Hierarchical TiO2 Nanostructures Research Articles

Controllable synthesis of TiO2hierarchical and their applications in lithium ion batteries

In this article, TiO2 has been successfully fa in the Ti(SO4)2–CH3COOH system via a simple hydrothermal method. The formation mechanism of the TiO2 hierarchical nanostructures was also investigated in detail, which involved the complex reaction between Ti(SO4)2 and CH3COOH, hydrolysis of the complex, nucleation and self-assembly of TiO2 nanocrystals and anisotropic growth of these subunits. SO42− plays an important role in the self-assembly of subunits. In addition, controllable synthesis of TiO2 hierarchical nanostructures with tunable size can be realized just by changing experimental parameters, such as reaction time or Ti(SO4)2 concentration. When used as the anode in lithium ion batteries, these TiO2 hierarchical nanostructures present much higher specific capacity and better rate performances than TiO2 nanoparticles, which are attributed to the stability of the secondary-structure during the charge and discharge processes.

Effects of non-polar solvent on the morphology and property of three-dimensional hierarchical TiO2 nanostructures by one-step solvothermal route

Three-dimensional (3D) hierarchical rutile TiO2 microspheres composed of nanorods with diameter of several-tens of nanometers, with different morphologies and with average size ranging from 1.3 to 1.8 μm, were successfully synthesized through a surfactant-free solvothermal route. The effects of the solvents n-hexane, chloroform, and cyclohexane on the microstructures of 3D hierarchical TiO2 nanostructures were investigated. Results of scanning electron microscopy showed that 3D sea-urchin like hierarchical TiO2 composed of nanorods with a diameter of ~10 nm can only be prepared in the cyclohexane-water system. The growth mechanism of 3D sea-urchin like hierarchical TiO2 composed of numerous nanorods was further examined and found to differ from the well-known “growth → assembly” mode. The effects of surface tension and polarity of solvents on the morphology and crystal strength of 3D hierarchical TiO2 nanostructure were also investigated. In addition, the prepared 3D sea-urchin like hierarchical TiO2 showed highest photocatalytic activity compared with other 3D hierarchical TiO2 nanostructures in this study and Degussa P25 for the degradation of Rhodamine B solution under UV light irradiation, which could be attributed to its special hierarchical superstructure, the increase of surface catalytic sites and its special composition units.

Superhydrophobic surface of TiO2 hierarchical nanostructures fabricated by Ti anodization

A hierarchical structure of TiO2 with disordered nanowires on top of regular nanotubes was fabricated. With organic monolayers, it showed stronger superhydrophobic properties than regular nanotubes. The surface roughness was further enhanced by removing the dense nanowires layer, resulting in 172° water contact angle. The potential application for aquatic devices was explored by the treated Ti mesh which showed striking floating stability.

CdS quantum dot sensitized anatase TiO2 hierarchical nanostructures for photovoltaic application

We report on solar cells based on CdS QD sensitized anatase TiO2 hierarchical nanostructures prepared via a facile and controlled hydrothermal process. To adjust the size of CdS QDs, the TiO2 surfaces were first treated with thiolactic acid which provides a binding site for Cd2+ ions. CdS QDs with controllable size are grown onto the TiO2 hierarchical nanostructures. The CdS QDs/TiO2 hierarchical heterojunction nanocomposites display a red shift from the ultraviolet to the visible light region with a longer wavelength for the absorption edge. The matching of band edges between CdS and TiO2 to form a semiconductor heterojunction plays an important role in effective separation of light induced electrons and holes, providing a promising photoanode for its wide absorption spectrum, high electron injection efficiency, and fast electron transference. Under UV-vis irradiation, both CdS and TiO2 are excited. Under visible light irradiation, photons are captured by CdS QDs, with the photogenerated electron–hole pairs rapidly separated into electrons and holes at the interface between TiO2 and CdS QDs. In addition to this, more photoelectrons are generated from TiO2 by harvesting UV photons. TiO2 collects the photoelectrons from CdS and passes them through to the back contact. The holes (from both CdS and TiO2) generated in the process are transferred to the solid–liquid interface. Under one sun illumination, the solar cells based on CdS QDs/TiO2 hierarchical heterojunction nanocomposites with a smaller average size of 7.6 nm for CdS QDs show a maximum short circuit current density of 6.09 mA cm−2 and a conversion efficiency of 3.06%. The performance improvement of the solar cells based on CdS QDs/TiO2 hierarchical nanostructures can be attributed to the unique microstructure characteristics and the bandgap energy matching between the CdS QDs and TiO2.

