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

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

In this Review, we aim to provide an updated summary of the research related to hollow micro- and nanostructures, covering both their synthesis and their applications. After a brief introduction to the definition and classification of the hollow micro-/nanostructures, we discuss various synthetic strategies that can be grouped into three major categories, including hard templating, soft templating, and self-templating synthesis. For both hard and soft templating strategies, we focus on how different types of templates are generated and then used for creating hollow structures. At the end of each section, the structural and morphological control over the product is discussed. For the self-templating strategy, we survey a number of unconventional synthetic methods, such as surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement. We then discuss the unique properties and niche applications of the hollow structures in diverse fields, including micro-/nanocontainers and rea...

Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures

One-dimensional TiO2 (1D TiO2) nanomaterials with unique structural and functional properties have been extensively used in various fields including photocatalytic degradation of pollutants, photocatalytic CO2 reduction into energy fuels, water splitting, solar cells, supercapacitors and lithium-ion batteries. In the past few decades, 1D TiO2 nanostructured materials with a well-controlled size and morphology have been designed and synthesized. Compared to 0D and 2D nanostructures, more attention has been paid to 1D TiO2 nanostructures due to their high aspect ratio, large specific surface area, and excellent electronic or ionic charge transport properties. In this review, we present the crystal structure of TiO2 and the latest development on the fabrication of 1D TiO2 nanostructured materials. Besides, we will look into some critical engineering strategies that give rise to the excellent properties of 1D TiO2 nanostructures such as improved enlargement of the surface area, light absorption and efficient separation of electrons/holes that benefit their potential applications. Moreover, their corresponding environmental and energy applications are described and discussed. With the fast development of the current economy and technology, more and more effort will be put into endowing TiO2-based materials with advanced functionalities and other promising applications.

A review of one-dimensional TiO2 nanostructured materials for environmental and energy applications

Hollow nanostructures offer promising potential for advanced energy storage and conversion applications. In the past decade, considerable research efforts have been devoted to the design and synthesis of hollow nanostructures with high complexity by manipulating their geometric morphology, chemical composition, and building block and interior architecture to boost their electrochemical performance, fulfilling the increasing global demand for renewable and sustainable energy sources. In this Review, we present a comprehensive overview of the synthesis and energy-related applications of complex hollow nanostructures. After a brief classification, the design and synthesis of complex hollow nanostructures are described in detail, which include hierarchical hollow spheres, hierarchical tubular structures, hollow polyhedra, and multi-shelled hollow structures, as well as their hybrids with nanocarbon materials. Thereafter, we discuss their niche applications as electrode materials for lithium-ion batteries and hybrid supercapacitors, sulfur hosts for lithium-sulfur batteries, and electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. The potential superiorities of complex hollow nanostructures for these applications are particularly highlighted. Finally, we conclude this Review with urgent challenges and further research directions of complex hollow nanostructures for energy-related applications.

Complex Hollow Nanostructures: Synthesis and Energy-Related Applications.

An alkaline electrolyzer fabricated by employing hollow CoP nanoparticles/N-doped graphene as bifunctional catalysts exhibits remarkable activity with a current density of 10 mA cm−2 at a cell voltage of 1.58 V and considerable stability over 65 h of continuous electrolysis operation, favorably comparable to the integrated performance of commercial Pt/C and IrO2.

Hollow CoP nanopaticle/N-doped graphene hybrids as highly active and stable bifunctional catalysts for full water splitting

Hierarchical nanosheet-based NiMoO4 nanotubes were synthesized by a hydrothermal treatment and a subsequent in situ diffusion reaction method. The nanotubes with a high surface area of 128.5 m2 g−1 were composed of highly ordered ultrathin nanosheets with a thickness of less than 10 nm. When used as electrodes in electrochemical capacitors, the hierarchical nanotubes exhibited excellent electrochemical performances. The specific capacitance of the hierarchical nanotubes was up to 864 F g−1 at a current density of 1 A g−1. The results indicate that the hierarchical nanosheet-based NiMoO4 nanotubes are very promising for applications in energy storage and other electrochemical devices. Furthermore, the strategy presented here is facile, and may be expended as a typical method to synthesize other kinds of metal molybdate nanotubes.

Hierarchical nanosheet-based NiMoO4 nanotubes: synthesis and high supercapacitor performance

National Natural Science Foundation of China [51072038, 51272050, 61205113, 51172275, 21001035]; Program for NCET in University [NECT-10-0049]; Outstanding Youth Foundation of Heilongjiang Province [JC201008]; Special Funds for Major State Basic Research Projects [2010CB934200]

Three‐Dimensional Hierarchical Architectures Constructed by Graphene/MoS2 Nanoflake Arrays and Their Rapid Charging/Discharging Properties as Lithium‐Ion Battery Anodes

The paper describes for the first time the successful synthesis of Fe2O3/TiO2 tube-like nanostructures, in which TiO2 shell is of quasi-single crystalline characteristic and its thickness can be controlled through adjusting the added amount of aqueous Ti(SO4)2 solution. The characterization of samples obtained at different stages using transmission electron microscope indicates that the outer TiO2 shell is changed gradually from amorphous and polycrystalline phase into quasi-single crystal under thermal actions through the Ostwald ripening process, accompanying the corrosion of the central parts of Fe2O3 nanorods, and the formation of small particles separating each other, leading to the special core/shell nanorods. Furthermore, Fe2O3/TiO2 tube-like nanostructures can be transformed into Fe2TiO5 nanostructures after they are thermally treated at higher temperatures. Those nanostructures exhibit enhanced ethanol sensing properties with respect to the monocomponent. Our results imply that not only hollow na...

Lihong Qi

Papers

Hollow CoP nanopaticle/N-doped graphene hybrids as highly active and stable bifunctional catalysts for full water splitting

Hierarchical nanosheet-based NiMoO4 nanotubes: synthesis and high supercapacitor performance

Three‐Dimensional Hierarchical Architectures Constructed by Graphene/MoS2 Nanoflake Arrays and Their Rapid Charging/Discharging Properties as Lithium‐Ion Battery Anodes

Fe2O3/TiO2 Tube-like Nanostructures: Synthesis, Structural Transformation and the Enhanced Sensing Properties

Electromagnetic absorption properties of graphene/Fe nanocomposites