Effect of Plasmon Coupling on Quantum Efficiencies of Plasmon

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Effect of Plasmon Coupling on Quantum Efficiencies of Plasmon-Induced Charge Separation Takuya Ishida, and Tetsu Tatsuma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07986 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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The Journal of Physical Chemistry

Effect of Plasmon Coupling on Quantum Efficiencies of Plasmon-Induced Charge Separation Takuya Ishida and Tetsu Tatsuma* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, Japan * Author to whom correspondence should be addressed, email: [email protected]

ABSTRACT

In order to clarify effects of plasmonic electric field intensity and distributions on the internal quantum efficiency (IQE) of plasmon-induced charge separation (PICS), we took advantage of interparticle plasmon coupling of Au nanoparticles (AuNPs) on TiO2. For the isolated AuNPs without coupling, the electric field localized at the Au-TiO2 interface contributes to the IQE value. As the plasmon coupling is formed, the plasmonic absorption peak of the AuNPs is red-shifted and the IQE value is increased. The electric field intensity at the AuNP surface is remarkably increased, whereas that at the interface is hardly changed, by the plasmon coupling. Therefore, it is concluded that the electric field at the AuNP surface is enhanced by the plasmon coupling, resulting in the improved IQE of PICS.

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INTRODUCTION In recent years, plasmonics has attracted much attention as a powerful technique for manipulation of light.1 Surface plasmons are excited through interaction between light and free electrons at the surface of a metal or a properly designed metal nanostructure. The plasmonic materials virtually confine light as plasmons beyond the diffraction limit and plasmons excite dyes and semiconductors efficiently as do photons.1,2 This plasmonic effect has been studied extensively and utilized for surface-enhanced Raman spectroscopy3 and enhancement of photoluminescence,4 photoelectric conversion,5 and photocatalysis.6 We have reported that charge separation occurs at the interface between a plasmonic metal nanoparticle in resonance with light and a semiconductor7 and that this phenomenon, plasmon-induced charge separation (PICS), involves electron transfer from the plasmonic nanoparticle to the semiconductor.7–9 PICS has been intensively studied by many scientists in wide research areas because it can convert surface plasmons directly to an electron flow and can also drive redox reactions.10,11 It has been applied to photocatalysis,7,12,13 photovoltaics,7,14,15 chemical sensing,16,17 photochromism,18,19 nanofabrication,20-22 and photon upconversion.23 Besides applications, mechanisms of PICS have also been investigated in order to control the reaction site,20,21,24-26 the potential,27 and the efficiency. Mechanisms such as energetic carrier transfer (Figure 1A-C)15,28 and interfacial charge-transfer transition (Figure 1D)29,30 have been proposed. In either case, it is reasonable to consider that PICS is facilitated by strong electric field, through which electrons are excited. For Au-TiO2 combination, which is the most typical PICS system,7 the charge separation process is often explained in terms of energetic electron injection from Au nanoparticle (AuNP) to TiO2. Electrons in a AuNP are excited via intraband


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transition,31 which is one of the non-radiative decay process of localized surface plasmon resonance (LSPR) to generate electron-hole pairs, and the energetic electrons (so-called hot electrons) that can cross the Schottky barrier are injected into the TiO2 conduction band (Figure 1A, C).15,28 It is sometimes pointed out that energetic holes contribute to oxidation reactions in PICS processes.13,32 We have revealed so far that AgNPs of various shapes carried on TiO2 oxidize and dissolve themselves, when their specific LSPR modes are excited, preferentially at resonance sites, where the plasmonic electric field is localized strongly.20,21,24 It is explained in terms of ejection of holes as Ag+ ions (Figure 1B, C) because this is observed even if the localized site is away from TiO2 by several tens of nanometers.21 Similar local anodic reactions have been observed for Pb2+ oxidation at AuNP-TiO2 systems.22,26

CB FL

VB Semiconductor Metal

Figure 1. Schematic representation of PICS mechanisms of (A)–(C) energetic carrier transfer and (D) interfacial charge transfer transition. Generation energetic electron-hole pairs and injection of (A) the electrons, (B) the holes or (C) both of them into a semiconductor or a molecule. Regardless whether the electron injection into a semiconductor or reaction of holes plays more essential role, electron-hole pairs are important for the energetic carrier-based mechanisms which should tend to generate at resonance sites, where the electric fields are localized.33,34 Govorov et


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al. predicted from the theoretical calculation that not only the external quantum efficiency (EQE) but also the internal quantum efficiency (IQE) of electron-hole pair generation increases as the electric field intensity is enhanced,33,34 for instance by forming plasmon coupling between plasmonic NPs.34 They explain that a quantum surface effect of the strong electric field provides an electron with a momentum necessary to generate an electron-hole pair. In the present work, we experimentally clarify possible effects of intensity and distribution of plasmonic electric field on IQE of PICS by controlling the electric field trough the plasmon coupling. Thus obtained knowledge would show ways to higher PICS efficiencies.

