Hierarchical TiO2 Microspheres Research Articles

Hierarchical N‐Doped TiO 2 Microspheres with Exposed (001) Facets for Enhanced Visible Light Catalysis

Hierarchical N-doped TiO2 microspheres with exposed (001) facets [N-TiO2-(001)] were synthesized through a simple, fluorine-free solvothermal reaction with subsequent thermal treatment. The results show that the hierarchical N-TiO2-(001) microspheres are made up of numerous TiO2 nanosheets, and that isopropylamine (IPAN) acts as both the nitrogen source and the capping and shape-controlling agent that generates the high-energy (001) facets. In addition, IPAN is also effective in increasing the onset temperature of the phase transformation of TiO2 from anatase to rutile. Compared with the commercially available P25 TiO2, the as-prepared TiO2 microspheres exhibit good photocatalytic activity and high stability under visible-light irradiation. The high photocatalytic performance is derived from the synergy effect of N-doping and the separation of photogenerated electrons and holes among different facets, as evidenced by surface photovoltage spectroscopy.

Coexistence of an anatase/TiO2(B) heterojunction and an exposed (001) facet in TiO2 nanoribbon photocatalysts synthesized via a fluorine-free route and topotactic transformation

In this work, we report a novel approach to fabricate hierarchical TiO2 microspheres (HTMS) assembled by ultrathin nanoribbons where an anatase/TiO2(B) heterojunction and high energy facet coexist. The as-adopted approach involves (1) nonaqueous solvothermal treatment of a mixture of tetrabutyl titanate and acetic acid and (2) topotactical transformation into HTMS via thermal annealing. By this approach, the TiO2(B) phase usually synthesized from an alkaline treatment route could be initially formed. Subsequently, phase transition from TiO2(B) to anatase TiO2 occurs upon thermal treatment. It is demonstrated that such phase transition is accompanied by crystallographic orientation along the c-axis of anatase and TiO2(B) crystals, resulting in not only a coherent interface between two phases but also oriented attachment of anatase mesocrystals along the [001] direction, and finally high-energy (001) facet exposure. Interestingly, this work provides an alternative fluorine-free route for the synthesis of TiO2 crystals with high-energy (001) facet exposure. The structural analysis reveals that lattice-match induced topotactic transformation from TiO2(B) to anatase is the sole reason for the (001) facet exposure of anatase TiO2. The photocatalytic test for acetaldehyde decomposition shows that HTMS with anatase/TiO2(B) heterojunction and high-energy (001) facet exhibits superior photocatalytic efficiency compared with the relevant commercial product P25, which can be ascribed to the synergistic effect of large surface area, anatase/TiO2(B) heterojunction as well as high-energy facet exposure.

Hierarchically porous anatase TiO2microspheres composed of tiny octahedra with enhanced electrochemical properties in lithium-ion batteries

Hierarchical porous anatase TiO2microspheres composed of ultrathin rod-like structures in the radial direction were fabricatedviaa growth model of oriented attachment. They exhibited high rate performance and excellent cycling stability.

Hydrothermal Synthesis of Flower-like Hierarchical TiO$lt;inf$gt;2$lt;/inf$gt; Microspheres from Titanium Sulfate and Hexafluorosilicic Acid

Flower-like hierarchical TiO2 microspheres assembled by nanosheets with exposed {001} facets were pre- pared through a cost-effective hydrothermal synthesis method, using cheap Ti(SO4)2 as the feedstock and hexafluoro- silicic acid as the capping agent. These hierarchical microspheres were assembled by numerous and cross-linked nanosheets. The fine structure of hierarchical microspheres could be easily controlled by adjusting the concentration of hexafluorosilicic ions. The growth process of the microspheres was examined through XRD and SEM characteriza- tions, and a possible formation mechanism was proposed. Compared with other solvothermal synthesis methods, this cost-effective hydrothermal synthesis route is attractive and can be potentially applied in the production of flower-like TiO2 hierarchical microspheres in large scale. The as-prepared flower-like TiO2 hierarchical microspheres exhibit good photocatalytic ability to degrade methyl orange dye.

A one-pot solvothermal synthesis of hierarchical microspheres with radially assembled single-crystalline TiO2-nanorods for high performance dye-sensitized solar cells

A simple solvothermal strategy is presented to prepare morphology-tunable hierarchical TiO2 microspheres, and the microsphere-based photoanode exhibited a power conversion efficiency of 7.95%.

