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Controlled Synthesis of Hydrogen Titanate-Polyaniline Composite Nanowires and Their Resistance-Temperature Characteristics Ligang Gai,†,‡ Guojun Du,† Zhiyuan Zuo,† Yanmin Wang,† Duo Liu,*,† and Hong Liu*,† State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China, and School of Chemical Engineering, Shandong Institute of Light Industry, Jinan 250353, People’s Republic of China ReceiVed: January 14, 2009; ReVised Manuscript ReceiVed: February 28, 2009
We report here an in situ polymerization method for the controlled synthesis of core-shell structured hydrogen titanate-polyaniline composite nanowires. The microstructure and physical properties of the composite nanowires were characterized with XRD, FESEM, TEM, FT-IR, thermal analysis, and UV-vis spectroscopy. It is found that the thermal stability of polyaniline in the composite is greatly improved. A hydrogen bondassisted formation mechanism is proposed to account for the experimental observation. In addition, the temperature dependence of electrical conductivity of an individual composite nanowire was measured as a function of temperature, which reveals improved conductivity compared with hydrogen titanate nanobelt. 1. Introduction Composite inorganic-organic nanostructures have recently attracted much attention due to their excellent physical, mechanical, and electrical properties, such as elevated conductivity, catalytic activity, gas sensitivity, and optoelectronic properties.1-16 In comparison with their bulk counterparts, one-dimensional (1D) nanostructures such as wires, rods, belts, and tubes have become the focus of intensive research because of their unique applications in mesoscopic physics and fabrication of nanoscale devices.17 Recently, TiO2-derived 1D structures, including tubes, rods, belts, and fibers etc.,18-26 have received extensive investigation because of their wide applications in photocatalysis, photoluminescence, gas sensors, and dye-sensitized solar cells. Therefore, the encapsulation of TiO2-derived 1D structures into the core of conducting polymers to obtain core-shell nanostructures is an appealing strategy for designing new types of functional materials. Although hybrid CdSe nanorod-poly-(3-hexylthiophene) composites have been proposed for being a promising low-cost candidate for inorganic solar cell materials,6,9 from the viewpoint of carrier mobility, the combination of 1D inorganic components with conjugated polymers in core-shell structure might be more efficient than those in a blending mode. Recently, a solutionbased in situ polymerization method has been developed to prepare oxide-polymer composite nanostructures.11 However, challenges still exist in the synthesis of 1D core-shell composites with high surface area and effective interfacial association which favor the charge transfer between two components.6,11 Among conducting polymers, p-type polyaniline (PANI) is unique for large scale applications due to its high conductivity, excellent environmental stability, and facile synthesis strategy.27,28 In view of the water-soluble property of PANI monomer, i.e., aniline, along with the hydrophilic surface of hydrogen titanate, we selected hydrogen titanate nanobelts and aniline to synthesize the desirous core-shell composites via an in situ polymerization procedure. Although several recent studies have presented the * Corresponding author. E-mail:
[email protected]. +86-531-88362807. Fax: +86-531-88362807. † Shandong University. ‡ Shandong Institute of Light Industry.
Tel.:
synthesisof1Dcompositescombinedwithconductingpolymers,11,14,15 no further study on electrical properties of individual composite nanowire has been described. In this paper, we report the fabrication of hydrogen titanate nanobelt-PANI core-shell composites, and the temperature-dependent current-voltage (I-V) characteristics of individual hydrogen titanate nanobelt and composite nanowire. 2. Experimental Section 2.1. Materials. Aniline (C6H5NH2, AN) of analytical grade was distilled under reduced pressure and stored at low temperature before usage. Analytical grade hydrochloric acid (HCl, 36-38%), ammonium peroxydisulfate ((NH4)2S2O8, APS), and p-toluenesulfonic acid monohydrate (p-CH3C6H4SO3H · H2O, TSA) were used directly without further purification. 2.1. Preparation of Hydrogen Titanate Nanobelts. An alkali hydrothermal method was employed for the synthesis.29,30 Briefly, 0.5 g of commercial TiO2 powders (P25) was dispersed in 70 mL of 10 M NaOH aqueous solution and placed into a Teflon-lined autoclave. The autoclave was heated to and maintained at 200 °C for 72 h. The precipitate was washed thoroughly with distilled water, followed by filtration, and then dipped in copious 0.1 M HCl aqueous solution for 24 h. Final product was obtained after filtration, washing, and air-drying process. 2.3. Preparation of Hydrogen Titanate Nanobelt-PANI Core-Shell Composites. In a typical procedure, 0.1 g of hydrogen titanate, 0.2 g of AN, 0.57 g of TSA, and 15 mL of anhydrous ethanol were added into a three-neck flask in sequence, sealed, and dispersed by ultrasonic treatment for 5 min. Then the mixture was kept at 5 °C for 24 h. Thereafter the flask was transferred to a thermostatic bath of 0 °C with constant stirring for 15 min, followed by dropwise addition of precooled APS aqueous solution (15 mL, 0.3 M) within ∼1 h and maintained at 0 °C for 24 h. The resulting dark blue product was collected by filtration and then doped in 1 M TSA aqueous solution for 24 h. The final dark green product was sequentially washed with distilled water and then anhydrous ethanol several times and finally dried in a vacuum oven at 50 °C for 12 h.
