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A novel conical structure of polyaniline nanotubes synthesized on ITO-PET conducting substrate by electrochemical method
Electrochimica Acta 89 (2013) 726–731
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A novel conical structure of polyaniline nanotubes synthesized on ITO-PET conducting substrate by electrochemical method Qi Qin a,b , Rui Zhang a,c,∗ a b c
School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China School of Materials and Chemical Engineering, Zhongyuan University of Technology, 41 Zhongyuan Road, Zhengzhou 450007, PR China Zhengzhou Institute of Aeronautical Industry Management, 2 Daxue Road, Zhengzhou 450015, PR China
a r t i c l e
i n f o
Article history: Received 27 July 2012 Received in revised form 6 November 2012 Accepted 25 November 2012 Available online 3 December 2012 Keywords: Polyaniline ITO-PET Conical nanotubes Flexible electrode
a b s t r a c t The conical nanotube structure of polyaniline (PANI) was facilely synthesized on indium tin oxide (ITO) conducting polyethylene terephthalate (PET) substrates via electrochemical polymerization method. The tubular morphology of PANI was confirmed by SEM and TEM images. It was observed that one side of PANI conical nanotube was a sharp closed tip, and the other was a circular open tube. In addition, the nanosheets, nanofibers and nanorods of PANI were observed by SEM with polymerization potentials varying from 2.0 V to 2.6 V. The resulting materials were also characterized by four-probe instrument, FTIR, cyclic voltammetry, and photoelectric measurement. Then the results showed that the optimal efficiency of dye sensitized solar cell (DSSC) with flexible PANI/ITO-PET counter electrode at 2.4 V reached to 0.86%, owing to the unique conical nanotube structure, excellent photoelectric property and high electrochemical activity of this PANI electrode. Therefore, the PANI/ITO-PET electrode can be applied as promising flexible electrode materials for DSSCs. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Polyaniline is one of the most potential conducting polymers as the electrode materials for dye sensitized solar cell, owing to its easy synthesis, high conductivity, and unique redox properties [1–4]. Among the previous research, there were many reports on this polyaniline can produce “one-dimensional” morphologies, like nanotubes [5–9], nanowires [10–17] or nanorods [18–20], coating on the different substrates. Khalid et al. reported on the nanotubes and nanofibers of polyaniline synthesized by a template-free and interfacial polymerization method respectively [21]. Qin et al. prepared a series of polyaniline nanofibers coating on stainless steel by electrodeposition for DSSC [19]. Qin and Guo researched on the nanorods of polyaniline deposited on conducting glasses by electrochemical polymerization [20]. However, there were few reports about the PANI electrodeposited on flexible PET substrates in the previous researches. Hence, in this study, polyaniline films are prepared on ITO-PET plastic by potentiostatic electropolymerization to construct a flexible electrodes for DSSCs, and the
∗ Corresponding author at: School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China. E-mail addresses: [email protected] (Q. Qin), [email protected] (R. Zhang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.107
“one-dimensional” morphology of PANI is also observed by SEM and TEM images. 2. Experimental 2.1. Materials Aniline (An, analytical grade from Sinopharm Chemical Reagent Co., Ltd.) was purified by distillation under reduced pressure prior to usage. Analytical grade reagents, H2 SO4 , HCl, ethanol and acetone (Sinopharm Chemical Reagent Co., Ltd.) were used without any pretreatment. All solutions were prepared from de-ionized water. ITO-PET films (Shenzhen Ang Zhi thin film Technology Co., Ltd. 50–100 /) were used for PANI deposition. Anhydrous lithium iodide, 4-tert-butylpyridine, 1methyl-3-propylimidazolium iodide, methoxypropionitrile were provided by Fluka Chemical Corporation. TiO2 colloid and dye (N719) were the commercial product purchased from Solaronix SA (Switzerland). Pt flexible counter electrode coating on ITO-PET substrate bought from Dalian Heptachroma SolarTech Co., Ltd. was used in the contrast test. 2.2. Preparation of polyaniline/ITO-PET electrodes First, conducting ITO-PET were rinsed with de-ionized water and immersed in ethanol ultrasonically for 10 min, then immersed
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in acetone ultrasonically for 10 min before PANI electropolymerization. Second, one ITO-PET sheet was used as working electrode for PANI deposition. The deposited area was 1 cm2 with other area insulated by adhesive tapes. And the counter electrode was a Ptfoiled electrode with a larger area. Saturated calomel electrode (SCE) was employed as reference electrode. Polyaniline films were electrodeposited on the surface of ITO-PET substrates from 0.5 M sulfuric acid (H2 SO4 ) electrolyte solution containing 0.3 M aniline by potentiostatic techniques. The polymerization potential was from 1.8 V to 2.6 V (1.8 V, 2.0 V, 2.2 V, 2.4 V, and 2.6 V) for 500 s. Third, PANI modified ITO-PET electrode was immersed in 0.1 M HCl statically in order to expel aniline monomer and oligomer PANI from the polymeric film and then rinsed with de-ionized water for several times and dried in a vacuum at 60 ◦ C for 24 h. Finally, the green conducting PANI/ITO-PET flexible electrode was obtained by this electrochemical method. 2.3. Assembling of DSSCs Nano-TiO2 colloid was dropped on the ITO glass plate by a doctor scraping technique to form a porous film. Then the TiO2 porous film was sintered by anneal at 450 ◦ C for 30 min. After cooling to 100 ◦ C, the TiO2 film was immersed in an ethanol solution of N719 dye (0.5 mM) for 24 h. Finally, DSSC was assembled by injecting a drop of electrolyte with I2 (0.05 M), LiI (0.5 M), 1-methyl3-propylimidazolium iodide (0.4 M), 4-tert-butylpyridine (0.5 M) in methoxypropionitrile (5 mL) into the aperture between the TiO2 porous film electrode and the PANI/ITO-PET or Pt flexible counter electrode. 