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Surface Characterization and Morphology of Conducting Polypyrrole Thin Films during Polymer Growth on ITO Glass Electrode Jalal Arjomandi, Davood Raoufi, and Fatemeh Ghamari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04913 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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The Journal of Physical Chemistry

Surface Characterization and Morphology of Conducting Polypyrrole Thin Films during Polymer Growth on ITO Glass Electrode Jalal Arjomandi*,1, Davood Raoufi2 and Fatemeh Ghamari2 1

Department of Physical Chemistry, Faculty of Chemistry, Bu Ali Sina University, 65178, Hamedan, Iran

2

Department of Physics, Faculty of Science, Bu Ali Sina University, 65174, Hamedan, Iran

* To whom correspondence should be addressed, E-mail: [email protected], Tel: +988138282807, Fax: +988138257407

ABSTRACT: Statistical, morphological and fractal analysis of the conducting polypyrrole (PPy) thin films during polymer growth on the indium tin oxide (ITO) glass electrode were investigated. Cyclic voltammetry (CV) was used to synthesis of polymer thin films with different thicknesses and calculation of fractal dimensions (Df). Atomic force microscopy (AFM) as a powerful technique was employed to statistical study and analysis the morphology of different type of thin films surfaces with parameters such as root mean square (RMS), kurtosis (Ku), skewness (Sk) and Df. For calculating the fractal dimensions from AFM images of different thin films, power spectral density, perimeter-area and box-counting methods were used. The results show that the fractal dimensions were increase with increasing the thicknesses of the films. RMS, Ku and Sk parameters of the films were changed with increasing the film thicknesses. Moreover, X-ray diffraction (XRD) analysis technique was confirmed process of growing polymers on polycrystalline indium tin oxide (ITO) and increasing the crystallinity of PPy during film growth.

INTRODUCTION Among well-known conductors and semiconductors, electrically conducting polymers (CPs) have a specific position. Polyaniline (PAn), polythiophene (PTh), polypyrrole (PPy) and other electronic CPs have generated great interest in various technological application such as

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metallization of dielectrics,1 batteries,2 antistatic coatings,3 shielding for electromagnetic interferences,4 sensors,5 actuators,6 and micro actuators,7 drug delivery,8 corrosion inhibition,9 solar cells,10 light emitting diodes,11 printing electronic circuits,12 gases,13,

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biomolecules,15-19 conductive yarns20 and supercapacitor.21-23 All this interest about CPs is due to their unique properties such as high electrical conductivity, chemical stability, flexibility and biocompatibility, low toxicity, simplistic synthesis procedure, ease of doping by both chemical and electrochemical polymerization, excellent environmental stability, ecofriendly nature, and good thermal stability.23–26 The study of the properties of the CPs materials and the ability to control their morphology is one of the most interesting fields for researchers, because these can lead to the improvement of the physical and chemical properties of conducting polymers.27-29 Moreover the study of changing film thickness as an important parameter in their applications was investigated.30, 31 Characterization and study of statistical properties as well as surface analysis of CPs thin films are affected by some parameters such as different temperatures, dopant size and changes in thickness during film deposition.32-34 Within those parameters, different thicknesses during film growth on the surface electrodes will lead to different electrical conductivity and their surface roughness.35, 36

Between CPs, PPy is one of the most promising polymers for many potential

applications.37 PPy film depositions can be prepared simply with one step procedure and exact control of thickness on electrode surfaces. PPy thicknesses can be estimated from the amount of charge passed during electrodeposition.38-40 Moreover, the preparation condition investigations of conducting polymers include deposition potential, current and composition of deposition media.39 To use the fractal approach for characterization of surface topography and surface roughness of CPs, atomic force microscopy (AFM) can be used as a powerful technique. The electrochemical techniques and AFM method were used to study the surface morphology of PPy by several researchers.33, 34, 39, 41 The statistically analysis the surface of PPy film in different synthesis temperatures was reported previously. Roughness parameters such as skewness, kurtosis and etc., were used to study the PPy surfaces.42 Also multi fractal dimension of PPy thin films in different film thicknesses and various synthesis temperatures were investigated previously.42,

