Photoregulated Nanopore Formation via Plasmon-Induced Dealloying

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Photoregulated Nanopore Formation via PlasmonInduced Dealloying of Au-Ag Alloy Nanoparticles Hiroyasu Nishi, and Tetsu Tatsuma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12131 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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The Journal of Physical Chemistry

Photoregulated Nanopore Formation via PlasmonInduced Dealloying of Au-Ag Alloy Nanoparticles Hiroyasu Nishi* and Tetsu Tatsuma* Institute of Industrial Science, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 1538505, Japan *Address correspondence to [email protected] and [email protected]

ABSTRACT: We propose and demonstrate the third method to dealloy and porosify plasmonic alloy nanoparticles (NPs). Au-Ag alloy NPs were deposited on a TiO2 thin film and irradiated with visible light in water. As a result, Ag was dissolved from the NPs and nanopores were formed, on the basis of plasmon-induced charge separation (PICS) at the alloy-TiO2 interface. The dissolution rate can be regulated by tuning the irradiation wavelength or intensity, resulting in different pore and ligament sizes. Porous NPs smaller than 35 nm can also be prepared by the aid of an auto-termination mechanism, which is unique to the present photoelectrochemical dealloying method.

INTRODUCTION

ACS Paragon Plus Environment

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Dealloying, one of the corrosion phenomena for metal alloys, is caused by dissolution of less noble components from the alloy and has been applied to fabrication of nanoporous metal structures. There have been two methods for dealloying, chemical and electrochemical methods. Less noble metal is oxidized by an oxidizing agent (e.g., nitric acid for dissolution of silver) in the chemical method,1‒10 and by applying a more positive potential than critical potential for nanopore formation to the alloy in the electrochemical method.11,12 The dissolution rate and pore size are controlled by the concentration of the oxidizing agent and the applied potential in the former and latter methods, respectively. Those two methods can be combined with each other for acceleration of the dissolution rate and formation of smaller pores.13 The most typical process, dealloying of Au-Ag alloy to nanoporous structures, has been studied extensively1‒6,8-16 since direct observation of the nanopores by Forty et al. through electron microscopy in 1979.2 Nanoporous Au was applied to electrochemical3 and other4,13,14 catalysts, sensors,10 and actuators15 because that has large specific surface area and many catalytic sites. Ding et al. reported that an unsupported nanoporous Au catalyst showed high CO oxidation activity even at ‒30 ºC.13 Since catalytic activities and other properties of nanoporous Au are dependent on pore and ligament sizes, their control is one of the most important issues in this field. According to previous literature, formation of pores and ligaments is dominated primarily by two factors: the dissolution rate of Ag and the surface diffusion rate of Au atoms.5,16 Smaller pores and thinner ligaments are formed at higher Ag dissolution and lower Au diffusion rates. The surface diffusion slows down as the temperature decreases or Pt is added.11 Meanwhile, porous nanoparticles (p-NPs) are also fabricated by dealloying of Au-Ag alloy nanoparticles (NPs). Wang et al. prepared Au-Ag NPs by dewetting of a Au/Ag bilayer film and dealloyed the particles chemically with nitric acid to yield p-NPs with size of 300 nm.6


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Wi et al. chemically dealloyed Au-Cu alloy nanodisks fabricated by lithography and applied the resulting porous Au nanodisks to surface enhanced Raman spectroscopy (SERS).7 Sieradzki et al. electrochemically dealloyed Au-Ag NPs prepared on an electrode through a dewetting process or a chemical synthesis and carefully investigated conditions such as applied potential, particle size, and alloy composition to give p-NPs.12 Hrelescu et al. studied localized surface plasmon resonance (LSPR) properties of individual p-NPs on the basis of scattering spectra obtained by dark-field microscopy and reported that even p-NPs with isotropic outlines exhibited polarization-dependent scattering spectra.8 Gao et al. synthesized p-NPs with controlled particle and pore sizes and applied them to SERS.9 The p-NPs exhibit unique LSPR and SERS properties with many hot spots, where strong electric field is localized upon photoirradiation.8,9 The large specific surface area of the p-NPs would also be advantageous for application to catalysts and sensors. Herein, we propose photoelectrochemical dealloying as the third method of dealloying. Our group previously reported plasmon-induced charge separation (PICS) at the interface between semiconductor such as TiO2 and plasmonic Au,17 Ag,18,19 and Cu20NPs. PICS is also observed for Au-Ag alloy NPs21,22 and we clarified that the PICS efficiency depends on the alloy composition.23 In PICS, electrons transfer from a NP in resonance with incident light to a semiconductor.24 Although the NP that ejected electrons takes electrons from electron donors nearby,17 the NP itself could be oxidized in the absence of the donors. In the case of Ag and Cu NPs, they are dissolved as ions into the surrounding electrolyte or adsorbed water.18‒20 Thus, it is expected that PICS can be applied to formation of p-NPs through photoelectrochemical dissolution of Ag from Au-Ag alloy NPs under appropriate conditions.


