Surface Chemistry in the Process of Coating Mesoporous SiO₂ onto Carbon Nanotubes Driven by the Formation of SiOC Bonds

Abstract

The deposition of mesoporous silica (SiO(2)) 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)SiO(x)(x-) 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, SiO(2) 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 SiO(2)-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 (NH(4)OH) in the synthesis, the SiO(2) 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.

Mechanisms of Colloidal Stabilization of Oxidized Nanocarbons in the Presence of Polymers: Obtaining Highly Stable Colloids in Physiological Media

“…On the other hand, the amount of OD adsorbed on the graphitic sheets (identified through event II, see Figure 2) is directly associated with the chemical oxidation process carried out for each sample as well as the nanocarbon morphology. It is know that chemical processing of MWCNTs tends to form less oxidation debris compared with SWCNT and graphene since much of the graphitic sheets of MWCNTs (i.e., inner walls) are preserved under chemical oxidation with inorganic acids 48 . In this way, considering the chemically oxidized samples used in this study (i.e., a-MWCNT and a-GO), the OD/nanocarbon weightratio for MWCNTs was smaller than for GO, thus resulting in the fact that sample a-MWCNT had much less oxidation debris than did sample a-GO (see event II in Figure 2).…”

“…In this way, considering the chemically oxidized samples used in this study (i.e., a-MWCNT and a-GO), the OD/nanocarbon weightratio for MWCNTs was smaller than for GO, thus resulting in the fact that sample a-MWCNT had much less oxidation debris than did sample a-GO (see event II in Figure 2). Furthermore, OD production is related to graphitic sheet cutting and unzipping processes induced by chemical oxidation with inorganic acids 31,44,48 . Both cutting and unzipping processes can be minimized in the oxidation through plasma treatment, thus preserving the integrity of nanotube morphology [49][50][51][52] .…”

Carbonate and sandstone reservoirs wettability improvement without using surfactants for Chemical Enhanced Oil Recovery (C-EOR)

“…118 In this work, the effect of MWCNT-SiO 2 nanohybrid based 119 nanofluids on the wettability alteration of carbonate and sand-120 stone reservoir rocks was experimentally studied. Therefore, this step must be controlled in order to produce 207 water-soluble MWCNT without destroying their morphological 208 characteristics [19]. Fig.…”

Redox‐enzymes, cells and micro‐organisms acting on carbon nanostructures transformation: A mini‐review

“…Furthermore, oxygenated surface functionalities in the sp2 graphitic structure had a substantial influence on the biodegradation of these materials by enzymes, cell or micro‐organisms 39, 41–46, 50–52, 55, 57. In a more detailed analysis, the surface microchemical environment of oxidized carbon nanotubes and graphenes comprises a myriad of oxygenated structures based on groups such as alcohols, ketones, ethers, esters, carboxylic acids, and anhydrides,85, 86 and other shorter oxidized fragments, which are strongly adsorbed on the graphitic sheets 87, 88. Even though the influence of each oxygenated group on the bioprocessing of these nanomaterials is difficult to be precisely probed, their particular chemical characteristics must be considered and surface simplifications must be avoided toward a better comprehension of their roles in the biodegradation process.…”