Low-density Ag–Au nanoparticle photodeposition on TiO ₂ thin film photocatalysts grown by atomic layer deposition

Heterogeneous TiO₂ photocatalysts, decorated with noble metal co-catalysts <i>via</i> 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 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 monometallic and bimetallic nanoparticles (NPs) were synthesized <i>via</i> 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 catalysts, which showed the highest activity towards photodegradation of methylene blue. Conversely, reversing the order (starting with Ag) resulted in a Au-Ag alloy/TiO₂ structure <i>via</i> a galvanic replacement reaction exhibiting bimodal particle size distributions of the 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 the UV-vis wavelength range <i>via</i> localized surface plasmon resonance (LSPR), but photocatalytic performance is improved due to the formation of a metal-semiconductor Schottky junction, which lowers recombination through spatial separation of charges. X-ray photoelectron spectroscopy results show the highest activity towards oxidation in an air atmosphere for Au-core-Ag-shell NPs/TiO₂, mirroring the results of the photocatalytic test. We conclude that the Au-core-Ag-shell is the optimal morphology for noble metal NP co-catalysts on an anatase TiO₂ support.