Iron sulphide formation in the ferric stearate Langmuir–Blodgett films

Applied Surface Science 257 (2011) 2000–2003

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Iron sulphide formation in the ferric stearate Langmuir–Blodgett films S. Kundu a,∗,1 , A.K.M. Maidul Islam b,1 , M. Mukherjee b a b

Department of Materials Science, S.N. Bose National Centre for Basic Sciences, JD Block, Sector III, Salt Lake City, Kolkata 700098, India Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India

a r t i c l e

i n f o

Article history: Received 9 June 2010 Received in revised form 30 June 2010 Accepted 16 September 2010 Available online 16 October 2010 Keywords: LB film Ferric stearate Iron sulphide XPS X-ray reflectivity AFM

a b s t r a c t Ferric stearate (FeSt) Langmuir–Blodgett (LB) films have been reacted chemically with H2 S gas for making iron sulphide within the organic matrix. Films, before and after the reaction with H2 S, have been analyzed with the X-ray reflectivity (XRR), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) studies. After sulphidation, more ‘pinhole’ defects form which changes the film morphology and the number of layers increases due to the rearrangement of the molecules. Formation of less ordered iron sulphide within the stearic acid multilayers after sulphidation increases the interfacial roughness that decreases the reflectivity. XPS analysis shows that polysulphide forms within the microenvironment of the FeSt LB films after reaction with H2 S whereas both mono and polysulphide are produced when the reaction occurs with FeSt in bulk. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Cluster like or layered structures of inorganic materials can be made by chemical reactions inside the microenvironment of Langmuir–Blodgett (LB) films. It has already been shown that the thin layers or clusters of Ag [1], HgS [2], PbS [3], CdS [4], CuS [5] and Fe2 O3 [6] formed inside the LB matrixes show different properties from the bulk substances. Moreover, better control over the size, shape and distribution of nanoclusters can be obtained performing the chemical reactions inside the LB multilayers [7]. Chemical reactions occurring in the microenvironment of LB films may have some special effects on the reaction process that helps to form products having unusual stoichiometry [8], unconventional structures [9] or special chemical or physical properties [10]. Usually LB multilayers [11] of divalent fatty acid salts like Zn/Cd/Cu/Co/Pb/Hg arachidate/stearate have been exposed to H2 S to grow the metal sulphide (e.g. CdS, CoS) within the LB matrix. Structures and morphology of LB films before and after the reaction with H2 S have been investigated by several techniques like X-ray diffraction (XRD) [12,13], X-ray reflectivity (XRR) [14], Fourier transform infrared spectroscopy (FTIR) [12,13], UV/vis spectroscopy [12,13], atomic force microscopy (AFM) [15] and transmission electron microscopy (TEM) [9]. On the other hand, Xray photoelectron spectroscopy (XPS), owing to its high sensitivity,

∗ Corresponding author. Tel.: +91 033 2335 5706/7/8; fax: +91 033 2335 3477. E-mail addresses: [email protected], [email protected] (S. Kundu). 1 Both authors have equal contributions. 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.042

is suitable for identifying the elements in LB films [5] and provides information about the chemical bonding [16] and the stoichiometry of the sulphide layers [8]. Very few works have been done on LB films of multivalent fatty acid salts [17,18]. Although the structures and growth of trivalent fatty acid salt like ferric stearate (FeSt) have been investigated [19–21] on water and solid substrate but their structures and chemical composition in LB films after the reaction with H2 S have not been done so far. Iron disulphide, Pyrite (FeS2 ), is the most abundant sulphide in earth’s crust and hence studied for mineral processing reasons. It has been studied for producing solar cell components as well as solid-state batteries [22]. Infrared spectra have revealed grains of pyrrhotite (Fe1−x S) in interplanetary dust [23]. Signs of troilite (FeS) have also been found from circumstellar dust [23]. In this paper, nine monolayer (9ML) LB films of FeSt have been prepared and then exposed to H2 S to complete the chemical reaction in the microenvironment of LB films. Structures and chemical compositions before and after the reaction with H2 S have been analyzed using XRR, AFM and XPS studies. 2. Experimental details Preparation and purification process of FeSt has been described elsewhere [18,19]. To transfer the FeSt film on hydrophilic silicon (001) substrates, FeSt molecules were spread from a 0.7 mg/mL chloroform solution in a Langmuir trough (KSV 5000) on Milli-Q water at room temperature (24 ◦ C). A platinum Wilhelmy plate was used to measure the surface pressure of the FeSt film. Film on water surface was compressed with a speed of ≈0.5 A˚ 2 /(molecule

