Article pubs.acs.org/Langmuir
Colloid Thermodynamic Effect as the Universal Driving Force for Fabricating Various Functional Composite Particles Yunxing Li,†,‡ Zhaoqun Wang,*,† Chunjian Wang,† Yunfei Pan,† Hao Gu,† and Gi Xue*,†,‡ †
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, and ‡State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China S Supporting Information *
ABSTRACT: The design and fabrication of functional nanocomposites is an active area of research because composite particles have significantly improved physical and chemical properties over those of their single-component counterparts. Traditionally, chemical pretreatments of the components were used to enhance their physicochemical or chemical interactions. Here, we propose a novel approach to taking advantage of the beauty of thermodynamics. A series of functional materials, including graphene nanosheets, carbon nanotubes, noble metals, magnetic materials, conducting polymers, attapulgite, and etc. were incorporated with polystyrene particles by a thermodynamic driving force. This unique approach is facile and versatile and shows the considerable significance of developments in both scientific methodology and particle engineering.
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INTRODUCTION In the area of particle engineering, there has been tremendous interest in the design and controlled fabrication of functional core/shell colloidal particles that consist of either inorganic or organic cores coated with shells of different chemical compositions.1 Such colloidal composite particles are well known for their advantages of incorporating the physical and chemical properties of their respective component counterparts. Moreover, the size, structure, and composition of such functional core/shell colloidal particles can be varied in a controllable way to adjust their thermal, electrical, optical, mechanical, catalytic, and magnetic properties over a broad range. Therefore, these hybrid materials have a variety of applications in immunoassays, coatings, catalysis, photonic crystals, medical imaging, thermal ablative cancer therapy, drug delivery, surface-enhanced spectroscopy, and so forth.2−14 A great deal of work has been devoted to the strategies of flexible deposition of various shell components on the surfaces of spherical colloidal substrates. The numerous strategies for constructing composite particles can be classified into two main categories: controlled adsorption and confined reaction. In general, they both rely on engineering the surface of substrate particles or shell components in order to enhance the physicochemical or chemical interaction between them. To name only a few, Armes et al. prepared a series of vinyl polymer−silica nanocomposite particles by in situ polymerization and heteroflocculation, relying on acid−base interaction © 2012 American Chemical Society
or electrostatic attraction between the surfaces of substrate particles and silica nanoparticles (NPs).15−19 Hyeon and coworkers reported a general procedure to assembly 2-bromo-2methylpropionic acid-stabilized Fe3O4 NPs first on aminofunctionalized silica spheres through covalent bonding, and then the preformed magnetic composite particles can be further modified with other functional NPs to obtain multifunctional assemblies.6,20 Ballauff’s group used spherical polyelectrolyte brushes consisting of a PS core and cationic long chains to adsorb a series of noble metal salt precursors by electrostatic interaction for the sake of generating stable, homogeneous single metal or bimetallic NPs on the surface of PS.5,10,14,21 Moreover, Caruso et al. have successfully used the layer-bylayer (LbL) technique to coat various metallic and silica NPs on either organic or inorganic cores on the basis of electrostatic interaction, and this strategy has also been adopted in various forms by others; for example, carbon nanotubes (CNTs) have been assembled on various types of colloidal particles.22−28 As a common feature of all of these strategies, surface functionalizations or modifications, according to the respective systems, were used to enhance the interactions between the substrate particles and shell components. Nevertheless, the targeted design, aimed at either the substrate particles or the Received: May 21, 2012 Revised: August 9, 2012 Published: August 13, 2012 12704
