NEWS OF THE WEEK SURFACE
SCIENCE
DEFECT DIFFUSION Study reveals details of 0 2 dance in surface-diffusion mechanism
P
AIRING UP AND BREAKING
up again and again sounds like the behavior of teenage couples. But atoms and molecules do it, too, and researchers in Denmark have discovered a new diffusion mechanism that shows that imperfections on crystal surfaces are quite mobile thanks to a process of rapid matchmaking and breaking. T h e surfaces of transitionmetal oxides play a key role in heterogeneous catalysis, photocatalysis, and other processes. And oxygen vacancies—crystal imperfections marked by missing oxygen atoms—often exert a strong influence on those processes. For example, the vacancies are common sites for preferential adsorption of molecules and immobilization and nucleation of metal nanoparticles. In addition, the imperfections alter surface bonding arrangements and electronic configurations. But despite the importance of oxygen vacancies in surface phenomena, little is known at the
atomic scale about basic dynamic processes involving surface defects. Now, University of Aarhus physics professor Flemming Besenbacher and research associates Renald Schaub and Erik Wahlstrôm and their coworkers have followed individual diffusion events on a T i 0 2 crystal surface using a rapid and high-resolution scanning tunneling microscopy (STM) method. In the presence of gaseous oxygen, the group finds, surface atoms undergo a type of rearrangement that "heals" the defects and creates new ones elsewhere—in effect causing oxygen vacancies to diffuse across the surface along specific crystal directions [Science, 299,377(2203)]. Based on a time-lapse series of STM images, each of which was recorded in a few seconds or less, the Aarhus group proposes that gas-phase oxygen adsorbs o n T i 0 2 and wanders around until the molecule reaches an oxygen vacancy. At the site of the
defect, the molecule dissociates. One of the oxygen atoms fills the vacancy. The lone oxygen atom quickly finds a new partner by abstracting a nearby oxygen atom from the surface lattice— thereby creating a vacancy at a new position. The researchers point out that the process leads to rapid vacancy migration across the oxide surface in a direction that is perpendicular to rows of bridging oxygen atoms. The simpler and more direct diffusion path—parallel to the rows —is a much slower process, they say 'Anyone studying this and related surfaces needs to control the number and distribution of oxygen vacancies," says Charles Τ Campbell, a professor of chemistry at the University of Washington, Seattle, who dis cusses the work in the same is sue of Science. "Therefore the ON T H E MOVE Time-lapse STM images (top and center) show that oxygen vacancies (circled) are mobile. The extent of the diffusion is seen in a difference image (top minus middle, shown at bottom). Original vacancy locations appear bright. Final locations appear dark.
new results will be of great prac tical importance." Campbell adds that the mechanism "prob ably operates on a wide range of transition-metal-oxide sur faces."—MITCH JACOBY
BUSINESS
Dow's Stavropoulos Details Plan For Cutting Costs àà
I am confident that we have the beginI nings of a comprehensive action plan I that will improve our company's profitability," Dow Chemical President and CEO William S. Stavropoulos says in his second message to employees since taking control of the company from Michael D. Parker in December. Among the steps Stavropoulos promises are an immediate freeze on all external hiring and geographic relocations; a dramatic reduction in external spending; tight control of travel; a six-month moratorium on all new, unauthorized, or engineering-
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only capital projects; and swift action, business-by-business, to raise prices to restore margins, especially in light of the volatile oil and energy environment. Dow, he says, will also "delay virtually every corporate initiative in order to reduce costs, keep money inside our company, and most importantly, give Dow people back the time they need to focus on what is needed in the near-term— especially spending more time with our customers." News of organizational changes and their impact on jobs is yet to come. "I
know many of you are anxiously awaiting detailed announcements on any organizational restructuring, portfolio changes, and, importantly, the impact our actions will have on jobs within our company," Stavropoulos, who is also Dow chairman, tells employees. "The goal of the management team is to get these answers to you as quickly as possible." Stavropoulos says that in the coming months, Dow will review all assets and will consider the divestiture or shutdown of those that are nonstrategic or underperforming.-WILLIAM STORCK
C&EN
/ JANUARY
20,
2003
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