Surface-to-Volume Ratio Drives Photoelelectron Injection from

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Surface-to-volume ratio drives photoelelectron injection from nanoscale gold into electrolyte Matthias Graf, Dirk Jalas, Jörg Weissmüller, Alexander Yu. Petrov, and Manfred Eich ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00384 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Surface-to-volume ratio drives photoelelectron injection from nanoscale gold into electrolyte Matthias Graf* †,‡, Dirk Jalas‡, Jörg Weissmüller†,§, Alexander Yu Petrov‡,∥, Manfred Eich†,‡ † Institute

of Materials Research, Helmholtz-Zentrum Geesthacht, Geesthacht, 21502, Germany

‡

Institute of Optical and Electronic Materials, Hamburg University of Technology, Hamburg, 21073, Germany §

Institute of Materials Physics and Technology, Hamburg University of Technology, Hamburg, 21073, Germany ∥

ITMO University, Saint Petersburg, 197101, Russia.

ABSTRACT

Hot charge carriers from plasmonic nanomaterials currently receive increased attention due to their promising potential in important applications such as solar water splitting. While a number of important contributions were made on plasmonic charge carrier generation and their transfer into the metal’s surrounding in the last decades, the local origin of those carriers is still unclear. With our study employing a nanoscaled bicontinous

ACS Paragon Plus Environment

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network of nanoporous gold, we take a comprehensive look at both subtopics in one approach and give unprecedented insights into the physical mechanisms controlling the broadband optical absorption and the generation and injection of hot electrons into an adjacent electrolyte where they enhance electrocatalytic hydrogen evolution. This absorption behavior is very different from the well-known localized surface plasmon resonance effects observed in metallic nanoparticles. For small ligament sizes the plasmon decay in our network is strongly enhanced via surface collisions of electrons. These surface collisions are responsible for the energy transfer to the carriers, thus, the creation of hot electrons from a broad spectrum of photon energies. As we reduce the gold ligament sizes below 30 nm, we demonstrate an occurring transition from absorption that is purely exciting 5d-electrons from deep below the Fermi level to an absorption which significantly excites “free” 6sp-electrons to be emitted. We differentiate these processes via assessing the internal quantum efficiency of the gold network photoelectrode as a function of the feature size providing a size-dependent understanding of the hot electron generation and injection processes in nanoscale plasmonic systems. We demonstrate that the surface effect - compared to the volume effect – becomes dominant and leads to


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significantly improved efficiencies. The most important fact to recognize is that in the surface photoeffect presented here, absorption and electron transfer are both part of the same quantum mechanical event.

KEYWORDS Photoemission, Nanoporous Au, Hot electron, Hydrogen evolution, Carrier injection, Surface damping, Water splitting

Introduction The phenomenon of “hot” electrons, i.e. electrons that are optically elevated well above the Fermi energy 𝐸F of a nanoscale metal, is currently under intense investigation especially due to its promising potential to enhance and kinetically modify chemical reactions at the interface.1–8 After extraction from the metal, these electrons were specifically and efficiently utilized for adsorbate reductions9,10, water splitting11 or industrially relevant organic conversions.12 Meanwhile, the detailed mechanisms of hot electron generation and emission remain a matter of debate. The understanding about


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where in a nanocrystal hot electrons are born (e.g. at the surface or in the volume) and how they are injected, e.g. into an electrolyte, appears central to further increasing the efficiency of hot-electron catalysis. Compared with other approaches (e.g. by quantitative analysis

5,13–16

or spectroscopic observation9,17 of the reaction products or by

spectroscopic observation of the injection process into adjacent layers18,19), the current associated to the charge transfer between metal and surrounding represents a precise metric to approach these questions. Relating this current to the absorption yields the

internal quantum efficiency which could reveal influences e.g. of the particle size on the origin of hot electrons. Here, we directly address this issue. Hot electrons can be created upon optical excitation of electrons in a metal. This process does not necessarily demand for the case where the plasmon oscillation is in resonance with the excitation wavelength. Therefore, also frequencies away from a potentially present localized surface plasmon resonance can create hot electrons, as long as photons are absorbed. The origin of hot electrons depends on where the damping of the plasmonic oscillation takes place so that hot electrons can be born inside (volume) or at the interface (surface) of a plasmonic nanometal and its surrounding (see below why in


