Jihan Zhou

Deciphering chemical order/disorder and material properties at the single-atom level

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.

GENFIRE: A generalized Fourier iterative reconstruction algorithm for high-resolution 3D imaging

Tomography has made a radical impact on diverse fields ranging from the study of 3D atomic arrangements in matter to the study of human health in medicine. Despite its very diverse applications, the core of tomography remains the same, that is, a mathematical method must be implemented to reconstruct the 3D structure of an object from a number of 2D projections. Here, we present the mathematical implementation of a tomographic algorithm, termed GENeralized Fourier Iterative REconstruction (GENFIRE), for high-resolution 3D reconstruction from a limited number of 2D projections. GENFIRE first assembles a 3D Fourier grid with oversampling and then iterates between real and reciprocal space to search for a global solution that is concurrently consistent with the measured data and general physical constraints. The algorithm requires minimal human intervention and also incorporates angular refinement to reduce the tilt angle error. We demonstrate that GENFIRE can produce superior results relative to several other popular tomographic reconstruction techniques through numerical simulations and by experimentally reconstructing the 3D structure of a porous material and a frozen-hydrated marine cyanobacterium. Equipped with a graphical user interface, GENFIRE is freely available from our website and is expected to find broad applications across different disciplines.

Composition tunable ternary Pt–Ni–Co octahedra for optimized oxygen reduction activity

Herein, we report a one-step synthesis method for octahedral Pt-Ni-Co ternary catalysts with tunable compositions and fixed shapes. Impressively, the composition optimized octahedral PtNi0.55Co0.1/C demonstrated a significant improvement in ORR activity compared to those of previously reported Pt-Ni-Co alloy octahedra, showing an outstanding specific activity of 5.05 mA cm-2 and a mass activity of 2.80 mA μgPt-1, which are around 20.2 times and 14.7 times higher than those of the commercial Pt/C catalyst (0.25 mA cm-2 and 0.19 mA μgPt-1).

Quantitative characterization of high temperature oxidation using electron tomography and energy-dispersive X-ray spectroscopy

We report quantitative characterization of the high temperature oxidation process by using electron tomography and energy-dispersive X-ray spectroscopy. As a proof of principle, we performed 3D imaging of the oxidation layer of a model system (Mo3Si) at nanoscale resolution with elemental specificity and probed the oxidation kinetics as a function of the oxidation time and the elevated temperature. Our tomographic reconstructions provide detailed 3D structural information of the surface oxidation layer of the Mo3Si system, revealing the evolution of oxidation behavior of Mo3Si from early stage to mature stage. Based on the relative rate of oxidation of Mo3Si, the volatilization rate of MoO3 and reactive molecular dynamics simulations, we propose a model to explain the mechanism of the formation of the porous silica structure during the oxidation process of Mo3Si. We expect that this 3D quantitative characterization method can be applied to other material systems to probe their structure-property relationships in different environments.

3D Imaging of Nanoalloy Catalysts at Atomic Resolution

Metal catalysts such as Pt, Pd, Cu, and Ni are critical components in many applications ranging from fuel cells in automobiles to mobile power generation. Recent studies have shown that introducing earthabundant metals such as Ni and Cu into Pt can not only significantly enhance the oxygen reduction reaction (ORR) activity of the catalysts, but also lower the cost [1, 2]. Among these nanoalloys, bimetallic platinum-nickel (PtNi) nanostructures represent an emerging class of candidates for ORR catalyst in fuel cells. More recently, a new nanoalloy catalyst of carbon-supported NiPt with Mo doped on the surface (PtNi-Mo) has been reported, demonstrating 80-fold enhancements compared with commercial Pt-C catalyst [3]. To understand the structure and performance of these nanoalloy catalysts, it is essential to fully characterize the surface structure of these nanoalloy catalysts in 3D with atomic resolution. An ideal method to achieve this challenging goal is atomic electron tomography (AET) [4], which retrieves 3D atomic structure information from a tilt series of high-resolution 2D images. In recent years, AET has been used to image the 3D structure of grain boundaries, stacking faults, and the core structure of edge and screw dislocations at atomic resolution [5, 6]. Furthermore, the combination of AET and atom tracing algorithms has enabled the determination of the coordinates of individual atoms and point defects in materials with a 3D precision of ~19 pm, allowing direct measurements of 3D atomic displacements and the full strain tensor [7]. More recently, the 3D coordinates of more than 23,000 atoms in an FePt nanoparticle have been determined by AET to correlate chemical order/disorder and crystal defects with material properties at the single-atom level [8]. Here, we apply AET to probe the 3D surface atomic structure of PtNi-Mo nanoalloy catalysts. By using the TEAM microscope at the National Center for Electron Microscopy at LBNL, we have acquired several tomographic tilt series from PtNi-Mo nanoalloys. After post processing and centre of mass alignment of the tilt series, we computed high-quality 3D reconstructions by using a Generalized Fourier Iterative Reconstruction (GENFIRE) algorithm [8]. Volume-renderings of a representative 3D reconstruction show individual atoms at different orientations (Fig. 1). The individual Pt, Ni and Mo atoms can be distinguished based on the intensity contrast. Figure 2 shows three perpendicular 1.7 Å thick central slices of the atomic resolution reconstruction, where red and blue arrows indicate Pt and Ni atoms, respectively. With further improvement of the tilt series alignment, denoising and tilt angle refinement, we expect the full determination of the 3D atomic structure of PtNi-Mo nanoalloys, including precise atomic coordinates and accurate classification of Pt, Ni, and Mo atoms. Coupled with first principles calculations, the full surface oxygen binding energy map can be derived from the 3D atomic structure, providing essential information to precisely tailor and optimize the catalytic behavior for various applications [9].

Atomic Electron Tomography: Probing 3D Structure and Material Properties at the Single-Atom Level

1. Dept. of Physics and Astronomy and California NanoSystems Institute, UCLA, CA, USA. 2. Dept. of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan. 3. NCEM, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 4. Dept. of Physics, University at Buffalo, the State University of New York, Buffalo, NY, USA. 5. Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, UK. 6. National Center for Computational Sciences, ORNL, Oak Ridge, TN, USA. 7. Computer Science and Mathematics Division, ORNL, Oak Ridge, TN, USA. 8. Center for Nanophase Materials Sciences, ORNL, Oak Ridge, TN, USA. 9. Dept. of Physics, University of Nebraska at Omaha, Omaha, NE, USA.