Carbon Quantum Dots Induced Ultrasmall BiOI Nanosheets with

Feb 12, 2016 - This was the first preparation of CQD-based photocatalysts at room temperature. The hydrogen bond and Coulomb force of the IL(51) could...
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Carbon Quantum Dots Induced Ultrasmall BiOI Nanosheets with Assembled Hollow Structures for Broad Spectrum Photocatalytic Activity and Mechanism Insight Jun Di,† Jiexiang Xia,*,† Mengxia Ji,† Bin Wang,† Sheng Yin,† Hui Xu,† Zhigang Chen,‡ and Huaming Li*,† †

School of Chemistry and Chemical Engineering, Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China ‡ School of the Environment, Jiangsu University, Zhenjiang, 212013, P. R. China S Supporting Information *

ABSTRACT: Carbon quantum dots (CQDs) induced ultrasmall BiOI nanosheets with assembled hollow microsphere structures were prepared via ionic liquids 1butyl-3-methylimidazolium iodine ([Bmim]I)-assisted synthesis method at room temperature condition. The composition, structure, morphology, and photoelectrochemical properties were investigated by multiple techniques. The CQDs/ BiOI hollow microspheres structure displayed improved photocatalytic activities than pure BiOI for the degradation of three different kinds of pollutants, such as antibacterial agent tetracycline (TC), endocrine disrupting chemical bisphenol A (BPA), and phenol rhodamine B (RhB) under visible light, light above 580 nm, or light above 700 nm irradiation, which showed the broad spectrum photocatalytic activity. The key role of CQDs for the improvement of photocatalytic activity was explored. The introduction of CQDs could induce the formation of ultrasmall BiOI nanosheets with assembled hollow microsphere structure, strengthen the light absorption within full spectrum, increase the specific surface areas and improve the separation efficiency of the photogenerated electron−hole pairs. Benefiting from the unique structural features, the CQDs/BiOI microspheres exhibited excellent photoactivity. The h+ was determined to be the main active specie for the photocatalytic degradation by ESR analysis and free radicals trapping experiments. The CQDs can be further employed to induce other nanosheets be smaller. The design of such architecture with CQDs/BiOI hollow microsphere structure can be extended to other photocatalytic systems.

1. INTRODUCTION Semiconductor photocatalysis is a very effective and promising technique for energy replacement and environmental pollutants removal.1 The exploration of novel visible-light response photocatalysts has been conducted by more and more researchers. A lot of efficient semiconductor materials such as CdS,2 g-C3N4,3 WO3,4 Bi2WO6,5 Bi2MoO66 have been developed under persistent efforts. Recently, BiOX (X = Cl, Br, I) has been demonstrated to be promising photocatalyst for the removal of toxic organic pollutants and metal ions.7−13 It is a layered structure with [Bi2O2] slabs interleaved with double halogen atom slabs along the [001] direction.14 This unique structure feature ensured the internal static electric fields along [001] direction, which relatively reduced the recombination probability of the photongenerated electron−hole pairs.15 Among the BiOX (X = Cl, Br, I), BiOI was a more promising photocatalyst, which could be motivated under most of the visible light range with a narrow band gap about 1.63−1.94 eV.16−18 However, the efficiency of pure BiOI was far from satisfaction due to the lack of bulk-to-surface channels for the electrons and the high recombination rate of photogenerated electron−hole pairs. In © XXXX American Chemical Society

order to improve its photocatalytic activity, many BiOI-based hybrid photocatalysts (BiOI/TiO2,19,20 ZnO/BiOI,21 AgI/ BiOI,22,23 ZnWO4/BiOI,24 MnOx-BiOI,25 BiOCl/BiOI,26 gC3N4/BiOI,27,28 MWCNT/BiOI,29 graphene/BiOI30) have been prepared and applied for the photodegradation of azo dyes. However, in the BiOI-based hybrid photocatalysts, the light harvesting ability and quantum efficiency of the materials were still poor. Studies showed that materials with hollow structures have certain advantages in photocatalysis.31,32 The materials with hollow structures could effectively harvest light through multiple scattering of light, and the high specific surface areas of hollow structures could adsorb more active species, which may endow them with remarkably improved photocatalytic activity. In our previous work, we have designed BiOI materials with hollow structures15,33 and constructed the g-C3N4/BiOI heterojunction28 to further improve the photocatalytic activity. However, the construction of g-C3N4/BiOI heterojunction lacks contact area for efficient charge transfer Received: November 24, 2015 Revised: January 24, 2016

