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Colloids and Surfaces A: Physicochem. Eng. Aspects 386 (2011) 141–150
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and assembly of catalytically active platinum-doped polymer nanocomposites at the liquid/liquid interface Lan-Jun Chen a , Huihui Ma a , Kuang-Cai Chen a,b , Weiliu Fan a , Hyeong-Rae Cha b , Yong-Ill Lee b , Dong-Jin Qian c , Jingcheng Hao a , Hong-Guo Liu a,∗ a
Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China Department of Chemistry, Changwon National University, Changwon 641-773, South Korea c Department of Chemistry, Fudan University, Shanghai 200433, China b
a r t i c l e
i n f o
Article history: Received 29 March 2011 Received in revised form 13 June 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Polymer Platinum nanoparticle Liquid/liquid interface Microcapsule Solid foam Catalysis
a b s t r a c t Platinum nanoparticle-doped polymer foam-like thin films were prepared via a synthesis and assembly process at the liquid/liquid interface of a chloroform solution of poly(2-vinylpyridine) and an aqueous solution of chloroplatinic acid hydrate and a subsequent UV-light irradiation process. Transmission electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy investigations indicated that the foam-like thin films formed at the interface were composed of polymer molecules, and Pt(II) and Pt(IV) ions. Platinum nanoparticles with average diameter of 2.70 ± 0.35 nm appeared after UV-light irradiation, which were embedded in and adsorbed on the walls of the foams. The formation of the composite nanostructures at the interface was attributed to selfassembly of the polymer molecules at the liquid/liquid interface, reduction of PtCl6 2− , and interaction between PtCl6 2− /intermediate PtCl4 2− ions and the protonated pyridine groups. The catalytic activity of the composite film for the reduction of methylene blue by potassium borohydride in aqueous solutions was evaluated. The apparent rate constant decreased gradually with increasing the number of runs due to the partially leaching of Pt nanoparticles and became stable after the fifth cycle, indicating that the composite film can be used as an effective, stable and reusable catalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Platinum nanoparticles exhibited excellent catalytic [1–12] and electrocatalytic [13–27] properties, and have important applications in chemical industry and the area of fuel cells. In addition, Pt-semiconductor hetero-nanostructures, such as Pt–CdSe [28], Pt–CdS [29] and Pt–TiO2 [30,31] showed enhanced photocatalytic performance and have potential applications in environmental treatment. So synthesis and catalytic studies of Pt nanoparticles have attracted increasing interests. It has been established that the catalytic activity of metal nanoparticles for heterogeneous catalytic systems enhances with decreasing the size as the result of the increase of the surface-to-volume ratio, the coverage of active surface atoms, and chemical potentials [32,33]. In order to control the size, Pt nanoparticles are usually capped with polymers, such as polyvinylpyrrolidone (PVP) [3–6,15], polyethylenimide [13], polyelectrolytes [14] and polyethylenimine [16], or alkylammonium ions, such as tetradecyltrimethylammonium bromide [2] and cetyltrimethylammonium bromide [17],
∗ Corresponding author. Tel.: +86 531 88362805; fax: +86 531 88564750. E-mail address: [email protected] (H.-G. Liu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.07.017
or oleic acid and oleylamine [18]. It has been revealed that PVP molecules adhere to Pt nanoparticles through a charge-transfer interaction between the pyrrolidone rings and the surface Pt atoms [34], and the interactions between them are very strong. So a significant number of active sites are blocked [2]. The interaction between alkylammonium ions and Pt surface are considerably weaker than that between the carbonyl group and Pt surface. It was expected that the catalytically active sites could be preserved by using alkylammonium ions as capping agents [2,17]. On the other hand, in order to avoid aggregation, Pt nanoparticles are often immobilized on matrix. The supporting materials include oxides, such as silica [8] and ceria [9], platinum and tungsten gauzes [27], carbons [10,13], carbon nanotubes [11,23,25], carbon nanospheres [26], polymer microgels [12], and polymer nanofibers [22]. In this paper, we described a new and facile approach to prepare Pt nanoparticle-doped poly(2-vinylpyridine) (P2VP) foam-like thin films. These composite nanostructures were formed at the liquid/liquid interface through self-assembly of P2VP molecules, the combination of the protonated pyridine groups with PtCl6 2− ions, and simultaneous reduction of PtCl6 2− ions at ambient temperature. Liquid/liquid interface has been utilized widely to synthesis inorganic nanoparticles [35,36] and to assemble thin membranes composed of inorganic nanoparticles [37–39] and monolayers of
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Fig. 1. TEM micrographs of the nanostructures formed at the liquid/liquid interface.
graphene nanosheets [40] or graphite nanoplatelets [41], and even coordination polymers [42,43] recently, however, to the best of our knowledge, study on the assembly of polymer thin films, especially composed thin film consisting of polymer and inorganic species at the liquid/liquid interface is very rare. We found that numerous Pt nanoparticles embedded in and adsorbed on the walls of the foams and capped with the protonated pyridine groups in the composites after UV-light irradiation. It should be noted that the interaction between Pt and P2VP is weaker than that between Pt and PVP due to the different molecular structures. It was also demonstrated that for some catalytic reactions, the catalytic efficiency of the nanoparticles depends mainly on two parameters. One is the available active surface area for adsorption and other is the number of nanoparticles present in per volume [44,45]. The formed composites fulfil the requirements, and exhibited high catalytic activity for the reduction of methylene blue by KBH4 .
characterization. The deposited films were further irradiated by UV-light with the wavelength of 254 nm using a UV-lamp with the power of 6 W for a certain time. The distance between the deposited film and the lamp was 15 cm. 2.3. General characterization The morphology, structure and compositions of the deposited and the UV-light irradiated films were investigated by using highresolution transmission electron microscopy (HRTEM, JEOL–2010) with the accelerating voltage of 200 kV and X-ray photoelectron spectroscopy (XPS, ESCALAB MKII) with Mg K␣ (h = 1253.6 eV) as the exciting source at a pressure of 1.0 × 10−6 Pa and a resolution of 1.00 eV. The optical properties of these films were characterized by using UV–vis spectroscopy (HP 8453E).
2. Materials and methods
2.4. Measurement of platinum content
2.1. Chemicals
Platinum content was analyzed by using microwave digestion inductively coupled plasma-atomic emission spectroscopy (ICPAES, Varian Inc. – Liberty Series II). A certain amount of the sample was dissolved in aqua regia for 15 min predigesting, then digested in a CEM MARS5 microwave accelerated reaction system for 30 min. Then the reaction system was evaporated to dryness by using rotary evaporator, cooled down to room temperature, washed with a small amount of concentrated HCl, and diluted with water. The prepared sample was analyzed by using ICP-AES with monitoring the emission wavelength of Pt at 214.423 nm. The standard curve was obtained by using the platinum standard solution. The content of platinum in the composite sample was calculated.
Poly(2-vinylpyridine) (P2VP. Mw : 5300, Mn : 5000), chloroplatinic acid hydrate (H2 PtCl6 ·xH2 O, 99.9+%) and platinum standard solution (981 g mL−1 (20 ◦ C)) were purchased from Aldrich. KBH4 (≥97.0%) was purchased from Shanghai Zhanyun Chem. Co. Ltd. Methylene blue (MB, ≥98.5%) and chloroform (≥99.0%) were obtained from Tianjin Guangcheng Chem. Co. The chloroform contains 0.3–1.0% of ethanol served as stabilizer. Concentrated hydrochloric acid (35%) and concentrated nitric acid (35%) were supplied by Matsunoen Chem. Ltd. Japan, and extra pure NaCl was obtained from Junsei Chem. Co., Ltd., Korea. The water used is highly purified with the resistivity ≥18.0 M cm.