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our case we do not consider the absorption in adsorbate layers as had been shown to lead to various decay and electron injection mechanisms in the past20–22).1,2,23–26 Consequently, emitting a hot electron will either be a three-step process (volume photoelectron emission)23,27 where, in the first step, the photon, storing electromagnetic energy, is absorbed by a free electron inside the metal. In order to conserve momentum, this requires the presence of a lattice defect or a phonon to create a hot electron. In the second step, the hot electron travels to the surface and, if it has not shared its energy with another electron via an electron-electron scattering event, needs to collide with the surface within a certain angle28 in order to transit with a certain probability through the boundary into the environment29, which concludes the third step. This exit cone condition is determined by the ratio of effective electron masses in the metal and the surrounding medium into which the electron is transferred.30 On the other hand, surface photoelectron emission is a one-step process, where photon absorption and the collision of an electron with the surface occur simultaneously.1 Under a certain probability the so-created hot electron will either stay in the metal or directly transit to the environment without a further possibility for thermalization.23 Nonetheless, this surface collision-promoted excitation is


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also subject to similar considerations on the allowed incident angles of the electron. Electron collisions with the surface render a well-known mechanism for the damping of plasmon excitations in metallic nanoparticles.31 For example, the size reduction of Au nanoparticles shows that surface-located damping becomes the dominant absorption mechanism for diameters < 10 nm.32 Although Au nanoparticles can be widely tuned in size33, using them to differentiate between the two hot electron generation mechanisms is problematic: First, with respect to charge extraction from the particle, the organic ligands commonly attached to dispersed nanoparticles34,35 represent significant energy barriers36,37 that may render conclusions on extraction efficiency difficult. Second, dispersed and thus isolated nanoparticles do not qualify as appropriate electrodes to directly assess the photocurrent because measuring the interfacial charge transfer between nanoparticles and electrolyte requires a support of the nanoparticles on a conductive and, at the same time, inert substrate. This can lead to high contact resistances. Direct photocurrent measurements have been done using such arrangements38–40, but should be evaluated with care due to


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these points. Therefore, a nanoscaled three-dimensionally connected metal network used as photoelectrode is more desirable. Nanoporous Au (npAu)41, a bi-continuous network of thin Au ligaments and pores42, fulfills the above criterion. Its most common synthesis by “dealloying“ starts from Au-Ag alloys42 by selectively dissolving the Ag component. This generates contiguous networks which are tunable in size by thermal coarsening (via Au surface diffusion41 leading to self-similar network structures with adjustable feature size43,44) and which expose clean and ligandfree Au surfaces.45 We have previously explained the underlying mechanism why npAu layers of only 100 nm thickness, discriminating from Au nanoparticles or bulk Au, show efficient and broadband visible light absorption.46 The unique feature to systematically vary the size and conserve the volume by thermal coarsening allows tuning the surfaceto-volume ratio and with this the site of optical absorption. This allows quantifying the fractions of hot electrons born in the volume and at the surface. A photocurrent can be measured as a contribution to the cell current when the npAu electrode is used as working electrode emitting hot electrons into an electrolyte. Here, we show how this effect


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enhances hydrogen evolution reaction (HER)47, which is chosen as a model reaction for electron injection. During HER no significant amounts of adsorbates are present on Au surfaces as long as cathodic potentials are held moderately low.48 Consequently, the hot electron generation by optical absorption in adsorbate states (by chemical interface damping26,49) can be neglected since also the optical properties of npAu turn out to be independent from the pore-infiltrating medium. As will be shown below, we observe a profound effect of the average ligament size on the internal quantum efficiency. This parameter turns out to be increased by a factor of ten when the size of the ligaments is reduced by a factor of four. While this observation cannot exclusively be explained with the increased surface area, we demonstrate that the process of surface photoelectron emission becomes increasingly dominant during size reduction below 30 nm.


