108
4 Ultrathin Layers
parallel with the change of hydrogen adsorption characteristics due to Pd atomic
terraces and their edges.
For tuning the catalytic properties of Pd and Pt, layered systems were synthesized by using 8–24 subsequent cycles first for one of the components and then for
the other [152]. The deposits were tested for oxygen reduction reaction. Another
multilayer approach to synthesize nanostructured catalysts was to deposit a Pd/Ru
sandwich structure with coating ex situ synthesized Pd particles with Ru [153] or
by performing 15 SLRR cycles for each metal on a foreign substrate [133]. In
these studies, the surprising finding was that the hydrogen absorption/desorption
into/from the Pd layers/particles was accelerated, although they were nearly fully
buried under another metal that does not adsorb/absorb hydrogen. Although no firm
conclusion could be drawn whether the elimination of the Pd/H adsorption state or
some competing absorption mechanism was responsible for the unexpected acceleration, these examples show the nature of nanostructures that is difficult to elucidate
on the basis of bulk properties.
The study of Pt–Pb SLRR layers is particularly interesting since these metals
form alloys with various compositions (Pt 3 Pb, PtPb, PtPb 4 ), but the atomic arrangement in the SLRR layers is probably completely different from that of the ordered
crystalline phases, and, in addition, they can also by synthesized as layers having
a non-equilibrium composition. The study of Pt–Pb SLRR layers with Pb mole
fraction less than 0.1 [114] indicated an immense decrease of the hydrogen adsorption, much higher than expected from the density of Pb atoms at the surface, which
was explained by the electronic interaction of the alloying element with Pt. For
CO oxidation, alloyed Pt–Ru SLRR layers proved to exhibit an outstanding activity
[107].
It is to be taken into account that the atomic spacing of the thin layer deposited on
a foreign substrate may substantially differ from that in the bulk form, which is due
to the stress and surface relaxation. This is also true for layer produced with SLRR.
Intentional modification of this spacing with intermediate layers like graphene has a
great perspective in the purposeful development of catalysts [135].
It is also a common feature of catalysts that they are applied in powder form
or as nanoparticles. Therefore, various techniques have been proposed for creating
deposit layers with even thickness by SLRR also on particles. The obvious case
is when a particle is formed in situ on a surface, possibly also by electrodeposition [138, 145] or electroless deposition [154], which makes it possible to perform
the SLRR process without another step of particle immobilization. If the preparation process starts with ex situ synthesized nanoparticles, the immobilization can be
based on a standard process in which the substrate surface is first modified with an
aminoalkyl trimethoxysilane compound. The substrate surface is hence terminated
with an assembly of ≡Si–R–NH 2 chains, which immobilize gold nanoparticles efficiently. The potential of the immobilized nanoparticles can be controlled, making it
possible to subject them SLRR cycles without a particle loss [125].
Various procedures have been developed without a chemical immobilization of
particles for SLRR. The preparation of catalyst-coated nanoparticles is also possible
with a sedimentation process in which the nanoparticles are collected on a horizontal
4 Ultrathin Layers
parallel with the change of hydrogen adsorption characteristics due to Pd atomic
terraces and their edges.
For tuning the catalytic properties of Pd and Pt, layered systems were synthesized by using 8–24 subsequent cycles first for one of the components and then for
the other [152]. The deposits were tested for oxygen reduction reaction. Another
multilayer approach to synthesize nanostructured catalysts was to deposit a Pd/Ru
sandwich structure with coating ex situ synthesized Pd particles with Ru [153] or
by performing 15 SLRR cycles for each metal on a foreign substrate [133]. In
these studies, the surprising finding was that the hydrogen absorption/desorption
into/from the Pd layers/particles was accelerated, although they were nearly fully
buried under another metal that does not adsorb/absorb hydrogen. Although no firm
conclusion could be drawn whether the elimination of the Pd/H adsorption state or
some competing absorption mechanism was responsible for the unexpected acceleration, these examples show the nature of nanostructures that is difficult to elucidate
on the basis of bulk properties.
The study of Pt–Pb SLRR layers is particularly interesting since these metals
form alloys with various compositions (Pt 3 Pb, PtPb, PtPb 4 ), but the atomic arrangement in the SLRR layers is probably completely different from that of the ordered
crystalline phases, and, in addition, they can also by synthesized as layers having
a non-equilibrium composition. The study of Pt–Pb SLRR layers with Pb mole
fraction less than 0.1 [114] indicated an immense decrease of the hydrogen adsorption, much higher than expected from the density of Pb atoms at the surface, which
was explained by the electronic interaction of the alloying element with Pt. For
CO oxidation, alloyed Pt–Ru SLRR layers proved to exhibit an outstanding activity
[107].
It is to be taken into account that the atomic spacing of the thin layer deposited on
a foreign substrate may substantially differ from that in the bulk form, which is due
to the stress and surface relaxation. This is also true for layer produced with SLRR.
Intentional modification of this spacing with intermediate layers like graphene has a
great perspective in the purposeful development of catalysts [135].
It is also a common feature of catalysts that they are applied in powder form
or as nanoparticles. Therefore, various techniques have been proposed for creating
deposit layers with even thickness by SLRR also on particles. The obvious case
is when a particle is formed in situ on a surface, possibly also by electrodeposition [138, 145] or electroless deposition [154], which makes it possible to perform
the SLRR process without another step of particle immobilization. If the preparation process starts with ex situ synthesized nanoparticles, the immobilization can be
based on a standard process in which the substrate surface is first modified with an
aminoalkyl trimethoxysilane compound. The substrate surface is hence terminated
with an assembly of ≡Si–R–NH 2 chains, which immobilize gold nanoparticles efficiently. The potential of the immobilized nanoparticles can be controlled, making it
possible to subject them SLRR cycles without a particle loss [125].
Various procedures have been developed without a chemical immobilization of
particles for SLRR. The preparation of catalyst-coated nanoparticles is also possible
with a sedimentation process in which the nanoparticles are collected on a horizontal