Pablo Guardia - Academia.edu (original) (raw)

Papers by Pablo Guardia

SSRN Electronic Journal, 2022

Nanoscale, 2021

Mixed transition metals ferrites nanocubes are here prepared. Their magnetic properties are evalu... more Mixed transition metals ferrites nanocubes are here prepared. Their magnetic properties are evaluated to assess their applicability as theranostic tools for magnetic hyperthermia treatment, magnetic resonance imaging and magnetic particles imaging.

Journal of Materials Chemistry A, 2022

Amorphous ultrathin MoCoxOy nanosheets with excellent OER catalytic performance were prepared, an... more Amorphous ultrathin MoCoxOy nanosheets with excellent OER catalytic performance were prepared, and the incorporation of molybdenum to enable a lattice oxygen oxidation mechanism (LOM).

Chemical Engineering Journal, 2021

Abstract The high processing cost, poor mechanical properties and moderate performance of Bi2Te3–... more Abstract The high processing cost, poor mechanical properties and moderate performance of Bi2Te3–based alloys used in thermoelectric devices limit the cost-effectiveness of this energy conversion technology. Towards solving these current challenges, in the present work, we detail a low temperature solution-based approach to produce Bi2Te3-Cu2-xTe nanocomposites with improved thermoelectric performance. Our approach consists in combining proper ratios of colloidal nanoparticles and to consolidate the resulting mixture into nanocomposites using a hot press. The transport properties of the nanocomposites are characterized and compared with those of pure Bi2Te3 nanomaterials obtained following the same procedure. In contrast with most previous works, the presence of Cu2-xTe nanodomains does not result in a significant reduction of the lattice thermal conductivity of the reference Bi2Te3 nanomaterial, which is already very low. However, the introduction of Cu2-xTe yields a nearly threefold increase of the power factor associated to a simultaneous increase of the Seebeck coefficient and electrical conductivity at temperatures above 400 K. Taking into account the band alignment of the two materials, we rationalize this increase by considering that Cu2-xTe nanostructures, with a relatively low electron affinity, are able to inject electrons into Bi2Te3, enhancing in this way its electrical conductivity. The simultaneous increase of the Seebeck coefficient is related to the energy filtering of charge carriers at energy barriers within Bi2Te3 domains associated with the accumulation of electrons in regions nearby a Cu2-xTe/Bi2Te3 heterojunction. Overall, with the incorporation of a proper amount of Cu2-xTe nanoparticles, we demonstrate a 250% improvement of the thermoelectric figure of merit of Bi2Te3.

Nanomaterials, 2021

The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to p... more The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to produce hydrogen and acetaldehyde from renewable resources. The optimization of this process requires the use of highly active, stable and selective photocatalytic materials based on abundant elements and the proper adjustment of the reaction conditions, including temperature. In this work, Cu2O-TiO2 type-II heterojunctions with different Cu2O amounts are obtained by a one-pot hydrothermal method. The structural and chemical properties of the produced materials and their activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. The Cu2O-TiO2 photocatalysts exhibit a high selectivity toward acetaldehyde production and up to tenfold higher hydrogen evolution rates compared to bare TiO2. We further discern here the influence of temperature and visible light absorption on the photocatalytic performance. Our results point toward the combination of ener...

ACS Applied Materials & Interfaces, 2018

We present a novel method to produce crystalline oxide aerogels which is based on the cross linki... more We present a novel method to produce crystalline oxide aerogels which is based on the cross linking of preformed colloidal nanocrystals (NCs) triggered by propylene oxide (PO). Ceria and titania were used to illustrate this new approach. Ceria and titania colloidal NCs with tuned geometry and crystal facets were produced in solution from the decomposition of a suitable

