AVS2015 Session AS-ThM: Practical Surface Analysis III: Multiple-technique Problem-solving and Structure-property Correlations

In the past few years, polymer composites of polytetrafluoroethylene (PTFE) and alumina particles have attracted a lot of interest as a promising Ultra-Low Wear (ULW) material. It has been discovered that by adding a small amount of alumina additive (<5wt%) to PTFE, the wear rate of the PTFE is enhanced dramatically by over four orders of magnitude. Although this polymer composite system has shown its uniqueness and importance in the tribological research, the tribochemical mechanism has not been well understood.

To fully understand the chemistry that occurs during the tribology/wearing, it requires the precise design of experiments as well as applying integrated techniques to study the tribochemical process. In our studies, we investigated the tribochemical products by studying the unique chemistry of the transfer film formed at the composite and stainless steel interfaces during wearing. XPS, FTIR, and ToF-SIMS techniques have been applied to determine the chemistry of these tribochemical products as a function of number of sliding cycles. These characterization techniques have allowed us to understand the ultralow wear behavior and help to develop a conceptual framework for the ultralow wear material system.

Time-based glide-force changes in pre-filled syringes built into autoinjector systems can lead to negative patient experience issues and dose accuracy problems. The long term performance of prefilled syringe systems is examined during stability testing study when monoclonal antibody solutions are stored at various temperatures for given time periods. Multiple parameters including lubricant amount, lubricant distribution, and surface chemistry contribute to friction between the plunger and barrel and consequently the glide force. Therefore, it is important to understand the impact of storage conditions on lubricant characteristics and surface properties so that the desirable long term performance can be achieved.

In the current study, a monoclonal antibody solution was filled in glass syringes coated with silicone lubricant. Increased glide forces were observed from syringes stored at 25 ^oC, comparing to those stored at 5 ^oC. Multiple analytical techniques, including ellipsometry, ICP-OES, contact angle, XPS and TOF-SIMS were utilized to characterize lubricant and surface properties of syringe interior surfaces to understand the root cause of undesirable glide force changes.

Polypropylene mesh has been used to surgically treat stress urinary incontinence and has shown promising short-term results. However, serious complications have been associated with longer term implanted meshes. We hypothesize that these complications are due to oxidative degradation of the mesh brought about by reactive oxygen species that are released by adherent macrophages on the surface of the polypropylene.

Polypropylene is known to oxidize through a stable hydroperoxide (-COOH) intermediate, followed by chain scission and formation of a carbonyl (-C=O) end group. Oxidation of polypropylene leads to a reduction in molecular weight, embrittlement, cracking, and eventually fracture and fragmentation. An in vitro study was performed to study the oxidation of polypropylene surgical mesh in a model environment that simulates conditions the mesh would experiences in the body. Samples cut from three commercially available surgical devices produced by two manufacturers and polypropylene control samples were placed in an oxidizing medium consisting of 20% H₂O₂ and 0.1 M CoCl₂.

Samples were placed in the oxidizing medium and were incubated at 37 °C on a shaker for up to 6 weeks. The oxidizing medium was replaced every 3 to 4 days. Six samples were removed every week, washed in DI water, and dried. XPS and FTIR were used to analyze the samples for the presence of hydroperoxide and carbonyl species. SEM micrographs were acquired at 0, 4, and 5 weeks of oxidation.

Over the past 30 years there has been a remarkable range of advances in the ability to understand the nature of many types of important surfaces and interfaces. X-ray photoelectron spectroscopy (XPS) has become nearly essential for characterizing many types of materials, verifying the quality of synthesis methods and understanding interactions of these materials in many environments and applications. This has become possible because of major improvements in instrumentation including stability, reproducibility, increases in spatial resolution, increases in count rate, major improvements in the ability to analyze insulating samples, digital control of instruments and advances in the ability to process, model and analyze the data. For Auger electron spectroscopy (AES) similar advances have occurred, with spatial resolution being an obvious differentiator. In the area of Secondary Ion Mass Spectrometry, the major excitement has been related to the evolution of new primary and sputter beams with Bucky ball and cluster sources. Thirty years ago scanning probe methods were just beginning to appear; now they serve as critical tools for many types of studies.

