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Applied Surface Science
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nteraction of non-equilibrium oxygen plasma with sintered graphite
ros Cvelbara,b,∗
Jozef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, SloveniaCenter of Excellence for Polymer Materials and Technologies, Tehnoloski Park 24, 1000 Ljubljana, Slovenia
r t i c l e i n f o
rticle history:vailable online 22 October 2012
eywords:lasmaraphite
a b s t r a c t
Samples made from sintered graphite with grain size of about 10 �m were exposed to highly non-equilibrium oxygen plasma created in a borosilicate glass tube by an electrodeless RF discharge. Thedensity of charged particles was about 7 × 1015 m−3 and the neutral oxygen atom density 6 × 1021 m−3.The sample temperature was determined by a calibrated IR detector while the surface modificationswere quantified by XPS and water drop techniques. The sample surface was rapidly saturated with car-
xygennteractions
bonyl groups. Prolonged treatment of samples caused a decrease in concentration of the groups what wasexplained by thermal destruction. Therefore, the created functional groups were temperature dependent.The heating of samples resulted in extensive chemical interaction between the O atoms and samples whatwas best monitored by decreasing of the O atom density with increasing sample temperature. The sat-uration with functional groups could be restored only after cooling down of the samples and repeated
t low
short plasma treatment a
. Introduction
Carbon is found in different allotropic forms including sootoften called amorphous carbon nanoparticles) [1,2], graphite [3],iamond [4], graphene [5], fullerene [6], multiwall carbon nano-ubes [7], and single-wall carbon nanotubes [8]. The properties ofarbon depend enormously on the allotropic form and so do itspplications. One application of graphite is in electrical and chemi-al industries as substitution for metals. Unlike most commerciallynteresting metals, graphite expresses extremely good chemicalnertness and is thus applied instead of metals in cases wherelectrically conductive material should be in long contact withggressive chemicals. The graphite electrode should be connectedo a metallic part in order to allow for functional properties. Theonnection between the graphite and the metal should be mechan-cally strong and electrically highly conductive in order to preventnwanted heating of the joint due to Joule effect. Obviously, thelectrical resistance of the joint should be smaller than the graphiteof metal) resistance. This requirement dictates pretty good adhe-ion of the metal to graphite that is not that easy to realize.everal techniques have been invented to improve the adhesion of aetal on graphite surface including mechanical brushing (in order
o increase the contact surface and thus allow for mechanicallytronger configuration), electrode-less deposition of a thin metal-ic film, and activation by palladium. The latter two procedures are
∗ Correspondence address: Jozef Stefan Institute, Jamova cesta 39, SI-1000 Lju-ljana, Slovenia. Tel.: +386 14773536; fax: +386 14773440.
ecologically unsuitable so alternative techniques are required. Pooradhesion of metals on graphite is due to the hydrophobic characterof graphite. The hydrophobicity is easily confirmed by determina-tion of the contact angle of a water drop. A typical value is around90◦. Such character prevents good wetting of the graphite with elec-trolytes suitable for electrochemical metallization so the surfaceproperties should be modified.
A suitable method for modification of the surface propertiesis application of gaseous plasma treatment. Numerous authorsreported good if not excellent results on a variety of polymersdue to formation of polar functional groups as well as increasedsurface roughness [9–13]. The rapid functionalization is explainedby interaction of reactive plasma particles with polymer surfaceresulting in bond breakage and incorporation of oxygen atoms intothe surface layer of treated polymers. The samples are often keptat floating potential (the typical order of magnitude is −10 V) inorder to prevent extensive bombardment by positive ions and thusmodification of structural properties of thin surface films. Typicaltreatment times depend largely on the type of discharge usedfor plasma generation and can be between well below a secondand several minutes. The type and concentration of functionalgroups depend also on the type of sample, but very typically avariety of different groups are found on the surface of polymersafter treatment with oxygen plasma and the majority of groupsinclude oxygen bonded to hydrogen atoms. Since graphite doesnot contain hydrogen such functional groups are not likely to
be formed during processing with oxygen plasma. Although, thefunctionalization of the pyrolitic graphite with oxygen plasmahas been done before [14,15], no one observed behavior of thecreated functional groups after longer treatment periods. Authors
ostly monitored degradation of the surface by erosion or etchingf graphite or graphite composites by oxygen plasma [16–18]. Inhe present paper, we address the processing of thick graphiteamples exposed to highly reactive oxygen plasma with respect tourface interactions and created functional groups.
. Experimental
Commercially available samples were prepared by sinteringraphite powder. The samples were pressed into the form of smalliscs 25 mm in diameter and 4 mm thick. Samples were pickedirectly after sintering and no pre-cleaning was performed prioro plasma treatment. Samples were characterized by water dropsin order to determine the surface character) and high resolution-ray photoelectron spectroscopy (XPS) (in order to determine sur-
ace functional groups). The XPS surface analyses were performedn the PHI-TFA XPS spectrometer produced by Physical Electronicsnc. The analyzed area was 0.4 mm in diameter and the analyzedepth (from which about 95% of XPS signal is coming) was about
nm. Sample surfaces were excited by X-ray radiation from Alonochromatic source at photon energy of 1486.6 eV. The survey
pectra were acquired over wide energy range to identify and quan-ify the concentration of elements. In order to determine differenthemical states of elements the narrow-scan spectra C 1s and O 1sere acquired with energy resolution of about 0.65 eV measured
t Ag 3d5/2 spectrum. The accuracy of binding energy was about.3 eV.
