Adhesion Mechanisms of the Contact Interface of TiO2 Nanoparticles in Films and Aggregates

S Salameh,P Facci,L Colombi Ciacchi,J Schneider,A Alessandrini,L Mädler,Jens Laube,J W Seo

doi:10.1021/la302242s

Abstract

Fundamental knowledge about the mechanisms of adhesion between oxide particles with diameters of few nanometers is impeded by the difficulties associated with direct measurements of contact forces at such a small size scale. Here we develop a strategy based on AFM force spectroscopy combined with all-atom molecular dynamics simulations to quantify and explain the nature of the contact forces between 10 nm small TiO(2) nanoparticles. The method is based on the statistical analysis of the force peaks measured in repeated approaching/retracting loops of an AFM cantilever into a film of nanoparticle agglomerates and relies on the in-situ imaging of the film stretching behavior in an AFM/TEM setup. Sliding and rolling events first lead to local rearrangements in the film structure when subjected to tensile load, prior to its final rupture caused by the reversible detaching of individual nanoparticles. The associated contact force of about 2.5 nN is in quantitative agreement with the results of molecular dynamics simulations of the particle-particle detachment. We reveal that the contact forces are dominated by the structure of water layers adsorbed on the particles' surfaces at ambient conditions. This leads to nonmonotonous force-displacement curves that can be explained only in part by classical capillary effects and highlights the importance of considering explicitly the molecular nature of the adsorbates.

Highlights

Adhesion forces between nanoparticles play a major role in unit operations such as fluidization,[1] agglomeration, and coating.[2]
Adhesion forces between micro- and nanoparticles can be directly quantified with force spectroscopy measurements using an atomic force microscope (AFM),[12] as performed for instance by Larson et al.[13] after functionalizing the AFM tips with single microparticles
We investigate the adhesion mechanisms of TiO2 nanoparticle aggregates in porous films using a combination of AFM force spectroscopy, in-situ AFM/TEM, and all-atom molecular dynamics (MD) techniques

Summary

Introduction

Adhesion forces between nanoparticles play a major role in unit operations such as fluidization,[1] agglomeration, and coating.[2] These processes are utilized in a wide range of technical fields, including the production of pharmaceutical powders,[2] paints,[3] and solar cells.[4] Currently, particle−particle adhesion is interpreted in terms of continuum models that are able to take into account the effects of humidity (capillary forces)[5−7] surface roughness and electrostatics (e.g., within DLVO-like theories)[8,9] and hold especially for particle sizes in the micrometer range. Ong and Sokolov[14] have presented a method to attach ceria (CeO2) nanoparticles (50 nm) on an AFM tip with epoxy glue. They used these functionalized tips to measure the adhesion force between the nanoparticles and a flat silica surface. Gluing a colloid on a tipless cantilever and measuring the adhesion force against a film of Received: June 1, 2012 Revised: July 10, 2012 Published: July 11, 2012

Methods

Results

Conclusion