- Email: [email protected]
WS2 nanodots-modified TiO2 nanotubes to enhance visible-light photocatalytic activity
Accepted Manuscript WS2 nanodots-modified TiO2 nanotubes to enhance visible-light photocatalytic activity Yongchuan Wu, Zhongmin Liu, Yaru Li, Jitao Chen, Xixi Zhu, Ping Na PII: DOI: Reference:
S0167-577X(18)32015-9 https://doi.org/10.1016/j.matlet.2018.12.056 MLBLUE 25457
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
18 September 2018 17 November 2018 12 December 2018
Please cite this article as: Y. Wu, Z. Liu, Y. Li, J. Chen, X. Zhu, P. Na, WS2 nanodots-modified TiO2 nanotubes to enhance visible-light photocatalytic activity, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet. 2018.12.056
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.
WS2 nanodots-modified TiO2 nanotubes to enhance visible-light photocatalytic activity Yongchuan Wu,a Zhongmin Liu,a Yaru Li,a Jitao Chena, Xixi Zhu b and Ping Na, a,* a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China
b
College of Chemistry and Environmental Engineering, Shandong University of Science and
Technology, Qingdao 266590, China *
Corresponding author: Ping Na
E-mail: [email protected]
Abstract WS2 nanodots-modified TiO2 nanotubes (TNT/WND) composites were fabricated via one-pot hydrothermal method for the first time. The physical and photophysical properties of the as-prepared photocatalysts were characterized by XRD, TEM, HRTEM, BET, UV-vis, and PL. The results demonstrated that WS2 nanodots have been successfully anchored on the inner wall of TiO2 nanotubes. TNT/WND
composites
showed
excellent
photocatalytic
activity
toward
photodegradation of rhodamine B (RhB) under visible-light, which mainly attributed to the synergistic effect from relative low recombination rate of the photogenerated electron-hole pairs and high specific surface area.
Key words: Semiconductors; Nanocomposites; Photocatalysis; WS2 nanodots; TiO2 nanotubes; Structural;
1 Introduction Since Fujishima and Honda reported the TiO2 electrodes as photocatalyst for water splitting, TiO2 has attracted considerable attention due to its excellent photocatalytic properties, chemical stability, nontoxicity and low-cost.[1,2] To date, various TiO2 nanostructures with different crystalline phase have been well studied, and special attention is paid to one-dimensional anatase TiO2 nanotubes (TNT) with high specific surface area, shorter photogenerated carrier diffusion lengths and excellent charge transport merits, enduing TNT as ideal candidate for photodegrading organic pollutant.[3] However, the wide band gap of TiO2 (3.2eV) make it hardly activated by visible light, which lead to inefficient utilization of sunlight.[4] Furthermore, the low quantum efficiency of bare TiO2 can’t satisfy the need of actual 1
application.[5] Therefore, it is highly desirable to overcome these inherent shortcomings of TiO2 to enhance its photocatalytic activity. Recently, transition metal dichalcogenide (TMD) nanodots, such as MoS2 and WS2 nanodots, have been extensively investigated as co-catalyst.[6,7] Wang et.al. [8] found that MoS2 nanodots modified TiO2 (P25) is able to photodegrade RhB under visible light. Lai [9] prepared MoS2 quantum dots@TiO2 nanotube arrays composites via electrodeposition method as a visible-driven photocatalyst for hydrogen evolution. These works confirm that MoS2 nanodots are an excellent co-catalyst which enhance the photocatalytic activity of TiO2. However, MoS2 nanodots directly loaded on the surface of TiO2 leads to poor stability. WS2 owns similar property as MoS but little work done related its co-catalyst property. Here, we presented a one-pot hydrothermal method to synthesize WS2 nanodots-modified TiO2 nanotubes (TNT/WND) composites with the sodium tungstate and L-cysteine as WS2 precursor source and TiO2 nanotubes as substrate. Due to high length-diameter ratio and mesoporous structure, TiO2 nanotubes acted as microreactor and impelled the WS2 to grow into nanodots instead of nanosheets. The as-prepared TNT/WND composites exhibited excellent photocatalytic activity for the degradation of RhB under visible-light irradiation.
