Role Of Elastic Strain Research Articles

(Invited) Controlling Metal Exsolution on Oxides By External Drivers – Role of Elastic Strain and Ion Irradiation

Exsolution is an effective approach to fabricating oxide-supported metal nanoparticle catalysts via phase precipitation out of a host oxide. A fundamental understanding and control of the exsolution kinetics are needed to engineer exsolved nanoparticles to obtain higher catalytic activity toward clean energy and fuel conversion. Since oxygen release via oxygen vacancy formation in the host oxide is behind oxide reduction and metal exsolution, we hypothesize that the kinetics of metal exsolution should depend on the kinetics of oxygen release. In this work, we probe the surface exsolution kinetics both experimentally and theoretically using thin-film perovskite oxide model systems, show its relation to the oxygen evolution kinetics. We also tune defect formation and the resulting metal exsolution, by external drivers including elastic strain and ion irradiation. Using both of these external drivers, we couple to the formation of point defects and defect clusters, that serve as nucleation sites for nanoparticle exsolution. As a result, we can controllably tune size, density, composition and position of the exsolved metal nanoparticles.

Role of Elastic Strain on Electrocatalysis of Hydrogen Evolution Reaction

We present a study on the effect of externally applied elastic strain on the catalytic activity of metal films in the context of hydrogen evolution reaction (HER). Thin metal films supported on elastic substrates are uniaxially strained in situ in compression and tension while they participate in the HER and their catalytic activity is measured through shifts in the cyclic voltammograms. We show that elastic strain tunes the catalytic activity in a controlled and predictable way; for each metal considered here, compressive and tensile strains have the opposite effect on the catalytic activity; also, the changes in the catalytic activity scale with the strain magnitude within the range of strain values accessed in our experiments. The experimental results show that Pt and Ni films show increased HER under compressive strain; while Cu's HER activity is retarded by compressive strain. The opposite was observed under tensile strain. The experimental observations are understood by considering the influence of elastic strain on hydrogen binding energy, which has been calculated through density functional theory (DFT). Compressive strain increases the hydrogen binding on Ni, shifting it towards the volcano peak, while tensile strain has the opposite effect. However, the same strains have the opposite effect on Cu since it is located on the other side of the volcano peak. By isolating elastic strain from the ligand effect, this study provides a better understanding of the processes that control electrocatalytic activity towards HER and can guide design of strained core-shell nano-particle catalysts.

Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt

The effect of elastic strain on catalytic activity of platinum (Pt) toward oxygen reduction reaction (ORR) is investigated through dealloyed Pt–Cu thin films; stress evolution in the dealloyed layer and the mass of the Cu removed are measured in real-time during electrochemical dealloying of (111)-textured thin-film PtCu (1:1, atomic ratio) electrodes. In situ stress measurements are made using the cantilever-deflection method, and nanogravimetric measurements are made using an electrochemical quartz crystal nanobalance. Upon dealloying via successive voltammetric sweeps between −0.05 and 1.15 V vs standard hydrogen electrode, compressive stress develops in the dealloyed Pt layer at the surface of thin-film PtCu electrodes. The dealloyed films also exhibit enhanced catalytic activity toward ORR compared with polycrystalline Pt. In situ nanogravimetric measurements reveal that the mass of dealloyed Cu is approximately 210 ± 46 ng/cm2, which corresponds to a dealloyed layer thickness of 1.2 ± 0.3 monolayers or 0.16 ± 0.04 nm. The average biaxial stress in the dealloyed layer is estimated to be 4.95 ± 1.3 GPa, which corresponds to an elastic strain of 1.47% ± 0.4%. In addition, density functional theory calculations have been carried out on biaxially strained Pt(111) surface to characterize the effect of strain on its ORR activity; the predicted shift in the limiting potentials due to elastic strain is found to be in good agreement with the experimental shift in the cyclic voltammograms for the dealloyed PtCu thin film electrodes.

