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The nature of plasmonically assisted hot electron transfer in a donor-bridge-acceptor complex Youngsoo Kim, Andrew J. Wilson, and Prashant K. Jain ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017
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The Nature of Plasmonically Assisted Hot Electron Transfer in a DonorDonorBridgeBridge-Acceptor Complex Youngsoo Kim,† Andrew J. Wilson,† and Prashant K. Jain*,†,§ † §
Department of Chemistry, University of Illinois Urbana-Champaign, Urbana, Illinois 61801 United States Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, Illinois 61801 United States
Supporting Information Placeholder ABSTRACT: This work provides mechanistic understanding of hot electron-based catalysis on Au nanoparticles (NPs) induced under plasmonic excitation. Plasmon excitation-induced hot electron transfer from an Au NP (donor) to a ferricyanide anion (acceptor) was studied as a function of the donor-acceptor distance set by a thiolate-based self-assembled monolayer (SAM). Hot electron transfer rates and activation barrier heights were measured as a function of the donor-acceptor distance, up to 20 Å. Hot electron transfer was found to be longer range than anticipated. The distance-dependent kinetics reveal that the hot electron transfer takes place via multi-step hopping in a “wire-like” manner across the insulating ligands, quite unlike the tunneling-dominated electron transfer known to take place across SAMs in the absence of plasmonic excitation. Field-assisted electron hopping may play a crucial role in hot electron extraction and catalysis involving plasmon-excited NPs. KEYWORDS: nanoparticle, LSPR, plasmon resonance, photocatalysis, artificial photosynthesis, Marcus theory, self-assembled monolayer, plasmonic catalysis Localized surface plasmon resonances (LSPRs), collective electronic resonances of noble metal nanoparticles (NPs), represent a strong form of light-matter interaction; their use in the production of electrical or chemical energy from light, therefore, constitutes a strategic opportunity.1-17 This is particularly the case for Au, Ag, and Cu NPs whose LSPRs lie in the visible region of the electromagnetic spectrum. Following its excitation by light, an LSPR decays either by radiation of light or non-radiatively via the excitation of electron-hole pairs.13,18 The latter process thus results in a transient population of electrons with energies above the Fermi level of the metal. These excited electrons, referred to as hot electrons, have been shown to drive chemical reactions19–21 such as the dissociation of H222,23 or the oxidation of ethylene24,25 at the surface of the NP. There is a long list of striking demonstrations of catalysis induced by plasmonic excitation of metal NPs, in some cases, with a semiconductor like TiO2 in contact. Yet, mechanistic questions remain about the exact nature of plasmon-induced or plasmon-enhanced photochemistry. Answers to these questions can equip us with design principles for engineering plasmonic photocatalysts bearing high efficiency or selectivity, rather than relying on empirical screening. A particularly important consideration is the short lifetime of excited carriers in a metal: for instance, electron-electron thermalization takes place within 100s of femtoseconds26; thereafter, decay via electronphonon coupling takes place on a one-picosecond timescale. What
Figure 1. (a) Idealized structures of thiolate-SAMs used as variable thickness spacers between the donor, the photoexcited Au NP, and the electron acceptor, [Fe(CN)6]3-. (b) Rate constant, k, of plasmonically-assisted hot electron transfer as a function of the SAM chain length at different temperatures: 27, 33, 37, 41, or 44 °C. The plotted value of k is an average over a set of at least three independent trials performed under the same conditions: excitation wavelength of 514.5 nm, laser power of 900 mW, and 5 M of EtOH as a hole scavenger. The error bars shown indicate the standard error of the mean (SEM) in k over those trials. special conditions allow excited electrons to be extracted from the NP, while the electrons are still sufficiently “hot”? Understanding
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Figure 2. Distance-dependent plasmon assisted hot electron transfer kinetics. (a) Plots of ln(k.T1/2) vs. 1/T for SAMs of different chain length: 3-MPA (black), 6-MHA (red), 8-MOA (blue), and 11-MUA (pink) along with straight-line fits. The average value of k obtained from several trials was used for this plot and the goodness of the linear fit is indicated by R2. (b) The activation free energy, ∆G‡, obtained from the straight-line fits is shown as a function of the donor-acceptor distance along with a curve to guide the eye. The error bars represent the standard error in the value of ∆G‡ obtained from the fits.
