Core–shell nanoheterodimers: laser-assisted deposition of single bimetallic Au@M (M = Au, Ag, Pd, Pt) nanodots on TiO2 nanoparticles

Abstract

We propose to synthesize well-controlled core–shell nanoheterodimers (NHDs) based on single bimetallic Au@M (M = Au, Ag, Pd, Pt) nanodots (BNDs) grown on bare TiO₂ nanoparticles (NPs) by a two-step laser-assisted deposition. The high photon flux emitted by a focused UV laser triggers nucleation and growth of a single core nanodot (ND) and its size is quantitatively controlled by varying the time exposure and the concentration of gold ions in the solution in the first step. A second laser deposition is performed after adding a new metal precursor to the Au–TiO₂ NHDs thus prepared. Due to the vectorial nature of the first Au–TiO₂ NHDs when photo-excited, and thus the subsequent efficient carrier separation, the second photodeposition strictly takes place on the first gold NDs. Growth is epitaxial for both Au@Au–TiO₂ and Au@Ag–TiO₂, while Au@Pt–TiO₂ shows a discontinuous shell with the formation of separate domains, which further rearrange. Au@Pd–TiO₂ exhibits an intermediate situation where the shell surface becomes progressively irregular, in agreement with surface energies and lattice mismatch between the two metals. Still, unlike conventional wet chemistry, where the metal atoms in the shell come from the bulk solution (external production), photodeposition produces carriers through the TiO₂ substrate and the gold ND (internal process). Such a process highlights the role of the work function (WF) mismatch between the two metals in electron transport at their interface for shell growth. It provides a unique opportunity to accurately control a thin shell with a thickness of typically two layers of metal atoms. This control is of significant interest to couple the synergy between catalysis and plasmonics. We, hence, propose a general methodology for synthesizing single BNDs with a core–shell structure loaded on semiconductor NPs, providing advanced asymmetric nano-texturing for Au–TiO₂-based photocatalysis and plasmonic photocatalysis applications.