Philosophiae Doctor - PhD / The first report of a functioning photo-electrochemical solar cell in 1991 attracted a lot of interest from scientists and industrial groups. From an industrial point of view these so-called dye-sensitised solar cells (DSCs) offered the promise of moderate efficiency devices at ultra-low costs, owing to simple processing methods and the use of inexpensive materials. From an academic viewpoint, DSCs raised important scientific questions around the fundamental processes governing their operation and how these processes influence the photon-to-electron conversion efficiency of the cell. Major successes have since been achieved in understanding these processes, however the conversion efficiency of the best manufactured DSCs remains around 11%, significantly lower than that of their silicon photovoltaic counterparts. In traditional DSCs, charge generation is achieved by ultrafast electron injection from a photo-excited ruthenium-based dye molecule into the conduction band of a film of TiO₂ nanoparticles, subsequent dye regeneration by an I⁻ /I⁻₃ containing redox electrolyte and finally hole transportation to a platinum-coated counter electrode. The low DSC efficiencies are attributed to scattering of electrons at the interface between two TiO₂ nanoparticles leading to recombination with holes present in the redox electrolyte. Recent studies have shown that the application of films of highly ordered TiO₂ nanotubes instead of nanoparticles has the potential to improve the overall conversion efficiency of the cell. This is ascribed to the one-dimensional nature of nanotubes, which provides a linear transportation route for electrons generated during operation of the DSC. As a result the recombination probability of the electrons with nearby holes in the device is decreased. This work investigated the synthesis of Al₂O₃-coated TiO₂ nanotubes via the anodisation technique for application in DSCs. TiO₂ nanotube arrays with an average length of 15 μm, diameter of 50 nm and wall thickness of 15 nm were synthesised via anodisation using an organic neutral electrolyte consisting of 2 M H₂O + 0.15 M NH₄F + ethylene glycol (EG) at an applied voltage of 60 V for 6 hours. In addition, scanning electron microscope (SEM) micrographs showed that anodisation at these conditions yields nanotubes with smooth walls and hexagonally shaped, closed bottoms. X-ray diffraction (XRD) patterns revealed that the as-anodised nanotubes were amorphous and as such were annealed at 450 °C for 2 hours in air at atmospheric pressure, which yielded crystalline anatase TiO₂ nanotubes. Highresolution transmission electron microscope (TEM) images revealed that the nanotube walls comprised of individual nano-sized TiO₂ crystallites. Photoluminescence (PL) spectroscopy showed that the optical properties, especially the bandgap of the TiO₂ nanotubes are dependent on the crystallinity, which in turn was dependent on the structural characteristics, such as the wall thickness, diameter and length. The PL measurements were supplemented by Raman spectra, which revealed an increased in the quantum confinement of the optical phonon modes of the nanotubes synthesised at low anodisation voltages, consequently yielding a larger bandgap The annealed nanotubes were then coated with a thin layer of alumina (Al₂O₃) using a simple sol-gel dip coating method, effectively used to coat films of nanoparticles. Atomic force microscopy (AFM) showed that the average nanotube diameter increased post sol-gel deposition, which suggests that the nanotubes are coated with a layer of Al₂O₃. This was confirmed with HR-TEM, in conjunction with selected area electron diffraction (SAED) and XRD analyses, which showed the coating of the nanotube walls with a thin layer of amorphous Al₂O₃ with a thickness between 4 and 7 nm. Ultraviolet-visible (UVvis) absorbance spectra showed that the dye-adsorption ability of the nanotubes are enhanced by the Al₂O₃ coating and hence is a viable material for solar cell application. Upon application in the DSC, it was found by means of photo-current density – voltage (I – V) measurements that a DSC fabricated with a 15 μm thick layer of bare TiO₂ nanotubes has a photon-to-light conversion efficiency of 4.56%, which increased to 4.88% after coating the nanotubes with a layer of alumina. However, these devices had poorer conversion efficiencies than bare and Al₂O₃-coated TiO₂ nanoparticle based DSCs, which boasted with efficiencies of 6.54 and 7.26%, respectively. The low efficiencies of the TiO₂ nanotube based DSCs are ascribed to the low surface area of the layer of nanotubes, which yielded low photocurrent densities. Electrochemical impedance spectroscopy (EIS) showed that the electron lifetime in the alumina coated nanotubes are almost 20 times greater than in a bare layer of nanoparticles. In addition, it was also found that the charge transfer resistance at the interface of the TiO₂/dye/electrolyte is the lowest for an Al₂O₃-coated TiO₂ layer.

