In situ transmission electron microscopy
In situ transmission electron microscopy(in situ TEM) is a technique that has recently allowed researchers to observe chemical reactions, interfacial phenomena, and mass transport in real time, thereby not only understanding the physical phenomena of materials but also analyzing how materials react in conditions under which catalysts, sensors, solar cells, semiconductor devices, and more operate.
This technique is used to observe the reactions of materials/devices to specific stimuli(such as temperature, gas, solution, voltage, mechanical pressure, etc.) in the range of resolutions from atomic to microscale in real time. Real-time experiments are recorded with fast temporal resolution(more than 30 frames per second), so in situ TEM facilitates the observation of phase transition and nucleation.
For example, it can quantify different characteristics including the growth and phase transition rates of individual nanoparticles and their effects on electrical conductivity and mechanical strength. In particular, the contribution of in situ TEM to the nucleation and growth of catalysts has been noticeable in recent years. In this article, some studies of catalysts in vacuum/gas and liquid environments will be introduced as follows.
[Figure1] Examples where in situ TEM is used; Source: MRS Bulletin 40,12(2015)
A study of the spontaneous generation of metal nanocatalyst particles in vacuum and gas environments using in situ TEM
The exsolution* phenomenon of metal catalyst particles with oxide supports has been considered critical in the application of high-temperature catalytic reactions (e.g., gas sensors, etc.) and renewable energy (e.g., gas reforming, fuel cells, etc.) as the catalyst particles are embedded on the surface of the support, so this phenomenon does not cause coarsening** at high temperatures. However, the existing studies have largely focused on the post-mortem analysis of eluted samples, which thus failed to understand the principle of controlling particle sizes, density, and distribution. As a result, it was impossible to maximize the activity and durability of catalysts.
However, the in situ TEM technique, which enables an atomic resolution and fast imaging (30 frames per second), allowed researchers to successfully identify and model the growth mechanism of cobalt (Co) by figuring out the kinetics of its exsolution on the polycrystalline SrTi0.75Co0.25O3-δ substrate. This study revealed that particle sizes could be controlled only by temperature. Specifically, growing temperature leads to an increase in the particle size and the total volume of the metal particles evolved on the surface by diffusion of the metal elements from the inside of the substrate.
In addition, all the exsolved metal particles were generated only at the grain boundary,*** lowering the elution temperature to 500°C, and the density and distribution of the particles could be optimized by controlling the grain size and distribution. Moreover, a thermodynamic model was developed to find out the rate-limiting step**** of particle growth and quantify the generation of cobalt vacancy (the location where an atom is missing from the crystal lattice), the exsolution enthalpy*****, and the activation energy****** for particle growth. Furthermore, the quantitative data obtained in real time and the experiments of the carbon monoxide (CO) reduction showed that the active site of the catalyst formed on the oxide substrate was the boundary between the metal and the oxide substrate.
* Exsolution: A phenomenon in which a metal element migrates to the surface of support doped with the element to form nanoparticles in a reducing atmosphere
** Coarsening: A phenomenon in which the size and density of catalyst particles change instantaneously as atoms move from small-sized nanocatalyst particles with higher chemical potential energy to large-sized nanocatalyst particles with lower chemical potential energy
*** Grain boundary: An area formed when different crystal grains meet, which has high free energy because of mis-oriented arrays of atoms
**** Rate-limiting step: A stage that determines a reaction rate, which is the slowest because of a high energy barrier
***** Enthalpy: Energy that can be derived from a thermodynamic system, which is defined as the sum of the internal energy and energy that can be gained from the volume it takes up
****** Activation energy: The minimum amount of energy required to activate a particular chemical reaction
[Figure2] The exsolution process of Co nanoparticles observed using in situ TEM and its growth kinetic data; Source: J. Am. Chem. Soc. 141, 6690(2019)
A study of the growth of gold nanoparticles in a liquid environment using in situ TEM
Spiky gold nanoparticles have the surface plasmon resonance that can adjust a wide range of light wavelengths from visible to ultraviolet depending on the sharpness of the spike located on the particle surface. This feature has allowed diverse fusion studies to be carried out. In addition, because the size of the gold particles, which increases as the thorns grow, changes light wavelengths, there has been an urgent need for the real-time observation of the overall growth process of gold nanoparticles produced in liquid. To do so, in situ TEM techniques have been developed and utilized to observe nanoparticles in liquid in real time at nanoscale resolutions.
In recent years, a study group has succeeded in transmitting a continuous electron beam into liquid by using bright field imaging,** as well as by creating a growth environment for single gold nanoparticles: circulating water sufficiently in the liquid cell to completely remove air bubbles and controlling the electron beam size and dose rate* and the concentration of HAuCl₄ solution. In this environment, the nucleation mechanism and kinetics of the growing gold nanoparticles were quantified through homogeneous nucleation*** in the liquid solution.
This study revealed that gold particles change from faceted to roughened particles when growing in certain conditions, and this transformation occurs in a wide range of wavelengths (530–1120 nm). In addition, theoretical modeling was applied to the data gained in real time to quantify the concentration of gold atoms on the particle surface that changes over time in conjunction with the morphology of gold particles, the electron beam dose rate, and the density and distribution of particles of liquid concentration.
* Electron beam dose rate: The number of electrons reaching a unit area (e.g., 1 nm2) for a second
** Bright field imaging: One of the diffractive imaging techniques using TEM, which selects transmitted beams with an objective aperture
*** Homogeneous nucleation: An event for creating a material using only the overconcentration of corresponding atoms in the material instead of using surfaces or defects in the material such as vacancy, dislocation, voids, etc.
[Figure3] The growth of gold nanoparticles in a liquid observed using in situ TEM and its kinetic data ; Source: J. Am. Chem. Soc. 141, 12601(2019)
Prof. Bong-Joong Kim | School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST)