The synergistic role of carbon quantum dots for the improved photocatalytic performance of Bi2MoO6.

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Nanoscale View Article Online

DOI: 10.1039/C5NR01350J

The synergistic role of carbon quantum dots for the improved

Jun Di, Jiexiang Xia*, Mengxia Ji, Hongping Li, Hui Xu, Huaming Li*, Rong Chen

School of Chemistry and Chemical Engineering, Institute of Energy, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China

*Corresponding author: Tel.:+86-511-88791108; Fax: +86-511-88791108; E-mail address: [email protected]; [email protected]


Published on 21 May 2015. Downloaded by North Dakota State University on 21/05/2015 12:06:41.

photocatalytic performances of Bi2MoO6

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DOI: 10.1039/C5NR01350J

Abstract Novel carbon quantum dots (CQDs) modified Bi2MoO6 photocatalysts were prepared via a facile hydrothermal process. The CQDs modified Bi2MoO6 materials

nm was distributed on the surface of Bi2MoO6 nanosheets. The photocatalytic activity of as-prepared CQDs modified Bi2MoO6 materials was investigated sufficiently by the photodegradation of four different kinds of pollutants, such as ciprofloxacin (CIP), bisphenol A (BPA), tetracycline hydrochloride (TC), and methylene blue (MB). The improved photocatalytic activity was observed for CQDs modified Bi2MoO6 samples compared with pure Bi2MoO6 under visible light irradiation. The CQDs modified Bi2MoO6 photocatalysts with a CQDs content of 2 wt% exhibited the optimum photocatalytic activity, which was found to increase by about 5 times than that of the pure Bi2MoO6 for the photodegradation of CIP. This improvement was attributed to the crucial role of CQDs, which acted as photocenter for absorbing solar light, charge separation center for suppressing charge recombination, and catalytic center for pollutant photo-degradation. The main active species were determined to be ·OH and O2•− by ESR technique and analyzed by calculation and XPS valence spectra, and a possible photocatalytic mechanism was also proposed.

Keywords: Bi2MoO6; CQDs; Photocatalytic; Active species



were characterized by multiple techniques. The CQDs with the average size about 7

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1. Introduction In order to exploit novel photocatalyst systems working under visible light, many efforts have been devoted by scientists and many efficient semiconductors working

with O 2p orbits to form a new preferable hybridized valence band (VB), thus the band gap of bismuth-based materials can be narrowed to harvest visible light.[2] Hence, bismuth-based materials have attracted more and more attentions due to the strong visible light absorption and excellent photocatalytic activity.[3, 4] Among various bismuth-based materials, Bi2MoO6 is especially attractive owing to its fascinating physicochemical properties, appropriate bandgap (2.5eV-2.8eV), as well as their excellent chemical stability, low cost, and corrosion resistance [5]. As a typical Aurivillius oxide, Bi2MoO6 is consisted of [Bi2O2]2+ layers sandwiched between MoO42− slabs, and is one of a new class of 2D layered materials for photocatalytic utilization of solar energy. Recent studies found that the Bi2MoO6 can display photocatalytic activity for water splitting and environmental decontamination under the visible light irradiation.[6, 7] Unfortunately, their practical applications are still remain a challenge by some inherent drawbacks, including poor quantum yield, inefficient charge separation and transportation, less activity sites, and poor selectivity of desired reaction. Up to now, many different materials have been applied to couple with Bi2MoO6, so as to improve its photocatalytic performance, such as semiconductors

like

TiO2,[8,

9]

ZnTiO3,[10]

Bi2O2CO3,[11]

Ag3PO4,[12]

Bi3.64Mo0.36O6.5,[13] MoS2[14] and Zn-Al layered double hydroxide[15] or couple with the noble metal Ag[16]. In addition, the nonmetallic carbon-based materials such as graphene/RGO,[17-19] C60, [20] carbon nanofibers,[21] g-C3N4, [22] has also been coupled with Bi2MoO6 to form a heterologous hybridization. These above modifications acting on Bi2MoO6 have been proved to be effective approachs to improve the photocatalytic activity of the Bi2MoO6. However, in the heterologous hybridization system, the quantum efficiencies of the materials are still poor. It is still of great importance and urgency to find appropriate hybrid materials to further



under visible light have been developed.[1] Due to the Bi 6s in Bi(III) could hybridize

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improve the photocatalytic activity of Bi2MoO6 for practical applications. Carbon quantum dots (CQDs), as a novel class of nanocarbons, have aroused extensive attentions in its own right.[23] It is benign, abundant, inexpensive, and