of this hot electron extraction process, its nature and kinetics, can provide mechanistic understanding of plasmonic catalysis, enable optimization of the energy harvesting efficiency, and allow the determination of fundamental limits of charge and energy harvesting from plasmonically excited NPs. Here, we describe the nature of hot electron extraction from plasmonically excited Au NPs. We measured the kinetics of hot electron transfer from an Au NP (donor) to an acceptor spaced by a bridge as a function of the donor-acceptor distance, up to 20 Å. The hot electron transfer was found to be longer range than anticipated. Secondly, from the distance-dependent rates, analyzed in the context of Marcus’ model,27–30 the hot electron transfer is found to take place via multi-step hopping, quite unlike the tunneling-dominated electron transfer known to take place in nonplasmon assisted electron transfer across SAMs. The strong electric field built up on the plasmonically excited, photocharged NP appears to stabilize this “wire-like” electron hopping across the insulating ligand. Our model experimental system is shown in Fig. 1a. Plasmonic excitation (λ = 514.5 nm) induces hot electron transfer from a thiolate self-assembled monolayer (SAM)-protected Au NP to an acceptor molecule adsorbed at the outer surface of the SAM layer. The thickness of the SAM layer controls the donoracceptor distance, similar to past studies of distance-dependent electron transfer.31–34 Spherical Au NPs, ~13 nm in diameter, were synthesized with citrate.35 The citrate capping was replaced with a SAM of carboxylate-terminated alkanethiol (see SI for details). Four different carbon chain lengths were employed: 3mercaptopropionic acid (3-MPA), 6-mercaptohexanoic acid (6MHA), 8-mercaptooctanoic acid (8-MOA), and 11-mercaptoundecanoic acid (11-MUA), allowing the donor-acceptor distance to be varied in the 5-20 Å range. The ligand chain lengths (plotted on the top axis in Fig. 1 B), were estimated by summing up the lengths of all chemical bonds of the thiolate ligand, from the sul-
fur end bound to the NP surface to the terminal carboxylate group as: d = 1.82 Å + (1.54 Å × (n-1)) + 1.32 Å, where, n is the number of carbon atoms in the SAMs and the following values were used for the bond lengths: S-C = 1.82 Å, C-C = 1.54 Å, and C-O = 1.32 Å. SAM thicknesses estimated on the basis of fully extended chain lengths provide a somewhat idealized view of the systematic variation of the donor-acceptor distance. However, if there are defects or “holes” in the SAM shell, then the donor-acceptor distance would not necessarily be set by the ligand chain length. Such defects typically appear in SAMs on small NPs and clusters at corners and edges of the surface. However, Murray and coworkers36 have shown that for Au NPs larger than 5 nm in diameter, the 3D thiolate SAMs approach the crystalline order and density of 2D SAMs on flat Au {111} surfaces. Unlike smaller Au clusters with a high percentage of corner and edge atoms, the effect of surface curvature is not substantial for the ~13 nm Au NPs employed here and the SAMs can be considered to be well packed and ordered. Although shorter chain thiols are thought to form more disordered/defective SAMs, Lämmerhofer and coworkers37 have determined the surface coverage of thiols to be 6.3 molecules nm-2 for 3-MPA, marginally higher than the coverage of 5.7 molecules nm-2 for 11-MUA on 10-25 nm diameter Au NPs. Thus, the ligand density or the prevalence of defects does not vary substantially across the chain lengths employed here. The use of thiolate SAMs as controlled-thickness insulating spacers is therefore similarly justified in our experiments as the SAMs employed in multiple electron transfer studies on bulk Au electrode surfaces.32,38,39 Ferricyanide ([Fe(CN)6]3- or Fe3+, for simplicity) was used as a model acceptor. We recently demonstrated40 that under visible light excitation in colloidal conditions, an electron can be transferred from a Au NP to a [Fe(CN)6]3- molecule at/near the NP surface, leading to the one-electron reduction of the [Fe(CN)6]3- to
