Advanced materials are those that have physical or chemical properties that can be controlled intelligently through external stimuli such as mechanical tension, temperature, electric and magnetic fields, among others. Several PPG-Nano research groups have been synthesizing and studying these materials focusing on the energetic, electrochemical, and optical properties of systems with photocatalytic activity, thermal stability, and biocompatibility.
Within this context, we present some of our most recent works.
Halide perovskite CsPbBr3 quantum dots (QDs) were synthesized via a supersaturated recrystallization process and deposited on the surface of TiO2 microtubes forming local nano-heterostructures.
Structural, morphological, and optical characterizations confirm the formation of heterostructures comprised of TiO2 microtube decorated with green-emitting CsPbBr3 nanocrystals. Optical characterizations reveal the presence of two band gap energies corresponding to CsPbBr3 (2.34 eV) and rutile-TiO2 (2.97 eV). Time-resolved photoluminescence decays indicate different charge dynamics when comparing both samples, revealing the interaction of CsPbBr3 QDs with the microtube surface and thus confirming the formation of local nano-heterostructures.
The figure on the right, (a) Representative SEM images of a TiO2 microtube surface with CsPbBr3 nanocrystals. (a) Low-magnification backscattered electron (BSE) image. (b–f) The sequence of SEM images showing particles with high size dispersity and different morphologies nearly spherical, cubic, rod-shaped, and some agglomerates.
In the figure on the left, SEM image of TiO2 microtubes synthesized by combining a thermal oxidation process at T = 1050 °C for 90 min with an applied electrical current I = 10 mA.
The voltage-current measurements in the dark show an abrupt decrease in the electrical resistivity of the CsPbBr3/TiO2 heterostructure reaching almost 95% when compared with the pristine TiO2 microtube. This significant increase in the electrical conductivity is associated with charge transfer from perovskite nanocrystals into the semiconductor microtube, which can be used to fine-tune its electronic properties. Besides controlling the electrical conductivity, decoration with semiconducting nanocrystals makes the hollow heterostructure photoluminescent, which can be classified as a multifunctionalization in a single device.
Spintronic devices that dissipate far less heat than electronic devices require spin current generation and control, both provided by the Rashba effect. But the discovery of compounds with large Rashba coefficient is rare and greeted as a pleasant surprise.
We establish the existence of anti-crossing between spin energy bands as a viable design principle for identifying compounds with large . We use this principle in quantum mechanical calculations as a filter, delineating 165 weak Rashba compounds from 34 strong Rashba compounds. Experimental testing of spin splitting in these compounds is called for and might significantly broaden the playing field of spintronics.
Surprisingly, this research also uncovered a new type of cross-functionality combining two hitherto separate functionalities: topological insulators (having surface states resilient to passivation) with Rashba spin splitting. Such topological Rashba insulators might offer a platform for robust surface spin currents.
The spin-orbit-induced spin splitting of energy bands in low-symmetry compounds (the Rashba effect) has a long-standing relevance to spintronic applications and the fundamental understanding of symmetry breaking in solids, yet the knowledge of what controls its magnitude in different materials is difficult to anticipate.
We advance the understanding of the “Rashba Scale” using the “inverse design” approach by formulating theoretically the relevant design principle and then identifying compounds satisfying it.
Since topological insulators must have band anti-crossing, this establishes an interesting cross-functionality of “topological Rashba insulators” that may provide a platform for the simultaneous control of spin splitting and spin polarization.
Shear thickening fluids (STFs) are smart materials that change from liquid to solid reversibly when undergoing critical stresses. These materials are good alternatives to improve applications where energy dissipation is important, for example, in the fabrication of liquid body armor and shock absorbing protective gear. However, as much as it is known about the effect of several variables on their properties, such as particle concentration and medium viscosity, the stability of these colloidal dispersions over time and over shearing is not yet well understood.
The development and design of new applications depend on predicting for how long the material will keep its properties. In this project, we studied the influence of fumed silica content, ultrasonication energy used during dispersion of the silica particles, and humidity during storage to analyze the changes in properties of STFs. The influence of shearing magnitude on their properties was also studied.
STFs with higher amounts of silica and produced using less dispersion energy showed the highest viscosity peak on initial tests, but they were also the least stable over time, due to flocculation of the particles. In stable samples, water absorption led to a large loss of maximum viscosity.
The figure on the right shows the visual aspect of 10% silica-STF, produced using 40% of amplitude, after 16 weeks stored in (a) regular humidity; (b) low humidity.
