Au Nanoparticles by the

LB films were deposited onto clean ITO-coated glass strips (termed ITO). Procedures. For the preparation of PANI LB films, 0.5 mg·mL−1 PANI was dis...
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Thin Nanocomposite Films of Polyaniline/Au Nanoparticles by the Langmuir-Blodgett Technique Golan Tanami,† Vitaly Gutkin,‡ and Daniel Mandler*,† †

Institute of Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel and ‡The Unit for Nanocharacterization, The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel Received September 2, 2009. Revised Manuscript Received October 28, 2009 The Langmuir-Blodgett (LB) method was used to deposit multilayers of polyaniline (PANI)- and mercaptoethanesulfonate (MES)-stabilized Au nanoparticles. The electrostatic interaction between the negatively charged nanoparticles in the subphase and the positively charged PANI at the air-water interface assisted the deposition of the nanocomposite film onto a solid support. These PANI/Au-NPs films were characterized using cyclic voltammetry, copper under potential deposition, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. We found that the nanocomposite layers were uniform and reproducible. The density of Au-NPs in the monolayer depended on the acidity of the subphase as well as on the nanoparticles concentration. Moreover, the AuNPs extrude above the PANI and therefore could be used as nanoelectrodes for the underpotential deposition (UPD) of copper.

Introduction During the past decade there has been a significant progress in the synthesis and characterization of nanocomposites.1-10 In most of these studies, the nanocomposites were formed by embedding nanoparticles (NPs) into a polymer matrix.3,8,9 These composite structures hold many advantages arising both from the size effects of the NPs as well as the unique properties of the polymer, which allow stacking of the particles and increase their stabilization. Careful selection of a polymer with certain properties can lead to a variety of potential applications, depending on the interactions between the NPs and the polymer. Organic conducting polymers exhibit, in addition to their organic skeleton, relatively high electrical conductivity. It is not surprising that numerous studies have focused on assembling nanocomposites based on conducting polymers and nano*Corresponding author. E-mail: [email protected]. (1) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559–3592. (2) Jang, J. Adv. Polym. Sci. 2006, 199, 189–259. (3) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608–622. (4) Fang, F. F.; Choi, H. J.; Joo, J. J. Nanosci. Nanotechnol. 2008, 8, 1559–1581. (5) Olad, A.; Rashidzadeh, A. Prog. Org. Coat. 2008, 62, 293–298. (6) Xiao, Y. H.; Li, C. M. Electroanalysis 2008, 20, 648–662. (7) Pomogailo, A. D. Polym. Sci., Ser. C 2006, 48, 85–111. (8) Maity, A.; Biswas, M. J. Ind. Eng. Chem. 2006, 12, 311–351. (9) Srivastava, S.; Kotov, N. A. Acc. Chem. Res. 2008, 41, 1841. (10) Hatchett, D. W.; Josowicz, M. Chem. Rev. 2008, 108, 746–769. (11) Neelgund, G. M.; Hrehorova, E.; Joyce, M.; Bliznyuk, V. Polym. Int. 2008, 57, 1083–1089. (12) Amaya, T.; Saio, D.; Hirao, T. Tetrahedron Lett. 2007, 48, 2729–2732. (13) Athawale, A. A.; Bhagwat, S. V. J. Appl. Polym. Sci. 2003, 89, 2412–2417. (14) Del Castillo-Castro, T.; Larios-Rodriguez, E.; Molina-Arenas, Z.; Castillo-Ortega, M. M.; Tanori, J. Composites, Part A 2007, 38, 107–113. (15) Drelinkiewicz, A.; Hasik, M.; Kloc, M. Catal. Lett. 2000, 64, 41–47. (16) Green, M.; Marom, G.; Li, J.; Kim, J. K. Macromol. Rapid Commun. 2008, 29, 1254–1258. (17) Houdayer, A.; Schneider, R.; Billaud, D.; Ghanbaja, J.; Lambert, J. Appl. Organomet. Chem. 2005, 19, 1239–1248. (18) Frydrychewicz, A.; Czerwinski, A.; Jackowska, K. Synth. Met. 2001, 121, 1401–1402. (19) Mourato, A.; Viana, A. S.; Correia, J. P.; Siegenthaler, H.; Abrantes, L. M. Electrochim. Acta 2004, 49, 2249–2257. (20) Li, X. W.; Wang, G. C.; Li, X. X.; Lu, D. M. Appl. Surf. Sci. 2004, 229, 395–401.

