Surface Electronic Structure and Mechanical Characteristics of Copper–Cobalt Oxide Thin Film Coatings: Soft X-ray Synchrotron Radiation Spectroscopic Analyses and Modeling

Abstract

Novel copper−cobalt oxide thin films with different copper/cobalt molar ratios, namely, [Cu]/[Co] = 0.5, 1, and 2, have been successfully coated on aluminum substrates via a simple and cost-effective sol−gel dip-coating method. Coatings were characterized using high resolution synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy, in combination with nanomechanical testing and field emission scanning electron microscopy (FESEM). The surfaces of both [Cu]/[Co] = 0.5 and 1 samples consisted primarily of fine granular nanoparticles, whereas the [Cu]/ [Co] = 2 has a smoother surface. The analyses reveal that the increase of copper concentration in the synthesis process tends to promote the formation of octahedral Cu 2+ which minimizes the development of octahedral Cu + , and these octahedral Cu 2+ ions substitute the Co 2+ site in cobalt structure host. The local coordinations of Co, Cu and O are not substantially influenced by the change in the copper to cobalt concentration ratios except for the [Cu]/[Co] = 2 coating where the local coordination appears to slightly change due to the loss of octahedral Cu + . The present film coatings are expected to exhibit good wear resistance especially for the [Cu]/[Co] = 1.0 sample due to its high hardness/elastic modulus (H/E) ratio. Finite element modeling (FEM) indicated that, under spherical loading conditions, the high stress and the plastic deformation were predominantly concentrated within the coating layer, without spreading into the substrate.

Tailoring the physicochemical and mechanical properties of optical copper–cobalt oxide thin films through annealing treatment

“…In addition, the obtained hardness values of the present study are generally consistent with those reported by other researchers [45,46]. [21], the results in this work were slightly higher after annealing. Further, following heat treatment, there is an increasing trend in both elastic modulus and hardness for the coatings, albeit this is not so pronounced for hardness.…”

“…Some variations, although not significant, were observed in the stress distributions (Fig. 10), compared to our previous work [21], due to the change in the mechanical properties of coatings after annealing at the temperature of 650 °C Therefore, two implications can be derived from the above analysis. a) Coating delamination would be suppressed, which typically occurs at the interface between the coating and plastically deformed substrate during unloading.…”

“…This is quite surprising, in view of the fact that mechanical strength and durability are important in extending their service life. In our previous work, the copper cobalt oxide thin films were deposited on aluminium substrate via the sol-gel dip-coating route [19,20,21]. The resulting coatings exhibited distinctive optical properties with a spectrally selective profile in the UV-Vis-NIR wavelengths region.…”

Developments in the synthesis of flat plate solar selective absorber materials via sol–gel methods: A review

“…The absorptance of 83.4% (prior to the addition of an antireflection layer) was accomplished using 0.25 M copper acetate and 0.25 M cobalt chloride (Cu/Co ratio=1) with dip-speed 120 mm/min (four cycles). Nanoindentation test using the Berkovich indenter showed that the coatings have a high wear resistance [115][116][117].…”

Optical and Morphological Characterization of BaSeThinFilms Synthesized via Chemical Bath Deposition

“…In the past few decades, various different techniques such as chemical bath deposition [6], cluster beam deposition [7], electro deposition [8], molecular beam epitaxy [9], thermal evaporation, spray pyrolysis [10], sputter deposition [11], metal organic chemical vapour deposition [12], sol-gel dip-coating technique [13][14][15], photo-enhanced chemical vapour deposition [16]and plasma polymerization method [17] have been extensively used to synthesize the thin film coatings for commercial and industrial applications. However, in the recent times, chemical bath deposition technique has been widely used for the development of thin film coatings because its cost effectiveness, minimum materials wastage, does not need sophisticated machineries and can be applied in large area deposition at low temperatures.…”