ARTICLE pubs.acs.org/JPCC
Photoelectrochemical and Optical Behavior of Single Upright Ag Nanoplates on a TiO2 Film Ichiro Tanabe,† Kazuki Matsubara,† Nobuyuki Sakai,† and Tetsu Tatsuma*,†,‡ † ‡
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan ABSTRACT: Changes in morphology and optical properties of single Ag nanoplates on a nanoparticulate TiO2 film were studied by means of combined atomic force microscopy and optical darkfield microscopy with the aid of discrete dipole approximationbased spectral simulation. Photocatalytic reduction of Agþ ions under diffusion-controlled conditions grows hexagonal and triangular Ag nanoplates mostly in vertical orientation. The Ag nanoplates absorb and scatter light at different wavelengths on the basis of three different localized surface plasmon resonance modes, the in-plane longitudinal (IPL), in-plane transverse (IPT), and out-of-plane modes. Typical upright nanoplates scatter orthogonally polarized green and near-infrared (NIR) light on the basis of the IPT and IPL modes, respectively. Sufficient irradiation with visible and/or NIR light topples the standing nanoplates over by photoelectrochemical reactions, and typical overturned nanoplates scatter NIR light. Such photoelectrochemical behavior is applied to orientation-selective overturn and polarization-selective control of the optical properties by irradiation with polarized visible or NIR light.
1. INTRODUCTION Metal nanoparticles (NPs) have various interesting properties as materials for optical, electronic, magnetic, and catalytic applications.1-4 Some of these properties depend on the particle size5-7 and shape,7 exposed crystal face,8,9 and orientation.10,11 In particular, NPs of noble metals such as Au and Ag exhibit a variety of colors due to localized surface plasmon resonance (LSPR), which gives rise to absorption and scattering of light at specific wavelengths (λLSPR).3,12,13 We have reported that Au and Ag NPs on TiO2 exhibit LSPR-assisted, photoinduced charge separation.14 We have also applied the charge separation to photocatalysis,14a,14b photovoltaic cells,14a-14d surface patterning,14e photoswellable and photoshrinkable hydrogels,14f and multicolor photochromism.14g,14h Recently, we have found in our preliminary study15 that hexagonal and triangular Ag nanoplates deposit photocatalytically on a nanoparticulate TiO2 film mostly in vertical orientation. These upright Ag nanoplates are toppled over by visible light because of the photoinduced charge separation. In the present report, we investigate the optical and photoelectrochemical properties of the upright Ag nanoplates extensively and systematically. In particular, the shape and scattered light of single Ag nanoplates are observed by combined atomic force microscopy (AFM) and dark-field microscopy. Optical and photoelectrochemical behavior of single nanoplates with different orientations is elucidated with the assistance of spectral simulation on the basis of discrete dipole approximation (DDA). r 2010 American Chemical Society
Polarization-selective changes in the optical properties of Ag nanoplates are also examined on the basis of the photoelectrochemical toppling of nanoplates by polarized visible and nearinfrared (NIR) light. In previous works,16 spectral control based on changes in size and shape of Ag NPs was carried out, but the control based on orientation change has never been performed. The orientation control of upright nanoplates would facilitate development of not only materials with controllable optical properties but also those for polarization-selective optical sensors, photovoltaic cells, and photocatalysts. In addition, the manipulation of nanoplate orientation would lead to control of mass transport, reactivity, and electronic levels of nanoplates, through changes in the topography, accessible crystal face, and contact area with the substrate, respectively. The polarization-selective changes in the optical properties would also be used for certification/anticounterfeit technologies.
