Hydrogen as an Unstable Shallow Donor in Oxides

Hydrogen is electrochemically introduced into a monoclinic Nb2O5 in a large amount, which donates electrons to Nb5+ and decreases the resistivity of Nb2O5 by more than six orders of magnitude. Both X-ray photoelectron spectroscopy and crystal structure analyses show that ionized hydrogen, namely, proton, is not connected to an oxygen ion by a strong O–H bond, and hydrogen can be easily driven out of this oxide as molecular hydrogen by annealing at 150 °C. This finding provides a relatively direct evidence for an unstable state of a hydrogen shallow donor in oxides and should contribute much to the general understanding of hydrogen in oxides.