Controlling the size and size distribution of colloidal nanoparticles have gained extraordinary attention as their physical and chemical properties are strongly affected by size. Ligands are widely used to control the size and size distribution of nanoparticles; however, their exact roles in controlling the nanoparticle size distribution and the way they affect the nucleation and growth kinetics are poorly understood. Therefore, understanding the nucleation and growth mechanisms and developing theoretical/modeling framework will pave the way towards controlled synthesis of colloidal nanoparticles with desired sizes and polydispersity. This dissertation focuses on identifying the possible roles of ligands and size on the kinetics of nanoparticle formation and growth using in-situ characterization tools such as small-angle X-ray scattering (SAXS) and kinetic modeling. The presented work further focuses on developing kinetic models to capture the main nucleation and growth reactions and examines how ligand-metal interactions could potentially alter the rate of nucleation and growth rates, and consequently the nanoparticle size distribution. Additionally, this work highlights the importance of using multi-observables including the concentration of nanoparticles, size and/or precursor consumption, and polydispersity to differentiate between different nucleation and growth models and extract accurate information on the rates of nanoparticle nucleation and growth. Specifically, during the formation and growth of colloidal nanoparticles, complex reactions are occurring and as such nucleation and growth can take place through various reaction pathways. Therefore, sensitivity analysis was applied to effectively compare different nucleation and growth models and identify the most important reactions and obtain a reduced model (e.g. a minimalistic model) required for efficient data analysis. In the following chapters, a more sophisticated modeling approach is presented (population balance model) capable of capturing the average-properties of nanoparticle size distribution. PBM allows us to predict the growth rate of nanoparticles of different sizes, the ligand surface coverage for each individual size, and the parameters involved in altering the size distribution. Additionally, thermodynamic calculations of nanoparticle growth and ligand-metal binding as a function of size and ligand surface coverage were conducted to further shed light on the kinetics of nanoparticle formation and growth. The combination of kinetic modeling, in-situ SAXS and thermodynamic calculations can significantly advance the understanding of nucleation and growth mechanisms and guide toward controlling size and polydispersity. / Doctor of Philosophy / The synthesis of colloidal metal nanoparticles with superior control over size and size distribution, and has attracted much attention given the wide applications of these nanomaterials in the fields of catalysis, photonics, and electronics. Obtaining nanoparticles with desired sizes and polydispersity is vital for enhancing the consistency and performance for specific applications (e.g., catalytic converters for automotive emission). Ligands are often employed to prevent agglomeration and also control the nanoparticle size and size distribution. Ligands can affect the precursor reactivity and therefore the reduction/nucleation by binding with the metal precursor. Nucleation refers to the assimilation of few atoms to form initial nuclei acting as templates for nanoparticle growth. Additionally, ligands can bind with the nanoparticle surface sites and change the rate of surface growth and therefore the final nanoparticle size. Despite strong effects of ligands in the colloidal nanoparticle synthesis, their exact role in the nucleation and growth kinetics is yet to be identified. Additionally, nucleation and growth models capable of unraveling the underlying mechanisms of nucleation and growth in the presence of ligands are still lacking in the literature. Therefore, obtaining nanoparticles with desired sizes and polydispersity mostly relies on trial-and-error approach making the synthesis costly and inefficient. As such, developing models capable of predicting suitable synthesis conditions is contingent upon understanding the chemistry and mechanism involved during nanoparticles formation. Therefore, in this study, novel kinetic models were developed to capture the nucleation and growth kinetics of colloidal metal nanoparticles under different synthetic conditions (different types of solvents, different concentrations of ligand and metal). In-situ SAXS was further employed to measure the average diameter, concentration of nanoparticles, and polydispersity during the synthesis and extract the nucleation and growth rates (evolution of concentration of nanoparticles and size). First, an average-property model was developed to account for ligand-metal bindings and capture the size and concentration of nanoparticles during the synthesis. Then, a more complex modeling approach; PBM, accompanied by the thermodynamic calculations of surface growth and ligand-nanoparticle binding enthalpies was implemented to capture the size distribution. As it will be shown later, the determination of the underlying mechanisms resulted in a highly predictive kinetic model capable of predicting the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can serve as a powerful tool to synthesize nanoparticles with specific sizes and polydispersity.

