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pH Dependent Electronic and Geometric Structures at the Water–Silica Nanoparticle Interface
journal contribution
posted on 2014-12-18, 00:00 authored by Matthew A. Brown, Marco Arrigoni, Florent Héroguel, Amaia Beloqui Redondo, Livia Giordano, Jeroen A. van Bokhoven, Gianfranco PacchioniElectronic
and geometric structures at the water-amorphous silica
nanoparticle (NP) interface are determined as a function of suspension
pH using a combination of X-ray photoelelectron spectroscopy (XPS)
from a liquid microjet, solid-state nuclear magnetic resonance (NMR),
and density functional theory (DFT). We provide direct spectroscopic
evidence of the existence of (de)protonated silanol groups at the
liquid–NP interface and give a microscopic description of the
interface structure. The (de)protonated silanol groups, Si–OH2+ and Si–(OH)(OH2+) in acidic suspension and Si–O– and Si–(OH)(O–) in basic, give
rise to well-resolved peaks in the Si 2p spectra that allow their
identification and subsequent assignment by DFT. The change in surface
potential at the silica NP surface as a function of pH can be directly
measured by XPS and allows for an estimate of the fraction of silanol
groups that become (de)protonated at the pH of the experiments. In
agreement with DFT calculations, NMR is unable to directly identify
the (de)protonated silanol species. DFT calculations, including solvent
effects indicate that protonation of bridging O atoms can compete
with protonation of silanol groups, and that (de)protonation strongly
affects the local geometry and stability of the silica network.