In Situ Observation of Single Au Triangular Nanoprism Etching to

Dec 22, 2016 - Hui XuPingping SongJin WangYukihide ShiraishiYukou DuQingyun Liu ... Jinlei Wu , Zhenyi Zhang , Benkang Liu , Yurui Fang , Li Wang , Bi...
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In Situ Observation of Single Au Triangular Nanoprism Etching to Various Shapes for Plasmonic Photocatalytic Hydrogen Generation Zaizhu Lou, Sooyeon Kim, Peng Zhang, Xiaowei Shi, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki Osaka 567-0047, Japan S Supporting Information *

ABSTRACT: In situ etching of single Au triangular nanoprism (TNP) was successfully monitored by the plasmonic photoluminescence (PL) spectra using single-particle microscopy, which provides clear results to understand the geometric and anisotropic dependence of surface plasmon resonance in Au nanostructures. Various Au nanostructures (TNP, obtuse TNP (O-TNP) and nanodisk) were obtained to synthesize anisotropic Pt−Au as plasmonic photocatalyts for hydrogen generation. Single-particle PL spectra and finite-difference time-domain simulations demonstrate that the Pt-edged Au O-TNP has larger tip area and higher plasmon enhanced electrical field for hot electron transfer and charge separation, leading to more efficient photocatalytic hydrogen generation. KEYWORDS: plasmonic photocatalysis, Au triangular nanoprism, hydrogen generation, in situ reaction, single-particle spectroscopy

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generated on Pt-loaded Au NRs and TNPs transfer from Au to Pt, leading to hydrogen generation.15,20 Photo-oxidization of CH3OH on individual Au NR has been monitored by the quenching of single-particle PL spectra.21 However, there are few reports on in situ etching of Au nanostructures based on single-particle PL spectroscopic measurement. As mentioned above, Au TNP is an ideal structure to study in situ etching reaction. Furthermore, plasmonic PL spectra during the transformation of single Au TNP will provide strong evidence to understand the relation between SPR and geometric and anisotropic nanostructure at the single-particle level. In this work, we successfully performed the confocal microscopic measurement of in situ etching of single Au TNP. Transformation from TNP to obtuse TNP (O-TNP) and nanodisk (ND) during the etching process was monitored by plasmonic PL spectra. Au TNP, O-TNP, and ND were used to synthesize anisotropic Pt-edged Au TNP, O-TNP, and ND as plasmonic photocatalysts for hydrogen generation. Singleparticle PL spectra demonstrated that in-plane SPR is the dominate channel for PL quenching in Pt-edged Au nanoplates. Finite-difference time-domain (FDTD) simulations showed

urface plasmon resonance (SPR) of metal nanoparticles (NPs) as a unique phenomenon has been applied in photothermal therapy,1,2 enhanced spectroscopies,3 sensor,4 and solar light harvesting.5,6 As a collective electronic oscillation, SPR is highly sensitive to the shapes, materials and surrounding medium of metal NPs.7 Among various Au shapes such as nanoshpere,8 nanorod (NR),9 nanowire,10 and nanoplate,11 two-dimensional (2D) triangular nanoprism (TNP) is the most attractive because of SPR sensitive to geometric and anisotropic structures with respect to tip, edge, and surface.12 Although there are some reports13,14 on Au TNPs with tunable SPR bands and Pt-loaded Au TNPs exhibit photocatalytic hydrogen generation under visible and nearinfrared light (NIR) irradiation,15 the detailed mechanism of SPR sensitive to the geometric and anisotropic structures is not clear. To date, special microscopies such as spatiotemporal ellipsometric microscopy,16 in situ liquid-cell electron microscopy,17 and shell-isolated nanoparticle-enhanced Raman spectroscopy18 have been used to monitor in situ reaction on Au NPs. On the other hand, single-particle PL microscopic measurement is the most powerful method to investigate SPR of single Au particle for understanding the generation, transfer, separation and recombination of plasmonic hot electron.19 For example, single-particle photoluminescence (PL) spectra have demonstrated that the hot electrons © 2016 American Chemical Society

Received: November 10, 2016 Accepted: December 22, 2016 Published: December 22, 2016 968

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Figure 1. TEM images of Au TNPs (a), O-TNPs (b), and NDs (c) obtained by Au TNPs etching and their visible-NIR extinction spectra (d) and FDTD simulated extinction spectra (e). Schematic illustration of the confocal microscopic measurement for monitoring in situ etching of single Au TNP (f).

