Yinkai Lei

Photocatalytic and degradation mechanisms of anatase TiO2: a HRTEM study

The photocatalytic process for degradation of methylene blue with P25 TiO2 was directly observed using a high-resolution transmission electron microscopy (HRTEM). It was found that: (1) the pristine anatase TiO2 nanoparticles exhibited a perfect crystal lattice with a clear HRTEM image; (2) after adsorption and degradation, there were many methylene blue crystals as 1 nm molecular adsorbed at the surface of TiO2 nanoparticles, which therefore resulted in a fuzzy HRTEM image; (3) when the TiO2 was exposed in air for a period of time, the methylene blue molecules disappeared and the TiO2 lattice image again became integrated as the pristine one; (4) however, if the TiO2 nanoparticles became deactivated after degradation of methylene blue for more than 20 cycles, the HRTEM lattice image was fuzzy fully and could not recover even it was exposed in air for a long time. The results reveal that the lattice distortion on the anatase TiO2 (101) surface induced by chemical adsorption of methylene blue molecules is a crucial intermediate step during photocatalyzing. The distorted lattice atoms absorb photons under illumination of light and tend to recover and cut the molecule bond which causes the degradation of methylene blue. Furthermore, we propose that there exists a surface lattice driving force forming on the surface distortion leads to the TiO2 lattice HRTEM image from fuzzy to integrate, and it decides the photocatalytic ability.

Surface faceting and elemental diffusion behaviour at atomic scale for alloy nanoparticles during in situ annealing

The catalytic performance of nanoparticles is primarily determined by the precise nature of the surface and near-surface atomic configurations, which can be tailored by post-synthesis annealing effectively and straightforwardly. Understanding the complete dynamic response of surface structure and chemistry to thermal treatments at the atomic scale is imperative for the rational design of catalyst nanoparticles. Here, by tracking the same individual Pt3Co nanoparticles during in situ annealing in a scanning transmission electron microscope, we directly discern five distinct stages of surface elemental rearrangements in Pt3Co nanoparticles at the atomic scale: initial random (alloy) elemental distribution; surface platinum-skin-layer formation; nucleation of structurally ordered domains; ordered framework development and, finally, initiation of amorphization. Furthermore, a comprehensive interplay among phase evolution, surface faceting and elemental inter-diffusion is revealed, and supported by atomistic simulations. This work may pave the way towards designing catalysts through post-synthesis annealing for optimized catalytic performance.

First-principles studies on vacancy-modified interstitial diffusion mechanism of oxygen in nickel, associated with large-scale atomic simulation techniques

This paper is concerned with the prediction of oxygen diffusivities in fcc nickel from first-principles calculations and large-scale atomic simulations. Considering only the interstitial octahedral to tetrahedral to octahedral minimum energy pathway for oxygen diffusion in fcc lattice, greatly underestimates the migration barrier and overestimates the diffusivities by several orders of magnitude. The results indicate that vacancies in the Ni-lattice significantly impact the migration barrier of oxygen in nickel. Incorporation of the effect of vacancies results in predicted diffusivities consistent with available experimental data. First-principles calculations show that at high temperatures the vacancy concentration is comparable to the oxygen solubility, and there is a strong binding energy and a redistribution of charge density between the oxygen atom and vacancy. Consequently, there is a strong attraction between the oxygen and vacancy in the Ni lattice, which impacts diffusion.

Bivalence Mn5O8 with hydroxylated interphase for high-voltage aqueous sodium-ion storage

Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn5O8 pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge–discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn5O8 suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn2+/Mn4+ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn5O8.

Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes

Oxygen adsorption energy is directly relevant to the catalytic activity of electrocatalysts for oxygen reduction reaction (ORR). In this study, we established the correlation between the O2 adsorption energy and the electronic structure of transition metal macrocyclic complexes which exhibit activity for ORR. To this end, we have predicted the molecular and electronic structures of a series of transition metal macrocyclic complexes with planar N4 chelation, as well as the molecular and electronic structures for the O2 adsorption on these macrocyclic molecules, using the density functional theory calculation method. We found that the calculated adsorption energy of O2 on the transition metal macrocyclic complexes was linearly related to the average position (relative to the lowest unoccupied molecular orbital of the macrocyclic complexes) of the non-bonding d orbitals (d(z(2)), d(xy), d(xz), and d(yz)) which belong to the central transition metal atom. Importantly, our results suggest that varying the energy level of the non-bonding d orbitals through changing the central transition metal atom and/or peripheral ligand groups could be an effective way to tuning their O2 adsorption energy for enhancing the ORR activity of transition metal macrocyclic complex catalysts.

