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J. Phys. Chem. C 2007, 111, 11560-11565
Hierarchical ZnO Nanostructures Obtained by Electrodeposition Lifen Xu, Qingwei Chen, and Dongsheng Xu* Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P.R. China ReceiVed: February 23, 2007; In Final Form: May 15, 2007
We present a two-step electrochemical deposition process to produce hierarchical ZnO nanostructures that involves the electrodeposition of ZnO crystals with different morphologies, followed by the electrochemical epitaxial growth of oriented nanorods on the surfaces of the primary ZnO nanostructures. A variety of hierarchical ZnO nanostructures, including 2-fold nanorod arrays on nanosheets, 6-fold nanorods on nanorods, and 6-fold nanoneedles on nanoneedles, were synthesized on a large scale. The hierarchical nanostructures obtained by this technique are pure and uniform in morphology. Furthermore, we demonstrate that the hierarchical ZnO nanostructure of nanorods on nanosheets provides a good candidate for photoanodes in dye-sensitized solar cells.
Introduction Hierarchical assembly of one-dimensional nanostructures (nanotubes, nanowires, or nanobelts) is essential for the success of bottom-up approaches toward future nanodevices.1 Many efforts have been devoted toward the assembly of nanorod/ nanowire building blocks into two- and three-dimensional ordered superstructures or complex functional architectures. Highly ordered arrays of vertically oriented nanorods and nanotubes have been obtained by vapor-phase deposition methods on patterned catalysts,2 templating against solid templates,3 and controlled hydrothermal deposition.4 Moreover, special architectures of nanorods, such as multipods, nanocombs, nanowindmills, nanowire and nanoribbon networks, and penniform structures, have recently been reported.5-9 However, the development of facile, mild, and effective methods for creating novel architectures based on nanowires remains an important challenge. Wurtzite-structured ZnO, a wide-band-gap (3.37 eV) semiconducting oxide with a large exciton binding energy (60 eV), has versatile properties that are important for applications in optoelectronics, solar cells, and sensors.10,11 ZnO nanostructures of different one-dimensional morphologies such as belts, wires, rods, and tubes have been synthesized by various approaches, including thermal evaporation, hydrothermal reaction, and electrochemical deposition.12-16 There is also a trend toward the fabrication of more hierarchical and intricate nanostructures of ZnO. Many interesting complex nanostructures of ZnO have been fabricated by thermal evaporation of oxide powders.6,17-19 In addition to vapor-phase deposition methods, a wet-chemistry approach using hydrothermal synthesis has proven to be a viable alternative for the synthesis of hierarchical ZnO nanostructures.20-23 Herein, we report for the first time an electrochemical route to the direct solution growth of hierarchical nanostructures based on ZnO nanorods on indium-doped tin oxide (ITO) covered glass substrates. Various hierarchical ZnO nanostructures including ZnO rod arrays on sheets, ZnO rods on rods, and ZnO * To whom correspondence should be addressed. E-mail: dsxu@ pku.edu.cn. Fax: +86 10 62760360. Tel.: +86 10 62753580.