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Figure 1: Properties of nanoporous Au (npAu) samples (film thickness: 100 nm, geometrical projection area: 2.5 cm²). A-H: SEM top-view images after the dealloying process (A) and after annealing for 2 (B), 3 (C), 4 (D), 5 (E), 6 (F) and 8 min (G) at 300 °C


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in air. H: Evolution of average ligament diameter (black) and ECSA (red) over the annealing time. I: Absorptivity spectra with different mean ligament diameter 𝐿 of npAu films soaked with 0.5 M H2SO4. Inset photograph shows a typical sample on a glass substrate and contrast background.

Results Samples of npAu were produced from 100 nm thick white gold leaves.50 The reproducibility of this process has been evaluated prior to this report.51 Scanning electron microscopy (SEM) images (Figures 1A-G) confirm an initial mean ligament diameter (for its determination see experimental details) 𝐿 = 9 ± 2 nm which is increased incrementally by thermal treatment (Figure 1H).52 Since the coarsening process produces self-similar porous structures, the width of the 𝐿 distribution approximately remains +/- 20% for all values of 𝐿 reported here. As confirmed by energy-dispersive X-ray spectroscopy (EDX), all samples contain 11 – 13 atomic-% of Ag and coarsening-induced shrinkage was found negligible, i.e. the solid fractions of all samples are comparable. npAu thin films were caught with a glass substrate which was pre-functionalized with an adhesion-promoting molecular layer53 exposing thiol functionality towards npAu and silane functionality


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towards glass. This prevented layer delamination without covering the ligament surface of npAu. We determined the electrochemically active surface area (𝐸𝐶𝑆𝐴, Figure 1H) by the capacitance ratio method.54 With increasing 𝐿 we observe a decrease in 𝐸𝐶𝑆𝐴 which is well consistent with values for the surface area of a cylinder with a diameter equal to 𝐿, i.e. 𝐸𝐶𝑆𝐴 ~ 𝐿 ―1. After dealloying, repetitive potential cycles were conducted in the dark and stopped after the cathodic run to strip oxygen adsorbates55,56 from the npAu surface which had in the past been identified as states for optical absorption.6 The equilibrium potential for HER was determined 𝐸0 = ― 50 mV vs. Ag/AgCl reference electrode and found 𝐿-independent. 100 nm thick layers of npAu show remarkable optical absorption (Figure 1I). The absorptivity strongly depends on 𝐿 with finest structures exhibiting the highest optical absorptivity. We find that the pore-filling medium only insignificantly affects the absorption behavior (see Figure S1) which justifies the above assumption of the npAu surface being nearly free of adsorbates. Similar to this, it had been shown that the adsorption properties of npAu can be varied by the application of a surface potential55, but these findings can


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Figure 2: Representative current transients for 100 nm thick nanoporous Au (ligament size 𝐿 = 11 nm) at different applied potentials 𝐸 and under switched visible (left column) and UV (right column) illumination with given wavelength and illumination power, illuminated sample area: 2.5 cm². be regarded negligible within the potential and wavelength window used in this study. More surprisingly, and different from both, Au nanoparticles and bulk Au, the absorption of npAu is truly broadband, stretching over the entire visible wavelength range.46 Only at longer wavelengths the coarse structures approach bulk Au behavior which leads to an increase of reflection, thus a decreased absorption.


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After characterization, npAu electrodes were subsequently applied as working electrodes for HER under switched illumination (given a certain wavelength 𝜆 and a defined power 𝑃). Therein the application of a certain external potential 𝐸 creates a dark current transient until a stable dark current is observed. Subsequent repetitive switching of the illumination also switches the absolute current (Figure 2). Abrupt increases of the (cathodic) current coincide with the switch-on of the illumination. We attribute the height of these switches to the photocurrent 𝐼ph (see Figure S2 for details of its determination). So-determined values of 𝐼ph are independent from the pulse duration (Figure S3), i.e. these do not necessarily require a stable dark current (as e.g. for 𝐸 > 𝐸0), and are linearly dependent on the applied illumination power (Figure S4) as one should expect for a reaction where one photon creates one hot electron57 which then creates one H(0) from one H+. Note also, that the occurring current transients do not result from changes in the potential which – if such changes occur (e.g. at open-circuit conditions, see Figure S5) and as demonstrated earlier58 – are fully compensated by the potentiostatic circuit (Figure S6). Depending on both, 𝐸 and 𝜆, different shapes of the photocurrent traces are observed. For visible light illumination under 𝐸 = ―450 mV larger time constants influenced by


