Preparation of Oriented Titanium Phosphate and Tin Phosphate

Jul 16, 2008 - Takahiro Takei , Kiyoaki Iidzuka , Akira Miura , Sayaka Yanagida ... Takahiro Takei , Qiang Dong , Yoshinori Yonesaki , Nobuhiro ... Pr...
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Langmuir 2008, 24, 8554-8560

Preparation of Oriented Titanium Phosphate and Tin Phosphate/ Polyaniline Hybrid Films by Electrochemical Deposition Takahiro Takei,* Yoshinori Yonesaki, Nobuhiro Kumada, and Nobukazu Kinomura Center for Crystal Science and Technology, UniVersity of Yamanashi, 7 Miyamae, Kofu 400-8511, Japan ReceiVed December 13, 2007. ReVised Manuscript ReceiVed March 3, 2008 The preparation of hybrid films of metal (Ti and Sn) phosphate nanosheets and polyaniline by simultaneous electrophoretic and electrolytic deposition was performed in an acetonitrile solvent. Emeraldine polyaniline was intercalated between the phosphate nanosheets with a monolayer arrangement. The obtained hybrid films were several tens of micrometers in thickness. The ratio of incorporated polyaniline to metal phosphate in the hybrid films reaches to around 0.45 and 0.30 at suitable concentrations of tetrabutylammonium hydroxide (TBAOH). These amounts correspond with occupancy of polyaniline in the interlayer gallery of several tens percent. Fractions of voids in a horizontal direction were around 22 and 1% in titanium phosphate/polyaniline and tin phosphate/polyaniline hybrid films, respectively. Thus, anodic electrodeposition makes it possible to form thick films of intercalation compounds of R-titanium and tin phosphates with polyaniline. These hybrid films were examined for redox activity. The cyclic voltammetry results of these films confirmed that the hybrid films have redox activity by polyaniline. For these voltammograms, the maximum current was observed in the tin phosphate/polyaniline hybrid deposited for 15 min. The redox activity of these hybrids possibly depends on the mesoscopic texture of the film, especially on the amount of voids in a horizontal direction.

Introduction Layered inorganic compounds have been examined for their soft chemical properties of intercalation, ion exchange, and exfoliation so far. Recently, exfoliation was applied for the preparation of highly oriented films,1–4 inorganic-organic nanohybrids,5,6 electrodeposited films,7,8 nanotubes,9–11 core-shell particles,12 and so on. Especially, the oriented hybrid films were prepared by the Langmuir-Blodgett method13,14 or by a layerby-layer sequential adsorption process;1–4 however, this process was very time-consuming to prepare thick films because only one layer can be adsorbed per immersion process. On the other hand, thick and free-standing films have been produced by casting suspensions or by spin coating of the exfoliated sheets of layered inorganic materials.15–17 These films have an ability for * To whom correspondence should be addressed. Telephone: +81-55220-8616. Fax: +81-55-254-3035. E-mail: [email protected]. (1) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H.-N.; Waraksa, C. C.; Miwa, T.; Fujishima, A.; Mallouk, T. E. Chem. Mater. 1999, 11, 1526–1532. (2) Kim, H.-N.; Keller, S. W.; Mallouk, T. E. Chem. Mater. 1997, 9, 1414– 1421. (3) Wang, L.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina, Y.; Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873–2878. (4) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M. Chem. Mater. 2001, 13, 4661–4667. (5) Liu, Z.-H.; Yang, X.; Makita, Y.; Ooi, K. Chem. Mater. 2002, 14, 4800– 4806. (6) Saruwatari, K.; Sato, H.; Idei, T.; Kameda, J.; Yamagishi, A.; Takagaki, A.; Domen, K. J. Phys. Chem. B 2005, 109, 12410–12416. (7) Sugimoto, W.; Terabayashi, O.; Murakami, Y.; Takasu, Y. J. Mater. Chem. 2002, 12, 3814–3818. (8) Koinuma, M.; Seki, H.; Matsumoto, Y. J. Electroanal. Chem. 2002, 531, 81–85. (9) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 3427–3434. (10) Saupe, G. B.; Waraksa, C. C.; Kim, H.-H.; Han, Y. J.; Kaschak, D. M.; Skinner, D. M.; Mallouk, T. E. Chem. Mater. 2000, 12, 1556–1562. (11) Wu, D.; Liu, J.; Zhao, X.; Li, A.; Chen, Y.; Ming, N. Chem. Mater. 2006, 18, 547–553. (12) Wang, L.; Ebina, Y.; Takada, K.; Sasaki, T. J. Phys. Chem. B 2004, 108, 4283–4288. (13) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618–620. (14) Umemura, Y.; Shinohara, E.; Koura, A.; Nishioka, T.; Sasaki, T. Langmuir. 2006, 22, 3870–3877.

