Plasmonic Nanochemistry Based on Nanohole Array

Plasmonic Nanochemistry Based on Nanohole Array Bin Ai,†,‡ Zengyao Wang,† Helmuth Möhwald,§ and Gang Zhang*,† †

State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P.R. China Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, United States § Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany ‡

S Supporting Information *

ABSTRACT: We show that the growth of Ag nanoparticles (NPs) follows the areas of maximum plasmonic field in nanohole arrays (NAs). We thus obtain Ag NP rings not connected to the metallic rim of the nanoholes. The photocatalytic effect resulting from the enhanced E-field of NAs boosts the reaction and is responsible for the site selectivity. The strategy, using plasmonics to control a chemical reaction, can be expanded to organic reactions, for example, synthesis of polypyrrole. After the NA film is removed, ordered ring-shaped Ag NPs are easily obtained, inspiring a facile micropatterning method. Overall, the results reported in this work will contribute to the control of chemical reactions at the nanoscale and are promising to inspire a facile way to pursue patterned chemical reactions. KEYWORDS: plasmonic nanochemistry, surface plasmon resonance, nanohole array, site selectivity, Ag nanoparticle

S

based on the fact that the particles with the largest plasmon absorption cross section at the laser wavelength initially grow fastest.5 Topographic modifications of a photosensitive azobenzene-dye polymer were created based on the enhanced near-field for a range of different nanostructures and illumination polarizations.6 By locally heating nanoscale metallic catalysts, growth of semiconductor nanowires and carbon nanotubes could be initiated and controlled at arbitrarily prespecified locations.7 Moskovits et al. reported that densely packed Au nanoparticles (NPs) in contact with aqueous Ag ions and exposed to red light rapidly photoreduced Ag ions in solution, producing radially symmetric metal deposits.8 They also observed the reduction of aqueous tetracholoroplatinate ions to zerovalent Pt metal at or near the surfaces. Following the photoexcitation of plasmons in the Au nanosystems,9 Kreiter et al. used gold crescents to guide Au colloid assembly onto substrates.10 The colloidal density also reflected the position and intensity of the near-field. Moreover, Au/Ag NPs have been successfully used to control photopolymerization and imprinting chemical functional groups in the hot spot regions with nanometer accuracy.11−13 While these impressive studies have demonstrated the great potential of using plasmonics to spatially control chemical

patially controlling chemical reactions enables the formation of products at a specific location, and this has attracted much attention. 1−3 Apart from its fundamental interest, once the reaction location can be controlled at the nanoscale, it may lead to a revolution in both traditional chemistry and nanofabrication techniques. This high spatial control below the diffraction limit of light is afforded by plasmonic fields, and therefore, this should lead to the field of “plasmonic nanochemistry”.4 Plasmonics, defined as light−metal interactions via coupling with the conduction electrons on the metal−dielectric interface, provides an enhanced electric field (E-field) at the nanometer level in the proximity of a plasmonic structure to cause valuable physical effects such as heat generation, optical near-field enhancement, and excitation of hot electrons. Hence, plasmonic materials can behave as efficient nanosources, remotely controllable by light, to selectively and locally boost chemical reactions. Simultaneously, the distribution of the enhanced E-field can be controlled by the shape of the plasmonic structure, the polarization and wavelength of incident light, as well as the surrounding environment. So, it is expected to provide a nanoscale spatial control of chemical reactions based on the control of the plasmonic performance. Based on this principle, some pioneering works have been reported to reveal the possibility of spatial control of chemical reactions. Brus et al. demonstrated photochemical Ag particle growth from adsorbed silver ions in the presence of citrate, © 2017 American Chemical Society

Received: July 11, 2017 Accepted: October 19, 2017 Published: October 19, 2017 12094

DOI: 10.1021/acsnano.7b04887 ACS Nano 2017, 11, 12094−12102

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Figure 1. (a) Experimental setup: a glass substrate coated with the Au NA film is immersed into the solution and irradiated by a white LED lamp. Scanning electron microscopy (SEM) images of the Au NAs with Ag NPs selectively growing in the interior of the holes with t = (b) 0 min, (c) 30 min, (d) 60 min, (e) 90 min, and (f) 120 min. The hole diameter is 500 nm. The insets show the 45° tilting SEM images of the corresponding samples. The scale bar in (b) corresponds to 1 μm and applies to all the SEM images. (g) Schematics showing the reaction process. t increases as the arrows point. The red dotted lines indicate the ring-shape distribution of the Ag NPs.

