PH-Dependent Surface Chemistry from First Principles: Application to the BiVO4(010)-Water Interface

We present a theoretical formulation for studying the pH-dependent interfacial coverage of semiconductor-water interfaces through ab initio electronic structure calculations, molecular dynamics simulations, and the thermodynamic integration method. This general methodology allows one to calculate the acidity of the individual adsorption sites on the surface and consequently the pH at the point of zero charge, pH(PZC) and the preferential adsorption mode of water molecules, either molecular or dissociative, at the semiconductor water interface. The proposed method is applied to study the BiVO4(010)-water interface, yields a pH(PZC) in excellent agreement with the experimental characterization. Furthermore, from the calculated pK(a) values of the individual adsorption sites, we construct an ab initio concentration diagram of all adsorbed species at the interface as a function of the pH of the aqueous solution. The diagram clearly illustrates the pH-dependent coverage of the surface and indicates that protons are found to be significantly adsorbed (similar to 1% of available sites) only in highly acidic conditions. The surface is found to be mostly covered by molecularly adsorbed water molecules in a wide interval of pH values ranging from 2 to 8. Hydroxyl ions are identified as the dominant adsorbed species at pH larger than 8.2.