Surface studies of halloysite nanotubes by XPS and ToF‐SIMS

This report provides detailed experimental results of thermal and surface characterization on untreated and surface‐treated halloysite nanotubes (HNTs) obtained from two geographic areas. Surface characterization techniques, including XPS and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) were used. ToF‐SIMS surface analysis experiments were performed with both atomic and cluster ion beams. Higher ion yields and more high‐mass ions were obtained with the cluster ion beams. Static ToF‐SIMS spectra were analyzed with principal component analysis (PCA). Morphological diversities were observed in the samples although they mainly contained tubular structures. Thermogravimetric data indicated that aqueous hydrogen peroxide solution could remove inorganic salt impurities, such as alkali metal salts. The amount of grafting of benzalkonium chloride of HNT surface was determined by thermogravimetic analysis. PCA of ToF‐SIMS spectra could distinguish the samples mined from different geographical locations as well as among surface‐treated and untreated samples. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of the surface chemistry of oxidized polyethylene: comparison of XPS and ToF‐SIMS

A series of low‐density polyethylene (LDPE) surfaces, chemically modified using a number of oxidative techniques employed for adhesion enhancement (pretreatments), have been studied by time‐of‐flight (ToF) SIMS and XPS. The methods consisted of corona discharge, flame, electrochemical, chromic acid, acid dichromate and acid permanganate treatment. All except flame treatment were performed under mild and fairly severe conditions to yield a range of surface chemistries. The XPS analysis, using high energy resolution and a refined approach to C 1s curve‐fitting, provided some new insights into the quantitative assessment of the type and concentration of functional groups. Both positive and negative ion ToF‐SIMS spectra were obtained at high mass resolution. The oxygen‐containing fragments were identified by accurate mass analysis and subjected to a detailed comparison with the XPS results. No convincing relative intensity correlations could be identified that would allow particular secondary ion fragments to be associated strongly with particular functional groups (in this multi‐functional surface situation). Inorganic residues resulting from wet chemical treatments were also investigated and here the two techniques were found to be more complementary. Copyright © 2003 John Wiley & Sons, Ltd.     

Quantitative analysis of styrene‐pentafluorostyrene random copolymers by ToF‐SIMS and XPS

Poly(styrene) (PS), poly(2,3,4,5,6‐pentafluorostyrene) (5FPS) and their random copolymers were prepared by bulk radical polymerization. The spin‐cast polymer films of these polymers were analyzed using X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). The surface and bulk compositions of these copolymers were found to be same, implying that surface segregation did not occur. The detailed analysis of ToF‐SIMS spectra indicated that the ion fragmentation mechanism is similar for both PS and 5FPS. ToF‐SIMS quantitative analysis using absolute peak intensity showed that the SIMS intensities of positive styrene fragments, particularly C₇H₇⁺, in the copolymers are higher than the intensities expected from a linear combination of PS and 5FPS, while the SIMS intensities of positive pentafluorostyrene fragments are smaller than expected. These results indicated the presence of matrix effects in ion formation process. However, the quantitative approach using relative peak intensity showed that ion intensity ratios are linearly proportional to the copolymer mole ratio when the characteristic ions of PS and 5FPS are selected. This suggests that quantitative analysis is still possible in this copolymer system. Copyright © 2005 John Wiley & Sons, Ltd.     

XPS and ToF‐SIMS characterization of a Finemet surface: effect of heating

X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) were used to study the surface composition and electronic structure of Finemet, Fe₇₃Si_15.8B_7.2Cu₁Nb₃, in the original amorphous state and after gradual heating in vacuum to a temperature of 400 °C and cooling back to room temperature. It was found that relaxation processes occurring during heat treatment well below the crystallization onset caused the physico‐chemical state of Finemet surface to change irreversibly. In the relaxed alloy, the surface originally covered with the native air‐formed oxide was significantly enriched with elemental iron and depleted of other alloy constituents compared with the original state. Yet in the as‐quenched state, clustering of copper atoms on the Finemet surface was detected which was enhanced by heating. The thermal treatment resulted in the selective reduction of iron oxides and caused noticeable changes in the valence band structure and the Fe L₃VV Auger spectrum associated with atomic redistribution. Copyright © 2009 John Wiley & Sons, Ltd.     