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min) at the time of deposition. FeSt films of 9ML were deposited on RCA cleaned hydrophilic Si (001) substrates by LB method. Depositions were done at 30 mN/m surface pressure with the speed of 2.5 mm/min of the trough-dipper for each up and down strokes. For the up stroke substrate goes from water to air, while for the down stroke it goes in the reverse direction, i.e., from air to water. 9ML FeSt film was then produced by using 5 up stokes and 4 down stokes. FeSt film was exposed to H2 S gas in a closed glass tube for ≈1 h. Surface topography of the FeSt films before and after H2 S exposition was studied through an AFM (Auto probe CP, Park Scientific) in contact mode using silicon nitride cantilever (with spring constant 0.05 N/m) and pyramidal tip [24]. Scans were performed in constant force mode over several portions of the film for different scan areas from 5 × 5 ␮m2 to 40 × 40 ␮m2 . To minimize the damage of the organic films low constant force (≈0.8 nN) was used. Reflectivity studies were carried out using a X-ray diffractometer (D8 Discover, Bruker AXS) [25] with Cu source (sealed tube) followed ˚ by Göbel mirror to select and enhance CuK␣ radiation (0 = 1.54 A). Data was taken in the specular condition, i.e., incident angle is equal to the exit angle and both are in the scattering plane. Under specular conditions the momentum transfer vector q = kf − ki (ki(f) = incident (scattered) wave vector) has only one non-vanishing component qz normal to the surface given by qz = (4␲/)sin , where  is the angle the incident X-ray beam makes with the surface [24]. XPS measurements were performed with an Omicron Multiprobe (Omicron NanoTechnology GmbH) spectrometer fitted with an EA125 hemispherical analyzer and a monochromatized Al K␣ (1486.6 eV) source [26]. All data were collected at normal take off angle keeping pass energy 40 eV with analyzer angular acceptance ±1◦ . The background correction of the data was done by Shirley [27] method. The curve fitting of the core XPS lines was carried out using Gaussian–Lorentzian area (GL) functions.

3. Results and discussion X-ray reflectivity data of the 9ML FeSt films before and after H2 S exposition were taken and are shown in Fig. 1a and b. Data analysis was done by the Parratt formalism [28] introducing finite interfacial width [29]. The calculated reflectivity curve for the best fit is presented in Fig. 1a with the corresponding EDP in the inset of Fig. 1a. The EDP follows the AML + 4SML model [21]. AML is the asymmetric molecular layer where all FeSt molecules are in the asymmetric configuration, i.e., all three tails of the amphiphilic molecules are on one side of the headgroup. SML is the symmetric molecular layer where all FeSt molecules are in the symmetric configuration, i.e., tails are on the both side of the headgroup as a ‘Y and inverted Y’ structures [19,20,21]. From the values of the electron densities of the individual layer, it is clear that very less transfer of molecules occurs after five strokes of the substrate [21]. SML layer thickness, ˚ AFM topograi.e., the head–head or tail–tail separation is ∼49.5 A. phy with the typical line profile of this film is also shown in the in the inset of Fig. 1a. From the line profile, it is clear that the varia˚ Thus the film is mostly tion in heights occur mainly after 100–115 A. compact up to AML + 2SML thickness, after that the coverage of each SML decreases [21]. Reflectivity data after H2 S treatment is shown in Fig. 1b. Reflectivity decreases rapidly and loses mostly all the peaks. The decrease of reflectivity implies the increase of interfacial roughness. From very weak Bragg peaks, SML layer thickness is obtained which is ˚ Thus, the molecules become tilted after the treatment with ∼40 A. H2 S. If only the molecular tilting is considered the tilt angle will be ∼36◦ . However, H2 S will react with the Fe-containing headgroups that will change the chemical structure of the molecules forming iron sulphide in the LB matrix. It has been observed that

