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Short CommunicationReceived: 18 June 2008 Revised: 24 October 2008 Accepted: 9 December 2008 Published online in Wiley Interscience: 20 January 2009

(www.interscience.wiley.com) DOI 10.1002/sia.3024

Photoelectron spectroscopy on organicsurfaces: X-ray degradation ofoxygen-plasma-treated and chemicallyreduced poly(propylene) surfaces incomparison to conventional polymersThomas Gross,∗ Gerhard Kuhn and Wolfgang E. S. Unger

The X-ray-induced sample damage during mono XPS analysis of an oxygen-plasma-oxidized and subsequently wet-chemicallyreduced poly(propylene) film was investigated as a showcase for plasma-modified or plasma-deposited samples. By doing this,the degradation index approach as introduced by Beamson and Briggs in the Scienta ESCA300 high-resolution XPS database oforganic polymers has been adopted. As to be expected, the sample degrades by loosing oxygen as revealed by observation ofdecreasing O/C and C–OR/Csum ratios. However, the X-ray degradation indices are definitely higher than those of conventionalreference polymers. Moreover, the C–OR/Csum degradation index is significantly higher in comparison with one obtained forthe O/C ratio. In that context, there is no difference between the plasma sample and a conventional poly(vinyl alcohol) polymer.It is concluded that for reliable quantitative surface chemical analysis, the quality of spectra in terms of acquisition times mustbe optimized aimed to a minimization of X-ray degradation. Finally, it is proposed to describe the photon flux of an X-ray gunin an XPS experiment, which defines the degradation rate at the end, by using the sample current simply measured with acarefully grounded sputter-cleaned reference silver sample. Copyright c© 2009 John Wiley & Sons, Ltd.

Keywords: photoelectron spectroscopy; ESCA/XPS; surfaces; surface modification; poly(propylene) (PP); plasma treatment; X-ray;degradation

Introduction

Nowadays, the technological relevance of plasma-modified orplasma-deposited organic surfaces increases continuously. Veryoften X-ray photoelectron spectroscopy (XPS) is used in testinglaboratories to determine quantitatively carbon atoms in a certainbonding state. There are no standards and reference materialsfor this analytical task and usually in-house protocols are usedin the laboratories. Driven by the technological importance,the problem how those analytical results obtained in differentlaboratories compare with each other becomes of interest. In thiscontext, initiatives are launched on the national level in Germany(AK Plasma) and internationally under ISO Technical Committee TC201 on Surface Chemical Analysis (ISO/TC201/SC7) with the goal ofreviewing the state of the art in identification of chemical speciesby XPS. As usual, interlaboratory comparisons are appropriatetools to discover the state of the art. However, interlaboratorycomparisons focussed on the estimation of functional groups onorganic surfaces by XPS have to deal with some special difficulties.The first is the development of a laterally homogeneous test sampleto be shared for the comparison which must be stable enough. Itis well known that plasma-modified or plasma-deposited organicsurfaces suffer from ageing. Another problem is that soft mattersamples sustain radiation damage when exposed to X-rays.

Currently, there is an interlaboratory comparison launchedunder AK Plasma aimed to the determination of the OHgroup surface concentration as the measurand at an plasma

oxidized poly(propylene) sample. In detail, the test sample wasoxygen-plasma-treated poly(propylene) that is subsequently wet-chemically reduced[1] and aged for a long time. Intentionally wetchemical reduction was applied to convert most of the plasma-formed oxygen species into C–OH groups. Results of the damageprovoked by a monochromatic X-ray source during XPS analysison the test sample are reported in this communication. Those dataare compared to results obtained with conventional polymersrevealing remarkable differences.

Experimental

An A4 formatted poly(propylene) foil (Hoechst, Germany)was treated by an radio frequency (r.f.) low pressure oxygenplasma at 100 W as described in detail elsewhere.[1] Subsequently,it was stirred in 12-ml dry tetrahydrofuran (THF) and 3 ml of 1M diborane (Aldrich, Germany) solution under N2 atmosphere atroom temperature for 18 h. After those treatments, the foil wasremoved from the bath and immersed in an alkaline H2O2 solutionof water and THF for 2 h. The foil was then washed with THF,

∗ Correspondence to: Thomas Gross, Bundesanstalt fur Materialforschungund–prufung (BAM), 12200 Berlin, Germany. E-mail: thomas.gross@bam.de

Bundesanstalt fur Materialforschung und–prufung (BAM), 12200 Berlin,Germany

Surf. Interface Anal. 2009, 41, 445–448 Copyright c© 2009 John Wiley & Sons, Ltd.

