Concha Fernández-Lorenzo

Sol-gel synthesis of SiO2P2O5 glasses

The synthesis of P2O5SiO2 glasses from solutions has been studied through gelation kinetics, Raman and nuclear magnetic resonance (NMR) spectroscopies. Different preparation methods are compared. The effect of the phosphorus precursor has been investigated. Phosphoric acid esters are not hydrolyzed and they do not yield condensation with silica. Reaction of anhydrous H3PO4 with SiOR groups leads to the formation of SiOP bonds which are easily hydrolyzed. Only a few remain after addition of water and gelation. Therefore, PSi homogeneity is difficult to achieve after gelation and the following drying and heat-treatment procedures. This absence of homogeneity is observed by solid-state magic angle spinning (MAS)-NMR and X-ray diffraction studies performed on gel samples heated at 300, 500 and 800°C in the case of xerogels and 500, 950 and 1140°C for aerogels.

ZnO-based dye solar cell with pure ionic-liquid electrolyte and organic sensitizer: the relevance of the dye–oxide interaction in an ionic-liquid medium

The use of non-volatile electrolytes and fully organic dyes are key issues in the development of stable dye-sensitized solar cells (DSCs). In this work we explore the performance of ZnO-based DSCs sensitized with an indoline derivative coded D149 in the presence of a pure ionic-liquid electrolyte. Commercial nanostructured zinc oxide and an electrolyte composed of iodine plus (1) pure 1-propyl-3-methyl imidazolium iodide (PMII) and (2) a blend of PMII with low-viscosity ionic liquids were employed to construct the devices. Without further additives, the fabricated devices exhibit remarkable short-circuit photocurrents and efficiencies under AM1.5 simulated sunlight (up to 10.6 mA cm−2, 2.9% efficiency, 1 sun, active area = 0.64 cm2) due to the high surface area of the ZnO film and the high absorptivity of the D149 dye. Impedance spectroscopy is used to characterize the devices. It is found that the addition of the low-viscosity ionic-liquid improves the transport features (leading to a better photocurrent) but it does not alter the recombination rate. The robustness of the dye–oxide interaction is tested by applying continuous illumination with a Xenon-lamp. It is observed that the photocurrent is reduced at a slow rate due to desorption of the D149 sensitizer in the presence of the ionic liquid. Exploration of alternative ionic-liquid compositions or modification of the ZnO surface is therefore required to make stable devices based on ZnO and fully organic dyes.

EXAFS, Raman and 31P NMR study of amorphous titanium phosphates

The reaction of phosphoric acid with titanium alkoxides for a PO4/Ti ratio ranging from 0.5 to 2 was investigated. For these compositions, amorphous precipitates are obtained. Their microstructure has been studied by means of Raman, nuclear magnetic resonance and X-ray absorption spectroscopies. The phosphatation of titanium alkoxide increases the Ti coordination number to 6 and the symmetry becomes purely octahedral for a P/Ti ratio of 2. For the composition range 0 < PO4/Ti ⩽ 2, phosphate anions are homogeneously dispersed in the precipitate and bound to titanium, the coordination of which is completed by oxo-, hydroxo- and remaining alkoxy- groups. Short TiO distances are observed in the materials having the lowest P/Ti ratios.

Introducing “UCA-FUKUI” software: reactivity-index calculations

A new software (UCA-FUKUI) has been developed to facilitate the theoretical study of chemical reactivity. This program can calculate global parameters like hardness, softness, philicities, and Fukui condensed functions, and also local parameters from the condensed functions. To facilitate access to the program we have developed a very easy-to-use interface. We have tested the performance of the software by calculating the global and local reactivity indexes of a group of representative molecules. Finite difference and frontier molecular orbital methods were compared and their correlation tested. Finally, we have extended the analysis to a set of ligands of importance in coordination chemistry, and the results are compared with the exact calculation. As a general trend, our study shows the existence of a high correlation between global parameters, but a weaker correlation between local parameters.

A precision method for laser focusing on laser beam induced current experiments

Using the laser beam induced current (LBIC) technique for the study of solar cells and photovoltaic devices, it is possible to obtain images representing the different degrees of quantum efficiency observed on the surface of these elements. Since the resolution of these images depends on the size of the spot used, the laser beam is directed through a focusing lens, which allows us to obtain significantly small sizes. This article puts forward a method for precise focusing of the laser beam used in the LBIC study of photovoltaic devices by analyzing line scans over sharp structures. The technique is applied to characteristic heterogeneities of solar cells such as the fingers used to collect the induced current, the grain boundaries characteristic of polycrystalline solar cells, and the small surface areas with a quantum efficiency different from that of its adjacent areas. The validity of the proposed method has been evaluated by carrying out virtual experiments where the focusing technique has been applie...

