As propriedades ópticas de nanocristais de silício (Si-ncs) têm sido extensivamente estudadas após a primeira demonstração em 1990 de fotoluminescência altamente eficiente em silício poroso. Apesar dos progressos no entendimento da natureza da alta eficiência da luminescência dos Si-ncs e da enorme versatilidade para aplicações optoeletrônicas, este campo ainda é um tema de controvérsia devido à complexidade destes materiais. Além disso, as condições de preparação ainda afetam as propriedades de emissão destes materiais que são de fundamental importância para as aplicações tecnológicas. O presente trabalho teve como objetivo o estudo das propriedades óticas dos Si-ncs e entender os processos fotofisicos envolvidos na recombinação radiativa de éxcitons altamente confinados nesse sistema. Si-ncs embebidos em matriz amorfa de SiO2 foram preparados a partir de filmes de oxido de silício SiyO1-y subestequiométricos (y≥1/3) depositados em substratos de quartzo utilizando um sistema deposição CVD na fase estimulada por plasma (electron cyclotron resonance-plasma enhanced chemical vapor deposition ou ECR-PECVD). Esta técnica oferece boa passivação e estabilidade interfacial Si/SiO2. O tratamento térmico a altas temperaturas (900°C≤Ta≤1100°C) promove a precipitação do silício dentro da matriz, favorecendo um processo de nucleação e crescimento dos Si-ncs. Foram realizados tratamentos térmicos nos filmes sob atmosferas de Argônio (Ar) ou (Ar+5%H2) por duas horas. As distintas atmosferas promoveram a passivação de defeitos superficiais, principalmente de ligações pendentes pelo Hidrogênio. As propriedades associadas diretamente à fabricação, tais como estrutura cristalina, morfologia, tamanho e química da superfície dos Si-ncs foram correlacionadas com os processos de emissão envolvendo éxcitons. A caracterização estrutural foi realizada por Raio-x (XRD), Microscopia de Transmissão de Alta Resolução (HRTEM), Retroespalhamento de Rutherford e Espectroscopia Raman. As medidas óticas foram basicamente Absorção, Excitação Seletiva, Fotoluminescência CW (PL) e Fotoluminescência Resolvida no Tempo. Os resultados da caracterização indicaram que efeitos de confinamento quântico e de estados de superfície dominam o processo de recombinação no Si-nc/SiO2. Em conclusão, os resultados obtidos neste trabalho mostram uma interessante e uma nova correlação entre as condições de fabricação da amostra e os processos de recombinação de éxcitons em Si-nc/SiO2. Todos estes resultados desafiam modelos anteriores propostos para explicar as propriedades ópticas do sistema de Si-nc/SiO2 e prevê ajudar na futura aplicação tecnológica dos mesmos. / The optical properties of silicon nanocrystals (Si-nc) have been extensively studied after the first demonstration in 1990 of highly efficient photoluminescence in porous silicon. Despite progress in understanding the nature of high luminescence efficiency of Si-ncs and versatility for optoelectronic applications, this field is still a subject of controversy due to its complexity. Furthermore, the preparation conditions still affect the emission properties of these materials that are of fundamental importance for technological applications. This work aimed to study the optical properties of Si-ncs and to understand the photophysical processes involved in the radiative recombination of excitons strongly confined in this system. Si-ncs embedded in amorphous SiO2 were prepared from silicon oxide films of substoichiometric SiyO1-y (y≥1/3) deposited on quartz substrates using a CVD deposition system in phase stimulated by plasma (electron cyclotron resonance-plasma enhanced chemical vapor deposition ou ECR-PECVD). This technique provides good passivation and Si/SiO2 interfacial stability. The thermal treatment at high temperatures (900°C≤Ta≤1100°C) promotes the precipitation of silicon within the matrix, favoring a process of nucleation and growth of Si-ncs. The thermal treatments were performed in the films under Argon atmosphere (Ar) or (Ar+5%H2) for two hours. The use of different atmospheres allowed the understand of the passivation process of surface defects, particularly of dangling bonds by Hydrogen. The properties directly related to fabrication such as crystalline structure, morphology, size and surface chemistry of Si-ncs were correlated with emission processes involving excitons. The structural characterization was performed by X-Ray Diffraction (XRD), High resolution transmission electron microscopy (HRTEM), Rutherford Backscattering and Raman spectroscopy. The optical measurements were basically Absorption, Selective excitation, CW photoluminescence (PL) and Time Resolved Photoluminescence. The characterization results indicate that both quantum confinement and surface states effects dominate the recombination process in Si-ncs/SiO2. In conclusion, the results obtained in this work show an interesting and a novel correlation between the sample fabrication conditions and the exciton recombination process in Si-ncs/SiO2. All these results challenges previous models proposed to explain the optical properties of Si-nc systems and are expected to help further technological applications of this system.

