15544
J. Phys. Chem. C 2009, 113, 15544–15547
Electrodeposition Growth of Vertical ZnO Nanorod/Polyaniline Heterostructured Films and Their Optical Properties Ming Chang,†,§ Xueli Cao,‡,§ and Haibo Zeng*,§ School of Materials Science and Engineering, Wuhan UniVersity of Technology, Wuhan 430063, People’s Republic of China, Department of Mathematics and Physics, Guilin UniVersity of Technology, Guilin, 541004, People’s Republic of China, and Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 24, 2009
Vertical ZnO nanorods/polyaniline (NRs/PAN) heterostructured films have been fabricated by a facile electrochemical deposition process and characterized by AFM, TEM, XRD, and photoluminescence spectroscopy. The predeposited PAN thin film on ITO substrates possesses porous surface morphology, which acts as a significant template-like function on the subsequent growth of ZnO NR arrays. The ZnO NRs on PAN exhibit excellent features, including good crystallinity, metal catalyst-free process, and strong ultraviolet (UV) photoluminescence, indicating potential applications in relevant optoelectronics devices. 1. Introduction ZnO nanostructures have attracted much attention due to their properties and potential applications in optics, electronics, optoelectronics, and photocatalysis.1-5 Recently, a combination of ZnO nanorods/nanowires (NRs/NWs) with polymer became a research focus due to its important role in related electronics and optoelectronics nanodevices. Wang’s group developed a microfiber-ZnO NW hybrid structure and fabricated the flexible nanogenerators.6,7 Through combining ZnO NRs and poly(methyl methacrylate) (PMMA), Sun et al. fabricated a heterostructure light-emitting diode (LED) and obtained electroluminescence at 342 nm.8 Nadarajah et al. deposited polystyrene on ZnO NRs grown on an ITO substrate for a flexible LED.9 Currently, most of these ZnO NR/polymer-based heterostructures are achieved through surface coating of the polymer layer. The direct growth of ZnO NRs on the conductive polymer is limited up to now. On the other hand, compared with n-type semiconductor ZnO, polyaniline (PAN) in its intrinsic state is a p-type semiconductor with low conductivity, and its conductivity can be further tailored through the length of the conjugate π-band.10,11 Therefore, the configuration of vertical ZnO nanorods/polyaniline (NRs/PAN) heterostructured films would benefit to the hybrid optoelectronics and electronics devices. In this paper, we present a two-step electrochemical deposition route to directly grow vertical ZnO nanorods/polyaniline (NRs/PAN) heterostructured films on conductive ITO substrates. The pre-electrodeposited porous PAN was found to assist the subsequent growth of ZnO NR arrays and to improve their qualities, including verticality, crystallinity, and ultraviolet (UV) photoluminescence. This hybridized film would have potential applications in electronics and optoelectronics devices. 2. Experimental Section The PAN film was electrodepositied from the 0.1 M aniline in 0.5 M H2SO4 solution by the potentiostatic mode with use * To whom correspondence should be addressed. E-mail: hbzeng@ issp.ac.cn. † Wuhan University of Technology. ‡ Guilin University of Technology. § Institute of Solid State Physics, Chinese Academy of Sciences.
