Dissertations / Theses on the topic 'Tin-oxygen'

Conventional power plants for the conversion of fossil fuels to electricity have low efficiencies and produce large amount of carbon dioxide, a greenhouse gas, which contribute to climate change. Hence, a molten tin reformer and methane-fuelled SOFC with molten tin anode (Sn(l)-SOFC) for easier CO2 capture and higher power efficiency were investigated. Both systems involved oxygen dissolution in molten tin and methane reaction with the dissolved oxygen, as well as gas bubbling, so oxygen dissolution and methane reaction at bubble | molten tin interface were investigated. Oxygen was separated successfully from a 10%O2-He blend through gas bubbling and dissolution in molten tin which suggests that oxygen may be separated from air in the molten tin reformer by bubbling air through molten tin in the first stage of the periodic process. An LSM-YSZ/LSM double-layered reference electrode and YSZ electrolyte potentiometric oxygen sensor was used to measure the concentration of dissolved oxygen in molten tin; hence, enabling derivation of the solubility limit and Gibbs energy change for the formation of SnO which was in equilibrium with oxygen at the solubility limit. The solubility of oxygen in molten tin in equilibrium with SnO in the temperature range 973-1123 K was ca. 0.019-0.107 atom%. The rate of oxygen dissolution in molten tin when 10%O2-He blend was bubbled through it was controlled by chemical reaction at the bubble | molten tin interface; the mechanism involved a first step of chemisorption to molten tin at the bubble | molten tin interface, forming SnO as the absorbed intermediate. The second step of the mechanism involved the dissociation of SnO to molten tin and oxygen atom incorporated in the molten tin. The rate limiting step was the dissociation of SnO into molten tin and oxygen atom. Likewise, the rate of deoxygenation of molten tin by 10%CH4-He was not limited by the diffusion of oxygen atoms in the molten tin but might be limited by surface reaction at the bubble | molten tin interface. The performance of the molten tin reformer and methane-fuelled Sn(l)-SOFC depends on bubble size and behaviour, so bubbles generated in molten tin were characterized by determining the sizes, shape, velocities, and behaviour under different operating conditions of nozzle diameter, gas flow rates and temperatures. A pressure pulse technique which incorporates a differential pressure transducer was employed successfully in the measurement of frequencies of bubble formation in molten tin at high temperatures in the range 973-1173 K while the bubbles were approximated as oblate spheroids which wobbled. LSM cathodes were deposited on micro-tubular YSZ electrolytes and the microstructures and electrical conductivities characterized by scanning electron microscopy (SEM) and four-point probe resistance measurement, respectively. SEM micrographs showed the densification of LSM cathodes with increased sintering temperature, which resulted in increased electrical conductivities. Potential difference-current density data and impedance spectra were determined for a methane-fuelled SOFC with molten tin anode. A peak power density of about 100 W m-2 at a current density of 222 A m-2 and potential difference of 0.45 V was obtained for the methane-fuelled SOFC with molten tin anode at 850 oC. Impedance spectra showed that ohmic potential losses controlled the reactor performance, with about half of those arising from the inherent difficulty in achieving a low resistance contact at the (Ag wire) Ag wool current collector | LSM cathode interface.

The generation of high purity hydrogen by renewable, sustainable means is a crucial building block towards the realisation of a carbon-free energy economy. Proton exchange membrane water electrolysis (PEMWE) offers a promising route for the generation of clean hydrogen, using renewable energy, for both stationary and mobile energy storage applications, and as a feedstock for the chemical industry. As water electrolysis is an electrochemical redox reaction, cathodic hydrogen evolution cannot occur without an efficient, and rapid anodic oxygen evolution reaction (OER). While both iridium and ruthenium oxides are state-of-the-art OER catalysts in acidic environment, the latter undergoes dissolution under anodic OER conditions much more rapidly than the former, and this makes iridium oxide the most suitable catalytic material for electrolyser anodes. Several strategies have been explored as a means to lower the iridium content in OER catalysts, and of these, the use of cheap, stable support materials has been seen as a promising means to produce highly active, durable catalysts, by enhancement of the electrocatalytically active surface area. In this thesis, the viability of an organometallic chemical deposition method for the deposition of IrOₓ nanoparticles on antimony-doped tin oxide (ATO) support is investigated. The effect of the gas environment (oxygen or argon) and the temperature used for the deposition was examined. The ex-situ OER performance of the synthesised electrocatalysts was evaluated using the rotating disk electrode technique. Using X-ray photoelectron spectroscopy (XPS) and high-resolution transmission scanning electron microscopy (HR-STEM), the physical properties of the synthesised IrOₓ/ATO catalysts were elucidated, in order to understand the observed oxygen evolution activity and stability of IrOₓ/ATO in relation to the OMCD technique. In addition to developing an understanding towards the physical and electrochemical properties of the synthesised materials, strategies to optimise the Ir yield achieved by the organometallic chemical deposition process were explored.

