Journal of Colloid and Interface Science 472 (2016) 84–89

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Journal of Colloid and Interface Science

journal homepage: www.elsevier .com/locate / jc is

High surface area electrodes by template-free self-assembledhierarchical porous gold architecture

http://dx.doi.org/10.1016/j.jcis.2016.03.0350021-9797/� 2016 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: Department of Chemistry, Ben-Gurion University ofthe Negev, Beer-Sheva 8410501, Israel.

E-mail address: razj@bgu.ac.il (R. Jelinek).

Ahiud Morag a,b, Tatiana Golub a, James Becker a, Raz Jelinek a,b,⇑aDepartment of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israelb Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 February 2016Revised 13 March 2016Accepted 16 March 2016Available online 17 March 2016

Keywords:Porous goldSupercapacitorsGold thiocyanatePlasma

a b s t r a c t

The electrode active surface area is a crucial determinant in many electrochemical applications anddevices. Porous metal substrates have been employed in electrode design, however construction of suchmaterials generally involves multistep processes, generating in many instances electrodes exhibitingincomplete access to internal pore surfaces. Here we describe fabrication of electrodes comprisinghierarchical, nano-to-microscale porous gold matrix, synthesized through spontaneous crystallizationof gold thiocyanate in water. Cyclic voltammetry analysis revealed that the specific surface area of theconductive nanoporous Au microwires was very high and depended only upon the amount of gold used,not electrode areas or geometries. Application of the electrode in a pseudo-capacitor device is presented.

� 2016 Elsevier Inc. All rights reserved.

1. Introduction

Electrode performance, particularly the electrode’s active sur-face area, is a critical factor in many applications, including super-capacitors [1–3], batteries [4,5], solar cells [6,7], and sensors [8,9].

Porous gold structures have attracted growing interest as a versa-tile electrode substance [10–14], and in related applications suchas sensing and catalysis [15–19]. A particular focus of currentresearch in this field is to increase the effective surface area ofporous gold matrixes by integrating morphologies in differentlength-scales, e.g. from nano- to macro-structures [20–22]. Whilesuch hierarchical Au scaffolds have promising functionalities asthey exhibit high surface areas, their fabrication methods are cum-bersome and involve complex processes. In some cases physical

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templates are required for gold deposition leading to non-optimaluse of the gold substance [15,23,24]. In particular, de-alloying, inwhich Ag/Au mixture are treated by strong acids and is likely themost widely-used method for production of porous gold matrixes,is often limited by inefficient mass transport and lack of completeexfoliation and exposure of surfaces within the gold matrix [16,25].Varied strategies have been reported to overcome those barriers,however they too generally comprise of multiple steps and requireadvanced instrumentations [21,22].

Here, we present a simple method for fabrication of a hierarchi-cal porous gold matrix through self-assembly of integrated nano/meso/macro-scale gold structures in aqueous solution. The fabrica-tion process is based upon spontaneous assembly of intertwinednetworks of Au microwires displaying nano-porous structuresformed through crystallization of KAuIII(SCN)4 followed by airplasma treatment. The hierarchical gold matrix exhibited excellentconductivity, and cyclic voltammetry measurements demonstratethe presence of extended Au surface areas. Particularly important,the experimental data indicate that electrode performance wasdirectly dependent upon the amount of Au deposited and not onthe substrate’s surface area, confirming highly effective ion acces-sibility into the porous matrix.

2. Materials and methods

2.1. Materials

HAuCl4�3H2O and KSCN were purchase from Sigma Aldrich andused as received. Acetonitrile was purchase from Bio Lab Ltd(jerusalem, Israel). The water used in the experiments were doublypurified by a Branstead D7382 water purification system(Branstead Thermolyne, Dubuque, IA), at 18.3 MX resistivity.

2.2. Synthesis of porous gold

1 mL solution of HAuCl4, 20 mg mL�1, was mixed with 1 mLsolution of KSCN, 24 mg mL�1. The precipitation formed was sepa-rated by centrifugation in 4000g for 20 min the precipitation wasdried and resuspended in 30 lL of acetonitrile. The acetonitrilewith the suspended Au complex was deposited on either glass, sil-icon (1 0 0) or gold evaporated on silicon. The substrate with theAu complex was treated with air plasma at 0.7 mbar and 85Wfor 10 min on a PCCE plasma cleaner (Diener electronics GmbH,Ebhausen, Germany). The substrate was then immersed in waterfor 15 min to remove residues from the plasma treatment. Afterthe immersion in water the substrate was treated for another10 min in air plasma. We determined that approximately 10 minof plasma exposure were required for complete reduction of thegold complex to metallic gold. This plasma exposure did not alterthe morphology of the porous Au structure.

