Synthesis of Ultrathin ZnO Nanofibers Aligned on a Zinc Substrate

communications ZnO nanofibers DOI: 10.1002/smll.200500379 Synthesis of Ultrathin ZnO Nanofibers Aligned on a Zinc Substrate** Yueping Fang, Qi Pang, Xiaogang Wen, Jiannong Wang, and Shihe Yang* synthetic challenges of nanowire materials is to reduce the nanowire diameter and, at the same time, align the nanowires in a controlled fashion. Here, we demonstrate the first synthesis of aligned ZnO nanofibers, which are no thicker than 10 nm and
500 nm in length, by hydrothermal treatment of a Zn foil in an ammonia/alcohol aqueous solution. It should be emphasized that the reaction was carried out at a relatively low temperature without any catalyst. We have established that the use of ammonia/alcohol is crucial to the growth of the aligned ultrathin ZnO nanofibers. Photoluminescence (PL) measurement at room temperature shows a prominent peak at 373 nm (3.32 eV), which is about 100 meV blue-shifted from the bulk ZnO emission (3.24 eV, 383 nm) due to the ultrafine dimension along the radial direction of the ZnO nanofibers. Scanning electron microscopy (SEM) images of the asprepared products on a Zn foil (Figure 1) show a tufted Dimensional reduction of ZnO semiconductors is expected to create wide-ranging application possibilities from roomtemperature ultraviolet (UV) lasers,[1] sensors,[2] photocatalysts,[3] solar cells,[4] to field-emission (FE) devices.[5] In general, ZnO nanowires as one-dimensional (1D) materials with diameters of less than 10 nm are expected to display novel and unique physical and chemical properties due to quantum confinement. Furthermore, it is very important to fabricate a dense array of ZnO nanowires on an appropriate substrate so that the nanowires can be directly addressed, surface area can be optimized, and/or collective properties may be induced. A host of techniques have been developed for the synthesis of aligned 1D ZnO nanostructures with sizes in the range of 10– 100 nm on a variety of substrates for nanodevice applications.[6–8] For example, well-aligned ZnO nanowires that are 10– 15 nm across have been grown from patterned, thin gold dots on sapphire substrates by a vapor–liquid–solid (VLS) process.[7] To our knowledge, however, little has been reported on the synthesis of well-aligned ZnO nanowires with diameters of less than 10 nm. Although 6nm ZnO nanobelts have been obtained Figure 1. SEM images of the nanofiber product layer grown on a Zn substrate: A) Low-magby solid–vapor deposition,[9] they are rannification image (top view); B) high-magnification image (top view) taken from the white domly oriented. Therefore, one of the square area in (A); C) view from
158 to the surface normal; D) side view of a cross section. [*] Dr. Y. Fang, X. Wen, Prof. Dr. S. Yang Department of Chemistry Institute of Nano Science and Technology The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong (China) Fax: (+ 852) 2358-1594 E-mail: [email protected] Dr. Q. Pang, J. Wang Department of Physics, Institute of Nano Science and Technology The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong (China) [**] We are grateful to the Hong Kong University of Science and Technology for financial support under the grant of HIA05/06.SC02. S.Y. wishes to thank the Hong Kong Young Scholar Cooperation Research Foundation of the NSFC. Supporting information for this article is available on the WWW under http://www.small-journal.com or from the author. 612 morphology. Overall, the product layer on the Zn substrate is quite uniform over a large area (Figure 1 A). A fiberlike morphology in a dense array is evident in the magnified image (Figure 1 B). The whole surface of the Zn foil is completely covered with the nanofibers. More important, the nanofibers grow in the form of bundles oriented perpendicular to the Zn substrate surface. This can be seen more clearly at even higher magnifications taken from
158 to the substrate normal and from the side (Figure 1 C and D, respectively). The nanofibers are clearly ultrathin with an estimated area density of