Figure 2. 3D AFM images of the nanoholes with selectively growing Ag NPs when t = (a) 0 min, (b) 30 min, (e) 45 min, (f) 60 min, and (g) 75 min. The black arrows in (b,g) indicate the NPs formed on the hole rim. (c) Top-view AFM image of the nanohole with t = 30 min. (d) Section analysis of the nanohole in (c). The position of the section is indicted by the white line in (c), and the red arrows correspond to those in (c). (h) Drawing of the nanohole with selectively growing Ag NPs and the main structural parameters. Roughness of the interhole surface, R1; roughness of the area of the NPs, R2; height between the interhole surface and the hole center, H; height between the NPs closest to the hole edge and the hole center, h; height between the NPs on the hole rim and interhole surface, T; diameter of the central blank circle, d.

reactions, this research field is only in its infancy as they do not make use of the large variety of plasmonic structures and chemical reactions. Most efforts are currently put into investigating the effect of metallic NPs, especially coated with TiO2, on chemical reactions. Until now, to the best of our knowledge, the function of another important form of plasmonic structureplasmonic filmsin controlling chemical reaction is still unexplored. Compared with NPs, plasmonic films possess two major features: (1) They support both surface plasmon polariton (SPP) and localized surface plasmon resonance (LSPR) modes, offering better opportunities to yield surprising optical effects by their interference.14−16 (2) The decay length of the enhanced E-field belonging to SPP is always hundreds of nanometers, much larger than that of the LSPR of metallic NPs (in the range of 10 nm). In terms of the difference between NPs and plasmonic film, it is important to explore chemical reactions in plasmonic films, which is expected to provide different phenomena and mechanisms. As in the film one can precisely locate the product, one can learn more about the mechanism; for example, one can discriminate if a reaction occurs at the metallic surface or at a distance of some 10 nm from the surface. In addition, plasmonic films are supposed to be able to form ordered chemical patterns which cannot be

achieved by NPs. To fill this huge gap, we investigated the plasmonic performance of a Au nanohole array (NA) film in the synthesis of Ag NPs and of polypyrrole. It is found that Ag NPs grow exclusively in nanoholes and form a ring shape with a distribution perfectly matching that of the enhanced E-field. The generality of the approach is demonstrated by extending the reaction to polypyrrole synthesis in nanoholes. Overall, the results reported in this work will advance the control of chemical reactions at the nanoscale and will inspire a facile way to pursue patterned chemical reactions.

RESULTS AND DISCUSSION The Au NA films were fabricated on glass substrates based on a simple and efficient colloidal lithography method. The detailed process can be seen in the Methods section and Figure S1. After the preparation, the Au NA films were immersed into a silver nitrate/sodium citrate (1 mM/34 mM, 100 mL/6 mL) solution and irradiated at 25 °C by a white light-emitting diode (LED) lamp with a power of 3 W over a circular area (dozens of cm2) larger than the sample (1 × 1 cm). The spectrum of the LED lamp can be seen in Figure S2. The experimental setup, shown in Figure 1a, is kept in a dark ambient environment during the whole process. The distance between the light 12095


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Figure 3. Plots of (A) R1 and R2, (B) d, (C) H and h, and (D) T as a function of t.

source and the substrate was controlled to be ∼1 cm, and the excitation intensity was kept at ca. 47 mW/cm2 on the substrate. Scanning electron microscopy (SEM) images of the Au NAs with different irradiation time (t) in Figure 1b−f reveal that there is a well-defined site selectivity for the growth of Ag NPs, which moreover experiences a clear trend with increasing t. For the NA films before immersion into the solution, the surfaces of the inside of the nanoholes and interhole are both smooth (Figure 1b). There are a few NPs appearing after t = 30 min (Figure 1c). When t = 60 min, the inside of the holes becomes clearly rougher compared with the interhole surface (Figure 1d). For larger t, the number, size, and deposition areas of the Ag NPs all become larger in the holes (Figure 1e,f), whereas the interhole film still keeps the same smoothness as the original NA with t = 0 min. These results demonstrate that Ag NPs only grow in the nanoholes rather than on the rougher Au interhole film. This is also supported by energy-dispersive X-ray spectroscopy (EDS) (Figure S3). In addition, it should be noted that the grown Ag NPs form a ring shape, leaving a blank gap near the hole edge and a blank circle at the center. The blank central circle shrinks as t increases. The whole process of the plasmon-induced reaction can be summarized in Figure 1g. Seeds first appear near the hole edge but not in close contact to the edge. Then Ag NPs continue growing based on the seeds and extend to the center of the nanoholes. Still the number and size of the NPs near the edge are largest. Different from previous reports, there are features that should be noted: (1) the experimental setup is very simple and easy to build and operate; (2) a cheap continuous-wave (CW) white LED lamp with very low power can trigger the site-selective chemical reaction instead of a laser source; (3) the Au NAs are not coated with any metal oxide; and (4) the products can be located in the arrays.