2.4. Characterization of PANI/ITO-PET electrodes The preparation of PANI/ITO-PET electrode was performed on an electrochemical workstation (CHI660A, CH Instrument, CHN). The morphology of PANI was characterized by scanning electron microscope (SEM, LEO1550, GER) and transmission electron microscope (TEM, JEM-2100, JEOL Co. Ltd., JAP). FTIR spectrum of the PANI was recorded in the range of 500–4000 cm−1 using FTIR spectroscopy (Perkin Elmer 1760, USA). The conductivity of PANI was measured by manual four probe instrument (MP 1008, WENTWORTH, UK). The cyclic voltammogram of samples was carried out in a three-electrode cell (PANI as a working electrode, Ptfoiled as a counter electrode and SCE as a reference electrode) using a CHI 660A electrochemical workstation (CH Instrument, CN). Photocurrent–voltage characteristics of the DSSCs were obtained by a Keithley model 2400 digital source meter using an Oriel 91192 solar simulator equipped with AM 1.5 filter and intensity of 100 mW cm−2 . 3. Results and discussion 3.1. Potentiostatic polymerization Fig. 1 illustrates the current–time curves traced during the potentiostatic polymerization of PANI at different constant potentials. It was shown that the oxidation current did not generate as PANI electrodeposited at 1.8 V, and when the constant potential reached to 2.0 V, a little current generated owing to the growth of PANI. That is, the growth of PANI electro-polymerized on the surface of ITO-PET conducting substrate occurs at 2.0 V by potentiostatic deposition, and polyaniline can not be electropolymerized on this ITO-PET conducting film below 2.0 V. Then with the rise of polymerization potential, the oxidation current density was increased several times. This increase of anodic current depends on the oxidation of aniline to PANI during the
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Fig. 1. Current–time transients traced during the PANI electrodeposited at (a) 1.8 V, (b) 2.0 V, (c) 2.2 V, (d) 2.4 V, and (e) 2.6 V.
growth process. Moreover, the current density increases more rapidly as the constant potential exceeds 2.2 V, and the current density increasing rate rises along with the constant potential moving to positive potential. This phenomenon might be attributed to the fact that the growth rate of PANI electropolymerized at high potential is much faster than that at low one. When the constant potential rose to 2.6 V, the oxidation current increased first within 120 s or so, but then decreased gradually. It indicated that the slight decrease of current should be related with the much higher thickness and resistance of this PANI film. The thickness of PANI film was much increased with the oxidation current rising, and the higher resistance of PANI electrodeposited at 2.6 V lead to the reduction of oxidation current. This result was also indicated in Table 1. In addition, in the polymerization process of aniline, meanwhile, the degradation phenomenon of polyaniline also happened. With the potential increasing (above 2.4 V), the process of the peroxidation degradation was dominated, and this degradation product was peroxide of polyaniline, benzoquinone (BQ) [22,23]. Thus, aniline can not be electrochemical polymerized on the surface of ITO-PET conducting substrate at 2.6 V, and the polymerization potential of PANI would be controlled from 2.0 V to 2.6 V. 3.2. Morphological properties The surface morphology of PANI/ITO-PET at different potentials for 500 s, respectively, was shown in Fig. 2 (a: 2.0 V, b: 2.2 V, c: 2.4 V, and d: 2.6 V). At the low potential (2.0 V), PANI nuclei grew on the surface of ITO-PET to form an open porous film, which closely adhered to the ITO-PET layer. It was indicated that in the initial nucleation stage, PANI nuclei particles with a diameter range of 10–30 nm were covered over the ITO-PET substrate uniformly and tightly to exhibit a compact basal layer. Then the growth process of PANI entered the rapid growth stage, a preferential vertical growth of PANI particles occurs at the nucleation sites, resulting in the formation of 1D dendrite-like PANI nanostructure. Thus, very short length and low density of dendritic PANI was observed on top of the compact basal layer, leading to a loosely porous structure.
Table 1 The thickness and conductivity of PANI film at different potential. Sample
Thinckness (m)
Conductivity (S cm−1 )
PANI-2.0 V PANI-2.2 V PANI-2.4 V PANI-2.6 V
0.0327 0.0716 0.355 0.812
77.10 35.01 13.81 1.35
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Fig. 2. SEM of PANI/ITO-PET electrodeposited at (a) 2.0 V, (b) 2.2 V, (c) 2.4 V, and (d) 2.6 V.
However, with the potential increasing, the shape and structure of PANI was changed. Increasing the potential to 2.2 V, both PANI nanosheets and nanofibrils appeared on the top of PANI nodules layer. Almost 80% of the area was covered by nanosheets and rest was covered by nanofibrils with an average diameter of 100 nm and the length of 1 m. Meanwhile, highly magnified image as shown in the inset showed that the nanosheets of PANI were similar to hexagon, with a diameter of 400–600 nm, and the thickness of PANI sheets was within 100 nm. At the higher potential (2.4 V), it can be seen that polyaniline formed a new conical nanotubes structure, which has not been reported by others. The short conical nanotubes of PANI also represented an open porous structure, which were benefited for adsorption. The length of nanotubes was observed by SEM to be about 1.2 m and the outer diameter of nanotubes varied in the range of 200–400 nm. The highly magnified view of nanotubes was shown in the inset of Fig. 2c. This sample was also examined by TEM for determination of length, diameter, and internal structure of nanotubes. Representative TEM micrograph of one short conical nanotube was shown in Fig. 3. It was clearly seen that PANI nanotube had the outer diameter of 250 nm, the inner diameter of 200 nm, and wall thickness of 20 nm or so. The length of PANI nanotube was about 800 nm. It was to be noticed that these short conical nanotubes have very sharp tips of diameter in the range of 20–50 nm. In general, tip diameter was estimated to be approximately one-fifth of the outer diameter of the tube body. Hence, PANI conical nanotubes had one side of sharp closed tips, and the other side of open nanotube which can enhance the specific surface area of PANI, formed to an open porous structure.
