Synthesis and Photocatalytic Properties of a Polyaniline-Intercalated

Mar 2, 2010 - Conjugated Polymer/Nanocrystal Nanocomposites for Renewable Energy Applications in Photovoltaics and Photocatalysis. Yu-Wei Su , Wei-Hao...
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J. Phys. Chem. C 2010, 114, 4765–4772

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Synthesis and Photocatalytic Properties of a Polyaniline-Intercalated Layered Protonic Titanate Nanocomposite with a p-n Heterojunction Structure Tao Guo, Lishi Wang, David G. Evans, and Wensheng Yang* State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China ReceiVed: June 13, 2009; ReVised Manuscript ReceiVed: February 6, 2010

The synthesis of p-type polyaniline (PAN) by the in situ polymerization of aniline in the confined interlamellar galleries of an n-type semiconducting layered protonic titanate (LPT) affords an intercalated PAN/LPT nanocomposite. The in situ polymerization of aniline can be initiated by oxygen in the air. The resulting nanocomposites have been characterized by XRD, FTIR, UV-visible spectroscopy, TG-DTG, SEM, and elemental analysis. The PAN/LPT nanocomposites show significant absorption in the visible region, whereas the pristine LPT absorbs only in the ultraviolet region. Under visible irradiation, the PAN π-π* transition delivers excited electrons into the conduction band of LPT, and the subsequent electron transfer to a substrate electrode contributes to the photocurrent. The PAN/LPT nanocomposites exhibit much higher photocatalytic activities for the degradation of methylene blue in aqueous solution under visible light irradiation than LPT itself. 1. Introduction The insertion of guests into layered inorganic host materials by the intercalation method allows the fabrication of novel organic/inorganic or inorganic/inorganic composite materials with a wide range of functionality.1 Such hybrid materials are receiving increasing attention because the synergism between host and guest often gives rise to properties that are superior to the sum of those of the individual components. For example, the photochemical properties of photofunctional materials intercalated in layered hosts can be markedly different from those of the free species in homogeneous solution. For such guest species, layered metal oxide semiconductors are of particular interest as host materials because not only the structure and stereochemistry of the host layers give influences on the confined guest materials but also the possibility of electron transfer between host and guest gives rise to additional functionality. Typical layered metal oxide semiconductors include cesium titanate (Cs0.67Ti1.83O4) and potassium niobate (K4Nb6O17).2,3 The alkali metal cations intercalated between the sheets of condensed transition-metal oxide polyhedra can be replaced by various other cations through ion-exchange reactions, as well as by the intercalation of protons and nalkylamines.2 By virtue of their layered structure and the advantages of ease of synthesis, low cost, and environmental friendliness, the applications of layered titanates, in particular, have been extensively studied in biosensing,4 humidity sensing,5 Li batteries,6 and other areas. Layered titanates and niobates have also been shown to act as photocatalysts.7 However, both classes of materials are wide band-gap semiconductors that only absorb photons in the nearUV region.8 Because, ideally, photocatalysts should be able to harvest the visible light resource of sunlight, there have been attempts to modify their properties by intercalating appropriate inorganic guest species.9,10 For example, Choy and co-workers have shown that hybridizing exfoliated titanate nanosheets with * To whom correspondence should be addressed. E-mail: yangws@ mail.buct.edu.cn.