The three-dimensional (3D) atomic force microscopy (AFM) images of the Au NA films in panels a,b and e,f in Figure 2 show that more Ag NPs form with increasing t (corresponding top-view AFM images can be seen in Figure S4). This result is consistent with the SEM images in Figure 1. In addition, the surface is slightly higher at the hole rim after the reaction, as indicated by the black arrows in Figure 2b,g, whereas this is not apparent for the initial Au NAs (Figure 2a). In the section analysis (Figure 2d), the red arrows also indicate the difference in height. Given all the information achieved from the SEM images and AFM profiles, the morphology of the Au NAs with Ag NPs can be observed in Figure 2h. The red dotted line shows the height change. Clearly, the red dotted line is in good agreement with the height curve derived from the actual structure, indicating that Figure 2h is a reasonable structure model. As shown in Figure 3a, the height R2 increases from 0.77 to 3.8 nm as the reaction proceeds, whereas the roughness R1 only slightly changes from 1.07 to 1.4 nm. The differences are due to the fact that Ag NPs exclusively grow in the holes and only a few NPs appear on the interhole surface. The diameter d of the central blank decreases from 210 to 65 nm as the reaction proceeds (Figure 3b), indicating that a higher area fraction in the nanohole is coated with Ag NPs and the Ag NP ring becomes larger. H remains similar at ∼25 nm (Figure 3c), as few Ag NPs appear on the interhole surface and the center. h dramatically increases from 0 to 30 min and remains stable at longer t. This indicates that the size and number of the Ag NPs at the position closest to the hole edge reach an equilibrium state. The increase of T from 0 to 6.3 nm (Figure 3d) demonstrates that a few small Ag NPs also formed at the hole rim whose position is indicated in Figure 2h. Control experiments were designed to conclude on the possible mechanisms. Ni NA films with the same structural parameters were fabricated and experienced the same experimental process. Contrary to the findings for Au NAs, 12096


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Figure 4. (a,b) Color map of |E/E0| as a function of wavelength. For each wavelength, the values are the E-field intensity along the diameter of the central hole, as indicated by the pink lines in (a1−a4, b1−b4). The calculated plane is along the x−y plane on the Au/glass (a) and Au/ water (b) interface, indicated by the black dotted lines in the schematic below. The white dotted lines indicate the position of the hole edge. Calculated distributions of normalized E-fields at wavelengths of (a1,b1) 430 nm, (a2,b2) 520 nm, (a3,b3) 580 nm, and (a4,b4) 670 nm. The dotted circles indicate the outline of the holes.

very weak surface plasmon resonance (SPR) can be excited by sunlight, which cannot affect the chemical reaction. So, few Ag NPs appeared in the hole or on the film. When the Au NAs were immersed in boiling solution, Ag NPs formed on the whole film (Figure S5e). If a glass substrate without NAs was placed into the solution under 45 min irradiation, Ag NPs appeared on the whole surface and did not show site selectivity (Figure S5f). The site selectivity cannot occur without either light irradiation or the plasmonic film. The results on the grown NPs are the same whether the Au NA films were inverse or obverse to the light source, indicating that the NPs in the hole are not due to the deposition induced by gravity and are not due to the shadow effect of the mask. The process has no relationship with the principle of photolithography. In summary, we can rule out some possible mechanisms: (1) heterogeneous nucleation, (2) gravity deposition, and (3) shadow effect. It should be noted that there are a few circular polystyrene (PS) residues at the hole center due to the fabrication method, with a diameter of ∼100 nm and height of ∼3 nm (Figure S7). However, the diameter of the central blank circle (d) changes from 210 to 65 nm, which is not matched to or limited by the size of any PS residue, indicating the minor role of the PS residual in the plasmonic reaction. In addition, the only way for the PS residue to affect the distribution is to provide a nucleation spot, but the effect of heterogeneous nucleation can be ruled out. Moreover, transferred Au NAs without the PS residual were used in the plasmonic reaction. The Au NAs were

Ag NPs mostly deposited on the interhole (Ni) surface (Figure S5a−c). This result can be explained by the classical notion that nucleation occurs preferentially on rough heterogeneous surfaces; herein, the roughness of the Ni surface (∼2 nm) is larger than that of the glass (0.77 nm). From the field of interfacial energy, it is also concluded that the Ag NPs prefer to grow on the metal surface (Ni and Au) rather than glass and silicon, which is discussed in section SIV in Supporting Information (SI). Based on the principle of interfacial energy, chemical reactions were spatially controlled on Au and Pt rather than TiO2 and silicon by localized heterogeneous nucleation driven by well-adjusted surface energies.17,18 However, in this work, although with the preference on the Au surface due to the roughness and interfacial energy, Ag NPs exclusively grow in the interior of the nanoholes on glass substrates, which is against the classical notion of heterogeneous nucleation. Due to the totally different phenomenon for Ni and Au NAs, the preference of chemical reaction on metals, and considering the result that NPs grow at the location with a ∼30 nm distance to the edge but not at the edge of a film, the site-selective chemical reaction on Au NAs is not due to heterogeneous nucleation. When the Au NAs were immersed in silver nitrate/sodium citrate solution in a dark room or in sunlight (Figure S5d), there were no NPs emerging in the hole or on the film. Ag+ can hardly be reduced without light at room temperature, so no Ag NPs formed. When silver nitrate solution was exposed to sunlight, the solution became black and turbid, indicating that Ag+ was reduced slowly to form very small Ag NPs. However, 12097


