Electrochemically Tunable Resistance of Nanoporous Platinum

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Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying Eva-Maria Steyskal, Zhen Qi, Peter Pölt, Mihaela Albu, Jörg Weissmüller, and Roland Wurschum Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01734 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying Eva-Maria Steyskal,∗,† Zhen Qi,‡ Peter Pölt, Mihaela Albu,¶ Jörg Weissmüller,‡,§ and Roland Würschum† Institute of Materials Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria, Institute of Materials Physics and Technology, Hamburg-Harburg University of Technology, Eißendorfer Straße 42, D-21073 Hamburg, Germany, Institute of Electron Microscopy and Fine Structure Research, Graz University of Technology, Steyrergasse 17, A-8010 Graz, Austria, and Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht, Germany E-mail: [email protected]

Abstract The extraordinary high surface-to-volume ratio of dealloying-prepared nanoporous platinum was applied for tuning electrical resistance by surface charging. In the asdealloyed state a characteristic sign-inversion of the charging-induced resistance variation occurs which can be associated with the electronic structure of PtO. After electrochemical reduction, relative resistance variations of nanoporous Pt up to 58 % can be ∗

To whom correspondence should be addressed Institute of Materials Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria ‡ Institute of Materials Physics and Technology, Hamburg-Harburg University of Technology, Eißendorfer Straße 42, D-21073 Hamburg, Germany ¶ Institute of Electron Microscopy and Fine Structure Research, Graz University of Technology, Steyrergasse 17, A-8010 Graz, Austria § Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht, Germany †

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generated by electrochemically induced adsorption and desorption, which is one order of magnitude larger compared to cluster-assembled nanocrystalline Pt. Although the maximum resistance variation is also higher than that of dealloyed nanoporous Au, the resistance variation related to the imposed charge is reduced owing to the higher bulk resistance of Pt compared to Au. The sign-inversion behaviour of the resistance can be recovered by re-oxidation.

Introduction Nanoporous metals produced by dealloying, an (electro-)chemical process in which the less noble component is removed from a master alloy by selective etching, 1 have attracted considerable scientific attention during the past few years. Due to their extraordinary inherent combination of three-dimensional macroscopic size and desirable properties associated with their nanostructure, dealloyed nanoporous materials exhibit attractive applications potentials such as sensing, 2,3 energy storage 4,5 or (electro-)catalysis. 6–11 The high surface-to-volume ratio also makes nanoporous materials predestined systems for property tuning by means of electrochemical charging since surface modifications may affect a considerable fraction of the bulk material. 12,13 Property tuning of dealloyed metals has already been demonstrated, e.g., with respect to various mechanical properties. 14–16 Dealloyed nanoporous metals have also been studied with respect to a tunability of the electrical resistance, 17–19 following the first studies of this kind on nanoporous materials made from compaction of nanoparticles. 20,21 By far the most intensively studied dealloyed material is nanoporous gold (np-Au) obtained from Ag-Au alloys which can even be dealloyed by free corrosion. 1 Among a great variety of experiments and simulations studying the dealloying process as well as the properties of the resulting nanoporous structures, 22 also charge-induced tuning of the electrical resistance of np-Au has been performed. 17,18 Reversible relative variations of the electrical resistance as high as 43 % could be achieved by electrochemically induced ad-/desorption. 18 Besides np-Au, a large variety of other dealloyable systems is known in literature. 23–25 2 ACS Paragon Plus Environment