Three-level hierarchical TiO2nanostructure based high efficiency dye-sensitized solar cells

Hierarchical structures are believed to be a viable approach to achieve higher conversion efficiency of dye-sensitized solar cells (DSSCs). In this report, a novel three-level hierarchical TiO2 nanostructure with the first level made up of nanourchins (NUs), the second level composed of nanowires (NWs), and the third level made up of nanoparticles (NPs) was synthesized using an ion-modified hydrothermal method and subsequent TiCl4 treatment. The hierarchical structure based DSSCs have achieved an overall conversion efficiency of 3.25%, which is 103.1% higher than that of their counterpart, pure NW based DSSCs. This improvement is attributed to the larger surface area and stronger light scattering effect of this new three-level hierarchical nanostructure, which reveals the significance and broad applications of hierarchical nanostructures.

Fabrication of black hierarchical TiO2nanostructures with enhanced photocatalytic activity

Here, we report the fabrication of hierarchical TiO2nanostructures with wire-in-tube architectures by hydrothermal treatment of anodic TiO2nanotubes.

Optical properties of hierarchical-nanostructured TiO2 and its time-dependent photo-degradation of gaseous acetaldehyde

The TiO2 hierarchical nanostructures (HNs) composed of rutile TiO2 nanowires on anatase TiO2 nanofibers had higher photocatalytic activities of 62% and 48% than the commercial TiO2 nanoparticles (∼21 nm diameter) in the continuous flow mode and closed-circulation mode, respectively, leading to an efficient degradation of gaseous acetaldehyde under UV-light irradiation. This behavior may be attributed to the effective TiO2 HNs with specific surface area of 85.1 m2/g and lower radiative recombination of self-trapped excitons, enabling an effective electron-hole separation.

Performance Improvement of Dye-Sensitized Solar Cells Using Room-Temperature-Synthesized Hierarchical TiO2Honeycomb Nanostructures

Highly ordered hierarchical TiO2 nanostructures involving primary honeycomb and secondary nanoparticles and nanowires are prepared by a two-step facile process. The TiO2 nanotube arrays grow first on Ti foil through anodization. After the wet-chemical reaction of the TiO2 nanotube arrays with alkaline and acid solution in turn, the hierarchical nanostructures wire-in-honeycomb and porous honeycomb are obtained by a dissolution-coagulation and dissolution-adsorption mechanism, respectively. The power conversion efficiency of the hierarchical TiO2 honeycomb nanostructures for the dye-sensitized solar cells (DSCs) shows a significant improvement, as high as 5.73%, increased by 1.42 times compared with that of TiO2 nanotube arrays. The performance improvement of DSCs based on the hierarchical nanostructures is attributed to the increase in the specific surface area.

Au or Ag nanoparticle-decorated 3D urchin-like TiO2 nanostructures: Synthesis, characterization, and enhanced photocatalytic activity

The semiconductors decorated with noble metals have attracted increasing attention due to their interesting physical and chemical properties. Here, 3D urchin-like hierarchical TiO2 nanostructures decorated with Au or Ag nanoparticles were prepared by wet-chemical process. The morphology and structure were characterized by different techniques. It shows that Au or Ag nanoparticles with narrow distribution are uniformly loaded on urchin-like TiO2 nanostructures, and the resulted composite nanostructures show distinct surface plasmon absorption band and quenched photoluminence compared to pure TiO2 nanostructures. Photocatalytic tests show both Au-decorated TiO2 and Ag-decorated TiO2 exhibit enhanced photocatalytic activity for photodegradation of methyl blue in water.

