polymer composite microspheres for adsorption and catalytic degradation of organic dyes in aqueous solutions

polymer composite microspheres for adsorption and catalytic degradation of organic dyes in aqueous solutions

Composites Science and Technology 107 (2015) 137–144 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: w...

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Composites Science and Technology 107 (2015) 137–144

Contents lists available at ScienceDirect

Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech

Porous Ag/polymer composite microspheres for adsorption and catalytic degradation of organic dyes in aqueous solutions Yu Yang, Huiling Liao, Zhen Tong, Chaoyang Wang ⇑ Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 14 November 2014 Received in revised form 18 December 2014 Accepted 23 December 2014 Available online 29 December 2014 Keywords: A. Functional composites A. Hybrid composites A. Nano composites A. Polymer-matrix composites (PMCs) B. Interface

a b s t r a c t Porous poly(styrene–divinyl benzene) (marked as PSD) microspheres with interconnected structure were facilely achieved by a suspension polymerization based on water-in-oil-in-water emulsion templates. The internal water-in-oil emulsion is a type of high internal phase emulsion, making it possible to fabricate porous open-cell microspheres. Furthermore, ascribe to the high concentration (much larger than critical micelle concentration) of hydrophobic surfactant in the oil phase, large amount of reverse micelles were generated and much more controllable and accessible pore structures of PSD microspheres were fabricated. Subsequently, sulfonated PSD microspheres and Ag-loading PSD hybrid microspheres were successfully achieved based on the above PSD microspheres. It was demonstrated that the sulfonated PSD microspheres could effectively adsorb dyes in an aqueous solution, and that the Ag-loading hybrid microspheres exhibited excellent catalytic degradation of the dyes. We believe our functional porous microspheres could be potential candidates as pollutant adsorbents, enzyme immobilization, and catalyst carriers. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Porous polymeric microspheres with high specific surface areas and physicochemical stability possess versatile important applications such as adsorbents [1], stationary phase for chromatography [2], supports for catalyst immobilization [3], and micro-vessels for drug delivery systems [4]. It is the pores on the surface and the interior that makes porous microspheres different from conventional microspheres which do not possess any pore structure. In most of applications, the porous structure plays an important role in determining the efficiency of adsorption and release [5,6]. To control the pore structure parameters is therefore of particular importance. Especially in the case of water treatment [7–10], surface area and pore structure remarkably influence the organic contaminant adsorption performance of the porous microsphere [11,12]. Traditionally, to fabricate porous microsphere with pore structure, suspension, dispersion, precipitation, multistage, membrane/ microchannel emulsification and microfluidic polymerization are the main techniques [13–16]. Among the above methods, suspension polymerization based on double emulsion templates has received more considerable attention as facile fabrication, tunable pore structures, and easy functionalization [17–20]. By ⇑ Corresponding author. Tel./fax: +86 20 22236269. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.compscitech.2014.12.015 0266-3538/Ó 2014 Elsevier Ltd. All rights reserved.

polymerizing the middle phase, the obtained microspheres consist of pore structure which is ascribed to the inner phase of the primary emulsion [21]. However, most of the double emulsion-templated microspheres possess close-cell structure as the inner droplets are independent each other, and the obtained structure would largely limit their practical applications especially in adsorption and catalysis [21,22]. In our previous work, we used styrene/divinyl benzene/hexadecane mixture as the middle polymerizable phase. As hexadecane is non-polymerizable leading to open-cell hierarchical porous structure [1]. However, the precise choose of middle phase mixture and the insufficient pore structure limit the wide applications. Herein, therefore, a much more general and versatile emulsion template should be exploited to obtain porous microsphere with an open-cell structure. In this work, a novel w/o/w double emulsion template, of which the primary emulsion is a type of w/o high internal phase emulsion (HIPE, >74% v/v internal droplet phase) and the surfactant concentration of the oil phase is much larger than the critical micelle concentration, was successfully introduced to fabricate porous poly(styrene–divinyl benzene) (PSD) microspheres with accessible pore structure for the first time. Moreover, the adsorption capacity of sulfonated PSD microspheres and the catalytic performance of Ag-loading PSD microspheres were both investigated. The comparative results of both experiments had demonstrated that the properties of porous material had seriously depended on its

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morphology, which was closely related to the surfactant concentration. We believe our porous microspheres would have a wide range of potential application, such as pollutant adsorbents, enzyme immobilization, and catalyst carriers.

