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拉曼Raman spectroscopy studies of carbide derived carbons

Raman spectroscopy studies of carbide derived carbons

S.Urbonaite a,*,L.Ha ¨lldahl b ,G.Svensson a

a Division of Structural Chemistry,Arrhenius Laboratory ,Stockholm University,SE 10691,Stockholm,Sweden b

K-analys AB,Salagatan 16F ,SE 75330Uppsala,Sweden

A R T I C L E I N F O Article history:

Received 12March 2008Accepted 6August 2008Available online 12August 2008

A B S T R A C T

The Raman spectra of a number of carbide derived carbons (CDCs)synthesised from TiC at 700,800,900,1000,1100and 1200°C and from VC,WC,TaC,NbC,HfC and ZrC made at 1000°C have been recorded using laser excitation wavelengths of 514and 785nm.The spectra show two main features,the D-and G-peaks situated around 1350cm à1and 1600cm à1,respectively.The peak positions,their intensities (I D /I G )and full width at half maximum (FWHM),as well as their wavelength dependent dispersion,were used to obtain information about the degree of disorder in CDCs.The increasing ordering with synthesis temperature was con?rmed by lower FWHM values obtained from CDCs made at higher synthesis temperatures.However,this parameter was not very sensitive to variation in ordering in CDCs made at 1000°C from different carbides.The I D /I G was used for determi-nation of the in-plane correlation length,which has shown to be independent of synthesis temperature and more sensitive to the choice of the precursor carbide.However,the changes in in-plane correlation length were small and barely accounted for the size of one sixfold ring.

1.Introduction

Carbon derived carbons (CDCs)are porous carbons made by chlorination of various metal carbides.Their specialty is that they exhibit several important advantages over traditional activated porous carbons,such as high purity,narrow pore size distribution,possibility to tune pore size and maintain shape and size of precursor particles after chlorination [1–4].These properties can be changed by varying synthesis condi-tions such as precursor,its grain size,chlorination tempera-ture,post treatment for chlorine removal.CDCs characterised by Raman spectroscopy in this work have been previously characterised by N 2and CO 2gas adsorption [5],electron energy loss spectroscopy (EELS),transmission elec-tron microscopy (TEM)[6]and scanning electron microscopy (SEM)[2].Structural studies of disordered materials are al-ways a challenge and require many analysis techniques.From adsorption and TEM studies,it was found that porosity and

microstructure are the most sensitive properties and vary with change of any of synthesis parameters,while from SEM it was seen that CDCs particle shape and size is depen-dent mostly on precursor.The change in bond hybridisation (sp 2/sp 3)and density of the particles is more sensitive to start-ing carbide than chlorination temperature as was concluded from EELS analysis.Raman spectroscopy is one of techniques which can compliment previous analysis and contribute to better understanding of CDCs structure.

Raman spectroscopy is a standard non-destructive analy-sis tool for characterisation of various carbon materials.In Raman spectra recorded in the near-infrared and visible light regimes,carbon materials typically exhibit two broad bands,called D (disordered)and G (graphitic).In HOPG (highly ori-ented pyrolytic graphite)only a G-peak appears in Raman spectra at 1582cm à1,while in disordered carbons,besides the G-peak,a D-peak appears at lower wavenumbers $1350cm à1.The presence and position of D-and G-peaks,

*Corresponding author:Fax:+46(0)8163118.

E-mail address:sigita@struc.su.se (S.Urbonaite).

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their intensity ratio (I D /I G )and full width at half maximum (FWHM)can be used to extract structural information of the materials.Appearance of both peaks in visible Raman spectra depends fundamentally on the ordering of sp 2sites and only indirectly on the fraction of sp 3sites [7].

