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Chemical reactivity of the gaseous species in a plasma discharge in air: An acid-base study
530
CHEMICAL ~ C T ~ ' ~ IN A PLASMA DIS~RGE
Applied Surface Science 36 (1989) 530-538 North-Holland, Amsterdam
OF THE GASEOUS SPECIES IlN A I R : A N A C i D - B A S E S T U D Y
J.L. B R I S S E T , A. D O U B L A , J. A M ~ O U R O U X Laboratoire des Rdacteurs Chimiques en Phase Plas.ma, ENSCP, II Rue P. et M. Curie, 75005 Paris, France
J. L E L I E V R E Laboratoire de Phys;.cochi;nie des Solutions, ENSCP, 11 Rue P. et M. Curie, 7~(,:7~ ~',ris. France
and M . GOLDIVL'~NN Physique des D~charg,:s, ESE, Plateau du Moulon, 91190 Gif,, France
Received 2 Jm~,e 1988; accepted for publication 14 July 1988
The ions and the chargeless particles which are present in a plasma discharge are responsible for the surface treatments of industrial interest. Their properties can be. considered in chemical terms of acid-base reactions of the relevant species with the substrate. The DC point-to-plane corona discharse at atmospheric pressure was selected as z, convenient source for excited gaseous species (and we focused on the chargeless species) to examine the acid-base properties of various gases or gas mixtures such as air. The substrate exposed to the disciiarge was a polya~:rylamide gel wetted with a basic or acid aqueous solution of coloured indicator. Tnis technique appears as a f~rst step to classify the plasma gases by means of their acid-base propertie~ and to forecast the acid-base behaviour of the gaseous chargeless excited species with respect tc the substrate molecules concerned in a given plasma treatment.
1. ~ n ~ o d u c ~ o n A n u m b e r o f c h e m i c a l e v e n t s w h i c h o c c u r in t h e liquid p h a s e a r e ~ o w n as acid-base reactions since they involve either a proton or an electron pair e x c h a n g e . S u c h r e a c t i o n s m a y also b e p e r f o r m e d in l e s s - c o m m o n c o n d i t i o n s s u c h as t h e p l a s m a p h a s e . T h e y a r e t h u s s t u d i e d f r o m b o t h a t h e o r e t i c a l a n d a t e c h n i c a l p o i n t o f view s i n c e t h e y a r e r e s p o n s i b l e for v a r i o u s a p p l i c a t i o n s , s u c h as e t c h i n g o f silicon d e r i v a t i v e s or t h e p r e p a r a t i o n o f n o n - s t o i c h i o m e t r i c c o m p o u n d s (e.g., WxCv, S i x N y . . . . ) w h i c h a r e d e v e l o p e d o n t h e i n d u s t r i a l scale. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 O Elsevier Science P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
J . L Brisset et al. / Chemical reactivity of gaseous species in plasma discharge
531
The reacting molecules involved in the plasma treatments which are mostly chargeless [1] species fl0 3 or 10 4 times the ion population in a non-equilibrium plasma at atmospheric pressure) are placed in a strong electric field. Therefore an important part of these heavy species is raised to some excited state: so that the gas phase is strongly enriched in excited chargeless species (further referred to as chargeless activated s0ecies or CAS) in which the subsequent electron repartition may differ significantly from that in the ground state, and this is connected with the improved chemical properties. In this paper we shall focus on the acid--base properties of the electronically excited species created in air at atmospheric pressure.
2. The ackl-base prope~es o| the excited specles We shall first consider oxygen as an introductory example. The theory of the molecular orbi~als leads to the MO diagram presented in fig. 1, in agreement with photoelectron spectroscopy. On this sketch, two unpaired electrons are in the high energy ~r*(2p) orbitals, which is related to the paramagnetie properties of the 0 2 molecule. The ground state of O2 is a triplet state (3Z~-). When excited, for example by exposure to the electric discharge, the molecule is raised to a higher energy level, with a new electron distribution which depends on the transferred energy (fig. 2). In the (~Ag) state for example, the two odd electrons are paired in one of ~he ~rg* orbitals and this gives rise to an empty orbital ~rg* which is then able to accept one electron pair given by a donor group. A good illustration of the acid character of O2(1Ag) as defined on the basis of Lewis concepts :s given by the Diels-Adler reaction. 3~
2p
O
.M~ u ~
0 2
2p
O
Fig. 1. Molecular orbit~s of 0 2 molecule from the atomic orbit~s.