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universal approach for effective light harvesting, shortened diffusion distance for charge transport, as well as excellent interfacial charge transfer and separation, which can be widely used in other photocatalytic systems.

across the interface due to the bulk g-C3N4 nanosheets and thus can not disperse uniformly and can not have sufficient contact with spherical BiOI. Therefore, it was significant to find an effective approach to enlarge the contact area and form a perfect interface, thus leading to enhancement of overall photocatalytic efficiency. Carbon quantum dots (CQDs) have greatly attracted research interests in recent years due to their superiority,34−36 such as water solubility, robust chemical inertness, biocompatibility, low toxicity, and resistance to photobleaching,37 making its promising application in fluorescent probes,38 photovoltaic devices,39 bioimaging,40 and so on. Thanks to the conjugated π structure of CQDs, they can act as electron transporters and acceptors in photoexcited states, which makes them promising for applications in photocatalysis fields. Very recently, Li et al. first designed the CQDs/semiconductor photocatalyst and applied for the photodegradation of MB.41 Subsequently, CQDs have been introduced to several semiconductors42−50 such as TiO2, C3N4, Cu2O, BiOCl, and BiVO4, and have been demonstrated to be an effective approach to improve photocatalytic activity. However, there have been few reports to study the coupling mode of CQDs and semiconductor hybrid photocatalysts. The functional mechanism of CQDs for the improvement of activity remains far from clear. And the photocatalytic mechanism of those CQDs-based photocatalysts for pollutant photodegradation still needs to be further explored. Considering the outstanding property of CQDs, the performance was worth the anticipation if the CQDs were introduced to the BiOI microspheres. Compared with the BiOI microspheres, which can only point contact with the bulk materials, the modified CQDs could have enough interface combining with BiOI microspheres owning to their small size. The ubiquitous CQDs shortened the charge transport time and distance. To the best of our knowledge, it was the first time for the preparation of CQD/hollow microsphere materials and employment in the photocatalysis field. Considering the usually high energy consumption for the preparation of hollow microsphere materials at high temperature conditions via methods such as the solvothermal method, it was more meaningful to develop the highly efficient photocatalysts at room temperature. Herein, the CQDs/BiOI hollow microspheres were prepared via an ionic liquids (ILs) 1butyl-3-methylimidazolium iodine ([Bmim]I)-assisted synthesis method at room temperature. This was the first preparation of CQD-based photocatalysts at room temperature. The hydrogen bond and Coulomb force of the IL51 could act on hydroxyl or carboxyl groups on the surface of CQDs, and thus result in a good combination of CQDs. Due to the [Bmim]I involved in the formation of BiOI by providing the I source, it could lead to the in situ combination of CQDs with the BiOI materials, with high dispersion and tight integration. The affect of CQDs on the morphology of BiOI microspheres was studied. The photocatalytic activities of the CQDs/BiOI microspheres were evaluated through the degradation of three different kinds of pollutants, such as phenol rhodamine (RhB), antibacterial agent tetracycline (TC), and the endocrinedisrupting chemical bisphenol A (BPA). At the same time, the photocatalytic activity of the CQDs/BiOI microspheres was further evaluated under the light above 580 nm and light above 700 nm, respectively. The key role of CQDs for the improvement of photoactivity was explored. The structure− activity relationships and the photocatalytic mechanism were investigated. The design of such architectures provided a

