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Hot Holes Assist Plasmonic Nanoelectrode Dissolution Alexander Al-Zubeidi, Benjamin S. Hoener, Sean S. E. Collins, Wenxiao Wang, Silke Regine Kirchner, Seyyed Ali Hosseini Jebeli, Anneli Joplin, Wei-Shun Chang, Stephan Link, and Christy F. Landes Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04894 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019
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Hot Holes Assist Plasmonic Nanoelectrode Dissolution Alexander Al-Zubeidi1§, Benjamin S. Hoener1§, Sean S.E. Collins1,3, Wenxiao Wang2, Silke R. Kirchner1, Seyyed Ali Hosseini Jebeli2, Anneli Joplin1, Wei-Shun Chang1¶, Stephan Link1,2,3*, Christy F. Landes1,2,3* Rice University, 1Department of Chemistry, 2Department of Electrical and Computer Engineering, 3Smalley-Curl Institute; 6100 Main Street; MS-60; Houston, TX 77005 * Corresponding authors, emails:
[email protected],
[email protected] § Both authors contributed equally to this work. ¶
Present address: Department of Chemistry and Biochemistry, University of Massachusetts
Dartmouth, 285 Old Westport Rd, North Dartmouth, MA 02747
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Abstract Strong light absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. Here, we demonstrate the role of photogenerated hot holes in oxidative dissolution of individual gold nanorods with millisecond time resolution while tuning charge carrier density and photon energy using snapshot hyperspectral imaging. We show that light induced hot charge carriers enhance the rate of gold oxidation and subsequent electrodissolution. Importantly, we distinguish how hot holes generated from interband transitions vs. hot holes around the Fermi level contribute to photooxidative dissolution. The results provide new insights into hot hole driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of non-equilibrium processes on innately heterogeneous nanoparticle supports.
Keywords: nanoantenna, photocatalysis, electrodissolution, hot carrier dynamics, interband transitions, snapshot hyperspectral imaging
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Plasmonic nanoparticles are excellent light absorbers, sparking great interest in their use as light harvesting antennas, driving processes such as carbon dioxide reduction, water splitting, and ethylene epoxidation.1-9 After photoexcitation of the plasmon, an athermal distribution of electron-hole pairs is created with a maximum energy separation corresponding to the photon energy.10-12 Energy redistribution via electron-electron scattering results in a Fermi-Dirac distribution of excited charge carriers around the Fermi level.10-11 Plasmonic photocatalysis bydesign is thus achievable if the optical excitation energies and initial carrier energies can be tuned relative to molecular orbital energies of adsorbed reactants through adjusting the nanoparticle geometry and applied electrochemical potentials, respectively.12-19 Such studies have been performed on nanoparticle ensembles by analyzing product yields or electrochemical currents.1-2, 4-8, 20-22 A mechanistic understanding of plasmonic photocatalysis requires a single particle approach, however, to address size and shape heterogeneity and to differentiate between single particles and aggregates.23-24 Here we show how highly energetic hot holes enhance a multi-step adsorptive redox reaction at gold nanoparticle surfaces and how reactivity rates among the nanoparticle distribution vary by as much as 100%. Furthermore we establish that hot holes from interband transitons are more efficient at photoassisted electrodissolution than warm holes in the sp-band. In Fig.1, photooxidative dissolution of single gold nanorods in a sodium chloride electrolyte is demonstrated. When anodic potentials are applied, soluble gold chloride complexes are formed,25-29 and as they diffuse into the electrolyte, the nanorod volume is decreased. The non-equilibrium dissolution of individual nanorods is quantified through changes in the localized surface plasmon resonance, which depends on nanorod size and shape.30-31 Due to the importance of high time resolution for monitoring non-equilibrium electrochemical processes,32 3 ACS Paragon Plus Environment
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we use our recently developed “snapshot hyperspectral imaging” method (Fig. 1a,b) to track heterogeneous size and shape changes during photooxidative dissolution in real time.27, 30-31, 33-34 Single nanorod spectra are compared before and after applying oxidative potentials with and without white light laser excitation for a 500 mM aqueous NaCl electrolyte (Fig. 1c,d). As shown in Fig. 1e, after three cycles in the dark, the scattering intensity decreases slightly, however after only one cycle under white light laser illumination the scattering intensity decreased by 90% for the same nanorod (Fig. S3). Correlated SEM confirms that photoexcited nanorods are almost 50% smaller and exhibit a greater size polydispersity than nanorods 4
