Electrochromic performance, wettability and optical study of copper manganese oxide thin films: Effect of annealing temperature
Graphical abstract
The nanostructured copper manganese oxide (CMO) thin films were prepared by acetate based sol–gel precursors dip-coating method with a Cu(ac)2/Mn(ac)2 molar ratio of 20/80. CMO thin films were deposited on glass and indium tin oxide (ITO) substrates and heat treated in ambient air at 300, 400 and 500 °C and named CMO_3, CMO_4 and CMO_5, respectively. With varying annealing temperature, the surface nature of thin films was changed from hydrophilic (CMO_3, CMO_4) to hydrophobic (CMO_5). The electrochromic performance including the anodic and cathodic charge density and capacitance of CMO electrode calculated by cyclic voltammetry measurements in potential range of −1–1 eV at different scan rates ranging from 10 to 100 mV/s. The annealed CMO thin film at 500 °C with maximum anodic and cathodic charge density and interfacial capacitance showed better electrochromic performance with respect to other electrodes at lower scan rate. Also, the increasing annealing temperature affected on crystal structure, optical constants and optical band gap.
Introduction
The development of nanostructured materials open up a new era in basic research and solid state technology. The nanostructured materials with notable increased surface area are expected to show novel physical and chemical properties. Among the nanostructured metal oxide materials, manganese oxides occupy a prominent place due to various valence states and crystalline structures such as MnO, Mn2O3, MnO2, and Mn3O4 [1]. Manganese oxides have attracted great research interest in the diverse field such as electrodes in batteries [3], [4] catalysts [5], [6], magnetic properties for removal of harmful inorganic contaminants from water [7] and biosensor [8]. Also, literature reports indicate that there have been considerable researches on the preparation of the manganese oxides in the form of thin films due to their applications in the electrochromic materials [9], [10], electrochemical supercapacitors and energy storage layers [2], [11]. Manganese oxide thin films have been prepared using various physical and chemical methods such as electron beam deposition [10], atomic layer deposition (ALD) [12], molecular beam epitaxy (MBE) [13], electrochemical deposition [14], potentiostatic anodic electrolysis [15], chemical spray pyrolysis technique [16], chemical bath deposition [17], sol–gel method [18], [19] and so on. The results indicated that the physical properties of manganese oxide thin films are very sensitive to the preparation method and conditions. Among the mentioned methods, sol–gel dip coating is a useful coating technique due to the low cost of preparation and homogeneity of the final products. Formation of a thin film with homogeneous surface is an important parameter for improvement of surface properties such as adhesion, wettability, surface reactivity and catalytic activity. In recent years, significant efforts have been made to develop nanostructured manganese oxides by doping or mixing with other materials. A number of experiments have investigated manganese oxides doped with Pd [20], V [21], Co [22], Ag [23] and Zn [24] to provide more sophisticated control over the catalytic, supercapacitor and magnetic applications. Also, there are experimental results of the mixed manganese oxides such as Mn3O4/MWCNT composite for lithium-ion batteries [25], Mn3O4/graphene (GM) composites and MnO2:La/MWCNTs composite for supercapacitors [26], [27], [28], Mn2O3/TiO2 and MnO2/TiO2 nanocomposites for photocatalytic activity and Ni and Co doped CuMn2O4 for magnetic application [29], [30], [31]. Copper manganese oxides (CMOs) compounds are also a kind of mixed manganese oxides that are widely studied for catalytic application. In recent decade, CuxMn3 − xO4/polypyrrole(PPy) composite, Au supported on Al2O3–CuO–Mn2O3 composite, Co/CuMnOx, CuxMn3 − xO4 compounds [32], [33], [34], [35], [36], [37], [38], [39] have been proposed for this application. To the author's knowledge, there is little information available in the literature on the optical properties of copper manganese oxides (CMOs) thin films [18], [40] and there have been no reports on the study of electrochromic performance and wettability of CMO thin films. Therefore, the preparation of CMO thin films for different applications such as unwettable layers and electrochromic electrode is worthwhile.
In the present work, the copper manganese oxide (CMO) thin films were prepared by a simple sol–gel process from metal acetates and organic solvent and deposited on glass and indium tin oxide (ITO) substrates by dip-coating technique. In this study, for the first time, the effects of heat treatment on electrochromic, wettability, optical, structural and morphological properties of CMO thin films were investigated. The mentioned properties were examined by cycling voltammetry, contact angle measurement (CA) and UV–vis spectrophotometry, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy analyses, respectively. Furthermore, the anodic and cathodic charge density was determined by CV measurements at different scan rate. The optical band gap and optical constants of the CMO thin films were calculated by transmittance and reflectance spectra.
Section snippets
Film preparation
The nanocomposite CMO thin films were prepared on glass and ITO substrates using the sol–gel dip-coating technique. In the preparation of the binary solution (0.4 M), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, Merck) and copper acetate monohydrate (Cu(CH3COO)2·H2O, Merck) were used as procurers with a Cu(ac)2/Mn(ac)2 molar ratio of 20/80. Firstly, Mn(CH3COO)2·4H2O and Cu(CH3COO)2·H2O were simultaneously dissolved into moderate amount absolute ethanol at room temperature under continuous
Structural studies
The XRD analyses indicated that the prepared CMO thin films are amorphous. Hence, in order to identify crystalline phases of CMOs at different annealing temperatures, the thicker copper manganese oxide (CMO) and manganese oxide (MO) samples were prepared and scratched. Then, X-ray diffractions were taken of the CMO and MO scratched films. The XRD patterns of the pure manganese oxide (MO) films, without additional dopant, were used to determine and identify the crystalline phases of the MO films
Conclusion
The nanostructured copper manganese oxide (CMO) thin films were deposited on glass substrates by sol–gel method and using the dip-coating technique. The CMO films are prepared with molar ratio of 20/80 of Cu(ac)2 to Mn(ac)2. Heat treatment of nanostructured CMO thin films was performed in ambient air at different temperatures (300, 400 and 500 °C). The XRD patterns of the CMO_3 (annealed at 300 °C), CMO_4 (annealed at 400 °C) and CMO_5 (annealed at 500 °C) thin films represent amorphous structure.
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