Nanoscale

PAPER

Cite this: Nanoscale, 2015, 7, 6007

Received 5th December 2014,Accepted 19th February 2015

DOI: 10.1039/c4nr07196d

www.rsc.org/nanoscale

Chemical modification of carbon nanomaterials(SWCNTs, DWCNTs, MWCNTs and SWCNHs) withdiphenyl dichalcogenides†

Z. Syrgiannis,*a A. Bonasera,a E. Tenori,a V. La Parola,b C. Hadad,a M. Gruttadauria,c

F. Giacalonec and M. Prato*a

Control over chemical functionalization is a crucial point in the field of nanotechnology. Herein, we

present the covalent functionalization of several carbon nanoforms (single-walled carbon nanotubes,

double-walled carbon nanotubes, multi-walled carbon nanotubes and carbon nanohorns) by means of

diphenyl dichalcogenides. These ones show different reactivity to the nanomaterials and are able to

modify their electronic properties depending on the electronegativity of the functionalizing heteroatom.

Theoretical calculations were also performed to support the experimental results. All the modified

structured nanocarbons were thoroughly characterized by TGA Raman, XPS, UV/Vis/nIR, IR and TEM

techniques. Our findings propose a simple approach to functionalize carbon nanomaterials and, in the

meantime, to tune their electronic properties.

Introduction

In the last few decades, different carbon nanoforms (CNFs)such as fullerenes, nanotubes, nanoonions, nanohorns and,most recently, nanodiamonds and graphene have attractedhuge interest from the scientific community due to theirpotential use in a wide range of applications, mainly in thefield of nanotechnology.1–5 Just to mention a few outstandingexamples of nanotechnological applications of CNFs, recentlyelastic transparent films of single-walled carbon nanotubes(SWCNTs) have been incorporated into wearable stretch andpressure sensors,6,7 carbon nanohorns (SWCNHs) have beenused for doping TiO2 electrodes in dye sensitized solar cells,8

nanodiamonds have been successfully employed for gene anddrug delivery,9,10 and graphene showed a great potential intransistors11,12 and supercapacitors.13 Despite their outstand-ing mechanical, physical and electronic properties, usuallyCNFs show poor solubility and bad dispersibility in solventsthat, to some extent, prevents their wider application andlimits their potential. A possible way to overcome such draw-

backs lies in their chemical modification. Although every yeara tremendous number of investigations are devoted to thenanotechnological application of CNFs, there is no direct cor-respondence in the development of proper chemical tools fortheir manipulation. In this regard, just a few general reliablesynthetic methods such as 1,3-dipolar cycloaddition,14–17

cyclopropanation reaction,18–21 diazonium salt coupling,22–24

or nitrene addition25–27 reaction are nowadays able to functio-nalize different CNFs both under classical heating and bymeans of non-conventional methods.28 Some of these proto-cols guaranteed a certain control over the CNF functionali-zation degree whereas the most employed manipulationstrategies involve an initial treatment with strong oxidantsunder harsh reaction conditions,29 which very often alter theirelectronic properties and present several limitations, especiallyin terms of reproducibility.30 We have recently developed a veryversatile method for the functionalization of SWCNT sidewalls,based on the easy and smooth addition of aliphatic, aromaticor even dendritic disulfides displaying a good control overthe functionalization degree as a function of reaction time.31

Therefore, it seemed natural to extend this reaction to thefunctionalization of other nanostructured carbons. Further-more, we were eager to understand whether other dichalco-genides, such as diselenides and ditellurides, which wereemployed in the doping of CNFs such as graphene oxide andcarbon nanotubes,32,33 were also able to chemically behavetoward CNFs similarly to disulfides.

