Advanced Transmission Electron Microscopy Studies of Complex Material Systems

This cumulative thesis explores the application of advanced transmission electron microscopy (TEM) techniques to investigate the structure, morphology and local chemical environments which govern the performance across a range of complex material systems. This work is motivated by the fact that many material functional properties are not only determined by the composition, but also the the nanoscale structure, chemical gradients, interfacial chemistry and structural reorganization across the length scales even up to the atomic level. These processes are often not fully accessible through bulk-averaged material characterization. The key question addressed in this thesis, therefore, is how scale-bridging electron microscopy and spectroscopy can reveal the local structural and chemical features which eventually control the macroscopic material function. To address this question, three challenging classes of materials with systematically evolving structure and chemistry are examined: I) gas sensing (porous carbon structures), II) energy conversion (transition-metal-boride-based electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)) and III) electrochemical energy storage (modified 𝛿-V2O5 host structure in Li-ion batteries and electrolyte-anode interface reactions in Li-metal batteries). Across these areas of application, an emerging common aspect is how the performance of these materials is controlled by the nanoscale structure and interfaces rather than their average bulk properties.
I) In gas sensing, carbon-rich organic molecular precursors are converted into open porous architectures through localized laser heating. The process has emerged as a versatile and scalable pathway for the production of functional materials for applications in gas sensing, catalysis and even energy storage. Through careful selection of the precursor materials and control of the conversion process, both the microstructure and chemical functionality of the resultant carbon networks are specifically tailored to produce a highly sensitive and selective gas sensing device. In that study, a one-step laser patterning technique is utilized to pyrolyze a solution-processed, nitrogen-containing organic ink into a highly porous and flexible architecture for selective CO2 sensing. The depth-dependent attenuation of the laser energy during processing leads to the formation of a vertical heterostructure across the sensor, comprising a highly graphitized, electrically conductive surface layer, followed by a narrow separation layer, a transition region, and an amorphous nitrogen-rich chemically active sensing layer at the bottom. Comprehensive scale-bridging electron microscopy analyses unraveled the emergence of this hierarchical architecture and how it controls the performance of the sensor. Correlative TEM and scanning transmission electron microscopy (STEM) – STEM electron energy-loss spectroscopy (STEM–EELS) analyses showed a clear structural, and chemical variation across the whole device, while multivariate statistical analysis allowed for the disentanglement of the complex mixture containing crystalline regions as well as the nitrogen-containing phases. Four-dimensional STEM (4D-STEM) was further used to provide spatially resolved information on the distribution, orientation and degree of alignment of the graphitic domains relative to the pore wall network. These analyses explained the link between the observed macroscopic electrical and sensing response of the device and the laser-induced processing gradients.
II) The development of efficient and durable electrocatalysts from earth-abundant elements is a key challenge in the sustainable production of hydrogen and oxygen in electrochemical energy conversion systems. Two boride-derived transition-metal catalyst systems are jointly developed to demonstrate how the deliberate engineering of nanoscale heterogeneity and controlled chemical reconstruction are exploited for the optimization of electrocatalytic properties. For efficient HER, a self-supported phosphite-modified cobalt boride catalyst (Co𝑥B-[0.2]P–O) is synthesized. It is found to comprise a robust seed layer upon which boron-depleted nanosheets grow along active mixed amorphous-crystalline domains (heterostructure) on the surface. Electron microscopy analyses reveal that this architecture is not a trivial coating system, but a complex interconnected volume with crystalline nanodomains embedded in a disordered matrix with spatially varying oxidation states and chemical composition. This structural hierarchy offers efficient charge transport, rapid transport of ions and gases while also providing high accessibility to the catalytically active sites. Simultaneously, they preserve the mechanical integrity of the catalyst during operation. On the other hand, for OER, chromium is introduced into an FeNi-boride structure which critically alters the catalyst evolution under the electrochemical operating conditions. Nanoscale imaging and spectroscopy provided insights to the structural reorganization of the parent boride lattice followed by partial chromium leaching during the activation process. This triggers a controlled surface reconstruction into an extended, oxyhydroxide-like nanosheets. This self-reorganization process precedes the redistribution of metal, boron, as well as, oxygen species. This chemically-driven transformation is invaluable to the stability and high activity of the catalytic sites.
III) The performance of electrochemical energy storage systems and their durability is strongly governed by the diffusivity of ions within the restrictive regions of the host structures. Moreover chemical reactions and transformations at electrode-electrolyte interfaces also play a critical role. Both aspects are tackled in this thesis by considering two complementary case studies: one which shows the role of geometric confinement and another focused on the interfacial layer chemistry control even up to the nanoscale. In one study, molecularly pillared 𝛿-V2O5 structures with whisker morphology were investigated and electron microscopy down to the atomic scale showed that the insertion of organic spacer molecules (alkyldiamine pillars) enabled the precise and systematic tuning of the inter-layer spacing while preserving the in-plane lattice structure. Such controlled geometric constraints are important as they determine the efficiency of Li+ intercalation and storage. The final study focused on a task that demonstrates the direction and outlook of comprehensive electron microscopy analyses of energy devices focusing on lithium metal–electrolyte interface reactions investigated using a combination of cryo–STEM imaging and spectroscopy. These revealed the stark differences between the degradation layers formed by carbonate-based and ionic-liquid-based electrolytes. Carbonate-based electrolytes form thick, porous heterogeneous layers while the ionic liquids resulted in a more compact interface. Taken together, these studies demonstrate that advanced transmission electron microscopy provides an invaluable analytical framework which links the material structure and chemistry up to the nanoscale and the macroscopic function. The thesis therefore ultimately establishes scale-bridging TEM analysis as an analytical framework for understanding complex functional materials whose properties mainly emerge at the nanoscale.