Istituto per l’Energetica e le Interfasi (IENI) – CNR, Corso Stati Uniti 4, 35127 Padova, ItalyDipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia,via Torino 155, 30172 Mestre (VE), Italy
r t i c l e i n f o
rticle history:eceived 16 May 2011eceived in revised form 12 July 2011ccepted 13 July 2011vailable online 23 July 2011
a b s t r a c t
A method for decorating the surface of disk electrodes with metal nanowires is presented. Cu and Ninanowires with diameters from 1.0 �m to 0.2 �m are directly deposited on the electrode surface usinga polycarbonate membrane filter template maintained in contact with the metal substrate by the softhomogeneous pressure of a sponge soaked with electrolyte. The morphologic and structural properties ofthe deposit are characterized by scanning electron microscopy (SEM) and electron backscatter diffraction
(EBSD). The latter shows that the head of nanowires with diameter of 0.4 �m is ordinarily polycrystalline,and that of nanowires with diameter of 0.2 �m is almost always monocrystalline for Cu and frequentlyalso for Ni. Cyclic voltammetries and impedance investigations recorded in alkaline solutions at repre-sentative Ni electrodes decorated with nanowires provide consistent values of roughness factor, in therange 20–25.
lectron backscatter diffraction
. Introduction
The preparation of submicro- and nanostructures with definedharacteristics or functional properties is a fast growing area ofaterials science, and the preparation procedures require some-
imes a series of complex physical/chemical steps [1,2]. In thisroad field, a relatively cheap technique like electrochemistryaintains a role of pre-eminence in the preparation of conductive
anowires or nanotubes since electrochemical deposition, per-ormed in templates providing lateral confinement, is suitable toroduce deposits with large aspect ratios, potentially almost one-imensional [3–5], taking advantage of the control over depositionate and deposited mass offered by current and integrated charge.
Electrodeposition of several metals and semiconductors haseen performed using alumina [5–8] and polymeric track-etched9–13] membrane templates. Electrodeposition is normally pre-eded by coating one side of the membrane with a noble metal orlloy – by electron beam evaporation, sputtering or other methods
to get the conductive substrate necessary to perform electrode-osition in the pores from the opposite, open pore mouth. Thisrocedure may prove convenient for certain aims, but it is not suit-ble to directly deposit on a metallic electrode nanostructures ofhe same material, in order to obtain a robust surface decoration
omogeneous in composition to the substrate.
In the present paper we propose an original electrodeposi-ion arrangement for the direct modification of a disk electrode
and investigate the morphological and structural properties ofthe deposit by scanning electron microscopy (SEM) and elec-tron backscatter diffraction (EBSD). The characterisation includeselectrochemical determination of the roughness factor fr of theresulting surface decorated with nanostructures.
2. Experimental
The deposition baths were prepared from deionized (by a Mil-lipore Milli-RO system) and triply distilled water and commercialchemicals (Sigma–Aldrich, ACS reagents, purity ≥98%). Cu deposi-tion was performed at room temperature from an acid sulfate bath(0.55 M H2SO4, 0.88 M CuSO4); Ni deposition was performed froma Watt’s bath [14,15] containing 1.126 M NiSO4·6 H2O, 0.185 MNiCl2·6 H2O and 0.485 M H3BO3, pH 4, at T = 55 ◦C.