A methodology for improving laser beam induced current images of dye sensitized solar cells

Using the laser beam induced current (LBIC) technique for the study of solar cells and photovoltaic devices, it is possible to obtain images representing the different degrees of quantum efficiency observed on the surface of these elements. Dye sensitized solar cells (DSSCs) or photoelectrochemical solar cells, in contrast to those based on solid-solid interfaces, show a slow response to irradiance variations--up to tens of seconds. This is basically due to both viscous matter transport processes and load transfer. This response is inappreciable when the device is functioning continuously but when a LBIC scan is performed, in which the laser moves quickly from one point to another, the slow response produces a memory effect and the signal generated at one given point depends on the conversion efficiency coefficients of the previously excited positions, resulting in diffuse images and a lack of sharpness. This work presents a methodology to correct high-resolution LBIC mappings of DSSCs using an algorithm based on the kinetics of the discharge process of the irradiated zone. The validity of the proposed method has been evaluated by carrying out experiments where the algorithm has been applied to LBIC mappings.

Electronic and structural properties of highly aluminum ion doped TiO(2) nanoparticles: a combined experimental and theoretical study.

This study presents the experimental and theoretical study of highly internally Al-doped TiO2 nanoparticles. Two synthesis methods were used and detailed characterization was performed. There were differences in the doping and the crystallinity, but the nanoparticles synthesized with the different methods share common features. Anatase to rutile transformation occurred at higher temperatures with Al doping. X-ray photoelectron spectroscopy showed the generation of oxygen vacancies, which is an interesting feature in photocatalysis. In turn, the band-gap energy and the valence band did not change appreciably. Periodic density functional calculations were performed to model the experimentally doped structures, the formation of the oxygen vacancies, and the band gap. Calculation of the density of states confirmed the experimental band-gap energies. The theoretical results confirmed the presence of Ti(4+) and Al(3+) . The charge density study and electron localization function analysis indicated that the inclusion of Al in the anatase structure resulted in a strengthening of the TiO bonds around the vacancy.

Tm-doped TiO2 and Tm2Ti2O7 pyrochlore nanoparticles: enhancing the photocatalytic activity of rutile with a pyrochlore phase

Summary Tm-doped TiO2 nanoparticles were synthesized using a water-controlled hydrolysis reaction. Analysis was performed in order to determine the influence of the dopant concentration and annealing temperature on the phase, crystallinity, and electronic and optical properties of the resulting material. Various characterization techniques were utilized such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and UV–vis spectroscopy. For the samples annealed at 773 and 973 K, anatase phase TiO2 was obtained, predominantly internally doped with Tm3+. ICP–AES showed that a doping concentration of up to 5.8 atom % was obtained without reducing the crystallinity of the samples. The presence of Tm3+ was confirmed by X-ray photoelectron spectroscopy and UV–vis spectroscopy: the incorporation of Tm3+ was confirmed by the generation of new absorption bands that could be assigned to Tm3+ transitions. Furthermore, when the samples were annealed at 1173 K, a pyrochlore phase (Tm2Ti2O7) mixed with TiO2 was obtained with a predominant rutile phase. The photodegradation of methylene blue showed that this pyrochlore phase enhanced the photocatalytic activity of the rutile phase.

Raman study of structural defects in SiO2 aerogels

The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups.The typical bands due to ring breathing also called defect bands D1 and D2 located at 490 and 610 cm−1 are present. However, the evolution of the D2 band compared to that of OH band (980 cm−1) seems apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300°C the D2 and the OH bands increase simultaneously. Generally, in silica glass the defect band D2 grows at the expense of the OH groups.This result is explained by the oxidation of the organic compounds which, in this temperature range, leads to the formation of the both species (OH) and those related to siloxane rings. 29Si MAS NMR results are in agreement with the Raman study.

High resolution laser beam induced current images under trichromatic laser radiation: Approximation to the solar irradiation

A laser beam induced current (LBIC) map of a photoactive surface is a very useful tool when it is necessary to study the spatial variability of properties such as photoconverter efficiency or factors connected with the recombination of carriers. Obtaining high spatial resolution LBIC maps involves irradiating the photoactive surface with a photonic beam with Gaussian power distribution and with a low dispersion coefficient. Laser emission fulfils these characteristics, but against it is the fact that it is highly monochromatic and therefore has a spectral distribution different to solar emissions. This work presents an instrumental system and procedure to obtain high spatial resolution LBIC maps in conditions approximating solar irradiation. The methodology developed consists of a trichromatic irradiation system based on three sources of laser excitation with emission in the red, green, and blue zones of the electromagnetic spectrum. The relative irradiation powers are determined by either solar spectrum distribution or Plancks emission formula which provides information approximate to the behavior of the system if it were under solar irradiation. In turn, an algorithm and a procedure have been developed to be able to form images based on the scans performed by the three lasers, providing information about the photoconverter efficiency of photovoltaic devices under the irradiation conditions used. This system has been checked with three photosensitive devices based on three different technologies: a commercial silicon photodiode, a commercial photoresistor, and a dye-sensitized solar cell. These devices make it possible to check how the superficial quantum efficiency has areas dependent upon the excitation wavelength while it has been possible to measure global incident photon-to-current efficiency values approximating those that would be obtained under irradiation conditions with sunlight.