needles on needles were fabricated. It is expected that these hierarchical ZnO nanostructures on conducting ITO substrates might be convenient for the fabrication of electrical devices such as dye-sensitized solar cells, sensors, and electroluminescent devices. Experimental Section Our synthesis of hierarchical ZnO nanostructures involves the electrodeposition of ZnO crystals with different morphologies, followed by the electrochemical epitaxial growth of oriented nanorods on the surfaces of the ZnO nanostructures obtained by the first electrodeposition. An ITO glass substrate with a sheet resistance of about 20 Ω/0 was cleaned ultrasonically in 0.1 M NaOH, distilled water, and acetone and then rinsed in distilled water again. All depositions were carried out in a three-electrode cell at 70 °C in which an ITO substrate, a platinum plate, and a saturated calomel electrode (SCE) served as the working electrode, counter electrode, and reference electrode, respectively. Hexagonal ZnO nanosheets, nanorods, and nanoneedles were electrodeposited in 0.05 M Zn(NO3)2 solutions mixed with 0.06 M KCl, 0.06 M KCl with 0.01 M ethylenediamine (EDA), and 0.01 M EDA, respectively, as reported in our previous paper.16 After the electrodeposition, the ZnO products on the ITO substrates were washed with deionized water and then used as the working electrodes for the second electrodeposition. The electrolyte for the second electrodeposition of ZnO nanorods was prepared by adding ammonia dropwise to the 0.05 M Zn(NO3)2 aqueous solution at 70 °C under continuous stirring until it turned clear. The second electrodeposition of ZnO nanoneedles was carried out in 0.05 M Zn(NO3)2/EDA solution, in which the dosage of EDA was also controlled until the solution turned clear. All electrodepositions were done at a potential of -1.10V vs SCE. The duration of the deposition was 1.5 h. The morphology and the phase identification of the product were characterized by scanning electron microscopy (SEM; Strata DB235 FIB) and power X-ray diffraction (XRD; D/MAXPC 2500 with Cu KR radiation and a normal θ-2θ scan), respectively. Further structural analysis of individual ZnO
10.1021/jp071536a CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
Electrodeposition of Hierarchical ZnO Nanostructures
Figure 1. SEM images of (a) primary ZnO nanosheets and (b-d) hierarchical ZnO nanorods on hexagonal nanosheets on ITO substrates.
microstructures was carried out using transmission electron microscopy (TEM; JEOL 200CX TEM). The fabrication and characterization of dye-sensitized solar cells were performed as follows: The hierarchical ZnO nanostructures on ITO substrates were annealed at 500 °C for 45 min in air first and then sensitized in a solution (3 × 10-4 M) of dye N719 [bis(tetrabutylammonium)-cis-bis(thiocyanato)bis(2, 2-bipyridine-4-carboxylic acid, 4′-carboxylate) ruthenium(II)] in dry ethanol for 30 min. A platinum-plated ITO glass counter electrode was used. The internal space of the two electrodes was 50 µm in thickness and was filled with a liquid electrolyte composed of 0.05 M LiI, 0.05 M I2, 0.6 M 1-propyl3-methylimidazolium iodiode (PMII), and 0.5 M 4-tert-butylpyridine in 3-methoxyproprionitrile. The irradiated area was 0.2 cm2. The cells were tested under AM 1.5G simulated sunlight (100 mW/cm2). The current and voltage were recorded with a CHI660A electrochemical instrument. Results and Discussion Twofold Hierarchical ZnO Nanostructures. Figure 1a presents a typical scanning electron microscopy (SEM) image of hexagonal ZnO nanosheets on ITO substrates as obtained by electrodeposition from 0.05 M Zn(NO3)2 containing 0.06 M KCl. These nanosheets were about 100 nm in thickness and 10 µm in diameter. Most of the nanosheets were vertical with respect to the substrate. After 1.5 h of secondary electrodeposition in 0.05 M [Zn(NH3)4]2+ solution prepared by adding ammonia dropwise to 0.05 M Zn(NO3)2 solution, highly oriented ZnO nanorod arrays were grown on the primary nanosheets. Figure 1b shows a top view of a large number of such nanostructures deposited on the ITO substrate. Each hierarchical nanostructure was ∼10 µm long and ∼1 µm thick (Figure 1c). A higher-magnification SEM image of the typical complex nanostructure in Figure 1d shows a 2-fold hierarchical ZnO nanostructure. It can be seen that uniform nanorods with diameters of ∼100 nm and lengths of ∼ 500 nm are standing perpendicularly and densely on the nanosheets. Figure 2a displays the X-ray diffraction (XRD) pattern of the nanosheets obtained by the first-step electrodeposition on the ITO substrate, which can be indexed to the hexagonal wurtzite structure of ZnO. The XRD pattern of the hierarchical ZnO nanostructures obtained by the second-step electrodepo-
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11561
Figure 2. XRD patterns of (a) primary ZnO nanosheets and (b) hierarchical ZnO nanorods on hexagonal nanosheets. The diffraction peaks marked with / corresponds to the ITO substrates.
