Bi2MoO6 composites with photocatalytic H2 evolution and near infrared activity

Accepted Manuscript Title: Carbon quantum dots/Bi2 MoO6 composites with photocatalytic H2 evolution and near infrared activity Authors: Zhijie Zhang, Tingting Zheng, Jiayue Xu, Haibo Zeng, Na Zhang PII: DOI: Reference:

S1010-6030(17)30389-1 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.05.029 JPC 10656

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

22-3-2017 11-5-2017 19-5-2017

Please cite this article as: Zhijie Zhang, Tingting Zheng, Jiayue Xu, Haibo Zeng, Na Zhang, Carbon quantum dots/Bi2MoO6 composites with photocatalytic H2 evolution and near infrared activity, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.05.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Carbon quantum dots/Bi2MoO6 composites with photocatalytic H2 evolution and near infrared activity Zhijie Zhanga,*, Tingting Zhenga, Jiayue Xua, Haibo Zenga,b, Na Zhanga

a School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, P. R. China b Institute of Optoelectronics and Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China *Corresponding author. Tel.: +86-21-6087-3447; Fax: +86-21-6087-3439 Email address: [email protected] Graphical abstract

Research Highlights > CQDs/Bi2MoO6 composite was prepared via a facile one-step hydrothermal reaction. > CQDs/Bi2MoO6 showed enhanced photo-activity in the degradation of RhB. > CQDs/Bi2MoO6 exhibited H2 production activity without loading any cocatalyst. 1

> CQDs/Bi2MoO6 displayed good photocatalytic performance under near infrared light.

Abstract: Carbon quantum dots modified Bi2MoO6 composites (CQDs/Bi2MoO6) with efficient photocatalytic activity were prepared via a facile one-step hydrothermal reaction. The CQDs/ Bi2MoO6 composite exhibited enhanced photocatalytic performance in the degradation of rhodamine B (RhB) under simulated solar light irradiation. Interestingly, different from pure Bi2MoO6, which has no H2 generation ability, the CQDs/Bi2MoO6 composite displayed photocatalytic activity for H2 production without loading any noble metal cocatalyst. Significantly, due to the excellent up-converted photoluminescence (UCPL) properties of CQDs, the CQDs/Bi2MoO6 composite also showed good photocatalytic performance under near infrared light irradiation. This study inspires a thought of utilizing the full spectrum of sunlight and provides a new strategy in the design of other high-performance photocatalysts. Keywords: CQDs; Bi2MoO6; Photocatalyst; Up-converted photoluminescence; H2 production

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1. Introduction With

the

urgent

environmental

concerns

and

increasing

energy

demand,

semiconductor-based photocatalysis has attracted much attention in the past few decades [1-4]. As is known, the light harvesting ability of the photocatalyst is a main factor determining its photocatalytic activity. In order to increase the utilization rate of the solar spectrum, great efforts have been made to develop visible-light-induced photocatalysts. However, there are few reports about the exploitation of the near infrared (NIR) and infrared (IR) light, which account for 53% of the solar spectrum. Another factor limiting the photocatalytic efficiency is the high recombination rate of the photo-generated electrons and holes in the present photocatalytic system. So the question is, how to extend the photo-response range and increase the charge separation efficiency of the photocatalyst simutaniously? Fortunately, carbon quantum dots (CQDs) as a rising star in the nanocarbon family can do the work. CQDs are a new class of carbon nanomaterials with sizes below 10 nm, and have attracted much attention in recent years because of their low cost, nontoxicity, abundance and environmental friendliness [5-8]. Recently, researchers have found that CQDs exhibit excellent up-converted photoluminescence (UCPL) properties, which can transfer the near infrared (NIR) or infrared (IR) light into ultraviolet and visible light [9, 10]. This exciting feature enables the CQDs/semiconductor composite photocatalyst to

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employ the longer NIR and IR light. For example, Kang et. al found that the CQDs/Cu2O composite can harvest the (near) IR light to enhance the photocatalytic activity [11]. Besides, CQDs also have electron reservoir and electron transfer properties, which can effectively inhibite the charge recombination rate of the CQDs/semiconductor composite [12].

