Preparation of Hybrid Film of Polyaniline and Organically Pillared

Dec 6, 2010 - Mosby, Díaz, Bakhmutov, and Clearfield. 2014 6 (1), pp 585–592. Abstract: Inorganic–organic hybrid materials were synthesized by co...
0 downloads 0 Views 4MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Preparation of Hybrid Film of Polyaniline and Organically Pillared Zirconium Phosphate Nanosheet by Electrodeposition Takahiro Takei,* Qiang Dong, Yoshinori Yonesaki, Nobuhiro Kumada, and Nobukazu Kinomura Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan Received May 31, 2010. Revised Manuscript Received November 16, 2010 R-Zirconium phosphate was chemically modified with 1,2-bis(dimethylchlorosilyl)ethane to graft organic chain, and then it was used as host material for inorganic nanosheet-polyaniline hybrid. The grafted R-zirconium phosphate was exfoliated in an acetonitrile solution with tetrabutylammonium salt and aniline. The electrodeposition in the presence of aniline was performed, and then it resulted in a formation of higher-order structure in which phosphate nanosheet was propped up by 1,2-bis(dimethylchlorosilyl)ethane with intrusion of polyaniline into the nanospace. The gravimetric capacitance of the R-zirconium phosphate without grafts and polyaniline hybrid film was around 194 F/g with the base on the amount of polyaniline mass. On the other hand, the R-zirconium phosphate nanosheet with grafts and polyaniline hybrid film provided larger capacitance of around 350 F/g in maximum. The nanospace formed by grafted phosphate nanosheet with 1,2-bis(dimethylchlorosilyl)ethane molecules gives increased amounts of polyaniline included and diffusion paths for ions.

1. Introduction Inorganic nanosheet solution can be prepared by exfoliating layered inorganic compounds. For the layered compounds composed of anionic inorganic layers and intercalated cations, protonation can be carried out by ion exchange from the intercalated cation to proton. Then, they are exfoliated by using quaternary ammonium salts, such as tetrabutylammonium hydroxide. The exfoliated inorganic nanosheet colloids are intriguing materials because of their large specific surface area, high flexibility, high reactivity, and so on. The ionic nanosheet colloid tends to be precipitated for the sake of electroneutrality. That is, the precipitation of the nanosheet will take place by combination with enough amounts of ions which have an opposite charge. The combination sometimes involves morphological change of the nanosheet. For example, some nanosheets will precipitate with rolling up to be nanotube by existence of ions with opposite charge.1,2 The inorganic nanosheet seems to be suitable for preparation of film because of its pellicular shape. In the prepared film from the nanosheets, large frictional force was generated between the nanosheets to reinforce the film due to wide contact areas of the nanosheets each other. In addition, some researchers have investigated preparation of organic-inorganic nanosheet hybrid films by means of the high reactivity of the nanosheet. Such hybrid film has been formed by the sequential adsorption method or electrochemical reaction process. Many reasearchers have been reported that layer-by-layer film can be prepared from inorganic *Corresponding author. Tel: þ81-55-220-8616. Fax: þ81-55-254-3035. E-mail: [email protected]. (1) Saupe, G. B.; Waraksa, C. C.; Kim, H.-N.; Han, Y. J.; Kaschak, D. M.; Skinner, D. M.; Mallouk, T. E. Chem. Mater. 2000, 12, 1556–1562. (2) Ma, R.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115–2119. (3) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M. Chem. Mater. 2001, 13, 4661–4667. (4) 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. (5) Wang, L.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina, Y.; Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873–2878. (6) Prasad, G. K.; Horiuchi, M.; Kumada, N.; Yonesaki, Y.; Takei, T.; Kinomura, N. J. Mater. Sci. 2007, 42, 10103–10105.

