Importance of Hot Spots in Gold Nanostructures on Direct Plasmon

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Importance of Hot Spots in Gold Nanostructures on Direct Plasmon Enhanced Electrochemistry Chen Wang, Xiao-Ping Zhao, Qiu-Yang Xu, Muhammad Rizwan Younis, Wen-Yuan Liu, and Xing-Hua Xia ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01436 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Applied Nano Materials

Importance of Hot Spots in Gold Nanostructures on Direct Plasmon Enhanced Electrochemistry

Chen Wang†‡, Xiao-Ping Zhao†, Qiu-Yang Xu†, Muhammad Rizwan Younis‡, Wen-Yuan Liu†, Xing-Hua Xia*‡

†

Key Laboratory of Drug Quality Control and Pharmacovigilance (China

Pharmaceutical University), Ministry of Education; Key Laboratory of Biomedical Functional Materials, School of Science, China Pharmaceutical University, Nanjing, 211198, China ‡

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry

and Chemical Engineering, Nanjing University, Nanjing 210023, China *

E-mail address: [email protected]

ABSTRACT Recently, the direct utilization of plasmonic metal nanostructures in accelerating the electrochemical reactions reveals the importance of hot charge carriers generated by localized surface plasmon resonance (LSPR). However, the effect of morphological forms of same metal element on direct plasmon enhanced electrocatalytic activity has not yet been well documented. Herein, four kinds of Au nanostructures with different morphologies of nanospheres (NSPs), nanorods (NRs), nanostars (NSs) and triangular 1

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nanoplates (NPLs) were synthesized. Shape-dependent plasmonic enhancement effect of Au nanostructures toward the electrooxidation of ascorbic acid (AA) was studied. We find that the electrochemistry of AA oxidation on these Au nanostructures can be enhanced upon light irradiation with the higher enhancement effect of the Au NPLs and NSs than the Au NSPs and NRs. This shape-dependent enhancement effect is suggested to be related to the number of “hot spots” in different NPs surfaces generated from Au LSPR. Thus, the present work would shed new insights on the direct plasmon enhanced electrochemistry, which helps in widening the potential applications of plasmonic materials in electrochemical sensors and electrochemical energy conversion. Keywords: Gold nanostructures, localized surface plasmon resonance (LSPR), hot spots, plasmon-enhanced electrochemistry, ascorbic acid

INTRODUCTION Plasmonic metallic nanostructures are well known for their unique properties of localized surface plasmon resonance (LSPR).1,2 The excited surface plasmons decay through two pathways, namely the radiative and non-radiative terms.1 The radiative scattering of resonant photons dissipates energy by heat, and the non-radiative energy relaxation

efficiently

generates

hot

charge

carriers

(electron-hole

pair).3

Simultaneously, the strong electromagnetic field and elevated electric fields are generated at the near surface of the nanostructures.4 The intensely localized field 2


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associated with the plasmonic hot charge carriers has resulted in their wide utilization in many respects, such as solar cells,5 surface-enhanced Raman spectroscopy,6 single molecule spectroscopy,7 electroanalysis,8 and enzyme-like study.9 It have been demonstrated by experimental and theoretical studies that LSPR of nanoparticles (NPs) allows the energy of visible light to be efficiently transferred into the adsorbed molecules. Accordingly, the activated chemical reactions on plasmonic nanostructures surface could be highly anticipated by LSPR excitation.1,9-14 In recent years, direct plasmon enhanced electrochemistry has been emerging as a cut-edge research field, showing unique and powerful potentials in energy conversion, electroanalysis,

and

electrochemical

devices.12-14

For

example,

the

direct

photocatalysis of water splitting on Au nanoparticles can be enhanced by using a plasmonic photoelectrode. A higher collected efficiency for the hot charge carriers has been achieved as compared to the meta/semiconductor composite.12 In a recent work, spherical Au NPs have been reported to be employed to catalytically accelerate the glucose oxidation reaction via the LSPR generated hot holes.14 We for the first time utilized the LSPR generated hot charge carriers to accelerate the sluggish electrochemical oxidation of glucose on Au NPs13 and found that the particle size had considerable effect on the accelerated electrochemical reaction, which confirms the importance of plasmon effect. On the other hand, it is well known that the LSPR of plasmonic NPs strongly depends on the shape, composition, interparticle distance and local dielectric environment.15-19 For example, it has been reported that the 3



nanostructures with anisotropy exhibit varied intensities of the electromagnetic field on the side, corner or surface.20 The electromagnetic intensity is much stronger at the sharp tips of nanostructures, called as “hot spot” regions.21 The anisotropic NPs with sharp tips can provide more significantly larger near-field enhancements.22 The importance of hot spots related to the generation of hot charge carriers has been confirmed.23 It is found that the light intensity can be enhanced by up to 106 times in the hot spots regions,24 which is expected to have great potential in electrocatalysis and electrochemical energy conversion. Recently, it has been proved that the hot spots can significantly generate obvious photothemal effect via the relaxation of hot electrons, elevating the local temperature at the nanostructure surface.25 However, how the hot spots influence the plasmon-enhanced electrochemistry yet remains unexplored. The effect of nanostructure shape on the LSPR property has been studied by theories before.21 However, the experimental investigation have been seldom exploited. In this work, direct plasmon enhanced electrocatalysis by Au nanoparticles (NPs) with different shapes was investigated using simple electrochemical method. Four Au NPs of nanospheres (NSPs), nanorods (NRs), nanostars (NSs) and triangular nanoplates (NPLs) were respectively synthesized (Scheme 1A). Using ascorbic acid (AA) as electrochemical probe, we find that the electrochemistry of AA at these Au NPs can be enhanced upon light irradiation. Interestingly, the Au NSs and NPLs show the higher enhancement effect toward the oxidation of AA as compared to the Au 4