2.3. Scanning Electron Microscopy (SEM)

For SEM measurements, Au matrixes were deposited on eitherglass or silicon (1 0 0). The SEM images were recorded using aJSM-7400 scanning electron microscope (JEOL LTD, Tokyo, Japan).

2.4. X-ray Photoelectron Spectroscopy (XPS)

XPS measurements were carried out following deposition of thegold matrix on silicon (1 0 0). Experiments were carried out in aThermo Fisher ESCALAB 250 instrument (Massachusetts, USA) witha basic pressure of 2⁄10�9 mbar. The samples were irradiated usinga beam size of 500 lm and the high energy resolution measure-ments were performed with pass energy of 20 eV. The core level

binding energies of the Au 4f peaks were normalized by settingthe binding energy for the C1 at 284.8 eV.

2.5. X-ray Diffraction (XRD)

XRD was performed on samples deposited on silicon (1 0 0). TheXRD pattern was obtain using Panalytical Empyrean powderdiffractometer (PANalytical, Almelo, Netherlands) equipped witha parabolic mirror on incident beam providing quasi-monochromatic Cu Ka radiation (k = 1.54059 Å) and X’celeatorlinear detector. Data were collected in the grazing geometry withconstant incident beam angle equal to 1� in a 2h range of 10–60�with a step equal to 0.05�.

2.6. Conductivity measurement

Conductivity measurements were conducted using 2 probesconnected directly on the gold structure at a distance of 1 cm.The measurement was done using Keithley 2400 source meter unit(SMU).

2.7. Cyclic voltammetry (CV)

Samples for CV measurements were prepared on a 100 nmTi/400 nm Au evaporated on a silicon (1 0 0) substrate. The mea-surements were performed in a 3 electrode configuration, in whichPt wire acted as counter electrode, Ag/AgCl as reference electrode,and porous gold on evaporated gold as the active electrode. CV wasrecorded in the range of 0–2 V in a 50 mV s�1 scan rate on a CHinstrument, electrochemistry workstation.

2.8. Pseudocapacitor preparation

Au porous electrode was coated with MnO2 using electrochem-ical method. The coating was perform in a solution containing0.05 M Mn(ac)2 and 0.05 M Na2SO4 using Ag/AgCl as referenceelectrode, Pt wire as counter electrode and porous Au as workingelectrode. The deposition was conducted at 0.9 V.

3. Results and discussion

Fig. 1 schematically depicts the preparation procedure of thehierarchical porous gold. The KAuIII(SCN)4 complex formed uponmixing chloroauric acid and potassium thiocyanate spontaneouslyassembled into microwires in an aqueous solution. Followingcentrifugation to remove water and resuspension in acetonitrile,the wires were deposited upon the target substrate. Completeremoval of the organic residues and gold reduction were subse-quently carried out by air plasma treatment [26–28], yielding theporous hierarchical metallic Au matrix.

Fig. 2 presents scanning electron microscopy (SEM) imagesdepicting the morphology of the gold matrix and illustrating thehierarchical nature of the porous structures. Specifically, the SEMimage in Fig. 2A reveals that a dense assembly of intertwined elon-gated microwires was formed through the procedure outlined inFig. 1. Closer microscopic inspection shows the microwires’ sur-faces comprise of highly porous mesh structures (Fig. 2B and C).Notably, the nanoporous morphology shown in Fig. 2B and C wasrecorded after plasma treatment of the surface-deposited micro-wires (SEM imaging of the microwires prior to plasma applicationfeatured significantly smoother surfaces, Fig. 1, SI). This result indi-cates that the nanoporous structures were likely formed throughremoval of the residual thiocyanate ligands from the Au matrixby the air plasma treatment. Ligand removal was accompaniedby an increase in density of the materials, from �4 g cm�3 in case

Fig. 1. Experimental scheme: (A) mixing chloroauric acid and potassium thiocyanate yielded the KAuIII(SCN)4 complex which self-assembled, forming a network ofmicrowires; (B) the wire were re-suspended in acetonitrile and deposited upon a solid substrate; (C) exposure of the microwires to air plasma resulted in nanoporous metallicgold nanowires.