In addition, since, during this process, new C–heteroatombonds would be formed, the comprehension of how differentheteroatom-based functionalizations should influence carbon

†Electronic supplementary information (ESI) available: TGA, Raman spectra ana-lysis. See DOI: 10.1039/c4nr07196d

aCentre of Excellence for Nanostructured Materials (CENMAT), INSTM, unit of

Trieste, Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste,

via L. Giorgieri 1, 34127 Trieste, Italy. E-mail: zsyrgiannis@units.it, prato@units.itbIstituto per lo Studio dei Materiali Nanostrutturati ISMN-CNR, Via Ugo La Malfa

153, 90146-Palermo, ItalycDipartimento Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche

(STEBICEF), Università degli Studi di Palermo, Viale delle Scienze, Ed. 17, 90128

Palermo, Italy

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nanoform properties becomes an interesting issue for possibleapplications. In this regard, the possibility that the interactionof decorating functional groups and nanotubes will bereflected in modifications of their electronic and phononstructures due to a shift in the Fermi level was firstlytheoretically hypothesized34–36 and then experimentally con-firmed.37,38 Moreover, the doping of CNFs with heteroatomssuch as chalcogenides allows us to fine-tune their properties,especially for the fabrication of efficient metal-free electrocata-lysts for oxygen reduction in fuel cells.32,33,39,40

Herein, we report the covalent modification of SWCNTsupon reaction with different diphenyl dichalcogenides –

(PhS)2, (PhSe)2 and (PhTe)2 – as well as the behavior of severalCNFs, namely DWCNTs, MWCNTs and SWCNHs, toward thesame processes (Fig. 1).

Results and discussion

Firstly, reactions among phenyl dichalcogenides and HiPcoSWCNTs were carried out in refluxing deoxygenated toluenefor 48 h (Table 1).

The reactions were analysed by thermogravimetric analysis(TGA), Raman spectroscopy and X-ray photoemission spec-troscopy (XPS), which showed marked changes in weight lossand in relative D- and G-band intensities, and in the surfacecomposition, respectively, compared to the starting pristineSWCNTs. These data are summarized in Table 1 and in ESI(Fig. S1–S7†).

TGA displays an increasing weight loss in the 250–700 °Crange from 4.35% [(PhSe)2] to 8.14% [(PhTe)2] (Fig. S1†),clearly indicating the detachment of organic fragments co-valently linked to the nanotube surface.41 For a better compari-son among all the chalcogenides, we have also repeated thereaction with diphenyl disulfide (the reaction was repeated in

order to have a homogeneous comparison of the reactivity onthe same batch of SWCNTs).31

The functionalization degree f in Table 1, calculated fromTGA data, gives an indication of how many organic groupsare attached per hundred carbons of the SWCNTs: 1.42% for(PhS)2, 0.35% for the case of (PhSe)2 and 0.52% for(PhTe)2.

42–44

Together with TGA, Raman spectroscopy represents apowerful and accurate technique for the characterization ofSWCNTs.45 In fact, the covalent attachment of functionalizinggroups on the CNTs, because of the rehybridization of thecarbon atoms from sp2 to sp3, can be detected from theenhancement of the area intensity of the D-band with respectto the G-band in the Raman spectrum. Based on this change,the normalized AD/AG ratio from the AD0/AG0 ratio can be usedas a more sensitive measure of the defect concentration. It isinteresting to note the good agreement between the values ofthe normalized AD/AG ratio for PhTe–SWCNTs and PhSe–SWCNTs and the heteroatom/carbon ratio (Table 1) measuredby X-ray photoemission spectroscopy (Fig. 2 and S2–S7 inESI†). A closer look at the RBM region may provide infor-mation on the portfolio of the reactions (Fig. 2, column (b)).Firstly, a blue shift of some of the peaks, mainly in the regionof the metallic nanotubes in comparison with the pristinenanotubes, is evident. A shift of the RBM modes in theSWCNTs can reveal important information about the inter-action of the adducts with the CNTs.45,46 The reason for thisshift may arise from changes in the electron density of CNTs,resulting in slight shifts of the resonance energy.38 Secondly,(PhS)2 causes the most significant spectral variations and itis assumed to be the most efficient functionalizing agent.Similar results are obtained for (PhSe)2, while (PhTe)2functionalization does not affect so deeply the Raman features.These statements are derived from a detailed analysis of theRBM peak intensities (column (b) of Fig. 2, S3 and S5†) andthe results are clearly elucidated in the chirality mappingdisplayed below (Fig. 3a); the diminished intensity of certainpatterns of peaks is related to a high level of functionalizationof the corresponding chiral tubes. So, in our study, the loss of

Table 1 Screening the different dichalcogenides on SWCNTs

Entry Substratea

Weightlossb

(%)f c

(%)

