Hot Carriers vs. Thermal Effects: Resolving the Enhancement Mechanisms for Plasmon … · 2018. 2....

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Article

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

Yun Yu, Vignesh Sundaresan, and Katherine A. WilletsJ. 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 19, 2018

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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*

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

Corresponding Author

[email protected]

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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 non-

radiatively 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 +

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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 steady-

state 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.

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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 P-

2000 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)6

4- 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, 532−50-

CDRH) 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 7-

µm-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)6

4- (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)6

3-, 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

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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 tip-

substrate 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) allows the collection

of O generated at the substrate (Figure 2A). Under these conditions, we expect the current at the

tip to increase upon plasmon-excitation due to both (1) a higher local concentration of O

generated by photo-induced oxidation at the surface and (2) faster mass transport of O to the tip

due to the local increase in temperature. Figure 2B shows the time traces of iT when a 260-nm-

radius tip electrode biased at a reducing potential is positioned at different tip-substrate

separation distances (d). When d = 10 µm, iT increases from 153 pA to 390 pA upon plasmon

excitation (red curve). Due to the fast diffusion of molecules to the tip at this d, the tip current

reaches steady-state nearly instantly. When d = 40 µm, iT gradually increases from 153 pA to 273

pA (green curve) with a longer transient time (~1.8 s). This time reflects the time required for the

molecules produced at the substrate to diffuse to the tip and agrees well with the predicted transit

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time, based on the diffusion coefficient of the molecule over this distance (e.g for d = (2Dt)1/2,

where D = 7×10-10 m2/s, t = 1.2 seconds when d = 40 µm). The lower magnitude of iT after

excitation is a result of lower concentration of O within the diffusion layer at this larger distance.

The response at d = 70 µm (blue curve) shows an even longer transient time (~3.5s) and a lower

iT after plasmon excitation.

Figure 2. (A) Schematic representation of an SECM generation-collection experiment and (B) the tip current vs time as the light is turned on at t = 10 s and off at t = 20 s. ET = 0 V, ES = open circuit. (C) Schematic representation of an SECM competition experiment and (D) tip current vs time as the light is turned on at t = 10 s and off at t = 20 s. ET = 0.4 V, ES = open circuit. The current-time curves were obtained with a 260-nm-radius Pt tip placed 10µm (red), 40µm (green) and 70µm (blue) away from the Au island film. Solution contains 2mM Fe(CN)6

3-, 2mM Fe(CN)6

4- and 0.5M KCl. The intensity of the 532-nm laser in all experiments was 335 W/cm2.

The previous example illustrates changes in tip current when plasmon-induced heating

and oxidation work synergistically to increase the current at the tip. We also created conditions

where the two effects compete, allowing us to confirm that both thermal effects and photo-

induced oxidation are present. By holding the tip at a high oxidative potential such that R is

converted to O, we expect a decrease in the tip current upon illumination due to consumption of

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R at the substrate (Figure 2C). At the same time, plasmonic heating effects should continue to

produce faster mass transport, yielding an increase in the tip current upon illumination. Figure

2D shows the time traces of iT with different tip-substrate separation distances when the tip is

held at an oxidizing potential. The red curve (d = 10 µm) shows a fast decrease of iT upon

plasmon excitation due to the decrease of the local concentration of R near the substrate because

of the photo-induced oxidation. The increase of iT by heating is not visible because the time

required to reach steady-state is very short due to the short tip-substrate distance. The green

curve (d = 40 µm) shows an abrupt increase of iT, followed by a gradual decrease. We attribute

the initial increase in iT to the enhanced mass transfer rate due to local heating, and the

subsequent decrease in iT to the relatively slower depletion of R, which ultimately dominates the

change in iT. The transient time to attain steady-state is ~1.9s, consistent with the observation in

Figure 2B. The blue curve (d = 70 µm) also shows an abrupt increase followed by a gradual

decrease of iT with a longer transient time (~3.7s), again due to the rapid effect of increased mass

transport, followed by the slower effect of the photo-enhanced oxidation reaction. When the

excitation is turned off, the fast drop of the tip current is due to the decrease of the mass transport

rate to the original value before excitation, and the relatively slow increase in iT afterwards is due

to the restoration of the concentration gradient at the tip-substrate gap. Thus, due to the

differences in the kinetics of the effects of local heating and photo-enhanced oxidation, we are

able to verify that both photo-induced effects impact the current measured at the tip electrode.

