The O 1s spectrum (Fig. 2b) was deconvoluted into two components using a Gaussian/Lorenzian mixed function employing Shirley background correction. The lower-energy component (A) with a binding energy of 528.8 eV corresponds to the Ce–O bond [10]. The higher-energy component (B) with a binding energy of 530.9 eV corresponds to adsorbed oxygen or surface hydroxide (Ce–OH) [5,10]. The weight of each component is obtained by dividing the area of that component by the total area of the O 1s spectrum. The weight of component A was 59%, and that of component B was 41%. The intensity of the B component is partly proportional to the amount of adsorbed water, which in turn is related to the porosity of the films. Thus, the relatively large weight of the B component indicates highly porous films