Colloidal nanoparticles are widely used for industrial and scientific purposes in many fields, including catalysis, biosensing, drug delivery, and electrochemistry. It has been reported that most of the functional properties and performance of the nanoparticles are highly dependent on the particle size and morphology. Therefore, controlled synthesis of nanomaterials with desired size and structure is greatly beneficial to the application. This dissertation presents a systematic study on the effect of ligands on the colloidal Pd nanoparticle synthesis mechanism, kinetics, and final particle size. Specifically, the research is focused on investigating how the ligand bindings to different metal species, i.e., metal precursor and nanoparticle surface, affect the nucleation and growth pathways and rates and connecting the binding thermodynamics to the kinetics quantitatively. The first part of the work (Chapters 4 and 5) is establishing isothermal titration calorimetry (ITC) methodology for obtaining the thermodynamic values (Gibbs free energy, equilibrium constant, enthalpy and entropy) of the ligand-metal precursor binding reactions, and the simultaneous metal precursor trimer dissociation. In brief, the binding products and reactions were characterized by nuclear magnetic resonance (NMR), and an ITC model was developed to fit the unique ITC heat curve and extract the thermodynamic properties of the reactions above. Furthermore, in Chapter 6, the thermodynamic properties, especially the entropy trend changing with the ligand chain length was investigated on different metal precursors based on the established ITC methodology, showing that the entropic penalty plays a significant role in the binding equilibrium. The second part of the dissertation (Chapter 7 and 8) presents the kinetic and mechanistic study on size-tuning of the colloidal Pd nanoparticles only by changing different coordinating solvents as ligands together with the trioctylphosphine ligand. In-situ small angle X-ray scattering was applied to characterize the time evolutions of size, size distribution, and particle concentration using synthesis reactor connected to a capillary flow cell. From the real-time kinetic measurements, the nucleation and growth rates were calculated and correlated with the thermodynamics, i.e., Gibbs free energies of solvent-ligand-metal precursor reactivity and ligand-nanoparticle surface binding which were modified by the coordination of different solvents. Higher reactivity leads to faster nucleation and high nanoparticle concentration, and stronger solvent/ligand-particle coordination energy results in higher ligand capping density and slower growth. The interplay of both effects reduces the final particle size. Furthermore, because of the significance of the ligand-metal interactions, the synthesis temperature and ligand to metal precursor ratio were systematically to modify the relative binding between the ligand and precursor, and the ligand and nanoparticle, and determine the effect on the nucleation and growth rates. The results show that the relative rates of nucleation and growth is critical to the final size. A methodology for using the in-situ measurements to predict the final size by developing a kinetic model based is discussed. / Doctor of Philosophy / Metal nanoparticles dispersed in solution phase, i.e., colloidal nanoparticles, are of great scientific interests due to their unique properties different from bulk metal materials. The size, shape and other morphology features can largely affect the nanomaterial properties and functional performances. Therefore, a successful synthesis of nanoparticles with desired structures is highly beneficial to the development of their application. Ligands, which are long-chain molecules that can cap on the surface of the nanoparticles, have been known as stabilizers of the nanoparticles in the solution phase. Whereas in recent studies, it has been found that changing the ligand type and concentration in the synthesis can result in different sizes and shapes of nanomaterials, which indicates that the ligands are playing critical roles in the synthesis mechanisms to control the kinetics. To have a better understanding on the control effects of the ligands, systematic studies were conducted on the ligand interactions (bindings) between the ligand-metal compound (as the metal source and initial agent in the nanomaterial synthesis) and ligand-nanoparticle surface, of which both can be quantified by thermodynamics. Using isothermal titration calorimetry, the ligand-metal precursor binding strength was measured and found to be dependent on ligand chain length and the metal precursors, which further affects the reactivity of the metal precursor based on the results of density functional theory calculations. On the other hand, the ligand-nanoparticle surface binding strength was found to affect the capping density of the ligands on the nanoparticle surface. In order to connect the thermodynamics to the kinetics, namely the nucleation (formation of new particles) and growth (particle size increase) rates, small angle X-ray scattering (SAXS) characterization was performed in real time during the synthesis on the nanoparticles. This technique allows the capture of the size, size distribution and concentration of nanoparticles changing with time, and the nucleation and growth rates were further calculated from the SAXS data. By changing solvents with the same functions of ligands but of different coordinating abilities, a correlation between the kinetics and thermodynamics was observed. The nucleation rate increases with the metal precursor reactivity, which corresponds to stronger solvent binding to the precursor. On the other hand, the stronger ligand-nanoparticle binding slows down the growth by lowering the surface capping density. To go deeper into the ligand-metal binding and kinetics correlation, the binding properties were tuned by changing other synthesis conditions, i.e., different temperatures and ligand to metal ratios (ligand concentration), and a qualitative discussion was given on the effects of these conditions on the synthesis kinetics and final particle size.