XPS and ToF‐SIMS characterization of a Finemet surface: effect of cooling

The surface composition of amorphous Finemet, Fe₇₃Si_15.8B_7.2Cu₁Nb₃, was studied by X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). The as‐received sample in the original state and after Ar⁺ sputter‐cleaning was analyzed at room temperature as well as after cooling to ? 155 °C. In the cooled state, the surface oxide layer composed of oxides of the alloy constituents was found to become enriched with elemental iron and depleted of elemental silicon, boron, oxygen and carbon as compared to the state at room temperature. Interaction of residual water vapor and hydrogen with the complex oxide layer occurring at low temperatures is believed to be responsible for the enhanced formation of surface hydroxides of the alloy constituents. The processes resulting in the observed redistribution of the elements on the surface of Finemet at low temperatures are discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Repair of defects created by Ar+ sputtering on graphite surface by annealing as confirmed using ToF‐SIMS and XPS

Defects were created on the surface of highly oriented pyrolytic graphite (HOPG) by sputtering with an Ar⁺ ion beam, then characterized using X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) at 500°C. In the XPS C1s spectrum of the sputtered HOPG, a sp³ carbon peak appeared at 285.3 eV, representing surface defects. In addition, 2 sets of peaks, the C_x⁻ and C_xH⁻ ion series (where x = 1, 2, 3...), were identified in the ToF‐SIMS negative ion spectrum. In the positive ion spectrum, a series of C_xH₂^+• ions indicating defects was observed. Annealing of the sputtered samples under Ar was conducted at different temperatures. The XPS and ToF‐SIMS spectra of the sputtered HOPG after 800°C annealing were observed to be similar to the spectra of the fresh HOPG. The sp³ carbon peak had disappeared from the C1s spectrum, and the normalized intensities of the C_xH⁻ and C_xH₂^+• ions had decreased. These results indicate that defects created by sputtering on the surface of HOPG can be repaired by high‐temperature annealing.     

Formation and electrochemical desorption of self‐assembled monolayers as studied by ToF‐SIMS

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) was used to study a number of processes involving thiol‐based self‐assembled monolayers (SAMs) on nontextured (polycrystalline) gold (Au) films deposited on Si wafers. ToF‐SIMS turned out to be a convenient and versatile semiqualitative technique which readily verified electrochemical desorption of a SAM and formation of another SAM on the same sample via reincubation with another thiol. The technique, allowing one to follow simultaneously more than one species on the surface, showed that any formation of a mixed SAM on surfaces which did not undergo electrolysis was negligible with the applied time scale (minutes). Copyright © 2010 John Wiley & Sons, Ltd.     

ToF‐SIMS and XPS surface characterization of novel perfluoropolyether–urethane ionomers from aqueous dispersions

In this work a new class of ionomeric block perfluropolyether (PFPE) polyurethanes are presented; these polymers are obtained in the form of aqueous dispersions due to the presence of hydrophilic sites (ionomeric groups such as acetates or trialkylammonium salts) along the macromolecular chain, offering the chance to combine PFPEs in a variety of possible structures for coating or surface treatments with an environmentally friendly use. X‐ray photoelectron spectroscopy analysis at two different sampling depths, as well as time‐of‐flight secondary ion mass spectrometry analysis modelled by the use of principal component analysis (PCA), were used to investigate the first nanometres of the surface samples. It resulted in a clear surface enrichment in fluorine, and the different extent of the fluorine stratification will be discussed in relation to the ionic character, film‐forming from water and cross‐linking. Copyright © 2003 John Wiley & Sons, Ltd.     

ToF‐SIMS studies of the oxidation of Fe by D2O vapour: comparison with XPS

The oxidation of iron (Fe) by water (D₂O) vapour at low pressures and room temperature was investigated using time‐of‐flight (ToF) SIMS. The results supported those found previously using XPS and the QUASES^? program in that a duplex oxide structure was found containing a thin outer surface hydroxide (Fe(OD)₂) layer over an inner oxide (FeO) layer. The extraordinary depth resolution of the ToF‐SIMS profiles assisted in identifying the two phases; this resolution was achieved by compensation for surface roughness. A substantial concentration of deuterium was found in the subsurface oxide layer. This observation confirmed previous assessments that the formation of FeO was from the reaction of Fe(OD)₂ with outward‐diffusing Fe, leaving deuterium as a reaction product. Copyright © 2005 John Wiley & Sons, Ltd.     

