Eletrosíntese de nanopartículas metálicas 1-D de DES usando modelos anódicos porosos

Doutoramento em Ciência e Engenharia de MateriaisO método de síntese de nanoparticulas 1-D assistido por um modelo tornou-se um tópico em voga na química após o desenvolvimento de filmes anódicos com poros bem ordenados. Contudo, a maioria dos trabalhos nesta área tem sido feita utilizando filmes porosos destacados devido à presença de uma barreira no fundo dos poros. No entanto, esta estratégia segue demasiados passos, o que aumenta o seu custo, torna mais difícil a execução e impõe várias limitações. Consequentemente, existe a necessidade de uma técnica que permita o enchimento (electrofilling) dos tubos sem remover a camada barreira – esta tese representa o nosso contributo para esse trabalho. Utilizámos uma técnica mais simples que permite a electrodeposição e “electrofilling” de nanoestruturas directamente nos modelos sobre o substrato metálico, utilizando solventes eutécticos profundos à base de cloreto de colina como electrólito. Relativamente à água, os solventes eutécticos profundos demonstram superior estabilidade térmica e uma janela electroquímica mais alargada, o que aumenta o número de materais secundários depositados. Como materiais a investigar foram escolhidos titânia e alumina dada a sua capacidade para formar estruturas porosas altamente ordenadas, propriedades eletroquímicas distintas e uso generalizado em síntese assistida por padrão. O estudo aqui apresentado encontra-se dividido em duas etapas. Primeiramente, a influência da camada barreira foi investigada em sistemas modelo através da utilização de filmes barreira densos na superfície dos elétrodos. Para os filmes de alumina e titânia, identificaram-se vários parâmetros que afectam a electrodeposição, dos quais se destacam a influência da voltagem de anodização, a espessura da camada de barreira, a dupla camada eléctrica e o perfil de corrente. Durante esta etapa detectaram-se efeitos nefastos, como a formação de uma densa camada orgânica na superfície do eléctrodo, que foram ultrapassados aumentando a temperatura ou alternando o potencial aplicado. A segunda etapa consistiu em passar de elétrodos planos (primeira etapa) para modelos porosos (segunda etapa). Foi realizado, com sucesso, o preenchimento dos poros de alumina e dos poros de titânia. Parâmetros como o perfil de corrente, temperatura de solução, entre outras, foram ajustadas para melhorar o fator de preenchimento e a homogeneidade do preenchimento. Foi desenvolvido um processo de preenchimento de moldes de alumina anódica em duas etapas, nucleação AC (1º passo) e preenchimento galvanostático (2º passo). Foram utilizadas três condições diferentes de modelos de titânia anódica porosa no “electrofilling”. O primeiro é sem modificação e demonstrou que a electroredução do zinco ocorre de forma aleatória ao longo de todo o comprimento do poro, o que leva ao fecho do poro e a um enchimento não homogéneo. A segunda modificação, cristalização total por têmpera, permite a preparação de estruturas coaxiais devido à deposição uniforme de zinco nas paredes dos poros. A última modificação foi a cristalização selectiva do fundo do poro. Foi descoberto que uma anodização adicional em eletrólitos não agressivos leva à cristalização da parte barreira dos tubos (fundo) e, consequentemente, a maior condutividade na parte inferior do que nas paredes. Este efeito permite um enchimento ascendente dos modelos porosos de titânia. As estratégias aqui apresentadas alargam a gama de possibilidades para a aplicação de modelos porosos anódicos na electrodeposição de diferentes nanoestruturas.The template assisted method of 1-D nanoparticles synthesis has become a hot topic in Chemistry after the development of high-ordered porous anodic films. Most studies in this field have focused on the use of detached porous films due to the presence of the barrier layer on the pore bottom. However, this strategy follows a great number of steps, which raises its cost while decreasing convenience of operation and imposing several limitations. Consequently, there is a need for a technique which allows electrofilling of tubes without removing the barrier layer – this thesis represents our contribution to that enterprise. We have devised a simpler technique which allows electrodeposition of nanostructures directly in the templates on metallic substrate, using choline chloride based deep eutectic solvents (DES) as electrolyte. Compared to water, DES have improved thermal stability and a wider electrochemical window, dramatically increasing the number of possible secondary materials deposited. Titania and alumina were chosen as materials under study due to their known capacity to form highly-ordered porous structures, different electrochemical profiles and widespread use in template assisted synthesis. The present work is divided in two parts. First, the influence of the barrier layer has been investigated by using dense barrier films on the electrode surface as a model system. For both alumina and titania films, several parameters affecting the electrodeposition of zinc have been identified, notably the influence of the anodization voltage, barrier layer thickness, electrical double layer and current profile. During this stage, some negative effects have been detected, such as a dense organic layer formation on electrode surface, a hurdle which has been overcome by either increasing the temperature or applying the alternating potential. The second stage consisted in transferring the method from the flat electrodes (the first stage) to the porous templates. The successful filling of both porous alumina and porous titania, has been achieved. Parameters such as current profile, solution temperature, among others, have been tuned to improve the fill factor and homogeneity of the filling. A two-step porous anodic alumina template filling with AC nucleation (1st step) and galvanostatic filling (2nd step) has been developed. Three different types of porous anodic titania templates have been used for electrofilling. The first one was used as-prepared, showing that zinc electroreduction occurs in random places along all pore length, resulting in pore sealing and non-homogeneous filling. The second modification, full crystallization by annealing, allows the preparation of coaxial structures due to uniform zinc deposition on the pore walls. The last modification is selective bottom crystallization. It has been found that additional anodization in unaggressive electrolytes leads to crystallization of the barrier (bottom) part of the tubes and, thus, to higher conductivity of the bottom part than that of the walls. This effect allows a bottom-up filling of the titania porous template. The strategies presented here widen the range of possibilities for the application of porous anodic templates in the electrodeposition of different nanostructures

