Are protons involved in the hydrogen-induced surface conductivity of diamond(001)?

We have investigated the mechanism underlying the surface conductivity observed for synthetic diamond(001) samples. It is found that (I) the surface conductivity depends on the pressure of the surrounding gas atmosphere, (2) the temperature dependence can be described as being due to a thermally activated process and (3) a chromogeneous WO3 cathode turns color from transparent to blue and gas bubbles develop underneath the cathode if an electrical de current is passed through such an arrangement. In contrast, the complementary WO3 anode neither changed color nor were gas bubbles observed. One concludes that hydrogen develops at the negatively biased WO3-diamond interface, recombining to gaseous H-2. However, most of the hydrogen invades the WO3 cathode coloring it blue, thereby clearly indicating the involvement of protons. Though the conductivity crucially depends on temperature, time and history of a sample, the resistances of various and even differently prepared diamond specimens covering eleven orders of magnitude can be scaled onto one master curve reflecting a thermally activated behavior. Taken together, we propose that the mechanism of the diamond(001) surface conductivity as observed under ambient conditions is at least partly due to proton conduction.