Franca Morazzoni

Photogenerated Defects in Shape-Controlled TiO2 Anatase Nanocrystals: A Probe To Evaluate the Role of Crystal Facets in Photocatalytic Processes

The promising properties of anatase TiO(2) nanocrystals exposing specific surfaces have been investigated in depth both theoretically and experimentally. However, a clear assessment of the role of the crystal faces in photocatalytic processes is still under debate. In order to clarify this issue, we have comprehensively explored the properties of the photogenerated defects and in particular their dependence on the exposed crystal faces in shape-controlled anatase. Nanocrystals were synthesized by solvothermal reaction of titanium butoxide in the presence of oleic acid and oleylamine as morphology-directing agents, and their photocatalytic performances were evaluated in the phenol mineralization in aqueous media, using O(2) as the oxidizing agent. The charge-trapping centers, Ti(3+), O(-), and O(2)(-), formed by UV irradiation of the catalyst were detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O(-) centers) increases with increasing {001} surface area and photoactivity, while the amount of Ti(3+) centers increases with the specific surface area of {101} facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites which do not directly assist the oxidative processes. Photoexcitation experiments in O(2) atmosphere led to the formation of Ti(4+)-O(2)(-) oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxidative processes. Although this work focuses on the properties of TiO(2), we expect that the presented quantitative investigation may provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.

Macroporous WO3 Thin Films Active in NH3 Sensing: Role of the Hosted Cr Isolated Centers and Pt Nanoclusters

Macroporous WO(3) films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous sol-gel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO(3) electrical response, was carried out by including Cr(III) and Pt(IV) centers in the oxide matrix. It turned out that Cr remains as homogeneously dispersed Cr(III) centers inside the WO(3) host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH(3), the electrical conductivity of transition metal doped-WO(3) increases, especially in the presence of Pt dopant, resulting in outstanding sensing properties (S = 110 ± 15 at T = 225 °C and [NH(3)] = 74 ppm). A mechanism was suggested to explain the excellent electrical response of Pt-doped films with respect to the Cr-doped ones. This associates the easy chemisorption of ammonia on the WO(3) nanocrystals, promoted by the inverted opal structure, with the catalytic action exerted by the surface Pt nanoclusters on the N-H bond dissociation. The overall results indicate that in Pt-doped WO(3) films the effects of the macroporosity positively combine with the electrical sensitization promoted by the metal nanoclusters, thus providing very lightweight materials which display high functionality even at relatively low temperatures. We expect that this synergistic effect can be exploited to realize other functional hierarchical metal oxide structures to be used as gas sensors or catalysts.

Layered Na0.71CoO2: a powerful candidate for viable and high performance Na-batteries

The present study reports on the synthesis and the electrochemical behavior of Na(0.71)CoO(2), a promising candidate as cathode for Na-based batteries. The material was obtained in two different morphologies by a double-step route, which is cheap and easy to scale up: the hydrothermal synthesis to produce Co(3)O(4) with tailored and nanometric morphology, followed by the solid-state reaction with NaOH, or alternatively with Na(2)CO(3), to promote Na intercalation. Both products are highly crystalline and have the P2-Na(0.71)CoO(2) crystal phase, but differ in the respective morphologies. The material obtained from Na(2)CO(3) have a narrow particle length (edge to edge) distribution and 2D platelet morphology, while those from NaOH exhibit large microcrystals, irregular in shape, with broad particle length distribution and undefined exposed surfaces. Electrochemical analysis shows the good performances of these materials as a positive electrode for Na-ion half cells. In particular, Na(0.71)CoO(2) thin microplatelets exhibit the best behavior with stable discharge specific capacities of 120 and 80 mAh g(-1) at 5 and 40 mA g(-1), respectively, in the range 2.0-3.9 V vs. Na(+)/Na. These outstanding properties make this material a promising candidate to construct viable and high-performance Na-based batteries.

NH3 interaction with chromium-doped WO3 nanocrystalline powders for gas sensing applications

Ammonia interaction with chromium-doped WO3 nanocrystalline powders was investigated. Chromium centres were characterised by EELS, Raman, XPS, EPR and UV-visible spectroscopy. Essentially, these studies revealed that chromium was well distributed on WO3, forming Cr(III) species and chromyl groups [CrO]3+. Test of thick-film gas sensors based on chromium-doped WO3 revealed that chromium addition boosted sensor response to NH3. Besides, it also avoided unsatisfactory dynamic behaviour previously found in gas sensors based on pure WO3. DRIFT spectroscopy and TPD were used to investigate the surface chemistry of ammonia over pure and chromium-doped WO3 nanocrystalline powders. It was concluded that chromium favours the combustion of ammonia into molecular nitrogen and nitrous oxide on chromium centres, which contributes to the improvement in the sensing properties of the material.