Nanomateriais apresentam propriedades ajustáveis pelo seu tamanho e forma, como o fenômeno de superparamagnetismo em nanopartículas magnéticas ou o confinamento quântico dos portadores de carga em pontos quânticos (quantum dots). Assim, a síntese de nanopartículas esféricas monodispersas torna-se um fator extremamente importante, haja visto que tais propriedades podem ser ajustáveis para diferentes aplicações na área de tecnologia e biomedicina. Nanopartículas magnéticas e quantum dots podem ser apontados como promissores materiais para diagnóstico e terapia de neoplasias (câncer), e o desenvolvimento desses sistemas busca, atualmente, intensificar a magnetização e a eficiência de emissão, respectivamente, relativo às propriedades magnéticas e ópticas, além de outros requisitos. Neste trabalho, nanopartículas esféricas de cobalto metálico foram sintetizadas com diâmetro médio de 5,3 nm e desvio padrão de 0,4 nm, distribuição de tamanhos lognormal. A equação de Langevin modificada pelo modelo de partículas interagentes foi utilizada no ajuste da curva de magnetização M(H) para obtenção do diâmetro magnético médio e desvio padrão, 4,7 nm e 1,0 nm, respectivamente. Comparando os dois diâmetros, encontra-se uma camada morta de magnetização de aproximadamente 3,0 Å a qual, praticamente, não contribui para a magnetização da amostra, sendo a magnetização de saturação de 125 emu g-1. Nanoestruturas core/shell de Co/Au apresentaram a propriedade de ressonância plamon de superfície, uma propriedade adicional também desejada para aplicações biomédicas, sendo este sistema denominado magnetoplasmônico. Quantum dots de CdSe foram sintetizados como elevado controle de tamanho e forma. Utilizando rotas de síntese diferentes dos clássicos procedimentos denominados TOP-TOPO, e dióxido de selênio como precursor, estudos mostraram que na presença de um agente redutor no meio de reação e do solvente 1-octadeceno, as amostras apresentaram melhores propriedades óticas. A estrutura cristalina das amostras de CdSe corresponde à formação da fase blenda de zinco, diferentemente das sínteses TOP-TOPO que levam à formação da fase hexagonal wurtzita. A cinética de crescimento dos quantum dots de CdSe também foram avaliadas através de alíquotas retiras com o tempo de reação mostrando um crescimento exponencial do diâmetro das partículas, como previsto pelas teorias de nucleação e crescimento. Estudos por microscopia de fluorescência mostraram que os quantum dots apresentaram o comportamento de intermitência de fluorescência relatado na literatura como um dos fatores que levam a uma diminuição do rendimento quântico de fluorescência. Nanoestruturas core/shell de CdSe/ZnS foram obtidas com elevado controle da espessura da camada de recobrimento e a intensificação das propriedades de fotoluminescência foram mostradas. Os objetivos do trabalho foram alcançados com sucesso, onde foi possível observar a estabilização e a intensificação da magnetização da fase de cobalto metálico, pouco relatado na literatura. Ainda, foi possível conferir maior estabilidade química, versatilidade de funcionalização da superfície e uma segunda propriedade de ressonância plasmônica com o recobrimento com ouro, sem grande prejuízo da propriedade magnética. Em relação aos sistemas ópticos, os semicondutores de CdSe foram obtidos por nova rota de síntese com expressivo controle de tamanho e forma, recobertos com ZnS intensificando as propriedades ópticas do sistema.  / Nanomaterials properties are size- and shape-controlled, such as the superparamagnetism phenomenon of magnetic nanoparticles or the quantum confinement of charge carriers of quantum dots. Therefore, synthesis of monodisperse spherical nanoparticles became extremely important over the past few deacades, since nanoparticles can be used for plenty of applications in technology and biomedicine. Magnetic nanoparticles and quantum dots are promising materials for diagnosis and therapy of cancer. Spherical nanoparticles of metallic cobalt were synthesized with mean diameter of 5,3 nm and standard deviation of 0,4 nm, lognormal distribution. A modified Langevin equation using the interacting superparamagnetic model was used to fit magnetization curves obtaining the mean magnetic diameter and standard deviation, 4,7 nm and 1,0 nm, respectively. The difference between these two diameters was assigned to the magnetic dead layer (∼3.0 Å), which does not contribute to the sample magnetization, being the saturation magnetization of cobalt nanoparticles around 125 emu g-1. Co/Au core/shell nanostructures were synthesized and the surface plasmon ressonance property was observed, an additional property also desired for biomedical applications, being the Co/Au core/shell system called magnetoplasmonic. CdSe quantum dots were synthesized with high size- and shape-controlled. Using different synthetic routes from the classic TOP-TOPO synthesis, and selenium dioxide as a precursor, the results show that and reducing agent is necessary and 1-octadecene solvent leads to better optical properties. CdSe samples showed a zinc blend (cubic phase) crystal structure, different from TOP-TOPO syntheses that leads to wurtzite structure (hexagonal phase). The growth kinetics of CdSe particles were also evaluated through aliquots from reaction showing exponential growth of particles diameter, as predicted on the theory of nucleation and growth. Fluorescence microscopy studies showed that quantum dots exhibited fluorescence intermittence behavior already reported in the literature as one fo the reasons for the quantum yield decrease. CdSe/ZnS core/shell nanostructures were obtained with high control of the coating layer thickness and the increase of the photoluminescence properties were shown.

