Real-Time Plasmonic Monitoring of Single Gold ... - ACS Publications

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Real-Time Plasmonic Monitoring of Single Gold Amalgam Nanoalloy Electrochemical Formation and Stripping Jun-Gang Wang,† John S. Fossey,§ Meng Li,†,‡ Tao Xie,† and Yi-Tao Long*,† †

Key Laboratory for Advanced Materials & Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China ‡ State Environmental Protection Key Laboratory of Risk Assessment and Control on Chemical Processes, East China University of Science and Technology, Shanghai 200237, P. R. China § School of Chemistry, University of Birmingham, Edgbaston, Birmingham, West Midlands B15 2TT, U.K. S Supporting Information *

ABSTRACT: Direct electrodeposition of mercury onto gold nanorods on an ITO substrate, without reducing agents, is reported. The growth of single gold amalgam nanoalloy particles and subsequent stripping was monitored in real-time monitoring by plasmonic effects and single-nanoparticle dark-field spectroelectrochemistry techniques. Time-dependent scattering spectral information conferred insight into the growth and stripping mechanism of a single nanoalloy particle. Four critical stages were observed: First, rapid deposition of Hg atoms onto Au nanorods; second, slow diffusion of Hg atoms into Au nanorods; third, prompt stripping of Hg atoms from Au nanorods; fourth, moderate diffusion from the inner core of Au nanorods. Under high Hg2+ concentrations, homogeneous spherical gold amalgam nanoalloys were obtained. These results demonstrate that the morphology and composition of individual gold amalgam nanoalloys can be precisely regulated electrochemically. Moreover, gold amalgam nanoalloys with intriguing optical properties, such as modulated plasmonic lifetimes and quality factor Q, could be obtained. This may offer opportunities to extend applications in photovoltaic energy conversion and chemical sensing. KEYWORDS: plasmon, single nanoparticle, gold amalgam nanoalloy, dark-field spectroelectrochemistry, electrochemical stripping

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INTRODUCTION Localized surface plasmon-based nanophotonics or plasmonics has drawn great attention with respect to photovoltaic conversion and chemical sensing, due to developing potential in the development of nanophotonic devices.1−5 Nanoalloys have come under close scrutiny due to the significantly different morphologies, electrocatalytic activities, and optical properties accessible from elemental nanostructures.6,7 In practical applications, optoelectronic elements are considered on a nanometre scale. Nanoalloys with optimized nanoplasmonic properties and structures will be crucial in the search for performance enhancements. Therefore, it is of fundamental significance to study the growth mechanisms of nanoalloys. Moreover, deeper understanding of growth mechanisms will improve the preparation of nanoalloys with precise tunablility. Amalgams have served as nanoalloys and have found novel applications in mercury(II) sensing and removal from polluted waters.8−11 However, only a few reports pay close attention to growth mechanisms and correlations between Hg−metal interactions and amalgamation.12−14 To develop a deeper © XXXX American Chemical Society

understanding of such amalgams’ use in chemical sensing and scavenging, gold amalgam nanoalloys were chosen as our model for real-time monitoring of gold amalgam nanoalloy electrochemical formation and stripping within a single-nanoparticle, by dark-field spectroelectrochemistry techniques (SN-DFS). Nanoparticles are commonly inhomogeneous, leading to heterogeneity in optical properties. Size and shape inhomogeneities of typical nanoparticles remain a major disadvantage in ensemble optical measurements, and only average values can be obtained. Ensemble averaging of optical properties due to inhomogeneous size distributions is eliminated by use of singleparticle spectroscopy techniques.15 Consequently, one approach that can obviate these deficiencies is to study amalgamation on a single noble metal nanoparticle. Dark-field microscopy has been exploited for its ability to observe dynamic scattering spectroscopy of a single plasmonic nanoparticle.15−17 Received: January 26, 2016 Accepted: March 4, 2016

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DOI: 10.1021/acsami.6b01029 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (A) Experimental setup of single-nanoparticle plasmon spectroelectrochemistry: Dark-field microscopy coupled with an electrochemical workstation. (B) Schematic depiction of the direct localized plasmon sensing strategy to real-time monitoring of single gold amalgam nanoalloy electrochemical formation (a-b-c; a-b-e) and stripping (a-b-c-d; a-b-e-f).

formed. The amalgamation and following stripping process had a dramatic effect on the plasmonic properties of the nanostructured particles. Moreover, the SN-DFS technique shed new light onto the real-time monitoring of single-particle nanoalloy formation and the corresponding mechanism.

