Anodically Grown Binder-Free Nickel Hexacyanoferrate Film: Toward

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Anodically Grown Binder-Free Nickel Hexacyanoferrate Film: Toward Efficient Water Reduction and Hexacyanoferrate Film Based Full Device for Overall Water Splitting Hoa Thi Bui,† Nabeen K. Shrestha,*,‡ Shubhangi Khadtare,† Chinna D. Bathula,‡ Lars Giebeler,§ Yong-Young Noh,‡ and Sung-Hwan Han† †

Department of Chemistry, Institute of Materials Design, Hanynag University, Seongdong-gu, 04763 Seoul, Republic of Korea Department of Energy and Materials Engineering, Dongguk University, 04620 Seoul, Republic of Korea § Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Helmholtzstraße 20, 01069 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: One of the challenges in obtaining hydrogen economically by electrochemical water splitting is to identify and substitute cost-effective earth-abundant materials for the traditionally used precious-metal-based water-splitting electrocatalysts. Herein, we report the electrochemical formation of a thin film of nickel-based Prussian blue analogue hexacyanoferrate (Ni-HCF) through the anodization of a nickel substrate in ferricyanide electrolyte. As compared to the traditionally used Nafion-binder-based bulk film, the anodically obtained binder-free NiHCF film demonstrates superior performance in the electrochemical hydrogen evolution reaction (HER), which is highly competitive with that shown by a Pt-plate electrode. The HER onset and the benchmark cathodic current density of 10 mA cm−2 were achieved at small overpotentials of 15 mV and 0.2 V (not iR-corrected), respectively, in 1 M KOH electrolyte, together with the long-term electrochemical durability of the film. Further, a metal-HCFelectrode-based full water-splitting device consisting of the binder-free Ni-HCF film on a Ni plate and a one-dimensional Co-HCF film on carbon paper as the electrodes for the HER and the oxygen evolution reaction (OER), respectively, was designed and was found to demonstrate very promising performance for overall water splitting. KEYWORDS: binder-free, Prussian blue analogue, nickel hexacyanoferrate, anodic film, hydrogen evolution, electrocatalyst electrocatalyst for the oxygen evolution reaction (OER).9,10 This implies that an efficient electrolysis device should be equipped with Pt-based cathode material and an Ir- or Ru-based anode material to decompose water and extract hydrogen effectively in terms of input electricity and output hydrogen production. However, because of the very high costs of these precious-metal-based catalysts, the development of low-cost electrocatalysts is in high demand from both the scientific and commercial points of view in this zero-emissions energyconversion technology. As a result, great attention has been paid to the development and investigation of new electrocatalysts that are highly efficient and robust for water decomposition.10−12 The ultimate goal of such investigations is to replace the precious-metal-containing electrocatalysts with earth-abundant low-cost materials that could provide superior intrinsic performance for water electrolysis.13 Various transition-metal chalcogenides have recently been demonstrated to be HER and/or OER electrocatalysts.9−24 However, gas-phase reactions at elevated temperatures and high-temperature/high-

1. INTRODUCTION Everyday consumption of fossil and atomic fuels is ultimately creating a global energy crisis in the near future on one hand, while on the other hand, the combustion of these fuels is causing environmental deterioration. Consequently, scientists are making enormous efforts to find or develop alternative sources of green and sustainable fuels.1 Among the alternatives as a renewable and clean fuel, hydrogen has recently been given a great deal of attention.2−5 Owing to its importance in several major industrial processes, hydrogen could, in the future, play a key role in the energy-carrier economy. Generally, fossil fuels are reformed to release hydrogen, and this technology is currently the key source of hydrogen being used and supplied. Although water might be an abundantly available renewable source of hydrogen and the generation of hydrogen from the electrolysis of water can potentially be a very promising approach, one of the challenges with electrolysis is being able to split water into its constituents efficiently and, thereby, extract the hydrogen economically.6,7 Traditionally, precious-metalbased electrocatalysts have been used to carry out efficient water electrolysis.8 To date, platinum is considered to be the state-of-the-art electrocatalyst for the hydrogen evolution reaction (HER), and IrO2 or RuO2 is the state-of-the-art © 2017 American Chemical Society