As the potential shifts to more positive potential, at 2.6 V, PANI deposition showed a more nanorods growth. The diameter of PANI rods was increased over 300 nm. The PANI nanorods grew shorter and shorter on the surface of PANI film and the length of rods was less than 500 nm. That is, the degradation of PANI quickened at higher potential, thus, the PANI particles would fall off and the PANI rods would be broken. This higher polymerization potential was benefit for preparing more loose film, and the degradation process of PANI happened more easily [23]. In addition, the open porous structure of PANI nanotubes was also broken due to this degradation process.
Fig. 3. TEM of PANI/ITO-PET electrodeposited at 2.4 V for 500 s.
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Fig. 5. The schematic diagram of four-point probes method.
Fig. 4. FTIR spectrums of (a) ITO-PET, PANI/ITO-PET at (b) 2.0 V, (c) 2.2 V, (d) 2.4 V, and (e) 2.6 V for 500 s.
As mentioned above, it is clearly indicated that the growth of PANI varies with the polymerization potential. At low polymerization potential, the anodic oxidization current density is low and hence slower growth rate of PANI is expected. Consequently, very short length and low density dendritic PANI nano-film was observed at low potential. With increase of potential, the morphology and structure of PANI varies due to the more nucleation and growth of PANI. The PANI nanosheets, nanofibers, nanotubes and nanorods are all observed at the different potential. 3.3. Fourier-transform infrared spectral analysis This FTIR spectrum shows aniline was polymerized on the surface of ITO conductive PET by electrochemical method at 2.0–2.6 V for 500 s. The FTIR spectrum of ITO-PET film was also shown in Fig. 4a. The characteristic peaks of ITO-PET were not obvious in this curve, only appeared at 1710 and 1238 cm−1 . As the potential was lower than 2.0 V, the polyaniline film was not formed on the ITO-PET, only some oligomers appeared on the surface. As the PANI electrodeposited at 2.0 V, it can be clearly seen that the characteristic peaks of PANI present near the wavenumbers of 1570 (C–C stretching mode for the quinoid ring), 1475 (benzenoid rings vibration), 1297 (C–N stretching mode), 1245 (C–N+ stretching vibration), 1103 (a vibration mode of the –NH+ = structure), and 798 (out-of-plane bending vibration of C–H) cm−1 (KBr, film, cm−1 ) [24–26,8,27,28]. Moreover, the characteristic peak at 1103 cm−1 corresponding to the stretching mode of protonated PANI showed a red shift at higher potential (from 2.0 V to 2.6 V), and the most red shift of this peak to 1030 cm−1 was observed for the PANI deposited at 2.4 V. It can be implied that the protonated degree of PANI deposited at 2.4 V is more than that at other potential. This obvious change suggested that 1D nanotubes structure of PANI had a higher doping degree and the effect of charge delocalization in molecule chain result in a red shift of the characteristic peaks. Thus, the characteristic peaks of PANI polymerized at 2.4 V had an intense red shifted owing to the decline of electron cloud density and the raise of charge delocalization caused by open nanotubes structure. 3.4. Conductivity measurement The conductivity of PANI deposited at different potential is measured by four-point probes instrument and then calculated by this equation: =
ln 2 I12 · d U34
(1)
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where is the conductivity, d is the thickness of films, I12 is the current from 1 to 2 point, and U34 is the potential difference between 3 and 4 point. The schematic diagram of four-point probes method is shown in Fig. 5. Meanwhile, polymer thickness is estimated from the amount of charge QA according to the equation [29]: d=
QA Mw zFA
(2)
where QA is the charge under the potentiostatic electrodeposition, Mw is molecular weight of aniline, z = 0.5 (number of electrons/aniline unit), A is area of the electrode, is specific density of aniline and F is Faraday’s constant. Table 1 presents the thickness and the conductivity of PANI layers at 2.0 V, 2.2 V, 2.4 V and 2.6 V. It can be found that the film thickness of PANI increases with the polymerization potential increasing, but the conductivity of PANI decreases. As the potential is 2.0 V, the PANI film is very thin (about 32 nm). It is contributed to the high sheet resistance of ITO-PET conducting film, which directly effects on the electropolymerization of PANI. The higher the resistance of ITO-PET substrate is, the lower anodic current on ITO-PET electrode is. And this small thickness of PANI film electrodeposited at 2.0 V is caused by the slower growth rate of PANI electrodeposited on ITO-PET electrode with poor conductivity. With the polymerization potential rising, the current density on the PANI/ITO-PET electrode is increased, leading to the thickness of PANI film increased too, however, the more thickness of PANI film also results in the reduction of conductivity. The best conductivity of PANI deposited on ITO-PET conducting substrate reached to 77.10 S cm−1 when the polymerization potential was 2.0 V. In addition, the conductivity of PANI films at 2.6 V reduced one magnitude due to the peroxidation degradation effect of PANI film, too high thickness and. Therefore, the appropriate potential of PANI film with excellent conductivity cannot exceed 2.4 V. 3.5. Cyclic voltammograms Fig. 6 shows the cyclic voltammograms of PANI electrode at different potential. It can be seen that the current density of PANI electrode at 2.4 V was much larger than that at other potential. This means PANI electrode at 2.4 V has a faster reaction rate due to the unique nanotube structure with high special surface area and remarkable chemical activity, which benefits for absorbing more electrolytes and increasing reaction rate. In other words, the charge-transfer resistance of PANI electrode at 2.4 V is lower than other PANI electrode. In addition, from the CV curve of PANI at 2.4 V, it can be seen that the curve symmetry is optimal, and the background current is the widest. Obviously, this reaction on
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Fig. 6. CV of different PANI electrodes at a scan rate of 20 mV s−1 in an acetonitrile solution of 0.1 M LiClO4 .
