Organic/inorganic hybrid nanostructures assembled on functional interfaces for catalytic applications

Abstract

Metal and semiconductor nanostructures have received much attention for energy and sensor applications due to a large active surface area and unique optical and electrical properties. Despite large surface area and unique properties, the nanostructures impose limitations to their practical uses because they tend to aggregate due to the high surface energy, decreasing surface area and denaturing functionalities originated from confined nanoscale. In general, surfactants, which reduce the surface tension at the interface between nanomaterials, are used to prevent aggregation and maintain intrinsic properties of the nanostructures. However, those surfactants reduce active surface area and cause steric hindrance, decreasing surface-related activity of the nanostructures. In this dissertation, I hypothesize that the assembly of inorganic nanostructures on supramolecular templates with tailored functional surfaces, producing organic/inorganic hybrid materials, is an effective means to improve the structural integrity of the nanostructures. In particular, this dissertation aims to investigate the impact of chemical properties of organic template interfaces on the synthesis, immobilization, charge transfer, and architecture of catalytic inorganic nanostructures with structural integrity and sustained activity. First, we report on the application of mussel-inspired adhesion chemistry to the formation of catalytic metal nanocrystal-polydopamine hybrid materials that exhibit a high catalytic efficiency during recycled uses. Electrospun polymer nanofibers are used as a template for in situ formation and immobilization of gold nanoparticles via polydopamine-induced reduction of ionic precursors. The prepared hybrid nanostructures exhibit a recyclable catalytic activity for the reduction of 4-nitrophenol with a turnover frequency of $3.2 - 5.1 μmol g^{-1} min^{-1}$. Repeated uses of the hybrid nanostructures do not significantly alter their morphology, indicating the excellent structural stability of the hybrid nanostructures. We expect that the polydopamine chemistry combined with the on-surface synthesis of catalytic nanocrystals is a promising route to the immobilization of various colloidal nanosized catalysts on supporting substrates for long-term catalysis without the physical instability problem. Second, we suggest a simple method to stabilize the catalyst/electrode interface using polydopamine as a redox-active adhesive layer. The reduction of the oxidation states of Co-based oxygen evolving catalyst from $Co^{3+} to Co^{2+}$ induces the dissociation of the Co-based oxygen evolving catalyst layer from the electrode. The adhesive polydopamine interlayer maintains the stable interface between Co-based oxygen evolving catalyst and electrode, sustaining the catalytic reaction under vigorous working conditions. Interestingly, the polydopamine layer also promotes water oxidation through proton-coupled electron transfer with the electrode through the redox reaction of catechol and (hydro)quinone. X-ray absorption near-edge structure analysis shows that Co-based oxygen evolving catalyst supported by the polydopamine layer maintains higher oxidation states than the catalyst without polydopamine at the same water oxidation potential. We expect that the redox-active polydopamine interlayer can be applied to various electrochemical catalysts for improving their microenvironments to enhance and sustain the catalytic activity. Third, we report a linker-free spontaneous binding of tetracobalt-based polyoxometalates $(Co_4POMs)$ on nitrogen-doped carbon nanotubes (NCNTs) via electrostatic hybridization. Protonated nitrogen-dopant sites at NCNTs enable linker-free immobilization of the $Co_4POMs$ and provide a fluent electron transfer in the resultant $Co_4POM/NCNT$ hybrid structures, as demonstrated by the low overpotential of 370 mV for the water oxidation at pH 7. Accordingly, the hybrids exhibit a fast reaction kinetics with a turnover frequency of $0.211 s^{-1}$ at 2.01 V vs. RHE. Density functional theory calculation propose that POMs vertically align at the NCNT surface exposing the maximal catalytic surfaces. This work suggests a reliable route to highly efficient water oxidation catalysis by employing POMs in neutral conditions and NCNTs as self-binding nanoelectrodes in a synergistic well-oriented hybrid structure. Lastly, we report a spontaneous formation of functional polycrystalline Pd nanowires (PdNWs) through the self-biomineralization of a genetically modified virus without reducing reagents under ambient conditions. A filamentous M13 virus, which displays a glutamate trimer on its major capsid proteins to increase the negative surface charge density, is employed to induce the electrostatic complexation with positively charged Pd complexes. Virus-enabled self-biomineralization minimizes the formation of Pd particles in the bulk phase, while only polydisperse larger Pd agglomerates are generated in the absence of virus. The virus-templated PdNWs are examined as catalysts for the Suzuki-coupling reaction. Owing to the well-entangled network structures and stabilizer-free clean surface of PdNWs, their catalytic activities are maintained after repeated catalysis, while conventional nanoparticle catalysts stabilized with polymers undergo the surface contamination and lose their catalytic activity during repeated uses. The high electronic connectivity and structural stability of the prepared PdNWs are also demonstrated through their application to a hydrogen gas sensing reaction at room temperature. The PdNWs exhibit about 9.8 times higher sensitivity and about 3.6 times faster response time compared to a Pd thin film. These feasibility tests demonstrate the high catalytic functionality of stabilizer-free, bio-templated nanostructures prepared through self-mineralization. In conclusion, structural integrities and sustained activities of the catalytic nanostructures are realized by functional interfaces. It is believed that our approach is expected to provide design principles for tailoring interfacial properties and, additionally, to realize toward practical for energy and sensor applications without the physical instability problem.