Before the etching reaction, three individual Au TNP particles were chosen as shown in single-particle PL image (Figure 2a). Figure 2c, d, and e shows the single-particle PL

that Pt-edged Au O-TNP has larger tip area and higher plasmon enhanced electric field to facilitate the hot electron transfer and charge separation, leading to higher efficiency than Pt-edged Au TNP and Pt-edged Au ND in the photocatalytic hydrogen generation.

RESULTS AND DISCUSSION Au TNPs (Figures 1a) with average size of 140 nm were synthesized by the modified seed-growth process used in our previous work.13−15 When 0.2 mL of HAuCl4 (0.5 mM) solution was injected into 10 mL of Au TNP solution, Au OTNPs with 100 nm length and obtuse tips were obtained after 1 h reaction. (Figures 1b) When 0.6 mL of HAuCl4 was injected, TNPs were completely etched to yield NDs with 78 nm diameter (Figures 1c). From visible-NIR extinction spectra (Figure 1d), two distinct bands were observed in Au TNPs. The stronger one at 1194 nm is assigned to in-plane dipole surface plasmon resonance (DSPR) of Au TNPs, while the other at 715 nm is assigned to in-plane multipole surface resonance (MSPR).15,22 Out-of-plane SPR (OP-SPR) is too weak to be observed in the spectrum. During the etching process, MSPR band became weaker for Au O-TNPs and disappeared for Au NDs.23 On the other hand, DSPR band showed a blue shift due to the decreased size. FDTD simulations (Figure 1e) support the experimental results. Detailed discussion is given in Supporting Information. Plasmonic PL spectra of individual Au TNP during the etching process were measured to illustrate the generation, separation, transfer and recombination of plasmonic hot electrons. Figure 1f shows the confocal microscopic measurement to observe in situ etching of single Au TNP particle. Au TNPs were coated on a quartz cover glass with single-particle dispersion and annealed at 100 °C for 1 h. SEM image was shown in Figure S1. A chamber was attached on the quartz cover glass as the reaction vessel (Figure S2). A circular-polarized 405 nm continuous wave (CW) laser was used as excitation source. Single-particle PL images were observed by a single-photon avalanche diode (SPAD) detector, while single-particle PL spectra were measured by an electron-multiplying chargecoupled device (EMCCD) camera.

Figure 2. Single-particle PL images of Au TNPs in air (a) and 20mM CTAB solution (b). Single-particle PL spectra of individual Au particles 1 (c), 2 (d), and 3 (e) corresponding to numbered points of Figures 2a and b. Scale bar = 500 nm.

spectra of individual Au TNP particles 1, 2, and 3 corresponding to Figure 2a and b, respectively. In air, a broad PL band around 630 nm was observed for both particles 1 and 2, while around 663 nm for particle 3, which are assigned to PL from MSPR of Au TNP. The broad MSPR PL band can be attributed to the overlapping of two PL bands from quadrupole and octupole SPR modes of Au TNP,22 consistent with FDTD simulations (Figure 1e). A weak PL band around 914 nm due to DSPR of Au TNP was observed for both particles 1 and 2. Energy difference (ΔE) between the initial excited state and DSPR state determines the population of excited electron transfer to DSPR mode.24 Large ΔE reduces the population of excited electron transfer to DSPR mode resulting in diminished PL. Compared to particles 1 and 2, 969

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single-particle PL images and PL spectra of three individual Au TNP particles at various reaction times during the etching process. From Figure 3a, it is clearly indicated that brightness of single-particle PL image became a slightly dark at 5 min, and then became much stronger at 20 min. It has no observable change from 20 to 40 min, while becomes slightly dark from 40 to 120 min. However, to our surprise, the single-particle PL spectra showed a significant change during the etching process. As shown in Figure 3b, at 5 min, the PL spectrum of Au particle 1 was completely different from the initial one. Two PL bands around 820 and 585 nm due to DSPR and MSPR modes of Au O-TNP were observed. The weak DSPR PL intensity is attributed to (i) large ΔE between the initial excited state and DSPR mode and (ii) the quenching by excess AuCl4− in solution. At 20 min, a PL band assigned to DSPR mode of Au ND was observed around 687 nm with the increased intensity. The weak peak around 550 nm is assigned to PL from OP-SPR mode. Disappeared MSPR PL is attributed to the absence of MSPR mode in ND. As the reaction time increased, DSPR PL band showed a blue shift because of the decreased size, while the OP-SPR PL band was kept around 550 nm. Increased DSPR PL intensity from 0 to 40 min is attributed to the decreased ΔE between the initial excited state and DSPR mode.25 However, as the size decrease, in-plane polarization of ND became weak resulting in weak DSPR mode. The excited electron−hole pairs transfer to OP-SPR mode competes with their transfer to DSPR mode. Therefore, the increased PL intensity from OP-SPR is resulted of the decrease in DSPR. At 120 min, both PL intensities of DSPR and OP-SPR were diminished due to the decreased size of ND. PL bands from DSPR and OP-SPR of ND became much closer, implying that in-plane polarization became weaker. Single-particle PL spectra of individual Au particles 2 and 3 (Figures 3c and 3d) showed the similar results during the etching process, further confirming that the results are credible. Slight difference in PL spectral shape of individual Au particles is due to their different sizes. The shapes of plasmonic PL spectra during etching of Au TNP are consistent with single-particle PL spectra of Au TNP, O-TNP, and ND (corresponding to TEM