The scattered light from a noble metal nanoparticle originating from the localized surface plasmon resonance (LSPR) is significantly affected by the composition, facets, structures, surface electron density, and local surroundings of the nanoparticles.3 Therefore, SN-DFS enables the exploration of the growth mechanism of nanoalloys on a single nanoparticle via plasmon band shifts and changes of full width at halfmaximum (fwhm, Γ). Gold nanorods were chosen for this study due to reduced plasmon damping and their strong optical extinction at visible and near-infrared wavelengths which can be tuned by adjusting the rod length and diameter.18,19 Typically, longitudinal localized plasmon resonance depends strongly on the Au nanorod aspect ratio with a tunability across a broad spectral range from 600 nm to the near-infrared (NIR) region.20,21 The purpose of this work is to investigate the growth mechanism of gold amalgam nanoalloys and their optical properties. Gold amalgam nanoalloys prepared by action of sodium borohydride or ascorbic acid on Hg2+ in the presence of gold colloids has been reported previously.8,22 Schopf and co-workers demonstrated Au−Hg alloy formation on individual substrateimmobilized Au nanorods exposed to Hg2+ solutions was accompanied by remarkable blue-shifts in the corresponding scattering spectra upon reduction of Hg2+.22 Herein, Hg2+ reduction by electrochemical manipulation, without any chemical reductant, and direct amalgamation of a single Au nanorod are monitored in real-time by the SN-DFS techniques (Figure 1). The gold amalgam nanoalloys formed were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FE-SEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Four critical stages were involved in the growth and stripping of single gold amalgam nanoalloys: First, rapid deposition of Hg atoms onto Au nanorods; second, sluggish diffusion of Hg atoms into Au nanorods; third, prompt stripping of Hg atoms from Au nanorods; fourth, moderate diffusion from the inner core of Au nanorods. Under high Hg2+ concentration, homogeneous distributed spherical gold amalgam nanoalloys were fabricated. Furthermore, tuning of the scattering properties of single gold amalgam nanoalloy particles was achieved through electrochemical stripping. We demonstrate, for the first time, deposition of Hg on a single Au nanorod and its stripping from the nanoalloy particle thus

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EXPERIMENTAL SECTION

Materials. Indium tin oxide (ITO)-coated glass (sheet resistances 150) as characterized by transmission electron microscopy (TEM, JEOL JEM-2100, Japan) at an accelerating voltage of 200 kV. Gold nanorods (10 optical density) were purchased from Nanoseedz (Hong Kong, China). All reagents were of analytical grade and used as received without any further purification. Ultrapure water, filtered by a Milli-Q reagent water system at a resistivity of >18MΩ cm, was used throughout the experiments. The ITO-coated glass was cleansed first by ethanol, and then it was washed successively in acetone, isopropanol and pure water with at least 30 min sonication. Lastly they were dipped in a mixture of pure water, hydrogen peroxide and ammonium hydroxide (volume ratio, 5:1:1) and heated to boiling for at least 30 min,25 and drieed under a flow of nitrogen gas. The gold nanorods used in the experiments were immobilized on an ITO electrode through electrostatic adsorption by placing the ITO electrode in a diluted gold nanorod solution (300 times) for 5 min.26 Then, the gold nanorod modified ITO electrodes were rinsed with copious amounts of water prior to the dark-field measurements. Characterization of Gold Amalgam Nanoalloy. Transmission electron microscopy (TEM) was performed on a JEOL2100 Transmission Electron Microscope. UV−vis absorption spectra were measured with a USB2000+ spectrophotometer at room temperature. B


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Figure 2. (A) Cyclic voltammetry (CV) curves of an Au NRs/ITO electrode with different Hg2+ concentrations (0, 2.5, 5.0, 6.2, 7.5, 10, 12.5, 13.7, 16.2, 17.5, 18.7, 22.5, and 25 μg/mL) in HNO3 (0.29 M). Scan rate: 50 mV/s. (B) Scan rate dependence of CV curves for an Au NRs/ITO electrode in a solution of HNO3 (0.29 M) with 5.0 μg/mL Hg2+. Scan rate: 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, and 600 mV/s. (C) The dependence of the peak currents of peaks a and b on square root of scan rate. (D) The dependence of peak voltage (peak a and peak b, respectively) on logarithm of scan rate. NRs/ITO electrode was prepared by spin-coating three drops of undiluted gold nanorods solution onto an ITO electrode. Experiments were carried out with 0.29 M HNO3 solution as the supporting electrolyte, the solutions were purged with nitrogen (20 min) before electrochemical experiments. All experiments were performed at room temperature 25 ± 2 °C. Growth of Gold Amalgam Nanoalloys. Dark-Field Scattering Microscopy. The foundation of the optical dark-field spectrum measurements was a Nikon eclipse Ti-U inverted microscope equipped with a dark-field condenser (0.8 < NA < 0.95) and a 40 × objective lens (NA = 0.6). Illumination was provided by a 100 W halogen lamp which was used to excite the gold nanorods to generate the local plasmon resonance scattered light. The scattered light was focused onto the entrance port of a monochromator (Acton SP2300i, Princeton Instruments, USA) that was equipped with a grating (grating density: 300 lines/mm; blazed wavelength: 500 nm) to disperse the scattered light. Then, the scattered light was recorded using a 400 × 1340 pixel cooled spectrograph CCD camera (Pixis 400, Princeton Instruments, USA). A true-color digital camera (Nikon, DSfi, Japan) was used to record the field of the microscope for coregistration with the monochromator. An adjustable entrance slit could be opened to retain only a single gold nanorod in the region of interest. Note that a distribution of spacings between gold nanorods can be produced in the sample preparation. The scattering spectra from single gold nanorods was corrected by subtracting a background spectrum taken from adjacent regions without gold nanorods and dividing by the calibrated response curve of the entire optical system. To ensure that only optical isolated gold nanorods were analyzed, only scattering with a smooth baseline, a single Lorentzian peak, and a nondistorted line shape was accepted.27 The integration time used in all experiments’ spectral acquisitions was 10 s. To monitor real-time electrochemical growth of single gold amalgam nanoalloy particles, a PDMS electrochemical microcell was used (Figure S1B).

