Pt–Au Triangular Nanoprisms with Strong Dipole Plasmon Resonance

May 22, 2016 - ... to H2 generation. SPR bands of Au TNPs depending on the size play an important role on the photocatalytic activity of Pt-edged Au T...
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Pt−Au Triangular Nanoprisms with Strong Dipole Plasmon Resonance for Hydrogen Generation Studied by Single-Particle Spectroscopy Zaizhu Lou, 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: Three anisotropic Pt-covered, Pt-edged, and Pt-tipped Au triangular nanoprisms (TNPs) were prepared by controlling the overgrowth of Pt as photocatalysts for H2 generation. With strong electric field and more interface for the hot electrons transfer, the H2 generation rate of Ptedged Au TNPs was 3 and 5 times higher than those of Pttipped and Pt-covered Au TNPs. Single-particle photoluminescence (PL) spectra and finite-difference-timedomain (FDTD) simulations demonstrated that dipole surface plasmon resonance (DSPR) of Au TNPs enhanced the hot electrons transfer from Au to Pt leading to H2 generation. SPR bands of Au TNPs depending on the size play an important role on the photocatalytic activity of Ptedged Au TNPs. KEYWORDS: Pt−Au triangular nanoprism, dipole plasmon resonance, plasmonic photocatalysis, hydrogen generation, single-particle spectroscopy

A

Pt(Pd)-tipped Au nanorods (NRs) demonstrate that the longitudinal surface plasmon resonance (LSPR) mode of Au NRs is the dominant channel for hot-electron transfer from Au to Pt, leading to efficient H2 generation under visible and nearinfrared (NIR) light irradiation. Such anisotropic structures play a great role on the hot electron transfer.16,18 Therefore, anisotropic growth of bimetallic nanostructures is a strategy to improve the efficiency of plasmonic photocatalysis. Surface plasmon resonance (SPR) of noble metal NPs is greatly determined by their compositions, shapes, and surrounding environment, especially for shapes.19 Although various Au nanostructures including nanorods,20 nanocages,21 nanodisks,22 nanoprisms,23 etc. have been reported to have strong SPR bands in visible and NIR region. To date, only NRs were used as metallic photocatalyts for hydrogen generation. Different from one-dimensional NRs, two-dimensional Au triangular nanoprisms (TNPs) have various modes of SPR including in-plane dipole surface plasmon resonance (DSPR), multipole surface plasmon resonance (MSPR), and out-of-plane SPR in visible-

s a green technology for solar energy conversion, photocatalysis has attracted significant attention because of its applications in chemical energy producing and environmental cleaning.1−5 However, low photocatalytic efficiency and limited visible light response hinder the applications of photocatalysis. For most semiconductor photocatalyts, fast charge recombination competes with charge separation resulting in low photocatalytic efficiency. Among various strategies6−8 for promoting charge separation, plasmonic photocatalysis is the most attractive for the efficient charge separation and strong visible light absorption.9−11 Most studied plasmonic photocatalyts are composed of noble metal nanoparticles (NPs) and semiconductors, in which electrons of metals are excited becoming hot electrons and transfer to semiconductors as the initial process.9,12−15 However, ultrafast energy relaxation of hot electrons leads to recombination of charge-carriers against the transfer of hot electrons, resulting in low quantum yield of plasmonic photocatalysis.16 Compared to monometal NPs, bimetal/multimetal NPs show unique performance in catalysis.17 Moreover, bimetallic structures facilitate the transfer of electrons between two metallic parts, leading to high efficient plasmonic photocatalysts. Single-particle photoluminescence (PL) spectra of © 2016 American Chemical Society

Received: April 13, 2016 Accepted: May 21, 2016 Published: May 22, 2016 6299

DOI: 10.1021/acsnano.6b02494 ACS Nano 2016, 10, 6299−6305

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ACS Nano NIR region24,25 and is a promising photocatalyst. However, detailed single-particle study of Au TNPs has few reports and it is not clear which plasmon resonance is the dominant channel for the hot electron transfer. Moreover, compared to NRs, TNPs with edges, tips and surface have more anisotropic bimetallic nanostructures as efficient photocatalyts for H2 generation. In this work, Au TNPs with alterable size and tunable SPR band were synthesized by modified seed-growth and purifications. Since the overgrowth of Pt NPs was successfully controlled, three anisotropic Pt-tipped, Pt-edged, and Ptcovered Au TNPs were prepared as photocatalyts for H2 generation under visible-NIR light irradiation. Pt-edged Au TNPs with strong electric field and more interface for the hot electron transfer exhibited higher activity than both Pt-tipped and Pt-covered Au TNPs for H2 generation. By single-particle PL spectra and simulations of finite-difference-time-domain (FDTD), DSPR of Au TNPs was demonstrated to be the dominant channel for the hot electron transfer from Au to Pt. The size of Au TNPs determines DSPR and MSPR bands as well as H2 generation rate of Pt-edged Au TNPs in visible-NIR light region.

tipped Au TNPs is that iodide ions absorbed on Au(111) facets bind Ag+ to form AgI on the surface of Au TNPs,26 limiting the seeding of Pt NPs. However, the corners of Au TNPs with high surface energy provide active sites for seeding and subsequently growth of Pt NPs. Finally, the dendritic structure of Pt NPs is formed on the corners under the effect of KI.27 For the overgrowth of Pt on edges of Au TNPs, CTAC was replaced by cetyltrimethylammonium bromide (CTAB). With addition of KI (60 μL, 10 mM), Pt NPs grew only on edges of Au TNPs to form Pt-edged Au TNPs (Figures 1c and S6). The growth mechanism of Pt-edged Au TNPs was explained based on the transfer of electrons.26 Figure 2a shows the visible-NIR extinction spectra of Au TNPs and Pt-loaded Au TNPs. Pure Au TNPs have two SPR

RESULTS AND DISCUSSION Au TNPs with average length of 141 nm and uniform morphology were prepared as shown in Figures 1a and S1

Figure 2. (a) Visible-NIR extinction spectra and (b) simulated extinction spectra of Au TNPs (black), Pt-covered (pink), Pt-edged (red), and Pt-tipped (blue) Au TNPs. In panel a, the noise around 1500 nm is from surfactant of CTAC in the background. (c) H2 generation over Pt-covered (▲), Pt-tipped (●) and Pt-edged (■) Au TNPs under visible-NIR light irradiation (>420 nm). (d) H2 generation rate normalized by the amount of Pt loaded for Pttipped, Pt-edged, and Pt-covered Au TNPs.

bands: one stronger band at 1170 nm is originated from inplane DSPR, while the other at 712 nm is in-plane MSPR.25 Because the thickness of Au TNPs is about 6.2 nm (Figure S2), out-of-plane DSPR is too weak to be observed in the spectrum. The simulations were performed to estimate the SPR bands of Au TNPs by using FDTD. Constructed models for simulations were shown in Figure S7. Figure 2b shows three distinct bands in the simulated extinction spectra of Au TNPs where the band at 1130 nm is assigned to in-plane DSPR. Another two bands around 660 and 730 nm are originated from in-plane octapole surface plasmon resonance (OSPR) and quadrupole surface plasmon resonance (QSPR).25 Due to nonuniform dispersion of actual Au TNPs in solution, OSPR and QSPR bands overlap to be one broad band assigned to MSPR of 712 nm in Figure 2a. When Pt was tipped on Au TNPs, both DSPR and MSPR bands showed a slight red shift because the variation in the collective oscillation of surface free electrons. Simulations of Pttipped Au TNPs showed both DSPR and QSPR bands have a little red shift which is consistent with the experimental results. When Pt was edged on Au TNPs, DSPR band showed a large red shift and intensity decrease, which was attributed to more

Figure 1. TEM images of (a) Au TNPs, (b) Pt-covered, (c) Ptedged, and (d) Pt-tipped Au TNPs. Scale bar, 200 nm.

and S2. After purification, Au TNPs covered by cetyltrimethylammonium chloride (CTAC) were used as precursors for Pt loading. With the absence of KI, Pt NPs grew randomly on the surface of TNPs to form Pt-covered Au TNPs which were observed in TEM (Figures 1b and S3). With addition of KI (60 μL, 10 mM), Pt NPs grew only on the tips of Au TNPs as shown clearly in Figures 1d and S4. For detailed synthetic process, see Experimental Section. The growth mechanism of Pt-tipped Au TNPs was investigated by tuning concentration of KI as shown in Figure S5. As iodide ion was increased, the growth sites of Pt moved from the surface to edges and then to corners of Au TNPs. However, it is difficult to obtain pure Ptedged Au TNPs in this case. A possible mechanism for Pt6300

