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The preparation of high-surface-area nanocrystalline TiO2 films using easy-reaggregation particles in solution
Materials Science and Engineering B 110 (2004) 227–232
The preparation of high-surface-area nanocrystalline TiO2 films using easy-reaggregation particles in solution Hongwei Han a , Ling Zan b , Jiasheng Zhong b , Lina Zhang b , Xingzhong Zhao a,∗ a b
Department of Physics, Wuhan University, Wuhan 430072, PR China Department of Chemistry, Wuhan University, Wuhan 430072, PR China Received 1 September 2003; accepted 16 March 2004
Abstract A high-surface-area nanocrystalline TiO2 films were successfully prepared from two kinds of easy-reaggregation primary nanoparticles with the mean size of 26 nm in ethanol solution by a novel technique of quickly volatilizing solvent to fix the nanoparticles. Structure and properties of the films were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that the mean size of nanoparticles does not change with heat-treatment at 450 ◦ C and the roughness factor about 86 and 80 was obtained for 1 m thickness films. A roughness factor of 82 was found for the commercial P25 TiO2 films with the mean size of 36 nm and the same thickness. The electrochemical properties of the bare interfaces of the TiO2 film electrodes in propylene carbonate (PC) containing 0.05 M tetrabutyl ammonium bromide (TBAB) was also measured and show that the electroactive of the TiO2 (I) film is similar to P25. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles for dye-sensitized nanocrystalline solar cells. © 2004 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Nanocrystalline; Thin films; Solar cells
1. Introduction Dye-sensitized nanocrystalline solar cells have attractive features in high-energy conversion efficiency and low production cost [1–5]. As the heart of this device, the use of mesoporous nanocrystalline films of the semiconductor in the place of compact signal increases considerably the effective surface area for dye adsorption [5]. They are typically fabricated from 5 to 100 nm size metal oxide nanocrystals, sintered together to yield an inter-connected, porous structure [6]. Titanium dioxide is widely used as semiconductor material in dye-sensitized nanocrystalline solar cells, which have achieved solar to electrical energy conversion efficiencies of up to 10.4%, owing to its favorable energetics stability, low price and simple processing [7,8]. In the method of preparation of titanium dioxide films electrode, the doctor blade technique shows more fascinating and efficient. It is a simple process leading to films with controllable thickness ∗
Corresponding author. Tel.: +86-2787642784; fax: +86-2787654569. E-mail addresses: [email protected] (H. Han), [email protected] (X. Zhao). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.03.019
and surface and more photoactivity [5,9]. The size of the particles and pores making up the film is controlled by the size of the particles in the colloidal solution [1]. But it is difficult to prepare smaller nanocrystalline and larger surface areas of films with this technique using the easy-reaggregation particles in the solution. However, a nice dispersal of the particles is necessary to obtain the effective surface area in this technique. In order to prevent reaggregation of the particles, a lot of stabilizers such as acids, bases, or titanium dioxide chelating agents was added into the solution of water system [3]. It is well known that the reaggregation of the particles increases with the size decrease for the large surface energy of the nanoparticles. In our early work a novel titanium dioxide nanoparticles with low production cost was developed. But unfortunately, these particles are easy-reaggregation and delaminate obviously to two layers after lying for about 30 min in solution. In this study, we try to prepare high-surface-area nanocrystalline thin films with these nanoparticles using a novel method in a good-volatility liquid system. All the results show that the roughness factor of the films prepared with the poor dispersing titanium dioxide nanoparticles is more
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than 80 for 1 m thick TiO2 films. A favorable electrochemical behavior of the bare interfaces of the films is also observed. A commercial P25 TiO2 (the mean size is about 36 nm) is used as comparison, which has excellent disperse in the ethanol and water system. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles for dye-sensitized nanocrystalline solar cells.
