Template Directed Synthesis and Characterization of Nanocrystalline Metal Oxides and Chalcogenides

Open Access
Author:
Dawood, Farah
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 21, 2010
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Raymond Edward Schaak, Committee Chair
  • Christine Dolan Keating, Committee Member
  • Ayusman Sen, Committee Member
  • Sridhar Komarneni, Committee Member
Keywords:
  • chalcogenides
  • oxides
  • nanoparticles
  • semiconductors
Abstract:
The increased interest in the use of inorganic nanomaterials in modern technology has stimulated many research efforts to expand the synthetic techniques that are currently available for the synthesis of inorganic nanocrystals. One such method uses nanocrystalline materials as reactive templates, where they serve as precursors for the controlled chemical synthesis of complex nanostructures. Here, the chemical composition, morphology, and crystal structure of the product can be controlled by using well-established and straightforward synthetic techniques. In addition, parameters such as phase purity, crystallinity, and dispersity can be tuned in order to access nanocrystalline solids that have properties that differ from their bulk analogues. We and others have been exploring the technique of templating in order to predict and better control the composition, morphology, and crystal structure of the final product. For this study, we use metal oxide and chalcogenide nanocrystals as prototypes due to their unique properties which in turn can be harnessed for a variety of applications. The first approach uses compositional templates where intermetallic nanocrystals of Bi2Pd and Bi-Pt synthesized via a modified polyol process are utilized as reactive precursors for the formation of multimetal oxides via a two-step thermal oxidation process. Once thermally oxidized, Bi2Pd nanoparticles form a Bi2O3/Pd nanocomposite which transforms into textured Bi2PdO4 upon further heating. This method provides an alternative strategy for the synthesis of multimetal oxides, supported nanoparticle catalyst systems and textured ceramics using low-temperature synthetic methods. We also use the process of morphology-based templating for the synthesis of nanoparticulate intermetallics and mixed metal sulfides. Here, spherical and platelet –like Bi nanoparticles are chemically transformed first to NiBi nanoparticles of similar morphology and then Ni3Bi2S2 nanoparticles via a two-step low temperature solution-based technique. Due to the limited reports on the synthesis of Ni3Bi2S2, this method provides an alternative route and helps expand the toolbox of synthetic techniques available for the formation of Ni3Bi2S2. The final approach is related to the method of structural templating where ZnO nanoparticles are used for the predictable and controllable formation of metastable wurtzite-type ZnS and ZnSe. These important wide-band gap materials crystallize in both the zinc blende and wurtzite structures. However, because of the subtle structural differences between the two crystal structures, subtle synthetic differences can favor the formation of one polymorph over the other. Through careful mechanistic studies, we show that ZnO nanoparticles, which adopt the wurtzite structure, form as intermediates therefore providing a structural template for the generation of the wurtzite polymorphs of ZnS and ZnSe.