80
R. Chen et al.
enhanced catalytic selectivity and activity have been shown for the core–shell structured NPs. In addition, the core–shell structured NPs cost less if the core material is replaced with a cheaper material, as the consumption of noble metal can be
reduced. The main challenge during the fabrication is to achieve selective deposition, to ensure the growth of shell materials exclusively on the preformed cores. In
typical metal ALD processes, the second metal will deposit on the substrate. As a
result, monometallic and core–shell NPs are both formed during this process. Up to
now, three types of ALD strategies have been reported to fabricate core–shell structured catalysts, which are focused on the selective deposition of the second metal
on the first metal core but not on the substrate to prevent the formation of mixture
compositions.
It was observed that the Pt growth via ALD using MeCpPtMe 3 and O 2 at 300 °C
on Al 2 O 3 substrate was suppressed when the O 2 partial pressure was decreased to
7.5 mTorr [71]. It was found that even after 600 cycles, Pt growth on Al 2 O 3 was
still inhibited. However, the growth of Pt on Pd was immediately occurred without
nucleation delay and the growth rate reached to 0.45 Å/cycle with the reduced O 2
partial pressure ALD process. After Pt deposition, the particles’ density per unit
area was not changed indicating no new nuclei were formed during Pt ALD process.
Thus, selective deposition of Pt on Pd was achieved. The presence of the Pt shell was
confirmed by the TEM images (Fig. 3.5a) [72].
Another process that could realize selective growth to fabricate core–shell NPs
was tuning the deposition temperature which strongly affected the nucleation and
growth rate. On the other hand, bimetallic alloy NPs could be fabricated by dosing two
metal precursors with an ABC-type ALD process. For example, Pd and Pt precursors
can be dosed on the substrate alternately to form Pd/Pt alloys [73]. The composition
and the ratio of the two metals in the alloy can be tuned by adjusting the ALD cycles
of two precursors.
Yet another approach to fabricate core–shell NPs was utilizing the area selective ALD (AS-ALD) method on the substrate modified with octadecyltrichlorosilane (ODTS) self-assembled monolayer (SAMs). SAMs was formed by immersing
the substrate in ODTS solution. The growth time of SAMs was controlled before
a continuous and defect-free SAMs layer was formed. In this way, SAMs with
nanoscale pinholes could be formed on the substrate, these pinholes were not covered
by ODTS group, and the reactive hydroxyl functional groups of the substrate were
exposed. Thus, these pinholes on the substrate could act as the nucleation sites in
the ALD reaction. On the contrary, the other part of the substrate was covered by
continuous ODTS SAMs that was inert toward ALD reaction, because the methyl
end groups of ODTS SAMs could block precursor chemisorption. The core and shell
materials growth could only take place at pinhole sites to form core–shell NPs as
shown in Fig. 3.5b [74]. Compared with the substrate without ODTS modification,
Pt also deposited homogeneously on the substrate rather than exclusively on Pd cores
without special ALD process control.
The ability of controlling the composition and structures of core–shell NPs with
atomic-scale accuracy opens new opportunities to gain fundamental understanding of
R. Chen et al.
enhanced catalytic selectivity and activity have been shown for the core–shell structured NPs. In addition, the core–shell structured NPs cost less if the core material is replaced with a cheaper material, as the consumption of noble metal can be
reduced. The main challenge during the fabrication is to achieve selective deposition, to ensure the growth of shell materials exclusively on the preformed cores. In
typical metal ALD processes, the second metal will deposit on the substrate. As a
result, monometallic and core–shell NPs are both formed during this process. Up to
now, three types of ALD strategies have been reported to fabricate core–shell structured catalysts, which are focused on the selective deposition of the second metal
on the first metal core but not on the substrate to prevent the formation of mixture
compositions.
It was observed that the Pt growth via ALD using MeCpPtMe 3 and O 2 at 300 °C
on Al 2 O 3 substrate was suppressed when the O 2 partial pressure was decreased to
7.5 mTorr [71]. It was found that even after 600 cycles, Pt growth on Al 2 O 3 was
still inhibited. However, the growth of Pt on Pd was immediately occurred without
nucleation delay and the growth rate reached to 0.45 Å/cycle with the reduced O 2
partial pressure ALD process. After Pt deposition, the particles’ density per unit
area was not changed indicating no new nuclei were formed during Pt ALD process.
Thus, selective deposition of Pt on Pd was achieved. The presence of the Pt shell was
confirmed by the TEM images (Fig. 3.5a) [72].
Another process that could realize selective growth to fabricate core–shell NPs
was tuning the deposition temperature which strongly affected the nucleation and
growth rate. On the other hand, bimetallic alloy NPs could be fabricated by dosing two
metal precursors with an ABC-type ALD process. For example, Pd and Pt precursors
can be dosed on the substrate alternately to form Pd/Pt alloys [73]. The composition
and the ratio of the two metals in the alloy can be tuned by adjusting the ALD cycles
of two precursors.
Yet another approach to fabricate core–shell NPs was utilizing the area selective ALD (AS-ALD) method on the substrate modified with octadecyltrichlorosilane (ODTS) self-assembled monolayer (SAMs). SAMs was formed by immersing
the substrate in ODTS solution. The growth time of SAMs was controlled before
a continuous and defect-free SAMs layer was formed. In this way, SAMs with
nanoscale pinholes could be formed on the substrate, these pinholes were not covered
by ODTS group, and the reactive hydroxyl functional groups of the substrate were
exposed. Thus, these pinholes on the substrate could act as the nucleation sites in
the ALD reaction. On the contrary, the other part of the substrate was covered by
continuous ODTS SAMs that was inert toward ALD reaction, because the methyl
end groups of ODTS SAMs could block precursor chemisorption. The core and shell
materials growth could only take place at pinhole sites to form core–shell NPs as
shown in Fig. 3.5b [74]. Compared with the substrate without ODTS modification,
Pt also deposited homogeneously on the substrate rather than exclusively on Pd cores
without special ALD process control.
The ability of controlling the composition and structures of core–shell NPs with
atomic-scale accuracy opens new opportunities to gain fundamental understanding of