Hole diffusion length in n-TiO2 single crystals and sintered electrodes: Photoelectrochemical determination and comparative analysis

The photoelectrochemical behavior of an n-TiO2 (rutile) single crystal under different treatments (mechanical polishing and chemical and photoelectrochemical etching) is analyzed and the conditions under which Gärtner’s model can be applied are determined. The hole diffusion length Lp obtained from the photoelectrochemical data is about 10−6 cm for the etched single crystal and seems to be governed by electron-hole recombination at centers associated to impurities introduced in the lattice during the manufacturing process. This value of the hole diffusion length determines the minimum value of the donor concentration Nd for an efficient separation of carriers within the semiconductor depletion layer. This explains the existence of a maximum of the quantum efficiency for Nd∼6×1018 cm−3, which has been found to be a value common to n-TiO2 single crystals of different origin. Lattice deffects introduced near the crystal surface by mechanical polishing behave as recombination centers which reduce Lp to values of the order of 10−7 cm. A comparison with Lp values of n-TiO2 sintered electrodes is made.