Effects of transition metal doping on surface properties and resistance to Cl− adsorption of α-Al2O3
Graphical abstract
Introduction
Aluminum (Al) alloys are protected from corrosion in a wide range of environments by passive oxide film. However, in some cases, the presence of halogen anions such as chloride ion (Cl−) can lead to local breakdown of passive films and pitting corrosion [1], [2], [3], [4]. As for the mechanism of interaction between Cl− and Alumina (Al2O3) film, three main views have been reported [4], [5], [6], [7]: i) Cl− migration/ penetration into the metal-oxide interface [7]; ii) Competitive adsorption of Cl− and O2− [8,9]; iii) Breakdown and repair of oxides [10]. The interaction between Cl− and oxide film is generally considered as the initial factor of pitting corrosion in these reports. Hence, an effective method to improve the surface pitting resistance is to prevent the reaction of Cl− with Al2O3 in the early stage.
Usually, using physical or chemical methods to dope other atoms (molecules) on the target surface is an efficient way to change its microstructure, charge distribution and reactivity. Experimentally, using sol gel method with new precursors, Khodadadi et al. synthesized Fe-doped Al2O3 nanoparticles with different doping percentage. The results showed that the doping of Fe3+ affects the size and uniformity of the particles. With increased iron dopant in alumina nanoparticles, the band gap decreases, and the photocatalytic activity, saturation magnetism, coercivity level of the samples increase [11]. Nguyen et al. synthesized Cr-doped Al2O3 using the solid-state method and investigated its thermochromic behavior. Their results indicated that, chromium doped Al2O3 shows a reversible color change from pink to gray/green, which depends on temperature and chromium concentration in the range of 25–600 °C [12]. Kim et al. studied nitrogen-doped Al2O3 thin films by atomic layer deposition. X-ray photoelectron spectroscopy and surface topography analysis verified that the nitrogen doping in Al2O3 reduces the formation of defects and surface roughness. For N-doped Al2O3 films deposited at 170 °C and annealed after 400 °C, the reduction of oxygen related defects reduces the leakage current to about 5 × 10−10 A/cm2 [13]. Moreover, it has also been reported in other studies that doping elements can change the structure, optical, magnetic, electronic and catalytic properties of Al2O3 [14], [15], [16], [17], [18].
From the perspective of simulation calculation, using periodic density functional theory (DFT) methods, electronic properties and reactivity of Fe3+/Cr3+ doped α-Al2O3 (0001) surfaces was studied by Baltrusaitis et al. The theoretical results demonstrated that the presence of Fe3+ and Cr3+ alters both structural and electronic properties, and weakens the adsorption tendency of CO on the surface [19]. The structural, electronic and magnetic properties of C-doped α-Al2O3 have been investigated by Ao et al. using an ab initio method based on DFT. It is found that the upper valence band of α-Al2O3 doped by C replaces Al has been greatly modified, and the band gap is narrowed observably. Both C substituted Al and O doping can induce magnetism, which is primarily originated from 2p states of polarized C [20]. Novita et al. established a 7-atoms model of CO3+ doped α-Al2O3 and estimated its optical properties. In estimated absorption spectra, it is showed that the CASTEP calculation results of the model considering lattice relaxation effect are in agreement with the experimental data [21]. Furthermore, the effects of doping elements on the properties of Al2O3 have also been calculated in other researches, in which the experimental results were studied from a microscopic perspective or predicted [22], [23], [24], [25], [26], [27].
Compared with the studies introduced above, the DFT simulation work about influences of doping atoms on the surface properties of Al2O3, which dedicated to improving the pitting corrosion resistance of Al2O3 surface, is rare. Liu et al. constructed a model of α-Al2O3 doped with Mg, Si and Cu, and investigated the migration behavior of Cl− inside the modified surface. The energy barrier of Cl− migration in α-Al2O3 decreases sharply from 2.85 eV to 0.88 and 0.35 eV when doped with Mg and Cu, respectively. While Si doping slightly increases the energy barrier, which indicated that it is more difficult for Cl− to migrate in Si doped surface [28]. Except for this, the interaction between Cl− and pure Al or Al2O3 has been reported by many other scholars. For instance, Liu et al. [29]. and Zhang et al. [30]. calculated the ingress of chloride into α-Al2O3 (0001) and Al (111) and the adsorption of Cl− on α-Al2O3 surface. Zhang et al. [31,32]. studied the erosion of layer-defect alumina by H2O and Cl− and the co-adsorption behavior of OH, Cl and H2O on the step-defect Al2O3 film. Marks et al. [33]. analyzed the effect of Cl− in aqueous solution on the surface structure of hydroxylated alumina and the thermodynamic corrosion. All these researches are splendid work and provide a basis for further studies.
The first step of chloride erosion of oxide film is the adsorption of ions, followed by further migration or reaction [4,6,9,19]. As a common alloying element in Al alloys, the fourth period transition metal element is also effective for the surface modification of Al2O3. Therefore, models of α-Al2O3 (0001) surface doped with fourth period transition metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) were established in this work to study the effects of doped atoms on the surface structure, electronic state and work function as well as the adsorption behavior of Cl− on the surface by DFT calculation method. With the change of nuclear charges of doped atoms, the alteration rules of surface properties and the adsorption behavior of Cl on α-Al2O3 (0001) surface were also discussed and contrastively analyzed.
Section snippets
Computational details and models
The calculations have been performed on Vienna Ab-initio Simulation Package (VASP) based on DFT [34], [35], [36]. The generalized gradient approximation (GGA) exchange-correlation function using the parametrization by Perdew, Burke, and Ernzerhof (PBE) were used [37]. The projector augmented wave (PAW) method was employed to describe the electron–ion interactions [38]. In all calculations of Cl− adsorption on pure surface and doped surfaces, a plane wave energy cutoff of 650 eV and a 4 × 4 × 1
Effect of doped atoms on surface structures
Fig. 2(a) and (b) - (k) are the side views of pure and Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn doped α-Al2O3 (0001), respectively. It is found that the final position of the first layer atom and the atomic interlayer spacing of the first and second layer on the doped surfaces changed compared with the pure surface, as well as the relative position between Al and O atoms nearby doped atoms. As is shown in Fig. 3, the first-layer Alc next to the doped atom and the Oc bonded with it were
Conclusions
In this work, we established and optimized the pure α-Al2O3 (0001) and fourth period transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) doped α-Al2O3 (0001) surface models. In an aqueous environment simulated by the method of VASPsol, surface properties of pure surface and doped surfaces and Cl− adsorption on surfaces were calculated by first principles method based on DFT.
The results show that the first layer atoms of Al2O3 to move up in different degrees in consequence of the transition
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We appreciate the High Performance Computing Center of Shanghai University, and Shanghai Engineering Research Center of Intelligent Computing System (No.19DZ2252600) for providing the computing resources and technical support.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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