Exploring Strategies toward Synthetic Precision Control within Core-Shell Nanowires

CONSPECTUS: Achieving precision and reproducibility in terms of physical structure and chemical composition within arbitrary nanoscale systems remains a "holy grail" challenge for nanochemistry. Because nanomaterials possess fundamentally distinctive size-dependent electronic, optical, and magnetic properties with wide-ranging applicability, the ability to produce homogeneous and monodisperse nanostructures with precise size and shape control, while maintaining a high degree of sample quality, purity, and crystallinity, remains a key synthetic objective. Moreover, it is anticipated that the methodologies developed to address this challenge ought to be reasonably simple, scalable, mild, nontoxic, high-yield, and cost-effective, while minimizing reagent use, reaction steps, byproduct generation, and energy consumption. The focus of this Account revolves around the study of various types of nanoscale one-dimensional core-shell motifs, prepared by our group. These offer a compact structural design, characterized by atom economy, to bring together two chemically distinctive (and potentially sharply contrasting) material systems into contact within the structural context of an extended, anisotropic configuration. Herein, we describe complementary strategies aimed at resolving the aforementioned concerns about precise structure and compositional control through the infusion of careful "quantification" and systematicity into customized, reasonably sustainable nanoscale synthetic protocols, developed by our group. Our multipronged approach involved the application of (a) electrodeposition, (b) electrospinning, (c) a combination of underpotential deposition and galvanic displacement reactions, and (d) microwave-assisted chemistry to diverse core-shell model systems, such as (i) carbon nanotube-SiO2 composites, (ii) SnO2/TiO2 motifs, (iii) ultrathin Pt-monolayer shell-coated alloyed metal core nanowires, and (iv) Cu@TiO2 nanowires, for applications spanning optoelectronics, photocatalysis, electrocatalysis, and thermal CO2 hydrogenation, respectively. In so doing, over the years, we have reported on a number of different characterization tools involving spectroscopy (e.g., extended X-ray absorption fine structure (EXAFS) spectroscopy) and microscopy (e.g., high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM)) for gaining valuable insights into the qualitative and quantitative nature of not only the inner core and outer shell themselves but also their intervening interface. While probing the functional catalytic behavior of a few of these core-shell structures under realistic operando conditions, using dynamic, in situ characterization techniques, we found that local and subtle changes in chemical composition and physical structure often occur during the reaction process itself. As such, nuanced differences in atomic packing, facet exposure, degree of derivatization, defect content, and/or extent of crystallinity can impact upon observed properties with tangible consequences for performance, mechanism, and durability.