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silica, and ferromagnetic guest nanoparticles were synthesized according to our previous work.29,31 Au, Ag, Pt, ZnS, Fe3O4, and graphene guest units were prepared by the methods of Frens, Meisel, Harriman, Velikov, Stroeve, and Li, respectively.32−37 Raw multiwalled carbon nanotubes were pretreated according to methods reported by Rosca.38 TiO2 and attapulgite were used as received. The assembly process of the preformed host particles and guest units was summarized as follows. The appropriate amount of aqueous PS host latex (0.2 g, 10 wt %) was added to a certain volume of the respective preformed functional guest colloidal aqueous dispersion mentioned above. These mixtures were homogenized via ultrasonic treatment for 1 min and then were allowed to stir at 300 rpm using a magnetic stirrer for 1 h at room temperature. If the guest was 0D (zero-dimensional), then the weight ratio of the guest unit to the host particles was controlled by the following equation
functional shell components, is time-consuming and is of limited application. The well-known LbL method was regarded as a strategy that can be applied in many systems. However, it is obviously laborious and cumbersome owing to the repeated coating/separation of polyelectrolytes. Moreover, the feasible materials and ultimate functionalities of composite particles are more or less restricted because of the requirements for polyelectrolytes and electrostatic properties of the shell components. Therefore, a facile, controllable, widely applicable method for incorporating various shell materials with colloidal substrates has not yet been achieved, and it remains a scientific challenge. Recently, we proposed a distinctly novel approach to fabricating functional composite particles.29,30 The thermodynamic effect, a very ordinary but probably disregarded one, is purposively adopted to drive and govern the coating of polymer particulate substrates with functional components, which are called host particles and guest units, respectively. Compared to the aforementioned methods, our method does not rely on electrostatic or chemical host−guest interactions, and thus surface functionalization and modification of the host particles or guest units are not required. A model system composed of PS host particles and silica NP guest units has been investigated, and detailed discussions have been carried out on the basis of the thermodynamic viewpoint. The results confirmed that the thermodynamic effect was rather powerful in driving the assembly of composite particles and further in controlling their morphology. In the present work, we explore the approach in depth, focusing on its broad applicability, to prepare various functional composite particles using the most common PS particles as host particles. As expected from the proposed mechanism, a series of successes have been achieved by adopting a large variety of functional guest units involving graphene nanosheets, carbon nanotubes, noble metals, magnetic materials, conducting polymers, attapulgite, and etc.
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Wg Wh
=
NsatMg Mh
=
4(R + r )2 rρg R3ρh
where Mh, Mg, R, r, rh, and rg are, respectively, the weight, radius, and density of individual hosts (subscript h) and guests (subscript g). If the guests were 1D and 2D, then the weight ratio of the guest unit to host particles was kept at 0.5. Route 2. The procedures are similar for the preparation of PS/Au, PS/Ag, and PS/Pt composite particles, other than their precursors. For example, first an HAuCl4 aqueous solution (100 mL, 0.5 × 10−3 M) was heated to boiling under magnetic stirring using an oil bath. Then, a mixture of PS latex (0.2 g, 10 wt %) and trisodium citrate solution (0.84 mL, 8 × 10−2 M) was added. The reaction was allowed to run 15 min after the solution reached a wine-red color, indicating that the reaction was completed. The procedures for the preparation of conducting composite particles are also similar, except for monomer and oxidant. For example, aniline (0.2 g) and hydrochloric acid (2 mL, 1 mol/L) were dissolved in water, followed by the addition of PS latex (12 g, 10 wt %). The monomer/latex reaction mixture was cooled to 0 °C prior to the addition of the APS solution (2 mL, 1 mol/L) via a syringe, and the polymerization temperature was maintained at 0 °C for the first 5 h. Then, the polymerization was carried out for another 18 h at room temperature. Characterization. The morphology of PS/NMNPs, PS/ MWCNTs, PS/ATNRs, PS/GNSs, and magnetic and conducting core/shell composite particles was observed by scanning electron microscope (SEM) using an S-4800 instrument (Hitachi Co., Japan), and samples were not sputter coated with gold prior to examination. Other samples were observed by transmission electron microscope (TEM) using a JEM-100 CX (JEOL Co., Japan). Fourier transform infrared (FTIR) analysis was performed with a Bruker VECTORTM 22 FT-IR spectrometer (Bruker Co., Germany). Raman spectra were obtained by use of the MultiRam spectrometer (Bruker Co., Germany).
