Mechanistic Understanding Of Plasmon-Enhanced Electrochemistry
SUPPORTING INFORMATIONMechanistic Understanding of Plasmon-EnhancedElectrochemistryAndrew J. Wilson†, Varun Mohan‡, and Prashant K. Jain*†# §†Department of Chemistry, ‡Department of Materials Science and Engineering, #MaterialsResearch Laboratory, Department of Physics, and §Beckman Institute of Advanced Science andTechnology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States*Corresponding author E-mail: jain@illinois.eduS1
Figure S1. Characterization of citrate-capped Au nanoparticles (NPs) used for the preparation ofthe electrocatalysts. (A) A representative transmission electron microscopy (TEM) image of AuNPs. A few drops of a Au NP colloid were drop-cast onto a Formvar C film on a 200-mesh CuTEM grid and allowed to dry. TEM images were acquired using a Hitachi H-9500 instrument at300 keV. (B) Histogram showing the distribution of diameters of 100 Au NPs estimated frommultiple TEM images. The distribution (fit to a normalized Gaussian function shown by the solidline) shows a mean diameter, μ, of 11.3 nm and a standard deviation, σ, of 0.9 nm. (C) Absorbancespectrum of an aqueous colloid of citrate-capped Au NPs showing a localized surface plasmonresonance (LSPR) band centered at 520 nm consistent with the measured diameter of the NPs.S2
Figure S2. Functionalization of a glassy carbon electrode (GCE) with Au NPs. Step 1:ethylenediamine dissolved in acetonitrile (MeCN) is grafted by electro-oxidation onto a GCE. Step2: negatively charged, citrate-capped (cit.) Au NPs from an aqueous colloid electrostatically attachto the terminal amine functional groups on the modified GCE.Ethylenediamine was required to immobilize the Au NPs on the GCE and prevent theirdelamination from the GCE during electrochemical measurements. Unfortunately, without theethylenediamine linker, photoelectrochemical measurements were unsuccessful because the AuNPs detached from the GCE and diffused into the electrolyte. All of our studies were conductedwith Au NP-coated GCEs prepared by this functionalization procedure and conditioned bypotential cycling under light excitation. As shown in Figure S7, the electrodes conditioned in thismanner exhibit a stable photoelectrochemical performance with no further change in the activity.From this stable photoelectrochemical performance, we can conclude that the ethylenediamineeither does not undergo electrochemical degradation during photoelectrochemical measurements,or, if such degradation does occur, it does not contribute to the measured electrochemical activity.Second, if ethylenediamine degradation were occurring then the Au NPs would no longer staylinked with the surface of the GCE and would have delaminated. We did not observe such an effectduring photoelectrochemical measurements on stabilized electrodes.S3
Figure S3. Electrochemical characterization of Au NP-coated GCE preparation. (A) Cyclicvoltammograms (CVs) showing electro-grafting of ethylenediamine from a MeCN solution ontoa bare GCE. The oxidation current decreases over successive potential cycles (indicated by thearrow) signifying an increase in the ethylenediamine coverage on the GCE to the point wheresaturation is reached. (B) Complex-plane plot of the impedance of a GCE (black curve), anethylenediamine-functionalized GCE (GCE/en, gray curve), and a Au NP-coated GCE(GCE/en/AuNP, orange curve). For the impedance measurement, a 10 mV rms ac potential wasapplied to the electrodes, which were immersed in a 0.05 M H2SO4 solution and held at a dc biasof -0.5 V vs. Ag/AgCl. The impedance was measured as the ac frequency was scanned from 200kHz to 1 Hz. (C) Magnified version of the plot in panel (B).S4
Figure S4. Optical characterization of Au NP-coated GCEs by photoluminescence (PL) emissionspectroscopy. PL emission spectrum of a representative Au NP-coated GCE (GCE/en/AuNP,orange curve) and a representative bare GCE (black curve) at an excitation wavelength (λexc) of(A) 457.9 nm, (B) 488.0 nm, and (C) 514.5 nm. The Au NP-coated GCE shows broad visiblerange PL emission centered around 530 nm. This PL emission is known to originate from radiativedecay of electron-hole pairs generated by the excitation of the Au NPs.S1–S3 Superposed on thebroad PL spectrum, sharp D and G Raman scattering modes originating from the glassy carbon areobserved. The bare GCE does not show such a PL emission, only the Raman scattering modes areobserved. PL emission spectra were measured by focusing a 10 mW laser line onto a small area ofeach electrode using a 10 microscope objective. The resulting light emission from this area wascollected with the same objective, filtered to remove the excitation wavelength, dispersed with a300 g/mm grating, and collected with a charge coupled device (CCD) camera using an integrationtime of 5 s and 10 accumulations.S5
