Role of Heterojunctions of Core–Shell Heterostructures in Gas Sensing

Abstract

Heterostructures made from metal oxide semiconductors (MOS) are fundamental for the development of high-performance gas sensors. Since their importance in real applications, a thorough understanding of the transduction mechanism is vital, whether it is related to a heterojunction or simply to the shell and core materials. A better understanding of the sensing response of heterostructured nanomaterials requires the engineering of heterojunctions with well-defined core and shell layers. Here, we introduce a series of prototypes CNT-nMOS, CNT-pMOS, CNT-pMOS- n MOS, and CNT- n MOS-pMOS hierarchical core–shell heterostructures (CSHS) permitting us to directly relate the sensing response to the MOS shell or to the p–n heterojunction. The carbon nanotubes are here used as highly conductive substrates permitting operation of the devices at relatively low temperature and are not involved in the sensing response. NiO and SnO2 are selected as representative p- and n-type MOS, respectively, and the response of a set of samples is studied toward hydrogen considered as model analyte. The CNT-n,pMOS CSHS exhibit response related to the n,pMOS-shell layer. On the other hand, the CNT-pMOS-nMOS and CNT-nMOS-pMOS CSHS show sensing responses, which in certain cases are governed by the heterojunctions between nMOS and pMOS and strongly depends on the thickness of the MOS layers. Due to the fundamental nature of this study, these findings are important for the development of next generation gas sensing devices.

Gas Sensing and Electrochemical Properties of CNT/WS2 Core–shell Nanostructures

“…Figure compares the response of the sensors fabricated with pristine CNTs and the CNT/WS 2 ( X ) heterostructures with varying thicknesses of the WS 2 shell layer toward NO 2 (5 ppm). The trend observed shows the typical behavior previously reported by us for CNTs/metal oxide-based sensors, , irrespective of the metal oxide (SnO 2 , TiO 2 , and NiO) or target gas (H 2 , ethanol, and acetone) used. The response increased as the ALD cycles increased up to 300 cycles (about 15 nm WS 2 shell thickness) The results reported here confirm that the metal disulfide shell thickness is a critical parameter for the design of the core–shell heterostructure-based conductometric sensors.…”

“…Owing to their discovery three decades ago, carbon nanotubes (CNTs) have emerged as an amazing class of nanomaterials due to their unique physical, chemical, optical, electrical, electrochemical, and magnetic properties . In many applications, they are also used to support the active phase(s), usually dispersed/coated on them, forming a shell layer. − It is well known that the functional properties of the shell layer depend significantly on the shape, morphology, as well as the thickness and coverage of the core . Further, the shell with a layered structure and tunable numbers of layers can have a wide range of mechanical, physical, and chemical properties, such as flexibility and strength, chemical reactivity, surface interactions, and electronic characteristics.…”

Metal–Organic Framework-Derived LaFeO3@SnO2/Ag p–n Heterojunction Nanostructures for Formaldehyde Detection

“…Besides, because of the porous structure, the gas molecules can reach the heterojunction interface directly, and the combination of LaFeO 3 and SnO 2 is equivalent to adding another barrier because of their different resistance. In a gas–solid reaction, electrons must pass through these two layers of walls, which undoubtedly increases the resistance in the air to produce more descending space after contact with formaldehyde . The mechanism is illustrated in Figure a.…”

Nanostructured metal oxide heterojunctions for chemiresistive gas sensors

“…For example, it has proven that although the conformal deposition of a second metal oxide onto already contacted (core-core junctions) one-dimensional SnO 2 nanowires dramatically affects the width and the thickness of the electron depletion layers, the gas-sensing characteristics are solely ascribed to the core materials. 123 In this case, the shell layer modies the surface of the core layer without signicantly altering the potential transduction mechanism. [124][125][126] The thin lm fabricated by ALD is conformably coupled with the underlying material, making it possible to optimize the subtle core-shell nanostructures.…”

“…The conduction behaviour and carrier concentration of coreshell heterojunctions can be precisely adjusted by changing the composition and proportion of the core-shell layer. 123,159 Zhang et al investigated the inuence of shell thickness on the gassensing performance of Cr 2 O 3 /TiO 2 nanobers fabricated by a coaxial electrospinning route. 160 The gas-sensing measurements revealed that the core-shell heterojunctions with the optimized shell thickness showed substantially enhanced gassensing performance as compared to pristine nanobers at their corresponding optimal working temperatures.…”