Thermal treatment and environment effect on transient photoconductivity behavior of anatase TiO2 with dominant {0 0 1} facets

Nanosized anatase TiO2 powders with dominant {0 0 1} facets were prepared by solvothermal reaction of titanium isopropoxide in the presence of hydrofluoric acid as a capping agent. Two kinds of samples, as prepared and calcinated at 600 °C were fabricated and their UV-Visible and transient photoconductivity were investigated in vacuum and in air. The photoconductivity reaches high values and is sensitive on the environment. Thermal treatment improves the crystalline quality and enhances the amount of created excess charge carriers

Electrical conductivity studies of anatase TiO2 with dominant highly reactive {0 0 1} facets

Nanostructured powders of titanium dioxide anatase nanoplates with dominant highly reactive {0 0 1} facets were fabricated using a solvothermal method. Two kinds of samples, as prepared and calcinated at 600 °C, were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and electrical conductivity in vacuum and in air. The dependence of the conductivity versus the inverse of temperature in the temperature range 150-440 K indicated the contribution of at least two conduction mechanisms in vacuum. The electron transport was controlled by partially depleted of charge carriers grains and adiabatic small polaron conduction in the high temperature regime and by Mott variable-range hopping (VRH) at lower temperatures. The environment was found from the experimental results to influence significantly the electrical conductivity values and its temperature dependence. A decrease with temperature in air is observed in the ranges 290-370 and 285-330 K for the as prepared and the calcinated sample respectively. Potential barriers caused by partial depletion of carriers at grain boundaries control the electrical conductivity behavior in air at high temperatures and VRH in the lower temperature regime

Novel synthesis of porous aluminium and its application in hydrogen storage

A novel approach for confining LiBH4 within a porous aluminium scaffold was applied in order to enhance its hydrogen storage properties, relative to conventional techniques for confining complex hydrides. The porous aluminium scaffold was fabricated by sintering NaAlH4, which was in the form of a dense pellet, under dynamic vacuum. The final product was a porous aluminium scaffold with the Na and H2 having been removed from the initial pellet. This technique contributed to achieving highly dispersed LiBH4 particles that were also destabilised by the presence of the aluminium scaffold. In this study, the effectiveness of this novel fabrication method of confined/destabilised LiBH4 was extensively investigated, which aimed to simultaneously improve the hydrogen release at lower temperature and the kinetics of the system. These properties were compared with the properties of other confined LiBH4 samples found in the literature. As-synthesised samples were characterised using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Nitrogen Adsorption measurements. The hydrogen storage capacity of all samples was analysed using temperature programmed desorption in order to provide a comprehensive survey of their hydrogen desorption properties. The porous aluminium scaffold has a wide pore size distribution with most of the porosity due to pores larger than 50 nm. Despite this the onset hydrogen desorption temperature (Tdes) of the LiBH4 infiltrated into the porous aluminium scaffold was 200 °C lower than that of bulk LiBH4 and 100 °C lower than that of nanosized LiBH4. Partial cycling could be achieved below the melting point of LiBH4 but the kinetics of hydrogen release decreased with cycle number

Solvothermal synthesis and photocatalytic performance of Mn4+-doped anatase nanoplates with exposed {001} facets

The photocatalytic activity of TiO2 and manganese doped TiO2 nanoplates with various manganese atomic percentages, in the range of 2-7%, was studied. The undoped and doped nanoplates with exposed {001} facets were produced by a solvothermal method. The crystal structure as well as the shape of the TiO2 and Mn4+/TiO2 anatase nanoparticles was determined with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Both techniques revealed that the nanocrystals are in the form of plates. Moreover, the anisotropic peak broadening of the X-ray diffraction patterns was studied using the Rietveld refining method. Chemical analysis of the photocatalyst that was carried out with X-ray photoelectron spectroscopy (XPS) showed the presence of manganese ions in the TiO2 anatase matrix. The Density Functional Theory (DFT) calculations exhibited a decrease in the energy gap and an increase in the density of the electronic stated inside the gap for the doped TiO2. These observations were in agreement with the results of the UV-visible diffuse reflectance spectroscopy (DRS) that demonstrated an adsorption shift towards the visible region for the same samples. The photocatalytic activity of the synthesized catalysts was investigated by the photocatalytic oxidation of the gaseous nitric oxide (NO) and decomposition of the gaseous acetaldehyde (CH3CHO) under visible light irradiation. The optimal concentration of dopant that improves the photocatalytic activity of the nanoplates was determined. © 2014 Elsevier B.V

Materials for hydrogen-based energy storage - past, recent progress and future outlook

Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered; porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage

Study of the photocatalytic activity of anatase TiO2 nano-microstructures with exposed {001} facets

TiO2 anatase nanoplates, hollow microspheres and microcrystals with exposed {001} crystal facets were fabricated via a solvothermal- hydrothermal method at 180 °C for 24 hours using titanium isopropoxide or titanium tetrafluoride as a titanium precursor, ethanol or distilled water as a solvent and lastly hydrofluoric acid as the enhancer for the formation of the TiO2 anatase {001} crystal facets. All samples were calcined at 600 °C in order to remove the fluoride atoms adsorbed on the photocatalysts surface. This paper presents the influence of different TiO2 anatase structures in their photocatalytic activity. The photocatalytic evaluation of all TiO2 anatase structures with exposed {001} crystal facets was obtained by oxidizing the NO gas to NO2 and NO3- and then calculating their photonic efficiency. Commercial TiO 2 photocatalyst P25 by Evonik Degussa was used as a reference. The TiO2 anatase nanoplates were the structure that exhibited the best photocatalytic activity of all TiO2 anatase structures, including the used reference P25 by Evonik Degussa

