Scanning Electrochemical Microscopy Study of Electron-Transfer

May 2, 2016 - Minjee Seo , Je Hyun Bae , Dae Woong Hwang , Bumju Kwak , Jeongse Yun , Sung Yul Lim , Taek Dong Chung. Electrochimica Acta 2017 258 ...
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Scanning Electrochemical Microscopy Study of Electron-Transfer Kinetics and Catalysis at Nanoporous Electrodes Je Hyun Bae,† Yun Yu,†,‡ and Michael V. Mirkin*,†,‡ †

Department of Chemistry and Biochemistry, Queens College, Flushing, New York 11367, United States The Graduate Center of CUNY, New York, New York 10016, United States

‡

S Supporting Information *

ABSTRACT: The complicated electrochemical properties of nanoporous electrodes arising from their geometry remain poorly understood because their complex structure defies easy interpretation of experimental results. The large surface area of a porous electrode results in a higher double-layer charging current and faster apparent heterogeneous rate constants, which are difficult to measure by voltammetry and other transient electrochemical techniques. In this article, we used a scanning electrochemical microscope equipped with a nanometer- or micrometer-sized tip to measure the rates of the same electron-transfer process at the flat and nanoporous Au electrodes. The origins and magnitude of the rate constant enhancement at the nanoporous surface (after the roughness factor correction) are discussed. Using the substrate generation/tip collection mode of the scanning electrochemical microscope operation, the higher catalytic activity of nanoporous Au for the oxygen reduction reaction was found from both substrate and tip voltammograms that can also be used for analyzing the reaction products.

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INTRODUCTION The remarkable characteristics of nanoporous electrodes arising from their geometry make them useful for a wide range of applications, including catalysis,1 energy conversion and storage systems (batteries,2 super capacitors,3 fuel cells,4 and dyesensitized solar cells5), electrochemical sensors,6 biosensors,7 and neural probes.8 Nanoporous electrodes exhibited unusually high catalytic activity that was attributed to the large surface area,9 specific crystal facets,10 and surface defects.11 It was also suggested that the increased activity of nanoporous catalysts can be due to various effects of nanoconfinement. Among them are the increased collision frequency and residence time of reactant molecules at the electrode surface,12,13 changes in the electronic structure of electroactive molecules,14,15 and solvent properties inside the nanopores.16 The increased rates of some relatively sluggish, one-step electron-transfer (ET) processes such as the oxidation− reduction of Fe2+/3+17 and ferro/ferricyanide18 at nanoporous carbon surfaces (as compared to the same kinetics measured at flat electrodes) have also been measured. The reliability of this data is uncertain in light of recent findings that the standard ET rate constants (k°) at carbon electrodes are much faster than the values measured previously,19 and the earlier experiments may have been affected by adsorption of low-level organic impurities.20 Moreover, several theoretical and experimental studies showed that the apparently enhanced kinetics could be attributed to the combination of the larger true surface area and mass-transfer effects,21−24 so that a smaller separation of the peak potentials in cyclic voltammograms (CVs) may reflect the electrode porosity rather than the enhanced kinetics.25 © XXXX American Chemical Society

Measuring fast ET reactions at Au or Pt porous electrodes is difficult. A large roughness factor ( f R) is supposed to result in significantly higher apparent rate constants. To determine large k° values at macroscopic porous electrodes, one has to use techniques with a sufficiently fast mass-transfer rate.26 The large surface area of a porous electrode results in a higher doublelayer charging current, further complicating the use of fast transient electrochemical techniques. Additionally, only a relatively slow reaction can be suitable for studying the effects of porosity on ET kinetics. If the ET rate is too fast, the electroactive species react at the pore mouth without entering deeply into the nanopore.24,27 Scanning electrochemical microscopy (SECM) has been extensively used for studying kinetics of outer-sphere ET reactions28 and electrocatalytic processes.29 Two modes of the SECM operation will be employed in our experiments. In a feedback mode experiment (Figure 1A), an SECM tip approaches the substrate surface in solution containing redox mediator (e.g., Fe2+), and the tip potential (ET) is such that the mediator oxidation occurs at a rate governed by diffusion. When the separation distance (d) becomes comparable to the tip radius (a), the oxidized form of the mediator (Fe3+) produced at the tip surface gets reduced at the substrate, and the tip current increases with decreasing d (positive feedback). Special Issue: Richard P. Van Duyne Festschrift Received: February 16, 2016 Revised: April 11, 2016