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ACS Nano amount of Pt loaded and larger size of TNPs. MSPR band also showed a red shift and intensity decrease, suggesting the overgrowth of Pt on edges of Au TNPs. Simulations support our results of Pt-edged Au TNPs. When Pt grew randomly and covered Au TNPs, DSPR band showed a blue shift and intensity decrease, which is attributed to decreasing free electrons and increasing TNPs thickness. These results were also supported by the simulations (Figure 2b). Red shift of Ptedged Au TNPs is larger than that of Pt-tipped Au TNPs. This implies the larger amount Pt loaded and the stronger interaction between Pt and Au. Strong interaction facilitates the hot electrons transfer between Au and Pt. However, for Ptcovered Au TNPs, randomly growth Pt NPs provide more recombination sites for charge carriers than in Pt-edged Au TNPs. Consequently, among the three Pt-loaded Au TNPs, Ptedged Au TNPs is the optimal structure for the hot electrons transfer from Au to Pt. Aqueous Au TNPs solution (10 mL, 0.163 mg Au0) with addtion of H2PtCl6 (40 μL, 10 mM) was used to prepare Ptcovered, Pt-tipped, and Pt-edged Au TNPs as photocatalysts for H2 generation in water/methanol (2:1 volume ratio) under visible-NIR light irradiation using a 300-W xenon lamp equipped with 420 nm cutoff filter. As shown in Figure 2c, the H2 generation rate increased in the order of Pt-covered, Pttipped, and Pt-edged Au TNPs. The H2 generation rate of Ptedged Au TNPs was 0.167 μmol h−1 which is 5 and 3 times higher than that of Pt-covered (0.031 μmol h−1) and tipped Au TNPs (0.048 μmol h−1). Considering the great different area of the surface, edge, and tip of Au TNPs, the actual amount of Pt loaded was measured by the EDX of TEM to be 45.3, 32.9, and 7.3% (atomic ratio) for Pt-covered, Pt-edged, and Pt-tipped Au TNPs, respectively. To compare role of Pt located on different sites, the H2 generation rate of three Pt-loaded Au TNPs was normalized by the actual Pt amount as shown in Figure 2d. Pttipped Au TNPs showed more than 7 times higher activity than that of Pt-covered Au TNPs. Pt-edged Au TNPs with part Pt loaded on corners exhibited a little difference with Pt-tipped Au TNPs in H2 generation. Therefore, Pt loading on the corners of Au TNPs plays the main role on H2 generation. However, considering actual amount of generated H2, Pt-edged Au TNPs are the optimal photocatalyts among three anisotropic structures. To investigate the influence of Pt amount on hydrogen generation, Pt-edged Au TNPs with various amounts of Pt loaded were synthesized and their TEM images of Pt-edged Au TNPs were shown in Figure 3a−d. As the amount of Pt loaded increases, edges of Au TNPs are covered by the Pt to form a thicker layer. Plasmon bands of Pt-edged Au TNPs with various amounts of Pt loaded were shown in visible-NIR extinction spectra of Figure 3e. As the amount of Pt loaded increases, DSPR band has a red shift gradually with the diminished intensity, which is attributed to the variation of free electrons and increasing size of Au TNPs, while MSPR band becomes weak and disappears gradually. Another illustration for red shift of DSPR band is that Au TNPs are thin during the loading of Pt, because of the oxidization of H2PtCl6, which can be confirmed by the holes formed in Pt-edged Au-TNPs (54.7%) (Figure S8). Simulation of FDTD was used to estimate the SPR bands of Pt-edged Au TNPs with various thicknesses (Figure S9). As the thickness of Au TNPs is decreased, SPR bands have red shift and diminished intensity, which is similar to the results shown in Figure 3e. As the Pt amount is increased, photocatalytic performance of Pt-edged Au TNPs is improved

Figure 3. TEM images of Pt-edged Au TNPs with various amounts of Pt loaded: (a) atomic 25.9, (b) 32.9, (c) 40.2 and (d) 54.7%. (e) Visible-NIR extinction spectra and (f) hydrogen generation rate of Pt-edged Au TNPs with various amounts of Pt loaded.

