Titanium dioxide (TiO2) is considered as one of the important semiconductor metal oxide due to its wide range of applications such as in solar cells, self cleaning surfaces, CO2 reduction and gas sensing [1-3]. TiO2 exits in three polymorphs anatase, rutile and brookite. Rutile is the most stable phase whereas anatase and brookite are metastable phases. Anatase and brookite can transform to rutile phase on heating. The energy band gap for anatase and rutile phases are 3.2 eV and 3.0 eV respectively .
In recent years mixed phase (anatase and rutile) nanostructured TiO2 was found to exhibit improved gas sensing properties [5, 6]. Therefore it is desirable to prepare mixed phase TiO2 nanopowder by a simple sol-gel method. In our investigation the preparation method of mixed phase TiO2 nanopowder is optimized by changing the concentration of titanium isopropoxide which acts as a starting material. The prepared TiO2 nanopowder shows lower band gap value as compared to data reported in the literature [6, 7]. The lower band gap value and the presence of mixed phase (anatase and rutile) in TiO2 nanopowder may lead to improved gas sensing properties [5-8]. Hence it will be beneficial to understand the effect of annealing temperature on its structural, electrical and optical properties.
MATERIALS AND METHODS
Titanium isopropoxide (TTIP) and methanol are used as starting materials. The chemicals used are of analytical research grade. 3.5 ml of titanium isopropoxide (TTIP) is added in 40 ml methanol. This results in a milky white solution. Magnetic stirrer is used to stir the solution vigorously for 1:30 hrs. at a temperature 57±30C. The gel thus produced is kept for 12 hrs. at room temperature. Therefore powder is obtained and annealed at temperatures 6000C, 7000C and 8000C for 1 hour in air . X-ray diffraction pattern (XRD) of TiO2 specimens annealed at different temperatures is recorded using CuKα radiation. The UV-absorption spectra are recorded on Shimadzu-1800 UV- spectrophotometer by dispersing TiO2 nanopowder in deionized water. In order to study electrical properties pellets having diameter 10 mm of TiO2 nanopowder specimens are formed by Hydraulic pellet press and copper wire with a silver paste is used to form metal contacts on specimens. Kiethley-2400 source meter with two probe set up is used to record I-V characteristics at room temperature.
RESULTS AND DISCUSSION
X-ray diffraction peaks show the presence of both anatase and rutile phases in various TiO2 specimens as shown in Fig 1. The diffraction angles are in good agreement with JCPDS no 21-1272 for anatase, JCPDS no 21-1276 for rutile and data reported in the literature [1, 9]. The average crystallite size for various TiO2 specimens annealed at 600 °C, 700 °C and 800 °C is calculated using Scherrer’s formula .
where D is crystallite size in nanometer, β is the full width at half maximum (FWHM) in radian, λ is the wavelength of the X-ray which is 0.15406 nm for Cu target Kα radiation and θ is the Bragg angle. The content of anatase and rutile phase is calculated using formula :
Fig.1. X-ray diffraction pattern of annealed TiO2 nanopowder specimens.
where Xa is the weight fraction of anatase in the mixture, Ia and Ir are intensities of anatase (101) and rutile (110) diffraction peaks. The average crystallite size and content of anatase and rutile phase for TiO2 specimens annealed at different temperatures are summarized in Table 1. It is observed (see Table 1) that the content of rutile phase and an average crystallite size increases with increase in annealing temperature. Moreover, it was reported that the presence of mixed anatase and rutile phase leads to improved gas sensing properties [5, 6]. This shows that the change in content of anatase and rutile phase present in TiO2 nanopowder after annealing may lead to interesting gas sensing properties. It is noteworthy here that the formation of rutile phase at an annealing temperature of 8000C was reported in several studies [10-13] but in our investigation the prepared TiO2 nanopowder contains both anatase and rutile phase after annealing at 8000C.
Table 1. The average crystallite size, content of anatase and rutile phase and energy band gap values for TiO2 specimens annealed at different temperatures.
Fig. 2 shows surface morphology of TiO2 specimens annealed at temperatures 600 °C-800 °C using scanning electron microscopy (SEM). SEM image of TiO2 specimen annealed at 600 °C show cylindrical shaped particles (Fig. 2 a) whereas SEM images of TiO2 specimens annealed at 700°C and 800 °C show particles in spherical shape (Fig. 2 b and Fig. 2 c) [2, 14]. Fig. 3 depicts linear I-V curves of TiO2 nanopowder specimens which confirm ohmic nature . It is observed that current increases with increase in annealing temperature. This is due to the fact that an increase in crystallite size leads to improvement in electron migration. Therefore annealing of the TiO2 nanopowder specimens increases average crystallite size which promises improved electrical properties .
Fig. 2. SEM images of TiO2 specimens annealed at (a) 600 °C (b) 700 °C and (c) 800 °C.
Fig. 3. I-V curves of TiO2 specimens annealed at (a) 600 °C (b) 700 °C and (c) 800 °C.
UV-absorption spectra and Tauc plot for TiO2 specimens annealed at different temperatures are shown in Fig. 4 (a) and Fig. 4 (b). The energy band gap values are obtained by extrapolating linear region to abscissa as shown in Fig. 4 (b) and are mentioned in Table 1. UV studies show that energy band gap increases with decrease in average crystallite size of TiO2 nanopowder specimens corresponding to blue shift of the optical absorption edge [1, 17]. It is also observed that (see Table 1) energy band gap decreases with increase in annealing temperature. This is due to the fact that an increase in annealing temperature lowers of interatomic spacing [18-20]. It is noteworthy here that values of energy band gap obtained are lower as compared to energy band gap 3.2 eV for pure anatase , 3.0 for pure rutile phase and the data reported for the mixed phase TiO2 nanopowder exhibiting as a capable candidate for gas sensing application [6, 7, 21].
Fig. 4. (a) UV absorption spectra and (b) Tauc plot for TiO2 specimens annealed at 600 °C, 700 °C and 800 °C.
1. TiO2 nanopowder exhibits lower energy band gap value which promises it as a suitable material for gas sensing application.
2. TiO2 nanopowder contains both anatase and rutile phase after annealing in the temperature range 600 °C to 800 °C.
Authors thank Science & Engineering Research Board (SERB) for providing financial grant vide no SERB/F/5303/2014-15 and MRC, MNIT, Jaipur for providing XRD and SEM facilities.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.