Academic literature on the topic 'Solvated metal atom dispersion'

Monometallic and bimetallic Ni and Sn catalysts were prepared in different ratios by the Solvated Metal Atom Dispersed (SMAD) method for the catalytic hydrogenation of acetophenone to 1-phenylethanol. The preparation of the catalysts was carried out by evaporation of Ni and Sn metal atoms and subsequent co-deposition at 77 K using 2- isopropanol as solvent on alumina and magnesium oxide as supports. X-ray photoelectron spectroscopy (XPS) analysis showed a high percentage of nickel atoms in zero valence, while the tin phases were founded in reduced and oxidized form. The average size of the nanoparticles measured by transmission electron microscopy (TEM) ranged from 8 to 15 nm while the metal dispersion on the surface measured by hydrogen chemisorption ranged from 0.07% for Ni1% Sn0.3%/MgO to 3.2% for Ni5%/MgO. Thermogravimetric analysis shows that γ-Al2O3 catalysts exhibit higher thermal stability than MgO catalysts. The catalysis results showed that the best support is MgO reaching 66% conversion in Ni5% Sn0.5%/MgO catalyst.

Doctor of Philosophy
Department of Chemistry
Kenneth J. Klabunde
This is an account of the synthesis of several drastically different forms of silver nanoparticles: Bare metal nanoparticles, dry nanoparticulate powders, aqueous soluble particles, and organic ligand coated monodisperse silver nanoparticles were all produced. The synthetic method was adapted from previous studies on gold nanoparticles and investigated to understand the optimal conditions for silver nanoparticle synthesis. Also the procedure for refinement of the nanoparticles was studied and applied to the formation of alloy nanoparticles. This extraordinary procedure produces beautifully colored colloids of spherical metal nanoparticles of the highest quality which under suitable conditions self-assemble into extensive three dimensional superlattice structures. The silver nanoparticle products were later tested against several biological pathogens to find dramatic increases in antimicrobial potency in comparison to commercially available silver preparations.