luminescence emission, robust chemical inertness, ease of bioconjugation,[24] which endow it wide applications such as light-emitting devices[25], optoelectronics[26], energy and charge transfer[27], bioimaging[28] and so on. Considering the excellent electron transfer and reservoir ability,[29, 30] CQDs has been used to modify the semiconductors to enhance their photocatalytic activities, such as TiO2,[31-34] Fe2O3, [35, 36] g-C3N4,[37] Ag3PO4,[38] Cu2O[39] and Bi2WO6[40]. However, in the most reported systems, the key role of CQDs for the enhanced photocatalytic activity of semiconductor have not been studied in detail, the photocatalytic activity of CQDs-based photocatalysts is only evaluated by the organ dye, which is far from enough, and the mechanism of degradation is also not been investigated. To the best of our knowledge, there has no report regarding the CQDs modified Bi2MoO6 materials or their application in the environment treatment field. In this system, the CQDs has been applied to modify the Bi2MoO6 nanosheets via a facile hydrothermal method. The -OH and carboxyl groups on CQDs surface could act as the nucleation sites for the reaction, which favors the formation of strong bonding between the Bi2MoO6 nanosheets and CQDs.[41] The nanosheets structure of Bi2MoO6 provide more surface of Bi2MoO6 which is advantageous for the uniform distribution of CQDs. Through the photocatalytic activity evaluation by the degradation of four different kinds of pollutants (such as CIP, BPA, TC and MB) under visible light irradiation, it is demonstrated that CQDs modification is an effective approach to improve the photocatalytic performance. In this photocatalyst system, CQDs not only served as electron mediator for shuttling electrons, leading to effective separation of photogenerated carriers at the junction interface, but also as photocenter for absorbing

solar light and catalytic center for pollutant

photo-degradation. The main active species during the photo-degradation process is investigated. The relationship between the structure and the photocatalytic activities



possess many fascinating properties such as size- and wavelength-dependent

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of the CQDs modified Bi2MoO6 is also discussed in details.

2.1. CQDs modified Bi2MoO6 photocatalysts CQDs powder was synthesized according to the literature followed by the freeze-drying.[42] The CQDs modified Bi2MoO6 photocatalysts were prepared through a facile hydrothermal treatment. The detail process was carried out as following steps. 1mmol Na2MoO4·2H2O was dissolved in 20 ml deionized water which contains a certain amount of CQDs. Then 2 mmol of Bi(NO3)3·5H2O was added into the solution above mentioned and regulated the pH value of the solution to 1 by using nitric acid. After being stirred for 2 h, the resultant suspension was sealed in a 25 mL teflon-lined stainless-steel autoclave. The autoclave was heated to 140 oC and maintained for 24 h, and then cooled down to room temperature. The product was collected after centrifugation, washed with water and ethanol for three times, and then dried in a vacuum at 50 °C. Pure Bi2MoO6 and CQDs modified Bi2MoO6 samples with different mass ratio (0.5%, 1%, 2%, 4%) were synthesized using the similar route by tuning the dosage of CQDs. 2.2. Characterization The X-ray diffraction (XRD) patterns of the samples were recorded on a Shimadzu XRD-6000

X-ray

diffractometer using Cu-Ka

radiation (λ=1.54Å).

X-ray

photoelectron spectroscopy (XPS) analysis was performed on an ESCALab MKII X-ray photo-electron spectrometer using the Mg Kα radiation. The field-emission scanning electron microscopy (FE-SEM) measurements were carried out with a field-emission scanning electron microscope (JEOL JSM-7001F) equipped with an energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) micrographs were taken with a JEOL-JEM-2010 (JEOL, Japan) operated at 200 kV. UV-vis absorption spectra of the samples were obtained on a UV-vis spectrophotometer (UV-2450, Shimadzu Corporation, Japan). The UV-vis absorption spectra (in the diffuse reflectance spectra



2. Experimental

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mode) were measured in solid state, and BaSO4 powder was used as the substrate. Fourier transform infrared (FT-IR) spectra (KBr pellets) were recorded on Nicolet Model Nexus 470 FT-IR equipment. The photoelectrochemical measurements were

three-electrode cell. The electron spin resonance (ESR) signals of radicals spin-trapped were examined on a Bruker model ESR JES-FA200 spectrometer by spin-trap reagent DMPO (Sigma Chemical Co.) in water and methanol. 2.3. Photocatalytic activity measurement The photocatalytic activities were evaluated by the decomposition of ciprofloxacin (CIP), bisphenol A (BPA), tetracycline hydrochloride (TC), and methylene blue (MB) under visible light irradiation. The pH value was not adjusted when reaction was conducted. Experiments were carried out at 30 oC with a circulating water system to prevent thermal catalytic effects. Aeration was performed using an air pump to ensure a constant supply of oxygen and full mixing of the solution and the photocatalysts during the photoreactions. Visible-light irradiation was provided by a Xe lamp (300 W), and UV irradiation was eliminated using a 400 nm cut filter. The distance between the light source and the Pyrex glass was about 10 cm. 100 mg of photocatalyst was totally dispersed in an aqueous solution of CIP (100 mL, 10 mg L-1), BPA (100 mL, 10 mg L-1), TC (100 mL, 20 mg L-1) and MB (100 mL, 10 mg L-1), respectively. Before irradiation, the suspensions were magnetically stirred in the dark for 30 min to get the absorption-desorption equilibrium between the photocatalyst and model pollutants. At certain time intervals, 3 mL aliquots were sampled and centrifuged to remove the particles. The concentrations of CIP, TC, MB were analyzed by recording the absorbance at the characteristic band of 276 nm, 356 nm, and 664 nm using a UV-vis spectrophotometer (UV-2450, Shimadzu), respectively. The remnant amount of BPA was analyzed through high performance liquid chromatography (HPLC). The HPLC setup was equipped with two Varian ProStar210 pumps, an Agilent TC-C (18) column, and a Varian ProStar325 UV-Vis Detector at 230 nm. The mobile phase was chose the mixed solution, which the volume ratio of methanol to H2O was 75:25. The flow rate was 1 mL/min, and 20 µL of the sample