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NRs and NSPs. To understand this shape-dependent plasmon enhancement effect, dithiol molecule is used to link nanostructures for creating anisotropy and generating more hot spots. The result shows that the shape-dependent enhancement effect is suggested to be related to the number of “hot spots” in different NPs surfaces generated from Au LSPR (as illustrated in Scheme 1B). The present work aims to shed new insights on the importance of hot spots in gold nanostructures in direct plasmon enhanced electrochemistry.

Scheme 1. (A) Design of different-shaped gold nanostructures. (B) Schematic illustration of the importance of hot spots on enhanced electrochemistry under light irradiation.

EXPERIMENTAL SECTION Materials and Reagents. Ascorbic acid (AA), sodium borohydride (NaBH4), potassium iodide (KI) and methoxy PEG thiol (mPEG-SH, MW=2000) were 5



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purchased from Sigma-Aldrich. Poly(vinylpyrrolidone) (PVP, average MW=10 000) was

purchased

from

Alfa

Aesar

(England).

Trisodium

citrate,

cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) were bought from the Sinopharm Group Chemical Reagent Co., Ltd. Hydrochloric acid (HCl), sodium oleate (NaOL), silver nitrate (AgNO3), monopotassium phosphate and dipotassium phosphate were from the Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Gold chloride (HAuCl4·4H2O) was purchased from the First Reagent Factory (Shanghai, China). Solutions were prepared using Millipore water (resistivity of 18.2 MΩ·cm). Synthesis of Au NSPs. Spherical Au NSPs were prepared by the well-established method as described by Frens.26 A 100 mL solution of HAuCl4 (1%) was heated to boiling, then the trisodium citrate solution (4 mL, 1% by wt) was immediately added under stirring. The solution color changed from faintly blue to blue within 70 s. Under continuous heating the blue color suddenly changed to brilliant red, and the Au NSPs were formed. After the reaction solution was kept boiling for another 10 min, it was gradually cooled to room temperature. During this course, Au NSPs of 15 nm size were achieved. Using the similar process, particles of 50 nm could be obtained by only changing the quantity of citrate solution. The seed solution for gold NRs growth was prepared as follows:27 Specifically, 103 µL of 24.28 mM HAuCl4 and 10 mL of 0.1 M CTAB solution was mixed in a 20 mL scintillation vial. Then, the diluted NaBH4 solution (0.6 mL of fresh 10 mM 6


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NaBH4 into 1 mL water) was dropped to the Au(III)-CTAB system under intense stirring. After the color of the reaction system changed from yellow to brownish yellow for 2 min, the stirring was stopped. Before use, the fabricated seed solution was aged at room temperature for half an hour. To prepare the growth solution, 7.0 g (0.037 M in the final growth solution) or 9.0 g (0.047 M in the final growth solution) of CTAB and a certain quantity of NaOL were dissolved in 250 mL of warm water (~50 °C) in a 1 L Erlenmeyer flask. After the solution was cool down to 30 °C, 4 mM AgNO3 solution was added. Then, the reaction solution was kept at 30 °C for 15 min, then 250 mL HAuCl4 solution (1 mM) was added. The solution changed to colorless after 90 min stirring (700 rpm). The solution pH was tuned by using HCl (37 wt. % in water, 12.1 M). After another 15 min of slow stirring at 400 rpm, ascorbic acid (AA, 1.25 mL, 0.064 M) was added with vigorously stirring for 30 s. After finishing the above procedures, the prepared seed solution was added into the growth solution, stirring for 30 s and then left undisturbed at 30 °C for 12 h, which enables the steady growth of Au NRs. The final product was purified by centrifugation at 7000 rpm for 30 min following by washing with water for three times. Au NSs were synthesized as described elsewhere.28 First, the PVP-coated gold seeds of 15 nm were synthesized according to the well-established sodium citrate reduction method.29 The PVP dissolved in water and the as-prepared gold colloidal solutions were mixed under stirring for 24 h (600 rpm). Then, the PVP–stabilized particles were transferred into ethanol. The as-prepared gold seeds were centrifuged at 7