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Among them, one particularly interesting material is nanoporous platinum (np-Pt), 25–27 due to the unique and well-known (electro-)chemical characteristics of platinum in combination with an extraordinarily fine porosity exceeding that of other dealloyed metals. 28 Despite its brittle nature, np-Pt has already proven as applicable for sensing 29,30 and electrocatalysis. 31 Moreover, the fine porosity may provide superior conditions for property tuning by electrochemically induced surface charging and modifications. First studies by our group of charging-induced tuning of the electrical resistance of np-Pt had to be restricted to a rather narrow potential range due to experimental limitations. 19 In the present study, this limitation could be overcome, enabling for the first time measurements of the electrochemical tunability of the electrical resistance of np-Pt in a wide potential range limited only by avoiding hydrogen and oxygen evolution in an aqueous electrolyte. The highest so far observed relative resistance variations of dealloyed nanoporous metals could be measured in this way. Moreover, deeper insight to the resistance characteristics of np-Pt, especially the extraordinary behavior of samples in freshly dealloyed condition, characterized by a strongly bound oxide, could be gained by reversible surface modifications, including electrochemical re-oxidation to PtO.

Experimental Procedures The nanoporous platinum sample material investigated in this work was produced by dealloying a Cu75 Pt25 master alloy, which was prepared in the same manner as described previously. 19 From this master alloy stripes were cut for dealloying and subsequent charging experiments. The electrochemical and resistance data are exemplary presented for a np-Pt sample with an initial alloy mass of 163 mg. The results were confirmed by means of test measurements on identically prepared further samples. An identically prepared separate sample was also used for electron microscopy of the as-dealloyed state. Our previous measurements of charging-induced resistance tuning of np-Pt had to be limited to a rather narrow poten-


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tial range, since the primary oxide of the as-dealloyed state could not be removed without sample damage or loss of the electrical contacts necessary for resistance measurements. 19 To overcome this experimental limitation, a different cell setup including an improved method of contacting the samples was employed in this work. Prior to any electrochemical treatment, five well-annealed flattened platinum wires were clenched around the master alloy in line and fixed by a droplet of glue (Crystalbond 509, Aremco) covering one face of the stripe. This bonding layer provides both electrical contact as well as a certain mechanical stabilization of the brittle material. For setting up the electrochemical cell in the following, the sample was immersed into the electrolyte hanging on the wires, by means of which any mechanical constraints on the sample are minimized (figure 1).

Figure 1: Experimental setup for resistance measurements upon electrochemical charging. All electrochemical procedures presented in this work were performed at room temperature with a PGZ-100 potentiostat (Radiometer Analytical) operating the np-Pt sample as the working electrode. The mid-positioned of the five sample contacts served as working electrode contact. As reference electrode a commercial Ag/AgCl reference filled with saturated KCl solution (Radiometer Analytical) was used, related to which all potentials UAg/AgCl will be stated in the following. 4 ACS Paragon Plus Environment

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Dealloying was performed at room temperature in 1 M H2 SO4 standardized solution under potentiostatic control, applying a potential UAg/AgCl =+1175 mV. During the dealloying process, copper deposition at the platinum wire counter electrode as well as a slight blueish colorization of the electrolyte could be observed, indicating copper dissolution and partial redeposition, while the sample material turned dark, losing its metallic shine. The procedure was stopped after several hours when the dealloying current had fallen below a threshold of 0.1 mA. To remove any traces of acid and dissolved copper, the dealloyed sample was subsequently cleaned carefully by immersion and rinsing with distilled water. All measuring runs on tuning the electrical resistance by electrochemical charging were performed in solution of 1 M KOH in ultrapure water; a highly porous carbon fabric contacted by a platinum wire served as counter electrode. In this cell, an open circuit potential of +140 mV and an initial resistance of 1.77 Ω were measured for the dealloyed sample upon first immersion. The resistance measurements were performed similar to our previous work. 19 To sum up briefly, the sample resistance was measured in four point geometry using a Keithley 2400 Source Meter, connected to the outermost contact wires for applying the four point measuring current and the remaining two wires for measuring the voltage drop. Resistance data points were taken in intervals of 2.5 seconds during the charging procedures. 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. Charging was performed both in cyclic voltammetry (CV) as well as chrono amperometry (CA) mode yielding consistent results. While cyclic voltammetry allows for dynamic insights to the charging processes, by means of chrono amperometry the signal can be corrected for minor leakage currents giving higher accuracy in integrating the imposed charge. Moreover a possible influence of the electrochemical charging current on the resistance measurements can be excluded by picking up the resistance data when the charging current has faded out. Occasionally visible spikes in otherwise smooth cyclic voltammograms are caused by coincidence with the measuring pulses from the four point ohm-meter since the amplitudes


