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Photoelectrochemical response from CdSe-sensitized anodic oxidation TiO2 nanotubes
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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
Photoelectrochemical response from CdSe-sensitized anodic oxidation TiO2 nanotubes Hua-Yan Si, Zhen-Hong Sun, Hao-Li Zhang ∗ State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China Received 31 October 2006; accepted 29 April 2007 Available online 7 June 2007
Abstract Uniform TiO2 nanotube arrays were fabricated on titanium substrates by self-organized anodic oxidation. CdSe quantum dots (QDs) were assembled onto TiO2 nanotube layers, which serve as sensitizers. The photocurrent measurement shows that annealing is essential to increase the anatase phase and hence increase the photocurrent. Attachment of CdSe QDs dramatically increased the photocurrent of TiO2 nanotubes in the visible region. The photo-response of CdSe sensitized TiO2 also shows strong dependence to the QDs size. The possible mechanism for energy transfer between the CdSe QDs and the TiO2 nanotubes has been discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 nanotubes; CdSe; Quantum dot; Sensitizer; Solar cell
1. Introduction Nanoscale titania (TiO2 ) is of great interest mainly due to its unique properties, such as high photoactivity and size dependent optical properties [1]. However, TiO2 is a wide band gap semiconductor (band gap around 3.2 eV), which adsorbs light only in the UV region. To overcome this problem, various materials, like dyes and metallic nanoparticles, have been used as sensitizer to increase the photoactivity of TiO2 in the visible range. Semiconductor quantum dots (QDs), such as CdS, PbS, and CdSe, have been the subject of considerable interest for sensitizers in dye-sensitized solar cells (DSSCs) as an alternative to organic dyes [2]. In the past, QD-sensitized TiO2 solar cells usually consist of a mesoporous anode formed by a sintered film of anatase TiO2 . In such a device, the photo-conversion efficiency is mainly limited by the slow percolation of electrons through a random polycrystalline network and the poor absorption of low energy photons by available QDs. The above problems may be solved by using an electrode consists of highly ordered TiO2 nanotubes. In this work, we explored the sensitizing effects of CdSe QDs to the TiO2 nanotubes. It
∗
Corresponding author. Tel.: +86 931 8912365; fax: +86 931 8912365. E-mail address: [email protected] (H.-L. Zhang).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.160
is expected that the CdSe QDs sensitized TiO2 nanotubes have some advantages compared with the conventional systems. First, the regular pore structure and relative big pores size allow the CdSe QDs to be uniformly coated in the porous TiO2 matrix. Second, the arrangement of the highly ordered perpendicular aligned titania nanotube-array permits directed charge transfer along the length of the nanotubes from the solution to the conductive substrate, thereby reducing the losses incurred by charge-hopping across the nanoparticles grain boundaries. The above features make this morphology desirable for photovoltaic applications [3,4]. 2. Experimental UV-vis absorption spectra were recorded using a T6 UV-Vis spectrometer (Purkinje General, China). Fluorescence measurements were made using a LS55 fluorescence spectrometer (PE, USA). The electrochemical set-up consisted of a three-electrode system in a quartz cell, using a saturated calomel electrode (SCE) reference electrode. The measurement was conducted using a CHI660B electrochemistry work station (CHI, USA). A 500 W Hg lamp was used to measure the photocurrent of TiO2 film in the UV region. Monochromatic light was obtained using a WDL 30 monochromator (BOIF, China) equipped with a 500 W Xe lamp and a mechanical chopping. Atomic force microscopy (AFM)
H.-Y. Si et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
measurement was performed on a P-47 SPM (NT-MDT, Russia) using tapping mode. Scanning electron microscopy (SEM) was measured using a JSM-6380LV (JEOL, Japan). The colloidal CdSe QDs were prepared using previously reported method [5]. TiO2 film was prepared by anodizing a 150 m thick titanium sheet (99.8%) in 0.5% HF for 0.5 h. The obtained nanotube arrays were subsequent annealed in ambient at 450 ◦ C for 3 h. The TiO2 films were dipped into an acetonitrile solution of thioglycolic acid for 12 h, then washed with both acetonitrile and toluene before being transferred to a glass vial containing a CdSe QDs toluene solution. After being immersed in the CdSe solution for 12 h, the TiO2 films were washed with toluene and dried in vacuum. 3. Results and discussion The sizes of the CdSe QDs are 3 nm and 4.5 nm in diameter, respectively, as measured by TEM (Fig. 1a inserts). The two QDs exhibit transitions in the visible range with sharp band edge absorption at 550 nm and 576 nm, respectively (Fig. 1a). The correlation between the particle sizes and their band edge positions are in good agreement with previous reported [6]. The fluorescence emission spectra shows that the two CdSe particles