43

In this research and in constant temperature for the first

time, we investigated the fractal and statistical analysis and electrochemical deposition of PPy film growth on indium tin oxide (ITO) electrodes. Several approach such as power spectral density, perimeter-area, peak-current or cyclic voltammetry and box-counting methods for investigating morphological details during the polypyrrole (PPy) film growth on ITO glass substrates were employed. The fractal analysis of different thicknesses type of PPy


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film was studied using cyclic voltammetry and AFM techniques. Statistical studies were performed by using of available concepts such as instance average roughness (Ra) and root mean square (RMS) during polymer growth on ITO glass electrode. Finally, XRD technique was used to confirm the process of growing PPy film on the ITO glass surface.

EXPERIMENTAL Pyrrole (Py) (Aldrich, 99%) was distilled under vacuum. Polymers were grown in an acetonitrile (Merck) solution containing 0.10 M pyrrole and 0.1 M LiClO4 (Aldrich Chemie, 99 %, electrolyte) potentiodynamically by scanning the different rang of applied potentials on ITO glass electrodes, separately. Cyclic voltammetry (CV) was performed using a Behpajoh model BHP/2062 and an Autolab model PGSTAT 20 potentiostat/galvanostat. A threecompartment electrochemical cell (H-cell) with glass frit separators between the cell compartments was used for voltammetric studies. ITO coated glass sheet (Praezisions Glas & Optik, Germany, R = 20 ± 5 Ωcm-2) was used as a working electrode. A gold wire (from AZAR electrode) was used as counter electrode. For electrosynthesis of PPy films on ITO coated glass, monomer concentrations was [Pyrrole] = 0.1 M and solutions were prepared in acetonitrile + 0.1 M LiClO4. After vigorous mixing and nitrogen purging for around 10 min, electropolymerization was effected by scanning the electrode potential between 0.00 < E SCE < 0.90 V at a scan rate of 50 mV·s-1. The electrosynthesis of different PPy thin films were stopped after 3, 7, and 20 cycles, respectively. The statistical and morphological properties of PPy thin films were characterized using atomic force microscopy (AFM) technique. The surfaces ex situ topographic measurements of the samples on a small scale have been measured using an atomic force microscopy (AFM) instrument (TM Microscopes Veeco Metrology Group) on contact mode. A commercial standard pyramidal Si3N4 tip was used. AFM images were obtained, at room temperature, and digitized into 256 × 256 pixels. A variety of scans were obtained at random positions on the films surfaces. Then, for analyzing the AFM images, the topographic image data were changed into ASCII data which were analyzed by the fractal analyses method to determine the micro roughness parameters of the thin films surfaces. The phase composition of PPy thin films and ITO thin film were characterized using XRD technique with a D8 Advanced Bruker X-ray diffractometer at room temperature, with monochromated CuKα (λ=0.154 nm) in the scan range of 2θ between 15° and 70° with a step size of 0.04 (2θ/s). Measurements were taken under beamacceleration conditions of 40 kV/35 mA.