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significant photo-thermal heating6,59,60 cause a slowly increasing electrolysis current which adds to the fast photoelectron injection yielding a higher apparent photocurrent. The definition of the photocurrent applied here however takes into account effects that occur on the short time scale, i.e. instantaneously with light switching, and mainly excludes secondary effects such as this photothermal heating. Throughout the entire investigation, 𝐼ph ≤ 0, i.e. either an enhancement of the HER or no influence from illumination was observed. Also, for the npAu electrodes, as shown in the top line of Figure 2, visible light does not lead to detectable photocurrents when 𝐸 > 𝐸0. In contrast, UV illumination causes a significant negative 𝐼ph even under >12 times lower intensity. However, a flat Au electrode used to derive the bulk value for the following discussion yielded a small measurable value of 𝐼ph under the highest power UV illumination (Figure S7) which is consistent with the literature.61

Discussion The optical absorption of Au is governed by electronic transitions either originating from the 6sp-band of free electrons (low photon energy 𝐸ph) or from the 5d-band (high 𝐸ph).62


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At 𝜆 > 500 nm (𝐸ph ≤ 2.5 eV) the 5d-band absorption in Au is very small so that the optical properties are dominated by “free” 6sp electrons. These are commonly desribed by the Drude model of metals.62 For reasons of momentum conservation the absorption of photons is only possible upon collisions of these free electrons with an obstacle. Such obstacles can either be lattice defects, phonons or the surface. How often such collisions occur (i.e. the collision frequency) predominantly determines the imaginary part of the dielectric constant of a Drude metal which also defines the absorptivity. A reduction of 𝐿 to nanometer scale, i.e. an increase of the surface-to-volume ratio, causes a dramatic increase in the probability for electron collisions with the surface.31,32 At longer wavelengths, we therefore attribute the increase in absorptivity when reducing 𝐿 to an increased collision frequency of 6sp electrons with the metal surface.31,32 For all npAu samples the total volume of Au is independent of 𝐿, thus absorption via 6sp electrons colliding with volume defects remains unchanged. Consequently, the summed frequency of collisions for those electrons generated in the Au volume during their travel to the surface must be independent of 𝐿, assuming that the phonon spectrum and lattice defect distribution is also largely independent of 𝐿.63,64 At the smallest ligament diameter


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investigated here, 𝐿 = 9 nm, the absorption at long wavelengths must thus be already influenced strongly by collisions of the free 6sp electrons with the surface. This result is in stark contrast to well-known bulk properties of Au.62 At 𝜆 < 500 nm, and increasingly so for shorter wavelengths, the absorption of bulk Au is dominated by the excitation of initially bound 5d-band electrons, which then can be lifted to energies slightly above 𝐸F and occupy states in the 6sp-band. At short wavelengths, the excitation of free 6sp electrons plays only an insignificant role in bulk Au.62 Most astonishingly, we observe from Figure 1I that a reduction of 𝐿 in npAu to 9 nm leads to absorption values at 400 nm which become comparable to those at 600 nm. This means that the damping by the free 6sp electrons is increased so strongly that even at UV wavelengths, the free 6sp electrons substantially contribute to the total absorption. The absorption through collisions of 6sp electrons is expressed by the contribution 𝜀′′6𝑠𝑝 to the total imaginary dielectric constant. The contribution from 5d electrons is given by 𝜀′′5𝑑. The first can be calculated from the electron collision frequency Γ and the plasma wavelength 𝜆P which corresponds to the Au plasma energy (ℏ𝜔𝑃 = 8.5 eV) 31,32


