Facile Synthesis of Carbon Dots@ 2D MoS2 Heterostructure with

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Facile Synthesis of Carbon Dots@2D MoS2 Heterostructure with Enhanced Photocatalytic Properties Ning Li,*,†,‡ Zhengtang Liu,‡ Ming Liu,† Chaorui Xue,† Qing Chang,*,† Huiqi Wang,† Ying Li,†,§ Zhenchao Song,† and Shengliang Hu*,† †

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School of Energy and Power Engineering & School of Material Science and Engineering, North University of China, Taiyuan 030051, P. R. China ‡ State Key Lab of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P. R. China § CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China S Supporting Information *

ABSTRACT: To better utilize carbon dots (CDs) as efficient photocatalysts, an excellent strategy of constructing CDs@ MoS2 heterostructure is presented. Here a facile sonication− hydrothermal method is utilized to synthesize CDs@MoS2. Such heterostructure regulates the energy level configuration, and visible light absorption and the separation and transfer of photogenerated charges are enhanced remarkably, which is propitious for the production of more photoinduced charges and improvement of the heterogeneous photocatalytic activity. Meanwhile, the photocatalytic performance of CDs@MoS2 was obviously improved in methylene blue degradation. On the basis of a series of contrast experiments, the possible mechanism of the photocatalytic reaction is proposed. Therefore, this work offers a facile route for the design of a zero-dimensional/twodimensional heterojunction for the adjustment of the energy level structure and the improvement in photocatalytic performance.

1. INTRODUCTION Carbon dots (CDs), due to their photoluminescence, photostability, biocompatibility, nontoxicity, low cost, and potential to capture, transfer, and convert light energy, have attracted a tremendous amount of attention in the fields of fluorescence imaging, sensing, and solar-to-energy conversion.1−13 For one thing, as we know, CDs typically exhibit strong optical absorption in the ultraviolet (UV) region (230−360 nm) with a tail extending into the visible range,14 yet UV light is only 5% of the whole solar spectrum, which restricts the photoabsorption and the amount of photoinduced charge.15 For another, highly efficient charge transfer is fundamental for photocatalytic solar-to-energy conversion. Nevertheless, there are plenty of recombination centers that work against the separation and transfer of the photogenerated charge.9,14 In view of these issues, many methods have been adopted to enhance the visible light absorption and charge transfer efficiency of CDs such as the creation of unique surface states, doping with heteroatoms, construction of heterostructure, etc.1,16−18 As part of these efforts, heterostructure not only can manipulate the absorption of sunlight together with the valence and conduction band positions but also possesses unique interfacial properties for the separation and transfer of photoinduced charges.4,12,19,20 For instance, Kang et al.4 fabricated the CDs-C3N4 composite. The optical properties, valence, and conduction band energies of this composite change, which is beneficial for the improvement of the light © XXXX American Chemical Society

absorbance and photocatalytic efficiency. Moreover, intimate interfacial contact in the layer-by-layer structure of CD-CdS quantum dot heterojunction films facilitates efficient photoinduced charge separation.19 By now, many CD-based heterostructures and hybrid systems such as TiO2, Ag3PO4, ZnFeO4, Fe2O3, C3N4, Bi2WO6, BiOCl, etc., have been designed.1,4,21−24 However, CDs act only as decorated accessories in these composites to heighten the photocatalytic activity of the other compound, which runs counter to the expectation of utilizing CDs adequately. Therefore, how to better utilize and design CDs in the heterostructure is still challenging. Two-dimensional (2D) transition metal dichalcogenides (TMDs) have advantages in terms of their large specific surface area, strong visible light absorption, high-edge active sites, and short migration distance of the photogenerated charge carrier, which can be used to form heterostructures (or composites) by combining with a metal or semiconductor to improve the photocatalytic activity.25−28 Consequently, 2D MoS2 does become the promotor of the CD-based hybridization. For example, Atkin et al.29 synthesized the 2D WS2/CDs hybrids by a two-step method of exfoliation followed by microwave irradiation. These hybrids have enhanced photocatalytic activity for Congo red (CR) dye. Moreover, Fang et al.30 Received: January 13, 2019

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DOI: 10.1021/acs.inorgchem.9b00111 Inorg. Chem. XXXX, XXX, XXX−XXX

