Intrinsic Au decoration of growing TiO2 nanotubes and formation of a high-efficiency photocatalyst for H2 production

Self-organized nanoporous or nanotubular systems produced by electrochemical anodization processes have in the past decade attracted much interest owing to remarkable potential applications, for example, in high-density recording media, biological nano-patterning, optical devices, functional electrodes, and templates for nanoscale geometry. Materials that exhibit self-organized anodic pore or tube growth include not only the well-studied cases of aluminum oxide1 and silicon,2 but also a variety of self-organized oxide nanotubes on valve metals such as titanium oxide,4-6 zirconium oxide,7 hafnium oxide,8 niobium oxide,9 tungsten oxide,10 tantalum oxide,11 vanadium oxide,12 and cobalt oxide.13 In spite of this wide available material range, by far the widest attention has been attracted by self-organized TiO2 nanotube layers due to their outstanding semiconducting properties that enable a number of applications such as electronic devices, sensors, and as anodes in dye-sensitized solar cells or for photoelectrochemical water splitting.14-16 Such TiO2 nanotube layers are typically formed by anodic oxidation of a Ti metal substrate in a fluoride containing electrolyte, and can be fabricated with a high control over geometry (length, diameter, wall thickness, surface area) and crystal structure (as-grown tubes are amorphous and can be easily crystallized to anatase or rutile by a thermal treatment.4, 5

A most straightforward and unique way to modify anodic tube properties is the use of Ti-X alloys for the self-organizing anodization process.17 X is typically another metal (such as Nb, Ta, Ru) that is simultaneously oxidized during the electrochemical treatment and is incorporated into the TiO2 structure as a secondary oxide (or in substitutional or interstitial position) giving the nanotubes a specific functionality (e.g. doping, band gap engineering).17

In the present work, we investigate the feasibility to form tube structures from a gold-titanium alloy – this is particularly interesting as noble metals such as Au are not expected to be oxidized during the anodization process. We show that over a certain Au concentration range TiO2 nanotube layers can be grown that show a homogeneous, well-defined and controllable decoration with elemental Au nanoclusters (Au nc) (Figure 1).