measured on an electrochemical system (CHI-660B, China), using a conventional

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solution was injected. 2.4. Photoelectrochemical measurements Bi2MoO6 and CQDs/Bi2MoO6 electrodes served as the working electrode. The counter

respectively. 0.1 M phosphate buffered saline (pH = 7.0) was used as electrolyte solution for the photocurrent measurement. A 500 W Xe arc lamp (CHF-XM35-500 W, Beijing Changtuo) was utilized as the light source for the PEC measurement. Electrochemical impedance spectra (EIS) were measured in a 0.1 M KCl solution containing 5 mM Fe(CN)63-/(Fe(CN)64-).

3. Results and discussion The phase structure of the prepared product is investigated by powder X-ray diffractometer. The diffraction pattern in Fig. 1 shows that the distinctive peaks at 2θ = 28.2°, 32.6°, 33.1°, 36.0°, 46.7°, 47.1°, 55.5°, 56.2°and 58.4° can be indexed to (131), (002), (060), (151), (202), (062)/(260), (331)/(133), (191) and (262) planes of orthorhombic phase Bi2MoO6 (JCPDS card no. 76-2388).[7] For the CQDs modified Bi2MoO6 samples, no characteristic peak of CQDs (about 26o) can be detected, which may attributed to the low CQDs content in the samples. The result can also be found in similar systems.[38, 43] No other peaks from possible impurities are detected, which indicating the high purity of the as-prepared samples. Fig. 2 displays the FT-IR spectra of the pure Bi2MoO6, and CQDs/Bi2MoO6 samples. The band at 448 cm-1 corresponds to the Bi-O stretching mode. The absorption bands located at 584 cm−1 and 728 cm−1 are attributed to the bending vibration and asymmetric stretching mode of MoO6, respectively. And the absorption bands at 838 cm−1 and 794 cm−1 can be assigned as the asymmetric and symmetric stretching mode of MoO6 involving vibrations of the apical oxygen atoms, respectively.[5] For the CQDs/Bi2MoO6 samples, the band at 1438 cm-1 in the spectrum is ascribed to the absorption peak of -COO-[44], and the 1650 cm-1 is attributed to the vibrational absorption band of C=O,[42] which is originate from the



and the reference electrodes were a platinum wire and a saturated Ag/AgCl electrode,

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CQDs. The result of FT-IR analysis indicates that CQDs and Bi2MoO6 have been coupled together successfully. XPS is used to study the components and surface properties of the CQDs

spectra of the Bi 4f, Mo 3d, O 1s and C 1s regions for 2wt % CQDs modified Bi2MoO6. The survey spectra (Fig. 3a) of 2wt % CQDs modified Bi2MoO6 exhibit that Bi, Mo, O and C exist on the surface of sample. In the Bi 4f spectra (Fig. 3b), the peaks centered at 159.1 and 164.4 eV are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3+ for Bi2MoO6, respectively.[12] In the high-resolution Mo 3d spectra (Fig. 3c) of CQDs modified Bi2MoO6, peaks at 232.3 and 235.5 eV are ascribed to Mo 3d5/2 and Mo 3d3/2 of Mo6+, respectively. The peak located at 530.1 eV can be ascribed to the O 1s (Fig. 3d) peak of oxygen in Bi2MoO6 crystals. It is noteworthy that the peaks of Mo 3d and O 1s in the CQDs modified Bi2MoO6 exhibit obvious shifts compared with pure Bi2MoO6. It implies that there exist interaction between the modified CQDs and Bi2MoO6. It can be seen from Fig. 5e that C peak are at 284.8 eV, 286.6 eV, and 288.4 eV, which could be assigned to the C-C bond with sp2 orbital, C-O-C bond, and C=O bond, respectively.[35] The XPS analysis indicate the coexistence of CQDs and Bi2MoO6, which is consistent with the FT-IR analysis. Fig. 4a, 4b shows the morphology of the CQDs modified Bi2MoO6 samples under a field-emission SEM. The prepared CQDs modified Bi2MoO6 sample is consisted of plenty of irregular nanosheet. The width of CQDs modified Bi2MoO6 materials is 100-200 nm in size. The microstructure of CQDs modified Bi2MoO6 material was further investigated by TEM analysis (Fig. 4c, 4d). From the Fig. 4c, it can be seen that some dark dots with the average size about 7 nm are distributed on the relatively bright nanosheets, which implying that CQDs are deposited on the surface of Bi2MoO6 nanosheets. Fig. 4d shows the arrangement of the CQDs and Bi2MoO6 via HR-TEM. It can be seen that the CQDs and Bi2MoO6 crystallites have distinct lattice spacings. The lattice spacing of 0.332 nm is corresponding to the (002) crystal plane of CQDs. The observed lattice fringe of the Bi2MoO6 crystallite is determined to be 0.316 nm, which correspond to the (131) crystal plane of Bi2MoO6



modified Bi2MoO6 sample. Fig. 3 shows XPS survey spectra and high-resolution XPS

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(JCPDS 76-2388). The TEM image of the CQDs modified Bi2MoO6 material reveals that CQDs is distributed on the surface of Bi2MoO6, coinciding with the above characterization results. The energy dispersive X-ray spectroscopy (EDS) analysis