Electrodepositions were performed on stationary disk elec-trodes with a 2 mm diameter (surface of 0.0314 cm2), of the typeused for RDE experiments (Tacussel), inserted in a cylindrical bodyof Teflon. The electrode was first abraded with fine emery paper4000 grit, then polished with graded alumina 1.0 �m and 0.3 �m(Buehler micropolish II), rinsed with water and dried. The pol-ished Cu electrode was used directly as a substrate for the templateassisted deposition of Cu nanowires. The substrate for deposition ofNi nanowires was a Au disk electrode, covered by a smooth Ni layerabout 2.5 �m thick and polished with the 0.3 �m alumina to elim-inate border defects. The cell used for deposition experiments was
a thermostated glass cell, with 50 mm inner diameter and 55 mmheight, partially filled with the deposition solution and equippedwith an L-shaped sheet counter electrode, made of the same metalto be deposited, positioned with the basis on the cell bottom and
A. Gambirasi et al. / Electrochimica Acta 56 (2011) 8582– 8588 8583
Fig. 1. Schematic illustration of the electrodeposition setup and procedure: risingtaa
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1
2
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1
2
IVIIIII
I
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A
III
-I / m
Atime / s
a
b
Fig. 2. Time evolution of cathodic current during electrodeposition of Cu from theacid sulfate bath. Membrane SPI-pore, pore diameter 1.0 �m. Applied potential: (a)
the electrode was removed, washed with water, blown dry in anitrogen stream and immersed in warm CH2Cl2 or CHCl3 until
he elevator, the membrane lying over the sponge soaked with electrolyte is pressedgainst the disk surface. The cell operates in a two-electrode geometry, cathodebove anode.
xposing to the solution a surface of at least 12 cm2, namely about00 times larger than that of the working electrode.
Most experiments were performed with polycarbonatePC) membranes purchased from SPI-pore, with the followingominal specifications: (i) pores of 1.0 �m diameter, den-ity 2 × 107 pores cm−2; (ii) pores of 0.4 �m diameter, density
× 108 pores cm−2; (iii) pores of 0.2 �m diameter, density × 108 pores cm−2; the membrane thickness was in all casesround 10 �m. Some experiments were made with PC membranesrom Whatman and Membrane Solutions, with nominal thick-esses of about 20 �m and other specifications similar to thoseeported. In order to perform the template assisted deposition,embrane disks with a diameter of about 1 cm were cut from the
ommercial sample and left to condition in the same solution usedor deposition for at least 2–3 h, to promote pore filling by the elec-rolyte. In the case of membranes with pore diameter, d = 0.2 �m,he conditioning included a few minutes in an ultrasonic bath: thisreatment had a positive effect on the reproducibility of the sub-equent deposition process, presumably due to promotion of porelling. The deposition set-up and procedure may be described witheference to the scheme in Fig. 1: a piece of commercial spongeade of a melamine foam with a tough microporous structure
BASF Basotect®), about 2 cm thick, was inserted in the bottomf the empty cell, above the counter electrode. The appropriateolution was added to the sponge level and the conditioned mem-rane disk was laid on the sponge with care, avoiding formationf wrinkles. The cell was then raised, making use of a laboratorylevator table, until the disk electrode surface (face down) madeome pressure on the membrane and the sponge supporting it,eaching the electrolyte level. The deposition was performed inhe two-electrode geometry, cathode (above) on anode (below),sing the large counter electrode with a low polarizability as aseudo-reference; a more rigorous three-electrode set-up was not
mployed to avoid mechanical troubles to the membrane (causing,.g., wrinkles formation) when the reference electrode is placedext to the working electrode. The applied voltages, referred to inhe following as �E, were typically �E = −0.4 V to −0.25 V for Cu
�E = −0.4 V; (b) �E = −0.4 V for the first 10 s, then �E = −0.25 V. The roman numbersI–IV refer to the different stages of nanowire formation, illustrated in Fig. 3.
deposition and �E = −0.7 V for Ni deposition; the current transientwas followed to identify an appropriate end time. After deposition
Fig. 3. Schematic illustration of the stages of nanowire formation: (I) nucleation ofa crystal at the pore base, with depletion of metal ions in solution; (II) pore fillingby a metal deposit; (III) metal deposition beyond pore mouth and caps formation;(IV) caps merging and metal deposition over the entire surface.
8584 A. Gambirasi et al. / Electrochimica Acta 56 (2011) 8582– 8588
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ct
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ig. 4. SEM images of Cu nanowires deposited in SPI-pore membranes: top line, (a) oottom line, (c) overview and (d) detail of deposit in a membrane with pores of 0oftware.
omplete dissolution of the polycarbonate (PC) membrane, thenhe deposit was characterised.
