- Nano Express
- Open Access
Growth of catalyst-free high-quality ZnO nanowires by thermal evaporation under air ambient
Nanoscale Research Letters volume 7, Article number: 220 (2012)
ZnO nanowires have been successfully fabricated on Si substrate by simple thermal evaporation of Zn powder under air ambient without any catalyst. Morphology and structure analyses indicated that ZnO nanowires had high purity and perfect crystallinity. The diameter of ZnO nanowires was 40 to 100 nm, and the length was about several tens of micrometers. The prepared ZnO nanowires exhibited a hexagonal wurtzite crystal structure. The growth of the ZnO nanostructure was explained by the vapor-solid mechanism. The simplicity, low cost and fewer necessary apparatuses of the process would suit the high-throughput fabrication of ZnO nanowires. The ZnO nanowires fabricated on Si substrate are compatible with state-of-the-art semiconductor industry. They are expected to have potential applications in functional nanodevices.
In the past decade, significant interest has emerged in the synthesis of one-dimensional semiconductor materials, such as Si [1–3], SiC [4, 5], GaN [6–8], SnO2  and ZnO [10–13]. Among these nanoscale semiconductors, ZnO has attracted a great deal of attention because of its potential as a large direct band gap semiconductor (Eg is about 3.35 eV at room temperature) with high exciton binding energy (60 meV). It can act as building blocks for nano-FET, nanolasers, photodetectors and gas sensors [8, 14]. In addition, ZnO nanowires have excellent field emission for its good hardness, thermal stability and resistance to oxidation [15, 16].
Recently, many methods have been developed to synthesize ZnO nanowires, for example, carbon thermal reduction [13, 17], chemical vapor deposition [12, 18], physical vapor deposition , electrodeposition , aqueous synthesis  and solvothermal technique . In this paper, we synthesized ZnO nanowires by thermal evaporation without a catalyst under air ambient. The reactions were carried out in a traditional horizontal furnace with one end open at 750°C. The gray-white product was successfully deposited on the Si substrate. The process does not need any metal catalyst, so it avoids catalyst contamination. Furthermore, the simplicity, low cost and fewer necessary apparatuses of the process would suit the high-throughput fabrication of ZnO nanowires.
The experiments were conducted in a horizontal furnace as schematically outlined in Figure 1. The raw Zn material (99.99%) was loaded into a quartz boat. The Si substrate was cleaned by the standard cleaning process, and then, it was laid above the Zn powders. The furnace was heated to 750°C under a constant flow of pure O2 gas, with the flow rate of 2 ml/min. Afterwards, the quartz boat was put in the central region of the horizontal quartz tube. After 2 h, the furnace was turned off and naturally cooled to room temperature. A gray-white layer was coated on the Si Substrate.
The as-synthesized products were characterized by X-ray diffraction (XRD) with CuKα radiation (wavelength, λ = 1.5406 Å), field emission scanning electron microscopy (SEM) (Hitachi S-4800, Hitachi Ltd., Tokyo, Japan), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (JEOL JEM2010F, JEOL Co., Ltd., Beijing, China).
Results and discussion
Phase analysis of as-synthesized products is shown in Figure 2. All the diffraction peaks can be indexed as wurtzite ZnO with lattice constants of a = 0.325 nm and c = 0.521 nm, agreeing well with the calculated diffraction pattern (JCPDs card no.03-1005). No other diffraction peaks are detected in the spectrum within the instrumental resolution, which indicates that the products are ZnO with high purity.
Figure 3a shows the typical low-magnification SEM images of the products on the Si substrate. It reveals that the products consist of high-density ZnO nanowires with typical lengths in the range of several tens of micrometers. Some aligned ZnO nanowires were also detected, as shown in Figure 3b. Figure 3c shows the larger magnification of the aligned ZnO nanowires. They reveal that the ZnOs with a diameter of 40 to 100 nm were nearly parallel and have a smooth surface. In order to investigate the further structure of the fabricated ZnO nanowires, TEM and HRTEM analyses were also performed.