Recent advancements in nanotechnology and emerging applications of nanomaterials in various fields have stimulated interest in fundamental scientific research dealing with the size and structure controlled synthesis of nanoparticles. The unique properties of nanoparticles are largely size dependent which could be tuned further by varying shape, structure, and surface properties, etc. The preparation of monodisperse nanoparticles is desirable for many applications due to better control over properties and higher performance compared to polydispersity nanoparticles. There are several methods for the synthesis of nanoparticles based on top-down and bottom-up approaches. The main disadvantage of top-down approach is the difficulty in achieving size control. Whereas, uniform nanoparticles with controllable size could be obtained by chemical methods but most of them are difficult to scale up. Moreover, a separate step of size separation is necessary in order to achieve monodispersed which may lead to material loss. In this context, a post-synthetic size modification process known as digestive ripening is highly significant. In this process, addition of a capping agent to poly disperse colloid renders it highly monodisperse either under ambient or thermal conditions. In addition to size control, digestive ripening is also effective in controlling the structure of nanoparticles in colloidal solution comprising two different elements. Use of co-digestive ripening strategy in conjunction with solvated metal atom dispersion (SMAD) method of synthesis resulted in hetero structures such as core–shell, alloy, and composite nanoparticles. Despite the versatility of digestive ripening process, the underlying mechanism in controlling size and structure of nanoparticles are not understood to date. The aim of this thesis is to gain mechanistic insight into size control of digestive ripening as well as to investigate structure control in various binary systems. Objectives  Study digestive ripening of Au nanoparticles using various alkyl amines to probe the mechanism  Study co-digestive ripening of binary colloids consisting of two metals, Pd and Cu prepared separately by SMAD method  Study co-digestive ripening of binary colloids consisting of a metal (Au) and a semiconductor (CdS) prepared separately by SMAD method  Study vaporization of bulk brass in SMAD reactor and analyse phase, structure, and morphology of various Cu/Zn bimetallic nanoparticles obtained from bulk brass under various experimental conditions Significant results In chapter 1, fundamental processes of nanoparticle formation and common synthetic techniques for the preparation of monodisperse nanoparticles are briefly discussed. Chapter 2 presents a mechanistic study of digestive ripening process with regard to size control using Au nanoparticles as a model system. Three long chain alkyl amine molecules having different chain length were used as digestive ripening agents. The course of digestive ripening process was analysed by UV-visible spectroscopy and transmission electron microscopy. The experimental conditions such as concentration of digestive ripening agent, time, and temperature were found to influence the size distribution of nanoparticles. The average particle size was found to be characteristic of metal-digestive ripening agent combination which is considered as the optimum size preferred during digestive ripening under a given set of experimental conditions. This study discusses stabilization of optimum sized particles, surface etching, and reversibility in digestive ripening. Chapter 3 describes the synthesis and characterization of PdCu alloy nanoparticles by co-digestive ripening method. Syntheses of individual Pd and Cu colloids were carried out by SMAD method. Pd nanoparticles obtained using THF as solvent and in the absence of any capping agent resulted in an extended small Pd nanowire network assembly. Morphological evolution of spherical Pd nanoparticles from Pd nanowire network structure was observed with the use of capping agent, hexadecyl amine (HDA) in SMAD method. Co-digestive ripening of Pd and Cu colloids was studied at various temperatures. This study revealed temperature dependent diffusion of Cu atoms into Pd lattice forming PdCu alloy nanoparticles. Next, co-digestive ripening of a colloidal system comprising a metal and a semiconductor was explored. Au-CdS combination was chosen for this study owing to its interesting photocatalytic properties. Chapter 4 deals with the synthesis of Au and CdS nanoparticles by SMAD method and Au/CdS nanocomposite by co-digestive ripening. CdS nanoparticles of size 4.0 + 1.2 nm and Au nanoparticles of size 5.6 + 1.1 nm were obtained as a result of digestive ripening process. Au/CdS nanocomposite obtained by co-digestive ripening was characterized by a matrix-like structure made up of CdS nanoparticles in which Au nanoparticles were embedded. CdS nanoparticles were found to establish an intimate surface contact with Au nanoparticles and the matrix of CdS surrounding Au was developed via aggregation during digestive ripening. Chapter 5 describes a comprehensive study on various Cu/Zn bimetallic nanoparticles obtained from bulk brass. Vaporization of bulk brass in SMAD reactor led to a deploying process and further growth of nanoparticles from phase separated Cu and Zn atoms formed a composite structure. The characterization of Cu/Zn nanocomposite revealed covering of composite surface with Cu resulting in a core-shell structure, Cu/Zn@Cu. Post-synthetic digestive ripening of these core-shell composite particles showed diffusion of Zn atoms to the composite surface in addition to size and shape modification. Annealing of Cu/Zn nanocomposites prepared in THF resulted in α-CuZn alloy nanoparticles via sequential transformation through η-CuZn5, γ-Cu5Zn8, and β-CuZn (observed as marten site) phases.

