Synthesis, Characterization and Applications of Plasmonic Gold Nanocrystals for Catalysis
Plasmonic metals are materials that strongly interact with incident light to produce oscillating waves of electrons. In nanomaterials, these plasmon waves are localized, having brilliant optical absorbance and scattering behavior. Harnessing this light capture can be key to drive many important chemical transformations. However, engineering stable and efficient catalytic material is not simple, due to synthesis challenges, and a lack of understanding of the true dominant mechanism at play. Heterogeneous plasmonic photocatalysis requires intact noble nanocrystals to maintain a stable localized surface plasmon resonance (LSPR) for solar energy conversion. Even in conventional catalysts, recovering and reusing nanoparticle is crucial to reducing industrial cost and waste as well as improving efficiency. Many studies successfully show supported plasmonic photocatalyze reactions but very few studies utilize precisely colloidally controlled plasmonic nanocrystals on support. Here, 12-14 nm colloidally synthesized Au nanospheres and 55-60 nm triangular nanoprisms are deposited onto Microcrystalline cellulose (MCC) and TiO2 by lyophilization. Transmission electron microscope (TEM) images show high dispersity of nanocrystals across the surface of MCC and TiO2. Sodium citrate additive enhances the dispersity of Au nanosphere (NS) and support material during the lyophilization process. UV-vis spectra of highly dispersed supported material show significant optical resemblance with their respective colloidal Au nanoparticles. Catalytic reduction of 4-Nitrophenol with sodium borohydride was utilized to show surface availability of Au. Additionally, platinum deposited on 5 wt% Au nanospheres on MCC demonstrate surface accessibility to reactive species for adsorption. 4-nitrophenol reduction was used as a model reaction for assessing Au nanocrystal (NC) performance. TiO2 supported Au showed higher degradation rate than MCC. This result was attributed to diffusion-limited in the zone near Au NCs. Citrate additive improved degradation for Au-TiO2 but not Au-MCC.Furthermore, we applied this method of immobilizing nanocrystals on supports to conduct photocatalytic reactions. For revealing mechanisms, we adapted a model reaction to experimentally measure the photoactivity of the catalysts. Moreover, decorating the Au nanocrystals with iridium and platinum showed significant enhanced catalytic activity. We have discovered that there are two dominant regimes of photocatalytic activity. Below ~5-6 W/cm2, thermal effects govern the rate of reaction while above this intensity, high energy charged carriers dictate the reaction rate. The core-shell design allows great flexibility in engineering light capture wavelength through nanocrystal size and geometry design while driving reactions can be tailored by decorating specific types of active metals to the Au particle. This designer core-shell catalyst can be optimized for many applications. Lastly, we briefly explored high valued applications for sustainable chemical production. We have investigated Au@Pt colloids for photocatalytic water reduction to hydrogen. Our mass spectrometer was able to detect hydrogen signals in real time upon illumination. Also, this system was applied to nitrogen fixation, a method to synthesize ammonia from nitrogen and water. The work conducted were only foundational experiments exploring real world applications but will be very important for influencing future work.