Superlattices and Microstructures 46 (2009) 745–751
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Fabrication of polyaniline/TiO2 heterojunction structure using plasma enhanced polymerization technique Sadia Ameen a , S.G. Ansari b , Minwu Song a , Young Soon Kim a , Hyung-Shik Shin a,∗ a
Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Chonju-561756, Republic of Korea b
Department of Physics, Faculty of Science, Najran University, Najran, Saudi Arabia
article
info
Article history: Received 4 June 2009 Received in revised form 29 June 2009 Accepted 1 July 2009 Available online 16 July 2009 Keywords: TiO2 Polyaniline Plasma polymerization IV properties
abstract The work reports on the fabrication of a p–n heterojunction structure comprised of polyaniline (PANI) and TiO2 nanoparticles. PANI was deposited by plasma enhanced polymerization on TiO2 thin film substrates. The structural and the crystalline properties demonstrated the coherence and the substantive interaction of the plasma polymerized PANI molecules with the TiO2 nanoparticle thin film. The UV–Vis studies of PANI/TiO2 thin film supported the internalization of PANI with TiO2 nanoparticles due to π –π ∗ transition of the phenyl rings with the lone pair electrons (e¯ ) of the nitrogen atom present in the PANI molecules. The I–V characteristics of the PANI/TiO2 heterojunction structure were obtained in the forward and the reverse biased at applied voltage ranging from −1 V to +1 V with a scan rate of 2 mV/s. The proficient current in the PANI/TiO2 heterojunction structure was attributed to the well penetration of PANI molecules into the pores of the TiO2 nanoparticle thin film. The I–V characteristics ensured an efficient charge movement at the junction of PANI/TiO2 interface and thus, behaved as a typical ohmic system. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The desire for inexpensive, renewable energy sources has led to the development of the photovoltaic devices based on the conjugated polymers [1,2] and small molecular organic materials [3,4].
∗
Corresponding author. Tel.: +82 63 270 4318; fax: +82 63 270 2306. E-mail address:
[email protected] (H.-S. Shin).
0749-6036/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2009.07.007
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Researchers have been attempting to make thin film solar cells, using inexpensive liquid based processing techniques such as spin coating [5], doctor blade [6], inkjet printing, and screen printing [7]. Often, the band gap and ionization potential could be tuned to the desired energies by modifying the chemical structure for the fabrication of flexible and lightweight solar cells [8]. Conducting polymers are promising materials because of the variety of structures with which the polymers could be synthesized and the electric charges could pass through the structures [9,10]. For these reasons, these semiconductor polymers have replaced many materials in the technological applications involving the transport of electric charges. Amongst several conducting polymers, (PANI) has gained immense importance due to its unique electrical, optical and photoelectric properties and being most significantly cheaper than other conducting polymers. PANI presents structural variants according to the degree in which its molecules are oxidized, resulting in the diversified electric conductivity [11–13]. Unlike several synthesis procedures for PANI like chemical route, electrochemical synthesis, interfacial polymerization etc., plasma polymerization has its own utility. The procedure is carried out by means of plasma, allowing the direct formation of the polymer from the vapor phase onto the surfaces of the substrates [14–16]. The plasma polymerizations show the importance over several other techniques by producing uniform and smooth polyaniline film, without using other reagents and thus, give a product with less contamination. Furthermore, plasma technique shows superb stability towards high temperature, intensive light and strong electric fields. Recently, conducting polymer/metal oxide based heterojunction structures have received much attention owing to their high current production in an external circuit by excitons dissociation at the metal oxide and π conjugated polymer interface [17–19]. These heterojunction structures confirmed high charge separation and charge carrier transport through the metal oxide and the hole conducting materials respectively. For efficient conducting polymer/metal oxide based heterojunction structure, an important challenge is preventing the recombination of excitons. The charge separation and charge transfer at the interface heterojunction is an essential factor for determining the performance of heterojunction structure. These factors are mainly related to the morphology, uniformity, and interconnection of both the layers of the heterojunction thin film. In this paper, we report the formation of an inorganic/organic heterojunction structure by depositing the plasma polymerized PANI on n-type nanocrystalline titania (TiO2 ) thin film substrate. The method employed here offers an extremely simple and scalable route for the polymerization of the aniline monomer. Results indicate that the effective interaction of PANI with TiO2 nanoparticles form a p–n heterojunction structure. 2. Experimental 2.1. Pulsed plasma polymerization of aniline Plasma polymerization was carried out in an experimental setup, shown in Fig. 1. The experimental setup was consisted of four parts—(1) a reactor chamber quartz tube (2 cm), (2) Cu coil (4 in.), (3) plasma system (R.F. generator: 0–600 W, matching network frequency of 13.56 MHz) and (4) mechanical vacuum pump (speed 600 1/min). Prior to the deposition, the fluorinated tin oxide glass (FTO glass, Hartford Glass Co., 8 /sq, 80% transmittance in visible spectrum) substrates were cleaned ultra-sonically in acetone, ethyl alcohol and distilled water. The glow discharges were introduced through RF amplifier with a resistive coupling mechanism at 13.5 MHz and power of 120 W. These discharges were set without any carrier gas or any other additional chemical elements to prevent the contamination. TiO2 thin layer substrates were placed just below the RF coil, placed inside the quartz tube. Initially, the chamber was evacuated to a base pressure of 10−3 Torr through a rotatory vacuum pump. After attaining the base pressure, aniline monomer (Aldrich, ACS reagent, ≥99.5%) was injected, using a hypodermic syringe (5 mL), fixed at the opposite end of the exhaust. The injector was open for the duration of 15 min. A three way stopper was mounted on the injector to intercept the aniline droplets, preventing from direct impinging on the substrate. The reaction was promoted by the collisions of the aniline monomer molecules with the ions/particles, present in the plasma. Aniline flowed into the reactor, was pumped by the vacuum system, using pressure gradient between the reactor and the container.
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Fig. 1. Reactor setup for plasma polymerization of aniline.
2.2. Fabrication of p–n heterojunction structure The nanocrystalline TiO2 slurry were prepared by the addition of 0.5 g TiO2 (P25 powder, approximately 70% anatase and 30% rutile, Degussa) nanoparticles powder (particles size ∼25 nm) to the incremental addition of 2 ml polyethylene glycol (4 wt% PEG, Fluka, average MW of 20,000) solution into a mortar and pestle by uniform grinding. Each 0.1 ml addition of the aqueous PEG solution was proceeded after achieving a uniform and lump-free slurry. The nanocrystalline TiO2 thin film was obtained by the deposition of slurry on the cleaned FTO glass substrate by simple doctor blade technique and dried at room temperature. The active area of deposited TiO2 thin film was calculated to be ∼0.25 cm2 . Finally, the TiO2 deposited FTO substrates were sintered at 450 ◦ C for 30 min in a muffle furnace. These substrates were later subjected to the deposition of PANI by plasma enhanced polymerization of aniline monomer for the fabrication of p–n heterojunction structure. Current (I)–voltage characteristics of the obtained p–n heterojunction structure were carried out at room temperature (298 K) with an applied voltage ranges from −1 V to +1 V. 2.3. Characterization High purity aniline monomer and other required chemicals were purchased from Aldrich Chemical Corporation and used without further purification. The morphological observation of PANI/TiO2 thin films was done by using field emission SEM (FESEM, Hitachi S-4700). The