EXPERIMENTAL Preparation of AuNPs. Citrate-protected AuNPs with average diameter of 14, 18, and 33 nm were prepared by using trisodium citrate as a reducing agent. A 0.010 wt% aqueous solution of hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4, 600 mL) was heated to the boiling temperature. Then, a 1.0 wt% aqueous solution of trisodium citrate (19.0, 13.4, or 7.40 mL) was added to the boiling solution of HAuCl4, giving AuNPs with the diameter of 14, 18, or 33 nm, respectively. The reaction mixture was kept at the boiling temperature for 60 min and was allowed to cool down to room temperature. Fabrication of Photoelectrodes. An anatase TiO2 layer ( ∼ 60 nm thick) was deposited on a fluorine-doped tin oxide (FTO)-coated glass substrate by a spray pyrolysis method35 at 550 °C from a 2-propanol containing 0.38 M titanium diisopropoxide bis(acetylacetonate) at a spray pressure of 0.12 MPa for 1 s × 2 cycles with 60 s intervals, followed by annealing at 550 °C for 30 min in air. The FTO/TiO2 substrate was irradiated with UV light for 1 h and was immersed in 45 mg mL-1 polyethylenimine (PEI, Mw = 50 000–100 000) aqueous solution including 0.20 M


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NaCl for 6 h. The FTO/TiO2/PEI substrate thus obtained was thoroughly rinsed with water and was immersed in the AuNP solution.36 The amount of adsorbed AuNPs was controlled by immersion time. The FTO/TiO2/PEI/AuNP substrate was rinsed with water and the AuNPs adsorbed on the back side were wiped off. Then, the surface was treated with oxygen plasma (100 W for 1 min, PR300, Yamato Scientific) to remove organic compounds, followed by thermal annealing at 500 °C for 1 h for better Au–TiO2 contact and cooling down to room temperature. Characterization. Scanning electron microscopy (SEM) images were taken on a field emission scanning electron microscope (FE-SEM, JSM-7500FA, JEOL). Optical spectra were obtained by a UV-vis spectrophotometer (V-670, Jasco) using an integrating sphere. Absorbance spectra of AuNPs were calculated by subtracting absorbance of FTO/TiO2 from that of FTO/TiO2/AuNP. Simulation. A finite-difference time-domain (FDTD) method (FDTD Solutions, Lumerical Solutions, Inc.) was used to simulate the absorption spectra of a AuNP on an anatase TiO2 substrate (60 nm thick). The bottom of each Au sphere (diameter d = 14, 18, or 33 nm) is partially removed so that the NP of 0.972d high is in plane contact with TiO2 as shown in Figure S1. The entire simulation domain was 350 × 350 × 350 nm composed of 2 nm cubic cells and the region close to the AuNP was further meshed in a three-dimensional box (50 × 80 × 50 nm) composed of 0.25 nm cubic cells, which are small enough to reproduce experimentally evaluated absorption cross section at the peak wavelength (Table S1). The electric field intensity was expressed as a ratio to incident light intensity (1 V m-1). Photoelectrochemical Measurements. The FTO/TiO2/AuNP working electrode was soaked with a Pt counter electrode in an air-saturated 0.1 M KNO3 aqueous solution containing 0.5 M


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ethanol (EtOH) as an electron donor. The working electrode was irradiated from the back side with monochromatic light (~5.5 × 1015 photons cm−2 s−1) using a Xe lamp (LAX-103, Asahi Spectra) through a band-pass filter (full width at half maximum = 10 nm) for measurement of short-circuit photocurrents. Action spectra of IQE for the photocurrent generation were obtained by dividing EQE (flux ratio of electron flow to photon irradiation) by absorption.