revealing the sample is CQDs modified Bi2MoO6 material. The CQDs content is about 1.87 wt% in the 2 wt% CQDs/Bi2MoO6 materials. EDS element mapping clearly shows the elements of C, O, Bi and Mo evenly distributed in the CQDs modified Bi2MoO6 materials (Fig. S1). The optical properties of CQDs modified Bi2MoO6 samples have been studied by UV-vis diffuse reflectance spectroscopy, and the result is shown in Fig. 5. The abrupt onset of absorption at 465 nm is attributed to the electronic transition from the valence band to the conduction band of Bi2MoO6 upon excitation by light. The absorption intensity increases with the introduction of CQDs, which corresponding to the optical absorption of CQDs. The enhanced light harvesting of CQDs modified Bi2MoO6 samples may endow the formation of more electron-hole pairs.[45] BET measurements are performed to gain further insight into the specific surface area of CQDs modified Bi2MoO6 materials, and the result is shown in Fig. 6. The BET specific surface area of pure Bi2MoO6 materials is calculated to be 6.08 m2 g-1. When the CQDs is introduced to Bi2MoO6, the BET specific surface area increased gradually and calculated to be 8.68 m2 g-1, 9.54 m2 g-1, 16.57 m2 g-1, 22.39 m2 g-1 for 0.5 wt%, 1 wt%, 2 wt%, 4 wt% CQDs modified materials, respectively. The remarkably higher specific surface areas of CQDs modified Bi2MoO6 materials permit it with adequate photocatalyst-polluants contact and absorbing more active species, thus further enabled them with dramatic improved photocatalytic activity. As a broad-spectrum antibiotic agent, ciprofloxacin (CIP) has been widely used for treating bacterial infections. CIP cannot be metabolized completely, and a significant fraction is discharged as the active form in pharmaceutically. The widespread use of CIP and the lack of treatment processes leading to their ubiquity in surface waters. Continued emission of CIP into aquatic environments may accelerate antibiotic resistance within native bacterial populations in impacted environments.[46]



(Fig. 4e) shows the sample is consist of C, O, Bi, Mo elements, which further

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Therefore, it is of great important for the removal of CIP. The photocatalytic activities of the CQDs modified Bi2MoO6 photocatalysts are evaluated by the degradation of CIP in solution under the visible light irradiation, which is shown in Fig. 7. The result

in the reaction system is negligible under visible light irradiation for 120 min, as shown in Fig. 7a. As for the pure Bi2MoO6, it shows poor activity, on which about 31% of CIP is decomposed after irradiation for 120 min. After the modification of CQDs, the photocatalytic activity of the hybrid material is significantly improved for the degradation of CIP compared with the pure Bi2MoO6. The photocatalytic activities of CQDs modified Bi2MoO6 increase monotonously when the mass ratios of CQDs increase from 0.5% to 2%. However, the photocatalytic activities decrease when the mass ratios further increase to 4%. After irradiation for 120 min, 71.4%, 76.8%, 88%, and 82.6% of CIP were photodegraded by using 0.5wt% CQDs/Bi2MoO6, 1wt% CQDs/Bi2MoO6, 2wt% CQDs/Bi2MoO6, and 4wt% CQDs/Bi2MoO6 sample, respectively. The 2wt% CQDs modified Bi2MoO6 photocatalyst exhibit the highest activity. Fig. 7b shows the time-dependent absorption spectra of CIP solution in the presence of 2wt% CQDs/Bi2MoO6 sample under visible light irradiation. As displayed in Fig. 8b, the absorption at λ = 276 nm of CIP evident deceases with the increase of irradiation time and nearly disappears after 120 min. To have a better understanding of the reaction kinetics of the CIP degradation, the experimental data are fitted by a pseudo-first-order kinetic model. Fig. 7c shows the pseudo-first-order kinetics data for the photodegradation of CIP using different photocatalysts. The pseudo-first-order kinetic parameters are calculated and summarized in the Table S1. All fitting curves of the irradiation time (t) against -ln(C/C0) are nearly linear and the correlation coefficients obtained are more than 0.99. Therefore, the reaction kinetics of the CIP degradation could be described properly by pseudo-first-order kinetic model. The 2 wt% CQDs modified Bi2MoO6 sample possess the maximum rate constant of 0.0188 min-1, which was almost 5.7 times as high as that of pure Bi2MoO6. At the same time, endocrine disrupting chemical bisphenol A (BPA) is chosen as



of the direct photodegradation experiment indicate that changes of CIP concentration

