Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy

Atomically Dispersed Mg-N-C Material Supported Highly Crystalline Pt3Mg Nanoalloys for Efficient Oxygen Reduction Reaction

Single-atom or atomically dispersed metal materials have emerged as highly efficient catalysts, but their potential as excellent supports has rarely been reported. In this work, we prepared Mg-N-C materials derived from annealing of a Mg-based metal-organic framework (MOF). By introducing Pt, Mg-N-C not only serves as a platform for anchoring Pt nanoparticles but also facilitates the integration of Mg into the Pt face-centered cubic lattice, resulting in the formation of highly crystalline Pt3Mg nanoalloys via the metal-support interfacial interaction. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) enables us to study the interfacial interaction and the surface electronic structure of this intricate system. The formation of Pt3Mg nanoalloys induces a downshift of the Pt d-band (gaining d-charge), as revealed by the decrease in the Pt L3-edge white-line (WL) area under the curve. This downshift can weaken the binding of oxygen reduction reaction (ORR) intermediates, hence improving the ORR performance.

Unraveling Origins of EPR Spectrum in Graphene Oxide Quantum Dots

Carbon nanostructures are utilized in a plethora of applications ranging from biomedicine to electronics. Particularly interesting are carbon nanostructured quantum dots that can be simultaneously used for bimodal therapies with both targeting and imaging capabilities. Here, magnetic and optical properties of graphene oxide quantum dots (GOQDs) prepared by the top-down technique from graphene oxide and obtained using the Hummers' method were studied. Graphene oxide was ultra-sonicated, boiled in HNO3, ultra-centrifuged, and finally filtrated, reaching a mean flake size of ~30 nm with quantum dot properties. Flake size distributions were obtained from scanning electron microscopy (SEM) images after consecutive preparation steps. Energy-dispersive X-ray (EDX) confirmed that GOQDs were still oxidized after the fabrication procedure. Magnetic and photoluminescence measurements performed on the obtained GOQDs revealed their paramagnetic behavior and broad range optical photoluminescence around 500 nm, with magnetic moments of 2.41 µB. Finally, electron paramagnetic resonance (EPR) was used to separate the unforeseen contributions and typically not taken into account metal contaminations, and radicals from carbon defects. This study contributes to a better understanding of magnetic properties of carbon nanostructures, which could in the future be used for the design of multimodal imaging agents.

Water Electrolysis Using Thin Pt and RuO x Catalysts Deposited by a Flame-Annealing Method on Pencil-Lead Graphite-Rod Electrodes

An inexpensive, simple, and high-activity catalyst preparation method has been introduced in this work. Pt and RuO _x catalysts were fabricated by soaking inexpensive graphite electrodes (pencil-lead graphite rod: PGR) in catalyst precursor solutions and using a simple flame-annealing method, which results in lower amount of Pt and RuO _x catalyst layers. From X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure analysis, it has been found that platinum and ruthenium were deposited as zero-valence metal (Pt) and oxide (RuO _x ), respectively. Catalytic activities of Pt/PGR and RuO _x /PGR for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were evaluated using neutral 1 M Na₂SO₄ aqueous electrolyte, respectively. Although HER and OER currents using PGR without catalysts were -16 mA cm^-2 (at -1.5 V vs Ag/AgCl) and +20 mA cm^-2 (at +2.0 V vs Ag/AgCl), they were improved to -110 and +80 mA cm^-2 with catalysts (Pt and RuO _x ), respectively. Such an inexpensive and rapid catalyst electrode preparation method on PGR using flame-annealing is a very significant method in the initial catalyst activity evaluation requiring a large amount of trial and error.

Origin of magnetic properties in carbon implanted ZnO nanowires

Various synchrotron radiation-based spectroscopic and microscopic techniques are used to elucidate the room-temperature ferromagnetism of carbon-doped ZnO-nanowires (ZnO-C:NW) via a mild C⁺ ion implantation method. The photoluminescence and magnetic hysteresis loops reveal that the implantation of C reduces the number of intrinsic surface defects and increases the saturated magnetization of ZnO-NW. The interstitial implanted C ions constitute the majority of defects in ZnO-C:NW as confirmed by the X-ray absorption spectroscopic studies. The X-ray magnetic circular dichroism spectra of O and C K-edge respectively indicate there is a reduction in the number of unpaired/dangling O 2p bonds in the surface region of ZnO-C:NW and the C 2p-derived states of the implanted C ions strongly affect the net spin polarization in the surface and bulk regions of ZnO-C:NW. Furthermore, these findings corroborate well with the first-principles calculations of C-implanted ZnO in surface and bulk regions, which highlight the stability of implanted C for the suppression and enhancement of the ferromagnetism of the ZnO-C:NW in the surface region and bulk phase, respectively.

Chemical Modification of Graphene Oxide by Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study

Nitrogen-doped graphene oxides (GO:N_x) were synthesized by a partial reduction of graphene oxide (GO) using urea [CO(NH₂)₂]. Their electronic/bonding structures were investigated using X-ray absorption near-edge structure (XANES), valence-band photoemission spectroscopy (VB-PES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). During GO:N_x synthesis, different nitrogen-bonding species, such as pyrrolic/graphitic-nitrogen, were formed by replacing of oxygen-containing functional groups. At lower N-content (2.7 at%), pyrrolic-N, owing to surface and subsurface diffusion of C, N and NH is deduced from various X-ray spectroscopies. In contrast, at higher N-content (5.0 at%) graphitic nitrogen was formed in which each N-atom trigonally bonds to three distinct sp²-hybridized carbons with substitution of the N-atoms for C atoms in the graphite layer. Upon nitrogen substitution, the total density of state close to Fermi level is increased to raise the valence-band maximum, as revealed by VB-PES spectra, indicating an electron donation from nitrogen, molecular bonding C/N/O coordination or/and lattice structure reorganization in GO:N_x. The well-ordered chemical environments induced by nitrogen dopant are revealed by XANES and RIXS measurements.

Plasma modification of the electronic and magnetic properties of vertically aligned bi-/tri-layered graphene nanoflakes

Saturation magnetization of vertically aligned bi/tri-layers is further enhanced by hydrogen, nitrogen plasma modification while organo-silane treatment reduces magnetization.

π-Bonding-dominated energy gaps in graphene oxide

The chemical bonding in graphene oxide with oxygen concentrations from 50% to 1% is investigated using first-principle calculations.