RESULTS AND DISCUSSION Plasmon Coupling of AuNPs. An FTO-coated glass substrate was further coated with TiO2, and AuNPs were adsorbed onto the TiO2 surface by immersing the substrate into a AuNP solution. As shown in SEM images (Figure S2) and particle size distributions (Figure S3), the average diameter of the AuNPs was 18 nm and the surface coverage of AuNP increased from 14.3% to 29.6% when the immersion time was extended from 2 h to 12 h. Figure 2a shows absorbance spectra of those AuNP ensembles obtained by subtracting absorbance of each FTO/TiO2 substrate from that of the FTO/TiO2/AuNP substrate. When the coverage was 26.7% or lower, a peak was observed at 522 nm. This is typical of AuNP LSPR.37 The peak was grown and red-shifted to 561 nm as the coverage was increased further. This shift may reflect formation of plasmon coupling37-40 due to decreasing interparticle gap length. We therefore normalized absorption spectra at 450 nm, at which the absorption is based on the interband transition of Au and is proportional to the total volume of loaded Au, so as to exclude possible direct effects of increases in the number and volume of AuNPs (Figure S4a). In addition, the difference spectra with reference to the sample with the lowest coverage (14.3%) are shown in Figure S4b. With increasing the number of coupled AuNPs, the absorbance at ~520 nm corresponding to isolated AuNPs decreased, and the absorbance in the range of 550 to 800 nm corresponding to coupled


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AuNPs increased which cannot be explained by the increase in the number or the size. We therefore studied the interparticle gap length at which plasmon coupling occurs and the electric field distribution around the coupled AuNPs by FDTD analysis.

Figure 2. (a) The experimental difference absorbance spectra of AuNPs on TiO2 with different coverage and (b) the simulated difference absorption cross section spectra of two Au NPs on TiO2 with different interparticle distance. Electric field distributions for (c, d) an isolated AuNP and (e) coupled AuNPs (gap = 1 nm) under light of different wavelengths. Characters λ, k, and E stand for wavelength, wave vector, and angle of electric field of incident light, respectively. Characters S and I indicate sites of the maximum electric field intensity at the Au surface and the Au-TiO2 interface, respectively. The simulated difference absorbance spectra are shown in Figure 2b for two AuNPs on TiO2 with different interparticle gap lengths. In the case where the gap is infinite, a peak at 533 nm


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and a shoulder around 580 nm are observed. From the electric field distribution at each resonance wavelength, the peak and the shoulder are assigned to the full-surface mode (Figure 2c), in which the electron oscillation and the electric field distribute over the whole AuNP surface and the interface mode (Figure 2d), in which the electron oscillation and the electric field are localized at the AuNP-TiO2 interface.20 In comparison with the simulated spectrum, the experimentally obtained spectrum shows much broader peak and shoulder, likely because the AuNP ensemble involves NPs that are not uniform in terms of the size, shape, and contact state with TiO2. When the interparticle gap is decreased, the maximum electric field intensity |E|2/|E0|2 (where E and E0 are the electric field at a given site in the vicinity of the AuNP and the electric field of the incident electromagnetic wave in vacuum, respectively) at Site S indicated in Figure 2c, e increases greatly (Figure S5a). We plotted the intensity ratio (|E|2/|E0|2)coupled/(|E|2/|E0|2)single (subscript "coupled" and "single" stand for coupled NPs and an isolated NP, respectively) as a function of the gap length in Figure 3a so as to clarify the theoretical threshold distance for plasmon coupling. This value reaches 10 when the gap is 4.9 nm. The calculated absorbance peak of the full-surface mode red-shifts when the interparticle gap length is decreased from this value (Figure 3a). These results indicate that plasmon coupling is formed and the two AuNPs behave as a single resonance pair when the gap is 4.9 nm or less. Incidentally, the electric field intensity at Site I (Figure 2d, e), which corresponds to the interface mode site, depends much more weakly on the gap length (Figure S5a).


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Figure 3. Intensity ratio (|E|2/|E0|2)coupled/(|E|2/|E0|2)single at Site S in Figure 2c, e (green symbols) and Site I in Figure 2d, e (red symbols) and the peak wavelength (black circles) for AuNPs with the diameter of (a) 18 nm, (b) 14 nm, and (c) 33 nm plotted against the interparticle gap length. All those data are calculated by a FDTD method. We therefore counted the number of coupled AuNPs with 20%). We therefore conclude that the plasmon coupling increased the IQE value.

