Basic features of surplus electrons of order 10²¹ - 10²² cm^-3 in bulk and at different (110) surfaces of tin dioxide are considered computationally, by using density functional theory. The additional charge is found to effect on surface energetics and surface relaxations considerably with respect to their charge neutral counterparts.

This is true especially in case of oxygen rich environments and surfaces, where oxygen can be adsorbed on-top of five-fold coordinated surface tin atoms. The highest occupied orbital of O^- corresponds to oxygen p-orbital oriented parallel to the surface normal and cause adsorbate to become spin polarized. Based on the evaluated total energy differences it is shown that its formation is favorable if there are initially surface oxygen vacancies, or alternatively, if there is initially surplus electrons present at the surface. It is shown that the adsorbed O^- can in principle decompose by transforming to negatively charged dioxygen species.

In cases of stoichiometric and slightly reduced surfaces the additional charge is accumulated increasingly at surface layers with a considerable amount of the surplus charge accumulated around Sn atoms. A pronounced charge accumulation around under-coordinated surface tin atoms is observed especially in the cases of oxygen deficient surfaces. Different charge accumulations are seen around five-fold coordinated surface Sn, four-fold coordinated surface Sn atom and around a single bridging oxygen vacancy. The Sn²⁺ type of surface Sn atom has a relatively strong additional charge trapping character, without decline when slab thickness was increased.

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