- Email: [email protected]
polyaniline nanocomposite
Journal of Colloid and Interface Science 356 (2011) 734–740
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Fabrication and characterization of core/shell structured TiO2/polyaniline nanocomposite Sitao Yang, Yoshie Ishikawa, Hiroshi Itoh, Qi Feng ⇑ Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu-shi 761-0396, Japan
a r t i c l e
i n f o
Article history: Received 21 October 2010 Accepted 22 January 2011 Available online 1 February 2011 Keywords: TiO2/polyaniline nanocomposite Chemical absorption Photoelectrochemical reaction Sensitization effect
a b s t r a c t A novel core/shell structured TiO2/polyaniline nanocomposite was fabricated by grafting aniline on aminobenzoate monolayer that is chemically adsorbed on the TiO2 nanocrystal surface. The formation and nanostructure of the nanocomposite were investigated by FT-IR and UV–Vis spectra, TEM, FE-SEM, and TG–DTA analysis. Adsorption of aminobenzoate on the TiO2 surface is an effective method to obtain the uniform nanocomposite. The thickness of polyaniline layer coating on the TiO2 nanocrystal surface can be controlled in a range of 2–5 nm by this method. A photoelectrochemical study was carried out on the TiO2/polyaniline nanocomposite, and found that polyaniline in the nanocomposite acted as a visible-light sensitizer in a photoelectrochemical reaction. The sensitization effect increased with increasing binding strength between polyaniline and TiO2. A dye-sensitized solar cell with a short circuit current density of 0.19 mA/cm2 and an open circuit voltage of 0.35 V was fabricated by using the TiO2/polyaniline nanocomposite film as a sensitized electrode. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Titanium dioxide is an important n-type semiconducting material, due to their excellent photocatalytic and electric properties. This material has been widely utilized as photocatalytic materials [1,2] and semiconducting electrode materials for dye-sensitized solar cells [3,4]. On the other hand, organic conducting polymer materials attracted widespread interest as functional device materials, due to their low cost, ease of processing, and compatibility with flexible substrates. Polyaniline (PANI), as a well known conducting polymer, has attracted a considerable interest in recent years because of its good electric, electrochemical, and optical properties and high stability [5,6]. Also this conducting polymer shows the hole conducting properties. By considering the properties of TiO2 and PANI, therefore, a nanocomposite of TiO2 and PANI will give a new type of organic–inorganic p–n junction material that would be expected to have a potential application in the electronic and optical devises. Up to now, some studies have been reported on the preparations of PANI/inorganic hybrid materials. A PANI/LiFePO4 hybrid material has been prepared by coating the LiFePO4 with PANI to increase the conductivity of LiFePO4 as cathodic material in lithium rechargeable battery [7]. PANI/TiO2 hybrid materials have been prepared as an electrode for a gas sensor etc. [8–10]. In the traditional hybrid processes, firstly PANI is prepared, and then coated on the surface of inorganic materials. Since PANI is very difficult ⇑ Corresponding author. Fax: +81 87 864 2438. E-mail address: [email protected] (Q. Feng). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.01.078
to be dissolved in the most solvents, a composite in nanoscale is almost impossible by this coating process. To solve this problem, chemical polymerization with blending process and sol–gel process have been developed [11–13]. By these methods, uniform PANI/inorganic hybrid materials can be obtained. However, the PANI/inorganic hybrid materials prepared by these methods are still difficult to be used as the high performance materials for the electronic and optical devises. Because PANI and the inorganic material is connected by the weak intermolecular forces, so that the electron transport through their junction is restricted. To give a strong binding between PANI and TiO2, Senadeera et al. have modified TiO2 surface with chemically adsorbed silane-bearing aniline compound (C6H5NHC3H6Si(OMe)3) monolayer and then grafted PANI on silane-bearing aniline compound monolayer [14]. The electron transport from PANI to TiO2 surface has been improved by this surface modification. In this nanocomposite, however, the distance between PANI and TiO2 surface is considerably long. Therefore, a further improvement can be expected by shorting the distance. In the present study, we describe a new chemical process to fabricate a core/shell structured TiO2/polyaniline nanocomposite and characterization of its photoelectronic properties to demonstrate its potential as the electronic and photoelectronic devise materials. In our process, first aminobenzoate is chemically adsorbed on the TiO2 nanoparticle surface using its carboxyl group to form an aminobenzoate monolayer on TiO2 nanoparticle surface, and then aniline molecules are grafted on the chemically adsorbed aminobenzoate monolayer.
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
2. Materials and methods 2.1. Material preparation In the preparation of TiO2/PANI nanocomposite process, first TiO2 nanoparticle sample was dried at 120 °C for 1 h, and then 2.0 g of the dried sample was put into a 0.2 M 4-aminobenzoic acid (AB) ethanol solution (100 mL) and reacted for 1 h under stirring conditions. The sample was filtered and washed with ethanol for several times to obtain AB absorbed TiO2 sample (TO-AB). After that 0.20 g of TO-AB sample was dispersed in 100 mL acetonitrile solvent in a beaker, followed by adding aniline–HCl mixed solution and NH4S2O8–HCl mixed solution into the beaker to obtain a reaction solution containing 2.5 mmol/L aniline, 2.5 mmol/L NH4S2O8, 5 mmol/L HCl, and then reacted for 6 h under stirring conditions. After the reaction, the product was filtered and washed with ethanol and water for several times to obtain the nanocomposite sample. The nanocomposite sample prepared under the conditions described above is named as TO-AB-PA-2.5, where the number of 2.5 indicates the concentrations (mmol/L) of aniline and NH4S2O8 in the polymerization reaction solution. Other samples of TO-ABPA-5.0 and TO-AB-PA-7.5 were prepared by the same method, but changed the concentrations of aniline and NH4S2O8 to 5.0 and 7.5 mmol/L, respectively. In the preparation, a commercial TiO2 nanoparticle sample (Dewassa P-25) with a particle size of 20 nm and BET surface area 63 m2 g 1 was used. The TiO2 sample is a mixed phases of anatase (70 wt.