Titanium dioxide, better known as a semiconductor oxide, is an important material for the study of solar energy conversion. It shows promising prospects in the research of preparation of organic solar cells, conversion of solar energy into chemical energy for the purpose of environmental protection, hydrogen formation from photocatalytic decomposition of water, and artificial photosynthesis, etc. It has become hot scientific issues to search for new catalytic materials and efficient energy transfer mechanism on the basis of this new material system-titanium dioxide.

Of the commonly investigated crystalline phases of titanium dioxide (TiO2), anatase is conventionally believed to be more catalytically active than rutile. A natural anatase crystal typically exhibits (101), (100)/(010) and minority (001) surfaces. Among them, the (001) surface has been suggested to be the most reactive one by several theoretical calculations. Motivated by this exciting hypothesis, although it is not yet experimentally verified, a great effort has already been devoted to the synthesis of (001)-rich anatase nanocrystals. It is known that a clean (001) surface of anatase TiO2 undergoes a (14) reconstruction in ultra-high vacuum (UHV). Theoretical predictions also suggest that this surface is highly catalytically reactive for waterand formic acid. However, the validity of these theoretical predictions is still controversial; in fact, they are in contrast to some recent experimental observations. For instance, a comparison of the activity of epitaxial anatase (001) and rutile (110) surfaces revealed nearly equal photochemical rate constants, and a clean anatase (001) surface exhibited a lower reactivity than the anatase (101) surface in photocatalytic reactions. It is thus essential to fully characterize the geometric and electronic structures of anatase TiO2 (001) surfaces to explicitly identify their active sites.

Here Prof. Bing Wang of Hefei National Laboratory for Physical Sciences at the Microscale of University of Science and Technology of China and his research team in single molecular study systematically investigated the structures and the reactivity of the oxidized and reduced (14) reconstructed surfaces of anatase TiO2 (001) thin films epitaxially grown on SrTiO3 using scanning tunneling microscopy (STM) or scanning tunneling spectroscopy, X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) and first-principles calculations. Quite unexpectedly, it is found that the anatase TiO2 (001)-(14) surface is not even active for water adsorption at room temperature. However, at a temperature of 80 K, the Ti-rich point defects at the ridge in the reduced surface can act as chemically active sites for H2O and O2 molecules. They thus proposed an oxidized ridge model for the reconstructed TiO2 (001)-(14) surface, where the Ti atoms at the normal ridge sites are six fold coordinated. The Ti-rich point defects on the reduced surface are four fold-coordinated, that is, Ti3+ sites. This model provides consistent explanations for their experimental findings from microscopic and spectroscopic measurements. Their findings provide very useful guidelines for the synthesis of TiO2 surfaces and the optimization of their chemical activity.

Source: Nature Communications, 2013, DOI: 10.1038/ncomms3214