2. Experiment TiO2 nanotubes were prepared by a facile hydrothermal process based on Ma, F’s work.[10] For TNT/WND composites. Typically, 1.2 mmol Na2WO4·2H2O was dissolved in 160 mL deionized water to form a transparent solution. The pH of the solution was adjusted to 3 by 2 M HCl. Then 12.0 mmol TNT was added and stirred for 6 h to ensure WO42- ion to enter into TiO2 nanotubes’ channel. Subsequently, 3.6 mmol L-cysteine was added and maintained stirring for 1h. The mixture was transferred into a Teflon-lined stainless steel autoclave and reacted at 200 oC for 24 h. Finally, black solid sample was collected, through washing by deionized water to remove extra precursor component and drying in a vacuum oven at 60 oC for 12 h. TNT/WND composites with different W:Ti molar ratios (0.20, 0.10 and 0.05) were prepared by adjusting the amount of Na2WO4·2H2O, which were named TNT/WND-0.20, TNT/WND-0.10 and TNT/WND-0.05, respectively. X-ray diffraction (XRD) (Bruker D8), Transmission electron microscopy (TEM) 2
(JEM-2100F,
JEOL,
Japan),
Brunauer-Emmett-Teller
(BET)
(NOVA-2000,
Quantachrome, USA), X-ray photoelectron spectroscopy (XPS) (PHI 1600 ESCA, PerkinElmer), UV–vis spectrophotometer (UV-2550, Shimadzu, Japan) and Photoluminescence spectra (PL) (Fluoromax-4P, Horiba Jobin Yvon, America) were used to characterize prepared TNT/WND composites. In the photocatalytic test, 300 W Xe lamp with 420 nm cut-off filter was applied as visible light source (>420 nm) and placed 15 cm above the liquid surface. 20 mg photocatalyst was dispersed in 100 mL 20 mg/L RhB solution. Prior to light irradiation, above suspension was vigorously stirred for 60 min in the dark to establish an adsorption-desorption equilibrium, and then turned on the light source. 5 mL solution was withdrawn in certain time intervals and removed solid catalyst by 0.45 m filter membrane. The RhB concentration was tested by UV-vis spectroscopy at wavelength 553 nm (Perkin Elmer, lambda 35).
3. Results and discussion Fig.1 (a) is the synthesis process of TNT/WND composites. The microstructure of TNT/WND-0.10 was investigated by TEM as shown in Fig. 1(b-d). Obviously, TNT/WND-0.10 (Fig.1b) maintained nanotube structure with the length-diameter ratio more than 30. The enlarge image in Fig.1c displays that much nanodots evenly distributed on the TiO2 nanotube. The HRTEM (Fig.1d) image shows that size of the nanodots is about 8 nm. The crystal lattice of nanodot can be observed and the d-spacing of 0.216 and 0.186 nm are assigned to (103) and (105) planes of hexagonal WS2, respectively.[11] The result confirmed that WS2 nanodots were successfully introduced into TiO2 nanotube. Fig.1e present N2 adsorption-desorption isothermals of TiO2 nanotube and TNT/WND-0.10. We can see that the specific surface area of TNT/WND-0.10 (256m2/g) has a slight increase compared with TiO2 nanotube (245m2/g). By contrast, the pore volume and size of TNT/WND-0.10 is markedly decreased. The result revealed that WS2 nanodots were deposited on the inner wall of TiO2 nanotubes so that sacrifice its pore volume and size. This complex nanostructure can inhibit abscission of WS2 nanodots and endow the composites with outstanding stability during photocatalytic reaction. In our previous work,[12] we synthesized WS2 with nanosheet using the same preparing method but adding TiO2 nanotube. So we can 3
draw such a conclusion that TiO2 nanotube plays a key role for the formation of WS2 nanodots. During the reaction, the Na2WO4 first entered into the TiO2 nanotube and then reacted with L-cysteine, and the TiO2 nanotube acted as a microreactor. Due to the confine effects of mesopores, the WS2 grew into nanodots instead of nanosheets.
Fig. 1 Synthesis procedures of TNT/WND composites (a), TEM (b, c) and HRTEM (d) images of TNT/WND-0.10, Nitrogen adsorption-desorption isothermals (e).
Fig.2a shows the XRD patterns of TiO2 nanotubes and TNT/WND composites with different W:Ti molar ratios. The diffraction peaks of both bare TiO2 nanotubes and TNT/WND composites match well with the standard diffraction patterns of anatase TiO2 (JCPDS: 73-1764).[13] No diffraction patterns associated with WS2 were observed in all TNT/WND composites, which may be attributed to the low concentration of the WS2 nanodots , as well 4
as its ultra-small size and high dispersion as confirmed in the HRTEM (Fig.1d).[14,15]
Fig.2 XRD patterns of as-prepared sample (a), core level XPS spectra of Ti 2p (b), W 4f (c) and S 2p (d).
