Oxidation-stable plasmonic copper nanoparticles in photocatalytic TiO₂ nanoarchitectures

Ultraporous copper/titanium dioxide (Cu/TiO₂) aerogels supporting <5 nm diameter copper nanoparticles are active for surface plasmon resonance (SPR)-driven photocatalysis. The extended nanoscale Cu‖TiO₂ junctions in Cu/TiO₂ composite aerogels-which arise as a result of photodepositing copper at the surface of the nanoparticulate-bonded TiO₂ aerogel architecture-stabilize Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after exposure to oxidizing conditions. The metallicity of the Cu nanoparticles within the TiO₂ aerogel is verified by aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, and infrared spectroscopy using CO binding as a probe to distinguish Cu(0) from Cu(i). In contrast, photoreduction of Cu(ii) at a commercial nanoscale anatase TiO₂ powder with primary particle sizes significantly larger than those in the aerogel results in a copper oxide/TiO₂ composite that exhibits none of the plasmonic character of Cu nanoparticles. We attribute the persistence of plasmonic Cu nanoparticles without the use of ligand stabilizers to the arrangement of Cu and TiO₂ within the aerogel architecture where each Cu nanoparticle is in contact with multiple nanoparticles of the reducing oxide. The wavelength dependence of the photoaction spectra for Cu/TiO₂ aerogel films reveals visible-light photocatalytic oxidation activity initiated by an SPR-driven process-as opposed to photo-oxidation initiated by excitation of narrow-bandgap copper oxides.