NormalizedAD/AG(633 nm)d

X/C(X = S,Se, Te)e

1 (PhS)−SWCNTs 11.45 1.42 5.72 0.012 (PhSe)−SWCNTs 4.35 0.35 4.57 0.0053 (PhTe)−SWCNTs 8.14 0.52 4.92 0.005

a Reaction conditions: 10 mg of p-SWCNTs (0.83 mmol) with thedichalcogenide (2.2 equiv.) in 100 mL of toluene (deoxygenated) at110 °C for 48 h. bMeasured by TGA in the temperature range between250 and 700 °C; from the estimated losses the loss of p-SWCNTs hasbeen subtracted (8.19%). cDegree of functionalization was evaluated asalready described in the previous study.31 dNormalized AD/AG(sample)ratios related to AD0/AG0 of p-SWCNTs. e Atomic ratio obtained by XPSanalysis.

Fig. 1 Schematic representation of the functionalization procedure forthe addition of phenyl-dichalcogenides to the CNFs surface.

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features in the RBM region has to be related to the deep modi-fication that a high degree of functionalization has on thebreathing modes of the carbon nanotubes, and the absence ofa peak means a successful functionalization of that specificchirality. As a tendency, we observe the highest decrease ofpeak intensity (and the highest degree of functionalization) forthe reaction in the presence of (PhS)2, minor for (PhSe)2 andfinally less significant for (PhTe)2.

The combination of all the information emerging fromRaman spectra analysis leads to the formulation of a chiralitymapping chart, in which it is possible to sum up at oncethe results provided by Raman spectra; the main evidence isthe comparable behaviour of S and Se, since almost the samechiralities are similarly affected in terms of functionalizationdegree. The mapping also reveals a preference in thefunctionalization of metallic CNTs and in a specific range ofdiameters (0.9–1.1 nm).

In relation to the observed blue shift of some of the bandsin the RBM region, what can be underlined is that the shiftis higher for the case of S, less for Se and much less for Te.This may indicate that the electronegativity of each of theseheteroatoms is responsible for the appearance of the shift.Particularly, a charge transfer between the tube and thefunctional group can explain this effect. We assume that theaforementioned effect arises from the difference in electro-negativity between the heteroatom (S, Se and Te) andcarbon. The polar bonding appears as a doping effect, in-troducing donor or acceptor levels within the gap of the Eiitransition.

Functionalized samples were analysed by the XPS techniquein order to evaluate surface modification. The typical SWCNTspectrum shows a main component at 284.4 eV arising from

the sp2 graphitic carbon and a tail at higher energy withcomponents at ca. 285–286 eV arising from sp3 C–C and C–Oand those at ca. 287–288 eV which stem from the CvObond.47 After functionalization the CvO componentdecreases and in the case of (TePh)2 totally disappears (Fig. 2,column (c)). The presence of covalent functionalization is con-firmed by the presence of the heteroatom on the surface.In Table 1, X/C atomic ratios, where X is the heteroatom,are indicated.

The high-resolution region of the chalcogen atom and theresults obtained by the fitting procedure are reported in theinsets of Fig. 2, column (c), and in Table S1,† respectively.Interestingly, none of the heteroatoms shows the reduced formtypical of dichalcogenides to be predominant. Sulfur and tell-urium are mostly present in the (IV) oxidation state. The S 2p3/2peak is centered at 169 eV and corresponds to sulfur in its

Fig. 2 Column (a) D- and G-bands from the Raman spectrum ofp-SWCNTs (black line) and the f-SWCNTs with the use of the threedichalcogenides (Ph–S–, red; Ph–Se–, blue; Ph–Te–, dark cyan)recorded at 633 nm. Column (b) RBM region from the Raman spectrumof p-SWCNTs (black line) and the f-SWCNTs with the three dichalco-genides (Ph–S–, red; Ph–Se–, blue; Ph–Te–, dark cyan) recorded at633 nm. Column (c) C 1s and in the inset S 2p (red line), Se 3d (blue) andTe 3d (dark cyan) high-resolution XPS spectra.

