target="_blank" rel="nofollow" href="#ulink_c457d3a0-43b0-5cf9-b82d-ab612d17206a">Figure 2.5 In-situ TEM experiments. (a) Schematics of structural evolution of bismuth to Na3Bi during electrochemical sodiation. Purple, Bi; Yellow, Na. (b) High-resolution TEM image of a pristine bismuth flake. (c)–(e) Time-lapse TEM images of sodiation in bismuth. Source: Reproduced with permission from Zhou et al. [77]. Copyright 2019, WILEY-VCH Verlag GMbH& Co. KGaA, Weinheim.
Apart from the TEM analysis on batteries, gas-phase reactions have also attracted a lot of attention. The degradation study during catalysis – Ostwald ripening – is a good example. Helveg et al. studied the shrinking behavior of Pt nanoparticles with amorphous Al2O3 as a support under O2 and N2 atmosphere [87]. As shown in Figure 2.6, by calculating the size change of a large amount of individual nanoparticles, they found that the larger particles grew larger while the smaller ones dissolved and finally disappeared, which undergoes the Ostwald ripening process. A similar phenomenon has also been observed in other catalysis systems, such as the system of Fe nanoparticles that worked for carbon nanotube growth [88]. Besides, photocatalysts and optical materials were also investigated under a continuous but less extreme irradiation condition. Crozier et al. observed the surface changes of TiO2 particles in ETEM [89]. The initial surface of the particles showed a good crystallinity. After treatment under H2O environment for one hour, a disordered layer appeared, which became more obvious after seven hours of light illumination. Based on the X-ray photoelectron spectroscopy (XPS) results, Ti3+ species can be found in the amorphous surface layer, indicating that the water-splitting was associated with the reduction of TiO2 during photocatalysis. Another example using the gas-phase ETEM is the CO oxidation with the metal/metal oxides as catalysts. Two typical phenomena occurred on the surface of these catalysts. One is the formation of a sublayer of oxide underneath the outer most layer of the metal nanoparticles because of the diffusion of oxygen into the metal [90]. The other is the thermodynamicallly driven surface reconstruction of nanoparticles because of the facet-preferential adsorption of CO molecules during catalysis [91]. Taking the Au/CeO2 system as an example [92], the absorbed CO molecules were bound to the on-top sites of Au atoms in a close-packed hexagonal orientation, leading to the surface restructuration. Because of this tensile bonding configuration between the surface layer and the sublayer, the surface reconstructed Au nanoparticles would absorb more CO molecules than the original nanoparticles. However, the challenge for gas-phase introduced TEM is the image resolution. Especially when the electron beams pass through the sample area and interact with gas, a large percentage of electrons would be scattered, resulting in the reduction of image resolution [93, 94]. The other challenges in observing the gas-phase chemical reaction in ETEM is that the ionized gases might generate the reactive gas species, giving rise to the unwanted chemical reaction at the material surface which therefore disturbs the reaction behavior.
Meanwhile, the dose rate in TEM is much higher than that of other external radiation sources [95, 96], making it easier to transfer energy to the specimens under the irradiation of 200–300 keV. Under such excitation, many ionic species would be generated to help initialize many side chemical reactions and produce various other species. When a solution of metal salt or precursor is under irradiation in TEM, the intermediated species will be produced during this process, e.g. hydrated electrons can act as a reducing agent to reduce metal cations into metal nuclei [97]. The metal nuclei are controllable within the TEM, as the dose rate can determine the speed to produce hydrated electrons and influence the reduction rate of metal cations, which then generates various morphologies either through single-atom deposition or through cluster-based oriented attachment. The observation under TEM can then provide a solid proof of the nucleation 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.