10.1021/jp900369y CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
Hydrogen Titanate-Polyaniline Nanowires
Figure 1. XRD patterns of hydrogen titanate (a), PANI (b), and the composite (c).
2.4. Characterizations. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu KR radiation (λ ) 1.5406 Å). Fourier transform infrared (FTIR) spectra were collected on a Nicolet Avatar 370 infrared spectrometer using pressed KBr discs. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) patterns were taken on a Hitachi S-4800 fieldemission scanning electron microscope. For the EDS analysis, samples were dispersed in ethanol by ultrasonic treatment and dropped onto a silicon wafer. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained on a Hitachi H-800 transmission electron microscope operating at 150 kV. Thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis were carried out on a Perkin-Elmer DSC-2C thermogravimetric analyzer at a heating rate of 10 °C · min-1 in air. Diffuse reflection ultraviolet-visible (UV-vis) spectra were collected on a SHIMADZU UV-2550 spectrometer equipped with an ISR-2200 diffuse reflection integrating sphere, using BaSO4 as reflection reference. The current-voltage (I-V) characteristics of individual nanowires were measured by a Keithley 2400 SMU electrometer. The samples used for I-V tests were fabricated on mica under a microscope with silver paste as the connecting electrodes. 3. Results and Discussion 3.1. Structure, Composition, and Morphology. Figure 1 shows the XRD patterns of hydrogen titanate (a), PANI (b), and the composite (c). The main diffraction peaks in Figure 1a can be indexed to the hydrogen titanate of H2Ti5O11 · xH2O (JCPDS 44-0131). The XRD pattern of PANI (Figure 1b) shows typical polymer characteristics.31 The diffraction peaks in Figure 1c are almost the same as those in Figure 1a, indicating that no phase transition in hydrogen titanate occurs during the polymerization process. An SEM image of the hydrogen titanate nanobelts (Figure 2a) shows that the nanobelts have smooth and clean surface with width in the range of 50-200 nm, thickness in the range of 20-50 nm, and length of up to a few millimeters. The PANI sample is composed of agglomerated short rodlike nanoparticles (Figure 2b). In the presence of hydrogen titanate nanobelts, PANI preferentially grows on the surfaces of nanobelts due to selective adsorption of free aniline molecules and/or anilinium cations on the hydrogen titanate substrate through formation of hydrogen bonds (Scheme 1). This leads to core-shell composite structures (Figure 2c and d). It is worth noting that the monodisperse composite along with the absence of PANI fragment indicate high quality of the composite in core-shell
J. Phys. Chem. C, Vol. 113, No. 18, 2009 7611 structure. The EDS pattern of the composite (Figure 2e) presents N, C, Ti, O, and S elements, confirming the presence of PANI doped by p-toluenesulfonic acid. The Si element is due to the underlying silicon wafer. Figure 3a shows a low-magnification TEM image of the composite nanowires, wherein undulations on the surface can be observed. The high-magnification TEM image of a single wire (Figure 3b) verifies the core-shell structure with shell thickness in the range of 30-100 nm. The corresponding SAED pattern (Figure 3c) exhibits a typical inorganic/organic composite feature, in which the halo rings are attributable to the noncrystalline organic shell and the spots stemming from the single-crystalline inorganic core are assigned to (110), (011), and (1j01) planes of monoclinic hydrogen titanate, with the electron beam projecting along the [1j11j] zone axis. In order to verify the combination of hydrogen titanate with PANI, the FTIR technique was therefore employed as shown in Figure 4. The IR spectrum of hydrogen titanate nanobelts (curve a) is similar to previous studies reported in the literature.20,32 The three peaks centered at 3260, 1645, and 920 cm-1 can be assigned to O-H stretching mode for interlayer water, oxonium ion, and hydroxyl groups, H-O-H bending for water and oxonium ions, and O-H bending for hydroxyls, respectively.32 The peaks centered at 675 and 466 cm-1 can be respectively attributed to Ti-O-Ti asymmetric and symmetric stretching mode. In curve b for PANI, the broad peak around 3438 cm-1 and the shoulder peak at 3231 cm-1 are due to the stretching vibrations of free and binding N-H in association with hydrogen bond.33 In curve c for the composite, the strong peaks centered at 1581, 1496, 1304, and 1141 cm-1 can be assigned to the stretching vibration of C)N along with deforming vibration of N-H, benzenoid, or