Page 80 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

66 Principles and Methods

TABLE 3.3 Guidelines for the Choice of Reducing Agents and Reaction Conditions
in the Precipitation of Metal Particles [117]
0
Metal species E (V) Reducing Agent Conditions Rate
3 4
Au , Au , Pt ,
0.7 organic acids,
70 C Slow
2
Pt , Pd 2 alcohols, polyols
3 2 3
Ag , Rh , Hg , Ir aldehydes, sugars 50 C Moderate
Ambient Fast
hydrazine, H 2 SO 3
NaBH 4 , boranes, Ambient Very fast
2
3
Cu , Re , Ru 3 0.7 and
0 polyols 120 C Slow
aldehydes, sugars 70–100 C Slow
hydrazine, hydrogen 70 C Moderate
Ambient Fast
NaBH 4
2
2
2
Cd , Co , Ni , Fe 2 0 and
polyols 180 C Slow
0.5
3
2
3
In , Sn , Mo , W 6 hydrazine 70–100 C Slow
NaBH 4 , boranes Ambient Fast

hydrated e , radicals Ambient Very fast
3 2 5 2
Cr , Mn , Ta , V 0.6 NaBH 4 , boranes T, P ambient Slow

hydrated e , radicals Ambient Fast
The driving force of the reaction is the difference, E , between the stan-
0
z
dard redox potentials of the two redox couples implicated, E(M /M ) and
E(Ox/Red). The value of E determines the composition of the system
through the equilibrium constant K given by:
ln K nF E /RT
The reaction is thermodynamically possible if E is positive, but prac-
tically, its value must be at least 0.3 to 0.4, otherwise the reaction pro-
ceeds too slowly to be useful. Thus, highly electropositive metals (Ag, Au,
0 0
Pt, Pd, Ru, Rh) with standard potentials E (Mn /M ) 0.7 V react with
mild reducing agents while more electronegative metals (Co, Fe, Ni)
0 0
with E (Mn /M ) 0.2 V need strongly reducing agents and have to
be manipulated with care because the metallic nanoparticles are very
sensitive to oxidation. Some widely used reactions are listed in Table 3.3.
Complexation of cations in solution plays an important role on their
reducibility and, as the stability of complexes increases, reduction is
more difficult (Table 3.4).
TABLE 3.4 Influence of Silver Ion Complexation on the Redox Potential
Couple logK E (V)
0
Ag e → Ag 0.80
+ 0
Ag(NH 3 ) 2 e → Ag 2 NH 3 7.2 0.38
3 0
Ag(S 2 O 3 ) 2 e → Ag 2 S 2 O 3 13.4 0.01
2 0
Ag(CN) 3 e → Ag 3 CN 3 22.2 0.51