Fig. 7. IV curves of DSSCs with Pt and PANI flexible counter electrodes.
PANI electrode at 2.4 V is a reversible reaction with high chargetransport speed. In other words, the PANI electrode at 2.4 V owns the fast reaction rate and excellent reaction activity, thus, is to be a promising electrode material in the application of electrochemical device. 3.6. Photoelctric performance of PANI/ITO-PET electrodes The photoelectric properties of DSSCs with different PANI/ITOPET and Pt flexible counter electrode are shown in Fig. 7, and the results are also listed in Table 2. The surface area of the PANI/ITOPET used in photoelectric measurements is 1 cm2 . Photoelectric conversion efficiency () is defined as the ratio of energy output from the solar cell to input energy from the sun. The fill factor (FF) is defined as the ratio of the maximum power from the solar cell to the product of open circuit voltage and short circuit current. Open circuit voltage (Voc) is the difference of electrical potential between two terminals of a device when disconnected from any circuit. The short-circuit current density (Jsc) is the current density Table 2 Photovoltaic parameter of DSSCs with Pt and PANI flexible counter electrode. Sample
Jsc (mA cm−2 )
Voc (V)
FF
(%)
PANI-2.0 V PANI-2.2 V PANI-2.4 V PANI-2.6 V Pt electrode
3.01 1.82 3.64 1.64 2.01
0.72 0.73 0.73 0.57 0.74
0.283 0.249 0.324 0.231 0.576
0.613 0.331 0.862 0.217 0.857
through the solar cell when the voltage across the solar cell is zero. The results indicated that the flexible PANI/ITO-PET counter electrode deposited at 2.4 V can obtain a better photoelectric conversion efficiency of 0.862%, compared with other PANI flexible electrodes at different potential. As the potential rising from 2.0 V to 2.2 V, the energy conversion efficiency of DSSC with PANI electrode descended from 0.613% to 0.331%, because of the reduction of conductivity of PANI at 2.2 V, leading to the obvious decrease of short circuit current. However, the efficiency did not continue decreasing with the polymerization potential rising to 2.4 V, on the contrary, this efficiency increased to 0.862%. It can be deduced that the unique conical nanotube of PANI at 2.4 V had higher electrocatalytic activity and lower charge transmission impedance, as a result, the fill factor and short circuit current of this DSSC enhanced. Thus, the energy conversion efficiency of DSSC with PANI/ITOPET counter electrode at 2.4 V had a remarkable increase. With the potential increasing to 2.4 V, the open circuit voltage and efficiency of DSSC would decline sharply, owing to the degradation of PANI electrode at too high potential. In one word, formation of a PANI electrode with the conical nanotubes structure on a flexible substrate resulted in the improvement of cell efficiency. The conical nanotube of PANI electrode at 2.4 V can provide very high electrocatalytic activity and good photoelectric property, leading to an improved fill factor and higher energy conversion efficiency of DSSC, compared to that of DSSC with PANI electrode at other potentials. Moreover, comparing to a Pt flexible counter electrode with a conversion efficiency of 0.857%, PANI flexible electrode had a higher short circuit current and the similar open circuit voltage. However, the fill factor of PANI flexible counter electrode was much lower than Pt electrode, leading to poorer conversion efficiency. This result was considered to be due to the increase of the charge transmission impedance of PANI electrode and a higher sheet resistance for this PANI counter electrode than that of the Pt electrode. The FF normally reveals energy losses coming from the inherent resistance of the photovoltaic device. The lower FF means more energy losses in the cell. An alternative way to decrease the ohmic resistance losses leading to improve the fill factor is to apply the higher conductive substrates in the DSSCs. Therefore, the sheet resistance of the counter electrode directly influences the fill factor of DSSC. The increase of the fill factor of DSSC using the Pt counter electrode should be attributed to its low sheet resistance; that is a higher charge transmission impedance of PANI/PET implies a lower fill factor. Hence, the ITO-PET substrate with much lower sheet resistance should be used in the PANI/ITO-PET counter electrode for increasing the FF and conversion efficiency of flexible DSSCs.