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RESULTE AND DISCUSSIONS

Cyclic Voltammetry

CVs during formation of PPy film in acetonitrile + 0.1 M LiClO4 and 0.1 M pyrrole at an ITO electrode in three different thicknesses, (a) 1, 2, 3 cycle, (b) 1, 2, 3..., 7th cycle and (c) 1, 2, 3,..., 20th cycle, are compared and displayed in Figure 1, respectively. Initial CVs of the polymer formation are shown a nucleation process followed by growth of nuclei to a continuous film.44-46 With increasing the number of cycles from 3 to 20, (Figure 1a to Figure 1c), the corresponding currents and thickness of the PPy film increases, respectively. The thicknesses of each film are determined by the electric charge passed during the electrosynthesis process.39 During PPy film grows on the ITO glass, the oxidation potentials for the curves show slightly different values. There is a plateau standing between 0.25 to 0.55 V (Figure 1c). The scan rate dependencies of different PPy films at ITO glass electrodes in acetonitrile + 0.1 M LiClO4 are displayed in Figure 2a-2c. Different scan rates (dE/dt) from 20 to 120 mV s-1 were employed. Different oxidation potential and current were observed for each film. The difference in oxidation potentials, i.e. the different irreversibility of the oxidation- reduction process are indicated the different behavior of the PPy films with various thicknesses on the surface of ITO glasses electrode. Kinetic results and in situ conducting measurements of PPy thin films during polymer growth on ITO glass electrode with different thicknesses revealed that with growing the thickness of the films conductivity increased.47

Statistical studies

For the characterization of PPy surfaces thin films on ITO glass by AFM, we used two dimensional matrix that each (xi,yj) array has a height of h(xi,yj).42, 47 It can be estimated that there are large numbers of points and applying of statistical analyses to investigate the characteristics of surface. In order to statistical analysis, average height of surface was calculated (see Equation S1 in the Supporting Information). Two main parameters of statistical studies are average roughness and interface width or root mean square (RMS). The average roughness (Ra) is defined as the average value of the absolute distances of the surface points from the mean plane (see Equation S2 in the Supporting Information).48,

49

The interface width (w) roughness or the root mean square

value of the surface height (RMS), is the standard deviation of the surface from the mean plane within the sampling area (see Equations S2 and S3 in the Supporting Information).48, 49


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Other important parameters for characterization asymmetry of the height distribution surface of PPy on ITO glass are skewness (Sk) and kurtosis (Ku). The skewness and kurtosis point out the third and fourth moments of the density function, respectively. For the Gaussian distribution, the skewness is equal to zero and the kurtosis is equal to three.50 The skewness and kurtosis were defined as equations. (See Equations S4 and S5 in the Supporting Information).42 More details about the skewness and kurtosis of thin films are available and described by researchers.42, 50-54 As demonstrated by them, the sign of Sk is positive or negative that reveals the further points are proportionately above or below the mean surface level, respectively.51 As well as, Ku is revels the randomness of the surface profile via Gaussian distributions that has a value of 3. The values of Ku < 3 and Ku > 3 demonstrate the distribution of mild and sharp peaks or platykurtic and leptokurtic, respectively.52 Typical AFM images of PPy thin films on ITO glass electrode are shown in Figure 3. The images with 1 µm×1 µm scan area were taken from some different randomly selected area of polymer films on ITO. Here, the AFM images with matrixes having 65536 points which calculated from 256 rows and 256 columns. The images clearly demonstrate that the different surface roughness for PPy at three conditions. Figure 3a, 3b and 3c shows the 8.3, 11.0 and 41.6 nm height for PPy films prepares after 3, 7 and 20 cycles, respectively. It can be seen that the surface roughness and actual height of the films increase with increasing the number of cycles. Values of average height, average roughness, root mean square roughness, skewness and kurtosis of PPy films on ITO glass electrode synthesized at different thicknesses from the 3D AFM images are shown in Table 1. The average height, average roughness and root mean square roughness of PPy films increases from thinner to thicker PPy films on ITO glass electrodes. AFM images and their corresponding cross section analyses obtained for PPy films on ITO glass electrode synthesized at different thicknesses are shown in Figure 4. The 2D AFM images and surface profiles of thin films clearly show the increasing of average height, average roughness and root mean square roughness. It can be seen that the average roughness and root mean square roughness increased as the thickness of the films is increased. These results are due to the aggregation of the native grains into the larger clusters upon growing the PPy films on ITO glass electrode. It is obvious that the degree of aggregation and cluster size of the films affect the roughness and root mean square roughness values. The various cluster size caused the difference surface morphology of the PPy films as can be seen from the AFM images. As a consequence, the thicker PPy films on ITO glass electrodes have larger grains and become rougher, as expected. These results are in



