Enhanced Charging-Induced Resistance Variations of Nanoporous

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Enhanced Charging-Induced Resistance Variations of Nanoporous Gold by Dealloying in Neutral Silver Nitrate Solution Eva-Maria Steyskal, Michael Seidl, Sanja Simic, and Roland Wurschum Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02082 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Enhanced Charging-Induced Resistance Variations of Nanoporous Gold by Dealloying in Neutral Silver Nitrate Solution Eva-Maria Steyskal,∗,† Michael Seidl,† Sanja Simic,‡ and Roland Würschum† Institute of Materials Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria, and Institute of Electron Microscopy and Nanoanalytics, Graz University of Technology, Steyrergasse 17, A-8010 Graz, Austria E-mail: [email protected]

Abstract Nanoporous gold (np-Au), produced by dealloying in silver nitrate solution exhibits extraordinary high surface-to-volume ratios of more than 20 m2 /g which represents an excellent prerequisite for property tuning by surface charging. Upon electrochemical charging in aqueous KOH solution, the electrical resistance is observed to vary reversibly by up to 88 %. The charge coefficient, thus the sensitivity of the resistance towards the imposed charge per mol, is however significantly smaller compared to conventionally prepared np-Au, prepared in nitric acid solution. While the strong resistance variation observed in the present work can directly be related to the high charge transfer due to extraordinary fine porosity, the charge coefficients can be understood with regards to the matrix resistance of the respective materials, which is strongly influenced by dealloying residuals. ∗

To whom correspondence should be addressed Institute of Materials Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria ‡ Institute of Electron Microscopy and Nanoanalytics, Graz University of Technology, Steyrergasse 17, A-8010 Graz, Austria †

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Introduction Porous nanophase metals are actively studied in various fields, ranging from basic research to technology applications such as energy storage, 1,2 sensing, 3,4 (electro-)catalysis 5–10 or biotechnologies. 11–14 One particularly attractive preparation method for nanoporous metal structures is (electro-)chemical dealloying, which allows for the fabrication of bulk quantities of sample material in quasi unrestrained shape. By this selective etching process, the less noble component is removed from a master alloy, leaving behind a sponge-like, bicontinuous network of metallic branches (ligaments) and pores. 15 Due to its high nobility and the superior properties of the generated nanostructure, by far the most intensively studied dealloyed material is nanoporous gold (np-Au), which is typically obtained from Ag-Au master alloys by a free corrosion process in nitric acid. However also a variety of other dealloyable systems (e.g. platinum 16 or palladium 17 ) are known in the literature. Owing to the high surface-to-volume ratios of nanoporous metals, electrochemical surface stimuli may affect a significant fraction of the material’s volume and in such a way influence its bulk properties. 18 Charging-induced variations have been presented in the literature with regards to the mechanical, 19,20 magnetic 21,22 and electrical properties 23–26 of porous nanophase metals. In the present work, charging-induced variations in the electrical resistance of np-Au produced by dealloying will be investigated. The origin of the resistance tunability of nanoporous metals by electrochemical charging can be considered to arise mainly from manipulations of the charge-carrier scattering at the metal-electrolyte interface. 23 For np-Au produced by corroding Ag-Au in nitric acid, a resistance increase ∆R/R0 of 43 % was reported upon oxygen adsorption in an aqueous electrolyte. 25 Even stronger variations up to almost 60 % could be achieved for dealloyed nanoporous platinum (np-Pt). 27 The higher values achieved in the latter case were assigned to the finer porosity and therefore higher surface-to-volume ratio of np-Pt compared to npAu, which is directly related to the slower atomic diffusivity associated with higher melting point of Pt. 28 2


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Nanoporous gold samples etched in nitric acid exhibit about 20 nm as typical poreand ligament-diameters 28 and about 10 m2 /g as specific surface areas. 29 The formation of smaller structural sizes (ligament diameters of 4-6 nm) was reported for an electrochemically assisted corrosion process in neutral silver nitrate solution, 30 along with a significant amount of residual impurities of the less noble component (Ag) remaining in the sample material. In spite of its high silver content of about 30 %, the resulting material will be referred to as nanoporous gold (np-Au) here, in conformity with its original presentation by Snyder et al. in the literature. 30 The dealloying route using AgNO3 solution provides ideal prerequisites for surface-charging induced property tuning due to the high specific surface areas achievable. In such a way, ∆R/R0 values as high as 88 % could be generated reversibly.

