Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Quality enhancement of copper oxide thin flm synthesized under elevated gravity acceleration by two-axis spin coating Soroosh Mahmoodi a,b,c,∗ , Dhia A. Hassan d , Akbar Hojjati-Najafabadi e , Wei Li a , Liefa Liao a , Ata Jahangir Moshayedi a , Xueyu Huang a , Mehrdad Nouri Khajavi f a School of Information Engineering, Jiangxi University of Science and Technology, Jiangxi Province, Ganzhou, 341000, PR China b Soroosh Khorshid Iranian Co, Abyek Industrial Zone, Qazvin, Iran c Department of Mechanical and Electrical Engineering, School of Aerospace Engineering, Xiamen University, Xiamen, Fujian Province, 361005, PR China d Department of Chemistry, College of Education for Pure Sciences, University of Basrah, Basrah, 61004, Iraq e Faculty of Materials, Metallurgy and Chemistry, School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, PR China f Faculty of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran ARTICLE INFO Keywords: Two-axis spin coating technology Elevated gravity acceleration Surface roughness Surface free energy Copper oxide thin flm ABSTRACT Two-Axis spin coating as a new modifed technique is employed to enhance the quality and surface leveling of thin flms. The modifed technology utilizes a synthetic centrifugal force perpendicular to the surface which generates an elevated gravity acceleration while spreading the coating on the entire wafer surface. In this paper, copper acetate sol-gel is coated by conventional and Two-Axis spin coating techniques. The coated layers are sintered in an air furnace at 275 °C. The fabricated layers are characterized by GIXRD, EDX, AFM and SEM devices. Wettability and Surface Free Energy (SFE) of sintered flms using the contact angle technique are measured, and evaluated by the Owens-Wendt method. XRD and EDX spectra show a higher intensity of copper oxide phase using Two-Axis spin coating technology. AFM micrographs showed an improvement in the surface leveling within the Two-Axis spin coated layer. A comparison between the SFE of conventional and Two-Axis spin coated layers shows an increase in SFE of the layer synthesized under 200g artifcial gravity acceleration. 1. Introduction Cuprite (CuO 2 ) and Tenorite (CuO), as inorganic metallic materials, have a wide range of usage in technological industries. Cuprite is a member of the cubic p-type semiconductors family with a band gap around 2.0 eV; tenorite is a member of monoclinic p-type with a narrow band gap of 1.2–-1.5 eV [1–4]. The oxygen absorption of cuprite, te- norite, and their nano phasic chemical reactions classifed them to be used as the catalyst devices for environmental reactions [5–13]. A change in the surface conductivity of copper oxide layers exposed by various types of gases and liquids, causes the layers to be used in the gas sensing instruments [14–19]. The contact angle of liquids is a method to measure the SFE and wettability of layers. A higher SFE of the copper oxide flm leads to a higher wettability and lower interfacial contact angles [20,21]. CuO 2 thin flms are one of the interest layers of pho- tovoltaic and solar cell industries. Recently, optical industries have paid great attention to transparent conductive oxide thin flms due to their low cost and high optical absorption efciencies. The surface leveling of layers leads to an increase in the transparency and efciency of photovoltaic cells [22,23]. Sintering temperature, curing time, rate of annealing, and applied protecting gas can be afected on the size of copper oxide particles and transform the nano phasic crystallites from cuprite to tenorite and vice versa [24]. Conventional spin coating is a low-cost method has been employed to fabricate conductive and semiconductive flms in a wide range of thicknesses such as copper oxide layers. However, the thickness of flms could not be well-controlled and leads to a probable porosity and cracks as well as secondary phases [25,26]. In this study, two diferent methods of spin coating, conventional and Two-Axis spin coating techniques, are employed to create copper oxide layers experimentally. The Two-Axis spin coating method bene- fts an elevated gravity acceleration while spreading the coating on substrate simultaneously. The elevation of gravity acceleration pro- poses penetration of copper microcrystals inside the coated layer before complete evaporation of solvents, and prevent the deposited sol to be thrown out by the horizontal centrifuge acceleration of conventional spinning. The elevation of gravity stimulates copper microcrystals to https://doi.org/10.1016/j.ceramint.2019.11.238 Received 6 October 2019; Received in revised form 26 November 2019; Accepted 26 November 2019 ∗ Corresponding author. School of Information Engineering, Jiangxi University of Science and Technology, Jiangxi Province, Ganzhou, 341000, PR China. E-mail address: soroosh.mahmoodi@yahoo.com (S. Mahmoodi). Ceramics International 46 (2020) 7421–7429 Available online 27 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved. T