NOVA

EQCM Tutorial

Version 1.11.0
 NOVA EQCM tutorial

1 – The Electrochemical Quartz Crystal Microbalance (EQCM)

The EQCM is an optional module for the Autolab PGSTAT and Multi Autolab. The
EQCM module provides the means to perform Electrochemical Quartz Crystal
Microbalance measurements. The EQCM module measures a mass change per unit
area by measuring the change in resonant frequency of a quartz crystal.

Quartz crystals belong to a group of materials displaying the so-called piezoelectric

effect. When a properly cut crystal (AT-cut) is exposed to an AC current, the crystal
starts to oscillate at its resonant frequency and a standing shear wave is generated.

In first approximation, the resonant frequency depends on the thickness of the

crystal. As mass is deposited on the surface of the crystal, the thickness increases;
consequently the frequency of oscillation decreases from the initial value. With some
simplifying assumptions, this frequency change can be quantified and correlated
precisely to the mass change using Sauerbrey's equation:

2𝑓𝑓02
∆𝑓𝑓 = − ∙ ∆𝑚𝑚
𝐴𝐴�𝜌𝜌𝑞𝑞 𝜇𝜇𝑞𝑞

Where ∆𝑓𝑓 is the change in frequency, in Hz, 𝑓𝑓0 is the nominal resonant frequency
of the crystal (6 MHz), ∆𝑚𝑚 is the change in mass, in g/cm2, 𝐴𝐴 is the area of the crystal
in cm2, 𝜌𝜌𝑞𝑞 is the density of quartz, in g/cm3 and 𝜇𝜇𝑞𝑞 is the is the shear modulus of
quartz, in g/cm·s2.

For a 6 MHz crystal, the same equation can be reduced to:

−∆𝑓𝑓 = ∆𝑚𝑚 ∙ 𝐶𝐶𝑓𝑓

Where 𝐶𝐶𝑓𝑓 is 0.0815 Hz/ng/cm2.

More information about the EQCM module can be found in the EQCM user manual.

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2 – Hardware setup
In order to use the EQCM module, the hardware setup in NOVA must be configured
accordingly (see Figure 1).

Figure 1 – Selecting the EQCM module

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3 – Measuring the EQCM signals

Regardless of the type of crystal connected to the EQCM oscillator, a total of three
electrochemical signals can be selected in the Sampler (see Figure 2):

• EQCM(1).Temperature
• EQCM(1).Driving force
• EQCM(1).∆Frequency

Figure 2 – The signals provided by the EQCM are available in the sampler

The temperature signal is measured by the sensor located at the bottom of the
EQCM cell. This signal is provided in °C. The driving force signal corresponds to a
voltage value between 0 V and 2.5 V. This value represents the amount of energy
required to sustain the oscillation of the crystal. When the loading of the crystal
increases, the driving force also increases. In air, the typical driving force is close to
0 V. In water, the driving force is about 0.85 V. The ∆Frequency signal corresponds
to the relative change in oscillation frequency of the quartz crystal. This variation is
expressed with respect to an arbitrary, user-defined reference frequency (zero Hz).

Note

The electrochemical signals provided by the EQCM are not sampled through the
ADC164 module. These signals are directly provided by the EQCM module. This
means that these signals cannot be sampled in ‘optimized mode’.

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Warning

The highest possible sampling rate of the EQCM module is 50 samples/s (20 ms
interval time). If measurements with shorter interval times are performed, the last
values of the EQCM signals will be recorded multiple times until new samples are
obtained for these signals.

4 – Using the EQCM

Before the signals from the EQCM module can be sampled in a procedure, the
EQCM module must be switched ON. The ON/OFF control of the oscillator is
available in the Autolab control command, on the EQCM tab (see Figure 3).

Figure 3 – The Autolab control window can be used to switch the EQCM module ON or OFF

Once the EQCM has been switched on, the electrochemical signals provided by the
EQCM module can be sampled, using any available measurement command (except
measurement commands using the FRA2/FRA32M module or the fast-sampling ADC
module 1).