The role of elastic strains at non-ferroelastic displacive phase transitions

The role of elastic strains at structural phase transitions is illustrated within the Landau theory and its first-order corrections due to critical fluctuations and defects are described. The Landau theory is sufficient to demonstrate the impossibility of bulk nucleation in a supercooled symmetrical phase and the absence of heterophase fluctuations in solids. The critical fluctuations are known to convert a second-order transition in an Ising-like system in a solid to a first-order one. Close to the mean-field tricritical point the effect can be described, for displacive systems, within a first-order perturbation theory and takes place for the Heisenberg systems as well. The influence of defects on these transitions is mediated essentially by the elastic strains. Defects smear the transition. For the “random local field” defects and an incommensurate (Heisenberg-like) transition this effect is so strong that first-order perturbation theory leads to a divergence.

Polymorphic transformations between olivine, wadsleyite and ringwoodite: mechanisms of intracrystalline nucleation and the role of elastic strain

AbstractKinetic models and rate equations for polymorphic reconstructive phase transformations in polycrystalline aggregates are usually based on the assumptions that (a) the product phase nucleates on grain boundaries in the reactant phase and (b) growth rates of the product phase remain constant with time at fixed P-T. Recent observations of experimentally-induced transformations between (Mg,Fe)2SiO4 olivine (α) and its high pressure polymorphs, wadsleyite (β) and ringwoodite (γ), demonstrate that both these assumptions can be invalid, thus complicating the extrapolation of experimental kinetic data. Incoherent grain boundary nucleation appears to have dominated in most previous experimental studies of the α–β–γ transformations because of the use of starting materials with small (<10–20 µm) grain sizes. In contrast, when large (0.6 mm) olivine single crystals are reacted, intracrystalline nucleation of both β and γ becomes the dominant mechanism, particularly when the P-T conditions significantly overstep the equilibrium boundary. At pressures of 18–20 GPa intracrystalline nucleation involves (i) the formation of stacking faults in the olivine, (ii) coherent nucleation of γ-lamellae on these faults and (iii) nucleation of β on γ. In other experiments, intracrystalline nucleation is also observed during the β-γ transformation. In this case coherent nucleation of γ appears to occur at the intersections of dislocations with (010) stacking faults in β, which suggests that the nucleation rate is stress dependent. Reaction rims of β/γ form at the margins of the olivine single crystals by grain boundary nucleation. Measurements of growth distance as a function of time indicate that the growth rate of these rims decreases towards zero as transformation progresses. The growth rate slows because of the decrease in the magnitude of the Gibbs free energy (stored elastic strain energy) that develops as a consequence of the large volume change of transformation. On a longer time scale, growth kinetics may be controlled by viscoelastic relaxation.

Atomic Level Observation of Early Events in Molecular Intercalation: Preintercalation Host-Layer “Loosening” and the Role of Elastic Strain

Dynamic high-resolution transmission electron microscopy (DHRTEM) has been used to follow the early events in ammonia/2H-TaS2 intercalation. This molecular intercalation process is characterized by two intriguing transient partial gallery expansions of 0.6 and 1.4 Å, prior to and during intercalation, respectively. These expansions do not persist throughout the intercalation process, eventually yielding to full gallery expansions of 3.0 Å. The former partial expansion was observed prior to intercalation during preintercalation ammonia adsorption on the basal planes and can be associated with a charge-transfer-induced host-layer “loosening” of the outermost guest gallery via octahedral-to-trigonal prismatic restacking of the adjacent host layers. The latter expansion is associated with the formation of a near planar ammonia species, which reduces the elastic strain energy for molecular intercalation.

Role of elastic strain and relaxation on the molecular-beam epitaxial growth of III-V alloy pseudomorphic layers

Elastic strain and relaxation are shown to play a central role in the local equilibrium properties of III-V alloys during molecular-beam epitaxy. An epitaxial alloy statistical model which accounts for these effects is proposed. The variation with temperature of the In-incorporation coefficient in (A1,In)As/InP and (A1,In)As/GaAs, measured from reflection high-energy electron-diffraction intensity oscillations, is discussed in the framework of the proposed model.

The role of elastic strains in the formation of stacks of hydride precipitates in zirconium alloys

The role of elastic strains in the formation of stacks of hydride precipitates in zirconium alloys