One-pot synthesis of porous Au-nanoparticles@polymer/reduced graphene oxide composite microspheres by γ-ray radiation and their application as a recyclable high-performance catalyst

Abstract

The synthesis of efficient and recyclable noble metal nanocatalysts has always been a hot research area in nanocatalysis. In this work, novel porous sulfonated polystyrene (SPS)/reduced graphene oxide (rGO) composite microspheres loaded with Au nanoparticles (AuNP@SPS/rGO) were synthesized through γ-ray radiation on a simple one-pot system, i.e. an aqueous solution containing cagelike porous SPS microspheres, GO, and HAuCl₄. rGO and AuNPs with a size of about 12 nm were formed in water simultaneously under γ-ray radiation, and in situ loaded on cagelike porous SPS microspheres. The prepared cagelike AuNP@SPS/rGO microspheres showed far better catalytic performance than conventional non-supported AuNP catalysts, which completed the reduction of o-nitroaniline by NaBH₄ within only 15 s. Furthermore, the prepared cagelike AuNP@SPS/rGO microspheres can be easily recycled from the reaction system by centrifugation with no loss of the catalytic activity. This work indicates that porous polymer/rGO composite microspheres can be used as a promising support to prepare recyclable metallic nanocatalysts with ultra-high catalytic activity.

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Introduction

Au nanoparticles (AuNPs) with a diameter of several nanometers exhibit excellent catalytic performance in the hydrogenation of unsaturated hydrocarbons,^1,2 the low-temperature oxidation of CO,^3,4 the selective oxidation of different alcohols,^5,6 the reduction of nitroaromatic compounds or dyes,^7,8 and several other important reactions.^9–11 However, AuNPs have a strong tendency to aggregate due to their high surface energy, which directly results in a sharp decrease in their catalytic activities.¹² Thus, how to stabilize AuNPs is a crucial problem for the development of highly efficient nano-Au catalysts. Immobilization of AuNPs on a suitable support, such as microspheres, is a good way to avoid the aggregation of AuNPs, and at the same time, helps to solve the problem of the separation and the recycling of the nano-Au catalyst.^13–16

Loading AuNPs on microspheres can be achieved commonly by two strategies. One is fixing the ready-prepared AuNPs onto the support; the other is in situ synthesis and loading of AuNPs by reducing the Au precursor in the presence of the support.^17–24 In the first strategy, extra stabilizers are needed for the stabilization and loading of AuNPs. For example, Yin et al. prepared Fe₃O₄@SiO₂ core–shell microspheres coated with a monolayer of the coupling agent 3-aminopropyltriethoxysilane as the support. Then, citrate-stabilized Au nanoparticles were adsorbed onto the silica surface through the strong affinity between Au and primary amines.¹⁷ In the second strategy, AuNPs are formed and immobilized simultaneously on the support by the reduction of a precursor containing Au(III). Ballauff et al. prepared polystyrene (PS) microspheres grafted with poly(aminoethyl methacrylate hydrochloride) to be used as the support by photo-initiated emulsion polymerization.²² The chloride counterions on the surface of the support were subsequently exchanged by AuCl₄⁻ ions, which were in situ reduced to form immobilized AuNPs. However, the capping ligands on the supports for immobilizing the nanoparticles will exert surface-blocking effect, which decrease the catalytic activity of AuNPs.²⁵ Recently, Hahn et al. found that AuNPs loaded on graphene oxide (GO) exhibited better catalytic performance than GO or free AuNPs in the reduction of o-nitroaniline by NaBH₄.²⁶ Zhang et al. also reported that when AuNPs were complexed with reduced graphene oxide (rGO), the rGO/AuNPs composite had much more better catalytic activities, even than free AuNPs and AuNPs/GO composite.²⁷ But the recycle of GO-based nanometal composites still encounters difficulties because of their small size and good hydrophilicity.