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Since photoelectrochemical dealloying takes place only at the irradiated area, it would allow two-dimensional patterning. Selective dealloying of NPs with specific size, anisotropy, or orientation may also be possible if light of appropriate intensity, wavelength, and polarization is selected; actually, selective etching of Ag NPs based on PICS has been reported.25‒27 In addition, since PICS-based oxidation of plasmonic metal NPs takes place preferentially at the locations where the electric field is strong,28‒30 localizing the electric field at a particular site by controlling irradiation wavelength and incident angle31‒33 would allow site-selective dealloying of a NP and three-dimensional nanofabrication of alloy NPs. In this study, we investigate PICS behavior of Au-Ag alloy NPs immobilized on TiO2 and demonstrate the feasibility of photoelectrochemical dealloying method. We also show advantages of the present method including the patterning and auto-termination of the pore formation. Although the p-NPs are formed on a solid substrate as in the most conventional dealloying systems for p-NPs,6-8,12 the p-NPs supported on a stable TiO2 substrate would be applied to SERS analysis, LSPR sensing, heterogeneous catalysts, and cocatalysts of TiO2 photocatalyst, besides PICS-based devices.

EXPERIMENTAL Preparation of TiO2 Electrode Modified with Au-Ag Alloy NPs. All the reagents were used without further purification. Water was purified by a Milli-Q system. Au-Ag alloy NPs ([Au]/([Au]+[Ag]) = 0.25) were prepared according to a method described in our previous paper.23 A compact anatase TiO2 film with thickness of ca. 60 nm was deposited on an indium tin oxide (ITO) electrode by a spray pyrolysis method.23 The electrode was immersed in the alloy


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NP solution, pH of which was adjusted to ca. 3 using dilute sulfuric acid. After 24 h, the electrode was washed with pure water and annealed at 400 ºC for 2 h in a nitrogen atmosphere. Photoelectrochemical Dealloying The NP-modified electrode was soaked in pure water and irradiated with broadband visible light (>480 nm, ca. 9 mW cm‒2) or monochromatic light (fullwidth at half maximum: 10‒12 nm, ca. 2 mW cm‒2). A Xe lamp (Asahi Spectra, LAX-102) with a long pass filter or a band pass filter, respectively, was used as a light source. Characterization. Extinction spectra were measured by a JASCO V-670 spectrophotometer. Scanning electron microscopy (SEM) images were obtained by using JEOL JSM-7500FA. Photopotential measurements were conducted using a two-electrode cell. The Au-Ag alloy NPmodified TiO2 electrode and platinum black were soaked in an aqueous solution of KNO3 (0.1 M) as working and counter electrodes, respectively. Open-circuit photovoltage was measured by a potentiostat (Solartron 1280Z) under intermittent irradiation (10-s light, 50-s dark) of monochromatic light (ca. 2 × 1015 photons cm‒2 s‒1). The measurement was carried out in the course of the photoinduced dealloying process in the electrolyte under exposure to broadband visible light (>480 nm, ca. 45 mW cm‒2) from the Xe lamp with the long-pass filter.

RESULTS AND DISCUSSION Photoelectrochemical Formation of Nanopores. The chemically synthesized Au-Ag alloy NPs were adsorbed onto TiO2 and annealed to be immobilized and homogenized34 in terms of chemical composition distribution in the NPs. The alloy NP ensemble thus obtained was exposed to broadband visible light (>480 nm, ~9 mW cm‒2) in water at 24 ºC for the photoelectrochemical dealloying. Figure 1 shows SEM images of the as-prepared and


