Investigating the Plasmonic Hot-Carrier Injection Mechanism for CO2 Reduction Using Nanostructured Ag Catalysts

Abstract

Converting CO2 into hydrocarbons or other liquid fuels and leveraging solar energy to do so are of great importance as an alternative energy source and as a potential pathway to slow climate change. However, CO2 reduction reactions suffer from high overpotentials and poor selectivity. Recently, localized surface plasmon resonance (LSPR) was suggested as a way to overcome these challenging barriers [1-3]. Nanostructured catalysts absorbing visible light at their plasmon frequency can excite hot carriers which can be transferred directly or indirectly into the molecular orbitals of adsorbates. This unique charge injection mechanism along with the ability to tune the plasmon frequency over a wide range of energies may allow plasmonic photocatalysts to unlock a new, unexplored CO2 reduction pathway resulting in increased selectivity and lower overpotentials. Silver (Ag) is a strong plasmonic material and well-known for selectively producing CO [4,5]. In the hopes of taking advantage of the plasmonic properties to make more reduced products on Ag, we have developed Ag nanopyramid arrays, resulting in strong plasmonic and catalytic behaviors. Photocurrent measurements were performed on the plasmonic nanopyramid catalysts in aqueousNaClO4 saturated with either CO2 or Ar. The photocurrent is greatly enhanced in the presence of CO2, and shows ‘resonant-like’ behavior at a specific potential (-1.1 V vs. RHE). In Ar-saturated electrolyte, the photocurrent was drastically reduced and no characteristic feature of photocurrent was observed. This result indicates that the hot carriers created by LSPR react specifically with CO2 molecules or other CO2 reduction intermediates. By measuring incident photon to current efficiency (IPCE), we find that the hot-carrier transfer mechanism can be modulated as a function of applied potentials. Further, we discuss the differences in product distributions measured with a gas chromatograph (GC) and high performance liquid chromatography (HPLC) under light illumination and in the dark.