3 Catalysts via Atomic Layer Deposition
93
In the following works, it was found that NiO x ALD performed with Ni(Cp) 2 and
O 3 as the precursors preferred to initiate growth on the edge sites of Pt, indicating the
complexity involved in the driving force of the selective ALD. During the nucleation
stage, Pt’s low coordinated sites were selectively passivated by NiO x and these sites
usually were the most unstable sites and tended to gasify at high temperature in
oxidizing environment [123]. The NiO x /Pt/Al 2 O 3 catalysts showed improved activity
toward CO oxidation. On the other hand, the thermal stability of the catalysts could
be also improved because of the selective passivation of the unstable edge sites. As
a result, after calcination at 700 °C, the Pt NPs size and catalytic activity could be
maintained.
Theoretical simulations are helpful to gain more insights of selective growth
origins. For example, Al 2 O 3 deposited with TMA showed different binding energies on Pd’s facets and low coordinated sites. It was found that TMA dissociative
adsorption preferably took take place thermodynamically at the low coordinated sites
of Pd rather than at the Pd (111) facets. On the other hand, the binding energy differences on Pt were much smaller than those on Pd surfaces. Thus, the selective growth
of Al 2 O 3 could be achieved on the Pd surface, and continuous coating layers would
form on Pt surface instead. The selective ALD process of MO x (M=Co, Ni, Fe) on
Pt based on DFT simulations is summarized in Fig. 3.9d [144]. The reaction barriers
and rates calculations showed that the M(Cp) 2 precursors prefer to start growth on
the edge sites of Pt. Based on the micro-kinetics analysis, the growth rate of MO x on
Pt sites followed the order of edge > (100) > (111). On the other hand, the activities
of M(Cp) 2 precursors on edge sites of Pt followed the order of Ni(Cp) 2 > Fe(Cp) 2 >
Co(Cp) 2 . The DFT calculations also indicated that this selectivity was temperature
dependent. Upon increasing the deposition temperature, edge sites selectivity would
decrease since the growth rate differences between edge and (100) decreased.
3.3.4 Catalysts Synthesis by ALD with Confined Structure
The confined catalytic structure was developed to achieve a trade-off between the
catalytic stability and activity. The structure could not only give rise to active metal
oxide interfaces, but also expose a large amount of metal NPs surfaces to reactants
compared with the overcoated structure. There were several methods to fabricate
the confined catalysts through ALD. For example, organic groups were utilized to
selectively chemisorb on the metal NPs before ALD; thus, the organic layer prevented
the deposition of oxide materials directly on the metal NPs. After ALD process, the
organic groups were removed to re-expose the metal NPs again and nanobowl-like
oxides around the metal NPs were formed (Fig. 3.10a). Stair et al. reported the
blocking ability of different organic groups during Al 2 O 3 ALD. The blocking ability
of the organic agents was influenced by chain lengths, functional groups, as well
as steric hindrance effects [145]. Chen et al. exploited ODT (1-octadecanethiol)
to selectively block Pt and successfully fabricated the Co 3 O 4 nanotrap structure
around Pt NPs that simultaneously improved the activity and sintering resistance
93
In the following works, it was found that NiO x ALD performed with Ni(Cp) 2 and
O 3 as the precursors preferred to initiate growth on the edge sites of Pt, indicating the
complexity involved in the driving force of the selective ALD. During the nucleation
stage, Pt’s low coordinated sites were selectively passivated by NiO x and these sites
usually were the most unstable sites and tended to gasify at high temperature in
oxidizing environment [123]. The NiO x /Pt/Al 2 O 3 catalysts showed improved activity
toward CO oxidation. On the other hand, the thermal stability of the catalysts could
be also improved because of the selective passivation of the unstable edge sites. As
a result, after calcination at 700 °C, the Pt NPs size and catalytic activity could be
maintained.
Theoretical simulations are helpful to gain more insights of selective growth
origins. For example, Al 2 O 3 deposited with TMA showed different binding energies on Pd’s facets and low coordinated sites. It was found that TMA dissociative
adsorption preferably took take place thermodynamically at the low coordinated sites
of Pd rather than at the Pd (111) facets. On the other hand, the binding energy differences on Pt were much smaller than those on Pd surfaces. Thus, the selective growth
of Al 2 O 3 could be achieved on the Pd surface, and continuous coating layers would
form on Pt surface instead. The selective ALD process of MO x (M=Co, Ni, Fe) on
Pt based on DFT simulations is summarized in Fig. 3.9d [144]. The reaction barriers
and rates calculations showed that the M(Cp) 2 precursors prefer to start growth on
the edge sites of Pt. Based on the micro-kinetics analysis, the growth rate of MO x on
Pt sites followed the order of edge > (100) > (111). On the other hand, the activities
of M(Cp) 2 precursors on edge sites of Pt followed the order of Ni(Cp) 2 > Fe(Cp) 2 >
Co(Cp) 2 . The DFT calculations also indicated that this selectivity was temperature
dependent. Upon increasing the deposition temperature, edge sites selectivity would
decrease since the growth rate differences between edge and (100) decreased.
3.3.4 Catalysts Synthesis by ALD with Confined Structure
The confined catalytic structure was developed to achieve a trade-off between the
catalytic stability and activity. The structure could not only give rise to active metal
oxide interfaces, but also expose a large amount of metal NPs surfaces to reactants
compared with the overcoated structure. There were several methods to fabricate
the confined catalysts through ALD. For example, organic groups were utilized to
selectively chemisorb on the metal NPs before ALD; thus, the organic layer prevented
the deposition of oxide materials directly on the metal NPs. After ALD process, the
organic groups were removed to re-expose the metal NPs again and nanobowl-like
oxides around the metal NPs were formed (Fig. 3.10a). Stair et al. reported the
blocking ability of different organic groups during Al 2 O 3 ALD. The blocking ability
of the organic agents was influenced by chain lengths, functional groups, as well
as steric hindrance effects [145]. Chen et al. exploited ODT (1-octadecanethiol)
to selectively block Pt and successfully fabricated the Co 3 O 4 nanotrap structure
around Pt NPs that simultaneously improved the activity and sintering resistance