Surface-Energy Control and Characterization of Nanoparticle Coatings

Abstract

Accurate and reproducible measurement of the structure and properties of high-value nanoparticles is extremely important for their commercialization. A significant proportion of engineered nanoparticle systems consist of some form of nominally core–shell structure, whether by design or unintentionally. Often, these do not form an ideal core–shell structure, with typical deviations including polydispersity of the core or shell, uneven or incomplete shells, noncentral cores, and others. Such systems may be created with or without intent, and in either case an understanding of the conditions for formation of such particles is desirable. Precise determination of the structure, composition, size, and shell thickness of such particles can prove challenging without the use of a suitable range of characterization techniques. Here, the authors present two such polymer core–shell nanoparticle systems, consisting of polytetrafluoroethylene cores coated with a range of thicknesses of either polymethylmethacrylate or polystyrene. By consideration of surface energy, it is shown that these particles are expected to possess distinctly differing coating structures, with the polystyrene coating being incomplete. A comprehensive characterization of these systems is demonstrated, using a selection of complementary techniques including scanning electron microscopy, scanning transmission electron microscopy, thermogravimetric analysis, dynamic light scattering, differential centrifugal sedimentation, and X-ray photoelectron spectroscopy. By combining the results provided by these techniques, it is possible to achieve superior characterization and understanding of the particle structure than could be obtained by considering results separately.

Determining nonuniformities of core‐shell nanoparticle coatings by analysis of the inelastic background of X‐ray photoelectron spectroscopy survey spectra

“…This conclusion is in agreement with the complementary analysis in Müller et al, 8 which showed that all PTFE‐PS CSNP samples deviate from ideality as depicted in Figure 3B. The incomplete encapsulation of the core by the shell material in the case of the PTFE‐PS samples was also confirmed by Cant et al 15 In this work, the uncoated area fraction of the core for sample PTFE‐PS(3) was determined from SEM micrographs to be 12%. This is in excellent agreement with the value cov 1 of 15% calculated by the presented QUASES analysis.…”

“…In Müller et al, 8 shell thicknesses were determined based on an analysis of XPS elastic peak intensities using the SESSA software, 2 and these values are similar to the thicknesses found here by the single island QUASES analysis. In a previous study, 15 Cant et al also determined a single PMMA overlayer thickness for the sample PTFE‐PMMA(1) by empirically fitting the XPS inelastic background and obtained d shell = 4.8 nm which is also close to the 5.5 nm found by the presented single‐island QUASES analysis.…”

“…The CSNPs in the current article have been extensively characterized in previous studies, 8,15,16 and the following information was confirmed so far. All samples have the same average core diameter of 45.4 nm but different average shell thicknesses of 7.5 and 20.3 nm for the two PTFE‐PMMA and of 3.9, 9.4, and 16.1 nm for the three PTFE‐PS CSNP samples as revealed by transmission scanning electron microscopy (T‐SEM) in Müller et al 8 (cf.…”

“…For the XSect(2) cross section, the results from the “single island” model are also presented. All values from “QUASES, two islands, XSect(1)” and from “T‐SEM” are copied from Müller et al 8 All values from “SEM” are copied from Cant et al 15 …”

Chemical in‐depth analysis of (Ca/Sr)F2 core–shell like nanoparticles by X‐ray photoelectron spectroscopy with tunable excitation energy

“…18 The importance of the method for this field is underlined by the number of articles and book chapters that have been published in recent years. [19][20][21][22][23][24][25][26][27][28] These publications not only report great advances but also point out the numerous challenges that must be considered in order to produce meaningful results with an XPS analysis of nanoparticles and nanoparticle coatings. SR-XPS has been successfully applied in the past to investigate the internal heterostructure of nanoparticles.…”