Person:
Maestre Varea, David

First Name

David

Last Name

Maestre Varea

URI

https://hdl.handle.net/20.500.14352/75109

Affiliation

Universidad Complutense de Madrid

Faculty / Institute

Ciencias Físicas

Department

Física de Materiales

Area

Ciencia de los Materiales e Ingeniería Metalúrgica

Identifiers

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Search Results

Now showing 1 - 9 of 9

In situ local oxidation of SnO induced by laser irradiation: a stability study
(Nanomaterials, 2021) Vázquez López, Antonio; Maestre Varea, David; Ramírez Castellanos, Julio; Cremades Rodríguez, Ana Isabel
In this work, semiconductor tin oxide (II) (SnO) nanoparticles and plates were synthesized at room conditions via a hydrolysis procedure. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the high crystallinity of the as-synthesized romarchite SnO nanoparticles with dimensions ranging from 5 to 16 nm. The stability of the initial SnO and the controlled oxidation to SnO_2 was studied based on either thermal treatments or controlled laser irradiation using a UV and a red laser in a confocal microscope. Thermal treatments induced the oxidation from SnO to SnO2 without formation of intermediate SnO_x, as confirmed by thermodiffraction measurements, while by using UV or red laser irradiation the transition from SnO to SnO_2 was controlled, assisted by formation of intermediate Sn3O4, as confirmed by Raman spectroscopy. Photoluminescence and Raman spectroscopy as a function of the laser excitation source, the laser power density, and the irradiation duration were analyzed in order to gain insights in the formation of SnO_2 from SnO. Finally, a tailored spatial SnO/SnO_2 micropatterning was achieved by controlled laser irradiation with potential applicability in optoelectronics and sensing devices.
Synergetic improvement of stability and conductivity of hybrid composites formed by PEDOT:PSS and SnO nanoparticles
(Molecules, 2020) Vázquez López, Antonio; Yaseen, Anisa; Maestre Varea, David; Ramírez Castellanos, Julio; Marstein, Erik S; Karazhanov, Smagul Zh; Cremades Rodríguez, Ana Isabel
In this work, layered hybrid composites formed by tin oxide (SnO) nanoparticles synthesized by hydrolysis and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) have been analyzed. Prior to the composite study, both SnO and PEDOT:PSS counterparts were characterized by diverse techniques, such as X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), photoluminescence (PL), atomic force microscopy (AFM), optical absorption and Hall effect measurements. Special attention was given to the study of the stability of the polymer under laser illumination, as well as the analysis of the SnO to SnO_2 oxidation assisted by laser irradiation, for which different laser sources and neutral filters were employed. Synergetic effects were observed in the hybrid composite, as the addition of SnO nanoparticles improves the stability and electrical conductivity of the polymer, while the polymeric matrix in which the nanoparticles are embedded hinders formation of SnO_2. Finally, the Si passivation behavior of the hybrid composites was studied.
Unravelling the role of lithium and nickel doping on the detect structure and phase transition of anatase TIO2 nanoparticles
(Journal of materials science, 2022) Vázquez López, Antonio; Maestre Varea, David; Martínez Casado, Ruth; Ramírez Castellanos, Julio; Pís, Igor; Nappini, Silvia; Cremades Rodríguez, Ana Isabel
Anatase TiO_2 nanoparticles doped either with Li or Ni have been synthesized via hydrolysis in variable concentrations. Microstructural analysis confirms the high crystallinity of the doped nanoparticles with sizes around 7 nm, while compositional analysis shows low doping below 2% at. Despite the low concentration of dopants, variations in the Raman and Photoluminescence signals were observed in the doped nanoparticles, mainly due to non-stoichiometry and oxygen deficiency promoted by Li or Ni doping. Doping effects associated with Li and Ni were observed by photoelectron spectroscopy and first principle calculations, which associate the appearance of states in the valence band region to oxygen deficiency and Li or Ni doping and lower n-type character induced by Ni doping. Finally, changes in the thermally induced anatase-to-rutile transition (ART) have been also observed in the doped samples, leading to a dopant-promoted faster ART which occurs at lower temperature boosted due to the dopant effect.
Effect of the synthesis method on the properties of lithium doped graphene oxide composites with tin oxide nanoparticles: Towards white luminescence
(Journal of physics and chemistry of solids, 2019) Prado Hurtado, Félix del; Taeño González, María; Maestre Varea, David; Ramírez Castellanos, Julio; González Calbet, José María; Cremades Rodríguez, Ana Isabel
Graphene oxide (GO) based composites functionalized with nanoparticles are emerging as potential candidates for multifunctional devices. As the synthesis route can strongly influence the way in which the nanoparticles are anchored to GO and the resulting properties, different routes have been employed and evaluated in this work for the preparation of composites formed by GO and undoped or Li doped SnO_2 nanoparticles. Promotion of synergetic effects, as well as changes in the structural and luminescent properties of the composites have been also analyzed. In comparison with GO, composite samples synthesized in this work involve enhanced luminescence while retaining nearly white emission, which could lead to wider applicability of GO based composites in free standing and emitting and sensing devices. Aspects, such as GO reduction, stabilization of different Sn-based oxides, variation in the concentration of oxygen related functional groups, changes in the sp^2 domain sizes, incorporation of lithium and enhancement of the luminescence, have been analyzed in this work by means of xray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) characterizations.