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Figure 1. XPS spectra obtained with oxygen-plasma-treated and wet-chemically reduced poly(propylene) films using monochromatic Al Kα

X-rays: (a) survey and (b) C1s spectrum after peak fitting.

three times with water and with methanol. Finally, it was driedin a desiccator and then stored in a envelope for more than 40days. It is reasonable to expect that the surface of the foil reacheda rather stable condition. XP spectra obtained with the oxygen-plasma-treated and wet-chemically reduced poly(propylene) aredisplayed in Fig. 1.

A poly(vinyl alcohol) (PVA) Celanese CELVOL#165SF PVOH25 KGBAG, Product No: 51002691 (hydrolysis of PVOH: 99.5 mol%, pH,4% solution: 6.2) was used as a reference sample.

XP spectra of these oxygen-plasma-treated and chemically re-duced poly(propylene) samples were recorded at monochromaticAl Kα excitation at pass energies of 80 eV for survey spectra and20 eV for core level spectra using an AXIS Ultra DLD electronspectrometer manufactured by Kratos Analytical, UK.

The photon flux was estimated in terms of a sample currentmeasured with a grounded sputter-cleaned silver sample of ca1 cm2. The X-ray intensity distribution across the X-ray spot inthe area as defined by the sample surface is unknown, but theirradiated area is surely smaller than the area of the silver sample.Careful positioning of the silver sample with its centre in thecentre of the X-ray spot enables a reproducible characterizationof the photon flux provided by the X-ray gun through themonochromator. The sample current determined as described

is unequivocally correlated to the photon flux in the area on theinvestigated sample viewed by the spectrometer. In addition, XPSdata displayed in Figs 1–4 are obtained with the gun powered at10 mA, 15 kV providing a sample current of 0.60–0.63 nA.

The electron emission angle was 0◦, and the source-to-analyzerangle was 54◦.

The binding energy scale of the instrument was calibratedfollowing a Kratos Analytical procedure that uses ISO 15472[2]

binding energy data. Spectra were taken by setting the instrumentto the hybrid lens mode and the slot mode. By doing this, electronsoriginating from an analysis area of approximately 300 × 700 µm2

are accepted by the electron optical system of the spectrometer.A charge neutralizer was used.

The binding energy scale was corrected for static charging,[3]

using an electron binding energy, BE, of 285.0 eV[4] for the C1slevel of aliphatic hydrocarbons.

Quantification of the survey spectra was carried out with thehelp of the CasaXPS software, version 2.3.12, by using relativesensitivity factors (RSF) taken from the Kratos element library. RSFsof 0.278 and 0.78 were used for C1s and O1s, respectively. Minorcomponents as Na (0.4 at.%), Zn (0.2 at.%), B (0.7 at.%), Si (0.1 at.%),Cu (0.1 at.%), and N (0.5 at.%) were detected, too. Those minorcomponents may originate from different sources as additives andcatalysts, insufficient washing and other impurities introduced byplasma processing and subsequent wet chemical treatments. It isassumed that those impurities do not influence the degradationbehavior of the sample under X-ray irradiation.

C1s peak fitting was also performed by the CasaXPS software.A Shirley background was removed in a BE window from 282.6to 291.1 eV. A Gaussian/Lorentzian product function peak shapemodel was used. Four components (cf Fig. 1(b)) were used forfitting of the C1s spectrum where the CHx BE was constrained to285.0 eV, and the BEs of the other three components, C–OR, C O,and COOR, were left free to run.

Results and Discussion

X-ray-induced sample damage is a well-known and generalproblem during XPS analysis of polymers and may cause thespectrum to alter with increasing exposure time. A survey on X-raydegradation of conventional polymers measured in terms of adegradation index has been undertaken in the Scienta ESCA300Database by Beamson and Briggs.[4] This X-ray degradation indexwas determined from graphs of X versus t, where X is a spectralparameter characteristic of the sample and t is the X-ray exposuretime. In Ref. [4], the X-ray degradation index is defined to be thevalue of 1 − X500/X0, that is, where X500/X0 is the ratio of the valuesof X obtained at t = 500 min and t = 0 min. Following Ref. [4],the X-ray degradation index is expressed on a percentage basisrounded up to the closest 5%. In Table 1, we call this index ‘X-raydegradation index 500 min’. Because 500 min is an unusual longexposition time, we used in our experiments an exposition timeof 250 min and the related X-ray degradation parameter is called‘X-ray degradation index 250 min’.

