Teses e Dissertações

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Navegando Teses e Dissertações por Autor "Almeida, Juliana de [UNIFESP]"

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    Fotoeletrorredução de CO2 empregando óxidos nanoestruturados
    (Universidade Federal de São Paulo, 2016-02-15) Almeida, Juliana de [UNIFESP]; Rodrigues, Christiane de Arruda [UNIFESP]; Universidade Federal de São Paulo (UNIFESP)
    The performance of the heterogeneous photocatalysis in wastewater treatment for the removal of organic pollutants, as well as for gaseous emissions, is associated to semiconductor photo-activity. TiO2 is a semiconductor commonly employed for photocatalytic processes, due to its corrosion resistance, chemical stability, low toxicity, and photoactivity. It has an energy band gap of 3.2 eV, which is equivalent to an absorption wavelength (?) shorter than 388 nm, which corresponds to UV radiation and becomes a major obstacle for its use under sunlight. In this study, TiO2 nanotubes (NT/TiO2) were modified with Cu nanoparticles, creating a p-n heterostructure in order to increase the photocatalytic activity, by the change of the absorption spectrum to longer wavelengths, the decrease of the rate of recombination of photogenerated charges, and the optimization of its application for photoreduction processes. The NT/TiO2 were synthesized on Ti substrate by anodic process and modified by electrodeposition. Voltammetric studies employing 0.01 mmol L-1 CuSO4 in different electrolytes, 0.1 mol L-1 of Na2SO4 or 0.01 mol L-1 C3H6O3 were performed to investigate the influence of the electrolyte and the potential range to be applied in electrodeposition process. In both electrolytes, the morphology, amount and distribution of Cu nanoparticles on NT/TiO2 showed strong influence of applied potential. High values of cathodic current was obtained in photocurrent experiments using NT/TiO2 modified with Cu at 0,70 and -0,85 V vs. Ag/AgCl, using Na2SO4 e C3H6O3, respectively, These samples were submitted to heat treatment to convert metallic Cu in Cu2O (NT/TiO2-Cu2O) and CuO (NT/CuO-TiO2). The NT/TiO2-Cu2O and NT/CuO-TiO2, synthesized in different electrolytes, were applied in CO2 photoelectroreduction in 0.1 mol L-1 NaHCO3, pH 8.0. Only the NT/TiO2-CuO electrode synthesized in C3H6O3 solution was able to produce acetone, ethanol and methanol. The result shows that the morphology and size of the cupric oxide nanoparticles are fundamental in the electrode efficiency, making it more attractive to contribute as a source for the production of fuels. The efficiency was certificated by its band gap energy which value was 46 % lower than that of undecorated NT/TiO2.
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    Síntese, caracterização e aplicação de semicondutores nanoescalares compostos por óxidos de Ti e Cu para a redução fotoeletrocatalítica de CO2
    (Universidade Federal de São Paulo (UNIFESP), 2021) Almeida, Juliana de [UNIFESP]; Rodrigues, Christiane de Arruda [UNIFESP]; Universidade Federal de São Paulo
    Copper species exhibit favorable positions in the conduction band and high potential for reducing compounds adsorbed on its surface, being widely studied on the CO2 conversion towards in multicarbon oxygenated products. However, when applied in photoelectrocatalytic processes, they are easily oxidized and susceptible to photocorrosion, so their stability is a challenge. Copper species can be coupled with other materials, such as TiO2, a n-type semiconductor with excellent charge transfer properties and chemical stability. In addition, it nanostructured morphology has their properties are enhanced, obtaining better photocatalytic efficiency. Therefore, this work evaluated the synthesis of semiconductors obtained from substrates of the Ti-xat.% Cu (x = 0.5 / 5.5 / 10 / 20 / 30 and 50) system after annealing and annealing / quenching heat treatment. The substrates phases were identified by optical microscopy, diffraction and X-ray dispersion and quantified using Rietveld method. These data revealed that the solid solution was absent only in the equiatomic alloy and that phases, such as Ti3Cu and Cu3Ti were detected. The nanostructured films of Ti– O - Cu were growth by anodic process from the manufactured alloys, it showed nantubular morphology, but the equiatomic composition was exception, which showed only formation of porous layer on its surface. This indicates, that Ti-α or Ti-β phases even doped with copper were essential for the formation of the nanotubes. The matrices were investigated by scanning electron microscopy and X-ray diffraction and revealed that the presence of intermetallic and Ti3Cu, when present in the substrates, leads to the formation of matrices with ternary oxides. The photoelectrochemical and impedance spectroscopy characterizations showed that the films of the Ti-O-Cu system have n-type behavior, but obtained an anodic shift in the potential for increasing the current density, characterizing the formation of a pn heterojunction. Therefore, these were used as a photocathode in the photoelectrocatalytic reduction of CO2 under 100 mW / cm2 illumination. The reduction reactions led, after 60 min, the formation of 0.10 mmol L-1 of methanol for the matrices composed only of binary oxides and a significant concentration of ethanol around 2.20 mmol L-1 for those rich in presence of ternary oxides. Therefore, the presence of Cu2Ti4O and Cu3TiO5 oxides promote significant concentrations of ethanol and high selectivity of the final products, as well as the stability of copper species.