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Figure 5. Spectra of the UV, blue, green, and red light for the plasmonic reaction. SEM images of the Au NAs after the plasmonic reaction illuminated by (b) UV, (c) blue, (d) green, and (e) red light.

transferred based on our previous work.19 In brief, the Au NA samples were immersed in HF/NH4F buffer solution for 10 s, before they were dipped slowly into deionized water, leaving free-standing films on the water/air interface. Then the floating films were lifted up by other glass substrates. After the transfer, there is no PS residual in the nanoholes (Figure S8a). After the plasmonic reaction for 2 h, Ag NPs only grew in the nanoholes, with a small blank circle in the center and a distance to the hole edge (Figure S8b), which is the same with the nontransfer Au NAs (Figure 1f). This result indicates that PS residue may have no effect on the distribution of Ag NPs. The distribution of Ag NPs is the opposite if the NAs change from Ni to Au. A significant difference between the two materials is that SPR of the Ni resonance is much weaker owing to the large imaginary part of its dielectric function compared with that of Au.20 Thus, we propose that the site-selective chemical reaction is related to the plasmonic performance of the Au NAs. Figure 4 shows the calculated distributions of the normalized E-fields |E|/|E0| of the Au NAs. Some examples from the cross-sectional view are shown in Figure S12. The profiles were calculated in water to be closer to the real experiment. The calculated spectra are presented in Figure S9 and show good agreement with the experimental spectra, demonstrating the model used for the simulation is reasonable. The E-fields were calculated as a function of wavelength from 400 to 700 nm because a CW light source (400−700 nm) was used in this work. Figure 4a shows the color map of |E|/|E0| as a function of the wavelength at the Au/glass interface. Because of the circularly symmetric E-field distribution, the values are plotted along the hole diameter versus wavelength to represent the whole distribution. Figure 4a shows that the E-field is enhanced between the hole edge and the center in a large wavelength range from about 400 to 560 nm. This E-field distribution on the x−y plane in this range is like those in Figure 4a1,a2, showing a ring shape. The ring shape perfectly matches the distribution of Ag NPs in the Au nanoholes (Figure 1 and Figure 2). At longer wavelengths from 590 to 700 nm, there are strong E-fields enhanced outside the hole. However, the area is very small (Figure 4a4), and they are mainly penetrating into the glass substrate. Thus, these enhanced E-fields are supposed to have little effect on the chemical reaction. Overall, the ring-shaped E-field distribution on the Au/glass interface is the key for formation of the ringshape distribution of the Ag NPs. The color map of |E|/|E0| as a function of wavelength at the Au/water interface is shown in

Figure 4b. There are also strong E-fields enhanced outside the hole on the Au surface from 590 to 700 nm. However, unlike those on the Au/glass interface, they are mainly distributed in air and show non-negligible influence on the chemical reaction. This enhanced E-field is responsible for the Ag NPs formed at the hole rim. The enhanced E-field between the white lines (in the hole) at the Au/water interface can hardly have an impact on the distribution of Ag NPs because Ag NPs grow from the glass substrate due to the lower energy barrier compared with that of the liquid phase. In conclusion, the distribution of Ag NPs follows exactly the localized E-field enhancement. The Efield enhancement of the Au NAs is generated by both SPP and LSPR modes.21,22 We further looked into the roles of the two plasmonic modes by comparing the simulated E-fields of the Au NAs and isolated holes (for details, see section SVII in SI). The same distribution of the Ag NPs and the localized E-field indicates that the site-selective chemical reaction is controlled by the localized E-field resulting from the LSPR. SPP also contributed to localized E-field enhancement up to 5% in the wavelength range of 400−700 nm by the interaction between SPP and LSPR, improving the reaction efficiency and site selectivity. SPP makes a well-defined ring-shape distribution of the localized E-field, contributing to the formation of ring-shape distribution of Ag NPs. In addition to the contribution of SPP to the E-field shape and intensity, the SPP makes the Au NAs possess resonance penetration depths of hundreds of nanometers, whereas the penetration depth of LSPR metallic nanoparticles is only in the range of 10 nm.23 Therefore, more chemicals in the solution are affected by the resonance, improving the reaction efficiency. To further demonstrate the relation between the plasmonic resonances and the chemical reaction, ultraviolet (UV), blue, green, and red light with full width at half-maximum (fwhm) < 50 nm were used to illuminate the Au NAs. The normalized spectra are shown in Figure 5a, and the excitation density on the substrates was 200, 17, 17, and 38 mW/cm2 for the UV, blue, green, and red light, respectively. Figure 5b−e shows the transferred Au NAs after the irradiation for 1, 10, 10, and 10 h for the UV, blue, green, and red light, respectively. For the UV light with the peak wavelength of λP = 365 nm, there are large Ag NPs emerging in the nanoholes with a distance to the edge and a black circle at the hole center, while no obvious Ag NPs are observed on the Au surface (Figure 5b). The distribution of the E-field in the wavelength range of 350−400 nm was calculated in Figure S13, showing the ring-shape resonance in 12098