126 DOI: 10.1021/la102198j

nanosheet and polycation by the sequential adsorption method.3-6 These hybrid films obtained were completely flat and had a perfect LBL structure; however, these films were quite thin. On the other hand, electrophoretic deposition provides a relatively thick film of around several tens of micrometers. For the electrochemical process, titanate, niobate, manganate, and other nanosheets have been examined for electrophoretic deposition to prepare oriented film.7,8 Takei et al. reported that inorganic phosphate nanosheet and polyaniline oriented hybrid film can be grown on the anode.9,10 The papers reported that electrophoresis of the nanosheet and electrolytic polymerization of polyaniline occurred simultaneously, and consequently oriented polyaniline-intercalated metal phosphate film was deposited. The grown hybrid films have a redoxability for using electrochemical capacitor due to emeraldine polyaniline. However, reactivity is not excellent because diffusion paths are possibly deficient. Generally, the electrochemical devices need large interface area for quick adsorption and desorption of electrolyte ions and large amount of nanospace which works as an ionic path. In addition, for preparation of electrode film, PTFE was usually added into the sample as a binder, in the case of the powder sample. Therefore, redoxable nanoporous film is considered to be one of the best solutions for the electrochemical devices. Using grafted nanosheet from pillared layered compound, such nanoporous film might be formed by our electrochemical simultaneous deposition. Layered compounds were examined for formation of pillared structure by many researchers. Ogawa et al. reported that control of interlayer microstructures of a layered silicate by organosilane.11 Shimojima et al. also reported that layered silicate was (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) Takei, T.; Kobayashi, Y.; Hata, H.; Yonesaki, Y.; Kumada, N.; Kinomura, N.; Mallouk, T. E. J. Am. Chem. Soc. 2006, 128, 16634–16640. (10) Takei, T.; Yonesaki, Y.; Kumada, N.; Kinomura, N. Langmuir 2008, 24, 8554–8560. (11) Ogawa, M.; Okutomo, S.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 7361– 7362.

Published on Web 12/06/2010

Langmuir 2011, 27(1), 126–131

Takei et al.

silylated by chloroalkylsilane to form grafting structure.12 Toriya et al. reported that surfactant-intercalated layered silicate and tetraethoxysilane were reacted to form pillared structure. They examined adsorption property of benzene and water.13 On the other hand, layered zirconium phosphates were also examined for preparation of pillared structure. Zirconium phosphate has been examined for soft chemical properties such as intercalation, ion exchange, and exfoliation.14,15 In addition, many layered zirconium organophosphonates were reported as follows. Kijima et al. have been reported that alkyldiamine-intercalated zirconium phosphate organophosphonate can be prepared.16 Alberti et al. reported zirconium phosphate-phosphonate. Especially, pillared derivatives of γ-zirconium phosphate containing nonrigid pillars were synthesized.17 Hanken et al. and Yang et al. reported preparation of film from zirconium organophosphonate.18,19 From these literatures, these products were composed of organophosphonate and zirconium; that is, since most of hydroxyphosphates were substituted with organophosphonate, the amount of nanospace within the interlayer space is prone to be small, especially for R-zirconium phosphate. On the other hand, we have reported that layered zirconium phosphate nanosheet is able to be modified by organosilane for expansion of interlayer space.20 In these silylated zirconium phosphate, several percent or several tens of percent of the hydroxyphosphates were reacted with organosilane. Such modification may provide nanospace which is effective for diffusion of ions. Therefore, the modification of zirconium phosphate nanosheet is examined and then is used for preparation of hybrid film with polyaniline, and the electrochemical behavior is examined for application to electrochemical capacitor in this paper.