2. EXPERIMENTAL AND SIMULATION SECTION Preparation of TiO2 Films and Deposition of Ag Nanoplates. Nanoparticulate TiO2 films were prepared on Pyrex
glass substrates from a titanium alkoxide ethanol solution (NDH510C, Nippon Soda) by a standard dip-coating technique Received: October 10, 2010 Revised: December 7, 2010 Published: December 21, 2010 1695
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(withdrawal rate was 2 mm s-1, calcined at 500 °C for 1 h). Their thickness was 70 ( 10 nm and the surface roughness was about 10 nm. An aqueous solution containing 10 mM AgNO3 was mixed with ethanol (1:1 by volume) and was cast on the TiO2 film (40 μL cm-2). The film was irradiated with UV and blue light (320-500 nm, 4.3 mW cm-2, spectrum is shown in part a of Figure 1B) for 15 min for the photocatalytic deposition of Ag NPs containing upright nanoplates. A Xe lamp (Luminar Ace LA-251Xe, Hayashi Watch Works) equipped with a wide bandpass filter (BLF-50S-390B, Sigma Koki) was used as the UV and blue light source. A Hg-Xe lamp (Luminar Ace LA-310UV,
Hayashi Watch Works) equipped with a band-pass filter (wavelength = 310 nm, fwhm = 10 nm, CVI Laser, LLC) was used for deposition of Ag NPs without nanoplates. Photoelectrochemical Reactions. The TiO2 film-coated glass plate with the deposited Ag nanoplates was rinsed with water, and water was removed by an air duster for 30 s. The sample was irradiated with visible and/or NIR light (460 nm, 600-700 nm, or 700-1000 nm, spectra are shown in parts b-d of Figure 1B) in air (relative humidity > 60%). The light source for the irradiation with 460 nm or 600-700-nm light was the Xe lamp equipped with a band-pass filter (wavelength = 460 nm, fwhm = 10 nm, CVI Laser, LLC) or long-pass (g600 nm, SCF50S-60R, Sigma Koki) and short-pass (e700 nm, CLDM-50S, Sigma Koki) filters, respectively. A Xe lamp (LAX-102, Asahi Spectra) equipped with a mirror reflecting 700-1000-nm light (LAX-IR, Asahi Spectra) was used for the 700-1000-nm light irradiation. Optical and Morphological Measurements. Optical extinction (absorption plus scattering) spectra of Ag NPs in the visible and NIR regions were collected by a spectrophotometer (V-670, Jasco). Morphologies and scattered light of the Ag NPs were observed by AFM (NanoNavi Station/SPA-400, SII Nanotechnology) combined with optical dark-field microscopy (BX51modified, Olympus-SII Nanotechnology) with a 50 field lens, a polarizer (U-AN360-3, Olympus, if necessary), a Xe lamp, and a CCD camera (ORCA-3CCD C6880, Hamamatsu Photonics, sensitivity spectra are shown in parts a-c of Figure 1C). AFM measurements were performed in a tapping mode using a silicon cantilever (SI-DF40P, SII Nanotechnology). Scattered NIR light was monitored by an optical microscope (BX51, Olympus) with a NIR tube (U-TA30IR, Olympus), a 50 field lens, a polarizer (U-AN360IR, Olympus, if necessary), a halogen lamp, and a NIR CCD camera (Xeva-1.7-320, Xenics, part d of Figure 1C). Simulation of Spectra. To support the analysis of experimental results, absorption and scattering spectra of Ag nanoplates were simulated on the basis of the DDA method.12,17 In the present DDA method, an object is modeled as a cubic array of infinitesimal polarizable units (spacing is 1 nm) that have a polarizability corresponding to the dielectric constant of the material.18 Spectra are calculated by computing the sum total over the models of induced dipole of each unit which is induced by the incident electric field.
Figure 1. (A) Visible-NIR extinction spectra of Ag NPs on TiO2 films (a) with and (b) without Ag nanoplates. (B) Light spectra of (a) UV and blue light for the Ag deposition and (b-d) visible and/or NIR light for the photoelectrochemical reactions. (C) Sensitivity spectra of the (a-c) visible and (d) NIR charge-coupled device (CCD) cameras.
3. RESULTS AND DISCUSSION Shapes and Spectra of Deposited Ag NPs. Ag NPs were deposited by irradiating the TiO2 film with UV and blue light (4.3 mW cm-2) in the presence of 5 mM AgNO3 and ethanol.
Figure 2. Typical (a, b, d) AFM, (c) SEM, and (e) dark-field images of Ag NPs deposited by the irradiation with (a-c, e) UV and blue light (320-500 nm, 4.3 mW cm-2, 5 mM AgNO3, 15 min) and (d) UV light (310 nm, 1 mW cm-2, 0.5 mM AgNO3, 5 min). 1696
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Figure 3. Typical (a) AFM images and (b) dark-field images obtained through a polarizer and (c) corresponding polar diagrams of the same Ag nanoplates.
