Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors
© Park et al.; licensee Springer. 2014
Received: 26 August 2014
Accepted: 14 November 2014
Published: 27 November 2014
TeO2-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In2O3, TiO2, Ga2O3, etc. TeO2/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO2 core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO2/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 μm in length. The multiple networked TeO2/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO2 at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO2 is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO2 nanorods over the same NO2 concentration range. The underlying mechanism for the enhanced NO2 sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism.
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In recent years, one-dimensional (1D) nanostructure-based sensors attracted considerable attention owing to their high surface-to-volume ratios [1–5]. Considerable effort has been made to develop 1D nanostructured gas sensors with good sensing performances, but further improvements in the sensitivity of 1D nanostructured sensors are needed. The fabrication of heterostructures [6–8] is a promising technique to improve the sensitivity of the 1D nanostructured sensors. The improved sensing performance of the heterostructured 1D sensors has been attributed to a range of factors including increased potential barriers at the interface of the heterostructure [9, 10], modulated depletion layer [11, 12], band bending due to equilibration of the Fermi energy levels , synergistic surface reactions , etc.
Paratellurite (α-TeO2) is a metal oxide semiconductor with a distorted rutile structure . TeO2 has applications in optical storage, laser devices and gas sensors, dosimeters, modulators, and deflectors owing to its unique properties such as high refractive index and high optical nonlinearity . TeO2-nanostructured sensors have attracted less attention compared to other metal oxide semiconductor materials such as ZnO, In2O3, TiO2, Ga2O3, etc. In 2007, Liu et al.  synthesized TeO2 nanowires that were sensitive to NO2, NH3, and H2S gases. According to their results, TeO2 1D nanostructures are promising for producing low power consumption gas sensors. The incorporation of a surface decoration or heterostructure formation technique can improve their sensing performance further. In this regard, a recent study reported the sensing properties of Pt-doped TeO2 nanorods . On the other hand, this paper reports the synthesis of TeO2-core/CuO-shell nanorods and the sensing properties of multiple networked TeO2-core/CuO-shell nanorod gas sensors toward NO2 gas. The underlying mechanism for the enhanced sensing performance of the core-shell nanorod sensors is also discussed.
TeO2/CuO core-shell nanorods were synthesized using a two-step process: thermal evaporation of Te powder followed by sputter deposition of CuO. TeO2 nanorods were synthesized on a p-type Si (100) substrate in a quartz tube furnace by thermal evaporation of Te powder at 400°C in air without a metal catalyst or the supply of other gas. The thermal evaporation process was conducted at room temperature for 1 h and the furnace was cooled to room temperature. Subsequently, the TeO2 nanorods were coated with a thin CuO layer by sputtering a CuO target by radio frequency (RF) magnetron sputtering from a CuO target. The base and working pressure was 5.0 × 10-6 Torr and 2.0 × 10-2 Torr, respectively, and the N2 gas flow rate was 20 cm3/min throughout the evaporation process. The RF sputtering power and sputtering time were 100 W and 20 min, respectively.
The structure and morphology of the nanorod samples were characterized by scanning electron microscopy (SEM, Hitachi S-4200, Billerica, MA, USA), transmission electron microscopy (TEM, Philips CM-200, Eindhoven, the Netherlands), and selected area electron diffraction. X-ray diffraction (XRD, Philips X’pert MRD, Eindhoven, the Netherlands) patterns were performed using Cu Kα radiation (0.15406 nm). Energy-dispersive X-ray spectroscopy (EDS) was carried out to examine the elemental composition of the core-shell nanorod samples. The resistance of multiple networked pristine TeO2 nanorod and TeO2/CuO core-shell nanorod sensors were measured using a Keithley source meter-2612 at a source voltage of 10 V at 150°C and 50% RH. The 50% relative humidity might be somewhat high for sensing tests. A flow-through technique was used to test the gas sensing properties. NO2 gas diluted with synthetic air at different ratios was injected into the testing tube at a constant flow rate of 200 cm3/min. The detailed procedures for sensor fabrication and the sensing test are reported elsewhere .
Results and discussion
Responses of the TeO 2 /CuO nanorod sensor to NO 2 gas at different concentrations at 150°C
Response (Ra/Rg, %)
Figure 3c compares the response to NO2 gas between pristine TeO2 nanorods and TeO2/CuO core-shell nanorods in the NO2 concentration range below 10 ppm. The response of an oxide semiconductor sensor can be expressed as R = A [C]n + B, where A and B, n, and [C] are constants, exponent, and target gas concentration, respectively . Data fitting gave R = 7.52 [C] + 132.5 and R = 27.48 [C] + 153.9 for the pristine TeO2 nanorod and TeO2-core/CuO-shell nanorod sensors, respectively. The core-shell nanorod sensor showed stronger response and higher increasing rate in response to NO2 gas at lower concentrations than the pristine nanorod sensor.
Comparison of the responses of the TeO 2 /CuO core-shell nanorod sensor with those of other oxide 1D nanostructure sensors
In-doped SnO2 nanoparticles
SnO2 hollow spheres
Ru-doped SnO2 nanowire
WO3-doped SnO2 thin film
Au-doped WO3 powders
Mesoporous WO3 thin film
180 to 300
CdO nanowire (porous)
70 to 2,000
20 to 320
TeO2/CuO core-shell nanorods were synthesized using a two-step process: the synthesis of TeO2 nanorods by thermal evaporation of Te powder and sputter deposition of CuO. The cores and shells of the nanorods were single crystal TeO2 and polycrystalline CuO, respectively. The responses of the TeO2 nanorods to NO2 were improved approximately 2.1- to 2.1-fold at NO2 concentrations of 0.5 to 10 by coating them with CuO. The responses of the core-shell nanorods to NO2 gas were also comparable or superior to those of the other metal oxide semiconductor nanostructured sensors reported previously. The enhanced response of the TeO2/CuO core-shell nanorods to NO2 gas may be due to modulation of the heights of the potential barriers formed at three different places in the multiple networked 1D nanostructure sensor: the TeO2 core-CuO shell interface, the CuO-CuO homojunction at the contact of two core-shell nanorods, and the grain boundaries in the polycrystalline CuO shell layers.
This study was supported by the 2010 Core Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
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