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electrode was Pt foil, the reference electrode was the Ag/AgCl reference electrode, and the working electrode was the prepared sample. The working electrodes were prepared by overlaying a slurry of the sample in ethanol onto an indium tin oxide (ITO) glass electrode, and then they were dried under vacuum at 80 °C for 2 h. The electrolyte solution was 0.5 M Na2SO4. Linear potential scans were performed at a rate of 5 mV s−1 within a given potential window. All of the electrochemical measurements were carried out on the BioLogic (SP 120) electrochemical workstation. 2.4. Photocatalysis Experiments. First, a 0.01 mg/mL methylene blue (MB) solution was prepared. Then, 2 mL of the sample (CDs or CDs@MoS2 sample) was added to 25 mL of MB and ceaselessly stirred for 1 h in the dark to establish an adsorption− desorption equilibrium. The solution was irradiated for 5 min with a 300 W xenon lamp placed 30 cm away (wavelengths of 90% (Figure S5a), indicating that the CDs@MoS2 sample remains photocatalytic and stable. As shown in Figures 1−3 and Figure S5b−g, the position and intensity of the (002) preferential peak, the binding energies of Mo 3d and S 2p, and the morphology of the CDs@MoS2 sample were almost the same before and after the photocatalytic experiments. Thus, the stability of CDs@MoS2 could be further validated through the XRD, TEM, and XPS results. In addition, PL spectra of the CDs, CDs@MoS2, and 2D MoS2 were measured. As one can see, an overt emission maximum at ∼415 nm is observed when CDs are excited at 340 nm (Figure S6a). However, the intensity of the emission peak maximum (∼413 nm) decreases significantly (Figure S6b) after combination with 2D MoS2 [the PL peak of the 2D MoS2 sample is extremely low (Figure S6c)], suggesting that the intimate interaction between CDs and 2D MoS2 promotes the effective separation and transfer of the photoinduced electron−hole and inhibits the recombination. This is an indirect explanation for the increment in the photocatalytic efficiency of CDs@MoS2. To thoroughly understand the behaviors of photogenerated charges and the oxidation mechanism, the band alignment [the conduction band minimum (CBM) and the valence band maximum (VBM) levels] and electrochemical property are estimated through linear potential scans and electrochemical impedance spectroscopy (EIS).6−8,45 Panels a and b, c and d, and e and f of Figure 5 show the cathodic and anodic scan results of 2D MoS2, CDs, and CDs@MoS2, respectively. The CBM and VBM values of these three samples are approximately −0.28 and 1.31 V, −1.57 and 0.94 V, and −0.56 and 1.01 V (vs Ag/AgCl), respectively, which are

Figure 5. Cathodic and anodic scans of (a and b) 2D MoS2, (c and d) CDs, and (e and f) CDs@MoS2, respectively. (g) Schematic energy level diagram of the three prepared samples. (h) EIS Nyquist plots of CDs and CDs@MoS2.

equivalent to −0.08 and 1.51 V, −1.37 and 1.14 V, −0.36 and 1.21 V (vs NHE), respectively. The energy gaps of the three samples determined from the potential scan are 1.59, 2.51, and 1.57 eV, respectively. In succession, we plot the schematic energy level diagram of 2D MoS2, CDs, and CDs@MoS2 (Figure 5g). This plot clearly shows that the energy level configuration has been regulated and a typical type II band alignment1,30,42,46−48 is formed between 2D MoS2 and CDs, which facilitates the transfer of photoexcited holes from MoS2 to CDs and photogenerated electrons from CDs to MoS2, resulting in an effective oxidation ability under visible light irradiation. Furthermore, EIS measurements were carried out to further investigate the separation efficiency of the charge carriers. The arc radius on the EIS Nyquist plot of CDs@MoS2 is smaller than that of CDs (Figure 5h), illustrating that the former sample has the lower interfacial charge transfer resistance and subsequently causes the effective separation of photogenerated electron−hole pairs.6−8,45,49 A reasonably theoretical simulation based on density functional theory can study the nature of the material properties and demonstrate experimental results. The just reported work50 can be the auxiliary support for the experimental results of energy level configuration and charge transfer. D

DOI: 10.1021/acs.inorgchem.9b00111 Inorg. Chem. XXXX, XXX, XXX−XXX

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can see that the photocatalytic activities are restrained most seriously by the BQ, suggesting that •O2− plays the most important role in photodegradation. Meanwhile, the active species h+ and •OH are also responsible for the final breakdown of the MB. Therefore, MB molecules are degraded by •O2−, •OH, and h+.22,35,47,51−53,55 On the basis of the analysis presented above, the photocatalytic activities of CDs@MoS2 can be significantly enhanced. The modulation of the valence band-edge position (from −0.08 to −0.36 eV vs NHE) enhances the ability to produce •O2− compared with that of 2D MoS2, and the increment in the absorption, separation, and transfer of photoinduced charges boosts the effective MB degradation in contrast to that of CDs. Therefore, we can conclude that the construction of the CDs@MoS2 heterostructure is propitious for the improvement of solar energy absorption and conversion.