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different type of model pollutant to further evaluate the photocatalytic activity of CQDs modified Bi2MoO6 sample. As it shown in Fig. 8a, only 21% of BPA is degraded over pure Bi2MoO6 under visible light irradiation for 120 min. However, the

degradation of BPA, which the degradation efficiency can reach at about 54% with the same irradiation time. Fig. 8b and 8c show the degradation process of BPA in the presence of pure Bi2MoO6 and CQDs modified Bi2MoO6 sample, respectively. For the CQDs modified Bi2MoO6 sample, the peak which directed to BPA deceases faster than that of pure Bi2MoO6. It clearly indicated that the CQDs modified Bi2MoO6 has higher photocatalytic activity than pure Bi2MoO6 material for the degradation of BPA. In order to study the selectivity of catalyst activity for CQDs modified Bi2MoO6 materials, the tetracycline hydrochloride (TC), and methylene blue (MB) are used to further evaluate the photocatalytic activity of the as-prepared photocatalysts. The CQDs modified Bi2MoO6 also exhibit higher photocatalytic performance than pure Bi2MoO6 for the degradation of TC and MB, as shown in Fig. 9. The photocatalytic degradation results of four different kinds of pollutants reveal that the CQDs modified Bi2MoO6 material is efficient visible-light-driven photocatalyst, which shows broad spectrum photodegradation activity. And the strategy of CQDs modification is efficient to improve the photocatalytic activity of semiconductor for the degradation of different pollutants. The charge separation is the most complex and key factor essentially determining the efficiency in photocatalysis.[47] To understand the photophysical behaviors of photogenerated electron-hole pairs, the time-resolved transient PL decay spectra of the pure Bi2MoO6 and 2 wt% CQDs/Bi2MoO6 materials are recorded. In general, a longer PL lifetime meant a lower recombination rate of the electron-hole pairs.[48] As shown in Fig. 10, the 2 wt% CQDs/Bi2MoO6 materials display the longer PL lifetime of charge carriers when compared to pure Bi2MoO6. This indicates that the CQDs modification favors the separation of photogenerated electron-hole pairs by effective transfer the electrons from Bi2MoO6 to CQDs and thus decreased the recombination rate.



CQDs modification can greatly improve the photocatalytic ability of Bi2MoO6 for the

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The transient photocurrent tests of the as-prepared pure Bi2MoO6 and 2 wt% CQDs/Bi2MoO6 materials are carried out, and the result is shown in Fig. 11. Both pure Bi2MoO6 and 2 wt%CQDs/Bi2MoO6 electrodes are prompt in generating transient

CQDs/Bi2MoO6 electrode exhibits a higher photocurrent than that of the pure Bi2MoO6 electrode (Fig. 11). The photocurrent of the 2 wt% CQDs/Bi2MoO6 electrodes is almost three times as higher as that of the pure Bi2MoO6 electrode. The noticeable improvement of photocurrent response reveal that the photogenerated electrons in the CB of Bi2MoO6 are transferred to CQDs (worked as electron collectors), which enhance the separation of the photogenerated charge carriers, effectively prolong the lifetime of the electron-hole pairs, and consequently may improve the photocatalytic activity.[49] To gain further insight into the electron-transport and -recombination properties of the pure Bi2MoO6 and CQDs modified Bi2MoO6 samples, the EIS analysis of different electrodes are carried out, and the result is shown in Fig. 12. Clearly, the diameter of the Nyquist circle in the high frequency region of the 2 wt% CQDs/Bi2MoO6/ITO electrode is smaller than that of the pure Bi2MoO6/ITO electrode, which implying that the 2 wt% CQDs/Bi2MoO6 material has lower electron-transfer resistance.[50, 51] The conjugatedπstructure of CQDs ensure it could acted as effective transporters to accelerate the interfacial charge transfer. Overall, both the electron-accepting and -transporting properties of CQDs in the hybrid material contribute to the suppression of charge recombination, and thus a higher of photocatalytic performance can be achieved. In order to identify the active species over CQDs modified Bi2MoO6 photocatalyst, the ESR spin-trap tests with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) are performed, and the result is shown in Fig. 13. No ESR signals are detected when the reaction is carried out in the dark. Under visible irradiation, the characteristic peaks from the DMPO-O2•− species can be observed in CQDs modified Bi2MoO6 dispersion, and the characteristic peaks of DMPO-·OH can also be obviously observed in the irradiated suspension. However, the intensity of O2•− radicals is higher



photocurrent with a reproducible response to on/off cycles, but the 2 wt%

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than that of ·OH radicals. Therefore, the O2•− radicals may play a more important role than ·OH radicals during the photodegradation process.[52] The absorption edge of Bi2MoO6 is about 465 nm, and the corresponding band

estimated according to the empirical equation EVB = X - Ee + 0.5Eg.[53] X is the electronegativity of the semiconductor, which the X value of Bi2MoO6 is 5.55 eV.[22] Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), and Eg is the band gap energy of the semiconductor. Thus, the EVB of Bi2MoO6 is estimated to be 2.39 eV. At the same time, the VB of Bi2MoO6 is further determined by using XPS valence spectra [54] and the result is shown in Fig. 14. The Bi2MoO6 shows the maximum energy edge of the VB at about 2.4 eV, which agree with the estimated value. Thus, the ECB can be acquired according to the formula ECB = EVB - Eg, which is estimated to be -0.28 eV. The VB of Bi2MoO6 (2.39 eV vs NHE) is more positive than E0(·OH/OH-) (2.38 eV vs NHE),[55] implying that the hVB+ of Bi2MoO6 can oxidize OH- to yield ·OH. At the same time, the CB (-0.28 eV) potential of Bi2MoO6 is less positive than E0(O2/O2•−) (-0.046 eV vs NHE),[56] which is able to reduce O2 to generate O2•−. However, due to the VB of Bi2MoO6 (2.39 eV vs NHE) is just slight more positive than E0(·OH/OH-) and the CB (-0.28 eV) potential of Bi2MoO6 is much less positive than E0(O2/O2•−) (-0.046 eV vs NHE), which imply that the generation of O2•− is more easier than ·OH. This above analysis is consistent with the result of ESR analysis that both the ·OH and O2•− dominate the photodegradation process and the O2•− radicals play a more important role than ·OH radicals. Based on the above results, the reaction mechanism diagram of CQDs/Bi2MoO6 photocatalysts is proposed in Scheme 1. When the Bi2MoO6 is irradiated by visible light, the electrons can be excited from the VB to the CB of Bi2MoO6 and leaving the holes on the VB. Then, the photogenerated electrons on the CB of Bi2MoO6 tend to transfer to the CQDs due to their excellent electronic conductivity, result in effective separation of photogenerated electron-hole pairs. The transferred electrons will accumulate on the CQDs and then capture the adsorbed O2 on Bi2MoO6 surface to form superoxide radical (O2•−). At the same time, the photogenerated holes in the VB



