Sebastian Calderon1 Tânia Ribeiro2 José Paulo Farinha2 Carlos Baleizão2 Paulo Ferreira1 3

1, International Iberian Nanotechnology Laboratory (INL), Braga, , Portugal
2, Universidade de Lisboa, Lisbon, , Portugal
3, The University of Texas at Austin, Austin, Texas, United States

Mesoporous silica nanoparticles have been intensively studied due to their potential use in catalysis and biomedicine, including cancer treatment and drug delivery applications. However, when mesoporous particles are produced at the nanoscale, the arrangement of pores is modified, hindering the characterization of the porous structure. In order to determine their morphology and structure, advanced microscopy techniques are usually required. However, the complexity of the pore structure makes the characterization very challenging, in particular, an accurate representation in 3D.

This work combines molecular dynamics techniques and electron microscopy computer simulations with experimental results to provide an insight into the structure of amorphous SiO2 NPs. The amorphous silica model is prepared using a simple melt-quench molecular dynamics (MD) method, while the reconstruction of the mesoporous nanoparticle is carried out using an isotropic unit cell to avoid false symmetry in the final model. For the high-resolution STEM simulations, the QSTEM software package is used based on a multislice technique. Finally, for comparison with the simulated images high angular annular dark field (HAADF) STEM images were taken using an aberration-corrected FEI Titan ChemiSTEM microscope, operated at 200 kV.

The amorphous models are analyzed using the radial distribution function (RDF) and mass density, demonstrating a good agreement with the experimental results. Depending on the quenching model, the local density can be modified obtaining isotropic values between 2.2 and 4 g cm-3, with radial distribution function similar to the bulk values reported experimentally. The highest probability of finding Si-O pairs is at 1.58 Å, O-O is at 2.62 Å and Si-Si 3.08 Å. The multislice STEM images demonstrate that the density of the models does not have a significant impact on the STEM images for isolated SiO2 phases. However, a detailed analysis reveals that the intensities of the systems show that denser SiO2 structures result in a more intense signal, due to the increase in the scattering power.
When constructing the nanoporous particles, an isotropic unit cell of the previously mentioned amorphous SiO2 model was utilized. In order to avoid false symmetry in the STEM images, the unit cell was randomly rotated finding good agreement with the experimental images obtained. The results show the possibility of accurately modeling amorphous SiO2 amorphous phases, and opens the possibility of simulating nanoparticles when functionalized for catalysis and biomedical applications.