Ab initio study of surface self-segregation effect on the adsorption of oxygen on the $\gamma \text{-TiAl}\left(111\right)$ surface

The effects of surface self-segregation on the adsorption of oxygen on the $\gamma \text{-TiAl}\left(111\right)$ surface are investigated by density-functional theory calculations. The oxygen binding-energy results show that on the pure $\gamma \text{-TiAl}\left(111\right)$ surface, the most stable adsorption sites of oxygen atoms are the sites with more Ti sites as their neighbors, which is in agreement with previous calculations. The defect formation energies and the relative surface energies as functions of chemical potential of Al are calculated. Furthermore, the phase diagram of $\gamma \text{-TiAl}\left(111\right)$ surfaces with different defects and different oxygen coverage is also calculated. The calculated results show that Al atom can segregate at the $\gamma \text{-TiAl}\left(111\right)$ surface. Especially, at high Al chemical potential, the first surface layer of the $\gamma \text{-TiAl}\left(111\right)$ surface can be composed of Al atoms. In this case, the oxygen adsorption behavior is very similar to the case of Al(111) surface: the binding energy per oxygen atom decreases with the increase in the oxygen coverage due to an attractive interaction between O atoms. These results show that the occurrence of Al self-segregation at the $\gamma \text{-TiAl}\left(111\right)$ surface can enhance the interaction between O and Al atoms. The Al surface segregation effect may be the reason why the growth of pure alumina layer resulting from the selective oxidation of aluminum formed on $\gamma \text{-TiAl}\left(111\right)$ surface at the first stage in the experiments.

Figure 1

(Color online) Top view of the $\gamma \text{-TiAl}\left(111\right)\text{−}\left(2\times 2\right)$ surface with possible oxygen adsorption sites for three cases: (a) pure $\gamma \text{-TiAl}\left(111\right)\text{−}\left(2\times 2\right)$ surface, (b) $\gamma \text{-TiAl}\left(111\right)\text{−}\left(2\times 2\right)$ surface with one Al antisite defect [denoted by $\gamma \text{-TiAl}\left(111\right)\text{−}\left(2\times 2\right)\text{−}1\text{Al}$], (c) $\gamma \text{-TiAl}\left(111\right)\text{−}\left(2\times 2\right)$ surface with two Al antisites defect [denoted by $\gamma \text{-TiAl}\left(111\right)\text{−}\left(2\times 2\right)\text{−}2\text{Al}$]. Large, medium, and small blue and gray balls represent the Ti and Al atoms in the first, second, and third surface layers, respectively. The cross “$+$” represents oxygen atom. The symbols $A$, $B$, $C$, and $D$ denote fcc-Al, fcc-Ti, hcp-Al, and hcp-Ti sites, respectively. The subscript numbers 1, 2, and 3 represent these three cases, and the prime “${}^{\prime }$” in (b) denotes the same sites as that on the pure surface, but the surrounding atom in the first surface layer is changed from Ti to Al.

Figure 2

(Color online) The calculated surface energies of the clean $\gamma \text{-TiAl}\left(111\right)$ surface with different surface defects as a function of the chemical potential of Al.

Figure 3

(Color online) The calculated average binding energies per oxygen atom for the O/Al(111), O/Ti(0001), $\text{O}/\gamma \text{-TiAl}\left(111\right)$, $\text{O}/\gamma \text{-TiAl}\left(111\right)\text{−}1\text{Al}$, and $\text{O}/\gamma \text{-TiAl}\left(111\right)\text{−}2\text{Al}$ systems at the most stable adsorption site as a function of the oxygen coverage.

Figure 4

(Color online) The relative surface energies of oxygen covered surfaces under (a) Ti-rich and (b) Al-rich conditions. (c) illustrates the calculated phase diagram for oxygen adsorption on the different $\gamma \text{-TiAl}\left(111\right)$ surfaces as a function of ${\mu }_{\text{Al}}$ and ${\mu }_{\text{O}}$. TiAl(111), TiAl(111)-1Al, and TiAl(111)-2Al represent the clean, with one, and with two Al antisite defects TiAl(111) surface in a $2\times 2$ unit cell, respectively. 1O, 2O, 3O, and 4O represent one, two, three, and four oxygen atom adsorptions on the corresponding TiAl surface in a $2\times 2$ unit cell, which correspond to 0.25, 0.50, 0.75, and 1.0 ML oxygen coverage, respectively. The dashed line in (c) represents the chemical potential of oxygen at the experimental conditions (Refs. 11, 12).

Figure 5

(Color online) The PDOS of oxygen and surface atoms in the different systems: (a) $\text{O}/\gamma \text{-TiAl}\left(111\right)$ with 0.25 ML, (b) $\text{O}/\gamma \text{-TiAl}\left(111\right)\text{−}1\text{Al}$ with 0.25 ML, (c) $\text{O}/\gamma \text{-TiAl}\left(111\right)\text{−}2\text{Al}$ with 0.25 ML, (d) $\text{O}/\gamma \text{-TiAl}\left(111\right)$ with 1.00 ML, and (e) O/TiAl(111)-2Al with 1.00 ML at the most stable adsorption sites. The $s$, $p$, and $d$ states are indicated by solid, dashed, and dotted lines, respectively. The zero energy corresponds to the Fermi level.