192
S. Luo et al.
of the latter reduced metal. As shown in Fig. 5.4, if the second metal atoms are
deposited symmetrically on the nucleus of the first metal, a core–shell structure will
be formed [53, 54]. If deposition occurs only on a specific surface of the nucleus of
the first metal, a heterostructured nanocrystal will be synthesized [55]. If the second
metal atoms diffuse into the lattice of the first metal to form a metal–metal bond, an
alloy or intermetallic compound will be obtained [56–58]. The chemical behavior
of metals is closely related to their intrinsic properties and that tends to be dynamically stable. Specially, reaction conditions such as solvents, ligands and surfactants
can change the dynamic equilibrium state of materials, thus affecting the structure
of as-synthesized products. For example, for the coreduction of metals A and B, if
the redox potential of A is higher than that of B, nanoparticles with A-core/B-shell
structure are usually formed. However, a B-core/A-shell structure can be produced by
adding surfactants that can strongly bind to A. In practical synthesis, the coreduction
method could easily fabricate alloys or intermetallic compounds [59].
By adopting appropriate reductant and reaction system, many researchers
prepared bimetallic catalysts with controllable components, uniform size and welldefined shape, which greatly promoted the development of coreduction method.
Sodium borohydride is a strong reductant, but the nucleation and growth of the two
metals can be well controlled by choosing appropriate reaction processes and conditions, and adding surfactants or ligands that can effectively adsorb on the surface
of nucleus [60]. High-quality nanoparticles with tunable size can be synthesized by
the introduction of surfactants and foreign ions, or adjusting other reaction parameters. In addition, the composition of bimetallic nanoparticles can be controlled by
changing the feeding molar ratio of two metal precursors, and their morphology can
be controlled with the coreaction path.
5.2.2 Seed-Mediated Growth Method
To prepare multimetallic catalysts with complex and tunable nanostructure, especially the core–shell structured or heterostructured catalysts, seed-mediated growth
method is an available and effective strategy, which can fabricate catalysts with
sophisticated structure that are usually difficult to achieve by other methods. Utilizing
the as-prepared nanoparticles as crystal seed, seed-mediated growth method ensures
the reduction of other metal ions and nucleation on the seed in solution, then the
deposition of atoms continuously, forming different structures [61–63].
If the metal atoms deposit uniformly and symmetrically on the surface of the
seed, a core–shell structure with separated components will be formed. If the metal
atoms deposit on a specific surface of the seed, a heterostructure will be formed.
If the metal atoms diffuse into the lattice of seed, a homogeneous alloy structure
will be formed. The size, composition, shape, facets and dispersity of seed will
affect the final structure of products. Generally, seeds should be uniform in size
and evenly dispersed. The surface of seed should be cleaned or properly treated,
ensuring their uninform dispersion in the reaction solution. The interaction between
S. Luo et al.
of the latter reduced metal. As shown in Fig. 5.4, if the second metal atoms are
deposited symmetrically on the nucleus of the first metal, a core–shell structure will
be formed [53, 54]. If deposition occurs only on a specific surface of the nucleus of
the first metal, a heterostructured nanocrystal will be synthesized [55]. If the second
metal atoms diffuse into the lattice of the first metal to form a metal–metal bond, an
alloy or intermetallic compound will be obtained [56–58]. The chemical behavior
of metals is closely related to their intrinsic properties and that tends to be dynamically stable. Specially, reaction conditions such as solvents, ligands and surfactants
can change the dynamic equilibrium state of materials, thus affecting the structure
of as-synthesized products. For example, for the coreduction of metals A and B, if
the redox potential of A is higher than that of B, nanoparticles with A-core/B-shell
structure are usually formed. However, a B-core/A-shell structure can be produced by
adding surfactants that can strongly bind to A. In practical synthesis, the coreduction
method could easily fabricate alloys or intermetallic compounds [59].
By adopting appropriate reductant and reaction system, many researchers
prepared bimetallic catalysts with controllable components, uniform size and welldefined shape, which greatly promoted the development of coreduction method.
Sodium borohydride is a strong reductant, but the nucleation and growth of the two
metals can be well controlled by choosing appropriate reaction processes and conditions, and adding surfactants or ligands that can effectively adsorb on the surface
of nucleus [60]. High-quality nanoparticles with tunable size can be synthesized by
the introduction of surfactants and foreign ions, or adjusting other reaction parameters. In addition, the composition of bimetallic nanoparticles can be controlled by
changing the feeding molar ratio of two metal precursors, and their morphology can
be controlled with the coreaction path.
5.2.2 Seed-Mediated Growth Method
To prepare multimetallic catalysts with complex and tunable nanostructure, especially the core–shell structured or heterostructured catalysts, seed-mediated growth
method is an available and effective strategy, which can fabricate catalysts with
sophisticated structure that are usually difficult to achieve by other methods. Utilizing
the as-prepared nanoparticles as crystal seed, seed-mediated growth method ensures
the reduction of other metal ions and nucleation on the seed in solution, then the
deposition of atoms continuously, forming different structures [61–63].
If the metal atoms deposit uniformly and symmetrically on the surface of the
seed, a core–shell structure with separated components will be formed. If the metal
atoms deposit on a specific surface of the seed, a heterostructure will be formed.
If the metal atoms diffuse into the lattice of seed, a homogeneous alloy structure
will be formed. The size, composition, shape, facets and dispersity of seed will
affect the final structure of products. Generally, seeds should be uniform in size
and evenly dispersed. The surface of seed should be cleaned or properly treated,
ensuring their uninform dispersion in the reaction solution. The interaction between