Samples were exposed to oxygen plasma created by induc-ively coupled radiofrequency discharge. The plasma chamber was
borosilicate glass tube of diameter 4 cm and length 60 cm. Theube was sealed into kovar rings that were welded onto standardacuum flanges. A 12 turn coil was mounted onto the glass tube ashown in Fig. 1 and connected to a 27.12 MHz radiofrequency gen-rator with internal matching network. The output power of theenerator was estimated to about 200 W. The discharge tube wasumped with a two stage rotary pump with the nominal pumpingpeed of 16 m3 h−1. On the other side, oxygen was leaked contin-ously through a rather precise manually adjustable leak valve.ontinuous gas leakage and pumping allowed for steady flow ofxygen through the system at 75 Pa. Pressure was measured with
calibrated Pirani gauge mounted away from plasma reactor inrder to prevent any degradation due to high frequency interfer-nces. Plasma parameters were measured with a double electricalrobe and a catalytic probe. The electrical probe was mounted intohe center of the discharge tube and the I(U) characteristics was
easured before experiments with graphite samples. The catalyticrobe with the fiber optic design [19] was mounted away from most
ntense plasma (as shown in Fig. 1) and the density of O atoms wasonitored during treatment of samples [20,21]. An optical fiber for
Fig. 2. Temperature of the sample treated in plasma for 10 min.
measuring sample temperatures was mounted into a small holedrilled into each sample and was placed into a narrow glass tubeperpendicular to the main discharge tube as shown in Fig. 1. TheIR detector was previously calibrated by placing a graphite sampleinto a furnace and measuring the signal versus the known furnacetemperature.
3. Results
Samples were exposed to oxygen plasma for different treatmenttimes. An example of the determination sample temperature evo-lution during plasma treatment is presented in Fig. 2 for the case ofthe sample treated in plasma for 10 min. The temperature of othersamples exposed to plasma for shorter time followed practically thesame heating curve. The lower detection limit of the detected tem-perature is about 380 K and the measurement is not very accurateat temperatures below about 400 K, what results in the observednoise.
The density of oxygen atoms versus treatment time was mon-itored with a catalytic probe. The operation of catalytic probe isbased on the calorimetric principle, where released energy due toneutral atom recombination on the sphere probe tip made fromNickel metal is converted into heat. Then the released heat is mea-sured with IR detector, where the IR transmission is transferredthrough optical fiber from coupled optical sphere with metal-lic catalytic cap [19]. A typical behavior of the catalytic probe isillustrated in Fig. 3 for the case of a sample exposed to oxygenplasma for 10 min. Similar behavior was measured for samplesexposed to plasma for different times. The probe response timeis few milliseconds. The density of charged particles was about7 × 1015 m−3 and maximum density of the neutral oxygen atomsabout 6.2 × 1021 m−3.
The samples were tested with a water drop before and afterplasma treatment. The contact angle of a water drop was about110◦ for untreated samples, which could advance due to roughnessand porosity of the sample. This value varied for few degrees foraccidently chosen samples. After treating samples for few seconds,the contact angle became very low, definitely below the detectionlimit of our device which has been estimated to few degrees. Thecontact angle remained below the detection limit also for prolongedtreatment times. A quantity that is presented instead of the contact
angle is the area on which the water drop spreads. Namely, wealways used the same water drop volume, i.e. 20 ml. The area inwhich the water drop is spread versus plasma treatment time ispresented in Fig. 4.
Fig. 3. The density of O atoms at the position of the probe tip versus plasma treat-ment time.
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Fig. 6. XPS spectra of the C 1s peak on the surface of graphite after plasma treatment.
ig. 4. The water drop area just after deposition versus plasma treatment time.
The appearance of the surface functional groups was deter-
ined by XPS method. The survey spectra were used to calculate
he concentration of elements in the probed surface film and theesults are shown in Fig. 5. Only carbon and traces of oxygen wasresented on the untreated samples. The small concentration of
ig. 5. Concentrations of oxygen and carbon on the surface of the graphite afterifferent treatment times in oxygen plasma.
Fig. 7. XPS spectra of the O 1s peak on the surface of graphite after plasma treatment.
oxygen is probably due to surface contaminants and not to chemicalbonds on untreated samples. This is confirmed by high resolutionC 1s peaks that show a rather narrow Gaussian peak for untreatedsamples. Selected C 1s peaks are shown in Fig. 6 while the corre-sponding O 1s peaks are shown in Fig. 7.
