1. Introduction

The construction of materials in different dimensions is always the significant topic in the field of material science. For example, the carbon-based spherical structures have attracted tremendous attention owing to the potential applications in adsorption, catalysis, energy conversion and storage, etc. ^[1]. In recent years, the fabrication of composite microspheres involving carbon nanotubes (CNTs) has also been widely studied, aiming to develop novel hierarchical composite materials with fascinating properties ^[2]. In general, the layer-by-layer (LbL) assembly, an efficient technique in fabricating multilayer films with controllable thickness and components ^[3], is one of the most commonly used methods in the preparation of CNTs composite spheres with core-shell structures. In a typical process, CNTs modified with carboxylic groups and the corresponding polyelectrolyte (PE) are alternately deposited on the colloidal templates through the electrostatic interactions. The thickness of the CNTs/PE multilayer shell on the template could be controlled by the number of the LbL cycles. By etching or dissolution of the template core, hollow CNmicrospheres or microcapsules could be obtained ^[4-9].

So far, the fabrication of CNT-based core-shell structures or capsules has got considerable progress, whereas the applications of such CNTs composites were barely investigated. Hao et al. reported the electrochemical behaviors of CNT-embedded microcapsules, indicating the potentials in the fields of biosensors and catalysis ^[8]. Actually, the introduction of functional components in these core-shell composites might afford the CNTs microspheres new applications. For example, electrode modified with spherical complexes incorporating metal nanoparticles and CNTs composite microspheres could exhibit satisfactory electrocatalytic performance ^[10]. In this letter, LbL assembly of multi-walled carbon nanotubes (MWNTs) and branched polyethyleneimine (PEI) is carried on the spherical polystyrene (PS) templates to fabricate (PEI/MWNTs)/PS composite microspheres. Thereafter, gold nanoparticles (GNPs) are deposited on the core-shell microspheres to prepare novel functional spherical GNP(PEI/MWNTs)PS complexes, which show good performance in the catalytic reduction of 4-nitrophenol.

2. Experimental

The pristine MWNTs (CVD method) were oxidized in nitricsulfuric acid with the aid of sonication for 6 h to attach carboxylic acid groups on the nanotubes. The monodisperse polystyrene (PS) microsphere templates, with the diameter of about 4 mm, were prepared by a dispersion polymerization method ^[7]. In a typical LbL assembly of PEI/MWNTs on the PS microspheres, 10 mg of PS microspheres in 20 mL of aqueous PEI solution (2.0 g/mL) was slightly stirred for 1 h before removing excess PEI by repeated centrifugation and water wash. Then a poly(sodium 4-styrenesulfonate) (PSS) layer and another PEI layer were deposited by using the same procedure. Thereafter the positively charged microspheres were stirred in 20 mL of aqueous MWNT suspension (1.0 g/mL) for 30 min before collected by centrifugation. After rinsed with deionized water, the microspheres covered by a layer of MWNTs were immersed in 20 mL of aqueous PEI solution (2.0 g/mL) for 30 min, followed by the same rinsing process as described above. By repeating the assembly steps for MWNTs and PEI alternately, the PEI/MWNTs multilayers were deposited on the PS microspheres.

The loading of GNPs on the microspheres was carried out as follows. 10 mg of (PEI/MWNTs)₃PS microspheres was dispersed in 20 mL of PEI solution (2.0 g/mL), and then 0.3 mL of 30 mmol/L HAuCl₄ solution was added into the mixture before stirred at 70 ℃ for 2 h. Finally, the resulting dispersion was treated through centrifugation and water wash for three times, which yielded GNP(PEI/MWNTs)₃PS composite microspheres.

To test the catalytic activity of the composites, 1.5 mL of aqueous 4-nitrophenol solution (0.87 mmol/L) and 10 mL of freshly prepared aqueous NaBH4 solution (0.015 mol/L) were mixed at room temperature. Then 3 mg of GNP(PEI/MWNTs)₃PS microspheres was added into the stirred solution to catalyze the transformation of 4-nitrophenol to 4-aminophenol, which was monitored by UV-vis absorption spectrophotometer in the reaction process.

3. Results and discussion

The LbL of MWNTs and PEI is realized through the electrostatic interaction between the carboxylic groups on the nanotubes and the amino groups of PEI, which is a typical process in the preparation of CNTs multilayer composites ^{[11, 12]}. To facilitate the subsequent assembly of MWNTs, the PS templates are pretreated with PEI and PSS, which would afford more negative charges on the colloids. Then the alternative adsorption of PEI and MWNTs can be proceeded to fabricate the core-shell structured (PEI/MWNTs)/PS microspheres. The classic LbL assembly procedure on the planar substrate is usually monitored by UV-vis spectrometry. While for the assembly on spherical template, the stepwise growth of the multilayer could be clearly visualized through SEM. As shown in Fig. 1, with the increase of the assembly number, more and more MWNTs are successfully deposited on the PS microspheres. When the first layer of MWNTs is assembled, the microspheres are almost fully covered by nanotubes. After three LbL assembly cycles, the overlapped and entangled nanotubes endow the resulting (PEI/MWNTs)₃PS microspheres with the rough and porous surfaces.