A

DOI: 10.1021/acs.jpcc.6b01620 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

as a substrate for preparing nanoporous Au electrodes. For electrochemical impedance spectroscopy (EIS), a programmed AC input of the CHI760E with a 10 mV amplitude over the frequency range from 1 Hz to 100 kHz was superimposed on the equilibrium potential of Fe2+/3+. The impedance data was analyzed using ZSimpWin 3.22 software (Echem Software). The two-electrode setup was used in experiments at micro- and nanoelectrodes with a 0.25 mm diameter Ag wire coated with AgCl serving as a reference. All experiments were carried out in a Faraday cage at room temperature (23 ± 2 °C). Fabrication of Nanoporous Au. Nanoporous Au electrodes were prepared by direct electrochemical etching to eliminate the possibility of a residual material (especially Ag) that may influence the electrocatalytic activity.36,37 Flat Au electrodes were anodized in oxalic acid, using the previously reported protocol.38,39 Briefly, a 2 mm Au disk was mechanically polished with slurries of 0.3 and 0.05 μm alumina on a Microcloth pad (Buehler) and sonicated in deionized water for 5 min between slurries. The polished electrode was anodized in a stirred 0.3 M oxalic acid solution at 0 °C (in the ice bath) using a three-electrode setup. The potential of the Au electrode was swept from 0 to 1.8 V vs Hg/Hg2SO4 reference at a scan rate of v = 1 mV/s and then biased at +1.8 V for 5 min. The pore depth increased linearly with etching time.38,39 The anodized electrode was electrochemically cleaned by cycling its potential between −0.43 V and +1.11 V vs Hg/Hg2SO4 in 1 M H2SO4 until reproducible identical CVs were obtained. Porous electrodes were characterized by scanning electron microscopy (SEM), using a Helios Dual Scanning Electron Beam/Focused Ion Beam System. Fabrication and Characterization of SECM Tips. Pt micro- and nanoelectrodes were fabricated as described previously.40 Briefly, disk-type electrodes were prepared by pulling 25 μm diameter annealed Pt wires (Goodfellow) into borosilicate capillaries (Drummond; OD, 1.0 mm; ID, 0.2 mm) with the help of a P-2000 laser puller (Sutter Instrument Co.). The pulled tips were polished on a 50 nm alumina disk (Precision Surfaces International) under video microscopic control. The electrode radius was evaluated from steady-state voltammograms, optical microscopy (micrometer-sized electrodes), and AFM images41 (nanoelectrodes; Figure S1 in Supporting Information). The RG value (i.e., the ratio of the glass insulator radius to that of the Pt disk) was about 10. The appropriate protection was used to avoid electrostatic damage to the nanotips.42 SECM Setup and Procedures. SECM experiments were carried out using a previously described home-built instrument.34 The nanoelectrode tip was positioned a few tens of micrometers above the substrate surface. A long-distance video microscope was used to monitor the initial approach of the SECM tip to the substrate. The tip was then brought closer to the substrate in an automated “surface hunter” mode until the tip current changed by ∼10%. The current versus distance curve was obtained during the subsequent fine approach. For each current−distance curve, the substrate was biased at the desired potential prior to acquiring data for 120 s to allow the concentration profile near the electrode surface enough time to equilibrate. In ORR experiments, the tip electrode was positioned about 500 nm above the substrate electrode in O2-saturated aqueous solution containing 10 mM HClO4 and 0.1 M NaClO4. The separation distance was determined from the SECM approach curve obtained using H+/H2 couple as a redox mediator.43 In

Figure 1. Schematic representation of the feedback (A) and substrate generation/tip collection (B) modes of the SECM operation.

