where co is a constant, Eg is the band gap, and is an exponent that depends on the nature (direct or indirect) of the band gap of the material. CeO2 was reported to have an indirect band gap [6], thus= 2. Thus, a plot of (˛E)1/2 versus E should be a straight line. Such a plot is shown in Fig. 2c. The linear portion of the curve was fitted using linear regression analysis, where the correlation coefficient was 0.998. An extrapolation of the linear region of the plot gives the value of the band gap as the intercept to the horizontal axis (where ˛= 0). The value of the band gap was 3.14 eV. This value is very close to the reported value of 3.15 eV for e-beam deposited CeO2 films [4].