Enhanced Electrorheological Activity of Mesoporous Cr-Doped TiO2 from Activated Pore Wall and High Surface Area

Synthesis of anatase TiO2 with exposed (100) facets and enhanced electrorheological activity

Herein, a simple hydrothermal method is employed to synthesize anatase TiO₂ with dominant (100) facets, as a precursor, using titanate nanofibers derived from alkali treatment. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) are carried out to confirm the surface morphology and phase structure of the TiO₂ product. The formation mechanism of TiO₂ enclosed by (100) and (101) facets is deduced to be the selective adsorption of OH^- on the (100) facets of anatase TiO₂. Electroheological (ER) experiments indicate that the tetragonal-facet-rod anatase TiO₂ with exposed (100) facets exhibits an excellent ER performance with a high ER efficiency of up to 52.5, which results from the anisotropy of its special morphology. In addition, the effect of shape on its dielectric property is investigated via broadband dielectric spectroscopy.

Wet-Chemical Preparation of TiO₂-Based Composites with Different Morphologies and Photocatalytic Properties

TiO₂-based composites have been paid significant attention in the photocatalysis field. The size, crystallinity and nanomorphology of TiO₂ materials have an important effect on the photocatalytic efficiency. The synthesis and photocatalytic activity of TiO₂-based materials have been widely investigated in past decades. Based on our group’s research works on TiO₂ materials, this review introduces several methods for the fabrication of TiO₂, rare-earth-doped TiO₂ and noble-metal-decorated TiO₂ particles with different morphologies. We focused on the preparation and the formation mechanism of TiO₂-based materials with unique structures including spheres, hollow spheres, porous spheres, hollow porous spheres and urchin-like spheres. The photocatalytical activity of urchin-like TiO₂, noble metal nanoparticle-decorated 3D (three-dimensional) urchin-like TiO₂ and bimetallic core/shell nanoparticle-decorated urchin-like hierarchical TiO₂ are briefly discussed.

Tunable resistance switching in solution processed chromium-doped strontium titanate nanoparticles films

In this work, resistance switching behaviours in solution processed chromium (Cr)-doped strontium titanate (SrTiO₃) films have been investigated. Undoped SrTiO₃ film shows I-V characteristics of typical nonlinear resistor and no resistance hysteresis loops are observed. On the contrary, Cr-doped SrTiO₃ films show stable and reversible hysteresis loops, which can be controlled by applying different voltage bias. Based on a series of characterization results, including X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), we infer that Ti⁴⁺ is substituted by Cr³⁺, giving rise to increased concentration of oxygen vacancies. Therefore, the observed resistance switching phenomenon is attributed to voltage driven oxygen vacancy migration. Furthermore, gradually decreased overall resistance is also realized under repeated sweeping cycles.

Complex doping chemistry owing to Mn incorporation in nanocrystalline anatase TiO2 powders

Mn-doped TiO2 powders with a wide range of nominal doping levels were fabricated using a one-step hydrothermal method followed by 400 °C annealing. Anatase powders with a uniform size distribution below 10 nm were obtained. The maximum solubility of Mn in the TiO2 lattice was around 30%, beyond which the Mn3O4 compound appeared as a secondary phase. The optical absorption edges for Mn-doped anatase TiO2 were red-shifted effectively through increasing Mn content. Alloying chemistry and associated elemental valences were elaborated through combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and theoretical simulation in the framework of density functional theory (DFT). The results showed that the Mn species exhibited mixed valence states of 3+ and 4+ in anatase TiO2, with the latter being the key to remarkable photocatalytic performance.

The application of low frequency dielectric spectroscopy to analyze the electrorheological behavior of monodisperse yolk-shell SiO2/TiO2 nanospheres

Monodisperse SiO2/TiO2 yolk-shell nanospheres (YSNSs) with different SiO2 core sizes were fabricated and adopted as dispersing materials for electrorheological (ER) fluids to investigate the influence of the gradual structural change of disperse particles on ER properties. The results showed that the ER performance of the YSNS-based ER fluid prominently enhanced with the decrease of SiO2 core size, which was attributed to the enhancement of electric field force between YSNSs. Combined with the analysis of dielectric spectroscopy, it was found that the increase of permittivity at low frequency (10(-2)-10(0) Hz) was due to the increase of polarized charges caused by secondary polarization (Psp). Moreover, the number of Psp closely related to the distributing change of polarized particles in ER fluid was a critical factor to assess the ER performance. Additionally, a parameter K (the absolute value of the slope of permittivity curves at 0.01 Hz) could be utilized to characterize the efficiency of structural evolution of polarized particles in ER fluid. Compared with the ER performance, it could be concluded that the value of Δε(100Hz-100kHz)' just demonstrated the initial intensity of the interface polarization in the ER fluid as the electric field was applied, which ignored the distributing evolution of polarized disperse particles in ER fluid. The polarizability Δε(0.01Hz-100kHz)' obtained in the frequency range of 10(-2)-10(5) Hz should be more suitable for analyzing the system of ER fluid. The relationships between polarizability of disperse particles, parameter K and ER properties were discussed in detail.

