Plasmonic light harvesting based on localized surface plasmon resonance (LSPR) of noble metal nanoparticles (NPs) has attracted much attention recently and is applied to photovoltaics and photocatalysis. There are two different ways of plasmonic light harvesting: (1) that based on a nanoantenna effect,1 in which a metal NP entraps a photon and transfers the obtained energy to a dye molecule or semiconductor via the optical near field generated in its vicinity, and (2) that based on plasmon-induced charge separation (PICS),2 in which an energetic electron is injected from a resonant NP to a semiconductor on contact. We reported PICS for the first time2 and indicated that it involves electron transfer from the plasmonic NPs to the semiconductor.18.104.22.168.–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,2,89.10.11.–12 photocatalysis,2,13,14 photochromism,1516.–17 photoactuation,18 and chemical sensing.19
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 deposition2 or thermal dewetting of an evaporated film9 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. In the case of Au nanorods (NRs) protected with cetyltrimethylammonium bromide in contact with a thin film, we addressed the issue by photoelectrochemical deposition of Au from aqueous at both ends of the NRs under UV irradiation for excitation of .22 The Au NRs were thus converted to nanodumbbells (NDs), and NDs exhibited improved efficiency of PICS-based electron transfer to via the two spherical caps at both ends of NDs. Here, we propose an easier method to improve the PICS-based photocurrents. We use plasmonic Ag nanocubes (NCs) protected by poly(vinylpyrrolidone) (PVP) in this work. Although Ag NCs exhibit nanoantenna effects2324.–25 and various interesting optical properties,2627.28.–29 their PICS efficiency when they are in contact with is low because of the insulating PVP layer. In the present work, in order to improve the PICS efficiency, we coupled the Ag NCs simply with an evaporated Au thin film, which is coated on the Ag NCs placed on or coated directly on as the underlayer of Ag NCs.
Preparation of Electrodes
Ag NCs were synthesized by the method reported by Im et al.30 with slight modifications28 [Fig. 1(a)]. The average edge length of the synthesized Ag NCs is about 80 nm. A transparent indium tin oxide (ITO) electrode was coated with a thin anatase film (60-nm thick, observed by scanning electron microscopy, SEM) by a spray pyrolysis method at 500°C from 2-propanol containing 0.028 M titanium diisopropoxide bis(acetylacetonate) as a precursor. The film was irradiated with UV light in order to make the surface sufficiently hydrophilic. The synthesized Ag NCs were drop-cast onto the surface (coverage ). If necessary, the substrate was coated with an Au film (ca. 5-nm thick, measured by quartz crystal microbalance) by vacuum evaporation before or after the drop-casting [Fig. 1(b)]. Thus, we prepared four different electrodes: NC, NC/Au, , and NC ().
Measurement of Photocurrents
One of the four electrodes was used as the working electrode. This electrode was connected to a potentiostat (SI 1280 B, Solartron) with an Ag|AgCl reference electrode and a Pt counter electrode and soaked in 0.1 M aqueous containing 0.5 M ethanol as an electron donor. The working electrode was polarized at versus Ag|AgCl and irradiated with monochromatic light (full width at , , and light ) using an optospectrum generator (Hamamatsu Photonics), and a steady-state photocurrent was measured at each wavelength. Virtually, the same photocurrents were observed even if the photoanode was short circuited to the counter electrode.
Results and Discussion
Photocurrent Responses of the ITO / TiO2 / Ag NC Electrode
Ag NCs dispersed in ethanol exhibit the main extinction peak based on dipolar LSPR at 506 nm. When an Ag NC is adsorbed onto a substrate with a high refractive index, the LSPR peak splits into two peaks due to the distal and proximal modes.26 In the distal mode, electron oscillation is localized at the top of the NC away from the substrate, and in the proximal mode, the oscillation is localized at the bottom of the NC in contact with the substrate. The Ag NCs on show two peaks [Fig. 1(c)], namely peak A at 456 nm due to the distal mode and peak B at 530 nm due to the proximal mode.
First, we measured photocurrent responses from the NC electrode irradiated from the backside with monochromatic light of different wavelengths in the electrolyte solution. Three independently prepared electrodes were examined and the average currents were plotted against the irradiation wavelength (Fig. 2). Small anodic photocurrents were observed in the wavelength range examined (430 to 670 nm). Conduction electrons in an Ag NC oscillate in resonance with electric field oscillation of the incident light, and an electron is injected from the resonant NC to the conduction band of , resulting in PICS.2 As a result, the anodic photocurrent flows [Fig. 3(a)]. However, the photocurrents were very low and no obvious current peaks corresponding to the distal and proximal modes were obtained. The low currents could be explained in terms of a blocking effect of insulating PVP, which covers and protects the NC surface. Capping agents like PVP are known to block access of reactant molecules.31,32 Likewise, the PVP layer should also inhibit the electron transfer from the Ag NC to .