2. Experiment 2.1. Materials Transparent conducting optically glass (TCO glass, fluorine-doped SnO2 coated glass, effective transmittance over 80%, sheet resistance is about 10–15 2 ) was purchased from Asahi Glass of Japan. The powders of TiO2 (I) (BET 57 m2 /g) and TiO2 (II) (BET 57 m2 /g) were prepared according to the patent [10]. Another commercial TiO2 P25 (BET 48 m2 /g) was purchased from Degussa AG of Germany. All of the other solvents and chemicals used in this work are reagent grade and used without further purification. 2.2. Preparation of nanocrystalline TiO2 films The nanocrystalline TiO2 films were prepared by the method described below [1,3]: TiO2 powders were ground in an attritor mill with ethanol containing acetylacetone for 1 h. Then more ethanol and Triton X-100 were added and continuously ground for 3 h. The conducting TCO glass was covered on two parallel edges with adhesive tape (about 40 m thickness) to control the thickness of the TiO2 film and to provide non-coated areas for electrical contact. Shaking the solution of the TiO2 adequately to produce tentatively a favorable separate, and then the suspension was applied onto the TCO glass by the doctor blade technique at 30 ◦ C. After air-drying, the TCO glass coating TiO2 films was immerged into dry ethanol for 10 min to improve the properties of the nanocrystalline films. Later, it was dried with a hot air and then fired for 30 min at 450 ◦ C in air. 2.3. Methods The X-ray diffraction (XRD) of the powders was carried out in Shimadzu CXD-3A diffractometer using Cu k␣ radiation at 40 kV and 20 mA in the region of 2θ = 20–50◦ . Apparent crystal size (ACS) was estimated through Scherrer’s equation [11] ACS =
kλ cos θβ
β = (B2 − b2 )1/2
(1) (2)
where k is the apparatus constant, and taken as 0.89; λ the wavelength of Cu k␣ line (1.542 Å); θ the Bragg’s angle; b
the instrumental constant (0.1◦ ), and B the half width in radians of the diffraction peak of the (1 0 1) planes of anatase and (1 1 0) planes of rutile. The anatase content, A, was determined as weight percentage by using the following equation [12]: A=
100 1 + 0.105 + 0.437F
(3)
where F represents the ratio of the integrated area of the rutile and the anatase diffraction peaks. The Brunauer–Emmett–Teller (BET) surface area of the powders was obtained with nitrogen adsorption in a Micromeritics ASAP 2010 nitrogen adsorption apparatus. The morphologies of nanopowders were observed in the transmission electron microscope (TEM) by using the JEM 2010 FEF (JEOL, Japan). The shape of the TiO2 nanocrystalline films were obtained with scanning electron microscope (SEM) by using the Sirion FEG (FEI, USA). The compositions of TiO2 films were analyzed by X-ray photoelectron spectroscopy (XPS), using a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh-vacuum (UHV) chambers. The pressure in the chambers during the experiments is approximately 10−7 Pa. An Mg K␣ X-ray source is used. The X-ray photoelectron spectra are referenced to the C 1s peak Eb = 284.80 eV resulting from the adventitious hydrocarbon (from the XPS instrument itself) present on the sample surface. A CHI601A electrochemical analyzer (CH Instruments Inc., USA) was used to perform cycle voltammeters in electrochemical cells with volumes of 50 mL at ambient temperature. A three-electrode cell was composed of a nanocrystalline TiO2 films on the TCO glass as working electrode (geometric surface area was 0.5 cm2 ), Pt as counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The electrolyte was 0.05 M tetrabutyl ammonium bromide (TBAB) in propylene carbonate (PC). It was maintained under an argon flow during measurements.
3. Results and discussion The XRD patterns of the three TiO2 powders before and after heat-treated at 450 ◦ C are shown in Fig. 1. The diffraction peaks were observed at 2θ = 25.3◦ , 27.2◦ , 36.1◦ , 37.8◦ , 39.3◦ , and 48.4◦ for TiO2 (I) and P25, and 2θ = 25.3◦ , 37.9◦ , and 48.4◦ for TiO2 (II). This indicates a mixture of two polymorphs of TiO2 , anatase and rutile, in TiO2 (I) and P25, but only pure anatase for TiO2 (II). The content of anatase and rutile of the powders is calculated with Eq. (3). The proportion of anatase is found to be approximately 71% for TiO2 (I), 100% for TiO2 (II) and 83% for P25. The content of anatase in the films does not change after sintering below 450 ◦ C. The apparent crystalline size (ACS) of (1 0 1) planes in anatase and (1 1 0) planes in rutile is summarized in Table 1. The result shows the ACS does not change after heat-treated. This indicates that the dimension of the particles does not
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Fig. 1. XRD patterns of the three kinds of powders TiO2 (I), TiO2 (II) and P25 before (bottom) and after heat-treated at 450 ◦ C (top).
