Author: Hoang (Bolton) Tran

Problem Statement

Previously (Post 2), the energetic and tilt angle of hydroxyl (OH) adsorption on Pt (111) was studied. In this study (Post 3), the OH bond stretching vibration \((\nu_{OH})\) is of primary interest. Firstly, the zero-point energy (ZPE) will be considered for adsorption energy. Secondly, the adsorption energy \((E_{ad})\) and \(\nu_{OH}\) will be computed and analyzed at different OH adsorption environments:

• Adsorption sites: Atop vs. bridge (see Fig. 1 in Post 2)
• Surface coverage: 1.0 ML vs. 0.25 ML (Monolayer – number of adatom per surface atom)
• Water solvation: introduce water molecules above the adsorbed molecule.

Methodology

Software:

DFT was implemented in both CASTEP and DMol3, a plane wave basis set software embedded inside BIOVIA Material Studio (MS). [1]

Calculation:

Geometric optimization (CASTEP): Please see Post 2

Vibrational analysis (DMol3):

• Exchange-Correlation Functional: Generalized Gradient approximations – Perdew-Burke-Ernzerhof (GGA-PBE) [2]
• Self-Consistent Field tolerance: 1E-05 eV
• Monkhost-Pack [3] grid size: 10x10x1 (50 irreducible kpoints) for 1×1 supercell & 5x5x1 (13 irreducible kpoints) for 2×2 supercell.
• Self-consistent dipole correction in z-direction (normal to surface)

The procedure of optimizing, cleaving and adsorbing OH onto Pt(111) surface are the same as outlined in Post 2. This procedure was carried out again for each adsorption environments outlined below. The optimized structure of each adsorption environment was then used as input for the vibrational frequency calculation.

ZPE correction:

The adsorption energy is defined as:

\begin{equation} E_{ad}=E_{OH/slab}-E_{OH}-E_{slab} \end{equation}

To calculate the ZPE correction for \(E_{ad}\), ZPE was calculated for both the free OH radical and the adsorbed OH. Since the free OH only has O-H stretching vibration, the ZPE of adsorbed OH also considered only the O-H stretching \(\nu_{OH}\).

Adsorption environments:

1.  Adsorption sites:

The two adsorption sites of interest are the high symmetry atop and bridge sites. Values of \(E_{ad}\) at 1.0 ML coverage were already calculated from previous study. See Post 2 for more details.

2. Surface coverage:

Adsorption energy at the atop site for two difference OH coverage values are considered: 1.0 ML and 0.25 ML.  In Post 2, \(E_{ad}\) was already calculated for 1.0 ML. To achieve 0.25 ML, 2×2 supercell with one absorbed OH was built (Fig. 1).

Figure 1. Visualization of OH surface coverage of 1.0 ML and 0.25 ML (click on tab to view)

3. Water solvation:

Solvation effect, or the interaction of water molecules near surface with the adsorbed OH was explored. Atop sites at 0.25 ML was used with 1 or 2 water molecules floating just above the adsorbed OH (Fig. 2).

Figure 2. Optimized structure of water floating above adsorbed OH (click on tab to view)

The adsorption energy for the system with extra water molecules were calculated differently:

\begin{equation} E_{ad}=E_{OH/slab/water}-E_{OH}-E_{slab/water} \end{equation}

In this case, \(E_{slab/water}\) was calculated by optimizing a bare slab with floating water(s).

Results and discussion

1. ZPE corrections:

The adsorption energies, calculated across different adsorption environments, with and without ZPE are shown in table 1 below.

Table 1. Comparing \(E_{ad}\) with and without ZPE:

Adsorption environments		\(E_{ad}\) without ZPE (eV)	\(E_{ad}\) with ZPE (eV)
1.0 ML	Bridge	-2.14	-2.15
	Atop	-3.02	-3.03
0.25 ML	Atop 0 water	-2.90	-2.90
	Atop 1 water	-2.99	-2.99
	Atop 2 waters	-3.21	-3.22

The difference in ZPE are all 0.01 eV or less, which is a lot smaller than the difference between \(E_{ad}\) at different environment. Therefore, it is concluded that ZPE does not affect the adsorption energy of OH on Pt (111) significantly.