Received: April 21, 2017 Accepted: May 9, 2017 Published: May 9, 2017 18015

DOI: 10.1021/acsami.7b05588 ACS Appl. Mater. Interfaces 2017, 9, 18015−18021

Research Article

ACS Applied Materials & Interfaces

2. EXPERIMENTAL DETAILS

anodization, the film was washed thoroughly in running water and then immersed for 24 h in water, with the water being changed every 4 h. Finally, the film was blow-dried in a stream of argon. To determine the amount of catalytic Ni-HCF film, the film was first employed for the HER. After the electrolysis, the film was washed again thoroughly and blow-dried. The nickel substrate with film was further dried under a vacuum overnight in a desiccator, and after removal of the film by ultrasonic agitation in water, the amount of film was determined from he weight difference. The amount of film was thus estimated to be nearly 1.4 mg cm−2. Further, for the bulk Ni-HCF film, first, the powdery mass of NiHCF was prepared by anodizing nickel substrates at 2 V against a Pt anode in 0.1 M K3[Fe(CN)6] aqueous electrolyte without glycerol. The loosely packed powdery mass of Ni-HCF was collected by scraping the films. Ten milligrams of vacuum-dried Ni-HCF powder was mixed with 1.25 mL of ethanol and 0.25 mL of Nafion as a binder. The mixture was ultrasonically agitated for 30 min to produce an ink. From the resulting ink, a Ni-HCF catalyst content of 210 μL cm−2 was loaded onto a nickel plate by drop-casting. The amount of film thus obtained was nearly 1.4 mg cm−2. This film is referred to here as the bulk Ni-HCF film. 2.3. Film Characterization. The surface topography of the film was examined by field-emission scanning electron microscopy (FESEM; Hitachi S-4800). The crystal structure of the thin film was studied by X-ray diffraction [XRD; Rigaku D/MAX 2600 V, Cu Kα (λ = 0.15418 nm)], high-resolution transmission electron microscopy (HR-TEM, Omega EM), and selected-area electron diffraction (SAED) analysis. Elemental mapping of the film was performed by scanning transmission electron microscopy (STEM). Electrochemical measurements were performed using a CompactStat Electrochemical Interface & Impedance Analyzer (Ivium Technologies) in a three-electrode electrochemical cell with Pt foil as the counter electrode and a Ag/AgCl (3 M KCl) reference electrode. For the measurements, aqueous 1 M KOH (pH 14) electrolyte was deaerated for 30 min by being bubbled with argon, and a circular area of the electrode having a 7-mm diameter was exposed to deaerated the electrolyte. The potential in voltammetry was swept at a rate of 5 mV s−1. All data were collected without iR compensation. The onset potential for the HER was estimated from the bias potential corresponding to the intersection of the voltammetry curves at j = 0 mA cm−2. A commercial Pt/C catalyst (20 wt % Pt, Premetek Co.) was used as a benchmark HER electrocatalyst. Further, the chargetransfer resistance for HER was studied by electrochemical impedance spectroscopy in the frequency range from 0.01 Hz to 1.5 MHz, and the measurements were performed under a bias of −0.25 V vs RHE. The experimental bias potential (EAg/AgCl) applied against Ag/AgCl reference electrode was converted into the potential on the reversible hydrogen electrode (RHE) scale using the equation E(RHE) = EAg/AgCl + 0.059pH + E0Ag/AgCl, where E0Ag/AgCl = 0.21 V at 25 °C. The electrochemically active surface areas of the materials were measured in terms of the double-layer capacitance (Cdl) using cyclic voltammetry at various sweeping rates. Cdl was estimated using the relation (Ia − Ic)/2 = Cdl(dE/dt) and from the slope of the straight line obtained by plotting (Ia − Ic)/2 versus dE/dt, where Ia, Ic, and dE/dt represent the cathodic current, the anodic current, and the potential sweeping rate, respectively.