The Influence of PSA Pre-Anodization of AA2024 on PEO Coating Formation: Composition, Microstructure, Corrosion, and Wear Behaviors

In the frame of the current work, it was shown that plasma electrolytic oxidation (PEO) treatment can be applied on top of phosphoric sulfuric acid (PSA) anodized aluminum alloy AA2024. Being hard and well-adherent to the substrate, PEO layers improve both corrosion and wear resistance of the material. To facilitate PEO formation and achieve a dense layer, the systematic analysis of PEO layer formation on the preliminary PSA anodized layer was performed in this work. The microstructure, morphology, and composition of formed PEO coatings were investigated using scanning electron microscopy (SEM), x-ray diffraction (XRD), and glow-discharge optical emission spectroscopy (GDOES). It was shown that under constant current treatment conditions, the PSA layer survived under the applied voltage of 350 V, whilst 400 V was an intermediate stage; and under 450 V, the PSA layer was fully converted after 5 min of the treatment. The comparison test with PEO formation on the bare material was performed. It was confirmed that during the "sparking" mode (400 V) of PEO formation, the PEO coatings, formed on PSA treated AA2024, were more wear resistant than the same PEO coatings on bare AA2024

Effect of fluoride-mediated transformations on electrocatalytic performance of thermally treated TiO2 nanotubular layers

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Electrocatalytic activity of Au nanoparticles onto TiO2 nanotubular layers in oxygen electroreduction reaction: size and support effects

Electrocatalytic activity of both bare high-ordered TiO2 nanotubes (TNTs) and gold nanoparticles (Au NPs) loaded TNTs toward oxygen reduction reaction (ORR) has been examined by cyclic voltammetry (CV). Cyclic voltammograms for Au NPs-TNT electrodes are characterized by an additional wave observed at less negative potentials which is responsible for oxygen electroreduction on the surface of gold NPs. The overpotential for O-2 reduction on the Au surface grows with increasing the temperature of TNT annealing and the Au NPs size. The nature of the effects observed was explained by peculiarities of the electron transport through Schottky barrier formed at the Au NPs - TiO2 interface. The width of the Schottky barrier, determined by semiconductor doping level and Au NPs size, plays a key role in the mechanism of electron transport through the space charge region. (C) 2016 Elsevier Ltd. All rights reserved

UV-assisted anchoring of gold nanoparticles into TiO2 nanotubes for oxygen electroreduction

Gold nanoparticles (AuNPs) have been deposited on titania nanotubular layers (TNT) via photocatalytic deposition, and the activity of the obtained AuNPs-TNT systems toward oxygen electroreduction reaction (ORR) in an alkaline medium has been studied and compared with the activity of AuNPs TNT composites prepared from Au colloidal solutions. Two photodeposition methods were utilized for anchoring of AuNPs: direct UV-irradiation of a TNT electrode immersed into a HAuCl4 containing solution (TNT-Au1 composites) and initial adsorption of AuCl4− ions on TNT followed by UV irradiation (TNT-Au2 composites). The size, spatial distribution, structure and surface of AuNPs deposited on TNT layers were studied via scanning and transmission electron microscopy, optical spectroscopy and underpotential deposition of lead ad-atoms on gold. It was found that these parameters depend on the photodeposition method. The TNT-Au2 composites have smaller size, higher surface concentration and more uniform distribution of AuNPs in the TNT layers as compared with the TNT-Au1 systems. The electrocatalytic efficiency of Au-TNT electrodes in ORR was found to depend on various factors such as doping level of TNT support (governed by annealing temperature), AuNPs size and their loading amount. The electroreduction of oxygen was observed at less negative potentials when Au nanoparticles were grown on the TNT surface by photoreduction in comparison with the TNT electrodes modified with AuNPs from sols. The enhanced activity of the photocatalytically prepared AuNPs-TNT composites can be explained by the consolidation of the interface between gold nanoparticles and TiO2 support and the absence of ligands on the AuNPs surface.publishe

Electrodeposition of Zinc Nanorods from Ionic Liquid into Porous Anodic Alumina

Electrochemical deposition of zinc nanorods into the anodic aluminium-oxide template from ionic-liquid electrolyte is reported. For the first time, the electrodeposition from an ionic liquid is performed in a porous alumina template, anodically grown on the aluminium substrate without complete removal of the barrier layer. The two-step process was applied; first, with alternating-current pulse nucleation of zinc nanoparticles on the bottom of the pores, followed by direct-current growth of the rods. The electrolyte consisted of 0.5 M ZnCl2 in choline chloride and ethylene glycol in a 1:2 molar ratiod. The resulting zinc nanorods are approximately 3 mu m in length and 70 nm in diameter. The achieved fill factor of the pores is in the 70-80 % range