1000 rpm for 30 min to remove the dissolved PVP. The precipitate was redispersed in an anhydrous ethanol. In the course of growth, 110 µL chloroauric acid (25 mM in water) was mixed with 10 mL PVP (10 mM in water), then 29 µL of PVP-coated gold seeds (15 nm) in ethanol was rapidly added under continuous stirring. Within 15 min, the solution color changed from pink to colorless, then to blue, indicating the successful formation of Au NRs. Triangular Au NPLs were synthesized using the one-pot seedless method.30 In a 20 mL flask, 1.6 mL of 0.1M CTAC was diluted with 8 mL of ultrapure water, followed by the sequential addition of 75 µL of 0.01M KI, 80 µL of 1% HAuCl4 and 20 µL of 0.1 M NaOH into the mixture. The mixed solution displayed light yellowish color. Then, ascorbic acid solution (80 µL, 0.064 M) was added with moderate shaking. Once addition of AA, the solution color turned from light yellowish to colorless gradually, indicating that Au3+ was quickly reduced to Au+. Finally, 10 µL of 0.1 M NaOH was injected with vigorously shaking for 2 s. The pH value was around 8.0. The colorless solution turned to red, purple and finally blue. At this time, the anisotropic Au NSs were formed. The growth process was completed within ~10 min. Synthesis of mPEG-Au NPs. To synthesize the mPEG coated Au NPS, a solution of mPEG2000-SH (20 mg/1 ml) was diluted in ultrapure water.31 PEG ligand (1 ml) was dropped into the diluted Au NPs (9 ml) with continuous stirring for 1 h (allowing the

8


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mPEG-SH to exchange with citrate ligands). The excess mPEG-SH was removed by centrifugation. Apparatus. The morphologies of the prepared samples were characterized by scanning electron microscope (SEM, S-4800, Japan) and transmission electron microscopy (TEM, JEM-2100, Japan). The UV-vis absorbance spectra of Au NPs suspensions were recorded on a Nanodrop-2000C spectrophotometer (Thermo Fisher Scientific Inc.). The UV-vis absorbance spectra of the Au NPs modified on glass substrates were performed by UV-VIS spectrophotometer (U-3900, HITACHI, Japan). The dark-field spectroscopic-electrochemical measurements were conducted on an inverted optical microscopy (eclipse Ti-E, Nikon, Japan). The image of scattering light was captured by a true color digital camera (Nikon digital sight DS-Ri1, Japan). At the same time, the scattering spectra were in-situ collected by a spectrometer CCD (PyLoN, Princeton Instrument, USA). The size distributions of the Au nanostructures were recorded on a Malver Zetasizer nano ZS90. Electrochemical Measurements. Electrochemical measurements were performed on a CHI 660E instrument (Chenhua, China). The working electrode was prepared by drop-casting a 10 µL of Au NPs onto a glassy carbon electrode (3 mm diameter), forming the Au catalysts/GC electrode. For all the electrochemical measurements, a platinum wire was used as the counter electrode and an Ag/AgCl electrode was used as the reference. A 300 W Xe lamp with wavelength from 350 to 1200 nm was used as the light source (Beijing zolix instruments Co., Ltd). Additional visible laser 9



sources with different wavelengths of 450, 532, 652, and 808 nm were used. The distance between the lamp and the electrochemical cell was controlled at 5 cm. Dark-field Scattering Spectroscopy. The prepared NPLs were modified on indium-tin-oxide (ITO) slide for in situ collection of dark-field scattering spectroscopy. Prior to use, the slide was sonicated in acetone and then water. The cleaned slide was blow-dried with nitrogen gas and then immersed into Au NPLs solution for 1 h for particles immobilization. After that the slide was blow-dried and used as the working electrode, with Pt wire as the counter electrode and Ag/AgCl electrode as the reference.

RESULTS AND DISCUSSONS Characterization of Au Nanostructures. The morphologies of the prepared Au NPs were characterized using transmission electron microscopy (TEM) (Figure 1A-D). The NSPs are of spherical structure with a diameter of ~15 nm (Figure 1A). The NRs are ~ 68 nm long and 22 nm wide with an aspect ratio of ~ 3 (Figure 1B). The NSs is a type of anisotropic nanostructure with branched thorn (Figure 1C), and the NPLs (Figure 1D) show clear tip structures with about 75 nm side length. The DLS size distributions of the Au NPs are presented in Figure 1E, which agree well with the TEM results. To rule out the effect of surface charges on experimental results, all the synthesized Au NPs were coated with a layer of polyethylene glycol (PEG). The zeta 10


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potentials of the Au NPs after PEG coating are close to zero (Figure 1F, measured by Malver Zetasizer), indicating nearly neutral surfaces for the PEG coated Au NPs. The UV-vis extinction spectra of the PEG-Au NPs and Au NPs immobilized on glass substrates are shown in Figure 1G and Figure S1 (ESI†), which agree well with those reported previously.32-35 There is no obvious difference in the UV-vis extinction spectra of the Au NPs after PEG coating. In addition, the UV-vis extinction spectra in solution were also collected (Figure S2 in ESI†). A minor red shift appears as the Au NPs are deposited on GC supports as compared to those in solutions, which might be caused by the change of dielectric environments. Specially, for the strongly scattered NSs and NPLs, shapes of the UV-vis extinction spectra of the NSs and NPLs on glass substrates are different from their absorbance spectra in solution.

