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Enhanced photocatalytic degradation of methyl orange in TiO2(B)@anatase heterostructure nanocomposites prepared by a facile hydrothermal method
Materials Letters 112 (2013) 173–176
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced photocatalytic degradation of methyl orange in TiO2(B)@anatase heterostructure nanocomposites prepared by a facile hydrothermal method Yuewei Zhang a, Jianfeng Xu a, Jingyao Feng b, Ao Yang a, Yi Liu a, Mingjia Zhi a, Zhanglian Hong a,n a b
State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Department of Physics, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2013 Accepted 3 September 2013 Available online 13 September 2013
In this work, one-dimensional TiO2(B)@anatase heterostructure nanocomposites were synthesized via a facile hydrothermal process in ethanol/water solution with the aid of polyvinylpyrrolidone. TiO2(B) nanobelts were uniformly coated with an anatase nanocrystals shell. The shell thickness can be tuned from 10 nm to 30 nm. The energy bandgap at the heterostructure interface favors the transferring of the photo-induced holes in anatase to TiO2(B) nanobelts, thus the charge recombination can be effectively suppressed. The nanocomposites showed superior photocatalytic degradation of methyl orange due to the enhanced charge separation efficiency. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Hydrothermal TiO2(B) Anatase Photocatalysis Semiconductors Nanocomposites
1. Introduction To date, most research on photocatalysis is based on TiO2 for its high physical/chemical stability, low price as well as high activity under ultraviolet (UV) light [1,2]. The photo-induced electron/hole pairs in TiO2 can effectively generate free radicals, which degrade the pollutant species. The main disadvantages of TiO2 photocatalyst lie on (1) large bandgap, which limits the utilization of light energy below the UV region and (2) the fast recombination rate of the photo-induced electron/hole pairs, which gives low quantum yielding. When a lot of work has been done to extend the light absorption [3–7], much attention is also paid to suppress the recombination by building heterostructure [8]. In the heterojunction method, TiO2 is combined with a secondary phase material with proper bandgap alignment. The photo-induced electron–hole pairs in TiO2 can be separated and the recombination can be suppressed, thus better photocatalytic performance can be achieved. Realizing effective charge separation depends on the morphology, the charge transfer capability as well as the interfacial property of the heterojunction. Among these heterostructures, binary TiO2 composites are of particular interest due to their low cost and good compatibility. Different types of combinations have been investigated [9–11]. Recently TiO2(B)/anatase composite has attracted much attention, since TiO2(B) is usually in one n
Corresponding author. Tel./fax: þ 86 571 87951234. E-mail address: [email protected] (Z. Hong).
dimensional (1-D) shape, which benefits the charge transfer and the recycling utilization. The photocatalytic activity of the anatase can be enhanced by combining with TiO2(B) [12]. However, preparing TiO2(B)/anatase heterostructure with controllable crystalline phase and morphology is of challenge. Crucial conditions (high temperature, long reaction time, complex procedures or sensitive reagents) were required, which limited its practical application [12–18]. Moreover, the 1-D nanostructure may degrade to 0-D nanoparticles during the synthesis, which hinders the fast charge transfer in the 1-D nanostructure [13]. Herein, in the present work, a facile one-step hydrothermal method is developed to prepare TiO2(B) @anatase nanocomposites under mild conditions. Anatase is coated on the surface of the TiO2(B) nanobelts to form the uniform 1-D heterostructure. The anatase shell thickness can be controlled by varying the TiF4 concentration in the coating precursor. The bandgap structure at the TiO2(B)/anatase interface allows the charge transfer from anatase to TiO2(B), which significantly reduces the recombination rate. It is found that with the optimized anatase coating, the photocatalytic activity of TiO2(B)@anatase in degradation of methyl orange (MO) can be greatly enhanced compared to either pure anatase or TiO2(B), due to the well aligned interfacial bandgap.