Effect of a Thin Gold Coating
We therefore coated the Ag NCs on with a thin Au layer (ca. 5 nm thick) by evaporation, envisaging promotion of the electron transfer. The photocurrent responses from the NC/Au electrode thus prepared were measured and are shown in Fig. 2. The Au coating greatly improved the anodic photocurrents and a broad peak was obtained at around 550 nm. This electrode showed an extinction peak at 580 nm [Fig. 1(c)]. There is a possibility that the influence of the high refractive index substrate is weakened by the Au coating and therefore the distal mode is suppressed. The photocurrent response at the peak was about 20 times higher than that without the Au coating.
We also coated the Au layer directly on and examined its photocurrent responses for comparison. This electrode also exhibited photocurrent responses that increased gradually with increasing wavelength from about 500 nm. The extinction spectrum of the Au film showed a similar trend [Fig. 1(c)]. In general, a thin Au film prepared by evaporation consists of interconnected fine Au NPs, and hence it exhibits not only the propagating surface plasmon resonance, but also LSPR.33 Therefore, the present Au film also injects electrons into and exhibits photocurrents based on PICS [Fig. 3(b)]. Actually, nanoparticulate structures of 10- to 20-nm size were observed by SEM [Fig. 1(b)]. With reference to the response of this electrode, the introduction of Ag NCs under the Au thin film enhanced the photocurrents by approximately five times.
Mechanisms of the Photocurrent Enhancement
There are two possible reasons that the response from the NC/Au electrode is much higher than those from the NC and electrodes. One of them is “PICS with accelerated electron transfer” shown in Figs. 3(c) and 4(a), in which electron injection from the Ag NC or Ag@Au core-shell NC to is promoted by good electronic contact between the Au film and . The other one is “PICS enhanced by the nanoantenna effect” shown in Figs. 3(d) and 4(b), in which the electron injection from Au to is enhanced by optical near field generated by the Ag NC; the Ag NC absorbs light and generates optical near field in its vicinity, and then the Au film is efficiently excited by the near field (i.e., the nanoantenna effect2324.–25) and causes PICS. In both cases, the current would be increased by increasing the amount of Ag NCs.34
In order to obtain further information about the mechanisms of photocurrent enhancement, we prepared another type of electrode by drop-casting the Ag NCs onto the Au thin film coated directly on . The photocurrent action spectrum of the obtained NC electrode was similar to that of the NC/Au electrode (Fig. 2), and a broad peak was observed at around 535 nm. The photocurrent enhancement factors at the peak wavelength are 23 and 5 with reference to the NC and the electrodes, respectively. These photocurrent enhancements can also be explained in terms of both of the two possible mechanisms, namely the PICS with accelerated electron transfer and PICS enhanced by the nanoantenna effect. However, it is more likely that the Au thin film is excited by the optical near field of the NC proximal mode (i.e., the nanoantenna effect) because the PVP may also block the electron transfer from Ag NCs to the Au film.
Photocurrent responses based on PICS from the NC/Au and the NC electrodes are much higher than those from the NC and electrodes. These enhancements are likely due to PICS with accelerated electron transfer or PICS enhanced by the nanoantenna effect. These results would provide an easier way to improve PICS efficiency of photovoltaics, photocatalysis, and other systems with commercially available or chemically synthesized plasmonic NPs with protecting layers.
The authors are grateful to Gyu Min Kim for his help with immobilization of the NPs. This work was supported in part by a Grant-in-Aid for Scientific Research (Nos. 16H02082 and 16K14017) from the Japan Society for the Promotion of Science.
Kazutaka Akiyoshi is a PhD candidate in University of Tokyo, Japan. He received his BSc degree from Tokyo University of Science in 2014 and MSc from University of Tokyo in 2016. His research focuses mainly on development of photoelectrochemical devices based on plasmonic nanoparticles.
Koichiro Saito is a PhD candidate in University of Tokyo, Japan. He received his BSc in 2013 and MSc in 2015 degrees from University of Tokyo. His research focuses mainly on fabrication and photo-manipulation of plasmonic nanomaterials and application to photonic materials and devices.
Tetsu Tatsuma is a professor at University of Tokyo, Japan. He received his BSc in 1988, MSc in 1990, and PhD in 1993 degrees in electrochemistry from University of Tokyo. His current research interests include electrochemistry and plasmonics, in particular mechanistic analysis of plasmon-induced charge separation and its application to photovoltaics, photocatalysis, photochromism, sensors, and actuators.