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3.1 Introduction of ALD for Catalyst Synthesis
Heterogeneous catalysts are essential and widely used in chemical industries, environmental protection, energy storage, and conversion, for example, Fischer–Tropsch
synthesis of olefin, catalytic converter for automobiles, and methane reforming reaction for hydrogen production [1–3]. Heterogeneous catalysts with designed structures perform a key role for many applications, whereas the precise fabrication of
such structures remains the Holy Grail in catalysis research. Generally, the performance of catalysts strongly depends on their composition and structure, especially
for the surface and hetero-interfaces. In the past decades, a variety of wet chemistry
synthesis methods, such as impregnation, precipitation, and hydrothermal method,
have been developed to prepare heterogeneous catalysts. Significant progress has
been made in this field, such as the size control of metal nanoparticles, morphology
control, and surface doping. These methods lead to remarkable improvement in the
activity, stability, and selectivity of catalysts, yet there is still a gap between current
synthesis technology and the demand of catalyst fabrication with atomic accuracy. To
drive the construction of precise configurations with direct reactive sites modulation
and understand the catalytic mechanism with a clear structure–property relationship
[4], methods for catalyst synthesis with control over catalytic structures with the
atomic-level precision are urgently needed [5, 6].
Atomic layer deposition (ALD) has been recently explored as an effective method
of heterogeneous catalysts synthesis. ALD technology, considered as a unique chemical vapor deposition method, was firstly reported as atomic layer epitaxy in the 1970s
by Dr. Tuomo Suntola in Finland to prepare ZnS thin film for electroluminescent
displays [7]. The self-limiting chemical reaction of alternative gaseous precursors
on the surface of substrate has been utilized to deposit thin films. A typical ALD
process consists of two self-limiting half reactions by precursor A and B as shown in
Fig. 3.1. The first half reaction occurs after precursor A is introduced into the reactor,
which will react with active sites on the substrate until reaching a saturated chemical
adsorption. Then, the inert gas is introduced to purge the excess unreacted precursor
A and reacted by-products. Subsequently, precursor B reacts with the residual ligand
of A on the substrate to complete the second half reaction, with the same purging
process of inert gas followed. Here, we give a typical ALD reaction of Al 2 O 3 as
listed in Eq. (3.1) and (3.2) for better understanding of ALD process [8]:
AlOH
∗
+ Al(CH 3 ) 3 → AlOAl(CH 3 )
∗
2 + CH 4
(3.1)
AlCH
∗
3 + H 2 O → AlOH
∗
+ CH 4
(3.2)
With the alternating saturated adsorption and reaction of precursors, the film thickness can be accurately controlled by adjusting the ALD cycles. The self-limiting
nature of ALD also facilitates the conformal deposition on complex, high aspect
ratio structures. Until now, more than a thousand ALD processes have been developed and the applications have been expanded for catalysis field [9–11]. ALD, with
R. Chen et al.
3.1 Introduction of ALD for Catalyst Synthesis
Heterogeneous catalysts are essential and widely used in chemical industries, environmental protection, energy storage, and conversion, for example, Fischer–Tropsch
synthesis of olefin, catalytic converter for automobiles, and methane reforming reaction for hydrogen production [1–3]. Heterogeneous catalysts with designed structures perform a key role for many applications, whereas the precise fabrication of
such structures remains the Holy Grail in catalysis research. Generally, the performance of catalysts strongly depends on their composition and structure, especially
for the surface and hetero-interfaces. In the past decades, a variety of wet chemistry
synthesis methods, such as impregnation, precipitation, and hydrothermal method,
have been developed to prepare heterogeneous catalysts. Significant progress has
been made in this field, such as the size control of metal nanoparticles, morphology
control, and surface doping. These methods lead to remarkable improvement in the
activity, stability, and selectivity of catalysts, yet there is still a gap between current
synthesis technology and the demand of catalyst fabrication with atomic accuracy. To
drive the construction of precise configurations with direct reactive sites modulation
and understand the catalytic mechanism with a clear structure–property relationship
[4], methods for catalyst synthesis with control over catalytic structures with the
atomic-level precision are urgently needed [5, 6].
Atomic layer deposition (ALD) has been recently explored as an effective method
of heterogeneous catalysts synthesis. ALD technology, considered as a unique chemical vapor deposition method, was firstly reported as atomic layer epitaxy in the 1970s
by Dr. Tuomo Suntola in Finland to prepare ZnS thin film for electroluminescent
displays [7]. The self-limiting chemical reaction of alternative gaseous precursors
on the surface of substrate has been utilized to deposit thin films. A typical ALD
process consists of two self-limiting half reactions by precursor A and B as shown in
Fig. 3.1. The first half reaction occurs after precursor A is introduced into the reactor,
which will react with active sites on the substrate until reaching a saturated chemical
adsorption. Then, the inert gas is introduced to purge the excess unreacted precursor
A and reacted by-products. Subsequently, precursor B reacts with the residual ligand
of A on the substrate to complete the second half reaction, with the same purging
process of inert gas followed. Here, we give a typical ALD reaction of Al 2 O 3 as
listed in Eq. (3.1) and (3.2) for better understanding of ALD process [8]:
AlOH
∗
+ Al(CH 3 ) 3 → AlOAl(CH 3 )
∗
2 + CH 4
(3.1)
AlCH
∗
3 + H 2 O → AlOH
∗
+ CH 4
(3.2)
With the alternating saturated adsorption and reaction of precursors, the film thickness can be accurately controlled by adjusting the ALD cycles. The self-limiting
nature of ALD also facilitates the conformal deposition on complex, high aspect
ratio structures. Until now, more than a thousand ALD processes have been developed and the applications have been expanded for catalysis field [9–11]. ALD, with