Electrodeposition experiments were controlled with an AMELorking station including a Model 2053 potentiostat, a Model 568
unction generator and a Model 731 integrator. A multipurposeaboratory interface (Vernier) allowed data acquisition in digitalorm. Electrochemical experiments aiming at estimating the sur-ace roughness factor fr of the decorated electrodes were performedith Ni electrodes in alkaline solutions, using either cyclic voltam-etry (1.0 M NaOH) or impedance spectroscopy (0.1 M NaOH), and
ontrolling the electrode potential versus a Hg/HgO/1 M KOH ref-rence electrode. CVs were performed with an Autolab PGSTAT02N. Impedance measurements were taken with a Solartron 1286lectrochemical interface and a Solartron 1254 frequency responsenalyzer, both controlled by a ZPlot-ZView commercial software,overing the frequency range 27 kHz to 0.01 Hz with 8 pointser decade. The electrode was polarized at the applied potential
= 0.10 V vs. (Hg/HgO), in the region of double-layer charging, tovoid any significant faradaic reaction; the potential modulationas 10 mV peak-to-peak, checked to be low enough to ensure linear
esponse.SEM analyses were performed with a FEI Quanta 200 FEG ESEM
nstrument, equipped with a field emission gun, operating in lowacuum condition at an accelerating voltage variable from 20 to0 kV, depending on the observation needs. Electron backscatteriffraction (EBSD) analyses, which can be used to find the crys-
al orientation of the material located within the incident electroneam’s interaction volume, were conducted in the scanning elec-ron microscope operating at an accelerating voltage of 20 kV, with
working distance of 10 mm and the sample tilted of 75◦ to hori-
ew and (b) detail of deposit in a membrane with pores of 0.4 �m nominal diameter; nominal diameter. The diameter of nanowires is estimated in (b) and (d) by SEM
zontal. The EBSD facility included the EDAX-TSL DigiView detectorconsisting of a DVC-1412M digital camera with CCD sensor anda circular phosphor screen coating as intensifier. The EBSD analy-ses were performed in two alternative ways: (i) sampling differentspots on the surface (minimum spot size was ca. 5 nm radius and20 nm depth), and considering/comparing their crystal orientationdefined by three points in a pole figure representation; (ii) per-forming an automated analysis over a selected area, according toan hexagonal grid with step size of 20 nm, resulting in the pro-duction of a map in which different shades of gray correspond todifferent crystal orientations. The TSL software OIM 5.31TM packagewas used to collect, index and analyse EBSD patterns.
3. Results and discussion
The current–time transient recorded during the electrodeposi-tion of Cu at an applied voltage �E = −0.4 V is shown in Fig. 2a: thecurve shape resembles to that already reported in the literature[11,12]. For the sake of simplification the curve may be divided in 4parts [11,12], denoted I–IV, associated to the intuitive sequence ofdeposition stages sketched in Fig. 3: immediately after closing thecircuit (stage I) the current shows a large peak and a sharp decay,due to depletion of the Cu2+ ions following initial fast Cu depositionand some associated increase in the electrolyte resistance insidethe pores. Subsequently, the current decays slowly (stage II), whilepore filling by metal deposition proceeds; then during stage III the
current increases due to emersion of the deposit from the mem-brane template and formation of caps with a typical mushroomshape; finally, caps growth leads to overlapping and the currentlevels out (stage IV) in correspondence of metal deposition over
A. Gambirasi et al. / Electrochimica Acta 56 (2011) 8582– 8588 8585
F ith pow the ars pots o
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ig. 5. SEM images of Cu nanowires deposited in Membrane Solutions template was performed on the head of the nanowires indicated in (b) and magnified in (c), in
hade of gray, each one indicating a monocrystalline domain. White dots indicate s
large surface. A major problem connected with the depositionn templates is the achievement of a high synchronisation in theucleation, in order to obtain nanowires of comparable length ando limit to a negligible extent the phenomenon of capping and over-rowth in areas where the growth starts earlier. Considering theomplications of our experimental system, including the imperfectotentiostatic control warranted by the two-electrode set-up, it isot advisable to discuss mechanistic details of the early stages ofhe deposition process; yet, we can observe that Cu is known to belectrodeposited at low pH according to the instantaneous nucle-tion mechanism [16], favourable to get deposition of structures ofegular length in templates.