Figures 2 and 3 show the X versus t dependences measuredfor the oxygen-plasma-treated and wet-chemically reducedpoly(propylene) sample where the parameter X is the O/Catomic ratio taken from XPS survey spectra on the one handand C–OR/Csum data determined from component peak areas incurve-fitted C1s spectra on the other. C–OR is the area of the

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X-ray degradation of plasma-treated poly(propylene)

Table 1. Values of the X-ray degradation indices for oxygen-plasma-treated and wet-chemically reduced poly(propylene) film and relevantoxygen-containing polymers (sample currents characteristic for therespective X-ray gun states and settings are given in the footnotes)

SampleParameter

X

X-raydegradation

index500 min (%)

X-raydegradation

index250 min (%)

Oxygen-plasma-treatedand wet-chemicallyreducedpoly(propylene)a

O/C 30 15

Oxygen-plasma-treatedand wet-chemicallyreducedpoly(propylene)a

C–OR/Csum 35 20

Poly(vinyl alcohol)b,c O/C 10 5

Poly(vinyl alcohol)c,d O/C 15 10

Poly(vinyl alcohol)e O/C 10 –

Poly(vinyl alcohol)b,c C–OH/Csum 20 10

Poly(vinyl alcohol)c,d C–OH/Csum 25 15

Poly(methylene oxide)e O/C 20 –

Poly(ethylene oxide)e O/C 5 –

Poly(propylene oxide)e O/C 5 –

Poly(tetramethyleneoxide)e

O/C 5 –

Poly(vinyl methyl ether)e O/C 10 –

Poly(vinyl ethyl ether)e O/C 10 –

Poly(vinyl isobutylether)e

O/C 10 –

a X-ray source powered at 10 mA, 15 kV (150 W) providing a samplecurrent of 0.60–0.63 nA, Kratos Analytical AXIS Ultra DLD.b X-ray source powered at 5 mA, 15 kV (75 W) providing a samplecurrent of 0.53 nA, Kratos Analytical AXIS Ultra DLD.c Celanese CELVOL#165SF PVOH25 KG BAG, Product No: 51002691.d X-ray source powered at 10 mA, 15 kV (150 W) providing a samplecurrent of 1.07 nA, Kratos Analytical AXIS Ultra DLD.e X-ray source power of 1400 W, Scienta ESCA300.[4]

C–OR component and Csum the total area under the C1s envelope.These experimental ratios were fitted with a straight line versus t.

X = A + B × t (1)

Subsequently, X0 and X250 were evaluated by using Eqn (1),and with these data, the ‘X-ray degradation index 250 min’ forthe sample under investigation was determined. In order tocompare results presented in this study to X-ray degradationdata reported in Ref. [4], we estimated ‘X-ray degradation index500 min’ data using Eqn (1) again and assuming linearity also forlonger exposition times. Linearity has been proved in Ref. [4],Fig. 27, for poly(alkylene oxides) and poly(vinyl alkyl ethers).

The X-ray degradation indices found with the oxygen-plasma-treated and wet-chemically reduced poly(propylene) sample arelisted in Table 1 in comparison with those of conventionalpolymers taken from Ref. [4] covering samples with hydroxyl–OH groups (poly(vinyl alcohol)), ether structures in the backbone(poly(alkylene oxides)), and ether structures as a side chain (poly(vinyl alkyl ethers)) and further experiments in our laboratory withpoly(vinyl alcohol). Table 1 reveals a number of relevant points:

1. As to be expected, an oxygen-plasma-treated and wet-chemically reduced poly(propylene) surface degrades by

Figure 2. Degradation of oxygen-plasma-treated and wet-chemicallyreduced poly(propylene) by monochromatic Al Kα X-rays as monitored byO/C atomic ratio versus time of X-ray exposition.

Figure 3. Degradation of oxygen-plasma-treated and wet-chemicallyreduced poly(propylene) by monochromatic Al Kα X-rays as monitored byC–OR/Csum data obtained from curve fittings of C1s spectra versus time ofX-ray exposition.

loosing oxygen expressed by a decrease of the O/C andC–OR/Csum ratios.

2. The X-ray degradation indices of the plasma sample aresignificantly higher in comparison with those characteristicof the reference polymers in Table 1. This phenomenon can beclearly observed in a comparison of datasets measured withthe plasma sample and bulk PVA using the Kratos AnalyticalAXIS Ultra DLD instrument with characterized photon fluxes.Comparison with bulk polymer data taken for the SCIENTAESCA 300 database[4] results in a similar result, but it is notso straightforward. It should be noted that the power usedfor X-ray excitation with the Scienta ESCA300 instrument’srotating anode is substantially higher (∼10 times) than usedin the Kratos instrument’s mono X-ray gun. However, wedo not know how the photon fluxes in the area viewed bythe respective spectrometer compare for both instruments.

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Figure 4. Means and uncertainties (2σ ) of O/C atomic ratios (closedsymbols and left scale) and 100% × C–OR/Csum data (open symbolsand right scale), respectively, consecutively measured with an oxygen-plasma-treated and wet-chemically reduced poly(propylene) surface atthree different positions in the sample. The sequence of measurementsleading to different exposition times is explained at the top (closed square,23.5 min; closed asterisk, 71 min; open square, 47.5 min; open asterisk,95 min). Monochromatic Al Kα X-rays were used.

Therefore, a discussion of that difference in terms of photonfluxes is not possible.