%) and rutile (30 wt.%). For comparison, the TiO2/PANI samples were prepared also under similar conditions, but used TiO2 nanoparticle sample without AB adsorption treatment in the polymerization reaction, and the samples were named as TO-PA-X, where X indicates the concentrations (mmol/L) of aniline and NH4S2O8 in the polymerization reaction solution. Polyaniline sample was also synthesized by the method described above but without addition of TiO2 nanoparticles into the reaction system. 2.2. Fabrication of TiO2/PANI film electrode A TiO2 nanocrystalline film on conducting glass plate (FTO coated glass) was prepared by a two steps process. Firstly a density TiO2 layer was prepared by dipping the conducting glass plate into a mixed solution of Ti[OCH(CH3)2]4, CH3COCH2COCH3 and CH(CH3)3OH (volume ratio = 1:2:8) for 10 min. Then the glass plate was dried in air and annealed at 450 °C for 30 min. Secondly, a P-25 TiO2 paste was coated on the density TiO2 layer surface of the glass plate by doctor blade method, and then calcined at 450 °C for 30 min. A TiO2/PANI nanocomposite film was prepared as follow steps: firstly, the TiO2 nanocrystalline film (TOF) described above was dipped into a 0.2 M 4-aminobenzoic acid (AB) ethanol solution for 1 h, and after that the AB-adsorbed TiO2 film (TOF-AB) was rinsed with ethanol. Secondly, the TOF-AB film was dipped into an acetonitrile solution containing 2.5 mM aniline, 2.5 mM NH4S2O8, and 5 mM HCl for 6 h under stirring conditions to graft the polyaniline on the TiO2 film surface, and then rinsed with ethanol. The TiO2/PANI nanocomposite film was names as TOF-AB-PAX, where X represents the concentrations of aniline and NH4S2O8 reactants. TOF-PA-X films were prepared by similar method to TOF-AB-PA-X, but without AB adsorption treatment. These films were used as the working electrodes in photoelectrochemical study. 2.3. Physical analysis FT-IR spectra of the samples were measured on a PERKIN ELMER SPECTRUM ONE spectrophotometer at a resolution of
735
2 cm 1 using the KBr technique. A Shimadzu UV-390 spectrophotometer equipped with an integrating sphere was used for measuring UV–Vis reflection spectra of the powder samples, and BaSO4 powder sample was used as the white standard. Absorption spectra were obtained by transformation of the reflection spectra to the absorption spectra. Transmission electron microscope (TEM) observation was performed on a JEOL JEM-3010 at 300 kV, and the sample was supported on a microgrid coated with carbon. Morphology of the samples was observed using FE-SEM (Hitachi, S900). 2.4. Photoelectrochemical measurement In the photoelectrochemical measurement, the TOF-AB-PA-X or TOF-PA-X film was used as the working electrode, and irradiated in a quartz cell containing 200 mL of 0.5 M Na2SO4 and 0.01 M NaI solution using a Xe lamp (Asahi Spectra USA LAX-Cute, Vis 400– 700 nm) with a light intensity of 2000 W/m2. The masked-off irradiated area was 4 cm2. A Pt plate and an Hg/Hg2Cl2/KClsat electrode (SCE) were used as counter and reference electrodes. An external bias of 0.24 V vs. SCE was applied, and photocurrent was measured using a Hokuto-Denko BAS100B electrochemical analyzer. In the I–V measurement, a dye-sensitized solar cell (DSSC) was constructed with TOF-AB-PA-X as working electrode, Pt deposited FTO glass as counter electrode, and a acetonitrile solution containing 0.05 M I2, 0.1 M LiI, and 0.5 M tertiary butyl pyridine as electrolyte. The current–voltage characteristics of the cell were recorded with the electrochemical analyzer under illumination of 100 mW/cm2 from a solar simulator (YSS-E40 YAMASHITA DENSO) and linear sweeping voltammetry. 3. Results and discussion 3.1. Preparation of TiO2/PANI nanocomposite In order to fix PANI firmly on TiO2 surface, we designed a new process for the preparation of TiO2/PANI nanocomposite. In this process, firstly a monolayer of 4-aminobenzoic acid (AB) was absorbed on the TiO2 surface, and then the PANI is grafted on the AB monolayer. We selected AB to modify TiO2 surface, because the carboxyl group of AB can combine to Ti(IV) on TiO2 surface by a coordinate bond [15,16], and the AArNH2 group of AB can be utilized in the polymerization with aniline to form a conjugated system with PANI, which will greatly decrease the energy barrier for the electron transfer from PANI to TiO2 particles. After the adsorption treatment of the TiO2 nanoparticles in the AB solution, the color of TiO2 nanoparticles changed from white to light yellow, suggesting the adsorption reaction occurred, because both the TiO2 and the AB have no absorption in the visible-light region. The adsorption reaction was investigated by FT-IR spectroscopy (Fig. 1). In the spectrum of AB, the strong peaks at 3460 and 3360 cm 1 are due to the asymmetric and symmetric stretching vibration of ANH2 group, the strong peaks at 1630, 1580, 1530 and 1450 cm 1 are assigned to the m(C@C) stretching vibration of aromatic ring, peak at 1320 cm 1 to the m(CAN), and the 1160 cm 1 to the in-plane d(CAH) of aromatic ring [17]. The two strong bands at 1680 and 1290 cm 1 are assigned to the m(C@O) and m(CAOH) of carboxylic acid group (ACOOH). In the spectrum of TiO2, a broad vibration band around 1630 cm 1 can be assigned to the d(OAH) of water adsorbed on the TiO2 surface. After AB adsorption on TiO2 surface (TO-AB sample), the m(C@O) and m(CAOH) bands of carboxylic acid group (ACOOH) around 1680 and 1290 cm 1 disappeared, and carboxylate asymmetric m(ACOOasym ) and symmetric m(ACOOsym ) bands were observed at 1630 and 1390 cm 1 (Fig. 1C), revealing that in
736
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
Fig. 1. FT-IR spectra of (A) aminobenzoic acid, (B) TiO2, (C) TO-AB, (D) PANI, and (E) TO-AB-PA-5.0 samples.
