Plasma polymerization of iron pentacarbonyl with C2 hydrocarbons

Thin Solid Films, I1 7 (1984) 33-51 PREPARATION

PLASMA POLYMERIZATION HYDROCARBONS N. MOROSOFF,

33

AND CHARACTERIZATION

D. L. PATEL,

Research Triangle Institute. A. L. CRUMBLISS

AND

P. M. Gross Chemical (U.S.A.)

OF IRON

A. R. WHITE*

AND

PENTACARBONYL

WITH

C,

M. UMA%A

P.O. Box 12194, Research Triangle Park. NC 27709 (U.S.A.) P. S. LUGG

Laborarory,

Department

qf Chemistry,

Duke

University,

Durham,

NC 27706

D. B. BROWN

Department

of Chemistry,

(Received March 23,1983;

University qf Vermont, Burlingron, accepted

VT05405

(U.S.A.)

May 8,1984)

Iron-containing plasma polymers are deposited from a plasma using iron pentacarbonyl (Fe(CO),) plus C, hydrocarbons as comonomers and are characterized using a variety of techniques. The plasma polymers are found to contain high proportions of highly dispersed iron oxide as well as lesser quantities of carboxylate and/or ketonate ions and aldehydic and/or ketonic groups. By decreasing the energy per unit mass W/FM part of the iron is deposited in a lower oxidation state complexed to CO and is soluble in organic solvents. The effects of hydrocarbon saturation and of the ratio of hydrocarbon to Fe(CO), are also described and a comparison is made with the product deposited when only Fe(CO), (or Fe(CO), plus H,) is used as the monomer.

1. INTRODUCTION

The ability to lay down thin highly cross-linked pinhole-free films by the process of plasma polymerization has led to a constantly increasing number of investigations of this method of surface modification. Such studies are described in a number of books”’ as well as in recent review articles3g4. A good deal is understood about the connection between the chemical nature of the monomer and the physical parameters of the glow discharge process, on the one hand, and the chemical and physical properties of the resultant plasma polymer, on the other. Rearrangement and decomposition may occur in the reaction pathway leading from monomer to plasma polymer because of the high energy of the electrons which initiate events in a glow discharge. This and the possibility of forming a highly crosslinked network makes it probable that some unusual metal-ligand arrangements may be produced when metal atoms are introduced into a glow discharge polymerization. This paper presents our initial results on the incorporation of iron in plasma polymers made using C, hydrocarbons (ethane, ethylene and acetylene) as monomers. The iron is introduced as iron pentacarbonyl (Fe(CO),) vapor. The * Present U.S.A.

address:

0040-6090/84/$3.00

Department

of Biological

Sciences,

Marshall

University,

Huntington,

0 Elsevier Sequoia/Printed

WV 25701,

in The Netherlands

34

N. MOROSOFF

et d.

nature of the resultant polymer is investigated by Fourier transform IR spectroscopy, electron spectroscopy for chemical analysis (ESCA) and Mossbauer spectroscopy as well as transmission electron microscopy. Previous studies of “metal-loaded” plasma polymers include an early documentation of the feasibility of the plasma polymerization of metal compounds and an investigation of their (non-exceptional) conductivity’. Recently there has been a reawakening of interest in the subject. Such recent studies include those by Kay and coworkers of the incorporation of metals during the plasma polymerization of fluorinated compounds. This is dependent on the formation of volatile fluorides group has studied the during the plasma polymerization process 6. Another production of organotin plasma polymers using volatile tin compounds as heavy metals (notably lead, using monomers’. Liepins et al.a have introduced tetramethyl lead comonomer) into plasma polymers for coatings on microballoons. The latter are designed as targets for lasers in order to achieve a controlled nuclear fusion process. Plasma polymerization of vinyl ferrocene has been used to coat the surface of an electrode for electrochemical applicationsg*“. Finally, Shuttleworth’ 1 has produced “metal polymer dispersions” using either ethylene or tetrafluoroethylene as monomers. Metals (chromium or tin) were introduced as a volatile organometallic comonomer (Cr(CO),) or by sputtering (tin metal). Noteworthy in these studies is the observation that metals are present as oxides or in a non-zero oxidation state (ESCA data)‘,” and transmission electron microscopy data that show metal particles 150-3000 A in diameter l1 . The fact that no particles were observed if sputtered tin and ethylene plasma polymer were codeposited” suggests that under the proper conditions metal atoms can be individually trapped in a plasma polymer matrix. In the following we present a survey of the effect of the ratio W/F (the ratio of the power W to the flow rate F) and of the composition of the monomer feed on the chemical and physical nature of the product formed when Fe(CO), plus a C, hydrocarbon are exposed to an r.f. plasma. The emphasis has been placed on the fullest possible characterization of the products of a limited number of preparative conditions as opposed to the partial characterization of many preparations. There has been no attempt to cover a continuum in the variation of any parameter. 2.

EXPERIMENTAL

DETAILS

Plasma polymerizations were carried out in the inductively coupled reactor previously described’ 2 and schematically illustrated in Fig. 1. When Fe(CO), was employed as a comonomer the monomer feed was introduced into the reactor vessel through an orifice of diameter 2 mm as larger orifices resulted in the deposition of products within the orifice. The pressure was measured using an MKS differential pressure transducer. The transducer was also used to obtain flow rates by measurements of the increase in pressure with time as the monomer is fed into a closed system (no pump-out) of known volume. An electrodeless r.f. discharge was used to initiate the plasma. The r.f. power supply is an r.f. transmitter (Heathkit model DX60B). The output is fed into a linear amplifier (Heathkit SB-200) with a 500 W capacity. The amplifier output is connected to the r.f. coil through a network of tunable capacitors. A Bendix coupler

PLASMA POLYMERIZATION

OF

Fe(CO),

WITH

C,

HYDROCARBONS

35

MONOMER

Fig. 1. Schematic representation of the plasma

circumferentially

in the reactor

reactor showing the location to collect plasma polymer for IR absorption

of aluminum spectra.

foil strips fitted

(model 262; 0.5-225 MHz) was used to measure the r.f. power. The r.f. power supply operates at 13.56 MHz and delivers a continuously variable output power from 0 to 200 W. A tuning circuit, located between the generator coil and the coupler, is used to match the impedance of the discharge vessel and the impedance of the amplifier output. The tuning circuit is adjusted so that the reflected power is maintained at the minimum. The Fe(CO), (Aldrich Chemical Co.) was stored as received at -76°C and warmed to 0 “C during polymerization. It was kept evacuated and in the dark at all times. The deposition rates were obtained gravimetrically. IR spectra were taken using the attenuated total reflection (ATR) technique and a Nicolet 7199 Fourier transform IR spectrometer. The plasma polymers were deposited onto aluminum foil which was pressed against a 45” KRS-5 internal reflection element. ESCA spectra were obtained using a Physical Electronics PHI model 548 AR spectrometer with an Mg Kcr X-ray source equipped with a model 15-255 precision electron energy analyzer. Plasma polymer was deposited onto aluminum foil blanks. A correction for charging was made by reference to the major contributor to the C 1s line at 284.6 eV. The 0 1s and C 1s peaks were resolved into spectral peaks representing various functional groups or molecules (see Section 3 for assignments) using a computer program written by Smith13. Peaks are defined by their height, width at half-height, binding energy at peak maximum and gaussian-lorentzian fraction. For the first several cycles, only the peak heights were allowed to vary. In subsequent cycles first the peak positions and then the peak widths (and positions) were allowed to vary as well. The gaussian-lorentzian fraction was 0.4. Samples for Miissbauer spectrometry were prepared by depositing ironcontaining plasma polymer onto a polyethylene sheet 10 cm x 13 cm in dimensions with the product collected from + 5 to -5 cm from the monomer inlet. The polyethylene sheet was then folded up to present a target of cross-sectional area 2.54cm x 2.54 cm to a “Co source. The Miissbauer spectrometer was of the constant acceleration type and was operated in connection with a 256-channel analyzer in the time scale mode. The source was “Co diffused in rhodium and was kept at room temperature at all times. Spectra were recorded in a horizontal transmission geometry, and each run typically lasted 24 h. Calibrations were made using the known hyperfine splittings in the metallic iron spectrum, and the isomer

