View Article Online View Journal
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. Gao, M. Wang, C. Ran and L. Li, Chem. Commun., 2014, DOI: 10.1039/C4CC08984G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/chemcomm
Page 1 of 3
Journal Name
ChemComm
Dynamic Article Links ► View Article Online
DOI: 10.1039/C4CC08984G
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
Facile one-pot synthesis of MoS2 quantum dots/Graphene/TiO2 composites for highly enhanced photocatalytic property† Weiyin Gao,a Minqiang Wang,a,* Chenxin Rana, Le Lia 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We reported a simple one-pot solvothermal approach to fabricate MoS2 quantuam dots (QDs)/Graphene/TiO2 (MGT) composite photocatalyst with significant improved photocatalysis property, which is caused by the increased charge separation, visible-light absorbance, specific surface area and reaction sites upon the introduction of MoS2 QDs.
50
55
15
20
25
30
35
40
45
Over the past few years, titanium dioxide (TiO2) has been chosen as a suitable and efficient photocatalyst owing to its unique characteristics, such as strong oxidizing power, reactive surface, chemically stable, nontoxic and low cost.1 However, despite these inherent and superior properties, TiO2, for photocatalysis application, mainly suffers from two disadvantages: a large band gap (3.2 eV) and low charge separation rate.2 To overcome these shortcomings of TiO2, researchers have made great efforts3-5, and all these methods can simultaneously enlarge the light absorption range and enhance the charge separation rate of the materials, which results in the improved photocatalysis performance. Recently, researchers have done a lot of works to develop materials as cocatalysts within TiO2-based photocatalyst.6-10 Specifically, as a typical two-dimensional (2D) layered transition metal sulfide, molybdenum disulfide (MoS2), with a structure composed of three stacked atom layers (S-Mo-S) held together by van der Waals forces, has attracted much attention in photocatalysis research activities as a cocatalyst due to its distinctive electronic, optical, and catalytic properties.11 Besides, MoS2 can also accept electrons and act as active sites for photocatalysis reactions.12,11(d) There are mainly two ways to prepare MoS2 for photocatalysis application, that is exfoliation from bulk MoS211(a)-(c) and synthesis from precursor of molybdate and sulfosalt11(d)-(k). It is worth noting that those works, involving of the synthesis processes, always require a high pressure and high temperature (~200°C) condition in a sealed autoclave, and the shape of the result MoS2 is strip-like layered sheet. In this communication, we demonstrate a simple one-pot solvothermal method for fabricating MoS2/Graphene/TiO2 (MGT) composite under atmospheric pressure, where Na2MoO4·2H2O and thiocarbamide precursor as well as P25 powder is reacted with a graphene oxide (GO) dispersion in mixed solvent (Dimethylacetamide(DMAc)/deionization(DI) H2O) at 150°C in a three-necked bottle (Scheme 1). Interestingly, our methods here can obtain MoS2 quantum dots instead of layered MoS2 nanosheets, and it is believed that the formation of MoS2 QDs is This journal is © The Royal Society of Chemistry [year]
60
attributed to the interactions between functional groups on GO sheets and Mo precursors in a suitable solvent environment.13. In the composite without graphene, no MoS2 QDs were observed (See Fig. S1), which demonstrate the key role of graphene during the formation of MoS2 QDs. The selective growth on GO was attributed to the interactions between functional groups on GO sheets and Mo precursors in a suitable solvent environment. 13 It is further found that the introduction of MoS2 QDs, as a cocatalyst, can efficiently increase the charge separation, visiblelight absorbance, specific surface area and reaction sites compared with pure P25, which results in an enhanced photocatalysis ability that is 4 times higher than P25. We found that holes left in TiO2 contribute more on the photodegradation reaction than transferred electrons.
Scheme 1 Scheme of the synthesis process of MGT. 65
70
75
80
85
The morphology of MGT was investigated by transmission electron microscopy (TEM) as shown in Fig. 1. It is observed in Fig. 1a that P25 nanoparticles are deposited on graphene sheets, wherein a large amount of small MoS2 QDs can be seen. Further, the high resolution TEM image in Fig. 1b displays the yellow arrows range in Fig. 1a, within which MoS2 QDs, in the size range of 2~3 nm, can be observed. While the lattice fringes of 0.315 nm and 0.345 nm are for rutile (101) and anatase (101) planes of TiO2, respectively.14 Besides, the edge of graphene can also be observed as pointed out in blue arrows in Fig. 1b. The presence of black dots on the background may be caused by either the “hole” area without P25 or exfoliation effect upon making TEM sample (See Fig. S2). Here, the presence of crystalline TiO2 makes MoS2 QDs difficult to observe clearly, so we also prepared the sample without TiO2, MoS2/graphene using the same method, and the HRTEM image of this composite in Fig. 1d clearly displays the MoS2 QDs on graphene.13 Besides, the TEM of the composite of TiO2/graphene (see Fig. S3) shows no black dots, which indicates that the black dots in MGT have a great possibility to be MoS2 QDs . The presence of MoS2 QDs is further confirmed by X-ray photoelectron spectroscopy (XPS) measurement. (See Fig. S4) [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
Published on 10 December 2014. Downloaded by Washington State University Libraries on 13/12/2014 02:21:58.
