Plasmonic Measurement of Electron Transfer between a Single Metal

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Plasmonic Measurement of Electron Transfer between a Single Metal Nanoparticle and an Electrode through a Molecular Layer Ruihong Liu,† Xiaonan Shan,‡ Hui Wang,*,† and Nongjian Tao*,†,§ †

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State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ Department of Electrical and Computer Engineering, University of Houston, Houston, Texas 77204, United States § Biodesign Center for Bioelectronics and Biosensors and School of Electrical, Energy and Computer Engineering, Arizona State University, Tempe, Arizona 85287, United States S Supporting Information *

ABSTRACT: We study electron transfer associated with electrocatalytic reduction of hydrogen on single platinum nanoparticles separated from an electrode surface with an alkanethiol monolayer using a plasmonic imaging technique. By varying the monolayer thickness, we show that the reaction rate depends on electron tunneling from the electrode to the nanoparticle. The tunneling decay constant is ∼4.3 nm−1, which is small compared to those in literature for alkanethiols. We attribute it to a reduced tunneling barrier resulting from biasing the electrode potential negatively to the hydrogen reduction regime. In addition to allowing study of electron transfer of single nanoparticles, the work demonstrates an optical method to measure charge transport in molecules electrically wired to two electrodes.

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INTRODUCTION Metal nanoparticles exhibit various interesting properties that are attractive for energy storage, solar cells, catalysis, chemical sensing, and molecular electronics applications.1−5 A welldefined system is metal nanoparticles immobilized on an electrode surface via an organic or inorganic insulation layer.6−25 Such a nanoparticle−insulator−electrode sandwich system can mediate electron transfer (ET) reactions of redox species in the solution phase and lead to large enhancement in the redox current and long-range ET.13,26 Another important application of this nanoparticle−insulator−electrode system is demonstration of molecular electronic functions and investigation of the electrical conductivity of the organic molecules serving as the “insulator”.27,28 In all these applications, measuring ET between the nanoparticles and the electrodes is key. The ET process of the nanoparticle−insulator−electrode structures has been studied with the traditional electrochemical approach, which measures electrochemical current summed over the entire electrode, including contributions from all the nanoparticles on the electrode and also the areas between the nanoparticles. This approach makes it difficult to study single nanoparticles, and the result may be complicated by interactions and lateral electron transfer between nanoparticles on the electrode.12,17 Furthermore, the areas between nanoparticles often lead to a large background current, especially when pinholes are present in the insulation layer. To minimize the background current, ultra-microelectrodes coated with metal oxides by atomic layer deposition have been used to as the insulation layer.21,22,24 Another way to reduce the background current is to use a scanning tunneling microscope © XXXX American Chemical Society

(STM) and conducting atomic force microscope (c-AFM). Both can image and measure ET between a single metal nanoparticle and the electrode,28 but the electrical contact between the scanning tip of the STM or c-AFM and nanoparticle has a profound impact on the charge transport mechanism through the insulation layer and thus the interpretation of the data.29

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RESULTS AND DISCUSSION Here we study ET between single platinum nanoparticles (PtNPs) and a gold electrode separated by a self-assembled monolayer of n-alkanethiol (Cn) with a plasmonic imaging technique (Figure 1a).30,31 The plasmonic technique was developed to image various electrochemical processes, including catalytic reactions and double layer charging,30−33 but it has not been applied to probe the electron tunneling phenomenon through a molecular layer and study molecular conductivity. We sweep the electrode potential to the regime of catalytic reduction of hydrogen on the PtNPs and determine the ET rate from the catalytic reaction current.32 By varying the Cn length, we systemically tune the tunneling distance between the nanoparticle and the electrode and study the corresponding effect on the ET rate. This allows us to determine the electron tunneling decay constant and examine theoretical models developed to describe the ET process of the nanoparticle−insulator−electrode structures. The plasmonic imaging technique can not only visualize single nanoparticles but also image local electrochemical Received: May 20, 2019 Published: July 1, 2019 A