intercalation or ion-exchange after deposition to give inorganic– organic hybrid or cation-exchanged films. However, there are some restrictions for insertion of foreign specimens. Films of inorganic layered compounds incorporating an organic polymer, especially a conductive polymer as a guest, have many applications in antistatic coating, organic solar batteries, secondary batteries, and redox capacitors. These applications require thick films that contain electroactive components such as polypyrrole, polyaniline, or polythiophene. Especially, polyaniline intercalation into inorganic layered materials has been examined using metal phosphate,18 vanadium oxide,19,20 and molybdenum oxide21 as host compounds; however, the polyaniline-intercalated samples were in the powder state. Meanwhile, we have reported the preparation of highly oriented zirconium phosphate and polyaniline-intercalated zirconium phosphate films via an electrodeposition method.22 In this reference, zirconium phosphate nanosheets were deposited orientationally on a platinum substrate with/ without intercalation of polylaniline. The thickness of the obtained films the reached with ease to several tens of micrometers. Therefore, this novel process has a great advantage for the preparation of thick and highly orientated inorganic-organic hybrid films. Generally, an R-metal phosphate has two hydrogen phosphate anions (HPO42-) per one tetravalent metal cation (Ti, Ge, Zr, Sn, Hf, Pb) in its structure. The hydroxyl group that protrudes in the interlayer space provides solid Brønsted acidity, proton conductivity, ion changeability, intercalation ability, and so on. The (15) Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem. 2004, 14, 165– 174. (16) Abe, R.; Hara, M.; Kondo, J. N.; Domen, K.; Shinohara, K.; Tanaka, A. Chem. Mater. 1998, 10, 1647–1651. (17) Abe, R.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Chem. Mater. 1998, 10, 329–333. (18) Liu, Y.-J.; Kanatzidis, M. G. Chem. Mater. 1995, 7, 1525–1533. (19) Wu, C.-G.; DeGroot, D. C.; Marcy, H. O.; Schindler, J. L.; Kannewurf, C. R.; Liu, Y.-J.; Hirpo, W.; Kanatzidis, M. G. Chem. Mater. 1996, 8, 1992–2004. (20) Kanatzidis, M. G.; Wu, C.-G. J. Am. Chem. Soc. 1989, 111, 4139–4141. (21) Kerr, T. A.; Wu, H.; Nazar, L. F. Chem. Mater. 1996, 8, 2005–2015. (22) Takei, T.; Kobayashi, Y.; Hata, H.; Yonesaki, Y.; Kumada, N.; Kinomura, N.; Mallouk, T. E. J. Am. Chem. Soc. 2006, 128, 16634–16640.

10.1021/la703886y CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

Phosphate Nanosheet/Polyaniline Electrodeposition

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Brønsted acidity might promote hybridization with a cationic polymer because of the negative charges of the phosphate nanosheet which acts as a conjugated base. We studied the preparation of hybrid films of R-metal phosphate (titanium and tin) and polyaniline hybrid films by simultaneous electrophoretic and electrolytic deposition, and examined the effect of tetrabutylammonium hydroxide concentration and effect of constituent metal in the phosphate for film preparation, their mesoscopic texture, and electrochemical properties of the film.