www.rsc.org/xxxxxx
ChemComm
Page 2 of 3 View Article Online
DOI: 10.1039/C4CC08984G
50
55
Fig. 1 TEM and HRTEM images of sample (a), (b)MGT-4 and (c), (d)MoS2/graphene. 60
10
15
20
25
30
35
40
45
are typical values for Mo4+ in MoS2.15 The EDS of MGT(Fig. S6) also demonstrates the presence of MoS2. Furthermore, powder Xray diffraction (XRD) of MGT (Fig. S7) matches the diffraction peaks those of the crystalline anatase phase of TiO2, and the appearance of diffraction peak around 13° is the typical structure of MoS2,14 which also confirms the presence of MoS2. The presence of graphene is failed to observe in XRD pattern since it is overlapped by (101) peak of TiO2, so we use Raman spectroscopy measurements (Fig. S8). The Raman spectrum for the P25 and MGT show several characteristic bands at 148, 399, 518, and 639 cm−1, corresponding to the Eg(1),B1g(1),A1g +B1g(2), and Eg(2) modes of TiO2, respectively.13 Clearly, two typical bands at about 1344 cm−1 (D band) and 1588 cm−1(G band) for the graphitized structures were also observed, which confirms the presence of graphene in the MGT composite.11(f) It is known that the introduction of cocatalyst into photocatalyst will change its electronic structure, which further changes the optical and surface properties.5(c) Usually, upon employing cocatalyst, like graphene or metal nanoparticles, it is expected that the absorption range of the composite will be enlarged, charge separation rate and specific surface area will be improved.2,3,11 Hence, the MGT was characterized by the photoluminescence (PL) spectroscopy, UV−vis absorption spectroscopy and Brunauer–Emmet–Teller (BET) measurement. The PL spectra (Fig. S9) of the P25 and the MGT excited at 320 nm show PL quenching effect, which suggests an efficient photoexcited electron transfer from the conduction band of TiO2 to the graphene and/or MoS2 QDs.16 The PL intensity of TiO2/graphene is weaker than P25, because the presence of graphene facilitate the charge transfer process. However, when MoS2 QDs are further introduced, the PL intensity of ternary composite MGT is even weaker than TiO2/graphene, indicating that MoS2 in the system has a positive synergistic effect, which can further improve the charge separation rate. From UV−vis spectra (Fig. S10) we can see that the absorption edge of MGT-4 underwent a red-shift of over 50 nm compared to P25, which indicates the interaction among MoS2, TiO2 and graphene. Also, the enhanced absorption in the visible-and near-IR regions due to the incorporation of MoS2 and graphene into TiO2 and would be 2 | Journal Name, [year], [vol], 00–00
65
70
75
80
85
90
95
beneficial for the efficiency of photocatalysis ability under a sunlight irradiation. From BET measurement we found that the specific surface area of MGT is as much as twice of P25, and is larger than TiO2/graphene (MGT-1) (Fig. S11, Table S2). These characteristics of MGT are expected to endow it with excellent phototcatalysis ability.
Fig. 2 (a) Photocatalytic degradation and (b) photocatalytic degradation reaction of Rh B under simulated sunlight irradiation over P25, MGT composites with different MoS2 content.
The enhanced photocatalytic property of the MGT composite was deducted from the photodegradation of Rh B. As shown in Fig. 2a, the relative photodegradation rate of MGT hybrid composites with varying MoS2 amount exhibited significant improvement compared to P25.(Experimental detail see Table S1) The amount of MoS2 has a significant influence on the photocatalytic activity of TiO2. Even with a small amount of MoS2 (MGT-2), the photodegradation rate of RhB noticeably increased. And as the MoS2 amount increases, the photodegradation rate increases first and reaches a maximum (MGT-4). A further increase in the amount of MoS2 led to a slight reduction of the activity. This is reasonable because more MoS2 QDs covering the surface of TiO2 particles will result in the light shielding effect, which reduces the UV absorption of TiO2.17 Under simulated solar light irradiation, it is observed that more than 80% of the initial pollutants were decomposed by MGT-4 after 80 min, which is even better than TiO2/graphene sample (MGT-1). From the photodegradation constant k in Fig. 4b we can see that the performance of MGT-4 is higher than P25graphene composite (MGT-1), and 4 times higher than that of bare P25. Besides, MGT-4 also exhibits excellent photochemical stability after 6 cycling photodegradation tests (Fig. S12). It is worth noting that the performance of MGT is more competitive than Ag/graphene/TiO2 shown in our previous work, 9c which is a valuable indication for further development of related composite materials with lower cost. Besides, using t-BuOH and EDTA-2Na as radical scavenger and hole scavenger, respectively, we found that photoinduced holes, rather than transferred electrons, were the main photodegradation oxidative species in the MGT photocatalytic systems. (Fig. S13) Based on the characterization and experimental data discussed above, a tentative mechanism proposed for the high photodegradation activity of the MGT-4 sample is illustrated in Fig. 3. Normally in pure P25, photogenerated electrons are quickly recombine and only a fraction of the electrons and holes participate in the photocatalytic reaction, resulting in low reactivity. However, when P25 is modified by MoS2 and graphene as cocatalyst, both UV and visible region of incident light can be utilized simultaneously. UV part of the incident light is absorbed by TiO2, these photogenerated electrons on the CB of TiO2 tend to transfer to MoS2 QDs directly or through graphene. At the same time, the MoS2 QDs, as a semiconductor, can be This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
Published on 10 December 2014. Downloaded by Washington State University Libraries on 13/12/2014 02:21:58.