sition also presents a hexagonal wurtzite structure, but the diffraction peaks are more intense than those of the primary nanosheets sample (Figure 2b). The sharp and intense diffraction peaks indicate that the sample is highly crystalline. Further structural characterization of the hierarchical ZnO nanostructures was performed by transmission electron microscopy (TEM). Figure 3a shows a typical TEM image of a single nanorod that was scratched from the hierarchical ZnO nanostructures. The inset in Figure 3a displays the electron diffraction pattern recorded from this rod, which reveals the single-crystal wurtzite structure of the nanorod with a [0001] growth direction along the wire axis. The HRTEM image shown in Figure 3b indicates a lattice spacing of ∼0.52 nm, corresponding to the (0001) planar spacing of ZnO in the wurtize phase. Furthermore, a detailed time-dependent morphology evolution study was conducted at a potential of -1.10 V (vs SCE) in 0.05 M [Zn(NH3)4]2+ solution (Figure 4a-f). It was observed that the nanorods grew on the surfaces of the primary ZnO nanosheets (Figure 4a-d). An interesting phenomenon observed in Figure 4e is that the dense and oriented nanorods on hexagonal ZnO sheets changed to a beltlike structure composed of closely packed nanorods when we prolonged the deposition or slightly decreased the pH of the solution. In Figure 4f, it is clearly seen that the nanorods in the center of these hierarchical structures have been removed, which leads to a hexagonal fence. However, for reaction time of more than 5 h, these hierarchical ZnO nanostructures could disappear completely. Sixfold Hierarchical ZnO Nanostructures. ZnO nanorods on nanorods could also be synthesized by this two-step electrochemical deposition process. Figure 5a shows an SEM image of hexagonal ZnO nanorods with diameters of about 100-200 nm as obtained by one-step electrodeposition from a mixed solution of 0.05 M Zn(NO3)2, 0.06 M KCl, and 0.01 M ethylenediamine (EDA).16 By the two-step electrochemical growth, hierarchical structures with ZnO nanorods on nanorods were obtained. Figure 5b displays a low-magnification SEM image of the resulting structures, indicating large amounts of pure 6-fold nanostructures with nanorods as the secondary branches. These secondary nanorods have diameters of ∼100 nm and lengths of ∼500 nm (Figure 5c,d). From the geometry of these 6-fold hierarchical structures, it is obvious that the
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Figure 3. Typical (a) TEM and (b) HRTEM images of a single nanorod. The inset in a shows the corresponding electron diffraction.
Figure 4. SEM images of hierarchical ZnO nanostructures electrodeposited at a potential of -1.10 V in 0.05 M [Zn(NH3)4]2+ solution for different deposition times: (a) 10 min, (b) 20 min, (c) 40 min, (d) 1.5 h, (e) 2.5 h, (f) 3.5 h.
Figure 5. SEM images of hierarchical ZnO nanorods on nanorods obtained by the two-step electrodeposition: (a) primary ZnO nanorods on ITO substrates, (b-c) hierarchical ZnO nanorods obtained after the second-step electrodeposition, (d) column of 6-fold hierarchical ZnO nanorods.