532
J . L Brisset et al. / Chemical reactitJity of gaseous sp.~ies in plasma discharge
STATE
~g ~g
leg÷
t
~
ENERGY lWkJ
1 Ag+
~
_
94.3kJ
3 Eg~ ~ 0kJ Fig. 2. Ground and excited states of O2 moleculegivingevidenceof the acid properties in the (~,~g)state. This acid character disappears when 02 is raised to the (tE~') excited state, since the two erg electrons are then unpaired with opposite spins in two ,~-~ orbitals. The example of 02 can be generalized to other species which present the same characteristic two odd electrons, such as organic diradicals in the ground state. Examples are numerous among carbenes, halogenocarbenes, nitrcnes. . . . which are granted with similar electrohic properties and are kl~own for their high reactivity (fig. 3). The carbenes are well known as reaction intermediates in orgarfic chemistry; they are also able to enter metal-carbon 0 bondings as illustrated by the several hundreds of metal-earbene complexes known (e.g. Cr(CO)sCHNMe~). The carbene ligand may be considered as being based on a s p 2 carbon atom with an empty orbital able to interact with a ~- base. When a species is raised from the ground state to an excited state which confers to it the acid character, the distribution of the electrons is modified in the bJgher orbitals as already mentioned. In addition the energy transfer implies a change in the orbital hybridation and hence in the molecular geometry of the species and this is directly related to the improved reactivity of the relevant species. The electric field which induces large changes in the electron densities of the heavy molecules entering reaction in the boundary layer, strongly affects their electron distribution which results in significant dipole formation. In the particular case of ~" bonding molecules, it backs up the acid or base character NH3
[ ] F~ ~
Fund.
Base
Fund.
Boso
Fund. Excited Excited
Radical Acid Radical
[~] ~'~ ~
Fund.
Base
[]
Excited
Radical
NH 2NH NH" NH '~
[] [] []
N N '~
[-~ [] [] []
~ ~ ~ ~ ]
Fig, 3. Electronicsketcb ot the electronrepartitionin some nitrogenderivatives,with evidenceof the Lewisacid-basecharacters.
J.L. Brisset et aL / Chemical reactioity of gaseot,~sspecies in plasma discharge
533
accoramg to its direction, and hence modifies the reactivity of the molecule. The relationship between acid-base behaviour and polarizability has been examined [2], and is supported by polymerization reactions electrochemically performed in convenient solvents [3]. Similar mechanisms may be considered in the gas phase and be related with the increased reactivity of the species. In the particular case of the plasma polymerization processes, a number of applications such as encapsulation or surface protection involve a polymerization step.
3. Strength of the acld-base systems The concepts of electron pair donor or acceptor in'Lroduced by Lewis have received new developments both by Pearson [4] with the '"hard and soft acid-base" theory and by Drago [5] who worked at classifying acids and bases according to their strength. This is in fact a very difficult operation, so ordering is uneasy alid of rather limited reliability. For the bases however, the question profited by sisnJficant progress by selecting the proton as a particular reference species. In this way, Bronsted's defhaitio~, of zcids and bases was resumed. It is thus possible to classify the various unsolvated species in the gas phase according to the value of the standard enthalpy of the associated reaction involving heterolytic cleavage: BH + --, B + H + (PA). The relevant thermodynamic functic.n is called the proton affinity (PA) and must not be confused vAth the hydrogen affinity (HA) which requires homolytic bond scission: BH + --, B + + H (HA). The two energies are related by the ionization potential difference between H and B: PA(B) = HA(D) - (I,~(B) - IP(H)). The proton affinity can also be related to the associated AG o value provided the entropy term T A S ° be calculated, which can be done on the basis of symmetry considerations [6]. The comparison between several bases :Jan be made from the relevant PA values. Up to now hundreds of PA values have been dete,.~-zir.ed and are available in the literature [7,8] which covers a large number of parent molecules and derivatives. Table 1 reports a limited number of illustrative exampies. In the above PA(B) expression, it must be pointed out that the IP(H) correction is the same for all donors. Thus the difference between two bases may be expressed in terms of orbital ep_ergies: the PA's difference depends not only on the difference of HA's - which reflects the H O M O / L U M O overlap
524
J . L Brisset et al. / Chemical reactwity of gaseous species in plasma discharge
Table 1 Selec.~ed proton affinity (PA) values (kJ tool - J ) NH ~ OH FCN HCHO HF N, NH 2 CHF3 F2CO CH3F
1690 1630 1550 1475 958 459 494.5 782 615 671 628
Pyridine Aniline NH 3 H 20 H2 F NO C6F6 NF3 Sill4 CH 2F~.
924 876 853.5 697 424 339 531 743 604 648 615
CO CH4 CF4 02 O Ar Xe HCOOH H 202 CH3NH 2
594 552 527 422 498 371 496 718 678 876
difference - but also in the difference of IP's which is related to the difference in H O M O / L U M O energy match. As developed above, the basic properties of gaseous species are classified by means of the associated P A values when they are matched against a given acid (i.e., the proton) b u t other references are available such as BH 3 or B(CH3) 3. The resulting classifications are relevant to gaseous species which are most often in the ground st:.te, b u t up to now, information on the a c i d - b a s e properties of excited species occurring in a plasma is rather scarce in the literature. D a t a o n the etching kinetics of various substrates, which are usually silicon derivatives, are available a n d a correlation between the etching ra:e a n d the basicity of the etching species was proposed [9]. W h e n the plasma gas is a mixture, the etching rate depends on the gas phase composition: for example the rate of Si etching by N F 3 + H2 mixtures increases with the mole fraction of the an~ne. F o r high hydrogen concentrations the N F 3 molecule dissociates into F and the fluororfitrene N F which is less reactive t h a n NF3 towards the acid substratc. Such a n approach appears as a first attempt to explain the collective properties of a plasma a n d leads one to consider a gas mixture in the same way as the solvent mb~tures with respect to the chemical properties. W e consider a more simple situation in two ways: first we focused o n the p r o t o n exchanges (i.e., the Bronsted acidity) a n d second we selected a point-to-plane corona [10] device working in the air as the plasma source because this discharge is a convenient model for a hot plasma.
