Plasmonic Au/TiO₂ Composite Nanoparticles: Synthesis, Structural and Optical Characterization and Functional Performance in The Photocatalytic Reduction of CO₂ to CO

Because of their localized surface plasmon resonance, metal nanoparticles are of interest for a broad variety of applications ranging from chemical and biological sensing to surface enhanced Raman spectroscopy and improved light extraction in LEDs. Here, we present the synthesis, structural and optical characterization of Au nanoparticles applied on TiO₂ for application as photocatalysts for the sunlight-powered reduction of CO₂ to CO.<br/>The Au/TiO₂composite nanoparticles were prepared by tailored deposition of Au(OH)₃ on the surface of 20 nm-sized anatase TiO₂ particles dispersed in water at a precisely controlled pH 9, and subsequent thermal anneal in an Ar:O₂ mixed atmosphere (80:20). The Au content was determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) obtaining a value of 3.12% Au. The particle size distribution was determined using High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM). The Au nanoparticles were randomly distributed on the surface of 20 nm-sized anatase nanoparticles, and their size followed a lognormal distribution with an average of 1.6 nm. The interplanar distance in the Au NP was 2.37 Å, corresponding to the (111) spacing. The (111) lattice planes were perfectly aligned with the (101) TiO₂ planes. The presence of Au and TiO₂ as sole crystalline materials was confirmed by XRD. The diffuse reflectance spectrum of the Au/TiO₂ powder displayed a broad minimum centered around 500 nm, which respresents the light absorption based on the plasmonic renonance of the Au nanoparticles. Since we intended to apply the Au/TiO₂ nanoparticles for photocatalysis, we also determined the light penetration depth (skin-depth) of the powder using UV-vis-NIR spectrophotometry. The resulting skin-depth was about 100 µm.<br/>We applied the resulting Au/TiO₂ composite nanoparticles for the sunlight-powered reverse water gas shift reaction, i.e. the reaction of CO₂ and green H₂ to CO and H₂O. Whilst this reaction, which is endothermic (Δ<i>H</i> = 41.2 kJ/mol) and limited by equilibrium, requires temperatures above 600^○C in conventional thermocatalysis, we achieved high production rate (7888 mmol/(m²h)) and CO selectivity (96.9%) at temperatures below 300^○C using mildly concentrated sunlight (up to 14 kW per m²) as sole and sustainable energy source. Reference experiments in dark at the same catalyst bed temperature yield more than 30% CH₄ as undesired side product. We proposed that the high selectivity of the catalyst could be attributed to promotion of CO desorption through charge transfer of plasmon generated charges, consequently avoiding further reduction of CO to unwanted CH₄.<br/>The TiO₂ nanoparticles without Au show no CO production when illuminated with sunlight. Furthermore, removal of all light below 400 nm (above the anatase TiO₂ bandgap) resulted in no loss of activity and selectivity for Au/TiO₂, demonstrating the direct excitation of the semiconductor plays no role in this catalytic process. The rate of the reaction exponentially increased with the solar irradiance, demonstrating that in addition to the above mentioned plasmon-induced desorption of CO, also photothermal heating contributes to the process. Studies with differently sized Au nanoparticles revealed a linear relationship between the catalytic activity and the number of Au-TiO₂ contact points, and studies with Au nanoparticles supported on dielectric materials such as Al₂O₃ and SiO₂ yielded no catalytic activity. In combination, these phenomena suggest that charge carriers generated in the Au nanoparticles by its localized surface plasmon resonance are injected into the semiconductor over the Schottky barrier, which prolongs their lifetime and makes them available for participation in the catalytic conversion of CO₂ to CO. Ergo, TiO₂ is not a simple catalyst support, and both Au and TiO₂ are essential components of this catalyst which operate synergistically.