One-Pot Surfactantless Route to Polyaniline Hollow Nanospheres with

Mar 23, 2012 - Herein, we report for the first time that PANI hollow nanospheres with incontinuous multicavities as a novel hollow nanostructure have ...
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One-Pot Surfactantless Route to Polyaniline Hollow Nanospheres with Incontinuous Multicavities and Application for the Removal of Lead Ions from Water Jie Han, Ping Fang, Jie Dai, and Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, Jiangsu, P. R. China S Supporting Information *

ABSTRACT: Polyaniline (PANI) hollow nanospheres with controllable incontinuous nanocavities ranging in size from 10 to 50 nm as a novel hollow nanostructure have been successfully fabricated by chemical polymerization of aniline with chloroaurate acid as the oxidant and citric acid as the doping acid. Experimental factors, such as concentration and kind of oxidant and doping acid, were investigated to illustrate their effect on morphology of PANI. According to experimental results and time-dependent investigations, a possible formation mechanism involved was then proposed. The adaptability of this route to hollow nanostructures with multicavities of other conducting polymer was also revealed. Furthermore, the adsorption properties of PANI hollow nanospheres toward lead ions in water were investigated. template.32 In our previous work, we have reported a general strategy for hollow spheres of PANI derivatives with typical one hole in each surfaces templated by monomer droplet.33−35 In general, PANI hollow structures with a single nanocavity are often formed. In recent years, polymer hollow spheres with multicavities have aroused great interest because of their novel structures and structure-related potential applications.36−38 For example, polydivinylbenzene latex particles with various cavity structures have been fabricated through an emulsion polymerization process;36 cagelike porous polymeric microspheres have been achieved by W/O/W emulsion polymerization of a surfactant monomer of N-(4-vinylbenzyl)-N,N-dibutylamine hydrochloride and a hydrophobic monomer of styrene in water.37 However, synthesis of PANI hollow structures with multicavities has been seldom seen. Herein, we report for the first time that PANI hollow nanospheres with incontinuous multicavities as a novel hollow nanostructure have been successfully fabricated by a surfactantless route through chemical polymerization of aniline monomer with chloroaurate acid as oxidant and citric acid as doping acid. Fourier-transform infrared (FTIR), UV−vis, and X-ray diffraction (XRD) techniques were used to characterize the products. In addition, effects of concentration and kind of oxidant and doping acid on the morphology of PANI were thoroughly investigated. Formation mechanisms involved had been proposed according to experimental observations. The adaptability of this route to hollow nanostructures with

1. INTRODUCTION As one of the mostly investigated conducting polymers, polyaniline (PANI) has continuing generated increasing interest for controllable synthesis of micro/nanostructures1−4 and applications as supporting material for catalysts,5−12 biosensors,13−15 electronic devices,16 and so forth. Morphology-dependent application of PANI has become the research focus in recent years. Hollow spheres have the potential for promising applications because of their advantageous properties as low effective density and high specific surface area.17−19 As a result, an enormously increasing research interest has been drawn for the design and synthesis of PANI hollow structures. Because of the simple synthetic procedures and uniform sizes and structures, polystyrene spheres are commonly considered as ideal templates to guide the formation of PANI hollow spheres.20−22 After removal of templates through selective dissolving, well-defined PANI hollow spheres can be obtained. It is noteworthy that MnO223,24 and Cu2O25,26 particles have been discovered as effective templates for PANI hollow spheres where dissolving of cores occurs during polymerization processes. More interestingly, solid particles of PANI derivatives can be transformed into hollow structures through a swelling-evaporation strategy.27,28 In addition, surfactants are often used as structure directors for the formation of PANI hollow spheres, where self-assembled micelles29,30 or vesicles31 formed by surfactants are considered as soft templates for PANI hollow spheres. Meanwhile, some complex structures such as rambutan-like hollow spheres with superhydrophobic property have also been fabricated where spherical micelles composed of perfluorooctane sulfonic acid (PFOSA) serve as a “microreactor” and PFOS/aniline salt micelles act as the soft © 2012 American Chemical Society

Received: February 12, 2012 Revised: March 21, 2012 Published: March 23, 2012 6468

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Figure 1. (a, b) FE-SEM and (c, d) TEM images of PANI hollow nanospheres. Synthetic conditions: [citric acid] = 0.020 mol L−1; [aniline] = 0.120 mol L−1; [HAuCl4] = 0.020 mol L−1; 8 h. 2, where Q was the adsorption capacity, q was the adsorptivity, C0 and C were Pb(II) ions concentrations before and after adsorption, respectively, V was the initial volume of the Pb(II) ions solution, and W was the weight of PANI hollow nanospheres added.

multicavities of other conducting polymer was also considered. Furthermore, the adsorption properties of PANI hollow nanospheres toward lead ions in water were also conducted.

