Interfacial synthesis of lollipop-like Au–polyaniline nanocomposites for catalytic applications

Abstract

Here we report a facile synthesis of lollipop-like Au–polyaniline (Au–PANI) nanocomposites through an interfacial polymerization method, with a lower-density oil phase (toluene) than the water phase. Simultaneous polymerization of aniline and reduction of AuCl₄⁻ ions lead to the formation of Au–PANI nanocomposites, and the action of gravity results in the final lollipop-like structure. The as-prepared Au–PANI nanocomposites show high catalytic activity for the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) by NaBH₄, which are also promising platforms for monitoring the plasmon-driven catalytic conversion of 4-aminothiophenol (4ATP) into 4,4′-dimercaptoazobenzene (DMAB). We believe such metal–polymer nanocomposites will be appealing for optical, plasmonic, and catalytic applications.

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Introduction

Noble metal nanoparticles (MNPs) have drawn much attention during the past decades, as they have high potential for various applications, such as catalysts, chemical and biochemical devices.^1,2 It is considered that the size and morphology of MNPs have a huge effect on their properties, and thus different methods have been developed to control size and morphology of them.^3,4 Among various approaches, fabrication of noble MNPs on polymer substrates is the most common, for its low cost, high stability and good performance in different applications.⁵ However, it still remains a challenging work to control the size of MNPs and morphology of metal-polymer nanostructures, as well as integration of different properties in a single nanoreactor.

Conducting polymers, with high electrical conductivity and tunable structure, are widely used in different chemical or biochemical fields. Polyaniline, comparatively cheap and easy to prepare, is one of the most promoted conducting polymers,^6,7 which has been greatly applied in metal-conducting polymer nanocomposites. For example, Pt–PANI nanocomposites, with different nanostructures, have been used as electrocatalyst in fuel cells,^8–11 highly sensitive sensors and biomedical applications,^12,13 while Au–PANI nanocomposites have been widely used for catalysis.^14–18 Recently, several methods to fabricate Au–PANI nanostructures have been explored. Liu et al. reported a scalable and continuous preparation route for fabrication of Au@nanosphere polymer composites through flash nanoprecipitation (FNP), which show high catalytic ability and stability for the reduction of 4-nitrophenol (4-NP).¹⁹ Wang et al. successfully synthesized Au/polyaniline/halloysite nanotubes (AuNPs/PANI/HNTs) through the combination of in situ polymerization of aniline and the facile reduction process of Au ions, and found high electrochemical performance as sensors to detect hydrogen peroxide.²⁰ Moreover, Bao et al. successfully decorated AuNPs on the surfaces of the as-prepared PANI nanofibers to achieve Au–PANI hybrid nanostructure with the assistance of thioglycolic acid (TA), which shows excellent degradation efficiency for the reduction of 4-NP and methyl blue (MB) by NaBH₄.²¹

It is well known that a metal ion having a higher reduction potential than that of a conducting polymer can be directly reduced by the conducting polymer to form MNPs.^22–25 Thus, in situ interfacial polymerization method has been applied to fabricate Au–PANI nanocomposites or AuNPs without additional oxidant or reducing agent. Huang et al. firstly developed an interfacial polymerization method to synthesize uniform PANI nanofibers.²⁶ In the past few years, we have focused on fabrication of noble MNPs and metal–polymer nanocomposites via the conducting polymer assisted route.^27,28 Au nanostructures synthesized on PANI membrane surfaces showed great potential in surface enhanced Raman spectroscopy (SERS) applications.²⁹ Au–PANI nanospheres synthesized by interfacial polymerization displayed high catalytic activity towards the reduction of rhodamine B (RhB) dye.³⁰ Generally, the diameter of AuNPs for efficient generation of SERS hot spots should be larger than 20 nm, while as a catalyst it is desired to be less than 10 nm, which makes it difficult to integrate SERS and catalytic applications.^31,32