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Photocatalysts. All the chemicals were analytical grade and used without further purification. The CQD solid was prepared according to the literature followed by freeze-drying.52 In a typical synthesis procedure of CQDs/BiOI microspheres, two different solutions were prepared at the beginning. Solution A: 1 mmol of Bi(NO3)3·5H2O was dissolved into a solution contained 9 mL distilled water, 1 mL acetic acid, and a certain amount of CQD solid. Solution B: 1 mmol of ionic liquid [Bmim]I was added into 10 mL of alcohol. The CQDs/ BiOI microsphere samples were synthesized by dropwise adding solution B to solution A over 5 min to complete under stirring conditions. Then, the mixing solution was further stirred for 1 h under room temperature conditions. The resulting powder was collected and washing with water and alcohol three times. The weight percentages of CQDs in the samples were 0, 0.5, 3, and 5 wt %, respectively. 2.2. Characterization. The crystal structure of samples was investigated using X-ray diffraction (Bruker D8 diffractometer) with Cu−Kα radiation at a scan rate of 7 min−1. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI5300 with a monochromatic Mg Kα source to determine the elements on the surface. The scanning electron microscopy measurements were carried out on a field emission scanning electron microscopy (SEM; (JEOL JSM-7001F; voltage = 10 kV). Transmission electron microscopy (TEM) measurements were obtained on a JEOL-JEM-2010 microscope. The structural information for CQDs/BiOI materials was measured using Fourier transforms pectrophotometer (FTIR, Nexus 470) using the standard KBr disk method. UV−vis diffuse reflection spectroscopy (DRS) was performed on a Shimadzu UV-2450 spectrophotometer using BaSO4 as the reference. The electron spin resonance (ESR) signals of spin-trapped oxidative radicals were obtained on a Bruker model ESR JES-FA200 spectrometer equipped with a Quanta-Ray Nd:YAG pulsed laser system. 2.3. Photoelectrochemical Characterization. The photoelectrochemical response was measured by using a CHI 660B electrochemical workstation with conventional three-electrode setup under visible-light illumination. The as-prepared pure BiOI and CQDs/BiOI microspheres samples paste was coated onto a slice of ITO glass with an area of 0.5 × 1 cm2 and employed as the working electrode. A platinum wire and a Ag/ AgCl (saturated KCl) were used as the counter and reference electrodes, respectively, and a 0.1 M phosphate buffer solution (pH = 7.0) was used as electrolyte. A 500 W Xe lamp was utilized as the visible light source. The electrochemical impedance spectroscopy (EIS) measurement was performed in 0.1 M KCl solution, which contained 5 mM Fe(CN)63−/ Fe(CN)64−. 2.4. Photocatalytic Test. The photocatalytic activity of the as-prepared pure BiOI and CQDs/BiOI microspheres with different CQDs contents were evaluated by photocatalytic degradation of 10 mg/L rhodamine B (RhB), 20 mg/L tetracycline (TC), and 10 mg/L bisphenol A (BPA), irradiated with visible light (300 W Xe lamp and a UV cutoff filter (λ > 400 nm)), light above 580 nm (300 W Xe lamp with a 580 nm B

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Langmuir cutoff filter), or light above 700 nm (300 W Xe lamp with a 700 nm cutoff filter). In a typical process, 30 mg, 50 mg, and 100 mg of the as-prepared sample as photocatalyst was added into 100 mL of RhB, TC, and BPA solution, respectively. Afterwards, the photocatalyst was dispersed in the solution and stirred for 0.5 h in the dark to reach adsorption equilibrium before being exposed to visible-light irradiation. The suspension was periodically sampled, and photocatalyst was removed by centrifugation at given time intervals, and the RhB, TC concentration was determined at 553 and 354 nm by using UV−vis spectroscopy. The concentration of BPA was analyzed through high performance liquid chromatography (HPLC).