2.1. Chemicals and Materials. All chemicals were of reagent grade and were used without further purification. Substrates for the deposition of catalysts were cleaned sequentially in acetone, ethanol, and deionized water for 10 min each in an ultrasonic bath and were dried in a stream of argon. Deionized water was used throughout the experiments. 2.2. Anodization and Ni-HCF Film Formation. Nickel sheet (ø = 0.3 mm, Nilaco, Tokyo, Japan) was cut into 2 × 2 cm2 pieces that were then cleaned. The cleaned nickel substrate (2 × 2 cm2) was exposed to electrolyte by pressing it against a rubber O-ring (ø = 10 mm) in an electrochemical cell using a copper back-contact electrode plate. The substrate was anodized at 2 V for 18 h against a Pt cathode in 0.1 M K3[Fe(CN)6] electrolyte (99% purity, Sigma-Aldrich) prepared in 80 vol % glycerol (≥99.5%, Sigma-Aldrich). After the

3. RESULTS AND DISCUSSION Anodization of a nickel substrate in an aqueous electrolyte containing 0.1 M K3[Fe(CN)6] prepared in 80 vol % glycerol led to the formation of a yellowish colored film, as shown in the left inset of Figure 1. A typical characteristic current transient curve, as shown in Figure S1a (Supporting Information), was recorded during the anodization process, which revealed that the high activation current observed in the initial stage of anodization gradually settled down as the substrate surface was covered by the deposited film. The electrolyte, under these anodizations, remained transparent even for a long anodization

pressure synthesis routes are not only harsh and complicated but also increase production costs. As a result, the development of a facile solution-based strategy that can advance the synthesis and practical application of transition-metal-based electrocatalysts is in high demand. Recently, we demonstrated an aqueous-solution-based chemical transformation approach for the formation of thin films of transition-metal chalcogenides and studied their electrocatalytic properties.25−27 Further, the intrinsic OER activity of cobalt−iron oxyhydroxide was reported to about 2 orders of magnitude higher than that of CoOOH.28 Therefore, on the basis of the synergic influence of Co and Fe metal centers on water oxidation and the above chemical transformation approach, we synthesized vertically aligned one-dimensional nanostructures of cobalt-based Prussian blue analogue hexacyanoferrate (Co-HCF) crystals that were found to be highly competitive with the recently developed best high-performance OER electrocatalysts in alkaline and neutral electrolytes.29 Being highly porous highsurface-area compounds, Prussian blue and its analogues are ideal for ion intercalation/deintercalation and catalysis. Consequently, in addition to being used for electrochemical water oxidation,29,30 Prussian blue-based hexacyanoferrate materials have also been employed in electrochemical energy storage and artificial enzyme peroxidase-based electrochemical biosensors.31,32 In a previous study,29 when a one-dimensional structured Co-HCF film was tested as a HER catalytic electrode, the performance was not satisfactory, particularly in terms of the kinetics and thermodynamics of the HER. Herein, we report the formation of a thin crystalline film of nickel-based Prussian blue analogue hexacyanoferrate (NiHCF) on a nickel substrate by the electrochemical anodization route. The thus-obtained Ni-HCF film exhibited an onset potential for the HER at an overpotential of 15 mV, and a benchmark current density of 10 mA cm−2 was achieved at a small potential of −0.2 V vs RHE (reversible hydrogen electrode). This key benchmarking HER current density of the anodically deposited binder-free Ni-HCF film was achieved at a potential similar to that demonstrated by a Pt-plate electrode, which was significantly smaller than the values of −0.29 and −0.38 V vs RHE demonstrated by a Ni-HCF bulk film on a nickel plate and a bare nickel plate, respectively. Further, a full water-splitting electrochemical device consisting of a binderfree Ni-HCF film grown on a nickel substrate and a vertically aligned one-dimensional nanostructured film of Co-HCF grown on a carbon paper as electrodes for the HER (cathode) and OER (anode), respectively, was designed and found to show promise for the overall splitting of water in alkaline media.

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DOI: 10.1021/acsami.7b05588 ACS Appl. Mater. Interfaces 2017, 9, 18015−18021

Research Article

ACS Applied Materials & Interfaces

Figure 2. XRD patterns of the Ni-HCF powder collected by scraping the anodically grown Ni-HCF film on a nickel substrate. Blue line patterns are reference data for Ni-HCF obtained from JCPDS card no. 51-1897.