Figure 5. (a) EQE and (b) IQE action spectra for PICS photocurrents of FTO/TiO2/AuNP (18 nm) electrodes. In order to corroborate this further, coverage dependencies of the internal PICS efficiency were also investigated for 14 nm and 33 nm AuNPs. In the case of 14 nm AuNPs, plasmon coupling occurred at the coverage of ~20% or higher as described above. Since the IQE value also increases in this range (Figure 4b), it is reasonable to ascribe this rise to plasmon coupling. In the case of 33 nm AuNPs, a small increase in IQE was observed in the range >20%, in which plasmon coupling formed, although the increase was not sufficiently larger than the experimental error (Figure 4b).


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Correlation between IQE and Light Confinement Index. On the basis of the results mentioned above, we concluded that the plasmon coupling gave rise to the increase in the IQE values. In general, plasmon coupling intensifies the electric field around the nanoparticles, in particular in the interparticle gap. Therefore, it is reasonable to assume that the intensified electric field enhances the IQE of PICS. We therefore examined the relationship between the electric field strength and the IQE value. Although the electric field enhancement factor (|E|2 |E0|2)/|E0|2 is appropriate for discussion of a possible effect of the electric field intensity on the EQE value, it cannot be directly compared with the IQE value, since it increases with light absorption. Therefore, here we define a light confinement index η by dividing the electric field enhancement factor by absorption A:

𝜂=

(|𝐸|2 ― |𝐸0|2) |𝐸0|2

(1)

𝐴

Note that the AuNPs used in this study are so small that their scattering intensity is negligibly small in comparison with their absorption. Figure 6a shows the dependence of the η value at Sites S and I indicated in Figure 2c-e at 580 nm (i.e. IQE peak wavelength) on the interparticle gap length for 18 nm AuNPs. When the distance between AuNPs is sufficiently long (e.g. 12 nm) and the plasmon coupling is not formed between them, the η value at Site I is higher than that at Site S. It reflects that the electric field strength for the interface mode is higher than that for the full-surface mode. As the interparticle distance is decreased (≤4.9 nm) and the coupling is formed, the η value at Site S increases remarkably while that at Site I decreases. Therefore, it is reasonable to conclude that the increase in the η value for the full-surface mode causes the improvement in the IQE value.


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Figure 6. Dependencies of the η value at Sites S (green) and I (red) indicated in Figure 2c-e at 580 nm on the interparticle gap length for (a) 18 nm, (b) 14 nm, and (c) 33 nm AuNPs. Dotted lines show the plasmon coupling distance. Similar tendencies are observed in calculation for both 14 nm and 33 nm AuNPs (Figure 6b, c). The η value at Site S increases and that at Site I decreases when the interparticle distance is 3.4 nm or less for the 14 nm NPs and 9.5 nm or less for the 33 nm NPs. In the case of the 14 nm AuNPs, the increase in IQE accompanying the plasmon coupling (Figure 4b) is also explained in terms of the increasing η value for the full-surface mode. In the case of the 33 nm NPs, the increment in the IQE value is not sufficiently large, as described above. For isolated AuNPs without plasmon coupling, we plotted the size-dependencies of the experimentally obtained IQE value and the calculated ηd2 values at 580 nm in Figure 7. Here, we multiply the η value by d2 so as to allow direct comparison between the value and IQE for AuNPs with different sizes. The ηd2 value at Site S increases as the size decreases, whereas that at Site I reaches the maximum value at the diameter of 18 nm, as does the IQE value. Those results mentioned above indicate that the η value of the interface mode and that of the fullsurface mode dominate the IQE values of the isolated and coupled AuNPs, respectively.


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Figure 7. Dependencies of the experimentally obtained IQE value (diamond) and the calculated ηd2 values at the Site S (green) and I (red) at 580 nm on the AuNP diameter d. PICS Mechanisms in the Present System. There are several possible PICS mechanisms, including those shown in Figure 1. In Mechanism A (Figure 1A), the overall process is dominated by injection of energetic electrons from a AuNP to the TiO2 conduction band, and oxidation reactions are caused by relaxed holes, which are no longer energetic but just positive charges, after accumulation in the AuNP.27 In Mechanism B (Figure 1B), energetic holes or those at a trap site drive oxidation reactions at the AuNP surface, and reduction reactions are caused by deactivated electrons, after accumulation in the AuNP and overflow into the TiO2 conduction band.42 In Mechanism C (Figure 1C), both of the energetic holes and electrons contribute to oxidation reactions and direct electron injection into TiO2, respectively, before relaxation. Mechanism D (Figure 1D) involves electron excitation at the interface, which is sometimes referred to as charge-transfer transition,29 from a band below the Fermi level of AuNP to the TiO2 conduction band. The rate of energetic carrier generation or electron transition should increase with increasing η value at a given site.