4. Separation of t ~ ions from file chargeless s F ~ i e s We must first recall that a corona discharge may be realized wtJert two conductors are raised to a convenient range of potentials and their shape is such that the associated electric field lacks for symmetry in ,'.he electrode gap.
J.L. Brisset et al. / Chemical reactivity of gaseous species in plasma discharge
535
vii~
.......
~ T
@ Fig. 4. Schematicdevice(see text) used to separate the ions from the CAS (DC coronadischarge at atmosphericpressurein air; T: target; d: electrodegap; D: point-to-targetdistance;the point is alwaysraisedto the highvoltageand the otherelectrodeearthed).
These conditions are fulfilled in a point-to-plane device. In such a device, the ions and the chargeless species are emitted by the point and are prominent in the ionization region around the point. In the drift region which lies in the electrode gap the prominent heavy species are the ions the sign of which is the same as the active electrode ar,d the chargeless species which are carried on by the electric wind in a thin beam. The acid-base properties of the species generated in tile plasma can be unambiguously demonstrated when the species of each kind are previously separated. This can be performed by means of the various devices sketched in fig. 4, in which the point electrode is always raised to the high voltage (positive or negative since we operate in the DC mode), and the other electrode earthed. Device I was first proposed by Goldmann. It consists in a vertical point arranged perpendicularly to a metallic grid which acts as an ion collector, and its transparency towards ions has been examined by physicists [11]. The chargeless species pass through the grid without noticeable deviation and then react with a target characterized by convenient chemical properties and disposed out of the electrode gap and under the grid. In our experiments, the electrode gap d was fixed in the range 5-15 mm and the point-to-target distance was varied around 20 rnm. Device H also suggested by Goldmann is characterized by a point inclined with respect to the plane which also holds the target (electrode gap close to 1 cm). The separation of the ion and CAS flows depends on the inclination angle: under the point the ions are the prominent species while the CAS are the major population along the point direction. Device 11I is developed in our laboratory from device II, with a point parallel to the plane.
536
J.L. Brisset et al. / Chemicalreactivity of gaseous species in plasma discharge
Device I V has just been tested; the point is arranged according to the axis of metallic frustum of a cone which is the earthed electrode. The C A S flow ca thus be observed along the common symmetry axis, under the cone. The substrate exposed to the discharge consists in an aqueous solution c coloured indicator trapped in a gel (e.g., a polyacrylamide gel prepared ftJ electrophoresis) to limit evaporation. For a number of experiments, th bromothymol blue was selected as the standard indicator.
5. Results and discussion Evidence of the acid-base reaction induced in the target by the ion flow i easily given if we use device II with a target under the point. For a positiv, point, the indicator turns to the acid medium while for a negative poin evidence is given of basic changes. These results illustrate the acid (basic properties of the cations (anions) emitted at the point according to the polarit~ ( + , - respectively) of the active electrode. The chemical effects of the CAS are demonstrated with device I, III am IV: the indicator turns to its acid form, whatever the polarity of the activ, electrode and in large way the nature of the plasma gas (i.e, N2, 02, CO2, CF4 .... ). At this juncture it is noticeable that an acid effect b, observed by exposure to the CAS of a NI-I3 plasma. Also, this acid changq takes place with a non-aqueous target (e.g., acetic anhydride). Most of the experiments were performed with device I, the point electrod~ being raised to the negative high voltage. We determined the influence o various parameters on the diameter of the acid spot observed on a gel targe after exposure to the CAS, and this was a first approach to the quantitativd aspect. The key parameters which were considered are of different kinds: we tint electric parameters (i.e., applied voltage U, current intensity/, exposure tired t), geometric parameters (i.e., electrode gap d, point-to-target distance D) ant chemical parameters (basicity of the target, acidity constant of the indicator) (a) The chemical parameters. For given experimental conditions, the diamete: of the acid spot of the target increases with the acidity of the trapped solutio~ (for a given indicator) or with the acidity constant of 'the indicator. (b) The electric parameters. The diameter ~ of the acid spot observed on th¢ target in given experimental conditions is an increasing function of th~ intensity I, the voltage (fig. 5) and the exposure duration t. As a firs approxim~ttion O is a linear function of I and x/t'. (c) The geometric parameters. If we now examine the influence of the electrod¢ gap (close to 1 cm) on the shape of the acid spot, we find that it is circular and that its diameter depends on the target-to-point distance D. By varying £
537
J.L Brisset et al. / Chemical reactivity of gaz'eous species in plasma discharge
mm Z
o
5
lO
u~
Fig. 5. Variations of the diameter r~ (mm) of the acidified spot versus the imposed voltage. The point is raised to the negative HV, and the grid earthed. Electrode gap: 5 nun; point-to-target distance: 20 ram; exposuretime: 600 s; indicator: bromothym~lblue. (from - 1 cm up to 20 cm) one obtains [12] a transversal profile of the area exposed to the CAS (fig. 6). This study was performed on liquid targets for demonstration purposes because evidence of the Brons::~ a c i d - b a s e properties are much easier to get than on solid targets. However, our results can be generalized and be applied to the treatments of solids. For example we found in treating Ti foils in a D C point-to-plane corona discharge (device II, with N H 3 as the plasrnagen gas) that exposure of the solid to the ions resulted in etclfing. Holes are observed as a result of the exposure to the ions: they are much larger and deeper with cations than with anions and their formation can be interpreted in terms of a c i d - b a s e reactions. In addition, these results are in gooa agreement with the plasma treatment of AI in humid air reported by G o l d m a n and Sigrnond [13], which is an argument in favour of our approach.