Electrical conductivity mechanisms in titania hollow microspheres with dominant {0 0 1} facets

Anatase titania hollow microspheres with dominant {0 0 1} facets were synthesized by a solvotherrmal-hydrothermal method. Two kinds of samples, as prepared and calcinated at 600 C, were fabricated. The temperature dependence of their electrical conductivity in vacuum and in air, in the temperature range 113-440 K, was investigated. The conductivity data were fitted using the grain boundary model (GB), the small polaron hopping (SPH) model and the Mott's variable range hopping model in the different temperature regimes, suggesting the contribution of several conduction mechanisms. A significant decrease of conductivity values in air is observed, indicating the influence of environment on titania hollow microspheres. In the temperature region 274-294 K a metal-like behavior in air is shown for the as prepared sample. © 2013 Elsevier B.V. All rights reserved

Thermochemical energy storage performance of zinc destabilized calcium hydride at high-temperatures

© the Owner Societies. CaH2 has 20 times the energy density of molten salts and was patented in 2010 as a potential solar thermal energy storage material. Unfortunately, its high operating temperature (>1000 °C) and corrosivity at that temperature make it challenging to use as a thermal energy storage (TES) material in concentrating solar power (CSP) plants. To overcome these practical limitations, here we propose the thermodynamic destabilization of CaH2 with Zn metal. It is a unique approach that reduces the decomposition temperature of pure CaH2 (1100 °C at 1 bar of H2 pressure) to 597 °C at 1 bar of H2 pressure. Its new decomposition temperature is closer to the required target temperature range for TES materials used in proposed third-generation high-temperature CSP plants. A three-step dehydrogenation reaction between CaH2 and Zn (1 : 3 molar ratio) was identified from mass spectrometry, temperature-programmed desorption and in situ X-ray diffraction studies. Three reaction products, CaZn13, CaZn11 and CaZn5, were confirmed from in situ X-ray diffraction studies at 190 °C, 390 °C and 590 °C, respectively. The experimental enthalpy and entropy of the second hydrogen release reaction were determined by pressure composition isotherm measurements, conducted between 565 and 614 °C, as ΔHdes = 131 ± 4 kJ mol-1 H2 and ΔSdes = 151 ± 4 J K-1 mol-1 H2. Hydrogen cycling studies of CaZn11 at 580 °C showed sufficient cycling capacity with no significant sintering occurring during heating, as confirmed by scanning electron microscopy, demonstrating its great potential as a TES material for CSP applications. Finally, a cost comparison study of known destabilized CaH2 systems was carried out to assess the commercial potential

Exploring the Magnetic and Electrocatalytic Properties of Amorphous MnB Nanoflakes

Two-dimensional (2D) metal borides are a class of ceramic materials with diverse structural and topological properties. These diverse material properties of metal borides are what forms the basis of their interdisciplinarity and their applicability in various research fields. In this study, we highlight which fundamental and practical parameters need to be taken into consideration when designing nanomaterials for specific applications. A simple one-pot chemical reduction method was applied for the synthesis of manganese mono-boride nanoflakes at room temperature. How the specific surface area and boron-content of the as-synthesized manganese mono-boride nanoflakes influence their magnetic and electrocatalytic properties is reported. The sample with the highest specific surface area and boron content demonstrated the best magnetic and electrocatalytic properties in the HER. Whereas the sample with the lowest specific surface area and boron content exhibited the best electric conductivity and electrocatalytic properties in the OER

Electrochemical Synthesis of Highly Ordered Porous Al Scaffolds Melt-Infiltrated with LiBH4 for Hydrogen Storage

Two highly ordered porous Al scaffolds were synthesized by applying a soft template assisted electrodeposition method, using an ionic liquid as the electrolyte. Polystyrene (PS) spheres with an average diameter of 399 ± 2 nm or 89 ± 20 nm were deposited on a polished Cu electrode using a dip-coater. An imidazolium-based ionic liquid mixed with aluminium chloride [EMIm]/AlCl3 (40/60 mol%) was used as the electrolyte for the Al electrodeposition. The PS spheres that were used as a soft template were removed after the Al electrodeposition method by chemically dissolving them in tetrahydrofuran (THF). Lithium borohydride (LiBH4) was then melt-infiltrated into the porous Al scaffold. Morphological observations of the dip-coated Cu electrodes with the PS spheres, the as-synthesized porous Al scaffolds, and the LiBH4 melt-infiltrated samples were carried out using Scanning Electron Microscopy (SEM). The scaffolds exhibited a highly ordered porous Al structure with an open network of pores and an average pore size of 355 ± 25 and 56 ± 20 nm respectively. The porous Al acts as a reactive scaffold which interacts with LiBH4 at elevated temperature. Temperature Programmed Desorption (TPD) experiments revealed that the melt-infiltrated LiBH4 samples exhibited faster H2 desorption kinetics in comparison to the bulk material. In particular, the 56 ± 20 nm Al scaffold showed a H2 desorption onset temperature (Tdes) at 100°C which is 250°C lower than for bulk LiBH4. This temperature drop can be attributed to the size reduction of LiBH4 down to the nanoscale, together with the high contact surface area with the Al scaffold