A study of the chemisorption of a series of aminophenols on plasma-grown aluminium and magnesium oxides by IETS

Inelastic electron tunnelling spectroscopy (IETS) has been applied to study the adsorption of a series of aminophenols on plasma-grown aluminium and magnesium oxides. Vapour-phase doping of these surfaces was used and spectra recorded for 1,2-aminophenol, 1,3-aminophenol and 1,4-aminophenol. The tunnel spectra of the 1,2- and 1,3-aminophenols show that their chemisorption at both oxide surfaces is via reaction between phenolic and surface hydroxyl groups. For the 1,4-aminophenol example, adsorbate chemisorption involves transfer of surface-bound protons from the oxide in amino-group protonation. Some of these protons are replaced by deprotonation of the phenol at surface oxide ions. Observed band intensities for all three systems on aluminium oxide are held to be indicative of an adsorbate orientation on the surface which is close to the vertical. In contrast, those for magnesium oxide are more consistent with a non-vertical configuration.     

Organic surface analysis with time-of-flight SIMS

In the last few years static secondary ion mass spectrometry (SSIMS) has proved to be a versatile and indispensable method for determining the composition and the structure of the outermost molecular layers of a surface. In particular when using a high-resolution time-of-flight (TOF) mass analyser a high sensitivity can be obtained with SSIMS. In this review it will be shown that the analysis of surfaces with a well-defined chemical structure by means of SSIMS has given detailed insight into the relation between the structure of the fragment ions formed by ion bombardment of the surface and the original surface structure. These studies have also improved the possibilities for quantifying the SSIMS results. In addition, the better knowledge about the ionformation process can be used for the analysis of surfaces of unknown composition and structure. Finally, some recent applications of SSIMS will be presented.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Threshold Al KLL Auger spectra of oxidized aluminium foils

Threshold Al KLL Auger electron spectroscopy and K‐edge x‐ray absorption fine structure spectroscopy have been used to examine technical purity (99.5%) aluminium foil before and after chemical treatment that altered the thickness and degree of hydroxylation of the oxidized layer. Comprehensive surface chemical characterization was effected by means of monochromatized Al Kα‐excited photoelectron spectroscopy. Threshold Al KL_{2, 3}L_{2, 3} spectra were obtained for three of the foils investigated and these spectra were in broad agreement with those observed previously for pure Al foil. The relative intensities of the spectral components for two of the foils were clearly consistent with the previously proposed assignment of the resonantly enhanced Auger component, situated between those arising from the metal and Al(III) oxide, to a thin interfacial layer. The threshold Auger spectra from the aluminium foil bearing the thickest and most hydroxylated oxidized layer were not obviously consistent with the interfacial layer model but O K‐edge spectra revealed that this surface layer was fundamentally different from the others and could have had a greater interfacial surface area. Copyright © 2003 John Wiley & Sons, Ltd.     

Study of the end‐group contribution to ToF‐SIMS and G‐SIMS spectra of poly (lactic acid) using deuterium labelling

In this study, polymeric (MW 50 000) and oligomeric (MW 2000) poly (lactic acid) (PLA), both with and without end‐group deuterium exchange, were analysed using static secondary ion mass spectrometry (SSIMS) to investigate the contribution of end‐group‐derived secondary ions to the SSIMS spectra. By monitoring the SSIMS intensities between the non‐deuterated and deuterated PLA, it is evident that the only significant end‐group‐derived secondary ions are [nM + H]⁺ (n > 1) and C₄H₉O₂⁺. The gentle‐SIMS (G‐SIMS) methodology was employed to establish that deuterated fragments were produced through low energy processes and were not the result of substantial rearrangements. It was noted that end‐group‐derived secondary ions had higher G‐SIMS intensities for oligomeric PLA than polymeric PLA, showing that these secondary ions are simple fragment products that are not the result of rearrangement or degraded product ions. Copyright © 2007 John Wiley & Sons, Ltd.     