Les électrocatalyseurs conventionnels utilisés dans les piles à combustibles à membrane échangeuse de protons (PEMFC) sont composés de nanoparticules de platine supportées sur des noirs de carbone de forte surface spécifique. A la cathode de la PEMFC, siège de la réaction de réduction de l’oxygène (ORR), le potentiel électrochimique peut atteindre des valeurs élevées - notamment lors de phases arrêt-démarrage - engendrant des dégradations irréversibles du support carboné. Une solution « matériaux » consiste à remplacer ce dernier par des supports à base d’oxydes métalliques. Ceux-ci doivent être résistants à la corrosion électrochimique, conducteurs électroniques et posséder une structure poreuse et nano-architecturée (permettant le transport des réactifs et produits et une distribution homogène de l’ionomère et des nanoparticules de platine). Dans ce travail, nous avons donc élaboré et caractérisé des électrocatalyseurs à base de nanoparticules de platine (Pt) déposées sur du dioxyde d’étain (SnO₂) et de titane (TiO₂) texturés (morphologies aérogel, nanofibres ou « loosetubes ») et conducteurs électroniques (dopés au niobium Nb ou à l’antimoine Sb). Le support permettant d’atteindre les meilleures propriétés électrocatalytiques est un aérogel de SnO₂ dopé à l’antimoine, noté ATO. En particulier, l’électrocatalyseur Pt/ATO présente une activité spécifique vis-à-vis de l’ORR supérieure à celle d’un électrocatalyseur Pt/carbone Vulcan® synthétisé dans les mêmes conditions, suggérant des interactions bénéfiques entre les nanoparticules de Pt et le support oxyde métallique (Strong Metal Support Interactions, SMSI).Des tests de durabilité simulant le fonctionnement d’une PEMFC en conditions automobile ont été effectués en électrolyte liquide à 80 °C sur ces deux électrocatalyseurs : cyclage entre 0,60 et 1,00 V vs l’électrode réversible à hydrogène (RHE) ou entre 1,00 et 1,50 V vs RHE. Le catalyseur Pt/ATO présente une durabilité accrue par rapport au catalyseur Pt/carbone Vulcan® de référence. Cependant, de nouveaux mécanismes de dégradation ont été mis en évidence dans cette étude : tout d’abord, l’élément dopant Sb est progressivement dissout au cours du vieillissement électrochimique, ce qui implique une perte de conductivité électronique. Cette perte est en partie liée à des incursions à bas potentiel, notamment durant les caractérisations électrochimiques. De plus, entre 5 000 et 10 000 cycles de vieillissement électrochimique (entre 0,60 et 1,00 V vs RHE ou entre 1,00 et 1,50 V vs RHE à 57 °C), le matériau support perd sa structure poreuse et forme un film amorphe peu conducteur
Conventional electrocatalysts used in proton exchange membrane fuel cells (PEMFC) are composed of platinum nanoparticles supported on high specific surface area carbon blacks. At the cathode side of the PEMFC, where the oxygen reduction reaction (ORR) occurs, the electrochemical potential can reach high values - especially during startup-shutdown operating conditions - resulting in irreversible degradation of the carbon support. A “material” solution consists of replacing the carbon with supports based on metal oxides. The latter have to be resistant to electrochemical corrosion, be electronic conductor and have a porous and nano-architectural structure (for the transport of reagents and products and the homogeneous distribution of the ionomer and platinum nanoparticles).In this work, we have developed and characterized electrocatalysts composed of platinum (Pt) nanoparticles based on tin dioxide (SnO2) and titanium dioxide (TiO2) with optimized textural (aerogel, nanofibres or loosetubes morphologies) and electron-conduction properties (doped with niobium Nb or antimony Sb). The best electrocatalytic properties are reached for an antimony-doped SnO2 aerogel support, denoted ATO. The Pt/ATO electrocatalyst has especially a higher specific activity for the ORR than a Pt/carbon Vulcan® electrocatalyst, synthesized in the same conditions, suggesting beneficial interactions between the Pt nanoparticles and the metal oxide support (Strong Metal Support Interactions SMSI).Durability tests simulating automotive operating conditions of a PEMFC were carried out in liquid electrolyte at 57 °C on these two electrocatalysts by cycling between 0.60 and 1.00 V vs the reversible hydrogen electrode (RHE) or between 1.00 and 1.50 V vs RHE. The Pt/ATO electrocatalyst has an increased stability compared to the reference Pt/carbon Vulcan® electrocatalyst. However, new degradation mechanisms were highlighted in this study: first, the doping element (Sb) is progressively dissolved during electrochemical ageing, which implies a loss of electronic conductivity. This loss is partly due to incursions at low potential, including during electrochemical characterizations. Moreover, between 5,000 and 10,000 cycles of the accelerated stress tests (between 0.60 and 1.00 V vs RHE or between 1.00 and 1.50 V vs RHE at 57 °C), the support loses its porous structure and forms a poorly conductive amorphous film