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particles.10-21 However, the processability of conducting polymers is one of the major obstacles in their wide-scale application in optoelectronics, microelectronics, and so forth.1-4,6,8 The methods of preparing conducting polymer nanocomposites have been reviewed by Gangopadhyay3 and Jang2 and researched by several groups.22-34 The most straightforward approach to assembling conducting polymer nanocomposites involves chemical13,17 or electrochemical polymerization in the presence of NPs that results in the formation of thin nanocomposite films (on the order of a few micrometers). For example, Visy and co-workers prepared polythiophene-magnetite composite layers through the electropolymerization of 3-thiopheneacetic acid in the presence of Fe3O4 nanoparticles.35 Additional methods comprised encapsulation and chemical copolymerization.1 (21) Wang, S. B.; Li, C.; Shi, G. Q. Sol. Energy Mater. Sol. Cells 2008, 92, 543–549. € (22) Laiho, A.; Majumdar, H. S.; Baral, J. K.; Jansson, F.; Osterbacka, R.; Ikkala, O. Appl. Phys. Lett. 2008, 93, 203309-1–203309-3. (23) De Azevedo, W. M.; De Mattos, I. L.; Navarro, M.; Da Silva, E. F. Appl. Surf. Sci. 2008, 255, 770–774. (24) Sharma, R. K.; Rastogi, A. C.; Desu, S. B. Electrochim. Acta 2008, 53, 7690–7695. (25) Zubillaga, O.; Cano, F. J.; Azkarate, I.; Molchan, I. S.; Thompson, G. E.; Cabral, A. M.; Morais, P. J. Surf. Coat. Technol. 2008, 202, 5936–5942. (26) Wu, C. G.; Lu, M. I.; Jhong, M. F. J. Polym. Sci., Part B: Polym. Phys. 2008, 46, 1121–1130. (27) Kaushik, A.; Kumar, J.; Tiwari, M. K.; Khan, R.; Malhotra, B. D.; Gupta, V.; Singh, S. P. J. Nanosci. Nanotechnol. 2008, 8, 1757–1761. (28) Tai, H. L.; Jiang, Y. D.; Xie, G. Z.; Yu, J. S.; Chen, X.; Ying, Z. H. Sens. Actuators, B 2008, 129, 319–326. (29) Chavali, M.; Lin, T. H.; Wu, R. J.; Luk, H. N.; Hung, S. L. Sens. Actuators, A 2008, 141, 109–119. (30) Kukhta, A. V.; Kolesnik, E. E.; Lesnikovich, A. I.; Nichik, M. N.; Kudlash, A. N.; Vorobyova, S. A. Synth. React. Inorg. Met.-Org. Nano-Met. Chem. 2003, 37, 333–339. (31) Tang, L. H.; Zhu, Y. H.; Xu, L. H.; Yang, X. L.; Li, C. Z. Electroanalysis 2007, 19, 1677–1682. (32) Patil, D.; Seo, Y. K.; Hwang, Y. K.; Chang, J. S.; Patil, P. Sens. Actuators, B 2008, 128, 374–382. (33) Yuan, J. H.; Han, D. X.; Zhang, Y. J.; Shen, Y. F.; Wang, Z. J.; Zhang, Q. X.; Niu, L. J. Electroanal. Chem. 2007, 599, 127–135. (34) Zhang, M.; Yamaguchi, A.; Morita, K.; Teramae, N. Electrochem. Commun. 2008, 10, 1090–1093. (35) Janaky, C.; Visy, C.; Berkesi, O.; Tombacz, E. J. Phys. Chem. C 2009, 113, 1352–1358.

