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Preparation of micropatterned polyaniline thin films with enhanced electrochromic properties by electrostatic field-assisted potentiostatic deposition
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Preparation of micropatterned polyaniline thin films with enhanced electrochromic properties by electrostatic field-assisted potentiostatic deposition Cheng-Lan Lin a,b,n, Li-Jun Liao a a b
Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan The Energy and Opto-Electronic Materials Research Center, Tamkang University, New Taipei City 25137, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 30 November 2014 Received in revised form 12 July 2015 Accepted 24 July 2015
Micropatterned polyaniline (mPani) thin films on indium tin oxide (ITO) conducting substrates are prepared by the electrostatic field-assisted potentiostatic deposition (ESaPD) and their electrochromic (EC) properties are investigated in this study. The mPani thin films can be directly electrodeposited onto the ITO substrates under the influence of a patterned electrostatic field induced by a micropatterned electrostatic film through contact electrification. It is found that the mPani thin film has faster coloring/ bleaching responses, small enhancement in transmittance modulation ability and higher coloration efficiency than the non-patterned Pani thin film prepared with the same deposition time. Electrochemical impedance spectroscopy analysis indicates that the charge-transfer resistance of the mPani thin film is smaller than that of the non-patterned Pani thin film, and which could be the reason for the enhancement of the EC properties of the mPani thin film. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polyaniline Micropattern Electrochromic material Electrostatic field
1. Introduction Electrochromic (EC) materials have the ability to change their absorbance properties of electromagnetic radiations upon the application of external potentials [1]. The theory of electrochromism was suggested by Platt [2] in 1961 and was first demonstrated by Deb [3] in 1969. Polyaniline (Pani) is a wellknown polymeric EC material with multiple color states and stable chemical and electrochemical properties, and its EC properties as well as applications have been extensively studied [1,4]. Pani can reversibly change its color between light yellow of the fully reduced state (Leucoemeraldine base, LB), green of the partially oxidized state (Emeraldine base, EB), and blue of the fully oxidized state (Pernigraniline base, PB) through its electrochemical redox reactions. Pani thin films can be prepared by electrochemical polymerization [5] on conducting substrates. The modifications of Pani nanostructures [6–8] or the developments of Pani composites with other materials [9–11] for improving the EC performances have also being widely explored. However, relatively few studies were concerning about the EC performances of micropatterned Pani (mPani) thin films. n Corresponding author at: Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan. Tel.: þ886 2 26215656 2723; fax:þ 886 2 26209887. E-mail address: [email protected] (C.-L. Lin).
Conventional methods for the preparation micropatterned thin films include photolithography [12], soft lithography [13], templating [14] or aqueous chemical growth [15]. Strategies of alternating the surface properties of the substrate such as hydrophobicity [16], photoconductivity [17], or static electricity [18] for deriving micropatterned thin films have also been studied. Most of these methods involved multiple pretreatment steps of the substrate. A novel electrostatic field-assisted potentiostatic deposition (ESaPD) method with one simple substrate pretreatment process has been demonstrated for the preparation of micropatterned inorganic thin film on indium tin oxide (ITO) conducting glass in our previous study [19]. By attaching a micropatterned electrostatic film (ESF) onto an ITO substrate through the contact electrification phenomenon [20], a patterned electrostatic field could be induced upon the removal of the ESF, and a micropatterned thin film could be directly electrodeposited on the ITO surface under the influence of the electrostatic field. In this study, mPani thin films were prepared on ITO substrates using the ESaPD method. EC properties including the coloring/ bleaching response time, transmittance modulation (ΔT%), and coloration efficiency (CE) of these mPani thin films were investigated and compared with the non-micropatterned Pani thin films prepared with the same deposition parameters. Electrochemical impedance spectroscopy (EIS) analysis was employed to characterize the kinetic parameters of the electrochemical reactions of these thin films.
http://dx.doi.org/10.1016/j.solmat.2015.07.035 0927-0248/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: C.-L. Lin, L.-J. Liao, Preparation of micropatterned polyaniline thin films with enhanced..., Solar Energy Materials and Solar Cells (2015), http://dx.doi.org/10.1016/j.solmat.2015.07.035i
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2. Materials and methods Aniline (99.5%) and hydrochloric acid (HCl, 37%) were purchased from Acros. Potassium chloride (KCl, 99.9%) was obtained from Shimakyu's Pure Chemicals. All chemicals were used as received without further purification. Deionized water was used throughout the electrochemical experiments. ITO conducting glass (7 Ω/sq) was purchased from Solaronix. ITO conducting glass was sequentially sonicated in diluted detergent solution, acetone and deionized water each for 5 min, dried by nitrogen flow and then kept in a desiccant case before use. Electrostatic film (PE-1101E) was obtained from Youlen Technology Co,. Ltd. Optical microscope (OM) image of the ESF is shown in Fig. 1. The ESF consisted of a polyethylene (PE) sheet with disk-shaped polyacrylate (PA) bumps orderly arranged on its surface. The height and diameter of the bumps were 5 μm and 220 μm, respectively. Surface morphology of the thin films was observed using a scanning electron microscope (SEM, Carl Zeiss, Leo 1530). Digital images of the thin films were obtained using an optical microscope (Olympus, BX51). A conventional three-electrode system with a potentiostat/galvanostat (CH Instrument, CHI-760D) was employed for the electrochemical experiments. A platinum coil was used as the counter electrode and the reference electrode was a Ag/AgCl electrode. The working electrode was an ITO glass for the Pani electrodeposition and was a mPani or Pani-coated ITO glass for other electrochemical experiments. The area of the working electrode was 1 2 cm2. The electrolyte for cyclic voltammetry (CV), double-potential-step (DPS) and electrochemical impedance spectra (EIS) experiments was an aqueous solution containing 0.1 M KCl and 0.01 M HCl. An ultraviolet–visible spectrophotometer (Thermo, EVO300PC) was used for the in-situ measurements of the absorbance changes during the electrochemical reactions of the mPai and Pani thin films. The procedure for the preparation of a mPani thin film is described as follows: The ESF was attached onto the ITO surface through contact electrification for at least 7 days and was peeled off right before the electrodeposition process. Pani was then potentiostatically deposited in an aqueous solution containing 2.0 M HCl and 1.0 M aniline. A potential of 0.8 V (vs. Ag/AgCl) was applied to the ESF-pretreated ITO substrate for various deposition times to deposit Pani. The mPani thin films prepared with the deposition time of 80 s, 100 s, 120 s, 140 s, and 160 s were named as mPani-80, mPani-100, mPani-120, mPani-140, and mPani-160 thin films, respectively. Non-micropatterned Pani thin films (Pani80, Pani-100, Pani-120, Pani-140, and Pani-160 thin films) were also prepared using the same procedures on pristine ITO substrates for comparisons. It may not be suitable for real fabrication if the preparation of a micropatterned film takes several days. The strength of an
Fig. 1. OM images of the ESF film.