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photoirradiated (1320 min) NP ensembles. The NPs were initially non-porous (Figure 1a), while almost all NPs turned into p-NPs with fine pores (~6 nm) after the irradiation (Figure 1b and c). The average diameter of the NPs decreased from 62 ± 18 to 53 ± 15 nm (n = 80 each) during the porosification. The volume reduction has also been reported for chemical dealloying of Au-Ag NPs12 and ascribed to a plastic deformation process.35 The number of the NPs per unit area remained almost unchanged (ca. 108 µm‒2). Meanwhile, micrometer-sized particles were deposited on TiO2 at density of around 1.0 × 10‒2 µm‒2 after irradiation (see Supporting Information, Figure S1). Those particles were identified as Ag by energy dispersive X-ray spectroscopy. The photoelectrochemical dealloying process is explained in terms of PICS (Figure 2a): energetic electrons are injected from photoexcited Au-Ag alloy NPs to the conduction band of TiO2, resulting in formation of p-NPs through oxidation of Ag to Ag+ ions, which dissolve into water, and subsequent surface diffusion of Au atoms (Figure 2b). The electrons injected into TiO2 recombine with the Ag+ ions dissolved in the electrolyte to form metallic Ag on the TiO2 surface. We reported PICS-based oxidative dissolution of pure Ag NPs and re-reduction of Ag+ on TiO2,25,28,36 in good accordance with the present results. The redeposition of Ag on TiO2 can be suppressed by leading the injected electrons to a counter electrode at which Ag+ ions are reduced.37 Efficient removal of Ag+ ions would be possible by constructing a thin-layer electrochemical cell with the counter electrode very close to the TiO2coated electrode with alloy NPs. Figure 3a shows changes in the extinction spectrum of the alloy NPs on TiO2 during photoirradiation. The initial extinction peak derived from LSPR at around 670 nm is redshifted and broadened in comparison with NPs suspended in water (the peak wavelength is typically 430‒440 nm23,38), likely because of contact with anatase TiO2 having high refractive index (n =


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ca. 2.539) including generation of the plasmonic interface mode29 localized at the particle-TiO2 interface. High particle density that gives rise to electrostatically coupled NP ensembles and fused anisotropic NPs may also be one of the reasons. The photoirradiation resulted in further redshift and broadening of the peak in the red and near infrared range, being consistent with previous reports,8,9 probably because of an increase in the surface area of the NPs and large distribution of pore and particle sizes. The spectral broadening and the NP size decrease mentioned above account for the extinction decrease in the whole wavelength range. As expected from the mechanism described above, the nanopore formation and the accompanying extinction change should take place only in the illuminated area. Actually, when the light was irradiated to the sample through a photomask with star-shaped windows, decoloration occurred only at the exposed area while bluish color of the Au-Ag alloy NPs remained in the masked area (Figure 3b). From these results, we can conclude that the Au-Ag alloy NPs were successfully dealloyed photoelectrochemically and transformed into p-NPs on the basis of PICS. It should be noted that 1320 min is not the prerequisite time for porous NP formation, but for saturation of the spectral change. Actually, we confirmed that p-NPs form within 340 min when using the thin-layer electrochemical cell. Changes in Photovoltage Responses. We carried out photoelectrochemical measurements to investigate the dealloying behavior. Figure 4 shows open-circuit photovoltage action spectra obtained in the course of the pore formation process driven by PICS under visible to nearinfrared light (>480 nm, ca. 45 mW cm‒2). The potential of the ITO/TiO2/alloy NP photoelectrode was more negative than that of a Pt counter electrode. This behavior is typical of a photoelectrode with plasmonic NPs deposited on TiO2.17 The initial peak wavelength in the action spectrum (ca. 550 nm) is shorter than the extinction peak wavelength (670 nm, Figure 3a),