From the analysis presented above, the possible degradation mechanism is shown in Figure 5g. First, samples of CDs, 2D MoS2, and CD@MoS2 could be excited, and simultaneously, the holes (h+) and electrons (e−) were produced under visible light irradiation (λ > 420 nm). However, the abilities of photoinduced e−−h+ pairs are different due to the absorption capacity. The better the absorption ability, the more the photoinduced charges produce. Then, the photogenerated electrons would participate in the following reaction:51,52 O2 + e− → •O2− •

O2− + 2H 2O + 2e− → 3OH− + •OH

(1) (2)

In view of the CBM values of 2D MoS2, CDs, and CDs@ MoS2 (−0.08, −1.37, and −0.36 V vs NHE, respectively), the photogenerated electrons reacted with absorbed O2 on the surface of photocatalysts to form •O2− (O2/•O2−, −0.046 eV vs NHE24). Due to the lower reduction potential of 2D MoS2 and the worse visible absorption of CDs, the capacities of reducing dissolved O2 are both poor. For CDs@MoS2, the separation and transfer of photoinduced charges promote the generation of enough •O2− radicals. To quantify the amount of formed •O2−, the nitro blue tetrazolium (NBT) was used as a probe for •O2− on the basis of the formation of the insoluble formazan dye.53,54 As displayed in panels a and b of Figure S7, the absorption bulge at ∼650 nm refers to the characteristic peak of formazan. When the CDs@MoS2 sample was added to the NBT/NaOH mixture, the adsorption intensity of formazan increased more evidently than those of the CD sample and 2D MoS2, indicating the more beneficial effect of CDs@MoS2 in the generation of •O2−. In addition, •O2− radicals of the three kinds of samples could react with H2O and e− to generate strongly oxidative •OH (•O2−/•OH, 0.9 V vs NHE52). The presence of •OH radicals can be determined by terephthalic acid (TA). After reaction, TA transforms to 2-hydroxyterephthalic acid (TAOH), which emits fluorescence at 426 nm.51 Panels c and d of Figure S7 illustrate the fluorescence spectra of the TA solution irradiated at different times after the addition of samples of CDs, CDs@MoS2, and 2D MoS2. The fluorescence intensity of 426 nm generated from CDs@MoS2 is much stronger than those of the CDs and 2D MoS2. To detect the main active species formed in the CDs@MoS2 samples and comprehend the degradation mechanism, a series of trapping experiments were performed by utilizing different scavengers in the photodegradation process: IPA for •OH, TEOA for h+, and BQ for •O2−.32,55 As shown in Figure 6, one

4. CONCLUSION In summary, we present a facile sonication−hydrothermal method for synthesizing CDs@MoS2 heterostructure. The results show that CDs loaded onto the 2D MoS2 not only can extend and enhance visible light absorption but also can produce higher separation and transfer efficiency of photoinduced charges compared with that of pure CDs. The linear potential scans illustrate that a staggered type II band alignment is formed near the CDs 2D MoS2 interface, and we know that electrons are transferred from the conduction band minimum of CDs to that of 2D MoS2. As a consequence, the proper band-edge positions of CDs@MoS2 are conducive to the production of adequate active radicals to participate in degradation. Meanwhile, the possible mechanism of the photocatalytic reaction is proposed. Finally, we can conclude that the construction of the CDs@MoS2 heterostructure enhances the solar energy absorption and conversion on the basis of this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00111.

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XRD patterns of MoS2 powder, TEM image of CDs@ MoS2, size distribution of CDs, dark adsorption curve, degradation rates, recycling experiments, XRD, XPS, and TEM images of the CDs@MoS2 sample after photodegradation, PL spectra, and quantification of active species (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ning Li: 0000-0002-4046-4825 Huiqi Wang: 0000-0003-1232-7572 Shengliang Hu: 0000-0002-2838-4072 Notes

Figure 6. Trapping of active species during the photocatalytic degradation of MB: IPA for •OH, TEOA for h+, and BQ for •O2−.