MSPR PL band of particle 3 shows a red shift (Figure S3) because of the different size. Therefore, the increased ΔE between the initial excited state and DSPR mode in particle 3 leads to the disappearance of DSPR PL. When 50-μL hexadecyltrimethylammonium bromide CTAB (20 mM) solution was injected into the chamber, brightness of singleparticle PL image became slightly dark. From the single-particle PL spectra, the MSPR PL band of Au TNPs showed a red shift because of the increased dielectric constant of medium surrounding Au TNPs (1.0 for air, 1.33 for water), consistent with FDTD simulations (Figure S4). The diminished MSPR PL and disappeared DSPR PL are attributed to increased ΔE. Etching reaction of Au TNPs began after injecting 50-μL HAuCl4 (1 μM) solution into the chamber. Figure 3 shows the

Figure 3. Single-particle PL images (a) and PL spectra of individual Au TNP particles 1 (b), 2 (c), and 3 (d) at various reaction times: 0, 5, 20, 40, 60, and 120 min after addition of 50-μL HAuCl4 (1 μM). Scale bar: 500 nm.

Figure 4. TEM images of Pt-edged Au TNPs (a), Pt-edged Au O-TNPs (b), and Pt-edged Au NDs (c); and their visible-NIR extinction spectra (d). Photocatalytic hydrogen generation over Pt-edged Au TNPs, Pt-edged Au O-TNPs, and Pt-edged Au NDs in methanol−water (25 vol %) solution under visible-NIR light (>420 nm) irradiation (e). Photocatalytic hydrogen generation rate over Pt-covered, Pt-edged and pure Au TNPs, O-TNPs, and NDs (f). 970

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Figure 5. TEM images of Au TNPs-2 (a), Pt-edged Au TNPs-2 (b), Au NDs-2 (c), and Pt-edged Au NDs-2 (d). Visible-NIR extinction spectra of Au (TNPs-2, O-TNPs, and NDs-2) (e) and Pt-edged Au (TNPs-2, O-TNPs and NDs-2) (f). Hydrogen generation rate of Pt-edged Au TNPs-2, O-TNPs, and NDs-2 in methanol−water (25 vol %) solution under visible-NIR light irradiation (>420 nm) (g).