Fig. 2. Morphology of the porous gold matrix. Scanning Electron Microscopy (SEM) images of the Au microwire network. (A) Scale bar 10 lm; (B) scale bar 5 lm; (C) scale bar500 nm. A highly nanoporous structure is apparent.

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of the gold thiocyanate complex [29], to around 19 g cm�3 formetallic gold. The density transformation further supports the con-traction process yielding the porous structure. Plasma-inducedextraction of organic substances was previously recorded in variedAu film morphologies [26–28]. SEM analysis of the surface-deposited cross-section confirms that the three-dimensionalmicrowire structure was retained after plasma treatment withinthe entire three-dimensional matrix (Fig. 2, SI).

Previous studies have reported assembly of Au wire inKAuIII(SCN)4 aqueous solutions, and showed that the gold ionswere reduced in such systems by the SCN� ligands. Specifically,the spontaneous reduction of the gold thiocyanate complexresulted in formation of KAuI(SCN)2 intermediate, as well asK2SO4 moieties produced through oxidation of the thiocyanateligands [26]. SEM analysis (Fig. 3, SI) and X-ray Diffraction (XRD)(Fig. 4, SI) indeed revealed formation of potassium sulfate domainswithin the microwire network after air-plasma treatment.

Spectroscopic experiments further illuminate the reduction/crystallization process, in particular highlighting the role of airplasma treatment in generating metallic porous Au framework(Fig. 3). X-ray Diffraction (XRD) patterns in Fig. 3A reveal that theas-prepared nanowires (prior to plasma treatment) gave rise toreflections ascribed to KAuIII(SCN)4 crystal planes [27], while thepeaks corresponding to metallic crystalline Au exhibited very lowintensity (Fig. 3A, top pattern). Application of air plasma, however,gave rise to diffraction peaks corresponding only to the (1 1 1) and(2 0 0) crystal planes of Au0 (Fig. 3A, bottom pattern). This resultdemonstrates that the hierarchical porous matrix constructedaccording to the process outlined in Fig. 1 comprised of metallicAu0.

X-ray Photoelectron Spectroscopy (XPS) experiments depictedin Fig. 3B corroborate the interpretation of the XRD data and

further illuminate the mechanism of metallic gold formationcomprising the porous matrix. Similar to the XRD pattern recordedprior to plasma treatment in Fig. 3A, the XPS spectrum of theas-synthesized microwires features a mixture of primarily AuIII-and AuI-containing species (Fig. 3B, top spectrum). Specifically,intense 4f5/2 and 4f7/2 peaks corresponding to AuIII (peaks at87.3 eV and 90.9 eV) and AuI (peaks at 86.5 eV and 90.1 eV)[26,27] were apparent in Fig. 3B (top spectrum), while only about5% of the Au was metallic (Au0; peaks at 84.5 eV and 88.6 eV)[26,27]. However, following plasma treatment almost 98% of thegold was reduce into Au0 (Fig. 3B, bottom spectrum).

The XRD and XPS data aid in illuminating the formation mech-anism of the porous structure. Specifically, both techniques indi-cate that, prior to plasma treatment, the matrix contained a highconcentration of the AuIII and AuI complexes which still compriseof the thiocyanate ligands. Subsequent removal of the ligandstogether with reduction of the ionic gold to Au0 led to the pro-nounced porous structure in the system (e.g. Fig. 2). This scenariois notable, since earlier studies of self-assembled KAu(SCN)4nanostructures revealed only plasma-induced reduction of goldspecies which was not accompanied by formation of porous archi-tectures [27].