Considering

these

remarkable

properties

of

CQDs,

the

design

of

CQDs/semiconductor composite would be an effective strategy to construct a high-performance photocatalyst. For example, CQDs/semiconductor composites such as CQDs/BiOX (X=Br, Cl) and CQDs/Bi2WO6 with enhanced photocatalytic activity have been reported [13, 14]. Bi2MoO6, as a typical Aurivillius oxide, has recently attracted much interest due to its visible-light-induced photocatalytic performance for pollutant degradation and O2 evolution [15-19]. However, the photo-absorption onset of Bi2MoO6 is located at ca. 500 nm, which occupies a small fraction of the solar spectrum. Moreover, the charge separation efficiency of pure Bi2MoO6 is not high enough for its practical application. Therefore, with the aim of broadening the photo-absorption range and promoting the charge separation rate of Bi2MoO6 simutaniously, we intend to introduce CQDs into Bi2MoO6 and design an efficient CQDs/Bi2MoO6 heterostructure photocatalyst. In a previous report, Xia et. al prepared CQDs/Bi2MoO6 composite and investigated its photocatalytic activity for pollutants degradation under visible light [20]. In this study,

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besides the photocatalytic degradation of organic pollutant, for the first time we found that the CQDs/Bi2MoO6 exhibited photocatalytic activity for H2 production without loading any noble metal cocatalyst, in contrast to pure Bi2MoO6. Also, we have demonstrated its good photocatalytic performance under near infrared light irradiation (λ > 700 nm). The crucial roles of CQDs in the enhancement of photocatalytic activity of the CQDs/Bi2MoO6 composite were discussed in detail.

2．Experimental 2.1. Preparation of CQDs/Bi2MoO6 composites CQDs were prepared according to the reference [21]. For the synthesis of CQDs/Bi2MoO6 composites, 2 mmol of Bi(NO3)3·5H2O and 1 mmol of Na2MoO4·2H2O were dissolved in nitric acid solution (2 M) and 20 mL of deionized water, respectively. Then these two solutions were mixed together to obtain a light yellow suspension. Subsequently, desired amount of CQDs solution (20 mg/mL) was added into the above mixture and stirred continuously for several hours. The resulting suspension was transferred into a Teflon-lined stainless steel autoclave and maintained at 160 °C for 24 h. After it was cooled to room temperature naturally, the products were washed with distilled water for several times and then dried at 60 °C for 12 h. CQDs/Bi2MoO6 composites prepared by adding 1.0, 2.0, and 3.0 mL of CQDs solution were denoted as C

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1.0/Bi2MoO6, C 2.0/Bi2MoO6 and C 3.0/Bi2MoO6, respectively. 2.2. Characterization The crystal structure of the products were characterized by X-ray diffraction (XRD) using an X-ray diffractometer at 40 kV and 100 mA with Cu Kα radiation (Rigaku Co. Ltd., Tokyo, Japan). Morphologies of the as-prepared products were examined with transmission electron microscopy (TEM) by a FEI tecnaiG2F30 electron microscope. Nitrogen adsorption–desorption measurements were conducted at 77 K on a Micromeritics Tristar apparatus. Specific surface areas were determined following the Brunauer–Emmet–Teller analysis. UV–vis diffuse reflectance spectra (DRS) of the samples were measured by using a PE Lambda 900 UV–vis spectrophotometer. The photocurrent was measured on an electrochemical system (CHI 650E, China) using a standard three-electrode cell. FT-IR measurements of the samples were performed on a FT-IR spectrometer (Nicolet 6700). X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific Escalab 250 with monochromated aluminum Kα X-rays at 1486.6 eV. The photoluminescence (PL) spectra were analyzed using a Fluoro Max-4 fluorescence spectrometer with a 450 W Xenon lamp as light source at room temperature. 2.3. Photocatalytic test The decomposition of rhodamine B (RhB) was performed to evaluate the photocatalytic

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performance of the CQDs/Bi2MoO6 nanocomposite. A 500 W Xe lamp was used as the light source to simulate the solar light. Typically, 0.05 g of CQDs/Bi2MoO6 composite immersed in 50 mL of RhB (10-5 mol/L) was kept in the dark for 1 h to achieve the adsorption/desorption equilibrium. Upon illumination, the concentration change of RhB was analyzed every 10 min intervals. The UV-vis adsorption spectrum of RhB solution was collected with a UV–vis spectrophotometer (PE Lambda 900). Photocatalytic H2 evolution was performed in a gas-closed circulation system. The reactant solution was obtained by dispersing 100 mg of the photocatalyst powder into 200 mL of aqueous solution containing 30 mL of methanol as the sacrificial agent. Prior to the reaction, the reaction vessel was evacuated several times to remove residual air. The light source was a 500 W Xe lamp. The evolved H2 was analyzed by an online gas chromatography equipped with a thermal conductivity detector (TCD).