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Electrons in the TiO2 valence band are excited by UV light to the conduction band (band gap ≈ 3.2 eV) and combined with Agþ ions in the solution, resulting in the deposition of Ag NPs.14g,14h The holes generated in the valence band are consumed by oxidation of ethanol in the solution. Parts a-c of Figure 2 show typical AFM and scanning electron microscopy (SEM) images of the deposited Ag NPs. A variety of Ag nanoplates were observed, mostly in vertical orientation. Length of the longest side of the nanoplates is 50-150 nm, and the thickness is about 10 nm. The shapes are regular triangle and hexagons (truncated regular triangle) and parts of these shapes (parts b and c of Figure 2); a nanoplate has two large Ag(111) facets and three or six edges with Ag(111), (100), and/or (110) facets. We have reported15 that upright Ag nanoplates deposit preferentially when the system is under diffusion-limited conditions and the TiO2 substrate has a certain surface roughness (about 10 nm) like the nanoparticulate TiO2 film used in this study. Actually, under milder conditions, at 1 mW cm-2 UV light (310 nm) in the presence of 0.5 mM AgNO3, more isotropic and featureless NPs deposit as shown in Figure 2d. Additionally, when the TiO2 nanoparticulate film is replaced with a TiO2 single crystal, no standing Ag nanoplates are observed, although the growth rate is almost the same.16a This indicates that the surface roughness of the substrate plays an essential role in the growth of upright Ag nanoplates.15 Figure 1A shows typical extinction spectra of the samples. The spectrum of the sample with the featureless Ag NPs (curve b) exhibits intense extinction at 380-500 nm. This is in line with our previous observation16 that spherical or hemispherical Ag NPs (diameter of ∼30 nm) on a TiO2 single crystal absorb light at around 400-500 nm. In contrast, the extinction spectrum of the sample with Ag nanoplates (curve a) exhibits a broad extinction band extending to the NIR region. Therefore, the extinction in the long-wavelength region (>500 nm) is predominantly ascribed to the nanoplates. This assignment is supported by the spectral simulation as described below. Observation of Scattered Light. Lights scattered from each single Ag NP was observed by dark-field microscopy with the color CCD camera (exposure time: red = 33.05 ms; green = 19.24 ms; blue = 8.21 ms) for visible light. As a result, scattered light spots with different colors were observed on the sample with upright Ag nanoplates (Figure 2e). It has been confirmed theoretically (e.g., by simulation based on a DDA method) that Ag NPs absorb and scatter light at almost the same wavelength.19 In contrast, no discrete light spot was observed on samples without Ag nanoplates at the same sensitivity. The bright spots can be scanned by AFM combined with darkfield microscopy (Figure 3). As a result, at least one upright Ag nanoplate was found at the center of all bright spots. This indicates that the visible light scattered from the nanoplates is much stronger than that from the other, featureless NPs. However, all the nanoplates were not necessarily bright. All the upright nanoplates found at the center of the bright spots were higher than 103 nm. Light spots for 30-nm-high nanoplates were observed only when the exposure time was 12 times longer. These results conform to the calculation data reported for metal NPs that the extinction intensity and scattering/extinction ratio increase as the particle size increases.20 Polarization Properties of the Upright Ag Nanoplates. To understand the LSPR-based optical properties of Ag nanoplates in further detail, their spectra were simulated by the DDA 1697
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Figure 4. (a-g) Absorption and scattering spectra simulated by DDA of the (a, e) IPL, (b, f) IPT, and (c, g) OP modes for (a-d) a standing and (e-g) an overturned Ag nanoplate. The incident angle to the TiO2 slab is 45° in part d. Solid curves are absorption spectra and dashed ones are scattering. (h) The model of the Ag nanoplate and the three LSPR modes examined.