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of these measuring pulses have to be sufficiently high for accurate resistance measurements. This causes certain irregularities in the CV, yet does not interfere with the property-tuning experiment. The concomitant drift in the imposed charge, calculated by continuously integrating the recorded charging current i, was corrected by a constant factor determined from the cycle-to-cycle offset. The novel contacting method presented above allowed for an investigation of the tunable sample resistance in a wide potential range limited only by hydrogen and oxygen evolution. The double layer window, the regime of oxygen ad-/desorption, as well as pseudocapacitive charging of strongly oxidized np-Pt became accessible. The extremely high surface area of np-Pt required long measuring time constants both for chronoamperometric steps and cyclic voltammetry. Cyclic voltammograms of the dealloyed samples were measured at scan rates between 1 mV/s and 0.1 mV/s and are presented in steady state if not stated otherwise.

Results For structural characterization of nanoporous platinum produced by electrochemical dealloying, studies by transmission electron microscopy (TEM) and scanning electron microscope (SEM) were performed. TEM micrographs taken for an as-dealloyed sample reveal ligament diameters of about 5 nm and pores in the range of 1 nm (see figure 2A). The as-prepared nanoscale structure is beyond the resolution of SEM (2B), only after heat-induced coarsening the pore structure becomes visible (figure 2C), signifying a high amount of pore volume in the initial structure. As already known from previous studies, nanoporous metals in as-dealloyed condition are covered by a primary oxide, associated with particular physical properties, 18,19,32 which, once removed by electrochemical reduction, cannot be entirely restored. For this reason in a first step the as-dealloyed state was investigated by cyclic voltammetry between potentials UAg/AgCl of +50 mV and +450 mV, corresponding to pseudocapacitive charging of the


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Figure 2: Electron-microscopical characterization of np-Pt. A: TEM micrograph revealing pore sizes in the range of 1nm. B and C: SEM images before and after thermal coarsening.


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oxidized sample avoiding any reduction of the primary oxide. In this potential range, as shown in figure 3A, the electrical resistance of np-Pt varies by about 5 % relative to the resistance R0 at open circuit potential in primary oxidized condition. The variation exhibits a sign inversion with an instantaneous resistance minimum at the lower and a delayed resistance minimum at the upper potential edge and resistance maxima in between (fig. 3B), a behaviour which was already discussed in detail in our previous work. 19

Figure 3: Relative variation ∆R / R0 of the electrical resistance of primary-oxide covered np-Pt in as-dealloyed state upon cyclic voltammetry in 1 M KOH. The potential UAg/AgCl is varied with a scan rate of 1 mV/s in the range between +50mV and +450mV, where the primary oxide is retained. The reference value R0 refers to the open circuit potential prior to the charging procedure. A: Current i (red) and ∆R / R0 (blue) as a function of UAg/AgCl . B: Variation of ∆R / R0 (blue) and UAg/AgCl (green) with time. To avoid any sample damage, the primary oxide was removed from the np-Pt sample by cyclic voltammetry at a very slow scan rate of 0.1 mV/s without measuring resistance (see


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figure 4). The potential range UAg/AgCl chosen between +450 mV and −1050 mV in order to avoid oxygen and hydrogen evolution. During the first cycle (red) starting from the upper potential limit of +450 mV, reduction of the strongly bound primary oxide occurs as signified by a prominent peak at about −550 mV. The reduction is also directly visible by the metallic gloss emerging during the process. Subsequent cycles are in good agreement with typical cyclic voltammograms of polycrystalline platinum well known from textbook literature. The noisy behaviour of the current i in the oxygen regime indicates temporary contact losses in this potential range which could yet be resolved by continuous cycling. From the charge flow associated with hydrogen desorption, corrected for the double layer charging current, an electrode surface area of 2.5 m2 was calculated for the sample after reduction. Assuming the np-Pt sample is free of residual copper, this corresponds to a surface-to-mass ratio of about 30 m2 /g.

