48
K. Illath et al.
surface faces. Figure 9d shows the schematic illustration of silver assisted Au NPs
synthesis using chloride and bromide ion. Addition of chloride ion leads to the
formation of {720} faceted concave cubes, while the addition of bromide leads to
the formation of a tetrahedron with {730} facets. Hence, the effect of halides has to
be investigated in detail. It should be studied in the presence and absence of silver
ions. The effect of halides in the absence of silver ions are further studied and found
that {100} faceted cubes are formed with bromide ion. In contrast, the addition
of iodide ion leads to the formation of {111} faceted octahedra and {111} faceted
truncated bitetrahedra with 10 and 75 μM sodium iodide, respectively. Introduction
of iodide or bromide ion in the presence of chloride ion reduces the rate of Au NPs
formation by two mechanisms; by lowering the reduction potential and solubility
of gold ion and inhibiting the growth by binding the halides strongly to the surface
of gold particles. However, these two mechanisms are difficult to separate from
obtaining specific shapes. Another important parameter in shape-controlled seeded
growth of Au NPs is silver ions. Silver prefers to deposit epitaxially onto gold sites,
where it has a high coordination number concerning gold. Hence, favourable growth
of high index facets happens by stabilizing the facets through increasing the silver
ions in the growth solution. This is due to the underpotential deposition (UPD)
of silver onto the growing surface of gold. The effect of halides in the presence
of silver ions is depicted in Fig. 9e–h. In the absence of bromide, a mixture of
{110}-faceted bipyramids and {110}-faceted rhombic dodecahedra are formed. By
increasing the amount of bromide, particle size, and yield of bipyramids decreased
while that of rhombic dodecahedra increased. Comparison of the effects of halides
with and without silver ions shows that surface effects are more influential with
silver assisted synthesis, while kinetic effects dominate in the absence of silver. But
increasing the amount of halide beyond some value inhibits the deposition of silver
due to the destabilization of the Ag UPD layer in silver assisted synthesis. This is not
possible with iodide. Figure 10 shows the schematic diagram of the summary of the
shape-controlled synthesis of Au NPs using halides in the presence or absence of
silver ions. A higher concentration of halide ions are more favourable for surface
effects and results in destabilization of the Ag UPD layer. This effect is observed in
increasing order with chloride, bromide, and iodide [62]. Hence, the concentration
of reducing agent, silver ions, and selection of halides has to be optimized for the
specific shape of Au NPs using a seeded growth method.
Critical parameters in controlling the size of MNP in the chemical reduction
method without the involvement of seed are the type of reagent and its concentration
[31, 63–69]. Other parameters like temperature, reaction time, reagent concentration,
molar ratio, pH, and type of additives can also influence, as explained previously [63,
68–74]. Effect of reducing agents trisodium citrate and NaBH 4 on the formation of
Au NPs and its absorbance is shown in Fig. 11. The observed trend is that as the
concentration of trisodium citrate increases, particle distribution becomes monodispersed. The reverse trend is observed with NaBH 4 as a reducing agent. It might be
because NaBH 4 is a strong reducing agent; hence as the concentration increases,
the availability of reducing agents to reduce the metallic salt is higher. So, nucleation is favoured compared to the growth process. Since nucleation is chaotic than
K. Illath et al.
surface faces. Figure 9d shows the schematic illustration of silver assisted Au NPs
synthesis using chloride and bromide ion. Addition of chloride ion leads to the
formation of {720} faceted concave cubes, while the addition of bromide leads to
the formation of a tetrahedron with {730} facets. Hence, the effect of halides has to
be investigated in detail. It should be studied in the presence and absence of silver
ions. The effect of halides in the absence of silver ions are further studied and found
that {100} faceted cubes are formed with bromide ion. In contrast, the addition
of iodide ion leads to the formation of {111} faceted octahedra and {111} faceted
truncated bitetrahedra with 10 and 75 μM sodium iodide, respectively. Introduction
of iodide or bromide ion in the presence of chloride ion reduces the rate of Au NPs
formation by two mechanisms; by lowering the reduction potential and solubility
of gold ion and inhibiting the growth by binding the halides strongly to the surface
of gold particles. However, these two mechanisms are difficult to separate from
obtaining specific shapes. Another important parameter in shape-controlled seeded
growth of Au NPs is silver ions. Silver prefers to deposit epitaxially onto gold sites,
where it has a high coordination number concerning gold. Hence, favourable growth
of high index facets happens by stabilizing the facets through increasing the silver
ions in the growth solution. This is due to the underpotential deposition (UPD)
of silver onto the growing surface of gold. The effect of halides in the presence
of silver ions is depicted in Fig. 9e–h. In the absence of bromide, a mixture of
{110}-faceted bipyramids and {110}-faceted rhombic dodecahedra are formed. By
increasing the amount of bromide, particle size, and yield of bipyramids decreased
while that of rhombic dodecahedra increased. Comparison of the effects of halides
with and without silver ions shows that surface effects are more influential with
silver assisted synthesis, while kinetic effects dominate in the absence of silver. But
increasing the amount of halide beyond some value inhibits the deposition of silver
due to the destabilization of the Ag UPD layer in silver assisted synthesis. This is not
possible with iodide. Figure 10 shows the schematic diagram of the summary of the
shape-controlled synthesis of Au NPs using halides in the presence or absence of
silver ions. A higher concentration of halide ions are more favourable for surface
effects and results in destabilization of the Ag UPD layer. This effect is observed in
increasing order with chloride, bromide, and iodide [62]. Hence, the concentration
of reducing agent, silver ions, and selection of halides has to be optimized for the
specific shape of Au NPs using a seeded growth method.
Critical parameters in controlling the size of MNP in the chemical reduction
method without the involvement of seed are the type of reagent and its concentration
[31, 63–69]. Other parameters like temperature, reaction time, reagent concentration,
molar ratio, pH, and type of additives can also influence, as explained previously [63,
68–74]. Effect of reducing agents trisodium citrate and NaBH 4 on the formation of
Au NPs and its absorbance is shown in Fig. 11. The observed trend is that as the
concentration of trisodium citrate increases, particle distribution becomes monodispersed. The reverse trend is observed with NaBH 4 as a reducing agent. It might be
because NaBH 4 is a strong reducing agent; hence as the concentration increases,
the availability of reducing agents to reduce the metallic salt is higher. So, nucleation is favoured compared to the growth process. Since nucleation is chaotic than