Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water-splitting catalysts with semiconductors. ...Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co3O4/Co(OH)2 biphasic electrocatalyst on Si by means of operando ambient-pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co(OH)2 and partial conversion of the spinel Co3O4 phases to CoO(OH) under precatalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core-level regions to emerge under oxygen evolution reaction conditions on CoO(OH). The operando photoelectron spectra support assignment of these newly observed features to highly active Co4+ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co3O4 catalyst supports this interpretation and reveals that the presence of Co(OH)2 enhances catalytic activity by promoting transformations to CoO(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design.
Anatase TiO
2
nanoparticles doped either with Li or Ni have been synthesized via hydrolysis in variable concentrations. Microstructural analysis confirms the high crystallinity of the doped ...nanoparticles with sizes around 7 nm, while compositional analysis shows low doping below 2% at. Despite the low concentration of dopants, variations in the Raman and Photoluminescence signals were observed in the doped nanoparticles, mainly due to non-stoichiometry and oxygen deficiency promoted by Li or Ni doping. Doping effects associated with Li and Ni were observed by photoelectron spectroscopy and first principle calculations, which associate the appearance of states in the valence band region to oxygen deficiency and Li or Ni doping and lower n-type character induced by Ni doping. Finally, changes in the thermally induced anatase-to-rutile transition (ART) have been also observed in the doped samples, leading to a dopant-promoted faster ART which occurs at lower temperature boosted due to the dopant effect.
Graphical abstract
Magnetic nanoparticles (MNPs) have shown exceptional potential for several biological and clinical applications. However, MNPs must be coated by a biocompatible shell for such applications. The aim ...of this study is to understand if and how the surface charge and coating can affect the electronic and magnetic properties of CoFe2O4 MNPs. The role of the surface on the total magnetic moment of MNPs is a controversial issue, and several effects can contribute to make it deviate from the bulk value, including the charge, the nature of the coating, and also the synthetic technique. Positively and negatively charged uncoated CoFe2O4 NPs as well as citrate-coated NPs were prepared by soft chemistry synthesis. The electronic properties and cationic distribution of CoFe2O4 NPs were probed by X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and X-ray photoemission spectroscopy (XPS) techniques and confirmed by theoretical simulations. The overall magnetic behavior and the hyperthermic properties were evaluated by magnetometry and calorimetric measurements, respectively. The results show that all of the investigated CoFe2O4 NPs have high magnetic anisotropy energy, and the surface charge and coating do not influence appreciably their electronic and magnetic properties. In addition, the citrate shell improves the stability of the NPs in aqueous environment, making CoFe2O4 NPs suitable for biomedical applications.
Grafting thiol-bearing molecules at the surface of silver nano-particles (AgNPs) is a successful strategy to tune their optical and antibacterial properties. The capping layer generated from ...self-assembly of the ligands at the nanoparticle surface determines the range of possible applications of the resulting material. In particular, direct grafting of the thiol heads to surface Ag(I) can occur, with various hybridizations of the S atom, sp versus sp 3. Alternatively, a passivating Ag 2 S layer can form. We make use of S K-edge X-ray absorption near edge structure (XANES) and synchrotron-based X-ray photoelectron spectroscopy (XPS) to probe the metal−ligand interface in different thiol-capped AgNPs. The use of cryogenic conditions for XAS analyses reveals a peculiar spectral signature for thiolates chemisorbed on the AgNPs surface, unambiguously distinguished from that of Ag 2 S. Ab initio simulations of XANES spectra and XPS analyses are used to predict the grafting mode, suggesting that different ligand architectures promote slightly different proportions of sp/sp 3 sites, and a dramatic variability in the stability of the nanomaterial that can evolve toward either self-assembly or dissolution of the AgNPs.
