Structural Tuning of Optical Antenna Properties for Plasmonic

Article pubs.acs.org/JPCC

Structural Tuning of Optical Antenna Properties for Plasmonic Enhancement of Photocurrent Generation on a Molecular Monolayer System Katsuyoshi Ikeda,*,†,‡,§ Kenji Takahashi,† Takuya Masuda,§ Hiromu Kobori,∥ Masayuki Kanehara,∥ Toshiharu Teranishi,∥,⊥ and Kohei Uosaki*,†,§,# †

Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan ∥ Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan ⊥ Institute of Chemical Research, Kyoto University, Uji 611-0011, Japan # International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan ‡

ABSTRACT: Metal nanostructures have the potential to increase photon− molecule interaction efficiency due to their plasmonic antenna effect. However, plasmonic enhancement of the efficiency of photochemical reactions such as photosynthesis is rather difficult because excited molecules are easily quenched near the metal surface. Sphere−plane metal nanostructures have an advantage to reduce such undesirable contribution due to their controllability of metal− molecule interfaces. Here, structural tuning of optical antenna properties, which is indispensable for selective enhancement of photochemical reactions, is demonstrated as plasmonic enhancement of photoinduced electron transfer reactions on porphyrin-based monolayer systems using the sphere−plane nanoantennas. The efficiency of photocurrent generation was substantially enhanced in good agreement with the plasmonic resonance properties of the optical antenna. The contribution of the excited state quenching was also evaluated by comparing the enhancement factor of the photocurrent and the degree of the field enhancement. This result provides useful insights for designing efficient plasmonic photochemical systems. plasmonically enhanced optical phenomena.12−16 Plasmonic resonance properties of metal nanostructures are explained by the electromagnetic theory. Hence, one of main issues in plasmonics is to design efficient antenna structures and precise shaping of such metal nanostructures.17,18 After the discovery of SERS, many attempts have been made to apply plasmonic enhancement to photochemistry.19−23 Despite the early developments on plasmonic enhancement of photochemical reactions such as photodecomposition,24−30 this area is actually still unexplored. In the case of more complicated photochemical reactions such as photoenergy conversion, the reaction efficiency depends not only on the photon capture efficiency but also on the charge separation or redox reaction efficiency. Unfortunately, the presence of nanostructured metals often reduces the efficiency of the latter processes. Especially, excited-state quenching is a serious problem in plasmonic photochemical systems.24−27 To minimize these undesirable effects, physicochemical interac-

1. INTRODUCTION Photon capture is a critical process in solar-energy conversion systems. However, even strongly photoabsorbing molecules have absorption cross sections that are only on the order of 10−2−10−3 nm2.1 Natural photosynthetic organisms contain optical antennas that promote efficient capture of photon energy.2 Because natural photosynthesis is considered to be the most elegant example of solar energy conversion, many researchers have attempted to construct artificial photosynthetic molecular systems.3−7 Molecular systems have the advantage of flexible design that enables the connections between various functional elements to be optimized. Unfortunately, it is currently too difficult to synthesize entire photosynthetic systems including antenna complexes. Metal nanostructures have recently been attracting much interest due to their potential to be used as optical antenna.8−11 Propagating optical radiation excites collective oscillations of free electrons known as surface plasmons on metal nanostructures. This localizes the photon energy near the metal nanostructure leading to efficiency enhancement of various optical events near the metal surface. Surface-enhanced Raman scattering (SERS) is known to be the first example of such © 2012 American Chemical Society

Received: August 21, 2012 Revised: September 5, 2012 Published: September 13, 2012 20806