Static SIMS studies of fatty alcohols,amines and esters on gold and aluminium–magnesium alloy surfaces

Static secondary ion mass spectrometry was used to study the chemical reactions and lateral distributions of fatty amines, alcohols and esters spin coated onto gold surfaces and commercial aluminium–magnesium (Al–Mg) alloy surfaces, cleaned using UV–ozone. The aim of this study is to develop an understanding of the interactions of model lubricants with metal surfaces, such as gold and aluminium. This static SIMS study of organic thin films has been able to identify specific reaction products on the aluminium surface for each functional group. This work demonstrates that organic molecules with alcohol, ester and amine functional groups undergo specific chemical reactions with oxidized Al–Mg alloy surfaces. For example, films composed of the fatty alcohol dodecanol were observed to emit monomers, dimers and trimers with discrete distributions. In addition, negative secondary ion mass spectra indicate that a surface carboxylate is formed from the alcohol. The formation of carboxylate reaction products was confirmed by Fourier transform infrared spectroscopy. On Al–Mg alloy surfaces, a direct interaction with the amine and aluminium oxide surface is observed by the detection of a molecular ion that corresponds to the mass of dodecylamine and AlO^?, characteristic of aluminium oxide. Ethyl laurate was shown to eliminate the ethyl group, leaving the laurate anion. This study demonstrates the ability of time‐of‐flight (ToF) SIMS to discriminate and detect chemical reaction products formed between model lubricant molecules and metal surfaces. As a result of this study, the use of ToF‐SIMS to identify reaction products of model lubricants can be extended to provide a better understanding of the interactions of lubricants and metal surfaces at high temperatures and pressures that more closely resemble the conditions encountered in industrial rolling processes. Copyright © 2005 John Wiley & Sons, Ltd.     

Static TOF‐SIMS — a VAMAS interlaboratory study. Part I. Repeatability and reproducibility of spectra

An interlaboratory study involving 32 Time‐of‐Flight Static SIMS instruments from 12 countries has been conducted. Analysts were supplied, by NPL, with a protocol for analysis together with three reference materials; a thin layer of polycarbonate (PC) on a silicon wafer, a thin layer of polystyrene (PS) oligomers on etched silver and poly(tetrafluoroethylene) (PTFE). The study involved static SIMS analysis of each reference material for both positive and negative polarity secondary ions. The option to test instrument suitability for G‐SIMS was also provided. The results of this study show that over 84% of instruments have excellent repeatabilities of better than 1.9%. Repeatabilities can be as good as 0.4%. A relative instrument spectral response (RISR) is calculated for each instrument for each reference material and ion polarity. The RISR is used to evaluate variations in spectral response between different generic types of SIMS instruments. Use of the RISR allows the identification of contamination, charge stabilisation problems and incorrectly functioning ion detectors. The high quality of the data presented here allows the RISR to reveal differences in individual operation of each instrument such as the use of apertures to remove metastables from the spectra and the use of different post‐acceleration voltages for ion detection. Spectral reproducibility can be measured, here, by the equivalence of RISRs between materials and ion polarities. It is found that reproducibilities are on average 10% but can be as good as 4% for the best instruments. This figure shows the consistency between instruments in measuring spectra from different samples. This study sets out the basic framework to develop static secondary ion mass spectrometry (SSIMS) as a reliable measurement method. © Crown Copyright 2005. Reproduced with the permission of Her Majestry's Stationery Office. Published by John Wiley & Sons, Ltd.     

Studies on surface structures of poly(ethylene oxide)‐segmented nylon films

The surface structures of three kinds of poly(ethylene oxide)‐segmented nylon (PEO‐Ny) molten films were investigated using a scanning electron microscopy (SEM), an electron spectroscopy for a chemical analysis (ESCA), and a static secondary ion mass spectrometry (SSIMS). The PEO‐Ny's used were high semicrystalline PEO‐segmented poly(iminosebacoyliminohexamethylene) (PEO‐Ny610), low semicrystalline PEO‐segmented poly(iminosebacoylimino‐m‐xylene) (PEO‐NyM10), and amorphous PEO‐segmented poly(iminoisophthaloyliminomethylene‐1,3‐cyclohexylenemethylene) (PEO‐NyBI). SEM observations show that the surfaces of the PEO‐Ny610 and PEO‐NyM10 films are composed of spherulite, and that PEO‐NyBI film has a smooth surface. The results of ESCA and SSIMS exhibit the significant enrichments of PEO segment at the surfaces of all the films regardless of the crystallinity. The reason for the enrichment of PEO segment was discussed in terms of the surface tension of the corresponding homopolymers in the melting state. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1045–1056, 2000     

