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Nanocomposites based on titanium dioxide and polythiophene: Structure and properties
Reactive & Functional Polymers 65 (2005) 69–77
REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react
Nanocomposites based on titanium dioxide and polythiophene: Structure and properties Quoc-Trung Vu a, Martin Pavlik b, Niels Hebestreit a, Ursula Rammelt a, Waldfried Plieth a, Jiri Pfleger b,* a
Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, Mommsenstrasse 13, D-01062, Germany b Institute of Macromolecular Chemistry, Academy of Science of the Czech Republic, Heyrovsky Sq. 2, CZ-16206 Prague 6, Czech Republic Received 1 September 2004; accepted 12 November 2004 Available online 20 July 2005
Abstract A composite of polythiophene (PTh) and nanoscopic titanium dioxide (TiO2), possessing core-shell structure, was prepared via oxidative polymerization of thiophene by iron (III) chloride in the presence of TiO2 particles. The morphology of the obtained composite particles was studied by transmission electron microscopy (TEM), proving the core-shell structure of the prepared nanocomposite. The chemical structure of the composites was investigated by Raman spectroscopy. The electrophoretic deposition (EPD) technique was used as a method to prepare composite layers on various conducting substrates (platinum and ITO plate). The characteristics of the composite layer after EPD were studied by electrochemical methods. The photoelectrical properties of PTh and TiO2 in the nanocomposites were studied as well. In the photoelectrochemical spectra (depending on the applied potential), the characteristic anodic peak of TiO2 at k = 340 nm and cathodic peak of PTh around k = 530 nm were observed. A bandgap energy, Eg = 1.95 eV and a direct electron transition of PTh were found. A cathodic peak in the photocurrent spectra was detected unexpectably also around 340 nm, and ascribed to the photoelectrical activity of the TiO2 core. The redox processes in PTh were investigated via cyclic voltammetry (CV) and electrochemical impedance microscopy (EIS). Ó 2005 Elsevier B.V. All rights reserved. Keywords: Polythiophene; Titanium dioxide; Nanocomposites; Electrophoretic deposition; Photoelectrochemical spectra
1. Introduction *
Corresponding author. Tel.: +420 222 511 696; fax: +420 222 516 969. E-mail address: pfl[email protected] (J. Pfleger).
After 20 years of maturation, the world of conjugated polymers and oligomers has become
1381-5148/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2004.11.011
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established as an important branch of materials science with many opportunities for applications in electronics and photonics. Among conducting polymers, various derivatives of polythiophene (PTh) have been investigated extensively because of their interesting semiconducting, electronic and optical properties, combined with processing advantages and good mechanical characteristics. Compared with many other p-conjugated polymers they show sufficient stability for practical applications. Depending on their doping level, the PThs behave either as metal-like conductors or semiconductors. When doped to metallic level, PThs become highly conducting and may find applications in batteries, electrochromic or smart windows, antistatic coatings and various types of sensors [1,2]. On the other hand, when in the semiconducting form they exhibit similar electrical and optical properties as inorganic semiconductors. High performance electrical or optoelectrical devices, such as light emitting diodes (LED), fieldeffect transistors (o-FET), photovoltaic cells, fabricated from conjugated polymers have been demonstrated [1,2]. Recently, the combination of semiconducting and mechanical properties of conjugated polymers with the properties of metals or semiconducting inorganic particles has brought new prospects for applications [3–7]. A number of different metallic and metal oxide particles have so far been encapsulated into the shell of conducting polymers giving rise to a host of nanocomposites [8–13]. In this paper, the morphology and chemical structure of the PTh/TiO2 core-shell nanocomposite particles, prepared via SugimotoÕs oxidative polymerization of thiophene in the presence of nanoscopic titanium dioxide, were studied by TEM and Raman spectroscopy. The properties of the nanocomposite layers were investigated using electrochemical and photoelectrochemical methods, and scanning electron microscopy as well. The electrophoretic deposition method [14] was adopted to provide thin nanocomposite layers of core-shell nanoparticles on conductive surfaces (metal or semitransparent ITO substrate) suitable for physical measurements.
2. Experimental 2.1. Preparation of composites Composites were prepared following the procedure described in [9–11]. As a core TiO2 powder (P25, Degussa) was used with average particle size 25 nm, surface area 50 m2/g. For the PTh preparation fresh distilled thiophene (Aldrich, 99+%) was used and water-free iron (III) chloride (Fluka) as an oxidant. An aliquot of 20 g of TiO2 (dried at 100 °C for 2 h before use) was dispersed together with 0.8 ml thiophene in 300 ml chloroform (Merck, max 0.03% water) and stirred for a few minutes. During stirring, the thiophene monomer was adsorbed on the surface of oxide particles. Meanwhile, the dispersion of 3.0 g FeCl3 in 50 ml CHCl3 was prepared and subsequently added to the dispersion of oxide in the monomer solution. The colour of the mixture changed from grey to black. After 3 h of stirring, the oxide particles covered by the oxidized PTh shell were filtered and dried. The composite was extracted with methanol for 100 h to remove the residual oxidant. During this procedure, the colour of the composite changed from black (PTh in oxidized state) to red (PTh in reduced state), indicating successful reduction of the polymer. The composite was dried at 60–80 °C for 2–3 h and kept in the dark. The amount of PTh (4.6%) in the composite was calculated from the data of the thermogravimetrical measurements (performed with a thermobalance, METTLER TG 50).
2.2. Material characterization The morphology of the obtained composite particles was studied by SEM and TEM. The SEM picture was measured by the Zeiss DSM 982 Gemini device and TEM picture was carried out with the Philips CM 200 device. The chemical structure of the composite was characterized by Raman spectra, recorded on the DILOR LabRAM 010 system equipped with 15 mW He–Ne and CCD detector.
Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77
The dark conductivity of the PTh/TiO2 nanocomposite, measured on the press-pellet samples at 300 K using the van der Pauw method with four Au electrodes vacuum deposited on the perimeter of the pellet, was found to be r = 4.107 Scm1. The electrophoretic deposition process was used to prepare the photoelectrically active nanocomposite layers. About 0.5 g of the composite material was first mixed with 100 ml of ethanol and the dispersion was homogenized by an ultrasonic treatment for a couple of minutes to form a stable suspension. The schematic arrangement of EDP was shown in Fig. 1. The EPD was carried out in a two-electrode cell using a DC power source (0–150 V, 3 A, Philips-PE 1527), at room temperature and in ambient atmosphere. The current was recorded with a multimeter (Keithley 175). The dispersion was stirred during the deposition. For the photocurrent measurements, the composite layer was deposited on ITO plate with an illuminated area of 0.5 cm2. Photocurrent measurements were recorded with a 1000 W Xenon lamp as a light source, a Zeiss grating monochromator and a lock-in amplifier (5208, EG&G Princeton Applied Research). The experiment was carried out in a three-electrode cell using a
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0.5 M LiClO4/CH3CN solution as an electrolyte. A platinum plate and a saturated calomel electrode (SCE) were used as a counter-electrode and a reference electrode, respectively. Cyclic voltammetry (CV) was performed using EG&G potentiostat model 264 A. Platinum plates were used as working and counter electrode. The saturated calomel electrode (SCE) was used as a reference electrode in the electrochemical experiment. The CV curves of the composite layers were measured in CH3CN (Fischer, 99.9+%) containing 0.5 M LiClO4 (Fluka, 98+%) with a scan rate of 10 mV/s in the interval 0–1.1VSCE, respectively. To measure the potential dependence of conductivity, the electrochemical impedance spectroscopy (EIS) was used. The nanocomposite layer deposited on platinum plate was used as a working electrode and the impedance was measured at different potentials after a waiting time of 30 min at each potential. The impedance spectra were recorded with the IMd5 system (Zahner–Elektrik). The impedance measurement was carried out in the 0.5 M LiClO4/CH3CN solution. The impedance of the composite layer was measured every 30 min in the potential range between 0 and 1VSCE in steps of 50 mV for the PTh/TiO2 composite layer which was deposited at 20 V for 2 min on platinum electrode.
- + A
3. Results and discussion 3.1. Morphological and chemical structure of composite
Fig. 1. Schematic cell arrangement for the electrophoretic deposition.
3.1.1. Morphology of composite particle Morphology of the obtained composite particles was investigated by TEM technique. In Fig. 2, the TEM micrograph of a single nanocomposite particle is presented. The clearly visible diffraction pattern represents the anatase crystalline modification of TiO2. The lighter non-patterned cloud on the left hand side of the oxide core belongs to the PTh shell, which is thus found to be several nanometres thick. The found structure relatively well corresponds to the theoretical calculations (taking complete monomer consumption during the polymerization and full coverage of
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Fig. 2. TEM-micrograph of TiO2 covered with polythiophene (reduced form).
all oxide particles as initial conditions), predicting 2–3 nm thick polymer shell. In reality, the coverage of the oxide is complicated by aggregation of the oxide particles before and during the polymerization, and the composite is also contaminated by granules of the pure polymer formed in the bulk of the solution. Nonetheless, the TEM observation gives the direct evidence that the covering of TiO2 via oxidative polymerization of thiophene is in principal possible.
1456
3.1.2. Chemical structure of composite The pure TiO2 absorbed weakly at around 700 cm1. Fig. 3 shows Raman spectra of the reduced and oxidized PTh/TiO2 composite. In oxidized state, peak at 1420 cm1 has been attributed to the quinoid units (radical cations). On the other hand, in reduced state this peak disappeared and a new peak at 1455 cm1 has arisen, attributed to the Ca = Cb ring stretching of the neutral PTh. This gave a clear evidence, that the reduction of the polymer from its oxidized form (positively charged polymer chain – see Fig. 4) to its reduced form (neutral chain in Fig. 4) proceeded via the extraction in methanol. The assigments of Raman bands of PTh in composite, listed in Table 1 (both oxidized state and reduced state), are in agreement with previous reports [15,16]. The bipolaron absorption, proposed to be at 1400 cm1 in the oxidized state [15] was not observed while the peak at 1420 cm1 was found, corresponding to Ca = Cb ring stretching of the quinoid units in the oxidized PTh chain, supporting that the oxidized PTh presents mainly in radical cation. This is in agreement with the results presented in [15] and the schematic representation for the state change of PTh (Fig. 4). By X-ray photoelectron spectroscopy (XPS) the doping level of the oxidized PTh in the analogous PTh/TiO2 composite (20%) was shown in [9]. The band of Cb–H bending (in oxidized state) appeared at 1049 cm1. This band showed a little shift to lower frequency [15]. The second band of C–S–C deformation (704 cm1) in the reduced
Intensity
1048
1153 1220
704
1495
S
1370
736
S 1420 1049 1148
688 731
S
1355
b
α S β
+e Cl(extraction)
1455
1223
β α
S
S
+
•
1495
a 800
1000
1200
1400
1600
S
1800
-1
Wavenumber / cm
Fig. 3. Raman spectra of PTh/TiO2 composite: (a) oxidized state and (b) reduced state.
S
S
S S
S
Fig. 4. Structure of the neutral and oxidized polythiophene during extraction.
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Table 1 Band assignment of the Raman spectra of polythiophene in the PTh/TiO2 composites Assigments
Reduced polythiophene
Oxidized polythiophene
Experimental
Literature
Experimental
Literature
Ca = Cb ring stretching (anti) Ca = Cb ring stretching Quinoid (radical cations) Cb–Cb 0 ring stretching Ca–Ca 0 stretching Cb–H bending Ca–Ca 0 stretching (anti) Ring deformation C–S–C
1495 1456 – 1370 1220 1048 1153 736 704
1495a 1499b 1455a 1458b – 1370a 1221a 1045a 1048b 1153a 736a 707b 693a
1495 1455 1420 1355 1223 1049 1148 731 688
1495a 1455a 1420a 1355a 1223a 1056a 1151a 729a 693a
a
Polythiophene was prepared by electropolymerization on stainless steel [15]. Polythiophene was prepared by chemical polymerization on silicon using 11-(3-thienyl) undecyltrichrosilan as adhesion promoter [16]. b
PTh was shifted positively (vs. 693 cm1 [15]), while that (688 cm1) in the oxidized state negatively (vs. 693 cm1 [15]). An additional small peak at 1463 cm1 (in the oxidized PTh) could not be attributed. It seams to belong to Cb–H bending in the quinoid units of the oxidized PTh chain. 3.2. Electrophoretic deposition Since the pressing technique was not suitable for the preparation of thin-layers for electrochemical and optical measurements, the electrophoretic deposition (EPD) technique was employed. Although, this technique allowed to fabricate thin-films only on conductive surfaces, for our
purposes, such a limitation was not critical, because the used conductive substrates neither complicated any measurements or even they were favourably used as working electrodes. In Fig. 5, the comparison of SEM micrographs of pressed layer and EPD layer is shown. It is perfectly readable that the pressed layer is denser with a few pores. On the other hand, the electrophoretically deposited layer possesses ‘‘foamy’’ porous structure (although the porosity, thus, the density is given by the voltage applied during the EPD). The main characteristics of such a porous layer are high surface area and high light scattering, which can be exploited e.g. in photovoltaic applications [17]. Nonetheless, as seen from below presented
Fig. 5. SEM-micrographs of PTh/TiO2 nanocomposite; (A) in a pressed pellet; (B) electrophoretically deposited layer.
Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77
physical measurements, the EPD can be easily used for successful deposition of otherwise unprocessable materials, yielding smooth thin layers with still preserved characteristic physical properties. 3.3. Photocurrent measurements The photocurrent spectra, acquired at various bias potentials applied, are shown in Fig. 6. The photocurrent spectra show a cathodic photocurrent maximum at the wavelength k = 530 nm corresponding with the optical absorption maximum [17] of the reduced PTh [17]. Anodic photocurrent peak obtained at 340 nm originates in the excitation of TiO2 core. It is close to the value 320 nm presented in [9]. An additional cathodic peak at 340 nm was observed at the negative voltage bias, analogous to the results shown in [9], where it was explained to be a new feature of the p/n interface between organic and inorganic semiconductor. To determine the origin of this peak, the dependence of the photocurrent on the composition of the material was studied: (i) PTh/TiO2 nanocomposite with lower amount of PTh (1.16%, prepared by reaction of 0.2 ml thiophene with 20 g TiO2) and (ii) pure TiO2. The layers were deposited on the ITO electrodes at the same condition above in order to get the same thickness of the PTh/
Photocurrent density /µAcm -2
4
- 0.3 V
0V + 0.3 V + 0.5 V
2
0
-2
-1
-2
-3
-4
PTh/TiO2 (4.6%) -5
PTh/TiO2 (1.16 %)
pure TiO2 -6 300
400
500
600
700
800
Wavelength /nm
Fig. 7. Photocurrent spectra of PTh/TiO2 composites and of pure TiO2 layers at 0.3 V.
TiO2 composite and pure TiO2 layer. The photocurrent spectra at 0.3 V (vs. SCE) of PTh/ TiO2(4.6%), PTh/TiO2(1.16%) and pure TiO2 layer are shown in Fig. 7. Obviously, the cathodic peak at 340 nm comes from TiO2. It is observed at negative electrode potentials (0.3VSCE) in the vicinity of the flatband potential of TiO2. Therefore it can be explained by the reversal of the current at the flatband potential. This might be connected with beginning reduction of the TiO2 layer and increased conductivity of the TiO2. When a p-semiconductor is immersed into an electrolyte, the band bending appears at the semiconductor surface. If this surface is illuminated by light with energy hv > Eg (bandgap energy), the photogenerated holes move to the bulk of the semiconductor whereas the photoelectrons towards the surface. Subsequently, a reduction occurs at the semiconductor surface that is observed as a cathodic photocurrent. This photocurrent was described by Go¨rtner and Butler [18] with the following equation: iph ¼ ½eU0 ð1 expðaW 0 ðE EFB Þ1=2 Þ=ð1 þ aLp Þ;
-4
-6 300
0
Photocurrent density /µAcm -2
74
ð1Þ 400
500
600
700
800
Wavelength /nm
Fig. 6. Photocurrent spectra of PTh/TiO2 (4.6%) composite layer electrophoretically deposited on ITO (20 V, 2 min).
where iph is the photocurrent density, a is the monochromatic absorption coefficient, W0 is the thickness of the depletion layer, U0 is the total photon flux, E is the applied voltage, EFB is the flatband potential and Lp is the diffusion length
Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77
3500 3000
-1
2500
-2
2000
(iphhν)2
Photocurrent density /µAcm -2
0
75
-3
1500 1000
-4 500
PTh/TiO2 (4.6%) PTh/TiO2 (1.16%)
-5
0 -500
-6 -0.4
1.8
-0.2
0.0
0.2
0.4
2.0
0.6
2.2
2.4
hν/eV
Applied potential / V SCE
Fig. 9. (Iphhv)2 vs. hv plot obtained from the action spectra for PTh/TiO2 (4.6%) composite.
Fig. 8. Photocurrent-potential dependence.
of the minority carriers. For Lp 1 and aW0(E EFB)1/2 1 the following approximation is obtained: 2
i2ph ¼ ðeU0 aW 0 Þ ðE EFB Þ;
ð2Þ
From Eq. (2), the flatband potential, EFB, can be obtained when plotting the photocurrent as a function of applied voltage, E as shown in Fig. 8 for 530 nm excitation wavelength. The values EFB = 0.6 VSCE and EFB = 0.55 VSCE were found for PTh/TiO2 composites with PTh content 4.6% and 1.16%, respectively. Obviously, photocurrent was found to be directionally proportional to the PTh content. The action spectrum measured at a fixed potential is a useful tool to determine the bandgap value of a semiconductor and the nature of the electronic transitions that lead to the photocurrent. The dependence of the absorption coefficient a on the excitation energy can be expressed as [19,20]: a ¼ Aðhv Eg Þ
1=n
=hv;
ð3Þ
where A is a constant and n depends on the nature of the optical transition: n = 2 for direct transition and n = 0.5 for indirect transition, respectively. The bandgap energy, Eg can be determined through the linear (iphhv)2 or (iphhv)1/2 vs. hv plots [16,20]. Fig. 9 shows that a good linearity is observed for plot of (iphhv)2 vs. hv. The photocurrent spectrum was measured with PTh/TiO2 (4.6%)
composite at 0.5 VSCE. Obviously, neither fit is fully satisfactory, however, the direct electron transition predominates because the (iphhv)2 plot yields better fit to the experimental data. The curve was extrapolated to the x-axis to determine the bandgap energy. The value Eg = 1.95 eV is coincident with that obtained from the absorption edge in the optical spectra [17] and shows a good agreement with the values for polymethythiophene (Eg = 1.9 eV, [19]) and polybithiophene (Eg = 2.03 eV, [16]). The deflection of the theoretical curve from the experimental points can be easily understood if we consider that certain distribution of the conjugation length and defects are always present in the polymer affecting its energy structure. 3.4. Cyclic voltammetry Fig. 10 shows the CV curve of the PTh/TiO2 nanocomposite layer deposited on platinum at 100 V. The observed oxidation potential of PTh (+0.8 vs. SCE) is in a good agreement with values published in the literatures (+0.76 VSCE) [9,10]. The anodic peak current-density decreased from 0.72 lA cm2 in the first cycle to 0.7 lA cm2) in the 10th cycle. The reduction potential of PTh was also determined to be +0.75 V vs. SCE. The TiO2 core showed no electrochemical activity. The onset oxidation potential of PTh at about
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Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77 1.0
st
1 cycle
10k
0.6
th
10 cycle 0.4 0.4 V SCE
Impedance/
Current density / mAcm -2
0.8
0.2 0.0 -0.2
1k
-0.4 -0.6 0.0
0.2
0.4
0.6
0.8
1.0
100 100m
Potential / VSCE
1.0 V SCE
1
10
A Fig. 10. Cyclovoltammogram of PTh/TiO2 (4.6%) composite layer prepared by EPD on platinum (E = 100 V, 3 min).
