Effects of the PPy layer thickness on Co–PPy composite films

Applied Surface Science 356 (2015) 817–826

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Effects of the PPy layer thickness on Co–PPy composite films Murside Haciismailoglu University of Uludag, Faculty of Science and Literature, Department of Physics, Gorukle 16059, Bursa, Turkey

a r t i c l e

i n f o

Article history: Received 4 May 2015 Received in revised form 1 August 2015 Accepted 25 August 2015 Available online 29 August 2015 Keywords: Magnetic materials Composite materials Magnetic properties XPS VSM

a b s t r a c t Co–PPy composite films were electrodeposited on ITO substrate from two different solutions potentiostatically. Firstly, the PPy layers with the thicknesses changing from 20 to 5000 nm were produced on ITO. Then Co was electrodeposited on these PPy/ITO substrates with a charge density of 1000 mC cm−2 . The electrochemical properties were investigated by the current density–time transients and the variation of the elapsed time for the Co deposition depending on the PPy layer thickness. X-ray photoelectron (XPS) spectra indicated the presence of both Co metal and its oxides on the surface. The weak reflections of the Co3 O4 , CoO and hcp Co were detected by the X-ray diffraction (XRD) technique. According to scanning electron microscopy (SEM) images, the thickness of the PPy layer strongly affects the Co nucleation. The composite films with the PPy layer thinner than 200 nm and thicker than 2000 nm have an isotropic magnetic behavior due to the symmetrical crystal field. The composite films with the PPy layer thicknesses between 200 and 2000 nm have an anisotropic magnetic behavior attributable to the deterioration of this symmetrical crystal field by the PPy bubbles on the surface. All films are hard magnetic material, since the coercivities are larger than 125 Oe. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Composite materials consisting of conductive polymers and metal particles were firstly produced in mid-80s [1–5]. Since then, they are being used in a wide area such as electrocatalysis, corrosion, gas sensing, biosensors, drug delivery and artificial muscles, and their new properties are still being discovered [6,7]. In these materials, polymers and metal particles provide mutual benefits. For example, metal can supply electronic and magnetic properties or mechanical strength to the conductive polymers. On the other hand, polymers can prevent agglomeration of the metal particles and supply substrate area larger than that a metal will be able to do, which are both significant for the electrocatalysis or the energy storage. One of the favorite conductive polymers is polypyrrole (PPy) due to its high conductivity, stability and redox behavior. The common synthesize method for the PPy is electrochemical polymerization (electropolymerization). In order to have PPy-based composite material, Pd, Au, Ag, Cu, Pb and Pt are mostly used metals [4,5,8–13]. These metals can be produced by electrochemical deposition (electrodeposition). Electrochemical deposition is a simple, low cost, fast technique, and allows the change of parameters affecting the sample properties. For a composite material,

E-mail address: [email protected] http://dx.doi.org/10.1016/j.apsusc.2015.08.217 0169-4332/© 2015 Elsevier B.V. All rights reserved.

the properties are influenced by the PPy film, metal particles and interaction between them. So that, in order to modify these properties, it is worth investigating the effect of the chemical parameters (electrolyte composition, doping ion, solvent, temperature, pH, deposition mode, etc.) and the physical parameters (substrate type, polymer thickness, etc.) [14]. The polymer thickness has a remarkable role for the deposition of a metal on polymer and the composite material properties [4,8–13]. According to Chandler and Pletcher, the deposition of the metals on a PPy film is affected by the conductivity of PPy, so that both the potentials of the metal deposition and the PPy reduction have crucial importance. Also, the PPy film should have a thickness that allows the metal nucleation. After nucleation, the polymer can supply required electrons for the metal deposition. They reported that Pd, Pt and Pb can be grown on PPy films, but the Ru deposition depends on the nucleation phase, since its deposition potential on a metal electrode is more negative than the formers [4]. Li and Lin studied on the catalytic effect of Pt–PPy structures, for which the PPy film thickness was changed from 0 to 120 nm. The catalytic activity of the films with the PPy thicknesses up to 60 nm increases due to more Pt deposition. For thicker films, Pt agglomeration cannot be prevented, and/or ohmic drops occurred on the PPy film surface, so that the catalytic activity decreases [8]. Similar results were observed by another research group as well [9]. It was found that the shape, size and density (particle number/cm2 ) of the electrodeposited Cu nanoparticles vary as a function of the