Static SIMS–VAMAS interlaboratory study for intensity repeatability,mass scale accuracy and relative quantification

An interlaboratory study involving 19 time‐of‐flight static secondary ion mass spectrometer (TOF‐SSIMS) instruments from 12 countries has been conducted. Analysts were supplied, by the National Physical Laboratory, with a protocol for analysis together with three reference materials: poly(tetrafluoroethylene), a thin layer of polycarbonate on a silicon wafer and a patterned sample with different amounts of Irganox 1010 in each of four quadrants on a silicon wafer. The objectives of the study are (i) to determine the repeatability and constancy of the relative intensity scale achievable using the draft ISO standard (DIS 23830), (ii) to evaluate the effectiveness of mass scale calibration and optimisation procedure and (iii) to evaluate the current capability of relative quantification using SSIMS. The results of this study show that the constancy of the relative intensity scale has an approximate scatter standard deviation of only 5%. This is excellent and demonstrates that SSIMS measurements are significantly more stable than often thought by analysts. The draft ISO standard (DIS) CD 13084 for calibration of the mass scale in TOF‐SIMS was evaluated and found to be consistent with our previous study. Four laboratories optimised the instrument mass calibration accuracy using this procedure leading to improvements in mass scale calibration by factors of 1.8, 2.2, 2.3 and 8.6. Using a novel patterned Irganox sample it is shown that the precision of relative quantification may be as good as a standard deviation of only 5% for 16 instruments—this is a remarkable result. Further work is required to develop more robust reference materials. © Crown copyright 2010. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

Hydration of calcium oxide surface predicted by reactive force field molecular dynamics

In this work, we present the parametrization of Ca-O/H interactions within the reactive force field ReaxFF, and its application to study the hydration of calcium oxide surface. The force field has been fitted using density functional theory calculations on gas phase calcium-water clusters, calcium oxide bulk and surface properties, calcium hydroxide, bcc and fcc Ca, and proton transfer reactions in the presence of calcium. Then, the reactive force field has been used to study the hydration of the calcium oxide {001} surface with different water contents. Calcium oxide is used as a catalyzer in many applications such as CO(2) sequestration and biodiesel production, and the degree of surface hydroxylation is a key factor in its catalytic performance. The results show that the water dissociates very fast on CaO {001} bare surfaces without any defect or vacancy. The surface structure is maintained up to a certain amount of water, after which the surface undergoes a structural rearrangement, becoming a disordered calcium hydroxyl layer. This transformation is the most probable reason for the CaO catalytic activity decrease.     

A comparison of the static SIMS and G‐SIMS spectra of biodegradable homo‐polyesters

Static SIMS (SSIMS) is a surface analytical technique capable of providing molecular chemical information from solids. A major barrier to the wider take‐up of the technique is the complexity associated with the interpretation of SSIMS spectra. Quality of the interpretation depends on the expertise of analysts and making references to the limited mass spectral libraries. For many materials, there are no SSIMS library spectra. A new library‐independent method, G‐SIMS, is capable of facilitating the interpretation of SSIMS data. G‐SIMS spectra contain parent fragments, which are formed without substantial degradation or rearrangements, and highlight molecular fragments, which are directly related to the surface. In our study, G‐SIMS has been tested on medically relevant biodegradable polyester series, including poly (glycolic acid) (PGA), poly‐l‐(lactic acid) (PLA), poly‐β‐(hydroxybutyrate) (PHB) and poly‐ε‐(caprolactone) (PCL). The polyester series chosen here have closely related structures, which allow us to explore the capabilities of G‐SIMS. The G‐SIMS spectra have facilitated the identification of different polyesters by exhibiting mainly characteristic ions, representative of the polymers' molecular structures. The results also indicated that for the chosen polyester series, the larger the repeating monomer structures, the smaller the maximum number of repeat units were seen in the G‐SIMS spectra. The G‐SIMS spectra for the homologous polyester series have provided an insight into the fragmentation mechanisms as a function of repeating monomer molecular weights and structures. Copyright © 2008 John Wiley & Sons, Ltd.     

