  Au nanoparticles at the TiO$_2$ surface can enhance the photocatalytic H$_2$
generation performances owing to their electron transfer co-catalytic ability.
Key to maximize the co-catalytic effect is a fine control over Au nanoparticle
size and placement on the photocatalyst, in relation to parameters such as the
TiO$_2$ morphology, illumination wavelength and pathway, and light penetration
depth in the photocatalyst. Here we present an approach for site-selective
intrinsic-decoration of anodic TiO$_2$ nanotubes (TNs) with Au nanoparticles:
we produce, by Ti and Au co-sputtering, Ti-Au alloy layers that feature
compositional gradients across their thickness; these layers, when anodized
under self-ordering electrochemical conditions, can form Au-decorated TNs where
the Au nanoparticle density and placement vary according to the Au
concentration profile in the metal alloy substrates. Our results suggest that,
the Au co-catalyst placement strongly affects the photocatalytic H$_2$
evolution performance of the TNs layers. We demonstrate that, when growing
Au-decorated TNs, the use of Ti-Au substrates with a suitable Au compositional
gradient can lead to higher H$_2$ evolution rates compared to TNs classically
grown with a homogenous co-catalyst decoration. As a side effect, a proper
placement of the co-catalyst nanoparticles allows for reducing the amount of
noble metal without dumping the H$_2$ evolution activity.
