ARTICLES nature materials | VOL 2 | DECEMBER 2003 | www.nature.com/naturematerials 821 E xtended and oriented nanostructures are desirable for many applications, including microelectronic devices, chemical and biological sensing and diagnosis, energy conversion and storage (photovoltaic cells, batteries and capacitors, and hydrogen storage devices), light-emitting displays, catalysis, drug delivery, separation, and optical storage. Previously, oriented carbon nanotubes 1,2 and oriented ZnO nanorods 3–10 have been prepared by high-temperature vacuum deposition techniques. Similar techniques were used for making nanowires of Si (ref. 11) and silicon carbide/nitride 12 , and nanobelts of a wide range of oxide materials 13 . Recently, hydrothermal solution synthesis 14,15 and electrochemical deposition in porous membranes 16–19 were also investigated to produce oriented ZnO nanorods and tubes. Although oriented nanowires and nanorods have attracted wide attention, the direct fabrication of large arrays of complex nanostructures with controlled crystalline morphology, orientation and surface architectures remains a significant challenge. We developed a low-temperature, environmentally benign, solution-based method to prepare complex ZnO nanostructures. ZnO is an important wide-bandgap semiconducting ceramic material with many useful properties (for reviews see ref. 20) such as piezoelectricity, conductivity, optical absorption and emission, high voltage–current nonlinearity, sensitivity to gases and chemical agents, and catalytic activity 21 . It has been extensively investigated for applications in luminescence 4,13,22,23 and as window and electrode material for solar cells 24 , phosphors 25 , piezoelectric transducers and actuators 26 , surface acoustic coatings 27 , varistors 28 , microsensors 29 , photocatalysts 30 , decontamination agents 27 and so on. The properties of the ZnO depend closely on the microstructures of the materials, including crystal size, orientation and morphology, aspect ratio and even crystalline density. The surface area and morphology (how the crystals are stacked) also have a crucial role in many applications (such as photo-emitters, transducers, actuators, varistors, sensors and catalysts). We used a seeded growth procedure that we developed 31–33 to control the nucleation event, and citrate anions to control the crystal morphologies. We chose citrate because it adsorbs strongly on metal 34 and mineral surfaces 35 and significantly alters the surface properties 36,37 and mineral growth behaviour 38 . The use of citrate anions will also allow us to develp molecular modelling and spectroscopic techniques in future to enhance our understanding of how these organic molecules bind to the different ZnO surfaces and affect crystal growth. Organic molecules are known to either promote or inhibit crystal growth.Although organic Extended and oriented nanostructures are desirable for many applications, but direct fabrication of complex nanostructures with controlled crystalline morphology, orientation and surface architectures remains a significant challenge. Here we report a low-temperature, environmentally benign, solution-based approach for the preparation of complex and oriented ZnO nanostructures, and the systematic modification of their crystal morphology. Using controlled seeded growth and citrate anions that selectively adsorb on ZnO basal planes as the structure- directing agent, we prepared large arrays of oriented ZnO nanorods with controlled aspect ratios, complex film morphologies made of oriented nanocolumns and nanoplates (remarkably similar to biomineral structures in red abalone shells) and complex bilayers showing in situ column-to-rod morphological transitions. The advantages of some of these ZnO structures for photocatalytic decompositions of volatile organic compounds were demonstrated. The novel ZnO nanostructures are expected to have great potential for sensing, catalysis, optical emission, piezoelectric transduction, and actuations. Complex and oriented ZnO nanostructures ZHENGRONG R. TIAN 1 , JAMES A. VOIGT 1 , JUN LIU 1 *, BONNIE MCKENZIE 1 , MATTHEW J. MCDERMOTT 1 , MARK A. RODRIGUEZ 1 , HIROMI KONISHI 2 AND HUIFANG XU 2 1 Materials and Process Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA 2 Department of Earth and Planetary Sciences, University of New Mexico, New Mexico 87131, USA *e-mail: jliu@sandia.gov Published online: 23 November 2003; doi:10.1038/nmat1014 ©2003 Nature Publishing Group