3. The C–OR/Csum degradation index is significantly higher incomparison with one obtained for the O/C ratio. In thatcontext, there is no difference between the plasma sample andthe conventional poly(vinyl alcohol) polymer. The conclusionis that the X-ray degradation does not lead only to oxygen lossbut also, to less extend, to the formation of higher oxidizedspecies, probably via scission of the O–H bonds with theformation of an alkoxy radical as initiation reaction. The decayof this radical via hydrogen abstraction or via beta scission caneasily explain the formation of carbonyl carbon atoms, esters,and ethers. Irradiation-induced oxidation was unequivocallyobserved with the PVA reference sample. Here, we found anincrease of the C O component peak in the highly resolvedC1s spectrum from 2% in the fresh sample to 6% after 253-minirradiation.

4. As to be expected, the degradation indices increase withincreasing photon flux expressed in terms of sample current.

Generally, a comparison of X-ray degradation indices fromdifferent sources (different instruments, different settings of anindividual instrument) is difficult because one has to relate thedegradation index, which is representing a reaction rate, to thephoton flux density of the X-ray gun. The photon flux itself relatesto the power used for X-ray excitation and the monochromatortransmission. At the present time, there is no standard guide onprocedures for measurement of photon flux density available.However, in Ref. [5], where results from a VAMAS interlaboratorycomparison on X-ray damage of polymers are reported, the authorsproposed a solution based on polymeric reference materials tobe used for a comparison of photon fluxes among different XPSinstruments. Unfortunately, this project was not developed to apractically relevant standard. As a workaround, the photon fluxcan be estimated in terms of a sample current simply measuredwith a carefully grounded sputter-cleaned silver sample.

Figure 4 summarizes the X-ray degradation of the oxygen-plasma-treated and wet-chemically reduced poly(propylene)

surface in dependence on typical times of exposure studied inthis communication. Here, results of measurements on threedifferent areas statistically distributed across the sample surface aredisplayed with their means and 2σ uncertainties. Obviously, boththe O/C atomic ratio and the C–OR/Csum value are underestimatedat higher X-ray exposition times. Therefore, acquisition times haveto be reduced as much as possible to minimize the bias ofmeasurement.

Conclusions

X-ray-induced sample damage during the XPS analysis of a oxygen-plasma-treated and wet-chemically reduced poly(propylene) filmwas investigated. This damage is an important feature tobe carefully considered in protocols of future interlaboratorycomparisons. Moreover, it is concluded that the quality of spectrain terms of acquisition times must be optimized aimed to aminimization of X-ray degradation. Participants of comparisonsmust receive enough sample material to determine the X-raydegradation index characteristic for their individual instrumentsettings. Protocols for direct or indirect characterization of thephoton flux of the X-ray gun used for the measurements haveto be developed and will be rather useful for a discussion of thescatter of quantitative XPS data obtained in comparisons with softmatter test samples as plasma-treated polymers.

Finally, it seems that a plasma-modified sample can be moresensitive against X-ray degradation than conventional polymersas displayed by higher degradation indices that represent higherdegradation reaction rates. This study proved this to be true forthe C–OR or C–OH bonds. A reason is probably the inherentstructural diversity of the plasma sample including cross-linksand branches leading to tertiary carbon species and radicalsas well. A reduction of bond scission energies required to releasehydroxyl and other oxygen species may be a result of those uniquestructures. A possible alternative explanation is that in the oxygen-plasma-treated and wet-chemically reduced poly(propylene) mostof the chemical functional groups are concentrated in a thin layer(1 . . . 2 nm) at the surface, whereas in the PVA reference theyare distributed more or less uniformly from the surface to thebulk. Hence, any free radicals or reactive intermediates formed bydegradation of the plasma PP are more easily able to escape intothe vacuum system than in the PVA reference sample.

Acknowledgements

The authors thank D. Treu for operating the instrument, Dr R.Mix, and Professor J. F. Friedrich for supplying samples andcollaboration in the framework of AK Plasma.

References

[1] G. Kuhn, St. Weidner, R. Decker, A. Ghode, J. Friedrich, Surf. Coat.Technol. 1999, 116–119, 796.

[2] ISO 15472 : 2001, Surface chemical analysis – X-ray photoelectronspectrometers – Calibration of energy scales.

[3] ISO 19318 : 2004, Surface chemical analysis – X-ray photoelectronspectroscopy – Reporting of methods used for charge control andcharge correction.

[4] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers, TheScienta ESCA300 Database, Wiley: Chichester, 1992.

[5] K. Yoshihara, A. Tanaka, Surf. Interface Anal. 2002, 33, 252.

www.interscience.wiley.com/journal/sia Copyright c© 2009 John Wiley & Sons, Ltd. Surf. Interface Anal. 2009, 41, 445–448