Figure S5. Characterization of the structure and composition of a Au NP-coated GCE afterstabilization by potential cycling under light irradiation. (A) A representative scanning electronmicroscopy (SEM) image of a Au NP-coated GCE. The contrast of the image was adjusted toallow easy visualization of the Au NPs; the original acquired image is available upon request. TheGCE is found to be coated with single Au NPs along with some aggregates or clusters of NPs. Atthis sparse coverage, we do not expect any bottlenecks in electrolyte diffusion.S4,S5 (B)Representative line profile (along the red line shown in panel A) shows features corresponding tosingle NPs coated on the GCE. (C) An energy-dispersive X-ray spectrum from a representativearea of the sample shows the Au composition of the features observed in the SEM image. Au M(2.1 keV) and Lα (9.7 keV) peaks (labeled) appear in the spectrum, but no peaks associated withPt are observed.For sample preparation for SEM and energy-dispersive X-ray spectroscopy, a Au NP-coated GCEwas subject to potential cycling using a Pt mesh counter electrode as per the procedure describedin Figure S7. Following stabilization, the Au NP-coated GCE was washed with deionized waterand dried with dinitrogen. SEM images were acquired using a JEOL 7000F field emission SEMoperated in secondary-electron mode. The accelerating voltage was 15 kV for imaging and 20 kVfor energy-dispersive X-ray spectroscopy. A Thermo Electron EDS detector was used for energydispersive X-ray spectroscopy.The strong electromagnetic fields within the hot spots formed by the NP aggregates can be sitesfor enhanced electron-hole pair formation and enhanced electrochemical HER under lightexcitation. However, these aggregates are expected to have significantly redder LSPRs due tostrong plasmon coupling; these hot spots are therefore unlikely to be resonantly excited by the457.9, 488, or 514.5 nm laser employed in our studies.S6
Figure S6. Effect of light irradiation on a Au NP-coated GCE and its components. (A) Hydrogenevolution reaction (HER) polarization curves for a bare GCE (black curves) and anethylenediamine-functionalized GCE (GCE/en, gray curves) in 0.05 M H2SO4. Under irradiationof 488 nm laser light of an intensity of 2.55 W/cm2, the total current density measured from GCEs(black and gray dashed curves) increases relative to dark conditions (black and gray solid curves),signifying some light absorption and photocurrent generation in the GCEs.S6,S7 Under both darkand light irradiation, the GCE/en shows a higher current density compared to the bare GCE, whichis likely due to the terminal amines on the former serving as proton shuttles. (B) HER polarizationcurves for a Au NP-coated GCE (Au/en/GCE, orange curves) in 0.05 M H2SO4 overlaid with thepolarization curves from (A), showing that the current densities measured from GCEs without AuNPs are near-negligible compared to the current density of a Au NP-coated GCE under both dark(solid curves) and light irradiation (dashed curves) conditions. Moreover, the comparison of thelight-excitation response of the Au NP-coated GCE with that of the non-Au-NP-bearing GCEsshows that the small light absorption of GCE contributes negligibly to the observed enhancementin electrochemical HER activity of Au NP-coated GCEs. The polarization curves were obtainedby linear sweep voltammetry (LSV) with a scan rate of 1 mV/s.Estimation of light intensity incident on Au NP-coated GCEs.The bottom surface of the GCE bearing the Au NP coating was irradiated with focused laser light.The circular surface had a diameter of 0.3 cm equivalent to an area, AGCE, of 0.071 cm2. Thediameter of the focused laser spot was 0.5 cm equivalent to a laser spot area, Alaser, of 0.196 cm2at the sample plane. As a result, the fraction of laser power incident on the GCE was determinedto be AGCE / Alaser 0.36. The measured laser power, Plaser, in W was multiplied with this factorand divided by the GCE area in cm2 to give the light intensity in W/cm2 (Equation S1). Losses dueto electrochemical cell reflections were assumed to be negligible and not included in the ��𝑡𝑦 (𝑐𝑚2 ) 𝑃𝑙𝑎𝑠𝑒𝑟 0.36𝐴𝐺𝐶𝐸(S1)S7