To avoid the phenomenon of capping and overgrowth, the endoint of the deposition was taken before the end of stage II. Inur experience, confirming literature reports [8], in order to getanowires of relatively homogeneous length it is helpful to use
ow deposition voltages and long deposition times. Hence, we per-ormed the deposition with a simple potential program (Fig. 2b):–10 s polarization at �E = −0.4 V (fairly large overpotential, pro-oting formation of metal nuclei at the pore base even on the less
ctive sites), followed by polarization at less negative potentials�E = −0.25/−0.20 V) in a slow stage II, interrupted before stage III.
The deposit morphology observed after dissolution of the tem-late is illustrated by SEM pictures. Fig. 4a and b shows Cu depositsbtained in a SPI-pore membrane with pores of 0.4 �m diame-
er: the nanowires show orientations with disordered tilting withespect to the normal to electrode surface, reproducing the direc-ion of pores in the original membrane; the diameters estimatedy SEM software on nanowire heads show a limited scattering
res of 0.4 �m diameter, (a) overview and (b) detail. Automated EBSD investigationea enclosed by the dark line: the map in (d) shows a few areas with an homogeneousf unidentified crystal orientation.
(Fig. 4b), with values mostly falling in the range 400–450 nm (theunrealistic accuracy of estimates is imposed by the software).The images of deposits obtained in a membrane with pores of0.2 �m diameter (Fig. 4c and d) show a more regular orientationof the nanowires; the diameters estimated by SEM on nanowireheads show again a limited scattering, with values mostly fallingin the range 200–240 nm. The nanowires, firmly anchored to thesubstrate, have typical lengths of 7–8 �m; they may cover homoge-neously macroscopic areas of the electrode, but in general surfacecovering is not total: the reasons for this incomplete success arenot obvious, and may include imperfections in both the procedure –e.g., non optimal cell arrangement, irregular sponge pressure on themembrane – and the commercial products – e.g., inhomogeneouswettability of membrane pores by the solution.
Fig. 5 shows Cu nanowires with ca. 0.4 �m diameter, depositedin PC membranes purchased from another supplier (MembraneSolutions). The nanowires appear well aligned and perpendicu-lar to the substrate (Fig. 5a and b). Automated EBSD analysis ofa nanowire head over a selected surface (Fig. 5c), shows severalareas of different homogeneous shades of gray (Fig. 5d), indicatinga polycrystalline structure. A few white dots represent positionsfor which the instrument could not recognize any crystal pattern.The response in Fig. 5d is typical of this nanowire size: sometimes adominant grain is observed, but a monocrystalline structure mustbe very unusual.
Fig. 6 shows a typical case of morphological and structural inves-tigation of Cu nanowires deposited in SPI-pore membrane withpores of 0.2 �m diameter. For this diameter EBSD investigationsof nanowire heads reveal in most cases a single crystal structure.
8586 A. Gambirasi et al. / Electrochimica Acta 56 (2011) 8582– 8588
Fig. 6. SEM image of Cu nanowires deposited in SPI-pore membrane with pores of 0.2 �m diameter. EBSD investigation was performed point by point on the head of thenanowires indicated in (a) and magnified in (b). The pole figure in (c) shows well superimposed sets of three points, indicating a constant crystal orientation, confirmed bythe map in (d) showing the same shade of gray over the entire sampled area.
Fig. 7. SEM image of Ni nanowires deposited in SPI-pore membranes with pores of 0.2 �m diameter. EBSD investigation was performed point by point on the head of thenanowire indicated in (a) and magnified in (b). The pole figure in (c) shows well superimposed sets of three points, indicating a constant crystal orientation, confirmed bythe map in (d) showing the same shade of gray over the entire sampled area.