o
~.o
20
3o
.
~
o
Fig. 6. Typical curves indicating the acidified spot diameter ~ = 2R (nun) measured at variou: point-to-target distances D for a given electrode gap d = 7 mm at controlled current intensit3 (60,120,170 ~A) in a~r at atmosphericpressure.
538
3".1- Brisset et al. / Chemical reactivity of gaseous species in plasma discharge
6. C o n c l ~ o n s T h e c h a r g e d a n d chargeless species w h i c h are the m a i n c o m p o n e n t s oi the p l a s m a p h a s e g e n e r a t e d in a D C c o r o n a discharge have b e e n s e p a r a t e d . B o t h k i n d s o f species are e m i t t e d f r o m the p o i n t into two b e a m s . E v i d e n c e o f the chemical p r o p e r t i e s o f these species is given: the a n i o n s are basic, the c a t i o n s a n d the chargeless s p e c i e s induce acid effects. T h e chemical p r o p e r t i e s o f the activated chargeless species have b e e n used to get i n f o r m a t i o n o n the relevant flow, w h i c h involves a n u m b e r o f w o r k i n g p a r a m e t e r s o f the d i s c h a r g e (intensity, voltage, e x p o s u r e d u r a t i o n to the discharge, the e l e c t r o d e gap o r t h e p o i n t - t o - t a r g e t distance) a n d chemical p a r a m e t e r s w h i c h characterize the initial a n d final acidity states o f the substrate. W e are n o w able to forecast t h e d i m e n s i o n s o f the treated area in a c o r o n a d i s c h a r g e a n d evaluate the energy b r o u g h t to the system b y the electric field a n d involved in t h e a c i d - b a s e reaction.
Acknowledgements W e are grateful to the C N R S - P I R S E M
for financial s u p p o r t o f this study.
References [1] It seems to us that the word "chargeless" should be preferred to "neutral" since species beating no charge (e.g., atoms, molecules, radicals) must not be confused with the well-known species with an overall charge 0 (e.g, zwitterionic forms of the aminoacids). [2] R.D. Hancock, B.S. Nakani and F. Marsicano, lnorg. Chem. 22 (1983) 2531. [3] G. Lecayon, Y. Bouizem~ C. Legressus, C. Raynaud, C. Boiziau and C. Juret, Chem. Phys. Letters 91 (1982) 506. [4] R.G. Pearson, Hard and Soft Acids and Bases (Dowden, Hutchinson and Ross, Stroudsburg, 1973). [5] R.S. Drago, Struct. Bonding 15 (1973) 73. [6] W.F. Bailey and A.'3. Monohan, J. Chem. Educ. 55 (1978) 489. [7] D.H. Aue and M.T. Bowers, Gas Phase Chemistry, Vol. 2, i:!d. M.T. Bowers (Academic Press, New York, 1979) pp. 2-51. [8] S.G. Liar, LF. Liebman and R.D. Lenin, J. Phys. Chem. Data ].3 (1984) 695. [91 J. Amouroux, J.L. Brisset, A. Doubla, A. Gicquel, A. Goldil~.a~' and F. Rouzbehi, Scanning Microsc. 1 (1987) 1715. [10] R.S. Sigmond and M. Goldman, Electric Breakdown and Discharges in Gases, Eds. E.E. Kunhardt and L.H. Luesseu, 'NATO ASI Series, Ser. B: Physics Vol. 89b (Plenum, New York, 1983) pp. 1-64. [11] R. Hang, J. Lebas and Y. Teisscyre, J. Phys. D (Appl. Phys.) 15 (1982) 1709. [12] J.L. Bris.;e~, A. Doubla and J. Amouroux, in: Proc. Intern. Symp. on Plasma Chemistry ISPC-8 (Tokyo, Japan, 1987) PI:.. 1793-1798. [131 A. Goldman and R.S. Sigmond, J. Electrochem. Soc. 132 (1985) 2842.