chromia clusters affords a nanohybrid that shows enhanced photocatalytic activity in the decomposition of organic compounds under visible light irradiation.11 In another example, increased photoelectrochemical activity was demonstrated when Ru(bpy)32+-intercalated layered niobate was used to produce H2 from water-methanol solution by irradiation with visible light.12 In the case of TiO2 nanoparticles, it has been shown that hybridization with a conducting polymer by taking the form of attracting polymer to the surface of nanoparticles, such as polyaniline (PAN), can increase their photocatalytic activity toward the decomposition of organic molecules when irradiated by visible light.34 PAN,13 a material with unique properties in respect to electron or hole transport, is a π-conjugated conductive polymer composed of two alternating units, namely, the reduced units and the oxidized units. Conducting PAN behaves as a p-type semiconductor with a narrow band gap and, because it has a band-gap absorption edge that can extend into the range of visible light,14 exhibits good stability and is environmentally benign, and has been used in gas sensing,15 batteries,16 electrochromic displays,17 microelectronic devices,18 and a variety of other applications. If a conducting polymer, such as PAN, can be incorporated into the interlayer galleries of a layered metal oxide semiconductor, it should be possible to realize a similar enhancement in photocatalytic activity to that observed for TiO2 nanoparticles,34 with the additional benefits that the molecular configuration of the polymer may be controlled by the host layers and that intercalation, rather than simple adsorption on the surface of nanoparticles, may enhance the stability of the polymer to leaching or decomposition. The p-n heterojunctions realized by intercalating the p-type conducting PAN into the interlayer galleries of an n-type layered metal oxide should allow the drawbacks of the latter, such as the poor response to visible light, the low efficiency of solar energy conversion, and high rate of electron-hole recombination, to be overcome.19,20 It has been demonstrated in photovoltaic cells and photoelectrochemical cells that p-n heterojunction structures made of n- and p-type semiconductors show greatly enhanced activities com-

10.1021/jp9055413  2010 American Chemical Society Published on Web 03/02/2010

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pared with devices consisting of a single semiconductor.39 Takagi et al. have recently reported the synthesis of PAN intercalated in a layered niobate material and shown that the resulting hybrid material exhibits a reversible redox-based color change under irradiation by UV and/or visible light,3 but there have been no previous reports of the use of conducting polymermodified layered metal oxide semiconductors in the photochemical degradation of organic pollutants. In this work, we describe how aniline may be inserted into a layered protonic titanate (LPT) host based on an acid-base reaction and how subsequent in situ polymerization in the interlayer galleries may be initiated by oxygen in the air. The confined environment, whereby the slabs of the inorganic host act as a template, should afford the ability to control the molecular configuration of the PAN guest molecules within the interlayer galleries.8 The resulting PAN/LPT nanocomposites are hybridized at the molecular level and behave as a p-n heterojunction structure. The PAN/LPT nanocomposites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, UV-visible spectroscopy, TG-DTG analysis, scanning electron microscopy, and elemental analysis and analyzed by photoelectrochemical measurements and electrochemical impedance spectroscopy (EIS) studies. The photocatalytic activity of PAN/LPT was also evaluated by employing the photodegradation of methylene blue (MB) as a probe reaction. 2. Experimental Section Materials. A layered cesium titanate (LCT) (CsxTi2-x/40x/ 0 ) vacancy) was synthesized from Cs2CO3 and TiO2, as outlined in the literature.21 The resulting LCT was converted into layered protonic titanate (LPT) (HxTi2-x/40x/4O4, 0 ) vacancy) by stirring the powder in 150 mL of 1 M HCl solution at room temperature. The acid leaching was repeated three times by renewing the acid solution every 24 h. The resulting acidexchanged product was filtered and then washed with distilled water, followed by drying over a saturated NaCl solution. Analytical grade aniline was purchased from Beijing Chemical Reagent Corp. Conductive PAN was purchased from Jilin Zhengji Corp (P. R. China). All other reagents were received as analytical grade and were used without further purification. Intercalation of Aniline into LPT Interlayer Galleries and in Situ Oxidative Polymerization of Aniline. The intercalation of aniline into LPT was carried out though an acid-base reaction, as follows: A 1 g portion of LPT was dispersed in 100 mL of distilled water, and 3.5 mL of aniline that had been distilled in vacuo was then added. The suspension was maintained with vigorous magnetic stirring in air for 6 days. During this process, the color of the resulting suspension turned from white to dark green. The precipitate was filtered and washed with distilled water and absolute ethanol several times, and finally, the PAN/LPT nanocomposites were obtained by drying at 50 °C in vacuo. Film Preparation by Electrophoretic Deposition. PAN/LPT nanocomposite films for the photoelectrochemical measurements were prepared by electrophoretic deposition (EPD), which is based on the electrophoresis of charged particles under the influence of an electric field.22,23 EPD was carried out using an acetonitrile suspension containing the PAN/LPT nanocomposite powders immediately after ultrasonication, with two 1 × 4 cm2 ITO plates used as electrodes. The distance between the two electrodes was fixed at 1 ( 0.1 cm. The concentration of the suspension was 2.5 g L-1. The EPD was carried out under a constant voltage of 50 V at room temperature. 4O4,

