Abstract
A polyterthiophene-titanium oxide (P3T+TiO2) composite material was electrochemically synthesized in dichloromethane-tetrabutylammonium perchlorate CH2Cl2/TBAP containing a monomer (terthiophene) and semiconductor (TiO2) nanoparticles. The obtained material was characterized using electrochemical methods (cyclic voltammetry and electrochemical impedance spectroscopy) and spectrometry analysis [scanning electron microscopy (SEM), spectrophotometer ultraviolet (UV)-visible, energy dispersive X-ray spectroscopy (EDX) and force atomic microscopy (AFM)]. The effect of TiO2 nanoparticles on the photoelectrochemical and optical characteristics of P3T composite films was also studied. The results confirmed the presence of TiO2 nanoparticles in the polyterthiophene matrix. The surface morphology of the (P3T+TiO2) composite films revealed that adding TiO2 nanoparticles increase the film’s roughness values. The addition of TiO2 nanoparticles improve the absorbance of P3T composite films. Moreover, the photocurrent of the composite increased with the TiO2 nanoparticles concentration and showed that this composite material could be used in photoelectrochemical applications such as photovoltaic cells.
1 Introduction
Conducting polymers have been studied during the last two decades as important semiconductor materials because of their excellent chemical and physical properties (1), (2), (3). Thus they can be used in various applications such as organic field-effect transistors (OFETs) (4), polymer light-emitting diodes (PLEDs) (5), solar cells (6) and chemical and electrochemical sensors (7). Recently, more interest has been focused on various conjugated polymers such as polythiophene, polypyrrole and conjugated polymers. These materials have rapidly gained significant attention, due to the existence of α, α′-linkages in their monomers, which makes the whole polyterthiophene-type chain grow regularly and leads in very interesting electronic, electrochromic and optical properties (8), (9), (10).
On the other hand, organic/inorganic polymer composite materials show a high potential to introduce original structural design in material sciences and to develop innovative derivative functions for device applications. Particularly, the hybrid association of an electronic conducting polymer and a semiconductor metal oxide is one of the most advantageous combinations for photo-electronic devices, including photovoltaics (11). The majority of the composite materials are based on TiO2, which is chemically stable and has a long life-time of electron-hole pairs generated by optical excitation. However, the band gap of TiO2~3.2 eV is so wide that it greatly limits the use of sunlight as an energy source for the photo-reaction, as only about 3%–4% of solar light falls in the UV range (12), (13). In response to this defect, many attempts have been made to improve the photocatalytic efficiency of TiO2 under visible-light irradiation by shifting its optical response from the ultraviolet (UV) to visible range, such as by noble metal deposition (14), (15), (16), metal (17), (18), (19) and non-metal doping (20), (21), (22), forming composites with narrow semi-conductors (23), (24) and surface dye sensitization (25). However, among these modification methods, photocatalysis applications of dye-sensitized TiO2 are still limited due to the dissolution and photocatalytic degradation of the dye itself during the photocatalysis process.
Among the conducting polymers, polyaniline, polythiophene and polypyrrole are widely used for the fabrication of conducting polymer/titanium oxide composite materials (26). Polythiophene and its derivatives combined with TiO2 show many advantages, and the large internal interface in the polythiophene/TiO2 composite permits an efficient separation of charge, which is very important for photovoltaic application (27), (28). Now, a considerable amount of reports have been published on the preparation of polythiophene and its derivatives/titanium oxide composites, including original in situ photopolymerization (29), (30), electrochemical polymerization and chemical solution method.
This work presents the synthesis, electrochemical and spectroscopic characterization of the composite: (P3T+TiO2) obtained from the terthiophene (3T) and titanium dioxide (TiO2) at a platinum electrode or indium tin oxide ITO glass electrodes. The so obtained films were characterized using cyclic voltamperometry (CV), impedance spectroscopy measurement (EIS), [UV-visible (UV-VIS), energy dispersive X-ray (EDX), scanning electron microscopy (SEM), and atomic force microscopy (AFM)]. to study their electrochemical properties. These modified electrodes can be used in various applications, such as light emitting diodes (LEDs) and photovoltaic cells.
