Unraveling the thermal evolution of TiO₂-supported Au@M (M = Ag, Pd, Pt) nanobipyramids: Impacts of shell composition on structural and plasmonic stability

Au nanobipyramids (NBPs) are a unique class of anisotropic plasmonic nanomaterial renowned for their highly efficient light absorption and tunable optical properties spanning the visible and near-infrared regions. Their functionality can be tailored for specific applications by coating them with thin shells of other metals. However, the elevated temperatures frequently employed during processing and application can lead to their structural degradation and a subsequent loss of their distinctive optical properties, ultimately diminishing their practical utility. In this study, we explore coating Au NBPs with different metals (Ag, Pd, and Pt) and compare their structural and spectral stability on TiO₂ support under thermal stress (100–400 °C). Our results reveal that for annealing periods of 1 h, pure Au and Au@Ag NBPs progressively lose their anisotropic shape and experience blue shifts in their plasmonic resonances even at moderate temperatures. In contrast, Au@Pd and Au@Pt NBPs exhibit remarkable thermal resilience, retaining their structural integrity and plasmonic character up to 200 and 250 °C, respectively, with stability extending to 4 h of annealing at 200 °C. The enhanced thermal stability of Au@Pd and Au@Pt NBPs underscores their suitability for high-temperature applications in optoelectronics, catalysis, and energy conversion systems. Furthermore, atomic-scale analysis reveals element-dependent alloying behavior in the bimetallic Au@M NBPs, offering valuable insights for designing thermally robust bimetallic nanostructures for challenging operating conditions.