Regulating thermal diffusion of gold thin films at solid-state interfaces for site-selective decoration of gold nanoparticles on titania nanotubes as an efficient SERS sensing platform

Decorating plasmonic Au nanoparticles on TiO2 nanotube arrays with high specific surface area is an efficient strategy for constructing high-performance surface-enhanced Raman scattering (SERS) substrates. Direct deposition of a Au thin film on a TiO2 support, followed by dewetting at elevated temperatures, is economical for scale-up production of Au nanoparticles. The thermal diffusion behavior of Au on a high-curvature surface, however, makes it difficult to meet an optimized distribution of Au nanoparticles that is required for forming electromagnetic SERS hotspot regions. Herein, we proposed a layer-by-layer deposition method to prepare Au-decorated TiO2 nanotube arrays using ZnO nanowires as structural templates. Au nanoparticles could be loaded on either the interior or exterior of the TiO2 nanotubes or buried in the walls simply by switching the sequence of magnetron sputtering of Au and atomic layer deposition of TiO2. We revealed that regulating the thermal diffusion of the Au thin film in the confined ZnO/TiO2 interlayer favors an optimal distribution of the dewetted Au nanoparticles at the interstices. By virtue of the size and interspace of the Au nanoparticles balancing each other, Au-TiO2 featuring Au nanoparticles dispersed on the inner walls of TiO2 nanotubes provided abundant localized electromagnetic field hot spots with a small amount of Au consumption. This Au-TiO2 substrate presented high SERS activity, e.g. a detection limit of 10(-9) M for the R6G molecule. This substrate could also achieve self-cleaning through photocatalytic degradation of adsorbed organic analytes, which is required for repeated SERS sensing. Regulating thermal diffusion and dewetting of Au thin films at the confined ZnO/TiO2 interface favors an optimal distribution of the produced Au nanoparticles for forming localized electromagnetic field hot spots.