Structure Transformation and Photoelectrochemical Properties of TiO2

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Structure Transformation and Photoelectrochemical Properties of TiO2 Nanomaterials Calcined from Titanate Nanotubes J. Qu, X. P. Gao,* G. R. Li, Q. W. Jiang, and T. Y. Yan Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed: December 4, 2008; ReVised Manuscript ReceiVed: January 1, 2009

Hydrothermally synthesized titanate nanotubes are calcined at different temperatures (400-700 °C) in air to obtain TiO2(B) nanotubes, anatase nanorods, and anatase nanoparticles. The morphology and structure of the prepared samples are characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). These samples with different morphologies and structures are used to fabricate photoelectrodes for dyesensitized solar cells (DSSCs). It is found from current-voltage curve (I-V) measurements that the DSSC with anatase nanorods calcined at 600 °C shows much better photoelectrochemical performance than those using other samples, with a photovoltaic conversion efficiency of 7.71%. Electrochemical impendence spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated voltage spectroscopy (IMVS) are used to further investigate the kinetics process of TiO2 film electrodes. The results indicate that the charge-transfer resistance and lifetime depend on the morphology and structure transformation of the synthesized TiO2 samples. The anatase nanorods, obtained from the calcination of titanate nanotubes at 600 °C, have a lower charge-transfer resistance and a longer electron lifetime, implying lower electronhole recombination and a higher charge-collection efficiency. I. Introduction Dye-sensitized solar cells (DSSCs) have received considerable attention as a potential cost-effective alternative to conventional silicon semiconductor solar cells.1-3 A typical DSSC consists of a dye-sensitized semiconductor electrode, redox electrolyte, and counter electrode. Once dye molecules absorb light, the excited dye injects electrons to the semiconductor. At the same time, the oxidized dye cation is reduced by the redox electrolyte, which competes with the recombination of injected electrons. Electrons are collected at the semiconductor electrode, pass through the external circuit, and then reenter the cell at the counter electrode to reduce the oxidized electrolyte.4 In these processes, electron-transfer kinetics, which mainly competes among electron injection, recombination, and regeneration, plays an important role in the efficiency of DSSCs. Certainly the electron-transfer kinetics depends strongly on the morphology and crystal structure of semiconductor materials. Therefore, titania with various polymorphs have been investigated as photoelectrode materials. It has been reported that the DSSC with oriented anatase nanotube arrays has significantly higher charge-collection efficiencies than that using nanoparticles, due to a much slower electron recombination.5-7 Also, it is proposed that the longer electron lifetime in TiO2 nanotube electrodes, compared to nanoparticle electrodes, can result in an increasing electron density to obtain higher efficiency.8 Nanorods and nanowires with cylindrical geometry can reduce intercrystalline contacts between grain boundaries to improve electron transport so that more electrons, surviving from the charge recombination, can lead to the increase of the photocurrent and the conversion efficiency.9-16 However, a better understanding of the potential effect of the morphology and structure of semiconductors on * To whom correspondence [email protected].

should

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addressed.

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electron-transfer kinetics is still necessary for further improving DSSC performance. In this work, we calcined protonated titanate nanotubes at different temperatures to make the morphology and structure transformation to TiO2(B) nanotubes, anatase nanorods, and anatase nanoparticles. The photoelectrochemical properties of the TiO2 polymorphs were investigated. It is found that the DSSC with anatase nanorods calcined at 600 °C have the highest photovoltaic conversion efficiency. This can be attributed to a lower charge-transfer resistance and a longer lifetime for the sample calcined at 600 °C, as proved by electrochemical impendence spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated voltage spectroscopy (IMVS).17-23 II. Experimental Section 1. Sample Preparation and Characterization. The samples were prepared via the hydrothermal reaction of titanium with NaOH solution according to our previous works.24,25 Anatase TiO2 was mixed with 10 M NaOH solution. After being sonicated in an ultrasonic bath for 0.5 h, the resulting suspension was transferred to a Teflon-lined autoclave and heated to 150 °C for 48 h. The solid product was recovered and rinsed with distilled water, 0.1 M HCl, and distilled water until pH = 7. After being dried at 60 °C for 2 days, the as-prepared sample was calcined at 400, 500, 600, and 700 °C for 2 h in air. The structure and morphology of the resultant samples were detected using X-ray diffraction (XRD, Rigaku D/max-2500) and transmission electron microscopy (TEM, FEI Tecnai 20). N2 adsorption data were measured using a NOVA 2000e (Quantachrome) instrument, and the specific surface area was evaluated by the BET method. UV-vis absorption spectra were recorded on a Varian Cary 100 spectrophotometer. 2. Photoelectrochemical Performance. The titanate nanotubes were mixed with ethanol and stirred to obtain a fluid

10.1021/jp810692t CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

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Figure 2. XRD patterns of titanate nanotubes calcined at different temperatures.

