Freezing-mediated polymerization of Ag nanoparticle-embedded polyaniline belts with polyoxometalate as doping acid exhibiting UV-photosensitivity

Abstract

Ag nanoparticle-embedded polyaniline belts have been synthesized via the freezing polymerization of aniline assisted by polyoxometalate and silver nitrate. The material exhibits a significant UV photoresponse, which can be attributed to the efficient dissociation of the photo-induced hole trapped by dispersed silver nanoparticles and the photo-induced electron exported by the polyoxometalate.

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Converting light into electricity provides various potential applications in lasers, photodetectors, photoswitch microdevices, and solar cells, among others.^1–7 Active research and development is being conducted on the use of ultraviolet (UV) or visible (vis) light as a convenient, environmentally friendly energy.^8,9 Previously reported UV- or vis-photosensitive materials are mainly based on using semiconducting metal oxides (e.g., ZnO, TiO₂, WO₃, or MnO₂) or functionalizing these metal oxides using polymers or dyes with high UV or vis absorption for enhancing their optoelectronic ability.^8,10–14 The demand for inexpensive optoelectronic materials is the driving force behind the development of advanced devices. Considerable effort has been directed toward the discovery of more affordable and more efficient materials that can be used in photovoltaic devices.¹⁵

Conductive polymers by virtue of their highly delocalized cyclic-electron systems show intense absorption in the UV or vis region; thus, conductive polymers have become suitable candidates for low-cost photo-sensitive devices. Polyaniline (PANI), as a potential low-cost and chemically stable functional polymer, exhibits favorable optical absorption in the UV-vis region.^16–18 If PANIs could obtain good UV- or vis-photosensitivity by using a simple method, the complexity of the operation involved would be reduced, and the study on photoelectric apparatus would be greatly accelerated. Condensed processing is also potentially less energy-consuming. However, PANI shows only weak vis and UV photoresponse as a result of the strong and quick recombination ability of the photogenerated electron–hole pair despite PANI exhibiting high optical absorption in the UV-vis region.^19,20 Light absorption leads to the generation of excitons in a photoresponse device, the efficient dissociation of strongly bound excitons into free charge carriers and the subsequent charge transfer should be crucial factors in obtaining substantial energy conversion efficiencies. To obtain the UV photoresponse of PANI, the design and synthesis of PANI composites are the main developing trend, which will synergistically improve performance or produce multifunctional properties.²¹ Thus, inducing UV photoresponse, aided by other molecules, is a significant strategy for developing photoresponse materials based on PANI. Studies on the combination of PANIs with semiconductors have been reported; regardless, these studies focus on the modification of the UV or vis photoresponse of semiconductors rather than that of PANI.^20–23

In the present study, we reported on green and facile freezing polymerization^24,25 by introducing silver nanoparticles and classical polyoxometalate (POM)–H₄SiW₁₂O₄₀ (abbreviated as SiW₁₂) into the PANI system to enhance its UV photoresponse.²⁶ We successfully obtained SiW₁₂-doped PANI belts with dispersed Ag nanoparticles by a one-step process. POM anions are good electron acceptors and can promote the electron transfer rate of the semiconductor conduction band by trapping photogenerated electrons.²⁷ Therefore, the incorporation of POM into PANI can facilitate photogenerated electron transfer and retard exciton recombination. Meanwhile, the remaining photogenerated holes can weaken the injection barrier between the Ag nanoparticles and PANI, improving the mobility of the injected charge by dispersing Ag nanoparticles into the PANI.²⁸ To our knowledge, the role of Ag nanoparticles and SiW₁₂ in inducing the UV photoresponse of PANI has thus far been undetermined.