We have structurally, chemically and electronically characterized the most stable (010) surface of a Mo-doped BiVO4 single crystal. Low energy electron diffraction (LEED) reveals that the surface is ...not significantly reconstructed from a bulk termination of the crystal. Synchrotron based X-ray spectroscopies indicate no surface enhancement of any of the crystal constituents and that the Mo dopant occupies tetrahedral sites by substituting for V at the surface. Using resonant photoemission to study the valence band structure as the V L3 edge is scanned we observe an intraband gap state associated with reduced vanadium formed by the Mo doping. This state is likely associated with small polaron formation. This feature is enhanced at a photon energy that is not resonant with any of the main features in the absorption spectrum of the pristine BiVO4. This indicates that the additional electron from Mo doping likely induces further distortion of the VO4 tetrahedral units and generates a new conduction band state either by splitting of the V d z 2 states or by hybridization of V d zx and V d z 2 states. We measure a work function of 5.15 eV for the BiVO4(010) surface. Measurement of the work function allows us to recast the electronic energy levels onto the normal hydrogen electrode scale for comparison to the standard reduction and oxidation potentials of water. This detailed study should provide a basis for future work aimed at a molecular level understanding of BiVO4/electrolyte interfaces used for photoelectrochemical water splitting.
The nature of the oxygen species active in ethylene epoxidation is a long-standing question. While the structure of the oxygen species that participates in total oxidation (nucleophilic oxygen) is ...known the atomic structure of the selective species (electrophilic oxygen) is still debated. Here, we use both
and UHV X-ray Photoelectron Spectroscopy (XPS) to study the interaction of oxygen with a silver surface. We show experimental evidence that the unreconstructed adsorbed atomic oxygen (O
) often argued to be active in epoxidation has a binding energy (BE) ≤ 528 eV, showing a core-level shift to lower BE with respect to the O-reconstructions, as previously predicted by DFT. Thus, contrary to the frequent assignment, adsorbed atomic oxygen cannot account for the electrophilic oxygen species with an O 1s BE of 530-531 eV, thought to be the active species in ethylene epoxidation. Moreover, we show that O
is present at very low O-coverages during
XPS measurements and that it can be obtained at slightly higher coverages in UHV at low temperature. DFT calculations support that only low coverages of O
are stable. The highly reactive species is titrated by background gases even at low temperature in UHV conditions. Our findings suggest that at least two different species could participate in the partial oxidation of ethylene on silver.
Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors ...in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS2 nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV−visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS2 nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS2 nanohybrids are seven times more efficient with respect to single MoS2 because of the formation of local p–n MoS2/N-doped graphene nanojunctions, which allow an efficient charge carrier separation.
The mesenchymal state in cancer is usually associated with poor prognosis due to the metastatic predisposition and the hyper-activated metabolism. Exploiting cell glucose metabolism we propose a new ...method to detect mesenchymal-like cancer cells. We demonstrate that the uptake of glucose-coated magnetic nanoparticles (MNPs) by mesenchymal-like cells remains constant when the glucose in the medium is increased from low (5.5 mM) to high (25 mM) concentration, while the MNPs uptake by epithelial-like cells is significantly reduced. These findings reveal that the glucose-shell of MNPs plays a major role in recognition of cells with high-metabolic activity. By selectively blocking the glucose transporter 1 channels we showed its involvement in the internalization process of glucose-coated MNPs. Our results suggest that glucose-coated MNPs can be used for metabolic-based assays aimed at detecting cancer cells and that can be used to selectively target cancer cells taking advantage, for instance, of the magnetic-thermotherapy.
Abstract
The effects of Cr on local environment and electronic structure of rutile TiO
2
are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional ...Cr impurities Cr
Ti
0
and Cr
Ti
1−
as well as Cr-oxygen vacancy complex 2Cr
Ti
+ V
O
are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO
2
at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the
t
2
g
states of the Cr ions in order to reach the stable oxidation state of Cr
3+
. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO
2
cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO
2
are very sensitive to the concentration of Cr.
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