Figs. 4 and 5 explore the electrochemical properties of the hier-archical porous gold matrix and its practical applicability. The cur-rent/voltage (I/V) curves in Fig. 4 attest to the excellentconductivity attained in the gold matrix following plasma treat-ment. Specifically, the I/V curve recorded before plasma treatmentindicates minimal electron transport (Fig. 4, curve i), consistentwith the XRD and XPS data discussed above (e.g. Fig. 3) whichreveal high abundance of ionic Au prior to plasma treatment, ratherthan the metallic species. In contrast, the plasma-treated goldmatrix was highly conductive, reflected in the electrical currents

Fig. 3. Spectroscopic characterization of the porous microwire network. (A) X-ray diffraction patterns recorded before (top) and after (bottom) plasma treatment. Formationof crystalline Au0 is apparent after plasma treatment. (B) X-ray photoelectron spectra (XPS) showing the Au 4f peaks before (top) and after (bottom) plasma treatment. Theexperimental spectra are traced by the black circles, while spectral deconvolution shows the AuIII (blue), AuI (green), and Au0 (orange) species. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Electrical conductance. Current/voltage curves recorded before (i) and after(ii) plasma treatment of the Au microwires.

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obtained in the I/V experiments (Fig. 4, curve ii), and confirmingthat the Au structures prepared through the procedure in Fig. 1were continuous and enabled efficient electron transport.

To evaluate the active gold area in the porous matrix andexplore its electrochemical profile we carried out cyclic voltamme-try (CV) measurements (Fig. 5). Fig. 5A compares the normalizedCV curves recorded in 1 M H2SO4 solution for the hierarchical por-ous gold matrix (black curve) and evaporated Au electrode (orangecurve). The magnified CV curve of the control Au electrode isshown in Fig. 5, SI. The CV data clearly show that the newly-developed porous Au matrix exhibited significantly larger activearea compared to conventionally-deposited Au electrode. Calcula-tions based upon the area of the Au reduction peak [30] yieldedan electrochemically active surface area of 63.43 cm2 (on a0.186 cm2 electrode), and corresponding ratio of 340, which waslarger by a factor of 150 than the ratio calculated for the referencegold electrode. This ratio is greater than previously reported por-ous gold structures deposited on substrates [10,15,17,23,31–33].

While the data in Fig. 5A attest to the dramatic enhancement ofactive surface area within the new hierarchical porous gold matrix,an important question one needs to address is whether the activearea is limited by the extent of gold deposition. Indeed, this issue is

pertinent to practical utilization of porous gold assemblies, and forassessment of whether the entire gold matrix participated in elec-tron transport. Accordingly, Fig. 5B depicts the surface areas (calcu-lated from the CV curves) and surface ratios, determined fordifferent samples for which we deposited the same amount of gold(2 mg) on electrodes exhibiting different surface areas. The exper-iments demonstrate that the measured surface areas exhibited asmall variation (blue circles; the broken line corresponds to theaverage value, variation among calculated surfaces was within±8%), reflecting the (equivalent) amount of deposited gold in allsamples. However, an experimentally-significant increase in theratios between the calculated surface of the porous gold and thatof the substrate was clearly apparent in Fig. 5B (red circles). Nota-bly, the linear relationship between the surface/electrode arearatio and electrode surface areas points to unhindered diffusionof ions within the porous gold matrix, enabling efficient electro-chemical reactions at the electrode surface. Experiments carriedout using a larger surface area (1 cm2) yielded an effective specificsurface area of 3.5 m2 g�1 for the porous gold matrix (Fig. 6, SI),which is at the upper echelon of porous Au electrodes reported[21,34–36].

To demonstrate applicability of the new hierarchical porousgold electrode in supercapacitor devices we constructed apseudo-capacitor and examined its electrical properties (Fig. 5C).Specifically, we deposited MnO2 upon the porous electrode usinga previously-reported electrochemical method [37], and measuredits capacitance through cyclic voltammetry in a 2 M Li2SO4 solutionat a scan rate of 1 mV s�1. The experiment depicted in Fig. 5Cyielded an areal capacitance of 1600 mF cm�2, which is compara-ble to recently-reported supercapacitor electrodes [10,38].

In conclusion, we fabricated a porous gold matrix through asimple self-assembly process involving crystallization and subse-quent reduction of KAuIII(SCN)4 in water, without co-addition ofreducing or templating agents. The one-step synthesis produceda hierarchical morphology comprised of a mesh of Au microwireswhich exhibited a nanoporous structure following air plasmaapplication. Cyclic voltammetry measurements revealed signifi-cantly high electrochemically-active surface areas that dependedonly upon the amount of gold used in the deposition process,rather than composition, size, or geometry of electrode substrate.The combination of readily-available reagents, facile synthesisroute, and pronounced surface areas point to potential uses ofthe multiscale porous gold architecture in diverse systems andapplications.

