks decrease, indicating that BPA was mineralized to CO2 and H2O in the process of photocatalytic reaction.  Combined with the above results, the Ag@AgCl/Bi 2 O 2 CO 3 composite photocatalyst showed a high photocatalytic activity for the photodegradation of MB dyes under visible light. The possible mechanisms for the enhancement of the photocatalytic activity of the Ag@AgCl/Bi 2 O 2 CO 3 composite photocatalyst are schematically illustrated in Figure 15. The surface plasmon resonance (SPR) of Ag nanoparticles in the Ag@AgCl/Bi 2 O 2 CO 3 composite will additionally enhance visible light absorption. Under visible light irradiation, Ag NPs can be excited. Excited state Ag on the catalyst surface can transfer its photo-generated electrons to the CB of AgCl. On the basis of the relative position of the conduction band, the photo-generated electrons will transfer from the CB of AgCl (´0.95 eV) [43] to the CB of Bi 2 O 2 CO 3 (0.16 eV) [44], making an efficient charge separation and reducing the probability of electron-hole recombination. Simultaneously, the holes in the VB of Bi 2 O 2 CO 3 can transfer to AgCl, which causes the increased separation rate of photo-generated electrons and holes. The photo-generated holes on the surface of AgCl then react with Cl´to form Cl˝. Cl˝is a reactive radical species that oxidizes the surface-adsorbed organic pollutants, reducing the Cl˝atoms back to Cl´. The resultant Cl´reacts with Ag + to form AgCl again, maintaining the stability of the photocatalyst sample under visible light irradiation. While a portion of the holes on the VB of Ag could react with OH´or H 2 O to form hydroxyl radicals (´OH), which have a strong oxidation ability to oxidize organic pollutant molecules, the holes could also oxidize the MB directly [45]. surface can transfer its photo-generated electrons to the CB of AgCl. On the basis of the relative position of the conduction band, the photo-generated electrons will transfer from the CB of AgCl (−0.95 eV) [43] to the CB of Bi2O2CO3 (0.16 eV) [44], making an efficient charge separation and reducing the probability of electron-hole recombination. Simultaneously, the holes in the VB of Bi2O2CO3 can transfer to AgCl, which causes the increased separation rate of photo-generated electrons and holes. The photo-generated holes on the surface of AgCl then react with Cl − to form Cl°. Cl° is a reactive radical species that oxidizes the surface-adsorbed organic pollutants, reducing the Cl° atoms back to Cl − . The resultant Cl − reacts with Ag + to form AgCl again, maintaining the stability of the photocatalyst sample under visible light irradiation. While a portion of the holes on the VB of Ag could react with OH − or H2O to form hydroxyl radicals (−OH), which have a strong oxidation ability to oxidize organic pollutant molecules, the holes could also oxidize the MB directly [45].

Conclusions
In summary, a novel flower-like Ag@AgCl/Bi2O2CO3 composite photocatalyst was synthesized by a facile oil-in-water self-assembly method. The introduction of Ag@AgCl nanoparticles into Bi2O2CO3 was found to induce an increased photocatalytic activity under visible light irradiation. The formation of Ag@AgCl/Bi2O2CO3 can improve the optical properties, the surface area, and charge transfer as well as hinder the recombination of electron-hole pairs, which is supposedly linked to the photocatalytic activity. Among the as-prepared samples, Ag@AgCl/Bi2O2CO3 composites with 10 wt % Ag@AgCl showed the highest photocatalytic performances with regards to the degradation of organic pollutants. The photocatalyst also displayed higher stability during the photoreactions and no obvious deactivation was found for the recycled catalyst after five test runs. The present results suggest that hierarchical flower-like Ag@AgCl/Bi2O2CO3 composites are a promising candidate for the photocatalytic degradation of organic dyes. Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used "Shuanglong Lin and Zisheng Zhang conceived and designed the experiments; Shuanglong Lin and Li Liu performed the experiments; Shuanglong Lin and Wenquan Cui analyzed the data; Yinghua Liang contributed reagents/materials/analysis tools; Shuanglong Lin wrote the paper." Authorship must be limited to those who have contributed substantially to the work reported.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In summary, a novel flower-like Ag@AgCl/Bi 2 O 2 CO 3 composite photocatalyst was synthesized by a facile oil-in-water self-assembly method. The introduction of Ag@AgCl nanoparticles into Bi 2 O 2 CO 3 was found to induce an increased photocatalytic activity under visible light irradiation. The formation of Ag@AgCl/Bi 2 O 2 CO 3 can improve the optical properties, the surface area, and charge transfer as well as hinder the recombination of electron-hole pairs, which is supposedly linked to the photocatalytic activity. Among the as-prepared samples, Ag@AgCl/Bi 2 O 2 CO 3 composites with 10 wt % Ag@AgCl showed the highest photocatalytic performances with regards to the degradation of organic pollutants. The photocatalyst also displayed higher stability during the photoreactions and no obvious deactivation was found for the recycled catalyst after five test runs. The present results suggest that hierarchical flower-like Ag@AgCl/Bi 2 O 2 CO 3 composites are a promising candidate for the photocatalytic degradation of organic dyes.