due to more active sites for hydrogen generation (Figure 3f). However, as SPR bands of Pt-edged Au TNPs are weak, this leads to low yield of hot electrons and low efficiency of hydrogen generation. Pt-edged Au TNPs with atomic 40.2% Pt loaded exhibit the optimal activities in hydrogen generation. SPR bands of Au nanostructures are determined by their size, especially for TNPs. Au-TNPs, Au-TNPs-400, Au-TNPs-600, Au-TNPs-800, and Au-TNPs-1000 with various sizes were prepared by the addition of 200, 400, 600, 800, and 1000 μL seed solution in the growth process. From TEM images (Figure S10), the average sizes of Au-TNPs (Figure 1a), Au-TNPs-400, Au-TNPs-600, Au-TNPs-800, and Au-TNPs-1000 are 141, 86, 82, 51, and 46 nm, respectively (Figure S11). Pt-edged Au TNPs with various sizes were prepared by loading of a constant amount of Pt NPs (atomic 33%, Figure 4a−d) to be used as photocatalyts for H2 generation. From Figure S12, both DSPR and MSPR band intensities of Au TNPs become weaker with decreasing the size. The ratio of length/thickness of Au-TNPs, Au-TNPs-400, Au-TNPs-600, Au-TNPs-800, and Au-TNPs-

Figure 4. TEM images of Pt-edged Au TNPs with various sizes synthesized by addition of various seed volumes: (a) 400, (b) 600, (c) 800, and (d) 1000 μL. The samples a−d were labeled to be AuTNPs-400, Au-TNPs-600, Au-TNPs-800, and Au-TNPs-1000, respectively. (e) Visible-NIR extinction spectra of Pt-edged Au TNPs with various sizes and (f) their H2 generation rate over the size of Au TNPs. 6301

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single-particle PL image of individual Au-TNPs-600. The single particle dispersion of Au-TNPs-600 on quartz cover glass was confirmed by SEM in Figure S15. The excitation source is a circularly polarized 405 nm laser. The single-particle PL spectra of individual Au-TNPs-600 are shown in Figure 5b. Two peaks were observed in six points in the PL spectra. (PL spectra of point number 3 exhibits characterization of two particles in Figure S26.) The peak in low energy region corresponds to inplane DSPR, while the other does to in-plane MSPR of Au TNPs. Peak position of the PL spectra depends on the ratio of length/thickness of Au TNPs. PL intensities of DSPR and MSPR are summarized for 10 individual Au-TNPs-600 and 10 individual Pt-edged Au-TNPs-600 particles as shown in Figure 5c. The DSPR PL intensity of Au-TNPs-600 decreases significantly when Pt is edged, while the MSPR PL intensity is similar for both Au-TNPs-600 and Pt-edged Au-TNPs-600. This proves that the DSPR PL is quenched by the Pt edged on Au-TNPs-600. The quenching efficiency is calculated to be 96%. Single-particle spectra of Au-TNPs, Pt-edged Au-TNPs, Au-TNPs-800, and Pt-edged Au TNPs-800 with sizes of 141 and 51 nm were measured as shown in Figures S15−32. As the size of Au TNPs increases from 51 to 82 then to 141 nm, the DSPR PL intensity is increased at first and then decreased (Figure 5d). On the other hand, the MSPR PL intensity is increased gradually (Figure S33). With increasing of the size and ratio of length/thickness of Au TNPs, the MSPR PL intensity is increased. The excited electron−hole pairs make rapid interconversion with MSPR and compete with DSPR resulting in increasing of the MSPR PL but decreasing of the DSPR PL. When Pt is edged, the DSPR PL is quenched efficiently. The quenching efficiency for Pt-edged Au TNPs800, Pt-edged Au-TNPs-600, and Pt-edged Au-TNPs was 91, 96, and 86%, respectively (Figure 5d). For Pt-edged Au-TNPs, the hot electrons transfer from MSPR to DSPR results in the low quenching efficiency of the DSPR PL. Photocatalytic mechanism over Pt-edged Au TNPs is illustrated in schematic diagram of Scheme 1. Similar with Au

