Many properties provided by supramolecular chemistry, nanotechnology and catalysis only appear in solids exhibiting large surface areas and regular porosity at the nanometre scale. In nanometre-sized particles, the ratio of the number of atoms in the surface to the number in the bulk is much larger than for micrometre-sized materials, and this can lead to novel properties. Here we report the preparation of a hierarchically structured mesoporous material from nanoparticles of CeO(2) of strictly uniform size. The synthesis involves self-assembly of these 5-nm CeO(2) pre-treated nanoparticles in the presence of a structure directing agent (poly(alkylene oxide) block polymer). The walls of this hexagonal structured CeO(2) material are formed from the primary nanoparticles. The material possesses large pore volumes, high surface areas, and marked thermal stability, allowing it to be easily doped after synthesis whilst maintaining textural and mechanical integrity. It also exhibits a photovoltaic response, which is directly derived from the nanometric particle size-normal CeO(2) does not show this response. We have constructed operational organic-dye-free solar cells using nanometric ceria particles (in both mesostructured or amorphous forms) as the active component, and find efficiencies that depend on the illuminating power.

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Development of microelectronic devices is driven by a large demand for faster and smaller systems. In the near future, colossal permittivity in nanomaterials will play a key role in the advances of electronic devices. We report on "colossal" permittivity values achieved in dense ceramics displaying ultrafine grain size ranging from 70 nm to 300 nm. Relative permittivity values of ∼ 10 6 at 1 kHz (0.1 < tand < 0.7) were obtained for Ba 0.95 La 0.05 TiO 3-x ceramics. The colossal effective permittivity is related to an interfacial polarization and is achieved in nanomaterials by the activation of a high number of carriers and their trapping at the interfaces. Polarization carriers involving Ti 3+ polaron is proposed to be at the origin of the observed colossal permittivity. These results may have an important technological impact since these ceramics display ultrafine grain size opening a new route to the fabrication of very thin dielectric films.High permittivity values such as e r ∼ 10 000 were reported in polycrystalline BaTiO 3 , a well known ferroelectric material.[1] Decreasing the grain size below 0.7 lm of the BaTiO 3 ferroelectric ceramic was shown to yield unrelieved stresses resulting in smaller permittivity values.[2] Indeed, colossal permittivity values up to 200000 at room temperature were achieved in BaTiO 3 micronic grain size materials in which preparation included incorporation of metallic layers in a complex multi step process. [3,4] The achievement of high permittivity values in this material was ascribed to interfacial polarization phenomena. Recently, giant permittivity values were reported in hexagonal barium titanate (h-BaTiO 3 ) single crystals. [5,6] High permittivity values around 10 5 measured on the oxygen deficient materials were explained by a MaxwellWagner interfacial polarization effect due to the presence of interfacial boundaries consisting of crystal defects such as screw dislocations. Nevertheless, the internal interfaces as well as the nature of the polarization carriers in the h-BaTiO 3 single crystals were not fully identified. Within the last few years, a large class of dielectric materials displaying colossal permittivity was proposed [7][8][9] with colossal dielectric con- [10] The large permittivity values of this material being attributed to the well established IBLC effect.[11] According to the brick layer model, the effective permittivity e eff of the microstructure can be expressed as e eff = e gb t g /t gb where e gb is the grain boundary permittivity and t g and t gb are the thickness of the grain and grain boundary, respectively.[12]Thus the IBLC mechanism is usually associated with enhanced grain size. In this context, a decrease of the average grain size should not favor the IBLC effect and colossal permittivity in ultrafine ceramics, to our knowledge, has not been reported. In this communication, we discuss the "colossal" permittivity values achieved in BaTiO 3-x and Ba 0.95 La 0.05 TiO 3-x materials exhibiting very small grain size (< 300 nm). Dense dielectrics...