Tuning Selectivity through Cobalt Doping in Spray Pyrolysis Deposited ZnO Thin Films

Research summary

Chemiresistive metal oxide gas sensors are widely used, but selectivity — distinguishing one gas from another in a mixed environment — remains a persistent weak point compared with their sensitivity. This study asks whether doping zinc oxide (ZnO) thin films with cobalt can be used deliberately to steer which gas the sensing film favors.

What the study examined

• Undoped and cobalt-doped ZnO thin films deposited on glass by spray pyrolysis at 523 K
• Cobalt doping concentration varied from 0.2 to 1 wt%
• Crystal structure (XRD), grain morphology (SEM), and optical band gap across the doping range
• Aroma-sensing response of undoped and doped films toward acetaldehyde and ethanol

Main findings

Across all doping levels the films kept a similar crystallite size (19-25 nm) and a preferred crystal orientation, but the optical band gap consistently decreased as cobalt concentration increased. Grain morphology also became less uniform with doping.

The gas-sensing tests showed that the doping concentration changed which vapor the film responded to more strongly: at some cobalt levels the film favored acetaldehyde, while at others ethanol produced the larger response. In other words, selectivity tracked with doping concentration rather than staying fixed.

Why it matters

Because doping level, not just the choice of dopant, could shift which gas a ZnO sensor favors, cobalt doping concentration is a practical design variable for engineers trying to build sensors that respond to one target gas rather than several similar ones — a step toward more selective, lower-cost metal oxide gas sensors.