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DDS films
European Polymer Journal 70 (2015) 118–124
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Short communication
Thickness dependence of the porosity of PPy/DDS films Allan Hallik a,⇑, Kaspar Roosalu b, Hugo Mändar b, Lauri Joosu c, Margus Marandi a,b, Jüri Tamm a a b c
Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
a r t i c l e
i n f o
Article history: Received 19 March 2015 Received in revised form 26 June 2015 Accepted 1 July 2015 Available online 2 July 2015 Keywords: Polypyrrole Dodecylsulfate Porosity GISAXS XRR AFM
a b s t r a c t Polypyrrole (PPy) films doped with large, low mobile ions (dodecylsulfate – DDS) were galvanostatically electrosynthesized. The effect of film thickness on its porosity was determined by a grazing incidence reflection small-angle X-ray scattering (GISAXS) technique. The Particle-/Pore-size analysis package of NANO-Solver v3.6 (Rigaku™) software was employed to quantify the volume distribution function of the pore sizes. The dopant concentration of the PPy/DDS films was determined by an Energy Dispersive X-ray microanalysis (EDX), with the morphology of these films investigated with atomic force microscopy (AFM). Our experimental results showed that the film thickness had quite a significant effect on the roughness and porosity of PPy film. The thicker PPy/DDS films were found to be significantly rougher than the thinner ones. Concerning porosity, whereas the 0.04 lm thick film possesses a predominantly microporous structure (57% of total porosity), the thicker films (0.2 lm and 1 lm) are mainly mesoporous (75%), or in the case of 5 lm thick film, almost entirely (98%) mesoporous. The GISAXS analysis showed that the average pore diameter (dAV) was smallest for the thinnest (0.04 lm) film – 2.3 nm. It increased slightly for thicknesses ranging from 0.2 to 1 lm – a few under 3 nm, but increased significantly for the thickest (5 lm) film – 7.9 nm. The results were analyzed on the basis of the correlation between PPy film thickness and its doping level, as well as its density. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Electrically conducting polymers (ECP) have been actively studied as promising materials for various applications. Among them, polypyrrole (PPy) has been one of the most extensively investigated ECP due to its easy preparation, good stability and relatively high conductivity. The properties (including porosity, roughness and size of aggregates or particles) of the referred-to nano-structured functional polymeric material – PPy – is strongly influenced by conditions used during its synthesis. In this way, it was ascertained that the nature and size of doping anions have an influence on the structure and morphology of PPy layers. And this influence emerges in both the cases of bulk PPy [1–3] and PPy nanofibers [4]. The effect of the size of the doping agent on the size of PPy particles can also be observed in the case of dispersed polypyrrole nanospheres [5]. In Ref. [6], the effect of solvent and deposition time on the morphology of PPy/ClO4 films was studied. The thick PPy films were found to show larger aggregates as compared to the thin films. Also, the roughness and pore size of these PPy films increased with film thickness. Here the roughness varied from 0.3 to 2.5 lm [6]. Yang et al. [7] established a variety of ⇑ Corresponding author. E-mail address: [email protected] (A. Hallik). http://dx.doi.org/10.1016/j.eurpolymj.2015.07.002 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
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structures in PPy, depending on the thickness of the deposited layer. This finding was applicable to both PPy/small dopant and PPy/large dopant films. At this point, from a certain film thickness, the morphology of PPy films acquires nodular amorphous structure, which is characteristic of bulk ECP. In the case of 1 lm thick PPy film doped with large poly(4-styrenesulfonate) – PSS anions (PPy/PSS), the nodules were 100–250 nm in size, and this size increased with increased film thickness [7]. According to the results presented by Tietje-Girault et al. [8] there can also be cases where the applied (synthesis) charge affects the smoothness of the emerging film only to a limited extent. This conclusion was found to be valid for PPy films with a thickness of up to 10 lm. The thicker films already showed rougher surfaces. Previously Silk et al. [9] established that average diameter and height of a PPy single globule were only slightly dependent on the dopant nature, upon the condition that the film thickness remained below 1 lm. In the case of PPy/DDS and PPy/SO4 films the above-mentioned tendency continued even for thicker layers. However, in the case of PPy/Cl and PPy/ClO4 films, there was a steep increase of the above-mentioned characteristics that was observed [9]. From an application’s point of view, it is important to know and/or control the porosity of synthesized PPy film. The porosity properties have been measured/ estimated using different methods, such as low-temperature N2 sorption [3,10,11], cyclic voltammetry [12], film density analysis [1], standard porosimetry (in the case of polyaniline (PANI) and polyparaphenylene (PPP) [13,14], and small-angle X-ray scattering SAXS [15]. SAXS and its modifications have been found to be an advanced scattering technique for investigation of the properties of nanostructured polymeric materials [15–22]. Polymers like PPy [15,19,20], PANI [17,18], poly(p-phenylene) [16], polypropylene [21], poly(amide) and poly(imide) [22] and properties like the degree of crystallinity [19], porosity [15,21,22] and fractal dimension [16–18,20,21] have been widely examined. Despite the achievements mentioned above, there is still very limited data regarding the dependencies between the thickness and porosity of ECP layers, especially of thin layers. The present study investigated the effect of layer thickness on the porosity of thin PPy/DDS films. Additionally, changes in the film density, doping level and surface roughness parameter (RMS) caused by film thickening or electrochemical treatment were measured and correlated with porosity data. On the basis of the results, we demonstrated that GISAXS together with X-ray reflectivity (XRR) form a useful technique for the porosity analysis of the thin layers of PPy films. 