(Invited) Plasmon-Driven Chemical Reactions

Abstract

Plasmonic metal nanocrystals can interact strongly with light, efficiently converting light into heat and generating hot charge carriers. Both plasmonic photothermal conversion and hot carrier generation can accelerate chemical reactions. The use of plasmons to drive chemical reactions has recently become a new and active research field (Adv. Mater. 2014, 26, 5274). Plasmonic hot charge carriers can not only enhance the reaction yield and selectivity, but also introduce new reaction pathways. The lifetime of plasmonic hot charge carriers is on the femtosecond scale. Without immediate usage, they will rapidly relax, converting their energy into heat. We have employed two approaches to enable the use of plasmonic hot charge carriers. The first is the integration of Au or Ag nanoparticles with Pd or Pt nanoparticles. We have applied this approach to enhance Suzuki coupling reactions by localized plasmons (J. Am. Chem. Soc. 2013, 135, 5588; Adv. Funct. Mater. 2017, 27, 1700016). The second is the integration of plasmonic metals with semiconductors. A barrier is formed at the interface. The generated hot electrons and holes can quickly inject into the conduction or valence band and therefore get separated. The injected charge carriers can then drive chemical reactions. We have synthesized Au/TiO2 (Energy Environ. Sci. 2014, 7, 3431; J. Am. Chem. Soc. 2018, 140, 8497), Au/CeO2 (ACS Nano 2014, 8, 8152) and Au/BiOCl hybrid structures (J. Am. Chem. Soc. 2017, 139, 3513) as plasmonic photocatalysts to drive the photo-generation of reactive oxygen species, the selective oxidation of alcohols, and N2 photofixation. Specifically, oxygen vacancies have been introduced in the metal oxides to “work in-tandem” together with plasmonic hot charge carriers. For example, during the use of Au/BiOCl hybrid structures for the photocatalytic selective oxidation of benzyl alcohol, oxygen vacancies on BiOCl facilitate the trapping and transfer of plasmonic hot electrons to adsorbed O2, producing superoxide anion radicals, while plasmonic hot holes remaining on the Au surface mildly oxidize benzyl alcohol to corresponding carbon-centered radicals. The ring addition between these two radical species leads to the production of benzaldehyde along with an unexpected oxygen atom transfer from O2 to the product. In contrast, the oxygen atom in the product is usually from the benzyl alcohol reactant. In another example, Au nanoparticles are anchored on ultrathin TiO2 nanosheets with oxygen vacancies. The oxygen vacancies on the nanosheets chemisorb and activate N2 molecules, which are subsequently reduced to ammonia by hot electrons generated from plasmon excitation of the Au nanoparticles. The hybrid photocatalyst can accomplish photodriven N2 fixation in the “working-in-tandem” pathway at room temperature and atmospheric pressure. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to ammonia is higher than those reported so far. The oxygen vacancies play three roles. They act as the active sites for N2 molecules; they cause defect states within the bandgap for trapping plasmonic hot electrons and therefore lengthening their lifetime; and they function as a bridge between plasmonic hot electrons and the activated N2 molecules. This work offers a new approach for the rational design of efficient catalysts towards sustainable N2 fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.