Langmuir, 2018

The assembly of colloidal nanocrystals (NCs) is a unique strategy to produce porous materials wit... more The assembly of colloidal nanocrystals (NCs) is a unique strategy to produce porous materials with high crystallinity and unmatched control over structural and chemical parameters. This strategy has been demonstrated mostly for single component nanomaterials. In the present work, we report the gelation of colloidal NC solutions driven by the electrostatic interaction of oppositely charged NCs. A key step for leading this strategy to success is to produce a stable colloidal solution of the positively charged component. We achieved this goal by means of functionalizing NCs with inexpensive and non-toxic amino acids such as glutamine. We demonstrate the combination of positively and negatively charged NCs in the proper concentrations to result in gels with a homogeneous distribution of the two compounds. In this way, porous nanocomposites with virtually any combination can be produced. We illustrate this approach by combining positively charged ceria NCs with negatively charged gold NCs to form Au-CeO2 gels. These gels were dried from supercritical CO2 to produce highly porous Au-CeO2 aerogels with specific surface areas of 120 m 2 g-1. The formation of a proper interface is confirmed through the evaluation of the nanocomposite catalytic activity toward the CO oxidation. We further demonstrate the versatility of this strategy to produce porous metal chalcogenide-metal oxide and metal-metal chalcogenide nanocomposites by the examples of PbS-CeO2 and Au-PbS.

Angewandte Chemie International Edition, 2018

In the present work, we detail a fast and simple solutionbased method to synthesize hexagonal nan... more In the present work, we detail a fast and simple solutionbased method to synthesize hexagonal nanoplates (NPLs) and their use to produce crystallographically textured SnSe2 nanomaterials. We also demonstrate that the same strategy can be used to produce orthorhombic SnSe nanostructures and nanomaterials. NPLs are grown though a screw dislocation-driven mechanism with additional detachment of the growing layers, resulting in flower-like structures. SnSe2 bulk nanomaterials with a significant crystallographic texture and obtained from the hot pressing of the SnSe2 NPLs display highly anisotropic charge and heat transport properties. The overall thermoelectric (TE) figures of merit of thus obtained SnSe2 nanomaterials is limited by their relatively low electrical conductivity. To improve this parameter, SnSe2 NPLs are blended with metal nanoparticles. The electrical conductivities of the blends are significantly improved with respect to bare SnSe2 NPLs and a threefold increase in the TE figure of merit is obtained, reaching unprecedented values up to ZT = 0.65 for this material.

Nanomaterials (Basel, Switzerland), Jan 5, 2018

The manufacturing of semiconducting films using solution-based approaches is considered a low cos... more The manufacturing of semiconducting films using solution-based approaches is considered a low cost alternative to vacuum-based thin film deposition strategies. An additional advantage of solution processing methods is the possibility to control the layer nano/microstructure. Here, we detail the production of mesoporous CuGaS₂ (CGS) and ZnS layers from spin-coating and subsequent cross-linking through chalcogen-chalcogen bonds of properly functionalized nanocrystals (NCs). We further produce NC-based porous CGS/ZnS bilayers and NC-based CGS-ZnS composite layers using the same strategy. Photoelectrochemical measurements are used to demonstrate the efficacy of porous layers, and particularly the CGS/ZnS bilayers, for improved current densities and photoresponses relative to denser films deposited from as-produced NCs.

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Langmuir : the ACS journal of surfaces and colloids, Jan 5, 2018

Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysi... more Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysis, offering easier recyclability and a number of potentially advantageous functionalities, such as tunable band gaps, plasmonic properties, or a magnetic moment. Using high-throughput printing technologies, colloidal NCs can also be supported onto substrates to produce cost-effective electronic, optoelectronic, electrocatalytic, and sensing devices. For both catalytic and technological application, NC surface chemistry and supracrystal organization are key parameters determining final performance. Here, we study the influence of the surface ligands and the NC organization on the catalytic properties of InS, both as a colloid and as a supported layer. As a colloid, NCs stabilized by inorganic ligands show the highest photocatalytic activities, which we associate with their large and more accessible surfaces. On the other hand, when NCs are supported on a substrate, their organization beco...