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ferrocyanide ([Fe(CN)6]4- or Fe2+ for simplicity). The left-over hole in the valence band (or deeper within the conduction band) of the Au NP is neutralized by a hole scavenger/proton donor such as ethanol (EtOH), which itself undergoes oxidation to acetaldehyde and protons. The SAM layer provides effective separation between the photoexcited NP and [Fe(CN)6]3- acceptor moieties. The [Fe(CN)6]3- complex is highly charged and strongly solvated by one or more hydration shells. Owing to its high polarity and large hydrodynamic radius, the [Fe(CN)6]3-complex is unlikely to be able to penetrate through the hydrophobic ligand shell. Furthermore, electrostatic repulsion by the carboxylate (-COO-) termini of the ligand chains prevent close approach of the [Fe(CN)6]3complex, precluding any penetration of [Fe(CN)6]3- moieties at defect sites in the SAM or exchange of thiolates by [Fe(CN)6]3-. Such a strategy was successfully employed in the work of Fermin and coworkers39 and is also validated by the high stability of our SAM spacer layer against any exchange with [Fe(CN)6]3- moieties (see Fig. S6). On the other hand, molecules of the hole scavenger EtOH, a good solvent for carboxylic acid thiols, can reach the NP surface either via diffusion through the ligand shell or via exchange with a thiolate. The latter ensures that hole scavenging by EtOH is not rate-limiting40 and the measured kinetics reflect the rate of hot electron transfer from the Au NP to [Fe(CN)6]3- separated by the insulating bridge. The plasmon excitation-catalyzed [Fe(CN)6]3- to [Fe(CN)6]4reduction process can be monitored spectrophotometrically (Fig. S1), allowing the measurement of the kinetics and the determination of the hot electron transfer rate constant, k, as shown in Figs. S2-S5. The photocatalytic measurements were performed in colloidal solution at a controlled temperature, while ensuring that the SAM spacer layer remained stable against the action of [Fe(CN)6]3- adsorption or laser excitation throughout the course of the measurement (Fig. S6). As anticipated, the hot electron transfer rate constant, k, decreased with increasing chain length, i.e., donor-acceptor distance (Fig. 1b). The fastest reaction was observed with the shortest ligand (3-MPA). There is a considerable drop in the rate constant from the 3-MPA case to 6-MHA, but thereafter the decrease with increasing ligand length is more gradual. In fact, the rate constant for the largest donor-acceptor distance (ca. 18 Å with 11-MUA) was only two-fold smaller than that at the smallest distance (ca. 6 Å with 3-MPA) under identical conditions and temperature (25 °C). In other words, the hot electron transfer seems to be operate even at considerably long donor-acceptor distances (ca. 20 Å), which qualifies as long-range.41 Typically, in donor-bridge-acceptor systems, electron transfer rate constants are found to decrease steeply with increasing donor-acceptor distance, d, consistent with the Marcus model for outer-sphere electron transfer:42 ɸ
∆‡
| | .
(1)
where, is the reduced Planck constant, kB is the Boltzmann constant, and is the reorganization energy involved in the electron transfer process. Since, the [Fe(CN)6]3- and the Au NP are not permanently bound, we modulate the rate by a pre-exponential factor of ɸ, which represents the fraction of [Fe(CN)6]3- molecules adsorbed transiently at the ligand-coated NP/solution interface. The application of the Marcus model to the electron transfer kinetics assumes that the rates are not simply limited by diffusion of [Fe(CN)6]3- molecules to this interface, but rather are limited by electron transport. The variation in rate as a function of ligand chain length ensures the validity of this assumption.
