93 CHAPTER 4 SYNTHESIS, CHARACTERIZATION OF TiO 2 NANOTUBES AND THEIR APPLICATION IN DYE SENSITIZED SOLAR CELL 4.1 INTRODUCTION TiO 2 -derived nanotubes are expected to be applicable for several applications, such as Dye sensitized solar cell [147-149], sensors [150], electrochemical capacitors [151], proton conduction [152], and lithium-inserting and ion-exchange materials [153]. It is widely reported that various TiO 2 -derived nanotubes with different microstructures have been synthesized by many techniques, namely template method [154], anodic oxidation [155] and hydrothermal method [156]. Among them, the hydrothermal method is the best method for the preparation of TiO 2 nanotubes along with concentrated aqueous NaOH, which is fairly simple and also enables the production of pure titanate nanotubes at low temperature [157,158]. It is very important to control the sodium ion concentration (Na/Ti) for the synthesis of TiO 2 -derived titanate nanotubes by the hydrothermal process [159]. Tsai et al., and Yang et al., have reported that the final ph value of the sample after the washing process has much effect on the structure of the nanotubes. Furthermore, their previous results indicated that TiO 2 -derived titanate nanotubes were synthesized by hydrothermal treatment of TiO 2 and the subsequent washing treatments with HCl solution after the hydrothermal treatments mainly led to the Na/H ion exchange in the structure for these titanate nanotubes [160,161]. TiO 2 nanotubes with diameters of 70 100 nm were produced using the sol-gel method reported by Kasuga et al., [162]. Premalal et al., reported the synthesis of vertically aligned, reasonably dense, about 500 nm long TiO 2 nanotubes
94 (NTs) that were prepared by a wet chemical method. DSSC fabricated using TiO 2 nanotubes as photoelectrodes and N719 dye with usual I /I 3 electrolyte that gave 2.46 % solar-to-electricity conversion efficiency. Ou et al., synthesized TiO 2 nanotubes with 9.9 nm diameter by microwave assisted hydrothermal method. [163]. Commercial TiO 2 nanoparticles like Degussa P25 were used to synthesis TiO 2 nanotubes, reported by Yuan et al., [164]. In the present work, we have synthesized TiO 2 nanotubes using the already prepared TiO 2 as a precursor by hydrothermal growth. The effects of growth parameter such as namely reaction temperature and reaction time on the preparation of TiO 2 nanotubes by hydrothermal route were discussed. The synthesized products were characterized by XRD, UV - vis, TEM and BET instruments. 4.2 SYNTHESIS OF TiO 2 NANOPARTICLES TiO 2 nanopowders were prepared via hydrothermal method using Tetra-n-butyl-titanate and deionized water as the starting materials with the volume ratio of 1.6 (tetra-n-butyl titanate: distilled water). Tetra-n-butyl titanate was added drop wise into the distilled water. The obtained gel was then transferred to an autoclave at 105 C for 10 hr. Finally the precipitate was washed with distilled water. Then the powder was dried in an air oven for 24 hours and then calcined at 450 C. 4.2.1 Synthesis of TiO 2 Nanotubes from the as Prepared TiO 2 Nanoparticles In a typical preparation procedure, 0.3 g of prepared TiO 2 power was placed in a beaker. 10M NaOH in 20 ml of distilled water was added into the beaker and stirred for 1 hr. The solution was transferred into autoclave and maintained at 110 C for 24 hr. Then the autoclave was allowed to cool naturally to room temperature. The obtained products were collected and washed with dil.hcl aqueous solution for several times until the ph value turned to 7. The products were then calcined at 400 C for 2 hr. The fabrication of DSSC was clearly explained in the previous section 2.9.
95 4.2.2 Structural Analysis Figure 4.1 displays the X-ray diffraction patterns of the as synthesized; (a) TiO 2 nanoparticles (TNP), (b), (c) and (d) TiO 2 nanotubes (TNT), prepared at 110 C, 130 C and 160 C, respectively. It is seen in Figure 4.1 (a) that the diffraction peaks indicate the TiO 2 (precursor) with a pure anatase phase. In Figure 4.1 (b) the diffraction peaks for the catalysts prepared at 110 C, are corresponding to anatase phase. The diffraction peaks of TNTs are weaker than the TNPs. This is because of NaOH treatment with TNPs. When NaOH is mixed with the TiO 2 nanoparticles, NaOH might rupture the Ti - O - Ti bonds, causing a lower crystallinity in the TNTs. Furthermore, when the temperature is increased to 130 C during the hydrothermal synthesis, the phase transformation is occured from anatase to rutile phase. Figure 4.1 Indexed Powder XRD patterns of the samples (a) synthesized TiO 2 nanoparticles, (b) 110 C, (c) 130 C, (d) 160 C
96 It is well known that usually more than 500 C temperature is required to calcine the prepared TiO 2 for the phase conversion from anatase to rutile. Yu et al., reported that anatase to rutile transformation was occurred at 700 C by [165]. Tsai et al., [166] also indicated that the temperature for anatase to rutile transformation was occurred at 900 C, whereas TNTs was synthesized at 130 C. In this study, by changing the preparation time, the anatase phase can easily be converted into rutile. Figure 4.2 UV - vis absorption spectra of samples prepared at (a) 110 C, (b) 130 C, (c) 160 C Figure 4.2 shows UV - vis spectra of the prepared samples at (a) 110 C, (b) 130 C, (c) 160 C and their corresponding absorption peaks at wavelengths of 428 nm, 401 nm, 385 nm, respectively. It is well known that the photocurrent of the flexible DSSC is correlated directly with the amount of the adsorbed dye molecule. More dye molecules are adsorbed means more incident light are harvested, as a result the enhanced photocurrent can be achieved [167]. Moreover, the TiO 2 prepared at 110 C has shown maximum absorption of light at 428 nm in comparison with other two temperatures as shown in UV-visible spectra (Figure 4.2). This in turn increases more light absorption and thereby increasing the overall efficiency of DSSC.
