Hierarchical or one-dimensional architectures are among the most exciting developments in material science these recent years. We present a nanostructured TiO(2) assembly combining these two concepts and resembling a forest composed of individual, high aspect-ratio, treelike nanostructures. We propose to use these structures for the photoanode in dye-sensitized solar cells, and we achieved 4.9% conversion efficiency in combination with C101 dye. We demonstrate this morphology beneficial to hamper the electron recombination and also mass transport control in the mesopores when solvent-free ionic liquid electrolyte is used.

A systematic study of the shift and linewidth of the Eg Raman peak at 144 cm−1 of anatase TiO2 nanopowders, produced by a flame aerosol technique, is here presented. The analysis was performed as a function of the crystal domain size and of the degree of oxidation. In the nanopowders, a clear contribution of the stoichiometry defects to the peak shift was evidenced, while the peak width seems to be less affected by the oxygen content. The Raman peak behavior due to size reduction has been interpreted in the framework of a phonon quantum confinement model. A critical review of the different approaches to this model, adopted in the literature to explain the behavior of the anatase Raman spectra as a function of the domain size, is presented. In particular, the hypothesis of a three-dimensional isotropic model for the dispersion relations is discussed. This analysis evidences general limits in the application of the phonon confinement model to the study and characterization of nanoparticles and nanostructured materials, showing how an uncritical use of the confinement theory can yield distorted results

Nanostructured carbon films produced by supersonic cluster beam deposition have been studied by in situ Raman spectroscopy. Raman spectra show the formation of a sp2 solid with a very large fraction of sp-coordinated carbyne species with a long-term stability under ultrahigh vacuum. Distinct Raman contributions from polyyne and cumulene species have been observed, as well as different stabilities under gas exposure. Our experiments confirm theoretical predictions and demonstrate the possibility of producing a carbyne-rich pure carbon solid. The stability of the sp2-sp network has important implications for astrophysics and for the production of novel carbon-based systems.

We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO 2 , ZrO 2 , and WO 3 . We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO 2 into Ti 2 O 3 , involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.

A template-free process for the synthesis of nanocrystalline TiO2 hierarchical microstructures by reactive pulsed laser deposition (PLD) is here presented. By a proper choice of deposition parameters a fine control over the morphology of TiO2 microstructures is demonstrated, going from classical compact/columnar films to a dense forest of distinct hierarchical assemblies of ultrafine nanoparticles (<10 nm), up to a more disordered, aerogel-type structure. Correspondingly, the film density varies with respect to bulk TiO2 anatase, with a degree of porosity going from 48% to over 90%. These structures are stable with respect to heat treatment at 400 degrees C, which results in crystalline ordering but not in morphological changes down to the nanoscale. Both as deposited and annealed films exhibit very promising photocatalytic properties, even superior to standard Degussa-P25 powder, as demonstrated by the degradation of stearic acid as a model molecule. The observed kinetics are correlated to the peculiar morphology of the PLD grown material. We show that the 3D multiscale hierarchical morphology enhances reaction kinetics and creates an ideal environment for mass transport and photon absorption, maximizing the surface area-to-volume ratio while at the same time providing readily accessible porosity through the large inter-tree spaces that act as distributing channels. The reported strategy provides a versatile technique to fabricate high aspect ratio 3D titania microstructures through a hierarchical assembly of ultrafine nanoparticles. Beyond photocatalytic and catalytic applications, this kind of material could be of interest for those applications where high surface-to-volume and efficient mass transport are required at the same time.

Nanostructured carbon films consisting of sp chains ~polyynes and polycumulenes! embedded in an sp2 matrix are grown using supersonic carbon cluster beam deposition in ultrahigh vacuum at room temperature. All the specimens have been analyzed by in situ Raman spectroscopy. The use of different excitation wavelengths ~532 and 632.8 nm! confirms the presence of distinct carbynoid species. Chemical stability of the sp species has been studied by exposing the as-deposited films to 500 mbar of H2, He, N2, and dry air. Gas exposure produces an exponential decay of the carbynoid fraction slightly affecting the sp2 component. Helium, hydrogen, and nitrogen do not chemically interact with the sp chains whereas oxygen reacts with the carbynoids species causing their fast and almost complete destruction. The films have been also thermally annealed at 20°, 100°, 150°, and 200 °C. The amount of carbynoid species is rapidly and strongly reduced at temperature larger than room temperature. The relevance for material science and interstellar chemistry of the production of a bulk form of carbon where sp and sp2 hybridizations coexist is addressed

Amorphous and nanostructured carbon films were grown by using two different techniques: ion sputtering and cluster beam deposition. The films were studied by near-edge x-ray absorption fine structure ͑NEXAFS͒ and Raman spectroscopy. Depending on the precursors, atoms, or clusters, the films are characterized by a different sp 2 /sp 3 ratio which influences the mechanical and the electronic properties. Due to the sensitivities of NEXAFS ͑local order͒ and Raman ͑medium-range order͒, we have characterized and compared the structure of the films over different length scales. The complementarity of NEXAFS and Raman techniques for the characterization of disordered forms of carbon is here presented and discussed. We also present an original method of NEXAFS spectra calibration allowing a better determination of peak positions.

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