Carbon Dots Sensitized BiOI with Dominant {001} Facets for Superior

Dec 10, 2015 - Defect Engineering of Bismuth Oxyiodide by IO3 Doping for Increasing Charge Transport in Photocatalysis. Yongchao Huang , Haibo Li ...
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Carbon Dots Sensitized BiOI with Dominant {001} Facets for Superior Photocatalytic Performance Bei Long,† Yongchao Huang,† Haibo Li,† Fengyi Zhao,† Zebao Rui,† Zili Liu,§ Yexiang Tong,*,† and Hongbing Ji*,†

Ind. Eng. Chem. Res. 2015.54:12788-12794. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/28/18. For personal use only.

†

MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, The Key Lab of Low-carbon Chem & Energy Conservation of Guangdong Province, Sun Yat-Sen University, 135 Xingang West Road, Guangzhou 510275, China § Guangzhou University, School of Chemistry and Chemical Engineering, Guangzhou, China S Supporting Information *

ABSTRACT: Degrading and removing harmful compounds by the use of semiconductor photocatalysts has been testified to be and effective and attractive green technique in wastewater treatment. Herein, carbon dots sensitized BiOI with highly exposed {001} facets has been prepared and used to study the photocatalytic degradation of methyl orange (MO). Due to the improved charge separation, transfer, and optical absorption, the photocatalytic performance for methyl orange degradation of the carbon dots/{001} BiOI nanosheets is 4 times higher than that of the {001} BiOI nanosheets under visible light irradiation. Additionally, the carbon dots/{001} BiOI nanosheets also have superior stability after 5 cyclings.

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INTRODUCTION

{001} facets had high photocatalytic activity in the degradation process of Rhodamine B upon visible and ultraviolet light irradiations.28 Additionally, results in density functional theory calculations have confirmed that the {001} facets of BiOXs nanosheets, in which clear boundaries of [Bi2O2] and halogen slabs are found, possess the highest thermodynamic stability and photoactivity among the {001}, {110}, and {010} facets.29,30 However, the performance of these semiconductors is quite unsatisfactory due to low charge separation efficiency. Therefore, it is urgent to enhance the activity of bismuth oxyhalides with exposed {001} facets. Carbon dots (CDs) exist as new carbonaceous nanomaterials that contain oxygen. They exhibited some captivating characteristics, such as low toxic properties, stable chemical properties, and the ability to functionalize easily with other materials.31−33 As a result of the quantum effect with various traps that can be emitted on the nanodots surface, the carbon dots could exhibit some exceptional characteristics, such as versatile absorption of visible light and strong photoluminescence, which is highly beneficial for photocatalytic reactions.34,35 Although some reports demonstrated the CDs sensitized semiconductors have fine abilities for photocatalytic hydrogen evolution and PEC, there are few reports of the study of CDs sensitized bismuth oxyhalides exposing dominant {001} facets for photocatalytic degradation.36−38 In this paper, CDs sensitized BiOI with highly exposed dominant {001} facets have been synthesized by a simple method and used to study the photocatalytic degradation of methyl orange (MO). This demonstrated that the CDs could serve (i) as electron surface trap sites, (ii) to enhance the absorption range of visible light,

Semiconductor photocatalysis has been employed as a possible and promising pathway to counter universal environmental predicaments.1−8 Compared to other photocatalysts, bismuth oxyhalides (BiOXs, X = F, Cl, Br, I) happen to be attractive to many researchers as a result of (i) their internal electric fields that exist between the [Bi2O2]2+ and (ii) the photoinduced electrons and holes that can be separated by the layers of the anionic halogen.9−15 However, their poor quantum yields and fast recombination rate of photoinduced electrons and holes has hindered them for practical application.16 Many strategies have been explored to enhance the photocatalytic activity of BiOXs, which include: (i) impurity element doping,17,18 (ii) facet-controlled fabrication,19 and (iii) semiconductor heterojunctions.20−23 The effect of facets on BiOXs has been studied generally in recent years because their intrinsic reactivity and chemical/surface physical properties can be enhanced by tailoring the surface atomic structures. Photocatalytic degradation can be achieved based on two conditions. The first one is the migration of photogenerated electrons and holes to the surface without any recombination, and the second is a surface-adsorbed species redox process.24−26 High photocatalytic activity can be further realized, as the thickness of the nanosheets allows rapid movement of the photogenerated charge carriers to the surface. For instance, Wang et al. reported that BiOCl with dominant {001} facets exhibits high photocatalytic performance for degradation of Rhodamine B (RhB).19 Ye and co-workers have demonstrated that BiOI nanosheets exhibit higher photocatalytic performance than irregular BiOI for decomposition of RhB.27 Additionally, vacancies existing on the surface of ultrathin BiOCl nanosheets composed of {001} facets were reported to be associated with the enhanced absorption capacity and effective separation of the electron−hole pairs.9 Zhang and co-workers showed that BiOCl nanodisks exposing © 2015 American Chemical Society