In this paper, highly efficient catalytic Au–PANI nanostructures, allowing us to monitor plasmon-driven reactions by in situ SERS as well, have been synthesized rapidly and easily using an interfacial method, during which aniline is not only an organic monomer for polymerization but also a reductant, while chloroauric acid (HAuCl₄) acts as an oxidant and is reduced to form AuNPs. Different from previous interfacial polymerization method, here an oil phase (toluene) with a lower density than water has been adopted, which allows the formation of novel lollipop-like Au–PANI nanocomposites that bear bifunctions of catalyzing the reduction of 4-NP into 4-aminophenol (4-AP) by NaBH₄ and in situ SERS monitoring the plasmon-driven reaction of 4-aminothiophenol (4ATP) dimerizing into 4,4′-dimercaptoazobenzene (DMAB). We believe this novel structure of Au–PANI nanocomposites can work as a nanoreactor both for Au-based catalysis and SERS monitoring plasmon-driven reactions.

Experimental

Materials

N-Methyl-2-pyrrolidone (NMP, 99% Aldrich), aniline (after distillation), gold(III) chloride hydrate (HAuCl₄ 99.999% Aldrich), 4-nitrophenol (4-NP), 4-aminothiophenol (4-ATP) and ethanol (99% Aldrich).

Synthesis of Au–PANI nanocomposites

In a 20 mL vial, 1 mL of 10 mM HAuCl₄ was dissolved into 5 mL of H₂O as the water phase. Then, 5 mL of toluene were added to the vial to form a water/oil interface, where the oil phase locates at the upper layer of the system. After 5 min, 1 mL of aniline was added, the colour of the water phase turned to be yellowish, indicating that the interfacial polymerization occurred immediately. With the proceeding of the reaction, the water phase got dark, and black products could be seen at the bottom of the vial. After a controlled reaction time period, the Au–PANI nanocomposites were collected by centrifugation and washed with H₂O and ethanol for both 3 times. Finally, Au–PANI nanocomposites were dispersed in 1 mL of H₂O for catalytic study and 1 mL of ethanol for SERS monitoring of the plasmon-driven reactions. A control experiment, in which 0.1 mL or 5 mL of 10 mM HAuCl₄ were added, was carried out to study the influence of HAuCl₄ concentration on the morphology and size of the Au–PANI nanocomposites. The conversion rate of aniline was not calculated in our experiments, as the amount of aniline used in the reaction was excessive as compared to that of HAuCl₄.

Characterization

The morphology of prepared Au–PANI nanocomposites was observed by S-570 scanning electron microscopic (SEM), and transmission electron microscope (TEM) images were acquired by IVIS Lumina Series TEM system. The characterization of the crystallite structure were tested using a D/max-rB X-ray diffractometer. UV-vis spectra were collected on an ultraviolet and visible spectrophotometer (TU-1901) for catalytic measurement, and a confocal Raman system (Renishaw inVia Raman Microscope) was used for SERS monitoring of plasmon-driven reactions.

Catalysis

The reduction of 4-NP into 4-AP by NaBH₄ was chosen as the model reaction for studying the catalytic activity of Au–PANI nanocomposites. To study the catalytic activity, 5 mg of Au–PANI nanocomposites after drying from the solution was re-dispersed into 15 mL of 4-NP aqueous solution (1 × 10⁻⁴ M). Then 1 mL of freshly prepared NaBH₄ (0.1 M) was added. The concentration of 4-NP was analyzed by UV-vis spectra.

In situ SERS monitoring of plasmon-driven reactions

0.1 mL of prepared Au–PANI nanocomposites were added into 1 × 10⁻³ M of 4-ATP ethanol solution for about 2 h. Then the Au–PANI nanocomposites were re-dispersed into ethanol to make a diluted suspension, and one drop of the suspension was transferred onto a glass wafer. After air drying, the sample was subjected to continuous laser illumination and Raman spectra were collected at controlled time intervals.

