In-situ Observation of Chemically Reacted Particles in Gas Atmosphere with an Aberration Corrected STEM/SEM

The in-situ observation for chemical reaction processes with environmental transmission electron microscopes (TEMs) or scanning TEMs (STEMs) has been demanded to evaluate specimen properties or to comprehend reaction mechanism in research and development of catalysts, fuel cell batteries, and various other novel materials [1, 2]. The HF5000 200 kV TEM/STEM [3, 4] (Figure 1) equipped with a cold field-emission gun and a probe-forming spherical aberration corrector has been developed to analyze elemental distributions and chemical states of the specimen with an atomic resolution, as well as specimen shape and internal structure by transmission electron images. Specimen surface morphology is also observable by scanning electron microscope (SEM) images. Its differential-pumping model has a gas introduction nozzle in the specimen chamber of the column. The nozzle tip is mounted pointing at the specimen from a distance of less than 5 mm. The introduced gas can maintain a fresh reaction gas atmosphere of up to several hundred Pascals at the specimen. Figure 2 shows a specimen heating holder (a) for this model, which specimen heater cartridge (b) has been developed by Microelectromechanical system (MEMS) technology [5] reaching to the maximum temperature of 1100 °C in a few seconds owing to its small heating material volume. With the developed TEM/STEM plus SEM under a controllable gas atmosphere, a technique for simultaneous in-situ observation of the surface morphology and internal structure of a sample at atomic-scale was established. It has been applied to analyses of oxidation-reduction mechanism of catalyst samples. Figures 3 and 4 (reproduced and modified from Lv, H. et al., [6]) are examples of in situ experimental results with the HF5000 differential-pumping model under the conditions of 200 kV acceleration voltage and 100 pA probe current at a pressure of ∼10 Pa at the specimen while introducing hydrogen and oxygen gases, respectively. The microscope was equipped with an Energy dispersive X-ray analysis system (EDX) and an Electron Energy loss Spectrometer (EELS). The specimen is SFRuM perovskite oxide (Sr2Fe1.4Ru0.5Mo0.5O6-σ) which is a material for the solid oxide electrolysis cell (SOEC) for carbon dioxide electrolysis [6, 7]. Figure 3 shows analytical results of the specimen once oxidized from raw material status after a reduction. The specimen was heated at 800 °C for 60 minutes, then 850 °C for 30 minutes under H2 atmosphere. Ru nanoparticles were segregated on the perovskite surface in an intermediate state at 800 °C, as shown by (Annular Dark-Field) ADF-STEM image (a) and spectrum #2, and finally particle surface was covered with Fe from matrix at 850 °C (spectrum #3). Figure 4 is simultaneous (a) (Bright Field) BF-STEM, (b) ADF-STEM and (c) SEM observation result of a Ru nanoparticle after an oxidation process. Ru particle facets could be observed with atomic resolution in image (c). The aberration corrected in-situ STEM/SEM can be a versatile tool for investigating specimen structures, compositions, and properties in or closer to the chemical reactions. The HF5000 (a) Photograph of the MEMS heating holder tip and (b) SEM image of the MEMS heater In-situ observation results of the SFRuM specimen once oxidized from raw material status after reduction; (a) ADF-STEM image of a segregated Ru particle on the perovskite matrix, (b) Ru-L and (c) Fe- K EDX maps, (d) EELS spectra obtained at the points 1-3 in image (a) Simultaneous observation result of a Ru nanoparticle after an oxygen atmosphere at 800 °C for 30 minutes; (a) BF-STEM, (b) ADF-STEM and (c) SEM images