3. Results and discussion 3.1. Crystal structure of the products The XRD patterns of CQDs/Bi2MoO6 composites with different amounts of CQDs are shown in Fig. 1. All the peaks could be indexed as orthorhombic Bi2MoO6 phase (JCPDS Card no. 77-1246). No characteristic diffraction peak of carbon is observed for the CQDs/Bi2MoO6 composites, which can be ascribed to the small amounts and low

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crystallinity of CQDs in the CQDs/Bi2MoO6 composites. 3.2. TEM Observation Fig. 2 reveals the morphology and microstructure of CQDs, Bi2MoO6 and the CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6). The TEM image of CQDs shown in Fig. 2(a) indicates that the CQDs are uniform and monodisperse with a diameter of about 5 nm. Fig. 2(b) shows the TEM image of pure Bi2MoO6, revealing the particles display nanosheet morphology and are about 100–200 nm in size. As shown in Fig. 2(c), the CQDs/Bi2MoO6 composite does not show obvious change in morphology and size compared with pure Bi2MoO6, indicating that the introduction of CQDs does not have obvious influence on the morphology of Bi2MoO6. To further confirm the formation of the composite structure, the high-resolution TEM (HRTEM) of the CQDs/Bi2MoO6 composite is recorded and shown in Fig. 2(d). The HRTEM image displays the crystal lattice spacings of 0.275 nm and 0.321 nm, which correspond to the (200) lattice planes of Bi2MoO6 and (002) lattice planes of graphitic carbon [22, 23], respectively. Moreover, the BET measurements are carried out and the specific surface areas are 14.3, 15.6, 18.2 and 20.1 m2/g for pure Bi2MoO6, C 1.0/Bi2MoO6, C 2.0/Bi2MoO6 and C 3.0/Bi2MoO6, respectively. This result indicates that the introduction of CQDs can increase the specific surface areas of Bi2MoO6, which is advantageous for absorbing more active species on their surface, thus leading to the enhancement of

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photocatalytic activity. 3.3. FT-IR spectra analysis FT-IR spectra are used to characterize the molecular structures of the resulting samples, as shown in Fig. 3. Within the spectrum of pure Bi2MoO6 sample, the absorption bands at 400–900 cm−1 are ascribed to Mo-O, Bi-O stretching and Mo-O-Mo bridging stretching modes. Peaks at 1636 and 3443 cm−1 are attributed to O–H vibrations. For CQDs/Bi2MoO6 composites, besides the characteristic absorption bands of Bi2MoO6, the characteristic absorption peaks of the CQDs at 1560 cm-1 associated with the O–C–O vibration is observed, which indicates the existence of CQDs in the composite photocatalyst. 3.4. X-ray photoelectron spectroscopic (XPS) analysis XPS measurement is carried out to study the surface composition and chemical states of the CQDs/Bi2MoO6 composites. The full survey spectrum in Fig. 4(a) displays the presence of Bi, Mo, O and C in the CQDs/Bi2MoO6 composite. In Fig. 4(b), the two peaks fitted at 159.2 and 164.6 eV are assigned to the Bi 4f7/2 and Bi 4f5/2, respectively. The Mo 3d orbital centered upon 232.3 eV and 235.4 eV is clearly resolved into Mo 3d5/2 and Mo 3d3/2 contributions (Fig. 4(c)), respectively. In the O 1s spectrum (Fig. 4(d)), wide asymmetric peaks can be deconvoluted into three component peaks at 529.7, 530.2 and 532.1 eV, which correspond to Bi–O, Mo–O and

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C–OH, respectively. In Fig. 4(e), the main peak of C 1s spectrum at 284.8 eV is assigned to the C–C bond with sp2 orbital. The peak at 287.3 and 286.3 eV are attributed to the C=O and C–O bonds, respectively. Both Fig. 4(d) and 4(e) suggest the existence of carbon–oxygen bonds in the CQDs/Bi2MoO6 composite. Moreover, the quantitative chemical compositions of the CQDs/Bi2MoO6 composites are analyzed by XPS, which demonstrate that the weight percentage of CQDs in the composites are 2.58%, 4.29% and 5.76% for C 1.0/Bi2MoO6, C 2.0/Bi2MoO6 and C 3.0/Bi2MoO6, respectively. 3.5. UV−vis diffuse reflectance spectra Fig. 5 shows the UV−vis diffuse reflectance spectra (DRS) of the CQDs/Bi2MoO6 composites with different amounts of CQDs. The DRS spectrum of pure Bi2MoO6 clearly displays the band gap absorption onset located at about 500 nm. Compared with pure Bi2MoO6, a red shift of the absorption edge is observed for the CQDs/Bi2MoO6 composites. Moreover, with increasing amount of CQDs in the composites, the absorption edges of the samples red shift gradually, which indicates that the introduction of CQDs can increase the utilization rate of solar energy. 3.6. Photocatalytic activity The photo-degradation of RhB under simulated solar light irradiation is carried out in order to evaluate the photocatalytic activities of the CQDs/Bi2MoO6 composites. As