Received: Revised: Accepted: Published: 12788

July 28, 2015 December 9, 2015 December 10, 2015 December 10, 2015 DOI: 10.1021/acs.iecr.5b02780 Ind. Eng. Chem. Res. 2015, 54, 12788−12794

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Figure 1. (a) PL emission spectra of CDs at the excitation wavelength from 350 to 650 nm. (b) XRD patterns of the as-prepared CDs/{001} BiOI composites.

Figure 2. (a) FT-IR patterns and (b) Raman spectra of the as-prepared CDs/{001} BiOI composites.

Characterization. The structure was characterized by X-ray diffractometry (XRD, D-MAX 2200 VPC), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. The morphologies of the materials were characterized by fieldemission SEM (JSM-6330F) and TEM (JEM2010-HR, 200 kV). UV−vis absorption spectra were acquired via a UV−vis− NIR spectrophotometer (UV−vis−NIR, Shimadzu UV 2450). Room-temperature photoluminescence (PL) spectra with excitation wavelength of 380 nm were obtained with a spectrofluorophotometer (RF-5301PC). Nitrogen adsorption/ desorption isotherms were measured at 77 K (ASAP 2020 V3.03 H). Before measurement, powders were outgassed using nitrogen at 100 °C for 5 h. A three-electrode cell was used for electrochemical measurements. An electrochemical workstation (CHI 760D) was used for all the electrochemical measurements at room temperature. Photocatalytic Activity Test. The degradation of MO was used to assess the photocatalytic performance under visible light (≥420 nm) irradiation. A 500 W xenon lamp with a 420 nm cutoff filter was used as the visible light source to ensure the desired irradiation light. First of all, catalyst (50 mg) was added into 100 mL aqueous suspension of MO (10 mg/L). Before photoreaction, the solution was stirred in the dark to achieve desorption−adsorption equilibrium between the dye and photocatalyst. At suitable intervals, 2−3 mL solution was withdrawn and centrifuged to deposit solid. The concentration of remanent MO, which has a maximum absorption wavelength at 465 nm, was tested by UV−vis spectrophotometry.

and (iii) to improve separation of charges at the heterojunction of CDs/{001} BiOI. All these merits contribute to the outstanding and improved photocatalytic performance for the removal of MO.

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EXPERIMENTAL SECTION

Preparation of {001} BiOI Nanosheets. One mmol Bi(NO3)3 and 3 mmol KI were dissolved in deionized water (15 mL) with continuous stirring at room temperature; then 2 M NaOH was added dropwise to adjust the pH to 6.0. The solution was poured into a 20 mL Teflon-lined stainless autoclave. The autoclave was heated at 160 °C for 2 h and then cooled to normal temperature. The obtained precipitates were collected and washed with ethanol and deionized water and dried at 60 °C for 8 h. Preparation of CDs. In a typical procedure, 0.75 g sucrose was dissolved in deionized water (30 mL) at normal temperature with continuous stirring for 30 min. The asprepared solution was subsequently loaded into a 40 mL Teflon lined stainless autoclave and heated to 180 °C for 5 h. After cooling to room temperature, the obtained products were collected and washed with deionized water and ethanol and dried at 60 °C. Preparation of CDs/{001} BiOI Nanosheets. Typically, 0.3 g {001} BiOI nanosheets were immersed in 0.5 mg/mL CDs solution for 5 h. Then, the obtained products were centrifuged and washed with ethanol and deionized water and dried at 80 °C for 8 h. CDs/{001} BiOI using various concentrations of CDs (0.2, 0.5, 1.0, 1.5 mg/mL) were prepared as well, which were labeled as CDs/BiOI-0.2, CDs/ BiOI-0.5, CDs/BiOI-1, and CDs/BiOI-1.5, respectively.

