Dissertations / Theses on the topic 'Electrochemical techniques'

Electrochemical Jet Processing (EJP) techniques have been traditionally limited in application by the inherent geometric inflexibility and limited process precision in comparison to alternative processes. It has been stated that process resultant geometries are defined by the Gaussian in-jet energy distribution and the hydrodynamic stagnation region formed under a jet on an impinging surface. This thesis reports upon investigations and innovations designed to challenge these assumptions. EJP is an emergent manufacturing process with a unique capability of subtraction and deposition of metals within a common machine tool. EJP demonstrates advantages beyond traditional electrochemical machining and electrochemical deposition including a high degree of flexibility, simplistic and therefore low-cost plant, requiring no complex, high-cost tooling and no masking requirement to achieve high fidelity geometries. These process traits are attractive to industry but EJP has yet to find significant commercial use. Electromechanical and electrochemical innovations are presented here demonstrated by electrochemical jet machining (EJM) the subtractive mode of EJP, which allow modulation of the properties of the inter-electrode gap leading to a paradigm shift in the functionality, precision and application of EJP. Electromechanical innovations demonstrate that the Gaussian energy distribution can be modified through the articulation of the jet angle of address and modified jet nozzles to manipulate the in-jet resistance. The outcome being the capability to produce bespoke removal profiles with increased precision and flexibility of form alongside refined surface finishes. Electrochemical innovations demonstrate an increase in precision through reducing overcut and reducing the feature shoulder radius when using a modified electrolyte. When these electromechanical and electrochemical innovations are coupled together, the overcut traditionally seen to be twice the nozzle diameter is reduced by 99%. Therefore, features can be created at kerfs approaching the nozzle diameter. Alongside this, a bespoke research platform has been built and developed to exploit these findings and incorporate features such as the rotational head for constant profiling and multiplexing of electrolytes to enhance the flexibility of the process. Process enhancements developed through this thesis have allowed the manipulation of the in-jet energy density profile and dissociation of the dissolution region from hydrodynamic phenomena thus allowing surface structuring by EJP to be developed well beyond the state-of-the-art.

This thesis presents work on the development of a number of scanned electrochemical probe microscopies. Such techniques have widespread applications, from materials science to the life sciences. Advances in flexible instrumentation, coupled with the theoretical description of electrochemical systems, are central themes which allowed for the fruitful investigation of a variety of experimental systems. Theoretical descriptions of scanning ion conductance microscopy (SICM) were developed, particularly to investigate the effect of tip-geometry on imaging resolution. This technique has already found a number of applications in the life sciences, but image resolution has not previously been addressed adequately. Images were recorded showing tip-convolution that were in agreement with theoretical predictions. The scanning microcapillary contact method (SMCM) was developed, as a method of assessing spatial heterogeneities in electrode activity on the submicron length-scale. An electrolyte filled microcapillary containing a reference/auxiliary electrode was approached to a substrate (working) electrode surface, via piezoelectric positioners. Contact of the electrolyte meniscus with the substrate electrode was sensed by a current flowing. Electrochemical measurements were performed before the microcapillary was retracted and another point on the sample was characterised. Spatial heterogeneities in electrode activity were imaged on a sub-micron length-scale and the activity of basal plane highly oriented pyrolytic graphite (HOPG) was demonstrated. Tip position modulation scanning electrochemical microscopy (SECM-TPM), where an ultramicroelectrode (UME) is oscillated perpendicularly to a surface and an amperometric current is recorded, was investigated experimentally and theoretically. A model including convective mass-transport was developed that gave an accurate description of the experimental situation. It was demonstrated that SECM-TPM is a potentially powerful approach for the measurement of the permeability of a sample. SECM experiments were performed investigating the growth of Ag particles at a liquid/liquid interface, which was caused through the electrodissolution of a Ag UME in an aqueous phase, and the reduction of the Ag+ ion by an electron donor in the organic phase. A model was created that allowed for the interpretation of data. Cyclic voltammetry investigations of HOPG covered with a Nafion film containing a redox mediator confirmed the activity of basal plane HOPG, as demonstrated by SMCM measurements. Nafion slowed diffusion sufficiently to allow the spatial-decoupling of surface sites with different activity.

The conventional method for synthesis of supported metal catalysts is a multi step reaction that produces large amounts of waste. The focus of this work has been to look at alternative methods of catalyst production which eliminate or lower the number of steps and therefore waste produced. The aim of the project was to synthesise supported metal catalysts by alternative (greener) techniques and then to test these catalysts alongside conventional catalysts for improved and novel activity. It was proposed that catalysts could be synthesized by a novel electrochemical route where the pure metal is electrochemically oxidized into solution to form a transient soluble complex. This complex is then reduced electrochemically/electrolessly and the metal is deposited onto a support with the ligand of the complex being recovered in solution for subsequent cycles. The development of such a system was studied for gold using ionic liquids, the stripping and depositing of gold was demonstrated using dicyanamide ligand ([DCAn but attempts to prove the recyclability of the ligand were not successful. However during these studies a set of active gold catalysts where prepared by the electroless deposition of gold from H[AuCI4].3H20 in [C4mim][NTf2] onto silica and titania. The activity of these catalysts was compared to standard wet impregnated catalysts, interestingly the preparation method was found to control the selectivity of the reaction. The standard catalysts showed activity for the oxidation of benzyl alcohol in toluene whereas the catalysts prepared by electroless deposition from ionic liquids showed Friedel-Crafts alkylation of benzyl alcohol with toluene. This is the first time that heterogeneously supported gold catalysts have been found to be active in Friedel--Crafts alkylations. Ag/AI203 and PtlAI203 catalysts have been prepared by means of solvent-free mechanochemistry using a ball mill. Remarkable catalytic activity was observed using a Ag/AI203 catalyst by ball milling (Ag20) for octane-SCR, compared with a conventionally prepared Ag/AI203 catalyst (wet impregnation) the ball milled catalyst shows an increase in activity with a reduction in the light off temperature of -150°C and NOx conversion below 200°C which is the first time this has been achieved in the absence of hydrogen.