97 4.2.3 Morphological Study Figure 4.3 (a - c) shows FESEM images of the TiO 2 samples prepared at 110 C, 130 C, and 160 C. From FESEM images, it is observed that the TiO 2 nanotubes are formed only at 110 C with 10 nm diameter and 60 nm length. Upon an increase of hydrothermal temperature to 130 C, the nanotubes are started to disintegrate into particles size in the range of 80-90 nm and some of them are aggregated together. At 160 C, the nanotubes are fully converted into spherical particles with size ranges from 150-180 nm. It is very interesting observation in this study i.e. just by changing the synthesis temperature, it is easy to covert nanotubes into nanoparticles [166]. Figure 4.4 depicts TEM images of TiO 2 nanoparticles and nanotubes. Figure 4.4 (b) shows the TEM image of TiO 2 nanotubes that was grown at 110 C for 24 hr. The width of the tube is about 20 nm. Figure 4.4 (c) shows the TEM image of the sample prepared at 130 C for 24 hr. It is clearly seen that, nanotubes are started to disintegrate into particles at 130 C. Figure 4.4 (d) exhibits the image of the sample prepared at 160 C, where there is no existence of nanotubes. The TiO 2 nanotubes are disintegrated into nanoparticles with average particle size of 60 nm. Figure 4.3 FESEM images of sample prepared at (a) 110 C, (b) 130 C (c) 130 C for 24 hr
98 Tsai et al., [161] reported such process during treatment of TiO 2 with NaOH, in which some Ti - O - Ti bonds broken into intermediates containing Ti - O - Na and Ti - OH. Then the intermediates formed as sheets by rearrangement of Na + and H + between the sheets. The variation of surface charge caused by ion exchange of Na + with H + led to the scrolling of sheets into nanotubes. The reasonable concept was proposed with the temperatures lower than 130 C, led to less cleavage of Ti - O - Ti bonds, which was the initial stage in synthesizing TNT [168-171]. High temperature (>130 C) treatment would destroy the intermediate structure of TiO 2, in the TNT formation process. The similar results were observed in this study. Figure 4.4 TEM images of sample prepared (a) TiO 2 nanoparticles, (b) 110 C, (c) 130 C (d) 130 C for 24 hr
99 Figure 4.5 (a) Nitrogen adsorption - desorption isotherm and pore size distribution curve. Inset: pore diameter distribution spectra of the TiO 2 NTs A typical isotherm of nitrogen adsorption and desorption on the surface of TiO 2 nanotubes is shown in Figure 4.5. The prepared nanotubes have the surface area of 20 m 2 /g and average pore volume is 0.1208 cm 3 /g. J - V characteristics of prepared TiO 2 nanotubes are shown in Figure 4.6. For the cell fabricated with TiO 2 nanotubes, the short circuit current density (J sc ) and the open circuit potential (V oc ) are 6.29 ma cm -2 and 0.59 V, respectively, resulting in a high energy conversion efficiency of 2.38 %.
100 Figure 4.6 J - V Characteristics curve of TiO 2 nanotubes In summary, anatase TiO 2 nanotubes with diameter of 10 nm were prepared from synthesized TiO 2 nanoparticles by hydrothermal method at 110 C for 24 hr. The TEM images clearly revealed the transformation from TiO 2 nanotube to spherical TiO 2 particles prepared at 160 C. Based on this result, optimized parameters were fixed for the preparation of TiO 2 nanotubes. Under the optimized condition, the flexible DSSC with light - to - electric energy conversion efficiency of 2.38 % was achieved. The conversion efficiency of the nanotubes was 2.38 % which was more than 2 times higher than the TiO 2 microflower discussed in Chapter 3. The higher conversion efficiency of TiO 2 nanotubes in comparison with other structures may be attributed to the enhancement of the dye adsorption. In the case of rutile nanoparticles the dye usually adsorb on the surface only, whereas in the nanotubes, the dye are also adsorbed inside as well as outside of the nantubes. This is the one of reasons for increasing the conversion efficiency of TiO 2 nanotubes apart from the fast transport of carriers in nanotubes structure rather than nanoparticles.