DOI: 10.1021/jacs.9b05388 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

creates imaging contrast, which is captured by a CCD imager (Figure 1b). Sweeping of the electrode potential negatively leads to catalytic reduction of hydrogen at the PtNPs, which changes the local refractive index around each PtNP, and thus scattering of the plasmonic wave. This change of scattering is reflected in the plasmonic image contrast, which is related quantitatively to the electrochemical current (see eq 1 and details herein).30,32 We extracted the changing scattering as a function of the electrode potential by subtracting the image frame captured at a potential far away from the hydrogen reduction regime (e.g., 0 V vs Ag/AgCl) from each of the subsequent images captured during sweeping of the potential to the hydrogen reduction regime. These images are referred to as differential plasmonic images, shown in Figure 1c, and the corresponding image contrast is referred to as plasmonic signal. The setup also records the electrochemical current of the entire working electrode vs potential following the traditional electrochemical method (Figure 1d), allowing us to compare results from the signal nanoparticle current measured with the plasmonic imaging and traditional methods. We measured the catalytic reduction of hydrogen on PtNPs by sweeping the electrode potential between 0 and −0.7 V while recording simultaneously the electrochemical current, and the differential plasmonic images over time. Figure 2a shows the plasmonic image intensity recorded for a single PtNP on 1-octanethiol (C8), where the large change in the differential image contrast is associated with hydrogen reduction. The average diameter of the PtNPs is 180 nm, and surface coverage of the PtNPs is ∼1.9 × 104 cm−2, as revealed by SEM and AFM (Figures s7 and s8). For this PtNP coverage, most of the electrode surface area is the blank Cn layer, including pinholes in the Cn layer, which results in a large background to the electrochemical current measured over the entire electrode with the traditional cyclic voltammetry. We examined the stability of the Cn layer by repeatedly sweeping

Figure 1. (a) Illustration of simultaneous electrochemical measurement and plasmonic imaging setup, where the inset shows electron transfer involved in electrocatalytic reaction at single platinum nanoparticles (PtNPs). (b) Plasmonic image of four PtNPs on thiol modified gold surface. (c) Differential plasmonic image (with false color) of the PtNPs at −0.7 V (vs Ag/AgCl) measures reduction of hydrogen. (d) Simultaneously recorded electrochemical current from the entire working electrode (area of 0.88 cm2). Potential sweeping rate, 0.1 V/s; electrolyte, 0.5 M H2SO4.

current, including charging and Faradaic processes associated with redox reactions.30−33 Its working principle and setup are illustrated in Figure 1a, where incident light is directed onto a gold thin film electrode to excite a plasmonic wave on the surface, and scattering of the plasmonic wave by a nanoparticle

Figure 2. (a) Plasmonic signal vs time during potential sweeping from 0 V to −0.7 V, and then back to 0 V (vs Ag/AgCl). (b) Snapshots of differential plasmonic images of a single 180 nm PtNP on C8 modified gold surface during the potential sweeping. Potential sweeping rate, 0.1 V/s. B

DOI: 10.1021/jacs.9b05388 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 3. (a) Distribution of plasmonic signal of individual PtNPs on C8 modified electrode at −0.7 V. (b) Distribution of corresponding current (obtained with eq 1) of individual PtNPs on C8 modified electrode at −0.7 V.

Figure 4. (a) Cyclic voltammograms of PtNPs on electrodes coated with self-assembled alkanethiols of different lengths. (b) Semilogarithmic plot of the electrochemical current measured from the cyclic voltammograms in panel a, which includes the contributions from all the PtNPs on the electrode but also regions without PtNPs, vs the alkanethiol monolayer thickness. (c) Plasmonic signal of single PtNPs on electrodes coated with self-assembled alkanethiols of different lengths during potential cycling. (d) Semilogarithmic plot of the plasmonic signal vs the alkanethiol monolayer thickness. (e) Electrochemical current of single PtNPs determined from the plasmonic images for electrodes coated with alkanethiols of different lengths.