Figure 1. TEM micrographs of titanate nanotubes calcined at 400 °C (a), 500 °C (b), 600 °C (c), and 700 °C (d) in air for 2 h.

mixture. Then a film was made by the doctor blade method on a FTO (fluorine-doped tin oxide) conductive glass (LOF, TEC15, 15 Ω/square). After being calcined at 400, 500, 600, and 700 °C in air for 2 h, the films were soaked in an ethanol solution of N-719 dye for about 24 h. The dye-adsorbed TiO2 electrode was assembled into a sandwich-type cell with a counter electrode (platinum-sputtered FTO glass) by clamps. A drop of electrolyte solution (0.5 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine in acetonitrile) was introduced into the clamped electrodes. The active area of the electrode was about 0.25 cm2. Photocurrent-voltage curves were measured with a Zahner IM6ex electrochemical workstation using a Trusttech CHF-XM500W source under simulated sun illumination (Global AM 1.5, 100 mW cm-2). Electrochemical impedance spectra (EIS) were conducted using a Zahner IM6ex electrochemical workstation. A perturbation of 10 mV was applied, and data were collected from 100 kHz to 0.1 Hz. Intensity-modulated photovoltage spectroscopy (IMVS) under open-circuit conditions and intensitymodulated photocurrent spectroscopy (IMPS) under short-circuit conditions were carried out using the Zahner CIMPS-2 system. The lamp house was provided with a blue-light-emitting diode (470 nm) driven by a PP210 (Zahner) frequency response analyzer. The LED provided both the DC and AC components of the illumination. The AC component of the current to the LED generated a modulation (10%) superimposed on the DC light intensity with the frequency range from 1000 to 0.01 Hz for IMPS and IMVS. III. Results and Discussion As shown in Figure 1a, the sample calcined at 400 °C keeps the nanotube morphology with a length of several hundred nanometers, outer diameter of about 10-15 nm, and inner diameter of about 6-8 nm. Hollow nanotubes of titanate are converted to solid nanorods when calcined at 500 °C (Figure 1b) with a shortened length, as reported in our previous work.25 The interference fringe spacing of the nanorods, which was measured from the high-resolution transmission electron microscopy (HRTEM) image of the sample (the insert in Figure

1b), is about 0.35 nm, corresponding to the interplanar distance of the (101) plane in the anatase phase. After calcination at 600 °C, the nanorods are much thicker and shorter with a diameter of 15-30 nm and a length of about 100 nm. Larger particles with a size of 20-80 nm are obtained when calcined at 700 °C (Figure 1d). The transformation of the crystal structure of the samples is detected by XRD (Figure 2). All the diffraction peaks of the sample calcined at 400 °C can be indexed to TiO2(B) (JCPDS 74-1940). With increasing calcination temperature, only the anatase phase is found in XRD patterns for the other three samples. Meanwhile, the peak intensities increase obviously with increasing temperature, indicating the improvement of crystallization of the anatase phase and the increment of grain sizes. UV-vis absorption spectra of the samples are shown in Figure 3a. As is well-known, TiO2 mainly absorbs UV light under 380 nm wavelength and has no photoresponse in the visible light region. The UV absorbance of TiO2(B) is obviously higher than that of anatase nanorods and nanoparticles. The enhanced light absorption of TiO2(B) is attributed to the fact that it has more surface defects (relatively poor crystallization as shown in XRD patterns) than the others. Figure 3b shows the UV-vis absorption spectra of N-719 dye adsorbed on the different sample films. Clearly, two main absorption peaks appear in all the UV-vis absorption spectra. The peak in the UV light region results from TiO2, and the other peak at about 535 nm is ascribed to the absorption of dye N-719. Dyeabsorbed TiO2(B) nanotubes show the highest absorption intensity in the visible light region, and the other samples show the following order: anatase nanorods (500 °C) > anatase nanorods (600 °C) > anatase nanoparticles (700 °C). The results suggest TiO2(B) can adsorb a relatively large amount of N-719 dye, due to more surface defects and higher specific surface area (Figure 4). For the samples calcined at higher temperatures, the specific surface area decreases rapidly, due to the crystal structure and morphology transformation from tubes to rods, as shown by XRD and TEM. Figure 5 shows the photocurrent-voltage characteristics (I-V) of the DSSC with the different samples calcined at 400, 500, 600, and 700 °C. It can be clearly found that both shortcircuit current (Jsc) and open-circuit voltage (Voc) of the corresponding DSSC are improved with the increase of the calcination temperature. Jsc and Voc of the DSSC with anatase nanorods (600 °C) are up to 15.55 mA/cm2 and 0.813 V, respectively, demonstrating the best performance. In the case of anatase nanoparticles (700 °C), open-circuit voltage is kept