11



Figure 1. (A-D) TEM images of Au NSPs (A), NRs (B), NSs (C) and NPLs (D). (E) Size distribution of different shaped Au NPs. (F) Zeta potential of different shaped Au NPs before and after PEG coating. (G) UV-vis-NIR spectra of the PEG-Au NPs immobilized on glass substrate. The upper blue coordinate axis is used for the blue curve of NSs due to the relative wider wavelength range. The other three curves (black, red and purple) are based on the bottom black coordinate axis. Electrochemistry of AA. AA participates in several metabolic reactions in human organis, which could efficiently prevent some mental illness and cancers. Thus, the precise measurement of AA concentration has attracted increasing interest in nowadays. In the present work, AA is chosen as the electrochemical probe to explore the LSPR effect on its electrochemical oxidation on plasmonic nanostructures with different shapes. For electrochemical investigations, glassy carbon (GC) electrode was chosen as the substrate due to the very low background response of light.13 After deposition of Au NSPs on the GC electrode, the electrocatalytic activity of NSPs modified GC electrode toward AA oxidation was first investigated in a N2 saturated AA solution (1 mM AA in 0.1 M PBS) using a typical three-electrode configuration (Figure 2). The cyclic voltammogram (CV) of the Au NSPs/GC electrode in PBS buffer shows a featureless curve (blank curve in Figure 2A) in the potential range from -0.1 V to 0.5 V. When AA is added, significant anodic current peak for AA electrocatalytic oxidation is observed (red curve in Figure 2A). As control, the electrochemical activity of a naked GC electrode toward AA oxidation was performed 12


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in dark (Figure 2A, pink line). The result indicates a negligible current of GC electrode toward the AA oxidation as compared to the Au NSP modified electrode. Upon light irradiation, the oxidation current increases (blue curve in Figure 2A). The increase in electrochemical current indicates more electrons injected from the Au modified electrode to external electric circuit. In addition, changes of the steady-state photocurrent changes were collected at 0.3 V (vs Ag/AgCl) with light irradiation on and off (Figure 2B). When light irradiation is turned on, the anodic current increases sharply, and then it returns to the initial value once the light is turned off. The result shows that AA electrocatalysis can be enhanced by the LSPR excitation of the Au NPs.

Figure 2. (A) CVs of AA (1 mM in 100 mM PBS, pH 7.4) on an Au NSPs/GC electrode. The scan rate was 50 mV/s. Blank curve: PBS; Red curve: AA with light off; Blue curve: AA with light on. Pink curve: naked GC electrode in 1mM AA solution. (B) I-t curve of the Au NSPs/GC electrode at 0.3 V (vs. Ag/AgCl) in a solution of 100 mM PBS (pH 7.4) containing 1 mM AA with light on and off (red arrow: light on; black arrow: light off). 13



Shape-Dependent Enhancement of Au NPs toward AA Electrocatalysis. In our previous study, it is confirmed that the hot charge carriers generated by light irradiation is responsible for the enhanced electrochemical performance.13 The importance of pH and particle size was investigated. However, in that work, only varied diameters of gold nanospheres was studied. Effect of different morphological forms of the same metal element on direct plasmon-enhanced electrocatalytic activity has not been exploited. Therefore we performed further investigation in this work. Figure 3A shows the CVs and corresponding I-t curves of the four different shaped Au NPs (from up to bottom: NSPs, NRs, NSs, NPLs) modified GC electrodes toward the electroxidation of AA. It is clear that all the Au NPs show enhanced catalytic performance upon light irradiation (for Au NSPs, the current at 0.3 V increases from 4.99 to 5.66 µA; NRs from 4.74 to 6.30 µA; NSs from 3.20 to 9.03 µA; NPLs from 7.66 to 14.28 µA). However, more significant enhancements is observed on the Au NSs and NPLs as compared to the Au NSPs and NRs. In order to clearly show the shape-dependence of the plasmon enhanced electrocatalysis, the current values at 0.3 V from the CVs in Figure 3A are subtracted. The real surface areas of each electrodes were calculated using the charge passing through the reduction peak of gold oxide in CVs in 0.5 M H2SO4 (from 0.8 V to 1.2 V), as indicated in Figure S3 and Table S1. The real surface area = Q/386. By dividing the current by the real surface area, the current densities with light on and off can be achieved (J=I/(Real surface area)). The 14


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difference between the values of Ion (light irradiation) and Ioff (in dark) divided by Ioff and the real surface areas of the Au nanostructures are used to characterize the plasmon enhancement effect (∆I/I)%, as shown in Figure 3B. It is clear that among the four shaped Au NPs, the Au NSPs show the weakest plasmon enhancement effect, while the Au NSs exhibit the strongest enhancement. To understand the current enhancement, the current densities of different electrodes toward AA electrooxidation in dark were calculated (the details refer to Figure S3 and Table S1, Supporting Information). The normalized current densities are shown in Figure 3C. It can be seen that a fluctuation ranges from 0.69 to 1.0 for the different Au NPs modified electrodes, which has a different change trend as compared to that in Figure 3B. The experimental results indicate that the shape-dependent enhancement effect is not caused by the NPs shapes themselves.

