Elsevier

Synthetic Metals

Volume 269, November 2020, 116544
Synthetic Metals

Poly(3-hexylthiophene)/titanium dioxide nanocomposites prepared by in-situ polymerization: structure, morphology and electrochemical properties

https://doi.org/10.1016/j.synthmet.2020.116544Get rights and content

Highlights

  • P3HT/TiO2 Nanocomposites prepared by polymerization in situ.

  • TiO2 anatase nanoparticles were obtained by microwave-assisted green synthesis.

  • Head-tail (HT) bonds in nanocomposites characterized regioregular polymer chains.

  • TiO2 improves the electron exchange between the P3HT and the working electrode.

ABSTRACT

In this work, poly(3-hexylthiophene) (P3HT) nanocomposites with different mass percentages of TiO2 nanoparticles (NPs) (1.0, 2.0 and 5.0% wt.) were prepared by in-situ polymerization to assess the effects of NPs on the physical and chemical properties of the P3HT polymer. Green TiO2 NPs in the anatase crystalline phase were obtained by microwave-assisted synthesis. Transmission electron microscopy images showed the presence of submicron agglomerates of TiO2 NPs inserted in the P3HT matrix. Cyclic voltammograms evidenced a quasi-reversible behavior at a 50 mV s-1 sweep rate for all samples, with the P3HT-TiO2 nanocomposite exhibiting a higher oxidation potential than P3HT. The nanocomposite containing 2.0% wt. TiO2 exhibited the highest thermal stability and the highest potential for application in photovoltaic devices due to its regiochemistry, which improved the charge transport properties of the P3HT polymer.

Introduction

In recent years, several studies have shown the use of organic compounds in the manufacture of solar cells based on conductive polymers (CPs) [1,2]. CPs retain the properties of conventional plastics, such as flexibility, thin film processing, lightness, and ability to manufacture devices over large areas, as well as strong absorption in the visible region of the electromagnetic spectrum [[3], [4], [5], [6]].

Among the most studied CPs are polythiophenes and their derivatives, such as poly(3-hexylthiophene) (P3HT), which presents high hole mobility, ease of donating electrons, band gap values close to 2 eV and solubility in some organic solvents, such as chloroform and tetrahydrofuran [[6], [7], [8]]. The regioregular P3HT structure and the light absorption properties in the solar spectrum contribute to its application in photosensitive layers of solar cells [[9], [10], [11]]. In addition, the literature has reported that P3HT-based photovoltaic devices using non-fullerene acceptors can lead to a high open-circuit voltage (Voc) of 1.2 V and can achieve a power conversion efficiency (PCE) of approximately 6% [12,13]. Conductive polymers combined with other materials show interesting optoelectronic properties for application in photovoltaic devices. The preparation of P3HT composites with MWCNTs (multiwalled carbon nanotubes) through covalent bonds showed a considerable increase in electrical conductivity, as reported by Alves and co-authors [14]. Patrício and co-authors [15] published a study showing the use of P3HT for the formation of blends with polyurethane in different concentrations, and they investigated the thermal, morphological and optical properties of these materials. Due to the presence of polyurethane, the films showed high elasticity and maintained the optical and electrical properties of P3HT, which made them interesting for applications in electro-optical devices.

The combination of CPs and inorganic nanoparticles (NPs) can produce nanocomposites with interesting properties that can be used in the manufacture of more economical and efficient devices, with improved charge separation and light collection [5]. In nanocomposites, the conducting polymer acts as the electron donor, and the NPs act as the electron acceptor and transporter [2,5]. The combination of these materials can improve cargo transport by increasing conductivity.

TiO2 NPs in the anatase crystalline phase have attractive optical and electronic properties, high absorption coefficients, low toxicity, high electronic mobility, ease of manufacture and well-known physical and chemical stability [5,7,11,16]. The synthesis of TiO2 nanoparticles or nanostructures has been performed by different methods, such as electrodeposition, direct oxidation, hydrothermal processing, and sol-gel synthesis [17]. The synthesis of green TiO2 nanoparticles in the crystalline anatase phase assisted by microwaves has been reported by Oliveira and co-authors [18] from the titanium tetrachloride precursor employing the precipitation process.