1011 cm2 and obtained in the form of bundles oriented approximately perpendicular to the Zn substrate surface. The bundles consist of multitudinous ultrathin nanofibers and usually terminate in sharp needles. The seemingly hazy image in (Figure 1 C) is due to A 2006 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim small 2006, 2, No. 5, 612 – 615 charging of the nanofibers. Coating with Au could ease the charging problem (Figure S1, see Supporting Information) but would increase the size and/or alter the morphology of the nanofibers. The side view shows most clearly that the nanofibers are indeed aligned approximately normal to the Zn substrate with a length of about 500 nm. X-ray diffraction (XRD) analysis of the product layer on the Zn substrate (Figure S2, see Supporting Information) suggests a hexagonal ZnO phase (JCPDS: 36-1451) with a space group of P63/mc (no. 186). The lattice constants calculated based on the XRD data (a = 3.25 C and c = 5.21 C) are in good agreement with the corresponding literature values (a = 3.249 C and c = 5.206 C). It is noticeable that the (0002) diffraction peak is enhanced and much stronger than the other peaks, suggesting a preferential orientation of the crystals along the (0002) plane of the ZnO nanofibers parallel to the Zn substrate. This is corroborated by the rockFigure 2. TEM images of a ZnO nanofiber array grown on a Zn substrate: A) Low-magnification ing curve of the (0002) plane difimage; B) higher-magnification image; C) two bundled nanofibers; D) HRTEM image of a single fraction of the ZnO nanofibers ZnO nanofiber together with the corresponding FFT pattern (inset). array (Figure S2, see Supporting Information), which exhibits a peak as narrow as 7.18 (full width at half maximum). The ZnO nanofibers have been further examined by (Figure S2, see Supporting Information), demonstrates that transmission electron microscopy (TEM), and some reprethe ultrafine ZnO nanofibers are aligned on the Zn substrate. sentative images are presented in Figure 2. Even after ultraIn fact, the importance of the alcohol (methanol)/water sonication in ethanol, the ZnO nanofibers are still bundled mixture in synthesizing small-diameter ZnO nanorods has together (Figure 2 A and B), with the bundles consisting of already been implicated previously.[8, 10] Here, the alcoholic numerous nanofibers with ultrathin tips of less than 10 nm. environment is crucial in ensuring the formation of Understandably, the ultrafine nanofibers have a strong ten[ZnO2]2 ions and a controlled release of this species from dency to form bundles simply to minimize their surface the alcohol/water mixed phase to the growing ZnO nanoenergy, a phenomenon that has been observed previously in rods.[10] However, the alcohol/water mixture alone is still in[9, 10] different systems. sufficient to shrink the nanorods to the size regime of less As a case in point, two nanofibers are than 10 nm; ammonia was used to achieve this. In our exattached to each other, shoulder to shoulder (Figure 2 C), periments, ammonia is not only a transporter of Zn2 + ions, each with a diameter of less than 6 nm. The high-resolution TEM (HRTEM) image of a single ZnO nanofiber (Figbut also an adsorbing ligand, which may preferentially cover ure 2 D) exhibits clear fringes perpendicular to the nanofibthe side walls, thus thwarting the radial growth of the nanoer axis, the spacing of which measures 0.255 nm and agrees rods relative to the longitudinal growth. We note that the well with the interplanar spacing of the (0002) planes of side walls are more or less neutralized owing to the alternatZnO. This is supported by the fast Fourier transform (FFT) ing planes of Zn2 + and O2 whereas the (0001) plane is pattern (inset in Figure 2 D). Clearly, the ZnO nanofiber highly charged; this striking difference appears to favor the grows along the [001] direction. This result is not surprising adsorption of neutral ammonia on the side walls and charggiven that 1D ZnO nanostructures synthesized by other ed species such as OH on the (0001) plane. methods often grow along this direction. This result, togethThe putative reactions relevant to the synthesis of our er with the SEM pictures