of a three-electrode system. Graphite and the Ag/AgCl (saturate KCl) were used as the counter and reference electrodes, respectively, and the ITO glass was used as the working electrode. The PAN film was electrodeposited on the ITO glass at 0.7 V versus Ag/AgCl for 10 min. After deposition, the film was washed and soaked with deionized water to get rid of the electrolyte and excess products, and then dried in air. Two ZnO samples were electrodeposited from the 0.1 M zinc nitride aqueous solution in a three-electrode system. The zinc sheet and the Ag/AgCl electrode (saturate KCl) were respectively used as the counter and reference electrodes for preparation of these two samples. Both electrodepositions were conducted in a water bath at 353 K with a current density of 1.0 mA/cm2 for 2 h. Field emission scanning electron microscopy (FESEM, JEOL 6700F or Sirion 200) and atomic force microscopy (Autoprobe CP) were used to examine the surface morphologies of the samples. TEM and HRTEM images were recorded with transmission electron microscopy (JEOL-2010 and HRTEM, JEOL-2010). Phase analysis of the products was carried out on a Philips X’Pert powder X-ray diffractometer, using Cu KR (0.15419 nm) radiation. Energy dispersive X-ray (EDX, IncaOxford) analysis was performed to determine the element composition. PL spectra were recorded on a LABRAM-HR spectrometer (Jobin-Yvon) excited with the 325 nm He-Cd laser at room temperature. 3. Results and Discussion After the deposition of PAN, the ITO surface was covered by an island-like PAN film. Figure 1a shows the obverse surface morphology of such a PAN film, which clearly exhibits the island and porous appearance. Each “island” consists of several branched structures and the bottoms of the “islands” are connected to form the pores. Figures 1b presents the SEM images of the back side of the PAN film. By contrast to the obverse side, the back side of the PAN film, which is the interface with the ITO substrate, is very compact and smooth, indicating the excellent interface state. From the cross section image in Figure 1c, it can be seen that the PAN islands stand vertically on ITO. A mass of pores are formed between these islands and the average pore diameter is about 200 nm. Further
10.1021/jp903881d CCC: $40.75 2009 American Chemical Society Published on Web 08/11/2009
ZnO Nanorod/Polyaniline Heterostructured Films
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15545
Figure 1. Electrodeposited PAN film on ITO substrate: obverse-side (a), back-side (b), and cross-section (c) SEM images, and AFM image and height profile (d).
morphology information is revealed by the AFM image in Figure 1d. According to the height profile, the thickness of the PAN film and the average depth of the pores are determined to be 400 and 200 nm, respectively. It should be noticed that the heights from the pore bottoms to the ITO surface are different due to the large surface roughness. Such novel surface morphology would result in the corresponding current fluctuation through PAN while the electrodeposition is implemented, and hance will greatly affect the subsequent growth of ZnO nanorod arrays. Figure 2 presents the electrodeposited ZnO results. Panels a and b of Figure 2 show the SEM images of ZnO NRs on predeposited PAN film. The ZnO NRs exhibit good vertical alighment, and have a regular hexagonal cross section and a relatively uniform size with an average diameter of 180 nm. By contrast, panels c and d of Figure 2 present the SEM images of electrodeposited ZnO NRs on pristine ITO (without the PAN intermediate layer). The sample surface is densely covered by the hexagonal structure ZnO NR bundles. The XRD patterns of ZnO NRs with and without PAN intermediate layers are compared in panels e and f of Figure 2. All of the diffraction peaks match the wurtzite ZnO structure. The relative intensity of the (002) diffraction peak is much stronger than that of the neighboring (100) and (101) peaks, which means strong c-axisoriented growth of the prepared ZnO nanostructure under the experiment condition. Obviously, the [002] preferential orientation is stronger on a large scale when PAN intermediate layers are adopted, as in Figure 2e. Figure 3 presents the TEM and HRTEM images of ZnO NRs by electrodeposition with assistance of PAN intermediate layers. The selected NR has a diameter and a length of 80 and 300 nm, respectively. The inserted SAED pattern in Figue 3a indicates the monocrystalline nature and [0002] growth direction. The EDX pattern of ZnO NR (not shown here) shows that only elements Zn and O are detectable with an element ratio of about