2.5. Catalytic experiments 2.2. Preparation 10 mL P2VP chloroform solution with the concentration of 0.2028 mg mL−1 was poured in a clean beaker, then 10 mL aqueous solution of H2 PtCl6 with the concentration of 1 × 10−3 mol L−1 was added carefully to form a clear liquid/liquid interface. The beaker was put in a sealed container that was placed in a dark oven. The temperature was controlled to be 25 ◦ C. 24 h later, thin films appeared at the liquid/liquid interface and air/aqueous solution interface, respectively. The films were transferred on carboncoated copper grids, silicon slides and quartz slides for further
0.5 mL MB aqueous solution with the concentration of 2 × 10−4 mol L−1 was poured in a quartz cuvette with the side length of 1 cm; then 1.0 mL aqueous solution of KBH4 with the concentration of 2 × 10−2 mol L−1 was added. The total volume of the reaction solution is 1.5 mL, and the concentrations of MB and KBH4 are 6.67 × 10−5 and 1.33 × 10−2 mol L−1 , respectively. A deposited thin film on a quartz slide after UV-light irradiation was immersed in the reaction system to catalyze the reduction of MB. The reaction was monitored by using UV–vis spectroscopy (HP-8453E) at room temperature (∼15 ◦ C).
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3. Results and discussion 3.1. Morphology, composition and structure Fig. 1 shows the TEM images of the nanostructures appeared at the liquid/liquid interface. A foam-like thin film can be seen from Fig. 1a, which are composed of hollow microcapsules with the diameter of about 300–500 nm. No particle was found in the high magnification image shown in Fig. 1b, indicating that no Pt nanoparticle or cluster formed at the liquid/liquid interface during the assembly process of the nanostructures. However, it was found that the weight content of Pt is 17.87% in the composite nanostructures by using ICP–AES technique with the recovery rate of 98.14%. This indicates that platinum do exist in the nanostructures. The limitation of detection (LOD) and limitation of quantification (LOQ) are 1.083 and 3.609 ppb, respectively. In order to clarify the composition of the nanostructures, the sample was investigated using X-ray photoelectron spectroscopy, as shown in Fig. 2. This spectrum could be decomposed into two pairs of peaks using Gaussian multi-peak fitting due to Pt4f7/2 and Pt4f5/2 spin–orbit coupling. The first pair at 72.6 and 75.9 eV was assigned to 4f7/2 and 4f5/2 of Pt(II), and the second pair at 74.7 and 78.1 eV was assigned to 4f7/2 and 4f5/2 of Pt(IV), respectively, according to the related literatures. For example, the binding energies located at 72.8 and 75.6 eV [13], or 71.9 and 75.1 eV [30] for 4f7/2 and 4f5/2 of Pt(II), respectively. It was also found that the binding energy for 4f7/2 of Pt(II) located at 72.1 [9] or 73 eV [31]. In addition, the binding energies of 4f7/2 and 4f5/2 of Pt(IV) located at 74.4 [9] and 77.5 eV [31], respectively. These values are close to those we obtained. In addition, the relative contents of Pt(II) and Pt(IV) ions were calculated to be 42.8% and 57.2% based on the peak areas of 4f7/2 and 4f5/2 of Pt(II), or 43.9% and 56.1% on the corresponding peak areas of Pt(IV), respectively. This means that platinum species in the composite formed at the liquid/liquid interface are mainly composed of platinum complex ions. In order to reduce the platinum ions in the composite nanostructure further to get Pt cluster-doped polymer nanostructure, the composite film was irradiated by UV-light at room temperature. Fig. 3 shows the TEM and HRTEM images of the thin film. Foam-like structure preserved, as shown in Fig. 3a, which are composed of microcapsules with the diameters of 200–600 nm. From Fig. 3b we can also see smaller, isolated, and opened microcapsules whose walls were interspersed with particles with the diameters of 6–8 nm. Furthermore, from the HRTEM image shown in Fig. 3c we can see that the walls were besprinkled with small particles whose average diameter is 2.70 ± 0.35 nm, as indicated by the size
Fig. 3. TEM (a and b) and HRTEM (c) micrographs of Pt–P2VP composites formed at the liquid/liquid interface and irradiated by UV-light for 1 h, and size distribution histogram of Pt nanoparticles in (c).
Fig. 2. XPS spectra of the composite thin film formed at the liquid/liquid interface.
distribution histogram shown in Fig. 3d. These particles dispersed homogeneously. The enlarged image inserted in Fig. 3c gives lat˚ indicating the tice fringes with the interplanar distance of 2.27 A, formation of Pt nanoparticles. The UV-light irradiated composite thin film was further investigated using XPS, as shown in Fig. 4. The spectrum was decomposed into three pairs of peaks using Gaussian multi-peak fitting. The second pair at 72.5 and 75.7 eV, and the third pair at 74.1 and 78.1 eV were attributed to Pt(II) and Pt(IV), respectively, and these values were in accordance with those obtained from Fig. 2. The first pair at 72.4 and 74.7 eV was assigned to 4f7/2 and 4f5/2 of Pt(0) by comparing with literature values, such as 71.1 and 74.4 eV [10], 71.0 and 74.6 eV [13], 71.1 and 74.5 eV [23], and 71.8 and
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Fig. 4. XPS spectra of the composite thin film formed at the liquid/liquid interface and irradiated with UV-light for 1 h.
74.9 eV [46]. It should be noted that the binding energy of 4f7/2 of Pt(0) is greater than the literature values. This may be related to the size effect of the nanoparticles. It has been demonstrated that the binding energies of electrons in metals, such as in Au(0) [47–49] and Ag(0) [50,51], increase with decreasing the particle size. The HRTEM image (Fig. 3c) exhibits clearly that the formed Pt nanoparticles are very small. The binding energies should therefore increase obviously. While the unreduced platinum complex ions still combined with the protonated pyridine groups, so the position of corresponding XPS peaks did not change. The relative contents of Pt(0), Pt(II) and Pt(IV) were calculated to be about 24%, 38% and 38%, respectively. So we can draw a conclusion that Pt nanoparticles formed after UV-light irradiation due to the reduction of the partial platinum ions, and the unreduced Pt(II) and Pt(IV) ions were still combined with the pyridine groups. It should be pointed out that the experimental conditions should be controlled. When increased the irradiation time or decreased the distance between the lamp and the sample, larger particles appeared. In addition, the UV-light irradiation led to cross-linking of the polymer molecules [52,53], which enhanced the stability of the composite films. UV–vis spectra shown in Fig. 5 also indicate the formation of Pt clusters. A peak at 266 nm appeared in the curve of pure P2VP film, which was attributed to the absorption of pyridine groups. In
Fig. 6. TEM (a) and HRTEM (b) images of Pt–P2VP composites appeared at the air/water interface and irradiated with UV-light for 1 h, and size distribution histogram of Pt nanoparticles in (b).
