Electroactivity of the hybrid material composed of poly(3-hexylthiophene) and titania nanoparticles synthesized by electrochemical process
DJAZIA CHALAL, AMINE KHERFI* , AHMED MADANI and ADEL BENGRINE
Laboratoire d’Electrochimie et Mate´riaux (LEM), De´partement de Ge´nie des Proce´de´s, Faculte´ de Technologie, Universite´ Ferhat Abbas, 19000 Se´tif, Alge´rie
*Author for correspondence (kherfiamine6@univ-setif.dz) MS received 30 July 2021; accepted 22 December 2021
Abstract. This work focuses on the electrochemical synthesis of polymer-inorganic semiconductor hybrid consisting of poly(3-hexylthiophene) (P3HT) and titania nanoparticles (TiO2). The P3HT–TiO2hybrid was prepared by electropoly- merization of the 3HT monomer in a dichloromethane-tetrabutylammonium perchlorate (CH2Cl2/TBAP) solution con- taining titania nanoparticles (TiO2). The hybrid material was electrodeposited on indium-tin oxide (ITO) substrates via cyclic voltammetry method. It was illustrated from scanning electron microscope observation that the incorporation of TiO2 nanoparticle within P3HT matrix created an interconnected network structure, which has improved interfacial transport charges. Furthermore, the potentiodynamic results revealed that the electropolymerization process of P3HT in the presence of TiO2was easier and occurred at an early value of 1 V (vs.Ag/AgCl) with a noticeable increase in current density. The impedance results showed lower charge-transfer resistance and higher capacitance values for the P3HT–TiO2 hybrid (Rct= 7 Xcm-2, CPE = 5.196lF cm–2) compared to pure P3HT (Rct= 37Xcm-2, CPE = 1.959lF cm–2), demonstrating the better conductivity and charge storage capacity for the P3HT–TiO2hybrid.
Keywords. Poly(3-hexylthiophene); titania; polymer/inorganic hybrids; electrochemistry.
1. Introduction
In recent years, hybrid materials consisting of polymer and inorganic components have attracted great interest in the development of low-cost, high electron mobility, good physical and chemical stability optoelectronic devices [1,2].
In general, hybrid material possess modified properties attributing to conjugated polymers, such as high air, thermal stability, facile synthesis, electrochemical properties, bio- compatibility, significant electrical conductivity and optical properties in combination with the relatively high electron mobility of inorganic nanoparticles. Zanget al[3] reported that the incorporation of RuO2 and V2O5 oxides into polymer matrix, such as polyaniline and polypyrrole [3,4], improved charge storage capacity for these formed hybrid materials. In this way, Keer et alshowed that the combi- nation of MoO3and Fe3O4[5,6] with conductor polymers led to materials with advanced sensing and catalytic properties.
So far a variety of metal oxides such as TiO2, ZnO, Fe3O4, NiO and MnO2 [7–11] have been blended with conductor polymers for polymer/inorganic hybrid devices.
Among these metal oxides, TiO2 has particular interest because of its non-toxicity, chemical stability, large surface-
to-volume ratios, high electron affinity, large electron mobility and low-cost processing [12]. Furthermore, TiO2 has been largely utilized in gas sensors [13], hydrogen production [14], dye-sensitized solar cells [15], photo- catalysis and catalysis [16,17].
Of the conductor polymer, poly(3-hexylthiophene) is one of the most extensively studied semiconducting polymers due to its high charge carrier mobility, excellent solution processability and environmental stability [18]. According to its interesting properties, P3HT was greatly used in organic solar cells [19,20], polymer light-emitting diodes [21,22], sensors [23–25] and a variety of other applications.
Hybrid materials were generally synthesized by chemical polymerization of the monomer in a dispersion that contains inorganic nanoparticles [26–28] or by grafting polymers on the surface of inorganic nanoparticles [29] using electro- chemical process [30]. However, chemical methods have significant drawbacks and limitations, two most important being the uncontrolled, random distribution of the nanoparticles within the polymer and inadequate electric contact. In contrast, by electrochemical polymerization (EP), in which the reaction occurs at the electrode surface with simultaneous polymerization and film formation on desired electrode in one step, hybrid materials with well- https://doi.org/10.1007/s12034-022-02659-8
defined morphologies and large area of organic/inorganic junction can be obtained. Traditionally, (EP) films exhibit good conductivity and charge-transport properties. The most commonly reported P3HT–TiO2 hybrids synthesis were performed using chemical in-situ polymerization process [31–36] and, according to our knowledge, there is no precedent in the literature for the electropolymerization of P3HT in the presence of TiO2nanoparticles.
