Nanoscale physics
The dual role of oxide film in hydrogenation of metals 
(on the basis of titanium)
Shkola A.A.
Alloys Technology Laboratory, G.V. Kurdyumov Institute for Metal Physics of the N.A.S. of Ukraine. Academician Vernadsky Boulevard, 36, Kyiv-03142, Ukraine. E-mail: svavil@imp.kiev.ua

The increased interest in the study of hydrogen interaction with the metals as long ago as the 40-50's years of the lust century meted with problem of the effect of oxygen on the process of hydrogenation [1, 2]. Both all the previous and subsequent literature has most likely stated about the negative effect of the oxygen film on the penetration of hydrogen atoms into the metal. There was a unanimous conviction: the oxide film on the metal surface becomes a barrier to the formation of hydrides in the volume of the metal. The only way to prevent this phenomenon is to anneal the hydrogenation object in a high vacuum. Nevertheless, it is known that annealing, even at 10-8 torr, does not annihilate the presence of oxygen on the surface, even formation of thin TiOx film (micron fraction).
Experiments confirm that TiOx film on a metal surface d≤0,45 μm thick does not prevents, but promotes the penetration of hydrogen [3]. Oxide on the metal becomes an obstacle to penetration or sudden deceleration from a certain thickness of the film.
The last decades are devoted to an active study of defective states on the surface of TiO2 in its varieties – the phases of rutile and anatase. The possible existence of these states is associated with Ti3+ – oxygen vacancy complexes, that is, Ti3+–V0 [4].
An estimate of the diffusion of the oxygen vacancies, if we use the typical value of the activation energy for it in TiOx EV0=0,84 eV from the literature, then DV0=3,3x10–9 (cm2/s) when moving through the film thickness of d=0,35–0,45 μm would require a time from τ>100 s to several hours, provided they are run-of to dislocations and bordering regions of grains. If the hydrogenation continues, then most likely, there are fast flowing electronic processes.
Very convincing is the electron transport through polaron states (jumps) in stoichiometric TiO2 in models from anatase to rutile, which differ in the binding and electron transfer energies in these modifications [5].
The mobility of an electron (and a polaron) in the TiO2 provides rapid transitions from Ti4+ to Ti3+ due to the Ti4++e–→Ti3+ reaction, forming Ti3+–V0 complexes on which the hydrogen molecules dissociate into two atoms. 
Thus, the thin oxide film of the indicated thickness does not prevent the penetration of hydrogen atoms into the metal.
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