Figure 1. Dissolution of gold nanorods under white light laser illumination. a, Prism total internal reflection of white light laser excitation of nanorods within an electrochemical cell. Scattered light is collected by the objective. b, Single nanoparticle scattering spectra were obtained from images of 0th and 1st order diffracted light. c, Electrochemical dissolution was compared with and without laser light excitation. d, Working electrode potential, E, was cycled from 0 V to 0.53 V without and with laser excitation (19 kW cm2, Fig. S1). Spectra were acquired before and after voltage cycling as indicated by the stars. e, Example scattering spectra for a nanorod before applying the potential, after three cycles in the dark, and after one cycle under illumination. f, Particle dimensions from SEM obtained for nanorods as prepared (control), inside the illumination area, and 1 mm away. Color coded insets show examples. The yellow star indicates the same nanorod as in e. Scale bars are 50 nm. All potentials were converted to standard hydrogen electrode (Fig. S2). ACS Paragon Plus Environment
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without photoexcitation and control samples without electrochemical potential and photoexcitation (Fig. 1f). Photoexcitation dramatically enhances both the extent and rate of oxidative dissolution, while the electrodissolution onset potential is unaffected (Fig. 2). The potential dependence of gold nanorod dissolution is obtained by cycling between 0 V and an increasing maximum potential, Emax, for the same nanorods for three cycles with a step Figure 2. Light dependent electrodissolution onset potential and rate. a, Normalized average scattering intensity and standard spectra (Fig. 2a). The dissolution onset deviation as a function of increasing potential is 0.48 V, and is unchanged by maximum cycle potential compares the onset potential of electrodissolution with and photoexcitation. The onset potential refers to without white light laser illumination for 27 and 12 nanorods, respectively. The inset the lowest potential at which an irreversible shows the normalized change in intensity between points. The gray line indicates the decrease in scattering intensity occurred, observed dissolution onset potential, above which the scattering intensity irreversibly indicating dissolution. However, the effect of decreased above the noise threshold. b, Rate of intensity decrease in the dark and under 7.3 light on the extent of dissolution at the onset kW cm-2 white light laser illumination for 27 nanorods. The inset shows a representative potential is apparent when comparing the example for the linear regressions applied to relative change in normalized scattering one nanorod. See Fig. S4 for all single particle trajectories. intensity with and without white light laser excitation (Fig. 2a inset). A similar behavior was time of 60 s while acquiring scattering
observed for gold nanospheres, which also showed dissolution well below the bulk oxidation 5 ACS Paragon Plus Environment
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potential and photoenhancement of oxidation (see Fig. S5). These results are consistent with studies by Zamborini and coworkers who showed that the dissolution onset potential is lowered significantly for gold and silver nanostructures compared to bulk materials.35 Photoexcitation increases the electrodissolution rate of gold nanorods by an order of magnitude (Fig. 2b). The change in 0th order scattering intensity over time is compared with and without white light laser excitation at 0.5 V, which is slightly above our experimentally determined dissolution onset potential. The rate of intensity decrease is determined by fitting a linear regression to the time dependent intensity change for each single nanorod (Fig. 2b, inset). Without photoexcitation, these rates are narrowly distributed around 0.03 ± 0.06 ks-1 compared to a broad distribution under illumination centered around 0.96 ± 0.17 ks-1. This broader distribution illustrates the heterogeneity in the plasmon mediated electrodissolution process. The fastest dissolving nanorods do so twice as quickly as the slowest. Our proposed mechanism for photoassisted electrodissolution is the injection of hot holes into the HOMO of adsorbed chloride ions, accelerating the formation of soluble Au(I) complexes. Bulk studies have shown that gold dissolution occurs by adsorption of chloride ions, followed by oxidation of Au0 to Au(I), according to Equation 1:36
