Abstract
The initial reaction of Si nanopowder with water to generate hydrogen is investigated using FT-IR and XPS measurements. Si nanopowder is fabricated using the simple beads milling method. For HF-etched Si nanopowder, strong peaks due to Si-H and Si-H2 stretching vibrational modes and a weak shoulder peak due to Si-H3 are observed. Although no peaks due to oxide is observed in the Si 2p XPS spectrum, weak vibrational peaks due to HSiO2 and HSiO3 species are observable. The hydrogen generation rate greatly increases with pH, indicating that the reacting species is hydroxide ions (OH− ions). After the reaction, the intensities of the peaks due to SiH and SiH2 species decrease while those for HSiO, HSiO2, and HSiO3 species increase. This result demonstrates that OH− ions attack Si back-bonds, with surface Si-H bonds remaining. After initial reaction of HF-etched Si nanopowder with heavy water, vibrational peaks for SiD, SiDH, and SiDH2 appear, and then, a peak due to DSiO3 species is observed, but no peaks due to DSiO2 and DSiO species are observable. This result indicates that SiD, SiDH, and SiDH2 species are formed by substitution reactions, followed by oxidation of back-bonds to form DSiO3 species. After immersion in D2O for a day, 37% H atoms on the surface are replaced to D atoms.
Highlights
Studies on Si nanostructures have attracted much interest because of their significant characteristics different from bulk Si, e.g., wider band-gap energies due to the quantum confinement effect[1,2] and higher reactivity
Bahruji et al.,[13] have found that the hydrogen generation reaction of Si with strong alkaline solutions is greatly enhanced by UV irradiation
Si nanopowder was etched with 5 wt% hydrofluoric acid (HF) aqueous solutions for 10 min
Summary
Studies on Si nanostructures have attracted much interest because of their significant characteristics different from bulk Si, e.g., wider band-gap energies due to the quantum confinement effect[1,2] and higher reactivity. Due to the wider band-gap energies,[3,4] Si nanostructures emit photoluminescence (PL) in the visible region, and they are applicable to biomarkers[5,6] and opto-devices.[7,8]. All the above studies concerning hydrogen generation from Si nanostructures use strong alkaline solutions. We have recently shown that Si nanopower with sizes less than ∼20 nm reacts with water in the neutral pH region between 7.0 and 8.6 to generate hydrogen,[11] which is useful especially for medical applications
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