The chemical states of Ti, W and O in TNT/WND-0.10 were also studied by XPS. The binding energy peaks located at approximately 465.5 and 459.8 eV (Fig. 2b), are associated with Ti2p1/2 and Ti2p3/2 peaks, respectively. The energy gap between Ti2p1/2 and Ti2p3/2 closes 5.7 eV, indicating a normal state of Ti4+ in TiO2.[16] In Fig. 2c, the W4f spectra is consist of six peaks and three of them which locate at about 36.2, 38.2 and 41.4 eV are associated with W4f7/2, W4f5/2 and W5p3/2 for the W4+ in the WS2 dots, respectively. The peaks at 36.8, 38.8 and 42.0 eV belong to the W4f7/2, W4f5/2 and W5p3/2, respectively, which originated from the W6+ in the WO3. Above results are accordance with the previously reported work.[17] Fig.2d shows the peaks of S2p appeared at 161.6 and 163.4 eV corresponding to S2p3/2 and S2p1/2, resepectively, indicating the presence of S2-.[18] Therefore, it is reasonable to speculate that we have introduce WS2 nanodots into TiO2 nanotubes successfully. The optical properties of pristine TiO2 nanotubes and TNT/WND composites were measured by UV-Vis and PL spectroscopy. The UV-Vis 5
spectra (Fig.3a) shows that the light absorption of TiO2 nanotubes start from 386 nm corresponding to the gap energy 3.21 eV. With the increasing of W:Ti molar ratios, the absorption edge of TNT/WND composites present significantly red-shift, and the absorption intensity increase sharply in the visible range. Clearly, the light absorption ability of TNT/WND composites were greatly enhanced comparing with bare TiO2 nanotubes, which leads to the increase on photocatalytic performance. The PL spectra of the as-prepared samples have been performed for both bare TiO2 nanotubes and TNT/WND composites. As seen in Fig.3b, TNT/WND composites present comparatively weaker intensity of the emission peaks than that of TiO2 nanotubes, indicating a lower rate of electron-hole recombination. It could be attributed to firming heterojunction between TiO2 nanotubes and WS2 nanodots, which can facilitate the transfer of electrons and holes to improve the photocatalytic activity of TNT/WND composites.
Fig.3 UV-vis spectra (a) and photoluminescence spectra (b) of as-prepared samples, photocatalytic degradation (c) and photocatalytic degradation rate (d) of RhB under visible irradiation.
Fig.3c shows the variation of RhB concentration (C/C0) with time evolved over different photocatalysts. No RhB degradation can be detected in the 6
absence of photocatalysts, suggesting that RhB is chemically stable under visible light irradiation. Only slight decrease of RhB was detected for bare TiO2 nanotubes. In contrast, all tested TNT/WND composites exhibit good performance in RhB degradation under light irradiation. After reacting for 120 min, about 86.1%, 70.3%, 54.5% and 29.8% of RhB were removed by TNT/WND-0.10, TNT/WND-0.20, TNT/WND-0.05 and TNT, respectively. The linear relationship of Ct vs. t (Fig.3d) indicates the photocatalytic process and reaction kinetics follows zero-order reaction. Obviously, TNT/WND-0.10 has the maximum apparent photodegradation reaction rate (0.1472 mg/L·min), which is 2.7 times than that of bare TiO2 nanotubes.
4. Conclusions In summary, TNT/WND composites were prepared by situ growing WS2 nanodots on the inner wall of TiO2 nanotube through hydrothermal method. The as-prepared TNT/WND composites exhibit superior activity toward photodegradation of RhB under visible light. The enhanced photocatalytic activity is ascribed to its high specific surface area and relative low recombination rate of electron-hole pairs resulted from firm heterojunction between TiO2 nanotubes and WS2 nanodots.
Acknowledgments The authors gratefully acknowledge financial support from National Natural Science Foundation of China (no.U1407116, 21511130020, 21276193).
