Author: Andrew Wong
1. Introduction
The goal of this post is to determine the preferred adsorption site of atomic oxygen (O) on the iron (Fe) (110) surface using DFT techniques. Due to their catalytic and magnetoelectric properties, iron and iron oxides are vital in various applications [1]. On the BCC Fe (110) surface, the most stable termination of Fe [2], there are four potential adsorption sites that O can adsorb onto; on top (OT), long bridge (LB), short bridge (SB), and the pseudo-three fold bridge (TB). To better understand adsorption of atomic oxygen on the Fe (110) surface, plane wave basis Density Functional Theory (DFT) was implemented with the Vienna Ab initio Simulation Package (VASP) [3] to calculate the corresponding adsorption energy of each of the four sites. By comparing these energies, the most preferred adsorption site of atomic oxygen on the Fe (110) surface can be determined by which site had the most negative adsorption energy.
2. Methodology
2.1 Calculation Parameters
The DFT calculations performed in this post utilized the plane wave basis set with pseudo potentials method in Vienna Ab initio Simulation Package (VASP). The Perdew, Burke, and Ernzerhof functional (PBE) was implemented to model the electron-electron exchange and correlation energies. To represent the ion-core electron interactions, the projector augmented-wave (PAW) method was implemented [4]. The Monkhorst-Pack was utilized to model the K-point grid. An Fe core radius of 2.4 Bohrs (1.27 Å) with a panel of 8 valence electrons ( 3d6 and 4s2) were implemented with an electornic convergence tolerance of 2E-06 eV in all calculations. For all calculation, a cutoff energy of 400 eV was implemented as this was seen to be optimal energy cutoff value during the geometry optimization calculations.
2.2 Optimization of Triplet Oxygen in Gas Phase
Before adsorption energies of atomic oxygen on Fe (111) surface can be calculated, the energy of diatomic oxygen must be calculated first. A geometry optimization of diatomic oxygen in the gas phase was performed in a 15 x 15 x 15 Å vacuum cube at an energy cutoff value of 400 eV and a 1 x 1 x 1 K Point set.
2.3 Construction and Optimization of Fe(110) Vacuum Slab
Based on literature that also utilized VASP for electronic structure calculations ,the 3 x 3 Fe (110) slab model was determined to be the most stable termination of Iron [2]. Specifically, a vacuum region of approximately 10 Å with a k-point mesh of 5 x 5 x 1 was utilized to constructed the slab model. A cutoff energy of 400 eV was also implemented as well. Dipole corrections and selective dynamics were implemented as the bottom two layers were fixed to imitate the bulk iron and the two top layers were relaxed. Figure 1 below presents the Fe (110) surface slab model constructed.
Figure 1a: Side View of 3 x 3 Fe (110) Slab Model
Figure 1b: Top View of 3 x 3 Fe (110) Slab Model
2.4 Adsorption of Atomic Oxygen on Fe(110) Surface
For the BCC Fe (110) surface, there are four potential sites of adsorption: On top (OT), Short bridge (SB), Long bridge (LB), and Pseudo-Three Fold Bridge (TB) [5]. A top view figure of these four sites are presented in figure 2 below.
Figure 2: Top View of Potential Adsorption Sites of BCC Fe (110)
In order to determine the most preferred site of atomic O on Fe (110) surface, adsorption energies of O onto the Fe (110) surface must be calculated from geometry optimization calculations. The adsorption site corresponding with the lowest adsorption energy is the most thermodynamically favorable site for O on Fe (110). In order to calculate the adsorption energies for each site, the following equation was used. [6]
Three terms are needed to calculate the adsorption energy. The energy of the Fe surface with O adsorb onto it will need to be determined first. To calculate the energy of a O adsorbate, half of the energy of diatomic oxygen will be utilized as it is easier to perform a geometry optimization on diatomic oxygen rather than atomic oxygen and there are more readily available experimental values for diatomic oxygen. Lastly, the energy of the bare Fe (110) surface without the adsorbate will be needed. As a result, the adsorption energies can then be compared to determine which adsorption site is the preferred site O to adsorb on.