Recent advancements in nanotechnology and emerging applications of nanomaterials in various fields have stimulated interest in fundamental scientific research dealing with the size and structure controlled synthesis of nanoparticles. The unique properties of nanoparticles are largely size dependent which could be tuned further by varying shape, structure, and surface properties, etc. The preparation of monodisperse nanoparticles is desirable for many applications due to better control over properties and higher performance compared to polydispersity nanoparticles. There are several methods for the synthesis of nanoparticles based on top-down and bottom-up approaches. The main disadvantage of top-down approach is the difficulty in achieving size control. Whereas, uniform nanoparticles with controllable size could be obtained by chemical methods but most of them are difficult to scale up. Moreover, a separate step of size separation is necessary in order to achieve monodispersed which may lead to material loss. In this context, a post-synthetic size modification process known as digestive ripening is highly significant. In this process, addition of a capping agent to poly disperse colloid renders it highly monodisperse either under ambient or thermal conditions. In addition to size control, digestive ripening is also effective in controlling the structure of nanoparticles in colloidal solution comprising two different elements. Use of co-digestive ripening strategy in conjunction with solvated metal atom dispersion (SMAD) method of synthesis resulted in hetero structures such as core–shell, alloy, and composite nanoparticles. Despite the versatility of digestive ripening process, the underlying mechanism in controlling size and structure of nanoparticles are not understood to date. The aim of this thesis is to gain mechanistic insight into size control of digestive ripening as well as to investigate structure control in various binary systems. Objectives  Study digestive ripening of Au nanoparticles using various alkyl amines to probe the mechanism  Study co-digestive ripening of binary colloids consisting of two metals, Pd and Cu prepared separately by SMAD method  Study co-digestive ripening of binary colloids consisting of a metal (Au) and a semiconductor (CdS) prepared separately by SMAD method  Study vaporization of bulk brass in SMAD reactor and analyse phase, structure, and morphology of various Cu/Zn bimetallic nanoparticles obtained from bulk brass under various experimental conditions Significant results In chapter 1, fundamental processes of nanoparticle formation and common synthetic techniques for the preparation of monodisperse nanoparticles are briefly discussed. Chapter 2 presents a mechanistic study of digestive ripening process with regard to size control using Au nanoparticles as a model system. Three long chain alkyl amine molecules having different chain length were used as digestive ripening agents. The course of digestive ripening process was analysed by UV-visible spectroscopy and transmission electron microscopy. The experimental conditions such as concentration of digestive ripening agent, time, and temperature were found to influence the size distribution of nanoparticles. The average particle size was found to be characteristic of metal-digestive ripening agent combination which is considered as the optimum size preferred during digestive ripening under a given set of experimental conditions. This study discusses stabilization of optimum sized particles, surface etching, and reversibility in digestive ripening. Chapter 3 describes the synthesis and characterization of PdCu alloy nanoparticles by co-digestive ripening method. Syntheses of individual Pd and Cu colloids were carried out by SMAD method. Pd nanoparticles obtained using THF as solvent and in the absence of any capping agent resulted in an extended small Pd nanowire network assembly. Morphological evolution of spherical Pd nanoparticles from Pd nanowire network structure was observed with the use of capping agent, hexadecyl amine (HDA) in SMAD method. Co-digestive ripening of Pd and Cu colloids was studied at various temperatures. This study revealed temperature dependent diffusion of Cu atoms into Pd lattice forming PdCu alloy nanoparticles. Next, co-digestive ripening of a colloidal system comprising a metal and a semiconductor was explored. Au-CdS combination was chosen for this study owing to its interesting photocatalytic properties. Chapter 4 deals with the synthesis of Au and CdS nanoparticles by SMAD method and Au/CdS nanocomposite by co-digestive ripening. CdS nanoparticles of size 4.0 + 1.2 nm and Au nanoparticles of size 5.6 + 1.1 nm were obtained as a result of digestive ripening process. Au/CdS nanocomposite obtained by co-digestive ripening was characterized by a matrix-like structure made up of CdS nanoparticles in which Au nanoparticles were embedded. CdS nanoparticles were found to establish an intimate surface contact with Au nanoparticles and the matrix of CdS surrounding Au was developed via aggregation during digestive ripening. Chapter 5 describes a comprehensive study on various Cu/Zn bimetallic nanoparticles obtained from bulk brass. Vaporization of bulk brass in SMAD reactor led to a deploying process and further growth of nanoparticles from phase separated Cu and Zn atoms formed a composite structure. The characterization of Cu/Zn nanocomposite revealed covering of composite surface with Cu resulting in a core-shell structure, Cu/Zn@Cu. Post-synthetic digestive ripening of these core-shell composite particles showed diffusion of Zn atoms to the composite surface in addition to size and shape modification. Annealing of Cu/Zn nanocomposites prepared in THF resulted in α-CuZn alloy nanoparticles via sequential transformation through η-CuZn5, γ-Cu5Zn8, and β-CuZn (observed as marten site) phases.