The incorporation of GNPs with the MWNTs composite microspheres is carried out through the in situ preparation of GNPs in the presence of (PEI/MWNTs)₃PS microspheres in a PEsolution. Herein, PEI acts as both reducing reagent and protecting reagent, avoiding the introduction of other reagent ^[13]. Moreover, PEI in the PEI/MWNTs shell would anchor the GNPs, enhancing the stability of the GNP(PEI/MWNTs)₃PS composite microspheres. Fig. 2a and b show the SEM images of the resulting sample at low and high magnification, respectively. Apparently, as seen in Fig. 2a, the deposition of GNPs seems to have little influence on the morphology of the microspheres. While from Fig. 2b, the location of GNPs could be clearly observed, which makes the surface of the nanotube much rougher. TEM characterization further exhibits the existence of GNPs on the microspheres. As shown in Fig. 2c, the GNPs are uniformly distributed on the nanotubes without obvious aggregation, with an average diameter of about 8 nm. The space fringes of the GNPs detected in the HRTEM image (Fig. 2d) is well matched with the (111) planes of Au. In addition, the SEM-EDX of GNP(PEI/MWNTs)₃PS (Fig. 2e) also clearly demonstrates the presence of Au peaks, which confirms the formation of the functional core-shell composites. From the EDX data, the mass ratio of MWNTs and GNP could be roughly estimated as about 1:1, which implies the high efficiency of GNP deposition.

The structure of GNP(PEI/MWNTs)₃PS microspheres is also characterized by X-ray diffraction (XRD), as well as PS microspheres and purified MWNTs for comparison. As shown in Fig. 3, PS microspheres present two characteristic peaks at 10.58 and 19.58, respectively ^[14]. The peak ofMWNTs at 26.2° is attributed to the (002) plane in the nanotube and the peak around 43° is assigned to reflections of impurities (iron phases) in the sample ^[15]. As for the patterns of GNP(PEI/MWNTs)₃PS, the absence of PS features indicates that the shell thickness is beyond the detection depth, while the peaks of MWNTs are obviously observed. The new peaks at 38.1° , 44.4° , 64.6° and 77.6° are ascribed to (111) , (200) , (220) and (311) reflections of crystalline Au, respectively ^[15].

It is well known that noble metal nanoparticles, such as Ag, Au, Pt and Pd, play critical roles in heterogeneous catalysis. However, such particles with the size of several nm, which provide highly active centers for the efficient catalysis, are apt to undergo aggregation under the reaction conditions, leading to decrease or loss of catalytic activity. In this case, the combination of nanoparticles and certain supports would solve this problem. In general, the supports with porous structure and high surface area could anchor the nanoparticles uniformly on the surface, which facilitates the catalytic activity of the catalyst without aggregation. Moreover, the catalyst recovery is easily performed through simple filtering or centrifuging ^[16]. Owing to their outstanding properties, CNTs are an ideal support for the metal nanoparticles ^[17]. Herein, the MWNTs-covered PS microspheres as the supports of GNP might present satisfactory catalytic behaviors. The catalytic reduction of 4-nitrophenol to 4-aminophenol with NaBH4, a representative reaction to evaluate the catalytic activity of supported GNPs ^[18-20], is used to test the performances of GNP(PEI/MWNTs)₃PS. The UV-vis spectra could clearly exhibit the reduction process of 4-nitrophenol to 4-aminophenol because of their feature absorption at 400 nm and 300 nm, respectively. Before the addition of the catalyst, the yellow-green mixture solution of 4-nitrophenol and NaBH4 shows a strong peak at 400 nm, assigned to the nitro group ^[21], whose intensity would not change without catalyst. Once the catalyst is introduced, the reduction reaction is activated and the peak at 400 nm begins to decrease, while the new peak at 300 nm appears and grows, attributed to the emergence of new compound 4-aminophenol. The whole reaction evolution procedure recorded by UV-vis spectra is illustrated in Fig. 4a, which clearly demonstrates the decrease of the peak at 400 nm, as well as the development of the peak at 300 nm, indicating the gradual transforming of 4-nitrophenol to 4-aminophenol. When the reaction finishes after 2700 s, the final solution turns colorless and the peak at 400 nm almost disappears, suggesting the complete reduction of the initial nitro compound. Fig. 4b shows the dependence of lnA (A is the absorbance value at 400 nm from Fig. 4a) on the reduction time, which could be well fitted as a linear correlation, revealing that the reduction reaction obeys the pseudo-first-order kinetics. The rate constant (K) obtained from the slope of the fitting straight line is 1.45 × 10^-3 s^-1. The ratio of constant K over the catalyst weight is 0.48 s^-1 g-1, similar to the reported ratio of spongy GNPs ^[22]. However, the value is much lower than some GNPs in the literatures ^[23-25], mainly due to low weight proportion of GNP(PEI/MWNTs) shell in the whole microsphere (less than 20% from TGA result). The morphology of the composite microspheres after catalytic reaction is investigated by SEM, which is shown in Fig. 4c. Compared with the microspheres before catalysis (Fig. 2b), the recycled GNP(PEI/MWNTs)₃PS presents a similar surface morphology, implying the good recyclability of the catalyst. To further improve the catalytic reactivity of this composite, the PS template with smaller size or more LbL cycles could be considered in the future work

4. Conclusion

In summary, we have demonstrated that LbL assembly of PEand MWNTs through electrostatic interaction could be carried out on spherical PS microspheres. Subsequent deposition of GNPs on the core-shell composites could produce functional GNP(PEI/ MWNTs)PS microspheres, which show satisfactory catalytic performance in the reduction reaction of 4-nitrophenol. Such hierarchical micro-structures with multi-functional components are expected to have potentials in the fields of adsorption, catalysis, sensor and battery electrode.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21173266 and 21473250) and the Fundamental Research Funds for the Central Universities, the Research Funds of Renmin University of China (Nos. 11XNJ021 and 15XNLQ04) .