2. EXPERIMENTAL SECTION Chemicals. Aniline and o-toluidine monomers (Shanghai Chemical Co.) were distilled under reduced pressure. Chloroaurate acid (HAuCl4, Shanghai Chemical Co.) and other reagents were purchased from Aldrich and used without further purification. The water used in this study was deionized by Milli-Q Plus system (Millipore, France), having 18.2 MΩ electrical resistivity. Preparation of PANI Hollow Nanospheres. In a typical synthesis, aniline (10 mg, 0.11 mmol) was added into 0.8 mL aqueous solution containing citric acid (0.1 mmol) with magnetic stirring at room temperature for 10 min. After that, 0.1 mol L−1 aqueous solution of HAuCl4 (0.2 mL, 0.020 mmol) was added at once. The resulting solution was stirred for another 0.5 min to ensure complete mixing and then the reaction was allowed to proceed without agitation for 8 h at 20 °C. Finally, products were washed with deionized water and ethanol until the filtrate became colorless and then dried in a vacuum at 60 °C for 24 h. Adsorption Experiments. It is observed that in addition to PANI nanospheres, Au particles also exist in products. In order to exactly evaluate the adsorption properties of PANI nanospheres, Au particles were removed from products by dissolving the products in excess amount of saturated KI/I2 solution for more than 12 h at 20 °C. After that, the products were washed with deionized water until the filtrate became colorless and then dried in a vacuum at 60 °C for 24 h for adsorption experiments. The adsorption of Pb(II) ions in aqueous solution on PANI hollow nanospheres was performed in a batch experiment. Aqueous solution (25 mL) containing Pb(II) ions with concentration ranges from 20 to 300 mg L−1 was incubated with PANI hollow nanospheres dosage ranges from 2 to 40 mg at a fixed temperature of 30 °C. After a desired treatment period, the polymer was filtered from the solution, and the concentration of Pb(II) ions in the filtrate after adsorption was measured by atomic absorption spectroscopy (AAS) analysis. The adsorbed amount of Pb(II) ions onto PANI hollow nanospheres was calculated according to eqs 1 and

Q=

(C0 − C)V W

(1)

q=

(C0 − C) × 100% C0

(2)

Characterization. The morphologies of products were examined by transmission electron microscopy (TEM, Tecnai-12 Philip Apparatus Co.) and field-emission scanning electron microscopy (FF-SEM, XL-30e Philip Co., Holland). FTIR spectra of PANI nanospheres were recorded in the range of 400−4000 cm−1 using FTIR spectroscopy (Tensor 27, Bruker, Germany). The samples were prepared in pellet form with spectroscopic-grade KBr. UV−vis spectra (UV-2501, Shimadzu Corporation, Japan) of products dissolved in dimethylformamide (DMF) were measured in the range between 300 and 900 nm. XRD patterns were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer. The products were recorded in the 2θ range from 5° to 90° in steps of 0.04° with a count time of 1 s each time.

3. RESULTS AND DISCUSSION 3.1. Morphology, Characterization, and Formation Mechanism of PANI Hollow Nanospheres. Figure 1 shows typical FE-SEM and TEM images of PANI nanostructures. It is clearly seen in Figure 1a that products are composed of spherical structures with an average diameter of 150 nm. Clear observation as given in Figure 1b reveals rough surfaces with prominences on surfaces of PANI nanospheres. The fractures on surfaces as indicated by arrows disclose that the individual nanosphere is made up of numerous interconnect smaller nanospheres. It is intriguing that almost all PANI nanospheres are hollow with incontinuous multicavities as illustrated in 6469