Le développement de catalyseurs de la réaction de dégagement de l’oxygène (OER) pour les électrolyseurs à membrane échangeuse de protons (PEM) dépend de la compréhension du mécanisme de cette réaction. Cette thèse est consacrée à l'application de la spectroscopie d’émission de photoélectrons induits par rayons X (XPS) et de la spectroscopie de structure près du front d'absorption de rayons X (NEXAFS) operando sous une pression proche de l'ambiante (NAP) dans le but d’étudier les mécanismes de la réaction d’oxydation de l’eau sur des anodes à base d’iridium et de ruthénium et leurs dégradation dans les conditions de la réaction. Cette thèse montre les mécanismes différents de la réaction OER pour les anodes à base d’Ir et de Ru impliquant respectivement des transitions anioniques (formation d’espèce OI- électrophile) ou cationiques (formation des espèces de Ru avec l’état d'oxydation supérieur à IV) quelle que soit la nature (thermique ou électrochimique) des oxydes
Development of oxygen evolution reaction (OER) catalysts for proton exchange membrane water electrolysis technology depends on the understanding of the OER mechanism. This thesis is devoted to the application of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and near edge X-ray absorption fine structure (NEXAFS) techniques for operando investigation of the Ir, Ru - based anodes. For Ru-based systems, we observe the potential-induced irreversible transition of Ru (IV) from an anhydrous to a hydrated form, while the former is stabilized in the presence of Ir. Regarding single Ir-based anodes, the analysis of O K edge spectra reveals formation of electrophilic oxygen OI- as an OER intermediate. Higher stability of Ir catalysts supported on antimony-doped tin oxide (ATO) is related to their lower oxidation. This work demonstrates different OER mechanisms on Ir, Ru-based anodes involving anion and cation red-ox chemistry, correspondingly, regardless the oxide nature

The mechanism of catalytic oxidative dehydrogenation (OXD) of butenes to 1,3-butadiene over Sn-P-O and Li-Sn-P-O catalysts was investigated in batch recirculation and microcatalytic pulse reactors. Mechanistic features of the reaction were examined using deuterium labeled butene (isotopic tracer technique) and $\sp{18}$O-labeled carbon dioxide (oxygen isotope exchange) experiments. Solid state changes in the catalyst were examined through BET surface area, electron microscopy, x-ray powder diffraction, and x-ray photoelectron spectroscopy. Reaction products include 1,3-butadiene, carbon dioxide, water, and butene isomers.Both Sn-P-O and Li-Sn-P-O catalysts have activity of about 11% conversion and about 98% initial selectivity for butadiene after 15 minutes reaction time at 300$\sp\circ$C. Catalyst deactivation is caused by the formation of coke, which decreases the catalyst surface area. The Sn-P-O catalyst forms coke more readily than the Li-Sn-P-O catalyst. The activity of the aged catalysts can be partially recovered with treatment in 150 torr oxygen at 500$\sp\circ$C. The rate of formation of butadiene is zero order in both oxygen and butene. The rate of formation of carbon dioxide is zero order in butene and about 0.5 order in oxygen. The OXD reaction is inhibited by product butadiene, where low conversion data can be modeled by a modified Langmuir-Hinshelwood type rate expression. But the rate expression does not fit CO$\sb 2$ well. The activation energy for butadiene formation is about 19 kcal/mole over the Li-Sn-P-O catalyst. Microcatalytic pulse experiments carried out in the absence of gas phase oxygen indicated that the reactions probably proceed by consuming catalyst surface oxygen. Two consecutive sets of $\sp{18}$O labeled CO$\sb 2$ pulse experiments demonstrate that only surface oxygen is relatively mobile, and that bulk diffusion of oxygen to the surface may not play a very important role in the OXD mechanisms. Perdeuterated butene is less reactive than non-deuterated butene. Comparison of the rates of formation and analysis of isotopic compositions of the products revealed a significant kinetic isotopic effect for the OXD reaction. Therefore, carbon-hydrogen bond cleavage is considered rate limiting. Isomerization may occur via a concurrent non-oxidative reaction over weak acid sites. Experimental data are consistent with an oxidation-reduction cycle involving a Sn$\sp{+4}$ cation active center.

碩士
國立交通大學
材料科學與工程研究所
86
This study investigated the effect of predeposited Ti or TiN on TiN structure and properties. The relationship was investigated betweeen oxygen content and TiN properties and structure by changing oxygen flow. The TiN film has the 〈111〉 highly preferred orientation when the sputtering conditions are set at uncollimated sputtering, low sputtering power, and low substrate temperature. And it has a lower film resistivity when the sputtering conditions are set at collimated sputtering, high sputtering power, and high substrate temperature. TiN film will grow along 〈111〉 when one Ti underlayer with 〈0002〉preferred orientation was predeposited. Finally, one novel "2-step TiN deposition process" was successfully developed to grow the TiN film with a high deposition rate, low resistivity (58.23u Ω-cm), 〈111〉 highly preferred orientation, and high bottom step coverage by predepositing one underlayer (about 100A∼200A) as a seed layer. It is one novel and excellent process which can be applied to sub-quatrer micron metallization. The TiN film would tend to be amorphous, and the increase resistivity and leakage current increased with oxygen content.