branched nanorods grow from the {101h0} surfaces of the primary nanorods along the [0001] direction. A substantially higher intensity was observed for the 〈002〉 diffraction peak in the XRD pattern of the primary ZnO
nanorods (Figure 6a), which indicates that the ZnO crystallites are oriented perfectly with their c axes perpendicular to the ITO substrates. Figure 6b shows the XRD pattern of the sample after the secondary electrodeposition. It can be seen that the sample with 6-fold hierarchical structures is also composed of the wurtzite ZnO structure and the intensities of the (100) and (101) diffraction peaks are significantly enhanced. The enhancement of the two diffraction peaks might be caused by the secondary rods with their 〈002〉 directions parallel to the substrate. Hierarchical structures of ZnO nanoneedles on nanoneedles were also synthesized by the same electrochemical strategy. Low-density ZnO nanoneedles with lengths of ∼4 µm were electrodeposited in solutions of 0.05 M Zn(NO3)2 and 0.01 M EDA.16 Figure 7a shows a top view of the hierarchical ZnO nanoneedles on ITO substrates prepared by the second electrodeposition. These structures present 6-fold symmetry and branching phenomena similar to those in Figure 5c and d, in which the hierachical nanoneedles retain their 6-fold arrays of parallel nanoneedles around the central nanoneedles. The highermagnification SEM image in Figure 7b indicates that these secondary nanoneedles have sizes of ∼100 nm and are around 1 µm in length. Growth Mechanism. Soluble Zn(II) species in the form of [Zn(NH3)4]2+ are generated upon addition of ammonia to Zn(NO3)2 solution according to eqs 1 and 2. In the present study, under the cathodic deposition conditions, reduction of nitrate
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Figure 6. XRD patterns of (a) primary ZnO nanorods and (b) hierarchical ZnO nanorods on nanorods.
ions can generate hydroxyl ions (eq 3).24 The generation of OHat the working electrode increases the local pH and results in the formation of hydroxyl complexes such as [Zn(OH)4]2according to eq 4. Then, the [Zn(OH)4]2- can react with OHgroups on the ZnO surface to allow nucleation and growth of ZnO (eq 5).
Zn2+ + 2NH3‚H2O f Zn(OH)2V + 2NH4+
(1)
Zn(OH)2 + 4NH3‚H2O T [Zn(NH3)4]2+ + 2OH- + 4H2O (2) NO3- + H2O + 2e- f NO2- + 2OH-
(3)
[Zn(NH3)4]2+ + 4OH- T [Zn(OH)4]2- + 4NH3
(4)
OH-(surface) + OH-(soln) T O2- + H2O
(5)
It is known that, when the surface energy of the substrate is much smaller than that of the deposits, the growth of isolated three-dimensional islands is favored. In our previous work, we demonstrated that the shape of the electrodeposited ZnO is strongly affected by the growth rate, controlled by capping agents.16 The formation of isolated nanostructures of ZnO on the ITO substrates in the first-step electrodepotion indicates that the potential barrier to crystal nucleation and subsequent growth into a perfect crystal of ZnO on ITO-coated glass is much higher than that on ZnO itself. Therefore, during the secondary electrodeposion, the ZnO deposits grew preferentially on the surface of the primary ZnO crystals rather than the ITO substrate, leading to the growth of hierarchical nanostructures. The growth of oriented ZnO nanorods on the ZnO nanocrystals can be attributed to a balance between [Zn(NH3)4]2+ and the growth unit of [Zn(OH)4]2-. In the beginning of the secondary electrodepostion, large amounts of Zn(NH3)42+ ions in the solution can easily turn into the growth units of [Zn(OH)4]2- (eq 4), which leads to the formation of crystal nuclei on the surfaces of the ZnO nanocrystals and subsequent growth into a perfect crystal of ZnO along the [0001] direction. In our experiments, control of the concentration of ammonia and the cathodic voltage is critical for allowing the correct sequences of reactions to proceed. When ammonia was not
Figure 7. SEM images of (a) hierarchical ZnO nanoneedles on nanoneedles, (b) column of 6-fold hierarchical ZnO nanoneedles.