The synthesis of poly(3-hexylthiophene) and TiO2 nanocomposites has been reported employing different methodologies [7,[19], [20], [21],22]. Boon and co-authors [20] reported the synthesis of TiO2 and P3HT nanocomposites from in-situ polymerization using iodobenzoic acid to modify TiO2 nanoparticles. They observed an efficient photoinduced electron transfer between the modified TiO2 and P3HT. To improve the interaction of TiO2 NPs with P3HT, Hunag and co-authors [21] used 2-thiophenecarboxylic acid to modify the TiO2 surface, which contributed to the in-situ polymerization, promoting an improvement in charge transfer and chemical stability of the nanocomposites. The authors observed a reduction in the energy barrier for charge transfer between TiO2 and P3HT.

The influence of TiO2 nanoparticles on the optical behavior of P3HT was investigated by Jiang and co-authors [23] using UV-Vis absorption spectroscopy, and the results allowed to establish the charge transfer process between the conjugated polymer and the inorganic semiconductors [23]. According to Zhang and co-authors [24], P3HT and TiO2 composites showed physical and chemical stability even after 10 hours of use in the photodegradation of methylene blue dye under visible light.

In this work, polymeric nanocomposites of poly(3-hexylthiophene) and TiO2 nanoparticles were synthesized in-situ with the objective of improving the charge transport and increasing the energy absorption range in the electromagnetic spectrum for future applications in photovoltaic devices. The influence of TiO2 concentration on the polymerization mechanism and optical and electrochemical properties was investigated. The physical-chemical properties were evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetry (TG), differential scanning calorimetry (DSC), UV-Vis absorption spectroscopy and fluorescence. Electrochemical characterization was performed by cyclic voltammetry (CV).

Section snippets

Preparation of TiO2 nanoparticles

Green TiO2 nanoparticles in the anatase crystalline phase were prepared employing a precipitation process from titanium tetrachloride (TiCl4), which is used as a precursor in the synthesis of TiO2 NPs in only approximately 5% of the studies analyzed that involve obtaining nanocomposites P3HT-TiO2 [18]. In the procedure, 2.0 mL of TiCl4 was slowly added to 50 mL of distilled water and kept in an ice-water bath with vigorous stirring. Ammonium hydroxide was added until the pH value reached 8.

X-ray diffraction, transmission electron microscopy

The diffractogram shown in Fig. 1a exhibits characteristic peaks attributed to TiO2 nanoparticles in the anatase phase [18,26]. The peaks appearing at 2θ (CuKα) = 25.3º; 37.9º; 48.0º; 54.1º; 54.6º; 62.8º; 68.8º; 70.1º and 75.2º were indexed comparing the experimental data with the reference crystallographic (PDF nº 4-577 - anatase). An average crystallite diameter of 7 nm was estimated from the width to half height of the diffraction peak (200) and employing the Scherrer equation [27].

The HRTEM

Conclusions

The in-situ polymerization process using different amounts of synthesized TiO2 nanoparticles proved to be efficient in the preparation of P3HT-TiO2 nanocomposites. X-ray diffraction and transmission electron microscopy techniques allowed the estimation of an average size for TiO2 nanoparticles and provided evidence of the presence of these nanoparticles in the nanocomposites.

Other techniques used, such as nuclear magnetic resonance spectroscopy and infrared spectroscopy, showed that the

Supplementary material

Figure S1a shows UV-Vis absorption spectra of methylene blue at different times and in the presence of TiO2 under UV light. Figure S1b shows the relationship between (C/Co) and the photodegradation time of TiO2 catalyzed methylene blue. Figure S2 shows the adsorption/desorption isotherm for the nanoparticulate TiO2 sample. Figure S3 shows the crystallographic rings characteristic of the diffraction pattern for TiO2 NPs in the anatase crystalline phase, and Table S1 shows the indexed diffraction

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank the Fundação de Amparo à Pesquisa de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 for the financial support received. The authors would like to acknowledge the Center of Microscopy at the Universidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) for providing the equipment and technical support for

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