Enhancement of Dye-Sensitized Photocurrents by Gold Nanoparticles

Jan 17, 2013 - I. Fratoddi , R. Matassa , L. Fontana , I. Venditti , G. Familiari , C. Battocchio , E. Magnano , S. Nappini , G. Leahu , A. Belardini ...
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Enhancement of Dye-Sensitized Photocurrents by Gold Nanoparticles: Effects of Plasmon Coupling Tokuhisa Kawawaki, Yukina Takahashi,† and Tetsu Tatsuma* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan S Supporting Information *

ABSTRACT: Plasmonic metal nanoparticles are known to work as light-harvesting antennae and to enhance photocurrents of photovoltaic cells and reaction rates of photocatalysts. The effects are expected to increase the energy conversion efficiency and to reduce the thickness of a light-absorbing layer and costs for materials. In this work, we examined the plasmonic enhancement of dye-sensitized photocurrents by Au nanoparticle ensembles with different particle densities to study the effects of interparticle plasmon coupling on the photocurrent enhancement. The coupling effects allow enhancement in a longer wavelength region. The optimum particle size for the enhancement by coupled nanoparticles is 100 nm, whereas that for isolated nanoparticles is 40 nm because the plasmon coupling effect is more significant for larger nanoparticles. Theoretical calculations reproduce those results.

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system.5 The dye-sensitized photocurrent enhancement is increased monotonically as the dye-NP spacing decreases from 8 to 5 nm and 5 to 2 nm for the system with anatase TiO2 and amorphous TiO2, respectively, prepared by atomic layer deposition (ALD). We prepared photoanodes with a 2-D Au NP/TiO2/dye system with anatase TiO2, in which the dye−NP spacing was controlled by changing the TiO2 thickness from 3 to 38 nm and found the optimum spacing to be ∼10 nm.2 We reported that an energy-transfer-based quenching effect and back electron transfer suppress the photocurrent enhancement when the spacing is less than the optimum value. In this study, we examined the dependence of the photocurrent enhancement on the particle density and the particle size to elucidate a plasmon coupling effect on the enhancement. When the interparticle spacing is less than the particle diameter, a localized electric field much stronger than that for a single NP is generated in the gap between the NPs (i.e., hot spot), and the resonance wavelength is red-shifted by the plasmon coupling effect.30,31 The strong electric fields due to the plasmon coupling effect are exploited for enhancement of the SERS intensity by four to eight orders of magnitude.21,22 It should therefore be important to take advantage of this effect for greater enhancement of photocurrents and more efficient use of long-wavelength light. However, there have been no systematic studies on the plasmon coupling effect on the photocurrent enhancement.28 For conventional DSSC photoelectrodes with a 3-D TiO2 network, changing of the NP concentration in the film for

INTRODUCTION Recently, it has been reported that Au1,2 and Ag3−7 nanoparticles (NPs) enhance photocurrents of dye-sensitized solar cells (DSSCs).8,9 The enhancement effect is believed to be based on localized surface plasmon resonance (LSPR) of the NPs. Oscillating conduction electrons of metal NPs resonate with the electric field of incident light, giving rise to strong absorption in the visible region, which is proportional to the particle volume in the case of spherical NPs,10,11 and oscillating electric fields (i.e., optical near field) that are localized in the vicinity of the metal NPs.12−14 The localized electric fields can enhance the photocurrents by exciting electrons of dye molecules (e.g., HOMO−LUMO transition) more efficiently than the incident far field light. The NPs thus function as lightharvesting antennae for dyes. The photocurrent enhancement is also possible in silicon solar cells15and organic solar cells.16−20 Similar processes are involved in surface-enhanced Raman scattering (SERS),21,22 enhancement of fluorescence intensity,23−25 and enhancement of photocatalytic reaction rate.26−29 The LSPR-based photocurrent enhancement in a solar cell is expected to improve incident photon to current conversion efficiency and energy conversion efficiency and to reduce the thickness of the light-absorbing layer. The thinner layer in turn allows development of flexible and low-cost cells. In many cases, photocurrents are enhanced by a factor of ≤6 by NPs.1−7,15−20 The enhancement factor depends on the intensity of oscillating electric fields in the vicinity of the metal NPs. The electric field intensity is, in turn, dependent on the metal species, the dye−NP and interparticle spacing, the size and shape of NPs, and the dielectric environment. Hupp et al. controlled the spacing between dye molecules and a Ag NP by changing the thickness of TiO2 for a 2-D Ag NP/TiO2/dye © XXXX American Chemical Society

