Models of Surface Morphology and Electronic Structure of Indium Oxide and Indium Tin Oxide for Several Surface Hydroxylation Levels

Indium oxide (IO) and indium tin oxide (ITO) are important metal oxide materials with a wide array of applications. Particularly, ITO is employed as a transparent conductive electrode in photovoltaic systems. While bulk metal oxides are typically well characterized, their surfaces, especially in real-life applications, can be hydroxylated and intrinsically disordered to a level that a structure-function prediction becomes a daunting task. We tackle this problem by carrying out simulations based on Density Functional Theory. We propose IO and ITO hydroxylated surfaces derived from the bcc and rombohedral IO polymorphs (100%, 66%, 33%, and 0% hydroxylation coverages were considered). By correlating computed quantities such as surface partial density of states, work functions, and surface dipole strength, a clear picture of the structure-function relationships in these model systems emerges. In line with conclusions drawn from experiments, we find that the density of states of 100% hydroxylated surfaces and bulk models are unaltered by Sn doping, with the only difference being the position of the Fermi level. The partially hydroxylated surfaces, instead show a rich array of behaviors, including appearance of surface states in the gap and appearance of interesting morphologies, such as chemisorbed molecular oxygen. We also find that the hydroxylation level affects surface dipoles in a systematic way, that is, the higher the hydroxylation level, the higher the surface dipole (screening/reducing the work function). Furthermore, models with In-atom vacancies show a relatively small decrease in surface dipole with hydroxyl coverage due to surface distortions.