Revised Manuscript 1 2 3 4 5 Structural, optical and mechanical 6 7 properties of cobalt copper oxides 8 9 coatings synthesized from low 10 11 concentrations of sol-gel process 12 13 Amun Amri 1,* , Zhong-Tao Jiang 2 , Chun-Yang Yin 3 , Ahmad Fadli 1 , M. Mahbubur Rahman 2 , Syaiful Bahri 1 , 14 Hantarto Widjaja 2 , Nicholas Mondinos 2 , Tutut Herawan 4 , M. Miftahul Munir 5 , Gadang Priyotomo 6 15 1 Department of Chemical Engineering, Universitas Riau, Kampus UR Panam, Pekanbaru, Riau 28293, Indonesia 16 2 School of Engineering & Information Technology, Murdoch University, 6150 WA, Australia 17 3 School of Science & Engineering, Teesside University, Middlesbrough, United Kingdom 18 4 Universitas Teknologi Yogyakarta, and AMCS Research Center, Yogyakarta, Indonesia 19 5 Department of Physics, Institut Teknologi Bandung (ITB), Bandung, Indonesia 20 6 Indonesian Institute of Sciences (LIPI), Serpong, Tangerang, Indonesia 22 23 Received ...., revised ...., accepted .... 24 Published online ..... 25 26 Keywords Cobalt copper oxides, low concentration sol-gel coating, structural, absorptance, nanoindentation. 27 28 29 *Corresponding author: amun.amri@eng.unri.ac.id , Phone: +62 823 8176 0767, Fax: +62 761 566 937 30 31 32 Abstract. Thin films of CoxCuyOz have been coated on 33 aluminium substrates via sol-gel route using low 34 concentration of copper and cobalt precursors at anneal- 35 ing temperatures in range of 500°C - 650°C. The coatings 36 were characterized by X-ray diffraction (XRD), X-ray 37 photoelectron spectroscopy (XPS), UV-Vis-NIR spectro- 38 photometry and nanoindentation. The XRD analysis in 39 2θ-range of 30°-42° revealed that the coatings exhibited 40 low crystallinity of CoCu2O3, CoCuO2 and CuCoO2. The surface bonding structure analyzed using XPS indicated 41 that the coating contained: Cu (tetrahedral Cu + and 42 octahedral Cu 2+ ), Co (octahedral Co 3+ , tetrahedral Co 2+ 43 and mixed Co 2+ and Co 3+ ), and O (lattice, surface and 44 sub-surface oxygens). The optical properties characteri- 45 46 47 48 49 1 Introduction Cobalt and copper mixed oxides have 50 found widespread applications such as catalyst for many 51 important reactions (e.g. oxygen evolution reactions 52 (OER), the Fischer−Tropsch process, etc.) and as thermoe- 53 lectric materials [1-4]. The physicochemical, electrochemi- 54 cal, magnetic, conductivity and thermal properties have 55 been substantially studied by various researches [5-8]. A 56 major conclusion reached is that the chemical nature and 57 the compositions of the precursors, as well as the annealing 58 treatments influence the properties of these mixed oxides 59 60 61 62 63 64 65 zed using UV-Vis-NIR showed that the reflectance spectra of coatings formed a spectrally solar selective absorber profile associated with the interference peaks and the absorption edges around wavelengths of below 1.2 μm. The maximum absorptance (α =75.8%) was shown by coating synthesized at 500 °C. The mechanical properties of coatings showed that the increase of anealling temperature increased the coating’s hardness (H) and the elastic modulus (E) due to the enhancement of the [CoCuO2/CuCoO2]:[CoCu2O3 ] oxide phases ratio, as the result, an excellent stability of the wear resistance (H/E) of around ~0.035 was recorded. Copyright line will be provided by the publisher i.e properties are influenced by the preparation method [1, 3, 5, 9, 10]. The term “Co and Cu mixed oxides” will be denoted by CoO/CuO for short in the rest of the manu- script. The surface chemical activity and selectivity of the CoO/CuO are markedly affected by the precursors composition that provide a high surface area as well as small and fairly uniform particle sizes which enhance the production of a well interdispersed CoO/CuO state. Further, the interdispersed state in the mixed oxides system Copyright line will be provided by the publisher