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X-ray diffraction and electron microscopy study of nanosize CeO2 as photoanode for dye-sensitized cell

Dye-sensitized cells differ from the conventional semiconductor devices in that they separate the function of light absorption from charge carrier transport. The device is based on a ť 1 mm thick, optically transparent film of cerium dioxide (Turković and Crnjak-Orel, 1997) particles a few nanometers in size, coated with a monolayer of charge-transfer dye to sensitise film for light harvesting. Following the procedure by Grätzel and co-workers (O’Regan and Grätzel, 1991), but using CeO2 or CeO2/SnO2films as photo anodes, we have produced new solar cells. They do not have efficiencies comparable to solar cells with dye-sensitized nanostructured TiO2, but they show photo sensitivity to solar light. Nanosize properties of CeO2 obtained by this preparation technique were modified to be more suitable for application in solar cells (Hagfeldt and Grätzel, 1995). The samples prepared by sol-gel method have been studied by X-ray diffraction analysis and analytical electron microscopy techniques including bright field (BF), dark field (DF), electron diffraction (ED), HREM and energy dispersive X-ray analysis (EDS). Following the preparation instructions of Hagfeldt and Grätzel of nanocrystalline TiO2 films we introduced a kind of autoclaving step in our CeO2 preparation procedure. The reason is that optical properties of the nanophase TiO2 can be drastically changed due to autoclaving step in the preparation procedure with implements for different device. As our previously prepared CeO2 (Crnjak-Orel,Leskovšek, Orel, Hutchins, 1996) and used in solar cell (Turković and Crnjak-Orel, 1997) was originally intended for electrocromic display it did not yield considerable efficiency in constructed solar cell. In the present preparation we intended to obtain better ion diffusion in electrolyte by obtaining agregates made of larger and smaller particles that could be used for preparation of colloidal paste of CeO2. X-ray diffraction (XRD) patterns of the sample 1 and sample 2 were assigned as CeO2 FCC structure according to JPCPDF , card no. 34-0394. The microstructure as well as the structure observed by HREM and ED did not reveal difference in two observed CeO2 samples having different preparation procedures. HREM image of sample 1 (Fig. 1) revealed grain sizes from 5 to 20nm, stacking faults F and pores P having sized from 2 to 5 nm. In the insert ED pattern reveals CeO2 FCC nanocrystalline material. Fig. 2 shows a region of sample 2. The agglomeration of small grains having sizes from 4 to 10 nm are attached to large CeO2 grain G having size of 40 nm. (111) CeO2 lattice image (d=0.312 nm) revealed a region of one particular the grain being the nucleus of agglomeration. Near the grain boundary GB, dislocation D is observed.

nanosize CeO_2; TEM; HREM; XRD

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