EXPERIMENTAL SECTION
Materials. Styrene (AR), aniline (AR), and pyrrole (AR) were obtained from Shanghai Chemical Reagent Co. and purified by distillation under reduced pressure. Graphite with an average size of 30 μm and a purity of >95% was obtained from Shanghai Chemical Reagent Co. Multiwalled carbon nanotubes (MWNTs) with a diameter of about 30 nm were provided by Shenzhen Nanotech Port Co. Attapulgite (ATNR) was supplied by Jiangsu Autobang International Co. Azodiisobutyronitrile (AIBN) of chemical reagent grade (Shanghai Chemical Reagent Co.) was purified by recrystallization in 95% ethanol. Poly(vinylpyrrolidone) with an average molar mass of 58 kg/mol (PVP K-30) was purchased from Acros Organics. HAuCl4, AgNO3, H2PtCl6, hydrogen peroxide, trisodium citrate, tetraethoxysilane, sodium dodecyl sulfate, and potassium iodine were obtained from the Shanghai Chemical Reagent Company and used as received. Ammonium persulfate (APS) and potassium persulfate (KPS) of analytical reagent grade (Shanghai Chemical Reagent Co.) were purified by recrystallization in water. FeCl3·6H2O, Fe2SO4·7H2O, CoCl2·6H2O, Fe(NO3)3·9H2O, MnCl2·4H2O, ZnCl2, NiCl2·6H2O, NaOH, Zn(NO3)2·6H2O, NaNO3, KMnO4, nitric acid, hydrazine hydrate, concentrated sulfuric acid, hydrochloric acid, ammonia (25 wt % NH3 in water), thioacetamide, acetic acid, absolute ethanol, 95% ethanol, and 2-propanol were purchased from Nanjing Chemical Reagent Co. and used as received. TiO2 powder (Aeroxide P25) was purchased from Degussa Co. Deionized water (18.2 MΩ·cm) was prepared in a Sartorius Arium 611 system and used throughout the experiment (unless otherwise specified). Deoxygenated water was obtained by bubbling deionized water with N2 gas for 1 h prior to use. Preparation of Various Functional Composite Particles. Route 1. PS host particles and small PS guest particles (PS′), AgI,
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RESULTS AND DISCUSSION It is easy to understand the colloidal steric stabilization theory on the basis of thermodynamics. The lowest Gibbs free energy is reached when unstable or metastable colloidal particles are covered by stabilizers, whether soluble macromolecules or nonsoluble solid powders. For our proposed approach, this thermodynamic effect is utilized as a unique driving force for coating polymer particles with different functional components, ultimately to obtain a variety of functional composite particles. In the three-phase colloidal system including the liquid medium, host particles, and guest units, the thermodynamically preferred state will depend on the respective interfacial area and interfacial tension. Therefore, great attention should be paid to the thermodynamic correlation among all the three phases. Moreover, the assembly of host particles and guest units can be 12705
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Scheme 1. Schematic Representation of the Synthesis of Various Functional Composite Particles
Figure 1. SEM images of (a, d) PS/AuNP, (b, e) PS/AgNP, and (c, f) PS/PtNP composite particles fabricated by blending the preformed NMNPs and PS latex.
same time, fully to illuminate the versatile advantages of the proposed approach. The whole preparation process of the proposed approach is represented in Scheme 1. The PS host particles employed in our system were prepared by dispersion polymerization using azodiisobutyronitrile (AIBN) as the initiator. The host particles thus bear no chemically bound ionic groups, which were solely stabilized by the amphiphilic and nonionic PVP. Through sufficient purification, the adsorption of PVP to PS particles is minimized, although the PVP was not completely removed. Thus, the PS host particles became metastable because most of the PVP was eliminated and the interfacial tension against the aqueous medium was enhanced. Then, two routes were exploited to achieve the incorporation of the nanosized hydrophilic guest units onto the metastable PS particles. The first route was simply to blend the preformed guest units and metastable PS host particles. The second route was to prepare guest units in situ by different chemical reactions such as redox and polymerization in the presence of the PS host particles. In addition to obtaining the anticipated composite particles, the rich diversity of their morphologies is also of considerable interest, and their guest parts can be generated in the form of 0D nanoparticles, 1D nanorods or nanotubes, and 2D nanooverlayers. As shown in Figure 1, a series of PS/noble metal NP composite particles are fabricated through route 1, including PS/AuNPs, PS/AgNPs, and PS/PtNPs. All of them had a mass
theoretically predicted under the guidance of detailed thermodynamic analyses. The thermodynamic effect of fabricating the composite particles is illuminated using the system of 0D guest units and host particles as an example shown in Scheme S1. On the basis of the assumption of the total interfacial free energy of a separated state in which one host particle and a certain number (Nt) of guest NPs exist individually and the heterocoagulated state in which all of the guest NPs are readily immobilized onto the host particle are G0 and Gt, respectively, the total free energy change, ΔG, from the separated state to the heterocoagulated state is expressed as ΔG = Gt − G0 = −NtAgh (γhm + γgm − γgh)
where Agh, γhm, γgm, and γgh denote the interfacial area between one guest NP and one host particle and the interfacial tension of the host particle and the medium, the guest NPs and the medium, the guest NPs and the host particle, respectively. Provided ΔG is negative, the process is thermodynamically favorable. Thus, any changes in the thermodynamic factors related to the medium, host, and guest solid phases will be capable of influencing the incorporation of the solid components and governing the morphology of the resultant composite particles. Obviously, the guest unit is a more significant and interesting thermodynamic factor in tuning the system state. Consequently, we can choose various materials as guest units to prepare a series of functional composite particles and, at the 12706
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Figure 2. SEM images of (a, d) PS/MWCNT, (b, e) PS/ATNR, and (c, f) PS/GNS composite particles fabricated by blending the nanotube-, nanorod-, and nanosheet-shaped guest units and PS latex.