The authors declare no competing financial interest. E

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Performance of N-Doped Carbon Quantum Dot/TiO2 Composites for Photocatalytic Hydrogen Evolution. ChemSusChem 2017, 10, 4650−4656. (14) Hu, S. Tuning Optical Properties and Photocatalytic Activities of Carbon-Based“Quantum Dots” through Their Surface Groups. Chem. Rec. 2016, 16, 219−230. (15) Shang, L.; Tong, B.; Yu, H.; Waterhouse, G. I. N.; Zhou, C.; Zhao, Y.; Tahir, M.; Wu, L. Z.; Tung, C.-H.; Zhang, T. CdS Nanoparticle-Decorated Cd Nanosheets for Efficient Visible LightDriven Photocatalytic Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1501241. (16) Wu, W.; Zhang, Q.; Wang, R.; Zhao, Y.; Li, Z.; Ning, H.; Zhao, Q.; Wiederrecht, G. P.; Qiu, J.; Wu, M. Synergies between Unsaturated Zn/Cu Doping Sites in Carbon Dots Provide New Pathways for Photocatalytic Oxidation. ACS Catal. 2018, 8, 747−753. (17) Hu, S.; Chang, Q.; Lin, K.; Yang, J. Tailoring Surface Charge Distribution of Carbon Dots through Heteroatoms for Enhanced Visible-Light Photocatalytic Activity. Carbon 2016, 105, 484−489. (18) Zhang, Q.; Xu, W.; Han, C.; Wang, X.; Wang, Y.; Li, Z.; Wu, W.; Wu, M. Graphene Structure Boosts Electron Transfer of DualMetal Doped Carbon Dots in Photooxidation. Carbon 2018, 126, 128−134. (19) Chai, N. N.; Wang, H. X.; Hu, C. X.; Wang, Q.; Zhang, H. L. Well-Controlled Layer-by-Layer Assembly of Carbon Dot/CdS Heterojunctions for Efficient Visible-Light-Driven Photocatalysis. J. Mater. Chem. A 2015, 3, 16613−16620. (20) An, X.; Wang, Y.; Lin, J.; Shen, J.; Zhang, Z.; Wang, X. Heterojunction: Important Strategy for Constructing Composite Photocatalysts. Sci. Bull. 2017, 62, 599−601. (21) Zhang, H.; Huang, H.; Ming, H.; Li, H.; Zhang, L.; Liu, Y.; Kang, Z. Carbon Quantum Dots/Ag3PO4 Complex Photocatalysts with Enhanced Photocatalytic Activity and Stability under Visible Light. J. Mater. Chem. 2012, 22, 10501−10506. (22) Huang, Y.; Liang, Y.; Rao, Y.; Zhu, D.; Cao, J. J.; Shen, Z.; Ho, W.; Lee, S. C. Environment-Friendly Carbon Quantum Dots/ ZnFe2O4 Photocatalysts: Characterization, Biocompatibility, and Mechanisms for NO Removal. Environ. Sci. Technol. 2017, 51, 2924−2933. (23) Zhang, H.; Ming, H.; Lian, S.; Huang, H.; Li, H.; Zhang, L.; Liu, Y.; Kang, Z.; Lee, S. T. Fe2O3/Carbon Quantum Dots Complex Photocatalysts and Their Enhanced Photocatalytic Activity under Visible Light. Dalton Trans 2011, 40, 10822−10825. (24) Xia, J.; Di, J.; Li, H.; Xu, H.; Li, H.; Guo, S. Ionic LiquidInduced Strategy for Carbon Quantum Dots/BiOX (X = Br, Cl) Hybrid Nanosheets with Superior Visible Light-Driven Photocatalysis. Appl. Catal., B 2016, 181, 260−269. (25) Di, J.; Xiong, J.; Li, H.; Liu, Z. Ultrathin 2D Photocatalysts: Electronic-Structure Tailoring, Hybridization, and Applications. Adv. Mater. 2018, 30, 1704548. (26) Cheng, Y.; Pang, K.; Wu, X.; Zhang, Z.; Xu, X.; Ren, J.; Huang, W.; Song, R. In Situ Hydrothermal Synthesis MoS2/Guar Gum Carbon Nanoflowers as Advanced Electrocatalysts for Electrocatalytic Hydrogen Evolution. ACS Sustainable Chem. Eng. 2018, 6, 8688− 8696. (27) Yu, H.; Xiao, P.; Wang, P.; Yu, J. Amorphous Molybdenum Sulfide as Highly Efficient Electron-Cocatalyst for Enhanced Photocatalytic H2 Evolution. Appl. Catal., B 2016, 193, 217−225. (28) Swain, G.; Sultana, S.; Moma, J.; Parida, K. Fabrication of Hierarchical Two-Dimensional MoS2 Nanoflowers Decorated upon Cubic CaIn2S4 Microflowers: Facile Approach To Construct Novel Metal-Free p-n Heterojunction Semiconductors with Superior Charge Separation Efficiency. Inorg. Chem. 2018, 57, 10059−10071. (29) Atkin, P.; Daeneke, T.; Wang, Y.; Carey, B. J.; Berean, K. J.; Clark, R. M.; Ou, J. Z.; Trinchi, A.; Cole, I. S.; Kalantar-zadeh, K. 2D WS2/Carbon Dot Hybrids with Enhanced Photocatalytic Activity. J. Mater. Chem. A 2016, 4, 13563−13571. (30) Li, Z.; Ye, R.; Feng, R.; Kang, Y.; Zhu, X.; Tour, J. M.; Fang, Z. Graphene Quantum Dots Doping of MoS2 Monolayers. Adv. Mater. 2015, 27, 5235−5240.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (U1510125, 51272301, 51502270, and 21703209) and the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201719). The authors also appreciate the financial support by the Shanxi Province Science Foundation (201601D021059), Key Research and Development (R&D) Projects of Shanxi Province (201803D121037 and 201803D421091), the Shanxi Province Science Foundation for Youths (201701D221087, 201701D221085, and 201801D221085), the Specialized Research Fund for Sanjin Scholars Program of Shanxi Province, the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, the Fund of CAS Key Laboratory of Carbon Materials (KLCMKFJJ1709), the North University of China Fund for Distinguished Young Scholars, and “333” talent project research. This work was part of the Ph.D. degree of Jin (2017) and North University of China Fund for Scientific Innovation Team.