On the other hand, the structure of the supports themselves also plays an important role on the loading of metal nanoparticles. Porous structured microspheres may be one of the most promising candidates for the synthesis of stable, recyclable, and high-performance composite catalyst because they possess larger specific surface area compared with the solid materials, and the pores can prevent the aggregation of nanoparticles, reduce the loss of the attached nanocatalyst, and provide good accessibility for reactants.^17,28

We herein describe a simple synthesis method to prepare recyclable porous sulfonated polystyrene (SPS)/rGO composite microspheres loaded with AuNPs (AuNP@SPS/rGO) through γ-ray radiation on a one-pot system, i.e. an aqueous solution containing cagelike porous SPS microspheres, GO, and HAuCl₄. Under the effect of γ-ray radiation, the reduction of GO and the formation of AuNPs could be done simultaneously, as well as the loading of rGO and AuNPs on cagelike porous SPS microsphere skeleton. The prepared novel porous AuNP@SPS/rGO composite microspheres showed ultra-high catalytic efficiency and excellent recyclability in the reduction of o-nitroaniline by NaBH₄, which indicates that the porous polymer/rGO composite has a potential application as a recyclable and efficient support for metallic nanocatalysts with ultrahigh catalytic activity.

Experimental

Materials

Styrene (St, CR) was distilled under reduced pressure to remove the inhibitor. 2,2′-Azobis(isobutyronitrile) (AIBN, CR) was purified by recrystallization in ethanol. Polyvinylpyrrolidone (PVP, K30, GR), o-nitroaniline (98%), N-hydroxysuccinimide (NHS, BR), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC, 99%), sodium borohydride (96%), and analytical reagents including chloroauric acid tetrahydrate, potassium permanganate, sodium hydroxide, hydrochloric acid (37%), hydrogen peroxide (30%), sulfuric acid (98%), ethanediamine (EDA), tetrahydrofuran (THF), ethanol, and isopropanol, were used as received. All of the above reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai. Flake graphite (1000 mesh) was purchased from Beijing Jixingsheng'an Industry & Trade Co., Ltd. Deionized water was used in all experiments.

Synthesis of cagelike porous sulfonated polystyrene (SPS) microspheres

The synthesis of cagelike porous SPS microspheres was described in the ESI (Part I)† according to our previous report.²⁹ Briefly, polystyrene (PS) microspheres were firstly prepared through the dispersion polymerization of St in the mixture of ethanol and water with PVP as the stabilizer. After the PS microspheres were sulfonated in concentrated sulfuric acid for a certain time, the obtained SPS microspheres were dispersed ultrasonically in a mixture of heptane, ethanol and water. The system was kept in an oil bath of 70 °C for 5 h, and finally cagelike SPS microspheres were fabricated.

Preparation of amino-functionalized graphene oxide (GO-NH₂)

(i) GO was synthesized according to the modified Hummers method.³⁰ Graphite (1.0 g) was dispersed in sulfuric acid (98%, 70.0 mL) in an ice-water bath. Potassium permanganate (10.0 g) was then added into the mixture below 20 °C. After being magnetically stirred for 1 h, the system was heated to 40 °C and kept for 1 h. Then, the mixture was cooled down to 20 °C in an ice-water bath, followed by adding water (80.0 mL) dropwise to ensure that the system temperature was below 20 °C. After that, the mixture was again heated to 95 °C, and kept for 1 h. Finally, H₂O₂ was added into the mixture until the color turned to orange. The orange suspension was dialyzed in deionized water. The dialysate was refreshed every 12 h until it became nearly neutral. The product, GO, was collected after the dispersion was dried at 50 °C in an oven.

(ii) The as-prepared GO (50.0 mg) was dispersed ultrasonically in water (50.0 mL). EDC (119.4 mg), NHS (72.4 mg), and EDA (415 μL) were added in order at every 15 min into the GO suspension in an ice-water bath. The reaction lasted for 24 h. The product, termed as GO-NH₂, was purified by dialysis in water. The dialysate was refreshed every 12 h for 3 d. The product was collected after being dried at 50 °C in an oven.

One-pot synthesis of cagelike porous AuNP@SPS composite microspheres under γ-ray radiation

Cagelike SPS microspheres (20.0 mg) were dispersed in the mixture of deionized water (8.5 mL), HAuCl₄ aqueous solution (1.0 mL, 5 mmol L⁻¹), and isopropanol (0.5 mL). The pH was adjusted to 3, 6, and 12 with HCl (1 mol L⁻¹) or NaOH solution (1 mol L⁻¹), respectively. The reduction of Au(III) was carried out in a γ-ray radiation field of ⁶⁰Co (2.03 × 10¹⁵ Bq, located in the University of Science and Technology of China) at a dose rate of 62 Gy min⁻¹ and a total absorbed dose of 55 kGy. The product was purified by alternative centrifugation (4500 rpm, 5 min) and redispersion in deionized water thrice. The centrifugate was also filtered by a 450 nm membrane filter to obtain the free AuNPs produced simultaneously in the solution.

