Agustín R. González-Elipe

Preparation and characterization of TiO2 photocatalysts supported on various rigid supports (glass, quartz and stainless steel). Comparative studies of photocatalytic activity in water purification

Abstract In the present study TiO 2 has been supported on several rigid substrates. Deposition on glass and quartz was carried out by a dip coating procedure and the deposition on stainless steel by an electrophoretic deposition process. The resulting materials have been characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy. The materials were then tested for the photocatalytic degradation of malic acid. For this reaction, the sample supported on quartz showed the highest catalytic activity. The photocatalytic activity pattern as a function of the nature of the support (fused silica, glass, stainless steel) followed the decreasing order: TiO 2 /quartz > TiO 2 /steel ≈ TiO 2 /glassaphotolysis. This decline in activity has been correlated with the presence of cationic impurities (Si 4+ , Na + , Cr 3+ , Fe 3+ ) in the layer as a consequence of the necessary thermal treatments to improve the cohesion of the titania layer and its adhesion onto the support.

Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag-TiO2 coatings on quartz

Abstract In the present study, TiO2 and Ag-TiO2 catalysts have been supported in the form of thin layers by a dip-coating procedure on quartz substrate. The resulting materials have been characterized by SEM/EDX, XRD, XPS and UV-vis absorption spectroscopy. The immobilized catalysts were tested in the photocatalytic degradation of malic acid. For this reaction, the presence of metallic silver does not produce an intrinsic increase in photocatalytic activity in comparison with pure titania. The apparent increase observed in activity is principally due to the increase in the exposed surface due to the textural characteristics of the Ag-TiO2 layer in comparison with TiO2. In addition, the presence of metallic silver always produces an increase in activity in comparison with oxidized Ag+ ions. This can be explained by the increase in the electron-hole pair-separation efficiency induced by trapping of electrons by metallic silver.

The state of the oxygen at the surface of polycrystalline cobalt oxide

Abstract XPS and factor analysis (FA) have been applied to characterize the surface state of three polycrystalline cobalt oxide samples with different crystallographic bulk structure (CO 3 O 4 and CoO) and surface characteristics. The study of the thermal behaviour of the O1s and Co2p spectra and of their changes as an effect of Ar + bombardment and exposure to O 2 have permitted verification of the existence of three components at the O1s spectra with binding energies at 529.6 (O I ), 531.1 (O II ) and 532.1 eV (O III ) and three components at the Co2p level. The shape of these components is similar to the Co2p spectra of Co 3 O 4 (Co III ), CoO (Co II ) and Co 0 (Co I ) compounds. In the three samples component O II yields component O III by heating at 473 K T III and O I decreases to a minimum at 923 K. Simultaneously, component Co III , the most abundant in the original samples, yields component Co II by outgassing at T > 473 K. Co II is the only one detected at 923 K. After Ar + sputtering of the samples heated at that temperature, components Co III and Co I were generated. Co I disappears, while the intensity of components O II and Co III increases when the samples are exposed to O 2 . The most intense O I species is attributed to oxygen atoms in sites with a well defined coordination. Species O II and O III are attributed to low coordinated oxygen atoms at special sites or domains of the surface where the covalence of the CoO bond is higher. Differences in the relative abundance of the three oxygen components, either during outgassing or after Ar + sputtering, are dependent on the texture of the three samples expressed in terms of their respective surface areas.

XPS study of oxidation processes of CeOx defective layers

Abstract A CeO 2 thin film has been subjected to Ar + bombardment at 298 K to induce the reduction of its outmost layers by preferential removal of oxygen. An XPS study of the altered layer at normal and grazing angle has been carried out. The Factor Analysis (FA) of the XPS spectra of this Ar + reduced film shows that it has a stoichiometry close to Ce 2 O 3 , being Ce 3+ the dominant species at both collection angles. Simultaneously, the O1s spectra depict a lateral peak whose relative intensity is higher for those spectra recorded at grazing angle. Exposure to successive doses of O 2 at 298 K of the reduced layers produces the increase of the O/Ce ratio and a progressive reoxidation of Ce 3+ into Ce 4+ as determined by FA of the Ce3d spectra. Simultaneously, the lateral component at the O1s peak also decreases, thus discarding that it is due to surface contamination by –OH or similar species, as previously suggested in the literature. After exposure to a high pressure of oxygen (ca. 1 Torr), the XPS spectrum obtained at a normal collection angle shows an almost complete oxidation of the film to CeO 2 . However, in the spectrum at grazing angle, Ce 3+ species and the lateral component of oxygen are still detected. The lateral O1s component is tentatively attributed to oxygen ions with unusual coordinations in a defective CeO x ( x 3+ ions might be due to fully coordinated species. Enrichment of the surface of the defective cerium oxide with these oxygen species seems to be a result of the same structural rearrangements that favour the observed stabilization of Ce 3+ species at the surface.

Spectroscopic characterization of Tio2/SiO2 catalysts

Abstract TiO2/SiO2 (7 wt% TiO2) has been prepared via modification of the SiO2 surface by reaction with Ti[OCH(CH3)2]4. Physical characterization of the material was performed by means of XRD, diffuse reflectance spectroscopy (DRS), XPS, and low-temperature IR spectroscopy of adsorbed CO. TiO2 exists in highly dispersed form on the SiO2 surface. Interestingly, a quantum size effect leads to a shift of the absorption edge of TiO2 toward higher energy. XPS difference spectra indicate the formation of Ti3+ during reduction. This is confirmed by CO adsorption in the temperature range 80

Response of Nanoparticle-Based One-Dimensional Photonic Crystals to Ambient Vapor Pressure

Herein we report an analysis of the variation of the optical properties of different nanoparticle-based one-dimensional photonic crystal architectures versus changes in the ambient vapor pressure. Gradual shift of the optical response provides us with information on the sorption properties of these structures and allow us to measure precise adsorption isotherms of these porous multilayers. The potential of nanoparticle-based one-dimensional photonic crystals as base materials for optical sensing devices is demonstrated in this way.

Structure determination of Ni(111)c(4 × 2)-CO and its implications for the interpretation of vibrational spectroscopic data

Abstract A detailed quantitative structure determination of the Ni(111)c(4 × 2)-CO structure has been undertaken using scanned energy mode photoelectron diffraction from the C 1s state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing of 1.29 ± 0.05 A. This structure is therefore essentially the same as that for Ni(111)c(4 × 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.

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.

A photoelectron diffraction study of the structure of PF3 adsorbed on Ni{111}

The technique of scanned energy photoelectron diffraction has been used to study the structure PF3 adsorbed on a Ni{111} surface. The molecule adsorbs in an atop site with a Ni-P separation of 2.07 (+/- 0.03) angstrom and gives rise to an expansion of the Ni-Ni outermost layer spacing of 0.03 (+/- 0.05) angstrom.

Single local site structure for vibrationally distinct adsorption states : NO on Ni(111)

Abstract Scanned energy mode N and O 1s photoelectron diffraction data collected from NO on Ni(111) under conditions corresponding to three distinct N—O vibrational frequencies, previously assigned to different (bridge and atop) local adsorption configurations, show that NO in all cases occupies the same site. Comparison of these data with the results of theoretical simulations indicates that the common site is a threefold coordinated hollow. These results indicate that there is a need to reevaluate the range of applicability of the widely used method of adsorption site assignment based on the numerical values of internal molecular vibrational frequencies.