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Preparation of sodium titanate nanotubes modified by CdSe quantum dots and their photovoltaic characteristics ZHANG HongMei, CHENG Ke, JI YanLing, LIU XiaoLan, LI LinSong, ZHANG XingTang & DU ZuLiang† Key Lab for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, China
Sodium titanate nanotubes have been prepared and modified chemically with CdSe quantum dots (QDs) using bifunctional modifiers (HS-COOH). Their photovoltaic characteristics have also been studied. The results indicate that the surface photovoltage response of nanotubes extends to the visible light region, and the intensity of surface photovoltage is enhanced after modification with CdSe QDs. The field-induced surface photovoltage spectroscopy (FISPS) shows that sodium titanate nanotubes have different photovoltaic response before and after modification. That is, the surface photovoltaic response of pure sodium titanate nanotubes increases with the enhancement of positive applied bias and decreases with the enhancement of negative applied bias. Meanwhile, the surface photovoltaic response of CdSe modified sodium titanate nanotubes is different from that of the pure sodium titanate nanotubes. The whole spectrum increases with the enhancement of applied bias at the first stage. However, when the applied bias reaches a certain value, the surface photovoltage response keeps increasing in some spectrum regions, while decreasing in other spectrum regions. This novel phenomenon is explained by using an electric field induced dipole model. sodium titanate nanotube, CdSe, surface photovoltage spectroscopy, diffusion mechanism, drift mechanism
1
Introduction
One-dimensional TiO2 nanomaterials have been widely investigated for their potential applications in photoca－ talysis, pollution purification, solar cells, gas sensor[1 4], and so on. Especially since the discovery of TiO2 nanotubes by Kasuga and his coworkers[5] by using the hydrothermal method in 1998, more and more researchers have paid much attention to these one-dimensional － TiO2 nanomaterials[6 13]. However, most researches focus on the structure, composition, and formation mechanism. Few reports have been found to study their prop－ erties[14 20]. Our lab prepared this kind of one-dimensional nanotubes with a small diameter of about 4－6 nm by an improved method. We have also confirmed this titanate nanotube structure with a general formula Na2Ti2O4(OH)2. The Na2Ti2O4(OH)2 nanotubes can be converted to H2Ti2O4(OH)2 nanotubes after treatment with an HCl solution[9]. H2Ti2O4(OH)2 nanotubes have
also been found to have a high photocatalyst activity[21], and they show a novel strong visible light absorption and luminescence[22,23]. A great deal of attention has been devoted to dyesensitized solar cells (DSSC) made from nanostructured TiO2 films because they have an energy conversion efficiency exceeding 10% and excellent stability[24]. Recently, Adachi and his coworkers reported that Ru-dye sensitized thin-film electrodes made of TiO2 nanotubes showed a doubled short-circuit current density than that of electrodes made from TiO2 nanoparticles (P-25)[25]. However, as we know, one-dimensional TiO2 nanomaterials as a wide band gap semiconductor material can absorb UV light only, and this limits its application to some extent. On the other hand, semiconductor Received June 17, 2007; accepted August 10, 2007 doi: 10.1007/s11426-008-0023-6 † Corresponding author (email:
[email protected]) Supported by the Program for New Century Excellent Talents in University (Grant No. NCET-04-0653), 973 Plan (Grant No. 2007CB616911) and the National Natural Science Foundation of China (Grant Nos. 20371015 and 90306010)
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QDs, such as CdS, PbS, and CdSe, have also been the subject of considerable interest as light harvesters in － DSSCs as an alternative to organic dyes[26 29]. The use of semiconductor QDs as the sensitizers in solar cell applications has some advantages[30]. Firstly, the energy gap of semiconductor QDs can be tuned by controlling their sizes so that the absorption spectra can be tuned to match the spectral distribution of sunlight. Secondly, semiconductor QDs have large extinction coefficients and large intrinsic dipole moments which may lead to rapid charge separation. Thirdly, a unique potential capability of a QD-sensitized solar cell is the production of quantum yields greater than unity due to the inverse Auger effect (impact ionization)[31,32]. However, there are few reports on modification of sodium titanate nanotubes with semiconductor quantum dots chemically. In this research, CdSe QDs were linked to sodium titanate nanotubes using bifunctional surface modifiers (HS-COOH), and their photoelectric properties were also demonstrated. We hope that this can provide new materials and experimental bases in the solar cell conversion application.