A remoção de compostos sulfurosos (sulfeto/sulfato) de águas residuárias tem grande importância para a saúde humana e para o ambiente. A biorremediação desses compostos, através da utilização de bactérias fototróficas anoxigênicas apresenta-se como alternativa viável ecológica e economicamente. Neste projeto de pesquisa realizaram-se ensaios em reatores em batelada expostos à iluminação fluorescente, com a finalidade de avaliar a utilização de sulfeto por cultura enriquecida de sulfubactérias fototróficas verdes, proveniente de sedimento de lagoa de estabilização. A avaliação da utilização de sulfeto e os resultados da cinética de crescimento da cultura visam a possibilidade de aplicação das bactérias fototróficas anoxigênicas no tratamento de águas residuárias. Na avaliação cinética de crescimento da cultura enriquecida de sulfubactérias fototróficas verdes foi verificado diminuição na velocidade de crescimento de 0,0346 h-1 para 0,0035 h-1 com o aumento da concentração inicial de sulfeto de 11,4 para 529,6 mg-S/L, respectivamente. O tempo de geração apresentou comportamento crescente com valores iguais a 19,98 h e 119,18 h, respectivamente. Os valores do coeficiente de conversão de sulfeto relativo ao crescimento de microrganismos (Yx/s), apresentaram diminuição progressiva com o aumento da concentração inicial de sulfeto, sendo esta queda mais acentuada no intervalo de concentrações de 40,5 a 147,2 mg-S/L. / The removal of sulphur compounds (sulfide/sulfate) from wastewaters is very important for human and for the environmental. The bioremediation of these compounds, applying anoxigenic phototrophic bacteria has been presented as a possible alternative. The aim of this work is to evaluate the utilization of sulfide by the enrichment of green sulphur bacteria proceeding from stabilization pond sediment, through the realization of batch reactors assays, which are exposed to fluorescent ilumination. The determination of the sulfide utilization and the results of the kinetic growth were done to clarify the possibility of the application of anoxigenic phototrophic bacteria in the wasterwater systems. In kinetic experiments it was observed that the specific rate of bacterial growth changed from 0.0346 h-1 to 0.0035 h-1. Then the initial sulfide concentration was increased form 11.4 mg-S/L to 529.6 mg-L. The doubling time showed an increase from 19.98 h to 119.18 h. The sulfide conversion to microorganism biomass (Yx/s) progressively decrease with the increase of sulfide concentration. Such a decrease was higher in between sulfide concentrations of 40.5 mg-S/L to 147.2 mg-S/L.

A remoção de compostos sulfurosos (sulfeto/sulfato) de águas residuárias tem grande importância para a saúde humana e para o ambiente. A biorremediação desses compostos, através da utilização de bactérias fototróficas anoxigênicas apresenta-se como alternativa viável ecológica e economicamente. Neste projeto de pesquisa realizaram-se ensaios em reatores em batelada expostos à iluminação fluorescente, com a finalidade de avaliar a utilização de sulfeto por cultura enriquecida de sulfubactérias fototróficas verdes, proveniente de sedimento de lagoa de estabilização. A avaliação da utilização de sulfeto e os resultados da cinética de crescimento da cultura visam a possibilidade de aplicação das bactérias fototróficas anoxigênicas no tratamento de águas residuárias. Na avaliação cinética de crescimento da cultura enriquecida de sulfubactérias fototróficas verdes foi verificado diminuição na velocidade de crescimento de 0,0346 h-1 para 0,0035 h-1 com o aumento da concentração inicial de sulfeto de 11,4 para 529,6 mg-S/L, respectivamente. O tempo de geração apresentou comportamento crescente com valores iguais a 19,98 h e 119,18 h, respectivamente. Os valores do coeficiente de conversão de sulfeto relativo ao crescimento de microrganismos (Yx/s), apresentaram diminuição progressiva com o aumento da concentração inicial de sulfeto, sendo esta queda mais acentuada no intervalo de concentrações de 40,5 a 147,2 mg-S/L. / The removal of sulphur compounds (sulfide/sulfate) from wastewaters is very important for human and for the environmental. The bioremediation of these compounds, applying anoxigenic phototrophic bacteria has been presented as a possible alternative. The aim of this work is to evaluate the utilization of sulfide by the enrichment of green sulphur bacteria proceeding from stabilization pond sediment, through the realization of batch reactors assays, which are exposed to fluorescent ilumination. The determination of the sulfide utilization and the results of the kinetic growth were done to clarify the possibility of the application of anoxigenic phototrophic bacteria in the wasterwater systems. In kinetic experiments it was observed that the specific rate of bacterial growth changed from 0.0346 h-1 to 0.0035 h-1. Then the initial sulfide concentration was increased form 11.4 mg-S/L to 529.6 mg-L. The doubling time showed an increase from 19.98 h to 119.18 h. The sulfide conversion to microorganism biomass (Yx/s) progressively decrease with the increase of sulfide concentration. Such a decrease was higher in between sulfide concentrations of 40.5 mg-S/L to 147.2 mg-S/L.