Chemistry of Materials, 2016

We report a facile synthesis of cube-shaped Co x Fe 3-x O 4 nanocrystals (NCs), which could be fi... more We report a facile synthesis of cube-shaped Co x Fe 3-x O 4 nanocrystals (NCs), which could be finely tuned in terms of NC size (from 15 to 27 nm) and cobalt stoichiometry (from 0.1 to 0.7). These particles exhibited high specific absorption rate (SAR) values, relevant for magnetic hyperthermia, and high relaxivity values, significant for magnetic resonance imaging (MRI) applications. The peculiarity of these NCs is that, already at low frequencies (such as 105 kHz, a working frequency used on human patients), they display SAR values that are three times as large as those of iron oxides nanocubes of comparable sizes (and which were already considered outstanding). The highest SAR value recorded on the NCs reported here (915± 10 Wg −1 (Co+Fe) at 105 kHz and 32 kAm −1) refers to particles with cube shape, 20±2 nm edge size and Co stoichiometry between 0.6 to 0.7. The highest r 2 value (958 mM −1 s −1) was instead recorded on nanocubes with Co stoichiometry around 0.5/0.6 and size of 20± 2 nm. Remarkably, only at this specific size and Co stoichiometry the NCs were not perfect cubes but had a slight concave shape, which together with their core-shell structure and magnetic parameters might account for the higher r 2 values recorded. NCs reported here, with optimized SAR and r 2 values, are promising tools for theranostic applications.

ACS Catalysis, 2015

In this work we applied colloidal preparation methods to synthesize AuCu nanocrystals (NCs) in th... more In this work we applied colloidal preparation methods to synthesize AuCu nanocrystals (NCs) in the ordered tetragonal phase with atomic composition close to 50:50. We deposited the NCs on a support (Al 2 O 3), studied their transformations upon different redox treatments and evaluated their catalytic activity in the CO oxidation reaction. The combined analyses by Energy Dispersive X-ray Spectroscopy (EDX)-Scanning Transmission Electron Microscopy (STEM), Selected Area Electron Diffraction (SAED) and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) highlighted a phase segregation between gold and copper upon the high temperature (350 °C) oxidizing treatment. While gold remained localized in the NCs, copper was finely dispersed on the support, likely in the form of oxide clusters. AuCu alloyed NCs, this time in the form of solid solution,

Nanoscale, 2015

Iron oxide nanocube-modified microcapsules as a platform for magnetically triggered molecular rel... more Iron oxide nanocube-modified microcapsules as a platform for magnetically triggered molecular release.

The Journal of Physical Chemistry C, 2011

ABSTRACT The influence of a variety of parameters on the synthesis of iron oxide nanoparticles (m... more ABSTRACT The influence of a variety of parameters on the synthesis of iron oxide nanoparticles (magnetite/maghemite Fe3O4/γ-Fe2O3) by thermal decomposition of a metal−organic iron precursor in an organic medium is reported. We study the role of both the surfactant and the reducing agent on the shape, the size distribution, and the magnetic properties. We aim at synthesizing magnetic nanoparticles with high crystal quality and good magnetic response. A narrow size distribution of pseudospherical and faceted particles (4−20 nm) with a high saturation magnetization (Ms ≈ 80−85 emu/g at 5 K) is obtained when using oleic acid as a surfactant. In contrast, decanoic acid yields much larger pseudocubic particles (45 nm) with a wider size distribution and a larger saturation magnetization (Ms = 92 emu/g at 5 K), close to the expected value for bulk magnetite. Besides, the use of a variety of reducing agents monitors the magnetic behavior. In the case of 1,2-hexadecanediol, magnetic characterization suggests that the nanoparticles have uniform oxidation. However, those particles prepared without the use of any reducing agent also show uniform oxidation just with a slightly smaller value of the saturation magnetization (Ms = 76 emu/g at 5 K). In contrast, hydrazine seems to promote a nonuniform oxidation that results in the appearance of the exchange bias phenomenon and in a smaller saturation magnetization (Ms = 67 emu/g at 5 K). New ways to tune the shape, the size, and the magnetic properties are discussed.