Published on Web 12/29/2009

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Polyaniline (PANI), polypyrrole (PPY), and polythiophene (PTH) are the most studied conducting polymers as the main matrix of these nanocomposites, although N-vinylcarbazole, polyphenylenevinylene, poly(aniline-co-aminobenzenesulfonic acid), and poly(3,4-ethylenedioxythiophene) have also been applied. A wide variety of metallic NPs such as Pd, Pt, Cu, Ag, Au, and Pb have been incorporated into these conducting polymers;2,3,9 however, different types of oxometalate NPs (e.g., ZnO, SiO2, TiO2, Fe2O3, Fe3O4, ZrO2, and CeO2) have also been reported.36-40 Of particular interest is the buildup of nanometer-thick nanocomposites. Such monomolecular and multimolecular films composed of conducting polymers and nanoparticles can be assembled by either the layer-by-layer (LbL) deposition, LangmuirBlodgett (LB), or Langmuir-Schaefer (LS) method or through vacuum deposition.9,10,23,27,28,33,41-49 For example, Ding et al. prepared ultrathin films of poly(thiophene-3-acetic acid)/TiO2 using the LbL method. The electrostatic interactions between the carboxylic groups of PTH and titanium chloroethoxide were the driving force for assembling the alternating layers.41 Amaya et al. have developed a two-stage template synthesis of PAN/Pd-NPs.12 However, the LbL method is limited to polyelectrolytes, thus making most of the conducting polymers not applicable. The LB (or LS) method, however, is simple and straightforward, which enables the assembly of monolayers and multilayers of a wide variety of conducting polymers. Nevertheless, only a limited number of studies45,49,51 have been reported that utilize the LB technique for the formation of nanocomposites of conducting polymers and NPs. Nicholson et al. studied the nanocomposite composed of poly(3-hexylthiophene)/Au-NPs prepared by the LS method.49 They investigated the effect of composition on the conductivity and morphology of the thin hybrid films. Vidya et al. deposited LB multilayers of poly(3-octylthiophene) and polyaniline with cadmium arachidate. The latter served as a precursor to the deposition of CdS nanoparticles.45 Wang et al. reported a nanocomposite of PbS NPs and poly(9-vinylcarbazole). Pyramid-shaped NPs were formed because of the interactions between the nitrogen atom of the polymer and Pb of the NPs.51 The goal of this study was to deposit and characterize monomolecular and multimolecular nanocomposite layers of polyaniline and Au-NPs (approximately 10 nm diameter) formed by the (36) He, J. Q.; Shao, W.; Zhang, L.; Deng, C.; Li, C. Z. J. Appl. Polym. Sci. 2009, 114, 1303–1311. (37) Sharma, M. K.; Roy, S.; Khilar, K. C. Colloids Surf., A 2009, 346, 123–129. (38) Zhang, D.; Karki, A. B.; Rutman, D.; Young, D. R.; Wang, A.; Cocke, D.; Ho, T. H.; Guo, Z. H. Polymer 2009, 50, 4189–4198. (39) Wang, B. B.; Sun, X. S.; Klabunde, K. J. Biobased Mater. Bioenergy 2009, 3, 130–138. (40) Garcia-Cerda, L. A.; Romo-Mendoza, L. E.; Quevedo-Lopez, M. A. J. Mater. Sci. 2009, 44, 4553–4556. (41) Ding, H. M.; Ram, M. K.; Nicolini, C. J. Mater. Chem. 2002, 12, 3585– 3590. (42) Ding, H. M.; Ram, M. K.; Nicolini, C. J. Nanosci. Nanotechnol. 2001, 1, 207–213. (43) Bertoncello, P.; Notargiacomo, A.; Erokhin, V.; Nicolini, C. Nanotechnology 2006, 17, 699–705. (44) Ram, M. K.; Yavuz, O.; Aldissi, M. Synth. Met. 2005, 151, 77–84. (45) Vidya, V.; Kumar, N. P.; Narang, S. N.; Major, S.; Vitta, S.; Talwar, S. S.; Dubcek, P.; Amenitsch, H.; Bernstorff, S. Colloids Surf., A 2002, 198, 67–74. (46) Berman, A.; Belman, N.; Golan, Y. Langmuir 2003, 19, 10962–10966. (47) Genson, K. L.; Holzmueller, J.; Jiang, C. Y.; Xu, J.; Gibson, J. D.; Zubarev, E. R.; Tsukruk, V. V. Langmuir 2006, 22, 7011–7015. (48) Li, F.; Bertoncello, P.; Ciani, I.; Mantovani, G.; Unwin, P. R. Adv. Funct. Mater. 2008, 18, 1685–1693. (49) Nicholson, P. G.; Ruiz, V.; Macpherson, J. V.; Unwin, P. R. Phys. Chem. Chem. Phys. 2006, 8, 5096–5105. (50) Lesiak, B.; Jablonski, A.; Zemek, J.; Trchova, M.; Stejskal, J. Langmuir 2000, 16, 1415–1423. (51) Wang, C. W.; Liu, H. G.; Bai, X. T.; Xue, Q. B.; Chen, X.; Lee, Y. I.; Hao, J. C.; Jiang, J. Cryst. Growth Des. 2008, 8, 2660–2664.

