Oxygen Partial Pressure Induced Mass Changes in Praseodymia Ceria (PCO) Thin Films Studied by Resonant Nanogravimetry
Authors: Wulfmeier Hendrik , Schlack Sebastian , Defferriere Thomas , Seo Han Gil , Tuller Harry L. , Fritze Holger 
Published: September 2023

Oxygen non-stoichiometry, δ, is a well-known phenomenon occurring in metal oxides (e.g., in MO2–
δ) that varies with the oxygen partial pressure, p
O2, in the material’s surrounding atmosphere. Various properties e.g., the electrical conductivity, changes correspondingly with δ, as utilized in gas sensing applications. However, changes in δ can also be driven electrochemically by pumping oxygen ions into the material of interest through a solid electrolyte such as yttria stabilized zirconia (YSZ). Although the associated mass changes may be low, the associated mechanical strains induced may induce cracks in the brittle oxides or other forms of degradations. The precise knowledge of the correlation of mass change with induced strain is crucial for the development of fuel and electrolysis cells, sensors and other metal oxide based electronic devices. While classical thermogravimetric analysis (TGA) is the obvious choice for bulk samples, its resolution is insufficient for the analysis of mass changes induced in thin films by variations in δ.

In this work, a direct characterization method based on resonant nanobalances is utilized for the detection of small mass changes in ceramic thin films due to changes in p
O2. These nanobalances consist of piezoelectric high-temperature stable single crystal blanks of the langasite family (langasite, La3Ga5SiO14, LGS or catangasite, Ca3TaGa3Si2O14, CTGS) coated with Pt90Rh10 electrodes and the oxide of interest. In this study, the authors focus on thin-film praseodymia ceria solid solution (PrxCe1–xO2–
δ, PCO) as a model system. Thin films of up to 1 µm thickness, and corresponding mass of a few 100 µg, are deposited by pulsed laser deposition on the nanobalance and simultaneously on high-resistivity sapphire substrates equipped with interdigitated Pt electrodes (digit and gap width 100 µm each). The latter serve for the detection of conductivity changes. The expected total mass changes in the PCO films are in the range of several hundred nanograms, well within the sensitivity limits of the nanobalance, that exhibits resolutions on the order of few ten nanograms or a Δδ of approximately 0.01.

The resonance frequency of the nanobalances is both mass load and temperature dependent. As the films in this work are characterized at different temperatures in the range from 300 °C to 700 °C, the change of the serial resonance frequency f
R can be directly converted to mass changes Δm using the Sauerbrey relation [1,2]:

Δm = (A
eff / Sm
) Δf
R

Here, A
eff and Sm
are the temperature dependent effective resonator area and mass sensitivity of the resonator. At high temperatures, the equilibration times are sufficiently fast to determine the mass change at single p
O2 steps after equilibration of the thin PCO films. The kinetics slow down substantially towards lower temperatures. The oxygen exchange rate decreases by slightly one-to-two decades per 100 K [3,4]. Slower kinetics result in correspondingly longer time constants for reaching equilibrium, so that external disturbances during equilibration contribute to diminished resolution.

At temperatures below 600 °C, the nanobalance is used in dynamic mode where the oxygen activity within the film is not in equilibrium with the external p
O2. The p
O2 in the furnace is altered over orders of magnitude from oxygen rich atmosphere to low p
O2 atmosphere by mass flow controllers, within several tens of minutes. The attached figure shows an example where the atmosphere is changed from 25 % O2/Ar (p
O2 = 0.25 bar) to pure Ar (purity: 99.9999 %; p
O2 approximately 10–5 bar) within 15 min. The oxygen non-stoichiometry changes by Δδ = 0.095. The process is reversible as illustrated. During these changes, the resonance frequency change of the nanobalance and the conductivity change of the film are monitored simultaneously. To follow the dynamics of this process, single frequency conductivity relaxation, instead of complete electrochemical impedance spectra, are monitored. As the conductivity dependence on δ for PCO is well known, these data can be used to correlate the dynamic mass change with the effective oxygen activity in the PCO thin films.

[1] Sauerbrey: Zeitschrift für Physik, 155, 206–222 (1959)

[2] Fritze: Measurement Science and Technology, 22, 012002–012030 (2011)

[3] Schaube et al.: Journal of Materials Chemistry A, 7, 21854-21866 (2019)

[4] Nicollet et al.: Nature Catalysis, 3, 913–920 (2020)

Figure 1