Guo et al. Photoelectrochemical Studies. All photoelectrochemical experiments were carried out in a conventional three-electrode electrochemical cell. The PAN/LPT nanocomposite film, Pt, and Ag/Ag+ were used as the working, counter, and reference electrode, respectively. The working electrode potential is referred to the reference electrode, unless otherwise stated. Tetrabutyl ammonium perchlorate (TBAP) (0.1 M) and 1 M triethanolamine (TEOA) in acetonitrile was used as the electrolyte solution. N2 saturation of the electrolyte was reached before the photoelectrochemical measurements in a quartz cell. A 300 W Xe lamp (Changtuo Photoelectronic Technology Ltd., Beijing, P. R. China) was used as the light source with a UV light filter film that was used to block the ultraviolet light with wavelengths below 400 nm. The photocurrent measurements of the PAN/LPT nanocomposite film were recorded with an electrochemical workstation (CHI660C, Chenhua Instruments Co., Shanghai, P. R. China) under illumination from the Xe lamp. Electrochemical Impedance Spectroscopy and Conductivity Studies. Electrochemical impedance spectroscopic measurements were carried out on an electrochemical ac impedance phase analyzer with a CHI660C electrochemical workstation to probe the impedance. The samples were ground and pressed into disks under a pressure of 100 kg cm-2 and assembled between two stainless steel electrodes. The range of the frequency ω used in the measurements was 0.1-105 Hz. The impedance data were fitted by equivalent circuits using Evolve Circuit software from Wuhan University, P. R. China. Visible Light Photocatalytic Reaction Tests. Visible light photocatalytic activity was evaluated by the degradation of MB in aqueous solution. An aqueous suspension (100 mL) of MB (1 × 10-5 M) and 30 mg of PAN/LPT nanocomposite particles was stirred for 30 min to establish an adsorption/desorption equilibrium before irradiation in a reaction cell made of quartz. Light from a 300 W Xe lamp passed through a UV light filter film (to remove radiation with λ < 400 nm) and was focused onto the reaction cell. Aliquots (3 mL) were removed at given time intervals and centrifuged to remove the particles. The filtrates were analyzed by recording the absorbance at the maximum at 662.5 nm in the UV-visible spectrum of MB. The degradation efficiency at time t was determined from the value of C/C0, where C0 is the initial concentration and C is the concentration of MB at the irradiation time t. Characterization. Powder X-ray diffraction (XRD) analysis was carried out with a Shimadzu XRD-6000 powder diffractometer, using Cu KR radiation (40 kV and 30 mA). Scanning electron microscope (SEM) images were recorded on an Hitachi S4700 SEM. Room-temperature Fourier transform infrared (FTIR) spectra were recorded in the range of 400-4000 cm-1 on a Bruker Vector 22 spectrometer using the KBr pellet technique. C,H,N elemental analysis for PAN/LPT (C, 7.205%; H, 1.068%; N, 1.168%) was carried out using an Elementar Vario El elemental analyzer. Thermogravimetric-differential thermogravimetric (TG-DTG) analysis was carried out on a thermogravimetric analyzer (Henven Instruments Co., Beijng, P. R. China). Samples were heated from 60 to 800 °C at a rate of 10 °C min-1 in ambient atmosphere. Diffuse reflectance UV-visible spectra were recorded on a Shimadzu-2501PC spectrometer equipped with an integrating sphere with a diameter of 60 mm using BaSO4 as a standard. 3. Results and Discussion Powder X-ray Diffraction (XRD) Analysis. The XRD patterns of the layered cesium titanate LCT and acid-exchanged

Intercalated PAN/LPT Nanocomposite

Figure 1. XRD patterns of (a) LCT, (b) LPT, and (c) the PAN/LPT nanocomposite.

