Enhanced photoelectrochemical activity of ZnO-coated TiO₂ nanotubes and its dependence on ZnO coating thickness

One-dimensional heterogeneous nanostructures in the form of ZnO-coated TiO₂ nanotubes (ZnO/TiO₂ NTs) were fabricated by atomic layer deposition of an ultrathin ZnO coating on electrochemical anodization-formed TiO₂ nanotubes (NTs) with the thickness of ZnO coating being precisely controlled at atomic scale, and the photoelectrochemical activity of the fabricated ZnO/TiO₂ NTs and the influence of ZnO coating and its thickness were studied. The structures of TiO₂ 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 TiO₂ NTs exhibit anatase structure, and the ZnO coatings are structured with hexagonal wurtzite. The photoelectrochemical activity of the ZnO/TiO₂ NTs is strongly dependent on the thickness of ZnO coating. ZnO/ TiO₂ 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/TiO₂ NTs with an about 2-nm-thick ZnO coating.