gap energy (Eg) is about 2.67 eV.[22] The valence band (VB) potentials can be

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could oxidize OH- to yield ·OH, and then participate in photocatalytic oxidation reaction. In this process, the CQDs play crucial role due to that these photogenerated electron-hole pairs will quickly recombine if without CQDs and only a fraction of

excellent adsorptive performance of CQDs could assure the adsorption of pollutant on its surface. Meanwhile, the O2•− is formed through O2 capture electron on the CQDs, and the ESR and above analysis indicate that the O2•− play more important role in the photocatalysis process, which meaning the CQDs could act as photocatalytic reaction centers. As a result, the introduction of CQDs greatly improved the photocatalytic activity.

4. Conclusions In summary, CQDs modified Bi2MoO6 photocatalysts were synthesized using a facile hydrothermal treatment process. After being modified by CQDs, the photocatalytic activities of hybrid materials on CIP, BPA, TC, and MB degradation under visible light irradiation increased greatly. The significant improvement on photocatalytic performance was attributed to the crucial role of CQDs in the CQDs modified Bi2MoO6 samples. The CQDs modification has several advantages, including enhanced light harvesting, improvement of interfacial charge transfer, suppression of charge recombination, and an increase in the number of active adsorption sites and photocatalytic reaction centers. The main reactive species were determined to be ·OH and O2•−. A possible photocatalytic mechanism is proposed based on the experimental results. This work provides useful information in design and fabricating other CQDs modified semiconductor materials.

Acknowledgements This work was financially supported by the National Nature Science Foundation of China (No. 21206060, 21476098 and 21471069), Jiangsu Province (1102118C), and



charge carriers could participate in the photocatalytic reaction. It is interesting that the

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DOI: 10.1039/C5NR01350J

the Special Financial Grant from the China Postdoctoral Science Foundation (2013T60506).

[1] A. Kubacka, M. Fernandez-García and G. Colon, Chem. Rev., 2012, 112, 1555. [2] H. F. Cheng, B. B. Huang and Y. Dai, Nanoscale, 2014, 6, 2009. [3] N. Zhang, R. Ciriminna, M. Pagliaro and Y. J. Xu, Chem. Soc. Rev., 2014, 43, 5276. [4] J. Li, Y. Yu and L. Z. Zhang, Nanoscale, 2014, 6, 8473. [5] L. W. Zhang, T. G. Xu, X Zhao and Y. F. Zhu, Appl. Catal. B, 2010, 98, 138. [6] M. Shang, W. Z. Wang, J. Ren, S. M. Sun and L. Zhang, Nanoscale, 2011, 3, 1474. [7] G. H. Tian, Y. J. Chen, W. Zhou, K. Pan, Y. Z. Dong, C. G. Tian and H. G. Fu, J. Mater. Chem., 2011, 21, 887. [8] G. H. Tian, Y. J. Chen, R. T. Zhai, J. Zhou, W. Zhou, R. H. Wang, K. Pan, C. G. Tian and H. G. Fu, J. Mater. Chem. A, 2013, 1, 6961. [9] M. Y. Zhang, C. L. Shao, J. B. Mu, Z. Y. Zhang, Z. C. Guo, P. Zhang and Y. C. Liu, CrystEngComm, 2012, 14, 605. [10] P. Zhang, C. L. Shao, M. Y. Zhang, Z. C. Guo, J. B. Mu, Z. Y. Zhang, X. Zhang and Y. C. Liu, J. Hazard. Mater., 2012, 217, 422. [11] Y. S. Xu and W. D. Zhang, Appl. Catal. B, 2013, 140, 306. [12] Y. S. Xu and W. D. Zhang, Dalton Trans., 2013, 42, 1094. [13] J. Ren, W. Z. Wang, M. Shang, S. M. Sun and E. P. Gao, ACS Appl. Mater. Interfaces, 2011, 3, 2529. [14] Y. J. Chen, G. H. Tian, Y. H. Shi, Y. T. Xiao and H. G. Fu, Appl. Catal. B, 2015, 164, 40. [15] H. P. Li, Q. H. Deng, J. Y. Liu, W. G. Hou, N. Du, R, J. Zhang and X. T. Tao, Catal. Sci. Technol., 2014, 4, 1028. [16] B. Yuan, C. H. Wang, Y. Qi, X. L. Song, K. Mu, P. Guo, L. T. Meng and H. M. Xi, Colloids and Surfaces A: Physicochem. Eng. Aspects, 2013, 425, 99.