TiO2 Nanomaterials Calcined from Titanate Nanotubes

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Figure 5. I-V curves for titanate nanotubes calcined at different temperatures. Illumination intensity of 100 mW cm-2 with Global AM 1.5 and an active area of 0.25 cm2 were applied.

TABLE 1: Detailed Photovoltaic Parameters of DSSC Nanomaterials Calcined at Different Temperatures calcination temperature

Jsc (mA/cm2)a

Voc (V)a

FFa

η (%)a

400°C 500°C 600°C 700°C

3.03 12.52 15.55 6.99

0.633 0.782 0.813 0.813

0.55 0.66 0.61 0.61

1.05 6.46 7.71 3.47

a Jsc is the short-circuit current, Voc the open-circuit voltage, FF the fill factor, and η the photovoltaic conversion efficiency.

Figure 3. (a) Absorption spectra of titanate nanotubes calcined at different temperatures. (b) Absorption spectra of N-719 dye adsorbed on different films.

Figure 6. Nyquist plots of the DSSC with titanate nanotubes calcined at different temperatures.

Figure 4. Specific surface area of samples calcined at different temperatures.

at 0.813 V, suggesting that the two samples calcined at 600 and 700 °C have the same Fermi level. More detailed parameters are summarized in Table 1. Although nanotubes have an advantage in morphology and surface area for enhancing photoelectrochemical performance, TiO2(B) nanotubes in this work show relatively poor photovoltaic properties, and the characterization parameters (Jsc, Voc, and η values) are lower than those of anatase nanorods and nanoparticles. The crystal structure transformation to the anatase phase (>500 °C) obviously improves the photoelectrochemical performance. Anatase nanorods obtained at 600 °C show the highest photovoltaic conversion efficiency. For the DSSC using anatase

nanoparticles (700 °C), Jsc and η decrease sharply to 6.99 mA/ cm2 and 3.47%, respectively. The photoelectrochemical performances will be further discussed in combination with the EIS, IMPS, and IMVS results in the following discussions. Figure 6 shows the electrochemical impedance spectra (EIS) of the DSSC with the samples calcined at different temperatures. The impedance spectrum of the DSSC with TiO2(B) consists of only one semicircle, which is related to the charge-transfer process occurring at the TiO2/dye/electrolyte interface.26 For the anatase samples calcined at 500, 600, and 700 °C, there are three semicircles in their EIS plots, in which the middle one has a much bigger radius than the other two. Similar to TiO2(B), the big semicircles are also assigned to the charge-transfer process occurring at the TiO2/dye/electrolyte interface. The semicircle in the higher frequency region corresponds to the resistance of the Pt/redox (I-/I3-) interface charge transfer, and the one in the lower frequency region corresponds to the diffusion process of I-/I3- redox electrolytes.26,27 In the case of TiO2(B), the charge-transfer resistance at the counter electrode

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Figure 7. Short-circuit IMPS response (a) and IMVS response (b) of dye-sensitized cells with titanate nanotubes calcined at different temperatures.

TABLE 2: Detailed IMPS/IMVS Parameters of the DSSC with Nanomaterials Calcined at Different Temperatures calcination temperature

τd (ms)a

τn (ms)a

Dn (cm2/s)a

Ln (µm)a

400°C 500°C 600°C 700°C

1.83 1.18 0.94 1.47

14.2 53 106 17.7

3.5 × 10-4 5.4 × 10-4 6.8 × 10-4 4.4 × 10-4

22.3 53.6 85 27.8

a τd is the electron collection time, τn the electron lifetime/ recombination time, Dn the electron diffusion coefficient, and Ln the electron diffusion length.