2. Experimental Section Grafting of Organic Group on Zirconium Phosphate. RZirconium phosphate (R-ZrP) was modified with organic groups by intercalation of octylamine and silylation of silane molecules as follows.14 The zirconim phosphate was put into 20 vol % of octylamine ethanol solution and was stirred vigorously for 48 h. Then, the sample was centrifuged and washed by ethanol to obtain intercalated intermediate. The obtained sample was designated as ZP-OA. The obtained ZP-OA was put into toluene in a harelip flask, and organic silane was added to the solution with the molar ratios of the silane to R-ZrP of 0.5, 2.0, and 3.0 in a nitrogen-filled glovebox. The organic silanes used in this paper were 1,2-bis(dimethylchlorosilyl)ethane (BMCE), chloropropyltrichlorosilane (CPTC), triphenylchlorosilane (TPhC), and diphenyldichlorosilane (DPDC). The solution was refluxed under dry nitrogen flow at 70 °C for 48 h. Then, the sample was washed by centrifugation with toluene and dried at 50 °C. The dried powder was put into distilled water and was shaken for 10 min to dissolve unwanted amine chloride salt which was formed during silylation reaction. The washed samples which were silylated by BMCE, CPTC, (12) Shimojima, A.; Mochizuki, D.; Kuroda, K. Chem. Mater. 2001, 13, 3603– 3609. (13) Toriya, S.; Tamura, Y.; Takei, T.; Fuji, M.; Watanabe, T.; Chikazawa, M. J. Colloid Interface Sci. 2002, 255, 171–176. (14) Kim, H.-N.; Keller, S. W.; Mallouk, T. E. Chem. Mater. 1997, 9, 1414– 1421. (15) Kaschak, D. M.; Johnson, S. A.; Hooks, D. E.; Kim, H.-N.; Ward, M. D.; Mallouk, T. E. J. Am. Chem. Soc. 1998, 120, 10887–10894. (16) Kijima, T.; Watanabe, S.; Machida, M. Inorg. Chem. 1994, 33, 2586–2591. (17) Alberti, G.; Costantino, U.; Dionigi, C.; Murcia-Mascaros, S.; Vivani, R. Supramol. Chem. 1995, 6, 29–40. (18) Hanken, D. G.; Naujok, R. R.; Gray, J. M.; Corn, R. M. Anal. Chem. 1997, 69, 240–248. (19) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S. L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855–11862. (20) Takei, T.; Kumada, N.; Kinomura, N.; Nakayama, H.; Tsuhako, M. Mater. Res. Bull. 2008, 43, 111–119.

Langmuir 2011, 27(1), 126–131

Article TPhC, and DPDC were designated as ZP-B, ZP-C, ZP-T, and ZP-D, respectively. There are both silane and amine molecules in the interlayer space of the silylated samples as reported previously.20 To remove amine molecules from the interlayer space, the water-washed sample was immersed and was shaken in 1 mol/ dm3 HCl aqueous solution for 12 h. These samples treated in HCl were designated as ZP-BHCl, ZP-CHCl, ZP-THCl, and ZP-DHCl, respectively.

Preparation of Nanosheet-Dispersed Solution and Film Growth from the Solution. After the HCl treatment, the

silylated R-ZrP was suspended in acetonitrile solution at concentration of 6.7 mmol/dm3, which contained tetrabutylammonium hydroxide at concentrations of 3.72, 4.96, 6.20, and 7.44 mmol/ dm3. Tetrabutylammonium chloride was added into all solutions at concentration of 4.96 mmol/dm3 for anionic dopant to polyaniline. Aniline was also put into the solution with the concentration of 10.0 mmol/dm3. The solutions were stirred at room temperature (RT) for 1 week and then centrifuged at 2000 rpm for 5 min to separate substance which did not exfoliate. Film was grown from the exfoliated nanosheet solution by electrodeposition established in the previous study.9,10 The typical condition is shown as follows. Two Pt plates (10  50 mm) with separation of 10 mm were immersed into the solution. Then, constant current of 1.25 mA/cm2 was loaded for 15 min to form electrodeposited film by dc power supply (7651, Yokogawa Electric Corp.). During the deposition, the current and potential between the electrodes were recorded by digital multimeter (7352A, ADC Corp.). The recorded potentials were almost constant within the deposition and were around 4-8 V, depending on the concentration of tetrabutylammonium hydroxide. After the deposition, the electrodes were pulled up promptly and were dried at RT. For all reactions, the film was formed on the anode and no deposition was observed on the cathode. Characterization. The electrodeposited films were examined for their layered structure, species of organic moieties, surface texture, and electrochemical activities. The layered texture was measured by X-ray diffraction using monochromated Cu KR radiation (RINT-2000, Rigaku) at room temperature. The organic components were examined by FT-IR/ATR (FT/IR-4100, Jasco) for their species and by CHN elementary analysis (MT-5, Yanaco) for their amounts in the films. The surface textures of the films were observed by FE-SEM (JSM-6500F, JEOL). Particle size and ζ potential distributions of the exfoliated nanosheet were measured by a light-scattering technique (Zetasizer Nano ZS, Sysmex). The electrochemical activities were measured in a threeelectrode cell filled with 1 mol/dm3 NaClO4 acetonitrile solution by a potentiostat/galvanostat (HAL-2001, Hokuto Denko) and function generator (HB-301, Hokuto Denko). The cyclicvoltammogram was measured within the range from -0.35 to 0.45 V versus Ag|Agþ as a reference electrode with 5 or 20 mV/s of sweep rate. The galvanostatic charge-discharge curve was also measured within same potential range with (100 μA of charge or discharge current. The capacitance was estimated from potential slope for discharge.