Experimental Procedures The nanoporous gold sample material was produced by dealloying an Ag73 Au27 master alloy, which was arc-melted from high purity wires of silver and gold and subsequently cold-rolled to a foil of 80 µm thickness in several rolling and annealing steps. All experimental data presented here were obtained using one single rectangular sample platelet sized 5×20 mm2 with an initial alloy mass of 86 mg. The results were confirmed by test measurements on identically prepared further samples. The setup for electrochemical measurements was arranged similar to our previous work on nanoporous platinum, 27 where a detailed description including a sketch can be found. Summing up briefly, since nanoporous metals are very brittle, all electrical contacts were attached to the sample material prior to dealloying. For the purpose of following resistance measurements under electrochemical control, five well-annealed, flattened gold wires were clenched around the platelet in line. Without further mechanical constraints, the sample was then immersed into the electrolyte hanging on the wires. All electrochemical measurements presented here were performed at room temperature

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with a PGZ-100 potentiostat (Radiometer Analytical), using the mid-positioned wire attached to the sample platelet as the working electrode contact. A commercial double junction Ag/AgCl electrode (Metrohm) served as reference, related to which all potentials UAg/AgCl will be stated in the following. In order to avoid precipiation during dealloying, filling solutions of 3 M KCl and 3 M KNO3 were used as the inner and outer chamber electrolyte (salt bridge) respectively. All electrolyte solutions used in this work were produced from ultrapure water. Dealloying was performed at room temperature in 0.1 M AgNO3 solution, applying a potential UAg/AgCl of + 1300 mV. At the graphite foil counter electrode, a strong dendritic growth of silver could be observed during the experiment run. The selective dissolution process was considered finished, when the etching current had fallen below a stable threshold of 1 mA. In order to remove the electrolyte from the pores, the np-Au sample was cleaned by careful rinsing and immersion into distilled water after dealloying. The resistance variations of np-Au upon electrochemical charging were investigated using 1 M KOH solution as measuring electrolyte. Resistance data were collected in four point geometry by a Keithley 2400 Source Meter in intervals of 2.5 seconds. The four point measuring current was applied via the outermost contact wires of the sample platelet. The remaining two wires served to measure the voltage drop. The relative variation ∆R/R0 of the electrical resistance is calculated with respect to the sample resistance R0 in the electrochemical double layer regime if not stated otherwise. Any possible influence of the charging current on the resistance measurements was carefully disproved in our previous work. 27 Following the measurements presented here, the np-Au material was again cleaned from electrolyte by rinsing and immersion into distilled water. After disassembly of the electrochemical cell, the sample was subjected to electron microscopy for structural and compositional characterization.

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Results Nanoporous gold produced by dealloying Ag-Au in neutral silver nitrate solution 30 is known to exhibit extraordinary small pore sizes. In fact, the structure of np-Au produced in such a way borders to the lower limit of SEM resolution. Two SEM images showing a fracture face of the sample platelet at different magnifications are presented in figure 1.

Figure 1: SEM characterization of a fracture face of the np-Au sample, recorded after the platelet had undergone the electrochemical charging procedures presented in this article. While the surface of the platelet, visible in the top left corner of the upper image (a), appears smooth, at the fracture face the sponge-structure of metal branches (bright) and pores (dark) becomes visible. At higher magnifications (b) small ligament and pore diameters of 10 nm can be estimated for the porous structure. Via EDX analysis a residual silver content 5


