Gray Group Home > CEE Home > NU Home
Photocatalysis Group
§ Yuan Yao
Current research efforts in our laboratory involve the synthesis, characterization, and application of novel titanium dioxide-based nanocomposites as highly active photocatalysts. By using electron paramagnetic resonance (EPR), we discovered that solid-solid interfaces between anatase and rutile, two crystallographic phases of TiO2, are responsible for the unique chemistry and high photocatalytic activity of mixed phase TiO2 materials (J. Electron Spectroscopy 2006, 150-155). We propose that TiO2-based nanocomposites with solid-solid interfaces are highly active photocatalysts. We use two major approaches for the preparation of these advanced nanocomposites: reactive DC magnetron sputtering and solvothermal processing.
Basic Principle of Photocatalysis
When a nanoparticle of TiO2 is illuminated with light of sufficient energy, an electron from the valence band is excited across the band gap to the conduction band, leaving a positively-charged hole behind. These charges may participate in redox reactions at the surface of the particle. Titanium dioxide photocatalysts have been demonstrated to perform a wide range of reactions, including the oxidation of organic pollutants. There are multiple crystallographic phases of titanium dioxide, most notably rutile and anatase. Rutile is excited by visible light, but has a fast charge recombination rate, leading to low efficiency. Anatase, however, is excited by near-UV light and has a slower rate of charge recombination, resulting in higher efficiency. Mixed phase catalysts with a high degree of rutile-anatase interfaces are highly efficient photocatalysts.
Synthesis: Magnetron Sputtering
We utilize reactive direct-current magnetron sputtering, a type of physical vapor deposition, for the synthesis of catalytic thin films with special interfacial structure. Control of sputtering parameters can result in pure-phase or mixed-phase catalysts with excellent homogeneity. The TEM images above (Copyright Le Chen) show the close contact between anatase and rutile nanoparticles in a sputtered sample. This work is done in collaboration with Professor Michael E. Graham, with the Institute for Catalysis in Energy Processes (formerly Institute for Environmental Catalysis).
Synthesis: Solvothermal Processing
Solvothermal processing is utilized for the development of novel catalysts in our lab. These catalysts include pure- and mixed-phase formulations as well as visible light responsive catalysts. Solvothermal processing is a low-cost, convenient, and robust method to synthesize TiO2-based nanocomposite materials. This work is done under the auspices of the Institute for Catalysis in Energy Processes (formerly Institute for Environmental Catalysis).
Composite Materials for Water Purification
We are currently studying two main types of composite materials: titania-coated ultrafiltration membranes (left image) and titania-carbon nanotube composites (right image).The application of pure- and mixed-phase photocatalysts to ceramic ultrafiltration membranes is being studied for the prevention of biofouling in drinking water applications. This work may have important implications for water reuse. We are also coupling titania catalysts with carbon nanotubes for enhanced reactivity and selectivity. These research endeavors are part of a collaboration with Professor Richard M. Lueptow of the Mechanical Engineering Department.
Dynamics of Photogenerated Charge Carriers in TiO2 Nanocomposites
Electron paramagnetic resonance is used to study the dynamics of charge carriers in a variety of our catalysts, for insight into their enhanced reactivity. This project is in collaboration with Dr. Tijana Rajh at Argonne National Laboratory.
Solar Fuel Production by Photoreduction of Carbon Dioxide
We are currently working on the photocatalytic reduction of carbon dioxide into methane and methanol fuel, in both aqueous and gaseous phases. The development of this technology is part of our efforts to work toward solutions to the coupled environmental problems of global warming and sustainable energy production.
Visible Light Responsive Catalysts
An important challenge in the next generation of TiO2-based photocatalysts is to develop catalysts with increased photoresponse in the visible light range. This achievement will have far-reaching implications for sustainable, affordable environmental treatment and energy production processes. We are currently studying the development of these catalysts through multiple synthetic routes.
Deterioration of Zinc Potassium Chromate Pigments
We have made progress in devising new strategies to probe the chemical phenomena involved in the deterioration of yellow zinc potassium chromate pigments used in Sunday at La Grande Jatte by Georges Seurat, above. This project is in collaboration with Professor Jean-Francois Gaillard and Richard Warta, of the Civil and Environmental Engineering Department at Northwestern, and Dr. Francesca Casadio, Andrew W. Mellon Conservation Scientist at the Art Institute of Chicago.
Selected Recent Publications
L. Chen, M.E. Graham, G. Li, K.A. Gray (2006) "Fabricating Highly Active Mixed Phase TiO2 Photocatalysts by Reactive DC Magnetron Sputter Deposition," Thin Solid Films, 515 (3), 1176-1181.
G. Li, K.A. Gray (2006). "Preparation of Mixed-phase Titanium Dioxide Nanocomposites via Solvothermal Processing." Chemistry of Materials, in revision.
S. Ciston, K.A. Gray (2007). "Photocatalysis for Water Recovery: Importance of nanostructure in reactive membrane filtration," G.I.T. Laboratory Journal, in press.
G. Li, S. Ciston, L. Chen, and K.A. Gray. "Enhanced Photoreduction of CO2 in Mixed Phase TiO2 with Solid-solid Interfaces," to be submitted.
G. Li, L. Chen, T. Rajh, K.A. Gray. "TiO2-based Nanocomposites with Solid-solid Interfaces as Second Generation Photocatalysts," invited article to appear in a special issue of Chemical Physics.
Professor, Civil and Environmental Engineering, Northwestern University
Fax: (847) 467-4011, Email: k-gray@northwestern.edu
Last modified: 12-15-06
NU CEE Northwestern
|
Northwestern Home | NU
Calendar: Plan-It Purple | NU
Sites A-Z | NU Search |