36

N. MOROSOFF

et al.

shifts reported here are relative to iron metal at room temperature. In calibrations with thin iron foils, linewidths were typically 0.38 mm s-r. Plasma polymer and iron-containing plasma polymer films were deposited onto thin carbon sheets on micronets, prepared as described elsewhere14, for transmission electron microscopy. All preparations were examined with a Hitachi HU-1 1E electron microscope. All micrographs and diffraction patterns were recorded on Kodak 4489 electron microscope film. For bright field imaging the instrument was operated at 75 kV. Micrographs were taken at a magnification of 72 000x and calibrated with a 52 000 lines in- ’ grating replica. For selected area electron diffraction the instrument was operating at 100 kV and calibrated with a thallous chloride diffraction standard. Diffraction patterns were recorded with lo-30 s exposures. Analyses for iron in an iron-containing plasma polymer or plasma deposit coated on a glass cover slip were carried out as follows. The coating was digested with 1 ml reagent grade nitric acid. The weight of the coating was obtained gravimetrically by comparing the weight of the coated cover slip with its original weight and with that obtained after digestion. (Agreement was obtained for the last two quantities.) The digest was diluted with 0.5% nitric acid. The reagent blank was obtained from a blank digestion carried out in parallel with that of the sample. Analysis was performed by inductively coupled argon plasma atomic emission spectroscopy on an Instrumentation Laboratories Plasma 100 spectrometer. The machine was calibrated with aqueous standards prepared as above and analysis was performed at the 238.20 nm line with a 1.0 s integration time. 3.

RESULTS

AND DISCUSSION

Coatings were formed in a glow discharge using Fe(CO), as the sole monomer (the resulting coating is called an iron-containing plasma deposit) or using Fe(CO), and one of the C, hydrocarbons (acetylene, ethylene or ethane) in various proportions (the resulting coatings are called iron-containing plasma polymers). Variable parameters for the plasma polymerizations include the r.f. power, the pressure in the glow discharge and the flow rates of the hydrocarbon and the Fe(CO),. The conditions are given in Table I, where P,,, and Pg denote the pressure in the plasma reactor prior to and during the glow discharge respectively. These conditions were chosen so as to study the effect of variations in the C, hydrocarbon saturation, the molar ratio of hydrocarbon to Fe(CO), and the r.f. power. The Fe(CO), molar fractions of the total flow rate used were Ox, lo%, 50% and 100%. R.f. powers of 50 and 100 W were employed. More important is the ratio of r.f. power to the flow rate which can be expressed in units of energy per molecule, per atom or per unit mass. The column headed W/F gives the power per molecule in units of joules per mole. It has been suggested that the ratio W/FM (energy per unit mass) is a basic quantity of importance in plasma polymerizations”. (It may be noted that for systems in which the monomers contain only hydrogen, carbon, oxygen and/or nitrogen WjFM may also be considered a measure of the energy per carbon, oxygen or nitrogen atom. This is not the case for our polymerizations as the atomic weight of iron is four times that of carbon.) The importance of this ratio of the r.f. power to the flow rate is most dramatically demonstrated by the observation that plasma

I

Hydrocarbon,

2

0.10

0.10

Fe&W, Hydrocarbon,

Fe(W, Fe(CO), Hydrogen,

5

6 7

Fe&W,

0.45

Hydrocarbon,

0.21

0.10 0.05

0.05

0.225

0.021

0.04s

0

0.45

0.45

rate (cm3 min-‘)

Fe(CO),flow

POLYMERIZATIONS

rate (cm3 min-‘)

4

WW,

Hydrocarbon,

WCO),

Hydrocarbon

3

OF PLASMA

Monomerfeed Totalflow

CONDITIONS

1

Row

EXPERIMENTAL

TABLE

50 50

50

50

100

50

50

6.1 6.7

6.1

1.49

6.4

1.49

1.49

WIF (x10-8Jmol~1)

R.J power (W)

34 68

60

13

142

33

53

W/FM (x10-* Jkg-‘)

Ethane Ethylene Acetylene Ethane Ethylene Acetylene Ethane Ethylene Acetylene Ethane Acetylene Ethane Acetylene

Hydrocarbon

28 15 3 28 12 5.5 10.5 9 4.5 48 53 I 7.2 11.6 8.9

Pg (mTorr)

2.4 9.2 3.5 3.8

20

20

20

20

Pm (mTorr)

38

N. MOROSOFF

et a/.

polymerization can be almost entirely avoided for organic molecules by using equipment and r.f. powers similar to those used by us but flow rates one to three orders of magnitude greater (see for example refs. 16). The product, chemically transformed by the plasma (plasma synthesis), is collected in a cold trap. In the following some general observations are made concerning the pressure and deposition rates obtained for the various plasma polymerizations. This is followed by a section dealing with the characterization of one of the preparations, obtained using a 1: 1 molar ratio of Fe(CO), to ethane, by a variety of techniques. The effects of the ratio of the power to the how rate, of the molar ratios of the reactants and of the degree of hydrocarbon saturation are then described. 3. I. Deposition rate as a function of position in the reactor Deposition rates of plasma polymers were obtained as a function of location within the reactor, These data are presented in Fig. 2. The results were obtained by mass difference for glass cover slips (2.2 cm square) placed underneath the monomer inlet and at distances of 6 and 11 cm from the monomer inlet as shown in Fig. 1. The deposition rates shown in Fig. 2 are expressed as per cent yield per strip 1 cm wide around the circumference of the reactor (i.e. the fraction of the mass fed into the reactor per unit time (obtainable from the flow rate) deposited onto a circumferential strip 1 cm in width (area, 13.8 cm’)). Since the main portion (widest part) of the reactor is 30 cm long, an even deposition rate of 3.3% per strip 1 cm wide along the whole length of the reactor would mean that 100% of the gaseous reactants had been transformed to a solid product within the reactor’s main portion. Expressing the deposition rate as per cent yield in this way allows a direct comparison to be made between plasma polymerizations that employ different monomer flow rates. The data in Fig. 2 indicate that for all hydrocarbons a much more even rate of polymer deposition is achieved in the absence of Fe(CO),. Increasing the ratio W/F for either 10% or 50% Fe(CO), increases the integrated deposition rate and for the

0 (4

11

0 W

0clisu2m

LA,Cml (4

11

0

8

11

(4

Fig. 2. Deposition rate as a function of distance from the monomer inlet for plasma polymerizations of Fe(CO), with (a) acetylene, (b) ethylene and (c) ethane and (d) without C, hydrocarbons: 0, row LO mol.% Fe(CO), in monomer mix; A, row 2,10mol.% Fe(CO),; 0, row 3,10 mol.o/, Fe(CO),; V, row4, SO mol.% Fe(CO),; I, row 5, 50 mol.% Fe(CO),; 0, row 6, 100 mol.% Fe(CO),; V, row 7, 50 mol.% Fe(CO),, 50 mol.% H,.