grow up even though being heat-treated at 450 ◦ C. The similar average ACS (26 nm) was found in (1 0 1) plane of anatase and in (1 1 0) planes of rutile for TiO2 (I), and ACS in (1 0 1) plane of anatase was 30 nm for TiO2 (II). Both of them are smaller than the mean ACS 36 nm, 40 nm of P25 in (1 0 1) of anatase and in (1 1 0) planes of rutile, respectively. However, this estimation is based on the supposition that the broadening of the peaks is only due to a size effect (lattice strain was neglected). TEM result shows that the particles strongly aggregated, especially for TiO2 (I) and TiO2 (II). However, TiO2 (I) and TiO2 (II) appear obviously delaminating after lying for about 30 min in ethanol. The high-resolution TEM micrographs of the powders before heat-treated are shown in Fig. 2, from which it can be determined that the mean size of nanoparticles is 26 nm for TiO2 (I), TiO2 (II) and 36 nm for P25. It can be seen that their values are smaller than that calculated from XRD. TEM has also show that the XRD coherent domains extend on domains larger than one particle. This explains the difference between XRD and TEM observation. The morphology of these particles seems to be approximately spherical. TiO2 films were deposited on TCO glass from the TiO2 solution in ethanol. Fig. 3 shows scanning electron microscope graph on the surfaces of these nanocrystalline films. The pattern reveals the titanium dioxide films to be com-
posed of a three-dimensional network of interconnected particles having an average size of approximately 26 nm for TiO2 (I) film and TiO2 (II) film, and 36 nm for P25 film, which are similar to the results of XRD and TEM and they do not change in the process of sintering. This indicates that a high-surface-area TiO2 films was produced in spite of reaggregation of the particles for TiO2 (I) and TiO2 (II). However, The size of the particles and the pores making up the film are controlled by the size of the particles in the solution. This phenomenon indicates a favorable high-surface-area film was prepared successfully for dye-sensitized nanocrystalline solar cells with the system of ethanol solution. In fact, it is well known that the reaggregation of the particles in other systems such as water is unsuitable because we cannot produce a high-surface-area film from these systems. Owing to a good-volatility liquid solution were used, the particles is fixed before reaggregation in the process of preparation. The roughness factor of the nanocrystalline TiO2 thin films was also calculated. It was carried through the number of bis(thio-cyanato)-2,2 -bipyridyl-4,4 -dicarboxylate) ruthenium(II) (known as the N3 dye) absorbing in the TiO2 films, which has been investigated extensively due to its unmatched performance in dye staff studied as solar cell sensitizer in dye-sensitized nanocrystalline solar cells. The TiO2 films absorbed N3 with monolayer were immerged into the solution of 0.5 M sodium hydroxide for about 3 h.
Table 1 The characters of the typical nanometer TiO2 powders Content of anatase (%)
TiO2 (I) TiO2 (II) P25
71 100 83
ACS (nm) (1 0 1) in anatase
(1 1 0) in rutile
26 30 36
26 40
The mean size (nm)
BET (m2 /g)
26 26 36
57 57 48
The content of anatase and ACS were calculated from X-ray diffraction patterns, the dimension of the particles was estimated by TEM, the specific surface area of the TiO2 nanoparticles was measured GEM INI 2360 with BET.
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Fig. 2. Transmission electron microscope of the TiO2 powders no treated. The powders were supported by carbon silkscreen, which were treated with ultrasonic in ethanol for 10 min.
Fig. 3. Scanning electron microscope pattern of nanocrystalline films based on different TiO2 powders.
UV-Vis spectrophotometer was recorded and the number of N3 in the solution was calculated through the optical density of the solution at 306 nm. As each dye molecule occupies an area of 1 nm2 , the inner surface of the films with 1 m thickness is 86, 80, 82 cm2 for each 1 cm2 of geometric surface for TiO2 (I), TiO2 (II) and P25, respectively. These values are approximate though they are different dispersing character in solution. It indicates a high-surface-area film was obtained through a process of quickly fix in spite of an inhomogeneous solution was used. Although those primary particles are easy-reaggregation particles in solution and delaminate obviously to two layers after lying for about 30 min in ethanol solution, the roughness factor of TiO2 (I) and TiO2 (II) thin films is close to the value of P25 thin films. However, a cubic close packing of the mean size about 26, 26 and 36 nm spheres to a 1 m thickness layer is expected to produce 115-, 115- and 83-fold increase in surface area for TiO2 (I), TiO2 (II) and P25, respectively. These are larger than the datum calculated from absorbing the dye N3 in the TiO2 films. The difference is attributed to the necking between TiO2 particles. In addition, the large size of N3 prevents its access to very small pores, reducing the apparent surface area [1]. Fig. 4 shows the XPS survey spectra for the surface of 0.5 m-TiO2 films deposited on the F doped SnO2 TCO glass and heat-treated at 450 ◦ C for 30 min. It can be seen that the three kinds of nanostructural films contain the same
elements of Ti, O, C, Na and Sn. The photoelectron peak for Ti 2p appears clearly at a binding energy, Eb , of 458 eV, O 1s at Eb = 531 eV and C 1s at Eb = 284 eV. The Na kll and Sn 3d photoelectron peaks are at binding energies 269 and 490 eV, respectively. The XPS peaks for Na and Sn are observed in the spectrum, implying that some chemical reactions occur at the interface between the films and the SnO2 doped F TCO glass substrates and sodium in leakage
Fig. 4. XPS survey spectrum for the surface of the TiO2 films heat-treated at 450 ◦ C for 30 min.
