Controlling the morphology and pore size of mesostructured silica nanoparticles: the role of the iron oxidation state

Abstract

We demonstrate morphology and pore size dependence of silica nanoparticles (SNPs) synthesized via control of the iron oxidation state. In the absence of any Fe species, only spherical SNPs are produced, whereas in the presence of Fe³⁺ and Fe²⁺ ions, SNPs with rod-like and nanosheet morphologies, respectively, are formed. The average pore size increases from 1.7 nm in the absence of iron to 3.2 and 5.9 nm as Fe³⁺ and Fe²⁺, respectively, were used during the synthesis. Both samples of SNPs synthesized in the presence of Fe²⁺ and Fe³⁺ have 0.2 wt% of tetrahedral iron in the silica framework, whereas most of the iron is in the silica extraframework, as verified by Mössbauer spectroscopy, UV-vis diffuse reflectance, FTIR, XRD data and TPR analysis. These Fe²⁺ and Fe³⁺ cations play a fundamental role in controlling these properties because they change the curvature and the surface charge density of CTAB micelles, thus favoring the spherical to rod-like transition. The rod-like shape was retained in Fe-containing samples, whereas a nanosheet-like morphology was produced in Fe²⁺-containing samples due to the breakage of silica walls during the thermal treatment to remove the template. The control of the textural properties is interesting to allow the fabrication of selective photocatalysts for oxidation of different organic substrates.