The size and shape of Au nanorods before and after growth were characterized by Field emission scanning electron microscopy (FESEM). FE-SEM images were obtained using Hitachi S-4800 Field emission scanning electron microscopy (Hitachi, Ltd., Japan) using an in-lens ion annular electron detector at an acceleration voltage of 15 kV. The chemical composition of the prepared gold amalgam nanoalloy on the ITO electrode (gold amalgam/ITO) was measured by energy dispersive X-ray analysis (EDX, Bruker AXS Microanalysis GmbH Berlin, Germany), and the structural properties were examined by X-ray diffractometry (XRD) (D/MAX 2550 VB/PC X-ray powder diffractometer, Rigaku Co., Japan) with a Cu anticathode (40 kV, 30 mA) using a Cu Kα radiation source (λ= 1.5406 Å). In a typical experiment, the range of diffraction (Bragg) angles were measured by scanning the RINT2000 vertical goniometer from 10° to 80° with a step angle of 0.01° and a scan rate of 0.15°min−1. The content of Hg in gold amalgam nanoalloys was determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, NexIon 300x, PerkinElmer, USA). Time-of-flight secondary ion mass spectra (ToF-SIMS) of ions were obtained on secondary ion mass spectrometer ToF-SIMS V (ION-TOF GmbH, Münster, Germany) equipped with a high mass resolution time-of-flight analyzer of a reflectron type. Bi3+ primary ion gun was used during analysis. Electrochemistry. All electrochemical experiments were performed with a CHI660E electrochemical station (CH Instruments, Inc., Shanghai, China) in a conventional one-compartment cell in conjunction with a standard three-electrode system. A PDMS microelectrochemical cell was used in the electrochemical experiments. The ITO electrode modified with gold nanorods served as the working electrode. A Pt wire electrode was used as the counter electrode and another Pt wire electrode functioned as reference electrode. The potential of the Pt reference electrode was calibrated through cyclic voltammetry scanning of potassiumferricyanide (Figure S1A, Supporting Information). The ΔE01/2 was 0.4 V between using Pt quasireference electrode and saturated calomel reference electrode (SCE) in 5.00 mM K3[Fe(CN)6] solution, respectively. The ensemble Au C


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Figure 3. FE-SEM images of gold amalgam nanoalloys: (A) low-resolution images; (B) high-resolution images, formed in 40 μg/mL Hg2+; (C) lowresolution images; (D) high-resolution images, formed in 150 μg/mL Hg2+. Representative TEM images (E) and high-resolution TEM images (F) of Au nanorod. Equal lattice spacing (0.20 nm) in the Au nanorod confirms epitaxial growth ([100] planes). (G) FE-SEM images of the ensemble gold amalgam nanoalloys, formed in 40 μg/mL Hg2+. (H) EDX mapping and corresponding Au (I) and Hg (J) element maps of the ensemble gold amalgam nanoalloys. (K) X-ray diffraction patterns of the ensemble gold amalgam nanoalloys (a) and Au nanorod (b). (L) EDX spectrum of the ensemble gold amalgam nanoalloys.

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H g 2 + + 2e− → Hg

RESULTS AND DISCUSSION

Electrochemical Deposition and Stripping of Hg. Electrochemical reduction and oxidation of Hg upon an ensemble Au NRs/ITO electrode was performed using cyclic voltammetry, shown in Figure 2A. The ensemble Au NRs/ITO electrode did not show any characteristic response in HNO3 solution. Interestingly, after the addition of Hg2+ (2.5 μg/mL), anodic and cathodic peaks located at 0.22 and 0.20 V, respectively, corresponding to the reduction of Hg and subsequent oxidation of Hg oxides, were observed. With an increase in the concentration of Hg2+, reduction and oxidation peak currents increase gradually, indicating the peak current originated from the reduction and oxidation of Hg. In control experiments, no peak current could be found on the bare ITO electrode in Hg2+ solution (Figure S2). Furthermore, in order to estimate the kinetics of the deposition and oxidation of Hg on the ensemble Au NRs/ITO electrode surface, the CVs of the electrode in HNO3 solution containing 5.0 μg/mL of Hg2+ at different scan rates were recorded (Figure 2B). Both the cathodic and anodic peak currents were proportional to the square root of the scan rate, indicating a typical diffusioncontrolled process with fast electron-transfer behavior (Figure 2C). Figure 2D depicts the dependence of the potential of peak a or b (in Figure 2B) on the logarithm of the scan rate. By using this dependence, it is possible to estimate the number of charges (n) during the charge transfer process. According to the equation ΔEp = 2.3RT/nF,28 we estimate that a two-electron transfer processes (n = 2) is involved in the deposition and oxidation of Hg in the negative and positive scans, respectively (eq 1).29,30