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𝜆3

𝜀′′6𝑠𝑝 ≈ Γ2𝜋𝑐𝜆2 = (Γ0 + P

𝜆3 4𝐵 ) 𝐿 2𝜋𝑐𝜆2P

(1)

with a collision frequency constant Γ0 describing collisions in the volume and a term related to electron collisions with the surface which inversely scales with 𝐿 (𝐵 is the constant slope of the collision frequency with surface-to-volume ratio 4 𝐿), Planck’s constant ℎ and speed of light 𝑐. For bulk Au and at photon energies 𝐸ph = 3.2 eV (390 nm), Olmon et al.62 estimate that 𝜀′′6𝑠𝑝 attains a value of approximately 0.1 while 𝜀′′5𝑑 reaches 6. The fraction of absorbed UV photons at roughly 400 nm wavelength to the total number of absorbed photons thus is 0.1 (0.1 + 6) = 0.016, i.e. at 400 nm only 1.6% of all electrons excited in bulk Au are 6sp electrons. For npAu with small 𝐿 the absorption values at 400 and 600 nm are equal, which indicates that the 6sp electron collision frequency is increased approximately 10 times compared to bulk Au.62 The fact that 𝜀′′6𝑠𝑝is increased to a value of 1 means that more frequent surface collisions occur in the nanoscaled bicontinous network and that this higher collision frequency depends on the surface-to-volume ratio. At 400 nm we observe an increase of the contribution of the 6sp electrons to the total absorption to approximately 15%.


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Figure 3: Dependence of the internal quantum efficiency 𝜂 of npAu on photon energy. Internal quantum efficiency 𝜂 of npAu with different ligament diameter during HER at 𝐸 = ―350 mV. To assess the efficiency of photoemission in dependence of 𝐿, the different absorptivity values have consequently to be taken into account. The internal quantum efficiency (𝜂, i.e. the ratio of the number of electrons 𝑁e from 𝐼ph derived via Faraday’s law, divided by the number of absorbed photons 𝑁abs) is derived by: 𝑁e

𝜂 = 𝑁abs =

𝐼ph 𝐸𝑝ℎ 𝑒

∙ 𝐴𝑃

(2)

with electron charge 𝑒, absorptivity 𝐴 (as taken from Figure 1I) and incident power 𝑃. As can be seen in Figure 3, for photon energies approximately above 3.1 - 3.4 eV, 𝜂 strongly


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increases when 𝐿 is decreased. This 𝐿-independent onset, which we denote as “red” energy 𝐸red, represents a minimum energy above which hot electrons can contribute to the overall current (Scheme 1A).65 Because there is no significant coverage of the surface with potentially electron-accepting H adsorbate states at the moderate potentials applied in this study (see Supporting Information Figure S8), the excited electrons will be directly injected into the electrolyte followed by their stabilization within a solvent shell66 e-hot

H2O

e-aq

(3a)

These freely moving electrons can cause proton reduction in the electrolyte which is in turn not strictly located at the electrode-electrolyte interface anymore. The state of free electrons in the liquid surrounding can be considered similar to that of free electrons in vacuum, except that they are stabilized by a solvent shell which gives an explanation for the lower work function value (for Au in vacuum this value is 5.3 eV67 in contrast to the 3.1-3.4 eV here) as well as the increased lifetime (few microseconds68) in comparison to hot electrons. The energy necessary for the above process (Eq. 3a) consequently corresponds to the work function of the metalelectrolyte system 𝐸B.65 The value found for 𝐸B to form e-aq species corresponds well to


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the energy barriers determined earlier for photoemission in other metal-electrolyte systems.2 However, the potential that needs to be applied in order to cause a measureable galvanic current (connected to the activation energy 𝐸A for this process) is smaller than 𝐸B. In the first step of HER, applying a cathodic potential 𝐸 < 𝐸0 leads to the adsorption reaction known typical for galvanic HER: + ― + eext →Had Haq

(3b)

On the other hand, negative photocurrents 𝐼ph are observed even when 𝐸 > 𝐸0 (see Figure 2) where this galvanic process (Eq. 3b) is unlikely to occur. Due to the absence of secondary electron acceptors, we must thus conclude on the electron injection directly into the electrolyte so that processes 3a and 3b can be considered independent from each other.