2. Experimental Synthesis of TiO2(B)@anatase nanocomposites: TiO2(B) was synthesized following previous reports and is denoted as TB, the
0167-577X/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.010
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detail procedure is shown in the supplementary data [12]. The TiO2(B)@anatase composite was prepared by hydrothermal method. In a typical reaction, 0.04 mmol of TiO2(B) nanobelts were added into 28 mL of 4.2% polyvinylpyrrolidone (PVP-40, Mw¼ 40,000, Sigma-Aldrich, AR) ethanol/water solution (11:3 in volume ratio), followed by sonication. Then 4 mL of TiF4 (SigmaAldrich, AR) aqueous solution with different concentrations was added. The mixture was stirred to achieve a homogeneous suspension. The suspension was moved into a 40 mL Teflon autoclave liner and heated at 180 1C for 3 h. After cooling, the products were washed by water for several times and dried at 100 1C. The samples derived from 0.01, 0.02 and 0.04 M TiF4 was named as TBA-1, TBA-2 and TBA-4. For comparison, pure anatase was synthesized via similar process without TiO2(B) nanobelts, and noted as TA. Characterization: Powder X-ray diffraction (XRD) was performed by a Rigaku D/max-3B X-ray diffractometer (Cu Kα line, λ ¼0.15406 nm, 40 kV, 35 mA). The morphology of the samples was observed on a JEOL-1200 transmission electron microscope (TEM) (120 kV) and a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) (5 kV). Diffuse reflectance spectra (DRS) and UV–visible absorption spectra were measured using a Hitachi U-4100 UV–visible spectrometer with an integrating sphere accessory. Raman spectra were collected by Thermo Fisher DXR Smart-Raman spectrometer (532 nm, 400 mW). Photoluminescence emission spectra (PL) were recorded using Hitachi F-4500 Fluorescence spectrophotometer (excitation wavelength: 340 nm). Photocatalysis testing: The photocatalytic activity of the as-prepared samples was evaluated by the degradation of MO under UV light. Twenty milligram of the as-prepared sample and 20 mL of MO aqueous solution (5 mg/L) were mixed under stirring. The solution was held for 30 min in dark to reach the adsorption equilibrium. Afterwards the mixture was subjected to the UV light irradiation (A metal halogen lamp (HQIBT, 400 W/D, Osram), 15 cm high above the suspension). After certain time, the suspension was centrifuged. The supernatant was subjected to the UV–visible absorption measurement. The concentration of residual MO was evaluated by monitoring absorbance at 464 nm.
3. Results and discussion Fig. 1(a) shows the XRD pattern of the as-prepared samples. The diffraction peaks of TB are indexed to TiO2(B) phase (JCPDs: 46-1237) with typical monoclinic structure (lattice constant of a ¼12.2 Ǻ, b¼ 3.7 Ǻ, c ¼6.54 Ǻ and β ¼107.41), which are composed of two-dimensional sheets of edge sharing TiO6 octahedra [15].
After TiF4 coating, diffraction peaks of tetragonal anatase (JCPDs: 21-1272) were observable in the patterns of TBA-1, TBA-2 and TBA-4. The (101) and (200) planes for anatase at around 25.31 and 48.11 overlapped with the (110) and (020) planes for TiO2(B). By increasing the amount of TiF4 precursor, the peak intensity for anatase increased while the ones of TiO2(B) diminished, indicating the increase of relative amount of anatase phase. The sharp appearance of the anatase peaks reveals its high crystallinity. Raman spectra were used to further analyze the structure evolution, which are shown in Fig. 1(b). In the TBA series samples, the bands at around 138, 190 and 633 cm 1 are attributed to the Eg vibration of anatase; The ones at 391 and 510 cm 1 are attributed to the B1g vibration of anatase and the ones at 116, 231 and 245 cm 1 are assigned to the vibration of TiO2(B) [12]. Therefore, the phase information obtained from the Raman spectra is consistent with the XRD results. Both anatase and TiO2(B) phases are confirmed in TiO2(B)@anatase nanocomposites. The morphology of the as-prepared samples is observed by TEM and SEM, and is shown in Fig. 2. TB presented typical 1-D nanobelts appearance with smooth surface as shown in Fig. 2(a). When 0.01 M TiF4 was added, the surface became rough and small nanocrystals started to appear in Fig. 2(b). As the concentration of TiF4 increased to 0.02 M, the TiO2(B) nanobelts were covered with closely packed anatase nanocrystals (Fig. 2(c)). Further increasing the concentration to 0.04 M, the TB nanobelts were fully covered by the anatase nanocrystals as shown in Fig. 2(d–f). It should be noted that the 1-D belt morphology remains and no destruction is observed, which is different from the one synthesized by other coating method [13]. As shown in Fig. 2(b–d), the diameters of TBA-1, TBA-2 and TBA-4 were about 140, 160 and 180 nm, respectively, which was larger than that of TB (120 nm), indicating the increasing amount of