Fig. 3 (a) Chirality mapping based on the analysis of RBM regions ofthe three excitation wavelengths (532, 633 and 785 nm) (red for (PhS)2,blue for (PhSe)2 and dark cyan for (PhTe)2), in accordance with Kataura’splots.66 (b) UV/Vis/nIR spectra of p-SWCNTs and the functionalizedSWCNTs with (PhS)2, (PhSe)2 and (PhTe)2 in NMP, normalized to thelocal minimum at 800 nm.

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sulfone form, –SO2–.48–50 On the other hand, in the Te 3d

spectrum, the main component is centered at 576 eV and cor-responds to telluroxide –TeO–,51,52 whereas a minor com-ponent (≤7%) at 573 eV is typical of Te(0) and may beattributed to Ph–Te–(SWCNT).53 The Se spectrum is morecomplex, even though the low intensity of the Se 3d signalmakes it hard to perform a good fitting. The spectrum showsat least three components which may be attributed to bothreduced and oxidized species of selenium.33,54,55

The dichalcogenide addition reaction yields materials withan enhanced solubility in organic solvents, like NMP. This factcan assist in the characterization of the optical properties withthe help of UV/Vis/nIR spectroscopy. The wide-range transitionspectroscopy is a complementary method to Raman spectro-scopy and the use of both techniques will make the resultsmore reliable. Transmission spectroscopy provides infor-mation about the electronic structure of the nanotubes. Thisenabled us to investigate the influence of the covalent sidewallfunctionalization on the excitonic transition features ofSWCNTs. It was mentioned in several studies that rehybridiza-tion of the sp2 carbons of CNTs to sp3 leads to the absence ofall the excitonic transitions which are related to the van Hovesingularities.22 This fact is used as an indication of thecovalent sidewall functionalization. On the other hand, it hasalso been reported that mild functionalization does not inducea complete depletion of these characteristic transitions.41,56–62

In addition, changes in the electronic structure have a directinfluence on the features of the optical spectrum.63 By investi-gating the spectra of samples functionalized with the differentdichalcogenides, we observed shifts in the transition energies(Fig. 3b). It should be mentioned that the absolute values forthe transition energies obtained from the different spectro-scopic methods cannot be compared, because the sampleshave been prepared either in solution or as bulk material. It iswell-known that these different environments lead to shifts ofthe optical transitions.64 After the functionalization withdichalcogenides, several peaks are blue-shifted. Althoughthese shifts have the same trends as those in the case ofRaman measurements, the magnitudes are considerablysmaller due to the different conditions of measurements(Raman in the solid state and UV/Vis/nIR in solution). Acharge transfer between the tube and the functional group canexplain the effect on the optical transition. The charge transferarises from the attachment of a heteroatom (sulfur, seleniumand tellurium) to the sidewalls of the tube due to the differ-ence in electronegativity. The polar bonding appears as adoping effect, introducing donor or acceptor levels within thegap of the Eii transition.

37,38,46,65

All the experimental data suggested that a charge transferphenomenon between the tube and the functionalized areascould explain the observed spectral shifts. In order to godeeper into the comprehension of such phenomena, someab initio calculation models have been developed. For the sim-plicity of the models and time-demanding issues, tubes havebeen designed with only 2 organic pendants in order to simu-late the addition of one dichalcogen molecule.67

As far as carbon nanotube models are concerned, tubeshave been designed with an approximate length of ∼2.5 nm;chirality (7,3) has been chosen for the model tubes on thebasis of chirality mapping (Fig. 3a) which shows that thistypology of tubes undergoes functionalization with all thedichalcogens. For geometry optimization of covalently functio-nalized SWCNTs, models have been optimized at theHSEh1PBE68/6-31G(d) level of density functional theory (DFT)in vacuo;69 in the presence of Te atoms, their optimizationrequired the introduction of pseudopotential values, and thecalculation for the heavy atoms has been performed using theLANL2DZ70 basis set; electrostatic potential-derived chargevalues have been produced according to the CHelp scheme.71

In order to assess exclusively the influence of the electro-negativity of the chalcogens on the CNTs and in order tosimplify the calculations, we have adopted the existence ofonly the unoxidised species on them. This represents the bestcompromise between the heteroatom and the input of theelectronegativity.

