Core–Shell Au@Metal-Oxide Nanoparticle Electrocatalysts for

Sep 25, 2017 - Enhanced catalysis for electrochemical oxygen evolution is essential for the efficacy of many renewable energy technologies, including ...
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Letter pubs.acs.org/NanoLett

Core−Shell Au@Metal-Oxide Nanoparticle Electrocatalysts for Enhanced Oxygen Evolution Alaina L. Strickler,† Marıá Escudero-Escribano,†,‡ and Thomas F. Jaramillo*,†,§ †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark § SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ‡

S Supporting Information *

ABSTRACT: Enhanced catalysis for electrochemical oxygen evolution is essential for the efficacy of many renewable energy technologies, including water electrolyzers and metal−air batteries. Recently, Au supports have been shown to enhance the activity of many 3d transition metal-oxide thin films for the oxygen evolution reaction (OER) in alkaline media. Herein, we translate the beneficial impact of Au supports to high surface area, device-ready core−shell nanoparticles consisting of a Au-core and a metal-oxide shell (Au@MxOy where M = Ni, Co, Fe, and CoFe). Through a systematic evaluation, we establish trends in performance and illustrate the universal activity enhancement when employing the Au-core in the 3d transition metal-oxide nanoparticles. The highest activity particles, Au@CoFeOx, demonstrate an overpotential of 328 ± 3 mV over a 2 h stability test at 10 mA cm−2, illustrating that strategically coupling Au support and mixed metal-oxide effects in a core−shell nanoparticle morphology is a promising avenue to achieve device-ready, high-performance OER catalysts. KEYWORDS: Oxygen evolution reaction, core−shell nanoparticles, electrocatalysts, nanostructures

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lysts with the promoting effect of Au is an interesting strategy to improve the performance of OER electrocatalysts. Very few reports have investigated nanoparticulate Au/metal-oxide systems for OER; previous studies have involved mixed Au and MnOx21,26 and Au and CoOx nanoparticles,30 Au/Co alloy nanoparticles,31 Co and Ni on Au nanoclusters,32 and Au@ CoOx core−shell nanoparticles.33,34 Of particular interest are core−shell nanoparticles, where a Au core is surrounded by an OER-active transition metal-oxide shell. Capitalizing on the beneficial effects of Au, the core−shell nanoparticle structure33 has the potential to achieve enhanced activity in a device-ready form for membrane electrode assemblies (MEAs), while also providing an increased number of active sites in close proximity to Au, higher conductivity, and greater material utilization than films. Herein we provide a systematic investigation of the Au-core 3d transition metal oxide-shell morphology for a broad set of OER catalysts. Upon synthesizing, characterizing, and evaluating electrochemical performance of Au-core metal oxide-shell nanoparticles (Au@MxOy where M = Ni, Co, CoFe, and Fe), we find that the Au-core provides a universal OER activity

he oxygen evolution reaction (OER) is important for sustainable electrochemical fuel generation and energy storage with major applications including water electrolysis, electrochemical CO2 reduction, and metal−air batteries.1−6 Currently, the slow kinetics of this reaction limit efficiencies and commercial implementation of many of these technologies, necessitating the development of high-performance OER catalysts.7,8 In acidic electrolyte, precious metal oxides based on Ir and Ru are the only materials that demonstrate activity and moderate stability for the OER.9−12 In contrast, in alkaline conditions many earth-abundant, first-row transition metal oxides and hydroxides (e.g., Ni, Co, Fe and Mn) exhibit both high activity and stability as anode catalysts.13−16 Nevertheless, high overpotentials are still required for the OER for all known catalysts in acid and in base.17,18 Catalyst supports have been shown to play a key role in the OER activity. In particular, Au substrates have been shown to enhance the OER catalytic activity of transition metal oxide films based on Fe, Ni, Co, and Mn.19−25 In these reports, Au was found to induce electronic effects,19−21,23,26−28 provide lower barrier mechanistic pathways,22 and/or form bimetallic interfacial oxides leading to higher OER activity.19,20,29 In many systems, the Au-induced enhancement decreases rapidly with increasing thickness of active metal-oxide limiting observed enhancement to low catalyst loadings in the thin film morphology.19,20,29 Developing advanced nanoparticulate cata© XXXX American Chemical Society

