Chemi-resistive sensors based on hybrid functional materials are promising candidates for gas sensing with high responsivity, good selectivity, fast response/recovery, great stability/repeatability, room-working temperature, low cost, and easy-to-fabricate, for versatile applications. This progress report reviews the advantages and advances of these sensing structures compared with the single constituent, according to five main sensing forms: manipulating/constructing heterojunctions, catalytic reaction, charge transfer, charge carrier transport, molecular binding/sieving, and their combinations. Promises and challenges of the advances of each form are presented and discussed. Critical thinking and ideas regarding the orientation of the development of hybrid material-based gas sensor in the future are discussed.

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Gas Sensors Based on Chemi-Resistive Hybrid Functional Nanomaterials

"Metal oxide -based heterostructures for gas sensors"- A review.

Quasi-one-dimensional (1D) heterostructures act as promising gas-sensing materials due to their high surface-to-volume ratio, large depletion area, and synergistic behaviors of the two components that contribute to dramatic change in resistance. To date, the noble metals and p-type metal oxides, a component of the heterostructure systems, have been extensively applied as good catalysts owing to their excellent selectivity and sensitivity to analyte gases. In particular, much effort has been devoted to the fabrication and processing of nanoscale quasi-1D metal-oxide-based heterostructural materials for a variety of practical applications especially as gas sensors. In this review, we first introduce the two-step preparation process of various quasi-1D nanostructures and then discuss how the primary parameters, e.g. the amount of activated adsorption sites, the gas diffusion ability, and the coverage the secondary materials, can have an impact on the gas-sensing performances. We finally review gas-sensing mechanism in terms of the species of heterojunctions, such as the Schottky-junction, p – n or n – p junction, and n – n or p – p junction.

Quasi-one-dimensional metal-oxide-based heterostructural gas-sensing materials: A review

Electronic Noses: From Advanced Materials to Sensors Aided with Data Processing

Here we present an innovative flexible ammonia (NH3) sensor based on S and N co-doped graphene quantum dots (S, N: GQDs)/polyaniline (PANI) hybrid loading on flexible polyethylene terephthalate thin film by chemical oxidative polymerization. The sensor exhibited not only excellent response, good selectivity and fast response and recovery time at room temperature but also flexibility, low cost and wearable characteristics. The experimental results reveal that the response of S, N: GQDs/PANI hybrid was about 5 fold higher than that of pure PANI at 100 ppm of NH3 and the attained gas-sensing performance may be attributed to the increased hole like carriers by S, N: GQDs and enhanced interactions between the hybrid sensing film and NH3 molecules via π electron networks. The NH3-sensing mechanisms of the flexible S, N: GQDs/PANI gas sensor based on acid-base doping/de-doping process, carriers mobility, and swelling process are highlighted.

Highly sensitive and flexible ammonia sensor based on S and N co-doped graphene quantum dots/polyaniline hybrid at room temperature

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.

Bu-Yu Yeh

Papers

Vapor-solid growth of p-Te/n-SnO2 hierarchical heterostructures and their enhanced room-temperature gas sensing properties.