Insight Into Electric Potential Distribution Within Mesoporous Titanium Dioxide Films

Charge transport and potential distribution in mesoporous semiconductor films operating in an electrolyte, especially those composed of TiO2 nanoparticles, are still the subject of wide debate. Herein we describe a series of experiments, performed under band-gap energy illumination of nanostructured TiO2 films, intended to shed new light on the actual electric potential profile across such three-dimensional electrode. Our approach stems from quite a general observation that addition of various electron acceptors to a solution containing an efficient hole scavenger (e.g., methanol, formic acid) results in a marked drop of the maximum photocurrent at the mesoporous TiO2 film electrodes whatever the applied anodic bias might be. We have chosen an electron acceptor, MV2+ dication, which-due to its negative redox potential, more negative than that of the bottom of conduction band of TiO2 in acidic media - causes a drop of the photooxidation current only in alkaline but not in acidic solutions of hole scavengers. Measurements of the incident photon-to-current efficiencies as a function of wavelength show that the drop of the photocurrent after the MV2+ addition, observed in alkaline formate solution extends practically over the whole range of wavelengths. As the optical penetration depth in TiO2 for the wavelengths close to its band edge is expected to match approximately the chosen film thickness, we can conclude that major part of the electric potential drop in the TiO2 electrode occurs actually close to the back contact.