The belt-like PANI samples were synthesized by the freezing polymerization of aniline assisted by SiW₁₂ and AgNO₃. In a typical experiment, 0.1 mL of aniline was added into the 40.0 mL mixture of H₄SiW₁₂O₄₀ (0.5 g) and AgNO₃ (0.05–0.089 g) solution. Then 0.350 g of Fe(NO₃)₃ as a mild oxidant was dissolved in the above solution, and the resulting solution stirred another 1 min at room temperature to ensure complete mixing. After that, the mixed solution was allowed to stand in freezer at −18 °C for 20 days.²⁹ Finally, after the freezing ice thawed, the green precipitate was filtered and washed with distilled water and ethanol for several times in order to remove excess ions and monomers thoroughly, and then dried in vacuum at 50 °C for 24 h. The synthesis of only SiW₁₂-doped PANI is similar to the above but no AgNO₃ was added in the all process.

Images obtained by typical scanning electron microscopy (SEM) of Ag nanoparticle-embedded PANI samples reveal a belt-like morphology (Fig. 1A–C), with average width is ca. 500 nm and length is ca. 90 μm. As a unique class of transition metal oxide complexes with nucleophilic oxygen-enriched surfaces and multihydrogen protons, SiW₁₂ plays a key role in determining the formation of the belt-like morphology.³⁰ In the current study, aniline monomers can be easily captured by SiW₁₂. As an interfacial adhesion between aniline and water molecules, SiW₁₂ directs aniline molecules polymerizing along the ice template and forms PANI blocks with nanobelts.^30,31 The magnification image of PANI belts shows uniform Ag nanoparticles decorated in the PANI belts (Fig. 1C and D), which can also be proven in the following images obtained by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), selected-area electron diffraction (SAED), and energy-dispersive X-ray (EDX).

Fig. 1 SEM images of PANI synthesized by freezing polymerization of aniline with the assistance of SiW₁₂ and AgNO₃ in different magnification size ((A)–(D): from small to large).

As shown in the TEM images in Fig. 2A to C, many dark-spot Ag nanoparticles with an average size of 50 nm are dispersed in the PANI belts. The HRTEM image of PANI nanoboards shows clear lattice fringes (Fig. 2D), indicating that Ag nanoparticles in PANI exhibit single-crystal characteristics. The inter-layer space is about 0.2353 nm, corresponding to the [111] plane of Ag. EDX (Fig. 3), EDX elemental mapping images, XRD and SAED patterns (see Fig. S1–S3†) all confirm the presence of Ag in PANI belts. In Fig. 3 and S1,† EDX pattern and EDX elemental mapping images of the PANI belts show the presence of C, N, O, Si, W and Ag elements, and the weight percent of Ag in PANI belts samples is about 1.1 percent as shown in Fig. 3A inset image. The structure of the resulting PANI synthesized using freezing polymerization has also been confirmed by Fourier transform infrared (FT-IR) spectroscopy and discussed in detail in ESI (Fig. S4†). The conductivities of Ag nanoparticle-embedded PANI belts was measured with a standard four-probe technique, the conductivity of PANI samples is 0.0365 S cm⁻¹ and the pure PANI is less than 10⁻⁶ S cm⁻¹.

Fig. 2 TEM (A)–(C) and HRTEM images (D) of PANI synthesized by freezing polymerization of aniline with the assistance of SiW₁₂ and AgNO₃.

Fig. 3 SEM images (A) used during EDX elemental mapping process (inset: the weight percent of every element in PANI belts samples) and EDX pattern (B) of the Ag nanoparticles-embedded PANI belts.

The UV photoresponse characteristics of PANI samples were investigated. The photosensitive device containing the Ag nanoparticle-embedded PANI belts was achieved using a similar method in our early reports.¹⁴ Electrochemical experiments were all performed in a conventional three-electrode electrochemical cell. An ITO glass was separated with a nonconducting gap into two pieces, and each piece was adopted as an electrode. The obtained dried products were re-dispersed in ethanol, and then dropped on the surface of the gap between the two electrodes, and dried in air at room temperature. In order to prevent interference with visible light and to decrease the thermal effect, the conductivity of PANI belts was measured in a dark box at all times. The ultraviolet lamp (365 nm) served as light source was 8 W and the distance of the lamp-to-device was greater than 5 cm. A voltage of 1 V is applied across the ITO–ITO electrodes.

