The importance of the majority carrier polarity and p-n junction in titanium dioxide films to their photoactivity and photocatalytic properties

Abstract

In this paper, undoped polycrystalline titanium dioxide films have been deposited by atomic layer deposition (ALD) using the titanium tetrachloride–water process. These films have been anatase, rutile, or mainly anatase with a varying content of rutile depending on the deposition conditions. The polarity of the majority carriers on the films has been measured with the hot-probe test and, over the range of deposition temperatures used, the anatase or anatase-dominant films have been found to be p-type whereas the rutile films have been n-type. X-ray diffraction (XRD), contact angle measurements, and methylene blue degradation tests were performed to investigate the crystallinity, photoactivity, and photocatalytic properties. All the p-type films showed good photoactivity as determined by their water contact angle while the best photocatalytic activity was shown with anatase films containing a small proportion of rutile. An explanation for the superior photoactivity of anatase due to its p-type polarity is given and the high photocatalytic property for anatase-dominant films is ascribed to the existence of p-n junctions leading to carrier separation.