the nanoparticle and the reactant, and electron tunneling from the electron (Au film) to the nanoparticle (Figure 1a, inset). Because the three processes are sequential (in series), the corresponding currents, iT(t), iet(t), and itun(t), are equal to each other (i.e., iT(t) = iet(t) = itun(t)). As explained previously, plasmonic imaging measures the refractive index change at the nanoparticle associated with the local concentrations of the reactant and product, which is iT(t), the mass-transport current (nF∇c). This allows us to determine iet and itun from the time and potential dependent plasmonic image of the nanoparticle. The quantitative relationship between the plasmonic signal, and the local electron transfer current of a single PtNP is given by30,32,34

potential over different ranges (Figure s2). When sweeping the potential to sufficiently negative potentials (e.g., below −0.8 V), instability in the Cn layer was observed, especially for short Cn (e.g., n = 6). For this reason, we focused on Cn with n = 8 or larger, and limited the potential to −0.7 V. The simultaneously recorded differential plasmonic images show the image contrast change of a PtNP during a full potential cycle (see Supporting Information for the movie). Figure 2b presents several snapshots of a movie of a single PtNP at different potentials, showing rapid change in the image contrast as the potential sweeps to −0.7 V, which measures the catalytic reaction on the PtNP. The hydrogen reduction at a single nanoparticle involves three steps: mass transport of reactant to and product out of the region near the nanoparticle surface, electron transfer from C

DOI: 10.1021/jacs.9b05388 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Δθ(t ) = A

∫0

t

iet(t )(t − t ′)−1/2 dt ′

redox species in the electrolyte on the nanoparticles and ET between the nanoparticles and the electrode.34 More recently, mass transport has been included explicitly.21 The three processes are sequential, and the currents involved are equal to each other, as we discussed above. In other words, the plasmonic image measures the local refractive index change associated with the mass transport, which is equal to the tunneling current via the organic layer. β extracted from the semilogarithmic plot of the plasmonic signal is 4.3 ± 0.2 nm−1. From the slope of the semilogarithmic plots of the electrochemical current, we obtained a β of 2.9 ± 0.1 nm−1, which is substantially smaller. The smaller β from the electrochemical current measurement can be attributed to be the large background current at the negative potentials, especially pinholes commonly found in self-assembled alkanethiol layers.18 The tunneling decay constant (β) for alkanes has been reported in literature14,15,35,36 and varies over a range of 7−9 nm−1. Compared to these literature values, β obtained for single PtNPs in this work is small. To understand the small decay constant observed here, we consider the energy diagram of the nanoparticle−insulator−electrode system for hydrogen reduction (Figure 5). Thermal power measurements revealed

(1)

where A = B(αR DR −1/2 − αODO−1/2)(nFπ 1/2)−1 (B is a constant related to the details of the optical setup, n is the number of electrons involved in the catalytic reaction, F is the Faraday constant, αO and αR are the changes in the local refractive indices per unit concentration of the oxidized and reduced molecules, DO and DR are the diffusion coefficients of the oxidized and reduced molecules). For hydrogen reduction, A was calibrated to be 0.0336 (mdeg s1/2)/C.32 Using eq 1, the local electron transfer current (iet(t)) of a single PtNP can be determined numerically from the time- and potential-dependent plasmonic signal, which leads to the measurement of the tunneling current (itun(t)). The above analysis neglects the contribution from the charging current, which is expected to be small compared to the electrocatalytic reaction at negative potentials. The present plasmonic imaging approach can resolve individual nanoparticles, allowing study of heterogeneity in different nanoparticles. We performed statistical analysis on plasmonic image intensity at −0.7 V, extracted the corresponding electrochemical current with eq 1 for different PtNPs (Figure 3), and observed significant variability for different PtNPs. Interactions of PtNPs with the Cn layer may induce mechanical deformation in the Cn layer and thus affect the tunneling current, which may contribute to the variability of different NPs in Figure 3. We also measured the catalytic reduction on 300 nm PtNPs (Figures s3−5). The overall results are similar to those of 180 nm PtNPs, but the plasmonic intensity and the corresponding current for the 300 nm PtNPs are larger. To study the ET mechanism between the PtNP and the electrode, we performed the measurement on PtNPs immobilized on electrodes coated with different lengths of Cn. The cyclic voltammograms obtained with the traditional electrochemical method show a rapid decrease in the cathodic current as the length of the organic layer increases (Figure 4a). As discussed above, the cathodic current contains contributions from the electrocatalytic reaction at the PtNPs and also reaction on the areas between the PtNPs. The corresponding plasmonic image intensity extracted from single PtNPs reveals a large change as the potential is swept negatively toward −0.7 V, where electrocatalytic reduction of hydrogen takes place (Figure 4c). The plasmonic signal versus potential plots show large hysteresis, which is due to transport of protons and hydrogen to and away from the nanoparticle surfaces. Using eq 1, we determined the electrochemical current from the plasmonic signal, leading to single nanoparticle-cyclic voltammograms, which reveal strong dependence of the ET rate on the thickness of the molecular layer (Figure 4e). Semilogarithmic plots of the reduction current measured with the cyclic voltammograms and plasmonic image intensity vs the Cn layer thickness are presented in Figures 4b,d, respectively. Both exhibit linear dependence, showing that the ET rate (k) varies according to k = k0 exp(−βL), where k0 is a constant, β is the decay constant, and L is the thickness of the Cn monolayer. We determined the thickness for each length of Cn from the capacitance measured by cycling the potential within the double layer-charging regime (Figure s1) and characterized the surface roughness by AFM (Figure s10). Redox reactions in nanoparticle−insulator−electrode structures have been described by a two-step process: reaction of