imica Acta 56 (2011) 8582– 8588 8587
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2
4
6
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smooth Ni decorated Ni
Fig. 8. Comparison of cyclic voltammograms (vscan = 10 mV s−1) in 1.0 M NaOH for asmooth Ni disk (dashed line, current density multiplied by 10) and a Ni disk deco-
A. Gambirasi et al. / Electroch
or instance, all the 18 spots sampled in Fig. 6b (dots) providehe response expected for a Cu face-centered cubic crystal and,dditionally, all the sets of three points in the (two dimensional)ole figure representation of Fig. 6c are superimposed, indicat-
ng that the same crystal orientation is present in every positionf the “interrogated” material surface. Likewise, the map result-ng from automated analysis (Fig. 6d) shows a constant shade ofray over the entire area, again indicating constant crystal orienta-ion. Occasionally, two different crystal orientations were detectedndicating formation of two grains. These findings are consistent
ith literature, reporting growth of single crystal wires in polycar-onate and anodic alumina membranes for Cu, Ag and other lowelting point metals like Pb and Bi (see e.g. [10] and references
herein).Deposition of Ni nanowires was performed (in SPI-pore mem-
ranes) from a Watt’s bath, at an applied voltage �E = −0.700 V.ore negative potentials were not used to avoid troubles result-
ng from possible hydrogen evolution. SEM images show the sameorphology observed for Cu deposition. EBSD investigations per-
ormed on the head of nanowires with a 0.4 �m diameter (noteported) show typically a polycrystalline structure, similar to thatbserved in Fig. 5 for Cu. This finding is expected on the basis ofiterature reports, according to which growth of single-crystallineigh melting point metals like Ni and Co is very difficult [10]; ondopting the usual procedure involving Pt sputtering on the back ofhe alumina membrane, single crystalline Ni nanowires could onlye obtained under particular conditions including very thin poreemplate (50 nm diameter), high deposition potentials and roomemperature [8].
Conversely, we could observe single crystal orientation of oureposits obtained under ordinary Watt’s deposition conditions, foranowires with diameter of 200 nm (Fig. 7a) and therefore, presum-bly, for any lower diameter: all the 18 spots (dots) shown in Fig. 7brovide perfectly coincident point sets in the (two dimensional)ole figure representation of Fig. 7c; the result is confirmed by auto-ated analysis, with a map showing a homogeneous shade of gray
ver the entire sampled area (Fig. 7d), indicating constant crystalrientation. The ability of our approach to provide single-crystal Nitructures under conditions in which this result is normally pre-luded [8] may depend on the use of a Ni substrate able to impose,rom the very beginning, a growth according to a few Ni orienta-ions and prevalence of low-surface-energy grains [17,18].
The roughness factor of Ni electrodes fr = Ar/Ag, where Ar is theeal surface (or interface) area of a rough sample and Ag is the geo-etric surface (interface) area of an ideally flat sample – can be
stimated in principle by measurement of electrochemical quanti-ies proportional to the surface area, in particular the redox charge
associated to the couple Ni(II)–Ni(III) [19,20] and the capacity C,n rough (Qr, Cr) and smooth flat (Qg, Cg) electrodes.
In order to obtain a modified electrode surface with a largerr value, deposition of nanowires was performed in a Whatman
embrane with pores of 0.2 �m diameter, which is about 20 �mhick. Fig. 8 reports cyclic voltammograms recorded in 1.0 M NaOHt smooth and decorated Ni electrodes. It is known that the redoxystem located at about 0.45 V is due to oxidation of Ni(II) to Ni(III),robably present in the respective species Ni(OH)2 and NiOOH,nd to the reverse reduction; the presence of pairs of peaks in theoltammetric pattern reflects the presence of at least two phasesf both Ni(II) and Ni(III) species [20]. Although the peak charge Qnvolved in a voltammetric sweep depends on electrode historynd sweep rate [19,20], the stabilized values obtained after pro-onged cycling at the same sweep rate may be used to estimate the
oughness factor as fr ∼= Qr/Qg. On the basis of the redox charges
estimated from the voltammogram in Fig. 8 we have obtainedr ∼= 21–25, where the range takes into account uncertainties in thestimation of peak areas.
rated with Ni nanowires deposited in Whatman membranes with pores of 0.2 �mdiameter (solid line).
