Paper link:
/articles/s 4 1467-020-20084-5
Substrate-supported nanoparticles have attracted great interest in industry because of their wide applications in biomedicine, energy storage and catalysts. Carbon is the most commonly used conductive substrate and is naturally abundant. This material has many unique forms from zero to three dimensions, which can be used to anchor nanoparticles and can be scaled up to form the required structure. So far, many methods have been developed to synthesize carbon-loaded nanoparticles. However, how to obtain nanoparticles with uniform size and dispersion is still a challenge. Perhaps this can be achieved by introducing dispersants or surfactants. However, the side effects of solvent synthesis residues may be problematic. In recent years, high-temperature dry synthesis technology has been successfully used to prepare nanoparticles, including pure metals, multicomponent alloys and even monoatomic. This method does not need additives in the synthesis process, which not only reduces the complexity of synthesis, but also realizes a "clean" synthesis strategy.
Although Joule heating at high temperature is universal and simple, the mechanism of forming nanoparticles with small particle size, good dispersibility and no agglomeration during high temperature processing is still unclear. Some atomic simulations attempt to study the interaction between metal nanoparticles and graphite substrates or defective substrates. However, the influence of high temperature is not considered in the simulation. In this case, this is even more important, because during Joule heating, at such a high temperature, nanoparticles will aggregate, thus reducing their surface energy. However, the stability of nanoparticles on carbon substrate at high temperature is still unknown. Nano-scale in-situ characterization technology, especially in-situ transmission electron microscope (TEM), has demonstrated the ability to monitor the dynamic process of various nano-scale materials with unprecedented high spatial resolution.
In this paper, the researchers used in-situ TEM device with electric bias to simulate the high temperature shock method, and studied the formation and stability of nanoparticles on carbon carriers during this process. It is found that the formation of metal nanoparticles is related to the turbulent graphite with high defects in amorphous carbon synchronous phase transition (T- graphite). Molecular dynamics (MD) simulation shows that the defect T- graphite provides many nucleation sites for the formation of nanoparticles. In addition, nanoparticles are partially embedded and rooted in the edge plane, which leads to higher binding energy on the scaffold. The interaction between nanoparticles and T- graphite matrix enhances the anchoring effect of nanoparticles and provides good thermal stability for nanoparticles.
Figure 1 The formation of platinum nanoparticles on CNF supported by H _ H2PtCl6 _ 6 was observed by Joule electric heating in situ transmission electron microscope.
Fig. 2 Structural evaluation of salt-containing amorphous CNF during Joule heating.
Fig. 3 TEM and EDS analysis of the evolution of original amorphous CNF during Joule heating.
Fig. 4 finite element analysis of Joule heating process of carbon fiber with input power of ~ 40μ W.
Fig. 5 HRTEM and STEM characterization of single-element and multi-element nanoparticles on Joule-heated S-CNFs.
Morphological inference of Pt clusters in the presence of prismatic t graphite.
Fig. 7 in-situ annealing method is used to study the thermal stability of nano-platinum on CNF carrier.
To sum up, the researcher's work provides a direct visualization of nanoparticles on amorphous carbon carrier, and expounds the graphitization details of amorphous carbon exposed to extremely high temperature during electric Joule heating. In-situ TEM study of Joule heating of amorphous carbon fiber shows that the volume of CNFs expands due to the amorphous-crystalline phase transition during Joule heating, which is also related to the formation and stability of metal nanoparticles on carbon support. These findings provide a mechanism for the rapid synthesis of metal nanoparticles on carbon carriers at high temperatures and the source of their stability. (Text: Aquatic)