With greenhouse gas emissions (of which CO2 is the major component) being a major environmental concern, mitigation of those emissions is becoming increasingly imperative. The ability to use a fast growing, photosynthetic organism like microalgae that can survive primarily on nutrients such as sunlight and air (with increased CO2 levels) makes it a desirable agent for CO2 sequestration. The primary goal of this project is the engineering of the sequestration of CO2 using the cultivation of the microalgae species <i>Chlorella vulgaris</i>. Secondary goals of the project are the exploration and development of valuable by-products of the cultivation and the determination of whether utilizing microalgae to capture CO2 could be integrated economically into an industrial facility.<p> The batch growth kinetics of the photosynthetic algal species <i>C. vulgaris</i> were investigated using a well-mixed stirred bioreactor. The growth rate was found to increase as the dissolved CO2 increased to 150 mg/L (10% CO2 by volume in the gas), but fell dramatically at higher concentrations. Increasing the radiant flux also increased growth rate. With a radiant flux of 32.3 mW falling directly on the 500 mL culture media, the growth rate reached up to 3.6 mg of cells/L-h. Both pH variation (5.5 - 7.0) and mass transfer rate of CO2 (KLa between 6 h-1 and 17 h-1) had little effect on growth rate.<p> The operation of continuously stirred tank bioreactors (CSTBs) at minimum cost is a major concern for operators. In this work, a CSTB design strategy is presented where impeller stirring speed and aeration rate are optimized to meet the oxygen demand of growing cells, simultaneously minimizing the capital and operating cost. The effect of microbial species, ions in the culture medium, impeller style, as well as changing CSTB size and biomass input density on the optimum operating conditions, is examined. A study of the effects of various parameters on the CSTB design is shown.<p> Using the kinetic data collected in the batch growth study, a novel external loop airlift photobioreactor (ELAPB) was designed and tested. A model was developed for <i>C. vulgaris</i> growth in the ELAPB that incorporated growth behaviour, light attenuation, mass transfer, and fluid dynamics. The model predicts biomass accumulation, light penetration, and transient CO2 concentrations, and compares predictions to experimental data for radiant fluxes of 0.075 1.15 W/m2 and 0 20% CO2 enrichment of feed air, with a 10% average error. The effect of radiant flux and CO2 concentration is presented with discussion of radial and vertical profiles along the column. For a fed-batch culture at a biomass density of 170 mg/L, the penetration of the radiant flux was found to decrease by 50% within the first 1 cm, and 75% at 2 cm. Theoretical optimum growth conditions are determined to be 0.30 W/m2 and 6% CO2 enrichment of inlet feed air.<p> The algal culture was observed to be a workable electron acceptor in a cathodic half cell. A net potential difference of 70 mV was achieved between the growing <i>C. vulgaris</i> culture acting as a cathode and a 0.02 M potassium ferrocyanide anodic half cell. Surge current and power levels of 1.0 µA/mg of cell dry weight and 2.7 mW/m2 of cathode surface area were measured between these two half cells. The recently developed photosynthetic cathode was also coupled to a fermentative anode to produce a completely microbial fuel cell. Loading effects and the effect of changing culture conditions on fuel cell operation are reported. The maximum power output measured was 0.95 mW/ m2 at 90 V and 5000 ohms. A significant increase in this output is achieved with the addition of supplemental glucose to the anodic half cell and the enrichment of the feed air bubbled into the cathodic half cell with 10% CO2.<p> Two economic feasibility studies were performed on the integration of ELAPBs into an industrial facility. These integration studies operated the ELAPBs continuously as biocathodes in coupled microbial fuel cells (MFCs) that capture CO2 from an existing 130 million L/yr bioethanol plant, while generating electrical power and yielding oil for biodiesel to provide operational revenue to offset costs. The anodes for the coupled MFCs are the existing yeast batch fermentors, and the CO2 to be sequestered comes from the existing bioethanol production. Two different design schemes were evaluated, in both cases the maximum profit was achieved with the maximum number of tall columns operated in parallel. The first design evaluated a batch bioethanol facility with off-site oil processing, and the economic feasibility is demonstrated by the positive Net Present Worth achieved over the 20 year life of the plant, at a 10% rate of return on investment. The second design, for a continuous bioethanol operation, processes both oil and algae biomass on-site, but the economics of this second process are only positive (Internal Rate of Return 9.93%.) if the government provides financial assistance in the form of generous carbon credits (a speculative $100 per tonne of CO2 not yet attained) and a 25% capital equipment grant.