2. Experimental The PPy/DDS films were prepared by electropolymerizing pyrrole at a constant current density (2 mA cm2) on plates of silicon monocrystal (1 0 0) covered with a platinum layer (50 nm thick) in aqueous solutions containing 0.1 mol dm3 of pyrrole monomer and 0.1 mol dm3 of supporting electrolyte (NaDDS). After polymerization, the polymer was thoroughly rinsed with Milli Q+ water. The film thickness was adjusted in accordance with the proportion 0.2 C cm2 per 1 lm for PPy/DDS [23]. The working electrode area used for electrodeposition was 3 cm2. In order to prepare the working electrodes, the silicon plates were placed in a sputtering chamber, and the coating of Pt was obtained by sputter-deposition with a Pt target in an argon atmosphere. A one compartment cell, equipped with Pt-sheet as a counter electrode and saturated Ag/AgCl as a reference electrode, was used for the electrochemical synthesis and treatment of PPy/DDS films. A PG581 potentiostat– galvanostat (Uniscan Instruments) was used in the electrochemical experiments. Pyrrole (Aldrich), which is used for the synthesis of PPy films, was purified by distillation over CaH2 under a vacuum and kept refrigerated in the dark. Analytical grade salts and Milli Q+ water were used for preparing the solutions. X-ray reflection (XRR) and grazing incidence small angle scattering (GISAXS) [24] patterns were recorded on a materials research diffractometer SmartLab (Rigaku™) operating at 9 kW with CuK-radiation (k = 0.15418 nm). Data were gathered in the 2h range of 0–6° with an incident angle of 0.3° and a sample to detector distance of 300 mm. The volume distribution function of pore sizes was calculated by the program NANO-Solver (version 3.6, Rigaku™) by least squares fitting of measured GISAXS curves. GISAXS curves were simulated using X-ray scattering theory [25]. Reflection and refraction effects owing to grazing incidence geometry were taken into account by distorted wave Born approximation. Scatterers were assumed to be spherical pores with C-distribution of radiuses. Calculated scattering intensity distributions were smeared with a slit function, specific for the Smartlab diffractometer optics. Goodness of fit was estimated from relative residual ‘‘R-factor’’, expressed in percentages:
PN R¼
obs Icalc j i i¼1 jIi PN obs I i¼1 i
100%
where Iobs and Icalc are observed and calculated scattering intensities for i-th data point. The thickness, roughness and density i i of the films were determined from the XRR data on the basis of Parratt’s recursive relation [26] and Simplex fitting algorithm used in the program AXES [27]. The chemical formula of PPy/DDS films, which is used for XRR simulations, was C4H5N. The surface roughness was modelled on the Névot–Croce approximation [28]. Refinement quality of model parameters (density and roughness) was assessed through the visual comparison of observed and calculated patterns as well as the values of residual errors. The surface morphology of investigated PPy/DDS modified electrodes was studied by an AFM/SPM measurement system 5500 (Agilent Technologies). All AFM images were recorded in non-contact mode using SSS-NCHR (NanosensorsTM) cantilevers. The scan area was 5 5 lm2. The GwyddionTM ver. 2.27 free software (Czech Metrology Institute) was employed for image processing and surface roughness calculations. All images were processed by 1st order flattening for background slope removing, and if necessary, the contrast and brightness were adjusted.
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A chemical composition analysis was performed by Energy Dispersive X-ray microanalysis (EDX) using a variable pressure Zeiss EVO MA15 scanning electron microscope (SEM) equipped with an Oxford X-MAX Energy Dispersive detector system (EDS). Spectra were processed afterward using Aztec software. Measurements were performed using internal standards for calibration. Samples were uncoated and studied in high vacuum mode. The smallest possible magnification was used to avoid the possible influence of minor chemical composition variations and gain a sample average composition. An electron beam was focused on the surface of the sample, and different accelerating voltages (mainly 5–10 keV) were used to avoid beam penetration through the surface layer. The signal acquisition time was not fixed: ‘‘auto mode’’ was used where the acquisition continues until enough counts are collected for spectrum quantification. Therefore, when using lower accelerating voltages, it took longer to collect sufficient counts. 3. Results and discussion 3.1. X-ray scattering data analysis By controlling the polymerization charge (according to the relation given above) the PPy/DDS films with a thickness of 0.04 lm, 0.2 lm, 1 lm and 5 lm were electrogenerated. The PPy/DDS films obtained were macroscopically smooth and glossy, and only the film with a thickness of 5 lm was matte. The freshly prepared polymer films were measured by GISAXS. The 1 lm thick film was additionally subjected to electrochemical reduction (at 0.8 V) in two different solutions: (a) 0.1 M aqueous solution of NaDDS, and (b) 0.05 M methanolic solution of NaDDS. After each electrochemical treatment, X-ray scattering measurements were carried out again. Fig. 1 shows the experimental and simulated SAXS profiles of porous PPy/DDS films of different thicknesses. The information about the average pore diameter and pore size distribution was derived from the simulated curves and presented in Fig. 2 and Table 1. The analysis of pore size distribution curves (Fig. 2) reveals that the porosity of PPy/DDS films significantly depends on the film thickness. The peculiarity of the thinnest film (0.04 lm) emerges in its predominantly microporous structure. Approximately 57% of the total porosity of this film must be attributed to pores with diameters less than 2 nm, i.e. to micropores. The pore dimension of the highest incidence (maximum of the corresponding curve) of this film remains around 0.4 nm, but the average pore diameter is 2.3 nm (see Table 1). It should be noted that due to a weak signal here, the indeterminacy of pore diameter is quite high. The proportion of (predominantly small) mesopores is 43%, in which the mesopores with diameters between 2 and 5 nm constitute the major portion – approximately 33% of total porosity. Larger mesopores with diameters between 5 and 10 nm give 9% of total porosity. The percentage of pores with d > 10 nm remains below 1%. According to the results obtained, the PPy/DDS films with a thickness of its polymer layer of 0.2 or 1 lm show very similar porosity properties. Thus both above-mentioned films have a predominantly mesoporous structure – the proportion of mesopores is 77% and 72% for 0.2 lm and 1 lm thick films, respectively. The average pore diameter is also equal, close to 3 nm (Table 1). Like the 0.04 lm thick film, the dimensions of mesopores in both previously described thicker films are quite small. Accordingly the micropores along with small mesopores (up to 5 nm) provide about 95% of the film’s total porosity for both (0.2 and 1 lm thick) films. The proportion of the pores with d > 7 nm is only 0.5% or less. From Fig. 2 it becomes evident that the pore size distribution curve for the thickest (5 lm) film differs from the others by its broadened and flattened shape. The average pore diameter calculated for this film is significantly higher, reaching 7.9 nm. This film is prevalently mesoporous (98%), also the variability of pore sizes is more extensive as compared to the thinner PPy films. Whereas the films with thicknesses up to 1 lm have an upper limit for its pore size at around 7–10 nm, the last indicator for 5 lm thick PPy/DDS film extends to roughly 20 nm value. This film also consists of pores with d > 20 nm but the percentage of such pores remains a little below 1%. The distribution of mesopores in 5 lm thick film was found to be
Fig. 1. Small angle X-ray scattering curves from PPy/DDS films of different thicknesses, noted in figure. Lines with unfilled marks – experimental data, lines with filled marks – calculated profile. Curves are offset for better viewing.