Figure 2e, the peak of C 1s binding energies at 284.6, 286.5, and 288.4 eV can be ascribed to sp2 C−C bonds, C−O−C and CO in CQDs, respectively. Based on the above XRD and XPS results analysis, it can be concluded that CQDs and BiOI coexist in the CQDs/BiOI microspheres. The structural information on CQDs/BiOI microsphere samples was further analyzed by FT-IR spectra, and the result is shown in Figure S1. The absorption band located at 487 cm−1 was ascribed to the Bi−O stretching mode, suggesting the existence of BiOI in CQDs/BiOI microspheres. The absorption bands located at 1649 cm−1 were associated with the vibrational absorption band of CO. The band at 1568 and 1336 cm−1 was attributed to the bending vibrations of N−H52 and the absorption peaks of −COO−, respectively.53 The results of the analysis above originated from the surface groups of CQDs and revealed that the CQDs were introduced to the BiOI successfully. The morphology of CQDs/BiOI microspheres was investigated by SEM and TEM analysis. From the SEM observations (Figure 3a), it can be found that the as-prepared 3 wt % CQDs/BiOI samples were mainly composed of numerous microspheres with average diameters of about 2−3 μm. View from the enlarged SEM image (Figure 3b), the microspheres were indeed constructed of numerous nanosheets, and the hollow feature can be revealed from the broken microsphere. The wall thickness of the hollow microspheres was about 0.6 μm. Figure 3c was the high-powered scanning electron micrograph of the CQDs/BiOI microspheres. It can be seen that the average size of the nanosheets was about 150 nm with a thickness of about 10 nm. Compared with pure BiOI microspheres (Figure S2), the nanosheets in CQDs/BiOI microspheres were much smaller and the self-assembly of nanosheets was more tight. This result indicated that the introduction of CQDs could adjust the surface microstructure of microspheres and induce the BiOI nanosheets to become smaller. The SEM images of 5 wt % CQDs/BiOI samples are presented in Figure S3. From Figure S3, the smaller nanosheets with more tight self-assembly can be seen for 5 wt % CQDs/ BiOI samples, and it was similar to the 3 wt % CQDs/BiOI samples. The hydrogen bond and Coulomb force of the IL51 could act on hydroxyl or carboxyl groups on the surface of CQDs, and thus result in a good combination of CQDs. Due to the [Bmim]I involved in the formation of BiOI by providing the I source, it could lead to in situ combination of CQDs with the formed BiOI nanocrystal. Due to the initial combination with BiOI nanocrystal, the CQDs could prevent the further growth of nanocrystal by creating the passivation layer around BiOI cores.54 This prohibited the agglomeration of BiOI nanocrystal along the c-axis (perpendicular [Bi2O2] and [X] layers) through the nature of the repulsive forces among the CQDs charged with the same electronegativity.54 As a result, the BiOI nanosheets with smaller structure can be obtained by the introduction of CQDs. The tight interface contact of nanosheets favored the faster interfacial charge transfer, and thus could endow the higher photocatalytic activity.55 The EDS analysis (Figure 3d) further indicated that the CQDs/BiOI microsphere sample consisted of Bi, O, I, and C elements. The CQD content was about 2.79 wt % in the 3 wt % CQDs/BiOI samples. At the same time, the EDS element mapping was employed to further indicate the existence and uniform distribution of CQDs. As shown in Figure S4, the EDS element mapping clearly presented the even distribution of Bi, O, I, and C elements in CQDs/BiOI materials.

3. RESULTS AND DISCUSSION 3.1. Structural and Morphology Information. In order to explore the effect of structure on photocatalytic activity of CQD/BiOI materials, multiple techniques were employed. Figure 1 presented the XRD patterns of as-prepared CQDs/

Figure 1. XRD patterns of BiOI and CQDs/BiOI microsphere photocatalysts.

BiOI microsphere photocatalysts. It can be seen that all the diffraction peaks of pure BiOI can be indexed as the tetragonal phase (JCPDS Card no. 10-0445), revealing the high purity and single phase. The positions of the diffraction peaks remain unchanged for CQDs/BiOI photocatalysts, which means the introduction of the CQDs does not change the phase structure of BiOI microspheres. However, no diffraction peak about CQDs can be detected, which can be attributed to the low loading amount of CQDs in the samples.47,50 XPS analysis was carried out to determine the presence of elements and their valence states in CQDs/BiOI samples. The survey XPS spectrum (Figure 2a) of the CQDs/BiOI microsphere sample contained Bi, O, I and C peaks, and the corresponding high resolution spectra are displayed in Figure 2b−e. As shown in Figure 2b, the peaks centered at 158.1 and 163.5 eV were ascribed to Bi 4f7/2 and Bi 4f5/2, respectively. This demonstrates that the bismuth species in the CQDs/BiOI microspheres were Bi3+. Compared with the pure BiOI sample,15 the peaks of Bi in CQDs/BiOI microspheres showed a slight shift, which revealed the existence of interaction between CQDs and the BiOI matrix. The peak of O 1s binding energies at 529.2 eV can be ascribed to the oxygen in BiOI microspheres (Figure 2c). For the I 3d spectra (Figure 2d), two peaks at about 618.3 and 629.9 eV can be ascribed to I 3d5/2 and I 3d3/2, respectively, indicating that the valence state of iodine in the CQDs/BiOI microspheres was −1. As shown in C