With greenhouse gas emissions (of which CO2 is the major component) being a major environmental concern, mitigation of those emissions is becoming increasingly imperative. The ability to use a fast growing, photosynthetic organism like microalgae that can survive primarily on nutrients such as sunlight and air (with increased CO2 levels) makes it a desirable agent for CO2 sequestration. The primary goal of this project is the engineering of the sequestration of CO2 using the cultivation of the microalgae species <i>Chlorella vulgaris</i>. Secondary goals of the project are the exploration and development of valuable by-products of the cultivation and the determination of whether utilizing microalgae to capture CO2 could be integrated economically into an industrial facility.<p> The batch growth kinetics of the photosynthetic algal species <i>C. vulgaris</i> were investigated using a well-mixed stirred bioreactor. The growth rate was found to increase as the dissolved CO2 increased to 150 mg/L (10% CO2 by volume in the gas), but fell dramatically at higher concentrations. Increasing the radiant flux also increased growth rate. With a radiant flux of 32.3 mW falling directly on the 500 mL culture media, the growth rate reached up to 3.6 mg of cells/L-h. Both pH variation (5.5 - 7.0) and mass transfer rate of CO2 (KLa between 6 h-1 and 17 h-1) had little effect on growth rate.<p> The operation of continuously stirred tank bioreactors (CSTBs) at minimum cost is a major concern for operators. In this work, a CSTB design strategy is presented where impeller stirring speed and aeration rate are optimized to meet the oxygen demand of growing cells, simultaneously minimizing the capital and operating cost. The effect of microbial species, ions in the culture medium, impeller style, as well as changing CSTB size and biomass input density on the optimum operating conditions, is examined. A study of the effects of various parameters on the CSTB design is shown.<p> Using the kinetic data collected in the batch growth study, a novel external loop airlift photobioreactor (ELAPB) was designed and tested. A model was developed for <i>C. vulgaris</i> growth in the ELAPB that incorporated growth behaviour, light attenuation, mass transfer, and fluid dynamics. The model predicts biomass accumulation, light penetration, and transient CO2 concentrations, and compares predictions to experimental data for radiant fluxes of 0.075 1.15 W/m2 and 0 20% CO2 enrichment of feed air, with a 10% average error. The effect of radiant flux and CO2 concentration is presented with discussion of radial and vertical profiles along the column. For a fed-batch culture at a biomass density of 170 mg/L, the penetration of the radiant flux was found to decrease by 50% within the first 1 cm, and 75% at 2 cm. Theoretical optimum growth conditions are determined to be 0.30 W/m2 and 6% CO2 enrichment of inlet feed air.<p> The algal culture was observed to be a workable electron acceptor in a cathodic half cell. A net potential difference of 70 mV was achieved between the growing <i>C. vulgaris</i> culture acting as a cathode and a 0.02 M potassium ferrocyanide anodic half cell. Surge current and power levels of 1.0 µA/mg of cell dry weight and 2.7 mW/m2 of cathode surface area were measured between these two half cells. The recently developed photosynthetic cathode was also coupled to a fermentative anode to produce a completely microbial fuel cell. Loading effects and the effect of changing culture conditions on fuel cell operation are reported. The maximum power output measured was 0.95 mW/ m2 at 90 V and 5000 ohms. A significant increase in this output is achieved with the addition of supplemental glucose to the anodic half cell and the enrichment of the feed air bubbled into the cathodic half cell with 10% CO2.<p> Two economic feasibility studies were performed on the integration of ELAPBs into an industrial facility. These integration studies operated the ELAPBs continuously as biocathodes in coupled microbial fuel cells (MFCs) that capture CO2 from an existing 130 million L/yr bioethanol plant, while generating electrical power and yielding oil for biodiesel to provide operational revenue to offset costs. The anodes for the coupled MFCs are the existing yeast batch fermentors, and the CO2 to be sequestered comes from the existing bioethanol production. Two different design schemes were evaluated, in both cases the maximum profit was achieved with the maximum number of tall columns operated in parallel. The first design evaluated a batch bioethanol facility with off-site oil processing, and the economic feasibility is demonstrated by the positive Net Present Worth achieved over the 20 year life of the plant, at a 10% rate of return on investment. The second design, for a continuous bioethanol operation, processes both oil and algae biomass on-site, but the economics of this second process are only positive (Internal Rate of Return 9.93%.) if the government provides financial assistance in the form of generous carbon credits (a speculative $100 per tonne of CO2 not yet attained) and a 25% capital equipment grant.

The mechanisms responsible for ice propagation on surfaces undergoing dropwise condensation have been determined and characterized. Based on experimental data acquired non-invasively with high speed quantitative microscopy, the freezing process was determined to occur by two distinct mechanisms: inter-droplet and intradroplet ice crystal growth. The inter-droplet crystal growth mechanism was responsible for the propagation of the ice phase between droplets while the intra-droplet crystal growth mechanism was responsible for the propagation of ice within individual droplets. The larger scale manifestation of these two mechanisms cooperating in tandem was designated as the aggregate freezing process. The dynamics of the aggregate freezing process were characterized in terms of the substrate thermal di usivity, the substrate temperature, the free stream air humidity ratio, and the interfacial substrate properties of roughness and contact angle, which were combined into a single surface energy parameter. Results showed that for a given thermal di usivity, the aggregate freezing velocity increased asymptotically towards a constant value with decreasing surface temperature, increasing humidity, and decreasing surface energy. The inter-droplet freezing velocity was found to be independent of substrate temperature and only slightly dependent on humidity and surface energy. The intra-droplet freezing velocity was determined to be a strong function of substrate temperature, a weaker function of surface energy, and independent of humidity. From the data, a set of correlational models were developed to predict the three freezing velocities in terms of the independent variables. These models predicted the majority of the measured aggregate, inter- and intra-droplet freezing velocities to within 15%, 10%, and 35%, respectively. Basic thermodynamic analyses of the inter- and intra-droplet freezing mechanisms showed that the dynamics of these processes were consistent with the kinetics of crystal growth from the vapor and supercooled liquid phases, respectively. The aggregate freezing process was also analyzed in terms of its constituent mechanisms; those results suggested that the distribution of liquid condensate on the surface has the largest impact on the aggregate freezing dynamics.

