Ab initio study of Ti0.5Al0.5N(001)—residual and environmental gas interactions

We have explored surface processes on Ti0.5Al0.5N(001) interacting with residual and environmental gases, namely O2, H2O and CO2, using ab initio molecular dynamics. Dissociative adsorption of O2 occurs on Ti sites, which are unusual sites, as Al2O3 is more stable than TiO2. This may be understood based on the electronic structure. We suggest that an increased Ti–O bond strength relative to Al–O surface bond strength is the electronic origin for the early stages of TiO2 formation on Ti0.5Al0.5N(001). Another unexpected atomic mechanism, identified as O covers the surface: Ti escapes from the Ti0.5Al0.5N(001)/O interface layer, generating vacancies, and hence enabling mobility at the interface. In the case of H2O and CO2, the dominating physical mechanism is dissociative adsorption, where O–H and N–H as well as C–O and Ti–O dipoles are formed, respectively. These fundamental surface processes are relevant for initial stages of oxidation, surface diffusion and nucleation of reaction layers upon exposure to residual and environmental gases.

ab initio calculations, it was reported that molecular H 2 O adsorption occurs on TiN(001) and TiN(111) surfaces, while dissociative H 2 O adsorption is observed for TiN(110) [35]. As soon as AlN is incorporated into cubic TiN forming Ti 1−x Al x N, these adsorption processes may be altered, but this has not yet been explored.
In this work, we use ab initio MD simulations to explore surface processes on Ti 0. 5 [29,30] and nucleation and growth of Ti-Al-O-N thin films [36].

Methodology
Ab initio MD simulations of Ti 0.5 Al 0.5 N(001) interacting with O 2 , H 2 O and CO 2 were performed using the OpenMX code [37], based on the density functional theory [38] and basis functions in the form of linear combination of localized pseudoatomic orbitals [39]. The electronic potentials were fully relativistic with partial core corrections [40,41] and the generalized gradient approximation was applied [42]. The basis functions used were generated by a confinement scheme [39,43] and specified as follows: Ti5.0-s2p2d1, Al6.0-s2p2, N4.5-s2p1, O4.5-s2p1, H4.5-s2 and C4.5-s2p1. Ti, Al, N, O, H and C designate the chemical name, followed by the cutoff radius (Bohr radius units) in the confinement scheme and the last set of symbols defines primitive orbitals applied. The confinement radii as well as the basis set were carefully checked with respect to basic elemental data, such as equilibrium volume (or bond length in the case of free molecules) and bulk modulus. The energy cut off 2040 eV (150 Ryd) and the 96 × 96 × 240 grid within the real-space grid technique [44] were adjusted to reach the accuracy of 3 × 10 −5 eV atom −1 (10 −6 H atom −1 ). Canonical ensembles at 300 K were used to simulate Ti 0.5 Al 0.5 N(001) surface (six atomic layers, surface area 12.536 × 12.536 Å 2 , 216 atoms, random distribution of Al and Ti [15]) interacting with O 2 , H 2 O and CO 2 gases as a function of coverage. The coverage was calculated based on ideal dissociative adsorption. These molecules were initially placed 3 Å from the Ti 0.5 Al 0.5 N(001) surface at ad hoc positions. The bottom layer of Ti 0.5 Al 0.5 N(001) slab was always frozen to mimic the infinitive bulk. The MD time step was 1.0 fs and the simulation time was 2000 fs for each coverage. The total MD time for all simulations performed was 54 000 fs. The adsorption energy ( E ad ) was calculated at 0 K as where E slab , E molecule , E TiAlN , n and m designate the total energy of a Ti 0.5 Al 0.5 N(001) slab with adsorbed molecules, total energy of a molecule, total energy of the pristine Ti 0.5 Al 0.5 N(001) surface, number of adsorbed molecules and number of atoms in a molecule, respectively. All total energies at 0 K were obtained after full structural relaxations by minimizing the interatomic forces.