References (1) Fujishima, A. A. H. K. Nature. 1972, 37-38. (2) Pan, L.; Zou, J.; Zhang, X.; Wang, L. J. Am. Chem. Soc. 2011, 133, 10000-10002. (3) Liu, S. M.; Gan, L. M.; Liu, et al. Chem. Mater. 2002, 14, 1391-1397. (4) Saha, A.; Sinhamahapatra, A.; Kang, T.; et al. Nanoscale. 2017, 9, 17029-17036. (5) Sun, L.; Han, X.; Jiang, Z.; et al. Nanoscale. 2016, 8, 12858. (6) Zhao, F.; Rong, Y.; Wan, J.; et al. Nanotechnology. 2018, 29, 105403. (7) Zhang, X.; Lai, Z.; Liu, Z.; et al. Angew. Chem. Int. Edit. 2015, 54, 5425-5428. 7
(8) Wang, D.; Xu, Y.; Sun, F.; et al. Appl. Surf. Sci. 2016, 377, 221-227. (9) Wang, Q.; Huang, J.; Sun, H.; et al. ChemSusChem. 2018, 11, 1708-1721. (10) Ma, F.; Geng, Z.; Yang, X.; Leng, J. RSC Adv. 2015, 5, 46677-46685. (11) Jin, H.; Baek, B.; Kim, D.; et al. Nano Lett. 2017, 17, 7471-7477. (12) Wu, Y.; Liu, Z.; Chen, J.; Cai, X.; Na, P. Mater. Lett. 2017, 189, 282-285. (13) Yang, L.; Zheng, X.; Liu, M.; et al. J. Hazard. Mater. 2017, 329, 230-240. (14) Xu, S.; Li, D.; Wu, P. Adv. Funct. Mater. 2015, 25, 1127-1136. (15) Ma, C.; Zhu, H.; Zhou, J.; et al. Dalton T. 2017, 46, 3877-3886. (16) Quan, Q.; Xie, S.; Weng, B.; et al. Small. 2018, 14, 1704531. (17) Lu, X.; Wang, R.; Hao, L.; et al. Phys. Chem. Chem. Phys. 2016, 18, 31211-31216. (18) Ghorai, A.; Bayan, S.; Gogurla, N.; et al. ACS Appl. Mater. Inter. 2016, 9, 558-565.
8
Highlights 1. TiO2 nanotubes/WS2 nanodots (TNT/WND) composites have been synthesized.
2. TNT/WND composites were formed by WS2 nanodots decorated on TiO2 nanotubes.
3. WS2 nanodots can effectively promote the charge separation of TiO2 nanotubes.
4. TNT/WND composites exhibited enhanced photocatalytic activity than TiO2 nanotubes.
9
S0167-577X(18)32015-9 https://doi.org/10.1016/j.matlet.2018.12.056 MLBLUE 25457
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
18 September 2018 17 November 2018 12 December 2018
Please cite this article as: Y. Wu, Z. Liu, Y. Li, J. Chen, X. Zhu, P. Na, WS2 nanodots-modified TiO2 nanotubes to enhance visible-light photocatalytic activity, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet. 2018.12.056
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.
WS2 nanodots-modified TiO2 nanotubes to enhance visible-light photocatalytic activity Yongchuan Wu,a Zhongmin Liu,a Yaru Li,a Jitao Chena, Xixi Zhu b and Ping Na, a,* a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China
b
College of Chemistry and Environmental Engineering, Shandong University of Science and
Technology, Qingdao 266590, China *
Corresponding author: Ping Na
E-mail: [email protected]
Abstract WS2 nanodots-modified TiO2 nanotubes (TNT/WND) composites were fabricated via one-pot hydrothermal method for the first time. The physical and photophysical properties of the as-prepared photocatalysts were characterized by XRD, TEM, HRTEM, BET, UV-vis, and PL. The results demonstrated that WS2 nanodots have been successfully anchored on the inner wall of TiO2 nanotubes. TNT/WND
composites
showed
excellent
photocatalytic
activity
toward
photodegradation of rhodamine B (RhB) under visible-light, which mainly attributed to the synergistic effect from relative low recombination rate of the photogenerated electron-hole pairs and high specific surface area.