Results and discussion

Au–PANI nanocomposites were prepared by an interfacial polymerization method, where HAuCl₄ solution was added into water, and toluene was selected as the oil phase because of its lower density than water, forming a water/oil phase that oil phase is above the water phase. As soon as aniline was added into the oil phase, the interface became yellowish, indicating the initiation of the polymerization of aniline. As the polymerization went on, the water phase turned black due to the formation of Au–PANI nanocomposites. Different from the spherical nanocomposites produced from an interfacial polymerization using CCl₄ (with a higher density than water) as the oil phase,^33,34 lollipop-like Au–PANI nanocomposites were obtained after a reaction time of 60 min (Fig. 1a). The spherical part has an average diameter of about 1 μm, and the rod part is 300–500 nm in diameter and at least 1 μm in length. It can be clearly seen that the surface of the lollipop-like structure is fully decorated with AuNPs, which can also be confirmed by the TEM image in Fig. 1b. It is believed that the AuNPs at the surface of the whole structure can limit the overgrowth of Au–PANI nanocomposites, so that uniform and size controllable Au–PANI nanocomposites are available. From the magnified TEM image (Fig. 1c) and size distribution diagram (Fig. 1d), one can find that the diameter of most AuNPs is less than 10 nm. Inset in Fig. 1c is a HR-TEM image of AuNPs on the surface of Au–PANI nanostructure. An interplanar spacing of 0.236 nm can be well indexed to the (111) plane of face-centered cubic (fcc) Au crystals.

Fig. 1 SEM (a), TEM (b and c) images and size distribution diagram (d) of the lollipop-like Au–PANI nanocomposites. Inset in c is an HR-TEM image of the AuNPs.

The lollipop structure is very unique in this study, where an oil phase with a lower density that water has been adopted. Without oil phase, the PANI–Au nanocomposites prepared just by mixing aniline aqueous solution with HAuCl₄ solution were either nanospheres or nanorods.³⁵ With CCl₄ as the oil phase, whose density is higher than that of water, the PANI–Au nanocomposites would be formed at the water/oil interface, and the structures were perfect microspheres.³⁰

The structure and composition of the Au–PANI nanocomposites were further explored by spectroscopic measurements. From the FT-IR spectrum of Au–PANI nanocomposites (Fig. 2a), one can find peaks at 1580 cm⁻¹ and 1498 cm⁻¹, which are related to the C=C stretching vibration of benzenoid and quinoid rings, respectively. The peak at 1296 cm⁻¹ was ascribed to the C–N stretching vibration with aromatic conjugation, and the peak at 1139 cm⁻¹ belongs to bending vibration of N=Q=N (where Q denotes the quinonoid ring).³⁶ Compared to the FT-IR spectrum of PANI, Au–PANI nanocomposites have much weakened characteristic peaks, resulted from the abundant existence of AuNPs on the surface of the whole structure. However, the absorption peaks of PANI can still be distinguished. UV-vis spectra (Fig. 2b) showed that the characteristic bands of pure PANI appear at 340 nm and 681 nm, attributed to the π–π* transition of benzenoid rings and polaron-π* transition, respectively.³⁶ AuNPs with similar size has a maximum absorption at 553 nm. With the co-existence of AuNPs and PANI, the absorption peak of Au–PANI nanocomposites was red-shifted to 570 nm owing to surface plasmon resonance (SPR) of small MNPs.³⁷ This confirms the interactions between AuNPs and PANI, which will bring excellent chemical or electrochemical properties to this nanocomposite.

Fig. 2 (a) FT-IR spectra of pure PANI and Au–PANI nanocomposites and (b) UV-vis absorption spectra of AuNPs, pure PANI and Au–PANI nanocomposites.

