Amorphous Nanomaterials. Lin Guo
and growth mechanims of nanomaterials. The coalescence of nanomaterials often occurs because of the exposure of the exposed surface with high surface energies. Zhang et al. reported the particle coalescence during the Pt nanoparticle growth under electron irradiation [98]. Through the observation of both morphology and size changes, the particles coalesced with other smaller particles during the growth. Besides, the dynamics of Pt3Fe nanorod undergoes an oriented attachment of nanoparticles [99]. Those nanoparticles can merge together into a relatively spherical shape and form chains through strong interparticle interactions. Moreover, our recent work demonstrated a growth process of a thin amorphous bismuth (Bi) metal nanosheet to unveil the nonclassical mechanism of crystal nucleation and growth from an amorphous metal to a crystal (Figure 2.7) [100]. We observed cluster coalescence-driven crystallization and identified the critical diameter of Bi metal for the amorphous to crystalline phase transformation of Bi metal. In addition, the coalescence mode of nanoparticles can be controlled by the dimension of the smaller particle in the two contacted nanoparticles and by their mutual orientation relationship. This observation with in-situ atomic resolution represents a significant step forward in understanding the nucleation and growth mechanisms at the atomic scale. The study of bismuth showed a nonclassical mechanism mediated by the particle coalescence. The coalescence pathway of two nanoparticles is governed by the dimension of the smaller particle and their orientation, which gives a better understanding of dynamic process of the phase transformation and nucleation.
Figure 2.6 Schematic view of placement of Pt/Al2O3 catalyst in the TEM. (a)–(e) TEM images of a Pt/Al2O3 catalyst (air pressure: 10 mbar; temperature: 650 °C). (f)–(j) Size distributions of Pt nanoparticles. Source: Reproduced with permission from Simonsen et al. [87]. Copyright 2010, American Chemical Society.
After reviewing the use of in situ or ex situ TEM in studying battery electrode, under gas phase or liquid phase with a tunable dose to study the mechanism of certain reactions or the nucleation mechanism of tiny crystals on 2D amorphous nanosheet, it is necessary to discuss several key challenges for further improvement of such a powerful technology. One important task is to push toward high resolutions because the liquid or gas phase or other stimuli along the beam path would largely decrease the accuracy of imaging or spectroscopy data. The key solution for this challenge is to reduce the inelastic electron scattering and guarantee atomic resolution imaging. In this case, an ultrathin graphene membrane with only one carbon atom thickness stands out as a good specimen support for TEM. Alivisatos et al. demonstrated laminated graphene to study the growth of colloid Pt nanoparticles [101]. The much-reduced thickness for both the window material and the specimen can greatly reduce the scattering from the electron beam, leading to a big improvement in resolution. Another challenge is to integrate the in situ TEM with other analytical techniques, for instance, the combination of STEM and EELS for structural and chemical investigations at the nanoscale. Muller et al. reported an integration of Cs-corrected STEM and EELS spectra to investigate a solution-based catalysis [102]. For new battery studies, it is quite challenging. For example, the observation of Li–O2 batteries in-situ is difficult to realize because the introduction of O2 is impossible in the TEM chamber. This question can be simplified by flowing an O2-rich electrolyte into the liquid cell in TEM [103]. The realization of in situ observation of Li–O2 batteries would then open up a new avenue for exploring the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics in batteries.
Figure 2.7 Nanoparticle-mediated crystal nucleation and growth in amorphous Bi to crystal-phase transformation. (a)–(d) HRTEM images showing the nanoparticle coalescence-mediated nucleation of nanocrystal. (e) and (f) HRTEM images showing the nanoparticle coalescence-induced growth of nanocrystal and the formation of grain boundaries. Inset images are the corresponding FFT patterns. The electron dose is 18000 eA−2 s−1. Source: Reproduced with permission from Li et al. [100]. Copyright 2018, WILEY-VCH Verlag GMbH& Co. KGaA, Weinheim.
Overall, in-situ and ex-situ study with spherical corrected TEM has shown its potential in direct visualization and spectroscopy of electrochemical processes at the atomic level. It is no doubt to foresee that in situ TEM would hold a promising future for the nanoscale electrochemistry.
2.1.5 Summary and Outlook
The progress of Cs-TEM has fulfilled the old dream of materials science: a direct link between atomic-level structural information and macroscopic properties. Indeed, people are now able to see the complexities of structure and chemistry at the atomic scale never before, enabling a better understanding of reaction and transformation pathways that fabricated desirable materials and creating new devices with enhanced properties. The use of aberration-corrected TEM is capable of imaging and analyzing materials at the sub-nano resolution in an easily managed way. Moreover, the acquisition of EELS by Cs-TEM can provide distinguishable information about bonding differences between dopant species, which can work with atomic imaging to carry out elemental and chemical analyses of site occupancy. This further paves the way for the study toward physical chemistry at the sub-nanolevel. In addition, based on in-situ TEM equipped with spherical aberration corrector, researchers are good at capturing localized information and inhomogeneity, which opens up the ways for investigating defects in the crystal lattice, charge transport, and phase boundary migration kinetics.
For the future study of Cs-TEM, several improvements are necessary. One is the irreversible structural changes especially at interfaces and surfaces when exposing to the irradiation of highly intense electron beam at higher magnification. This may possibly be alleviated by operating the microscope at a lower voltage or carried out experiments with cryo-TEM. The second is the long-term stability of the aberration corrector; this would require further improvement in corrector hardware to allow aberration control over probe size for longer periods. The third is the sample preparation, which has become more demanding, aiming to eliminate the surface oxide or contamination layers to avoid the background noise to the image signal. For EELS, the aim is to overcome the dechanneling, delocalization, and absorption in electron scattering of each atomic column. In addition, for in situ TEM, more effort should be devoted to high-speed computers for real-time recording to obtain aberration parameters, and then combining faster data acquisition with STEM, which would then open up new chances for three-dimensional tomographic chemical imaging and real-time imaging of STEM-based environmental microscopy in different atmospheres or liquid cells.
2.2 X-ray Absorption Fine Structure Spectrum
2.2.1 Introduction
X-ray absorption fine structure spectroscopy (XAFS) is a unique tool for studying the local structure around the selected elements that are contained within a material, at the atomic and molecular scale. Owing to its element-specific and short-range nature, the core-level XAFS is now routinely used to elucidate the local structural, vibrational, and other physical properties of complex, aperiodic materials. The XAFS encompasses both the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge spectra (XANES). The XANES refers to the structure