Figure 4: Cyclic voltammetry of as-dealloyed np-Pt with a scan rate of 0.1 mV/s in 1 M KOH between +450 mV and −1050 mV, limited by avoiding oxygen and hydrogen evolution. Current i as a function of potential UAg/AgCl . The first cycle (red) strongly differs from successive cycles (black) due to the irreversible removal of the strongly bound primary oxide. The reduction of the primary oxide is associated with a decrease of the electrical resistance by more than a factor of 10 as deduced from resistance measurements performed at open circuit potential before and after reduction. Figure 5 shows the variation of R of np-Pt during CV cycling after reduction; cycling was performed with the same param9 ACS Paragon Plus Environment

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eters used for removing the primary oxide. Qualitatively, the charging-induced resistance variation behaves similar to that observed recently for porous nanocrystalline platinum pellets produced by compaction of commercial Platinum Black powder (nc-Pt) under similar charging conditions. 33 While only small resistance variations can be generated by hydrogen adsorption and double layer charging, a significantly higher tunability with a hysteresis in the resistance-potential-characteristics can be observed in the oxygen regime (fig. 5A). As discussed recently in more detail, 33 this hysteresis stems from the direct relation of R to oxygen ad-/desorption. The hysteresis-free R-variation with imposed charge (fig. 5B) is in line with this notion (see ref. 33). From a quantitative point of view, however, the relative variation ∆R/R0 of np-Pt by 58% exceeds that of nc-Pt (6%) by nearly one order of magnitude. This is due to extremely high surface-to-volume ratio of the samples produced by dealloying. To exclude any influence of the potentiostat’s charging current on the resistance measurements, the R variation was confirmed using a chronoamperometric charging program. The potential of the nanoporous sample was varied in CA steps with a steptime of 60 minutes and a stepwidth of 50 mV between −1000 mV and +450 mV. Data points of the resistance were then taken at the end of each 60-minute-charging step when the charging currents are faded away. The results of this control measurement, which are shown in figure 6, are in perfect agreement with the data acquired by cyclic voltammetry. CV cycling in the double layer regime between −510 mV and −360 mV (scan rate of 0.1 mV/s) causes a linear, hysteresis-free variation of R with potential U as well as with imposed charge Q (see figure 7), as expected due to the capacitive nature of charging. A relative variation ∆R/R0 of ca. 1 % is achieved for an imposed charge of about 0.5 C. Since the states with and without primary oxide show fundamentally different characteristics of the charging-induced resistance variation (compare figures 3 and 5), it appears obvious to check to what extent the behavior associated with the as-dealloyed state can be restored by strong oxidation. For this purpose, the np-Pt sample, from which the primary


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Figure 5: Relative variation ∆R/R0 of electrical resistance of np-Pt upon cyclic voltammetry in 1 M KOH with a scan rate of 0.1 mV/s between −1050 mV and +450 mV, limited by avoiding hydrogen and oxygen evolution. The reference value R0 refers to the sample resistance in the double layer region at -450mV. A: Current i (red) and ∆R / R0 (blue) as a function of UAg/AgCl . B: Presentation of ∆R/R0 data in dependence of imposed charge Q.