In order to understand the structure evolution of the lollipop-like Au–PANI nanocomposites, time-dependent SEM images were taken to study this interfacial polymerization process. At a reaction time of 5 min (Fig. 3a), one can see that these Au–PANI nanocomposites are spherical in morphology at this stage. They are heterogeneous with different size, from less than 100 nm to nearly 1 μm. At the beginning of the reaction, aniline and Au³⁺ ions encounter at the interface to form nuclei with a smaller mass density than water, so that they can float on the interface. With prolonged reaction time period, growth of the spherical structure is accompanied by the increase of mass density (with more AuNPs deposited). This will last for at least 5 min until Au–PANI nanospheres are large enough to drop away from the oil/water interface. Then it comes to a transition state, during which smaller nanospheres keep growing and some of them are heavy enough to leave the interface, hauling out a small “tail” to become a lollipop-like structure. SEM image shows that Au–PANI nanocomposites are a mixture of relatively large nanospheres and lollipop-like materials at a reaction time of 20 min. After 1 h, nanospheres have disappeared and large amount of lollipop-like Au–PANI nanocomposites have been synthesized (Fig. 3c). We believe the formation of the lollipop-like structure is greatly influenced by the gravity, where the “tail” is formed when the spherical structure is heavy enough to leave the interface. Also, the oil phase plays an essential role in the formation of such structures, as no lollipop-like structure was obtained by just mixing the HAuCl₄ with aniline or through an interfacial synthesis technique with CCl₄ as the oil phase.^30,35 Therefore, the lollipop-like structure would be formed by a synergetic effect of the gravity and the selected oil phase. XRD pattern (Fig. 3d) of this lollipop-like Au–PANI nanostructure shows diffraction peaks that can be well indexed to the (111), (200), (220), (311), and (222) crystal planes of face-centered cubic (fcc) Au crystal. EDX spectrum can also validate the existence of Au on the lollipop-like Au–PANI nanocomposites (see Fig. S1†).

Fig. 3 SEM images of Au–PANI nanocomposites at a reaction time of (a) 5 min, (b) 20 min, (c) 1 h, and XRD pattern (d) of the lollipop-like Au–PANI nanocomposites obtained at 1 h.

During our experimental work, some attempts have been done to have the structure of Au–PANI nanocomposites tunable. When more HAuCl₄ solution is added into water phase, it is excited to find that the structure of Au–PANI nanocomposites can be greatly changed. With 5 mL of 10 mM HAuCl₄ added into the water phase and at a reaction time of 1 h, one can only see a large quantity of nanospheres (Fig. 4a), which have a similar morphology to that obtained with 1 mL of 10 mM HAuCl₄ added at a reaction time of 5 min (Fig. 3a), but the minimum diameter of the Au–PANI nanocomposites is more than 800 nm. Interestingly, we cannot even find a single “tail” interconnected to the nanosphere part. TEM images show that surface of Au–PANI nanospheres are covered with branched nanorods, with a length from 10 to 20 nm and a width of about 10 nm (Fig. 4b and c). A statistical study shows that these Au nanorods are mainly 10–15 nm in length (Fig. 4d). One possible reason that lollipop-like structures are not produced is that more Au³⁺ ions in the water phase leads to large amount of nucleation, so that there is not enough aniline near the interface that can encounter with Au³⁺ ions to form the “tails”. One may wonder what if much less HAuCl₄ solution is used. In a control experiment, when 0.1 mL of 10 mM HAuCl₄ was added into the water phase for a reaction time of 1 h, we also found lollipop-like structures and the difference in the diameters of the spherical and rods parts was not that significant as seen previously (see Fig. S2†). This confirms that the growth of the “tail” should be under a condition with sufficient aniline monomers present in the oil phase.

Fig. 4 (a) SEM image, (b) low- and (c) high-magnification TEM images and (d) size distribution diagram of Au–PANI nanocomposites. The reaction time is 1 h, with 5 mL of 10 mM HAuCl₄ added into the water phase.

