Rubén Arroyo

Influence of manganese ions on the anatase–rutile phase transition of TiO2 prepared by the sol–gel process

Abstract The anatase–rutile phase transformation of TiO 2 containing various amounts of Mn 2+ ions was studied by X-ray diffraction, electron spin resonance spectroscopy, and surface area measurements. The samples were prepared by the sol–gel process from titanium(IV) isopropoxide and manganese(II) acetate tetrahydrate. The amorphous powders were calcined for 2 h at different temperatures. Significant structural changes were observed during the various stages of the phase transformation. It was concluded that at low dopant concentrations, manganese ions are incorporated in the TiO 2 structure, and the anatase phase is stabilized, but at larger amounts, part of the dopant is segregated on the surface of TiO 2 and the rutile formation is accelerated. This was clearly confirmed by electron spin resonance. This experiment allowed the authors to follow the manganese oxidation and the phase transformation in detail.

Synthesis, characterization and luminescence properties of Tb3+ and Eu3+-doped poly(acrylic acid)

Abstract Tb 3+ and Eu 3+ -doped poly(acrylic acid) (PAA) was synthesized by polymerization in aqueous solution of the monomer partially neutralized with various amounts of Tb 2 O 3 and Eu 2 O 3 , using potassium persulfate as initiator. The monomer modification was confirmed by 1 H-NMR. The doped polymer was characterized by 1 H-NMR, Fourier transform infrared (FT-IR) and luminescent spectroscopy. The NMR and FT-IR results show clear evidence of the coordination between the dopant cations and the carboxylate anions, which are formed during the partial neutralization of the monomer prior to polymerization. On the other hand, the polymer phosphorescence intensity decreased as the Eu 3+ concentration was increased due to the complex formation. Additionally, the inhomogeneous broadening and the relative intensity of the 5 D 0 → 7 F 1,2 transitions of Eu 3+ ions, and also, the relatively high intensity of 5 D 4 → 7 F 5 green emission of Tb 3+ , indicate that the dopant ions are uniformly distributed in low symmetry coordination sites. Besides, when both dopants are incorporated to the polymer, the Eu 3+ ions quench the luminescence of Tb 3+ , indicating an efficient energy transfer from Tb 3+ to Eu 3+ .

Synthesis and spectroscopic characterization of Eu3+-doped poly(acrylic acid)

Abstract Eu3+-doped poly(acrylic acid) was synthesized by polymerization in aqueous solution of the monomer partially neutralized with different amounts of Eu2O3 (0.5–5.0 mol% of Eu3+) using potassium persulfate as initiator. The Eu3+-doped polymer was characterized by 1 H - and 13 C -NMR, Fourier transform infrared (FT-IR) and luminescent spectroscopy. The NMR and FT-IR results show clear evidence of the coordination between Eu3+ cations and the carboxylate anions which result of the partial neutralization of the monomer prior to polymerization. On the other hand, the polymer phosphorescence intensity was observed to decrease with the increasing of the Eu3+ concentration due to the complex formation. Additionally, the homogeneous broadening and the relative intensity of the 5 D 0 → 7 F 1,2 transitions indicate that the dopant ions are uniformly distributed in low symmetry coordination sites.

On the Reactivity of Sulfosalts in Cyanide Aqueous Media: Structural, Bonding and Electronic Aspects

The reactivity of the ruby silver minerals proustite (3Ag(2)S⋅As(2)S(3)) and pyrargyrite (3Ag(2)S⋅Sb(2)S(3)) was studied with two types of electrodes: a carbon-paste electroactive electrode (CPEE) and a paraffin-impregnated graphite electrode (PIGE). Polycrystalline samples of α-Ag(2)S (acanthite), As(2)S(3) (orpiment), Sb(2)S(3) (stibnite), Ag(3)AsS(3) (proustite), Ag(3)SbS(3) (pyrargyrite), and three samples of the proustite-pyrargyrite solid solution series were synthesized from pure elements by a solid-state reaction method. Phase identification of samples was carried out by XRD and chemical homogeneity was checked by SEM-EDS. Besides, sulfosalts were characterized by diffuse reflectance spectroscopy (DRS). Flat-band and formal potentials of sulfosalts were determined by the Mott-Schottky method and differential pulse abrasive stripping voltammetry, respectively. Band structure, bonding and solid-state structure are considered to investigate the oxidation and reduction of the solids. A ligand-to-metal charge transfer (LMCT) transition from the AsS(3) (or SbS(3)) group to Ag is related to ease of reducing the pyrargyrite-proustite series. Despite the increase in the amount of As (Sb) in Ag(3)SbS(3) (Ag(3)AsS(3)), reactivity is similar due to the similarity of the solid-state structures, and the same oxidation states of S, As, Sb and Ag species in the lattice. However, the nature of the pnictogen (As or Sb) changes the position of the conduction and valence band edges and modulates the reactivity of the pyrargyrite-proustite series. Anodic dissolution occurs by hole transfer from the top of the valence band that is formed mainly by the states of the AsS(3) and SbS(3) groups. Meanwhile, silver reduction occurs by electron transfer from the Ag 5s orbitals located at the bottom of the conduction band. The difficulty in dissolving proustite and pyrargyrite in cyanide is related to the presence of pyramidal AsS(3) and SbS(3) groups in these sulfosalts.