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tions between metal and molecules at optical antenna surfaces need to be carefully managed through controlling the atomic surface arrangements of metal nanostructures. Plasmonic studies have mostly focused on engineering the electromagnetic response of optical antennas. This is partly because it is still difficult to control atomic surface features of optical antennas. We have recently reported that both the electromagnetic and physicochemical properties of optical antennas can be controlled simultaneously using sphere− plane metal nanostructures.31−35 This antenna structure36,42 can capture photon energy even at atomically controlled planar metal surfaces. We have then demonstrated that photoinduced uphill electron transfer was plasmonically enhanced in a molecular-monolayer system.43 Because photodriven electron transfer is the main process in photosynthetic energy conversion, this system should be a good model for studying plasmonic enhancement of photoenergy conversion. One advantage of plasmonic optical antennas is that their antenna resonances can be tuned by the shape and size of metal nanostructures.17,18 This is widely accepted in the field of plasmonics, but experimental confirmation in photochemical systems is still unclear. In the sphere−plane system, the plasmonic resonance can be well controlled by the use of an atomically planar single crystalline metal substrate. This feature is suitable to elicit purely plasmonic effects from the complicated behavior of photochemical systems with plasmonic optical antennas. Here, we present convincing evidence that photocurrent enhancement conforms well with the plasmonic resonance properties of the optical antennas.

2. EXPERIMENTAL SECTION Part a of Figure 1 shows a schematic of a porphyrin-ferrocenethiol linked molecule and energy diagram of photoinduced uphill electron transfer in the molecule bound to an electrode surface. The details of the electron transfer process will be explained later. Synthesis of the molecule and formation of the molecular monolayer have been described before.5,43,44 The monolayer was formed on an Au(111)-like gold electrode with roughness factor of less than 1.1 (left panel of part b of Figure 1). The sphere−plane optical antenna structures were constructed by immobilizing Au nanoparticles (Au NPs) onto the monolayer-covered Au substrate (bottom of part b of Figure 1). In this hybrid system, the incident photon energy is confined within the sphere−plane gap as a result of excitation of the hybridized plasmon mode (right panel of part b of Figure 1) and then delivered to the porphyrin layer efficiently. We used Au NPs coated with poly(diallyldimethylammonium chloride) (PDDA)45−47 to avoid direct contact of porphyrins with metal surfaces, which may cause undesirable energy or electron transfers. The incident photon-to-current efficiency (IPCE) of the system was measured in 0.1 M NaClO4 solution with 0.5 mM MV under an applied electrochemical potential of −200 mV versus Ag/AgCl. Photoirradiation was provided by a Xe lamp through a monochromator (light intensity of 0.2 mW/cm2 at 650 nm) with the incident angle of 45 degree to excite the hybridized plasmon mode on the electrode surface.5,43

Figure 1. (a) Schematic diagram of a porphyrin-ferrocene-thiol linked molecule and an energy diagram for photoinduced uphill electron transfer in the molecular system (P, porphyrin; excited P*, porphyrin; Fc, ferrocene; MV, methyl viologen). (b) Schematic illustrations of a photofunctionalized electrode (upper left panel), a sphere−plane optical antenna system explaining that the electromagnetic interaction between the Au NP and Au electrode creates the hybridized dipole plasmon mode (upper right panel), and optical antenna−molecule hybrid system (bottom panel).

cathodic photocurrent from the electrode to the electron acceptors (methyl viologen, MV) via the redox center of ferrocene (Fc). The photocurrent generation efficiency is generally sensitive to the degree of reverse electron transfer. The present system has three main reverse electron transport pathways: (i) from MV to P, (ii) from MV to the electrode, and (iii) from P* to the electrode. Reverse electron transfer (i) can be negligible when the electrode potential is negative with respect to the redox potential of Fc. The other two pathways, (ii) and (iii), are reduced when the electrode surface is completely covered with molecules. Therefore, these two pathways are expected to be sensitive to the atomic arrangement of the electrode surface because the atomic arrangement affects the molecular organization. In fact, when an atomically defined gold electrode surface is utilized as a substrate for highly organized molecular monolayers, an internal quantum efficiency of over 10% can be realized in this molecular system; this efficiency decreases rapidly with increasing atomic-scale surface roughness.44