Herein, hybrid based on P3HT and TiO2was prepared by in-situ electropolymerization using cyclic voltammetry (CV) method. Our aim in this study is creating an intimately mixed structure of the two materials to tune the morphology and to investigate the electrochemical properties of the resultant organic/inorganic junction. This hybrid can be used as cathode material in battery-supercapacitor and in hybrid polymer solar cells. Electrochemical properties and electrical conductivity were investigated using different analysis techniques.
2. Experimental 2.1 Materials
The monomer 3-hexylthoiphene (3HT) with 98.8% purity was purchased from BASF (Germany). Titania (TiO2) 99.9% with an average diameter of 40 nm were purchased from (Aldrich, USA). Dichloromethane (CH2Cl2) used as the electrolyte solution was obtained from Aldrich, and tetra-butyl-ammonium perchlorate (TBAP) used as the supporting electrolyte was purchased from (Fluka product, Switzerland). Indium-tin oxide (ITO)-coated glass sub- strates were purchased from Visiontek Systems Ltd.
(Chester, UK) with sheet resistance of 15Xper square. All materials of analytical grade were used as received.
2.2 Electrochemical synthesis of P3HT and P3HT–TiO2 hybrid
P3HT–TiO2hybrid films were electrochemically prepared by CV method in a conventional three-electrode electrochemical cell. ITO substrate (1lm thickness) was used as the working electrode, a platinum wire and Ag/AgCl were used as counter and reference electrodes, respectively. The deposit electrolyte solution consisted of CH2Cl2/TBAP 10–1M?10–2 M 3HT monomer ? TiO2 at different concentrations. Before elec- tropolymerization, the electrolyte solution was sonicated for 60 min and purged with N2gas. P3HT–TiO2composite films were deposited on ITO substrate by cycling the potential from 0 to
?1.6 Vvs.Ag/AgCl for 10 cycles with a scan rate of 10 mV s-1. During the electrodeposition process, the electrolyte solution was kept under stirring with a speed rate of 300 rpm to ensure the arrival of TiO2nanoparticles and to be continuously in contact with the electrode surface where the hybrid film is being deposited. Furthermore, pure P3HT was electropolymerized in
the same condition as mentioned above, but no TiO2particles were added in the polymerization electrolyte. Finally, P3HT and P3HT–TiO2electrodes obtained were rinsed with deion- ized water for several times and dried with N2flow, then used directly for electrochemical characterizations.
2.3 Characterization techniques
P3HT and P3HT–TiO2hybrid films were characterized by various techniques. The morphology was determined using a Hitachi S-5000H field-emission scanning electron microscope (SEM) at an accelerating voltage of 20 kV. The elemental analysis was done by using energy dispersive X-ray (EDX). The electrochemical measurements were investigated by CV and electrochemical impedance spec- troscopy (EIS). In EIS measurements, the AC frequency range was extended from 100 kHz to 10 MHz using amplitude of 10 mV. All electrochemical experiments were carried out on PGZ-301 potentiostat/galvanostat from Tacussel controlled by a personal computer via Voltamaster 4 software at ambient temperature (25±1°C).
3. Results and discussion
3.1 Surface morphology (SEM) and EDX analysis
Figure 1a and b shows the surface morphology of pure P3HT and P3HT–TiO2 hybrid films, respectively. Pure P3HT electrodeposited on ITO substrates, presents a form of a mesh of fibrillary structure as indicated in figure 1a.
Similar fibril-like P3HT crystallites were observed by Klimovet al[37]. However, the morphology of the P3HT–
TiO2(figure 1b) was modified after incorporation of TiO2 nanoparticles. As can be seen, white granules of TiO2were uniformly dispersed in the P3HT matrix. This homogeneous dispersion suggests the formation of an interconnected network of donor–acceptor phases, which is very advanta- geous for increased charges carrier dissociation and trans- portation transfer yield [38,39].
The EDX elemental spectra of P3HT and P3HT–TiO2are presented in figure1c and d. Well-defined signals of sulphur (S) were depicted at 2.4 keV in both spectra resulting from P3HT polymer [40]. In the case of P3HT–TiO2, two intense signals of titanium (Ti) located at 0.4 and 4.5 keV were detected, which match well with the ones available in the literature [41,42], indicating the successfully incorporation of TiO2 into P3HT matrix. In addition, signal of oxygen (O) was also detected on the hybrid spectrum.