Heterogenous interfacial electron transfer processes are of fundamental and applied importance to electrochemists and are extensively studied by a wide range of electrochemical techniques. This thesis focuses on the development of analysis strategies and electrochemical methodologies for more detailed quantitative investigations of electron transfer kinetics at a plethora of electrode materials, with an emphasis on carbon-based materials. Of interest are the techniques of Fourier-transformed large amplitude alternating current voltammetry (FTACV) and scanning electrochemical microscopy (SECM). The complementary electrochemical techniques of FTACV and SECM are used for measurements of fast electron transfer to reveal the impact of the complex heterogeneous surface of degenerately-doped polycrystalline boron-doped diamond electrode surfaces compared to conventional electrode materials such as platinum and gold. This part of the work highlights the importance of understanding the influence of measurement technique and further demonstrates how electron transfer at semi-metallic electrodes differ from conventional metallic electrodes. The oxidation of a ferrocene-derivative at highly oriented pyrolytic graphite is used to demonstrate the effects of reversible reactant adsorption on the SECM response. The high surface area-to-solution volume ratio of nanogap SECM measurements depicts the importance of understanding the impact of such surface effects. Precise quantitative kinetic analysis requires understanding of the mass transport between the SECM probe and electrode surface. Finite element method modelling was used to extensively investigate the effects of electrode reactant processes and the results of the models shed light on important factors that need to be accounted for in quantitative analysis of nanogap voltammetric measurements. FTACV is further developed as a tool for kinetic selectivity at heterogeneous electrode surfaces. This is achieved by taking advantage of the harmonic-dependent measurement timescale of FTACV to deconvolute a dual-heterogeneity electrochemical response into its individual components. Protocols are developed for this application and demonstrated experimentally using the ruthenium hexamine and ferrocene methanol redox couples.

A new system for the generation of hydrodynamic modulation voltammetry (HMV) is presented. This system consists of an oscillating jet produced through the mechanical vibration of a large membrane/piston. The structure of the cell is such that a relatively small vibration is transferred to a large (~ 1 m s-1) fluid flow at the jet outlet. High-speed imaging of the system shows vortex behaviour of the flow at the exit of the jet. Positioning of an electrode over the exit of this jet enables the detection of the modulated flow of liquid. The periodic character of the signal recorded at the electrode allows a “lock-in” approach to be employed. This enables discrimination of the background processes signal from the mass transport component. This is demonstrated for Fe(CN)6 3-/4- 3-/4-. Here “lock-in” to the modulated hydrodynamic signal is achieved through the deployment of bespoke software. The apparatus and procedure is shown to produce a simple and efficient way to obtain the desired signal. In addition the spatial variation of the HMV signal, phase correction and time averaged current with respect to the jet orifice is presented. The detection limit for the analysing system is shown to be 45 × 10-9 mol dm-3. The HMV method is employed to study the reduction of molecular oxygen at high surface area (HI-Pt) modified electrodes. The successful elimination of background signals is achieved for the 0.5 mm diameter nanostructured Pt electrode with roughness factor (RF) of 42.4. Employment of higher roughness factors (>50) HIPt electrodes revealed an anomalous “drop off” effect characterising these electrodes. It is demonstrated that the “drop off” is not caused by the hydrogen peroxide production at the electrode or pH change near the electrode surface. However, a clear dependence of the current deflection on the roughness factor of the electrodes is observed. The shape of the “drop off” followed the shape of the hydrogen adsorption region. It is suggested that the surface characteristics of the electrodes are important in these investigations. It is proposed here, that the capacitance of the electrode influences the HMV signal. Another type of hydrodynamic modulation method, specifically a vibrating 50 μm diameter Pt or Au wire or “tight-rope” electrode was studied. High frequency modulation (80 Hz) is employed. FFT data processing was employed to extract the desired signal from the total current. This technique was applied to study reduction of molecular oxygen at the modulated electrodes. A current “drop off” in hydride region was again observed. This is shown to be related to the uncompensated resistance of the cell. In particular, the resistance of the reference electrode is demonstrated to contribute to this effect

The synthesis of the gramicidin A (gA) analogue gram-2-(nicotinamido)ethyl carbamate (gAN) has been performed. Like gA itself, monomers of the nicotinamide analogue undergo self-assembly in lipid membranes to form K+ selective transmembrane ion channels. Scanning electrochemical microscopy (SECM) has been used to monitor the movement of permeable ions across these channels under a range of physiological conditions. Feedback curve results suggest that K+ ions only pass across the channels when a certain external potential, referred to as the switching potential, is applied to the membrane. This implies that ion transport in the system is controlled by the redox properties of the nicotinamide sub-unit, which acts as a pendant able to occlude the channel opening in the 'ball and chain' model of inactivation. Chronoamperometric measurements demonstrate that the gAN channel can be repetitively cycled between the opened and closed states. This controllable, predictable switching behaviour has possible applications in membrane transport and drug delivery systems. The permeability of gAN channels to Tl+ and Eu 3+ ions has also been tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with gold electrode supported lipid bilayers. Results indicate that the channel is permeable to the Tl+ ion although not to the multicharged Eu3+ ion, which is believed to bind occluding the channel entrance. It was also possible to directly monitor the passage of Tl+ ions across the gAN channel by using Prussian Blue as a dual-purpose electrolyte and electrochromic indicator.

This thesis describes the development of an electrochemical scanned probe microscope, SECCM, outlining the need for such a development, by highlighting previous techniques and their limitations. SECCM consists of a double barrel capillary pulled to small dimensions, filled with electrolyte solution and a redox mediator of choice, with a QRCE is inserted into each channel. A potential is applied between the QRCEs, whilst modulating the pipet normal to the surface. The probe is translated towards the surface and once contact is established, a modulation in the ion current arises due to the physical oscillation of the probe, which is then used as a feedback parameter for imaging. The potential at the working electrode substrate is also controlled. SECCM is introduced using a model test substrate, gold bands on glass, showing that the probe is able to track topographical features, making simultaneous electrochemical measurements. Ion conductance measurements between the two QRCEs, are shown to be sensitive to the nature of the substrate investigated. The fundamental electrochemical behaviour of CVD graphene and SWNT is investigated. A multimicroscopy approach is used for CVD graphene studies, correlating surface structure and activity, deducing heterogeneous electron transfer kinetics through simulation. The SWNT samples are studied in two different morphologies: as 3D forests; and, as a 2D network. In the forests, the probe is positioned at the ends and sidewalls, making spot measurements. The voltammetric behaviour shows very similar responses, whilst in the network, a nanosized probe is scanned across the surface, showing relatively uniform activity across an entire tube. These new insights indicate that SWNTs are highly active electrode materials. The fabrication and characterisation of SECM-SICM probes, in a straightforward manner is also presented. These types of probes were found to be ideal for the investigation of biological samples, being extremely easy and quick to fabricate.