Figure S7. Electrochemical stabilization of the Au NP-coated GCE. (A-C) Representative CVs offreshly prepared Au NP-coated GCEs irradiated with 2.55 W/cm2 of 488 nm laser light. Theworking electrode potential is cycled 25 in 0.05 M H2SO4 using either (A) a Pt mesh or (B) agraphite counter electrode. (A, B) The currents change and the onset potentials shift with repeatedcycling (indicated by the arrows) until the current and onset potential stabilize. With the Pt counterelectrode, the onset potential shifts to more positive values and the currents increase. This effectis due to the deposition of Pt onto the working electrode. With a graphite counter electrode, suchan effect is not observed; instead, only a small decrease in the current and a small shift in the onsetpotential to negative values is observed. (C) A Au NP-coated GCE potential is cycled 10 in darkconditions (black curves) followed by 10 under light irradiation with 2.55 W/cm2 of 488 nm laserlight (blue curves) in 0.05 M H2SO4 using a Pt mesh counter electrode. The CVs do not showappreciable change upon cycling under dark conditions, but they shift under light irradiation,which suggests plasmon-excitation-induced deposition of Pt onto the Au NPs. Scan rate is 5 mV/sfor all CVs.Table S1. Au NP electrocatalyst metrics (onset overpotential η, Tafel slope, and exchange currentdensity j0) for the HER when the electrode is irradiated with 2.55 W/cm2 of 488 nm laser light atthe start of electrode stabilization compared with those measured post-stabilization.Initial (1st cycle)Final (post 25th cycle)η (mV)191.2 12.846.3 3.0Tafel slope (mV/dec)105.5 8.376.0 4.0j0 (A/cm2)5.5 x 10-6 3.5 x 10-66.3 x 10-5 3.5 x 10-5A Au NP-coated GCE immersed in 0.05 M H2SO4 is irradiated with 2.55 W/cm2 of 488 nm laserlight during potential cycling. A representative example is shown in Figure S7A. Electrocatalystchanges observed over the course of repeated cycling are in line with a previous report of Au NPelectrodes subject to potential cycling in H2SO4.S8 Each tabulated value is an average ofmeasurements from four similarly prepared electrodes. The range listed represents the standarddeviation.S8
Figure S8. Reproducibility of electrochemical characteristics of a single Au NP-coated GCE in0.05 M H2SO4 after current stabilization (see Figure S7). (A, C) LSV scans of a Au NP-coatedGCE. Scan rate is 1 mV/s. (B, D) Complex-plane plots of the impedance of a Au NP-coated GCE.Impedance was measured by electrochemical impedance spectroscopy with an ac potential of 10mV rms while holding the applied dc bias at -0.5 V vs. Ag/AgCl. The ac frequency was scannedfrom 200 kHz to 1 Hz. Three trials each are shown for both electrochemical measurements withthe electrode subject to (A, B) irradiation with 0.51 W/cm2 of 488 nm laser light, or (C, D) darkconditions with the bulk solution temperature maintained at 28 C.S9
Figure S9. Example showing how the onset potential is extracted from a HER polarization curve(black curve). The third-order derivative of the current (red curve) was determined using aSavitzky-Golay filter (31-point, polynomial order 1) for smoothing the current data before andafter each differentiation step. The peak (vertical blue line) of the smoothed third-order derivativewas determined. The potential at this peak represents the change from a capacitive current to acurrent dominated by Faradaic processes and is therefore representative of the onset of hydrogenevolution.S10
Figure S10. Temperature measurements to guide dark control experiments that account forphotothermal heating. (A) Measured bulk temperature of an aqueous solution of 0.05 M H2SO4with an immersed Au NP-coated GCE irradiated with different intensities of 488 nm laser light.The temperature was measured in the bulk electrolyte ca. 2 mm from the electrode surface. Theplot of temperature (y) vs. light intensity (x) is fit to a straight line to yield the fit equation indicated.(B) The rise in the temperature, relative to dark conditions, as a function of the laser intensity. TheAu NP-coated GCE was immersed in 0.05 M H2SO4 and irradiated with 488 nm laser light and thelocal temperature was measured by a thermocouple probe placed near the GCE/en/AuNP surface(red data points) and in the bulk electrolyte ca. 2 mm away from the electrode surface (grey datapoints). Each plot (y vs. x) was fit to a straight line. The respective fit equation is listed for eachcase.S11