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Figure 2. XPS spectra of 3 wt % CQDs/BiOI microspheres. (a) Survey of the sample; (b) Bi 4f; (c) O 1s; (d) I 3d; and (e) C 1s.

pore volume of the pure BiOI,15 0.5 wt % CQDs/BiOI, 3 wt % CQDs/BiOI, and 5 wt % CQDs/BiOI samples were calculated to be 0.113, 0.132, 0.204, and 0.252 cm3 g−1, respectively. And the specific surface area was calculated to be 15.81, 23.67, 43.12, and 51.47 m2 g−1 for pure BiOI, 0.5 wt % CQDs/BiOI, 3 wt % CQDs/BiOI, and 5 wt % CQDs/BiOI, respectively. The pore volume and specific surface areas gradually increased when the content of CQDs increased from 0.5 wt % to 5 wt %. As the SEM images show, the introduction of CQDs resulting in the CQDs/BiOI samples with improved pore quantity of microsphere’s surface. The result of increased pore volume from BET analysis was consistent with the SEM analysis. The advantage of CQDs/BiOI materials having high specific surface areas is that it permits adequate photocatalyst-polluants contact and absorption of more active species, thus endowing them with remarkably increased photoactivity. These mesoporous channels were anticipated to enhance photocatalytic activity by facilitating the diffusion of pollutants into the CQDs/BiOI microspheres.56 Figure 6a shows UV−vis absorption spectra of the CQDs/ BiOI microspheres and pure BiOI. For the pure BiOI microspheres, a significant absorption at wavelength shorter than 650 nm can be assigned to the intrinsic bandgap absorption. After the CQDs were introduced, the light absorption strengthened in both the ultraviolet and visible

TEM analysis was applied to further reveal the morphology and microstructure of the CQDs/BiOI microspheres sample. The obvious contrast between the dark edge and the relatively bright center from the TEM image (Figure 4a) also confirmed the formation of the nanosheet self-assembly hollow structure. The width of the dark edge was about 0.5−0.6 μm, which implying the wall thickness of the hollow microspheres was 0.5−0.6 μm. This was consistent with the SEM analysis. It can be seen from Figure 4b that numerous dark dots were dispersed on the nanosheets, suggesting the CQDs have been introduced to the BiOI. Figure 4c showed the arrangement of the CQDs and BiOI via HRTEM. Two different kinds of lattice fringes were clearly observed. One fringe with d = 0.321 nm matched the (002) crystallographic plane of CQDs.49 The lattice space of the BiOI crystallites was determined to be 0.301 nm, corresponding to the (110) crystallographic planes (JCPDS 100445). The result clearly showed that CQDs has been introduced to the BiOI phase, which would favor the charge transfer from BiOI to CQDs. To get more insight into the roles of CQDs playing for tuning the superficial microstructure of BiOI hollow microspheres, the samples were characterized by N2 adsorption− desorption analysis at 77 K. As it is shown in Figure 5, the N2 adsorption−desorption isotherm for the CQDs/BiOI samples can be classified as type IV with a hysteresis loop observed. The D

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Figure 3. SEM images of the 3 wt % CQDs/BiOI microspheres. (a) The low magnification SEM image; (b) high magnification SEM image; (c) top view SEM image; (d) EDS of the 3 wt % CQDs/BiOI microspheres.

