Metallic nanostructures which show localized surface plasmon resonances (LSPRs) have received considerable attention as a light harvesting optical antenna for light-energy conversion systems such as solar cells as well as artificial photosynthesis [1,2]. To construct efficient light harvesting optical antennae, an optimization of structural design is one of the most important research topics in plasmon-induced light energy conversions. Coupled plasmonic systems such as nanogap heptamer, dolmen, metal-insulator-metal nanostructures, and so on is promising as a photoelectrode design because of their strong near-field enhancement and wide wavelength responsibility. In this study, a coupled plasmonic system based on a waveguide-LSPR coupling system is employed to investigate whether the photocurrent response extends over a wide wavelength range and is promoted by near-field enhancement in the plasmon-induced photocurrent generation using gold nanostructured titanium dioxide (TiO₂) photoelectrodes. Near-field spectrum and photocurrent action spectrum are compared to elucidate the effect of near-field enhancement on the photocurrent generation.

[Experimental]

TiO₂ photoelectrodes supporting periodic gold nanogratings (AuNGs) with different pitch sizes were fabricated by a deposition of TiO₂ on a glass substrate with a thickness of 250 nm using an atomic layer deposition (ALD) reactor, and subsequent electron beam lithography and lift-off processes. A conventional photoelectrochemical measurement was performed. The AuNGs/TiO₂ photoelectrode, a platinum wire and a saturated calomel electrode (SCE) were employed as working, counter and reference electrodes, respectively. An aqueous Ar-gas-bubbled KClO₄ (0.1 mol/dm³) solution was used as a supporting electrolyte solution. A plasmon-induced water oxidation as a half reaction of water splitting was explored [3].

[Results and Discussion]

Figure 1(a) shows a scanning microscope (SEM) image of AuNGs/TiO₂ photoelectrode with a pitch size of 300 nm. Extinction spectra of the AuNGs/TiO₂ photoelectrode with a different pitch size are shown in Fig. 1(b). There is only one peak corresponding to the LSPR mode of periodic Au-NGs with 200 nm and 225 nm pitch sizes. Starting with a pitch size of 250 nm, three peaks can be observed, and the peaks show a spectral shift with increasing the pitch size. The two outer peaks can be assigned as coupled modes (P₊ and P_-) deriving from the symmetric waveguide mode E_w and LSPR mode E_p as shown in the illustration of the waveguide-LSPR coupling system (Fig. 1(c)). The center peak can be considered as a coupling mode between the asymmetric waveguide mode and the waveguide-LSPR coupling system.

Internal quantum efficiency (IQE) spectra and simulated near-field spectra of the AuNGs/TiO₂ photoelectrodes with 300 nm and 350 nm pitch sizes are shown in Fig. 2. The near-field intensity in the spectra was calculated by monitoring at the interface between the AuNGs and the TiO₂ film by the electromagnetic simulations using a finite-difference time-domain (FDTD) method. It was clearly elucidated that IQE spectrum has successfully reproduced the near-field spectrum under the coupling conditions. This indicates that the photocurrent response extended over a wide wavelength range utilizing the coupled plasmonic systems and near-field enhancement effects promoted the plasmon-induced water oxidation because IQE values increased responding to the near-field spectra [4].

References

1. Ueno, K., Oshikiri, T., Sun, Q., Shi, X., Misawa, H. Chem. Rev., DOI: 10.1021/acs.chemrev.7b00235.
2. Ueno, K., Oshikiri, T., H. Misawa, ChemPhysChem, 2016, 17, 199−215.
3. Nishijima, Y., Ueno, K., Yokota, Y., Murakoshi, K., Misawa, H. J. Phys. Chem. Lett. 2010, 1, 2031−2036.
4. Guo, J., Ueno, K., Shi, X., Oshikiri, T., Misawa, H. et al., J. Phys. Chem. C 2017, 121, 21627−21633.