the adsorption reaction, ACOOH group of AB was deprotonated to ACOO group, and the ACOO group anchored to the TiO2 surface [18,19]. When TO-AB sample was reacted in the mixed solution of aniline–NH4S2O8–HCl, the color of the sample changed to light green, revealing formation of polyaniline by an oxidation reaction [20], as shown in Fig. 6 that will be discussed later in detail. The polymerization reaction can be confirmed by FT-IR spectroscopic analysis. In the spectrum of PANI sample (Fig. 1D), the bands at 1580, 1485, 1300, 1250 and 1120 cm 1 are attributed to the m(C@C) in quinoid phenyl ring (Q), m(C@C) in benzenoid phenyl ring (B), m(CAN) in QBQ unit, m(CAN) in BBB unit, and d(CAH) in benzene plane, respectively, which corresponds to the IR character of a polyaniline–HCl salt (PANI–HCl) [5,21]. In the IR spectrum of TOAB-PA-5.0 sample (Fig. 1E), the bands corresponding to the m(C@C) in quinoid phenyl ring (Q), m(C@C) in benzenoid phenyl ring (B), m(CAN) in QBQ unit, m(CAN) in BBB unit, and d(CAH) in benzene plane of PANI–HCl were observed at 1580, 1480, 1300, 1250 and 1120 cm 1, respectively, revealing the formation of PANI in the TO-AB-PA-5.0 sample. The formation of the TiO2/PANI nanocomposite was studied using TEM analysis. Fig. 2 shows the TEM images of the TiO2 nanoparticle, TO-AB, TO-AB-PA-X, and TO-PA-5.0 samples. The TiO2 particles are nanocrystals with an average particle size of about 20 nm. After the adsorption of AB, no obvious change was observed; suggesting AB monolayer on the TiO2 surface was very thin. In the TO-AB-PA-2.5 sample, the TiO2 nanocrystals were coated by an amorphous PANI layer with a uniform thickness of
about 2 nm, forming a core/shell-structured TiO2/PANI nanocomposite. The thickness of the PANI shell increased in the TO-ABPA-5.0 and -7.5 samples, which corresponds to the increase of the concentration of aniline in the polymerization reaction solution. On the other hand, the core/shell-structured TiO2/PANI nanocomposite was not observed in the TO-PA-5.0 sample, where a simple mixture of amorphous PANI particles and TiO2 nanocrystals was observed (Fig. 2F). The particle size of PANI is much larger than that of TiO2. These results revealed that the AB adsorption treatment of TiO2 nanoparticles is very important to obtain the core/ shell-structured TiO2/PANI nanocomposite. The morphology difference between TO-AB-PA-5.0 and TO-PA5.0 samples was observed also by the FE-SEM (Fig. 3). TO-AB-PA5.0 sample has uniform particle size similar to TiO2 nanoparticles, but the size is slightly larger than that of TiO2 nanoparticles, due to TiO2 nanoparticles are coated by PANI layer. In the TO-PA-5.0 sample, however, two kinds of particles were observed. The small one with size of about 20 nm corresponds to TiO2 nanoparticles, and the large one with a size of about 100 nm to the PANI particles. 3.2. Characterization of TiO2/PANI nanocomposite Fig. 4 shows the TG–DTA curves of PANI and TO-AB-PA-X samples. In the TG–DTA curves of PANI, endothermic peaks around 80 °C with a weight loss belong to the vapor of the adsorption water in the PANI. The large exothermic peak around 450 °C is due to the decomposition of PANI by oxidation reaction, which accompanies with a large weight loss in a temperature range of
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
737
Fig. 2. TEM images of (A) TiO2 nanoparticle, (B) TO-AB, (C) TO-AB-PA-2.5, (D) TO-AB-PA-5.0, (E) TO-AB-PA-7.5, (F) TO-PA-5.0 samples.
300–600 °C. For the TO-AB-PA-X samples, the weight loss corresponded to the decomposition of PANI was observed in a temperature range of 250–550 °C, revealing PANI in the nanocomposite has a lower thermal stability than the pure PANI. The PANI content in the nanocomposite can be calculated from the weight loss, which increases in an order of TO-AB-PA-2.5 (11.3%) < TO-AB-PA5.0 (15.6%) < TO-AB-PA-7.5 (24.8%), agreeing with the results of TEM study (Fig. 2). Fig. 5 shows the UV–Vis spectra of the TiO2, TO-AB, TO-AB-PA-X, and PANI samples. The TiO2 have absorption edges at about 410 nm. When AB was adsorbed on the surface of the TiO2 particles, the absorption edge shifts to the long wavelength, suggesting a red-shift occurs after combining AB on TiO2 surface by a strong chemical adsorption. The free PANI shows three characteristic absorption peaks at 310 nm, 430 nm and 680 nm. These character-
istic peaks are attributed to the p p, polaron-p, p-polaron transitions [5,22]. With adsorption of PANI onto the TiO2 surface in the TO-AB-PA-X samples, the peak at 680 nm shows a red-shift, and the red-shift increases with decreasing of PANI content in the nanocomposite. This result indicates that the interaction between PANI and TiO2 causes the red-shift of the 680 nm peak, and the interaction increases with decreasing PANI content in the nanocomposite. The similar red-shift has been also observed in SnO2/ PANI composites [22]. It is well known that when a p-type semiconductor is in contacted with an n-type semiconductor, the energy band edge of the p-type semiconductor shifts to positive potential, while that of the n-type semiconductor shifts to negative potential at around their contacting interface, which causes decrease of the band gap of the p-type semiconductor at around the interface. Therefore, the red-shift of PANI has been explained
738
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
Fig. 3. FE-SEM images of (A) TiO2 nanoparticle, (B) TO-AB-PA-5.0, and (C) TO-PA-5.0 samples.
Fig. 6. (a) Schematic model of formation PANI layer on TiO2 nanoparticle surface and (b) polymerization reaction of aniline.
Fig. 4. TG–DTA curves of (A) PANI and (B) TO-AB-PA-X samples. (a) TO-AB-PA-2.5, (b) TO-AB-PA-5.0, (c) TO-AB-PA-7.5.
by the decrease of the band gap of PANI at around the interface between PANI and an n-type semiconductor [23]. In the case of the TiO2/PANI nanocomposite, when the PANI layer is very thin, the red-shift is significant, but it decreases with increasing the thickness of the PANI layer. 3.3. Formation mechanism of TiO2/PANI nanocomposite
Fig. 5. UV–Vis spectra of (A) TiO2, (B) TO-AB, (C) PANI, (D) TO-AB-PA-2.5, (E) TO-ABPA-5.0, and (F) TO-AB-PA-7.5.