Catalytic properties of Au–PANI nanocomposites are studied by conducting the reduction of 4-NP in the presence of NaBH₄. Without the existence of catalyst, the reduction of 4-AP cannot occur even with a large excess of NaBH₄.³⁸ Also, PANI itself is not catalytically active for this reaction except for adsorption of 4-NP on its surface. Here, Au–PANI nanocomposites with different structures are studied, and the amount of the nanocomposites is controlled at 5 mg for the reactions. Typical UV-vis absorption spectra of the reaction process can be found in Fig. S3,† where the characteristic absorption of 4-NP at 400 nm decreases while the characteristic absorption of 4-AP at 230 nm increases appreciably as the reaction proceeds.³⁹ From Fig. 5, one can see that catalysts synthesized at different reaction time periods and conditions have distinguishable catalytic properties. Au–PANI nanocomposites obtained at 5 min reveal worse catalytic property than those obtained at a comparatively longer reaction time. The longer time the reaction lasts for, the better catalytic property can be obtained for Au–PANI nanocomposites. Notably, the lollipop-like nanostructure is most efficient and can finish the catalytic reaction in about 12 min. The size of most AuNPs at the surface of lollipop-like nanostructure is about 7 nm, making it highly effective in catalyzing the reduction of 4-NP by NaBH₄. In contrast, the spherical Au–PANI nanocomposites prepared with 5 mL of 10 mM HAuCl₄, with the diameter of AuNPs at the surface of about 10–15 nm, the remaining 4-NP is about 20% after 30 min (also see Fig. S4†). This confirms that AuNPs with size of about 7 nm have better catalytic properties than those with 10–15 nm, for the reason that the well crystallized AuNPs with smaller size would especially facilitate the electron transfer from NaBH₄ to 4-NP.^40,41 As the size of AuNPs increased, the surface area of active Au for catalysis will be reduced, and it would take more time to have 4-NP reduced.

Fig. 5 Reduction of 4-NP catalyzed by Au–PANI nanocomposites synthesized at different reaction conditions. The concentrations of 4-NP and NaBH₄ are kept constant, and mass of Au–PANI nanocomposites is controlled at 5 mg.

The dimerization reaction of 4-ATP to DMAB was chosen as a model plasmon-driven reaction for studying the catalytic property of Au–PANI nanocomposites. Again, PANI itself is not catalytically active for this reaction due to the absence of plasmonic hot electrons upon laser irradiation. Here, lollipop-like Au–PANI nanostructure was used as a single nanoreactor for SERS monitoring of plasmon-driving reactions, with the advantages that it can be located by optical microscope coupled on the Raman instrument, and more importantly, the reaction can be focused on a fixed position.^42,43 To have 4-ATP well adsorbed, as-prepared Au–PANI nanocomposites were added into 4-ATP ethanol solution for 2 h, and then dispersed on a glass wafer. After air drying, SERS spectra were collected to evaluate the reaction in this single nanoreactor under continuous 633 nm laser excitation (Fig. 6). The new β(C–H) band at 1141 cm⁻¹ is owing to the connection of two benzene rings, while the two peaks at 1390, 1435 cm⁻¹ are related to the N=N stretching of DMAB.⁴⁴ All these peaks occur immediately under laser excitation, which confirms the formation of DMAB. Optical photograph and SERS mapping of a single lollipop-like Au–PANI nanostructure are shown in Fig. S5.† One can easily find a single nanoreactor with a micron size. Moreover, most of the hot spots are concentrated on the body of lollipop-like nanostructure, which avoids the noise from surrounding substrate, further indicating that this lollipop-like Au–PANI nanostructure is suitable for SERS monitoring of plasmon-driven reactions. Au–PANI nanospheres synthesized with 5 mL of 10 mM HAuCl₄ were also used for SERS monitoring of plasmon-driven reactions. However, the reaction process of 4-ATP to DMAB is too fast to be recorded by Raman spectra, as the characteristic peaks of DMAB appeared immediately after laser excitation (Fig. S6†). This may be due to the light absorption of Au–PANI nanospheres closer to 633 nm as seen from the UV-vis spectrum (Fig. S7†). By using a 532 nm laser instead of 633 nm laser, we also found the reaction process of 4-ATP to DMAB on both substrates would be too fast to be captured.

