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Silica, not that neutral…

Silica: an "electron pump" which ionises nanosilver!

by Guy Molénat - published on

Amorphous silica is commonly used as a coating matrix to protect metals from oxidation because it is considered to be inert. But is it true? By DFT modelling, we have shown that a silver nanoparticle surrounded by silica becomes cationic. Not only does it form chemical bonds with the silica atoms but it also transfers electrons to silica over a distance of 4 Å. Moreover, if defects are present in silica, they act as "electron pumps" and the nanoparticle loses even more electrons!

Silver nanoparticles (AgNP), extensively studied in recent years due to their remarkable biological and optical properties, find many applications in sensing, optoelectronics and as antimicrobials. However, because of their toxicity and their high propensity for oxidation and sulfidation, these nanoparticles require to be incorporated into a matrix for many applications. Nevertheless, despite their frequent use, the effect of the matrix on the nanoparticle properties remains largely unknown.

By means of dispersion-corrected Density Functional Theory (DFT) calculations, we have examined the effect of an amorphous silica matrix on the structure and charge distribution of 55- and 147-atom AgNPs. We observe that covalent bondings between the AgNP and the matrix occur at the interface involving the breaking of Si–O bonds, which systematically leads to the formation of Ag–Si bonds, and in some cases, to the formation of Ag–O ones. Interestingly, these interface reconstructions are accompanied by electron depletion of the nanoparticles, a substantial number of electrons being transferred from the two outer shells of the AgNP to the surrounding silica medium. The electrons lost by the nanoparticles are captured by the Si atoms involved in the interface bonds, but also, unexpectedly, by the atoms of the silica network inside a few angströms spherical shell around the AgNP, this electronic extension going beyond that attributable to the AgNP spill-out. The numbers of interface bonds and electrons transferred to the surrounding silica shell appear to be proportional to the surface area of the AgNP. Another noteworthy point is the electron pump effect of the undercoordinated silica atoms when silica is defective. The presence of additional electrons in the matrix, especially on defects, is consistent with the experimental literature.

Implications of this finding are important for the AgNP chemical, electrochemical and optical properties, in particular for its surface reactivity which drives the AgNP dissolution mechanisms. In addition, the presence of electrons trapped on defects in the matrix predicted by our simulations will impact the conduction properties of such nanocomposite devices.

This work is part of a transverse project between the NeO and SINAnO groups in collaboration with J. Puibasset (ICMN, Orléans). It is supported by the ANR BENDIS “Interaction of biological targets with solid dielectric layers consisting of silver nanoparticles embedded in silica matrices: Towards tailored antimicrobial surfaces”.

 

Publication:
This work has been published in Nanoscale, 2022,14, 7280-7291
https://doi.org/10.1039/D1NR05884C

 

Contacts:
Magali Benoit – magali.benoit[at]cemes.fr
Nathalie Tarrat – natahlie.tarrat[at]cemes.fr
Caroline Bonafos – caroline.bonafos[at]cemes.fr