3 Catalysts via Atomic Layer Deposition
77
ALD method have a narrower size distribution than spherical ones of the catalysts
prepared by impregnation methods. Flat cubic cobalt nanoparticles in ALD catalysts
exhibit more uniform cobalt species and a stronger interaction with Al 2 O 3 .
3.2.2 Monometallic Nanoparticle, Cluster, and Single Atom
Synthesis
ALD was utilized to fabricate highly dispersed monometallic (Ru, Pd, Pt, Ir, Ni, etc.)
nanoparticles utilizing the nucleation growth stage of the ALD process [48–52]. The
nanoparticle size (typically 1–3 nm with small deviations) and density distribution
can be precisely tuned through the deposition temperature and the number of deposition cycles. It has also become an important method to synthesize single atom
catalysts. The size optimization of metal catalysts is a key parameter to evaluate
the utilization of catalysts since catalytic reactions occur almost exclusively on the
surface. Moreover, the utilizations for the noble metals are extraordinarily important
due to their high costs and limited reserves. It has been reported that downsizing
the metal catalysts to nanoscale can greatly increase the number of surface sites for
reactants’ adsorption and reaction [53, 54]. The metal nanoparticles can also possess
unique catalytic performances due to the interfacial interactions between metal and
supports [55]. Due to the stronger cohesive energy of metal atoms as compared to
the binding energy between metal and supports, the initial stage of metal ALD is
usually in the island growth mode instead of layer-by-layer growth. Aroused by such
nucleation phenomena, many researchers have utilized the first few ALD cycles to
synthesize metal nanoparticles [50, 56]. Compared to conventional wet chemistry
methods, the gas-phase-based ALD method with the self-limiting nature of surface
reactions can satisfy the high dispersion requirement of supported metal nanoparticles. Moreover, the size of metal nanoparticles can be readily controlled by ALD
recipes.
Up to now, several parameters in ALD process such as ALD cycles, deposition
temperature, ALD precursor, and surface structure of substrate (shown in Fig. 3.3)
have been utilized to control the size of nanoparticles. Varying ALD cycles is the most
straightforward way of controlling the size of nanoparticles at the nucleation stage
[56]. The average sizes of Pt and Pd nanoparticles have been reported to show linear
correlation from 1 to 5 nm by selecting the appropriate number of ALD cycles [57,
58]. Besides the number of ALD cycles, the deposition temperature can also affect
the average size of deposited metal nanoparticles by controlling the aggregation
process of metal nanoparticles on the substrate. It has been reported that a low
deposition temperature usually results in narrow particle size distribution [59]. Since
the oxidizing atmosphere could lead to atom or cluster mobility on the surface,
the ripening of metal nanoparticles can be promoted during ALD growth. It has
been reported that the surface diffusion phenomena can be suppressed by replacing
the oxidative precursor with reductive or inert precursors. Consequently, the size
77
ALD method have a narrower size distribution than spherical ones of the catalysts
prepared by impregnation methods. Flat cubic cobalt nanoparticles in ALD catalysts
exhibit more uniform cobalt species and a stronger interaction with Al 2 O 3 .
3.2.2 Monometallic Nanoparticle, Cluster, and Single Atom
Synthesis
ALD was utilized to fabricate highly dispersed monometallic (Ru, Pd, Pt, Ir, Ni, etc.)
nanoparticles utilizing the nucleation growth stage of the ALD process [48–52]. The
nanoparticle size (typically 1–3 nm with small deviations) and density distribution
can be precisely tuned through the deposition temperature and the number of deposition cycles. It has also become an important method to synthesize single atom
catalysts. The size optimization of metal catalysts is a key parameter to evaluate
the utilization of catalysts since catalytic reactions occur almost exclusively on the
surface. Moreover, the utilizations for the noble metals are extraordinarily important
due to their high costs and limited reserves. It has been reported that downsizing
the metal catalysts to nanoscale can greatly increase the number of surface sites for
reactants’ adsorption and reaction [53, 54]. The metal nanoparticles can also possess
unique catalytic performances due to the interfacial interactions between metal and
supports [55]. Due to the stronger cohesive energy of metal atoms as compared to
the binding energy between metal and supports, the initial stage of metal ALD is
usually in the island growth mode instead of layer-by-layer growth. Aroused by such
nucleation phenomena, many researchers have utilized the first few ALD cycles to
synthesize metal nanoparticles [50, 56]. Compared to conventional wet chemistry
methods, the gas-phase-based ALD method with the self-limiting nature of surface
reactions can satisfy the high dispersion requirement of supported metal nanoparticles. Moreover, the size of metal nanoparticles can be readily controlled by ALD
recipes.
Up to now, several parameters in ALD process such as ALD cycles, deposition
temperature, ALD precursor, and surface structure of substrate (shown in Fig. 3.3)
have been utilized to control the size of nanoparticles. Varying ALD cycles is the most
straightforward way of controlling the size of nanoparticles at the nucleation stage
[56]. The average sizes of Pt and Pd nanoparticles have been reported to show linear
correlation from 1 to 5 nm by selecting the appropriate number of ALD cycles [57,
58]. Besides the number of ALD cycles, the deposition temperature can also affect
the average size of deposited metal nanoparticles by controlling the aggregation
process of metal nanoparticles on the substrate. It has been reported that a low
deposition temperature usually results in narrow particle size distribution [59]. Since
the oxidizing atmosphere could lead to atom or cluster mobility on the surface,
the ripening of metal nanoparticles can be promoted during ALD growth. It has
been reported that the surface diffusion phenomena can be suppressed by replacing
the oxidative precursor with reductive or inert precursors. Consequently, the size