PLASMA

OF

POLYMERIZATION

Fe(CO),

C,

WITH

HYDROCARBONS

39

10% Fe(CO), plasma polymerization decreases Pp (the pressure in the glow discharge). (In the case of the 50% Fe(CO), plasma polymerizations (Table I, rows 4 and 5) the values of P, vary and so a direct comparison of Pg for rows 4 and 5 is not meaningful.) Deposition rates obtained for the plasma deposit formed when Fe(CO), is used as the sole monomer or in combination with hydrogen are also given in Fig. 2. The same preferential deposition close to the monomer inlet is observed for these cases as is obtained to a lesser degree for the plasma polymerizations of Table I, rows 2-5. The product of the Fe(CO), plus C, hydrocarbon plasma polymerization therefore may be a mixture of the plasma products of the individual components. The identification of the preferred deposition rate with the presence of iron in the plasma polymer is bolstered by the ESCA results shown in Fig. 3. This figure shows that for a given preparation the deposit formed nearest the monomer inlet contains the highest ratio of iron to carbon. The ratios of oxygen to carbon fall in the same sequence, for the various preparation conditions, as the ratios of iron to carbon. This suggests that oxygen from the Fe(CO), or from the atmosphere is incorporated in the plasma deposit. The former would be incorporated during the process of plasma deposition, the latter after exposure of the plasma deposit to the atmosphere. The ESCA results are discussed in detail later.

s

NJ

.30 0

.20 0

A

n

A

.lO 0 I

I 0

4

6

8

10

12

Distance From Monomer Inlet km)

I

0

4

6

8

10

12

Distance From Monomer Inlet km)

Fig. 3. Elemental ratios obtained from ESCA spectra for plasma polymers made using various ethaneFe(CO), monomer mixtures vs. distance from the monomer inlet: 0, ethane, W/FM = 5300 MJ kg- ’ ; 0, 10% Fe(CO),, W/FM = 3300 MJ kg.‘; 0,10x Fe(CO),, W/FM = 14200 MJ kg-‘; a,5077 Fe(CO),, W/FM = 1300 MJ kg-‘; & 50% Fe(CO),, W/FM = 6000 MJ kg-‘.

3.2. Characterization of iron-containing plasma polymer formed at high W/F using a I : 1 Fe( CO), : C, hydrocarbon monomer feed ( Table I, row 5) The iron-containing plasma polymer deposited under the conditions of Table I, row 5 (row 5 plasma polymer) was chosen for detailed characterization. The techniques employed are IR spectroscopy, transmission electron microscopy, Mbssbauer spectroscopy, ESCA, cyclic voltammetry and analyses for iron by

40

N. MOROSOFF

et al.

emission spectroscopy. As the deposition rate data suggest that the product of Fe(CO), plus C2 hydrocarbon plasma polymerization may be a mixture of the plasma products of the individual components, some of the characterization techniques employed on the row 5 iron-containing plasma polymer were also used for the row 6 and row 7 iron-containing plasma deposits. (The term plasma deposit will signify any material deposited in a plasma, the term plasma polymer being reserved for the material deposited when an organic monomer, i.e. a C2 hydrocarbon, is a component of the monomer feed.) 3.2.1. IR spectroscopy IR spectra were taken of plasma polymer laid down on strips of aluminum foil (5 cm x 1.3 cm). The strips were placed along the circumference of the reactor (length of the foil strip perpendicular to reactor’s long axis) at several positions along the reactor length in order to detect variations in the chemical nature of the ironcontaining plasma polymer as a function of the distance from the monomer inlet (see Fig. 1). The IR spectrum for a row 5 (1: 1 ethane: Fe(CO),) plasma polymer is given in Fig. 4 along with the corresponding spectrum for a row 6 (100% Fe(CO),) plasma deposit. The most prominent features of the spectrum for the iron-containing plasma polymer are three broad peaks at approximately 1565,140O and 1040 cm- ‘. Of these, the peaks at 1565 and 1400 cm-’ are the only dominant feature of the IR spectrum for the iron-containing plasma deposit (row 6). These bands correspond to the intense absorptions assigned to the carboxylate ion (at about 1570 and 1400cm-1)‘7-19. A s confirmation of the assignment of these two peaks to the carboxylate ion, the spectrum obtained for iron formate (KBr pellet) is shown at the bottom of Fig. 4 for comparison. Other possible assignments or contributions are discussed later.

3800

3200

2600

Zoo0

1700

1400

1100

800

500

WAVENUMBERS

Fig. 4. Fourier transform IR absorbance spectra obtained using the ATR technique for row 5 (Table I) plasma polymer (curve b) and row 6 plasma deposit (curve c) as deposited under the monomer inlet as well as spectra for iron(H) formate (curve d) and iron(III) acetylacetonate (curve a) both obtained by the transmission method.

Both the literature and other experimental data (see below) suggest that iron oxide will be one of the components present in the iron-containing plasma polymer. Although the IR spectra shown in Fig. 4 for the iron-containing plasma polymer and