Examination of the interface of a model adhesive joint by surface analysis: a study by XPS and ToF‐SIMS

Model samples of the interface of an adhesive joint containing small levels of aminopropyl triethoxysilane (APS) have been prepared in order to examine the interface formed with an aluminium substrate. X‐ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF‐SIMS) have been used to analyse and image the interface region in between the aluminium and an epoxy adhesive in order to ascertain the reactions by the organosilane which is present as a minor component within the system. It was found that APS was present at the interface between the adhesive and the substrate and that it had reacted with the substrate forming a covalent bond and was also crosslinked within the adhesive. Evidence of near to full hydrolysis of APS is also present within the spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Image fusion combining SEM and ToF‐SIMS images

Image fusion allows for the combination of an image containing chemical information but low spatial resolution with a high‐spatial resolution image having little to no chemical information. The resulting hybrid image retains all the information from the chemically relevant original image, with improved spatial resolution allowing for visual inspection of the spatial correlations. In this research, images were obtained from two sample test grids: one of a copper electron microscope grid with a letter ‘A’ in the center (referred to below as the ‘A‐grid’), and the other a Tantalum and Silicon test grid from Cameca that had an inscribed letter ‘C’ (referred to below as the ‘Cameca grid’). These were obtained using scanning electron microscopy (SEM) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Image fusion was implemented with the Munechika algorithm. The edge resolution of the resulting hybrid image was calculated compared with the edge resolution obtained for both the individual ToF‐SIMS and SEM images. The challenges of combining complimentary datasets from different instrumental analytical methods are discussed as well as the advantages of having a hybrid image. The distance across the edge for hybrid images of the A‐Grid and the Cameca grid were determined to be 21 µm and 8 µm, respectively. When these values were compared to the original ToF‐SIMS, SEM and optical microscopy measurements, the fused image had a spatial resolution nearly equal to that obtained in the SEM image for both samples. Copyright © 2014 John Wiley & Sons, Ltd.     

XPS and ToF‐SIMS study of a chalcopyrite–pyrite–sphalerite mixture treated with xanthate and sodium bisulphite

We present data from the surface analysis of a mineral mixture of chalcopyrite, pyrite, and sphalerite, elucidating surface reactions occurring during grinding and flotation. Flotation tests are also performed on the mixture, carried out in the presence of collector (SIBX) and also in the absence and presence of sodium bisulphite (NaHSO₃), a gangue sulphide mineral depressant. X‐ray photoelectron spectroscopy (XPS) studies on the ground mineral sample prior to flotation indicate that the mineral feed is heavily oxidised, especially the sphalerite in the mixture. Flotation recovery data clearly shows the effect of this oxidation, with the mineral recoveries of all three phases being lower than those observed in single mineral studies. In addition, the flotation recoveries show the effect of the inadvertent copper activation of pyrite and sphalerite, and the effect of bisulphite in reducing the flotation of sphalerite and pyrite in the mixture. Time of flight secondary ion mass spectrometry(ToF‐SIMS) data indicates that the depressing effect of bisulphite is due to the removal of copper and sulphur‐like species from the surface of pyrite and sphalerite and a consequent increase in the oxidation of these minerals. ToF‐SIMS data also indicates that the low recovery of pyrite and chalcopyrite in the absence of collector is most likely due to precipitation of zinc hydroxide on the surfaces of these minerals, formed in solution due to copper activation of sphalerite. Copyright © 2005 John Wiley & Sons, Ltd.     

Data and device protection: A ToF‐SIMS,wetting, and XPS study of an Apple iPod nano

An important aspect of the robustness of an electronic device is its ability to resist water, fingerprints, dirt, and smudges that may compromise its ability to function and/or the information within it. Here, we report a chemical analysis by ToF‐SIMS, wetting, and XPS of the surfaces in a commercially available Apple iPod nano (8GB, MC525LL/A), which showed good resistance to its environment. This analysis reveals that the front panel (touchscreen) of the device is coated with a low free energy fluorinated polymer that may consist of short segments of a fluorinated hydrocarbon connected through ether linkages. No other part of the device appears to have this hydrophobic coating. A plasma treatment of the device leads to a deterioration of its performance. This work demonstrates how different analytical techniques can complement each other and contribute to a better understanding of a surface or a material. Copyright © 2013 John Wiley & Sons, Ltd.     