3. RESULTS AND DISCUSSION 3.1. Photofunctional Molecular System. First, we briefly describe our photofunctional molecular system. As shown in part a of Figure 1, optical excitation of porphyrin (P) induces a 20807

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3.2. Plasmonic Resonances of Au Sphere-Au Plane Nanostructures. Planar metal surfaces are unsuitable for surface plasmon excitation,48 but they are suitable for preparing well-ordered monolayers. Conversely, roughened metal surfaces are commonly utilized as plasmonic substrates, but they are not suitable as a support for well-organized molecular layers; the net efficiency decreases with increasing the contributions of reverse reaction pathways such as (ii) and (iii) in part a of Figure 1. As we have already pointed out, optical antennas that have the potential to overcome this problem are the sphere− plane nanostructures (part b of Figure 1).31−43 The resonance properties of sphere−plane systems are determined by the sphere−plane distance (which in this case is equal to the SAM thickness) and the Au NP size.36,37 It is then expected that the resonance is tunable by alternating the size of the Au NPs. To investigate this possibility, we utilized highly monodispersed Au NPs.45−47 Figure 2 shows TEM images of

Figure 3. (a) Typical SEM image of 50 nm diameter octahedral Au NPs, which were deposited on a monolayer-covered Au electrode. Insert shows enlarged image of Au NPs. (b) Normalized extinction spectra of theoretically calculated Au NP/Au-plane system with a separation of 5 nm. (c) Reflectance spectra of Au NP/SAM/Au substrate systems with respect to an Ag mirror. Figure 2. (a) TEM images of the octahedral Au NPs with different mean sizes: edge lengths of 30 nm (30.0 ± 2.0 nm), 40 nm (39.5 ± 1.8 nm), 50 nm (49.5 ± 2.1 nm), and 60 nm (58.3 ± 2.9 nm). (b) Normalized extinction spectra of experimentally obtained colloidal Au NPs in aqueous solution.

resonance because interparticle plasmon coupling may affect the sphere−plane plasmon resonances.36,37 Part b of Figure 3 shows theoretically calculated extinction spectra of an Au sphere−Au plane system. They were calculated for p-polarized illumination by taking into account multipolar contributions. The detailed calculation procedures based on the Wind method have been described elsewhere.32,49,50 The Au NP was assumed to be spherical to simplify the calculation. The sphere−plane separation was assumed to be 5 nm by considering the porphyrin-monolayer thickness of 4.4 nm and the PDDA layer of the Au NP.51 In part b of Figure 3, there are two distinctive plasmon modes in the wavelength ranges 550−600 nm and 600−700 nm; the longer wavelength peak is greatly red-shifted from the particle plasmon resonance peak in the Au colloidal solution. The amount that the peak is shifted increases with increasing Au NP diameter. This resonance peak shift indicates that the Au NP is plasmonically interacting with the Au substrate. Hence, the longer wavelength peak can be assigned to the hybridized dipolar plasmon mode while the shorter

the Au NPs and their extinction spectra measured in aqueous solution. One can see that the Au NPs are well controlled to octahedral shape with mean sizes of 30 nm (30.0 ± 2.0 nm), 40 nm (39.5 ± 1.8 nm), 50 nm (49.5 ± 2.1 nm), and 60 nm (58.3 ± 2.9 nm). With increasing diameter, the resonance wavelength near 550 nm is slightly red-shifted. Part a of Figure 3 shows a typical SEM image of the Au NPs/ SAM/Au electrode. The insert shows that the triangular crystal facets of the Au NPs are oriented parallel to the electrode surface, indicating that the sphere−plane nanostructures are almost perfectly controlled. More importantly, the deposited Au NPs are spatially well separated from each other on the SAM. This is needed for precise control of the antenna 20808

