Thin mercury film electrodes in dynamic speciation studies of metals

No presente trabalho, foi avaliado o desempenho e a aplicabilidade do eléctrodo de filme fino de mercúrio, em estudos de especiação dinâmica de metais vestigiais. Para tal, foram utilizadas duas técnicas electroanalíticas de redissolução: a clássica Voltametria de Redissolução Anódica (ASV) e a recentemente desenvolvida, Cronopotenciometria de Redissolução com varrimento do potencial de deposição (SSCP). As propriedades de troca-iónica e de transporte de massa de películas mistas preparadas a partir de dois polímeros com características distintas, o Nafion (NA) e o 4-Poliestireno sulfonato de sódio (PSS), foram avaliadas, antes da sua aplicação no âmbito da especiação de metais. Estas películas de NA-PSS demonstraram uma elevada sensibilidade, reprodutibilidade, estabilidade mecânica, bem como, propriedades de anti-bloqueio adequadas na modificação química do eléctrodo de filme fino de mercúrio (TMFE) e, na sua aplicação na determinação de catiões metálicos vestigiais em amostras complexas, por ASV. Para além disso, o desempenho de membranas do polielectrólito PSS em estudos de voltametria de troca-iónica (IEV) foi estudado. O objectivo desta investigação foi reunir as condições ideais na preparação de películas de PSS estáveis e com uma densidade de carga negativa elevada, de modo a aumentar a acumulação electrostática de catiões metálicos no filme polimérico e por conseguinte, conseguir incrementos no sinal voltamétrico. O desempenho e aplicabilidade do TMFE em estudos de especiação de metais vestigiais foram extendidos à SSCP como técnica analítica. Dada a elevada sensibilidade e resolução evidenciada pelo TMFE, este revelou ser uma alternativa adequada aos eléctrodos de mercúrio convencionais, podendo ser utilizado durante um dia de trabalho, sem degradação aparente do sinal analítico de SCP. As curvas de SSCP obtidas experimentalmente utilizando o TMFE estavam em concordância com aquelas previstas pela teoria. Para além disso, a constante de estabilidade (K) calculada a partir do desvio do potencial de meia-onda, para dois sistemas metal-complexo lábeis, aproxima-se não só do valor teórico, como também daquele obtido utilizando o eléctrodo de mercúrio de gota suspensa (HMDE). Adicionalmente, o critério experimental de labilidade inerente a esta técnica foi validado e o grau de labilidade para um dado sistema metal-complexo foi determinado, utilizando o filme fino de mercúrio depositado sob um eléctrodo rotativo (TMF-RDE). Este eléctrodo é muito útil na determinação de parâmetros cinéticos, como é o caso da constante de velocidade de associação (ka), uma vez que as condições hidrodinâmicas, durante a etapa de deposição, se encontram bem definidas.

Modified electrodes with Keggin-type polyoxotungstates

O objectivo deste trabalho é a produção de novos eléctrodos modificados com polioxotungstatos (POMs) do tipo Keggin, incluindo POMs lacunares e substituídos por metais de transição. A preparação e caracterização dos polioxotungstatos encontram-se descritas no Capítulo 2. No Capítulo 3 descreve-se a produção de eléctrodos de carbono vítreo funcionalizados com sais híbridos de tetra-n-butilamónio de vários silicotungstatos pelo método de evaporação da gota. As propriedades electroquímicas dos polioxotungstatos imobilizados foram comparadas com as das espécies solúveis correspondentes. A morfologia dos depósitos foi avaliada por microscopia óptica e por microscopia electrónica de varrimento. No capítulo 4 descreve-se a preparação de novos eléctrodos compósitos de carbono e poli(hexilmetacrilato) com fosfotungstatos. Os estudos electroquímicos revelaram que as principais características dos POMs são mantidas e que os processos de redução são controlados por difusão, dependendo da difusão dos protões da solução. O Capítulo 5 descreve a construção de filmes em multicamadas ultrafinos contendo POMs e polietilenimina, preparados pelo método de auto-montagem camada-sobre-camada em eléctrodos de carbono vítreo. Os filmes em multicamada foram caracterizados por voltametria cíclica e por microscopia electrónica de varrimento e foi usada a espectroscopia de absorção de UV-Vis em placas de quartzo para monitorar o crescimento de filme. Os resultados voltamétricos revelaram que os processos de redução dos POM são confinados à superfície. Alguns destes eléctrodos modificados revelaram propriedades electrocatalíticas relativamente à redução dos aniões nitrito, bromato e/ou iodato. A espectroscopia de impedância electroquímica também foi usada na caracterização destes filmes e os resultados revelaram que a resistência à transferência de carga aumenta com o aumento do número de bicamadas para ambas as espécies redox, indicando que a espessura do filme tem um efeito importante sobre a cinética de reacções de transferência de carga. No capítulo 6 descreve-se a síntese de filmes híbridos orgânicos/inorgânicos compostos por poli(3,4-etilenodioxitiofeno) (PEDOT) e por silicotungstatos do tipo Keggin através da polimerização electroquímica, em condições aquosas, na superfície de electrodos de carbono vítreo. A voltametria cíclica revelou que as características principais dos POMs são mantidas nos filmes. Verificou-se que estes filmes são muito estáveis, possivelmente devido a fortes interacções electrostáticas entre os POMs aniónicos e o polímero positivamente carregado. A espectroscopia de impedância electroquímica foi também utilizada e os resultados mostraram que a resistência de transferência de carga aumenta com o aumento do pH e para valores de potenciais mais elevados. O capítulo 7 apresenta as conclusões finais e possíveis trabalhos futuros.

