Читать книгу Amorphous Nanomaterials - Lin Guo - Страница 29

The recent progress of Cs-TEM provides a solid foundation to investigate the structures and compositions at the atomic level or in a complex environment with gas or liquid. This can then be integrated with energy-dispersive X-ray spectroscopy (EDS) and EELS techniques to observe the structure evolution of the materials and to investigate the mechanism of the composition changes. Notably, one achievement in the last few decades is the application of in situ TEM that involves various stimuli to nanomaterials with high-resolution imaging and spectroscopy. These stimuli may include heat, stress, electrical biasing, and ultrashort photon pulses to the materials. However, the high vacuum of TEM within the column to protect the electron gun and to avoid the electron scattering by gases and liquids makes it not compatible with gaseous or liquid environments. The development of environmental transmission electron microscopy (ETEM) has offered great chances to study the dynamic changes in materials with ultrahigh resolution in complex gaseous or liquid environments.

Since the inception of in situ TEM techniques for battery research in 2010 [69], continuous efforts have been made to give a better understanding of material dynamics during electrochemical reactions. In the battery analysis by using TEM, the scientific challenges include how cathodes experience thermal degradation with compromised battery safety, what is the charge storage mechanism for electrodes with different elements, and how Li dendrites evolve during Li intercalation. As a stable electrode, intercalation should work without obvious structural degradation during ion insertion/extraction. To evaluate the structural changes with the high spatial and temporal resolution, the advantages of in situ TEM with aberration corrector are obvious. One typical example is the case of MnO₂ cathode, which possesses a one-dimensional tunneled structure. An asynchronous lattice expansion was found to be driven by a sequential Jahn–Teller distortion of [MnO₆] octahedral [70]. The dynamic observation of structure degradation demonstrates the powerful capability of in situ TEM in studying the localized reaction mechanisms. Inspired by these observations, the battery performance can be improved by structure modification, such as minimizing the particle size or tracing the dopant component to reduce the structural degradation. Interestingly, nanosized transitional metal oxides, sulfides, and fluorides (MX) showed that lithium storage through a conversion reaction between metal oxide and LiX is reversible, but the intermediate steps involve multiphase reactions, resulting in a low Coulombic efficiency (CE), a large overpotential, and a fast capacity fading upon cycling [71]. In addition, even for materials with the same metal cations, there is still a difference in conversion kinetics. Taking Fe as an example, an intermediate phase can be confirmed as LixFe₃O₄ before the conversion of nanosized Fe₃O₄ to Fe [72], while no similar phenomena can be observed with nanosized Fe₂O₃ [73]. In addition, for iron sulfide as a sodium battery electrode, the coexistence of Fe₃S₄, FeS₂, and FeS can be observed by HRTEM, and those quantum-sized FeSx ensured a synergistic and highly reversible conversion reaction, leading to a superior cyclability and rate capability [74–76]. Recently, we also investigated the charge storage mechanism of bismuth as a promising anode material for the state of the art rechargeable batteries [77]. In our work, a 2D structure of few-layer bismuthene was designed (as shown in Figure 2.5), which undergoes a two-step mechanism of ion intercalation, followed by a reversible crystalline phase evolution. This structure can alleviate the stress accumulated along the critical z-axis and allow sodium ions to rapidly diffuse due to a shorter diffusion distance, which is very helpful to develop high-performance batteries. Meanwhile, ex-situ TEM can also be a powerful tool to study the structural evolution of an amorphous electrode transition. For example, we prepared a Prussian blue analog (PBA) Co₃[Co(CN)₆]₂ as nonaqueous potassium-ion anode material [78]. The HRTEM image revealed that tiny crystallites of metallic Co of 5 nm in size are dispersed in the amorphous matrix. This observation is quite similar to the lithiation behavior of metal oxides, in which metal nanoparticles are found in the Li₂O matrix [71]. The metallic Co formed during the lithiation process may enhance the electronic conductivity. The amorphous matrix might contribute to the good cycling performance because the isotropic nature of the amorphous materials can tolerate homogeneous volume changes and accommodate volume strain. Besides, we have also carried out research with amorphous FeVO₄, which is a bimetallic element oxide for K-ion battery [79]. Local structural information of amorphous FeVO₄ after potassiation can be obtained based on the HRTEM images, which displayed tiny crystallites of VO₂, V₂O₃, and FeO with sizes below 5 nm. These tiny crystallites are surrounded by amorphous materials with solid electrode interface (SEI) or other potassiated products. The conversion was reversible because those crystalline phases partially recover to amorphous FeVO₄ after subsequent depotassiation. The particle size for the crystalline counterpart is much larger, showing lower potassiation/depotassiation capacities. This observation suggested that particle size plays a role in determining the electrochemical performance of amorphous materials. Another important issue is the observation of SEI and Li (de)plating on the electrode/electrolyte interface with high spatial resolution. By using in-situ TEM, Li dendrite growth and SEI formation or decomposition in a LiPF₆/ethylene carbonate (EC)/diethyl carbonate (DEC) electrolyte can be recorded at nanoscale resolution [80]. The SEI formation is not uniform but in the shape of dendrites. The growth kinetic of SEI can further be a valuable reference for understanding battery failure. Moreover, the beam irradiation with hundreds of keVs can cause side reactions in targeted materials and affect the imaging process, including atomic displacement, e-beam sputtering due to the elastic scattering, and heating or contamination damage due to inelastic scattering [81]. For a solid-state open cell, the main concern is the stability of Li₂O under the electron beam and the subsequent effect on the battery electrochemical performance. An effective solution to reduce the electron dosage to a safe value (approximately 1 A cm⁻²) is to suppress the chemical lithiation [82]. Meanwhile, in situ liquid cell TEM is subject to more side reactions such as the electrolyte breakdown [83] and the nanoparticles’ precipitation/dissolution [84]. The LiPF₆-based electrolyte has proven to be stable as the formation of fewer and smaller nanoparticles, and the SEI nucleation and growth can be captured on the Li deposit [85]. In addition, similar cyclic voltammetry (CV) curves were observed under and without electron beam irradiation, which demonstrates the suitability of applying liquid TEM in a real battery system [86].

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.

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.