(1)

Characterization of Nanoalloys. The FE-SEM images of the gold amalgam nanoalloys fabricated using various Hg2+ concentrations are shown in Figure 3A−D. Before the formation of gold amalgam nanoalloys the Au nanorods have a regular cylinder shape, capped by two hemispherical termini (Figure 3E). High-resolution TEM images (HRTEM) revealed that Au nanorods have an equal lattice spacing of 0.20 nm (Figure 3F), this confirms epitaxial growth ([100] planes). Under 40 μg/mL Hg2+, the formed nanoparticles had a similar shape to the Au nanorod (Figure 3A, B). No significant changes to the shape and size of the nanorods after electrodeposition of Hg could be found, which is similar to previous work.31 Elemental mapping of the formed gold amalgam nanoalloys was performed to demonstrate the distribution of Hg and Au. It showed that Hg and Au were homogeneously distributed in the amalgam nanoalloy (Figure 3I, G). For further phase analysis, XRD was performed and XRD patterns of the gold amalgam nanoalloys is shown in Figure 3K. The diffraction lines are indexed as cubic gold with space group Fm3m and the corresponding lattice parameter a is 4.0786 Å.12,32 The 2θ peak at 44.3° corresponding to the [100] planes of the Au nanorod, confirmed by the HRTEM (Figure 3F).33,34 A broadening of full width at half-maximum (fwhm) in this peak could be observed, which indicated that there was an increase in the dimension of the nanorod according to the Scherrer equation after deposition of Hg2+.12 The formation of gold amalgam nanoalloys was further confirmed by EDX shown in Figure 3L. Moreover, ToF-SIMS measurements were performed to verify the presence of mercury species on the surface of the D


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Figure 4. Schematic illustration of shape changes of a single Au nanorod during the electrochemical growth of gold amalgam nanoalloys (stages not to scale).

Figure 5. (A) Full scattering spectra (contour plots) of the single Au nanorod exposed to the electrolyte containing 40 μg/mL Hg2+ under −0.4 V for 400 s. (B) Plasmon shift (a) and fwhm (b) as a function of the reaction period during growth of gold amalgam nanoalloy from a single Au nanorod. (C) Scattering spectra of a typical single Au nanorod after amalgamation: (a) original spectrum; (b) spectrum at 50 s and (c) spectrum at 410 s at open circuit potential. (D) Contour plots showing the evolution of the scattering spectra of the single Au nanorod in the absence of Hg2+ under 0.4 V for 400 s. (E) Plasmon shift (a) and fwhm (b) as a function of the stripping period. (F) Scattering spectra of a typical single Au nanorod after stripping of Hg: (a) original spectrum; (b) spectrum at 30 s; and (c) spectrum at 410 s at open circuit potential.

nanoalloys were formed under 150 μg/mL Hg2+, no rods, only spheres were found (Figure 3C, D). The formed nanoparticles had a similar diameter, ca. 84 nm, and a similar scattering spectrum to Au nanorods after deposition of Hg. This is due to the fact that active sites are mainly located at the tips of Au nanorods, where a shielding effect of cetyltrimethylammonium bromide (CTAB) is minimized, and amalgamation lead to a decrease of the aspect ratio of the Au nanorod from ca. 2.4 to 0.9 and a rounding of the corners. Furthermore, due to the much lower cohesive energy, 0.67 eV/atom for Hg, compared to that of Au, 3.81 eV/atom, the formed nanoparticles increased atom diffusivity after deposition of Hg. Then, amalgamation reduced the stability of the crystal and the diffusivity of the Au atoms in the interfacial region was enhanced by a factor of 5.7.14 Therefore, Hg atoms, with high diffusivity, have a greater possibility to diffuse into the interior of the nanoparticles. As a consequence, dramatic structural transitions upon change of the aspect ratio of the nanorods occurred with diffusion of Hg into the Au nanorods (Figure 4). Amalgamation increased the volume of nanoparticles due to Hg atom deposition and an increase of the number of atoms per nanoparticle. Barrosse-Antle and co-workers had demonstrated that amalgam microparticles were larger than the original

nanostructures (Figure S6 and Table S1) by the presence of diagnostic mercury isotope patterns CsXHg (X = 196, 198, 199, 200, 201, 202 and 204). The FE-SEM image of the gold amalgam nanoalloys fabricated under 80 μg/mL Hg2+ is shown in Figure S5. A gradual shape transition from nanorod to sphere of gold amalgam nanoalloys was observed with a calculated aspect ratio (AR) of ca. 1.9. Assuming that the alloying metals have the same molar volume in the alloy as they do in pure form,12 the size increase upon mercury incorporation could be estimated by the following equation: 4R13 + 3L1R12 3