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of 30.0 % was determined at the fracture face. These results are well in line with similarly prepared samples presented in the literature, 30 where also high resolution imaging of the material can be found. Due to porosity evolution and the concomitant formation of a strongly bound oxide during dealloying the electrical resistance of nanoporous metals increases by about three orders of magnitude compared to the initial resistance of the master alloy. 31 This so-called ’primary oxide’ condition is associated with peculiar physical properties regarding the electrical resistance 25,26 as well as actuation. 29 Therefore, in order to observe a metallic behavior of the np-Au material, the sample platelet was carefully reduced by cyclic voltammetry (CV) in 1 M KOH solution between potentials UAg/AgCl of -400 mV and +800 mV with a scan rate of 1 mV/s prior to any resistance tuning experiments, which is shown in figure 2. Obviously the first CV cycle after dealloying (plotted in red) strongly differs from the following ones, showing as most prominent feature an oxygen desorption peak, which is not only stronger pronounced but also shifted towards more negative potentials. The shape of the cyclic voltammogram exhibits significant changes during the first few cycles and clearly differs from conventional, acid-etched np-Au. This can be assigned to the higher Ag concentration remaining after dealloying with silver oxide in different oxidation states present at the surface as described by Snyder et al. 30 The observed influence on the CV is very similar to an experiment, presented recently by Innocenti et al., where even very small amounts of silver microparticles deposited on glassy carbon electrodes lead to significant CV changes in KOH solution. 32 Directly after dealloying the np-Au platelet, an electrical resistance of R=1.10 Ω is recorded between the sensing contacts. Upon passing the primary oxide desorption peak in the first CV cycle, the resistance drops by more than one order of magnitude to a value of 0.03 Ω after reduction. The concomitant resistance variation is plotted as a function of time in figure 3, along with the applied potential for the first three CV cycles. The corresponding cyclic voltammograms are those shown in figure 2. Both the appearance of the

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Figure 2: Cyclic voltammetry of the freshly dealloyed np-Au sample, performed with a scan rate of 1 mV/s in 1 M KOH solution. The current i is plotted as a function of UAg/AgCl . In the first cycle (red) stripping of the strongly bound primary oxide can be observed, in the following cycles (black) the curve approaches steady state. oxygen desorption peak as well as the irreversible, strong drop in sample resistance indicate that a primary oxide is initially present and can subsequently be removed by this stripping procedure, successfully yielding a sample platelet in metallic condition. Clearly, the strong resistance drop in the primary oxide reduction step is irreversible. However, also after this process the CV is far from a steady state. As already mentioned, the present material hosts a high amount of residual silver, remaining after dealloying, with silver oxide in different oxidation states present at the surface, 30 which have to be resolved, causing strong influence on the electrical resistance. Therefore, during the first few cycles, the electrical resistance varies by more than 100%, which is however only partly reversible, superimposed by a considerable drift, as visiblie in figure 3. When the cyclic voltammogram finally reaches steady state, also the resistance variations are observed reversibly. The steady state CV of the now metallic np-Au platelet (without primary oxide), recorded again between potentials of UAg/AgCl of -400 mV and +800 mV with a scan rate of 1 mV/s in 1 M KOH solution, is shown red in figure 4 along with the relative variation in electrical resistance (blue). Simliar to previous results on different porous nanophase metals, 22,25,27 the variations due to double layer charging are rather small 7


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Figure 3: Electrical resistance variation of the np-Au sample, recorded during the first three cycles of the CV shown in figure 2. Sample resistance R (blue) and applied potential UAg/AgCl (gray) are plotted as a function of measurement time t. while a strong, reversible resistance increase occurs upon oxygen adsorption. Along with a charge transfer of 9.4 As, a ∆R/R0 value of 88 % is observed in this steady state condition. The double layer capacitance of the np-Au sample is directly related to its active surface area A via

A=

id sC d

(1)

for cyclic voltammetry in the double layer regime with the scan rate s, the current id and the specific capacitance C d . 33 In order to determine the surface area of the well reduced np-Au platelet, CVs were recorded between potentials UAg/AgCl of -200 mV and -100 mV with different scan rates. The average currents recorded during these scans in the middle of this potential window at UAg/AgCl = -150 mV are plotted as a function of scan rate in figure 5. Using the specific double layer capacitance of pure porous gold electrodes, 34 which 8


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Figure 4: Steady state cyclic voltammetry of np-Au, recorded with a scan rate of 1 mV/s between potentials of UAg/AgCl of -400 mV and +800 mV in 1 M KOH solution. Current i (red) and relative variation of the electrical resistance ∆R/R0 (blue) are plotted as a function of applied potential UAg/AgCl amounts to 30 µF/cm2 , from the linear fit slope id /s=0.29 As/V, a specific surface area S of 22.5 m2 /g (with S=A/m) can be estimated for the np-Au material. This specific surface area is related to the ligament diameter d via