605
exhibit band edge emission (480 nm excitation) at 567 nm and 609 nm (Fig. 1b), respectively. The TiO2 nanotube films absorb UV light at 232 nm and 309 nm (Fig. 1c), corresponding to a band gap of 3.2 eV. SEM measurement (Fig. 2b insert) shows that the TiO2 layers formed by anodization treatment consists of nanotube arrays with a uniform tube size around 80 nm in diameter and have a wall thickness around 10 nm. Cross section image shows that the thickness of the TiO2 nanotube film is approximately 500 nm. The crystal structure of the TiO2 layer was identified using X-ray diffraction (XRD). Fig. 2 shows the XRD patterns of the as prepared TiO2 film and that after annealing. The as-prepared TiO2 shows only peaks corresponding to the Ti substrate, suggesting that the TiO2 layer is mostly in an amorphous state. The annealed sample shows clearly crystalline signature of anatase. The inset top-view SEM image confirms that the TiO2 nanotube arrays retained its structural integrity after annealing. TiO2 has a strong affinity for the carboxylate group, as demonstrated previously with a variety of sensitizing dyes [7]. Thiol and amine groups, on the other hand, bind strongly to CdSe nanoparticles [8]. Therefore, HOOCCH2 SH was used as a bifunctional linker to attach CdSe QDs onto the TiO2 films. The assembly of CdSe QDs onto TiO2 tubes was confirmed by
Fig. 1. UV-vis absorption spectra (a) and fluorescence emission spectra (b) of CdSe QDs in toluene, with the particle size of 3 nm (solid line) and 4.5 nm (dotted line). The absorption spectra of the TiO2 film (c). The inset in (a) is the TEM images of CdSe QDs with size of 3 nm (left) and 4.5 nm (right).
Fig. 2. XRD patterns of as prepared TiO2 film (a) and that after being annealed at 450 ◦ C for 3 h (b). Reference peaks for Ti substrate are annotated T and reference peaks for anatase are annotated with A. The inset in (b) shows an SEM top-view image of the TiO2 nanotubes after annealing.
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H.-Y. Si et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
Fig. 3. AFM topographic images of a TiO2 nanotube array (a) and QDs-coated TiO2 film (b).
Fig. 4. The voltage dependence of the photocurrent measured using a 500 W Hg lamp for the unannealed (a) and annealed (b) samples of TiO2 .
AFM. The AFM image of bare TiO2 film (Fig. 3a) shows regular pore topography. The average pore size found in AFM is around 100 nm which is slight larger than the pore size obtained from SEM. The slight difference in pore size obtained from AFM and SEM may be attributed to the tip-convolution effect of the AFM. After assembly of the 4.5 nm CdSe QDs, the topography of the TiO2 films becomes granular like (Fig. 3b), confirming the formation of a full layer of CdSe QDs. The particle sizes of assembled QDs were estimated by AFM to be between 30 nm and 50 nm, which is much larger than the particle sizes. The large particle sizes obtained by AFM are mainly attributed to the tip convolution effect, while slight aggregation of the QDs might also contributed the apparent large particle size. A few large spots were observed in the Fig. 3(b) which is due to contamination. The photocurrent measurement under UV irradiation (Fig. 4) shows a dramatic increase of photocurrent after annealing, indicating that it is essential to convert the TiO2 into anatase phase for photo-energy conversion. Fig. 5 illustrates the photo-response of TiO2 films after modified by the CdSe QDs. Upon light illumination, very little photocurrent was recorded for the pure TiO2 film at wavelengths longer than 300 nm. In contrast, the CdSe QDs-coated TiO2 films show a prompt rise in photocurrent. For comparison, at 500 nm wavelength, the unmodified TiO2 film gives rise a photocurrent only around 0.07 mA/cm2 , while the TiO2 –CdSe (4.5 nm) sample is around 1.5 mA/cm2 and the TiO2 –CdSe (3.0 nm) reaches 1.8 mA/cm2 . The reason for the very small photocurrent response from unmodified TiO2 film is
that it adsorbs light only below 380 nm while our illumination system gives very low light intensity below 400 nm. Fig. 5 also shows a higher photocurrent response from the 3.0 nm CdSe QDs than that from the 4.5 nm CdSe QDs. The reason for the size dependent sensitizing effect is not fully understood at the moment. One possible reason is that the smaller QDs may coat the TiO2 better than the bigger ones, and have a higher efficiency for electron injection to the TiO2 . Another possible reason is that the conduction band of the 3.0 nm CdSe QDs is higher than that of the 4.5 nm CdSe QDs due to the quantum size effect. The
Fig. 5. Photocurrent action spectra of (a) TiO2 and 4.5 nm (b) and 3.0 nm (c) QDs-coated TiO2 .