Photocatalytic Deposition of Hydroxyapatite onto a Titanium Dioxide Nanotubular Layer with Fine Tuning of Layer Nanoarchitecture

A new effective method of photocatalytic deposition of hydroxyapatite (HA) onto semiconductor substrates is proposed. A highly ordered nanotubular TiO2 (TNT) layer formed on titanium via its anodization is chosen as the photoactive substrate. The method is based on photodecomposition of the phosphate anion precursor, triethylphosphate (TEP), on the semiconductor surface with the following reaction of formed phosphate anions with calcium cations presented in the solution. HA can be deposited only on irradiated areas, providing the possibility of photoresistfree HA patterning. It is shown that HA deposition can be controlled via pH, light intensity, and duration of the process. Energy-dispersive X-ray spectroscopy profile analysis and glow discharge optical emission spectroscopy of HA-modified TNT prove that HA deposits over the entire TNT depth. High is proven by protein adsorption and pre-osteoblast cell growth

Degradation behavior of PEO coating on AM50 magnesium alloy produced from electrolytes with clay particle addition

Amorphous plasma electrolytic oxidation (PEO) coating with sealed discharge channels can be formed in alkaline phosphate electrolyte containing montmorillonite clay particles. The effect of various concentrations of phosphate and hydroxide ions in the clay-containing electrolyte on the microstructure of the coatings was studied in the present work and correlated with the corrosion behavior. The clay particles were reactively incorporated into the coating. Single amorphous phase appears in PEO coatings produced from electrolytes containing higher concentration of phosphate. These amorphous coatings are degrading within a relatively short period in 0.5 wt% NaCl solution. Electrolytes containing higher concentration of KOH tend to produce mixed PEO coatings composed of crystalline and amorphous phases. These layers demonstrate higher corrosion resistance and degradation stability. Thus, the degradation rate of PEO coatings is governed mostly by the stability of their phase composition, which might be controlled by varying electrolyte composition. (C) 2014 Elsevier B.V. All rights reserved

Formation and structure of ZIF-8@PEO coating on the surface of zinc

Recently, plasma electrolytic oxidation (PEO) found broad application as a multi-purpose process to create effective corrosion and wear resistant coatings on various metallic substrates. The exceptional properties of metal organic frameworks (MOFs) put them also in focus as perspective materials for corrosion protection. In this work, the formation of a novel ZIF-8@PEO coating is reported for the first time. It was synthesized by controllable recrystallization of a PEO layer formed on zinc alloy Z1 into ZIF-8 in the presence of 2-methylimidazole organic linkers. The multi-stage mechanism of PEO to ZIF-8 rearrangement is proposed. Cross section, glow discharge optical emission spectroscopy and nano-focused synchrotron X-ray diffraction demonstrated that varying of synthesis parameters, the ZIF-8@PEO coating with different distribution of ZIF-8 through PEO layer can be prepared. Based on the results of laser scanning microscopy, the surface smoothing was observed with increasing the degree of the PEO-to-ZIF-8 rearrangement. Containing two components, the novel ZIF-8@PEO coating is expected to combine admirable physical-chemical properties of both PEO and ZIF-8. Such a feature can open the way for its potential application not only for corrosion protection, but also for photo- and heterogeneous catalysis

Layered Double Hydroxide Clusters as Precursors of Novel Multifunctional Layers: A Bottom-Up Approach

The specific microstructure of aluminum alloys is herein explored to grow spatially-resolved layered double hydroxide (SR-LDH) clusters on their surface. Upon chemical modification of LDHs via intercalation, adsorption and grafting with different functional molecules, novel surface-engineered surfaces were obtained. Crystal structure and phase composition were analyzed by X-ray diffraction (XRD) and surface morphology was observed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectrometry (GDOES) were used to correlate structural changes upon ion-exchange and interfacial modifications with chemical composition and surface profiles of the SR-LDH films, respectively. The protection conferred by these films against localized corrosion was investigated at microscale using the scanning vibrating electrode technique (SVET). LDH-NO3 phase was obtained by direct growth onto AA2024 surface, as evidenced by (003) and (006) XRD diffraction reflections. After anion exchange of nitrate with 2-mercaptobenzothiazole (MBT) there was a decrease in the SR-LDH thickness inferred from GDOES profiles. The subsequent surface functionalization with HTMS was confirmed by the presence of Si signal in XPS and GDOES analyses, leading to an increase in the water contact angle (c.a 144° ± 3°). SVET measurements of the SR-LDH films revealed exceptional corrosion resistance, whereas the bioluminescent bacteria assay proved the anti-microbial character of the obtained films. Overall the results obtained show an effective corrosion protection of the SR-LDHs when compared to the bare substrate and the potential of these films for biofouling applications as new Cr-free pre-treatments