Selectivity of Photoelectrochemical Water Splitting on TiO₂ Anatase Single Crystals

The photoelectrochemical behaviors of (101)- and (100)-oriented anatase single crystals were compared in water oxidation in acidic as well as alkaline solutions. Both crystallographic orientations show comparable flat-band potential close to −0.1 V versus reversible hydrogen electrode. The activity of anatase single crystals in water oxidation (at constant bias) decreases with increasing pH. This activity drop demonstrates shift of the photocurrent onset potential in regions close to the point of zero charge. This trend is ascribed to the gradual deprotonation of the surface OH groups and resulting negative surface charge. The activity is further affected by the cations forming the supporting electrolyte—in particular, by their structure-making/breaking abilities. The apparent suppression of the photoelectrochemical water oxidation in alkaline media can be lifted in the presence on oxygen, which shifts the selectivity of the process toward ozone formation. This trend is more pronounced for (101)- than for (100)-oriented anatase, which reflects the role of surface orientation in selectivity control.