H.-Y. Si et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
higher energy of conduction band makes it more effective to inject charge to the conduction band of TiO2 . 4. Conclusion Highly ordered TiO2 nanotube films were fabricated by a simple anodic oxidization process. The TiO2 nanotubes can be converted into anatase phase by annealing in air. Attachment of CdSe QDs significantly extends the photo-response of the TiO2 nanotubes in the visible region. The smaller CdSe QDs studied in this work serves as a better sensitizer than the larger ones. The reason for such size effect requires further investigation. It is believed that the CdSe QDs sensitized TiO2 nanotubes structures can be used in fabricating high efficiency photovoltaic devices. Acknowledgments The authors acknowledge the financial support from the Program for New Century Excellent Talents in University (NCET), National Natural Science Foundation of China (NSFC,
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20503011, 20621091), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20050730007), Key Project for Science and Technology of the Ministry of Education of China (106152), Fund of National Key Lab. of Vacuum and Cryogenics Technology and Physics, Lanzhou Institute of Physics (5145020105JW2301). References [1] Fujishima, Nature 238 (1972) 37. [2] R. Plass, S. Pelet, J. Krueger, M. Gratzel, U. Bach, J. Phys. Chem. B 106 (2002) 7578. [3] M. Paulose, K. Shankar, O.K. Varghese, G.K. Mor, B. Hardin, C.A. Grimes, Nanotechnology 17 (2006) 1446. [4] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. [5] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 183. [6] W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854. [7] M.K. Nazeeruddin, A. Kay, I. Rodicio, B.H. Baker, E. Mueller, P. Liska, N. Vlachopoulos, M. Graetzel, J. Am. Chem. Soc. 115 (1993) 6382. [8] T. Cassagneau, T.E. Mallouk, J.H. Fendler, J. Am. Chem. Soc. 120 (1998) 7848.
Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
Photoelectrochemical response from CdSe-sensitized anodic oxidation TiO2 nanotubes Hua-Yan Si, Zhen-Hong Sun, Hao-Li Zhang ∗ State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China Received 31 October 2006; accepted 29 April 2007 Available online 7 June 2007
Abstract Uniform TiO2 nanotube arrays were fabricated on titanium substrates by self-organized anodic oxidation. CdSe quantum dots (QDs) were assembled onto TiO2 nanotube layers, which serve as sensitizers. The photocurrent measurement shows that annealing is essential to increase the anatase phase and hence increase the photocurrent. Attachment of CdSe QDs dramatically increased the photocurrent of TiO2 nanotubes in the visible region. The photo-response of CdSe sensitized TiO2 also shows strong dependence to the QDs size. The possible mechanism for energy transfer between the CdSe QDs and the TiO2 nanotubes has been discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 nanotubes; CdSe; Quantum dot; Sensitizer; Solar cell
1. Introduction Nanoscale titania (TiO2 ) is of great interest mainly due to its unique properties, such as high photoactivity and size dependent optical properties [1]. However, TiO2 is a wide band gap semiconductor (band gap around 3.2 eV), which adsorbs light only in the UV region. To overcome this problem, various materials, like dyes and metallic nanoparticles, have been used as sensitizer to increase the photoactivity of TiO2 in the visible range. Semiconductor quantum dots (QDs), such as CdS, PbS, and CdSe, have been the subject of considerable interest for sensitizers in dye-sensitized solar cells (DSSCs) as an alternative to organic dyes [2]. In the past, QD-sensitized TiO2 solar cells usually consist of a mesoporous anode formed by a sintered film of anatase TiO2 . In such a device, the photo-conversion efficiency is mainly limited by the slow percolation of electrons through a random polycrystalline network and the poor absorption of low energy photons by available QDs. The above problems may be solved by using an electrode consists of highly ordered TiO2 nanotubes. In this work, we explored the sensitizing effects of CdSe QDs to the TiO2 nanotubes. It
∗
Corresponding author. Tel.: +86 931 8912365; fax: +86 931 8912365. E-mail address: [email protected] (H.-L. Zhang).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.160