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Fig. 2. Pore size distribution plots for PPy/DDS films of different thicknesses, noted in figure.
Table 1 Characteristics of the PPy/DDS films. Thickness of the film (lm)
Electrochemical treatment
Average pore diameter (nm)
Residual R-factor (%)
0.04 0.2 1.0 1.0 1.0 5.0
Non-treated Non-treated Non-treated Reduced in aq Reduced in MeOH Non-treated
2.3 2.9 2.8 2.9 2.9 7.9
7.4 1.7 1.8 1.3 1.7 1.8
Percentage of pores with selected diameter(s) (%) d < 2 nm
d = 2–5 nm
d > 5 nm
57 23 28
33 72 66
10 5 6
2
23
75
Reduced in aq – electrochemically reduced at 0.8 V in 0.1 M aqueous solution of NaDDS; Reduced in MeOH – electrochemically reduced at 0.8 V in 0.05 M methanolic solution of NaDDS; Residual R-factor (%) – see the formula in Section 2.
as follows: mainly there are pores with d = 5–10 nm (50%), following pores with d < 5 nm and with d > 10 nm 25% equally in both cases. As mentioned before, the 1 lm thick film was additionally subjected to electrochemical reduction at 0.8 V in two different solutions: (a) 0.1 M aqueous solution of NaDDS, and (b) 0.05 M methanolic solution of NaDDS. The aim of these treatments was to establish the possible changes in the PPy/DDS film structure (in porosity and density) accompanying such treatments. It has been previously established that in the case of PPy films containing large, immobile anions (including DDS) the electroneutrality of such films during electrochemical reduction is realized mainly by the insertion of cations in an aqueous solution and by the expelling of the anions in certain organic solutions (herewith methanolic). Because of this, changes take place in the doping level (f), defined as the ratio of incorporated anion to the PPy unit, as well as in the film thickness and morphology [29,30]. Thus in the present case, the Na+ ions were incorporated into the polymer matrix during electrochemical reduction in the aqueous solution of NaDDS due to pseudo cation doping. The electrochemical reduction in the methanolic solution caused the partial emptying of the film from bulky DDS ions. The EDX analysis of a 1 lm thick film showed a sixfold decrease of f value after treatment in an alcoholic solution of NaDDS: from f = 0.32 (freshly prepared film) to f = 0.053 (reduced in CH3OH solution film). Surprisingly, all the pore size distribution plots for 1 lm thick PPy/DDS films were practically coinciding, independently of the treatment (Fig. 3). It means that at least dry PPy film porosity (likewise the surface morphology) is mainly determined by synthesis conditions, and that even a significant decrease in the mass and f of the PPy film, caused by the leaving of the bulky anions, cannot change the original structure of the film. Research on the porosity of PPy films by the low-temperature N2 sorption measurement method needs quantities of polymer about several tens of milligrams and hence films of greater thicknesses. Therefore our previous report [3] in this field comprised PPy films with thicknesses that were approximately from 50 to 100 lm. Presumably the total porosity and pore size distribution of those films and of the films with reduced thicknesses (up to three orders of magnitude) do not have to coincide. Despite this, the comparison of the pore sizes of (relatively) thin PPy films (present study) with the corresponding values of thick PPy films [3,10,15] shows reasonable correlation. It should be mentioned here that significant synthesis parameters as current density, also the concentration of pyrrole monomer and supporting electrolyte were the same in both our cases. Different was the deposition time i.e. film thickness and the measurement method. As with thick PPy films, where neither N2 sorption measurements [3,10] nor SAXS measurements [15] showed any considerable microporosity, the thin PPy/DDS films of the present study also exhibited a predominantly mesoporous structure. The only exception was the 0.04 lm film. The 5 lm thick films were already almost completely mesoporous. However, the thick PPy films differ in their
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Fig. 3. Pore size distribution plots for 1 lm thick PPy/DDS films of different treatments. Electrochemically non-treated (green/dotted curve), electrochemically reduced in 0.1 M aqueous solution of NaDDS (red/solid curve) and electrochemically reduced in 0.05 M methanolic solution of NaDDS (blue/labeled curve) films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
wider range of pore sizes, also as a result by a higher number of bigger pores. For example, in 5 lm thick film the proportion of the pores with d > 20 nm is marginal (<1%) but in thick films this value comes close to 10% [3] or more [10,15]. Concerning the average pore diameter of thick PPy/DDS film – dAV = 3.6 nm [3], it is essentially compatible with the corresponding values for significantly thinner PPy/DDS films – see Table 1. The dAV value for thick film exceeds the dAV values for thin films of up to 1.0 lm film thickness, i.e. up to a thickness with a significant proportion of microporosity. The difference between dAV values for (relatively) thin film (5 lm) and thick film [3] can originate from different experimental methods and conditions, including different temperatures (room temperature/196 °C, correspondingly). As a conclusion from the comparisons presented above, the GISAXS method seems to be applicable to obtain reliable results relating to the porosity of (relatively) thin PPy films. 3.2. Atomic force microscopy, X-ray reflectivity analysis and Energy Dispersive X-ray microanalysis Thickness-induced morphology changes in PPy/DDS films were estimated by AFM. Besides using the RMS values in the additional characterization of PPy/DDS films of different thicknesses, they were also used in the modelling of XRR patterns. The typical AFM images are presented in Fig. 4. It can be seen that in all cases, the surface of Si/Pt electrodes are fully covered with a PPy layer. The AFM images of PPy films clearly show that the increase of film thickness causes an increase of surface roughness. Thus, the calculated RMS roughness values were as follows: 1.4 nm, 8.2 nm and 114 nm for 0.04, 1 and 5 lm thick PPy films respectively. Fig. 5 shows the results of the X-ray reflection (XRR) analysis for three selected samples (0.04, 0.2 and 1 lm thick films), which provided a relatively good match with the calculated patterns. The XRR pattern of the 5 lm thick sample did not allow for finding an acceptable model for pattern matching. XRR and AFM roughness parameters are physically different in nature. XRR roughness also reflects density gradients inside the film. Since the XRR and AFM roughness have a bit different physical background, it is not possible to compare these values in absolute numbers, although AFM and XRR roughness values usually correlate well with each other. This is confirmed also for our samples where the XRR refined roughness values (1.2 nm for 0.04 lm thick and above 7 nm for 1 lm thick film) were in good correlation with the values obtained from the AFM measurements 1.4 and 8.2 nm, correspondingly. For the rest of the PPy films studied, the roughness parameters were above the detection limits of XRR analysis, and were calculated using the AFM measurements.