anatase. To be mentioned, PVP played a key role in the coating process, which stabilized the nanobelts in the hydrothermal solution, since PVP is amphiphilic and can introduce viscosity [19]. Without adding PVP, the nanobelts quickly precipitated and TiF4 hydrolyzed to anatase agglomerates (the detail is shown in the supplementary data). The HRTEM image of TBA-4 sample is shown in Fig. 2(g). The lattice spacing fringes of 0.36 nm for anatase (101) plane indicate that these surface nanocrystals are of polycrystalline within highly crystallinity, while the ones of 0.61 nm and 0.37 nm in the right region were referred to the (200) and (110) planes of TiO2(B). The interface between anatase and TiO2(B) was observed. The small lattice mismatch between the (001) plane of anatase and the (100) plane of TiO2(B) leads to the coherent interface, which possesses few interface defects and benefits the charger transfer [17]. UV–visible absorption spectra in Fig. 3(a) show that all the samples have absorption band in the UV region. The adsorption
Fig. 1. (a) XRD patterns and (b) Raman spectra of as-prepared samples.
Y. Zhang et al. / Materials Letters 112 (2013) 173–176
175
Fig. 2. TEM images of (a) TB, (b) TBA-1, (c) TBA-2, (d) TBA-4, (e,f) SEM images of TBA-4 and (g) HRTEM image of TBA-4.
Fig. 3. (a) Converted UV–vis absorption spectra of as-prepared samples, (b) photocatalytic decomposition efficiency of MO in the presence of different samples, (c) Schematic mechanism of the enhanced charger carriers separation and photocatalysis and (d) PL spectra of as-prepared samples.
edge shows blue-shift when the relatively amount of anatase nanocrystals increases, as anatase owned larger bandgap than TiO2(B). The bandgap (Eg) for TB, TBA-1, TBA-2 and TBA-4 sample gradually increased from 3.18, 3.20, 3.20 to 3.23 and finally reached to 3.25 eV for TA (pure anatase). Fig. 3(b) shows the photo-degradation plots of MO within different samples under UV illumination, which demonstrates the improvement of photocatalytic activity caused by the formation of the heterostructure. The
pristine TB nanobelts showed the lowest catalytic activity. When the anatase nanocrystals were decorated, the activity of TBA samples is enhanced compared with TB. TBA-1 had weaker activity than TA, due to the low amount and incomplete anatase coating. When the TiO2(B) was completely covered by anatase in TBA-2 and TBA-4, the activity was higher than that of either TB or TA. The high photocatalytic activity of TBA-2 and TBA-4 could be attributed to the enhanced interfacial charge transfer caused by the
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Y. Zhang et al. / Materials Letters 112 (2013) 173–176
heterostructure. The conduction and valence band edge of both TiO2(B) and anatase were estimated and demonstrated in Fig. 3(c), by the equation provided below [20] c
ECB 1 ¼ X E 0:5 Eg
ð1Þ
ECB 1 is the conduction band edge at zero charge; X is the absolute electronegativity (5.81 eV for TiO2); Ec is the energy of free electrons on the hydrogen scale (4.50 eV); Eg is the band gap. The calculated conduction band edge and valence band edge for anatase are 0.32 and 2.93 eV, while the ones for TiO2(B) are 0.28 and 2.90 eV. The estimated bandgap of TiO2(B) located in the ones of anatase, which agreed with the values reported in literature [12–14,17]. When UV light irradiated on the surface anatase, the excited electrons were trapped by the surface adsorbed oxygen and larger amount of holes in anatase may spontaneously transfer to the inner TiO2(B). As the result, the recombination of the photo-induced electron–hole pairs in anatase was well suppressed [17]. The trapped electrons and the remaining holes on anatase may react to generate free radicals and degrade MO. Therefore the better charge carrier separation is achieved in the nanocomposites and the significant increase of the photocatalytic activity is obtained. It is proved by the PL spectra shown in Fig. 3(d). The emission intensity decreases when the heterostructure is obtained, which implies the recombination was effectively suppressed [7]. 4. Conclusion A facile method was developed to prepare 1-D TiO2(B)@anatase heterostructure nanocomposites photocatalysts. The anatase nanocrystals were uniformly coated on the surface of TiO2(B) nanobelts via simple hydrothermal process. The shell thickness can be tuned from 10 nm to 30 nm. The bandgap alignment between the TiO2(B) and anatase allows the charge separation in the active anatase phase. The photocatalytic experiment shows that with the optimized anatase loading, the TiO2(B) @anatase heterostructure composite has higher photocatalytic activity than that of its each component, due to the high charger separation ability of the heterostructure. It is concluded that this heterostructure nanocomposite is a promising photocatalytic material.