Fig. 6 Time-dependent SERS spectra of 4-ATP dimerizing to DMAB under continuous 633 nm laser excitation with a laser power of 1.5 mW.

Conclusions

In conclusion, we have successfully synthesized Au–PANI nanocomposites with novel and tunable nanostructure through an interfacial polymerization method. Especially, an oil phase (toluene) with a lower density than water leads to a lollipop-like nanostructure due to the special interface structure and the influence of gravity. Catalytic study shows that the as-prepared lollipop-like Au–PANI nanocomposites are highly efficient for the reduction of 4-NP by NaBH₄. Moreover, this structure can also be a good nanoreactor for SERS monitoring of plasmon-driven reactions. We believe the Au–PANI nanocomposites will be also useful for other catalytic, sensing, and optical applications.

Acknowledgements

We thank the financial support from NSFC (No. 21471039, 21571043, 21671047), Fundamental Research Funds for the Central Universities (PIRS of HIT A201502 and HIT. BRETIII. 201223), China Postdoctoral Science Foundation (2014M560253), Postdoctoral Scientific Research Fund of Heilongjiang Province (LBH-Q14062, LBH-Z14076), Natural Science Foundation of Heilongjiang Province (B2015001), Open Project Program of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China (PEBM 201306), and Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201401).

Notes and references

1. S.-E. Bae, K.-J. Kim, Y.-K. Hwang and S. Huh, J. Colloid Interface Sci., 2015, 456, 93–99.
2. C. K. K. Choi, J. Li, K. Wei, Y. J. Xu, L. W. C. Ho, M. Zhu, K. K. W. To, C. H. J. Choi and L. Bian, J. Am. Chem. Soc., 2015, 137, 7337–7346.
3. M. Zhang, T. P. Otanicar, P. E. Phelan and L. L. Dai, Langmuir, 2015, 31, 13191–13200.
4. V. Kumar, V. Patil, A. Apte, N. Harale, P. Patil and S. Kulkarni, Langmuir, 2015, 31, 13247–13256.
5. P. Puthiaraj and K. Pitchumani, Green Chem., 2014, 16, 4223–4233.
6. J. Pecher and S. Mecking, Chem. Rev., 2010, 110, 6260–6279.
7. J. Huang and R. B. Kaner, Angew. Chem., Int. Ed., 2004, 116, 5941–5945.
8. L. Niu, Q. Li, F. Wei, S. Wu, P. Liu and X. Cao, J. Electroanal. Chem., 2005, 578, 331–337.
9. S. Guo, S. Dong and E. Wang, Small, 2009, 5, 1869–1876.
10. S. Peng, L. Li, H. Tan, M. Srinivasan, S. G. Mhaisalkar, S. Ramakrishna and Q. Yan, Electrochim. Acta, 2013, 105, 447–454.
11. P. Yang, J. Duan, D. Liu, Q. Tang and B. He, Electrochim. Acta, 2015, 173, 331–337.
12. W. Gao, S. Sattayasamitsathit, J. Orozco and J. Wang, J. Am. Chem. Soc., 2011, 133, 11862–11864.
13. D. Zhai, B. Liu, Y. Shi, L. Pan, Y. Wang, W. Li, R. Zhang and G. Yu, ACS Nano, 2013, 7, 3540–3546.
14. Y. Xia, T. Li, C. Ma, C. Gao and J. Chen, RSC Adv., 2014, 4, 205–216.
15. S. G. Song, C. Satheeshkumar, J. Park, J. Ahn, T. Premkumar, Y. Lee and C. Song, Macromolecules, 2014, 47, 6566–6571.
