Surface characterization of Langmuir-Blodgett polypyrrole films obtained by a solid-state reaction

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Synthetic Metals 87 (1997) 19-22

Surface characterization of Langmuir-Blodgett polypyrrole films obtained by a solid-state reaction E. Milella *, F. De Riccardis, C. Gerardi, C. Massaro PASTIS-CNRSM,

Centro Nazionale

per la Ricerca

e Svilrrppo

Materiali,

SS 7 Appia Km 714, 72100 Brindisi,

Italy

Received 18 June 1996; accepted 9 October 1996

Abstract We have examined polypynole obtained by a solid reaction from a ferric stearate Langmuir-Blodgett film by means of X-ray photoelectron spectroscopy, static secondary ion mass spectrometry and scanning electron microscopy. These surface-specific techniques enable us to probe the surface composition of the conducting polymer. All the analyses show the presence of the polypyrrole at the surface. The morphology of the samples appears to be formed by small clusters of polymer with dimensions in the range 50-300 nm in a steak acid matrix with a quite porous surface. Moreover, by measuring the specimen current, we observe that, in accordance with polypynole clusters, the current value is higher with respect to the remaining film; this result further confirms the formation of the conducting polymer. Keywords:

Langmuir-Blodgett

film; Poiypynole; X-ray photoelectron spectroscopy; Secondary ion massspectrometry; Scanning electron microscopy

1. Introduction In recent years there has been a great concern about conducting polymers due to their multiple potential applications [ 1,2]. The actual challenge in this field is the fabrication of thin films of electroactive polymers with desiredproperties. The Langmuir-Blodgett (LB) technique hasbeen shown to be a suitable method for the preparation of low-dimensionalmaterials [ 3,4], The insertion of inorganic compounds into LB films [5-81 has been used with successand this processhasbeen extended to confine polypyrrole (PPy) in a LB matrix [ 9, lo]. PPy is a poly( heterocycle) that represents one of the more interesting polymeric materials owing to its electroactive properties and air stability. We have prepared PPy in LB film by meansof a solidstate reaction with the method reported in Ref. [9]. The method consistsof exposing ferric stearateLB films to chloridic acid gas; such a processinvolves the precipitation of ferric chloride within the planesof a multilayer. The precipitated salt acts to polymerize the pyrrole monomers,which are introduced into the film by vapour diffusion and dopethe resultant PPy. In the present paper we characterized PPy-containing LB film by using surface-specific techniquessuch asX-ray photoelectron spectroscopy (XPS), secondary ion massspectrometry (SIMS) and scanningelectron microscopy (SEM) . * Corresponding author. 0379-6779/97/$17.00 PIISO379-6779(96)04780-7

0 1997 Elsevier Science S.A. All rights reserved

The aim of this work is to examine the nature and the morphology of the surface of the samplein order to confirm the formation of the conducting polymer in the LB films.

2. Experimental

2.1. Preparation of the PPy-containing

LBJiLms

For LB film deposition 1 mg/ml solution of stearic acid in chloroform was spread onto a subphaseof deionized water containing 4 X lo-’ M ferric chloride. The subphasepH was kept at a value of 5.6 and the temperaturewas fixed at 15 “C. The pressure-area(7-A) isothermsand multilayer film deposition were performed by using a KSV 5000 LB system. The monolayer was compressedwith a barrier rate of 5 mm/ min up to a pressureof 35 mN/m. Silicon (100) slideswere usedassubstratesafter treatment with hydrofluoric acid. The Y-type layers were depositedwith a dipping rate of 1 mml min. The transferratio wascloseto 0.9. After a proper drying, the film was exposed to hydrochloric acid vapour for a few minutes and then kept over vapour of pyrrole for 24 h.

2.2. XPS XPS measurementswere performed with aVG ESCALAB 210-D electron spectrometer(in ultrahigh vacuum, 5 X lo-’

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mbar) , using non-monochromatized radiation. During the analysis the X-ray gun operatedat 15 kV and 300 W. Pass energiesof 20 eV for narrow scanand 50 eV for survey scan were used.Overall scanspectra, in the range of O-l 100 eV, and narrow scans, in the C(ls), O(ls), C1(2p), Fe(2p), N( Is) regions,were recorded.

C(KLL) ClS I

O(KLL)

2.3. Static SIMS SIMS analyseswere carried out on a modified CAMECA ims4f magnetic sector able to detect up to m/z = 1000a.m.u. A probing Csf beam acceleratedat 14.5 keV was scanned over an areaof 500 X 500 km2 achieving a current density of about 5 nA/cm’. Under theseconditions the surfacedamage is negligible allowing for the acquisition of static SIMS spectra [ll]. Negative secondaryionswere detectedby an electronmultiplier detector. The maximum instrumentaltransmissionfactor wasusedin order to optimize the secondaryion yields. An electron beamimpinging on the samplewith a normal incidence and a very low energy was used to passivatethe eventual charging of the surface.

0

I

200

400

I

I

600

,

,

800

,

,

,

1000

BindingEnergy (eV) CIS

c

2.4. SEM

0

200

400

600

800

1000

Binding Energy (ev)

SEM observationswere taken by using a Philips XL40 La B6 scanning electron microscope,equipped with an energy dispersive spectrometer (EDS) EDAX Dx-4i. Before the examination, the sampleswere coatedwith 15 nm Au film to ensurea better analysis.

Fig. 1. XPS suwey spectra for (a) ferric stearate LB film and (b) PPycontaining stexic acid LB film.

metallic ion begins to hydrolyse in a high pH region and occursin gel form. Only at pH 5.6 doesthe surfacepressurearea (T-A) isotherm exhibit the usualbreak in the slopeat G-TT= 20 n-&I/m, correspondingto a transition from liquid condensedto solid (LS transition). Since the viscoelasticpropertiesstrongly influence the film transfer onto a solid substrate [ 121, hysteresis tests were performed. Under the above-mentionedconditions, the hys-

3. Results and discussion 3.1. T-A isotherm The difficulty of fabrication of ferric stearateis due to a limited narrow region of deposition conditions, becausethe

Binding

(a)

k

Energy

(eV)

Fig. 2. N( 1s) XPS core-level spectrum of PPy-containing LB film.

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teresis area is almost negligible suggesting a good elasticity of the monolayer. 3.2. XPS experiments

Cl-

!-

mass (amu)

0.4

I mass (amu) Fig. 3. Negative ion static SIMS spectra in O-40 a.m.u. mass range for (a) ferric stearate LB film and (b) PPy-containing stearic acid LB film.

By means of XPS we studied both the chemical composition and the oxidation of the conducting polymer contained in the first monolayers of the sample. The comparison of the spectra resulting from the ferric stearate LB film and those from the PPy-containing film produced very useful information on the formation of the polymer. Fig. l(a) shows the XPS survey spectrum related to the ferric stearate LB film, where no sign of nitrogen species is present. Conversely, after the solid reaction (Fig. 1(b)) a N( Is) peak is clearly observed, suggesting the formation of PPy. Moreover, the N( Is) line (Fig. 2) exhibits a double peak localized at 399.6 and 401.4 eV, respectively, that can be assigned to different electrostatically inequivalent groups of nitrogen atoms [ 131. In fact PPy is in an oxidized form containing the Cl- anions located between PPy chains. The anions, bearing a localized charge, can induce different electric fields at the nitrogen sites, thus explaining the occurrence of the splitting of N( 1s) signals.

Fig. 4. SEM micrographs: (a) fine grain structure of ferric stearate LB film (10 000 X ); (b) small clusters of PPy contained (c) small clusters on porous surface of the LB film (20 000 X ) (reduced in reproduction by 65%).

in steak

acid LB film (5000 x );

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3.3. Static SIMS measurements

The secondary ion emissionin static SIMS involves the first monolayersof the surfaceand the identification of chemical compoundsis possibleby the detection of characteristic fragments from which the molecular structure can be deduced. We performed static SIMS analysis on the ferric stearate LB film as well as on the PPy-containing LB film. Fig. 3(a) showsthe negative massspectrain them/z. range IO-45 a.m.u. recorded on the ferric stearatesample,where no nitrogen-containing fragments are observed.The signals at 35-37 a.m.u. related to Cl- suggestthat, during the LB deposition, the chloride ion has been incorporated into the film. Fig. 3(b) showsthe massspectra in the samem/z range for the PPy-containing LB film. The peaksat 26 and40 a.m.u. correspondingto C-N- and (C,H,N) - fragments, respectively, can be directly attributed to the conducting polymer [ 141, It should be noted that the presenceof the peak at 36 a.m.u. related to HCl- and the increaseof the intensity of the chloride peaks are due to the treatment of the iron( III) stearate LB film with chloridic acid vapour during the polymerization reaction. 3.4. SEA! experiments Fig. 4(a) reports the high-magnification micrograph (M= 10 000 X ) of the surfaceof the ferric stearateLB film. The LB film appearssmoothwith a very fine grain structure. After the solid-statereaction, it is possibleto distinguishon the surfacesmallclustersof PPy whosedimensionsare in the range 50-300 nm (Fig. 4(b) and (c)). The film surface between the small clusters appearsquite porous. Moreover, lessfrequently, some larger clusterscan be observed.EDS analysisshowsthat suchclustersare Fe and Cl based. Finally, by measuringthe specimencurrent inducedon the sampleby the electron beam,we have observedthat, in accordance with PPy clusters, the current value is higher with respect to the remaining film; this result confirms that the conducting polymer hasformed in thoseregions.

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4. Conclusions The XPS and SIMS surfaceanalyseshave confirmed unequivocally the formation of polypyrrole in a matrix of stearic acid. Furthermore, the XPS N( 1s) lineshapespectrumhas indicated that the polypyrrole is presentas oxidized form. The morphology of the surfaceby SEM measurementshas evidenced small clustersof polymer with dimensionsin the range 50-300 nm in a stearic acid matrix with a quite porous surface. Moreover the clusters, irradiated by the electron beam, exhibit a conductive behaviour compared with the insulating remaining part of the sample. All these results confirm that clustersof conducting polypyrrole are formed in the examinedfilm.

Acknowledgements The authorswould like thank M.B. Alba, P. Rotolo and A. Cappellofor their skilful technical contribution.

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