Tuning of the Surface-Exposing and Photocatalytic Activity for AgX (X = Cl and Br): A Theoretical Study

The underlying physical mechanism and dynamics of crystal growth in tuning the crystal morphology and surface exposing by adjusting the concentration of the precursor NaCl are elucidated by examining the surface energies, adsorption energy, and total energy for several surfaces upon adsorption of Cl in AgCl based on first-principles calculations. Our results indicate that the specific surface structures of different surfaces and the nature of ionic interactions between Ag and adsorbed Cl atoms in AgCl facilitate the reversal of relative surface energy upon adsorption of Cl, enabling the possibility of tuning the crystal morphology and surface exposing by adjusting the concentration of precursor NaCl. Two critical concentrations of NaCl at which the morphology of crystal changes are predicted, in satisfactory accordance with the experimental observations. Moreover, for the first time, we propose that the morphology and surface exposing of AgBr can be controlled by subtly tuning the concentration of precursor NaBr; however, the adjustment is quite demanding due to the weaker interaction between Br and Ag on the surfaces in AgBr. In addition, the analysis of electronic structures indicates that the (111) surface with Ag exposure exhibits higher photocatalytic activities due to the reduction of band gap resulting from the dangling bonds of surface Ag atoms without altering the oxidation potential. On the other hand, because adsorption of Cl on all surfaces decreases the oxidation potential, one should eliminate the surface adsorbates as much as possible to obtain higher photocatalytic activities.