is expected that the CdSe QDs sensitized TiO2 nanotubes have some advantages compared with the conventional systems. First, the regular pore structure and relative big pores size allow the CdSe QDs to be uniformly coated in the porous TiO2 matrix. Second, the arrangement of the highly ordered perpendicular aligned titania nanotube-array permits directed charge transfer along the length of the nanotubes from the solution to the conductive substrate, thereby reducing the losses incurred by charge-hopping across the nanoparticles grain boundaries. The above features make this morphology desirable for photovoltaic applications [3,4]. 2. Experimental UV-vis absorption spectra were recorded using a T6 UV-Vis spectrometer (Purkinje General, China). Fluorescence measurements were made using a LS55 fluorescence spectrometer (PE, USA). The electrochemical set-up consisted of a three-electrode system in a quartz cell, using a saturated calomel electrode (SCE) reference electrode. The measurement was conducted using a CHI660B electrochemistry work station (CHI, USA). A 500 W Hg lamp was used to measure the photocurrent of TiO2 film in the UV region. Monochromatic light was obtained using a WDL 30 monochromator (BOIF, China) equipped with a 500 W Xe lamp and a mechanical chopping. Atomic force microscopy (AFM)
H.-Y. Si et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
measurement was performed on a P-47 SPM (NT-MDT, Russia) using tapping mode. Scanning electron microscopy (SEM) was measured using a JSM-6380LV (JEOL, Japan). The colloidal CdSe QDs were prepared using previously reported method [5]. TiO2 film was prepared by anodizing a 150 m thick titanium sheet (99.8%) in 0.5% HF for 0.5 h. The obtained nanotube arrays were subsequent annealed in ambient at 450 ◦ C for 3 h. The TiO2 films were dipped into an acetonitrile solution of thioglycolic acid for 12 h, then washed with both acetonitrile and toluene before being transferred to a glass vial containing a CdSe QDs toluene solution. After being immersed in the CdSe solution for 12 h, the TiO2 films were washed with toluene and dried in vacuum. 3. Results and discussion The sizes of the CdSe QDs are 3 nm and 4.5 nm in diameter, respectively, as measured by TEM (Fig. 1a inserts). The two QDs exhibit transitions in the visible range with sharp band edge absorption at 550 nm and 576 nm, respectively (Fig. 1a). The correlation between the particle sizes and their band edge positions are in good agreement with previous reported [6]. The fluorescence emission spectra shows that the two CdSe particles
605
exhibit band edge emission (480 nm excitation) at 567 nm and 609 nm (Fig. 1b), respectively. The TiO2 nanotube films absorb UV light at 232 nm and 309 nm (Fig. 1c), corresponding to a band gap of 3.2 eV. SEM measurement (Fig. 2b insert) shows that the TiO2 layers formed by anodization treatment consists of nanotube arrays with a uniform tube size around 80 nm in diameter and have a wall thickness around 10 nm. Cross section image shows that the thickness of the TiO2 nanotube film is approximately 500 nm. The crystal structure of the TiO2 layer was identified using X-ray diffraction (XRD). Fig. 2 shows the XRD patterns of the as prepared TiO2 film and that after annealing. The as-prepared TiO2 shows only peaks corresponding to the Ti substrate, suggesting that the TiO2 layer is mostly in an amorphous state. The annealed sample shows clearly crystalline signature of anatase. The inset top-view SEM image confirms that the TiO2 nanotube arrays retained its structural integrity after annealing. TiO2 has a strong affinity for the carboxylate group, as demonstrated previously with a variety of sensitizing dyes [7]. Thiol and amine groups, on the other hand, bind strongly to CdSe nanoparticles [8]. Therefore, HOOCCH2 SH was used as a bifunctional linker to attach CdSe QDs onto the TiO2 films. The assembly of CdSe QDs onto TiO2 tubes was confirmed by
Fig. 1. UV-vis absorption spectra (a) and fluorescence emission spectra (b) of CdSe QDs in toluene, with the particle size of 3 nm (solid line) and 4.5 nm (dotted line). The absorption spectra of the TiO2 film (c). The inset in (a) is the TEM images of CdSe QDs with size of 3 nm (left) and 4.5 nm (right).
Fig. 2. XRD patterns of as prepared TiO2 film (a) and that after being annealed at 450 ◦ C for 3 h (b). Reference peaks for Ti substrate are annotated T and reference peaks for anatase are annotated with A. The inset in (b) shows an SEM top-view image of the TiO2 nanotubes after annealing.