Figure 3. SEM images of (a, e) PS/Fe3O4, (b, f) PS/CoFe2O4, (c, g) PS/Mn0.6Zn0.4Fe2O4, and (d, h) PS/Ni0.7Zn0.3Fe2O4 composite particles fabricated by blending the preformed magnetic NPs and PS latex.
of individual noble metal NPs (NMNPs) uniformly distributed on the surfaces of the PS particles. Moreover, the host/guest interparticle adhesion was sufficiently strong because there was no change in guest coverage during the purification process, including repeated ultrasonic dispersion and centrifugation. AuNPs and PtNPs supported on the PS particles possess an ultrafine size and a narrow size distribution, whereas the size of the AgNPs is larger and some nanosized Ag rods are found on the substrate surface. In essence, it is worth noting that their morphologies could be governed by flexibly selecting the size, size distribution, and shape of the used NMNPs, which mainly depend on the existing and developing techniques of preparing the NMNPs. Figure 2 illustrates the composite particles having 1D and 2D guest units fabricated by blending multiwalled carbon nanotubes (MWCNTs), attapulgite nanorods (ATNRs), and
graphene nanosheets (GNSs) with the PS latex, respectively. According to the SEM images in Figure 2a,d, the chemically pretreated MWCNTs lie along the spherical surfaces of PS particles and behave much like water-soluble, long, flexible polymer chains in stabilizing the unstable PS particles. In Figure 2b,e, the guest unit is ATNR, a kind of natural, fibrous silicate clay. It plays the role of stabilizer for the PS microspheres because there are many hydrophilic hydroxy groups on its surface. Unlike the long, flexible MWCNTs, the short, rigid ATNRs densely and desultorily accumulate on the surfaces of PS particles, making the composite particles appear to be canyballs. Figure 2c,f displays SEM images of the PS/GNS composite particles, providing a rather interesting example for illustrating typical 2D guest units. Though the GNSs are a kind of ultrathin, flexible nanosheet that keeps its spatial conformation under control with difficulty, they almost 12707
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Figure 4. SEM images of (a, d) PS/AuNP, (b, e) PS/AgNP, and (c, f) PS/PtNP composite particles prepared by generating the NMNP's in the presence of the PS latex.
perfectly wrapped around the PS particles by simply blending the host particles and the GNSs. Some wrinkles and faults, irregularly distributed on the spherical surface of PS particles, are observed in their SEM images. Nevertheless, these lessthan-perfect domains provide visual evidence for the coating of PS particles with ultrathin GNSs. Further evidence of the formation of composite structure is provided by the Raman spectra shown in Figure S2.39 It is reasonable to consider that the facile fabrication of perfect, uniform PS/GNS composite particles is a remarkable and intriguing success because they should exploit the greatest possible advantage of the GNSs. Figure 3 shows SEM images of a group of magnetic composite particles containing superparamagnetic PS/Fe3O4 composite particles and ferromagnetic PS/CoFe2O4, PS/ Mn0.6Zn0.4Fe2O4, and PS/Ni0.7Zn0.3Fe2O4 composite particles fabricated via route 1. Evidently, magnetic NPs attached densely to the surfaces of PS particles, some irregular agglomerations, and especially a multilayer stack of Fe3O4 NPs were observed on these composite particles. This should rather be related to their colloidal instability and unavoidable homocoagulation tendency, apart from the thermodynamically preferred attachment of magnetic NPs to the surfaces of PS particles. The above-mentioned composite particles were all prepared by obtaining the guest units beforehand and then making them attach onto the surface of PS host particles. These can exhibit clearly and straightforwardly the assembly process of the two dissimilar preformed parts. In fact, there potentially exists the thermodynamic driving effect in such a case of synthesizing the corresponding guest units in the presence of preformed PS host particles, namely, the in situ generation of the guest units as route 2 shown in Scheme 1. By employing the same series of noble metals (NMs) as guest candidates, the PS particles were added to a boiling solution of NM precursors, followed by the addition of reductant. As displayed in Figure 4, the AuNPs, AgNPs, and PtNPs have been formed with the respective packing density on the surfaces of PS particles. In comparison to Figure 1, it is apparent that the two groups of PS/NMNP composite particles prepared by routes 1 and 2 have similar morphology. In addition, more or less agglomerations on these composite particles are probably attributed to the relatively high reaction temperature. Drawing a comparison between the