form LPT are shown in Figure 1, patterns a and b, respectively. The XRD features for the LCT correspond to the lepidocrocite structure with orthorhombic symmetry. After treatment with HCl, the d value of the (020) reflection increased, from 8.4 Å for LCT to 9.2 Å for LPT, due to the proton-exchange reaction and the concomitant intercalation of water molecular into the interlayer galleries of LPT. The LPT was used as the starting material to carry out the intercalation and polymerization of aniline in the interlayer galleries because its acidic nature allows the intercalation of aniline by an acid-base reaction. The XRD pattern of the material obtained by vigorous stirring in air of an aqueous suspension of LPT with aniline for six days is shown in Figure 1, pattern c. The (020) reflection is shifted to a lower angle relative to that for LPT and corresponds to a basal spacing (d020) of 12.0 Å. Because the thickness of the titanante sheet is about 7.5 Å,11 the gallery height is, therefore, 4.5 Å, which is comparable to the reported values for a monolayer of PAN in other layered solids.8,24 The color of the resulting material turned from white to dark green, which confirms the formation of fully protonated PAN in the interlayer galleries of LPT.25 To further study the process of intercalation and polymerization of aniline, portions of the mixture were removed at given time intervals and the basal spacings d020 of the resulting materials were determined. The variation in the value of d020 with time, shown in Figure 2, suggests that aniline molecules first diffuse into the interlayer galleries of LPT to form anilinium ions by reaction with H+ present in the galleries, giving a material with a maximum d020 value of 12.3 Å, and subsequent polymerization initiated by oxygen in the air leads to a reduced d020 value of 12.0 Å.26 This result is in accord with that of HRTEM analysis (Figure 3) for the PAN/LPT nanocomposite. When the reaction was carried out under N2 protection, the resulting material was white in color with the d020 value of 12.3 Å, which demonstrates that oxygen in the air is acting as the oxidizing agent that initiates the polymerization of aniline. The process of intercalation and in situ polymerization of aniline is illustrated in Scheme 1. Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectra of the PAN/LPT nanocomposite, LCT, and LPT are shown in Figure 4, spectra a-c, respectively. All the characteristic bands of the protonated emeraldine salt (conductive form) of PAN,27 such as the CdC stretching deformation of the quinoid (1576 cm-1) and benzenoid (1474 cm-1) rings, the C-N stretch of secondary aromatic amines (1305 cm-1),

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Figure 2. Variation of basal spacing (d020) vs time during the six-day reaction of aniline with LPT.

Figure 3. HRTEM image of the PAN/LPT nanocomposite.

SCHEME 1: Process for the Intercalation and in Situ Polymerization of Aniline

the aromatic C-H in-plane bending (1140 cm-1), and the outof-plane deformation of C-H in the 1,4-disubstituted benzene ring (822 and 506 cm-1), can be observed in Figure 4, spectrum a. Furthermore, a band at 1248 cm-1 is clearly observed; this is also often interpreted as characteristic of the emeraldine salt form and related to the bipolaron structure.28 On comparison with Figure 4, spectrum a, a broad band around 3400 cm-1 and an absorption at 1630 cm-1 in Figure 4, spectra b and c, which are diagnostic of stretching and bending vibrations of H2O absorbing on the surface of PAN/LPT nanocomposites, have disappeared due to drying the PAN/LPT in vacuo. According to the FTIR spectrum of PAN/LPT prepared under N2 protection shown in Figure 5, the characteristic absorption peaks of the monosubstituted benzene ring (756 and 691 cm-1) merely reveal the structure of aniline monomers, which indicates that aniline is only intercalated into the interlayer space of LPT without polymerization. UV-visible Spectroscopy. Figure 6 shows the diffuse reflectance UV-visible spectrum of the PAN/LPT nanocomposite, with that of the LPT for comparison. The absorption band centered at 278 nm is characteristic of pristine LPT and

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Figure 4. FTIR spectra of (a) the PAN/LPT nanocomposite, (b) LCT, and (c) LPT.

Figure 5. FTIR spectrum of PAN/LPT prepared under N2 protection.

Figure 7. SEM images of (a) the PAN/LPT nanocomposite film obtained by EPD on an ITO substrate, (b) PAN, and (c) LPT. The inset of (a) shows a cross section of the PAN/LPT nanocomposite film.