On the basis of the results of above, we give a mechanism for the formation of PANI layer on TiO2 surface, as shown in Fig. 6a. When TiO2 nanoparticles are added into AB-ethanol solution, the carboxyl group of AB can coordinate to Ti (IV) on TiO2 surface. This causes a chemical adsorption of AB on TiO2 surface and formation of AB monolayer on the surface. Such adsorption of organic molecules on TiO2 or other metal oxide surface with the carboxyl group has been reported also for other organic acids and complexes [19,23–25]. The aniline molecules can be polymerized to the PANI by an oxidation polymerization reaction in the (NH4)2S2O8 solution, as shown in Fig. 6b. When the AB-adsorbed TiO2 particles are added into the oxidation polymerization reaction system, the
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
739
AB-adsorbed on TiO2 surface can polymerize with aniline in the solution using its AArNH2 group, which grafts PANI on the AB monolayer adsorbed on TiO2 surface and form the TiO2/PANI nanocomposite. Since the adsorption of AB on TiO2 surface is strong and uniform, the PANI layer coating on the TiO2 surface is strong and uniform. Therefore the formation of the AB monolayer on TiO2 surface is very important to obtain the uniform nanocomposite. 3.4. Photoelectrochemical properties of TiO2/PANI nanocomposite To study the photoelectrochemical properties of TiO2/PANI nanocomposite, a TiO2/PANI nanocomposite film was fabricated on a conducting glass. A FE-SEM analysis revealed that the uniform PANI layer was coated on TiO2 nanoparticles in the nanocomposite film sample (Fig. 7). This result indicated that method developed in this study is also useful to fabricate the uniform nanocomposite film. The visible-light sensitization effect of PANI in the nanocomposite was investigated by using the nanocomposite film as working electrode and Pt as counter electrode in a Na2SO4 solution containing NaI. Fig. 8 shows the photocurrent character of TOFAB-PA-2.5 and TOF-PA-2.5 electrodes under intermittent irradiation with Xe lamp (400–700 nm). In the case of TOF-AB-PA-2.5 electrode, the photocurrent increase and decrease responded sensitively to the light on and off, and the photocurrent was large. In the case of TOF-PA-2.5 electrode, however, a small and unstable photocurrent was observed. The results suggest PANI on TiO2 surface shows the visible-light sensitization effect, where the electrons of PANI can be excited by absorption of visible-light, and injected into TiO2 conducting band. The fact of larger photocurrent on the TOF-AB-PA-2.5 electrode than that on the TOF-PA-2.5 electrode can be explained by stronger binding between TiO2 and PANI in TOF-AB-PA-2.5 electrode than that in TOF-PA-2.5 electrode, which improves the transport rate of excited electron from PANI to TiO2. The above results suggest that the photoinduced interfacial charge separation can occur on the polymer/semiconductor nanocomposite, so TOF-AB-PA-2.5 electrode can be utilized as a sensitized electrode for a dye-sensitized solar cell. To confirm this, we constructed a solar cell using the TOF-AB-PA-2.5 electrode, as illustrated in Fig. 9. It shows the I–V characteristics of the solar cell in dark and under light irradiation conditions (Fig. 10). In dark a good p–n junction character I–V curve was observed, which means that a good contact was formed between the TiO2 and PANI. Under the light irradiation conditions, a short circuit current density (Jsc) of 0.19 mA/cm2 and an open circuit voltage (Voc) of 0.35 V were generated. Although the Jsc and Voc are still low, it proves that the
Fig. 8. Current–time curves for (a) TOF-AB-PA-2.5 electrode and (b) TOF-PA-2.5 electrode in a solution containing 0.1 M NaI and 0.5 M Na2SO4 under intermittent irradiation with Xe lamp (400–700 nm).
Fig. 9. Structure of a dye-sensitized solar cell using TiO2/PANI nanocomposite electrode.
Fig. 10. I–V curves of dye-sensitized solar cell using TOF-AB-PA-2.5 electrode.
Fig. 7. FE-SEM images of cross sections of (A) the TiO2 film and (B) TiO2/PANI composite film (TOF-AB-PA-2.5).
740
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
nanocomposite of conducting polymer and TiO2 has a potential application in solar cells, and strong binding between the conducting polymer and TiO2 can improve the electron transport from conducting polymer to TiO2 in the solar cell system. 4. Conclusion The uniform core/shell-structured TiO2/PANI nanocomposite can be fabricated by adsorbing AB on TiO2 surface and then grafting aniline on the AB monolayer. PANI in the nanocomposite can act as sensitizer in the photoelectrochemical reaction. The sensitization effect increases with increasing binding strength between PANI and TiO2. The TiO2/PANI nanocomposite film can be utilized as the sensitized electrode for the dye-sensitized solar cell. Acknowledgment This work was supported by Grants-in-Aid for Scientific Research (B) (No. 20350096) from Japan Society for the Promotion of Science, and Kagawa University. References [1] A. Fujishima, K. Honda, Nature 23 (1972) 37–38. [2] A. Housa, H. Lachheb, M. Ksibi, E. Elaloui, J.-M. Herrmann, Appl. Catal. B 31 (2001) 145–157. [3] B. O’Regan, J. Moser, M. Anderson, M. Grätzel, J. Phys. Chem. 94 (1990) 8720– 8726. [4] Seigo. Ito, S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K. Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida, M. Gratzel, Adv. Mater. 18 (2006) 1202–1205.
[5] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277–324. [6] S. Bhadra, D. Khastgir, N.K. Singha, J.H. Lee, Prog. Polym. Sci. 34 (2009) 783– 810. [7] G.T. Lei, X.H. Yi, L. Wang, Z.H. Li, Zhou, J. Polym. Adv. Technol. 20 (2009) 576– 580. [8] X. Wei, L.F. Jiao, J.L. Sun, S.C. Liu, H.T. Yuan, J. Solid State Electrochem. 14 (2010) 197–202. [9] X.F. Ma, M. Wang, G. Li, H.Z. Chen, R. Bai, Mater. Chem. Phys. 98 (2006) 241– 247. [10] V. Gupta, N. Miura, Mater. Lett. 60 (2006) 1466–1469. [11] P. Xu, X.J. Han, J.J. Jiang, X.H. Wang, X.D. Li, A.H. Wen, J. Phys. Chem. C 111 (2007) 12603–12608. [12] D. Ahn, Y.M. Koo, M.G. Kim, N. Shin, J. Park, J. Eom, J. Cho, T.J. Shin, J. Phys. Chem. C 114 (2010) 3675–3680. [13] D.C. Schnitzler, M.S. Meruvia, I.A. Hummelgen, A.J.G. Zarbin, Chem. Mater. 