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indicating that PICS is more efficient at shorter wavelengths, for NPs with higher Ag content, smaller size, or less significant plasmon coupling. We actually reported that Au-Ag alloy NPs with higher Ag content show greater photon-to-current conversion efficiencies5,16 and that higher maximum photocurrents are observed for smaller Au NPs on TiO2.40 Meanwhile, Ng et al. demonstrated experimentally and theoretically that lower energy photons are less advantageous for PICS at the Au-TiO2 interface.41 The photopotential response decreased and the edge wavelength of the action spectrum was blueshifted gradually from >900 to ~700 nm in the course of the dealloying process for 970 min (Figure 4). Taking the redshift of the extinction edge intensity into account (Figure 3a), we conclude that the PICS efficiency of the p-NPs produced as a result of the dealloying is lower than that of the initial alloy NPs. The decreased efficiency can be explained in terms of the increased Au fraction,23 reduced absorption, and deteriorated electrical contact of the NPs with TiO2 as a result of the dissolution of Ag and porosification. Effects of the Irradiation Wavelength. Next, we investigated effects of the irradiation wavelength on the photoelectrochemical dealloying behavior. It is expected that porosity of the NPs is able to be controlled photoelectrochemically by changing the irradiation wavelength since the onset wavelength of the photopotential response blueshifts along with the dealloying (Figure 4). Under irradiation at 700 nm, for instance, the photoresponse edge wavelength decreases gradually and the dealloying ceases automatically when the edge wavelength becomes shorter than 700 nm. Although similar photoetching methods were proposed and used for size control of CdS NPs42 and Ag clusters,43 there has been no report on photoregulation of the particle porosity. We therefore examined photoelectrochemical dealloying processes under illumination at 500, 600, and 700 nm (ca. 3.9, 6.9, and 6.4 × 1015 photons s‒1 cm‒2, respectively, Figure 5). As the


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irradiation wavelength decreases, the dealloying process is accelerated and the final peak is more redshifted, broadened, and suppressed. In addition, it is noteworthy that the dealloying process is saturated in ~1500 min at all those wavelengths examined, as well as the case of the broadband visible light irradiation (Figure 5g). Thus we conclude that the rate and degree of dealloying can be controlled by tuning the irradiation wavelength. In order to take a closer look at the dependence on the light wavelength, the dealloyed NPs were subjected to SEM observation (Figure 5b, d, and f). After irradiation of 500-nm light, most of the NPs were similar to the p-NPs obtained by the broadband irradiation (ca. 9 mW cm‒ 2

) as shown in Figure 5b, while p-NPs were hardly observed after exposure to 700-nm light

(Figure 5f). The conduction band edge potential at the TiO2 surface (flat band potential) is around ‒0.20 V vs. NHE because pH of the solution is 5‒6.44 Since the photon energy of 700-nm light is 1.77 eV, the potential of NP is estimated to be ca. +1.57 V or less positive.44 Meanwhile, the standard electrode potential for Ag/Ag+ couple is +0.799 V vs. NHE and the activity of Ag in the present alloy (Au:Ag = 25:75) is close to unity. Those values allow us to conclude that Ag dissolution should take place at the alloy NPs under 700-nm light. The spectrum actually altered as shown in Figure 5e. Nevertheless, the spectral change was small and pore formation did not occur. These results suggest that the PICS is not efficient enough at 700 nm, and that the oxidative dissolution of Ag is so slow that comparatively fast diffusion of Au atoms at the particle surface gives rise to surface passivation by Au condensation and termination of Ag dissolution. The slight blueshift of the extinction peak can therefore be explained in terms of small reduction of the particles size without pore formation. Actually, passivation caused by the surface diffusion of Au was observed during electrochemical dealloying of Au-Ag alloy with relatively small overvoltage.12


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Irradiation of 600-nm light yielded larger pores than those obtained under 500-nm light (Figure 5d). This can also be explained in terms of relatively slow dealloying; the effect of surface diffusion of Au may be more significant and leads to formation of the larger pores. In the meantime, the surface diffusion rate increases with temperature. Therefore, photoelectrochemical dealloying at 60 ºC is slower than that at room temperature (Figure S2). Even so, a possibility of a local temperature increase due to a plasmonic photothermal effect is ruled out because of light with low power density of around 2 mW cm‒2: it is known that the temperature increase of 50nm Au NP was calculated to be ~1 ºC even under far stronger light of 103 W cm‒2.45 Effects of the Photon Flux. The discussion described in the preceding section suggest that the dependence on the irradiation wavelength could be due not only to the blueshift of the photoresponse onset wavelength but also to dependence on the Ag dissolution rate. In order to confirm this, we examined the effect of the dissolution rate on the dealloying behavior by decreasing the photon flux at 500 nm from 3.9 × 1015 to 1.2 × 1015 and 0.2 × 1015 photons s‒1 cm‒ 2

. Figure S3a shows changes in the extinction peak height at different photon fluxes. The reaction

slowed down and the extinction change was suppressed as the photon flux was decreased. The extinction change was saturated even at the lower light intensity, indicating that the reaction was terminated automatically. After the saturation, the NPs were observed by SEM. When the photon flux was 0.2 × 1015 photons s‒1 cm‒2 (Figure S3c), p-NPs were rarely observed. At 1.2 × 1015 photons s‒1 cm‒2 (Figure S3b), the pore size was slightly larger than the case of 3.9 × 1015 photons s‒1 cm‒2 (Figure 5b). The dependence on the photon flux observed here is thus similar to that on the irradiation wavelength shown in Figure 5. Therefore, we conclude that the photoregulation of the degree of dealloying and the pore size is based on the dependence on the Ag oxidation rate, at least in part.