3.2 Electropolymerization of P3HT and P3HT–TiO2 Figure2shows voltammogramms for the electrodeposition of P3HT in a dichloromethane solution containing 10-1M
TBAP and 10-2M 3HT. The electropolymerization process was performed by scanning the potential between 0 and
?1.6 Vvs.Ag/AgCl at a sweep rate of 10 mV s-1. During the first cycle, an irreversible oxidation peak appeared at 1.35 V, indicating the production of radical cations of 3HT by the oxidation of the 3HT monomer, and no cathodic peak was noticed, which indicate that the radicals were promptly employed by the subsequent reactions.
In the following scans, a new peak appeared around 0.9 V vs. Ag/AgCl. And the oxidation peak current gradually increased with cycle numbers demonstrating the successful polymerization of 3HT on ITO electrode and confirming oxidation of most of the repeat units of the polymer, as reported by several studies [43]. After 10 cycles of potential scanning, a brick red film covered the surface of the elec- trode. The resulting film was washed with dichloromethane to remove the supporting electrolyte, then dried in a vacuum oven at room temperature.
Figure3illustrates the cyclic voltammograms during the electropolymerization of P3HT in the presence of different concentrations of TiO2(10–4, 10–3, 5910–2, 10–2M). The voltammograms show an increase in current density as the TiO2 concentration increases. However, with a concentra- tion (10–2M) of TiO2, the redox current was greatest, which suggest an enhanced electroactivity of the prepared com- posite. Therefore, the synthesis of the composite P3HT–
TiO2was performed with this optimal concentration.
As can be seen from figure 4, the oxidation of the poly- mer P3HT in the presence of (10–2 M) TiO2 occurs at an early value of 1 V with a noticeable increase in the current density, which indicated a beneficial effect of the TiO2on the electropolymerization process. In addition, in the inverse scan a cathodic peak appeared more pronounced at 0.65 V, assigned to the reduction of the polymer formed.
This peak was not detected in the medium exempt of TiO2 nanoparticles. The improvement in redox currents in the presence of TiO2could be attributed to the better quality of Figure 1. SEM images of (a) P3HT, (b) P3HT–TiO2 and EDX spectra of (c) P3HT and (d) P3HT–TiO2 films
deposited on ITO substrates.
Figure 2. Electropolymerization of (10-2 M) 3HT during 10 potentiodynamic cycles at a sweep rate of 10 mV s-1in CH2Cl2/ TBAP (10-1M) solution.
the interfaces between the TiO2 and P3HT, which led to more-efficient charges transfer. This finding could poten- tially offer an approach to adjust the morphology of the hybrid films to improve performance, for example, for new hybrid solar devices.
3.3 CV characterization
The electrochemical activity of the polymer P3HT and the P3HT–TiO2 hybrid material was studied by CV in the electrolyte without monomer and TiO2 and the corre- sponding cyclic voltammograms are recorded in figure 5.
As can be seen, the electrochemical behaviour of the polymer and the hybrid was similar to that observed in the
presence of monomer and TiO2(figures2and4), in which the intensity of oxidation and reduction peaks remain unchanged indicating the good stability and satisfying electroactivity of the P3HT–TiO2and P3HT.
Furthermore, from the parts (a) and (b) of the voltammograms in figure 5, the difference in the Figure 3. Cyclic voltammograms during the polymerization of
(10-2M) 3HT in CH2Cl2/TBAP (10-1M) solution obtained for different concentrations of TiO2 nanoparticles. (C= 10-4, 10-3, 5910-2and 10-2M), at a sweep rate of 10 mV s-1
Figure 4. Electrodeposition of P3HT–TiO2 hybrid during 10 potentiodynamic cycles at a sweep rate of 10 mV s-1in CH2Cl2/ TBAP (10-1 M) solution containing (10-2 M) 3HT ? TiO2
nanoparticles.
Figure 5. Cyclic voltammograms relating to (a) P3HT/ITO and (b) P3HT–TiO2/ITO electrodes in CH2Cl2/TBAP (10-1 M) solution, without monomer and TiO2, at sweep rate of 10 mV s-1and between –0.2 and 1.8 Vvs.Ag/AgCl.