Properties of nanocrystals are extremely sensitive to their sizes when their sizes are smaller or of the order of the excitonic diameter due to the quantum confinement effect. The interest in this field has been concentrated basically in understanding the size-property relations of nanocrystals, for example, the pronounced variation in the bandgap of the material or the fluorescence emission properties, by tuning the sizes of the nanocrystals. Thus, the optical and electronic properties of semiconductor nanocrystals can be tailor-made to suit the needs of the specific application and hence is of immense importance. One of the major aspects necessary for the actual realization of the various applications is the ability to synthesize nanocrystals of the required size with a controlled size distribution. The growing demand to obtain such nanocrystals with the required size and controlled size distribution is met largely by the solution route synthesis of nanocrystals, that constitutes an important class of synthesis methods due to their ease of implementation and the high degree of flexibility. The main difficulty of this method is that the dependence of the average size and the size distribution of the generated particles on parameters of the reaction are not understood in detail and therefore, the optimal reaction conditions are arrived at essentially in an empirical and intuitive manner. From a fundamental point of view, understanding the growth kinetics of various nanocrystals can provide a deeper insight into the phenomena. The study of growth kinetics can be experimentally achieved by measuring the time evolution of diameter using several in-situ techniques like UV-absorption and small angle X-ray scattering. Having understood the mechanism of growth of nanocrystals, it is possible to obtain the required size of the nanocrystal using optimized synthesis conditions. The properties of these high quality nanocrystals can be further tuned by doping with a small percentage of magnetic ions. The optical and magnetic properties of these nanocrystals play an important role in the various technological applications. The first part of the thesis concentrates on the theoretical methods to study the electronic structure of semiconductor nanocrystals. The second part describes the studies performed on growth of various nanocrystal systems, both in the presence and absence of capping agents. The third part of the thesis describes the studies carried out on ZnO and doped ZnO nanocrystals, synthesized using optimal conditions that were obtained in the earlier part of the thesis. The thesis is divided into five chapters which are described below. Chapter 1 provides a brief overall perspective of various interesting properties of semiconductor nanocrystals, including various concepts relevant for the study of such systems. Chapter 2 describes experimental and theoretical methods used for the study of nanocrystals reported in this thesis. In Chapter 3 of this thesis, we report results of theoretical studies carried out on III-V and II-VI semiconductors using the tight-binding (TB) methodology. Chapter 4 presents the investigations on the growth kinetics of several nanocrystal systems. Chapter 5 presents experimental investigations carried out on undoped and various transition metal (TM) doped ZnO nanocrystals. In summary, we have performed electronic structure calculations on various nanocrystal systems, devised a novel method to obtain the size distribution from UV-absorption spectrum and studied the mechanism of growth in the presence and absence of capping agents in various II-VI semiconductors. Using the optimal conditions obtained from the growth studies, we prepare high quality ZnO nanocrystals of required size, both in free-standing and capped states and doped it with small percentages of various transition metals like Mn, Cu and Fe. We have then studied their optical and magnetic properties.

Το θειικό βάριο είναι ένα κρυσταλλικό στερεό το οποίο απαντάται ως ορυκτό. Οι χρήσεις του στην βιομηχανία είναι πολλές, καθώς χρησιμοποιείται σε ένα ευρύ φάσμα εφαρμογών από την κατασκευή πυράντοχων βαφών έως την βιομηχανία παραγωγής πετρελαίου για την αποφυγή αύξησης της πίεσης κατά την διάρκεια των γεωτρήσεων. Ωστόσο, κατά τη χρήση του στην άντληση πετρελαίου, η χρήση του έχει ως αποτέλεσμα το σχηματισμό επικαθίσεων οι οποίες είναι δύσκολο να απομακρυνθούν λόγω της μικρής τους διαλυτότητας. Για την απομάκρυνση των επικαθίσεων θειικού βαρίου δεν είναι δυνατόν να χρησιμοποιηθούν κοινά οξέα, καθώς το θειικό βάριο αφενός είναι δυσδιάλυτο σε αυτά αλλά παράλληλα δημιουργούν σοβαρά προβλήματα διάβρωσης του εξοπλισμού (σωληνώσεις, reservoir αποθήκευσης νερού). Για την αντιμετώπιση των καθαλατώσεων αυτού του είδους, χρησιμοποιούνται υδατοδιαλυτές ενώσεις οι οποίες προστίθενται στα ρευστά στα οποία λαμβάνει χώρα καταβύθιση του θειικού βαρίου και έχουν την ικανότητα να παρεμποδίσουν ή να επιβραδύνουν το σχηματισμό του θειικού βαρίου. Ενώσεις αυτού του τύπου είναι οι πολύ-φωσφονικές, πολυηλεκτρολύτες όπως τα πολύ-καρβοξυλικά, πολυσουλφονικά οξέα κ.