Progress in the synthesis and applications of hierarchical flower-like TiO_2 nanostructures

Flower-like TiO_2 materials,with their advantages of high specific surface area,developed pore structure,and high photocatalytic activity,have been widely used in environmental management and air purification,sterilization,and surface self-cleaning,among other areas.This paper summarizes several methods used to fabricate the flower-like TiO_2 nanostructures,such as the hydrothermal,solvothermal,microemulsion,sol-gel,hydrolysis,and electrodeposition oxidation methods.In addition,the morphologies,properties,and performance of different flower-like TiO_2 structures are discussed.Meanwhile,the application progresses of different flower-like TiO_2 structures are also analyzed.     

Hydrodesulfurization catalyst prepared by urea-matrix combustion method

Co–Mo/γ-Al₂O₃–TiO₂ hydrodesulfurization (HDS) catalyst samples prepared by a urea matrix combustion (UMxC) method, were evaluated in a stainless tubular fixed-bed reactor, with thiophene, benzothiophene and dibenzothiophene in xylene as model feedstocks. The samples were pre-sulfurized using a cyclohexane solution of 3% CS₂ and then tested for the HDS reaction. The test results were compared with catalysts prepared by conventional methods involving sequential impregnation (SI) and co-impregnation (CI). The catalysts were characterized using X-ray diffraction (XRD), laser Raman spectroscopy (LRS), high resolution transmission electron microscopy (HRTEM) and N₂ physisorption, showing that the UMxC catalyst had higher pore volume and surface area than those prepared by the CI and SI methods. The UMxC method increased metal loading and avoided formation of inert phase, e.g., β-CoMoO₄, for the HDS reaction, suggesting that UMxC method is superior to the conventional impregnation techniques. TiO₂ promoter made particles on the catalyst surface closer and alleviated the interaction between molybdenum oxide and the support, and facilitated the formation of well-dispersed Co- and Mo-oxo species on catalyst surface, thus resulting in higher HDS catalytic activity than pure γ-Al₂O₃ support without modifiers. Consequently, the addition of TiO₂ obviously improved the HDS conversion of dibenzothiophene.     

Preparation of SiO2/TiO2 Janus particles by electrostatic assembly,hydrolysis and calcination

This paper describes a novel chemical method for preparing SiO₂/TiO₂ Janus particles. First, polystyrene (PSt)/SiO₂ particles with a raspberry-like structure are prepared by electrostatic assembly. The influences of the reaction time of sulfonation and the treatment times of polyelectrolyte solutions (PDADMAC, PSS) on PSt and SiO₂ are investigated with respect to the surface charge density of the particles. SiO₂/TiO₂ Janus particles are then obtained by hydrolysis of butyl titanate on the surface of PSt/SiO₂ particles followed by a calcination process. Particle size analyzer, Zeta potential instrument, FTIR, TEM and SEM are used to characterize the particle size, the amount of charge on the surface of PSt and SiO₂ particles and the compositions and morphologies of PSt/SiO₂, SiO₂/TiO₂ and PSt/SiO₂/TiO₂. The diameters of the PSt, SiO₂, PSt/SiO₂ and SiO₂/TiO₂ particles are 2.0 μm, 303 nm, 2.7 μm and 330 nm, respectively.     

Growth characteristics of spherical titanium oxide nanoparticles during the rapid gaseous detonation reaction

The nanosize grain growth characteristics of spherical single-crystal titanium oxide (TiO₂) during the rapid gaseous detonation reaction are discussed. Based on the experimental conditions and the Chapman–Jouguet theory, the Kruis model was introduced to simulate the growth characteristics of spherical TiO₂ nanoparticles obtained under high pressure, high temperature and by rapid reaction. The results show that the numerical analysis can satisfactorily predict the growth characteristics of spherical TiO₂ nanoparticles with diameters of 15–300 nm at different affecting factors, such as concentration of particles, reaction temperature and time, which are in agreement with the obtained experimental results. We found that the increase of the gas-phase reaction temperature, time, and particle concentration affects the growth tendency of spherical nanocrystal TiO₂, which provides effective theoretical support for the controllable synthesis of multi-scale nanoparticles.     