3. Results and Discussion
Using the optimal K-point grid of 5 x 5 x 1 and cutoff energy of 400 ev, Figure 3 was constructed below to represent the top view of the four adsorption sites of Fe (110) with 1/9 ML surface coverage of O after geometric optimization done in VASP.
Figure 3. Optimization of atomic O on Fe (110) top view for the following four sites a. On Top b. Long Bridge c. Short Bridge d. Pseudo-Three Fold Bridge
The table below lists the results of the adsorption energy and distance of the O atom from the Fe (110) surface for the four potential adsorption sites. By comparing the adsorption energies between each site, it can be seen that the adsorption O favors the long bridge site as this site significantly had the most negative adsorption energy compared to the other three sites. It is important to note that, although not as favorable by 0.42 eV, the short bridge site is also a thermodynamically favorable site due to its negative adsorption energy value. This may be due to the multiple bond interactions between multiple oxygen atoms seen in the short and long bridge sites. Although the Pseudo-Three Fold bridge has multiple oxygen atom interactions, it required multiple attempts to converge exactly to this position, as it requires it to be between three oxygen atoms. As a result, the TB site is thermodynamically unfavorable for the adsorption of O onto the Fe (110) surface due to it having the highest adsorption energies. The OT Adsorption site was also favorable but it tend to converge towards the long bridge and short bridge adsorption site and did not have nearly as strong of an adsorption energy by approximately 1 – 1.5 eV.
| Adsorption Sites | Adsorption Energy (eV) | O Distance from Fe Surface (Å) |
|---|---|---|
| On Top (OT) | -1.2582 | 1.652 |
| Long Bridge (LB) | -2.8135 | 1.850, 2.215 |
| Short Bridge (SB) | -2.3922 | 1.795 |
| Pseudo-Three Fold Bridge (TB) | -0.9553 | 1.873, 1.928 |
4. Conclusion
The thermodynamically favorable adsorption sites for the 1/9 ML surface coverage of atomic O on the Fe (110) were the long bridge, short bridge sites, and the on top sites, with the long bridge adsorption site being the most preferred adsorption site. Additionally, the pseudo-three fold bridge sites was not a thermodynamically favorable site due to their positive adsorption energies. As a result, they converged to the local minimums of the long bridge and short bridge sites during the geometric optimization of the adsorbed Fe (110) surface, rarely converging towards the on top configuration. Although the DFT calculations creates a notion that the preferred adsorption site of O on the Fe (110) is the long bridge site, additional considerations, such as surface coverage variations, non-vacuum conditions, and impurities within the bulk surface model, should be noted when studying this particular adsorption chemistry in a more practical and experimental setting. An experimental study examining the adsorption of atomic oxygen on the Fe (110) surface with the use of standard surface characterization methods, such as Auger electron spectroscopy and conversion electron Mossbauer spectroscopy, has confirming agreement that the long bridge site is the preferred adsorption site [5].
5. Citations
[1] Ossowski, Tomasz, and Adam Kiejna. “Oxygen Adsorption on Fe(110) Surface Revisited.” Surface Science, North-Holland, 13 Mar. 2015, www.sciencedirect.com/science/article/pii/S0039602815000618.
[2] Maheshwari, Sharad, et al. “Elementary Kinetics of Nitrogen Electroreduction on Fe Surfaces.” AIP Publishing, AIP Publishing LLC, 28 Jan. 2019, aip.scitation.org/doi/full/10.1063/1.5048036.
3] Init.at. “Vienna Ab Initio Simulation Package.” VASP, www.vasp.at/.
[4] Rostgaard, and Carsten. “The Projector Augmented-Wave Method.” ArXiv.org, 12 Oct. 2009, arxiv.org/abs/0910.1921.
[5] Freindl, K., et al. “Oxygen on an Fe Monolayer on W(110): From Chemisorption to Oxidation.” Surface Science, North-Holland, 13 July 2013, www.sciencedirect.com/science/article/pii/S0039602813002021.
[6] Nguyen, Angela, and Angela Nguyen. Density Functional Theory and Practice Course, 2 Apr. 2019, sites.psu.edu/dftap/2019/04/02/exploring-the-effects-of-surface-coverage-on-the-binding-site-and-adsorption-energy-of-atomic-o-on-pt111/.