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LB method. Au-NPs stabilized by 2-mercapoethanesulfonate (MES) were added to the aqueous subphase and electrostatically attracted to the PANI monolayer in the air-water interface. The films were characterized using cyclic voltammetry (CV), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). We found that highly reproducible and uniform nanocomposite multilayers were deposited and that the Au-NPs extrude above the PANI and therefore could be used as nanoelectrodes for the underpotential deposition (UPD) of copper.

Experimental Section Instrumentation. A Teflon Langmuir trough with an area

of 240 cm2 (Nima Technology, Coventry, England, model 312DMC) was used. Electrochemical measurements (CV) were performed with a potentiostat (CHI-750B, CH Instruments Inc.). A standard three-electrode cell was used for the electrochemical experiments where the working electrode was either indium tin oxide (ITO-coated glass, 25  7 mm2, Delta Technologies, Stillwater, MN) coated with polyaniline (ITO/PANI) or ITO coated with polyaniline and gold nanoparticles (ITO/PANI/Au-NPs). The reference electrode was Ag/AgCl (KCl sat.), and a platinum rod served as an auxiliary electrode. Images of the various electrodes were acquired by high-resolution scanning electron microscopy (HR-SEM Sirion, FEI Company). The chemical and morphology analysis of the layers was obtained by X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical, U.K.) and by atomic force microscopy (AFM, Dimension 3100, Nanoscope V, Veeco, CA), respectively. AFM images were acquired using Si tips (Si TESP probe, Veeco). Materials. Emeraldine base polyaniline (PANI, MW 10 000) was purchased from Aldrich. Hydrogen tetrachloroaurate (HAuCl4, 99.9%) was obtained from Strem Chemicals, sodium borohydride (NaBH4, 98þ%) was received from Acros Organics, and sodium 2-mercaptoethanesulfonate (MES, 98.0%) was purchased from Fluka BioChemika. Chloroform (99.8%, HPLCanalyzed) and acetonitrile (ACN, 99.9%, HPLC-analyzed) were purchased from Merck. LB films were deposited onto clean ITO-coated glass strips (termed ITO). Procedures. For the preparation of PANI LB films, 0.5 mg 3 mL-1 PANI was dissolved in a mixture of chloroform/ m-cresol solution (9:1 v/v). The resulting solution (75 μL) was spread over the subphase in the Langmuir trough, which contained 10 mM HCl (pH 2) and 1-8 mL of Au-NPs. The latter were synthesized following our recent procedure.52 Specifically, 0.0394 g (0.1 mmol) of HAuCl4 and 0.0164 g (0.1 mmol) of MES were dissolved in 36 mL of water (EasyPure UV, Barnstead) and stirred for 10 min at room temperature. NaBH4 (0.2 mmol, 7.6 mg) in 10 mL of deionized water was added over a period of 2 min. The solution slowly turned deep purple. The solution was stirred for 2 h. Then, the Au-NPs were precipitated by adding 3 times the sample volume of ACN. After precipitation (overnight) and decantation of the ACN, the sample was rinsed with 5 mL of ethanol twice. This procedure assures the removal of excess reducing agent and stabilizer. After decantation of the ethanol, the sample was resuspended in water before use. The NPs were characterized using the Nanosizer (Nanozs, Malvern, Worcestershire, U.K.) as well as TEM. TEM gave a diameter of ca. 6 ( 1 nm, whereas the Nanosizer gave a diameter of 10 nm. This difference is due to the fact that whereas TEM provides the size of the metallic core of the nanoparticles the Nanosizer (which is based on measuring the Brownian motion) is affected by the hydrodynamic size and shape. ITO was cleaned with deionized water and ethanol, sonicated for 10 min, rinsed again with deionized water, and dried using (52) Gofberg, I.; Mandler, D. J. Nanopart. Res. 2009, DOI 10.1007/s11051-0099738-3.

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Kimwipes before deposition. Small (1  1 cm2) silicon wafers (used for SEM and AFM analysis of the PANI/Au-NPs layers because of their low roughness as opposed to ITO) were etched for 5 min in a 5% HF aqueous solution, rinsed with deionized water, and dried using Kimwipes. The monolayers formed at the interface were left for at least 2 h before compression. The monolayer was compressed (15 cm2 3 min-1) until a steady surface pressure of 22 mN 3 m-1 was reached. Films were transferred onto ITO or Si under constant pressure by Z-type deposition. The speed of deposition was 5 mm 3 min-1 (upward). The time interval between each monolayer deposited was 1 min. Cyclic voltammetry (CV) of the films was carried out in 0.1 M KCl solution. Underpotential deposition (UPD) was performed in a solution containing 2 mM CuSO4, 0.1 M K2SO4, and 10 mM H2SO4 by sweeping the potential between 0.5 and 0 V at a rate of 50 mV 3 s-1.