Experimental Section Synthesis of r-Ti(HPO4)2 · H2O and r-Sn(HPO4)2 · H2O. R-Ti(HPO4)2 · H2O was synthesized by the reflux process reported by Bruque et al.23 A total of 4.0 g of titanium dioxide (anatase) was refluxed in 100 cm3 of H3PO4 (14.7 mol/dm3) at 100 °C for 6 h. The solution into which 60 cm3 of H2O was added was then refluxed again for 60 h. The obtained precipitate was washed with distilled water, separated by centrifugation, and dried at 50 °C. On the other hand, R-Sn(HPO4)2 · H2O was prepared by a hydrothermal process. SnCl4 · 5H2O aqueous solution (0.54 mol/dm3) was evaporated to dryness completely above 150 °C. The obtained powder (1.0 g) was put into 50 cm3 of H3PO4 aqueous solution (8.8 mol/dm3). This solution was then poured into a Teflon-lined vessel and hydrothermally treated at 180 °C for 168 h. The deposition was centrifuged, washed with distilled water, and dried at 50 °C. Exfoliation of Titanium Phosphate and Tin Phosphate, and Electrochemical Deposition. The metal phosphates were suspended in acetonitrile as a solvent at a concentration of 6.7 × 10-3 mol/dm3. Tetrabutylammonium hydroxide was added to this solution at concentrations of 2.48-7.44 × 10-3 mol/dm3 as an exfoliator. Tetrabutylammonium chloride was also added to this solution at a concentration of 4.96 × 10-3 mol/dm3 as an electrolyte for a supply of conductivity. The mixed solutions were stirred for more than 12 h. Aniline was also added to the solution at a molar ratio of aniline to metal phosphate of 1.5 as a source of polyaniline (PAni) according to the process reported.22 Electrochemical deposition was carried out by using a direct current (DC) power supply (AE-8750, ATTO Corp.). Platinum plates (10 mm × 50 mm × 0.1 mm) were used as both electrodes of the two-electrode cell. The electrodes were fixed in parallel at an interval of 10 mm. These Pt plates were immersed vertically in the solution, such that the lower 20 mm was in the solution. The electrodeposition was performed at a current density of 1.25 mA/cm2 for 15, 30, 60, and 90 min. In all electrodeposition treatments, there was no deposit on the cathode. Characterizations. XRD patterns of the obtained films were measured by X-ray powder diffraction (XRD; RINT-2000, Rigaku) with Cu KR radiation. Fourier transform infrared (FT-IR) spectra (FT/IR-4100, Jasco) were taken to examine the exfoliated state of metal phosphate nanosheets and the polymerization of aniline using an attenuated total reflection (ATR) attachment with a ZnSe prism. Amounts of PAni in the hybrid films were estimated by CHN elemental analysis (MT-5, Yanaco). Surface charge properties of the exfoliated nanosheet on which aniline molecules might be adsorbed electrostatically were examined by ζ-potential measurements (Zetasizer Nano-Z, Malvern Instruments). The film thickness was measured by micrometer gauge several times for each sample. The electrochemical properties of the films were measured by cyclic voltammetry using a potentiostat and a function generator (HA-301 and HB-104, Hokuto Denko) with a saturated calomel electrode (SCE) reference electrode in a three-electrode cell. The voltammograms were obtained in acetonitrile solution containing 1.0 mol/ dm3 NaClO4. Sample, counter, and reference electrodes were immersed in the electrolyte solution, and the cyclic voltammograms were measured between -0.48 and 7.52 V vs SCE at sweep rates of 5 and 20 mV/s. (23) Bruque, S.; Aranda, M. A. G.; Losilla, E. R.; Oliver-Pastor, P.; MairelesTorres, P. Inorg. Chem. 1995, 34, 893–899.

Figure 1. XRD patterns and FE-SEM photographs of synthesized TiP and SnP.

Figure 2. XRD patterns and photographs of electrodeposited TiP and TiP/PAni hybrid films depending on TBAOH concentration.