structural information of the heterojunction devices was obtained by the X-ray powder diffractometer (Rigaku, Cu Kα , λ = 1.54178 Å) in the Bragg angle ranging between 20◦ and 80◦ . The structural characterization of the fabricated p–n heterojunction structure was studied by the Fourier transform infrared (FTIR, Nicolet, IR300) spectroscopy in the wavelength range of 400–4000 cm−1 . UV–Visible was used to observe the optical properties of the fabricated heterojunction structure and was studied by UV–Vis Spectrophotometer (JASCO, V-670, Japan). A scanning potentiostat (EG & G 273) was applied across the voltage from −1 V to +1 V at a scan rate of 2 mV/s for I–V measurements. 3. Results and discussion Fig. 2 shows the cross-sectional and surface FESEM images of pristine TiO2 and PANI/TiO2 thin films substrates. Fig. 2(a) demonstrates a porous TiO2 formed on the FTO substrates. The film is composed of the well crystalline TiO2 nanoparticles of size ∼25 nm, as shown in Fig. 2(c). It is seen in Fig. 2(b) that the PANI molecules are well penetrated into the porous TiO2 thin film upon the plasma enhanced polymerization. In addition, the particles size of TiO2 nanoparticles get altered and increased ∼10–15 nm from its original particle size upon the deposition of plasma enhanced PANI. Moreover, the accumulation of TiO2 nanoparticles occurs after the deposition of PANI on the surface of TiO2 thin film, and could be seen in Fig. 2(d). This assemblage confirms the internalization of plasma deposited PANI into porous TiO2 nanoparticles thin film substrates. The FTIR of plasma polymerized PANI, shown in Fig. 3(b), depicts a slight up-shift of the peaks at 1496 and 1626 cm−1 , unlike, the peaks appeared at 1410 and 1520 cm−1 , generally due to the chemical polymerization of PANI. These peaks are attributed to C=C interactions of benzenoid and
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Fig. 2. Cross-sectioned FE-SEM images of (a) pristine TiO2 (b) PANI/TiO2 thin film and surface FESEM images of (c) pristine TiO2 (d) PANI/TiO2 thin film.
quinoid rings respectively [20]. These shifting in the peak positions could be due to the cross-linking interactions in the plasma treated polymers. The conjecture of a cross-linked structure of PANI films is supported by the appearance of peaks in the range of 2300–3400 cm−1 . The power input for an effective plasma polymerization offers a wider range of discharge conditions under which the ring structure of aniline can possibly be preserved, and are indicated by the absorption bands at 1496 and 1626 cm−1 , which are the characteristic of the benzenoid and quinoid units, respectively. The appearance peak at 3380 cm−1 might due to N–H vibration. This medium and small peak suggests the presence of aliphatic group in the main chain and appeared probably due to the breaking of benzene ring. A peak belonging to the C–H aliphatic vibration is located at 2353 cm−1 . In addition, the intensity of pristine TiO2 peaks, as shown in Fig. 3(c) gets changed after the interaction of TiO2 with PANI. Moreover, the intensity of the peak at 3452 cm−1 increase, shown in Fig. 3(a), which probably due to the effect of plasma polymerization of aniline on TiO2 thin film substrates. From Fig. 4(b), the appearance of peaks at 25.2◦ , 36.0◦ , 37.8◦ , 48.0◦ , 54.1◦ and 55.0◦ match well with JCPDS 21-1272 and confirm the existence of TiO2 structure in PANI/TiO2 thin film. The existence of small intensity peaks lie between ∼20◦ –25◦ is ascribed to the periodicity parallel and perpendicular to the polymer (PANI) chain. However, prominent peaks of PANI at 28.2◦ , 38.6◦ , 48.8◦ , 55.1◦ are clear in the XRD patterns of PANI/TiO2 thin film (Fig. 4(a)). The PANI/TiO2 thin film demonstrates that the peaks present at 26.1◦ , 27.4◦ , 54.1◦ , matches to JCPDS 21–1276 and confirm the presence of TiO2 nanoparticles in the PANI/TiO2 , shown in Fig. 4(a). However, it can be noted that due to the PANI deposition on the surface of TiO2 nanoparticles thin film, the diffraction peaks of TiO2 gets slightly shifted from their original positions at 25.2◦ , 37.8◦ , 48.0◦ , 55.0◦ . Thus, due to PANI deposition on the surface of the TiO2 nanoparticles substrate, the molecular chains of the PANI gets tethered and interacted with TiO2 nanoparticles.
