(27e) Design of Band Engineered Photocatalysts Using Titanium Dioxide

Difficulties in achieving control over carrier concentration have impeded progress toward tailoring the electric fields in semiconducting oxide photocatalysts based upon principles of electronic band engineering drawn from classical optoelectronics. The present work demonstrates such principles using the model case of methylene blue photo-oxidation over thin-film anatase TiO₂ grown by atomic layer deposition. The carrier concentration in the polycrystalline semiconductor is controlled over a range of 2.5 orders of magnitude via an unconventional means - film thickness, which indirectly influences the concentration of electrically active donor defects at grain boundaries. Over this range, the reaction rate constant varies by more than a factor of 10, and is well described by a quantitative one-dimensional model for photocurrent.  The model suggests that the 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 techniques, confirm this picture.  Prospects for better control of grain boundary donor defects through film synthesis procedures are discussed.