Triphenylphosphine-Passivated Aluminum Nanoparticles: Synthesis and Reactivity in Al/CuO Nanothermites

This work reports the synthesis of triphenylphosphine (PPh₃)-passivated aluminum nanoparticles (Al@PPh₃) via a one-pot wet chemical reduction of AlCl₃ with LiAlH₄ in refluxing mesitylene at a 1:1 PPh₃/AlCl₃ molar ratio under nitrogen, and evaluates their reactivity in Al/CuO nanothermites. High-resolution transmission electron microscopy (HR-TEM) revealed a conformal ∼4.6 nm PPh₃-derived shell on the crystalline Al core. No distinct oxide interlayer was resolved within the spatial and contrast limits of the images. X-ray diffraction (XRD) confirmed crystalline fcc Al with no detectable Al₂O₃, and X-ray photoelectron spectroscopy (XPS) resolved an Al 2p component at ∼74.7 eV, consistent with an interfacial interaction between the PPh₃-derived layer and the Al surface. Solid-state ³¹P NMR showed that the shell is a PPh₃-derived organophosphorus layer with partial oxidation to phosphine oxide. Dynamic light scattering (DLS) gave an effective particle diameter of ∼65 nm. In air, thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) showed PPh₃ removal up to ∼450 °C followed by two-stage aluminum oxidation peaking at 631 and 880 °C. Under argon, Al@PPh₃/CuO nanothermites exhibited a thermite onset near 543 °C, approximately 50 °C above the native oxide Al/CuO baseline. Strand burner tests in 2 mm ID capillary tubes showed a nonmonotonic burn rate dependence on Al@PPh₃ replacement fraction: partial substitution raised the burn rate from ∼330 m/s at 0% to 387 m/s at 30%, whereas higher loadings caused a sharp decline to ∼95 m/s at 50% and prevented self-sustained propagation above 50%. The higher burn rate at 30% is attributed to a trade-off between partial removal of the native oxide diffusion barrier, augmented by shell-derived gas release, and the endothermic cost of shell decomposition at higher loadings.