Figure 5. Nitrogen absorption−desorption isotherms of CQDs/BiOI microspheres. Figure 4. (a,b) TEM image of 3 wt % CQDs/BiOI microspheres. (c) HRTEM image of 3 wt % CQDs/BiOI microspheres.

different kinds of pollutants, namely, RhB, TC, and BPA, were used as model organic pollutants for the photodegradation reaction under visible light irradiation. The photocatalytic performance of CQDs/BiOI microsphere photocatalysts was first evaluated by the degradation of RhB. The adsorption analysis of the CQDs/BiOI hollow microspheres in the dark for the RhB has been provided (Figure S5). An absorption− desorption balance was achieved between the catalyst and dye for the 30 min absorption process. It can be seen that 34.1%, 37.8%, 43.1%, and 47.5% of RhB were respectively adsorbed by pure BiOI, 0.5 wt % CQDs/BiOI, 3 wt % CQDs/BiOI, and 5 wt % CQDs/BiOI within 30 min, which should be attributed to their different adsorption capacity. As shown in Figure 7a, 29% RhB can be degraded after 30 min irradiation for the pure BiOI sample. When the irradiation time was extended to 150 min, the degradation rate of RhB was increased to 70%. The photocatalytic activity of CQDs/BiOI microspheres was higher

light regions when compared to the pure BiOI microspheres, and CQDs further expand the light absorption region to 800 nm. The UV−vis results suggested that the CQDs/BiOI microspheres samples were able to absorb more light to produce more electron−hole pairs and thus improve the photocatalytic activity. Figure 6b showed the band gaps of the CQDs/BiOI microsphere samples, which was calculated by means of Kubelka−Munk theory. The band gap of the pure BiOI sample was about 1.68 eV, while the introduction of CQDs lessened the band gaps, changing from 1.68 to 1.45 eV gradually. The reduced band gaps facilitated the electronic transition and thus accelerated the photocatalytic degradation process.57 3.2. Photocatalytic Tests. To evaluate the photocatalytic activities of the CQDs/BiOI microsphere photocatalysts, three E

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than that of pure BiOI. The photocatalytic activities of CQDs/ BiOI microspheres increased when the content of CQDs increased from 0.5% to 3%. However, the photocatalytic activity decreased when the content of CQDs further increased to 5%. This was attributed to the fact that, although modification of CQDs facilitates the charge separation, too many CQDs dispersed on the surface of BiOI would shield the BiOI from absorbing visible light. As is well known, the semiconductor needs to absorb light to generate charge carriers and then participate in the subsequent redox reactions. As a result, too much content of CQDs results in a decrease of photocatalytic performance. The 3 wt % CQDs/BiOI sample showed the best photocatalytic activity and has an improvement of 61% when compared with pure BiOI after 30 min irradiation. This indicated that the CQD modification was an effective approach to improve the photocatalytic performance of BiOI material. From the UV−vis absorption spectra, the CQDs/BiOI sample displayed the light absorption at near-infrared field, and the introduction of CQDs could further improve the light absorption. Thus, the CQDs/BiOI materials may exhibit photocatalytic activity at near-infrared field. Under the light above 580 nm irradiation (Figure 7b), the CQDs/BiOI materials also exhibited higher activity than BiOI, and 23.4% and 76.8% RhB can be photodegraded by pure BiOI and 3 wt % CQDs/BiOI materials after 4 h irradiation, respectively. Additionally, under the light above 700 nm irradiation by using 50 mg photocatalyst (Figure S6), 26.6% and 60.4% of RhB can be removed by the pure BiOI and 3 wt % CQDs/BiOI materials after 5 h irradiation, respectively. This result indicated that the as-prepared CQDs/BiOI materials displayed broad spectrum photocatalytic activity. Moreover, antibiotic TC and endocrine disrupting chemical BPA were chosen as model organic pollutant to further evaluate the photocatalytic activity of the CQDs/BiOI sample, as shown

Figure 6. (a) UV−vis absorption spectra of CQDs/BiOI microspheres. (b) (αEphoton)1/2 vs Ephoton curves of CQDs/BiOI microspheres.