15 (2003) 4658–4665. [14] G.K.R. Senadeera, T. Kitamura, Y. Wada, S. Yanagida, J. Photochem. Photobiol. A 164 (2004) 61–66. [15] M.K. Nazeeruddin, R. Humphry-Baker, P. Liska, M. Grätzel, J. Phys. Chem. B 107 (2003) 8981–8987. [16] F. Gajardo, A.M. Leiva, B. Loeb, A. Delgadillo, J.R. Stromberg, G.J. Meyer, Inorg. Chim. Acta 361 (2008) 613–619. [17] T. Inomata, Takao Moriwaki, Bull. Chem. Soc. Jpn. 46 (1973) 1148–1154. [18] K. Konstadinidis, B. Thakkar, A. Chakraborty, L.W. Potts, R. Tannenbaum, M. Tirrell, J.F. Evans, Langmuir 8 (1992) 1307–1317. [19] F. Jones, J.B. Farrow, W.V. Bronswijk, Langmuir 14 (1998) 6512–6517. [20] J.X. Huang, J.A. Moore, J.H. Acquaye, R.B. Kaner, Macromolecules 38 (2005) 317–321. [21] T.A. Kerr, H. Wu, L.F. Nazar, Chem. Mater. 8 (1996) 2005–2015. [22] D.C. Schnitzler, M.S. Meruvia, I.A. Hümelgen, A.J.G. Zarbin, Chem. Mater. 15 (2003) 4658–4665. [23] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, M. Gratzel, J. Phys. Chem. B 107 (2003) 8981–8987. [24] K. Konstadinidis, B. Thakkar, A. Chakraborty, L.W. Potts, R. Tannenbaum, M. Tirrell, Langmuir 8 (1992) 1307–1317. [25] M.J. McGuire, J. Addai-Mensah, K.E. Bremmell, J. Colloid Interface Sci. 299 (2006) 547–555.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Fabrication and characterization of core/shell structured TiO2/polyaniline nanocomposite Sitao Yang, Yoshie Ishikawa, Hiroshi Itoh, Qi Feng ⇑ Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu-shi 761-0396, Japan
a r t i c l e
i n f o
Article history: Received 21 October 2010 Accepted 22 January 2011 Available online 1 February 2011 Keywords: TiO2/polyaniline nanocomposite Chemical absorption Photoelectrochemical reaction Sensitization effect
a b s t r a c t A novel core/shell structured TiO2/polyaniline nanocomposite was fabricated by grafting aniline on aminobenzoate monolayer that is chemically adsorbed on the TiO2 nanocrystal surface. The formation and nanostructure of the nanocomposite were investigated by FT-IR and UV–Vis spectra, TEM, FE-SEM, and TG–DTA analysis. Adsorption of aminobenzoate on the TiO2 surface is an effective method to obtain the uniform nanocomposite. The thickness of polyaniline layer coating on the TiO2 nanocrystal surface can be controlled in a range of 2–5 nm by this method. A photoelectrochemical study was carried out on the TiO2/polyaniline nanocomposite, and found that polyaniline in the nanocomposite acted as a visible-light sensitizer in a photoelectrochemical reaction. The sensitization effect increased with increasing binding strength between polyaniline and TiO2. A dye-sensitized solar cell with a short circuit current density of 0.19 mA/cm2 and an open circuit voltage of 0.35 V was fabricated by using the TiO2/polyaniline nanocomposite film as a sensitized electrode. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Titanium dioxide is an important n-type semiconducting material, due to their excellent photocatalytic and electric properties. This material has been widely utilized as photocatalytic materials [1,2] and semiconducting electrode materials for dye-sensitized solar cells [3,4]. On the other hand, organic conducting polymer materials attracted widespread interest as functional device materials, due to their low cost, ease of processing, and compatibility with flexible substrates. Polyaniline (PANI), as a well known conducting polymer, has attracted a considerable interest in recent years because of its good electric, electrochemical, and optical properties and high stability [5,6]. Also this conducting polymer shows the hole conducting properties. By considering the properties of TiO2 and PANI, therefore, a nanocomposite of TiO2 and PANI will give a new type of organic–inorganic p–n junction material that would be expected to have a potential application in the electronic and optical devises. Up to now, some studies have been reported on the preparations of PANI/inorganic hybrid materials. A PANI/LiFePO4 hybrid material has been prepared by coating the LiFePO4 with PANI to increase the conductivity of LiFePO4 as cathodic material in lithium rechargeable battery [7]. PANI/TiO2 hybrid materials have been prepared as an electrode for a gas sensor etc. [8–10]. In the traditional hybrid processes, firstly PANI is prepared, and then coated on the surface of inorganic materials. Since PANI is very difficult ⇑ Corresponding author. Fax: +81 87 864 2438. E-mail address: [email protected] (Q. Feng). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.01.078
to be dissolved in the most solvents, a composite in nanoscale is almost impossible by this coating process. To solve this problem, chemical polymerization with blending process and sol–gel process have been developed [11–13]. By these methods, uniform PANI/inorganic hybrid materials can be obtained. However, the PANI/inorganic hybrid materials prepared by these methods are still difficult to be used as the high performance materials for the electronic and optical devises. Because PANI and the inorganic material is connected by the weak intermolecular forces, so that the electron transport through their junction is restricted. To give a strong binding between PANI and TiO2, Senadeera et al. have modified TiO2 surface with chemically adsorbed silane-bearing aniline compound (C6H5NHC3H6Si(OMe)3) monolayer and then grafted PANI on silane-bearing aniline compound monolayer [14]. The electron transport from PANI to TiO2 surface has been improved by this surface modification. In this nanocomposite, however, the distance between PANI and TiO2 surface is considerably long. Therefore, a further improvement can be expected by shorting the distance. In the present study, we describe a new chemical process to fabricate a core/shell structured TiO2/polyaniline nanocomposite and characterization of its photoelectronic properties to demonstrate its potential as the electronic and photoelectronic devise materials. In our process, first aminobenzoate is chemically adsorbed on the TiO2 nanoparticle surface using its carboxyl group to form an aminobenzoate monolayer on TiO2 nanoparticle surface, and then aniline molecules are grafted on the chemically adsorbed aminobenzoate monolayer.
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
