Chapter 9
Si Nanopowder for Photoluminescence
and Hydrogen Generation Materials
Yuki Kobayashi and Hikaru Kobayashi
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, SiO 2 , and
electrons in the SiO 2 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 SiO 2 layer is the rate-determining step. The hydrogen generation
reaction stops when the SiO 2 thickness reaches to ∼5 nm.
Keywords PL enhancement · Vibrational excited-state · Neutral water ·
Hydroxyl ions · Internal hydrogen generation · Oxidative stress
Y. Kobayashi · H. Kobayashi ()
Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan
e-mail: h.kobayashi@sanken.osaka-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
T. Onishi (ed.), Theoretical Chemistry for Advanced Nanomaterials,
https://doi.org/10.1007/978-981-15-0006-0_9
353
Si Nanopowder for Photoluminescence
and Hydrogen Generation Materials
Yuki Kobayashi and Hikaru Kobayashi
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, SiO 2 , and
electrons in the SiO 2 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 SiO 2 layer is the rate-determining step. The hydrogen generation
reaction stops when the SiO 2 thickness reaches to ∼5 nm.
Keywords PL enhancement · Vibrational excited-state · Neutral water ·
Hydroxyl ions · Internal hydrogen generation · Oxidative stress
Y. Kobayashi · H. Kobayashi ()
Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan
e-mail: h.kobayashi@sanken.osaka-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
T. Onishi (ed.), Theoretical Chemistry for Advanced Nanomaterials,
https://doi.org/10.1007/978-981-15-0006-0_9
353