quinonoid CdC, stretching vibration of C-N, and in-plane bending vibration of benzenoid or quinonoid C-H, respectively.14,31,34,35 The peaks around 881∼682 cm-1 correspond to the out-of-plane bending vibrations of benzenoid or quinonoid C-H and N-H.31,33 In addition, coexistence of the peaks centered at 1380 and 1247 cm-1, corresponding to the deforming vibration of C-H in -CH3 and stretching vibration of SdO in -SO3H, respectively, confirms the doping of p-toluenesulfonic acid molecules in PANI chains.31 Similar to the reported results in TiO2-PANI nanocomposite powders33 and TiO2-PANI composite nanofibers,14 the main peaks in curve c related to PANI (1581, 1496, 1304, and 1141 cm-1) blue-shift in comparison with those (1570, 1491, 1298, and 1136 cm-1) in curve b. However, the peaks related to hydrogen titanate (653 and 456 cm-1) red-shift in comparison with those (673 and 465 cm-1) in curve a. It is believed that the N atoms with lone electron pairs in PANI are apt to attach to the surface hydroxyls of hydrogen titanate by hydrogen bonds, thereby decreasing the electron cloud density of CdC, C)N, and C-N bonds in the conjugated benzenoid and/or quinonoid rings along with weakening the surface Ti-O bonds, indicating compact combination rather than blend of two components. At the same time, the peak in curve c corresponding to the free N-H is lower in intensity than that in curve b and red-shifts from 3438 to 3400 cm-1, because linear arrangement of atoms in each hydrogen bond (O-H...N or O...H-N) (Scheme 1) confines the vibration and reduces the intensity of N-H bond. 3.2. Formation Mechanism of the Core-Shell Structure. On the basis of the above analysis, a hydrogen bond-assisted formation mechanism of the core-shell structures is proposed as illustrated in Scheme 1. At the first stage, when hydrogen titanate nanobelts, aniline, and TSA are dispersed in anhydrous ethanol and kept at 5 °C for 24 h, aniline molecules and/or
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Figure 2. SEM images of hydrogen titanate (a), PANI (b), the composite (c), and high-magnification SEM image of the composite (d). The EDS pattern (e) corresponds to the area squared in (c).
anilinium cations attach to the surface of the belt through hydrogen bonds (Scheme 1A and B). During the chemical oxidation process, a PANI layer grows on the surface of the belt, accompanied by doping of TSA in the PANI chain, resulting in formation of the core-shell structure (Scheme 1C and D). As mentioned above, due to the selective adsorption of aniline molecules and/or anilinium cations on the surface of the belt, it is reasonable for the absence of PANI fragment in the sample. Such a core-shell structured nanocomposite is beneficial to photoelectric and photovoltaic applications, because of much easier charge transfer between PANI and hydrogen titanate.6,11 3.3. TG/DSC Analysis. Thermal behavior of the samples was investigated by TG and DSC as shown in Figure 5. At temperatures lower than ∼200 °C, weight losses of the samples are mainly assigned to the elimination of adsorbed water and
ethanol. An increasing weight loss of PANI (curve b) occurring at temperatures ranging from 200 to 280 °C is attributed to the loss of acid dopant.36,37 A sharp weight loss appearing at 280-570 °C and continuing until 700 °C for PANI is probably assigned to large scale thermal degradation of PANI chains.36,37 The stepwise weight losses of PANI are correlated with two broad exothermal peaks centered at 280 and 450 °C in its DSC curve (curve b′). It is worth noting that thermal behavior of the composite (curve c) is very close to that of hydrogen titanate (curve a) at temperatures below 300 °C, indicating tight combination between two components, and thereafter strong weight loss takes place revealing decomposition of PANI and dehydroxylation of hydrogen titanate. Unlike the DSC curve of hydrogen titanate nanotubes,20 only one broad peak at temperatures ranging from 150 to 800 °C occurs in hydrogen titanate nanobelts (curve a′) due to continu-
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SCHEME 1: Illustration of the Proposed Formation Mechanism of the Core-Shell Structure: (A) Hydrogen Titanate Nanobelt with Surface Hydroxyls; (B) Aniline and/or Anilinium Cations Attach to the Surface of the Belt through Hydrogen Bonds (A-1 Represents the p-Toluenesulfonic Acid Anion); (C) Growth of Conducting PANI on the Surface of the Belt; (D) Formation of the Core-Shell Structure
Figure 5. TG (solid lines) and DSC (dotted lines) curves of hydrogen titanate (a and a′), PANI (b and b′), and the composite (c and c′).