We simulated the time-dependent iT response using a diffusion model to qualitatively

verify our experimental data. The shape of the current-time curves in Figure 2B and 2D are

consistent with the simulated results (Figure S4). Moreover, we performed controls on both bare

glass and bare ITO and found the change in iT 1−2 orders of magnitude lower than that obtained

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with an Au island film under the same irradiation conditions, thus verifying that the response of

iT is not significantly affected by the supporting substrate or by laser illumination of the Pt tip

electrode (Figure S5). The light absorption by either Fe(CN)63- or Fe(CN)6

4- is at much shorter

wavelength than that used in the experiments (Figure S6) and therefore the current response is

only attributed to light absorption by the Au film.

The responses of iT in Figures 2B and 2D illustrate conditions where we can create

synergy or competition between photo-induced oxidation and plasmon-induced heating effects

on the tip current, respectively. However, we would ideally like to isolate the two effects and

probe their relative contributions to the measured currents. This is a challenge because the

steady state, mass-transport-limited anodic tip currents (ia,∞) and cathodic tip currents (ic,∞) that

we measure under highly oxidizing (ET>>E0) or reducing (ET<<E

0) tip potentials, respectively,

are a function of both the mass transport coefficient (which is temperature dependent) and the

local concentration of reactant (which depends on the substrate potential), as given by Eqs. 1 and

2 below:37

, = ∗ (1)

, = ∗ (2)

Here F is the Faraday constant, A is the area of the tip electrode, mO and mR are the mass transfer

coefficients of O and R, respectively, and cO* and cR

* are the bulk concentrations of O and R,

respectively. Because two parameters in the limiting current expressions are impacted by

plasmon excitation, comparing the magnitudes of the current when the light is on vs. off does not

provide insight into the relative contributions of local heating vs. photo-induced oxidation. Thus,

we need to design experiments that allow us to isolate each of the two effects in order to

determine their impact on plasmon-mediated photoelectrochemistry.

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To isolate the heating effect, we apply a potential either much higher or lower than E0 at

the plasmonic substrate in order to dominate the effect of photo-induced oxidation. For example,

biasing the Au island substrate at a high oxidative potential (ES = 0.4 V vs Ag/AgCl) results in

oxidation of R at the Au surface with a diffusion-controlled rate. The complete depletion of R

near the substrate (Figure 3A) allows the effects of photo-induced oxidation to be minimized;

thus any increase in iT can be attributed to heating-enhanced mass transport. Figure 3B shows the

time traces of iT when a 260-nm-radius tip electrode is positioned at different distances from the

substrate with ES = 0.4 V and ET = 0 V. Upon illumination, iT increases from 260 pA to 390 pA

(~50% increase) when the tip is positioned 10 µm away from the substrate. When d changes to

40 µm and 70 µm, iT increases 45% and 19% after illumination, respectively. The enhancement

of the mass transport by local heating is weaker at longer distance, and results in a smaller

increase in iT. Figure 3C shows the reverse scenario, in which a reducing potential of 0 V is

applied to the substrate, reducing O at the diffusion-controlled rate, and an oxidizing potential is

applied at the tip (ET = 0.4 V). This experimental condition results in complete depletion of O

near the substrate, and again the response of iT is only affected by the local heating. Under these

conditions, the increased magnitude of iT after plasmon excitation is 49%, 40%, and 15% at tip-

substrate distances of 10 µm, 40 µm, and 70 µm, respectively (Figure 3D). The similar response

of the anodic iT (ET = 0.4 V) and the cathodic iT (ET = 0 V) suggests that the mass transfer rates of

O and R are enhanced by the same magnitude.