(1) Dichloroaurate(I) can either desorb from the surface into the solution, or oxidize further to form tetrachloroaurate(III) (see Supporting Information for further discussion).36-39 Previous work has shown that chloride ions start to chemisorb at an anodic potential of 0.48 V (Eq. 1, step 1),30 consistent with our measured electrodissolution onset potential. At larger potentials increased
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dissolution is observed (Fig. 2a), indicating that adsorbed chloride ions are always present on the gold surface above 0.48 V and, hence, that the reaction is not limited by mass transport.40 The oxidation of Au0 to Au(I) must be the rate determining step (eq. 1, step 2), consistent with the lack of a shift in electrodissolution onset potential under photoexcitation (Fig. 2a). Instead, the onset potential shifts as a function of NaCl concentration (Fig. S6), confirming that the first, rate limiting step is chloride ion adsorption. This mechanism is furthermore supported by control experiments, which demonstrate that nanorods are stable after 1 hour of continuous excitation without applied potentials (Fig. S7). Photon energy-dependent experiments suggest a mechanism in which nonthermalized, highly energetic hot holes in the d-band drive the photoelectrodissolution process in addition to hot holes around the Fermi level. (Fig. 3). The extent of electrodissolution is compared in the dark vs. when exciting mainly the longitudinal surface plasmon resonance with 600 – 1000 nm red light or predominantly interband transitions
Figure 3. Dependence of electrodissolution on hot carrier distribution. a, Cumulative probability distributions of the final scattering intensity decrease for 69 gold nanorods in the dark, 23 nanorods under red light illumination, and 46 nanorods under green light illumination. Fig. S10 plots the individual particle time trajectories used to create these distributions. b, Energy level diagram showing dissolution in the dark and dissolution driven by photo-generated holes created by red light mainly through intraband transitions and by green light via excitation of interband transitions. Note that this cartoon presents a simplified picture as the red light used here is still above the interband transition threshold, but longitudinal plasmon excitation and decay is expected to initially create a larger number of hot holes around the Fermi level in comparison to green light excitation.
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with 450- 550 nm green light (Fig. 3a). The excitation power is adjusted to ensure similar amounts of total absorbed energy under red and green illumination based on the calculated absorption cross-section of a nanorod with average dimensions for this sample (Figs. S8 and S9). Specifically, we calculate that on average under the green light excitation conditions 10 % fewer photons and 10 % more energy is absorbed compared to red light excitation (see Supporting Information). In contrast, the data shown in Figs. 1 and 2 were acquired with the white light laser spectrum pre-filtered to allow excitation at all wavelengths between 450 and 1000 nm.41-42 As shown in the cumulative probability distributions in Fig. 3a, nanorods excited by green light show stronger photoassisted electrodissolution than those excited by red light. Green light mainly excites interband transitions, either directly or through the decay of the transverse plasmon resonance (Fig. 3b), while the red light is strongly absorbed by the longitudinal plasmon. As the resonance wavelength increases from the green region, these gold nanorod plasmons mostly decay into excited electron-hole pairs within the conduction band (Fig. 3b). Fast electron-electron scattering on the sub-picosecond time scale, however, leads to the same thermalized Fermi-Dirac distributions around the Fermi level for both excitation conditions.11 That scenario should result in the same electrodissolution independent of excitation wavelength, clearly in contradiction to our experiments. We therefore must conclude that hot holes in the dband, excited more efficiently with green light, must play a dominant role here. This conclusion is consistent with reports by Zhao et al. and Kim et al., who found that hot carriers from interband transitions drive oxidations and reductions more efficiently.43-44 At the same time, hot holes around the Fermi level, created via initial intraband excitation or decay of the d-band holes,9, 43 are expected to contribute to the photoassisted electrodissolution as well, based on the 80 mV difference in applied potentials needed to achieve complete electrodissolution under our 8 ACS Paragon Plus Environment