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RESULTS AND DISCUSSION PL Emission Spectra and XRD Patterns of CDs. The CDs were synthesized using a previously reported pyrolysis 12789

DOI: 10.1021/acs.iecr.5b02780 Ind. Eng. Chem. Res. 2015, 54, 12788−12794

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Industrial & Engineering Chemistry Research method.39 A broad peak centered at 650 nm is observed in the absorption spectrum of CDs, as demonstrated by Figure S1. The emission wavelength of CDs shows a gradual red shift and variation of emission intensity upon increasing excitation wavelength from 350 to 650 nm due to the characteristic surface-defect emission (Figure 1a). The fabrication process of CDs/{001} BiOI photocatalysts was described in the Experimental Section. Structural characterization of the CDs/ {001} BiOI samples is shown in Figure 1b. Figure 1b shows the XRD patterns of the CDs/{001} BiOI samples, where all peaks can be assigned to the diffraction pattern of the tetragonal BiOI phase (PDF# 10-0445). Note that the diffraction intensity ratios of {110}/{001} nanosheets are much higher than expected; such a phenomenon might indicate that the sheetlike building blocks were oriented along the {001} direction, with a relatively larger lateral size oriented along the {110} direction. However, no typical patterns of the CDs were observed in CDs/{001} BiOI samples, which was probably due to small amounts of the CDs as well as a high degree of CDs dispersion in the nanocomposite samples. FT-IR Patterns and Raman Spectra. To investigate the presence of CDs, the CDs/{001} BiOI samples with different CDs contents were further characterized by Fourier transform infrared (FT-IR) spectroscopy. Figure 2a shows the results of FT-IR spectra analysis of the CDs/{001} BiOI samples. The absorption peak at 514 cm−1 of BiOI can be observed due to the Bi−O stretching mode. The peaks around 3430 and 1630 cm−1, which are commonly related to O−H bending vibrations (ν(O−H) and δ(O−H)), possibly resulted from the trace amount of surface absorbed water according to a previous report.40,41 With regard to the CDs/{001} BiOI samples, the peaks at 1380 cm−1 correspond to the typical stretching mode of the CDs. This exhibited the existence of CDs in the CDs/ {001} BiOI samples. The FT-IR results showed that BiOI and CDs have been coupled together successfully. Furthermore, to detect the formation of CDs/{001} BiOI composites, Raman spectra of the CDs/{001} BiOI samples were also recorded at room temperature. Two typical vibrational peaks at 85 and 148.5 cm−1 are observed for the BiOI sample, which are assigned to A1g and Eg of the Bi-X stretching mode, respectively.42 It is noted that the broad peak regions (1000− 3000 cm−1) appearing after the CDs were coupled with the BiOI. As for the CDs/{001} BiOI composites, the peak intensity of BiOI decreased and the peak intensity of CDs increased drastically when increasing CDs amount, corroborating the formation of CDs/{001} BiOI composites. Morphology and Microstructure. Scanning electron microscopy (SEM) was used in order to investigate the surface nanostructures of samples. Figure 3a and S2 show the SEM images of pure {001} BiOI and CDs/BiOI with different CDs contents, respectively. We can find that the samples have a flower-like construction of a diameter of 1−2 μm. These microflowers are made up of loose and packed flake-like subunits which have an extremely thin thickness. However, compared with the BiOI nanosheets, there is no change in the morphologies of CDs/{001} BiOI with different CDs contents. Given the limited resolution of our instrument, the quantumsized CDs cannot be directly observed from SEM images. The SEM data coupled with the XRD dates strongly substantiated that the single nanosheet grows along the {110} direction and the {001} facets located along the side surface, resulting in a sheet-like structure with a long side of 1 μm and flat surface exposing {001} facets. Furthermore, detailed crystal structures

