Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

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

ZnS nanostructures: From synthesis to applications

The porous Fe3O4/carbon core/shell nanorods were fabricated via a three-step process. α-Fe2O3 nanorods were first obtained, and α-Fe2O3/carbon core/shell nanorods were subsequently fabricated using glucose as a carbon source by a hydrothermal method, in which the thickness of the carbon coating was about 3.5 nm. Fe3O4/carbon core/shell nanorods were synthesized after an annealing treatment of the product above under a mixture of Ar/H2 flow. After the H2 deoxidation process, the Fe3O4 core exhibited a character of porosity; the thickness of the carbon shell was decreased to about 2.5 nm, and its degree of graphitization was enhanced. The interesting core/shell nanostructures are ferromagnetic at room temperature, and the Verwey temperature was about 120 K. Electromagnetic properties of the core/shell nanorod–wax composite were investigated in detail. The maximum reflection loss was about −27.9 dB at 14.96 GHz for the composite with a thickness of 2.0 mm, and the absorption bandwidth with the reflection los...

Porous Fe3O4/Carbon Core/Shell Nanorods: Synthesis and Electromagnetic Properties

α-Fe 2 O 3 /ZnO heteronanostructures were synthesized via a three-step process. XRD, SEM, TEM and EDS analyses indicated that their diameters and lengths were ∼40 nm and 0.35–1.2 μm, respectively, in which the largest thickness of ZnO shells was about 10 nm. The heteronanostructures exhibited a dramatic improvement in ethanol sensing characteristics compared to the pure α-Fe 2 O 3 nanorods. Based on the space-charge layer model, such enhanced sensing properties were attributed to small thickness of ZnO shell. Our results demonstrate that One-dimensional (1D) metal oxide heteronanostructures, whose shell thickness is comparable to Debye length, are very promising materials for fabricating gas sensors with good performances.

Synthesis and enhanced ethanol sensing properties of α-Fe2O3/ZnO heteronanostructures

?-Fe2O3/SnO2 core?shell nanorods are synthesized via a three-step process. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses reveal that their diameters and lengths are respectively in the ranges 35?120?nm and 0.35?1.2??m, and the thickness of the shell composed of 3.5?nm SnO2 nanoparticles is about 10?nm. The core?shell nanostructures exhibit a dramatic improvement in ethanol sensing characteristics compared to pure ?-Fe2O3 nanorods. The sensor response is up to 19.6 under 10?ppm ethanol exposure at 220??C. Both the response time and the recovery time of the core?shell structures are less than 30?s. Based on the space?charge layer model and semiconductor heterojunction theory, the small thickness of the SnO2 shell and the formation of heterojunctions contribute to the enhanced ethanol sensing characteristics. Our results demonstrate that one-dimensional metal oxide core?shell nanostructures whose shell thickness is smaller than the Debye length are very promising materials for fabricating gas sensors with good performances.

Synthesis and enhanced ethanol sensing characteristics of alpha-Fe2O3/SnO2 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

SnO2/α-Fe2O3 hierarchical nanostructures, in which the SnO2 nanorods grow on the side surface of α-Fe2O3 nanorods as multiple rows, were synthesized via a three-step process. The diameters and lengths of the SnO2 nanorods are 6–15 nm and about 120 nm. The growth direction of SnO2 nanorods is [001], significantly affected by that of α-Fe2O3 nanorods. The hetero-nanostructures exhibit very good selectivity to ethanol. The sensing characteristics are related to the special heterojunction structures, confirmed by high-resolution transmission electron microscopy observation. Therefore, a heterojunction barrier controlled gas sensing mechanism is realized. Our results demonstrate that the hetero-nanostructures are promising materials for fabricating sensors and other complex devices.

https://iopscience.iop.org/article/10.1088/0957-4484/19/20/205603/pdf

Yu-Jin Chen

Papers

Porous Fe3O4/Carbon Core/Shell Nanorods: Synthesis and Electromagnetic Properties

Synthesis and enhanced ethanol sensing properties of α-Fe2O3/ZnO heteronanostructures

Synthesis and enhanced ethanol sensing characteristics of alpha-Fe2O3/SnO2 core-shell nanorods.

Ethanol sensing characteristics of ambient temperature sonochemically synthesized ZnO nanotubes

The synthesis and selective gas sensing characteristics of SnO(2)/α-Fe(2)O(3) hierarchical nanostructures.