method. We used the particle model shown in Figure 4h, a semihexagonal nanoplate (side length = 60 nm, thickness = 10 nm), considering shape and size of typical Ag nanoplates observed in our experiments. This plate has three main LSPRmodes, in-plane longitudinal (IPL) mode, in-plane transverse (IPT) mode, and out-of-plane (OP) mode. The IPL, IPT, and OP modes are based on the electron oscillation in parallel direction to the longest side of the plate, that in the perpendicular direction in plane to the longest side of the plate, and that in the parallel direction to the thickness of the plate, respectively (Figure 4h). Parts a-c of Figure 4 show the calculated absorption and scattering spectra of the upright Ag nanoplate on a cylindrical TiO2 slab (diameter =150 nm, thickness = 60 nm) for those LSPR modes. The IPL mode exhibits a strong peak at 940 nm and a smaller peak at 480 nm. The IPT mode exhibits a strong peak at 540 nm, and the OP mode exhibits a very small peak at 500 nm. With the theoretical behavior in mind, we experimentally examined optical properties of upright Ag nanoplates by observation of the scattered light through a polarizer. Typical results are shown in Figure 3. It is obvious that the scattered light intensity strongly depends on the polarizer angle (panel b). Corresponding polar diagrams are also shown in panels c; the relative light intensity is plotted as a function of the polarizer angle (the strongest signal among blue, green, and red light was plotted, unless otherwise noted). It is clear from the figure that the visible scattered light is the strongest when the polarizer angle is perpendicular to the plane of the plate and is the weakest in the parallel geometry, for all the nanoplates examined (23 plates). That is to say, scattered visible light on the basis of the IPT and/ or OP mode is more intense than that of the IPL mode. By considering that the calculated intensity of the OP mode is far much lower than that of the IPT mode, it is reasonable to ascribe the perpendicular scattered light mainly to the IPT mode. If the plane of an upright plate was completely parallel to the direction of incident light, the IPT mode would not be excited. However, the most of the plates were not perfectly vertical, and the incident angle for the dark-field observation was about 30°, so that the IPT mode should have been excited. Actually, even in simulation, both of the IPT and OP modes of
the upright nanoplate is able to be excited by oblique light (incident angle is 45°), and the calculated spectra almost solely reflect those for the IPT mode (Figure 4d). It is noteworthy that some of the upright nanoplates change their colors as the polarizer is rotated. In the case of the nanoplate shown in Figure 3E, scattered green light is polarized perpendicularly to the plane of the plate, and red light is polarized parallel. This scattered red light is likely due to the IPL mode. In the DDA calculation, the scattered light for the IPL mode is chiefly in the NIR region (Figure 4a). However, some plates seem to have weak resonance even in the red region. So we studied the scattering by dark-field microscopy using a NIR CCD. As a result, NIR light spots were also observed. Also, some of the NIR spots were observed at the same locations where bright visible spots were observed (Figure 5A). In these cases, the polarization angle of the scattered NIR light was perpendicular to that of the scattered visible light (Figure 5B). On the basis of these experimental and calculated results, we conclude that upright Ag nanoplates typically scatter visible light based chiefly on the IPT mode, and NIR light based on the IPL mode. Photoelectrochemical Behavior of the Ag Nanoplates. Here we examined photoelectrochemically induced changes in the orientation and optical properties of the upright Ag nanoplates. Figure 6a shows a difference spectrum of a sample without Ag nanoplates after irradiation with 600-700-nm light for 30 min. The observed spectral change is ascribed to oxidative dissolution of Ag NPs which resonate with the incident light. As we have reported previously,14c,14d electrons transfer from the plasmon-excited Ag NPs to TiO2, and Ag is oxidized to Agþ ions which dissolve in adsorbed water on TiO2.21 The electron transfer has been verified by electrochemical means,14c,14d and its mechanisms have been discussed elsewhere.14a-14d The oxidative dissolution of the resonant NPs decreases the particle size, as evidenced by AFM observation,16a resulting in a blueshift of the resonant wavelength λLSPR until the NPs no longer resonate with the incident light. As a result, the extinction decreases at around the irradiation wavelength and increases at a shorter wavelength. In the meantime, the electrons in TiO2 recombine with the Agþ ions in the adsorbed water at the surface of TiO2 or nonresonant Ag NPs, resulting in the redeposition of 1698
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Figure 6. (a, b) Difference and (c, d) polarized difference extinction spectra of Ag NPs on TiO2 (a, c) without and (b, d) with nanoplates after irradiation with (a, b) nonpolarized or (c, d) polarized (600-700 nm) light. Figure 5. (A) Typical (a) visible and (b) NIR dark-field images of the upright Ag nanoplates on TiO2 at the same location. (B) Typical (a) visible and (b) NIR dark-field images obtained through a polarizer and (c) corresponding polar diagram (green, visible; dark red, NIR) of the same scattered light spot.
