Oxidation of pure and potassium-doped NiTi shape memory surface: A density functional theory investigation

The oxidation of Ti-terminated NiTi surfaces is investigated with density functional theory. The surfaces considered are (001) and (110) of the austenite phase (B2) and (010) of the martensite phase $\left(\text{B}19^{\prime }\right)$. The favored adsorption sites for atomic oxygen are fourfold hollow on B2 (001), and twofold bridged between two surface Ti atoms on B2 (110) and $\text{B}19^{\prime }$ (010). Adsorption of one oxygen molecule per surface Ti atom results in the formation of a rutilelike protective layer of ${\text{TiO}}_2$. The oxidation results in outward and inward movement of surface Ti and Ni atoms, respectively, signaling depletion of Ni atoms from the surface region. Substitution of Ni by K near the surface is found to enhance the formation of ${\text{TiO}}_2$ at the expense of TiO, and increase the stability and facilitate the growth of thicker layers of titanium oxides on the NiTi surfaces. This is of importance for the biofunctionality of the alloy.

Figure 1

(Color online) Adsorption of 1 ML O on B2 NiTi; (a) and (b) side and top view, fourfold hollow site on the (001) surface; (c) and (d) side and top view, twofold bridge site on the (110) surface. Colors (size, grayscale): Ti-blue (large gray), O-red (small, gray), Ni-white (large, white).

Figure 2

(Color online) Partial density of states of Ti and O atoms: (I) clean Ti-terminated B2 NiTi(001) surface; (II) $\text{I}+1\text{ML}$ O adsorbed [Figs. 1a, 1b]; (III) one layer of TiO(001); (IV) $\text{I}+1\text{ML}$ ${\text{O}}_2$ adsorbed [Figs. 3c, 3d]; (V) one layer of ${\text{TiO}}_2\left(110\right)$.

Figure 3

(Color online) Adsorption of 1 ML ${\text{O}}_2$ on B2 NiTi; (a) and (b) side and top view, hollow and top sites on the (001) surface; (c), and (d) side and top view, hollow and bridge sites on the (001) surface; (e) and (f) side and top view, bridge and in-plane sites on the (110); (g) and (h) side and top view, bridge and bridge sites on the (110) surface. Colors (size, grayscale): Ti-blue (large, gray), O-red (small, gray), Ni-white (large, white).

Figure 4

(Color online) Partial density of states of Ti and O atoms: (I) clean Ti-terminated B2 NiTi(110) surface; (II) $\text{I}+1\text{ML}$ O adsorbed [Figs. 1c, 1d]; (III) one layer of TiO(110); (IV) $\text{I}+1\text{ML}$ ${\text{O}}_2$ adsorbed [Figs. 3e, 3f]; (V) one layer of ${\text{TiO}}_2\left(001\right)$.

Figure 5

(Color online) Adsorption on B2 NiTi with K doping; (a) and (b) side and top view, 1 ML O, fourfold hollow site on the (001) surface; (c) and (d) side and top view, 1 ML O , twofold bridge site on the (110) surface; (e) and (f) side and top view, 0.5 ML O, twofold bridge site on the (110) surface ($2\times 1$ supercell with only half of Ni in top layer substituted by K; K-O distance is $5.02\text{Å}$); (g) and (h) side and top view, 0.5 ML O, twofold bridge site on the (110) surface, ($2\times 1$ supercell with only half of Ni in top layer substituted by K; K-O distance is $2.57\text{Å}$). Colors (size, grayscale): K-green (huge, light gray), Ti-blue (large, gray), O-red (small, gray), Ni-white (large, white).

Figure 6

(Color online) Partial density of states (B2 (001)) of Ti, O, and K atoms: (I) clean K-doped NiTi(001); (II) $\text{I}+1.0\text{ML}$ O adsorbed [Figs. 5a, 5b]; (III) 1.0 ML O adsorbed on NiTi(001) [Figs. 1a, 1b]; (IV) $\text{I}+1\text{ML}$ ${\text{O}}_2$ adsorbed [Figs. 7a, 7b]; V) 1 ML ${\text{O}}_2$ adsorbed on NiTi(001) [Figs. 3c, 3d].

Figure 7

(Color online) Adsorption of 1 ML ${\text{O}}_2$ on B2 NiTi with K doping; (a) and (b) side and top view, hollow and bridge sites on the (001) surface; (c) and (d) side and top view, bridge and in-plane sites on the (110) surface. Colors (size, grayscale): K-green (huge, light gray), Ti-blue (large, gray), O-red (small, gray), Ni-white (large, white).

Figure 8

(Color online) Partial density of states (B2 (110)) of Ti, O, and K atoms; (I) clean K-doped NiTi(110); (II) $\text{I}+1\text{ML}$ O adsorbed [Figs. 5c, 5d]; (III) 1 ML O adsorbed on NiTi(110) [Figs. 1c, 1d]; (IV) $\text{I}+1\text{ML}$ ${\text{O}}_2$ adsorbed [Figs. 7c, 7d]; (V) 1 ML ${\text{O}}_2$ adsorbed on NiTi(110) [Figs. 3e, 3f].

Figure 9

(Color online) Adsorption of oxygen on $\text{B}19^{\prime }$ NiTi(010); (a) and (b) side and top view, 1 ML O, twofold bridge site; (c) and (d) side and top view, 1 ML ${\text{O}}_2$, bridge and in-plane sites. Colors (size, grayscale): Ti-blue (large, gray), O-red (small, gray), Ni-white (large, white).

Figure 10

(Color online) Partial density of states ($\text{B}19^{\prime }$ phase) of Ti, O, and K atoms; (I) clean NiTi(010); (II) $\text{I}+1\text{ML}$ O adsorbed [Figs. 9a, 9b]; (III) $\text{I}+1\text{ML}$ ${\text{O}}_2$ adsorbed [Figs. 9c, 9d]; (IV) clean K-doped NiTi(010); (V) $\text{IV}+1\text{ML}$ O adsorbed [Figs. 11a, 11b]; (VI) $\text{IV}+1\text{ML}$ ${\text{O}}_2$ adsorbed [Figs. 11c, 11d].

Figure 11

(Color online) Adsorption of oxygen on K-doped $\text{B}19^{\prime }$ NiTi(010); (a) and (b) side and top view, 1 ML O, twofold bridge site; (c) and (d) side and top view, 1 ML ${\text{O}}_2$, bridge and in-plane sites. Colors (size, grayscale): K-green (huge, light gray), Ti-blue (large, gray), O-red (small, gray), Ni-white (large, white).

Figure 12

(Color online) Oxygen coverage dependence of the relaxation of surface atoms for undoped (solid lines) and K-doped (dashed lines) surfaces of B2 (001) (black), B2 (110) (blue), and $\text{B}19^{\prime }$ (010) (red), i.e., change in vertical Ti-Ni separation upon oxidation. At 1 ML O coverage (one O pr surface Ti), the data points correspond to the geometries of Figs. 1a, 1b (black star, solid), 1c, 1d (blue plus, solid), 9a, 9b (red triangle, solid), 5a, 5b (black star, dashed), 5c, 5d (blue plus, dashed), 11a, 11b (red triangle, dashed). At 1 ML ${\text{O}}_2$ coverage (two O pr surface Ti), the data points correspond to the geometries of Figs. 3c, 3d (black star, solid), 3e, 3f (blue plus, solid), 9c, 9d (red triangle, solid), 7a, 7b (black star, dashed), 7c, 7d (blue plus, dashed), 11c, 11d (red triangle, dashed).