Dynamic light scattering studies of the stability and growth of silica particles

Abstract The kinetics of silica sol growth under different chemical conditions was observed by measuring the particle size of solds size profile shows damped and periodic oscillations under a steady state condition is reached. This oscillatory behavior of the profile is associated with the unstable character of the chemical reaction. The instability arises because one reaction of the process is auto-catalytic. This instability produces a wide variety of possible structures that are formed during this part of the reaction. By appropriatly changing some chemical parameters such as pH, concentration, temperature, etc., it was possible to obtain a stable reaction from the beginning of the process. All the predictions of the effect of growth parameters on the stability of the reaction were fulfilled, this giving good support to the assumptions about the unstable character of the chemical reaction.

Time dependence of particle size and particle number of silica sols under reflux conditions

Abstract In this paper some results of work on the kinetics of aggregation of silica particles obtained by the sol-gel method are reported. The silica gel was prepared by mixing tetraethyl orthosilicate, ethanol, water, and a cobalt salt (cobalt (II) acetylacetonate). The chemical reaction was carried out under reflux conditions (T = 75°C). The aggregation was followed using dynamic light scattering from the onset of the chemical reaction. A systematic and reproducible behavior of the growth of the silica sols was obtained. The system begins with sols of relatively small sizes (around 100 nm), but within a short time the particle size increases to around 1 μm. After this increase, damped oscillations in size are observed until the radius remains constant. The particle size in this regime is similar to the initial values of the sols. The oscillations in the particle size profile can be explained as being a non-linear effect in the chemical process because one stage of the reaction is autocatalytic.

Aggregation profiles of silica sols in a sol-gel process

Abstract The kinetics of aggregation of sols in a sol-gel system was followed by measuring the particle size profile of sols from the start of the chemical reaction. The particle size was measured using dynamic light scattering. The particle size profiles show three different regions. The first one corresponds to an unstable regime characterized by reproducible oscillations in the size of the sols. This is due to a wide variety of different dissipative structures formed during the early stage of the reaction because of the autocatalytic character of the polymerization reaction. The second region is a steady-state regime which is characterized by the fact that the particle size remains practically constant for a long period of time. In this regime, the relative scattered intensity grows in an approximately linear way with the reaction time, i.e. the number of sols is increasing with time. In the last region, the particle size profile shows a rapid increase of the size of sols, and this corresponds to gelation. The number concentration of sols is so large that they begin to touch each other to form the gel. This is the mechanism by which the system reaches gelation.

An Approach to the Reactivity of Isomorphous Proustite (Ag3AsS3) and Pyrargyrite (Ag3SbS3) in Cyanide Solutions

Cyanidation is the main process for recovering silver from its ores. Its impact on hydrometallurgy over the last 100 years is very great. The complexity of the silver mineralogy is the main problem during cyanidation, since each ore is a problem in itself due to mineralogical and physical differences. Ruby silver ores, proustite (Ag3SbS3) and pyrargyrite (Ag3SbS3) are refractory to leaching with cyanide. Unfortunately, the chemistry of recovery of silver-bearing minerals in cyanide has been few understood. In order to understand the main factors associated to the reactivity of the ruby silver ores in the cyanide solution, synthesis and characterization of proustite and pyrargyrite has been performed.

Optical spectroscopy of Nd3+ ions in poly(acrylic acid)

Nd3+ dissolved in solid poly(acrylic acid) was synthesized by polymerization of the monomer partially neutralized with neodymium hydroxide in aqueous solution. The monomer modification and the coordination of ligands to Nd3+ were confirmed by 1H NMR spectroscopy. The measured oscillator strengths for transitions from the ground state to the main excited state manifolds compared favourably with calculated electric dipole oscillator strengths. The spontaneous emission rates, the fluorescence branching ratios and the stimulated emission cross sections of the , and transitions, as well as the radiative lifetime and the quantum efficiency of the 4F3/2 emitting level, were determined.

Intramolecular charge transfer model in fluorescence processes

The absorption–emission processes that characterize the fluorescent phenomena are analyzed by assuming that the changes in the ground state electronic density, when the system is promoted to the first excited state, and that the changes in the first excited state electronic density, when the system decays to a ground state with a distorted geometry, can be interpreted as intramolecular charge transfer processes. Thus, it is shown that in this context the hardness is the property that describes the absorption and the emission energies. Additionally, it is indicated that the substituent effects on the oscillator strengths may be described through the frontier molecular orbital condensed Fukui functions for the charge donating process.