Received: June 3, 2017 Revised: August 20, 2017

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Figure 1. TEM micrographs of (a) NiOx, (b) CoOx, (c) CoFeOx, and (d) FeOx nanoparticles. TEM micrographs of (e,i) Au@NiOx, (f,j) Au@ CoOx, (g,k) Au@CoFeOx, and (h,l) Au/FeOx nanoparticles.

respectively. For simplicity, transition metals are referred to with the generic formula MOx as oxidation and protonation states are known to change throughout electrochemical testing.14,19 Pure transition metal oxide nanoparticles of NiOx, CoOx, CoFeOx, and FeOx are depicted in Figure 1(a− d), whereas larger CoFeOx and FeOx nanoparticles are depicted in Figure S2. Monodisperse 3.3 ± 0.4 nm diameter Au nanoparticles (Figure S1) were used as the Au cores. Au@ NiOx, Au@CoOx, Au@CoFeOx, and Au/FeOx nanoparticles are shown in Figure 1(e−h) with corresponding highresolution micrographs depicted in Figure 1(i−l). The core− shell structure is visibly apparent by the Z-contrast. Au nanoparticles for the Ni and Fe systems have apparently undergone ripening due to their higher temperature syntheses, whereas the lower temperature syntheses for the Co and CoFe systems maintain the original Au size and dispersion. It should be noted that Au/FeOx particles are not core−shell but have a peanut-like morphology. This is likely a result of incomplete water removal during synthesis leading to preferential single site nucleation. Because of the high resistivity of iron oxide, the peanut morphology where gold can make a direct contact between the iron oxide particle and the conductive support is interesting for catalysis applications. TEM energy dispersive spectroscopy (TEM-EDS) and inductively coupled plasma mass spectrometry (ICP-MS) of core−shell nanoparticles were used to verify the bulk catalyst composition. EDS spectra for the four core−shell materials are shown in Figure S3 and confirm that expected elements are present. The ratios of the integrated peaks between the K transition of the 3d-TMs and the Au(L) transition are consistent between catalysts indicating a similar 3d-TM to Au molar ratio. ICP-MS confirms that core−shell catalysts have a 3d-TM to Au molar ratio of 4 ± 1 to 1. Because of the similarity in k-factors between the Fe(K) and Co(K) transitions, the bulk Co/Fe ratios can be reliably estimated from EDS peak ratios. Consistent with synthesis reagent ratios, the Co/Fe ratios for

enhancement. Wet chemical methods were used to synthesize single metal and mixed metal-oxide nanoparticles with and without Au cores with high uniformity. To avoid the effects of incidental Fe incorporation, Fe-free electrochemical methods were employed, a successful translation to carbon-supported nanoparticulate systems. We also combine the effect of mixed metal-oxide nanoparticles with the enhancing effect of Au for the first time, by synthesizing and testing Au@CoFeOx core− shell nanoparticles. Au cores were found to enhance the OER activity of all the transition metal oxide materials tested with Au@CoFeOx core−shell nanoparticles demonstrating both high activity and stability for the OER. This work demonstrates that the core−shell nanostructure is a promising generalized strategy for translating thin film support effects into a highsurface area, device-ready architecture and that by combining support effects with mixed metal-oxides high-performance OER catalysts can be achieved. Nanoparticle catalysts were synthesized via wet-chemical methods (see Supporting Information for details). Briefly, core−shell nanoparticles were synthesized by first forming the Au core using a method developed previously.35 Ni, Co, CoFe, and Fe oxides were subsequently grown on the preexisting cores following adapted methods from literature.33,36−39 The appropriate solvent, capping ligands, and reaction conditions were found to be heavily dependent on shell material. While most materials required a unique synthesis, the generalized procedures involve decomposing or reducing a transition metal precursor under inert atmosphere onto the preexisting Au-core in the presence of an organic solvent and one or more capping agents. Using this generalized method, nanoparticles were synthesized with precursor molar ratios of 5:1 Ni/Au, 10:1 Co/ Au, 5:5:1 Co/Fe/Au, and 5:1 Fe/Au. Corresponding Au-free 3d-transition metal (TM) oxide nanoparticles were synthesized following the same generalized procedure. TEM micrographs of the resulting nanoparticles and particle size distributions are shown in Figure 1 and Table S1, B