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experimental dark vs. light conditions (Fig. 2a) in comparison to the excitation photon energy of multiple eV.” A laser power dependence of the photoassisted electrodissolution proves that the observed process is indeed driven by hot charge carriers. We find that the dissolution rate depends linearly on laser power and hence the number of initially created hot carriers (Fig. S11).45 This result also excludes the possibility that the 10% difference in absorbed energy for the two colors alone could account for the almost two-fold enhancement in electrodissolution rate for green vs. red light. Our results are furthermore consistent with previous ensemble measurements, which concluded that hot carriers from interband transitions drive photoelectrochemical reactions more efficiently than from plasmon decay.13, 46 Local heating due to plasmon excitation was shown to increase the rate of electrochemical reactions as well.45, 47 However, in that case an exponential pump power dependence was observed.45 We can further rule out local heating as a contributor to the observed photoassisted electrodissolution by estimating the expected temperature increase. As shown in the Supporting Information, under our excitation conditions a local nanorod surface temperature increase of 2 K is expected. Furthermore, the Govorov group found that at 50 kW cm-2 a solution temperature increase of less than 3 K is found in the surroundings of nanorods of similar dimensions.48 Local temperature differences and bulk electrolyte heating hence cannot explain a ten-fold increase in dissolution rate based on bulk temperature-dependent studies.49-50 Additional control experiment ruling out heating and the role of surfactants are discussed in the supporting information (sections 12 and 13). In conclusion, we demonstrated the role of hot holes in the photoassisted electrodissolution of single gold nanorods. The onset of nanorod dissolution occurs at similar 9 ACS Paragon Plus Environment
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potentials with and without laser excitation, but the rate of electrodissolution is enhanced by photoexcitation. This reaction is best described by a plasmon generated hot hole driven mechanism in which the applied electrochemical potential is required to drive chloride ion adsorption. Wavelength dependent studies show that hot holes in the d-band drive electrodissolution more efficiently than hot holes around the Fermi level. These results are significant for the development of plasmonic photocatalysts because they establish that hot dband holes are able to participate in chemical reactions despite their ultrashort lifetimes. In particular, our findings illustrate that holes in the d-band are able to drive electrochemical oxidation reactions despite their ultrafast decay, and importantly overcome reaction barriers that are otherwise too high for thermalized holes around the Fermi level. Consequently, the use of hot d-band holes expands the utility of plasmonic photooxidation catalysts.
Associated Content Calibration of the Pt pseudo reference electrode, the white light laser excitation spectrum, 0th order change in intensity with and without white light laser excitation, excitation wavelength dependent dissolution, dissolution of gold nanospheres, calculation of change in temperature at the gold nanorod surface, calculations of relative absorbed photons and energies under red and green light illumination, and the effect of laser light heating the electrolyte solution can be found in the Supporting Information. Author Contributions A.A., B.H., S.S.E.C. and S.R.K. designed the experiments. A.A., B.H. and W.W. analyzed data. S.A.H.J. carried out simulations. S.R.K. and W.S.C. designed the instrument. A. J. contributed to 10 ACS Paragon Plus Environment
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data visualization. A.A., B.H., S.S.E.C., W.S.C., S.L., and C.F.L. interpreted the results and wrote the manuscript with contributions from the other authors. S.L. and C.F.L. conceived and supervised the project. Acknowledgements C.F.L. and S.L acknowledge funding from the DOE BES (DE-SC0016534) and the Robert A. Welch Foundation [Grant C-1787 to C.F.L, Grant C-1664 to S.L.]. S.S.E.C. acknowledges support from the Smalley-Curl Institute at Rice University through a Carl & Lillian Illig Fellowship. We thank Professor Peter Nordlander for stimulating discussions. Competing Interests The authors declare no conflict of interest.