This thesis presents the development of electrochemical etch-stop techniques (ECES) to achieve high precision 3-dimensional integrated MEMS sensors with wet anisotropic etching by applying proper voltages to various regions in silicon. The anisotropic etchant is selected as tetra methyl ammonium hydroxide, TMAH, considering its high silicon etch rate, selectivity towards SiO2, and CMOS compatibility, especially during front-side etching of the chip/wafer. A number of parameters affecting the etching are investigated, including the effect of temperature, illumination, and concentration of the etchant over the etch rate of silicon, surface roughness, and biasing voltages. The biasing voltages for passivating the n-well and enhancing the etching reactions on p-substrate are determined as -0.5V and -1.6V, respectively, after a series of current-voltage characteristic experiments. The surface roughness due to TMAH etching is prevented with the addition of ammonium peroxodisulfate, AP. A proper etching process is achieved using a 10wt.% TMAH at 85°
C with 10gr/lt. AP. Different silicon etch samples are produced in METU-MET facilities to understand and optimize ECES parameters that can be used for CMOS microbolometers. The etch samples are fabricated using various processes, including thermal oxidation, boron and phosphorus diffusions, aluminum and silicon nitride layer deposition processes. Etching with the prepared samples shows the dependency of the depletion layer between p-substrate and n&
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well, explaining the reason of the previous failures during post-CMOS etching of CMOS microbolometers from the front side. Succesfully etched CMOS microbolometers are achieved with back side etching in 6M KOH at 90 °
C, where &
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3.5V and 1.5V are applied to the p-substrate and n-well. In summary, this study provides an extensive understanding of the ECES process for successful implementations of integrated MEMS sensors.

Scanning Kelvin Nanoprobe (SKN) microscopy is a new technique, based on Lord Kelvin's theory of contact electrification, which is able to measure changes in the work function of a surface with nanometre- scale precision. This technique has great potential in the analysis of surface chemistry, especially that of self-assembled monolayers and biochemical interactions. This thesis examines the potential of SKN microscopy in analytical chemical applications. SKN microscopy is used and contrasted against a range of other analytical techniques, including atomic force microscopy (AFM), confocal Raman spectroscopy and common electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. A short background is given on SKN microscopy, as well as the other analytical techniques used. This is followed by a chapter discussing the theory behind basic electrochemistry and the electroanalytical techniques used in the thesis. Work is then presented on the hydrogen bonding environment in mixtures of dimethylsulfoxide (DMSO) in water. Confocal Raman spectroscopy showed that the hydrogen bonding environment in DMSO:water mixtures had a profound effect on the S-H (thiol) group of the amino acid cysteine, as well as on the thiol groups of the small-molecule protein analogue BMC. As molecular biologists use DMSO:water mixtures extensively in studies, this work brings up important issues concerning these experiments. SKN microscopy was used with a range of other analytical techniques including AFM, Raman spectroscopy and cyclic voltammetry to probe the formation and characteristics of a new derivative of phthalocyanine synthesised to avoid crystalline formation and to naturally form an amorphous thin-film. Films were deposited on a variety of substrates, including gold, high-order pyrolytic graphite and glassy carbon, and were compared with a range of other phthalocyanine compounds. Simple computer modelling was also carried out on the compound. The derivative was found to form nanoporous films which allowed the passage of positively-charged molecules less then 7Å in diameter. Self-assembling monolayers of organothiols on gold were then probed using the SKN and electrochemical impedance spectroscopy. Selections of linear, branched, cyclic, aromatic and biological organothiols were tested. The SKN was capable of directly measuring the length of a linear alkanethiol from the change in work function of the monolayer. The SKN also proved capable of measuring the degree of organisation of the monolayer - branched and cyclic alaknethiols, which are expected to form looser-packed layers, recorded more significant changes in work function. These results were confirmed by the use of electrochemical impedance spectroscopy to measure the effects of a monolayer on an electrode surface.

This thesis documents studies directed towards the development of electrochemical measurement procedures capable of quantifying corrosion resistance and/or the corrosion rate in Tin Mill products, and in particular steel food/beverage cans. Chapters 3 and 4 describe electrochemical studies carried out using commercially produced cans. Conventional corrosion evaluation techniques were compared with Electrochemical Impedance Spectroscopy (EIS) data. EIS was found to provide useful information regarding container corrosion performance, however statistical variation from can to can required the use of large sample populations unless cans could be conserved between measurements. This was achieved by the development of "sealed-in-electrodes", which were used to identify trends in can coating performance. Sealed-in-electrodes form the basis of a predictive corrosion test method, whereby short-term electrochemical measurements may be extrapolated to predict long-term corrosion performance. Conventional electrochemical techniques provide a surface averaged measurement of the corrosion performance of coated metals, giving no information regarding the spatial distribution of penetrative defect sites in the organic coating. For this reason, a novel method for defect detection was developed, exploiting the phenomenon of electrogenerated chemiluminescence (ECL) in conjunction with digital (CCD) photography. The development, optimisation and evaluation of techniques for the spatially resolved characterisation of penetrative coating defect populations using luminol ECL is described in chapters 5 - 7. A proposed mechanism and kinetic model for the ECL process is described in chapter 5. Chapter 6 describes the application of luminol ECL to the detection of induced defects in a polymer laminate material, and an estimation of the limits of the technique. Chapter 7 describes the application of luminol ECL to the detection of defect populations in commercially produced polymer laminate cans. ECL images were suitable for computerised image analysis, providing information on the frequency, size distribution and location of penetrative defects.

Electrochemical techniques have potential for use in conservation, both to evaluate the protectiveness of existing coatings on metal artefacts and to evaluate potential new conservation coatings. Three electrochemical methods have been examined in this study for their applicability to conservation problems. Corrosion Potential Measurement is simple but provides only minimal information on the corrosion processes occurring in an electrochemical system. Electrochemical Impedance Spectroscopy provides both mechanistic and predictive information on coating performance, but the data are complex to interpret and measurements require equipment that is at present too bulky for effective on-site use and beyond the budget of most conservation laboratories. Electrochemical Noise Measurement can be performed using cheap, portable instrumentation and theoretically requires relatively simple statistical processing and interpretation, making it attractive for conservation applications. This project looks at the development of a simple, low cost electrochemical noise measurement system for conservation needs, and uses it to compare Electrochemical Noise Measurement with the other two techniques.

Aniline was chosen as an atmospheric pollutant which might be monitored using an electrochemical sensor. The effect of pH and of different organic solvents on electrode poisoning was investigated for the analysis of aniline by voltammetry and it was concluded that it was not possible to prevent poisoning of the electrode by the reaction products. The analysis of aniline by flow injection analysis (fia) with DC (constant potential) and pulsed (double pulse) amperometric detection also suffered from electrode poisoning and the latter had a relatively high detection limit. Secondary and tertiary substituted anilines with similar volatilities to aniline at room temperature were examined as suitable alternatives to aniline using voltammetry. Dimethyl-p-toluidine poisoned the electrode to a small extent when analysed by voltammetry. Detection of this compound by fia with pulsed amperometric detection showed improved electrode stability but was not judged suitable for long term monitoring of atmospheric samples of the amine. A satisfactory method for monitoring aniline on line was developed using fia with triple pulse amperometric detection (PAD). The PAD waveform was optimised with respect to a low detection limit and a degree of selectivity towards possible atmospheric interferents for the detection of aniline in dilute aqueous acid at a platinum electrode. A wall jet cell was designed for the analysis of aniline vapour in air which was continuously trapped in dilute acid and periodically injected into a fia system. The cell was not affected by small gas bubbles and was reasonably portable.

