Transparent-conducting, gas-sensing nanostructures (nanotubes, nanowires, and thin films) of titanium oxide synthesized
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A template-based, electroless wet-chemical method for synthesis of nanotubes and nanowires of nanocrystalline anatase titanium oxide (titania) at 45 °C is reported. Single-nanowire electrical property measurements reveal low dc resistivities (7–21 × 10−4 ⍀ cm) in these titania nanowires. In the presence of 1000 parts per million of CO gas at 100 °C, the resistivity is found to increase reversibly, indicating low-temperature gas-sensing capability in these titania nanowires. Thin films of nanocrystalline anatase titania, deposited using a similar wet-chemical method, also have low room-temperature dc resistivities (6–8 × 10−3 ⍀ cm), and they are transparent to visible light. Nanostructure-properties relations, together with possible electrical conduction, optical absorption, and gas-sensing mechanisms, are discussed. The ability to fashion transparent-conducting and gas-sensing nanocrystalline anatase titania into nanotubes/nanowires and thin films at near-ambient conditions could open a wider field of applications for titania, including nanoelectronics, chemical sensing, solar cells, large-area windows and displays, invisible security circuits, and incorporation of biomolecules and temperature-sensitive moieties.
I. INTRODUCTION
Titanium oxide (titania) is used in a wide range of consumer applications such as paint, paper, plastics, cement, toothpaste, and sunscreen. In the past few years, nanostructured titania has found use in many advanced applications such as photocatalysis (water cleavage and detoxification),1,2 photovoltaics,3 chemical sensing,4,5 electrochromics,6 protein immobilization,7 and biomedical implants.8 Titania is a wide-band gap semiconductor (e.g., 3.0 eV for rutile phase and 3.2 eV for anatase phase9), and in pure, bulk form, it has a very high electrical resistivity at room temperature (∼1013 ⍀ cm).10 It has been shown that the resistivity of titania can be decreased dramatically by making it nanostructured11,12 and by doping.13 Being able to tailor the electrical conductivity of nanostructures of titania is likely to enhance its present applications and lead to new sets of applications. a)
Present address: Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. b) Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy DOI: 10.1557/JMR.2006.0352 2894
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J. Mater. Res., Vol. 21, No. 11, Nov 2006 Downloaded: 07 Jun 2014
Motivated by the new paradigm of “bottom up” nanoelectronics,14–19 nanowire/nanotube building blocks of titania have been synthesized using a variety of methods, including precipitation,20 hydrothermal synthesis,21 template-based chemical deposition,22 template-based electroless23,24 and electrodeposition of sols,25,26 electrochemical deposition,27–29 chemical vapor deposition,30 and physical-vapor deposition.3
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