DOI: 10.1002/adfm.200700038 Co 3 O 4 Nanostructures with Different Morphologies and their Field-Emission Properties** By Binni Varghese , Teo Choon Hoong, Zhu Yanwu, Mogalahalli V. Reddy , Bobba V. R. Chowdari, Andrew Thye Shen Wee, Tan B. C. Vincent, Chwee Teck Lim,* and Chorng-Haur Sow* 1. Introduction Nanostructured materials with tunable characteristics, such as chemical composition, relative size, and morphological structure, have great potential from a fundamental as well as a technological viewpoint. Recent demonstrations of the applica- tion of nanostructures as electron field emitters, [1] functional nanoelectronic components, [2] and chemical or biological sen- sor elements [3] have an invigorating influence on nanomaterials research. Among the wide variety of research activities on nanomaterials, nanostructure-based field emitters have at- tracted much attention because of their great commercial po- tential. Carbon nanotubes (CNT) have long been recognized as an excellent field emitter with a low turn-on field and high current density. [4] However, some of the challenges involved in the development of CNT emitters include uncertainty in their electronic properties, and difficulties involved in their manipu- lation and integration into devices. Several researchers have fo- cused their effort on the search for alternate field-emitting ma- terials. Studies on semiconducting oxide nanostructures have shown great promise in this direction. [5] Cobalt oxides belong to the family of transition-metal oxides and the most stable phase, Co 3 O 4 , is an intrinsic p-type semi- conductor (direct optical bandgaps at 1.48 and 2.19 eV). [6] The chemical stability of Co 3 O 4 over a wide temperature range and its high mechanical strength (Young’s modulus ≈ 116–160 GPa) render this material a potential candidate for field emitters. The field-emission (FE) properties of Co 3 O 4 nanostructures, however, have not been investigated in detail. Cobalt oxide nanoparticles have exhibited outstanding electrochemical, [7] magnetic, [8] catalytic, [9] and gas sensing [10] capabilities. In recent years, the synthesis and structural characterization of cobalt ox- ide nanostructures have been reported. [11] More recently, Nam et al. demonstrated the feasibility of employing viruses to synthesize and assemble cobalt oxide nanowires as an elec- trode material (anode) for lithium-ion batteries. [12] In an earlier report, our group demonstrated an approach to synthesize aligned cobalt oxide 2D nanostructures (nanowalls) by direct heating of a cobalt foil using a hotplate under ambient condi- tions and we investigated their FE properties. [13] However, the hotplate technique is limited to the formation of simple 2D nanostructures. Rational synthetic approaches to form differ- ent nanostructures are highly desirable to fully exploit the po- tential offered by the reduced dimensionality. In this Full Paper, we report a facile method to synthesize vertically aligned, single-crystalline 1D (nanowires) or 2D 1932 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 1932–1939 – [*] Prof. C. T. Lim, Prof. C.-H. Sow, B. Varghese,T. C. Hoong, Dr. Z. Yanwu,Prof. A. T. S. Wee,Prof. T. B. C. Vincent National University of Singapore Nanoscience and Nanotechnology Initiative (NUSNNI) 2 Science Drive 3, Singapore 117542 (Singapore) E-mail: mpelimct@nus.edu.sg Prof. C.-H. Sow, B. Varghese,T. C. Hoong,Dr. Z. Yanwu, Dr. M. V. Reddy,Prof. B. V. R. Chowdari,Prof. A. T. S. Wee Department of Physics, Faculty of Science National University of Singapore 2 Science Drive 3, Singapore 117542 (Singapore) E-mail: physowch@nus.edu.sg Prof. C. T. Lim Division of Bioengineering National University of Singapore Blk E3A, 9 Engineering Drive 1, Singapore 117576 (Singapore) Prof. C. T. Lim,Prof. T. B. C. Vincent Department of Mechanical Engineering National University of Singapore 9 Engineering Drive 1, Singapore 117576 (Singapore) [**] Supporting Information is available online from Wiley InterScience or from the authors. We report an efficient method to synthesize vertically aligned Co 3 O 4 nanostructures on the surface of cobalt foils. This synthe- sis is accomplished by simply heating the cobalt foils in the presence of oxygen gas. The resultant morphologies of the nano- structures can be tailored to be either one-dimensional nanowires or two-dimensional nanowalls by controlling the reactivity and the diffusion rate of the oxygen species during the growth process. A possible growth mechanism governing the formation of such nanostructures is discussed. The field-emission properties of the as-synthesized nanostructures are investigated in detail. The turn-on field was determined to be 6.4 and 7.7 V lm –1 for nanowires and nanowalls, respectively. The nanowire samples show superior field-emission characteristics with a lower turn-on field and higher current density because of their sharp tip ge- ometry and high aspect ratio. FULL PAPER