Atomic Structure of the Fe3O4/Fe2O3 Interface During Phase Transition from Hematite to Magnetite.

Phase transition between iron oxides practically defines their functionalities in both physical and chemical applications. Direct observation of the atomic rearrangement and a quantitative description of the dynamic behavior of the phase transition, however, are rare. Here, we monitored the structure evolution from a rod-shaped hematite nanoparticle to magnetite during H2 reduction at elevated temperatures. Environmental transmission electron microscopy observations, along with selected area electron diffraction experiments, identified that the reduction preferentially commenced with Fe3O4 nucleation on the surface defective sites, followed by laterally growing into a Fe3O4 film until fully covering the particle surface. The Fe3O4 phase then propagated toward the bulk particle via a Fe3O4/α-Fe2O3 interface with the relationship α-Fe2O3(0001)//Fe3O4(111) in an aligned orientation of [112]Fe3O4||[112̅0]α-Fe2O3. Upon this Fe3O4/α-Fe2O3 interface, the Fe-O octahedra in Fe3O4(111) (as layer A) matches that of α-Fe2O3(0001) at a rotation angle of 30°, and the reduction proceeds in such a pattern that two-thirds of the FeOh in the adjacent layer (layer B) is transformed into FeTe.

Precise Drift Tracking for In Situ Transmission Electron Microscopy via a Thon-Ring Based Sample Position Measurement.

Visualizing how a catalyst behaves during chemical reactions using in situ transmission electron microscopy (TEM) is crucial for understanding the activity origin and guiding performance optimization. However, the sample drifts as temperature changes during in situ reaction, which weakens the resolution and stability of TEM imaging, blocks insights into the dynamic details of catalytic reaction. Herein, a Thon-ring based sample position measurement (TSPM) was developed to track the sample height variation during in situ TEM observation. Drifting characteristics for three commercially available nanochips were studied, showing large biases in aspects of shifting modes, expansion heights, as well as the thermal conduction hysteresis during rapid heating. Particularly, utilizing the TSPM method, for the first time, the gas layer thickness inside a gas-cell nanoreactor was precisely determined, which varies with reaction temperature and gas pressure in a linear manner with coefficients of ~8 nm/°C and ~50 nm/mbar, respectively. Following drift prediction of TSPM, fast oxidation kinetics of a Ni particle was tracked in real time for 12 s at 500°C. This TSPM method is expected to facilitate the functionality of automatic target tracing for in situ microscopy applications when feedback to hardware control of the microscope.

Endogenous Symbiotic Li3 N/Cellulose Skin to Extend the Cycle Life of Lithium Anode.

Nitrocellulose (NC) is proposed to stabilize the electrolytes for Li metal batteries. The nitro group of NC preferentially reacts with Li metal, and along with the cellulose skeleton is tightly wrapped on the surface, so that the polymer-inorganic double layer is formed on the Li surface. XPS profile analysis and corroborative cryo-environmental TEM reveal that the flexible outer layer of the bilayer is a C-O organic layer, while the dense inner layer is mainly composed of crystalline lithium oxide, lithium oxynitride, and lithium nitride. The Li deposition process was observed via in situ optical microscopy, which indicated that the NC-derived bilayer facilitates the uniform deposition of Li ions and inhibits the growth of dendrites. After the introduction of NC into the electrolyte, the cycle life of the Li battery is twice than that of the Li battery without NC at 1.0 and 3.0 mA cm-2 .

Diffusion-Limited Formation of Nonequilibrium Intermetallic Nanophase for Selective Dehydrogenation.

Nonequilibrium intermetallic phases in the nanoscale were realized by diffusion-controlled solid-state transformation, forming SiO2 supported NPs with Pd core and a CsCl type Pd1M1 shell, where M is Sn or Sb. The core-shell geometry is identified from scanning transmission electron microscopy and infrared spectroscopy and the crystal structure is confirmed from in situ synchrotron X-ray diffraction and X-ray absorption spectroscopy. The highly symmetric Pd1M1 intermetallic phase has not been reported previously and contains catalytic ensembles with high selectivity toward dehydrogenation of propane. The kinetically limited solid-state reaction is generally applicable to nanoparticle synthesis and could produce materials with desired structures and properties beyond conventional structural limits.

Oxygen activation on Ba-containing perovskite materials.

Oxygen activation, including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is at the heart of many important energy conversion processes. However, the activation mechanism of Ba-containing perovskite materials is still ambiguous, because of the complex four-electron transfer process on the gas-solid interfaces. Here, we directly observe that BaO and BaO2 segregated on Ba-containing material surface participate in the oxygen activation process via the formation and decomposition of BaO2. Tens of times of increase in catalytic activities was achieved by introducing barium oxides in the traditional perovskite and inert Au electrodes, indicating that barium oxides are critical for oxygen activation. We find that BaO and BaO2 are more active than the B-site of perovskite for ORR and OER, respectively, and closely related to the high activity of Ba-containing perovskite.

Dynamic structural evolution of iron catalysts involving competitive oxidation and carburization during CO2 hydrogenation.

Identifying the dynamic structure of heterogeneous catalysts is crucial for the rational design of new ones. In this contribution, the structural evolution of Fe(0) catalysts during CO2 hydrogenation to hydrocarbons has been investigated by using several (quasi) in situ techniques. Upon initial reduction, Fe species are carburized to Fe3C and then to Fe5C2. The by-product of CO2 hydrogenation, H2O, oxidizes the iron carbide to Fe3O4. The formation of Fe3O4@(Fe5C2+Fe3O4) core-shell structure was observed at steady state, and the surface composition depends on the balance of oxidation and carburization, where water plays a key role in the oxidation. The performance of CO2 hydrogenation was also correlated with the dynamic surface structure. Theoretical calculations and controll experiments reveal the interdependence between the phase transition and reactive environment. We also suggest a practical way to tune the competitive reactions to maintain an Fe5C2-rich surface for a desired C2+ productivity.