Thesis (M.S.)--University of Cincinnati, 2009.
Advisor: Margaret J. Kupferle. Title from electronic thesis title page (viewed Apr. 26, 2010) Includes abstract. Keywords: phenol; electrochemical oxidation; chlorate; boron doped diamond anode. Includes bibliographical references.

This thesis is concerned with the construction of new electrochemical scanning probe microscopes. Designed to support a wide variety of existing techniques as well as to develop new techniques. This exibility was achieved by basing the equipment around a field programmable gate array card (FPGA), which allows for a software program to be configured on the physical FPGA card as hardware. This technology provides the efficiency and high speed of bespoke hardware and can be reprogrammed like software. The instrumentation was programmed in-house using the graphical programming language, LabVIEW, which allowed for changes and upgrades to be made when necessary. Two branches of projects were studied with the FPGA instrumentation, crystal dissolution and surface charge mapping. For the crystal dissolution studies, dual-barrel conductance micropipettes were used to investigate the dissolution of NaCl and calcite, with microscale spacial resolution. This technique had many advantages over traditional methods. For instance, high temporal resolution in the order of sub-milliseconds was achieved through the employment of in-house built current followers. In addition, fast mass transport inside the pipette allowed the study of surface kinetic processes. The implementation of finite element method simulations complemented the experimental findings, by enabling the quantitative analysis of the data to extract intrinsic dissolution rate constants. The technique is also complemented by atomic force microscopy, which provides an alternative method for analysing the etch pits. The same equipment is used as a scanning ion conductance microscope (SICM) to investigate the surface charge of both conductive and non-conductive surfaces. A double electric layer forms at the solid-liquid interface of a sample that is immersed in electrolyte solution, at low ionic strength the thickness of the double layer increases, which enables its detection via the SICM pipette. The formation of the double layer at the pipette walls induces ion current rectification, at the same time a surface induced rectification arises as the nanopipette approaches the substrate surface. The combination of these results in the creation of an ion perm-selective region, which results in an increase or decrease in current that is proportional to surface charge. Point measurements, maps and CVs of dynamically changing surfaces have been recorded.

Le présent travail porte sur l’étude de deux protéines de la famille des oxydases à hème-fer par des techniques de spectroscopie et d’électrochimie. Le premier chapitre décrit l’étude du cytochrome bo3 oxydase issue d’E. coli. Nous nous sommes intéressés à l’étude des interactions enzyme-quinone par l’utilisation de quinones avec des longueurs chaines isoprenyl différentes. Notre but est de mieux comprendre le rôle de la longueur de la chaine des quinones sur l’activité catalytique de l’enzyme et sur les propriétés redox des cofacteurs à hème. Dans l’étape suivante, on a étudié les résidus impliqués dans le site de liaison des quinones (haute affinité, QH). Plusieurs mutations de ces résidus sont étudiées pour mieux comprendre l’importance de chacun des résidus dans cette liaison. Dans la dernière partie de ce premier chapitre, la spectroscopie SEIRAS «spectroscopie d’absorption infrarouge exaltée de surface» est introduite comme une technique alternative pour l’étude des protéines membranaires. Dans le second chapitre, la protéine membranaire cNOR issue de P. denitrificans est étudiée. Nous nous sommes focalisés sur l’effet de différents environnements (pH, présence de protéo-liposomes) sur la stabilité de la cNOR. Pour ce faire, trois valeurs de pH (6.5, 7.5 et 8.5) sont choisies et quelques échantillons de cNOR sont reconstitués dans des protéo-liposomes. Enfin, le donneur de proton terminal (au centre binucléaire) dans la protéine cNOR était étudié. De plus, nous avons étudié les ligands des ions Ca2+ puisqu’il est proposé que le donneur de proton est situé proche de cette région
This thesis is focused on the study of two members of the heme-copper oxidase family by using spectroscopic and electrochemical techniques. In the first chapter cytochrome bo3 oxidase from E. coli was studied. We focused on the quinone-enzyme interactions by using quinones with different isoprenyl chains. Our aim was to better understand the role of isoprenyl chain on the catalytic activity of the enzyme and the redox properties of the heme cofactors. In the next step we studied the residues that are suggested to be in the high-affinity (QH) quinone binding site. Several site-directed mutants of these residues were investigated in order to better understand the position of QH binding site and the importance of each residue. In the last part of this chapter surface-enhanced infrared absorption spectroscopy (SEIRAS) was introduced as an alternative technique to study the membrane proteins. In the second chapter cytochrome c dependent nitric oxide reducates (cNOR) from P. denitrificans was studied. We focused on the effect of different environment (pH, proteoliposomes) on the stability of cNOR. For that purpose three pH values (6.5, 7.5 and 8.5) was selected and some of the cNOR samples were reconstituted in liposomes. Finally, the terminal proton donor (to the binuclear center) in cNOR was investigated. We studied the ligands of the Ca2+ site in cNOR since it was suggested that the proton donor may be close to this area

This work investigated the protection of mechanism of organic coatings on steel exposed to 3% sodium chloride solution at 50°C, coupled with the use of electrochemical impedance spectroscopy (EIS) to monitor progress of corrosion and degradation of coating. Unlike Walter, EIS measurement was conducted at 50°C as well as after cooling, and measurements at intermediate temperatures have been used to characterize the dependence of the process involved. The proposition that corrosion rate is controlled by the ionic resistance of an organic coating has been tested. EIS results were fitted to a model circuit and changes in the film resistance and charge-transfer resistance with temperature were analyzed to deduce activation energies for the processes involved. Surprisingly, the calculated activation energy for coating resistance is significantly lower than the activation energy for the charge transfer resistance. This suggests that ion conduction in the coating, as apparent in an AC measurement, cannot be controlling the corrosion rate. Potentiostatic pulse tests on coated metal enable iR-corrected polarization curves to be plotted at different temperatures. From this, the activation energy determined from the corrosion currents also higher matches the higher activation energy value calculated from the charge transfer resistance. However, measurements of coating resistance on free films of the same coating also generate higher activation energy values, leaving two possible models that can account for the results.

