Roughness in Surface Force Measurements: Extension of DLVO Theory To Describe the Forces between Hafnia Surfaces

The interaction between colloidal particles is commonly viewed through the lens of DLVO theory, whereby the interaction is described as the sum of the electrostatic and dispersion forces. For similar materials acting across a medium at pH values remote from the isoelectric point the theory typically involves an electrostatic repulsion that is overcome by dispersion forces at very small separations. However, the dominance of the dispersion forces at short separations is generally not seen in force measurements, with the exception of the interaction between mica surfaces. The discrepancy for silica surfaces has been attributed to hydration forces, but this does not explain the situation for titania surfaces where the dispersion forces are very much larger. Here, the interaction forces between very smooth hafnia surfaces have been measured using the colloid probe technique and the forces evaluated within the DLVO framework, including both hydration forces and the influence of roughness. The measured forces across a wide range of pH at different salt concentrations are well described with a single parameter for the surface roughness. These findings show that even small degrees of surface roughness significantly alter the form of the interaction force and therefore indicate that surface roughness needs to be included in the evaluation of surface forces between all surfaces that are not ideally smooth

Investigation of Ligand-Stabilized Gold Clusters on Defect-Rich Titania

Chemically synthesized atomically precise gold clusters stabilized by triphenylphosphine ligands [Au₉(PPh₃)₈]­(NO₃)₃] were deposited onto the surface of titania fabricated via atomic layer deposition. The titania surface was pretreated by heating and sputtering. After deposition of the clusters onto pretreated titania, the samples were heated at 200 °C for 20 min under ultrahigh vacuum and subsequently investigated using metastable-induced electron spectroscopy to study the electronic structure of the outermost layer of the sample and X-ray photoelectron spectroscopy to determine the chemical composition of the surface of the sample. The former study revealed that two reference spectra are needed to explain the electronic structure of the sample. One reference spectrum is related to the titania substrate, while the second spectrum is related to the presence of the Au cluster cores and the ligands removed from the cluster cores. The latter study found that the Au 4f peak is shifted to lower binding energy and the P 2p peak to higher binding energy after heating. These are interpreted in the light of ligand removal and size evolution of Au particles upon heating of the clusters on titania. The important outcome of the present work is that defects introduced at the ALD titania surface via sputtering and heating strongly reduce the agglomeration of the Au clusters adsorbed to the surface