Figure 3. The reorganization energy, λ, is found to increase as a function of the donor-acceptor distance, d. Inset shows the nearlinear decrease in λ as a function of 1/d, which agrees with the two-sphere polarization model. In the typical non-adiabatic scenario, the electron transfers from the donor to acceptor (either via tunneling or hopping across the bridge) when a thermal fluctuation brings the system to an isoenergetic point of the initial and final potential energy surfaces. H in eq. 1 represents the transfer integral or matrix element between donor and acceptor states (and often including intermediary bridge states).43–46 For electron tunneling, the transfer integral depends on the spatial overlap between donor and acceptor orbitals involved in tunneling, which decays exponentially as a function of donor-acceptor distance, as: | | | ! | "#$ (2) thereby contributing to the distance-decay of the electron transfer rate constant. In eq. 2, ! is the matrix element for the donor/acceptor complex at contact and β is the distance-attenuation constant, which depends on the nature of the bridge connecting the donor and acceptor. For electron transfer processes across rigid insulating alkanethiol spacers on both flat Au surfaces and Au NPs, β has been measured to be ca. 1 Å-1,47–51 which amounts to a ca. three-fold drop in the rate constant for every angstrom increase in the donor-acceptor distance. It is apparent that the distance-dependence for hot electron transfer in our case is nowhere as steep. While it is quite common to extract the value of β from a plot of lnk vs. d, it must be realized that there is another source of distance dependence that must first be taken into account:52,53 the second term in the product of eq. 1 (the Frank-Condon factor), which represents the contribution of the activation free energy, ∆G‡, where:54 &'°)
∆% ‡ (3) * Here, ∆G° is the Gibbs free energy of the reaction (the free energy difference of the reactant and product) and is the reorganization energy, both of which can vary with the donor-acceptor distance due to Coulombic interactions between the donor and acceptor and their polarized solvation shells. These distance dependence contributions can be quite important in the overall electron transfer kinetics, as shown rigorously by Fayer and coworkers52 and by Rӧsch and coworkers.55 Fayer and coworkers used a model to
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simulate the distance dependencies involved in ∆G‡. Here, we directly measured the free energy of activation as a function of the ligand chain length, i.e., donor-acceptor distance. From the hot electron plots of ln(kT1/2) vs. 1/T (Fig. 2a), ∆G‡ was determined for 3-MPA, 6-MHA, 8-MOA, and 11-MUA from which we found a strong distance dependence (Fig. 2b). ∆G‡ is 10 kJ/mol higher for the 11-MUA case than for the 3-MPA case. From eq. 3, the trend in the barrier height, ∆G‡, can be explained by the distance dependence of the reorganization energy, λ. While Δ%° can depend on the distance, the effect is generally weaker.42 The reorganization energy involved in electron transfer has two contributions: λ - . / (4) where λi is the inner-sphere contribution, which corresponds to the energy cost of nuclear and bond reorganization of the donoracceptor complex to the configuration of the product. λo is the outer-sphere contribution, also known as the solvent reorganization energy, which corresponds to the energetic cost for reorganizing solvent molecules and counter-ions in the environment of the donor and acceptor entities to the product state configuration. While - is typically distance-independent. λo increases with increasing donor-acceptor distance as per the Marcus two-sphere polarization model42: /
∆0
*12
.3
4
56
.