The tip current can be recorded as a function of d (approach curve) or tip x−y position (imaging). The feedback mode offers fast mass-transfer rate and high spatial resolution, essential for addressing nanometer-sized pores. Importantly, the effects of the double-layer charging current and resistive potential drop in solution are practically negligible in steady-state feedback-mode SECM measurements. The feedback mode of the SECM operation cannot be used for probing low signal sources (e.g., a kinetically slow electrochemical reaction in a nanopore) that produce feedback current lower than the tip current in the bulk solution (iT,∞).30 Due to its higher sensitivity, the substrate generation/tip collection mode (SG/TC mode) is more suitable for probing relatively slow electrocatalytic processes.29 In a SG/TC experiment, the tip collects electroactive species generated at the substrate surface (e.g., H2O2 in Figure 1B). The SG/TC mode was previously employed in quantitative studies of the H2O2 production in the oxygen reduction reaction (ORR) at noble metal UMEs31,32 and for detecting the products of CO2 reduction at Au electrodes.33 We used this mode for mapping hydrogen fluxes produced at single Au nanoparticles.34 Here we employ the SG/TC mode to compare the catalytic activities of the ordered mesoporous Au (i.e., with the pore size between 2 and 50 nm, according to the IUPAC classification35) and flat Au substrates toward the ORR and detecting hydrogen peroxide species produced at both surfaces.

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EXPERIMENTAL SECTION Chemicals. Ferrocenemethanol (FcMeOH, 97%, Alfa Aesar) was sublimed before use. Potassium chloride (99%, Alfa Aesar), iron(II) perchlorate hydrate (98%), iron(III) perchlorate hydrate, sodium perchlorate (ACS reagent, ≥ 98%), perchloric acid (ACS reagent, 70%), sulfuric acid (99.999%,), and oxalic acid (98%), all from Sigma-Aldrich, were used as received. Aqueous solutions were prepared using deionized water from the Milli-Q Advantage A10 system equipped with Q-Gard T2 Pak, a Quantum TEX cartridge, and a VOC pak. The total organic carbon (TOC) was 50 nm, the concentration of this species (cO) is close to 0. In contrast, at ES = E°′ − 90 mV, the slow reduction rate allowed the oxidized form of the mediator to diffuse all the way to the bottom of the pore (profile 1; the pore depth, dp = 320 nm, was obtained for f R = 19.5, as discussed in Supporting Information). More exactly, the effective penetration depth was evaluated from the current distribution along the pore wall (Figure 6C). The effective penetration depth (dpenetration) was defined as the z-coordinate of the point such that the integral of the diffusion flux over the pore surface between the pore mouth (z = 0) and this point equals 95% of total current flowing in the pore. The simulations showed that dpenetration is essentially independent of the tip radius as long as a ≫ ap and the tip−substrate separation distance (Figure 6D). Thus, all dpenetration values were simulated for d/a = 0.6. The decrease in dpenetration with increasing cathodic overpotential (Figure 6B,C) corresponds to the smaller surface area D

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Figure 6. Simulations of the penetration of tip-generated species into the 20 nm diameter conductive pore. (A) Geometry of the simulation space and parameters defining the diffusion problem for a larger disk-shaped SECM tip approaching a nanopore (not to scale). (B) Simulated concentration profiles of the oxidized species inside the pore. (C) Integrated diffusion flux at the pore surface as a function of the vertical distance from the pore orifice. (D) Normalized effective penetration depth as a function of the normalized tip/substrate distance. (B−D) a = 500 nm, RG = 10, and ES − E°′, mV = −90 (1), −190 (2), and −290 (3). (B, C) d/a = 0.6.

of porous Au involved in Fe3+ reduction. The red curve in Figure 5B shows the potential dependence of the rate constant corrected for the variable dpenetration. For each point (red symbols in Figure 5B), the kf value was obtained iteratively, as follows. The initial dpenetration value was obtained from the simulation of the flux distribution inside the pore (Figure 6C) for a given ES and corresponding kf,eff. The first approximation for kf was calculated by dividing the corresponding kf,eff (black symbols in Figure 5B) by the roughness factor found from dpenetration (see Supporting Information). The next approximation for dpenetration was produced from the simulation with the kf value found as the first approximation; this dpenetration was used to obtain the second approximation for the kf. The process continued until the kf and dpenetration values were found such that the product of the kf and the corresponding roughness factor was equal to kf,eff measured at the given potential. The transfer coefficient value obtained from the red curve in Figure 5B (α = 0.44) is close to those reported in the literature48,49 and to the value measured here at the flat Au substrate. The extrapolation of this dependence to ES = E°′ yields k° = 3.3 × 10−3 cm2 s−1, which is almost three times the value measured at the flat Au. The extraction of the rate constants from SECM current− distance curves (Figures 4 and 5) was based on the theory developed for the irreversible substrate reaction46,51 and therefore could be done only at a significant cathodic overpotential. An alternative approach based on the analysis of the iT versus ES curves enables the measurement of k° and α in the vicinity of formal potential.52 Figure 7 shows four iT