818

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

PPy thickness. Reducing electric field obtained at the PPy surface boost the nucleation barrier, and hence decreases the nucleation sites. That is why the crystal size increases and the number of density decreases as the PPy film gets thicker [10,11]. Au–PPy structures also showed similar behavior, namely, as the PPy thickness increases from 0 to 30 nm, larger Au particles with low number of density grow. This was attributed to thicker PPy films having fewer pinholes for nucleation [12]. Lee and Tan investigated the deposition of Cu, Ni, Pb, Sn and Ag metals on two different PPy films which are called as thin (5 min polymerization time) and thick (10 min polymerization time) films by cyclic voltammetry. It was observed that the PPy film causes slow metal deposition at higher negative potentials and this effect become more noticeable as the PPy thickness increases. The metals like Ni and Sn electrodeposited on PPy films have high current efficiency as a consequence of the hydrogen evolution inhibition, which is very promising result for the electrodeposited metals [13]. Composite materials with ferromagnetic metals (Co, Ni, Fe and alloys) and PPy are also investigated by several groups [15–18]. Heining et al. specified that Ni nanoparticles having almost same size can be uniformly deposited on PPy films with the thickness range of 90–110 nm. For the PPy films with thickness less than 90 nm, the Ni nanoparticles did not appear, while for thicker than 110 nm, the Ni nanoparticles with different shapes grew irregularly [15]. Such kind of structures can be used in nanocatalysis and quantum electronics. In our previous work, the effect of PPy film thickness (which was changed from 20 to 100 nm) on Co metal deposition was studied. As the film thickness increases, the PPy film resistance increases, and hence Co deposition gets slow and difficult. Also, the magnetic moment increases with decreasing PPy thickness [16]. Conducting ferromagnetic polymer materials have potential applications as magnetic semiconductors and data storage even though mostly the applications of electrocatalysis and gas sensing are emphasized. In this work, Co–PPy composite films were produced by electrochemical deposition and their PPy thicknesses was changed from 20 to 5000 nm. The PPy was chosen due to its stability, conductivity and self-standing properties, Co was determined because of its ferromagnetic and metallic behavior. The priority was to investigate the magnetic behavior of these films. From the hysteresis curves, the coercivity, saturation and remnant magnetizations were determined. The effects of the electrochemical, chemical and structural properties on the magnetic behavior were examined. To our knowledge, there are very limited studies on the magnetic properties of such composite materials. 2. Material and methods Acetonitrile, pyrrole, tetrabutylammonium hexafluorophosphate (TBA·PF6 ), cobalt sulphate (CoSO4 ·7H2 O) were purchased from Sigma–Aldrich and used as received. The pyrrole was stored at 4 ◦ C before use. The water purified by Elga Felix system (resistivity = 18.2 M) was employed for the experiments. 2.1. Electropolymerization of PPy layers A solution (Py solution) containing 0.1 M pyrrole, 0.5 M tetrabutylammonium hexafluorophosphate and acetonitrile was prepared. Indium tin oxide (ITO) substrates were used as working electrode. A platinum sheet and saturated calomel electrode (SCE) were served as counter and reference electrode, respectively. Prior to polymerization, the working electrodes were cleaned in ethanol and acetone ultrasonically then, masked with a kapton tape except 0.36 cm2 area. After rinsing with acetonitrile, they were immersed in the Py solution. The electropolymerization was performed with

VersaStat4 potentiostat/galvanostat. A deposition potential of 0.9 V vs. SCE was applied. The charge density was changed between 6.4 and 945 mC cm−2 , to vary the layer thicknesses from 20 to 5000 nm. The PPy thicknesses were chosen in a wide range to examine and optimize the properties of the Co particles on the PPy layers. The thickness was calculated according to the Faraday law [19], assuming a current efficiency of 100%. After the polymerization, PPy/ITO films were rinsed in Py solution and acetonitrile gently to get rid of the excess PPy monomers and oligomers attached to the surface, then cleaned with ultrapure water. 2.2. Electrodeposition of Co PPy/ITO films were used as a new substrate (working electrode) for the Co deposition. The counter electrode was a platinum sheet and the reference electrode was a SCE. All electrodes were placed into a cell having an aqueous electrolyte containing 0.3 M CoSO4 (Co solution). The Co deposition was carried out under an electrode potential of −2.0 V vs. SCE until a charge density of 1000 mC cm−2 was reached. Another Co film was also produced on bare ITO under similar condition and its nominal thickness was adjusted to be 260 nm. This Co film was used to analyze the difference between the properties of Co and Co–PPy composite films. 2.3. Characterization The current density–time transients were recorded to investigate the deposition process of Co on bare ITO and PPy/ITO substrates. The surface analysis was performed by X-ray photoelectron spectrometer (XPS) by using monochromatic Al K␣ radiation as an X-ray source. The survey spectra and high resolution spectra of C1s, N1s, O1s, F1s, P2p and Co2p were recorded. Each core spectra were deconvoluted by Gaussian curves into the peaks to get best fit with Shirley background subtraction. X-ray diffraction (XRD) technique was employed by using Cu K␣ radiation. The diffraction angle (2) was changed from 20 to 60◦ with a step of 0.02◦ , since the reflection planes with the highest intensity of In2 O3 , Co and Co oxide is in this range. The morphology was analyzed by a scanning electron microscope (SEM). The images were taken at an accelerating voltage of 20.00 kV from 11.0 mm working distance. For 500 nm PPy and 260 nm (grown with 1000 mC cm−2 charge density) Co films, to be clear 5000× magnification images were presented. For all studied composite films, the magnification is 1000×. The chemical composition was studied by energy dispersive X-ray spectrometer (EDX). The magnetic properties were investigated by a vibrating sample magnetometer (VSM). The hysteresis curves were recorded by applying a magnetic field of 20 kOe parallel and perpendicular to the film plane. From the hysteresis curves, the coercivity (Hc ), the saturation (Ms ) and remnant magnetization (Mr ) were found. 3. Results and discussion Fig. 1 shows the development of the current density–time transients of Co deposition on (a) bare ITO (without PPy) and PPy/ITO substrates with the PPy layer thicknesses of (b) 20, (c) 50, (d) 200, (e) 500 and (f) 2000 nm in 5 s. All curves were plotted in cathodic direction. The deposition of Co on bare ITO takes place in 16 s with the initial current density of 70 mA cm−2 . The shape of the curve is very similar to those of obtained by other groups and the deposition can be explained as three dimensional (3D) (Fig. 7b) and diffusion controlled nucleation [20]. For the composite films, the applied negative potential is used not only for Co deposition and the hydrogen evolution but also for PPy reduction, since the potential is high enough for PPy reduction by releasing doping anion (PF6 − ). Based on this, it is difficult to determine