Fig. 1. (a) Observed (open circles) and calculated (lines) X-ray reflectivity profiles of 9ML LB film of FeSt molecules on silicon before sulphidation. Left inset: Electron density profiles, i.e., average electron density () versus depth (z) extracted from the fit of reflectivity profile. AML and SML are described in the text. Right inset: AFM image of FeSt film before sulphidation of scan area 30 ␮m × 30 ␮m and the typical line profile. (b) Observed (open circles) X-ray reflectivity profile of 9ML LB film of FeSt molecules on silicon after sulphidation. Arrows indicate the Bragg peaks. Inset: AFM image of FeSt film after sulphidation of scan area 30 ␮m × 30 ␮m and the typical line profile.

the hydrocarbon chains in the CoSt2 LB film are packed in a hexagonal fashion with all-trans (zigzag) conformation, and the chains are aligned normal to the film plane with long spacing being ≈50.0 A˚ [30]. After the reaction with H2 S two-dimensional CoS monolayers has formed in the LB matrix and the hydrocarbon chains change into orthorhombic subcell packing and tilt on the substrate, and the long spacing becomes ≈40.0 A˚ [30]. Multilayer stearic acid (SA) LB film has a very similar structure to that of C-form crystal of SA with its hydrocarbon chains packed in an orthorhombic unit cell. The orientation angle of the chain axis from the normal direction of the film surface in multilayer SA LB film is about 30◦ [31]. FeSt LB film thus changes to SA LB film with the formation of iron sulphide within the SA layers. Iron sulphides are not organized in layers, i.e., not exactly forming two-dimensional layered structures within the SA LB film, but are rather distributed randomly within the organic film that enhances the film roughness and decreases the reflectiv-

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ity. Other groups have also been observed the same type of random distribution of sulphide particles within the LB films [32,33]. AFM topography with the line profile is shown in the inset of Fig. 1b. Image shows that compact and relatively less defect (typical ‘pinhole’ type) morphology has been modified after the treatment with H2 S and more defects has formed. Line profile shows that ‘pinhole’ type defects have started to form from the substrate surface and the ˚ As the SML thickness is film thickness has been increased by ≈38 A. ˚ the maximum height (≈262 A) ˚ of the film indicates that now ≈40 A, molecular reorganization has been taken place from in-plane to out-of-plane direction and two extra SML layers has formed. More ‘pinhole’ type defects has generated due to this reorganization. X-ray photoelectron spectroscopy studies have been performed on 9ML LB films of FeSt on Si substrate before and after sulphidation. Fig. 2 shows the Fe2p and C1s XPS core level spectra of 9ML FeSt LB film before sulphidation. Fig. 2a shows that C1s spectra of 9ML FeSt LB film before sulphidation contains two peaks, a strong peak at lower binding energy corresponding to CH2 and CH3 carbon atoms and a very weak peak at higher binding energy (290.6 eV) corresponding to the carboxyl carbon atoms of the stearate chains. Two GL functions at binding energy positions 285.7 and 286.7 eV are required to fit the main peak of C1s spectra as shown in Fig. 2a. The smaller peak at lower binding energy appears due to partially neutralized differentially charged sample [26]. For energy calibration, the intense peak of C1s spectra was fixed to 285 eV. After energy calibration, the peak position of higher binding energy component (288.9 eV) matches well with the binding energy position of carboxyl carbon atoms. The XPS spectra of Fe2p shows two strong peaks that correspond to Fe2p3/2 and Fe2p1/2 respectively due to the spin–orbit coupling as shown in Fig. 2b with satellite peaks at higher binding energies that are common for Fe2p peaks in the Fe3+ state of

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Binding energy (eV) Fig. 3. (a) Fe2p and (b) S2p core level lines measured for 9ML LB film of iron stearate after sulphidation. (c) S2p core level line measured for bulk iron stearate after sulphidation.

