New DOI: 10.1002/cphc.201200789 Preparation and … · 2015. 5. 22. · carbon peak (C 1s component...

DOI: 10.1002/cphc.201200789

Preparation and Characterization of Carbon Nano-Onion/PEDOT:PSS CompositesMarta E. Plonska-Brzezinska,*[a] Mikołaj Lewandowski,[c] Małgorzata Błaszyk,[c]

Agustin Molina-Ontoria,[b] Tadeusz Luciński,[c] and Luis Echegoyen*[b]

1. Introduction

The unique physical properties of nanomaterials have placedthem at the forefront of emerging technologies. Significant en-hancement of electrical, optical, mechanical, and structuralproperties is commonly found for novel nanomaterials. Carbonnanostructure materials are potentially useful in energy storageand conversion devices due to their favorable mechanical andelectrical properties.[1] A lot of attention has been paid to thecapacitive properties of thin films of carbon nanotubes(CNTs),[2] and of graphene,[3] due to their porous structures andlarge surface areas. These materials exhibit properties of typicaldouble-layer capacitors, in which energy storage arises mainlyfrom the separation of electronic and ionic charges at the in-terfaces between the electrode materials and the electrolytesin solution.

Carbon nano-onions (CNOs) are one of the ideal platformsfor the preparation of new materials due to their unique physi-cochemical properties. Recently, the electrochemical propertiesof thin films formed from small CNOs (5–6 nm in diameter, sixto eight fullerene shells) were investigated.[4] They show highchemical stability and large specific surface area.[5] The nonco-valent and covalent functionalization of CNOs changes theirphysicochemical properties.[6] The integration of CNOs withpolymers,[7] biomolecules,[8] or polyelectrolytes[9] has led to thedevelopment of new hybrid materials and sensors. CNOs havea variety of potential applications, including optical limiting,[10]

field emission in solar cells,[11] fuel cell electrodes,[12] chargestorage devices,[13] and hyperlubricants.[14]

Composite materials based on the integration of carbonstructures with substances can lead to materials possessingproperties of the individual components. The development ofcomposites for use as supercapacitor electrodes can provideenhanced potential electronic and ionic conductivity, and canconsiderably improve charge storage and delivery. Their pseu-

docapacitive behavior has been studied extensively owing totheir high power capability, long cycle lifetime, and fast chargeand discharge rates. Materials used for such purposes includecarbon materials,[15] conducting polymers,[16] and metal oxidenanoparticles.[17]

The use of poly(3,4-ethylenedioxythiophene):poly(styrenesul-fonate) (PEDOT:PSS) as a viable electrode material due to itsconjugated backbone and processing ability has been report-ed.[18] PEDOT:PSS has a conjugated backbone that allows goodtransport of delocalized electrons through the p-orbital sys-tems.[19] Combining a relatively high conductivity with opticaltransparency in the doped state, it is suitable for applicationssuch as electrodes in light-emitting diodes,[20] transistors,[21]

photovoltaics,[22] sensors,[23] ion-selective electrodes,[24] electro-chromic devices,[25] polymer electrolyte fuel cells,[26] and highpower-density supercapacitors.[27] Recently, several strategieshave been applied to increase the conductivity of PEDOT:PSS.It has been reported that incorporation of carbon nanostruc-tures, such as CNTs[28] or graphene,[29] into conventional con-ducting polymers improves their conductivity. Increased con-

Composites of unmodified or oxidized carbon nano-onions(CNOs/ox-CNOs) with poly(3,4-ethylenedioxythiophene):poly(s-tyrenesulfonate) (PEDOT:PSS) are prepared with different com-positions. By varying the ratio of PEDOT:PSS relative to CNOs,CNO/PEDOT:PSS composites with various PEDOT:PSS loadingsare obtained and the corresponding film properties are studiedas a function of the polymer. X-ray photoelectron spectroscopycharacterization is performed for pristine and ox-CNO samples.The composites are characterized by scanning and transmis-

sion electron microscopy and differential scanning calorimetrystudies. The electrochemical properties of the nanocompositesare determined and compared. Doping the composites withcarbon nanostructures significantly increases their mechanicaland electrochemical stabilities. A comparison of the resultsshows that CNOs dispersed in the polymer matrices increasethe capacitance of the CNO/PEDOT:PSS and ox-CNO/PE-DOT:PSS composites.

