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Coherent diffrAction foR a look Inside Nanostructures towards atomic rEsolution: catalysis and interface

Periodic Reporting for period 1 - CARINE (Coherent diffrAction foR a look Inside Nanostructures towards atomic rEsolution: catalysis and interface)

Reporting period: 2019-11-01 to 2021-04-30

To help optimising (electro- or photo-) catalyst and reactor operations simultaneously, it is important to develop tools for in situ characterisation of nanocatalysts under realistic reaction conditions to monitor the dynamics of catalysts with high spatial, temporal and energy resolutions. How a catalyst works is rather complex: molecules adsorb to surface of catalyst and onto support, catalyst sits on support; different molecules move on facets and interact; product molecule desorbs, ... Of particular interest for catalysis chemists is the simultaneous dynamic in situ characterization of the chemical, morphological and structural (strain, defects, …) dynamical evolution of individual nanoparticles at high resolution and (near) operational conditions.

Among all the x-ray diffraction techniques, coherent x-ray diffraction (CDI) is one of the most promising. When applied under Bragg conditions, CDI has a unique sensitivity to atomic displacement and strain. The local symmetry of reciprocal space can be broken giving rise to a complex object in direct space, whose modulus represents the electron density of the object, and the phase, i.e. the displacements of these electron density portions relative to one another (with pm sensitivity), corresponding to strain. Progress in the catalytic issues requires the creation and development of new groundbreaking CDI methodology and techniques.

The development of in situ and operando CDI under well-defined catalytic conditions is thus of highest priority and extremely timely as the technique can probe structural changes in individual nanocrystals under conditions where up to now, no other technique could probe the relevant structural parameters.

The CARINE project will include two important advances:
1) the development of new in situ and operando environments (including temperature, gas and mass spectrometry analysis) compatible with nano-focused x-ray beams,
2) algorithm development for improved resolution and reproducibility in reconstructions.
We have developed the methodology around Bragg coherent x-ray diffraction.

- We have recently published a paper in Scientific Reports to give clues for a quantitative determination of strain in Bragg coherent x-ray diffraction imaging. We have demonstrated how to calculate the displacement field inside a nanocrystal from its reconstructed phase depending on the mathematical convention used for the Fast Fourier transform. We have used numerical simulations to quantify the influence of experimentally unavoidable detector deficiencies such as blind areas or limited dynamic range as well as post-processing filtering on the reconstruction. We have also proposed a criterion for the isosurface determination of the object, based on the histogram of the reconstructed modulus. Finally, we have studied the capability of the phasing algorithm to quantitatively retrieve the surface strain (i.e. the strain of the surface voxels). This work emphasizes many aspects that have been neglected so far in BCDI, which need to be understood for a quantitative analysis of displacement and strain based on this technique. It concludes with the optimization of experimental parameters to improve throughput and to establish BCDI as a reliable 3D nano-imaging technique.

- We have published another paper in Scientific Reports to demonstrate continuous scanning for Bragg coherent x-ray imaging. We have explored the use of continuous scanning during data acquisition for Bragg coherent diffraction imaging, i.e. where the sample is in continuous motion. We show a reduction of 30% in total scan time compared to conventional step‑by‑step scanning. The reconstructed Bragg electron density, phase, displacement and strain fields are in excellent agreement with the results obtained from conventional step‑by‑step scanning. Continuous scanning will allow to minimise sample instability under the beam and will become increasingly important at diffraction‑limited storage ring light sources.

- We have published an article in Small to demonstrate the use of variable-wavelength quick scanning nano-focused x-ray microscopy for in situ strain and tilt mapping. Variable-wavelength quick scanning X-ray microscopy opens the route for in situstrain and tilt mapping towards more diverse and complex materials environments, especially where sample manipulation is difficult.

We have applied Bragg coherent x-ray diffraction to map the evolution of structural defects along a nanowire. The results have been published in ACS Nano. This work provides an accurate inner view of planar defects inside small crystals.
This research is aimed at implementing and developing new infrastructures and methodologies for state-of-the-art x-ray based in situ experiments to probe in 3D catalytic nanostructures, i.e. materials of next-generation nanometric devices for future sustainable energy supply as well as for eco-technology and “green” chemistry. If we are to make the processing-structure-property relationships necessary for technological progress, it is crucial that we have the ability to probe the evolution of catalysts with atomic resolution under realistic conditions.

Our expected results are:
- to gather essential knowledge to control the effects of both surface and interface at the nanometer scale for catalysis via advanced in situ and operando characterization methods
- to provide clues to control in situ and operando the surface/interface effects and new possibilities for tuning and optimizing the functional physical and chemical properties of nanomaterials
- to image deep inside matter in complex, active environments
- to enable strain-engineered catalysis
- to extract parameters to be incorporated into models to overcome the “cook and look” approach (i.e. to develop a predictive science)
- to deliver new breakthroughs and new models in the design of catalytic materials.

The project should also impact other fields, e.g. nanoparticle synthesis / thermodynamic properties of alloyed nanoparticles / interface energies and Wulff shapes in different chemical environments.
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