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

819

Fig. 1. The current density–time transients of (a) Co film on ITO, Co–PPy composite films with the PPy layer thicknesses of (b) 20, (c) 50, (d) 200, (e) 500 and (f) 2000 nm.

exactly the amount of the deposited Co from total passed charge. As seen from Fig. 1b, however the curve of the composite film with 20 nm PPy layer is similar to that of the Co film, the initial current density is smaller and reaches to a maximum value in a time longer than that of the Co film. This can be attributed to occurring the PPy reduction and Co deposition simultaneously. For 50 nm PPy layer (Fig. 1c), the initial current density is less than that of the formers and reaches a constant value with in the 4 s. For thicker PPy layers (>200 nm), an increase in the current density cannot be observed for the first few seconds, conversely it decreases and reaches a steady state. The discrepancies seen in the curves arise from different nucleation mechanism of Co on these PPy layers and/or inhibition of approaching Co2+ ions in solution to the PPy layer by released PF6 − ions. Fig. 2 displays the variation of the elapsed time with PPy thickness for Co deposition. The difference between Co nucleation mechanisms deposited on various PPy layers is seen obviously in that figure. This variation can be considered as three different regions (R), which are R1 (0–400 nm), R2 (500–1000 nm) and R3 (2000–5000 nm). It will be more reasonable to interpret Figs. 1 and 2 together. In R1, as the PPy thickness increases the initial current density becomes lower than the value of 70 mA cm−2 , caused by increasing resistance on the PPy surface and hence decreasing Co overpotential. In R2, the elapsed time decreases and for 1000 nm PPy layer, the deposition occurs in almost 40 s which is the same with the time recorded for PPy layers thinner than 200 nm. So that, the initial current density increases as the PPy layer gets thicker in R2. Although the thicker PPy layers (500–1000 nm) possibly have higher resistance, they have larger deposition area due to the surface roughness (Fig. 7) and this provides more nucleation sites for Co deposition. In R3, the elapsed time increases monotonically with increasing PPy layer thickness. Even though the initial current density is higher than that of the films in R2, after 5 s the current density decreases

Fig. 2. The variation of the elapsed time for Co deposition on ITO and PPy/ITO substrates as a function of PPy thickness.

down to below 5 mA cm−2 . The increase in the initial current density presumably arises from a decrease in the resistance of the PPy layer with high doping level [21]. Fig. 3 indicates the survey spectra of Co–PPy composite films having PPy layer with the thicknesses of (a) 20, (b) 200, (c) 500 and (d) 2000 nm. The peaks belonging to C1s, O1s and Co2p are dominant, and F2p, F2s, F1s, Co3p, Co3s, S2p and S2s are detectible. P2p, P2s and N1s peaks have almost same level with the device noise but the core spectra for N1s were obtained. The peaks labeled as S2p and S2s are appeared at the binding energy of 169 and 233 eV

820

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

Fig. 3. The survey spectra of Co–PPy composite films having PPy layer with the thickness of (a) 20, (b) 200, (c) 500 and (d) 2000 nm.

Fig. 4. C1s core level spectra of Co–PPy composite films with (a) 20, (b) 200, (c) 500 and (d) 2000 nm PPy layers.

respectively, which are both resulting from the presence of SO4 2− ions. These ions may locate on/into the PPy layers, attach to Co2+ ions and generate CoSO4 , since the value of these binding energies is close to that of the metal sulfate [22a,b]. Moreover, there could be a possible attraction and/or interaction between N+ and SO4 2− ions. As the PPy layer thickness increases the intensity of S2p and S2s peaks decreases. The core spectra of C1s and Co2p are depicted in Figs. 4 and 5, respectively. The results obtained from the deconvolution of the core spectra of C1s, N1s, F1s, O1s and Co2p are given in Table 1. C1s core level spectra of each Co–PPy composite film are deconvoluted into four curves (Table 1). The composite film with 20 nm

PPy layer has a peak centered at 284.2 eV, representing ␤ carbon type in PPy and similar peaks are observed for the PPy layers of 200, 500 and 2000 nm thicknesses. As the PPy thickness increases the intensity of the peaks decreases. For 200 nm and thicker PPy layers, the peak at 284.6 eV corresponds to ␣ carbon type in PPy. This peak does not appear in the spectrum of the composite film with 20 nm PPy layer, and the intensity increases with increasing PPy thickness. As seen, the linewidth of ␣ carbon type is wider than that of ␤ carbon type for each film [23,24]. The dominance of the ␤ carbon type for the composite films with the PPy layers thinner than 200 nm indicates that these films have more structural disorder on the PPy surface. The 20 nm PPy layer is very thin to establish

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

821

Fig. 5. Co2p core level spectra of Co–PPy composite films with (a) 20, (b) 200, (c) 500 and (d) 2000 nm PPy layers.

