Elucidating the Role of Surface Ligands on the Oxidative Etching of Au Bipyramids During Photothermia Using Liquid Cell Transmission Electron Microscopy

Gold bipyramids (AuBPs) are emerging as high‐performance anisotropic plasmonic nanoheaters, yet maintaining their structural integrity under photothermia (PT) remains a critical challenge. Here, we demonstrate the role of oxygen bubbles formed during PT in reshaping AuBPs irradiated with an 808 nm continuous‐wave laser. We elucidate how surface ligands dictate AuBPs stability by comparing cetyltrimethylammonium bromide (CTAB), citrate, and polystyrene sulfonate (PSS). The morphological evolution of individual nanoparticles is monitored by ex situ TEM, and their optical response by UV–vis–NIR spectroscopy. We show that CTAB‐ and citrate‐coated AuBPs undergo significant tip shortening and facet reconstruction due to heat‐enhanced oxidative etching, which leads to strong blueshifts and decreased LSPR intensity. In stark contrast, PSS imparts remarkable structural and optical stability, enabling persistent heat generation during prolonged irradiation. To uncover the underlying mechanisms, we combine in situ liquid cell TEM tracking of single‐particle reshaping with optical modeling and Pb‐UPD. We demonstrate that PSS provides enhanced protection of the lateral {111} facets and suppresses tip shortening by preventing oxygen nanobubbles from locally destabilizing the Au surface. Our findings reveal that nanoparticle durability arises from a synergistic interplay between ligand chemistry and interfacial gas dynamics, offering an unexplored route to engineer long‐term plasmonic materials.