Nickel oxide (NiO) thin film optimization by reactive sputtering for highly sensitive formaldehyde sensing

•Optimization of NiOx thin film using reactive sputtering.•Four samples with different Ar:O2 ratio during reactive sputtering were prepared.•It was observed that gas sensing behavior strongly depends on the Ar:O2 ratio during deposition.•Highly sensitive thin film with limit of detection (LOD) of 50 ppb is reported.•Developed sensing film exhibited Fast response and recovery time with long term stability. Trace level detection of formaldehyde (HCHO) is of utmost importance due to its harmful effects (carcinogenic) on humans. In the present work, we have deposited nickel oxide (NiO) using reactive sputtering technique with varying Ar:O2 ratio. It was found that NiO developed with Ar:O2 ratio of 70:30 exhibited the best response (operating temperature 200 °C) for formaldehyde. Developed metal oxide material is highly sensitive to formaldehyde with limit of detection as low as 50 ppb. Developed films were characterized for crystal structure using XRD, and surface morphology using AFM and SEM. Crystallographic assessment confirms the presence of face centered cubic phase of NiO and surface morphology of the film clearly shows the granular structure of the metal oxide film. Deposited NiO is found to be p-type which is confirmed by hotpoint probe, hall measurement as well as gas sensing behavior. The developed material was tested for various other indoor gases such as benzene (C6H6), carbon monoxide (CO), toluene (C7H8), and ammonia (NH3) and the material exhibited high selectivity towards HCHO. HCHO gas concentration ranging from 0.3 ppm to 2.5 ppm was tested on the sample. The material also showed good stability over the period of 3 months.