Electrochemical crystal frequency shift

The quartz crystal microbalance (QCM) consists of a quartz crystal that is electrically driven into oscillation. The resonance frequency of the crystal is monitored. This frequency is highly dependent on any mass added to the crystal surface. Hence the mass dependence of the QCM resonance frequency can be, in air, used to weigh minute amounts of material with a sensitivity of the order of 1 ng/cm. QCM can also be coupled with electrochemistry here, the quartz crystal surface is coated with an appropriate electrode material, for example, thin film gold. This electrochemical QCM (EQCM) configuration can be used to monitor electrochemically triggered surface processes associated with the deposition (or loss) of material at the working electrode surface. However, in liquid medium the frequency shift of the QCM crystal is not solely sensitive to added mass but is also influenced by changes in the local property of the medium associated with the surface electrochemical process of interest. For example, density or viscosity variation of the medium in the electrode vicinity, in addition to variation in the viscoelastic properties of the deposited layer, can cause shifts in the resonant frequency of QCM. 

The main interest in coupling QCM with SECM is that the SECM tip can specifically monitor the flux of electroactive species associated with the surface electrochemical process. This additional information can help to quantify the actual contribution of electroactive flux to the apparent total mass change derived from the QCM frequency shift The combination of SECM with EQCM was initially described by Hillier and Ward [160]. The authors used an SECM tip to trigger the electrodeposition of a small amount of copper on predefined locations of the QCM surface. By monitoring the ensuing frequency shift of the QCM, the radial sensitivity of the QCM crystal was mapped. 

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