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REFERENCES

(1) Yu, H.; Shi, R.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. Smart Utilization of Carbon Dots in Semiconductor Photocatalysis. Adv. Mater. 2016, 28, 9454−9477. (2) Sun, Y. P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; Luo, P. G.; Yang, H.; Kose, M. E.; Chen, B.; Veca, L. M.; Xie, S. Y. QuantumSized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756−7757. (3) Li, H.; He, X.; Kang, Z.; Huang, H.; Liu, Y.; Liu, J.; Lian, S.; Tsang, C. H.; Yang, X.; Lee, S. T. Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem., Int. Ed. 2010, 49, 4430−4434. (4) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway. Science 2015, 347, 970−974. (5) Lim, S. Y.; Shen, W.; Gao, Z. Carbon Quantum Dots and Their Applications. Chem. Soc. Rev. 2015, 44, 362−381. (6) Song, Z.; Chang, Q.; Trinchi, A.; Li, N.; Wang, H.; Yang, J.; Hu, S. A Simple, Scalable Approach for Combining Carbon Dots with Hexagonal Nanoplates of Nickel-based Compounds for Efficient Photocatalytic Reduction. Dalton Trans 2018, 47, 12694−12701. (7) Han, X.; Chang, Q.; Li, N.; Wang, H.; Yang, J.; Hu, S. In-situ Incorporation of Carbon Dots into Mesoporous Nickel Boride for Regulating Photocatalytic Activities. Carbon 2018, 137, 484−492. (8) Hu, S.; Yang, W.; Li, N.; Wang, H.; Yang, J.; Chang, Q. CarbonDot-Based Heterojunction for Engineering Band-Edge Position and Photocatalytic Performance. Small 2018, 14, 1803447. (9) Bao, L.; Liu, C.; Zhang, Z. L.; Pang, D. W. PhotoluminescenceTunable Carbon Nanodots: Surface-State Energy-Gap Tuning. Adv. Mater. 2015, 27, 1663−1667. (10) Wang, X.; Cao, L.; Lu, F.; Meziani, M. J.; Li, H.; Qi, G.; Zhou, B.; Harruff, B. A.; Kermarrec, F.; Sun, Y. P. Photoinduced Electron Transfers with Carbon Dots. Chem. Commun. 2009, 46, 3774−3776. (11) Huang, J.; Lu, W.; Wang, J.; Li, Q.; Tian, B.; Li, C.; Wang, Z.; Jin, L.; Hao, J. Strategy to Enhance the Luminescence of Lanthanide Ions Doped MgWO4 Nanosheets through Incorporation of Carbon Dots. Inorg. Chem. 2018, 57, 8662−8672. (12) Li, Y.; Feng, X.; Lu, Z.; Yin, H.; Liu, F.; Xiang, Q. Enhanced Photocatalytic H2-Production Activity of C-Dots Modified g-C3N4/ TiO2 Nanosheets Composites. J. Colloid Interface Sci. 2018, 513, 866− 876. (13) Shi, R.; Li, Z.; Yu, H.; Shang, L.; Zhou, C.; Waterhouse, G. I. N.; Wu, L. Z.; Zhang, T. Effect of Nitrogen Doping Level on the F

