Size-Dependent Electrochemical Stability of Silver Nanoparticles

Abstract

Understanding the electrochemical stability or corrosion behavior of metallic nanoparticles in aqueous environments is of central importance in the fields of catalysis, sensing, nanoelectronics and photonics [1]. The advantages of nanoparticles, such as increased solubility of nanosuspensions and their low production costs have resulted in their rapid commercialisation for medical applications (e.g. drug delivery) [2]. In addition, metal ion release is a major pathway underlying their potential toxicity, and the cellular environment can be considered as an electrochemical cell. Therefore, stability against oxidation or loss of effective surface area is necessary for metallic nanoparticles (NPs) to retain their useful properties in long-term applications and also important for understanding the safety of NPs. The objective of this study was to investigate the electrochemical stability of silver nanoparticles (AgNPs) as a function of applied potential, pH and particle size. AgNPs with controlled size and size distribution were deposited on to boron-doped diamond electrodes via magnetron sputtering, with size-filtering capability. The anodic behavior of AgNPs in perchlorate buffer pH solutions (ranging from pH 2-12) were examined by linear sweep voltammetry. The surface chemistry and oxidation state of Ag after each treatment were characterized using X-ray photoelectron spectroscopy (XPS). The size distribution of AgNPs in response to electrochemical polarization was examined using atomic force microscopy (AFM). Pourbaix diagrams for bulk Ag suggest that the equilibrium condition is not pH-dependent in acidic conditions. However, we observe that, for nanoparticles with the average size of 4.4 ± 2.1 nm, the oxidation potential deviates from expectation in acidic conditions by approximately 0.015V / pH. By contrast, the relationship between potential and pH follows that of bulk Ag for alkaline solutions. XPS confirmed the expected electrochemical changes in surface chemistry, with binding energy (BE) of Ag3d5/2 photoelectron signal and O1s/C1s ratio correlating with the observed changes in the silver oxidation state. These direct voltammetric measurements of the Ag oxidation potential, as a function of size and pH, indicate the electrochemical stability of NPs is different from their bulk metal. This study supports the findings of Tang et.al., [3] suggesting that theoretically derived energy diagrams for a bulk material might not always accurate to NPs, especially AgNPs which has been shown in our studies.