Understanding the nature of photogenerated carriers in a photocatalyst is central to understanding its photocatalytic performance. Based on density functional theory calculation we show that for TiO₂, the most popular photo-catalyst, the electron hole self-trapping leads to band gap states which position is dependent on the type of surface termination. Such variations in hole state energies can lead to differences in photocatalytic activity among rutile and anatase surface facets. We find that the calculated hole state energies correlate with photo-deposition and photo-etching rates. We anticipated that our results can aid the design of more reactive photo-catalysts based on TiO₂ and our approach can be utilized for other relevant photo-catalysts as well.