Elsevier

Electrochimica Acta

Volume 279, 20 July 2018, Pages 34-43
Electrochimica Acta

The influence of polarization of titania nanotubes modified by a hybrid system made of a conducting polymer PEDOT and Prussian Blue redox network on the Raman spectroscopy response and photoelectrochemical properties

https://doi.org/10.1016/j.electacta.2018.05.068Get rights and content

Highlights

  • We focus on the influence of different polarization conditions onto the properties of composite.

  • Raman spectroscopy measurements were carried out to verify the crystal phase of materials.

  • At low and intermediate doping levels polarons populate the polymer chain.

Abstract

In this work we show the impact of applied potential on network vibrations and photoelectrochemical properties of a composite material containing hydrogenated titania nanotubes and poly (3,4-ethylenedioxythiophene) with iron hexacyanoferrate (H-TiO2/pEDOT:Fehcf) acting as a redox centre. For this purpose, Raman spectroscopy measurements under the working electrode (WE) polarization were carried out, allowing investigation of changes in the structure of the obtained heterojunction. The photoelectrochemical behaviour of the H-TiO2/pEDOT:Fehcf composite was also studied at different potentials of WE. Both, in-situ Raman spectroelectrochemical and transient photocurrent measurements were performed in aqueous 0.1 M K2SO4 electrolyte. The reduction and oxidation of the electrode material enabled control of the organic matrix doping level and in consequence processes occurring at the electrode/electrolyte interface. The intensity of bands typical for the organic part of the junction strongly depends on the applied potential: the highest intensity of Raman bands characteristic for the pEDOT chain was observed in the cathodic potential range, whereas under anodic polarization pEDOT signals diminish. On the contrary, the intensity and the positions of anatase active modes remain almost unchanged independently of the applied potential. Furthermore, the effect of various polarization conditions within the anodic and cathodic potential ranges on the photocurrents was also observed. The maximum value of the photocurrent is reached at +0.8 V vs. Ag/AgCl/0.1 M KCl and equals 290 μA/cm2.

Introduction

Composite materials, consisting of an organic electron donor and an inorganic oxide semiconductor electron acceptor, have attracted much attention in the past decades [1,2]. Such composites exhibit novel properties derived from the successful combination of the characteristics of parent constituents [3]. Usually this material is composed of an inorganic metal oxide in the form of nanoparticles (Fe2O3, V2O5), nanowires (ZnO) or nanotubes (TiO2) overgrown by a conducting polymer network (CPs) [4]. Among different CPs, poly (3,4-ethylendioxythiophene) (pEDOT) is regarded as a very stable electroactive material even during electrochemical charging and discharging and exhibits relatively high electrical conductivity [5]. Furthermore, its electrochemical properties and photoactivity could be easily tuned by modification with different metal hexacynanoferrates, for example Prussian Blue (Fehcf) [6]. Such great interest paid to the organic-inorganic junctions results from their unique properties allowing for potential use in electrocatalysis, photoelectrocatalysis, sensors and energy storage and conversion devices [[7], [8], [9], [10]].

Most possible applications of such composites require electrode polarization. According to [11,12], materials under applied voltage may change their structure, band bending and in consequence electronic properties are modulated. Such control of the electrode potential has a large influence on its optical, electrochemical and photoelectrochemical properties [5,[12], [13], [14]]. Among others, in-situ Raman measurements could be used as a powerful method of investigating the crystallographic structure of a material during its polarization [5,11,12,15]. Raman spectroscopy (RS) allows for verification of the phase composition and the presence of secondary phases, as well as lattice dynamics and phase transitions of materials [[16], [17], [18]]. RS is ideally suited also for in-situ studies because there are no inherent limitations to the temperature, polarization, pressures or the presence of reaction gases during investigations [19]. Raman spectroscopy has been also helpful for obtaining detailed information about the molecular structure of the metal oxide overlayer on oxide supports and electrochemical reductive doping [12,19]. Moreover, it is known that in-situ Raman spectroscopy is often used for studying the doping process in conjugated polymers [15,20]. In polyconjugated systems oxidative doping results not only in the increase of their electronic conductivity, but also in significant modification of their electronic and vibrational properties [15].

Apart from the registration of Raman spectra under material polarization, important information could be obtained from photoactivity measurements when the transient photocurrent is registered in various conditions. According to Spadavecchia et al. [21], the shape of the chronoamperometry curve recorded when the sample is periodically illuminated could be assigned to photoinduced electron-hole separation, trapping, recombination and scavenging processes. Until now, such investigations were applied to titania nanotubes and their combinations with a conducting polymer [22], metal [23] or metal oxide [24] nanoparticles and even CdSe quantum dots [25]. However, in the vast majority of performed research the photocurrent is registered at a single one potential and only its value is discussed without any special attention paid to its shape.

In our previous reports, generally we focus on the optimization of the synthesis procedure, characterization of sample morphology and structure; also some photoactivity measurements or charge-discharge tests were reported [26,27]. Furthermore, the impact of different redox centres being the Prussian Blue analogues onto the material photoresponse was also verified [28] and the synergistic effect between both organic and inorganic elements was described [29]. Similar experimental approaches covering the studies of surface, structure and electrochemical properties are often applied to other titania NTs - conducting polymer composites, but more detailed investigation of the role of individual components and the material behaviour in various polarization conditions is usually neglected. Thus, knowing the great performance and the stability of H-TiO2/pEDOT:Fehcf upon illumination, this composite was stated to be an appropriate model organic-inorganic composite for further, more complex research. Following that, in this manuscript, we would like to report the relationship between the applied potential to WE and the crystallographic structure and photoelectrochemical properties of the heterojunction composed of hydrogenated titania nanotubes and a conducting polymer with Prussian Blue species imbedded in the polymer matrix. The uniform infiltration and direct contact between the organic part and the metal oxide support was inspected by transmission electron microscopy (TEM) together with energy dispersive X-ray spectroscopy (EDX). Performed Raman spectroscopy measurements during working electrode polarization allow to follow the changes in the structure of the obtained composite: H-TiO2/pEDOT:Fehcf. Furthermore, we present that by changing the potential of the electrode, we could modulate the photoactivity of the whole heterojunction affected by processes occurring at the electrode/electrolyte interface. We hope that the presented results will encourage researchers to perform more insightful investigations on other organic-inorganic materials allowing for understanding the synergy between both parts of the junction.