4. Conclusions Polyaniline counter electrodes were successful fabricated on the ITO-PET substrate by potentiostatic electrochemical method. The results showed that the morphology and structure of PANI changed with increase of potential. The dendritic PANI, nanosheets, nanotubes, and nanorods were prepared by increasing potential from 2.0 V to 2.6 V. Especially, as PANI electrodeposited at 2.4 V, the novel conical nanotube of PANI was obtained, meanwhile, this PANI electrode showed the excellent charge transport property and high electrochemical activity, leading to the improvement of the energy conversion efficiency of DSSC, compared with PANI electrodes at lower potential. Therefore, the PANI/ITO-PET counter electrode with the simple preparation procedure, low production cost, and predominant electrochemical property will be a credible alternative in the application of flexible counter electrode in DSSCs.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A novel conical structure of polyaniline nanotubes synthesized on ITO-PET conducting substrate by electrochemical method Qi Qin a,b , Rui Zhang a,c,∗ a b c
School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China School of Materials and Chemical Engineering, Zhongyuan University of Technology, 41 Zhongyuan Road, Zhengzhou 450007, PR China Zhengzhou Institute of Aeronautical Industry Management, 2 Daxue Road, Zhengzhou 450015, PR China
a r t i c l e
i n f o
Article history: Received 27 July 2012 Received in revised form 6 November 2012 Accepted 25 November 2012 Available online 3 December 2012 Keywords: Polyaniline ITO-PET Conical nanotubes Flexible electrode
a b s t r a c t The conical nanotube structure of polyaniline (PANI) was facilely synthesized on indium tin oxide (ITO) conducting polyethylene terephthalate (PET) substrates via electrochemical polymerization method. The tubular morphology of PANI was confirmed by SEM and TEM images. It was observed that one side of PANI conical nanotube was a sharp closed tip, and the other was a circular open tube. In addition, the nanosheets, nanofibers and nanorods of PANI were observed by SEM with polymerization potentials varying from 2.0 V to 2.6 V. The resulting materials were also characterized by four-probe instrument, FTIR, cyclic voltammetry, and photoelectric measurement. Then the results showed that the optimal efficiency of dye sensitized solar cell (DSSC) with flexible PANI/ITO-PET counter electrode at 2.4 V reached to 0.86%, owing to the unique conical nanotube structure, excellent photoelectric property and high electrochemical activity of this PANI electrode. Therefore, the PANI/ITO-PET electrode can be applied as promising flexible electrode materials for DSSCs. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Polyaniline is one of the most potential conducting polymers as the electrode materials for dye sensitized solar cell, owing to its easy synthesis, high conductivity, and unique redox properties [1–4]. Among the previous research, there were many reports on this polyaniline can produce “one-dimensional” morphologies, like nanotubes [5–9], nanowires [10–17] or nanorods [18–20], coating on the different substrates. Khalid et al. reported on the nanotubes and nanofibers of polyaniline synthesized by a template-free and interfacial polymerization method respectively [21]. Qin et al. prepared a series of polyaniline nanofibers coating on stainless steel by electrodeposition for DSSC [19]. Qin and Guo researched on the nanorods of polyaniline deposited on conducting glasses by electrochemical polymerization [20]. However, there were few reports about the PANI electrodeposited on flexible PET substrates in the previous researches. Hence, in this study, polyaniline films are prepared on ITO-PET plastic by potentiostatic electropolymerization to construct a flexible electrodes for DSSCs, and the
∗ Corresponding author at: School of Materials Science and Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China. E-mail addresses: [email protected] (Q. Qin), [email protected] (R. Zhang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.107
“one-dimensional” morphology of PANI is also observed by SEM and TEM images. 2. Experimental 2.1. Materials Aniline (An, analytical grade from Sinopharm Chemical Reagent Co., Ltd.) was purified by distillation under reduced pressure prior to usage. Analytical grade reagents, H2 SO4 , HCl, ethanol and acetone (Sinopharm Chemical Reagent Co., Ltd.) were used without any pretreatment. All solutions were prepared from de-ionized water. ITO-PET films (Shenzhen Ang Zhi thin film Technology Co., Ltd. 50–100 /) were used for PANI deposition. Anhydrous lithium iodide, 4-tert-butylpyridine, 1methyl-3-propylimidazolium iodide, methoxypropionitrile were provided by Fluka Chemical Corporation. TiO2 colloid and dye (N719) were the commercial product purchased from Solaronix SA (Switzerland). Pt flexible counter electrode coating on ITO-PET substrate bought from Dalian Heptachroma SolarTech Co., Ltd. was used in the contrast test. 2.2. Preparation of polyaniline/ITO-PET electrodes First, conducting ITO-PET were rinsed with de-ionized water and immersed in ethanol ultrasonically for 10 min, then immersed
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in acetone ultrasonically for 10 min before PANI electropolymerization. Second, one ITO-PET sheet was used as working electrode for PANI deposition. The deposited area was 1 cm2 with other area insulated by adhesive tapes. And the counter electrode was a Ptfoiled electrode with a larger area. Saturated calomel electrode (SCE) was employed as reference electrode. Polyaniline films were electrodeposited on the surface of ITO-PET substrates from 0.5 M sulfuric acid (H2 SO4 ) electrolyte solution containing 0.3 M aniline by potentiostatic techniques. The polymerization potential was from 1.8 V to 2.6 V (1.8 V, 2.0 V, 2.2 V, 2.4 V, and 2.6 V) for 500 s. Third, PANI modified ITO-PET electrode was immersed in 0.1 M HCl statically in order to expel aniline monomer and oligomer PANI from the polymeric film and then rinsed with de-ionized water for several times and dried in a vacuum at 60 ◦ C for 24 h. Finally, the green conducting PANI/ITO-PET flexible electrode was obtained by this electrochemical method. 2.3. Assembling of DSSCs Nano-TiO2 colloid was dropped on the ITO glass plate by a doctor scraping technique to form a porous film. Then the TiO2 porous film was sintered by anneal at 450 ◦ C for 30 min. After cooling to 100 ◦ C, the TiO2 film was immersed in an ethanol solution of N719 dye (0.5 mM) for 24 h. Finally, DSSC was assembled by injecting a drop of electrolyte with I2 (0.05 M), LiI (0.5 M), 1-methyl3-propylimidazolium iodide (0.4 M), 4-tert-butylpyridine (0.5 M) in methoxypropionitrile (5 mL) into the aperture between the TiO2 porous film electrode and the PANI/ITO-PET or Pt flexible counter electrode. 