Effect of gold and silver nanoislands on the electrochemical properties of carbon nanofoam

Electrochimica Acta 111 (2013) 305–313

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Effect of gold and silver nanoislands on the electrochemical properties of carbon nanofoam E. Spanakis a,b , M. Pervolaraki c , J. Giapintzakis c , N. Katsarakis b,d,e , E. Koudoumas d,f , D. Vernardou d,e,∗ a

Department of Materials Science and Technology, University of Crete, 710 03 Heraklion, Crete, Greece Institute of Electronic Structure and Laser, Foundation for Research & Technology-Hellas, P.O. Box 1527, Vassilika Vouton, 711 10 Heraklion, Crete, Greece c Nanotechnology Researcher Center and Department of Mechanical and Manufacturing Engineering, 75 Kallipoleos Av., P.O. Box 20537, 1687 Nicosia, Cyprus d Center of Materials Technology and Photonics, School of Applied Technology, Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece e Science Department, School of Applied Technology, Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece f Electrical Engineering Department, Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece b

a r t i c l e

i n f o

Article history: Received 21 May 2013 Received in revised form 30 July 2013 Accepted 30 July 2013 Available online xxx Keywords: Carbon nanofoam Metal nanoislands Electrochemical properties

a b s t r a c t Carbon nanofoam electrodes were fabricated by picosecond ultrafast pulsed laser deposition at room temperature. Silver and gold nanoislands were deposited onto the electrodes by sputtering. Compared with bare carbon, a substantial enhancement in the electrochemical performance, both in terms of magnitude and of temporal response was clearly observed only for the case of the gold covered electrodes. The specific capacitance of these electrodes reached 130 F g−1 . This is a value among the best ever reported for low temperature grown, non-activated carbon based hosts and it was observed to be stable up to 500 continuous charge intercalation/deintercalation scans. The effect of the noble metal and its nanoisland form on the enhancement of the electrochemical behaviour of the carbon nanofoams is discussed. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Carbon represents a very attractive candidate for electrochemical applications due to its 0–3D dimensionality and its ability to exist under different forms (from powders to fibres, foams, fabrics and composites [1,2]). In particular, a great interest has been focused on its application as electrode material in electrochemical capacitor because of its chemical inertness, high surface area, low mass density and relatively low cost [3]. Two types of electrochemical capacitors exist, depending on the kind of accumulated energy: electrical double layer capacitors (EDLC), where only a pure electrostatic attraction between ions and the charged surface of an electrode takes place and supercapacitors (SC), where faradaic redox reactions increase the active dipole concentration in an otherwise called pseudocapacitive effect [4,5]. In the case of EDLC, the capacitance is associated with charge double layer accumulated at the electrode/solution interface. Thus a crucial improvement in the energy density of the materials’ capacitive properties comes from maximisation of the exposed surface.

∗ Corresponding author at: Center of Materials Technology and Photonics, School of Applied Technology, Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece. Fax: +30 2810379157. E-mail address: [email protected] (D. Vernardou). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.222

This led to finding and optimising processes for post-fabrication increase of the porosity of the material, usually referred to as “carbon activation”. In porous carbon, the magnitude of the capacitance varies with the type of porosity and the conditions of its preparation. Taking an average value of 25 ␮F cm−2 and a specific area of 1000 m2 g−1 , the attainable capacitance would be 250 F g−1 [1]. Critical to the overall performance is the size of the pores usually ranging from micro to macro pores. Although the micropores provide greater surface areas, they are either less easily wetted by electrolytes or not accessed at all by large ionic species, compared with the macropores. Thus a large part of the surface exposed may not be utilised for charge storage [7]. In addition, the ionic motion is not facilitated in small pores resulting in low charging rate capability of the corresponding EDLCs [8]. From the above it comes obvious that actual experimental capacitance values depend also on the size of the ionic species of the electrolyte and the type of the solvent utilised (aqueous or non-aqueous) [2]. Non-aqueous solvents are preferred for practical small-scale applications because they have lower melting points therefore retaining functionality at wider temperature ranges. They are also more viscous and do not require deaeration. For large scale charge storage, low-cost aqueous solutions are utilised. Although the most common specific capacitances, obtained from porous carbon in non-aqueous electrolyte solutions, range between 30 and 160 F g−1 (with a mean value of ∼90 F g−1 ) [2], there are reports, like in the case of onion like carbon (OLC)