One-dimensional isomeric and hierarchical TiO2nanostructures: novel air stable semiconducting building blocks

One-dimensional (1D) semiconducting nanostructures, as both interconnections and functional units in fabricating electronic, biological/chemical sensors, optoelectronic, electrochemical, and electromechanical devices, attract immense interest. However, all those 1D semiconducting nanostructures are sensitive to both volatile organic compounds (VOFs) and relative humidity (RH), causing the instability of those devices operated in air (except in biological/chemical sensors). Herein, we demonstrated an effective route to not only stabilize 1D semiconducting nanostructures in air but also maintain their electrical properties by constructing 1D isomeric semiconducting nanorods on 1D semiconducting nanostructures to form 1D isomeric and hierarchical semiconducting nanostructures. 1D isomeric and hierarchical TiO2 nanostructures (IHTNs) were chosen as a model, both excellent air stability and good electrical properties can be achieved. With such IHTNs as building blocks, a stable field-effect transistor has been realized.

Improved performance of flexible dye-sensitized solar cells based on hierarchical TiO2 nanostructures with high surface area

One-dimensional TiO2 nanostructures display remarkable performance in dye-sensitized solar cells by reducing the charge recombination; however, their relatively low surface area restricts both the dye absorption amount and solar energy conversion efficiency. Hydrothermal treatment of anodic TiO2 nanotube arrays in 0.01 M (NH4)2TiF6 increased the surface area of TiO2 photoanodes by generating hierarchical architecture. Hierarchical TiO2 nanostructures obtained by optimal hydrothermal treatment conditions exhibited a wire-in-tube architecture decorated by nanoparticles on both the inner nanowire and the outer nanotube, which further increased the surface area. Flexible dye-sensitized solar cells based on these hierarchical nanostructures yield significantly improved solar energy conversion efficiencies due to enlarged surface area, increased dye loading, and superior light scattering capacity.

Synthesis of hierarchical TiO2 nanowires with densely-packed and omnidirectional branches

In this study, a hierarchical TiO2 nanostructure with densely-packed and omnidirectional branches grown by a hydrothermal method is introduced. This morphology is achieved via high-concentration TiCl4 treatment of upright backbone nanowires (NWs) followed by hydrothermal growth. Secondary nanobranches grow in all directions from densely distributed, needle-like seeds on the jagged round surface of the backbone NWs. In addition, hierarchical, flower-like branches grow on the top surface of each NW, greatly increasing the surface area. For dye-sensitized solar cell (DSSC) applications, the TiO2 nanostructure demonstrated a photoconversion efficiency of up to 6.2%. A parametric study of the DSSC efficiency showed that branched TiO2 DSSCs can achieve nearly four times the efficiency of non-branched TiO2 nanowire DSSCs, and up to 170% the efficiency of previously-reported sparsely-branched TiO2 NW DSSCs.

Microwave-assisted deposition of metal sulfide/oxide nanocrystals onto a 3D hierarchical flower-like TiO2 nanostructure with improved photocatalytic activity

An effective and general microwave-assisted strategy has been developed for deposition of monodisperse metal sulfide/oxide nanocrystals on the surface of a type of 3D hierarchical flower-like nanostructure to form TiO2-based metal sulfide/oxide hetero-nanostructures. We choose the as-prepared TiO2–CdS hetero-nanostructure as an example, which exhibits obviously enhanced photocatalytic activity for the photodegradation of methyl orange (MO) compared with TiO2 nanoflowers, P25 and pure CdS under visible-light irradiation.