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Comparison

with

the

Chemical

Dealloying.

We

finally

compared

the

present

photoelectrochemical dealloying process with the conventional chemical dealloying using nitric acid. Figure 6 shows changes in the extinction spectra of the alloy NPs on TiO2 during immersion in concentrated or dilute (10 times) nitric acid and corresponding SEM images. The spectrum changed dramatically and the reaction was almost saturated within 10 min in concentrated nitric acid (Figure 6a). As can be seen in the SEM image shown in Figure 6b, there are ~100 nm NPs with large pores and thick ligaments and small NPs without pores. It is known that small pores generate initially in a fast chemical and electrochemical dealloying process of bulk Au-Ag alloy, but the pores grow and finally result in micrometer-sized pores and ligaments.16 Thus, in the case of alloy NPs, it is reasonable that the thick ligaments turn into small nonporous NPs eventually. In the case of dilute nitric acid, extinction decreased in the wavelength range of 600‒1000 nm and the peak was blueshifted in the initial stage (Figure 6c, 0‒10 min). The blueshift is explained in terms of Ag oxidation at the whole surface to Ag+ ions, resulting in NP size reduction and/or consequent disappearance of plasmon coupling. In this process, which is the rate-determining step for the chemical dealloying,16 nanopores are not yet formed. Porosification takes place gradually along with further Ag dissolution, resulting in redshift of the peak (Figure 6c, 50‒1365 min). Pores were found scarcely on small NPs (< ca. 35 nm) even after immersion for 1365 min (Figure 6d). Actually there have been no reports on fabrication of p-NPs from Au-Ag or Au-Cu alloy NPs smaller than 25 nm by the chemical or electrochemical method, to the best of our knowledge. In literature regarding electrochemical dealloying processes,12 it is described that Au-Ag alloy NPs with diameter less than 25 nm do not transform into p-NPs. Because the chemical and electrochemical dealloying processes should


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proceed faster for smaller NPs having larger specific surface area, excessive dealloying, which leads to nonporous NPs, takes place preferentially in smaller NPs. In the marked contrast, in the photoelectrochemical dealloying process with appropriate conditions, small p-NPs can also be produced (e.g., Figure 1b and c). The differences in the porosification behavior can be attributed to an auto-termination mechanism involved in the photoelectrochemical dealloying process. Schematic illustration of possible reaction mechanisms for the photoelecrochemical dealloying is shown in Figure 2b. Even under continuous illumination, the photoreaction slows down because of the efficiency decrease due to the Au fraction increase,23 the absorption decrease, and the deterioration of electrical contact between NPs and the electrode resulted from the Ag dissolution and porosification, as mentioned above. As a result, the surface diffusion of Au becomes more significant than the slow dissolution and finally the dealloying process is terminated through passivation of NPs by the Au-rich surface (Figure 2b) before (at long wavelength and/or weak light intensity) or after (all the other conditions examined) the pore formation.

CONCLUSIONS In the present work we developed the third method of dealloying, namely the photoelectrochemical dealloying technique based on PICS. It enables us to dealloy Au-Ag alloy NPs and make them nanoporous remotely, only in the irradiated region. The degree of dealloying and pore size can be controlled by tuning the irradiation wavelength and/or light intensity. Nanopores can be formed regardless of the NP size on the basis of the auto-termination mechanism, which is unique to the present method. In the future, combination of the present


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method with the conventional dealloying techniques, anisotropic alloy NPs, or polarized light may allow us to fabricate more sophisticated three dimensional metal nanostructures.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxxxx SEM images of Au-Ag alloy NP ensembles and redeposited Ag on TiO2 electrode after irradiation of broadband visible light in water at room temperature taken at low magnification (Figure S1), SEM images and time course of extinction of the NPs after irradiation of broadband visible light in water at 60 ºC (Figure S2), and those after irradiation of 500-nm light of different photon fluxes (Figure S3).