Fig. 4. Typical non-contact AFM images of PPy/DDS films deposited onto surface of Si/Pt electrodes. The PPy film thicknesses are: (a) 0.04 lm; (b) 1 lm; and (c) 5 lm.
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Fig. 5. First part of XRR patterns for PPy/DDS films of different thicknesses, noted in figure. Lines with unfilled marks – experimental data, lines with filled marks – calculated XRR profile. All patterns were measured and refinement performed in the range of reflection angle 0–6° (2h).
Concerning the determination of the density of the films investigated, it should be taken into account that the first cut-off for the XRR patterns shows the existence of low density PPy films on high density Pt film. The position and extent of this cut-off together reflect the density of films that was refined during a fitting of experimental patterns to modelled patterns. The cut-off at higher 2-theta values occurs due to the Pt layer. The average density of the films, estimated from XRR analysis, was mostly in the range of 1.2–1.3 g/cm3. Here, the standard deviation of the densities was ±0.2 g/cm3. These values are slightly smaller compared to the common PPy density (1.4–1.5 g/cm3) presented in relevant literature and ordinarily found in ‘‘wet’’ flotation density experiments [1,31]. Only the thinnest film of the present study (0.04 lm thick) showed the film density 1.4–1.5 g/cm3. Modelling of the XRR pattern showed that introducing a density gradient into the layer gave better match to the experimental patterns as compared to the single layer, constant density model. According to this model, it was established that the density of 1 lm thick film decreased inside the film from the surface of the substrate to the top of the film. This tendency also appeared for the film after its electrochemical reduction in the aqueous solution. In accordance with these results, the use of a single layer model also revealed the scarce decrease of the average film density from 1.3 to 1.2 g/cm3 by a switch from 0.2 lm to 1 lm thick film. The greater density of the thinner film is also ascertained for PPy films doped with dodecyl-benzenesulfonate (PPy/DBS) [29]. The moderate trend of the film density increase with a decreased film thickness seem to be the result of the increased microporosity of the thinner films and higher doping levels: both can result in less void volume in the film. Thus according to the EDX results the f values calculated for 0.2 and 1 lm thick films were around 0.31–0.32, but in the case of 0.04 lm film, it was f = 0.58. However, it should be mentioned here that the last value may be somewhat elevated due to a strong background signal from the Pt substrate. The greater f values of PPy near-substrate sublayers were also established in our recent study of PPy/DDS/Cl and PPy/Cl/DDS bilayers [32]. These higher f values were ascertained for PPy lamellae, prepared by an in situ slice technique and analyzed as consecutive polymer sublayers with 100 nm steps. However according to our present study, the higher f values alone without the dominance of smaller pores, as is the case with the thickest (5 lm) film (f = 0.61), cannot guarantee a higher film density. Here the influence of high f value is counterbalanced by the existence of significantly greater pores that change the layer sparser. 4. Conclusions PPy/DDS films with different thicknesses of 0.04, 0.2, 1 and 5 lm were electrogenerated onto smooth Si/Pt electrodes. Their porous structure characteristics and surface morphology (RMS roughness) were investigated by reflection small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) methods. Energy Dispersive X-ray microanalysis (EDX) was used to determine the doping level of the dopant in the film. The analysis of pore size distribution curves reveals that the porosity of PPy/DDS films depends quite significantly on the film thickness. It was established that only the thinnest (0.04 lm) PPy/DDS film had a prevalently microporous structure. All the thicker films investigated were mainly mesoporous (0.2 and 1 lm thick films) or almost entirely mesoporous (5 lm thick film). It was found that the increase of film thickness up to 5 lm brought along not only an increase of the average pore diameter but also the greater variability of pore sizes. The electrochemical treatments (reduction) of the PPy/DDS film (1 lm) caused very few if any changes in the film’s porous structure. The analysis of the chemical composition and density of the films have suggested a reason for this behaviour: the higher doping levels, established in the case of thinner PPy/DDS films, result (due to long and ‘‘flexible’’ tail of DDS) in the more compact packing of PPy chains, and therefore in smaller pores and increased densities of the films. The surface roughness
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parameter (RMS) of the films investigated also showed thickness dependent behaviour – the RMS value increased significantly as the film thickness increased. The porosity characteristics that were obtained and their comparison with the corresponding values from divergent experiments (different method, different film thickness) let us conclude that the GISAXS method along with the proper porosity analysis software is suitable for characterizing porosity – thickness relations of (relatively) thin PPy films. Acknowledgements The authors are grateful to Peeter Ritslaid for preparation of Si/Pt electrodes. This work was supported by Estonian Centre of Excellence in Research Project TK117 ‘‘High-technology Materials for Sustainable Development’’ and Estonian Ministry of Education and Research Project IUT2-24. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Short communication
Thickness dependence of the porosity of PPy/DDS films Allan Hallik a,⇑, Kaspar Roosalu b, Hugo Mändar b, Lauri Joosu c, Margus Marandi a,b, Jüri Tamm a a b c
Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia Institute of Physics, University of Tartu, Ravila 14c, 50411 Tartu, Estonia Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
a r t i c l e
i n f o
Article history: Received 19 March 2015 Received in revised form 26 June 2015 Accepted 1 July 2015 Available online 2 July 2015 Keywords: Polypyrrole Dodecylsulfate Porosity GISAXS XRR AFM