3. Results and Discussion Formation of Silane-Grafted r-ZrP as a Raw Material of Nanosheet. X-ray diffraction patterns of R-ZrP, ZP-OA, and the silylated compounds with silane/ZrP ratio of 3.0 before and after HCl treatment are included in the Supporting Information. For the silylated samples, ZP-B exhibits bimodal diffraction peaks at ∼1.9 and 2.5 nm, while the others, ZP-C, ZP-T, and ZP-D, show monomodal peak at around 2.5 nm, which is derived from octylamine-intercalated phase, at low angle region. After the HCl treatment, the diffraction peak from octylamine-intercalated phase vanishes for all silylated compounds. On the other hand, for ZP-BHCl, the diffraction peak at ∼1.5 nm still remains after HCl treatment, corresponding to the interlayer spacing with silylated DOI: 10.1021/la102198j

127

Article

Takei et al.

Figure 1. Particle size distributions and ζ potential of 1,2-bis(dimethylchlorosilyl)ethane-modified zirconium phosphate nanosheet.

phase. These XRD patterns confirm that only BMCE molecules can be grafted on ZrP nanosheet. Therefore, we have used ZPBHCl for further experiments: exfoliation, electrodeposition, and characterizations (see Supporting Information). We investigate dependence of the BMCE content included in ZP-BHCl on the amount of loaded BMCE. Here, the general formula of ZP-B and ZP-BHCl can be expressed as ZrP 3 mOA 3 nBMCE, where OA means octylamine. The amounts of octylamine (m) and BMCE (n) molecules in ZP-B and ZP-BHCl calculated from CHN elemental analysis on the loaded amount of BMCE are included in the Supporting Information. For ZP-B and ZPBHCl, n increased linearly in proportion to the loaded amount with slope of around 0.13. However, m decreased with increase of the loaded BMCE in ZP-B. This decrease apparently results from the substitution of octylamine with BMCE during silylation treatment. For ZP-BHCl, the m plot confirms that the HCl treatment completely eliminated intercalated octylamine molecules which are unwanted for following experiments. Preparation of Exfoliated Nanosheet Solution and Growth of Films from the Solution. The ZP-BHCl was put into an acetonitrile solution with TBAOH and stirred to exfoliate. Figure 1a,b shows particle size and ζ potential distributions for ZP-BHCl exfoliated by addition of TBAOH and TBACl with typical concentration of 4.96 mmol/dm3. From the particle size distributions, the sizes show tailed distributions toward large range direction. The ZrP nanosheet distributes from 100 to 700 nm, whereas ZP-BHCl were from 200 to 700 nm. BMCE probably worked as not only graft but also adhesive for ZP-OA particles. On the other hand, the ζ potential of ZP-BHCl distributed over negative region and absolute values of the potential were relatively smaller than that of R-ZrP. After silylation process, these samples were washed by distilled water to remove an unwanted salt as mentioned above. This washing treatment is supposed to hydrolyze unreacted chlorine to form Si-OH. Then ZP-BHCl has two kinds of hydroxyl groups: an intrinsic P-OH on a plane and an acral Si-OH within the graft bound by consumption of a P-OH. Generally, the degree of proton dissociation for P-OH is larger than that for Si-OH. The Si-OH in the graft certainly provides lower surface charge. Moreover, the ζ potential is affected by not only the surface charge but also adsorption of counterions. Since the graft has an ethylene group which is hydrophobic, they influence adsorption of TBAþ on the surface. In this paper, the potential of ZP-BHCl seems to be constant irrespective of amount of BMCE included. Such constant potential might be determined by cancellation of the surface charge and the adsorption of ions each other. Consequently, the substitution of Si-OH for P-OH by grafting of BMCE surely provides the relatively smaller surface potentials. Figure 2 shows the XRD patterns of electrodeposited films grown from the nanosheet solution containing typical TBAOH 128 DOI: 10.1021/la102198j

