J. M. Sanz

Induced changes in ceria by thermal treatments under vacuum or hydrogen

The reduction of CeO2 either under vacuum or hydrogen at temperatures between 295 and 773 K has been investigated by gravimetric methods and IR, 1H-NMR, and ESR spectroscopies. Thermal treatments of the samples under vacuum produce mainly the progressive dehydroxylation of the samples and a weak surface reduction. However, treatments under hydrogen modify substantially this behavior. The gravimetric results show two well-differentiated processes: a fast weight gain due to incorporation of H2 in the oxide bulk, followed by an important weight loss due to lattice oxygen extraction. Infrared data indicate that H2-treatments at temperatures below 673 K do not significantly change the surface concentration of hydroxyl groups, but they induce an important decrease above that temperature. The magnetic resonance spectra confirm the hydrogen incorporation in the oxide, mainly in the bulk, when the sample is heated under H2 in the 373–773 K range.

Preferential sputtering of oxides: a comparison of model predictions with experimental data

Abstract Experimental data on the composition of the altered layer of some oxides bombarded with noble gas ions are compared with different model predictions based on Sigmunds sputtering theory. It is shown that although the difference in mass of the metal atoms is the dominant factor in the preferential sputtering of oxygen, the additional consideration of the surface binding energies yields an improved agreement between theory and experiment.

Compositional changes induced by 3.5 keV Ar+ ion bombardment in Ni-Ti oxide systems: A comparative study

Abstract Compositional changes induced by 3.5 keV Ar + sputtering in TiO 2 , NiO, NiTiO 3 and a (TiO 2 + NiO) mixture have been quantitatively studied by XPS. Although all the samples show important changes in their stoichiometry, the extent of the decomposition depends on the compound. The stability of Ti 4+ appears to be enhanced by the presence of Nisu2+ cations which, on the other hand, are more easily reduced to Ni 0 than in pure NiO. To explain these results a redox solid state reaction between the intermediate phases formed during sputtering is proposed, which tends to preserve the most stable phases.

Auger electron spectroscopy and X-ray photoelectron spectroscopy studies of the oxidation of polycrystalline tantalum and niobium at room temperature and low oxygen pressures

Abstract Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to characterize the initial stages of the oxidation of polycrystalline tantalum and niobium in an oxygen atmosphere at low pressures (10−6 Pa ⩽ po2 ⩽ 10−3 Pa) androomtemperature. Theoxidationofbothmetalsproceedsinasimilarway: thegrowthoftheoxidelayersiscomparableandfollowsthesamekinetics. Twodistinctstagesareobserved. Atfirstrapidoxygenuptaketakesplaceuptoanoxygenexposureofabout 1.5L (1L = 10[su−6 Torr s = 1.32 × 10−4 Pa s) followed by a continuous decrease in the oxygen sorption rate until an oxygen exposure of 7–10 L is reached. This stage is characterized by the formation of an oxide film 2–3 monolayers thick (TaO and TaO2 on tantalum and NbO and NbO2 on niobium). At oxygen exposures above 10 L a second stage with a low reaction rate is observed which is characterized by the formation of Ta2O5 (Nb2O5) according to a logarithmic growth law until an oxygen exposure of 104 L is attained.

The statistical sputtering contribution to resolution in concentration-depth profiles

Abstract The statistical contribution to resolution in concentration-depth profiles by sputtering combined with Auger electron spectroscopy and secondary ion mass spectrometry is evaluated using a modified form of the sequential layer sputtering model. The modification makes allowance for the fact that each surface atom has a sputtering probability which is inversely proportional to its instantaneous bonding with the matrix, as predicted by Sigmunds theory, instead of being constant. Thus the sputtering is site dependent with the most exposed atoms having the highest sputtering probability. The theory shows that the statistical contribution to the depth resolution for depths greater than 10 nm is approximately constant and, depending on the precise choice of parameters, will probably be in the range 1–2 nm instead of increasing with the square root of depth. Thus the statistical contribution, instead of being the dominant term in the resolution of concentration-depth profiles, may often have only a minor effect when compared with the atomic mixing, diffusional and instrumental terms.