CHEMICAL ~ C T ~ ' ~ IN A PLASMA DIS~RGE
Applied Surface Science 36 (1989) 530-538 North-Holland, Amsterdam
OF THE GASEOUS SPECIES IlN A I R : A N A C i D - B A S E S T U D Y
J.L. B R I S S E T , A. D O U B L A , J. A M ~ O U R O U X Laboratoire des Rdacteurs Chimiques en Phase Plas.ma, ENSCP, II Rue P. et M. Curie, 75005 Paris, France
J. L E L I E V R E Laboratoire de Phys;.cochi;nie des Solutions, ENSCP, 11 Rue P. et M. Curie, 7~(,:7~ ~',ris. France
and M . GOLDIVL'~NN Physique des D~charg,:s, ESE, Plateau du Moulon, 91190 Gif,, France
Received 2 Jm~,e 1988; accepted for publication 14 July 1988
The ions and the chargeless particles which are present in a plasma discharge are responsible for the surface treatments of industrial interest. Their properties can be. considered in chemical terms of acid-base reactions of the relevant species with the substrate. The DC point-to-plane corona discharse at atmospheric pressure was selected as z, convenient source for excited gaseous species (and we focused on the chargeless species) to examine the acid-base properties of various gases or gas mixtures such as air. The substrate exposed to the disciiarge was a polya~:rylamide gel wetted with a basic or acid aqueous solution of coloured indicator. Tnis technique appears as a f~rst step to classify the plasma gases by means of their acid-base propertie~ and to forecast the acid-base behaviour of the gaseous chargeless excited species with respect tc the substrate molecules concerned in a given plasma treatment.
1. ~ n ~ o d u c ~ o n A n u m b e r o f c h e m i c a l e v e n t s w h i c h o c c u r in t h e liquid p h a s e a r e ~ o w n as acid-base reactions since they involve either a proton or an electron pair e x c h a n g e . S u c h r e a c t i o n s m a y also b e p e r f o r m e d in l e s s - c o m m o n c o n d i t i o n s s u c h as t h e p l a s m a p h a s e . T h e y a r e t h u s s t u d i e d f r o m b o t h a t h e o r e t i c a l a n d a t e c h n i c a l p o i n t o f view s i n c e t h e y a r e r e s p o n s i b l e for v a r i o u s a p p l i c a t i o n s , s u c h as e t c h i n g o f silicon d e r i v a t i v e s or t h e p r e p a r a t i o n o f n o n - s t o i c h i o m e t r i c c o m p o u n d s (e.g., WxCv, S i x N y . . . . ) w h i c h a r e d e v e l o p e d o n t h e i n d u s t r i a l scale. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 O Elsevier Science P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
J . L Brisset et al. / Chemical reactivity of gaseous species in plasma discharge
531
The reacting molecules involved in the plasma treatments which are mostly chargeless [1] species fl0 3 or 10 4 times the ion population in a non-equilibrium plasma at atmospheric pressure) are placed in a strong electric field. Therefore an important part of these heavy species is raised to some excited state: so that the gas phase is strongly enriched in excited chargeless species (further referred to as chargeless activated s0ecies or CAS) in which the subsequent electron repartition may differ significantly from that in the ground state, and this is connected with the improved chemical properties. In this paper we shall focus on the acid--base properties of the electronically excited species created in air at atmospheric pressure.
2. The ackl-base prope~es o| the excited specles We shall first consider oxygen as an introductory example. The theory of the molecular orbi~als leads to the MO diagram presented in fig. 1, in agreement with photoelectron spectroscopy. On this sketch, two unpaired electrons are in the high energy ~r*(2p) orbitals, which is related to the paramagnetie properties of the 0 2 molecule. The ground state of O2 is a triplet state (3Z~-). When excited, for example by exposure to the electric discharge, the molecule is raised to a higher energy level, with a new electron distribution which depends on the transferred energy (fig. 2). In the (~Ag) state for example, the two odd electrons are paired in one of ~he ~rg* orbitals and this gives rise to an empty orbital ~rg* which is then able to accept one electron pair given by a donor group. A good illustration of the acid character of O2(1Ag) as defined on the basis of Lewis concepts :s given by the Diels-Adler reaction. 3~
2p
O
.M~ u ~
0 2
2p
O
Fig. 1. Molecular orbit~s of 0 2 molecule from the atomic orbit~s.