1000 is estimated to be 22.6, 12.9, 12.4, 8.1, and 7.3, respectively. The ratio of length/thickness decreases with decreasing of the size, leading to the weak in-plane polarization of Au TNPs, therefore, resulting in the blue shift and decreased intensity of DSPR. On the other hand, MSPR appears only in Au TNPs with the ratio larger than 8, and the intensity increases with increasing of the ratio (Figure S12). After Pt edged, SPR bands of Au TNPs have a red shift attributed to the variation of free electrons on surface (Figure 4e). The position and intensity of SPR bands of Au TNPs and Pt-edged Au TNPs were summarized in Figure S13. The order of DSPR intensity of Pt-edged Au TNPs was similar to that of Au TNPs with decreasing of the size. Compared to others, Pt-edged Au-TNPs have the largest red shift implying the strong interaction between Pt and Au. From Figure 4f, the H2 generation rate for Pt-edged Au TNPs (with same amount of Au0) decreased with decreasing the size, to be 0.167, 0.160, 0.146, 0.058, and 0.037 μmol h−1 for Pt-edged Au-TNPs, Pt-edged Au-TNPs-400, Ptedged Au-TNPs-600, Pt-edged Au-TNPs-800, and Pt-edged Au-TNPs-1000, respectively. The relation of the H2 generation rate to the size of Au TNPs is similar to that of their extinction peak intensity and the size (Figures 4e and S13b). Similar results are also found in hydrogen generation of Pt-tipped and Pt-covered Au TNPs with various sizes (Figure S14). Consequently, the photocatalytic activity of Pt-edged Au TNPs with various sizes is attributed to the intensity of their SPR bands. It is necessary to clarify the detailed mechanism of photocatalytic reaction by Pt-loaded Au. Although we have proved the hot electrons transfer in Pt loaded Au NRs by single-particle spectroscopy in the previous work,16 there is no report on the electron transfer in Pt loaded Au TNPs. Here, we used single-particle spectroscopy to study the hot electron transfer in individual Pt-edged Au TNPs. First, samples were spin coated on quartz cover glass. Figure 5a showed a typical

Scheme 1. Schematic Diagram for Radiative Decay of DSPR and MSPR (Left) And Reaction Mechanism for H2 Generation over Pt-Edged Au TNPs (Right)

NRs,16,18 PL of Au TNPs is attributed to the radiative decay from MSPR and DSPR, and the latter is the dominant channel for the decay. In single-particle PL experiment, 405 nm laser light excited d−sp interband transition of Au TNPs to generate electron−hole pairs (green line in Scheme 1).28,29 Due to rapid interconversion between electron−hole pairs and MSPR, radiatively decay (purple line) occurs to generate short wavelength peak in PL spectrum. Meanwhile, in the electron−hole pairs, the energy transfer occurs from MSPR to DSPR. In addition, excitons of d−sp interbands transfer directly to DSPR by the nonradiative energy loss, which

Figure 5. (a) PL images of single Au-TNPs-600 on quartz glass. (b) Normalized PL spectra of each of the corresponding single AuTNPs-600 as numbered in PL image. (c) PL intensity of DSPR and MSPR of 10 single Au-TNPs-600 and 10 Pt-edged Au-TNPs-600 particles. (d) Average PL intensity of DSPR of Au TNPs and Ptedged Au TNPs and their quenching efficiency depending on their sizes of 141, 82, and 51 nm. 6302