2 Experimental 2.1 Materials and equipments Raw materials, including TiO2, NaOH, ethanol, thiolacetic acid, toluene, and acetonitrile, are commercially available and of reagent grade, which were used directly without further purification. The morphology of the nanotubes was recorded on a JEM-100 CX Ⅱ transmission electron microscope (TEM). Photoluminescence (PL) properties of samples were obtained on an SPEX F212 spectrometer. The surface photovoltaic characteristics were measured by surface photovoltaic spectroscopy (SPS) using a light source-monochromator-lockin detection technique. The light source is a 500W xenon lamp and the tin-doped indium oxide (ITO) conductive glass used as the detection electrode. The samples were measured with a solid junction photovoltaic cell with a sandwich structure (ITO/sample/ITO). 2.2 Preparation of CdSe/ sodium titanate nanotube composite TiO2 nanoparticles were treated in a 10 mol·L−1 NaOH solution at 120℃ for 24 h. When the dispersion cooled down to room temperature, it was washed with de-
ionized water to pH = 8.46 and dried at 60℃. We obtain the pure sodium titanate nanotubes. CdSe QDs with a diameter about 5.5 nm were prepared using the method of Peng Xiaogang et al.[33]. Sodium titanate nanotubes were modified by CdSe QDs following the method in ref. [27]. Firstly, 0.023 g pure sodium titanate nanotubes were dissolved in the solution of 20 mL thiolacetic acid and 40 mL acetonitrile by ultrasonic dispersion. After being stirred for 48 h, sodium titanate nanotubes modified with thiolacetic acid were obtained by washing with toluene. Secondly, this sample was mixed in a toluene solution with CdSe QDs by ultrasonic dispersion. After being stirred for 48 h, this solution was centrifuged and then the sample obtained was dissolve in ethanol and water. Finally, sodium titanate nanotubes modified by CdSe QDs were obtained by filtration.
3 Results and discussion 3.1 Morphology and composition Figure 1 is the TEM images of pure sodium titanate nanotubes (a) and sodium titanate nanotubes modified by CdSe QDs (b). From Figure 1(b), it can be seen that there are nanoparticles on sodium titanate nanotubes while there is no nanoparticle that can be seen in Figure 1(a). These nanoparticles can be assumed as CdSe QDs which were linked to sodium titanate nanotubes through the chemical modification process. However, the diameter of these nanoparticles is larger than the single CdSe QD used. This is may be due to congregation of some CdSe QDs during the modification process. The energy dispersive X-ray spectroscopy (EDS) indicates that there are Cd and Se elements in sodium titanate nanotubes modified by CdSe QDs, which confirms that the nanoparticles observed in the TEM images are CdSe QDs. The EDS also displays that there are C and S elements. The C and S elements may be from the thiolacetic acid modifier used. Due to a great number of OH groups on the sodium titanate nanotube surface[9,17,34], sodium titanate nanotubes can easily be bonded to the carboxylate group. CdSe nanoparticles can easily be bonded with thiol and － amine groups[35 37]. Thus, we can modify CdSe QDs on the sodium titanate nanotubes using the bifunctional surface modifier HS-COOH. Scheme 1 shows that CdSe QDs are chemically linked to sodium titanate nanotubes. Pure sodium titanate nanotubes, sodium titanate nanotubes modified by thiolacetic acid, and sodium titanate
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Figure 1 TEM images of pure sodium titanate nanotubes (a) and sodium titanate nanotubes modified with CdSe QDs (b).
nanotubes modified by CdSe QDs were characterized by FT-IR spectroscopy. Figure 2 shows their FT-IR spectra. It can be seen that the deformation and stretching vibrations of the OH group of physisorbed water at 1618 and 3423 cm−1 are present in the FT-IR spectrum of pure sodium titanate nanotubes. Because of the strong interactions between Ti ions and OH groups within the tubular structure, a shoulder peak at 3208 cm−1 from Ti-OH bonds is observed[38]. Comparing curves 1 and 2 in Figure 2, it can be seen that there are stretching vibrations of the S-H group of thiolacetic acid at 2561 cm−1 and there is a dissymmetric peak of carboxylate at 1576 cm−1[39 40]. These results indicate that thiolacetic acid is linked to sodium titanate nanotubes through –COOH. Meanwhile, the stretching vibrations of the S-H group of thiolacetic acid at 2561 cm−1 disappear in the FT-IR spectra in sodium titanate nanotubes modified with CdSe QDs, which indicates that thiolacetic acid is linked to CdSe through —SH. The FT-IR results prove that the CdSe QDs are chemically modified on the sodium titanate nanotubes through the HS-COOH.