2. Experimental 2.1. Materials Styrene (St) and divinyl benzene (DVB) (Beijing Chemical Reagents Co., China) were purified under a vacuum to remove the inhibitor before use. Sorbitan monooleate (Span 80) was obtained from Shanghai Shenyu pharmaceutical chemical Co., China. Poly(vinyl alcohol) (PVA-217, degree of polymerization 1700) was used as the external oil-in-water emulsion stabilizer. Polyvinylpyrrolidone (PVP, MW = 40,000), H2SO4 (98%), concentrated ammonia (28%), NaBH4, AgNO3, and rhodamine 6G (R6G) were all used as received. Water used in all experiments was deionized and filtrated by a Millipore purification apparatus with resistivity more than 18 MO cm. 10% PVA (w/v%) aqueous solution was prepared by stirring the corresponding amount of PVA in water that was heated up to 90 °C.

2.5. Adsorption capacity of PSD–SO3H microspheres The adsorption isotherms were obtained in batch equilibrium experiments with 10 mg of PSD–SO3H samples dispersed in 5 mL solution of R6G with concentrations ranging from 0 to 1  103 mol/L. The PSD–SO3H samples were allowed to adsorb R6G at 25 °C under a vibration condition until reaching equilibrium. The adsorption capacity versus time curves among PSD–SO3H samples was obtained by fixing the concentration of R6G on 2  105 mol/L. The left procedure was the same as the aforementioned adsorption isotherms experiment. The adsorption percentage value was gained on corresponding time. The adsorption behavior was traced by UV–vis spectra, and the R6G concentration was calculated by the absorbance at the maximum adsorption (526 nm). The amounts of the adsorbed R6G were calculated with the following equation [24].

Qe ¼

Co  Ce V M

where Qe (mg/g) is the amount adsorbed per gram of PSD–SO3H microsphere at equilibrium. Co is the initial dyes concentration (mg/mL). Ce is the equilibrium concentration (mg/mL). V is the R6G solution volume (mL). M is the mass of PSD–SO3H used (g).

2.2. Preparation of porous PSD microspheres 2.6. Catalytic properties of PSD/Ag hybrid microspheres The preparation of w/o/w emulsion was formed by a two-step process. 0.4 mL of St, 0.6 mL of DVB, 10 mg of AIBN, and a certain amount of Span 80 (0.1, 0.25 or 0.4 g) were mixed as the oil phase. Afterward, 4 mL of water was emulsified into the oil phase, producing the primary w/o HIPE. And then, the novel double emulsion template was obtained by adding the w/o HIPE to 10% PVA aqueous solution under slightly hand-shaking. Subsequently, the resultant emulsion was heated to 65 °C promptly and kept for 20 h at N2 atmosphere. After washing with ethanol and dried in vacuum at 40 °C, the products of PSD-1 (0.1 g Span 80), PSD-2 (0.25 g Span 80), and PSD-3 (0.4 g Span 80) were obtained.

In a typical catalytic experiment, both the dye (R6G, 2  105 mol/L) and NaBH4 (1  102 mol/L) were freshly prepared as aqueous solutions. Subsequently, a series of PSD/Ag hybrid microsphere samples (5 mg) was mixed with 30 mL of 2  105 mol/L R6G aqueous solution, and 1 mL of 1  102 mol/L NaBH4 solution was rapidly injected into this mixture while stirring. The color of the mixture vanished gradually, indicating the reduction of the dye. The concentration changes of R6G were monitored with a UV–vis spectrophotometer. The calibration curve of R6G was determined by taking absorbance versus R6G concentration between 0 and 2  105 mol/L. In this interval, the calibration curve fits the Lambert and Beer’s law [25]:

2.3. Preparation of sulfonated PSD microspheres (PSD–SO3H)

A ¼ 83; 750  C þ 0:0097 Sulfonated PSD was achieved by the following process. Dried PSD microspheres (0.1 g) were dispersed into the concentrated sulfuric acid (10 mL) at 50 °C for 5 h, then, the resultant microspheres were washed with a large amount of ethanol/water repeatedly, centrifuged and dried in vacuum at 40 °C for 24 h. The suffix ‘‘SO3H’’ was added to the PSD name to indicate sulfonated microspheres.

2.4. Preparation of PSD/Ag hybrid microspheres The typical strategy to prepare PSD/Ag composite microspheres was based on the previously reported method [23]. In brief, 40 mg of dried PSD–SO3H microspheres were quickly added to 4 mL of a freshly prepared aqueous solution of [Ag(NH3)2]+ (5  101 mol/ L), and the dispersion was vibrated at room temperature for 1 h. The surfaces of microspheres would be surrounded by the [Ag(NH3)2]+ ions because of the electrostatic attraction between [Ag(NH3)2]+ ions and SO3H groups. Subsequently, the dispersion was mixed with 12 mL of PVP aqueous solution (5  104 mol/L). Finally, this mixture was kept at 70 °C and slightly stirred for around 6 h at a speed of 100 rpm. The final products were collected by centrifugation, washed with an excess amount of deionized water several times, and dried in vacuum at 40 °C for 24 h.

where A is the absorbance and C is the concentration of R6G.

2.7. Characterization UV–vis spectra were recorded on a UV-3010 spectrophotometer (Hitachi, Japan) at room temperature. Infrared absorption spectra were measured in the range 400–4000 cm1 by Fourier transformed infrared (FT-IR) spectrometer (Nicolet 6700) with a resolution of 4 cm1. XRD pattern was collected using a Bruker D8 Advance X-ray diffractometer with Cu target (40 kV, 40 mA) from 10° to 90°. Thermogravimetric analysis (TGA) was conducted on a Netzsch TG 209 thermal analysis system from 40 to 800 °C at a heating rate of 10 °C/min under nitrogen. Morphological observation of the microspheres was surveyed by scanning electron microscopy (SEM, Zeiss EVO-18) at the acceleration voltage of 10 kV. The specimen was firstly scattered onto a conductive adhesive, then sliced by a razor blade under the observation of optical microscope, finally sputtered with gold. An energy-dispersive Xray spectroscopy (EDX) was conducted on the SEM to examine the surface composition of PSD/Ag hybrid microsphere. All measurement studies were carried out in triplicate. The obtained values were averaged from repeated experimental results.

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3. Results and discussion In this work, the w1/o/w2 double emulsions are obtained by the hydrophobic surfactants absorbed at the primary w1/o interface and the hydrophilic surfactants coating at the outer interface (Fig. 1a). PVA and Span 80 were used as two kinds of particulate stabilizers. The hydrophobic Span 80 was used as the stabilizer for the primary w1/o emulsions while the hydrophilic PVA stabilized the secondary o/w2 emulsions. Fig. 1b and c is digital photos of the primary w1/o emulsion and the corresponding w1/o/w2 double emulsion, respectively. It is well known that surfactants in the preparation of the porous materials have played an important role. Suitable surfactants can effectively reduce the interfacial tension of the emulsion and improve the stability of the emulsion [26]. Thus, it is reasonable for us to pay more attention to the surfactant in this work. It was observed that the content of surfactant (Span 80) in the oil phase had a significant impact on the morphology of the microspheres, not only on the surface but also in the interior, as shown in Fig. 2. When the Span 80 content was 0.1 g, the as-prepared PSD1 microspheres appeared polydispersity with diameters in the range 7–17 lm (the inset in Fig. 2a1), as well as with a big hole in the interior (Fig. 2a2), due to Ostwald ripening effect in which inner emulsion droplets grew at the expense of smaller ones [27]. And the half surface of the microsphere exhibited smooth and the left one shown rough containing a lot of smaller than 400 nm macro-/giga-pores. And as to its magnified cross section, its skeleton exhibited a core–shell structure, where the edges of the shell grew in alignment with those of the core. Moreover, the core was as solid as usual particles prepared by emulsion polymerization and the incomplete shell representing the rough half-surface pointed above was messy with a thickness ranging from 0 to 2 lm. Fig. 2b1 and b2 has shown that when the Span 80 content was 0.25 g, there was a smaller hole in PSD-2 microspheres than that in the resulting PSD-1 microspheres (Fig. 2b2). These microspheres, with diameters of 5–20 lm (the inset in Fig. 2b1), seem like gigaporous polymeric particles for its whole messy and macro-/gigaporous skeleton from the surface to the hollow interior (Fig. 3). Comparing with the PSD-1 and PSD-2, the thing became different when the Span 80 content was up to 0.4 g. PSD-3 microspheres,