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Figure 3. (A) Left panel: (a), (c), (e), (g) show the CVs of Au NSPs, NRs, NSs, NPLs modified GC electrodes, respectively (Black curves: in dark; Red curves: light on). Right panel: (b), (d), (f), (h) show the corresponding I-t curves of the Au NPs modified electrodes at 0.3 V (vs. Ag/AgCl) with light irradiation on and off in 1 mM AA solution. (B) Shape-dependent current enhancements upon light irradiation. (C) The normalized current density (J) of different shaped Au NPs/GC electrodes toward the electrooxidation of AA in dark. Once surface plasmons are excited by the incident light, the plasmonic energy relax through two ways: radiative decay and nonradiative decay. The hot carriers can be generated via Landau damping during the nonradiative decay process. To verify the efficient generation of hot electrons and holes, wavelength-dependent 16


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electrochemical measurements of the Au NSPs under illumination were performed. As shown in Figure S4, the maximum current response appears under 532 nm light irradation, which is in accordance with the plasmonic excitation peak of 15 nm AuNPs (~531 nm). The result suggestes that LSPR contributes to the the enhanced current.25,36 Thus, the results confirm the efficient generation of hot charge carriers. As known, different morphologies of nanostructures have different excitation spectra. The nanostructures with anisotropy have varied intensities of the electromagnetic field on the side, corner or surface.20 For anisotropic nanoparticles of Au NSs and triangular Au NPLs, the electromagnetic intensity is much stronger at the corner or edge of nanostructures, where we called it as “hot spot”.22 In the hot spots, hot electrons and holes can be generated more efficiently upon LSPR excitation.23 Recently, it has been proved that the hot spots can significantly generate obvious photothemal effect via the relaxation of hot electrons, elevating the local temperature at the nanostructure surface.25 The hot charge carriers and photothermal would generate simultaneously by light irradiation. Therefore, an accelerated reaction kinetics upon LSPR excitation can thus be expected. The increased reaction rate in turn need more hot holes as the oxidation reagent, resulting in more hot electrons entering the external circuit. Both the hot charge carriers and photothermal effect generated from LSPR contribute to the enhanced performance for AA electrochemical oxidation. For Au NSs, the number of tips and hot spots is the largest on the surface. Based on the mechanism of direct plasmon-enhanced electrochemistry,13 the more the 17



hot charge carriers, the larger the electrochemical current. Therefore a higher concentration of hot charge carriers could be expected on Au NSs with more sharp tips, and the strongest enhancement in electrochemical activity is observed on Au NSs. Mechanism of Plasmon-Enhanced Electrocatalysis. Based on the above analysis and experimental results, the shape-dependent enhancement mechanism of Au NPs toward AA electrocatalysis is displayed in Figure 4A. Upon LSPR excitation, hot charge carriers generate on Au NPs surface. In the hot spots region, efficient photothermal and photocatalytic effects occur, which leads to increased temperature and hot charge carriers number. Upon light irradiation, the electrons filled in the d-band are excited to high energy level which is above the Fermi level of the Au NPs.37 The plasmon-induced charge separation is concentrated on the Au NPs surface. Due to their matched energy levels, the generated hot holes can assist the oxidation of AA. This process can effectively inhibit the recombination of electron with hole. Under the action of a potential bias, the generated hot electrons could be effectively transferred into the external circuit. Combing with the heat effect originated from hot spots, an obviously enhanced current response can be expected. Among the four different shaped Au NPs, the anisotropic NSs have the best capacity to generate hot spots upon LSPR excitation. Accordingly, the strongest plasmon enhancement effect toward the electrooxidation of AA appears in the case of Au NSs. It is worthy to note that in addition to the different nanostructure shapes, the effect of gold surface facets 18


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on the electrocatalytic performance is also a very important and interesting question, which we will perform the related study later.

Figure 4. (A) Schematic illustration of the mechanism for the direct LSPR enhanced electrocatalysis. (B) LSPR spectra of a single Au NPL immobilized on an ITO slide in different mediums. Black curve: 100 mM PBS at open circuit potential; Red curve: 1 mM AA (in 100 mM PBS, pH 7.4) at open circuit potential ; Blue Curve: 1 mM AA (in 100 mM PBS, pH 7.4) with applied positive potential of 0.3 V (vs. Ag/AgCl).

Recently, the nonradiative decay process for generation of hot carriers has been extensively investigated in numerous articles.10,38-40 To obtain direct evidence of generated hot charge carriers on nanostructures, the dark field spectroscopy integrated with electrochemistry technique was used to collect LSPR spectra of single NPL under the same experimental condition (Figure 4B). After the diluted NPLs solution was modified on an indium-tin-oxide (ITO) slide, the LSPR scattering spectroscopy was collected at the open circuit potential with peak appears at 633 nm (black curve). When AA was added, the LSPR scattering peak has a blue shift (~16.0 nm) after 10 19



min illumination (shown as the red curve in Figure 4B). Then a positive potential of 0.3 V is applied, and the LSPR scattering peak shifts to nearly its initial state (blue curve in Figure 4B). The first blue shift is caused by an increased hot electron density at hot spots, which is generated upon illumination with hot holes scavenged by AA. When a positive potential is applied, the excess hot electrons were transferred to the external circuit, resulting in a reduced electron density of the initial state. The dark field LSPR spectroscopy result proves the formation of hot electrons and hot spots upon light irradiation.