Formation of iron complexes from trifluoroacetic acid based liquid chromatography mobile phases as interference ions in liquid chromatography/electrospray ionization mass spectrometric analysis

Two unexpected singly charged ions at m/z 1103 and 944 have been observed in mass spectra obtained from electrospray ionization mass spectrometric analysis of liquid chromatography effluents with mobile phases containing trifluoroacetic acid (TFA) that severely interfered with sample analysis. Accurate mass measurement and tandem mass spectrometry studies revealed that these two ions are composed of three components; clusters of trifluoroacetic acid, clusters of mass 159 and iron. Formation of these ions is inhibited by removing TFA from the mobile phases and using formic acid in its place, replacing the stainless steel union with a titanium union or by adding a small blank fused-silica capillary column between the chromatography column and the electrospray tip via a stainless steel union without any adverse effects to chromatographic separation, peak broadening or peptide identifications.     

Probing the electronic structure of early transition-metal oxide clusters: polyhedral cages of (V2O5)n(-) (n = 2-4) and (M2O5(2)(-) (M = Nb, Ta)

Vanadium oxide clusters, (V2O5)n, have been predicted to possess interesting polyhedral cage structures, which may serve as ideal molecular models for oxide surfaces and catalysts. Here we examine the electronic properties of these oxide clusters via anion photoelectron spectroscopy for (V2O5)n(-) (n = 2-4), as well as for the 4d/5d species, Nb4O10(-) and Ta4O10(-). Well-resolved photoelectron spectra have been obtained at 193 and 157 nm and used to compare with density functional calculations. Very high electron affinities and large HOMO-LUMO gaps are observed for all the (V2O5)n clusters. The HOMO-LUMO gaps of (V2O5)n, all exceeding that of the band gap of the bulk oxide, are found to increase with cluster size from n = 2-4. For the M4O10 clusters, we find that the Nb/Ta species yield similar spectra, both possessing lower electron affinities and larger HOMO-LUMO gaps relative to V4O10. The structures of the anionic and neutral clusters are optimized; the calculated electron binding energies and excitation spectra for the global minimum cage structures are in good agreement with the experiment. Evidence is also observed for the predicted trend of electron delocalization versus localization in the (V2O5)n(-) clusters. Further insights are provided pertaining to the potential chemical reactivities of the oxide clusters and properties of the bulk oxides.     

An IETS study of the adsorption of 2,3-dihydroxynaphthalene, 1,2-dihydroxybenzene, 1,3-dihydroxybenzene and 1,4-dihydroxybenzene on thin-film plasma-grown aluminium and magnesium oxides

Inelastic Electron Tunnelling Spectroscopy (IETs) has been applied to study the adsorption of 2,3-dihydroxynaphthalene, 1,2-dihydroxybenzene, 1,3-dihydroxybenzene and 1,4-dihydroxybenzene onto plasma-grown thin-film partially hydroxylated magnesium and aluminium oxides. For both 2,3-dihydroxynaphthalene and 1,2-dihydroxybenzene on aluminium oxide it is found that adsorbate chemisorption involves reaction of the two hydroxyl groups present in the adsorbate to form a di-anion in the case of the former and both the mono- and di-anion for the latter. The tunnel spectra for both compounds on magnesium oxide indicate that the di-anion is formed. Adsorption at the oxide surfaces for these two adsorbates involves adsorbate deprotonation with the formation, at the oxide surface, of molecular water which is subsequently desorbed and pumped away during sample junction preparation. For the 1,3- and 1,4-dihydroxy systems, on both oxides, the presence of a strong ν(OH) band at ≈3650 cm⁻¹ suggests that only one of the hydroxyl groups present in both systems is involved in adsorbate deprotonation interactions at the respective oxide surfaces, with the second hydroxyl group present contributing to the enhanced substrate oxide ν(OH) envelope observed.     