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with diameters of 5–15 lm (the inset in Fig. 2c1), had like-irregular nanoparticle’s agglomeration structure, not only on the surface, but also in the whole interior (Fig. 2c2), or that was to say the macro-giga-porous structure went homogeneously through around the microspheres. From the SEM results, it could be safe to say that there were three different porous morphologies were achieved successfully by changing the high surfactant concentration in the oil phase. To our best knowledge, when the surfactant concentration in the oil phase was increased to the critical micelle concentration, a micelle could be emerged through the surfactant molecules gathering, which had an ability to absorb water from inner or outer water phase to form the swollen micelle, seems like a new water-in-oil emulsion. With the surfactant concentration increasing further, the micelle could absorb a good deal of water to probably form a water–oil bio-continuous structure, which was a common phenomenon in microemulsion polymerization system [28,29]. Finally, the phase separation between absorbed water and polymer proceeded further by polymerization, where gigapores were generated by water channels. Hence, the reason for our resulting PSD microspheres morphology would be discussed which was relevant to the existence of reverse micelles. On the basis of the continuous observation of the process, the probable diagram of emulsion evolution, which would influence the formation of various pore morphologies on PSD microspheres, was illustrated in Fig. 4. When the concentration of Span 80 was lower than 40% as to PSD-1 and PSD-2, destruction was likely to occur among the primary emulsions in its instable state, where the droplet changed from multiple-core globule to a single-cell globule, as shown in Fig. 4A. Reverse micelles, formed by the aggregation of overmuch surfactant molecules, should have stayed possibly around the outer water–oil interface phase at the beginning to effectively lower the interfacial energy and kept a kinetic equilibrium, and then absorbed water from outer water phase to produce the swollen micelles. When the Span 80 concentration was up to 40%, a new stable multiple-core globule containing a lot of reverse micelles were formed as shown in Fig. 4B. A whole bio-continuous structure could be generated in the globule. When the polymerization started, since our research belonged to the vinyl monomers and cross-linkers of conventional free radical polymerization (CFRP) system, the growing polymer chains would tend to

Fig. 1. (a) Illustration of preparation of the (W/O HIPE)/W double emulsions. (b) and (c) are the optical images of the W/O HIPE and (W/O HIPE)/W double emulsion droplets, respectively. The scale bars in (b) and (c) are 50 lm.

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a1

c1

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20 µ m

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5 µm

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Fig. 2. SEM images of the microspheres influenced by high Span 80 concentrations. Left column: surface morphology of a whole microsphere (a1) 0.1 g Span 80, PSD-1; (b1) 0.25 g Span 80, PSD-2; (c1) 0.4 g Span 80, PSD-3; the insets: light microscopy image of corresponding microspheres; right column: cross section images of left column samples.

a

1

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3 2 c

2 µm

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Fig. 3. SEM images of the different part of PSD-2 microspheres. (b)–(d) are the corresponding magnified image of the zone of (1) (2) (3) in (a).

aggregate each other, and the segregated polymer chains soon developed tiny particles, then those particles aggregated to the typical heterogeneous porous structures composed of micron size particles [30]. Since the water–oil bio-continuous existed in all changed emulsion, it could facilitate this phenomenon pointed above. What is more, according to our careful observation of SEM images, it was not difficult to find that the pores were not interconnected in all instances. It was potentially because the degree of biocontinuous structure was low to emerge a heterogeneous phase distribution. To explain more specifically, an incomplete outer messy shell emerged in the PSD-1 was ascribed to a small amount of reverse micelles, leading to lower water absorbing capacity under 10% Span 80. While the condition was different as to 25%, in which

the value was high enough to absorb water to form a whole biocontinuous structure in the bulk oil phase. Subsequently, it was reasonable to explain the formation of hollow hole and messy shell exhibited in PSD-1 and PSD-2, which were due to the bulk water phase and the water channel absorbed by reverse micelles, respectively. Comprehensively, we provided a simple method to prepare more accessible and multiple morphologies with gigapores than the traditional single emulsion method [31]. To make the microspheres functional, we made sulfonated treatment and Ag-loading hybridization, and chose PSD-2 as a representative to make a modification analysis. Firstly, the morphology of the PSD-2/Ag was examined by SEM (see Fig. 5a). Comparing the original PSD-2 in Fig. 2b2 and combining the PSD-2/Ag’s corresponding EDX spectra in Fig. 5b, respectively, Ag

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Multiple-core globule with high Span 80 concentration

a

b

Single-cell globule with less reverse micelles (Span 80 <40%) Reverse micelle

Multiple-core globule with more reverse micelles (Span 80 =40%)

Water phase

Oil phase

Fig. 4. Schematic illustration of emulsion’s morphology evolution with high surfactant concentration.

nanoparticles were found on the surfaces of the sulfonated PSD after the loading process. The macro-giga-porous structure had become unapparent due to the existence of Ag. Note, C and S element signals in high intensity observed in the EDX spectra were originated from PSD and sulfonate group. In addition, as shown in Fig. 5b, the presence of Ag element signal could be observed in the EDX spectra of the PSD-2/Ag hybrid microsphere. Therefore, these evidences illustrated that PVP has successfully induced the reduction of [Ag(NH3)2]+ ions to Ag nanoparticles. Meanwhile, PVP can also stabilize the system against aggregation in aqueous media. Moreover, the FT-IR measurements of PSD-2 derivatives were performed to examine the synthesis of PSD-2–SO3H and the chemical bonding of Ag(I) onto the polymer. The effects of sulfonation on the chemical structure were reflected in the FT-IR spectra (Fig. 5c), where the in-plane bending vibration of CAH bond in benzene ring could have been affected by the symmetric stretching vibrations of

a

[email protected] bond in sulfonate group. It was observed that the absorption bands at 1016 cm1 of poly(St–DVB) was split into 1008 cm1 and 1033 cm1 due to the existence of sulfonate group. Besides, the strong absorption bands at 1179 cm1 well suggest the existence of asymmetric stretching vibration of [email protected] bond in sulfonate group. On the other hand, the existence of Ag had several diversities on the absorption spectra comparing to the PSD-2–SO3H, such as the disappearance of absorption bands at 469 cm1 and 646 cm1 and the appearance of absorption bands at 1126 cm1 and 1783 cm1. Next, the typical XRD pattern of the composite PSD-2/Ag microspheres, as illustrated in Fig. 5d, exhibited peaks at 2h of 38.1°, 43.9°, 64.2°, 77.4°, corresponding to the reflections of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystalline planes of the fcc structure of Ag (JCPDS No. 04-0783), respectively. It indicated that the Ag nanoparticles with crystallinity could be obtained by PVP reduction of [Ag(NH3)2]+ ions. These results revealed that the product was a composite consisting of a polymer phase and silver nanocrystallites. Moreover, the diffraction peaks reveal that the as-prepared samples have a poor crystallinity. To sum up, our resultant PSD microspheres had been smoothly modified by a sulfonation process and a metal-loading hybridization. For the PSD/Ag hybrid microspheres, the true loading mass of Ag component was generally obtained from TGA characterization (Fig. 6). Firstly, it was obvious that thermostability of all PSD/Ag microspheres had been improved due to the existence of inorganic particle. The mass of Ag loaded in the PSD-1/Ag, PSD-2/Ag, PSD-3/ Ag was 37%, 41%, 34%, respectively, collected from the different value between residual masses. Since the sulfonation process does not have a significant influence on the morphologies of PSD spheres, because the sulfonation reaction usually occurs homogeneously on the surfaces of polymer spheres [32]. So, the SEM images (Figs. 2 and 3) can stand for the morphologies of PSD– SO3H. Comparing PSD-1 and PSD-2, we can find that the pore hardly exhibited on the surface of PSD-3, but a lot of obvious water channels as its water–oil bio-continuous structure (Fig. 4). It can help us to deduce that the PSD-3–SO3H potentially shown a smallest surface area, resulting to impede the SO3H group grafting on the surface and the electrostatic attraction with [Ag(NH3)2]+, consequently leading to Ag-loading failure. Conversely, the complete