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Figure 6: Relative variation ∆R/R0 of resistance of np-Pt upon chronoamperometric charging steps in 1M KOH at potentials UAg/AgCl with a step time of 60 minutes and a step width of 50 mV between −1000 mV and +450 mV. R-values are measured at the end of each potential step. The reference value R0 refers to the open circuit potential before the charging procedure. oxide had been removed before, was held at a potential UAg/AgCl =+450 mV for 12 hours. From the charge flow of about 28 C at this potential step, a transfer of 0.7 oxygen species per platinum atom in the sample can be approximated. After this process of re-oxidation, a resistance about twice as high was measured at open circuit potential. Obviously, both the degree of oxidation and the associated resistance increase do not reach the state of as-dealloyed np-Pt with the primary oxide. Nevertheless, the particular charging-induced resistance variation (CV range +50 mV and +450 mV) with the characteristic sign inversion of ∆R/R0 is recovered after re-oxidation (compare figures 3 and 5), although with a strongly reduced amplitude of ca. 0.5 %. Finally, the potential range was again extended to the regime of electrochemical reduction (results not shown). The reduction behavior of the re-oxidized np-Pt resembles that of the asdealloyed sample with the primary oxide, exhibiting the pronounced oxygen desorption peak during the first CV cycle. Ongoing cycling of this twice reduced sample furthermore shows qualitatively the same CV- and ∆R/R0 -characteristic as after the first reduction process.


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Figure 7: Relative variation ∆R/R0 of electrical resistance of np-Pt upon cyclic voltammetry in the double layer regime between −510 mV and −360 mV with a scan rate of 0.1 mV/s. The reference value R0 refers to the sample resistance at −450 mV. A: Current i (red) and ∆R/R0 (blue) as a function of potential UAg/AgCl . For charge determinations the spikes in the CV, caused by four point current pulses, were corrected by a linear approximation (dashed line). B: Presentation of ∆R/R0 data in dependence of imposed charge Q.


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Figure 8: Relative variation ∆R/R0 of electrical resistance of np-Pt after oxidation for 12 hours at +450mV in 1 M KOH. Cyclic voltammetry is restricted to the potential regime (+50 · · · + 450 mV) above which oxygen desorption is avoided (scan rate of 0.1mV/s). The reference value R0 refers to the sample resistance after re-oxidation. A: Current i (red) and ∆R/R0 (blue) as a function of potential UAg/AgCl . B: Variation of ∆R/R0 (blue) and UAg/AgCl (green) with time.


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Discussion The presented measurements on np-Pt prepared by dealloying reveal as most remarkable result that the charging-induced resistance variation shows distinct and characteristic changes upon removal of the primary oxide. The particular behaviour in the as-prepared state can even be recovered by re-oxidation. We start the discussion with the charging-induced resistance variation in the state after removal of the primary oxide which could be measured here for the first time. The results of the relative resistance variations are summarized in Table 1 for the two regimes of double charging and specific ad-/desorption in comparison to previous findings for dealloyed nanoporous gold (np-Au) 17,18 and porous Pt made by compaction of nanocrystalline powder (nc-Pt). 20,33 The origin of the resistance tunability of porous nanoscaled metals was discussed extensively in previous articles. 18,33 The variations are considered to arise mainly from manipulations of the charge-carrier scattering at the metal-electrolyte interface. The effect of a charging-induced actuation of nanoporous the metals 13–16,34 or variation of the charge carrier density on the resistance turned out be insignificant. While in comparison to previous studies 17,18,20 only small resistance variations are observed upon double layer charging, the maximum value of ∆R/R0 of np-Pt in the regime of specific ad-/desorption exceeds that of np-Au 17,18 by about 35% and that of nc-Pt 33 by almost a factor of ten (Table 1). Relating ∆R/R0 to the imposed charge ∆Q per mol, the charge coefficients (∆R/R0 )/∆Q deduced for np-Pt and nc-Pt are rather similar. This shows that the ten-fold increase of (∆R/R0 )max of np-Pt compared to nc-Pt arises from the much higher surface area and, related to that, from the much higher charging capacity of more than 300 C/g upon ad-/desorption. In contrast to the rather similar (∆R/R0 )/∆Q values obtained for Pt, nanoporous gold shows a much higher sensitivity to imposed charge. 17,18 The electron mean free path λf in pure coarse-grained Pt at ambient temperature is by factor of 3 lower than in Au (λf,Au = 15 ACS Paragon Plus Environment