Dielectric analysis of TiO2-based electrorheological suspensions

Dielectric behaviors of titanium dioxide (TiO₂)-based electrorheological (ER) suspensions with different particle concentrations and TiO₂ polymorphs were investigated in the frequency range of 40 Hz to 110 MHz. Two relaxations in kilohertz and megahertz frequency range were attributed to interface polarization between TiO₂ and silicone oil and ion pair polarization between dissociated counterions and fixed charges on TiO₂ surfaces, respectively. Dipolar coefficient D, which is related to the construction or structure of the colloid, changes after critical volume fraction $\phi _{\rm c} \approx $ 0.05, indicating that chain-like or network structures are formed by particles. Based on percolation model, the values of critical exponent suggest that particles may form two-dimensional percolation network. Furthermore, the effective dielectric mismatch parameter, $\beta _{\rm eff}$ , was calculated based on the obtained phase parameters. We found that rutile should have better ER activity than anatase. The main reason for weak ER activity of pure TiO₂ ER suspensions may due to poor conductivity properties of TiO₂ crystals.     

Visible light photodegradation of methylene blue by AgBr-TiO2/SiO2@Fe3O4 magnetic photocatalysts

Visible photoactive AgBr/TiO2 was immobilized on a SiO2@Fe3O4 magnetic support by solvother- rnal and sol-gel methods to form a AgBr-TiO2/SiO2@Fe3O4 magnetic photocatalyst. Samples were characterized by X-ray diffraction, high-resolution transmission electron microscopy and magne- tometry. Hereto-structured AgBr/TiO2 was well seeded on the shell-core SiO2@Fe3O4 structure. The AgBr-TiO2/SiO2@Fe3O4 magnetic photocatalyst exhibited high photocatalytic activity in the degrada- tion of methylene blue under visible light. The photocatalyst was superparamagnetic, which is beneficial for facile magnetic separation.     

Hydrothermal synthesis and characterization of NiS flower-like architectures

Under the influence of thiocyanate anions (SCN^?) and cetyltrimethyl ammonium bromide (CTAB), NiS flower-like architectures were successfully synthesized by a one-step hydrothermal method. The synthesized flower-like architectures, with a multilayered and highly ordered texture, have diameters of several micrometers. X-ray powder diffraction (XRD) shows that the NiS flower-like architectures are rhombohedral crystalline. On the basis of condition-dependent experiments, the diffusion-limited aggregation (DLA) model and cage effect were used to explain the growth process of rhombohedral crystalline NiS flower-like architectures. Magnetic measurements showed that the coercivity (H_c) of the as-obtained NiS flower-like architectures was 102.14 Oe.     

Preparation of ternary Cr2O3–SiC–TiO2 composites for the photocatalytic production of hydrogen

Novel ternary Cr₂O₃–SiC–TiO₂ composites were synthesized by implanting Cr³⁺ into SiC–TiO₂ via sol–gel and impregnation approaches. The results from X-ray diffraction, scanning electron microscopy and transmission electron microscopy show that the Cr³⁺ species were doped onto the surface of the SiC–TiO₂ carrier. The diffuse reflectance ultraviolet–visible absorption spectra revealed that the absorption edges of the ternary Cr₂O₃–SiC–TiO₂ composites were gradually shifted red with increasing chromium content. The luminescence intensities of the composites decreased with increasing doped Cr³⁺ content due to the reduction in the number of recombination sites of electron–hole pairs. The ternary Cr₂O₃–SiC–TiO₂ composites showed high hydrogen-producing activities, which probably results from the formation of donor levels of the Cr³⁺ species in the forbidden band of the SiC semiconductor.     

