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Mass changes measurements

Investigation of the ammonia sorption mechanism by gas sensor on the base of “Quartz Crystal Microbalance with Dissipation Monitoring - Atomic Force Microscope” integrated system

The experiment shows possibilities of gas sensor based on QCM with dissipation monitoring and AFM integrated system. The bromocresol purple (BCP) and ammonia interaction is chosen as a modeling system because BCP is well-known ammonia indicator. Hafnium dioxide nanocrystals are used for increasing the indicator surface area and for easy monitoring of surface topography changes.

Equipment:

  • System for ammonia/nitrogen mixture preparing
  • Gas flow-through cell
  • QCM with dissipation monitoring, consisting of a crystal resonator, built-in gas cell, and electric circuit vector analyzer (control unit)
  • AFM with probe, approached to the QCM with dissipation monitoring crystal resonator
  • Ammonia gas analyzer

Probe NanoLaboratory NTEGRA Aura performs under controlled atmosphere environments.

Sample preparing

The sample was prepared by the following steps:

  1. Hafnium dioxide nanocrystals were dispersed in bromocresol purple aqueous-alcoholic solution by volumetric dispersant.
  2. The colloidal solution was deposited on the gold surface of the quartz resonator electrode and dried out. As a result the insular agglomerates were formed scaled 500 nm to 1500 nm, which consisted of clusters of 20-80 nm scale at the surface.
An appropriate surface area was chosen with the help of the AFM’s optical microscope.


Optical microscope image of the chosen surface area Area size: 500 х 350 um

AFM topography of the chosen area Size: 10 х 10 um

Basic frequency of oscillation with the deposited substance: 5,007,536.3 Hz Quartz frequency stability on 5 minutes interval: about +/-0.06 Hz

The experiment

The crystal resonator with the sensor material coating is placed in a gas flow-through cell. The ammonia and nitrogen mixture is injected into the cell. The gas analyzer shows an ammonia concentration equal to 279 mg/m3 on the output of the cell. The BCP film adsorbs the ammonia that is confirmed by the resonator mass increasing (see the graphs below).

After the ammonia was stopped the system is purged with nitrogen. The desorption occurred at this stage is confirmed by the resonator coating mass decreasing and corresponding quartz resonator oscillation frequency increasing. Some of the ammonia was adsorbed irreversibly, as shown at the graphs.

The graphs below represent oscillation frequency and mass changes vs time. The mass was calculated according to the Sauerbrey model.

Quartz resonator frequency changes during the experiment

Mass changes during the experiment

Maximum mass of the adsorbed ammonia in the experiment is 86 ng

The images below illustrate three experiment stages.

In the AFM images it is clearly seen that the topography of hafnium dioxide coated with BCP film changes under the ammonia influence.

For further verification, each AFM image is coupled by cross-section profile of the same surface area at the different experiment stages.

AFM images of cluster topography: 3.5 х 3.5 um

Cluster section profile: 2 um

Stage 1. t = 0 ÷ 1200 sec
Initial condition. Ammonia absence

Stage 2. t = 1200 ÷ 2200 sec
Sorption during ammonia and nitrogen mixture gas injection

Stage 3. t = 2200 ÷ 3200 sec
Desorption while pure nitrogen injection after ammonia was stopped.

The experiment shows possibilities of the gas sensor on the base of QCM with dissipation monitoring and AFM integrated system. AFM and QCM with dissipation monitoring integration allows one to carry out simultaneous monitoring of the surface structure changes and corresponding changes of the sorbed analyte gas integral mass.

The experiment was carried out by Dr. Sevastyanov V.G., Popov V.S., Shelaev. A.V. (N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences)

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