Figure S11. Tafel analysis of the HER on Au NP-coated GCEs. (A) Representative Tafel plots,(B) Tafel slopes, and (C) exchange current densities of the HER as a function of intensity of 488nm laser light. (D) Representative Tafel plots, (E) Tafel slopes, and (F) exchange current densitiesof the HER in dark conditions as a function of the measured bulk solution temperature. Tafel plotswere constructed from LSVs collected at a scan rate of 1 mV/s in 0.05 M H2SO4. The Tafel slopeand exchange current density were determined from each Tafel plot by straight-line fitting. Eachdata point in (B), (C), (E), and (F) is an average of measurements from four different electrodeswith the error bars representing the standard deviation. The plots (y vs. x) in (B), (C), and (E) werefit to a straight line. The fit equation is listed in each case.S12
Figure S12. Effect of the excitation-photon-energy on Au NP-coated GCEs for the HER.Excitation-photon-energy-dependence of the change in the (A) Tafel slope, b, and (B) exchangecurrent density, j0, from the dark value to that under light irradiation of an intensity of 1.02 W/cm2.Each data point of the Tafel slope and exchange current density was normalized by the absorbanceof the Au NP colloid (Figure S1C), which serves as the proxy for the total absorption cross-section,at the corresponding photon energy. Tafel slopes were measured using LSV at a scan rate of 1mV/s in 0.05 M H2SO4. Data points are the average of measurements from three differentelectrodes with the error bars representing the standard deviation. The plot (y vs. x) in (B) was fitto a straight line. The resulting fit equation is listed.S13
Figure S13. Comparison of the HER on a Au NP-coated GCE and a Pt disc ( 5 mm) electrodein dark conditions. (A) HER polarization curve from a Pt disc electrode (violet curve) in 0.05 MH2SO4. (B) HER polarization curves from a GCE (black curve), an ethylenediamine-functionalizedGCE (GCE/en, gray curve), and a Au NP-coated GCE (GCE/en/AuNP, orange curve) in 0.05 MH2SO4 overlaid with the polarization curve from panel (A). Data for the GCE, ethylenediaminefunctionalized GCE, and Au NP-coated GCE are reproduced from Figure 1A for comparison.Polarization curves were obtained by LSV at a scan rate of 1 mV/s. The onset potential, Eonset(dotted vertical line), for each polarization curve was determined using the method described inFigure S9. (C) Tafel plot of the HER on a Pt disc electrode in 0.05 M H2SO4. The Tafel slope andexchange current density extracted from the plot by a straight-line fit are listed (along with thestandard deviation from three trials).S14
Figure S14. Randles equivalent circuit (see Scheme 1) parameters determined from the complexplane electrochemical impedance of Au NP-coated GCEs. (A) Solution resistance (Rs), (B) chargetransfer resistance (Rct), and (C) double-layer capacitance (Cdl) measured for Au NP-coated GCEsin a 0.05 M H2SO4 solution as a function of the applied dc bias relative to Ag/AgCl. Impedancemeasurements were performed in dark conditions with the ac potential set at 10 mV rms and theac frequency scanned from 200 kHz to 1 Hz. Each data point is an average of measurements fromtwo different electrodes, with the error bar representing the standard deviation. The solid greenline in B is a guide to the eye, whereas the one in (C) is a straight-line fit to the plot, the fitexpression for which is listed.Scheme 1. Randles equivalent circuit with a solution resistance (Rs) element in series with acomponent consisting of a constant phase element (CPE) in parallel with charger transfer (Rct) andWarburg diffusion (W) elements.The CPE impedance (ZCPE) is given by:1𝑍𝐶𝑃𝐸 (𝑖𝜔)𝛽𝑄0(S2)where Q0 is the capacitance, ω is the frequency, and the exponent, β, is the capacitor ideality factor(1 β 0; β 1 represents an ideal capacitor whereas β 0 represents an ideal resistor). The Cdlmodeled by a CPE is given by:1𝐶𝑑𝑙 (𝐹) (𝑄𝑅𝑐𝑡 ) β𝑅𝑐𝑡(S3)S15
Figure S15. CPE exponent (β in Equation S2) as a function of (A) the light intensity underirradiation of 488 nm laser light and (B) the measured bulk solution temperature in dark conditions.Exponents were obtained by fitting potentiometric electrochemical impedance measurements fromAu NP-coated GCEs to a Randles equivalent circuit with a CPE (Scheme 1). Each data point is anaverage of measurements from four different electrodes, with the error bar representing thestandard deviation. The solid blue line in (A) is a straight-line fit to the plot, the fit expression forwhich is listed.S16
