Raül Díaz

Perovskite-type BaSnO3 powders for high temperature gas sensor applications

Abstract Perovskite-type barium stannate obtained from a simple wet chemical route has been structural and electrical characterised. The resulting powders calcined at different temperatures have been studied by XRD, Raman and TEM. Thick films of BaSnO3 screen printed on an alumina substrate have been electrically studied as a function of the temperature and as a function of the ambient composition.

Synthesis of perovskite-type BaSnO3 particles obtained by a new simple wet chemical route based on a sol-gel process

Abstract A new wet chemical route to produce BaSnO 3 nanoparticles is reported. The method, based on a sol–gel process is characterized by its simplicity, reduced sintering time, mass production, high level of repeatability and a low industrialization implementation cost. Structural and microstructural analyses by TEM, XRD and Raman spectroscopy of the obtained BaSnO 3 were presented.

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO3 Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO3 quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO3, surface covered by unstable W(V) species, slowly oxidized upon standing in room conditions. The WO3 nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics.

Atomic Force Microscopy Study of the Silicon Doping Influence on the First Stages of Platinum Electroless Deposition

The first stages of platinum electroless deposition on (100) Si from hydrogen fluoride solutions are studied by tapping mode atomic force microscopy (AFM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Tapping mode AFM and TEM provide a morphological characterization of the samples, while XPS accounts for the compounds present on the surface. During immersion in an aqueous HF solution containing a platinum salt, platinum nucleates on the silicon substrate while the surface is etched. The deposited nuclei are polycrystalline, highly pure, and strongly silicidated at room temperature. Metal deposition takes place by means of a redox reaction in which silicon atoms oxidize, supplying the electrons for the metal to reduce. For all substrates, platinum silicide is formed during deposition at room temperature, suggesting a competition between the deposition of pure metal by an electrochemical mechanism and the formation of the silicide by direct reaction. For equal deposition times, more platinum deposits on p-type substrates than in n + . p-Type substrates, moreover, undergo a general increase in roughness in the bare silicon areas, while n-type substrates present a lower and more local etching. This seems to indicate that electroless platinum deposition is somehow hindered on n + substrates.

New method to obtain stable small-sized SnO2 powders for gas sensors

Abstract A new method based on the pyrolytic reaction of SnCl4⋅5(H2O) in the range of 400–900°C is reported to produce stable small grains of SnO2 from 6 to 34 nm. The characterisation of these nanocrystallites is discussed, giving important results about oxygen incorporation to the SnO2 lattice and grain growth. This allows the understanding of the crystalline features of the samples. The influence of this new method on grain growth topics (nucleation, Ostwald ripening, coalescence) is discussed. Examples of the CO and NO2 detection are reported, showing its relation with structural parameters.

Simultaneous platinum deposition and formation of a photoluminescent porous silicon layer

A method is presented for simultaneously producing porous silicon and depositing platinum on silicon from a platinum and fluoride solution operating at the rest potential. The resulting layers display visible photoluminescence. Quantitative electrochemical measurements demonstrate that the platinum ions act as an oxidizing agent for silicon, and that the silicon oxidation reaction proceeds in the porous silicon regime when the solution parameters are properly chosen.

Different Behavior in the Deposition of Platinum from HF Solutions on n‐ and p‐Type (100) Si Substrates

Platinum electroless deposition on silicon from HF solutions was studied by scanning electron microscopy and transmission electron microscopy, focusing the interest on the different evolution of the deposit on n- and p-doped samples. In both cases the final result was that a complete platinum layer was eventually formed, but the process was hindered on n + substrates which exhibited an induction period and displayed a more local behavior compared to p substrates. The results are discussed in terms of a global electrochemical process in which silicon is oxidized and platinum reduces injecting holes to the silicon valence band.

An electrochemical study of tin oxide thin film in borate buffer solutions

The electrochemical behavior of tin polycrystals in borate buffer solutions at pH 7.5 was systematically investigated using cyclic voltammetry, EC-STM and electrochemical impedance measurements. A systematic shift to more negative values of the potentials corresponding either to the major reduction peak either to the hydrogen evolution was measured when the anodization potential used for film growth was increased. A discussion about the different oxides there formed was included and a reassignation of tin oxidation and tin oxides reduction processes was given.

Surface Modification of TiO2 Nanocrystals by WOx Coating or Wrapping: Solvothermal Synthesis and Enhanced Surface Chemistry

TiO2 anatase nanocrystals were prepared by solvothermal processing of Ti chloroalkoxide in oleic acid, in the presence of W chloroalkoxide, with W/Ti nominal atomic concentration (R(w)) ranging from 0.16 to 0.64. The as-prepared materials were heat-treated up to 500 °C for thermal stabilization and sensing device processing. For R(0.16), the as-prepared materials were constituted by an anatase core surface-modified by WO(x) monolayers. This structure persisted up to 500 °C, without any WO3 phase segregation. For R(w) up to R(0.64), the anatase core was initially wrapped by an amorphous WO(x) gel. Upon heat treatment, the WO(x) phase underwent structural reorganization, remaining amorphous up to 400 °C and forming tiny WO3 nanocrystals dispersed into the TiO2 host after heating at 500 °C, when part of tungsten also migrated into the TiO2 structure, resulting in structural and electrical modification of the anatase host. The ethanol sensing properties of the various materials were tested and compared with pure TiO2 and WO3 analogously prepared. They showed that even the simple surface modification of the TiO2 host resulted in a 3 orders of magnitude response improvement with respect to pure TiO2.

Corrigendum: Inorganic Photocatalytic Enhancement: Activated RhB Photodegradation by Surface Modification of SnO 2 Nanocrystals with V 2 O 5 -like species

SnO2 nanocrystals were prepared by precipitation in dodecylamine at 100 °C, then they were reacted with vanadium chloromethoxide in oleic acid at 250 °C. The resulting materials were heat-treated at various temperatures up to 650 °C for thermal stabilization, chemical purification and for studying the overall structural transformations. From the crossed use of various characterization techniques, it emerged that the as-prepared materials were constituted by cassiterite SnO2 nanocrystals with a surface modified by isolated V(IV) oxide species. After heat-treatment at 400 °C, the SnO2 nanocrystals were wrapped by layers composed of vanadium oxide (IV-V mixed oxidation state) and carbon residuals. After heating at 500 °C, only SnO2 cassiterite nanocrystals were obtained, with a mean size of 2.8 nm and wrapped by only V2O5-like species. The samples heat-treated at 500 °C were tested as RhB photodegradation catalysts. At 10−7 M concentration, all RhB was degraded within 1 h of reaction, at a much faster rate than all pure SnO2 materials reported until now.