Complementary 2D/3D Imaging of Functional Materials using X-ray & Electron Microscopy

Abstract

Catalysts and other functional materials are generally hierarchically structured materials. Hence, the detailed characterization at different length scales, and especially under reaction conditions, are necessary to unravel their mechanisms and to improve their performance and catalytic activities. Besides, a combination of several techniques is required to acquire complementary information owing to the lack of a single technique able to cover all the length scales. With respect to length, the best way to investigate is by microscopy either in 2D or more preferably in 3D. The work began with an exploration of three different 3D imaging techniques, i.e. ptychographic X-ray computed tomography, electron tomography, and focused ion beam slice-and view. Using nanoporous gold as the model, this study aimed to exhibit the versatility of 3D microscopy as a method beyond imaging as well as to confirm the necessity of complementary nature between them, where electron offers better spatial resolution and X-ray provides larger field of view. The study then continued by utilizing ptychographic X-ray computed tomography for quasi in situ thermal treatment of the same materials under atmospheric pressure. This study highlighted its ease of use of implementing in situ studies, complemented by electron tomography to prove its powerful ability to resolve what ptychographic tomography cannot. The resulting 3D volumes were then used for air permeability and CO2 diffusion simulations, along with material’s electrical and thermal conductivity simulations in order to further expose another excellent benefit from 3D microscopy. Ultimately, the work proceeded into developing two cells in order to perform full in situ investigations under controlled temperatures and atmospheres, where one cell was built for 2D only (X-ray) ptychography experiments with simultaneous X-ray fluorescence mapping, and the other was constructed with an additional capability for 3D limited-angle ptychographic tomography experiments. The feasibility tests were conducted using several functional materials, i.e. nanoporous gold, zeolite, and cobalt-manganese-oxides hollow sphere, as the models for thermal annealing process under specific atmospheres. This work eventually attests the importance of in situ studies in precisely determining the onset annealing temperatures under particular environments, to visualize the morphology online either in 2D or 3D, and to simultaneously map elemental distributions live. Moreover, a complementary technique via transmission electron microscopy was also demonstrated on the same sample, adding up another advantage in using the cells. Despite the preliminary results from in situ limited-angle ptychographic tomography experiments for limitation in data reconstruction, a new tomographic reconstruction technique was recently developed as a solution to acquire 3D images with shortened acquisition times. In conclusions, the work here converges into the ideal case of performing all-around in situ 3D imaging of functional materials for quantitative analysis and simulation.