Table 1 Results obtained from deconvoluted core spectra of C1s, N1s, F1s, O1s and Co2p for different PPy layer thicknesses. 20 nm PPy

200 nm PPy

500 nm PPy

2000 nm PPy

B.E. (eV)

%

B.E. (eV)

%

B.E. (eV)

%

B.E. (eV)

%

Assignment

284.2 – 285.9 287.2 288.3

82.1

51.3 44.1

6.0 3.4

283.9 284.3 – 287.0 288.2

11.2 56.5

3.0 1.5

284.0 284.6 – 287.5 288.9

35.7 54.9

7.4 6.6 3.9

283.9 284.6 – 287.7 288.8

31.0 1.3

␤ Type carbon ␣ Type carbon C N+ , C N and C OH C O and C N+ COOH

N1s

396.9 398.8 399.5 405.3

8.4 14.3 60.7 16.6

397.3 398.5 400.9 403.7

49.5 31.4 16.4 2.7

– 398.1 – 407.5

– 82.7 – 17.3

– 398.1 – 400.4

– 25.7 – 74.3

Co N N NH Oxidized N

F1s

684.2 686.2 689.4

42.6 37.9 19.5

685.1 685.8 –

15.9 84.1 –

– 686.0 –

– 100.0 –

– 686.4 –

– 100.0 –

O1s

530.8 530.9

52.7 47.3

530.5 530.8

54.6 45.4

530.3 530.5

52.4 47.6

530.4 530.7

45.4 54.6

C O, Co O C O, Co O

Co2p

777.7 780.0 781.9 785.0 796.2 801.8

12.8 24.0 10.2 20.8 15.1 17.1

777.8 780.0 782.0 785.0 796.1 801.7

28.1 25.6 5.4 11.2 15.2 14.6

778.5 779.9 781.9 784.9 795.9 801.6

17.2 28.8 5.5 17.2 16.4 15.0

777.1 779.9 781.7 784.8 796.0 801.7

21.5 25.4 7.9 16.3 15.7 13.2

Metallic Co Co oxide Co(OH)2 Shake up satellite Co oxide Shake up satellite

C1s

enough structural order, and for 200 nm PPy layer, the released PF6 − ions and the attached SO4 2− ions may cause the disorderliness. The difference of the PPy layer surface is also distinguishable from the current density–time transients (Fig. 1d) and SEM image (Fig. 7d), which are both proving different Co nucleation mechanisms. The peak at 285.9 eV is assigned to C N+ , C N and C OH bonds and obtained for only 20 nm PPy layer [24,25]. The peak of 287.2 eV is due to C O and C N+ bonds for 20 nm PPy layer [24,26]. This peak is observed for other films and the intensity is very low exception for 2000 nm PPy layer. It means that the amount of positively charged nitrogen (N+ ) ions on the surface is low. These results are compatible with that found in the core spectra of N1s, in which the peak corresponding to the N+ ions cannot be detected. For PPy films, N+ ions represent the doping level and hence the conductivity of the

PF6 − PF6 − C F

Co2+

film [27]. The conductivity of the 2000 nm PPy layer may higher than the thinner ones due to high doping level as found in the current density–time transients. The last peak seen at the spectrum of 20 nm PPy layer is 288.3 eV as a result from COOH bonds [28]. The similar peaks with very low intensity are obtained for other films. N1s core level spectra of the composite films with 20 and 200 nm PPy layer are decomposed into 4 peaks while the 500 and 2000 nm layers into 2 peaks to get the best fit. The intensity for the films with 500 and 2000 nm PPy layer is very low. The peaks at 396.9 and 397.3 (200 nm PPy) eV are attributed to Co N bonds, which both arise from the interaction between Co and N atoms [18]. Even though this peak does not appeared for thicker PPy films, the absence of the peaks (at 401–402 eV) belonging to N+ ions may prove an interaction between Co and N atoms for them [29–31]. The peak at

822

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

Fig. 6. XRD spectra of ITO substrate and Co–PPy composite films with different PPy thicknesses.

398.8 eV is assigned to imine like ( N ) nitrogen for 20 nm PPy layer and similar peaks are observed for other films as well. These peaks indicate the structural defects on the PPy layer surface. The intensity of them increases up to 500 nm and then decrease [32,33]. The peaks of 399.5, 400.9 and 400.4 eV are due to the neutral amine like ( NH ) nitrogen for the PPy layers with the thicknesses of 20, 200 and 2000 nm, respectively [34]. It does not appear for 500 nm PPy layer. The peak seen at 405.3 eV for 20 nm PPy layer represents oxidized nitrogen, and there are also similar peaks for thicker films. In order to get information about the doping anion in the films, the core spectra of F1s and P2p were recorded. The intensity of the P2p is very low to study and the intensity of F1s decreases as the thickness of the PPy film increases. F1s spectra are deconvoluted into 3 peaks (20 nm PPy), 2 peaks (200 nm PPy), and 1 peak (500 and 2000 nm PPy) to get the best fit (Table 1). The peaks at 684.2 (20 nm PPy) and 685.1 eV (200 nm PPy) are assigned to a possible interaction between PF6 − and Co2+ ions [35]. The peak seen at 686.2 eV which is common for all studied PPy films, indicates the presence of PF6 − ions [36]. The peak at 689 eV observed for only 20 nm PPy layer may arise from the C F bonds [23]. The one of the dominant peaks for the spectra of all films is O1s due to easiness oxidative character of PPy and Co metal. The spectra are decomposed into two peaks located very near to each other as given in Table 1. These peaks are attributed to C O and Co O bonds [37]. The core spectra of Co2p are deconvoluted into six peaks as seen in Fig. 5 (see Table 1). In the film with 20 nm PPy layer, the peaks at 777.7, 780.0, 781.9, 796.2 eV are assigned to Co metal, 2p3/2 Co oxide (CoO, Co3 O4 ), Co(OH)2 and 2p1/2 Co oxide (CoO, Co3 O4 ) respectively [37,38]. These peaks are also detected for thicker films. The intensity of the oxide peaks is stronger than that of the metal peaks, which confirms the presence of the Co oxides on the surface, as found from O1s core spectra. The separation of 2p3/2 and 2p1/2 Co oxide peaks is about 16 eV for all samples. This shows that Co oxide is in the form of CoO dominantly, but XRD results verify the presence of Co3 O4 . The peaks appeared at 785.0 and 801.8 eV are attributed to shake up satellite peaks. These peaks locate at a value of 5–6 eV higher than those of 2p3/2 and 2p1/2 Co oxide peaks. As a result of this, on the surface of the composite films, Co exits at high spin Co2+ state rather than low spin Co3+ state (from Co3 O4 ). The high spin Co2+ state has the paramagnetic effect while the low spin Co3+ state is diamagnetic [37,39,40]. Fig. 6 shows XRD patterns of the ITO substrate, and Co–PPy composite films with different PPy layer thicknesses. ITO substrate has clear peaks at around 22, 31, 36, 38, 45 and 51◦ corresponding to the Bragg reflections from the planes of (2 0 0), (2 2 2) (4 0 0), (4 1 1), (1 3 4) and (4 4 0) respectively [41,42]. These are the characteristic