S=

c ρd

(2)

with the material’s bulk density ρ and a dimensionless constant c, which was found to be 3.7 for disordered nanoporous structures such as np-Au. 35 Using ρ = 16.06 g/cm3 , determined for an Ag3 Au7 alloy, 36 as an estimate for the bulk density of the sample material, equation (2) yields a ligament size of 10 nm. This value is significantly smaller than diameters typically observed for acid-etched np-Au and, given the fact that it was determined for a sample in well reduced (double layer) condition, agrees well with previous findings. 30

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Figure 5: Surface area determination of the np-Au sample by cyclic voltammetry in the double layer regime between potentials UAg/AgCl of -200 mV and -100 mV in 1 M KOH solution. The current id recorded at UAg/AgCl = -150 mV is plotted for different scan rates s, resulting in a linear fit slope id /s=0.29 As/V. Table 1: Maximum relative resistance variation (∆R/R0 )max and charge coefficient (∆R/R0 )/∆Q of relative resistance variation of the sample material compared to previous studies. For nanoporous gold the respective dealloying electrolyte is given in brackets.

np-Au (AgNO3 ), this work np-Au (HNO3 ) 25 np-Pt 27 nc-Pt 22

(∆R/R0 )max [%] 88 43 58 15

(∆R/R0 )/∆Q [10−5 mol/As] 1.5 4.6 1.0 1.6

Discussion With a reversible electrical resistance increase up to almost 90 % in the oxygen regime, the np-Au material presented here exhibits the strongest adsorption-driven resistance variation reported for a metal so far, which will now be discussed in context to the literature. Table 1 gives an overview of the available data on the tunable electrical resistance of compacted metallic nanopowder (nc) and dealloyed nanoporous metals (np) upon oxygen adsorption. In addition to the maximum resistance variation, the table also features the charge coefficients (∆R/R0 )/∆Q, which relate the resistance variation to the imposed charge per mol ∆Q, assuming a perfectly etched sample without any residues of the less noble component. 10


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A detailed comparison of the resistance tuning for the present material with nanoporous gold produced via the more conventional route of dealloying in nitric acid electrolyte 25 shows, that even though significantly higher R variations can be obtained in the present case of npAu (AgNO3 ), its charge coefficient (∆R/R0 )/∆Q is inferior. To understand these trends, which may appear contradictory at first glance, the most striking differences between the two materials, namely the structural sizes and the concentration of impurities (dealloying residuals), have to be considered: It was found in the literature 30 as well as here, that the AgNO3 dealloying route leads to significantly smaller ligament diameters, however with higher amounts of silver remaining in the dealloyed structure. After a brief summary about the origins of charging-induced resistance variations of nanoporous structures in general, the consequences of these characteristic features will be discussed and compared to findings for platinum nanostructures in the following. As already mentioned in the introduction, the resistance tunability of porous nanoscaled metals can be considered to originate mainly from manipulations of the charge carrier scattering at the metal-electrolyte interface. Possible effects of a charging-induced actuation of nanoporous metals 19,20,37,38 or of a variation of the charge carrier density on the electrical resistance were demonstrated to be insignificant in previous studies. 23,25 Instead, the resistance variation is assigned to an increasing scattering probability of electrons at the crystal-electrolyte interface with positive charging, similar as described for thin films by Tucceri and Posadas in 1990. 39 In the free electron model of conductivity 40

σ=

e2 nτ m

(3)

with the elementary charge e, charge carrier density n and the electron mass m, the bulk conductivity of a metal is inversely proportional to the scattering rate τ −1

