Addressing the Key Challenges of Light Penetration and Charge Separation for Plasmonic Catalysis

Abstract

Research into high temperature photocatalysis has revealed advantages to using light to augment or replace thermal energy for the synthesis of feedstock chemicals. However, poor penetration of light into a typical powder catalyst bed poses a challenge for efficient high temperature photo-driven heterogenous catalysis. To address this issue, we present a 3D porous optical diffuser loaded with plasmonic Rh nanoparticles enabling volumetric illumination of the nanoparticle catalysts. Within the field of plasmonic catalysis, much attention has been given to the tuning of the catalysts optical properties by manipulating of the plasmonic nanoparticles itself. However, much less attention has been given to the impact which the plasmonic particles dielectric supporting substrate has upon the system’s optical profile. This dissertation presents a monolithic plasmonic Rh/SiO2 structure which exhibits dramatically increased response to light compared to a powdered catalyst due to the control exerted over the dielectric substrate scattering profile. Chapter 1 presents an introduction to plasmonic catalysis. Chapter 2 shows how thermal non-thermal mechanisms can be distinguished with accessible and common place laboratory tools and procedures. Two of these methods, the mass-dependent method and the cover/uncover method revolve around embracing the poor penetration of light into a powder catalyst bed compared to the deeper penetration of photothermal heat. In addition to describing accessible means to differentiate between bulk thermal and local thermal, and nonthermal effects, Chapter 2 also highlights some previous misconception and misused terminology regarding quantum efficiency claims in plasmonic catalysis. Chapter 3 shows how changing from a powder supported catalyst to a monolithic aerogel supported catalyst homogenizes the optical environment, makes the catalyst more responsive to light, and drives selectivity toward a single product. In a powdered photocatalyst the intensely illuminated surface is followed by an abrupt transition to a dark but thermally active subsurface, creating a dichotomy of dueling thermal and non-thermal reaction paradigms. However, in an aerogel supported catalyst a broad gradient of optical intensity extending throughout the entirety of the catalyst bed was achieved. This broadening of the optical gradient between catalyst bed surface and subsurface improves the selectivity of the CO2 reduction reaction. The creation of a continuous and more uniform optical environment also provides greater reaction efficiency compared to the powdered catalyst with a sharper illumination gradient. The broad illumination gradient can be further tuned by augmenting the loading scheme to broaden or sharpen the optical distribution. This dynamic illumination paradigm is achieved through minimizing the supports optical scattering by employing an aerogel where the particle size is reduced to a size where only weak Rayliegh scattering is relevant. Support optical scattering can then be reintroduced in a controlled manner through the use of a sacrificial template composed of fused zinc oxide tetrapods. The pores added by this template-based approach provide interfaces upon which light scatters to create a highly effective optical diffuser. These pores also improve mass transport over conventional aerogel supported catalysts. The ZnO scaffolding implemented to improve mass flow also provides easier nanoparticle loading as well as the potential for more facile adjustments to the aerogel's surface chemistry compared to an unmodified aerogel were tight native pores heavily restrict surface accessibility. This approach to supported photocatalyst design provides exceptional flexibility for tuning the balance between optical properties, mass flow considerations, and available metal surface area. Chapter 4 presents work which chronologically preceded Chapter 3 and established the potential influence of dielectric scattering on the plasmonic particles’ local electric field strength. To probe this interaction silica microlenses and Mie resonators were synthesized and used to combine these two types of light matter interactions to further concentrate light at the nanoscale beyond what the plasmonic and dielectric scattering modes achieved separately. While the local field was enhanced in line with simulations, it was concluded that altering the dielectric support to be more diffusive as opposed to more concentrating would provide greater benefit. This directly informed the approach of Chapter 3. Chapter 5 focuses on a second challenge facing plasmonic catalysis, the short charge separated lifetime of hot carriers produced by plasmon dephasing, typically on the order of femtoseconds. Previous works have leveraged charge injection into the supporting material to prevent recombination due to the barrier posed by the Schottky junction The final portion of this dissertation explores the introduction of a type-II heterojunction to the supporting structure to further separate carriers and prolong excited lifetimes. The junction is achieved by oxidizing ZnS to form a layer of ZnO on the surface and then vacuum annealing to convert both phases to wurtzite, and to promote interfacial connectivity. Materials characterization including TEM, XRD, Raman, and uv-vis reflectance measurements support the formation of ZnO layer on the ZnS surface. Differences in the photoluminescent spectra and lifetimes and EPR spectra of oxidized Zns and unoxidized ZnS suggest that photoexcited electrons reside in ZnO phase for an extended period before recombining, compared to ZnS samples where photoexcited electrons persist in sulfur vacancy traps. Chapter 6 presents the conclusion from this body of research into designing the supporting material architecture to better complement the plasmonic catalyst in terms of both optical and electronic properties.