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good agreement with those reported in elsewhere.33, 34, 55, 56 With increasing the grain size, grain boundaries are decreased and thus, the conductivity of the electrodeposited PPy films on ITO glass electrodes increases as mentioned before.47, 49 Different and positive values of Sk was observed for PPy films on ITO glass electrodes. As mentioned elsewhere,53 Sk values shows the surface with more holes and bumps. More holes and more bumps on the surfaces can be determined by more positive or negative Sk values. Positive Sk values shows a distribution of bumps with an asymmetric tail extending out toward more positive height.54 Therefore, the positive and different values of Sk for PPy in different condition may shows the variety of a distribution of bumps on the surfaces. In the case of negative Sk, a distribution of holes with a tail extending out toward more negative height can be observed.54 Like Sk parameters, different Ku parameters values were measured and observed for PPy films. The values of the Ku for PPy formed after 3 and 20 cycles are 2.4 and 0.032 (Ku< 3). These values show the distribution of mild peaks. On the other hand, the value of PPy formed after 7 cycle (Ku>3) show the distribution of sharp peaks. Height distribution of surfaces for PPy films on ITO glass electrode synthesized at different thicknesses with respect to the Figure 3 and values of Sk and Ku are shown in Figure 5. The Sk and Ku values for each PPy thin films on ITO glass electrode were calculated for each matrix for 1 µm×1µm area of surfaces. It is noteworthy to mention that the values of Sk for PPy synthesized after 20 cycles with thicker thickness is around to zero which demonstrated that height distribution is near to Gaussian distributions (see Figure 5c). In various surfaces and for each PPy thin films, the values of Sk and Ku are different due to the different distribution of bumps on a surfaces.

Morphological studies

Fractal analysis was performed experimentally using both AFM images and cyclic voltammograms of the PPy thin films with different thicknesses. Power spectral density (PSD), perimeter-area, peak-current or cyclic voltammetry and box counting methods are used for characterizing the surface as following.

Power spectral density method. For PSD studies, several algorithms have been used to determine the value of fractal dimension (Df) based upon AFM images. In this case, the fractal features of PPy thin films with different thicknesses, the PSD functions of all the surface profiles of each film have been calculated and combined into a single PSD profile


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covering large spatial frequency band width. For calculating the Df parameters from AFM images of PPy thin films with different thicknesses, we used the equations 1 and 2, which described elsewhere.57, 58 =

γ

(1)

Where γ is defined asγ = + 2α, E is dimension of the system (E=2), and α is the roughness exponent, representing the logarithmic spatial increase rate of the system saturated roughness. ( ) ∝ γ

(2)

Where P(k) is defined as the one dimensional averaged PSD, k is frequency, k=1/L, L is the entire edge length of AFM images (see Equation S6 in the Supporting Information). Logarithm of power spectra density (log P(k)) versus logarithm of frequency (log k) for PPy thin films in different thicknesses is shown in Figure 6. It can be seen from curves that with increasing the thicknesses of PPy thin films, the slope γ of PSDs spectra decreases and thus the fractal dimensions from thinner to thicker PPy film increases.

Perimeter-area method. The second method for the studies of the surfaces fractal of thin layer films is perimeter-area (P-A) which described elsewhere.59, 60 The fractal dimension of the cross-sectioned original surfaces (Df,sa) were determined through a fractal dimension (df), area (A) and perimeter (P) which is given as equations 3 and 4: , = + 1

(3)

= ⁄

(4)

where is a proportionality constant. The relation of the Df,sa, P and A is calculated clearly (see Equation S7 in the Supporting Information). Two dimensions AFM images of PPy thin films on ITO glass electrode prepared at three different thicknesses description of perimeters (white pixels) and areas (red pixels-white pixels) are shown in Figure 7. The figures reveal different characteristics such as their size, separation and irregular shapes. The white pixels point to the grain boundaries. The red pixels show hills. Moreover, it can be seen from images that with growing the polymer films on ITO glass electrodes, the perimeters (P) and the areas (A) of the self-similar lakes relate to their fractal dimension df increase. The logarithmic plot of fractal analysis of perimeter (P) versus area (A) for PPy films on ITO glass electrode in different thicknesses is shown in Figure 8. The slope df of (lnP-lnA) curves increases with PPy thicknesses and thus cross-sectioned original surfaces increase.