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and 3.2 mL of A solution was added into B solution rapidly, and the mixture was stirred for several seconds. Then, B solution was left alone for 1 h for complete growth of AuTNPs. Purification of Au TNPs was carried out by tuned concentration of CTAC: 0.62 mL of CTAC (25 wt %) solution was added to above B solution, and after that, the B solution was left alone overnight (12 h). Then, supernatant liquid of B solution was removed and precipitate was left on the bottom. Twenty milliliters of CTAC (0.02 M) was added into the bottle with precipitation, and green solution was obtained after ultrasonic, which was pure Au TNPs containing about 0.33 mg Au0. Au TNPs with various sizes were obtained by following above process with addition of various volumes of seed solution (400, 600, 800, 1000 μL) and purified by addition of CTAC 25 wt % (5, 10, 15, 20 mL). Synthesis of Pt Tipped and Pt Covered Au TNPs. Twenty microliters of AgNO3 (20 mM) was added into 10 mL of Au TNPs solution (0.163 mg) with a gentle shaking, and 0.2 mL of L-ascorbic acid (0.1 M) was added into above solution; then, the solution was transferred into water bath at 70 °C for 20 min. When the color of solution became blue with Ag loaded on Au TNPs, 60 μL of KI (10 mM) and 40 μL of H2PtCl6 (10 mM) were added, and then the solution was left alone overnight, and Pt tipped Au TNPs were obtained. For synthesis of Pt covered Au TNPs, the above process was followed without addition of KI solution, and then Pt covered Au TNPs were obtained. Synthesis of Pt Edged Au TNPs. Ten milliliters of Au TNPs CTAC solution was centrifuged at 5000 rpm for 20 min, and then the precipitation was dispersed into 10 mL of CTAB (0.02 M) solution. Sixty microliters of KI (10 mM), 20 μL of AgNO3 (20 mM), 30 μL of NaOH (0.1 M), and 0.2 mL of L-ascorbic acid (0.1 M) were added into Au TNPs solution. Above solution was transferred into water bath at 70 °C for 1 h. Then, the solution was kept in water bath at 70 °C for 2 h after 40 μL of H2PtCl6 (10 mM) was added, and Pt loaded on edge of Au TNPs were obtained. For synthesis of Pt-edged Au TNPs with various sizes, 10 mL of Au TNPs with the same amount of Au0 which can be confirmed by the UV−visible absorption of Au0 in 400 nm were used as precursor following the above process. FDTD Calculations. The computational simulations were performed by using the finite-difference-time-domain (FDTD) 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 Johnson and Christy.30 The size of TNPs was matched 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 tipped Au-TNPs, Pt spheres with radius of 3 nm were located at each corner of Au-TNPs. For model of Pt-edged Au-TNPs, Pt cuboid with size of 160 × 10 × 6 nm3 was located along edges based on model of Pt-tipped Au-TNPs, and thickness of Au-TNPs was 4 nm. For model of Pt-covered Au-TNPs, a thin layer of Pt with thickness of 1 nm covered the surface Au-TNP. For TNPs dispersed in aqueous solutions, the refractive index of the medium was set to be 1.33. Photocatalytic Hydrogen Production Activity Test. The 10 mL of aqueous solution of Pt loaded Au TNPs solution was centrifuged at 5000 rpm to remove excess surfactant and washed with Milli-Q water three times. Then, the sample was further washed with 2 mL of 60% HClO4 solution under ultrasonic condition for 15 min, and the samples were thoroughly washed using Milli-Q water and used as photocatalysts for hydrogen generation. In typical process of photocatalytic reaction, the above obtained samples were dispersed into 2 mL of Milli-Q water, and 1 mL methanol was mixed, sealed with a rubber stopper in a tube, and degassed for 15 min using argon. Then, the tube was irradiated under visible-NIR light (Asahi Spectra, LAX-C100) with magnetic stirring at room temperature. A 420 nm cutoff filter was used to remove UV light. Hydrogen evolution was measured by using Shimadzu GC-8A gas chromatograph equipped with an MS-5A column and a thermal conductivity detector.

induces a radiative decay with emitting a photon leading to a PL peak in low energy level (yellow line in Scheme 1). When Pt is edged on Au TNPs, the hot electron transfer from Au to Pt competes with the DSPR radiative decay. Our single-particle experiment has demonstrated that the DSPR PL spectra of AuTNPs are efficiently quenched by Pt edged, proving the hot electrons transfer from Au to Pt. Under visible-NIR light irradiation, the excited hot electrons including d−sp interband excitation and MSPR/DSPR excitation transfer from Au to Pt and react with H+ to generate H2. Since electron transfer generates a charge separation (CS) state,16 the holes stay on Au to oxidize methanol. However, quantum yield of Pt-edged Au TNPs is still low, suggesting the charge recombination (CR) as the dominant relaxation (orange line in Scheme 1). Pt-covered Au-TNPs, with Pt randomly loaded on the surface of Au TNPs, provide more sites for charge recombination, resulting in the low efficiency of H2 generation. Furthermore, simulated results of electric field (Figure S34) suggest that the Pt-edged AuTNPs have strong electric field on corners and edges covered by Pt which can promote charge separation leading to efficient photocatalytic H2 generation.

