INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 73)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
APPLICATIONS 6-1
6 APPLICATIONS
The RQCM will respond very sensitively to minute stress changes on its vibrating surface
resulting from mass deposits or frictional forces. This makes it a powerful tool for a wide variety
of applications, including: biofilms formations on surfaces, bio-sensing, specific gas detection,
environmental monitoring, and basic surface-molecule interaction studies.
A full spectrum of its potential applications is beyond the scope of this manual. This section only
describes a few typical applications. The user is advised to consult the publications listed in
Section 12 for further information.
6.1 ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE
The basic principles and applications of the QCM to electrochemical processes have been
extensively reviewed in the electrochemical literature 29 30 .
In most electrochemical experiments, mass changes occur as material is deposited or removed
from the “working” electrode. It is of interest to monitor those changes simultaneously with the
electrochemical response, and the RQCM is the standard means of doing so. As a gravimetric
probe, the QCM has been used in many types of electrochemical studies, including:
underpotential deposition of metals 31 32 33 34 , corrosion 35 , oxide formation 36 , dissolution studies 37
38 39 40 , adsorption/desorption of surfactants 41 42 43 and changes in conductive polymer films
during redox processes 44 .
6.1.1 CALIBRATION
Many published literature has demonstrated that when experiments involve only relative
frequency shifts which are measured in a fixed solution, the offset caused by the viscous loading
of the liquid, has negligible effect on the accuracy of the Sauerbrey equation for the determination
of small mass changes in rigid deposits 45 . Quantitative interpretation of the EQCM data in those
cases is based on the combination of the Sauerbrey equation and Faraday’s law.
The Sauerbrey equation relates change in frequency to change in mass for thin, lossless deposited
films, whereas Faraday’s law relates charge passed in an electrochemical experiment to the
number of moles of material electrolyzed. Therefore, frequency changes can be related to the total
charge passed.
An example would be the electrodeposition of Ag on a Pt electrode QCM crystal. The charge, Q,
is an integral measure of the total number of electrons delivered at the interface during the
reduction process. To the extent, that each electron supplied results in the deposition of one atom
of Ag, there should be a linear relationship between Q and ∆f as is given by this next equation:
Equation 11
r
f W
A F n
Q C M
f
⋅ ⋅
⋅ ⋅ ⋅
= ∆
6 10
where:
∆f
= frequency change in Hz,
M
W
= apparent molar mass of the depositing species in grams/mole,
C
f
= Sauerbrey’s sensitivity factor for the crystal used,