Endothelial cells (ECs) form a dynamic surface within blood vessels, constantly monitoring and responding to their local environment by synthesising and releasing diffusible and cell surface bioactive molecules. The regulated secretion of a range of such molecules is mediated by the exocytosis of secretory granules (SGs) called Weibel-Palade bodies (WPBs). Optical data has suggested that WPBs undergo different forms of exocytosis whereby subsets of cargo molecules are released or retained based on their size. In other cell types the SG fusion pore is implicated in regulating selective release of specific molecules, however almost nothing is known about the WPB fusion pore. A highly effective method for studying fusion pore formation and expansion is carbon fibre amperometry, an electrochemical technique, allowing each stage of exocytosis to be quantified with submillisecond resolution. The primary aim of this thesis was therefore to establish carbon fibre amperometry as a technique to measure the WPB fusion pore in order to analyse its dynamics for the first time. Because WPBs did not contain endogenous oxidisable molecules suitable for amperometric detection, methods were developed to specifically load WPBs with suitable molecules. Amperometry was then used in combination with live cell imaging of WPB exocytosis to provide direct characterisation of the properties of the WPB fusion pore during Ca2+-driven WPB exocytosis. WPB fusion pore parameters were comparable to those reported for SGs in other cell types (e.g. chromaffin cells), indicating that WPBs may share similar processes controlling membrane fusion and the mobilisation and release of oxidisable species. Half of the exocytotic events demonstrated a pre-spike foot (PSF) signal prior to the current spike indicating that a restricted fusion pore had initially formed. In rare cases PSF signals showed step changes or fluctuations suggesting that during expansion the fusion pore may transition through different configurations before fully opening. Following characterisation of the WPB fusion pore under control conditions, factors which may affect the behaviour of the fusion pore were investigated, including the role of PM cholesterol. In line with previous studies, depletion of PM cholesterol increased the rate of fusion pore expansion and decreased the duration of the lifetime of the restricted fusion pore. This has been attributed to the promotion of the formation of a restricted fusion pore due to the intrinsic negative curvature of cholesterol, which is subsequently destabilised upon removal of cholesterol. Results presented here therefore support this idea. However, in contrast to results obtained from a range of cell types, which reported an inhibition of SG exocytosis following PM cholesterol depletion, WPB exocytosis remained largely unperturbed providing evidence for the insensitivity of the EC response to removal of PM cholesterol. In conclusion, the work presented in this thesis provides a detailed characterisation of the WPB fusion pore and has begun to address factors which may be important for its regulation. The extensive characterisation under control conditions now allows further elements potentially involved in the regulation of the WPB fusion pore to be investigated.

Electron Spectroscopy for Chemical Applications (ESCA) and Cyclic Voltaimnetry (CV) have been used to study the surface and electro chemistries of electroactive, thin organic films. A method is developed for the preparation of such films by the synthesis of a molecule containing both an electroactive centre (ferrocene or iron (II) or ruthenium (II) trisbipyridyl) bonded to an electro-polymerizable unit (pyrrole). The surface chemistries of polypyrrole, polyaniline and electrodes potentiostatted in ferrocene solutions were also investigated and results corpared to previous studies where possible. The poly-Ru(II)- and particularly the poly-Fe(II)- films are stable to electrochemical cycling between the +2 and +3 oxidation states. The poly-Fe(II)-, and probably the poly-Ru(II)-, trisbipyridyl films have charge transfer diffusion coefficients similar to those previously reported for other films containing similar redox centres. Redox conductivity has been demonstrated for an aged sample of the poly-Fe(II)- film. Electroactive films, prepared by two different methods of plasma polymerization of substituted ferrocene moncsners are described. In each case the ferrocene/ferricenium electroactivity is degraded, by repeated potential cycling. The incorporation of ferrocene into a plasma polymer does not necessarily produce an electroactive film. A preliminary investigation of the surface and electrochemistry of electrode deposits, prepared by cathodic reduction of perfluorocyclo-pentene, is described. The deposits appear similar in some respects to cathodically reduced PTFE.

Cluster cavitation erosion, generated by piston-like emitter (PLE) vibrating at ultrasonic frequencies, was measured using a novel aluminium erosion sensor in conjunction with optoisolation techniques, in order to bridge the electrochemical detection of cavitation erosion with a counting device possessing USB connectivity, while minimising electrical noise. High-speed imaging assisted in correlating the periodicity of cluster collapse with the frequency of erosion events detected by the sensor. The consistency of the shape and duration of current-time transients associated with erosion were shown to be dependent on drive voltage amplitude and drive frequency of the PLE. Erosion in the presence of silicon carbide particles, agitated by the PLE, was measured in an identical manner. Cluster cavitation dynamics were shown to be affected by the presence of silicon carbide, making it difficult to ascertain whether erosion events were due to silicon carbide grazing the sensor surface or whether the events were bubble-driven. Subsequent high-speed imaging and analysis of the shapes of current-time transients, suggested that under certain experimental conditions, erosion of the sensor surface could be attributed to silicon carbide particles. Analysis of the acoustic noise spectrum generated during inertial cavitation showed that an increase in the magnitude of the 2f component and the presence of the f/2 component corresponded with the onset of non-inertial bubble collapse and inertial cavitation erosion respectively. However, no subsequent correlation could be made between the magnitude of the f/2 component and the extent of erosion detected. Measurements from both a hydrophone and microphone gave similar results. The effect of a liquid’s physical properties on the sonoluminescence intensity was investigated by analysing the results of prior studies and experiments conducted here using an image intensifier. Sonoluminescence intensity was found to increase with normal boiling point of a liquid assuming identical experimental conditions (drive frequency, voltage, ambient temperature), and provided the boiling point did not exceed c. 200 oC.

