Discriminating adsorption mechanisms in liquid-phase photocatalysis <i>via</i> high-intensity laser-induced metallic nanodot deposition on TiO ₂ anatase

Light intensity plays a crucial role in liquid-phase semiconductor-based heterogeneous photocatalysis. It impacts reactant chemi- or physisorption 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 single metallic nanodots (silver, gold, and palladium) photodeposited on the surface of bare TiO₂ anatase nanoparticles at varying intensities, as a special case of a liquid-phase semiconductor heterogeneous photocatalytic reaction. We then developed three distinct models, the first establishing the theoretical background of so-called intensity-dependent disrupted-Langmuir adsorption (classically associated with chemisorption), the second outlining a new Eley-Rideal-like adsorption mechanism (physically-assisted adsorption), and the third considering a hybrid of both. When confronted with the 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 to a single master model for behavior, regardless of exposure time, reactant concentration, and light intensity. High-intensity-dependent experiments thus provide a quantitative understanding for distinguishing the underlying adsorption mechanism. Finally, considering the general framework of the present modeling approach, it is broadly applicable to other redox reactions on photoexcited semiconductor surfaces. As such, it also predicts how reaction rates, at constant light exposure, should evolve with reactant concentration and shows how adjusting the concentration can also be used to discriminate between adsorption mechanisms.