Fig. 11 shows the electrochemical impedance spectra (EIS) of the PTh/TiO2 layer deposited on platinum at 20 V. The analysis of the impedance spectra of the PTh/TiO2 pellet electrodes were presented in [9]. The resistance of the TiO2 core was observed at low frequencies (<1 Hz) and the impedance of the PTh shell at higher frequencies (1–1000 Hz) (Fig. 11). In the potential range of 0 to 0.55VSCE, the resistance of PTh/TiO2 system did not change, indicating that in this potential regime PTh was present in the reduced state. From 0.6 to 0.9VSCE, the PTh resistance reduced, supporting that PTh in the composite changed from reduced semiconducting form to oxidized conducting one. Consequently, the impedance of the whole composite layer decreased. It is in a good agreement with the results from CV (Fig. 10) and flatband potential measurements (Fig. 8). It can be shown that in this potential range, TiO2 presents the semiconducting properties so that it is not influenced by the impedance of system [9]. In the highest potential range (0.9–1.0VSCE) the impedance of the PTh/TiO2 system kept at constant value (Fig. 11). It means that the PTh shell remained in the highest doped state and its resistance did not
1k
10k
100k
1k
10k
100k
-90
-75
+ 0.6VSCE was also observed. It was in a good agreement with the EFB value determined by photocurrent measurements.
0.4 V SCE
-60 Phase/
3.5. Electrochemical impedance microscopy
100
Frequency / Hz
-45
-30
-15 1.0 V SCE
0 100m
B
1
10
100
Frequency / Hz
Fig. 11. Bode plot of the impedance measurements of the PTh/ TiO2 (4.6%) composite layer prepared by EPD on platinum (20 V, 2 min) in the potential region of 0.4–1.0 VSCE; (A) – impedance and (B) – phase shift.
change. The equivalent circuit and Mott–Schottky relation presented in [9] were used for analysing the impedance spectra. Unfortunately, the linear Mott–Schottky plot was not found. Therefore, the charge carrier density (ND) and EFB of PTh could not be determined via EIS measurements. It is similar to the results obtained from the EIS measurements on the PTh/TiO2 composite pellets [9]. It can be explained if we consider that although the whole impedance of the PTh/TiO2 system decreased (with increasing applied voltage), no change in the capacitance of PTh was observed because the amount of PTh was very low. (see Fig. 11(A) and (B)).
Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77
4. Conclusion The PTh/TiO2 nanocomposites composed of nanoparticles with TiO2 core and a thin PTh shell (2–3 nm) were prepared by the chemical polymerization of thiophene in the presence of TiO2 nanoparticles. By means of transmission electron microscopy, the expected core-shell structure of the prepared nanocomposite was confirmed. The electrophoretic deposition was found to be a feasible method for the preparation of thin nanocomposite layers on various conductive substrates (Pt, ITO), with higly porous structure. Such prepared thin layers were applicable for the investigation of electrochemical and photoelectrochemical properties of the nanocomposites. The cyclic voltammetry confirmed that the PTh is still electrochemically active after the electrophoretic deposition, showing the oxidation potential of the PTh at +0.8VSCE. The oxidation process of PTh (from +0.6VSCE to +0.9VSCE) was observed via electrochemical impedance microscopy measurements as well. The acquisition of the photocurrent spectra was performed at several bias potentials, discovering the separate photoelectrochemical signals from titanium dioxide and from polythiophene (each compound represented different type of semiconductor), and showing that the both components of the composite are as well photoelectrically active after electrophoretic deposition. A direct transition (with bandgap energy 1.95 eV) and a flatband potential (0.6VSCE) were obtained via photocurrent measurements. An additional cathodic peak observed in photocurrent spectra at 340 nm was explored to be originating in a changed surface state of the titanium dioxide.
Acknowledgements We would like to thank Mrs. Kern (Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology) and Prof. LichteÕs group (Triebenberg Lab, Institute of Structure Physics, Dresden University of Technology) for
77
the SEM- and TEM-pictures, respectively. We are especially grateful to Mr. Anders for Raman spectrum measurements. This work is financially supported by the Deutsche Forschung Germeinshaft (DFG) and the European Graduate School ‘‘Advanced Polymeric Materials’’ (IGK 720) and partially by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. IAA4050406).