532
J . L Brisset et al. / Chemical reactitJity of gaseous sp.~ies in plasma discharge
STATE
~g ~g
leg÷
t
~
ENERGY lWkJ
1 Ag+
~
_
94.3kJ
3 Eg~ ~ 0kJ Fig. 2. Ground and excited states of O2 moleculegivingevidenceof the acid properties in the (~,~g)state. This acid character disappears when 02 is raised to the (tE~') excited state, since the two erg electrons are then unpaired with opposite spins in two ,~-~ orbitals. The example of 02 can be generalized to other species which present the same characteristic two odd electrons, such as organic diradicals in the ground state. Examples are numerous among carbenes, halogenocarbenes, nitrcnes. . . . which are granted with similar electrohic properties and are kl~own for their high reactivity (fig. 3). The carbenes are well known as reaction intermediates in orgarfic chemistry; they are also able to enter metal-carbon 0 bondings as illustrated by the several hundreds of metal-earbene complexes known (e.g. Cr(CO)sCHNMe~). The carbene ligand may be considered as being based on a s p 2 carbon atom with an empty orbital able to interact with a ~- base. When a species is raised from the ground state to an excited state which confers to it the acid character, the distribution of the electrons is modified in the bJgher orbitals as already mentioned. In addition the energy transfer implies a change in the orbital hybridation and hence in the molecular geometry of the species and this is directly related to the improved reactivity of the relevant species. The electric field which induces large changes in the electron densities of the heavy molecules entering reaction in the boundary layer, strongly affects their electron distribution which results in significant dipole formation. In the particular case of ~" bonding molecules, it backs up the acid or base character NH3
[ ] F~ ~
Fund.
Base
Fund.
Boso
Fund. Excited Excited
Radical Acid Radical
[~] ~'~ ~
Fund.
Base
[]
Excited
Radical
NH 2NH NH" NH '~
[] [] []
N N '~
[-~ [] [] []
~ ~ ~ ~ ]
Fig, 3. Electronicsketcb ot the electronrepartitionin some nitrogenderivatives,with evidenceof the Lewisacid-basecharacters.
J.L. Brisset et aL / Chemical reactioity of gaseot,~sspecies in plasma discharge
533
accoramg to its direction, and hence modifies the reactivity of the molecule. The relationship between acid-base behaviour and polarizability has been examined [2], and is supported by polymerization reactions electrochemically performed in convenient solvents [3]. Similar mechanisms may be considered in the gas phase and be related with the increased reactivity of the species. In the particular case of the plasma polymerization processes, a number of applications such as encapsulation or surface protection involve a polymerization step.
3. Strength of the acld-base systems The concepts of electron pair donor or acceptor in'Lroduced by Lewis have received new developments both by Pearson [4] with the '"hard and soft acid-base" theory and by Drago [5] who worked at classifying acids and bases according to their strength. This is in fact a very difficult operation, so ordering is uneasy alid of rather limited reliability. For the bases however, the question profited by sisnJficant progress by selecting the proton as a particular reference species. In this way, Bronsted's defhaitio~, of zcids and bases was resumed. It is thus possible to classify the various unsolvated species in the gas phase according to the value of the standard enthalpy of the associated reaction involving heterolytic cleavage: BH + --, B + H + (PA). The relevant thermodynamic functic.n is called the proton affinity (PA) and must not be confused vAth the hydrogen affinity (HA) which requires homolytic bond scission: BH + --, B + + H (HA). The two energies are related by the ionization potential difference between H and B: PA(B) = HA(D) - (I,~(B) - IP(H)). The proton affinity can also be related to the associated AG o value provided the entropy term T A S ° be calculated, which can be done on the basis of symmetry considerations [6]. The comparison between several bases :Jan be made from the relevant PA values. Up to now hundreds of PA values have been dete,.~-zir.ed and are available in the literature [7,8] which covers a large number of parent molecules and derivatives. Table 1 reports a limited number of illustrative exampies. In the above PA(B) expression, it must be pointed out that the IP(H) correction is the same for all donors. Thus the difference between two bases may be expressed in terms of orbital ep_ergies: the PA's difference depends not only on the difference of HA's - which reflects the H O M O / L U M O overlap
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J . L Brisset et al. / Chemical reactwity of gaseous species in plasma discharge
Table 1 Selec.~ed proton affinity (PA) values (kJ tool - J ) NH ~ OH FCN HCHO HF N, NH 2 CHF3 F2CO CH3F
1690 1630 1550 1475 958 459 494.5 782 615 671 628
Pyridine Aniline NH 3 H 20 H2 F NO C6F6 NF3 Sill4 CH 2F~.
924 876 853.5 697 424 339 531 743 604 648 615
CO CH4 CF4 02 O Ar Xe HCOOH H 202 CH3NH 2
594 552 527 422 498 371 496 718 678 876
difference - but also in the difference of IP's which is related to the difference in H O M O / L U M O energy match. As developed above, the basic properties of gaseous species are classified by means of the associated P A values when they are matched against a given acid (i.e., the proton) b u t other references are available such as BH 3 or B(CH3) 3. The resulting classifications are relevant to gaseous species which are most often in the ground st:.te, b u t up to now, information on the a c i d - b a s e properties of excited species occurring in a plasma is rather scarce in the literature. D a t a o n the etching kinetics of various substrates, which are usually silicon derivatives, are available a n d a correlation between the etching ra:e a n d the basicity of the etching species was proposed [9]. W h e n the plasma gas is a mixture, the etching rate depends on the gas phase composition: for example the rate of Si etching by N F 3 + H2 mixtures increases with the mole fraction of the an~ne. F o r high hydrogen concentrations the N F 3 molecule dissociates into F and the fluororfitrene N F which is less reactive t h a n NF3 towards the acid substratc. Such a n approach appears as a first attempt to explain the collective properties of a plasma a n d leads one to consider a gas mixture in the same way as the solvent mb~tures with respect to the chemical properties. W e consider a more simple situation in two ways: first we focused o n the p r o t o n exchanges (i.e., the Bronsted acidity) a n d second we selected a point-to-plane corona [10] device working in the air as the plasma source because this discharge is a convenient model for a hot plasma.