Fig. 5. Cyclic voltammetry (CV) analysis. (A) Normalized CV curves of the hierarchical porous gold matrix (black) and evaporated gold electrode (orange) recorded in 1.0 MH2SO4 solution. (B) Surface areas recorded for identical amounts of gold deposited on substrates having different surface areas. The blue circles depict surface areas calculatedfrom the CV curves (e.g. data in A), the broken blue line corresponds to the average value. The red circles indicate the ratios between the calculated surface areas and thesubstrate surface areas used for deposition. The broken red line reflect the linearity of the recorded ratio values. (C) CV of porous Au electrode coated with MnO2 in a 2 MLi2SO4 solution at a scan rate of 1 mV s�1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Acknowledgements

The Israel Science Foundation (Grant no. 201/14) is acknowl-edged for generous financial support. The authors would like tothanks Dimitry Mogiliansky for helping with the XRD experimentsand Natalya Froumin for assistance with XPS measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2016.03.035.

References

[1] T. Kim, G. Jung, S. Yoo, K.S. Suh, R.S. Ruoff, Activated graphene-based carbons assupercapacitor electrodes with macro- and mesopores, ACS Nano 7 (2013)6899–6905, http://dx.doi.org/10.1021/nn402077v.

[2] Z. Tang, C. Tang, H. Gong, A high energy density asymmetric supercapacitorfrom nano-architectured Ni(OH)2/carbon nanotube electrodes, Adv. Funct.Mater. 22 (2012) 1272–1278, http://dx.doi.org/10.1002/adfm.201102796.

[3] S. Mondal, U. Rana, S. Malik, Graphene quantum dot-doped polyanilinenanofiber as high performance supercapacitor electrode materials, Chem.Commun. 51 (2015) 12365–12368, http://dx.doi.org/10.1039/C5CC03981A.

[4] J. Schuster, G. He, B. Mandlmeier, T. Yim, K.T. Lee, T. Bein, et al., Sphericalordered mesoporous carbon nanoparticles with high porosity for lithium-sulfur batteries, Angew. Chem. Int. Ed. 51 (2012) 3591–3595, http://dx.doi.org/10.1002/anie.201107817.

[5] B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou, et al., Hierarchical three-dimensional ZnCo2O4 nanowire arrays/carbon cloth anodes for a novel class ofhigh-performance flexible lithium-ion batteries, Nano Lett. 12 (2012) 3005–3011, http://dx.doi.org/10.1021/nl300794f.

[6] J. Chen, K. Li, Y. Luo, X. Guo, D. Li, M. Deng, et al., A flexible carbon counterelectrode for dye-sensitized solar cells, Carbon N. Y. 47 (2009) 2704–2708,http://dx.doi.org/10.1016/j.carbon.2009.05.028.

[7] J.D. Roy-mayhew, D.J. Bozym, C. Punckt, I.A. Aksay, Functionalized graphene asa catalytic counter electrode in dye sensitized Solar Cells, ACS Nano 4 (2010)6203–6211.

[8] X. Dong, H. Xu, X. Wang, Y. Huang, M.B. Chan-Park, H. Zhang, et al., 3Dgraphene-cobalt oxide electrode for high-performance supercapacitor andenzymeless glucose detection, ACS Nano 6 (2012) 3206–3213, http://dx.doi.org/10.1021/nn300097q.

[9] Y. Lü, W. Zhan, Y. He, Y. Wang, X. Kong, Q. Kuang, et al., MOF-templatedsynthesis of porous Co3O4 concave nanocubes with high specific surface areaand their gas sensing properties, ACS Appl. Mater. Interfaces 6 (2014) 4186–4195, http://dx.doi.org/10.1021/am405858v.

[10] A. Ferris, S. Garbarino, D. Guay, D. Pech, 3D RuO 2 microsupercapacitors withremarkable areal energy, Adv. Mater. 27 (2015) 6625–6629, http://dx.doi.org/10.1002/adma.201503054.

[11] X. Lang, A. Hirata, T. Fujita, M. Chen, Nanoporous metal/oxide hybridelectrodes for electrochemical supercapacitors, Nat. Nanotechnol. 6 (2011)232–236, http://dx.doi.org/10.1038/nnano.2011.13.