electrostatic field generated by contact electrification depends on the two materials brought in contact. According to the triboelectric series, it is expect that greater electrostatic field could be generated in a shorter time by contacting an ITO surface with other materials such as polyurethanes or polyvinylchloride. Studies in reducing the preparation time are ongoing in our laboratory.
3. Results and discussion 3.1. Procedures and mechanisms of the ESaPD The mPani thin films were prepared using the ESaPD method. Procedures of the ESaPD and the proposed mechanism are illustrated in Fig. 2. The ESF was firstly attached onto the ITO surface through contact electrification. The ESF was stored in roll and in that way the PA bumps were in contact with the backside of the PE sheet. According to the tribo-electric series reported in previous studies [21,22], equal amount of positive and negative charges could be developed on the PA and the PE sides, respectively. Therefore there were positive charges initially on the PA bumps surface. When the positive-charged PA bumps were brought in contact with the ITO surface, negative charges could be attracted and accumulated at the ITO side of the PA/ITO interface. These negative charges should induce an equal amount of positive charges gathered at the opposite side of the ITO layer (the ITO side of the ITO/glass interface). Glass is an electret material [23] which could retain quasi-permanent charge separations. The positive charges at the ITO side of the ITO/glass interface might induce the formation of charge separations inside the glass, as shown in Fig. 2. When the ESF was removed from the ITO surface, a positive electrostatic field was thus built up by the positive charges at ITO side of the ITO/glass interface. The surface areas that have been contacted with the PA bumps were thus experiencing the positive electrostatic field. There should be no local negative charges accumulated on the ITO surface since it is a conductor. The locally distributed electrostatic field on the ITO surface could affect the potentiostatic deposition process and a mPani thin film might be obtained. The ITO surface areas with and without the influence of the underlying positive electrostatic field were designated as A þ and A0, respectively (Figs. 2 and 3(f)). A þ was the area that has been attached with the PA bumps, and A0 was the pristine ITO surface area. The electrodeposition of Pani is an oxidative process
Fig. 2. Illustrations of the procedures and the proposed mechanism of the ESaPD of a mPani thin film on an ITO substrate.
Please cite this article as: C.-L. Lin, L.-J. Liao, Preparation of micropatterned polyaniline thin films with enhanced..., Solar Energy Materials and Solar Cells (2015), http://dx.doi.org/10.1016/j.solmat.2015.07.035i
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Fig. 3. SEM images of (a) mPani-80, (b) mPani-100, (c) mPani-120, (d) mPani-140, (e) mPani-160 thin films and (f) Illustration of the A0 and A þ areas on the ITO surface.
and consequently a thick Pani layer was obtained on top of A þ under the influence of the positive electrostatic field. 3.2. Surface morphology Fig. 3 shows the SEM images of the mPani thin films prepared with different deposition times. The brighter part in a SEM image should indicate a higher surface profile for the surface consisted of a single material. Clear micropatterns could be observed for the mPani-80 and mPani-100 thin films (Fig. 3(a) and (b)), and fuzzy but still distinguishable micropatterns were observed for the
mPani-120 and mPani-140 thin films (Fig. 3(c) and (d)), but there was no apparent micropatterns could be defined for the mPani160 thin film (Fig. 3(e)). Fig. 3(f) shows the distribution of A þ (affected by the electrostatic field) and A0 (pristine ITO surface) areas on the ITO surface. The diameter of the circle-shaped areas in Fig. 3(a) and (b) were about the same size (220 μm) as the PA bumps of the ESF, suggesting that the effect of the electrostatic field could be well-confined within the A þ areas. High resolution SEM images of the A þ and A0 areas on the mPani-80 thin film surface are shown in Fig. 3(g) and (h), respectively. More pronounced microstructures with comparatively clearly-defined
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Fig. 4. (a) Absorbance spectra of the mPani-120 thin film at different applied potentials (vs. Ag/AgCl). OM images of the mPani-120 thin film at its (b) bleached state (at 0.3 V vs. Ag/AgCl) and (c) colored state (at 0.5 V vs. Ag/AgCl). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
contours were observed on the A þ area than on the A0 area. The observations also suggested that the Pani deposition rate was affected by the presence of the electrostatic field. The SEM images indicated that mPani thin films could be successfully prepared by the ESaPD method if the deposition time shorter than 140 s was used. The electrostatic field strength is inversely proportional to the distance. It is deduced that as the Pani grew thicker with the longer deposition time, the influence of the electrostatic field on the deposition become weaker and therefore distinct micropatterns can no longer be obtained. 3.3. Optical and electrochemical properties Absorbance spectra of the mPani-120 thin film at different applied potentials and the OM images of its bleached and colored states are shown in Fig. 4. Similar absorption spectra were recorded when the applied potential was between 0.3 and 0.1 V (vs. Ag/AgCl), the mPani–120 thin film was at its bleached state with low absorbance in the visible range. When the applied potential was increased from 0.1 to 0.5 V (vs. Ag/AgCl), the absorbance of the mPani-100 thin film between 400 and 480 nm and 550 and 800 nm increased. Color of the mPani-120 thin film gradually switched from light-yellow to green, which suggested that the Pani changed from its LB to EB form. The absorption spectrum at the applied potential of 0.6 V (vs. Ag/AgCl) was apparently different from that at 0.5 V (vs. Ag/AgCl), indicating that the Pani started to switch from the EB to PB (blue) form. By comparing Fig. 4(b) and (c), a denser color change from yellow to green could be observed within the circle-shaped areas (A þ ) than the area outside them (A0), which suggested that the presence of the ordered Pani micropatterns. The mPAni thin film could be successfully prepared on an