2 Experimental section
2.1 Materials
Dichloromethane (CH2Cl2) is used as solvent. The supporting electrolyte used is tetrabutylammonium perchlorate (TBAP) (Fluka product, Switzerland), which is a pure salt for analysis. This electrolyte is chosen because of its solubility in organic solution, and of its electrochemical stability on a large domain of potential. The reagents (Aldrich, USA) are: titanium dioxide (TiO2) 99.9%, a powder used as a doping semiconductor, and terthiophene (3T) with 98% purity as a monomer.
2.2 Instrumentations
All the electrochemical measurements (cyclic voltammetry and electrochemical impedance spectroscopy) were performed using a Voltalab 402 type PGZ from Radiometer Analytical, coupled with a computer equipped with a software (voltamaster 4) to select the electrochemical technique and fix the desired parameters.
A three-electrode cell was used to carry out the electrochemical experiments. A platinum working electrode (diameter 2 mm) was used as a working electrode for the deposition of (P3T+TiO2) composite films. For photoelectrochemistry and UV-VIS characterization, ITO coated glass electrodes (SOLEMS) were used as working electrodes (area of 2 cm2), a platinum wire served as an auxiliary electrode and a saturated calomel electrode (SCE) as reference, all the potential values were referred to this electrode.
The Nyquist diagrams were recorded in a frequency range of 100 kHz–10 mHz, with a perturbation of 10 mV. In order to insure the inert effect of the P3T electrode during the experiment, an equilibrium potential was chosen. UV-vis spectroscopy measurements were performed on a Shimadzu UV-spectrophotometer UV-1800 (Japan) coupled with UV Probe software. AFM images were extracted using a Pico Scan 5.3 from Molecular Imaging. SEM and EDX measurements were carried out with a Zeiss ultra 55 microscope, the operating voltage used was 1 KV. The apparatus was coupled with EDX. The EDX parameters are HV:10 KV and pulx:12.84 kcps, where cps was the current gross count rate.
The P3T/Pt films were washed then dried before applying the electron beams. The photoelectrochemical experiments were performed by the potentiostatic method in a three-electrode configuration cell at room temperature. The polymer and composite films were irradiated through the ITO side (substrate/electrode interface). The cell was placed in a home-made optical bench consisting of a 100 mW·cm−2 polychromatic lamp.
2.3 Preparation of the (P3T+TiO2) composite film
The smoothing pre-treatment of the surface was proceeded by mechanical polishing, using emery paper down to 1200 grade, then degreasing with acetone/ethanol mixture, washing with distilled water and drying. Then the electropolymerization of terthiophene in the presence of TiO2 was performed in potentiodynamic conditions in a cell containing CH2Cl2/TBAP (10−1m) and terthiophene (10−2m) in the presence of different concentrations of TiO2. The composite material was obtained on the electrode by cyclic voltammetry (cycling) in the potential range between −0.3 and 1.6 V/SCE, at a scan rate of 10 mV/s. Before each experiment, the electrolytic solution was degased by argon bubbling for 15 min.
3 Results and discussion
3.1 Electrochemical polymerization of terthiophene
Figure 1 compares the cyclic voltammograms recorded during the electrochemical polymerization of terthiophene in the absence and in the presence of TiO2. Figure 1A shows the cyclic voltammograms correspond to terthiophene (10−2m) in CH2Cl2/TBAP (10−1m), using potential cycling between −0.3 and 1.6 V/SCE, at a scan rate of 10 mV/s.
Electrochemical polymerization of 3T: (A) 10 cycles, (a′) 1st cycle and electrochemical polymerization of 3T in presence of TiO2: (B) 10 cycles, (b′) 1st cycle, in a solution of CH2Cl2/TBAP (10−1m) with scan rate of 10 mV/s, by using a Pt electrode (Ø=2 mm).
On the first cycle (inset), an oxidation peak was observed at +1.07 V assigned to the oxidation of terthiophene especially to its radical cation followed by coupling via α-α′ which are bonding to form the insoluble polymer, we subsequently deposit these ones onto the electrode. In the reverse cathodic scan, a reduction peak was noted at a potential of 0.40 V due to the reduction of polyterthiophene deposit. An additional broader cathodic shoulder observed at 0.1 V was attributed to the characteristic of hydrogen reduction.