Figure 2. XRD patterns of the electrodeposited hybrid film from 1,2-bis(dimethylchlorosilyl)ethane-modified zirconium phosphate nanosheet solution with and without aniline.

concentration of 4.96 mmol/dm3. Bottom 4 patterns show the films grown from the ZP-BHCl nanosheet solution which does not include aniline and top 4 patterns from the solution including aniline. For R-ZrP film, the addition of aniline posed increase the interlayer spacing by 0.4 nm to around 1.17 nm, corresponding to thickness of a monolayer PAni arrangement with the molar ratio of around 0.22 on ZrP. Such increase must be same as the previous case of ZrP-PAni hybrid films reported.9 For the ZP-BHCl film without addition of aniline, the interlayer spacings of around 1.4 nm are almost same to those for ZP-BHCl powders as shown in the Supporting Information. For the BMCE/ZrP = 0.5, the intensity of diffraction peak seemed to be apparently smaller than the others. Such small peaks are difficult to clarify; a plausible explanation is smaller amount of paralleled nanosheet-restacking structure to the substrate due to small amount of grafts in the BMCE/ZrP = 0.5. However, the interlayer spacing does not change substantially. That is, BMCEpillared structure becomes more stable as BMCE increase. On addition of aniline, PAni was formed within interlayer space with the molar ratio of around 0.3-0.4 on ZrP, while no significant change of the interlayer spacing can be observed because the polyaniline probably deposited within the nanospace created by addition of BMCE. FT-IR spectra are included in the Supporting Information. For the sample without BMCE, the absorption band at 3150 cm-1 for OH and 3590 and 3510 cm-1 for two kinds of crystallization H2O ; however, there are no such obvious bands for the others. Formation of PAni can be confirmed by at 1580 and 1490 cm-1 attributed to CdN stretching of quinoid and CdC stretching of benzenoid. No N-H stretching vibration is observed at 3020 cm-1 because N-H groups were bound with OH groups. Thus, These spectra confirm that the PAni composed of quinoid and benzenoid rings and that is formed witnin the interlayer space (see Supporting Information). Figure 3 shows FE-SEM micrographs of surface of the deposited films. Insets show photographs of entire film. For the FESEM micrograph, (a), (b), (c), and (d) show the films which do not Langmuir 2011, 27(1), 126–131

Takei et al.

Article

Figure 3. FE-SEM micrographs and film photographs of the electrodeposited hybrid film from 1,2-bis (dimethylchlorosilyl)ethane-modified zirconium phosphate nanosheet solution with and without aniline. Insets: photographs of film appearance.

Figure 4. Schematic illustrations for cross section of the electrodeposited hybrid films.

include PAni, and (e), (f), (g), and (h) show PAni hybrid films prepared from R-ZrP and ZP-BHCl with the BMCE/R-ZrP loaded ratio of 0.5, 2.0, and 3.0, respectively. From the photographs of entire film, the grown deposits without addition of aniline are transparent except translucent R-ZrP. The transparency apparently depends on their film mass and thickness; the mass of deposited film of R-ZrP is around 3 times as large as the films of ZP-BHCl. Thickness of the films was around 5 μm and 2.0-2.5 μm for the film from R-ZrP and ZP-BHCl, respectively. For PAniincluded films, the films have green color which results from emeraldine state of PAni. From FE-SEM micrographs, these films are not perfectly flat in sub-micrometer level. Most of the nanosheets which seem to lie paralell on the substrate. These micrographs confirm that there are no textural difference between ZP-BHCl and ZP-BHCl/PAni hybrid film. To observe compositional segregation for ZrP, BMCE, and PAni deposited, compositional distribution of the surface was examined for elements of C, O, Si, P, and Zr by dot-mappings. The surface micrograph and dot maps of C, O, Si, P, and Zr for ZP-BHCl segregations in the film with the BMCE/R-ZrP ratio of 3.0 are included in the Supporting Information. This result is collateral evidence for grafting of BMCE and intercalation of PAni into interlayer space. Figure 4 shows schematic illustrations showing a cross section of the electrodeposited hybrid films. Nanostructure of the Grown Film and Electrochemical Property. As mentioned previously, the BMCE works as a pillar and provides the nanospace, hereafter designated as “nanocavity”. Langmuir 2011, 27(1), 126–131

Figure 5. Relationships between the amount of 1,2-bis(dimethylchlorosilyl)ethane and amount of polyaniline in the hybrid film and nanocavity per unit cell.