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networks deposited on interdigitated micro-electrodes, where an order of magnitude higher specific capacitance was achieved [6]. One way to increase the capacitance by more than an order of magnitude is by entering into the supercapacitive regime [1] through the functionalisation of the nanostructured carbon surface [2]. Deliberate chemical oxidation or spontaneous one, during activation, have been shown to be extremely beneficiary with aqueous solvents. This is not generally the case with non-aqueous ones because side effects such as increased resistivity due to the presence of the surface oxide [1] and reduced reliability due to self discharge and leakage currents [2] degrade overall performance. Another way to functionalise the surface for the same purpose is by covering it with metal nanostructures bearing multiple redox moieties [3]. Metal nanoparticles such as Au, Ag, Pt, Pd, Ni and Cu have been coated onto the side walls of carbon nanotubes [9–12]. Those involving Au were of special interest due to the facility of modification and biocompatibility along with some unique electrochemical properties of the resulting carbon composites [13]. Electrochemical reduction [14–16] and electroless deposition [17,18] are two common chemical techniques used for the deposition of metal nanoparticles on carbon nanotubes providing adequate control over their nucleation and growth. Nevertheless, prior to metallization, carbon nanotubes require surface modification using environmentally toxic chemicals [19,20] in order to increase binding sites for anchoring the metal nanoparticles [21,22]. Physical methods including sputtering, ion, electron or pulsed laser beam deposition and thermal evaporation can also be utilised to prepare metal nanoparticles on carbon without post surface modification. This paper compares the electrochemical performance of asreceived, non-activated carbon nanofoams before and after the growth of metallic silver and gold nanoislands on top of them. The nanoislands are grown by sputtering and subsequent annealing of the samples at 200 ◦ C. We report on the way by which the speed and reversibility of the charge transfer and the charge storage capability are affected with respect to the type of metal and the thickness of the underlying carbon nanofoam. 2. Experimental 2.1. Samples preparation Carbon nanofoam (CNF) was grown on corning glass substrates using high repetition rate picosecond ultrafast pulsed laser deposition technique (UFPLD) at room temperature. A train of 120 × 106 and 60 × 106 laser pulses from the fundamental wavelength (1064 nm) of a Nd:YVO4 laser (Duetto, Time-Bandwidth Products AG) was employed for the deposition of a thick and a thin carbon layer (named MPC75 and MPC76 respectively). The individual pulse duration (full width at half maximum) was 10 ps at a pulse repetition rate of 200 kHz. Evaporation of graphite was ensured at an intensity ∼2.3 × 1011 W cm−2 . The target was placed 40 mm away from the substrate. All depositions runs were under an Ar pressure of 300 Pa. Above conditions resulted in the preparation of high purity materials that are not post-activated.

Fig. 1. XRD patterns of Ag and Au nanoislands grown onto MPC75 by the sputtering method.

