Electro-synthesis, characterization and photoconducting performance of ITO/polybithiophene–MnO2 composite

https://doi.org/10.1016/j.mseb.2016.02.007Get rights and content

Highlights

  • PBTh–MnO2 composites are prepared by electro-polymerization of bithiophene on ITO.

  • Photocurrent of ITO/PBTh–MnO2 films is three times higher than that of ITO/PBTh substrate.

  • Electrochemical gap, HOMO and LUMO potentials are determined.

  • ITO/PBTh–MnO2 films can be used as a new active material in solar cells.

Abstract

Manganese dioxide is synthesized by reduction reaction of potassium permanganate with hydrogen peroxide. The as-synthesized α-MnO2 is characterized by powder X-ray diffraction and infrared spectroscopy. The MnO2 particles are used to prepare composite films containing polybithophene (PBTh) on indium tin oxide (ITO) glass substrates. The composite films ITO/PBTh–MnO2 are obtained by electro-polymerization of bithiophene in the presence the α-MnO2 particles dispersed in the electrolytic solution. The XRD and SEM analyses show that the α-MnO2 particles of size in the range 100–300 nm are incorporated in the polymer. The films are characterized by cyclic voltammetry impedance spectroscopy, UV–vis spectroscopy and scanning electron microscopy. As a result, the electrochemical gap and the optical gap are shifted by the incorporation of MnO2 from 2.15 eV for ITO/PBTh to 1.88 eV for ITO/PBTh–MnO2, while the electrical conductivity decreases from 195.35 S/cm for ITO/PBTh down to 0.047 S/cm for ITO/PBTh–MnO2 at the highest MnO2 investigated. The photo-electrochemical measurements also indicate that the ITO/PBTh–MnO2 films show a photocurrent that is three times higher than that of ITO/PBTh substrate to reach 20.6 μA cm−2, so that such a composite can be used as a new active material in solar cells.

Introduction

Conjugated conducting polymers, such as polyaniline, polythiophene, polypyrrole, can be synthesized electrochemically and deposited onto metal electrodes [1], [2], [3] or with oxide metal [4]. These polymers can be used in a wide range of applications such as electric conductors, batteries [5], solar cells [6], light emitting diodes [7], field effect transistors [8], electrochromics [9] and biosensors [10].

The electrical conductivity of polymers is usually small. This problem, however, can be solved by mixing the polymer with a conducting material, in particular multi-walled carbon nanotubes (MWCNT). For example, a conductivity of 4.8–5.0 S/cm could be achieved with polyaniline (PANI)-functionalized MWCNT/Au or Ag [11]. The conductivity of poly(3,4-ethylene-dioxythiophene) (PEDOT) with 27% MWCNT was raised to 2.9 S/cm [12]. Less expensive and more scalable synthesis processes are obtained by submerging nanoparticles of oxides of transition metals in the polymer. The conductivity, however, is decreased by an order of magnitude in that case. For instance, a conductivity of 0.2 S/cm at room temperature was obtained by submerging Fe3O4 in poly(aniline-co-8-amino-2-naphthalenesulfonic acid) (PANSA) [13] and in poly(3,4-ethylene-dioxythiophene) PEDOT [14], or by submerging SiO2 in poly(3-aminophenylboronic acid) (PAPBA) [15].

Polythiophenes constitute a particularly important class of conjugated polymers, which has been extensively studied for the relation between the geometrical structure and the optic and electronic properties. They are, furthermore, chemically and thermally stable materials, and are very attractive for exploitation of their physical properties [16]. In photo-electrochemical reactions the observation of photocurrents from polythiophene-modified metal electrodes has been reported by several groups of investigators [17], [18]. The photocurrents were either cathodic or anodic depending on the potential applied to the electrode [19], and polythiophenes are being intensively investigated for use in polymer/inorganic bulk-heterojunction photovoltaic (PV) devices.

The configurations for all devices are based on the mechanism of photoinduced electron transfer across the internal or external donor–acceptor n–p heterojunctions [20], [21], [22], [23], [24], [25], [26], [27].

Notable examples include organic/organic [28], organic/inorganic [29] and polymer/inorganic [30].

Recent developments in photoelectric cells process polymers in conjunction with TiO2 to separate and trap photoexcited charge in mesoporous coated devices [31]. In particular, polybithiophene–TiO2 films were electro-polymerized to evaluate their catalytic performance, their photonic conversion efficiency [32]. The films were stable, and showed promise as a viable photocathode material as they reduced the cathodic overpotential needed to promote hydrogen generation up to a current density of 75 μA cm−2. A drawback of TiO2, however, is the difficulty to synthesize it in the pure anatase phase, and avoid the conversion to the rutile phase, and the anatase phase is known to be a better photocatalyst than rutile [33]. In the present work, we consider a polybithiophene–MnO2 composite, as MnO2 can be synthesized and stabilized in the pure α-phase.