References [1] P. Chandrasekhar, Conducting Polymer, Fundamentals and Applications, Kluwer Academic Publishers, Boston, Dordrecht, London, 1999. [2] Dais Fichou, Handbook of Oligo – and Polythiophene, Wiley-VCH, Weinheim, New York, Chichester, Brisbane, Singapore, Toronto, 1999. [3] Pedro Gomez-Romero, Adv. Mater. 13 (2001) 163. [4] Phuong Tuyen Nguyen, Ursula Rammelt, Waldfried Plieth, Macromol. Symp. 187 (2002) 929. [5] H. Nguyen Cong, M. Dieng, C. Sene, P. Chartier, Solar Energy Mater. Solar Cells 63 (2000) 23. [6] B. Garcia, A. Lamzoudi, F. Pillier, H. Nguyen Thi Le, C. Deslouis, J. Electrochem. Soc. 149 (2002) 560. [7] S. Tagmouti, A. Outzourhit, A. Oueriagli, M. Khaidar, M. Elyacoubi, R. Evrard, E.L. Ameziane, Thin Solid Films 379 (2000) 272. [8] Xingwei Li, Wei Chen, Chaoqing Bian, Jinbo He, Ning Xu, Gi Xue, Appl. Surf. Sci. 217 (2003) 16. [9] N. Hebestreit, J. Hofmann, U. Rammelt, W. Plieth, Electrochim. Acta 48 (2003) 1779. [10] J. Hofmann, Diplomarbeit, TU Dresden, 2000. [11] W. Plieth, N. Hebestreit, German Patent Application No. 19919261.8. [12] Rupali Gangopadhyay, Amitabha De, Chem. Mater. 12 (2000) 608. [13] S. Maeda, S.P. Armes, Synth. Metals 73 (1995) 151. [14] A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci. 6 (2002) 251. [15] Gaoquan Shi, Jingkun Xu, Mingxiao Fu, J. Phys. Chem. B 106 (2002) 288. [16] Axel Fikus, PhD thesis, TU Dresden, 1999. [17] J. Pfleger, M. Pavlik, N. Hebestreit, W. Plieth, Macromol. Symp. 212 (1) (2004) 539. [18] W. Zhang, P. Schmidt-Zhang, G. Kossmehl, W. Plieth, J. Solid State Electrochem. 3 (1999) 135. [19] L. Micaroni, C.N. Polo da Fonseca, F. Decker, M.-A. De Paoli, Solar Energy Mater. Solar Cells 60 (2000) 27. [20] M. Martini, M.-A. De Paoli, Solar Energy Mater. Solar Cells 60 (2000) 73.
REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react
Nanocomposites based on titanium dioxide and polythiophene: Structure and properties Quoc-Trung Vu a, Martin Pavlik b, Niels Hebestreit a, Ursula Rammelt a, Waldfried Plieth a, Jiri Pfleger b,* a
Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, Mommsenstrasse 13, D-01062, Germany b Institute of Macromolecular Chemistry, Academy of Science of the Czech Republic, Heyrovsky Sq. 2, CZ-16206 Prague 6, Czech Republic Received 1 September 2004; accepted 12 November 2004 Available online 20 July 2005
Abstract A composite of polythiophene (PTh) and nanoscopic titanium dioxide (TiO2), possessing core-shell structure, was prepared via oxidative polymerization of thiophene by iron (III) chloride in the presence of TiO2 particles. The morphology of the obtained composite particles was studied by transmission electron microscopy (TEM), proving the core-shell structure of the prepared nanocomposite. The chemical structure of the composites was investigated by Raman spectroscopy. The electrophoretic deposition (EPD) technique was used as a method to prepare composite layers on various conducting substrates (platinum and ITO plate). The characteristics of the composite layer after EPD were studied by electrochemical methods. The photoelectrical properties of PTh and TiO2 in the nanocomposites were studied as well. In the photoelectrochemical spectra (depending on the applied potential), the characteristic anodic peak of TiO2 at k = 340 nm and cathodic peak of PTh around k = 530 nm were observed. A bandgap energy, Eg = 1.95 eV and a direct electron transition of PTh were found. A cathodic peak in the photocurrent spectra was detected unexpectably also around 340 nm, and ascribed to the photoelectrical activity of the TiO2 core. The redox processes in PTh were investigated via cyclic voltammetry (CV) and electrochemical impedance microscopy (EIS). Ó 2005 Elsevier B.V. All rights reserved. Keywords: Polythiophene; Titanium dioxide; Nanocomposites; Electrophoretic deposition; Photoelectrochemical spectra
1. Introduction *
Corresponding author. Tel.: +420 222 511 696; fax: +420 222 516 969. E-mail address: pfl[email protected] (J. Pfleger).
After 20 years of maturation, the world of conjugated polymers and oligomers has become
1381-5148/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2004.11.011
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established as an important branch of materials science with many opportunities for applications in electronics and photonics. Among conducting polymers, various derivatives of polythiophene (PTh) have been investigated extensively because of their interesting semiconducting, electronic and optical properties, combined with processing advantages and good mechanical characteristics. Compared with many other p-conjugated polymers they show sufficient stability for practical applications. Depending on their doping level, the PThs behave either as metal-like conductors or semiconductors. When doped to metallic level, PThs become highly conducting and may find applications in batteries, electrochromic or smart windows, antistatic coatings and various types of sensors [1,2]. On the other hand, when in the semiconducting form they exhibit similar electrical and optical properties as inorganic semiconductors. High performance electrical or optoelectrical devices, such as light emitting diodes (LED), fieldeffect transistors (o-FET), photovoltaic cells, fabricated from conjugated polymers have been demonstrated [1,2]. Recently, the combination of semiconducting and mechanical properties of conjugated polymers with the properties of metals or semiconducting inorganic particles has brought new prospects for applications [3–7]. A number of different metallic and metal oxide particles have so far been encapsulated into the shell of conducting polymers giving rise to a host of nanocomposites [8–13]. In this paper, the morphology and chemical structure of the PTh/TiO2 core-shell nanocomposite particles, prepared via SugimotoÕs oxidative polymerization of thiophene in the presence of nanoscopic titanium dioxide, were studied by TEM and Raman spectroscopy. The properties of the nanocomposite layers were investigated using electrochemical and photoelectrochemical methods, and scanning electron microscopy as well. The electrophoretic deposition method [14] was adopted to provide thin nanocomposite layers of core-shell nanoparticles on conductive surfaces (metal or semitransparent ITO substrate) suitable for physical measurements.
2. Experimental 2.1. Preparation of composites Composites were prepared following the procedure described in [9–11]. As a core TiO2 powder (P25, Degussa) was used with average particle size 25 nm, surface area 50 m2/g. For the PTh preparation fresh distilled thiophene (Aldrich, 99+%) was used and water-free iron (III) chloride (Fluka) as an oxidant. An aliquot of 20 g of TiO2 (dried at 100 °C for 2 h before use) was dispersed together with 0.8 ml thiophene in 300 ml chloroform (Merck, max 0.03% water) and stirred for a few minutes. During stirring, the thiophene monomer was adsorbed on the surface of oxide particles. Meanwhile, the dispersion of 3.0 g FeCl3 in 50 ml CHCl3 was prepared and subsequently added to the dispersion of oxide in the monomer solution. The colour of the mixture changed from grey to black. After 3 h of stirring, the oxide particles covered by the oxidized PTh shell were filtered and dried. The composite was extracted with methanol for 100 h to remove the residual oxidant. During this procedure, the colour of the composite changed from black (PTh in oxidized state) to red (PTh in reduced state), indicating successful reduction of the polymer. The composite was dried at 60–80 °C for 2–3 h and kept in the dark. The amount of PTh (4.6%) in the composite was calculated from the data of the thermogravimetrical measurements (performed with a thermobalance, METTLER TG 50).