the hydrogen generation rates were 1.5, 2.1, and 1.9 mmol g−1 h−1 for Pt-edged Au TNPs, Pt-edged Au O-TNPs and Pt-edged Au NDs, respectively. Light energy conversion efficiencies of three different structures were given in Supporting Information. As shown in Figure 4d, visible-NIR extinction spectra of Ptedged Au TNPs, O-TNPs, and NDs are much different, leading to different light absorption cross-section. To make clear which (light absorption or conversion) plays a main role on the enhanced photocatalytic hydrogen generation, Au TNPs-2 and NDs-2 with the similar extinction spectra of Au O-TNPs were synthesized and their TEM images were shown in Figure 5a and c. Average size of Au TNPs-2 and Au NDs-2 were about 94 and 114 nm, respectively. By tuning their concentration, Au TNPs-2 and NDs-2 solution exhibited the similar light absorption with that of Au O-TNPs solution (Figure 5e). After Pt was edged, Pt-edged NDs-2 (Figure 5d) and Pt-edged O-TNPs (Figure 4b) exhibited similar visible-NIR extinction spectra (Figure 5f). However, Pt-edged TNPs-2 (Figure 5b) exhibited a more red shift due to higher sensitivity of TNPs for Pt loading. Considering the absorption of Pt-edged TNPs-2 in 600−800 nm, total absorption cross-section of Pt-edged TNPs2 is larger than those of Pt-edged O-TNPs and Pt-edged NDs2. Figure 5g shows the photocatalytic hydrogen generation rate of Pt-edged TNPs-2 and Pt-edged NDs-2. The hydrogen generation rates of Pt-edged TNPs-2 and Pt-edged NDs-2 are 0.30 and 0.35 μmol h−1, which are 59% and 69% for Pt-edged Au O-TNPs (0.51 μmol h−1), respectively. Compared with Ptedged TNPs-2 and Pt-edged NDs-2, higher activity of Pt-edged Au O-TNPs is not due to the light absorption but due to more efficient light conversion. Consequently, among the three anisotropic Pt-edged Au nanostructures, Pt-edged O-TNPs showed the optimal hydrogen generation. Single-particle study was carried out to clarify the photocatalytic mechanism in Pt−Au nanostructures. Figure 6a shows the single-particle PL spectra of individual Au TNP, O-TNP, and ND (corresponding to TEM images of Figure 1a−c), which are similar to those observed during Au TNP etching process (Figure 3b−d). Detailed single-particle PL image and PL spectra are given in Figures S7−S18. For Au TNP, the PL spectra are divided to four separated bands assigned to PL from DSPR, two MSPR (MSPR1 and MSPR2) and OP-SPR from long to short-wavelength region (Figure S8). For Au O-TNP, three bands assigned to DSPR, MSPR, and OP-SPR were

images of Figure 1a−c) as shown in Figure 6a. Consequently, plasmonic PL spectra of individual Au particles were found to be clearly related to their geometric and anisotropic structural change from TNP to ND, providing a strong evidence for the geometric and anisotropic dependence of SPR in Au nanostructures, and in situ etching of single Au TNP was successfully observed. The single-particle PL spectra demonstrated that the geometric and anisotropic structures of Au nanoplates have a strong relation with generation, transfer and recombination of plasmonic hot electrons, leading to different efficiencies in light harvesting. In our previous work, Pt-edged Au TNPs showed efficient hydrogen generation under visible-NIR irradiation,15 however, the photocatalytic activities of other Pt-edged Au nanoplates with different geometric and anisotropic structures are still not clear. So, herein, anisotropic Pt-edged Au TNPs, Ptedged Au O-TNPs, and Pt-edged Au NDs were synthesized (Figure 4a−c) as plasmonic photocatalysts for hydrogen generation under visible-NIR light (>420 nm) irradiation. After Pt was edged, SPR bands of Au TNPs, O-TNPs, and NDs became broad with a red shift (Figure 4d), consistent with FDTD simulations (Figure S5). The hydrogen generation for Pt-edged Au TNPs, O-TNPs, and NDs were shown in Figure 4e, and Pt-edged O-TNPs exhibits higher activity than other ones. Figure 4f shows Hydrogen generation rates of Pt-covered (Figure S6), Pt-edged and pure Au TNPs, O-TNPs, and NDs. For Pt-covered Au TNPs (0.046 μmol h−1), O-TNPs (0.056 μmol h−1) and NDs (0.051 μmol h−1), the photocatalytic hydrogen generation rates have slight differences indicating that Au nanostructures have little effect on the photocatalytic activities. The hydrogen generation rate of Pt-edged O-TNPs was 0.51 μmol h−1, 38% and 19% higher than those of Pt-edged TNPs (0.37 μmol h−1) and Pt-edged NDs (0.43 μmol h−1), respectively. The error bars were calculated by results from three sets of samples synthesized in the same process, demonstrating the reproducible experiments. It is clear that Au nanostructures have great influence on the photocatalytic activity of Pt-edged Au nanoplates. Atomic ratios of Pt were measured by the EDX of TEM to be 33%, 33%, and 34% for Ptedged Au TNPs, Pt-edged Au O-TNPs, and Pt-edged Au NDs, respectively. Actual amounts of Pt-edged Au TNPs, Pt-edged Au O-TNPs, and Pt-edged Au NDs were estimated to be 0.24, 0.24, and 0.23 mg by the amount of Pt (0.4 μmol). Normalized 971

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Scheme 1. Schematic Diagram for Radiative Decay of SPR of Au TNP and Reaction Mechanism for Photocatalytic Hydrogen Generation of Pt−Aua

