Notes Bull. Korean Chem. Soc. 2009, Vol. 30, No. 12 3073 DOI 10.5012/bkcs.2009.30.12.3073
Production of Hydrogen-Capped Polyynes by Laser Ablation of Graphite in Neat Water
Yeong Kyung Choi, JaeKyuSong,* and Seung MinPark*
Departmentof Chemistry,Kyunghee University,Seoul 130-701,Korea
*E-mail: jaeksong@khu.ac. kr (JKS); smpark@khu. ac. kr (SMP) Received July15, 2009, AcceptedAugust 24, 2009 KeyWords: Polyyne, Liquid laser ablation,Graphite
Laser ablationof a solidgraphite targethas been a topic of utmost research interest sincethe advent oflasers in 1960s.1 Carbonclusters ofvarioussizes were formed simplyby irra
diating a graphitesurface in vacuum or gas-phase andremained highly stable dueto theirhigh binding energies.2,3 Accordingly, laser ablationof graphite has been widely employed as an effective tool for the synthesisof carbonclusters and it has ulti
mately brought about thediscovery ofC60.
Recently, there has beena growinginterest inlaser ablation of agraphite target in liquid phase aiming atthe growth of polyynes,4-9 which are hardlyproducedemploying conventional organic synthesis methods. Hydrogen-cappedpolyynes,(H(C 三 C)nH), are linearcarbonchain molecules consisting of sphybri dized orbitals, beingobserved in the interstellar space. Due to theirtypicalone-dimensionalelectronicstructures, polyynes haveattracted considerable attention in theirsize-dependent band gap10 and nonlinear optical properties.9
Largepolyynes withn >5, in particular,have been prepared by laser ablation of graphitemostly in organicsolvents, while we present experimental results onthe formation of C10H2 (namelyn =5) by laser irradiation of a graphite targetin neat water. Although larger polyynescanbeformed in organicsol vents, laser ablation in water has a uniqueadvantage in that it can greatlyreducethe contaminant carbon materialsincluding graphiteandgraphite-likecarbonparticulates (in short,carbon contaminants hereafter) compared totheorganic solvents.
Thekey feature oflaser ablation ofa solid target in liquid phaseis certainly the formation of adense plasma whichgen
erateslocal and temporalnon-equilibrium conditions. There
fore, physicalparameters including laser fluence,wavelength, and ablation time as well as chemicalparameters including solventundoubtedlydetermine the characteristics of theresul tant polyynes significantly. InthisNote, wediscuss the effects of aging and post-irradiation on the polyynesolution,together with the influence of laser fluence on theformationrate and size of polyynes.
Expeiimental
Polyynes were produced by laser ablationof a graphite target (diameter = 20mm, thickness =6.0 mm,99.99%)placed onthebottom ofa glass vessel filled with30mL of deionized water. The graphite targetwas irradiated vertically by a Q- switchedNd-YAG laser (Quantel 980C,入=1064nm) operating
at 10 Hz. Thelaserenergyranged from 60 to200mJ/pulse.
The laserbeam waslooselyfocused using a lens witha focal length of 300 mm.Thespot sizeof the focusedlaser beam was 2.0 mm in diameter. The vesselwas continuously rotated to minimize thetarget aging effect and to givesomestirringeffect.
The optical properties of the nascent polyyne solution was examined atroom temperatureby a UV-Vis absorption spectro photometer (HP 8452A). The irradiation timeandlaser fluence were variedto investigatethe influence on theoptical properties.
The post-irradiation wasperformed using pulsed UV laser (Z= 266 nm, pulse duration= 7 ns) for 30 min. The laserbeam diameter was7 mm and energywas2 mJ/pulse.
Results and Discussion
The UV absorption spectrum ofa solution prepared bylaser ablation ofa graphite target in neat water andthat ofa graphite powder solutionare displayedin Figure 1. The laser energy was 160mJ/pulse(5.1 J/cm2) and theablationtime was 5 min.