Figure 6. Nyquist plots of P3HT/ITO and the (P3HT–TiO2)/ITO hybrid electrodes in CH2Cl2/TBAP (10-1 M) solution without monomer and TiO2.
Figure 7. Equivalent circuit models for P3HT/ITO and P3HT–
TiO2/ITO electrodes.
electropolymerization phenomena is observable in which the addition of TiO2 significantly increases the recorded currents, then facilitates electropolymerization process at the same applied potentials. Chen et al [44] found the similar behaviour with PPy–ZnO hybrids. As can be seen the electropolymerization process occurs at a lower poten- tial in the presence of TiO2 nanoparticles, thus elec- tropolymerization on P3HT–TiO2 initial layers is easier than on pure P3HT initial layers. Additionally, P3HT–TiO2 exhibited better electroactivity with respect to pure P3HT, implying strongest electrochemical exchange reaction. This result suggests the feasibility of employing such hybrids in photocatalysis [31,35,45] applications.
3.4 EIS characterization
EIS is an experimental technique that can provide useful information about kinetics and mechanism of monolayer formation [46–48], as well as quantitative estimate of coverage at the electrode active area [49].
To investigate the kinetics of electron transfer at the P3HT–TiO2interface, the hybrid films were characterized using EIS and the results are expressed via Nyquist plots.
Figure 6illustrates the corresponding Nyquist plots for the P3HT–TiO2 and pure P3HT electrodes. The films were analysed in CH2Cl2/TBAP (10-1 M) solution, free of monomer and TiO2.
The impedance response is characterized by the presence of a semi-circle arc in the higher frequency region and a straight line in the lower frequency region. As usual, the semi-circle arc at high frequency corresponds to the double- layer capacitance (CPEdl) in parallel with the charge- transfer resistance (Rct) at the interface between electrode and electrolyte solution. The constant phase element (CPE) is used instead of a pure capacitance because of the non- ideal behaviour capacitor. However, at lower frequencies, the straight line having an angle with the real axis corre- sponds to Warburg impedance (Zw), which is characteristics of the semi-infinite diffusion.
EIS parameters (Rs, Rct, CPEdl, Zw) issue from the equivalent circuit (figure 7) were obtained by fitting impe- dance curves using the ZSimpWin 3.21 Software. From the listed parameters in table1, the Rsof P3HT–TiO2electrode is smaller than that of pure P3HT revealing the lower internal resistance of P3HT–TiO2 electrode. The charge- transfer resistance value for P3HT–TiO2electrodes (Rct= 7 X.cm-2) was relatively low with respect to pure P3HT samples (Rct = 37 X.cm-2), which is associated to the
surface morphology of P3HT–TiO2. This decrease in charge-transfer resistance results in a faster kinetics process and easier electrochemical reaction to happen, due to the conjunction and synergistic effect of P3HT and TiO2 [50,51].
Furthermore, P3HT–TiO2 featured smallest semicircle, which is consistent with the interfacial modification pro- moting charge transfer between TiO2and P3HT, and lead- ing to higher conductivity. This finding is advantageous for utilization of such hybrids in polymer/inorganic hybrid solar cells [52,53]. In addition, the value of capacitance for the hybrid P3HT–TiO2(CPE = 5.196lF cm-2) was found to be higher compared to pure P3HT (CPE = 1.959 lF cm-2), confirming its ability for charge storage. On the other hand, in the low frequency range the P3HT–TiO2 hybrid electrode exhibits a more vertical line with an angle of 45°approximatively, demonstrating faster ion diffusion process. Based on the acquired results, it can be concluded that P3HT–TiO2 hybrid electrode exhibits improved con- ductivity and charge storage capability.
4. Conclusion
In this work, we developed a new approach to form P3HT–TiO2 hybrid electrodes by in-situ electropolymer- ization using CV method. The electrodeposition of TiO2 nanoparticles within P3HT matrix provides a well-defined morphology with more interfaces owing to fast ion diffusion and electron-transfer rate during electrochemical reaction.
The lower oxidation potential values for P3HT–TiO2 revealed by CV suggest the feasibility of using this hybrid in energy conversion devices and photocatalysts. In addi- tion, the enhanced conductivity and capacity of P3HT–TiO2 hybrid attributed to the interfacial modification promoting large charge transfer between TiO2and P3HT. This kind of hybrid electrodes can be used as promising materials for new energy storage applications.
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