PLASMA

POLYMERIZATIONOF

Fe(CO),

WITH

C, HYDROCARBONS

41

plasma deposit do not contain any sharp bands specific to any crystalline form of an iron oxide, the very broad absorption maximum at 800 cm-’ and below and the broad absorption maxima at 3200-3500 cm- ’ are compatible with the presence of iron oxide. Iron oxides generally have a series of absorption maxima in the region 850-500 cm-’ 20-22. A published IR spectrum for amorphous ferric of the spectra we obtained for the oxyhydroxide 20*21 is similar to the background iron-containing plasma polymer and plasma deposit with maxima at high and low frequencies. Differences between the spectra for the iron-containing plasma polymer and iron-containing plasma deposit (shown in Fig. 4) include a broad peak at at 1700 and 1280 cm-’ (locations approximately 1040 cm- 1 and shoulders determined from difference spectra) which are present for the iron-containing plasma polymer but not for the iron-containing plasma deposit. The shoulder at 1700 cm- ’ suggests the presence of an aldehydic or ketonic C=O group and is seen to be particularly pronounced for a high concentration of saturated hydrocarbons in the monomer feed (see later). It appears to be related to the presence of hydrogen in the plasma as it also appears in the spectrum obtained for the row 7 plasma polymer (Fe(CO), plus H, monomer feed). One possible cause for the 1040 cm-’ peak is the presence of Fe,O, 23 or an iron oxide hydroxide in the plasma polymer but not in the plasma deposit. Another possible cause is the C-OH group (alcohol). In addition to a strong absorption at 1050-1150 cm-‘, alcohols should exhibit an OH in-plane deformation in the region 1300-1400 cm-’ and an OH stretch in the region 3200-3700 cm- ‘. Finally, it may be noted that the methyl rocking vibration of acetates occurs at 1009 cm - ’ (ref. 24, p. 23 1). A particularly intriguing possibility is that some of the iron in the ironcontaining plasma polymers may be complexed to P-diketones. The IR spectrum of tris(acetylacetonato)iron(III) is shown at the top of Fig. 4. Intense absorption bands at 1608-1524 and 1390-1309 cm- ’ are attributed to carbonyl groups by Bellamy and Branch25 for a series of P-diketones. However, Nakamato (ref. 24, p. 251) provides a complete table of assignments for tris(acetylacetonato)iron(III) with the intense peaks at 1360 and 1385 cm-’ attributed to a methyl deformation. The propensity of the 1700 cm ’ peak to show up under conditions where hydrocarbon groups would show up in the plasma polymer chain (saturated hydrocarbons) might thus be related to a tendency to intersperse more than one carbon between ketone groups. The more unsaturated C, hydrocarbons would be expected to accept more CO groups leading to a closer spacing and hence more CO groups involved in p-diketone binding of iron atoms. The presence of C-H groups in the iron-containing plasma polymer is denoted by small peaks at 2925 and 2865 cm- ’ (C-H stretch), which are also present in the row 7 product (Fe(CO), plus H, monomer). In the latter case the hydrocarbon may either be the result solely of the presence of CO and H, in a plasma or require the catalytic effect of an iron oxide surface in contact with the plasma. It may be noted that formic acid chemisorbed onto ZnO has been detected as an intermediate in the water gas shift reaction catalyzed by ZnO 26. In the chemisorbed form the carbon of formic acid is bound to the oxygen of ZnO yielding IR absorptions at 1572 and 1369 cm ’ assigned to O-C-O stretching vibrations26. This intermediate form may thus contribute to the broad peaks at 1565 and 1400cm-’ for the row 5 and

42

N. MOROSOFF

et a/.

PLASMA

OF

POLYMERIZATION

Fe(CO),

C, HYDROCARBONS

WITH

43

row 7 products but not for the row 6 product (no hydrogen present in this plasma deposit). 3.2.2. Transmission electron microscopy Row 5 plasma polymer was deposited onto thin carbon sheets (30-100 8, in thickness) supported by micronets on top of copper microscope grids. Plasma polymer was deposited onto the carbon in three thicknesses: area1 densities, 1,5 and 10ngcm~2. Transmission electron micrographs were obtained for all three preparations and showed the presence of a grainy structure with dense particles of diameter 20-30 A. Such a transmission electron micrograph is shown in Fig. 5 together with one obtained for plasma polymer prepared in the same way using ethane (no Fe(CO),) as the monomer. It is clear that the inclusion of Fe(CO), in the monomer feed causes the growth of dense particles. An electron diffraction pattern was obtained for the row 5 film of area1 density 5 ng cm-’ and found to be consistent with that of iron oxide (Fig. 6). An analysis of the packing density and size of the dark spots in the photographs for the 1 and 5 pgcm-’ films revealed that such heterogeneities account for only So/, of the iron in the film, if it is assumed that the heterogeneities are iron oxide. The remainder of the iron can therefore be assumed to be dispersed evenly throughout the matrix in a non-crystalline form.

!

1.5

t

IJ 0.5

3;

1

u

’ r I

f 0.5

lj 1.5 I

L

L-0.1

0.2

0.3

0.5

0.6

v

0.7

0.8

0.9

1.0

1.1

1.2

0.4 2

l/d

(ii’)

Fig. 6. Plot of log ! vs. l/d for y-Fe,O, as given in ref. 27 and for electron diffraction patterns of row 5 iron-containing plasma polymers. For the latter log I is measured from the top of the figure for intensities estimated as very strong, strong, medium and weak.

3.2.3. Miissbauer spectroscopy The Mossbauer spectrum for the row 5 iron-containing plasma polymer (Fig. 7) is consistent with the presence of high spin iron(II1) as the sole iron component. Although crystalline Fe,O, normally produces a six-line Mijssbauer spectrum, it has been shown that Fe,O, spectra are sensitive to particle size. FezO, crystallites smaller than 100 A in diameter are found to be superparamagnetic yielding a

44

N. MOROSOFF et a[.

I

-4

I

I

1

-3

-2

-1

I

I

1 0 Source Velocity Immlsecl

I

I

I

2

3

4

I

Fig. 7. Miissbauer spectrum obtained for row 5 iron-containing plasma polymer at room temperature (C, monomer, ethane). Plasma polymer was deposited at locations ranging from - 5 cm to + 5 cm from the monomer inlet onto polyethylene film.

Miissbauer spectrum identical with that shown in Fig. 7 28. Mossbatter spectra taken at several temperatures can be used to derive the size of the iron oxide crystallites 28mm32 . The larger the crystallite size, the higher is the temperature up to which the six-line spectrum predominates. However, for iron oxide crystallites of size 40 A a Miissbauer spectrum with a very small contribution from the six-line spectrum has been reported even at 55 K 32. Thus the Mijssbauer results reported here confirm that iron(III) oxide present in the iron-containing plasma polymer is highly dispersed. 3.2.4. Electron spectroscopy for chemical analysis ESCA spectra were obtained for selected plasma polymerizations. Data (elemental ratios and binding energies by inspection) are presented in Table II following the sequence used in Table I. Every row except rows 6 and 7 in Table I is represented. ESCA spectra obtained for one of the row 5 preparations are shown in Fig. 8. These results yield information regarding the top 50-100 8, of the plasma polymer film in contrast with the Fourier transform IR ATR results which sample the entire thickness of the 1000 A thick films. In particular, the ESCA results may give greater weight to elements of the film highly accessible to oxygen (in the ambient air) than the Fourier transform IR ATR results do. The Fe 2p,,, binding energy for all samples is approximately 7 11 eV indicating that the iron atoms are electron deficient relative to metallic iron. The binding energy agrees with that observed for iron(III), e.g. 711 eV for Fe,O, 33-35 and the binding energy observed for Fe0 is 711.5eV for Fe30, 34. For comparison 709.7 eV 33*34,that for Fe(CO), is 709.4 eV 36 and that for metallic iron is 707 eV 33. The C is spectrum consists of a major peak at 284.6 eV (as observed for graphite and alkanes) with a small peak at 288 eV (indicating binding to oxygen). The 0 IS

as,strong;

5

4

ROW

ELECTRON

TABLE

SPECTROSCOPY

FOR CHEMICAL

0.21

Fe@), Ethane,

w, weak.

Fe(W,

Ethane,

Fe0X

Ethane,

0.1

0.47

0.45

FeW), Acetylene,

FtiCO),

0.045

0.45 0.45 0.45

Ethane Acetylene Ethylene,

0.05

0.25

0.02 1

0.045

(cm’min-‘)

(cm3 min-‘)

50

50

100

50

50 50 50

(W

power

R.f.