Surfactant bound stable colloids of Au, Ag, and Pd were prepared by the solvated Metal Atom Dispersion (SMAD) method, a method involving co-condensation of metal and solvent vapors on the walls of a reactor at 77 k. The as=prepared dodecanethiol-capped Au and Ag colloids consisting of polydisperse nanoparticles were transformed into colloids consisting of highly monodisperse nanoparticles by the digestive ripening process. In the case of Pd colloids, digestive ripening led to the formation of thiolate complexes. The [Pd(SC12H25)2]6 complex formed from the dodecanethiol-capped Pd nanoparticles was found to be a versatile precursor for the synthesis of a variety of Pd nanophases such as Pd(0), PdS, and Pd@PdO by soventless thermolysis. Co-digestive ripening of as-prepared dodecanethiol-capped Au or Ag colloids with Pd colloid resulted in Au@Pd and Ag@Pd core-shell nanoparticles, respectively; attempts to transform the core-shell structures into alloy phases even at high temperatures were unsuccessful. Phosphine-capped Au nanoparticles were also prepared by the SMAD method and refluxing of this colloid resulted in an Ostwald ripening process rather than the expected digestive ripening due to the labile nature of bound PPh3. The labile nature of the bound phosphine was studied using 31P NMR spectroscopy and utilized in the adsorption of CO. Palladium nanoparticles obtained from the SMAD Pd-butanone colloids and Pd@PdO nanoparticles prepared by the solventless thermolysis of Pd-dodecanethiolate complex were found to be good catalysts for the generation of H2 from AB via either hydrolysis and methanolysis. The active hydrogen atoms produced during the hydrolysis and methanolysis diffuse into the Pd lattice. It was also noticed that hydrogen atoms that were buried deep inside the Pd lattice cannot be removed completely by heating the sample even at 600°C. Wet chemical reduction method was employed for the synthesis of PVP capped, nearly monodisperse, spherical Ir nanoparticles which undergo a polymer driven self-assembly at 80°C to afford rectangular structures and interlinked particles.

Surfactant bound stable colloids of Au, Ag, and Pd were prepared by the solvated Metal Atom Dispersion (SMAD) method, a method involving co-condensation of metal and solvent vapors on the walls of a reactor at 77 k. The as=prepared dodecanethiol-capped Au and Ag colloids consisting of polydisperse nanoparticles were transformed into colloids consisting of highly monodisperse nanoparticles by the digestive ripening process. In the case of Pd colloids, digestive ripening led to the formation of thiolate complexes. The [Pd(SC12H25)2]6 complex formed from the dodecanethiol-capped Pd nanoparticles was found to be a versatile precursor for the synthesis of a variety of Pd nanophases such as Pd(0), PdS, and Pd@PdO by soventless thermolysis. Co-digestive ripening of as-prepared dodecanethiol-capped Au or Ag colloids with Pd colloid resulted in Au@Pd and Ag@Pd core-shell nanoparticles, respectively; attempts to transform the core-shell structures into alloy phases even at high temperatures were unsuccessful. Phosphine-capped Au nanoparticles were also prepared by the SMAD method and refluxing of this colloid resulted in an Ostwald ripening process rather than the expected digestive ripening due to the labile nature of bound PPh3. The labile nature of the bound phosphine was studied using 31P NMR spectroscopy and utilized in the adsorption of CO. Palladium nanoparticles obtained from the SMAD Pd-butanone colloids and Pd@PdO nanoparticles prepared by the solventless thermolysis of Pd-dodecanethiolate complex were found to be good catalysts for the generation of H2 from AB via either hydrolysis and methanolysis. The active hydrogen atoms produced during the hydrolysis and methanolysis diffuse into the Pd lattice. It was also noticed that hydrogen atoms that were buried deep inside the Pd lattice cannot be removed completely by heating the sample even at 600°C. Wet chemical reduction method was employed for the synthesis of PVP capped, nearly monodisperse, spherical Ir nanoparticles which undergo a polymer driven self-assembly at 80°C to afford rectangular structures and interlinked particles.

Nanoparticles have gained great interest in various fundamental and applied areas of science and technology due to their tunable size dependent properties which is not achievable by their bulk counterparts. Nanoparticles are extremely reactive by virtue of their high surface energy arising from their large surface-to-volume ratio. Generally, bare nanoparticles need to be stabilized using capping agents or surfactants to prevent aggregation and coalescence, which could influence their unique properties. Therefore, the synthesis of non-aggregated nanoparticles without capping agents becomes very important. Metal organic frameworks (MOFs), a promising class of porous materials have attracted great attention because of their fascinating properties such as high surface area, uniform pores, and flexible structures. Therefore, combinations of unprotected nanoparticles with a porous support material, for example, MOF as a stabilizer without affecting the properties of the nanoparticles will be quite interesting. Benefits of both MOFs and nanoparticles can be effectively integrated to obtain novel hybrids with new or enhanced properties which will have interesting applications in various fields such as catalysis, chemical sensing, gas adsorption and storage etc.