a b s t r a c t Polypyrrole (PPy) films doped with large, low mobile ions (dodecylsulfate – DDS) were galvanostatically electrosynthesized. The effect of film thickness on its porosity was determined by a grazing incidence reflection small-angle X-ray scattering (GISAXS) technique. The Particle-/Pore-size analysis package of NANO-Solver v3.6 (Rigaku™) software was employed to quantify the volume distribution function of the pore sizes. The dopant concentration of the PPy/DDS films was determined by an Energy Dispersive X-ray microanalysis (EDX), with the morphology of these films investigated with atomic force microscopy (AFM). Our experimental results showed that the film thickness had quite a significant effect on the roughness and porosity of PPy film. The thicker PPy/DDS films were found to be significantly rougher than the thinner ones. Concerning porosity, whereas the 0.04 lm thick film possesses a predominantly microporous structure (57% of total porosity), the thicker films (0.2 lm and 1 lm) are mainly mesoporous (75%), or in the case of 5 lm thick film, almost entirely (98%) mesoporous. The GISAXS analysis showed that the average pore diameter (dAV) was smallest for the thinnest (0.04 lm) film – 2.3 nm. It increased slightly for thicknesses ranging from 0.2 to 1 lm – a few under 3 nm, but increased significantly for the thickest (5 lm) film – 7.9 nm. The results were analyzed on the basis of the correlation between PPy film thickness and its doping level, as well as its density. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Electrically conducting polymers (ECP) have been actively studied as promising materials for various applications. Among them, polypyrrole (PPy) has been one of the most extensively investigated ECP due to its easy preparation, good stability and relatively high conductivity. The properties (including porosity, roughness and size of aggregates or particles) of the referred-to nano-structured functional polymeric material – PPy – is strongly influenced by conditions used during its synthesis. In this way, it was ascertained that the nature and size of doping anions have an influence on the structure and morphology of PPy layers. And this influence emerges in both the cases of bulk PPy [1–3] and PPy nanofibers [4]. The effect of the size of the doping agent on the size of PPy particles can also be observed in the case of dispersed polypyrrole nanospheres [5]. In Ref. [6], the effect of solvent and deposition time on the morphology of PPy/ClO4 films was studied. The thick PPy films were found to show larger aggregates as compared to the thin films. Also, the roughness and pore size of these PPy films increased with film thickness. Here the roughness varied from 0.3 to 2.5 lm [6]. Yang et al. [7] established a variety of ⇑ Corresponding author. E-mail address: [email protected] (A. Hallik). http://dx.doi.org/10.1016/j.eurpolymj.2015.07.002 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
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structures in PPy, depending on the thickness of the deposited layer. This finding was applicable to both PPy/small dopant and PPy/large dopant films. At this point, from a certain film thickness, the morphology of PPy films acquires nodular amorphous structure, which is characteristic of bulk ECP. In the case of 1 lm thick PPy film doped with large poly(4-styrenesulfonate) – PSS anions (PPy/PSS), the nodules were 100–250 nm in size, and this size increased with increased film thickness [7]. According to the results presented by Tietje-Girault et al. [8] there can also be cases where the applied (synthesis) charge affects the smoothness of the emerging film only to a limited extent. This conclusion was found to be valid for PPy films with a thickness of up to 10 lm. The thicker films already showed rougher surfaces. Previously Silk et al. [9] established that average diameter and height of a PPy single globule were only slightly dependent on the dopant nature, upon the condition that the film thickness remained below 1 lm. In the case of PPy/DDS and PPy/SO4 films the above-mentioned tendency continued even for thicker layers. However, in the case of PPy/Cl and PPy/ClO4 films, there was a steep increase of the above-mentioned characteristics that was observed [9]. From an application’s point of view, it is important to know and/or control the porosity of synthesized PPy film. The porosity properties have been measured/ estimated using different methods, such as low-temperature N2 sorption [3,10,11], cyclic voltammetry [12], film density analysis [1], standard porosimetry (in the case of polyaniline (PANI) and polyparaphenylene (PPP) [13,14], and small-angle X-ray scattering SAXS [15]. SAXS and its modifications have been found to be an advanced scattering technique for investigation of the properties of nanostructured polymeric materials [15–22]. Polymers like PPy [15,19,20], PANI [17,18], poly(p-phenylene) [16], polypropylene [21], poly(amide) and poly(imide) [22] and properties like the degree of crystallinity [19], porosity [15,21,22] and fractal dimension [16–18,20,21] have been widely examined. Despite the achievements mentioned above, there is still very limited data regarding the dependencies between the thickness and porosity of ECP layers, especially of thin layers. The present study investigated the effect of layer thickness on the porosity of thin PPy/DDS films. Additionally, changes in the film density, doping level and surface roughness parameter (RMS) caused by film thickening or electrochemical treatment were measured and correlated with porosity data. On the basis of the results, we demonstrated that GISAXS together with X-ray reflectivity (XRR) form a useful technique for the porosity analysis of the thin layers of PPy films. 2. Experimental The PPy/DDS films were prepared by electropolymerizing pyrrole at a constant current density (2 mA cm2) on plates of silicon monocrystal (1 0 0) covered with a platinum layer (50 nm thick) in aqueous solutions containing 0.1 mol dm3 of pyrrole monomer and 0.1 mol dm3 of supporting electrolyte (NaDDS). After polymerization, the polymer was thoroughly rinsed with Milli Q+ water. The film thickness was adjusted in accordance with the proportion 0.2 C cm2 per 1 lm for PPy/DDS [23]. The working electrode area used for electrodeposition was 3 cm2. In order to prepare the working electrodes, the silicon plates were placed in a sputtering chamber, and the coating of Pt was obtained by sputter-deposition with a Pt target in an argon atmosphere. A one compartment cell, equipped with Pt-sheet as a counter electrode and saturated Ag/AgCl as a reference electrode, was used for the electrochemical synthesis and treatment of PPy/DDS films. A PG581 potentiostat– galvanostat (Uniscan Instruments) was used in the electrochemical experiments. Pyrrole (Aldrich), which is used for the synthesis of PPy films, was purified by distillation over CaH2 under a vacuum and kept refrigerated in the dark. Analytical grade salts and Milli Q+ water were used for preparing the solutions. X-ray reflection (XRR) and grazing incidence small angle scattering (GISAXS) [24] patterns were recorded on a materials research diffractometer SmartLab (Rigaku™) operating at 9 kW with CuK-radiation (k = 0.15418 nm). Data were gathered in the 2h range of 0–6° with an incident angle of 0.3° and a sample to detector distance of 300 mm. The volume distribution function of pore sizes was calculated by the program NANO-Solver (version 3.6, Rigaku™) by least squares fitting of measured GISAXS curves. GISAXS curves were simulated using X-ray scattering theory [25]. Reflection and refraction effects owing to grazing incidence geometry were taken into account by distorted wave Born approximation. Scatterers were assumed to be spherical pores with C-distribution of radiuses. Calculated scattering intensity distributions were smeared with a slit function, specific for the Smartlab diffractometer optics. Goodness of fit was estimated from relative residual ‘‘R-factor’’, expressed in percentages:
PN R¼
obs Icalc j i i¼1 jIi PN obs I i¼1 i
100%
where Iobs and Icalc are observed and calculated scattering intensities for i-th data point. The thickness, roughness and density i i of the films were determined from the XRR data on the basis of Parratt’s recursive relation [26] and Simplex fitting algorithm used in the program AXES [27]. The chemical formula of PPy/DDS films, which is used for XRR simulations, was C4H5N. The surface roughness was modelled on the Névot–Croce approximation [28]. Refinement quality of model parameters (density and roughness) was assessed through the visual comparison of observed and calculated patterns as well as the values of residual errors. The surface morphology of investigated PPy/DDS modified electrodes was studied by an AFM/SPM measurement system 5500 (Agilent Technologies). All AFM images were recorded in non-contact mode using SSS-NCHR (NanosensorsTM) cantilevers. The scan area was 5 5 lm2. The GwyddionTM ver. 2.27 free software (Czech Metrology Institute) was employed for image processing and surface roughness calculations. All images were processed by 1st order flattening for background slope removing, and if necessary, the contrast and brightness were adjusted.
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A chemical composition analysis was performed by Energy Dispersive X-ray microanalysis (EDX) using a variable pressure Zeiss EVO MA15 scanning electron microscope (SEM) equipped with an Oxford X-MAX Energy Dispersive detector system (EDS). Spectra were processed afterward using Aztec software. Measurements were performed using internal standards for calibration. Samples were uncoated and studied in high vacuum mode. The smallest possible magnification was used to avoid the possible influence of minor chemical composition variations and gain a sample average composition. An electron beam was focused on the surface of the sample, and different accelerating voltages (mainly 5–10 keV) were used to avoid beam penetration through the surface layer. The signal acquisition time was not fixed: ‘‘auto mode’’ was used where the acquisition continues until enough counts are collected for spectrum quantification. Therefore, when using lower accelerating voltages, it took longer to collect sufficient counts. 3. Results and discussion 3.1. X-ray scattering data analysis By controlling the polymerization charge (according to the relation given above) the PPy/DDS films with a thickness of 0.04 lm, 0.2 lm, 1 lm and 5 lm were electrogenerated. The PPy/DDS films obtained were macroscopically smooth and glossy, and only the film with a thickness of 5 lm was matte. The freshly prepared polymer films were measured by GISAXS. The 1 lm thick film was additionally subjected to electrochemical reduction (at 0.8 V) in two different solutions: (a) 0.1 M aqueous solution of NaDDS, and (b) 0.05 M methanolic solution of NaDDS. After each electrochemical treatment, X-ray scattering measurements were carried out again. Fig. 1 shows the experimental and simulated SAXS profiles of porous PPy/DDS films of different thicknesses. The information about the average pore diameter and pore size distribution was derived from the simulated curves and presented in Fig. 2 and Table 1. The analysis of pore size distribution curves (Fig. 2) reveals that the porosity of PPy/DDS films significantly depends on the film thickness. The peculiarity of the thinnest film (0.04 lm) emerges in its predominantly microporous structure. Approximately 57% of the total porosity of this film must be attributed to pores with diameters less than 2 nm, i.e. to micropores. The pore dimension of the highest incidence (maximum of the corresponding curve) of this film remains around 0.4 nm, but the average pore diameter is 2.3 nm (see Table 1). It should be noted that due to a weak signal here, the indeterminacy of pore diameter is quite high. The proportion of (predominantly small) mesopores is 43%, in which the mesopores with diameters between 2 and 5 nm constitute the major portion – approximately 33% of total porosity. Larger mesopores with diameters between 5 and 10 nm give 9% of total porosity. The percentage of pores with d > 10 nm remains below 1%. According to the results obtained, the PPy/DDS films with a thickness of its polymer layer of 0.2 or 1 lm show very similar porosity properties. Thus both above-mentioned films have a predominantly mesoporous structure – the proportion of mesopores is 77% and 72% for 0.2 lm and 1 lm thick films, respectively. The average pore diameter is also equal, close to 3 nm (Table 1). Like the 0.04 lm thick film, the dimensions of mesopores in both previously described thicker films are quite small. Accordingly the micropores along with small mesopores (up to 5 nm) provide about 95% of the film’s total porosity for both (0.2 and 1 lm thick) films. The proportion of the pores with d > 7 nm is only 0.5% or less. From Fig. 2 it becomes evident that the pore size distribution curve for the thickest (5 lm) film differs from the others by its broadened and flattened shape. The average pore diameter calculated for this film is significantly higher, reaching 7.9 nm. This film is prevalently mesoporous (98%), also the variability of pore sizes is more extensive as compared to the thinner PPy films. Whereas the films with thicknesses up to 1 lm have an upper limit for its pore size at around 7–10 nm, the last indicator for 5 lm thick PPy/DDS film extends to roughly 20 nm value. This film also consists of pores with d > 20 nm but the percentage of such pores remains a little below 1%. The distribution of mesopores in 5 lm thick film was found to be
Fig. 1. Small angle X-ray scattering curves from PPy/DDS films of different thicknesses, noted in figure. Lines with unfilled marks – experimental data, lines with filled marks – calculated profile. Curves are offset for better viewing.