Figure 3. (a) SEM images of CDs/BiOI-1 nanosheets. (b) TEM images of CDs/BiOI-1 nanosheets. (c) HRTEM image of CDs/BiOI-1 nanosheets. (d and e) HRTEM images of the CDs and BiOI. (f) SAED of CDs/BiOI-1 nanosheets.

of CDs/BiOI-1 samples were studied using transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Figure 3b displays a closer TEM image of CDs/BiOI-1, further displaying the ultrathin sheet. The nanoflower is indeed made up of self-organized flake-like subcells which have a mean thickness of 10 nm. As displayed by high-resolution transmission electron microscopy (HRTEM) imaging at the edge of the microflower in Figure 3c, the CDs are attached to the surfaces of the BiOI nanosheets simply by a soaking method. Figure 3d and 3e show the HRTEM images of the CDs and BiOI, respectively, collected from Figure 3c. The lattice spacings of 0.664 and 0.322 nm correspond to the BiOI (001) planes and (002) planes of graphitic carbon, respectively. According to the XRD results and structural analyses, it comes down to the conclusion that the prepared BiOI nanoplates exhibited in Figure 3a are primarily made up of {001} facets and that a single nanosheet is bounded by {001} facets at the top and bottom. Optical Properties. It is very important for the photocatalysts to extend the light adsorption range under visible light irradiation. Figure 4a shows the UV−vis diffuse reflectance spectra of the as-prepared CDs/{001} BiOI samples. From the UV light to visible light shorter than 655 nm, BiOI has strong absorption intensity. When BiOI was mixed with CDs, the absorption edge of CDs/{001} BiOI composites shifted red compared to the BiOI sample, revealing that the CDs/{001} BiOI composites could utilize more solar light. Correspondingly, the samples show different colors, ranging from wine red to black (Figure S3). The optical band gap of the crystalline semiconductor could be estimated via the classic Tauc method and using the following equation: αhν = A(hν − Eg)1/2, where α, ν, Eg, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. Therefore, Eg could be calculated by the intercept of a tangent with the x-axis of the plot. From Figure 4b, the estimated band gaps of CDs/BiOI composites are smaller than that of the BiOI sample. This convincingly supports that the introduction of CDs can intrinsically improve the optical absorption property of BiOI, which may be the main reason for the elevated photocatalytic performance of CDs/BiOI-1 Electron−Hole Analysis. To study the high separation efficiency of photoinduced charge in the CDs/{001} BiOI composites, photocurrents were measured for the CDs/{001} BiOI composites as shown in Figure 4c. It was observed that 12790

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Figure 4. (a) UV−vis absorption spectra of the as-prepared CDs/{001} BiOI composites. (b) Plots of the (αhν)1/2 vs photon energy (hν) for the asprepared CDs/{001} BiOI composites. (c) Photocurrent responses of as-prepared CDs/{001} BiOI composites under visible light irradiation. (d) Photoluminescence spectra of as-prepared CDs/{001} BiOI composites with an excitation wavelength of 380 nm.

Figure 5. (a) Photocatalytic activity of as-prepared CDs/{001} BiOI composites for degradation of MO in visible light. (b) Pseudo-first-order reaction kinetics of as-prepared CDs/{001} BiOI composites. (c) Cyclic test of photodegradation of MO over CDs/BiOI-1 samples. (d) Degradation of MO of CDs/BiOI-1 samples in the presence of different scavengers.

efficiency shows an elevating trend with increasing concentration of CDs from 0 to 1 mg/mL, and drops when concentration rises to 1.5 mg/mL. That is, the optimal concentration for CDs soaking treatment is about 1 mg/mL, indicating that a proper amount of CDs is required to achieve effective improvement in visible light response.43 Moreover,

fast photocurrent responses through on−off cycles were detected in these photocatalysts under visible light irradiation. This directly corresponds with the separation efficiency of photogenerated carriers. Interestingly, samples with CDs attached on the surface of BiOI nanosheets were more photoactive in degradation of Rhodamine B. The degradation 12791

