Francisco Hernandez-Ramirez

Layered perovskites as promising cathodes for intermediate temperature solid oxide fuel cells

The suitability of GdBaCo2O5+δ as a cathode material for intermediate temperature solid oxide fuel cells has been evaluated. The 18O/16O isotope exchange depth profile (IEDP) method has been used to obtain the oxygen surface exchange and oxygen tracer diffusion coefficients yielding optimum values for applicability in fuel cells (k* = 2.8 × 10−7 cm s−1 and D* = 4.8 × 10−10 cm2 s−1 at 575 °C) especially in terms of low activation energies (EAk = 0.81(4) and EAD = 0.60(4) eV). The same material has been characterized electrically as a part of a symmetrical electrochemical system (GdBaCo2O5+δ/Ce0.9Gd0.1O2−x/GdBaCo2O5+δ), by means of impedance spectroscopy measurements, corroborating an excellent performance in the classical intermediate temperature range for solid oxide fuel cells (500–700 °C). An area specific resistance (electrode–electrolyte interface) of 0.25 Ω cm2 at 625 °C was achieved for a cell processing temperature of 975 °C. Finally, layered perovskites are presented as a promising new family of materials for cathode use in solid oxide fuel cells at low temperatures.

Ultralow power consumption gas sensors based on self-heated individual nanowires

Dissipated power in metal oxide nanowires (rNW<45 nm) often causes important self-heating effects and as a result, undesired aging and failure of the devices. Nevertheless, this effect can be used to optimize the sensing conditions for the detection of various gaseous species, avoiding the requirement of external heaters. In this letter, the sensing capabilities of self-heated individual SnO2 nanowires toward NO2 are presented. These proof-of-concept systems exhibited responses nearly identical to those obtained with integrated microheaters, demonstrating the feasibility of taking advantage of self-heating in nanowires to develop ultralow power consumption integrated devices.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Portable microsensors based on individual SnO2 nanowires

Individual SnO(2) nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Enhanced photoelectrochemical activity of an excitonic staircase in CdS@TiO2 and CdS@anatase@rutile TiO2 heterostructures

TiO2 nanorod arrays grown on conductive substrates were converted using chemical strategies into CdS@TiO2 and CdS@anatase@rutile TiO2 heterostructures to fabricate visible-light harvesting assemblies. Compared to pure TiO2 nanorods, CdS@TiO2 heterostructures evidently extended the absorption edge and exhibited enhanced photoelectrochemical (PEC) response in the visible region. Further enhancement of PEC performance was achieved by introducing an intermediate anatase TiO2 layer in the CdS@rutile TiO2 heterostructures. An excitonic cascade of band alignment (CdS, anatase-TiO2 and rutile-TiO2) was constituted by arranging different semiconductors in order to align the edges of their conducting band, which improved charge separation and suppressed the recombination processes by facilitating the transfer of forward electrons and limiting the reverse processes due to spatial separation of the electron and hole in different material regions.

On the photoconduction properties of low resistivity TiO2 nanotubes

TiO(2) nanotubes were synthesized by anodic oxidation of titanium foils using dimethyl sulfoxide and hydrofluoric acid as the electrolyte. The electrical properties of individual nanotube-based devices were evaluated and modeled after exposing some of them to different gas and illumination conditions. Resistivity values fully comparable to those of TiO(2) single crystal anatase (ρ(SA) = 1.09 ± 0.01Ω cm) were found, and their photoconductive characteristics, explained in terms of the Shockley-Read-Hall model for non-radiative recombination in semiconductors, were found to be strongly influenced by the applied experimental conditions such as the surrounding atmosphere. These devices may have potential applications in photocatalytic processes, such as CO(2) reduction or H(2)O splitting, avoiding the interfering effects typical of nanotube arrays.

A Highly Selective and Self‐Powered Gas Sensor Via Organic Surface Functionalization of p‐Si/n‐ZnO Diodes

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

Quantitative analysis of CO-humidity gas mixtures with self-heated nanowires operated in pulsed mode

Self-heating effect in individual metal oxide nanowires can be used to activate their response to gases with power consumptions below tenths of microwatts. The thermal response time of these devices is extremely fast (a few milliseconds) and it makes it possible to observe the kinetics of the interactions between the gas molecules and the metal oxide. In this work we demonstrate that such effects enable an experimental methodology to improve the selectivity of metal oxide-based sensors based on the analysis of their fast response dynamics. Specifically, this work jointly analyzes the magnitude and response time of SnO2 nanowire-based sensors to carbon monoxide (CO) and humidity (H2O) mixtures, proving that a quantitative analysis of CO–H2O gas blends can be achieved by modulating their work temperature through the self-heating effect.

Direct observation of the gas-surface interaction kinetics in nanowires through pulsed self-heating assisted conductometric measurements

Dynamics of gas-surface interactions determine the limits of the fastest response times of sensors based on metal oxides. Here, the kinetics of adsorption and desorption of gaseous molecules onto the surface of metal oxide nanowires was analyzed through pulsed self-heating assisted conductometric measurements. This approach overcomes gas diffusion, which is typical of conventional porous film based devices, and provides thermal response times fast enough to evaluate the fundamental gas-surface reactions kinetics. Experimental response and recovery times of individual SnO2 nanowires toward oxidizing and reducing gases obtained with the here-proposed methodology were related to the reaction barriers predicted by theoretical models and other experimental techniques.

Characterization of individual barium titanate nanorods and their assessment as building blocks of new circuit architectures

In this work, we report on the integration of individual BaTiO(3) nanorods into simple circuit architectures. Polycrystalline BaTiO(3) nanorods were synthesized by electrophoretic deposition (EPD) of barium titanate sol into aluminium oxide (AAO) templates and subsequent annealing. Transmission electron microscopy (TEM) observations revealed the presence of slabs of hexagonal polymorphs intergrown within cubic grains, resulting from the local reducing atmosphere during the thermal treatment. Electrical measurements performed on individual BaTiO(3) nanorods revealed resistivity values between 10 and 100 Ω cm, which is in good agreement with typical values reported in the past for oxygen-deficient barium titanate films. Consequently the presence of oxygen vacancies in their structure was indirectly validated. Some of these nanorods were tested as proof-of-concept humidity sensors. They showed reproducible responses towards different moisture concentrations, demonstrating that individual BaTiO(3) nanorods may be integrated in complex circuit architectures with functional capacities.