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

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 Cu²⁺ which minimizes the development of octahedral Cu⁺, and these octahedral Cu²⁺ ions substitute the Co²⁺ 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 (<i>H</i>/<i>E</i>) 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

Relative proportions of the major compound classes and chain length of dragonfly wing epicuticle components.

<p>Relative proportions of the major compound classes and chain length of dragonfly wing epicuticle components.</p

Atomic proportions of oxygen and nitrogen in the wings of <i>Hemianax papuensis</i>.

<p>Atomic proportions of oxygen and nitrogen in the wings of <i>Hemianax papuensis</i>.</p

Proposed model of the epicuticle of <i>Hemianax papuensis</i> wing membranes.

<p>Three layers are contained within the epicuticle: the outer epicuticle, the meso epicuticle and the inner epicuticle.</p

Scanning electron micrographs of <i>Hemianax papuensis</i> wing membranes.

<p>Cross-sectional (a, c, e) and surface view (b, d, f) images were taken of each wing before chloroform extraction (a, b), after 10 s extraction (c, d), and 1 hr extraction (e, f). Loss of surface structure is visible on the wings subjected to chloroform extraction; no structure is evident after extraction for 1 hr, whereas the internal wing structure appears unchanged. Scale bars = 400 nm.</p

Frequencies and assignments of the major absorption peaks found in the IR spectra of <i>Hermianax papuensis.</i>

<p>Frequencies and assignments of the major absorption peaks found in the IR spectra of <i>Hermianax papuensis.</i></p