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Thermodynamics of Alkanethiol Self-Assembled Monolayer Assembly on Pd Surfaces

Summary

Uses dispersion-corrected DFT to compute alkanethiolate SAM energetics and preferred coverages on Pd(111), Pd(100), and Pd(110) as a function of chain length and thiol chemical potential, linking dispersion interactions among alkyl tails to coverage transitions. Temperature-dependent CO DRIFTS experiments probe weakened CO binding as thiolate coverage increases, and the paper discusses preliminary multiscale modeling with ReaxFF for the Pd–thiol system. The Langmuir study is positioned at the interface of surface science and catalysis: thiolate SAMs poison Pd sites for small-molecule adsorption, yet the thermodynamic phase diagram of SAM coverage versus chemical potential is facet-dependent, so nanoparticle shapes under saturating thiol environments may differ from vacuum Wulff constructions without adsorbate effects.

Methods

  • DFT: vdW-corrected supercell calculations of thiolates on Pd facets; binding energies decomposed into covalent vs noncovalent contributions as reported; Wulff constructions used to discuss particle shape under thiol saturation.
  • Experiment: CO DRIFTS vs temperature for CO on thiol-covered Pd surfaces at varying SAM densities.
  • Reactive FF: ReaxFF results described as preliminary multiscale coupling (see article text for scope).
  • Coverage models: Grand-potential-style comparisons across facets identify transitions (e.g., toward ½ ML) where tail packing and metal–S bonding trade off.

The article also references preliminary ReaxFF/multiscale coupling for Pd–thiol chemistry; N/A — full LAMMPS protocol tables (timestep, production ns, thermostat) are not the main deliverable—use pdf_path if extended MD appears beyond the DFT core. For the DFT-led SAM phase diagram, three-dimensional periodic boundary conditions (PBC) enclose Pd(111), Pd(100), and Pd(110) slab supercells. N/A — NVT/NVE/NPT molecular dynamics production runs are not documented for the primary coverage models—static 0 K DFT minimizations supply the energies. DFT stress tensors and cell relaxation follow the plane-wave program setup in Methods; N/A — constant-stress NPT classical MD is not part of the summarized workflow. Barostat: N/A — NPT MD not used in the DFT core. Electric field: N/A — bias MD not reported. Replica / enhanced sampling: N/A — umbrella / metadynamics / replica exchange MD not reported.

Findings

Outcomes (DFT). Pd(111) favors ⅓ ML thiolate coverage in the equilibrium construction; Pd(100)/(110) show coverage increases with thiol chemical potential, including moves toward ½ ML depending on facet and chain length.

Comparisons. CO DRIFTS experiments track weakened CO binding as SAM density rises, matching weakened CO/O/H adsorption in DFT with increasing thiolate coverage.

Sensitivity. Chain length tunes when vdW tail interactions compete with facet-specific metal–S bonding, shifting predicted Wulff shapes (cubic dominance on Pd(100) at saturation).

Limitations and PDF grounding. ReaxFF portions are explicitly preliminary; DFT dispersion treatment and coverage models should be taken from the Langmuir PDF and SI sibling 2017kumar-venue-paper when needed.

Limitations

  • ReaxFF discussion is explicitly preliminary relative to the DFT core of the paper; quantitative agreement across all facets may require further parameterization and validation.

Curation note: the SI-only corpus sibling 2017kumar-venue-paper holds extra tables referenced in the article; keep DFT coverages and DRIFTS interpretations on this primary article page unless the SI adds genuinely new numerical data.

Relevance to group

Group-authored Pd–SAM thermodynamics bridging DFT, spectroscopy, and ReaxFF-oriented multiscale follow-on.

Citations and evidence anchors

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