Reduced Ensemble Plasmon Line Widths and Enhanced Two-Photon Luminescence in Anodically Formed High Surface Area Au–TiO₂ 3D Nanocomposites

Abstract

Localized surface plasmon resonances (LSPR) in TiO nanorod and nanotube arrays decorated by gold nanoparticles can be exploited to improve photocatalytic activity, enhance nonlinear optical coefficients, and increase light harvesting in solar cells. However, the LSPR typically has a low quality factor, and the resonance is often obscured by the Urbach tail of the TiO band gap absorption. Attempts to increase the LSPR extinction intensity by increasing the density of gold nanoparticles on the surface of the TiO nanostructures invariably produce peak broadening due to the effects of either agglomeration or polydispersity. We present a new class of hybrid nanostructures containing gold nanoparticles (NPs) partially embedded in nanoporous/nanotubular TiO by performing the anodization of cosputtered Ti-Au thin films containing a relatively high ratio of Au:Ti. Our method of anodizing thin film stacks containing alternate layers of Ti and TiAu results in very distinctive LSPR peaks with quality factors as high as 6.9 and ensemble line widths as small as 0.33 eV even in the presence of an Urbach tail. Unusual features in the anodization of such films are observed and explained, including oscillatory current transients and the observation of coherent heterointerfaces between the Au NPs and anatase TiO. We further show that such a plasmonic NP-embedded nanotube structure dramatically outperforms a plasmonic NP-decorated anodic nanotube structure in terms of the extinction coefficient, and achieves a strongly enhanced two-photon fluorescence due to the high density of gold nanoparticles in the composite film and the plasmonic local field enhancement.

The Polarization Effect in Surface‐Plasmon‐Induced Photocatalysis on Au/TiO2 Nanoparticles

“…High‐resolution X‐ray photoelectron spectroscopy (XPS) showed that the Au 4f of the Au/TiO 2 sample shifted to a lower binding energy after the annealing treatment (Figure b), suggesting the occurrence of fine interfacial contact and electron transfer from TiO 2 to Au NPs. The high‐resolution transmission electron microscopy (HRTEM) image revealed that intimate physical contact was formed at the Au NPs/TiO 2 interface (Supporting Information, Figure S3), which is important for hot electron injection from Au to TiO 2 upon SPR excitation . Figure c shows the absorption spectra of the pure TiO 2 and AuNPs/TiO 2 photocatalysts.…”

Hot Electrons in TiO2–Noble Metal Nano-Heterojunctions: Fundamental Science and Applications in Photocatalysis

“…Two-photon luminescence (TPL) is a photoluminescence technique utilized to analyze the plasmonic response of noble metal nanoparticle systems [ 195 , 196 , 197 ] in which the noble metal nanoparticle is excited by the simultaneous absorption of two photons, usually in the infrared regime, with sufficiently intense laser illumination [ 198 ]. Single ultraviolet photon excited photoluminescence in plasmonic nanostructures exhibits a linear dependence of the emission intensity on the excitation power, and is understood to proceed through the recombination of d -band holes with sp -band electrons, as previously mentioned.…”

“…In principle, steady-state and time-resolved TPL could also be used to probe the dynamics of carriers created by interband transitions, specifically to understand whether photoexcited electrons in the several micrometer-thick noble metal–semiconductor nanocomposites are transferred to the semiconductor or not (i.e., by observing if some or all of the two-photon photoluminescence of the isolated noble metal nanoparticle is quenched in the presence of the semiconductor), and to measure the time-scales of competing processes, such as plasmon-mediated radiative recombination; however, very few reports have actively used TPL to examine phenomena specifically related to hot electrons. TPL was used by Farsinezhad et al [ 195 ] in demonstrating the effectiveness of a novel plasmonic nanoparticle-embedded nanotube structure in generating a high local field enhancement that outperformed a plasmonic nanoparticle-decorated anodic nanotube structure. ( Figure 17 ) Localized surface plasmon resonances, and surface plasmon modes are both influenced by the local dielectric properties, and fluctuations in the charge density of the given plasmonic material.…”

“…Apart from the electronic structure of the metal nanoparticle, numerous other properties, including the loading amount and size [ 151 ], architecture of the noble metal nanoparticle [ 58 , 60 , 181 , 238 ], the amplitude of UV irradiation [ 151 ], the structural distribution of the noble metal nanoparticle forming the semiconductor heterojunction (composite plasmonic nanostructure systems where the plasmonic noble metal is embedded within the semiconductor nanostructure as opposed to decorating the nanostructure have resulted in greater performance efficiencies), all influence the mechanism of hot electron injection in noble metal nanoparticle–semiconductor heterojunctions [ 195 , 249 , 289 ].…”

“…These conditions are codependent on the nanoparticle geometry and size. As Ratchford et al [ 130 ] point out, in the case of Au nanoparticles embedded in TiO 2 (a system realized by Farsinezhad et al [ 195 ]), wherein the nanoparticle dimensions are less than the mean free path of electron–electron scattering most of the excited electrons are allowed to reach the interface. Similarly, the fact that the nanoparticles are fully embedded within the semiconductor provides greater opportunities for the excited electron to be injected into the semiconductor, and last but not least, the small size of the nanoparticles assists in relaxing momentum conservation requirements at the interface.…”