Key words: Semiconductors; Nanocomposites; Photocatalysis; WS2 nanodots; TiO2 nanotubes; Structural;
1 Introduction Since Fujishima and Honda reported the TiO2 electrodes as photocatalyst for water splitting, TiO2 has attracted considerable attention due to its excellent photocatalytic properties, chemical stability, nontoxicity and low-cost.[1,2] To date, various TiO2 nanostructures with different crystalline phase have been well studied, and special attention is paid to one-dimensional anatase TiO2 nanotubes (TNT) with high specific surface area, shorter photogenerated carrier diffusion lengths and excellent charge transport merits, enduing TNT as ideal candidate for photodegrading organic pollutant.[3] However, the wide band gap of TiO2 (3.2eV) make it hardly activated by visible light, which lead to inefficient utilization of sunlight.[4] Furthermore, the low quantum efficiency of bare TiO2 can’t satisfy the need of actual 1
application.[5] Therefore, it is highly desirable to overcome these inherent shortcomings of TiO2 to enhance its photocatalytic activity. Recently, transition metal dichalcogenide (TMD) nanodots, such as MoS2 and WS2 nanodots, have been extensively investigated as co-catalyst.[6,7] Wang et.al. [8] found that MoS2 nanodots modified TiO2 (P25) is able to photodegrade RhB under visible light. Lai [9] prepared MoS2 quantum dots@TiO2 nanotube arrays composites via electrodeposition method as a visible-driven photocatalyst for hydrogen evolution. These works confirm that MoS2 nanodots are an excellent co-catalyst which enhance the photocatalytic activity of TiO2. However, MoS2 nanodots directly loaded on the surface of TiO2 leads to poor stability. WS2 owns similar property as MoS but little work done related its co-catalyst property. Here, we presented a one-pot hydrothermal method to synthesize WS2 nanodots-modified TiO2 nanotubes (TNT/WND) composites with the sodium tungstate and L-cysteine as WS2 precursor source and TiO2 nanotubes as substrate. Due to high length-diameter ratio and mesoporous structure, TiO2 nanotubes acted as microreactor and impelled the WS2 to grow into nanodots instead of nanosheets. The as-prepared TNT/WND composites exhibited excellent photocatalytic activity for the degradation of RhB under visible-light irradiation.
2. Experiment TiO2 nanotubes were prepared by a facile hydrothermal process based on Ma, F’s work.[10] For TNT/WND composites. Typically, 1.2 mmol Na2WO4·2H2O was dissolved in 160 mL deionized water to form a transparent solution. The pH of the solution was adjusted to 3 by 2 M HCl. Then 12.0 mmol TNT was added and stirred for 6 h to ensure WO42- ion to enter into TiO2 nanotubes’ channel. Subsequently, 3.6 mmol L-cysteine was added and maintained stirring for 1h. The mixture was transferred into a Teflon-lined stainless steel autoclave and reacted at 200 oC for 24 h. Finally, black solid sample was collected, through washing by deionized water to remove extra precursor component and drying in a vacuum oven at 60 oC for 12 h. TNT/WND composites with different W:Ti molar ratios (0.20, 0.10 and 0.05) were prepared by adjusting the amount of Na2WO4·2H2O, which were named TNT/WND-0.20, TNT/WND-0.10 and TNT/WND-0.05, respectively. X-ray diffraction (XRD) (Bruker D8), Transmission electron microscopy (TEM) 2
(JEM-2100F,
JEOL,
Japan),
Brunauer-Emmett-Teller
(BET)
(NOVA-2000,
Quantachrome, USA), X-ray photoelectron spectroscopy (XPS) (PHI 1600 ESCA, PerkinElmer), UV–vis spectrophotometer (UV-2550, Shimadzu, Japan) and Photoluminescence spectra (PL) (Fluoromax-4P, Horiba Jobin Yvon, America) were used to characterize prepared TNT/WND composites. In the photocatalytic test, 300 W Xe lamp with 420 nm cut-off filter was applied as visible light source (>420 nm) and placed 15 cm above the liquid surface. 20 mg photocatalyst was dispersed in 100 mL 20 mg/L RhB solution. Prior to light irradiation, above suspension was vigorously stirred for 60 min in the dark to establish an adsorption-desorption equilibrium, and then turned on the light source. 5 mL solution was withdrawn in certain time intervals and removed solid catalyst by 0.45 m filter membrane. The RhB concentration was tested by UV-vis spectroscopy at wavelength 553 nm (Perkin Elmer, lambda 35).