16. T. M. Abdel-Fattah and A. Wixtrom, ECS J. Solid State Sci. Technol., 2014, 3, 18–20.
17. T. Amaya, T. Isaji, M. Abe and T. Hirao, J. Inorg. Organomet. Polym., 2015, 25, 145–152.
18. J. Yu, W. Guo, M. Yang, Y. Luan, J. Tao and X. Zhang, Sci. China: Chem., 2014, 57, 1211–1217.
19. R. Liu, C. Sosa, Y.-W. Yeh, F. Qu, N. Yao, R. K. Prud'homme and R. D. Priestley, J. Mater. Chem. A, 2014, 2, 17286–17290.
20. P. Wang, M. Du, M. Zhang, H. Zhu, S. Bao, M. Zou and T. Yang, Chem. Eng. J., 2014, 248, 307–314.
21. S. Bao, M. Zhang, M. Du, H. Zhu and P. Wang, Soft Mater., 2014, 12, 179–184.
22. P. Xu, X. Han, B. Zhang, Y. Du and H. L. Wang, Chem. Soc. Rev., 2014, 43, 1349–1360.
23. E. T. Kang, Y. P. Ting and K. L. Tan, J. Appl. Polym. Sci., 1994, 53, 1539–1545.
24. W. S. Huang, M. Angelopoulos, J. R. White and J. M. Park, Mol. Cryst. Liq. Cryst., 1990, 189, 227–235.
25. Y. P. Ting, K. G. Neoh, E. T. Kang and K. L. Tan, J. Chem. Technol. Biotechnol., 1994, 59, 31–36.
26. J. Huang and R. B. Kaner, J. Am. Chem. Soc., 2004, 126, 851–855.
27. P. Xu, E. Akhadov, L. Wang and H. L. Wang, Chem. Commun., 2011, 47, 10764–10766.
28. L. Kang, X. Han, J. Chu, J. Xiong, X. He, H.-L. Wang and P. Xu, ChemCatChem, 2015, 7, 1004–1010.
29. S. Li, P. Xu, Z. Ren, B. Zhang, Y. Du, X. Han, N. H. Mack and H. L. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 49–54.
30. B. Zhang, B. Zhao, S. Huang, R. Zhang, P. Xu and H.-L. Wang, CrystEngComm, 2012, 14, 1542–1544.
31. P. Li, B. Ma, L. Yang and J. Liu, Chem. Commun., 2015, 51, 11394–11397.
32. V. Joseph, C. Engelbrekt, J. Zhang, U. Gernert, J. Ulstrup and J. Kneipp, Angew. Chem., Int. Ed., 2012, 51, 7592–7596.
33. F. G. Moore and G. L. Richmond, Acc. Chem. Res., 2008, 41, 739–748.
34. Q. Sun and Y. Deng, J. Am. Chem. Soc., 2005, 127, 8274–8275.
35. Y. F. Huang, Y. I. Park, C. Kuo, P. Xu, D. J. Williams, J. Wang, C. W. Lin and H. L. Wang, J. Phys. Chem. C, 2012, 116, 11272–11277.
36. A. A. Rakić, S. Trifunović and G. Ćirić-Marjanović, Synth. Met., 2014, 192, 56–65.
37. N. G. Bastús, J. Comenge and V. Puntes, Langmuir, 2011, 27, 11098–11105.
38. S. Xuan, Y.-X. J. Wang, J. C. Yu and K. C.-F. Leung, Langmuir, 2009, 25, 11835–11843.
39. J. Zhang, G. Chen, D. Guay, M. Chaker and D. Ma, Nanoscale, 2014, 6, 2125–2130.
40. L. Fensterbank and M. Malacria, Acc. Chem. Res., 2014, 47, 953–965.
41. M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114, 1827–1870.
42. P. Xu, L. Kang, N. H. Mack, K. S. Schanze, X. Han and H.-L. Wang, Sci. Rep., 2013, 3, 2997.
43. L. Kang, P. Xu, D. Chen, B. Zhang, Y. Du, X. Han, Q. Li and H.-L. Wang, J. Phys. Chem. C, 2013, 117, 10007–10012.
44. X. Tang, W. Cai, L. Yang and J. Liu, Nanoscale, 2014, 6, 8612–8616.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S7. See DOI: 10.1039/c6ra15446h

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