Uniform, spherical-shaped TiO2:Eu nanoparticles with different doping concentrations have been synthesized through controlled hydrolysis of titanium tetrabutoxide under appropriate pH and temperature in the presence of EuCl3·6H2O. Through air annealing at 500°C for 2 h, the amorphous, as-grown nanoparticles could be converted to a pure anatase phase. The morphology, structural, and optical properties of the annealed nanostructures were studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy [EDS], and UV-Visible diffuse reflectance spectroscopy techniques. Optoelectronic behaviors of the nanostructures were studied using micro-Raman and photoluminescence [PL] spectroscopies at room temperature. EDS results confirmed a systematic increase of Eu content in the as-prepared samples with the increase of nominal europium content in the reaction solution. With the increasing dopant concentration, crystallinity and crystallite size of the titania particles decreased gradually. Incorporation of europium in the titania particles induced a structural deformation and a blueshift of their absorption edge. While the room-temperature PL emission of the as-grown samples is dominated by the 5D0 - 7Fj transition of Eu+3 ions, the emission intensity reduced drastically after thermal annealing due to outwards segregation of dopant ions.

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Effects of crystallization and dopant concentration on the emission behavior of TiO2:Eu nanophosphors

Tubular and fibrous nanostructures of titanates have recently been
synthesized and characterized. Three general approaches (template assisted, anodic oxidation, and alkaline hydrothermal) for the preparation of nanostructured titanate and TiO2 are reviewed. The crystal structures, morphologies, and mechanism of formation
of nanostructured titanates produced by the alkaline hydrothermal method are critically discussed. The physicochemical properties of nanostructured titanates are highlighted and the links between properties and applications are emphasized. Examples of early applications of nanostructured titanates in catalysis, photocatalysis, electrocatalysis, lithium batteries, hydrogen storage, and solar-cell technologies are reviewed. The stability of titanate nanotubes at elevated temperatures and in acid media is considered.

Protonated Titanates and TiO2 Nanostructured Materials: Synthesis, Properties, and Applications

The advent of negative index materials has spawned extensive research into metamaterials over the past decade. Metamaterials are attractive not only for their exotic electromagnetic properties, but also their promise for applications. A particular branch–the metamaterial perfect absorber (MPA)–has garnered interest due to the fact that it can achieve unity absorptivity of electromagnetic waves. Since its first experimental demonstration in 2008, the MPA has progressed significantly with designs shown across the electromagnetic spectrum, from microwave to optical. In this Progress Report we give an overview of the field and discuss a selection of examples and related applications. The ability of the MPA to exhibit extreme performance flexibility will be discussed and the theory underlying their operation and limitations will be established. Insight is given into what we can expect from this rapidly expanding field and future challenges will be addressed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201200674

Metamaterial Electromagnetic Wave Absorbers

Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Controlling sound with acoustic metamaterials

Dynamics of Phononic Materials and Structures: Historical Origins, Recent Progress, and Future Outlook

Electrorheological fluids based on nano-fibrous polyaniline

We design an ultra-thin terahertz metamaterial absorber based on graphene/MgF2 multilayer stacking unit cells arrayed on an Au film plane and theoretically demonstrate a dual-band total absorption effect. Due to strong anisotropic permittivity, the graphene/MgF2 multilayer unit cells possess a hyperbolic dispersion. The strong electric and magnetic dipole resonances between unit cells make the impedance of the absorber match to that of the free space, which induces two total absorption peaks in terahertz range. These absorption peaks are insensitive to the polarization and nearly omnidirectional for the incident angle. But the absorption intensity and frequency depend on material and geometric parameters of the multilayer structure. The absorbed electromagnetic waves are finally converted into heat and, as a result, the absorber shows a good nanosecond photothermal effect.

Terahertz dual-band metamaterial absorber based on graphene/MgF(2) multilayer structures.

Coaxial cable-like conducting polyaniline (PANI)@titania nanofibers were synthesized and used as novel electrorheological materials. Under electric fields, the suspension of PANI@titania nanofibers showed notable yield stresses but significantly lower current densities compared to the suspension of pure PANI nanofibers.

Coaxial cable-like polyaniline@titania nanofibers: facile synthesis and low power electrorheological fluid application

We developed a novel electrorheological (ER) suspension composed of Cr-doped titania particles with a sea-urchin-like hierarchical morphology, and demonstrated a distinct enhancement in ER properties. These hierarchical ER particles, which are easy to synthesize by a solvothermal approach, are nearly monodisperse and consist of high-density rutile Cr-doped titania nanorods assembled radially on the surfaces of particles. The specific surface area of the hierarchical Cr-doped titania particles is ∼ 65 m2/g, which is higher than that (5 m2/g) of the smooth non-hierarchical Cr-doped titania particles. Under electric fields, the ER properties of the suspension containing such hierarchical particles are investigated by steady shear viscosity and dynamic viscoelastic experiments. It is found that the suspension of hierarchical Cr-doped titania particles possesses a stronger ER effect compared to the corresponding suspension of smooth non-hierarchical Cr-doped titania particles. Its yield stress and ER efficiency are approximately twice as high as those of the smooth Cr-doped titania suspension. Meanwhile, a higher ER effect is also demonstrated for the hierarchical pure titania suspension in comparison with the smooth pure titania suspension. Furthermore, under electric and shearing fields the rheological and structuring processes of the hierarchical particle suspension is significantly different from that of the smooth particle suspension. In terms of the dielectric analysis and microscopy observation, the enhancement of ER effect of the suspension of hierarchical Cr-doped titania is likely attributed to the combined effects including improved interfacial polarization and increased interparticle interaction due to the presence of urchin-like nanostructures on the ER microparticles.

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Enhanced electrorheology of suspensions containing sea-urchin-like hierarchical Cr-doped titania particles

We designed a two-dimensional acoustic metamaterial with periodical array of split hollow sphere (SHS) and investigated its transmission behaviors in the impedance tube system. The results showed that the acoustic metamaterial exhibited a transmission dip and inverse phase close to 5 kHz, which indicated the local resonance of SHS. Based on the homogeneous-media theory, the effective modulus of the acoustic metamaterial was calculated to be negative. The finite element method and resonant model simulation results also confirmed the local resonance and negative response in acoustic metamaterial, which agreed well with experiment results. This metamaterial is able to achieve the double negative acoustic metamaterial conveniently by arraying acoustic “atoms” with negative mass density in the matrix.

Xiaopeng Zhao

Papers

Electrorheological fluids based on nano-fibrous polyaniline

Terahertz dual-band metamaterial absorber based on graphene/MgF(2) multilayer structures.

Coaxial cable-like polyaniline@titania nanofibers: facile synthesis and low power electrorheological fluid application

Enhanced electrorheology of suspensions containing sea-urchin-like hierarchical Cr-doped titania particles

Two-dimensional acoustic metamaterial with negative modulus