1. Introduction
Photovoltaics has received increasing attention over the past decades as a feasible way to replace the diminishing fossil fuels and reduce environmental damage.
Inorganic-organic heterojunction photovoltaic devices have been elicited because of their advantages, such as low cost and light weight. Inorganic semiconductor particles, such as TiO2 [1] [2] , have been used as electron acceptor in solar cells. The sol-gel method has been selected to allow the sample preparation of high-purity films at low cost. Conducting polymers used as hole transporting layers have been recently applied on photovoltaic (PV) cells. We have investigated the effect of TiO2 nanoparticle concentration on thin film morphology and the performance of PANI/TiO2 solar cells.
Conducting PANI is important and has exhibited great potential for commercial applications because of its unique electrical, optical, and photoelectrical properties, as well as its easy preparation and excellent environmental stability [3] [4] . Nanocrystalline TiO2 has also been frequently used for preparing various nanocomposites with conducting polymers because of its excellent physical and chemical properties and promising applications in advanced coatings, solar cells, gas sensors, and photo catalysts [5] . Therefore, PANI/TiO2 nanocomposites have been the most intensively studied among various nanocomposites, because they combine the merits of PANI and nanocrystalline titanium dioxide (TiO2) particles within a single material and could be applied in electronic devices, nonlinear optical system, gas sensors, and photoelectrochemical devices [6] [7] . Most of the properties of these materials are based on the synergy between the properties of the components, which are a direct result of their chemical and structural compositions, and thus, can be tailored. For instance, coatings based on organic-inorganic hybrid materials have the capability to combine the flexibility and easy processing of polymers with the interesting properties of the inorganic part: hardness, thermal stability, as well as electrical and electrochemical distinguished properties. The combination of nanocrystalline titanium dioxide (TiO2) and polyaniline (PANI) is attractive because the combination of PANI and metal oxide exhibits excellent electrical, mechanical, and optical properties, such as surface hardness, modulus, strength, transparency, high refractive index, and acids; their derivatives are highly promising coupling molecules that allow the anchoring of organic groups to inorganic solids [8] [9] . Thus, the preparation of PANI-nano-TiO2 has been a subject of interest in many studies. Feng et al. synthesized a composite of PANI encapsulating TiO2 nanoparticles through in situ emulsion polymerization [10] .
The authors explained the nature of chain growth and interaction between PANI and nano-TiO2 particles by Fourier transform infrared (FTIR) spectroscopic analyses [10] . Xia and Wang prepared PANI nanocrystalline titanium dioxide (TiO2) composite through ultrasonic irradiation, which is a novel method for the preparation of 1D to 3D conducting polymer nanocrystalline composites [7] . Somani et al. reported the preparation of highly piezoresistive conducting PANI-TiO2 composite through in situ deposition technique at low temperature (0˚C) [9] . The technological relevance of both conducting PANI and semiconducting material TiO2 in nano form leads to the preparation of a composite of PANi and TiO2 at molecular-level interaction. Such molecular-level interaction may lead to novel properties in these two dissimilar chemical components [11] -[13] . In this paper, we report the synthesis of PANi/TiO2 composite by sol-gel method. Their morphological, structural, electrical, and optical properties are also studied.
2. Materials and Methods
2.1. Materials
Aniline (C6H5NH) from Merck (Schuchardt, Germany) was purified through distillation at reduced pressure before it was used. Ammonium peroxydisulfate (APS) was purchased from Merck (KGaA, Germany). Nanodimensional titanium dioxide (TiO2, 99.7%) and anatase nanoparticles with size <25 nm were also used.
2.2. Synthesis of Polyaniline
PANI was synthesized through the polymerization of aniline in the presence of hydrochloric acid as a catalyst and ammonium peroxydisulfate as an oxidant by chemical oxidative polymerization method. For the synthesis, 50 ml of 1 M HCl was taken, and 2 ml of aniline was added together into a 250 ml equipped with electromagnetic stirrer. Then, 5 mg of ammonium peroxydisulfate ((NH4)2S2O8) in 50 ml and 1 M HCl were suddenly added into the above solution. The polymerization temperature at 0˚C was maintained for 5 h to complete the polymerization reaction. Then, the obtained precipitate was filtered.