2.4. Characterization of PANI/ITO-PET electrodes The preparation of PANI/ITO-PET electrode was performed on an electrochemical workstation (CHI660A, CH Instrument, CHN). The morphology of PANI was characterized by scanning electron microscope (SEM, LEO1550, GER) and transmission electron microscope (TEM, JEM-2100, JEOL Co. Ltd., JAP). FTIR spectrum of the PANI was recorded in the range of 500–4000 cm−1 using FTIR spectroscopy (Perkin Elmer 1760, USA). The conductivity of PANI was measured by manual four probe instrument (MP 1008, WENTWORTH, UK). The cyclic voltammogram of samples was carried out in a three-electrode cell (PANI as a working electrode, Ptfoiled as a counter electrode and SCE as a reference electrode) using a CHI 660A electrochemical workstation (CH Instrument, CN). Photocurrent–voltage characteristics of the DSSCs were obtained by a Keithley model 2400 digital source meter using an Oriel 91192 solar simulator equipped with AM 1.5 filter and intensity of 100 mW cm−2 . 3. Results and discussion 3.1. Potentiostatic polymerization Fig. 1 illustrates the current–time curves traced during the potentiostatic polymerization of PANI at different constant potentials. It was shown that the oxidation current did not generate as PANI electrodeposited at 1.8 V, and when the constant potential reached to 2.0 V, a little current generated owing to the growth of PANI. That is, the growth of PANI electro-polymerized on the surface of ITO-PET conducting substrate occurs at 2.0 V by potentiostatic deposition, and polyaniline can not be electropolymerized on this ITO-PET conducting film below 2.0 V. Then with the rise of polymerization potential, the oxidation current density was increased several times. This increase of anodic current depends on the oxidation of aniline to PANI during the
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Fig. 1. Current–time transients traced during the PANI electrodeposited at (a) 1.8 V, (b) 2.0 V, (c) 2.2 V, (d) 2.4 V, and (e) 2.6 V.
growth process. Moreover, the current density increases more rapidly as the constant potential exceeds 2.2 V, and the current density increasing rate rises along with the constant potential moving to positive potential. This phenomenon might be attributed to the fact that the growth rate of PANI electropolymerized at high potential is much faster than that at low one. When the constant potential rose to 2.6 V, the oxidation current increased first within 120 s or so, but then decreased gradually. It indicated that the slight decrease of current should be related with the much higher thickness and resistance of this PANI film. The thickness of PANI film was much increased with the oxidation current rising, and the higher resistance of PANI electrodeposited at 2.6 V lead to the reduction of oxidation current. This result was also indicated in Table 1. In addition, in the polymerization process of aniline, meanwhile, the degradation phenomenon of polyaniline also happened. With the potential increasing (above 2.4 V), the process of the peroxidation degradation was dominated, and this degradation product was peroxide of polyaniline, benzoquinone (BQ) [22,23]. Thus, aniline can not be electrochemical polymerized on the surface of ITO-PET conducting substrate at 2.6 V, and the polymerization potential of PANI would be controlled from 2.0 V to 2.6 V. 3.2. Morphological properties The surface morphology of PANI/ITO-PET at different potentials for 500 s, respectively, was shown in Fig. 2 (a: 2.0 V, b: 2.2 V, c: 2.4 V, and d: 2.6 V). At the low potential (2.0 V), PANI nuclei grew on the surface of ITO-PET to form an open porous film, which closely adhered to the ITO-PET layer. It was indicated that in the initial nucleation stage, PANI nuclei particles with a diameter range of 10–30 nm were covered over the ITO-PET substrate uniformly and tightly to exhibit a compact basal layer. Then the growth process of PANI entered the rapid growth stage, a preferential vertical growth of PANI particles occurs at the nucleation sites, resulting in the formation of 1D dendrite-like PANI nanostructure. Thus, very short length and low density of dendritic PANI was observed on top of the compact basal layer, leading to a loosely porous structure.
Table 1 The thickness and conductivity of PANI film at different potential. Sample
Thinckness (m)
Conductivity (S cm−1 )
PANI-2.0 V PANI-2.2 V PANI-2.4 V PANI-2.6 V
0.0327 0.0716 0.355 0.812
77.10 35.01 13.81 1.35
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Fig. 2. SEM of PANI/ITO-PET electrodeposited at (a) 2.0 V, (b) 2.2 V, (c) 2.4 V, and (d) 2.6 V.
However, with the potential increasing, the shape and structure of PANI was changed. Increasing the potential to 2.2 V, both PANI nanosheets and nanofibrils appeared on the top of PANI nodules layer. Almost 80% of the area was covered by nanosheets and rest was covered by nanofibrils with an average diameter of 100 nm and the length of 1 m. Meanwhile, highly magnified image as shown in the inset showed that the nanosheets of PANI were similar to hexagon, with a diameter of 400–600 nm, and the thickness of PANI sheets was within 100 nm. At the higher potential (2.4 V), it can be seen that polyaniline formed a new conical nanotubes structure, which has not been reported by others. The short conical nanotubes of PANI also represented an open porous structure, which were benefited for adsorption. The length of nanotubes was observed by SEM to be about 1.2 m and the outer diameter of nanotubes varied in the range of 200–400 nm. The highly magnified view of nanotubes was shown in the inset of Fig. 2c. This sample was also examined by TEM for determination of length, diameter, and internal structure of nanotubes. Representative TEM micrograph of one short conical nanotube was shown in Fig. 3. It was clearly seen that PANI nanotube had the outer diameter of 250 nm, the inner diameter of 200 nm, and wall thickness of 20 nm or so. The length of PANI nanotube was about 800 nm. It was to be noticed that these short conical nanotubes have very sharp tips of diameter in the range of 20–50 nm. In general, tip diameter was estimated to be approximately one-fifth of the outer diameter of the tube body. Hence, PANI conical nanotubes had one side of sharp closed tips, and the other side of open nanotube which can enhance the specific surface area of PANI, formed to an open porous structure.
