Top-Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces

Direct observation of strong surface reconstruction in partially reduced nickelate films

The polarity of a surface can affect the electronic and structural properties of oxide thin films through electrostatic effects. Understanding the mechanism behind these effects requires knowledge of the atomic structure and electrostatic characteristics at the surface. In this study, we use annular bright-field imaging to investigate the surface structure of a Pr0.8Sr0.2NiO2+x (0 < x < 1) film. We observe a polar distortion coupled with octahedral rotations in a fully oxidized Pr0.8Sr0.2NiO3 sample, and a stronger polar distortion in a partially reduced sample. Its spatial depth extent is about three unit cells from the surface. Additionally, we use four-dimensional scanning transmission electron microscopy (4D-STEM) to directly image the local atomic electric field surrounding Ni atoms near the surface and discover distinct valence variations of Ni atoms, which are confirmed by atomic-resolution electron energy-loss spectroscopy (EELS). Our results suggest that the strong surface reconstruction in the reduced sample is closely related to the formation of oxygen vacancies from topochemical reduction. These findings provide insights into the understanding and evolution of surface polarity at the atomic level.

Defect-Engineered Charge Transfer in a PtCu/PrxCe1-xO2 Carbon-Free Catalyst for Promoting the Methanol Oxidation and Oxygen Reduction Reactions

Platinum (Pt) and Pt-based alloys have been extensively studied as efficient catalysts for both the anode and cathode of direct methanol fuel cells (DMFC). Defect engineering has been revealed to be practicable in tuning the charge transfer between Pt and transition metals/supports, which leads to the charge density rearrangement and facilitates the electrocatalytic performance. Herein, Pr-doped CeO2 nanocubes were used as the noncarbon support of a PtCu catalyst. The concentration and structure of oxygen vacancy (Vo) defects were engineered by Pr doping. Besides the Vo monomer, the oxygen vacancy with a linear structure is also observed, leading to the one-dimensional PtCu. The Vo concentration shows the volcanic scenario as Pr increased. Accordingly, the activities of PtCu/PrxCe1-xO2 toward methanol oxidation and oxygen reduction reactions exhibit the volcanic scenario. PtCu/Pr0.15Ce0.85O2 exhibits the optimal catalytic performance with the specific activity 3.57 times higher than that of Pt/C toward MOR and 1.34 times higher toward ORR. The MOR and ORR mass activities of PtCu/Pr0.15Ce0.85O2 reached 1.05 and 0.12 A·mg-1, which are 3.09 and 0.92 times the values of Pt/C, respectively. The abundant Vo afforded surplus electrons, which tailored the electron transfer between PtCu and PrxCe1-xO2, leading to enhanced catalytic performance of PtCu/PrxCe1-xO2. DFT calculations on PtCu/Pr0.15Ce0.85O2 revealed that Pr doping reduced the band gap of CeO2 and lowered the overpotential.

Applications of a novel electron energy filter combined with a hybrid-pixel direct electron detector for the analysis of functional oxides by STEM/EELS and energy-filtered imaging

The performance and suitability of a new electron energy filter in combination with a hybrid pixel, direct electron detector for analytical (scanning) transmission electron microscopy are demonstrated using four examples. The STEM-EELS capabilities of the CEOS Energy Filtering and Imaging Device (CEFID) were tested with focus on weak signals and high spatio-temporal resolution. A multiferroic, multilayer structure of REMnO₃ (RE = Yb, Er, Tb, Y), grown on yttria-stabilized zirconia (YSZ), is used to exemplify that this new instrumental setup produces valuable electron energy-loss spectroscopy (EELS) data at high energy losses even when using short acquisition times, providing detailed chemical information about the interfaces in this complex multilayer sample. Another functional oxide, namely a ferromagnetic La₂NiMnO₆ thin film grown on SrTiO₃, demonstrates that atomically resolved spectrum images can be recorded, using short dwell times and moderate beam currents in order to warrant the integrity of the sample. In a third example, inhomogeneously Er-doped YSZ shows by EELS spectrum imaging that elements at low concentrations can be detected semi-quantitatively, uncovering the expected layered Er distribution but revealing substantial interdiffusion. In a final example, we simply demonstrate that the hybrid pixel detector in combination with the energy filter can also be used for energy-filtered imaging and thus for elemental mapping complementary to EELS in scanning transmission mode.