Novos materiais com polioxotungstatos substituídos com metais d e f

The present work reports studies on the new compounds obtained by the combination of polyoxoanions derived from the Keggin and Lindquist structures with several cations. The studies were first focused on the monolacunary Keggin polyoxoanions [PW11O39M(H2O)]n- (M = FeIII, MnIII and n = 4; M = CoII and n = 5) and its combination with the organic cation 1-butyl-3-methylimidazolium (Bmim+). The association of Bmim+ cation with the polyoxoanion [PW11O39Fe(H2O)]4- allowed to isolate for the first time both the monomeric and the dimeric [PW11O39Fe)2O]10- anions, with the same cation and using simple bench techniques by pH manipulation. Studies regarding the stability of these inorganic species in solution indicated that both species are present in solution in equilibrium. However, the inability to up until now isolate the dimeric unit through simple bench methods, lead to the hypothesis that the cation had a role to play in the selective precipitation of either the monomer or the dimer. Repetition of the same procedures with the polyoxoanions [SiW11O39Fe(H2O)]5- and [PW11O39M(H2O)]n- (M = FeIII, MnIII and n = 4; M = Co and n = 5), afforded only the corresponding monomeric compounds, (Bmim)5[SiW11O39FeIII(H2O)]· 4H2O (3), (Bmim)5[PW11O39CoII(H2O)]· 0.5 H2O, (4) and (Bmim)5[PW11O39MnIII(H2O)]· 0.5 H2O (5). Moreover, the combination of Bmim+ and the polyoxotungstate [PW11O39Co(H2O)]5- afforded two different crystal structures, depending on the synthetic conditions. Thus, a ratio Bmim+:POM of 5:1 and the presence of K+ cations (due to addition of KOH) led to a formula Na2K(Bmim)2[PW11.2O39Co0.8(H2O)]·7H2O (4a), whilst a ratio Bmim:POM of 7:1 led to the formation of a crystal with the chemical formula Na2(Bmim)8[PW11O39Co(H2O)]2·3H2O (4b). Electrochemical studies were performed with carbon paste electrodes modified with BmimCl to investigate the influence of the Bmim+ cation in the performance of the electrodes. The voltametric measurements obtained from solutions containing the anions [PW11O39]7- and [SiW11O39]8- are presented. Results pointed to an improvement of the acquired voltametric signal with a slight addition of BmimCl (up to 2.5% w/w), specially in the studies regarding pH variation. Additional synthesis were carried out with both the cations Omim+ and THTP+.

