Hot Carriers vs. Thermal Effects: Resolving the Enhancement

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Hot Carriers vs. Thermal Effects: Resolving the Enhancement Mechanisms for Plasmon-Mediated Photoelectrochemical Reactions Yun Yu, Vignesh Sundaresan, and Katherine A. Willets J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12080 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry

Hot Carriers vs. Thermal Effects: Resolving the Enhancement Mechanisms for Plasmon-Mediated Photoelectrochemical Reactions

Yun Yu, Vignesh Sundaresan, and Katherine A. Willets*

Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States

Corresponding Author [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT Non-radiative decay of localized surface plasmons results in the production of hot charge carriers and the generation of heat, both of which can affect the efficiency of plasmon-mediated photoelectrochemical processes. Unfortunately, decoupling the impact of each effect on measured photocurrents is extremely challenging because the relative contribution of the two plasmon decay pathways cannot be controlled or easily measured.

Here we present a

methodology for exploring the roles of hot carriers and heat generation on plasmon-mediated photoelectrochemical processes using scanning electrochemical microscopy (SECM). Light is used to drive a redox reaction at a plasmonic substrate, while an ultramicroelectrode tip is positioned close to the substrate to read out both the reaction products and the mass transfer rate of the redox species. By controlling the potential at the tip and substrate electrodes, the roles of photo-induced reactions at the substrate and enhanced mass transport to the tip due to local heating can be isolated and investigated independently. We observe enhanced photo-oxidation at the substrate that is due to both plasmon-generated hot holes as well as a thermal-induced change in the equilibrium potential of the redox molecules. The concentration of the reaction products changes as a function of excitation intensity, showing a linear dependence on hot carrier effects and an exponential dependence for thermal effects, and allowing us to quantify the relative contributions of the two plasmon decay pathways to enhanced photo-oxidation. This SECM approach is suitable for probing a variety of photoactive structures used in photovoltaic and photocatalytic devices.


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The search for efficient light harvesting and energy conversion materials has resulted in significant research interest in plasmonic metal nanostructures, due to their ability to support localized surface plasmons.1-7 Plasmons occur in metal nanostructures with high free-electron mobility (Au, Ag, etc) due to the collective oscillations of surface conduction electrons driven by an applied electromagnetic field (e.g. light) at the localized surface plasmon resonance (LSPR).8 After excitation, plamons decay either radiatively through re-emitted photons9-10 or nonradiatively by generating hot electrons and hot holes.1,

11-14

The ability to harvest these hot

carriers allows charge-transfer photochemistry of adsorbed molecules at the surface of the nanostructures, including H2 dissociation,15-16 water splitting,17-19 and directed nanoparticle growth.20-21 In another non-radiative pathway, the excited electrons relax through electron– electron and electron–phonon collisions, and the photon energy converts into heat.22-25 When designing photovoltaic and photocatalytic devices that exploit plasmon-generated hot carriers, it is imperative to gain insight into the efficiency of hot carrier production and extraction relative to other plasmon-decay mechanisms, such as thermal relaxation. Unfortunately, it is not straightforward to isolate these different effects, and independently studying their roles in plasmon-mediated processes remains challenging.26 Scanning electrochemical microscopy (SECM) is evolving as a powerful tool to probe charge-transfer reactions,27-28 electrocatalysis,29-31 and photoelectrochemical processes32-36 at liquid/solid or liquid/liquid interfaces. In an SECM experiment, an ultramicroelectrode (UME) or nanoelectrode employed as a tip is brought close to a region of interest of a substrate (as in Figure 1A). The products of any electrochemical reactions occurring at the substrate surface (such as R → O + e-, Figure 1A) are monitored by the UME/nanoelectrode tip by holding the tip at a fixed potential and measuring the current associated with a complementary reaction (e.g. O +



