Enhanced Photoelectrochemical Activity of ZnO-Coated TiO(2) Nanotubes and Its Dependence on ZnO Coating Thickness

One-dimensional heterogeneous nanostructures in the form of ZnO-coated TiO(2) nanotubes (ZnO/TiO(2) NTs) were fabricated by atomic layer deposition of an ultrathin ZnO coating on electrochemical anodization-formed TiO(2) nanotubes (NTs) with the thickness of ZnO coating being precisely controlled at atomic scale, and the photoelectrochemical activity of the fabricated ZnO/TiO(2) NTs and the influence of ZnO coating and its thickness were studied. The structures of TiO(2) NTs and ZnO coatings were characterized by X-ray diffraction, Raman backscattering spectroscopy, and transmission electron microscopy. The photoelectrochemical activity was studied through the measurements of electrochemical impendence, flat-band potential, and transient photocurrent density. The TiO(2) NTs exhibit anatase structure, and the ZnO coatings are structured with hexagonal wurtzite. The photoelectrochemical activity of the ZnO/TiO(2) NTs is strongly dependent on the thickness of ZnO coating. ZnO/TiO(2) NTs with a thinner rather than a thicker ZnO coating exhibit better photoelectrochemical activity with reduced charge transfer resistance, increased negative flat-band potentials, and enhanced photocurrent densities. Under visible illumination, an increase of about 60 % in the photoelectrochemical activity is obtained for ZnO/TiO(2) NTs with an about 2-nm-thick ZnO coating.