Solid State Chemistry

Research efforts in my group are devoted to synthesis and characterization of solid state inorganic materials. Our end goal is to discover new materials with interesting and technologically useful physical properties. Students in my group will be exposed to a mixture of experiment and theory. Experimental techniques include solid state synthetic techniques, crystallography (x-ray, neutron and electron diffraction) and physical property measurements (optical, electrical, magnetic, dielectric, and catalytic properties). Theoretical efforts are concerned not so much with the development of new theories, but rather with the application of existing theories in order to understand the relationship between elemental properties (electronegativity, ionic radius, electronic energy levels, etc.), crystal structure and physical properties. The aim of this approach is to use computers to predict the structure and properties of previously unknown materials. We then take the information gained in the computer modeling process and use it to direct synthetic efforts. Specific areas of research in my group can be divided into roughly two fields at this time: synthesis and characterization of new photocatalytic materials and predictive modeling of new perovskite materials.

Photocatalysis

Photocatalytic materials use the energy of a photon of light to catalyze a chemical reaction. Applications of photocatalysts include the decomposition of water into hydrogen and oxygen and the complete oxidation of organic contaminants in aqueous environments. The first process is used to split water into hydrogen and oxygen, which can then be recombined in a fuel cell to produce electricity. The second process has numerous environmental applications, ranging from elimination of organic contaminants (such as gasoline, fertilizers and pesticides) in natural water systems, to treatment of residential and industrial wastewater. Current photocatalysts, such as titanium dioxide, need UV photons in order to initiate the photocatalytic process. This has the distinct disadvantage that UV lamps are needed to supply an adequate flux of high energy photons. Our goal is to find materials which can effectively use photons in the visible region of the spectrum to catalyze chemical reactions. The obvious advantage of such materials is that sunlight can act as the photon source.

The first step in photocatalysis is for the catalyst to absorb a photon of light in order to excite an electron from the valence band (VB) to the conduction band (CB), thus creating an electron-hole pair.  Each species must then migrate to the surface before recombination occurs.  If this happens the electron can be transferred to a surface adsorbed molecule, reducing it.  In the same manner the photogenerated hole is capable of catalyzing an oxidation reaction at the surface.  The overall process is illustrated in the figure below.  In order to avoid excessive recombination, the rates of reduction and oxidation must be comparable.  The positions of the band edges are critical for each step of this process.  First of all, only light with energy greater than the band gap (E_g) will be absorbed.  Secondly, reduction will only occur if the bottom of the CB is higher in energy than the reduction potential of the reductive species (A/A^-), while oxidation can only occur if the top of the VB is lower in energy than the oxidation potential of the oxidative species (D/D⁺).  An additional requirement of the photocatalyst is that it must be stable in water. Our research program centers around investigating the relationships between crystal structure, composition and the band edge positions.  The aim of this work is ultimately to be able to control the band edge positions through changes in structure and/or composition.  Research in this area involves synthesis of solid state materials, structure determination using diffraction techniques (primarily x-ray and neutron powder diffraction), electrochemical, spectroscopic and catalytic characterization of materials as well as band structure calculations.

Perovskites

The perovskite structure class is one of the most commonly occurring and important in all of materials science. Physical properties of interest among perovskites include superconductivity, colossal magnetoresistance, ionic conductivity, and a multitude of dielectric properties, which are of great importance in microelectronics and telecommunication. Because of the great flexibility inherent in the perovskite structure there are many different types of distortions which can occur from the ideal structure.  These include tilting of the octahedra, displacements of the cations out of the centers of their coordination polyhedra, and distortions of the octahedra driven by electronic factors (i.e. Jahn-Teller distortions).  Many of the physical properties of perovskites depend crucially on the details of these distortions, particularly the electronic, magnetic and dielectric properties which are so important for many of the applications of perovskite materials.  We are currently working on computer modeling approaches to predict the stability, detailed structure and select physical properties of hypothetical perovskite and perovskite related materials (including the Ruddlesden-Popper, Aurivillius phases and structures derived from perovskite by ordering of anion vacancies, such as the brownmillerite and the so called double and triple perovskite phases). Using these predictions as a starting point for subsequent synthetic attempts, greatly increases the success ratio of our synthetic effort, while at the same time providing deeper insight into structure-property relations in extended inorganic materials.
 

e-mail =  woodward@chemistry.ohio-state.edu
telephone = (614) 688-8274