e- → R, Figure 1A). A change in the local concentration of product molecules at the substrate is therefore reported as a change in the current measured at the tip. The tip current reaches steadystate rapidly due to the small dimension of the UME/nanoelectrode.37 The fast transport of the molecules in the gap between the tip and substrate provides a way to characterize electrochemical reactions occurring at the substrate with both high spatial and temporal resolution. Herein we demonstrate our strategy of employing a Pt UME/nanoelectrode as an SECM tip to probe and quantify the plasmon-induced effects on photoelectrochemical reactions at an illuminated plasmonic nanoparticle substrate. The gold island substrates are prepared by thermal evaporation of Au onto indium tin oxide (ITO) coated glass coverslips (see Experimental Section for details), forming closely-packed individual Au disks with diameters ranging from 20-100 nm, with an LSPR peak at ~565 nm (Figure S1). To excite plasmons in the substrate, a 532-nm laser is introduced through a 60× objective of an inverted optical microscope, illuminating an area ~68 µm in diameter. We use a well-defined, one-electron transfer, reversible redox couple, Fe(CN)63(the oxidized form, O) and Fe(CN)64- (the reduced form, R), as a model system to probe the plasmon-mediated reaction. Photo-induced oxidation/reduction of the redox probe molecules occurs only at the excited portion of the Au islands. With the tip electrode positioned within the laser spot and close to the excited Au island substrate, the change of the local concentration of O or R due to plasmon excitation can be quantitatively measured by recording the tip current when the tip is held at either a reducing or oxidizing potential relative to the standard potential of the redox couple, E0. At the same time, plasmon-mediated heating induces an increase of the local temperature at the Au. The heat transfer from the Au to the solution results in faster diffusion rates of the dissolved species and convection of the fluids, thus resulting in enhanced mass


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transfer rates of the redox molecules to the tip electrode.

Both the change in the local

concentration of O and R as well as the increased mass transfer due to local heating effects are expected to impact the current measured at the tip electrode. By controlling the potentials applied to both the substrate and tip electrodes as well as the tip-substrate distance, we obtain spatially- and temporally-resolved electrochemical data that allow us to probe the relative contributions of heating and hot carriers on plasmon-mediated photoelectrochemical processes. EXPERIMENTAL SECTION Chemicals. Potassium hexacyanoferrate (II) trihydrate (K4Fe(CN)6·3H2O, 99.95%), potassium ferricyanide (K3Fe(CN)6, 99%), potassium chloride (KCl, 99%), and sodium hydroxide (NaOH, >98%) were purchased from Sigma-Aldrich and used as received. All aqueous solutions were prepared using deionized water from the arium pro ultrapure water systems (Sartorius). Substrate Preparation and Characterization. Au nano-islands supported by indium tin oxide (ITO) or glass were prepared by thermal evaporation. ITO-coated glass coverslips (15–30 Ω, SPI Supplies) or glass coverslips (Fisher Scientific) were sonicated in acetone, ethanol, and nanopure water for 15 min in each solvent before deposition. Gold (99.95%, Ted Pella, Inc.) was thermally evaporated (Nano 36, Kurt J. Lesker) onto the cleaned ITO or glass surface at a rate of 0.5 Å/s to a final thickness of 10 nm. The deposited Au was annealed in air at 400 °C for 1 h. All Au samples were either used immediately or stored in vacuum before use. A copper wire was attached to the ITO with silver epoxy (MG Chemicals) for electrical contact. A Quanta 450 FEG scanning electron microscope (SEM) was used to characterize surface morphology of the Au film. A Lamda 35 UV/Vis spectrophotometer (PerkinElmer) was used to obtain the spectra of the Au film.