peaks of the cubic structure of indium oxide (In2 O3 ). The reflections of the composite films cannot be distinguished clearly from the substrate but some slight changes are observed. As the PPy layer gets thicker, the intensity of the peak at 22◦ decreases due to the influence of the amorphous structure of the PPy between 20 and 30◦ . The intensity of the other peaks decreases as well. The peak at 31◦ shifts to slightly higher degree, which represents the (2 2 0) reflections of spinel Co3 O4 . From the peak positions, the lattices parameter was calculated to be 8.13 A˚ for all studied films. The result is in well ˚ [43]. agreement with the lattice parameter of Co3 O4 (a = 8.083 A) The peak at 36◦ for the PPy layer thicknesses of 20, 200 and 500 nm is at the same position with that of ITO, but for 2000 nm PPy layer the position barely moves to a higher value. This is possibly due to the (1 1 1) reflection from cubic CoO [43]. The increase of the peak intensity at 38◦ with PPy layer thickness can be attributed to the (2 2 2) reflection of Co3 O4 [43]. The peak at 42◦ is clear for 20 and 2000 nm PPy film, which belongs to the reflections from the (1 0 0) planes of hexagonal closed packet (hcp) Co or the (2 0 0) planes of cubic CoO [43]. For 20 nm PPy layer, the peak seen at 48◦ represents the (1 0 1) reflection of hcp Co. Despite the surface of the films contain CoO amount higher than Co metal and Co3 O4 according to XPS, the reflections from CoO do not appear clearly. Also, there is not any reflection for the face centered cubic (fcc) Co which can be adopted by Co on a cubic substrate, possibly by inhibition of the PPy layer. Fig. 7 shows SEM images of a 500 nm PPy film (a), a Co film deposited with the charge density of 1000 mC cm−2 which corresponds to thickness of 260 nm (b), and the composite films with the PPy thicknesses of (c) 20, (d) 200, (e) 500 and (f) 2000 nm. The PPy film has a smooth surface with very small hydrogen bubbles which are characteristic property for thick PPy films. Thanks to these bubbles the PPy layer can provide larger surface area for the Co deposition. The Co film has a uniform granular morphology. On the surface of the composite film with 20 nm PPy layer, there are light, spherical, Co rich regions (by EDX, one of them is shown in a yellow circle). In the narrow, smooth and dark areas, little amount of Co was detected as well. For most of the locations, Co atoms prefer to deposit along the line, which are likely Co nucleation sites of the polymer chains. The surface of the composite film with 200 nm PPy layer is very disorder and far from the homogeneity. This can be estimated from the current density–time transients (Fig. 1d), since the recorded current density values for the few seconds are not very regular. For some areas Co grows as dendritic and cauliflower shaped particles seen from Fig. 7d. Some other areas (not shown) are very smooth and in these areas small amount of Co is recorded. This can be attributed to increasing in the resistance and/or disorderliness of the PPy film, which does not allow the Co to deposit homogenously. For the composite film with 500 nm PPy, as seen from Fig. 7e, the surface is smoother than those of the formers and the Co deposition is observed all around the surface. This can be arise from 500 nm PPy layer having a surface area larger than those of the thinners due to hydrogen bubbles. Co prefers to grow as clusters (by EDX, shown as yellow circles) and disordered lines (by EDX, shown as yellow ellipse) in different regions. For the composite film with the 2000 nm PPy, there are both large and small PPy islands due to its thickness. On these islands Co clusters are seen (by EDX, shown yellow circles), but they favor to deposit very close to each other. Fig. 8 displays the EDX spectra of the composite films having 20, 200, 500 and 2000 nm PPy layer. The measurements were made from a wide square surface with 100× magnification. The peaks belonging to Au and Pd arise from the top surface coating made to study with SEM and EDX. The peaks of Si, In and Sn are arise from ITO substrate and their intensity decreases with increasing the PPy layer thickness as found in XRD results. The peaks assigned to C, N, O and F were labeled as 1, 2, 3 and 4, respectively. The N and F peaks are detectible but the P peaks do not appear in the spectra.

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

823

Fig. 7. SEM images of (a) 500 nm PPy film, (b) 260 nm Co film, and the composite films with the PPy thicknesses of (c) 20, (d) 200, (e) 500 and (f) 2000 nm.

Fig. 8. EDX spectra of the composite films with 20, 200, 500 and 2000 nm PPy layer.