Figure 7. Experimental (solid lines) and theoretical (symbols) iT versus ES curves obtained at a 500 nm radius Pt tip positioned near a nanoporous Au substrate in solution containing 10 mM Fe(ClO4)2 and 1 M H2SO4. v = 50 mV/s; ET, V = 0.8 (curves 1 and 2) and −0.6 (curves 3 and 4). d, nm = 365 (curves 1 and 3) and 600 (curves 2 and 4).

versus ES curves obtained with a 500 nm radius tip positioned at different heights above two different spots of the porous Au substrate. Curves 1 and 2 were recorded with the tip biased at ET = +0.8 V vs Hg/Hg2SO4 reference, where Fe2+ was oxidized at the diffusion-controlled rate; the diffusion-controlled reduction of Fe3+ occurred at the tip at ET = −0.6 V (curves 3 and 4). The experimental voltammograms (symbols in Figure 7) were fitted to the theory (solid curves52) to determine the k° and α values (Table 1). E

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Table 1. Kinetic, Thermodynamic, and Mass-Transport Parameters Used for Calculating Theoretical iT versus ES Dependences in Figure 7 d (nm) 365 600 a

keff ° (cm/s) 0.15 0.13

k° (cm/s) −3

7.5 × 10 6.7 × 10−3

α

DR (cm2/s) −6

4.2 × 10 4.2 × 10−6

0.41 0.40

DO (cm2/s)a −6

4.1 × 10 4.1 × 10−6

E0′ (V) −0.013 −0.013

Ref 47.

Figure 8. Probing ORR at flat (1) and nanoporous (2) Au surfaces by SECM in the SG/TC mode. Substrate voltammograms for oxygen reduction (A) and corresponding iT versus ES curves (B) were obtained in O2-saturated solution containing 10 mM HClO4 and 0.1 M NaClO4. v = 2 mV/s; a = 400 nm; ET = 0.5 V vs Hg/Hg2SO4; d ≈ 500 nm.

The analysis of the iT versus ES curves (Figure 8B) helps to identify ORR products at both flat (curve 1) and porous (curve 2) gold substrates. With a relatively long separation distance (d ≈ 500 nm) and strongly acidic pH, the only electroactive ORR product that could cross the tip−substrate gap was hydrogen peroxide. At ET = 0.5 V vs Hg/Hg2SO4, the tip current was due to the H2O2 oxidation. The close correspondence between the first waves in iT versus ES and iS versus ES curves obtained at both flat and porous substrates indicates that H2O2 was the principal ORR product at both types of Au surfaces at small cathodic overpotentials (i.e., 0 to −0.3 V vs Hg/Hg2SO4). The increase in the iS at higher overpotentials corresponding to the second reduction wave (Figure 8A) is accompanied by the decrease in the tip current (Figure 8B) due to the reduction of H2O2 to H2O at the substrate.32 With the nanoporous Au substrate, this decrease is much steeper and the tip current decays to the offset level (∼1−2 pA; curve 2) at ES ≈ −0.6 V, while the iT remains measurable even at more negative potentials of the flat Au substrate (curve 1). These results suggest a higher catalytic activity of nanoporous Au toward 4e− ORR, in agreement with the earlier studies at the rotating ring− disk electrode.53