After the second cycle, the current intensity of the oxidation and reduction peaks progressively increased with the number of cycles, indicating the formation and the growth of the conducting polymer film and suggesting a systematic increase in the electrode area as a result of the actual deposition of P3T (31).
Figure 1B displays the electropolymerization of terthiophene in the conditions mentioned above but in presence of TiO2. The formation and growth of (P3T+TiO2) composite material film can be seen in this figure (Figure 1B). The oxidation peak of the film was slightly shifted to 1.48 V, indicating that the polymer hosts titanium oxide (TiO2) particles, which were trapped in the polymer matrix during the electropolymerization process. This is confirmed also by the increase of the current intensity of oxidation and reduction peaks with the increase of TiO2 content (10−4, 10−3, 5·10−3, 10−2m) as shown in Figure 2. The first voltammogram of each concentration was recorded on the platinum electrode, where it was cleaned before each recording and in the presence of a slight stirring to have a better homogeneity of the solution before each experiment. Thus, during the positive potential scan, an anodic shoulder was observed at 1.48 V/SCE, which was characteristic of the terthiophene oxidation. While, during the negative potential scan, two waves located at 0.5 and 0.6 V/SCE were observed, attributed to the reduction of the formed polymer. The oxidation and reduction waves were shifted slowly towards higher voltages when the TiO2 was added to the solution (32), (33). This confirms that the presence of TiO2 influences the electrochemical behavior of terthiophene, mainly on the kinetics of the polymer deposition process. So, the P3T film containing TiO2 grows faster compared to the pure P3T film formation.
Cyclic voltammograms during the polymerization of (10−2m) 3T in CH2Cl2/TBAP (10−1m) solution obtained for different concentrations of TiO2 (C=0, 10−4, 10−3, 5·10−3 and 10−2m), obtained with scan rate of 10 mV/s, over a potential range of −0.3–1.6 V/SCE by using a Pt electrode (Ø=2 mm).
3.2 Characterization by electrochemical impedance spectroscopy (EIS)
Figure 3 gives the Nyquist diagrams corresponding to the platinum electrode coated by P3T and (P3T+TiO2) composite films obtained in CH2Cl2/TBAP (10−1m) solution, in a frequency range between 100 kHz and 10 mHz. It is apparent from Figure 3 that the Nyquist diagrams have similar shape, semicircles at high-frequency region, followed by a straight line at low frequencies. The high-frequency part gives the electrolyte resistance (Re) by the distance from the original value, and the width of the semicircle gives the estimated impedance of the formed film. It corresponds to the ohmic resistance or ionic charge transfer resistance (Rct) in the polymer-solution interface by the intercept of the semicircle with the real axis (34). This behavior is typical to impedance diagram of polymer film-coated metals in the asymmetric metal/film/electrolyte configuration (35).
Nyquist diagrams relative to the P3T/Pt and the (P3T+TiO2)/Pt films in CH2Cl2/TBAP (10−1m) solution exempt of monomer and TiO2, run on a frequency range of 100 kHz–10 mHz, obtained at the open circuit potential (Eocp), with a disturbance of 10 mV.
It is important to note that after the incorporation of low concentration of TiO2 particles (10−4m) in the polymer (Figure 3), the resistance of charge transfer radically decreases from 3.04 to 1.13 kΩ cm2, consequently, the conductivity increases. The electrolyte resistance (RΩ) and double layer capacitance (Cdl) of (P3T+TiO2) composite, evaluated after fitting the Nyquist diagrams, were given in Table 1. The results show that the incorporation of TiO2 improves the electric and the electrochemical properties of P3T and consequently give a modified (P3T+TiO2) composite material. So, this result implies that the TiO2 particles present in the film increase the capacity and the electronic conductivity of the P3T.