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Figure 6. (a) Schematic showing the principle of the growth of Ag NPs induced by the plasmonic field. On the left, enhanced plasmonic field (red ring) is excited inside of the Au nanoholes (yellow structure) by light illumination. On the right, Ag NPs (white ring) grow in the Au nanoholes (yellow structure) following the plasmonic field distribution. (b) SEM images of the Ag NA film with selectively growing Ag NPs with t = 20 min. (c) SEM image of the Au NA growing with polypyrrole. (d) SEM image of Ag NP patterns after removing the NA. The topleft inset shows the boundary with/without the NA.

distributions of Ag NPs and enhanced E-field; (3) results are not due to heterogeneous nucleation and shadow effect. All of these features can be explained by combining the plasmonic effect with the mechanisms of nucleation and growth of NPs. Synthesis of Ag NPs experiences two relatively separated processes: nucleation and growth. Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth. In the liquid phase, the process of homogeneous nuclei formation can be considered thermodynamically by looking at the total free energy ΔG of a nanoparticle, which can be described by the equation:24

the whole wavelength range, which is consistent with the distribution of Ag NPs. When blue light (λP = 433 nm) was used, the nanoholes were filled with large Ag NPs and no Ag NPs appeared on the Au surface, again showing the site selectivity (Figure 5c). However, there are no blank circles at the hole center, which might be due to the long reaction duration (10 h) and the effect of the formed Ag NPs on the following reduction of Ag+ (discussed later). For the Au NAs illuminated by the green light (λP = 531 nm), sparse Ag NPs were formed in the nanohole, with a few smaller Ag NPs emerging on the Au surface. The large Ag NPs in the nanoholes form a ring shape with a large blank circle at the hole center. There are no Ag NPs formed in the nanoholes (Figure 5e) for the red light (λP = 646 nm), even with a density larger than that of the blue and green light. Instead, small Ag NPs were formed on the Au surface, making it rougher. The different results for the blue, green, and red light can be demonstrated based on the distribution of the E-field in Figure 4. The ring-shape E-field of the blue light (Figure 4a1) is stronger than that of the green light (Figure 4a2), leading to more Ag NPs formed in Figure 5c. For the red light, the E-field of the hole edge is much stronger than that in the nanoholes. The difference between the E-fields of the nanoholes and Au surface is no longer large enough to overcome the preference of Ag NPs on the Au surface, leading to the Ag NPs on the Au surface (Figure 5e). In summary, the UV and blue light are more efficient to trigger the site-selective chemical reaction than the green and red light due to the different resonances in the four wavelength ranges. This again shows the key function of plasmonic resonances in the site-selective chemical reaction. Based on the results discussed above, we can summarize the features of the growth of Ag NPs on the Au NAs as follows: (1) well-defined site selectivity; (2) great match between the

ΔG = ΔGv + ΔGs

where ΔGv is the free energy gain for the phase transformation and ΔGs is the free energy penalty for the formation of a solid surface. Considering that ΔGv depends on the volume and ΔGs depends on the surface area, it is possible to find a maximum free energy as a function of diameter, which a nucleus has to overcome to form a stable nucleus, giving a critical free energy ΔGcrit. For heterogeneous nucleation, ΔGhetero < ΔGhomo crit crit where ϕ < 1 is a factor dependent on the contact angle between the NPs and the support. According to previous reports25 and the LaMer mechanism,26,27 Ag0 monomers are generated in solution under illumination. Ag0 cannot spontaneously condense into solid nuclei until the total free energy reaches a critical value (i.e., ΔGcrit). Because ΔGhetero < ΔGhomo crit crit , seeds first are formed on inhomogeneities. This explains the results for Ni NAs in Figure S5a−c. For Au NAs, upon illumination, the E-field in the proximity of the nanostructure is enhanced (Figure 4) and re-emitted. The enhanced E-fields at the metal surface dramatically increase the number of available photons per unit volume at these optical “hot areas”. The enhanced Efields provide additional energy to facilitate reaching. Seeds first 12099


