Bridging the gap with a "stepping stone" for more solar energy

Converting the abundant energy from the sun into a form convenient for human use has been a long standing dream for sustainable generation of environmentally clean energy.  With the seminal discovery of water splitting by Fujishima and Honda in the early 1970s [1], titanium dioxide (TiO2), an inexpensive white pigment widely used in our daily life, emerged as the premier photocatalysts for enabling solar energy utilization. However, because of its wide intrinsic band gap, TiO2 can absorb only ultraviolet light. This results in less than 1% efficiency for solar energy conversion. Reducing the band gap of TiO2 is the main avenue for boosting the conversion efficiency. In a recent paper to appear in Phys Rev. Lett. [2], Zhu et
al report on a potential conceptual breakthrough for controllably narrowing the band gap of TiO2. A commonly used approach is to dope the host material with trace amounts of foreign elements. Until now numerous attempts at doping TiO2 with either cations or anions have failed to produce a breakthrough. In this paper, they report on a conceptually novel doping scheme, termed non-compensated anion-cation codoping. This new approach, in addition to enhancing dopant incorporation (due to electrostatic attraction within the n-p copant pairs), also ensures the creation of intermediate bands in the gap region of the host semiconductor to effectively narrow the band gap. The resulting materials in the form of nanoparticles show dramatically enhanced photoactivity in the visible light region. This concept  may  find applicability in developing other types of
advanced materials for catalysis, optoelectronics, spintronics, and clean energy.

1. A. Fujishima andK. Honda, Nature 238, 37 (1972).

2. W. G. Zhu et al., Phys Rev. Lett. (in press).