Discriminating Adsorption Mechanisms in Liquid-Phase Photocatalysis via High-Intensity Laser-Induced Metallic Nanodot Deposition on TiO₂ Anatase

Abstract

Light intensity plays a crucial role in liquid-phase semiconductor-based heterogeneous photocatalysis. It impacts reactant chemi- or physi- sorption at the semiconductor surface and subsequent photocatalytic reaction rates. Despite its recognized importance, the relationship between light intensity and adsorption remains controversial, necessitating further theoretical and dedicated experimental strategies to clarify the underlying mechanisms at work. To address this, we designed two laser-based experimental setups, respectively operating at high and very high intensities, to investigate the growth of a single metallic nanodot (silver, gold, and palladium) photodeposition at the surface of bare TiO2 anatase nanoparticles at varying intensities, as a special case of liquid phase semiconductor heterogeneous photocatalytic reaction. We then developed three distinct models, the first establishing the theoretical background of the so-called intensity-dependent Disrupted-Langmuir Adsorption (classically associated with chemisorption), the second setting a new Eley-Rideal-like Adsorption mechanism (physically-assisted adsorption), and the third considering a hybrid of both. When confronted with experimental results, the Eley-Rideal-like mechanism was the only one to successfully account for all the observed behaviors, enabling a universal rescaling of data onto single master behaviors regardless of exposure time, reactant concentration, and light intensity. High-intensity-dependent experiments thus provide quantitative understandings to distinguish the underlying adsorption mechanism. Finally, considering the general frame of the present modeling, it is broadly applicable to other redox reactions on photo-excited semiconductor surfaces. As such, it also predicts how reaction rates, at constant light exposure, should evolve with reactant concentrations and shows how playing with concentration can also discriminate between adsorption mechanisms.