Plasmon‐Driven Phase Engineering of TiO 2 Nanoshells via Collective Thermal Accumulation in Plasmonic Core–Shell Nanoparticle Assemblies

ABSTRACT Plasmonic photothermal heating enables localized energy deposition in nanoscopic volumes, yet harnessing this capability to reach the extreme temperatures required for phase engineering of inorganic solids has remained an outstanding challenge. Here we report two central findings. First, focused 785 nm continuous‐wave laser irradiation of gold nanorod–TiO 2 core–shell nanoparticles (AuNR@TiO 2 ) drives amorphous TiO 2 nanoshells through the complete amorphous → anatase → rutile phase sequence within one second. Achieving such extreme local temperatures through plasmonic photothermal heating is exceptional. The transformations are unambiguously confirmed by in situ Raman spectroscopy and ex situ selected‐area electron diffraction. Wavelength‐selective crystallization reveals that plasmonic absorption of AuNR cores leads to the phase transition. Second, nanoparticle assembly geometry is a decisive design parameter: isolated particles on silicon nitride membrane grids transform predominantly to anatase, whereas dense multilayer assemblies on glass substrates produce rutile under identical conditions. Heat‐transfer simulations reveal that overlapping thermal fields in closely packed assemblies elevate local temperatures far beyond those achievable by single‐particle heating—a collective thermal accumulation effect that determines the final crystalline phase. These findings establish plasmonic nanoheating as a viable route for spatially selective phase engineering of oxide nanomaterials and identify nanoparticle assembly as a tunable handle for controlling nanoscale crystallinity.