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Figure 3. (A) Schematic representation of an SECM generation-collection experiment with substrate biased for diffusion-controlled oxidation reactions. (B) Current vs time trace obtained with ET = 0 V, ES = 0.4 V. (C) Schematic representation of an SECM generation-collection experiment with substrate biased for diffusion-controlled reduction reactions. (D) Current vs time trace obtained with ET = 0.4 V, ES = 0 V. The data was obtained with a 260-nm-radius Pt tip placed 10 µm (red), 40 µm (green), and 70 µm (blue) away from the Au island film. A 532-nm laser with the intensity of 335 W/cm2 was switched on at t = 10s and off at t = 20s. The solution contains 2mM Fe(CN)6

3-, 2mM Fe(CN)64- and 0.5M KCl.

We note that the heat-induced change of iT is very fast, as evidenced by the rapid increase

of iT in Figures 3B and 3D at all separation distances, most likely due to the faster kinetics of

heat transfer than the diffusion rate of molecules in an aqueous solution.41 The thermal

diffusivity of water, 1.43×10−7 m2/s,41 is several orders of magnitude higher than the diffusion

coefficient of the redox species, ~7×10−10 m2/s.42 The rapid heating effect is consistent with the

data in Figure 2D where the effect of local heating led to an initial sharp rise in current, but is

less clear in Figure 2B, where the increase in current due to heating is dominated by the change

in the concentration of O. To further verify that the observed changes in the current are

dominated by plasmon-induced heating, we perform a control experiment using Au island films

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on glass, where the current flowing to the external circuit is prevented, and find similar rapid

changes in iT as the excitation light is modulated (Figure S7). The discrepancy in the trends of

the relative current as a function of distance between Figure S7A and Figure 3B, 3D may be

attributed to the heterogeneity of the Au substrates.

Next we investigate the effects of the laser intensity on the plasmon-induced heating at

the Au substrate. The heating effect is quantified as the ratio of iT with and without substrate

illumination (ion/ioff). As before, the experiments were carried out under mass transport-limited

conditions with the substrate biased at high overpotentials, thus minimizing contributions from

the photo-induced oxidation reaction. The intensity-dependent time traces of the cathodic iT (ET =

0 V, ES = 0.4 V) and the anodic iT (ET = 0.4 V, ES = 0 V) for a 220-nm-radius tip positioned 10

µm away from the substrate are shown in Figures 4A and 4B respectively. The ratio of ion/ioff

shown in Figure 4C indicates that excitation with higher intensity results in larger heat-induced

changes in both ia,∞ and ic,∞ at the tip, as expected. Figure 4D shows a plot of ion/ioff vs. laser

intensity, and the ratio of iT with and without illumination is linearly dependent on the

illumination intensity at 532 nm. The slopes of the linear dependence for both cathodic and

anodic iT are essentially identical, suggesting that the mass transfer rate of O and R is enhanced

by the same order of magnitude. However, we do note a slight deviation between the anodic and

cathodic current ratios at higher laser intensities (Figure 4, C and D), as well as small differences

in the shape of the current transients (Figure 4, A and B), indicating that the effect of photo-

induced oxidation at the highest laser intensities is not completely suppressed, though the

quantification of the heating effects is not significantly affected.

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Figure 4. (A-B) Current vs time trace obtained with a 220-nm-radius Pt tip placed 10µm away from the Au islands film with (A) ET = 0 V, ES = 0.4 V (schematic shown in Figure 3A), and (B) ET = 0.4 V, ES =0 V (schematic shown in Figure 3C ). The laser was switched on at t = 10s, 30s and off at t = 20s, 40s. The laser intensities were 335 W/cm2 (red), 268 W/cm2 (orange), 184 W/cm2 (green), 112 W/cm2 (blue) and 56 W/cm2 (black). (C-D) Plots of i,on/i,off versus laser intensity for ET = 0 V, ES = 0.4 V (blue) and ET = 0.4 V, ES =0 V (green). The solution contains 2mM Fe(CN)6

3-, 2mM Fe(CN)64- and 0.5M KCl.