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Figure 1c. Most of the cavities are about 20 nm in diameter (Figure 1d). It should be noted that gold nanoparticles with irregular morphology resulted from reduction of HAuCl4 are also observed apart from PANI hollow nanospheres. To elucidate the formation mechanism of novel PANI hollow nanospheres with multicavities, the influence of the synthesis conditions, such as kind and concentration of reactants, and polymerization time on the morphology of the resulting PANI nanostructures was systematically investigated. Figure 2 shows TEM images of PANI nanostructures with increasing citric acid concentration where aniline monomer and HAuCl4 oxidant concentration are maintained at 0.120 and 0.020 mol L−1, respectively. As shown in Figure 2a, if citric acid is not introduced, PANI solid nanospheres with size of about 100 nm that interconnected with each other together with some fibrous structures in small proportion are seen in products. The magnified image in Figure 2b discloses that all of the PANI nanospheres and nanofibers (inset in Figure 2b) are decorated with highly dispersed black dots with diameter of 2 nm that should be ascribed to gold nanoparticles. This indicates that uniform PANI/gold nanoparticle composites can be formed by simply mixing aniline monomer and HAuCl4 oxidant without using any doping acid. If citric acid is introduced at relatively low concentration (0.020 mol L−1), PANI hollow nanospheres with multicavities can be identified as shown in Figure 1. When increasing the citric acid concentration from 0.020 to 0.050 mol L−1, PANI hollow nanospheres with multicavities also can be verified; however, obvious increase in cavity size from about 20 nm to about 30 nm can be observed (Figure 2c and d). Further increasing citric acid concentration to 0.100 mol L−1 leads to increase in cavity size to about 50 nm (Figure 2e and f). However, cavity size decreases to about 15 nm (Figure 2g and h) and even solid PANI nanospheres (Figure 2i and j) are formed, when increasing citric acid to 0.200 and 0.800 mol L−1, respectively. Results hint that cavity size of PANI hollow nanospheres can be tuned by changing the citric acid concentration; however, higher citric acid concentration is unfavorable for the formation of PANI hollow nanospheres with multicavities. For comparison, similar polymerization reaction using commonly used doping acid HCl instead of citric acid is also conducted. Results show that polymerization of aniline in HCl aqueous solution using HAuCl4 as the oxidant leads to the formation of PANI nanofibers (Figure 3a), which is consistent with results reported.39−41 Therefore, it is believed that citric acid as a special doping acid plays a determining role in the formation of PANI hollow nanospheres with multicavities. In this study, HAuCl4 instead of commonly used ammonium persulfate (APS) is chosen as oxidant. It is well-established that oxidation of aniline using APS in the presence of a doping acid will lead to the formation of PANI nanofibers.42 Our preliminary experiment also confirms the formation of PANI nanofibers under similar synthetic conditions for PANI hollow nanospheres with multicavities except for using APS instead of HAuCl4 as the oxidant (Figure 3b). This indicates that HAuCl4 as the oxidant is another important factor for the formation of PANI hollow nanospheres with multicavities. Thereafter, the influence of HAuCl4 oxidant concentration on PANI morphology is investigated for further understanding the function of oxidant. In comparison with Figure 1, if HAuCl4 concentration decreases from 0.020 to 0.015 mol L−1, significant decrease in density of nanocavities can be seen, and the size of nanocavities also decreases from 20 to about 10

Figure 2. TEM images of PANI hollow nanospheres synthesized under different citric acid concentration (mol L−1): (a, b) 0, (c, d) 0.050, (e, f) 0.100, (g, h) 0.200, and (i, j) 0.600. Other synthetic conditions: [aniline] = 0.12 mol L−1; [HAuCl4] = 0.02 mol L−1; 8 h.

nm (Figure 4a). The nanocavities almost disappear when HAuCl4 concentration is maintained at 0.008 mol L−1 (Figure 4b). Results show the transformation of PANI hollow nanospheres with multicavities to solid nanospheres by 6470

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Figure 3. (a) TEM image of PANI nanofibers using HAuCl4 as oxidant and HCl as doping acid. Synthetic conditions: [aniline] = 0.12 mol L−1; [HCl] = 0.10 mol L−1; [HAuCl4] = 0.02 mol L−1; 25 °C, 8 h. (b) TEM image of PANI nanofibers using APS as oxidant and citric acid as doping acid. Synthetic conditions: [aniline] = 0.12 mol L−1; [citric acid] = 0.10 mol L−1; [APS] = 0.02 mol L−1; 25 °C, 8 h.