Importance of Hot Spots on Direct Plasmon Enhanced Electrochemistry. To confirm the proposed assumption, a linker reagent, 1,3-propyldimercaptan was used to connect the Au NSPs (15 nm), which would be able to produce gap structures among Au NSPs with more hot spots.19 The UV-vis characterizations of the Au NSPs before and after lingking with 1,3-propyldimercaptan are shown in black and blue curves (Figure 5A), respectively. The inset show the SEM image of the coupled Au NSPs (15 nm). The red shift in the spectrum indicates that the nanoparticles are connected together with the linking effect of dimercap. Then, the connected NSP-S-NSP (named as Au-S-Au in the following part) was used to perform the same experiment as described above. The electrochemical active area of the Au-S-Au/GC electrode was estimated using CV in 0.5 M H2SO4 (Figure S5 and Table S2 in ESI†), which shows the similar active area as the NSPs/GC electrode. The LSPR 20


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enhancement effects of the Au-S-Au/GC and NSPs/GC electrodes are compared (Figure 5B). The I-t curves at a potential of 0.3 V (vs. Ag/AgCl) are shown in Figure 5C. It is clear that the linked Au-S-Au/GC electrode shows the more enhanced current response as compared to the NSPs/GC electrode. In addition, we find that the nanostructure of Au-S-Au (15 nm) has similar LSPR absorption band as the 50 nm Au NSPs (red curve in Figure 5A), besides appearance of a larger absorption at longer wavenumbers. To rule out the size influence on the enhancement effect due to the increased size of the Au-S-Au via linking connection, a control experiment using 50 nm NSPs was performed to compare with the Au-S-Au (15 nm). The results are shown in Figure 5C,D and Figure S6. It can be seen that although the 50 nm Au NSPs have similar LSPR absorption band as the Au-S-Au (15 nm), the electrochemical enhancement on the 50 nm Au NSPs (8.0 %) is much smaller than that on the Au-S-Au (62.1 %) structure. The result confirms that the enhancement effect is not caused by the increased size, but originates from the generated hot spots among Au nanoparticles.

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Figure 5. (A) UV-vis spectra of the different Au nanostructures, Black curve: 15 nm Au NSPs; Blue curve: connected Au-S-Au (15 nm). Red curve: 50 nm Au NSPs. The inset shows the SEM image of the connected Au-S-Au. (B) CVs of the different Au nanostructures modified GC electrodes toward 1 mM AA electrocatalytic oxidation. Black solid curve: 15 nm Au NSPs with light off; Red solid curve: 15 nm Au NSPs with light on; Black dot curve: Au-S-Au (15 nm) with light off; Red dot curve: Au-S-Au (15 nm) with light on. (C) I-t curves of the different Au nanostructures modified GC electrodes at 0.3 V (vs. Ag/AgCl) with light on and off in 1 mM AA, Black curve: Au-S-Au NSP (15 nm); Red curve: 50 nm Au NSP; Blue curve: 15 nm Au NSP. (D) CVs of the different Au nanostructures modified GC electrodes toward 1 mM AA electrocatalytic oxidation. Black solid curve: 50 nm Au NSPs with light off;

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Red solid curve: 50 nm Au NSPs with light on; Black dot curve: Au-S-Au (15 nm) with light off; Red dot curve: Au-S-Au (15 nm) with light on.

CONCLUSIONS In summary, direct plasmon enhanced electrocatalysis using four different shapes of Au NPs has been investigated using the electrochemical oxidation of AA as model. It is observed that the triangular Au NPLs and NSs show the better enhancement effect as compared to the Au NRs and NSPs. The shape-dependent hot spots in the Au nanostructures can be used to explain the experimental results, which is confirmed by the results obtained using dithiol molecules linked NSPs. The experimental results indicate that the dithiol molecules linked Au NSPs exhibit the stronger LSPR enhancement effect as compared to the scatterd Au NSPs, which well explains the shape-dependent enhancement in the electrocatalytic activity of Au nanoctructures. The present work provides further insights into the effect of nanostructure morphologies on the LSPR enhanced electrochemistry, offering new avenues for designing novel electrochemical devices and ultrasensitive assays using plasmonic metals.

Corresponding Author *E-mail: [email protected], Fax: (+86)-25-89686106. ORCID 23



Chen Wang: 0000-0001-6544-4065 Xing-Hua Xia: 0000-0001-9831-4048 Notes The authors declare no competing financial interest. Supporting Information. Figure S1. UV-vis-NIR spectra of the Au NPs and PEG-Au NPs immobilized on glass substrates. Figure S2. UV-vis-NIR spectra of the Au NPs suspensions and Au NPs immobilized on glass substrates. Figure S3. CVs of the different shaped Au NPs modified GC electrodes in 0.5 M H2SO4. Figure S4. Effect of light wavelength on the current responses of AA oxidation at 0.3 V (black column) and the LSPR spectrum (blue dot line) of the 15 nm Au NSPs (530 nm). Figure S5. (A) CVs of the Au NSPs modified GC electrodes in 0.5 M H2SO4. The scan rate was 50 mV/s. (B) The normalized current density (J) of different Au nanostructures modified on GC electrodes toward the electrooxidaiton of AA in dark. The reference electrode was Ag/AgCl electrode. Figure S6. Increased percentage in current for different Au nanostructures at 0.3 V upon light irradiation. Table S1. Calculation of the current density of different electrodes. 24


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Table S2. Calculation of the current density of different electrodes.