in Figure 1 and the XRD data aligned upright nanofibers are as follows: small 2006, 2, No. 5, 612 – 615 A 2006 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim www.small-journal.com 613 communications Zn þ O2 þ 2 H2 O ! Zn2þ þ 4 OH ð1Þ Zn2þ þ 4 NH3 ! ½ZnðNH3 Þ4 2þ ð2Þ ½ZnðNH3 Þ4 2þ þ 4 OH ! ½ZnO2 2 þ 4 NH3 þ 2 H2 O ð3Þ ½ZnO2 2 þ H2 O ! ZnO þ 2 OH ð4Þ We believe that spontaneous oxidation is accelerated in the ammonia/alcohol aqueous solution at the reaction temperature used for the nucleation of ZnO on the Zn substrate. The subsequent growth [Eq. (4)] is sustained by the continuous Zn oxidation in the inter-nucleus regions [Eq. (1)]. As mentioned above, the ultrathin ZnO nanofibers grow preferentially along the [001] direction. Although the ZnO nuclei may not have preferred crystalline orientations on the Zn substrate, only the approximately vertical growth could carry on and dominate whereas growth along other directions could not continue in a crowded environment and would be suppressed. It appears that the growth is self-organized in the sense that the nanofibers grown in the upright direction support each other, often in the form of bundles, as observed in Figure 2, whereas the growth-hindered nanocrystals may reposition or reorient themselves through dynamic equilibration in favor of the approximately vertical growth. Control experiments have demonstrated the dramatic influence of the solution on the morphology of the product layer above the Zn substrate. For example, an interesting hierarchical ZnO nanostructure was obtained when 20 mL of dilute aqueous ammonia (1 mL NH3) solution replaced the 20 mL of ammonia (1 mL)/ethanol (10 mL)/water (9 mL) solution under the same conditions (Figures S2 and S3, see Supporting Information). On the other hand, a ZnO nanorod array could also be obtained on the Zn substrate when the solution consisted of ethanol (10 mL) and water (10 mL), see Figure S4 in the Supporting Information, but the resulting nanorods are relatively thick (20–40 nm). Further addition of ammonia brought the ZnO nanofibers to a regime less than 10 nm in thickness. Therefore, the ammonia/alcohol/water mixed phase appears to be essential for the growth of the ultrathin ZnO nanofibers. Because the Bohr exciton radius of ZnO is known to be 2.34 nm,[11] a nanoscale entity of this material when approaching this size regime is expected to show quantum confinement effects. Indeed, a blue shift of the exciton emission was observed for ZnO nanobelts or nanorods with the smallest dimension being less than 10 nm.[9, 12] We also recorded the PL spectrum of an ultrathin ZnO nanofiber array at room temperature, which is shown in Figure 3. The ultrathin ZnO nanofibers exhibit a near-band-edge emission at 373 nm (3.324 eV). Compared with the PL spectra of bulk ZnO crystals at room temperature, a blue shift of about 100 meV is observed, which possibly indicates quantum confinement arising from the reduced size of the ultrathin nanofibers. The full width at half-maximum of the PL peak is about 30 nm, which is somewhat broader than usual 614 www.small-journal.com Figure 3. A PL spectrum of the ultrathin ZnO nanofiber array at room temperature. The inset is an enlarged portion between 350 nm and 400 nm. due perhaps to a broad diameter distribution or, more interestingly, to an inter-nanofiber interaction of the dense (even bundled) array. It should be noted that we also observed a defect-related emission between 450 and 650 nm but this was weaker than the UV emission. In summary, we have successfully synthesized aligned ZnO nanofibers in a dense array from and on a Zn substrate by hydrothermal treatment of Zn foil in an ammonia/ alcohol/water mixed solution. Notably, the ZnO nanofibers are ultrathin (3–10 nm) with a length of
