Substrate-friendly synthesis of metal oxide nanostructures using a hotplate.

Nanostructures with large surface areas and possible quantum-confinement effects exhibit distinct electronic, optical, mechanical, and thermal properties and are believed to be essential to much of modern science and technology. Some of the most important and widely studied candidates in the family of nanomaterials are metal oxide nanostructures. This is due to their great potential in addressing some fundamental scientific issues on low dimensionality and applications thereof. These include chemical or biological sensors, electron-field emitters, electrodes of lithium-ion batteries, lasers, and optical switches for nanoscale memory and logic devices. Many techniques have been developed to synthesize metal oxide nanostructures, including an ethylene glycol mediated synthesis, a carbothermal reduction process, a vapor–liquid–solid (VLS) process, and a vapor– solid (VS) process. Among these methods, the VS and VLS processes are well suited for the synthesis of metal oxide nanostructures with single-crystalline structures and in relatively large quantities. For example, Ga2O3 and ZnO nanowires have been synthesized by heating powders of these materials to elevated temperatures in a tube furnace; oxide nanowires of low-melting-point metals, such as Zn, Mg, and Ge, could be fabricated by directly heating metal powders in a tube furnace under appropriate oxygen atmospheres. Besides wire-shaped nanostructures, semiconducting metal oxide nanobelts were also successfully created. Both VS and VLS processes require careful control of the growth conditions. For example, to generate vapor sources, normally elevated formation temperatures are necessary ( 1000 8C). The appropriate atmosphere, flow rate, and substrate position must be carefully controlled. For the VLS process, an appropriate catalyst must be selected, and subsequently the properties of the nanostructures may be modified because of the catalyst introduced. As new techniques for the synthesis of nanomaterials are continuously being discovered, it is also important to develop a method with more practical attributes. Some of the desirable, practical attributes of a new synthesis technique include mass production, rapid growth, catalyst-free, diversity of materials made, low-formation temperature, reasonably low costs, and ease of nanomaterials assembly onto various substrates for further characterization and applications. Here, we report a novel and yet surprisingly simple method to synthesize metal oxide (a-Fe2O3, Co3O4, and ZnO) nanostructures by directly heating metal-foilor metal-film-coated substrates in air using a hotplate (hereafter called the hotplate method). Successful attempts have been achieved on a wide variety of substrates, such as a plain silicon wafer, a glass slide, quartz, a silica microsphere, atomic force microscopy (AFM) tips, and electrochemically etched W tips. Our prior attempts with the hotplate technique created CuO nanowires on Cu foil, ZnO nanostructures on Zn foil, and Co3O4 on Co foil. [12] In this work, we extend this method to another high-melting-point metal, iron (Fe, melting point: 1538 8C), and synthesize single-crystalline aFe2O3 (hematite) nanoflakes by heating Fe foil directly in air. More importantly, we further develop this method to be able to directly synthesize the nanostructures on a wide variety of substrates. In this report, we demonstrate this hotplate method by focusing on a-Fe2O3 nanoflakes. a-Fe2O3 has been extensively used in the production of gas sensors, catalysts, and pigments. Nanoscale a-Fe2O3 with different morphologies, such as nanoparticles, nanorods, and nanotubes, has been successfully synthesized and exhibits promising applications. However, to the best of our knowledge, there has been no report for the preparation of aFe2O3 two-dimensional (2D) nanostructures. As an illustration of how a new morphology may generate new properties for nanostructures, we investigate the field-emission properties of a-Fe2O3 nanoflakes for the first time. Optical pictures of a piece of iron foil (10 cm?10 cm) during the heating process were captured. Figures 1a–c show the foil before heating, after heating at 300 8C for 10 min, and after heating at 300 8C for 24 h, respectively. The shiny surface of the Fe foils rapidly became dull and gray, and eventually became dark for long heating durations. Figures 1d–f show the corresponding SEM images of the [*] Dr. T. Yu, Y. Zhu, Dr. P. Chen, Prof. C.-H. Sow Department of Physics, Blk S12, Faculty of Science National University of Singapore 2 Science Drive 3, Singapore 117542 (Singapore) Fax: (+65)6777-6126 E-mail: physowch@nus.edu.sg