Page - 176 - in Photovoltaic Materials and Electronic Devices

are relatively low-cost and easy to prepare with a simple chemical synthesis procedurewithasimplecoatingtechnique. 2. ResultsandDiscussion 2.1.NanoparticlesCharacterization The optical absorption spectra of the synthesized REDC NPs are plotted in Figure 1a. Consequently, the corresponding values for the calculated allowed direct band gaps of the annealed samples are shown in Figure 1b, using Equation(1) [12,13]. 9E“A`E´Eg˘1{2 (1) whereα is the absorbance coefficient, A is a constant dependent on the effective masses of electrons and holes of the material, E is the absorbed photon energy, and Eg is the direct allowed band gap. Thus, from the absorbance dispersion results in Figure 1a, (9E)2 is presented versus the photon energy (E) as shown in Figure 1b. Then, the intersection between the extension of the linear region of the resultedcurvewiththeE-axisindicatesthebandgap. Comparedtothenon-reduced nanoparticles, it can be observed that the band gap of the annealed nanoparticles is biased towards 3eV, which is approximately the band gap energy for Ce2O3. Thus, there is evidence for the formation of a higher concentration of Ce3+ with corresponding oxygen vacancies [14]. The annealed REDC NPs are imaged using TEMasshowninFigure2a. Themeandiameter is foundtobe10nm,whichshows that our synthesized REDC NPs are smaller than other optical nanoparticles that have been studied as an optical active medium for down- or up-conversion [15,16], which can lead to both better conductive and optical properties due to a higher surface-to-volume ratio. The XRD pattern is presented in Figure 2b, measured on a sample of the REDC NPs annealed at 700 ˝C, to demonstrate that the predominant nanostructure of the REDC NPs is cerium dioxide [17,18]. From the measurement of the width of individual intensity peaks, the average size (tXRD) of nanoparticles, or the diameter in case of spherical-shaped nanoparticles, can be calculated from Scherrer’sequation[13] tXRD“ 0.9λ βcosθ (2) whereλ is the wavelength of the incident X-rays (0.15406 nm),β is the full-width half-maximum (FWHM), andθ is the diffraction angle. From the first peak, which represents the most stable plane of ceria (111), the average size of the nanoparticles is foundtobe~10nm. 176