Electrochemical and SECM Setup. Pt disk UME/nanoelectrodes were prepared by pulling and heat sealing 25 µm-diameter Pt wires (Goodfellow) into borosilicate glass capillaries with a P2000 laser pipette puller (Sutter Instrument Co.) and polishing under video microscopic control, as described previously.38 The electrode radii varied from 200 nm to 10 µm and the RG (i.e., the ratio of glass sheath radius to the electrode radius) varied from 10 to 20. All electrochemical measurements were performed using a CH750E bipotentiostat (CH Instruments). A Pt wire and an Ag wire coated with AgCl were used as a counter and reference electrode respectively. The solution contains equal amount of Fe(CN)63- and Fe(CN)64- unless otherwise specified. The tip electrode was positioned ∼20 µm above the substrate using a stepper motor (Microdrive, Mad City Labs Inc.). The process is continuously monitored with an inverted optical microscope (Olympus IX-73). A piezo controller (Thorlabs) is employed for further approach of the tip towards the substrate with a step size of 40 nm. A 532-nm laser (Spectra-Physics, 532−50CDRH) was introduced through a 60× oil immersion objective (Olympus PlanoApo N) to excite the Au substrate. The laser was chopped with a controlled frequency using an optical shutter (Uniblitz Electronic). Finite Element Simulation. The finite element simulations were performed using COMSOL Multiphysics v5.2a (COMSOL) to model the concentration profile and tip current response. Simulation details are provided in the Supporting Information.

RESULTS AND DISCUSSION We first measured the current at the tip (iT) as a function of tip-substrate distance for a 7µm-radius Pt tip approaching an Au island film at open circuit (O.C.) in a solution containing 1 mM Fe(CN)63- (O) and 1 mM Fe(CN)64- (R). The tip potential (ET) is biased at 0 V vs Ag/AgCl,


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corresponding to reduction of O at the tip at the diffusion-controlled rate (Figure S2). The tip was positioned at the approximate center of the laser spot and gradually brought closer to the substrate (decrease of the axial position, z) to create a feedback loop between molecules produced at the substrate and consumed at the tip (Figure 1A). In Figure 1B, the current measured at the tip as it approaches the non-illuminated unbiased substrate is shown in black. The current at the tip increases by a factor of 2 as the tip moves from z = 10 µm to z = 0.8 µm. The higher tip current at a shorter tip-substrate distance is caused by redox cycling of the molecules upon regeneration of the O at the Au and the underlying ITO (Figure 1A). This positive feedback response at an unbiased conductor is a result of bipolar electrochemical processes at the substrate.39

Figure 1. (A) Schematic representation of an SECM feedback experiment with a reducing potential applied at the tip and oxidation occurring at the substrate. (B) The tip current (iT) vs displacement (effective tip-substrate distance) curve obtained from a 7-µm-radius Pt tip approaching an Au island film without (black) and with (red) plasmon excitation. Solution contains 1mM Fe(CN)63-, 1mM Fe(CN)64- and 0.5M KCl. Substrate was unbiased and ET = 0 V vs Ag/AgCl. Upon plasmon excitation, we observe both an overall increase in iT (as indicated by the black arrow) as well as a sharper rise in iT as the tip electrode approaches the substrate (red curve in Figure 1B). We attribute the current jump to two independent effects: (1) the increase in the local concentration of O due to photo-enhanced oxidation of R at the substrate; and (2) the 7 ACS Paragon Plus Environment


enhanced mass transport rate of O to the tip induced by heating at the Au surface. Similar to the curve obtained in the dark, the red approach curve also shows a positive feedback response at smaller values of z, but with a much larger magnitude, i.e. iT increases by a factor of 3.9 when the tip travels from z = 10 µm to z = 0.8 µm. The sharper curvature of the red curve at small tipsubstrate distances is indicative of a higher apparent oxidation rate at the Au substrate,40 presumably caused by a photo-induced oxidative potential in addition to the increase of the local temperature. For comparison, approach curves obtained with an oxidizing potential biased at the tip are shown in Figure S3, where a higher oxidation rate at the substrate after illumination is also revealed. To verify that both photo-induced local heating and enhanced oxidation impact the measured current at the tip, we next performed chronoamperometry experiments, in which we measured iT as the excitation light was modulated. In the first set of experiments, the substrate bias is at open circuit, while a high reduction potential at the tip (ET >E0) or reducing (ET

Hot Carriers vs. Thermal Effects: Resolving the Enhancement

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