As the potential shifts to more positive potential, at 2.6 V, PANI deposition showed a more nanorods growth. The diameter of PANI rods was increased over 300 nm. The PANI nanorods grew shorter and shorter on the surface of PANI film and the length of rods was less than 500 nm. That is, the degradation of PANI quickened at higher potential, thus, the PANI particles would fall off and the PANI rods would be broken. This higher polymerization potential was benefit for preparing more loose film, and the degradation process of PANI happened more easily [23]. In addition, the open porous structure of PANI nanotubes was also broken due to this degradation process.
Fig. 3. TEM of PANI/ITO-PET electrodeposited at 2.4 V for 500 s.
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Fig. 5. The schematic diagram of four-point probes method.
Fig. 4. FTIR spectrums of (a) ITO-PET, PANI/ITO-PET at (b) 2.0 V, (c) 2.2 V, (d) 2.4 V, and (e) 2.6 V for 500 s.
As mentioned above, it is clearly indicated that the growth of PANI varies with the polymerization potential. At low polymerization potential, the anodic oxidization current density is low and hence slower growth rate of PANI is expected. Consequently, very short length and low density dendritic PANI nano-film was observed at low potential. With increase of potential, the morphology and structure of PANI varies due to the more nucleation and growth of PANI. The PANI nanosheets, nanofibers, nanotubes and nanorods are all observed at the different potential. 3.3. Fourier-transform infrared spectral analysis This FTIR spectrum shows aniline was polymerized on the surface of ITO conductive PET by electrochemical method at 2.0–2.6 V for 500 s. The FTIR spectrum of ITO-PET film was also shown in Fig. 4a. The characteristic peaks of ITO-PET were not obvious in this curve, only appeared at 1710 and 1238 cm−1 . As the potential was lower than 2.0 V, the polyaniline film was not formed on the ITO-PET, only some oligomers appeared on the surface. As the PANI electrodeposited at 2.0 V, it can be clearly seen that the characteristic peaks of PANI present near the wavenumbers of 1570 (C–C stretching mode for the quinoid ring), 1475 (benzenoid rings vibration), 1297 (C–N stretching mode), 1245 (C–N+ stretching vibration), 1103 (a vibration mode of the –NH+ = structure), and 798 (out-of-plane bending vibration of C–H) cm−1 (KBr, film, cm−1 ) [24–26,8,27,28]. Moreover, the characteristic peak at 1103 cm−1 corresponding to the stretching mode of protonated PANI showed a red shift at higher potential (from 2.0 V to 2.6 V), and the most red shift of this peak to 1030 cm−1 was observed for the PANI deposited at 2.4 V. It can be implied that the protonated degree of PANI deposited at 2.4 V is more than that at other potential. This obvious change suggested that 1D nanotubes structure of PANI had a higher doping degree and the effect of charge delocalization in molecule chain result in a red shift of the characteristic peaks. Thus, the characteristic peaks of PANI polymerized at 2.4 V had an intense red shifted owing to the decline of electron cloud density and the raise of charge delocalization caused by open nanotubes structure. 3.4. Conductivity measurement The conductivity of PANI deposited at different potential is measured by four-point probes instrument and then calculated by this equation: =
ln 2 I12 · d U34
(1)
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where is the conductivity, d is the thickness of films, I12 is the current from 1 to 2 point, and U34 is the potential difference between 3 and 4 point. The schematic diagram of four-point probes method is shown in Fig. 5. Meanwhile, polymer thickness is estimated from the amount of charge QA according to the equation [29]: d=
QA Mw zFA
(2)
where QA is the charge under the potentiostatic electrodeposition, Mw is molecular weight of aniline, z = 0.5 (number of electrons/aniline unit), A is area of the electrode, is specific density of aniline and F is Faraday’s constant. Table 1 presents the thickness and the conductivity of PANI layers at 2.0 V, 2.2 V, 2.4 V and 2.6 V. It can be found that the film thickness of PANI increases with the polymerization potential increasing, but the conductivity of PANI decreases. As the potential is 2.0 V, the PANI film is very thin (about 32 nm). It is contributed to the high sheet resistance of ITO-PET conducting film, which directly effects on the electropolymerization of PANI. The higher the resistance of ITO-PET substrate is, the lower anodic current on ITO-PET electrode is. And this small thickness of PANI film electrodeposited at 2.0 V is caused by the slower growth rate of PANI electrodeposited on ITO-PET electrode with poor conductivity. With the polymerization potential rising, the current density on the PANI/ITO-PET electrode is increased, leading to the thickness of PANI film increased too, however, the more thickness of PANI film also results in the reduction of conductivity. The best conductivity of PANI deposited on ITO-PET conducting substrate reached to 77.10 S cm−1 when the polymerization potential was 2.0 V. In addition, the conductivity of PANI films at 2.6 V reduced one magnitude due to the peroxidation degradation effect of PANI film, too high thickness and. Therefore, the appropriate potential of PANI film with excellent conductivity cannot exceed 2.4 V. 3.5. Cyclic voltammograms Fig. 6 shows the cyclic voltammograms of PANI electrode at different potential. It can be seen that the current density of PANI electrode at 2.4 V was much larger than that at other potential. This means PANI electrode at 2.4 V has a faster reaction rate due to the unique nanotube structure with high special surface area and remarkable chemical activity, which benefits for absorbing more electrolytes and increasing reaction rate. In other words, the charge-transfer resistance of PANI electrode at 2.4 V is lower than other PANI electrode. In addition, from the CV curve of PANI at 2.4 V, it can be seen that the curve symmetry is optimal, and the background current is the widest. Obviously, this reaction on
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Fig. 6. CV of different PANI electrodes at a scan rate of 20 mV s−1 in an acetonitrile solution of 0.1 M LiClO4 .