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wavelength one to the higher-order mode.52−54 One can expect that the incident photon energy is delivered to porphyrin monolayers within the sphere−plane gap in the wavelength region of the hybridized dipolar mode. Importantly, this peak position coincides with the Q-bands of porphyrins.5 Thus, the photocurrent is expected to be enhanced in the Q-band region in the presence of Au NPs. (As for 50 nm Au NPs, we have indeed succeeded the enhancement of the photocurrent.43) The calculated results were then experimentally examined by measuring reflectance of the Au NPs/SAM/Au system with respect to a silver mirror reference. As shown in part c of Figure 3, there is a dip in the spectrum, which position is consistent with that of the calculated dipolar plasmon mode; the dip position was certainly shifted by alternating the Au NP diameter. This is direct evidence that sphere−plane metal nanostructures were constructed on the electrode in the bottom-up manner. 3.3. Photocurrent Enhancement. In the absence of Au NPs, there appear distinct photocurrent peaks in the action spectrum of porphyrin−SAMs, which agree with the absorption bands of porphyrin.5 As we have reported in the previous preliminary work, the deposition of Au NPs can increase the photocurrent in the entire visible region. Here, we focus on the plasmonic enhancement in the Q-band region. Figure 4 shows

Figure 5. (a) EF per optical antenna for various antenna sizes as a function of excitation wavelength. (b) Calculated field enhancements, | Eloc/E0|2, for Au NPs of various sizes as a function of excitation wavelength.

NPs enhanced mainly the Qx(0−1) and Qx(0−0) bands. The 40 nm Au NPs enhanced the Qx(0−0) band, whereas the 50 and 60 nm Au NPs significantly enhanced the low-energy tail of the Qx(0−0) band. These results clearly indicate that the most enhanced wavelength is red-shifted with increasing Au NP size; this behavior agrees well with the shift in the plasmon resonances in Figure 3. The observed photocurrent enhancement is ascribed to the optical antenna effect of the sphere− plane nanostructures. To discuss the antenna effect more quantitatively, it is essential to estimate the enhancement factor (EF) of a single optical antenna because the net photocurrent is affected by the density of the optical antennas or adsorbed chromophores. Such estimation is generally difficult in conventional plasmonic systems such as an irregularly roughened metal surface or colloid aggregates. In the sphere−plane system, however, the surface density of the optical antennas is simply equal to that of the adsorbed Au NPs, which can be evaluated for each sample from SEM photographs. Moreover, the density of porphyrins is well controlled on the atomically defined electrode surfaces. Part a of Figure 5 shows the EF per optical antenna as a function of the excitation wavelength, which was obtained from Figure 4. When the 30 nm Au NPs were deposited on the SAM, the maximum EF was estimated to be about 20 at λ = 630 nm. When the 60 nm Au NPs were utilized, the maximum EF reached to 140 at λ = 700 nm. Such peak shift of the EF is in good agreement with the reflectance spectra in part c of Figure 3. It shows that the plasmonic optical antenna effect clearly depends on the Au NP size. This is the first experimental demonstration of the resonance tuning of optical antennas in photoelectrochemical molecular systems.

Figure 4. IPCE spectra measured without and with variously sized Au NPs. These were measured in 0.1 M NaClO4 electrolyte solution containing 5 mM MV as an electron acceptor under an applied potential of −200 mV vs Ag/AgCl. Photoirradiation was provided by a Xe lamp through monochromator with the incident angle of 45 degree to excite the hybridized plasmon mode on the electrode surface. The spectra are normalized with respect to the highest peak to clearly show changes in relative peak height.

action spectra in the Q-band region of the porphyrin−SAM before and after immobilization of variously sized Au NPs. To facilitate comparison of the spectral features, each spectrum is normalized with respect to the highest peak (the net enhancement is provided in part a of Figure 5). When no Au NPs were present on the SAM, the Qy(1−0) peak was the highest of the four peaks. However, the presence of Au NPs clearly altered the relative peak height. For example, the 30 nm Au 20809

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itself is well established in the plasmonic theory, this experimental achievement in the photoelectrochemical system should contribute to the development of plasmonic photochemistry. Because the photon energy is effectively confined to the sphere−plane gap and delivered to the molecular layer, the efficiency of photon−molecule interactions is substantially improved. Moreover, well-organized molecule−electrode interfaces allow both electromagnetic and physicochemical properties of optical antennas to be controlled leading to substantial photocurrent enhancement in the resonance wavelength region. Sphere−plane optical antennas offer a simple method for maximizing the efficiencies of various photochemical reactions on a metal electrode. This strategy is expected to open up new possibilities in designing novel molecular devices.