Figure 1. Π-A isotherms of a monolayer of PANI spread over 10 mM HCl in the (a) absence and presence of (b) 1, (c) 2, and (d) 4 mL of Au-NPs.

Results and Discussion Incorporating nanoparticles (NPs) into Langmuir-Blodgett (LB) films can be accomplished by adding water-soluble NPs to the subphase, provided that they strongly associate with the Langmuir film. Hence, we synthesized Au-NPs stabilized by mercaptoethanesulfonate (MES), which are negatively charged at pH >1.5. Alkyl and aryl sulfonates (e.g., dodecylbenzenesulfonic acid) are known for their high affinity for PANI.14,53-55 Thus, we anticipated that a monolayer of MES-stabilized Au-NPs would be formed at the air-water interface in the presence of PANI. Figure 1 shows the surface pressure-mean molecular area (Π-A) isotherms of a monolayer of PANI, which was spread as the emeraldine base over 10 mM HCl in the absence and presence of Au-NPs. The isotherms are typical for PANI56-59 and do not exhibit any phase transition. The mean molecular area was calculated on the basis of a polyaniline repeat unit consisting of a benzenoid, a quinoid, and an imine unit with a total molecular weight of 362. The isotherm of PANI is very stable, and hysteresis could not be detected upon decompressing the Langmuir film. The collapse pressure, Pcollapse, is higher than 35 mN/m, and Alim (at Pcollapse) area per molecule and the zero-pressure molecular area, A0, of the PANI film are ca. 17 and 10 A˚2 3 molecule-1, respectively. The values of Alim and A0 are slightly higher than in previous reports, presumably because of the presence of a small concentration of m-cresol, which is known to extend the polymer chains.56,60 Figure 1 also shows the isotherms of PANI upon adding an increasing number of Au-NPs. The isotherm is shifted to larger areas per molecule. Alim is ca. 23 A˚2 3 molecule-1, indicating that the Au-NPs interact with the PANI Langmuir film. Such a small change suggests that the Au-NPs do not penetrate into the Langmuir film because their average size is ca. 10 nm. Moreover, it can be seen that increasing the concentration of Au-NPs in the (53) Haba, Y.; Segal, E.; Narkis, M.; Titelman, G. I.; Siegmann, A. Synth. Met. 1999, 106, 59–66. (54) Zou, X. Q.; Bao, H. F.; Guo, H. W.; Zhang, L.; Li, Q.; Jiang, J. G.; Niu, L.; Dong, S. J. J. Colloid Interface Sci. 2006, 295, 401–408. (55) Yue, J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; Macdiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665–2671. (56) Choi, B. Y.; Chung, I. J.; Chun, J. H.; Ko, J. M. Synth. Met. 1999, 99, 253–256. (57) Ramanathan, K.; Ram, M. K.; Malhotra, B. D.; Murthy, A. S. N. Mater. Sci. Eng., C 1995, 3, 159–163. (58) Ram, M. K.; Adami, M.; Sartore, M.; Salerno, M.; Paddeu, S.; Nicolini, C. Synth. Met. 1999, 100, 249–259. (59) Chowdhury, A. N.; Saleh, F. S.; Rahman, M. R.; Rahim, A. J. Appl. Polym. Sci. 2008, 109, 1764–1771. (60) Zhang, J.; Burt, D. P.; Whitworth, A. L.; Mandler, D.; Unwin, P. R. Phys. Chem. Chem. Phys. 2009, 11, 3490–3496.