Under conditions when photo-induced oxidation is also present (e.g. when ES is at open

circuit), the tip current ratio ion/ioff is not suitable for quantification of the photo-induced

oxidation reaction because the current is also affected by thermal effects. To illustrate this,

Figure 5, A-B, shows the time traces of the cathodic iT (ET = 0 V) and the anodic iT (ET = 0.4 V)

with the substrate at open-circuit while a 220-nm-radius tip is positioned 10 µm away from the

substrate. The trends in ion/ioff with ET = 0 V and ET = 0.4 V at different laser intensities are

shown in Figure 5C. As the laser intensity increases, the cathodic current (ic,∞, ET=0 V) increases

monotonically; however, we note that the magnitude of the current change under conditions in

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which both effects are present is higher than what was observed under conditions when only

mass transport enhanced heating effects occurred (Figure 4C, blue data). This is due to the

synergistic effects of photo-induced oxidation and local heating, which increase both cO and mO,

respectively. Alternatively, the anodic current ratio (ia,∞, ET=0.4 V) in Figure 5C decreases

monotonically as a function of intensity, showing that the increase in current due to local heating

(i.e. increased mR) is dominated by the loss of current caused by photo-induced oxidation

(decrease in cR). While these results are consistent with the data presented in Figure 2B and 2D,

they also illustrate the challenge of using ion/ioff as a metric to quantify the roles that different

plasmon-induced effects have on the light-induced current.

Figure 5. (A-B) Current vs time trace obtained with a 220-nm-radius Pt tip placed 10µm away from the Au islands film with (A) ET = 0 V, ES = open circuit (schematic shown in Figure 2A) and (B) ET = 0.4 V, ES = open circuit (schematic shown in Figure 2C). The 532-nm laser was switched on at t = 10s, 30s and off at t = 20s, 40s respectively. Solution contains 2mM Fe(CN)6

3-, 2mM Fe(CN)64- and 0.5M KCl. The laser intensities were 335 W/cm2 (red), 268

W/cm2 (orange), 184 W/cm2 (green), 112 W/cm2 (blue) and 56 W/cm2 (black). The plot of i,on/i,off and cO/cR versus laser intensity are shown in panels (C) and (D) respectively.

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An alternative approach is to obtain the ratio of the local concentrations of O and R

(cO/cR) using Eqs. (1) and (2),

=

,

, (3)

Here the ratio of the mass transport rates of the two species (mR/mO=0.92)42 remains constant as a

function of laser intensity, as evidenced by Figure 4D. Thus, by comparing the magnitudes of ic,∞

and ia,∞ when the substrate is at open circuit, the effects of photo-induced oxidation can be

isolated from the interference of the heating effects. The cO/cR ratios at different laser intensities

are calculated using Eq. 3 and presented in Figure 5D. Impressively, these data show that

efficient photooxidation at the substrate is achieved. The value of cO/cR increases from 1 to 12,

indicating that over 80% of R near the substrate is oxidized at the highest illumination intensities.

Next, we discuss the possible mechanisms for photo-enhanced oxidation at the plasmonic

substrate. The first possible pathway is depicted in Figure 6, panel i. Upon LSPR excitation,

energetic charge carriers are generated on the Au island surface. The hot electrons are injected

into the conductive support (ITO) adjacent to the Au islands and transferred to the external

circuit or to redox molecules at other locations on the substrate. The hot holes remaining at the

gold are able to oxidize adsorbed redox molecules with matched energy levels. Though this hot-

hole generation process has been described in previous reports,43-45 we must also consider

another mechanism that can lead to the observed photo-induced oxidation. The equilibrium

potential (Eeq) of Fe(CN)63-/4- shifts with temperature due to the entropy change associated with

this redox process.46-48 Figure S8A shows that temperature affects both the mass transport

limiting current and Eeq of the Fe(CN)63-/4- system. At higher temperatures, Eeq (the potential at

zero-current) shifts towards more negative values with a slope of -1.6mV/K, indicating that E0

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also shifts negatively as the temperature increases (Figure S8B). We note that the slope of the

temperature-dependent response of the standard potential that we measure is close to the -1.53

mV/K value reported in literature46 (the small difference between the values is attributed to the

potential shift of the reference electrode in an isothermal electrochemical cell). Thus, in addition

to hot carrier production, light-driven temperature effects can also generate enhanced oxidation

at the substrate.

Figure 6. Proposed mechanisms for the photo-induced oxidation reaction. (i) hot-hole generation at Au surface and (ii) thermal-induced shift in the equilibrium potential of the redox molecules.