Cationisation of oligomeric alkylethoxylate surfactants in ToF‐SIMS analysis

Variable cationisation of the alkylethoxylate surfactant Synperonic A7^? has been observed during a ToF‐SIMS study of its interaction with a leaf surface, either by itself or as a component in a simple herbicide formulation. Depending on the conditions, cationisation predominantly by K⁺, Na⁺ and H⁺ was found. By deliberate surfactant solution doping with alkali bromides, the cationisation channel could be manipulated. Comparison of the apparent molecular weight dependence, derived from the ToF‐SIMS spectra, with that measured by HPLC‐MS, showed a further dependence on the cationising species, with Na⁺ giving the best match. An understanding of these effects is critical to the correct interpretation of the spectra from this surfactant. Copyright © 2007 John Wiley & Sons, Ltd.     

An investigation of hydrogen depth profiling using ToF‐SIMS

Hydrogen depth distributions in silicon, zinc oxide, and glass are of great interest in material research and industry. Time‐of‐flight SIMS has been used for hydrogen depth profiling for many years. However, some critical information, such as optimal instrumental settings and detection limits, is not easily available from previous publications. In this work, optimal instrumental settings and detection limits of hydrogen in silicon, zinc oxide, and common glass were investigated. The recommended experimental settings for hydrogen depth profiling using time‐of‐flight SIMS are: (i) keeping pressure in the analysis chamber as low as possible, (ii) using a cesium beam for sputtering and monitoring the H^– signal, (iii) employing monatomic ion analysis beams with the highest currents, and (iv) using interlace mode. In addition, monatomic secondary ions from a matrix are recommended as references to normalize the H^– signal. Detection limits of hydrogen are limited by the pressure of residual gases in the analysis chamber. The base pressure of the analysis chamber (with samples) is about 7 × 10^?10 mbar in this study, and the corresponding detection limits of hydrogen in silicon, zinc oxide, and common glass are 1.3 × 10¹⁸ atoms/cm³, 1.8 × 10¹⁸ atoms/cm³, and 5.6 × 10¹⁸ atoms/cm³, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Multivariate statistical analysis of non‐mass‐selected ToF‐SIMS data

Cluster LMIGs are now regarded as the standard primary ion guns on time‐of‐flight secondary ion mass spectrometers (ToF‐SIMS). The ToF‐SIMS analyst typically selects a bombarding species (cluster size and charge) to be used for material analysis. Using standard data collection protocols where the analyst uses only a single primary bombarding species, only a fraction of the ion‐beam current generated by the LMIG is used. In this work, we demonstrate for the first time that it is possible to perform ToF‐SIMS analysis when all of the primary ion intensity (clusters) are used; we refer to this new data analysis mode as non‐mass‐selected (NMS) analysis. Since each of the bombarding species has a different mass‐to‐charge ratio, they strike the sample at different times, and as a result, each of the bombarding species generates a spectrum. The resulting NMS ToF‐SIMS spectrum contains contributions from each of the bombarding species that are shifted in time. NMS spectra are incredibly complicated and would be difficult, if not impossible, to analyze using univariate methodology. We will demonstrate that automated multivariate statistical analysis (MVSA) tools are capable of rapidly converting the complicated NMS data sets into a handful of chemical components (represented by both spectra and images) that are easier to interpret since each component spectrum represents a unique and simpler chemistry. Copyright © 2008 John Wiley & Sons, Ltd.     

Identifying and minimizing buffer interferences in ToF‐SIMS analyses of lignocellulose

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) has demonstrated applicability to the analysis of lignocellulosic samples including pulp, paper, plants, and wood. One such application is to use ToF‐SIMS as a tool for detecting the activity of enzymes applied to degrade or modify plant biomass. The use of buffers for pH control of these enzymatic reactions can pose problems due to the nature of the ToF‐SIMS measurement. Specifically, inorganic species (e.g. salts) from buffer components could introduce several concerns for quantitative or semi‐quantitative ToF‐SIMS analysis. First, salts can produce additional peaks in the mass spectra, which may overlap with lignocellulose peaks of interest (mass interference). Second, salts can alter the chemical environment, or ‘matrix’, altering the ionization probability of lignocellulose‐related secondary ions during the sputtering mechanism of the ToF‐SIMS measurement (matrix effects). Third, salts may physically coat the lignocellulose surface, decreasing the signal from the lignocellulose, causing poor signal‐to‐noise in the analysis. The current work presents a simple approach for identifying interferences due to buffers, using both principal component analysis (PCA) and previously established lignocellulose‐relevant peak ratios. Furthermore, a simple acetic acid rinsing protocol is compared to distilled water rinsing and is evaluated and for its effectiveness in removing buffer‐related salts. The data shows that briefly rinsing lignocellulose samples in dilute acetic acid can be effective in restoring the validity of lignocellulose composition interpretations using ToF‐SIMS. Copyright © 2014 John Wiley & Sons, Ltd.     