Figure 7. (a) The degradation of RhB with different photocatalysts under visible light irradiation. (b) Photocatalytic degradation of RhB in the presence of BiOI and 3 wt % CQDs/BiOI materials under light above 580 nm irradiation. (c) The degradation of TC with pure BiOI and 3 wt % CQDs/BiOI sample under visible light irradiation. (d) The degradation of BPA with pure BiOI and 3 wt % CQDs/BiOI sample under visible light irradiation. F

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S7).58 This result further confirmed that the CQDs/BiOI microspheres have good electronic conductivity, which would contribute to the improvement of photocatalytic performance.59 This was ascribed to the fact that the introduction of CQDs could tune the surface microstructure of microspheres, ultrasmall BiOI nanosheets could be formed, and the tight interface contact of nanosheets could be accomplished, which favored the faster interfacial charge transfer. What’s more, the CQDs/BiOI microspheres exhibited much lower PL intensity than pure BiOI, (Figure S8) which means the CQDs/BiOI microspheres have a lower recombination possibility of photogenerated electrons and holes. The result of PL analysis was in accordance with the photocurrent and EIS analysis. To explore the degradation mechanism of pollutant, the ESR spin-trap with DMPO technique was employed to determine the reactive oxygen species generated over 3 wt % CQDs/BiOI microspheres under visible light irradiation (Figure 9). Both the

in Figure 7c,d. As the typical antibiotic agent and endocrine disrupting chemical, the presence in natural environments may cause various adverse effects on aquatic organisms even at low exposure levels. Therefore, the high efficiency removal of antibiotic and endocrine disrupting chemical was of great importance. From Figure 7c, it can be found that the TC was not degraded in the absence of catalysts, indicating that the photolysis of TC can be ignored. After visible light irradiation for 120 min, the pure BiOI could degrade TC by 50%. Compared with the pure BiOI sample, the photocatalytic degradation efficiency of TC by 3 wt % CQDs/BiOI microspheres was enhanced about 18%. With respect to the photodegradation of BPA (Figure 7d), the 3 wt % CQDs/BiOI microspheres material also exhibited the enhanced photocatalytic activity than pure BiOI, and 99% BPA was degraded by 3 wt % CQDs/BiOI microspheres after 120 min irradiation. These results of photodegradation for RhB, TC, and BPA revealed that the CQDs/BiOI microspheres were efficient photocatalysts and may become valuable photocatalytic material for the potential applications of environmental protection. 3.3. Mechanism Analysis. Electrochemistry analysis was applied to explore the separation, migration, and trapping of photogenerated charge carriers. Figure 8 displayed the transient

Figure 8. Photocurrent response of the as-prepared pure BiOI and 3 wt % CQDs/BiOI microsphere samples under visible light irradiation.

photocurrent responses for each turn-on and turn-off event in multiple 40 s on−off cycles under the irradiation of visible light. The photocurrent of the CQDs/BiOI microspheres electrode was about 1.6 times higher than that of the pure BiOI electrode. The enhanced photocurrent response of the as-prepared CQDs/BiOI microspheres indicated higher separation efficiency of the photogenerated electron−hole pairs. This can be attributed to the fact that the favorable transfer of electrons from BiOI to CQDs reduced the recombination possibility of electron−hole pairs. It was worth noting that it seems to be a charge accumulation during the turn-off condition. This may be attributed to the fact that when the illumination was performed, the CQDs could trap the photogenerated electrons to realize certain accumulation. When the light was turned off, the CQDs would release some trapped electrons. Therefore, the current was first a little rise and then decreased rapidly and seem to be a charge accumulation during dark time. Moreover, the CQDs/BiOI microspheres exhibited much lower resistance than pure BiOI, as evidenced by the reduced diameter of the semicircle in the high-frequency region in the electrochemical impedance spectroscopy (EIS) profiles (Figure

Figure 9. DMPO spin-trapping ESR spectra of 3 wt % CQDs/BiOI microsphere photocatalyst in (a) methanol and (b) water under visible light irradiation.

characteristic peaks of DMPO-superoxide radical (O2•−) and DMPO-hydroxyl radical (•OH) were not observed under visible light irradiation, confirming that the photogenerated electrons of BiOI could not be trapped by molecular oxygen to generate O2•−, and the hVB+ on the VB of BiOI cannot oxidize OH− to yield •OH under visible light irradiation.60 The band edges of the BiOI material were calculated by the empirical formulas EVB = X − Ee + 0.5Eg and ECB = EVB − Eg.57 Since the band gap energy of BiOI was about 1.68 eV, the conduction band (CB) and valence band (VB) of BiOI material were G

