Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2011, 40, 10961 www.rsc.org/dalton PAPER Synthesis, field-emission and electric properties of metastable phase VO 2 (A) ultra-long nanobelts Ming Li, Fengyu Kong, Liang Li, Yunxia Zhang, Li Chen, Weiwei Yan and Guanghai Li* Received 19th May 2011, Accepted 4th August 2011 DOI: 10.1039/c1dt10941c High quality single crystalline metastable phase VO 2 (A) ultra-long nanobelts were synthesized by hydrothermal method using inorganic V 2 O 5 sol as precursor and polyethylene glycol (PEG) as both surfactant and reducing agent. It was found that the oriented attach growth mechanism is responsible for the formation of VO 2 (A) nanobelts. In addition to an endothermic peak, an unusual exothermic peak was detected in DSC curve of the nanobelts. The temperature dependence of the lattice parameters have been studied, and it was found that the a-axis expands while the c-axis contracts in the high-temperature XRD test. The VO 2 (A) nanobelt has a low turn-on field of 3.8 V mm -1 and a high field enhancement factor of 1739 in the field emission measurement. Electrical transport measurement of a single VO 2 (A) nanobelt gives a relative low hoping activation energy of 0.28 eV. 1. Introduction One-dimensional nanostructures, such as nanobelts and nan- otubes, have stimulated great interest due to their fundamental research importance and wide range of potential technique appli- cations in nanodevices. 1–6 Vanadium dioxide (VO 2 ) nanostructures have many promising properties for various applications in the nanometre regime, and have been the research focus of recent years. 7–11 VO 2 can adopt a number of polymorphic forms, such as VO 2 (M), VO 2 (R), VO 2 (B) and VO 2 (A) as well as recently reported VO 2 (C). 12 Among them, VO 2 (R), the most thermodynamic stable phase, has been attracting a lot of attention for its reversible phase transition (VO 2 (M)↔VO 2 (R)) at about 68 ◦ C. By contrast, metastable phase VO 2 (A), which also exhibits a similar reversible phase transition at a temperature around 162 ◦ C, has been scarcely reported due to the harsh growth conditions (obtained exclusively by a hydrothermal method). VO 2 (A) was first synthesized by hydrothermal reaction of a V 2 O 3 –V 2 O 5 –H 2 O system. 13 The crystal structure, phase transition mechanism and electrical properties of VO 2 (A) have been investigated, 14–18 and a simple crystallographic slip mechanism was proposed to explain the phase transition process from VO 2 (B) to VO 2 (A). 19 To the best of our knowledge, there were no reports on VO 2 (A) nanostructures during the past decade. Until recently, VO 2 (A) nanorods were fabricated via reduction of V 2 O 5 by oxalic acid through hydrothermal treatment at the relative high temperature of 270 ◦ C, 20 and thus a facile preparation method and property exploration of the metastable phase VO 2 (A) is essential for practical applications. In addition, V 2 O 5 sol has been known for a long time, and different synthesis methods have been reported in the literature, Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomate- rials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, P.R. China. E-mail: ghli@issp.ac.cn including inorganic, metal–organic precursors and ion exchange methods. 21 This sol, which can remain stable for months and even years when kept in a closed vessel, exhibits some amazing properties. Because of cost-saving and ease of handling, inorganic V 2 O 5 sols have been widely used to prepare VO 2 films. In comparison, there are few studies on hydrothermal synthesis of metastable phase VO 2 (A) nanostructures using inorganic V 2 O 5 sols as precursors. In this paper, VO 2 (A) nanobelts were synthesized by a facile and economical method combing sol–gel and hydrothermal techniques. In our approach an inorganic V 2 O 5 sol was used as the vanadium source and polyethylene glycol (PEG) was used as both the reducing agent and surfactant. The formation mechanism, the thermal effect related to the phase transition, the variable- temperature XRD studies, the field emission (FE) and electrical transport properties of the nanobelts are presented and discussed. Our primary results are helpful in the fabrication and exploration of the physical properties of the metastable phase VO 2 (A). 2. Experimental 2.1. Preparation of VO 2 (A) nanobelts V 2 O 5 sol was obtained through a melt quenching process. The V 2 O 5 powder (99.9%) was heated up to 850 ◦ C in a crucible until molten, and then was quickly poured into 600 ml of distilled water at room temperature. After vigorous stirring for one day, a red V 2 O 5 sol was obtained. In a typical reaction, a certain amount of polyethylene glycol (PEG-6000) (0.3–1.5 g) was added into 40 ml of V 2 O 5 sol to form a homogeneous solution after stirring, and then the solution was placed in a 50 mL Teflon cup that was heated in a sealed autoclave with a stainless steel shell at 220 ◦ C for 4 to 6 days. The precipitate obtained upon cooling to room temperature This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 10961–10965 | 10961 Downloaded by Hefei Institutes of Physical Science, Chinese Academy of Sciences on 22 October 2011 Published on 14 September 2011 on http://pubs.rsc.org | doi:10.1039/C1DT10941C View Online