The product was washed successively by 1 M HCl followed by distilled water and washed until the solution turned colorless. Then, the product was re-filtered and thoroughly washed once again by distilled water to obtain the emeraldine salt (ES) form of PANI. To obtain the emeraldine base (EB) form of PANI, the ES form of PANI with 0.1 M NH4OH solution was dried at 60˚C in vacuum oven for 24 h. Thus, the powder of insulating PANI EB polymer was obtained [9] .
2.3. Synthesis of (TiO2/PANI) Nanocomposite
Figure 1 schematically shows that the photovoltaic device structure is ITO glass/TiO2/PANI/Ag.
The device dimension for this measurement was 1 cm2. Titanium dioxide was used as a material for the thin films of the nanocomposite. About 4 ml of m-cresol (C7H8O) 97.7% from Acros, USA was added to the TiO2 nanoparticle powder under vigorous stirring for 12 h for peptization. The sols were deposited on ITO conducting glass through spin-coating method at 1500 rpm for 60 s. Then, the sols were annealed at 450˚C for 2 h in a tube furnace (Model: LENTON VTF/12/60/700). The TiO2 nanoparticle with thickness of 120 was prepared through sol-gel method and annealed at 450˚C and had a perfect crystalline structure. The formation of the Ti-O-Ti bonds in the films was observed after thermal treatment. However, the film became crystalline at anatase phase after annealing at 450˚C.
PANI (emeraldine base, EB) powder was dissolved in 1:1 m-cresol deposit on the obtained TiO2 thin films through spin-coating method at 3000 rpm for 60 s, and then dried at 100˚C for 10 min. Afterward, the film was dried at 60˚C for 24 h in an oven vacuum. Ag, an electrode, was evaporated in high vacuum with 10−4 Pa pressure during evaporation.
2.4. Characterization and Measurement Methods
X-ray diffraction (XRD) studies were carried out using high resolution X-ray diffractometer (Model: PANalytical X pert Pro MRD PW3040). The XRD patterns were recorded in the 2θ range of 20˚ - 70˚ with step width of 0.02˚ and step time of 1.25 s by using CuKα radiation (λ = 1.5406 A˚). XRD patterns were analyzed by matching the observed peaks with the standard pattern provided by JCPDS file. FTIR spectroscopy (Model: Perkin Elmer Spectrum Gx) of iO2, PANi, and PANi:TiO2 (20%) composite was studied in the frequency range of 400 - 4000 cm−1. Morphological study of the films of PANi and PANi:TiO2 composite was carried out using field effect scanning electron microscopy (Model: FEI Nova NanoSEM 450) operated at 20 kV. UV-vis spectra of the samples, which were dispersed in deionized water under ultrasonication, were recorded on a Shimadzu 1800 UV-vis spectrophotometer.
The I/V characteristic measured by a Keithley 2400 current-voltage source in the dark indicated that no barrier was apparent at the Ag/PANI or ITO/TiO2 interface.
3. Result and Discussion
Figures 2(a)-(c) show the XRD patterns of pure PANI in EB form, TiO2, and PANI:TiO2 (20%) composite. The XRD pattern of PANI shows a broad peak at 2θ = 22.68˚, which corresponds to 112 planes of PANI [10] . In Figure 2(b) and Figure 2(c), the patterns show sharp and well-defined peaks, indicating the crystallinity of the synthesized materials. The observed 2θ values were consistent with the standard values and showed the tetragonal structure of TiO2 [8] . Figure 2(b) shows that a = 3.78 A˚ and c = 9.51 A˚ [14] .
The intensity of the diffraction peaks for PANI:TiO2 composites was lower than that for TiO2 (Figure 2(c)). Noncrystalline PANI reduced the volume fraction percentage of TiO2, and thus, weakened the diffraction peaks of TiO2 in the composite.
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Figure 1. Structure of the (1 cm × 1 cm) ITO glass/TiO2/PANI/Ag.
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Figure 2. X-ray diffraction (a) PANi (EB), (b) TiO2, and (c) PANI:TiO2 nanocomposite.
Figures 3(a)-(c) show the FESEM of pure PANI, pure TiO2, PANI:TiO2 (20%), and nanocomposite. FESEM image of the composite shows a uniform distribution of the TiO2 particles in the PANI chains without any agglomeration. According to the FESEM images, the nanostructure TiO2 particles are embedded within the netlike structure built by PANI chains. The composite is highly microporous and is able to increase the liquid-solid interfacial area [15] [16] .