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Fig. 2. Pore size distribution plots for PPy/DDS films of different thicknesses, noted in figure.
Table 1 Characteristics of the PPy/DDS films. Thickness of the film (lm)
Electrochemical treatment
Average pore diameter (nm)
Residual R-factor (%)
0.04 0.2 1.0 1.0 1.0 5.0
Non-treated Non-treated Non-treated Reduced in aq Reduced in MeOH Non-treated
2.3 2.9 2.8 2.9 2.9 7.9
7.4 1.7 1.8 1.3 1.7 1.8
Percentage of pores with selected diameter(s) (%) d < 2 nm
d = 2–5 nm
d > 5 nm
57 23 28
33 72 66
10 5 6
2
23
75
Reduced in aq – electrochemically reduced at 0.8 V in 0.1 M aqueous solution of NaDDS; Reduced in MeOH – electrochemically reduced at 0.8 V in 0.05 M methanolic solution of NaDDS; Residual R-factor (%) – see the formula in Section 2.
as follows: mainly there are pores with d = 5–10 nm (50%), following pores with d < 5 nm and with d > 10 nm 25% equally in both cases. As mentioned before, the 1 lm thick film was additionally subjected to electrochemical reduction at 0.8 V in two different solutions: (a) 0.1 M aqueous solution of NaDDS, and (b) 0.05 M methanolic solution of NaDDS. The aim of these treatments was to establish the possible changes in the PPy/DDS film structure (in porosity and density) accompanying such treatments. It has been previously established that in the case of PPy films containing large, immobile anions (including DDS) the electroneutrality of such films during electrochemical reduction is realized mainly by the insertion of cations in an aqueous solution and by the expelling of the anions in certain organic solutions (herewith methanolic). Because of this, changes take place in the doping level (f), defined as the ratio of incorporated anion to the PPy unit, as well as in the film thickness and morphology [29,30]. Thus in the present case, the Na+ ions were incorporated into the polymer matrix during electrochemical reduction in the aqueous solution of NaDDS due to pseudo cation doping. The electrochemical reduction in the methanolic solution caused the partial emptying of the film from bulky DDS ions. The EDX analysis of a 1 lm thick film showed a sixfold decrease of f value after treatment in an alcoholic solution of NaDDS: from f = 0.32 (freshly prepared film) to f = 0.053 (reduced in CH3OH solution film). Surprisingly, all the pore size distribution plots for 1 lm thick PPy/DDS films were practically coinciding, independently of the treatment (Fig. 3). It means that at least dry PPy film porosity (likewise the surface morphology) is mainly determined by synthesis conditions, and that even a significant decrease in the mass and f of the PPy film, caused by the leaving of the bulky anions, cannot change the original structure of the film. Research on the porosity of PPy films by the low-temperature N2 sorption measurement method needs quantities of polymer about several tens of milligrams and hence films of greater thicknesses. Therefore our previous report [3] in this field comprised PPy films with thicknesses that were approximately from 50 to 100 lm. Presumably the total porosity and pore size distribution of those films and of the films with reduced thicknesses (up to three orders of magnitude) do not have to coincide. Despite this, the comparison of the pore sizes of (relatively) thin PPy films (present study) with the corresponding values of thick PPy films [3,10,15] shows reasonable correlation. It should be mentioned here that significant synthesis parameters as current density, also the concentration of pyrrole monomer and supporting electrolyte were the same in both our cases. Different was the deposition time i.e. film thickness and the measurement method. As with thick PPy films, where neither N2 sorption measurements [3,10] nor SAXS measurements [15] showed any considerable microporosity, the thin PPy/DDS films of the present study also exhibited a predominantly mesoporous structure. The only exception was the 0.04 lm film. The 5 lm thick films were already almost completely mesoporous. However, the thick PPy films differ in their
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Fig. 3. Pore size distribution plots for 1 lm thick PPy/DDS films of different treatments. Electrochemically non-treated (green/dotted curve), electrochemically reduced in 0.1 M aqueous solution of NaDDS (red/solid curve) and electrochemically reduced in 0.05 M methanolic solution of NaDDS (blue/labeled curve) films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
wider range of pore sizes, also as a result by a higher number of bigger pores. For example, in 5 lm thick film the proportion of the pores with d > 20 nm is marginal (<1%) but in thick films this value comes close to 10% [3] or more [10,15]. Concerning the average pore diameter of thick PPy/DDS film – dAV = 3.6 nm [3], it is essentially compatible with the corresponding values for significantly thinner PPy/DDS films – see Table 1. The dAV value for thick film exceeds the dAV values for thin films of up to 1.0 lm film thickness, i.e. up to a thickness with a significant proportion of microporosity. The difference between dAV values for (relatively) thin film (5 lm) and thick film [3] can originate from different experimental methods and conditions, including different temperatures (room temperature/196 °C, correspondingly). As a conclusion from the comparisons presented above, the GISAXS method seems to be applicable to obtain reliable results relating to the porosity of (relatively) thin PPy films. 3.2. Atomic force microscopy, X-ray reflectivity analysis and Energy Dispersive X-ray microanalysis Thickness-induced morphology changes in PPy/DDS films were estimated by AFM. Besides using the RMS values in the additional characterization of PPy/DDS films of different thicknesses, they were also used in the modelling of XRR patterns. The typical AFM images are presented in Fig. 4. It can be seen that in all cases, the surface of Si/Pt electrodes are fully covered with a PPy layer. The AFM images of PPy films clearly show that the increase of film thickness causes an increase of surface roughness. Thus, the calculated RMS roughness values were as follows: 1.4 nm, 8.2 nm and 114 nm for 0.04, 1 and 5 lm thick PPy films respectively. Fig. 5 shows the results of the X-ray reflection (XRR) analysis for three selected samples (0.04, 0.2 and 1 lm thick films), which provided a relatively good match with the calculated patterns. The XRR pattern of the 5 lm thick sample did not allow for finding an acceptable model for pattern matching. XRR and AFM roughness parameters are physically different in nature. XRR roughness also reflects density gradients inside the film. Since the XRR and AFM roughness have a bit different physical background, it is not possible to compare these values in absolute numbers, although AFM and XRR roughness values usually correlate well with each other. This is confirmed also for our samples where the XRR refined roughness values (1.2 nm for 0.04 lm thick and above 7 nm for 1 lm thick film) were in good correlation with the values obtained from the AFM measurements 1.4 and 8.2 nm, correspondingly. For the rest of the PPy films studied, the roughness parameters were above the detection limits of XRR analysis, and were calculated using the AFM measurements.