Figure 2 FT-IR spectra of pure sodium titanate nanotubes (1), sodium titanate nanotubes modified with HS-COOH (2), and sodium titanate nanotubes modified with CdSe QDs (3).
3.2 Photovoltaic characteristics of the nanocomposite Figure 3 shows the absorption spectra of pure sodium titanate nanotubes, sodium titanate nanotubes modified with CdSe and pure CdSe QDs. The absorption edge of sodium titanate nanotubes is at 375 nm, which can be
Scheme 1 Illustration of the CdSe QDs modification to the sodium titanate nanotubes surface with a bifunctional modifier. 978
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Figure 3 Absorption of pure sodium titanate nanotubes (1), sodium titanate nanotubes modified with CdSe (2) and pure CdSe QDs (3).
attributed to the band-to-band transition. While CdSe has three peaks centered at 473 nm, 538 nm, and 570 nm, which are three exciton absorption peaks. After being modified with CdSe QDs, the absorption region of sodium titanate nanotubes extends from UV to the visible light region. A red-shift of the absorption onset of CdSe in sodium titanate nanotubes modified with CdSe contrast to pure CdSe QDs is observed because of the congregation of some CdSe QDs during the preparation process, which is consistent with our TEM result. Surface photovoltaic spectroscopy (SPS) is a sensitive tool to investigate the charge transfer process at the surface and interface. There is no SPS signal that can be obtained for the pure CdSe QDs. The SPS of pure sodium titanate nanotubes and sodium titanate nanotubes modified with CdSe QDs without an external electrical field are illustrated in Figure 4(a). It shows that the
photovoltaic response of sodium titanate nanotubes modified with CdSe QDs extends from the UV light region to the visible light region contrast to the pure sodium titanate nanotubes. Moreover, the intensity of the photovoltaic response is also enhanced. CdSe is a semiconductor with a narrow band gap which can absorb visible light, so the photovoltaic response can extend to the visible region accordingly. At the same time, the valence band and conduction band of CdSe are both higher than those of sodium titanate nanotubes. When the energy of incident light is larger than the band gap energy of CdSe, the electrons in the valence band of CdSe can be excited to the conduction band. Then the excited electrons can transfer from the conduction band of CdSe to the conduction band of sodium titanate nanotubes, while holes are left in the valence band of CdSe. Therefore, the separation efficiency of electron-hole pairs can be improved effectively. As we know, PL and SPS are two opposite photo-generated charge carriers’ transition processes. That is, SPS response will be obtained by the efficient separation of electron-hole pairs while PL is achieved by the recombination of electron-hole pairs; the PL efficiency will be enhanced by more and more electron-hole pairs recombined. Figure 4(b) shows the PL spectra of sodium titanate nanotubes modified with CdSe and pure CdSe QDs. As can be seen from Figure 5(b), the PL intensity of CdSe modified sodium titanate nanotubes decreases greatly compared with pure CdSe QDs. This result indicates that the separation efficiency of electron-hole pair is improved effectively when CdSe is linked to sodium titanate nanotubes, which is consistent with our SPS results.
Figure 4 (a) SPS of pure sodium titanate nanotubes (1) and sodium titanate nanotubes modified with CdSe QDs (2); (b) PL of sodium titanate nanotubes modified with CdSe QDs (1) and pure CdSe QDs (2).