5
The high-resolution XPS spectrum of C 1s of MGT shows less content of surface oxygenic functional groups, which proves the reduction of GO to graphene compared to the C 1s pattern of GO (see Fig. S5). In addition, the binding energies of the Mo 3d5/2 and Mo 3d3/2 peaks at 228.9 and 231.9 eV, respectively, which
Page 3 of 3
ChemComm View Article Online
excited by visible light, and the excited state electron in MoS2 QDs can also transfer onto graphene.11(g),18 These electrons can all take part in the photodegradation process.5 Most importantly, this charge transfer restrains the recombination of electron and
50
Published on 10 December 2014. Downloaded by Washington State University Libraries on 13/12/2014 02:21:58.
55
60
65
5
Fig. 3 Proposed mechanism for the photodegradation of RhB by MGT under simulated sunlight irradiation.
10
15
20
25
30
35
hole, which further makes much more holes can participate in the photodegradation process. So, the introduction of MoS2 QDs not only increases the charge separation rate and decreases the possibility of charge recombination, but also creates much more photochemical reaction sites as well. As a result, the MoS2 QDs, as cocatalysts, synergistically leads to an improved charge separation and efficient chemical degradation process that macroscopically enhances the photocatalysis ability of the product. In summary, we have demonstrated a simple and efficient onepot approach to prepare MoS2 QDs/Graphene/TiO2 composites using solvothermal method under atmospheric pressure and low temperature. The shape of MoS2 obtained using this method is quantum dots instead of layered sheet because of the interaction between functional groups on GO sheets and Mo precursors in a suitable solvent environment. And it shows significant increased photodegradation performance even without a noble-metal cocatalyst, which is on the account of the the increased charge separation, visible-light absorbance, specific surface area and reaction sites as the presence of MoS2 QDs. Besides, the enhancement mainly came from holes left in the TiO2 crystals rather than electrons transferring to RGO. We believe that this simple one-pot approach to fabricate noble-metal-free composites would open up many windows for further applications of the material. The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 91123019 and 61176056), the International Collaboration Program and the "13115" Innovation Engineering Project of Shaanxi Province (Grant Nos. 2013KW-12-05 and 2010ZDKG-58).
70
75
80
85
90
95
100
105
Notes and references a
40
45
Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education, School of Electronic and Information Engineering, International Centers for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China China. E-mail: [email protected] † Electronic Supplementary Information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/b000000x/ 1. (a)M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69. (b)J. Du, X. Y. Lai, N. L. Yang, J. Zhai. D.
This journal is © The Royal Society of Chemistry [year]
110
115
Kisailus, F. B. Su, D. Wang and L. Jiang, ACS Nano, 2011, 5, 590. (c)H. H. Chen. C. E. Nanayakara and V. H. Grassian, Chem. Rev., 2012, 112, 5919. 2. (a)G. Liu, Y. N. Zhao, C. H. Sun, F. Li, G. Q. Lu and H. M. Cheng, Angew. Chem. Int. Ed., 2008, 47, 4516. (b)X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503. (c)A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253. 3. (a)X. B. Chen and C. Burda, J. Am. Chem. Soc., 2008, 130, 5018. (b)T. Q. Lin, C. Y. Yang, Z. Wang, H. Yin, X. J. Lü, F. Q. Huang, J. H. Lin, X. M. Xie and M. H. Jiang, Energy Environ. Sci., 2014, 7, 967. 4. (a)L. Gu,; J. Y. Wang, H. Cheng, Y. Z. Zhao, L. F. Liu and X. J. Han, ACS Appl. Mater. Interfaces, 2013, 5, 3085. (b)S. Liu, J. Yu and M. Jaroniec, Chem. Mater., 2011, 23, 4085. 5. (a)B. F. Xu, L. Q. Jing, Z. Y. Ren, B. Q. Wang and H. G. Fu, J. Phys. Chem. B, 2005, 109, 2805. (b)M. Teranishi, S. I. Naya and H. Tada, J. Am. Chem. Soc., 2010, 132, 7850. (c)W. Gao, M. Wang, C. Ran, X. Yao, H. Yang, J. Liu, D. He and J. Bai, Nanoscale, 2014, 6, 5498. (d)H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS nano, 2010, 4, 380. 6. M. D. Hernández-alonso, F. Fresno, S. Suárez and J. M. Coronado, Energy Environ. Sci., 2009, 2, 1231. 7. (a)M. Luo, Y. Liu, J. C. Hu, H. Liu and J. L. Li, ACS Appl. Mater. Interfaces, 2012, 4, 1813.(b)Y. J. Wang, Q. S. Wang, X. Y. Zhan, F. M. Wang, M. Safdar and J. He, Nanoscale, 2013, 5, 8326. 8. (a)P. Tongying, V. V. Plashnitsa, N. Petchsang, F. Vietmeyer, G. J. Ferraudi, G. Krylova and M. Kuno, J. Phys. Chem. Lett., 2012, 3, 3234. (b)C. Harris and P. V. Kamat, ACS Nano, 2010, 4, 7321. 9. (a) X. Qiu, L. Li, J. Zheng, J. Liu, X. Sun and G. Li, J. Phys. Chem. C, 2008, 112, 12242.(b)H. B. Zeng, W. P. Cai, P. S. Liu, X. X. Xu, H. J. Zhou, C. Klingshirn and H. Kalt, ACS Nano, 2008, 2, 1661. 10. (a)H. S. Jung, Y. J. Hong, Y. R. Li, J. H. Cho, Y. J. Kim and G. C. Yi, ACS Nano, 2008, 2, 637. (b)W. Tong, L. Li, W. Hu, T. Yan and G. Li, J. Phys. Chem. C, 2010, 114, 1512. (c)F. Dong, Z. W. Zhao, T. Xiong, Z. L. Ni, W. D. Zhang, Y. J. Sun and W. K. Ho, ACS Appl. Mater. Interfaces, 2013, 5, 11392. 11. (a)D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, B. B. Shenoy, G. Eda and M. Chhowalla, Nano Lett., 2013, 13, 6222. (b)M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274. (c)T. Wang, L. Liu, Z. Zhu, P. Papakonstantinou, J. Hu, H. Liu and M. Li, Energy Environ. Sci., 2013, 6, 625. (d)T. Jia, A. Kolpin, C. Ma, R. C. Chan, W. M. Kwok and S. C. E. Tsang, Chem. Commun., 2014, 50, 1185. (e)X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L. Wang and C. Li, J. Am. Chem. Soc., 2008, 130, 7176. (f)Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575. (g)Y. Liu, Y. X. Yu and W. D. Zhang, J. Phys. Chem. C, 2013, 117, 12949. (h)K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang and J. Ye, ACS Nano, 2014, 8, 7078. (i)U. Maitra, U. Gupta, M. De, R. Datta, A. Govindaraj and C. N. R. Rao, Angew. Chem. Int. Ed., 2013, 52, 13057. (j)W. Zhou, Z. Yin, Y. Du, X. Huang, Z. Zeng, Z. Fan, H. Liu, J. Wang and H. Zhang, Small, 2013, 9, 140. (k) J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. D. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807. 12. H. Sabbah, L. Biennier, I. R. Sims, Y. Georgievskii, S. J. Klippenstein and I. W. M. Smith, Science, 2007, 317, 100. 13. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc. 2011, 133, 7296. 14. D. Gopalakrishnan, D. Damien and M. Shaijumon, ACS Nano, 2014, 8, 5297. 15. B. A. Vanchura, P. G. He, V. Antochshuk, M. Jaroniec, A. Ferryman, D. Barbash, J. E. Fulghum and S. D. Huang, J. Am. Chem. Soc., 2002, 124, 12090. 16. (a)C. Ran, M. Wang, W. Gao, J. Ding, Y. Shi, X. Song, H. Chen and Z. Ren, J. Phys. Chem. C, 2012, 116, 23053. (b) C. Ran, M. Wang, W. Gao, Z. Yang, J. Deng, J. Ding and X. Song, Phys. Chem. Chem. Phys., 2014, 16, 4561. 17. B. Xin, Z. Ren, B. Wang and H. Fu, J. Phys. Chem. B, 2005, 109, 2805. 18. M. D. J. Quinn, N. H. Ho and S. M. Notley, ACS Appl. Mater. Interfaces, 2013, 5, 12751. (b)X. Xu, J. Hu, Z. Yin and C. Xu, ACS Appl. Mater. Interfaces, 2014, 6, 5983.
Journal Name, [year], [vol], 00–00 | 3
ChemComm Accepted Manuscript
DOI: 10.1039/C4CC08984G
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. Gao, M. Wang, C. Ran and L. Li, Chem. Commun., 2014, DOI: 10.1039/C4CC08984G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/chemcomm
Page 1 of 3
Journal Name
ChemComm
Dynamic Article Links ► View Article Online
DOI: 10.1039/C4CC08984G
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
Facile one-pot synthesis of MoS2 quantum dots/Graphene/TiO2 composites for highly enhanced photocatalytic property† Weiyin Gao,a Minqiang Wang,a,* Chenxin Rana, Le Lia 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We reported a simple one-pot solvothermal approach to fabricate MoS2 quantuam dots (QDs)/Graphene/TiO2 (MGT) composite photocatalyst with significant improved photocatalysis property, which is caused by the increased charge separation, visible-light absorbance, specific surface area and reaction sites upon the introduction of MoS2 QDs.
50
55
15
20
25
30
35
40
45
Over the past few years, titanium dioxide (TiO2) has been chosen as a suitable and efficient photocatalyst owing to its unique characteristics, such as strong oxidizing power, reactive surface, chemically stable, nontoxic and low cost.1 However, despite these inherent and superior properties, TiO2, for photocatalysis application, mainly suffers from two disadvantages: a large band gap (3.2 eV) and low charge separation rate.2 To overcome these shortcomings of TiO2, researchers have made great efforts3-5, and all these methods can simultaneously enlarge the light absorption range and enhance the charge separation rate of the materials, which results in the improved photocatalysis performance. Recently, researchers have done a lot of works to develop materials as cocatalysts within TiO2-based photocatalyst.6-10 Specifically, as a typical two-dimensional (2D) layered transition metal sulfide, molybdenum disulfide (MoS2), with a structure composed of three stacked atom layers (S-Mo-S) held together by van der Waals forces, has attracted much attention in photocatalysis research activities as a cocatalyst due to its distinctive electronic, optical, and catalytic properties.11 Besides, MoS2 can also accept electrons and act as active sites for photocatalysis reactions.12,11(d) There are mainly two ways to prepare MoS2 for photocatalysis application, that is exfoliation from bulk MoS211(a)-(c) and synthesis from precursor of molybdate and sulfosalt11(d)-(k). It is worth noting that those works, involving of the synthesis processes, always require a high pressure and high temperature (~200°C) condition in a sealed autoclave, and the shape of the result MoS2 is strip-like layered sheet. In this communication, we demonstrate a simple one-pot solvothermal method for fabricating MoS2/Graphene/TiO2 (MGT) composite under atmospheric pressure, where Na2MoO4·2H2O and thiocarbamide precursor as well as P25 powder is reacted with a graphene oxide (GO) dispersion in mixed solvent (Dimethylacetamide(DMAc)/deionization(DI) H2O) at 150°C in a three-necked bottle (Scheme 1). Interestingly, our methods here can obtain MoS2 quantum dots instead of layered MoS2 nanosheets, and it is believed that the formation of MoS2 QDs is This journal is © The Royal Society of Chemistry [year]
60
attributed to the interactions between functional groups on GO sheets and Mo precursors in a suitable solvent environment.13. In the composite without graphene, no MoS2 QDs were observed (See Fig. S1), which demonstrate the key role of graphene during the formation of MoS2 QDs. The selective growth on GO was attributed to the interactions between functional groups on GO sheets and Mo precursors in a suitable solvent environment. 13 It is further found that the introduction of MoS2 QDs, as a cocatalyst, can efficiently increase the charge separation, visiblelight absorbance, specific surface area and reaction sites compared with pure P25, which results in an enhanced photocatalysis ability that is 4 times higher than P25. We found that holes left in TiO2 contribute more on the photodegradation reaction than transferred electrons.
Scheme 1 Scheme of the synthesis process of MGT. 65
70
75
80
85
The morphology of MGT was investigated by transmission electron microscopy (TEM) as shown in Fig. 1. It is observed in Fig. 1a that P25 nanoparticles are deposited on graphene sheets, wherein a large amount of small MoS2 QDs can be seen. Further, the high resolution TEM image in Fig. 1b displays the yellow arrows range in Fig. 1a, within which MoS2 QDs, in the size range of 2~3 nm, can be observed. While the lattice fringes of 0.315 nm and 0.345 nm are for rutile (101) and anatase (101) planes of TiO2, respectively.14 Besides, the edge of graphene can also be observed as pointed out in blue arrows in Fig. 1b. The presence of black dots on the background may be caused by either the “hole” area without P25 or exfoliation effect upon making TEM sample (See Fig. S2). Here, the presence of crystalline TiO2 makes MoS2 QDs difficult to observe clearly, so we also prepared the sample without TiO2, MoS2/graphene using the same method, and the HRTEM image of this composite in Fig. 1d clearly displays the MoS2 QDs on graphene.13 Besides, the TEM of the composite of TiO2/graphene (see Fig. S3) shows no black dots, which indicates that the black dots in MGT have a great possibility to be MoS2 QDs . The presence of MoS2 QDs is further confirmed by X-ray photoelectron spectroscopy (XPS) measurement. (See Fig. S4) [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
Published on 10 December 2014. Downloaded by Washington State University Libraries on 13/12/2014 02:21:58.
www.rsc.org/xxxxxx
ChemComm
Page 2 of 3 View Article Online
DOI: 10.1039/C4CC08984G
50
55
Fig. 1 TEM and HRTEM images of sample (a), (b)MGT-4 and (c), (d)MoS2/graphene. 60
10
15
20
25
30
35
40
45
are typical values for Mo4+ in MoS2.15 The EDS of MGT(Fig. S6) also demonstrates the presence of MoS2. Furthermore, powder Xray diffraction (XRD) of MGT (Fig. S7) matches the diffraction peaks those of the crystalline anatase phase of TiO2, and the appearance of diffraction peak around 13° is the typical structure of MoS2,14 which also confirms the presence of MoS2. The presence of graphene is failed to observe in XRD pattern since it is overlapped by (101) peak of TiO2, so we use Raman spectroscopy measurements (Fig. S8). The Raman spectrum for the P25 and MGT show several characteristic bands at 148, 399, 518, and 639 cm−1, corresponding to the Eg(1),B1g(1),A1g +B1g(2), and Eg(2) modes of TiO2, respectively.13 Clearly, two typical bands at about 1344 cm−1 (D band) and 1588 cm−1(G band) for the graphitized structures were also observed, which confirms the presence of graphene in the MGT composite.11(f) It is known that the introduction of cocatalyst into photocatalyst will change its electronic structure, which further changes the optical and surface properties.5(c) Usually, upon employing cocatalyst, like graphene or metal nanoparticles, it is expected that the absorption range of the composite will be enlarged, charge separation rate and specific surface area will be improved.2,3,11 Hence, the MGT was characterized by the photoluminescence (PL) spectroscopy, UV−vis absorption spectroscopy and Brunauer–Emmet–Teller (BET) measurement. The PL spectra (Fig. S9) of the P25 and the MGT excited at 320 nm show PL quenching effect, which suggests an efficient photoexcited electron transfer from the conduction band of TiO2 to the graphene and/or MoS2 QDs.16 The PL intensity of TiO2/graphene is weaker than P25, because the presence of graphene facilitate the charge transfer process. However, when MoS2 QDs are further introduced, the PL intensity of ternary composite MGT is even weaker than TiO2/graphene, indicating that MoS2 in the system has a positive synergistic effect, which can further improve the charge separation rate. From UV−vis spectra (Fig. S10) we can see that the absorption edge of MGT-4 underwent a red-shift of over 50 nm compared to P25, which indicates the interaction among MoS2, TiO2 and graphene. Also, the enhanced absorption in the visible-and near-IR regions due to the incorporation of MoS2 and graphene into TiO2 and would be 2 | Journal Name, [year], [vol], 00–00
65
70
75
80
85
90
95
beneficial for the efficiency of photocatalysis ability under a sunlight irradiation. From BET measurement we found that the specific surface area of MGT is as much as twice of P25, and is larger than TiO2/graphene (MGT-1) (Fig. S11, Table S2). These characteristics of MGT are expected to endow it with excellent phototcatalysis ability.
Fig. 2 (a) Photocatalytic degradation and (b) photocatalytic degradation reaction of Rh B under simulated sunlight irradiation over P25, MGT composites with different MoS2 content.
The enhanced photocatalytic property of the MGT composite was deducted from the photodegradation of Rh B. As shown in Fig. 2a, the relative photodegradation rate of MGT hybrid composites with varying MoS2 amount exhibited significant improvement compared to P25.(Experimental detail see Table S1) The amount of MoS2 has a significant influence on the photocatalytic activity of TiO2. Even with a small amount of MoS2 (MGT-2), the photodegradation rate of RhB noticeably increased. And as the MoS2 amount increases, the photodegradation rate increases first and reaches a maximum (MGT-4). A further increase in the amount of MoS2 led to a slight reduction of the activity. This is reasonable because more MoS2 QDs covering the surface of TiO2 particles will result in the light shielding effect, which reduces the UV absorption of TiO2.17 Under simulated solar light irradiation, it is observed that more than 80% of the initial pollutants were decomposed by MGT-4 after 80 min, which is even better than TiO2/graphene sample (MGT-1). From the photodegradation constant k in Fig. 4b we can see that the performance of MGT-4 is higher than P25graphene composite (MGT-1), and 4 times higher than that of bare P25. Besides, MGT-4 also exhibits excellent photochemical stability after 6 cycling photodegradation tests (Fig. S12). It is worth noting that the performance of MGT is more competitive than Ag/graphene/TiO2 shown in our previous work, 9c which is a valuable indication for further development of related composite materials with lower cost. Besides, using t-BuOH and EDTA-2Na as radical scavenger and hole scavenger, respectively, we found that photoinduced holes, rather than transferred electrons, were the main photodegradation oxidative species in the MGT photocatalytic systems. (Fig. S13) Based on the characterization and experimental data discussed above, a tentative mechanism proposed for the high photodegradation activity of the MGT-4 sample is illustrated in Fig. 3. Normally in pure P25, photogenerated electrons are quickly recombine and only a fraction of the electrons and holes participate in the photocatalytic reaction, resulting in low reactivity. However, when P25 is modified by MoS2 and graphene as cocatalyst, both UV and visible region of incident light can be utilized simultaneously. UV part of the incident light is absorbed by TiO2, these photogenerated electrons on the CB of TiO2 tend to transfer to MoS2 QDs directly or through graphene. At the same time, the MoS2 QDs, as a semiconductor, can be This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
Published on 10 December 2014. Downloaded by Washington State University Libraries on 13/12/2014 02:21:58.