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distribution of Ag NPs is the same as that of the Au NAs on glass substrates (Figure S16). The substrate with Au NAs was immersed into a higher concentration of silver nitrate (10 mM) aqueous solution and still showed good site selectivity (Figure S17). This also indicates that the kinetics may play a minor role in the plasmonic reaction (for details, see section SXI in SI). Ag NAs were also used in the chemical reaction of growing Ag NPs and showed good site selectivity (Figure 6b). The Au NA was placed into pyrrole (10 mM) solution with irradiation from the white LED lamp for 20 h. Some products were formed and mainly emerge in the nanoholes, showing again site selectivity (Figure 6c). Next, a quantitative EDS surface analysis of the sample in Figure 6c was carried out over an area of ∼18 μm × 24 μm. The values of the atom percent are shown in Table S1. The component of O and Au is from the glass substrate (SiO2) and Au NA, respectively. There is no the component of N before irradiation. The N1s peak, which can be based on polypyrrole, is observed from the substrate after the irradiation. Therefore, the generated product was confirmed to be polypyrrole.17 The site selection of polypyrrole indicates that the site-selective chemical reaction induced by the plasmonic film is not limited to inorganic reactions but can be extended to a broad range of organic reactions. In addition, after the siteselective reaction, by using tape to remove the NAs, one can easily obtain ordered ring-shaped Ag NPs left on the substrate (Figure 6d). Thus, the site-selective chemical reaction induced by plasmonic films is promising to directly yield patterned materials.

form in the hot area, experiencing a burst-nucleation process (LaMer mechanism). Through an Ostwald ripening process,28 that is, smaller particles redissolve and, in turn, allow the larger particles to grow even more, the Ag NPs are exclusively synthesized in the area of enhanced E-field (Figure 6a). The resonance of formed Ag NPs may facilitate the following site-selective reduction of Ag+. At first, the Ag NPs are too small to have a significant resonance to affect the reaction. The resonance of the Au NAs dominates the reaction. As the Ag NPs grow larger, their resonance becomes stronger. The size of the Ag NPs is 10−40 nm after 1−2 h irradiation, whose resonance wavelength is in the wavelength of 400−410 nm.29 The ring-shape resonance of the Au NAs is excited in this wavelength range. Moreover, the Ag NPs are distributed in the ring-shape resonance. The couple between the resonances of the Ag NPs and Au NAs is supposed to lead to stronger E-field enhancement. Herein, we measured surface-enhanced Raman scattering (SERS) to demonstrate the resonance property of the pristine Au NAs and the Au NAs with Ag NPs. The SERS intensity continues increasing as more Ag NPs are produced in the holes (Figure S14). At the 1140 cm−1 peak, the SERS intensity of the composite particle-in-hole array with t = 90 min can be 70-fold higher than the intensity of the NA with t = 0 min. This indicates that the Ag NPs will contribute to the enhancement of the electric field, making stronger SP energy and favoring the following site-selective reduction of Ag+. The effect of the Ag NPs on the following reduction of Ag+ may be responsible for the result that the Ag NPs extend to the hole center and fill up the hole for a long-time irradiation (Figure 1f, Figure 5c, and Figure S8b) and higher concentration (Figure S17). At the beginning of the plasmonic reaction, the Ag NPs follow the ring-shape resonance exactly before 90 min (Figure 1c−e). As the reaction goes on, the resonance of the old Ag NPs leads to fresh NPs near the old ones, resulting in the extending growth to the hole center. SPR also can lead to other physical effects such as heat generation and excitation of hot electrons. It has been reported that localized heating by a laser is irrelevant in the photoreduction of Ag NPs.30−32 Moreover, the light power used in this work is very low, and no temperature changes were measured in the process. Thus, the effect of heat generation is very limited. The hot-electron transfer induced by SPR can only happen on the interface.11,33 However, the Ag NPs formed in the nanoholes have a distinct distance to the Au hole edge (Figure 1). Thus, hot-electron transfer does not play a role. In addition, the site-selective chemical reaction induced by the bare Au NA films should be separated from the photocatalysis process using a plasmonic structure coated with a metal oxide (e.g., TiO2).34,35 In the latter process, the plasmonic structure serves as a hot charge carrier. The growing Ag NPs in the nanohole also lead to corresponding changes of optical properties. The changes of transmission and reflection spectra with increasing t are shown in Figure S15. As t increases, the transmission peak and dips red shift, and the transmission intensity decreases (Figure S15a); the reflection peak red shifts, and the reflection intensity increases as t increases (Figure S15b). The well-defined evolution in optical absorption reflects the changes in morphology of the NAs and also indicates that the growth of Ag NPs is promising to be used to tune the optical performance of the NAs. Additional experiments were perfromed to demonstrate the generality of the strategy: silicon substrates with transferred Au NAs were used in the reaction, and the

