The work function of a material is commonly used as an intrinsic reference for band alignment; however, it is notoriously susceptible to extrinsic conditions. Following the classification of Bardeen we calculate values for the bulk binding energy of electrons in rutile-structured SnO2 and the effect of the surface on the work function, thus highlighting the role of the surface in determining the energy levels of a material. Furthermore we demonstrate how, through the use of ultrathin heteroepitaxial oxide layers (SiO2, TiO2, PbO2) at the surface, the work function can be tuned to achieve energy levels commensurate with important technological materials. This approach can be extended from transparent conducting oxides to other semiconducting materials.