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Porosimetry of Mesoporous MO03 Environmental Ellipsometry


IGERT trainee Alan May supervised by Prof. Keith Stevenson has published in Journal of Physical Chemistry C (2007, 111(49), 18251-18257) a paper detailing the optical properties of mixed MoxW1-xO3 thin films showing a grain size dependence of the optical band gap related to quantum size effects. New methods based on variable angle spectroscopic ellipsometry (VASE) have been used to measure optical constants of MoO3, WO3, and mixed MoxW1-xO3 with 0 < x < 1 electrodeposited onto ITO substrates from peroxo-polymolybdotungstate solutions. A Tauc-Lorentz dispersion model was employed to determine the real and imaginary components of the complex refractive index as a function of Mo-doping in MoxW1-xO3. The refractive index increased (2.07 – 2.20 at 800 nm) while the optical band gap decreased (3.38 – 2.95 eV) in a linear fashion for with increasing Mo fraction (Fig. 1). These trends correlate chiefly with Mo-doping induced grain size changes, as supported by X-ray diffraction measurements, and not just composition or the formation of color centers. In the case of MoxW1-xO3 materials, Mo facilitates crystallization of orthorhombic WO3 at lower temperatures. We feel observations of this nature have not been made because other studies of mixed MoxW1-xO3 films because they have focused on analysis of the optical properties while ignoring structural contributions. Some studies have performed structural analysis, but have not included structural information or estimated grain sizes for amounts of Mo below x= 0.75. Future studies with VASE will more closely examine the correlation between structural properties and optical band gap (widening or narrowing) as a function of doping content. Additionally, the collaboration, begun last year, with Professor Keith Johnston in Chemical Engineering has resulted in the submission of two papers. Alan used the optical constants derived from spectroscopic ellipsometry to determine the band gap and porosity of mesoporous TiO2 films as well as the orientation of Au nanocrystals electrophoretically fused into those same films. A third paper detailing characterization of mesoporous TiO2 infused with Pt nanocrystals by ellipsometric porosimetry is in preparation. Alan presented preliminary results from this work at the Fall 2007 ACS meeting with support from the IGERT travel award. Prof. Stevenson also delivered a presentation on Alan’s work at the Spring 2008 ACS meeting in New Orleans. Through this training program Alan has gained experience and understanding in several areas including electrochemistry, materials science, surface analytical characterization methods, and optical spectroscopies. Collectively the research and training will lead to a thorough appreciation of the challenges associated with discovering advanced metal oxide materials and in developing new surface analytical techniques for characterization of their properties on nanoscopic length scales. Publications: May, R. Alan; Kondrachova, Lilia; Hahn, Benjamin P.; Stevenson, Keith J. Optical Constants of Electrodeposited Mixed Molybdenum-Tungsten Oxide Films Determined by Variable-Angle Spectroscopic Ellipsometry. Journal of Physical Chemistry C (2007), 111(49), 18251-18257.

Hahn, Benjamin P.; May, R. Alan; Stevenson, Keith J. Electrochemical Deposition and Characterization of Mixed-Valent Rhenium Oxide Films Prepared from a Perrhenate Solution. Langmuir (2007), 23(21), 10837-10845.

Patel, Mehul N.; Williams, Ryan D.; May, R. Alan; Uchida, Hiroshi; Stevenson, Keith J.; Johnston, Keith P. Electrophoretic Deposition of Au Nanocrystals inside Perpendicular Mesochannels of TiO2. Submitted (2008).

Uchida, Hiroshi; Patel, Mehul N.; May, R. Alan; Gupta, Gaurav; Stevenson, Keith J.; Johnston, Keith P. Mesoporous titania thin films with perpendicular mesochannels prepared on conducting ITO/glass substrate. Submitted (2008).

May, R. Alan; Patel, Mehul N.; Johnston, Keith P.; Stevenson, Keith J.; Ellipsometric Porosimetry of Perpendicularly Oriented Mesoporous TiO2. In preparation (2008).

Address Goals

The primary goal of Alan’s work is the development of new high-resolution optical and electrochemical techniques to elucidate structure property relationships at nanostructured, mesoporous metal oxide electrodes that offer promise as batteries, photovoltaics and sensors. At present, there is insufficient understanding of the structure-property relationships of binary mixed metal oxides as they are challenging materials to characterize due to their disordered and complex structural and compositional nature. New characterization tools will advance fundamental understanding of these materials.