Anatase TiO2 nanorod-decoration for highly efficient photoenergy conversion

In recent studies of inorganic materials for energy applications, surface modification processes have been shown to be among the most effective methods to enhance the performance of devices. Here, we demonstrate a facile nano-decoration method which is generally applicable to anatase TiO2 nanostructures, as well as a nano-decorated hierarchical TiO2 nanostructure which improves the energy conversion efficiency of a dye-sensitized solar cell (DSSC). Using a facile sol-gel method, 0-D, 1-D, and 2-D type anatase TiO2 nanostructures were decorated with 200 nm long anatase TiO2 nanorods to create various hierarchical nanostructures. A structural analysis reveals that the branched nanorod has a highly crystalline anatase phase with anisotropic growth in the [001] longitudinal direction. When one of the hierarchical structures, a chestnut bur-like nanostructure, was employed in a dye-sensitized solar cell as a scattering layer, offering increased dye-loading properties, preserving a sufficient level of light-scattering ability and preserving superior charge transport and recombination properties as well, the energy conversion efficiency of the cell improved by 19% (from 7.16% to 9.09%) compared to a cell with a 0-D TiO2 sphere as a scattering layer. This generally applicable anatase nanorod-decorating method offers potential applications in various energy-conversion applications, especially in DSSCs, quantum-dot solar cells, photoelectrochemical water-splitting devices, photocatalysis, and lithium ion batteries.

Morphology change and band gap narrowing of hierarchical TiO2 nanostructures induced by fluorine doping

A series of TiO2 hierarchical nanostructures were synthesized by a facile template-free hydrothermal method in the presence of sodium fluoride (NaF) using titanium tetrachloride (TiCl4) as precursor. The pompon-like and football-like TiO2 hierarchical microspheres are composed of aligned rutile and anatase nanoparticles. The TiO2 nanostructures are comprised of oriented anatase nanoparticles. It is found that the doping concentration of NaF plays an important role in controlling the morphology and structure of the hierarchical nanoarchitectures. A sequential phase transition is observed with increasing dopant concentration: rutile → rutile + anatase → anatase. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) results indicated that F ions physically absorbed on the surface of TiO2 and incorporated into TiO2 crystal lattice. For the first time, a sequential red shift of absorption band edge of F-doped TiO2 is observed with increasing the concentration of NaF as compared to pure TiO2. The band gap could be tuned from 3.0 eV to 2.2 eV, which depends on the fluorine doping concentration. It is found that F-doped TiO2 nanostructures exhibit higher photocatalytic activity than that of pure TiO2 under visible light irradiation.

Construction of hierarchical titanium dioxide nanomaterials by tuning the structure of polyvinylpyrrolidone–titanium butoxide complexes from 2- to 3-dimensional

An approach to prepare TiO2 nanomaterials with favorable morphologies and hierarchical structures is reported. It is seen that polyvinylpyrrolidone (PVP) and titanium butoxide (TBT) can combine together to form PVP@TBT complexes through a water molecule, changing from a 2-dimensional (2D) linear structure to a 3-dimensional (3D) network structure in the presence of a small amount water and/or by aging. The structure of the complexes significantly affects the morphologies of the resulting electrosprayed materials. With the change of the PVP@TBT complexes from a 2D to a 3D structure, the morphologies of the electrosprayed materials transform from core–shell spheres, to bowls, to ink-bottles, to rings, and to microfibers. Moreover, the electrosprayed materials can be converted to porous TiO2 hierarchical nanostructures with tunable specific surface area, pore size, and pore volume, through facile hydrothermal treatment. Our findings provide an effective strategy to fabricate hierarchical TiO2 nanomaterials that are suitable for TiO2-based applications. It is shown that the TiO2 materials with the morphologies of bowls, ink-bottles, and rings have higher photocatalytic activity due to the multi-scattering of light.