ITO surface by the ESaPD method with more Pani deposited in the circle-shaped areas. It is deduced that the presence of the electrostatic field within the A þ areas resulted in an uneven deposition of Pani on the ITO surface, and a mPani thin film was thus formed. CVs of the mPani and Pani thin films are shown in Fig. 5(a). Redox current responses of the mPani thin films were smaller than that of the Pani thin films prepared with the same deposition times. The results suggested that there might be more Pani deposited in the Pani thin films than in the mPani thin films prepared with the same deposition time. Fig. 5(b) shows the colored state absorbance spectra of the mPani and Pani thin films at the applied potential of 0.5 V (vs. Ag/AgCl). With the same deposition time, the mPani thin films have lower absorbance than the non-patterned Pani thin films because they contained fewer amount of Pani. The electrostatic field affected about 42% of the ITO surface area (A þ /(A þ þA0)¼0.42), and there
Fig. 5. (a) CVs of the mPani and Pani thin films. The scan rate was 50 mV/s. (b) Colored state absorbance spectra of the mPani and Pani thin films at an applied potential of 0.5 V (vs. Ag/AgCl).
were more Pani deposited on A þ than A0 as indicated in Fig. 4(b) and (c). It is supposed that such non-uniform distribution of Pani might also responsible for the lower absorbance observed for the mPani thin films. The CV and absorption results suggested that although Pani
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micropatterns could be generated by the ESaPD method, the presence of the electrostatic field might decrease the overall Pani deposition efficiency.
3.4. Electrochromic properties Fig. 6(a) shows the current–time responses of the mPani and Pani thin films during the DPS experiments. The applied potential was switched from 0.2 to 0.5 V (vs. Ag/AgCl) for the coloring
Fig. 6. (a) i–t curves of the double-potential-step experiments of the mPani and Pani thin films. (b) The in-situ absorbance changes (at 700 nm) of the mPani and Pani thin films during the double-potential-step experiments.
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process and then switched back to 0.2 V (vs. Ag/AgCl) for the bleaching process. The corresponding absorbance-time responses of the thin films were in-situly recorded and are shown in Fig. 6(b). As can be seen from Fig. 6(a), the current responses of the mPani thin films after the potential switches (at 60 s for coloring and 120 s for bleaching) dropped faster than that of the Pani thin films, which implied faster color change rates for the mPani thin films. The charge capacities involved in the coloring and bleaching processes of the thin films, Qc and Qb, were calculated by integrating the area under the i–t curves and are listed in Table 1. Qc and Qb of the mPani thin films were smaller than that of the Pani thin films with the same deposition time. The ratio of Qc/Qc could be used as an index for the assessment of the redox reversibility the mPani and Pani thin films. All of the Qc/Qc values were close to unity, indicating that the redox reactions of these mPani and Pani thin films were highly reversible. It is desired that an EC material could be rapidly switched between its colored and bleached states. The coloring and bleaching response times (tc and tb) is defined as the time needed to achieve 95% of the maximum absorbance change of the processes. tc and tb of the mPani and Pani thin films were estimated from their absorbance-time responses during the DPS experiments (Fig. 6(b)) and are listed in Table 1. All of the thin films have larger tc than tb, which indicated that it required longer time for the coloring than the bleaching reactions. It is noted that both tc and tb of the mPani thin films were shorter than that of the Pani thin films. tc than tb of the mPani-80 thin film (2.13 s and 0.75 s) was 53% and 62% faster than that of the Pani-80 thin film (4.56 s and 1.98 s). The mPani-160 thin film has 16% and 21% faster coloring and bleaching rates than the Pani-160 thin film. The results suggested that the micropatterned structure could promote the colorswitching rates of the Pani, and a more well-defined micropattern might result in faster coloring/bleaching responses. Transmittance modulation ability, ΔT%, of an EC material corresponds to the transmittance difference between its colored and bleached states. A higher ΔT% represents a better ability for an EC material in modulating the electromagnetic radiation. ΔT% of the mPani and Pani thin films at 700 nm are listed in Table 1. ΔT% of the mPani and Pani thin films increased with the increasing of their deposition time. ΔT% of the mPani thin films were slightly higher than that of the Pani thin films with the same deposition time because of the mPani thin films have lower bleached state absorbance. CE of an EC material is defined as the absorbance change it can achieve by unit amount of charge density consumption during its coloring reaction. CE can be estimated by ΔOD/Qc, where ΔOD is the absorbance difference of the colored and bleached states of the EC material (Ac and Ab). CE of the mPani and Pani thin films are listed in Table 1. CEs of the mPani thin films were averagely higher than that of the Pani thin films, indicating that the mPani thin films could exhibit larger absorbance changes with the same amount of charge consumption. The result implied
Table 1 Electrochromic performances (at 700 nm) and electrochemical properties of the mPani and Pani thin films. Sample
Ac
Ab
ΔT%
CE (cm2/C)
tc (s)
tb (s)
Qc (mC/cm2)
Qb (mC/cm2)
Qc/Qb
Pani-160s Pani-140s Pani-120s Pani-100s Pani-80s mPani-160s mPani-140s mPani-120s mPani-100s mPani-80s
0.772 0.634 0.578 0.490 0.355 0.692 0.562 0.493 0.402 0.283
0.127 0.089 0.085 0.081 0.077 0.065 0.053 0.048 0.042 0.037
63.45 58.24 55.80 50.63 39.60 65.73 60.97 57.25 51.15 39.17
54.47 50.00 48.05 46.48 43.57 56.04 54.75 60.77 53.25 58.84
5.89 5.72 5.41 5.15 4.56 4.89 4.70 4.19 3.64 2.13
2.91 2.73 2.50 2.23 1.98 2.31 2.13 1.93 1.48 0.75
11.83 10.90 10.26 8.80 6.38 11.17 9.26 7.29 6.76 4.15
11.53 10.67 10.12 8.74 6.45 11.18 9.24 7.37 6.84 4.21
1.02 1.02 1.01 1.01 0.99 1.00 1.00 0.99 0.99 0.99
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Fig. 8. Nyquist plot of the mPani and Pani thin film and the equivalent circuit for fitting. Table 2 Fitting results of the impedance spectra in Fig. 8.