Received: December 8, 2012 Revised: January 17, 2013

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Figure 1. Models of (a) an isolated Au NP, (f) coupled NPs with 2 nm gap, and (k) coupled NPs with −2 nm gap coated with TiO2 (10 nm thick) and (b−e, g−j, and l−o, respectively) corresponding electric field distributions calculated by a FDTD method for NPs of 15, 40, 100, and 150 nm diameter at the wavelength that gives the maximum electric field intensity at the arrows.

thickness of the TiO2 film, which was measured by scanning electron microscopy (JSM-7500FA, JEOL), was adjusted to 10 nm, at which the photocurrent enhancement was maximum in our previous work.2 The TiO2-coated electrodes were immersed for 18 h in ethanol containing 0.5 mM N749 (black dye) (Solaronix), followed by thorough rinsing with acetonitrile. Evaluation Methods. Photocurrents of the obtained ITO/ Au NP/TiO2/dye or ITO/TiO2/dye electrode (electrode area = 0.196 cm2) were examined by constructing a two-electrode cell with a platinum-coated ITO counter electrode. The gap between the electrodes (0.5 mm) was filled with N2-saturated acetonitrile containing 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine. LSPR-based absorption of the Au NPs coated with TiO2 does not decrease in the electrolyte solution, which is very corrosive to Au, during the photoelectrochemical measurements, indicating that the TiO2 films are sufficiently dense and that contact of the Au NPs with the dye and the redox species (i.e., I−/I3−) is negligible. Short-circuit photocurrents were measured by a potentiostat (SI1280B, Solartron) under light at various wavelengths (460−900 nm) at a constant photon flux (6.0 × 1015 photons cm−2 s−1) or white light (480−700 nm, 100 mW cm−2) from a Xe lamp (LAX-102, Asahi Spectra) equipped with a band-pass filter (full width at half-maximum = 10 nm) or a long-pass filter (SCF-50S-48Y, Sigma Koki), respectively. We simulated the localized electric fields around the NPs by a FDTD (finite-difference time-domain) method (Lumerical Solutions) for the models of a spherical Au NP of 15−150 nm diameter on an ITO (150−500 × 150−500 × 150 nm)/glass (150−500 × 150−500 × 150 nm) substrate covered with an anatase TiO2 layer (10 nm thick). The simulation domain (1200 × 1200 × 1200 nm) consisted of 100 nm cubic cells, and the central region (120−450 × 120−450 × 120−450 nm) was further meshed with a three-dimensional grid of 1 nm spacing. The dielectric functions of Au and anatase TiO2 were extracted from the data of Johnson and Christy34 and Jellison,35 respectively.

tuning of the interparticle spacing inevitably changes light penetration depth as well as electron conductivity and transport ability of a redox species (e.g., I−/I3−) in the nanoporous TiO2. Therefore, the coupling effect cannot be separated from the other effects. To address this issue, we employed the 2-D Au NP/TiO2/dye system described above (Figure 1a) and regulated the interparticle spacing by changing the NP density on the electrode surface so that the coupling effect on the photocurrent enhancement is studied experimentally by the aid of the spectral simulation. As a result, we confirmed the contribution of the plasmon coupling effect to enhancement of dye-sensitized photocurrents. The effect allows further enhancement of the photocurrents in a long-wavelength range. In addition, the optimum particle size for the enhancement by coupled NPs was found to be 100 nm, whereas that for isolated NPs was 40 nm.

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EXPERIMENTAL METHODS Preparation of Working Electrodes. Smooth indium tin oxide (ITO)-coated glass plates (Kuramoto) were cleaned by sonication in a detergent solution for 30 min and in a 1 M NaOH aqueous solution for 30 min, followed by rinsing with water. The hydrophilic plates thus obtained were immersed in an ethanol solution of (3-aminopropyl)triethoxysilane (10% v/ v) for 18 h at room temperature.32 The plates were rinsed with ethanol and left to dry for 2 h at 120 °C. A colloidal suspension of Au NPs (diameter = 15, 40, 100, or 150 nm, Tanaka Kikinzoku, 0.0065 wt %, 300 μL) was applied to the silanized plate and left for 7.5−360 min, followed by rinsing with water. The occupancy of the NPs, ANP/AE, was controlled by changing the treatment time or repeating the treatment (one to three times). The occupancy of NPs was evaluated by atomic force microscopy (NanoNavi Station, SPA-400, SII Nanotechnology). The ITO surface with Au NPs and a bare ITO surface were coated with thin TiO2 films prepared by a spray pyrolysis method2,33 from a 2-propanol solution of 55.3 mM titanium diisopropoxide bis(acetylacetonate) (spray pressure was 0.12 MPa, spray time was 1 s, calcined at 500 °C for 30 min). The B

