Synthesis and Characterization of NiO Nanostructured Thin Films for Gas Sensing Applications

ABSTRACT

          In this work, nanostructured Nickel Oxide thin films were successfully synthesized on glass substrates via spray pyrolysis technique.

          Effects of different molarities (0.05, 0.1 and 0.2M) of hydrated nickel chloride salt precursors solution (NiCl2.6H2O) at different substrate temperatures  were optimized for constructing NiO nanostructured based gas sensor with highest gas sensitivity.

          Characterization of synthesized nanostructured nickel oxide thin films were investigated by using X-Ray Diffractometer (XRD), UV-Visible Spectrophotometer (UV–Vis), and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectrometer (EDX).

          Based on XRD analysis, the molar concentration and substrate temperature for the prepared thin films were optimized as 0.2 M and 400 °C respectively. The XRD result displayed the formed cubic polycrystalline structure films with preferred orientation along (111) direction.    

          The UV-Vis spectrophotometry analyzing showed the energy gap for NiO thin films varied from 3.37 eV to 3.85 eV.

          The morphology from SEM analysis results revealed that the nanostructured nickel oxide thin films were in different shapes and oriented randomly with sizes ranging from 36.05 to 195.56 nm, with an average particle size was found to be 83.97 nm.        

          The stoichiometric elemental composition ratio of NiO was confirmed by EDX analysis. The overall results from this investigation suggested that the synthesized good quality crystalline nanostructured NiO thin films could be suitable for gas sensing applications.

A thermal evaporation coating system was used to make an electrical Ohmic contact for the fabricated Al/NiO/Al gas sensor.

          The gas sensitivity due to Al/NiO/Al device for detecting oxidizing gases (oxygen and nitrogen) and reducing gases (acetone and ethanol) were investigated as a function of the bias voltage in both forward and reverse of (-10 to 10) V. The fabricated Al/NiO/Al device sensor has shown high response and selectivity towards oxidizing gasses.

          At forward voltage of 10 V, the percentage in the sensitivity of the sensor increased by about 250.2 % and 112.0% when exposed to oxide gas O2 and N2, and 198.8% and 24.0% when exposed to reduce gas acetone and ethanol, respectively. Furthermore, the gas sensing mechanisms were also elucidated.