Um eletrodo de carbono vítreo foi modificado pela deposição de uma camada de nanotubos de carbono de paredes múltiplas, funcionalizados e decorados com nanopartículas de ouro. Este eletrodo foi caracterizado por microscopia ótica, mostrando uma superfície homogeneamente recoberta. Além disto, a sua morfologia foi investigada por microscopia eletrônica de transmissão, onde observou-se a distribuição e o tamanho médio aproximado de 20 nm das nanopartículas de ouro. Estas nanopartículas metálicas também foram caracterizadas por espectroscopia de absorção na região do UV-vis, mostrando um máximo de absorção em aproximadamente 525nm, o que confirma o seu tamanho médio de 20 nm. Os eletrodos modificados foram caracterizados eletroquimicamente pelo seu comportamento voltamétrico em uma solução de H2SO4 0,1 mol L-1, com uma velocidade de varredura de 0,100 V s-1. Nestes experimentos, ficou evidente os picos de formação e redução do óxido de ouro em potenciais acima de 0,8 V vs Ag/AgCl. Ainda foi observado o bom funcionamento dos eletrodos pela resposta voltamétrica do par redox [Ru(NH3)6]Cl2 / [Ru(NH3)6]Cl3 em meio de KCl. O desempenho deste eletrodo modificado para a oxidação dos pesticidas carbaril, etil-paration, malation e carbendazim foi investigado por voltametria de onda quadrada em tampão fosfato pH 7, porém apenas o inseticida e fungicida carbendazim mostrou eletroatividade. Desta forma, os estudos posteriores se focaram neste pesticida. O voltamograma cíclico do carbendazim mostrou um pico de oxidação e, na varredura reversa, um pico bem menor de redução. Isto sugeriu um mecanismo EC e um esquema da reação de oxidação foi proposto. Com o perfil voltamétrico estabelecido, a voltametria de onda quadrada foi utilizada para a determinação da curva analítica para o pesticida. Com todos os parâmetros da voltametria de onda quadrada otimizados, uma dependência linear da corrente de pico de oxidação com a concentração de carbendazim foi obtida, com a equação: Ip = 0,1 + 4,30 [carbendazim], com r2 = 0,9911 (n = 5). Esta curva analítica mostrou que a metodologia apresenta um limite de detecção de 17 x 10-8 mol L-1. Esta metodologia foi empregada na determinação de carbendazim em amostras de suco de laranja contaminadas artificialmente. A utilização de um teste t, de Student, mostrou que os valores recuperados pela voltametria não apresentaram qualquer diferença significante em relação àqueles adicionados às amostras. Assim, esta metodologia foi validada para a utilização na análise de suco de laranja contaminado com carbendazim
A glassy carbon electrode was modified by depositing a layer of multi-walled carbon nanotubes, functionalized and decorated with gold nanoparticles. This electrode was characterized by optical microscopy, showing a evenly coated surface. In addition, the morphology was investigated by transmission electron microscopy, where the distribution and the average size of 20 nm of the gold nanoparticles were observed. These metal nanoparticles were also characterized by absorption spectroscopy in the UV-vis region showing an absorption maximum at approximately 525 nm, which confirms their average size of 20 nm. The modified electrodes were electrochemically characterized by its voltammetric behavior in a 0.1 mol L-1H2SO4 solution, with a scanning rate of 0.100 V s-1. In these experiments, it became clear the formation and reduction of gold oxide peaks at potentials above 0.8 V vs. Ag/AgCl. It was also observed the proper functioning of the electrodes for the voltammetric response of the redox couple [Ru(NH3)6]Cl2/[Ru(NH3)6]Cl3 in a KCl electrolyte. The performance of the modified electrode for the oxidation of the pesticides: carbaryl, ethyl-parathion, malathion and carbendazim was investigated by square wave voltammetry in phosphate buffer, pH 7, but only the insecticide and fungicide carbendazim showed electroactivity. Thus, future studies focused on this pesticide. The cyclic voltammogram of carbendazim in phosphate buffer showed an oxidation peak and in the reverse scan, a much smaller reduction one. It suggested an EC mechanism and an oxidation reaction scheme was proposed. With the voltammetric profile established, square wave voltammetry was used to determine the calibration curve for the pesticide. With all square wave voltammetric parameters optimized, a linear dependence of the oxidation peak current with the concentration of carbendazim was obtained, with equation: Ip = 0.1 ± 4.30 [carbendazim] with r2 = 0, 9911 (n = 5). This calibration curve showed that the method has a detection limit of 17 x 10-8mol L-1. This methodology was used in the determination of carbendazim in orange juice samples artificially contaminated. A t-test of Student showed that the amounts recovered by voltammetry showed no significant difference in relation to those added to the samples. Thus, this methodology has been validated for use in analysis of orange juice contaminated with carbendazim

There is a great demand for low-cost disposable sensors in a variety of markets, such as the food chainand health care. No assay is performed more than that of glucose and approximately 85 % of the entirebiosensor market accounts for glucose biosensors. Each year, 6 billion glucose assays are performed andthe majority of them are based on electrochemical detection. Organic electrochemical transistors(OECTs) have favorable properties in terms of low operating voltages and have previously been used asbase for electrochemical detection of glucose. A low-cost disposable biosensor can be achieved by theuse of high throughput printing techniques. Up until now, no printable biosensors based on organic electrochemicaltransistors have been developed. In this thesis a printable miniaturized prototype for a glucose biosensor based on an OECT with a platinizedgate electrode has been designed, developed and evaluated. The biosensor has been functionalizedwith the enzyme glucose oxidase. Different platinum deposition techniques have been used to depositplatinum onto the printed carbon gate electrode: electrodeposition, platinum nanoparticle solutiondeposited either by inkjet printing or pipetting and thermal evaporation. The gate electrodes were characterized with cyclic voltammetry in hydrogen peroxide, ferricyanide andglucose. The characterizations revealed no significant differences between the different deposition techniques.However, with gate electrodes produced by printed carbon followed by electrodeposition ofplatinum it was possible to sense glucose in a concentration in the range of the values for diabetic persons.Thus, the electrodes are a promising option as gate electrodes in a glucose biosensor based on anOECT. The characteristics of the OECT revealed that the responses resembled a transistor.

Polymer electrolyte fuel cells (PEFCs) are a key technology as the world strives for a low carbon future. The main obstacles facing mass-market uptake are the high cost and the longevity of the units; as such, research is needed to enhance performance and understand the degradation mechanisms. In this study, dynamic compression is applied using a cell compression unit (CCU) to study the effect on performance of a membrane electrode assembly (MEA) and its individual components with dimension change. Electrochemical impedance spectroscopy (EIS) is used to delineate the effect of compression on contact resistance, membrane resistance and mass transfer losses. Derived parameters such as the ‘displacement factor’ are used to characterise a representative range of commercial gas diffusion layers (GDLs). Increasing compaction pressure leads to a non-linear decrease in resistance for all GDLs. Different GDLs have different intrinsic resistance; however, all GDLs of the same class share a common compaction profile (change in resistance with pressure). Cyclic compression of Toray GDL leads to progressive improvement in resistance and reduction in thickness that stabilises after ~10 cycles. During initial hydration of Nafion membranes there is a direct relationship between membrane conductivity and dimensional change (swelling) of MEAs. Electrode flooding is found to result in membrane hydration and an increase in stress or strain, depending on the compression mode of the fuel cell. Results suggest that hydration cycles and flooding events can lead to cell degradation due to the stresses imposed. With increasing compression, a significant reduction in net performance is observed, with the most significant differences occurring in the mass transport regions of the performance curves. As the compression increases, the high-frequency resistance reduces with the improvement in contact resistance between the GDL and bipolar plate material, concurrently the low frequency resistance increases with increasing compression.