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Table 1. Transfer Ratio (TR) as a Function of pH, Speed, and Type of Deposition pH

speed (mm 3 min-1)

path

TR

4

5 5 30 5 5 30

down up down down up down

23.7 ( 13.0% 48.8 ( 15.3% 3.6 ( 0.0% 36.9 ( 26.2% 61.6 ( 29.8% 12.0 ( 10.5

2

subphase had almost no influence on the isotherm. The fact that the isotherm did not change dramatically upon increasing the concentration of Au-NPs in the subphase also implies that PANI does not entrap the Au-NPs and change its 2D structure. It is worth mentioning that no hysteresis was observed upon decompressing these films. The Langmuir films were transferred onto indium tin oxidecoated glass (ITO). The transfer ratio (TR, eq 1), namely, the ratio between the changes in the trough area (ΔSbarriers) upon transferring the LB film divided by the area of the ITO that was exposed to the air-water interface (Ssubstrate), was recorded (Table 1).   ΔSbarriers  100% TR ¼ Ssubstrate

ð1Þ

We found that the TR was greatly affected by the pH of the subphase as well as by the type (upward or downward) of deposition. As the acidity was increased to pH 2.0, TR attained a value of ca. 60%. Furthermore, higher TR values were achieved upon withdrawing ITO from the subphase (Z-type deposition) than by downward deposition (X-type deposition). This suggests that the negatively charged Au-NPs bridge between the ITO (that is positively charged at low pH) and the positively charged PANI. Previous reports using dodecylbenzenesulfonic acid-doped PANI clearly showed that the surfactant bridged between the substrate (ITO) and the polymer.60 Figure 2A shows the cyclic voltammetry (CV) of one, two, and three layers of deposited PANI/Au-NPs. The electrolyte was a 0.1 M KCl solution. The CV shows two sets of distinct peaks. The set of peaks at potentials from 0.2 to 0.5 V is characteristic of PANI and is attributed to the doping and undoping of the polymer by chloride ions.61 As expected, increasing the number of layers increases the current. The potential of the peaks is (61) Goldenberg, L. M.; Petty, M. C.; Monkman, A. P. J. Electrochem. Soc. 1994, 141, 1573–1576.

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Figure 2. (A) Cyclic voltammetry of one, two, and three layers of PANI/Au-NPs deposited on ITO in 0.1 M KCl (at a scan rate of 50 mV 3 s-1). (B) Charge of the peaks at 1.0 V as a function of the number of LB layers. Table 2. Peak Potential of the Anodic Dissolution of Au-NPs as a Function of the Number of LB Layers layers 1 2 3

potential (mV) 994.3 997.9 1002.5

independent of the scan rate, but the current increases linearly with the scan rate (not shown), which is indicative of a surface process. In addition to the doping/undoping peaks, a clear anodic wave at 1.0 V is observed that is assigned to the oxidation and dissolution of gold in a chloride medium. This peak has the clear shape of a surface-confined process, and its potential is slightly shifted toward positive potentials as the number of layers increases (Table 2). This suggests that the distance of the AuNPs from the ITO increases with the number of layers, which slightly slows down the rate of electron transfer and oxidation. Figure 2B plots the charge associated with the oxidation of gold versus the number of layers. A good linear correlation is found, implying that the amount of gold transferred onto ITO varies linearly with the number of layers. The charge of gold per layer is 4.5  10-5 C, which corresponds to 5.8  10-5 mol 3 cm-2 gold. It should be noticed that the extrapolated curve in Figure 2B does not cross the origin, presumably because of the further oxidation of PANI, which occurs at more or less the same potentials and for excess NPs deposited in the first layer. Figure 3 shows the effect of the pH of the subphase on the oxidation of the Au-NPs in the nanocomposites. Not only are the doping and undoping of PANI less prominent but also the charge associated with the oxidation of gold is significantly lower with the rising pH. These findings are independent of the number of layers that was transferred and are in good agreement with the TR reported above. Thus, additional LB films were deposited at pH 2.0. Figure 4A shows the CV of three layers of PANI/Au-NPs deposited on ITO as a function of the volume of Au-NPs added to the subphase. It is evident that the amount of gold that was oxidized increased with the concentration of Au-NPs in the subphase. This clearly indicates that the PANI at the air-water interface was not saturated by the gold nanoparticles. The dependence of the charge of the anodic peaks as a function of the volume of Au-NPs added to the subphase is plotted in Figure 4B. The linear dependence clearly alludes to the relatively weak interaction between the nanoparticles and the PANI film. This is in accordance with the isotherms shown in Figure 1. 4242 DOI: 10.1021/la903284g

Figure 3. Cyclic voltammetry of three layers of PANI/Au-NP LBs deposited on ITO at different pH values: (a) 4.0 and (b) 2.0. All other parameters are the same as in Figure 2.

