Experimental and Theoretical Studies on Oxidation of Cu-Au Alloy Surfaces: Effect of Bulk Au Concentration

We report results of our experimental and theoretical studies on the oxidation of Cu-Au alloy surfaces, viz., Cu3Au(111), CuAu(111), and Au3Cu(111), using hyperthermal O2 molecular beam (HOMB). We observed strong Au segregation to the top layer of the corresponding clean (111) surfaces. This forms a protective layer that hinders further oxidation into the bulk. The higher the concentration of Au in the protective layer formed, the higher the protective efficacy. As a result, of the three Cu-Au surfaces studied, Au3Cu(111) is the most stable against dissociative adsorption of O2, even with HOMB. We also found that this protective property breaks down for oxidations occurring at temperatures above 300 K.


Electron Inelastic Mean Free Path (IMFP)
We calculated the electron inelastic mean free path length (IMFP) using the method of Seah and Dench [S1]. Here, we show in detail the procedures for calculating the IMFP in Cu 3 Au, CuAu, and Au 3 Cu. For the materials considered in this study, we used the following expression for the IMFP, expressed in monolayer, i.e., λ m λ m = 538 E 2 + 0.41(aE) 1/2 .
The kinetic energy of the electron E [eV] can be obtained from the Au-4f peak position. The monolayer thickness a [nm] can be obtained from the atomic or molecular weight A, the bulk density ρ, and the number of atoms in the molecules n, via the equation where N A is Avogadro's number. Conversion of λ m to the IMFP expressed in nm, i.e., λ n , can be done via the equation For Cu 3 Au, A = 387.6, ρ = 12.3 g · cm −3 , and n = 4. From equations (S1) and (S3), we get λ m = 6.30 and λ n = 1.48 nm, respectively.
And again, for Au 3 Cu, A = 654.5, ρ = 17.2 g · cm −3 , and n = 4, we get λ n = 1.63 nm. Figure S1: Detection angle dependence of Au-4f SR-XPS spectra on clean CuAu(111) at 0.60 ML (left panel) and Au 3 Cu(111) 0.20 ML (right panel). Surface normal detection: 0 • . Oblique detection: 35 • and 70 • . The XPS spectra can be clearly separated into bulk (B), surface (S), and interface (I) components, corresponding to green, blue, and orange lines, respectively (see also text). The background was already subtracted by the Shirley method [S2]. Intensities given in arbitrary units and intensity scales differ between panels (i.e., differ between samples and detection angles). Figure S1 shows the Au-4f SR-XPS spectra of CuAu(111) at Θ = 0.60 ML and Au 3 Cu(111) at Θ = 0.20 ML, measured at 0 • , 35 • , and 70 • from the surface normal. We see that we can now separate both the Au-4f 7/2 and Au-4f 5/2 XPS peaks into three components, viz., the bulk (B) and surface (S) components, and an additional interface-layer (I) component. Here, we adopt the same peak shape and position for the S and B components for both the clean and oxidized surfaces. For CuAu(111) at Θ = 0.60 ML, the S component decreases in intensity with increasing Θ and disappears at Θ = 0.60 ML. At the same time, we observe a newly developed I component (CLS = -331 meV), which most probably comes from the Au atoms in the interface-layer, situated between the topmost oxidized Cu-O layer and the third (sub-surface) metallic bulk layer. Previous studies on Cu 3 Au(100) [S3, S4], Cu 3 Au(110) [S5, S6], and Cu 3 Au(111) [S7] also report observing similar I components. This suggests that, during HOMB irradiation, the Au-atoms in the top-layer become almost depleted due to the strong Cu segregation on the surface, as we have also observed on Cu 3 Au.

Au Layer Profile after Surface Oxidation
We approximate the peak intensity ratio of I to B (A I /A B ) using the following simple equation, x n gives the Au fraction of the n-th layer from the surface. d gives the interlayer distance. The corresponding Au-4f photoelectron IMFP λ in each Cu-Au alloy can be obtained using the method discussed in Section above [S1]. θ is the photoelectron detection angle from the surface normal. From A I /A B measured at θ = 0 • , 35 • , and 70 • , we obtained x 1 = 0, x 2 = 1.0, and x 3 = 0.56, assuming d to be the bulk interlayer distance, ignoring layer relaxation and taking x n ≥ 4 to be the bulk value. For Au 3 Cu(111) at Θ = 0.20 ML, the surface was oxidized at a surface temperature setting of 500 K. Similar to CuAu(111), the B, S, and I components also appears on the Au-4f spectra for the oxidized Au 3 Cu(111). The CLS of I component is −193 meV. From A I /A B measured at θ = 0 • , 35 • , and 70 • , we obtained x 2 = 0.43 and x 3 = 0.51. Also, we obtained x 3 = 0.31 from the peak intensity ratio of S to that of the clean surface.

Surface Energy
Following some detailed thermodynamic derivations found in the literature (cf., e.g., [S8, S9, S7, S10, S11]), we address the surface segregation of clean and oxidized Cu-Au alloys, viz., CuAu(111) and Au 3 Cu(111), by calculating the corresponding surface free energy γ (using the slab model) given by S slab is the surface area of the slab. G slab is the Gibbs free energy of the slab. µ i is the chemical potentials of each atomic species i (i = Cu, Au, O). N i are the number of each atomic species i in the slab. Assuming that the alloy surface is in equilibrium with the underlying bulk reservoir, we have where x bulk i is the mole fraction of the atomic species i (i = Cu, Au) in the Cu-Au alloys. For example, for Au 3 Cu, x bulk Cu = 0.25 and x bulk Au = 0.75. As an estimate, the range of ∆µ Au−Cu spans the two limits corresponding to the phase separation of Cu (Cu-rich limit, when the bulk reservoir is rich in Cu) and the phase separation of Au (Au-rich limit, when the bulk reservoir is rich in Au). From equation (S7) ∆µ where and µ fcc Cu and µ fcc Au are the chemical potentials of Cu and Au in the fcc bulk, respectively.
Similarly, assuming that the alloy surface is in equilibrium with the surrounding gas phase (O 2 at temperature T and partial pressure p O 2 ), we have and E gas O 2 is the total energy of an isolated O 2 at T = 0 K, which gives the upper limit of µ gas O 2 . Assuming an ideal gas, the second term on the right hand side of equation (S12) depends on the difference in chemical potential of O 2 at T = 0 K and the temperature of interest T , at the reference pressure p • , i.e., where k B is the Boltzmann constant. H O 2 (T, p • ) and S O 2 (T, p • ) are the enthalpy and entropy of O 2 at the temperature T and reference pressure p • , respectively [S12]. Finally, we can recast equation (S5) into the following form N slab and x slab Au are the total number of metal atoms and the mole fraction of Au in the slab of the Cu-Au alloys, respectively. N O is the number of O atoms in the slab.