碩士
國立清華大學
材料科學工程學系
94
Abstract Ti/SiO2/Si, TiN/Si and TiN/Ti/SiO2/Si structures were annealed in vacuum to observe the ability of Ti absorbing oxygen and the stress relaxation of TiN layer. The stress of the film was determined in situ by measuring the curvature of the sample during the annealing process. The phases and the microstructure of the film after annealing process were identified using XRD, AES, and TEM. A clear correlation was between the evolution of stress and the absorption of oxygen atoms by the Ti film. From the XRD data, we can find that the (002) peak of Ti shifts to low angle at 280oC~400oC in the Ti/SiO2/Si system. It is due to the oxygen-induced Ti lattice expansion. The content of oxygen increases as the annealing temperatures and time increase, and this process let the stress become more compressive. The experiment finds that the content of oxygen can be monitored by in-situ curvature measurement. Moreover, diffusivity and the activation energy of oxygen in Ti film can be extracted. The stress relaxation of TiN is due to grain growth proved by Dark Field TEM and XRD intensity. The starting temperatures for the stress relaxation are different between TiN/Si and TiN/Ti/SiO2 system because of different TiN textures in these two systems.

碩士
國立交通大學
電子研究所
107
In this study, a novel material ethanol gas sensor was developed, which uses tin dioxide as a sensing layer to make a resistive gas sensor, and uses lithography process technology to transfer to the interdigitated electrode in the mask, by measuring the current signal as an indicator of the gas response of the sensor, a gas sensor capable of detecting ethanol gas in a general environment is successfully produced with Dual E-Gun Evaporation System and used to realize the process of growing tin dioxide by physical vapor deposition (PVD) and the process of oxygen plasma treatment by atomic layer chemical vapor deposition system, due to the technology of thin film deposition or the technology of the end plasma treatment, etc., causes the surface roughness of the material and to be different from the hydrophilicity of the gas molecules. By changing the time of the oxygen plasma treatment, it is inferred from the literature that the oxygen plasma is treated with the gas of tin dioxide. Sensing mechanisms play an important role and are supported by the relationship between material analysis and electrical measurements. In this study, the optimal processing time was found by oxygen plasma treatment. The tin dioxide gas sensor by oxygen plasma treatment has good sensing performance and has a wide range of applications in life.

碩士
國立交通大學
光電系統研究所
102
Recently, the thin film transistors (TFTs) with transparent amorphous conductive thin film as active layer perform higher mobility and better reliability than conventional hydrogenated amorphous silicon TFT (a-Si: H TFT). In addition, the uniformity of a-IGZO TFT is also superior to low temperature polycrystalline silicon TFT (LTPS TFT). However, the usage of the rare element(Ga) and further enhancement of device mobility will be an important issue for the long-term applications. In this work, we developed new amorphous oxide semiconductor--- amorphous In-Zn-Sn-O thin film transistors (a-IZTO TFTs). We investigated the effect of oxygen incorporation on a-IZTO thin film transistors. For the devices with different oxygen flow rate incorporation, the small quantity of oxygen can repair defects and optimize the characteristic, however, the trap states increase when oxygen flow rate increases, leading to the decreasing mobility. Moreover, the mobility of the optimized devices after annealing has reached 15 cm2/V.s~ 20cm2/V.s. For the application of flat panel displays, we have successfully transferred the optimized device from Si wafer to the ultra thin glass (0.13mm) which has bending potential. Devices on ultra thin glasses have stable basic electrical characteristics and their mobilities were also comparable (12~13 cm2/V.s). With the advantages of high mobility and ultra thin glass, these results show the future application potentials of a-IZTO TFT devices on flat panel display technology.

碩士
國立交通大學
電子工程系所
97
Recently, since nonvolatile memories acquire a lot of attention and flash memories are facing with the scale limit issue, the next generation nonvolatile memory has been carried out to discover extensively. The resistive random access memories (ReRAMs) that have the strengths of high cell density array, high operation speed, low power consumption, high endurance, lower scale limit and non-destructive readout, are one of the most potential candidate for flash memories. In this thesis, a physical model and mechanism which is about the role of oxygen vacancies and phase change in TiN/SiO2/PtFe resistance nonvolatile random access memories is proposed. This study can be categorized into three parts, different structures, different thermal treatments and small size devices, all of these electrical results can support the model and mechanism. In the first part, replacing metal electrode materials and SiO2 thickness with different structures was found the results which the effective resistance switching region is at interface region, and Fe element plays an important role to cause resistance switching behavior. In the second part, with different thermal treatments to examine the resistance switching characteristics, was discovered that amount of Fe2O3 and oxygen vacancies would affect endurance reliability and electric characteristics. In the third part, using small size cells to examine the resistance switching characteristics was found the results which are similar with the electric faucet theory and the proposed model. Moreover, a possible model about electric faucet is proposed by physical and mathematical methods. Further investigation, including interfacial electric faucet structure and electrode effects, would help to achieve a better understanding.