Fig. 4A shows the photocurrent of the device for the repetitive switching characteristic of 365 nm UV light illumination during light switching on/off in air. The photocurrent of the device containing the belt-like PANI unexpectedly increases and decreases in accordance with the UV light source on/off, which exhibits good photosensitivity. After scores of cycles, the photocurrent only slightly changes with the turning on/off of the UV light, exhibiting excellent reversibility and stability of the device. The aforementioned results prove that PANI can indeed be induced to show UV photosensitivity after it has been functionalized by SiW₁₂ and Ag nanoparticles. After extended illumination of the UV light, the photocurrent tends to finally become steady, revealing the largest increase in photocurrent, with about 1 μA (Fig. 4B). The inset in Fig. 4B is the schematic of the device containing the belt-like PANI samples. Due to PANI strongly absorb the light at 365 nm, the thermal effect on the photosensitivity of samples were examined (the detail experiment were described in ESI†), we tested the I-T curves of SiW₁₂-doped PANI/Ag belts under different temperature (30 °C and 40 °C) as shown in Fig. S5,† the current both have a little increase compared with UV photosensitivity current test. These results reveal that the thermal effect have a little influence on the photosensitivity of samples and can be ignored.

Fig. 4 Photocurrent curves of the device containing the PANI belts synthesized by freezing polymerization of aniline with the assistance of SiW₁₂ and AgNO₃. (A) Light switching on/off and (B) long time illumination by using an UV lamp (λ = 365 nm, 8 W). The inset is the schematic of the device containing the PANI belts.

In our system, the ice template plays an important role in controlling the dispersion of Ag nanoparticles and the PANI morphology. Compared with SiW₁₂-doped PANI/Ag composites obtained using room-temperature polymerization, Ag nanoparticles easily reunite with each other and become the large reunion body (see Fig. S6, ESI†). From the SiW₁₂-doped PANI/Ag obtained using the ice template method, we can see that the Ag nanoparticles can be easily dispersed in the PANI matrix, as observed from the SEM and TEM images in Fig. 1 and 2. However, the morphology of PANI products have some changes (see Fig. 5) as the amount of AgNO₃ increases to 0.089 g, with the PANI sheets becoming thicker and irregular, and the Ag particles tending to increase in size and beginning to agglomerate (Fig. 5C). Aided by ice templates, SiW₁₂ as a doping acid controls the formation of PANI belts. It can provide a “ligament” between the ice crystal and the aniline monomer by a strong bonding action, as revealed in previous reports, favoring aniline monomer polymerization and formation of a one-dimensional structure. The Ag nanoparticle-embedded sheet structure of PANI can facilitate the charge transport.^10,32 The result can be also proven in the following comparative UV photosensitivity test by vary AgNO₃ concentration (Fig. S7†). The photocurrent of PANI samples are obviously decreased (the largest increase in photocurrent is about 0.4 μA and 0.25 μA) compared with the result as we has mentioned above (1 μA). These result showed that aggregation of Ag nanoparticles don't help the mobility of the charge.

Fig. 5 SEM images of the PANI products synthesized by adding different quality of AgNO₃. (A) 0.045 g; (B) 0.067 g; (C) 0.089 g (aniline = 0.1 mL, aniline : oxidant : acid = 6 : 7 : 1, −18 °C; 20 days).