The vapor-liquid-solid (V-L-S) and vapor-solid (V-S) formation mechanisms are usually responsible for the one-dimensional (1-D) semiconductor nanowires. For the V-L-S mechanism, the metal nanoclusters act as catalyst and guide the nanowire to grow towards the 1-D direction. It is evident that the nanowire tip will have an alloy droplet. In our experiment, no impurity metal particles were detected in the ZnO nanowires (from the SEM and TEM images). Hence, the V-S growth process could be well accepted in our work. Firstly, a reaction occurs between Zn and O2 to form ZnOx (x < 1); the ZnOx vapor is transferred by the O2 to the nuclei at the Si substrate. The continuously introduced O2 oxidizes ZnOx to ZnO. Due to the high supersaturation of Zn, ZnOx vapor and oxygen, the ZnO nanostructures can easily nucleate and grow along the  direction, which was substantiated by the HRTEM image (Figure 4b). The already-formed ZnO nucleation continues to grow along the direction of the O2 gas flow, so some aligned ZnO nanowires on the Si substrate are formed.
ZnO nanowires with high purity and perfect crystallinity were fabricated by simple thermal evaporation of pure Zn powders under air ambient without any catalyst. The diameter of the ZnO nanowires was 40 to 100 nm, and the length was about several tens of micrometers. Some aligned ZnO nanowires with smooth surface were also detected. The growth of ZnO nanostructure was explained by the V-S mechanism. The prepared ZnO nanowires exhibited a hexagonal wurtzite crystal structure. The as-fabricated ZnO nanowires are expected to find applications in nanosensors and nanodetectors.
Dr. Ping Liu got her PhD degree in 2010. She has devoted her effort in the research of one-dimensional semiconductor materials for 7 years. Her research interest lies in the fabrication and application of zinc oxide nanowires. She has published her work in several important international journals.
Shiu SC, Lin SB, Hung SC, Lin CF: Influence of pre-surface treatment on the morphology of silicon nanowires fabricated by metal-assisted etching. Appl Surf Sci 2011, 257: 1829–1834. 10.1016/j.apsusc.2010.08.086
Cheng YK, Chie G, Bau TD: Photovoltaic characteristics of silicon nanowire arrays synthesized by vapor-liquid-solid process. Sol Energ Mat Sol C 2011, 95: 154–157. 10.1016/j.solmat.2010.04.028
Kumar D, Srivastava SK, Singh PK, Husain M, Kumar V: Fabrication of silicon nanowire arrays based solar cell with improved performance. Sol Energ Mat Sol C 2011, 95: 215–218. 10.1016/j.solmat.2010.04.024
Zhou WM, Yang B, Yang ZX, Zhu F, Yan LJ, Zhang YF: Large-scale synthesis and characterization of SiC nanowires by high-frequency induction heating. Appl Surf Sci 2006, 252: 5143–5148. 10.1016/j.apsusc.2005.07.031
Li XT, Chen XH, Song HH: Preparation of silicon carbide nanowires via a rapid heating process. Mater Sci Eng B 2011, 176: 87–91. 10.1016/j.mseb.2010.09.007
Wang X, Sun XY, Fairchild M, Hersee SD: Fabrication of GaN nanowire arrays by confined epitaxy. Appl Phys Lett 2006, 89: 233115. 10.1063/1.2402893
Navamathavan R, Ra YH, Song KY, Kim DW, Lee CR: Different growth behaviors of GaN nanowires grown with Au catalyst and Au + Ga solid solution nano-droplets on Si(111) substrates by using MOCVD. Curr Appl Phys 2011, 11: 77–81. 10.1016/j.cap.2010.06.022
