Effect of porosity and crystallinity on the electrochemical, photoelectrochemical and photonic crystal properties of titania and barium titanate

In this thesis the focus of the research is on the effect of length scale and crystallinity on the electrochemical, photoelectrochemical and photonic crystal properties of mesoporous and macroporous forms of titania and barium titanate. Titania is a well-studied wide-band gap metal oxide and its crystalline anatase form has been intensely studied as a material for solar cells and batteries. If titania is synthesized with pore sizes in the range of 3 nm and with a purely amorphous framework this leads to large changes in the electronic structure of the material. In particular, defect states are found to control the electrical transport properties of this form of titania and a complete reversal of behavior from a n-type to a p-type semiconductor is observed in a dye sensitized solar cell made of this material. Also, lithium cations and electrons that are able to be electrochemically inserted into nanocrystalline titania upon electrochemical cycling in a non-aqueous Li+ electrolyte are found not to insert into this type of mesoporous titania. Instead a large capacitive current contribution and completely irreversible charge insertion dominates the electrochemical behavior of mesoporous titania with an amorphous framework. This behavior is a direct consequence of the high density of trap states in the amorphous framework of the material.; Another interesting feature of materials with crystalline porosity emerges when the length scale of the porosity is comparable to the wavelength of light—the material can function as a 3-D Bragg optical diffraction grating. Materials of this type are referred to as photonic crystals and have the ability to control the flow of photons as semiconductors like silicon control the transport of electrons. In this context the optical properties of ferroelectric barium titanate with the lattice structure of an inverted colloidal photonic crystal have been explored. The colloidal photonic properties were found to be dependent on the thermal treatment of the material, which controls the size of the nanocrystals that comprise the photonic lattice and ultimately its ferroelectric and optical properties.