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Box-counting method. The third method for the studies of the surfaces fractal of PPy thin films is box counting method. This method is one of the simple techniques to estimate the fractal dimension of surfaces which described in detailed elsewhere.61-63 The technique is based on estimating the number of boxes (N(ε)) of size ε ( ≤ 1) with pixels of any nonvanishing heights from the AFM image used for calculated the Df by equation 5: (ε) ∝ ε

(5)

Figure 9 represents the ln N(ε) versus ln ε from box counting method for the PPy films on ITO glass electrode synthesized at different thicknesses. The slope of ln N(ε)–ln ε curves reveal the Df . The curves show that with increasing the thicknesses of the polymer films the Df increase.

Peak-current method. Fourth method for calculating the Df is peak-current method used cyclic voltammetry technique which described elsewhere.60, 64, 65 For calculating the Df via peak-current method for PPy in different thicknesses, the equation 6 and cyclic voltammograms (Figure 2) were used: 60

!

∝ "#

(6)

where Ipeak represent the peak current; ν is the scan rate (from Figure 2), and α is related to Df according to equation 7:60 $=

&

(7)

Dependence of anodic peak current versus scan rate obtained from the cyclic voltammograms of PPy films (Figure 2) on ITO glass electrode in different thicknesses are shown in Figure 10. The slope of log I-logν increased from 0.84 for thinner PPy to 0.919 for thicker PPy films on ITO glass electrode and thus Df increase. The values of Df calculated by the power spectral density, perimeter-area, box counting and peak-current methods represented in Table 2. The results of all procedure indicate that with PPy film growth on the surface of ITO glass electrode, fractal dimensions increase. The changes in the surface morphology is related to the fractal dimensions. A smaller value of fractal dimensions demonstrate a locally smooth surface morphology, while a larger one shows a more jagged surface structure. These results show that by growing the thickness of the PPy films on ITO glass electrode, the fractal dimensions increase, and hence caused the more jagged surface morphology. These results are in good agreements with other reports.50, 66, 67


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Structural Characterization

In order to study the microstructure of PPy thin films on ITO glass electrode, the X-ray diffraction (XRD) measurements were performed. Figure 11 and 12 Shows the XRD patterns of ITO thin film and sample electrodes prepared at different thicknesses. The XRD of ITO glass (Figure 11) shows crystalline phase. It can be seen clearly that, there are a single broad background peak centered on 2θ=25° and well-deﬁned peaks positions. Moreover, the figure shows the diffraction peaks from the (2 1 1), (2 2 2), (4 0 0), (4 1 1), (4 3 1), (2 4 0) and (6 2 2) crystalline planes for the ITO film. The appearance of the (2 2 2) diffraction peak in the XRD patterns clearly indicates the formation of In2O3 polycrystalline domains.68 The XRD's pattern of PPy thin films on ITO glass electrode with different thicknesses was shown in Figure 12. This figure clearly demonstrates the PPy thin films on ITO glass electrode have a peak at about 2θ=24°. The result is in good agreements with other reports elsewhere.69-71 The appearance of this peak arise from the inter planer Van der Waals spacing between pyrrole groups scattering.72 During the initial stage of electropolymerization, PPy polymers are poorly crystalline.73 This crystalline micro-islands also have been observed in AFM image of PPy thin films on glass electrode after 3th cycle (Figure 7). Beside other well-defined peaks, this figure confirm growing of PPy thin films on ITO crystalline glass electrode. Moreover the image shows that the ﬁlms microstructure and orientation were both sensitive to the thickness of PPy thin films on ITO glass electrode. So that the relative intensity of the peaks corresponding to the (104) plane increased with increasing the thickness of the films.