Fig. 7. (a) Plots of tc or tb vs. Qc or Qb of the mPani and Pani thin films. (b) Plots of tc or tb vs. ΔT% of the mPani and Pani thin films.
that the presence of the micropatterns might facilitate the redox reaction of the Pani. Fig. 7(a) shows the plots of tc or tb versus Qc or Qb of the mPani and Pani thin films. From the plots it is suggested that if the same amount of charges (for example, 10 mC/cm2) was required the coloring or bleaching processes, the mPani thin film could complete the processes faster than the non-patterned Pani thin film. The micropatterns might provide a more efficient way for the charge transfers during the coloring and bleaching reactions of Pani. The plot of tc and tb versus ΔT% of the mPani and Pani thin films are shown in Fig. 7(b). It indicated that for the thin films providing the same ΔT% (for example, 60%), the mPani thin film completed the color-switching processe faster than the Pani thin film. 3.5. Electrochemical impedance spectroscopy analysis Nyquist plots from the EIS analysis of the mPani and Pani thin films and the equivalent circuit for simulating the impedance plots [24] are shown in Fig. 8. The plots consist of a distorted semicircle in the high frequency region and its intercept with the real axis is the solution resistance (Rs). The diameter of the semicircle is equal to the charge transfer resistance (Rct) in the Pani thin film. Cdl is the double layer capacitance at electrode surfaces and W1 is the Warburg impedance for the ionic diffusion process. The fitting results of the impedance spectra are listed in Table 2. The Rct of the
Sample
Rs (Ω)
Rct (Ω)
mPani-80 PAni-80 mPAni-120 Pani-120 mPAni-160 Pani-160
50.59 52.17 51.80 55.58 53.81 53.19
31.41 61.55 85.03 90.76 139.01 150.10
Cdl (F) 1.94 10 5 2.45 10 5 3.65 10 5 3.97 10 5 4.49 10 5 5.32 10 5
W1 (Ω/s0.5) 0.044 0.104 0.157 0.164 0.184 0.189
mPani thin films were smaller than the Pani thin films, which suggested that the formation of micropatterned thin film might decrease the charge transfer resistance of Pani. It is noted that the difference between the Rct values of the mPani and Pani thin films decreased if the thin films were prepared with longer deposition time. The Rct value of the mPani-80 thin films (31.41 Ω) was approximately half of that of the Pani-80 thin film (61.55 Ω), but the mPani-160 and Pani-160 thin films have similar Rct values (139.01 Ω and 150.10 Ω). As shown in the SEM images Fig. 3(e), distinct micropatterns can be observed for the mPani-80 thin film but not for the mPani-160 thin film, which implied that such micropatterned structure might facilitated the charge transfer processes during the redox reactions of Pani.
4. Conclusions Micropatterned Pani thin films with different deposition times were prepared by ESaPD method and their EC properties were investigated and compared with the non-micropatterned Pani thin films in this study. SEM and OM images evidenced that the mPani thin films can be directly deposited under the influence of a patterned electrostatic field induced by the ESF through contact electrification process. EIS studies indicated that the Rct of the mPani thin films were smaller than that of the non-patterned Pani thin films prepared with the same deposition time. It is deduced that such micropatterned structure might facilitate the redox reactions of Pani, and therefore enhanced EC properties including coloring/bleaching response times, ΔT%, and CE were observed for the mPani thin films. The increase in ΔT%, and CE were moderate at the present stage, which should because of the electrostatic field affected only 42% of the area and limited difference in the film thickness between A þ and A0 areas. Accordingly, it is expected that a higher bleaching/coloring contrast might be achieved by the film having smaller patterns with larger A þ area and being
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deposited under the influence of a stronger electrostatic field. It is envisaged that the ESaPD method could serve as a new straightforward method for the preparation of micropatterned thin films, and the micropatterned EC thin films might find their potential applications for EC devices or sensors.
Acknowledgments This work has been supported by the National Science Council of the Republic of China (NSC 102-2221-E-032-051).