碩士
義守大學
材料科學與工程學系
89
Metallization of oxide surfaces using aluminum（Al）or gold（Au）is a key process in the fabrication or packaging of electronic and optoelectronic devices. Since these metal thin films provide the electrical and mechanical connection, the adhesion between metal thin films and oxide layer is one of the main concerns in the processing. Higher adhesion strength of metal thin film to oxide surface normally leads to high mechanical reliability of devices. Al thin film with Cr interlayer has been used as a composite layer to metallize ITO-coated glass. The top Al layer provides bondability and electrical conductivity, while the Cr layer is inserted to provide adhesion strength . From a previous study, it is well known that the adhesion between Al/Cr and ITO glass needs to be increased in order to sustain some thermal and mechanical tests. In order to enhance the adhesion between the Al/Cr and ITO glass, oxygen interface doping at the Cr - ITO interface has been proposed in this study. Various deposition parameters（bias, oxygen flow rate）have been used to determine the optimal condition for thin film deposition. From the results of compressive stress and crystalline size, they indicate the optimal bias voltages are in the range of —40 to —80V. The maximum improvement in adhesion strength can be obtained as the oxygen flow rate at 6 sccm. The structure and morphology of this change can be studied by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).

碩士
國立中山大學
電機工程學系研究所
104
With the development of integrated circuits, the transistors dimension has been scaling. In order to maintain the electrical behavior of transistors, miniature engineering faced many challenges. For example, reduced of gate control ability and increased of subthreshold swing. There are many ways to keep the transistors performance. One of the option is using high dielectric constant material instead of the traditional gate oxide layer. In the thesis, LTPS-TFTs are fabricated with Hafnia oxide gate dielectric and the impact of oxygen plasma induced interfacial layer on electrical behavior of P-type Poly-Si thin film transistors were investigated. First discuss the electrical behavior at room temperature. The transistors with oxygen plasma have less dangling bonds and more strain bonds, leading to smaller subthreshold swing and lower transconductance. The oxygen plasma will grow plasma-induced interfacial layer, it contains negative fix oxide charges, resulting in smaller threshold voltage. To study the effect of reliability, it was divided into negative bias stress, negative bias temperature instability and temperature effects. In the experiment, the traditional transistors have serious degradation in subthreshold swing. Since the dangling bonds generated on the surface of channel. The transistors with oxygen plasma occurred hole injection in HfO2, producing positive fix oxide charges.

碩士
崑山科技大學
電機工程研究所
101
Indium tin oxide (ITO) films were grown by in-line DC sputtering with unheated substrate. The oxygen concentration and substrate moving speed was varied during sputtering. The result indicate ITO films grown at 1.3% oxygen concentration can reach the lowest electrical resistivity 3.6×10-4 Ω-cm among these of the varied oxygen contraction test. In addition , the lowest electrical resistivity and high mobility of ITO films among test of varied substrate moving speed corresponds to low substrate moving speed. It may be related with formed during sputtering. Low substrate moving speed cause low interface stress.

碩士
國立中山大學
電機工程學系研究所
104
The application of polysilicon thin-film transistors in active matrix liquid crystal displays has been the main driver of the development of polysilicon thin-film transistors technology. The conventional poly-Si TFTs with SiO2 gate dielectric has been scaling down to meet the requirements of high performance, but hard to achieve this goal. Using high-κ materials as gate dielectric layer can improve gate capacitance density and induce more carriers to enhance the device characteristics. Among the high-κ materials, HfO2 is a promising alternative to be the gate dielectric. On the other side, there could be defects in the grain boundary of poly-Si channel film, which would capture carriers to form potential barrier and affect device performance. Oxygen plasma surface treatment is capable of passivating these defects and improving the gate dielectric/poly-Si interface quality. In this paper, impacts of oxygen plasma surface treatment on performance and reliability of n-type poly-Si thin-film transistors with TiN/HfO2 gate stack have been researched: To study the impact of oxygen plasma surface treatment on performance of n-type poly-Si thin-film transistors with TiN/HfO2 gate stack, measurements of the transfer characteristics and output characteristics have been performed on HfO2 poly-Si N-type TFTs of various channel length without and with O2 plasma surface treatment at room temperature. Enhancement of device performance has been observed through all the channel length of 20μm, 10μm, 5μm, 2μm and 1μm with O2 plasma surface treatment. To study the impact of oxygen plasma surface treatment on reliability of n-type poly-Si thin-film transistors with TiN/HfO2 gate stack, measurements of the transfer characteristics and output characteristics have been performed on HfO2 poly-Si N-type TFTs of W/L = 100m/10m without and with O2 plasma surface treatment at the temperature of 125℃, the PBTI stress condition being set as VOC = VG-VTH = 5V, 6V, 7V. The PBTI degradation characteristics have been observed, while the O2 plasma surface treatment has reduced the degradation and enhanced the device reliability.

