Visible Plasmon-Enhanced Water Splitting

The interaction between photon and molecule is weak. The probability of interaction between photon and molecule is roughly estimated as low as 10^-7 because the focal spot size of visible light (~1×10^-8 cm²) is 10⁷ times larger than the general molecular absorption cross-section (~1×10^-15 cm²) due to the relationship between the diffraction limit of the light and the molecular size. Thus far, the number of excited-state molecules has been increased by photon density using a laser as an excitation source, although there is a loss of energy since the probability of interaction does not change. We have recently found that metallic nanostructures exhibiting localized plasmon resonance are promising in the photochemical reaction field, which make it possible to increase the interaction between photons and molecules. As one of the most important achievements, we have observed significant photopolymerization via two-photon absorption in photoinitiator molecules of negative photoresists in the nanogaps of closely spaced gold nanoparticles irradiated by a weak incoherent light source [1]. The key technology and outstanding feature of our photochemical reaction fields is the fabrication of nanogap metallic nanostructures with nanometric accuracy and use of the localization of electromagnetic waves within nanogaps. Near-field enhancement effects localized on nanogap gold structures were studied by two-photon photoluminescence from gold [2], surface-enhanced Raman scattering [3] and plasmon-enhanced photochemical reactions [4,5]. Thus, gold nanostructures are useful in photochemical reaction fields, which make it possible to promote strong coupling between photons and molecules with the aim of effectively using photons. From this standpoint, we have recently demonstrated plasmonic photoelectric conversion from visible to near-infrared wavelengths using electrodes in which gold nanorods are arrayed on TiO₂single-crystal and photocatalytic water splitting [6,7].

References

[1] K. Ueno, H. Misawa et al. J. Am. Chem. Soc. 2008, 130, 6928.

[2] K. Ueno, H. Misawa et al. Adv. Mater. 2008, 20, 26.

[3] Y. Yokota, K. Ueno, H. Misawa, Chem. Commun. 2011, 3505.

[4] K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 2010, 1, 657.

[5] B. Wu, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 2012, 3, 1443.

[6] Y. Nishijima, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 2010, 1, 2031.

[7] Y. Nishijima, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 2012, 3, 1248.