This result also has significant implementation later.

2. Adsorption environments:

Calculated values of \(\nu_{OH}\) and \(E_{ad}\) for each environments are shown in Figure 3 and 4 below:

Figure 3. Frequency of OH stretching at different adsorption environments and of free OH.

Figure 4. Adsorption energy of OH at different environments.

The hypothesis of this study is: The adsorption energy is stronger (more negative) when the adsorption environment (site, coverage, solvation) allows the adsorbed OH to form stronger hydrogen bonds.

The strength of hydrogen bond is characterized by \(\nu_{OH}\), weaker OH stretching (lower \(\nu_{OH}\)) means stronger hydrogen bonds. The presented results put this hypothesis to the test.

Adsorption sites:

Looking first at the 1.0 ML (blue section) in Fig. 3, it is observed that the OH vibrations at atop and bridge sites are both smaller than that of free OH radical, indicating some degree of hydrogen bonds formed at the sites. The hydrogen bond strength is greater at the atop site. Looking now at 1.0 ML section in Fig. 4, atop adsorption is stronger than that of the bridge site (results of Post 2).

Surface coverage:

Results at “1.0 ML Atop” and “0.25 ML Atop 0w” represent the direct comparison for surface coverage. \(\nu_{OH}\) at 0.25 ML is about the same as free OH, indicating the absence hydrogen bonds. Fig. 4 then shows that OH adsorbs weaker at 0.25 ML.

Water solvation:

Focusing now at the 0.25 ML sections (orange sections). Surprisingly, adding one water (Atop 1w) does not lower \(\nu_{OH}\), indicating that there are still no hydrogen bonding. This, however, makes sense by observing the position of this water molecule in Fig. 2 above. The placement of this water molecule did not seem to produce hydrogen bonding, which would be water’s O atom pointing toward OH’s H atom.

Nevertheless, OH adsorption is still slightly stronger with one water. This can be attributed to the solvation effect on the slab itself, because this effect is the only difference left between the two equations for \(E_{ad}\) (\(E_{slab}\) and \(E_{slab/water}\)).

Considering now the case with two water, \(\nu_{OH}\) is now lower, indicating formation of hydrogen bonding, which is also confirmed by looking at position of the second added water in Fig. 2. Finally, the adsorption with 2 waters is much stronger than with both 0 water and 1 water by looking at Fig. 4.

Conclusion

Despite the above analysis seems to indicate that stronger hydrogen bonds lead to stronger adsorption, the aforementioned hypothesis is false. The most obvious evidence is from the ZPE correction. It was already shown that ZPE does not contribute significantly to the adsorption energy. The ZPE correction only comes from the difference in frequency of OH stretching \(\nu_{OH}\). Therefore, \(\nu_{OH}\), despite varying between different environments, does not contribute significantly to \(E_{ad}\).

In other words, the most that one can conclude from these results is that there is a correlation between hydrogen bond strength \(\nu_{OH}\) and adsorption energy \(E_{ad}\). The reason behind this correlation can be thought of as hydrogen bonds formation lowering the energy of adsorbed OH \(E_{OH/slab}\) or \(E_{OH/slab/water}\), which subsequently lowers adsorption energy \(E_{ad}\). However, it would be wrong to conclude that \(\nu_{OH}\) is solely responsible for the difference in \(E_{ad}\), because the magnitude does not check out.

Finally, another interesting takeaway is that water solvents increase the adsorption tendency, not merely by interacting directly with the adsorbed species (through hydrogen bonding), but potentially through other electronic interaction with the surface as well. This is why understanding of the solvation effects on solid-liquid interfacial processes (e.g. electrocatalysis) is of great interest.

References

[1] “First principles methods using CASTEP”, Zeitschrift fuer Kristallographie 220(5-6) pp. 567-570 (2005) S. J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. J. Probert, K. Refson, M. C. Payne

[2] J. P. Perdew, K. Burke, M. Enzerhof (1996). Generalized Gradient Approximation Made Simple, Phys, Rev. Lett., 77, 3865.

[3] H. J. Monkhorst and J. D. Pack (1976), Special points for Brillouin-zone integrations, Phys. Rev. B 13, 5188.