| dc.description | The extensive use of hydrogen gas for numerous applications is a short term goal in the development of a hydrogen based-economy [1]. Hydrogen production, storage, perm-selective membranes and sensors are some of the active areas of research.
In this work we studied the Nb reduc-tion under H2 gas exposure promoted by a Pd thin film capping. Oxide reduction was moni-tored by the change in optical transmittance of the films using a light spectrometer [2] and reduction of mass of the films using a quartz crystal microbalance [3], respectively. For both measurements identical samples were fabricated; 13 nm of Nb were evaporated using an e-beam technique on substrates of 4nm of Ti over glass and a quartz crystal. Then, oxidation of the Nb film was achieved heating the film up to 150°C for 30 minutes in air, obtaining layers with 10 nm thick of Nb2O5. Finally, 5 nm of Pd were evaporated over the oxide. The thicknesses of the differ-ent layers were calculated from X-ray Reflec-tivity (XRR) measurements.
We show the transmittance variation ( =600 nm) during two cycles of H2 exposure in Figure 1. In the first cycle, an initial de-crease of transmittance, due to Nb oxide re-duction, was observed up to a pressure of 6 torr. Then at higher pressures, when no more oxide remained in the sample, an increase of the transmittance is observed due to the hy-drogen absorption by the Pd film. Hydrogen saturating occurred at 20 torr approximately. In the case of the second cycle an increase of the transmittance was observed without ini-tial decreased, because the oxide was already reduced. The only effect present corresponds to the hydrogen absorption.
Figure 1. Transmittance variation as a function of hydrogen pressure during two exposure cycles.
Similar results were found using the quartz crystal microbalance measurement. Exposures at 5 torr of H2 gas were applied to the sample obtaining a mass decrease in time due to the Nb reduction.
These preliminary results suggest that the thin Pd film promotes the Nb reduction when exposed to H2. The hydrogen molecules are dissociated on the surface of the Pd layer and then the atomic H diffused in, reaching the Nb oxide, reducing it to form H2O.
Funding from VRI Puente 10/2012 is greatly acknowledge.
References:
[1] B.D. Adams and A. Chen, Materials Today, 14, 6 (2011).
[2] C.P. Romero, R.A. Trabol, J.I. Avila, P. Lievens and A.L. Cabrera, Int. J. Hydrogen En-ergy, 36, 21 (2011).
[3] R. El Far, D.E. Diaz-Droguett, S. Rojas, J.I. Avila, C.P. Romero, P. Lievens, A.L. Cabrera, Thin Solid Films, 552 (2012). | |