2.2. Material characterization The morphology of the obtained composite particles was studied by SEM and TEM. The SEM picture was measured by the Zeiss DSM 982 Gemini device and TEM picture was carried out with the Philips CM 200 device. The chemical structure of the composite was characterized by Raman spectra, recorded on the DILOR LabRAM 010 system equipped with 15 mW He–Ne and CCD detector.
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The dark conductivity of the PTh/TiO2 nanocomposite, measured on the press-pellet samples at 300 K using the van der Pauw method with four Au electrodes vacuum deposited on the perimeter of the pellet, was found to be r = 4.107 Scm1. The electrophoretic deposition process was used to prepare the photoelectrically active nanocomposite layers. About 0.5 g of the composite material was first mixed with 100 ml of ethanol and the dispersion was homogenized by an ultrasonic treatment for a couple of minutes to form a stable suspension. The schematic arrangement of EDP was shown in Fig. 1. The EPD was carried out in a two-electrode cell using a DC power source (0–150 V, 3 A, Philips-PE 1527), at room temperature and in ambient atmosphere. The current was recorded with a multimeter (Keithley 175). The dispersion was stirred during the deposition. For the photocurrent measurements, the composite layer was deposited on ITO plate with an illuminated area of 0.5 cm2. Photocurrent measurements were recorded with a 1000 W Xenon lamp as a light source, a Zeiss grating monochromator and a lock-in amplifier (5208, EG&G Princeton Applied Research). The experiment was carried out in a three-electrode cell using a
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0.5 M LiClO4/CH3CN solution as an electrolyte. A platinum plate and a saturated calomel electrode (SCE) were used as a counter-electrode and a reference electrode, respectively. Cyclic voltammetry (CV) was performed using EG&G potentiostat model 264 A. Platinum plates were used as working and counter electrode. The saturated calomel electrode (SCE) was used as a reference electrode in the electrochemical experiment. The CV curves of the composite layers were measured in CH3CN (Fischer, 99.9+%) containing 0.5 M LiClO4 (Fluka, 98+%) with a scan rate of 10 mV/s in the interval 0–1.1VSCE, respectively. To measure the potential dependence of conductivity, the electrochemical impedance spectroscopy (EIS) was used. The nanocomposite layer deposited on platinum plate was used as a working electrode and the impedance was measured at different potentials after a waiting time of 30 min at each potential. The impedance spectra were recorded with the IMd5 system (Zahner–Elektrik). The impedance measurement was carried out in the 0.5 M LiClO4/CH3CN solution. The impedance of the composite layer was measured every 30 min in the potential range between 0 and 1VSCE in steps of 50 mV for the PTh/TiO2 composite layer which was deposited at 20 V for 2 min on platinum electrode.
- + A
3. Results and discussion 3.1. Morphological and chemical structure of composite
Fig. 1. Schematic cell arrangement for the electrophoretic deposition.
3.1.1. Morphology of composite particle Morphology of the obtained composite particles was investigated by TEM technique. In Fig. 2, the TEM micrograph of a single nanocomposite particle is presented. The clearly visible diffraction pattern represents the anatase crystalline modification of TiO2. The lighter non-patterned cloud on the left hand side of the oxide core belongs to the PTh shell, which is thus found to be several nanometres thick. The found structure relatively well corresponds to the theoretical calculations (taking complete monomer consumption during the polymerization and full coverage of
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Fig. 2. TEM-micrograph of TiO2 covered with polythiophene (reduced form).
all oxide particles as initial conditions), predicting 2–3 nm thick polymer shell. In reality, the coverage of the oxide is complicated by aggregation of the oxide particles before and during the polymerization, and the composite is also contaminated by granules of the pure polymer formed in the bulk of the solution. Nonetheless, the TEM observation gives the direct evidence that the covering of TiO2 via oxidative polymerization of thiophene is in principal possible.
1456
3.1.2. Chemical structure of composite The pure TiO2 absorbed weakly at around 700 cm1. Fig. 3 shows Raman spectra of the reduced and oxidized PTh/TiO2 composite. In oxidized state, peak at 1420 cm1 has been attributed to the quinoid units (radical cations). On the other hand, in reduced state this peak disappeared and a new peak at 1455 cm1 has arisen, attributed to the Ca = Cb ring stretching of the neutral PTh. This gave a clear evidence, that the reduction of the polymer from its oxidized form (positively charged polymer chain – see Fig. 4) to its reduced form (neutral chain in Fig. 4) proceeded via the extraction in methanol. The assigments of Raman bands of PTh in composite, listed in Table 1 (both oxidized state and reduced state), are in agreement with previous reports [15,16]. The bipolaron absorption, proposed to be at 1400 cm1 in the oxidized state [15] was not observed while the peak at 1420 cm1 was found, corresponding to Ca = Cb ring stretching of the quinoid units in the oxidized PTh chain, supporting that the oxidized PTh presents mainly in radical cation. This is in agreement with the results presented in [15] and the schematic representation for the state change of PTh (Fig. 4). By X-ray photoelectron spectroscopy (XPS) the doping level of the oxidized PTh in the analogous PTh/TiO2 composite (20%) was shown in [9]. The band of Cb–H bending (in oxidized state) appeared at 1049 cm1. This band showed a little shift to lower frequency [15]. The second band of C–S–C deformation (704 cm1) in the reduced
Intensity
1048
1153 1220
704
1495
S
1370
736
S 1420 1049 1148
688 731
S
1355
b
α S β
+e Cl(extraction)
1455
1223
β α
S
S
+
•
1495
a 800
1000
1200
1400
1600
S
1800
-1
Wavenumber / cm
Fig. 3. Raman spectra of PTh/TiO2 composite: (a) oxidized state and (b) reduced state.
S
S
S S
S
Fig. 4. Structure of the neutral and oxidized polythiophene during extraction.