From ESP charge distribution charts, sulfur atoms presenta residual negative charge due to the higher value of electro-negativity when compared with C atoms: a decrease in absolutevalues of residual charges for the C–Y fragment (Y = S/Se/Te) isobserved when passing from S to the less electronegativeelements, which have diminished electronegativity and a lesspolarized covalent bond, till the case of Te atoms, which havea negligible positive charge; moreover, sp3-C atoms present aresidual negative charge that, in a similar manner, is decreas-ing on passing to less electronegative chalcogens.

From the molecular orbitals description, we observe thatthe presence of chalcogen atoms produces modifications ofthe electron density distribution; from the images provided, itis evident that the high contribution to HOMO and LUMOorbitals is from chalcogen p-orbitals (Table S3†), with anapparent difference for the different chalcogens that is simplydue to the contribution of p-orbitals which are lying indifferent energetic levels for each heteroatom (3p, 4p and 5porbitals respectively). In our hypothesis, the localization ofelectron density over the organic pendants (mainly over S/Se/Te atoms) is subtracting electrons from the tube surface, theremaining electron density is more localized and electronictransitions require higher energy to take place.

Given the successful functionalization of SWCNTs withdiphenyl dichalcogenides, we decided to explore the reactivityof such substrates to other CNFs such as DWCNTs, MWCNTsand SWCNHs in order to discern whether these processes havegeneral application to CNFs. The reaction procedure followedwas the same as that in the case of SWCNTs (48 h in deoxyge-nated refluxing toluene with the addition of the same molarratio of (PhS)2, (PhSe)2 and (PhTe)2, and the resulting data arepresented in Table 2.

Interestingly, DWCNTs, namely the simplest MWCNTs,react to a lesser extent with respect to SWCNTs with (PhS)2 and(PhTe)2, whilst diphenyldiselenide seems not to react underthese conditions (see entries 1–3). In fact, XPS analyses ofthese samples showed the presence of S and Te (S/C and Te/C

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0.004 and 0.007, respectively) but were unable to disclose thepresence of selenium. On the other hand, although all thereactions performed on MWCNTs (entries 4–6) gave weightlosses in TGA, only in the sample treated with (PhTe)2 it waspossible to find the heteroatom signal in the XPS spectrum(Te/C 0.02). According to these results we can argue that thediameter of the nanotubes plays an important role in thesuccess of the reactions, even if additional experiments have tobe carried out in order to understand the discrepancy between

TGA, Raman and XPS techniques. Finally, reactions betweendiphenyl dichalcogenides and SWCNHs (entries 7–9) gave riseto functionalized materials, especially in the case of (PhS)2and (PhTe)2, whereas once again we were unable to detect theselenium signal in the XPS spectrum.

As an example, in Fig. 4 the Raman spectrum as well ashigh-resolution XPS spectra of C 1s and S 2p peaks arecollected for the nanocarbons treated with diphenyl disulfide.In the case of DWCNTs and SWCNHs the sulfur atom ispresent both in the reduced and in the oxidized form, as pre-viously found,31 while tellurium is present mainly as oxidized–Te(vO)– in all functionalized nanocarbons. In Fig. S26 andS27,† the same data are reported for samples treated with(PhSe)2 and (PhTe)2.

Finally, TEM analyses of all the functionalized SWCNTsamples as well as those of DWCNT, MWNT and CNHs func-tionalized with (PhS)2 show that the method we propose is notdestructive for the scaffold of the carbon nanoforms (seeESI†).