Figure 6. Diffuse reflection UV-visible spectra of (a) the PAN/LPT nanocomposite and (b) LPT.

is not shifted in PAN/LPT. The spectrum of the PAN/LPT nanocomposite shows two new bands at ca. 400 and 800 nm, which arise from the polaron band transition in PAN in the

interlamellar galleries of LPT. The positions of these two peaks are characteristic of the emeraldine salt form,29 whereas the emeraldine base form shows absorption maxima at around 320 and 600 nm. The absorption band at 800 nm is attributed to the n-π* transition of quinoid rings, and the other band at 400 nm is assigned to the π-π* transition of benzenoid rings of the intercalated PAN. The results confirm that PAN intercalated in LPT has been completely transformed into the emeraldine salt form by protonation of PAN with the interlayer protons of the host, with Ti1-σO24σ- slabs acting as the counteranions. Scanning Electron Microscopy (SEM). An SEM image of the PAN/LPT nanocomposite film on an ITO substrate is shown in Figure 7a. The PAN/LPT nanocomposite has a platy morphology with platelet diameters of 100-800 nm. The surface of the PAN/LPT nanocomposite has an identical appearance to that of LPT, confirming that the PAN was intercalated into the interlayer galleries of LPT, rather than adsorbed on the surface

Intercalated PAN/LPT Nanocomposite

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4769 dehydration of water in the interlayer galleries. Subsequent weight loss arises from collapse of the titanate layers to produce an anatase phase after complete dehydration. The weight loss for conductive PAN (Figure 8b) occurs in three steps.10 At temperatures lower than 131 °C, the weight loss is mainly due to the loss of adsorbed water molecules. The conductive PAN is stable between 131 and 213 °C, whereas degradation occurs at higher temperatures and becomes extensive above 297 °C.30 The TG-DTG curve of the PAN/LPT nanocomposite is shown in Figure 8c. The first step is similar to that of the pure LPT and is due to the dehydration of water in the interlayer galleries, and the second step between 348 and 461 °C is attributed to the decomposition of PAN intercalated into the interlayer galleries of LPT. The third step above 461 °C is assigned to the collapse of the titanate layers. The second step involves a weight loss of ca. 7.40 wt %, which is similar to the amount of PAN in PAN/LPT determined by elemental analysis (C,H,N) (7.76 wt %). It is noteworthy that the temperature of the onset of decomposition of PAN is increased significantly by intercalation in the LPT host. This indicates that there is a strong interaction between intercalated PAN molecules doped by H+ of LPT and the negatively charged slabs of the host. Optical Properties of PAN/LPT. UV-visible absorption measurements are a convenient and effective method for investigating some important features of the band structures of semiconductor materials. By analyzing the absorption coefficients of LPT behaving as an n-type semiconductor, and PAN behaving as a p-type semiconductor, in the PAN/LPT nanocomposites, the so-called optical band-gap energies can be estimated using a classical Tauc equation approach.31 It has been well-established that, for a large number of semiconductors, the dependence of the absorption coefficient, R, for the highfrequency region on the photon energy, Ep, for optically induced transitions, is given by the following expression32

REp ) K(Ep - Eg)n

Figure 8. TG-DTG curves for (a) LPT, (b) conductive PAN, and (c) the PAN/LPT nanocomposite.

of the material. The cross-sectional view in the inset of Figure 7a shows that the PAN/LPT nanocomposite can be deposited on the ITO to form a uniform film. In addition, in Figure 7b,c, comparing the SEM image of PAN with that of LPT, we have obviously noted that no PAN is absorbed on the surface of LPT, indicating that PAN has been inserted into the interlayer space of LPT. This result is also in accord with that of XRD characterization. Thermogravimetric-Differential Thermogravimetric (TGDTG) Analysis. TG-DTG curves of LPT, conductive PAN, and the PAN/LPT nanocomposite are shown in Figure 8. For LPT (Figure 8a), the first weight loss step originates from the

(1)