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Article

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with Excellent Photocatalytic Activity for E. Coli Disinfection and Organic Pollutant Degradation. Appl. Surf. Sci. 2018, 457, 846−855. (49) Jiang, D.; Du, X.; Zhou, L.; Li, H.; Wang, K. New Insights toward Efficient Charge-Separation Mechanism for High-Performance Photoelectrochemical Aptasensing: Enhanced Charge-Carrier Lifetime via Coupling Ultrathin MoS2 Nanoplates with Nitrogen-Doped Graphene Quantum Dots. Anal. Chem. 2017, 89, 4525−4531. (50) Li, N.; Liu, Z.; Hu, S.; Chang, Q.; Xue, C.; Wang, H. Electronic and Photocatalytic Properties of Modified MoS2/Graphene Quantum Dots Heterostructures: A Computational Study. Appl. Surf. Sci. 2019, 473, 70−76. (51) Hu, S.; Han, X.; Zhou, Y.; Xue, C.; Chang, Q.; Yang, J. Hybrid Carbon Ddot/Ni3S2 Architecture Supported on Nickel Foam for Effective Light Collection and Conversion. Chem. Eng. J. 2017, 321, 608−613. (52) Sawyer, D. T.; Valentine, J. S. How Super Is Superoxide? Acc. Chem. Res. 1981, 14, 393−400. (53) Dou, X.; Lin, Z.; Chen, H.; Zheng, Y.; Lu, C.; Lin, J. M. Production of Superoxide Anion Radicals as Evidence for Carbon Nanodots Acting as Electron Donors by the Chemiluminescence Method. Chem. Commun. 2013, 49, 5871−5873. (54) Han, C.; Li, J.; Ma, Z.; Xie, H.; Waterhouse, G. I. N.; Ye, L.; Zhang, T. Black Phosphorus Quantum Dot/g-C3N4 Composites for Enhanced CO2 Photoreduction to CO. Sci. China Mater. 2018, 61, 1159−1166. (55) Bhati, A.; Anand, S. R.; Gunture; Garg, A. K.; Khare, P.; Sonkar, S. K. Sunlight-Induced Photocatalytic Degradation of Pollutant Dye by Highly Fluorescent Red-Emitting Mg-N-Embedded Carbon Dots. ACS Sustainable Chem. Eng. 2018, 6, 9246−9256.