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Scheme 1: Photoelectron injection from nanoporous Au into electrolyte at𝐸 = 0 (A) with barrier energy 𝐸B being surmounted by photoelectrons (blue arrow). Magnified image shows the activation energy 𝐸A for the galvanic electron injection to a level at the equilibrium potential for hydrogen evolution 𝐸0. Galvanic injection creates a background electrocatalytic current (purple arrow) when a bias potential 𝐸 is applied (B) besides a lift of the Fermi level 𝐸F of Au and increased photoelectron injection. Electrons originating from the 5d-band of Au, although they contribute the strongest to the absorption in the UV, cannot be transferred into the electrolyte and do not contribute to HER (Scheme 1A). 5d electrons are located at least 1.96 eV below 𝐸F.69 Even absorbing the highest photon energy of 4.3 eV used in our experiments is insufficient to achieve both, interband transition of 5d to 6sp electrons and insertion into the electrolyte over 𝐸B. Because 5d electrons absorb nonetheless, 𝜂 is consequently reduced by 5d


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absorption. On the other hand, hot electrons excited from the 6sp band close to 𝐸F will overcome 𝐸B. These show very low absorption in the UV range.32,62 Consequently, some of them can overcome 𝐸𝐵 and contribute to 𝜂. This fraction is determined by absorption and relates to 𝜂 via62

𝜂 = 𝛼(𝜆, 𝐿)𝜀′′

𝜀′′6𝑠𝑝

5𝑑

+ 𝜀′′6𝑠𝑝

(4)

The pre-factor 𝛼(𝜆, 𝐿) determines the probability of free electrons to exit from the metal upon excitation. Whether an electron will leave the metal into the electrolyte or not, will depend on 𝐸ph23 so that 𝛼(𝜆, 𝐿)~(𝐸ph ― 𝐸B)5/2 will apply for both, volume and surface photoelectron emission.2,65 For volume photoelectron emission this expression is derived when considering the transmission of the excited electron through the metal-electrolyte interface.70


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Figure 4: Influence of applied bias potential 𝐸 on internal quantum efficiency 𝜂 of npAu electrodes (𝐿 = 11 nm). A: 𝜂 spectra for different 𝐸 (given by the respective numbers in the graph) with parabola fits according to Eq. (5). Inset B: Red edge energy 𝐸red and wavelength 𝜆red (from parabola fits using Eq. (5)) vs. 𝐸 (error bars from linear regression over at least three data points from 291 nm) and theoretical (ideal) slope = 1 due to potential variation. C: 𝜂

25

vs. 𝐸 after Brodsky2 for different illuminating wavelengths with


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linear regression lines (regression based on data 𝐸 ≥ ―350 mV to avoid misinterpretation during 𝐼ph determination at lower 𝐸, see Figure 2). In surface photoelectron emission, the photon is absorbed via electron-surface collision which leads to the direct occurrence of the electron outside the metal. The probability of this event shows the same functional dependency2 while its derivation is more complex and beyond the scope of this manuscript. For volume photoelectron emission an additional factor defines the probability of an excited electron to reach the surface which is added to the L-dependency (as discussed below).28,70 We use the dependence of 𝜂 on 𝐸 to check the validity of the expected power law of photoelectron emission employing the exponent 5/2. The internal quantum efficiency 𝜂 is expected to scale according to 𝜂~(𝐸B + 𝑒𝐸 ― 𝐸ph)5/2

(5)

with the elementary charge 𝑒.2 Figure 4A displays values of 𝜂 under illumination with different wavelengths when the electrode is biased by different 𝐸. As can be seen from there, increasing 𝐸 increases the entire internal quantum efficiency since the Fermi level of Au is lifted (d-electrons do not respond accordingly) and the effective barrier height between 𝐸F and 𝐸B is decreased (Scheme 1B). Longer wavelength photons, however,