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Field-induced surface photovoltage spectroscopy (FISPS) is a technique which combines the field-effect principle with SPS. With an external voltage applied to the two sides of the sample, the mobile direction and the diffusion length of the photogenerated charge carriers can be altered. Moreover, the space charge density and the electronic state of the sample can be changed. Thus, the external bias will have direct effects on the SPS intensity and the photovoltaic characteristics. Figure 5 shows the FISPS results of pure sodium titanate nanotubes and sodium titanate nanotubes modified with CdSe QDs. As can be seen from Figure 5, the FISPS results of pure sodium titanate nanotubes and sodium titanate nanotubes modified with CdSe QDs are different. The higher the external electric field applied, the higher intensity of FISPS of pure sodium titanate nanotubes may be obtained. At the same time, the photovoltage re-
sponse extends to the visible region. While the FISPS of sodium titanate nanotubes modified with CdSe QDs increases with the enhancement of the external electric field at the first stage. However, when the external electric field reaches a certain value, the FISPS increases in some wavelength regions while it decreases in other wavelength regions. We can explain this phenomenon using an electric field induced dipole model[41], as shown in Figure 6. The nanotubes easily absorb oxygen and water in air due to many surface states existing in the sodium titanate nanotubes surface[20], which results in so many — OH and O2− groups on the surface of nanotubes. Under incident light, electrons in the valence band of sodium titanate nanotubes transferred to the conduction band and then move toward the un-illuminated side, while holes accumulate at the illuminated side of the sample
Figure 5 FISPS of pure sodium titanate nanotubes (a) and sodium titanate nanotubes modified with CdSe QDs (b).
Figure 6 Diagram of change of the built-in field of dipoles with the change of a positive electric field. (a) Diffusion photovoltage; (b) built-in field of dipole enhanced by a positive electric field, where the photovoltage is controlled by both diffusion and drift mechanisms; (c) built-in field of dipole decreased by a positive electric field and the diffusion photovoltage.
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localized by —OH and O2 . Under this situation, the photovoltage is controlled by the diffusion mechanism (see Figure 6(a)). The difference in concentration of photogenerated carriers in sodium titanate nanotubes is the driving force to generate photovoltage. The photogenerated electron-hole pairs are captured on the surface of sodium titanate nanotubes. Moreover, the localized electron-hole pairs can be induced by the applied external electric field, and then form a dipole whose built-in field direction is reverse to that of the applied external electric field (see Figure 6(b)). Under a positive electric field, photogenerated electrons diffuse towards the un-illuminated side. At the same time, they drift towards the direction of the built-in field of the dipole. Therefore, the intensity of SPS is enhanced. That is, the diffusion mechanism is controlled by the difference in concentration of photogenerated charge, while the drift mechanism is controlled by the built-in field of sodium titanate nanotubes. Furthermore, under this situation, the directions of diffusion and drift are consistent. While under a negative electric field, the diffusion direction and the drift direction of photogenerated electrons are opposite, so the intensity of SPS is weakened. The amount of photogenerated electron-hole pairs is determined by the state density under certain wavelengths of incident light. That is, there is a maximum for the built-in field intensity of dipoles under certain wavelength of incident light. When the induced built-in field intensity of dipoles reaches its maximum, it gradually decreases as the applied external electric field increases (see Figure 6(c)). Comparing Figure 5(a) and Figure 5(b), we can find that the SPS intensity of sodium titanate nanotubes modified by CdSe QDs increases in some light regions while decreases in some light regions when the applied bias is 1.5 V. However, the SPS intensity of sodium titanate nanotubes is enhanced with the increase of applied bias. 1
This illustrates that the density of surface states of sodium titanate nanotubes is reduced. At the same time, Li TieJin[42] and his coworkers reported that the density of surface states can be reduced when the TiO2 film was sensitized with QDs. Based on the dipole model, under different wavelengths of incident light, the applied bias is different when the build-in field reaches the maximum. Thus, the applied bias is different when the SPS reaches its maximum under different wavelengths of incident light. This can explain the FISPS of sodium titanate nanotubes modified with CdSe QDs. On the other hand, according to the principle of FISPS[43,44], the charge carriers populating in surface states are localized, and the electronic transition is forbidden. Consequently, the SPS response originating from surface states is weak and even undetectable without an external field. However, if an external field is applied, the energy band can tilt, and its optical constant changes for the surface states. These two effects enlarge the transition momentum of local states and increase the probability of electronic transitions. As a result, the enhanced surface photovoltage responses can be observed under the external field. Thus, the new SPS peak under an external field can be attributed to the surface state transition of sodium titanate nanotube.
4 Conclusions CdSe QDs were modified to sodium titanate nanotubes using bifunctional surface modifiers (HS-COOH). The photoelectric results indicate that the SPS of sodium titanate nanotubes after being modified with CdSe QDs extends to a long wavelength, and the intensity of SPS is enhanced compared with that of pure sodium titanate nanotubes. Furthermore, their FISPS results are different. We interpret this phenomenon by the diffusion mechanism and the drift mechanism.
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