Results and Discussion Dependence of Film Texture on the Concentration of Tetrabutylammonium Hydroxide. Figure 1 shows XRD patterns and field emission scanning electron microscopy (FESEM) photographs of the prepared titanium phosphate and tin phosphate powder. All diffraction lines were attributed to R-titanium phosphate (TiP) or R-tin phosphate (SnP). As seen in these patterns, since the diffraction line from the perpendicular direction to the layered structure of SnP is slightly broader than that of TiP, crystallinity with such a direction of SnP is relatively poorer than that of TiP. The 002j diffraction line in these patterns indicates interlayer spacing of around 0.77 for TiP and 0.79 nm for SnP. From the photographs, since the nanosheet sizes seem to be very similar of around several micrometers, hybrid structures, processes of formation for hybrid films from the nanosheet, and redox activities will be comparable. Figure 2 shows XRD patterns and macroscopic photographs of TiP and TiP/PAni hybrid films electrodeposited on a Pt anode from solutions with various concentrations of tetrabutylammonium hydroxide (TBAOH). For the XRD patterns of TiP films, a sharp peak can be observed at around 0.77 nm for the film prepared from the solution containing 2.48 mmol/dm3 TBAOH,

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Figure 4. FE-SEM photographs of surfaces on (a) electrodeposited TiP, (b) TiP/PAni, (c) SnP, and (d) SnP/PAni hybrid films.

Figure 3. XRD patterns and photographs of electrodeposited SnP and SnP/PAni hybrid films depending on TBAOH concentration.

whereas a broad peak emerged at around 0.83 nm for the film from the solution containing 4.96 mmol/dm3 TBAOH. From the solution containing 7.44 mmol/dm3 TBAOH, the diffraction peak at 0.77 nm vanished and the broad peak shifted slightly to around 0.89 nm. The shift and broadening of the diffraction line possibly correspond to a mismatch of deposited TiP nanosheet direction (twisted and parallel displacement) and to turbostratic restacking. From the macroscopic photographs, the surfaces of the obtained films seem to be flat and smooth. The films prepared are white and turn from opaque to translucent and finally to transparent with an increase of TBAOH concentration from 2.48 to 4.96 and 7.44 mmol/dm3. For TiP/PAni hybrid films, expanded interlayer spacing at around 1.6-1.8 nm was observed as well as the peak at around 0.77 nm and the latter peak diminished almost at the TBAOH concentration of 7.44 mmol/dm3. The increase of TBAOH concentration promotes exfoliation of TiP in acetonitrile solution, and the exfoliation seems to be completed at 7.44 mmol/ dm3. The deposited PAni is probably emeraldine from their color. Figure 3 shows XRD patterns and photographs of SnP and SnP/PAni films electrodeposited on a Pt anode from solutions containing various TBAOH concentrations. The general trend is similar to the cases of TiP. For the films without aniline, the diffraction peak at around 0.79 nm tends to shift to a lower angle and to broaden with an increase of TBAOH concentration. The obtained film grown from the solution containing 2.48 mmol/ dm3 TBAOH is white and translucent. However, the film becomes clear with increasing TBAOH concentration. For the SnP/PAni hybrid films, the diffraction peak emerged at around 1.5 nm for those from the solution containing 6.94 mmol/dm3 TBAOH or more. The color of the obtained films was dark green, corresponding to emeraldine PAni. The surface of the hybrid film grown from the solution containing 6.94 mmol/ dm3 is flat and tight, whereas that grown from the solution containing 7.44 mmol/ dm3 seems to not cover whole region. Therefore, we confirm that the optimum concentration of TBAOH for SnP exfoliation is

Figure 5. FT-IR spectra of electrodeposited TiP, TiP/PAni, SnP, and SnP/PAni hybrid films with an ATR attachment.