[12] L.Y. Chen, Y. Hou, J.L. Kang, A. Hirata, T. Fujita, M.W. Chen, Toward thetheoretical capacitance of RuO2 reinforced by highly conductive nanoporousgold, Adv. Energy Mater. 3 (2013) 851–856, http://dx.doi.org/10.1002/aenm.201300024.

[13] J. Patel, L. Radhakrishnan, B. Zhao, B. Uppalapati, R.C. Daniels, K.R. Ward, et al.,Electrochemical properties of nanostructured porous gold electrodes inbiofouling solutions, Anal. Chem. 85 (2013) 11610–11618, http://dx.doi.org/10.1021/ac403013r.

[14] U. Salaj-Kosla, S. Pöller, Y. Beyl, M.D. Scanlon, S. Beloshapkin, S. Shleev, et al.,Direct electron transfer of bilirubin oxidase (Myrothecium verrucaria) at anunmodified nanoporous gold biocathode, Electrochem. Commun. 16 (2012)92–95, http://dx.doi.org/10.1016/j.elecom.2011.12.007.

[15] B.J. Plowman, A.P. O’Mullane, P.R. Selvakannan, S.K. Bhargava, Honeycombnanogold networks with highly active sites, Chem. Commun. 46 (2010) 9182–9184, http://dx.doi.org/10.1039/c0cc03696j.

[16] A. Wittstock, V. Zielasek, J. Biener, C.M. Friend, M. Baumer, Nanoporous goldcatalysts for selective gas-phase oxidative coupling of methanol at lowtemperature, Science (80-.) 327 (2010) 319–322, http://dx.doi.org/10.1126/science.1183591.

[17] Y. Li, Y.Y. Song, C. Yang, X.H. Xia, Hydrogen bubble dynamic template synthesisof porous gold for nonenzymatic electrochemical detection of glucose,Electrochem. Commun. 9 (2007) 981–988, http://dx.doi.org/10.1016/j.elecom.2006.11.035.

A. Morag et al. / Journal of Colloid and Interface Science 472 (2016) 84–89 89

[18] J. Zhang, P. Liu, H. Ma, Y. Ding, Nanostructured porous gold for methanolelectro-oxidation, J. Phys. Chem. C 111 (2007) 10382–10388, http://dx.doi.org/10.1021/jp072333p.

[19] X. Zhang, Y. Ding, Unsupported nanoporous gold for heterogeneous catalysis,Catal. Sci. Technol. 3 (2013) 2862, http://dx.doi.org/10.1039/c3cy00241a.

[20] M.N. Lee, M.A. Santiago-Cordoba, C.E. Hamilton, N.K. Subbaiyan, J.G. Duque, K.A.D. Obrey, Developing monolithic nanoporous gold with hierarchicalbicontinuity using colloidal bijels, J. Phys. Chem. Lett. 5 (2014) 809–812,http://dx.doi.org/10.1021/jz5001962.

[21] Z. Qi, J. Weissmüller, Hierarchical nested-network nanostructure bydealloying, ACS Nano 7 (2013) 5948–5954, http://dx.doi.org/10.1021/nn4021345.

[22] A. Sharma, J.K. Bhattarai, A.J. Alla, A.V. Demchenko, K.J. Stine, Electrochemicalannealing of nanoporous gold by application of cyclic potential sweeps,Nanotechnology 26 (2015), http://dx.doi.org/10.1088/0957-4484/26/8/085602. 085602.

[23] S. Sattayasamitsathit, A.M. O’Mahony, X. Xiao, S.M. Brozik, C.M. Washburn, D.R. Wheeler, et al., Highly ordered tailored three-dimensional hierarchicalnano/microporous gold–carbon architectures, J. Mater. Chem. 22 (2012)11950–11956, http://dx.doi.org/10.1039/c2jm31485a.

[24] W. Gao, X.-H. Xia, J.-J. Xu, H.-Y. Chen, Three-dimensionally orderedmacroporous gold structure as an efficient matrix for solid-stateelectrochemiluminescence of Ru(bpy)32+/TPA system with high sensitivity,J. Phys. Chem. C 111 (2007) 12213–12219, http://dx.doi.org/10.1021/jp0722814.

