Hydrogen production by photocatalytic water splitting on TiO2 nanotubes

The photocatalytic activity of a series of TiO2 nanotube arrays obtained under different anodization conditions is discussed in relation to the morphology and phase composition changes occurring during anodization. TiO2 nanotubes of different lenghts were prepared by anodization in NH4F-H2O-formamide solutions and characterized mainly by SEM, XRD and photocurrent measurements. Their photoactivity in hydrogen production by water splitting was investigated by evaluating the amount of hydrogen evolved in a two compartment PEC cell for separate H2 and O2 production. Phase compsition and nanotubes length were strictly related to the anodization time. After the formation of a well grown porous morphology, an ordered tubular structure was produced, followed by the formation of a compact TiO2 layer on the nanotubes top at long anodization time. An increase of the anatase phase content occurred in parallel. During 60 s-long lightdark cycles, a fast raise and decay of photocurrent was observed with fully open nanotube arrays with well defined, regular architecture, wheres slower photocurrent raise and decay were observed in the presence of a compact top oxide surface layer. In all photocatalytic tests water photosplitting into hydrogen and oxygen occurred at constant rate, with no applied external bias. Highest photoactivity was achieved with ordered nanotube arrays obtained under optimized anodization conditions and composed of mixed anatase and rutile phases.