Dear David, In experiments , neutral O2 gas leaves the TiO2 crystal causing the creation of oxygen vacancies behind. However, these vacancies can have any charge state. Neutral in which the two electrons (reminiscent of O2- ion) set around the vacancy forming polarons, 2+ in which the two electrons leaves the vacant site and go somewhere else, or 1+ which is intermediate between the past two cases.
Have a look at the following paper for theoretical analysis of the oxygen vacancy in TiO2: J. Phys. Chem. C 2010, 114, 21694–21704 For an experimental point view, have a look at figure 2.4 (and the associated discussion) in the following thesis: https://scholar.google.com.eg/citations?view_op=view_citation&hl=en&user=1Ga4WncAAAAJ&sortby=pubdate&citation_for_view=1Ga4WncAAAAJ:u-x6o8ySG0sC Regards, Mostafa MIT
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