CONCLUSIONS In summary, by combining Au nanohole arrays and light irradiation, Ag NPs and polypyrrole can selectively be synthesized in the resonant area, and the distributions of chemical products and SP energy are well matched. This demonstrates the key function of the plasmonic field for the site-selective chemical reaction. Compared with NP-based plasmonic chemistry, film-based plasmonic chemistry shows better spatial control to achieve ordered arrays of chemical products and may make a contribution in fabricating chemical patterns. Moreover, because the growth of Ag NPs follows the distribution of the enhanced E-field, more chemical patterns can be fabricated by controlling the E-field distribution based on different incident light and plasmonic structures. The combination of plasmonics and chemical reactions is promising to inspire another progressing way for both traditional chemistry and nanofabrication techniques. METHODS Materials. In all experiments, deionized water was ultrapure (18.2 MΩ·cm) from a Millipore water purification system. The glass slides (15 × 30 mm2) used as substrates were cleaned in an O2 plasma cleaner for 4 min to create a hydrophilic surface. PS spheres (700 nm) were purchased from Wuhan Tech Co. Ltd. Au (99.999%) for vapor deposition, silver nitrate electrolyte, and pyrrole were purchased from Sinopharm Chemical Reagent Co. Ltd. Ethanol and toluene were purchased from Beijing Chemical Works. The solutions of paminothiophenol (PATP, Sigma-Aldrich) were prepared by using 97% purity, diluted to 10−3 M in ethanol, and all solvents were used as received without any further purification. Fabrication of Au NAs. The schematic of the fabrication process is shown in Figure S1. First, polystyrene (PS) sphere (700 nm) monolayers were prepared by an interface method.36 After that, oxygen reactive ion etching (RIE) was carried out for 240 s, accompanied by reduction of the size of PS spheres. The oxygen RIE 12100


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ACS Nano was performed on a Plasma Lab Oxford 80 Plus (ICP 65) system (Oxford Instrument Co. UK), and the RIE procedure was operated at a pressure of 10 mTorr, a flow rate of 50 sccm, a radio frequency power of 100 W, and an inductively coupled plasma power of 100 W. Then the samples were mounted in a thermal evaporator to vertically deposit 25 nm Au (99.999%). Finally, the PS spheres were removed by sonication in toluene, forming a Au NA. Chemical Reactions Induced by the Au NAs. The substrate with Au NAs was immersed into an aqueous solution containing silver nitrate/sodium citrate (1 mM/35 mM, 100 mL/6 mL) and illuminated by a white LED lamp (ca. 47 mW/cm2) for different durations t. Then the samples were taken out from the solution, washed by deionized water, and blown-dry with N2 gas. The substrates (1 cm × 1 cm) with Au NAs were immersed into a pyrrole (10 mM) aqueous solution and illuminated by a white LED lamp (ca. 47 mW/cm2) for 20 h. Then the samples were taken out from the solution, washed by deionized water, and blown-dry with N2 gas. Finite-Difference Time-Domain Calculations. A commercial software package, FDTD Solutions (Lumerical Solutions, Inc. Canada), was used to calculate transmission spectra and near-field E-field distribution of the NA with the same structural parameters as those extracted from the actual fabricated samples. A rectangular unit cell consisting of one nanohole in the center and four quartering nanoholes at the four corners was used with periodic boundary conditions in two dimensions to simulate an infinite array of periodic nanoholes. The auto nonuniform mesh was chosen in the entire simulation domain for higher numerical accuracy. The mesh refinement is conformal variant 2. Monitors of frequency domain field profile were placed to calculate the E-field distributions and the transmission spectra in the continuous-wave normalization state. The magnitude of the incident E-fields was taken to be unity, and the enhancement of electromagnetic fields was evaluated. To simulate a nonpolarized beam or plane wave source, two simulations with orthogonal polarizations are required. Results for a nonpolarized source can then be calculated by incoherently summing results from the two polarized simulations. The whole structure was immersed in H2O. The optical parameters of Au, SiO2, and H2O were taken from Palik’s handbook.37 Characterization. Scanning electron microscopy images were taken with a JEOL JSM 6700F field emission scanning electron microscope with a primary electron energy of 3 kV, and the samples were sputtered with a layer of Pt (ca. 2 nm thick) prior to imaging to improve conductivity. Height analyses were taken under tapping mode with a Nanoscope IIIa scanning probe microscope from Digital Instruments under ambient conditions. A Maya 2000PRO optics spectrometer and a model DT 1000 CE remote UV/vis light source (Ocean Optics) were used to measure the transmission spectra as well as the reflection spectra. SERS spectra were measured by using a Lab RAM HR Evolution Raman spectrometer. All SERS spectra were excited by a λ = 633 nm laser line. The transmission and accumulation time were set to 5% and 10 s, respectively.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Bin Ai: 0000-0002-0502-9166 Helmuth Möhwald: 0000-0001-7833-3786 Gang Zhang: 0000-0001-6486-501X Author Contributions