Electrochemical film parameters obtained from electrochemical impedance spectroscopy.
| (P3T+TiO2) composite material | |||||
| [TiO2] (m) | 0 | 10−4 | 10−3 | 5·10−3 | 10−2 |
| Rct (k Ω·cm2) | 3.041 | 0.201 | 0.858 | 1.272 | 1.136 |
| C (μF/cm2) | 2.61 | 2.200 | 2.93 | 2.89 | 4.985 |
3.3 UV-vis spectroscopy of the modified P3T/ITO and (P3T+TiO2)/ITO electrode
Figure 4 illustrates UV-visible spectra of P3T and composite films obtained by cycling on ITO electrodes in a CH2Cl2/TBAP (10−1m) solution at potential scanning range between −0.3 and 1.6 V/SCE with scan rate of 10 mV/s. As can be seen from the UV-visible absorption spectra, two absorption bands were observed in the absence of TiO2. The first at λmax=465 nm specific to π–π* P3T transition (36), and the second broad and badly defined at λmax=796 nm and this was attributed to the oxidized state of the film (p-doped) (37), while the absorption spectra of the composite shows a new band at 400–350 nm and this is attributed to the absorption of TiO2. This result is in agreement with the work of Deng et al. (38). Also, we noted an increase in the absorbance over all the wave-lengths. This spectrum evolution may be due to the interaction between TiO2 nanoparticles and the P3T molecules because the incorporation of TiO2 nanoparticles increases the real area, which significantly changes the structure of the material resulting in a composite with new optical properties.
UV-visible absorbance spectra of (P3T)/ITO and (P3T+TiO2)/ITO films, obtained for various contents of TiO2 (0, 10−4, 5·10−3, 10−3 and 10−2m).
3.4 Morphological characterization of the P3T and composites film (P3T+TiO2) by SEM and EDX
The surface morphology was examined using SEM analysis. Figure 5A and B show the SEM image of virgin P3T without any modification as a reference sample, and the rest of SEM images (Figure 5a′, b′and 5C) present the morphology of the surface of P3T film electrodeposited on the ITO electrode, impregnated with titanium oxide nanoparticles. Comparing the surfaces of the three films, it can be seen that titanium dioxide nanoparticles with different sizes in the nanoscale (between 200 nm and 400 nm), estimated using Visiometre Software, were distributed on the whole surface of the electrode (Figure 5C). These results match those observed by Abaci et al. (39). The presence of TiO2 in the P3T was confirmed by EXD and the analyzed elements are shown in Figure 6B. Intense rays of titanium were observed at 4.60 and 0.20 keV. This is in good agreement with literature values (40), (41). In addition, the EDX spectrum (Figure 6A) of the electrochemically prepared composite material film shows a signal of a carbon atom (C) and sulfur (S) at 0.2 and 2.36 keV, respectively, resulting from P3T films. The signals of chlorine (Cl) at 2.63 keV, 0.15 keV and of the oxygen (O) at 0.53 keV indicate that the P3T film is doped by the perchlorate (ClO4−) ions. This anion results from tetrabutylamonium perchlorate (TBAP), which was used as a supporting electrolyte. Also, the presence of another element, mercury (Hg), indicates the presence of trace amounts of impurities in titanium oxide. The noted signals at 3.28, 0.4 keV, are attributed to indium doped in tin oxide (ITO) and the 1.75 keV signal refers to silicon which is a glass substrate. Thus, the incorporation of TiO2 particles in the polymer, during the electropolymerization of terthiophene, was confirmed by SEM and EDX analysis. This incorporation results in a pigmentation of the polymer film and the deposition of the (P3T+TiO2) composite on the electrode. Therefore, interesting electrochemical properties are obtained, allowing the use of this composite as an electrode material in a wide range of fields such as electrochemistry, photoelectrochemistry (material photoconductivity), electronic and electrocatalysis applications.
Micrographs of unmodified P3T/ITO (A, B), and (P3T+TiO2)/ITO films (A′, B′, C).
EDX analysis of P3T/ITO (A), and (P3T+TiO2)/ITO composite material films (B).