Facile aqueous synthesis and thermal insulating properties of low-density glass/TiO2 core/shell composite hollow spheres

Anatase TiO₂ shells assembled on hollow glass microspheres (HGM) with tunable morphologies were successfully prepared through a controllable chemical precipitation method with urea as the precipitator. Thus, glass/TiO₂ core/shell composite hollow spheres with low particle density (0.40 g/cm³) were fabricated. The phase structures, morphologies, particle sizes, shell thicknesses, and chemical compositions of the composite microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The morphology of the TiO₂ shell can be tailored by properly monitoring the reaction system component and parameters. The probable growth mechanism and fabrication process of the core/shell products involving the nucleation and oriented growth of TiO₂ nanocrystals on hollow glass microspheres was proposed. A low infrared radiation study revealed that the radiation properties of the products are greatly influenced by the unique product shell structures. A thermal conductivity study showed that the TiO₂/HGM possess low thermal conductivity that is similar to that of the pristine HGMs. This work provides an additional strategy to prepare low-density thermal insulating particles with tailored morphologies and properties.     

等离子体辅助球磨制备表面修饰纳米TiO2的摩擦学性能分析

以季铵盐和月桂酸钠为过程处理剂,利用等离子体辅助球磨制备表面修饰纳米TiO_2粉体,并测试其摩擦学性能.结果表明:在等离子体的热爆效应及脉冲电子轰击效应的协同作用下,辅助球磨11 h制备的表面修饰纳米TiO_2粉体粒径在20 nm左右,晶型发生由锐钛矿型向金红石型的转变.等离子体辅助球磨使得纳米TiO_2获得了良好的亲油疏水表面特性,在40CA船用润滑油中表现出稳定的分散性.由于纳米TiO_2粉体的"微轴承"作用,复合润滑油的摩擦系数降低,摩擦副的磨损失重量减少.纳米TiO_2粉体在摩擦过程中容易吸附沉积在摩擦副表面并修补磨痕,使得复合润滑油具备良好的减摩及自修复性能.     

TiO2-loaded activated carbon fiber: Hydrothermal synthesis,adsorption properties and photo catalytic activity under visible light irradiation

TiO₂-loaded activated carbon fibers (ACF) were prepared by a hydrothermal method. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry and UV–vis diffuse reflectance spectra (DRS). SEM images showed that the TiO₂ nanoparticles were deposited on the surface of ACF, and the particle size and loading amount of TiO₂ were varied by changing the initial concentration of tetrabutyl titanate (TBOT). The results of an ash experiment showed that the loading amounts of TiO₂ were 18.4%, 43.3%, 52.5%, 75.1%, and 91.1% for initial concentrations of TBOT of 0.07, 014, 0.21, 0.28, and 0.35 mol/L, respectively. Physical interactions played an important role in the formation of TiO₂/ACF composite fibers that absorb UV and visible light. Compared with those of ACF, improved adsorption and photocatalytic activity toward Rhodamine B (RhB) were observed for TiO₂/ACF composite fiber. The Rhodamine B could be removed efficiently by TiO₂/ACF composite fibers, and the TiO₂ loading amount had a significant effect on the photocatalytic activity of TiO₂/ACF composite fibers.     

A novel method for fast and continuous preparation of superfine titanium dioxide nanoparticles in microfluidic system

Previously we had developed a microfluidic system that can be easily fabricated by bending a stainless-steel tube into large circular loops. In this study, a fast and continuous preparation method for superfine TiO₂ nanoparticles (TiO₂-NPs) was developed for the aforementioned microfluidic system. The proposed method can yield anatase TiO₂ in 3.5 min, in contrast to the traditional hydrothermal reaction method, which requires hours or even days. Different reaction conditions, such as reaction temperature (120–200 °C), urea concentration (20–100 g/L), and tube length (5–20 m) were investigated. X-ray diffraction and Brunauer–Emmett–Teller analysis indicate that the as-prepared TiO₂-NPs have crystalline sizes of 4.1–5.8 nm and specific surface areas of 250.7–330.7 m²/g. Transmission electron microscopy images show that these TiO₂-NPs have an even diameter of approximately 5 nm. Moreover, because of their small crystalline sizes and large specific surface areas, most of these as-prepared TiO₂-NPs exhibit considerably better absorption and photocatalytic performance with methylene blue than commercial P5 TiO₂ does.     