Namely, the addition of Au-NPs to the subphase shifts the equilibrium at the air-water interface toward the binding of the nanoparticles, yet saturation of the PANI with Au-NPs was not achieved as would have been observed in the nonlinear dependence in Figure 4B. Scanning electron microscopy (SEM) supports these findings. Figure 5 shows SEM images of one to three layers of PANI/ Au-NPs on a Si surface. It is obvious that the number of nanoparticles increases drastically with the number of layers. Clearly, the electron beam (5 kV) penetrates across the layers and therefore the image of the three layers shows the total number of nanoparticles in all layers. The relatively low density of Au-NPs in one layer (Figure 5A) is in complete accordance with the observations discussed above. That is, the electrostatic attachment of the Au-NPs to the PANI film is relatively weak, and because the concentration of nanoparticles in the subphase is low, we cannot expect full coverage. Further information about the structure of these nanocomposite films was obtained by atomic force microscopy (AFM). Figure 6 shows the AFM images of one, two, and three layers of PANI/Au-NPs on a Si wafer. Interestingly, we also observe in the AFM images an increase in the density of nanoparticles with the number of deposited layers. This could not be explained if the layers were homogeneously layered as shown schematically in Figure 7A. Taking into account the relatively low density of Au-NPs per layer cannot yield such a structure, and we anticipate a more condensed assembly as shown schematically in Figure 7B. Such a structure will clearly be revealed by AFM because the underlying nanoparticles will also be imaged. Indeed, a cross Langmuir 2010, 26(6), 4239–4245

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Figure 4. (A) Cyclic voltammetry of three layers of PANI/Au-NPs deposited on ITO in 0.1 M KCl (at a scan rate of 50 mV 3 s-1) in the presence of increasing volumes of Au-NPs added to the subphase (2, 4, and 8 mL). (B) Charge of the peaks at 1.0 V as a function of the volume of Au-NPs added to the subphase.

Figure 5. SEM images of (A) one, (B) two, and (C) three layers of PANI/Au-NPs deposited on a silicon wafer.

Figure 6. AFM images (topography) of the samples shown in Figure 5. The scan area is 1  1 μm2, and the Z scale is 9.0 nm.

Figure 7. Possible arrangements of the Au-NPs within the PANI. (A) Homogeneous layers, (B) condensed assembly, and (C) extrusion across the polymer.

section of the AFM image of the two layers discloses two or more populations of nanoparticle heights. Langmuir 2010, 26(6), 4239–4245

The Au-NPs have a diameter of ca. 10-20 nm. This means that they are too large to be accommodated within the PANI film, DOI: 10.1021/la903284g

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which is estimated to have a thickness of ca. 1 nm, vide infra. From the TR (discussed above), we conclude that the layers are deposited by the Z-type deposition. This suggests that the PANI is expected to cover the Au-NPs. However and as shown in Figure 7, the size of the nanoparticles is significantly larger than the thickness of the PANI, which might result in their extrusion across the polymer (Figure 7C). The interfacial structure of the PANI/Au-NPs nanocomposite was determined by studying the underpotential deposition (UPD) of copper on these systems. Figure 8 shows the UPD of Cu2þ on four systems: three layers of ITO/PANI and one, two, and three layers of ITO/PANI/ Au-NPs. We used a solution that consisted of 2 mM CuSO4, 0.1 M K2SO4, and 10 mM H2SO4. The UPD of copper on Au(111) has been studied by Kolb and others.62,63 Our results obtained on a flame-annealed polycrystalline gold surface (not shown) are in good agreement with these previous reports. Not surprisingly, no UPD was detected on bare ITO (also not shown) or on ITO covered with three layers of PANI (Figure 8A). However, a distinct set of UPD peaks can be seen on the ITO coated with layers of PANI/Au-NPs. Because UPD requires the presence of a metallic surface (such as gold), this clearly implies that the Au-NPs are exposed to the electrolyte solution. Furthermore, it can be seen that the amount of copper that is electrochemically deposited increases with the number of PANI/Au-NPs layers. This indicates that there is more gold that is exposed to the

Figure 8. (A) Cyclic voltammetry of Cu2þ (2 mM) in 0.1 M K2SO4 and 10 mM H2SO4 recorded with (a) three layers of ITO/PANI and (b-d) one, two, and three layers of PANI/Au-NPs deposited on ITO. The scan rate was 50 mV 3 s-1. (B) Charge associated with the UPD as a function of the number of layers.