The general formula of the hybrid including PAni can be expressed as ZrP 3 lPAni 3 nBMCE without regard to the very small amount of octylamine. The nanocavity is estimated by subtraction of the volume of inserted PAni and BMCE molecules from the expanded interlayer space. The nanocavity per unit cell, VC, can be calculated using following formula VC ¼ 2ðabh - 2nVBMCE - 2lVPAni Þ

ð1Þ

where a and b are lattice parameters of R-ZrP, 0.9076 and 0.5298 nm, h is increment of interlayer spacing from 0.76 nm of asreceived R-ZrP, n and l are amounts of grafted BMCE and PAni as mentioned above, and VBMCE and VPAni are the volume of a partly hydrolyzed BMCE molecule, 0.187, and a unit of emeraldine PAni, 0.097 nm3, calculated from van der Waals radii, respectively. The nanocavity surely affects electrochemical property and is possibly changes by exfoliation and deposition condition. Figure 5 shows relationship between TBAOH concentration and the nanocavity of the hybrid films including BMCE and PAni. These plots show plateau at larger than around 5 mmol/dm3 of TBAOH concentration. For R-ZrP, exfoliation deficiency poses a small amount of the nanocavity due to small amount of intercalated PAni. However, for ZP-BHCl with a lot of BMCE, the volume of the nanocavity is rather large at low exfoliation degree because BMCE molecule created the nanocavity beforehand. Thus, the plots confirm that larger that around 5 mmol/dm3 is necessary for the exfoliation. At larger than 5 mol/dm3 of TBAOH concentration, DOI: 10.1021/la102198j

129

Article

Takei et al.

Figure 6. Relationships between the amount of 1,2-bis(dimethylchlorosilyl)ethane and amount of polyaniline in the hybrid film and gravimetric capacitance per polyaniline.

the nanocavities of the hybrid from ZP-BHCl with BMCE/ZrP of 0.5 and 2.0 are larger than that from R-ZrP ; however, that with BMCE/ZrP of 3.0 become apparently smaller. From the actual amount of BMCE, n, as shown in the Supporting Information, the nanocavity tends to increase by graft of BMCE with the amount lower than 0.25 of BMCE/ZrP. Figure 6 shows relationships between TBAOH concentration in the nanosheet solutions from which PAni hybrid film grown and electrochemical capacitance, CA, for the hybrid films. These CAs are calculated from slope of galvanostatic charge-discharge curves which are omitted in this paper. In these plots, CA shows smallest value at low TBAOH concentration (3.72 mmol/dm3), and it jumps up at around 5 mmol/dm3. The CAs reach to around 194 and 350 F/g for R-ZrP/PAni and ZP-BHCl/PAni hybrids, respectively. From Figure 6, the grafting of BMCE apparently results in enlargement of electrochemical capacitance. Such trends have a good agreement with exfoliation state; however, the calculated CA seems to have no relation to the amount of nanocavity. This discrepancy may be explained by using hydrophobicity. In this case, solvent used is acetonitrile which is polar molecule. B€oes reported that the free energy of salvation for ClO4- in acetonitrile is around -214 kJ/mol, which indicates association can be formed readily between ClO4- and acetonitrile.21 The electrolyte anion complex, ClO4- with several acetonitrile molecules, must be isolated when the ClO4- anions are intercalated into interlayer space. Since acetonitrile molecule has a polar character, hydrophobic circumstance may be preferable for the desorption of acetonitrile and the intercalation of electrolyte. In this paper, hydrophobicity, h, is defined by dividing the amount of consumed OH group by the amount of originally intrinsic OH group in a chemical formula h ¼