Hollow spherical titanium dioxide nanoparticles for energy and environmental applications

Hollow spherical titanium dioxide (TiO₂) nanoparticles possess unique properties toward energy and environmental applications, because of the intrinsic properties of TiO₂ and benefits induced by their hollow structure. A detailed understanding of TiO₂ hollow spheres will promote their use in sustainable energy and environmental applications. This perspective details current methods for synthesizing hollow spherical TiO₂ nanoparticles, and their performance in dye-sensitized solar cells, photocatalysts, and batteries. This perspective will promote the design and innovative thinking on the application of hollow spherical metal oxide nanoparticles.     

A core-shell composite pigment with rutile TiO2 intensification for UV inhibition

Organic pigments generally have bright colors but poor ultraviolet (UV) resistance. To improve the UV resistance and extend the applications, a core-shell composite pigment with rutile TiO₂ intensification for UV inhibition is proposed by a facile sol-gel method in this work. A core-shell structure, with a homogeneous sol-gel TiO₂ shell containing additional nanosized rutile TiO₂ particles and with the pigment as the core, was established taking advantage of UV resistance of TiO₂ and binding ability of sol-gel. While the sol-gel TiO₂ shell alone has already shown obvious ultraviolet shielding effect, as tested over different sol-gel aging times and TiO₂ loadings, the UV resistance of the fluorescent pigments was further enhanced by binding the nanosized rutile TiO₂ in the sol-gel shell. At a sol-gel TiO₂ to rutile TiO₂ ratio of 2:1, the UV exposure time is extended about eight times compared with that of the original pigment and twice as that of the modified pigment with pure sol-gel TiO₂ for the same color change. Therefore, the novel core-shell composite pigment intensified with nanosized rutile TiO₂ particles is proved to be efficient in improving the UV resistance of organic pigments.     

Liquid phase heterogeneous photocatalytic ozonation of phenol in liquid–solid fluidized bed: Simplified kinetic modeling

The isotherms of original AC (activated carbon) and photocatalysts (TiO₂-AC) calcined at 500 °C for phenol were measured. The results showed a reversible adsorption of phenol onto both kinds of particles at 25 °C, and could be fitted well to the Freundlich adsorption equation for the dilute solution. Five oxidation processes, namely O₃, O₃/UV, O₃/UV/AC, O₂/UV/TiO₂ and O₃/UV/TiO₂, for phenol degradation in fluidized bed were evaluated and compared, and the photocatalytic ozonation was found to give the highest phenol conversion because of the combined actions of homogenous ozonation in the liquid phase, heterogeneous ozonation on the surface of the catalyst support, i.e. activated carbon, and heterogeneous photocatalytic oxidation on the TiO₂ catalyst surface. With the simplified kinetic model, photolytic ozonation was confirmed to predominantly take place on the particle surface in comparison with the heterogeneous and homogeneous photolytic ozonation. Additionally, the heterogeneous photocatalytic oxidation constant was found to be enhanced by 3.73 times in photocatlaytic ozonation process with ozone as the scavenger compared to the photocatalytic oxidation process with oxygen as the scavenger.     

2D reduced graphene oxide–titania nanocomposites synthesized under different hydrothermal conditions for treatment of hazardous organic pollutants

Two-dimensional reduced graphene oxide–titania (RGO–TiO₂) composites were prepared using a single-step hydrothermal method under various hydrothermal reaction conditions. The morphological and surface characteristics of the RGO–TiO₂ composites and reference materials were determined. The RGO–TiO₂ composites showed photocatalytic activity for the decomposition of two target pollutants that was superior to both pure TiO₂ and RGO under fluorescent daylight lamp illumination. The photocatalytic activity of the RGO–TiO₂ composite increased as the hydrothermal treatment time increased from 1 to 24 h, but then it decreased as the time increased to 36 h, which indicated the presence of an optimal treatment time. RGO–TiO₂ composites activated by violet light-emitting diodes (LEDs) displayed lower decomposition efficiency than those activated by a daylight lamp, likely because of the lower light intensity of violet LEDs (0.2 mW/cm²) when compared with that of the daylight lamp (1.4 mW/cm²). However, the photocatalytic decomposition of the target pollutants using the RGO–TiO₂ composite was more energy-efficient using the violet LEDs. The photocatalytic reaction rates increased as the residence time decreased, whereas the reverse was true for the decomposition efficiency.     