500 nm. This is the first time that uniform, aligned, and ultrathin ZnO nanofibers have been obtained via a hydrothermal method in the absence of catalysts and at a relatively low temperature. The PL measurements at room temperature revealed a significantly blue-shifted near-band-edge emission at 373 nm (3.32 eV), whish was ascribed to quantum confinement arising from the reduced size of the ultrathin ZnO nanofibers. Further experiments are under way to investigate the detailed mechanisms of the formation of the ultrathin nanofibers, to grow the nanofibers in a dense array on other devicerelated substrates, and to explore potential applications of this unique nanostructured material. Experimental Section In a typical synthesis, a piece of an ethanol-washed Zn foil (10 1 10 1 0.25 mm3, 99.9 %, Aldrich) was immersed in a solution of ammonia (1 mL, 30 %), ethanol (10 mL), and water (9 mL) in a teflon-lined stainless steel autoclave (25 mL) followed by heating at a constant temperature of 95 8C for 24 h. After the hydrothermal treatment, the Zn foil was rinsed with ethanol and dried in air for further characterization. The nanostructured products grown on the Zn substrate were directly subjected to scanning electron microscopy characterization (SEM, JEOL JSM-6300 at an accelerating voltage of 15 kV; FEG-SEM, JEOL JSM-6700F at an accelerating voltage of 5 kV; Au coating was sometimes used to mitigate the charging), powder XRD (Philips PW-1830 and Bruker AXS D8 ADVANCE X-ray diffractometer), and PL measurement (with a 325 nm He-Cd laser as the excitation source). For the TEM (JEOL 2010F microscope operated at an accelerating voltage A 2006 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim small 2006, 2, No. 5, 612 – 615 of 200 kV) observations, the Zn foil covered with the product was sonicated in ethanol for 20 min and the suspension was dropped onto a carbon-coated Cu grid, followed by evaporation of the solvent in the ambient environment. [5] [6] Keywords: arrays · hydrothermal synthesis · nanofibers · quantum confinement · zinc oxide [7] [1] a) H. Cao, J. Y. Xu, D. Z. Zhang, S. H. Chang, S. T. Ho, E. W. Seelig, X. Liu, R. P. H. Chang, Phys. Rev. Lett. 2000, 84, 5584; b) D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, T. Goto, Appl. Phys. Lett. 1997, 70, 2230; c) R. F. Service, Science 1997, 276, 895. [2] a) T. Shibata, K. Unno, E. Makino, Y. Ito, S. Shimada, Sens. Actuators A 2002, 102, 106; b) S. C. Minne, S. R. Manalis, C. F. Quate, Appl. Phys. Lett. 1995, 67, 3918; c) C. Baratto, G. Sberveglieri, A. Onischuk, B. Caruso, S. D. Stasio, Sens. Actuators B 2004, 100, 261; d) K. D. Mitzner, J. Sternhagen, D. W. Galipeau, Sens. Actuators B 2003, 93, 92; e) G. Sberveglieri, S. Groppelli, P. Nelli, A. Tintinelli, G. Giunta, Sens. Actuators B 1995, 25, 588. [3] H. Yumoto, T. Inoue, S. J. Li, T. Sako, K. Nishiyama, Thin Solid Films 1999, 345, 38. [4] a) K. Keis, L. Vayssieres, S. E. Lindquist, A. Hagfeldt, Nanostruct. Mater. 1999, 12, 487; b) K. Hara, T. Horiguchi, T. Kinoshita, K. small 2006, 2, No. 5, 612 – 615 [8] [9] [10] [11] [12] Sayama, H. Sugihara, H. Arakawa, Sol. Energy Mater. Sol. Cells 2000, 64, 115. Y. W. Zhu, H. Z. Zhang, X. C. Sun, S. Q. Feng, J. Xu, Q. Zhao, B. Xiang, R. M. Wang, D. P. Yu, Appl. Phys. Lett. 2003, 83, 144. a) M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, P. D. Yang, Science 2001, 292, 1897; b) Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 2001, 291, 1947; c) Y. Ding, P. X. Gao, Z. L. Wang, J. Am. Chem. Soc. 2004, 126, 2066; d) J. J. Wu, S. C. Liu, Adv. Mater. 2002, 14, 215; e) L. Vayssieres, K. Keis, S. E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 2001, 105, 3350; f) L. Guo, Y. L. Ji, H. B. Xu, J. Am. Chem. Soc. 2002, 124, 14 864. E. C. Greyson, Y. Babayan, T. W. Odom, Adv. Mater. 2004, 16, 1348. L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson, Y. F. Zhang, R. J. Saykally, P. D. Yang, Angew. Chem. 2003, 115, 3139; Angew. Chem. Int. Ed. 2003, 42, 3031. a) X. D. Wang, Y. Ding, C. J. Summers, Z. L. Wang, J. Phys. Chem. B 2004, 108, 8773; b) Z. L. Wang, Mater. Res. Materials Today 2004, 7, 26. B. Liu, H. C. Zeng, J. Am. Chem. Soc. 2003, 125, 4430. Y. Gu, I. L. Kuskovsky, M. Yin, S. O’Brien, G. F. Neumark, Appl. Phys. Lett. 2004, 85, 3833. R. T. Senger, K. K. Bajaj, Phys. Rev. B 2003, 68, 045 313. A 2006 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim Received: October 4, 2005 Revised: December 27, 2005 Published online on March 15, 2006 www.small-journal.com 615