Charged vacancy diffusion in chromium oxide crystal: DFT and DFT+U predictions

In this work, we computationally studied the lattice diffusion through the ion-vacancy exchange mechanism in α-Cr2O3 crystal using the first-principles density functional theory (DFT) and DFT+U calculation methods. For both O and Cr vacancies, we have identified four elementary diffusion paths in α-Cr2O3 crystal. Our DFT+U calculations predict that the O vacancy with charge +2 ( VO2+) is stable when Fermi energy is near to valence band maximum, whereas the Cr vacancy with charge −3 ( VCr3−) is stable when Fermi energy is close to conduction band minimum. Moreover, the DFT+U calculations predict that the migration energy for VO2+  diffusion varies from 1.18 to 2.98 eV, whereas that for VCr3− diffusion varies from 2.02 to 2.59 eV, close to experimental data. Both DFT and DFT+U results indicate that the migration energy of neutral vacancies ( VO0 and VCr0) is higher than that of the charged vacancies ( VO2+ and VCr3−) along any diffusive path. Importantly, it is found that the DFT+U method describes α-Cr2O3 ...

Influence of surface segregation on magnetic properties of FePt nanoparticles

Surface segregation leads to chemical disordering in magnetic alloy nanostructures and thus could have profound impact upon the magnetic properties of these nanostructures. In this study, we used the first-principles density functional theory calculation method to determine how Pt surface segregation (exchanging interior Pt with surface Fe atoms) would affect the magnetic properties of L10 ordered FePt nanoparticles. For both cuboid and cuboctahedral FePt nanoparticles, we predicted that the Pt surface segregation process could cause a decrease in total magnetic moments, a change in (easy and/or hard) magnetization axes, and a reduction in magnetic anisotropy.

Dislocation nucleation facilitated by atomic segregation

Surface segregation-the enrichment of one element at the surface, relative to the bulk-is ubiquitous to multi-component materials. Using the example of a Cu-Au solid solution, we demonstrate that compositional variations induced by surface segregation are accompanied by misfit strain and the formation of dislocations in the subsurface region via a surface diffusion and trapping process. The resulting chemically ordered surface regions acts as an effective barrier that inhibits subsequent dislocation annihilation at free surfaces. Using dynamic, atomic-scale resolution electron microscopy observations and theory modelling, we show that the dislocations are highly active, and we delineate the specific atomic-scale mechanisms associated with their nucleation, glide, climb, and annihilation at elevated temperatures. These observations provide mechanistic detail of how dislocations nucleate and migrate at heterointerfaces in dissimilar-material systems.

Linking Initial Microstructure to ORR Related Property Degradation in SOFC Cathode: A Phase Field Simulation

Microstructure evolution driven by thermal coarsening is an important factor for the loss of oxygen reduction reaction rates in SOFC cathode. In this work, the effect of an initial microstructure on the microstructure evolution in SOFC cathode is investigated using a recently developed phase field model. Specifically, we tune the phase fraction, the average grain size, the standard deviation of the grain size and the grain shape in the initial microstructure, and explore their effect on the evolution of the grain size, the density of triple phase boundary, the specific surface area and the effective conductivity in LSM-YSZ cathodes. It is found that the degradation rate of triple phase boundary density and specific surface area of LSM is lower with less LSM phase fraction (with constant porosity assumed) and greater average grain size, while the degradation rate of effective conductivity can also be tuned by adjusting the standard deviation of grain size distribution and grain aspect ratio. The implication of this study on the designing of an optimal initial microstructure of SOFC cathodes is discussed.

Oxygen Electroreduction on M-N4 Macrocyclic Complexes

Inspired by biological catalysts, such as myoglobin and hemoglobin, many M-N4 macrocycles have been investigated as promising catalysts for the oxygen reduction reactions (ORRs) in alkaline and acid media for several decades. Such macrocyclic complexes include transition-metal porphyrins (MPs) or phthalocyanines (MPcs)-like molecules, and nitrogen-chelated transition metal clusters in a carbon matrix (pyrolyzed M-N4/C). Although extensive research has been carried out to acquire understanding on how the ORRs progress on these M-N4 macrocyclic complexes, there are still several key fundamental aspects to be clarified. It is still debatable about the nature of the active sites of M-N4 complex catalysts for the ORRs and the ORR reaction mechanisms on the M-N4/C catalysts. In this chapter, we reviewed the studies of ORRs on M-N4 macrocyclic complexes up to date from both experimental and computational perspectives. First, we surveyed the experimental results about the various factors affecting the catalytic performance of the M-N4 macrocyclic complexes for ORR. Then, we specifically discussed how heat treatment and carbon nanostructured substrates would significantly enhance the catalytic performance of the M-N4 macrocyclic catalysts. As the focus of this paper, we summarized the advancements on application of quantum mechanical calculations to gain insights into the ORRs on the M-N4 macrocyclic catalysts at an electronic and atomistic scale.