1
ADC750 or ADC10M.

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5 – Zeroing the ∆Frequency

It is common practice to measure the relative frequency change with respect to a
control situation. In order to do this, the user has to control the potential relative to
the reference electrode or control the current flowing through the working
electrode coated on the quartz crystal with the Autolab potentiostat/galvanostat
and set the frequency change to 0 Hz. All consequent frequency changes will then
be measured with respect to the defined control situation. A dedicated command,
Reset EQCM ∆Frequency, is available for this purpose.

This command is located in the Measurement – General group of commands (see

Figure 4).

Figure 4 – The Reset EQCM ∆Frequency command can be found in the Measurement –
General group

Note

The Reset EQCM ∆Frequency command is an intermediate command and will

not be visible if the Basic profile is active. Please adjust the Profile scheme
accordingly.

This command can be added anywhere in a procedure. Whenever the Reset EQCM
∆Frequency command is used, a window allowing fine tuning of the EQCM driving
force and zeroing of the ∆Frequency value will be displayed (see Figure 5).

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Figure 5 – The Determine EQCM zero frequency window

The Determine EQCM zero frequency window contains three sections:

The topmost section, shown in Figure 6, displays real time information on the
measured signals provided by the EQCM:

Figure 6 – The topmost section of the Determine EQCM zero frequency window displays the
measured values in real time

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• EQCM(1).ΔFrequency (Latest value, Averages): these fields show the

latest and averaged ΔFrequency values measured by the EQCM,
respectively. The average is obtained from a moving average over the last
ten values.
• EQCM(1).Driving force (Latest, Averages, Minimum): these fields show
the latest, the averaged and the minimum value of the driving force,
respectively. The driving force is a measure of the sustainability of the
oscillation. The average value is obtained from a moving average over
the last ten values. The measurable range of driving force is between 0 V
and 2.5 V. The minimum field shows the absolute minimum value
recorded.
• EQCM(1).Temperature (Latest, Averages): these fields show the latest
and averaged temperature values measured by the EQCM, respectively.
The average is obtained from a moving average over the last ten values.
• Time to average: this field indicates the number of values used in the
moving average determination (hardcoded to 10).

The middle section, shown in Figure 7, displays the recorded values of the driving
force plotted versus time.

Figure 7 – The middle section of the Determine EQCM zero frequency window displays the
measured driving force plotted versus time

The bottom section, shown in Figure 8, displays a series of buttons that can be used
to control the behavior of the Determine EQCM zero frequency:

Figure 8 – The bottom section of the Determine EQCM zero frequency window displays the
measured values in real time

• Zero Δf: set the value of the measured ΔFrequency to zero. Setting the
value to zero requires several iterations. During this adjustment, the
button will be grayed out.
• Clear plot: this button can be used to clear the plot displayed in the
window. The measurement resumes after the button is pressed. The
values shown in the topmost part of the window are not cleared.

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• Abort: closes the Determine EQCM zero frequency window and

terminates the complete procedure.
• OK: closes the Determine EQCM zero frequency window and proceeds
with the rest of the procedure.

Using the provided screw driver, the trimmer on the EQCM oscillator should always
be adjusted in order to minimize the driving force of the QCM crystal. This should
always be performed every time the environment of the crystal is changed. The
driving force value is displayed in the plot area of the Determine EQCM zero
frequency window.

The Clear plot button can be pressed at any time to clear the display. The
button can be used to zero the ∆Frequency signal. Pressing the Abort button stops
the measurement.

Note

Pressing the button will perform five consecutive adjustments to 0 Hz.

While the ∆Frequency is zeroed, the button will be grayed out.

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6 – Measurements with the EQCM module

An EQCM tutorial folder is located in the Program Files\Metrohm Autolab\Nova
1.11\Shared Databases\Tutorials folder (see Figure 9). Using the database
manager, set this folder as the Standard database.

