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

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

Carbon (C) is a crucial material for many branches of modern technology. A growing number of demanding applications in electronics and telecommunications rely on the unique properties of C allotropes. The need for microwave absorbers and radar-absorbing materials is ever growing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference among components and circuits, reduction of the back-radiation of microstrip radiators). Whatever the application for which the absorber is intended, weight reduction and optimization of the operating bandwidth are two important issues. A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (C black, C nanotube, C fiber, graphene) and the embedding matrix (rubber, thermoplastic). This paper offers a perspective on the experimental efforts toward the development of microwave absorbers composed of carbonaceous inclusions in a polymer matrix. The absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and volume fraction of the filler particles. Polymercomposites filled with carbonaceous particles provide a versatile system to probe physical properties at the nanoscale of fundamental interest and of relevance to a wide range of potential applications that span radar absorption, electromagnetic protection from natural phenomena (lightning), shielding for particle accelerators in nuclear physics, nuclear electromagnetic pulse protection, electromagnetic compatibility for electronic devices, high-intensity radiated field protection, anechoic chambers, and human exposure mitigation. Carbonaceous particles are also relevant to future applications that require environmentally benign and mechanically flexible materials.

A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles

Core–shell composites, Fe3O4@C, with 500 nm Fe3O4 microspheres as cores have been successfully prepared through in situ polymerization of phenolic resin on the Fe3O4 surface and subsequent high-temperature carbonization. The thickness of carbon shell, from 20 to 70 nm, can be well controlled by modulating the weight ratio of resorcinol and Fe3O4 microspheres. Carbothermic reduction has not been triggered at present conditions, thus the crystalline phase and magnetic property of Fe3O4 micropsheres can be well preserved during the carbonization process. Although carbon shells display amorphous nature, Raman spectra reveal that the presence of Fe3O4 micropsheres can promote their graphitization degree to a certain extent. Coating Fe3O4 microspheres with carbon shells will not only increase the complex permittivity but also improve characteristic impedance, leading to multiple relaxation processes in these composites, thus the microwave absorption properties of these composites are greatly enhanced. Very inte...

Shell thickness-dependent microwave absorption of core-shell Fe3O4@C composites.

Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Fe3O4/TiO2 core/shell nanotubes are fabricated via a three-step process. α-Fe2O3 nanotubes are first obtained, and α-Fe2O3/TiO2 core/shell nanotubes are subsequently fabricated using Ti(SO4)2 as a Ti source by a wet chemical process. The thickness of the amorphous TiO2 shell is about 21 nm. After a H2 deoxidation process, the amorphous TiO2 layer changes into crystalline structures composed of TiO2 nanoparticles with an average diameter of 2.5 nm, and its thickness is decreased to about 18 nm. At the same time, α-Fe2O3 transforms into cubic Fe3O4. Consequently, crystalline Fe3O4/TiO2 core/shell nanotubes can be fabricated through the process above. The measurements of the magnetic properties demonstrate that the Fe3O4/TiO2 core/shell nanotubes exhibit ferromagnetic behavior at room temperature, and the Verwey temperature is about 120 K. The eddy current effect is largely reduced and the anisotropy energy is improved significantly for the core/shell nanotubes due to the presence of the TiO2 shells. The max...

Fe3O4/TiO2 Core/Shell Nanotubes: Synthesis and Magnetic and Electromagnetic Wave Absorption Characteristics

Fe3O4/ZnO core/shell nanorods are successfully fabricated by combing an inorganic-phase reaction with a hydrogen annealing process. The transmission electron microscopy analysis indicates that the diameter and the length of the core/shell nanorods are 25−80 and 0.35−1.2 μm, respectively. Electromagnetic properties of the core/shell nanorod−wax composites are investigated. The permittivity of the composites shows four dielectric resonant peaks in 2−18 GHz, which can be explained by the transmission line theory. The resonant behavior mainly results from interface polarization induced by the special core/shell structures, dipole polarization of both Fe3O4 and ZnO, and electron transfer between Fe2+ and Fe3+ ions in Fe3O4. The maximum reflection loss is about −30 dB at 10.4 GHz for the composites with a thickness of 1.5 mm, and the absorption bandwidth with the reflection loss below −20 dB is up to 11 GHz for an absorber with the thickness in 2−4 mm. Thus, our results demonstrate that the Fe3O4/ZnO core/shell...

Synthesis, Multi-Nonlinear Dielectric Resonance, and Excellent Electromagnetic Absorption Characteristics of Fe3O4/ZnO Core/Shell Nanorods

ZnO nanotubes were synthesized by a sonochemical method at low temperature. The length and the diameter of the obtained nanotubes were 1.5–2 μm and 250 nm, respectively, and the walls were about 30 nm in thickness. The sensors fabricated from the nanotubes exhibited excellent ethanol sensing properties at a working temperature of 300 °C. The nanotubes can detect ethanol vapor with concentration down to 1 ppm and also showed good sensing characteristics at relatively low temperature. Our results demonstrated that the ZnO nanotubes were very promising for gas sensors with good sensing characteristics.

Ethanol sensing characteristics of ambient temperature sonochemically synthesized ZnO nanotubes

Crystalline CeO 2 /TiO 2 core/shell nanorods were fabricated by a hydrothermal method and a subsequent annealing process under the hydrogen and air atmosphere. The thickness of the outer shell composed of crystal TiO 2 nanoparticles can be tuned in the range of 5–11 nm. The crystal core/shell nanorods exhibited enhanced gas-sensing properties to ethanol vapor in terms of sensor response and selectivity. The calculated sensor response based on the change of the heterojunction barrier formed at the interface between CeO 2 and TiO 2 is agreed with the experimental results, and thus the change of the heterojunction barrier at different gas atmosphere can be used to explain the enhanced ethanol sensing properties.

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

Crystalline α-MoO 3 /TiO 2 core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H 2 /Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO 2 particles with a diameter of 2–6 nm, and its thickness can be easily controlled in the range of 15–45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare α-MoO 3 nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO 2 nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between α-MoO 3 and TiO 2 in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance.

Gang Xiao

Papers

Fe3O4/TiO2 Core/Shell Nanotubes: Synthesis and Magnetic and Electromagnetic Wave Absorption Characteristics

Synthesis, Multi-Nonlinear Dielectric Resonance, and Excellent Electromagnetic Absorption Characteristics of Fe3O4/ZnO Core/Shell Nanorods

Ethanol sensing characteristics of ambient temperature sonochemically synthesized ZnO nanotubes

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

α-MoO3/TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties