Eider Goikolea

Chemically Induced Permanent Magnetism in Au, Ag, and Cu Nanoparticles: Localization of the Magnetism by Element Selective Techniques

We report a direct observation of the intrinsic magnetization behavior of Au in thiol-capped gold nanoparticles with permanent magnetism at room temperature. Two element specific techniques have been used for this purpose: X-ray magnetic circular dichroism on the L edges of the Au and 197Au Mössbauer spectroscopy. Besides, we show that silver and copper nanoparticles synthesized by the same chemical procedure also present room-temperature permanent magnetism. The observed permanent magnetism at room temperature in Ag and Cu dodecanethiol-capped nanoparticles proves that the physical mechanisms associated to this magnetization process can be extended to more elements, opening the way to new and still not-discovered applications and to new possibilities to research basic questions of magnetism.

Nanoporous carbons from natural lignin: study of structural–textural properties and application to organic-based supercapacitors

Lignin-derived nanoporous carbon with narrow and tuneable pore size distribution has been produced by activation with potassium hydroxide (KOH). The results manifest the competition between the oxidation reaction of carbon and the intriguing C–C bond re-organization provoked by the chemical activation. A trend between the average pore size and the in-plane crystal size of few-layer graphene is observed. The ability for non-Faradaic charge storage is negatively affected by the graphenization degree. The ion-sieving effect is detected for carbon materials with an average pore size below 0.9 nm, suggesting at least partial solvation of electrolyte ions inside pores. Capacitance values up to 87 F g−1 in an organic based electrolyte are obtained.

Thiol-capped ferromagnetic Au nanoparticles investigated by Au L3 x-ray absorption spectroscopy

Different dodecanethiol capped Au nanoparticles (NP) with similar sizes (∼2nm) but different ferromagnetic signals at room temperature have been investigated by means of x-ray absorption spectroscopy at the Au L3-edge. The reversion of the x-ray magnetic circular dichroism signal with the change of sign of the external applied magnetic field confirms the location of the magnetism at the Au atoms. In comparison with the Au foil, all the samples present accentuated white lines at the x-ray absorption near-edge structure (XANES) indicating generation of 5d holes in the Au atoms located at surface of the NPs as consequence of a localized charge transfer from the Au surface atoms to the S atoms of the capping agent. XANES spectra reflect differences among the electronic structure of the Au NPs which are compared with the observed different macroscopic magnetic signals.

Ferromagnetism of polythiophene-capped Au nanoparticles

The magnetic and electrical transport properties of regioregular poly(3-hexylthiophene)-capped Au nanoparticles (NPs) doped with iodine have been investigated to clarify the effectiveness of conductive polymer capping on the induction of ferromagnetism in Au. The room-temperature magnetization curve of the undoped polythiophene-capped Au NPs exhibits a clear hysteresis behavior with a coercive force of 160 Oe. The spontaneous magnetization normalized by the mass of Au is 2.0 × 10−2 emu/g. The spontaneous magnetization was found virtually unaffected by iodine doping, whereas the electrical conductivity is enhanced dramatically to ∼10 S/cm. Our results show that polythiophene capping could lead to spontaneous magnetic polarization in Au NPs, and the conductivity of the polymer capping does not affect the ferromagnetism of the Au nanoparticles, opening a possibility for further investigation into the magnetotransport behavior of ferromagnetic Au NPs.

One-pot synthesis of highly activated carbons from melamine and terephthalaldehyde as electrodes for high energy aqueous supercapacitors