Nanomateriais apresentam propriedades ajustáveis pelo seu tamanho e forma, como o fenômeno de superparamagnetismo em nanopartículas magnéticas ou o confinamento quântico dos portadores de carga em pontos quânticos (quantum dots). Assim, a síntese de nanopartículas esféricas monodispersas torna-se um fator extremamente importante, haja visto que tais propriedades podem ser ajustáveis para diferentes aplicações na área de tecnologia e biomedicina. Nanopartículas magnéticas e quantum dots podem ser apontados como promissores materiais para diagnóstico e terapia de neoplasias (câncer), e o desenvolvimento desses sistemas busca, atualmente, intensificar a magnetização e a eficiência de emissão, respectivamente, relativo às propriedades magnéticas e ópticas, além de outros requisitos. Neste trabalho, nanopartículas esféricas de cobalto metálico foram sintetizadas com diâmetro médio de 5,3 nm e desvio padrão de 0,4 nm, distribuição de tamanhos lognormal. A equação de Langevin modificada pelo modelo de partículas interagentes foi utilizada no ajuste da curva de magnetização M(H) para obtenção do diâmetro magnético médio e desvio padrão, 4,7 nm e 1,0 nm, respectivamente. Comparando os dois diâmetros, encontra-se uma camada morta de magnetização de aproximadamente 3,0 Å a qual, praticamente, não contribui para a magnetização da amostra, sendo a magnetização de saturação de 125 emu g-1. Nanoestruturas core/shell de Co/Au apresentaram a propriedade de ressonância plamon de superfície, uma propriedade adicional também desejada para aplicações biomédicas, sendo este sistema denominado magnetoplasmônico. Quantum dots de CdSe foram sintetizados como elevado controle de tamanho e forma. Utilizando rotas de síntese diferentes dos clássicos procedimentos denominados TOP-TOPO, e dióxido de selênio como precursor, estudos mostraram que na presença de um agente redutor no meio de reação e do solvente 1-octadeceno, as amostras apresentaram melhores propriedades óticas. A estrutura cristalina das amostras de CdSe corresponde à formação da fase blenda de zinco, diferentemente das sínteses TOP-TOPO que levam à formação da fase hexagonal wurtzita. A cinética de crescimento dos quantum dots de CdSe também foram avaliadas através de alíquotas retiras com o tempo de reação mostrando um crescimento exponencial do diâmetro das partículas, como previsto pelas teorias de nucleação e crescimento. Estudos por microscopia de fluorescência mostraram que os quantum dots apresentaram o comportamento de intermitência de fluorescência relatado na literatura como um dos fatores que levam a uma diminuição do rendimento quântico de fluorescência. Nanoestruturas core/shell de CdSe/ZnS foram obtidas com elevado controle da espessura da camada de recobrimento e a intensificação das propriedades de fotoluminescência foram mostradas. Os objetivos do trabalho foram alcançados com sucesso, onde foi possível observar a estabilização e a intensificação da magnetização da fase de cobalto metálico, pouco relatado na literatura. Ainda, foi possível conferir maior estabilidade química, versatilidade de funcionalização da superfície e uma segunda propriedade de ressonância plasmônica com o recobrimento com ouro, sem grande prejuízo da propriedade magnética. Em relação aos sistemas ópticos, os semicondutores de CdSe foram obtidos por nova rota de síntese com expressivo controle de tamanho e forma, recobertos com ZnS intensificando as propriedades ópticas do sistema.  / Nanomaterials properties are size- and shape-controlled, such as the superparamagnetism phenomenon of magnetic nanoparticles or the quantum confinement of charge carriers of quantum dots. Therefore, synthesis of monodisperse spherical nanoparticles became extremely important over the past few deacades, since nanoparticles can be used for plenty of applications in technology and biomedicine. Magnetic nanoparticles and quantum dots are promising materials for diagnosis and therapy of cancer. Spherical nanoparticles of metallic cobalt were synthesized with mean diameter of 5,3 nm and standard deviation of 0,4 nm, lognormal distribution. A modified Langevin equation using the interacting superparamagnetic model was used to fit magnetization curves obtaining the mean magnetic diameter and standard deviation, 4,7 nm and 1,0 nm, respectively. The difference between these two diameters was assigned to the magnetic dead layer (∼3.0 Å), which does not contribute to the sample magnetization, being the saturation magnetization