and the diffusion impedance of I-/I3- redox electrolytes are too small, as compared with the charge-transfer resistance at the TiO2/dye/electrolyte interface in the EIS plot. The chargetransfer resistance at the TiO2/dye/electrolyte interface for the DSSC is clearly different, showing a temperature-dependent order: 400 °C > 700 °C > 500 °C > 600 °C. Furthermore, the charge-transfer resistance at the Pt/redox (I-/I3-) interface and diffusion impedance of I-/I3- redox electrolytes, estimated from Figure 6, have the same order. Thus, the reaction activity for the cells also follows the temperature-dependent order: 600 °C > 500 °C > 700 °C > 400 °C. This is in agreement with the order of the photoelectrochemical performance as presented in the photocurrent-voltage characteristic measurement. IMPS and IMVS were used to investigate further the electron transport and recombination processes, as shown in Figure 7. The IMPS and IMVS plots display a semicircle in the complex plane (the IMVS response curves for the samples obtained at 500 and 600 °C show an incompleted semicircle, due to the measurement range). The electron collection time (τd) and the recombination time (τn) can be estimated from the IMPS plots and the IMVS plots, respectively. Meanwhile, the electron diffusion coefficient (Dn) and the diffusion length (Ln) can be determined on the basis of film thickness, τd, and τn.28,29 The detailed parameters are listed in Table 2. It is found that τn is greatly different in the DSSC with the samples calcined at different temperatures, showing a clear order: 600 °C > 500 °C > 700 °C > 400 °C. In the case of the anatase nanorods (600 °C), the electron lifetime reaches the longest value, 106

ms (using the expressions τn ) 1/ωmin ) 1/2πfmin and fmin ) 1.5 Hz), which is over 10-fold longer than that for TiO2(B) nanotubes. On the other hand, τd changes little in the different DSSCs. Furthermore, electrons in the anatase nanorods (600 °C) have large Dn and Ln, compared to those in the other samples. This indicates that anatase nanorods with a good crystallinity are beneficial to a faster electron transport and a longer electron lifetime. In fact, it is considered that the good crystallinity and the cylindrical geometry could allow the nanorods to support radial electric fields that could keep the electrons away from the nanorods surface, thereby reducing surface electron densities and recombination.14 It is noted that both the samples calcined at 500 and 600 °C are nanorods. However, it is clear from Table 2 that the recombination time (τn) of the sample calcined at 600 °C is about two times longer than that of the sample calcined at 500 °C, due to better crystallinity and fewer defects of the nanorods obtained at 600 °C. As is well-known, fast electron transport and long electron lifetime can improve charge-collection efficiency and, thus, increase photovoltaic conversion efficiency.17,30 All these above results indicate the shorter anatase nanorods at 600 °C have the unique morphology and structure to present much better performance. However, after calcination at 700 °C, the rodlike morphology is destroyed and converted to larger anatase nanoparticles without cylindrical geometry, resulting in the poor photoelectrochemical performance. In particular, the recombination time (τn) for larger anatase nanoparticles is reduced sharply down to 17.7 ms, due to the configuration confinement of nanoparticles for charge-carrier diffusion processes, as compared with nanoparticle/nanorod electrodes.31,32 For TiO2(B) nanotubes calcined at 400 °C, the poor performance is mainly related to the serious recombination in numerous surface defects and a relatively open tunnel structure of TiO2(B).25 Therefore, the high photoelectrochemical performance of anatase nanorods can be attributed to the unique morphology and structure. IV. Conclusion After calcination at different temperatures, hydrothermally synthesized titanate nanotubes can be converted to TiO2(B) nanotubes (400 °C), anatase nanorods (500-600 °C), and anatase nanoparticles (700 °C). The DSSCs fabricated with the different TiO2 samples demonstrate the effect of the morphology and structure transformation on the photoelectrochemical properties. Anatase nanorods calcined at 600 °C show much better photoelectrochemical performance than TiO2(B) nanotubes and anatase nanoparticles, and the corresponding DSSC has the photovoltaic conversion efficiency of 7.71%. The results from EIS, IMPS, and IMVS demonstrate that the anatase nanorods calcined at 600 °C have a lower charge-transfer resistance at the TiO2/dye/electrolyte interface and provide faster electron transport and longer electron lifetime, due to its unique morphology and structure. The results are very significant for further understanding the effects of the morphology and crystalline structure of photoelectrode materials on photoelectrochemical properties. Acknowledgment. This work is supported by the 973 Program (2009CB220100) of China. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨eller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834.

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