abstract: Redox reactions are crucial to energy transduction in biology. Protein film electrochemistry (PFE) is a technique for studying redox proteins in which the protein is immobilized at an electrode surface so as to allow direct exchange of electrons. Establishing a direct electronic connection eliminates the need for redox­active mediators, thus allowing for interrogation of the redox protein of interest. PFE has proven a versatile tool that has been used to elucidate the properties of many technologically relevant redox proteins including hydrogenases, laccases, and glucose oxidase. This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from Pyrococcus furiosus (PfSHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis.
Dissertation/Thesis
Doctoral Dissertation Biochemistry 2014

Global warming and depletion in fossil fuels have forced the society to search for alternate, clean sustainable energy sources. An obvious solution to the aforesaid problem lies in electrochemical energy storage systems like fuel cells and batteries. The desirable properties attributed to these devices like quick response, long life cycle, high round trip efficiency, clean source, low maintenance etc. have made them very attractive as energy storage devices. Compared to many advanced battery chemistries like nickel-metal hydride and lithium - ion batteries, metal-air batteries show several advantages like high energy density, ease of operation etc. The notable characteristics of metal - air batteries are the open structure with oxygen gas accessed from ambient air in the cathode compartment. These batteries rely on oxygen reduction and oxygen evolution reactions during discharging and charging processes. The efficiency of these systems is determined by the kinetics of oxygen reduction reaction. Platinum is the most preferred catalyst for many electrochemical reactions. However, high cost and stability issues restrict the use of Pt and hence there is quest for the development of stable, durable and active electrocatalysts for various redox reactions. The present thesis is directed towards exploring the electrocatalytic aspects of titanium carbonitride. TiCN, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrochemical studies. The work function as well as the bonding in the material can be tuned by varying the composition of carbon and nitrogen in the crystal lattice. The current study explores the versatility of TiCN as electrocatalyst in aqueous and non-aqueous media. One dimensional TiC0.7N0.3 nanowires are prepared by simple one step solvothermal method without use of any template and are characterized using various physicochemical techniques. The 1D nanostructures are of several µm size length and 40 ± 15 nm diameter (figure 1). Orientation followed by attachment of the primary particles results in the growth along a particular plane (figure 2). (a) (b) (c) Figure 1. (a) SEM images of TiC0.7N0.3 nanowires (b) TEM image and (c) High resolution TEM image showing the lattice fringes. (a) (b) (d) Figure 2. Bright field TEM images obtained at different time scales of reaction. (a) 0 h; (b) 12 h; (c) 72 h and (d) 144 h. The next aspect of the thesis discusses the electrochemical performance of TiC0.7N0.3 especially for oxygen reduction. Electrochemical oxygen reduction reaction (ORR) reveals that the nanowires possess high activity for ORR and involves four electron process leading to water as the product. The catalyst effectively converts oxygen to water with an efficiency of 85%. A comparison of the activity of different (C/N) compositions of TiCN is shown in figure 3. The composition TiC0.7N0.3 shows the maximum activity for the reaction. The catalyst is also very selective for ORR in presence of methanol and thus cross-over issue in fuel cells can be effectively addressed. Density functional theory (DFT) calculations also lead to the same composition as the best for electrocatalysis, supporting the experimental observations. Figure 3. Linear sweep voltammetric curves observed for different compositions of titanium carbonitride towards ORR. The next chapter deals with the use of TiC0.7N0.3 as air cathode for aqueous metal - air batteries. The batteries show remarkable performance in the gel- and in liquid- based electrolytes for zinc - air and magnesium - air batteries. A partial potassium salt of polyacrylic acid (PAAK) is used as the polymer to form a gel electrolyte. The cell is found to perform very well even at very high current densities in the gel electrolyte (figures 4 and 5). Figure 4 Photographs of different components of the gel - based zinc - air battery. (a) (b) Figure 5. a) Discharge curves at different current densities of 5, 20, 50 and 100 mA/cm2 for zinc-air system with TiC0.7N0.3 cathode b) Charge – discharge cycles at 50 mA/cm2 for the three electrode configuration with TiC0.7N0.3 nanowire for ORR and IrO2 for OER and Zn electrode (2h. cycle period). Similarly, the catalytic activity of TiC0.7N0.3 has also been explored in non-aqueous electrolyte. The material acts as a bifunctional catalyst for oxygen in non- aqueous medium as well. It shows a stable performance for more than 100 cycles with high reversibility for ORR and OER (figure 6). Li-O2 battery fabricated with a non-aqueous gel- based electrolyte yields very good output. (a) (b) (c) Figure 6. Galvanostatic charge –discharge cycles. (a) at 1 mA/cm2 (b) specific capacity as a function of no. of cycles (c) photographs of PAN-based gel polymer electrolyte. Another reaction of interest in non –aqueous medium is I-/I3-. redox couple. TiC0.7N0.3 nanowires show small peak to peak separation, low charge transfer resistance and hence high activity. The catalyst is used as a counter electrode in dye sensitized a solar cell that shows efficiencies similar to that of Pt, state of the art catalyst (figure 7). (a) (b) (c) Figure 7 (a) Cyclic voltammograms for I-/I3 - redox species on TiC0.7N0.3 nanowires (red), TiC0.7N0.3 particle (black) and Pt (blue). (b) Photocurrent density - voltage characteristics for DSSCs with different counter electrodes. TiC0.7N0.3 nanowire (black), TiC0.7N0.3 particle (blue), Pt (red). (c) Photograph of a sample cell. (a) (b) (c) (d) Figure 8 a) Comparison ORR activity for (i) NiTiO3(black), (ii) N-rGO (red), (iii) NiTiO3 – N-rGO (green) and (iv) Pt/C (blue) (b) Linear sweep voltammograms for OER observed on NiTiO3 – N-rGO composite (black), NiTiO3 (brown), N-rGO (blue), glassy carbon (red) in 0.5 M KOH. (c) Galvanostatic discharge curves of NiTiO3 – N-rGO as air electrode (d) Charge – discharge cycle at 5 mA/cm2 for the rechargeable battery with 10 min. cycle period. The last part of the thesis discusses about a ceramic oxide, nickel titanate. The electrocatalytic studies of the material towards ORR and OER reveal that the catalyst shows remarkable performance as a bifunctional electrode. A gel - based zinc - air battery fabricated with nickel titanate – reduced graphene oxide composite shows exceptional performance of 1000 charge-discharge cycles in the rechargeable mode (figure 8). Of course, the primary battery configuration works very well too The thesis contains seven chapters on the aspects mentioned above with summary and future perspectives given as the last chapter. An appendix based on TiN nanotubes and supercapacitor studies is given at the end.

