The development of improved hydrogen storage concepts is crucial for the advancement of hydrogen and fuel cell technologies in applications including stationary power, portable power, and transportation. Since hydrogen has the highest energy per mass of any fuel, but has a low density under ambient conditions, advance storage methods are needed to obtain a sufficient energy density. To date, most research into hydrogen storage is focussed on storing hydrogen as a lightweight, compact energy carrier for mobile applications.
Pd is an archetypical hydrogen storage metal and an effective catalyst for hydrogen-related reactions in a variety of industrial processes and stores hydrogen under ambient conditions. The interaction between hydrogen and Pd is relevant not only from the perspective of basic research, but also for applications in hydrogen storage technology. In the bulk material, the hydrogen occupies interstitial lattice sites of the face-centred cubic Pd forming two different PdHx phases. It has been found that the hydrogen loading saturates at about x = 0.7, At this concentration, the Pd lattice is already expanded by a value of about 6%. For Pd nanoparticles, even larger lattice constants were reported and discussed in terms of an enhanced hydrogenation of nanoparticles with respect to thin films or bulk materials due to additional surface and subsurface adsorption sites.Further reading:
Magnetic nanoparticles can be used for many medical applications, e.g. cell labelling, targeted drug delivery, as contrast agents in magnetic resonance imaging (MRI) or in hyperthermia cancer treatment. For all these applications, the nanoparticles have to biocompatible like e.g. the Fe oxide nanoparticles we investigate.
MRI contrast agents. Magnetic resonance imaging (MRI) is based on nuclear magnetic resonance (NMR) which describes the resonant absorption of an alternating magnetic field applied perpendicularly to a static magnetic field. Resonant absorption occurs if the frequency of the alternating magnetic field equals the Larmor precession frequency of the nuclear magnetic moments. For MRI, usually NMR of hydrogen nuclei, i.e. protons, is used because hydrogen is present in all biological tissues.
The macrospin of the nanoparticles is connected to a large magnetic stray field that alters the relaxation time of the protons in the nearest environment. In general, there three different relaxation times exist: T1, T2 and T2*. The longitudinal relaxation time T1, is the decay constant for the recovery of the nuclear spin magnetisation component along the direction of the external magnetic field towards its thermal equilibrium value. It is commonly called spin-lattice relaxation since it involves the exchange of energy with its surroundings. The transverse relaxation time T2 is the decay constant for the magnetisation component perpendicular to the external field and corresponds to a phase decoherence of the transverse nuclear spin magnetisation. Since T2 is affected only by the dynamics of the nuclear spins, it is called spin-spin relaxation time. Magnetic field inhomogeneities yield a distribution of resonance frequencies resulting in a dephasing of nuclear spins as well. The latter decay constant is denoted T2*. The use of magnetic nanoparticles as contrast agents indirectly influences the T2 and T2* relaxation. This gives rise to a higher contrast in the spin echo MRI which is used as a method to visualise the transverse relaxation behaviour. Besides biocompatibility, a high net magnetic moment of the nanoparticles is an important requisite for a high MRI contrast. Therefore, magnetite (Fe3O4) nanoparticles are favoured over maghaemite (γ-Fe2O3) as contrast agents.
Hyperthermia. The cancer treatment using hyperthermia was already successfully tested for functionalised Fe oxide nanoparticles and is approved in the EU for the treatment of brain tumours. The iron oxide nanoparticles - γ-Fe2O3 (maghaemite) or Fe3O4 (magnetite) - can be injected directly into a tumour similar to a biopsy procedure, injected into the arterial supply of tumour tissue, and/or it will be enriched at tumour sites by an appropriate antibody-conjugation. The latter is advantageous, if the hyperthermia treatment must be repeated, while the direct injection is usually connected to a higher local concentration of nanoparticles.
By application of an alternating magnetic field, the particles generate heat that may destroy the surrounding cancer cells or support chemotherapies where already a moderate tissue heating leads to a more effective cell destruction.Further reading:
A new application of magnetite (Fe3O4) nanoparticles is the removal of heavy metals or actinides from waste water. The conventional way to clean waste water from metal ions is by the formation of metal hydroxides that usually have a low solubilities. However, the metal hydroxides can form gelatinous precipitates, which are difficult to filter and more chemicals have to be added frequently to facilitate the filtration process.
To clean the waste water using nanopartilces instead, small seeds of magnetite nanoparticles are dispersed in the water. After adding additional Fe2+ ions, ferrites start to grow that include the heavy metal or actinide ions. It was shown, that a large variety of ions can be incorporated in one step, e.g. Ag, As, Cd, Co, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, Ti, U, V, W, Zn, and Zr without noticeable removal of natural components like Ca and Mg ions. When the growth process is finished, the ferrite nanoprticles can be easily removed in one step from the water. This can be done by conventional filtration, because the ferrites are small crystalline solids and do not form gelatinous precipitates as it can happen in the conventional removal process. Another possibility is to remove the ferrites by the so-called magnetic filtration. By application of a magnetic field, the ferrite nanoparticles are concentrated and can be easily separated from the clean water. In contrast to common filtration, this method works also for very small nanoparticles.Further reading:
Magnetic data storage. One idea to further increase the data storage density is to represent a bit by the magnetisation direction of one nanoparticle that is much smaller than the bit size to date. This approach gives the possbility to reach a storage density in the range of Tbit/cm2. However, to achieve a magnetisation that is stable over ten years as desired for data storage applications, materials with a high magnetocrystalline anisotropy is needed that enhances the energy barrier to switch the magnetisation direction. FePt is one of the materials with the highest known anisotropy and is a prime candidate for magnetic data storage using nanoparticle ensembles.
The use of FePt nanoparticles as magnetic storage media has been discussed for more than a decade. However, there are still several obstacles that have to be overcome. In particular, there seems to be a reduced anisotropy in the nanoparticles with respect to the corresponding bulk material, the arrangement of nanoparticles in dense regular superlattices over large areas is not satisfactorily solved and the alignment of the easy axes of magnetisation is another delicate task.
Magnetoelectric data storage. If the magnetic nanoscale bits are made of a magnetostrictive material like CoFe2O4 and embedded in an electrically insulating piezoelectric matrix like BaTiO3, this device could be used as an energy saving data storage device, in which the bits can be written be application of a voltage without any electric current flowing.