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Table 1: Maximum relative resistance variation (∆R/R0 )max and charge coefficient (∆R/R0 )/∆Q of relative resistance variation of np-Pt compared to dealloyed np-Au and cluster-assembled nc-Pt. ∆Q: imposed charge per mol (C/mol). (∆R/R0 )max [%]

(∆R/R0 )/∆Q [10−5 mol/C]

1 4 6 8

0.9 11.2

58 43 6

1.0 4.6 1.6

Double Layer Charging np-Pt this work np-Au Wahl et al. 18 np-Au Mishra et al. 17 nc-Pt Sagmeister et al. 20 Specific Ad/-Desorption np-Pt this work np-Au Wahl et al. 18 nc-Pt Steyskal et al. 33

1.5

31 nm); this is also reflected by the resistivity which is by a factor of 5 higher compared to Au. 35 On the other hand, the relative resistivity increase due to residual Cu or Ag in np-Pt or np-Au, respectively, may be considered to be similar if one assumes that similar amounts of Cu or Ag remain after dealloying. This is because bulk Pt and Au exhibit a similar relative resistivity increase of about 10% per 1 at.% of doping. 36,37 Therefore, irrespective of dopants, the higher bulk resistivity and the shorter electron mean free path presumably causes the much lower charging sensitivity of the relative resistance in np-Pt compared to np-Au. Due to the shorter mean free path, scattering processes inside the ligands of the porous structure of np-Pt add more strongly to the overall scattering rate and, therefore, the scattering rate is less sensitive to the charging-induced variations of the scattering at the metal-electrolyte interfaces compared to np-Au. Comparing, on the other hand, np-Pt with nc-Pt, the slightly reduced sensitivity of np-Pt with respect to charging (cf. (∆R/R0 )/∆Q values, Table 1) may be caused by a higher matrix resistance of np-Pt due to residual copper atoms. Now we will discuss the resistance variation of the oxidized samples which exhibits the characteristic sign inversion. Even though of slightly different shape, the resistance variation shows similar trends as observed in our previous work: 19 upon positive as well as negative


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scan, the electrical resistance first increases and subsequently decreases. Thus relative resistance minima near the reversal points of the CV are observed with maxima in between. Yet the exact position of the maxima slightly varies from sample to sample. This sign inversion behavior of (∆R/R0 ) is considered to arise from the electronic structure of PtO which is present in the oxidized state. In fact, PtO exhibits a minimum of the density of states at the Fermi edge 38 similar to graphite. In the case of graphite, due to this minimum, the density of states increases and, hence, the resistance decreases upon charging in both directions. The same appears to apply to charging of the oxidized Pt sample: the resistance maximum observed upon positive as well as negative scan can be explained by passing through the DOS minimum. The exact potential of the maximum depends on the Fermi edge of the uncharged sample and thus the degree of oxidation. An alternative interpretation of the (∆R/R0 )-behaviour in analogy to the effect of gasadsorption on metallic films, that was discussed in our previous paper, 19 can be ruled out on the basis of the present studies. An initial increase and subsequent decrease of the resistance of a Cu film upon oxygen adsorption was associated with diffuse charge carrier scattering at the surface, attaining a maximum for semi-monolayer coverage and a minimum for monolayer coverage due to smoothening of the surface. 39 Such a notion for np-Pt now reveals as incompatible with the results of the present resistance measurements after removing of the primary oxide. Within the scanned potential range the oxygen adsorption on the sample clearly goes beyond one monolayer, 40 yet no maximum followed by a decrease in resistance can be observed associated to semi-monolayer oxygen coverage. The fact that the charging-induced property tuning substantially changes upon removal or generation of a surface oxide is not restricted to the present case of electrical resistance, but has been known before also for mechanical properties. Viswanath et al. 34 reported on opposite sign of the surface stress-charge coefficients of porous nanocrystalline Pt when reversibly switching from double layer behaviour (positive strain-charge response) to the oxide-layer behaviour (negative strain-charge response) by appropriate electrochemical treatment. Like