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Fig. 5. SEM images (a) and the corresponding EDX spectra (b) of PSD-2/Ag hybrid microspheres prepared by PVP in aqueous media. Sulfonated PSD-2 microspheres, 40.0 mg; PVP, 5  104 mol/L; [Ag(NH3)2]+ ions, 5  101 mol/L. (c) FITR spectra of (1) PSD-2; (2) PSD-2-SO3H; (3) PSD-2/Ag microspheres. (d) XRD patterns of PSD-2 (1) and PSD-2/Ag (2) microspheres.

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PSD-1 PSD-1/Ag

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Temperature (°C) Fig. 6. TGA curves of microspheres: (a) PSD-1 and PSD-1/Ag; (b) PSD-2 and PSD-2/Ag; (c) PSD-3 and PSD-3/Ag.

sum up, PSD-2–SO3H developed a best adsorption property for its largest adsorption rate and maximum Qe. Undoubtedly, it should be mainly the porous morphology that made the adsorption behavior various among PSD–SO3H microspheres. The specific surface area of microspheres could account for their different adsorption behavior. That is to say better adsorption property benefits from larger specific surface area. Then, judging from the SEM images (Figs. 2 and 3), it was easy to find that the size of tiny particle and the grooves (generated by the water channels) exhibited on the surface of PSD-3–SO3H were obviously larger than that of PSD-1–SO3H and PSD-2–SO3H, which would help to estimate that the PSD-3–SO3H potentially shown the smallest specific surface area resulting to impair its adsorption property. Moreover, comparing the PSD-1–SO3H, the completed messy skeleton and macro-/giga-porous structure of PSD-2–SO3H can contribute PSD-2–SO3H to a better adsorption property than that of PSD-1–SO3H. Since the sulfonation process would not change the original morphology, it was safe to conclude that the structure of PSD-2 was conducive to develop an optimal adsorption performance. It has been experimentally demonstrated that metal nanoparticles have high catalytic activities for hydrogenation, hydroformylation, carbonylation, and so forth [34]. Most of the organic dyes are 40

a

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b 32

Qe (mg/g)

Adsorption percentage (%)

messy and macro-/giga-porous skeleton of PSD-2 has played a more positive effect to the Ag loading than that of PSD-1. We studied the adsorption behavior of the obtained three hybrid microspheres samples to a hydrophilic dye (R6G) in water. Firstly, to investigate the adsorption rate, we began by fixing the initial concentrations of dyes at 2  105 mol/L. The comparative results are shown in Fig. 7a. It was clear that sample PSD-1– SO3H and PSD-2–SO3H shown a similarly higher adsorption rate, where R6G was absorbed by 100% quickly within 1.5 h while it took closed for 5 h for PSD-3–SO3H. Next, the saturated adsorption capacities (Qe) were studied on initial concentrations of dyes ranging from 0 to 1  103 mol/L. As shown in Fig. 7b, PSD-2–SO3H exhibited the largest maximum Qe, slightly higher than to that of PSD-1–SO3H, and PSD-3–SO3H possessed the lowest maximum Qe. The maximum Qe of all samples had emerged around at 5  104 mol/L. After that, the Qe trended to decline under the excessively high R6G concentration. There were probably two reasons for this phenomenon. One was due to the inevitable test error through diluting the R6G solution by 50 times when testing the absorbance of 1  103 mol/L, the other was that with increasing the concentration of R6G, R6G would be likely to aggregate to become in the dimeric form, impairing the adsorption to the microspheres surface [33]. To

80 60 40 PSD-3-SO3H PSD-1-SO3H PSD-2-SO3H

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0 0.0

0.1

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Ce (mg/ml)

Fig. 7. Adsorption performance of PSD–SO3H. (a) Adsorption rate at 25 °C for R6G (2  105 mol/L). (b) Adsorption isotherms at 25 °C for R6G (0–1  103 mol/L).

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0 min 5 min 15 min 30 min 60 min 90 min

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d 75 50 25 0 PSD-1/Ag

PSD-2/Ag PSD-3/Ag

Fig. 8. UV absorbance of the R6G aqueous solution reduced by NaBH4 combined with (a) PSD-1/Ag, (b) PSD-2/Ag, (c) PSD-3/Ag as a function of reaction time. All PSD/Ag samples: 5 mg; R6G aqueous solution: 2  105 mol/L, 30 mL; NaBH4: 1  102 mol/L, 1 mL. (d) Degradation percentage of different hybrid microspheres.

synthetic compounds, which present a potential danger to the environment due to their toxicity and resistant to the aerobic degradation. Herein, the catalytic degradation property of PSD/Ag hybrid microspheres for organic dyes was investigated. R6G was selected as a model dye and its evolution of UV–visible spectra at the wavelength of absorbance maximum (kmax) at 526 nm during the reduction is illustrated in Fig. 8. When the reaction system was in the presence of PSD/Ag hybrid microspheres and NaBH4, the absorbance at kmax of R6G was quickly decreased with the reaction time (Fig. 8a–c), compared to the absence of NaBH4. However, the absence of PSD/Ag hybrid microspheres (but in the presence of NaBH4) in the R6G solution only resulted in a slight decrease of kmax after 4 h, which strongly confirmed the nucleophilic reagent role of NaBH4 and the catalyst role of PSD/Ag, respectively. The catalytic mechanism could be explained as follows. The silver nanoparticles supported on PSD served as an electronic relay in the system for an oxidant and a reductant, and electron transfer occurred via the supported metal nanoparticles. Usually, dyes are electrophilic and BH 4 ions are nucleophilic with respect to the silver nanoparticles. In the reaction, the nucleophile NaBH4 could donate electrons to silver nanoparticles, and the electrophile dyes (R6G) would capture electrons from silver nanoparticles to take a reduction reaction. So the silver nanoparticles served as an electronic relay for the catalytic reduction of dyes in NaBH4 solution [35]. Here, we have succeeded in immobilizing silver onto polymer microspheres effectively to protect metal particles from aggregation, thus avoiding deactivation and poisoning of the catalysts during the catalytic reaction. It was worth to notice that the UV absorbance intensity decreased sharply to zero within only 15 min, indicating an extraordinary catalytic rate of PSD-2/Ag (Fig. 8b), while the degradation of R6G in PSD-1/Ag and PSD-3/Ag had not finished yet after 1.5 h. To exactly quantify the catalyst property, we figured out catalytic average rate (mg/h) and degradation percentage (%) within 1.5 h (Fig. 8d). The results illustrated that PSD-2/Ag exhibited the best catalyst performance for its fastest rate (10.3  104 mg/h)

and degradation percentage (100%), followed by PSD-3/Ag (1.5  104 mg/h, 91% respectively) and PSD-1/Ag (1.3  104 mg/h, 81.4% respectively). Obviously, PSD-2/Ag hybrid microspheres with a highest concentration of Ag resulted in the fastest catalytic reaction rate. On the other hand, poor distribution and aggregation of Ag nanoparticles would also play a significant negative influence on the catalyst activity. So, due to this potential reason, PSD-1/Ag with a higher concentration of Ag emerged an inferior catalytic performance than that of PSD-3/Ag, contrarily. These evidences simultaneously confirmed that the PSD-2 substrate plays an important role in decreasing the aggregation of the metal nanoparticles and the concentration of surfactant in the oil phase had a deep influence on the catalytic performance of resulting products for their different obtained structures. 4. Conclusions We successfully provided a sample fabrication method to PSD microspheres with a hierarchical pore structure through suspension polymerization based on emulsion templates, in which focusing on controlling high Span 80 concentrations in the oil phase. This strategy enabled the morphology of PSD to be controlled and multiple. The key to manipulating this multiple hierarchical pore structure was to balance the competition of the emulsion stability and reverses micelles concentration. The comparative results of the adsorption capacity of sulfonated PSD microspheres and the catalytic performance of PSD/Ag hybrid microspheres had both demonstrated that the properties of porous material had seriously depended on its morphology which was closely related to the surfactant concentration. Acknowledgements This work was supported by the National Natural Science Foundation of China (21274046 and 21474032) and the Natural Science Foundation of Guangdong Province (S20120011057).

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