  Au and Pt do not form homogeneous bulk alloys as they are thermodynamically
not miscible. However, we show that anodic TiO$_2$ nanotubes (NTs) can in-situ
be uniformly decorated with homogeneous AuPt alloy nanoparticles (NPs) during
their anodic growth. For this, a metallic Ti substrate containing low amounts
of dissolved Au (0.1 at%) and Pt (0.1 at%) is used for anodizing. The matrix
metal (Ti) is converted to oxide while at the oxide/metal interface direct
noble metal particle formation and alloying of Au and Pt takes place;
continuously these particles are then picked up by the growing nanotube wall.
In our experiments the AuPt alloy NPs have an average size of 4.2 nm and, at
the end of the anodic process, are regularly dispersed over the TiO$_2$
nanotubes. These alloyed AuPt particles act as excellent co-catalyst in
photocatalytic H2 generation - with a H2 production of 12.04 {\mu}L h-1 under
solar light. This represents a strongly enhanced activity as compared with
TiO$_2$ NTs decorated with monometallic particles of Au (7 {\mu}L h-1) or Pt
(9.96 {\mu}L h-1).