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Fig. 5. High-resolution XPS spectra of the Ti 2p region for the surface of the TiO2 films heat-treated at 450 ◦ C for 30 min.
into the films from the glass substrates. The element C in the films is attributed to the residual carbon. The high-resolution XPS spectra of the Ti 2p taking on the surface of TiO2 films shows in Fig. 5. The Ti 2p3/2 region may be decomposed into two contributions corresponding to the different oxidation states of titanium. The symmetric peak situated at about Ti 2p3/2 = 458.8 eV is assigned to Ti(IV) (titanium in the IV oxidation state). Another small contributing peak situated at about Ti 2p3/2 = 458.1 eV, which corresponding to titanium in the III oxidation state (Ti(III)). However, the areas of binding energy at Eb = 458.8–458.1 eV of the TiO2 (I), TiO2 (II) and P25 films are 2.75, 1.64 and 2.59, respectively. It indicates that there is the largest content of Ti(III) in the films corresponding to TiO2 (II), but that in the TiO2 (I) is the lowest. In fact, this trend is corresponds to the content of anatase (Table 1). With the increase of content of anatase in the films, the rate of Ti(III) is increased. However, in the case of nanocrystalline TiO2 , electron transport is strongly influenced by trapping, resulting in remarkably slow electron transit times across the film [13]. Electron traps are usually assigned to Ti(III) sites and are believed to result from intrinsic defects as well as surface related and intercalated species [14]. This indicated that electron transport is more slow in TiO2 (II) nanostructural films than P25 and TiO2 (I). TiO2 films were cycled in propylene carbonate solutions to study charge transfer with electrolytes. Voltammograms of TiO2 nanocrystalline films in propylene carbonate solutions containing 0.05 M TBAB are presented in Fig. 6. The voltammograms show that the curve of TiO2 (II) films, which containing 100% anatase, is not distinct change. This result is similar to Koelsch’s report [15]. However, a redox reaction peak is exhibited at E = −1.07 V/SCE for TiO2 (I) and P25 films, which can be attributed to the reduction of surface Ti(IV) [7]. These indicate that a close electroactive is obtained for TiO2 (I) and P25 films in spite of their difference reaggregation primary nanoparticles in propylene carbonate containing 0.05 M tetrabutyl ammonium bromide. As showed in the high-resolution of XPS, the rate of Ti(III) is increased with the increase of content of anatase in the films
Fig. 6. Cyclic voltammetry of 1 m TiO2 (I) (solid), TiO2 (II) (dash) and P25 (dot) electrodes collected in three-electrode at a scan rate of 50 mV s−1 . The experiment employed in propylene carbonate containing 0.05 M tetrabutyl ammonium bromide. Geometric surface area was 0.5 cm2 . All voltages are shown relative to an Ag/AgCl reference electrode.
and electron transport in TiO2 (II) nanostructural films exhibits more slow than that of P25 and TiO2 (I). This can be explained for the difference of electroactive for these kinds of TiO2 films in this solution. 4. Conclusion A novel technique of preparing nanocrystalline films using a low-cost and easy-reaggregation primary nanoparticles was developed. A high-surface-area film was obtained with easy-reaggregation TiO2 solution. A roughness factor of 86 and 80 was obtained for TiO2 (I) and TiO2 (II) films with the mean size 26 nm and thickness 1 m, respectively. These are close to the roughness factor of 82 for the commercial P25 TiO2 films with the mean size of 36 nm and the same thickness. Owing to a good-volatility liquid solution was used, the particles is fixed before reaggregation in the process of preparation. Their structure and properties were investigated and show that TiO2 (I) nanocrystalline films have excellent electroactive property in the solution of 0.05 M tetrabutyl
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ammonium bromide in propylene carbonate, which is similar to P25 films. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles. Acknowledgements Transmission electron microscope and Scanning electron microscope were experimented in the center of highresolution electron microscope in Wuhan University. References [1] D. O’Regant, G.M. Grätzel, Nature 353 (1991) 737. [2] G.M. Grätzel, Nature 414 (2001) 338.
[3] M.K. Nazeeruddin, A. Kay, I. Rodicio, et al., J. Am. Chem. Soc. 115 (1993) 6382. [4] K. Kalyanasundaram, M. Gratzel, Coord. Chem. Rev. 77 (1998) 347. [5] A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 267. [6] M.K. Nazeeruddin, P. Pechy, T. Renouard, et al., J. Am. Chem. Soc. 123 (2001) 1613. [7] G. Boschloo, D. Fitzmaurice, J. Phys. Chem. 103 (1999) 2228. [8] R.L. Willis, C. Olson, B. O’Regan, et al., J. Phys. Chem. B 106 (2002) 7605. [9] K.J. Jiang, T. Kitamura, H. Yin, S. Ito, Chem. Lett. 9 (2002) 872. [10] L. Zan, J. Zhong, Q. Luo, Chinese Patent 1373089A (2002). [11] G.N. Scherrer, Gottinger Nachr. 2 (1918) 98. [12] R.A. Spurr, H. Myers, Anal. Chem. 29 (1957) 760. [13] K. Schwarzburg, F. Willig, Appl. Phys. Lett. 58 (1991) 2520. [14] R.L. Willis, C. Olson, B. O’Regan, et al., J. Phys. Chem. B 106 (2002) 7605. [15] M. Koelsch, S. Cassaignon, J.F. Guillemoles, et al., Thin Solid Films 403 (2002) 312.