As a comparison, AuNPs loaded on pure solid SPS microspheres had also been prepared by the same method.

One-pot synthesis of porous AuNP@SPS/rGO composite microspheres under γ-ray radiation

The as-prepared cagelike SPS microspheres (20.0 mg) and GO-NH₂ (2.0 mg) were dispersed in a mixture of deionized water (8.5 mL), HAuCl₄ aqueous solution (1.0 mL, 5 mmol L⁻¹), and isopropanol (0.5 mL). The pH of the solution was adjusted to 12 by NaOH solution (1 mol L⁻¹). The system was exposed in the γ-ray radiation field of ⁶⁰Co (2.03 × 10¹⁵ Bq, located in University of Science and Technology of China) at an absorbed dose rate of 62 Gy min⁻¹ and a total absorbed dose of 55 kGy. Then, the pH of the system was adjusted to 4 by HCl (1 mol L⁻¹). The product was collected by centrifugation (4500 rpm, 5 min) and purified by filtration through 450 nm membrane filter and washing with the aqueous solution of HCl (pH = 4) alternatively thrice.

Characterization

The morphology of the products was investigated by field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F, 5 kV) and transmission electron microscopy (TEM, H-7650, 100 kV). The nitrogen adsorption–desorption isotherms were measured at 77.3 K on a Micromeritics Tristar II 3020M V1.03 after the samples were outgassed at 60 °C for 5 h. The specific surface area was analyzed by Brunauer–Emmett–Teller (BET) method. Thermal gravimetric analysis (TGA) was conducted on a TGAQ5000IR analyzer in an air atmosphere at a heating rate of 10 °C min⁻¹ in a temperature range from 30 °C to 700 °C. X-ray photoelectron spectroscopy (XPS) was carried out on Thermo ESCALAB 250 using monochromatic Al Kα radiation. Raman spectra were obtained by Raman spectrometer (LABRAM-HR) with the excitation laser irradiation of 325 nm. Fourier transform infrared spectra (FTIR) were conducted on a Bruker Tensor 27 FTIR spectrometer. Zeta potentials of cagelike SPS microspheres and rGO in aqueous dispersion (0.1 mg mL⁻¹) were obtained respectively by a Malvern Zetasizer ZS90.

Investigation on the catalytic activity of AuNP@SPS/rGO composite microspheres

The catalytic activity of AuNP@SPS/rGO composite microspheres was evaluated based on a model reaction, i.e. the reduction of o-nitroaniline to 1,2-benzenediamine by NaBH₄ at room temperature. A certain amount of AuNP@SPS/rGO composite microspheres was dispersed in NaBH₄ solution (0.68 mol L⁻¹). The theoretical concentration of AuNPs was controlled to be 5 × 10⁻³ mg mL⁻¹. Put 1 mL of the above dispersion into an optical cell settled in UV-Vis spectrometer (Shimadzu UV-2600). The UV-Vis spectrum was recorded immediately after 1 mL of the aqueous solution of o-nitroaniline (1.67 mmol L⁻¹) was added in. For the study on the recyclability of the composite nanocatalyst, the reacted dispersion containing AuNP@SPS/rGO composite microspheres was acidized to pH = 4 with HCl (1 mol L⁻¹), and centrifuged (5000 rpm, 5 min) to collect the composite microspheres. The collected composite microspheres were then dispersed in NaBH₄ solution again to catalyze the same reaction according to the above same procedure.