Polyaniline/Gold Nanocomposites J. Anthony Smith 141 Pages Directed by Dr. Ji and #345;?anata The expectation that it is possible to create a range of new materials from two basic components, polyaniline fibers and gold particles is explored. Three synthetic methods were employed each of which created different materials and required different investigation techniques. The methods are: chemical, one step aniline oxidation / AuCl4- reduction; electrochemical/chemical, a two-step composite growth achieved by electrochemical polyaniline thin film growth followed by film immersion in AuCl4- solution and spontaneous reduction to gold particles; electrochemical, resulting in freestanding polyaniline thin film/Au nanoparticles carried out by electrochemical stripping of a polyaniline thin film grown over a sacrificial gold layer in the presence halide solutions. The incorporation of particles was shown to affect film morphology and electrical properties in all synthetic methods. The changes are in large part attributed to the development of a contact potential between the polyaniline and the gold particles. Applications for the composites include use as chemically sensitive layers, corrosion inhibition materials, and use as probes to evaluate nanoparticle substrate interactions.

The use of microporous materials with ordered porous structures as the hosts to stabilize metal nanoclusters has attracted particular interest in the catalysis because the pore size restriction could confine the growth of nanoclusters and lead to an increase in the percentage of catalytically active surface atoms. In this dissertation we report the preparation, characterization and the investigation of the catalytic performance of zeolite framework stabilized ruthenium(0) nanoclusters in the hydrolysis of sodium borohydride and the hydrogenation of aromatics. The zeolite framework stabilized ruthenium(0) nanoclusters were prepared by borohydride reduction of ruthenium(III)-exchanged zeolite-Y in aqueous solution at room temperature and isolated as black powders. Their characterization by using ICP-OES, XRD, TEM, ZC-TEM, HR-TEM, TEM-EDX, SEM, XPS, DR-UV-vis, far-IR, mid-IR, Raman spectroscopy, N2 adsorption-desorption technique and (P(C6H11)3)/(PC6H11O3) poisoning experiments reveal the formation of ruthenium(0) nanoclusters within the zeolite cages as well as on the external surface of zeolite without causing alteration in the framework lattice or loss in the crystallinity. The catalytic performance of zeolite framework stabilized ruthenium(0) nanoclusters depending on the various parameters was tested in the hydrolysis of sodium borohydride and the hydrogenation of aromatics. The important results obtained from these experiments can be listed as follows: (i) the zeolite framework stabilized ruthenium(0) nanoclusters provide a record total turnover number (103200 mol H2/mol Ru) and turnover frequency (33000 mol H2/mol Ru&bull / h) in the hydrolysis of sodium borohydride at room temperature, (ii) they also catalyze the same reaction in the basic medium (in 5 wt % NaOH solution) at room temperature with the unprecedented catalytic activity (4000 mol H2/mol Ru&bull / h) and lifetime (27200 mol H2/mol Ru), (iii) the isolated and vacuum dried samples of zeolite framework stabilized ruthenium(0) nanoclusters are active catalysts in the hydrogenation of cyclohexene, benzene, toluene and o-xylene in cyclohexane, they provide TOF values of 6150, 5660, 3200, and 1550 mol H2/mol Ru&bull / h, respectively under mild conditions (at 22.0 &plusmn / 0.1 &deg / C, and 40 &plusmn / 1 psig of initial H2 pressure), (iv) more importantly, the zeolite framework stabilized ruthenium(0) nanoclusters are the lowest temperature, most active, most selective (100 % selectivity with complete conversion) and longest lifetime catalyst hitherto known for the hydrogenation of benzene to cyclohexane in the solvent-free system (TTON of 2420 and TOF of 1040 mol benzene/mol Ru&bull / h) under mild conditions (at 22.0 &plusmn / 0.1 &deg / C, and 40 &plusmn / 1 psig of initial H2 pressure), (v) moreover, the resultant ruthenium(0) nanoclusters exhibit high durability throughout their catalytic use against agglomeration and leaching. This significant property makes them reusable catalyst without appreciable loss of their inherent activity.