Comparative study of the implementation of tin and titanium oxide nanoparticles as electrodes materials in Li-ion batteries
(Scientific reports, 2020) Prado, Félix del; Andersen, Hanne Flaten; Taeño González, María; Mhlen, Jan Petter; Ramírez Castellanos, Julio; Maestre Varea, David; Karazhanov, Smagul; Cremades Rodríguez, Ana Isabel
Transition metal oxides potentially present higher specific capacities than the current anodes based on carbon, providing an increasing energy density as compared to commercial Li-ion batteries. However, many parameters could influence the performance of the batteries, which depend on the processing of the electrode materials leading to different surface properties, sizes or crystalline phases. In this work a comparative study of tin and titanium oxide nanoparticles synthesized by different methods, undoped or Li doped, used as single components or in mixed ratio, or alternatively forming a composite with graphene oxide have been tested demonstrating an enhancement in capacity with Li doping and better cyclability for mixed phases and composite anodes.
Improved silicon surface passivation by hybrid composites formed by PEDOT:PSS with anatase TiO_2 nanoparticles
(Materials letters, 2020) Vázquez López, Antonio; Yaseen, Anisa; Maestre Varea, David; Ramírez Castellanos, Julio; Karazhanov, Smagul Zh; Marstein, Erik S; Cremades Rodríguez, Ana Isabel
In this work, hybrid composites formed by PEDOT:PSS (poly(3,4-ethylene dioxythiophene):-polystyrene sulfonate) and anatase TiO_2 nanoparticles synthesized by hydrolysis were spin-coated on Si substrates in order to evaluate their potential implementation in Si-hybrid solar cells, based on their passivation behaviour. X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy, quasi-steady state photoconductance (QSS-PC) based on PL imaging, and Hall effect measurements were carried out for the structural, optical and electrical characterization. Enhanced carrier lifetime values of tau similar to 0.5 ms were obtained by adding a controlled amount of TiO_2 nanoparticles (0.5% wt.) to the PEDOT:PSS matrix.
Understanding the effects of Cr doping in rutile TiO₂ by DFT calculations and X-ray spectroscopy
(Scientific reports, 2018) Vásquez, G. C.; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Ramírez Castellanos, Julio; Magnano, Elena; Nappini, Silvia; Karazhanov, Smagul Zh.
The effects of Cr on local environment and electronic structure of rutile TiO₂ are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities Cr_(Ti)(0)* and Cr_(Ti)(-1)* as well as Cr-oxygen vacancy complex 2Cr_(Ti) + V₀ are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO₂ at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t_(2g) states of the Cr ions in order to reach the stable oxidation state of Cr(3+)*. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO₂ cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO₂ are very sensitive to the concentration of Cr.
Towards control of the size, composition and surface area of NiO nanostructures by Sn doping
(Nanomaterials, 2021) Taeño González, María; Maestre Varea, David; Ramírez Castellanos, Julio; Li, Shaohui; Lee, Pooi See; Cremades Rodríguez, Ana Isabel
Achieving nanostructures with high surface area is one of the most challenging tasks as this metric usually plays a key role in technological applications, such as energy storage, gas sensing or photocatalysis, fields in which NiO is gaining increasing attention recently. Furthermore, the advent of modern NiO-based devices can take advantage of a deeper knowledge of the doping process in NiO, and the fabrication of p-n heterojunctions. By controlling experimental conditions such as dopant concentration, reaction time, temperature or pH, NiO morphology and doping mechanisms can be modulated. In this work, undoped and Sn doped nanoparticles and NiO/SnO_2 nanostructures with high surface areas were obtained as a result of Sn incorporation. We demonstrate that Sn incorporation leads to the formation of nanosticks morphology, not previously observed for undoped NiO, promoting p-n heterostructures. Consequently, a surface area value around 340 m^2/g was obtained for NiO nanoparticles with 4.7 at.% of Sn, which is nearly nine times higher than that of undoped NiO. The presence of Sn with different oxidation states and variable Ni^(3+)/Ni^(2+) ratio as a function of the Sn content were also verified by XPS, suggesting a combination of two charge compensation mechanisms (electronic and ionic) for the substitution of Ni^(2+) by Sn^(4+). These results make Sn doped NiO nanostructures a potential candidate for a high number of technological applications, in which implementations can be achieved in the form of NiO-SnO_2 p-n heterostructures.
Influence of doping and controlled sn charge state on the properties and performance of SnO nanoparticles as anodes in li-ion batteries
(Journal of physical chemistry C, 2020) Vázquez López, Antonio; Maestre Varea, David; Ramírez Castellanos, Julio; González Calbet, José María; Pís, Igor; Nappini, Silvia; Yuca, Neslihan; Cremades Rodríguez, Ana Isabel
Li-ion batteries (LiB) play nowadays a major role in several technological fields. In addition to enhanced high capacity and long cyclability, some other issues regarding safety, materials sustainability, and low cost remain unsolved. Tin oxide (SnO_2) presents several of those advantages as an anode material; however, some aspects still require to be investigated such as capacity fading over cycles. Herein, tin oxide nanoparticle-based anodes have been tested, showing high capacities and a significant cyclability over more than 150 cycles. A complementary strategy introducing doping elements such as Li and Ni during the synthesis by hydrolysis has been also evaluated versus the use of undoped materials, in order to assess the dependence on SnO_2 quality and properties of battery performance. Diverse aspects such as the Sn charge state in the synthesized nanoparticles, the variable incorporation of dopants, and the structure of defects have been considered in the understanding of the obtained capacity.