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In the case of isolated AuNPs, the η value of the interface mode, rather than that of the fullsurface mode, dominates IQE, as described above. In Mechanisms A and C, which involve the injection of energetic electrons into TiO2, the interface mode should be advantageous for the injection because energetic carriers are tend to be generated at around the interface. Likewise, the electron transition occurs at the interface in Mechanism D, so that excitation of the interface mode may contribute to IQE than does the full-surface mode. For Mechanism B, in which energetic holes dominate the overall reactions, the full-surface mode may be more advantageous than the interface mode for higher IQE. Even so, the latter mode exhibits a larger η value than the former for the isolated AuNP. Since the mean free path of energetic holes is 10-40 nm,43 the holes generated at around the interface could also contribute to the overall process. Energy loss of the energetic holes during transport from Site I to the AuNP surface may suppress the IQE value for larger AuNPs, if Mechanism B or C dominates. This could explain the negative dependence of the IQE/ηd2 ratio on the NP size (Figure 7). Ethanol oxidation reactions, which is observed in the present system, could be driven by one-electron reactions.44 In addition, energetic holes could be trapped at oxides, hydroxides, or clusters of Au at the NP surface. Therefore, the one-electron oxidation or even multi-electron oxidation reactions could be driven by the energetic holes. Hence, none of Mechanisms A-D has been ruled out so far. In the case of the coupled AuNPs, the η value of the full-surface mode dominates IQE as described above. Therefore, we have to rule out Mechanism D, in which electric field localization at the interface is essential. There still is a possibility that Mechanisms B and C, in which energetic holes react at the NP surface around Site S, contribute IQE mainly. Mechanism A is also not excluded, considering that the mean free path of energetic electrons is 10-40 nm,43,45 which is comparable to or longer than the distance from Site S to the Au-TiO2 interface


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for 14 nm and 18 nm AuNPs. In the case of 33 nm AuNP, the IQE value is not increased significantly by plasmon coupling (Figure 4b), whereas the η value for the full-surface mode increases greatly (Figure 6c), as described above. These results can be explained for Mechanisms A and C in terms of energy attenuation of electrons during travel from Site S to the Au-TiO2 interface. We therefore conclude that energetic electrons and/or holes play essential role in PICS of the present system. Here, the plasmon coupling enhances the η value, and the IQE value of PICS is improved. This improvement may be explained in terms of (i) enhanced generation efficiency of energetic carriers, (ii) enhanced accumulation of relaxed holes in AuNPs or relaxed electrons in AuNPs or TiO2 for higher overpotential, or (iii) enhanced entrapment of holes at trap sites for multi-electron oxidations.

CONCLUSIONS In order to clarify experimentally the effect of the electric field intensity and distribution on the PICS efficiency, we studied an effect of plasmon coupling on PICS photocurrents. The IQE of isolated and coupled AuNPs on TiO2 are dominated chiefly by the interface mode and the fullsurface mode, respectively. It was also experimentally clarified that the formation of plasmon coupling increases IQE of PICS. Both of the η value for the interface mode and the IQE value depend on the AuNP size for the isolated AuNPs, and their maximum values are reached when the particle diameter is 18 nm. In the case of the coupled AuNPs, the η value for the full-surface mode and the IQE value increase as the interparticle gap length decreases. Therefore, the IQE value would be improved by increasing the η value through, for instance, plasmon coupling.

ACKNOWLEDGMENT


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This work was supported in part by a Grant-in-Aid for Scientific Research (A), (JP16H02082) from the Japan Society for the Promotion of Science (JSPS). SUPPORTING INFORMATION Schematic model for FDTD simulation (Figure S1). Experimental and simulated absorption cross section of AuNPs (Table S1). SEM images of the AuNPs and their surface coverage (Figure S2). Particle size distributions of the AuNPs (Figure S3). Normalized absorption spectra and corresponding difference spectra of AuNPs (Figure S4). Electric field intensities at Site S and Site I of AuNPs (Figure S5). SEM image of coupled AuNPs (Figure S6).

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TOC Graphic


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Effect of Plasmon Coupling on Quantum Efficiencies of Plasmon

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