2. Materials and methods 2.1. Material preparation In the preparation of TiO2/PANI nanocomposite process, first TiO2 nanoparticle sample was dried at 120 °C for 1 h, and then 2.0 g of the dried sample was put into a 0.2 M 4-aminobenzoic acid (AB) ethanol solution (100 mL) and reacted for 1 h under stirring conditions. The sample was filtered and washed with ethanol for several times to obtain AB absorbed TiO2 sample (TO-AB). After that 0.20 g of TO-AB sample was dispersed in 100 mL acetonitrile solvent in a beaker, followed by adding aniline–HCl mixed solution and NH4S2O8–HCl mixed solution into the beaker to obtain a reaction solution containing 2.5 mmol/L aniline, 2.5 mmol/L NH4S2O8, 5 mmol/L HCl, and then reacted for 6 h under stirring conditions. After the reaction, the product was filtered and washed with ethanol and water for several times to obtain the nanocomposite sample. The nanocomposite sample prepared under the conditions described above is named as TO-AB-PA-2.5, where the number of 2.5 indicates the concentrations (mmol/L) of aniline and NH4S2O8 in the polymerization reaction solution. Other samples of TO-ABPA-5.0 and TO-AB-PA-7.5 were prepared by the same method, but changed the concentrations of aniline and NH4S2O8 to 5.0 and 7.5 mmol/L, respectively. In the preparation, a commercial TiO2 nanoparticle sample (Dewassa P-25) with a particle size of 20 nm and BET surface area 63 m2 g 1 was used. The TiO2 sample is a mixed phases of anatase (70 wt.%) and rutile (30 wt.%). For comparison, the TiO2/PANI samples were prepared also under similar conditions, but used TiO2 nanoparticle sample without AB adsorption treatment in the polymerization reaction, and the samples were named as TO-PA-X, where X indicates the concentrations (mmol/L) of aniline and NH4S2O8 in the polymerization reaction solution. Polyaniline sample was also synthesized by the method described above but without addition of TiO2 nanoparticles into the reaction system. 2.2. Fabrication of TiO2/PANI film electrode A TiO2 nanocrystalline film on conducting glass plate (FTO coated glass) was prepared by a two steps process. Firstly a density TiO2 layer was prepared by dipping the conducting glass plate into a mixed solution of Ti[OCH(CH3)2]4, CH3COCH2COCH3 and CH(CH3)3OH (volume ratio = 1:2:8) for 10 min. Then the glass plate was dried in air and annealed at 450 °C for 30 min. Secondly, a P-25 TiO2 paste was coated on the density TiO2 layer surface of the glass plate by doctor blade method, and then calcined at 450 °C for 30 min. A TiO2/PANI nanocomposite film was prepared as follow steps: firstly, the TiO2 nanocrystalline film (TOF) described above was dipped into a 0.2 M 4-aminobenzoic acid (AB) ethanol solution for 1 h, and after that the AB-adsorbed TiO2 film (TOF-AB) was rinsed with ethanol. Secondly, the TOF-AB film was dipped into an acetonitrile solution containing 2.5 mM aniline, 2.5 mM NH4S2O8, and 5 mM HCl for 6 h under stirring conditions to graft the polyaniline on the TiO2 film surface, and then rinsed with ethanol. The TiO2/PANI nanocomposite film was names as TOF-AB-PAX, where X represents the concentrations of aniline and NH4S2O8 reactants. TOF-PA-X films were prepared by similar method to TOF-AB-PA-X, but without AB adsorption treatment. These films were used as the working electrodes in photoelectrochemical study. 2.3. Physical analysis FT-IR spectra of the samples were measured on a PERKIN ELMER SPECTRUM ONE spectrophotometer at a resolution of
735
2 cm 1 using the KBr technique. A Shimadzu UV-390 spectrophotometer equipped with an integrating sphere was used for measuring UV–Vis reflection spectra of the powder samples, and BaSO4 powder sample was used as the white standard. Absorption spectra were obtained by transformation of the reflection spectra to the absorption spectra. Transmission electron microscope (TEM) observation was performed on a JEOL JEM-3010 at 300 kV, and the sample was supported on a microgrid coated with carbon. Morphology of the samples was observed using FE-SEM (Hitachi, S900). 2.4. Photoelectrochemical measurement In the photoelectrochemical measurement, the TOF-AB-PA-X or TOF-PA-X film was used as the working electrode, and irradiated in a quartz cell containing 200 mL of 0.5 M Na2SO4 and 0.01 M NaI solution using a Xe lamp (Asahi Spectra USA LAX-Cute, Vis 400– 700 nm) with a light intensity of 2000 W/m2. The masked-off irradiated area was 4 cm2. A Pt plate and an Hg/Hg2Cl2/KClsat electrode (SCE) were used as counter and reference electrodes. An external bias of 0.24 V vs. SCE was applied, and photocurrent was measured using a Hokuto-Denko BAS100B electrochemical analyzer. In the I–V measurement, a dye-sensitized solar cell (DSSC) was constructed with TOF-AB-PA-X as working electrode, Pt deposited FTO glass as counter electrode, and a acetonitrile solution containing 0.05 M I2, 0.1 M LiI, and 0.5 M tertiary butyl pyridine as electrolyte. The current–voltage characteristics of the cell were recorded with the electrochemical analyzer under illumination of 100 mW/cm2 from a solar simulator (YSS-E40 YAMASHITA DENSO) and linear sweeping voltammetry. 3. Results and discussion 3.1. Preparation of TiO2/PANI nanocomposite In order to fix PANI firmly on TiO2 surface, we designed a new process for the preparation of TiO2/PANI nanocomposite. In this process, firstly a monolayer of 4-aminobenzoic acid (AB) was absorbed on the TiO2 surface, and then the PANI is grafted on the AB monolayer. We selected AB to modify TiO2 surface, because the carboxyl group of AB can combine to Ti(IV) on TiO2 surface by a coordinate bond [15,16], and the AArNH2 group of AB can be utilized in the polymerization with aniline to form a conjugated system with PANI, which will greatly decrease the energy barrier for the electron transfer from PANI to TiO2 particles. After the adsorption treatment of the TiO2 nanoparticles in the AB solution, the color of TiO2 nanoparticles changed from white to light yellow, suggesting the adsorption reaction occurred, because both the TiO2 and the AB have no absorption in the visible-light region. The adsorption reaction was investigated by FT-IR spectroscopy (Fig. 1). In the spectrum of AB, the strong peaks at 3460 and 3360 cm 1 are due to the asymmetric and symmetric stretching vibration of ANH2 group, the strong peaks at 1630, 1580, 1530 and 1450 cm 1 are assigned to the m(C@C) stretching vibration of aromatic ring, peak at 1320 cm 1 to the m(CAN), and the 1160 cm 1 to the in-plane d(CAH) of aromatic ring [17]. The two strong bands at 1680 and 1290 cm 1 are assigned to the m(C@O) and m(CAOH) of carboxylic acid group (ACOOH). In the spectrum of TiO2, a broad vibration band around 1630 cm 1 can be assigned to the d(OAH) of water adsorbed on the TiO2 surface. After AB adsorption on TiO2 surface (TO-AB sample), the m(C@O) and m(CAOH) bands of carboxylic acid group (ACOOH) around 1680 and 1290 cm 1 disappeared, and carboxylate asymmetric m(ACOOasym ) and symmetric m(ACOOsym ) bands were observed at 1630 and 1390 cm 1 (Fig. 1C), revealing that in
736
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
Fig. 1. FT-IR spectra of (A) aminobenzoic acid, (B) TiO2, (C) TO-AB, (D) PANI, and (E) TO-AB-PA-5.0 samples.