To determine whether hot carriers, thermal effects, or both impact the photo-enhanced

oxidation in our experiments, we consider how the cO/cR ratio is expected to vary with excitation

intensity. The relationship between cO/cR and the potential shift, ∆E, can be evaluated based on

the Nernst equation,

∆ = =

(4)

In the case of local heating, E0 will decrease linearly as the temperature increases, as shown in

Figure S8B. Because temperature scales linearly with excitation intensity,22-23 we expect E0 to

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have a linear dependence, and thus cO/cR to have an exponential dependence, with excitation

intensity. We note that under conditions in which only local heating is present, ES is not expected

to change. Because the laser spot is significantly smaller than the entire substrate, the rest of the

Au substrate remains unheated; thus the substrate potential (ES) at open circuit is determined by

the equilibrium potential of the non-illuminated substrate at room temperature, Eeq,RT (Figure 6,

panel ii). On the other hand, if hot carriers are present, we expect ES to also change with laser

intensity, due to the build-up of hot holes locally within the laser spot. In this case, we expect

the substrate photovoltage to vary logarithmically with intensity, consistent with reports

describing hot-hole photochemistry,45, 49 and thus, cO/cR will show a linear dependence on

excitation intensity. As shown in Figure 5D, the measured cO/cR in our data shows a linear

dependence on intensity, suggesting that hot carriers play a role in the enhanced photo-oxidation

at the gold island film substrate.

To further probe this, we identified a gold island film on ITO substrate that did not show

evidence of hot carrier effects. Figure 7A shows the measured cO/cR for this substrate (referred

to as substrate 2). Unlike the data in Figure 5D for the original substrate (referred to as substrate

1), we clearly observe a non-linear relationship between the concentration ratio of the redox

species and the excitation intensity. Using the measured cO/cR values for substrate 2 and the

dependence dEeq/dT= –1.53 mV/K46 (assuming no intensity dependence on ES), we calculate ∆E

using equation 4 and convert it to the temperature change, ∆T. ∆E shows a linear response with

intensity (Figure 7B, blue data), which is inconsistent with reports in the literature in which hot-

hole photochemistry is observed, further supporting our claim that no hot carrier photochemistry

is present in substrate 2.45, 49 ∆T also shows a linear dependence with excitation intensity (Figure

7B, green data), consistent with expectations due to the local heating.22-23 Moreover, both

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relationships show a y-intercept value of zero at zero laser intensity, as expected. Thus, for

substrate 2, all evidence of light-enhanced oxidation can be attributed to thermal effects. More

discussion of this sample and further validation of this mechanism will be presented in a separate

publication.

Figure 7. (A) cO/cR vs. laser intensity and an exponential fit obtained at a second Au island sample (substrate 2). (B) Intensity dependence of ∆Eeq (blue) and ∆T (green) calculated from the cO/cR values in panel A. (C) Potential shifts for substrate 1 calculated from the cO/cR values in Figure 5D (1, black) which have contributions from both thermal and hot carrier effects. Subtracting the thermal contribution to the potential shift from substrate 2 shown in panel B (2, blue), we obtain an approximate value for the substrate potential due to hot carrier effects (1-2, red). (D) Photovoltage vs. log (intensity) calculated from curves 1 and 1-2 in panel C.

Next we perform a similar analysis on the intensity dependent behavior of substrate 1.

The black curve in Figure 7C shows the calculated change in ∆E = ES – E0 under different

irradiances calculated from the measured cO/cR values (Figure 5D) using Eq.4. A potential

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change of ~76 mV at 335 W/cm2 suggests the strong oxidizing power of this sample. More

importantly, ∆E appears non-linear with intensity. If curve 1 is fitted to a line, the y-intercept is

significantly greater than zero. The plot of curve 1 as a function of the log of the excitation

intensity (Figure 7D, black) is also non-linear. This suggests that both photo-induced hot carriers

(which change ES) and thermal effects (which change E0) are possible mechanisms behind the

photo-enhanced oxidation. To decouple the effect of the thermal-induced shift in potential, we

make the approximation that a constant laser intensity leads to a similar ∆T between different

samples, and thus we treat the relationship of ∆Eeq vs. intensity for substrate 2 (Figure 7B and

