Titanium dioxide is the go-to material for a host of technological applications including self-cleaning glass, antifogging coatings, and ways to split water to make hydrogen fuel. To understand why TiO2 works well and possibly develop strategies to enhance its performance, researchers have now determined the first detailed mechanism of initial steps of oxidations catalyzed by TiO2’s less common mineral phase, anatase—reactions central to many TiO2 applications.
Most experimental surface studies have focused on the more typical mineral phase of TiO2, rutile, which is relatively easy to prepare as high-quality single crystals—ideal samples for surface analyses. Yet the anatase phase, which has greater photocatalytic activity, has remained largely unexplored by experimental methods. The main reason is that the anatase phase is metastable, making it difficult to form perfect crystals.
That challenge hasn’t prevented Vienna University of Technology’s Ulrike Diebold from studying anatase crystals. Her research group, which includes Martin Setvín, developed methods for cleaving and cleaning naturally occurring mineral crystals—some of which they purchased on eBay—in a way that exposes pristine anatase crystal faces.
Armed with reproducible sample preparation methods, Diebold’s team has now used scanning tunneling microscopy and other techniques to investigate the atomic scrambling that occurs at anatase surfaces as oxygen molecules adsorb and interact with subsurface defects known as oxygen vacancies—lattice positions where oxygen atoms “should” reside. By teaming up with Princeton University theoretician Annabella Selloni, the group has deduced the step-by-step anatase lattice restructuring process that underlies catalytic oxidation reactions (Science 2013, DOI: 10.1126/science.1239879).
One of the enabling advances was figuring out how to use the microscope tip to create oxygen vacancies below the surface and draw them toward the surface. Diebold explains that in the absence of these common defects the surface is unlikely to bind oxygen, which is necessary for oxidations. The group also worked out ways to use the tip to alter the charge state of adsorbed O2 and nudge oxygen into reacting with electron-rich vacancies.
With this combined experimental and theoretical know-how, the group determined that on anatase crystals with subsurface lattice vacancies, O2 adsorbs in an anionic state and sets off a sequence of atom “jumping” events. The rejiggering relocates the vacancy to the surface, where it is filled by O2, key first steps in catalytic oxidation reactions.
“This is a beautiful example of how high-quality surface science experiments and theory work together to understand complex reaction mechanisms that are very relevant for technology,” comments Stanford University’s Jens K. Nørskov, a catalysis specialist. People have tried for a long time to understand the difference between TiO2 rutile and anatase, he says. The results of this study provide important new insights that can help explain why anatase works better than rutile in photocatalysis, he adds.