References

Nanoscale


DOI: 10.1039/C5NR01350J

[17] Y. Zhang, Y. K. Zhu, J. Q. Yu, D. J. Yang, T. W. Ng, P. K. Wong and J. C. Yu, Nanoscale, 2013, 5, 6307. [18] G. H. Tian, Y. J. Chen, J. Zhou, C. G. Tian, R. Li, C. J. Wang and H. G. Fu,

[19] P. F. Wang, Y. H. Ao, C. Wang, J. Hou and J. Qian, Carbon, 2012, 50, 5256. [20] X. Zhao, H. J. Liu, Y. L. Shen and J. H. Qu, Appl. Catal. B, 2011, 106, 63. [21] M. Y. Zhang, C. L. Shao, J. B. Mu, X. M. Huang, Z. Y. Zhang, Z. C. Guo, P. Zhang and Y. C. Liu, J. Mater. Chem., 2012, 22, 577. [22] H. P. Li, J. Y. Liu, W. G. Hou, N. Du, R. J. Zhang and X. T. Tao, Appl. Catal. B, 2014, 160, 89. [23]S. N. Baker and G. A. Baker, Angew. Chem. Int. Ed., 2010, 49, 6726. [24] S. Y. Lim, W. Shen and Z. Q. Gao, Chem. Soc. Rev., 2015, 44, 362. [25] D. I. Son, B. W. Kwon, D. H. Park, W. S. Seo, Y. Yi, B. Angadi, C. L. Lee and W. K. Choi, Nature Nanotechnol., 2012, 7, 465. [26] Y. Li, Y. Hu, Y. Zhao, G. Q. Shi, L. E. Deng, Y. B. Hou and L. T. Qu, Adv. Mater., 2011, 23, 776. [27] Y. R. Zhu, X. B. Ji, C. C. Pan, Q. Q. Sun, W. X. Song, L. B. Fang, Q. Y. Chen and C. E. Banks, Energy Environ. Sci., 2013, 6, 3665. [28] S. J. Zhu, J. H. Zhang, C. Y. Qiao, S. J. Tang, Y. F. Li, W. J. Yuan, B. Li, L. Tian, F. Liu, R. Hu, H. N. Gao, H. T. Wei, H. Zhang, H. C. Sun and B. Yang, Chem. Commun., 2011, 47, 6858. [29] Z. Ma, Y. L. Zhang, L. Wang, H. Ming, H. T. Li, X. Zhang, F. Wang, Y. Liu, Z. H. Kang and S. T. Lee, ACS Appl. Mater. Interfaces, 2013, 5, 5080. [30] H. T. Li, Z. H. Kang, Y. Liu and S. T. Lee, J. Mater. Chem., 2012, 22, 24230. [31] X. Zhang, F. Wang, H. Huang, H. T. Li, X. Han, Y. Liu and Z. H. Kang, Nanoscale, 2013, 5, 2274. [32] H. J. Yu, Y. F. Zhao, C. Zhou, L. Shang, Y. Peng, Y. H. Cao, L. Z. Wu, C. H. Tung and T. R. Zhang, J. Mater. Chem. A, 2014, 2, 3344. [33] J. Tian, Y. H. Leng, Z. H. Zhao, Y. Xia, Y. H. Sang, P. Hao, J. Zhan, M. C. Li and H. Liu, Nano Energy, 2015, 11, 419.



CrystEngComm, 2014, 16, 842.

Page 17 of 18


DOI: 10.1039/C5NR01350J

[34] H. T. Li, X. D. He, Z. H. Kang, H. Huang, Y. Liu, J. L. Liu, S. Y. Lian, C. H. A. Tsang, X. B. Yang and S. T. Lee, Angew. Chem. Int. Ed., 2010, 49, 4430. [35] B.Y. Yu and S. Y. Kwak, J. Mater. Chem., 2012, 22, 8345.