The analysis of the iT versus ES dependences obtained at a nanoporous Au substrate ( f R = 19.5) is more straightforward because most kinetic information is contained in the middle portion of such a curve corresponding to ES = E°′ ± ∼ 50 mV, where the electroactive species can diffuse all the way though the pore; therefore, k°eff = k°f R. Two very similar sets of k° and α values were obtained at the tip facing different spots on the substrate surface and at different d. This indicates that the nanoporous structure is uniform over the electrode surface, consistent with the SEM image of porous Au in Figure 2 (inset). Both α values are in excellent agreement with that determined from the ln kf versus Es dependence. The k° values extracted from the iT versus ES dependences are slightly higher, though the agreement is quite good, keeping in mind the approximate nature of the dpenetration evaluation in Figure 5B. The roughness-corrected rate constants obtained at nanoporous substrates are ∼3−5 times higher than the value determined at the flat Au electrodes, suggesting the possibility of the nanoconfinement effect on the ET rate. With the average pore size of about 20 nm in the nanoporous Au, the measured rate constants could be influenced by doublelayer effects. However, a very high electrolyte concentration (1 M) was used in this work to minimize such effects. This electrolyte concentration corresponds to the thickness of the diffuse layer of ∼1 nm, which is much smaller than the pore diameter; therefore, the double-layer effects should be negligible. ORR at Nanoporous Au Electrodes. The effect of porosity on electrocatalytic activity of Au for ORR was probed in the SG/TC mode of the SECM operation. Figure 8A shows voltammograms of ORR obtained at the flat (curve 1) and nanoporous (curve 2) 2 mm diameter disk-shaped Au substrate. Nanoporous Au is more active toward ORR, as evidenced by the about 150 mV positive shift of the first 2e− reduction wave and the increase in the peak current in Figure 8A. The second wave corresponding to H2O2 to H2O also appears at less negative potentials at nanoporous Au.

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CONCLUSIONS We used the SECM to measure kinetics of the Fe3+ reduction at nanoporous Au surfaces. Kinetic experiments at nanoporous interfaces are challenging because the measured rate constant is proportional to the microscopic surface area of the electrode at which ET occurs; hence, the effective rate constants at porous electrodes are significantly higher. To ensure consistency of our results, we first determined very similar (within 10%) k° values for the Fe3+ reduction at the same flat Au electrode by two completely different techniques, SECM and EIS. The use of a submicrometer-sized SECM tip allowed relatively slow kinetics of Fe3+ reduction to be measured at nanoporous Au at cathodic overpotentials up to approximately −300 mV. To evaluate the true surface area involved in the interfacial ET, one has to know F