Fig. 5. UV–vis spectra of the composite film without (black) and with UV-light irradiation for 20 (red), 40 (blue) and 60 min (green) and a pure P2VP (pink) thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the curve of the composite film without UV-light irradiation, two intense peaks appeared at 210 and 272 nm, respectively. The first peak should be attributed to PtCl4 2− ions [16,54,55] and the second one should be the superposition of the absorption of PtCl6 2− ions [14,17,56,57] and the pyridine groups. The intensity of these two peaks decreases gradually upon UV-light irradiation, indicating the
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Scheme 1. Schematic illustration of the formation process of the foamlike thin film at the liquid/liquid interface. (a) The formation of the liquid/liquid interface; (b) adsorption of polymer molecules, combination of P2VP with the platinum complex ions and possible reduction of PtCl6 2− ions to PtCl4 2− ions; (c) formation of the composite thin layers; (d and e) self-assembly of the thin layer to a microcapsule; and (f) formation of a foamlike thin film.
transformation of platinum ions to atoms. In addition, this curve exhibited a significant increase of the baseline absorbance around 400 nm, indicating the formation of Pt(0) species [14,16,56]. This means that PtCl6 2− , PtCl4 2− and Pt(0) coexisted in the composite. These results are consistent with that of XPS. 3.2. Formation mechanism In order to illuminate the formation mechanism, two problems should be clarified. How do the P2VP molecules self-assemble into foam structure at the liquid/liquid interface? And how do PtCl6 2− ions be reduced to PtCl4 2− ions? As we know, P2VP can dissolve into organic solvents, such as chloroform. It can also dissolve into water. We have investigated the interfacial behavior of P2VP at the liquid/liquid interface formed by chloroform solution of P2VP and pure water with different pH values. We found that no film formed at the liquid/liquid interface, while some structures appeared at the air/water interface, and the structure depended on the pH values of water. The formation of the structures at the air/water interface should be resulted from the phase transfer of P2VP molecules from the organic phase to water phase, and the adsorption and self-organization of the P2VP molecules at the air/water interface. However, when the pure water was replaced by aqueous solution
of H2 PtCl6 , thin films were formed at the liquid/liquid interface. It is clearly that the presence of platinum ions is a key factor for the formation of the thin films. The formation process of the thin films may be described as follows. As soon as the liquid/liquid interface formed, P2VP molecules adsorbed at the interface to reduce the interfacial free energy. The nitrogen atoms of pyridine groups protonated at the interface because the aqueous solution of chloroplatinic acid supplied plenty of H+ ions. The protonated pyridyls would extend the polymer chains due to the electrostatic repulsion between them and combine PtCl6 2− ions due to the electrostatic attraction between pyridinium groups and the anions. The combination of PtCl6 2− ions with pyridyl groups attracted more and more polymer molecules to the interface rapidly. It is reasonable to suppose that the side of the polymer chain bearing the ion pairs faced the water phase, and the other side composed of methylene groups faced the organic phase. More and more polymer molecules migrated to and adsorbed at the interface. These polymer chains intertwined and intersected with each other to form a thin layer, and these layers overlapped to form a relatively thick film in which numerous PtCl6 2− ions were embedded. The interaction between the film and the water phase is greater than that between the film and the oil phase. So the film inclined to enter the water phase. However, from the view point of surface free energy, it is unfavor-
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Fig. 7. Time-dependent absorption spectra for the catalytic reduction of MB by KBH4 in aqueous solution in the presence of the UV-light irradiated thin composite film for the first (a), second (c), third (e), fourth (g) and fifth cycles (i), and the corresponding linear relationship between ln(At /A0 ) and reduction time (b, d, f, h and j). The weight of the composite is about 0.1 mg.
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Fig. 8. Relation between the apparent rate constants and the reaction cycles.
able for the film entering the water phase, because the interface free energy between the hydrophobic face and water is high. So the film self-assembled into a capsule in which the organic phase was incorporated. We found a thin film also appeared at the air/water interface. Fig. 6 shows TEM and HRTEM images of the deposited thin film after UV-light irradiation. It can be seen a foam-like nanostructure composed of capsules whose walls were embedded with Pt nanoparticles. This means that microcapsules formed at the liquid/liquid interface can enter the aqueous phase due to the stronger hydrophilicity of the outer surface of the capsule and can be captured by the air/water interface. This further confirms the formation mechanism. In the initial stage, some isolated capsules formed and entered into water phase due to the stronger interaction between the exterior surface of the capsules and water. These capsules adsorbed finally at the air/water interface and contacted each other to form a foam structure. More and more capsules formed with time at the liquid/liquid interface, which congregated to form a stable foam structure that prevented the capsule from escaping from the interface. At last, a thin foam film composed of numerous capsules was formed. The average diameter of the Pt particles in Fig. 6b was found to be 2.75 ± 0.44 nm, as indicated by the histogram (Fig. 6c). This average diameter is very close to that of the Pt nanoparticles in Fig. 3c. The formation process was illustrated in Scheme 1. In order to avoid the decomposition of chloroform under light irradiation, small amount of ethanol with the mass fraction of 0.3–1.0% is usually added in the commercial available chloroform as stabilizer. We measured the content of ethanol in the chloroform used by using gas chromatography and found the content is 0.2% (v/v). It is the ethanol that acts as reductant in the reaction system to reduce PtCl6 2− ions to PtCl4 2− ions [14]. When the liquid/liquid interface formed, ethanol molecules moved to the interface and entered to water phase due to their strong polarity. When they came into contact with PtCl6 2− ions adsorbed by P2VP at the interface or in the aqueous phase, redox reaction took place, resulting in the formation of PtCl4 2− ions. These ions combined with the protonated pyridyls and subsequently embedded into the foam-like nanostructures. Why no Pt(0) was in the composite formed at the liquid/liquid interface? This should be related to the weak reduction ability and the low percentage of ethanol in the reaction system. It was revealed that the reduction of PtCl6 2− to Pt(0) via Pt2+ [58]. The further reduction of Pt2+ to Pt(0) is a slow process compared with the reduction of Pt4+ to Pt2+ [16,58], and Pt(0) metal formation does not occur until a ∼90% yield of PtCl4 2− has accumulated [59]. So the ethanol in the system just reduces PtCl6 2− to PtCl4 2− . It is difficult to reduce PtCl4 2− to Pt(0) further. We tried to increase the fractional
conversion of Pt(0) by adding a certain amount of ethanol in the organic phase. We found larger particles formed at the interface. This confirmed that ethanol acted as reductant in this process.
3.3. Catalytic performance The catalytic activity of the composite was evaluated by using the reduction of MB by KBH4 as a model reaction. MB is a watersoluble cationic dye that has a dye skeleton of a thiazine group. The reduction of MB to colorless leucomethylene blue (LMB) has been used to evaluate the catalytic activities of various nanoparticulate catalysts, including Ag [60,61], Au [62,63], Pd [64,65], Pt [8], and metal oxides [66]. It is convenient to monitor the reaction by using UV–vis spectroscopy. Fig. 7 shows the absorption spectra for successive decolorization of MB. Two peaks appeared at 665 and 610 nm in the curve of mixed aqueous solution of MB and KBH4 . The shoulder peak at 610 nm resulted from the exciton coupling of the transition moments due to the formation of aggregates [67]. After immersion of the UV-light irradiated composite film deposited on a quartz slide, the intensity of these peaks decreased quickly with time, indicating the production of LMB [66]. The blue color of MB faded away rapidly, and a colorless solution resulted after 40 s for the first cycle, as shown in Fig. 7a. It was tested that the blue color of MB faded away very slowly in the mixed aqueous solution with NaBH4 in the absence of catalysts [60,62,63]. The blue color was bleached completely within just 40 s in the presence of the composite film for the first cycle, indicating high catalytic activity of the nanocomposite. This high catalytic activity should arise from the small size and high density of platinum particles in the composite film. As can be seen from the TEM and HRTEM images, this foam-like thin film was composed of microcapsules doped with a large numbers of Pt nanoparticles. In addition, the thin porous walls of the capsules allowed diffusion of the reactants and products freely. Considering that the concentration of KBH4 is larger greatly than that of MB, the reduction rate is assumed to be independent of the KBH4 concentration and only dependent of the concentration of MB. Since the absorbance of MB is proportional to its concentration, the absorbance of MB at t = 0 (A0 ) and the time t (At ) should corresponds to the concentration of t = 0 (C0 ) and the time t (Ct ), respectively. So the ratio Ct /C0 equals At /A0 . Fig. 7b gives the relation between ln(At /A0 ) and the reaction time. A0 and At were taken from the peak at 665 nm. The linear relation between them indicated that the catalytic reduction of MB follows pseudo first-order rate kinetics with respect to MB concentration. The rate constant
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Fig. 9. TEM micrographs of the composite film after the eighth cycle and size distribution histogram of Pt nanoparticles in (b) and (d).
was calculated from the slope of the fitting line. The apparent rate constant was found to be 3.7686 min−1 . However, the catalytic activity decreased gradually for the second, third, fourth and fifth cycles, as shown in Fig. 7, and did not decrease further after the fifth cycle. It was also found from Fig. 7d, f and h that two lines were fitted, indicating two apparent rate constants were obtained for the second, third and fourth cycles, and the rate constant corresponding to prior period is less than that corresponding to later period of the cycle. The experimental procedure on catalytic activity was described as follows. When the UV-light irradiated composite film deposited on a quartz slide was inserted into the mixed aqueous solution of MB and KBH4 , timing
began. After a certain period, such as 10 s, 15 s, 20 s, 2 min and 5 min for the first, second, third, fourth and fifth cycles, respectively, the composite film was removed, timing stopped, and the spectrum was recorded rapidly within several seconds using a HP 8453 spectrometer. Then the composite film was reinserted in the solution immediately, and timing restarted. So the successive spectra were recorded. The appearance of two apparent rate constants in the same cycle indicates the possible detachment of the Pt nanoparticles from the composite film. This is also the reason why the catalytic activity decreased gradually. Another possible reason why two apparent rate constants appeared may be the further reduction or coalescence of the Pt species at a slower rate during the
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initial stages of these cycles. As indicated by XPS results, there are plenty of platinum complex ions in the composite films. These ions reduced gradually in the catalytic reaction systems because KBH4 is a strong reductant. More and more Pt nanoparticles generated with time, leading to the increase of rate constant in the later stage of the cycle. Fig. 8 shows the plot of the apparent rate constants against number of successive reductions of MB with the composite P2VP–Pt film as catalyst. From Fig. 8a we can see that the apparent rate constant decreased rapidly from the first cycle to the second cycle, and decreased continuously to the third and fourth cycles. It can be clearly seen from the enlarged figure (Fig. 8b) that the rate constant did not change any longer after the fifth cycle, indicating that the composition and structure of the film became stable after this cycle. It can be also seen from Fig. 7j that only one line was obtained, and the correlation coefficient is 0.99963, indicating no Pt nanoparticle detached or generated further from the film, and the film could be used as a reusable catalyst. The average apparent constant from the fifth to the eighth cycle was calculated to be 0.0242 min−1 . The weight of the film was measured to be about 0.1 mg, and the initial content of the platinum in the film was 17.87%. Considering the small amount of Pt used, this film still exhibited effective catalytic activity. The film after the eighth cycle was further investigated using TEM, as shown in Fig. 9. Form Fig. 9a and c we can see that the foam-like structure became a multilayer structure. From the high magnification images shown in Fig. 9b and d we can see that Pt nanoparticles dispersed in the film, but the number of the particles is much less than that appeared in Fig. 3. The size distribution histogram was shown in Fig. 9e. The average diameter of these particles was calculated to be 2.75 ± 0.83 nm. Although the size distribution of the particles became broader than that before use, the average value did not change. This further confirms the detachment of nanoparticles during the catalytic reaction cycles. As revealed in the formation mechanism of the composite film, platinum complex ions were embedded in capsule walls and adsorbed on the outer surface of the capsules. When irradiated, Pt nanoparticles formed which were embedded in the walls and adsorbed on the surface, respectively. The particles adsorbed on the surface would detach from the surface easily during the catalytic reaction process because there was no sufficient capping agent around them. Some particles embedded in the walls would leach from the film, too, because the interaction between the pyridyls and Pt is not stronger enough [2,34]. Although the catalytic activity decreases in the initial cycles, the nanoparticles remained in the film after the fifth cycle would be stable, and acted as an effective catalyst for the reduction of MB. Comparing with other methods for fabricating metal nanoparticle/polymer composite films, such as Ag/PVA film [68], the fabrication method described here is more convenient and simple. The improvement of stability of the catalyst is in progress.
4. Conclusion In summary, we developed a convenient and facile method to prepare Pt nanoparticle-doped polymer foamlike nanostructures by making use of the self-assembly of polymer molecules, reduction of Pt complex ions and combination of the polymer molecules with the complex ions at the liquid/liquid interface and a subsequent UV-light irradiation process. The formed nanostructures are composed of polymer capsules whose walls were embedded and adsorbed numerous small particles that were dispersed homogeneously. Although some Pt nanoparticles detached or leached from the composite film during the catalytic reaction, the composite film still exhibited effective and stable catalytic activity after the fifth
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cycles, and was used as a reusable catalyst. This presents a convenient and facile way to prepare metal nanoparticle-doped polymer composites with effective catalytic activity. Acknowledgments We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 20873078, 21033005 and 50802056), the National Basic Research Program of China (973 Program, No. 2009CB930103) and Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF 20090094066). References [1] R. Narayanan, M.A. El-Sayed, Catalysis with transition metal nanoparticles in colloidal solution: nanoparticle shape dependence and stability, J. Phys. Chem. B 109 (2005) 12663–12676. [2] H. Lee, S.E. Habas, S. Kweskin, D. Butcher, G.A. Somorjai, P. Yang, Morphological control of catalytically active platinum nanocrystals, Angew. Chem. Int. Ed. 45 (2006) 7824–7828. [3] R. Narayanan, M.A. El-Sayed, Shape-dependent catalytic activity of platinum nanoparticles in colloidal solution, Nano Lett. 4 (2004) 1343–1348. [4] R. Narayanan, M.A. El-Sayed, Changing catalytic activity during colloidal platinum nanocatalysis due to shape changes: electron-transfer reaction, J. Am. Chem. Soc. 126 (2004) 7194–7195. [5] M.A. Mahmoud, C.E. Tabor, M.A. El-Sayed, Y. Ding, Z.L. Wang, A new catalytically active colloidal platinum nanocatalyst: the multiarmed nanostar single crystal, J. Am. Chem. Soc. 130 (2008) 4590–4591. [6] R. Narayanan, M.A. El-Sayed, Effect of catalytic activity on the metallic nanoparticle size distribution: electron-transfer reaction between Fe(CN) and thiosulfate ions catalyzed by PVP platinum nanoparticles, J. Phys. Chem. B 107 (2003) 12416–12424. [7] Y. Li, J. Petroski, M.A. El-Sayed, Activation energy of the reaction between hexacyanoferrate(III) and thiosulfate ions catalyzed by platinum nanoparticles, J. Phys. Chem. B 104 (2000) 10956–10959. [8] J.H. Kim, H.J. Woo, C.K. Kim, C.S. Yoon, The catalytic effect of Pt nanoparticles supported on silicon oxide nanowire, Nanotechnology 20 (2009) 235306. [9] L. Pino, V. Recupero, S. Beninati, A.K. Shukla, M.S. Hegde, P. Bera, Catalyticpartial-oxidation of methane on a ceria-supported platinum catalyst for application in fuel cell electric vehicles, Appl. Catal. A 225 (2002) 63–75. [10] T. Zheng, N. Nishiyama, Y. Egashira, K. Ueyama, Ionic surfactant-mediated synthesis of Pt nanoparticles/nanoporous carbons composites, Colloids Surf. A 262 (2005) 52–56. [11] M. Sanles-Sobrido, M.A. Correa-Duarte, S. Carregal-Romero, B. RodriguezGonzalez, R.A. Alvarez-Puebla, P. Herves, L.M. Liz-Marzan, Highly catalytic single-crystal dendritic Pt nanostructures supported on carbon nanotubes, Chem. Mater. 21 (2009) 1531–1535. [12] A. Biffis, L. Minati, Efficient aerobic oxidation of alcohols in water catalysed by microgel-stabilised metal nanoclusters, J. Catal. 236 (2005) 405–409. [13] P.-L. Kuo, W.-F. Chen, H.-Y. Huang, I.-C. Chang, S. Dai, Stabilizing effect of pseudo-dendritic polyethylenimine on platinum nanoparticles supported on carbon, J. Phys. Chem. B 110 (2006) 3071–3077. [14] Z.Q. Tian, S.P. Jiang, Z. Liu, L. Li, Polyelectrolyte-stabilized Pt nanoparticles as new electrocatalysts for low temperature fuel cells, Electrochem. Commun. 9 (2007) 1613–1618. [15] B. Lim, X. Lu, M. Jiang, P.H.C. Camargo, E.C. Cho, E.P. Lee, Y. Xia, Facile synthesis of highly faceted multioctahedral Pt nanocrystals through controlled overgrowth, Nano Lett. 8 (2008) 4043–4047. [16] L. Bai, H. Zhu, J.S. Thrasher, S.C. Street, Synthesis and electrocatalytic activity of photoreduced platinum nanoparticles in a poly(ethylenimine) matrix, ACS Appl. Mater. Interfaces 1 (2009) 2304–2311. [17] H. Ullah, W.-S. Chung, I. Kim, C.-S. Ha, pH-Selective Synthesis of monodisperse nanoparticles and 3D dendritic nanoclusters of CTAB-stabilized platinum for electrocatalytic O2 reduction, Small 2 (2006) 870–873. [18] C. Wang, H. Daimon, Y. Lee, J. Kim, S. Sun, Synthesis of monodisperse Pt nanocubes and their enhanced catalysis for oxygen reduction, J. Am. Chem. Soc. 129 (2007) 6974–6975. [19] N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, Z.L. Wang, Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity, Science 316 (2007) 732–735. [20] N.M. Markovic, H.A. Gasteiger, P.N. Ross Jr., Oxygen reduction on platinum lowindex single-crystal surfaces in sulfuric acid solution: rotating ring-Pt (hkl) disk studies, J. Phys. Chem. 99 (1995) 3411–3415. [21] F.J. Vidal-Iglesias, J. Solla-Gullon, P. Rodriguez, E. Herrero, V. Montiel, J.M. Feliu, A. Aldaz, Shape-dependent electrocatalysis: ammonia oxidation on platinum nanoparticles with preferential (1 0 0) surfaces, Electrochem. Commun. 6 (2004) 1080–1084. [22] S. Guo, S. Dong, E. Wang, Polyaniline/Pt hybrid nanofibers: high-efficiency nanoelectrocatalysts for electrochemical devices, Small 5 (2009) 1869–1876.
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis and assembly of catalytically active platinum-doped polymer nanocomposites at the liquid/liquid interface Lan-Jun Chen a , Huihui Ma a , Kuang-Cai Chen a,b , Weiliu Fan a , Hyeong-Rae Cha b , Yong-Ill Lee b , Dong-Jin Qian c , Jingcheng Hao a , Hong-Guo Liu a,∗ a
Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China Department of Chemistry, Changwon National University, Changwon 641-773, South Korea c Department of Chemistry, Fudan University, Shanghai 200433, China b
a r t i c l e
i n f o
Article history: Received 29 March 2011 Received in revised form 13 June 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Polymer Platinum nanoparticle Liquid/liquid interface Microcapsule Solid foam Catalysis
a b s t r a c t Platinum nanoparticle-doped polymer foam-like thin films were prepared via a synthesis and assembly process at the liquid/liquid interface of a chloroform solution of poly(2-vinylpyridine) and an aqueous solution of chloroplatinic acid hydrate and a subsequent UV-light irradiation process. Transmission electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy investigations indicated that the foam-like thin films formed at the interface were composed of polymer molecules, and Pt(II) and Pt(IV) ions. Platinum nanoparticles with average diameter of 2.70 ± 0.35 nm appeared after UV-light irradiation, which were embedded in and adsorbed on the walls of the foams. The formation of the composite nanostructures at the interface was attributed to selfassembly of the polymer molecules at the liquid/liquid interface, reduction of PtCl6 2− , and interaction between PtCl6 2− /intermediate PtCl4 2− ions and the protonated pyridine groups. The catalytic activity of the composite film for the reduction of methylene blue by potassium borohydride in aqueous solutions was evaluated. The apparent rate constant decreased gradually with increasing the number of runs due to the partially leaching of Pt nanoparticles and became stable after the fifth cycle, indicating that the composite film can be used as an effective, stable and reusable catalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Platinum nanoparticles exhibited excellent catalytic [1–12] and electrocatalytic [13–27] properties, and have important applications in chemical industry and the area of fuel cells. In addition, Pt-semiconductor hetero-nanostructures, such as Pt–CdSe [28], Pt–CdS [29] and Pt–TiO2 [30,31] showed enhanced photocatalytic performance and have potential applications in environmental treatment. So synthesis and catalytic studies of Pt nanoparticles have attracted increasing interests. It has been established that the catalytic activity of metal nanoparticles for heterogeneous catalytic systems enhances with decreasing the size as the result of the increase of the surface-to-volume ratio, the coverage of active surface atoms, and chemical potentials [32,33]. In order to control the size, Pt nanoparticles are usually capped with polymers, such as polyvinylpyrrolidone (PVP) [3–6,15], polyethylenimide [13], polyelectrolytes [14] and polyethylenimine [16], or alkylammonium ions, such as tetradecyltrimethylammonium bromide [2] and cetyltrimethylammonium bromide [17],
∗ Corresponding author. Tel.: +86 531 88362805; fax: +86 531 88564750. E-mail address: [email protected] (H.-G. Liu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.07.017
or oleic acid and oleylamine [18]. It has been revealed that PVP molecules adhere to Pt nanoparticles through a charge-transfer interaction between the pyrrolidone rings and the surface Pt atoms [34], and the interactions between them are very strong. So a significant number of active sites are blocked [2]. The interaction between alkylammonium ions and Pt surface are considerably weaker than that between the carbonyl group and Pt surface. It was expected that the catalytically active sites could be preserved by using alkylammonium ions as capping agents [2,17]. On the other hand, in order to avoid aggregation, Pt nanoparticles are often immobilized on matrix. The supporting materials include oxides, such as silica [8] and ceria [9], platinum and tungsten gauzes [27], carbons [10,13], carbon nanotubes [11,23,25], carbon nanospheres [26], polymer microgels [12], and polymer nanofibers [22]. In this paper, we described a new and facile approach to prepare Pt nanoparticle-doped poly(2-vinylpyridine) (P2VP) foam-like thin films. These composite nanostructures were formed at the liquid/liquid interface through self-assembly of P2VP molecules, the combination of the protonated pyridine groups with PtCl6 2− ions, and simultaneous reduction of PtCl6 2− ions at ambient temperature. Liquid/liquid interface has been utilized widely to synthesis inorganic nanoparticles [35,36] and to assemble thin membranes composed of inorganic nanoparticles [37–39] and monolayers of
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Fig. 1. TEM micrographs of the nanostructures formed at the liquid/liquid interface.