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Preparation of micropatterned polyaniline thin films with enhanced electrochromic properties by electrostatic field-assisted potentiostatic deposition Cheng-Lan Lin a,b,n, Li-Jun Liao a a b
Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan The Energy and Opto-Electronic Materials Research Center, Tamkang University, New Taipei City 25137, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 30 November 2014 Received in revised form 12 July 2015 Accepted 24 July 2015
Micropatterned polyaniline (mPani) thin films on indium tin oxide (ITO) conducting substrates are prepared by the electrostatic field-assisted potentiostatic deposition (ESaPD) and their electrochromic (EC) properties are investigated in this study. The mPani thin films can be directly electrodeposited onto the ITO substrates under the influence of a patterned electrostatic field induced by a micropatterned electrostatic film through contact electrification. It is found that the mPani thin film has faster coloring/ bleaching responses, small enhancement in transmittance modulation ability and higher coloration efficiency than the non-patterned Pani thin film prepared with the same deposition time. Electrochemical impedance spectroscopy analysis indicates that the charge-transfer resistance of the mPani thin film is smaller than that of the non-patterned Pani thin film, and which could be the reason for the enhancement of the EC properties of the mPani thin film. & 2015 Elsevier B.V. All rights reserved.
Keywords: Polyaniline Micropattern Electrochromic material Electrostatic field
1. Introduction Electrochromic (EC) materials have the ability to change their absorbance properties of electromagnetic radiations upon the application of external potentials [1]. The theory of electrochromism was suggested by Platt [2] in 1961 and was first demonstrated by Deb [3] in 1969. Polyaniline (Pani) is a wellknown polymeric EC material with multiple color states and stable chemical and electrochemical properties, and its EC properties as well as applications have been extensively studied [1,4]. Pani can reversibly change its color between light yellow of the fully reduced state (Leucoemeraldine base, LB), green of the partially oxidized state (Emeraldine base, EB), and blue of the fully oxidized state (Pernigraniline base, PB) through its electrochemical redox reactions. Pani thin films can be prepared by electrochemical polymerization [5] on conducting substrates. The modifications of Pani nanostructures [6–8] or the developments of Pani composites with other materials [9–11] for improving the EC performances have also being widely explored. However, relatively few studies were concerning about the EC performances of micropatterned Pani (mPani) thin films. n Corresponding author at: Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan. Tel.: þ886 2 26215656 2723; fax:þ 886 2 26209887. E-mail address: [email protected] (C.-L. Lin).
Conventional methods for the preparation micropatterned thin films include photolithography [12], soft lithography [13], templating [14] or aqueous chemical growth [15]. Strategies of alternating the surface properties of the substrate such as hydrophobicity [16], photoconductivity [17], or static electricity [18] for deriving micropatterned thin films have also been studied. Most of these methods involved multiple pretreatment steps of the substrate. A novel electrostatic field-assisted potentiostatic deposition (ESaPD) method with one simple substrate pretreatment process has been demonstrated for the preparation of micropatterned inorganic thin film on indium tin oxide (ITO) conducting glass in our previous study [19]. By attaching a micropatterned electrostatic film (ESF) onto an ITO substrate through the contact electrification phenomenon [20], a patterned electrostatic field could be induced upon the removal of the ESF, and a micropatterned thin film could be directly electrodeposited on the ITO surface under the influence of the electrostatic field. In this study, mPani thin films were prepared on ITO substrates using the ESaPD method. EC properties including the coloring/ bleaching response time, transmittance modulation (ΔT%), and coloration efficiency (CE) of these mPani thin films were investigated and compared with the non-micropatterned Pani thin films prepared with the same deposition parameters. Electrochemical impedance spectroscopy (EIS) analysis was employed to characterize the kinetic parameters of the electrochemical reactions of these thin films.
http://dx.doi.org/10.1016/j.solmat.2015.07.035 0927-0248/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: C.-L. Lin, L.-J. Liao, Preparation of micropatterned polyaniline thin films with enhanced..., Solar Energy Materials and Solar Cells (2015), http://dx.doi.org/10.1016/j.solmat.2015.07.035i
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2. Materials and methods Aniline (99.5%) and hydrochloric acid (HCl, 37%) were purchased from Acros. Potassium chloride (KCl, 99.9%) was obtained from Shimakyu's Pure Chemicals. All chemicals were used as received without further purification. Deionized water was used throughout the electrochemical experiments. ITO conducting glass (7 Ω/sq) was purchased from Solaronix. ITO conducting glass was sequentially sonicated in diluted detergent solution, acetone and deionized water each for 5 min, dried by nitrogen flow and then kept in a desiccant case before use. Electrostatic film (PE-1101E) was obtained from Youlen Technology Co,. Ltd. Optical microscope (OM) image of the ESF is shown in Fig. 1. The ESF consisted of a polyethylene (PE) sheet with disk-shaped polyacrylate (PA) bumps orderly arranged on its surface. The height and diameter of the bumps were 5 μm and 220 μm, respectively. Surface morphology of the thin films was observed using a scanning electron microscope (SEM, Carl Zeiss, Leo 1530). Digital images of the thin films were obtained using an optical microscope (Olympus, BX51). A conventional three-electrode system with a potentiostat/galvanostat (CH Instrument, CHI-760D) was employed for the electrochemical experiments. A platinum coil was used as the counter electrode and the reference electrode was a Ag/AgCl electrode. The working electrode was an ITO glass for the Pani electrodeposition and was a mPani or Pani-coated ITO glass for other electrochemical experiments. The area of the working electrode was 1 2 cm2. The electrolyte for cyclic voltammetry (CV), double-potential-step (DPS) and electrochemical impedance spectra (EIS) experiments was an aqueous solution containing 0.1 M KCl and 0.01 M HCl. An ultraviolet–visible spectrophotometer (Thermo, EVO300PC) was used for the in-situ measurements of the absorbance changes during the electrochemical reactions of the mPai and Pani thin films. The procedure for the preparation of a mPani thin film is described as follows: The ESF was attached onto the ITO surface through contact electrification for at least 7 days and was peeled off right before the electrodeposition process. Pani was then potentiostatically deposited in an aqueous solution containing 2.0 M HCl and 1.0 M aniline. A potential of 0.8 V (vs. Ag/AgCl) was applied to the ESF-pretreated ITO substrate for various deposition times to deposit Pani. The mPani thin films prepared with the deposition time of 80 s, 100 s, 120 s, 140 s, and 160 s were named as mPani-80, mPani-100, mPani-120, mPani-140, and mPani-160 thin films, respectively. Non-micropatterned Pani thin films (Pani80, Pani-100, Pani-120, Pani-140, and Pani-160 thin films) were also prepared using the same procedures on pristine ITO substrates for comparisons. It may not be suitable for real fabrication if the preparation of a micropatterned film takes several days. The strength of an
Fig. 1. OM images of the ESF film.
