Molecular Dynamics Beyond the Monolayer Adsorption as Derived from Langmuir Curve Fitting

Langmuir adsorption model is a classic physical-chemical adsorption model and is widely used to describe the monolayer adsorption behavior at the material interface in environmental chemistry. Traditional adsorption dynamic modeling solely considered the surface physiochemical interaction between the adsorbent and adsorbate. The surface reaction dynamics resulting from the heterogeneous surface and intrinsic electronic structure of absorbents were rarely considered within the reported adsorption experiments. Herein, by employing the chlorine hybrid graphene oxide (GO-Cl) to adsorb Ag⁺ in an aqueous solution, complicated molecular dynamics significantly deviated from the monolayer adsorption mechanism, as suggested by Langmuir adsorption curve fitting, has been elucidated down to atomic scale. In the time-dependent Ag adsorption experiments, both Ag single atoms and Ag/AgCl nanoparticle heterostructures are observed to be formed sequentially on GO-Cl. These observations indicate that for the surface adsorption dynamics, not only the surface chemical adsorption process involved but also photoreduction and the C-Cl bond cleavage reaction has been heavily engaged within the GO-Cl interface, suggesting a much more complicated vision rather than the monolayered adsorption algorithm as derived from curve fitting. This study uses GO-Cl as a simple example to disclose the complicated adsorption dynamic process underneath Langmuir adsorption curve fitting. It advocates the necessity of imaging the interfacial atomic-scale dynamic structure with high-resolution microscopy techniques in modern adsorption studies, rather than simply explaining the adsorption dynamics relying on the curve fitting results due to the complicated physiochemical reactivity of the adsorbents.