Spray Deposited Nanostructured Zinc Oxide Thin Film as Room Temperature Ethanol Sensor: Role of Annealing

Research summary

Room-temperature gas sensors are attractive because they avoid the extra power draw and complexity of a heating element, but achieving a useful response without heating is harder to engineer. This short study looks at whether the post-deposition annealing temperature of a spray-deposited, zinc-dominant ZnO thin film can be used to unlock a room-temperature ethanol response.

What the study examined

• A nanostructured, zinc-dominant thin film deposited on glass by spray pyrolysis, then annealed separately at 523 K, 623 K, and 723 K for 6 hours in air
• Crystal structure and crystallite size across the three annealing temperatures, using X-ray diffraction
• Surface morphology of the as-deposited and annealed films, using field-emission electron microscopy
• Optical transmittance and band gap of the annealed films
• Room-temperature ethanol sensing response of each annealed film, including its response and recovery behavior

Main findings

Annealing converted the initially zinc-dominant film into polycrystalline ZnO with a hexagonal wurtzite structure, and crystallite size grew steadily with annealing temperature, from about 12 nm at 523 K to 21 nm at 723 K. The surface morphology also changed markedly: the as-deposited film’s sprout-like nanostructures gave way to more uniformly distributed nanospherical grains as annealing temperature increased, though grain boundaries widened further at the highest temperature.

Despite all three annealed films being well-formed ZnO, only the film annealed at 623 K produced a meaningful room-temperature response to ethanol vapor; the films annealed at 523 K and 723 K showed essentially no response. The 623 K film reached a response of 3.73 toward 50 ppm ethanol and 1.69 toward 10 ppm, recovering back to baseline resistance once the ethanol vapor was removed. The authors attribute this to the uniformly distributed, well-connected nanospherical grains specific to that annealing condition.

Why it matters

The results show that annealing temperature alone, without any dopant or elevated operating temperature, can be the deciding factor in whether a ZnO thin film senses ethanol at room temperature at all. That makes annealing a simple, low-cost parameter for tuning spray-deposited ZnO sensors, and the reported response compares favorably with other room-temperature ethanol sensors from the literature.