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Table 1 Band assignment of the Raman spectra of polythiophene in the PTh/TiO2 composites Assigments
Reduced polythiophene
Oxidized polythiophene
Experimental
Literature
Experimental
Literature
Ca = Cb ring stretching (anti) Ca = Cb ring stretching Quinoid (radical cations) Cb–Cb 0 ring stretching Ca–Ca 0 stretching Cb–H bending Ca–Ca 0 stretching (anti) Ring deformation C–S–C
1495 1456 – 1370 1220 1048 1153 736 704
1495a 1499b 1455a 1458b – 1370a 1221a 1045a 1048b 1153a 736a 707b 693a
1495 1455 1420 1355 1223 1049 1148 731 688
1495a 1455a 1420a 1355a 1223a 1056a 1151a 729a 693a
a
Polythiophene was prepared by electropolymerization on stainless steel [15]. Polythiophene was prepared by chemical polymerization on silicon using 11-(3-thienyl) undecyltrichrosilan as adhesion promoter [16]. b
PTh was shifted positively (vs. 693 cm1 [15]), while that (688 cm1) in the oxidized state negatively (vs. 693 cm1 [15]). An additional small peak at 1463 cm1 (in the oxidized PTh) could not be attributed. It seams to belong to Cb–H bending in the quinoid units of the oxidized PTh chain. 3.2. Electrophoretic deposition Since the pressing technique was not suitable for the preparation of thin-layers for electrochemical and optical measurements, the electrophoretic deposition (EPD) technique was employed. Although, this technique allowed to fabricate thin-films only on conductive surfaces, for our
purposes, such a limitation was not critical, because the used conductive substrates neither complicated any measurements or even they were favourably used as working electrodes. In Fig. 5, the comparison of SEM micrographs of pressed layer and EPD layer is shown. It is perfectly readable that the pressed layer is denser with a few pores. On the other hand, the electrophoretically deposited layer possesses ‘‘foamy’’ porous structure (although the porosity, thus, the density is given by the voltage applied during the EPD). The main characteristics of such a porous layer are high surface area and high light scattering, which can be exploited e.g. in photovoltaic applications [17]. Nonetheless, as seen from below presented
Fig. 5. SEM-micrographs of PTh/TiO2 nanocomposite; (A) in a pressed pellet; (B) electrophoretically deposited layer.
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physical measurements, the EPD can be easily used for successful deposition of otherwise unprocessable materials, yielding smooth thin layers with still preserved characteristic physical properties. 3.3. Photocurrent measurements The photocurrent spectra, acquired at various bias potentials applied, are shown in Fig. 6. The photocurrent spectra show a cathodic photocurrent maximum at the wavelength k = 530 nm corresponding with the optical absorption maximum [17] of the reduced PTh [17]. Anodic photocurrent peak obtained at 340 nm originates in the excitation of TiO2 core. It is close to the value 320 nm presented in [9]. An additional cathodic peak at 340 nm was observed at the negative voltage bias, analogous to the results shown in [9], where it was explained to be a new feature of the p/n interface between organic and inorganic semiconductor. To determine the origin of this peak, the dependence of the photocurrent on the composition of the material was studied: (i) PTh/TiO2 nanocomposite with lower amount of PTh (1.16%, prepared by reaction of 0.2 ml thiophene with 20 g TiO2) and (ii) pure TiO2. The layers were deposited on the ITO electrodes at the same condition above in order to get the same thickness of the PTh/
Photocurrent density /µAcm -2
4
- 0.3 V
0V + 0.3 V + 0.5 V
2
0
-2
-1
-2
-3
-4
PTh/TiO2 (4.6%) -5
PTh/TiO2 (1.16 %)
pure TiO2 -6 300
400
500
600
700
800
Wavelength /nm
Fig. 7. Photocurrent spectra of PTh/TiO2 composites and of pure TiO2 layers at 0.3 V.
TiO2 composite and pure TiO2 layer. The photocurrent spectra at 0.3 V (vs. SCE) of PTh/ TiO2(4.6%), PTh/TiO2(1.16%) and pure TiO2 layer are shown in Fig. 7. Obviously, the cathodic peak at 340 nm comes from TiO2. It is observed at negative electrode potentials (0.3VSCE) in the vicinity of the flatband potential of TiO2. Therefore it can be explained by the reversal of the current at the flatband potential. This might be connected with beginning reduction of the TiO2 layer and increased conductivity of the TiO2. When a p-semiconductor is immersed into an electrolyte, the band bending appears at the semiconductor surface. If this surface is illuminated by light with energy hv > Eg (bandgap energy), the photogenerated holes move to the bulk of the semiconductor whereas the photoelectrons towards the surface. Subsequently, a reduction occurs at the semiconductor surface that is observed as a cathodic photocurrent. This photocurrent was described by Go¨rtner and Butler [18] with the following equation: iph ¼ ½eU0 ð1 expðaW 0 ðE EFB Þ1=2 Þ=ð1 þ aLp Þ;
-4
-6 300
0
Photocurrent density /µAcm -2
74
ð1Þ 400
500
600
700
800
Wavelength /nm
Fig. 6. Photocurrent spectra of PTh/TiO2 (4.6%) composite layer electrophoretically deposited on ITO (20 V, 2 min).
where iph is the photocurrent density, a is the monochromatic absorption coefficient, W0 is the thickness of the depletion layer, U0 is the total photon flux, E is the applied voltage, EFB is the flatband potential and Lp is the diffusion length
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3500 3000
-1
2500
-2
2000
(iphhν)2
Photocurrent density /µAcm -2
0
75
-3
1500 1000
-4 500
PTh/TiO2 (4.6%) PTh/TiO2 (1.16%)
-5
0 -500
-6 -0.4
1.8
-0.2
0.0
0.2
0.4
2.0
0.6
2.2
2.4
hν/eV
Applied potential / V SCE
Fig. 9. (Iphhv)2 vs. hv plot obtained from the action spectra for PTh/TiO2 (4.6%) composite.
Fig. 8. Photocurrent-potential dependence.