electrostatic field generated by contact electrification depends on the two materials brought in contact. According to the triboelectric series, it is expect that greater electrostatic field could be generated in a shorter time by contacting an ITO surface with other materials such as polyurethanes or polyvinylchloride. Studies in reducing the preparation time are ongoing in our laboratory.
3. Results and discussion 3.1. Procedures and mechanisms of the ESaPD The mPani thin films were prepared using the ESaPD method. Procedures of the ESaPD and the proposed mechanism are illustrated in Fig. 2. The ESF was firstly attached onto the ITO surface through contact electrification. The ESF was stored in roll and in that way the PA bumps were in contact with the backside of the PE sheet. According to the tribo-electric series reported in previous studies [21,22], equal amount of positive and negative charges could be developed on the PA and the PE sides, respectively. Therefore there were positive charges initially on the PA bumps surface. When the positive-charged PA bumps were brought in contact with the ITO surface, negative charges could be attracted and accumulated at the ITO side of the PA/ITO interface. These negative charges should induce an equal amount of positive charges gathered at the opposite side of the ITO layer (the ITO side of the ITO/glass interface). Glass is an electret material [23] which could retain quasi-permanent charge separations. The positive charges at the ITO side of the ITO/glass interface might induce the formation of charge separations inside the glass, as shown in Fig. 2. When the ESF was removed from the ITO surface, a positive electrostatic field was thus built up by the positive charges at ITO side of the ITO/glass interface. The surface areas that have been contacted with the PA bumps were thus experiencing the positive electrostatic field. There should be no local negative charges accumulated on the ITO surface since it is a conductor. The locally distributed electrostatic field on the ITO surface could affect the potentiostatic deposition process and a mPani thin film might be obtained. The ITO surface areas with and without the influence of the underlying positive electrostatic field were designated as A þ and A0, respectively (Figs. 2 and 3(f)). A þ was the area that has been attached with the PA bumps, and A0 was the pristine ITO surface area. The electrodeposition of Pani is an oxidative process
Fig. 2. Illustrations of the procedures and the proposed mechanism of the ESaPD of a mPani thin film on an ITO substrate.
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Fig. 3. SEM images of (a) mPani-80, (b) mPani-100, (c) mPani-120, (d) mPani-140, (e) mPani-160 thin films and (f) Illustration of the A0 and A þ areas on the ITO surface.
and consequently a thick Pani layer was obtained on top of A þ under the influence of the positive electrostatic field. 3.2. Surface morphology Fig. 3 shows the SEM images of the mPani thin films prepared with different deposition times. The brighter part in a SEM image should indicate a higher surface profile for the surface consisted of a single material. Clear micropatterns could be observed for the mPani-80 and mPani-100 thin films (Fig. 3(a) and (b)), and fuzzy but still distinguishable micropatterns were observed for the
mPani-120 and mPani-140 thin films (Fig. 3(c) and (d)), but there was no apparent micropatterns could be defined for the mPani160 thin film (Fig. 3(e)). Fig. 3(f) shows the distribution of A þ (affected by the electrostatic field) and A0 (pristine ITO surface) areas on the ITO surface. The diameter of the circle-shaped areas in Fig. 3(a) and (b) were about the same size (220 μm) as the PA bumps of the ESF, suggesting that the effect of the electrostatic field could be well-confined within the A þ areas. High resolution SEM images of the A þ and A0 areas on the mPani-80 thin film surface are shown in Fig. 3(g) and (h), respectively. More pronounced microstructures with comparatively clearly-defined
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Fig. 4. (a) Absorbance spectra of the mPani-120 thin film at different applied potentials (vs. Ag/AgCl). OM images of the mPani-120 thin film at its (b) bleached state (at 0.3 V vs. Ag/AgCl) and (c) colored state (at 0.5 V vs. Ag/AgCl). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
contours were observed on the A þ area than on the A0 area. The observations also suggested that the Pani deposition rate was affected by the presence of the electrostatic field. The SEM images indicated that mPani thin films could be successfully prepared by the ESaPD method if the deposition time shorter than 140 s was used. The electrostatic field strength is inversely proportional to the distance. It is deduced that as the Pani grew thicker with the longer deposition time, the influence of the electrostatic field on the deposition become weaker and therefore distinct micropatterns can no longer be obtained. 3.3. Optical and electrochemical properties Absorbance spectra of the mPani-120 thin film at different applied potentials and the OM images of its bleached and colored states are shown in Fig. 4. Similar absorption spectra were recorded when the applied potential was between 0.3 and 0.1 V (vs. Ag/AgCl), the mPani–120 thin film was at its bleached state with low absorbance in the visible range. When the applied potential was increased from 0.1 to 0.5 V (vs. Ag/AgCl), the absorbance of the mPani-100 thin film between 400 and 480 nm and 550 and 800 nm increased. Color of the mPani-120 thin film gradually switched from light-yellow to green, which suggested that the Pani changed from its LB to EB form. The absorption spectrum at the applied potential of 0.6 V (vs. Ag/AgCl) was apparently different from that at 0.5 V (vs. Ag/AgCl), indicating that the Pani started to switch from the EB to PB (blue) form. By comparing Fig. 4(b) and (c), a denser color change from yellow to green could be observed within the circle-shaped areas (A þ ) than the area outside them (A0), which suggested that the presence of the ordered Pani micropatterns. The mPAni thin film could be successfully prepared on an ITO surface by the ESaPD method with more Pani deposited in the circle-shaped areas. It is deduced that the presence of the electrostatic field within the A þ areas resulted in an uneven deposition of Pani on the ITO surface, and a mPani thin film was thus formed. CVs of the mPani and Pani thin films are shown in Fig. 5(a). Redox current responses of the mPani thin films were smaller than that of the Pani thin films prepared with the same deposition times. The results suggested that there might be more Pani deposited in the Pani thin films than in the mPani thin films prepared with the same deposition time. Fig. 5(b) shows the colored state absorbance spectra of the mPani and Pani thin films at the applied potential of 0.5 V (vs. Ag/AgCl). With the same deposition time, the mPani thin films have lower absorbance than the non-patterned Pani thin films because they contained fewer amount of Pani. The electrostatic field affected about 42% of the ITO surface area (A þ /(A þ þA0)¼0.42), and there