Global warming and depletion in fossil fuels have forced the society to search for alternate, clean sustainable energy sources. An obvious solution to the aforesaid problem lies in electrochemical energy storage systems like fuel cells and batteries. The desirable properties attributed to these devices like quick response, long life cycle, high round trip efficiency, clean source, low maintenance etc. have made them very attractive as energy storage devices. Compared to many advanced battery chemistries like nickel-metal hydride and lithium - ion batteries, metal-air batteries show several advantages like high energy density, ease of operation etc. The notable characteristics of metal - air batteries are the open structure with oxygen gas accessed from ambient air in the cathode compartment. These batteries rely on oxygen reduction and oxygen evolution reactions during discharging and charging processes. The efficiency of these systems is determined by the kinetics of oxygen reduction reaction. Platinum is the most preferred catalyst for many electrochemical reactions. However, high cost and stability issues restrict the use of Pt and hence there is quest for the development of stable, durable and active electrocatalysts for various redox reactions. The present thesis is directed towards exploring the electrocatalytic aspects of titanium carbonitride. TiCN, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrochemical studies. The work function as well as the bonding in the material can be tuned by varying the composition of carbon and nitrogen in the crystal lattice. The current study explores the versatility of TiCN as electrocatalyst in aqueous and non-aqueous media. One dimensional TiC0.7N0.3 nanowires are prepared by simple one step solvothermal method without use of any template and are characterized using various physicochemical techniques. The 1D nanostructures are of several µm size length and 40 ± 15 nm diameter (figure 1). Orientation followed by attachment of the primary particles results in the growth along a particular plane (figure 2). (a) (b) (c) Figure 1. (a) SEM images of TiC0.7N0.3 nanowires (b) TEM image and (c) High resolution TEM image showing the lattice fringes. (a) (b) (d) Figure 2. Bright field TEM images obtained at different time scales of reaction. (a) 0 h; (b) 12 h; (c) 72 h and (d) 144 h. The next aspect of the thesis discusses the electrochemical performance of TiC0.7N0.3 especially for oxygen reduction. Electrochemical oxygen reduction reaction (ORR) reveals that the nanowires possess high activity for ORR and involves four electron process leading to water as the product. The catalyst effectively converts oxygen to water with an efficiency of 85%. A comparison of the activity of different (C/N) compositions of TiCN is shown in figure 3. The composition TiC0.7N0.3 shows the maximum activity for the reaction. The catalyst is also very selective for ORR in presence of methanol and thus cross-over issue in fuel cells can be effectively addressed. Density functional theory (DFT) calculations also lead to the same composition as the best for electrocatalysis, supporting the experimental observations. Figure 3. Linear sweep voltammetric curves observed for different compositions of titanium carbonitride towards ORR. The next chapter deals with the use of TiC0.7N0.3 as air cathode for aqueous metal - air batteries. The batteries show remarkable performance in the gel- and in liquid- based electrolytes for zinc - air and magnesium - air batteries. A partial potassium salt of polyacrylic acid (PAAK) is used as the polymer to form a gel electrolyte. The cell is found to perform very well even at very high current densities in the gel electrolyte (figures 4 and 5). Figure 4 Photographs of different components of the gel - based zinc - air battery. (a) (b) Figure 5. a) Discharge curves at different current densities of 5, 20, 50 and 100 mA/cm2 for zinc-air system with TiC0.7N0.3 cathode b) Charge – discharge cycles at 50 mA/cm2 for the three electrode configuration with TiC0.7N0.3 nanowire for ORR and IrO2 for OER and Zn electrode (2h. cycle period). Similarly, the catalytic activity of TiC0.7N0.3 has also been explored in non-aqueous electrolyte. The material acts as a bifunctional catalyst for oxygen in non- aqueous medium as well. It shows a stable performance for more than 100 cycles with high reversibility for ORR and OER (figure 6). Li-O2 battery fabricated with a non-aqueous gel- based electrolyte yields very good output. (a) (b) (c) Figure 6. Galvanostatic charge –discharge cycles. (a) at 1 mA/cm2 (b) specific capacity as a function of no. of cycles (c) photographs of PAN-based gel polymer electrolyte. Another reaction of interest in non –aqueous medium is I-/I3-. redox couple. TiC0.7N0.3 nanowires show small peak to peak separation, low charge transfer resistance and hence high activity. The catalyst is used as a counter electrode in dye sensitized a solar cell that shows efficiencies similar to that of Pt, state of the art catalyst (figure 7). (a) (b) (c) Figure 7 (a) Cyclic voltammograms for I-/I3 - redox species on TiC0.7N0.3 nanowires (red), TiC0.7N0.3 particle (black) and Pt (blue). (b) Photocurrent density - voltage characteristics for DSSCs with different counter electrodes. TiC0.7N0.3 nanowire (black), TiC0.7N0.3 particle (blue), Pt (red). (c) Photograph of a sample cell. (a) (b) (c) (d) Figure 8 a) Comparison ORR activity for (i) NiTiO3(black), (ii) N-rGO (red), (iii) NiTiO3 – N-rGO (green) and (iv) Pt/C (blue) (b) Linear sweep voltammograms for OER observed on NiTiO3 – N-rGO composite (black), NiTiO3 (brown), N-rGO (blue), glassy carbon (red) in 0.5 M KOH. (c) Galvanostatic discharge curves of NiTiO3 – N-rGO as air electrode (d) Charge – discharge cycle at 5 mA/cm2 for the rechargeable battery with 10 min. cycle period. The last part of the thesis discusses about a ceramic oxide, nickel titanate. The electrocatalytic studies of the material towards ORR and OER reveal that the catalyst shows remarkable performance as a bifunctional electrode. A gel - based zinc - air battery fabricated with nickel titanate – reduced graphene oxide composite shows exceptional performance of 1000 charge-discharge cycles in the rechargeable mode (figure 8). Of course, the primary battery configuration works very well too The thesis contains seven chapters on the aspects mentioned above with summary and future perspectives given as the last chapter. An appendix based on TiN nanotubes and supercapacitor studies is given at the end.

