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C: Surfaces, Interfaces, Porous Materials, and Catalysis
2D Ni Nanoclusters on Ultrathin MgO/Ag(100)
Letizia Savio, Marco Smerieri, Jagriti Pal, Edvige Celasco, Mario Agostino Rocca, and Luca Vattuone
J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b08684 • Publication Date (Web): 12 Dec 2019
Downloaded from pubs.acs.org on December 18, 2019
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1
2D Ni Nanoclusters on Ultrathin MgO/Ag(100)
Letizia Savio
1,*
, Marco Smerieri
1
, Jagriti Pal
2,+
, Edvige Celasco
1,2
, Mario Rocca
1,2
, Luca
Vattuone
1,2
1
IMEM-CNR, UOS Genova, Via Dodecaneso 33, 16146 Genova, Italy.
2
Dipartimento di Fisica, Universita` di Genova, Via Dodecaneso 33,
16146 Genova, Italy
ABSTRACT:
Ni nanoclusters up to 30 Å in diameter are grown by Ni deposition on ultrathin MgO/Ag(100) films
at different temperature and characterized by combining low temperature scanning tunnelling
microscopy with photoemission and vibrational spectroscopies. At 200 K both small Ni
x
O
y
aggregates and 2D Ni nanoparticles of average size close to 12 Å form. The latter have a metallic
nature and efficiently catalyze CO dissociation at 200 K. When Ni is deposited at 300 K, only larger
3D Ni clusters are observed.
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2
Introduction
Metal nanoparticles (NPs) dispersed on metal oxide supports have been a matter of studies for decades
due to their technological and industrial relevance for applications in fields such as sensoristics and
heterogeneous catalysis
1,2
. In addition, they are considered as model systems able to bridge the so-
called structure gap between single crystal surfaces
3
, usually employed for a fundamental
understanding of the reaction processes and identification of the active sites, and real catalysts, often
consisting of oxides impregnated with (or supporting) nanoparticles of different size. The availability
of size-selected clusters has further boosted research in this field by enabling to study the reactivity
of nanoparticles for a selected reaction as a function of the number of atoms in the cluster
4
.
In this frame, Ni NPs have been the subject of a particular attention both for their magnetic properties
and for their high catalytic activity for economically relevant reactions such as methanation
5,6
.
Indeed, Ni nanoclusters have been deposited on different oxide supports, including MgO
7–9
, TiO
2
10–
13
, SrTiO
3
14
and Al
2
O
3
15
. In particular, due to its simple structure and wide bandgap, MgO became
a model both as an active system in heterogeneous catalysis
16,17
and as a substrate for deposition of
NPs. In fact, its non-reducible nature guarantees that it is a relatively inert support, though the edges
of monolayer films were shown to dissociate water molecules
18,19
. On the other hand, the MgO
substrate can influence the shape and size of the nanoparticles during their growth process and,
consequently, affect their chemical reactivity.
In order to take advantage of electron based spectroscopies and imaging methods, usually thin or
ultrathin oxide films deposited on metal supports are used in model studies. Large Ni NPs were
deposited on a 10 ML film of MgO/Mo(001)
7
. At room temperature (RT), 1 ML-equivalent of Ni
arranges in three dimensional (3D) NPs of 2 to 6 nm in diameter and 0.5 to 1.5 nm in height containing
up to several hundred atoms. Due to the 16% lattice mismatch between the (100) face of MgO and
the Ni fcc lattice, small Ni clusters take a hcp structure to reach commensurability with the MgO
substrate and have interfacial Ni atoms in registry with surface oxygen atoms of the MgO layer. Only
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3
for clusters larger than 2000 atoms in size, i.e. exceeding 4.5 nm in diameter and 2.5 nm in height,
the fcc structure becomes energetically convenient with respect to the hcp one
20
.