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containing catalysts were tested in a glass cell with a Ag/ AgCl reference electrode. To avoid the effects of Fe contamination on OER performance, Fe-free catalysts were tested in a Teflon cell with a Teflon Hg/HgO reference electrode and purified electrolyte following procedures developed previously.41 In addition, Fe was removed from the carbon black support and glassy carbon disks by stirring in 2 M HCl at 50 °C and soaking in 10 vol % nitric acid overnight, respectively. Figure S6 demonstrates that the purification of all components is crucial to completely eliminate the effects of Fe contamination and achieve performance commensurate with Fe-free NiOOH thin films.41 We first present an overview of measured activity across all samples and subsequently discuss details on each material system. Electrochemical OER activity of nanoparticle catalysts with and without Au was evaluated with cyclic voltammetry at 10 mV s−1 as summarized in Figure 3 and Table S1. Without Au, catalyst activity in terms of geometric current density and ICP-MS normalized 3d-TM mass activity trends are as follows: CoFeOx > CoOx > NiOx > FeOx. The finding of CoFeOx as the most active catalyst is consistent with previous results of electrodeposited thin films tested in similar conditions.15,42 Activity trends of these materials vary throughout literature; whereas the trend found here is consistent with that found previously for a controlled study of electrodeposited thin film catalysts,15 it differs from another systematic report of electrodeposited films on Au and Pt supports: FeOxHy > CoOxHy > NiOxHy.42 Differences in synthesis methods, catalyst morphology, and supports may potentially account for observed differences in activity trends. In particular, noble metal supports have been shown to significantly affect the activity of related metal oxide catalyst systems, e.g., MnOx and CoOx.21,28 It should be noted that during nanoparticle synthesis trace Fe impurities may be introduced; however, this effect is likely insignificant as the NiOx and CoOx activities measured here match well with Fe-free catalysts.42 With the addition of Au, the activity of all catalyst materials is enhanced. Activity is observed to follow the same trend as with the base materials with Au@CoFeOx > Au@CoOx > Au@NiOx > Au/FeOx. The only slight improvement in activity of Au@ CoOx over CoOx on a current density basis can be attributed to lower Co yield resulting from the low-temperature shell growth synthesis. On a mass normalized basis, both Au@CoOx and Au@NiOx demonstrate 5-fold improvements in mass activity at 350 mV overpotential over CoOx and NiOx, respectively (530 versus 110 A/g3d TM for Au@CoOx and CoOx, respectively, and 170 versus 35 A/g3d TM for Au@NiOx and NiOx, respectively). Au@CoFeOx has the highest mass activity of 1350 A/g3d TM at 350 mV overpotential and demonstrates approximately a 10fold enhancement over 6 nm CoFeOx (120 A/g3d TM). The addition of Au to FeOx results in the greatest boost in activity with a 20-fold increase in mass activity at 450 mV overpotential (580 A/g3d TM versus 30 A/g3d TM for Au/FeOx and 3 nm FeOx, respectively). Tafel plots of the catalysts (Figure 3d and S12− S14) show that for Ni, Co, and CoFe systems, the addition of Au reduces the onset potential significantly but does not substantially change the Tafel slope possibly indicating that Au reduces the limiting potential of the catalyst without altering the rate limiting step of the reaction. For the Fe system, Au both reduces the onset potential and the Tafel slope. The comparatively high Tafel slope of FeOx is likely due to poor conductivity and stability of the FeOx nanoparticles.

Au@CoFeOx and CoFeOx particles were found to be near unity (see Figure S4). In agreement with EDS, ICP-MS confirms that Co/Fe ratios are near optimal40 with CoFeOx nanoparticles having a 0.4:0.6 molar ratio of Co/Fe and Au@ CoFeOx having a 0.6:0.4 molar ratio of Co/Fe. The spatial composition of Au@CoFeOx nanoparticles was analyzed using scanning TEM-EDS (STEM-EDS). Figure 2 depicts the dark-

Figure 2. (a) Dark-field STEM micrograph of Au@CoFeO x nanoparticles indicting in yellow where the line scan was performed. (b) STEM-EDS line scan of a Au@CoFeOx nanoparticle showing integrated counts for Co(K), Fe(K), and Au(L) peaks fitted using TIA software. Results are consistent with an approximately 4 nm Au particle surrounded by a 1−2 nm CoFeOx shell.