Among the new hydrogen storage materials, ammonia borane (AB) appears to be the most promising one as it has high hydrogen content, high stability, and being environmentally benign. Dehydrogenation of AB can be achieved via hydrolysis, thermolysis or methanolysis. Methanolysis of AB eliminates some drawbacks of other dehydrogenation reactions of AB. The use of colloidal and supported particles as more active catalyst than their bulky counterparts for the hydrolysis of AB implies that reducing the particle size can cause an increase in the catalytic activity as the fraction of the surface atoms increases by decreasing the particle size. Similarly, transition metal nanoclusters can be utilized as catalyst for the methanolysis of AB as well. For this purpose transition metal nanoclusters need to be stabilized to a certain extent. Actually in the catalytic application of transition metal nanoclusters one of the most important problems is the aggregation of nanoclusters into bulk metal, despite of using the best stabilizers. In this regards, the use of metal nanoclusters as catalysts in systems with confined void spaces such as inside mesoporous and microporous solids appears to be an efficient way of preventing aggregation. In this dissertation we report for the first time the use of intrazeolite rhodium(0) nanoclusters as a catalyst in the methanolysis of ammonia borane. Rhodium(0) nanoclusters could be generated in zeolite-Y by a two-step procedure: (i) incorporation of rhodium(III) cations into the zeolite-Y by ion-exchange, (ii) reduction of rhodium(III) ions within the zeolite cages by sodium borohydride in aqueous solution, followed by filtration and dehydration by heating to 550 &deg / C under 10-4 Torr. Zeolite confined rhodium(0) nanoclusters are stable enough to be isolated as solid materials and characterized by ICP-OES, XRD, SEM, EDX, HRTEM, XPS and N2 adsorption-desorption technique. The zeolite confined rhodium(0) nanoclusters are isolable, bottleable, redispersible and reusable. They are active catalyst in the methanolysis of ammonia-borane even at low temperatures. They provide exceptional catalytic activity with an average value of TOF = 380 h-1 and unprecedented lifetime with 74300 turnovers in the methanolysis of ammonia-borane at 25 &plusmn / 0.1 &deg / C. The work reported here also includes the full experimental details of previously unavailable kinetic data to determine the rate law, and activation parameters (Ea, &amp / #916 / H&amp / #8800 / and &amp / #916 / S&amp / #8800 / ) for the catalytic methanolysis of ammonia-borane.

More attention has been paid to find new type renewable energy sources because of increasing concern about the environmental problems arising from the combustion of fossil fuels as energy sources. The development of new storage materials will facilitate the use of hydrogen as a major energy carrier. Several possibilities exist for &lsquo / &lsquo / solid-state&rsquo / &rsquo / storage: the hydrogen can be trapped in metal organic frameworks, carbon nanotubes and certain alloys / or one can use materials in which hydrogen is already present in the composition (e.g., chemical hydrides). The latter option seems to be the most promising since it permits a higher mass ratio of hydrogen. Recently, ammonia-borane complex (NH3BH3, AB) has been considered as solid hydrogen storage material since it possess one of the highest hydrogen contents (19.6 wt. %) and high stability under the moderate conditions. Hydrolysis and methanolysis are the two reactions liberating hydrogen from AB. However, a catalyst is needed for hydrogen generation from methanolysis of AB. In this context, we aim to develop PVP-stabilized palladium(0) and ruthenium(0) nanoclusters as catalyst for the methanolysis of AB. The PVP-stabilized palladium(0) and ruthenium(0) nanoclusters were prepared from the in-situ reduction of palladium(II) acetylacetonate and ruthenium(III) chloride respectively in the methanolysis of AB. The prepared palladium(0) nanoclusters were isolated as solid materials by removing the volatile in vacuum and characterized by using TEM, SAED, XPS, FT-IR, XRD and UV-visible electronic absorption spectroscopy techniques while and ruthenium(0) nanoclusters were characterized by TEM, XPS, XRD, FT-IR and UV-visible electronic absorption spectroscopy techniques. The kinetics of methanolysis of AB catalyzed by palladium(0) and ruthenium(0) nanoclusters were studied depending on the catalyst concentration, substrate concentration and temperature. The activation parameters of the catalytic methanolysis reaction obtained from the evaluation of kinetic data.

