Reversal of polarity in YSZ-based thin film potentiometric oxygen sensors

The negative electric potential difference observed in a YSZ-based thin film potentiometric sensor at a low operating temperature of 673 K is investigated. The multilayered sensor is grown sequentially using pulsed laser deposition at 998 K with Cu-Cu₂O reference electrode, YSZ electrolyte and Pt as working electrode respectively. The thickness of the Cu-Cu₂O reference layer is a critical factor in thin film sensors as the Pt₃O₄ phase stabilized along with Cu-Cu₂O can reach the YSZ interface. XRD, μ- Raman and XPS confirm the formation of the Pt₃O₄ phase on the reference side along with the Cu-Cu₂O layer. However, the pure Pt₃O₄ phase could not be stabilized in the thin film without Cu-Cu₂O. The presence of the Pt₃O₄ phase on the reference side generates a Pt-Pt₃O₄ reference couple with an equilibrium oxygen partial pressure of ~ 10⁴ ppm higher than the sample oxygen pressure of 10-10³ ppm giving rise to a negative electric potential difference with a sensitivity of -7.5 mV/decade.