Figure 6. Single-particle PL spectra of individual Au (TNP, O-TNP and ND corresponding to TEM images of Figure 1a−c (a), Ptedged Au TNP (b), Pt-edged Au O-TNP (c), and Pt-edged Au ND (d), respectively.

a

ET: Energy transfer. CS: Charge separation. CR: Charge recombination.

of maximum electric field of DSPR moved from the tips of TNP to the round side of ND. From TNP to ND, the tip area increased gradually and more Pt located on tip. However, the intensity of electric field was diminished. Therefore, Pt-edged Au O-TNP has larger tip area than Pt-edged Au TNP and higher electrical field than Pt-edged Au ND, which facilitates the electron transfer and charge separation leading to efficient photocatalytic hydrogen generation.

observed in PL spectra (Figure S12), while only two bands due to DSPR and OP-SPR were observed in PL spectra of Au ND (Figure S16). The DSPR PL intensity of Au ND was 2 and 50 folds higher than those of Au O-TNP and TNP, respectively, which is attributed to the reduced ΔE between the initial excited state and DSPR mode of Au ND leading to the increased population of excited electron transfer to DSPR mode.24 After Pt was edged, PL intensities of individual Au TNP, O-TNP, and ND were diminished due to the quenching (Figures 6b−d). From the averaged PL intensity of ten individual Au (Pt−Au) particles (Figure S19), the PL quenching efficiencies of DSPR, MSPR1, MSPR2, and OPSPR modes of Pt-edged Au TNPs were 100%, 85%, 69%, and 50%, while those for DSPR, MSPR, and OP-SPR modes of Ptedged Au O-TNPs were 94%, 76%, and 32%, and those for DSPR and OP-SPR modes of Pt-edged Au NDs were 97% and 11%, respectively. For Pt-edged Au TNPs, O-TNPs, and NDs, the PL quenching efficiencies for in-plane SPR were much higher than that for OP-SPR. Therefore, in-plane SPR is the dominant channel for PL quenching in Pt-edged Au nanoplates. Scheme 1 shows the mechanism of radiative decay of SPR in Au TNP. In our single-particle experiment, under irradiation of 405 nm laser, the interband transition (black line) of Au TNP is excited to generate electron−hole pairs. Excited electron−hole pairs on original excited state have a nonradiative relax and transfer to OP-SPR mode by energy transfer (ET) to occur a radiative decay, emitting a short-wavelength peak in the PL spectrum (light blue line). Meanwhile, an ET occurs from OPSPR to MSPR and DSPR mode. Furthermore, direct conversion of excited electron−hole pairs from original excited state into MSPR and DSPR mode by the nonradiative decay and ET, produces long-wavelength emissions in PL spectrum (light green line and light red line).22 For Pt-edged Au, the hot electron transfer from Au to Pt competes with the radiative decay resulting in the PL quenching.15,20,21 It has been demonstrated that in-plane SPR is the dominant channel for the PL quenching of Pt-edged Au and the hot electron can transfer from Au to Pt leading to hydrogen generation, and the holes left on Au can oxidize methanol.15,20,21 From FDTD simulations (Figure S20), plasmon enhanced electric fields of Pt-edged Au TNP, Pt-edged O-TNP, and Pt-edged ND were similar to those of Au TNP, O-TNP, and ND with the locations

CONCLUSIONS In summary, we successfully observed in situ etching of Au TNPs by the plasmonic PL spectra using single-particle microscopy, and a clear result was provided to understand the dependence of SPR on the geometric and anisotropic Au nanostructures at single-particle level. Various Au structures of TNPs, O-TNPs, and NDs were obtained by etching Au TNPs to synthesize the anisotropic Pt-edged Au TNPs, Pt-edged OTNPs, and Pt-edged NDs as photocatalysts for hydrogen generation. Single-particle PL spectra demonstrate that the generation, transfer, separation and recombination of plasmonic hot electron depend on geometric and anisotropic structure of Au and in-plane SPR is the dominant channel for PL quenching in Pt-edged Au nanoplates. FDTD simulations show that Ptedged Au O-TNPs with larger tip area and higher electrical field are the optimal structure for electron transfer and charge separation, leading to higher activity than Pt-edged TNPs and NDs in photocatalytic hydrogen generation. This work clearly shows the geometric and anisotropic dependence of SPR enhanced photocatalytic hydrogen generation using various Pt−Au 2D nanostructures. EXPERIMENTAL SECTION Materials. Hexadecyltrimethylammonium chloride (CTAC, ≥ 95%, Wako), hexadecyltrimethylammonium bromide (CTAB, ≥ 95%, Wako), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4· H2O, ≥ 99.999%, Sigma-Aldrich), hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6·nH2O, ≥99.9%, Sigma-Aldrich), L-ascorbic acid (≥99.5%, Sigma-Aldrich), potassium iodide (≥99.5%, Sigma-Aldrich), sodium borohydride (NaBH4, ≥99%, Sigma-Aldrich), silver nitrate (≥99%, Sigma-Aldrich), and methanol (>99.8%, nacalai tesque) were used after purchase without any further purification. Milli-Q water at 25 °C was used in all experiments. Synthesis of Au TNPs, O-TNPs, and NDs. Au TNPs were synthesized by the modified seed-growth method.13 After purification, 972