graphene nanosheets [40] or graphite nanoplatelets [41], and even coordination polymers [42,43] recently, however, to the best of our knowledge, study on the assembly of polymer thin films, especially composed thin film consisting of polymer and inorganic species at the liquid/liquid interface is very rare. We found that numerous Pt nanoparticles embedded in and adsorbed on the walls of the foams and capped with the protonated pyridine groups in the composites after UV-light irradiation. It should be noted that the interaction between Pt and P2VP is weaker than that between Pt and PVP due to the different molecular structures. It was also demonstrated that for some catalytic reactions, the catalytic efficiency of the nanoparticles depends mainly on two parameters. One is the available active surface area for adsorption and other is the number of nanoparticles present in per volume [44,45]. The formed composites fulfil the requirements, and exhibited high catalytic activity for the reduction of methylene blue by KBH4 .
characterization. The deposited films were further irradiated by UV-light with the wavelength of 254 nm using a UV-lamp with the power of 6 W for a certain time. The distance between the deposited film and the lamp was 15 cm. 2.3. General characterization The morphology, structure and compositions of the deposited and the UV-light irradiated films were investigated by using highresolution transmission electron microscopy (HRTEM, JEOL–2010) with the accelerating voltage of 200 kV and X-ray photoelectron spectroscopy (XPS, ESCALAB MKII) with Mg K␣ (h = 1253.6 eV) as the exciting source at a pressure of 1.0 × 10−6 Pa and a resolution of 1.00 eV. The optical properties of these films were characterized by using UV–vis spectroscopy (HP 8453E).
2. Materials and methods
2.4. Measurement of platinum content
2.1. Chemicals
Platinum content was analyzed by using microwave digestion inductively coupled plasma-atomic emission spectroscopy (ICPAES, Varian Inc. – Liberty Series II). A certain amount of the sample was dissolved in aqua regia for 15 min predigesting, then digested in a CEM MARS5 microwave accelerated reaction system for 30 min. Then the reaction system was evaporated to dryness by using rotary evaporator, cooled down to room temperature, washed with a small amount of concentrated HCl, and diluted with water. The prepared sample was analyzed by using ICP-AES with monitoring the emission wavelength of Pt at 214.423 nm. The standard curve was obtained by using the platinum standard solution. The content of platinum in the composite sample was calculated.
Poly(2-vinylpyridine) (P2VP. Mw : 5300, Mn : 5000), chloroplatinic acid hydrate (H2 PtCl6 ·xH2 O, 99.9+%) and platinum standard solution (981 g mL−1 (20 ◦ C)) were purchased from Aldrich. KBH4 (≥97.0%) was purchased from Shanghai Zhanyun Chem. Co. Ltd. Methylene blue (MB, ≥98.5%) and chloroform (≥99.0%) were obtained from Tianjin Guangcheng Chem. Co. The chloroform contains 0.3–1.0% of ethanol served as stabilizer. Concentrated hydrochloric acid (35%) and concentrated nitric acid (35%) were supplied by Matsunoen Chem. Ltd. Japan, and extra pure NaCl was obtained from Junsei Chem. Co., Ltd., Korea. The water used is highly purified with the resistivity ≥18.0 M cm.
2.5. Catalytic experiments 2.2. Preparation 10 mL P2VP chloroform solution with the concentration of 0.2028 mg mL−1 was poured in a clean beaker, then 10 mL aqueous solution of H2 PtCl6 with the concentration of 1 × 10−3 mol L−1 was added carefully to form a clear liquid/liquid interface. The beaker was put in a sealed container that was placed in a dark oven. The temperature was controlled to be 25 ◦ C. 24 h later, thin films appeared at the liquid/liquid interface and air/aqueous solution interface, respectively. The films were transferred on carboncoated copper grids, silicon slides and quartz slides for further
0.5 mL MB aqueous solution with the concentration of 2 × 10−4 mol L−1 was poured in a quartz cuvette with the side length of 1 cm; then 1.0 mL aqueous solution of KBH4 with the concentration of 2 × 10−2 mol L−1 was added. The total volume of the reaction solution is 1.5 mL, and the concentrations of MB and KBH4 are 6.67 × 10−5 and 1.33 × 10−2 mol L−1 , respectively. A deposited thin film on a quartz slide after UV-light irradiation was immersed in the reaction system to catalyze the reduction of MB. The reaction was monitored by using UV–vis spectroscopy (HP-8453E) at room temperature (∼15 ◦ C).
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3. Results and discussion 3.1. Morphology, composition and structure Fig. 1 shows the TEM images of the nanostructures appeared at the liquid/liquid interface. A foam-like thin film can be seen from Fig. 1a, which are composed of hollow microcapsules with the diameter of about 300–500 nm. No particle was found in the high magnification image shown in Fig. 1b, indicating that no Pt nanoparticle or cluster formed at the liquid/liquid interface during the assembly process of the nanostructures. However, it was found that the weight content of Pt is 17.87% in the composite nanostructures by using ICP–AES technique with the recovery rate of 98.14%. This indicates that platinum do exist in the nanostructures. The limitation of detection (LOD) and limitation of quantification (LOQ) are 1.083 and 3.609 ppb, respectively. In order to clarify the composition of the nanostructures, the sample was investigated using X-ray photoelectron spectroscopy, as shown in Fig. 2. This spectrum could be decomposed into two pairs of peaks using Gaussian multi-peak fitting due to Pt4f7/2 and Pt4f5/2 spin–orbit coupling. The first pair at 72.6 and 75.9 eV was assigned to 4f7/2 and 4f5/2 of Pt(II), and the second pair at 74.7 and 78.1 eV was assigned to 4f7/2 and 4f5/2 of Pt(IV), respectively, according to the related literatures. For example, the binding energies located at 72.8 and 75.6 eV [13], or 71.9 and 75.1 eV [30] for 4f7/2 and 4f5/2 of Pt(II), respectively. It was also found that the binding energy for 4f7/2 of Pt(II) located at 72.1 [9] or 73 eV [31]. In addition, the binding energies of 4f7/2 and 4f5/2 of Pt(IV) located at 74.4 [9] and 77.5 eV [31], respectively. These values are close to those we obtained. In addition, the relative contents of Pt(II) and Pt(IV) ions were calculated to be 42.8% and 57.2% based on the peak areas of 4f7/2 and 4f5/2 of Pt(II), or 43.9% and 56.1% on the corresponding peak areas of Pt(IV), respectively. This means that platinum species in the composite formed at the liquid/liquid interface are mainly composed of platinum complex ions. In order to reduce the platinum ions in the composite nanostructure further to get Pt cluster-doped polymer nanostructure, the composite film was irradiated by UV-light at room temperature. Fig. 3 shows the TEM and HRTEM images of the thin film. Foam-like structure preserved, as shown in Fig. 3a, which are composed of microcapsules with the diameters of 200–600 nm. From Fig. 3b we can also see smaller, isolated, and opened microcapsules whose walls were interspersed with particles with the diameters of 6–8 nm. Furthermore, from the HRTEM image shown in Fig. 3c we can see that the walls were besprinkled with small particles whose average diameter is 2.70 ± 0.35 nm, as indicated by the size
Fig. 3. TEM (a and b) and HRTEM (c) micrographs of Pt–P2VP composites formed at the liquid/liquid interface and irradiated by UV-light for 1 h, and size distribution histogram of Pt nanoparticles in (c).
Fig. 2. XPS spectra of the composite thin film formed at the liquid/liquid interface.
distribution histogram shown in Fig. 3d. These particles dispersed homogeneously. The enlarged image inserted in Fig. 3c gives lat˚ indicating the tice fringes with the interplanar distance of 2.27 A, formation of Pt nanoparticles. The UV-light irradiated composite thin film was further investigated using XPS, as shown in Fig. 4. The spectrum was decomposed into three pairs of peaks using Gaussian multi-peak fitting. The second pair at 72.5 and 75.7 eV, and the third pair at 74.1 and 78.1 eV were attributed to Pt(II) and Pt(IV), respectively, and these values were in accordance with those obtained from Fig. 2. The first pair at 72.4 and 74.7 eV was assigned to 4f7/2 and 4f5/2 of Pt(0) by comparing with literature values, such as 71.1 and 74.4 eV [10], 71.0 and 74.6 eV [13], 71.1 and 74.5 eV [23], and 71.8 and
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Fig. 4. XPS spectra of the composite thin film formed at the liquid/liquid interface and irradiated with UV-light for 1 h.