Figures 4(a)-(c) show the FTIR spectra of the undoped PANi, PANi-TiO2 composite, and TiO2 nanoparticles. The origins of the vibration bands are as follows: 3365, 2922, and 622 cm−1, which are caused by the NH stretching of aromatic amine, CH-stretching, and CH out-of-plane bending vibration, respectively. The CH out-of- plane bending mode has been used as a key to identify the type of substituted benzene.
The bands at 1665 and 1489 cm−1 are attributed to the C=N and C=C stretching mode of vibration for the quinonoid and benzenoid units of PANI. The peaks at 1296 and 1155 cm−1 are assigned to the C-N stretching mode of benzenoid ring.
The bands in the region 1000 - 1115 cm−1 are caused by the in-plane bending vibration of C-H mode. The bend at 850 cm−1 originates from the out-of-plane C-H bending vibration.
The low wavenumber region exhibits a strong vibration around 621 cm−1, which corresponds to the antisymmetric Ti-O-Ti mode of the titanium oxide [8] .
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Figure 3. FESEM morphology of (a) pure PANI, (b) pure TiO2, and (c) PANI:TiO2 (20% wt).
The absorption of PANI pure film at the visible spectrum, which was measured on a Shimadzu UV1700 ultraviolet visible spectrophotometer, is shown in Figure 5.
Notably, two peaks lie at about 426 and 805 nm. These peaks indicate that the insertion of nanoparticles TiO2 has the effect of doping the conducting PANI, and hence, should lead to an interaction at the interface of PANI and nanoparticles TiO2. Strong terrestrial solar photon flux between 400 - 900 nm was noted. A primary factor influencing the photo-induced carrier mechanism of solar cells should also be considered. Therefore, the limiting factor of TiO2/PANI and solar cell devices is the low absorption of photons. We could solve this issue by increasing the absorption spectrum of polyaniline in the visible zone using suitable dopants.
A built-in electric field at the nanocrystalline TiO2/polyaniline interface has been created. Figure 6 shows the I/V characteristics obtained from the devices under 50 mW/cm2. A short-circuit current density of 3.15 mA/cm2 and an open-circuit voltage of 0.656 V were obtained from device. The efficiency of the solar cell was very minimal (η = 0.0004%) because of the increased resistance of the device, leading to the reduction of the open-cir- cuit voltage. Figure 6 shows the absorption spectrum of the polyaniline in the visible spectrum. The strong terrestrial solar photon flux between 400 and 900 nm should be considered a primary factor influencing the photo-induced carrier mechanism of a solar cell. This result suggests that the low absorption of photons, which is the limiting factors of TiO2/polyaniline solar cells, might be solved by the increase of the absorption spectrum of polyaniline in the visible region by using suitable dopants.
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Figure 4. FTIR spectra of (a) PANI (EB), (b) PANI:TiO2, and (c) TiO2.
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Figure 5. The absorption spectrum of polyaniline (EB) in the visible spectrum.
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Figure 6. I/V characteristic of the sandwich-type structure of PANI, TIO2, and ITO/TiO2/PANI/Ag.
4. Conclusion
Thin films of conducting polymer (PANI), TiO2 nanoparticles, and PANI/TiO2 nanocomposites were synthesized through sol-gel method. The absorption peaks in the FTIR and UV-vis spectra of PANI/TiO2 composite films were found to shift around higher wavenumber compared with those in pure PANI. The observed shifts were attributed to the interaction between the TiO2 particles and polymer molecular chains PANI. A change in the value of the lattice parameter of TiO2 in the PANi/TiO2 composite was observed, which also indicated the presence of interaction between TiO2 particles and PANI matrix polymer. FESEM analysis of PANI/TiO2 composite films revealed uniform distribution of TiO2 particles in the PANI matrix. The I/V characteristic for the device under simulated solar radiation (50 mw/cm2) has the largest open-circuit voltage of 0.656 V and short- circuit current density of 315 mA/cm2.
Acknowledgements
We gratefully acknowledge the support of the School of Physics, USM, under the short-term grant nos. 203.PSF.6721001 and 304/PFIZIK/6312076.