Enhanced Visible-Light Photoelectrochemical Performance of Titana Nanotubes Modified by Atomic Layer Deposited Cobalt Oxide

Abstract

TiO2 is widely considered as one of the promising materials to fulfill photoelectrochemical water splitting requirements for its low cost, nontoxicity, photostablity, and strong photo oxidation ability. Several synthesis approaches have been reported to fabricate TiO2 nanostructures, including template assisted hydrothermal, anodization, and sol-gel. Among them, anodization method is capable of producing highly ordered, well aligned TiO2 nanotubes (NTs) with controlled diameters and tube length. The unidirectional tubular structure provides the charge transport channels from electrolyte/photoanode interface to conductive substrate. The intrinsic band gap of TiO2 (3.2eV for anatase and 3.0eV for rutile), however, limits its optical absorption only within UV region. The rapid recombination rate of photo-excited electrons and holes also severely hinders its practical application.1 Many approaches have been developed to promote the photo response of TiO2 in visible light region. For example, by hetero coupling, the holes and electrons can be photo excited from the narrow-gap semiconductor and separated effectively, which significantly promotes the visible light response. To form hetero coupling, cobalt oxide, a typical P-type semiconductor with band gap of ~2.1eV is chosen. Recent report shows cobalt oxide coupled with TiO2 exhibit could enhance the photo oxidation activity under visible light illumination.2 CoOx/TiO2 nanoparticles also show great photocatalytic activity on water splitting.Here we report the formation of CoOx/TiO2 NTs hetero coupling by atomic layer deposition (ALD). ALD is ultrathin film deposition technique to have sub-nanometer control of film thickness. As a promising method to passivate and modify photoanode,3 ALD could form chemical bonds on substrate and operate at a relatively low temperature. We will show that by uniformly depositing a layer of cobalt oxide film onto the TiO2 nanotubes, CoOx/TiO2 NTs system exhibits one fold improvement on photocurrent response under visible light illumination compared with pristine TiO2 NTs, and about two times the efficiency of CoOx/TiO2 synthesized via wet-chemistry method. The structure was detailed studied and optimized, and mechanism will also be discussed.