74.9 eV [46]. It should be noted that the binding energy of 4f7/2 of Pt(0) is greater than the literature values. This may be related to the size effect of the nanoparticles. It has been demonstrated that the binding energies of electrons in metals, such as in Au(0) [47–49] and Ag(0) [50,51], increase with decreasing the particle size. The HRTEM image (Fig. 3c) exhibits clearly that the formed Pt nanoparticles are very small. The binding energies should therefore increase obviously. While the unreduced platinum complex ions still combined with the protonated pyridine groups, so the position of corresponding XPS peaks did not change. The relative contents of Pt(0), Pt(II) and Pt(IV) were calculated to be about 24%, 38% and 38%, respectively. So we can draw a conclusion that Pt nanoparticles formed after UV-light irradiation due to the reduction of the partial platinum ions, and the unreduced Pt(II) and Pt(IV) ions were still combined with the pyridine groups. It should be pointed out that the experimental conditions should be controlled. When increased the irradiation time or decreased the distance between the lamp and the sample, larger particles appeared. In addition, the UV-light irradiation led to cross-linking of the polymer molecules [52,53], which enhanced the stability of the composite films. UV–vis spectra shown in Fig. 5 also indicate the formation of Pt clusters. A peak at 266 nm appeared in the curve of pure P2VP film, which was attributed to the absorption of pyridine groups. In
Fig. 6. TEM (a) and HRTEM (b) images of Pt–P2VP composites appeared at the air/water interface and irradiated with UV-light for 1 h, and size distribution histogram of Pt nanoparticles in (b).
Fig. 5. UV–vis spectra of the composite film without (black) and with UV-light irradiation for 20 (red), 40 (blue) and 60 min (green) and a pure P2VP (pink) thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the curve of the composite film without UV-light irradiation, two intense peaks appeared at 210 and 272 nm, respectively. The first peak should be attributed to PtCl4 2− ions [16,54,55] and the second one should be the superposition of the absorption of PtCl6 2− ions [14,17,56,57] and the pyridine groups. The intensity of these two peaks decreases gradually upon UV-light irradiation, indicating the
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Scheme 1. Schematic illustration of the formation process of the foamlike thin film at the liquid/liquid interface. (a) The formation of the liquid/liquid interface; (b) adsorption of polymer molecules, combination of P2VP with the platinum complex ions and possible reduction of PtCl6 2− ions to PtCl4 2− ions; (c) formation of the composite thin layers; (d and e) self-assembly of the thin layer to a microcapsule; and (f) formation of a foamlike thin film.
transformation of platinum ions to atoms. In addition, this curve exhibited a significant increase of the baseline absorbance around 400 nm, indicating the formation of Pt(0) species [14,16,56]. This means that PtCl6 2− , PtCl4 2− and Pt(0) coexisted in the composite. These results are consistent with that of XPS. 3.2. Formation mechanism In order to illuminate the formation mechanism, two problems should be clarified. How do the P2VP molecules self-assemble into foam structure at the liquid/liquid interface? And how do PtCl6 2− ions be reduced to PtCl4 2− ions? As we know, P2VP can dissolve into organic solvents, such as chloroform. It can also dissolve into water. We have investigated the interfacial behavior of P2VP at the liquid/liquid interface formed by chloroform solution of P2VP and pure water with different pH values. We found that no film formed at the liquid/liquid interface, while some structures appeared at the air/water interface, and the structure depended on the pH values of water. The formation of the structures at the air/water interface should be resulted from the phase transfer of P2VP molecules from the organic phase to water phase, and the adsorption and self-organization of the P2VP molecules at the air/water interface. However, when the pure water was replaced by aqueous solution
of H2 PtCl6 , thin films were formed at the liquid/liquid interface. It is clearly that the presence of platinum ions is a key factor for the formation of the thin films. The formation process of the thin films may be described as follows. As soon as the liquid/liquid interface formed, P2VP molecules adsorbed at the interface to reduce the interfacial free energy. The nitrogen atoms of pyridine groups protonated at the interface because the aqueous solution of chloroplatinic acid supplied plenty of H+ ions. The protonated pyridyls would extend the polymer chains due to the electrostatic repulsion between them and combine PtCl6 2− ions due to the electrostatic attraction between pyridinium groups and the anions. The combination of PtCl6 2− ions with pyridyl groups attracted more and more polymer molecules to the interface rapidly. It is reasonable to suppose that the side of the polymer chain bearing the ion pairs faced the water phase, and the other side composed of methylene groups faced the organic phase. More and more polymer molecules migrated to and adsorbed at the interface. These polymer chains intertwined and intersected with each other to form a thin layer, and these layers overlapped to form a relatively thick film in which numerous PtCl6 2− ions were embedded. The interaction between the film and the water phase is greater than that between the film and the oil phase. So the film inclined to enter the water phase. However, from the view point of surface free energy, it is unfavor-
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Fig. 7. Time-dependent absorption spectra for the catalytic reduction of MB by KBH4 in aqueous solution in the presence of the UV-light irradiated thin composite film for the first (a), second (c), third (e), fourth (g) and fifth cycles (i), and the corresponding linear relationship between ln(At /A0 ) and reduction time (b, d, f, h and j). The weight of the composite is about 0.1 mg.
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Fig. 8. Relation between the apparent rate constants and the reaction cycles.
able for the film entering the water phase, because the interface free energy between the hydrophobic face and water is high. So the film self-assembled into a capsule in which the organic phase was incorporated. We found a thin film also appeared at the air/water interface. Fig. 6 shows TEM and HRTEM images of the deposited thin film after UV-light irradiation. It can be seen a foam-like nanostructure composed of capsules whose walls were embedded with Pt nanoparticles. This means that microcapsules formed at the liquid/liquid interface can enter the aqueous phase due to the stronger hydrophilicity of the outer surface of the capsule and can be captured by the air/water interface. This further confirms the formation mechanism. In the initial stage, some isolated capsules formed and entered into water phase due to the stronger interaction between the exterior surface of the capsules and water. These capsules adsorbed finally at the air/water interface and contacted each other to form a foam structure. More and more capsules formed with time at the liquid/liquid interface, which congregated to form a stable foam structure that prevented the capsule from escaping from the interface. At last, a thin foam film composed of numerous capsules was formed. The average diameter of the Pt particles in Fig. 6b was found to be 2.75 ± 0.44 nm, as indicated by the histogram (Fig. 6c). This average diameter is very close to that of the Pt nanoparticles in Fig. 3c. The formation process was illustrated in Scheme 1. In order to avoid the decomposition of chloroform under light irradiation, small amount of ethanol with the mass fraction of 0.3–1.0% is usually added in the commercial available chloroform as stabilizer. We measured the content of ethanol in the chloroform used by using gas chromatography and found the content is 0.2% (v/v). It is the ethanol that acts as reductant in the reaction system to reduce PtCl6 2− ions to PtCl4 2− ions [14]. When the liquid/liquid interface formed, ethanol molecules moved to the interface and entered to water phase due to their strong polarity. When they came into contact with PtCl6 2− ions adsorbed by P2VP at the interface or in the aqueous phase, redox reaction took place, resulting in the formation of PtCl4 2− ions. These ions combined with the protonated pyridyls and subsequently embedded into the foam-like nanostructures. Why no Pt(0) was in the composite formed at the liquid/liquid interface? This should be related to the weak reduction ability and the low percentage of ethanol in the reaction system. It was revealed that the reduction of PtCl6 2− to Pt(0) via Pt2+ [58]. The further reduction of Pt2+ to Pt(0) is a slow process compared with the reduction of Pt4+ to Pt2+ [16,58], and Pt(0) metal formation does not occur until a ∼90% yield of PtCl4 2− has accumulated [59]. So the ethanol in the system just reduces PtCl6 2− to PtCl4 2− . It is difficult to reduce PtCl4 2− to Pt(0) further. We tried to increase the fractional
conversion of Pt(0) by adding a certain amount of ethanol in the organic phase. We found larger particles formed at the interface. This confirmed that ethanol acted as reductant in this process.