added to the electrolyte, the ZnO nanoparticles grew on the bare ITO substrates instead of the ZnO surface. In addition, as the reaction progressed, the concentration of Zn2+ decreased, resulting in a relative acceleration of dissolution. Prolonging the reaction led to the dissolution of the hierarchical structure detailed with nanorods standing on nanosheets and formed a coiled-up beltlike structure composed of closely packed nanorods (Figure 4e) or a hexagonal fence shape (Figure 4f). Photoelectrochemical Behaviors of Dye-Sensitized Solar Cells with ZnO Hierarchical Nanostructure Photoanodes. Dye-sensitized solar cells (DSSCs), in which TiO2 nanocrystallines are usually used to fabricate the photoanode,25-27 have attracted great attention since the 1990s. ZnO is also expected to be a fascinating material that can be competitive with TiO2 in dye-sensitized solar cells because of its fast process of electron injection. Hagfeldt et al. reported that DSSCs fabricated with ZnO nanoparticles had a conversion efficiency analogous to TiO2 DSSCs, considering the surface area of ZnO particles and the amount of dye absorbed.28-29 However, electron transport in nanoparticles relies on trap-detrap diffusion, which is a limited process. It was recently reported that an increase of the electron diffusion length in the anode can be obtained by replacing the nanoparticle film with an array of oriented ZnO nanowires.20,30 The hierarchical ZnO nanostructures of nanorods on nanosheets might provide a good candidate for photoanodes in DSSCs. Figure 8 shows the photocurrent density vs voltage curves of the ZnO DSSCs. All experimental parameters used here were not optimized. As can be seen, when the nanorods were deposited on the nanosheets, the short-circuit current density (Jsc) and the conversion efficiency (η) of the ZnO DSSCs increased from 2.64 mA‚cm-2 and 0.48% to 4.40 mA‚cm-2 and 0.74%, respectively, and at the same time, the open-circuit voltage (Voc) and the fill factor (ff) decreased from 476 mV and 0.38 to 442 mV and 0.37, respectively. The secondary deposited nanorods should increase the surface area of the photoanode, which results in an increase in the amount of dye
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Xu et al. formation of hierarchical nanostructures. First, this process is a simple, low-temperature, and efficient route to the formation of hierarchical ZnO nanostructures on a large scale. Second, the structures of the hierarchical products can be predesigned by controlling the electrodeposition parameters in both the first and second steps. Third, the hierarchical nanostructures obtained by this technique are pure and uniform in morphology, in contrast to the mixtures obtained by other solution syntheses. In addition, these hierarchical nanostructures were grown directly on conductive glass substrates and might be convenient for use in electrical devices. Furthermore, we have demonstrated that the hierarchical ZnO nanostructures of nanorods on nanosheets provide a good candidate for use as photoanodes in DSSCs.
Figure 8. Photocurrent versus voltage plots for DSSCs: (a) ZnO nanosheet film and (b) hierarchical ZnO nanorods on nanosheets. The irradiated area was 0.2 cm2. The cells were tested under AM 1.5G simulated sunlight (100 mW/cm2).
Figure 9. Dark performance of ZnO DSSCs: (a) ZnO nanosheet film, (b) hierarchical ZnO nanorods on nanosheets.