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damping).30,31 However, it is reported that the electric field intensity due to the plasmon coupling effect exhibits different NP size dependence from that for the isolated NPs.36−40 Theoretical Evaluation of Coupled NPs. We therefore modeled coupled NPs (Figure 1f) and investigated the dependence of the electric field intensity on interparticle spacing x (= spacing between the TiO2 surfaces) at the outside and inside of the coupled particles (points indicated by the blue and red arrows, respectively, Figure 1f) at the wavelength giving the maximum intensity. In the case of the coupled NPs embedded in a single TiO2 shell (i.e., x < 0), we calculated the intensity at the locations indicated by the arrows shown in Figure 1k. Figure 3a,b shows the results obtained by the simulation. The electric-field intensity at the outside surface (blue arrow)

RESULTS AND DISCUSSION Theoretical Evaluation of Isolated NPs. In this study, we prepared the 2-D ITO/Au NP/TiO2/dye photoanodes2 and controlled the interparticle spacing by changing the surface density of the NPs. Prior to the experiments, we modeled the present system as shown in Figure 1 and calculated localized electric fields around the spherical Au NPs of 15, 40, 100, and 150 nm diameter by the FDTD method. The TiO2 thickness was set to 10 nm in all of the following simulations and experiments because we found in the previous work2 that it was the optimum thickness in terms of the photocurrent enhancement by Au NPs of 40 and 100 nm diameter. We calculated electric field distributions for isolated NPs at different wavelengths, and Figure 1b-e shows the cases in which the electric field intensity at the TiO2 surface (point indicated by the arrow in Figure 1a) is the highest. In the case of the Au NP model of 15 nm diameter, the LSPR-based electric field is almost negligible outside the TiO2 layer (Figure 1b). In the model of 40, 100, and 150 nm diameter, the electric fields penetrate out of the TiO2 layer (Figure 1c−e). Figure 2a (black circles) shows the dependence

Figure 3. Relationships between the electric field intensity and the interparticle spacing at (a) outside and (b) inside of coupled Au NPs (15, 40, 100, and 150 nm diameter, with a TiO2 coating) (simulated), and (c) the dependence of the photocurrent enhancement factor of the photoelectrode with TiO2-coated Au NPs (40 and 100 nm diameter) on the particle occupancy, ANP/AE (experimental). In both graphs, the maximum values in the examined wavelength region are plotted.

Figure 2. (a) Intensity of localized electric fields at the TiO2 surface of isolated Au NPs and at the surface in the gap between the coupled NPs (indicated by the red arrow in Figure 1f; gap = 2, 10, and 30 nm) (simulated) and (b) photocurrent enhancement factor of the photoelectrode with TiO2-coated Au NPs (ANP/AE = 0.05) under monochromatic light (experimental), both as a function of the NP size. In both graphs, the maximum values in the examined wavelength region are plotted.

of the electric field intensity at the location indicated by the arrow (Figure 1a) on the NP size. The Au NP model of 40 nm diameter gives the highest intensity. These results are reasonable because it is reported that a Au NP with a diameter of a few tens of nanometers exhibits the strongest electric field.14 Both absorption and the electric field intensity are suppressed for smaller NPs due to electron-surface scattering and for larger NPs owing to light scattering (i.e., radiation

exhibits no prominent dependence on the interparticle spacing. The intensity at the inside surface of the coupled NPs (red arrow) is also close to that for the isolated particle when the interparticle spacing is 30 nm, indicating that the electromagnetic interaction between the NPs is weak. However, as the spacing of the two NPs decreases, the electric-field intensity gradually increases and the maximum value is reached at ∼2 nm C