6.94 mmol/dm3. The configuration of the intercalated PAni will be discussed later. Figure 4 shows FE-SEM photographs of surfaces of TiP, TiP/PAni, SnP, and SnP/PAni films. The observation confirms that their surfaces are not completely smooth on a micrometer level and especially nanosheets can be distinguished on the surfaces of the SnP related films. Hybridization Mechanism. Figure 5 shows FT-IR spectra of TiP, the electrodeposited TiP film, the TiP/PAni film, SnP, the electrodeposited SnP film, and the SnP/PAni film. In the spectra of TiP and SnP powders, doublet absorption of asymmetric and symmetric stretching vibrations of OH attributed to H2O are observed at around 3555 and 3478 cm-1 in wavenumber. On the other hand, these absorption bands for the electrodeposited films from nanosheet-including acetonitrile solutions were eliminated. This disappearance possibly results from dehydration during exfoliation by adsorption of TBA+. The broad absorption attributed to P-OH at around 3000 cm-1 decreased in the electrodeposited films. Upon electrodeposition, the nanosheets turbostratically restack with a mismatch of in-plane orientation as mentioned above. Such restacking possibly provides disordered directions of hydroxyl groups to decrease the absorption peak. The adsorptions for νas(P-OH), νas(P-O), and νs(P-O) are seen in the range of 900-1100 cm-1 for all samples in Figure 5. The band at 1001 cm-1 in the TiP powder can be assigned to νs(P-O)

Phosphate Nanosheet/Polyaniline Electrodeposition

Figure 6. Relationship between TBAOH concentration and the ratio of PAni on the hybrid.

of HPO4 according to Stanghellini et al.24 Compared with those for TiP and SnP powders, those for electrodeposited films are degraded, especially those for hybrid films. This fact indicates that the nanosheets stacked with the turbostratic structure and mismatch orientation on the deposition, irrespective of whether they include PAni. It is worthwhile to mention that ν(NdQdN) and ν(CdCB) adsorptions (Q, quinoid; B, benzenoid) are observed at 1610 and 1496 cm-1 in the TiP/PAni and SnP/PAni hybrid films, respectively. Figure 6 shows the molar ratio of aniline monomer of PAni incorporated in the hybrids per tetravalent metal (Ti or Sn). The molar ratio of SnP increased steeply at 7.44 mmol/dm3. From the macroscopic photograph of this film in Figure 3, the hybrid seems to not cover the whole region of the substrate. The abundant TBA+ cation probably poses a decrease of the negative charge of the SnP nanosheet. Aniline may be polymerized without the SnP nanosheet on the region which is not covered by the hybrid. Therefore, we made an exception for the sample at 7.44 mmol/ dm3. These ratios increased gradually with an increase of TBAOH and reached to around 0.45 for the TiP/PAni film and 0.30 for the SnP/PAni film. There are no TBA+ cations within the dried films from the XRD patterns and CHN elemental analysis. The electrophoretic treatment separates the TBA+ cations from the exfoliated nanosheets. Since PAni emeraldine salt has 0.5 valence per aniline unit, the ratios of 0.45 and 0.30 correspond to 11.5 and 7.5% of hydroxyl groups in the hybrid films. The interlayer spacing increased by 0.76 nm for TiP/PAni and by 0.70 nm for SnP/PAni films compared with TiP and SnP films, respectively. Generally, increases in d-spacing are reported to be around 0.5-0.7 nm for monolayer PAni and 1.3-1.4 nm for bilayer PAni in intercalated compounds, metal phosphates,18 vanadium oxide,19,20 and molybdenum oxide.21 The net increases of 0.76 and 0.70 nm are consistent with the reported increases for the monolayer of PAni. The crystallographic area per tetravalent cation is around 0.22 and 0.21 nm2; the interlayer volume per tetravalent cation can be calculated from the product of the interlayer spacing and the crystallographic area to be 0.16 and 0.15 nm3, respectively. Meanwhile, the ratios of aniline unit per trivalent cation are 0.45 and 0.30 for TiP/PAni and SnP/PAni films, respectively. Since the volume of aniline monomer is around 0.10 nm3, incorporated PAni volumes per trivalent cation are 0.045 and 0.030 nm3. Thus, the PAni molecules were filled to occupy around 27 and 20% of the interlayer gallery for TiP/PAni and SnP/PAni hybrids, respectively. These low occupancies probably result from the temporary intercalation of solvent and/ or twisted shape of PAni. These considerations also support the formation of a monolayer of PAni intercalated between phosphate (24) Stanghellini, P. L.; Boccaleri, E.; Diana, E.; Alberti, G.; Vivani, R. Inorg. Chem. 2004, 43, 5698.