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ACS Nano pure Au TNPs dispersed in 0.02 M CTAB solution was used as the precursor to synthesize the O-TNPs and NDs. In typical procedure, 0.2 mL of a solution containing HAuCl4 (0.5 mM) and CTAB (50 mM) was added into 10 mL of a Au TNPs solution (26.4 mg/L). Au TNPs were etched by HAuCl4 in solution to be O-TNPs after reaction for 1 h. Au NDs were synthesized by following above process with the addition of 0.6 mL of a HAuCl4 solution for 1 h. After reaction, OTNPs and NDs were separated from solution by centrifuged (7000 rpm) for 20 min and dispersed into 20 mM CTAB solution for next step. Au TNPs-2 was synthesized by following the synthetic process of Au TNPs using 400 μL of seed solution. Au NDs-2 was synthesized by the etching of Au TNPs which was synthesized by using 100 μL of seed solution. Synthesis of Pt-Edged Au TNPs, O-TNPs, and NDs. The Ptedged Au TNPs, O-TNPs, and NDs were synthesized by the reported method.25 In typical procedure, 60 μL of KI (10 mM), 20 μL of AgNO3 (20 mM), 30 μL of NaOH (0.1 M), and 0.15 mL of L-ascorbic acid (0.1 M) were added into 10 mL of Au TNPs solution, which was transferred into water bath for 1 h at 70 °C. Then, 40 μL of H2PtCl6 (10 mM) was added into above solution for 2 h at 70 °C to obtain Ptedged Au TNPs. Pt-edged O-TNPs and NDs were synthesized by following above process using 10 mL of Au O-TNPs and NDs as precursor. Pt-covered Au O-TNPs were synthesized by adding 0.15 mL of L-ascorbic acid (0.1 M) and 40 μL of H2PtCl6 (10 mM) into 10 mL of Au O-TNPs solution for reaction of 2 h at 70 °C. FDTD Calculations. The computational simulations were performed by using the FDTD (finite-difference-time-domain) method with perfectly matched layers boundary condition. A software package, FDTD Solution (Lumerical Solution, Inc.) was used to carry FDTD calculations. The override mesh cell size used was 1 × 1 × 1 nm3. The optical constant of Au was adopted from tabulated values for bulk gold measured by Johnsom amd Christy.26 The size of TNPs was match with the average values. Specifically, Au triangular nanoprism was modeled as a triangular nanoplate with length of 140 nm and thickness of 6 nm. For model of Pt-edged Au-TNPs, Pt cuboid with size of 160 × 10 × 6 nm located along edges and Pt spheres with radius of 3 nm located at each corner of Au TNPs model with thickness of 4 nm. One obtuse triangular nanoprisms with the size of 100 nm and the thickness of 6 nm were constructed as model of Au OTNPs. For model of Pt-edged Au O-TNPs, a thin layer of Pt with the size of 10 × 6 covered the sides of Au O-TNP model with 1 nm overlap. For model of NDs, a disk with the diameter of 78 nm and the thickness of 6 nm were constructed. A ring with outer radius of 50 nm, inner radius of 38 nm and the thickness of 6 nm were located on model of NDs for model of Pt-edged NDs. For Pt−Au and Au structures dispersed in aqueous solution, the refractive index of the medium was set to be 1.33. Photocatalytic Hydrogen Production Activity Test. The 10 mL aqueous solution of Pt-edged Au TNPs solution was centrifuged at 5000 rpm to remove excess surfactant and washed with Milli-Q water for three times. The sample was further washed with 2 mL of 60% HClO4 solution under ultrasonic condition for 15 min, and then, the samples were thoroughly washed using Milli-Q water and used as photocatalysts for hydrogen generation. In typical process of photocatalytic reaction, above obtained samples were dispersed into 3 mL of Milli-Q water, and 1 mL of methanol, mixed, sealed with a rubber stopper in a tube, and degassed for 20 min using argon. Then the tube was irradiated under visibleNIR light (Asahi Spectra, LAX-C100) with magnetic stirring at room temperature. A 420 nm cutoff filter was used to remove UV light and light density is 900 mW cm−2. Hydrogen evolution was measured by using Shimadzu GC-8A gas chromatograph equipped with an MS-5A column and a thermal conductivity detector. Photocatalytic hydrogen generation of other samples was carried by following above process. Single-Particle PL Measurements by Confocal Microscopy. Sample preparation for single-particle photoluminescence (PL) experiments: The quartz cover glasses were purchased from DAICO MFG CO., Ltd. (Japan) and cleaned by sonication in a 20% detergent solution (As One, Cleanace) for 6 h, followed by repeated washings