Ag. Since TiO2 does not absorb visible light, the spectral changes are induced by photoexcited Ag NPs. The upright nanoplates on TiO2 were irradiated with 460 nm (fwhm = 10 nm, 5 mW cm-2), 600-700 nm (5 mW cm-2), or 700-1000 nm (10 mW cm-2) light for 30 min. As a result, the extinction decreased at around the excitation wavelength and increased at a shorter wavelength (Figure 6b), as was the case for the spheroidal NPs. Additionally, extinction increased at wavelengths longer than the irradiation wavelength. Those spectral changes were accompanied by toppling of upright nanoplates (Figure 7A). This is also explained in terms of the LSPR-assisted oxidative dissolution of Ag, which proceeds in this system in the interfacial region consisting of Ag nanoplates, TiO2, and adsorbed water. Dissolution of the upright Ag nanoplates at the bottom results in the overturn. Further irradiation dissolves some of the toppled nanoplates, as evidenced by AFM observation.15 The nanoplates were not toppled by the AFM tip because no change was observed over four repeated scans in the absence of illumination. The toppling direction of the nanoplates was
random, even though the substrate was set up vertically during the light irradiation. Therefore, the dominant force for toppling Ag nanoplates is not gravity, but most likely the capillary force of adsorbed water. The above-mentioned spectral changes suggest that the resonant wavelength λLSPR of Ag nanoplates red-shifts due to the overturn. The DDA calculation indicates that the toppling of a Ag nanoplate gives rise to a red-shift by >100 nm in λLSPR of the IPL and IPT modes (parts e and f of Figure 4), likely due to an increase in the contact area with TiO2, which has a high refractive index. It is well-known that an increase in the refractive index of the surrounding medium red-shifts λLSPR of a metal nanoparticle.22,23 In the dark-field observation after 480-700nm light irradiation (10 mW cm-2, 30 min), about 2/3 of the red spots (80/120 spots) and the green ones (68/100 spots) disappeared. Furthermore, a few green spots (3/100 spots) changed into red ones (Figure 7B). These changes are explained in terms of red-shifts in λLSPR of the IPT mode from green to red or NIR and that from red to NIR by toppling (from part b to f of Figure 4). Actually, toppling of nanoplates was observed at the disappearing red spots (Figure 7C). As described above, the upright plates scatter polarized visible and NIR light, so that the NIR dark-field image of the sample strongly depends on the polarizer angle (Figure 7D). After the 1699
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extinction in the visible region. Therefore, visible light polarized perpendicularly to the plane of the plate would selectively excite the IPT mode. So it is expected that standing nanoplates of particular orientation are toppled over under visible light of a specific polarization angle. Figure 6d shows the difference polarization spectra after irradiation with polarized visible light (600-700 nm, 5.0 mW cm-2, 30 min). The irradiation decreased the parallel extinction (0°) much more significantly than the perpendicular extinction (90°). On the other hand, in the case of a sample without Ag nanoplates, no significant dependence on the polarization angle was observed (Figure 6c). The orientation-selective overturn of the nanoplates was observed by the dark-field microscopy (Figure 7F). After the irradiation with the polarized (0°) visible light (480-700 nm, 15 mW cm-2, 60 min), the visible scattered light spots observed through a parallel polarizer (0°, panel a) were more than 5 times fewer than those observed through a perpendicular polarizer (90°, panel b). On the contrary, after the irradiation with the polarized (0°) NIR light (1000-1700 nm, 30 mW cm-2, 180 min), the light spots observed through a parallel polarizer (0°, part a of Figure 7G) were more than 3 times as many as those observed through a perpendicular one (90°, part b of Figure 7G). These results indicate that polarized visible light chiefly topples Ag nanoplates in the perpendicular orientation through excitation of the IPT mode and that polarized NIR light preferentially overturns nanoplates in the parallel orientation by exciting the IPL mode. Thus we controlled polarization properties of the Ag nanoplates by polarized visible and NIR light.
Figure 7. (A, C) AFM and (B) visible dark-field images of Ag nanoplates (a) before and (b) after the visible light irradiation (480-700 nm, 10 mW cm-2, 30 min). Inset images for panel C are the corresponding scattering light from the same nanoplate. (D, E) Typical NIR dark-field images obtained through a polarizer (angle a, 0°; b, 90°) (D) before and (E) after the visible light irradiation (480-700 nm, 15 mW cm-2, 60 min). (F, G) Typical visible dark-field images obtained through a polarizer (angle a, 0°; b, 90°) after polarized (angle = 0°) (F) visible (480-700 nm, 15 mW cm-2, 60 min) or (G) NIR (1000-1700 nm, 30 mW cm-2, 180 min) light irradiation.
irradiation with visible light (480-700 nm, 15 mW cm-2, 60 min), however, the NIR dark-field image stayed almost the same even when the polarizer was turned 90° (Figure 7E). Although the nanoplates should have the IPL and IPT modes even after the toppling, those modes could not be distinguished by the NIR CCD, wavelength region of which is much wider (ca. 700 nm) than that for the color CCD (