In this work we report the preparation of porous carbons with very large specific surface areas (over 3000 m2 g−1) by a simple all-in-one route that involves the simultaneous polymerization, carbonization and in situ activation of a mixture of melamine and terephthalaldehyde. The influence that different activating agents (KOH and a eutectic mixture of KOH and NaOH) have on the polymerization process and thus the final textural properties of the carbons is also explored. Materials were characterized by X-ray diffractometry (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal analysis (TG/DTA) and nitrogen adsorption–desorption at −196 °C. It was found that carbons prepared in the presence of KOH showed a hierarchical multimodal pore-size distribution that combines large micropores and medium-size mesopores while those carbons obtained in the presence of the KOH–NaOH mixture exhibited a narrower distribution within the micropore range and small mesopores. Both materials were tested as electrodes for symmetric supercapacitors using three different aqueous electrolytes, namely 6 M KOH, 1 M Li2SO4 and 5 M LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), which allowed their steady cycling at 1.2, 1.8 and 2.2 V, respectively. The different performance between both carbons working in different electrolytes is discussed and related to their textural features. The hierarchical micro–mesoporosity favored a good diffusion of ions when working with LiTFSI, which allows achieving very high energy densities of 21 W h kg−1 at 0.14 kW kg−1. For moderate requirements in terms of energy and power density, the same micro/mesoporous material can provide 12.4 W h kg−1 at 3.3 kW kg−1 for 104 cycles using Li2SO4 as the electrolyte. Finally, both mesopore-containing and mesopore-free materials can provide very high capacitance values up to 360 F g−1, a very fast response and excellent cycling performance when working in 6 M KOH, being suitable candidates for high power applications.

Magnetic and structural characterization of thiol capped ferromagnetic Ag nanoparticles

Dodecanethiol capped Ag nanoparticles (NPs) have been independently synthesized by the well-known Brust method under the same physical-chemical conditions. The obtained NP present similar sizes (∼2 nm) but different magnetic behaviors. The extended x-ray absorption fine structure analyses at the K-edge of Ag did not reveal any noticeable structural nor topological differences among the samples. In clear contrast with the structure provided for thiol capped ferromagnetic Au NPs, the analysis also brings out the existence of Ag–S bonds in a diffuse region surrounding a reduced Ag core where the magnetism of the Ag NPs would be located.

Highly packed graphene–CNT films as electrodes for aqueous supercapacitors with high volumetric performance

The increasing complexity of portable electronics demands the development of energy storage devices with higher volumetric energy and power densities. In this work we report a simple strategy for the preparation of partially reduced graphene oxide/carbon nanotube composites (prGO–CNT) as highly packed self-standing binder-free films suitable as electrodes for supercapacitors. These carbon-based films are easily obtained by the hydrothermal treatment of an aqueous suspension of graphene oxide and CNTs at 210 °C and then compacted under pressure. The prGO–CNT films, which had an apparent density as high as 1.5 g cm−3, were investigated as binder-free electrodes for aqueous supercapacitors using 6 M KOH solution as the electrolyte. The results show that the presence of merely 2 wt% of CNTs produces a significant enhancement of the capacitance retention at high current densities compared to the CNT-free samples, and this improvement is especially relevant in systems formed using electrodes with high mass loadings. Volumetric capacitance values of 250 F cm−3 at 1 A g−1 with outstanding capacitance retention (200 F cm−3 at 10 A g−1) were achieved using the prGO–CNT electrodes with an areal mass loading above 12 mg cm−2.

A two-step process for preparation of dodecanethiol-capped Au nanoparticles with room-temperature spontaneous magnetization

Thiol-capped Au nanoparticles with a permanent magnetic moment were prepared using a new two-step chemical process. The two-step process opens up a new facet of fundamental research on the mechanism of the induced ferromagnetism in intrinsically nonmagnetic materials by investigating independently the nano-size and chemical effects on the magnetism of thiol-capped Au nanoparticles.

Effect of Organic Capping on the Magnetic Properties of Au Nanoparticles

The magnetic and optical properties of regioregular poly(3-hexylthiophene)-capped Au nanoparticles (NPs) have been investigated in order to clarify the effectiveness of polythiophene capping on the induction of ferromagnetism in Au. The room-temperature magnetization curve of the polythiophene-capped Au NPs exhibits a clear hysteresis behavior with a spontaneous magnetization of 8.5 x 10-3 emu/g. The average magnetic moment of the surface Au atoms is estimated to be 2.6 x 10-3 B. The ultraviolet-visible spectrum shows a clear sign of the surface plasmon absorbance of metallic Au which reflects the week character of the chemical bond between the Au and S atoms on capping surface. Our results show clearly that Au NPs capped with polythiophene can be ferromagnetic.