# The Interaction of Sub-Monolayer Ta Adatoms and Clusters with Oxygen at the Pt(111) Interface

## TL;DR

This study explores how tantalum clusters interact with oxygen on a platinum surface, revealing oxidation mechanisms and structural changes under different conditions.

## Contribution

The paper presents a detailed oxidation mechanism for tantalum clusters on Pt(111) and identifies distinct Ta 4f photoemission features influenced by surface interactions.

## Key findings

- Ta clusters fragment in the presence of oxygen and form a common oxidation product.
- Ta-oxide islands with Ta in +5 oxidation state agglomerate on the Pt surface.
- Annealing leads to oxygen loss and formation of a Ta–Pt alloy through intermixing.

## Abstract

The interaction of submonolayer quantities of size-selected
and
soft-landed Tan (n =
4, 5, 6, 8, 13) clusters with Pt(111) is investigated employing high-resolution
X-ray photoelectron spectroscopy (HR-XPS), scanning tunneling microscopy
(STM), and density functional theory (DFT) simulations. The deposited
clusters are monodispersed and stable under ultrahigh vacuum (UHV)
conditions at 40 K. They display a size-specific trend in photoemission
spectra, which is reasoned in terms of the distinct in plane coordination
of Ta atoms in the clusters. Both the Ta coordination number and distance
from the Pt surface influence its Bader charge and, accordingly, the
oxidation state of the atoms in the Ta cluster. They already fragment
in the presence of low amounts of oxygen and form a common oxidation
product observed for all cluster sizes. Based on our observations,
we propose an oxidation mechanism in the example of Ta8 clusters, which is closely comparable to the one discussed in gas-phase
studies on the oxidation of cationic Ta clusters of similar size.
Concomitant to oxidation-induced fragmentation, the agglomeration
into Ta-oxide islands with Ta in an oxidation state of +5 is observed.
However, the strong interaction with the Pt surface leads to Ta 4f
orbital photoemission features that differ from those commonly observed
for Ta2O5. Computational insights concerning
the structure of the Ta-oxide islands indicate flat agglomerates that
agree with STM observations. They suggest distinct Ta 4f photoemission
contributions from interfacial and surface-related Ta configurations.
The respective HR-XPS spectra display specific core-level shifts as
a function of bonding configuration and vicinity to the Pt surface.
By annealing at 900 K in UHV, we observe oxygen loss and concomitant
intermixing of Ta atoms with the Pt subsurface lattice to which results
in the formation of a Ta–Pt alloy. These species, Ta-oxide
islands, and Ta–Pt alloy, can reversibly interconvert by oxidative
surface segregation and reductive intermixing.