where Eg represents the optical band gap, Ep is the photon energy, and K is a constant, which depends on the nature of the transition. The parameter n takes into account different possible electronic transitions responsible for light absorption and has a value of 1/2, 3/2, 2, and 3 for allowed direct, forbidden direct, allowed indirect, and forbidden indirect transitions, respectively. Figure 9 shows the Tauc plot of (REp)2 versus Ep for LPT and PAN/LPT nanocomposites. For both materials, the best fit of (REp)2 versus Ep was obtained for n ) 1/2, suggesting allowed direct transitions across the energy band gaps of LPT and PAN/LPT. The extrapolated value (the straight lines to the x axis) of Ep at R ) 0 gives the value of the optical band-gap energy. For LPT (Figure 9a), the value is Eg ) 3.64 eV. PAN/ LPT (Figure 9b) clearly exhibits dual band-gap character with Eg ) 3.66 and 2.76 eV, corresponding to the values for LPT and PAN, respectively, in the nanocomposite. The band gap for pure LPT is close to the value for LPT in the PAN/LPT nanocomposite, showing that the intercalated PAN has no obvious effect on the optical band-gap energy of LPT. The band gap for PAN in PAN/LPT is also comparable to the reported value for PAN in the literature.18 Photoelectrochemical Properties. Pure LPT has a large band-gap energy, and therefore, no photoresponse occurs in the visible region. The photoresponse of a PAN/LPT nanocomposite film prepared by electrophoretic deposition (EPD) on an ITO electrode was examined under visible light irradiation. Figure

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Guo et al. SCHEME 2: Mechanism of the Visible Light Photoelectrochemical Process

Figure 9. Tauc plots for (a) LPT and (b) the PAN/LPT nanocomposite.

Figure 10. Photocurrent response of the PAN/LPT nanocomposite film.

10 illustrates a typical photocurrent response of the PAN/LPT nanocomposite electrode, with triethanolamine (TEOA) as a sacrificial regent. A steady-state photocurrent of ca. 10 µA cm-2 was observed under visible light irradiation. This suggests that a photocurrent is generated by the photoexcitation of intercalated PAN in the interlamellar galleries of LPT and that the intercalated PAN is acting as a visible light photosensitizer. The relative energy levels of p-type PAN and n-type LPT (conduction band, CB, and valence band, VB) are shown in

Scheme 2, in which the potentials for CB and VB of PAN were determined by employing the method in the literature.33 A mechanism for photocurrent generation under visible light irradiation can be proposed as follows: p-type PAN absorbs visible light, inducing a π-π* transition, exciting electrons to the π* orbital; the CB of LPT and π* orbital of PAN are wellmatched in energy level and have a chemical bonding interaction, which causes a synergic effect.34 On the basis of this synergic effect, the photoinduced electrons are transferred from the π* orbital of PAN to the CB of LPT, accompanied by injection of electrons into the ITO substrate electrode. Meanwhile, the holes generated in the oxidized PAN are reduced by the TEOA in solution. Because the nanocomposite forms a p-n heterojunction structure at the molecular level, the recombination of electron-hole pairs is prevented effectively. Electrochemical Impedance Spectroscopy (EIS) and Conductivity Studies. EIS has been widely used for the investigation of charge transport processes in conducting polymers. A Nyquist plot for LPT at room temperature is shown in Figure 11a; the semicircle at high frequencies is followed by a Warburg tail related to the ionic conductivity of interlayer H+ with diffusion favored by the presence of interlayer water molecules.8,35 The Nyquist plots for conductive PAN at room temperature (Figure 11b) show the expected electrochemical reaction character at high frequency rather than H+ transfer character. After PAN was intercalated into LPT, the EIS plots of the resulting nanocomposites at different temperatures present regular curves, as shown in Figure 11c. In contrast, disordered Nyquist plots were obtained from a mechanical mixture of PAN and LPT with the same ratio, which further confirms that the PAN was intercalated into the interlamellar galleries of LPT, rather than being present as a simple mixture or adsorbed on the surface of LPT.8 The impedance plots at different temperatures were fitted using the equivalent circuit model, and the fitted impedance plots are also shown in Figure 11c. The circuit consists of one body resistance Rb, one interfacial resistance Ri, one charge transfer

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Figure 12. Plot of the conductivity of the PAN/LPT nanocomposite vs temperature fitted by the Arrhenius equation.

observation of semicircles whose radius (related to the conductivity) diminished with increasing temperature. From these diagrams and taking account of the geometric characteristics of samples formed into pellets, one can deduce the evolution of the specific conductivity by plotting log σdc versus 1000/T for PAN/LPT nanocomposites in the temperature range of 20-90 °C, as shown in Figure 12. We find that the conductivity of PAN/LPT is dependent on temperature and follows the Arrhenius equation37,38

σ ) σ0 exp(-Ea /kT)