Conclusions

In conclusion, a novel class of reactions among different CNFsand diphenyl dichalcogenides has been reported. In particular,SWCNTs seem to be more prone to react with all the dichalco-genides as shown by TGA, Raman, XPS and UV/Vis/nIR tech-niques. Analysis of RBM bands in the Raman spectra reveals acomparable behaviour of S and Se, which show preference inthe functionalization of metallic CNTs and in a specific rangeof diameters (0.9–1.1 nm). Moreover, a blue shift of some ofthe bands in the RBM region of Raman spectra as well as insome of the transition energies in the UV/Vis/nIR spectra canbe detected. Such a blue shift is bigger for the case of S, lessfor Se and much less for Te and indicates the interaction ofadducts with the CNT backbone, very likely due to thedifferent electronegativity of these heteroatoms and carbonatoms. In particular, a charge transfer between the tube andthe functional group can explain this effect. From ab initio cal-culations, the analysis of the theoretical data provided sup-ports the hypothesis of charge transfer phenomena by asimple analysis of electrostatic potential values and from adescription of the electron density. Chalcogen atoms withdrawcharge density from the tube surface and localize it on theorganic pendant, with a consequent shift of Raman frequen-cies and UV/Vis/nIR features as experimentally observed.Finally, the same reactions have been applied to other CNFssuch as DWCNTs, MWCNTs and SWCNHs. Whilst (PhS)2reacted with both DWCNTs and SWCNHs, (PhTe)2 is able tofunctionalize all the studied CNFs. Additional experiments areneeded in order to discern the behavior of (PhSe)2, whichseems to interact to a lower degree for SWCNTs or not at allfor the rest of the CNFs. The present approach paves the way tothe use of these “hybrids” chalcogen–CNF as materials withmodulated energetic levels which can be used in differentapplications such as catalysis.

Table 2 Screening the different dichalcogenides on MWCNTs,DWCNTs and SWCNHs

Entry Substratea

Weightlossb

(%)f c

(%)

NormalizedAD/AG(532 nm)d

X/C(X = S,Se, Te)e

1 (PhS)–DWCNTs 7.35 0.87 3.02 0.0042 (PhSe)–DWCNTs 4.62 0.37 0.85 n.d3 (PhTe)–DWCNTs 8.93 0.57 0.94 0.0074 (PhS)–MWCNTs 6.45 0.76 1.1 n.d.5 (PhSe)–MWCNTs 9.66 0.81 1.12 n.d6 (PhTe)–MWCNTs 5.40 0.33 1.08 0.027 (PhS)–SWCNHs 11.13 1.38 1.46 0.0038 (PhSe)–SWCNHs 3.62 0.29 0.95 n.d.9 (PhTe)–SWCNHs 3.00 0.18 0.73 0.002

a Reaction conditions: 10 mg of CNFs (0.83 mmol) with thedichalcogenide (2.2 equiv.) in 100 mL of toluene (deoxygenated) at110 °C for 48 h. bMeasured by TGA in the temperature range between250 and 600 °C; from the estimated losses the value of 1.38% forMWCNTs, 0.00% for the DWCNTs and 0.05% for the SWCNHs hasbeen subtracted (loss of p-DWCNTs, p-MWCNTs and p-SWCNHs).cDegree of functionalization was evaluated as already described in theprevious study.31 d Values are the normalized ratios of AD/AG(sample)related to AD0/AG0 of p-SWCNTs. e Atomic ratio obtained by XPSanalysis.

Fig. 4 Column (a) D and G bands from the Raman spectrum recordedat 532 nm of p-DWCNTs, p-MWCNTs and p-SWCNHs (black line) andthe corresponding f-SWCNTs with (Ph-S)2 (red line). Column (b) C 1sand column (c) S 2p (red) high-resolution XPS spectra.

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Experimental procedures

In a 250 mL flask, 10 mg p-CNFs (0.83 mmol) and 2.2 equi-valents of the dichalcogenide were suspended in 100 mL ofdegassed toluene. The mixture was treated under sonic bathirradiation (under an Ar atmosphere) for 15 min followed byreflux for the indicated time under an Ar atmosphere. Theresulting dispersion was diluted with 100 mL of THF, filteredon a Millipore membrane (PTFE 0.45 μm) and washed withDMF, THF, MeOH and diethyl ether. The resulting black solidwas dried under vacuum overnight.

Acknowledgements

This work was supported by the University of Trieste, Con-sorzio Interuniversitario Nazionale per la Scienza e Tecnologiadei Materiali (INSTM), Ministero dell’Università e della Ricerca(MIUR) (FIRB prot. RBAP11ETKA and Cofin. Prot.2010N3T9M4), ERC Advanced Grant Carbonanobridge(ERC-2008-AdG-227135), and Associazione Italiana per laRicerca sul Cancro (project: Application of Advanced Nano-technology in the Development of Innovative Cancer Diagnos-tics Tools, AIRC 5 per mille, Rif.12214).

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