DOI: 10.1021/acs.iecr.5b02780 Ind. Eng. Chem. Res. 2015, 54, 12788−12794

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By adding various scavengers, active species conducive to the oxidation process triggered by the irradiation process are studied, and Figure 5d shows the results.44 The photocatalytic activity was restrained with the addition of 10 mmol triethanolamine (TEOA) serving as scavenger for the photoinduced hole in the reactive system, which means that removing the hole may increase the electron−hole separation efficiency.45 However, while adding tert-butylalcohol (TBA) as scavenger for • OH, the photocatalytic performance of CDs/BiOI-1 remained stable without obvious changes, which means that it is not the main oxidative species in the photocatalytic reactive process of CDs/BiOI-1, while when benzoquinone (BQ), serving as a scavenger of superoxide radical species, was injected in the reactive system, the photocatalysis was obviously restrained. Such a phenomenon indicates that superoxide radicals may be a main active species contributing to the photocatalytic process. Thus, it is believed that the direct hole oxidation process mainly controls the degradation of MO over the CDs/BiOI-1 sample and then the oxidative process of the generated •O2− radicals, but the difference of the •OH reactions is negligible taking place on the surface of the photocatalyst. Benefiting from the high quality of the sp2 conjugated bond from the carbon lattice of CDs, the electrons easily and ballistically move in a carbon dots layer, which makes carbon dots perform as an outstanding electron acceptor.46 The conduction band position of BiOI is −0.68 V (vs SHE), and the potential of graphene/graphene•− is −0.08 V (vs SHE) in the present system.47,48 Therefore, the photogenerated electrons could be easily shifted from the conduction band of BiOI to carbon dots under visible light irradiation. The electronaccepting properties of carbon dots provide an easy way to direct the flow of photogenerated charge carriers; therefore, elevated photocatalytic activity can be attained by the structure of CDs/BiOI. The reaction mechanism of improved photoactivity of the CDs/BiOI composites is shown in a schematic illustration (Figure 6).

photoluminescence (PL) spectra can be utilized for efficiency exploration of the charge carrier trapping, charge migration, and transfer. The PL emission originates from free carriers recombination, which is beneficial for understanding of electron−hole pairs in semiconductor particles. Figure 4d displays the PL spectra of the CDs/{001} BiOI composites and BiOI with 380 nm excitation wavelength. Two emitting peaks around 550 and 702 nm are observed for the pure BiOI sample. The shapes and peaks positions of CDs/{001} BiOI composites samples are similar to those of the untreated BiOI, while the emission intensity of the CDs/{001} BiOI composites decreases. In addition, the emission band intensities of the spectra vary for the different amounts of CDs. The CDs/ BiOI-1 causes the biggest decrease in the intensity of the photoluminescence peak. It is generally accepted that the PL spectrum of a semiconductor is related to the radiative recombination mechanism of self-trapped excitations. Hence, the existence of the CDs in the photocatalyst is capable of suppressing the radiative recombination process, thereby creating a weak recombination of the electron/hole pairs and high photon efficiency. Photocatalytic Activity. The photocatalytic degradation of MO over the samples was evaluated under visible light irradiation. Before the photocatalytic tests, all the CDs/{001} BiOI samples were magnetically stirred in the dark for 60 min to reach the adsorption−desorption equilibrium. The MO adsorption capabilities of all the CDs/{001} BiOI samples are shown in Figure S4, and the pH value of the MO solution was adjusted to 9. The MO adsorptions over untreated BiOI sample were 14.1%, while the adsorption of CDs/BiOI samples decreased. These results revealed that the introduction of CDs could slightly decrease the adsorption capability of BiOI toward MO. The specific surface areas of CDs/BiOI samples were further verified by the Brunauer−Emmett−Teller (BET) test. The results obtained from the N2 absorption displayed that the CDs/BiOI-1 samples exhibited the largest BET values (9.88 m2 g−1), indicating the surface area has a large effect on the performance of the samples (Figure S5). Indeed, as shown in Figure 5a, the content of CDs had a significant influence on the photocatalytic activity. In comparison with untreated BiOI, all the CDs/{001} BiOI samples have higher photocatalytic activity in visible light. When increasing the proportion of CDs, the photocatalytic activity is improved gradually. After 50 min of visible light irradiation, 98.5% of MO was degraded over the CDs/BiOI-1, while the BiOI, CDs/BiOI-0.2, CDs/BiOI0.5, and CDs/BiOI-1.5 were only 30%, 55%, 78%, and 60%, respectively. The photolysis of MO itself was negligible under the same conditions. Moreover, the maximum k (0.080 min−1) was achieved with the CDs/BiOI-1 composite, the maximum k is about 10 times that of untreated BiOI (0.008 min−1) and 3 times of CDs/BiOI-0.5 (0.027 min−1). This is because of the segregation of photoinduced carriers by introduction of CDs and BiOI heterojunctions. When more CDs exist in the composite, the absorption intensity of visible light may decrease. By means of cycling tests for the photocatalysis of MO in visible light, the stability of the CDs/BiOI-1 composites was assessed, which was displayed in Figure 5c. After five cycling tests, no obvious change in the photocatalytic performance was observed, which indicated that the CDs/ BiOI-1 photocatalysts are highly stable in the process of photocatalysis. The good catalytic stability may be closely related to the facile restoration of the microsized assembly in the reactive system.

