Structural engineering of nanomaterials : from optical properties to energy conversions

Abstract

Multiscale structural engineering is pivotal in optimizing the properties of nanomaterials and improving their performance in various technological applications. This thesis explores the structural design of nanomaterials across microscopic, nanoscopic, and atomic scales, focusing on three key aspects: morphology transformation, heterostructure designs, and atomic-scale modification. The developed strategies address the key challenges in nanomaterials' optical properties and energy conversion performance. At the microscopic scale, I demonstrate the transformation of two-dimensional semiconducting transition metal dichalcogenides (TMDs) nanoflakes into three-dimensional nanoscrolls via an acetone-induced scrolling process. Comprehensive microscopy and spectroscopy characterizations show that the nanoscrolls exhibit expanded interlayer spacing, likely due to the trapping of acetone molecules. Low-frequency Raman scattering experiments and theoretical calculations confirm that this expansion weakens the interlayer van der Waals interactions, thus preserving the direct band gap feature in multilayer structures. Consequently, these nanoscrolls exhibit photoluminescence signals up to 11 times more intense than the starting monolayer, showcasing their potential for optoelectronic applications such as light-emitting and light-harvesting devices. At the nanoscale, I investigate heterostructure design to address the poor charge utilization efficiency of graphitic carbon nitride (CN) in photocatalytic water reduction. Targeting CN’s poor carrier mobility, I combine the optimized exfoliated CN (ECN) with bismuth oxyselenide (BOS), a material with high electron mobility, via an electrostatically driven self-assembly process. Multiple time-resolved characterizations reveal that the electric field at the heterointerface facilitates the separation of photogenerated charges and prolongs their lifetime. The optimized heterostructure photocatalyst achieves a 75% increase in apparent quantum efficiency and a 41 % improvement in photocatalytic hydrogen generation rate compared to ECN. These findings highlight heterostructure engineering as an effective strategy for improving photocatalytic efficiency in solar-fuel production. At the atomic scale, I study the structural modification of manganese oxybromide (MOB) with Ru single atoms (SA) to elucidate the structure-mechanism relationship of electrocatalysts in acidic water oxidation. Using in situ Raman spectroscopy, I unveil that the inherent, bias-induced surface reconstruction of MOB toward γ-MnO2 is intensified by the incorporated Ru SA, which lower the transformation onset potential by ~100 mV. Interestingly, by combining various in situ and operando characterizations and computational analysis, I show that the reconstructed surface of the Ru SA-modified MOB catalyst drives water oxidation predominantly via the adsorbate evolution mechanism, instead of the lattice oxygen mechanism observed for pristine MOB. This optimized catalyst requires a ~90 mV lower overpotential to reach 10 mA cm-2 in water oxidation than pristine MOB. It also operates with negligible activity loss for > 1400 h, owing to the mechanistic regulation. The strategy of utilizing atomic structure modifications to control the electrochemical behaviors of catalysts offers a promising pathway toward realizing better electrocatalyst designs for sustainable hydrogen production. Collectively, the studies presented in this thesis establish structure-property relationships of nanomaterials across multiple scales, demonstrating how structural modifications can optimize their optical properties and catalytic performance. The insights pave the way for the rational design of next-generation light-emitting and renewable energy technologies.