7C, blue curves) as a thermal background term which can be subtracted from ∆E to obtain an

estimate for ES. The resultant ES intensity dependence (curve 1-2, red) after subtraction of the

thermal background (curve 2, blue) from the original ∆E data (curve 1, black) is non-zero,

indicating that additional photovoltages are generated upon plasmon excitation. The extracted ES

is replotted as a function of the log of the excitation intensity in Figure 7D (red), and shows a

linear dependence of photovoltage on log (irradiance), consistent with those reports describing

hot-hole photochemistry.45, 49 The x-intercept is 0.74, corresponding to an intensity value close to

zero, as expected, suggesting that our approximation of the thermal background contribution is

reasonable. While it is challenging to verify the origins of the photovoltage, we tentatively

attribute it to the generation of hot-holes at Au.

The origin of the heterogeneous responses from the Au island samples remains as an

open question (e.g. substrate 1 appears to have contributions from hot carriers and thermal

effects, while substrate 2 only shows evidence of thermal effects). We hypothesize that the

underlying ITO substrate may be responsible for these differences. We have recently noted

substantial slide-to-slide variability in the ITO-coated coverslips used for electrodissolution of

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silver nanoparticles.50 Given that hot carrier extraction in our samples requires hot electrons to

be injected into the conductive support, thereby preventing charge recombination and allowing

us to harvest hot holes (Figure 6, panel i), we hypothesize that the variability in the ITO

substrates may also affect the charge separation process. This suggests that both the plasmonic

nanoparticles as well as the substrate play key roles in dictating the efficiency of hot carrier-

driven photoelectrochemistry.

A second challenge we face is to obtain more meaningful quantitative information of the

photovoltage as a function of the excitation intensity. In the analysis described above, we

approximated that the temperature change at a given irradiance is roughly the same across

different samples and used this approximation to extract an intensity-dependent photovoltage

(Figures 7C and 7D). However, we would ideally prefer to have a more quantitative

measurement of the temperature increase—and thus shift in equilibrium potential—for individual

substrates in order to generate a more accurate value for the photovoltage. Fortuitously, the

change in mass transport due to local temperature effects (e.g. Figure 4) provides us a route to

assess the light-induced temperature change of our samples, without the influence of hot carrier

effects. Future work will focus on quantifying the relationship between the mass transport rates

and temperature changes at the substrate surface.

CONCLUSION

In summary, we present an approach to probe plasmon-mediated photoelectrochemical

processes with SECM that allows us to quantify the extent to which redox-active species are

photo-oxidized, while also evaluating the effect of plasmon-induced local heating on measured

currents. By controlling the potential at a tip electrode, we have a unique tool to define regimes

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where photo-induced electrochemical reactions and local heating work synergistically to boost

measured photocurrents or competitively, allowing us to compare the relative kinetics. We have

shown that enhanced photo-oxidation can be attributed to both plasmon-generated hot carriers as

well as thermal effects, with different dependences on excitation intensity, allowing each effect

to be uniquely identified. Although hot-hole photochemistry has been previously proposed for

Au-ITO structures, the thermal-induced change of the thermodynamic properties of redox

molecules also plays a significant (and under-appreciated) role. To gain more insights of the

relative contribution of the two effects, the response of different redox molecules is being

explored in our laboratory and will be discussed in future reports. Future work is also aimed at

extracting quantitative heating profiles by heat/mass transfer modeling. The SECM approach will

enable new experiments to probe photoelectrochemical processes at a variety of nanoparticle

structures and substrates used in photovoltaic and photocatalytic devices.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS

Publications website: characterization of the Au substrate, voltammograms of the redox species,

approach curves with an oxidizing tip potential, simulated current response, responses from a

bare ITO, a glass slide, and a glass slide deposited with Au, voltammograms at different

temperatures, and descriptions of the finite-element simulation. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected].

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ACKNOWLEDGMENTS

The support of this work by the AFOSR MURI (FA9550-14-1-0003) is gratefully acknowledged.

The authors thank Prof. Michael V. Mirkin for helpful discussions.

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