absorbed and the corresponding value of Jsc. On the other hand, an increase in the number of absorbed dye molecules would lead to the aggregation of dye molecules on the surface of ZnO. It has been reported that the carboxyl group of the dye N3 (or N719) could react with Zn surface atoms to form Zn2+/dye complexes in the pores of the ZnO films, which would suppress the solar cell performance.29 This would result in an increase of the dark current (Figure 9) and a decrease of Voc and ff of hierarchical ZnO DSSCs. Although ZnO hierarchical structures showed better performance than ZnO nanosheets in DSSCs, the smaller thickness of the film (about 8 µm) limited them from providing a better dye loading. As ITO substrate was unstable especially when calcined at 500 °C, the marked increase of the sheet resistance resulted in the low ff and η values. By enhancing the film thickness and replacing the conducting ITO substrates with FTO, it can be expected that the performance of the hierarchically structured ZnO DSSCs would be markedly improved. Conclusion In summary, we have demonstrated the ability of a two-step electrodeposition technique to produce various hierarchical ZnO nanostructures. This strategy offers many advantages for the
Acknowledgment. This work was supported by NSFC (Grants 20433010, 20525309, 20521201, 20673008) and MSTC (MSBRDP, Grant 2006CB806102, 2007CB936201). References and Notes (1) Wang, D.; Qian, F.; Yang, C.; Zhang, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (2) Wu, Y. Y.; Yan, H. Q.; Yang, P. D. Top. Catal. 2002, 19, 197. (3) (a) Mu, C.; Yu, Y. X.; Wang, R. M.; Wu, K.; Xu, D. S. AdV. Mater. 2004, 16, 1550. (b) Liang, Y. Q.; Zhen, C. G.; Zou, D. C.; Xu, D. S. J. Am. Chem. Soc. 2004, 126, 16338. (c) Liang, Y. Q.; Zhai, L.; Zhao, X. S.; Xu, D. S. J. Phys. Chem. B 2005, 109, 7120. (4) Vayssieres, L. AdV. Mater. 2003, 15, 464. (5) (a) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (b) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (6) Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 4728. (7) (a) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502. (b) Ma, C.; Moore, D.; Li, J.; Wang, Z. L. AdV. Mater. 2003, 15, 228. (8) (a) Wang, Z. L.; Pan, Z. W. AdV. Mater. 2002, 14, 1029. (b) Nguyue, P.; Ng, H. T.; Kong, J.; Cassell, A. M.; Quinn, R.; Li, J.; Han, J.; McNeil, M.; Meyyappan, M. Nano Lett. 2003, 3, 925. (9) Shi, H. T.; Qi, L. M.; Ma, J. M.; Cheng, H. M. J. Am. Chem. Soc. 2003, 125, 3450. (10) (a) Minne, S. C.; Manalis, S. R.; Quate, C. F. Appl. Phys. Lett. 1995, 67, 3918. (b) Shibata, T.; Unno, K.; Makino, E.; Ito, Y.; Shimada, S. Sens. Actuators A 2002, 102, 106. (c) Huang, M. H.; Mao, S.; Feick, H. N.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (11) Yoshida, T.; Minoura, H. AdV. Mater. 2002, 12, 1219. (12) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625. (c) Kong, X. Y.; Ding, Y.; Yang, R. S.; Wang, Z. L. Science 2004, 303, 1348. (13) Zhang, X. H.; Zhang, Y.; Xu, J.; Wang, Z.; Chen, X. H.; Yu, D. P.; Zhang, P.; Qi, H. H.; Tian, Y. J. Appl. Phys. Lett. 2005, 87, 123111. (14) (a) Zhang, J.; Sun, L. D.; Liao, C. S.; Yan, C. H. Chem. Commun. 2002, 262. (b) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (15) Liu, R.; Vertegel, A. A.; Bohannan, E. W.; Sorenson, T. A.; Switzer, J. A. Chem. Mater. 2001, 13, 508. (16) Xu, L. F.; Guo, Y.; Liao, Q.; Zhang, J. P.; Xu, D. S. J. Phys. Chem. B 2005, 109, 13519. (17) (a) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano. Letter. 2002, 2, 1287. (b) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. J. Mater. Chem. 2004, 14, 770. (18) Park, J. H.; Park, J. G. Appl. Phys. A 2005, 80, 43. (19) (a) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (b) Gao, P. X.; Wang, Z. L. J. Phys. Chem B 2004, 84, 2883. (20) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (21) Koh, Y. W.; Lin, M.; Tan, C. K.; Foo, Y. L.; Loh, K. P. J. Phys. Chem. B 2004, 108, 11419. (22) (a) Gao, X. P.; Zheng, Z. F.; Zhu, H. Y.; Pan, G. L.; Bao, J. L.; Wu, F.; Song, D. Y. Chem. Commun. 2004, 1428. (b) Zhang, H.; Yang, D. R.; Li, D. S.; Ma, X. Y.; Li, S. Z.; Que, D. L. Cryst. Growth Des. 2005, 2, 547.
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