High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity, type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity, and parameter's stability in sensing material choice as well.

Metal oxides for solid-state gas sensors: What determines our choice?

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

Over the last decade, significant effort has been devoted to the applications of hierarchically structured porous materials owing to their outstanding properties such as high surface area, excellent accessibility to active sites, and enhanced mass transport and diffusion. The hierarchy of porosity, structural, morphological and component levels in these materials is key for their high performance in all kinds of applications. The introduction of hierarchical porosity into materials has led to a significant improvement in the performance of materials. Herein, recent progress in the applications of hierarchically structured porous materials from energy conversion and storage, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine is reviewed. Their potential future applications are also highlighted. We particularly dwell on the relationship between hierarchically porous structures and properties, with examples of each type of hierarchically structured porous material according to its chemical composition and physical characteristics. The present review aims to open up a new avenue to guide the readers to quickly obtain in-depth knowledge of applications of hierarchically porous materials and to have a good idea about selecting and designing suitable hierarchically porous materials for a specific application. In addition to focusing on the applications of hierarchically porous materials, this comprehensive review could stimulate researchers to synthesize new advanced hierarchically porous solids.

Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine

High-surface area mesoporous CuO/Ce 0.8 Zr 0.2 O 2 catalysts were prepared by a surfactant-assisted method of nanocrystalline particle assembly, and characterized by XRD, N 2 adsorption, TEM, H 2 -TPR, TG-DTA and XPS techniques. The catalytic properties of the CuO/Ce 0.8 Zr 0.2 O 2 nanocatalysts were evaluated by low-temperature carbon monoxide oxidation using a microreactor-GC system. XRD and TEM analysis indicated that the catalyst particles were nanoscaled with cubic, fluorite structure. N 2 adsorption–desorption isotherms revealed a mesoporous nanocatalyst system with high-surface area and uniform pore-size distribution. The results of catalytic activity measurements showed that these mesoporous nanostructured CuO/Ce 0.8 Zr 0.2 O 2 catalysts were very active for low-temperature CO oxidation. The catalytic behavior depended on the CuO loading amount, the calcination temperature, the surface area and the particle size of the catalyst. The catalyst with 25 mol% CuO loading and calcined at 400 °C exhibited the highest catalytic activity.

Preparation, characterization and catalytic behavior of nanostructured mesoporous CuO/Ce0.8Zr0.2O2 catalysts for low-temperature CO oxidation

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

CuO catalysts supported on attapulgite clay for low-temperature CO oxidation

A facile solution approach was employed to synthesize α-FeOOH nanorods by using FeSO4·7H2O and CH3COONa without templates at low temperature (40 °C). The porous α-Fe2O3 nanorods were successfully obtained by calcining the α-FeOOH precursors at 300 °C for 2 h. The as-prepared products were characterized by thermogravimetry-differential thermal analysis, X-ray powder diffraction, transmission electron microscopy (TEM), high-resolution TEM, and N2 adsorption−desorption analysis techniques. The as-prepared porous α-Fe2O3 nanorods have a tiny crystal size (5 nm) and a pore size distribution of 1−10 nm, resulting in a high specific surface area of 221.9 m2·g−1. A possible growth mechanism of the porous α-Fe2O3 nanorods was proposed. The gas-sensing measurement results demonstrated that the porous α-Fe2O3 nanorods presented a much higher response than the α-Fe2O3 nanoparticles and showed excellent selectivity and stability to ethanol vapor. Due to the fact that it has exciting gas-sensing properties and can be o...

Yan Wang

Papers

Preparation, characterization and catalytic behavior of nanostructured mesoporous CuO/Ce0.8Zr0.2O2 catalysts for low-temperature CO oxidation

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

NO2 sensing performance of SnO2 hollow-sphere sensor

CuO catalysts supported on attapulgite clay for low-temperature CO oxidation

Facile Synthesis of Porous α-Fe2O3 Nanorods and Their Application in Ethanol Sensors