5
The high-resolution XPS spectrum of C 1s of MGT shows less content of surface oxygenic functional groups, which proves the reduction of GO to graphene compared to the C 1s pattern of GO (see Fig. S5). In addition, the binding energies of the Mo 3d5/2 and Mo 3d3/2 peaks at 228.9 and 231.9 eV, respectively, which
Page 3 of 3
ChemComm View Article Online
excited by visible light, and the excited state electron in MoS2 QDs can also transfer onto graphene.11(g),18 These electrons can all take part in the photodegradation process.5 Most importantly, this charge transfer restrains the recombination of electron and
50
Published on 10 December 2014. Downloaded by Washington State University Libraries on 13/12/2014 02:21:58.
55
60
65
5
Fig. 3 Proposed mechanism for the photodegradation of RhB by MGT under simulated sunlight irradiation.
10
15
20
25
30
35
hole, which further makes much more holes can participate in the photodegradation process. So, the introduction of MoS2 QDs not only increases the charge separation rate and decreases the possibility of charge recombination, but also creates much more photochemical reaction sites as well. As a result, the MoS2 QDs, as cocatalysts, synergistically leads to an improved charge separation and efficient chemical degradation process that macroscopically enhances the photocatalysis ability of the product. In summary, we have demonstrated a simple and efficient onepot approach to prepare MoS2 QDs/Graphene/TiO2 composites using solvothermal method under atmospheric pressure and low temperature. The shape of MoS2 obtained using this method is quantum dots instead of layered sheet because of the interaction between functional groups on GO sheets and Mo precursors in a suitable solvent environment. And it shows significant increased photodegradation performance even without a noble-metal cocatalyst, which is on the account of the the increased charge separation, visible-light absorbance, specific surface area and reaction sites as the presence of MoS2 QDs. Besides, the enhancement mainly came from holes left in the TiO2 crystals rather than electrons transferring to RGO. We believe that this simple one-pot approach to fabricate noble-metal-free composites would open up many windows for further applications of the material. The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 91123019 and 61176056), the International Collaboration Program and the "13115" Innovation Engineering Project of Shaanxi Province (Grant Nos. 2013KW-12-05 and 2010ZDKG-58).
70
75
80
85
90
95
100
105
Notes and references a
40
45
Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education, School of Electronic and Information Engineering, International Centers for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China China. E-mail: [email protected] † Electronic Supplementary Information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/b000000x/ 1. (a)M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69. (b)J. Du, X. Y. Lai, N. L. Yang, J. Zhai. D.