3.3. Catalytic performance The catalytic activity of the composite was evaluated by using the reduction of MB by KBH4 as a model reaction. MB is a watersoluble cationic dye that has a dye skeleton of a thiazine group. The reduction of MB to colorless leucomethylene blue (LMB) has been used to evaluate the catalytic activities of various nanoparticulate catalysts, including Ag [60,61], Au [62,63], Pd [64,65], Pt [8], and metal oxides [66]. It is convenient to monitor the reaction by using UV–vis spectroscopy. Fig. 7 shows the absorption spectra for successive decolorization of MB. Two peaks appeared at 665 and 610 nm in the curve of mixed aqueous solution of MB and KBH4 . The shoulder peak at 610 nm resulted from the exciton coupling of the transition moments due to the formation of aggregates [67]. After immersion of the UV-light irradiated composite film deposited on a quartz slide, the intensity of these peaks decreased quickly with time, indicating the production of LMB [66]. The blue color of MB faded away rapidly, and a colorless solution resulted after 40 s for the first cycle, as shown in Fig. 7a. It was tested that the blue color of MB faded away very slowly in the mixed aqueous solution with NaBH4 in the absence of catalysts [60,62,63]. The blue color was bleached completely within just 40 s in the presence of the composite film for the first cycle, indicating high catalytic activity of the nanocomposite. This high catalytic activity should arise from the small size and high density of platinum particles in the composite film. As can be seen from the TEM and HRTEM images, this foam-like thin film was composed of microcapsules doped with a large numbers of Pt nanoparticles. In addition, the thin porous walls of the capsules allowed diffusion of the reactants and products freely. Considering that the concentration of KBH4 is larger greatly than that of MB, the reduction rate is assumed to be independent of the KBH4 concentration and only dependent of the concentration of MB. Since the absorbance of MB is proportional to its concentration, the absorbance of MB at t = 0 (A0 ) and the time t (At ) should corresponds to the concentration of t = 0 (C0 ) and the time t (Ct ), respectively. So the ratio Ct /C0 equals At /A0 . Fig. 7b gives the relation between ln(At /A0 ) and the reaction time. A0 and At were taken from the peak at 665 nm. The linear relation between them indicated that the catalytic reduction of MB follows pseudo first-order rate kinetics with respect to MB concentration. The rate constant
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Fig. 9. TEM micrographs of the composite film after the eighth cycle and size distribution histogram of Pt nanoparticles in (b) and (d).
was calculated from the slope of the fitting line. The apparent rate constant was found to be 3.7686 min−1 . However, the catalytic activity decreased gradually for the second, third, fourth and fifth cycles, as shown in Fig. 7, and did not decrease further after the fifth cycle. It was also found from Fig. 7d, f and h that two lines were fitted, indicating two apparent rate constants were obtained for the second, third and fourth cycles, and the rate constant corresponding to prior period is less than that corresponding to later period of the cycle. The experimental procedure on catalytic activity was described as follows. When the UV-light irradiated composite film deposited on a quartz slide was inserted into the mixed aqueous solution of MB and KBH4 , timing
began. After a certain period, such as 10 s, 15 s, 20 s, 2 min and 5 min for the first, second, third, fourth and fifth cycles, respectively, the composite film was removed, timing stopped, and the spectrum was recorded rapidly within several seconds using a HP 8453 spectrometer. Then the composite film was reinserted in the solution immediately, and timing restarted. So the successive spectra were recorded. The appearance of two apparent rate constants in the same cycle indicates the possible detachment of the Pt nanoparticles from the composite film. This is also the reason why the catalytic activity decreased gradually. Another possible reason why two apparent rate constants appeared may be the further reduction or coalescence of the Pt species at a slower rate during the
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initial stages of these cycles. As indicated by XPS results, there are plenty of platinum complex ions in the composite films. These ions reduced gradually in the catalytic reaction systems because KBH4 is a strong reductant. More and more Pt nanoparticles generated with time, leading to the increase of rate constant in the later stage of the cycle. Fig. 8 shows the plot of the apparent rate constants against number of successive reductions of MB with the composite P2VP–Pt film as catalyst. From Fig. 8a we can see that the apparent rate constant decreased rapidly from the first cycle to the second cycle, and decreased continuously to the third and fourth cycles. It can be clearly seen from the enlarged figure (Fig. 8b) that the rate constant did not change any longer after the fifth cycle, indicating that the composition and structure of the film became stable after this cycle. It can be also seen from Fig. 7j that only one line was obtained, and the correlation coefficient is 0.99963, indicating no Pt nanoparticle detached or generated further from the film, and the film could be used as a reusable catalyst. The average apparent constant from the fifth to the eighth cycle was calculated to be 0.0242 min−1 . The weight of the film was measured to be about 0.1 mg, and the initial content of the platinum in the film was 17.87%. Considering the small amount of Pt used, this film still exhibited effective catalytic activity. The film after the eighth cycle was further investigated using TEM, as shown in Fig. 9. Form Fig. 9a and c we can see that the foam-like structure became a multilayer structure. From the high magnification images shown in Fig. 9b and d we can see that Pt nanoparticles dispersed in the film, but the number of the particles is much less than that appeared in Fig. 3. The size distribution histogram was shown in Fig. 9e. The average diameter of these particles was calculated to be 2.75 ± 0.83 nm. Although the size distribution of the particles became broader than that before use, the average value did not change. This further confirms the detachment of nanoparticles during the catalytic reaction cycles. As revealed in the formation mechanism of the composite film, platinum complex ions were embedded in capsule walls and adsorbed on the outer surface of the capsules. When irradiated, Pt nanoparticles formed which were embedded in the walls and adsorbed on the surface, respectively. The particles adsorbed on the surface would detach from the surface easily during the catalytic reaction process because there was no sufficient capping agent around them. Some particles embedded in the walls would leach from the film, too, because the interaction between the pyridyls and Pt is not stronger enough [2,34]. Although the catalytic activity decreases in the initial cycles, the nanoparticles remained in the film after the fifth cycle would be stable, and acted as an effective catalyst for the reduction of MB. Comparing with other methods for fabricating metal nanoparticle/polymer composite films, such as Ag/PVA film [68], the fabrication method described here is more convenient and simple. The improvement of stability of the catalyst is in progress.
4. Conclusion In summary, we developed a convenient and facile method to prepare Pt nanoparticle-doped polymer foamlike nanostructures by making use of the self-assembly of polymer molecules, reduction of Pt complex ions and combination of the polymer molecules with the complex ions at the liquid/liquid interface and a subsequent UV-light irradiation process. The formed nanostructures are composed of polymer capsules whose walls were embedded and adsorbed numerous small particles that were dispersed homogeneously. Although some Pt nanoparticles detached or leached from the composite film during the catalytic reaction, the composite film still exhibited effective and stable catalytic activity after the fifth
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