Polythiophene–graphene oxide doped epoxy resin nanocomposites with enhanced electrical, mechanical and thermal properties

Polythiophene–graphene oxide nanoparticle hybrids as high performance multifunctional reinforcement for epoxy resin.

Secondary polarization in electrorheological phenomenon

The Schematic illustration of the distribution of the electric field between two testing parallel-plates after the formation of chainlike structure in electrorheological fluids.

Enhanced Electrorheological Performance of Nb-Doped TiO2 Microspheres Based Suspensions and Their Behavior Characteristics in Low-Frequency Dielectric Spectroscopy

Titanium dioxide and Nb-doped titanium dioxide microspheres with the same size were fabricated by a simple sol-gel method, and the formation mechanism of Nb-doped titanium dioxide microspheres was proposed. Titanium dioxide and Nb-doped titanium dioxide microspheres were adopted as dispersed materials for electrorheological (ER) fluids to investigate the influence of the charge increase introduced by Nb doping on the ER activity. The results showed that Nb doping could effectively enhance the ER performance. Combining with the analysis of dielectric spectroscopy, it was found that the interface polarization of Nb-doped TiO2 ER fluid was larger than that of TiO2 ER fluid, which might be caused by more surface charges in Nb-TiO2 microspheres due to Nb(5+) doping and resulting in enhancement of electric field force and strengthening of fibrous structure. In addition, by comparing and analyzing the permittivity curves of Nb-TiO2/LDPE solid composite and Nb-TiO2/silicone-oil fluid composite, it could be concluded that the enhancement of permittivity at low frequency resulted from the increase of the order degree of dispersed particles in ER fluid rather than from the quasi-dc (QDC) behavior. Moreover, the absolute value of slope of permittivity curves (K) at 0.01 Hz could be utilized as the standard for judging the ability to maintain the chainlike structure. The relationships between polarizability of dispersed particles, dielectric spectrum, parameter K, and ER properties were discussed in detail.

A detailed study on the working mechanism of a heteropoly acid modified TiO2 photoanode for efficient dye-sensitized solar cells

The use of SiW₁₁Ni as a photoanode in a DSSC results in the photocurrent of 7.82%, 31% higher than that of pristine P25 DSSCs.

Synthesis and characterization of guar-alginate hybrid bead templated mercury sorbing titania spheres

Present communication reports on the synthesis and characterization of Hg(II) sorbing millimeter sized porous titania spheres (TSP). The synthesis utilizes guar gum-alginate hybrid beads as sacrificial template to polymerize titanium(IV) isopropoxide. The hybrid beads are crafted by pouring guar-alginate mixed solution to calcium bath. The mechanical strength of the beads depended on guar to alginate ratio in the mixed solution. The equal weight ratio of the two polysaccharides is appropriate for adequate mechanical strength beads. The unique performance of the templating beads is attributed to the synergistic interaction between guar gum and sodium alginate. FTIR, BET, SEM, TEM, XRD, TGA, and DTG analyses have been used for the characterization of the optimum performance TSP (TSPAG2). TSPAG2 is a mesoporous material that has higher surface area and narrower pore size distribution than pure alginate derived titania spheres (TSPA). TEM study demonstrated that TSPAG2 spheres are constituted of aggregated TiO2 nanoparticles of ∼ 10 nm size. TSPAG2 is able to capture >95% Hg(II) from synthetic Hg(II) solution in 10h at pH 5 as opposed to only 68% removal by TSPA.

Enhanced dielectric polarization and electro-responsive characteristic of graphene oxide-wrapped titania microspheres

Electric field-induced particle polarization is essential to the electro-responsive electrorheological (ER) effect of particle suspensions. In this work, we use graphene oxide (GO) as a soft and polar coating shell to prepare GO-wrapped titania dielectric microspheres for use as the dispersal phase of an ER suspension. Under a DC electric field, the ER characteristic of GO-wrapped titania microspheres dispersed in silicone oil is investigated by rheological tests, and then compared with that of a suspension of bare titania microspheres. The results show that the suspension of GO-wrapped titania microspheres possesses an enhanced ER characteristic. Its field-induced shear yield stress and storage modulus are much higher than those of the suspension of bare titania microspheres. The soft and polar GO shell is regarded as the origin of the ER enhancement. Dielectric analysis indicates that wrapping GO can enhance the interfacial polarization and thus improve the ER characteristics of titania microspheres. Wrapping GO onto the surface of titania microspheres can also reduce the particle sedimentation velocity of the suspension.

