Anisotropic II-VI semiconductor nanostructures are important photoactive materials for various energy conversion and optical applications. However, aside from the many available surface chemistry studies and from their ubiquitous photodegradation under continuous illumination, the general chemical reactivity and thermal stability (phase and shape transformations) of these materials are poorly understood. Using CdSe and CdS nanorods as model systems, we have investigated the behavior of II-VI semiconductor nanorods against various conditions of extreme chemical and physical stress (acids, bases, oxidants, reductants, and heat). CdSe nanorods react rapidly with acids, becoming oxidized to Se or SeO 2 . In contrast, CdSe nanorods remain mostly unreactive when treated with bases or strong oxidants, although bases do partially etch the tips of the nanorods (along their axis). Roasting (heating in air) of CdSe nanorods results in rock-salt CdO, but neither CdSe nor CdO is easily reduced by hydrogen (H 2 ). Another reductant, n-BuLi, reduces CdSe nanorods to metallic Cd. Variable temperature X-ray diffraction experiments show that axial annealing and selective axial melting of the nanorods precede particle coalescence. Furthermore, thermal analysis shows that the axial melting of II-VI nanorods is a ligand-dependent process. In agreement with chemical reactivity and thermal stability observations, silica-coating experiments show that the sharpest (most curved) II-VI surfaces are most active against heterogeneous nucleation of a silica shell. These results provide valuable insights into the fate and possible ways to enhance the stability and improve the use of II-VI semiconductor nanostructures in the fields of optics, magnetism, and energy conversion. KeywordsCadmium compounds, Cadmium sulfide, Chemical stability, Energy conversion, melting, oxidants, surface chemistry, thermoanalysis, thermodynamic stability, X ray diffraction, chemical decomposition, heterogeneous nucleation, optical applications, photoactive materials, semiconductor nanorods Disciplines Chemistry CommentsReprinted (adapted) with permission from Chemistry of Materials 26 (13) ABSTRACT: Anisotropic II−VI semiconductor nanostructures are important photoactive materials for various energy conversion and optical applications. However, aside from the many available surface chemistry studies and from their ubiquitous photodegradation under continuous illumination, the general chemical reactivity and thermal stability (phase and shape transformations) of these materials are poorly understood. Using CdSe and CdS nanorods as model systems, we have investigated the behavior of II−VI semiconductor nanorods against various conditions of extreme chemical and physical stress (acids, bases, oxidants, reductants, and heat). CdSe nanorods react rapidly with acids, becoming oxidized to Se or SeO 2 . In contrast, CdSe nanorods remain mostly unreactive when treated with bases or strong oxidants, although bases do partially etch the tips of the nanorods (along th...

High resolution 29Si NMR spectroscopy has been used to study the hydrolysis and condensation reactions of TEOS and related silicon alkoxides under a range of conditions selected to model initial steps in the formation of ceramic precursors. Kinetic results have led to a detailed description of the role of various reaction parameters and to differences in reactivity of various polyethoxysiloxanes. Reactivity studies have provided insight into the processes that form multicomponent alkoxide/oxide materials. Titanium alkoxides in particular can act as catalysts for silanol condensation in addition to forming Si-O-Ti linkages, and the former role may predominate in certain systems.

The hydrolysis and initial condensation reactions of Si(OC2H5)4 and low order polyethoxysiloxanes have been studied with high resolution 29Si NMR and size exclusion chromatography/FTIR spectroscopy. The effects of various parameters such as H2O/TEOS mole ratio, catalyst, pH, solvent, temperature and aging have been observed in the different nature of the reaction products. The products include partially hydrolyzed and unhydrolyzed Si(OC2H5)4, Si2O(CH2H5)6, and Si3O2(OC2H5)8. The rate of formation and concentration of each product obtained under different conditions provides information about the reactivity of various silanol-containing species and the factors that affect the overall reaction scheme.

This is a long and delightful journey. When it comes to the end, it is time to part away. I truly believe I could not complete this task without the company and assistance from people during the past five years. I would like to express my sincere gratitude to these people. First, I want to thank my research advisor, Prof. Javier Vela, for his support and guidance. Under his instruction, I learned how to be a good chemist. I would also like to thank my committee members, Prof. Gordon Miller, Prof. Keith Woo, Prof. Ning Fang and Prof. Wenyu Huang for all the helpful and valuable suggestions. Equally importantly, I want to thank all the Vela group members for your company. I would also like to thank all my friends. With you, life in Ames is always interesting. Last, but not the least, I want to thank my family. Without their support, I could not accomplish this.