[a] Dr. M. E. Plonska-BrzezinskaInstitute of ChemistryUniversity of BialystokHurtowa 1, 15-399 Bialystok (Poland)E-mail : [email protected]

[b] Dr. A. Molina-Ontoria, Prof. Dr. L. EchegoyenDepartment of ChemistryUniversity of Texas at El Paso500 W. University Ave., El Paso, TX 79968 (USA)E-mail : [email protected]

[c] Dr. M. Lewandowski, Dr. M. Błaszyk, Prof. Dr. T. LucińskiInstitute of Molecular PhysicsPolish Academy of SciencesM. Smoluchowskiego 17, 60-179 Poznan (Poland)

4134 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13, 4134 – 4141

ductivities have been achieved by doping with polyoxometal-late,[30] metal particles,[23] inert solvents such as sorbitol,[31] man-nitol,[32] dimethyl sulfoxide,[33] and tetrahydrofuran,[34] or poly-mers.[35]

Composite films of CNTs and PEDOT:PSS were prepared viaelectrochemical co-deposition from solutions containing CNTsand the corresponding monomer or by chemical methods.Carbon nanostructures acted as both the backbone of thethree-dimensional micro- and nanoporous structures and theeffective charge balancing dopand within the polymer. All thecomposites showed improved mechanical integrity, higherelectronic and ionic conductivity, and exhibited larger elec-trode specific capacitance. In addition, the CNT/PEDOT-PSSnanocomposites (50 wt % PEDOT) had a specific capacitance of198.2 F g�1 and a capacitance degradation of 26.9 % after2000 cycles, much better than those of pristine PEDOT andCNTs/PEDOT (50 wt % PEDOT).[36] For example, Lota et al. haverevealed that the CNTs/PEDOT nanocomposite electrode canexhibit excellent electrochemical performance when operatingin acidic, alkaline, and organic media.[37] Frackowiak et al. re-ported multiwalled CNT and PEDOT:PSS composites that ach-ieved capacitance values of 100 F g�1.[38] Similarly, excellent me-chanical and electrochemical stability was observed for gra-phene oxide and PEDOT:PSS composites, which exhibit capaci-tance values of 108 F g�1.[39]

The challenge is the fabrication of composite materials con-taining conducting polymers and carbon material with theright ratio of the two components and a homogeneous struc-ture. To address this, we synthesized novel composites includ-ing CNO particles and PEDOT:PSS. The advantage of smallCNOs over CNTs, or bigger CNOs, is their easier dispersion inaqueous solutions, which improves the homogeneity of thecomposites. X-ray photoelectron spectroscopy (XPS) studies re-vealed oxygen-containing groups on the CNO surface. Thescanning electron microscopy (SEM) and transmission electronmicroscopy (TEM) characterization of the synthesized compo-sites showed good dispersion of CNO particles throughout thepolymer matrix. Differential scanning calorimetry (DSC) ther-mograms of the composites confirmed the presence of the PE-DOT:PSS layer on the carbon nanostructure’s surface. There-fore, the electrochemical properties, especially the capacitiveproperties, of the composites were investigated.

2. Results and Discussion

2.1. XPS Characterization of Oxidized CNOs

We performed XPS studies to confirm the presence of oxygen-containing functional groups on the CNOs’ surface, which wereoxidized in HNO3 solution. Figure 1 presents the XPS C 1s spec-tra of oxidized (top) and reference (pristine; bottom) CNO sam-ples. The measured curves are presented as solid lines. Forboth samples the main C 1s peak was unsymmetric, which ischaracteristic of conducting forms of carbon (for example,graphite).[40] For the reference sample (CNO) the main C 1speak could be fitted with four components centered at 285.2,285.6, 287.8, and 291.0 eV. The first two peaks were assigned

to sp2 and sp3 hybridized carbon atoms, respectively.[41] Theother two peaks were interpreted as relaxation shake-uppeaks,[42] as no oxygen or other elements (except the gold sub-strate) were observed in the survey spectrum (not shown). Forthe HNO3-oxidized sample (ox-CNO) a prominent signal (�4 %versus 96 % carbon signal) appeared in the O 1s region(Figure 2). The C 1s peak obtained for this sample could befitted with six components: 285.2, 285.6, 287.4, 288.4, 291.5,

Figure 1. XPS C 1s spectra of modified (top) and reference (bottom) CNOs.