Acknowledgments This work was supported partially by Natural Science Foundation of China (No. 51072180) and the Fundamental Research Funds for the Central Universities (No. 2009QNA4005, 585000*172210311[02]). Zhang is grateful to China Scholarship Council for the financial support.
Appendix A. Supplementary Material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.09.010. References [1] Fujishima A, Honda K. Nature 1972;238:37–8. [2] Chen X, Mao SS. Chemical Reviews 2007;107:2891–959. [3] Wang J, Tafen DN, Lewis JP, Hong Z, Manivannan A, Zhi M, et al. Journal of the American Chemical Society 2009;131:12290–7. [4] Li M, Hong Z, Fang Y, Huang F. Materials Research Bulletin 2008;43:2179–86. [5] Li M, Zhou S, Zhang Y, Chen G, Hong Z. Applied Surface Science 2008;254:3762–6. [6] Zhang Y, Hong Z. Nanoscale 5 (19), 2013, 8930–8933. [7] Khalid NR, Hong Z, Ahmed E, Zhang Y, Chan H, Ahmad M. Applied Surface Science 2012;258:5827–34. [8] Qu Y, Duan X. Chemical Society Reviews 2013;42:2568–80. [9] Ding Z, Lu GQ, Greenfield PF. Journal of Physical Chemistry B 2000;104:4815–20. [10] Kandiel TA, Feldhoff A, Robben L, Dillert R, Bahnemann DW. Chemistry of Materials 2010;22:2050–60. [11] Huberty J, Xu H. Journal of Solid State Chemistry 2008;181:508–14. [12] Zhou W, Gai L, Hu P, Cui J, Liu X, Wang D, et al. Crystal Engineering Communications 2011;13:6643–9. [13] Paul B, Locke A, Martens WN, Frost RL. Journal of Colloid and Interface Science 2012;386:66–72. [14] Yang D, Zhao J, Liu H, Zheng Z, Adebajo MO, Wang H, et al. Chemistry – A European Journal 2013;19:5113–9. [15] Liu B, Khare A, Aydil ES. ACS Applied Materials and Interfaces 2011;3:4444–50. [16] Pan K, Dong Y, Zhou W, Wang G, Pan Q, Yuan Y, et al. Electrochimica Acta 2013;88:263–9. [17] Yang D, Liu H, Zheng Z, Yuan Y, Zhao JC, Waclawik ER, et al. Journal of the American Chemical Society 2009;131:17885–93. [18] Zhou W, Du G, Hu P, Li G, Wang D, Liu H, et al. Journal of Materials Chemistry 2011;21:7937–45. [19] Lou XW, Archer LA. Advanced Materials 2008;20:1853–8. [20] Lin X, Xing J, Wang W, Shan Z, Xu F, Huang F. Journal of Physical Chemistry C 2007;111:18288–93.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced photocatalytic degradation of methyl orange in TiO2(B)@anatase heterostructure nanocomposites prepared by a facile hydrothermal method Yuewei Zhang a, Jianfeng Xu a, Jingyao Feng b, Ao Yang a, Yi Liu a, Mingjia Zhi a, Zhanglian Hong a,n a b
State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Department of Physics, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2013 Accepted 3 September 2013 Available online 13 September 2013
In this work, one-dimensional TiO2(B)@anatase heterostructure nanocomposites were synthesized via a facile hydrothermal process in ethanol/water solution with the aid of polyvinylpyrrolidone. TiO2(B) nanobelts were uniformly coated with an anatase nanocrystals shell. The shell thickness can be tuned from 10 nm to 30 nm. The energy bandgap at the heterostructure interface favors the transferring of the photo-induced holes in anatase to TiO2(B) nanobelts, thus the charge recombination can be effectively suppressed. The nanocomposites showed superior photocatalytic degradation of methyl orange due to the enhanced charge separation efficiency. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Hydrothermal TiO2(B) Anatase Photocatalysis Semiconductors Nanocomposites
1. Introduction To date, most research on photocatalysis is based on TiO2 for its high physical/chemical stability, low price as well as high activity under ultraviolet (UV) light [1,2]. The photo-induced electron/hole pairs in TiO2 can effectively generate free radicals, which degrade the pollutant species. The main disadvantages of TiO2 photocatalyst lie on (1) large bandgap, which limits the utilization of light energy below the UV region and (2) the fast recombination rate of the photo-induced electron/hole pairs, which gives low quantum yielding. When a lot of work has been done to extend the light absorption [3–7], much attention is also paid to suppress the recombination by building heterostructure [8]. In the heterojunction method, TiO2 is combined with a secondary phase material with proper bandgap alignment. The photo-induced electron–hole pairs in TiO2 can be separated and the recombination can be suppressed, thus better photocatalytic performance can be achieved. Realizing effective charge separation depends on the morphology, the charge transfer capability as well as the interfacial property of the heterojunction. Among these heterostructures, binary TiO2 composites are of particular interest due to their low cost and good compatibility. Different types of combinations have been investigated [9–11]. Recently TiO2(B)/anatase composite has attracted much attention, since TiO2(B) is usually in one n