The preparation of high-surface-area nanocrystalline TiO2 films using easy-reaggregation particles in solution Hongwei Han a , Ling Zan b , Jiasheng Zhong b , Lina Zhang b , Xingzhong Zhao a,∗ a b
Department of Physics, Wuhan University, Wuhan 430072, PR China Department of Chemistry, Wuhan University, Wuhan 430072, PR China Received 1 September 2003; accepted 16 March 2004
Abstract A high-surface-area nanocrystalline TiO2 films were successfully prepared from two kinds of easy-reaggregation primary nanoparticles with the mean size of 26 nm in ethanol solution by a novel technique of quickly volatilizing solvent to fix the nanoparticles. Structure and properties of the films were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that the mean size of nanoparticles does not change with heat-treatment at 450 ◦ C and the roughness factor about 86 and 80 was obtained for 1 m thickness films. A roughness factor of 82 was found for the commercial P25 TiO2 films with the mean size of 36 nm and the same thickness. The electrochemical properties of the bare interfaces of the TiO2 film electrodes in propylene carbonate (PC) containing 0.05 M tetrabutyl ammonium bromide (TBAB) was also measured and show that the electroactive of the TiO2 (I) film is similar to P25. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles for dye-sensitized nanocrystalline solar cells. © 2004 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Nanocrystalline; Thin films; Solar cells
1. Introduction Dye-sensitized nanocrystalline solar cells have attractive features in high-energy conversion efficiency and low production cost [1–5]. As the heart of this device, the use of mesoporous nanocrystalline films of the semiconductor in the place of compact signal increases considerably the effective surface area for dye adsorption [5]. They are typically fabricated from 5 to 100 nm size metal oxide nanocrystals, sintered together to yield an inter-connected, porous structure [6]. Titanium dioxide is widely used as semiconductor material in dye-sensitized nanocrystalline solar cells, which have achieved solar to electrical energy conversion efficiencies of up to 10.4%, owing to its favorable energetics stability, low price and simple processing [7,8]. In the method of preparation of titanium dioxide films electrode, the doctor blade technique shows more fascinating and efficient. It is a simple process leading to films with controllable thickness ∗
Corresponding author. Tel.: +86-2787642784; fax: +86-2787654569. E-mail addresses: [email protected] (H. Han), [email protected] (X. Zhao). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.03.019
and surface and more photoactivity [5,9]. The size of the particles and pores making up the film is controlled by the size of the particles in the colloidal solution [1]. But it is difficult to prepare smaller nanocrystalline and larger surface areas of films with this technique using the easy-reaggregation particles in the solution. However, a nice dispersal of the particles is necessary to obtain the effective surface area in this technique. In order to prevent reaggregation of the particles, a lot of stabilizers such as acids, bases, or titanium dioxide chelating agents was added into the solution of water system [3]. It is well known that the reaggregation of the particles increases with the size decrease for the large surface energy of the nanoparticles. In our early work a novel titanium dioxide nanoparticles with low production cost was developed. But unfortunately, these particles are easy-reaggregation and delaminate obviously to two layers after lying for about 30 min in solution. In this study, we try to prepare high-surface-area nanocrystalline thin films with these nanoparticles using a novel method in a good-volatility liquid system. All the results show that the roughness factor of the films prepared with the poor dispersing titanium dioxide nanoparticles is more
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H. Han et al. / Materials Science and Engineering B 110 (2004) 227–232
than 80 for 1 m thick TiO2 films. A favorable electrochemical behavior of the bare interfaces of the films is also observed. A commercial P25 TiO2 (the mean size is about 36 nm) is used as comparison, which has excellent disperse in the ethanol and water system. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles for dye-sensitized nanocrystalline solar cells.