47:53 without any trace of impurity. The lattice fringes in the HRTEM image in Figure 3b exhibit an interspace of 0.52 nm, matching the (0001) planes of the wurtzite ZnO. These results demonstrate that the highly qualified ZnO NRs can be synthesized on porous PAN intermediate layers with conductive ITO substrates. PAN is a typical conductive polymer material and can be polymerized based on radical cations.10,11 The conductivity of PAN can be tailored according to the length of the conjugate π-band within it. During electrodeposition of PAN, soluble low molecular weight oligomers appear first, and then the deposition will occur when the unsolvable products are produced after further dimerization.10,11 In our experiment, the potentiostatic mode is adopted, and the ITO glass substrate working electrode is positively charged. Since the radical cations and oligomers are also positively charged in the electrolyte, electrostatic repulsion impendes the direct deposition on the substrate. Another reason for the formation of porous structure is its conjugate polymer characteristic. The conductivity of a conjugate polymer depends on the length of the conjugate π-band. During the electrodeposition process, the growth of PAN tends to increase its conjugate π-band length and the alignment direction of its molecular chains will be mainly along the direction of the current.12 Liu et al. have synthesized PAN nanowire arrays on a Pt electrode according to this characteristic by the electrodeposition method.13 PAN in its intrinsic state can be regard as a p-type semiconductor with low conductivity. Under our experiment conditions, the PAN film was deposited on ITO glass as the working electrode (cathode), and the distribution of the electric field on the surface of substrates will be greatly affected by the surface morphology of the PAN films. As stated above, when the electrodeposition of ZnO is implemented, the uneven distribution of electric field on the electrode surface will induce the different overpotential at
15546
J. Phys. Chem. C, Vol. 113, No. 35, 2009
Chang et al.
Figure 2. SEM images of ZnO NR/PAN films (a, b) and pure ZnO NRs (c, d), and the corresponding XRD patterns with (e) and without (f) intermediate PAN layers.
Figure 3. TEM (a) and HRTEM (b) images of ZnO NR by PAN-assisted deposition. The insert shows the corresponding EDS pattern.
different locations (on the islands and in the pores). In view of kinetics, the deposition of ZnO surely prefers to occur in pores rather than on the “islands”. Namely, the nucleation positions will be confined in those pores. ZnO has a wurtzite structure with a net dipole moment along the c-axis, nonpolar planes have the lower surface energy compared to the polar basal plane, and the difference in growth speed between planes results in the orientation of the final product.14-16 In our experiment, usual nitric solution was used, whereas the bath temperature was relatively high. Elevation of the temperature will increase the
growth speed of all the planes. From the XRD result of the contrasted sample, the growth of the ZnO nanostructure under this condition is in the [001] growth direction. But the growth of other planes of ZnO results in intercrossing of the [001] growth ZnO nanocrystallines. As for the case of ZnO deposition on the PAN/ ITO glass substrates, the ZnO nanoarrys can be formed. The existence of the island-like polyaniline film confined the nucleation positions and reduced the density of the ZnO rod-like structure. This enabled us to synthesize high-quality ZnO NR arrays in a simple way, as demonstrated in Figure 4.
ZnO Nanorod/Polyaniline Heterostructured Films
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15547 effective catalyst-free method to obtain ZnO nanorod arrays with high structural and optical qualities. 4. Conclusion
Figure 4. Schematic diagram of the growth process of ZnO NR/PAN films.
[001] oriented ZnO NR arrays with large areas were synthesized on porous PAN film with ITO glass substrates by using a two-step electrodeposition route to form vertical ZnO NRs/PAN heterostructured films. The excellent crystallization, strong [001] orientation, metal catalyst-free process, and strong UV PL property indicate that this route is a facile method and such heterostructured films would be useful in future optoelectronics applications. The fluctuation of the electric field caused by the porous PAN film can result in the site-selective nucleation and growth during electrochemical deposition. Acknowledgment. This work was supported by the NSFC (10604055) and the National Major Project of Fundamental Research (973 Program, Grant No. 2005CB623603). References and Notes
Figure 5. PL spectra of ZnO films with (a) and without (b) the PAN interlayer.