4. Discussion
The exposure of graphite to oxygen plasma leads to creation offunctional groups, but also to heating of the sample. Taking intoaccount that our oxygen plasma has predominantly ions and neu-tral atoms which interact with the surface and recombine on it,they are the major source for surface heating. The density of ions7 × 1015 m−3 with recombination probability of impinging ion onthe surface is approximately 1, since their energies can be up to10 eV. This means that every ion that comes to surface recombinesand releases its kinetic energy to the surface. Whereas the initialdensity of neutral O atoms is 6.2 × 1021 m−3 and the probability fortheir recombination on graphite approximately 1 × 10−3 [22]. Therecombination mechanism of neutral atoms and release of their
energy is predominantly based on Eley–Rideal mechanism, sincethe surface of graphite is saturated with adsorbed O atoms in firstseveral seconds. If we look at the ratio between incoming atomsand ions, we have 103 recombined O atoms per one ion heating
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he surface. The dissociation energy of the O2 molecule is 5.13 eV,hich means that during dissociation each of two O atoms takes theotential energy of 2.56 eV. Although the energy of ions can be upo 10 eV compared to 2.56 eV of O atoms, the neutral O atoms aretill predominant source of heating through their surface recom-ination. The heating of the samples therefore highly depends onensity of neutral O atoms in the plasma as well as determinesaximum sample surface temperature reached. In our case this
emperature reached is around 600 K.Most interesting is dynamic of increasing temperature as well as
orresponding decrease of O atom density seen from Figs. 2 and 3.s soon as oxygen plasma is turned on the density of O atoms rises
o the initial value typical for empty reactor, which is in our case.2 × 1021 m−3. However the reactor is loaded with samples, andhe oxygens start interacting with surface and consumption as wells heating occurs [23]. Up to 200 s of plasma treatment, we do notave so pronounced heating, but there is significant consumptionf O atoms. This behavior can be contributed to functionalizationnd removal of impurities and loosely bonded grains from sinteredraphite surface. The trend of O atom consumption slows downfter 200 s. The reason for this is that loosely bounded grains haveeen removed, surface temperature increases which can affect Otom recombination probability, and plasma is locally modifiedue to etching products. The drop in oxygen concentration forbout 3 × 1021 m−3 can be attributed to enlarged surface area forecombination, since surface roughness increases for a magnitudeue to removal of grains and etching as well as increased surfaceemperature. Whereas the brake in O atom consumption trendccurs due to stabilization of the recombination area, the surface-emperature dependent O atom recombination probability andlightly changed surrounding plasma due etching products whichre mostly CO molecules.
The surface functionalization of graphite mainly occurs in therst 30 s, where the sample surface becomes rapidly saturatedith carbonyl groups. The prolonged treatment of samples causes
decrease in concentration of the groups, predominantly C Oonded oxygen. The decrease occurs after etching of loosely bondedrains and additional heating of the sample surface. This can belearly observed by water wettability area in Fig. 4. When we loseaturation of created functional groups and the impurities andoosely bonded grains are removed, the wettability area shrinks.he wettability starts increasing again only after continues expo-ure. This local minimum in wettability can be attributed only toncreased surface roughness, since longer treatments show the con-tant trend in decrease of carbonyl groups. And more the decreasef these functional groups can be explained by thermal destruc-ion. The heating of samples result of extensive chemical interactionetween the O atoms and samples destroys less stable bonds andnly small quantity of functional groups still persists.
The saturation with functional groups could be restored onlyfter cooling down of the samples and repeated short plasma treat-ent at low temperature. This can be clearly seen from comparison
etween C/O concentration (Fig. 5) and high resolution peaks C 1snd O 1s (Figs. 6 and 7), where we functionalized samples withdditional 20 s plasma treatment.
. Conclusions
The interactions of thermally non-equilibrium oxygen plasmaith samples made from sintered graphite are determined by
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ience 269 (2013) 33– 36
plasma species created inside plasma. In our case, the density ofcharged particles was about 7 × 1015 m−3 and the neutral oxygenatom density 6 × 1021 m−3. Although the probability for recombi-nation of ions is higher than the one for neutral oxygen atoms, thehigher density of O atoms determines surface reactions. As a resultof those interactions, we observed extensive heating after contin-ues exposure of the graphite to plasma. These reactions heat up thesurface of sintered graphite in 10 min up to 600 K.
The sample surface was rapidly saturated with carbonyl groupsafter initial 10–30 s treatment. But prolonged treatment of samplescaused a decrease in concentration of the groups, due to ther-mal degradation. The saturation with functional groups could berestored only after cooling down of the samples and repeated shortplasma treatment. Therefore, when processing graphite sampleswith oxygen plasma we always have optimal treatment times forfunctionalization of the surface, where the surface is saturated withcarbonyl groups. Prolonged treatments lead to more extensive sur-face interactions which cause etching and roughen the surface aswell as increase the temperature and destroy functional groups.The optimal plasma processing of graphite for material bondingcan be therefore achieved by mixed plasma regimes where thematerial is treated for longer periods and then sequentially sat-urated with functional groups after plasma activation for severalseconds.
Acknowledgements
The author acknowledges the financial support from the Min-istry of Higher Education, Science and Technology of the Republicof Slovenia through the contract No. 3211-10-000057 (Center ofExcellence Polymer Materials and Technologies) and SlovenianResearch Agency (ARRS).
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