Recovering LSPR coloration in dried metal-nitride nanoparticle powders by sol-gel silica embedding

Plasmonic transition-metal nitrides such as TiN and HfN display localized surface plasmon resonance (LSPR)-driven color in dilute dispersion, but their dried powders are nearly black. Here we show that embedding TiN or HfN nanoparticles in amorphous SiO₂ by a one-pot sol-gel process recovers a visible LSPR-derived coloration of HfN in the dry state, despite a heterogeneous product in which single-core, multi-core, and core-free SiO₂ particles coexist. CIELAB colorimetry quantifies a shift from b* ≈ -1.4 (bare HfN) to b* ≈ -12.3 (HfN/SiO₂, 3 h), a CIE 1976 color difference ΔE₇₆ = 13.4 accompanied by a large increase in chroma; TiN, whose LSPR lies near the visible-NIR border, serves as a boundary case and shows only a small change that is dominated by lightness. A single-parameter (ω_p) quasi-static dipole/Kubelka-Munk reflectance model of isolated bare and silica-coated particles-calibrating only the nitride plasma frequency to the dispersion-phase LSPR-reproduces both the featureless reflectance of bare nitride powders and the LSPR-related reflectance dip of the silica-embedded particles (∼690 nm for HfN; beyond the measured range for TiN). Within this model, the recovery arises from scattering dilution: the transparent silica shell raises the scattering cross section so that the LSPR absorption re-emerges as a wavelength-selective dip. Physical separation of the nitride domains by SiO₂ may additionally limit contact-induced damping, but this contribution is not resolved here. Sol-gel silica embedding thus offers a simple route to solid-state LSPR coloration of metal-nitride nanoparticles.