The colloidal synthesis of nanoparticles is governed by both thermodynamic and kinetic factors, yet the role of reduction potential, a key thermodynamic parameter, remains underexplored. In this study, we systematically investigate how the reduction potential of phenol derivatives affects the seed-mediated growth of gold nanobipyramids (Au NBPs). Four structurally similar 4-substituted phenols with varying electron-donating or withdrawing groups (-Cl, -H, -CH₃, -OCH₃) were used as reductants. Their reduction potential was modulated by adjusting the pH of the growth solution, enabling a controlled investigation of redox thermodynamics on product morphology and plasmonic properties. For each reductant, there exists a specific pH window where high-quality Au NBPs are formed, with the longitudinal localized surface plasmon resonance (L-LSPR) peak red-shifting as pH increases. While these effective pH ranges differ across reductants, plotting L-LSPR peak positions against the calculated reduction potentials (via the Nernst equation) collapses all data onto a single trend. This demonstrates that the reaction outcome under the investigated conditions and for the tested reductants is governed by reduction potential, not the identity of the reductant. Further validation using hydroquinone, a structurally distinct reductant with a two-electron oxidation, yields consistent results, underscoring the generality of this relationship. Our findings establish reduction potential as a potential dominant descriptor for guiding the shape-controlled synthesis of Au NBPs, offering a pathway to improve reproducibility and rational design in colloidal nanomaterial synthesis.

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.