Hierarchical TiO2 Nanostructures Research Articles

Synthesis of hierarchical anatase TiO2 nanostructures with tunable morphology and enhanced photocatalytic activity

A facile one-pot method to prepare three-dimensional hierarchical nanostructures of titania with good control over their morphologies without the use of hydrofluoric acid is developed. The reaction is performed under microwave irradiation conditions in pure water, and enables enhanced photocatalytic activity. This study indicates that photocatalytic activity depends not only on the surface area but also on the morphology of the titania.

Controlled Synthesis of TiO2 Hierarchical Nanofibre Structures via Electrospinning and Solvothermal Processes: Photocatalytic Activity for Degradation of Methylene Blue

The present article describes a new titanium oxide-based (TiO2) photocatalyst that shows promise for acceleration of dye degradation. A hierarchical TiO2 nanostructure comprising nanorods on-nanofibres has been prepared using a sol-gel route and electrospinning. Calcination of electrospun nanofibre mats was performed in air at 500 °C. The TiO2 nanofibre surface was then exploited as a ‘seeding ground’ to grow TiO2 nanorods by a solvothermal process in NaOH. The nanofibres had a diameter of approximately 100 nm while the nanorods were evenly distributed on the nanofibre surface with a mean diameter of around 50–80 nm. The hierarchical nanostructure showed enhanced photocatalytic activity when compared to pure TiO2 nanofibres. This improved efficiency in degrading methylene blue through the photocatalytic process was attributed to the larger specific surface area of the TiO2 nanostructures, as well as high surface-to-volume ratio and higher reactive surface resulting in enhanced surface adsorption and interfacial redox reaction.

Facile fabrication of a dual hierarchical TiO2 nanostructure

A dual hierarchical TiO2 nanostructure is directly fabricated by electrochemically anodizing of an electropolished Ti substrate in an ethylene glycol electrolyte containing small amounts of fluoride with a two-step process, which is carried out at room temperature in a two-electrode electrochemical cell under 30V for 3h plus 10min. The formed nanostructure consists of a nanoporous top layer (~60nm thickness) and a nanotubular underneath layer (~600nm thickness). The highly regular nanopores (~100nm diameter) in hexagonal shape distribute over the top layer and the underneath layer is arranged with the well-aligned nanotubes (~50nm diameter). The dual parts of the hierarchical nanostructure are both composed of TiO2, which exhibits anatase phase after being annealed at 450°C under ambient air for 3h. The facile fabrication of this dual hierarchical TiO2 nanostructure inspires to prepare other nanoscale materials with alike complex geometries by the similar approach.

Template-Free Synthesis and Photocatalytic Application of Rutile TiO2 Hierarchical Nanostructures

A simple template-free hydrothermal method was employed to synthesize flower-shaped single-crystal rutile TiO2 hierarchical nanostructures without calcination process. Scanning electron microscope and transmission electron microscope images show that rutile TiO2 nanostructures with diameters of 1–1.5 μm are composed of nanorods with a wimble shape. The band gap of the as-prepared rutile TiO2 is about 3.02 eV by ultraviolet–visible absorption spectrum. The photocatalytic performance of the as-obtained samples as catalysts for Rhodamine B (RhB) degradation under simulated solar light was greatly enhanced with the assistance of a small amount of H2O2. In the H2O2-containing system, the as-prepared rutile TiO2 photocatalyst was more efficient in the photodegradation of RhB than commercial P25. The stability and recycle of the rutile TiO2/H2O2 system were also investigated.

3D hierarchical flower-like TiO2 nanostructure: morphology control and its photocatalytic property

3D hierarchical flower-like TiO2 nanostructures were synthesized via a facile solvothermal method combining a calcination process. A series of electron microscopy characterization results suggest that the growth of 3D hierarchical flower-like TiO2 nanostructures is governed by a nucleation and nuclei growth–dissolution–recrystallization growth mechanism from time-dependent morphology evolution. The influence of the experimental parameters such as reaction species, temperature, and time on the morphology growth was systematically studied. Importantly, the prepared 3D hierarchical flower-like TiO2 nanostructures showed enhanced photocatalytic activity compared with Degussa P25 for the degradation of phenol under UV light irradiation, which is attributed to its special hierarchical porous structure, good permeability, high light-harvesting capacity and more surface active sites. Electron spin resonance (ESR) detection of active oxygen species (hydroxyl radicals) and the total organic carbon (TOC) changes are also investigated to evaluate the degradation efficacy.