(31) Li, Z.; Meng, X.; Zhang, Z. Recent Development on MoS2Based Photocatalysis: A Review. J. Photochem. Photobiol., C 2018, 35, 39−55. (32) Wang, D.; Xu, Y.; Sun, F.; Zhang, Q.; Wang, P.; Wang, X. Enhanced Photocatalytic Activity of TiO2 under Sunlight by MoS2 Nanodots Modification. Appl. Surf. Sci. 2016, 377, 221−227. (33) Hu, L.; Ren, Y.; Yang, H.; Xu, Q. Fabrication of 3D Hierarchical MoS2/Polyaniline and MoS2/C Architectures for Lithium-Ion Battery Applications. ACS Appl. Mater. Interfaces 2014, 6, 14644−14652. (34) Wang, Y.; Ou, J. Z.; Balendhran, S.; Chrimes, A. F.; Mortazavi, M.; Yao, D. D.; Field, M. R.; Latham, K.; Bansal, V.; Friend, J. R.; Zhuiykov, S.; Medhekar, N. V.; Strano, M. S.; Kalantar-zadeh, K. Electrochemical Control of Photoluminescence in Two-Dimensional MoS2 Nanoflake. ACS Nano 2013, 7, 10083−10093. (35) Deng, Y.; Tang, L.; Feng, C.; Zeng, G.; Wang, J.; Lu, Y.; Liu, Y.; Yu, J.; Chen, S.; Zhou, Y. Construction of Plasmonic Ag and Nitrogen-Doped Graphene Quantum Dots Codecorated Ultrathin Graphitic Carbon Nitride Nanosheet Composites with Enhanced Photocatalytic Activity: Full-Spectrum Response Ability and Mechanism Insight. ACS Appl. Mater. Interfaces 2017, 9, 42816−42828. (36) Tan, L. K.; Liu, B.; Teng, J. H.; Guo, S.; Low, H. Y.; Loh, K. P. Atomic Layer Deposition of a MoS2 Film. Nanoscale 2014, 6, 10584− 10588. (37) Chen, Y.; Zheng, M.; Xiao, Y.; Dong, H.; Zhang, H.; Zhuang, J.; Hu, H.; Lei, B.; Liu, Y. A Self-Quenching-Resistant Carbon-Dot Powder with Tunable Solid-State Fluorescence and Construction of Dual-Fluorescence Morphologies for White Light-Emission. Adv. Mater. 2016, 28, 312−318. (38) Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11, 5111−5116. (39) Nurdiwijayanto, L.; Ma, R.; Sakai, N.; Sasaki, T. Stability and Nature of Chemically Exfoliated MoS2 in Aqueous Suspensions. Inorg. Chem. 2017, 56, 7620−7623. (40) He, J. N.; Liang, Y. Q.; Mao, J.; Zhang, X. M.; Yang, X. J.; Cui, Z. D.; Zhu, S. L.; Li, Z. Y.; Li, B. B 3D Tungsten-Doped MoS2 Nanostructure: A Low-Cost, Facile Prepared Catalyst for Hydrogen Evolution Reaction. J. Electrochem. Soc. 2016, 163, H299−H304. (41) Li, N.; Su, J.; Feng, L.; Li, D.; Liu, Z. Composition and Optical Properties of Amorphous Al-MoSexOy Thin Films and Electrical Characteristics of Al-MoSexOy Field Effect Transistors. Vacuum 2015, 121, 42−47. (42) Hu, X.; Zhao, H.; Tian, J.; Gao, J.; Li, Y.; Cui, H. Synthesis of Few-Layer MoS2 Nnanosheets-Coated TiO2 Nanosheets on Graphite Fibers for Enhanced Photocatalytic Properties. Sol. Energy Mater. Sol. Cells 2017, 172, 108−116. (43) Ouyang, Y.; Ling, C.; Chen, Q.; Wang, Z.; Shi, L.; Wang, J. Activating Inert Basal Planes of MoS2 for Hydrogen Evolution Reaction through the Formation of Different Intrinsic Defects. Chem. Mater. 2016, 28, 4390−4396. (44) Xu, S.; Li, D.; Wu, P. One-Pot, Facile, and Versatile Synthesis of Monolayer MoS2/WS2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2015, 25, 1127−1136. (45) Meng, X.; Li, Z.; Zeng, H.; Chen, J.; Zhang, Z. MoS2 Quantum Dots-Interspersed Bi2WO6 Heterostructures for Visible Light-Induced Detoxification and Disinfection. Appl. Catal., B 2017, 210, 160−172. (46) Niu, M.; Cheng, D.; Cao, D. Understanding the Mechanism of Photocatalysis Enhancements in the Graphene-like Semiconductor Sheet/TiO2 Composites. J. Phys. Chem. C 2014, 118, 5954−5960. (47) Paul, K. K.; Sreekanth, N.; Biroju, R. K.; Narayanan, T. N.; Giri, P. K. Solar Light Driven Photoelectrocatalytic Hydrogen Evolution and Dye Degradation by Metal-Free Few-Layer MoS2 Nanoflower/ TiO2(B) Nanobelts Heterostructure. Sol. Energy Mater. Sol. Cells 2018, 185, 364−374. (48) Ma, X.; Xiang, Q.; Liao, Y.; Wen, T.; Zhang, H. Visible-LightDriven CdSe Quantum Dots/Graphene/TiO2 Nanosheets Composite G

DOI: 10.1021/acs.inorgchem.9b00111 Inorg. Chem. XXXX, XXX, XXX−XXX