the adsorption reaction, ACOOH group of AB was deprotonated to ACOO group, and the ACOO group anchored to the TiO2 surface [18,19]. When TO-AB sample was reacted in the mixed solution of aniline–NH4S2O8–HCl, the color of the sample changed to light green, revealing formation of polyaniline by an oxidation reaction [20], as shown in Fig. 6 that will be discussed later in detail. The polymerization reaction can be confirmed by FT-IR spectroscopic analysis. In the spectrum of PANI sample (Fig. 1D), the bands at 1580, 1485, 1300, 1250 and 1120 cm 1 are attributed to the m(C@C) in quinoid phenyl ring (Q), m(C@C) in benzenoid phenyl ring (B), m(CAN) in QBQ unit, m(CAN) in BBB unit, and d(CAH) in benzene plane, respectively, which corresponds to the IR character of a polyaniline–HCl salt (PANI–HCl) [5,21]. In the IR spectrum of TOAB-PA-5.0 sample (Fig. 1E), the bands corresponding to the m(C@C) in quinoid phenyl ring (Q), m(C@C) in benzenoid phenyl ring (B), m(CAN) in QBQ unit, m(CAN) in BBB unit, and d(CAH) in benzene plane of PANI–HCl were observed at 1580, 1480, 1300, 1250 and 1120 cm 1, respectively, revealing the formation of PANI in the TO-AB-PA-5.0 sample. The formation of the TiO2/PANI nanocomposite was studied using TEM analysis. Fig. 2 shows the TEM images of the TiO2 nanoparticle, TO-AB, TO-AB-PA-X, and TO-PA-5.0 samples. The TiO2 particles are nanocrystals with an average particle size of about 20 nm. After the adsorption of AB, no obvious change was observed; suggesting AB monolayer on the TiO2 surface was very thin. In the TO-AB-PA-2.5 sample, the TiO2 nanocrystals were coated by an amorphous PANI layer with a uniform thickness of
about 2 nm, forming a core/shell-structured TiO2/PANI nanocomposite. The thickness of the PANI shell increased in the TO-ABPA-5.0 and -7.5 samples, which corresponds to the increase of the concentration of aniline in the polymerization reaction solution. On the other hand, the core/shell-structured TiO2/PANI nanocomposite was not observed in the TO-PA-5.0 sample, where a simple mixture of amorphous PANI particles and TiO2 nanocrystals was observed (Fig. 2F). The particle size of PANI is much larger than that of TiO2. These results revealed that the AB adsorption treatment of TiO2 nanoparticles is very important to obtain the core/ shell-structured TiO2/PANI nanocomposite. The morphology difference between TO-AB-PA-5.0 and TO-PA5.0 samples was observed also by the FE-SEM (Fig. 3). TO-AB-PA5.0 sample has uniform particle size similar to TiO2 nanoparticles, but the size is slightly larger than that of TiO2 nanoparticles, due to TiO2 nanoparticles are coated by PANI layer. In the TO-PA-5.0 sample, however, two kinds of particles were observed. The small one with size of about 20 nm corresponds to TiO2 nanoparticles, and the large one with a size of about 100 nm to the PANI particles. 3.2. Characterization of TiO2/PANI nanocomposite Fig. 4 shows the TG–DTA curves of PANI and TO-AB-PA-X samples. In the TG–DTA curves of PANI, endothermic peaks around 80 °C with a weight loss belong to the vapor of the adsorption water in the PANI. The large exothermic peak around 450 °C is due to the decomposition of PANI by oxidation reaction, which accompanies with a large weight loss in a temperature range of
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
737
Fig. 2. TEM images of (A) TiO2 nanoparticle, (B) TO-AB, (C) TO-AB-PA-2.5, (D) TO-AB-PA-5.0, (E) TO-AB-PA-7.5, (F) TO-PA-5.0 samples.
300–600 °C. For the TO-AB-PA-X samples, the weight loss corresponded to the decomposition of PANI was observed in a temperature range of 250–550 °C, revealing PANI in the nanocomposite has a lower thermal stability than the pure PANI. The PANI content in the nanocomposite can be calculated from the weight loss, which increases in an order of TO-AB-PA-2.5 (11.3%) < TO-AB-PA5.0 (15.6%) < TO-AB-PA-7.5 (24.8%), agreeing with the results of TEM study (Fig. 2). Fig. 5 shows the UV–Vis spectra of the TiO2, TO-AB, TO-AB-PA-X, and PANI samples. The TiO2 have absorption edges at about 410 nm. When AB was adsorbed on the surface of the TiO2 particles, the absorption edge shifts to the long wavelength, suggesting a red-shift occurs after combining AB on TiO2 surface by a strong chemical adsorption. The free PANI shows three characteristic absorption peaks at 310 nm, 430 nm and 680 nm. These character-
istic peaks are attributed to the p p, polaron-p, p-polaron transitions [5,22]. With adsorption of PANI onto the TiO2 surface in the TO-AB-PA-X samples, the peak at 680 nm shows a red-shift, and the red-shift increases with decreasing of PANI content in the nanocomposite. This result indicates that the interaction between PANI and TiO2 causes the red-shift of the 680 nm peak, and the interaction increases with decreasing PANI content in the nanocomposite. The similar red-shift has been also observed in SnO2/ PANI composites [22]. It is well known that when a p-type semiconductor is in contacted with an n-type semiconductor, the energy band edge of the p-type semiconductor shifts to positive potential, while that of the n-type semiconductor shifts to negative potential at around their contacting interface, which causes decrease of the band gap of the p-type semiconductor at around the interface. Therefore, the red-shift of PANI has been explained
738
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
Fig. 3. FE-SEM images of (A) TiO2 nanoparticle, (B) TO-AB-PA-5.0, and (C) TO-PA-5.0 samples.
Fig. 6. (a) Schematic model of formation PANI layer on TiO2 nanoparticle surface and (b) polymerization reaction of aniline.
Fig. 4. TG–DTA curves of (A) PANI and (B) TO-AB-PA-X samples. (a) TO-AB-PA-2.5, (b) TO-AB-PA-5.0, (c) TO-AB-PA-7.5.
by the decrease of the band gap of PANI at around the interface between PANI and an n-type semiconductor [23]. In the case of the TiO2/PANI nanocomposite, when the PANI layer is very thin, the red-shift is significant, but it decreases with increasing the thickness of the PANI layer. 3.3. Formation mechanism of TiO2/PANI nanocomposite
Fig. 5. UV–Vis spectra of (A) TiO2, (B) TO-AB, (C) PANI, (D) TO-AB-PA-2.5, (E) TO-ABPA-5.0, and (F) TO-AB-PA-7.5.