The widespread increase in the range and types of portable electronic devices in the past decades has resulted in higher requirements for energy storage and conversion modules. Most of these devices use rechargeable batteries as energy storage elements. No matter what type of batteries are used (Ni-Cd, Ni-MH, Li-Ion, etc.) they all have one serious drawback in common, in terms of charging time. Electrochemical double layer capacitors (EDLCs) also known as ultracapacitors or supercapacitors seem to have overcome this disadvantage, at the cost of lower energy storage capacity. This work aims to explore the design of fast and compact integrated charging techniques for ultracapacitors using the AC mains network as the source. The main constraints that arise are the power dissipation on-chip and in the magnetic components due to the large amount of energy that has to be transferred in a very short time interval. Two other limitations come from the EDLC side due to the device parasitics and the widely varying voltages over the operational envelope. This will impose the need for a flexible control system providing high efficiency over the whole output voltage range. The structure of this thesis comprises five main parts: literature review, behavioural modelling of the control system (including matlab simulations); implementation of the device with discrete components; design of an analogue circuit implementation and design of a mixed signal circuit implementation. As ultracapacitors represent one of the newest solutions in the field of electrical energy storage there are very few designs for chargers from the mains network. Therefore the literature review will also examine the properties and the modelling of EDLCs, as well as the choice of converter topologies available and the characteristics of the magnetic devices required for the system. The behavioural model of the control module gives a preview of the system parameters, while the design chapter introduces a series of new control techniques. The simulations and measurements of the breadboard circuit come as a first confirmation of the design approach and make it a viable starting point for an IC implementation. The analogue IC design presents the integration of the algorithms in a medium-voltage process using the current mode approach, as a demonstrator for a fully monolithic high-voltage IC. Once the functionality of the system is demonstrated at IC level, the mixed-signal system aims to optimize the device and provide a broader flexibility for the system parameters and control algorithms.

The improvement of low cost, efficient photovoltaic devices is a leading technological challenge in the recent decade. There is a need to develop scalable and high-throughput manufacturing techniques that could reduce costs and improve manufacturing of chalcogenide solar cells. Copper, indium, gallium, and selenium (CIGS) Thin films polycrystalline heterojunction solar cells appear to be most appropriate with to cost and ease of manufacture. Currently Cu (In,Ga) (Se, S)2 materials hold the highest record cell efficiency of 22.3% in laboratory scale for thin films solar cells and the efficiency still be boosted by improving the different layers of the photovoltaic devices. CIGS chalcogenide absorber layers has been a leading candidate material in photovoltaic devices for thin films solar cells and space applications due to its unique optical-electronic properties as well as its radiation resistance. In the present work, thin films of Cu (In, Ga) (Se, S)2 were deposited at room temperature on glass substrates coated with ITO and Mo by electrodeposition techniques. The obtained polycrystalline thin films were characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analysis. Thin films of Cu (In, Ga) (Se, S)2 grown by electrodeposition were subsequently processed into several sets of conditions including vacuum heat treatment, heat treatment in the presence of selenium or sulfur, heat treatment in nitrous gas atmosphere (N2H2) at different temperature and processing times. To improve the composition and the crystalline structure of the thin layers and to optimize the electro-optical properties a heat treatment of the thin films was developed in two stages after the electrodeposition. It was observed that the first annealing step (heating treatment at 450 °C in a selenium atmosphere 40 minutes) produced an appreciable improvement in the crystalline structure in the thin layer composition. In a second stage a sulfurization of the CuGaSe2 films was performed at 400 °C for 10 min in the presence of molecular sulfur and under the forming gas atmosphere. The effect of sulfurization was the complete conversion of selenium to sulfur and, therefore, the transformation of CuGaSe2 into CuGaS2. The formation of CuGaS2 thin films was evidenced by the by the displacement of the diffraction peaks of the CuGaSe2 towards higher angles to which makes the X-Ray diffraction 18 pattern which makes it coincide with the diffraction pattern of the CuGaSe2 films, and by the shift towards the blue (higher energies) of the optical gap. The optical gap found for the CuGaSe2 layer was 1.66 eV, while the optical gap for the CuGaS2 was raised up to 2.2 eV. CdS thin films have been widely used as buffer layer in CIGS solar cells. However, when alloyed with Zn, ZnCdS can still improve its performance as buffer layer. ZnCdS can be used as buffer and as window material in photoconductive devices and in heterojunction thin film solar cells due the possibility to tune the bandgap with the content of Zn. The band spacing of this ternary material can be from 2.42 to 3.50 eV, depending on the Cd/Zn ratio.
La obtención de dispositivos fotovoltaicos más eficientes y de bajo coste es uno de los desafíos tecnológicos más importantes de las últimas décadas. Existe la necesidad de desarrollar técnicas de fabricación escalables y de alto rendimiento que puedan reducir los costos y mejorar la fabricación de células solares de capa fina. Las células solares de heterounión de capas finas de seleniuro (o sulfuro) de cobre, indio y galio (CIGS) parecen estar bien adaptadas lograr este reto debido a su bajo costo, facilidad de fabricación y elevado rendimiento de los dispositivos. En la actualidad, Cu(In, Ga)Se2 ostenta el record de eficiencia de células solares con 22,3% a escala de laboratorio y esta eficiencia todavía puede ser acrecentada si se mejoran las diferentes capas de los dispositivos fotovoltaicos. Además, las capas absorbedoras de calcogenuros CIGS son un material candidato importante en dispositivos fotovoltaicos para capas delgadas celdas solares para aplicaciones espaciales debido a sus propiedades electrónicas, así como a su resistencia a la radiación. En el presente trabajo, las películas delgadas de Cu(In, Ga)(Se, S)2 se depositaron a temperatura ambiente sobre sustratos de vidrio recubiertos con ITO y Mo mediante técnicas electroquímicas. Las películas finas policristalinas obtenidas se caracterizaron por espectroscopia óptica UV-Vis, difracción de rayos X (XRD), microscopía electrónica de barrido (SEM), microscopía de fuerza atómica (AFM), microscopía electrónica de transmisión (TEM) y espectroscopia de energía dispersiva (EDS). Las películas finas de Cu(In, Ga)(Se, S)2 crecidas por electrodeposición se procesaron posteriormente en varios conjuntos de condiciones que incluían tratamiento térmico en vacío, tratamiento térmico en presencia de selenio o de azufre, tratamiento térmico en atmósfera gas nidrón (N2H2) a diferentes temperaturas y tiempos de procesado. Para mejorar la composición y la estructura cristalina de las capas finas y para optimizar las propiedades electro-ópticas se desarrolló un tratamiento térmico de las películas finas en dos etapas posterior a la electrodeposición. Se observó que la primera etapa de recocido (tratamiento térmico a 450 ºC en una atmósfera de selenio durante 40 minutos) producía una mejora apreciable en la estructura cristalina y en la composición de la capa fina. 20 En una segunda etapa se realizó una sulfuración de las películas de CuGaSe2 se realizó a 400 °C durante 10 min en presencia de azufre molecular y bajo la atmósfera reductora de gas nidrón. El efecto de la sulfuración fue la completa conversión del selenio en azufre y, por tanto, la transformación de CuGaSe2 en CuGaS2. La formación de películas delgadas de CuGaS2 se evidenció por el desplazamiento de los picos de difracción de las capas de CuGaSe2 hacia ángulos más altos hasta lo que hace que el patrón de difracción de rayos X lo que hace que coincida con el patrón de difracción del CuGaS2 y por el desplazamiento hacia el azul (energías más altas) del gap óptico. El gap óptico encontrado para las capas de CuGaSe2 era de 1,66 eV, mientras que el gap óptico para las capas de CuGaS2 se elevó hasta 2,2 eV. Las películas delgadas de CdS se han utilizado ampliamente como capa tampón en células solares CIGS. Sin embargo, cuando se alea con Zn, para formar el ternario ZnCdS, todavía puede mejorar su rendimiento como capa buffer. ZnCdS puede utilizarse como tampón y como ventana óptica en dispositivos fotoconductores y en células solares de capa fina de heterounión debido a la posibilidad de ajustar el bandgap con el contenido de Zn.
L'obtenció de dispositius fotovoltaics més eficients i més barats és un dels reptes tecnològics més importants de les últimes dècades. Hi ha la necessitat de desenvolupar tècniques de fabricació que siguen escalables i d'alt rendiment i que permeten reduir els costos de fabricació i millorar el rendiment de les cèl·lules solars de capa fina. Les cèl·lules solars de heterounió de capes fines de seleniur (o sulfur) de coure, indi i gal·li (CIGS) semblen estar ben adaptades per assolir aquest repte degut a del seu baix cost, facilitat de fabricació i elevat rendiment dels dispositius. En l'actualitat, el Cu(In, Ga)Se2 ostenta el rècord d'eficiència de cèl·lules solars amb 22,3% a escala de laboratori i aquesta eficiència encara pot ser augmentada si es milloren les característiques de les diferents capes dels dispositius fotovoltaics. Les capes absorbidores de calcogenurs CIGS són un candidat important per dispositius fotovoltaics per a pel·lícules primes en cel·les solars i aplicacions espacialles degut a les seues propietats electròniques així com a la seua resistència a la radiació. En el present treball, les pel·lícules primes de Cu(In, Ga)(Se, S)2 es van dipositar a temperatura ambient sobre substrats de vidre recoberts amb ITO i Mo mitjançant tècniques electroquímiques. Les pel·lícules fines policristal·lines obtingudes es van caracteritzar per espectroscòpia òptica UV-Vis, difracció de raigs X (XRD), microscòpia electrònica de rastreig (SEM), microscòpia de força atòmica (AFM), microscòpia electrònica de transmissió (TEM) i espectroscòpia d'energia dispersiva (EDS). Les pel·lícules fines de Cu(In, Ga)(Se, S)2 crescudes per electrodeposició es van processar posteriorment en diversos conjunts de condicions que incloïen tractament tèrmic en buit, tractament tèrmic en presència de seleni o de sofre, tractament tèrmic en atmosfera reductora de gas nidró (N2H2) a diferents temperatures i temps de processat. Per millorar la composició i l'estructura cristal·lina de les capes fines i per optimitzar les propietats electro-òptiques es va desenvolupar un tractament tèrmic de les pel·lícules fines en dues etapes posterior a la electrodeposició. Es va observar que la primera etapa de recuit (tractament tèrmic a 450 º C en una atmosfera de seleni durant 40 minuts) produïa una millora apreciable en l'estructura cristal·lina i en la composició de la capa fina. 24 En una segona etapa es va dur a terme una sulfuració de les pel·lícules de CuGaSe2 que es va realitzar a 400 °C durant 10 min en presència de sofre molecular i sota l'atmosfera reductora de gas nidró. L'efecte de la sulfuració va ser la completa conversió seleni en sofre i, per tant, la transformació de CuGaSe2 a CuGaS2. La formació de pel·lícules primes de CuGaS2 es va evidenciar pel desplaçament dels pics de difracció de les capes de CuGaSe2 cap angles més alts fins el que fa que el patró de difracció de raigs X el que fa que coincideixi amb el patró de difracció del CuGaS2 i pel desplaçament cap al blau (energies més altes) del gap òptic. El gap òptic trobat per a les capes de CuGaSe2 era de 1,66 eV, mentre que el gap òptic per a les capes de CuGaS2 es va elevar fins a 2,2 eV. Les pel·lícules primes de CdS s'han utilitzat àmpliament com a capa amortidora en cèl·lules solars de CIGS. No obstant això, quan s'alea amb Zn per formar ZnCdS encara pot millorar el seu rendiment com a capa d'amortiment. ZnCdS pot utilitzar-se com capa tampó i com a finestra òptica en dispositius fotoconductors i en cèl·lules solars de pel·lícula fina d'heterounió degut a la possibilitat d'ajustar el seu bandgap que depoen del contingut de Zn.
Ullah, S. (2017). THIN FILM SOLAR CELLS BASED ON COPPER-INDIUMGALIUM SELENIDE (CIGS) MATERIALS DEPOSITED BY ELECTROCHEMICAL TECHNIQUES [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86290
TESIS