The effect of fabrication method of hierarchical 3D TiO2 nanorod spheres on photocatalytic pollutants degradation

The effect of fabrication method on the properties of hierarchical 3D TiO2 nanostructures was investigated by employing and developing both hydrothermal method and calcination method to synthesize hierarchical 3D TiO2 nanorod spheres in this study. A comprehensive comparison in terms of morphologies, crystallization, specific surface areas, light absorption capabilities, and photoluminescence spectrum was conducted between 3D TiO2 nanorod spheres synthesized via hydrothermal method and that synthesized via calcination method. A better photocatalytic activity was demonstrated over the TiO2 nanorod spheres synthesized through calcination method. This was ascribed to the better crystallization and monodispersion of the hierarchical 3D TiO2 nanorod spheres resulted from the calcination method; thus rendering it with more superior characteristics such as larger specific surface area, enhanced light absorption capability and faster transfer of electrons which suppress the recombination of photogenerated electrons and holes. This study is thus significant not only in promoting the development of hierarchical 3D TiO2 nanorod spheres via different methods, but also in revealing the effect of fabrication method on the photocatalytic activity of hierarchical 3D TiO2 nanorod spheres. The calcination method is proposed to be a facile and promising method for scale-up production of the hierarchical 3D TiO2 nanorod spheres with high photocatalytic activity for efficient pollutants degradation.

Hierarchically Assembled One-Dimensional TiO2 Nanostructures: Echinoid- and Labyrinth-Shaped TiO2 Crystals for Dye Sensitized Solar Cell

TiO2 nanostructures have been studied intensively for decades for their photocatalytic properties. Recently, several interesting TiO2 nanostructures with controlled surface facets or shapes were reported. However, systemic approaches to obtain designed nanostructures are still rare. Here, various hierarchical 1D TiO2 nanostructures, including TiO2 nanorods, echinoid-shaped TiO2, and labyrinth-structured TiO2, were fabricated by a one-pot hydrothermal process. Concentrated HCl was added to a solution having a Ti4+/H+ ratio that ranged from 1/4 to 1/8. The highly concentrated acid stabilized hydrolysis and hindered condensation, thereby balancing nucleation and growth of TiO2 nanostructures in the hydrothermal treatment step. Dye-sensitized solar cells prepared using the hierarchical 1D TiO2 nanostructures have shown higher photon to current conversion efficiency in the wavelength over 600 nm compared to those prepared with TiO2 nanoparticles.

Formation of hierarchical TiO2 nanoporous structure from free-standing TiO2 nanotubes layers

Hierarchical TiO2 nanoporous structures with tailored porosity have been fabricated by simply annealing of free-standing TiO2 nanotubes (TNTs) layers in the presence of in situ trapped F-species. The hierarchical nanostructures consist of concatenated anatase TiO2 nanoparticles (NPs). TiO2 with different hierarchical nanostructure are obtained by changing the annealing rate of the free-standing TNTs layers. The hierarchical structure with nanopores formation in the walls of TNTs is achieved when the annealing rate is 3 °C/min. Increase in the annealing rate of the free-standing TNTs layers leads to the formation of hierarchical nanostructure of anatase TiO2 NPs morphologically resembling beads threaded into linear strings. The TiO2 hierarchical nanostructure undergoes two evolution steps, which include formation of nanopores in the walls of nanotubes and finally to concatenated anatase TiO2 NPs when the annealing rate is increased. Results show that hierarchical nanoporous structure of anatase TiO2 consist of nanopores whose diameter varies from 20 to 100 nm. EDX analysis confirms the presence of C and F-species in the as-prepared free-standing TNTs layer in significant amount. It is found that the hierarchical TiO2 nanostructure formation mainly depends upon the annealing rate and presence of F-species. GAXRD, Raman analysis and SAED pattern exhibits enhancement in the crystallinity of the hierarchically TiO2 nanostructure. This study provides an easy method for the synthesis of the hierarchical anatase TiO2 nanostructure on a large scale.

Non-hydrolytic synthesis of hierarchical TiO2 nanostructures using natural cellulosic materials as both oxygen donors and templates

Non-hydrolytic reactions between TiCl4 and cellulosic natural materials acting as both oxygen-donors and templates lead to hierarchical TiO2 nanostructures by a mineralization process.