Fig. 5. (a) CVs of the mPani and Pani thin films. The scan rate was 50 mV/s. (b) Colored state absorbance spectra of the mPani and Pani thin films at an applied potential of 0.5 V (vs. Ag/AgCl).
were more Pani deposited on A þ than A0 as indicated in Fig. 4(b) and (c). It is supposed that such non-uniform distribution of Pani might also responsible for the lower absorbance observed for the mPani thin films. The CV and absorption results suggested that although Pani
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micropatterns could be generated by the ESaPD method, the presence of the electrostatic field might decrease the overall Pani deposition efficiency.
3.4. Electrochromic properties Fig. 6(a) shows the current–time responses of the mPani and Pani thin films during the DPS experiments. The applied potential was switched from 0.2 to 0.5 V (vs. Ag/AgCl) for the coloring
Fig. 6. (a) i–t curves of the double-potential-step experiments of the mPani and Pani thin films. (b) The in-situ absorbance changes (at 700 nm) of the mPani and Pani thin films during the double-potential-step experiments.
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process and then switched back to 0.2 V (vs. Ag/AgCl) for the bleaching process. The corresponding absorbance-time responses of the thin films were in-situly recorded and are shown in Fig. 6(b). As can be seen from Fig. 6(a), the current responses of the mPani thin films after the potential switches (at 60 s for coloring and 120 s for bleaching) dropped faster than that of the Pani thin films, which implied faster color change rates for the mPani thin films. The charge capacities involved in the coloring and bleaching processes of the thin films, Qc and Qb, were calculated by integrating the area under the i–t curves and are listed in Table 1. Qc and Qb of the mPani thin films were smaller than that of the Pani thin films with the same deposition time. The ratio of Qc/Qc could be used as an index for the assessment of the redox reversibility the mPani and Pani thin films. All of the Qc/Qc values were close to unity, indicating that the redox reactions of these mPani and Pani thin films were highly reversible. It is desired that an EC material could be rapidly switched between its colored and bleached states. The coloring and bleaching response times (tc and tb) is defined as the time needed to achieve 95% of the maximum absorbance change of the processes. tc and tb of the mPani and Pani thin films were estimated from their absorbance-time responses during the DPS experiments (Fig. 6(b)) and are listed in Table 1. All of the thin films have larger tc than tb, which indicated that it required longer time for the coloring than the bleaching reactions. It is noted that both tc and tb of the mPani thin films were shorter than that of the Pani thin films. tc than tb of the mPani-80 thin film (2.13 s and 0.75 s) was 53% and 62% faster than that of the Pani-80 thin film (4.56 s and 1.98 s). The mPani-160 thin film has 16% and 21% faster coloring and bleaching rates than the Pani-160 thin film. The results suggested that the micropatterned structure could promote the colorswitching rates of the Pani, and a more well-defined micropattern might result in faster coloring/bleaching responses. Transmittance modulation ability, ΔT%, of an EC material corresponds to the transmittance difference between its colored and bleached states. A higher ΔT% represents a better ability for an EC material in modulating the electromagnetic radiation. ΔT% of the mPani and Pani thin films at 700 nm are listed in Table 1. ΔT% of the mPani and Pani thin films increased with the increasing of their deposition time. ΔT% of the mPani thin films were slightly higher than that of the Pani thin films with the same deposition time because of the mPani thin films have lower bleached state absorbance. CE of an EC material is defined as the absorbance change it can achieve by unit amount of charge density consumption during its coloring reaction. CE can be estimated by ΔOD/Qc, where ΔOD is the absorbance difference of the colored and bleached states of the EC material (Ac and Ab). CE of the mPani and Pani thin films are listed in Table 1. CEs of the mPani thin films were averagely higher than that of the Pani thin films, indicating that the mPani thin films could exhibit larger absorbance changes with the same amount of charge consumption. The result implied
Table 1 Electrochromic performances (at 700 nm) and electrochemical properties of the mPani and Pani thin films. Sample
Ac
Ab
ΔT%
CE (cm2/C)
tc (s)
tb (s)
Qc (mC/cm2)
Qb (mC/cm2)
Qc/Qb
Pani-160s Pani-140s Pani-120s Pani-100s Pani-80s mPani-160s mPani-140s mPani-120s mPani-100s mPani-80s
0.772 0.634 0.578 0.490 0.355 0.692 0.562 0.493 0.402 0.283
0.127 0.089 0.085 0.081 0.077 0.065 0.053 0.048 0.042 0.037
63.45 58.24 55.80 50.63 39.60 65.73 60.97 57.25 51.15 39.17
54.47 50.00 48.05 46.48 43.57 56.04 54.75 60.77 53.25 58.84
5.89 5.72 5.41 5.15 4.56 4.89 4.70 4.19 3.64 2.13
2.91 2.73 2.50 2.23 1.98 2.31 2.13 1.93 1.48 0.75
11.83 10.90 10.26 8.80 6.38 11.17 9.26 7.29 6.76 4.15
11.53 10.67 10.12 8.74 6.45 11.18 9.24 7.37 6.84 4.21
1.02 1.02 1.01 1.01 0.99 1.00 1.00 0.99 0.99 0.99
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Fig. 8. Nyquist plot of the mPani and Pani thin film and the equivalent circuit for fitting. Table 2 Fitting results of the impedance spectra in Fig. 8.