ACKNOWLEDGEMENTS This work was supported by a Grant-in-Aid of for Young Scientists (B) (No. 26810043) and a Grant-in-Aid for Scientific Research on Innovative Areas "Artificial Photosynthesis" (15H00863) from the Japan Society for the Promotion of Science.

REFERENCE 1. Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki K. Evolution of Nanoporosity in Dealloying. Nature 2001, 410, 450‒453.


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2. Forty, A. J. Corrosion Micromorphology of Noble Metal Alloys and Depletion Gliding. Nature 1979, 282, 597‒598. 3. Zhang, J.; Liu, P.; Ma, H.; Ding, Y. Nanostructured Porous Gold for Methanol ElectroOxidation. J. Phys. Chem. C 2007, 111, 10382‒10388. 4. Zielasek, V.; Jürgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Bäumer, M. Gold Catalysts: Nanoporous Gold Foams. Angew. Chem. Int. Ed. 2006, 45, 8241‒8244. 5. Erlebacher, J.; Sieradzki, K. Pattern Formation During Dealloying. Scr. Mater. 2003, 49, 991‒ 996. 6. Wang, D.; Schaaf, P. Nanoporous Gold Nanoparticles. J. Mater. Chem. 2012, 22, 5344 ‒5348. 7. Wi, J.-S.; Tominaka, S.; Uosaki, K.; Nagao, T. Porous Gold Nanodisks with Multiple Internal Hot Spots. Phys. Chem. Chem. Phys. 2012, 14, 9131‒9136. 8. Vidal, C.; Wang, D.; Schaaf, P.; Hrelescu, C.; Klar, T. A. Optical Plasmons of Individual Gold Nanosponges. ACS Photonics 2015, 2, 1436‒1442. 9. Liu, K.; Bai, Y.; Zhang, L.; Yang, Z.; Fan, Q.; Zheng, H.; Yin, Y.; Gao, C. Porous Au−Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis. Nano Lett. 2016, 16, 3675‒3681. 10. Qiu, H.; Xu, C.; Huang, X.; Ding, Y.; Qu, Y.; Gao, P. Adsorption of Laccase on the Surface of Nanoporous Gold and the Direct Electron Transfer between Them. J. Phys. Chem. C 2008, 112, 14781‒14785. 11. Snyder, J.; Asanithi, P.; Dalton, A. B.; Erlebacher, J. Stabilized Nanoporous Metals by Dealloying Ternary Alloy Precursors. Adv. Mater. 2008, 20, 4883‒4886.


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12. Li, X.; Chen, Q.; McCue, I.; Snyder, J.; Crozier, P.; Erlebacher, J.; Sieradzki, K. Dealloying of Noble-Metal Alloy Nanoparticles. Nano Lett. 2014, 14, 2569‒2577. 13. Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. Low Temperature CO Oxidation over Unsupported Nanoporous Gold. J. Am. Chem. Soc. 2007, 129, 42‒43. 14. Biener, J.; Biener, M. M.; Madix, R. J.; Friend, C. M. Nanoporous Gold: Understanding the Origin of the Reactivity of a 21st Century Catalyst Made by Pre-Columbian Technology. ACS Catal. 2015, 5, 6263−6270. 15. Wittstock, A.; Bienerb, J.; Bäumer, M. Nanoporous Gold: A New Material for Catalytic and Sensor Applications. Phys. Chem. Chem. Phys. 2010, 12, 12919‒12930. 16. McCue, I.; Benn, E.; Gaskey, B.; Erlebacher, J. Dealloying and Dealloyed Materials. Annu. Rev. Mater. Res. 2016, 46, 263‒286. 17. Tian, Y.; Tatsuma, T. Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632‒7637. 18. Ohko, Y.; Tatsuma, T.; Fujii, T.; Naoi, K.; Niwa, C.; Kubota, Y.; Fujishima, A. Multicolor Photochromism of TiO2 Films Loaded with Ag Nanoparticles. Nature Mater. 2003, 2, 29‒31. 19. Kawahara, K.; Suzuki, K.; Ohko, Y.; Tatsuma, T. Electron Transport in SilverSemiconductor Nanocomposite Films Exhibiting Multicolor Photochromism. Phys. Chem. Chem. Phys. 2005, 7, 3851‒3855. 20. Yamaguchi, T.; Kazuma, E.; Sakai, N.; Tatsuma, T. Photoelectrochemical Responses from Polymer-Coated Plasmonic Copper Nanoparticles on TiO2. Chem. Lett. 2012, 41, 1340‒1342.