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Methods Electrochemical Snapshot Hyperspectral Microscopy. Multiple gold nanorod spectra were measured simultaneously using a snapshot hyperspectral technique recently developed by us (Fig. 1a).51 Gold nanorods were excited with a white light laser (WhiteLase SC480, Fianium) using prism total internal reflection to create an evanescent wave at the indium tin-oxide (ITO) working electrode interface. A 1000 nm shortpass dichroic mirror (DMSP1000, ThorLabs) and a 1 cm path length cuvette filled with water were added to the beam path to prevent heating of water within the electrochemical cell. Light scattered from gold nanorods was collected by a 40x air-spaced objective (Zeiss) and directed through a transmission grating (70 grooves/mm, Edmund Optics) to a CMOS camera (ORCA-flash 4.0 V2, Hamamatsu). The camera was calibrated with halogen gas lamps so that wavelength information could be obtained from the relationship between the 0th order and the 1st order position on the camera (Fig. 1a, left). The working (top ITO coverslip), counter (bottom ITO coverslip), and pseudoreference electrode (polymer coated Pt wire) were connected to a potentiostat (630D, CHI) to control the working electrode potential and thus the nanoparticle potential. For wavelength dependent excitation measurements, the white light laser was split using a 550 nm short pass dichroic filter (Edmund Optics) for green light excitation and a 600 nm long pass dichroic filter (Edmund Optics) in combination with neutral density filters (ND501B and ND506B, Thor Labs). When the full white light spectrum or red light were used, videos during laser excitation could be taken to track the process. The power densities used here were 13 kW cm-2 and hence about 1000 times higher than in typical photocatalytic and photoelectrolytic studies.21 he relatively high power densities were needed to achieve detectable photodissolution since a significant extent of reaction is needed to
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optically detect a size change. When we lower the light intensity by a factor of 100 we still see the same behavior (Fig. S11). Working Electrode Preparation. Two holes were cut into ITO (~56 Ω/sq., Evaporated Coatings Inc.) coated glass coverslips using a CO2 laser. A pattern was etched into the ITO surface using a focused ion bean for experiments requiring scanning electron microscopy (SEM, Helios NanoLab 660, FEI) to be correlated with single gold nanorod scattering spectroscopy. ITO coverslips were O2 plasma treated before drop casting 76 nm x 38 nm gold nanorods, synthesized with a bi-surfactant seeded growth method,52 onto the ITO surface. Characterization of the gold nanorods can be found in our previous work.53 The ITO coverslips were O2 plasma treated again to remove ligands from the gold nanorod surface. These ITO coverslips were used as the working electrode. Electrochemical Cell Preparation. Two adhesive spacers (Grace Bio-Labs) were laser cut with a custom cell pattern matching the distance between the two holes cut in the ITO coverslip. A polymer coated Pt wire pseudoreference electrode (A-M Systems) was placed near the ITO working electrode on top of the first adhesive spacer. A blank ITO coverslip counter electrode was placed on top of a second adhesive layer, and a custom aluminum clamp with a set screw hole was used to secure the cell to an aluminum plate with the set screw holes matching the holes in the ITO coverslip. Silcon Med-X tubing (U.S. Plastic) was threaded through a hollow set screw and screwed into the clamp to form a seal over the hole in the ITO coverslip. The tubing was connected to a vial of electrolyte solution through one hole and a syringe through the other hole so that electrolyte solution could be pulled through the cell. The Pt pseudoreference electrode was calibrated using the ferrocyanide redox couple (Fig. S1). Metal nanoparticles form
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Ohmic contacts with semiconductors.54 Therefore, it is assumed that the potential of the nanoparticle is similar to that of the ITO substrate and behaves linearly with potential changes.
