Optical characterization of Au-supported anatase TiO₂ by using s-SNOM

抄録

Titanium dioxide (TiO₂) have crystalline structures to indicate photocatalytic effects, that are Anatase and Rutile. Anatase is more photocatalytic than Rutile. Loading Au nanoparticles (Au NPs) onto Anatase TiO₂, photocatalysis is enhanced [1, 2]. This phenomenon is contributed to by SPR (Surface Plasmon Resonance) [3]. SPR phenomenon is the resonance excitation of localized free charge oscillations (surface plasmons) on a metal surface by incident light interacting with the metal surface. As a consequence, SPR phenomenon leads to an increase in light absorption and enhancement of electric field intensity at the surface [2, 4]. In previous studies, Au deposited on silicon substrate, and on TiO₂ thin film of sapphire substrate were performed simultaneous Scanning Near-field Optical Microscopy (SNOM) / Atomic Force Microscopy (AFM) measurements. From the measurements conducted, it was observed that the electric field intensity of Au was smaller than that of the substrate. The reduction in Au electric field intensity was attributed to a decrease in the near-field signal of Au [4, 5]. However, the reasons for the decrease in Au near-field signal remain uncertain, although it has been determined to exhibit dependency on nanoparticle size. In this study, we focus to elucidate the relationship between the size and height of Au NPs obtained from AFM and the near field light intensity obtained from SNOM. To further clarify, we intend to understand the mechanism by which Au NPs contribute to the enhancement of catalytic activity in Anatase TiO₂.

In this experiment, Anatase TiO₂ was epitaxially grown on Nb doped SrTiO₃ (100) (0.05 wt%) substrate by Pulsed Laser Deposition (PLD). Following the deposition, measured by Reflection High-Energy Electron Diffraction (RHEED), Anatase TiO₂ (001) - (1×4) surface reconstruction is observed. RHEED image of Anatase TiO₂ (001) - (1×4) reconstruction surface shows Figure1 (a). Subsequently, Au NPs were deposited in vacuum and anneal at 400 degrees in a furnace, Au -supported Anatase TiO₂ was made. The produced Anatase TiO₂ was characterized by using Scanning Near-Field Optical Microscopy (SNOM) / Atomic Force Microscopy (AFM). SNOM involves bringing a probe into contact with the sample surface, detecting the scattering light of near-field light generated at the probe's tip, and performing Fourier series expansion on the scattered near-field light to map the intensity of higher-order harmonics. On the other hand, AFM oscillates the probe at a constant frequency and makes contact with the sample surface to measure the topography of the sample surface. The SNOM/AFM measurement setup used in this experiment shows Figure1 (b). A incident laser with a wavenumber (k) of 1634 cm^-1 detects through a beam splitter, focusing the laser through an off-axis parabolic mirror. The scattered near-field light from the interaction of the probe and the sample surface is directed to a detector, allowing for the observation of the near-field intensity (third harmonic frequency spectrum). The obtained distributions of near-field intensity and topography from SNOM/AFM were plotted to illustrate the relationship between Au NPs size, height, and near-field light intensity. Figure1 (c, d) illustrates the SNOM/AFM simultaneous measurement images of Anatase TiO₂ with a 2 Å deposition of Au NPs. Figure1 (c) is the image displays the third harmonic near-field intensity (SNOM Image), while Figure1 (d) is the topography image obtained from AFM (AFM Image). From Figure1 (c, d), tendency is observed that the intensity of near-field intensity decreases as Au NPs size increases.

References

[1]: Jingtao Zhang, et al., Material Letters, 162, 235 (2016)

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