PHYSICAL REVIEW B 83, 184111 (2011)
Local structure at interfaces between hydride-forming metals: A case study of Mg-Pd nanoparticles
by x-ray spectroscopy
L. Pasquini,
1
F. Boscherini,
1,2
E. Callini,
1
C. Maurizio,
2,3
L. Pasquali,
4,5
M. Montecchi,
4,5
and E. Bonetti
1
1
Department of Physics and CNISM, University of Bologna, v. Berti-Pichat 6/2, I-40127 Bologna, Italy
2
Consiglio Nazionale delle Ricerche, IOM-OGG, c/o ESRF GILDA CRG, BP 220, F-38043 Grenoble, France
3
Department of Physics, University of Padova, via Marzolo 8, I-35131 Padova, Italy
4
Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, v. Vignolese 905,
I-41125 Modena, Italy
5
CNR-IOM, Area Science Park, s.s.14, km 163.5, I-34012 Basovizza (Trieste), Italy
(Received 10 December 2010; published 19 May 2011)
The structure at the interface between elements or phases that exhibit different hydrogen (H) binding energies
exerts a profound influence on the thermodynamics of H in nanophase materials. In this paper, we study the local
structure at the Mg/Pd interface in Mg nanoparticles with partial Pd coating, and we map its evolution in response
to annealing and H sorption. This task is accomplished by x-ray photoelectron spectroscopy and x-ray absorption
spectroscopy, also including in situ experiments, with the support of crystallographic information from x-ray
diffraction. It is shown that the initial Pd surface layer reacts with Mg at relatively low temperatures, leading
to irreversible formation of a Mg-rich intermetallic phase Mg
6
Pd. Due to the high Mg-H binding energy, this
phase reversibly transforms, upon H absorption, into a nanophase mixture of magnesium hydride and a Pd-rich
intermetallic with H in solid solution, MgPdH
δ
. These reversible structural changes are discussed with reference
to recent calculations that highlight their relevance to the thermodynamics of the metal-hydride transition.
The picture drawn here might be relevant to other multiphase materials presently investigated in the field of
hydrogen-related science and technology.
DOI: 10.1103/PhysRevB.83.184111 PACS number(s): 61.46.-w, 61.05.cj, 64.70.-p, 68.35.Fx
I. INTRODUCTION
The metal-hydride reversible transformation stands out as
a prominent example of first-order phase transition in solids.
Many elements exhibit this phenomenon at temperatures and
hydrogen (H) pressures whose equilibrium values are mostly
determined by the metal-H bond energy. The corresponding
enthalpies of hydride formation
f
H
hyd
span from the large
negative values of extremely stable hydrides such as TiH
2
,
ZrH
2
, and LaH
2
(-126 to -210 kJ/mol H
2
), to the typical
values of interstitial metallic hydrides PdH
0.5
, VH
2
, and NbH
2
(-40 to -60 kJ/mol H
2
), up to the slightly negative enthalpies
of high-pressure hydrides such as NiH
0.5
and AlH
3
(-6 to
-11 kJ/mol H
2
). Among all the elements, Pd was the first that
attracted research activity, with the work of Graham nearly
150 years ago,
1
and still remains the prototypical and most
studied metal-hydrogen system.
2
On the other hand, Mg is an
appealing hydride former on account of its elevated H-storage
capacity (7.6 wt %), abundance, and low cost, although its
relatively high stability (
f
H
MgH
2
=-75 kJ/mol H
2
) poses
severe constraints on the temperature for H release. In order
to tailor the thermodynamics and the stability of hydride
formation, intermetallic compounds (IMCs) of metals with
different
f
H
hyd
were developed and extensively studied in
the past.
3
Nowadays, the desired advent of hydrogen as a future
energy carrier has given new impetus to the research on its
interaction with advanced materials, aimed at the development
of efficient H-storage media, H sensors, and smart devices.
In this context, it is of particular interest to study artificially
created nanostructures where two or more phases having
different
f
H
hyd
coexist on a nanometric length scale. The
thermodynamics of these nanophase materials does not simply
result from a weighted average of the components, since new
physics emerges due to various interactions between them.
For instance, it has been demonstrated that the stability of
thin MgH
2
films can be tuned by the elastic interaction with a
constraining layer, in particular, Pd or Ni.
4
The extent to which
such an appealing effect takes place strongly depends on the
structure of the interface between the two metals: If they are
immiscible, such as Mg and Ti, the resulting elastic coupling
is very weak and a quasifree behavior of Mg films occurs.
5
Therefore, the local structure at the interface between the two
hydride-forming metals plays a very important role in tailoring
the overall H-material interaction. The guiding rules adopted
nowadays in the architecture of nanostructured hydrides
resemble to some degree the principles of physical chemistry
that lead to the discovery of conventional hydride-forming
binary IMCs such as Mg
2
Ni, LaNi
5
, FeTi, and ZrCr
2
.
6
One
rule is the combination of elements with markedly different
f
H
hyd
values. The second rule is to include one element
which provides the ability to dissociate and/or recombine
the H
2
molecule, so that atomic H can be chemisorbed and
diffused into the material. This issue is especially relevant
for nanostructures based on Mg, the catalytic properties of
which are rather poor. Following this rule, Pd capping layers
and Pd surface decoration have been applied to different
Mg-based nanostructures, e.g., thin films and multilayers,
4,7
nanoparticles (NPs),
8
and nanoblades.
9
Furthermore, Pd NPs
serve as model materials to test size and quantum effects on the
thermodynamics
10
as well as on the kinetics
11,12
of H sorption
in metals. In fact, the microstructural features of modern
nanostructures imply very short critical diffusion lengths for
solid-state transformations such as the metal-hydride transition
184111-1 1098-0121/2011/83(18)/184111(12) ©2011 American Physical Society