2. Experiment 2.1. Materials Transparent conducting optically glass (TCO glass, fluorine-doped SnO2 coated glass, effective transmittance over 80%, sheet resistance is about 10–15 2 ) was purchased from Asahi Glass of Japan. The powders of TiO2 (I) (BET 57 m2 /g) and TiO2 (II) (BET 57 m2 /g) were prepared according to the patent [10]. Another commercial TiO2 P25 (BET 48 m2 /g) was purchased from Degussa AG of Germany. All of the other solvents and chemicals used in this work are reagent grade and used without further purification. 2.2. Preparation of nanocrystalline TiO2 films The nanocrystalline TiO2 films were prepared by the method described below [1,3]: TiO2 powders were ground in an attritor mill with ethanol containing acetylacetone for 1 h. Then more ethanol and Triton X-100 were added and continuously ground for 3 h. The conducting TCO glass was covered on two parallel edges with adhesive tape (about 40 m thickness) to control the thickness of the TiO2 film and to provide non-coated areas for electrical contact. Shaking the solution of the TiO2 adequately to produce tentatively a favorable separate, and then the suspension was applied onto the TCO glass by the doctor blade technique at 30 ◦ C. After air-drying, the TCO glass coating TiO2 films was immerged into dry ethanol for 10 min to improve the properties of the nanocrystalline films. Later, it was dried with a hot air and then fired for 30 min at 450 ◦ C in air. 2.3. Methods The X-ray diffraction (XRD) of the powders was carried out in Shimadzu CXD-3A diffractometer using Cu k␣ radiation at 40 kV and 20 mA in the region of 2θ = 20–50◦ . Apparent crystal size (ACS) was estimated through Scherrer’s equation [11] ACS =
kλ cos θβ
β = (B2 − b2 )1/2
(1) (2)
where k is the apparatus constant, and taken as 0.89; λ the wavelength of Cu k␣ line (1.542 Å); θ the Bragg’s angle; b
the instrumental constant (0.1◦ ), and B the half width in radians of the diffraction peak of the (1 0 1) planes of anatase and (1 1 0) planes of rutile. The anatase content, A, was determined as weight percentage by using the following equation [12]: A=
100 1 + 0.105 + 0.437F
(3)
where F represents the ratio of the integrated area of the rutile and the anatase diffraction peaks. The Brunauer–Emmett–Teller (BET) surface area of the powders was obtained with nitrogen adsorption in a Micromeritics ASAP 2010 nitrogen adsorption apparatus. The morphologies of nanopowders were observed in the transmission electron microscope (TEM) by using the JEM 2010 FEF (JEOL, Japan). The shape of the TiO2 nanocrystalline films were obtained with scanning electron microscope (SEM) by using the Sirion FEG (FEI, USA). The compositions of TiO2 films were analyzed by X-ray photoelectron spectroscopy (XPS), using a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh-vacuum (UHV) chambers. The pressure in the chambers during the experiments is approximately 10−7 Pa. An Mg K␣ X-ray source is used. The X-ray photoelectron spectra are referenced to the C 1s peak Eb = 284.80 eV resulting from the adventitious hydrocarbon (from the XPS instrument itself) present on the sample surface. A CHI601A electrochemical analyzer (CH Instruments Inc., USA) was used to perform cycle voltammeters in electrochemical cells with volumes of 50 mL at ambient temperature. A three-electrode cell was composed of a nanocrystalline TiO2 films on the TCO glass as working electrode (geometric surface area was 0.5 cm2 ), Pt as counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The electrolyte was 0.05 M tetrabutyl ammonium bromide (TBAB) in propylene carbonate (PC). It was maintained under an argon flow during measurements.
3. Results and discussion The XRD patterns of the three TiO2 powders before and after heat-treated at 450 ◦ C are shown in Fig. 1. The diffraction peaks were observed at 2θ = 25.3◦ , 27.2◦ , 36.1◦ , 37.8◦ , 39.3◦ , and 48.4◦ for TiO2 (I) and P25, and 2θ = 25.3◦ , 37.9◦ , and 48.4◦ for TiO2 (II). This indicates a mixture of two polymorphs of TiO2 , anatase and rutile, in TiO2 (I) and P25, but only pure anatase for TiO2 (II). The content of anatase and rutile of the powders is calculated with Eq. (3). The proportion of anatase is found to be approximately 71% for TiO2 (I), 100% for TiO2 (II) and 83% for P25. The content of anatase in the films does not change after sintering below 450 ◦ C. The apparent crystalline size (ACS) of (1 0 1) planes in anatase and (1 1 0) planes in rutile is summarized in Table 1. The result shows the ACS does not change after heat-treated. This indicates that the dimension of the particles does not
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Fig. 1. XRD patterns of the three kinds of powders TiO2 (I), TiO2 (II) and P25 before (bottom) and after heat-treated at 450 ◦ C (top).
