Imaging and characterization of activated ${\text{CO}}_2$ species on Ni(110)

By means of a joint experimental and theoretical approach we provide single molecule imaging and characterization of ${\text{CO}}_2$ on Ni(110), chemisorbed with high charge transfer from the substrate in an activated state that plays a crucial role in the hydrogenation process. Low-temperature scanning tunneling microscopy images of single ${\text{CO}}_2$ molecules are combined with ab initio density-functional-theory calculations, where dispersive forces and zero-point energy estimates have been explicitly accounted for. We obtain a detailed characterization of the adsorption geometries and an estimate of the energies corresponding to the different adsorbed states. A consistent picture of ${\text{CO}}_2$ chemisorption on Ni(110) is provided on the basis of the newly available information, yielding a deeper insight into the previously existing spectroscopic and theoretical data.

Figure 1

(Color online) STM image (top left, $5.2\times 5.2{\text{nm}}^2$) of ${\text{CO}}_2$ (0.05 ML coverage) adsorbed on Ni(110) at 110 K. Two nonequivalent ${\text{CO}}_2$ species can be distinguished from their apparent height. Closeups $\left(1.4\times 1.0{\text{nm}}^2\right)$ of (a) the three-molecule, (b) the two-molecule, and (c) the single-molecule units are shown in the bottom row. In the dashed line inset (top right, $1.6\times 2.2{\text{nm}}^2$) the crossing points of the grid correspond to the positions of the substrate Ni atoms. (sample bias $V=-10\text{mV}$, $I_t=1\text{nA}$).

Figure 2

Coverage of the dim (black) and the bright (gray) ${\text{CO}}_2$ species on Ni(110), as a function of the total ${\text{CO}}_2$ coverage.

Figure 3

(Color online) Energy diagram for different ${\text{CO}}_2$ adsorption configurations on Ni(110) and relevant adsorption and diffusion barriers, calculated using DFT both with standard GGA and with DFT-D-corrected GGA. DFT-D energies including the ZPE contributions are also reported. The shaded region highlights the energy range of all the chemisorbed configurations in the latter approximation. The zero of the energy is set to the nondissociated molecule at infinite distance from the surface. The bottom rows depict the ball models of all the stable ${\text{CO}}_2$ configurations; only Ni atoms of the two outer layers are shown but a five-layers-thick slab was used in the calculations.

Figure 4

(Color online) Closeups from Fig. 1 showing the experimental (top row) and DFT calculated (middle row) STM images of (a) the three-molecule, (b) the two-molecule, and (c) the single-molecule units. The bottom row shows ball models of the calculated geometries of the three units.