Section snippets

Sample pretreatment

A high purity Ti foil (0.1 mm thick, 99.97% in purity, Strem) was used as the substrate material for anodization. At first, the Ti substrate was cut into rectangular plates (2.0 × 2.5 cm) degreased in acetone and ethanol, then dried in air.

Fabrication of TiO2 nanotubes

The anodization procedure was previously optimized basing on multiple experimental approaches and already utilized for fabrication of ordered titania NTs substrates [9,29,30]. All anodization experiments were carried out in a double-walled electrochemical

Morphology and structure studies

In Fig. 1 the top-view and cross-section images of hydrogenated titania nanotubes (Fig. 1a) and the composite material (Fig. 1b and c) are presented. The H-TiO2 sample is composed of regular nanotubes with the internal radius of ca. 50 nm, wall thickness of ca. 15 nm and the length equal to 2.5 μm. In the case when only 50 mC cm−2 had been consumed during the electropolymerization process, the polymer layer was very thin and hardly seen on SEM images (Fig. 1b). However, as it was presented in

Conclusions

In this work we focus on the influence of different polarization conditions on the properties of the H-TiO2NTs/pEDOT:Fehcf composite. The heterojunction was fabricated using various electrochemical techniques including anodization, hydrogenation and electropolymerization leading to the direct contact between the organic and inorganic material as was confirmed using transmission electron microscopy together with EDX inspection. The provided EDX maps of titanium, oxygen, sulfur, iron and carbon

Acknowledgement

This work received financial support from the Polish National Science Centre: Grant No. 2012/07/D/ST5/02269. K.S. and M.S. research were supported by the Foundation for Polish Science. M.S. and A. L.-O. gratefully acknowledges the financial support from National Science Centre, Poland under grant no. 2016/23/N/ST5/02071 and Gdańsk University of Technology DS 032406.

References (56)

  • K. Siuzdak et al.

    Synthesis and photoelectrochemical behaviour of hydrogenated titania nanotubes modified with conducting polymer infiltrated by redox active network

    Electrochim. Acta

    (2016)
  • A. Lisowska-Oleksiak et al.

    Poly(3,4-ethylenedioxythiophene)-Prussian Blue hybrid material: evidence of direct chemical interaction between PB and pEDOT

    Electrochem. Commun.

    (2006)
  • A. Lisowska-Oleksiak et al.

    Ex situ XANES, XPS and Raman studies of poly(3,4-ethylenedioxythiophene) modified by iron hexacyanoferrate

    Synth. Met.

    (2010)
  • M. Szkoda et al.

    Facile preparation of extremely photoactive boron-doped TiO2 nanotubes arrays

    Electrochem. Commun.

    (2015)
  • K. Trzciński et al.

    Enhanced photoelectrochemical performance of inorganic–organic hybrid consisting of BiVO4 and PEDOT: PSS

    Appl. Surf. Sci.

    (2016)
  • T.P. Nguyen et al.

    An investigation into the effect of chemical and thermal treatments on the structural changes of poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate and consequences on its use on indium tin oxide substrates

    Appl. Surf. Sci.

    (2004)
  • J.L. Duvail et al.

    Transport and vibrational properties of poly(3,4-ethylenedioxythiophene) nanofibers

    Synth. Metall.

    (2002)
  • P.R. Somani et al.

    Electrochromic materials and devices: present and future

    Mater. Chem. Phys.

    (2003)
  • W.W. Chiu et al.

    Spectroscopic and conductivity studies of doping in chemically synthesized poly(3,4-ethylenedioxythiophene)

    Synth. Met.

    (2005)
  • C. Langlade et al.

    Characterization of titanium oxide films with Magnéli structure elaborated by a sol-gel route

    Appl. Surf. Sci.

    (2002)
  • K.M. Coakley et al.

    Conjugated polymer photovoltaic cells

    Chem. Mater.

    (2004)
  • R. Gangopadhyay et al.

    Conducting polymer nanocomposites: a brief overview

    Chem. Mater.

    (2000)
  • S. Garreau et al.

    In situ spectroelectrochemical Raman studies of poly (3, 4-ethylenedioxythiophene)(PEDT)

    Macromolecules

    (1999)
  • Y. Wang et al.

    Preparation, characterization and sensitive gas sensing of conductive core-sheath TiO2-PEDOT nanocables

    Sensors

    (2009)
  • K. Siuzdak et al.

    Highly stable organic–inorganic junction composed of hydrogenated titania nanotubes infiltrated by a conducting polymer

    RSC Adv.

    (2016)
  • S. Devaraj et al.

    Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties

    J. Phys. Chem. C

    (2008)
  • C. Janáky et al.

    Electrochemical grafting of poly(3,4-ethylenedioxythiophene) into a titanium dioxide nanotube host network

    Langmuir

    (2010)
  • V. Likodimos et al.

    Phase composition, size, orientation, and antenna effects of self-assembled anodized titania nanotube arrays; a polarized micro-Raman investigation

    J. Phys. Chem. C

    (2008)
  • View full text