Deep Insights into the Processes Occurring during Early Stages of the Formation and Room-Temperature Evolution of the Core (Amorphous SiO₂)@Shell (Organocations) Nanoparticles

Abstract

Following the assumption that the crucial processes governing the formation, properties and evolution of the core(amorphous silica)@shell(organocations) nanoparticles take place during short-time, room-temperature (rt) stirring/aging of the homogeneous reaction mixtures (HmRMs) formed by hydrolysis of TEOS (tetraethyl orthosilicate) in solutions of Org(OH)n, we investigated these processes by various experimental methods (pH, ionic conductivity, 29 Si-NMR, dynamic light scattering and atomic force microscopy). The analysis of the data obtained by detail and careful investigation of the "model" HmRMs having the starting chemical composition: xTEOS:0.25TPAOH:20H2O (TPAOH = tetrapropylammonium hydroxide; x = 0.05 -1), offer some new elements for the understanding of the mechanisms of formation and rt evolution of the core@shell silica nanoparticles: (1) There is a resolute evidence of the formation of the stable, about 1.2 nm sized core(amorphous SiO2)@shell(TPA + ions) nanoparticles below the critical aggregation concentration (CAC). (2) Due to the intensive particulate processes (growth, aggregation, disaggregation, dissolution) which take place during the rt aging of the investigated HmRMs, the equilibrated core@shell silica nanoparticles do not exist as individual primary ones, but as the aggregates (about 2 nm to about 20 nm), composed of 1 -2 nm sized "primary" nanoparticles. (3) In spite of the most frequent meaning that the nanoparticle shell is composed of the "free" TPA + ions adsorbed on the surface of the nanoparticle core, the results of this study show that the nanoparticle shell can be formed mainly by attachment of the polysilicate anions (silicate oligomers), associated with TPA + ions, on the surfaces of the nanoparticles cores.

Impact of Organic Templates on the Selective Formation of Zeolite Oligomers

“… kMC results using the extended set of reactions illustrated in Scheme 2 and 29 Si NMR experimental data with templates TMA + [28] (a) or TPA + [29] (b) for the different silica oligomers in terms of their Si‐O‐Si connectivity expressed as Q n where n is the number of bridging oxygen atoms per SiO 4 . In (a), experimental results are reported after 0.67 h and 1 h of reaction while in (b), experimental results are reported at two initial reactant/template/water proportions (with x [Tetraethyl orthosilicate]/0.25[TPAOH]/20[H 2 O] and x =0.145 and x =0.24).…”

“…This is typically expressed as Q n following the notation used in 29 Si‐NMR spectroscopy where n is the number of “Si” units attached through a bridging oxygen to an individual SiO 4 unit: Q 0 for monomers, Q 1 for Si‐O‐Si dimers (in black), Q 2 for Si‐O‐Si‐O‐Si chains (in red), Q 3 (in orange) and Q 4 (in green) for branched oligomers. We used this connectivity descriptor for all oligomers in this study since this is the essential one monitored by 29 SI NMR experiments [28, 29] …”

Controlled aggregation of core(amorphous silica)@shell(TPA+-polysilicate) nanoparticles at room temperature by selective removal of TPA+ ions from the nanoparticle shell

“…In spite of widely spread meaning that the silica nanoparticles cannot be formed below the CAC, 15,16,21,27,30,35,[47][48][49]51 our recent study resolute evidenced the formation of stable, about 1 -2 nm-sized core(amorphous SiO 2 )@shell(TPA + ions) nanoparticles, below the CAC. 52 Earlier models of the sub-colloidal PNPs included about 5 nm-sized globular units (GUs) composed of several tetrapods, that may be either amorphous or crystalline 40 and about 3 nmsized, predominantly amorphous primary precursor species (PPSs), formed by aggregation of several inorganic-organic composite species. 25,36,42 Both these models assume homogeneous distribution of TAA + ions in the nanoparticle.…”

Regulating Nonclassical Pathways of Silicalite‐1 Crystallization through Controlled Evolution of Amorphous Precursors

“…Silicalite-1 and its aluminosilicate isostructure ZSM-5 both have aM FI framework consisting of 3-dimensional pores.ZSM-5 is one of the most widely used zeolites in commercial applications, [10] whereas silicalite-1 is commonly used as am odel system for mechanistic studies of zeolite growth. [13] Upon heating,nanoparticles evolve in both size and microstructure,b ut still lack longrange order. [13] Upon heating,nanoparticles evolve in both size and microstructure,b ut still lack longrange order.…”

“…[11] Silicalite-1 growth solutions are sols of nanoparticles (1-6 nm) [11a, 12] with ac ore-shell structure (Scheme 1) comprised of solvated amorphous silica and ashell of physisorbed OSDA. [13] Upon heating,nanoparticles evolve in both size and microstructure,b ut still lack longrange order. [14] It has been hypothesized that the degree to which nanoparticles structurally evolve leads to differences in their kinetic rates of attachment to silicalite-1 surfaces (k i , Scheme 1).…”