3. Results and discussion Fig.1 (a) is the synthesis process of TNT/WND composites. The microstructure of TNT/WND-0.10 was investigated by TEM as shown in Fig. 1(b-d). Obviously, TNT/WND-0.10 (Fig.1b) maintained nanotube structure with the length-diameter ratio more than 30. The enlarge image in Fig.1c displays that much nanodots evenly distributed on the TiO2 nanotube. The HRTEM (Fig.1d) image shows that size of the nanodots is about 8 nm. The crystal lattice of nanodot can be observed and the d-spacing of 0.216 and 0.186 nm are assigned to (103) and (105) planes of hexagonal WS2, respectively.[11] The result confirmed that WS2 nanodots were successfully introduced into TiO2 nanotube. Fig.1e present N2 adsorption-desorption isothermals of TiO2 nanotube and TNT/WND-0.10. We can see that the specific surface area of TNT/WND-0.10 (256m2/g) has a slight increase compared with TiO2 nanotube (245m2/g). By contrast, the pore volume and size of TNT/WND-0.10 is markedly decreased. The result revealed that WS2 nanodots were deposited on the inner wall of TiO2 nanotubes so that sacrifice its pore volume and size. This complex nanostructure can inhibit abscission of WS2 nanodots and endow the composites with outstanding stability during photocatalytic reaction. In our previous work,[12] we synthesized WS2 with nanosheet using the same preparing method but adding TiO2 nanotube. So we can 3
draw such a conclusion that TiO2 nanotube plays a key role for the formation of WS2 nanodots. During the reaction, the Na2WO4 first entered into the TiO2 nanotube and then reacted with L-cysteine, and the TiO2 nanotube acted as a microreactor. Due to the confine effects of mesopores, the WS2 grew into nanodots instead of nanosheets.
Fig. 1 Synthesis procedures of TNT/WND composites (a), TEM (b, c) and HRTEM (d) images of TNT/WND-0.10, Nitrogen adsorption-desorption isothermals (e).
Fig.2a shows the XRD patterns of TiO2 nanotubes and TNT/WND composites with different W:Ti molar ratios. The diffraction peaks of both bare TiO2 nanotubes and TNT/WND composites match well with the standard diffraction patterns of anatase TiO2 (JCPDS: 73-1764).[13] No diffraction patterns associated with WS2 were observed in all TNT/WND composites, which may be attributed to the low concentration of the WS2 nanodots , as well 4
as its ultra-small size and high dispersion as confirmed in the HRTEM (Fig.1d).[14,15]
Fig.2 XRD patterns of as-prepared sample (a), core level XPS spectra of Ti 2p (b), W 4f (c) and S 2p (d).
The chemical states of Ti, W and O in TNT/WND-0.10 were also studied by XPS. The binding energy peaks located at approximately 465.5 and 459.8 eV (Fig. 2b), are associated with Ti2p1/2 and Ti2p3/2 peaks, respectively. The energy gap between Ti2p1/2 and Ti2p3/2 closes 5.7 eV, indicating a normal state of Ti4+ in TiO2.[16] In Fig. 2c, the W4f spectra is consist of six peaks and three of them which locate at about 36.2, 38.2 and 41.4 eV are associated with W4f7/2, W4f5/2 and W5p3/2 for the W4+ in the WS2 dots, respectively. The peaks at 36.8, 38.8 and 42.0 eV belong to the W4f7/2, W4f5/2 and W5p3/2, respectively, which originated from the W6+ in the WO3. Above results are accordance with the previously reported work.[17] Fig.2d shows the peaks of S2p appeared at 161.6 and 163.4 eV corresponding to S2p3/2 and S2p1/2, resepectively, indicating the presence of S2-.[18] Therefore, it is reasonable to speculate that we have introduce WS2 nanodots into TiO2 nanotubes successfully. The optical properties of pristine TiO2 nanotubes and TNT/WND composites were measured by UV-Vis and PL spectroscopy. The UV-Vis 5
spectra (Fig.3a) shows that the light absorption of TiO2 nanotubes start from 386 nm corresponding to the gap energy 3.21 eV. With the increasing of W:Ti molar ratios, the absorption edge of TNT/WND composites present significantly red-shift, and the absorption intensity increase sharply in the visible range. Clearly, the light absorption ability of TNT/WND composites were greatly enhanced comparing with bare TiO2 nanotubes, which leads to the increase on photocatalytic performance. The PL spectra of the as-prepared samples have been performed for both bare TiO2 nanotubes and TNT/WND composites. As seen in Fig.3b, TNT/WND composites present comparatively weaker intensity of the emission peaks than that of TiO2 nanotubes, indicating a lower rate of electron-hole recombination. It could be attributed to firming heterojunction between TiO2 nanotubes and WS2 nanodots, which can facilitate the transfer of electrons and holes to improve the photocatalytic activity of TNT/WND composites.
