Abstract

We have recently demonstrated a method for the preparation of gold [1], platinum [2], and gold−silver alloy nanoparticles [3] by high intensity laser irradiation of metallic salts solution. The formation of metal nanoparticles in the absence of any reducing agent can be attributed to the active species generated by high-intensity laser irradiation of water molecules (i.e., H₂O → e_aq−, H^•, OH^•, etc.). Solvated electrons (e_aq−) and hydrogen radicals (H^•) are known to act as strong reducing agents in the solution. Accordingly, metal ions in the solution are readily reduced to rezo-valent metal. This technique enables us to produce metal nanoparticles with a narrow size distribution as well as small particle size down to single nanometer and is applicable to a large number of metal solutions. However, for a copper solution, Cu²⁺ ion has an absorption peak around 800 nm, which corresponds to the wavelength of a Ti:sapphire laser beam. Therefore, photo-induced reduction can hardly occur at the focal point because the laser beam is absorbed by the solution and then the laser intensity is attenuated before reaching the focal point. When ammonia is added to a solution of Cu²⁺ ion, complex tetraammine copper (II) is formed and the absorption peak of Cu ion moves to 600 nm. (e.g., CuSO₄ + 4NH₃ → [Cu(NH₃)₄]²⁺ + SO₄2⁻). In this study, we fabricate Cu nanoparticles by high-intensity laser irradiation using tetraammine copper (II) ions as precursors. Complex ion solution was prepared by dissolving the copper (II) sulfate pentahydrate (CuSO₄·5H₂O) in 1.0 wt% NH₃ solution with the concentration of 0.05 wt%. 2-propanol ((CH₃)₂CHOH) was added to the solution as a scavenger of hydroxyl radical (OH˙) to promote the reduction process. A quartz glass cuvette was filled with the solution of 3 ml. Femtosecond laser beam (wavelength: 800 nm; pulse energy: 5 mJ; pulse width: 100 fs; repetition rate: 100 Hz) was tightly focused in the solution by an aspheric lens (focal length: 8 mm; numerical aperture: 0.5). The laser intensity was estimated to be 2.1×10¹⁴ W/cm². The irradiation time was 1 hour in the experiment.

© 2011 Optical Society of America