Pseudo n-type behaviour of nickel oxide thin film at room temperature towards ammonia sensing

Sensors are part of a safe laboratory, working space and closed environment. In view of this, a sensing material for ammonia (NH₃) vapour, nickel oxide (NiO) has been investigated to improve its quality as a sensor. The transparent nanostructured NiO thin films were deposited on glass substrates by sol-gel spin coating method at different molar concentrations (0.4 M, 0.6 M and 0.8 M). The NH₃ sensing studies reveal that 0.6 M film shows admirable response of 403 (75 ppm), and also it has good response and recovery times (110 s and 33 s) for 25 ppm at room temperature. These features of the film are attributed to low surface roughness, small grain size and higher surface to volume ratio compared to other films. In presence of air ambience, the electrons at film surface are chemisorbed by oxygen molecules and thereby cause an increase of the depletion layer. Once the film is exposed to the analyte gas NH₃, the depletion layer decreases because of electrons returning back to the conduction band (CB). These electrons are then de-excited to the valence band (VB) and recombine with holes (annihilation of holes). At the same time, the trapping of electrons by Ni(OH)₂ and creation of holes by oxidation of Ni²⁺ into Ni³⁺ increases the hole concentration at VB, followed by a reduction of recombination rate of electrons and holes. All these processes decrease the potential barrier between the grains and thereby cause the Fermi level (E_F) to shift towards VB. These processes remarkably enhance the sensing behaviour of the film. Importantly, the prepared NiO thin film behaves as a n-type sensor at room temperature for NH₃; and therefore, termed as pseudo-n-type.