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Figure 7. Dependence of the ζ-potential of the solution containing exfoliated TiP or SnP coexisting with aniline on TBAOH concentration.

Figure 8. Dependence of the pH of the solution containing exfoliated TiP or SnP coexisting with aniline on TBAOH concentration.

nanosheets. The molar ratio indicated that around 10% of hydroxyl groups can be consumed for charge compensation with PAni, which is too small to form a bilayer of PAni. Figure 7 shows the mean ζ-potential of exfoliated nanosheets in acetonitrile, and Figure 8 shows the pH of the solution coexisting with or without aniline against TBAOH concentration. For TiP, on addition of aniline, the pH jumped up, while the ζ-potential slightly increased for all of the TBAOH concentrations. For SnP, the addition of aniline caused no pH change but decreased the ζ-potential at dilute TBAOH concentrations (2.48 mmol/ dm3). At a high TBAOH concentration (7.44 mmol/dm3), the addition of aniline results in an increase of the pH like that of TiP and a slight increase of the ζ-potential. The reason for the increase of the pH is attributed to aniline adsorption mechanism. That is, aniline molecules adsorbed on SnP nanosheets at only dilute concentrations of TBAOH. In the solution with such a dilute concentration, SnP was not exfoliated completely as shown in the XRD patterns (Figure 3) due to small amounts of TBAOH, though TBA+ cations were probably intercalated into SnP interlayer spaces. There were some hydroxyl groups which did not adsorb TBA+. Such hydroxyl groups may combine with the aniline monomer. On the other hand, sufficient TBA+ cations which are adsorbed on the nanosheets prevent adsorption of the aniline monomer at the high concentration of TBAOH. Thus, the increases of the pH confirm that aniline did not adsorb on TiP and SnP nanosheets at the optimum concentration of TBAOH (7.44 mmol/dm3 for TiP and 6.94 mmol/dm3 for SnP). From these results, hybridization of metal phosphate and PAni is found to occur during electrodeposition rather than before the deposition by adsorption of aniline. Dependence of Film Texture on Electrodeposition Duration. Figure 9 shows XRD patterns and macroscopic photographs of TiP related films grown in the optimum solution for 15, 30, 60, and 90 min. For TiP deposited from the solution without aniline,

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Figure 9. XRD patterns and photographs of electrodeposited TiP and TiP/PAni hybrid films depending on deposition duration.

a diffraction peak emerged at around 0.89 nm in the XRD patterns, while the films were cracked and peeled after electrodeposition for longer than 30 min. For the TiP/PAni hybrids, films electrodeposited for longer than 15 min corrugated and shattered partially on drying. In their XRD patterns, the diffraction peak attributed to the interlayer spacing was not observed due to their shattering. Figure 10 shows the XRD patterns and macroscopic photographs of SnP related films grown for 15, 30, 60, and 90 min. For SnP deposited films, the diffraction line at 0.87 nm grew depending on the deposition duration. The deposited films were translucent and had no cracks whatsoever. In terms of the hybrids, the deposited films which have a deep green color began to crack at 60 min or longer. The diffraction peak diminished when the film cracked. On electrodeposition for long time, the cracks were formed during drying because the stress on drying cannot be reduced in the thick films. Masses of deposited films were plotted versus the thicknesses of TiP and SnP films as shown in Figure 11. For the cracked film, the edges of the cracked pieces are curled, and the area and thickness of the cracked piece may be constant. Therefore, their thicknesses were measured at the center area which was not curled in the piece. Since areas of deposited film are constant (20 mm × 10 mm), the slopes of these plots are in proportion to their densities. For TiP related films, the maximum thickness of the TiP film (solid line and open circle) was around 10 µm while that of the TiP/PAni hybrid (dotted line and filled circle) was around 20 µm. These plots provide their bulk densities of 1.75 and 1.24 g/cm3, respectively. The density of the TiP film indicates that exfoliation and restacking of TiP nanosheets poses the formation of a slightly loose (turbostratic) structure. Since the expansion degree at the restacking direction was 116% (interlayer spacing increment: 0.12 nm), the theoretical density of deposited TiP (thin solid line) is 2.25 g/cm3 calculated from the density of

Takei et al.

