CHEMICAL SYNTHESIS OF SIZE- AND SHAP-CONTROLLED INTERMETALLIC AND METAL NANOCRYSTALS

Open Access
Author:
Chou, Nam Hawn
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 06, 2009
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Raymond Edward Schaak, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Karl Todd Mueller, Committee Member
  • Sridhar Komarneni, Committee Member
Keywords:
  • chemical synthesis
  • size- and shape-controlled nanocrystals
  • intermetallic
Abstract:
The interest in using nanocrystalline inorganic solids for nanoscale devices and technologies has fueled a tremendous research effort for developing syntheses of inorganic nanocrystals. Even though much progress has been made for the development of synthetic approaches, synthetic methods to generate size- and shape-controlled nanocrystalline materials are still challenging, particularly for multi-metal and metal systems that have been less explored due to the lack of robust synthetic approaches. This dissertation presents robust and facile solution based approaches to synthesize size- and shape-controlled intermetallic and metal nanocrystals. We have been exploring the concept of chemical conversion for synthesizing a variety of nanomaterials with size- and shape-control. In particular, this approach has shown to be an effective chemical route for synthesizing shape-and size-controlled intermetallic nanocrystals. It has been known that size-and shape-controlled intermetallic compounds are not easily attainable since intermetallic compounds normally consist of elements possessing notably different reduction potentials, reduction kinetics, and reactivity. This chemical conversion strategy utilizes a single metal as a reactive template for synthesizing more complex nanomaterials. We have shown that â-Sn nanocrystals can chemically transform into size- and shape-controlled M-Sn intermetallic nanocrystals. This chemical conversion method also affords compounds that can be hard to obtain by traditional solid state synthetic methods. We have also established general and predictive guidelines for accessing dense and hollow single crystal nanorods in M-Sn systems. Through this study, we have realized that reaction temperature plays a vital role in maintaining the morphology of the â-Sn nanorod templates in the products. For the synthesis of a variety of nanomaterials by the exploitation of the established chemical conversion strategy, robust and general chemical approaches have been developed for the synthesis of shape-controlled In nanoparticles by understanding reduction kinetics. Interestingly, metal precursors that have negative reduction potential vs standard hydrogen electrode have previously been synthesized by only harsh chemical and physical methods (high reaction temperatures and strong reducing agents), which can be inadequate for yielding precisely shape-controlled nanoparticles. We have shown a simple and robust kinetically controlled borohydride reduction process for synthesizing shape-controlled In nanocrystals at room temperature. By controlling the reduction rates via the rate of addition of sodium borohydride solution and controlling several reaction parameters, including reaction solvents, additives, solvents for sodium borohydride, and alcohol solutions containing metal precursors in the presence of poly(vinyl pyrrolidone), indium nanoparticles are formed that include shapes of high aspect ratio nanowires, uniform octahedra, truncated octahedra, decahedra, triangles, spheres, and star-like shapes. We have shown that a simple kinetically controlled reduction process can also be applied to the synthesis of size- and shape-controlled Ge nanocrystals. Again, representative chemical and physical approaches have been previously developed using harsh reaction conditions (high temperatures, high pressures, and strong reducing agents). The kinetically controlled reduction process by sodium borohydride at room temperature leads to the formation of spherical Ge nanocrystals with high monodispersity, as well as cubic shape. By varying concentration of metal precursors and reaction solvents, different sizes of germanium nanocrystals were obtained.