Corresponding author. Tel./fax: þ 86 571 87951234. E-mail address: [email protected] (Z. Hong).
dimensional (1-D) shape, which benefits the charge transfer and the recycling utilization. The photocatalytic activity of the anatase can be enhanced by combining with TiO2(B) [12]. However, preparing TiO2(B)/anatase heterostructure with controllable crystalline phase and morphology is of challenge. Crucial conditions (high temperature, long reaction time, complex procedures or sensitive reagents) were required, which limited its practical application [12–18]. Moreover, the 1-D nanostructure may degrade to 0-D nanoparticles during the synthesis, which hinders the fast charge transfer in the 1-D nanostructure [13]. Herein, in the present work, a facile one-step hydrothermal method is developed to prepare TiO2(B) @anatase nanocomposites under mild conditions. Anatase is coated on the surface of the TiO2(B) nanobelts to form the uniform 1-D heterostructure. The anatase shell thickness can be controlled by varying the TiF4 concentration in the coating precursor. The bandgap structure at the TiO2(B)/anatase interface allows the charge transfer from anatase to TiO2(B), which significantly reduces the recombination rate. It is found that with the optimized anatase coating, the photocatalytic activity of TiO2(B)@anatase in degradation of methyl orange (MO) can be greatly enhanced compared to either pure anatase or TiO2(B), due to the well aligned interfacial bandgap.
2. Experimental Synthesis of TiO2(B)@anatase nanocomposites: TiO2(B) was synthesized following previous reports and is denoted as TB, the
0167-577X/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.010
174
Y. Zhang et al. / Materials Letters 112 (2013) 173–176
detail procedure is shown in the supplementary data [12]. The TiO2(B)@anatase composite was prepared by hydrothermal method. In a typical reaction, 0.04 mmol of TiO2(B) nanobelts were added into 28 mL of 4.2% polyvinylpyrrolidone (PVP-40, Mw¼ 40,000, Sigma-Aldrich, AR) ethanol/water solution (11:3 in volume ratio), followed by sonication. Then 4 mL of TiF4 (SigmaAldrich, AR) aqueous solution with different concentrations was added. The mixture was stirred to achieve a homogeneous suspension. The suspension was moved into a 40 mL Teflon autoclave liner and heated at 180 1C for 3 h. After cooling, the products were washed by water for several times and dried at 100 1C. The samples derived from 0.01, 0.02 and 0.04 M TiF4 was named as TBA-1, TBA-2 and TBA-4. For comparison, pure anatase was synthesized via similar process without TiO2(B) nanobelts, and noted as TA. Characterization: Powder X-ray diffraction (XRD) was performed by a Rigaku D/max-3B X-ray diffractometer (Cu Kα line, λ ¼0.15406 nm, 40 kV, 35 mA). The morphology of the samples was observed on a JEOL-1200 transmission electron microscope (TEM) (120 kV) and a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) (5 kV). Diffuse reflectance spectra (DRS) and UV–visible absorption spectra were measured using a Hitachi U-4100 UV–visible spectrometer with an integrating sphere accessory. Raman spectra were collected by Thermo Fisher DXR Smart-Raman spectrometer (532 nm, 400 mW). Photoluminescence emission spectra (PL) were recorded using Hitachi F-4500 Fluorescence spectrophotometer (excitation wavelength: 340 nm). Photocatalysis testing: The photocatalytic activity of the as-prepared samples was evaluated by the degradation of MO under UV light. Twenty milligram of the as-prepared sample and 20 mL of MO aqueous solution (5 mg/L) were mixed under stirring. The solution was held for 30 min in dark to reach the adsorption equilibrium. Afterwards the mixture was subjected to the UV light irradiation (A metal halogen lamp (HQIBT, 400 W/D, Osram), 15 cm high above the suspension). After certain time, the suspension was centrifuged. The supernatant was subjected to the UV–visible absorption measurement. The concentration of residual MO was evaluated by monitoring absorbance at 464 nm.