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21. Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J., Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H-2 on Au. Nano Lett. 2013, 13 (1), 240-247. 22. Christopher, P.; Xin, H.; Linic, S., Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures. Nat. Chem. 2011, 3, 467. 23. Qian, J.; Shen, M.; Zhou, S.; Lee, C.-T.; Zhao, M.; Lyu, Z.; Hood, Z. D.; Vara, M.; Gilroy, K. D.; Wang, K.; Xia, Y., Synthesis of Pt nanocrystals with different shapes using the same protocol to optimize their catalytic activity toward oxygen reduction. Mater. Today 2018, 21 (8), 834-844. 24. Byers, C. P.; Hoener, B. S.; Chang, W. S.; Yorulmaz, M.; Link, S.; Landes, C. F., Single-Particle Spectroscopy Reveals Heterogeneity in Electrochemical Tuning of the Localized Surface Plasmon. J. Phys. Chem. B 2014, 118 (49), 14047-14055. 25. Sannomiya, T.; Dermutz, H.; Hafner, C.; Voros, J.; Dahlin, A. B., Electrochemistry on a Localized Surface Plasmon Resonance Sensor. Langmuir 2010, 26 (10), 7619-7626. 26. MacKenzie, R.; Fraschina, C.; Dielacher, B.; Sannomiya, T.; Dahlin, A. B.; Voros, J., Simultaneous electrical and plasmonic monitoring of potential induced ion adsorption on metal nanowire arrays. Nanoscale 2013, 5 (11), 4966-4975. 27. Jing, C.; Rawson, F. J.; Zhou, H.; Shi, X.; Li, W. H.; Li, D. W.; Long, Y. T., New Insights into Electrocatalysis Based on Plasmon Resonance for the Real-Time Monitoring of Catalytic Events on Single Gold Nanorods. Anal. Chem. 2014, 86 (11), 5513-5518. 28. Dondapati, S. K.; Ludemann, M.; Muller, R.; Schwieger, S.; Schwemer, A.; Handel, B.; Kwiatkowski, D.; Djiango, M.; Runge, E.; Klar, T. A., Voltage-Induced Adsorbate Damping of Single Gold Nanorod Plasmons in Aqueous Solution. Nano Lett. 2012, 12 (3), 1247-1252. 29. Zhumaev, U.; Rudnev, A. V.; Li, J.-F.; Kuzume, A.; Vu, T.-H.; Wandlowski, T., Electro-oxidation of Au(111) in contact with aqueous electrolytes: New insight from in situ vibration spectroscopy. Electrochim. Acta 2013, 112, 853-863. 30. Hoener, B. S.; Byers, C. P.; Heiderscheit, T. S.; Indrasekara, A. S. D.; Hoggard, A.; Chang, W. S.; Link, S.; Landes, C. F., Spectroelectrochemistry of Halide Anion Adsorption and Dissolution of Single Gold Nanorods. J. Phys. Chem. C 2016, 120 (37), 20604-20612. 31. Oikawa, S.; Minamimoto, H.; Li, X. W.; Murakoshi, K., Nanoscale control of plasmon-active metal nanodimer structures via electrochemical metal dissolution reaction. Nanotechnology 2018, 29 (4). 32. Zong, C.; Chen, C.-J.; Zhang, M.; Wu, D.-Y.; Ren, B., Transient Electrochemical Surface-Enhanced Raman Spectroscopy: A Millisecond Time-Resolved Study of an Electrochemical Redox Process. J. Am. Chem. Soc. 2015, 137 (36), 11768-11774. 33. Minamimoto, H.; Oikawa, S.; Hayashi, T.; Shibazaki, A.; Li, X.; Murakoshi, K., Electrochemical Fine Tuning of the Plasmonic Properties of Au Lattice Structures. J. Phys. Chem. C 2018, 122 (25), 1416214167. 34. Thambi, V.; Kar, A.; Ghosh, P.; Khatua, S., Light-Controlled in Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3. J. Phys. Chem. C 2018. 35. Masitas, R. A.; Zamborini, F. P., Oxidation of Highly Unstable