DATA FOR PLASMA

fiott

Fe(CO),

rate

TotalJo\+

ANALYSIS

rate

Plasma polymer identification

II

11

0.30

0.03

0.40 0.14

0.09

0

0.65

0.80

0.20

‘0.26

0.36

0.37

0.12 0.10 0.34

PI [Cl

0.02

0.15

0.14

0

2

0.014

II

0.07

0.04

0.05

cc1

[Fe]

0.03

inlet

Elemental ratio

7

0

3

3

0

0

(cm)

mon*mer

Disranwfrom

POLYMERS

9.1

6.3

4.2

3.8

5.6

14.7

10.0

5.0

9.1

6.7

PI WeI

-

Is

284.6 (s) 288.4 (w)

284.6 284.6 284.6 (s) 288.1 (w) 284.6 (s) 286.65 (w) 284.6 (s) 287.5 (w) 284.6 (s) 287.8 (w) 284.6 (s) 287.5 (w) 284.6 (s) 288.5 (w) 284.6 (s) 288.3 (w) 284.6 (s) 287.9 (w) 284.6

c

711.6

710.7

710.7

711.2

711.2

711.6

711.5

711.2

711

710.5

Fe 2p,,,

Binding energy” (eV)

-

53 1.65 526.95 532

531.6 530.4 531.7

531.8

532.15 (s) 530.35 (s) 532 (s) 529.9 (w) 532.4

531.65

533 532.7 531.9

0 Is

2.3

2.9

3.6

2.9

2.5

2.8

2.8 2.8 3.2

(eV)

0 Is peak halfiidrh

46

Le-5e-G (a)

N. MOROSOFF et U/.

I

I

6

545

(b)

I

535

Binding

Energy

525

(ev)

740

730

720

710

700

(c)

Fig. 8. ESCA spectra obtained for the row 5 iron-containing plasma polymer monomer inlet and assigned to (a) C Is,(b) 0 1s and(c) Fe 2p, ,z and Fe 2p,.,.

deposited

under

the

spectrum consists of a peak at approximately 532 eV with a shoulder at 530 eV, the latter indicating a charged oxygen atom. In the following, only the results for the row 5 iron-containing plasma polymer will be discussed, reference being made to the other preparations later. There is an obvious decrease in the molar ratios of iron to carbon and oxygen to carbon with increasing distance from the monomer inlet, as noted earlier. We have interpreted peak shapes so as to obtain a semiquantitative indication of how oxygen is distributed in the iron-containing plasma polymer. The C 1s and 0 1s peaks were resolved into the minimum number of peaks of halfwidth 1.5-2.3 eV required to reproduce the peak shape with the results given in Table III. In interpreting these results, peak positions noted for carbon bonded to oxygen and for oxygen bonded to carbon or iron are as given below. For C 1s this resulted in three peaks. Of these the peak at 284.6 eV represents an aliphatic and/or graphitic carbon, the peak at 286 eV is indicative of hydroxyl groups attached to the carbon while the peak at 288 eV denotes aldehydic or ketonic carbonyl groups or a carboxylate. The 0 1s peak was easily resolved into two peaks of which that at 530.2 eV is considered to be indicative of iron oxide and that at 532 eV of oxygen bonded to carbon or of surface-adsorbed water. Justifications for these assignments from the literature and by ESCA analysis of a model compound (iron formate) are as follows. The C 1s peak is conventionally resolved into four components at 284.6eV (aliphatic or graphitic carbon), 286 eV (C-O), 287.5 eV (C=O) and 288 eV (carboxylate group in an ester or carboxylate ion)37P39. The 0 1s peak would be conventionally resolved into four components at 530.2 eV (bulk Fe,03)40941, ion), 533.1 eV (C=O in the 532.3 eV (surface-adsorbed water4’ or carboxylate carboxylate group of an ester) and 534eV (C-O in the carboxylate group of an ester)38,39. The assignment for the carboxylate ion was made by obtaining an ESCA spectrum for iron formate, in which a C 1s peak at 288.3 eV (halfwidth, 2 eV) and an 0 1s peak at 532 eV (halfwidth, 2.3 eV) were observed. On the basis of these assignments and the data in Table III, it is concluded that at locations close to the monomer inlet most of the iron is bonded directly to oxygen.

III

Fe(CO),

Ethane,

FeW),

Ethane,

rate

rate

0.1

0.47

0.05

0.25

(cm3 min _ ‘) (cm3 min

Fe/CO),

.flOW

Tom/

.pow

SPECTROSCOPY

Monomer

OF ELECTRON

feed

CLIRVE ANALYSIS

TABLE

‘)

50

50

;;,w

R$

1s AND 0

87 67

11

80

82

71

284.6 e V

7

0

2

0

inlet (cm)

monomer

from

c

1s PEAKS

18

285.4 eV)

286.2 eV) lO(at

14 (at

9

5 286 e V

Area (“A) at energies of

ANALYSIS

Disrunce

FOR CHEMICAL

to Fe peaks

1.6 288.4 eV) 9 (at 0.6 288.4 eV) 1 (at 0.78 288.2 eV) 3 (at 1.5 287.5 eV) 14 (at 4.7 288.4 eV)

23 (at

z288eV

Area ratio qfC(288eV)

6 (at 531.2 eV)

24

25 (at 530 eV) 37

30

z530.2eV

eV

94

76

75 (at 531.6eV) 63

70

-532

Area (:,A) at mergie.r of __

0.5

1.5

1.6

0.95

1.7

to Fe peaks

0 1530.2 e VI

Area ratio of

48

N. MOROSOFF et a/.

The ratio of oxygen under the 530.2 eV peak to iron is close to 1.5 as would be expected for Fe,O,. There may be some contribution from FeOOH and, if so, not all iron is in the form of Fe,O,. However, the ratio of carbon at 288 eV to iron is only 0.8 suggesting that not more than 25% of the ferric iron is in the form of iron carboxylate (or 13% as the P-diketonate). (The ratio of carbon to iron in Fe(-COO), is 3/l while that in Fe( -COCHCO-), is 6/l.) At 11 cm from the monomer inlet the ratio of carbon under the peak at 288 eV to iron increases to the point that at least some of this carbon must be in the form of carbonyl groups not bonded to iron. There is also a drop in the relative amount ofoxygen directly bonded to iron. The ratio of carbon under the 286 and 288 eV peaks to oxygen under the 532 eV peak is close to unity (1.2-0.8) for all three row 5 samples, suggesting that oxygen is bonded to carbon either as C-OH or as C=O. This, in turn, implies that carboxylate groups (oxygen-to-carbon ratio, 2) are a relatively minor component of the C-O population. 3.2.5. Analysis,for iron by atomic emission spectrometry The deposits formed on glass cover slips, placed under the monomer inlet, at the conditions of Table I, row 5, row 6 and row 7 respectively, were digested in nitric acid with subsequent analysis for iron by atomic emission spectroscopy. The weights per cent of iron for the various monomer feeds are as follows: Fe(CO)S plus C2H,