4. Separation of t ~ ions from file chargeless s F ~ i e s We must first recall that a corona discharge may be realized wtJert two conductors are raised to a convenient range of potentials and their shape is such that the associated electric field lacks for symmetry in ,'.he electrode gap.
J.L. Brisset et al. / Chemical reactivity of gaseous species in plasma discharge
535
vii~
.......
~ T
@ Fig. 4. Schematicdevice(see text) used to separate the ions from the CAS (DC coronadischarge at atmosphericpressurein air; T: target; d: electrodegap; D: point-to-targetdistance;the point is alwaysraisedto the highvoltageand the otherelectrodeearthed).
These conditions are fulfilled in a point-to-plane device. In such a device, the ions and the chargeless species are emitted by the point and are prominent in the ionization region around the point. In the drift region which lies in the electrode gap the prominent heavy species are the ions the sign of which is the same as the active electrode ar,d the chargeless species which are carried on by the electric wind in a thin beam. The acid-base properties of the species generated in tile plasma can be unambiguously demonstrated when the species of each kind are previously separated. This can be performed by means of the various devices sketched in fig. 4, in which the point electrode is always raised to the high voltage (positive or negative since we operate in the DC mode), and the other electrode earthed. Device I was first proposed by Goldmann. It consists in a vertical point arranged perpendicularly to a metallic grid which acts as an ion collector, and its transparency towards ions has been examined by physicists [11]. The chargeless species pass through the grid without noticeable deviation and then react with a target characterized by convenient chemical properties and disposed out of the electrode gap and under the grid. In our experiments, the electrode gap d was fixed in the range 5-15 mm and the point-to-target distance was varied around 20 rnm. Device H also suggested by Goldmann is characterized by a point inclined with respect to the plane which also holds the target (electrode gap close to 1 cm). The separation of the ion and CAS flows depends on the inclination angle: under the point the ions are the prominent species while the CAS are the major population along the point direction. Device 11I is developed in our laboratory from device II, with a point parallel to the plane.
536
J.L. Brisset et al. / Chemicalreactivity of gaseous species in plasma discharge
Device I V has just been tested; the point is arranged according to the axis of metallic frustum of a cone which is the earthed electrode. The C A S flow ca thus be observed along the common symmetry axis, under the cone. The substrate exposed to the discharge consists in an aqueous solution c coloured indicator trapped in a gel (e.g., a polyacrylamide gel prepared ftJ electrophoresis) to limit evaporation. For a number of experiments, th bromothymol blue was selected as the standard indicator.
5. Results and discussion Evidence of the acid-base reaction induced in the target by the ion flow i easily given if we use device II with a target under the point. For a positiv, point, the indicator turns to the acid medium while for a negative poin evidence is given of basic changes. These results illustrate the acid (basic properties of the cations (anions) emitted at the point according to the polarit~ ( + , - respectively) of the active electrode. The chemical effects of the CAS are demonstrated with device I, III am IV: the indicator turns to its acid form, whatever the polarity of the activ, electrode and in large way the nature of the plasma gas (i.e, N2, 02, CO2, CF4 .... ). At this juncture it is noticeable that an acid effect b, observed by exposure to the CAS of a NI-I3 plasma. Also, this acid changq takes place with a non-aqueous target (e.g., acetic anhydride). Most of the experiments were performed with device I, the point electrod~ being raised to the negative high voltage. We determined the influence o various parameters on the diameter of the acid spot observed on a gel targe after exposure to the CAS, and this was a first approach to the quantitativd aspect. The key parameters which were considered are of different kinds: we tint electric parameters (i.e., applied voltage U, current intensity/, exposure tired t), geometric parameters (i.e., electrode gap d, point-to-target distance D) ant chemical parameters (basicity of the target, acidity constant of the indicator) (a) The chemical parameters. For given experimental conditions, the diamete: of the acid spot of the target increases with the acidity of the trapped solutio~ (for a given indicator) or with the acidity constant of 'the indicator. (b) The electric parameters. The diameter ~ of the acid spot observed on th¢ target in given experimental conditions is an increasing function of th~ intensity I, the voltage (fig. 5) and the exposure duration t. As a firs approxim~ttion O is a linear function of I and x/t'. (c) The geometric parameters. If we now examine the influence of the electrod¢ gap (close to 1 cm) on the shape of the acid spot, we find that it is circular and that its diameter depends on the target-to-point distance D. By varying £
537
J.L Brisset et al. / Chemical reactivity of gaz'eous species in plasma discharge
mm Z
o
5
lO
u~
Fig. 5. Variations of the diameter r~ (mm) of the acidified spot versus the imposed voltage. The point is raised to the negative HV, and the grid earthed. Electrode gap: 5 nun; point-to-target distance: 20 ram; exposuretime: 600 s; indicator: bromothym~lblue. (from - 1 cm up to 20 cm) one obtains [12] a transversal profile of the area exposed to the CAS (fig. 6). This study was performed on liquid targets for demonstration purposes because evidence of the Brons::~ a c i d - b a s e properties are much easier to get than on solid targets. However, our results can be generalized and be applied to the treatments of solids. For example we found in treating Ti foils in a D C point-to-plane corona discharge (device II, with N H 3 as the plasrnagen gas) that exposure of the solid to the ions resulted in etclfing. Holes are observed as a result of the exposure to the ions: they are much larger and deeper with cations than with anions and their formation can be interpreted in terms of a c i d - b a s e reactions. In addition, these results are in gooa agreement with the plasma treatment of AI in humid air reported by G o l d m a n and Sigrnond [13], which is an argument in favour of our approach.
