In Situ Nanoimaging of Photoinduced Charge Separation at the Plasmonic Au Nanoparticle‐TiO₂ Interface

Abstract

Nb-doped rutile TiO 2 (100), unless otherwise noted) in the presence of a Na [AuCl 4 ] aqueous solution mixed with ethanol (refer to SI for details of the experiments). Electrons in the TiO 2 valence band (VB) are excited by UV light to the conduction band (CB), and holes are generated in the VB. Au 3+ ions are reduced to Au NPs by the excited electrons, and ethanol, a sacrifi cial electron donor, is oxidized by the holes simultaneously. In addiation to polydisperse spheroids (diameter ≈20-110 nm, height/diameter ≈0.5), truncated triangular nanoplates (sides ≈60-280 nm, height ≈7-16 nm) consisting mainly of Au(111) faces were deposited ( Figures 1 a, S2a). Similar NPs were also deposited on a non-doped rutile TiO 2 (100) ( Figure S2b) and the NPs exhibited LSPR-based extinction in the visible and NIR regions (500-1600 nm) ( Figure S2c).We evaluated photoinduced surface potential changes of the sample with KFM (Nanonavi Station/E-sweep, SII Nanotechnology) in non-contact mode at a scan rate of ≈0.5−1.0 Hz by using a gold-coated silicon cantilever (SI-DF3-A, SII Nanotechnology) with a normal spring constant of 3 N m −1 and tip curvature radius of 30 nm. Tip was scanned at a constant distance (10 nm) from the sample surface. The sample was irradiated with s-polarized light, and the incident angle was 45°. Measurements were performed in a closed KFM chamber after the chamber was degassed (≤3.0 Pa) and refi lled with dry N 2 gas (including 0.3 ppm O 2 and 1.0 ppm H 2 O) until atmospheric pressure was reached, to avoid dissociation of oxygen from the TiO 2 surface under high vacuum.[ 20 ] In a KFM measurement, an offset bias voltage ( V off ) is applied between the sample and the tip so as to cancel out the electrostatic force between them. The surface potential of the sample vs. tip ( E sample vs. tip ) equals − V off . In the present work, the potential of a clean Pt plate vs. tip ( E Pt vs. tip ) is measured and the potential of the sample surface is reported in reference to that of Pt ( E sample = E sample vs. tip -E Pt vs. tip ). Incidentally, E Pt vs. tip was not changed by light irradiation, suggesting that the photoinduced potential change of the tip was negligible. On the basis of a fi nite-difference time-domain (FDTD) calculations, LSPR of the KFM tip is excited only in the 400−600 nm region. Therefore, the results for TiO 2 excitation by UV light and Au nanoplate excitation by NIR light are not affected by the tip plasmon. In addition, electric fi eld is not localized at the tip under s-polarized light, and it is known that PICS occurs at sites where electric fi eld is localized. [ 15,16 ] When the TiO 2 substrate without Au NPs was excited by 310 nm UV light from a Hg-Xe lamp (0.48 mW cm −2 ), the surface potential of TiO 2 ( E TiO2 ) shifted uniformly by about +280 mV ( Figure S3), whereas topographic images remained unchanged. Since the average exciton lifetime in TiO 2 (<1 ns) [ 21 ] is much shorter than the average interval of photon incidence (13 μs even for a 10 4 nm 2 region), the observed potentia...

Plasmon-induced charge separation at two-dimensional gold semishell arrays on SiO2@TiO2 colloidal crystals

“…Those photoelectrochemical responses were highly reproducible for on-off cycles. These results suggest that PICS is caused at the Au-TiO 2 interface, as is the case for metal nanoparticle PICS; 28 electrons are transferred from photoexcited Au semishells to conduction band of the TiO 2 layer. On the other hand, Au-coated flat TiO 2 electrode without SiO 2 @TiO 2 spheres showed very small responses because of no significant absorption in the wavelength range examined.…”

Metal and Metal Oxide Nanoparticles for Photoelectrochemical Materials and Devices

“…9,37 In contrast, anodic photocurrents flow when nanoparticles are deposited on a TiO 2 electrode. 35,38 These results indicate that PICS is based on electron transfer 39 from the plasmonic nanoparticle to TiO 2 . PICS attracts much attention, because of its wide range of applications.…”

Enhancement of plasmon-induced charge separation efficiency by coupling silver nanocubes with a thin gold film

“…We reported PICS for the first time 2 and indicated that it involves electron transfer from the plasmonic NPs to the semiconductor. [2][3][4][5][6][7] The electron transfer involved is explained in terms of external photoelectric effect, hot electron injection, or interfacial electron transition, and recently some groups call it plasmonic hot electron (or carrier) injection. PICS has been widely applied to photovoltaics, 2,[8][9][10][11][12] photocatalysis, 2,13,14 photochromism, [15][16][17] photoactuation, 18 and chemical sensing.…”

“…A wide variety of plasmonic NPs have been used for PICS such as nanospheres, 20 nanoplates, 7 and halfshell arrays. 21 NPs prepared by photocatalytic deposition 2 or thermal dewetting of an evaporated film 9 are suitable for PICS because of direct contact with the semiconductor, while chemically synthesized NPs may exhibit lower charge separation efficiency since they are covered with a protective agent.…”