Manganese dioxide is one of the important materials used in primary dry cells as depolariser and in rechargeable lithium batteries as active cathode [34], [35], [36]. Another application is the addition of MnO2 to zinc–bismuth–borate glasses that affects the luminescence properties due to redox reaction with the various valence states of bismuth ions [37]. In previous works, we have coated a conducting polymer on the surface of β-MnO2, γ-MnO2 particles that we had synthesized. These composites were used for oxygen reduction reaction [38], Zn//MnO2 cells [39] and supercapacitors [40]. From the analysis of the magnetic measurements in [39], we know that MnO2 interacts with the polybithophene (PBTh) by oxidation of the polymer at the interface, accompanied by an equivalent reduction of γ-MnO2 to conserve the electrical neutrality, i.e. change of the valence state from Mn4+ to Mn3+ in MnO2 for the manganese ions at the interface with PBTh [39]. This charge transfer associated to the ionicity of the bonding between PBTh and MnO2 is at the origin of the stability of the PBTh–MnO2 composite. This will be confirmed later in this work by the optical spectroscopy measurements.

The manganese dioxide obtained by electrochemical process (EMD) or by chemical process (CMD) possesses a structure of polymorphs involving various packing of MnO6 octahedra with different motifs of edge- and corner-sharing [41]. Several routes have been developed for the synthesis of MnO2 by the oxidation of Mn(II) nitrates or carbonates [42], aqueous routes involving the reduction of permanganate solution [43], [44] or reaction of Mn2O3 with sulfuric acid [45]. In the present work, MnO2 has been synthesized by the hydrothermal method, which is known to be a cheap and environmentally friendly method to prepare materials in different nano-architectures such as nanorods, nanowires and nanoparticles [46], [47]. Our samples have been analyzed by X-ray diffraction and infrared spectroscopy. The (ITO/PBTh–MnO2) composite films were electrochemically prepared by cyclic voltammetry on a ITO (indium–tin-oxide) glass substrate (100 nm thick, from SOLEMS) used as working electrode and immersed in a support electrolyte containing the MnO2 powder dispersed by a weak agitation at different concentrations in acetonitrile solution containing 10−1 M LiClO4 as the supporting electrolyte and 10−2 M bithiophene as monomer. The electrochemical synthesis has been preferred to the chemical synthesis, because it leads to more conductive polymers, for a review on synthetic principles for band-gap control in linear π-conjugated systems, see [48].

In the process, PBTh is doped with (ClO4) ions so that it is a p-type semiconductor, while MnO2 is n-type, so that PBTh–MnO2 realizes a p–n heterojunction. The composite films were characterized by SEM/EDS, UV–vis spectroscopy, FTIR spectroscopy, cyclic voltammetry, impedance spectroscopy and photocurrent tests.

Section snippets

Experimental

All chemicals used in this study were of analytical reagent grade and used without further purification. The MnO2 was prepared by a redox reaction between potassium permanganate (KMnO4) (ACROS) and hydrogen peroxide (H2O2) (MERK MILLIPORE) in an acidic medium. A 45 ml H2O2 (10%) solution was slowly added from the burette, drop by drop, to 50 ml of 10−2 M KMnO4 solution at continuous stirring until complete reduction of the potassium permanganate, detected by the disappearance of the purple colour

Structural characterization of synthesized MnO2

Fig. 1 shows the XRD pattern of the as-prepared MnO2 sample. All the peaks can be indexed according to the crystallographic α-MnO2 variety after the JCPDS 044-0141. The crystal structure of α-MnO2 consists in a combination of octahedral MnO6 units. Double chains of MnO6 edge-sharing octahedra are linked to form double octahedral zigzag chains along the c-axis. These chains share their corners with each other to form approximately square (2 × 2) tunnels parallel to the c-axis. The structure is

Discussion

The incorporation of MnO2 particles into the polymer films highly increases the generated photocurrent from 5.9 μA cm−2 for ITO/PBTh up to 16, 16.6 and 20.6 μA cm−2 for the ITO/PBTh–MnO2 films with 30, 50, 100 mg MnO2 incorporated, respectively. Two effects contribute to this effect. First, the addition of MnO2 increases importantly the absorbance due to the π–π* transition, also at the origin of the photocurrent in polymer solar cells that are currently under investigation. The PBTh–MnO2 composite

Conclusion

The MnO2 particles were successfully synthesized via a hydrothermal method. The XRD and FTIR spectroscopy characterization indicate that the crystalline α-MnO2 polymorph was obtained. During polymer growth on ITO, α-MnO2 was incorporated at different weight to prepare ITO/PBTh–MnO2 composite films. The composite films analysis by cyclic voltammetry revealed a decrease of the current intensity near the cathodic and anodic peaks upon increasing the MnO2 quantity incorporated, which is attributed

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