Geometrical and orientation investigations on the electronic structures of elements adsorption on graphene via density functional theory

Nano-sized materials have promising contemporary and novel technological applications as they possess favourable properties due to quantum effects. The nano-sized graphene material exhibits remarkable electrical, optical, thermal and mechanical characteristics. Adding impurities or doping constitutes an effective way in fine-tuning properties of graphene for specific applications. This study aims to investigate the geometrical aspects of elements adsorption on graphene to produce more accurate models of the electronic structure of graphene as a result of the doping. Previous models investigated mainly the adsorption sites (bridge, hollow, top); however, they could not systematically explain certain phenomena, e.g. nonlinearity of band gaps to atomic ratios in oxygen-adsorbed graphene. We hypothesise that this is attributed to the positions and orientation of the adatoms (adsorbed elements) relative to one another, which is, in essence, a geometrical phenomenon. In the present study, geometrical investigations of elemental adsorption on graphene focused on side (single-, double-sided), site (bridge, hollow, top) and orientation (the position of adatom relative to one another and graphene). The computational simulations were conducted by using the generalized gradient approximation (GGA) functional within the density functional theory (DFT) framework. The VASP (Vienna Ab initio Simulation Package) software was utilised for all simulations. Trends in the elemental adsorption on graphene in terms of sides/sites/orientations are presented in terms of: binding energy (stability); migration (barrier) energy; adatom height; graphene distortion; Fermi energy; magnetization; charge transfer and energy band gap. The calculated results of 10 elements (Na, Mg, Al, Si, P, S, F, Cl, Br and I) adsorbed on pristine graphene indicate that the geometrical combination of side, site and orientation is vital in determining the most stable configuration of the adsorbed systems. This study reinforces the notion that the involvement of site/orientation of element (or functional group) is essential in future models of adsorption on graphene

Phenol Dissociation on Pristine and Defective Graphene

Phenol (C6H5O‒H) dissociation on both pristine and defective graphene sheets in terms of associated enthalpic requirements of the reaction channels was investigated. Here, we considered three common types of defective graphene, namely, Stone-Wales, monovacancy and divacancy configurations. Theoretical results demonstrate that, graphene with monovacancy creates C atoms with dangling bond (unpaired valence electron), which remains particularly useful for spontaneous dissociation of phenol into phenoxy (C6H5O) and hydrogen (H) atom. The reactions studied herein appear barrierless with reaction exothermicity as high as 2.2 eV. Our study offers fundamental insights into another potential application of defective graphene sheets

Understanding Local Bonding Structures of Ni-Doped Chromium Nitride Coatings through Synchrotron Radiation NEXAFS Spectroscopy

CrN has widespread applications as protective coatings, for example, in aircraft jet engines whereby their high hardness and good oxidation resistance render metal components resistant to harsh operating conditions. Alloying elements are commonly incorporated (doped) into the coatings to further enhance their thermomechanical properties. However, the effect of dopants on the electronic properties and their roles in modifying the grain boundary configurations remain unclear. Lack of such critical knowledge has hindered the development of design strategies for high performance CrN-based coatings. To address this challenging issue, in the present study near-edge X-ray absorption fine structure (NEXAFS) investigations of Cr1-yNiyN coatings at the Cr L3,2-edge (570-610 eV), Ni L3,2-edge (840-890 eV), and N K-edge (380-450 eV) regions were conducted using synchrotron radiation soft X-ray (SXR) spectroscopy in both Auger electron yield (AEY) and total fluorescence yield (TFY) modes. The chemical states in CrNiN were found to change with the increase of Ni content, manifested as a small chemical shift and moderate change of shapes of various absorption edges. The CrN grain size also became smaller with increasing Ni concentration. These findings help improve our understanding of local bonding structures, which could potentially lead to improved coating designs for highly demanding applications

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