CONCLUSIONS In summary, three anisotropic Pt-loaded Au TNPs nanostructures such as Pt-covered, Pt-edged, and Pt-tipped Au TNPs were successfully prepared as plasmonic photocatalyts. Ptedged Au TNPs exhibited 3 and 5 times higher activity than Pttipped and Pt-covered ones in the photocatalytic H2 generation under visible-NIR light irradiation. By single-particle PL spectra, we found that the DSPR PL of Au TNPs is efficiently quenched by the Pt loaded on edges. This proves that DSPR is the dominant channel for the PL quenching and the hot electrons can transfer from Au to Pt. FDTD simulations prove that Pt-edged Au TNPs have strong electric field and interaction between Au and Pt, resulting in the efficient charge separation and H2 generation of Pt-edged Au TNPs. SPR bands of Au TNPs depending on the size play an important role on the photocatalytic activity of Pt-edged Au TNPs. This work clearly shows Pt−Au TNPs with the anisotropic structures as novel plasmonic photocatalysts based on single-particle spectroscopy. EXPERIMENTAL SECTION Materials. Materials used were hexadecyltrimethylammonium chloride (CTAC, ≥95%, Wako), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·H2O, ≥99.999%), hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6·nH2O, ≥99.9%), L-ascorbic acid (≥99.5%), potassium iodide (≥99.5%), sodium borohydride (NaBH4, ≥99%), and silver nitrate (≥99%). All chemicals were used after purchased without any further purification. Milli-Q water at 25 °C was used in all experiments. All glassware was washed with aqua regia, Milli-Q water, and dried before used. Synthesis of Gold Triangular Nanoprisms (Au TNPs). Pure Au TNPs were synthesized by the reported seed-growth method with some modifications. In typical procedure, first, Au seed@CTAC was synthesized by following reported works.24 A total of 8 mL of Milli-Q water and 1.6 mL of CTAC (0.1 M) were mixed, and then 80 μL of KI3 (10 mM) and 40 μL of HAuCl4 (50 mM) were added into above solution which was labeled as A solution. Then, 400 μL of KI3 (10 mM) and 500 μL of HAuCl4 (50 mM) were added into 40 mL of CTAC (50 mM) solution which was labeled as B solution. Forty and 400 μL of L-ascorbic acid (0.1 M) was added into above A and B solution, respectively. With gentle stirring, both of A and B solution become transparent solution. Then, 200 μL of 10 times diluted Au seed@CTAC solution was added into A solution with shaking for 1 s, 6303

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ACS Nano 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 7 h, followed by repeated washings with warm water 5 times. Finally, the quartz cover glasses were washed again with Milli-Q ultrapure water (Millipore). Pt loaded Au TNPs and pure Au TNPs aqueous suspensions were centrifuged at 6000 rpm (Hitachi, himac CF16RX) to remove excess surfactant and then washed with Milli-Q ultrapure water one time, and finally redispersed in Milli-Q ultrapure water. The well-dispersed aqueous suspensions of 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× , 1.4 NA), and a circular-polarized 405 nm continuous wave laser controlled by a PDL800B driver (PicoQuant). Typical excitation powers for the PL measurements were 350 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) and high-resolution transmission microscopy (HRTEM) measurements were carried out on JEOL-2100 operated at 100 kV and JEM-3000F operated at 300 kV (JEOL). Extinction spectra were measured using quartz cuvettes of 0.2 cm path length on Shimadzu UV-3600 UV−vis-NIR spectrophotometer.

the JSPS for a Postdoctoral Fellowship for Foreign Researchers (No. P15073).

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02494. TEM images, Figures S1−S6, S8, and S10; models for calculations, Figure S7; calculated extinction spectra, Figure S9; size dispersion, Figure S11, and extinction spectra, Figure S12, of Au TNPs with various sizes; hydrogen generation of Pt-tipped and Pt-covered Au TNPs with various sizes, Figure S14; SEM images, Figures S15−S20; detailed results of single-particle microscopy, Figures S21−S33; electric field, Figure S34 (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

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 6304

DOI: 10.1021/acsnano.6b02494 ACS Nano 2016, 10, 6299−6305

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DOI: 10.1021/acsnano.6b02494 ACS Nano 2016, 10, 6299−6305