The G-peak appears due to the in-plane bond-stretching motion of pairs of C sp 2bonded atoms.This mode does not require the presence of sixfold rings,it occurs at all sp 2sites.The G-peak width can be used as a measure of quality of the graphene planes [8].A broadening of the G-peak can be inter-preted as an increase in bond angle disorder [9].The D-peak is symmetry forbidden in perfect graphite and is activated only in the presence of disorder.Its intensity is directly dependent on the presence of sixfold aromatic rings.The width of the D-peak is correlated to a distribution of sp 2bonded clusters with different ring sizes.Therefore,the information about the less distorted aromatic rings is in the intensity maximum and not in the width of the peak,which depends on the disorder.Ring order,other than six,tends to decrease the peak height and increase the width [7].

In several articles,Robertson and Ferrari [7,10,11]have dis-cussed how the peak parameters mentioned above can be used to describe the structure of carbon materials.They have identi?ed three stages depending on the disorder in the car-bon structure.Stage 1is characterised by a transition from perfect sp 2bonded graphene units towards an in-plane corre-lation length of 2nm.Stage 2is described by a transition from the topological disordering of graphene units and loss of aro-matic bonding with a purely sp 2network to the introduction of up to 20%of sp 3bonds.Stage 3is described by a transition from 20%of sp 3bonded to the purely sp 3bonded carbon.

The ?tting of the Raman spectra of amorphous carbons is not always straightforward as the D-and G-peaks normally are distorted and there is no particular function for Raman spectra ?tting.The simplest ?ts are done by applying two Gaussians or two Lorentzians pro?les.Sometimes 2+2Gaussian pro?les are used to ?t the peaks since the two main

D-and G-peaks at 1300–1360cm à1and 1580–1600cm à1often are accompanied by two minor peaks at 1150and 1450cm à1[12].An alternative method of spectra treatment is to ?t an asymmetry shifted Breit–Wigner–Fano (BWF)pro?le for the G-peak and a symmetric Lorentzian for the D-peak as sug-gested by Ferrari and Robertson [7].Asymmetric BWF line shape,which arises from the coupling of a discrete mode to continuum is given by I ew T?

I 0?1t2ex àx 0T=Q C 21t?2ex àx 0T=C ;

e1T

where I 0is the peak intensity at the peak position x 0,C corre-sponds to the full width at half maximum (FWHM)and Q à1is the BWF coupling coef?cient.The Lorentzian shape is ob-tained when Q à1!0.A positive respectively negative sign of Q gives the relative position of the asymmetry.The maxi-mum of a BWF peak is not at x 0as the peak pro?le is asym-metric.Therefore,in this study we are using x max ,which is shifted according to

x max ?x 0t

2áQ :e2TAs Q <0,all G-peaks will be shifted towards lower x although this effect is largest when FWHM is large,i.e.more disordered carbons.

The intensity ratio of D-and G-peaks (I D /I G )is proportional to the in-plane correlation length.I D /I G is increasing with de-cay of size of the perfect graphene units,as the disorder is growing and the D-mode is becoming more active as pre-dicted by the T–K relationship [13]I D I G ?C ek TL a

;e3T

where I D and I G are heights or integrated intensities of the D-peak and G-peak,respectively,L a is the in-plane correlation length.The parameter C is wavelength dependant and can be described by the following equation:C %C 0tk C 1;

e4T

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where C0=à12.6nm and C1=0.033,valid in region400nm< k<700nm[14].

The T–K method was developed for disordered graphite, i.e.for100%sp2bonded carbons,with graphene unit sizes lar-ger than2nm.For more disordered carbons the connection between I D/I G and L a is better described by the Ferrari–Robert-son(F–R)relationship[7]

I D G ?C0ekTáL2

:e5TParameter C0is also wavelength dependant and is derived

from combination of Eqs.(3)and(5),when L a=2nm,which is considered to be a transition region for validity of the afore-mentioned equations

