Copyright © 2018 Authors. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. International Journal of Engineering & Technology, 7 (4.18) (2018) 277-282 International Journal of Engineering & Technology Website: www.sciencepubco.com/index.php/IJET Research paper Fabrication of Nickel Oxide Nanowall Network Films at Different Annealing Temperatures for Humidity Sensing Applications M.H. Mamat 1,2 *, N. Parimon 1 , M.A.R. Abdullah 1, , A.S. Ismail 1 , M.F. Malek 2 , W.R.W. Ahmad 1 , A.S. Zoolfakar 1 , A.B. Suriani 3 , M.K. Ahmad 4 , N. Nayan 4 , I. B. Shameem Banu 5 , R. Amiruddin 5 , M. Rusop 1,2 1 NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia 2 NANO-SciTech Centre (NST), Institute of Science (IOS), Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia 3 Nanotechnology Research Centre, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris (UPSI), 35900 Tanjung Malim, Perak, Malaysia 4 Microelectronic and Nanotechnology – Shamsuddin Research Centre (MiNT-SRC), Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia (UTHM), Batu Pahat, Johor, Malaysia 5 Department of Physics, B.S. Abdur Rahman Crescent Institute of Science & Technology, Vandalur, Chennai 600 048, India *Corresponding author E-mail: mhmamat@salam.uitm.edu.my Abstract Nickel oxide (NiO) nanowall network films were successfully prepared on indium tin oxide (ITO) glass substrates by sonicated sol-gel immersion method using a precursor solution of nickel nitrate hexahydrate. The NiO nanowall network films were annealed at different annealing temperature that ranges from 300 ⁰C to 500 ⁰C. The effects of annealing temperature on the structural, optical and humidity sensing properties of NiO nanowall network films were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), ultraviolet-visible (UV-vis) spectroscopy and humidity sensor measurement system. The X-ray diffraction patterns revealed that the grown NiO nanowall network films have a crystalline cubic structure. The UV-vis spectra demonstrates that the average transmittance value of all samples in the visible region are high and exceeded 90% transmission. The optical bandgap energy of NiO nanowall network films ranged from 3.76 to 3.77 eV. Results obtained showed that the humidity sensing performance of NiO nanowall network films are very promising and could be tuned by annealing temperatures. Keywords: nickel oxide; nanowall network film; structural properties; optical properties; humidity sensing. 1. Introduction Humidity sensors have received great attention in various fields including meteorology, chemical, agriculture, and medicine, which require the continuous monitoring of water vapour concentration. The humidity level is normally measured as relative humidity (RH), which defines as the pressure of actual vapour over the pressure of saturated vapour at a specified temperature. Among the desirable properties of humidity sensors are high sensitivity and reliability. Various materials have been explored for humidity sensor applications. These materials include polymer, photonic crystal, electrolytes, and metal oxides [1-5]. Particularly metal oxide-based humidity sensor, the performance of the device is very promising. Since the performance of the humidity sensor is closely related with availability of the surface area to react with water molecules, the metal oxide that has size in nano is favorable [6, 7]. The performance of sensors is also influenced by the morphology and structure of the metal oxide [8, 9]. Thus, the development of outstanding nanostructured metal oxide with good humidity sensing capability is very important to achieve high device performance. Various metal oxides have shown great response to humidity such as zinc oxide (ZnO), tin oxide (SnO), titanium dioxide (TiO 2 ) and nickel oxide (NiO) [10-13]. NiO is one of the attractive materials with potential applications in solar cell, water splitting, and sensors [14-16]. NiO has p-type conductivity in native and it has structure similar with sodium chloride (NaCl). NiO can be fabricated by various physical and chemical approaches including sputtering, evaporation, chemical vapour deposition, and sol-gel [17-23]. Particularly in nanowall network configuration, the humidity response using NiO film is expectedly to enhance due to high surface area availibity and porous surface. The three- dimensional mesoporous nanowall with well-connected structure also offers a numerous active sites on the exposed surface for humidity sensing applications. There are several techniques used to enhance the surface properties of NiO nanostructures. Among them are doping process, metal-coating, and heat treatment [24- 26]. Among these approaches, tailoring of NiO nanostructures by heat treatment presents a facile and good potential for tuning the response and sensitivity during humidity sensing. Accordingly, NiO nanowall network films with different annealing temperatures were prepared in the present study and their humidity sensing properties were investigated. The nanowall