2

4R + 3LR

=1+

Vm , HgXHg Vm , AuXAu

(2)

where R and L are the initial cap radius and length of the Au nanorod, respectively, R1 and L1 are the cap radius and length of gold amalgam nanoalloys, respectively; XHg and XAu are the mole fractions of Hg and Au atoms, and Vm,Hg and Vm,Au are the molar volumes of metal Hg and Au, respectively. For R = 20 nm, L = 56 nm, R1 = 28 nm, and L1 = 50 nm, and taking Vm,Au = 10.21 cm3/mol and Vm,Hg = 14.81 cm3/mol,35 the molar ratio of the gold amalgam nanolloys was estimated to be XHg/XAu = 0.738:1. The Hg had an average content of ca. 42.4 atom % in gold amalgam nanoalloys. However, when the gold amalgam E


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Figure 6. (A) Representative plasmon scattering spectra of single gold nanorod: (a) before Hg deposition; (b) after Hg deposition; and (c) after Hg stripping. (B) Line width of gold nanorods (without exposure to Hg (orange square, a); after deposition of Hg (purple dots, b); and after stripping of Hg (green triangle, c)) plotted against their plasmon resonance wavelength λ. Line width (fwhm) and λ were both extracted directly from the scattering spectrum of a single nanorod (inset). The data points are clustered into three regions, as indicated by the ellipses. (C) The corresponding statistical data (standard deviations and mean value) of the line width of gold nanorods versus the peak wavelength. Data comes from (B).

during amalgamation. A blue-shift and damping caused by the amalgamation could be observed (Figure 5B, C), which is in accordance with previous results.8,9 During cathodic polarization, Hg deposition occurred, and the peak wavelength had an obvious blue-shift from 689 to 682 nm with a sensitivity of 0.11 nm/s and the line width of the scattering peak exhibited a distinct widening from 72.6 to 81.2 meV with a sensitivity of 0.14 meV/s in the initial period (within 60 s). The initial peak blue-shift and plasmon damping could be reconciled as an inward diffusion behavior of Hg atoms within the nanorods.12 Subsequently, a slight red-shift of the peak wavelength to 684 nm was found. A similar phenomenon was found for single Ag nanostructures.40 Furthermore, the line width of the scattering spectra decreased remarkably at the early deposition stage and then reached a plateau at ca. 82.2 meV. The damping of the nanoparticle was stable after initial Hg deposition, which indicated that the interfacial region of gold naonrods had been saturated by doped Hg atoms. Hg doping-induced line broadening could provide a strategy for controlling plasmon line width and might have the potential to modify the relative decay channels for localized nanoparticle surface plasmons.41 The mechanism of the amalgamation can be explained as follows: Hg atoms were deposited on the surface of the Au nanorods. The amalgamation process preferentially proceeded on the tips of the Au nanorod, because the active sites for Hg deposition were mainly located at the tips of the Au nanorod rather than the side regions. Meanwhile, a decrease in aspect ratio of the single Au nanorods occurred, which led to a blueshift of the scattering spectra. Then, with saturation of the Hg atoms at the interfacial regions of the nanoparticle, the diffusion of the Hg atoms into a Au nanorod gradually increased the effective volume of the nanoparticle. This resulted in a slight red-shift of the spectra and stable damping of the localized plasmon. During cathodic polarization, the number of Hg atoms and average content of Hg by electrochemical deposition at a single gold nanorod were ca. 2.71 × 1010 (0.045 pmol) and 13.7 atom %, respectively, under 40 μg/mL Hg2+ (Figure S3), which contributed to the change of plasmon scattering properties of the single gold amalgam nanoalloy. Under anodic polarization, Hg stripping occurred, and the peak wavelength experienced a red-shift from 686 to 689 nm with a sensitivity of 0.14 nm/s and the line width of the scattering peak exhibiting an obvious narrowing from 77.4 to 72.8 meV with a sensitivity of 0.23 meV/s in the initial period (within 60 s, Figure 5D, E).

microparticles due to the electrochemical deposition of Hg at gold microparticles.36 Nanoparticle dimensions were statistically evaluated through analysis of FE-SEM images, and an average size of 84 nm after deposition of Hg was determined. The volume of the nanoparticles increased by ca. 198%, which could be attributed to a “swelling effect”.37 The size increase upon mercury incorporation could be estimated by the following equation: 4R13 3