27% Fe

Fe(CO)S

46% Fe

Fe(CO)S plus H,

31% Fe

In all cases the amount of iron is below that expected for Fe,O, (70%) or FeO(OH) (64%). This indicates that iron oxide is diluted with a hydrocarbon-containing organic polymer (rows 5 and 7) and with a non-hydrogen-containing entity such as an oxalate or acetylene dicarboxylate (row 6). The iron content obtained by this method for row 5 (27%) is comparable with a value of 22% obtained from ESCA data in Table III. The fact that the ESCA value is smaller agrees with the known sources of error in the ESCA estimate. 3.2.6. Electrochemical activity and solubility of iron-containing plasma pol>lmers Plasma deposits made under the conditions of row 5 and row 6 have been found to be redox active42.43 with redox potentials consistent with the oxidation and reduction of iron oxide44,45 by electrochemical means. Graphite electrodes coated with these plasma deposits have been found to induce electrochemical deposition of highly adherent Prussian blue from a hexacyanoferrate solution42.43. Such electrodes were exposed to the same electrochemical treatment in a solution of KNO, electrolyte (no hexacyanoferrate) to determine the extent of leaching of iron from the electrode under such circumstances. A major difference between plasma deposition in the presence and absence of ethane was discovered: the row 5 plasma polymer (monomer feed, Fe(CO), plus ethane) exhibited negligible leaching of iron to the electrolyte; the row 6 plasma deposit (monomer feed, Fe(CO),) exhibited leaching of 25% of all the iron on the electrode under similar experimental conditions. 3.2.7. Origin of oxygen in iron oxide The presence of iron oxide in the iron-containing plasma polymer raises the

PLASMA

POLYMERIZATIONOF

Fe(CO),

WITH

C, HYDROCARBONS

49

question of whether the iron oxide is formed in the plasma reaction or after exposure of the plasma polymer to air. This question is currently under investigation. Preliminary results indicate that reaction with atmospheric oxygen does contribute. The evidence includes the observation of an irreversible weight gain in a freshly deposited iron-containing plasma polymer on exposure to oxygen and an increase in the IR absorption over the wavelength range 800-400 cm- ’ during the course of 1 day. 3.3. The effect of W/FM The ratio of the power to the flow rate can have a dramatic effect on the nature of the metallated plasma polymer as seen in the contrast in the properties of the products produced under the conditions of row 4 and row 5 defined in Table I. The energy fed into the plasma per atom or per gram is the lowest in the table for row 4. Contrasting row 4 and row 5 the residence time in the plasma is shorter for the row 4 conditions than for those of row 5. Finally the pressure in the plasma is the highest in Table I for row 4. As the average electron energy is proportional to the inverse of the pressure46 (other conditions being equal), the average electron energy would be expected to be the lowest for row 4. The result is that only the row 4 product has CO complexed to iron, albeit IR and ESCA spectroscopy indicate that this moiety exists in a matrix chemically similar to the row 5 iron-containing plasma polymer. Supporting evidence is given in this section, in which the detailed characterization of the row 4 iron-containing plasma polymer (i.e. the product deposited under the conditions of row 4 of Table I) is presented. Appropriate comparisons are made with the row 5 iron-containing plasma polymer, whose characterization was described above. The other opportunity to note the effect of the power is in the comparison of the row 2 and row 3 products where no major change between the resulting two ironcontaining plasma polymers is noted. The IR characterization of these products will be described in Section 3.4. 3.3.1. IR spectroscopy The IR spectra obtained for the row 4 plasma polymer are shown for the case of ethane in Fig. 9. There is obviously a pronounced variation in the chemical nature with position. A very heavy deposit forms underneath the monomer inlet (see Fig. 2). This polymer contains CO complexed to iron as evidenced by the intense absorption peak at 1970 cm- ‘. As the plasma polymer is formed in a partial vacuum (less than 0.1 Torr) and then exposed to a vacuum for 15 min after formation of the polymer, it is unlikely that the complexed carbonyl is indicative of trapped Fe(CO), which is relatively volatile. It is more likely that the CO is bound to oligomers or polymers containing iron. The breadth of the peak and the fact that its tail extends to 1760 cm- 1 indicates that CO is complexed to iron in a variety of environments including bridging adjacent iron sites. The material containing CO complexed to iron adheres poorly to aluminum or glass substrates, in contrast with the iron-containing plasma polymers prepared according to the conditions of rows 2,3 and 5 in Table I, in which complexed CO is non-existent or at best a minor component. The latter are strongly adherent to glass and aluminum substrates and insoluble in organic solvents, attesting to their highly cross-linked nature. The solubility of the poorly adherent plasma polymer (ethane as

50

3800

N. MOROSOFF et a/.

3200

2600

2ooo~

1700

1400 _L

1100 AP700A-

500

WAVENUMBERS

Fig. 9. Fourier transform IR absorbance spectra obtained for row 4 (Table I) plasma polymer 2 cm from the monomer inlet (curve c), 4 cm from the monomer inlet (curve b) and 11.5 cm from the monomer inlet (curve a). The baselines of the spectra have been straightened and the spectra therefore show only relatively sharp peaks. The C, monomer was ethane.

comonomer, row 4 of Table I) could not be determined gravimetrically because of the loss of solid polymer in the solvent (as an insoluble powder). However, the material was found to be more soluble in acetone than in toluene. At sites removed from the monomer inlet the “row-5-type” iron-containing plasma polymer matrix (of the row 4 iron-containing plasma polymer) becomes more predominant as seen in the other two spectra in Fig. 9 (plasma polymer formed with ethane comonomer). The row 4 iron-containing plasma polymer made using acetylene yielded similar results, a heavy deposit close to the monomer inlet yielding an IR spectrum indicating the presence of CO complexed to iron. 3.3.2. Miissbauer spectroscopy A Miissbauer spectrum was obtained for row 4 plasma polymer deposited directly below the monomer inlet (Fig. 10). It reflects the two-phase nature of the product indicating that the high spin iron(II1) found for row 5 is present. The other component may be iron(I1) or zerovalent iron in an organometallic complex. 3.3.3. Electron spectroscopy for chemical analysis The analysis of the ESCA spectra obtained for the row 4 plasma polymer is presented in Tables II and III. The concentration of iron and of oxygen relative to carbon is considerably greater in the row 4 preparation than in the row 5 ironcontaining plasma polymer (see Table II). The results of the analysis of the C 1s and 0 1s peak shapes (Table III) indicate that for the material deposited directly under the monomer inlet there is a greater proportion of the C 1s peak at 288 eV than for any of the other samples. This is the expected position for coordinated CO. The C 1s peak position for Fe(CO), in the solid phase is 287.8 eV (the C 1s peak position for hexane is 284.6 eV) and the 0 1s position is 533.8 eV 36. There is no peak at 533.8 eV in our spectrum and it was initially assumed that the carbonyl oxygen is included in the 532 eV peak. Conversion of data for gaseous carbonyl in the literature4’ to binding energies to be expected in the solid by the method described in ref. 36 reveals that the 0 1s position is as low as 532 eV for some carbonyl complexes but that there is a corresponding decrease in the C 1s position. However, the closest agreement (for

PLASMA

Fe(CO),

OF

POLYMERIZATION

51

C, HYDROCARBONS

WITH

6 = 1.2 mm/set

AE = 2.5mmhc

AE = 0.8mm/s%

, .. .