0:5l þ n 2

ð2Þ

where l and n are ratio of included PAni and BMCE as mentioned above. Since emeraldine PAni has 0.5 positive charge per aniline unit, the PAni must consume 0.5l of intrinsic OH. In this formula, OH group within BMCE molecule is ignored because very little amount of the OH group in BMCE probably dissociates. Figure 7 shows the relationships between the amount of BMCE (n) and the amount of PAni (l), nanocavity (VC), hydrophobicity (h), and the product of nanocavity and hydrohobicity (hVC) of the hybrid films prepared from the solution containing 4.96 mmol/dm3 (21) B€oes, E. S.; Livotto, P. R.; Stassen, H. Chem. Phys. 2006, 331, 142–158.

130 DOI: 10.1021/la102198j

Figure 7. Dependence of the amount of PAni, nanocavity per unit cell, hydrophobicity, and product of nanocavity and hydrophobicity on the amount of BMCE included in the hybrid film.

Figure 8. Cyclic voltammograms of the electrodeposited hybrid film from 1,2-bis(dimethylchlorosilyl)ethane-modified zirconium phosphate nanosheet solution with aniline measured at 20 mV/s.

TBAOH. The nanocavity increase by modification of BMCE; however, that decrease with increase of BMCE and consequently become lower than that of the sample prepared from nonmodified R-ZrP. On the other hand, the hydrophobicity increases linearly depending on the amount of BMCE. The product, hVC, of the sample obtained from ZP-BHCl is apparently larger than that of the sample from R-ZrP, like as the CA trend. This factor, hVC, indicating not only amount of nanocavity but also hydrophobic character apparently influences the electrochemical activity. Thus, large amount of hydrophobic nanocavity is found to be preferable for increase of electrochemical capacitance. Figure 8 shows the cyclic voltammograms of R-ZrP and ZPBHCl/PAni hybrid with different the BMCE ratio prepared under the condition which provides maximum electrochemical capacitance at the sweep rate of 20 mV/s. The voltammogram loop apparently become larger by the pillaring interlayer space with BMCE. The broad peak at 0.27 V on positive sweep and that at 0 to -0.1 V on negative sweep can be assigned to oxidation and reduction of PAni, respectively. These peaks probably result from adsorption and desorption of ClO4- ions to PAni. Such ClO4adsorption and desorption in nonaqueous system were reported by some researchers.22,23 Actually, the peak shifts of around -0.1 V in potential were observed owing to irreversibility of the sample without BMCE. The cyclic voltammograms measured at (22) Schemid, A. L.; Torresi, S. I. C.; Bassetto, A. N.; Carlos, I. A. J. Braz. Chem. Soc. 2000, 11, 317–323. (23) Fusalba, F.; Gouerec, P.; Villers, D.; Belanger, D. J. Electrochem. Soc. 2001, 148, A1–A6.

Langmuir 2011, 27(1), 126–131

Takei et al.

