Design of band-engineered photocatalysts using TiO2

Difficulties in achieving control over carrier concentration have impeded progress toward tailoring the electric fields in semiconducting oxide photocatalysts based on principles of electronic band-engineering drawn from classical microelectroncis. The present work demonstrates such principles using the model case of methylene blue photooxidation over thin-film anatase TiO2 grown by atomic layer deposition. The near-surface electric field was controlled by tuning the space charge layer width through varying the bulk carrier concentration and the surface potential. The carrier concentration in the polycrystalline semiconductor was controlled over a range of 2.5 orders of magnitude via an unconventional method - film thickness, which indirectly influenced the concentration of electrically active donor defects at grain boundaries, through which the reaction rate constant varied by about a factor of 5. The surface potential of the TiO2 was controlled by treating the surface with remote oxygen plasma, changing the surface Fermi level by as much as 0.4 V through which the reaction rate constant varied by about 35%. Both trends are well-described by a quantitative one-dimensional model for photocurrent. The model suggests that changes in rate result fundamentally from variations in the width of the space charge layer near the surface. Electrical characterization of the films by capacitance-voltage measurements and ultraviolet photoelectron spectroscopy, together with detailed physical characterization by a variety of other techniques, confirm this picture