The high temperature adsorption of oxygen and the room temperature adsorption of CO on the Pt3Sn(111) and Pt3Sn(110) surfaces have been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Beforehand the structure of the clean surfaces has been reviewed. After exposure to several 1000 L O2 at sample temperatures of about 750 K on both Pt3Sn(111) and (110) an ultra-thin Sn-O surface layer is formed. For the (111) X-ray photoelectron spectroscopy (XPS) indicates that this layer does not yet exhibit oxide properties. STM topographs of the Sn-O phase show on both surfaces meshes of highly corrugated protrusions commensurate with the substrate. In the case of the (111), after additional thermal annealing with STM and LEED a (4 × 4) reconstruction is observed, that is due to a (2 × 2) supermesh of depressions in the p(2 × 2) mesh of protrusions. This structure is similar to findings reported for the oxidation of Sn/Pt(111) surface alloys. X-ray photoelectron diffraction (XPD) measurements in comparison with simulations yield a tentative model for the (111) Sn-O layer. On the Pt3Sn(110) surface after oxygen exposure a c(2 × 2) hexagonal grid of protrusions with regard to the (2 × 1) substrate is observed with STM and LEED. STM reveals the existence of domains due to two equivalent positions of the Sn-O layer relative to the substrate. The domain boundaries zigzag around the [1-10] direction. The Sn-O layer can on both surfaces be removed by thermal annealing to more than 1050 K. After CO adsorption at room temperature on both Pt3Sn(111) and (110) adsorbate structures are observable with the STM. On the (111) two different types of structures are found: ordered patches of protrusions and unordered clusters. These structures are seen only on (√3 × √3)R30° substrate regions, not on p(2 × 2) regions. Surprisingly on the (110) the CO molecules mostly arrange in dimers. For both (111) and (110) saturation coverage is already reached at about 30% of a closed monolayer. The CO can be desorbed by slightly heating the samples to about 400 K. STM topographs show that on both surfaces CO adsorbes in Pt sites, not on Sn. It was possible to observe the CO adsorption on the (110) directly live with the STM. The observed adsorption processes hint to a dimer formation mechanism where a preadsorbed monomer and a CO molecule form the gas phase or a precursor phase stick together. When on partially Sn-O phase covered Pt3Sn(111) and (110) surfaces CO is adsorbed at room temperature, the respective structures coexist. Neither is CO observed on the Sn-O phase nor does a reaction between CO and O occur.