The S peaks are seen as a shoulder of Au. The peaks rising at 0.78 and 6.9 keV belong to Co L␣ and K␣ respectively and very clear for all films. The integrated intensities of the C, N, F, S, O and Co peaks were calculated and evaluated within the limit error. The C intensity increases with increasing PPy film thickness up to 500 nm and then remains stable. The N intensity is very low and almost the same for all analyzed films. The F intensity of 20 nm PPy layer has the highest value, while for thicker layers (>20 nm), it firstly decreases and then

becomes constant. The S intensity increases for 200 nm PPy layer, and then decreases. The highest S intensity for the film with 200 nm PPy layer could be an indicative of forming the irregular surface of SO4 2− ions and hence the inhibiting homogeneity of Co deposition as seen in the SEM image (Fig. 7d). The O intensity decreases with increasing PPy layer thickness. The Co intensity is the same for all studied films. Fig. 9 indicates the hysteresis curves of the Co film deposited on ITO substrate and the Co–PPy composite films with different

824

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

Fig. 9. Hysteresis curves of (a) the Co film on ITO substrate, and the composite films with the PPy layer thicknesses of (b) 20, (c) 200, (d) 500, (e) 2000 and (f) 4000 nm.

Fig. 10. The variation of Hc (a) and Ms and Mr (b) depending on PPy thickness.

PPy layers. Although the XPS results showed that the composite films contain Co oxide abundantly, they have ferromagnetic behavior dominantly. For all composite films, the value of the magnetic moment reduces after saturates as a result of PPy layers which have diamagnetic behavior. For Co film on ITO substrate (Fig. 9a), when the magnetic field applied parallel to the film plane the magnetic moment saturates at the value lower than that of the perpendicular, namely, the easy axis is parallel to the film plane. As expected, Co thin films have anisotropic behavior. For the composite film with 20 nm PPy layer (Fig. 9b), the saturation of the magnetic moment obtained at the same magnetic field with both parallel and perpendicular to the film plane, which shows isotropic behavior. Similar hysteresis curves were observed for the PPy layers with 50 and 100 nm. The composite film with 200 nm PPy layer (Fig. 9c), displays weak magnetic anisotropy and this is more distinctive for 500 nm PPy layer (Fig. 9d). For thicker films, this anisotropy diminishes gradually and the isotropic behavior comes out, as seen from the hysteresis curves of 2000 nm PPy film (Fig. 9e). The composite films with 3000 and 4000 nm (Fig. 9f) PPy have isotropic and

5000 nm PPy anisotropic behavior. As known, in microscopic scale, the magnetic anisotropy depends on the crystal electric field and the spin–orbit interaction. Generally, in the 3d transition metals and alloys the former one is dominant [44]. In our case, Co was deposited on/in an amorphous PPy and the composite films have reflections as a sign of weak crystal structure, even it is difficult to determine any preferred orientation, as given in XRD spectra (Fig. 6). The Co atoms deposited on the PPy layers, which are thinner than 200 nm and thicker than 2000 nm, may expose to symmetrical crystal field, therefore, have an isotropic behavior. For the PPy layers with the thicknesses between 200 and 2000 nm, the characteristic bubbles on the surface possibly change the crystal field and cause an anisotropic behavior. Such kind of effects was reported for Co nanowires which do not have any preferred orientation as well [45]. Fig. 10a displays the coercivity (Hc ) depending on PPy thickness. The Hc value calculated for the Co film is 417 Oe. For the composite films, the Hc values indicate fluctuations between 260 and 530 Oe. All films are hard magnetic material since their Hc values are

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

higher than 125 Oe. The variation of Ms and Mr values with the PPy thickness is given in Fig. 10b. The Ms and Mr values were calculated by dividing the saturation moment (emu) to the nominal mass (calculated according to Faraday law) of Co (g). The Ms of the Co film was found to be 123 emu/g, which is smaller than that of the bulk Co (162.5 emu/g). This is mostly obtained for one or two dimensional structures due to the finite size effect. For Co–PPy composite films, the Ms decreases drastically up to 200 nm PPy layer, then increases up to 2000 nm PPy layer. It fluctuates for thicker PPy layers (>2000 nm). According to the EDX results, the Co content of the composite films is the same, but the Ms values are different. The reason for this may arise from the different interactions between Co and Co, and/or Co and PPy for each film. The Mr value was calculated to be 50 emu/g for the Co film, but for all composite films, it is under a value of 25 emu/g. Because the PPy layer may weaken the interaction between Co atoms and/or hinder large Co grain sizes. 4. Conclusion Co–PPy composite films were deposited on ITO substrate from two separate solutions under the potentiostatic control. The Co deposition was made by applying a charge density of 1000 mC cm−2 on the PPy layers. The layer thicknesses were changed from 20 to 5000 nm. A Co film with the same charge density was electrodeposited on ITO for comparison. The current density–time transients revealed that the nucleation of the Co on ITO and each PPy layer is different. It was found that, both the Co metal and Cooxides (Co3 O4 and CoO) on the composite surfaces were detected by XPS. The XRD spectra showed that, the films have weak reflections from the planes of Co3 O4 , CoO and hcp Co. According to SEM images, the Co particles prefer to grow along the lines on 20 nm PPy layer. The Co sites seen on 200 nm PPy layer is very irregular, possibly due to high resistance of the PPy layer, and PF6 − and/or SO4 2− ions on the surface. The characteristic bubbles on the surface of 500 and 2000 nm PPy provide a larger surface area for the Co deposition. The Co content of composite films was found to be the same within the error limits by EDX. The magnetic isotropic behavior was observed for the composite films having PPy layer thinner than 200 nm and thicker than 2000 nm. This behavior could arise from exposing of Co to a symmetrical crystal field. The magnetic anisotropic behavior was obtained for the composite films with the PPy layer thicknesses between 200 and 2000 nm, owing to the fact that the degradation of the symmetrical crystal field. All studied films are hard magnetic material because the Hc value is higher than 125 Oe. Acknowledgements This work was supported by Uludag University under Grant No. UAP(F)-2010/56 and State Planning Organization, Turkey under Grant No. 2005K120170 for VSM system. The author is grateful to Middle East Technical University, Central Lab. Turkey for XPS measurements. References [1] G. Tourillon, E. Dartyge, H. Dexpert, A. Fontaine, A. Jucha, P. Lagarde, D.E. Sayers, Electrochemical inclusion of metallic clusters in organic conducting polymers, J. Electroanal. Chem. 178 (1984) 357–366. [2] G. Tourillon, F. Gamier, Inclusion of metallic aggregates in organic conducting polymers. A new catalytic system, [poly(3-methylthiophene)-Ag–Pt], for proton electrochemical reduction, J. Phys. Chem. 88 (1984) 5281–5285. [3] D.E. Weisshaar, T. Kuwana, Electrodeposition of metal microparticles in a polymer film on a glassy carbon electrode, J. Electroanal. Chem. 163 (1984) 395–399. [4] G.K. Chandler, D. Pletcher, The electrodeposition of metals onto polypyrrole films from aqueous solution, J. Appl. Electrochem. 16 (1986) 62–68.