Flame-retardant electrical conductive nanopolymers based on bisphenol F epoxy resin reinforced with nano polyanilines

Both fibril and spherical polyaniline (PANI) nanostructures have successfully served as nanofillers for obtaining epoxy resin polymer nanocomposites (PNCs). The effects of nanofiller morphology and loading level on the mechanical properties, rheological behaviors, thermal stability, flame retardancy, electrical conductivity, and dielectric properties were systematically studied. The introduction of the PANI nanofillers was found to reduce the heat-release rate and to increase the char residue of epoxy resin. A reduced viscosity was observed in both types of PANI-epoxy resin liquid nanosuspension samples at lower loadings (1.0 wt % for PANI nanospheres; 1.0 and 3.0 wt % for PANI nanofibers), the viscosity was increased with further increases in the PANI loading for both morphologies. The dynamic storage and loss modulii were studied, together with the glass-transition temperature (T(g)) being obtained from the peak of tan δ. The critical PANI nanofiller loading for the modulus and T(g) was different, i.e., 1.0 wt % for the nanofibers and 5.0 wt % for the nanospheres. The percolation thresholds of the PANI nanostructures were identified with the dynamic mechanical property and electrical conductivity, and, because of the higher aspect ratio, nanofibers reached the percolation threshold at a lower loading (3.0 wt %) than the PANI nanospheres (5.0 wt %). The PANI nanofillers could increase the electrical conductivity, and, at the same loading, the epoxy nanocomposites with the PANI nanofibers showed lower volume resistivity than the nanocomposites with the PANI nanospheres, which were discussed with the contact resistance and percolation threshold. The tensile test indicated an improved tensile strength of the epoxy matrix with the introduction of the PANI nanospheres at a lower loading (1.0 wt %). Compared with pure epoxy, the elasticity modulus was increased for all the PNC samples. Moreover, further studies on the fracture surface revealed an enhanced toughness. Finally, the real permittivity was observed to increase with increasing the PANI loading, and the enhanced permittivity was analyzed by the interfacial polarization.

Preparation of highly ordered mesoporous Al2O3/TiO2 and its application in dye-sensitized solar cells

Highly ordered mesoporous Al(2)O(3)/TiO(2) was prepared by sol-gel reaction and evaporation-induced self-assembly (EISA) for use in dye-sensitized solar cells. The prepared materials had two-dimensional, hexagonal pore structures with anatase crystalline phases. The average pore size of mesoporous Al(2)O(3)/TiO(2) remained uniform and in the range of 6.33-6.58 nm while the Brunauer-Emmett-Teller (BET) surface area varied from 181 to 212 m(2)/g with increasing the content of Al(2)O(3). The incorporation of Al content retarded crystallite growth, thereby decreasing crystallite size while simultaneously improving the uniformity of pore size and volume. The thin Al(2)O(3) layer was located mostly on the mesopore surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The Al(2)O(3) coating on the mesoporous TiO(2) film contributes to the essential energy barrier which blocks the charge recombination process in dye-sensitized solar cells. Mesoporous Al(2)O(3)/TiO(2) (1 mol % Al(2)O(3)) exhibited enhanced power conversion efficiency (V(oc) = 0.74 V, J(sc) = 15.31 mA/cm(2), fill factor = 57%, efficiency = 6.50%) compared to pure mesoporous TiO(2) (V(oc) = 0.72 V, J(sc) = 16.03 mA/cm(2), fill factor = 51%, efficiency = 5.88%). Therefore, the power conversion efficiency was improved by approximately 10.5%. In particular, the increase in V(oc) and fill factor resulted from the inhibition of charge recombination and the improvement of pore structure.

Fabrication of uniform core-shell structural calcium and titanium precipitation particles and enhanced electrorheological activities

A simple co-precipitation route was developed to synthesize uniform core-shell structured calcium and titanium precipitation (CTP) particles with ideal morphology and no aggregation. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), and interface tension/contact angle (CA) measurement were utilized to characterize the components, structure, morphology, and wettability of the SiO(2)-CTP materials. The obtained core-shell structural SiO(2)-CTP particles were well dispersed spherical nanoparticles with a narrow size distribution. The electrorheological (ER) properties were studied by the shear stress under various electric fields. The SiO(2) (2.3 wt%)-CTP ER fluid showed notable ER activity with a shear stress of about 109 kPa (at 5 kV mm(-1)), which outclassed the shear stress (65 kPa) of the CTP ER fluid. The ER properties of samples can be tuned by a few factors in the experimental process, such as the concentration of SiO(2) particles and citric acid, pointing out the great potential for application of this route in bulk synthesis of many other types of ER materials.

The direct synthesis of mesoporous structured MnO(2)/TiO(2) nanocomposite: a novel visible-light active photocatalyst with large pore size

A series of visible-light-driven mesoporous structured MnO(2)/TiO(2) nanocrystal photocatalysts have been synthesized through a modified sol-gel method, and the N(2) adsorption-desorption isotherm confirms that the mesoporous materials possess large pore size (up to 9.2 nm) and a narrow pore size distribution. X-ray powder diffraction (XRD) analyses and complementary x-ray photoelectron spectroscopy (XPS) measurements reveal that the doping of the transition metal Mn inhibits the growth of TiO(2) anatase nanocrystals and the Mn species are highly dispersed on the surface of TiO(2). The ultraviolet (UV)-vis spectrum demonstrates the excellent adsorption properties of MnO(2)/TiO(2) over the whole region of visible light, which enables this novel photocatalysis material to possess remarkable activity in the photocatalytic degradation of methylene blue under visible light radiation. Moreover, a 'coating mechanism' based on the nucleation of titania nanocrystals along with the interaction between the dopant precursors and titania clusters has been suggested.