Hierarchical and hollow oxide nanostructures are very promising gas sensor materials due to their high surface area and well-aligned nanoporous structures with a less agglomerated configurations. Various synthetic strategies to prepare such hierarchical and hollow structures for gas sensor applications are reviewed and the principle parameters and mechanisms to enhance the gas sensing characteristics are investigated. The literature data clearly show that hierarchical and hollow nanostructures increase both the gas response and response speed simultaneously and substantially. This can be explained by the rapid and effective gas diffusion toward the entire sensing surfaces via the porous structures. Finally, the impact of highly sensitive and fast responding gas sensors using hierarchical and hollow nanostructures on future research directions is discussed.

Gas sensors using hierarchical and hollow oxide nanostructures: Overview

Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high-performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high-temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room-temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room-temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure-property correlations. Finally, some future research perspectives and new challenges that the field of room-temperature sensors will have to address are also discussed.

Nanostructured Materials for Room-Temperature Gas Sensors

SnO2: A comprehensive review on structures and gas sensors

Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

The development of new electrode materials for lithium-ion batteries (LIBs) has always been a focal area of materials science, as the current technology may not be able to meet the high energy demands for electronic devices with better performance. Among all the metal oxides, tin dioxide (SnO₂) is regarded as a promising candidate to serve as the anode material for LIBs due to its high theoretical capacity. Here, a thorough survey is provided of the synthesis of SnO₂-based nanomaterials with various structures and chemical compositions, and their application as negative electrodes for LIBs. It covers SnO₂ with different morphologies ranging from 1D nanorods/nanowires/nanotubes, to 2D nanosheets, to 3D hollow nanostructures. Nanocomposites consisting of SnO₂ and different carbonaceous supports, e.g., amorphous carbon, carbon nanotubes, graphene, are also investigated. The use of Sn-based nanomaterials as the anode material for LIBs will be briefly discussed as well. The aim of this review is to provide an in-depth and rational understanding such that the electrochemical properties of SnO₂-based anodes can be effectively enhanced by making proper nanostructures with optimized chemical composition. By focusing on SnO₂, the hope is that such concepts and strategies can be extended to other potential metal oxides, such as titanium dioxide or iron oxides, thus shedding some light on the future development of high-performance metal-oxide based negative electrodes for LIBs.

SnO2‐Based Nanomaterials: Synthesis and Application in Lithium‐Ion Batteries

Pure and Au-doped WO3 powders for NO2 gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO2 sensing properties of the sensors based on pure and Au-doped WO3 powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO3 sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO3 sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO2 than the others at relatively low operating temperature (150 °C).

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Ag-doped α-Fe 2 O 3 nanoparticles were synthesized by a chemical coprecipitation method and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), thermogravimetry-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller specific surface area analysis (BET) techniques. Obtained results indicated that spherical Ag grains with size of about 5 nm are highly dispersed on the surface of α-Fe 2 O 3 nanoparticles. The surface area of the Ag/Fe 2 O 3 nanoparticles is several times as large as that of pure α-Fe 2 O 3 . The H 2 S sensing properties of these Ag/Fe 2 O 3 sensors were systematically investigated. In comparison with pure α-Fe 2 O 3 , all of the Ag-doped sensors showed better sensing performance in respect of response, selectivity and optimum operating temperature. The effects of Ag content, calcination and operation temperature on the sensing characteristics of the Ag/α-Fe 2 O 3 sensors were also investigated. The sensor containing 2 wt% Ag and calcined at 400 °C exhibited the maximum response to H 2 S at 160 °C. A possible mechanism for the influence of Ag on the H 2 S-sensing properties of Ag/α-Fe 2 O 3 sensors was proposed.

Low-temperature H2S sensors based on Ag-doped α-Fe2O3 nanoparticles

This work examined the possibility of using SnO 2 hollow spheres mediated by carbon microspheres for the efficient detection of NO 2 gas. The influences of gas concentration, operating temperature, crystallite (grain) size and structure feature on the sensing performance of the sensors based on SnO 2 hollow spheres were comprehensively studied. The SnO 2 hollow spheres had a mean crystallite size of 12.7, 14.1 and 22.8 nm when calcined at 450, 500 and 550 °C, respectively. It was found that the SnO 2 hollow spheres calcined at 450 °C with a crystallite size of 12.7 nm were the most sensitive to NO 2 gas. The sensor also had a better selectivity to NO 2 than to ethanol, methanol, gasoline, acetone, CCl 4 and NH 3 when operated at 160 °C.

NO2 sensing performance of SnO2 hollow-sphere sensor

Nanocomposites of SnO 2 and polythiophene (PTP) were synthesized by the in situ chemical oxidative polymerization method. These nanocomposites were characterized by FTIR, transmission electron microscope (TEM), X-ray diffraction (XRD) and thermogravimetric and differential thermal analysis (TG–DTA) techniques, which proved the polymerization of thiophene monomer and the strong interaction between polythiophene and SnO 2 . The composites were used for gas sensing to methanol (MeOH), ethanol (EtOH), acetone, and NO x at different working temperature. It was found that PTP/SnO 2 materials with different PTP mass percent (1%, 5%, 10%, 20% and 30%) could detect NO x with very higher selectivity and sensitivity at much lower working temperature than the reported SnO 2 . The PTP/SnO 2 nanocomposites responded to NO x at concentration as low as 10 ppm. PTP/SnO 2 composite containing 5 mass% PTP showed the highest sensitivity at room temperature. The sensing mechanism of PTP/SnO 2 nanocomposites to NO x was presumed to be the effects of p–n heterojunction between PTP and SnO 2 .

Yanmei Wang

Papers

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Low-temperature H2S sensors based on Ag-doped α-Fe2O3 nanoparticles

NO2 sensing performance of SnO2 hollow-sphere sensor

The preparation and gas sensitivity study of polythiophene/SnO2 composites

A DRIFTS study of low-temperature CO oxidation over Au/SnO2 catalyst prepared by co-precipitation method