grow up even though being heat-treated at 450 ◦ C. The similar average ACS (26 nm) was found in (1 0 1) plane of anatase and in (1 1 0) planes of rutile for TiO2 (I), and ACS in (1 0 1) plane of anatase was 30 nm for TiO2 (II). Both of them are smaller than the mean ACS 36 nm, 40 nm of P25 in (1 0 1) of anatase and in (1 1 0) planes of rutile, respectively. However, this estimation is based on the supposition that the broadening of the peaks is only due to a size effect (lattice strain was neglected). TEM result shows that the particles strongly aggregated, especially for TiO2 (I) and TiO2 (II). However, TiO2 (I) and TiO2 (II) appear obviously delaminating after lying for about 30 min in ethanol. The high-resolution TEM micrographs of the powders before heat-treated are shown in Fig. 2, from which it can be determined that the mean size of nanoparticles is 26 nm for TiO2 (I), TiO2 (II) and 36 nm for P25. It can be seen that their values are smaller than that calculated from XRD. TEM has also show that the XRD coherent domains extend on domains larger than one particle. This explains the difference between XRD and TEM observation. The morphology of these particles seems to be approximately spherical. TiO2 films were deposited on TCO glass from the TiO2 solution in ethanol. Fig. 3 shows scanning electron microscope graph on the surfaces of these nanocrystalline films. The pattern reveals the titanium dioxide films to be com-
posed of a three-dimensional network of interconnected particles having an average size of approximately 26 nm for TiO2 (I) film and TiO2 (II) film, and 36 nm for P25 film, which are similar to the results of XRD and TEM and they do not change in the process of sintering. This indicates that a high-surface-area TiO2 films was produced in spite of reaggregation of the particles for TiO2 (I) and TiO2 (II). However, The size of the particles and the pores making up the film are controlled by the size of the particles in the solution. This phenomenon indicates a favorable high-surface-area film was prepared successfully for dye-sensitized nanocrystalline solar cells with the system of ethanol solution. In fact, it is well known that the reaggregation of the particles in other systems such as water is unsuitable because we cannot produce a high-surface-area film from these systems. Owing to a good-volatility liquid solution were used, the particles is fixed before reaggregation in the process of preparation. The roughness factor of the nanocrystalline TiO2 thin films was also calculated. It was carried through the number of bis(thio-cyanato)-2,2 -bipyridyl-4,4 -dicarboxylate) ruthenium(II) (known as the N3 dye) absorbing in the TiO2 films, which has been investigated extensively due to its unmatched performance in dye staff studied as solar cell sensitizer in dye-sensitized nanocrystalline solar cells. The TiO2 films absorbed N3 with monolayer were immerged into the solution of 0.5 M sodium hydroxide for about 3 h.
Table 1 The characters of the typical nanometer TiO2 powders Content of anatase (%)
TiO2 (I) TiO2 (II) P25
71 100 83
ACS (nm) (1 0 1) in anatase
(1 1 0) in rutile
26 30 36
26 40
The mean size (nm)
BET (m2 /g)
26 26 36
57 57 48
The content of anatase and ACS were calculated from X-ray diffraction patterns, the dimension of the particles was estimated by TEM, the specific surface area of the TiO2 nanoparticles was measured GEM INI 2360 with BET.
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Fig. 2. Transmission electron microscope of the TiO2 powders no treated. The powders were supported by carbon silkscreen, which were treated with ultrasonic in ethanol for 10 min.
Fig. 3. Scanning electron microscope pattern of nanocrystalline films based on different TiO2 powders.
UV-Vis spectrophotometer was recorded and the number of N3 in the solution was calculated through the optical density of the solution at 306 nm. As each dye molecule occupies an area of 1 nm2 , the inner surface of the films with 1 m thickness is 86, 80, 82 cm2 for each 1 cm2 of geometric surface for TiO2 (I), TiO2 (II) and P25, respectively. These values are approximate though they are different dispersing character in solution. It indicates a high-surface-area film was obtained through a process of quickly fix in spite of an inhomogeneous solution was used. Although those primary particles are easy-reaggregation particles in solution and delaminate obviously to two layers after lying for about 30 min in ethanol solution, the roughness factor of TiO2 (I) and TiO2 (II) thin films is close to the value of P25 thin films. However, a cubic close packing of the mean size about 26, 26 and 36 nm spheres to a 1 m thickness layer is expected to produce 115-, 115- and 83-fold increase in surface area for TiO2 (I), TiO2 (II) and P25, respectively. These are larger than the datum calculated from absorbing the dye N3 in the TiO2 films. The difference is attributed to the necking between TiO2 particles. In addition, the large size of N3 prevents its access to very small pores, reducing the apparent surface area [1]. Fig. 4 shows the XPS survey spectra for the surface of 0.5 m-TiO2 films deposited on the F doped SnO2 TCO glass and heat-treated at 450 ◦ C for 30 min. It can be seen that the three kinds of nanostructural films contain the same
elements of Ti, O, C, Na and Sn. The photoelectron peak for Ti 2p appears clearly at a binding energy, Eb , of 458 eV, O 1s at Eb = 531 eV and C 1s at Eb = 284 eV. The Na kll and Sn 3d photoelectron peaks are at binding energies 269 and 490 eV, respectively. The XPS peaks for Na and Sn are observed in the spectrum, implying that some chemical reactions occur at the interface between the films and the SnO2 doped F TCO glass substrates and sodium in leakage
Fig. 4. XPS survey spectrum for the surface of the TiO2 films heat-treated at 450 ◦ C for 30 min.