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τ −1 =

vF λf

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(4)

which in this simple model only depends on the Fermi velocity vF and the electron mean free path λf , which is approximately λf,Au ≈ 31 nm for pure gold and λf,P t ≈ 11 nm for pure platinum in bulk. 41 Possible alloying effects by impurities due to dealloying residuals will be discussed below. For nanoporous metals with structural sizes in the range of or even below λf , scattering at the interfaces gives a dominant contribution to the scattering rate. Therefore interfacial manipulations, such as electrochemical charging, may strongly influence the electrical resistance. This picture is particularly supported by the fact, that for nanoporous samples generally (here as well as in the literature) the strongest resistance variations were observed in the oxygen regime. Since anions show a stronger tendency for specific adsorption due to their larger ionic radii and therefore weaker bound solvation shells, they may strongly contribute to interfacial scattering. Prior to this study, the strongest adsorption-driven resistance variation had been observed for nanoporous platinum (np-Pt) produced by dealloying, 27 even though nanoporous gold investigated in earlier studies exhibited a much higher charge coefficient, 24,25 thus a stronger sensitivity of the electrical resistance to the imposed charge per mol. The reduced charge coefficient of platinum compared to gold occurs due to the much higher matrix resistance ρP t , which arises from the scattering of conducting s-electrons into d-band states. Therefore the strong resistance variation reported for np-Pt can clearly be assigned to the extremely high charging capacity of more than 300 As/g of the platinum structures, 27 which is directly related to its extraordinary small pore sizes. Interestingly, upon comparing dealloyed np-Pt with resistance tuning results for compacted platinum nanopowder (nc-Pt), differences similar to those described above for np-

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Au(AgNO3 ) and np-Au(HNO3 ) can be found: In spite of the much smaller resistance variations achievable for for nc-Pt due to the lower specific surface areas compared to np-Pt, the compacted nanopowder shows a higher sensitivity of the resistance to imposed charge, thus a higher charge coefficient. 22 This finding was attributed to a higher matrix resistance of the dealloyed material due to additional scattering centers given by residuals of the less noble component. 27 A similar condition is now created by the alternative dealloying routes for np-Au: As described above, the neutral silver nitrate solution (AgNO3 ) used for the dealloying process in this work is known to result not only in smaller ligament sizes but also in higher amounts of residual silver, 30 compared to conventional systems such as free corrsion in HNO3 or electrochemical etching in HClO4 that were studied earlier with respect to resistance tuning. 24,25 The residual silver in np-Au, etched in different acids, was recently found to be located in silver-rich clusters, being relics of the master alloy that have never been exposed to corrosion. 42 With residual Ag present also in silver oxide at the surface, 30 the situation may be more complex in the present case of np-Au dealloyed in AgNO3 solution. Nevertheless, this composite-like architecture of regions with different Ag-content may give an even more severe influence to charge carrier scattering than a similar concentration of silver atoms, distributed homogeneously in an Au matrix. Irrespective of their distribution, a higher content of dealloying residues in a material will generally lead to an increased matrix resistance and in further consequence a lower sensitivity towards surface charging, since a larger fraction of scattering events occurs in the ligament interior not affected by charging. This is the reason for the inferior charge coefficients observed for np-Pt (compared to nc-Pt) as well as np-Au etched in AgNO3 (compared to HNO3 ). The strong resistance variations in these systems are simply achieved due to the small structural sizes (ligaments) and thus high surface-to-volume ratio associated with high charging capacities: For both materials, similarly high charge densities up to about 300 As/g could be imposed.

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These findings open up the perspective of maximising the adsorption-driven tunability of the electrical resistance by engineering nanoporous metals towards small pore sizes while carrying a minimum of dealloying residuals. Owing to an exceptionally low bulk matrix resistance and therefore high charge coefficient (∆R/R0 )/∆Q, nanoporous gold appears as an ideal host structure for such endeavours. However, due to the well known tendency for atomic rearrangement once the primary oxide of pure np-Au is removed, which leads to the typical ligament sizes around 20 nm, 28 a certain amount of impurities in the material appears necessary to prevent coarsening. The desirable composition for resistance tuning is then represented by an ideal tradeoff between a structural stabilization and an (unwanted) increase of the matrix resistance, which may be achieved by using alloying elements or choosing dealloying routes with a suitable concentration of residuals in the structure. One option for such an optimization may lie in varying the dealloying sequence in the np-Au (AgNO3 ) system. By a step-wise dealloying program with alternating etching (in AgNO3 solution) and oxide dissolution (in Na2 SO4 solution) intervals, the Ag concentration in the resulting structure could be decreased below 20% in the literature, 30 which however also lead to structural coarsening. Another particularly promising candidate might be the ternary Au(Pt)-Ag alloy system, where a small amount of platinum is added to the classical Ag-Au system in order to stabilize the nanostructure at ligament sizes below 4 nm. 43 An excellent actuator performance has been reported for this material in the literature, 38 which may be point towards a similarly well tunability of the electrical resistance.