I

.

‘. .

i I

‘.

I

1

I

-4

-3

-2

I

I

I

1 -1 0 SourceVelocity Imm/secl

1

1

I

2

3

4

Fig. 10. Miissbauer spectrum obtained for a row 4 iron-containing monomer, ethane). Plasma polymer was deposited at locations ranging monomer inlet onto polyethylene film.

I

plasma polymer at 77 K (C, from - 5 cm to + 5 cm from the

both C 1s and 0 1s peak positions) is found for CO complexed to iron to which a hydrocarbon is also complexed, i.e. for such compounds as C,H,Fe(CO),, C,H,Fe(CO), and C(CH,),Fe(CO),. For these compounds the observed C 1s peak position is 0.1-0.5 eV less than 288 eV and the observed 0 1s position is 0.8-1.0 eV greater than 532 eV. Even better agreement of the 0 1s position for CO complexed to iron is observed for neutral iron carbido clusters whose ESCA spectra have been reported recently 48. For [Fe,C(CO),,] and [Fe,(CH)(CO),,H] the carbonyl C 1s peak is observed at 284.6 eV and the 0 1s peak is observed at 532.2 eV. For these complexes the Fe 2p 3,2 peak is observed at 709.2 eV. Comparison of the Fe 2p,,, peaks for the row 4 and row 5 plasma polymers reveals a shoulder at about 709 eV present for the row 4 sample but not the row 5 sample. Some o-bonded complexes of mercury, tellurium, bismuth, cadmium, tin and germanium with methyl, trifluoromethyl, trifluorosilicon or trifluorosulfur groups have been prepared by the deposition of the required metal atoms in a methane plasma (or in a plasma of substituted methane or methyl compound, or of the corresponding silicon or sulfur analog) environment49, so that the formation of iron carbido clusters or carbonyl alkyl clusters in an Fe(CO), plus C2H, plasma is not unreasonable. Additional support for a structure similar to the carbido clusters in the row 4 plasma polymer comes from the ratio of oxygen atoms under the 532 eV peak to carbon atoms under the 288 and 286 eV peaks. This ratio is 2.4 for both row 4 samples. Since this is higher than can be accounted for on the basis of oxygen bound to carbon, one possible explanation is that since the carbonyls are present in iron carbido clusters, their contribution to the total C 1s spectrum is at 284.6 eV 48 not at 288 eV. The relatively intense peak at 288 eV is then indicative of carbido carbons4’

52

N. MOROSOFF

et al.

or of a larger relative population of P-diketone, alcohol or carboxylate groups for the row 4 product than for the row 5 product. The alternative explanation is that some of the oxygen atoms giving rise to the 532 eV ESCA peak may have their origin in water adsorbed tenaciously on iron oxide41. If this is so, the row 4 sample may be more accessible to water than the row 5 preparation. The ratio of oxygen to iron in iron oxide or iron oxyhydroxide (i.e. 0 1s at 530.2 eV) is 1.7 for the sample deposited directly under the monomer inlet, the corresponding ratio for carbon in C=O or C-0 groups (i.e. at 288 eV) to iron is 1.6 (Table III). This means that considerably more iron is bonded directly to oxygen than CO, carbonyls or carboxylates as there should be 1.5-2 atoms of oxygen per iron atom in iron oxide but 3-4 CO, carbonyl or carboxylate groups per iron atom. However, this reasoning is invalid if the carbidocarbonyl groups are predominant as appears to be likely. The ejfkct of’ the molar ratio of’C, to Fe(CO), and qf hydrocarbon unsaturation The characterization of the iron-containing plasma polymers made using 10% Fe(CO), in the monomer feed (rows 2 and 3 of Table I) was carried out exclusively by IR spectroscopy (except for the use of ESCA data for elemental ratios; see Table II). The IR spectra ofthese preparations were found to contain features common to both the row 5 iron-containing plasma polymer and the C2 hydrocarbon plasma polymers (row 1 of Table I). The IR spectra of the latter are discussed first. Spectra for the purely hydrocarbon plasma polymers (row 1 in Table I) are shown in Fig. 11. The most prominent absorption peaks include one at approximately 2900 cm - ’ caused by the C-H stretch (2925cm-’ and 2865cm-’ indicating C-H stretch in CH, and CH,), one at 1376cm-’ attributed to symmetrical C-H deformation ofa methyl branch and one at 1458 cm-’ attributed both to the asymmetrical C-H deformation of a methyl branch and to the C-H deformation of a methylene group 17*18. Assignments for other minor peaks of such plasma polymers are given in the literature50*5’. The IR spectra obtained for polymers obtained using 10% Fe(CO), in the 3.4.

WAVENUMBERS

Fig. 11. IR spectra obtained for C, hydrocarbon plasma polymers (plasma polymerization conditions as given in Table I, row 1; plasma polymer deposited onto aluminum foil strips laid along the length of the reactor): curve a, ethane; curve b, ethylene; curve c, acetylene

PLASMA

POLYMERIZATIONOF

Fe(CO),

WITH

C,

HYDROCARBONS

53

monomer feed (rows 2 and 3 of Table I) yielded spectra (Fig. 12) with features characteristic of both the purely hydrocarbon plasma polymers (Fig. 11) and those made with a 100% or 50% Fe(CO), feed (Fig. 4). It may be noted that the peaks in the range 1500-1775 cm- 1 are more intense relative to the other absorption peaks in the spectrum than is the case for the hydrocarbon plasma polymer spectra in Fig. 11. In part peaks in this region of the spectrum may be ascribed to the presence of carboxylate or P-diketone groups as was done for plasma polymers made with a 500/, Fe(CO), feed. However, comparison with Fig. 4 demonstrates the presence of an additional peak for the 10% Fe(CO), plasma polymers at 1700 cm-‘. This may be assigned to the C=O stretch for ketones, aldehydes and carboxylic acids or possibly to a C=C bond. Since the peak is most intense (relative to other peaks in the spectrum) when the comonomer is ethane and least intense when acetylene is the comonomer, the assignment of the peak to a carbonyl group appears to be correct. As the supply of hydrogen is decreased, either by using a hydrogen-poor hydrocarbon or by increasing the molar ratio of Fe(CO), to hydrocarbon, the number of carboxylate ions or I%diketone groups increases, and the number of carbonyl groups in aldehydes, free ketones or carboxylic acids decreases. This trend is also consistent with the observation that a shoulder at 1700cm-’ (location obtained from a difference spectrum) is observed for the row 7 iron-containing plasma polymer (monomer feed, Fe(CO), plus HZ) but not for the row 6 ironcontaining plasma deposit (monomer feed, Fe(CO),). It has also been noted that the carbonyl peak is most intense furthest from the monomer inlet (see for example Fig. 9).

WAVENUMBERS

Fig. 12. IR spectra obtained for iron-containing plasma polymers (plasma polymerization conditions as given in Table I, row 2; plasma polymer was deposited onto aluminum foil strips positioned along the length of the reactor): curve a, ethane; curve b, ethylene; curve c, acetylene. The baselines of the spectra have been straightened and the spectra therefore show only sharp peaks.