field STEM micrograph and resulting EDS analysis of a line scan across a single particle. Integrated counts for the fitted Fe(K), Co(K), and Au(L) peaks are consistent with a 4 nm Au core surrounded by a 1−2 nm CoFeOx shell. Nanoparticle inks were prepared by loading 20 wt % total metal on carbon black (Vulcan XC-72) and dispersing in isopropyl alcohol with an ion-exchanged Nafion binder (see Supporting Information for details). TEM micrographs of supported nanoparticles are shown in Figure S5. Catalyst inks were loaded onto polished glassy carbon disks (GCDs) at a loading of 1 μmol of 3d-TM per cm2. This loading was determined by the amount of metal used during synthesis and is therefore a conservative estimate since 100% synthetic yield was not achieved. Catalysts were tested in a three-electrode rotating disk electrode (RDE) setup rotated at 1600 rpm in O2 saturated 1 M KOH with a Pt wire counter electrode. All catalysts were tested at least three times and activities are reported as mean values with standard deviations. FeC

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Figure 3. (a) OER activity summary of nanoparticle catalysts quantified as overpotential required to achieve 10 mA cm−2 on a geometric basis. (b) OER activity summary of nanoparticle catalysts quantified as the overpotential required to achieve 300 A g−1 of 3d-TM. All Au containing catalysts exceed their pure metal-oxide counterparts in OER activity on both a geometric and mass activity basis. (c) Cyclic voltammograms of Au-containing catalysts and (d) mass activity Tafel plots performed in O2 saturated 1 M KOH (purified for Fe-free catalysts) at 10 mV s−1 and 1600 rpm with solution series resistance compensation.

synthesized and electrochemically evaluated (Figure S7). The Au3nm/FeOx9nm nanoparticles show decreased OER activity compared to the Au7nm/FeOx11nm particles with a larger interfacial area. This suggests that maximizing Au/FeOx interfacial area is important for achieving activity enhancement beyond that achieved by minimizing conductivity limitations via decreasing FeOx particles size. This supports the notion of a Au/FeOx synergistic interaction, potentially due to the formation of a hydrated Au−Fe mixed surface oxide.29,43 Au@CoOx and CoOx nanoparticulate systems (Figure 4c) require overpotentials of 367 ± 6 and 380 ± 7 mV, respectively, to achieve 10 mA cm−2, consistent with reports of similar catalyst systems.17,28,33,44 The modest activity enhancement on a geometric current density basis can be rationalized by the decreased Co yield in the Au@CoOx synthesis compared to the CoOx nanoparticles. The observed significant enhancement in Co mass activity with the Au core is consistent with previous reports of thin film and nanoparticulate Au/CoOx architectures19,28,30,32 and has been attributed to Au-facilitated oxidation to CoIV.19 While variations in shell thickness and morphology were not explored here to maintain comparable catalysts between material systems, further activity enhancements can likely be achieved though optimization of shell thickness and particle morphology as activity has been found to be dependent on CoOx film thickness19,28 and degree of undercoordination.45 For CoFeOx (Figure 4d), reducing the nanoparticle diameter from 12 to 6 nm decreases the overpotential at 10 mA cm−2 by

Cyclic voltammograms comparing the geometric current density of individual material systems are shown in Figure 4 whereas ICP-MS determined mass normalized activities are shown in Figure S11. The activities of Fe-free NiOx systems are compared in Figure 4a. While Au@NiOx activates upon electrochemical cycling as a result of increased electrochemically accessible Ni sites (increased Ni redox features), NiOx nanoparticles deactivate possibly due to a decreased amount of active defect/edge sites.41 After activation, Au incorporation reduces the overpotential required to achieve 10 mA cm−2 by 90 ± 20 mV over pure NiOx. The overpotentials and enhancement observed here are consistent with computational predictions23 and experimental observations of thin films of NiOx on Au20,23,41 and may potentially result from Aufacilitated oxidation of Ni or the formation of a AuNi interfacial oxide.20 Enhancement is not likely a result of increased conductivity as the resistance loss expected for NiOx is negligible.20,41 In the case of FeOx (Figure 4b), reducing the nanoparticle diameter from 14 to 3 nm lowers the overpotential by 160 ± 10 mV at 10 mA cm−2, likely due to conductivity limitations associated with a larger size.42 Compared to 3 nm FeOx particles, Au/FeOx peanut nanoparticles (with 11 ± 3 nm FeOx particles) require a lower overpotential by 120 ± 7 mV at 10 mA cm−2 despite their larger average FeOx particle size. To investigate the nature of this enhancement, Au/FeOx peanut nanoparticles with a smaller FeOx nanoparticle size (9 nm) and decreased Au/FeOx interfacial area (Au3nm/FeOx9nm) were D