Figure 2. XPS O 1s spectrum of modified CNOs.

ChemPhysChem 2012, 13, 4134 – 4141 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 4135

Carbon Nano-Onion/PEDOT:PSS Composites

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and 295.5 eV. The first two peaks were again assigned to sp2

and sp3 carbon atoms. The integrated intensity of the 285.2 eVpeak dropped (by �5 %) relative to the reference sample (pris-tine CNO), which we assigned to carbon–carbon bonds break-ing and bonding to oxygen-containing functional groups. Thebroad signal at 287.4 eV could arise from carbon atoms boundto oxygen in the form of oxygen bridges -C-O-C- or carbonylgroups >C=O.[43] The 288.4 and 291.5 eV peaks could arisefrom carboxyl O=C�OH groups, while the 295.5 eV peak couldarise from adsorbed CO and/or CO2.

[44] Note that all the peaks>286 eV were superimposed by satellite peaks. The surveyspectrum obtained for this sample (not shown) gave no evi-dence for the presence of nitrogen (N 1s) or other elementsexcept carbon, gold, and oxygen; thus, the presence of nitro-gen-containing functional groups on the surface of ox-CNOswas excluded. The O 1s spectrum could be fitted with threepeaks centered at 531.6, 533.4, and 536.6 eV. The 531.6 eVpeak could arise from carbonylgroups >C=O,[45] with somepossible contribution of oxygenfrom carboxyl O=C�OHgroups.[44] The 533.4 eV peakcould also arise from carboxylgroups (as these groups containtwo differently bound oxygenspecies) and from oxygenbridges -C-O-C-.[45] The 536.6 eVpeak was assigned to adsorbedO2, H2O,

[45] CO, and CO2.[46] The

533.4 eV peak could also poten-tially arise from hydroxyl �C�OH groups;[45] however, this wasexcluded as no correspondingcarbon peak (C 1s componentcentered around 286.4–286.9 eV) was observed in therecorded XPS spectra.[43, 45, 47] Theresults confirmed modificationof CNOs by attachment ofoxygen-containing functionalgroups. The data are consistentwith our previous findings byBoehm titration methods, whichsuggested the presence of car-boxyl groups on the surface ofHNO3-oxidized CNOs.

[47]

2.2. Morphology Characteriza-tion and Colorimetric Studiesof the ox-CNO/PEDOT:PSS andCNO/PEDOT:PSS Composites

Electrochemical capacitorsbased on carbon electrodes areof two types: electrical doublelayer capacitors and supercapa-citors.[48] The capacitance values

are strictly correlated with the nature and surface of the elec-trode/electrolyte interface. Generally, the more developed thespecific surface the higher the charge accumulation.[49]

The PEDOT:PSS and CNO nanocomposites were obtained vianoncovalent modification of the carbon nanostructures withpolymer layers. Solutions were prepared by dispersing CNOs orox-CNOs in the PEDOT:PSS solution in water using ultrasonica-tion without any oxidation process. The morphology androughness of the synthesized CNO composites were studiedby SEM, and the results for ox-CNO/PEDOT:PSS and CNO/PE-DOT:PSS films are shown in Figure 3. The morphology of thecomposites differs significantly from the morphology of thinfilms formed only by the filler, PEDOT:PSS. At the micrometerscale, PEDOT:PSS forms smooth and uniform layers on the goldsurface (Figure 3 a). The morphology of the CNO/PEDOT:PSSfilms was highly dependent on the concentration of the poly-mer in the composites. The CNO/PEDOT:PSS composites with

Figure 3. SEM images of the Au foil covered with a) PEDOT:PSS; b)–e) CNOs/PEDOT:PSS with mass ratio b) 1:0.1,c) 1:0.25, d) 1:0.5, e) 1:1; and f) ox-CNOs/PEDOT:PSS with mass ratio 1:1.

4136 www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13, 4134 – 4141

M. E. Plonska-Brzezinska, L. Echegoyen et al.


very low concentration of PEDOT:PSS (1:0.1 mass ratio; Fig-ure 3 b) exhibit porous morphologies with many channels andoutcroppings. The roughness of the composite surface de-creases with increasing amounts of PEDOT:PSS on the CNOsurface (Figure 3 b–e).

Composites prepared using ox-CNOs were also obtained.SEM studies did not show significant differences between thestructures of the composites prepared from unmodified andoxidized CNOs. The TEM images in Figure 4 clearly reveal PE-

DOT:PSS polymer wrapping of CNO and show a significant in-crease of PEDOT:PSS in the composite. Increasing the concen-tration of PEDOT:PSS copolymer during the synthesis (see Fig-ure 4 a, b) resulted in the formation of island-like structures ofthe composite (structures 1 and 2 in Figure 4 b). Homogeneousstructures were obtained for low concentrations of PEDOT:PSSin the composite of about 10 % (Figure 4 a, structure 1). Re-quired characteristics of carbon materials for electrochemicalapplications include mainly wettability and high conductivity.Wettability is generally improved by the presence of surfacefunctionalities, whereas electrical conductivity depends mainlyon the microstructures.[50] The nanotextural properties ofcarbon are characterized by its specific surface area, the pres-ence of micro- and mesopores, and their shapes. High surfacearea is required for high-performance supercapacitors. These

microstructural variations have a profound influence on the re-sulting capacitive behavior of the CNO/PEDOT:PSS composites.

Analysis of the pure components of the composite (CNOsand PEDOT:PSS), and the CNO/PEDOT:PSS and ox-CNO/PE-DOT:PSS derivatives was performed to determine the presenceof polymer in the composite structures. DSC thermograms ofthe composites are presented in Figure 5. A curve with

a broad melting peak with a minimum around 50–70 8C wasobserved, due to solvent.[51] Weak endotherms were observedbetween 320 and 450 8C, ascribed to the presence of the poly-mer layer on the carbon nanostructure’s surface. The degrada-tion peak at 320 8C is due to the decomposition of PSS via rup-ture of the sulfonate groups from the styrene.[52] The meltingpeak around 450 8C is due to another PEDOT/PSS polymerbackbone decomposition. These results also indicate that theCNO/PEDOT:PSS nanocomposite is fairly stable up to a temper-ature of around 300 8C and subsequently starts to decompose,whereas pristine PEDOT:PSS copolymer undergoes decomposi-tion in the temperature range of 200–300 8C.[53] As shown inFigure 5, the DSC curves clearly showed that the mass of PE-DOT:PSS increased in the composites (see curves b–e) and con-firmed the presence of the PEDOT:PSS layer on the carbonnanostructure’s surface.

2.3. Voltammetric Study of the CNO/PEDOT:PSS and ox-CNO/PEDOT:PSS Composites

The electrochemical characteristics of CNO/PEDOT:PSS were in-vestigated and correlated with the nature of the CNO surface,microtextural characterization, and the composition of films. Toelucidate the effect of the unmodified and oxidized CNO struc-tures on the electrochemical properties of the PEDOT:PSS com-posites, cyclic voltammetric studies were performed. Solidfilms of ox-CNO/PEDOT:PSS and CNO/PEDOT:PSS were pre-pared by drop coating. The carbon nanostructure composites(3 mg mL�1) were sonicated in ethanol solution. A drop of solu-tion containing the dispersed functionalized carbon nanostruc-tures was then deposited on the electrode surface. After sol-

Figure 4. TEM images of CNOs/PEDOT:PSS with mass ratio a) 1:0.1 andb) 1:1.

Figure 5. DSC curves of a) ox-CNO/PEDOT:PSS (1:1 mass ratio) and b)–e) CNOs/PEDOT:PSS of mass ratio b) 1:0.1, c) 1:0.25, d) 1:0.5, and e) 1:1 ata 10 8C min�1 heating rate under static nitrogen atmosphere.




vent evaporation, the electrode covered with a thin film ofmodified carbon nanostructures was transferred to an aqueoussolution containing 0.1 mol L�1 NaCl as the supporting electro-lyte.

The voltammetric responses of ox-CNO/PEDOT:PSS andCNO/PEDOT:PSS (1:1 mass ratio) were compared with those ofthe references, namely pristine CNO and PEDOT:PSS (seeFigure 6). All voltammograms exhibit pseudorectangular pro-

files that are typical for double-layer capacitors. These resultsindicate that the charge–discharge responses of the electricdouble layer are highly reversible and kinetically facile.[54] Thespecific capacitance value of the CNO/PEDOT:PSS compositeelectrode is larger than that of pristine CNO or PEDOT:PSS, re-sulting from the structure of CNO derivatives which reducesthe difficulty for electron transfer. However, all films exhibit ex-cellent electrochemical stability under cyclic voltammetric con-ditions within the potential range between 0 and + 600 mVversus Ag/AgCl. Doping the composites with carbon nano-structures significantly increases their mechanical stability. Pro-longed potential cycling does not affect the electrochemicalproperties of the composites. A rapid loss in specific capaci-tance is observed during the initial 300 cycles, of 34 and 19 %for the pristine PEDOT:PSS and CNO/PEDOT:PSS, respectively.

However, PEDOT:PSS prefers hydrophilic surfaces for efficientattachment on the carbon nanostructures. The capacitance ofthe CNO composites prepared using the same concentrationof the PEDOT:PSS copolymer was higher for the unmodifiedcarbon nanostructure than for the oxidized one (see Figure 6 cand d, respectively). The difference between the observed ca-pacitances could be due to the more porous structure of theunmodified CNOs.[55] Therefore, the oxidation process causesdamage and structural changes on the carbon surfaces by dis-rupting their graphene sheet structure and decreasing theirconductivity.

Four different composites were examined, each differing inthe thickness of the PEDOT:PSS layer adsorbed on the CNOsurfaces. The PEDOT:PSS to CNO mass ratios were 0.1:1; 0.25:1;0.5:1, and 1:1, respectively. Based on the results in Figure 7,

the specific capacitance increases as the composition changesfrom 10 to 50 % CNOs. The specific capacitance reaches a maxi-mum at approximately 50 % CNOs (see Table 1). These resultssuggest that a 1:1 mass ratio composition provides a perfectbalance between pore size and conductivity of the polymermatrix. The results are in good agreement with those obtainedfor the CNT/PEDOT:PSS systems.[56] Faster charging/dischargingof the faradaic reactions are observed for the composites withthinner PEDOT:PSS layers on the CNO surface.

Figure 6. Cyclic voltammograms of glassy carbon (GC) electrodes coveredwith a) PEDOT:PSS, b) CNOs, c) ox-CNO/PEDOT:PSS (1:1 mass ratio), andd) CNO/PEDOT:PSS (1:1 mass ratio) in 0.1 mol l�1 NaCl. The sweep rate was50 mV s�1.

Figure 7. Cyclic voltammograms of GC electrodes covered with CNOs/PE-DOT:PSS with mass ratio a) 1:0.1, b) 1:0.25, c) 1:0.5, and d) 1:1 in 0.1 mol L�1

NaCl. The sweep rate was 50 mV s�1, 10 cycles.

Table 1. Specific capacitances Cs of CNO/PEDOT:PSS composites obtainedon the basis of voltammetric studies.

Composition Cs [F g�1][b]

From the integration of ic–E vol-tammogram[c]

From the slope ofic–V relation

CNO/PE-DOT:PSS[a]

1:0.1 35.16 36.521:0.25 39.19 45.091:0.5 75.08 77.271:1 95.26 96.47ox-CNO/PE-DOT:PSS1:1 46.53 49.05CNO 7.18 –ox-CNO 2.79[d] –PEDOT:PSS 22.59 24.12

[a] mCNO to mPEDOT:PSS mass ratio in composites. [b] Capacitance per gramof the composite. [c] At sweep rate 50 mV s�1. [d] From ref. [57] .




The specific capacitance, Cs, ofthe electrode can be estimatedfrom the cyclic voltammetrycurves by using Equation (1):

Cs ¼R

E2E1

i Eð ÞdE2 vm E2 � E1ð Þ

ð1Þ

where E1 and E2 are the cutoffpotentials in cyclic voltammetry,i(E) is the instantaneous current,i(E)dE is the total voltammetriccharge obtained by integrationof the positive and negativesweeps in the cyclic voltammo-grams, v is the scan rate, and mis the mass of the individualsample. The specific capacitan-ces of PEDOT, CNOs, CNO/PE-DOT:PSS, and ox-CNO/PE-DOT:PSS composite electrodeswere calculated using Equa-tion (1), and the values are col-lected in Table 1.

The specific capacitance canalso be calculated by Equa-tion (2):

ic ¼ Csvm ð2Þ

where ic is the capacitive cur-rent, m is the mass of materialdeposited on the electrode sur-face, and v is the potentialsweep rate. Figure 8 shows thevoltammetric behavior at differ-ent sweep rates. The capacitivecurrent varies linearly with thesweep rate at 300 mV versusAg/AgCl for all concentrationsof PEDOT:PSS in the composite(Figure 8, curves a–d). A lineardependence of the capacitivecurrent on the sweep rate is ob-served over a large sweep rate range (up to 300 mV s�1), asshown in Figure 8 e.

The values of the specific capacitances calculated usingEquation (2) from the dependence of the current on the scanrate for the different compositions are collected in Table 1. Thevalues of the specific capacitances obtained by integration ofI–E curves and those calculated from the linear relationship ofI–v plots are in good agreement. In both cases, the capacitanceof the films is controlled by the amount of CNO in the compo-sites. CNOs improve the composite film properties by makingthem more active for faradaic reactions, and conferring larger

specific capacitances and better stabilities than for pristine PE-DOT:PSS.

3. Conclusions

Our studies show that uniform PEDOT:PSS layers were success-fully adsorbed on CNO surfaces. Two main parameters, theamount of CNOs in the composites and the porosity of thematerial, influence the capacitance of the systems. The PE-DOT:PSS polymer network and CNO structures significantly in-crease the roughness and specific surface area of the nano-

Figure 8. Cyclic voltammograms of GC electrodes covered with CNOs/PEDOT:PSS; the mass ratio of CNOs to PE-DOT:PSS was: a) 1:0.1, b) 1:0.25, c) 1:0.5, and d) 1:1, in 0.1 mol L�1 NaCl. The sweep rate was 1) 5, 2) 10, 3) 20, 4) 50,5) 75, 6) 100, 7) 150, 8) 200, 9) 250, and 10) 300 mV s�1. e) Dependence of the capacitive current on the sweep ratefor CNOs/PEDOT:PSS at + 300 mV in 0.1 mol L�1 NaCl. The mass ratio of CNOs to PEDOT:PSS was: (^) 1:0.1,(*) 1:0.25, (~) 1:0.5, and (&) 1:1.




composite electrodes. The electrochemical data of the compo-sites showed a specific capacitance of about 96 F g�1 forCNOs/PEDOT:PSS (1:1 mass ratio). The simple synthesis ofthese materials, their cation-exchange properties, and theirfacile electrochemistry make them attractive for applications aselectrodes for supercapacitors.

Experimental Section

Materials : All chemicals and solvents used were commercially avail-able and used without further purification: poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS; Aldrich Company,1.3 wt % dispersion in H2O, conductive grade), nitric acid (AldrichCompany), sodium chloride (POCh Company). Aqueous solutionswere prepared with deionized water with a resistivity of 18.4 MWfrom Millipore.

Small CNOs were obtained by annealing nanodiamond powder(Molto, 5 nm average particle size) under a positive pressure ofhelium at 1650 8C for 1 h.[58] After separation and purification, theCNOs were heated at 200 8C to remove the residue from thecarbon surface. High-resolution TEM images of the CNOs were re-ported elsewhere.[5]

Crude six- to eight-shell CNOs (20 mg) were dispersed by ultrasoni-cation for 30 min and refluxed for 3 h in 3.0 m aqueous nitricacid.[5] The mixture was then centrifuged for 10 min and the blackpowder collected in the bottom of the test tube was washed sev-eral times with 0.1 m NaOH and deionized water, and dried over-night in a vacuum oven (T = 120 8C) to yield ox-CNOs.

Four different volumes of a PEDOT:PSS solution (1.3 wt % disper-sion in H2O: 90, 180, 360, or 720 mL) were added to CNOs (3 mg)and sonicated using an ultrasonic bath for 2 h. Nanocompositeswere then isolated by filtration and washed with a large amount ofwater followed by several washings with ethanol. Finally, the com-posites were washed with acetone and dried at 60 8C overnight.

Preparation of CNO/PEDOT:PSS Films: CNO/PEDOT:PSS (3 mg; 1:0.1,1:0.25, 1:0.5, or 1:1 mass ratio) or ox-CNO/PEDOT:PSS (1:1 massratio) was dispersed with the aid of ultrasonic agitation in ethanol(1 mL) to give a black suspension (3 mg mL�1). The CNO/PEDOT:PSSand ox-CNO/PEDOT:PSS films were prepared by the drop-castingmethod. The CNO/PEDOT:PSS or ox-CNO/PEDOT:PSS solution(10 mL) was then transferred to the GC (1.5 mm diameter) surfaceand solvent was evaporated under an argon atmosphere.

Methods: XPS measurements were performed in an ultrahigh-vacuum chamber (base pressure: 6 � 10�10 mbar) using a non-mon-ochromatic MgKa (1253.7 eV) radiation source (RS 40B1; PREVAC)and a semispherical electron energy analyzer (R3000; VG Scienta).The CNO powder samples, pristine and oxidized, were placed onnaturally oxidized Si wafers (ITME, Poland) precovered with�50 nm of Au (purity 99.999 %) by using molecular beam epitaxy(PREVAC). The measurements were performed at room tempera-ture. The spectra were recorded using a pass energy (PE) of 50 eV(except the survey spectra, which were obtained at PE = 100 eV).The exact binding energies were determined by referencing thespectra with respect to the value for the Au 4f components (84and 88 eV). The peaks were fitted using the CasaXPS software(Casa Software Ltd). A Voigt function (linear combination of Gaussand Lorentz functions) and Shirley background subtraction wereused for the fittings.

A Mettler DSC system with STARe analysis software was used toconduct DSC experiments. The thermal curve from the calorimeterwas used to evaluate the thermal properties of the materials. Thefollowing measuring parameters were used: surrounding atmos-phere: nitrogen; heating rate: + 10 8C min�1.

Voltammetric experiments were performed using a potentiostat/galvanostat model AUTOLAB (Utrecht, The Netherlands) witha three-electrode cell. Electrochemical impedance spectroscopy(EIS) measurements were performed on an AUTOLAB (Utrecht, TheNetherlands) computerized electrochemistry system equipped witha PGSTAT 12 potentiostat and frequency response analyzer (FRA)expansion card with a three-electrode cell. The AUTOLAB systemwas controlled with the GPES 4.9 software of the same manufactur-er.

A glassy carbon (GC) disk with a diameter of 1.6 mm (BioanalyticalSystems Inc.) was used as the working electrode. The surface ofthe electrode was polished using extra-fine carborundum paper(Buehler) followed by 0.3 mm alumina and 0.25 mm diamond polish-ing compound (Metadi II, Buehler). The electrode was then sonicat-ed in water to remove traces of alumina from the metal surface,washed with water, and dried. The counter electrode was madefrom platinum mesh (0.25 mm) and was cleaned by heating ina flame for approximately 30 s. A silver wire immersed in 0.1 mAgCl and separated from the working electrode by a ceramic tip(Bioanalytical Systems Inc.) served as the reference electrode.

The studied films were imaged by secondary electron SEM withthe use of an S-3000N scanning electron microscope from FEI(Tokyo, Japan). The accelerating voltage of the electron beam waseither 15 or 20 keV and the working distance was 10 mm. TEMimages were recorded using the FEI Tecnai instrument. The acceler-ating voltage of the electron beam was 200 keV.

Acknowledgements

We gratefully acknowledge the financial support of the NCN,Poland, grant no. 2011/01/B/ST5/06051 to M.E.P.-B. L.E. thanksthe Robert A. Welch Foundation for an endowed chair, grant no.AH-0033 and the US NSF, grant CHE-1110967. TEM, SEM, and DSCwere funded by EFRD, as part of the Operational Programme De-velopment of Eastern Poland 2007–2013, project : POPW.01.03.00-20-034/09-00.

Keywords: carbon · composites · electrochemistry ·nanostructures · polymers

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Received: September 24, 2012Published online on November 21, 2012


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