Abstract
The deposition of mesoporous silica (SiO2) on carbon nanotubes (CNTs)
has opened up a wide range of assembling possibilities by exploiting the
sidewall of CNTs and organosilane chemistry. The resulting systems may
be suitable for applications in catalysis, energy conversion,
environmental chemistry, and nanomedicine. However, to promote the
condensation of silicon monomers on the nanotube without producing
segregated particles, (OR)(4-x)SiOxx- units must undergo nucleophilic
substitution by groups localized on the CNT sidewall during the
transesterification reaction. In order to achieve this preferential
attachment, we have deposited silica on oxidized carbon nanotubes
(single-walled and multiwalled) in a sol-gel process that also involved
the use of a soft template (cetyltrimethylammonium bromide, CTAB). In
contrast to the simple approach normally used to describe the attachment
of inorganic compounds on CNTs, SiO2 nucleation on the tube is a result
of nucleophilic attack mainly by hydroxyl radicals, localized in a very
complex surface chemical environment, where various oxygenated groups
are covalently bonded to the sidewall and carboxylated carbonaceous
fragments (CCFs) are adsorbed on the tubes. Si-O-C covalent bond
formation in the SiO2-CNT hybrids was observed even after removal of the
CCFs with sodium hydroxide. By adding CTAB, and increasing the
temperature, time, and initial amount of the catalyst (NH4OH) in the
synthesis, the SiO2 coating morphology could be changed from one of
nanoparticles to mesoporous shells. Concomitantly, pore ordering was
achieved by increasing the amount of CTAB. Furthermore, preferential
attachment on the sidewall results mostly in CNTs with uncapped ends,
having sites (carboxylic acids) that can be used for further localized
reactions.
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