M. Mohai

Surface characterisation of plasma-nitrided titanium: an XPS study

DC plasma nitriding was applied to titanium metal sheets in a commercial cell using Ar + NH3 as the admixture and the nitrided surface investigated by means of XPS. As a comparison, in situ nitriding of a chemically pure Ti surface at room temperature was performed by N2+ ion bombardment (1–5 keV) in the electron spectrometer. Synthesis of the complex Ti2p envelope was accomplished using two sets of loss peaks, separated by 1.6 and 3.0 eV from the major TiN-type Ti2p32 and Ti2p12 components. A doublet at 458.8 and 464.5 eV was also included to account for a TiO2-type oxide. The sum of the main Ti TiN peak and the two loss peaks were taken to be representative of nearly stoichiometric TiNx with x ranging from 0.85 to 1.15. Further components derived from the peak synthesis were assigned to TiNxOy and Ti2O3. The stoichiometric nitride is represented by a Ti2p doublet at binding energies of 454.7 and 460.6 eV and a single sharp N 1s peak at 396.7 ± 0.1 eV. On the superstoichiometric samples, especially after N2+ bombardment, a second peak appears at about 395.8 eV with a positive correlation between this peak concentration and the relative amounts of species (TiNxOy, Ti2O3, TiO2) derived from Ti2p components and the surface O and N content. Consecutive Ar+, N2+ and (N2+ + O2+) bombardment leads to significant changes in composition together with rearrangement of short-range chemical structure which is reflected in peak-shape changes of the Ti2p and N 1s lines.

Influence of cations on the corrosion inhibition efficiency of aminophosphonic acid

Abstract The iron/electrolyte interphase has been studied in the presence of N , N -di(phosphonomethyl)glycine (DPMG) inhibitor with or without the bivalent cations. In electrochemical measurements, inhibition was assumed by the formation of a complex in the presence of DPMG. In situ atomic force microscopy provided direct insight into changes in the surface morphology at several hundred nanometres when topographical changes owing to the breakdown of the passive layer, and initiation of corrosion were monitored. Section analysis has revealed the degree of deterioration. Bivalent cations (Ba 2+ , Sr 2+ , Ca 2+ and Zn 2+ ) synergically improved the activity of DPMG, though the mechanism was different. Barium, strontium and calcium ions in blends hindered the anodic iron dissolution, while zinc ions influenced both the anodic and cathodic processes. Composition of the surface layer was analysed by X-ray photoelectron spectroscopy (XPS). The 2:1 Ba 2+ /Sr 2+ /Ca 2+ /DPMG molar ratio in the solution resulted in a surface layer with a composition of about 1.3:1 of cation/DPMG. In the case of Zn 2+ /DPMG mixture the surface layer was composed of a mixture of slightly soluble Zn 2+ /DPMG (∼3:l) and zinc hydroxide.

Thermal plasma synthesis of zinc ferrite nanopowders

Ž. Nanosized zinc ferrite spinel powders of various compositions were produced in a radiofrequency RF thermal plasma from oxide mixtures and from co-precipitated hydroxides. Bulk and surface chemical compositions of the products were measured by ICP-AES and XPS. Phase conditions were determined by XRD. Morphology was investigated by SEM and TEM. In spite of the short residence time of reagents in the plasma reactor, a high degree of spinel formation was achieved. The Zn enrichment on the surface determined by quantitative XPS is attributed to a thin ZnO layer on the surface of the zinc ferrite particles. In zinc ferrous ferrites, an inhomogeneous distribution of Zn inside the grains may also exist. q 2001 Elsevier Science B.V. All rights reserved.

