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

Abstract

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.