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- Title
- TUNING THE PROPERTIES OF NANOMATERIALS AS FUNCTION OF SURFACE AND ENVIRONMENT.
- Creator
-
Karakoti, Ajay, Seal, Sudipta, University of Central Florida
- Abstract / Description
-
Nanotechnology has shaped the research and development in various disciplines of science and technology by redefining the interdisciplinary research. It has put the materials science at the forefront of technology by allowing the researchers to engineer materials with properties ranging from electronics to biomedical by using materials as diverse as ceramics to just plain carbon. These exceptional properties are achieved by minimizing the dimension of particles in such smaller domains that...
Show moreNanotechnology has shaped the research and development in various disciplines of science and technology by redefining the interdisciplinary research. It has put the materials science at the forefront of technology by allowing the researchers to engineer materials with properties ranging from electronics to biomedical by using materials as diverse as ceramics to just plain carbon. These exceptional properties are achieved by minimizing the dimension of particles in such smaller domains that the boundary between the individual atoms, ions or cluster of particles is very small. This results in a change in conventional properties of particles from continuum physics to quantum physics and hence the properties of nanoparticles can be tuned based upon their size, shape and dimensionality. One of the most apparent changes upon decreasing the particle size is the increase in surface area to volume ratio. Thus nanoparticles possess greater tendency to interact with the environment in which they are present and similarly the environment can affect the properties of nanomaterials. The environment here is described as the immediate solid, liquid or gaseous material in immediate contact with the external surface of the nanoparticles. In order to control the physico-chemical properties of nanoparticles it is important to control the surface characteristics of nanoparticles and its immediate environment. The current thesis emphasizes the role of tuning the surface of nanoparticles and/or the environment around the nanoparticles to control their properties. The current approach in literature uses nanoparticles as a platform that can be used for a myriad of applications by just changing the surface species which can tune the properties of nanoparticles. Such surface modification can provide nanomaterials with hydrophilic, hydrophobic, biocompatible, sensing, fluorescence and/or electron transfer properties. The current thesis demonstrates the interaction between nanoparticles and the environment by changing the surface characteristics of nanomaterials through the use of oxide nanoparticles as examples. The first part of the thesis discusses the synthesis, modification and properties of cerium oxide nanoparticles (CNPs), a versatile material used in wide range of applications from catalysis to glass polishing, for their potential use in biomedical applications as a function of medium. The thesis starts by projecting the effect of environment on the properties of nanomaterials wherein it is shown that simple medium, such as, water can influence the optical properties of nanoparticles. It was shown that the strong polarizing effect of water on the non-bonding f electrons can cause a blue shift in the optical properties of CNPs as a function of increase in trivalent oxidation state of cerium in CNPs. This phenomenon, contradictory to existing literature in solid state where a red shift is observed upon increasing the trivalent oxidation state of cerium in CNPs, is purely attributed to the medium-inflicted change in properties of nanoparticles. This concept is built upon in the first half of thesis by increasing the colloidal stability of nanoparticles by surface and/or medium modification. It is shown that the narrow range of pH in which the colloidal CNPs are stable can be extended by changing the medium from water to polyhydroxy compounds such as glucose and dextran. The synthesis was designed specially to avoid the traditional precipitation and re-dispersion strategy of synthesis of nanoparticles to preserve the surface activity. The complex forming ability of cerium with polysaccharides was employed to synthesize the CNPs in a one step process and the pH stability of the NPs was extended between 2.0 to 9.5. The difference in the complexing ability of the monomer - glucose and its anhydro glucose polymer - dextran is reflected in the ability of cerium to form super-agglomerates with the monomer while anhydro gluco polymer forms extremely disperse 3-5 nm nanoparticles through steric modification. It is shown that the antioxidant activity of nanoparticles remain unchanged by surface modification by demonstrating the cycling of the oxidation state of cerium in CNPs, with time, through hydrogen peroxide mediated transition of oxidation states of cerium. It is demonstrated that the polymeric coatings, generally considered as passive surface coatings, can also play an active role in tuning the properties of nanomaterials and increasing their biocompatibility as well as bio-catalytic activity. It is demonstrated that the antioxidant activity of CNPs can be increased as a function of polyethylene glycol (PEG) while the biocompatibility is unaltered due to the biocompatible nature of PEG. The antioxidant activity of CNPs involves an electron transfer (ET) from the CNPs to the reactive oxygen species or vice versa. This heterogeneous ET system is further complicated by the presence of surface adsorbed species. Interfacial charge/electron transfer (ET) between a particle and adsorbed (or covalently bonded) molecule presents significant complexity as it involves a solid state electron transfer over long distance. Unlike a free ion, in solid state, the conducting electrons can be temporarily trapped by the coupling lattice sites. Adsorption/attachment of surface species to nanoparticle can disturb the electronic levels by further polarizing the electron cloud thereby localizing the electron and facilitating the charge transfer. Such an interfacial electron transfer between NPs and adsorbed organic species can be compared to the single electron transfer carried by organometallic systems with a metal ion core modified with electron delocalizing porphyrin ligands. It is demonstrated that in this PEGyltaed CNPs system, the PEG essentially forms a complex with CNPs in the presence of hydrogen peroxide to facilitate this electron transfer process. The superoxide dismutase (SOD) and catalase mimetic ability of CNPs is described and special emphasis is given to its biocompatibility. The second half of the thesis emphasizes the role of synthesis and surface modification in influencing the catalytic performance of cerium oxide modified titanium dioxide catalysts for decomposition of methanol. Noble metals supported on oxide nanoparticles have been an area of active research in catalysis. It is demonstrated that the modification of surface of the oxide nanoparticles by noble metals is a function of the synthesis process. By keeping the size of the nanoparticles constant, it was demonstrated that the differences in the oxidation state of noble metals can lead to change in the activity of noble metals. This contribution adds to the already existing controversy in the open literature about the role of the oxidation state of platinum in catalysis. The core level shifts in the binding energy of the 4f electrons of platinum was used as a guide to the gauge the oxidation state. Results from an in-house built catalytic reactor coupled to mass spectrometer and in-situ diffuse reflectance infra-red spectroscopy are used to quantify the catalytic performance and identify the mechanism of reaction as well as products of methanol decomposition.
Show less - Date Issued
- 2010
- Identifier
- CFE0003189, ucf:48590
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003189