Influence of preparation route and slip casting conditions on titania and barium titanate ceramics

Titania (TiO₂) and barium titanate (BaTiO₃) were synthesized using three different dicarboxylates, which included oxalate, malate and tartarate. These powders were characterized by X-ray powder diffraction, scanning electron micrographs, BET specific surface area and particle size distribution. Their properties depended to a great extent on the nature of the precursor. The titania and barium titanate powders obtained from the tartarate precursor were found to be good for slip casting. Slips of these oxides with different solids contents were prepared at different pH values using both distilled water and ethanol as the dispersing agent and also with and without deflocculant. The rheological behaviors of the suspensions were then determined, and the slip, green and sedimentation bulk densities were measured. The minimum viscosities were observed at pH 8.2 for the TiO₂–water and pH 10.2 for the BaTiO₃–water system.     

Ti、TiN、TiO2改性层的纳米力学性能测试与分析

采用等离子表面合金化技术,分别在316L不锈钢表面制备出渗Ti改性层、渗TiN改性层和TiO2改性层薄膜.使用连续刚度法,从截面方向和表面方向对改性层进行纳米压痕实验,研究改性层的纳米力学性能.实验测得材料在压痕过程中的载荷—位移曲线以及硬度和模量随压入深度的连续变化值.结果表明,改性层纳米力学特性表现为各向异性;TiN改性层的力学性能表现良好.TiO2改性层由渗Ti改性层经氧化制成,二者的弹性模量和硬度在截面方向上变化规律相似,在表面方向上数值相近.     

Green synthesis and enhanced photocatalytic activity of Ce-doped TiO2 nanoparticles supported on porous glass

A facile and green method to prepare Ce-doped TiO₂ nanoparticles supported on porous glass beads is reported. An ion exchange process and subsequent calcination yielded Ce-doped TiO₂ nanoparticles with a mean size of 4.8 ± 0.3 nm. The nanoparticles were dispersed on the surface of porous glass beads. The addition of Ce enhanced the visible light absorption of the TiO₂ nanoparticles in the 400–500 nm spectral window. The band gap of the as-prepared catalyst was 2.80 eV. The Ce-doped TiO₂ nanoparticles immobilized on porous glass beads exhibited excellent photocatalytic activity for the visible-light-degradation of methyl orange (MO) and rhodamine B (RhB); with rate constants of 0.095 and 0.230 min⁻¹; respectively. The effects of Ce dosage; reaction duration; and initial solution pH on the conversion of MO and RhB dyes were investigated. The green synthesis and favorable photocatalytic activity makes the Ce-doped TiO₂ nanoparticles immobilized on porous glass an attractive alternative for the efficient degradation of organic pollutants.     

Non-isobaric Marangoni boundary layer flow for Cu,Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and TiO<Subscript>2</Subscript> nanoparticles in a water based fluid

In this paper, a non-isobaric Marangoni boundary layer flow that can be formed along the interface of immiscible nanofluids in surface driven flows due to an imposed temperature gradient, is considered. The solution is determined using a similarity solution for both the momentum and energy equations and assuming developing boundary layer flow along the interface of the immiscible nanofluids. The resulting system of nonlinear ordinary differential equations is solved numerically using the shooting method along with the Runge-Kutta-Fehlberg method. Numerical results are obtained for the interface velocity, the surface temperature gradient as well as the velocity and temperature profiles for some values of the governing parameters, namely the nanoparticle volume fraction φ (0≤φ≤0.2) and the constant exponent β. Three different types of nanoparticles, namely Cu, Al₂O₃ and TiO₂ are considered by using water-based fluid with Prandtl number Pr =6.2. It was found that nanoparticles with low thermal conductivity, TiO₂, have better enhancement on heat transfer compared to Al₂O₃ and Cu. The results also indicate that dual solutions exist when β<0.5. The paper complements also the work by Golia and Viviani (Meccanica 21:200–204, 1986) concerning the dual solutions in the case of adverse pressure gradient.