Characterization of size‐segregated aerosols using ToF‐SIMS imaging and depth profiling

Size‐segregated particles were collected with a ten‐stage micro‐orifice uniform deposit impactor from a busy walkway in a downtown area of Hong Kong. The surface chemical compositions of aerosol samples from each stage were analyzed using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) operated in the static mode. The ToF‐SIMS spectra of particles from stage 2 (5.6–10 µm), stage 6 (0.56–1 µm), and stage 10 (0.056–0.1 µm) were compared, and the positive ion spectra from stage 2 to stage 10 were analyzed with principal component analysis (PCA). Both spectral analysis and PCA results show that the coarse‐mode particles were associated with inorganic ions, while the fine particles were associated with organic ions. PCA results further show that the particle surface compositions were size dependent. Particles from the same mode exhibited more similar surface features. Particles from stage 2 (5.6–10 µm), stage 6 (0.56–1 µm), and stage 10 (0.056–0.1 µm) were further selected as representatives of the three modes, and the chemical compositions of these modes of particles were examined using ToF‐SIMS imaging and depth profiling. The results reveal a non‐uniform chemical distribution from the outer to the inner layer of the particles. The coarse‐mode particles were shown to contain inorganic salts beneath the organics surface. The accumulation‐mode particles contained sulfate, nitrate, ammonium salts, and silicate in the regions below a thick surface layer of organic species. The nucleation‐mode particles consisted mainly of soot particles with a surface coated with sulfate, hydrocarbons, and, possibly, fullerenic carbon. The study demonstrated the capability of ToF‐SIMS depth profiling and imaging in characterizing both the surface and the region beneath the surface of aerosol particles. It also revealed the complex heterogeneity of chemical composition in size and depth distributions of atmospheric particles. Copyright © 2014 John Wiley & Sons, Ltd.     

Detection of protein coatings on nanoparticles surfaces by ToF‐SIMS and advanced electron microscopy

In this study, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and advanced electron microscopy (scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM)) were applied to detect and analyse different nano‐scaled protein coatings on gold nanoparticles (NP). The NP were coated with collagen type I and fibronectin as well as different combinations of these proteins. These two main proteins in human cell organization and tissue formation were identified with the aid of ToF‐SIMS by typical amino acid mass peak detection. In addition, the protein‐coated particles were investigated by TEM and SEM to get information about the protein structure, the protein layer thickness on the particle surfaces and the reaction of NP in different protein solutions. In this study, a differentiation of diverse protein induced particle agglomeration was proven. The investigations of this study were part of the Specific Targeted Research Project CellNanoTox (project no. NMP4‐CT‐2006‐032731) funded by the European Commission under the 6th EU Framework Programme for Research and Technological Development. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigation of ionic liquids under Bi‐ion and Bi‐cluster ions bombardment by ToF‐SIMS

A systematic study of five different imidazolium‐based room temperature ionic liquids, 1‐butyl‐3‐methylimidazolium acetate, 1‐butyl‐3‐methylimidazolium nitrate, 1‐butyl‐3‐methylimidazolium iodide, 1‐butyl‐3‐methylimidazolium hexafluorophosphate and 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide were carried out by means of time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) in positive and negative ion mode. The compounds were measured under Bi‐ion and Bi‐cluster ions (Bi_2–7⁺, Bi_{3, 5}²⁺) bombardment, and spectral information and general rules for the fragmentation pattern are presented. Evidence for hydrogen bonding, due to high molecular secondary cluster ions, could be found. Hydrogen bonding strength could be estimated by ToF‐SIMS via correlation of the anionic yield enhancement with solvent parameters. Copyright © 2010 John Wiley & Sons, Ltd.