Fig.3 UV-vis spectra (a) and photoluminescence spectra (b) of as-prepared samples, photocatalytic degradation (c) and photocatalytic degradation rate (d) of RhB under visible irradiation.
Fig.3c shows the variation of RhB concentration (C/C0) with time evolved over different photocatalysts. No RhB degradation can be detected in the 6
absence of photocatalysts, suggesting that RhB is chemically stable under visible light irradiation. Only slight decrease of RhB was detected for bare TiO2 nanotubes. In contrast, all tested TNT/WND composites exhibit good performance in RhB degradation under light irradiation. After reacting for 120 min, about 86.1%, 70.3%, 54.5% and 29.8% of RhB were removed by TNT/WND-0.10, TNT/WND-0.20, TNT/WND-0.05 and TNT, respectively. The linear relationship of Ct vs. t (Fig.3d) indicates the photocatalytic process and reaction kinetics follows zero-order reaction. Obviously, TNT/WND-0.10 has the maximum apparent photodegradation reaction rate (0.1472 mg/L·min), which is 2.7 times than that of bare TiO2 nanotubes.
4. Conclusions In summary, TNT/WND composites were prepared by situ growing WS2 nanodots on the inner wall of TiO2 nanotube through hydrothermal method. The as-prepared TNT/WND composites exhibit superior activity toward photodegradation of RhB under visible light. The enhanced photocatalytic activity is ascribed to its high specific surface area and relative low recombination rate of electron-hole pairs resulted from firm heterojunction between TiO2 nanotubes and WS2 nanodots.
Acknowledgments The authors gratefully acknowledge financial support from National Natural Science Foundation of China (no.U1407116, 21511130020, 21276193).
References (1) Fujishima, A. A. H. K. Nature. 1972, 37-38. (2) Pan, L.; Zou, J.; Zhang, X.; Wang, L. J. Am. Chem. Soc. 2011, 133, 10000-10002. (3) Liu, S. M.; Gan, L. M.; Liu, et al. Chem. Mater. 2002, 14, 1391-1397. (4) Saha, A.; Sinhamahapatra, A.; Kang, T.; et al. Nanoscale. 2017, 9, 17029-17036. (5) Sun, L.; Han, X.; Jiang, Z.; et al. Nanoscale. 2016, 8, 12858. (6) Zhao, F.; Rong, Y.; Wan, J.; et al. Nanotechnology. 2018, 29, 105403. (7) Zhang, X.; Lai, Z.; Liu, Z.; et al. Angew. Chem. Int. Edit. 2015, 54, 5425-5428. 7
(8) Wang, D.; Xu, Y.; Sun, F.; et al. Appl. Surf. Sci. 2016, 377, 221-227. (9) Wang, Q.; Huang, J.; Sun, H.; et al. ChemSusChem. 2018, 11, 1708-1721. (10) Ma, F.; Geng, Z.; Yang, X.; Leng, J. RSC Adv. 2015, 5, 46677-46685. (11) Jin, H.; Baek, B.; Kim, D.; et al. Nano Lett. 2017, 17, 7471-7477. (12) Wu, Y.; Liu, Z.; Chen, J.; Cai, X.; Na, P. Mater. Lett. 2017, 189, 282-285. (13) Yang, L.; Zheng, X.; Liu, M.; et al. J. Hazard. Mater. 2017, 329, 230-240. (14) Xu, S.; Li, D.; Wu, P. Adv. Funct. Mater. 2015, 25, 1127-1136. (15) Ma, C.; Zhu, H.; Zhou, J.; et al. Dalton T. 2017, 46, 3877-3886. (16) Quan, Q.; Xie, S.; Weng, B.; et al. Small. 2018, 14, 1704531. (17) Lu, X.; Wang, R.; Hao, L.; et al. Phys. Chem. Chem. Phys. 2016, 18, 31211-31216. (18) Ghorai, A.; Bayan, S.; Gogurla, N.; et al. ACS Appl. Mater. Inter. 2016, 9, 558-565.
8
Highlights 1. TiO2 nanotubes/WS2 nanodots (TNT/WND) composites have been synthesized.
2. TNT/WND composites were formed by WS2 nanodots decorated on TiO2 nanotubes.
3. WS2 nanodots can effectively promote the charge separation of TiO2 nanotubes.
4. TNT/WND composites exhibited enhanced photocatalytic activity than TiO2 nanotubes.
9