Figure 5. Tunneling model for the nanoparticle−insulator−electrode system showing changing of the tunneling barrier with the applied potential (see text for more details).

that highest occupied molecular orbital (HOMO)37 of Cn is closer to the Fermi energy than the lowest unoccupied molecular orbital (LUMO), indicating hole transport with an effective tunneling barrier of ϕB, determined by the Fermi energy levels of the electrode and Pt nanoparticles and the HOMO. 38−40 In the absence of applied potential, 2( 2m * )

β= α ϕB , according to the Simmons model, where ℏ m* is the electron effective mass and α is an adjustable parameter to account for nonideal tunneling barrier.38−41 According to the literature,34,40,41 α = 0.65 and ϕB = 1.43 eV (determined from the conductance of Cn sandwiched between two gold electrodes). When the electrode’s potential is swept negatively to the hydrogen reduction regime, we expect tilting of the barrier by an amount proportional to the potential change (ΔE). Consequently, the decay constant is reduced, 2( 2m * )

α ϕB − eζ ΔE , where ζ = 0.5 for given by, β = ℏ uniform tilting of the barrier.36 Independent determination of ΔE requires details of the molecular energy level alignment with respect to the electrode and PtNP Fermi levels, as well as its dependence on the applied potential.42 β in literature varies within a range of 7 to 9 nm−1.14,15,35,36 Using the above simple model, one would expect a potential change of >1.7 V. The model offers a simple interpretation of the smaller tunneling decay constant observed here, but a more accurate model including molecular scale characteristics of the molecules and detailed alignment of the molecular energy levels and electrode Fermi levels will be needed. Correct prediction of β will also D

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need to consider the interactions of the PtNPs with alkanethiols, which can affect the actual distance between the PtNPs and the gold electrodes.

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CONCLUSIONS We have studied catalytic reduction of single PtNPs immobilized on an alkanethiol self-assembled layer on an electrode with a plasmonic electrochemical imaging technique. The technique allows us to resolve hydrogen reduction on single PtNPs and study electron transfer between the PtNPs and the electrode. From the thickness dependence of the plasmonic image intensity, electron tunneling through the alkanethiol layer has been quantified with the decay constant of ∼4.3 nm−1 at −0.7 V. The result indicates a reduction in the work function of the electrode at negative potentials. The work demonstrates a capability to unravel electron transfer of nanoparticles on electrodes, which is critical to various applications, including electrocatalysis, energy conversion, and sensing. It also points to a new way to measure charge transport in molecules for molecular electronics, and thus contribute to the understanding of the fundamentals of electronic functions of molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b05388.

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Materials and methods and related experimental data (PDF) Movie of single nanoparticle (corresponding to the plasmonic images in Figure 2) during electron transfer reaction (AVI)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Xiaonan Shan: 0000-0001-7521-5573 Hui Wang: 0000-0003-1094-8826 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work was provided by NSFC (Nos. 21773117 and 21575062), China Postdoctoral Science Foundation (No. 2017M621695), and China Postdoctoral Science Special Foundation (No. 2018T110476). We thank Prof. Justin Gooding for many helpful discussions.

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DOI: 10.1021/jacs.9b05388 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX