Stability and Surface Functionalization of Plasmonic Group 4 Transition Metal Nitrides

Abstract

Plasmonic transition metal nitrides (TMNs) have emerged as a low-cost and thermally and chemically robust alternatives to noble metals. While their superior thermal properties have been established, their chemical properties on the nanoscale haven't been as well investigated. Herein, the oxidative stability over time under ambient conditions and colloidal stability as function of pH was explored for plasmonic TiN, ZrN, and HfN nanoparticles. It was discovered that the TMN nanoparticles made via solid-state method had a narrow pH stability range between 2–3. Under highly acidic conditions, the particles underwent dissolution and at pH ≥4, they aggregate and precipitate from the solution. Additionally, TiN nanoparticles had poor oxidative stability and oxidized to TiO₂ after ~40 days. 3-Aminopropyltriethoxysilane (APTES) and dimethylsilane coated TMNs were synthesized to yield water and organic solvent dispersible particles, respectively. These functionalized colloidal suspensions showed enhanced oxidative stability over 60 days and the APTES coating widened the pH stability window of TMNs to include physiological pH. This study shows that surface functionalization using M−O−Si linkages (where M=Ti, Zr, or Hf) can greatly enhance the stability, dispersibility and therefore applicability of plasmonic TMN nanoparticles.