2D-Confinement Approach for Catalytic Nanomaterials

Abstract

The distinct anisotropy and electronic properties of ultrathin 2D materials have sparked huge interest in their fundamental electrochemistry and diverse range of applications. Especially, the utilization of 2D electrocatalysts, which take advantage of their maximum mass utilization, expanded surface area, and a multitude of exposed sites, presents an appealing approach to creating a superior and cost-effective electrolyzer and fuel cell. However, a challenge arises from the irreversible stacking of individual nanosheets, which leads to the blockage of active sites, reduced electrolyte diffusion rates, and consequently, poor durability during electrocatalysis. Additionally, this irreversible conversion to a 3D structure occurs not only in the electrochemical field but also in other applications, such as solid state thermal conversion reactions. When a solid 2D material is exposed to high temperatures, it undergoes a transformation, losing its original 2D structure and transitioning into a single, large bulk structure. This transformation hampers the utilization of the unique advantages offered by the 2D structure and restricts expansion of the its range of applications. On the other hand, wet chemical synthesis, a bottom-up approach, can provide high-quality ultrathin 2D materials. Nevertheless, it requires the use of solid support or surfactants to control the material's growth direction anisotropically. The use of such supports causes the blocking of active sites, leading to reduced catalytic activity. To overcome these limitations, we developed a novel 2D confinement approach which was space confinement with layered double hydroxides (LDHs) via an inert silica shell and surface confinement of LDH. Remarkably, this approach allows for the synthesis of various nanomaterials with unique structures, all of which exhibited outstanding performance in electrochemical reaction or high-temperature gas conversion reaction. In this thesis, we introduced the concept of 2D confinement involving LDH and silica encapsulation, focusing on one particular 2D material. We examined the structures of catalysts created using our approach and demonstrated their exceptional performance across diverse applications in various fields. In chapter 2, we describe a method called the lamellar confinement strategy, which allowed us to convert nanosheets into ultrasmall NCs within a closely fitting 2D silica enclosure. This transformation resulted in the conversion of individual Ni(OH)2 nanosheets into a horizontal arrangement of ultrasmall Ni-NCs arrays, resembling a cartridge called Ni-SiCart. The 2D silica casing effectively maintained the flat and uniform configuration of these NCs by limiting the movement of atoms in both horizontal and vertical directions. In an impressive solid-state transformation, the silica enclosure displayed a flexible behavior during repeated cycles of heating and cooling while securely enclosing the Ni-NCs, similar to how eggs are packaged in a carton. This flexibility contributed significantly to the thermal stability of Ni-SiCart. We subsequently demonstrated the potential application of catalyst in the dry reforming of methane (DRM), a process demanding a resilient catalyst system capable of withstanding extreme thermal conditions. Owing to its unique 2D-enclosed and rigid structure that effectively preserves nanoscale features, Ni-SiCart prevented the formation of coke deposits while enduring high temperatures exceeding 700 ºC for extended periods. Consequently, it achieved high conversion rates to syngas during over 100 hours of continuous operation. As an extension of above study, we unexpectedly discovered upon a novel edge- to-edge self-assembly (EE-SA) process while silica encapsulating with 2D hexagonal-shaped single layered NiCo-hydroxide nanosheets. In chapter 3, we introduce a unique process known as EE-SA. In this process, we showcase the assembly of 2D silica nanosheets (2D-SiNSs) into hollow micron-sized shells that resemble soccer balls, and we refer to them as SA-SiMS. What makes this process truly remarkable is that these flat 2D- SiNS units naturally come together in an edge-to-edge manner to create the necessary curved surface, forming complete monolayer hollow shells. All of this occurs without the need for additional organic linkers, surfactants, or complex emulsions. The self-assembled edge-to-edge connection of these sheets and the resulting closed curved structure is both unexpected and unique in 2D material research. The ultra-thin edge regions of the silica nanosheets create an electrostatic bias that initiates irreversible bonding between the sheet edges. Furthermore, the flexibility of the individual 2D-SiNSs, which varies with their silica thickness, is essential for inducing the curvature required for the assembly process to occur and reach completion. Remarkably, SA-SiMS structures, achieved through the EE-SA process, exhibit exceptional mechanical stability even in their dry powder form, enduring exposure to high temperatures and reactive gases without deconstruction. When these SA-SiMS structures incorporate metallic NiCo nanocrystals (NCs), they demonstrate excellent performance in a bed-reactor used for the dry reforming of methane (DRM). They endure temperatures exceeding 700 °C for extended periods of approximately 100 hours while achieving high conversion rates into syngas. The outstanding high-temperature mechanical stability of NiCo@SA-SiMS during DRM plays a crucial role by facilitating access to active sites and enhancing mass transport. This ensures efficient diffusion of reactants and products, surpassing the performance of other assembled structures. Moreover, the thin silica shell provides a confinement effect, effectively preventing the metal NCs from sintering and deactivation during the reaction. Additionally, the macroporous structure of SA-SiMS enhances heat transfer, maintaining a uniform temperature distribution and preventing the formation of hotspots. This improved resistance to mechanical stress and stability at high temperatures ultimately leads to a significantly prolonged operational life of the catalyst. And in the last chapter 4, we present a novel strategy that allows us to finely control the lateral growth of 2D-Pt islands in a solution, enabling us to regulate their coverage on a functional template based on layered double hydroxides (LDHs). This technique results in the formation of a distinct structure called LDH@2D-Pt, where an ultrathin and nanoporous layer of 2D-Pt intimately encloses a NiFe-LDH sheet. This unique LDH@2D-Pt structure is created through a Pt growth process that involves attracting Pt precursors to the 2D surface of LDH. It benefits from a carefully engineered 2D-2D interface with a molecularly-accessible design and the synergistic integration of active sites from both NiFe-LDH and porous 2D- Pt components. These active sites play a crucial role in various reactions, including water adsorption, dissociation, hydrogen adsorption, and desorption, thus promoting a cascade of reactions. Due to these remarkable features and the solution-based fabrication of isolated colloidal nanosheets, LDH@2D-Pt exhibits an exceptionally high mass activity for the hydrogen evolution reaction (HER). In fact, it surpasses the performance of commercial 20% Pt/C catalysts by a factor of up to 6.1 times, setting a new record among all carbon-free electrocatalysts used for HER in alkaline conditions. Furthermore, owing to the precise chemical bonding between LDH and 2D-Pt, LDH@2D-Pt catalysts demonstrate outstanding structural and functional stability during electrocatalysis. They remain stable for several days and can endure 5000 cyclic voltammetry cycles and 50 hours of prolonged electrochemical testing.