τ.λ., με ορισμένες από αυτές να είναι αρκετά δραστικές και να μπορούν να περιορίσουν σε μεγάλο βαθμό τον σχηματισμό επικαθίσεων του θειικού βαρίου. Στην παρούσα εργασία μελετήθηκε η κινητική της καταβύθισης του θειικού βαρίου σε υδατικά υπέρκορα διαλύματά του, στα οποία η αναλογία πλεγματικών ιόντων Ba:SO4 1:1 στους 25οC, απουσία και παρουσία πρόσθετων. Πραγματοποιήθηκαν πειράματα αυθόρμητης καταβύθισης για την εύρεση του εύρους της μετασταθούς ζώνης με την τεχνική “free drift”. Από την συσχέτιση του χρόνου επαγωγής που μετρήθηκε, συναρτήσει του υπερκορεσμού και βάσει της κλασσικής θεωρίας της πυρηνογένεσης υπολογίσθηκε ότι η επιφανειακή ενέργεια του θειικού βαρίου ήταν 17,4 mJ•m-2. Η τιμή αυτή, η οποία είναι σημαντικά διαφορετική από τις τιμές οι οποίες αναφέρονται στην βιβλιογραφία, αντανακλά τη σημασία του τρόπου παρασκευής των υπέρκορων διαλυμάτων στις μετρήσεις αυτές. Στη σταθερή περιοχή των υπέρκορων διαλυμάτων, και προκειμένου να διερευνηθεί ο μηχανισμός κρυσταλλικής ανάπτυξης του θειικού βαρίου, έγινε σειρά πειραμάτων στα οποία μετρήθηκε ο ρυθμός κρυσταλλικής ανάπτυξης του θειικού βαρίου σε κρυσταλλικά φύτρα θειικού βαρίου. Στα υπέρκορα διαλύματα, η αναλογία πλεγματικών ιόντων Ba:SO4 1:1 στους 25oC. Οι μετρήσεις αυτές πραγματοποιήθηκαν με την τεχνική διατήρησης σταθερού του υπερκορεσμού κατά την διάρκεια της καταβύθισης. Ως παράμετρος παρακολούθησης της εξέλιξης της κρυσταλλικής ανάπτυξης χρησιμοποιήθηκε η ειδική αγωγιμότητα των υπέρκορων διαλυμάτων, η οποία εμετρείτο με τον αντίστοιχο αισθητήρα, το σήμα από τον οποίο, ενεργοποιούσε αυτόματο τιτλοδότη για την προσθήκη αντιδραστηρίων κατάλληλης συγκέντρωσης. Οι μετρήσεις του ρυθμού κρυσταλλικής ανάπτυξης έδειξαν παραβολική εξάρτηση από τον υπερκορεσμό των αντίστοιχων διαλυμάτων ενώ δεν παρουσιάστηκε εξάρτηση του ρυθμού από την συγκέντρωση των φύτρων για τις συγκεντρώσεις κρυστάλλων μεταξύ 0,026 – 0,19 mg.L. Η εξάρτηση του ρυθμού κρυσταλλικής ανάπτυξης του θειικού βαρίου από τον υπερκορεσμό των αντίστοιχων διαλυμάτων έδειξε ότι ο μηχανισμός καθορίζεται από την επιφανειακή διάχυση των δομικών μονάδων. Οι αναστολείς που χρησιμοποιήθηκαν για την μελέτη στην επίδραση της παρουσίας τους στα υπέρκορα διαλύματα στο ρυθμό της κρυσταλλικής ανάπτυξης φύτρων θειικού βαρίου ήταν το βενζοϊκό- 1,3,5 τρις φωσφονικό οξύ (BTP) και το άμινο τρις-μεθυλενοφωσφονικό οξύ (AMP). Η κυριότερη διαφορά των δυο ενώσεων έγκειται στη μοριακή τους γεωμετρία: Το πρώτο χαρακτηρίζεται από σχετική ακαμψία των δεσμών ενώ το δεύτερο από ευκινησία . Κατά τη διάρκεια των πειραμάτων αυτών διαπιστώθηκε ότι η παρουσία των αναστολέων στα υπέρκορα διαλύματα είχε σημαντική επίδραση (αύξηση) στη διαλυτότητα του θειικού βαρίου, ενώ ταυτόχρονα παρατηρήθηκε μείωση των ρυθμών κρυσταλλικής ανάπτυξης. Το AMP βρέθηκε ότι ήταν περισσότερο αποτελεσματικό στην αναστολή της καταβύθισης του θειικού βαρίου, προκαλώντας μείωση στο ρυθμό σε ποσοστό μεγαλύτερο του 90% σε συγκεντρώσεις της τάξης των 30 ppm. Το BTP ήταν και αυτό αρκετά αποτελεσματικό, παρατηρήθηκε όμως ότι η αποτελεσματικότητα της παρουσίας του ήταν αρκετά υψηλή (>50%) σε χαμηλές (10 ppm) και σε υψηλές συγκεντρώσεις (>50 ppm) συγκεντρώσεις, ενώ στις ενδιάμεσες συγκεντρώσεις η αποτελεσματικότητά του ήταν σημαντικά μικρότερη. Επιπλέον, το AMP είχε ανασταλτική δράση στο ρυθμό καταβύθισης του θειικού βαρίου τόσο σε αλκαλικές (pH=9,5) όσο και σε όξινες (pH=3,6) τιμές pH στα υπέρκορα διαλύματα. Η αποτελεσματικότητα του AMP ήταν αρκετά υψηλή, μεγαλύτερη από 70%, σε συγκεντρώσεις >30 ppm. / Barium sulfate is a crystalline solid encountered as mineral and precipitated in numerous applications from analytical chemistry to the of fire resistant paints up to the oil industry to avoid the pressure increase during the drilling. However, in oil industry form deposits which are difficult to remove due to the low solubility. The removal of barium sulfate scale deposits is not possible through the use of common acids, because the solubility of this salt does not change significantly with increasing acid concentration. Moreover the use of mineral acids result in the severe corrosion of the metal parts of the equipment involved (pipes, water storage reservoir). Alternative descaling and scale prevention techniques have been desighed and are widely applied. In these techniques, a number of compounds have been used which, when added at very low concentrations in scale prone aquatic media result in the inhibition or cancellation of the formation of barium sulfate scale deposits. In these compounds which include poly-phosphonates or polyelectrolytes with sulfonated or carboxyl functional groups, have shown impressive results. The issue of structure of