Response of oxides to ion bombardment: the difference between inert and reactive ions

Abstract The loss of O from oxides bombarded by chemically neutral ions, i.e. inert gas ions, is widely documented. It is, however, still not possible to relate with certainty such loss to the basic properties of the oxides, namely to such factors as mass, surface binding energy, bulk binding energy, volatility, mass transport, Gibbsian segregation, or electric fields. Even the most basic response of all, namely chemically random behavior [Vertoti et al., Surf. Interf. Anal. 19 (1992) 291), has an unestablished importance. The present work further develops our approach to the problem. The effects of 1–5 keV bombardment with Ar + , O 2 + , N 2 + , and N 2 O + are compared using X-ray photoelectron spectroscopy for a number of oxide pairs with similar stoichiometries and electronic structures: B 2 O 3 and Al 2 O 3 , SiO 2 and GeO 2 , ZrO 2 and TiO 2 , V 2 O 5 and Nb 2 O 5 . Minor O loss occurred with the first five and major loss with the last three. What is important, however, is that the loss was consistently higher for N 2 + than Ar + impact, yet the overall anion content, expressed as the sum O + N, remained largely unchanged. We take this result as demonstrating a prominent role for chemically random behavior, i.e. the relative strengths of the MO and MN bonds did not enter.

XPS characterization of the composition and bonding states of elements in CNx layers prepared by ion beam assisted deposition

Abstract CN x layers were grown on polished Si(100) wafers by the ion beam assisted deposition (IBAD) technique at temperatures varying between 200 and 600 °C. A Kaufman type ion source fed by Ar and N 2 was applied together with an e-beam heated evaporation source of graphite for the deposition of the CN x layers. The composition and chemical bonding state of elements were studied by X-ray photoelectron spectroscopy. The N-content varied in the range of 8–16 at.% and showed a decrease with the increase of deposition temperature. The broad C1s and N1s XPS lines manifested several bonding states. The relative intensities of the component peaks varied with the preparation conditions. The two main components of the N1s peak situated at BE=398.2 eV and 400.6 eV were assigned to sp 2 (CNC) and sp 3 (NC) type bonding states, respectively. The thermal stability of the sp 3 states was higher than that of the sp 2 ones, because the intensity of the 398.2 eV component decreased preferentially with increasing deposition temperature. A post-deposition treatment with low energy N 2 + ion beam resulted in a significant increase in the overall N-content on the surface, with a preferential increase in the concentration of sp 2 type nitrogen.

Interfacial properties of hydrophilized poly(lactic-co-glycolic acid) layers with various thicknesses

Biodegradable polyesters such as poly(lactic-co-glycolic acid) copolymers (PLGA) are preferred materials for drug carrier systems although their surface hydrophobicity greatly limits their use in controlled drug delivery. PLGA thin films on a solid support blended with PEG-containing compound (Pluronic) were used as model systems to study the interfacial interactions with aqueous media. Degree of surface hydrophilization was assessed by wettability, and X-ray photoelectron spectroscopy (XPS) measurements. Protein adsorption behavior was investigated by in situ spectroscopic ellipsometry. The degree of protein adsorption showed a good correlation with the hydrophilicity, and surface composition. Unexpectedly, the layer thickness was found to have a great impact on the interfacial characteristics of the polymer films in the investigated regime (20-200 nm). Thick layers presented higher hydrophilicity and great resistance to protein adsorption. That special behavior was explained as the result of the swelling of the polymer film combined with the partial dissolution of Pluronic from the layer. This finding might promote the rational design of surface modified biocompatible nanoparticles.

Selective hydrogenation of pentynes over PdZr and PdCuZr prepared from amorphous precursors

The selective semi-hydrogenation of isomeric pentynes was studied in the gas phase in a static circulation system over PdZr and PdCuZr alloys activated by HF dissolution. The catalysts were characterized by X-ray photoelectron spectroscopy and X-ray probe microanalysis. 2-Pentyne was found to be more reactive over PdZr, which was attributed to strong self-poisoning by 1-pentyne. The two reactants, in turn, exhibited similar reactivities over PdCuZr. The reactivity, however, changed drastically in competitive hydrogenations: the compounds reacted more slowly and 1-pentyne was always more reactive. The reactivity difference was particularly pronounced over PdCuZr. These observations could be interpreted by competitive adsorption of the reactants. Attempts to reactivate the self-poisoned (aged) PdZr sample used in the hydrogenation of 1-pentyne by applying various treatments (evacuation and treatment in hydrogen at different temperatures) were unsuccessful. This is due to the accumulation of carbonaceous deposits, Pd dissolution, and crystallization of the amorphous matrix.