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the resistance behaviour, the negative strain-charge response of the oxide-covered sample is considered to be associated with the particular electronic properties of PtO. 34 As demonstrated in figure 8, the behaviour before removing the primary oxide can, at least partially, be recovered by re-oxidation. At the potential applied for strong oxidation (+450mV) site exchange steps of adsorbed oxygen with underlying platinum atoms can be expected. 41 Taking into account the fine porosity of the sample and the recorded transfer of 0.7 oxygen species per Pt atom, the resulting nanoporous structure may again contain a substantial amount of PtO, exhibiting a qualitatively similar resistance behavior as in dealloyed state. This notion may also explain the fact that for cluster-assembled nanocrystalline Pt (ncPt) resistance tuning, performed under similar conditions, does not exhibit the sign-inversion characteristics as in the present case of np-Pt. 33 np-Pt exhibits a much finer porosity and higher specific surface area than nc-Pt. Due to the higher surface-to-volume ratios of np-Pt, site exchange steps of O and Pt in atomic layers close to the surface may affect a substantial portion of the sample. Therefore, upon oxidation np-Pt becomes governed by the behavior of PtO, while the coarser structured nc-Pt, despite superficial oxidation, preserves metallic conduction behavior. Finally, we will compare np-Pt with np-Au with respect to the as-dealloyed state. Also for np-Au strongly different property-to-charge-responses have been reported before and after reduction of the primary oxide. 18,32 For the resistance variation of np-Au a change in magnitude, yet not in sign, of the charge coefficient upon removing the primary oxide was observed. 18 Electrochemically induced length change of np-Au even showed opposed sign behaviour of the surface stress-charge coefficients before and after reduction. 32 In both experiments on np-Au, however, once the primary oxide had been removed the as-dealloyed behavior could not be regenerated by re-oxidation in contrast to np-Pt. This different behaviour may be related with strongly different atomic diffusivities which scale with the strongly different melting temperatures. While dealloyed gold is known for structural rear-


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rangement during and after dealloying, 1,42,43 coarsening processes are suppressed in np-Pt due to the significantly lower surface diffusivity. This property causes not only the extraordinarily fine porosity of dealloyed np-Pt, 28 but also makes Pt applicable as minor component in ternary dealloying systems in order to yield finer porosities. 24 Therefore it can be reasonably assumed that in the case of np-Pt a reduction of the primary oxidized state may lead to minor reordering only, enabling re-oxidation conditions similar to those upon dealloying, at least partially regenerating the characteristics of the primary oxide, in contrast to np-Au.

Summary and Conclusions In the present work the tunable electrical resistance of nanoporous platinum produced by dealloying of Cu75 Pt25 was investigated in a wide potential range, leading to the following major results: • Relative resistance variations (∆R/R0 ) of nanoporous Pt as high as 58 % could be generated by electrochemically induced ad-/desorption processes, representing the highest value reported for a nanostructured metal. • The sign inversion characteristic of the resistance of the as-dealloyed nanoporous Pt can be assigned to the electronic structure of PtO. • The primary oxide of freshly dealloyed samples can be removed while retaining the high specific surface area. • Re-oxidation of reduced nanoporous Pt regenerates the sign-inverted resistance behavior typical for the oxidized state.

Acknowledgement Financial support by the Graz inter-university cooperation on natural sciences (NAWI Graz) is appreciated. 19 ACS Paragon Plus Environment

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Electrochemically Tunable Resistance of Nanoporous Platinum

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