Nanotechnology, 2008

The effect of surface anisotropy on the distribution of energy barriers in magnetic fine particle... more The effect of surface anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the T ln(t/τ 0) scaling approach. The comparison between the distributions of the anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core anisotropy, and of the total anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable T ln(t/τ 0), enables the determination of the surface anisotropy as a function of the particle size. We show that the contribution of the particle surface to the total anisotropy energy can be well described by a size-independent value of the surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe 3−x O 4 particles of 4.9 nm in average size and x about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective surface anisotropy constant K s = 2.9 × 10 −2 erg cm −2 gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.

Nano Letters, 2013

Materials and Methods Azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate (VA057) was purcha... more Materials and Methods Azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate (VA057) was purchased from Wako Chemicals and all other chemicals were purchased from Sigma-Aldrich and used as received. 1 H-NMR spectra were recorded on a Bruker DRX 400 spectrometer. For the determination of iron concentration, Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, ThermoFisher, CAP 6000) was utilized. Samples for ICP-AES were prepared by incubating 25 µL of sample overnight in 2 mL of aqua regia, and finally milliQ water was added to get a final volume of 25 mL. TEM images were obtained by a JEOL 1011 microscope operated at an accelerating voltage of 100 kV. TEM samples were prepared by drop-casting the solution on a carbon-coated copper grid and letting the solvent evaporate. For purification of the IONPs from free reagents after the synthesis, Amicon centrifuge filters (15 mL, 100 kDa molecular weight cutoff (MWCO), Millipore) were used in a temperature controlled centrifuge (Sigma, 3-16PK). For ultrapure samples, free FA or DOX was removed by two times gel filtration on PD10 desalting columns (GE Healthcare) with ice cold milliQ water as eluent. Incubation experiments were carried out in a temperature-controlled water bath (Memmert). Application of AMF to the samples was carried out in a magneTherm® AC device (Nanotherics Corp.). Separation of release FA from the particles was achieved by centrifugal filtration in Amicon tubes (0.5 mL, 100 kDa MWCO, Millipore) in a fixed angle, temperature-controlled centrifuge (Hettich, Mikro 200R). Fluorescence spectra were recorded on a Cary Eclipse Photospectrometer (Varian) in quartz 3-window micro-cuvettes (Hellma). Synthesis of IONPs Monodisperse IONPs with 15 nm core diameter were synthesized following a published procedure [1]. Briefly, 180 mg (2 mmol) of Fe 2 O 3 hydrate (catalyst grade, 30-50 mesh), 2.82 g (10 mmol) of oleic acid (technical grade) and 5 g of octadecene were mixed in a 50 mL 3-neck flask equipped with a chiller and a N 2 /vacuum inlet. The mixture was degassed under high vacuum and magnetic stirring for 1h and afterwards heated under N 2 atmosphere to 320°C for 1h. After cooling to room temperature, the

Langmuir, 2010

We report on the effect of using decanoic acid as capping ligand on the synthesis of iron oxide n... more We report on the effect of using decanoic acid as capping ligand on the synthesis of iron oxide nanoparticles by thermal decomposition of an organic iron precursor in organic medium. This procedure allowed us to control the particle size within 5 nm and about 30 nm by modifying the precursor-to-capping ligand ratio in a systematic fashion and to further expand the particle size range up to about 50 nm by adjusting the final synthesis temperature. The nanoparticles also showed high saturation magnetization of about 80-83 emu/g at low temperature, almost sizeindependent and close to the value for the bulk counterpart. Decanoic acid-coated nanoparticles were transferred to water by using tetramethylammonium hydroxide, which allowed further coating with silica in a tetraethyl orthosilicate solution. Consequently, these iron oxide nanoparticles are tunable in size and highly magnetic, and they could become suitable candidates for various biomedical applications such as contrast agents for magnetic resonance imaging and magnetic carriers for drug delivery.

Journal of the American Chemical Society, 2009

Figure S1. Scheme of the reaction of A particles with an element B in solution to form an AB shel... more Figure S1. Scheme of the reaction of A particles with an element B in solution to form an AB shell/particle.

Journal of the American Chemical Society, 2013