Fig. 7. (a) Plots of tc or tb vs. Qc or Qb of the mPani and Pani thin films. (b) Plots of tc or tb vs. ΔT% of the mPani and Pani thin films.
that the presence of the micropatterns might facilitate the redox reaction of the Pani. Fig. 7(a) shows the plots of tc or tb versus Qc or Qb of the mPani and Pani thin films. From the plots it is suggested that if the same amount of charges (for example, 10 mC/cm2) was required the coloring or bleaching processes, the mPani thin film could complete the processes faster than the non-patterned Pani thin film. The micropatterns might provide a more efficient way for the charge transfers during the coloring and bleaching reactions of Pani. The plot of tc and tb versus ΔT% of the mPani and Pani thin films are shown in Fig. 7(b). It indicated that for the thin films providing the same ΔT% (for example, 60%), the mPani thin film completed the color-switching processe faster than the Pani thin film. 3.5. Electrochemical impedance spectroscopy analysis Nyquist plots from the EIS analysis of the mPani and Pani thin films and the equivalent circuit for simulating the impedance plots [24] are shown in Fig. 8. The plots consist of a distorted semicircle in the high frequency region and its intercept with the real axis is the solution resistance (Rs). The diameter of the semicircle is equal to the charge transfer resistance (Rct) in the Pani thin film. Cdl is the double layer capacitance at electrode surfaces and W1 is the Warburg impedance for the ionic diffusion process. The fitting results of the impedance spectra are listed in Table 2. The Rct of the
Sample
Rs (Ω)
Rct (Ω)
mPani-80 PAni-80 mPAni-120 Pani-120 mPAni-160 Pani-160
50.59 52.17 51.80 55.58 53.81 53.19
31.41 61.55 85.03 90.76 139.01 150.10
Cdl (F) 1.94 10 5 2.45 10 5 3.65 10 5 3.97 10 5 4.49 10 5 5.32 10 5
W1 (Ω/s0.5) 0.044 0.104 0.157 0.164 0.184 0.189
mPani thin films were smaller than the Pani thin films, which suggested that the formation of micropatterned thin film might decrease the charge transfer resistance of Pani. It is noted that the difference between the Rct values of the mPani and Pani thin films decreased if the thin films were prepared with longer deposition time. The Rct value of the mPani-80 thin films (31.41 Ω) was approximately half of that of the Pani-80 thin film (61.55 Ω), but the mPani-160 and Pani-160 thin films have similar Rct values (139.01 Ω and 150.10 Ω). As shown in the SEM images Fig. 3(e), distinct micropatterns can be observed for the mPani-80 thin film but not for the mPani-160 thin film, which implied that such micropatterned structure might facilitated the charge transfer processes during the redox reactions of Pani.
4. Conclusions Micropatterned Pani thin films with different deposition times were prepared by ESaPD method and their EC properties were investigated and compared with the non-micropatterned Pani thin films in this study. SEM and OM images evidenced that the mPani thin films can be directly deposited under the influence of a patterned electrostatic field induced by the ESF through contact electrification process. EIS studies indicated that the Rct of the mPani thin films were smaller than that of the non-patterned Pani thin films prepared with the same deposition time. It is deduced that such micropatterned structure might facilitate the redox reactions of Pani, and therefore enhanced EC properties including coloring/bleaching response times, ΔT%, and CE were observed for the mPani thin films. The increase in ΔT%, and CE were moderate at the present stage, which should because of the electrostatic field affected only 42% of the area and limited difference in the film thickness between A þ and A0 areas. Accordingly, it is expected that a higher bleaching/coloring contrast might be achieved by the film having smaller patterns with larger A þ area and being
Please cite this article as: C.-L. Lin, L.-J. Liao, Preparation of micropatterned polyaniline thin films with enhanced..., Solar Energy Materials and Solar Cells (2015), http://dx.doi.org/10.1016/j.solmat.2015.07.035i
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deposited under the influence of a stronger electrostatic field. It is envisaged that the ESaPD method could serve as a new straightforward method for the preparation of micropatterned thin films, and the micropatterned EC thin films might find their potential applications for EC devices or sensors.
Acknowledgments This work has been supported by the National Science Council of the Republic of China (NSC 102-2221-E-032-051).
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Please cite this article as: C.-L. Lin, L.-J. Liao, Preparation of micropatterned polyaniline thin films with enhanced..., Solar Energy Materials and Solar Cells (2015), http://dx.doi.org/10.1016/j.solmat.2015.07.035i