Electrochemical characterization of PbS quantum dots capped with oleic acid and PbS thin films — a comparative study

Abstract

This research aims at probing electrochemical response of oleic acid capped PbS quantum dots deposited on a Pt electrode in 0.1 M aqueous sodium hydroxide solution by cyclic voltammetry and chronoamperometry. Quantum dots were also characterized by photoluminescence and IR spectroscopy. Cyclic voltammetry of bulk PbS thin films obtained via chemical bath deposition is investigated in order to interpret the data on PbS quantum dots. It was found that the two materials exhibit essentially similar voltammetric behavior; however, oxidation of PbS quantum dots tends to start at slightly more cathodic potentials than that of bulk PbS films. This effect is attributed to the influence of capping oleic acid that binds lead ions into an insoluble lead oleate thereby causing the cathodic shift of the formal redox potential. The procedures designed to partially remove oleic acid from PbS quantum dots result in the anodic shift of the PbS quantum dots oxidation towards the values characteristic for the bulk material. A possibility of determining absolute positions of conduction and valence bands in quantum dots by cyclic voltammetry is discussed but the influence of the energy level structure of PbS quantum dots on their voltammetric response was not revealed under the conditions of the present study. However, PbS quantum dots can withstand multiple redox cycles whereas bulk PbS films dissolve readily upon the first oxidation. The effect was attributed to the oleic acid layers on the PbS quantum dots surface, those layers preventing soluble oxidation products from diffusing into the bulk of solution. Certain PbS quantum dots samples showed remarkable stability against oxidation typical for PbS and starting at −0.2 V vs Ag/AgCl (sat.) and a stable response at oxidation and reduction at higher (0.55 V) and lower (−0.8 V) potentials, respectively.