Chen J, Xue CS: Catalytic growth of large-scale GaN nanowires. J Mater Eng Perform 2010, 19: 1054–1057. 10.1007/s11665-009-9574-8
Zhou ZH, Wu J, Li HD, Wang ZM: Field emission from in situ-grown vertically aligned SnO2nanowire arrays. Nanoscale Res Lett 2012, 7: 117. 10.1186/1556-276X-7-117
Ma CY, Zhou ZH, Wei H, Yang Z, Wang ZM, Zhang YF: Rapid large-scale preparation of ZnO nanowires for photocatalytic application. Nanoscale Res Lett 2011, 6: 536. 10.1186/1556-276X-6-536
Wang ZL, Song JH: Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006, 312: 242–246. 10.1126/science.1124005
Wang XH, Li RB, Fan DH: Control growth of catalyst-free high-quality ZnO nanowire arrays on transparent quartz glass substrate by chemical vapor deposition. Appl Surf Sci 2011, 257: 2960–2964. 10.1016/j.apsusc.2010.10.100
Zhou ZH, Zhan CH, Wang YY, Su YJ, Yang Z, Zhang YF: Rapid mass production of ZnO nanowires by a modified carbothermal reduction method. Mater Lett 2011, 65: 832–835. 10.1016/j.matlet.2010.12.032
Khan R, Ra HW, Kim JT, Jang WS, Sharma D, Im YH: Nanojunction effects in multiple ZnO nanowire gas sensor. Sens Actuators B 2011, 150: 389–393.
Luo L, Sosnowchil BD, Lin LW: Room temperature fast synthesis of zinc oxide nanowires by inductive heating. Appl Phys Lett 2007, 90: 093101. 10.1063/1.2709618
Ramanathan S, Chen YC, Tzeng Y: Zinc oxide nanowire based field emitters. Physica E 2010, 43: 285–288. 10.1016/j.physe.2010.07.072
Wang FF, Cao L, Pan AL, Liu RB, Wang X, Zhu X, Wang HQ, Zou BS: Synthesis of tower-like ZnO structures and visible photoluminescence origins of varied-shaped ZnO nanostructures. J Phys Chem C 2007, 111: 7655–7660. 10.1021/jp067151u
Wu JJ, Liu SC: Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition. Adv Mater 2002, 14: 215–218. 10.1002/1521-4095(20020205)14:3<215::AID-ADMA215>3.0.CO;2-J
Zhu G, Yang R, Wang S, Wang ZL: Flexible high-output nanogenerator based on lateral ZnO nanowire array. Nano Lett 2010, 10: 3151–3155. 10.1021/nl101973h
Zhang Z, Meng GW, Xu QL, Hu YM, Wu Q, Hu Z: Aligned ZnO nanorods with tunable size and field emission on native Si substrate achieved via simple electrodeposition. J Phys Chem C 2010, 114: 189–193. 10.1021/jp9087223
Breedon M, Rahmani MB, Keshmirii SH, Wlodarski W, Kalantarzadeh K: Aqueous synthesis of interconnected ZnO nanowires using spray pyrolysis deposited seed layers. Mater Lett 2010, 64: 291–294. 10.1016/j.matlet.2009.10.065
Sarkar S, Patra S, Bera SK, Paul GK, Ghosh R: Water repellent ZnO nanowire arrays synthesized by simple solvothermal technique. Mater Lett 2010, 64: 460–462. 10.1016/j.matlet.2009.11.047
The authors thank the Foundation of He'nan Educational Committee (no. 2011A470015) and the Henan province science and technology tackling key project (no. 102102210444).
The authors declare that they have no competing interests.
PL prepared the manuscript and supervised all of the study. YBL performed the experiment. YQG helped in the technical support for the experiments. ZHZ participated in the measurements. All the authors discussed the results and approved the final manuscript.
About this article
Cite this article
Liu, P., Li, Y., Guo, Y. et al. Growth of catalyst-free high-quality ZnO nanowires by thermal evaporation under air ambient. Nanoscale Res Lett 7, 220 (2012). https://doi.org/10.1186/1556-276X-7-220
- zinc oxide
- thermal evaporation