Well-aligned ZnO nanowires with excellent field emission and photocatalytic properties† Fu-Hsuan Chu, a Chun-Wei Huang, a Cheng-Lun Hsin, a Chun-Wen Wang, a Shih-Ying Yu, a Ping-Hung Yeh b and Wen-Wei Wu * a Received 12th July 2011, Accepted 6th September 2011 DOI: 10.1039/c1nr10796h Well-aligned ZnO nanowires (NWs) were successfully synthesized on Si(100) by the process of carbothermal reduction and vapor– liquid–solid method. Scanning electron microscopy and transmission electron microscopy results confirmed that ZnO NWs were single crystalline wurtzite structures and grew along the [0001] direction. The influences of substrate temperature and total pressure on the growth were discussed. The well-aligned ZnO NWs show good field emission properties, and the emitter constructed of pencil-like ZnO NWs exhibited a low turn-on field (3.82 V mm 1 ) and a high field enhancement factor (b ¼ 2303). Finally, we demonstrated that the as-prepared ZnO NWs with small diameter on the substrate have good photocatalytic activity toward degradation of methylene blue. Using ZnO NWs with Au nanoparticles (NPs) would decrease the recombination rate of hole–electron pairs due to the great shift of the Fermi level to the conduction band. Hence, adding Au NPs was a promising method to enhance the photocatalytic performance of ZnO NWs. It is significant that photocatalyst fabricated by ZnO NWs can apply to the degradation of organic pollution, and solve the environmental issues. Introduction ZnO nanostructures, such as nanobelts, 1,2 nanohelixes, 3 nano- combs, 4,5 nanosprings, 6 nanotubes, 7 nanowires, 8 and nanorings 9 have attracted increasing attention in recent years due to a wide band gap of 3.37 eV and a large bonding energy of 60 meV. The growth of ZnO nanostructures can be synthesized by various methods, such as vapor phase transport, 10 metal–organic chemical vapor deposition (MOCVD), 11 pulse-laser ablation, 12 a hydrothermal method, 13 and an electrochemical deposition technique. 14 Among the nano- structures, ZnO nanowires (NWs) have great potential applications in photoelectronic devices, 15,16 electronic devices, 17,18 dye-sensitized solar cells, 19 nanosensors, 20–22 and nanogenerators, 23 attributing to their high aspect ratio and chemical stability. The well-aligned ZnO NWs tend to improve the emission efficiency compared with randomly grown NWs. And the tipped structures of NWs could be beneficial for the application of field emission display, cath- odoluminescence, antireflection and self-cleaning. 24–26 The surface effect also plays an important role in the device efficiency including solar cells and sensors. 27 Moreover, ZnO NWs could have potential applications as photocatalyst in environmental purification, for example, the organic contaminants can be decomposed by the pho- tocatalytic reaction of ZnO NWs. In this paper, we present the growth of well-aligned ZnO NWs using a carbothermal reduction method and a vapor–liquid–solid (VLS) mechanism on a Si substrate with a Au catalyst film. The shape of the ZnO NWs can be controlled by tuning growth parameters and the field-emission property can be enhanced. The pencil-like structure showed excellent field emission properties with a low turn-on field and a high field enhancement factor. Furthermore, Au-NP-func- tionalized ZnO NWs were synthesized. By controlling the diameter of Au NPs, the photodegradation will be enhanced due to the decrease in built-in potential and increase of the residual holes. With these results, we may provide some further information for ZnO nano- structure applications. The photocatalytic activity of ZnO NWs was studied by degradation of methylene blue (MB) with various diam- eters. For enhancement of the photocatalytic property, Au NPs were synthesized on the surface of ZnO NWs using a physical method. The effect of Au NP size on photodegradation of MB has been studied. Experimental The growth of well-aligned ZnO NWs was performed in a three-zone furnace using a carbothermal reduction method and VLS mecha- nism. The 3 nm-thick Au film was deposited on Si(100), which serves as a catalyst. The temperatures at different zones of the furnace were set at 950, 750 and 600  C, respectively. ZnO and graphite powder were mixed with a weight ratio of 2 : 1 in an alumina boat at an upstream zone (950  C) and the Si substrate was place at the midstream zone (750  C). The flow rates of Ar and O 2 were adjusted to 100 sccm and 10 sccm, respectively. The chamber was pumped to the pressure of 0.75 Torr. The temperature was elevated at the rate of 10  C min 1 and maintained at the highest temperature for 1.5 hour. a Department of Materials Science and Engineering, National Chiao Tung University, No. 1001, University Rd, East Dist, Hsinchu City, 300, Taiwan. E-mail: wwwu@mail.nctu.edu.tw; Fax: +886 3 5724727; Tel: +886 3 5712121 55395 b Department of Physics, Tamkang University, No. 151 Yingzhuan Rd, Danshui Dist, New Taipei City, 25137, Taiwan † This article was submitted as part of a collection highlighting papers on the ’Recent Advances in Semiconductor Nanowires Research’ from ICMAT 2011. This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 1471–1475 | 1471 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 1471 www.rsc.org/nanoscale COMMUNICATION Published on 07 October 2011. Downloaded by National Chiao Tung University on 28/04/2014 23:28:58. View Article Online / Journal Homepage / Table of Contents for this issue