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shown in Fig. 6, all the CQDs/Bi2MoO6 composites exhibit superior photocatalytic performance to pure Bi2MoO6, with the C 2.0/Bi2MoO6 sample displaying the highest photocatalytic activity, which could degrade 97.1% of RhB after 50 min of irradiation under simulated solar light. However, further increasing the amount of CQDs leads to the decrease of the photocatalytic activity, which can be ascribed to the reason that the higher content of CQDs in the composites can compete for light absorption, and the availability of light for RhB degradation is decreased [24, 25]. It has been reported that pure Bi2MoO6 is not active for H2 evolution from an aqueous methanol solution [26], because its conduction band level is not high enough for water reduction to generate H2. However, after hybridizing with CQDs, the CQDs/Bi2MoO6 composite exhibits obvious photocatalytic activity for H2 production under simulated solar light irradiation. As shown in Fig. 7, continuous H2 evolution from water is observed in the presence of CQDs/Bi2MoO6 composite. After 6 h of simulated solar light irradiation, 29.3 μmol of H2 is generated by the CQDs/Bi2MoO6 composite, which is much higher than that of pure CQDs. As expected, no H2 evolution is detected with pure Bi2MoO6 under the same conditions. The enhanced photocatalytic activity of the CQDs/Bi2MoO6 composite for H2 production could be ascribed to sufficiently negative conduction band edge of CQDs, which provides enough reduction power for hydrogen generation, as well as the efficient charge separation rate of the CQDs/Bi2MoO6 composite (as will be

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discussed later). 3.7. Mechanism of enhanced photo-activities Based on the above results, we can see that introducing CQDs into Bi2MoO6 can enhance the photocatalytic activity of the composites. In order to investigate the role of CQDs in the composites, the photocurrent responses of pure Bi2MoO6 and CQDs/Bi2MoO6 after deposition on FTO electrodes are measured under simulated solar light irradiation. As shown in Fig. 8, the photocurrent over CQDs/Bi2MoO6 is about 2.0 times as high as that of pure Bi2MoO6, which indicates a more efficient charge separation efficiency of the CQDs/Bi2MoO6 composite. The facilitated charge separation rate is favorable to an enhanced photocatalytic activity. PL spectra can also reflect the charge transfer and recombination processes, and a higher PL intensity indicates higher recombination rate of the photo-induced charge carriers. As shown in Fig. 9, a strong emission peak centered at about 440 nm is observed for pure Bi2MoO6. While after the modification of CQDs, the emission intensity becomes much lower than that of pure Bi2MoO6, indicating that the recombination of electron−hole pairs is inhibited effectively. In order to further explore the photocatalytic mechanism, the main oxidative species in the photocatalytic process are detected through the trapping experiments of radicals, in which benzoquinone is used as superoxide radical (·O2−) scavenger, EDTA-2Na as holes

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radical scavenger, and tert-butyl alcohol (tBuOH) as hydroxyl radical scavenger [27, 28], respectively. As shown in Fig. 10, with the addition of tBuOH in the reaction system, a slight decrease of the photocatalytic activity is observed, while the addition of EDTA-2Na and benzoquinone cause a severe depression of the photocatalytic activity. Such phenomenon indicates that holes and superoxide radicals are the main active species for the CQDs/Bi2MoO6 composite, rather than hydroxyl radicals. Previous literatures indicate that CQDs possess up-converted behavior, which can absorb visible and NIR light, and then convert them into shorter wavelengths [29-31]. To verify whether the CQDs/Bi2MoO6 composite has NIR light harnessing ability, the photocatalytic degradation of RhB is conducted under NIR light (Xe lamp with a cutoff filter to remove light of λ<700 nm). Fig. 11(a) shows the absorption spectra changes of RhB with irradiation time in the presence of the CQDs/Bi2MoO6 composite. As time goes on, the absorption band at 552 nm decreases steadily, accompanied with a blue shift of the absorption band. This result indicates that CQDs/Bi2MoO6 composite can harvest NIR light to realize pollutant degradation. Fig. 11(b) shows a comparison of photocatalytic activity between pure Bi2MoO6 and CQDs/Bi2MoO6 composite under NIR light. After 10 h of irradiation, 93.6% of RhB is decomposed by the CQDs/Bi2MoO6 composite. In contrast, no degradation of RhB is observed in the presence of pure Bi2MoO6. Therefore, the introduction of CQDs can make the composite catalyst sensitive to the NIR spectrum

