Low-density Ag–Au Nanoparticle Photodeposition on TiO2 Thin Film Photocatalysts Grown by Atomic Layer Deposition

Abstract

Heterogeneous TiO₂ photocatalysts, decorated with noble metal co-catalysts via photodeposition have been recognized for their efficacy across a spectrum of photocatalytic applications. Improved performance is often attributed to either plasmonic enhancement or improved separation of photoinduced charge carriers by the noble metal nanoparticles. The differentiation between the two typically co-existing mechanisms is challenging and calls for investigations using model systems in which either mechanism is suppressed. To bridge this gap, low number density (<100 µm⁻²) Au and Ag mon-ometallic and bimetallic nanoparticles (NPs) were synthesized via photodeposition from liquid precursors on anatase titania (TiO₂) thin films fabricated by atomic layer deposition (ALD). Initiating bimetallic NP deposition with Au first created bimetallic Au-core–Ag-shell NP/TiO₂ thin film catalyst, which showed the highest activity towards photodegradation of methylene blue. Conversely, reversing the order (starting with Ag) resulted in Au–Ag alloy/TiO₂ structure via galvanic replacement reaction exhibiting bimodal particle size distributions of Au–Ag alloy and Au alone - showing the lowest photocatalytic activity. The presence of low number density metal nanoparticles does not impact the optical absorption of TiO₂ in UV-Vis wavelength range via Localized surface plasmon resonance (LSPR), but photocatalytic performance is improved due to the formation of metal-semiconductor Schottky junction, which lowers recombination through spatial separation of charges. X-ray photoelectron spectroscopy results show the highest activity towards oxidation in air atmosphere for Au-core–Ag-shell NPs/TiO₂, mirroring the results of photocatalytic test. We conclude that Au-core–Ag-shell is the optimal morphology for noble metal NP co-catalysts on anatase TiO₂ support.