Imaging G-SIMS: a novel bismuth-manganese source emitter

G-SIMS (gentle-SIMS) is a powerful method that considerably simplifies complex static secondary ion mass spectrometry (SSIMS) analysis of organics at surfaces. G-SIMS uses two primary ion beams that generate high and low fragmentation conditions at the surface. This allows an extrapolation to equivalent experimental conditions with very low fragmentation. Consequently, the spectra are less complex, contain more structural information and are simpler to interpret. In general, G-SIMS spectra more closely resemble electron ionisation mass spectra than SSIMS spectra. A barrier for the wider uptake of G-SIMS is the requirement for two ion beams producing suitably different fragmentation conditions and the need for their spatial registration (spatial alignment) at the surface, which is especially important for heterogeneous samples. The most popular source is the liquid metal ion source (LMIS), which is now sold with almost every new time-of-flight (TOF)-SIMS instrument. Here, we have developed a novel bismuth-manganese emitter (the 'G-tip') for the popular LMISs. This simplifies the alignment and gives excellent G-SIMS imaging and spectroscopy without significantly compromising the bismuth cluster ion beam performance.     

Effects of applied anodic potential and pH on the repassivation kinetics of pure aluminium in aqueous alkaline solution

The repassivation kinetics of pure aluminium have been explored in aqueous alkaline solutions as functions of applied anodic potential and pH by using an abrading electrode technique and a rotating disc electrode. The repassivation rate of the abraded bare surface of pure aluminium increased with increasing applied anodic potential in aqueous alkaline solutions, while it decreased with increasing pH. These results revealed that the growth rate of the passivating oxide film is enhanced by an applied electric field, but it is lowered due to the chemical attack by hydroxyl ions. A potentiostatic anodic current decay transient obtained from the abraded electrode surface showed a constant repassivation rate in neutral and weakly alkaline solutions. In contrast, in concentrated alkaline solutions it was observed to consist of three stages: a high repassivation rate in the initial stage due to a high formation rate of the oxide film on the abraded bare surface; a zero value of the repassivation rate in the second stage due to the dissolution of the oxide film by the attack of OH⁻; a high repassivation rate in the third stage due to a lowered dissolution rate of the oxide film. The dissolution rate of the passivating oxide film was observed to depend on the removal rate of aluminate ions from the oxide/solution interface. Received: 1 April 1998 / Accepted: 3 July 1998     

FTIR spectroscopy combined with quantum chemical calculations to investigate adsorbed nitrate on aluminium oxide surfaces in the presence and absence of co-adsorbed water

Surface reactions of nitrogen oxides with aluminium oxide particles result in the formation of adsorbed nitrate. Specifically, when alpha-Al(2)O(3) and gamma-Al(2)O(3) particles are exposed to gas-phase NO(2) and HNO(3) adsorbed nitrate forms on the surface. In this study, Fourier transform infrared (FTIR) spectroscopy is combined with quantum chemical calculations to further our understanding of the adsorbed nitrate product on aluminium oxide particle surfaces in the presence and absence of co-adsorbed water at 296 K. FTIR spectra of adsorbed nitrate on alpha-Al(2)O(3) and gamma-Al(2)O(3) particles are interpreted using calculated vibrational frequencies of nitrate coordinated to binuclear Al oxide cluster models. Comparison of the calculated and experimental vibrational frequencies of adsorbed nitrate establishes different modes of coordination (monodentate, bidentate and bridging) of the nitrate ion to the surface in the absence of adsorbed water. In the presence of co-adsorbed water, the nitrate ion becomes fully solvated, as shown by a comparison of the experimental nitrate infrared spectra as a function of relative humidity with the calculated nitrate vibrational frequencies for binuclear Al cluster compounds which contain both coordinated nitrate ions and water molecules. These calculations also suggest that adsorbed water can displace nitrate from direct coordination to the surface, leading to an outer-sphere nitrate adsorption complex as well as an inner-sphere complex. Furthermore, the relative humidity dependence of the spectra suggest that water does not evenly wet the surface even at high relative humidity, as there are open or bare surface sites where nitrate ions are not solvated. Besides adsorbed mondendate, bidendate, bridging and solvated nitrate, the presence of ion bound nitrate ion, partially solvated nitrate, molecular nitric acid, hydronium ion and H(3)O(+):NO(3)(-) ion pairs on the oxide surface are also discussed.     