2

4R + 3LR

=1+

Vm , HgXHg Vm , AuXAu

(3)

where R and L are the initial cap radius and length of the Au nanorod, respectively, R1 is the particle radius of gold amalgam nanoalloys; XHg and XAu are the mole fractions of Hg and Au atoms, and Vm,Hg and Vm,Au are the molar volumes of metal Hg and Au, respectively. For R = 20 nm, L = 56 nm, and R1 = 42 nm, and taking Vm,Au = 10.21 cm3/mol and Vm,Hg = 14.81 cm3/ mol,35 the molar ratio of the gold amalgam nanolloys and average content of Hg were estimated to be XHg/XAu = 1.365:1 and 57.7 atom %, respectively. In order to confirm the mechanism of amalgamation on a single Au nanorod, the distribution of local electric fields surrounding a single Au nanorod was simulated by the discrete dipole approximation (DDA) simulation (Figure S4).38 The region of the strongest local electric fields was located at the maximum curvature points around the hemispheres cap, and then the tip progressed toward the center of the gold nanorod. This supports the hypothesis that amalgamation leads to a transformation of shape from a nanorod into a nanosphere.39 Scattering Spectra of Single Nanoalloys. A typical scattering spectrum of a single gold nanorod at an open circuit potential, DDA, and the UV−vis absorption spectrum of Au nanorods are shown in Figure S4. The typical scattering spectrum of a gold nanorod fit well (solid line) to a single Lorentzian function H(ω) = AΓ/[(ω − ω0)2 + Γ2/4], as expected for a single-particle scattering spectrum. The values of the LSPR energy (ω0) and the full width at half-maximum (fwhm, Γ) could be obtained from the fitting results. For characterizing the amalgamation, real-time monitoring of changes in optical properties of single gold nanorods during the deposition of Hg was applied, based on single-nanoparticle dark-field spectroelectrochemistry (SN-DFS). Figure 5A shows a time-dependent scattering spectrum of a single Au nanorod F


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region of gold by the adsorption of Hg.47 Plasmon damping of a single nanoparticle was caused by a lowering of the energy required to excite d-band electrons into the conduction band in Hg compared to Au.48,49 After deposition of Hg, a decrease of the standard deviation of the resonance wavelength of the nanoparticles from 25 to 12 nm accompanied by an increase of the standard deviation of the particles line width fwhm from 29.2 to 211 meV was observed (Figure 6C). A transformation of the plasmon-shape relation contributed to this change. Here, Au nanorods with a higher aspect ratio had a stronger plasmon resonance shift than those of rods with lower aspect ratios (Figure 7, bottom panel). Meanwhile, a longer plasmon

The high sensitivity of spectral parameters demonstrated that the Hg atoms doped on the interfacial region were readily stripped from the nanoparticle. Moreover, a slight spectral blueshift and a substantial narrowing of the homogeneous plasmon resonance line width of the nanoparticle were found. For the more sluggish diffusion of Hg atoms from the interior of the gold amalgam nanoalloy rather than diffusion into the nanoalloy, there was a lower rate of spectral blue-shift (ca. 2.2e-3 nm/s) than that of the spectral red-shift (ca. 4.9e-3 nm/ s). The narrowing of the scattering spectra could be attributed to the slight increase in the aspect ratio of the nanoparticle and the decrease of the effect of doped-Hg atoms on the plasmon line shape of individual Au nanorods.27,42 After Hg stripping, there was a ca. 1.1 nm blue-shift of the scattering spectrum of a single Au nanorod before Hg deposition in spite of no obvious change of the aspect ratio of the gold amalgam nanoalloy (Figure S7). According to the empirical formula ΔR = Δλmax/ 90.6, a plasmon resonance band shift results from a decrease ca 0.01 in aspect ratio of the single nanoparticles which cannot be distinguished in Figure S7.43,44 Furthermore, this demonstrated that the amalgamation at 40 μg/mL Hg2+ had only a slight influence on the plasmon properties of a single Au nanorod. The plasmon properties of the single Au nanorod were quasireversible after adsorption of mercury even at low Hg concentration within ppb levels.45 These results indicated that deposition and stripping analysis of mercury using a single Au nanorod, monitored by plasmon spectra, need to be taken into account and better understood before realization of a sensor at the single-particle level can enable quantification of mercury in the field. Plasmonic properties of single Au nanorods were examined after deposition of Hg at 150 μg/mL. Representative plasmon scattering spectra of single gold nanorods before and after deposition of Hg, and after stripping of Hg are shown in Figure 6A. The corresponding resonance wavelength and the fwhm were extracted from each particle’s spectrum (Figure S8). A plot of single nanoparticle fwhm as a function of the scattering resonance wavelength is shown in Figure 6B. Interestingly, a dramatic blue-shift of the spectrum ca. 119 nm from 705 to 586 nm and an increase of the fwhm ca. 858 meV from 186 to 1044 meV occurred after deposition of Hg. Then, a red-shift ca. 52 nm from 586 to 638 nm and a narrowing of the fwhm ca. 462 meV from 1044 to 582 meV occurred after stripping of Hg. The LSPR spectral shift with the controllability by doping and stripping of Hg atoms indicated that it could be used in electrical nanodevices, for instance in spatial light modulators, modern optical communication, and display systems and photovoltaics.46 Most importantly, a blue-shift (ca. 90 nm) accompanied by an increase of the single-particle line width fwhm (ca. 422 meV) after deposition of Hg was observed from region a to region b (Figure 6B). A dramatic scattering spectral shift contributed to a decrease in the aspect ratio of the single Au nanorods (Figure 3C, D) while a contribution from amalgamation cannot be precluded. Under 150 μg/mL Hg2+, the number of Hg atoms electrodeposited at a single gold nanorod and average content of Hg were ca. 1.14 × 1011 (0.189 pmol) and 57.7 atom % (Figure S3), respectively, which is more than that reported by Schopf et al.22 The higher concentration of Hg2+ and a long deposition time adopted here might contribute to the higher Hg content in the gold amalgam nanoalloys. The work function of mercury is lower than that of Au (ca. 0.3 eV); thus, the observed scattering spectral shift may be related to a change in the polarization of the near surface