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of the solar light, which significantly increases the utilization rate of the sunlight. On the basis of the above experimental results, the photocatalytic process of the CQDs/ Bi2MoO6 composite under NIR light is proposed, as illustrated in Fig. 12. When the CQDs/Bi2MoO6 composite is illuminated, CQDs with up-converted PL behavior can transform NIR light into shorter wavelength light, which then excites Bi2MoO6 to generate electrons and holes. Moreover, electrons on the conduction band of Bi2MoO6 can be trapped by CQDs, which act as an electron reservoir. The special conducting network of CQDs can shuttle the electrons freely, thus the life time of photogenerated holes of Bi2MoO6 is prolonged. According to the previous report [32], the conduction band (CB) and valence band (VB) positions of Bi2MoO6 are calculated to be -0.5 and 2.05 eV, respectively. Therefore, the VB of Bi2MoO6 is less positive than Eɵ(·OH/OH−) (2.38 eV vs. NHE), which indicates that the holes on the VB of Bi2MoO6 can not oxidize OH− to yield ·OH. On the other hand, the CB potential of Bi2MoO6 is more negative than Eɵ (O2/·O2−) (−0.046 eV vs. NHE), implying that the photo-generated electrons can reduce O2 to produce ·O2−. The above analysis is consistent with the result of trapping experiments that both holes and ·O2− dominate the photodegradation process. Overrall，the up-converted PL behavior of CQDs can increase the utilization rate of the solar spectrum, while the electron reservoir property of CQDs can inhibite the recombination rate of the charge carriers, which

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lead to the enhanced photocatalytic activity of the CQDs/Bi2MoO6 composite. 4. Conclusions In summary, CQDs/Bi2MoO6 composite with efficient photocatalytic activity for pollutant degradation and H2 evolution has been successfully prepared by a facile one-step hydrothermal strategy. More significantly, it also displays good photocatalytic performance under NIR light irradiation. The excellent photocatalytic activity of the CQDs/Bi2MoO6 composite can be ascribed to the excellent up-converted PL, photoinduced electron transfer and reservoir properties, as well as the sufficiently negative conduction band edge of CQDs. This study provides a new strategy to design other high-performance CQDs-based photocatalysts, which has potential applications in catalytic and new energy fields. Acknowledgements This work was supported by the National Natural Science Foundation of China (51402194, 51572128), the Shanghai Science and Technology Committee (14YF1410700), and Shanghai Institute of Technology (BJPY2015-1).

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Figure Captions Fig. 1 XRD patterns of the CQDs/Bi2MoO6 composites with different amounts of CQDs. Fig. 2 (a) TEM image of CQDs; (b) TEM image of Bi2MoO6; (c) TEM image of CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6); (d) High resolution TEM image of the CQDs/Bi2MoO6 composite. Fig. 3 FT-IR spectra of CQDs/Bi2MoO6 composites with different amounts of CQDs. Fig. 4 XPS spectra of CQDs/Bi2MoO6 composite for (a) full survey, (b) Bi 4f, (c) Mo 3d, (d) O 1s and (e) C 1s spectra. Fig. 5 UV-vis diffuse reflectance spectra of the as-prepared samples. Fig. 6 Photocatalytic degradation of RhB by the CQDs/Bi2MoO6 composites with different amounts of CQDs under simulated solar light irradiation. Fig. 7 Hydrogen evolution by pure Bi2MoO6 and CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6) in 15% methanol aqueous solution. Fig. 8 Transient photocurrent response curves of Bi2MoO6 and CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6). Fig. 9 PL spectra of CQDs, Bi2MoO6 and CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6) excited at 343 nm. Fig. 10 Photocatalytic degradation of RhB by CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6) with different scavengers. Fig. 11 (a) The temporal evolution of the spectra during the photodegradation of RhB

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mediated by CQDs/Bi2MoO6 composite (C 2.0/Bi2MoO6) under NIR irradiation (λ>700 nm); (b) Comparison of the degradation rate of RhB by Bi2MoO6 and CQDs/Bi2MoO6 composite under NIR irradiation (λ>700 nm). Fig. 12 Schematic illustration for the photocatalytic degradation process over CQDs/Bi2MoO6 composite under NIR irradiation.

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