C0?C=L3

?C=8:e6TLately reports concerning carbide derived carbons(CDCs) are often accompanied by Raman spectroscopy studies and the integrated intensities of the peaks without referring to the integration method,or a simple?tting of two Lorentzian or two Gaussian peaks are used[15–19].In this work the BWF in combination with Lorentzian?t have been used for data evaluation?tting only two main peaks(D-and G-)since, according to Ferrari and Robertson,gives good?ts for all car-bon materials at a broad laser energy range[7,10].Peak heights were used since information about the aromatic rings is in the intensity of the peak as mentioned above.Moreover, previous EELS studies showed that CDCs possess a small amount of sp3bonded carbon[6]and therefore,they belong to stage2of the amorphisation trajectory described above. Consequently,the F–R method was used in this work to esti-mate the in-plane correlation length of studied CDCs.

2.Experimental

The CDCs were made by heating(T=700–1200°C)the carbide under a?ow of20ml/min of Cl2(g)in a silica tube furnace.The synthesis can be described by the general chemical reaction

MeC xesTty=2Cl2egT)MeCl yegTtx CesT:

During the reaction metal(Me)is transported away from the reactor as metal chloride(MeCl x(g))together with excess of chlorine gas,leaving behind pure carbon.The reaction time for0.5g carbide was40min.The CDCs were post-treated in Ar at1000°C for1h to remove remaining chlorine gas from car-bon and reactor,whereupon samples were allowed to cool down to room temperature in Ar atmosphere.

The Raman spectra were collected with a Renishaw inVia Raman microscope instrument equipped with an Argon ion la-ser(k=514nm)and a diode laser(k=785nm).Before each ses-sion,the instrument was calibrated against a Si crystal. T ypical collection times were60s and120s per scan with50 objective.CDCs from TiC prepared at700–1200°C,and from

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MeC prepared at 1000°C were studied,where Me =V ,W ,Ta,Nb,Hf,and Zr.The BWF in combination with Lorentzian ?t have been used for data evaluation ?tting only two main peaks (D-and G-)with goodness of ?t in range of R 2=0.998–0.994.

Transmission electron microscopy (TEM)studies were made in a JEOL UHR3010operated at 300kV .For these studies,carbon powder was dispersed in n-butanol using an ultra-sonic bath and transferred to a copper grid coated by a holey carbon ?lm.During the EELS (electron energy loss spectros-copy)analysis the samples were checked for residual chlorine and presence of metals,but no signal was detected [6].

Results and discussion

3.1.

TiC derived carbons

Raman spectra,recorded using lasers of two different wave-lengths,are presented in Fig.1.All spectra exhibit the two typ-ical distinct bands for disordered carbons,the G-band at $1580–1600cm à1and the D-band at $1300–1350cm à1.Peaks of the spectra recorded using the Ar ion laser (k L =514nm)are more narrow than those from the diode laser (k L =785nm),in accordance to previous report for disordered carbons [10].

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Increase in CDC synthesis temperature results in more or-dered CDCs without change in bond hybridisation[6].At both studied wavelengths,D-and G-peaks are becoming narrower with increasing synthesis temperature,see Figs.1and2.This can be explained by increased ordering of carbon bonds as sup-ported by TEM and EELS studies[5,6].The decrease in FWHM is accompanied by a growth of I D/I G values and can be attributed to the material ordering.Change in I D/I G is usually related to amorphisation or graphitisation[7,10,20],normally described as change in the sp2/sp3ratio.The CDCs under study have been shown to have a more or less constant sp2/sp3ratio[6],and therefore the differences in Raman spectra are only due to changes in bond ordering,not their hybridisation.FWHM of both bands and the I D/I G ratio are shifted to higher values with increase in excitation wavelength,which is consistent with previously published energy dispersive behaviour[10].