Figure 6. Schematic illustration of the CDs/BiOI-1 photocatalytic reaction process under visible light irradiation.

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CONCLUSIONS In summary, CDs sensitized BiOI with highly exposed {001} facets was prepared via a simple method, and the corresponding photocatalytic activity was further explored. Three times enhancement in photocatalytic performance was achieved by incorporating BiOI with CDs as light absorber. This demonstrated that the CDs not only serve as electron surface trap sites but also enhance the visible light absorption range and the charge separation at the heterojunction of CDs/{001} BiOI, which can superiorly improve the photocatalysis of MO. Besides, it is believed that the direct hole oxidation process and the oxidative process of the generated •O2− radicals control the degradation of MO over the CDs/BiOI-1 sample. Generally, 12792

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(11) Gnayem, H.; Sasson, Y. Hierarchical nanostructured 3D flowerlike BiOClxBr1−x semiconductors with exceptional visible light photocatalytic activity. ACS Catal. 2013, 3, 186. (12) Jiang, J.; Zhao, K.; Xiao, X.; Zhang, L. Synthesis and facetdependent photoreactivity of BiOCl single-crystalline nanosheets. J. Am. Chem. Soc. 2012, 134, 4473. (13) Zhao, K.; Zhang, L.; Wang, J.; Li, Q.; He, W.; Yin, J. J. Surface structure-dependent molecular oxygen activation of BiOCl singlecrystalline nanosheets. J. Am. Chem. Soc. 2013, 135, 15750. (14) Ma, J.; Ding, J.; Yu, L.; Li, L.; Kong, Y.; Komarneni, S. BiOCl dispersed on NiFe-LDH leads to enhanced photo-degradation of Rhodamine B dye. Appl. Clay Sci. 2015, 109, 76. (15) Xia, J.; Yin, S.; Li, H.; Xu, H.; Yan, Y.; Zhang, Q. Self-assembly and enhanced photocatalytic properties of BiOI hollow microspheres via a reactable ionic liquid. Langmuir 2011, 27, 1200. (16) Huang, Y.; Long, B.; Li, H.; Balogun, M. S.; Rui, Z.; Tong, Y.; Ji, H. Enhancing the photocatalytic performance of BiOClxI1−x by introducing surface disorders and Bi nanoparticles as cocatalyst. Adv. Mater. Interfaces 2015, 2, DOI: 10.1002/admi.201500249. (17) Jiang, G.; Wang, R.; Wang, X.; Xi, X.; Hu, R.; Zhou, Y.; Wang, S.; Wang, T.; Chen, W. Novel highly active visible-light-induced photocatalysts based on BiOBr with Ti doping and Ag decorating. ACS Appl. Mater. Interfaces 2012, 4, 4440. (18) Dash, A.; Sarkar, S.; Adusumalli, V. N.; Mahalingam, V. Microwave synthesis, photoluminescence, and photocatalytic activity of PVA-functionalized Eu3+-doped BiOX (X= Cl, Br, I) nanoflakes. Langmuir 2014, 30, 1401. (19) Wang, D. H.; Gao, G. Q.; Zhang, Y. W.; Zhou, L. S.; Xu, A. W.; Chen, W. Nanosheet-constructed porous BiOCl with dominant {001} facets for superior photosensitized degradation. Nanoscale 2012, 4, 7780. (20) Wang, S.; Guan, Y.; Wang, L.; Zhao, W.; He, H.; Xiao, J.; Yang, S.; Sun, C. Fabrication of a novel bifunctional material of BiOI/ Ag3VO4 with high adsorption-photocatalysis for efficient treatment of dye wastewater. Appl. Catal., B 2015, 168, 448. (21) Kong, L.; Jiang, Z.; Xiao, T.; Lu, L.; Jones, M. O.; Edwards, P. P. Exceptional visible-light-driven photocatalytic activity over BiOBrZnFe2O4 heterojunctions. Chem. Commun. 