3. Results and discussion Fig. 1(a) shows the XRD pattern of the as-prepared samples. The diffraction peaks of TB are indexed to TiO2(B) phase (JCPDs: 46-1237) with typical monoclinic structure (lattice constant of a ¼12.2 Ǻ, b¼ 3.7 Ǻ, c ¼6.54 Ǻ and β ¼107.41), which are composed of two-dimensional sheets of edge sharing TiO6 octahedra [15].
After TiF4 coating, diffraction peaks of tetragonal anatase (JCPDs: 21-1272) were observable in the patterns of TBA-1, TBA-2 and TBA-4. The (101) and (200) planes for anatase at around 25.31 and 48.11 overlapped with the (110) and (020) planes for TiO2(B). By increasing the amount of TiF4 precursor, the peak intensity for anatase increased while the ones of TiO2(B) diminished, indicating the increase of relative amount of anatase phase. The sharp appearance of the anatase peaks reveals its high crystallinity. Raman spectra were used to further analyze the structure evolution, which are shown in Fig. 1(b). In the TBA series samples, the bands at around 138, 190 and 633 cm 1 are attributed to the Eg vibration of anatase; The ones at 391 and 510 cm 1 are attributed to the B1g vibration of anatase and the ones at 116, 231 and 245 cm 1 are assigned to the vibration of TiO2(B) [12]. Therefore, the phase information obtained from the Raman spectra is consistent with the XRD results. Both anatase and TiO2(B) phases are confirmed in TiO2(B)@anatase nanocomposites. The morphology of the as-prepared samples is observed by TEM and SEM, and is shown in Fig. 2. TB presented typical 1-D nanobelts appearance with smooth surface as shown in Fig. 2(a). When 0.01 M TiF4 was added, the surface became rough and small nanocrystals started to appear in Fig. 2(b). As the concentration of TiF4 increased to 0.02 M, the TiO2(B) nanobelts were covered with closely packed anatase nanocrystals (Fig. 2(c)). Further increasing the concentration to 0.04 M, the TB nanobelts were fully covered by the anatase nanocrystals as shown in Fig. 2(d–f). It should be noted that the 1-D belt morphology remains and no destruction is observed, which is different from the one synthesized by other coating method [13]. As shown in Fig. 2(b–d), the diameters of TBA-1, TBA-2 and TBA-4 were about 140, 160 and 180 nm, respectively, which was larger than that of TB (120 nm), indicating the increasing amount of anatase. To be mentioned, PVP played a key role in the coating process, which stabilized the nanobelts in the hydrothermal solution, since PVP is amphiphilic and can introduce viscosity [19]. Without adding PVP, the nanobelts quickly precipitated and TiF4 hydrolyzed to anatase agglomerates (the detail is shown in the supplementary data). The HRTEM image of TBA-4 sample is shown in Fig. 2(g). The lattice spacing fringes of 0.36 nm for anatase (101) plane indicate that these surface nanocrystals are of polycrystalline within highly crystallinity, while the ones of 0.61 nm and 0.37 nm in the right region were referred to the (200) and (110) planes of TiO2(B). The interface between anatase and TiO2(B) was observed. The small lattice mismatch between the (001) plane of anatase and the (100) plane of TiO2(B) leads to the coherent interface, which possesses few interface defects and benefits the charger transfer [17]. UV–visible absorption spectra in Fig. 3(a) show that all the samples have absorption band in the UV region. The adsorption
Fig. 1. (a) XRD patterns and (b) Raman spectra of as-prepared samples.