4
57
4
8 9. $
4
1:;
8
4
1
/@ is the radius of the acceptor/donor entity, ∆A is the magnitude of the charge transferred from donor to acceptor, B/C is the optical dielectric constant of the solvent and BD is the static dielectric constant of the medium. As per the model, an increase in the donor-acceptor distance results in a decrease in the stabilizing Coulomb interactions between the donor, acceptor, and their polarized solvation shells in the reorganized (non-equilibrium) configuration. We estimated λ from the measured ∆G‡ (Fig. 3) and found that it increases substantially with increasing donoracceptor distance. The observed trend in λ as a function of d is similar to prior studies52,55 and is consistent with the two-sphere polarization model of Marcus’ electron transfer theory: as the inset of Fig. 3 shows, λ decreases near-linearly as a function of 1/d (see eq. 5). Thus, the distance dependence of the hot electron kinetics can be understood in detail (see Table S2). Photoexcitation of the Au NP produces electron-hole pairs, a fraction of which survive fast recombination within the NP. As described previously,40 holes react with EtOH to produce acetaldehyde and protons. Formed protons and water molecules solvate the charge-polarized NP with stored hot electrons. For the transfer of a hot electron from the NP to the [Fe(CN)6]3-, the anion has to reorganize to the nuclear configuration of [Fe(CN)6]4-. Polarized water molecules and the carboxy-thiol backbone around both entities need to rearrange and a proton also needs to be transferred from the solvation shell of the NP to that of the distorted [Fe(CN)6]3-. The energetic cost of this solvent reorganization increases with increasing distance between the photoexcited Au NP and the [Fe(CN)6]3- due to Coulomb effects described above. A longer carbon backbone, stabilized by stronger inter-chain interactions, may also require more energy to reorganize. Due to these effects, the reorganization energy and therefore the activation barrier height increase with increasing chain length and the hot electron transfer rate decreases. In fact, this trend in the Frank-Condon factor fully accounts for the decrease in k with increasing chain length and none of the decrease results from the electronic coupling term, |Hab|2. We estimated |Hab|2 as a function of chain length (Table S2) from the intercepts of plots in Fig. 2a. |Hab|2 does not decrease
Figure 4. Mode of electron transport from Au NP to [Fe(CN)6]3(denoted simply as Fe3+) across the insulating SAM bridge a) under low-bias, non-plasmon-assisted conditions and b) for plasmonassisted electron transfer. exponentially as a function of the donor-acceptor distance. In other words, |Hab|2 does not follow the form (eq. 2) expected for electron tunneling from donor to acceptor. As detailed below, this finding implies that the mode of plasmon-assisted hot electron transfer across the insulating spacer is in stark contrast to the wellestablished nature of non-plasmonic electron transfer from SAMcoated Au NPs or flat surfaces47,48 (Fig 4). In donor-bridge-acceptor systems,28,33,45,53,56–58 at distances as large as 18 Å, wave-function overlap between donor and acceptor (“through-space” coupling) is too small in magnitude for direct electron tunneling.28,46 Long-range electron transfer follows one or a combination of two mechanisms.45,53,57 Coherent electron tunneling from donor to acceptor can take place via virtual states in the bridge. This mechanism is referred to as super-exchange or “through-bond” coupling.28 The LUMO (or HOMO) orbitals of the bridge mediate electron (or hole) coupling between the donor and acceptor, but the electron does not occupy these bridge LUMO orbitals during the transfer. On the other side, there can be incoherent multi-step hopping of the electron from the donor to acceptor,59 in which case the electron is located at the bridge states for a short time during its transfer. While coupling via super-exchange has an exponential distance dependence, incoherent hopping shows weak or no distance dependence. The energy gap between the donor states and the bridge LUMO states dictates the relative contribution of electron hopping and super-exchange. For insulating, saturated carbon chain-based SAMs, where bridge LUMO states are considerably higher in energy than the donor states, super-exchange-mediated coherent tunneling is accepted to be the prevalent mechanism for electron transfer with a distance