Fig. 7. IV curves of DSSCs with Pt and PANI flexible counter electrodes.
PANI electrode at 2.4 V is a reversible reaction with high chargetransport speed. In other words, the PANI electrode at 2.4 V owns the fast reaction rate and excellent reaction activity, thus, is to be a promising electrode material in the application of electrochemical device. 3.6. Photoelctric performance of PANI/ITO-PET electrodes The photoelectric properties of DSSCs with different PANI/ITOPET and Pt flexible counter electrode are shown in Fig. 7, and the results are also listed in Table 2. The surface area of the PANI/ITOPET used in photoelectric measurements is 1 cm2 . Photoelectric conversion efficiency () is defined as the ratio of energy output from the solar cell to input energy from the sun. The fill factor (FF) is defined as the ratio of the maximum power from the solar cell to the product of open circuit voltage and short circuit current. Open circuit voltage (Voc) is the difference of electrical potential between two terminals of a device when disconnected from any circuit. The short-circuit current density (Jsc) is the current density Table 2 Photovoltaic parameter of DSSCs with Pt and PANI flexible counter electrode. Sample
Jsc (mA cm−2 )
Voc (V)
FF
(%)
PANI-2.0 V PANI-2.2 V PANI-2.4 V PANI-2.6 V Pt electrode
3.01 1.82 3.64 1.64 2.01
0.72 0.73 0.73 0.57 0.74
0.283 0.249 0.324 0.231 0.576
0.613 0.331 0.862 0.217 0.857
through the solar cell when the voltage across the solar cell is zero. The results indicated that the flexible PANI/ITO-PET counter electrode deposited at 2.4 V can obtain a better photoelectric conversion efficiency of 0.862%, compared with other PANI flexible electrodes at different potential. As the potential rising from 2.0 V to 2.2 V, the energy conversion efficiency of DSSC with PANI electrode descended from 0.613% to 0.331%, because of the reduction of conductivity of PANI at 2.2 V, leading to the obvious decrease of short circuit current. However, the efficiency did not continue decreasing with the polymerization potential rising to 2.4 V, on the contrary, this efficiency increased to 0.862%. It can be deduced that the unique conical nanotube of PANI at 2.4 V had higher electrocatalytic activity and lower charge transmission impedance, as a result, the fill factor and short circuit current of this DSSC enhanced. Thus, the energy conversion efficiency of DSSC with PANI/ITOPET counter electrode at 2.4 V had a remarkable increase. With the potential increasing to 2.4 V, the open circuit voltage and efficiency of DSSC would decline sharply, owing to the degradation of PANI electrode at too high potential. In one word, formation of a PANI electrode with the conical nanotubes structure on a flexible substrate resulted in the improvement of cell efficiency. The conical nanotube of PANI electrode at 2.4 V can provide very high electrocatalytic activity and good photoelectric property, leading to an improved fill factor and higher energy conversion efficiency of DSSC, compared to that of DSSC with PANI electrode at other potentials. Moreover, comparing to a Pt flexible counter electrode with a conversion efficiency of 0.857%, PANI flexible electrode had a higher short circuit current and the similar open circuit voltage. However, the fill factor of PANI flexible counter electrode was much lower than Pt electrode, leading to poorer conversion efficiency. This result was considered to be due to the increase of the charge transmission impedance of PANI electrode and a higher sheet resistance for this PANI counter electrode than that of the Pt electrode. The FF normally reveals energy losses coming from the inherent resistance of the photovoltaic device. The lower FF means more energy losses in the cell. An alternative way to decrease the ohmic resistance losses leading to improve the fill factor is to apply the higher conductive substrates in the DSSCs. Therefore, the sheet resistance of the counter electrode directly influences the fill factor of DSSC. The increase of the fill factor of DSSC using the Pt counter electrode should be attributed to its low sheet resistance; that is a higher charge transmission impedance of PANI/PET implies a lower fill factor. Hence, the ITO-PET substrate with much lower sheet resistance should be used in the PANI/ITO-PET counter electrode for increasing the FF and conversion efficiency of flexible DSSCs.
4. Conclusions Polyaniline counter electrodes were successful fabricated on the ITO-PET substrate by potentiostatic electrochemical method. The results showed that the morphology and structure of PANI changed with increase of potential. The dendritic PANI, nanosheets, nanotubes, and nanorods were prepared by increasing potential from 2.0 V to 2.6 V. Especially, as PANI electrodeposited at 2.4 V, the novel conical nanotube of PANI was obtained, meanwhile, this PANI electrode showed the excellent charge transport property and high electrochemical activity, leading to the improvement of the energy conversion efficiency of DSSC, compared with PANI electrodes at lower potential. Therefore, the PANI/ITO-PET counter electrode with the simple preparation procedure, low production cost, and predominant electrochemical property will be a credible alternative in the application of flexible counter electrode in DSSCs.
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