Density Functional Theory Study of Adsorption of Benzotriazole on Cu₂O Surfaces

The adsorption of benzotriazole, an outstanding corrosion inhibitor, was characterized on Cu₂O surfaces using density functional theory (DFT) calculations. Cu₂O surfaces contain two distinct Cu sites: coordinatively saturated (CSA) and coordinatively unsaturated (CUS). The following surfaces were considered: (i) Cu₂O­(111); (ii) Cu₂O­(111)-w/o-Cu^CUS, which lacks the CUS copper sites; and (iii) Cu₂O­(110):CuO, which is CuO-terminated Cu₂O­(110). Neutral benzotriazole (BTAH) binds weakly (by about −0.4 and −0.7 eV as calculated with the PBE functional) to CSA copper sites on Cu₂O­(111)-w/o-Cu^CUS and Cu₂O­(110):CuO, respectively. These binding energies are comparable to those obtained on metallic Cu(111) and Cu(110). In contrast, BTAH binds rather strongly to CUS copper sites (about −1.5 eV), which is even stronger than the bonding to very-low-coordinated surface defects on metallic Cu surfaces. Indeed, this bonding is so strong that it overcompensates for a thermodynamic deficiency of stoichiometric Cu₂O­(111). Hence, the BTAH-covered Cu₂O­(111) is thermodynamically more stable than the BTAH-covered Cu₂O­(111)-w/o-Cu^CUS. Similar to results reported previously for metallic Cu surfaces, a deprotonated benzotriazole also bonds much more strongly on Cu₂O surfaces than the neutral BTAH does. Its binding energies on CSA sites range from −1.7 to −2.0 eV (depending on details), and on CUS sites they are about −2.8 eV (as measured with respect to the BTA^• radical in the gas phase). Despite this relatively strong bonding, the dissociation of BTAH (N–H bond cleavage) is slightly endothermic on Cu₂O surfaces. The impact of van der Waals dispersion interactions on adsorption bonding was also addressed, and their main effect is to strengthen the molecule–surface bonding on average by about 0.4 eV, but otherwise the relative stability trends among various adsorption forms largely remain the same. The observation that benzotriazole binds much more strongly to coordinatively unsaturated copper sites on oxidized surfaces and to undercoordinated defects on metallic Cu surfaces (compared to regular sites) may indicate its ability to passivate reactive surface sites