  It is shown that in nanoporous titanium dioxide films, sensitivity to
atmospheric hydrogen exposure and electroforming can coexist and are
interdependent. The devices work as conventional hydrogen sensors below a
threshold electric field while above it, the well-known electroforming is
observed. Offering hydrogen in this regime accelerates the electroforming
process, and in addition to the usual reversible increase of the conductance in
response to the hydrogen gas, an irreversible conductance decrease is
superimposed. The behavior is interpreted in terms of a phenomenological model
where current carrying, oxygen-deficient filaments with hydrogen-dependent
conductivities form inside the titanium dioxide matrix.