To explain the UV photosensitive mechanism of Ag nanoparticle-embedded PANI belts, the following discussion is divided into two parts for ease in understanding: the roles of Ag nanoparticles and the POM in the UV photoresponse of PANI. Diffusion of Ag nanoparticles embedded into the PANI matrix may create a large number of traps in the bulks of the PANI.^28,33 These traps in the PANI influence the mobility of the charge injected by Ag nanoparticles. Filling of these traps by charges from Ag nanoparticles creates an electric field near the junction, which adds to the charge injection barrier and limits the charge transport.^28,33 Meanwhile, SiW₁₂ anion in the PANI is an electron acceptor and can promote the electron transfer rate of the semiconductor conduction band by trapping photogenerated electrons.²⁷ Thus, SiW₁₂ as the acceptor in the current can make charge transfer from PANI to SiW₁₂.²⁷ Under UV illumination, although the recombination of the photogenerated electron–hole pairs in PANI are considerably strong, they can still dissociate and excite into free charge carriers within a short time. Under this condition, the remaining photogenerated holes become trapped by Ag nanoparticles and weaken the injection barrier between Ag nanoparticles and PANI, which enhances the mobility of the charge injected by Ag into PANI. Meanwhile, the photogenerated electrons become easily trapped and exported by SiW₁₂, as illustrated in Fig. 6. In this method, the cyclic electron transfer may enhance electron–hole separation and migration, rendering interfacial electron or hole transport more feasible by cooperating with Ag nanoparticle and SiW₁₂. We determined the photoresponse for PANI containing Ag nanoparticles or SiW₁₂ only under the same UV light condition (see Fig. S8, ESI†); the aforementioned observation was confirmed. However, we didn't observe obvious UV photocurrent for them. Evidently, the UV photocurrent of PANI belonged to neither Ag or SiW₁₂; instead, it was a synergic result of the Ag nanoparticles and SiW₁₂. Only under the synergic effect of SiW₁₂ and Ag nanoparticles does PANI exhibit excellent UV photoresponse. The synergy of Ag nanoparticles and SiW₁₂ facilitates the efficient dissociation of excited electron–hole pairs. The photo-induced hole trapped by Ag nanoparticles and the photo-induced electron exported by POM resulted in efficient dissociation of excited electron–hole pairs. The photocurrent for the PANI in the UV region is thus achieved and enhanced.

Fig. 6 Scheme mechanism of the UV photoinduced electron transfer in the belt-like PANI assisted with Ag nanoparticles and POM.

In conclusion, Ag nanoparticle-embedded PANI belts were successfully synthesized by freezing polymerization. The dispersion of Ag nanoparticles in PANI and PANI morphology were well-controlled and improved when aided with the ice template. The UV photocurrent response of the PANI belts was realized and enhanced. This effect can be attributed to the occurrence of photo-induced electron transfer from PANI to SiW₁₂, thereby increasing the efficient dissociation of excitons. Meanwhile, the diffusion of Ag nanoparticles into PANI could help strongly bound excitons into becoming free charge carriers and the subsequent charge transfer, which maintained the cyclic electron transfer. Given the synergic effect as a result of the combination of SiW₁₂ and Ag nanoparticles, the photocurrent of PANI in the UV region was achieved and enhanced, opening a new trend in the design and development of advanced photosensitive conductive polymers in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51303076, 21403102, and 51172102), Shandong Province Higher Educational Science and Technology Program (J15LA10), and Liaocheng University Funds for Young Scientists (31805).