For nanoclusters of few Ni atoms on MgO(001), ab initio calculations predict a 3D shape already for
the Ni
4
cluster
21
, for which the tetrahedral configuration is proved to be most stable. In fact, since
the binding energy.
of a single Ni atom onto the MgO surface is 1.4 eV
22,23
while the addition of a Ni atom to a pre-
existing small Ni cluster is exothermic (by 2.6-4.1 eV, depending on the size of the cluster and the
nature of the MgO site) the formation of large 3D clusters is energetically favoured. Surface energy
arguments lead to the same conclusions; under equilibrium conditions, or at least when
thermodynamics dominates over kinetics, the surface energy determines the growth mode of the
cluster. For Ni, such quantity varies from 2.011 J/m
2
for Ni(111) to 2.368 J/m
2
for Ni(100)
24–26
, while
for MgO it is only 1.15 J/m
2
27
. Therefore, when the mobility is high enough, formation of 3D clusters
through a Vollmer Weber growth mode is expected.
At variance with this picture, if the mobility is not high enough to overcome the diffusion barriers,
kinetics dominates over thermodynamics and 2D structures or small isolated clusters may form. For
0.03 ML of Ni deposited on Al
2
O
3
at 300 K, e.g., 85 % of the NPs have an average diameter of 15 Å
and an apparent height of 2-3 Å, reasonably corresponding to a single Ni layer. Only a minor fraction
of the clusters shows a clear 3D shape
28
. Similarly, 0.1 ML of Ni deposited on ZrO
2
/Pt
3
Zr at RT
leads to 90% of the clusters with apparent height lower than 2 Å, also compatible with a single layer
of Ni atoms
29
.
We showed that, if Ni is deposited on an ultra-flat monolayer MgO film at 200 K
30,31
, the scenario
can be even different
9
. At low (0.2 ML) Ni coverage, 2D nanoclusters of four to six atoms coexist
with larger clusters. Both the shape and the interatomic distance between neighbouring Ni atoms of
the former NPs are indicative of a non-metallic character consistent with Ni
y
O
x
aggregates
9
. The
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4
spontaneous oxidation of NPs is enabled by the availability of oxygen atoms at the MgO/Ag interface;
such atoms segregate to the surface through the MgO monolayer film, directly or - more probably -
via an exchange mechanism involving the oxygen atoms of the MgO. In presence of Ni adatoms,
these oxygens are captured to form the Ni
x
O
y
aggregates. DFT calculations allow to identify tetramers
and pentamers with Ni
4
O
5
and Ni
5
O
12
clusters, respectively. Therefore, under the investigated
conditions, the formation of Ni
y
O
x
structures represents a competitive channel with respect to the
growth of larger metallic Ni clusters. Indeed, the surface energy of NiO, calculated using ab initio
methods, turns out to be 0.38 J/m
2
for (100), 0.82 J/m
2
for (110) and 1.14 J/m
2
for (111) surfaces
32
to be compared with the above given value of 2 J/m
2
for bare Ni
24
. Therefore, if enough oxygen is
available and its mobility is high enough, formation of Ni
x
O
y
is energetically favoured.
In the present work, we focus on Ni clusters grown on monolayer MgO and made of more than 6 Ni
atoms. We demonstrate that they have a metallic nature and show how the deposition temperature
affects their shape and size.
Experimental
Experiments were carried out in two different ultra-high vacuum apparatuses. The former consists of
an analysis chamber, hosting a low temperature scanning tunneling microscope (LT-STM by Createc)
and of a preparation chamber. The latter is equipped with a high resolution electron energy loss
spectrometer (HREELS – Delta0.5 by SPECS) and with a conventional setup for X-ray Photoelectron
spectroscopy (XPS - non monochromatized DAR400 Omicron X-ray source and EA125 Omicron
hemispherical analyser).
Both UHV chambers are equipped with a Knudsen cell and an O
2
doser for reactive Mg evaporation,
with a quartz microbalance (QMB) for Mg flux measurements, with an e-beam evaporator (Focus
EFM 3) mounting a high purity (99.99%) Ni rod, with an ion-gun plus gas inlet for sample cleaning
and with a quadrupole mass spectrometer for residual gas analysis. Finally, a four degrees of freedom
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