Figure 1. SEM image (top view) of the Ni-HCF film grown anodically on a nickel substrate for 18 h at 2 V against a Pt cathode in 0.1 M K3[Fe(CN)6] aqueous electrolyte with 80 vol % glycerol. The right inset image shows a magnified SEM view, and the left inset image shows a photograph of the anodized film.

process (i.e., 18 h), revealing that the precipitation reaction for film formation took place preferentially at the electrode− electrolyte interface. The thus-obtained film was uniform and adhered strongly to the substrate. This film is referred to here as the anodically deposited binder-free film. In contrast, when the glycerol content in the electrolyte was decreased or when no glycerol was used, in addition to the significantly higher current due to the spontaneous dissolution of the nickel substrate (Figure S1b), the electrolyte became turbid, and a loosely agglomerated powdery mass with poor adherence to the anodized surface was deposited. For comparison, the anodically formed powdery mass was collected and was subsequently deposited on the nickel substrate using Nafion as a binder, which is referred to here as the bulk film. Figure 1 shows the top-view SEM image of the anodically deposited film, and the right inset SEM image revealed that the film consisted of an assembly of microflakes. The intrinsically open-framework structure together with the porous morphology resulting from the microflake assembly of Ni-HCF allow for the rapid diffusion of electrolyte to the catalytic sites, which can kinetically enhance the charge-transfer reaction. Figure 2 shows the XRD patterns of powder collected by scraping the anodically deposited films with or without the addition of glycerol to the electrolyte. Both of these XRD patterns are well-matched with each other, and they are in agreement with the JCPDS card no. 51-1897, suggesting that both of these anodically grown films consist of Ni-HCF crystals. The Ni-HCF films exhibited three major distinct diffraction peaks corresponding to the (200), (220), and (400) crystallographic planes of the fcc structure. The anodically deposited film shown in Figure 1 was further characterized by highresolution TEM, which revealed a d spacing of 3.62 Å corresponding to (220) planes (Figure 3). Further, the SAED image of the Ni-HCF film in the inset of Figure 3 shows diffraction rings, which are also in line with the results of the XRD patterns shown in Figure 2. These findings confirm the formation of a Ni-HCF film on the nickel substrate upon anodization in ferricyanide electrolyte. The chemical composition of the Ni-HCF film was studied by X-ray photoelectron spectroscopy (XPS), and the XPS survey spectrum is shown in

Figure 3. HR-TEM image of anodically grown Ni-HCF film on a nickel substrate. Inset shows SAED image of the Ni-HCF film. dspacing of 3.62 Å corresponds to (220) planes.

Figure S2a, revealing that the material consisted of K, C, N, Fe, and Ni elements. Further, the high-resolution N 1s XPS spectrum of the film shows a peak at about 397.7 eV (Figure S2b), suggesting the existence of bonding between C and N from the cyano-coordinated compound.30 In addition, the locations of the 2p peaks in the high-resolution Ni 2p and Fe 2p XPS spectra (Figure S2c,d) reveal the existence of Ni(II) and Fe(III) in coordination with the cyanide group.33−35 Thus, the XPS results are in good agreement with the above XRD fingerprint identification of Ni-HCF and suggest the formation of KNi[Fe(CN)6] by the aforementioned electrochemical anodization process. Figure 4 shows the STEM elemental mapping images of the anodically deposited binder-free NiHCF film, which represents the uniform distribution of all elements throughout the film. To study its potential value, the anodically deposited NiHCF film was tested as an electrocatalyst for the HER. Figure 5 shows the linear sweep voltammograms obtained from a barenickel substrate, a Ni-HCF bulk film on a nickel substrate, an anodically deposited binder-free Ni-HCF film on a nickel substrate, a Pt-plate electrode, and the state-of-the-art Pt(20%)/C HER catalyst electrodes in 1 M KOH solution 18017

DOI: 10.1021/acsami.7b05588 ACS Appl. Mater. Interfaces 2017, 9, 18015−18021

Research Article

ACS Applied Materials & Interfaces

Figure 6. Nyquist plots demonstrating the charge-transfer resistances for HER under a bias of −0.25 V vs RHE for various cathodic materials in 1 M KOH aqueous electrolyte. Figure 4. Scanning transmission electron microscopy (STEM) images showing the elemental mapping of anodically grown Ni-HCF film. The scale bar in each image represents 1 μm.

