Freely Dispersible Au@TiO₂, Au@ZrO₂, Ag@TiO₂, and Ag@ZrO₂ Core−Shell Nanoparticles:  One-Step Synthesis, Characterization, Spectroscopy, and Optical Limiting Properties

We report a one-step route for the synthesis of Au@TiO 2, Au@ZrO 2, Ag@TiO 2, and Ag@ZrO 2 particles in nanometer dimensions, with controllable shell thickness. This scalable procedure leads to stable and freely dispersible particles, and bulk nanocomposite materials have been made this way. The procedure leads to particles of various morphologies, with a crystalline core in the size range of 30−60 nm diameter and an amorphous shell of ∼3 nm thickness in a typical synthesis. The core diameter and shell thickness (in the range of 1−10 nm) can be varied, leading to different absorption maxima. The material has been characterized with microscopic, diffraction, and spectroscopic techniques. The metal particle growth occurs by the carbamic acid reduction route followed by hydrolysis of the metal oxide precursor, resulting in the oxide cover. The particles could be precipitated and redispersed. The shell, upon thermal treatment, gets converted to crystalline oxides. Cyclic voltammetric studies confirm the core−shell structure. The E 1/2 value is 0.250 V (Δ E ≈ 180 mV) for the quasi-reversible Ag m /Ag m + couple and 0.320 V (Δ E ≈ 100 mV) for the Au n /Au n + couple for Ag and Au particles, respectively. Adsorption on the oxide surface blocks electron transfer partially. Nonlinear optical measurements in solutions show that these materials are strong optical limiters with a high laser damage threshold.