Transition metal nanoclusters are more active and selective catalysts than their bulk counterparts as the fraction of surface atoms increases with the decreasing particle size. When stabilized in organic or aqueous solutions, they can catalyze many reactions. The catalytic activity of metal nanoclusters depends on the particle size and size distribution. Particle size can be controlled by encapsulating the nanoclusters in the cavities of highly ordered porous materials such as zeolites. In this project, osmium(0) nanoclusters were formed within the void spaces of zeolite. Thus, nanoclusters of certain size were prepared as supported catalyst. Osmium(III) cations were introduced into the cavities of zeolite by ion exchange and were reduced partially or completely to form intrazeolite osmium(0) nanoclusters. The intrazeolite osmium(0) nanocusters were characterized by HRTEM, TEM, EDX, XPS, XRD, ICP-OES spectroscopic methods and N2 adsorption-desorption technique. Intrazeolite osmium(0) nanoclusters were employed as catalyst in the aerobic oxidation of alcohols in organic solution.

Because of the growing concerns over the depletion of fossil fuel supplies, environmental pollution and global warming caused by a steep increase in carbon dioxide and other greenhouse gases in the atmosphere, much attention has been given to the development of renewable energy sources that are the only long-term solution to the energy requirements of the world&rsquo / s population, on the way towards a sustainable energy future. Hydrogen has been considered as a clean and environmentally benign new energy carrier for heating, transportation, mechanical power and electricity generation. However, the lack of effective, safe, and low-cost hydrogen storage materials for mobile, portable, and stationary applications is one of the major hurdles to be overcome for the implementation of hydrogen economy. Among various solid state hydrogen storage materials, chemical hydrogen storage materials such as sodium borohydride (NaBH4) and ammonia borane (H3NBH3) have received much attention as promising candidates for fuel cell applications under ambient conditions due to their high gravimetric and volumetric hydrogen storage capacities. Both sodium borohydride and ammonia borane generate hydrogen upon hydrolysis in the presence of suitable metal catalysts. Transition metal nanoclusters can be used as active catalysts to catalyze the hydrolysis reactions of sodium borohydride and ammonia borane for hydrogen generation since they exhibit unique properties that differ from their bulk counterparts. Although the catalytic activity of metal nanoclusters increases with decreasing particle size, they are unstable with respect to agglomeration into the bulk metal leading to a significant decrease in activity in their catalytic applications. Therefore, the exploitation of microporous and mesoporous materials with ordered porous structures as hosts to encapsulate metal nanoclusters has attracted great interest since the pore size restriction of these host materials could limit the growth of nanoclusters leading to an increase in the percentage of the catalytically active surface atoms. In this dissertation, we report the preparation, characterization and the investigation of the catalytic activities of zeolite confined cobalt(0) nanoclusters in the hydrolysis of sodium borohydride and ammonia borane. The zeolite confined cobalt(0) nanoclusters were prepared by the reduction of cobalt(II)-exchanged zeolite-Y by sodium borohydride in aqueous solution at room temperature with no alteration in the framework lattice or loss in the crystallinity. The characterization of zeolite confined cobalt(0) nanoclusters were done by using inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-visible spectroscopy (DR-UV-Vis), infrared spectroscopy (IR), Raman spectroscopy, and N2 adsorption-desorption technique. The catalytic activity of zeolite confined cobalt(0) nanoclusters and the kinetics of hydrogen generation from the hydrolysis of sodium borohydride and ammonia borane were studied depending on catalyst concentration, substrate concentration and temperature. The rate laws and the activation parameters (Arrhenius activation energy, Ea / activation enthalpy, &Delta / H# / and activation entropy, &Delta / S#) for both catalytic hydrolysis reactions were calculated from the obtained kinetic data.