Figure 10. XRD patterns and photographs of electrodeposited SnP and SnP/PAni hybrid films depending on deposition duration.

Figure 11. Relationship between film thickness and their weight for (a) TiP and TiP/PAni, and (b)SnP and SnP/PAni.

R-titanium phosphate of 2.60 g/cm3. Therefore, the fraction of the void in a horizontal direction can be estimated at around 22% for the TiP film. For the TiP/PAni hybrid film, the density (1.24 g/cm3) is very small, and a looser structure is found to be built up via hybridization with PAni. This small density probably results from the low occupancy of PAni within the interlayer volume. Using the volume of the aniline subunit (0.10 nm3), the theoretical density of PAni molecule can be calculated at 1.48 g/cm3. Assuming that PAni is combined only with the surface of the nanosheet, the fraction of the void in a horizontal direction can be calculated at around 22% from the theoretical densities of the deposited TiP and PAni. For SnP related films, maximum thicknesses of the SnP and SnP/PAni films were around 23 and 35 µm, respectively. The plots in Figure 11b provide their bulk densities of 2.05 and 1.96 g/cm3, respectively. The theoretical density of deposited SnP (thin solid line) is 2.97 g/cm3 calculated from the density of R-tin phosphate of 3.27 g/cm3 dividing by the expansion degree at a restacking direction of around 110% (interlayer spacing increment: 0.08 nm) for the deposited SnP film. Therefore, the fraction of the void in a horizontal direction can be estimated at around

Phosphate Nanosheet/Polyaniline Electrodeposition

Figure 12. Schematic illustrations of (a) TiP and SnP, and (b) TiP/PAni and SnP/PAni.

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deposition time because amounts of PAni in the hybrid film increased. Since the fraction of the void in a horizontal direction of TiP/PAni is 22%, the voids seemed to not prevent the diffusion of ions. On the other hand, for SnP related films, the currents for SnP/PAni hybrid films are much larger than that for the SnP film. In the hybrid deposited for 15 min, the voltammogram in which redox peaks are observed at 0.45 V on the anodic sweep and at 0.28 V on the cathodic sweep is much larger than those of others deposited for longer times. Since the fraction of the void in a horizontal direction of the SnP/PAni hybrid is very small (≈1%), the redox reactions are controlled by the diffusion of ions. Since the film at 15 min, which is thinner than the others, has an advantage for such reactions, the current in the voltammogram tends to increase. For the films of SnP/PAni grown for 30, 60, and 90 min, the areas of the voltammograms are very similar, while the redox peaks diminish and the voltammogram eventually becomes ohmic shaped on deposition for long times due to the diffusion-limited redox reaction. For the film deposited for 15 min, the voltammogram area of SnP/PAni was much larger than that of TiP/PAni. The redox reaction of PAni generally involves doping of anionn and protonation of imine parts in the PAni polymer. The redox reaction of the intercalated PAni needs diffusion paths for anion and proton conductive ability. Thus, the voltammogram areas might be affected by the proton conductivity of the metal phosphate. The proton conductivities of TiP and SnP have been reported as 9.0 × 10-7 and 5.4 × 10-5 S/cm, respectively.25,26 Therefore, it is found that a large proton conductivity of the host material tends to result in a large current to expand the voltammogram area, which is good for electrochemical capacitor applications.

Conclusion Figure 13. Cyclic voltammograms of (a) TiP and TiP/PAni, and (b) SnP and SnP/PAni at a sweep rate of 5 mV/s in a 1 mol/dm3 NaClO4 acetonitrile solution.

