Transition metal compounds are famous for the large variety in electronic structure. They can be metallic or semiconducting. Compounds of first row transition metals often also show magnetic and orbital ordering. However in almost all cases the bonding is sufficiently ionic to retain the amphiphilic acid/base activity characteristic of the surfaces of main group metal oxides. At low pH the basic sites are protonated and at high pH the acidic sites deprotonated creating a surface potential. The alignment of the electronic energy levels is therefore pH dependent. In fact, even in absence of protonic surface charge, the coordination with water molecules is already capable of shifting the electronic energy levels by 1 eV or more. The alignment of the electronic energies in transition metal compounds requires therefore detailed modeling of their aqueous surface chemistry. In this talk we outline the Density Functional Theory based Molecular Dynamics (DFTMD) method we have developed for this purpose[1]. The central computational tool is a free energy perturbation method for the computation of the solvation energy of the proton which is the common energy reference for ionization potentials and electron affinities on the Standard Hydrogen Electrode (SHE) scale as well as the surface acidity constants (pKa). As an illustration we discuss the application to the aqueous interface of two model photoelectrodes, TiO2 and GaN, focusing on the energy of the conduction and valence band edge relative to the SHE and the shift in the vacuum level of the solid relative to the electrolyte. Both energies are observable manifestations of the interface dipole and can be directly compared to experiment.[1] Cheng, J.; Sprik, M. Phys. Chem. Chem. Phys. 14  , (2012) 11245.

Electronic energy level alignment at the water interface of a model oxide and nitride photoelectrode

248th National Meeting (2014)

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