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

Recent developments in cathode materials for lithium ion batteries

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

Recovery and reuse of expensive catalysts after catalytic reactions are important factors for sustainable process management. The aim of this Review is to highlight the progress in the formation and catalytic applications of magnetic nanoparticles and magnetic nanocomposites. Directed functionalization of the surfaces of nanosized magnetic materials is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.

Magnetically Separable Nanocatalysts: Bridges between Homogeneous and Heterogeneous Catalysis

Hollow micro-/nano-structured materials are now playing an important role in cutting edge innovations for energy conversion and storage technologies such as solar cells, fuel cells, lithium ion batteries and super capacitors. These materials show great promise in addressing growing environmental concerns for cleaner power sources at a time of increasing global demand for energy. In this perspective, we show that complex multi-shelled micro-/nano-materials show significant material advantages in many applications over conventional simple hollow structures. We also summarize the vast array of synthetic strategies used to create multi-shelled hollow structures, and discuss the possible application of these novel materials for power generation and storage. Finally, the emergent challenges and future developments of multi-shelled hollow structures are further discussed.

Recent advances in micro-/nano-structured hollow spheres for energy applications: From simple to complex systems

This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO2 fibers. Unloaded and Pd-loaded SnO2 nanofiber mats are synthesized by electrospinning followed by hot-pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd-loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO2 crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO2 crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd-loaded specimen, indicating that Pd-loading could effectively suppress the SnO2 grain growth during the calcination step. The Pd-loaded SnO2 sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H2 and lower sensitivity to NO2 compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO-decorated SnO2 crystallites and catalytic effect of PdO in promoting the oxidation of H2 into H2O. These phenomena appear to have a much larger effect on the sensitivity of the Pd-loaded sensors than the reduction in grain size.

Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading

Departamento de Fisico-Quimica Instituto de Quimica Universidade Estadual Paulista (UNESP), Araraquara, SP, 14800-900

The role of hierarchical morphologies in the superior gas sensing performance of CuO-based chemiresistors

In this work, we report a remarkably improved toluene response and superior cross-sensitivity against H 2 S molecules by combining Pd catalysts and highly porous WO 3 nanofibers (NFs). We functionalized Pd catalysts inside and/or outside of WO 3 NFs synthesized by electrospinning, which is a facile and versatile process for producing webs of metal oxide NFs. Pd-embedded WO 3 NFs were obtained by the electrospinning of a Pd and W composite precursor/poly(methyl methacrylate) (PMMA) solution followed by calcination at 700 °C. Pd nanoparticles (NPs) (6–10 nm) synthesized by a polyol method were decorated on the WO 3 NFs (Pd-NPs/WO 3 NFs) by the attachment of the Pd NPs on as-prepared WO 3 NFs. The gas sensing characteristics of pure WO 3 , Pd-embedded WO 3 , Pd-NPs/WO 3 , and Pd-NPs/Pd-embedded WO 3 NFs were tested using H 2 S and toluene gases in a highly humid atmosphere (RH 90%), which is similar to human exhaled breath. The results showed that the Pd-NPs/Pd-embedded WO 3 NFs whose inner and outer layers were decorated by Pd catalysts, exhibited a high toluene response ( R air / R gas  = 5.5 at 1 ppm) and remarkable selectivity against H 2 S ( R air / R gas  = 1.36 at 1 ppm) at 350 °C, whereas pristine WO 3 NFs showed superior H 2 S response ( R air / R gas  = 11.1 at 1 ppm) along with a negligible response toward toluene ( R air / R gas  = 1.27 at 1 ppm). The highly porous WO 3 NFs decorated with Pd catalysts, exhibited potential feasibility, i.e., a low limit of detection (LOD) of 20 ppb ( R air / R gas  = 1.32) at 350 °C, for application in VOCs sensors, particularly for diagnoses of lung cancer.

Highly sensitive and selective hydrogen sulfide and toluene sensors using Pd functionalized WO3 nanofibers for potential diagnosis of halitosis and lung cancer

Hollow TiO2 hemispheres obtained by colloidal templating for application in dye-sensitized solar cells

Nanostructured semiconducting metal oxides and particularly nanofiber-based photoelectrodes can provide enhanced energy conversion efficiencies in dye-sensitized solar cells (DSSCs). In this study ZnO/poly(vinyl acetate) composite nanofiber mats were directly electrospun onto a glass substrate coated with F:SnO2, then hot pressed at 120°C and calcined at 450°C. This resulted in multiple nanofiber networks composed of a twisted structure of 200–500nm diameter cores with ∼30nm single grains. The DSSCs using ZnO nanofiber mats exhibited a conversion efficiency of 1.34% under 100mW∕cm2 (AM-1.5G) illumination.

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Dae-Jin Yang

Papers

Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading

The role of hierarchical morphologies in the superior gas sensing performance of CuO-based chemiresistors

Highly sensitive and selective hydrogen sulfide and toluene sensors using Pd functionalized WO3 nanofibers for potential diagnosis of halitosis and lung cancer

Hollow TiO2 hemispheres obtained by colloidal templating for application in dye-sensitized solar cells

Dye-sensitized solar cells using network structure of electrospun ZnO nanofiber mats