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Fig. 5. High-resolution XPS spectra of the Ti 2p region for the surface of the TiO2 films heat-treated at 450 ◦ C for 30 min.
into the films from the glass substrates. The element C in the films is attributed to the residual carbon. The high-resolution XPS spectra of the Ti 2p taking on the surface of TiO2 films shows in Fig. 5. The Ti 2p3/2 region may be decomposed into two contributions corresponding to the different oxidation states of titanium. The symmetric peak situated at about Ti 2p3/2 = 458.8 eV is assigned to Ti(IV) (titanium in the IV oxidation state). Another small contributing peak situated at about Ti 2p3/2 = 458.1 eV, which corresponding to titanium in the III oxidation state (Ti(III)). However, the areas of binding energy at Eb = 458.8–458.1 eV of the TiO2 (I), TiO2 (II) and P25 films are 2.75, 1.64 and 2.59, respectively. It indicates that there is the largest content of Ti(III) in the films corresponding to TiO2 (II), but that in the TiO2 (I) is the lowest. In fact, this trend is corresponds to the content of anatase (Table 1). With the increase of content of anatase in the films, the rate of Ti(III) is increased. However, in the case of nanocrystalline TiO2 , electron transport is strongly influenced by trapping, resulting in remarkably slow electron transit times across the film [13]. Electron traps are usually assigned to Ti(III) sites and are believed to result from intrinsic defects as well as surface related and intercalated species [14]. This indicated that electron transport is more slow in TiO2 (II) nanostructural films than P25 and TiO2 (I). TiO2 films were cycled in propylene carbonate solutions to study charge transfer with electrolytes. Voltammograms of TiO2 nanocrystalline films in propylene carbonate solutions containing 0.05 M TBAB are presented in Fig. 6. The voltammograms show that the curve of TiO2 (II) films, which containing 100% anatase, is not distinct change. This result is similar to Koelsch’s report [15]. However, a redox reaction peak is exhibited at E = −1.07 V/SCE for TiO2 (I) and P25 films, which can be attributed to the reduction of surface Ti(IV) [7]. These indicate that a close electroactive is obtained for TiO2 (I) and P25 films in spite of their difference reaggregation primary nanoparticles in propylene carbonate containing 0.05 M tetrabutyl ammonium bromide. As showed in the high-resolution of XPS, the rate of Ti(III) is increased with the increase of content of anatase in the films
Fig. 6. Cyclic voltammetry of 1 m TiO2 (I) (solid), TiO2 (II) (dash) and P25 (dot) electrodes collected in three-electrode at a scan rate of 50 mV s−1 . The experiment employed in propylene carbonate containing 0.05 M tetrabutyl ammonium bromide. Geometric surface area was 0.5 cm2 . All voltages are shown relative to an Ag/AgCl reference electrode.
and electron transport in TiO2 (II) nanostructural films exhibits more slow than that of P25 and TiO2 (I). This can be explained for the difference of electroactive for these kinds of TiO2 films in this solution. 4. Conclusion A novel technique of preparing nanocrystalline films using a low-cost and easy-reaggregation primary nanoparticles was developed. A high-surface-area film was obtained with easy-reaggregation TiO2 solution. A roughness factor of 86 and 80 was obtained for TiO2 (I) and TiO2 (II) films with the mean size 26 nm and thickness 1 m, respectively. These are close to the roughness factor of 82 for the commercial P25 TiO2 films with the mean size of 36 nm and the same thickness. Owing to a good-volatility liquid solution was used, the particles is fixed before reaggregation in the process of preparation. Their structure and properties were investigated and show that TiO2 (I) nanocrystalline films have excellent electroactive property in the solution of 0.05 M tetrabutyl
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ammonium bromide in propylene carbonate, which is similar to P25 films. Moreover, this method is adapted to prepare nanostructural films using other materials and smaller primary nanoparticles. Acknowledgements Transmission electron microscope and Scanning electron microscope were experimented in the center of highresolution electron microscope in Wuhan University. References [1] D. O’Regant, G.M. Grätzel, Nature 353 (1991) 737. [2] G.M. Grätzel, Nature 414 (2001) 338.
[3] M.K. Nazeeruddin, A. Kay, I. Rodicio, et al., J. Am. Chem. Soc. 115 (1993) 6382. [4] K. Kalyanasundaram, M. Gratzel, Coord. Chem. Rev. 77 (1998) 347. [5] A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 267. [6] M.K. Nazeeruddin, P. Pechy, T. Renouard, et al., J. Am. Chem. Soc. 123 (2001) 1613. [7] G. Boschloo, D. Fitzmaurice, J. Phys. Chem. 103 (1999) 2228. [8] R.L. Willis, C. Olson, B. O’Regan, et al., J. Phys. Chem. B 106 (2002) 7605. [9] K.J. Jiang, T. Kitamura, H. Yin, S. Ito, Chem. Lett. 9 (2002) 872. [10] L. Zan, J. Zhong, Q. Luo, Chinese Patent 1373089A (2002). [11] G.N. Scherrer, Gottinger Nachr. 2 (1918) 98. [12] R.A. Spurr, H. Myers, Anal. Chem. 29 (1957) 760. [13] K. Schwarzburg, F. Willig, Appl. Phys. Lett. 58 (1991) 2520. [14] R.L. Willis, C. Olson, B. O’Regan, et al., J. Phys. Chem. B 106 (2002) 7605. [15] M. Koelsch, S. Cassaignon, J.F. Guillemoles, et al., Thin Solid Films 403 (2002) 312.