the additive molecules both in solution but most important upon adsorption on the surface of the nuclei of the crystalline deposit forming under the favorable friving force created by the solution supersaturation, is very important for obtaining a better understanding of the factors underlying the efficiency of inhibition of inorganic scale formation. In the present work, we investigate the kinetics of precipitation of barium sulfate from supersaturated solutions both in the absence and in presence of additives was investigated. The kinetics of crystal growth were investigated using the seeded growth techiwue at sustained supersaturation. The molar ratio of total barium : sulfate (Ba:SO4) in the supersaturated solutions was 1:1 and all experiments were done at 25oC in the absence and presence of additives. The width of the metastable zone for the barium sulfate system was determined from spontaneous precipitation experiments involving unstable supersaturated solutions with the “free drift” technique. From the dependence of the inhibition times preceding precipitation on the solution supersaturation and using the classical nucleation theory (CNT) models the surface energy of the precipitated phase was estimated. The kinetics of crystallization of barium sulfate were investigated in stable supersaturated solutions which were seeded with well-aged and characterized barium sulfate crystals prepared from slow mixing of equimolar barium chloride and sodium sulfate solutions. The molar ratio Ba:SO4 was in these experiments 1:1. The rates of crystal growth were measured at conditions of constant supersaturation using a specific conductivity probe, which through the synchronized burettes of an automatic titrator triggered the addition of equimolar barium chloride and sodium sulfate solutions. The added titrants had the appropriate composition to compensate for the respective quantities transferred to the solid phase forming. The rate of titrants addition yieleded the rates of crystal growth at the respective conditions. The measured crystal growth rates showed parabolic dependence on the solution supersaturation suggesting the prevalence of a surface diffusion controlled mechanism. Moreover, the independence of the measured crystal growth rates (moles precipitated per unit time and seed crystals surface area) on the mass of the seed crystals. Confirmed that crystal growth took place exclusively on the seed crystals. The effect of the presence of benzene-1,3,5-triyltris phosphonic acid (BTP) and amino-tris(methylenephosphonic) acid (AMP) in the supersaturated solutions on the rates of crystal growth of barium sulfate was investigated by measurements of the respective crystal growth rates at sustained supersaturation as in the additives free solutions. The main structural difference of the two molecules tested is that the former has a flat conformation because of the aromatic ring while the latter has a significantly higher freedom of motion. The presence of the test additives in the supersaturated solutions had a significant effect on the solubility of barium sulfate. The modified solubilities were calculated from measurements of the concentrations of free Ba2+ and SO42- ions concentrations and solution supersaturations were calculated accordingly. The presence of the test additives resulted in the significant reduction of the respective crystal growth rates. The presence of AMP in the ssupersaturated solutions caused reduction of the crystal growth rates as high as 90% at 30 ppm. BTP was efficient as well in inhibiting barium sulfate crystallization. However, it was found that rates were reduced by more than 50% at concentrations as low as 10 ppm and high concentrations in the range of 50 ppm. At intermediate concentration (20-30 ppm) the efficiency of BTP in the reduction of crystal growth of barium sulfate was significantly lower. AMP inhibited barium sulfate scale not only at alkaline pH (pH=9,5) values and at acidic values (pH=3,6). At pH=3,6 and for AMP conentrations of 30 ppm the rates of crystal growth of barium sulfate were reduced by 70% with respect to the values in its absence.

The synthesis of nanocrystals is unique compared to the formation of larger micron-sizesspecies as the final crystal sizes are not much larger than the primary nuclei. As a consequencethe final outcome of a nanocrystal synthesis i.e mean crystal size, concentrationand standard deviation is almost solely determined by the end of the nucleation phase. Directingthe growth of crystals beginning from aggregates of only tens of atoms into maturemonodisperse nanocrystals requires that the governing kinetics are strictly controlled at everymoment of the reaction. To effect this task various different ligands need to be employed,each performing a particular function during both nucleation and growth. (For complete abstract open document)