Unlikethegraphite powder solution,there appear distinctive features in thespectrum for thesolutioncontaininglaser-gene rated species. The peaksat 205 nm, 215.5 nm,and 226nm repre sent C8H2 and those at 239.5 nmand251.5nm belongtoC10H2.9 It is gratifying that polyynes as large as C10H2 can be gene ratedby laser ablation of graphite in neatwater, for thefirst
Figure 1. The UV absorption spectrum of a solution prepared by laser ablation of a graphite target in neat water at 1064 nm (160 mJ/pulse) and that of a graphite powder solution.
3074 Bull.KoreanChem. Soc. 2009, Vol. 30, No. 12 Notes
①
°
드 은
。요
①° 드 은
。요 5 4
3 . .
. 0 0
0
2 4 6 8 10 20
Ablation Time (min) (b) 0.25
60 120 180 240
①° 드 은
。요 0.02
Laser Energy (mJ)
0.5
0.4
.3
Figure 2. The effects of (a) the irradiation time, and (b) the laser energy on the formation of polyynes (log-log plots). In Fig. 2(b), the base line was subtracted when the absorbance at 215.5 nm was measured to select contribution from C8H2 only, while this still may include some intensities from smaller or larger polyynes. Since the concen
tration of polyynes larger than C10H2 is negligible, the absorbance at 300 nm is loosely indicative of the content of the carbon contaminants.
0.1
0.0
200 220 240 260 280 300
Wavelength (nm)
Figure 3. The aging and post-radiation effects on the polyyne solu
tion.
timeto our knowledge.WhileCompagnini et al6 reported syn thesis of C8H2 by laser ablation of graphite in waterat 532 nm, they could not detect any largerpolyynesatthelaser fluence of
0.4 J/cm2. Since the density of C2, which is consideredtobe the major building block of polyynes, determines in essence the size ofpolyynes, the shorterablationlaser wavelengthwas often preferred toenrichC2 molecules in liquidlaser ablation of graphite.4It is, however, of note that the laser fluence as well as the laser wavelength is a critical parameterfor thegrowth of largepolyynes.
Figure 2 showstheeffects of the irradiation time and laser energyon the formationof polyynes using log-logplots. The absorbance of thepolyyne(morestrictly, polyyneplus other carbon contaminants) solution at 215.5 nmincreased with ablationtime as shown in Fig. 2(a),but the rate was drastically reduced after 5 min.This indicates that theself-absorptionby thesolutiondiminishes theeffective laser fluenceonthe target surface. Figure2(b) illustrates therelativeconcentrations of a polyyne (■), C8H2, andcarboncontaminants (。)，whichwere estimated from the absorbance data at 215.5 nm (afterthe correction ofbaseline toeliminatethecontribution of carbon contaminants) and300 nm, respectively, as a function oflaser energy. At first glance, polyynes and carbon contaminants show similar trendin their absorption intensities asthe laser energy increases.Nevertheless, the slight difference in theslope maypossiblyimply that polyynes arepreferablyproducedat higherlaser energies.Thisisin line with theresult of Fig. 1, which showes theeffect oflaser fluence onthe growth of large polyynes.
The aging andpost-radiation effectson the polyynesolution are depicted in Fig.3. Two days afterthe preparation ofpolyyne solution by laserablation, the absorptionpeaksrepresenting C8H2 decreased as little as 〜25% while those belong to C10H2 werenotquite detectable. Thisleadsustoconclude thatthe larger polyynes areless stable inliquid environment containing carbon-containingspecies. Asthe polyyne solutionwasirra diated for 30 min at 266 nm, thepeakintensities relatedto poly
ynes weredrasticallyreduced. Since theband gapenergies of C8H2 and C10H2 are largerthan the photon energy of C =266 nm,10 thedisappearance ofpolyynes byirradiation at266 nm is presumably related with a 2-photonabsorption process.
Details are tobefurther investigated by moreelaboratestudies.
Acknowledgments. This work was supportedby a grant from the Kyung Hee University in2008. (KHU-20080381)
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