Figure 3. TEM images and SAED pattern of the composite: (a) a low-magnification image; (b) a high-magnification image corresponding to the wire arrowed in (a); (c) SAED pattern corresponding to the area squared in (b).
Figure 4. IR spectra of hydrogen titanate (a), PANI (b), and the composite (c).
ous dehydration and phase transition of hydrogen titanate to anatase and to rutile TiO2 in sequence. A sharp exothermic peak centered at 550 °C (curve c′) reveals phase transition of
hydrogen titanate to anatase TiO2, and a plateau appearing at 510-540 °C is related to the decomposition of PANI shell, which is higher than that of 450 °C for pure PANI. Several reports have demonstrated that the thermal stability of PANI is decreased by encapsulating inorganic components, because of the weakened interaction between PANI chains which results from the interaction between two components.14,36,38 However, divergence still exists on the thermal stability of PANI in composite.12 In the present case of core-shell structure, the thermal stability of PANI together with that of hydrogen titanate are improved according to the TG and DSC curves, because the polymer shell tightly binds to the surface of the core by hydrogen bonds, leading to a relatively ordered structure and then delaying the degradation of PANI as well as the dehydration of hydrogen titanate. The increased decomposition temperature enables the composite structure to be used at elevated temperatures. The weight percent of PANI in the composite sample is ∼48.8% by stipulating the weight loss in the temperature range of 300-570 °C on TG curves, close to that of 46.9% estimated by CHNS elemental analysis. Under the condition that the weight loss before 200 °C is assigned to the loss of water adsorbed on the surface and the weight loss from 200 to 500 °C is attributed to the loss of structural water,39 then the stoichiometry of the hydrogen titanate is H2Ti5O11 · 2.56H2Oads. 3.4. UV-Vis Spectra. Figure 6 shows the diffuse reflection UV-vis absorbance spectra of the samples. The band gap energy of hydrogen titanate nanobelts (curve a) is determined to be 3.1 eV corresponding to the absorption edge of 400 nm, similar to that of hydrogen titanate nanotubes.20 The appearance of peaks centered at 216 and 293 nm together with whole
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Figure 6. UV-vis spectra of hydrogen titanate (a), PANI (b), and the composite (c). The magenta lines and green lines correspond to the Gaussian fitting of PANI and the composite, respectively.
transmittance in the visible region indicates ultraviolet absorption characteristic of hydrogen titanate nanobelts. However, a dramatic absorption in the range of 200-800 nm is observed in the spectra of PANI (curve b) and composites (curve c). Gaussian fitting of curve b (magenta lines) reveals four peaks centered at 310, 418, 565, and 780 nm, attributable to π-π*, polaron-π*,exciton,andπ-polarontransitions,respectively,15,37,40,41 whereas five peaks centered at 290, 418, 540, 635, and 770 nm occur in curve c by Gaussian fitting (green lines). The peaks appearing at 310, 418, and 780 nm (curve b) confirm the doping state of PANI15 in accordance with the IR results. In curve c, the characteristic peaks of hydrogen titanate cannot be discerned due to the coating of PANI. At the same time, a weak peak around 290 nm appears. This peak blue-shifts ∼20 nm in comparison with that of PANI, which arises from the interaction between two components.40 The peaks centered at 540 and 635 nm correspond to exciton transitions, and the latter is attributed to the exciton transition from a localized benzenoid highest occupied molecular orbital (HOMO) to a quinoid lowest unoccupied molecular orbital (LUMO).42 It is worthy of note that the excitonic peak at 540 nm blue-shifts ∼25 nm in comparison with that in curve b. It is known that the position of this peak is sensitive to the nature of solvent and counterions and to the chemical structure of polymer.42 In the present case, the observed shift together with the dividing of the red band is associated with the core-shell structure of the composite. It should be pointed out that, in curve c, the absorption bands concentrated in the visible region which stem from the PANI shell will cause a decrease in resistivity of the composite nanowires and promise a potential application of the composite in solar cells. 3.5. Resistance-Temperature Dependence of Individual Hydrogen Titanate Nanobelt and Composite Nanowire. The I-V characteristics of individual hydrogen titanate nanobelt and composite nanowire were measured from room temperature to 200 °C. I-V diagrams of the composite nanowire (Figure 7B) show linear behavior, which indicates ohmic contact.43 However, linear behavior can only be observed on hydrogen titanate nanobelt at temperatures above 100 °C (Figure 7A, curves d and e). Figure 7C shows resistance variations of the both 1D structures as a function of temperature. It is found that the resistance of hydrogen titanate nanobelt (Figure 7C, curve a) dramatically decreases with increasing temperature until 100 °C, and thereafter decreases slowly with increasing temperature until 200 °C. However, a steady decrease and a sharp increase in resistance occur in the composite nanowire (Figure 7C, curve b) at temperatures before and after 100 °C.
