WO₃-Based Thin Films Grown by Pulsed Laser Deposition as Gas Sensors for NO₂ Detection

Thin films based on tungsten oxide (WO₃) were grown by nanosecond pulsed laser deposition on alumina printed-circuit boards to fabricate electrochemical sensors for nitrogen dioxide (NO₂) detection. Samples exposed to thermal annealing (400 °C for 3 h) were also produced to compare the main properties and the sensor performance. Before gas testing, the morphology and structural properties were investigated. Scanning electron microscopy and atomic force microscopy showed the formation of granular films with a more compact structure before the thermal treatment. Features of the main WO₃ phases were identified for both as-deposited and annealed samples by Raman spectroscopy, whereas X-ray diffraction evidenced the amorphous nature of the as-deposited samples and the formation of crystalline phases after thermal annealing. The as-deposited samples showed a higher W/O ratio, as displayed by energy-dispersive X-ray spectroscopy. An Arrhenius plot revealed a lower activation energy (0.11 eV) for the as-deposited thin films, which are the most electrically conductive samples, presenting a better gas response (30% higher than the response of the annealed ones) in the investigated NO₂ concentration range of 5-20 ppm at the moderate device operating temperature of 75 °C. This behavior is explained by a larger quantity of oxygen vacancies, which enhances the sensing mechanism.