Abstract:
Carbon nanotubes (CNTs) are the most extensively investigated carbonaceous
material for hydrogen storage. Chemisorption of atomic hydrogen has already been
reported for graphite as a planer model system of CNTs with regard to the degree of
hydrogenation and the preferable adsorption structure. Boron nitride (BN) has
attracted much research attention as a promising candidate for hydrogen storage and
it has been reported that BN nanotubes are more superior compared to CNTs for
storing hydrogen. But, it is not clear yet how the adsorption mechanism works on BN
materials. One of the most basic arguments is the site dependence of atomic hydrogen
adsorption. However, there are a number of theoretical reports that contradict with
each other. In this study, we have investigated the site dependence of atomic
deuterium adsorption on a thin film of BN, using near-edge X-ray absorption fine
structure (NEXAFS), X-ray photoelectron spectroscopy (XPS), and photo stimulated
ion desorption (PSID) in order to verify the appropriate theoretical models.
All experiments were performed with linearly-polarized synchrotron radiation at the
BL-11A beam line of the Photon Factory in the High Energy Accelerator Research
Organization, Japan. A BN thin film was formed on Ni (111) by chemical vapor
deposition with borazine gas. Then the film was exposed to an atomic deuterium beam
produced by a tungsten hot filament in an ultra-high vacuum chamber. NEXAFS and
XPS spectra were measured before and after the exposure to atomic deuterium. A
clear spectral change was observed in the B K-edge region after deuteration and only
slight change was observed in the N K-edge region. The XPS spectrum of B 1s showed
a prominent shoulder on the low binding energy side of the main peak after
deuteration, while the N 1s spectrum showed only peak broadening at the high
binding energy side. After deuteration, D+ ion yield was measured as a function of the
incident photon energy at the B and N K-edges in the PSID measurements. It was
observed that there was a clear enhancement of D+ signal at the B K-edge but no such
enhancement at the N K-edge. The experimental results of NEXAFS and XPS were
analyzed by the DV-Xα molecular orbital calculation method with core-hole effect.
Finally, it was concluded that hydrogen atoms are preferentially adsorbed on the B
sites of BN materials, via a single hydrogen adsorption model.