the sweep rate of 5 mV/s are included in the Supporting Information. The peak on positive sweep becomes obvious and shifts negatively in potential especially for sample without BMCE or BMCE/ZrP = 0.5. These results confirm that slow sweep (low frequency) pose improvement of adsorption of ClO4- for the sample with low hVC (see Supporting Information). Thus, these results confirmed that the pillars composed of ethylene group and Si may dissociate ClO4- anion from acetonitrile-ClO4- assembly. Such dissociation phenomena are very important for intercalation and deintercalation of electrolyte into electrode with layered structure. For similar hybrid structure, layered inorganic nanosheet with intercalated polyaniline reported by us, the obtained capacitance in this study is relatively larger than reported values. The PAni and metal phosphate nanosheet hybrid films have showed around 150 F/g or less.10 The reported structure of these hybrid films may be too rigid to diffuse ions, though the HPO4 group in the metal phosphate nanosheet has proton conductivity acting as ion diffusion path. For other composites of polyaniline/inorganic layered materials, preparation of some composites such as PAni/ Nb2O5,24 PAni/vermiculite,25 PAni/V2O5,26 PAni/MoO3,27 PAni/ MnO2,28 and so on were reported. However, there are little reports examining for behavior of electrochemical capacitor because these materials might have little amounts of ion diffusion paths. Our approach, use of metal phosphate nanosheet and expansion of interlayer space in this paper, may promote redox reaction. On the other hand, carbon-related PAni hybrids were prepared by relatively many researchers.29-33 The reported gravimetric capacitance in these papers were widely distributed from approximately one to several hundred F/g. From these papers, shape and size of PAni particle were found to be important for enhancement of redox activity because many electrolyte ions can be adsorb easily if the surface area of PAni is high. In addition, carbon-related materials may support electronic conductivity of hybrid. (24) Yang, G.; Hou, W.; Feng, X.; Jiang, X.; Guo, J. Adv. Funct. Mater. 2007, 17, 3521–3529. (25) Liu, D.; Du, X.; Meng, Y. Mater. Lett. 2006, 60, 1847–1850. (26) 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. (27) Kerr, T. A.; Wu, H.; Nazar, L. F. Chem. Mater. 1996, 8, 2005–2015. (28) Wang, Y.-G.; Wu, W.; Cheng, L.; He, P.; Wang, C.-X.; Xia, Y.-Y. Adv. Mater. 2008, 20, 2166–2170. (29) Ko, J. M.; Song, R. Y.; Yu, H. J.; Yoon, J. W.; Min, B. G.; Kim, D. W. Electrochim. Acta 2004, 50, 873–876. (30) Chen, W.-C.; Wen, T.-C.; Teng, H. Electrochim. Acta 2003, 48, 641–649. (31) Gupta, V.; Miura, N. Electrochim. Acta 2006, 52, 1721–1726. (32) Khomenko, V.; Frackowiak, E.; Beguin, F. Electrochim. Acta 2005, 50, 2499–2506. (33) Yang, M.; Cheng, B.; Song, H.; Chen, X. Electrochim. Acta 2010, 55, 7021– 7027.

Langmuir 2011, 27(1), 126–131

Article

4. Conclusions Hybrid film for polyaniline and organically modified zirconium phosphate nanosheet were prepared by electrophoretic and electrolytic depositions. For the modification, four kinds of silanes, 1,2-bis(dimethylchlorosilyl)ethane, chloropropyltrichlorosilane, triphenylchlorosilane, and diphenyldichlorosilane, were used as the graft for preventing the nanosheet from sticking each other within a subsequent electrodeposited film. The silylated samples were washed in HCl for removal of remained octylamine within the interlayer spaces. However, most of the silylated samples gave up the silane via the HCl treatment except for the case of 1,2-bis(dimethylchlorosilyl)ethane. The trychlorosilane of which reactivity is too high, and di- and triphenylsilane which are too large probably resulted in such unmodification. Consequently, in the case of 1,2-bis(dimethylchlorosilyl)ethane, grafted R-ZrP was synthesized and HCl treatment certainly removed octylamine remained in the samples. The ratio of effective 1,2-bis(dimethylchlorosilyl)ethane, which indicates the grafted one per the loaded one, is around 0.13. XRD patterns confirmed that small amounts of grafted silane molecules, at least around 5 mol %, can work as props between nanosheets, and then intercalation of polyaniline does not provide increase of interlayer spacing. There are many researches for preparation of powder sample with pillared layered structure. However, there are very little reports for preparation of oriented film with pillared layered structure. In this paper, we report the succession of preparing hybrid film with pillared layered structure with and without polyaniline intercalation. The grafting for the nanosheet tends to increase the gravimetric capacitance. The hydrophobic nanocavity formed from the propping-up structure by the grafts promotes diffusion of ClO4anions within the film because hydrophobic circumstance may be preferable for dissociation of acetonitrile from ClO4- anion. Such reaction is very important for adsorption and desorption of electrolyte ions on the active materials to promote capacitive behavior. Consequently, for ZP-BHCl, the gravimetric capacitance achieved to around 350 F/g, which is around 1.7 times as large as that without graft. Acknowledgment. This work was supported by the Grant-inAid for Young Scientists Grant (B) 19760465. Supporting Information Available: XRD patterns of octylamine intercalated intermediate and silylated phases; amounts of included octylamine and silanes; and EDX mapping results for deposited hybrid film. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la102198j

131