In spectra recorded using the Ar ion laser,the positions of D-and G-bands are nearly constant,situated at$1340cmà1and $1590cmà1,respectively.In the diode laser spectra of the same series,the D-peak position is constant,while the position of the G-peak is slightly increasing with temperature,since its posi-tion is more sensitive to ordering at higher excitation wave-length[10].However,even if the D-peak position is constant at a given wavelength,the clear jump to lower wavenumbers from$1340cmà1to$1300cmà1is observed by changing laser wavelength from514nm to785nm,see Fig.2.This is in agree-ment with a Raman study of SiC based CDCs[4]as well as with studies of D-peak position dependence on excitation wave-length and is attributed to double-resonance effect[10,20].

The calculated in-plane correlation length is presented in Fig.3as a function of synthesis temperature.The in-plane corre-lation length is slightly increasing from1.3to1.5nm for an exci-tation wavelength of514nm,while for a785nm wavelength it is nearly constant.It seems that while ordering is increasing with temperature(FWHM and TEM studies,see Fig.7),the in-plane correlation length stays constant,because an increase in 0.2nm does not even account for the size of one sixfold ring.

3.2.Carbon derived from various carbides

Raman spectra from VC,WC,TaC,NbC,HfC and ZrC based CDCs made at1000°C were recorded by two lasers with differ-ent wavelengths,see Fig.4.In all?gures the carbides are arranged by increasing unit cell volume per C atom in precur-sor carbide.

The longer the wavelength,the better the peaks are re-solved.The same dispersion effects as for TiC based CDCs are observed for these carbons,although a stronger wave-length effect is observed for the peak positions.The position of the D-peak is shifting to the lower wavenumber with longer excitation wavelength due to double-resonance effect.The G-peak position moves slightly from$1595cmà1at k L=514nm to over1600cmà1at k L=785nm.

The FWHM is quite constant$75cmà1in spectra recorded with an excitation wavelength k L=514nm,though at k L=785nm,a larger spread is observed for CDCs from

some Fig.7–TEM of CDCs from various carbides made at temperatures indicated on images.

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of the carbides,see Fig.5.Narrowest FWHM is observed for VC based CDC,which is the most ordered-like according to TEM studies,see Fig.7.

The calculated in-plane correlation length is presented in Fig.6as a function of precursor carbide.The values of in-plane correlation length are similar,although the spread is larger for shorter excitation wavelength.This is in accordance with observations for TiC based CDCs discussed above.The trends of in-plane correlation length and as I D/I G ratio are very similar to the change in sp2/sp3ratio determined by the EELS studies[6],see Fig.6.

4.Conclusions

Raman spectra were recorded for CDCs made from VC,WC, TaC,NbC,HfC and ZrC at1000°C and for temperature series of TiC based CDCs.The F–R relationship was used for calculat-ing the in-plane correlation length L a since the T–K relation-ship is not valid for disordered carbons possessing some fraction of sp3hybridised C atoms with graphene units smal-ler than2nm,as it has been developed for disordered graph-ite,i.e.100%sp2bonded carbon.

Since EELS studies of TiC based CDCs had shown that there is no change in sp2/sp3ratio,all changes in Raman spec-tra are due to bond ordering,not hybridisation.The increased bond ordering,observed in TEM studies and con?rmed by de-creased FWHM of both D-and G-peaks with increasing syn-thesis temperature,does not affect the in-plane correlation length of these CDCs.

CDCs made from other carbides at1000°C have different amounts of sp2bonded carbon and therefore can be con-cluded that the information re?ected in Raman spectra de-pends on both ordering and hybridisation.FWHM in spectra of CDCs from various carbides made at1000°C is almost con-stant at k L=514nm;despite clear differences in structural changes observed by TEM.However,the most disordered-like CDC from NbC has the largest FWHM in spectra recorded with an excitation wavelength of k L=785nm,which seems to be more sensitive to structural changes.

The in-plane correlation length was found to be$1–1.5nm and the trend of its variation is very similar to the changes in sp2/sp3ratio(EELS)of the studied CDCs.This indicates that despite variations observed in CDCs Raman spectra,TEM images and by other analysis techniques,the size of in-plane correlation length only very slightly differ with change of pre-cursor or chlorination temperature.

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