Quantum yields of hydroxyl radicals in illuminated TiO₂ nanocrystallite layers

The yield of hydroxyl radicals has been determined by illumination of TiO₂ layers immersed in air saturated aqueous methanol solutions. This yield is equal to half the measured formaldehyde yield in the pH range 7-13. A detailed mechanism is proposed, accounting for the lack of accumulation of hydrogen peroxide. The effect of changing methanol concentration, pH and light intensity (the latter by three orders of magnitude) is in agreement with a very simple mechanism. In contrast to hydroxyl radicals, which react via hydrogen abstraction, leading to formation of HCHO, there is no sign for reaction of methanol with mobile holes. Thus, the limiting quantum yield observed at high methanol concentration is related to the maximum yield of ^•OH_ads under air saturated conditions at the given pH and light intensity. The effect of light intensity shows the expected inverted square root dependency. The yield of ^•OH_ads is nearly constant in the range 7 < pH < 12. This system may be useful for comparative tests of different TiO₂ preparations.