Summary and Conclusions Nanoporous gold, produced by electrochemically dealloying an Ag-Au alloy in AgNO3 was found to show an excellent tunability of the electrical resistance, which can be assigned to its extraordinary small ligament size (high surface-to-volume ratio). The porous structure exhibits an active surface area of about 22.5 m2 /g, corresponding to ligament diameters of

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about 10 nm, and a residual silver content of 30.3 %. Upon electrochemical charging in KOH solution the sample material a resistance tunability up to 88 % in the oxygen regime, representing the strongest adsorption-driven resistance variation reported so far for a metal. However the charge coefficient (∆R/R0 )/∆Q, thus the sensitivity of the material towards imposed charge per mol, is reduced by a factor of 3 compared to np-Au with higher purity. In good agreement with earlier results for nanoporous platinum, the reason for the excellent resistance tunability lies in the high specific surface area of the sample material, while the inferior charge coefficient is assigned to the high amount of residual silver in the porous structure which strongly increases the matrix resistance compared to pure Au. In order to achieve even higher resistance tunabilities in future studies, suitable materials need to exhibit fine porosities and yet a matrix resistance as low as possible. This demands an optimum compromise of structural stabilization and resistance increase by impurities in nanoporous gold, which may possibly achieved by stepwise dealloying programmes or the use of ternary alloys such as Au(Pt)-Ag.

Acknowledgement The authors would like to thank Dr. Matthias Graf from the Institute of Materials Physics and Technology, Hamburg-Harburg University of Technology, for kindly preparing the master alloys by arc melting.

References (1) Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors. Nature Nanotechnology 2011, 6, 232–236. (2) Lang, X.; Yuan, H.; Iwasa, Y.; Chen, M. Three-dimensional nanoporous gold for electrochemical supercapacitors. Scripta Materialia 2011, 64, 923 – 926.

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(3) Huang, J.-F.; Sun, I.-W. Fabrication and Surface Functionalization of Nanoporous Gold by Electrochemical Alloying/Dealloying of Au-Zn in an Ionic Liquid, and the SelfAssembly of L-Cysteine Monolayers. Advanced Functional Materials 2005, 15, 989–994. (4) Meng, F.; Yan, X.; Liu, J.; Gu, J.; Zou, Z. Nanoporous gold as non-enzymatic sensor for hydrogen peroxide. Electrochimica Acta 2011, 56, 4657 – 4662. (5) Zielasek, V.; Jürgens, B.; Schulz, C.; Biener, J.; Biener, M.; Hamza, A.; Bäumer, M. Gold Catalysts: Nanoporous Gold Foams. Angewandte Chemie International Edition 2006, 45, 8241–8244. (6) Zeis, R.; Lei, T.; Sieradzki, K.; Snyder, J.; Erlebacher, J. Catalytic reduction of oxygen and hydrogen peroxide by nanoporous gold. Journal of Catalysis 2008, 253, 132 – 138. (7) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Bäumer, M. Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature. Science 2010, 327, 319–322. (8) Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N.; Ishikawa, Y.; N., A.; Yamamoto, Y.; Erlebacher, J.; Chen, M. Atomic origins of the high catalytic activity of nanoporous gold. Nature Materials 2012, 11, 775–780. (9) Ding, Y.; Chen, M.; Erlebacher, J. Metallic Mesoporous Nanocomposites for Electrocatalysis. Journal of the American Chemical Society 2004, 126, 6876–6877. (10) Zeis, R.; Mathur, A.; Fritz, G.; Lee, J.; Erlebacher, J. Platinum-plated nanoporous gold: An efficient, low Pt loading electrocatalyst for PEM fuel cells. Journal of Power Sources 2007, 165, 65 – 72. (11) Chen, L.; Fujita, T.; Chen, M. Biofunctionalized nanoporous gold for electrochemical biosensors. Electrochimica Acta 2012, 67, 1–5. 16


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Graphical TOC Entry

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Enhanced Charging-Induced Resistance Variations of Nanoporous

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