Ferroelectric : CNTs structures fabrication for advanced functional nano devices

This work is about the combination of functional ferroelectric oxides with Multiwall Carbon Nanotubes for microelectronic applications, as for example potential 3 Dimensional (3D) Non Volatile Ferroelectric Random Access Memories (NVFeRAM). Miniaturized electronics are ubiquitous now. The drive to downsize electronics has been spurred by needs of more performance into smaller packages at lower costs. But the trend of electronics miniaturization challenges board assembly materials, processes, and reliability. Semiconductor device and integrated circuit technology, coupled with its associated electronic packaging, forms the backbone of high-performance miniaturized electronic systems. However, as size decreases and functionalization increases in the modern electronics further size reduction is getting difficult; below a size limit the signal reliability and device performance deteriorate. Hence miniaturization of siliconbased electronics has limitations. On this background the Road Map for Semiconductor Industry (ITRS) suggests since 2011 alternative technologies, designated as More than Moore; being one of them based on carbon (carbon nanotubes (CNTs) and graphene) [1]. CNTs with their unique performance and three dimensionality at the nano-scale have been regarded as promising elements for miniaturized electronics [2]. CNTs are tubular in geometry and possess a unique set of properties, including ballistic electron transportation and a huge current caring capacity, which make them of great interest for future microelectronics [2]. Indeed CNTs might have a key role in the miniaturization of Non Volatile Ferroelectric Random Access Memories (NVFeRAM). Moving from a traditional two dimensional (2D) design (as is the case of thin films) to a 3D structure (based on a tridimensional arrangement of unidimensional structures) will result in the high reliability and sensing of the signals due to the large contribution from the bottom electrode. One way to achieve this 3D design is by using CNTs. Ferroelectrics (FE) are spontaneously polarized and can have high dielectric constants and interesting pyroelectric, piezoelectric, and electrooptic properties, being a key application of FE electronic memories. However, combining CNTs with FE functional oxides is challenging. It starts with materials compatibility, since crystallization temperature of FE and oxidation temperature of CNTs may overlap. In this case low temperature processing of FE is fundamental. Within this context in this work a systematic study on the fabrication of CNTs - FE structures using low cost low temperature methods was carried out. The FE under study are comprised of lead zirconate titanate (Pb1-xZrxTiO3, PZT), barium titanate (BaTiO3, BT) and bismuth ferrite (BiFeO3, BFO). The various aspects related to the fabrication, such as effect on thermal stability of MWCNTs, FE phase formation in presence of MWCNTs and interfaces between the CNTs/FE are addressed in this work. The ferroelectric response locally measured by Piezoresponse Force Microscopy (PFM) clearly evidenced that even at low processing temperatures FE on CNTs retain its ferroelectric nature. The work started by verifying the thermal decomposition behavior under different conditions of the multiwall CNTs (MWCNTs) used in this work. It was verified that purified MWCNTs are stable up to 420 ºC in air, as no weight loss occurs under non isothermal conditions, but morphology changes were observed for isothermal conditions at 400 ºC by Raman spectroscopy and Transmission Electron Microscopy (TEM). In oxygen-rich atmosphere MWCNTs started to oxidized at 200 ºC. However in argon-rich one and under a high heating rate MWCNTs remain stable up to 1300 ºC with a minimum sublimation. The activation energy for the decomposition of MWCNTs in air was calculated to lie between 80 and 108 kJ/mol. These results are relevant for the fabrication of MWCNTs – FE structures. Indeed we demonstrate that PZT can be deposited by sol gel at low temperatures on MWCNTs. And particularly interesting we prove that MWCNTs decrease the temperature and time for formation of PZT by ~100 ºC commensurate with a decrease in activation energy from 68±15 kJ/mol to 27±2 kJ/mol. As a consequence, monophasic PZT was obtained at 575 ºC for MWCNTs - PZT whereas for pure PZT traces of pyrochlore were still present at 650 ºC, where PZT phase formed due to homogeneous nucleation. The piezoelectric nature of MWCNTs - PZT synthesised at 500 ºC for 1 h was proved by PFM. In the continuation of this work we developed a low cost methodology of coating MWCNTs using a hybrid sol-gel / hydrothermal method. In this case the FE used as a proof of concept was BT. BT is a well-known lead free perovskite used in many microelectronic applications. However, synthesis by solid state reaction is typically performed around 1100 to 1300 ºC what jeopardizes the combination with MWCNTs. We also illustrate the ineffectiveness of conventional hydrothermal synthesis in this process due the formation of carbonates, namely BaCO3. The grown MWCNTs - BT structures are ferroelectric and exhibit an electromechanical response (15 pm/V). These results have broad implications since this strategy can also be extended to other compounds of materials with high crystallization temperatures. In addition the coverage of MWCNTs with FE can be optimized, in this case with non covalent functionalization of the tubes, namely with sodium dodecyl sulfate (SDS). MWCNTs were used as templates to grow, in this case single phase multiferroic BFO nanorods. This work shows that the use of nitric solvent results in severe damages of the MWCNTs layers that results in the early oxidation of the tubes during the annealing treatment. It was also observed that the use of nitric solvent results in the partial filling of MWCNTs with BFO due to the low surface tension (<119 mN/m) of the nitric solution. The opening of the caps and filling of the tubes occurs simultaneously during the refluxing step. Furthermore we verified that MWCNTs have a critical role in the fabrication of monophasic BFO; i.e. the oxidation of CNTs during the annealing process causes an oxygen deficient atmosphere that restrains the formation of Bi2O3 and monophasic BFO can be obtained. The morphology of the obtained BFO nano structures indicates that MWCNTs act as template to grow 1D structure of BFO. Magnetic measurements on these BFO nanostructures revealed a week ferromagnetic hysteresis loop with a coercive field of 956 Oe at 5 K. We also exploited the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO3 (BFO) films were in-situ deposited on the surface of VA-MWCNTs by RF (Radio Frequency) magnetron sputtering. For in situ deposition temperature of 400 ºC and deposition time up to 2 h, BFO films cover the VA-MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by PFM. A week ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700 Oe. Our systematic work is a significant step forward in the development of 3D memory cells; it clearly demonstrates that CNTs can be combined with FE oxides and can be used, for example, as the next 3D generation of FERAMs, not excluding however other different applications in microelectronics.