We have synthesized brushlike p-Te/n-SnO2 hierarchical heterostructures by a two-step thermal vapor transport process. The morphologies of the branched Te nanostructures can be manipulated by adjusting the source temperature or the argon flow rate. The growth of the branched Te nanotubes on the SnO2 nanowire backbones can be ascribed to the vapor-solid (VS) growth mechanism, in which the inherent anisotropic nature of Te lattice and/or dislocations lying along the Te nanotubes axis should play critical roles. When exposed to CO and NO2 gases at room temperature, Te/SnO2 hierarchical heterostructures changed the resistance in the same trend and exhibited much higher responses and faster response speeds than the Te nanotube counterparts. The enhancement in gas sensing performance can be ascribed to the higher specific surface areas and formations of numerous Te/Te or TeO2/TeO2 bridging point contacts and additional p-Te/n-SnO2 heterojunctions.
We have synthesized two highly sensitive, room-temperature operating TeO/SnO gas sensors with hierarchical nanowire structures. One is a brush-like nanostructure, from a two-step thermal vapor-transport route, and the other one is a TeO/SnO bead-like nanostructure, from annealing of the former. The TeO/SnO nanostructures exhibit a greatly enhanced room-temperature gas-sensing response compared to pristine TeO nanowires in the sequence: TeO/SnO bead-like structure > brush-like structure > pristine TeO nanowire. The response of the TeO/SnO bead-like structure is in a range of 10 to 20 against NO gas of ppm levels (3-100 ppm) at room temperature. This compares favorably to the response, smaller than 2, for the pristine TeO nanowires. Interestingly, the TeO/SnO bead-like structure exhibits a typical n-type gas-sensing behavior, in contrast to the p-type behavior from the brush-like and the pristine TeO structures. Possible hybrid growth and sensing mechanisms are discussed.
Vanadium pentoxide (V 2 O 5 ) nanowires decorated with CuO nanoparticles on their surface have been prepared by a facile chemical route. The gas-sensing performance of the CuO/V 2 O 5 nanohybrids has been examined against H 2 S, CO, and NO 2 gases over a range of gas concentrations from 7 to 60 ppm and working temperatures from 100 to 400 C, and compared with that of pristine V 2 O 5 nanowires without the decoration. The CuO/V 2 O 5 nanohybrids exhibit a greatly enhanced sensitivity toward H 2 S gas selectively and are relatively indifferent to CO and NO 2 gases. The gas-sensing response of the nanohybrids increases by nearly 18 times (from 1.84 to 31.86) when tested against 23 ppm of H 2 S gas at 220 C. The nanohybrids remain stable when detecting H 2 S gas for a period of two weeks. This selective enhancement is attributable to the local p-n junction formed at the interface together with the reversible chemical reaction that occurs when CuO is exposed to H 2 S gas at the temperature employed.
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