This journal is © The Royal Society of Chemistry [year]
110
115
Kisailus, F. B. Su, D. Wang and L. Jiang, ACS Nano, 2011, 5, 590. (c)H. H. Chen. C. E. Nanayakara and V. H. Grassian, Chem. Rev., 2012, 112, 5919. 2. (a)G. Liu, Y. N. Zhao, C. H. Sun, F. Li, G. Q. Lu and H. M. Cheng, Angew. Chem. Int. Ed., 2008, 47, 4516. (b)X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503. (c)A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253. 3. (a)X. B. Chen and C. Burda, J. Am. Chem. Soc., 2008, 130, 5018. (b)T. Q. Lin, C. Y. Yang, Z. Wang, H. Yin, X. J. Lü, F. Q. Huang, J. H. Lin, X. M. Xie and M. H. Jiang, Energy Environ. Sci., 2014, 7, 967. 4. (a)L. Gu,; J. Y. Wang, H. Cheng, Y. Z. Zhao, L. F. Liu and X. J. Han, ACS Appl. Mater. Interfaces, 2013, 5, 3085. (b)S. Liu, J. Yu and M. Jaroniec, Chem. Mater., 2011, 23, 4085. 5. (a)B. F. Xu, L. Q. Jing, Z. Y. Ren, B. Q. Wang and H. G. Fu, J. Phys. Chem. B, 2005, 109, 2805. (b)M. Teranishi, S. I. Naya and H. Tada, J. Am. Chem. Soc., 2010, 132, 7850. (c)W. Gao, M. Wang, C. Ran, X. Yao, H. Yang, J. Liu, D. He and J. Bai, Nanoscale, 2014, 6, 5498. (d)H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS nano, 2010, 4, 380. 6. M. D. Hernández-alonso, F. Fresno, S. Suárez and J. M. Coronado, Energy Environ. Sci., 2009, 2, 1231. 7. (a)M. Luo, Y. Liu, J. C. Hu, H. Liu and J. L. Li, ACS Appl. Mater. Interfaces, 2012, 4, 1813.(b)Y. J. Wang, Q. S. Wang, X. Y. Zhan, F. M. Wang, M. Safdar and J. He, Nanoscale, 2013, 5, 8326. 8. (a)P. Tongying, V. V. Plashnitsa, N. Petchsang, F. Vietmeyer, G. J. Ferraudi, G. Krylova and M. Kuno, J. Phys. Chem. Lett., 2012, 3, 3234. (b)C. Harris and P. V. Kamat, ACS Nano, 2010, 4, 7321. 9. (a) X. Qiu, L. Li, J. Zheng, J. Liu, X. Sun and G. Li, J. Phys. Chem. C, 2008, 112, 12242.(b)H. B. Zeng, W. P. Cai, P. S. Liu, X. X. Xu, H. J. Zhou, C. Klingshirn and H. Kalt, ACS Nano, 2008, 2, 1661. 10. (a)H. S. Jung, Y. J. Hong, Y. R. Li, J. H. Cho, Y. J. Kim and G. C. Yi, ACS Nano, 2008, 2, 637. (b)W. Tong, L. Li, W. Hu, T. Yan and G. Li, J. Phys. Chem. C, 2010, 114, 1512. (c)F. Dong, Z. W. Zhao, T. Xiong, Z. L. Ni, W. D. Zhang, Y. J. Sun and W. K. Ho, ACS Appl. Mater. Interfaces, 2013, 5, 11392. 11. (a)D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, B. B. Shenoy, G. Eda and M. Chhowalla, Nano Lett., 2013, 13, 6222. (b)M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274. (c)T. Wang, L. Liu, Z. Zhu, P. Papakonstantinou, J. Hu, H. Liu and M. Li, Energy Environ. Sci., 2013, 6, 625. (d)T. Jia, A. Kolpin, C. Ma, R. C. Chan, W. M. Kwok and S. C. E. Tsang, Chem. Commun., 2014, 50, 1185. (e)X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L. Wang and C. Li, J. Am. Chem. Soc., 2008, 130, 7176. (f)Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575. (g)Y. Liu, Y. X. Yu and W. D. Zhang, J. Phys. Chem. C, 2013, 117, 12949. (h)K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang and J. Ye, ACS Nano, 2014, 8, 7078. (i)U. Maitra, U. Gupta, M. De, R. Datta, A. Govindaraj and C. N. R. Rao, Angew. Chem. Int. Ed., 2013, 52, 13057. (j)W. Zhou, Z. Yin, Y. Du, X. Huang, Z. Zeng, Z. Fan, H. Liu, J. Wang and H. Zhang, Small, 2013, 9, 140. (k) J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. D. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807. 12. H. Sabbah, L. Biennier, I. R. Sims, Y. Georgievskii, S. J. Klippenstein and I. W. M. Smith, Science, 2007, 317, 100. 13. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc. 2011, 133, 7296. 14. D. Gopalakrishnan, D. Damien and M. Shaijumon, ACS Nano, 2014, 8, 5297. 15. B. A. Vanchura, P. G. He, V. Antochshuk, M. Jaroniec, A. Ferryman, D. Barbash, J. E. Fulghum and S. D. Huang, J. Am. Chem. Soc., 2002, 124, 12090. 16. (a)C. Ran, M. Wang, W. Gao, J. Ding, Y. Shi, X. Song, H. Chen and Z. Ren, J. Phys. Chem. C, 2012, 116, 23053. (b) C. Ran, M. Wang, W. Gao, Z. Yang, J. Deng, J. Ding and X. Song, Phys. Chem. Chem. Phys., 2014, 16, 4561. 17. B. Xin, Z. Ren, B. Wang and H. Fu, J. Phys. Chem. B, 2005, 109, 2805. 18. M. D. J. Quinn, N. H. Ho and S. M. Notley, ACS Appl. Mater. Interfaces, 2013, 5, 12751. (b)X. Xu, J. Hu, Z. Yin and C. Xu, ACS Appl. Mater. Interfaces, 2014, 6, 5983.
Journal Name, [year], [vol], 00–00 | 3
ChemComm Accepted Manuscript
DOI: 10.1039/C4CC08984G