The presence of humidity on samples diminished the overall viscosity, but did not prevent the sample from becoming a gel if the parameters used resulted in an unstable STF. Shearing the STF reduced its maximum viscosity, being more evident in samples with higher viscosity.
Passing current at given threshold voltages through a metal/insulator/metal sandwich structure device may change its resistive state. Such switching has been rationalized by ion drift models, or changes in electronic states, but the underlying physical mechanism is poorly understood. We propose a new model based on electrostatics to explain multiple resistive states in memristors that contain large defect densities. The different resistive states are due to spontaneously charged states of the insulator ‘storage medium’, characterized by different ‘band bending’ solutions of Poisson’s equation. For an insulator with mainly donor type defects, the low-resistivity state is characterized by a negatively charged insulator due to convex band bending, and the high-resistivity state by a positively charged insulator due to concave band bending; vice versa for insulators with mainly acceptor type defects. We show that these multiple solutions coexist only for nanoscale devices and for bias voltages limited by the switching threshold values, where the system charge spontaneously changes and the system switches to another resistive state. We outline the general principles how this functionality depends on material properties and defect abundance of the insulator ‘storage medium’.
A comprehensive study on the correlations of structural, magnetic, and electronic properties of a new disordered Nd2CoFeO6 double perovskite has been conducted.
The lack of strong divergence of the magnetic susceptibility suggests competition between magnetic interactions at the magnetic phase transition TN = 246 K, which is confirmed by the absence of a heat capacity peak. The magnetic susceptibility results indicate that the Fe/Co spins form a classical noninteracting paramagnetic state above T ≈ 2.2TN, while deviations are found at intermediate temperatures indicating the presence of strong short-range magnetic interactions.
AC and DC electrical resistivity results reveal a melting of insulating polaronic behavior to a metallic-like conductivity, establishing an electronic crossover closely related to both local magnetic moment and lattice-parameter evolution. We show from density functional calculations that the magnetic configurations have a strict relation to this crossover, being associated to a transition from low to high spin states of Co3+ ions. This insulator–metal transition has its origin driven by a local increase in the magnetic moment of Co3+ ions. Our results point to a scenario in which a continuous spin-state transition triggers a crossover between distinct electronic states from the insulating polaronic behavior to permanent metallic states.
In recent years, there has been growing interest in developing technologies for direct ethanol fuel cells (DEFCs) in an alkaline medium [, , , ]. The use of ethanol has received great attention because it can easily be produced in large quantities from biological processing of agricultural products and is considered a renewable energy source due to low pollutant emission characteristics . Moreover, ethanol is a strategic fuel because of its low toxicity, since its complete oxidation to CO2 involves 12 electrons per molecule oxidized, resulting in high energy density (8.01 kW h Kg−1 or 6.34 kW h L−1, b.p. 78.4 °C) when compared to hydrogen [, , ,], as well as other alcohols, such as methanol (6.09 kW h Kg−1, b.p. 64.7 °C) [9,10], glycerol (5.0 kW h Kg−1, b.p. 290.0 °C)  and ethylene glycol (5.2 kW h Kg−1, b.p. 197.3 °C) . However, the greatest obstacle to the commercialization of DEFCs is the lack of catalysts that can trigger the organic oxidation at a favorable rate . The search for an active catalyst that yields high current densities during the ethanol oxidation reaction (EOR) is a key goal in research into DEFCs.
In this work, Carbon-supported Pd, Au@Pd core–shell and Au1–xPdx-alloyed nanoparticles were prepared by a chemical reduction method and characterized by different experimental techniques, including X-ray powder diffraction, transmission electron microscopy, scanning-transmission electron microscopy using bright-field and high-angle annular dark field detectors and X-ray energy dispersive spectroscopy. The catalytic mass activity toward ethanol oxidation was assessed by cyclic voltammetry and chronoamperometry at room temperature. The measurements showed that the addition of Au enhances remarkably the electrocatalytic activity of the material, due to the bifunctional effect of Au1–xPdx/C alloys, and the synergetic effect on Au@Pd/C, resulting in a dissolution resistance of core–shell catalysts at potentials of 1.5 V versus reversible hydrogen electrode. In situ Fourier transform infrared spectroscopy measurements showed that the mechanism for ethanol oxidation depends on the electrocatalyst structure and morphology. Acetate was identified as the main product of ethanol electro-oxidation on the studied electrocatalysts. However, the presence of a core–shell structure on Au@Pd/C resulted in enhanced ethanol oxidation selectivity toward CO2. The improvement of activity is attributed to the interaction between Pd shell and Au core.