The plasma polymerizations of 10% Fe(CO), were run at two ratios of power to flow rate. The IR spectra show no clearly discernible effect of this parameter. One other feature characteristic of the IR absorption spectra of 10% Fe(CO), plasma polymers (Fig. 12) is worthy of note. These are peaks characteristic of metal carbonyls. An absorption peak at 2000 cm -i is observed for the plasma polymer obtained using 10% Fe(CO), in acetylene (50 W). In this case a pulsating glow

54

N. MOROSOFF

et al.

discharge was obtained as a result of the extremely low pressure obtained in the glow discharge. Fe(CO), may have been trapped in the polymer during periods when the plasma was not lit. Another feature characteristic of polymers made with 10% Fe(CO), is a small sharp absorption peak at 2200 cm- ‘. This is characteristic of CO adsorbed onto metal oxides”. When CO is adsorbed onto iron, an adsorption band is observed at 1960 cm-‘. However, on exposure of this system to oxygen this band is replaced by a band at 2 128 cm - 1 (ref. 53). A peak at 2250 cm _ 1 has been reported recently for CO adsorbed on an Ni/AI,O, catalyst at 6 Torr and 150 “C 54. 4.

CONCLUSIONS

(1) Iron-containing plasma polymer made using Fe(CO), plus C, hydrocarbon in a l/l molar ratio and at a sufficiently high W/FM (row 5) contain high spin iron(III) predominantly in the form of iron oxide. Some of the iron appears to be bound to groups containing carbon and oxygen as a counterion to a carboxylate ion or complexed to a P-diketone. This conclusion is based on IR, Mossbauer, ESCA and atomic emission data. ESCA data suggest that complexation to a P-diketone is more probable. (2) The hydrocarbon part of the above iron-containing plasma polymer contains C-H, C-OH and C=O bonds as shown by IR and ESCA data. It is probable that the l3-diketone or carboxylate groups that bind iron are part of the cross-linked hydrocarbon plasma polymer matrix. (3) Some of the iron oxide (of the order of 5%) is present as crystallites of maximum diameter 20 A. The rest is evenly dispersed throughout the plasma polymer matrix. This conclusion is based on results obtained using electron microscopy and Miissbauer spectroscopy. (4) The iron-containing plasma polymer described above (row 5) was deposited onto graphite electrodes and shown to be redox active. It also promotes the electrochemical deposition of a highly adherent deposit of Prussian blue which presumably binds to the iron atoms in the iron-containing plasma polymer42,43. (5) The hydrocarbon portion of the iron-containing plasma polymer tends to bind the iron to the graphite electrode in water. Leaching experiments described elsewhere42a43 have shown that negligible leaching of iron occurs in a cyclic voltammogram cell when the electrode is coated with row 5 plasma polymer, in contrast with the extensive leaching for a row 6 (sole monomer, Fe(CO),) plasma deposit. (6) A dramatically different product, containing iron coordinated to CO in a row-5-type matrix is obtained by decreasing the ratio W/FM of the power to the flow rate. The portion of this product containing iron complexed to CO is soluble in organic solvents. The iron complexes may be similar to iron carbido clusters. These conclusions are based on IR, ESCA and Mijssbauer data. It may be noted that the ability to form a product in which a portion of the CO remains complexed to the metal by working at sufficiently low W/FM values has also been noted on exposure of n5-cyclopentadienylcobaltdicarbonyl to a glow discharge, resulting in the formation of a trimeric cluster compound”. (7) The effect of increasing the molar ratio of C, hydrocarbon to Fe(CO), is twofold. The first effect is that the row 5 iron-containing plasma polymer is diluted in

PLASMA

POLYMERIZATIONOF

Fe(CO),

WITH

C, HYDROCARBONS

55

a hydrocarbon plasma polymer matrix. This is shown by IR and deposition rate data. The second effect is related to the increase in H, in the plasma which accompanies the increase in C, hydrocarbon and is described next. (8) The effect of H, in the plasma is to produce aldehydic or ketonic carbonyl groups not bound to metal. Thus the product formed in an Fe(CO), plus H, plasma (row 7) yields an absorption at 1700 cm-’ (IR) not found for the row 6 product (sole monomer, Fe(CO),). This peak is also present in the IR spectrum of the row 5 ironcontaining plasma polymer but is increased (reiative to other absorptions) by increasing the molar ratio in the monomer feed of C2 hydrocarbon to Fe(CO), or by increasing the saturation of the C, hydrocarbon. In addition, the iron-containing plasma polymers (Fe(CO), plus Cz hydrocarbon feed) appear to contain OH groups not present in the row 6 or row 7 products (monomer, Fe(CO), or Fe(CO), plus HZ). These conclusions are based on IR data. (9) All iron-containing involatile species are formed relatively rapidly in the plasma and are therefore preferentially deposited close to the monomer inlet. The product formed at points furthest from the monomer inlet is relatively iron poor and contains a larger proportion of aldehydic or ketonic carbonyl groups. This conclusion is based on deposition rate, IR and ESCA data.

ACKNOWLEDGMENTS

The authors gratefully acknowledge useful discussions with Professor K. S. Spartalian, Department of Physics, University of Vermont (where Mossbauer spectra were run), regarding the effect of crystallite size on Mossbauer spectra. ESCA spectra were obtained and the curves analyzed at the Department of Chemistry, University of North Carolina at Chapel Hill. X. B. Cox and D. Griffis obtained ESCA spectra. The authors are particularly grateful to S. Simko and X. B. Cox for help in curve analysis of ESCA spectra. Analyses for iron using atomic absorption and atomic emission spectra were run by D. F. Natschke at the Research Triangle Institute. The very careful experimental work of G. Cessna who prepared many of the plasma polymer samples and obtained their IR spectra is acknowledged. This work was made possible by the U.S. National Science Foundation, Grant CPE-8006805.

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I 2

3 4 5 6 7

J. R. Hollahan and A. T. Bell. Techniques and Applications of Plasma Chemistry, Wiley,New York, 1974. M. Shen (ed.), Plasma Chemistry of Polymers, Dekker,New York, 1976;J. Macromol. Sci., Chem., IO (1976) 367. M. Shen and A. T. Bell (eds.), Plasma Polymerization, Am. Chem. Sot. Sympos. Ser., 108 (1979). H. Yasuda, Glow discharge polymerization, in J. L. Vossen and W. Kerns (eds.), Thin Film Processes, Academic Press, New York, 1978. A. T. Bell, Top. Cut-r. Chem., 94 (1980) 43. A. Bradley and J. P. Hammes, J. Electrochem. Sot., IlO(1963) 15. E. Kay and A. Dilks, J. Vat. Sci. Technol., I8 (1981) 1. E. Kny, L. L. Levenson, W. J. James and R. A. Auerbach, J. Var. Sci. Technol., 16 (1979) 359.

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37

C. W. Miller, D. H. Karweik

10

11 12

13 14 15 16

17 18 19 20 21 22 23 24 25 26 27

28 29

30 31 32 33

34 35

Sot.. Faraday

38 39 40

41 42 43 44 45 46

47 48 49

50

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