Notes and references

1. X. Wang and Y. Li, J. Am. Chem. Soc., 2002, 124, 2880.
2. F. Davar, A. Majedi and A. Mirzaei, J. Am. Ceram. Soc., 2015, 98, 1739.
3. W. W. Xiong, J. Q. Chen, X. C. Wu and J. J. Zhu, J. Mater. Chem. C, 2015, 3, 1929.
4. Z. Wei and M. Wan, Adv. Mater., 2002, 14, 1314.
5. Y. R. Tao, X. C. Wu and W. W. Xiong, Small, 2014, 10, 4905.
6. A. Qurashi, K. S. Subrahmanyam and P. Kumar, J. Mater. Chem. C, 2015, 3, 11959.
7. A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S. F. Chichibu, S. Fube, Y. Segaua, H. Ohno, H. Koinuma and M. Kawasaki, Nat. Mater., 2005, 4, 42.
8. C. S. Lao, M. C. Park, Q. Kuang, Y. L. Deng, A. K. Sood, P. L. Polla and Z. L. Wang, J. Am. Chem. Soc., 2007, 129, 12096.
9. Y. Huang, G. Jahreis, C. Lucke, D. Wildemann and G. Fischer, J. Am. Chem. Soc., 2010, 132, 7578.
10. T. Y. Wei, P. H. Yeh, S. Y. Lu and Z. L. Wang, J. Am. Chem. Soc., 2009, 131, 17690.
11. C. G. Silva, R. Juarez, T. Marino, R. Molinari and H. Garcıa, J. Am. Chem. Soc., 2011, 133, 595.
12. X. Zhang, X. Lu, Y. Shen, J. Han, L. Yuan, L. Gong, Z. Xu, X. Bai, M. Wei, Y. Tong, Y. Gao, J. Chen, J. Zhou and Z. L. Wang, Chem. Commun., 2011, 47, 5804.
13. X. G. Li, H. Feng, M. R. Huang, G. L. Gu and M. G. Moloney, Anal. Chem., 2012, 84, 134.
14. S. X. Yang, H. Y. Yang, H. Y. Ma, S. Guo, F. Cao, J. Gong and Y. Deng, Chem. Commun., 2011, 47, 2619.
15. F. Yang, M. Shtein and S. R. Forrest, Nat. Mater., 2004, 4, 37.
16. A. J. Heeger, J. Phys. Chem. B, 2001, 105, 8475.
17. M. Lahav, C. Durkan, R. Gabai, E. Katz, I. Willner and M. E. Welland, Angew. Chem., Int. Ed., 2001, 40, 4095.
18. M. Wohlgenannt, K. Tandon, S. Mazumdar, S. Ramsesha and Z. V. Vardeny, Nature, 2001, 409, 494.
19. Y. J. Noh, S. S. Kim, T. W. Kim and S. I. Na, Sol. Energy Mater. Sol. Cells, 2014, 120, 226.
20. J. Gong, Y. H. Li and Y. L. Deng, Phys. Chem. Chem. Phys., 2010, 12, 14864.
21. D. P. Geraghty, S. B. Mazzone, C. Carter and D. A. Kunde, Catal. Sci. Technol., 2011, 1, 1393.
22. X. Li, D. Wang, G. Cheng, Q. Luo, J. An and Y. Wang, Appl. Catal., B, 2008, 81, 267.
23. X. Wang, Y. Shen, A. Xie, L. Qiu, S. Li and Y. Wang, J. Mater. Chem., 2011, 21, 9641.
24. P. Bober, J. Stejskal, M. Trchová and J. Prokeš, J. Solid State Electrochem., 2011, 15, 2361.
25. E. Alekseeva, P. Bober, M. Trchová, I. Šeděnková, J. Prokeš and J. Stejskal, Synth. Met., 2015, 209, 105.
26. J. Stejskal, Chem. Pap., 2013, 67, 814.
27. Y. B. Yang, L. Xu, F. Y. Li, X. G. Du and Z. X. Sun, J. Mater. Chem., 2010, 20, 10835.
28. J. B. M. Krishnal, A. Saha, G. S. Okram, S. Purakayastha and B. Ghosh, J. Phys. D: Appl. Phys., 2009, 42, 115102.
29. P. Bober, J. Stejskal, M. Trchová and J. Prokeš, Polymer, 2011, 52, 5947.
30. H. Y. Ma, Y. Q. Luo, S. X. Yang, Y. W. Li, F. Cao and J. Gong, J. Phys. Chem. C, 2011, 115, 12048.
31. M. J. Janik, J. Macht, E. Iglesia and M. Neurock, J. Phys. Chem. C, 2009, 113, 1872.
32. F. Qian, G. M. Wang and Y. Li, Nano Lett., 2010, 10, 4686.
33. S. C. K. Misra, M. K. Ram, S. S. Pandey, B. D. Malhotra and S. Chandra, Appl. Phys. Lett., 1992, 61, 1219.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06216d

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