alignment of the rods [61]. It is also possible, however, that the different crystal
faces of gold nanorods might react differently with reagents, and also form the
basis for rational assembly strategies of gold nanorods.
9.4
Reactivity of Metallic Nanoparticles Depends on Aspect Ratio
Understanding the chemical reactivity of nanoparticles as a function of size and
shape is a research area which is likely to lead to many useful and perhaps surprising results. We have already mentioned that gold nanoparticles @3 nm in diameter are no longer ‘‘noble’’ and can catalyze chemical reactions [4]. Poisoning of
platinum nanoparticle catalysts by sulfur compounds results in different crystal
facets of Pt becoming exposed to the environment, and the subsequent reactivity
of the Pt nanoparticles is compromised [47]. We have found that the reactivity of
gold with cyanide and persulfate depends on the size and shape of the gold nanoparticles [62].
The reaction of cyanide with gold, in the presence of air and water, produces the
gold–cyanide complex ion [63]:
4 Au þ 8 CN
À þ O 2 þ 2 H 2 O ! 4 [Au(CN) 2 ]
À þ 4 OH
À
ð1Þ
This reaction is the industrially important cyanide process for recovering gold from
rocks [63]; in that case, elemental gold is precipitated out by adding Zn dust to reduce the Au(I) back to Au(0).
Gold spheroids have a transverse plasmon band, at @530 nm and a longitudinal
plasmon band at 600–1000 nm, depending on the aspect ratio. Various aspect ratio
spheroids (aspect ratio 2–5) were used for cyanide dissolution experiments, and
all gave similar results; typical results for spheroids of aspect ratio 2.5 G 0.5 will
be described here [62]. Figure 9.9 shows the ultraviolet–visible absorption spectra
of the 2.5 G 0.5 aspect ratio gold spheroids upon reaction with cyanide. In the
absence of cyanide, the solution appears violet and has two absorption peaks at
approximately 526 nm and 685 nm. Upon addition of cyanide solution to the
nanoparticle solution, the absorption spectra changed, depending on the cyanide
concentration (Figure 9.9b–g). The violet color gradually turned red from a to d as
the cyanide concentration increased; in set e (5 Â 10
À4 M cyanide) the solution
appeared deep red; and in f and g, it was colorless. For cyanide concentrations below 2.5 Â 10
À4 M, both of the band positions decreased in absorbance, with the
band maximum at 526 nm remaining unchanged and the 685 nm band blue
shifting by @20 nm. At 5 Â 10
À4 M CN
À the long wavelength band completely
disappeared within 1 min, and the intensity of the 526 nm band decreased by
@30% within 2 min. At 10
À3 M CN
À only a weak band at 526 nm was observed. As
9.4 Reactivity of Metallic Nanoparticles Depends on Aspect Ratio 299
faces of gold nanorods might react differently with reagents, and also form the
basis for rational assembly strategies of gold nanorods.
9.4
Reactivity of Metallic Nanoparticles Depends on Aspect Ratio
Understanding the chemical reactivity of nanoparticles as a function of size and
shape is a research area which is likely to lead to many useful and perhaps surprising results. We have already mentioned that gold nanoparticles @3 nm in diameter are no longer ‘‘noble’’ and can catalyze chemical reactions [4]. Poisoning of
platinum nanoparticle catalysts by sulfur compounds results in different crystal
facets of Pt becoming exposed to the environment, and the subsequent reactivity
of the Pt nanoparticles is compromised [47]. We have found that the reactivity of
gold with cyanide and persulfate depends on the size and shape of the gold nanoparticles [62].
The reaction of cyanide with gold, in the presence of air and water, produces the
gold–cyanide complex ion [63]:
4 Au þ 8 CN
À þ O 2 þ 2 H 2 O ! 4 [Au(CN) 2 ]
À þ 4 OH
À
ð1Þ
This reaction is the industrially important cyanide process for recovering gold from
rocks [63]; in that case, elemental gold is precipitated out by adding Zn dust to reduce the Au(I) back to Au(0).
Gold spheroids have a transverse plasmon band, at @530 nm and a longitudinal
plasmon band at 600–1000 nm, depending on the aspect ratio. Various aspect ratio
spheroids (aspect ratio 2–5) were used for cyanide dissolution experiments, and
all gave similar results; typical results for spheroids of aspect ratio 2.5 G 0.5 will
be described here [62]. Figure 9.9 shows the ultraviolet–visible absorption spectra
of the 2.5 G 0.5 aspect ratio gold spheroids upon reaction with cyanide. In the
absence of cyanide, the solution appears violet and has two absorption peaks at
approximately 526 nm and 685 nm. Upon addition of cyanide solution to the
nanoparticle solution, the absorption spectra changed, depending on the cyanide
concentration (Figure 9.9b–g). The violet color gradually turned red from a to d as
the cyanide concentration increased; in set e (5 Â 10
À4 M cyanide) the solution
appeared deep red; and in f and g, it was colorless. For cyanide concentrations below 2.5 Â 10
À4 M, both of the band positions decreased in absorbance, with the
band maximum at 526 nm remaining unchanged and the 685 nm band blue
shifting by @20 nm. At 5 Â 10
À4 M CN
À the long wavelength band completely
disappeared within 1 min, and the intensity of the 526 nm band decreased by
@30% within 2 min. At 10
À3 M CN
À only a weak band at 526 nm was observed. As
9.4 Reactivity of Metallic Nanoparticles Depends on Aspect Ratio 299