Ultrathin organized films of organic electronic materials, such as phthalocyanines (Pc), are promising for both fundamental and applied studies due to their special optical, electronic and photoconductive properties. The studies presented in this dissertation include fabrication of ultrathin molecular assemblies by molecular beam epitaxy and Langmuir-Blodgett techniques. The degree of molecular order, extent of charge transfer and the morphology within these films, assessed by methodologies, such as photoelectrochemistry, electrochemistry, surface analysis and optical spectroscopy were discussed and characterized. Under high vacuum condition, a wide range of ordered structures of some trivalent metal phthalocyanines, such as GaPc-Cl, InPc-Cl and AlPc-F, can be fabricated. These materials exhibit "layer-by-layer" growth on the single crystal SnS₂ surface when deposited by molecular beam epitaxy (MBE). The MBE technique allows for closer packing of these highly ordered phthalocyanines than in self-assembled (SA) or Langmuir-Blodgett (LB) thin films, due to the lack of hydrocarbon side chains which are necessary for control of molecular architecture during SA or LB depositions. Several new solution processable substituted phthalocyanines are introduced, which due to their strong self-assembled tendency, may be suitable for the formation of well organized thin films by SA and LB techniques. It is found that the types of the substituents attached to the Pc rings play a significant role in determining both the aggregation tendency and the electrochemical properties of Pcs. Surface pressure-area isotherms of these substituted phthalocyanines show that there can be one or two stable phase transition regimes for monomolecular film at the air/water interface. On-trough spectroscopic studies of benzylalkoxy substituted phthalocyanines show that in the pressure-area region prior to the formation of the first stable phase extensive aggregation has occurred. Electrochemical studies of fully compressed films of substituted phthalocyanines on certain substrates show the presence of multiple electroactive domains, controlling the oxidation or reduction process of the Pc rings. Spectroelectrochemical studies of LB films of CuPcOC₂OBz suggest that the presence of both monomer and aggregates leads to the two separate oxidation processes.