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21. Zielińska-Jurek, A.; Kowalska, E.; Sobczak, J. W.; Lisowski, W.; Ohtani, B.; Zaleska, A. Preparation and Characterization of Monometallic (Au) and Bimetallic (Ag/Au) ModifiedTitania Photocatalysts Activated by Visible Light. Appl. Catal., B 2011, 101, 504‒514. 22. Verbruggen, S. W.; Keulemans, M.; Filippousi, M.; Flahaut, D.; Van Tendeloo, G.; Lacombe, S.; Martens, J. A.; Lenaerts, S. Plasmonic Gold–Silver Alloy on TiO2 Photocatalysts with Tunable Visible Light Activity. Appl. Catal., B 2014, 156–157, 116‒121. 23. Nishi, H.; Torimoto, T.; Tatsuma, T. Wavelength- and Efficiency-Tunable Plasmon-Induced Charge Separation by the Use of Au-Ag Alloy Nanoparticles. Phys. Chem. Chem. Phys. 2015, 17, 4042‒4046. 24. Kazuma, E.; Tatsuma, T. In-Situ Nanoimaging of Photoinduced Charge Separation at the Plasmonic Au Nanoparticle-TiO2 Interface. Adv. Mater. Interfaces 2014, 1, 1400066. 25. Matsubara, K.; Tatsuma, T. Morphological Changes and Multicolor Photochromism of Ag Nanoparticles Deposited on Single-Crystalline TiO2 Surfaces. Adv. Mater. 2007, 19, 2802‒2806. 26. Kazuma, E.; Tatsuma, T. Photoinduced Reversible Changes in Morphology of Plasmonic Ag Nanorods on TiO2 and Application to Versatile Photochromism. Chem. Commun. 2012, 48, 1733‒1735. 27. Tanabe, I.; Matsubara, K.; Sakai, N.; Tatsuma, T. Photoelectrochemical and Optical Behavior of Single Upright Ag Nanoplates on a TiO2 Film. J. Phys. Chem. C 2011, 115, 1695‒ 1701. 28. Kazuma, E.; Sakai, N.; Tatsuma, T. Nanoimaging of Localized Plasmon-Induced Charge Separation. Chem. Commun. 2011, 47, 5777‒5779.


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Page 17 of 25

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29. Tanabe, I.; Tatsuma, T. Plasmonic Manipulation of Color and Morphology of Single Silver Nanospheres. Nano Lett. 2012, 12, 5418‒5421. 30. Saito, K.; Tanabe, I.; Tatsuma, T. Site-Selective Plasmonic Etching of Silver Nanocubes. J. Phys. Chem. Lett. 2016, 7, 4363‒4368. 31. Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410‒.8426. 32. Sherry, L. J.; Chang, S.-H.; Schatz, G. C.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy of Single Silver Nanocubes. Nano Lett. 2005, 5, 2034‒2038. 33. Ringe, E.; McMahon, J. M.; Sohn, K.; Cobley, C.; Xia, Y.; Huang, J.; Schatz, G. C.; Marks, L. D.; Van Duyne, R. P. Unraveling the Effects of Size, Composition, and Substrate on the Localized Surface Plasmon Resonance Frequencies of Gold and Silver Nanocubes: A Systematic Single-Particle Approach. J. Phys. Chem. C 2010, 114,12511‒12516. 34. Shore, M. S.; Wang, J.; Johnston-Peck, A. C.; Oldenburg, A. L.; Tracy, J. B. Synthesis of Au(Core)/Ag(Shell) Nanoparticles and their Conversion to AuAg Alloy Nanoparticles. Small 2011, 7, 230‒234. 35. Parida, S.; Kramer, D.; Volkert, C. A.; Rösner, H.; Erlebacher, J.; Weissmüller, J. Volume Change During the Formation of Nanoporous Gold by Dealloying. Phys. Rev. Lett. 2006, 97, 035504. 36. Matsubara, K.; Kelly, K. L.; Sakai, N.; Tatsuma, T. Effects of Adsorbed Water on PlasmonBased Dissolution, Redeposition and Resulting Spectral Changes of Ag Nanoparticles on SingleCrystalline TiO2. Phys. Chem. Chem. Phys. 2008, 10, 2263‒2269.


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Page 18 of 25

37. Tatsuma, T.; Suzuki, K. Photoelectrochromic Cell with a Ag-TiO2 Nanocomposite: Concepts of Drawing and Display Modes. Electrochem. Commun. 2007, 9, 574‒576. 38. Link, S.; Wang, Z. L.; El-Sayed, M. A. Alloy Formation of Gold-Silver Nanoparticles and the Dependence of the Plasmon Absorption on their Composition. J. Phys. Chem. B 1999, 103, 3529‒3533. 39. Takahashi, Y.; Matsuoka, Y. Dip-Coating of TiO2 Films Using a Sol Derived from Ti(O-iPr)4-Diethanolamine-H2O-i-PrOH System. J. Mater. Sci. 1988, 23, 2259‒2266. 40. Yu, K.; Tian, Y.; Tatsuma, T. Size Effects of Gold Nanaoparticles on Plasmon-Induced Photocurrents of Gold-TiO2 Nanocomposites. Phys. Chem. Chem. Phys. 2006, 8, 5417‒5420. 41. Ng, C.; Cadusch, J. J.; Dligatch, S.; Roberts, A.; Davis, T. J.; Mulvaney, P.; Gómez, D. E. Hot Carrier Extraction with Plasmonic Broadband Absorbers. ACS Nano 2016, 10, 4704‒4711 42. Torimoto, T.; Kontani, H.; Shibutani, Y.; Kuwabata, S.; Sakata, T.; Mori, H.; Yoneyama, H. Characterization of Ultrasmall CdS Nanoparticles Prepared by the Size-Selective Photoetching Technique. J. Phys. Chem. B 2001, 105, 6838‒6845. 43. Kogo, A.; Sakai, N.; Tatsuma, T. Photoelectrochemical Etching and Energy Gap Control of Silver Clusters. Nanoscale 2015, 7, 14237‒14240. 44. Nishi, H.; Tatsuma, T. Oxidation Ability of Plasmon-Induced Charge Separation Evaluated on the Basis of Surface Hydroxylation of Gold Nanoparticles. Angew. Chem. Int. Ed. 2016, 55, 10771‒10775. 45. Govorov, A. O.; Richardson, H. H. Generating Heat with Metal Nanoparticles. Nano Today 2007, 2, 30‒38.


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FIGURES

Figure 1. SEM images of Au-Ag alloy NP ensembles on TiO2 electrodes (a) before and (b, c) after irradiation of broadband visible light (>480 nm, ca. 9 mW cm‒2) in water at room temperature. Panel c was obtained at a tilt angle of 45 degrees.


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Figure 2. Schematic illustrations of the (a) photoelectrochemical dealloying and (b) accompanying nanopore formation process of Au-Ag NPs based on PICS.


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Figure 3. (a) Time course of the extinction spectrum of the TiO2-coated ITO electrodes with AuAg alloy NPs during photoirradiation (>480 nm, ca. 9 mW cm‒2). (b) Photograph of the electrode after phoroelectrochemical dealloying through a photomask.


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0.3

−∆Potential / V

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0 min

0.2

3 min 40 min

0.1 970 min

0.0

400 500 600 700 800 900

Wavelength / nm Figure 4. Open-circuit photovoltage action spectra of the TiO2-coated ITO electrode with Au-Ag alloy NPs in the course of photoelectrochemical dealloying by the broadband visible light irradiation (>480 nm, ca. 45 mW cm‒2).


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Figure 5. Extinction spectral changes of the TiO2-coated ITO electrodes with Au-Ag alloy NPs upon irradiation of (a) 500-nm, (c) 600-nm, and (e) 700-nm light. The corresponding SEM images after photoirradiation are shown in panels b, d, and f, respectively. (g) Time courses of extinction at the peak wavelengths upon photoirradiation of the different wavelengths (data from panels b, d, and f and Figure 3a).


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Figure 6. Extinction spectral changes of the TiO2-coated ITO electrodes with Au-Ag alloy NPs in (a) concentrated and (c) dilute nitric acid. The corresponding SEM images after chemical dealloying are shown in panels b and d, respectively. (e) Time courses of extinction at the peak wavelengths in nitric acid (data from panels a and c).


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TOC Graphic


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Photoregulated Nanopore Formation via Plasmon-Induced Dealloying

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