3.5 Analysis of the P3T and (P3T+TiO2) composite films by atomic force microscopy
In general, AFM images provide information about the height differences and the roughness of the composite thin film surface. As the nanoparticles are tough, they will be easily observable in the soft polymer. The root mean square (RMS) roughness data obtained from the AFM analysis are summarized in Table 2. It can be seen that the roughness of the polymer film increases, due to the incorporation of titanium dioxide (n-TiO2) nanoparticles, from 188.0335 nm for unfilled P3T to 194.689 nm for (P3T+TiO2) composite (42). Moreover, the AFM image (Figure 7) shows dispersed TiO2 nanoparticle structure and as a result provides more organic/inorganic interface forming their own networks in the active layer. The results show that the interpenetrating network and the small feature size of the (P3T+TiO2) composite film facilitate more efficient exciton dissociation, providing more conducting channels for charge transfer and then enhancing the device performance.
Root mean square (RMS) and average roughness (RA) of P3T (10−2m) and (P3T+TiO2) (10−2m) determined from AFM characterization of the films.
| Sample | RMS (nm) | RA (nm) |
|---|---|---|
| P3T | 188.0335 | 144.379 |
| (P3T+TiO2) | 194.689 | 137.655 |
AFM images of P3T/ITO (A), and (P3T+TiO2)/ITO composite material films (B).
3.6 Photoelectrochemistry measurements
In order to test the photoelectrochemical performance of P3T and (P3T+TiO2) composite films as well as their hybrid with TiO2, these films were used as a photoelectrode and submitted to polychromatic light irradiation with an intensity of 100 mW cm−2. Photocurrent measurements were performed in CH2Cl2/TBAP (10−1m) solution, in a three-compartment photoelectrochemical cell.
During the measurement the polymer and the composite films were polarized at −400 mV. After the stabilization of the current, the working electrode was irradiated by polychromatic light. The recorded plots of photocurrents versus bias time are illustrated in (Figure 8). As shown, significant cathodic photocurrent peak was noted directly after the irradiation. This response indicates that recombination processes are taking place in the film; this could be attributed to the presence of charge carriers in the polymer bulk, mainly due to structural disorder. Thus, the prepared films have the conducting behavior of p-type polymers (43), (44). The presence of the space-charge region suggests that these polymers may produce photocurrents when illuminated. Moreover, the incorporation of TiO2 in the polymer films increased the generated photocurrent. Thus, it can be suggested that the semiconducting TiO2 nanoparticles act as dissociation centers for the polymer excitons, which increase the number of charge carriers that get to (P3T+TiO2) composite interface. Therefore, when assembling photovoltaic devices based on a conducting polymer and an inorganic semiconductor, it is important to remember that the device performance is closely related to the content of the latter.
Current density-time plots of P3T/ITO and (P3T+TiO2)/ITO films with different concentrations of TiO2 (C=0, 10−4, 10−3, 5.10−3 and 10−2m) (A), P3T/ITO and (P3T+TiO2)/ITO films of (10−2m) in concentration (B), in CH2Cl2/TBAP (0.1 m) at a potential of −400 mV, observed on switching the light on and off for 100 mW·cm−2 light intensity.
4 Conclusion
The analyses conducted on composite films, which were prepared from a (TiO2) filled conjugated conducting polymer P3T in a solvent electrolyte support system CH2Cl2/TBAP (10−1m), were performed by cyclic voltamperometry and impedance spectrometry. The study showed a variation of the cyclic voltamperogram shape when titanium dioxide was added to the polymer.
The presence of the semi-conducting species in the composite material was confirmed by SEM, EDX and AFM. Consequently, the titanium dioxide nanoparticles can be incorporated in polyterthiophene during the electropolmerization of 3T, in the presence of a slight stirring. This leads to a pigmentation of the polymer film resulting in a (P3T+TiO2) composite material deposited on the electrode surface.
UV-visible spectra of the polyterthiophene films with and without titanium dioxide show that the composite absorbance is more important compared to that of the pure polymer.
The addition of (TiO2) nanoparticles improved the photocurrent of (P3T+TiO2) composite, and this one was higher than that of P3T films without (TiO2). Moreover, the photocurrent increased with the (TiO2) concentration. Our results demonstrate that (TiO2) nanoparticles enhance the optical and photoelectrochemical properties of P3T films.
This work conclusively provides a simple and efficient approach for making conductive polymer-immobilized quantum dot coatings of importance in photovoltaic cells.
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