Supporting References(S1) Mooradian, A. Photoluminescence of Metals. Phys. Rev. Lett. 1969, 22, 185–187.(S2) Tcherniak, A.; Dominguez-Medina, S.; Chang, W.-S.; Swanglap, P.; Slaughter, L. S.;Landes, C. F.; Link, S. One-Photon Plasmon Luminescence and Its Application to CorrelationSpectroscopy as a Probe for Rotational and Translational Dynamics of Gold Nanorods. J.Phys. Chem. C 2011, 115, 15938–15949.(S3) Fang, Y.; Chang, W.-S.; Willingham, B.; Swanglap, P.; Dominguez-Medina, S.; Link, S.Plasmon Emission Quantum Yield of Single Gold Nanorods as a Function of Aspect Ratio.ACS Nano 2012, 6, 7177–7184.(S4) Gara, M.; Ward, K. R.; Compton, R. G. Nanomaterial Modified Electrodes: EvaluatingOxygen Reduction Catalysts. Nanoscale 2013, 5, 7304–7311.(S5) Huang, J.; Zhang, J.; Eikerling, M. H. Particle Proximity Effect in NanoparticleElectrocatalysis: Surface Charging and Electrostatic Interactions. J. Phys. Chem. C 2017,121, 4806–4815.(S6) Modestov, A. D.; Gun, J.; Lev, O. Graphite Photoelectrochemistry Study of Glassy Carbon,Carbon-Fiber and Carbon-Black Electrodes in Aqueous Electrolytes by PhotocurrentResponse. Surf. Sci. 1998, 417, 311–322.(S7) Modestov, A. D.; Gun, J.; Lev, O. Graphite Photoelectrochemistry 2. PhotoelectrochemicalStudies of Highly Oriented Pyrolitic Graphite. J. Electroanal. Chem. 1999, 476, 118–131.(S8) Wang, Y.; Sun, Y.; Liao, H.; Sun, S.; Li, S.; Ager, J. W.; Xu, Z. J. Activation Effect ofElectrochemical Cycling on Gold Nanoparticles towards the Hydrogen Evolution Reactionin Sulfuric Acid. Electrochimica Acta 2016, 209, 440–447.S17
Electrochemistry Andrew J. Wilson † , Varun Mohan ‡ , and Prashant K. Jain *†# § † Department of Chemistry, ‡ Department of Materials Science and Engineering, # Materials
INTRODUCTION Surface plasmon energy can be transferred from Au nano-particles (AuNPs) as donors to dye molecules or semi-conductors as acceptors, through so-called plasmon-induced resonance energy transfer (PIRET) or plasmon resonance energy transfer (PRET).1 7 Compared with dye molecule donors in Förster resonance energy transfer (FRET), AuNP
Fluorescence Infrared optoelectronics abstract Photonic junctions with their plasmonic components can outstandingly amplify the light-matter in-teractions, boost the localized surface plasmon resonance and tightly concentrate the optical electro-magnetic fields. The mechanisms of plasmon-enhanced metal photoluminescence (PL) and especially
doping, we blue-shift the plasmon resonance toward the two-photon absorption edge. We observed a 3-fold enhancement of emission in these samples and report two-photon action cross sections that are an order of magnitude greater than conventional fluorophores. These nanomaterials offer a novel "all-in-one" platform for engineering plasmon .
In this work, we investigate surface plasmon-driven electron transfer processes at an atomically sharp M-S nanojunction of a monolithic Al Ge nanowire (NW) heterostructure. Figure 1a,b shows the schematic and a false-color scanning electron microscopy (SEM) image of the gated plasmon transfer device (GPTD) comprising a focused grating coupler .
mechanistic strategies. Three new mechanistic strategies were introduced onto the market in the last decade. Pipeline: The 84 clinical programs for type 2 diabetes encompass 36 novel chemical drug programs with new mechanistic strategies. However
1. It uses a definition of evidence based on inferential effect, not study design. 2. It separates evidence based on mechanistic knowledge from that based on direct evidence linking the intervention to a given clinical outcome. 3. It represents the minimum sufficient set of steps for building an indirect chain of mechanistic evidence. 4.
Selective protein sensor based on Surface Plasmon Resonance and Surface Enhanced Raman Spectroscopy Eric Finot Nanomedecine 2010, Beijing- China October 2010 Plasmon nano-optics: towards novel nanotools for biomedicine Romain Quidant Passion for Knowledge, Donostia, Spa in September 2010 The numerical modelling of optical nanostructures
DEGREE COURSE: DATE OF BIRTH: FOR TEST SUPERVISORS USE ONLY: [ ] Tick here if special arrangements were made for the test. Please either include details of special provisions made for the test and the reasons for these in the space below or securely attach to the test script a letter with the details. Signature of Invigilator FOR OFFICE USE ONLY: Q1 Q2 Q3 Q4 Q5 Q6 Q7 Total. 1. For ALL .
