Highly sensitive and selective room temperature alcohol gas sensors based on TeO2 nanowires
Introduction
Low-dimensional nanomaterials with different morphologies, such as nanowires, nanorods, nanotubes, and nanobelts, have received considerable attention recently due to their unique physical and chemical properties and potential applications in nanoscale devices [1], [2], [3], [4]. Various inorganic nanomaterials including metals, such as Pt and Pd [5], [6]; oxides, such as SnO2, WO3, and ZnO [7], [8], [9]; sulfides, such as CdS and PbS [10], [11]; nitrides, such as GaN and Si3N4 [12], [13]; and also complicate oxide, such as ITO and WO3/SnO2 [14], [15] have been fabricated and characterized. Such nanomaterials have attracted particular interest for their special technological applications due to not only the superior optical, electrical, thermal, and mechanical properties, but also their high efficiency and activity caused by their large surface-to-volume ratio and high porosity [16].
In the past decades, the solid-state gas sensors based on metal oxide semiconductors have played an important role in environment monitoring, domestic safety, and chemical process controlling due to their distinct advantages such as simple implementation, low cost, high reproducibility, and compatibility with micro-fabrication processes [17], [18]. Various materials such as SnO2, WO3, In2O3, ZnO, TiO2, and mixed oxides have been investigated and showed promising application for detecting toxic and harmful gases [17], [19]. Although some of gas sensors based on metal oxide semiconductors have been commercialized for years, many problems still need to be solved in order to improve sensitivity, selectivity, and stability, requiring the further development in high-performance gas sensing devices [20].
Tellurium dioxide (TeO2), as a versatile wide band gap semiconductor material, is a significant acousto-optical and electro-optical material with a variety of desirable characteristics including elastic behaviour, high refractive index, and good optical quality [21]. Some works have been carried out to prepare TeO2 materials and investigated their gas sensing properties in recent years. Liu et al. and Shen et al. [22], [23], [24] investigated the gas sensing properties of TeO2 nanowires synthesized by thermal evaporation of Te metal in air. They showed that TeO2 nanowires with p-type conduction had good sensing performance to toxic gases such as NO2, NH3, and H2S at room temperature. TeO2 nanomaterials synthesized by Kim et al. [21] and Guan et al. [25] were reported to show p-type semiconductor and a promising NO2 sensing at room temperature. However, Siciliano et al. [26] investigated NH3 sensing properties of sputtered TeO2 thin films. The increase in the conductance suggested that these films had an n-type conduction at the operating temperatures ranging from 130 to 220 °C. Moreover, Siciliano et al. [27] also synthesized TeO2 nanowires by thermal evaporation of Te metal in oxygen atmosphere without the presence of any catalysts. The results clearly showed that the transition from n-type to p-type conduction of TeO2 nanowires was induced by the variation of the ethanol concentration. Although these works have been performed to clarify the gas sensing properties of TeO2 materials, their conduction properties are entirely different. Therefore, it is necessary to further confirm the conduction and gas sensing properties of TeO2 materials in order to enhance their practical application.
In the present study, TeO2 nanowires were synthesized by thermal evaporation of Te powders in air at ambient pressure. The structure, morphology, and composition of TeO2 nanowires were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy with energy dispersive X-ray spectroscopy. The alcohol sensing properties demonstrated that TeO2 nanowires showed an n-type conduction and reversible response at room temperature. The results indicate a possibility of developing low power consumption gas sensors based on TeO2 nanowires.
Section snippets
Experimental
TeO2 nanowires were synthesized on Au-coated glass substrates by thermal evaporation of Te powders. The schematic diagram of the apparatus used for the synthesis of TeO2 nanowires is shown in Fig. 1. In a typical experimental procedure, 0.5 g of Te powders with a high purity of 99.999% was placed in an alumina boat and positioned in the central part of a 45 cm long horizontal quartz tube in a tubular electric furnace. The glass substrates coated with an Au film with a thickness of 10 nm by ion
Microstructure characterization
Fig. 2 illustrates the XRD pattern of the obtained TeO2 nanowires. All the diffraction peaks can be indexed to the tetragonal TeO2 structure with lattice constants of a = b = 0.4810 nm and c = 0.7613 nm according to JCPDS card no. 11-0693 [28]. The strongest diffraction peak due to the reflection from the (102) crystal plane reveals that TeO2 nanowires grow with a strong preferred orientation. More importantly, no other crystal forms were observed on the as-prepared product, confirming the
Conclusions
TeO2 nanowires were synthesized by thermal evaporation of Te powders in air at ambient pressure. The structure, morphology, and composition of TeO2 nanowires were characterized by XRD, FESEM, TEM and EDX. TeO2 nanowires were about 70–200 nm in diameter and several hundreds of micrometers to 2 mm in length, which grew based on the VS growth mechanism. Ethanol gas sensing properties demonstrated that TeO2 nanowires had an n-type conduction and showed a reversible response to ethanol gas at room
Acknowledgement
The project was supported by the National Natural Science Foundation of China (51422402), Fundamental Research Funds for the Central Universities (N140105002, N130301003, L1501003), Program for Liaoning Excellent Talents in University (LJQ2013025), Specialized Research Fund for the Doctoral Program of Higher Education of China (20130042120033), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (47-3). The authors also wish to gratefully thank
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