Figure 5. TEM images of PANI hollow nanospheres synthesized at different reaction time (min): (a, b) 7, (c) 20, and (d) 120. Other synthetic conditions: [citric acid] = 0.020 mol L−1; [aniline] = 0.120 mol L−1; [HAuCl4] = 0.020 mol L−1. Inset shows corresponding magnified TEM image.

polymerized, meanwhile gold ions are reduced into metallic states as gold nanoparticles.42,47−49 At the beginning, redox reactions happen in micelles that lead to the formation of gold nuclei and PANI oligomers. Due to interactions between gold nuclei and functional groups of amine and π electrons in benzene rings in PANI oligomers, stable PANI oligomer nanospheres embedded with gold nanoparticles that are templated by micelles will be formed. With polymerization ongoing, PANI polymer derived from PANI oligomers will be formed. The transformation of amine groups into imine groups with polymerization of PANI weakens the interaction between gold nuclei and PANI. In addition, with polymerization ongoing, citric acid will be doped in PANI, which further weakens the interaction between gold nuclei and PANI. It is well-known that citric acid is also a good candidate as stabilizer for gold nanoparticles.50 Therefore, gold nuclei are inclined to migrate into micelle shells where hydrophilic −COOH groups exist because the interaction between gold nuclei and citric acid are superior to that between gold nuclei and PANI with polymerization proceeding, which is contrast with previous reported results where p-toluenesulfonic acid instead of citric acid is used as doping acid under similar synthetic conditions.43 As a result, gold nanoparticles will leave the PANI matrix that leads to the formation of PANI hollow nanospheres with multicavities. According to this proposal, the reasons for the morphology change of PANI with reactant concentrations can be rationalized. As for the influence of citric acid concentration given in Figure 2, it can be deduced that PANI nanospheres embedded with gold nanoparticles will be formed at first. Due to the absence of citric acid, competition interaction with gold nanoparticles disappears, and then PANI nanospheres embedded with stable gold nanoparticles will be formed (Figure 2a and b). As for the influence of HAuCl 4 concentration given in Figure 4, it is believed that decreasing

Figure 4. TEM images of PANI hollow nanospheres synthesized under different HAuCl4 concentration (mol L−1): (a) 0.015 and (b) 0.008. Other synthetic conditions: [aniline] = 0.120 mol L−1; [citric acid] = 0.020 mol L−1; 8 h.

decreasing HAuCl4 oxidant concentration, which is similar to the function of citric acid where similar phenomenon can be evidenced by increasing citric acid concentration. Examining the morphological development of PANI hollow nanospheres with multicavities will provide more positive evidence for understanding the formation mechanisms. As shown in Figure 5, PANI hollow nanospheres containing gold nanoparticles (2 nm in size) embedded are first formed at reaction time of 7 min (Figure 5a and b). Conglomeration of gold nanoparticles to form large particulates that are apart from PANI hollow nanospheres can also been found (Figure 5a). With polymerization reaction proceeding, the density of gold nanoparticles embedded in PANI hollow nanospheres decreases (Figure 5c) and eventually PANI hollow nanospheres with multicavities are formed (Figure 5d). It seems that the outlet of gold nanoparticles embedded in PANI hollow nanospheres leads to formation of PANI hollow nanospheres with multicavities. Although the exact mechanisms involved in the formation of PANI hollow nanospheres with multicavities are not fully understood, the above-mentioned results allow us to suggest a possible process responsible for the formation of such unusual hollow nanospheres as shown in Scheme 1. It is believed that micelles composed of citric acid−aniline salt will be formed in solution owing to the hydrophilic −COOH groups, and free aniline will diffuse into micelles to form monomer filled micelles.43−46 When HAuCl4 is added, redox reaction between aniline and HAuCl4 will happen. Aniline monomers are 6471

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Scheme 1. Illustration of the Proposed Formation Mechanism of PANI Hollow Nanospheres