Comparison of composition and bonding states of constituents in CNx layers prepared by d.c. plasma and magnetron sputtering

Layers of CN x were grown on polished Si(100) wafers and cleaved NaCl(100) substrates by reactive sputtering of graphite in d.c. nitrogen plasma and also by d.c. or r.f. magnetron sputtering of graphite in N 2 . The deposited layers were studied in situ by XPS and ex situ by Fourier transform infrared spectroscopy (FTIR). Direct current plasma deposition resulted in brown transparent layers of high nitrogen content, close to CN with 1:1 stoichiometry, as determined by XPS. Direct current magnetron sputtering resulted in layers with slightly lower nitrogen content. The grey opaque CN x layers deposited by r.f. magnetron sputtering contained only 33-38 at.% N. The broad C 1s and N 1s lines manifested several bonding states. The relative intensities of the component peaks varied with the preparation conditions. Differences were observed also in the 1000-1700 cm -1 region of the FTIR spectra. Assignments of the various photoelectron peak components were proposed, based on published results and measurements on model compounds and binding energy separations between the related C 1s and N 1s lines. It was deduced that the concentration ratio of sp 3 -type C-N clusters to that of the sp 2 -type clusters was significantly higher for the r.f. magnetron-sputtered layers than for the d.c. plasma-deposited layers. The observed differences can be explained by the specificity of unbalanced magnetron sputtering, providing an increased amount of plasma ions and intensive bombardment of the as-deposited material at the growing surface. The increase of the ratio of sp 3 -type C-N clusters to the sp 2 -type C=N clusters can be enhanced further for the r.f. magnetron-sputtered samples by applying a negative bias to the substrate.

Ion beam induced chemical effects in organosilicon polymers

Abstract Poly(vinyltrimethylsilane) (PVTMS), poly(dimethylsiloxane) (PDMSO), poly(methylsilsesquioxane) (PMeSSO), and poly(phenylsilsesquioxane) (PPhSSO) were bombarded by 2.5 keV Ar+ ions and the beam induced chemical effects were studied by XPS and XAES. For PVTMS the gradual development of a new component on the high kinetic energy side of the original Si KL23L23 peak was observed, which was attributed to the formation of [SiC4] units present in a densely crosslinked matrix. For the poly(organosiloxanes) PDMSO, PMeSSO and PPhSSO a kind of disproportionation of Si was inferred from the evolution of their Si 2p and Si KL23L23 peaks, involving the transformation of a part of the original [SiCxOy] units to [SiC4]-type and [SiO4]-type ones surrounded by a densely crosslinked organosilicon matrix.

XPS investigation of ion beam treated MoS2–Ti composite coatings

MoS2 coatings incorporating Ti were deposited on Si substrates primarily for compositional characterisation and chemical structure studies. For comparison, MoS2 layers and natural molibdenite crystals were also involved in this work. The MoS2 and the MoS2–Ti composite layers were co-deposited by unbalanced close field DC magnetron sputtering using simultaneously operating MoS2 and Ti targets in Ar plasma. The coatings are almost amorphous by XRD. The overall composition and the in-depth homogeneity of the coatings were determined by NBS. XPS studies show that the surface of the coatings stored in ambient air was partially oxidised and carbon contaminated. The majority of Mo and S were found to be in sulphide environment and part of the Ti in oxide (TiO2) and part in sulphide states. A small part of the S proved to be in a newly discovered sulphidic environment, different from that found in MoS2. No evidence of such state could be detected for the molibdenite crystal but developed in it at Ar+ treatment. The major effect of the Ar+ ion bombardment was the severe preferential sputtering of the sulphide type S lowering the ratio of sulphide S to Mo-S states from the initial value of 2 down to approx. 1.5 for molibdenite. This effect was more pronounced for both coatings lowering the S/Mo-S ratio to about 1. The experimental results demonstrate that the atomic scale chemical structure of the MoS2 coatings was also affected by energetic particles in agreement of their amorphous microstructure.