Page 62 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

48 Principles and Methods

are formed. Therefore, the morphology of the particles is heavily depend-
ent upon the conditions of acidity in which condensation takes place. The
catalysis of silica condensation may also be affected by nucleophilic acti-
vation using additives such as 4-dimethylaminopyridine (DMAP, see
Table 3.1). Particles and polymers may remain dispersed in the medium,
forming sols, or they can agglomerate and gel more or less rapidly,
depending on the surface charge density of particles and consequently
on the pH of the medium. On either side of pH 2, gelation is faster
because acid or base catalysis accelerate the condensation rate of Si-OH
groups between particles. At pH 2, the surface charge is too small to
provide efficient repulsion between particles. At pH 2, base catalysis of
oxolation has the same effect, which is maximum for pH 6. For pH 6,
the surface charge is high enough for the sol to remain stable.
The reactivity of metal alkoxides is also deeply influenced by their
molecular structure and complexity that depends on the steric hin-
drance of the alkoxo ligands, OR, especially for the transition element
alkoxides. Due to the fact that the oxidation state, z, is generally smaller
than the coordination number of the metal, it inhibits coordination of
species. For instance, this occurs
the metal in the monomeric M(OR) z
i
in the case of titanium alkoxide Ti(O Pr) , which is a monomer in iso-
4
propanol. The coordination of titanium is only four and the reaction with
water leads to instantaneous precipitation of heterogeneous and amor-
phous titania particles. With ethoxy ligands, titanium forms oligomeric
species [Ti(OEt) ] (n 3 in benzene, n 2 in EtOH) in which the tita-
4 n
nium coordination is higher, n 5 in the trimer, n 6 in the dimer
because of the formation of a solvate [Ti(OEt) ] (EtOH) . Monodispersed
4 2
2
spherical particles have been synthesized by controlled hydrolysis of a
diluted solution of Ti(OEt) in EtOH [47]. The monodispersity clearly
4
results from slower hydrolysis and condensation reactions with less
reactive precursors allowing decoupling of the nucleation and growth
steps. It is however possible to control the reactivity of low coordinated
titanium in the presence of specific ligands. For instance, hydrolysis at
60 C of titanium butoxide Ti(OBu) 4 in the presence of acetylacetone
forms monodispersed 1–5 nm TiO 2 anatase nanoparticles [48]. A very ele-
gant design of the shape of anatase nanospheres and nanorods is
i
obtained by controlling the rate of hydrolysis of Ti(O Pr) at 80 C in the
4
presence of oleic acid.
In a general way, the rate of reactions and the nature of condensed
species obtained depend also on the hydrolysis ratio defined as h H O/M.
2
■ Molecular clusters are formed with very low hydrolysis ratios (h 1).
The condensation reactions are relatively limited. Hydrolysis of
[Ti(OEt) ] 2(EtOH) forms soluble species such as Ti O (OEt) 20
4 2
4
7
(h 0,6), Ti O (OEt) 24 (h 0,8) or Ti O (OEt) 32 (h 1). A variety
8
10
16
16