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attenuation coefficient, β, of ca. 1.0 Å−1. With π-conjugated, conductive organic linkers, electron hopping becomes a contributor and smaller β values (0.2-0.6 Å−1) are measured.56 When incoherent hopping dominates, β is smaller than 0.2 Å−1 and may even approach zero when forward hops are greatly favored over backward hops, i.e., a wire-like transfer is achieved.60 Thus, the lack of distance decay measured for the transfer integral |Hab|2 here indicates that hot electron transfer takes place via multiple incoherent hops across the insulating carbon chain in a wire-like manner. This mode of transfer is in contrast to electrically/electrochemically driven electron transfer across SAM coatings on Au NPs,47,48 where β has been measured to be 0.8-1.0 Å−1. In such non-plasmonic, low-bias electron transfer (Fig. 4a), the Fermi level, EF, of Au {111} lies well within the large HOMOLUMO gap of the carbon backbone of the alkanethiol.61,62 Due to the large ca. 3 eV gap between the Fermi level of Au {111} and the LUMO of the carbon backbone (n = 4-16 C atoms63), electrons transfer over long distances primarily by super-exchange coupling. On the other hand, the scenario is markedly different here (Fig. 4b). Under constant visible light plasmonic excitation, in the presence of an efficient hole scavenger, the Au NP is considerably photocharged or cathodically polarized.40 At the conditions employed here (900 mW of 514.5 nm excitation, 5 M EtOH), a steady state photocharge (N) as high as 7500 electrons is built up on the Au NP, as estimated in our previous work.40 This photocharge is equivalent to an excess of 11% with respect to the free electron density of Au, which is sustained due to the high doublelayer capacitance of Au NPs in a solvating medium.40 The resulting quasi-Fermi level rise can enable energy matching between a hot electron, occupying a higher lying sp orbital in the Au, and the LUMO orbitals of the bridging carbon backbone, facilitating hopping of the electron onto the ligand. In fact, the hybridization of these higher lying Au sp orbitals and the LUMO orbitals of the strongly adsorbed thiolate, akin to findings of Linic and coworkers,64,65 may be a key to facile electron injection into the thiolate. In addition, the cathodic photocharge on the plasmonically excited Au NP amounts to a strong electrostatic (to be distinguished from the alternating electrodynamic) field E built up on the surface of the NP of radius R = 6.5 nm: G NO E F (6) M M HIJK JL F
HIJK JL F
The field at the Au NP/SAM interface is calculated from eq. 6 to be 109 V/cm, sufficiently large for field emission of electrons.66 This large electric field extending across the ligand shell can result in a downward energy cascade across the carbon backbone, such that forward electron transport is favored over backward electron transport.56 The strong bias produced under plasmonic excitation is thus conducive for wire-like transfer via multiple hops across the bridging ligand. In conclusion, we studied the distance dependence of hot electron transfer in a model system consisting of an Au NP donor separated from a [Fe(CN)6]3- acceptor by an insulating SAM spacer. In contrast to conventional electron transfer on Au NPs, hot electrons transport by hopping across the insulating ligand in a wire-like manner. The strong electrostatic field built up on the plasmonically excited, photocharged NP is responsible for the atypical nature of the electron transfer, which deserves further theoretical attention.67 Field-assisted electron hopping may play a role in ultrafast, efficient hot electron extraction from plasmon excited NPs.68,69
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details including synthesis and ligand exchange of Au NP; procedure of temperature-dependent photocatalytic experiments; photoconversion-time plots for trials performed; determination of rate constant; plots of the temperature dependence of the rate constant; table of average rate constant for each condition and associated error in rate constant over multiple trials; and determination of activation free energies, reorganization energies, and electronic coupling strengths from analysis. (PDF)
AUTHOR INFORMATION Corresponding Author *
[email protected] Author Contributions Y.K. conducted experiments, performed data analysis and cowrote the manuscript. A.J.W. performed spectroscopy studies. P.K.J conceived the project, designed experiments, performed data analysis, and wrote the manuscript.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This research was supported by the National Science Foundation under Grant (NSF CHE-1455011). A.J.W. was supported by a Springborn postdoctoral fellowship.
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