Materials based on nanoparticles are often used in various applications, for example, catalysis, energy production, and drug delivery.
The size, shape, and composition of nanoparticles have a huge impact on their performance. The creation of manageable manufacturing systems is essential to the commercialization of the most recent nanomaterials. Strong microscopes, for example, the transmission electron microscope (TEM) should be utilized for the characterization of nanoparticles.
Nanoscale features can be resolved by the TEM using electron diffraction and x-ray (EDS) analysis, it can extract chemical and structural information, which is critical in understanding the features of nanoparticles. Most nanoparticles are synthesized in liquid. The high vacuum environment inside the TEM means that it is more challenging to image samples that are stored in liquid.
To precisely understand the natural behavior and dynamic growth processes of nanoparticles in liquid, the use of the latest in situ techniques, such as the Protochips Poseidon liquid cell, is essential. Professor Wen-Wei Wu, a Distinguished Professor of Materials Science and Engineering at the National Chiao Tung University in Taiwan, focuses his research on the growth and characterization of nanomaterials.
The Poseidon liquid cell was used by his research group in order to conceptualize the growth dynamics of Au-Cu2O core-shell nanoparticles and to characterize the resulting structure. The group produced various Cu2O shapes in a controlled fashion, and watched the growth process materialize in real-time by tuning the surface functionalization of the original Au nanoparticle.
Revealing the Dynamics of Core-Shell Nanoparticle Synthesis
Core shell nanoparticles have been utilized in many applications, such as sensors and solar cells. Similar to most nanoparticles, the surface, or shell, structure and composition strongly affect performance. In situ liquid TEM alone can establish which factors influence how the shell is created at the nanoscale and in real-time.
It is also the only method to provide guidance for controlled synthesis, even though many methods are used before and after reaction to characterize the particles in liquid. The mixture of three solutions in the experiment is how the nanoparticles were grown. Firstly, a solution of gold nanoparticles was flowed into the Poseidon cell, following a mixture of CuSO4/NaOH/H2O, and finally ascorbic acid (a reducing agent) was used.
The researchers established that two types of core-shell structures were created: cubic and multifaceted. Two factors informed the type of structure that was created: the uniformity of citrate ligands on the seed Au nanoparticles and the dispersion of Au nanoparticles. Dynamic and high resolution imaging over multiple seconds allowed for the quantitative determination of the shell growth rate in time (Figure 1) at 210 nm2/s.
Figure 1. The growth of Au-Cu2O particles in liquid as a function of time. The yellow arrows indicate steric hindrance effects, and the red arrows indicate gold nanoparticles that lacked ligands.
Figure 2. The impact of growth conditions on the structure of Au-Cu2O particles
Further ex-situ investigations of the Au/Cu2O interface demonstrated that for the structure that was multi-faceted, the growth of Cu2O on Au is epitaxial. The core and the shell basically have the same crystalline orientation as presented in Figure 3(a)-(e) underneath.
For the cubic core-shell structure, the core and the shell have different crystalline orientations. The shell grows separately from the gold core, and its facets are made on low-index planes to decrease surface energy as shown in Figure 3(f)-(j) below.
Figure 3. Core/shell interface structure for multi-faceted and cubic Au-Cu2O particles
Dynamic and high-resolution imaging using the Poseidon Select liquid cell allowed for the fast characterization of the phenomenon of synthesis for Au/Cu2O core-shell nanoparticles in the TEM.
The experimental parameters that were responsible for the rate of growth and the structure of the particles were found, and a model was made to outline the growth. Important insight into the mechanisms of growth and which factors affected the ensuing structure was obtained by the researchers at NCTU.
This helped them to design and create materials with an extensively enhanced performance, working faster than ever before. Liquid cell TEM is a successful tool for the fast and accurate characterization of the features of nanomaterials, whether they are to be created in-house or used within a system. The accurate measurement of the composition, size, and shape of nanomaterials in their native environment has no replacement.
References and Further Reading
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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