iron [34,35]. The Fe2p peaks are fitted with a function defined as a sum of two GL functions where the intensity corresponding to 2p1/2 was half of 2p3/2 and are separated by 14 eV. It was observed earlier that multilayer LB films of cadmium arachidate show differential charging where discrete cadmium layers at different heights are differentially charged [26]. A combination of two sets of functions as defined above was required to fit the peaks as shown in Fig. 2b. The requirement of the lower binding energy peak was due to the differential charging in Fe2p spectra. After energy calibration, the binding energy positions of Fe2p3/2 and Fe2p1/2 found from the neural component were at 711.2 and 725.2 eV respectively. The shake up satellite peaks were fitted by separate GL function at 718.0 and 732.9 eV. Compositional analysis of the material was done from Fe2p and C1s spectra. The obtained value of the atomic ratio of carbon is to iron was 51.8 which is close to the theoretical value 54 when one iron ion is attached with three stearate chains (CH3 (CH2 )16 COO− ). This result confirms the presence of Fe3+ state in FeSt LB films. Similar to the unreacted sample, compositional analysis of H2 S reacted sample was also performed. Fig. 3 shows Fe2p and S2p spectra of the sample after sulphidation. It may be noted that in this case no differential as well as total charging was observed. A single set of function (described before) was required to fit the data properly with binding energy positions of Fe2p3/2 and Fe2p1/2 components at 711.5 and 725.5 eV respectively. The higher binding energy asymmetries due to shake up satellite peaks were fitted by separate GL function at 714.5 and 728.1 eV. These satellite peaks are observed at lower energies compared to the unreacted LB films. The S2p spectra were fitted with single spin–orbit-split doublet, S2p3/2 and S2p1/2 ,

S. Kundu et al. / Applied Surface Science 257 (2011) 2000–2003

separated by 1.1 eV [36] and the peak position of S2p3/2 was found to be 163.6 eV. There is a small hump close to 162 eV (indicated in Fig. 3b) which could not be fitted well with an additional set of peaks, which may indicate presence of small quantity of sulphur in a different chemical environment. On the other hand, if the sulphidation occurs with bulk FeSt, two distinguishable components are observed in S2p spectra as shown in Fig. 3c whereas the Fe2p spectrum is similar but broader compared to that of LB films (not shown). Binding energy positions of S2p3/2 spectra for the two components were at 161.8 and 163.6 eV respectively. The position of the higher binding energy peak of S2p3/2 matches with the same when iron sulphide formation takes place in the LB matrix. The above results suggest that two different types of sulphur species were present in the bulk sample after sulphidation. It is known from the literature that within the binding energy range from 160.3 to 163.9 eV, sulphur element exists as sulphide (S2− ) with minus 2 valences [37,38]. It has been reported in literature that iron sulphide may exist in the form of monosulphide, disulphide and polysulphide with increasing binding energy of S2p3/2 [39]. Generally, peaks appear for monosulphide at ∼ 161.6 eV, disulphide (S2 2− ) at ∼ 162.9 eV and polysulphide (Sn 2− , 3 ≤ n ≥ 7) at ∼ 163–163.7 eV. The binding energy position of major S2p3/2 spectra in our case falls at higher side of the above range. It thus indicates the formation of polysulphides in both, LB matrix as well as bulk FeSt. Whereas formation of monosulphide together with polysulphide occurs when sulphidation takes place in bulk FeSt. This indicates that the microenvironment within LB films affect differently on the reaction process compared to that in the bulk. 4. Conclusion FeSt LB film after sulphidation shows more ‘pinhole’ defects that changes the film morphology and the film thickness slightly increases due to the rearrangement of the molecules. Formation of less ordered iron sulphide within the stearic acid multilayers after sulphidation increases the interfacial roughness that decreases the reflectivity. XPS analysis shows that mainly polysulphide forms inside the microenvironment of FeSt LB films after the reaction with H2 S, whereas both polysulphide and monosulphide forms after sulphidation in bulk. Acknowledgment The authors thankfully acknowledge Prof. Satyajit Hazra for providing X-ray diffractometer facility for X-ray reflectivity measurements.

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