825

[5] S. Holdcroft, B.L. Funt, Preparation and electrocatalytic properties of conducting films of polypyrrole conducting platinum microparticulates, J. Electroanal. Chem. 240 (1988) 89–103. [6] L. Dai, Intelligent Macromolecules for Smart Devices from Materials Synthesis to Device Applications, 1st ed., Beijing and Springer-Verlag GmbH, Berlin, 2004. [7] S. Cosnier, A. Karyakin, Electropolymerization: Concepts, Materials and Applications, 1st ed., Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim, 2010. [8] J. Li, X. Lin, Electrocatalytic reduction of nitrite at polypyrrole nanowire–platinum nanocluster modified glassy carbon electrode, Microchem. J. 87 (2007) 41–46. [9] L. Tian, Y. Qi, B. Wang, Electrochemical preparation and structural characterization of platinum thin film on a polypyrrole film modified ITO electrode, J. Colloid Interface Sci. 333 (2009) 249–253. [10] D.K. Sarkar, X.J. Zhou, A. Tannous, M. Louie, K.T. Leung, Growth of self-assembled copper nanostructure on conducting polymer by electrodeposition, Solid State Commun. 125 (2003) 365–368. [11] X.J. Zhou, A.J. Harmer, N.F. Heinig, K.T. Leung, Parametric study on electrochemical deposition of copper nanoparticles on an ultrathin polypyrrole film deposited on a gold film electrode, Langmuir 20 (2004) 5109–5113. [12] Y. Li, G. Shi, Electrochemical growth of two-dimensional gold nanostructures on a thin polypyrrole film modified ITO electrode, J. Phys. Chem. B 109 (2005) 23787–23793. [13] J.Y. Lee, T.C. Tan, Cyclic voltammetry of electrodeposition of metal on electrosynthesized polypyrrole film, J. Electrochem. Soc. 137 (1990) 1402–1408. [14] V. Tsakova, How to affect number, size, and location of metal particles deposited in conducting polymer layers, J. Solid State Electrochem. 12 (2008) 1421–1434. [15] N.F. Heinig, N. Kharbanda, M.R. Pynenburg, X.J. Zhou, G.A. Schultz, K.T. Leung, The growth of nickel nanoparticles on conductive polymer composite electrodes, Mater. Lett. 62 (2008) 2285–2288. [16] M. Haciismailoglu, M.C. Haciismailoglu, M. Alper, W. Schwarzacher, Electrodeposition and characterization of Co particles on ultrathin polypyrrole films, J. Supercond. Nov. Magn. 11 (2014) 2599–2606. [17] M. Chipara, R. Skomski, D.J. Sellmyer, Electrodeposition and magnetic properties of polypyrrole–Fe nanocomposites, Mater. Lett. 61 (2007) 2412–2415. [18] N. Watanabe, J. Morais, S.B.B. Accione, A. Morrone, J.E. Schmidt, M.C. Martins Alves, Electronic, structural, and magnetic properties of cobalt aggregates embedded in polypyrrole, J. Phys. Chem. B 108 (2004) 4013–4017. [19] E. Hermelin, J. Petitjean, S. Aeiyach, J.C. Lacroix, P.C. Lacaze, Industrial polypyrrole electrodeposition on zinc-electroplated steel, J. Appl. Electrochem. 31 (2001) 905–911. [20] R.D. Armstrong, J.A. Harrison, Two-dimensional nucleation in electrocrystallization, J. Electrochem. Soc. 116 (1969) 328–331. [21] F. Chen, G. Shi, M. Fu, L. Qu, X. Hong, Raman spectroscopic evidence of thickness dependence of the doping level of electrochemically deposited polypyrrole film, Synth. Met. 132 (2003) 125–132. [22] (a) T.I. Koranyi, I. Manninger, Z. Paal, O. Marks, J.R. Günter, Activation of unsupported CoMo catalysts in thiophene hydrodesulfurization, J. Catal. 116 (1989) 422–439; (b) D.S. Zingg, D.M. Hercules, Electron spectroscopy for chemical analysis studies of lead sulfide oxidation, J. Phys. Chem. 82 (1978) 1992–1995. [23] P. Pfluger, G.B. Street, Chemical, electronic, and structural properties of conducting heterocyclic polymers: a view by XPS, J. Chem. Phys. 80 (1984) 544–553. [24] L. Sabbatini, C. Malitesta, E. De Giglio, I. Losito, L. Torsi, P.G. Zambonin, Electrosynthesised thin polymer films: the role of XPS in the design of application oriented innovative materials, J. Electron Spectrosc. Relat. Phenom. 100 (1999) 35–53. [25] Y.S. Lim, Y.P. Tan, H.N. Lim, N.M. Huang, W.T. Tan, M.A. Yarmo, Y. Chun-Yang, Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors, Ceram. Int. 40 (2014) 3855–3864. [26] M. Omastova, K. Boukerma, M.M. Chehimi, M. Trchova, Novel silicon carbide/polypyrrole composites; preparation and physicochemical properties, Mater. Res. Bull. 40 (2005) 749–765. [27] W. Liang, J. Lei, C.R. Martin, Effect of synthesis temperature on the structure, doping level and charge-transport properties of polypyrrole, Synth. Met. 52 (1992) 227–239. [28] C. Malitesta, I. Losito, L. Sabbatini, P.G. Zambonin, Applicability of chemical derivatization – X-ray photoelectron spectroscopy (CD–XPS) to the characterization of complex matrices: case of electrosynthesized polypyrroles, J. Electron Spectrosc. Relat. Phenom. 97 (1998) 199–208. [29] K. Hyun, J.H. Lee, C.W. Yoon, Y.H. Cho, L.H. Kim, Y. Kwon, Improvement in oxygen reduction activity of polypyrrole-coated PtNi alloy catalyst prepared for proton exchange membrane fuel cells, Synth. Met. 190 (2014) 48–55. [30] H.Y. Qin, K.N. Zhu, L.Q. Ye, Z.P. Li, Oxygen reduction reaction on a polypyrrole-modified, carbon-supported cobalt hydroxide catalyst, J. Power Sources 208 (2012) 203–209. [31] D.K. Sarkar, X.J. Zhou, A. Tannous, K.T. Leung, Growth mechanisms of copper nanocrystals on thin polypyrrole films by electrochemistry, J. Phys. Chem. B 107 (2003) 2879–2881.