Results and discussion

In situ synthesis and loading of AuNPs in cagelike porous SPS microspheres by γ-ray radiation

To in situ load AuNPs on a porous polymer support, a prerequisite is that the porous polymer microspheres can be dispersed uniformly in the aqueous solution of HAuCl₄. Here, cagelike porous SPS microspheres shown in Fig. 1-A1 and A2 were fabricated by a swelling-osmosis process of solid SPS microspheres in the ternary mixture composed of water, ethanol, and heptane, according to our previous work.²⁹ The diameter of the whole cagelike microspheres is 4.8 (±0.2) μm. The pore size on the surface ranges between 200 and 400 nm. A typical type II N₂ adsorption–desorption isotherms of the cagelike SPS microspheres according to BDDT classification³¹ is also shown in Fig. 1-A2 (inset). The BET surface area can be calculated to be 17.89 m² g⁻¹. There is nearly no change in the adsorbed quantity of nitrogen in the middle P/P₀ regime (0.3–0.7) and no hysteresis during the desorption process, which means no existence of mesopores. However, the rapid increase of the adsorption observed when P/P₀ is above 0.9 implies the presence of macropores.

Fig. 1 SEM and TEM images of cagelike SPS microspheres (A1 and A2), cagelike AuNP@SPS microspheres prepared at a condition of pH = 3 (B1 and B2), 6 (C1 and C2), and 12 (D1 and D2) in the presence of isopropanol. The red circles indicate typical AuNPs loaded in the cagelike SPS microspheres.

The mechanism of pore formation involves three processes: SPS seed microspheres swollen by heptane, water osmosis into SPS seed microspheres to form inner water phase, and water evaporation to form the pores. According to the mentioned pore formation mechanism, the strong hydrophilic –SO₃H groups should be distributed on the pore interfaces and the surface of the microspheres as proved by the XPS spectrum in Fig. S3 (ESI),† so that the cagelike porous SPS microspheres should have enough good hydrophilicity. When the cagelike SPS microspheres are dispersed in the aqueous solution of HAuCl₄, the Au precursor, i.e., AuCl₄⁻, can penetrate into the pores of cagelike SPS microspheres with water. Therefore, when the system was exposed under γ-ray radiation, nanoparticles can be observed to be formed and in situ loaded in the pores of the cagelike SPS microspheres at different pH, as shown in Fig. 1B–D. A typical XPS spectrum of the nanoparticle-loaded cagelike porous SPS is shown in Fig. 2A. The peaks assigned to Au(0) 4f verify that AuNPs were formed under γ-ray radiation. This should be attributed to the effect of the radiolysis of water. It is well-known that the main active species produced by the radiolysis of water are hydrated electrons (e_aq⁻), ˙H and ˙OH radicals.³² e_aq⁻ and ˙H have strong reducing ability (the reduction potentials of e_aq⁻ and ˙H are −2.87 V and −2.3 V respectively), which can be utilized to reduce metal ions to prepare metal nanoparticles.^32–36 The oxidative ˙OH radicals will hinder the reduction, but it can be eliminated by the addition of radical scavengers, such as isopropanol. The formation mechanism of AuNPs through the radiation reduction of Au(III) has been investigated extensively.^34–36 Firstly, Au(III) is reduced to Au(II), then Au(II) readily disproportionates into Au(III) and Au(I). Finally, Au(I) disproportionates slowly to Au(0). Au(0) can in turn catalyze the disproportionation of Au(I), which was crucial for the growth of AuNPs.

Fig. 2 A typical XPS spectrum (A) and TGA curves (B) of cagelike AuNP@SPS microspheres. The inset in (A) is the corresponding high resolution XPS spectra of Au(0) 4f.

Generally, the size of AuNPs synthesized by γ-ray radiation depends on the pH of the HAuCl₄ solution.³⁷ Here, the size of AuNPs formed in the presence of cagelike SPS microspheres had been investigated.

In an acid solution (pH = 3), free Au particles with a size over 300 nm were basically formed, as shown in Fig. 1-B2. Nearly no AuNPs can be observed to be loaded on the cagelike porous microspheres. But when the pH of HAuCl₄ solution was changed to 6, no large-sized aggregates appeared in the system. Instead, AuNPs with a size of 40 ± 22 nm are formed and loaded on the cagelike SPS microspheres (Fig. 1-C2). The weight content of AuNPs is 0.95%, determined by TGA analysis, as shown in Fig. 2B. In an alkaline solution (pH = 12), it can be found that AuNPs with a much smaller size of 11 ± 3 nm are uniformly loaded in cagelike SPS microspheres (Fig. 1-D2), which is termed as AuNP@SPS. The weight content of AuNPs is 1.58% (Fig. 2B), a little higher than that prepared at pH = 6. The results indicate that alkaline condition favors to synthesize smaller-sized AuNPs and achieve higher loading capacity of AuNPs on the cagelike SPS microspheres.