also coated along with each CNF at every sputter deposition run. The samples were subsequently annealed for 30 min, in ambient conditions, at a temperature of 200 ◦ C i.e. low enough to maintain the structural integrity of the underlying CNFs. Even at this low temperature the limited surface mobility of the noble metal atoms drives reshaping and agglomeration in an attempt to minimise surface energy. The maximum initial film thickness, at which annealing results to electrically discontinuous i.e. isolated nanoislands, is 5 nm and 2 nm for silver and gold respectively. 2.3. Characterisation X-ray diffraction (XRD) measurements were carried out using a Siemens D5000 Diffractometer for 2 = 10.00–50.00◦ , with a step of 0.02◦ and integration time of 30 s per step. Raman measurements were performed in the 500–3500 cm−1 range, using the 473 nm laser line of a Nicolet Almega XR micro-Raman system at an incident intensity of 8 mW ␮m−2 . This intensity is low enough to prevent fluorescence from or any structural change to either the film or the underlying substrate. Optical measurements were done between 400 and 1100 nm using a Perkin-Elmer Lambda 950 UV-Vis spectrophotometer with an integrating sphere. Surface photos were captured in a Jeol JSM-7000F field-emission electron microscope. Electrochemical measurements were performed with a tri-electrode cell and a computer-controlled AUTOLAB potentiostat/galvanostat [26–29]. CNF (with or without the metal nanoislands) coated glass substrates acted as the working electrode. The cyclic voltammetry (CV) tests were conducted using the inorganic electrolyte LiClO4 (1 M) because it provides small size cations that favour maximisation of the transferred charge density. Propylene carbonate was selected as the non-aqueous solvent because its high dielectric constant [2] contributes to increased capacitance. CV operating parameters were: 10 mV s−1 scan rate, potential window of −500 mV to +250 mV for a number of scans up to 500. To study Li+ kinetics, chronoamperometry was carried out at −500 mV and +250 mV with a step of 200 s. 3. Results and discussion 3.1. Structure of carbon nanofoams

2.2. Metal nanoisland formation Silver and gold metal nanoislands were formed on top of MPC75 and MPC76 CNFs by dc-magnetron sputter coating, followed by annealing, as reported earlier in the literature [23–25]. 99.9% purity metal targets were sputtered at a Baltec MED020 chamber reaching a base pressure of 1 × 10−3 Pa. The deposition rate was 0.15 and 0.028 nm s−1 for Ag and Au respectively as measured by a quartz crystal (QC) thickness monitor. A bare corning glass substrate was

Fig. 1 presents the XRD patterns for MPC75, MPC75-Ag and MPC75-Au. They are similar to those of the corresponding MPC76 samples (not shown here for brevity). In all of them, there is a broad peak at about 23.4◦ indicating that CNFs mainly consist of amorphous carbon [30]. On the other hand, the Raman spectra of the samples (Fig. 2) show two bands centred at 1353 cm−1 (D-band) and 1580 cm−1 (G-band), which are characteristic of nanostructured carbon materials including carbon nanofoams [31],

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Fig. 2. Raman spectra of MPC75 and MPC76 with and without the presence of Au nanoislands.

Fig. 3. UV–vis absorbance of gold (a) and silver (b) nanoislands on glass substrates. Insets: absorbance of MPC76 carbon nanofoams with (w/) and without (wt/) the nanoislands. Difference between the two (w/–wt/) is also shown on (a) and (b) for comparison.

carbon aerogels [32] and multi-walled carbon nanotubes [12]. The D-band corresponds to either a disordered or small crystallites of sp2 networks [33], while the G-band originates mainly from the Raman active in-plane E2g vibration mode [34]. In addition, a sharp feature at 2709 cm−1 is observed, which may be attributed to the second order Raman peak appearing due to overtone of the first order mode at 1353 cm−1 [34]. According to the relative intensity of the D and G bands, the materials could be placed in an intermediate state between nanocrystalline graphite and sp2 -bonded amorphous carbon in the limit of validity of the Tuinstra and Koenig expression [35]. This relates the relative intensity of the D- to G-bands (ID /IG ) ratio with the in-plane size of the graphitic nanocrytalline regions (L) as shown below: L(nm) =

 I −1 D

IG

In order to apply this expression, the Raman spectra were fitted using two Lorentzian lines. Hence, the L values (based on the area under the peaks) for the MPC75 and the MPC76 samples were derived to be 6.9 and 3.9 nm respectively. The higher intensity ratio of the D- to G-bands measured in the Raman spectrum corresponding to the MPC76 sample is indicative of the higher degree of carbon disorder. The higher degree of carbon disorder, together with the lower carbon content account for the lower signal-to-noise ratio recorded for the particular sample.

Fig. 2 also shows the Raman spectra for MPC75-Au and MPC76Au, which are similar to those obtained for MPC75-Ag and MPC76Ag (not shown in the paper for brevity). As revealed in Fig. 2, there is no distinct difference in the Raman spectra including the ratio of intensities of D- to G-bands, despite the presence of Ag and Au nanoislands. The fact that the NPs do not affect the structure of carbon nanofoam is expected since their deposition is only on the surface of the carbon. Furthermore, the absence of any change in the Raman signal gives a good indication on the absence of possible damage produced to the CNFs upon annealing. Although, XRD analysis gave no specific information, Raman study revealed the presence of nanostructured carbon materials. Raman spectroscopy is sensitive to short-range order vibrational modes of bond configurations, while XRD responds to long range order crystallinity of materials [36]. Hence, the observed behaviour suggests that the samples are primarily amorphous retaining however a short range, within a few neighbouring atoms, crystalline ordering. 3.2. Silver and gold nanoislands UV/Visible light of certain wavelength is known to excite collective electron oscillations, called surface plasmons, upon incidence on silver and gold nanostructures having average sizes less than that wavelength [37,38]. The result is a resonant scatteringabsorption that is absent on the bulk spectrum. Plasmon absorption

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peaks at a wavelength which depends on the metal and surrounding medium, the proximity of nearby nanostructures and their size, provided that they are near enough or large enough to produce depolarisation effects [39]. Fig. 3(a) and (b) presents on linear axes the absorption spectra of both nanoislands on glass. Resonance peaks are located at 440 nm and 550 nm for silver and gold, respectively [37,38]. In the insets of those figures we plot the absorption spectra of the carbon nanofoam film MPC76 before and after the formation of the noble metal nanoislands. We studied only the thinnest CNF of the two that had absorption low enough to be measurable by our spectrophotometer. A clear increase in the absorption values is observed for both cases due to the presence of the metal. The second curve in Fig. 3(a) and (b) represents the difference between the absorption values after and before the nanoisland formation. This difference reveals a distinct absorption peak at 414 nm and 546 nm for MPC76-Ag and MPC76-Au, respectively. The clear, albeit wider, absorption peaks compared to the glass substrates, evidence the formation of metal nanoislands on top of the CNFs for both metals. The peak wavelengths are very similar to those found before for silver [40] and gold [13,41] nanoparticle coated grapheme or nanotubes. The fact that those peaks are located at very similar wavelengths to those found for the glass substrates indicates that the mean size of the nanoislands does not differ substantially between the two. The difference in the width of the peaks however evidences a corresponding difference in the width of the nanoisland size distribution; CNFs do not facilitate nanoisland uniformity as one may expect from the 3D porous character of their surface. 3.3. Morphology SEM images present the exterior surface of MPC76 before and after the addition of the Au nanoislands (Fig. 4). SEM observations of the thicker foam and of the Ag nanoislands are similar to those of Fig. 4. Bare CNFs exhibit a foam-like porous network with at least double scale porosity, which is similar to that observed in other “spongy materials” [42–44]. Low magnification imaging shows that CNFs are composed of open, 3D interconnected macropores, ranging in size from 800 to 1600 nm. Higher magnification (insets of Fig. 4) reveals a mesoporosity in the 50 nm scale. It is thus clear that this as-received material is porous enough to render, in a first approach, further activation unnecessary. The presence of the nanoislands on the metal covered CNFs can be indicated only by the resolution/contrast improvement of the corresponding image, due to the secondary electron emission enhancement imposed by the increased conductivity of the metallic nanoislands, with respect to the bare carbon host. 3.4. Cyclic voltammetry Since, carbon nanofoams are adequately conductive nanomaterials, they could be incorporated into electrochemical systems as matrices that facilitate the charge exchange between the electrode and the electrochemical probe. We have performed comparative studies between bare and Ag or Au coated CNFs in order to investigate the effect of the metals on the electrochemical behaviour of the glass-supported CNF electrodes. Fig. 5(A) and (B) shows the first cyclic voltammograms of the Ag and Au nanoislands onto MPC75 and MPC76 respectively. An ideal double-layer capacitance at the CNF electrode/electrolyte interface, would result in a rectangular shape for the voltammetry curves [45]. In our case the shape of the curves indicates that a significant electrical resistance is present and affects the efficiency of double-layer formation mechanism [45]. Fig. 5 also indicates that the electrochemical activity, as represented by the absolute value of current, is enhanced only under the presence

of Au nanoislands [46,47]. This enhancement is better in the case of the thickest MPC75-Au CNF. The performance of the bare CNFs does not seem to depend on their thickness. The maximum current value attained is ∼1 A g−1 . This value is higher than that of the modified porous carbon electrodes with ferrocenated 2 nm Au nanoparticles (0.4 mA g−1 ) [3] and of the porous carbon materials (0.15 mA g−1 ) [48] while it is comparable to that of hierarchical porous carbon (1 A g−1 ) [49] and MnO2 embedded in mesoporous carbon walls (1.5 A g−1 ) [45]. We thus find that our non-activated, room-temperature grown and Au-nanoislandcovered carbon nanofoams, can give similar or better peak-current performance to more hierarchical carbon structures blended with gold nanoparticles. Increased resistance however may come from the absence of a dense conductive buffer layer underneath the CNFs. As bare MPC75 and MPC76 CNFs, with different thickness, give similar currents in the voltammograms, the smaller Au-directed enhancement observed for the thin CNF may be due to the lack of requisite ion injection sites resulting from this film’s lower carbon content. 3.5. Temporal response The temporal response of the specific current was recorded using chronoamperometry and is shown in Fig. 6(I) for MPC75 and (II) for MPC75-Au. In this measurement, the potential was repetitively switched between −500 mV and +250 mV at an interval of 200 s. Based on these curves, absolute current vs time, log–log plots of all temporal segments corresponding to charging and discharging processes are obtained. Both processes, for both bare CNF electrodes (MPC75 and MPC76), are found to decay in time approximately as t−1/2 , which is associated with a proton diffusionlimited process [50]. An earlier report places part of this diffusion along the graphitic planes differentiating it from intercalation occurring at their edges [51]. If we define a “time constant”, tc as the time needed for excess current to reduce to 10% of its absolute maximum value [28], then charging (discharging) proceeds with a tc of 37 ± 2 s (43 ± 2 s) and 60 ± 2 s (62 ± 2 s) for MPC75 and MPC76, respectively. We thus find that in absolute numbers both processes occur at the same speed, which can be expected when the diffusion path and spatial ion concentration conditions remain the same. On the other hand both processes are faster for the thick CNF. This may either be due to smaller diffusion paths associated to a more accessible network of intercalation sites (not confirmed however by SEM analysis) or due to the somewhat improved electrode conductivity expected from the thickest CNF host. Silver coating resulted always in the same overall temporal behaviour (not presented here for brevity) to that of bare carbon, indicating that the charge transport mechanism is not affected by the presence of the silver nanoislands [52]. The presence of Au however on both MPC75 (Fig. 6(II)) and MPC76 resulted in a faster charging process in which the current decays in time as t−2/3 . On the other hand gold does not change the discharge power law exponent. Li+ transfer dynamics is most probably affected by the presence of added adsorption and/or intercalation sites compared to those of bare CNFs (e.g. the graphitic edges). These sites, lying along the corresponding planes and probably spatially correlated to the NPs, may be better (and thus faster) accessible to cations. In terms of absolute time constants we always find lower values with respect to bare CNFs: 28 ± 1 s (35 ± 1 s) and 47 ± 1 s (54 ± 1 s) for charging (discharging) of MPC75-Au and MPC76-Au, respectively. 3.6. Specific charge and capacitance The amount of Li+ charge interchanged between the samples and the electrolyte was calculated by integration of the excess specific current measured upon switching the bias voltage with time

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Fig. 4. SEM images of MPC76 without (a) and with (b) the gold nanoislands. Scale bars are 1 ␮m. Magnifications with 100 nm scale bars are shown in the inset.

(see Fig. 6). We found that the integrated charges during intercalation and deintercalation are similar to each other within 10% per scan and are fairly stable upon repetitive cycling for a total amount of time of 1800 s, showing a less than 5% reduction from their initial values. Charge density, is 97 ± 2 C g−1 for MPC75-Au i.e. five times larger than that of bare carbon MPC75. Once again Au nanoislands are confirmed to enhance significantly the electrochemical performance of the carbon electrodes. The specific capacitance C (in F g−1 ) of all samples was calculated according to the following equation [53]

C=

Q Vm

where Q/m (in C g−1 ) is the average charge density interchanged between the electrode and the electrolyte during the intercalation and deintercalation process, and V (in V) is the potential window employed for the measurement. The mass for the MPC75 and MPC76 electrodes was measured by a 5-digit analytical grade scale to be 40 ␮g and 20 ␮g, respectively. The potential window was −500 mV to +250 mV. Q was calculated by averaging the integrals of excess specific current versus time as described previously. The specific capacitances were estimated to be 73 and 130 F g−1 for the thin MPC76-Au and the thick MPC75-A respectively i.e. 3–5 times higher than that of bare carbon. These values are comparable to those reported for modified porous carbon with ferrocenated 2 nm Au nanoparticles (66 F g−1 ) [3], hierarchical porous carbons (120 F g−1 ) [49], carbon nanotubes (110 F g−1 ) [54] and activated

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Fig. 5. The first cyclic voltammetric scan for the Ag and Au nanoislands onto MPC75 (A) and MPC76 (B). Cyclic voltammograms as a function of the scan number for the Au layers onto MPC75 and MPC76 as insets in (A) and (B), respectively.

carbons (132 F g−1 ) [55]. 130 F g−1 is actually in the upper end of the values reported for porous carbon materials tested with non-aqueous solvents as already mentioned in the introduction. Reported specific capacitance usually refers to post-activated carbon i.e. an electrode with porosity that was deliberately introduced or increased on the material’s pristine form. Chemical activation and/or chemical or electrochemical post-oxidation of the carbon surface are thought to improve specific capacitance due to improved wetting of the surface by the electrolyte solution [2]. The drawback is that chemical treatment of the surface contributes also to increased resistance due to the presence of the oxide layer [1]. Our material has not received any post-activation. The effect however of such chemical treatments on the CNF performance before and after Au NP formation will be a matter of future research. The electrochemical stability of all electrodes was investigated by studying the relationship between the specific capacitance and

the scan number as shown in Fig. 7. After 500 scans, the specific capacitance of the MPC75-Au decreased only by 2% revealing that this material has excellent electrochemical stability. On the other hand, the specific capacitance of the MPC76-Au decreased by 15%, a behaviour that may have risen from a partial dissolution of the foam by the electrolyte as traces were observed in the solution after 500 scans and affected also the maximum current of the corresponding voltammograms (compare insets of Fig. 5(A) and (B)). 3.7. Discussion Gold nanoparticles have been reported to catalytically enhance electrochemical activity in carbon nanotubes [47]. The exact mechanism remains however elusive. A plausible contribution to the enhancement of the charge exchange under the presence of gold is the mild modification of the exchange mechanism from the

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Fig. 6. The chronoamperometric response recorded at −500 mV and +250 mV at an interval of 200 s for the MPC75 (I) and MPC75-Au (II) electrodes (a). Logarithmic plots of the chronoamperometric response of MPC75 and MPC75-Au corresponding to intercalation (b) and deintercalation (c) processes.

pure double-layer to faradaic. A careful study of Fig. 5(A) indicates the presence of a broad oxidation peak associated with the presence of Au, in the range of −200 mV to −16 mV. On the same figure we plotted a dotted line as a guide to the eye for the baseline under that peak The peak was found to move to the range of −340 mV to −117 mV as the number of scans increased from 100 to 500 (inset of Fig. 5(A)). A similar peak clearly emerged for the thin CNF in the range of −280 mV to −87 mV only after 500 scans (inset of Fig. 5(B)). This behaviour suggests that ion intercalation is to some extend irreversibly metastable probably due to some alteration of mechanical or chemical nature of the environment around Au.

The magnitude of electrochemical enhancement with respect to bare CNFs may suggest that enhanced charge storage is not limited in the vicinity of gold nanoislands but is extended to a greater part on or under the CNF surface. Au is known to diffuse along graphitic planes and get incorporated in the carbon lattice, adhering selectively to imperfections e.g. dangling bonds of disordered or amorphous regions [56,57]. This means that upon deposition and annealing (a) gold nanoislands may preferentially settle on top of disordered areas of the surface and (b) a significant amount of gold may be incorporated in either atomic form or as clusters inside disordered regions of the mostly amorphous CNF network. Li+ intercalation on the other hand is thought to get facilitated

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References

Fig. 7. Specific capacitance of all electrodes in 1 M LiClO4 electrolyte at 10 mV s−1 as a function of the scan number (1, 100, 250 and 500).

predominantly at such disordered or amorphous regions of the carbon network [58,59]. We thus suggest that gold could catalyse excess charge storage by occupying carbon network areas, along or below the surface, that are susceptible to ion intercalation. Silver on the other hand does not produce any measurable improvement in the charge storage properties of CNFs. It is indeed not a priori expected that it should. Silver is known to be almost insoluble to carbon and thus cannot be incorporated easily into its network [60,61]. It is thus plausible that the presence of silver along the CNF surface is mainly limited to the nanoislands. Those nanoislands may weakly interact with the underlying material and even if they do this interaction will be limited strictly to the CNFnanoisland interface. Thus even if silver can catalyse Li+ storage its effect may be insignificant due to the correspondingly small fraction of the CNF network that it can affect.

4. Conclusions Using the sputtering method, Ag and Au nanoislands were deposited onto carbon nanofoams of different thickness grown using the picosecond ultrafast pulsed laser deposition technique at room temperature. It was demonstrated that the type of the metal nanoisland and the thickness of carbon nanofoam can have a significant influence on the electrochemical performance of the electrodes. Silver nanoislands do not affect the performance of carbon nanofoam, while gold nanoislands do. The best electrochemical activity was obtained for the thicker carbon nanofoam coated with the Au nanoislands. This composite presented the fastest charge transfer processes combined with the highest charge storage and correspondingly the best capacitive performance. All these are accompanied by considerable stability over time upon successive charge loading-unloading scans. This performance, obtained with no activation of the pristine material, may be attributed to the occurrence of Au-associated reactions that increase the availability and/or improve the accessibility of ion injection sites correlated to the corresponding nanoislands. We expect that the addition of gold nanoparticles on high performance carbon-based devices as the onion like carbon micro capacitors [6] will substantially increase their performance.

Acknowledgements The authors thank Mrs. Aleka Manousaki for her invaluable help with the SEM characterisation. M.P. and J.G. would like to acknowledge support by the Cyprus Research Promotion Foundation under grant Carbon Nanofoam-EPYAN/0205/06.

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