826

M. Haciismailoglu / Applied Surface Science 356 (2015) 817–826

[32] H. Zhao, L. Pan, J. Jin, L. Li, J. Xu, PtCo/polypyrrole-multiwalled carbon nanotube complex cathode catalyst containing two types of oxygen reduction active sites used in direct methanol fuel cells, Fuel Cells 12 (2012) 876–882. [33] V.W.L. Lim, S. Li, E.T. Kang, K.G. Neoh, K.L. Tan, In situ XPS study of thermally deposited aluminium on chemically synthesized polypyrrole films, Synth. Met. 106 (1999) 1–11. [34] H.S. Oh, J.G. Oh, B. Roh, I. Hwang, H. Kim, Development of highly active and stable non-precious oxygen reduction catalysts for PEM fuel cells using polypyrrole and a chelating agent, Electrochem. Commun. 13 (2011) 879–881. [35] B.P. Lochel, H.H. Strehblow, Breakdown of passivity of nickel by fluoride: II. Surface analytical studies, J. Electrochem. Soc. 131 (1984) 713–723. [36] C. Oumahi, J. Lombard, S. Casale, C. Calers, L. Delannoy, C. Louis, X. Carrier, Heterogeneous catalyst preparation in ionic liquids: titania supported gold nanoparticles, Catal. Today 235 (2014) 58–71. [37] A. Amri, X.F. Duan, C.Y. Yinc, Z.T. Jiang, M.M. Rahman, T. Pryor, Solar absorptance of copper–cobalt oxide thin film coatings with nano-size, grain-like morphology: optimization and synchrotron radiation XPS studies, Appl. Surf. Sci. 275 (2013) 127–135. [38] Y.T. Hsieh, M.C. Lai, H.L. Huang, I.W. Sun, Speciation of cobalt–chloride-based ionic liquids and electrodeposition of Co wires, Electrochim. Acta 117 (2014) 217–223.

[39] A. Mekki, D. Holland, Kh. Ziq, C.F. McConville, XPS and magnetization studies of cobalt sodium silicate glasses, J. Non-Cryst. Solids 220 (1997) 267–279. [40] S.C. Petitto, E.M. Marsh, G.A. Carson, M.A. Langell, Cobalt oxide surface chemistry: the interaction of CoO(1 0 0), Co3 O4 (1 1 0) and Co3 O4 (1 1 1) with oxygen and water, J. Mol. Catal. A Chem. 281 (2008) 49–58. [41] B.C. Kim, J.Y. Kim, D.D. Lee, J.O. Lim, J.S. Huh, Effects of crystal structures on gas sensing properties of nanocrystalline ITO thick films, Sens. Actuators B Chem. 89 (2003) 180–186. [42] Y. Wang, J. Liu, X. Wu, B. Yang, Adhesion enhancement of indium tin oxide (ITO) coated quartz optical fibers, Appl. Surf. Sci. 308 (2014) 341–346. [43] J.S. Gwag, Y. Sohn, Interfacial natures and controlling morphology of Co oxide nanocrystal structures by adding spectator Ni ions, Bull. Korean Chem. Soc. 33 (2012) 505–510. [44] R.C. O’Handley, Modern Magnetic Materials Principles and Applications, 1st ed., John Wiley & Sons Inc., New York, 2000. [45] D. Kaur, S. Chaudhary, D.K. Pandya, R. Gupta, R.K. Kotnal, Magnetization reversal studies in structurally tailored cobalt nanowires, J. Magn. Magn. Mater. 344 (2013) 72–78.