In spite of the significant progress, many important materials information needs remain, providing opportunities for continued evolution of traditional surface analysis tools and the development and applications of other methods. These might be framed in the context of some “what if” questions that are not out of the range of the possible. What if we could monitor in real-time the evolution of surfaces and buried interfaces (and the actual compositional and chemical information at those interfaces) in “operational” environments? What if we could obtain quantitative compositional and chemical information at the resolution of an AFM? What if micro-technology could be used to create an XPS unit that produced quality data and operated without or with only trivial need for vacuum? What would be possible if NMR could be easily used to quantitatively characterize surfaces, interfaces or individual particles? Examples of progress along these directions will be described. All indications are that the next 30 years will be as dynamic and productive as the past 30 years, if not more so.

Catalysts have traditionally been characterized before or after reactions and analyzed based on static representations of surface structures. It is shown here how dynamic changes on a catalyst’s chemical state and morphology can be followed during a reaction by a combination of in situ microscopy (AP-STM) and spectroscopy (AP-XPS and AP-IRRAS). In addition to determining the active phase of the catalyst by in situ methods, the presence of weakly adsorbed surface species or intermediates generated only in the presence of reactants can be determined, allowing in turn the comparison of experimental results with first principle modeling of specific reaction mechanisms. Three reactions are used to exemplify the approach: CO oxidation (CO + 1/2O₂ → CO₂), water gas shift reaction (CO + H₂O → CO₂ +H₂) and methanol synthesis (CO₂ + 3H₂ → CH₃OH + H₂O). During CO oxidation, the full conversion of Cu⁰ to Cu⁺² deactivates an initially outstanding catalyst. This can be remedied by the formation of a TiCuO_x mixed-oxide that protects the presence of active partially oxidized Cu⁺ cations. We also show the switch from a redox mechanism on Cu(111) to a more efficient associative mechanism pathway for the WGSR at the interface of ceria nanoparticles deposited on Cu(111) [1]. Similarly, the activation of CO₂ at the ceria/Cu(111) interface allows its facile hydrogenation to methanol [2]. Our combined studies emphasize the need of searching for optimal metal/oxide interfaces, where multifunctional sites can lead to new efficient reaction pathways.

[1] Angew. Chem. Int. Ed. 52, 5101–5105 (2013)

[2] Science, 345, 546-550 (2014)

A single geological sample may have multiple phases, each with different chemical bonding environments. Geologists will typically have access to SEM-EDS, giving them elemental information from the different grains (from bulk depths). When it comes to chemical information, however, XPS provides a unique ability to easily acquire data from these small grains. It offers quantitative chemical bonding data with a probe size small enough to acquire data from individual phases.

In this work, rock cross-sections were analysed with XPS to quantify the elements in different phases and to identify and quantify the oxidation states of elements in these regions. The analysis can be divided into four main sections: 1) Finding the features of interest, 2) Aligning the X-ray spot on the features, 3) Acquiring the XPS data and 4) Processing the XPS data to answer the key questions about chemical bonding states.

Any XPS tool used for this analysis needs to have a high quality optical system, enabling to geologist to rapidly find the features of interest, with a live optical feedback system. The grains were typically 10s of microns in size, so a small X-ray analysis area is required. This small area analysis was achieved in two different ways: shrinking the

X-ray spot to match the feature size or collecting spectroscopic parallel imaging data. In the latter case, every pixel in the image has full XPS data allowing the analyst to perform sub-10mm area analysis directly from the X-ray image.

Techniques complementary to XPS can also help provide a more complete characterization of geological samples. SEM/Auger can offer analysis with a much smaller probe size than XPS, but it is necessary to charge compensate during the experiment. Charge compensation methodology will be discussed in this work.