Figure 9 – Loading the EQCM tutorial database

The EQCM tutorial contains two procedures that can be used as examples of
electrochemical quartz crystal microbalance measurements (see Figure 10).

Figure 10 – Two procedures are provided in the EQCM tutorial

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6.1 – Cyclic voltammetry with EQCM

Load the Cyclic voltammetry with EQCM procedure in the procedure editor. This
procedure is designed to perform a cyclic voltammetry staircase measurement in
potentiostatic mode. At the beginning of the procedure, the EQCM module is
switched on, using the Autolab control command (see Figure 11).

Figure 11 – The cyclic voltammetry with EQCM tutorial procedure

During the cyclic voltammetry experiment, the three signals provided by the EQCM
module are sampled alongside the WE(1).Current signal. The measurement starts by
applying a pre-conditioning potential. Just before the cyclic voltammetry
measurement starts, the Reset EQCM ∆Frequency is used to fine tune the settings
of the EQCM module and to zero the ∆Frequency signal (see Figure 11).

Figure 12 shows an example of a cyclic voltammogram recorded using the Cyclic

voltammetry with EQCM procedure. This example corresponds to the bulk
deposition (over-potential deposition) of lead on a gold-coated QCM crystal, from a
0.01 M lead (II) perchlorate solution (HClO4 0.1 M). All frequency changes are
measured with respect to the zero ∆Frequency which was set while pre-
conditioning the working electrode at 0.8 V vs Ag/AgCl (KCl 3 M).

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Figure 12 – An example of EQCM cyclic voltammetry recorded using the Cyclic voltammetry
with EQCM procedure (blue curve: current, red curve: ∆Frequency)

6.2 – Chrono amperometry with EQCM

Load the Chrono amperometry with EQCM procedure in the procedure editor. This
procedure is similar to the previous one. It is designed to perform a potential step
experiment during which the signals provided by the EQCM module are sampled.
At the beginning of the procedure, the EQCM module is switched on, using the
Autolab control command (see Figure 13).

Figure 13 – The chrono amperometry with EQCM tutorial procedure

After the first potential value has been applied and the response of the cell has been
recorded using the Record signals (> 1 ms) command, the Reset EQCM ∆Frequency
is used to fine tune the settings of the EQCM module and to zero the ∆Frequency
signal (see Figure 13). After the ∆Frequency signal has been zeroed, the procedure
proceeds with the potential step and the response of the cell is sampled.

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Figure 14 shows an example of a chrono amperometry experiment recorded using

the Chrono amperometry with EQCM procedure. This example corresponds to
under-potential deposition (UPD) of lead on a gold-coated QCM crystal, from a 0.01
M lead (II) perchlorate solution (HClO4 0.1 M). All frequency changes are measured
with respect to the zero ∆Frequency which was set while pre-conditioning the
working electrode at 0.8 V vs Ag/AgCl (KCl 3 M).

Figure 14 – An example of chrono amperometry experiment recorded using the Chrono

amperometry with EQCM procedure (blue curve: current, orange curve: ∆Frequency)

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Hardware specifications
The EQCM module is an option module for the modular Autolab PGSTAT, with the
exception of the PGSTAT100, PGSTAT100N and PGSTAT302F 2. The EQCM module
is fitted with a 6 MHz crystal oscillator and it can be used to monitor changes in
frequency, with a dynamic range of 80000 Hz.

The module is also fitted with a temperature probe, which can be connected to the
temperature sensor embedded in the standard electrochemical cell provided with
the module.

Table 1 provides an overview of some of the EQCM module specifications.

Specification Value
Oscillation frequency 6 MHz
Frequency resolution 0.07 Hz
Relative accuracy 1 Hz
Absolute accuracy 10 Hz
Frequency range 80 kHz
Temperature sensor accuracy 1 °C
Temperature sensor resolution 0.1 °C

Table 1 – Overview of the specifications of the EQCM modules

Note

More information about the EQCM module can be found in the EQCM User
Manual.

2
The EQCM module cannot be fitted in the µAutolab II/III and in the PGSTAT101.

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