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Figure 4. Electrochemical cyclic voltammograms of nanoparticle catalysts in O2 saturated 1 M KOH (purified for Fe-free catalysts) at 10 mV s−1 and 1600 rpm with solution resistance compensation. (a) Au@NiOx compared to NiOx nanoparticles initially and after 100 activation cycles from 0.25 to 1.6 V vs RHE at 200 mV s−1. (b) Au/FeOx compared to 3 and 14 nm FeOx particles initially and after repeated cycling. (c) Au@CoOx and CoOx nanoparticles initially and after 10 cycles. (d) Au@CoFeOx compared to 6 and 12 nm CoFeOx nanoparticles initially and after 20 cycles. Au containing nanoparticles demonstrate enhanced OER activity compared to their corresponding pure 3d metal-oxide counterparts.

74 ± 13 mV (from 441 ± 13 to 367 ± 2 mV, respectively). Unlike FeOx, the lower overpotential for smaller particle size likely results from an increased number of undercoordinated sites45 or greater surface area46 rather than a conductivity enhancement.40 The activity of 6 nm CoFeOx exceeds that of CoO x, FeO x, and Au/FeOx illustrating the synergistic interaction of Co and Fe, where CoOx is hypothesized to serve as a high surface area, conductive scaffold.14,40 For these particular morphologies, the Co matrix is more influential on FeOx activity than Au incorporation alone, consistent with previous results for CoFeOOH thin films.29,42 The synergy of the binary CoFeOx system has been explained by beneficial electronic interactions of the metal oxides leading to near optimal metal−hydroxide (MOH) bond strength, a property that correlates well with OER intrinsic activity.15 The incorporation of the Au core into CoFeOx reduces the overpotential at 10 mA cm−2 to 328 ± 3 mV, a 39 ± 4 mV reduction relative to 6 nm CoFeOx. This could be due to the Au-facilitated oxidation of the active transition metals or the formation of an active mixed metal-oxide interface. The enhanced activity of Au@CoFeOx demonstrates that both the synergistic role of Au as well as the Co matrix play a role to increase activity. The stability of the core−shell nanoparticle catalysts was evaluated by a chronopotentiometric 2 h hold at 10 mA cm−2 in O2 saturated 1 M KOH (purified for Fe-free catalysts) at 1600 rpm. The initial overpotential versus the overpotential required after the 2 h hold is depicted in Figure 5 where the dashed line indicates stable catalytic activity. Because of transient behavior

Figure 5. Catalyst stability after a 2 h chronopotentiometric hold at 10 mA cm−2 in O2 saturated 1 M KOH (purified for Fe-free catalysts) at 1600 rpm summarized as initial overpotential versus the overpotential after 2 h. The inset depicts the chronopotentiometry data for the highest performing catalyst, Au@CoFeOx.

at the start of the experiments, the initial overpotential was taken after 3 min (see Figure S9 for raw chronopotentiometry (CP) data). Au@NiOx and Au@CoOx slightly decrease in activity during stability testing likely due to morphological or structural rearrangements reducing the number of active sites or inhibiting access to them.40 Additionally, some increase in E

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work demonstrates that thin film support effects can be successfully translated to high surface area, device-ready nanostructures for a variety of materials. Furthermore, Aucore mixed metal oxide-shell nanoparticles that combine support and mixed metal-oxide effects provide a promising route to achieve optimal OER kinetics.