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Transmittance (%)
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Fig. 3. (a) Typical FTIR spectrum of PANI/TiO2 thin film (b) plasma polymerized PANI (c) and pristine TiO2 .
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Fig. 4. XRD patterns of (a) PANI/TiO2 thin film (b) pristine TiO2 films.
To understand the absorption and optical properties, UV–Vis spectroscopy of plasma polymerized PANI and PANI/TiO2 thin film substrate are examined. For pristine plasma polymerized PANI, two electronic bands are found at 264 nm and 292 nm, which are assumed to originate from the π –π ∗ transition of the phenyl rings of PANI [21]. Interestingly, PANI/TiO2 thin film substrate shows a broader peak at ∼292 nm and the shifting of the 264 nm peak is noticed at 267 nm, shown in Fig. 5. It indicates that the degree of the orbital overlap between the π -electrons of the phenyl rings with the lone pair of the nitrogen atom in the PANI macromolecules decreases due of its interaction with TiO2 . Conclusively, the alteration in the conjugation of PANI chains causes the shifting of the peaks in PANI/TiO2 thin films. On comparison with pristine PANI, the PANI/TiO2 thin film has successfully extended the absorption of the spectrum. Fig. 6 shows the forward and reverse biased I–V characteristics of plasma polymerized PANI and PANI/TiO2 heterojunction structure. The PANI/TiO2 heterojunction is created by the film deposition of n-type TiO2 nanoparticles and p-type plasma polymerized PANI. I–V measurements were carried out at room temperature (298 K) with an applied voltage ranging from −1 V to +1 V. The I–V characteristics of pristine PANI substrate show almost symmetrical behavior both in the reverse and forward bias, in which the current increases linearly with the increase of the applied voltage. It is indicated that the formation of efficient charge carriers leads the generation of excitons in the
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Fig. 5. UV–Vis of plasma polymerized PANI and PANI/TiO2 thin film.
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Fig. 6. I–V characteristic curves of (a) plasma polymerized PANI and (b) PANI/TiO2 thin film.
plasma polymerized PANI interfacial layer, and thus, contributes to the increased current in PANI at the high voltage. The I–V characteristics for PANI/TiO2 heterojunction structure indicate that no barrier is apparently present because the I–V characteristic curve is almost linear. Moreover, the forward biased current increases with an increased applied voltage and have attained the almost linear curve. The proficient current in PANI/TiO2 is attributed to well penetration of PANI molecules into the pores of TiO2 nanoparticles layers which improves the degree of contact with TiO2 and thus decreases the series resistance of the cell while increasing the current. Therefore, the good conducting and mobility properties associated with PANI provide suitable conducting pathway and reduce the degree of excitons recombination. This result indicates the efficient charge movement at the junction of PANI and TiO2 interfaces has made the heterojunction structure which behaves as a typical ohmic system. In the reverse bias, the lower current is related to the decrease of the depletion layer because of the presence of the charge carriers at TiO2 nanoparticles thin film. Hence, it is confirmed that a p–n heterojunction at PANI/TiO2 interface has been created. 4. Conclusions PANI/TiO2 p–n heterojunction structures are prepared using the plasma enhanced polymerization of aniline monomer on the surface of TiO2 thin film substrates. The structural and the absorption
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studies of PANI/TiO2 thin film supports the efficient interaction between polymerized PANI and TiO2 nanoparticles. The shifting and the changes in the intensity of the peaks, obtained from FTIR and XRD results, are credited to the substantive internalization between PANI and TiO2 thin films. The I–V characteristics show that the typical ohmic phenomenon is present in the fabricated PANI/TiO2 heterojunction structure. The results from I–V studies indicate the absence of barriers at the heterojunction of PANI and TiO2 interfaces because the I–V characteristic curves are almost linear. It could be concluded that further optimization of the results by performing certain modifications in the synthesis and the deposition techniques could enhance the performance of the heterojunction structure. Acknowledgement This work is supported by the grant of Post Doc program, Chonbuk National University (2008). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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