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electrolyte by each additional layer that is deposited. Hence, it can be concluded that the Au-NPs, which are in fact deposited by Z-type deposition, form holes across the PANI layer as schematically depicted in Figure 7C. The charge due to copper deposition as a function of the number of PANI/Au-NPs layers is shown in the inset of Figure 8. The fact that the amount of copper levels off upon increasing the number of layers suggests that an almost full coverage of gold nanoparticles is reached upon depositing three layers. The composition of the nanocomposite layers was examined by X-ray photoelectron spectroscopy (XPS). Figure 9A shows the atomic concentration percentage of the elements originating from the ITO (i.e., In and Sn) and those that were contributed by the PANI/Au-NPs layers, namely, N, C, and Au. A general trend can be observed. That is, the atomic concentration percentage of those elements constituting the substrate decreases with the number of layers, whereas the percentage of elements stemming from the nanocomposite layers increases. Furthermore, the ratio between the atomic concentration percentage of N (which is associated with PANI) and Au (originates only from the nanoparticles) remains constant for two and four layers. This provides additional evidence of the progressive growth of the nanocomposite with the number of layers. Figure 9B presents some of the results obtained by angleresolved XPS. Specifically, the effect of the ratio between Au and C as a function of the takeoff angle is shown for six layers. Angle-resolved XPS is very sensitive to the depth of the element inside the layer. The signal increases with angle for those elements located closer to the surface, whereas it decreases for those buried deeply inside the layer. Clearly, the ratio between Au and C increases with increasing takeoff angle, which indicates that the gold occupies the uppermost part of the interface, in excellent agreement with our suggested model shown in Figure 7C. Further information is presented in Table 3. The thickness of the layers can be determined from the attenuation of the elements of the ITO. Figure 9C shows that a linear correlation is obtained by plotting the natural logarithm of the atomic percent of Sn versus the number of layers deposited. The Sn signal is expected to decrease exponentially because of the effect of film thickness on the inelastic mean path of the ejected electrons. The thickness of each layer can readily be calculated from the slope. We obtain 8 A˚ per PANI/Au-NP layer, assuming an inelastic mean free path of 15 A˚50 across PANI. This also indicates that it is the PANI layer that attenuates the ejected electrons from the ITO rather than the scattered Au nanoparticles.

Figure 9. (A) Atomic concentration of the elements that emerged from the ITO and the nanocomposite. (B) Ratio between Au and C as a function of the takeoff angle. (C) Natural logarithm of the atomic percent of Sn vs the number of layers deposited. 4244 DOI: 10.1021/la903284g

Langmuir 2010, 26(6), 4239–4245

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Table 3. Atomic Concentration of the Elements as a Function of the Angle of the Substrate Relative to the Incident XPS Beam angle (deg) 0 35 75

In

Sn

C

N

Au

O

total (%)

1.81 1.14 0.45

0.22 0.12 0.07

75.38 77.28 80.46

2.92 3.23 2.83

2.07 2.27 2.57

15.07 14.02 12.17

99.30 98.06 98.55

Conclusions Nanocomposites composed of multilayers of polyaniline- and mercaptoethanesulfonate-stabilized Au nanoparticles were assembled by the LB technique. The water-dispersed nanoparticles in the subphase that are negatively charged were attracted by the positively charged polymer film at the liquid-air interface and therefore transferred through Z-type deposition on various substrates. Different methods, including cyclic voltammetry, scanning electron microscopy, atomic force microscopy, and X-ray (62) Nichols, R. J.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1991, 313, 109–119. (63) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275–291.

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photoelectron spectroscopy were used to characterize the samples. The film grows progressively, as was proven by the linear correlation between charge associated with the oxidation of the gold nanoparticles and the number of layers as well as by XPS measurements. Moreover, we noticed a strong pH effect of the subphase (i.e., increasing the acidity of the subphase increased the number of Au nanoparticles extracted by polyaniline). Finally, the underpotential deposition of copper clearly proved the extrusion of NPs above the polyaniline film. These assemblies are likely to find interesting applications in many fields such as catalysis, corrosion inhibition, tribology, and photonic devices. This approach is not limited to conducting polymers or to Au-NPs but can be implemented in many other systems. The fact that the Au-NPs are ohmically connected to the substrate underneath could also be used in sensing applications. Finally, we are currently examining this approach to imprinting NPs in various matrices. Acknowledgment. This work is supported by the Israel Science Foundation (contract 485-06). The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University is acknowledged.

DOI: 10.1021/la903284g

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