CONCLUSION Conducting polypyrrole with different thicknesses were synthesized electrochemically on ITO glass electrode successfully. Morphological, statistical and fractal analysis of the films during polymer growth were reported by cyclic voltammetry, atomic force microscopy and X-ray diffraction analysis technique. The parameters such as root mean square, kurtosis, skewness and fractal dimensions of different thin films were calculated and used to confirm the differences of polymer film thicknesses and polymer surfaces on ITO glass electrode. The result of the power spectral density, perimeter-area and box-counting parameters indicated that with increasing the thicknesses of the polypyrrole films fractal dimensions were increased. Finally, the process of growing polymers on ITO glass electrode and increasing the crystallinity of PPy during film growth were confirmed by X-ray diffraction analysis.



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ASSOCIATED CONTENT Supporting Information Equations of average height, average roughness, interface width or RMS, skewness, kurtosis, the one dimensional averaged PSD and relation between P and A. This materials is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Fax: +98-811-8380709.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was sponsored by the Bu Ali Sina University which is gratefully appreciated.

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Figure and Table Captions Figure 1. CVs during formation of PPy film inaqueous solution containing 0.1 M LiClO4 and 0.1 M pyrrole at an ITO electrode in three different thicknesses, (a) 1, 2, 3cycle, (b) 1, 2, 3..., 7th cycle and (c) 1, 2, 3..., 20th cycle, respectively. Scan rate was 50 mV·s−1. Figure 2. CVs obtained from PPy films on ITO glass electrode synthesized in different thicknesses, (a) after 3th, (b) 7th and (c) 20th cycles, at different scan rates (dE/dt) of (1) 20, (2) 40, (3) 50, (4) 60, (5) 80 (60) 100 and (7) 120 mV.s-1 Figure 3. AFM topographic images of PPy thin films on ITO glass electrode prepared at different thicknesses (a) after 3th cycle, (b) after 7th cycle, (c) after 20th cycle.


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Figure 4. AFM images and their corresponding cross section analyses obtained of PPy films on ITO glass electrode synthesized at different thicknesses (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Figure 5. Height distribution of surfaces of PPy films on ITO glass electrode synthesized at different thicknesses (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Figure 6. Two dimension AFM images of PPy thin films on ITO glass electrode description of perimeters (white pixels) and areas (red pixels-white pixels), (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Figure 7. Logarithmic plot of power spectra density versus frequency for PPy thin films in different thicknesses, (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Figure 8. Logarithmic plot of fractal analysis of perimeter versus area for PPy films on ITO glass electrode in different thicknesses, (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Figure 9. Logarithmic plot of N(ε) versus ε from box counting method for the PPy films on ITO glass electrode synthesized at different thicknesses, (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Figure 10. Dependence of anodic peak current (Ipeak) on scan rate (ν) obtained from the cyclic voltammograms for PPy films on ITO glass electrode in different thicknesses, (a) after 3th (b) 7th and (c) 20th cycles. Figure 11. XRD patterns of ITO glass electrode. Figure 12. XRD patterns of PPy thin films on ITO glass electrode prepared at different thicknesses (a) after 3th cycle, (b) after 7th cycle and (c) after 20th cycle. Inside Figure represents the processes of PPy growing on polycrystalline ITO and increasing the crystallinity of PPy during film growth. Table 1 Average height, average roughness, root mean square roughness, skewness and kurtosis of PPy films on ITO glass electrode synthesized at different thicknesses from the 3D AFM images (Figure 3). Table 2 Fractal dimensions of PPy films on ITO glass electrode synthesized at different thicknesses determined by the power spectral density, perimeter-area, box counting and peakcurrent or cyclic voltammetry methods.



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