with warm water for 5 times. Finally, the quartz cover glasses were washed again with Milli-Q ultrapure water (Millipore). Monitoring in situ etching process of Au TNPs: Au TNPs aqueous suspensions were centrifuged at 6000 rpm (Hitachi, himac CF16RX) to remove excess surfactant and then washed with Milli-Q ultrapure water for two times, finally redispersed in Milli-Q ultrapure water. The well-dispersed aqueous suspensions of Au TNPs were spin-coated on the cleaned quartz cover glass. The quartz cover glass was annealed at 100 °C for 1 h to immobilize the particles on the surface. A chamber was attached on the center of quartz cover glass, HAuCl4 (1 μM) was injected into the chamber to etch the Au TNPs. The etching process of Au TNPs was monitored by the single-particle PL image and PL spectra using single-particle microscopy as detailed description in following section. Single-Particle PL Measurement of Au TNPs, Pt-Edged Au TNPs, Au O-TNPs, Pt-Edged Au O-TNPs, Au NDs, and Pt-Edged Au NDs. Au TNPs, Pt-edged Au TNPs, Au O-TNPs, Pt-edged Au O-TNPs, Au NDs, and Pt-edged Au NDs aqueous suspensions were centrifuged at 6000 rpm (Hitachi, himac CF16RX) to remove excess surfactant and then washed with Milli-Q ultrapure water for two times, finally redispersed in Milli-Q ultrapure water. The well-dispersed aqueous suspensions of O-TNPs were spin-coated on the cleaned quartz cover glass. The quartz cover glass was annealed at 100 °C for 1 h to immobilize the particles on the surface. Single-particle PL images and spectra of samples were recorded by using an objective scanning confocal microscope system (PicoQuant, MicroTime 200) coupled with an Olympus IX71 inverted fluorescence microscope. The samples were excited through an oil-immersion objective lens (Olympus, UplanSApochromat, 100×, 1.4NA) a circular-polarized 405 nm continuous wave laser controlled by a PDL-800B driver (PicoQuant). Typical excitation powers for the PL measurements were 250 μW at the sample. The emission from the sample was collected by the same objective and detected by a single photon avalanche photodiode (Micro Photon Devices, PDM 50CT) through a dichroic beam splitter (Chroma, 405rdc) and long pass filter (Chroma, HQ430CP). For the spectroscopy, only the emission that passed through a slit entered the imaging spectrograph (Acton Research, SP-2356) that was equipped with an electron-multiplying charge-coupled device (EMCCD) camera (Princeton Instruments, ProEM). The spectra were typically integrated for 30 s. The spectrum detected by the EMCCD camera was stored and analyzed by using a personal computer. All the experimental data were obtained at room temperature. Characterization of Materials. Transmission electron microscopy (TEM) measurements were carried out on JEOL-2100 operated at 100 kV and JEM-3000F operated at 300 kV (JEOL). Atomic ratio of Pt and Au in samples was measured by the EDX of JEM-3000F. Extinction spectra were measured using quartz cuvettes of 0.2 cm path length on Shimadzu UV-3600 UV−vis−NIR spectrophotometer.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07581. SEM image, cover glass with chamber, experimental and FDTD calculated extinction spectra, TEM images, singleparticle PL images and PL spectra, PL quenching efficiency, and FDTD calculated electric field (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Mamoru Fujitsuka: 0000-0002-2336-4355 Tetsuro Majima: 0000-0003-1805-1677 973