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H.-Y. Si et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
Fig. 3. AFM topographic images of a TiO2 nanotube array (a) and QDs-coated TiO2 film (b).
Fig. 4. The voltage dependence of the photocurrent measured using a 500 W Hg lamp for the unannealed (a) and annealed (b) samples of TiO2 .
AFM. The AFM image of bare TiO2 film (Fig. 3a) shows regular pore topography. The average pore size found in AFM is around 100 nm which is slight larger than the pore size obtained from SEM. The slight difference in pore size obtained from AFM and SEM may be attributed to the tip-convolution effect of the AFM. After assembly of the 4.5 nm CdSe QDs, the topography of the TiO2 films becomes granular like (Fig. 3b), confirming the formation of a full layer of CdSe QDs. The particle sizes of assembled QDs were estimated by AFM to be between 30 nm and 50 nm, which is much larger than the particle sizes. The large particle sizes obtained by AFM are mainly attributed to the tip convolution effect, while slight aggregation of the QDs might also contributed the apparent large particle size. A few large spots were observed in the Fig. 3(b) which is due to contamination. The photocurrent measurement under UV irradiation (Fig. 4) shows a dramatic increase of photocurrent after annealing, indicating that it is essential to convert the TiO2 into anatase phase for photo-energy conversion. Fig. 5 illustrates the photo-response of TiO2 films after modified by the CdSe QDs. Upon light illumination, very little photocurrent was recorded for the pure TiO2 film at wavelengths longer than 300 nm. In contrast, the CdSe QDs-coated TiO2 films show a prompt rise in photocurrent. For comparison, at 500 nm wavelength, the unmodified TiO2 film gives rise a photocurrent only around 0.07 mA/cm2 , while the TiO2 –CdSe (4.5 nm) sample is around 1.5 mA/cm2 and the TiO2 –CdSe (3.0 nm) reaches 1.8 mA/cm2 . The reason for the very small photocurrent response from unmodified TiO2 film is
that it adsorbs light only below 380 nm while our illumination system gives very low light intensity below 400 nm. Fig. 5 also shows a higher photocurrent response from the 3.0 nm CdSe QDs than that from the 4.5 nm CdSe QDs. The reason for the size dependent sensitizing effect is not fully understood at the moment. One possible reason is that the smaller QDs may coat the TiO2 better than the bigger ones, and have a higher efficiency for electron injection to the TiO2 . Another possible reason is that the conduction band of the 3.0 nm CdSe QDs is higher than that of the 4.5 nm CdSe QDs due to the quantum size effect. The
Fig. 5. Photocurrent action spectra of (a) TiO2 and 4.5 nm (b) and 3.0 nm (c) QDs-coated TiO2 .
H.-Y. Si et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 604–607
higher energy of conduction band makes it more effective to inject charge to the conduction band of TiO2 . 4. Conclusion Highly ordered TiO2 nanotube films were fabricated by a simple anodic oxidization process. The TiO2 nanotubes can be converted into anatase phase by annealing in air. Attachment of CdSe QDs significantly extends the photo-response of the TiO2 nanotubes in the visible region. The smaller CdSe QDs studied in this work serves as a better sensitizer than the larger ones. The reason for such size effect requires further investigation. It is believed that the CdSe QDs sensitized TiO2 nanotubes structures can be used in fabricating high efficiency photovoltaic devices. Acknowledgments The authors acknowledge the financial support from the Program for New Century Excellent Talents in University (NCET), National Natural Science Foundation of China (NSFC,
607
20503011, 20621091), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20050730007), Key Project for Science and Technology of the Ministry of Education of China (106152), Fund of National Key Lab. of Vacuum and Cryogenics Technology and Physics, Lanzhou Institute of Physics (5145020105JW2301). References [1] Fujishima, Nature 238 (1972) 37. [2] R. Plass, S. Pelet, J. Krueger, M. Gratzel, U. Bach, J. Phys. Chem. B 106 (2002) 7578. [3] M. Paulose, K. Shankar, O.K. Varghese, G.K. Mor, B. Hardin, C.A. Grimes, Nanotechnology 17 (2006) 1446. [4] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. [5] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 183. [6] W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854. [7] M.K. Nazeeruddin, A. Kay, I. Rodicio, B.H. Baker, E. Mueller, P. Liska, N. Vlachopoulos, M. Graetzel, J. Am. Chem. Soc. 115 (1993) 6382. [8] T. Cassagneau, T.E. Mallouk, J.H. Fendler, J. Am. Chem. Soc. 120 (1998) 7848.