Tunable hierarchical TiO2 nanostructures by controlled annealing of electrospun fibers: formation mechanism, morphology, crystallographic phase and photoelectrochemical performance analysis

Highly crystalline hierarchical TiO2 nanostructures of morphology ranging from one-dimensional regular fibers, hollow tubes, porous rods and spindles were achieved from electrospun TiO2/composite fibers by annealing at temperatures ranging from 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C, with a ramp rate of 5 °C min−1, and at a pressure of 1 mbar. Crystallographic structure, crystallite size, surface morphology and surface area of annealed TiO2 nanostructures were analysed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Brunauer–Emmett–Teller (BET) method. The analysis of post-annealing process on electrospun TiO2 nanofibers showed an orderly change in the crystallographic phase transformation with corresponding change in their surface morphologies. XRD and HRTEM analysis confirmed the phase transformation of highly crystalline anatase phase to rutile with crystallite size varied from 11 nm to 36 nm upon tuning the annealing temperature. Interestingly, TiO2 nanostructures annealed at 700 °C showed the formation of biphasic TiO2 hollow tubes with stoichiometry phase compositions of 45.74% anatase and 54.25% rutile. A possible formation mechanism was proposed based on series of temperature-dependent experiments. To evaluate the potential use of these TiO2 nanostructures, dye sensitized solar cell (DSSC) was fabricated using the post-annealed TiO2 nanostructures as photoanode. A higher conversion efficiency (η) of 4.56% with a short circuit current (Jsc) of 8.61 mA cm−2 was observed for highly ordered porous anatase TiO2 nanorods obtained upon annealing at 500 °C under simulated AM1.5 G (100 mW cm−2), confirming that surface area of TiO2 resulted out of porous structure played dominant role.

Growth of hierarchical TiO2 nanostructures on anatase nanofibers and their application in photocatalytic activity

Three-dimensional hierarchical nanostructures of TiO2, consisting of high density nanorods on nanofibers, were synthesized by the combination of electrospinning, thermal annealing and hydrothermal methods. The morphologies, crystal structures, surface area, band gap and photocatalytic activity of the hierarchical nanostructures were characterized by Raman microscopy, X-ray diffraction, UV-vis spectroscopy, scanning electron microscopy, transmission electron microscopy and surface area analysis. The results revealed that the nanofibers, synthesized by the electrospinning method followed by the thermal annealing, were of anatase phase. Similarly, the TiO2 nanorods grown on the anatase nanofibers by hydrothermal reaction in Ti-HCl solution were of rutile phase, with tetragonal shape, and had top square facet morphologies. The diameter and the length of these rutile nanorods could be varied over the range, 10 nm to 400 nm, and 20 nm to 1 um, respectively, by changing the parameters of the hydrothermal reaction. The growth mechanisms of these rutile rods are discussed on the basis of (i) surface roughness of the anatase nanofibers, (ii) presence of the twin planes of anatase layer with structure similar to rutile phase and (iii) Cl/Ti ratio in the solution. The estimated band gap energies of the nanofibers and nanorods were close to 3.2 eV and 3.0 eV. The total surface area of the hierarchical nanostructures could reach up to ∼20.41 m2 g−1; while, the pure the anatase nanofibers surface area is ∼19.79 m2 g−1. For the first time, such TiO2 hierarchical nanostructures, rutile nanorods on anatase nanofibers, have been synthesized. The photocatalytic activity of the TiO2 hierarchical nanostructures was found to be superior to that of pure anatase nanofibers.

Large Scale Synthesis and Gas-Sensing Properties of Anatase TiO2 Three-Dimensional Hierarchical Nanostructures

Three-dimensional (3D) crystalline anatase titanium dioxide (TiO(2)) hierarchical nanostructures were synthesized through a facile and controlled hydrothermal and after-annealing process. The formation mechanism for the anatase TiO(2) 3D hierarchical nanostructures was investigated in detail. The 3D hierarchical nanostructures morphologies are formed by self-organization of several tens of radially distributed thin petals with a thickness of several nanometers with a larger surface area. The surface area of TiO(2) hierarchical nanostructures determined by the Brunauer-Emmett-Teller (BET) adsorption isotherms was measured to be 64.8 m(2) g(-1). Gas sensing properties based on the hierarchical nanostructures were investigated. A systematic study on sensitivity as a function of temperatures and gas concentrations was carried out. It reveals an improved ethanol gas sensing response property with a sensitivity of about 6.4 at 350 degrees C upon exposure to 100 ppm ethanol vapor for the TiO(2) hierarchical nanostructures. A gas sensing mechanism based on the adsorption-desorption of oxygen on the surface of TiO(2) is discussed and analyzed. This novel gas sensor can be multifunctional and promising for practical applications. Furthermore, the hierarchical nanostructures with high surface area can find variety of potential applications such as solar cells, biosensors, catalysts, etc.