IPCE depends on both of the photon capture efficiency and the internal quantum efficiency. In the case of the present porphyrin−SAM system without Au NPs, IPCE is lower than 0.2% in the Q-band region, despite the high internal quantum efficiency of over 10%, because of the limited absorption in the monolayer.5 Although the presence of Au NPs increases the photon capture efficiency, it may affect to the internal quantum efficiency. Next, we discuss the contribution of the optical antennas to the latter. This competitive contribution is thought to be mainly due to the excited-state quenching.23−27 This issue has been extensively studied in enhancement of fluorescence.55,56 Accounting for the effect, the local EF in the overall photoelectrochemical process can be written as: EF = (η /η0)|E loc /E i|2

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(1)

where η and η0 respectively denote the internal quantum efficiencies with and without optical antennas, and Eloc and Ei represent the localized and incident fields, respectively. Here, the purely plasmonic contribution, |Eloc/Ei|2, can be estimated theoretically.43,49,50 Figure. 5b shows the calculated |Eloc/Ei|2 per optical antenna; the spatial distribution of |Eloc/Ei|2 in the nanogap was averaged in the calculation. Compared with part a of Figure 5, the wavelength dependence of the calculated results well follows the resonance features of the antenna systems although the absolute value of |Eloc/Ei|2 were about 10 times larger than the experimental EFs. Because the increase of the photon capture efficiency is proportional to |Eloc/Ei|2, this significant difference in the magnitude could be ascribed to the decrease of the internal quantum efficiency in the presence of Au NPs; that is, η /η0 is approximately on the order of 10−1. This reduction in the internal quantum efficiency is presumably due to the energy transfer from excited molecules to Au NPs (i.e., excited-state quenching). To maximize the antenna effect, it is essential to reduce excited-state quenching in metal nanostructures. In principle, both of the plasmonic field enhancement and quenching are sensitive to the distance between chromophores and metal surfaces.55,56 For example, SiO2-coated Au NPs have been utilized to enhance fluorescence efficiency.57,58 In the present system, the porphyrin−Au NP distance is very close as a result of the molecular structure although the Au NPs utilized in the experiments were covered with PDDA layer. Therefore, separation of porphyrins from Au NPs is essentially important to increase η /η0. In the sphere−plane system, the localized field is not strongly dependent on the spatial position within the gap.36,37,59 Hence, if porphyrins are located halfway between the sphere and the plane, excited-state quenching would be minimized without reducing the plasmonic enhancement. Such precise control of metal−molecule structures is only possible on atomically defined electrode surfaces. There is still room for further enhancement of the IPCE by optimizing the position of dye units in molecular monolayers even though the photocurrent enhancement was considerably larger than previously reported values.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.I.), UOSAKI. [email protected] (K.U.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Grant-in-Aids for Scientific Research on Priority Area “Strong Photon−Molecule Coupling Fields” (21020001 and 19049007), a Grant-in-Aid for Scientific Research (A) (23245028), a Grant-in-Aid for Young Scientists (B) (22750001), the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, the Global COE program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science), and the MEXT Program for Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The SEM measurements were supported by Hokkaido Innovation through NanoTechnology Support (HINTS, Nanotechnology Network Project supported by MEXT, Japan).

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REFERENCES

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4. CONCLUSION Sphere−plane nanostructures are promising as optical antennas for photosensitizing molecular monolayer system. While the procedure for fabricating these antenna is rather simple, their resonance properties can be tuned by alternating the Au NP size, which is indispensable for selective enhancement of photochemical reactions. Although plasmonic resonance tuning 20810

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