Hydrogenation of carbon monoxide: evidence of a strong metal-support interaction in Rh/ZrO2 catalysts

Abstract Temperature-programmed desorption (TPD) studies are reported on a series of Rh catalysts dispersed on ZrO2 and γ-Al2O3 and activated under H2 flow at 250, 400, and 600°C. While the high temperature of activation did not show any significant difference in the Rh γ-Al 2 O 3 samples, in the case or Rh ZrO 2 a pronounced effect is quite evident. The experimental observations have been associated to a modification of the metal-support interface, for which a possible model is suggested. Parallel experiments in the hydrogenation of carbon monoxide at 220°C and P = 1 atm have shown a sharp decrease in the overall activity while the selectivity was virtually unaffected by the temperature of reduction.

Molecular oxygen interaction with Bi2O3: a spectroscopic and spectromagnetic investigation

The interaction of molecular oxygen with polycrystalline Bi2O3 was investigated by infrared (IR) and thermogravimetric (TGA) analyses, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results indicate the formation of “end-on” and “bridging” superoxide species, which are trapped in the β-Bi2O3 vacant oxygen sites, Bi(IV) and Bi(V) centres also being produced. O2− interacts with Bi(IV) centres giving two superoxide adducts with temperature-dependent relative amounts, Bi(IV)–O2− and Bi(IV)–O2−–Bi(III), both having a triplet spin state.

Ultraviolet photoluminescence of porous silica

Excitation pattern and decay kinetics of ultraviolet photoluminescence of porous silica are investigated between 4.5 and 10 eV by means of synchrotron radiation. Spectra are dominated by a 3.7 eV emission similar to the recently observed ultraviolet emission of oxidized porous Si and Si nanostructures. Emission intensity is found to be controlled by the material specific surface. Other emissions are observed at 2.9, 3.8, and 4.2 eV. All emissions show lifetimes of a few nanoseconds. Spectral and kinetic features are sensibly different than in glassy SiO2, suggesting a revision of previous assignments of ultraviolet emissions in oxidized porous Si and Si nanostructures.

NH 3 Interaction with Catalytically Modified Nano- WO 3 Powders for Gas Sensing Applications

Nanocrystalline powders of WO 3 , pure and with catalytic additives such as copper and vanadium, for ammonia gas detection are analyzed in detail. Material was annealed at two different temperatures (400 and 700 ° C) and catalytic additives were introduced in two different concentrations (0.2 and 2%)in order to study the gas sensor performances of these WO 3 -based materials. Crystalline structure characterization shows that a mixture of triclinic and monoclinic structure was present in the materials analyzed. Additive characterization reveals that catalytic metals were located as cations in the matrix lattice. Thick-film gas sensors based on pure WO 3 show an abnormal sensor response, which is attributed to a complex process originated by the oxidation of ammonia to NO. On the other hand, catalyzed WO 3 -based gas sensors show a more direct and simple sensor response. Interaction of ammonia with WO 3 was studied by diffuse reflectance infrared spectroscopy. Only pure WO 3 presented a W=O overtone band decrease and some nitrosil bands. In this case, NH 3 would react with the surface oxygen of terminal W=O bonds and would lead to the formation of NO. Catalyzed WO 3 avoided this reaction and so the unselective catalytic oxidation of NH 3 , improving sensor response. Influence of introduced additives on ammonia oxidation and thus on sensor response is discussed.

Surface reactivity of nanostructured tin oxide and Pt-doped tin oxide as studied by EPR and XPS spectroscopies

Abstract Nanostructured (3–6 nm) thin films (80 nm) of SnO 2 and Pt-doped SnO 2 were obtained by a new sol–gel route using tetra( tert -butoxy)tin(IV) and bis(acetylacetonato)platinum(II) as precursors. EPR and XPS investigations, performed on thin films after interaction with CO, demonstrated that singly ionized oxygen vacancies (V o ) fully transferred their electrons to the noble metal and reduced Pt(IV) to Pt(II). Contact with air at room temperature led to the reduction of O 2 to O 2 − , therefore, re-oxidizing metal centers. The reaction mechanism concords with the high electrical sensitivity of this material.

Rubber–silica nanocomposites obtained by in situ sol–gel method: particle shape influence on the filler–filler and filler–rubber interactions

Silica–natural rubber composites were prepared by in situ sol–gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler–filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler–rubber interactions. Both filler–rubber and strong filler–filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler–filler interaction, and it is maximum when also the filler–rubber interaction occurs. Strong silica–rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.