Figure 7. (A) Correlation plots of the single Au nanorod resonance wavelength shift (λ0 − λ1) after Hg deposition (orange dots) and (λ1 − λ2) after Hg stripping (purple dots). λ0: the resonance wavelength of a single nanorod before Hg deposition. λ1: the resonance wavelength of a single nanorod after deposition of Hg. λ2: the resonance wavelength of a single nanorod after Hg stripping. (B) Histograms give the distributions of the single nanopaticles’ change of plasmoic properties with a Gaussian distribution shape after Hg deposition (red color) and stripping (blue color).

resonance wavelength was found from larger aspect ratio single Au nanorods (Figure S9). Therefore, the amalgamation process homogenized the particles’ aspect ratio, leading to a narrower distribution of plasmon resonances (Figure 3B, C). However, due to the differentiation of the morphology present in the original Au nanorods, the Hg content in the gold amalgam nanoalloys might be heterogeneous. Hence, the fwhm variation was more distinct after Hg deposition. Systematic broadening of the plasmon resonance with amalgamation is a property of the combination of Hg and Au dielectric functions. Halas reported that the width of the scattering resonance increases with the percentage of Co increases.41 No plasmonic focusing effect was found in the gold amalgam nanoalloys rather than the Au@Ag bimetallic structures.50 Intriguingly, a red-shift (ca. 28 nm) accompanied by a decrease of the single-particle line width fwhm (ca. 128 meV) was found after stripping of Hg (region b versus region c Figure 6B, respectively). The scattering spectral red-shift was due to the increase of the relative content of Au in the interfacial region of the gold amalgam nanoalloys rather than a contribution from a change in aspect ratio.17 After stripping, no obvious change on aspect ratio of the nanoparticles was observed (Figure S7). In addition, the resonance wavelength shift of single nanoalloys after stripping indicated a similar degree which could preclude the marked change of the aspect ratio of the gold amalgam nanoalloys (Figure 7, upper panel). After deposition of Hg, an increase of the standard G


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shown in Figure S11. The distinct scattering spectral blue-shift caused by electrodeposition of Hg2+ demonstrates that Au nanorods exhibit a significant specificity to Hg0, the results were further confirmed by the scattering spectra shown in Figure S11A. Hg0 possess lower cohesive energy of 0.67 eV compared to other metals, such as Mn, Cu, Sn, Ni, Cd, Pb, Zn, Bi, Li, Co and Fe, holding cohesive energies of 2.92, 3.49, 3.14, 4.44, 1.16, 2.03, 1.35, 2.18, 1.63, 4.39, and 4.28 eV/atom, respectively.14,56 In addition, the Hg2+ (E0(Hg2+/Hg) = 0.85 V) poses higher standard electrode potential than other metals (Table S2).57 Hence, this could account for the higher specificity of Hg0 to the Au nanorod. Furthermore, the higher specificity of Hg0 to the Au nanorod lead to the formation of gold amalgam nanoalloys. Amalgamation was confirmed by the time-dependent scattering spectra and morphology changes of Au nanorods. Huang and co-worker had demonstrated that a nanostructure’s size, shape, electron density and dielectric (electronic) properties as well as its dielectric environment change can have influence on LSPR’s properties.58 Thus, the LSPR properties of Au nanorods could be influenced significantly by deposition of foreign metals.48,59−61 However, no significant scattering spectral changes could be found when Au nanorods were exposed to growth solutions containing various metals cations (Figure S11). The presence of other metals could not induce noteworthy scattering spectral changes of a single Au nanorod. This suggests that single Au nanorods might find utility as sensitive and selective Hg2+ sensors.