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by visible light, the BiOI can be easily excited with photogenerated electrons on the CB and leaving the holes on the VB. Then the photogenerated electrons transfer to the CQDs and construct the effective separation of photogenerated electron−hole pairs. The photogenerated holes on the VB of BiOI could oxidize the pollutants directly.

calculated to be 0.6 and 2.28 eV (vs normal hydrogen electrode (NHE)). Because the CB value of BiOI was more positive than E0 (O2/O2•−) (−0.046 eV vs NHE) and the VB value of BiOI was less positive than E0 (•OH/OH−) (2.38 eV vs NHE),23 the photogenerated electrons cannot reduce O2 to generate O2•− and the photogenerated holes of BiOI cannot oxidize OH− to yield •OH. Based on the above analysis, it can be inferred that the O2•− and OH− were not the main active species and the direct hole oxidation may dominate the photodegradation process. Active species trapping experiments (Figure S9) were carried out to further confirm the above results. When tertbutanol (scavenger of •OH)61 was added, the degradation of RhB was not affected. This indicated that the •OH was not the main active species. However, the photocatalytic activity was greatly inhibited when the disodium ethylenediaminetetraacetate (EDTA-2Na) and ammonium oxalate (AO) (quencher of h+) were added,62 which indicated that h+ played the key role in the degradation process. The result of active species trapping experiments was consistent with the above analysis. To understand the improvement of the photocatalytic performance of the CQDs/BiOI microspheres, the key role of CQDs was considered. First, the introduction of CQDs could tune the surface microstructure of microspheres, induce the formation of smaller BiOI nanosheets, and the more tightly interface contact of nanosheets favored the faster interfacial charge transfer of BiOI materials. Second, the CQDs strengthened the light absorption of photocatalyst and extend the light absorption to full spectrum, which was advantageous for producing more electron−hole pairs. Third, CQDs/BiOI microspheres showed larger specific surface areas than pure BiOI, which could possess more active centers and thus benefit for the photodegradation. Fourth, as the excellent electron transporter and acceptor, the CQDs could provide bulk-tosurface channels for the electrons in the CB of BiOI and thus improve the separation efficiency of the photogenerated electron−hole pairs. Based on the results of photodegradation and reactive oxygen species during the degradation process, a proposed schematic mechanism of the CQDs/BiOI microspheres was shown in Figure 10. When the CQDs/BiOI microspheres were irradiated

4. CONCLUSIONS Novel CQDs/BiOI hollow microspheres were prepared in the presence of [Bmim]I at room temperature. The CQDs/BiOI photocatalysts exhibited improved photocatalytic activity for the degradation of TC, BPA, and RhB under visible and nearinfrared light irradiation. The modification of CQDs could tune the surface microstructure of microspheres, strengthen the light absorption, increase the specific surface areas, and enhance the separation efficiency of the photogenerated electron−hole pairs, which was believed to be responsible for the improved photocatalytic activity. Such an effective method to improve the photocatalytic activity can be widely used in other photocatalytic systems.

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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.langmuir.5b04308. FT-IR spectra of CQDs/BiOI photocatalysts, SEM images of the pure BiOI, SEM images of the 5 wt % CQDs/BiOI microspheres, elemental mapping images of the 3 wt % CQDs/BiOI microspheres, Nyquist plots of pure BiOI and 3 wt % CQDs/BiOI microspheres, PL spectra of pure BiOI and 3 wt % CQDs/BiOI microspheres, and free radical trapping experiments (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.:+86-511-88791108; Fax: +86-511-88791108; E-mail address: [email protected] (J.X.). *E-mail address: [email protected] (H.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (No. 21206060, 21471069 and 21476098), China Postdoctoral Science Foundation (2013M541619), and the science and technology support program of Zhenjiang (SH2014018).

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REFERENCES

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Figure 10. Schematic drawing illustrating the mechanism of the charge separation and photodegradation process over CQDs/BiOI photocatalysts under visible light irradiation. H

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