Impact of annealing temperature on the response and sensitivity of spinel ZnFe₂O₄ thin film to ammonia gas sensing at room temperature

The lack of highly efficient, cost-effective, and stable ammonia (NH₃) gas sensors capable of operating at room temperature with trace-level detection capabilities remains a significant challenge for the development of next-generation gas sensors. One of the primary obstacles is the excessive sensitivity required for detecting low concentrations of NH₃. In this work, nanostructured ZnFe₂O₄ (ZF) thin films with favorable surface characteristics have been developed to enable the trace-level detection of NH₃ at room temperature. ZF film was deposited via the chemical spray pyrolysis method, and the deposited film was annealed at different temperatures (300–500 °C) to evaluate their gas sensing performance. A systematic investigation was conducted to explore the relationship between the morphology of ZF films and their sensor performance. The film annealed at 400 °C (ZF400) exhibited a remarkable NH₃ sensing response, achieving a value of 6.2 at 1 ppm, which improved five-fold compared to the as-deposited film (1.15). Particular attention is paid to nanorods with angular morphology, reduced crystallite size, and enhanced surface roughness, all of which strongly influence the gas-sensing potential of ZF400. Additionally, the sensor's selectivity (90 % relative selectivity), sensitivity (5.56 ppm⁻¹), repeatability (1.74 %), stability, and humidity tolerance (3.5 % coefficient of variation) were evaluated. This work demonstrates the potential of using morphological tuning as a strategy to enhance sensor response in a time-efficient manner.