Size Determination of Metal Nanoparticles Based on Electrochemically Measured Surface-Area-to-Volume Ratios

Here we report the electrochemical determination of the surface-area-to-volume ratio (SA/ V) of Au nanospheres (NSs) attached to electrode surfaces for size analysis. The SA is determined by electrochemically measuring the number of coulombs of charge passed during the reduction of surface Au₂O₃ following Au NS oxidation in HClO₄, whereas V is determined by electrochemically measuring the coulombs of charge passed during the complete oxidative dissolution of all of the Au in the Au NSs in the presence of Br⁻ to form aqueous soluble AuBr₄⁻. Assuming a spherical geometry and taking into account the total number of Au NSs on the electrode surface, the SA/ V is theoretically equal to 3/radius. A plot of the electrochemically measured SA/ V versus 1/radius for five different-sized Au NSs is linear with a slope of 1.8 instead of the expected value of 3. Following attachment of the Au NSs to the electrode and ozone treatment, the plot of SA/ V versus 1/radius is linear with a slope of 3.5, and the size based on electrochemistry matches very closely with those measured by scanning electron microscopy. We believe the ozone cleans the Au NS surface, allowing a more accurate measurement of the SA.