Results and discussion
We start the discussions on the Ti 0.5 Al 0.5 N(001) surface interaction with residual and environmental gases by considering the O 2 case. Figure  This Al-O bond formation may be due to a decreased population of Ti atoms in the surface exposed to additional O 2 molecules. In this case, there is a broader distribution of O-metal bond lengths, from 1.65 to 2.18 Å. This structural disorder (broad bond length distribution) was also observed for the oxidation of Al(111) [48]. At an O 2 coverage of 0.44 ML, a Ti atom coordinated with five nearest O neighbours is observed. This is consistent with TiO 2 [47]. Our data also support the suggestion by Gnoth et al [49] that surface oxidation of cubic Ti-Al-N thin films is caused by reaction with atmospheric oxygen and/or residual gas immediately after synthesis [49]. to Baben et al [50] showed that the incorporation of O into cubic Ti-Al-N is energetically favourable and Sjölen et al [36] have observed the incorporation experimentally. Hence, the above discussed phenomena are also relevant for nucleation and growth of Ti-Al-O-N thin films. This may be a general feature of cubic transition metal Al nitrides, as O incorporation is energetically also favoured for Sc-, V-and Cr-containing phases [51]. We continue to discuss the structure evolution as a function of coverage in terms of energetics. Figure 2 contains the corresponding adsorption energy data for O 2 molecules. As the O 2 coverage is increased from 0.06 to 0.50 ML, the adsorption energy increases from −5.8 to −3.5 eV per adatom. Clearly, the affinity of O towards Ti 0.5 Al 0.5 N(001) decreases with larger O 2 coverages. This trend as well as the absolute values is consistent with the literature [52,53].
The fact that Ti-O bond formation is preferred at low O 2 coverages is counter-intuitive as Al 2 O 3 is known to be more stable than TiO 2 [54]. This apparent inconsistency can be understood based on the electronic structure. Figure 3  The last molecule interacting with Ti 0.5 Al 0.5 N(001) to be considered is CO 2 (see figure 4). Non-dissociative adsorption of CO 2 occurs for the lowest coverage of 0.06 ML. Already at a CO 2 coverage of 0.11 ML, CO 2 dissociates and C-O and Ti-O dipoles are formed. Evidently, surface oxidation can also be CO 2 driven. The observed dissociative adsorption of CO 2 on Ti 0.5 Al 0.5 N(001) is consistent with the interaction of CO 2 and many metal surfaces, such as Al, Bi, Fe, Mg, Ni, Pd, Re, Rh and Ru [58]. At a CO 2 coverage of 0.22 ML, Ti escapes from the Ti 0.5 Al 0.5 N(001)/O interface layer, induced by an increased Ti-O coordination. Here, Al-O bonds already coexist. This is the same physical mechanism as in the case of Ti 0.5 Al 0.5 N(001)-O 2 interaction, which is discussed above. Dissociative adsorption is continued for all coverages probed except of a CO 2 coverage of 0.44 ML, where a non-dissociative adsorption of CO 2 occurs at very large distance from the surface, namely ∼5 Å which is in the order of two interplanar distances. Since a relatively large CO 2 coverage is reached, Coulomb repulsion may occur with already adsorbed CO 2 molecules. However, already at a CO 2 coverage of 0.50 ML, dissociative adsorption is continued including the dissociation of the non-dissociated molecule at a CO 2 coverage of 0.44 ML. Clearly, dissociative adsorption is the dominating physical mechanism. Figure 2 also contains the corresponding adsorption energy data for CO 2 molecules. As CO 2 coverage is increased from 0.06 to 0.50 ML, the adsorption energy increases from −1.6 to −1.0 eV per adatom. This is the same trend as in the case of O 2 adsorption and the affinity of CO 2 towards Ti 0.5 Al 0.5 N(001) is in the range between those of O 2 and H 2 O. This implies that the Ti 0.5 Al 0.5 N(001) surface would mainly be covered with O, while C-containing species should be seen as minor impurities.

Conclusions
Using ab initio MD simulations at 300 K, we have studied fundamental surface processes on Ti 0. 5 [35], while dissociative adsorption occurs on Al 2 O 3 (0001) [57]. In the case of CO 2 , non-dissociative adsorption occurs for the lowest coverage of 0.06 ML. Already at a CO 2 coverage of 0.11 ML, CO 2 dissociates and C-O and Ti-O dipoles are formed. At a CO 2 coverage of 0.22 ML, Ti escapes from the Ti 0.5 Al 0.5 N(001)/O interface layer, induced by an increased Ti-O coordination. This is the same physical mechanism as in the case of Ti 0.5 Al 0.5 N(001)-O 2 interaction. Larger CO 2 coverages are dominated by dissociation. The identified physical mechanisms are relevant for various processes, ranging from initial stages of oxidation and surface diffusion to nucleation of reaction layers upon exposure to residual and environmental gases.