Extracting information on the surface monomer unit distribution of PLGA by ToF‐SIMS

The surface chemistry of a range of random poly l‐lactide‐co‐glycolide (PLGA) materials has been investigated using XPS, static secondary ion mass spectrometry (SSIMS) and gentle secondary ion mass spectrometry (G‐SIMS). The estimated mole fraction of lactide units provided by SSIMS was in good agreement with bulk composition and appeared not to have been affected by contamination. Conversely, XPS assessment of lactide compositions was unreliable due to hydrocarbon contamination contributions. In this study, we propose a novel model to demonstrate that by using SSIMS it is possible to infer the degree of trans‐esterification for PLGA co‐polymers synthesised from a mixture of lactide and glycolide homo‐dimers. This was determined by introducing two independent parameters, the ratio of trans‐esterified bonds to the total number of ester bonds, P_T, and the lactide composition. The model has indicated that, for this set of polymers, P_T was approximately 0.25. Furthermore, we have demonstrated that G‐SIMS successfully identified the structurally important key fragments leading to direct identification. Analysis by G‐SIMS showed that the glycolic acid units from all PLGA compositions are emitted in a lower energy‐fragmentation process than lactic acid units. Copyright © 2008 John Wiley & Sons, Ltd.     

Static SIMS studies of carboxylic acids on gold and aluminium–magnesium alloy surfaces

Static secondary ion mass spectrometry was used to study the surface reactions and lateral distributions of fatty carboxylic acid molecules on sputter‐deposited gold and aluminium surfaces, as well as commercial aluminium–magnesium alloy surfaces, cleaned using UV/ozone. Films were prepared by spin coating dilute solutions of stearic acid and lauric acid onto the above surfaces. These carboxylic acids were shown to react with the oxide formed on the aluminium and aluminum–magnesium alloy substrates to produce a deprotonated acid anion, stabilized by the formation of a magnesium soap on the aluminium–magnesium alloy surface. Secondary ion imaging of stearic acid films revealed the formation of C‐type crystals. Copyright © 2003 John Wiley & Sons, Ltd.     

Laser ablation mass spectrometry of inorganic transition metal compounds. Additional knowledge for the understanding of ion formation

Laser ablation of transition-metal oxides have been investigated to better understand the formation processes of inorganic cluster ions. The study of binary oxide mixtures and the relative distribution of the ions produced suggest three salient mechanisms that occur after laser/matter interaction, that function to produce the observed ensemble of ionic species. Molecular recombination reactions, unimolecular dissociation processes, emission of small neutrals, including molecular oxygen from transition-metal oxide samples, or from species expelled in gas phase appear to be a significant mechanism, especially under high laser irradiance conditions. These processes are used to propose a set of pathways to rationalize the envelope of ionic clusters formed under photon bombardment.     

Infrared spectroscopic studies of siderophore-related hydroxamic acid ligands adsorbed on titanium dioxide

The adhesion of bacteria to metal oxide and other mineral surfaces may involve bacterial siderophores, many of which contain hydroxamic acid (Ha) ligands. The adsorption behavior of the siderophore-related ligands acetohydroxamic acid, N-methylformohydroxamic acid, N-methylacetohydroxamic acid, and 1-hydroxy-2-piperidone on titanium dioxide thin films has been investigated using in situ ATR-IR spectroscopy with variation of concentration and pH. All the ligands were found to adsorb strongly on the TiO2 surface as hydroxamate ions and form bidentate surface complexes. Vibrational modes involving C=O stretching and N-O stretching of these ligands were assigned by comparing observed IR spectra with those calculated by the density functional method at the B3LYP/6-31+G(d) level. Calculated spectra of the complex [Ti(Ha)(OH)4]-, used to model the TiO2 surface, were compared with observed spectra of adsorbed hydroxamic acids. These results suggest that hydroxamic acid ligands in siderophores would be expected to bind to metal (oxide) and mineral surfaces during bacterial adhesion processes.