The property transition metal nanoclusters are more active catalysts than their bulk counterparts because of increasing proportion of surface atoms with decreasing paricle size. The development of efficient and economical catalysts to further improve the kinetic properties under moderate conditions is therefore important for the practical application of nanoclusters as catalyst in the hydrogen generation from hydrolysis of ammonia borane this. In this regard, the development of active iron catalysts is a desired goal because it is the most ubiquitous of the transition metals, the fourth most plentiful element in the Earth&rsquo / s crust. In this dissertation, we report the preparation, characterization and investigation of the catalytic activity of the water soluble polymer stabilized iron(0) nanoclusters. They were prepared from the reduction of iron(III) chloride by a mixture of sodium borohydride (NaBH4, SB) and ammonia borane (H3NBH3, AB) mixture in the presence of polyethylene glycol (PEG) as stabilizer and ethylene glycol as solvent at 80 &deg / C under nitrogen atmosphere. PEG stabilized iron(0) nanoclusters were isolated from the reaction solution by centrifugation and characterized by SEM, EDX, TEM, HRTEM, XRD, UV-Vis, ICP-OES and FT-IR techniques. PEG stabilized iron(0) nanoclusters have almost uniform size distribution with an average particle size of 6.3 &plusmn / 1.5 nm. They were redispersible in water and yet highly active catalyst in hydrogen generation from the hydrolysis of AB. They provide a turnover frequency of TOF = 6.5 min-1 for the hydrolysis of AB at 25.0 &plusmn / 0.5 &deg / C. The TOF value is the best ever reported among the Fe catalyst and comparable to other non-noble metal catalyst systems for the catalytic hydrolysis of AB. Kinetics of hydrogen generation from the hydrolysis of AB in the presence of PEG stabilized iron(0) nanoclusters were also studied by varying the catalyst concentration, substrate concentration, and temperature. This is the first kinetic study on the hydrolysis of AB in the presence of an iron catalyst. Moreover, PEG stabilized iron(0) nanoclusters can be separated magnetically from the catalytic reaction solution by using a magnet and show catalytic activity even after tenth run.

Sodium borohydride (NaBH4) offers a simple and safe technology for storage and on demand production of hydrogen being a promising and a feasible method for fuel cell applications. The objectives of the present study are to emphasize the role of sodium borohydride as a part of future hydrogen energy system, to investigate the kinetics of the catalytic hydrolysis reaction of NaBH4 in a batch and flow system with respect to temperature, concentration, catalyst and flow rate for flow reactor by proposing a kinetic model and finally based on kinetic analysis, to design a prototype reactor to meet the hydrogen requirement for a 100W PEM fuel cell and operate it in steady state conditions. To express hydrolysis reaction by a kinetic model, series of batch experiments was performed in a glass flask (30mL) where the following parameters were systematically changed: the solution temperature varied as 20, 30 and 50&deg / C, the NaBH4 concentration changed as 0.17, 0.23 and 0.3M, NaOH concentration varied as 0.27, 1.32 and 2.85M and catalyst amount was changed as 0.048, 0.07 and 0.1g Pt/C (ETEK&reg / ). In the kinetic model catalyst effect proposed within the rate constant. The kinetic model was purposed as: For flow reactor system, in a differential glass reactor (5mL) concentration, catalyst amount, catalyst type and flow rate was systematically analyzed at a constant temperature. For Pt/C catalyst the purposed model was: Also, for intrazeolite Co(0) nanoclusters, as a result of controlled experiments, the rate expression was found as: Based on these data prototype reactor (recycle) with internal volume of 122cm3 and storage volume of 1336 cm3 was designed, manufactured from Delrin&reg / and operated.

Photophysical characterization of several species of fluorescent silver nanoclusters, encapsulated in oligonucleotide scaffolds, was achieved at the bulk and single molecule level. These studies reveal the presence of a short-lived microsecond blinking component which leads to higher emission rates than exhibited by common organic dyes. This dark state was found to be photo-accessible with a very efficient depopulation transition leading to repopulation of the emissive state. Secondary excitation on resonance with this transition significantly shortens the residence time in the dark state giving rise to as much as 5-fold fluorescence enhancement. Manipulation of the secondary laser can be used to impose a regularly modulated waveform onto the fluorescent signal. Signal processing techniques can be employed to extract the modulated signal from large backgrounds, leading to drastically improved sensitivity. This new imaging concept can be extended, beyond Ag nanoclusters, to common organic fluorophores that demonstrate large dark state quantum yields. These fluorophores simultaneously illustrate the utility of this technique and help to define a general set of parameters for engineering ideal dyes for modulated signal extraction. Ideally suited for fluorescence enhancement, FRET pairs can be used to engineer a wide range of modulatable systems, based on detecting donor emission in the presence of a laser directly exciting the acceptor. The utility of Ag nanoclusters, organic dyes, and FRET systems for improved sensitivity are investigated in this work.