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
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.
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