relatively smaller. This could be due to the intimate contact of the Ni-HCF film with the substrate, which thus allows electron transport from the substrate to the film, whereas such electron transport is relatively retarded in the case of the Ni-HCF bulk film deposited on a nickel substrate. However, as compared to the blank nickel substrate, the comparatively smaller chargetransfer resistance for the HER of the Ni-HCF film on the nickel substrate could be due to the fact that the Ni-HCF film has a comparatively larger number of active sites for the adsorption of atomic hydrogen with ideally zero free energy and, hence, these sites act as catalytic active centers for the HER. This presumption was justified by measuring cyclic voltammograms (Figure S3), which showed a larger current density and double-layer capacitance for the Ni-HCF film than for the nickel substrate, suggesting that the electrochemically active surface areas of the samples are in the same order. It is worth noting that the onset potential of the anodically deposited binder-free Ni-HCF film competes closely with that of the Pt-plate electrode, showing an onset overpotential of about 15 mV, as shown in Figure 5 (curves iii and iv). Although the rate of the HER is lower at the beginning of the bias up to about −0.115 V, the anodically deposited binder-free Ni-HCF film achieved the benchmark HER current density of 10 mA cm−2 at the same overpotential of 0.2 V (not iR-corrected), as demonstrated by the Pt-plate electrode. In addition, the HER performance of the binder-free Ni-HCF film is highly competitive with that of the best high HER performance of Pt-free catalysts found recently in alkaline (pH 14) electrolyte. Details of comparative studies on the HER performances of the anodically grown binder-free Ni-HCF film and other recently reported HER electrocatalysts are provided in Table S1. Without exception, the binder-free Ni-HCF film competes favorably with the reported high-performance earth-abundant HER electrocatalysts. The durability of the binder-free Ni-HCF film against the electrochemical HER was evaluated potentiostatically at a bias of −0.2 V vs RHE. For this measurement, the corresponding benchmark current density (i.e., 10 mA cm−2) was monitored as a tracking parameter. Chronoamperometry traces are shown in Figure 7a, which demonstrate a fairly stable current density of 10 mA cm−2 during the HER. In addition, after the potentiostatic stability test, the Ni-HCF film electrode exhibited a linear sweep voltammogram similar to that shown by the film before the stability test (Figure 7b). Further, the same Ni-HCF film electrode was employed for the HER galvanostatically for 5 h at 10 mA cm−2, and subsequently, a

Figure 5. Cathodic linear sweep voltammograms obtained using various electrode materials in deaerated 1 M KOH aqueous electrolyte, showing performance in the HER. The loading of Ni-HCF in the bulk and binder-free films was 1.4 mg cm−2.

(pH 14). The bare-nickel substrate showed the onset potential of the HER at an overpotential of 0.22 V, and a HER current density of 10 mA cm−2 can be observed at a potential of −0.38 V vs RHE. However, the onset overpotential was reduced to 0.15 V, and a benchmark current density of 10 mA cm−2 was observed at a relatively smaller potential of −0.29 V vs RHE when the Ni-HCF powder was deposited on the nickel substrate as a thin film using Nafion as a binder (i.e., Ni-HCF bulk film). This finding suggests that the Ni-HCF film works as HER electrocatalyst. Strikingly, the onset overpotential and the potential for achieving the benchmark current density of 10 mA cm−2 were further reduced to 15 mV and −0.2 V vs RHE, respectively, when the anodically deposited binder-free Ni-HCF film on a nickel substrate was employed as the working electrode for the HER. To understand the better HER performance demonstrated by the anodically deposited binder-free Ni-HCF film, the charge-transfer resistance for the HER was studied by electrochemical impedance spectroscopy (EIS) at an HER overpotential of 250 mV (i.e., at a bias of −0.25 V vs RHE). Figure 6 shows the Nyquist plots, demonstrating that the charge-transfer resistance for the HER of the anodically deposited binder-free Ni-HCF film on a nickel substrate is 18018

DOI: 10.1021/acsami.7b05588 ACS Appl. Mater. Interfaces 2017, 9, 18015−18021

Research Article

ACS Applied Materials & Interfaces

Figure 8. Cathodic linear sweep voltammograms measured in deaerated 1 M KOH aqueous electrolyte using the following HER cathode and OER anode pairs: (i) Pt (20% C)/GC cathode and Ptwire anode, (ii) Ni-HCF/Ni-plate cathode and Pt-wire anode, (iii) NiHCF/Ni-plate cathode and IrO2(20 wt %/C)/GC anode, (iv) NiHCF/Ni-plate cathode and Co-HCF/carbon paper anode.

anode (i.e., counter electrode) and the reference electrode (Figure S5). As the bias was increased further, the HER current density and the rate of evolution of gas bubbles from the electrodes also increased proportionally and finally reached a current density of 62 mA cm−2 at 3.0 V. Based on this result, a pair of AA-sized alkaline batteries was used to power the device, which exhibited the aggressive evolution of H2 and O2 from the Ni-HCF and the Co-HCF electrodes, respectively (as shown by the movie clip in Figure S6). Such a device powered by a smallsized battery could find practical applications in the onsite production of hydrogen by water electrolysis, as the transport of hydrogen gas itself is costly and complicated. As a result, a device based on Ni-HCF and Co-HCF electrodes for the overall splitting of water could be promising for real practice.