The chemical structures of PANI hollow nanospheres were characterized by FTIR (Figure 7a) and UV−vis spectra (Figure 7b). In the FTIR spectrum, the CC stretching deformation of the quinoid (1574 cm−1) and benzenoid rings (1494 cm−1), C−N stretching of secondary amine in polymer main chain (1308 cm−1), C−N stretching in bipolaron structure (1246 cm−1), and out-of-plane deformation of C−H in the 1,4disubstituted benzene ring (821 cm−1) are observed. In addition, the stretching vibration of the CH2 group (2923 cm−1) and CO stretching vibration band (1719 cm−1) indicate the existence of citric acid in the nanostructures. The UV−vis absorption spectrum of PANI hollow nanospheres dissolved in DMF was also studied. The peaks at 304 and 570 nm, which are assigned to the π−π* benzenoid transition and the benzenoid to quinoid excitotic transition, respectively, are observed. Results from FTIR and UV−vis are in consistent with results of traditional synthesized PANI using APS as oxidant.1,2 3.2. Adsorption Properties of PANI Hollow Nanospheres with Multicavities toward Lead Ions. Recently, PANI and its derivatives have attracted considerable attention for removal of various pollutions from water due to its unique properties of easy of preparation, high environmental stability, special doping mechanisms, and particular low cost.51−54 Herein, the adsorption properties of the novel PANI hollow nanospheres with high surface areas toward Pb(II) ions are also conducted. In order to exactly evaluate the adsorption properties of Pb(II) ions onto PANI hollow nanospheres, Au particles should be removed by dissolving the products in excess saturated KI/I2 solution. It has been confirmed from XRD patterns (Figure S1, Supporting Information) and TEM images (Figure S2, Supporting Information) that Au particles are almost completely removed from products resulting in pure PANI hollow nanospheres. In addition, it is found that the morphology of PANI hollow nanospheres with multicavities can survive this process (Figure S2, Supporting Information). Figure 8a shows the adsorption capacity and adsorptivity of Pb(II) ions as a function of PANI dosage. It is seen that the Pb(II) ion adsorptivity increases whereas the Pb(II) ion adsorption capacity decreases with increasing adsorbent dosage. It is shown that PANI hollow nanospheres are efficient adsorbents for removal of Pb(II) ions, as the adsorptivity of Pb(II) ions can reach as high as 97.0% with Pb(II) ions concentration of 20 mg L−1. In order to examine the effect of solution pH on Pb(II) ion adsorption onto PANI hollow nanospheres, the initial pH solution was adjusted between 3.0−7.0 by adding a certain amount aqueous solution of 0.1 M HCl. Figure 8b shows the

density of gold nuclei is produced with HAuCl4 concentration, which leads to decreased density and size of nanocavities. It is interesting to find that PANI hollow nanospheres with multicavities (Figure 6a, b) also can be fabricated when another

Figure 6. (a, b) TEM images of PANI hollow nanospheres Synthetic conditions: [citric acid] = 0.020 mol L−1; [aniline] = 0.120 mol L−1; [H2PtCl6] = 0.020 mol L−1; 8 h. (c, d) TEM images of poly(otoluidine) hollow nanospheres. Synthetic conditions: [citric acid] = 0.020 mol L−1; [o-toluidine] = 0.120 mol L−1; [HAuCl4] = 0.020 mol L−1; 8 h.

oxidant, such as H2PtCl6 instead of HAuCl4, is applied under similar synthetic conditions. Besides the formation of PANI hollow nanospheres, this strategy is also applicable to hollow nanospheres of PANI derivatives. For example, when the typical aniline derivative of o-toluidine instead of aniline is introduced, uniform poly(o-toluidine) hollow nanospheres can be fabricated as given in Figure 6c. However, the nanocavities seem to be connected with each other (Figure 6d), which is in contrast with that of PANI hollow nanospheres where incontinuous nanocavities can be found (Figure 1d). Results confirm the generality of the proposed strategy for conducting polymer hollow nanopsheres with multicavities. 6472

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Figure 7. (a) FTIR and (b) UV−vis spectra of PANI hollow nanospheres. Synthetic conditions: [citric acid] = 0.020 mol L−1; [aniline] = 0.120 mol L−1; [HAuCl4] = 0.020 mol L−1; 8 h.

Figure 8. (a) Effect of dosage of PANI hollow nanospheres on adsorption of Pb(II) ions. Conditions: initial Pb(II) ion concentration = 20 mg L−1, pH = 7.0, 30 °C; 24 h. (b) Effect of pH value on adsorption of Pb(II) ions onto PANI hollow nanospheres. Conditions: initial Pb(II) ions concentration = 20 mg L−1, adsorbent dosage = 2 mg, 30 °C; 24 h.