Figure 11. Nyquist plots at room temperature for (a) LPT and (b) conductive PAN. (c) Nyquist plots at different temperatures and the corresponding fitting data for the PAN/LPT nanocomposite.

resistance Rct, and two constant phase elements (CPE). The inclusion of a conducting polymer gives a different electrical response from that of the starting LPT, resulting in Nyquist diagrams that show semicircles without Warburg diffusion. Similar observations have been reported for a variety of composites of conducting polymers, such as polypyrrole and polythiophene, and other layered solids.36 A similar trend to that observed for pure PAN has been found in the Nyquist plots of PAN/LPT nanocomposites, with the

(2)

where k is the Boltzmann constant, T is the temperature, σ0 is the pre-exponential factor, and the Ea is the activation energy. The low activation energy, ca. 0.14 eV, obtained by fitting with the Arrhenius equation can be ascribed to electrical conductivity mainly based on electronic transport, and not to ionic conductivity, as has been discussed above. Interparticle connectivity assured by extra-framework PAN cementing the intercalated particles leads to percolation behavior favoring the transport of the electrical signal. Furthermore, the low activation energy is favorable as far as promotion of the photodegradation of target molecules adsorbed on the surface of PAN/LPT is concerned.15 Visible Light Photocatalytic Activity Performance. Photocatalytic processes based on electron-hole pairs separated by virtue of the formation of the conducting PAN in the interlayers of LPT should give rise to redox reactions with species adsorbed on the surface of the PAN/LPT. The photoelectrochemical properties of PAN/LPT nanocomposites discussed above suggest that the material should exhibit photocatalytic properties under irradiation by visible light. Photodegradation of methylene blue (MB) was selected as a probe reaction. As shown in Figure 13, curve c, the rapid decrease in absorption intensity at 662.5 nm shows that MB is efficiently degraded under visible light irradiation in the presence of PAN/LPT, with degradation being essentially complete after 240 min. In contrast, the concentration of MB decreased much more slowly when irradiated in the presence of pure LPT (Figure 13, curve b). In a control experiment (Figure 13, curve a), no decomposition of MB was observed on irradiation in the absence of LPT or PAN/LPT. The results confirm that the visible light response of LPT has been significantly enhanced by formation of the PAN/LPT nanocomposites.

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Figure 13. Plots of degradation of MB over different photocatalysts under visible light irradiation with λ > 400 nm: (a) blank, (b) LPT, and (c) the PAN/LPT nanocomposite.

4. Conclusions Novel PAN-intercalated LPT nanocomposite materials have been successfully prepared through the intercalation of aniline and its subsequent in situ polymerization, initiated by O2 in the air, in the interlamellar galleries of the LPT. FTIR and UV-visible spectroscopy, as well as the dark green color of the resulting samples, confirm that the interlayer PAN is doped by protons from the layered host, resulting in the conductive emeraldine salt form of the polymer. The PAN/LPT nanocomposites exhibited superior thermal stability to that of pure PAN. According to diffuse reflection UV-visible spectroscopy and the Tauc equation, the optical band-gap energies of the LPT host and intercalated PAN guest are Eg ) 3.66 and 2.76 eV, respectively. By virtue of the p-n heterojunction structure of PAN/LPT nanocomposites, a photocurrent was observed under visible light illumination. In contrast to the disordered EIS plots observed for a mechanical mixture of PAN and LPT, the regular Nyquist plots of the PAN/ LPT nanocomposites suggest that the PAN and LPT are hybridized at the molecular level. The variation of conductivity with temperature follows Arrhenius behavior. The p-n heterojunction structure of PAN/LPT contributes to the remarkable enhancement in the visible light photodegradation efficiency of MB compared with that for pure LPT. The properties of the PAN/LPT nanocomposites suggest that the materials have potential applications in novel electronic devices, photovoltaic cells, and sensors, in addition to their activity as photocatalysts for environmental remediation. Acknowledgment. This work was supported by the National Natural Science Foundation of China, the National 863 Project (No. 2006AA03Z343), the 111 Project (No. B07004), and the Program for New Century Excellent Talents in University. References and Notes (1) Gomez-Romero, P.; Sanchez, C. Functional Hybrid Materials; Wiley-VCH: Weinheim, Germany, 2004; Chapters 1 and 8.

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