deviation in the resonance wavelength of the nanoparticles from 12 to 15 nm accompanied by an increase of the standard deviation in the particles line width fwhm from 211 to 126 meV was observed in Figure 6C. The single-nanoparticle line width and the relative standard deviation decreased after stripping Hg. The line width and relative standard deviation decreased through electrochemical rather than the chemical focusing during nanocrystal growth.18,51 Such electrochemical plasmonic focusing (EPF) is a novel characteristic observed during the stripping of the gold amalgam nanoalloys. EPF could reduce the fwhm of single gold amalgam nanoalloys by about 21.5%. Therefore, this strategy has the potential to produce plasmonic nanoparticles with high quality factors and tunable plasmon resonance wavelengths. Another important parameter follows immediately from the results in Figure S10, the quality factor of the resonance (Q = ω0/Γ). It has been reported that the quality factor is the enhancement of the oscillation amplitude of a driven oscillating system with respect to the driving amplitude, in the case of particle plasmons, and it is proportional to the local-field enhancement; high quality factors indicated large local-field enhancements and low values mean small local-field enhancement.52,53 In our results, the quality factor decreased dramatically from 10.6 to 3.9 after amalgamation, followed by a slight increase to 4.5 after stripping of Hg in Figure S10A, D. With amalgamation, the aspect ratio of the single gold nanorod decreased which resulted in a blue-shift of scattered light. However, an increase in fwhm of single nanoparticle after amalgamation was observed. The increase of plasmon damping was due to the deposition of Hg on the interface of Au nanorod.41 Thus, higher plasmon damping means a short plasmon lifetime (T2) and a lower local-field enhancement. After stripping of Hg, the slight red-shift of scattered light accompanied lower plasmon damping, contributed to the increased quality factor. Plasmon dephasing is characterized by the time constant T2, and it is associated with the inelastic population time constant T1, according to the equation T2−1 = T1−1/2 + T*−1; herein, T* described the elastic dephasing process.54 The simple equation T1 = T2/2 for a gold nanorod could be obtained by neglecting the pure dephasing process that did not contribute to the overall line width.52 In our results, T2 decreased from 8.1 to 2.6 fs after amalgamation, and then increased to 3.2 fs after stripping of Hg. Meanwhile, T1 decreased from 4.1 to 1.3 fs after amalgamation with a slight increase to 1.6 fs after Hg stripping. Since the volume of a single Au nanorod increased after Hg deposition, it effectively increased the radiation and interband damping of a single gold amalgam nanoalloy, which contributed to a decrease of T1 and T2.55 Furthermore, the suppression of the damping in a single nanoparticle, after stripping of Hg, could give rise to the increase in T1 and T2. The amalgamation lead to the ca. 68% decrease of plasmon lifetime compared to the Au nanorods; the stripping of Hg produced ca. 23% increase of plasmon lifetime of nanoalloys. This makes the prepared nanostructures promising candidates for application in nanosensing, optoelectronic conversion, and materials science. Affinity of Hg0 to Single Au Nanorod. To demonstrate the specificity of a single Au nanorod to Hg2+ over other cations, the effect of other metal ions on the scattering spectra of single Au nanorods was studied, at same concentration and under the same growth conditions. After the deposition, none of the cations probed (Mn2+, Cu2+, Sn2+, Ni2+, Cd2+, Pb2+, Zn2+, Bi3+, Li+, Co2+ and Fe3+) caused a significant blue-shift, as

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CONCLUSIONS

In summary, we have explored an electrochemical protocol to fabricate gold amalgam nanoalloys and manipulated the plasmonic properties and geometry of the nanoalloys through amalgamation by direct electrochemical deposition of Hg, rather than chemical reduction of Hg2+ by reducing reagents. According to the changes in resonance wavelength and line width of the resonance peak, the proposed growth and stripping mechanism of single gold amalgam nanoalloys involved the following phases: first, Hg atoms were deposited rapidly on the interfacial region of the Au nanorod; second, Hg atoms at the surface diffused sluggishly into the interior of the Au nanorod. For Hg stripping, Hg atoms were stripped from the surface quickly, and then with a slower diffusion rate compared to the deposition, Hg atoms diffused from the interior to the exterior of the nanoalloys. Under high concentration of Hg, the diffusion of Hg atoms resulted in the formation of gold amalgam nanoalloys with a homogeneous distributed sphere geometry. After Hg stripping, a dramatic redshift and a narrowing of the plasmon line width were observed. In addition, lower plasmon damping, higher quality factor Q, and a longer plasmon lifetime T2 could be obtained. Thus, the present work enables us to fabricate gold amalgam nanoalloys with controllable geometry. Furthermore, the plasmonic properties of the gold amalgam nanoalloys could be tuned by applied anodic polarization. The fabricated nanoalloys have promising features for potential applications in optical electronic devices and chemical and biological sensing. Moreover, the SN-DFS technique was used to monitor realtime growth and stripping of gold amalgam nanoalloys and to increase the plasmon life and improve the quality factor of the nanoalloys. H


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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/acsami.6b01029. Detailed gold nanorod characterization and DDA simulation of single gold nanorods; scattering spectra of gold nanorod before and after mercury deposition and stripping, respectively; the specificity of Hg. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: 86-21-64252339. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Chinese National Foundation of Natural Science Research (21327807), National Science Found for Creative Research Groups (21421004), and 973 Program (2013CB733700) is gratefully acknowledged. The CASE network is thanked for networking opportunities.62 J.S.F. thanks ECUST for a Guest Professorship, the Royal Society for an Industry Fellowship, and the University of Birmingham for support.

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