In order to determine the optical energy band gap of sample,
the UV–vis absorption spectrum was recorded. The sample
shows a strong absorption peak (max) at 238 nm in the UV region.
Fig. 5a shows the UV–vis absorption spectrum of sample. This can
be attributed to photoexcitation of electron from valence band to
conduction band. The optical energy band gap (Eg) was estimated
Q4 (Fig. 5b) by the method proposed by Wood and Tauc [37] according
to the following equation; where ‘˛’ is the absorbance, ‘h’ is the Planck’s constant, ‘’ is the
frequency, ‘Eg’ is the optical energy band gap and ‘n’ is a constant
associated to the different types of electronic transitions (n = 1/2, 2,
3/2 or 3 for direct allowed, indirect allowed, direct forbidden and
indirect forbidden transitions, respectively). Eg value for -Al2O3
nanoparticles is ∼5.25 eV.