Figure 5 shows the PL result of the ZnO nanorod arrays. A strong UV emission peak at about 380 nm and a very weak green emission band were observed. The UV peak was the characteristic emission of ZnO and was attributed to the band edge emission or the exciton transition.17,18 The green emission is usually attributed to deep energy level defects related to singly ionized oxygen vacancy.19,20 Remarkably, for the sample with predeposited polyaniline, the UV emission becomes much stronger than those without the PAN interlayer, and the emission ratio of UV to green is also improved. The enhancement of UV emission could be induced by the electronic passivation effect of PAN. This indicates the potential applications of such heterostructure films on optics and optoelectronics devices. These results demonstrate that by electrodeposition of ZnO on polynailine film, well-aligned ZnO NR arrays can be obtained. The polymer film guided growth method surely is a very
(1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (2) Zeng, H. B.; Cai, W. P.; Liu, P. S.; Xu, X. X.; Zhou, H. J.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. (3) Shen, G.; Bando, Y.; Liu, B.; Golberg, D.; Lee, C. AdV. Funct. Mater. 2006, 16, 410. (4) Liu, J.; Li, Y.; Ding, R.; Jiang, J.; Hu, Y.; Ji, X.; Chi, Q.; Zhu, Z.; Huang, X. J. Phys. Chem. C 2009, 113, 5336. (5) Usui, H. J. Phys. Chem. C 2007, 111, 9060. (6) Qin, Y.; Wang, X.; Wang, Z. Nature 2008, 451, 809. (7) Yang, R.; Qin, Y.; Dai, L.; Wang, Z. Nat. Nanotechnol. 2008, 4, 34. (8) Sun, X. W.; Zhang, J. Z.; Huang, J. X.; Xu, Z. Nano Lett. 2008, 8, 1219. (9) Nadarajah, A.; Word, R. C.; Meiss, J.; Konenkamp, R. Nano Lett. 2008, 8, 534. (10) Ma, Y.; Zhang, J.; Zhang, G.; He, H. J. Am. Chem. Soc. 2004, 126, 7079. (11) Gupta, V.; Miura, N. Electrochem. Commun. 2005, 7, 995. (12) Parthasarathy, R. V.; Matin, C. R. Chem. Mater. 1994, 6, 1627. (13) Liu, J.; Lin, Y.; Liang, L.; Voigt, J. A.; Huber, D. L.; Tian, Z. R.; Coker, E.; Mckenzie, B.; Mcdermott, M. J. Chem.sEur. J. 2003, 9, 604. (14) Kong., X. Y.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 975. (15) Cao, B.; Cai, W.; Sun, F.; Li, Y.; Lei, Y.; Zhang, L. Chem. Commun. 2004, 1604. (16) Li, L.; Pan, S.; Dou, X.; Zhu, Y.; Huang, X.; Yang, Y.; Li, G.; Zhang, L. J. Phys. Chem. C 2007, 111, 7288. (17) Meyer, B. K.; Alyes, H.; Hofmann, D. M.; Kriegseis, W.; Forster, D.; Bertram, F.; Christen, J.; Hoffmann, A.; Straburgs, M.; Dworzak, M. L.; Haboeck, U.; Rodina, A. V. Phys. Status Solidi B 2004, 241, 231. (18) Zhou, H. J.; Fallert, J.; Sartor, J.; Dietz, R. J. B.; Klingshirn, C.; Kalt, H.; Weissenberger, D.; Gerthsen, D.; Zeng, H. B.; Cai, W. P. Appl. Phys. Lett. 2008, 92, 13211. (19) Zeng, H. B.; Cai, W. P.; Hu, J. L.; Duan, G. T.; Liu, P. S.; Li, Y. Appl. Phys. Lett. 2006, 88, 171910. (20) Zeng, H. B.; Li, Z. G.; Cai, W. P.; Cao, B. Q.; Liu, P. S.; Yang, S. K. J. Phys. Chem. B 2007, 111, 14311.
JP903881D
