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Published in:
2020 | OriginalPaper | Chapter
9. Si Nanopowder for Photoluminescence and Hydrogen Generation Materials
Authors : Yuki Kobayashi, Hikaru Kobayashi
Published in: Theoretical Chemistry for Advanced Nanomaterials
Publisher: Springer Singapore
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Abstract
Si nanopowder fabricated from Si swarf using the beads milling method exhibits two kinds of photoluminescence (PL), green-PL and blue-PL. Green-PL arises from band-to-band transition of Si nanopowder with band-gap enlarged by the quantum confinement effect. Blue-PL, on the other hand, is attributable to adsorbed 9,10-dimethylanthracene (DMA) impurity in hexane because the structure of the observed PL spectra is nearly identical to that of DMA solvent. The peaked PL spectra arise from vibronic interaction of DMA, and nearly the identical separation energies between the neighboring peaks correspond to the vibrational energy of DMA in the electronic ground-state. The PL intensity of DMA is enhanced by 60,000 times due to adsorption of DMA on Si nanopowder. For excitation photon energies higher than 4.0 eV, new peaks appear in the energy region higher than the (0, 0) band, attributable to transition from vibrational excited-states.
Si nanopowder reacts with water in the neutral pH region between 7 and 9. The hydrogen generation rate strongly depends on pH, while pH doesn’t change after the reaction. Si nanopowder reacts with OH− ions, generating hydrogen, SiO2, and electrons in the SiO2 conduction band. Electrons are accepted by water molecules, generating hydrogen and OH− ions. Since OH− ions act as a catalyst, the hydrogen generation rate greatly increases with pH. The generated hydrogen volume vs. the reaction time follows a logarithmic relationship, indicating that migration of OH− ions through the SiO2 layer is the rate-determining step. The hydrogen generation reaction stops when the SiO2 thickness reaches to ∼5 nm.