RT info:eu-repo/semantics/bachelorThesis
T1 Nanopartículas luminiscentes para aplicaciones en dispositivos ópticos. Obtención, caracterización estructural y estudio espectroscópico.
A1 Medina Alayón, Francisco Miguel
AB Despite the apparent relationship of nanoparticles with modern science, it is knownthat the first records of their use date back to the Middle Ages, where they were madeusing primitive methods for the purpose of embellishing artistic representations. However,it is not until today that the potential applications of nanostructured materials are beingused, covering scientific fields ranging from medicine, with the development of luminescent biocapsules for the treatment of some serious diseases, to the energy and ligthingsectors, with the substantial improvement of the efficiency in photovoltaic panels and thequality of illumination. In particular, if these materials are doped with rare earth ions,important luminescent properties and photonic effects emerge from them, as is the caseof the up-conversion phenomenon, which can be especially interesting in photocatalytical applications, as for example, the hydrogen production through water-splitting or thetreatment of wastewater.In the present work, different nanostructured materials based on N aY bF4 and dopedwith ions of Eu3+ (5 % and 2 %) or Tm3+ (1 %) through different synthesis methods havebeen elaborated, so that its advantages over traditional methods have been reaffirmed: onthe one hand, different nanocristal samples (NCs) were synthesized using the solvothermalmethod, which involves organic solvents and surfactants and working with high pressuresand relativelly low temperatures through the use of an autoclave. On the other hand, several nano-glass-ceramics (nGCs) were produced using the sol-gel technique, which allowsto obtain nanostructured materials with optical quality, controlling their composition precisely and working at lower temperatures than conventional melt-quenching techniques.Additionally, the importance to dope these materials with rare earths lies in the electronic configuration of those ions, since the phenomenon called lanthanide contractionproduces electronic mismatch effects that magnify their magnetic and optical propertieswhen interacting with visible, ultraviolet and infrared radiation.The synthesized nanostructured materials were then subjected to structural characterization using X-ray diffraction (XRD), transmission electron microscopy (TEM-HRTEM),and X-ray dispersive spectroscopy (EDS):First, XRD measurements confirmed the formation of solvothermal cubic and hexagonal N aY bF4 NCs, with average sizes of 17 nm and 80 nm, respectively (calculated usingthe Scherrer equation), showing the possibility to select the nanocrystalline phase in function of the selected heat treatment. Second, the images obtained through TEM-HRTEMalso confirmed the formation of those NCs, with spherical morphology and similar sizesto the estimated from XRD measurements. Third, the EDS measurements confirmed theexistence of the expected chemical elements within the different samples. Moreover, it wasalso confirmed the precipitation of N aY bF4 cubic NCs into the SiO2 matrix for all thenGCs, with average sizes around 7 nm. Once the structural characterization was completed, a spectroscopic study was carriedout for the different nanostructured materials through different excitation and emissionspectra.On the one hand, and with the purpose of complementing the structural characterization, an Eu3+ ion environment analysis was carried out, taking advantage of theirproperties as a spectroscopic probe to determine the site symmetry of dopant RE3+ ions.Emission spectra of the solvothermal NCs exciting at 393 nm, corresponding to thetransition 7F0 ⇒5 L6 showed that the ratio of the emissions associated with 5D0 ⇒7 F1and 5D0 ⇒7 F2, known as the asymmetry ratio, present a value of R = 1.19 for cubicNCs and R = 1.91 for hexagonal NCs, related with different symmetry sites occupied byRE3+ ions. On the other hand, a R = 1.31 for the 95SiO2 − 5N aY bF4 : 2 %Eu3+ nGCwas obtained, suggesting an effective distribution of these ions into cubic N aY bF4 NCs.Moreover, a R = 1.87 when exciting at 464 nm, suggest that some Eu3+ ions remain intothe glassy matrix.On the other hand, intense UV up-conversion emissions were observed when exciting at 980 nm in cubic and hexagonal Tm3+ doped solvothermal NCs. The overall UCemissions for the hexagonal NCs result more intense than the corresponding to the cubicNCs, which can be related with different symmetry sites for the RE3+ ions. Moreover, corresponding intense up-conversion emissions for the Tm3+ doped nGC were also observed.Furthermore, the proportionality of the intensity of the up-conversion emissions withthe laser pumping power was analyzed from logarithmic representations.Thus, the resultsobtained showed saturation effects, since these values were lower than the expected theoretical ones. This phenomenon can be related to the competition between linear decayand UC processes for the depletion of the intermediate excited levels, and in particular,in all the studied materials, is related with high amount of Y b3+ ions.Finally, and taking advantage of the observed intense UV up-conversion emissions,a methylene blue photocatalysis experiment was carried out in order to evaluate potential industrial applications of these nanostructured materials. On one hand, degradationcurves of the methylene blue obtained from Tm3+ solvothermal NCs showed an important degradation, more intense in hexagonal NCs than in the cubic ones. On the otherhand, a degradation of 31 % was obtained for the Tm3+ doped nGC. These results suggest the possible application of these nanostructural materials in promising applicationsin photocatalytic processes, such as the hydrogen generation through water-splitting orthe treatment of wastewater.
YR 2021
FD 2021
LK http://riull.ull.es/xmlui/handle/915/25742
UL http://riull.ull.es/xmlui/handle/915/25742
LA es
DS Repositorio institucional de la Universidad de La Laguna
RD 18-may-2024