Y. Zhang et al. / Materials Letters 112 (2013) 173–176
175
Fig. 2. TEM images of (a) TB, (b) TBA-1, (c) TBA-2, (d) TBA-4, (e,f) SEM images of TBA-4 and (g) HRTEM image of TBA-4.
Fig. 3. (a) Converted UV–vis absorption spectra of as-prepared samples, (b) photocatalytic decomposition efficiency of MO in the presence of different samples, (c) Schematic mechanism of the enhanced charger carriers separation and photocatalysis and (d) PL spectra of as-prepared samples.
edge shows blue-shift when the relatively amount of anatase nanocrystals increases, as anatase owned larger bandgap than TiO2(B). The bandgap (Eg) for TB, TBA-1, TBA-2 and TBA-4 sample gradually increased from 3.18, 3.20, 3.20 to 3.23 and finally reached to 3.25 eV for TA (pure anatase). Fig. 3(b) shows the photo-degradation plots of MO within different samples under UV illumination, which demonstrates the improvement of photocatalytic activity caused by the formation of the heterostructure. The
pristine TB nanobelts showed the lowest catalytic activity. When the anatase nanocrystals were decorated, the activity of TBA samples is enhanced compared with TB. TBA-1 had weaker activity than TA, due to the low amount and incomplete anatase coating. When the TiO2(B) was completely covered by anatase in TBA-2 and TBA-4, the activity was higher than that of either TB or TA. The high photocatalytic activity of TBA-2 and TBA-4 could be attributed to the enhanced interfacial charge transfer caused by the
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Y. Zhang et al. / Materials Letters 112 (2013) 173–176
heterostructure. The conduction and valence band edge of both TiO2(B) and anatase were estimated and demonstrated in Fig. 3(c), by the equation provided below [20] c
ECB 1 ¼ X E 0:5 Eg
ð1Þ
ECB 1 is the conduction band edge at zero charge; X is the absolute electronegativity (5.81 eV for TiO2); Ec is the energy of free electrons on the hydrogen scale (4.50 eV); Eg is the band gap. The calculated conduction band edge and valence band edge for anatase are 0.32 and 2.93 eV, while the ones for TiO2(B) are 0.28 and 2.90 eV. The estimated bandgap of TiO2(B) located in the ones of anatase, which agreed with the values reported in literature [12–14,17]. When UV light irradiated on the surface anatase, the excited electrons were trapped by the surface adsorbed oxygen and larger amount of holes in anatase may spontaneously transfer to the inner TiO2(B). As the result, the recombination of the photo-induced electron–hole pairs in anatase was well suppressed [17]. The trapped electrons and the remaining holes on anatase may react to generate free radicals and degrade MO. Therefore the better charge carrier separation is achieved in the nanocomposites and the significant increase of the photocatalytic activity is obtained. It is proved by the PL spectra shown in Fig. 3(d). The emission intensity decreases when the heterostructure is obtained, which implies the recombination was effectively suppressed [7]. 4. Conclusion A facile method was developed to prepare 1-D TiO2(B)@anatase heterostructure nanocomposites photocatalysts. The anatase nanocrystals were uniformly coated on the surface of TiO2(B) nanobelts via simple hydrothermal process. The shell thickness can be tuned from 10 nm to 30 nm. The bandgap alignment between the TiO2(B) and anatase allows the charge separation in the active anatase phase. The photocatalytic experiment shows that with the optimized anatase loading, the TiO2(B) @anatase heterostructure composite has higher photocatalytic activity than that of its each component, due to the high charger separation ability of the heterostructure. It is concluded that this heterostructure nanocomposite is a promising photocatalytic material.
Acknowledgments This work was supported partially by Natural Science Foundation of China (No. 51072180) and the Fundamental Research Funds for the Central Universities (No. 2009QNA4005, 585000*172210311[02]). Zhang is grateful to China Scholarship Council for the financial support.
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