of cobalt nanoparticles around 125 emu g-1. Co/Au core/shell nanostructures were synthesized and the surface plasmon ressonance property was observed, an additional property also desired for biomedical applications, being the Co/Au core/shell system called magnetoplasmonic. CdSe quantum dots were synthesized with high size- and shape-controlled. Using different synthetic routes from the classic TOP-TOPO synthesis, and selenium dioxide as a precursor, the results show that and reducing agent is necessary and 1-octadecene solvent leads to better optical properties. CdSe samples showed a zinc blend (cubic phase) crystal structure, different from TOP-TOPO syntheses that leads to wurtzite structure (hexagonal phase). The growth kinetics of CdSe particles were also evaluated through aliquots from reaction showing exponential growth of particles diameter, as predicted on the theory of nucleation and growth. Fluorescence microscopy studies showed that quantum dots exhibited fluorescence intermittence behavior already reported in the literature as one fo the reasons for the quantum yield decrease. CdSe/ZnS core/shell nanostructures were obtained with high control of the coating layer thickness and the increase of the photoluminescence properties were shown.

Philosophiae Doctor - PhD / This work investigated the synthesis of Al2O3-coated TiO2 nanotubes via the anodisation technique for application in DSCs. TiO2 nanotube arrays with an average length of 15 μm, diameter of 50 nm and wall thickness of 15 nm were synthesised via anodisation using an organic neutral electrolyte consisting of 2 M H2O + 0.15 M NH4F + ethylene glycol (EG) at an applied voltage of 60 V for 6 hours. In addition, scanning electron microscope (SEM) micrographs showed that anodisation at these conditions yields nanotubes with smooth walls and hexagonally shaped, closed bottoms. X-ray diffraction (XRD) patterns revealed that the as-anodised nanotubes were amorphous and as such were annealed at 450 °C for 2 hours in air at atmospheric pressure, which yielded crystalline anatase TiO2 nanotubes. Highresolution transmission electron microscope (TEM) images revealed that the nanotube walls comprised of individual nano-sized TiO2 crystallites. Photoluminescence (PL) spectroscopy showed that the optical properties, especially the bandgap of the TiO2 nanotubes are dependent on the crystallinity, which in turn was dependent on the structural characteristics, such as the wall thickness, diameter and length. The PL measurements were supplemented by Raman spectra, which revealed an increased in the quantum confinement of the optical phonon modes of the nanotubes synthesised at low anodisation voltages, consequently yielding a larger bandgap The annealed nanotubes were then coated with a thin layer of alumina (Al2O3) using a simple sol-gel dip coating method, effectively used to coat films of nanoparticles. Atomic force microscopy (AFM) showed that the average nanotube diameter increased post sol-gel deposition, which suggests that the nanotubes are coated with a layer of Al2O3. This was confirmed with HR-TEM, in conjunction with selected area electron diffraction (SAED) and XRD analyses, which showed the coating of the nanotube walls with a thin layer of amorphous Al2O3 with a thickness between 4 and 7 nm. Ultraviolet-visible (UVvis) absorbance spectra showed that the dye-adsorption ability of the nanotubes are enhanced by the Al2O3 coating and hence is a viable material for solar cell application. Upon application in the DSC, it was found by means of photo-current density – voltage (I – V) measurements that a DSC fabricated with a 15 μm thick layer of bare TiO2 nanotubes has a photon-to-light conversion efficiency of 4.56%, which increased to 4.88% after coating the nanotubes with a layer of alumina. However, these devices had poorer conversion efficiencies than bare and Al2O3-coated TiO2 nanoparticle based DSCs, which boasted with efficiencies of 6.54 and 7.26%, respectively. The low efficiencies of the TiO2 nanotube based DSCs are ascribed to the low surface area of the layer of nanotubes, which yielded low photocurrent densities. Electrochemical impedance spectroscopy (EIS) showed that the electron lifetime in the alumina coated nanotubes are almost 20 times greater than in a bare layer of nanoparticles. In addition, it was also found that the charge transfer resistance at the interface of the TiO2/dye/electrolyte is the lowest for an Al2O3-coated TiO2 layer.