As a comparison, the loading of AuNPs on the primary solid seed SPS microspheres had also been investigated, as shown in Fig. S4 (ESI).† It is clear that little amount of AuNPs is loaded on the solid microspheres whatever at a pH of 6 or 12. The weight content of the loaded AuNPs is below 0.2%. Evidently, the porous structure of the cagelike SPS microspheres can improve the load capacity of AuNPs greatly.

One-step synthesis of porous AuNP@SPS/rGO composite microspheres by γ-ray radiation

In order to achieve much higher load capacity and catalytic efficiency of AuNPs in cagelike SPS microspheres, rGO was herein introduced in complex with cagelike SPS microspheres due to its excellent electric conductivity and highly efficient adsorption ability on metal nanoparticles.²⁷ The strategy to prepare porous AuNP@SPS/rGO composite microspheres is illustrated in Scheme 1.

Scheme 1 Preparation process of porous AuNP@SPS/rGO composite microspheres.

The prepared GO were sheetlike with ∼500 nm square (Fig. S5A, ESI†). The FTIR spectrum of GO is shown in Fig. S5B (ESI†). The characteristic peaks assigned to –C=O (1736 cm⁻¹), –C(=O)–O (1224 cm⁻¹), and –OH (3500–3300 cm⁻¹) indicate the existence of oxygen-containing groups on GO,³⁸ which makes GO have a negative zeta potential (−41.4 ± 6.19 mV) in an alkali solution, and repel to the negatively charged cagelike SPS microspheres. Thus –NH₂ groups were introduced on the surface of GO through the amidation reaction between EDA and the –COOH groups, which can be activated by the widely used EDC/NHS system.³⁹ The XPS spectrum of GO-NH₂ in Fig. 3 clearly shows the signal of N 1s compared with that of the original GO, indicating that amine groups have been successfully introduced on GO sheets.

Fig. 3 XPS spectra of GO, GO-NH₂, AuNP@SPS/rGO composite microspheres, and AuNP@SPS/rGO extracted by THF. The insets are the corresponding high resolution XPS spectra of Au 4f and C 1s.

GO-NH₂ and cagelike porous SPS microspheres can disperse in the aqueous solution of HAuCl₄ homogeneously. After the pH of the dispersion was adjusted to 12, the system was exposed under γ-ray radiation for a certain time to achieve the reduction of GO and in situ synthesis of AuNPs on cagelike SPS microspheres. The zeta potentials of the produced rGO and cagelike SPS microspheres are −39.5 ± 8.67 and −26.6 ± 0.8 mV respectively. Both of them can be stably dispersed in water so that the combination of rGO and AuNPs-loaded SPS microspheres won't occur. However, if the pH of the dispersion was adjusted to 4, the zeta potential of rGO turned to 24.0 ± 4.10 mV due to the protonation of –NH₂, while the zeta potential of cagelike SPS microspheres are still negative, i.e. −24.3 ± 0.4 mV respectively. Thus, rGO and AuNPs-loaded cagelike SPS microspheres will be in complex with each other spontaneously through the electrostatic interaction, resulting in the formation of neutralized AuNP@SPS/rGO composite microspheres. The formed AuNP@SPS/rGO composite microspheres are unstable in water due to the neutralization effect, and finally spontaneously precipitate from the dispersion. The Raman spectrum of the formed AuNP@SPS/rGO composite microspheres is shown in Fig. 4A, compared with that of GO-NH₂ before γ-ray radiation. Both of them exhibit two intense peaks, i.e., the D band (1350 cm⁻¹, related to the vibration of carbon atoms reflecting the disorder of the structure) and the G-band (1594 cm⁻¹, associated with the in-plane vibration of sp²-bonded carbon domains).^40,41 The ratio of I_D/I_G, is considered as an indicator of the internal structure defects, increases from 0.93 for GO-NH₂ to 0.99 for AuNP@SPS/rGO composite microspheres, indicating the reduction of GO under γ-ray radiation.⁴² The XPS spectrum of the formed AuNP@SPS/rGO composite microspheres shown in Fig. 3 verifies the formation of AuNPs since the signals of Au(0) can be detected. In order to investigate the variation in the chemical components of GO after γ-ray radiation, AuNP@SPS/rGO composite microspheres were extracted with THF to remove the SPS microsphere skeletons. The XPS spectrum of the extracted product is also displayed in Fig. 3. It is noted that the relative intensity of the peak assigned to C–C (non-oxygenated graphite carbon) to C–O (alkanol and epoxide) and C=O (carbonyl and carboxylate) bonds of the THF-extracted AuNP@SPS/rGO sample is obviously higher than those of GO and GO-NH₂, indicating the loss of the oxygen-containing groups from GO-NH₂ after γ-ray radiation. The result gives an indirect evidence to the reduction of GO under γ-ray radiation. Another notable result is that the signals of Au(0) also can be detected on the XPS spectrum of the THF-extracted AuNP@SPS/rGO sample. This means that a part of AuNPs are also anchored on the rGO sheets so that they cannot be removed during the extraction of cagelike SPS microspheres by THF.