ACKNOWLEDGMENTS This work was supported by the grants from the National Natural Science Foundation of China (21327902, 21575163, 21635004, 81573557) and the Natural Science Foundation of Jiangsu Province (BK20151437).

REFERENCE: (1) Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Photochemical Transformations on Plasmonic Metal Nanoparticles. Nat. Mater. 2015, 14, 567-576. (2) Wang, C.; Shi, Y.; Yang, D. R.; Xia, X. H. Combining Plasmonics and Electrochemistry at the Nanoscale. Curr. Opin. Electrochem. 2018, 7, 95-102. (3) McFarland, E. W.; Tang, J. A Photovoltaic Device Structure Based on Internal Electron Emission. Nature 2003, 421, 616-618. (4) Marchuk, K.; Willets, K. A. Localized Surface Plasmons and Hot Electrons. Chem. Phys. 2014, 445, 95-104. (5) Qiao, L.; Wang, D.; Zuo, L.; Ye, Y.; Qian, J.; Chen, H.; He, S. Localized Surface Plasmon Resonance Enhanced Organic Solar Cell with Gold Nanospheres. Appl. Energy 2011, 88, 848-852. (6) Li, K. Q.; Yi, J.; Zhong, J. H.; Hu, S; Liu, B. J.; Liu, J. Y.; Zong, C.; Lei, Z. C.; Wang, X.; Aizpurua, J.; Esteban, R.; Ren, B. Plasmonic Photoluminescence for 25



Recovering Native Chemical Information from Surface-Enhanced Raman Scattering. Nat. Commun. 2017, 8, 14891. (7) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102-1106. (8) Shi, Y.; Wang, J.; Wang, C.; Zhai, T. T.; Bao, W. J.; Xu, J. J.; Xia, X. H. Chen, H. Y. Hot Electron of Au Nanorods Activates the Electrocatalysis of Hydrogen Evolution on MoS2 Nanosheets. J. Am. Chem. Soc. 2015, 137, 7365-7370. (9) Wang, C.; Shi, Y.; Dan, Y.; Nie, X. G.; Li, J.; Xia, X. H. Enhanced Peroxidase-Like Performance of Gold Nanoparticles by Hot Electrons. Chem. Eur. J. 2017, 23, 6717-6723. (10) Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology. Nat. Nanotechnol. 2015, 10, 25-34. (11) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911-921. (12) Robatjazi, H.; Bahauddin, S. M.; Doiron, C.; Thomann, I. Direct Plasmon-Driven Photoelectrocatalysis. Nano Lett. 2015, 15, 6155-6161. (13) Wang, C.; Nie, X. G.; Shi, Y.; Zhou, Y.; Xu, J. J.; Xia, X. H.; Chen, H. Y. Direct Plasmon-Accelerated Electrochemical Reaction on Gold Nanoparticles. ACS Nano 2017, 11, 5897-5905.

26


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Peng, T. H.; Miao, J. J.; Gao, Z. S.; Zhang, L. J.; Gao, Y.; Fan, C. H.; Li, D. Reactivating Catalytic Surface: Insights into the Role of Hot Holes in Plasmonic Catalysis. Small 2018, 14, 1703510. (15) Zhao, Y.; He, Y. K.; Zhang, J.; Wang, F. B.; Wang, K.; Xia, X. H. Conformational Change and Biocatalysis-Triggered Spectral Shift of Single Au Nanoparticles. Chem. Commun. 2014, 50, 5480-5483. (16) Xie, T.; Jing, C.; Long, Y. T. Single Plasmonic Nanoparticles as Ultrasensitive Sensors. Analyst 2017, 142, 409-420. (17) Wang, K.; Li, S. G.; Liu, Y. J.; Jiang, L.; Zhang, F.; Wei, Y. Q.; Zhang, Y. J.; Qi, Z. J.; Wang, K.; Liu, S. Q. In Situ Detection and Imaging of Telomerase Activity in Cancer Cell Lines via Disassembly of Plasmonic Core–Satellites Nanostructured Probe. Anal. Chem. 2017, 89, 7262-7268. (18) Qian, G. S.; Kang, B.; Zhang, Z. L.; Li, X. L.; Zhao, W.; Xu, J. J.; Chen, H. Y. Plasmonic Nanohalo Optical Probes for Highly Sensitive Imaging of Survivin mRNA in Living Cells. Chem. Commun. 2016, 52, 11052-11055. (19) Besteiro, L. V.; Govorov, A. O. Amplified Generation of Hot Electrons and Quantum Surface Effects in Nanoparticle Dimers with Plasmonic Hot Spots. J. Phys. Chem. C 2016, 120, 19329-19339. (20) Zhang, H.; Govorov, A. O. Optical Generation of Hot Plasmonic Carriers in Metal Nanocrystals: the Effects of Shape and Field Enhancement. J. Phys. Chem. C 2014, 118, 7606-7614. 27



