Single-nanoparticle phase transitions visualized by four-dimensional electron microscopy

The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structural dynamics in real and reciprocal space. Here, we demonstrate the selectivity and sensitivity of the technique by visualizing the spin crossover dynamics of single, isolated metal–organic framework nanocrystals. By introducing a small aperture in the microscope, it was possible to follow the phase transition and the associated structural dynamics within a single particle. Its behaviour was observed to be distinct from that imaged by averaging over ensembles of heterogeneous nanoparticles. The approach reported here has potential applications in other nanosystems and those that undergo (bio)chemical transformations

Temperature- and Light-Induced Spin Crossover Observed by X-ray Spectroscopy on Isolated Fe(II) Complexes on Gold

Using X-ray absorption techniques, we show that temperature- and light-induced spin crossover properties are conserved for a submonolayer of the [Fe(H2B(pz)2)2(2,2′-bipy)] complex evaporated onto a Au(111) surface. For a significant fraction of the molecules, we see changes in the absorption at the L2,3 edges that are consistent with those observed in bulk and thick film references. Assignment of these changes to spin crossover is further supported by multiplet calculations to simulate the X-ray absorption spectra. As others have observed in experiments on monolayer coverages, we find that many molecules in our submonolayer system remain pinned in one of the two spin states. Our results clearly demonstrate that temperature- and light-induced spin crossover is possible for isolated molecules on surfaces but that interactions with the surface may play a key role in determining when this can occur

Trans effect and inverse trans effect in MLX5 complexes (M=Mo, U; L=O, S; X=Cl, Br): A rationalization within density functional theory study

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Nanocomposite electrodes based on pre-synthesized organically capped platinum nanoparticles and carbon nanotubes. Part II: Determination of diffusion area for oxygen reduction reflects platinum accessibility

International audienceIn this paper we report the determination of the diffusion area for oxygen reduction in porous electrode structure having a controlled platinum loading and based on capped platinum electrocatalysts and carbon nanotubes. Such a parameter is expected to be higher than the macroscopic geometrical area of the active porous layer. The oxygen diffusion area is determined by cyclic voltammetry after impregnation of the electrode structure by the electrolyte, and using the equations available for peak potential and peak current as a function of scan speed for irreversible redox couple. First it is found first that the oxygen diffusion area is dependent on the total amount of platinum in the electrode. Second, for a given platinum loading, the diffusion area is higher when the mass ratio of platinum to carbon nanotube decreases. This point indicates that the accessibility of platinum capped electrocatalyst is better in such cases. It is thus concluded that the oxygen diffusion area determination in porous electrode structures may be used to characterize the accessibility of the capped electrocatalysts for oxygen reduction. Even if this area is different in nature from the one calculated by Hydrogen Underpotential Deposition, we believe that its determination might be of interest for the characterization of porous electrodes structures in which the electrocatalyst is combined with a finely divided carbon suppor

Porous electrodes based on platinum capped electrocatalyst: Combining thermal treatment XPS analysis and electrochemistry give evidence for the stabilizing role of the thiol capping agent on the Pt dispersion and core feature

International audienceIn previous work we reported oxygen reduction reaction (ORR) studies on porous electrodes based on capped platinum (Pt) electrocatalyst and carbon nanotubes. These structures exhibited a significant activity but a very low platinum electrochemically active surface area (Pt-EASA) due to the grafted molecules on the platinum core nanoparticle surface. The present paper reports on thermal pre-treatment of such electrodes at moderate temperature aiming at degrading the organic capping without changing the nanoparticle Pt core feature. Using X-ray diffraction, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) it is shown that a treatment at 100 C under air fit these requirements. This results in a strong increase of the Pt-EASA. However, it is evidenced by TEM and XRD that, as soon as the organic capping is modified by partial oxidation of sulfur atoms involved in initial strong Pt-S bond, the electrochemical measurement triggers dramatic changes on the Pt dispersion and Pt core feature. Much bigger size Pt regions are formed, complete oxidation of the sulfur atoms is observed and organic capping molecules are significantly eliminated in the electrolyte. Finally, although it was not possible to prove systematically that we get rid of organic contamination after the involved treatments, the dramatic changes of Pt catalyst nanoparticles compared to the initial organically capped ones are clearly established. These original results demonstrate the essential stabilizing role of the grafted thiol molecules in the initial system and allow proposing a scenario for the ageing of these capped electrocatalyst when submitted to prolonged ORR