Fig. 4 Raman spectra of GO-NH₂ and porous AuNP@SPS/rGO composite microspheres (A) and TGA curve of porous AuNP@SPS/rGO composite microspheres (B).

The morphology of AuNP@SPS/rGO composite microspheres is shown in Fig. 5. It is seen in Fig. 5A that the composite microspheres still remains the porous skeleton of the cagelike SPS microspheres, although the surfaces of the composite microspheres become wrinkled, caused by the enwrapping of the flexible rGO sheets as clearly displayed in the magnified TEM image of Fig. 5B. At the same time, it can be found that AuNPs have also been effectively loaded on rGO sheets, which is in accord with the above mentioned XPS result. This should be attributed to the affinity between gold atoms and the defects as well as the amide groups on rGO sheets.^43–46 The inset magnified TEM image in Fig. 5B exhibits that the size of AuNPs attached on rGO sheets with a size of 12 ± 2 nm. The weight content of the loaded AuNPs on AuNP@SPS/rGO composite microspheres measured by TGA (Fig. 4B) is 2.68%, further higher than that loaded on pure cagelike SPS microspheres.

Fig. 5 SEM (A) and TEM (B) images of porous AuNP@SPS/rGO composite microspheres. The inset in (B) is the magnified TEM image of a part of the surface of the composite microsphere.

The preparation of the porous AuNP@SPS/rGO composite microspheres greatly depends on the amino-functionalized GO. As a comparison, the aqueous dispersion containing cagelike SPS microspheres, GO, and HAuCl₄ was also irradiated by γ-ray under the same conditions. The morphology of the product was shown in Fig. S6 (ESI†). Only cagelike SPS microspheres with smooth surface and individual AuNPs-loaded non-aminated rGO sheets can be observed, indicating that pure rGO cannot be combined with cagelike SPS microspheres. This should be attributed to the relative weaker electrostatic interaction between cagelike SPS microspheres and non-aminated rGO since the zeta potential of non-aminated rGO is only 3.89 ± 9.29 mV at pH = 4.