Figure 7. Temperature-dependent current-voltage (I-V) diagrams of individual hydrogen titanate nanobelt (A) and composite nanowire (B) ((a) 25 °C; (b) 50 °C; (c) 100 °C; (d) 150 °C; (d) 200 °C) and resistance-temperature plots (C) for the measured belt (a) and wire (b).
It has been reported that protonic conduction dominates the conductivity of hydrogen titanate nanotubes at temperatures lower than ∼130 °C.39 With removal of water during heating process, the number of protons decreases. This leads to a disruption of the protonic conduction pathway in particular at temperatures above 130 °C, and hence the conductivity is determined by electronic conduction.39 The humidity induced enhancement of conductivity has been reported on PANI/Co3O4 composite nanopowders, where water molecules were found to facilitate the charge transfer process of conducting species.35 In the case of hydrogen titanate nanobelts, it is known that protons reside in the interlayer spaces of titanate layers.44 Therefore, the decrease in resistance with increasing temperature is assigned to the protons as well as the binding and adsorbed water molecules confirmed by the IR spectrum (Figure 4, curve a) and TG curve (Figure 5, curve a). At temperatures lower than ∼100 °C, proton mobility across the belt is enhanced due to less lattice scattering, corresponding to a sharp decrease in resistance. On the other hand, water molecules are released from
Hydrogen Titanate-Polyaniline Nanowires the belt in particular at temperatures above 100 °C, which hinders the decrease in resistance. With further increase of temperature above 150 °C, the conductivity turns out to be determined by the electronic conduction, leading to a little change in resistance.39 As for the composite nanowire, when temperature is lower than ∼100 °C, a steady decrease in resistance occurs with increasing temperature, due to the adsorbed water molecules which make the polymer more p-type in nature.35 In other words, the hole concentration is increased by donation from the conducting polymer toward the water molecules attaching to hydrogen titanate nanobelt, resulting in an increase in the charge transfer efficiency between two components.35 In addition, the thermal curing effect as well as the molecule rearrangement on heating,45 which cause the molecular conformation favorable for electron delocalization, also contribute to the decrease in resistance. However, at temperatures above 100 °C, a relatively sharp increase in resistance occurs. Similar phenomenon has been observed in NaFe4P12-PANI composite nanowires in pellet form at 160 °C, which has been assigned to the decomposition of PANI polymer.34 In the present case, apart from the favorable role of temperature in contribution to conductivity, the release of water molecules and the partial loss of acid dopant together with the disturbed chain alignment in polymer should be responsible for the increase in resistance with increasing temperature. The I-V measurements of the composite nanowires reveal distinct electrical behaviors from hydrogen titanate nanobelts, which may find applications in electronic and photoelectronic devices. 4. Conclusions In summary, core-shell structured hydrogen titanate nanobelt-conducting polyaniline composite nanowires have been fabricated by a solution-based in situ polymerization method. The core-shell structures are monodisperse with the absence of polyaniline fragment in the sample. A hydrogen bond-assisted formation mechanism is suggested. It is shown that the thermal stabilities of the composite nanowires are improved, enabling their applications at elevated temperature. The composite nanowires show strong absorption in the visible region, which may facilitate their applications as solar cell materials. Individual composite nanowire shows ohmic behavior with lower resistance in comparison with hydrogen titanate nanobelt. In addition, temperature dependent resistance variation is explained in terms of adsorption and desorption of water molecules, partial dopant loss, and polymer chain alignment. The high-quality 1D core-shell composite presented here promises potential applications in sensor and solar cells. Acknowledgment. This work is supported by NSFC (5087070, 50702031, 50721002), National Basic Research Program of China(973ProgramofChina)(G2007CB613302,G2004CB619002), Natural Science Foundation (Y2007F17) of Shandong Province of China, and the Program of Introducing Talents of Discipline to Universities in China (111 program). References and Notes (1) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (2) Calvert, P. Nature (London) 1999, 399, 210. (3) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (4) Chen, R. J.; Zhang, Y. G.; Wang, D. W.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 3838.
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