Kang and S. T. Lee, Dalton Trans., 2011, 40, 10822. [37] J. Liu, Y. Liu, N. Y. Liu, Y. Z. Han, X. Zhang, H. Huang, Y. Lifshitz, S. T. Lee, J. Zhong and Z. H. Kang, Science, 2015, 347, 970. [38] H. C. Zhang, H. Huang, H. Ming, H. T. Li, L. L. Zhang, Y. Liu and Z. H. Kang, J. Mater. Chem., 2012, 22, 10501. [39] H. T. Li, R. H. Liu, Y. Liu, H. Huang, H. Yu, H. Ming, S. Y. Lian, S. T. Lee and Z. H. Kang, J. Mater. Chem., 2012, 22, 17470. [40] J. Di, J. X. Xia, Y. P. Ge, H. P. Li, H. Y. Ji, H. Xu, Q. Zhang, H. M. Li and M. N. Li, Appl. Catal. B, 2015, 168-169, 51. [41] X. Huang, C. L. Tan, Z. Y. Yin and H. Zhang, Adv. Mater., 2014, 26, 2185. [42] S. J. Zhu, Q. N. Meng, L. Wang, J. H. Zhang, Y. B. Song, H. Jin, K. Zhang, H. C. Sun, H. Y. Wang and B. Yang, Angew. Chem. Int. Ed., 2013, 52, 3953. [43] D. Tang, H. C. Zhang, H. Huang, R. H. Liu, Y. Z. Han, Y. Liu, C. Y. Tong and Z. H. Kang, Dalton Trans., 2013, 42, 6285. [44] J. Z. Li, N. Y. Wang, T. T. Tran, C. A. Huang, L. Chen, L. J. Yuan, L. P. Zhou, R. Shen and Q. Y. Cai, Analyst, 2013, 138, 2038. [45] J. Di, J. X. Xia, S. Yin, H. Xu, M. Q. He, H. M. Li, L. Xu and Y. P. Jiang, RSC Adv., 2013, 3, 19624. [46] T. Paul, P. L. Miller and T. J. Strathmann, Environ. Sci. Technol., 2007, 41, 4720. [47] J. Di, J. X. Xia, S. Yin, H. Xu, L. Xu, Y. G. Xu, M. Q. He and H. M. Li, J. Mater. Chem. A, 2014, 2, 5340. [48] P. Niu, L. L. Zhang, G. Liu and H. M. Cheng, Adv. Funct. Mater., 2012, 22, 4763. [49] J. Di, J. X. Xia, S. Yin, H. Xu, L. Xu, Y. G. Xu, M. Q. He and H. M. Li, RSC Adv., 2014, 4, 14281. [50] J. X. Xia, J. Di, S. Yin, H. Xu, J. Zhang, Y. G. Xu, L. Xu, H. M. Li and M. X. Ji, RSC Adv., 2014, 4, 82.



[36] H. C. Zhang, H. Ming, S. Y. Lian, H. Huang, H. T. Li, L .L. Zhang, Y. Liu, Z. H.

Nanoscale


DOI: 10.1039/C5NR01350J

[51] S. W. Zhang, J. X. Li, M. Y. Zeng, G. X. Zhao, J. Z. Xu, W. P. Hu and X. K. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 12735. [52] Y. H. Lv, Y. Y. Zhu and Y. F. Zhu, J. Phys. Chem. C, 2013, 117, 18520.

Trans., 2014, 43, 15429. [54] Z. Y. Zhao, Y. Zhou, F. Wang, K. H. Zhang, S. Yu and K. Cao, ACS Appl. Mater. Interfaces, 2015, 7, 730. [55] J. Di, J. X. Xia, Y. P. Ge, L. Xu, H. Xu, M. Q. He, Q. Zhang and H. M. Li, J. Mater. Chem. A, 2014, 2, 15864. [56] L. Q. Ye, J. N. Chen, L. H. Tian, J. Y. Liu, T. Y. Peng, K. J. Deng and L. Zan, Appl. Catal. B, 2013, 130, 1.



[53] J. Di, J. X. Xia, Y. P. Ge, L. Xu, H. Xu, J. Chen, M. Q. He and H. M. Li, Dalton

Improving photocatalytic performance of ZnO via synergistic effects of Ag nanoparticles and graphene quantum dots.

Carbon quantum dots and applications in photocatalytic energy conversion.

Hydrothermal growth of TiO2 nanowire membranes sensitized with CdS quantum dots for the enhancement of photocatalytic performance.

Photocatalytic carbon disulfide production via charge transfer quenching of quantum dots.

Size-dependent penetration of carbon dots inside the ferritin nanocages: evidence for the quantum confinement effect in carbon dots.

CuAlO2 films composites.

Three-dimensional multichannel aerogel of carbon quantum dots for high-performance supercapacitors.

Fluorescent carbon nanomaterials: "quantum dots" or nanoclusters?

Carbon quantum dots and their applications.

Carbon Quantum Dots Induced Ultrasmall BiOI Nanosheets with Assembled Hollow Structures for Broad Spectrum Photocatalytic Activity and Mechanism Insight.

Nitrogen-Doped Carbon Dots for "green" Quantum Dot Solar Cells.

Graphene quantum dots, graphene oxide, carbon quantum dots and graphite nanocrystals in coals.

Facile one-pot synthesis of MoS2 quantum dots-graphene-TiO2 composites for highly enhanced photocatalytic properties.

Ratiometric fluorescent paper sensor utilizing hybrid carbon dots-quantum dots for the visual determination of copper ions.

Solvent Engineering for High-Performance PbS Quantum Dots Solar Cells.

Novel Cu₂O quantum dots coupled flower-like BiOBr for enhanced photocatalytic degradation of organic contaminant.

A carbon dots-CdTe quantum dots fluorescence resonance energy transfer system for the analysis of ultra-trace chlortoluron in water.

Visible-Light-Activated Bactericidal Functions of Carbon "Quantum" Dots.

Pt and 2-phenyl-4-(1-naphthyl)quinolinium ion.

Thermal treatment of hair for the synthesis of sustainable carbon quantum dots and the applications for sensing Hg2.

Assembling carbon quantum dots to a layered carbon for high-density supercapacitor electrodes.

Probing the Cytotoxicity Of Semiconductor Quantum Dots.

Enhanced Photocatalytic Activity of the Carbon Quantum Dot-Modified BiOI Microsphere.

Designing quantum dots for solotronics.