Thickness Dependent Room Temperature Sensing Properties of Spray Pyrolysis Deposited Nanostructured ZnO Thin Films

Research summary

Film thickness is known to influence how well a metal oxide thin film performs as a gas sensor, but it is often changed indirectly, as a side effect of other deposition parameters, making it hard to isolate its role. This study deposits ZnO thin films by spray pyrolysis using zinc nitrate hexahydrate, a precursor salt less commonly used for this application than the standard zinc acetate, and varies substrate temperature specifically to control film thickness and examine its effect on room-temperature gas sensing.

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

What the study examined

• ZnO thin films deposited by spray pyrolysis with zinc nitrate hexahydrate as the precursor salt
• Crystal structure and preferred plane orientation across the three deposition temperatures, using X-ray diffraction
• Grain size and film thickness, measured using a weighing method and confirmed by profilometry
• Optical band gap of each film
• Room-temperature sensing response and detection limit toward acetaldehyde vapor

Main findings

All films were preferentially oriented along the (0 0 2) crystal plane, though the strength of that orientation varied with substrate temperature. Grain size increased with substrate temperature, while film thickness decreased, giving the researchers a way to tune thickness through a single, easily controlled deposition parameter. Optical band gap values were close together across the samples, ranging from 3.12 to 3.19 eV.

The film deposited at 723 K was both the thinnest and had the highest electrical resistance among the three samples. This film gave the best room-temperature response to acetaldehyde vapor and the lowest detection limit measured in the study, 5 ppm. The authors attribute this to the higher-resistance film acting as a better electron acceptor, which makes it more sensitive to reducing gases like acetaldehyde, and to the thinner film’s larger surface-to-volume ratio.

Why it matters

By deliberately linking substrate temperature to film thickness and then to sensing performance, the study identifies film thickness as a practical, controllable design variable for spray-pyrolysis ZnO sensors, even when made with a less-studied precursor salt. This gives engineers a straightforward route to lower detection limits for acetaldehyde, a common indoor air pollutant, using an inexpensive and scalable deposition method.