On the basis of the results of above, we give a mechanism for the formation of PANI layer on TiO2 surface, as shown in Fig. 6a. When TiO2 nanoparticles are added into AB-ethanol solution, the carboxyl group of AB can coordinate to Ti (IV) on TiO2 surface. This causes a chemical adsorption of AB on TiO2 surface and formation of AB monolayer on the surface. Such adsorption of organic molecules on TiO2 or other metal oxide surface with the carboxyl group has been reported also for other organic acids and complexes [19,23–25]. The aniline molecules can be polymerized to the PANI by an oxidation polymerization reaction in the (NH4)2S2O8 solution, as shown in Fig. 6b. When the AB-adsorbed TiO2 particles are added into the oxidation polymerization reaction system, the
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
739
AB-adsorbed on TiO2 surface can polymerize with aniline in the solution using its AArNH2 group, which grafts PANI on the AB monolayer adsorbed on TiO2 surface and form the TiO2/PANI nanocomposite. Since the adsorption of AB on TiO2 surface is strong and uniform, the PANI layer coating on the TiO2 surface is strong and uniform. Therefore the formation of the AB monolayer on TiO2 surface is very important to obtain the uniform nanocomposite. 3.4. Photoelectrochemical properties of TiO2/PANI nanocomposite To study the photoelectrochemical properties of TiO2/PANI nanocomposite, a TiO2/PANI nanocomposite film was fabricated on a conducting glass. A FE-SEM analysis revealed that the uniform PANI layer was coated on TiO2 nanoparticles in the nanocomposite film sample (Fig. 7). This result indicated that method developed in this study is also useful to fabricate the uniform nanocomposite film. The visible-light sensitization effect of PANI in the nanocomposite was investigated by using the nanocomposite film as working electrode and Pt as counter electrode in a Na2SO4 solution containing NaI. Fig. 8 shows the photocurrent character of TOFAB-PA-2.5 and TOF-PA-2.5 electrodes under intermittent irradiation with Xe lamp (400–700 nm). In the case of TOF-AB-PA-2.5 electrode, the photocurrent increase and decrease responded sensitively to the light on and off, and the photocurrent was large. In the case of TOF-PA-2.5 electrode, however, a small and unstable photocurrent was observed. The results suggest PANI on TiO2 surface shows the visible-light sensitization effect, where the electrons of PANI can be excited by absorption of visible-light, and injected into TiO2 conducting band. The fact of larger photocurrent on the TOF-AB-PA-2.5 electrode than that on the TOF-PA-2.5 electrode can be explained by stronger binding between TiO2 and PANI in TOF-AB-PA-2.5 electrode than that in TOF-PA-2.5 electrode, which improves the transport rate of excited electron from PANI to TiO2. The above results suggest that the photoinduced interfacial charge separation can occur on the polymer/semiconductor nanocomposite, so TOF-AB-PA-2.5 electrode can be utilized as a sensitized electrode for a dye-sensitized solar cell. To confirm this, we constructed a solar cell using the TOF-AB-PA-2.5 electrode, as illustrated in Fig. 9. It shows the I–V characteristics of the solar cell in dark and under light irradiation conditions (Fig. 10). In dark a good p–n junction character I–V curve was observed, which means that a good contact was formed between the TiO2 and PANI. Under the light irradiation conditions, a short circuit current density (Jsc) of 0.19 mA/cm2 and an open circuit voltage (Voc) of 0.35 V were generated. Although the Jsc and Voc are still low, it proves that the
Fig. 8. Current–time curves for (a) TOF-AB-PA-2.5 electrode and (b) TOF-PA-2.5 electrode in a solution containing 0.1 M NaI and 0.5 M Na2SO4 under intermittent irradiation with Xe lamp (400–700 nm).
Fig. 9. Structure of a dye-sensitized solar cell using TiO2/PANI nanocomposite electrode.
Fig. 10. I–V curves of dye-sensitized solar cell using TOF-AB-PA-2.5 electrode.
Fig. 7. FE-SEM images of cross sections of (A) the TiO2 film and (B) TiO2/PANI composite film (TOF-AB-PA-2.5).
740
S. Yang et al. / Journal of Colloid and Interface Science 356 (2011) 734–740
nanocomposite of conducting polymer and TiO2 has a potential application in solar cells, and strong binding between the conducting polymer and TiO2 can improve the electron transport from conducting polymer to TiO2 in the solar cell system. 4. Conclusion The uniform core/shell-structured TiO2/PANI nanocomposite can be fabricated by adsorbing AB on TiO2 surface and then grafting aniline on the AB monolayer. PANI in the nanocomposite can act as sensitizer in the photoelectrochemical reaction. The sensitization effect increases with increasing binding strength between PANI and TiO2. The TiO2/PANI nanocomposite film can be utilized as the sensitized electrode for the dye-sensitized solar cell. Acknowledgment This work was supported by Grants-in-Aid for Scientific Research (B) (No. 20350096) from Japan Society for the Promotion of Science, and Kagawa University. References [1] A. Fujishima, K. Honda, Nature 23 (1972) 37–38. [2] A. Housa, H. Lachheb, M. Ksibi, E. Elaloui, J.-M. Herrmann, Appl. Catal. B 31 (2001) 145–157. [3] B. O’Regan, J. Moser, M. Anderson, M. Grätzel, J. Phys. Chem. 94 (1990) 8720– 8726. [4] Seigo. Ito, S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K. Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida, M. Gratzel, Adv. Mater. 18 (2006) 1202–1205.
[5] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277–324. [6] S. Bhadra, D. Khastgir, N.K. Singha, J.H. Lee, Prog. Polym. Sci. 34 (2009) 783– 810. [7] G.T. Lei, X.H. Yi, L. Wang, Z.H. Li, Zhou, J. Polym. Adv. Technol. 20 (2009) 576– 580. [8] X. Wei, L.F. Jiao, J.L. Sun, S.C. Liu, H.T. Yuan, J. Solid State Electrochem. 14 (2010) 197–202. [9] X.F. Ma, M. Wang, G. Li, H.Z. Chen, R. Bai, Mater. Chem. Phys. 98 (2006) 241– 247. [10] V. Gupta, N. Miura, Mater. Lett. 60 (2006) 1466–1469. [11] P. Xu, X.J. Han, J.J. Jiang, X.H. Wang, X.D. Li, A.H. Wen, J. Phys. Chem. C 111 (2007) 12603–12608. [12] D. Ahn, Y.M. Koo, M.G. Kim, N. Shin, J. Park, J. Eom, J. Cho, T.J. Shin, J. Phys. Chem. C 114 (2010) 3675–3680. [13] D.C. Schnitzler, M.S. Meruvia, I.A. Hummelgen, A.J.G. Zarbin, Chem. Mater. 15 (2003) 4658–4665. [14] G.K.R. Senadeera, T. Kitamura, Y. Wada, S. Yanagida, J. Photochem. Photobiol. A 164 (2004) 61–66. [15] M.K. Nazeeruddin, R. Humphry-Baker, P. Liska, M. Grätzel, J. Phys. Chem. B 107 (2003) 8981–8987. [16] F. Gajardo, A.M. Leiva, B. Loeb, A. Delgadillo, J.R. Stromberg, G.J. Meyer, Inorg. Chim. Acta 361 (2008) 613–619. [17] T. Inomata, Takao Moriwaki, Bull. Chem. Soc. Jpn. 46 (1973) 1148–1154. [18] K. Konstadinidis, B. Thakkar, A. Chakraborty, L.W. Potts, R. Tannenbaum, M. Tirrell, J.F. Evans, Langmuir 8 (1992) 1307–1317. [19] F. Jones, J.B. Farrow, W.V. Bronswijk, Langmuir 14 (1998) 6512–6517. [20] J.X. Huang, J.A. Moore, J.H. Acquaye, R.B. Kaner, Macromolecules 38 (2005) 317–321. [21] T.A. Kerr, H. Wu, L.F. Nazar, Chem. Mater. 8 (1996) 2005–2015. [22] D.C. Schnitzler, M.S. Meruvia, I.A. Hümelgen, A.J.G. Zarbin, Chem. Mater. 15 (2003) 4658–4665. [23] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, M. Gratzel, J. Phys. Chem. B 107 (2003) 8981–8987. [24] K. Konstadinidis, B. Thakkar, A. Chakraborty, L.W. Potts, R. Tannenbaum, M. Tirrell, Langmuir 8 (1992) 1307–1317. [25] M.J. McGuire, J. Addai-Mensah, K.E. Bremmell, J. Colloid Interface Sci. 299 (2006) 547–555.