overpotential may be attributed to ink delamination from the electrode as a result of constant gas production, high electrode rotation rates, and inherent carbon instability at high potentials. Sudden increases in overpotential seen in the CP data for Au@ CoOx (Figure S9) are a result of these phenomena. The activity of Au/FeOx continually decreases over the 2 h CP as seen in Figures 5 and S9. Instability is a result of Fe dissolution as FeO42−(aq)47 and concomitant carbon corrosion at high potentials leading to activity loss and delamination of the carbon support. Au/FeOx demonstrates remarkable stability, however, compared to pure 3 nm FeOx particles that exhibit a complete loss in OER activity within 5 min of testing (Figure S10). While the incorporation of Au is not sufficient to completely stabilize Fe, the activity retention of Au/FeOx is substantially improved by (1) Au lowering the overpotential required to achieve 10 mA cm−2 thus decreasing the driving force for Fe dissolution and carbon corrosion/ delamination and (2) the preservation of the active Au/FeOx interface by the primary dissolution of the nanoparticle surface. This activity/stability relation is observed during cyclic voltammetry as well (Figure 4b) where higher activity resulted in less degradation with cycling. In contrast to Au/FeOx particles, Au@CoFeOx nanoparticles show no decay in activity (see inset of Figure 5 and Figure S8 for CVs before and after stability testing) illustrating that the Co matrix serves to stabilize Fe from dissolution.40,42 ICP-MS of Au/FeOx and Au@CoFeOx confirms that while there is an 80% loss in Fe and a 50% loss in Au for the Au/FeOx due to dissolution, detachment, and/or carbon corrosion, there is no significant loss in Co, Fe, or Au for the Au@CoFeOx particles after stability testing. The activity and stability of the Au@CoFeOx particles demonstrate the ability of the core−shell nanostructure to take advantage of support and mixed metal-oxide effects to yield high performance OER catalysts. Core−shell Au@MxOy nanoparticles were synthesized via wet chemical methods and examined for the OER. TEM analysis revealed that Au@NiOx, Au@CoOx, and Au@CoFeOx display a distinctive core−shell structure whereas Au/FeOx has a peanut morphology. To deconvolute the effects of the Aucore and incidentally incorporated Fe, Fe-free testing methods were successfully extended to supported nanoparticulate systems. All catalysts demonstrate enhanced activity with Au compared to their non-Au containing nanoparticle counterparts. Activity enhancement due to the Au-core is greater than that resulting from decreased particle size supporting the notion of a Au/MOx synergistic interaction that goes beyond conductivity effects. For the FeOx system, the significant activity enhancement with Au and dependence on Au/FeOx interfacial area reflect the synergistic interaction with Au and the high resistivity of FeOx at low overpotentials that limit activity to sites near a conductive interface (i.e., Au or carbon black). This suggests that tuning of nanoparticle morphology and active oxide thickness is a promising method for activity optimization. While Au/FeOx is not perfectly stable, it demonstrates remarkable stability compared to FeOx without Au due to its higher activity which decreases the driving force for Fe dissolution. The highest performance catalyst, Au@ CoFeOx, shows both high activity and stability for the OER with an overpotential of 328 ± 3 mV at 10 mA cm−2 after 2 h of stability testing. The activity of Au@CoFeOx exceeds that of CoFeOx and Au/FeOx, illustrating that both the synergistic effect of Co as a high surface area, conductive, and stabilizing support and the Au-core play a role to enhance activity. This

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02357. Details on catalyst synthesis, preparation, and additional material and electrochemical characterization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas F. Jaramillo: 0000-0001-9900-0622 Author Contributions

All authors have given approval to the final version of the manuscript. Funding

U.S. Department of Energy, Basic Energy Sciences, through the SUNCAT Center for Interface Science and Catalysis National Science Foundation Graduate Research Fellowship. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Basic Energy Sciences, through the SUNCAT Center for Interface Science and Catalysis. A.S. acknowledges fellowship support from the National Science Foundation Graduate Research Fellowship Program (NSF-GRFP). M.E.-E. acknowledges the Danish Council for Independent Research for her Individual Postdoctoral and Sapere Aude: Research Talent grants. The authors would like to thank Jakob Kibsgaard for the graphical abstract figure.

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DOI: 10.1021/acs.nanolett.7b02357 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.7b02357 Nano Lett. XXXX, XXX, XXX−XXX