B.A. and Z.W. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673085, 51373066, and 51173068) and “111” project (B06009). REFERENCES (1) Assion, A.; Baumert, T.; Bergt, M.; Brixner, T.; Kiefer, B.; Seyfried, V.; Strehle, M.; Gerber, G. Control of Chemical Reactions by Feedback-Optimized Phase-shaped Femtosecond Laser Pulses. Science 1998, 282, 919−922. (2) Wang, L.; Nemoto, Y.; Yamauchi, Y. Direct Synthesis of Spatiallycontrolled Pt-on-Pd Bimetallic Nanodendrites with Superior Electrocatalytic Activity. J. Am. Chem. Soc. 2011, 133, 9674−9677. (3) Lv, W.; Lee, K. J.; Li, J.; Park, T. H.; Hwang, S.; Hart, A. J.; Zhang, F.; Lahann, J. Anisotropic Janus Catalysts for Spatially Controlled Chemical Reactions. Small 2012, 8, 3116−3122. (4) Baffou, G.; Quidant, R. Nanoplasmonics for Chemistry. Chem. Soc. Rev. 2014, 43, 3898−3907. (5) Maillard, M.; Huang, P. R.; Brus, L. Silver Nanodisk Growth by Surface Plasmon Enhanced Photoreduction of Adsorbed [Ag+]. Nano Lett. 2003, 3, 1611−1615. (6) Hubert, C.; Rumyantseva, A.; Lerondel, G.; Grand, J.; Kostcheev, S.; Billot, L.; Vial, A.; Bachelot, R.; Royer, P.; Chang, S. H.; Gray, S. K.; Wiederrecht, G. P.; Schatz, G. C. Near-Field Photochemical Imaging of Noble Metal Nanostructures. Nano Lett. 2005, 5, 615−619. (7) Cao, L. Y.; Barsic, D. N.; Guichard, A. R.; Brongersma, M. L. Plasmon-Assisted Local Temperature Control to Pattern Individual Semiconductor Nanowires and Carbon Nanotubes. Nano Lett. 2007, 7, 3523−3527. (8) Lee, S. J.; Piorek, B. D.; Meinhart, C. D.; Moskovits, M. Photoreduction at a Distance: Facile, Nonlocal Photoreduction of Ag Ions in Solution by Plasmon-mediated Photoemitted Electrons. Nano Lett. 2010, 10, 1329−1334. (9) Kim, N. H.; Meinhart, C. D.; Moskovits, M. Plasmon-Mediated Reduction of Aqueous Platinum Ions: The Competing Roles of Field Enhancement and Hot Charge Carriers. J. Phys. Chem. C 2016, 120, 6750−6755. (10) Dostert, K. H.; Alvarez, M.; Koynov, K.; del Campo, A.; Butt, H. J.; Kreiter, M. Near Field Guided Chemical Nanopatterning. Langmuir 2012, 28, 3699−3703. (11) Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. Nanoparticle Plasmon-Assisted Two-photon Polymerization Induced by Incoherent Excitation Source. J. Am. Chem. Soc. 2008, 130, 6928−6929. (12) Volpe, G.; Noack, M.; Acimovic, S. S.; Reinhardt, C.; Quidant, R. Near-Field Mapping of Plasmonic Antennas by Multiphoton Absorption in Poly(methyl methacrylate). Nano Lett. 2012, 12, 4864− 4868. (13) Galloway, C. M.; Kreuzer, M. P.; Acimovic, S. S.; Volpe, G.; Correia, M.; Petersen, S. B.; Neves-Petersen, M. T.; Quidant, R. Plasmon-Assisted Delivery of Single Nano-Objects in an Optical Hot Spot. Nano Lett. 2013, 13, 4299−4304.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04887. Fabrication of the Au nanohole arrays; spectrum of LED; EDS analysis; AFM images of Au NAs; corresponding control experiments; effect of the PS residual; calculated and experimental transmission spectra; contributions of the SPP and LSPR; SERS measurement; optical properties; effect of the substrate; effect of the concentration of silver nitrate; EDS analysis of polypyrrole (PDF) 12101


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Plasmonic Nanochemistry Based on Nanohole Array

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