Core-and-shell nature of Stöber silica particles

Abstract

Duas amostras diferentes de partículas monodispersas de sílica de Stöber foram examinadas por microscopia eletrônica de transmissão analítica, utilizando-se diferentes tipos de imagens: campo-claro, campo escuro, imagens espectrais e mapas de distribuição elementar. As partículas (141 e 36 nm de diâmetro efetivo) contêm domínios de relação elementar O/Si e, portanto, de grau de hidratação muito variável, que coexistem com partículas medindo poucos nanômetros de diâmetro e com relação O/Si elevada, que aparecem dispersas no fundo das imagens. As imagens de campo claro e de perda de energia revelam que as partículas maiores possuem uma morfologia do tipo caroço-casca e as cascas das partículas possuem uma quantidade maior de domínios com razão O/Si maior, comparada com o interior da partícula, o que é atribuído ao acúmulo de domínios mais hidratados na casca, e também à presença de compostos de carbono, contaminantes da sílica. Por outro lado, as partículas menores (diâmetro efetivo = 36 nm) não são esféricas nem têm morfologia caroço-casca, embora também sejam formadas por domínios de composições químicas muito diferentes. Os vários mecanismos de formação de partículas apresentados na literatura são discutidos, considerando-se estes novos resultados.Two different samples of monodisperse Stöber silica particles were examined in the analytical transmission electron microscope, using different imaging modes: bright-field, dark-field, energyloss and elemental distribution maps. The particles (effective diameters = 141 and 36 nm) are formed by domains of variable O/Si ratio, which is consistent with a variable degree of hydration, and they coexist with particles with a high O/Si ratio measuring a few nanometers only, which appear dispersed in the picture background. Bright-field and energy-loss images of the larger particles show a core-and-shell morphology, and the shells have a higher amount of high-O/Si domains as well as contaminating carbon compounds. On the other hand, the smaller particles (effective diameter = 36 nm) are also formed by distinct domains, but their morphology is neither spherical or core-and-shell. The mechanisms for particle formation presented in the literature are discussed, considering the present findings.Keywords: colloidal silica, transmission electron microscopy, energy-filtered imaging, silica particle microchemistry J. Braz. Chem. Soc., Vol. 12, No. 4, 519-525, 2001. Printed in Brazil ©2001 Soc. Bras. Química 0103 -5053 $6.00+0.00 IntroductionUniform fine particles are interesting as model systems in the study of adsorption and catalysis, as well as in the study of size-dependent solid state properties such as quantum confinement 1 and superplasticity 2,3 . There have been significant achievements concerning the preparation of uniform colloid dispersions, both inorganic 4 and organic 5 . However, uniformity has often been considered only in relationship to particle size and shape, and little information is currently available concerning the uniformity of chemical composition of particl...

Ar+ ion irradiation-induced reorganization of colloidal silica nanoparticles in Langmuir–Blodgett monolayers

“…Considering our experiments, we can state that the contamination of the silica particles by Si, C, and O components is present due to the Stöber formation process [21], but their concentration is larger for 450 nm than for 220 nm diameter. On the other hand, the contamination due to the Ar+ ion beam deposited residual gas molecules is expected to linearly increase with The measured AFM data were processed using the Gwyddion software, type 2.26 (publisher: David Necas and Petr Klapetek, Czech Metrology Institute).…”

“…The contamination in the preparation process is induced by the tetraethyl orthosilicate hydrolysis, in ethanolic medium in the presence of ammonia [21]. Measurements by various diagnostic techniques discovered the presence of Si, O and C elemental contaminants in the SiO 2 particles, as well as of some hydrogen-carbon compounds [21].…”

“…Measurements by various diagnostic techniques discovered the presence of Si, O and C elemental contaminants in the SiO 2 particles, as well as of some hydrogen-carbon compounds [21]. The silica particles of larger diameter have a higher O/Si ratio at their surface, as compared to their cores [21].…”

Hydrous non-crystalline phosphates: structure, function and a new white pigment

“…The variety and complexity of their structures and properties have been often overlooked and many unexpected results were produced in the past few years. [42][43][44] A recent contribution from this laboratory shows that the sorption of surfactants and NaCl in silica nanosized particles creates unexpected spatial distributions of solutes, that were evidenced by electron spectroscopy imaging in the transmission electron microscope (ESI/TEM). The particles absorb both Na + and Cl -ions in similar amounts, from aqueous solutions, thus confirming that Stöber silica nanoparticles are highly porous and have a potential usefulness as carriers of small molecules and ions, due to the small particle size, exceptional colloidal stability added to this newly found sorption behavior.…”

“…crystalline iron phosphates. This is easily understood considering that the particles are swollen under water, behaving as for instance the nanosized Stober silica particles, 43,44 and probably sharing some of their colloidal and sorption properties. Crystallization of the amorphous particles under water has not been observed, under usual handling conditions.…”