Abstract: Electrocatalysts play a pivotal role in advancing the application of water splitting for hydrogen production. This research unveils the potential of defective biphenylenes as high-efficiency catalysts for the hydrogen evolution reaction. Using first-principles simulations, we systematically investigated the structure, stability, and catalytic performance of defective biphenylenes. Our findings unveil that defect engineering significantly enhances the electrocatalytic activity for hydrogen evolution. Specifically, biphenylene with a double-vacancy defect exhibits an outstanding Gibbs free energy of -0.08 eV, surpassing that of Pt, accompanied by a remarkable exchange current density of -3.08 A cm(-2), also surpassing that of Pt. Furthermore, we find the preference for the Volmer-Heyrovsky mechanism in the hydrogen evolution reaction, with a low energy barrier of 0.80 eV. This research provides a promising avenue for developing novel metal-free electrocatalysts for water splitting with earth-abundant carbon elements, making a significant step toward sustainable hydrogen production.

Abstract: Accurate geometrical calibration between the scan coordinates and the camera coordinates is critical in four-dimensional scanning transmission electron microscopy (4D-STEM) for both quantitative imaging and ptychographic reconstructions. For atomic-resolved, in-focus 4D-STEM datasets, we propose a hybrid method incorporating two sub-routines, namely a J-matrix method and a Fourier method, which can calibrate the uniform affine transformation between the scan-camera coordinates using raw data, without a priori knowledge about the crystal structure of the specimen. The hybrid method is found robust against scan distortions and residual probe aberrations. It is also effective even when defects are present in the specimen, or the specimen becomes relatively thick. We will demonstrate that a successful geometrical calibration with the hybrid method will lead to a more reliable recovery of both the specimen and the electron probe in a ptychographic reconstruction. We will also show that, although the elimination of local scan position errors still requires an iterative approach, the rate of convergence can be improved, and the residual errors can be further reduced if the hybrid method can be firstly applied for initial calibration. The code is made available as a simple-to-use tool to correct affine transformations of the scan-camera coordinates in 4D-STEM experiments.

Abstract: The nucleation and growth mechanisms of Ni5Al3 precipitates and microtwinned plates in B2 austenite and 2M (3R) martensite phases are described on the basis of conventional and high resolution electron microscopy. In the Ni62.5Al37.5 B2 austenite matrix short annealings at 550 degrees C introduce three-pointed star shaped precipitates consisting of twin related parts of different variants of the Ni5Al3 structure. Longer annealings result in plates growing separately from these wings and developing microtwinning in order to accommodate stress built-up at the interfaces with the surrounding matrix. Annealing of Ni65Al35 2M martensite plates induces simple reordering into the Ni5Al3 phase, increasing the fct c/a ratio by about 1%. As a result stracking faults are introduced in the smallest twin variants.

Abstract: Homogenised and quenched Ni65Al35 samples were heated and studied in situ in a CM20 electron microscope up to 900 degrees C. The Ni5Al3 phase first forming around 550 degrees C in the quenched L1(0) microtwinned martensite starts to decompose around 800 degrees C yielding B2 precipitates in a twinned L1(2) matrix. The latter twinning is a remainder of the microtwinning in the original room temperature martensite. Also the crystallographic relations between precipitates and matrix can be traced back to the original formation of twinned martensite plates within the austenite. Some aspects of the dynamics of the process are discussed on the basis of snap shots and video recordings.

Abstract: This contribution describes conventional and high resolution electron microscopy results on the different twinning arrangements in NisAl3 precipitates grown inside the B2 austenite phase. Short annealings introduce self-accommodating three-pointed star shaped precipitates consisting of twin related parts of different variants of the NisAl3 structure. Longer annealings result in plates growing separately from these wings and developing microtwinning in order to accommodate stress built-up at the interfaces with the surrounding matrix.