o
~.o
20
3o
.
~
o
Fig. 6. Typical curves indicating the acidified spot diameter ~ = 2R (nun) measured at variou: point-to-target distances D for a given electrode gap d = 7 mm at controlled current intensit3 (60,120,170 ~A) in a~r at atmosphericpressure.
538
3".1- Brisset et al. / Chemical reactivity of gaseous species in plasma discharge
6. C o n c l ~ o n s T h e c h a r g e d a n d chargeless species w h i c h are the m a i n c o m p o n e n t s oi the p l a s m a p h a s e g e n e r a t e d in a D C c o r o n a discharge have b e e n s e p a r a t e d . B o t h k i n d s o f species are e m i t t e d f r o m the p o i n t into two b e a m s . E v i d e n c e o f the chemical p r o p e r t i e s o f these species is given: the a n i o n s are basic, the c a t i o n s a n d the chargeless s p e c i e s induce acid effects. T h e chemical p r o p e r t i e s o f the activated chargeless species have b e e n used to get i n f o r m a t i o n o n the relevant flow, w h i c h involves a n u m b e r o f w o r k i n g p a r a m e t e r s o f the d i s c h a r g e (intensity, voltage, e x p o s u r e d u r a t i o n to the discharge, the e l e c t r o d e gap o r t h e p o i n t - t o - t a r g e t distance) a n d chemical p a r a m e t e r s w h i c h characterize the initial a n d final acidity states o f the substrate. W e are n o w able to forecast t h e d i m e n s i o n s o f the treated area in a c o r o n a d i s c h a r g e a n d evaluate the energy b r o u g h t to the system b y the electric field a n d involved in t h e a c i d - b a s e reaction.
Acknowledgements W e are grateful to the C N R S - P I R S E M
for financial s u p p o r t o f this study.
References [1] It seems to us that the word "chargeless" should be preferred to "neutral" since species beating no charge (e.g., atoms, molecules, radicals) must not be confused with the well-known species with an overall charge 0 (e.g, zwitterionic forms of the aminoacids). [2] R.D. Hancock, B.S. Nakani and F. Marsicano, lnorg. Chem. 22 (1983) 2531. [3] G. Lecayon, Y. Bouizem~ C. Legressus, C. Raynaud, C. Boiziau and C. Juret, Chem. Phys. Letters 91 (1982) 506. [4] R.G. Pearson, Hard and Soft Acids and Bases (Dowden, Hutchinson and Ross, Stroudsburg, 1973). [5] R.S. Drago, Struct. Bonding 15 (1973) 73. [6] W.F. Bailey and A.'3. Monohan, J. Chem. Educ. 55 (1978) 489. [7] D.H. Aue and M.T. Bowers, Gas Phase Chemistry, Vol. 2, i:!d. M.T. Bowers (Academic Press, New York, 1979) pp. 2-51. [8] S.G. Liar, LF. Liebman and R.D. Lenin, J. Phys. Chem. Data ].3 (1984) 695. [91 J. Amouroux, J.L. Brisset, A. Doubla, A. Gicquel, A. Goldil~.a~' and F. Rouzbehi, Scanning Microsc. 1 (1987) 1715. [10] R.S. Sigmond and M. Goldman, Electric Breakdown and Discharges in Gases, Eds. E.E. Kunhardt and L.H. Luesseu, 'NATO ASI Series, Ser. B: Physics Vol. 89b (Plenum, New York, 1983) pp. 1-64. [11] R. Hang, J. Lebas and Y. Teisscyre, J. Phys. D (Appl. Phys.) 15 (1982) 1709. [12] J.L. Bris.;e~, A. Doubla and J. Amouroux, in: Proc. Intern. Symp. on Plasma Chemistry ISPC-8 (Tokyo, Japan, 1987) PI:.. 1793-1798. [131 A. Goldman and R.S. Sigmond, J. Electrochem. Soc. 132 (1985) 2842.