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ACS Catalysis

can contribute and cause electron injection so that 𝐸red is decreased (visible from a shift of the 𝜂 values to lower photon energies, Figure 4B). Consequently, for 𝐸 = 𝐸0 we find a minimum 𝐸ph to cause a measurable current signal (the already mentioned “red edge energy”, 𝐸red) of approximately 3.2 eV, well in accordance with the already found value for 𝐸B and shifting with 𝐸 as expected. For various 𝐸ph the plot of 𝜂

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vs. 𝐸 clearly (Figure

4C) yields parallel straight line fits as one would expect from Eq. (5) (Figure 4B). Note, that for lower 𝐸ph, points obtained at higher 𝜂 were not useful for linear regression due to larger errors in the low photocurrents. Figure 4 C experimentally validates the 5/2 exponent which was initially derived by Brodsky and Pleskov2 for the emission of photoelectrons into a solvated state. This fact represents another confirmation that electrons are first released in a solvated state into the electrolyte und then react with the electroactive species which is in contrast to often assumed photon absorption into adsorbate states followed by their reaction.6,20–22


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Figure 5: Size dependence of internal quantum efficiency 𝜂 for wavelength 𝜆 = 291 nm measured at 𝐸 = ―350 mV. Dashed line represents 𝜂 scaling with 𝐿 ―1 as it would be expected from sole volume photoelectron emission. Error bars were derived from mean deviation in 𝐿 (from SEM) and from the distribution width of 𝐼ph derived from the repetition


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ACS Catalysis

of illumination pulses. Since the confirmed exponent will be the same for volume and surface photoelectron emission it is thus not capable of differentiating between them.23 Moreover, the dependence of 𝜂 on the surface area, i.e. on 𝐿 ―1 (Figure 5), can be used. It displays an increase when the surface area is increased. 𝜂 is expected to profit from both, volume and surface photoelectron emission contributions: Photoelectron emission from the volume will benefit from a smaller travel distance of hot electrons to the surface and thus from a smaller probability for hot electrons colliding with other electrons (within which they lose part of their kinetic energy).28,71 Taking into account a mean free path for electronelectron collision of roughly 10 𝑛𝑚 at 𝐸B > 3.5 𝑒𝑉28,72, the probability for electron extraction, and with this 𝜂 from volume photoelectron emission, should scale inversely proportional with the ligament diameter.23 In Figure 5 this trend is represented by the dashed line which was calculated assuming that photoelectron emission from bulk Au and npAu with the highest 𝐿 = 39 nm occurs under a negligible surface effect. Surface photoelectron emission will benefit if more 6sp electrons absorb photons by collisions with the surface, (Eq. (4)). Also, if ligaments are smaller, those hot electrons


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that appear at the surface but are reflected back into the volume are likely to collide with the opposite surface a second time without losing their energy in an electron-electron scattering event. Thus, surface photoemission should scale stronger with an increasing surface area than volume photoemission because both factors in Eq. (4) are increased. From Figure 5 we observe an improvement in 𝜂 from 0.01% for bulk Au to 0.015% for npAu with 𝐿 = 39 𝑛𝑚 and to 0.14% for 𝐿 = 9 𝑛𝑚. Most importantly, measured values for 𝜂 increase much faster than those predicted by sole volume photoelectron emission via 𝐿 ―1 (for the sake of brevity, values for the lowest wavelength are illustrated, refer to Figure S9 for the other wavelengths). We find that this deviation becomes evident when ligament sizes are reduced to 𝐿 < 30 𝑛𝑚 , so that a surface photoelectron emission process can be ascribed to occur when ligament diameters undercut this value. As observable from Figure 5, the total internal quantum efficiency increases significantly (by a factor up to 4) when 𝐿 is reduced down to 9 nm. Together with the fact that only 15% of all excited electrons are 6sp electrons that are excited dominantly via surface collisions (the others are 5d electrons that do not lead to a photocurrent) reveals that the surface photoelectron emission mechanism becomes dominant, as expected, when ligaments diameters are

Surface-to-Volume Ratio Drives Photoelelectron Injection from

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