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spacing. A plasmon coupling effect must be responsible for the high electric field intensity in the gap between the coupled NPs. Figure 2a shows dependencies of the electric field intensity on the NP size at the inside surface in the gap between the coupled NPs (indicated by the red arrow in Figure 1f, gap = 2, 10, and 30 nm). A Au NP of 100 nm diameter exhibits the highest intensity when the interparticle spacing is 10 nm or less. It is reported that the decay length of localized electric fields increases as the NP size increases for NPs larger than ∼50 nm in diameter.40 Consequently, the plasmon coupling is enhanced for larger NPs, and the optimum NP size in terms of the electric field intensity shifts from 40 to 100 nm. We also calculated the electric field distributions at the wavelength that gives the maximum electric field intensity at the arrows when the interparticle spacing is 2 and −2 nm (Figure 1g−j and l−o, respectively). Note that the scale bar of the electric field intensity for these Figures is significantly different from that for Figure 1b−e. In the case of the 40−150 nm NPs with 2 nm spacing, the highest electric field is observed in the interparticle gap. In the case of −2 nm spacing, the maximum intensity of electric fields is observed at the TiO2 surface (around the point indicated by the red arrow in Figure 1k), rather than the gap in the TiO2 phase. This is advantageous for near-field excitation of dye molecules on the TiO2 surface. Next, we investigated systems with Au NPs of 40 and 100 nm diameter, which exhibit high electric field intensity, in further detail. We calculated the localized electric field intensities as functions of the irradiation wavelength at outside (Figure 4a) and inside (Figure 4b) of coupled 40 nm NPs with 2 or 10 nm spacing. Calculated results for the isolated NP are also plotted in Figure 4. The isolated NP exhibits the intensity peak of localized electric fields at 620 nm. As the interparticle spacing decreases, the intensity peak at the outside surface red-shifts by ∼50 nm and the peak is suppressed (Figure 4a). In the gap between the coupled NPs, the intensity peak red-shifts and becomes larger with decreasing interparticle spacing (Figure 4b). In comparison with the isolated NP, the red shift is about 40 nm and the peak enhancement is 2.9 fold for the coupled NPs with 2 nm spacing. We also calculated for coupled NPs with 100 nm diameter (Figure 5a,b). The isolated NP has the intensity peak of localized electric fields at 680 nm. The peak at 580 nm is likely due to a quadrupolar plasmon mode, which is often observed for NPs of ∼100 nm or larger.11 When two NPs are coupled, the peak at 580 nm red-shifts by ∼25 nm, and that at 680 nm further red-shifts by ∼140 nm at the outside surface of the coupled NPs (Figure 5a). The peak heights were comparable to or smaller than those for the isolated NP. In the gap between the coupled NPs, the electric field intensity is increased by the coupling (Figure 5b), as is the case for 40 nm NPs. The red shifts for the peaks at 580 and 680 nm are 35 and 140 nm, respectively. On the basis of those calculations, we anticipate that the coupling effects would contribute to further enhancement of photocurrents, particularly at longer wavelengths. For the systems based on the coupling effects, 100 nm NPs are expected to be more effective than 40 nm NPs. Experimental Evaluation of Photocurrent Enhancements by Dense NP Ensembles. We examined experimentally the plasmon coupling effects on the photocurrent enhancement by controlling the surface density of NPs. Particle occupancy ANP/AE, where ANP is projected area of Au NPs and

Figure 4. Spectra of the electric field intensity at (a) outside and (b) inside of coupled TiO2-coated 40 nm Au NPs (simulated) and (c) absorption spectra and (d) action spectra of photocurrent enhancement factor for the TiO2-coated 40 nm Au NP electrodes with different particle occupancies (ANP/AE = 0.01 to 0.19) (experimental). The data for isolated NPs are also shown in panels a and b.

AE is that of the electrode surface, was changed in the range 0.01 to 0.19. Au NPs of 40 nm diameter were adsorbed onto an ITO electrode and covered with a TiO2 layer of ∼10 nm thick. Scanning electron micrographs of the TiO2-coated NPs are shown as insets of Figure 4c. As the NP density increased, the number of NPs with narrow spacing was increased, and some of the coupled NPs were covered with single TiO2 shells. We investigated absorption (= extinction − specular reflection − forward and backward scattering; measured by using an integrating sphere) spectra (Figure 4c) and action spectra for photocurrent enhancement factor (= photocurrent ratio of the electrode with Au NPs to that without NPs) (Figure 4d). We also show photocurrent action spectra in Figure S1 of the Supporting Information. The difference in the photocurrents between the electrodes with and without Au NPs was sufficiently larger than the photocurrents of an ITO/Au NP/ TiO2 electrode without dye (peak IPCE < 0.01%).2 As the ANP/AE value is increased from 0.01 to 0.19, the LSPR-based absorption peak grew and red-shifted gradually from 580 to 605 nm. The enhancement factor of photocurrents has a peak at 620 nm. The peak also grew and red-shifted by about several tens of nanometers with increasing surface density of NPs (Figure 4d). Note that the maximum enhancement factor of photocurrents is ∼4.5, which is significantly higher than the maximum increase ratio of the TiO2 surface area of the electrode, 1.48. Because the photocurrents are not enhanced at