EIS investigations performed on the same electrode of Fig. 8, atE = 0.1 V vs. Hg/HgO, in a potential region free from interference byredox processes, gave the response reported in Fig. 9. The hf rangeof the Nyquist plot, above ca. 200 Hz, has a shape which appearsdistorted as compared to that typical of electrodes with cylindri-cal pores [21], presumably due to irregular shape and size of pores[21,22] and to contribution of flat, non decorated portions of theelectrode area [23]. The lf pattern (below ca. 100 Hz) is typical ofa blocking electrode and is characterised by a constant-phase ele-ment (CPE) behaviour, with a CPE exponent ˛ ∼= 0.91, determinedas the slope of the lf straight line in the plot of the logarithm of theimaginary part vs. the logarithm of frequency [24] (Fig. 9c). Since
is markedly different from 1, the capacity cannot be simply eval-uated as C = [−ωZ ′′]−1 at any frequency. On the other hand, theuse of formulas proposed for the calculation of capacity from CPEparameters [25,26] might allow a sound determination of C onlyif the reasons for the CPE behaviour were clearly identified [27],which is not the case in the present study. Considering that similar
values were observed for both the decorated and the smoothNi electrodes, fr was directly estimated as the ratio fr ∼= Z ′′
g /Z ′′r ,
where Z ′′g and Z ′′
r represent the imaginary parts of the impedance ofthe smooth and decorated electrode, respectively – which is littledependent on frequency. In the frequency range f = 5–100 Hz, onefinds fr ∼= 21–23, close to the value obtained as the ratio of the redoxcharges. On the basis of the good agreement between the voltam-metric and the impedance measurements, a surface roughnessfactor fr ∼= 23 ± 2 can be reasonably ascribed to this representa-tive sample. The approximate value of fr expected for a complete
filling of all the membrane pores, on the basis of geometric con-siderations based on the parameters provided by the supplier (ca3–4 × 108 pores cm−2, 0.2 �m nominal diameter, 20 �m membrane
8588 A. Gambirasi et al. / Electrochimica
10-2 10-1 100 10 1 10 2 10 3 10 4 10 5100
101
102
103
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101
102
103
104
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10
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2
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c
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-Z'' /
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Z' / Ω cm2
Fig. 9. Impedance response of a Ni disk decorated with Ni nanowires depositediid
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4
d
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[[[
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[[[[[25] G.J. Brug, A.L.G. van den Eeden, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal.
n Whatman membranes with pores of 0.2 �m nominal diameter (same sample asn Fig. 8), recorded in 0.1 M NaOH: (a) Nyquist plot, (b) dependence of Z
′ and (c)ependence of −Z
′′ on frequency.
hickness), is fr ∼= 38–50. Considering that (i) these parameters arendicative values known with some uncertainties; (ii) the electrodeurface was not completely decorated; (iii) the real wires lengthas lower than 20 �m since deposition was stopped before com-lete filling of the pores to avoid overgrowth, the determined fralue is in substantial agreement with expectations.
. Conclusions
We have proposed a cell arrangement suitable to directlyeposit metal nanowires on the electrode surface using a PC
[[
Acta 56 (2011) 8582– 8588
membrane as template. The porous surface structure has beencharacterised for a representative deposit of Ni nanowires with adiameter of about 0.2 �m, estimating a surface roughness factorfr ∼= 23 ± 2. EBSD investigations have shown that nanowires with adiameter of about 0.2 �m or smaller have frequently a monocrys-talline head, a result valid for both Cu and Ni; the latter result, notobserved in the previous literature attempts based on deposition onan amorphous noble metal thin layer, is presumably due to directdeposition on a Ni substrate, favouring nucleation and growth oflow-surface-energy Ni grains. At the present stage, homogeneoussurface decoration by nanowires occurs, as a rule, only over part ofthe electrode area. Further work is in progress, trying to improvethe procedure and to achieve more complete control of the result.
Acknowledgments
Financial support of the Regione Veneto to the 12 months stay ofA. Gambirasi at IENI-CNR (Asse “Capitale Umano”, DGR 2215/2009)is gratefully acknowledged.
The authors are indebted to FILA INDUSTRIA CHIMICA SPA, SanMartino di Lupari, Padova, Italy, owner of the Fei-ESem FEI Quanta200 FEG instrument, for allowing its use for the research workdescribed in this article, and to Mr. F. Montagner (IENI-CNR) fortechnical assistance.
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