2011, 47, 5512. (22) Li, N.; Hua, X.; Wang, K.; Jin, Y.; Xu, J.; Chen, M.; Teng, F. In situ synthesis of uniform Fe2O3/BiOCl p/n heterojunctions and improved photodegradation properties for mixture dyes. Dalton Trans. 2014, 43, 13742. (23) Weng, B.; Xu, F.; Xu, J. Hierarchical structures constructed by BiOX (X= Cl, I) nanosheets on CNTs/carbon composite fibers for improved photocatalytic degradation of methyl orange. J. Nanopart. Res. 2014, 16, 1. (24) Su, R.; Shen, Y.; Li, L.; Zhang, D.; Yang, G.; Gao, C.; Yang, Y. Silver-modified nanosized ferroelectrics as a novel photocatalyst. Small 2015, 11, 202. (25) Luo, Z.; Poyraz, A. S.; Kuo, C. H.; Miao, R.; Meng, Y.; Chen, S. Y.; Jiang, T.; Wenos, C.; Suib, S. L. Crystalline mixed phase (anatase/ rutile) mesoporous titanium dioxides for visible light photocatalytic activity. Chem. Mater. 2015, 27, 6. (26) Ye, K. H.; Yu, X.; Qiu, Z.; Zhu, Y.; Lu, X.; Zhang, Y. Facile synthesis of bismuth oxide/bismuth vanadate heterostructures for efficient photoelectrochemical cells. RSC Adv. 2015, 5, 34152. (27) Ye, L.; Tian, L.; Peng, T.; Zan, L. Synthesis of highly symmetrical BiOI single-crystal nanosheets and their {001} facetdependent photoactivity. J. Mater. Chem. 2011, 21, 12479. (28) Zhang, X.; Wang, X. B.; Wang, L. W.; Wang, W. K.; Long, L. L.; Li, W. W.; Yu, H. Q. Synthesis of a highly efficient BiOCl single-crystal nanodisk photocatalyst with exposing {001} facets. ACS Appl. Mater. Interfaces 2014, 6, 7766. (29) Huang, W. L.; Zhu, Q. DFT calculations on the electronic structures of BiOX (X= F, Cl, Br, I) photocatalysts with and without semicore Bi 5d states. J. Comput. Chem. 2009, 30, 183. (30) Yang, W.; Wen, Y.; Chen, R.; Zeng, D.; Shan, B. Study of structural, electronic and optical properties of tungsten doped bismuth

this work provides a platform to understand the effect of the CDs, which will support a novel method to design progressive catalysts for dealing with worldwide environmental issues.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02780. UV−vis absorption spectra of CDs solution, SEM images of BiOI, and photoimages of BiOI nanosheets. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 86-20-84110071. Fax: 86-20-84112245. E-mail: [email protected] (Y.X.T). *Phone: 86-20-84110071. Fax: 86-20-84112245. E-mail: jihb@ mail.sysu.edu.cn (H.B.Ji). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was preliminarily supported by the National Science Fund for Distinguished Young Scholars (21425627), the Science and Technology Plan Project (2013B090600036), the Natural Science Foundation of China (21461162003 and 395 21425627), and Natural Science Foundation (2014KTSCX004 and 2014A030308012) of Guangdong Province, China.

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DOI: 10.1021/acs.iecr.5b02780 Ind. Eng. Chem. Res. 2015, 54, 12788−12794