Figure 7. (a) Chronoamperometry and (b) linear sweep voltammetry traces measured for an anodically grown binder-free Ni-HCF film on a nickel substrate in deaerated 1 M KOH aqueous electrolyte. Voltammograms were measured at a sweep rate of 5 mV s−1.

cathodic linear sweep voltammogram for the HER was measured. The fairly stable traces of the chronopotentiometry monitored during the HER (Figure S4a) and the similar linear sweep voltammograms (Figure S4b) demonstrated by the NiHCF film electrode in 1 M KOH electrolyte before and after the galvanostatic stability test confirm that the binder-free NiHCF film is electrochemically stable against the HER in alkaline electrolyte for practical applications. To explore the overall splitting of water using metal-HCFbased HER and OER electrodes, the HER performance of the anodically deposited binder-free Ni-HCF film was evaluated using a vertically aligned one-dimensional nanostructured film of Co-HCF grown on carbon paper as the OER electrode29 (i.e., counter electrode in a three-electrode configuration system). Figure 8 shows the linear sweep voltammograms exhibited by the binder-free Ni-HCF film electrode in 1 M KOH electrolyte when counter electrodes consisting of different OER catalysts were employed. Notably, as demonstrated by the onset potential and the slope of the voltammogram curves in Figure 8, the HER performance of the Ni-HCF electrode was nearly the same as when the state-ofthe-art IrO2 (20%)/C OER electrode or the Co-HCF electrode was employed as a counter electrode. Based on the above findings, a full electrochemical water-splitting device was designed by assembling the Ni-HCF and the Co-HCF electrodes as the cathode and anode, respectively, in 1 M KOH aqueous electrolyte. Interestingly, when a bias was applied gradually between the two electrodes, the visually noticeable evolution of gas bubbles from both electrodes was realized at an HER current density of 10 mA cm−2. Note that the Co-HCF electrode, in this three-electrode configuration measurement system, was employed simultaneously as the

4. CONCLUSIONS The present work demonstrates a facile electrochemical anodization route for the formation of binder-free Ni-HCF film on a nickel substrate. The obtained Ni-HCF film demonstrated high performance for the electrochemical hydrogen evolution reaction at a small onset overpotential of 15 mV. Further, the binder-free Ni-HCF film competes favorably with the reported high-performance earth-abundant HER electrocatalysts. Most importantly, the Ni-HCF catalytic film demonstrated a highly competitive HER performance when used with IrO2 (20 wt %/C) or Co-HCF/carbon paper as the OER anode. These findings suggest that metal-HCF materials represent promising electrocatalytic water-splitting electrode materials for the overall splitting of water, making the HER feasible and energy-efficient. In addition, the present electrochemical anodization route can be generalized to the formation of metal-HCF film of various transition metals. Thus, the frameworks can be decorated with fascinating catalytically active metal centers that can find high potential value in the field of electrocatalysis.

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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/acsami.7b05588. 18019

DOI: 10.1021/acsami.7b05588 ACS Appl. Mater. Interfaces 2017, 9, 18015−18021

Research Article

ACS Applied Materials & Interfaces

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Current transient obtained during anodization, XPS spectra, traces of chronopotentiometry, and additional linear sweep voltammograms (PDF) Movie clip showing hydrogen and oxygen gases aggressively evolving from a water-splitting device based on metal-HCF film electrodes (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nabeen K. Shrestha: 0000-0002-4849-4121 Lars Giebeler: 0000-0002-6703-8447 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013009768). N.K.S. acknowledges the Alexander von Humboldt Foundation for support during a research stay at Leibniz Institute for Solid State and Materials Research (IFW) in Dresden, Germany.

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DOI: 10.1021/acsami.7b05588 ACS Appl. Mater. Interfaces 2017, 9, 18015−18021