of the minority carriers. For Lp 1 and aW0(E EFB)1/2 1 the following approximation is obtained: 2
i2ph ¼ ðeU0 aW 0 Þ ðE EFB Þ;
ð2Þ
From Eq. (2), the flatband potential, EFB, can be obtained when plotting the photocurrent as a function of applied voltage, E as shown in Fig. 8 for 530 nm excitation wavelength. The values EFB = 0.6 VSCE and EFB = 0.55 VSCE were found for PTh/TiO2 composites with PTh content 4.6% and 1.16%, respectively. Obviously, photocurrent was found to be directionally proportional to the PTh content. The action spectrum measured at a fixed potential is a useful tool to determine the bandgap value of a semiconductor and the nature of the electronic transitions that lead to the photocurrent. The dependence of the absorption coefficient a on the excitation energy can be expressed as [19,20]: a ¼ Aðhv Eg Þ
1=n
=hv;
ð3Þ
where A is a constant and n depends on the nature of the optical transition: n = 2 for direct transition and n = 0.5 for indirect transition, respectively. The bandgap energy, Eg can be determined through the linear (iphhv)2 or (iphhv)1/2 vs. hv plots [16,20]. Fig. 9 shows that a good linearity is observed for plot of (iphhv)2 vs. hv. The photocurrent spectrum was measured with PTh/TiO2 (4.6%)
composite at 0.5 VSCE. Obviously, neither fit is fully satisfactory, however, the direct electron transition predominates because the (iphhv)2 plot yields better fit to the experimental data. The curve was extrapolated to the x-axis to determine the bandgap energy. The value Eg = 1.95 eV is coincident with that obtained from the absorption edge in the optical spectra [17] and shows a good agreement with the values for polymethythiophene (Eg = 1.9 eV, [19]) and polybithiophene (Eg = 2.03 eV, [16]). The deflection of the theoretical curve from the experimental points can be easily understood if we consider that certain distribution of the conjugation length and defects are always present in the polymer affecting its energy structure. 3.4. Cyclic voltammetry Fig. 10 shows the CV curve of the PTh/TiO2 nanocomposite layer deposited on platinum at 100 V. The observed oxidation potential of PTh (+0.8 vs. SCE) is in a good agreement with values published in the literatures (+0.76 VSCE) [9,10]. The anodic peak current-density decreased from 0.72 lA cm2 in the first cycle to 0.7 lA cm2) in the 10th cycle. The reduction potential of PTh was also determined to be +0.75 V vs. SCE. The TiO2 core showed no electrochemical activity. The onset oxidation potential of PTh at about
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Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77 1.0
st
1 cycle
10k
0.6
th
10 cycle 0.4 0.4 V SCE
Impedance/
Current density / mAcm -2
0.8
0.2 0.0 -0.2
1k
-0.4 -0.6 0.0
0.2
0.4
0.6
0.8
1.0
100 100m
Potential / VSCE
1.0 V SCE
1
10
A Fig. 10. Cyclovoltammogram of PTh/TiO2 (4.6%) composite layer prepared by EPD on platinum (E = 100 V, 3 min).
Fig. 11 shows the electrochemical impedance spectra (EIS) of the PTh/TiO2 layer deposited on platinum at 20 V. The analysis of the impedance spectra of the PTh/TiO2 pellet electrodes were presented in [9]. The resistance of the TiO2 core was observed at low frequencies (<1 Hz) and the impedance of the PTh shell at higher frequencies (1–1000 Hz) (Fig. 11). In the potential range of 0 to 0.55VSCE, the resistance of PTh/TiO2 system did not change, indicating that in this potential regime PTh was present in the reduced state. From 0.6 to 0.9VSCE, the PTh resistance reduced, supporting that PTh in the composite changed from reduced semiconducting form to oxidized conducting one. Consequently, the impedance of the whole composite layer decreased. It is in a good agreement with the results from CV (Fig. 10) and flatband potential measurements (Fig. 8). It can be shown that in this potential range, TiO2 presents the semiconducting properties so that it is not influenced by the impedance of system [9]. In the highest potential range (0.9–1.0VSCE) the impedance of the PTh/TiO2 system kept at constant value (Fig. 11). It means that the PTh shell remained in the highest doped state and its resistance did not
1k
10k
100k
1k
10k
100k
-90
-75
+ 0.6VSCE was also observed. It was in a good agreement with the EFB value determined by photocurrent measurements.
0.4 V SCE
-60 Phase/
3.5. Electrochemical impedance microscopy
100
Frequency / Hz
-45
-30
-15 1.0 V SCE
0 100m
B
1
10
100
Frequency / Hz
Fig. 11. Bode plot of the impedance measurements of the PTh/ TiO2 (4.6%) composite layer prepared by EPD on platinum (20 V, 2 min) in the potential region of 0.4–1.0 VSCE; (A) – impedance and (B) – phase shift.
change. The equivalent circuit and Mott–Schottky relation presented in [9] were used for analysing the impedance spectra. Unfortunately, the linear Mott–Schottky plot was not found. Therefore, the charge carrier density (ND) and EFB of PTh could not be determined via EIS measurements. It is similar to the results obtained from the EIS measurements on the PTh/TiO2 composite pellets [9]. It can be explained if we consider that although the whole impedance of the PTh/TiO2 system decreased (with increasing applied voltage), no change in the capacitance of PTh was observed because the amount of PTh was very low. (see Fig. 11(A) and (B)).
Q.-T. Vu et al. / Reactive & Functional Polymers 65 (2005) 69–77
4. Conclusion The PTh/TiO2 nanocomposites composed of nanoparticles with TiO2 core and a thin PTh shell (2–3 nm) were prepared by the chemical polymerization of thiophene in the presence of TiO2 nanoparticles. By means of transmission electron microscopy, the expected core-shell structure of the prepared nanocomposite was confirmed. The electrophoretic deposition was found to be a feasible method for the preparation of thin nanocomposite layers on various conductive substrates (Pt, ITO), with higly porous structure. Such prepared thin layers were applicable for the investigation of electrochemical and photoelectrochemical properties of the nanocomposites. The cyclic voltammetry confirmed that the PTh is still electrochemically active after the electrophoretic deposition, showing the oxidation potential of the PTh at +0.8VSCE. The oxidation process of PTh (from +0.6VSCE to +0.9VSCE) was observed via electrochemical impedance microscopy measurements as well. The acquisition of the photocurrent spectra was performed at several bias potentials, discovering the separate photoelectrochemical signals from titanium dioxide and from polythiophene (each compound represented different type of semiconductor), and showing that the both components of the composite are as well photoelectrically active after electrophoretic deposition. A direct transition (with bandgap energy 1.95 eV) and a flatband potential (0.6VSCE) were obtained via photocurrent measurements. An additional cathodic peak observed in photocurrent spectra at 340 nm was explored to be originating in a changed surface state of the titanium dioxide.
Acknowledgements We would like to thank Mrs. Kern (Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology) and Prof. LichteÕs group (Triebenberg Lab, Institute of Structure Physics, Dresden University of Technology) for
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the SEM- and TEM-pictures, respectively. We are especially grateful to Mr. Anders for Raman spectrum measurements. This work is financially supported by the Deutsche Forschung Germeinshaft (DFG) and the European Graduate School ‘‘Advanced Polymeric Materials’’ (IGK 720) and partially by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. IAA4050406).
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