Water has an exceptional ability to dissolve minerals. It is safe and chemically stable, and it remains liquid over a wide temperature range. Thus, it is the best solvent and reaction medium for both laboratory and industrial purposes. Water is able to dissolve ionic and ionocovalent solids because of the high polarity of the molecule (dipole moment μ = 1.84 Debye) as well as the high dielectric constant of the liquid (ε = 78.5 at 25°C). This high polarity allows water to exhibit a strong solvating power: that is, the ability to fix onto ions as a result of electrical dipolar interactions. Water is also an ionizing liquid able to polarize an ionocovalent molecule. For example, the solvolysis phenomenon increases the polarization of the HCl molecule in aqueous solution. Finally, owing to the high dielectric constant of the liquid, water is a dissociating solvent that can decrease the electrostatic forces between solvated cations and anions, allowing their dispersion as H+solvated and Cl−solvated through the liquid. (The attractive force F between charges q and q′ separated by the distance r is given by Coulomb’s law, F = qq′/εr2.) These characteristics are rarely found together in common liquids. The dipole moment of the ethanol molecule (μ = 1.69 Debye) is close to that of water, but the dielectric constant of ethanol is much lower (ε = 24.3). Ethanol is a good solvating liquid, but a poor dissociating one; consequently, it is considered a bad solvent of ionic compounds. Dissolution in water of an ionic solid such as sodium chloride is limited to dipolar interactions with Na+ and Cl− ions and their dispersion in the liquid as solvated ions, regardless of the pH of the solution. Cations with higher charge, especially cations of transition metals, retain a fixed number of water molecules, thereby forming a true coordination complex [M(OH2)N]z+ with a well-defined geometry. In addition to the dipolar interactions, water molecules behave as true ligands because they are Lewis bases exerting an electron σ-donor effect on the empty orbitals of the cation.

During the past decade, the study of photoinitiated reactive and inelastic processes within weakly bound gaseous complexes has evolved into an active area of research in the field of chemical physics. Such specialized microscopic environments offer a number of unique opportunities which enable scientists to examine regiospecific interactions at a level of detail and precision that invites rigorous comparisons between experiment and theory. Specifically, many issues that lie at the heart of physical chemistry, such as reaction probabilities, chemical branching ratios, rates and dynamics of elementary chemical processes, curve crossings, caging, recombination, vibrational redistribution and predissociation, etc., can be studied at the state-to-state level and in real time. Inevitably, understanding the photophysics and photochemistry of weakly bound complexes lends insight into corresponding processes in less rarefied surroundings, for example, molecules physisorbed on crystalline insulator and metal surfaces, molecules residing on the surfaces of various ices, and molecules weakly solvated in liquids. However, such ties to the real world are not the main driving force behind studies of photoinitiated reactions in complexed gaseous media. Rather, it is the lure of going a step beyond the more common molecular environments. Theoretical modeling, which in many areas purports to challenge experiment, must rise to the occasion here if it is to offer predictive capability for even the simplest of such microcosms. Subtleties abound. Roughly speaking, two disparate regimes can be identified which are accessible experimentally and which correspond to qualitatively different kinds of chemical transformations. These are distinguished by their reactants: electronically excited versus ground state. For example, it is possible to study the chemical selectivity that derives from the alignment and orientation of excited electronic orbitals, albeit at restricted sets of nuclear coordinates. This is achieved by electronically exciting a complexed moiety, such as a metal atom, which then undergoes chemical transformations that depend on the geometric properties of the electronic orbitals such as their alignments and orientations relative to the other moiety (or moieties) in the complex.

The barium vapour laser, as well as being a high average power source of near to mid-IR radiation, is also a useful test system for studying kinetics of metal vapour lasers. The barium atom and ion ground states have visible upward transitions of sufficient oscillator strength suitable for hook spectroscopy. In contrast, the more popular copper vapour laser requires broadband uv sources and detectors to access the copper ground state, and the copper ion density is impossible to measure by conventional techniques (absorption, emission, and anomalous dispersion spectroscopy). Hook studies of metal vapour lasers have been popular in the last ten years, however, there has been little work on spatially resolved density measurements, particularly of the ground state.