Fig. 4. Typical non-contact AFM images of PPy/DDS films deposited onto surface of Si/Pt electrodes. The PPy film thicknesses are: (a) 0.04 lm; (b) 1 lm; and (c) 5 lm.
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Fig. 5. First part of XRR patterns for PPy/DDS films of different thicknesses, noted in figure. Lines with unfilled marks – experimental data, lines with filled marks – calculated XRR profile. All patterns were measured and refinement performed in the range of reflection angle 0–6° (2h).
Concerning the determination of the density of the films investigated, it should be taken into account that the first cut-off for the XRR patterns shows the existence of low density PPy films on high density Pt film. The position and extent of this cut-off together reflect the density of films that was refined during a fitting of experimental patterns to modelled patterns. The cut-off at higher 2-theta values occurs due to the Pt layer. The average density of the films, estimated from XRR analysis, was mostly in the range of 1.2–1.3 g/cm3. Here, the standard deviation of the densities was ±0.2 g/cm3. These values are slightly smaller compared to the common PPy density (1.4–1.5 g/cm3) presented in relevant literature and ordinarily found in ‘‘wet’’ flotation density experiments [1,31]. Only the thinnest film of the present study (0.04 lm thick) showed the film density 1.4–1.5 g/cm3. Modelling of the XRR pattern showed that introducing a density gradient into the layer gave better match to the experimental patterns as compared to the single layer, constant density model. According to this model, it was established that the density of 1 lm thick film decreased inside the film from the surface of the substrate to the top of the film. This tendency also appeared for the film after its electrochemical reduction in the aqueous solution. In accordance with these results, the use of a single layer model also revealed the scarce decrease of the average film density from 1.3 to 1.2 g/cm3 by a switch from 0.2 lm to 1 lm thick film. The greater density of the thinner film is also ascertained for PPy films doped with dodecyl-benzenesulfonate (PPy/DBS) [29]. The moderate trend of the film density increase with a decreased film thickness seem to be the result of the increased microporosity of the thinner films and higher doping levels: both can result in less void volume in the film. Thus according to the EDX results the f values calculated for 0.2 and 1 lm thick films were around 0.31–0.32, but in the case of 0.04 lm film, it was f = 0.58. However, it should be mentioned here that the last value may be somewhat elevated due to a strong background signal from the Pt substrate. The greater f values of PPy near-substrate sublayers were also established in our recent study of PPy/DDS/Cl and PPy/Cl/DDS bilayers [32]. These higher f values were ascertained for PPy lamellae, prepared by an in situ slice technique and analyzed as consecutive polymer sublayers with 100 nm steps. However according to our present study, the higher f values alone without the dominance of smaller pores, as is the case with the thickest (5 lm) film (f = 0.61), cannot guarantee a higher film density. Here the influence of high f value is counterbalanced by the existence of significantly greater pores that change the layer sparser. 4. Conclusions PPy/DDS films with different thicknesses of 0.04, 0.2, 1 and 5 lm were electrogenerated onto smooth Si/Pt electrodes. Their porous structure characteristics and surface morphology (RMS roughness) were investigated by reflection small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) methods. Energy Dispersive X-ray microanalysis (EDX) was used to determine the doping level of the dopant in the film. The analysis of pore size distribution curves reveals that the porosity of PPy/DDS films depends quite significantly on the film thickness. It was established that only the thinnest (0.04 lm) PPy/DDS film had a prevalently microporous structure. All the thicker films investigated were mainly mesoporous (0.2 and 1 lm thick films) or almost entirely mesoporous (5 lm thick film). It was found that the increase of film thickness up to 5 lm brought along not only an increase of the average pore diameter but also the greater variability of pore sizes. The electrochemical treatments (reduction) of the PPy/DDS film (1 lm) caused very few if any changes in the film’s porous structure. The analysis of the chemical composition and density of the films have suggested a reason for this behaviour: the higher doping levels, established in the case of thinner PPy/DDS films, result (due to long and ‘‘flexible’’ tail of DDS) in the more compact packing of PPy chains, and therefore in smaller pores and increased densities of the films. The surface roughness
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parameter (RMS) of the films investigated also showed thickness dependent behaviour – the RMS value increased significantly as the film thickness increased. The porosity characteristics that were obtained and their comparison with the corresponding values from divergent experiments (different method, different film thickness) let us conclude that the GISAXS method along with the proper porosity analysis software is suitable for characterizing porosity – thickness relations of (relatively) thin PPy films. Acknowledgements The authors are grateful to Peeter Ritslaid for preparation of Si/Pt electrodes. This work was supported by Estonian Centre of Excellence in Research Project TK117 ‘‘High-technology Materials for Sustainable Development’’ and Estonian Ministry of Education and Research Project IUT2-24. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
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