31% for the SnP film. For the SnP/PAni hybrid film, the density of around 1.96 g/cm3 is similar to that of the SnP film, and a relatively tight texture seems to be deposited via hybridization with PAni. In fact, the fraction of the void in a horizontal direction can be calculated at around 1% from theoretical densities of the deposited SnP and PAni. These results indicate that the TiP/ PAni film has larger amount of voids in a horizontal direction than SnP/PAni in which the amount of void is very small. This difference of the void fraction in a horizontal direction may result from difference of their ζ-potential as shown in Figure 7. The absolute value of the ζ-potential for SnP is smaller than that of TiP nanosheets, especially after the addition of aniline. Since the addition of aniline decreased the surface charge, a slightly larger amount of SnP nanosheets might deposit for temporal charge compensation on the anode to decrease the void fraction in a horizontal direction. Schematic models of cross sections of electrodeposited metal phosphate and metal phosphate/PAni hybrid films are showed in Figure 12. Electrochemical Measurement of Deposited Hybrid Films. Figure 13 shows cyclic voltammograms of TiP, TiP/PAni, SnP, and SnP/PAni films measured at a sweep rate of 5 mV/s. For the TiP or SnP film, there are flattened voltammograms which show a small current. The large voltammograms were shown by the hybrids, which indicate the existence of emeraldine PAni in the hybrids. For the TiP/PAni hybrid, there are two gradual redox shoulders at around 0.4-0.5 V on the anodic sweep and at around 0.2-0.3 V on the cathodic sweep. The currents increased with

The preparation of layered tetravalent metal (Ti and Sn) phosphate and PAni hybrid films by simultaneous electrophoretic and electrolytic deposition was demonstrated in acetonitrile. The obtained films in which emeraldine PAni was intercalated as a monolayer between metal phosphate nanosheets were much thicker (several tens of micrometers in thickness) than the inorganic-organic hybrid films grown by polycation/polyanion layer-by-layer deposition and Langmuir-Blodgett films. The optimum concentrations of tetrabutylammonium hydroxide for the preparation of TiP and SnP related films were 7.44 and 6.94 mmol/dm3, respectively. The ratio of PAni to the hybrid gently increased with increasing tetrabutylammonium cation concentration and reached to around 0.45 and 0.30 for TiP/PAni and SnP/ PAni, respectively. The PAni occupancies within the interlayer gallery were around 27 and 20% in volume fraction. The fractions of the void in a horizontal direction were around 22 and 1% for TiP/PAni and SnP/PAni, respectively. These results indicate that TiP/PAni has some pore spaces within the interlayer gallery and has some gaps between the nanosheets, whereas SnP/PAni has some spaces only within the interlayer gallery. To examine the mechanism of hybridization, the dependence of the ζ-potential and pH change on TBAOH concentration was measured. On addition of the aniline monomer, the ζ-potentials of the exfoliated nanosheets which have a negative charge slightly increased at a high concentration of TBAOH (7.44 mmol/dm3). On the other hand, the pH jumped up after addition of the aniline monomer. The results confirm that most of the aniline monomer (25) Szirtes, L.; Megyeri, J.; Riess, L.; Kuzmann, E.; Havancsa´k, K. Radiat. Phys. Chem. 2005, 73, 39–44. (26) Stenina, I. A.; Aliev, A. D.; Glukhov, I. V.; Spiridonov, F. M.; Yaroslavtsev, A. B. Solid State Ionics 2003, 162-163, 191–195.

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did not adsorb on the nanosheet in the solutions, while the hybrid films grew anodically with intercalation of PAni preformed near the anode. The existence of emeraldine PAni was confirmed via cyclic voltammograms. On hybridization of PAni, the voltammograms with redox peaks expanded. The current in the voltammograms increased with the amount of PAni in the hybrid when the voids in a horizontal direction acting as diffusion paths were enough. In the case of the deposition with small amounts of the void, thinner hybrid films showed a higher redox active property than thick films. That is, the voids in a horizontal direction are found

Takei et al.

to act as diffusion paths to ions in the solution of the cell. The ion conductivity of the metal phosphate is also an effective factor for the voltammogram. A large ion conductivity of the host material is considered to be preferable for ion diffusion, which is necessary for the redox reaction of polyaniline. Acknowledgment. This study was partially supported by the Grant-in-Aid for Young Scientists Grant (B) 19760465 and by the Association for the Progress of New Chemistry. LA703886Y