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

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

It is conventionally assumed that the growth of monodisperse colloidal nanocrystals requires a temporally discrete nucleation followed by monomer attachment onto the existing nuclei. However, recent studies have reported violations of this classical growth model, and have suggested that inter-particle interactions are also involved during the growth. Mechanisms of nanocrystal growth still remain controversial. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution onto the existing particles or by coalescence between the particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow. We suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems.

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

Templating is one of the most important techniques for the controlled synthesis of nanostructured materials. This powerful tool uses a pre-existing guide with desired nanoscale features to direct the formation of nanomaterials into forms that are otherwise difficult to obtain. As a result, templated synthesis is capable of producing nanostructures with unique structures, morphologies and properties. In this review, we summarize the general principles of templated synthesis and cover recent developments in this area. As a wide variety of synthesis techniques are utilized to produce nanomaterials using template-based methods, the discussion is organized around the various types of common templates. We examine the use of both physical and chemical hard colloidal templates, soft templates, and other non-colloidal templates, followed by our perspective on the state of the field and potential future directions.

Templated synthesis of nanostructured materials

Inorganic hollow spheres have attracted considerable interest due to their singular properties and wide range of potential applications. In this critical review, we provide a comprehensive overview of the preparation and applications of inorganic hollow spheres. We first discuss the syntheses of inorganic hollow spheres by use of polymers, inorganic nonmetals, metal-based hard templates, small-molecule emulsion, surfactant micelle-based soft-templates, and the template-free approach. For each method, a critical comment is given based on our knowledge and related research experience. We go on to discuss some important applications of inorganic hollow spheres in 0D, 2D, and 3D arrays. We conclude this review with some perspectives on the future research and development of inorganic hollow spheres (235 references).

Fabrication and application of inorganic hollow spheres

Octahedral Cu2O crystals with all edge length about I pill were synthesized by reducing it copper-citrate complex With glucose. The morphology and Structure Of Cu2O particles were greatly affected by the concentration of glucose, reaction temperature, and time. When the concert tralion of glucose increased from 0.6 to 1.6 M, the morphology of Cu2O Could be changed from octahedral single crystals to spherical polycrystals. When the reaction time was prolonged from 6 to 36 11, solid Cu2O Octahedra could be changed to it mixture of hollow Cu2O octahedra and irregular Cu particles. After removing Cu particles. pure Cu2O hollow Octahedra could be obtained. The formation mechanism Of hollow Cu2O octahedra was discussed. Similarly, hollow Cu2O spheres could also be obtained using this method. The prepared hollow Octahedra[ Cu2O Particles exhibited higher pholoocatalytic activity for photodegradation of p-nitrophenol aqueous Solution under visible-light illumination thim other Cu2O particles with different morphologies (hollow spheres, solid octahedra, and solid spheres).

Morphology Evolution of Cu2O from Octahedra to Hollow Structures

In this work, we report a simple one-pot approach to prepare hollow Co microspheres via self-assembly of octahedron-like nanocrystals. Uniform hollow Co spherical assemblies, prepared via a facile solvothermal reduction approach in the presence of NaCl, were composed of ordered nanooctahedra with a thickness of about 80-120 nm. The formation mechanism (NaCl-assisted Ostwald ripening) has been investigated with TEM/HRTEM/SEM/XRD methods. Surface morphologies of Co hollow spheres have been finely modulated from rough to smooth surface through altering reaction conditions (such as time, temperature, and amount of NaCl). The interior-cavity size of the Co hollow sphere has been well tunable by controlling reaction time or temperature. These materials exhibit good ferromagnetic characteristics, showing their potential applications for catalysts and other related devices. The influence of surface morphology, the interior cavity size, and surface modification upon the magnetic properties of hollow Co spheres have been systematically studied. The possible reasons for diverse magnetic properties of products obtained under different conditions are also discussed.

Liuyang Bai

Papers

Morphology Evolution of Cu2O from Octahedra to Hollow Structures

Self-Assembled Growth of Hollow Spheres with Octahedron-like Co Nanocrystals via One-Pot Solution Fabrication

Development of new NLO crystals for UV and IR applications

Synthesis of nickel nanoparticles with uniform size via a modified hydrazine reduction route

Constructing continuous networks by branched alumina for enhanced thermal conductivity of polymer composites