Read the full article at https://www.sciencedirect.com/science/article/abs/pii/S0926337319302899#fig0045 and access the numerical references.
In this paper is explored the diffusion of lithium ions decoupled from a solid polymer electrolyte matrix is the key for high-energy electrochemical devices with the safety needed for commercial use. This Letter reports how the ion mobility in a single-phase hybrid polyelectrolyte (SPHP) matrix can be tuned by changing an inorganic coordinating atom from silicon (Si) to germanium (Ge).
Nuclear Magnetic Resonance (NMR) results show that the lithium ion activation barrier in the polyelectrolyte with Si can be modulated from 0.26 eV to the unprecedented value of 0.12 eV in the polyelectrolyte with Ge. Density functional theory is used to show that the electronic structures of both polymers are very different, although their chemical structures are very similar, except for the coordinating atom.
As a starting point, a model was built for the synthesized SPHP matrix, containing the Si/Ge tetrahedral core and the subsequent C chains with different oxygen groups. The optimized geometry and symmetry of both polymers are very similar, but their electronic structures are rather different (Figure a–d).
While the LUMO is mostly localized at specific C and O sites in SPHP/Si (a), for SPHP/Ge it is mostly localized around the Ge atom and the neighboring oxygen sites (b).
The origin of this difference can be understood by analysis of their projected density of states. From this, schematic representation of the molecular orbital diagrams of sp hybridization for SPHP/Si and SPHP/Ge are shown in panels (e and f), respectively. SPHP/Si and SPHP/Ge have the same s2p2 electronic configuration in valence, with the Si 3p orbitals having very similar energy as the Ge 4p orbitals.
However, the source of the LUMO difference is mainly related to the energy of the Si and Ge s orbitals; the Ge 4s atomic orbitals are ∼0.8 eV deeper than the Si 3s orbitals. As a consequence, the energy difference between the O p and Ge s orbitals is larger compared to O p and Si s. Taking also into account that the lengths of the covalent radius are distinct, with 1.11 and 1.20 Å for Si and Ge, respectively, the sp hybridization–repulsion in the Ge polymer is decreased.
FT-Raman spectroscopy was performed to observe the “fingerprints” of SPHP/Ge and SPHP/Si, to properly index their molecular vibrations, and to present the main differences between these two systems.
FT-Raman spectra of SPHP/Si and SPHP/Ge and the structural formula of SPHP/Ge (figure). The blue and green shaded regions indicate the differences found ascribed by unequal electronic structure. The red shaded region represents the vibration of the ester group, which is the preferred ion site.
The possibility of large scale utilization of hydrogen as energy carrier depends on the convenient solution of several technological problems. One of the most challenging is to find a safe and feasible way of storing this gas for subsequent utilization in fuel cells [1,2]. Metal hydrides have been considered as interesting candidates for hydrogen storage, but the search for hydrides which fulfil all the technical requirements for hydrogen storage applications is far to be a simple task, remaining an open question so far. Among others, magnesium hydride (MgH2) has attracted great attention from the scientific community as potential hydrogen storage material mainly because of its high gravimetric hydrogen storage capacity (7.6 wt%) and relative low cost of magnesium. In opposition, the slow reaction kinetics and high temperatures needed for hydrogen absorption and release have been hindering its practical use [1e3]. This slow kinetics is especially important during the first absorption (activation) of hydrogen by magnesium. The activation of magnesium normally takes several hours, even days and requires high hydrogen pressures and temperatures above 400 C.
In this paper we report an investigation on the processing of magnesium samples by filing and the hydrogen storage properties of the obtained chips. Filing of Mg was performed manually and mechanized. The effect of the filing tool design on the morphology and hydrogen storage properties of Mg chips was evaluated. The hydrogen absorption and desorption kinetics of manual filed Mg is strongly influenced by the geometry of the filing tool. Chips obtained by using a fine tool have larger surface areas and faster hydrogenation and dehydrogenation kinetics. Moreover, these chips show high resistance to air contamination and can absorb and release hydrogen even after long periods of exposition to air. Chips obtained by mechanized filing also showed suitable hydrogen storage properties and a slight increase in incubation period for hydrogen absorption. After this initial delay, the reaction kinetics of hydrogen absorption was similar for both mechanized and manually filed chips. The hydrogen storage properties of the chips are not affected by the machining speed adopted during mechanized filing. These results clearly indicate the feasibility of adopting filing processing to produce Mg for hydrogen storage and open possibilities to scale-up the production of such material using a simple and economical approach.
Kinetic curves for the (a) first hydrogen absorption (activation) and (b) desorption, and second absorption of manual filed magnesium samples.
SEM images of as-prepared coarse (a and b), medium (c and d), and fine (e and f) magnesium chips.
Kinetic curves of hydrogen absorption (a) and desorption (b) for: freshly processed (signed as “air”) sample; processed and stored in air for 30 days; and after one cycle of hydrogen absorption/desorption followed by air exposure (reactivation).
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Niobium oxide films have attracted much attention in materials science in the past few years. NbO, NbO2 and Nb2O5 are the most stable phases, being niobium pentoxide the one with the lowest Gibbs free energy of formation between them and, hence, the most thermodynamically favorable to form. The growing interest in Nb2O5 arises from its applicability in several advanced devices such as sensors, solar cells, capacitors and smart windows and. In these devices electronic and optical properties of Nb2O5 films such as photoelectric and photocatalytic activity, high permittivity, high refractive index and transparency in the UV–vis–NIR region have been advantageously exploited to produce a variety of optical devices. Furthermore, high wear resistance, good thermal stability and biocompatibility have been reported as attributes that widen the engineering applications of Nb2O5 layers to additional areas such as biomedical devices and barrier coatings.
SEM micrographs of the top surfaces of the uncoated and Nb2O5-coated 316 stainless steel samples: (A) uncoated substrate, before immersion; (B) 15′-coated sample, before immersion; (C) 30′-coated sample, before immersion; (D) uncoated substrate, after polarization; (E) 15′-coated sample, after polarization; (F) 30′-coated sample, after polarization.
CSLM images of corrosion pit formed on the surface of uncoated 316 stainless steel sample: (A) 2D-image; (B) 3D-image; (C) transverse profile.
CSLM images of corrosion pit formed on the surface of 15 min-coated sample: (A) 2D-image; (B) 3D-image; (C) transverse profile
CSLM images of corrosion pit formed on the surface of 30 min-coated sample: (A) 2D-image; (B) 3D-image; (C) transverse profile.
Ceria based compounds are used in a broad range of technol- ogies such as catalysis, environmental, electromechanical, electrochemical, and various emerging energy technologies. Such an extensive use of these materials is due to the unique combination of different properties such as high ionic conductivity, mixed electronic and ionic conductivity, and large oxygen storage and exchange capabilities, which are all linked to the ability of the materials to undergo rapid Ce3+/Ce4+ redox cycles and form oxygen vacancies. Moreover, ceria solid solutions usually show exceptionally high chemical stability, even in extremely harsh and corrosive environments, in the presence of sulphur or at very high operating temperatures.
The valence and size of cations influence mass diffusion and oxygen defects in ceria. Here we show that reduction of Ce4+ to Ce3+, at high temperatures and low oxygen activity, activates fast diffusion mechanisms which depend on the aliovalent cation concentration. As a result, polycrystalline solid solutions with enhanced electrochemical properties are formed.
The mass diffusion in aliovalent (i.e. Gd in 0, 0.1, 10 and 30 mol%) doped ceria is investigated in this work. Since mass diffusion in ceria is drastically inuenced by the solute drag effect, the combined effects of chemical reduction from Ce4+ to Ce3+ and Gd3+ dopant concentration result in signicant vari- ations of mass diffusion mechanisms. In particular, consistent with the solute drag theory, in highly defective ceria obtained at high temperatures (1400 C) and low oxygen activity (10 31 < pO2 < 10 12 atm), these lead to opposite effects of inhibition of densication and grain growth in pure ceria and low dopant concentration (0.1 mol% Gd2O3) and fast densication and grain growth at a high dopant concentration (10 and 30 mol% of Gd2O3). The nal result of the thermal and chemical treatments is a set of solid solutions whose electrochemical properties do not depend on the microstructure or on the nominal dopant concentration.
SEM images of CeO2, CGO0.1, CGO10 and CGO30 compacts after sintering in air and 9% H2–N2 at 1400 C for 4 h. Arrhenius plots for the total conductivity in air calculated by EIS data of CeO2, CGO0.1, CGO10 and CGO30 compacts sintered in air and 9% H2–N2 at 1400 C for 4 h.