Substrate Temperature Effects on Room Temperature Sensing Properties of Nanostructured ZnO Thin Films

Research summary

Ammonia is produced and used at a huge industrial scale, and accidental exposure can cause severe harm, so reliable low-temperature ammonia sensors are valuable for safety monitoring. This study looks at zinc oxide (ZnO) thin films deposited by chemical spray pyrolysis, a simple and cost-effective deposition method, and asks how the substrate temperature used during deposition shapes the resulting film’s structure and its ability to sense ammonia at room temperature.

ZnO thin films were deposited on glass substrates at three substrate temperatures: 523 K, 623 K, and 723 K.

What the study examined

• Crystal structure and preferred orientation of ZnO films deposited at 523, 623, and 723 K, using X-ray diffraction
• Grain shape and surface texture using scanning electron microscopy
• Optical transmittance and band gap, and electrical conductivity, across the three deposition temperatures
• Surface wettability (contact angle) of the films
• Room-temperature sensing response and selectivity toward ammonia versus other gases

Main findings

All three films were polycrystalline with a hexagonal wurtzite crystal structure, but their preferred crystal-plane orientation and crystallite size (ranging from about 13 to 58 nm) changed with deposition temperature. Grain morphology shifted from uniformly distributed spherical grains at lower temperature to a pebble-like texture at the highest temperature. Optical transmittance reached 85% for the film deposited at 723 K, and the optical band gap increased from 3.15 to 3.23 eV as deposition temperature rose. Electrical conductivity also improved with higher substrate temperature, and the films became increasingly hydrophobic, with contact angles rising from 92 degrees to 128 degrees across the temperature range.

Ammonia-sensing tests showed the films were highly selective for ammonia over other gases. Response values of roughly 1175, 5263, and 27 were measured toward 100 ppm ammonia for films deposited at 523, 623, and 723 K respectively, with the 623 K film giving by far the strongest response. That film also showed practical response and recovery times of 27 and 19 seconds. The drop in response at the highest deposition temperature is attributed to the larger crystallite size and associated changes in the film’s electronic structure at that condition.

Why it matters

The results show that substrate temperature alone, without any change in precursor chemistry, is enough to shift a ZnO thin film from a mediocre to a strongly selective ammonia sensor. Because spray pyrolysis is inexpensive and scalable, identifying 623 K as the optimal deposition condition gives a straightforward, low-cost route toward better room-temperature ammonia sensors for industrial and environmental safety monitoring.