Dans cette thèse nous avons étudié en détail les propriétés mécaniques de l’or sous forme de nanofils et de structures nanoporeuses revêtues ou non de silicium amorphe (a-Si). Ces travaux ont été effectués par dynamique moléculaire. Nous avons dans un premier temps étudié la déformation cyclique de nanofils d’or (NF-Au) et de nanofils cœur-coquille or-silicium amorphe (NF-AuSi). Ces simulations ont montré que le NF-Au est déformé au cours des cycles par deux mécanismes prépondérants : le maclage extensif puis le glissement d’un unique plan atomique. Le cyclage a pour effet d’altérer progressivement la morphologie de la structure en augmentant le nombre et la taille des défauts créés en surface. La déformation cyclique du NF-AuSi montre que le revêtement de a-Si délocalise la plasticité le long de la structure et permet de mieux conserver la morphologie initiale du cœur. Nous avons ensuite développé une méthode originale de génération de l’or nanoporeux. Cette méthode a été validée par la comparaison structurale et mécanique avec des résultats expérimentaux. Puis nous avons étudié la déformation en traction et en compression de différentes structures générées par cette méthode. Nous avons dans les deux cas mis en évidence les mécanismes de déformation des ligaments. En traction, nous avons apporté de nouveaux résultats permettant de mieux comprendre pourquoi l’or nanoporeux est fragile alors que l’or massif est ductile. En particulier, nous avons étudié comment s’opère la fracture en cascade des ligaments par transfert de contrainte entre ceux-ci. En compression nous avons entre autres montré que l’effondrement des pores et la création de joints de grains est responsable de l’augmentation de la contrainte à la transition écoulement-densification. Les simulations de traction et de compression des mêmes structures mais revêtues de silicium amorphe montrent plusieurs résultats intéressants. Par exemple, la résistance des structures est augmentée d’un facteur 2 à 4. De plus, le revêtement a pour effet de délocaliser la plasticité ce qui augmente la ductilité notamment en traction. En compression, la transition écoulement-densification est avancée probablement en raison de la diminution de la taille des pores causée par le revêtement. / In this thesis we have studied in detail the mechanical properties of gold nanowires and nanoporous gold with and without an amorphous silicon coating (a-Si). This work was done using molecular dynamics simulation. We first studied the cyclic deformation of gold nanowires (Au-NW) and gold-silicon core-shell nanowires (AuSi-NW). These simulations showed that the Au-NW is deformed during cyclic loading by two main mechanisms: extensive twinning and the slip of a single atomic plane. Cycling gradually alters the morphology of the structure by increasing the number and size of defects created on the surface. The cyclic deformation of the AuSi-NW shows that the a-Si coating delocalizes the plasticity along the structure and allows to better preserve the initial morphology of the core. We then developed an original method for generating nanoporous gold. This method was validated by structural and mechanical comparison with experimental results. Then we studied the tensile and compressive deformation of different structures generated by this method. In both cases, we have highlighted the deformation mechanisms of ligaments. In tension, our simulations have brought new results to better understand why nanoporous gold is brittle while bulk gold is ductile. In particular, we studied how the catastrophic failure of ligaments occurs by stress transfer between them. In compression we have shown, for example, that pore collapse and the creation of grain boundaries are responsible for the increase of stress at the transition from flow to densification. Tensile and compression tests simulations on the same structures but coated with amorphous silicon show several interesting results. For example, the strength of the structures is increased by a factor of 2 to 4. In addition, the coating has the effect of delocalizing the plasticity, which increases ductility, particularly in tension. In compression, the transition from flow to densification is advanced probably due to the decrease in pore size caused by the coating.