two routes, this one has an obvious advantage in further simplifying the preparation process, whereas the size, morphology, and function of guest units can be flexibly and extensively selected in route 1. In nature, route 2 could be applied to other reaction systems for the in situ generation of guest units, for instance, the undermentioned chemical oxidative polymerization of conjugated monomers. As shown in Figure 5, the smooth, uniform
Figure 5. SEM images of (a, c) PS/PANi and (b, d) PS/PPy conducting composite particles fabricated by the chemical oxidative polymerization of aniline or pyrrole in the presence of the PS latex using a 1:6 weight ratio of monomer/PS.
polyaniline (PANi) and polypyrrole (PPy) overlayers were formed on the PS host particles. Further evidence of the core/ shell structure can be seen in Figure S3. Significantly, the particulate substrate used in our system was the very common PS particles going without any surface modifications and functionalizations, in sharp contrast to previous work that prevalently utilized specific functional groups on the surfaces of the substrate particles to induce the coating. Moreover, it is worth noting that the conducting composite particles, especially 12708
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Langmuir PS/PANi, possessed a uniform, smooth surface morphology even at a relatively high monomer concentration. To both highlight and further demonstrate the broad applicability of the proposed approach, a broad range of materials involving oxides, iodides, sulfides, and polymer were adopted as guest candidates, and as expected, they were successfully attached to the PS host particles. Figure S4 illustrates the TEM images of PS/SiO2, PS/PS′, PS/AgI, PS/ TiO2, and PS/ZnS composite particles. According to the micrographs of all of the composite particles, the large PS particles were encircled with numerous small particles. The PS/ PS′ composite particles (Figure S4b) obtained from the regular assembly of the two types of PS particles are of considerable interest for demonstrating the marvelous thermodynamic driving effect. There was a distinction between the large host PS particles and the small guest PS particles (named PS′) only in their polymerization methods. The small PS′ particles were synthesized by emulsifier-free emulsion polymerization and thus had charged surfaces. In other words, the charged PS particles played the role of stabilizer for the uncharged metastable PS particles. The regular assembly of the two types of PS particles should minimize the Gibbs free energy of the colloidal system. Therefore, together with all of the abovementioned results, it can be expected that any hydrophilic particles with an appropriate size could attach to the surfaces of the metastable colloidal particles as the stabilizer and thus drive the assembly between the two parts.
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REFERENCES
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CONCLUSIONS A distinct approach was proposed on the basis of a thermodynamic mechanism, which is an ordinary, easily neglected, but indeed marvelous driving force for fabricating composite particles. A great variety of functional composite particles were fabricated by adopting this thermodynamic driving approach. It has the following advantages: (a) straightforward and facile, eliminating the need for surface functionalizations and modifications of both host and guest materials; (b) versatile, being capable of preparing a broad range of functional composite particles because of a wide range of options on both guest materials and substrates; (c) multiform, having a rich variety of morphology of guest units involving 0D nanoparticles, 1D nanotubes or nanorods, and 2D nanooverlayers. These plentiful results show considerable significance in the development of the scientific methodology, particle engineering, and synthesis technique of nanocomposite materials. ASSOCIATED CONTENT
S Supporting Information *
Detailed thermodynamic analyses of the assembly process of the composite particles. Raman characterization of PS particles and PS/GNSs composite particles. FTIR characterization of PS particles and conducting composite particles. TEM characterization of composite particles with the guest units involving SiO2, TiO2, AgI, ZnS, and PS′. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (no. 51133002).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest. 12709
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dx.doi.org/10.1021/la302068c | Langmuir 2012, 28, 12704−12710
