Surface analytical study of self-assembled nanophase particle (SNAP) surface treatments

Abstract

The recent discovery of a Self-assembled NAnophase Particle (SNAP) process of forming functionalized silica nanoparticles in situ from hydrolyzed tetramethoxysilane (TMOS) and glycidoxypropyltrimethoxysilane (GPTMS) in an aqueous sol–gel process, and then crosslinking the nanoparticles to form a thin, dense, protective film on Al aerospace alloys, is an excellent example of a nanoscience approach to coatings. To investigate the surface chemistry of the SNAP films, X-ray photoelectron spectroscopy (XPS) was utilized to obtain detailed chemical state information on the coating constituents. During the course of these surface analytical studies, a charge referencing method from which accurate and reliable photoelectron peak binding energies could be determined was developed. Use of an internal standard allowed the spectra to be charge referenced, and the referenced data enabled accurate identification of chemical states in the SNAP coatings. Results indicate that the Si bonds present in the SNAP film are a combination of the bonds in the individual precursors TMOS and GPTMS. These data support the concept that the nanosized siloxane macromolecule structures of the precursors are retained through the coating application process and basically comprise the film. There is little evidence of other chemical reactions occurring. This chemical state information was verified by a silicon chemical state plot and the calculated modified Auger parameter, which fell between the precursors’ Auger parameter values. The XPS peak data and Auger parameter data are both self-consistent and consistent with the presented model of the SNAP film. The XPS analysis provides an overview of the film chemistry and changes that could occur during the processing. Researchers involved in the XPS analysis of organic coatings could benefit greatly from the demonstration and application of good charge referencing techniques.