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(6) Evans, S. A. G.; Elliott, J. M.; Andrews, L. M.; Bartlett, P. N.; Doyle, P. J.; Denuault, G. Detection of Hydrogen Peroxide at Mesoporous Platinum Microelectrodes. Anal. Chem. 2002, 74, 1322− 1326. (7) Qiu, H.; Xue, L.; Ji, G.; Zhou, G.; Huang, X.; Qu, Y.; Gao, P. Enzyme-Modified Nanoporous Gold-Based Electrochemical Biosensors. Biosens. Bioelectron. 2009, 24, 3014−3018. (8) Heim, M.; Rousseau, L.; Reculusa, S.; Urbanova, V.; Mazzocco, C.; Joucla, S.; Bouffier, L.; Vytras, K.; Bartlett, P.; Kuhn, A.; et al. Combined Macro-/Mesoporous Microelectrode Arrays for Low-Noise Extracellular Recording of Neural Networks. J. Neurophysiol. 2012, 108, 1793−1803. (9) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Ordered Nanoporous Arrays of Carbon Supporting High Dispersions of Platinum Nanoparticles. Nature 2001, 412, 169−172. (10) Seo, B.; Kim, J. Electrooxidation of Glucose at Nanoporous Gold Surfaces: Structure Dependent Electrocatalysis and Its Application to Amperometric Detection. Electroanalysis 2010, 22, 939−945. (11) Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N.; et al. Atomic Origins of the High Catalytic Activity of Nanoporous Gold. Nat. Mater. 2012, 11, 775−780. (12) Han, J.-H.; Lee, E.; Park, S.; Chang, R.; Chung, T. D. Effect of Nanoporous Structure on Enhanced Electrochemical Reaction. J. Phys. Chem. C 2010, 114, 9546−9553. (13) Bae, J. H.; Han, J.-H.; Han, D.; Chung, T. D. Effects of Adsorption and Confinement on Nanoporous Electrochemistry. Faraday Discuss. 2013, 164, 361−376. (14) Martínez de la Hoz, J. M. M.; Balbuena, P. B. Geometric and Electronic Confinement Effects on Catalysis. J. Phys. Chem. C 2011, 115, 21324−21333. (15) Martinez de la Hoz, J. M. M.; Balbuena, P. B. Small-Molecule Activation Driven by Confinement Effects. ACS Catal. 2015, 5, 215− 224. (16) Senapati, S.; Chandra, A. Dielectric Constant of Water Confined in a Nanocavity. J. Phys. Chem. B 2001, 105, 5106−5109. (17) Acevedo, D.F.; Reisberg, S.; Piro, B.; Peralta, D. O.; Miras, M. C.; Pham, M. C.; Barbero, C. A. Fabrication of an Interpenetrated Network of Carbon Nanotubes and Electroactive Polymers To Be Used in Oligonucleotide Biosensing. Electrochim. Acta 2008, 53, 4001−4006. (18) Poh, H. L.; Pumera, M. Nanoporous Carbon Materials for Electrochemical Sensing. Chem. - Asian J. 2012, 7, 412−416. (19) Patel, A. N.; Guille Collignon, M.; O’Connell, M. A.; Hung, W. O. Y.; McKelvey, K.; Macpherson, J. V.; Unwin, P. R. A New View of Electrochemistry at Highly Oriented Pyrolytic Graphite. J. Am. Chem. Soc. 2012, 134, 20117−20130. (20) Nioradze, N.; Chen, R.; Kurapati, N.; Khvataeva-Domanov, A.; Mabic, S.; Amemiya, S. Organic Contamination of Highly Oriented Pyrolytic Graphite as Studied by Scanning Electrochemical Microscopy. Anal. Chem. 2015, 87, 4836−4843. (21) Menshykau, D.; Streeter, I.; Compton, R. G. Influence of Electrode Roughness on Cyclic Voltammetry. J. Phys. Chem. C 2008, 112, 14428−14438. (22) Streeter, I.; Wildgoose, G. G.; Shao, L. D.; Compton, R. G. Cyclic Voltammetry on Electrode Surfaces Covered with Porous Layers: An Analysis of Electron Transfer Kinetics at Single-Walled Carbon Nanotube Modified Electrodes. Sens. Actuators, B 2008, 133, 462−466. (23) Henstridge, M. C.; Dickinson, E. J. F.; Aslanoglu, M.; BatchelorMcAuley, C.; Compton, R. G. Voltammetric Selectivity Conferred by the Modification of Electrodes Using Conductive Porous Layers or Films: the Oxidation of Dopamine on Glassy Carbon Electrodes Modified with Multiwalled Carbon Nanotubes. Sens. Actuators, B 2010, 145, 417−427. (24) Punckt, C.; Pope, M. A.; Aksay, I. A. On the Electrochemical Response of Porous Functionalized Graphene Electrodes. J. Phys. Chem. C 2013, 117, 16076−16086.

the effective penetration depth of the electroactive species into the nanopore, which depends on the ET rate and thus changes with the electrode potential. The self-consistent values of surface area-corrected kinetic parameters were determined using different tips and two different SECM-based approaches. The k° values measured at the nanoporous Au were 3−5 higher than those obtained at flat Au electrodes, suggesting that the ET rate at the nanoporous interface can be increased by the effect of nanoconfinement. SG/TC mode SECM experiments showed the enhanced ORR catalysis at nanoporous Au. The comparison of the tip and substrate signals enabled the identification of H2O2 and showed that the nanoporous Au is significantly more active toward the 4e− ORR than the flat gold. In agreement with a recent study,54 SECM is a useful technique for studying electrocatalysis at nanoporous interfaces. Additional advantages can be expected from measuring the electrocatalytic activity of single conducting nanopores by using smaller SECM tips; such experiments are currently underway in our laboratories.

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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/acs.jpcc.6b01620. Tip characterization, impedance measurements, evaluation of the roughness factor and effective penetration depth, formulation of the diffusion problem for SECM with a nanoporous substrate, and COMSOL model report (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 718-997-4111. Fax: 718997-5531. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the National Science Foundation (CHE-1416116). We thank Drs. Huolin Xin and Kim Kisslinger (Center for Functional Nanomaterials at Brookhaven National Laboratory) for assistance with SEM imaging.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b01620 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b01620 J. Phys. Chem. C XXXX, XXX, XXX−XXX