[25] A. Wittstock, B. Neumann, A. Schaefer, K. Dumbuya, C. Kübel, M.M. Biener,et al., Nanoporous Au: an unsupported pure gold catalyst?, J Phys. Chem. C 113(2009) 5593–5600, http://dx.doi.org/10.1021/jp808185v.

[26] A. Morag, N. Froumin, D. Mogiliansky, V. Ezersky, E. Beilis, S. Richter, et al.,Patterned transparent conductive Au films through direct reduction of goldthiocyanate, Adv. Funct. Mater. 23 (2013) 5663–5668, http://dx.doi.org/10.1002/adfm.201300881.

[27] A. Morag, V. Ezersky, N. Froumin, D. Mogiliansky, R. Jelinek, Transparent,conductive gold nanowire networks assembled from soluble Au thiocyanate,Chem. Commun. 49 (2013) 8552, http://dx.doi.org/10.1039/c3cc44397c.

[28] S.W. Lee, D. Liang, X.P.A. Gao, R.M. Sankaran, Direct writing of metalnanoparticles by localized plasma electrochemical reduction of metal cations

in polymer films, Adv. Funct. Mater. 21 (2011) 2155–2161, http://dx.doi.org/10.1002/adfm.201100093.

[29] N.L. Coker, J.A.K. Bauer, R.C. Elder, Emission energy correlates with inverse ofgold-gold distance for various [Au(SCN)2]-salts, J. Am. Chem. Soc. 126 (2004)12–13, http://dx.doi.org/10.1021/ja037012f.

[30] L.D. Burke, P.F. Nugent, The electrochemistry of gold: I the redox behaviour ofthe metal in aqueous media, Gold Bull. 30 (1997) 43–53, http://dx.doi.org/10.1007/BF03214756.

[31] H. Wei, W. Minghua, Z. Jufang, L. Zelin, Facile fabrication of multifunctionalthree-dimensional hierarchical porous gold films via surface rebuilding, J.Phys. Chem. C 113 (2009) 1800–1805, http://dx.doi.org/10.1021/jp8095693.

[32] S. Reculusa, M. Heim, F. Gao, N. Mano, S. Ravaine, A. Kuhn, Design ofcatalytically active cylindrical and macroporous gold microelectrodes, Adv.Funct. Mater. 21 (2011) 691–698, http://dx.doi.org/10.1002/adfm.201001761.

[33] H.J. Qiu, X.R. Huang, Effects of Pt decoration on the electrocatalytic activity ofnanoporous gold electrode toward glucose and its potential application forconstructing a nonenzymatic glucose sensor, J. Electroanal. Chem. 643 (2010)39–45, http://dx.doi.org/10.1016/j.jelechem.2010.03.011.

[34] E. Rouya, S. Cattarin, M.L. Reed, R.G. Kelly, G. Zangari, Electrochemicalcharacterization of the surface area of nanoporous gold films, J. Electrochem.Soc. 159 (2012) K97, http://dx.doi.org/10.1149/2.097204jes.

[35] Y.H. Tan, J.A. Davis, K. Fujikawa, N.V. Ganesh, A.V. Demchenko, K.J. Stine,Surface area and pore size characteristics of nanoporous gold subjected tothermal, mechanical, or surface modification studied using gas adsorptionisotherms, cyclic voltammetry, thermogravimetric analysis, and scanningelectron microscopy, J. Mater. Chem. 22 (2012) 6733, http://dx.doi.org/10.1039/c2jm16633j.

[36] M.M. Biener, J. Biener, A. Wichmann, A. Wittstock, T.F. Baumann, M. Bäumer,et al., ALD functionalized nanoporous gold: thermal stability, mechanicalproperties, and catalytic activity, Nano Lett. 11 (2011) 3085–3090, http://dx.doi.org/10.1021/nl200993g.

[37] L.Y. Chen, J.L. Kang, Y. Hou, P. Liu, T. Fujita, A. Hirata, et al., High-energy-density nonaqueous MnO2@nanoporous gold based supercapacitors, J. Mater.Chem. A 1 (2013) 9202, http://dx.doi.org/10.1039/c3ta11480e.

[38] L. Zhang, P. Zhu, F. Zhou, W. Zeng, H. Su, G. Li, et al., Flexible asymmetricalsolid-state supercapacitors based on laboratory filter paper, ACS Nano 10(2016) 1273–1282, http://dx.doi.org/10.1021/acsnano.5b06648.