Page 28 of 32

(21) Wei, H.; Xu, H. Hot Spots in Different Metal Nanostructures for Plasmon-Enhanced Raman Spectroscopy. Nanoscale 2013, 5, 10794-10805. (22)

Sousa-Castillo,

A.;

Comesaña-Hermo,

M.;

Rodríguez-González,

B.;

Pérez-Lorenzo, M.; Wang, Z. M.; Kong, X. T.; Govorov, A. O.; Correa-Duarte, M. A. Boosting Hot Electron-Driven Photocatalysis through Anisotropic Plasmonic Nanoparticles with Hot Spots in Au–TiO2 Nanoarchitectures. J. Phys. Chem. C 2016, 120, 11690-11699. (23) Harutyunyan, H.; Martinson, A. B. F.; Rosenmann, D.; Khorashad, L. K.; Besteiro, L. V.; Govorov, A. O.; Wiederrecht, G. P. Anomalous Ultrafast Dynamics of Hot Plasmonic Electrons in Nanostructures with Hot Spots. Nat. Nanotechnol. 2015, 10, 770-774. (24) Christopher, P.; Xin, H.; Marimuthu, A.; Linic, S. Singular Characteristics and Unique Chemical Bond Activation Mechanisms of Photocatalytic Reactions on Plasmonic Nanostructures. Nat. Mater. 2012, 11, 1044-1050. (25) Wen, L.; Chen, Y. F.; Liang, L.; Chen, Q. Hot Electron Harvesting via Photoelectric Ejection and Photothermal Heat Relaxation in Hotspots-Enriched Plasmonic/Photonic Disordered Nanocomposites. ACS Photonics 2018, 5, 581-591. (26) Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse Gold Suspensions. Nature 1973, 241, 20-22.

28


Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Ye, X. C.; Zheng, C.; Chen, J.; Gao, Y. Z.; Murray, C. B. Using Binary Surfactant Mixtures to Simultaneously Improve the Dimensional Tunability and Monodispersity in the Seeded Growth of Gold Nanorods. Nano Lett. 2013, 13, 765-771. (28) Kumar, P. S.; Pastoriza-Santos, I.; Guez-Gonzáalez, B. R.; Abajo, J.; Liz-Marzán, L. M. High-Yield Synthesis and Optical Response of Gold Nanostars. Nanotechnology 2007, 19, 015606. (29) Graf, C.; Vossen, D. L. J.; Imhof, A.; Blaaderen, A. V. A General Method to Coat Colloidal Particles with Silica. Langmuir 2003, 19, 6693-6700. (30) 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. (31) Zámbó, D.; Radnóczi, G. Z.; Deák, A. Preparation of compact nanoparticle clusters from polyethylene glycol-coated gold nanoparticles by fine-tuning colloidal interactions. Langmuir 2015, 31, 2662-2668. (32) Yamada, K.; Miyajima, K.; Mafune, F. Thermionic emission of electrons from gold nanoparticles by nanosecond pulse-laser excitation of interband. J. Phys. Chem. C 2007, 111, 11246-11251. (33) Chen, H. J.; Shao, L.; Lia, Q.; Wang, J. F. Gold nanorods and their plasmonic properties. Chem. Soc. Rev. 2013, 42, 2679-2724.

29



(34) Nehl, C. L.; Liao, H. W.; Hafner, J. H. Optical properties of star-shaped gold nanoparticles. Nano Lett. 2006, 6, 683-688. (35) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A. Observation of a quadrupole plasmon mode for a colloidal solution of gold nanoprisms. J. Am. Chem. Soc. 2005, 127, 5312-5313. (36) Christopher, P.; Xin, H.; Linic, S. Visible-Light-Enhanced Catalytic

Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3, 467-472. (37) Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M. Ultrafast Plasmon-Induced Electron Transfer from Gold Nanodots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 14852-14853. (38) Bernardi, M.; Mustafa, J.; Neaton, J. B.; Louie1,S. G. Theory and Computation of Hot Carriers Generated by Surface Plasmon Polaritons in Noble Metals. Nat. Commun. 2015, 6, 7044. (39) Brown, A. M.; Sundararaman, R.; Narang, P.; Goddard III, W. A.; Atwater, H. A. Nonradiative Plasmon Decay and Hot Carrier Dynamics: Effects of Phonons, Surfaces, and Geometry. ACS Nano 2016, 10, 957-966. (40) Besteiro, L. V.; Kong, X. T.; Wang, Z. M.; Hartland, G.; Govorov, A. O. Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms. ACS Photonics 2017, 4, 2759-2781 30


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Schematic illustration for the importance of hot spots on plasmon-enhanced electrochemistry 157x155mm (150 x 150 DPI)


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Importance of Hot Spots in Gold Nanostructures on Direct Plasmon

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