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ACS Nano Notes

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been partly supported by a Grant-in-Aid for Scientific Research (Projects 25220806, 25288035, and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Z. L. thanks the JSPS for a Postdoctoral Fellowship for Foreign Researchers (no. P15073). REFERENCES (1) Huang, X. Q.; Tang, S. H.; Mu, X. L.; Dai, Y.; Chen, G. X.; Zhou, Z. Y.; Ruan, F. X.; Yang, Z. L.; Zheng, N. F. Freestanding Palladium Nanosheets with Plasmonic and Catalytic Properties. Nat. Nanotechnol. 2011, 6, 28−32. (2) Lal, S.; Clare, S. E.; Halas, N. J. Nanoshell-Enabled Photothermal Cancer Therapy: Impending Clinical Impact. Acc. Chem. Res. 2008, 41, 1842−1851. (3) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (4) Larsson, E. M.; Langhammer, C.; Zoric, I.; Kasemo, B. Nanoplasmonic Probes of Catalytic Reactions. Science 2009, 326, 1091−1094. (5) Bian, Z. F.; Tachikawa, T.; Zhang, P.; Fujitsuka, M.; Majima, T. Au/TiO2 Superstructure-Based Plasmonic Photocatalysts Exhibiting Efficient Charge Separation and Unprecedented Activity. J. Am. Chem. Soc. 2014, 136, 458−465. (6) Liu, G. G.; Li, P.; Zhao, G. X.; Wang, X.; Kong, J. T.; Liu, H. M.; Zhang, H. B.; Chang, K.; Meng, X. G.; Kako, T.; Ye, J. H. Promoting Active Species Generation by Plasmon-Induced Hot-Electron Excitation for Efficient Electrocatalytic Oxygen Evolution. J. Am. Chem. Soc. 2016, 138, 9128−9136. (7) Cobley, C. M.; Chen, J.; Cho, E. C.; Wang, L. V.; Xia, Y. Gold Nanostructures: A Class of Multifunctional Materials for Biomedical Applications. Chem. Soc. Rev. 2011, 40, 44−56. (8) Bastus, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-stabilized Gold Nanoparticles of Up to 200 nm: Size Focusing Versus Ostwald Ripening. Langmuir 2011, 27, 11098−11105. (9) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 1957−1962. (10) Huo, Z. Y.; Tsung, C. K.; Huang, W. Y.; Zhang, X. F.; Yang, P. D. Sub-Two Nanometer Single Crystal Au Nanowires. Nano Lett. 2008, 8, 2041−2044. (11) Hong, S.; Choi, Y.; Park, S. Shape Control of Ag Shell Growth on Au Nanodisks. Chem. Mater. 2011, 23, 5375−5378. (12) O’Brien, M. N.; Jones, M. R.; Kohlstedt, K. L.; Schatz, G.; Mirkin, C. A. Uniform Circular Disks With Synthetically Tailorable Diameters: Two-Dimensional Nanoparticles for Plasmonics. Nano Lett. 2015, 15, 1012−1017. (13) Scarabelli, L.; Coronado-Puchau, M.; Giner-Casares, J. J.; Langer, J.; Liz-Marzan, L. M. Monodisperse Gold Nanotriangles: Size Control, Large-Scale Self-Assembly, and Performance in SurfaceEnhanced Raman Scattering. ACS Nano 2014, 8, 5833−5842. (14) Chen, L.; Ji, F.; Xu, Y.; He, L.; Mi, Y. F.; Bao, F.; Sun, B. Q.; Zhang, X. H.; Zhang, Q. High-Yield Seedless Synthesis of Triangular Gold Nanoplates Through Oxidative Etching. Nano Lett. 2014, 14, 7201−7206. (15) Lou, Z. Z.; Fujitsuka, M.; Majima, T. Pt-Au Triangular Nanoprisms with Strong Dipole Plasmon Resonance for Hydrogen Generation Studied by Single-Particle Spectroscopy. ACS Nano 2016, 10, 6299−6305. (16) Munteanu, S.; Garraud, N.; Roger, J. P.; Amiot, F.; Shi, J.; Chen, Y.; Combellas, C.; Kanoufi, F. In Situ, Real Time Monitoring of Surface Transformation: Ellipsometric Microscopy Imaging of Electro974

DOI: 10.1021/acsnano.6b07581 ACS Nano 2017, 11, 968−974