Figure 9. (a) Effect of initial Pb(II) ions concentration on Pb(II) ions adsorption onto PANI hollow nanospheres. Conditions: adsorbent dosage = 20 mg; pH = 7.0; 30 °C; 24 h. (b) Langmuir and Freundlich plots of the adsorption data in the concentration range from 20 to 300 mg L−1.

pH value. Therefore, with increasing pH, protons are released from the amine/imine groups, leaving more binding sites available for Pb(II) ions adsorption. The effect of the initial Pb(II) ion concentration on the adsorptive ability of Pb(II) ions onto PANI hollow nanospheres is also given in Figure 9a. The adsorption capacity can reach as high as 1589 mg g−1, and the highest adsorptivity of lead ions is up to 93%. For comparison, PANI nanofibers, which are the most intensively investigated one-dimensional nanostructures with high surface areas, are also selected as

variation of Pb(II) ions adsorption onto PANI hollow nanospheres with the pH value of Pb(II) solution at initial Pb(II) ions concentration of 20 mg L−1 and adsorbent dosage of 2.0 mg. It is clearly seen that both adsorptivity and adsorption capacity increase in the overall pH range. Herein, the adsorption behavior has been primarily suggested as competitive adsorption of Pb(II) ions over H+ ions to the nitrogen atom on the surface of the PANI hollow nanospheres.51 As PANI reserves acid/base doping/dedoping properties, the level of doping will be increased with decreasing 6473

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nanostructures of other conducting polymers. The adsorption capacity of PANI hollow nanospheres toward lead ions can reach as high as 1589 mg g−1, and the highest adsorptivity of lead ions is up to 93%, which evidences their potential applications as adsorbents for heavy metal ions.

adsorbent for removal of lead ions. PANI nanofibers assynthesized by the rapid-mixing route41 with diameter of 50 nm are chosen. It is revealed that the adsorptivity of PANI nanofibers toward lead ions is 68%, where that of PANI hollow nanospheres with multicavities is as high as 93% under similar adsorption conditions. The Pb(II) ion adsorptivity almost linearly rises with an increase in Pb(II) ion concentration, while the Pb(II) ion adsorption capacity decreases significantly with increasing Pb(II) ion concentration. Therefore, both the Pb(II) ion adsorption capacity and adsorptivity reach a high level at the optimal initial Pb(II) ion concentration of around 80 mg L−1. Two mathematical models proposed by Langmuir and Freundlich have been used to describe and analyze the adsorption isotherm and equilibrium as listed in eqs 3 and 4, where Qm is the adsorption capacity at saturation (mg g−1), Ka is the equilibrium constant indicating adsorption capacity, and n is the adsorption equilibrium constant. Ce C 1 = e + Qe Qm K aQ m log Q e =

■

S Supporting Information *

Figures showing XRD patterns of PANI nanospheres; TEM images of PANI nanospheres. This material is available free of charge via the Internet at http://pubs.acs.org.

■

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Funding is acknowledged from the National Natural Scientific Foundation of China (No. 20903079 and 21073156) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

(3)

(Freundlich)

(4)

These constants are evaluated from the intercept and the slope, respectively, of the linear plots of Ce/Qe versus Ce, and log Qe versus log Ce, based on experimental data through a regression analysis. The adsorption data in the concentration range from 20 to 300 mg L−1 is selected to be modeled. The modeled quantitative relationships between Pb(II) ion concentration and the adsorption process are shown in Figure 9b, and the calculated correction coefficients and standard derivations are listed in Table 1. It can be seen that the adsorption isotherm

■

equation

correlation coefficient

standard deviation

Langmuir Freundlich

Ce/Qe = 5.73 × 10−4Ce + 0.022 log Qe = 0.38log Ce+2.28

0.947 0.986

0.013 0.050

REFERENCES

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Table 1. Isotherm Model Equations for Pb(II) Ion Adsorption onto PANI Hollow Nanospheres mathematical mode

AUTHOR INFORMATION

Corresponding Author

(Langmuir)

1 log Ce + log KF n

ASSOCIATED CONTENT

behaviors of Pb(II) ions onto PANI hollow nanospheres are better described by the Freundlich isotherm than Langmuir isotherm because the Freundlich models yield a higher correction coefficient.

4. CONCLUSIONS In conclusion, PANI hollow nanospheres with controllable incontinuous nanocavities ranging in size from 10 to 50 nm as a novel hollow nanostructure have been successfully fabricated by chemical polymerization of aniline with HAuCl4 instead of commonly used ammonium peroxydisulfate as oxidant and citric acid as doping acid. A possible formation mechanism was then proposed according to experimental observations; that is, micelles composed of citric acid−aniline salt may act as template for gold/PANI core/shell nanospheres, followed by the aggregation and fusion processes of gold nuclei embedded in PANI that should be attributed to the formation of multicavities in PANI matrix. Moreover, the proposed strategy is also applicable to hollow nanospheres with multicavities of poly(o-toluidine), indicating its potential for novel hollow 6474

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dx.doi.org/10.1021/la300619d | Langmuir 2012, 28, 6468−6475