Catalytic activity of porous AuNP@SPS/rGO composite microspheres

The reduction of o-nitroaniline by NaBH₄ to 1,2-benzenediamine at room temperature is often used as the model reaction to investigate the catalytic activities of AuNPs.²⁷ The UV-Vis spectrum of o-nitroaniline solution measured at different time in the presence of porous AuNP@SPS/rGO composite microspheres is recorded in Fig. 6A. The absorbance at 415 nm was recorded to trace the reaction process.^33,47 As a comparison, the UV-Vis spectra of the reaction solution in the presence of free AuNPs and cagelike AuNP@SPS microspheres are also shown in Fig. 6B and C, respectively. In all experiments, the concentration of AuNPs were controlled to be 5 × 10⁻³ mg mL⁻¹ according to the weight content of AuNPs calculated from the TGA analysis. Obviously, the porous AuNP@SPS/rGO composite microspheres show particularly high catalytic performance on the reaction between o-nitroaniline and NaBH₄ since the reduction reaction is completed within only 15 s. While the same reaction would take 7 min in the presence of free AuNPs, and 24 min in the presence of cagelike AuNP@SPS. This similar high catalytic activity towards the reduction of o-nitroaniline had been reported when a relatively high concentration of rGO–AuNPs (3.3 × 10⁻² mg mL⁻¹) was used.²⁷ But here, only 5 × 10⁻³ mg mL⁻¹ of the composite microspheres was used in this work. The results indicate that loading AuNPs in cagelike SPS microspheres would affect the catalytic activity of AuNPs since the diffusion of the reactants and products will be limited by the porous structure of the supports. However, the introduction of rGO on the cagelike porous SPS microsphere support results in a dramatic enhancement in the catalytic activity of the loaded AuNPs. It is well-known that metallic nanoparticles can promote the decomposition of BH₄⁻, and transfer the produced reductive active species to aromatic nitro groups so as to speed the reduction of aromatic nitro compounds.^48,49 When conductive materials are used as the supports of metal nanoparticles, electron transfer process can be further promoted.⁵⁰ rGO has been proved to have good conductive property.^38,41 Therefore, the combination of rGO and AuNPs will alter the work function of the rGO/AuNPs composites,⁵¹ which enhances the chemisorption and dissociation of BH₄⁻ ions on the surface of AuNPs and improves the catalytic performance. The enhancement in the catalytic activity caused by electronic effect due to the charge transfer between AuNPs and rGO²⁵ seems to be much stronger than the spatial limitation induced by the macropores structure. On the other hand, it was reported that the carbon atoms at the zigzag edges of rGO might weaken the N–O bond of the adsorbed o-nitroaniline,^26,52,53 which may further enhance the catalytic performance accordingly.

Fig. 6 Time-dependent UV-Vis spectra of the aqueous solutions of o-nitroaniline and NaBH₄ in the presence of porous AuNP@SPS/rGO (A), free AuNPs (B), and cagelike AuNP@SPS microspheres (C) prepared from the solution with pH = 12.

Furthermore, the porous AuNP@SPS/rGO composite microspheres can be successfully recycled simply by adjusting the system to be acidic and then centrifugation. The recycled AuNP@SPS/rGO composite microspheres still have the excellent catalytic performance. As shown in Fig. 7, a conversion over 88% within 15 s can be achieved for at least 4 repeated cycles, indicating the excellent recyclability and stability in catalytic activity of the prepared AuNP@SPS/rGO composite microspheres.

Fig. 7 The conversion of o-nitroaniline at t = 15 s in successive cycles catalyzed by recycled porous AuNP@SPS/rGO composite microspheres.

Conclusions

In this work, novel porous sulfonated polystyrene/reduced graphene oxide composite microspheres loaded with Au nanoparticles (AuNP@SPS/rGO) were synthesized through γ-ray radiation on a simple one-pot system, i.e. an aqueous solution containing cagelike porous SPS microspheres, amino-functionalized GO, and HAuCl₄. The XPS, Raman spectra, and TEM analysis verified that rGO and AuNP with a size of about 12 nm were formed in water simultaneously under γ-ray radiation, and in situ loaded on cagelike porous SPS microspheres. The weight content of the loaded AuNPs on AuNP@SPS/rGO composite microspheres can reach 2.68%. The prepared porous AuNP@SPS/rGO microspheres showed far better catalytic performance than conventional non-supported AuNP catalyst, which made the reduction of o-nitroaniline by NaBH₄ be completed nearly within 15 s. Furthermore, the prepared porous AuNP@SPS/rGO microspheres can be easily recycled from the reaction system simply by acidizing the system and centrifugation, with no loss of the catalytic activity. This work indicates that porous polymer/rGO composite microspheres can be expectedly used as the promising support to prepare recyclable metallic nanocatalysts with ultra-high catalytic activity.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51103143, 51173175, 51473152, 51573174, and 21377122), the Fundamental Research Funds for the Central Universities (WK2060200012, WK3450000001, and WK2140000009), and China Postdoctoral Science Foundation (2016M592069).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of GO, cagelike SPS, AuNP@SPS, AuNP@SPS/rGO composite microspheres. See DOI: 10.1039/c6ra11205f

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