Comparing Micro and Nano Spectroscopy

Tools: Raman, PiFM, ToF-SIMS, & SEM

Background

Many analytical techniques can provide researchers with crucial insights into how our world works at the micro and nano scales. Since each technique has its own strengths, using many analytical techniques in concert can be an extremely effective approach. PiFM (photo-induced force microscopy) is one of the only imaging techniques that can provide direct chemical data with sub-5 nm resolution. Similarly, PiF-IR (photo-induced force infrared) spectroscopy is one of the only options for getting IR absorption spectra on monolayer materials. These specifications sound impressive on their own, but the real utility lies in an instrument’s ability to help people solve important scientific questions.

A Nature Communications paper used a variety of techniques, named the micro-spectroscopy toolbox, to analyse polyolefin catalysts [1]. The goal was to explore the formation of polyethylene on a catalyst model that is more relevant to the industrial production of polyethylene. These data provide an excellent showcase of how different advanced nanoscale analytical tools can complement one another to efficiently study complex chemical systems.

The Model

The authors focused on polyolefin catalysts which, due to their hierarchically complex nature, are usually studied using a simplified planar model system whereas highly spherical catalyst particles are used industrially. To bridge the gap between the spherical catalysts and planar catalyst model, the authors introduce a spherical cap model that can be analysed by all the techniques in their microspectroscopy
toolbox.

The material discussed is a Ziegler-type catalyst spherical cap model which is based on a moisture-stable LaOCl framework designed to support a TiCl4 pre-active site. This is created to be as consistent as possible with the industrially relevant MgCl2 framework that is technically and experimentally limited due to high moisture sensitivity. Therefore, the authors were inspired to design the LaOCl support matrix because it would provide strong SEM imaging contrast due to the high atomic weight of the lanthanide, and because of its stability in ambient conditions. The primary goal behind this new spherical cap model is to provide a system in which to study ethylene polymerisation process ex-situ, but in a system more like the highly spherical industrial framework.

Advantage of PiFM

PiFM provides:

• Sub-5 nm spatial resolution
• Sub-monolayer sensitivity
• Chemical identification via IR absorption
• Operation in ambient conditions

This makes it ideal for detecting chemical composition and uniformity at the nanoscale.

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Summary

The authors of this paper were able to use their microspectroscopy toolbox to make some impressive observations about the polymerisation of ethylene in this LaOCl catalyst matrix. Some of the techniques in their toolbox, like XPS and ToF-SIMS were most helpful during the synthesis of the LaOCl spherical caps. ToF-SIMS did help them understand the polymerisation rate of polyethylene, but the most impressive results about the rate of crystallisation were made using PiF-IR spectroscopy. Raman played a key role in the authors’ understanding of where the strongest formation of polyethylene was early in the polymerisation times.

However, the high resolution of the PiFM images were able to not only corroborate that finding but also show the structure of the polyethylene fibres themselves. They even quantified the thickness of the polyethylene layer via PiFM and the AFM. Besides the insights from these vibrational techniques, the FIB-SEM images played a key role in understanding the fragmentation behavior of the samples as a function of polymerisation time. This was made easy due to the stark contrast of the atomic weights of their materials. However, without that advantage they would have likely needed to use chemical techniques like Raman or PiFM to observe the morphologies of different materials with similar atomic weights. Overall, these data show an impressive array of results from each technique, and the authors do an excellent job of highlighting the strengths of the tools in their micro-spectroscopy toolbox.

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See also…