In the last decades TiO₂ has received large attention due to its ability to “photocatalytically” split H₂O into H₂ and O₂. Under “open-circuit” conditions, noble metal-modified (mainly Pt, Au or Pd) TiO₂ is needed for H₂ photo-generation.¹ Not only the presence but also the size, amount and distribution of the noble metal particles affect the photocatalytic efficiency of TiO₂.^2,3

In this contribution we show that a most effective way to control the size and distribution of co-catalytic nanoparticles deposited on ordered TiO₂ nanotube arrays is a controlled sputter-dewetting process.⁴

For this, we first grow electrochemically highly-ordered arrays of short aspect-ratio TiO₂ nanotube in a o-H₃PO₄/HF electrolyte.⁵ The tubes growth is based on a simple anodization of a Ti metal substrate in self-ordering electrochemical conditions, and the morphology of the formed tubular oxide structures can be simply adjusted by the experimental conditions. These arrays of TiO₂ nanotubes are coated with sputtered Au and Ag films, and a following controlled thermal treatment is carried out that leads at the same time to Au-Ag alloying and thermal dewetting of the conformal metallic film, that is, the sputtered metals are converted into an array of orderly distributed alloyed Au-Ag nanoparticles.

By adjusting the sputtering conditions, we are able to selectively deposit the Au-Ag nanoparticles at the rim and/or within the cavity of the TiO₂ nanotubes. Subsequently, we perform a dealloying process, consisting in the selective chemical dissolution of Ag in a HNO₃ solution, this leaving behind nanoporous Au particles on/into the tubes. Compared to tubes decorated by conventional approaches, remarkably enhanced photocatalytic H₂ production is obtained upon porousification of the embedded Au nanoparticles. These porous Au/TiO₂ nanotube structures show a remarkable photocatalytic enhancement particularly when the Au porous particles are selectively formed at the rims of the tubes (“crown” position).

[1] K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 2014, 114, 9385–9454.

[2] N. T. Nguyen, J. Yoo, M. Altomare, P. Schmuki, Chem. Commun. 2014, 50, 9653–9656.

[3] N. T. Nguyen, M. Altomare, J. E. Yoo, N. Taccardi, P. Schmuki, Adv. Energy Mater. 2015, doi: 10.1002/aenm.201501926.

[4] N. T. Nguyen, M. Altomare, J. Yoo, P. Schmuki, Adv. Mater. 2015, 27, 3208–3215.

[5] J. E. Yoo, K. Lee, M. Altomare, E. Selli, P. Schmuki, Angew. Chem. Int. Ed. 2013, 52, 7514–7517.