XRD, XPS and 119Sn NMR study of tin sulfides obtained by using chemical vapor transport methods

Application of the chemical vapor transport method to the Sn–S system allowed three different phases (viz. SnS, SnS2 and Sn2S3) to be synthesized. No evidence of the formation of other, previously reported mixed valence compounds such as Sn3S4 or Sn4S5 was found, whichever the Sn:S atomic ratio and temperature gradient used. Except for SnS2, which was always obtained as a pure phase as a result of starting from the required stoichiometry, a mixed phase was invariably obtained. The XPS spectrum for Sn2S3 was only slightly different from those for SnS and SnS2 in spite of the presence of Sn(II) and Sn(IV) in the former compound, which hindered the identification of mixed valence compounds in the Sn–S system by the use of this spectroscopic technique. By contrast, the chemical shifts, anisotropy and skew parameters for Sn2S3 as obtained by 119Sn NMR were markedly different from those for SnS and SnS2, and reflected the severe distortion of Sn(IV) and, especially, Sn(II) in the former compound relative to the latter two. The time scale for this resonance technique is shorter than the lifetime of the valence states, which allows one to unambiguously distinguish the two oxidation states of Sn.

Chemical changes induced by sputtering in TiO2 and some selected titanates as observed by X-ray absorption spectroscopy

Abstract We studied the chemical changes induced by Ar+-bombardment in TiO2 and some selected titanates by means of soft X-ray absorption spectroscopy. The O 1s and Ti 2p spectra provide unique information on hybridization, valency and crystal-field effects. In TiO2, we found that some Ti ions are partly reduced but no metallization is observed. On the other hand, the effect of ion-bombardment in titanates depends on the counter-ions. In Al2TiO5, the central Ti ions are strongly reduced while Al2O3 remains unaffected. In NiTiO3, by contrast, we found the opposite behaviour with strong reduction of the counter Ni ions. This contrasting behaviour seems to be related to differences in the relative heat of formation of the sub-species. In PbTiO3, both central- and counter-ions are affected by sputtering. Finally, the electronic structure of BaTiO3 presents very little changes upon ion-bombardment.

The O 1s x-ray absorption spectra of transition-metal oxides: The TiO2−ZrO2−HfO2 and V2O5−Nb2O5−Ta2O5 series

Abstract We present the O 1 s x-ray absorption spectra of two series of transition-metal oxides including all the d 0 oxides of the groups IVa and Va elements, namely TiO 2 −ZrO 2 −HfO 2 and V 2 O 5 −Nb 2 O 5 −Ta 2 O 5 . The spectra correspond to transitions to the conduction band and are related to the O p unoccupied density of states. The results provide direct information about crystal-field splitting, hybridization strengths and band dispersions.

Low energy ion assisted film growth

This is an introductory manual for Ion Assisted Deposition (IAD) procedures of thin films. It is addressed to researchers, post-graduates and even engineers with little or no experience in the techniques of thin film deposition. It reviews the basic concepts related to the interaction of low energy ion beams with materials. The main procedures used for IAD synthesis of thin films and the main effects of ion beam bombardment on growing films, such as densification, stress, mixing, surface flattening and changes in texture are critically discussed. A description of some of the applications of IAD methods and a review of the synthesis by IAD of diamond-like carbon and cubic-boron nitride complete the book.

Thermal oxidation of TiN studied by means of soft x‐ray absorption spectroscopy

The oxidation of TiN in an oxygen flow at temperatures in the range 300–500 °C has been studied by means of soft x‐ray absorption spectroscopy. The analysis of the experimental results indicates that O progressively displaces N to form TiO2. The process appears to be controlled by the temperature dependence of the oxygen diffusion. Some oxidation is observed to take place even at room temperature. No evidence of oxynitride formation was found in thermally oxidized TiN, instead complete phase separation is observed. The interface between TiO2 and TiN seems to be very abrupt. The appearance of a sharp absorption peak in the N 1s spectrum of TiN is believed to be due to nitrogen atoms which are displaced during the oxidation process and remain unbounded within the TiO2 matrix. For temperatures above 400 °C, this peak disappears as the interstitial N atoms migrate to the surface and desorb.