Elsevier

Surface Science

Volume 642, December 2015, Pages 51-57
Surface Science

Potential of lateral interactions of CO on Pt (111) fitted to recent STM images

https://doi.org/10.1016/j.susc.2015.08.018Get rights and content

Highlights

  • The potential of long-range lateral interactions of CO on Pt(111) was developed.

  • The potential was fitted to recently published STM images of CO/Pt(111) monolayer.

  • The potential includes long-range surface-mediated interactions and short-range dipole–dipole repulsions.

  • The potential is oscillating and non-monotonic.

Abstract

Monolayers of carbon monoxide (CO) on Pt(111) surfaces are one of the most studied adsorption systems. However, molecular models of this system still do not take into account the reliable potential of lateral interactions between adsorbed CO molecules. Recent advances in experimental technique have brought high-resolution real-space images of CO/Pt(111) monolayers. For example, Yang et al. (J. Phys. Chem. C 117 (2013) 16429–16437) found island structures for coverages from 0.11 to 0.25 ML. In this study we have shown that these island structures can be explained with long-range oscillating lateral interactions. Parameters of the proposed potential were fitted to experimental scanning tunneling microscopy images with a series of Monte Carlo simulations.

Introduction

CO adsorption on a Pt surface has been a subject of numerous experimental and theoretical studies due to the high catalytic activity of Pt in CO oxidation reactions. In addition to this classical reaction, the CO/Pt(111) adsorption system is interesting for nanotechnology because it has a very high adsorption energy per unit surface area. NO, H2 and atomic oxygen also bind strongly to some metal surfaces, but carbon monoxide on Pt(111) is the most extensively studied system. High adsorption energies can be used in novel methods to control the shapes of nanoscale objects such as nanoparticles or nanowires. One can find examples of such processes in Refs. [1], [2], [3], [4], [5]. Development of molecular models of adsorption systems with high adsorption energy is crucial for progress in the controlled production of nanoscale devices. One of the most important aspects of molecular models of adsorption systems is the potential of lateral interactions between adsorbed species.

Numerous studies have investigated the lateral interaction potential of CO on Pt(111) adsorption systems [6], [7], [8], [9], [10], [11], [12]. The potentials proposed in these studies can be divided into two distinct types: monotonically decreasing repulsive interactions [6], [10], [12] and non-monotonic interactions with local energy minima at some distance or energy oscillations [7], [8], [9], [10], [11].

Density functional theory (DFT) studies of CO adsorbed onto Pt(111) have been conducted since the late 1990s [13]. Early DFT calculations had significant problems with prediction of adsorption site preference, which was unambiguously known from experiments. A notable paper by Feibelman et al. [14] underlined this puzzle and stimulated research efforts in this direction [15], [16], [17]. Today, the problem of site preference seems to be solved [16], but calculation of lateral interactions, especially long-range interactions, is computationally very expensive, and they still have not been conducted at an appropriate theoretical level [12].

Section snippets

Indirect experimental data

Experimental techniques used up to the 1990s for studying CO adsorption on Pt(111) gave only indirect information about the monolayer structure: different spectra or isotherms. Technologies commonly used at that time included thermal desorption spectroscopy, work-function measurements, angle-resolved photoelectron emission spectroscopy, infrared reflection–absorption spectroscopy, high-resolution electron energy loss spectroscopy, and analysis of low-energy electron diffraction (LEED). A

Model

Based on the monolayer model from Yang et al. [36], we have developed the potential of lateral interactions of CO on Pt(111) adsorbed on atop sites. STM images from [36] show that for θ < 0.25 ML after room temperature annealing, almost all molecules are bound to atop sites. Hence, we suggest that for θ < 0.25 ML, a monolayer structure can be described with a simple lattice model. As atop sites are the most energetically favorable, for low coverages and low temperature all other sites can be ignored.

Computation details

To fit potential parameters to experimental images, equilibrium monolayer structures were calculated for different parameter values. Parameter δF was explored in a range 12π,32π because EHP is periodic by δF with period π. The value of parameter qF in [11] was equal to 1 Å 1, and so we investigated the range, including the value [0.5, 2]. An interval of explored μ values was defined from limitations (6), (7), (8) for particular values of δF and qF. Most δF, qF pairs give a zero length interval

Results and discussion

The best results were obtained for the following parameter values: μ = 0.399075, δF = 3.312389, and qF = 0.845 Å 1 (see plot in Fig. 4).

With these values, island phases were reproduced for coverages of 0.16 ML (Fig. 6) and 0.25 ML (Fig. 7). For coverage of 0.11 ML (Fig. 5) almost all molecules were isolated and no islands were observed as expected from repulsive nature of the potential in correspondence with experimental image (Fig. 1a). The average island size obtained for θ = 0.16 ML was 3.84 molecules

Conclusion

In this study, the potential of long-range lateral interactions of CO on Pt(111) was developed. The potential is consistent with recently published experimental real-space images. The potential catches qualitative features of the adsorption monolayer for coverages below 0.25 ML. To the best of our knowledge, this is the first potential that reproduces the island structures of a CO/Pt(111) monolayer. The potential form was adopted from earlier studies [26], [11], and it has a simple and clear

Acknowledgment

This study was supported by the Ministry of Education and Science of the Russian Federation on a budget-funded basis for 2014–2016 (project no. 16.2413.2014/K) and president grants for government support of the leading scientific schools of the Russian Federation (5998.2014.10).

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