Electrochemical Synthesis of Nanorods
this chapter: micelle-stabilized Au nanorods and the core-shell Au nanorod@ silica structures. A comparison of the calculated and experimental absorption spectra will then be made. We also demonstrate that the value of the dielectric constant of the silica shell can be extracted from the spectral measurements by observing the spectral shift of the SPl band as a function of the shell thickness.
II. SYNTHESIS OF AU NANORODS IN AQUEOUS SOLUTION A. Experimental Setup We have described elsewhere a method for synthesizing suspensions of Au nanorods in aqueous solutions (16,17), wherein a mixed surfactant system was employed to define the size and shape of the nanorods. Since these first studies, we have developed an improved method (in terms of yield and control over size and shape). Briefly, the synthesis is performed within a simple two-electrode electrochemical cell. A gold metal plate (3 1 0.05 cm) and platinum plate (3 1 0.05 cm) serve as the anode and cathode, respectively. Both electrodes, fixed face-to-face at distance of 0.25 cm (determined by a Teflon spacer), are immersed into the electrolyte solution to a depth of ca. 1.5 cm. The electrolyte solution volume is typically 3 mL and contains 0.08 M of the cationic surfactant hexadecyltrimethylammonium bromide (C16TABr, 99%, Sigma), and 12.6 mg of the more hydrophobic cosurfactant tetradodecylammonium bromide (TC12ABr, 98%; Fluka). The C16TABr-TC12ABr system serves as the supporting electrolyte and as the stabilizer for the Au nanorods. The glass electrochemical cell containing the electrodes and mixed surfactant solution is then placed into an ultrasonic bath (Branson model 1210) whose water solution is held at 36째C. The cell temperature is allowed to equilibrate with the bath for 5 min. Immediately prior to the electrolysis, 65 L of acetone and 45 L of cyclohexane are added to the electrolyte solution. Acetone is used to loosen the micelle framework, and cyclohexane is necessary for enhancing the formation of rodlike C16TABr micelles (18). Meanwhile, one (or two) silver plate(s) of similar dimensions to the anode and cathode is (are) placed above the solution near the cathode and is (are) not connected to an electrode lead. The electrolysis is done under constant-current mode, with the typical setting being 5 mA. The electrolysis time is typically 20 min under constant sonication (again, the bath temperature is held at 36째C). Gold metal is then oxidized to form a soluble complex, which then diffuses to the Pt cathode where it is reduced. Also, the silver plates are gradually immersed into the electrolytic solution during the electrolysis to constantly provide fresh Ag surface (vide infra). The gold nanorods form in the solution with the aid of stabilizers directing their growth.
Chemistry of metal nanoparticles