Room-temperature optical absorbance spectra of TiO2 nanoparticles sample thermally
decomposed at 400, 500 and 600oC for 2hr are shown in Fig. 7. The absorption spectra of all TiO2
samples exhibit strong absorption below 400 nm.
The direct band gap (Eg) of the samples is determined by fitting the absorption data to the direct
transition Eq. (7):
αhν=Ed(hν-Eg)
1/2
(7)
where α is the optical absorption coefficient, hν is the photon energy, Eg is the direct band gap,
and Ed is a constant[26]. The band gab of TiO2 sample was measured by plotting (αhν) 2
as a function
of photon energy, and extrapolation the linear portion of the curve to absorption equal to zero as given
in Fig.7. The spectrum of TiO2 sample at 400 and 500oC indicates the absorption onset at around 393
nm which is in excellent agreement with band gab of anatase phase [27]. When the calcination
temperature increases, the band gap gradually decreased from 3.13 to 3.0 eV as reported in Table 1.
At high calcined temperature of 600oC, the band gap was at the lowest value (3.0eV) because the rutile
phase and size of particle becomes bigger.