Template-free synthesis of hierarchical porous anatase TiO2 microspheres with carbon coating and their electrochemical properties

Hierarchically porous anatase titanium oxide (TiO2) microspheres were synthesized using a green supercritical methanol route over a very short reaction time of 15min without using templates or surfactants. Primary nano-sized particles with diameters of 20–55nm with organic coverage on the surface were loosely aggregated and formed secondary micron-sized particles 1.0–2.5μm in diameter, creating a porous structure with average pores 9–15nm in diameter. When the as-synthesized microspheres were calcined under a Ar/5% H2 condition, carbonization of the organic groups formed an ultrathin and uniform carbon layer on the nano-sized primary particles with a thickness of 0.5–1nm and reduced some of the Ti4+ to Ti3+. Both the hierarchically porous structure and the conductive layer coating had positive effects by increasing Li ion storage capacity. The prepared TiO2 microspheres exhibited a high reversible discharge capacity of 212.3mAhg−1 at 0.1C, a high-rate performance of 77.9mAhg−1 at 8C, and an excellent capacity retention of >97% at the end of 100 cycles at 1.0C, whereas TiO2 nanoparticles without porous structure and surface modification exhibited lower discharge capacities of 161.8mAhg−1 at 0.1C and 5.2mAhg−1 at 8C, and poorer capacity retention of 26%. The considerable improvement in the electrochemical performance was attributed to the nano-sized TiO2 primary particles, porous structure, and carbon coating and Ti3+ incorporation.

A novel 3D structure composed of strings of hierarchical TiO2 spheres formed on TiO2 nanobelts with high photocatalytic properties

A novel hierarchical titanium dioxide (TiO2) composite nanostructure with strings of anatase TiO2 hierarchical micro-spheres and rutile nanobelts framework (TiO2 HSN) is successfully synthesized via a one-step hydrothermal method. Particularly, the strings of hierarchical spheres are assembled by very thin TiO2 nanosheets, which are composed of highly crystallized anatase nanocrystals. Meanwhile, the HSN has a large surface area of 191m2/g, which is about 3 times larger than Degussa P25. More importantly, the photocatalytic activity of HSN and P25 were evaluated by the photocatalytic oxidation decomposition of methyl orange (MO) under UV light illumination, and the TiO2 HSN shows enhanced photocatalytic activity compared with Degussa P25, as result of its continuous hierarchical structures, special conductive channel and large specific surface area. With these features, the hierarchical TiO2 may have more potential applications in the fields of dye-sensitized solar cells and lithium ion batteries.

Tripartite Layered Photoanode from Hierarchical Anatase TiO2 Urchin-Like Spheres and P25: A Candidate for Enhanced Efficiency Dye Sensitized Solar Cells

A trilaminar layer photoanode for dye-sensitized solar cells (DSSCs) was constructed using urchin-like TiO2 hierarchical microspheres and P25. The top layer made of hierarchical microspheres enhanced light scattering; the middle layer consisting of P25 and as-prepared microspheres is a multifunctional layer for DSSCs, high adsorption ability to the dye, light scattering ability, and slow recombination rates coexistence; the bottom layer used P25. The DSSCs based on the photoanode with tripartite-layers structure exhibited a much higher short-circuit photocurrent density of 18.97 mA cm–2 and energy conversion efficiency of 8.80%, which indicated a 36% increase in the conversion efficiency compared to those of the P25 electrode (14.51 mA cm–2, 6.50%). The great improvements of photocurrent density and energy conversion efficiency for hierarchical TiO2 microspheres were mainly attributed to a considerable surface area, a higher light scattering ability, and slower electron recombination rates for the former.

Controllable synthesis of uniformly distributed hollow rutile TiO2 hierarchical microspheres and their improved photocatalysis

Abstract We reported the controllable synthesis of uniformly distributed hierarchical hollow microspheres composed of rutile TiO2 nanorods as building blocks, prepared by a hydrothermal method without employing any templates/substrates or surfactants. The homogenous hollow microspheres were obtained by optimizing the experimental conditions including hydrothermal temperatures and tetrabutyl titanate (TBOT) concentrations. A detailed formation mechanism was also proposed. The samples were analyzed by Brunauer–Emmett–Teller (BET) specific surface area analysis and ultraviolet–visible (UV–Vis) diffuse reflectance spectra (DRS). The photocatalytic results for methylene orange (MO) degradation showed that the hollow hierarchical microspheres exhibited the best photocatalytic activity among the as-synthesized products. Further optimum photocatalytic conditions were determined to study the degradation rate, decolorization and TOC (total organic carbon) removal efficiencies, and reaction kinetics in detail. Under optimum conditions, the contrastive photocatalytic experiments indicated that the photocatalytic activity was enhanced markedly when assembling the single-crystal rutile TiO2 nanorods into hollow hierarchical microstructures.

Facile synthesis of TiO2 hierarchical microspheres assembled by ultrathin nanosheets for dye-sensitized solar cells

TiO2 hierarchical microspheres assembled by ultrathin nanosheets were prepared via solvothermal route for dye-sensitized solar cells (DSSCs). The performance of cells was investigated by diffuse and reflectance spectra, photocurrent–voltage measurement, incident-photon-to-current conversion efficiency and electrochemical impedance spectra. Photoanodes with different thickness of TiO2 hierarchical spheres were studied, which proves that the photoanode with thickness of 15.9μm exhibits higher performance (short-circuit current density of 12.36mAcm−2, open-circuit voltage of 0.73mV, fill factor of 61.95, and conversion efficiency of 5.56%) than that of P25-based DSSC due to the excellent particle interconnections, low electron recombination and high specific surface area (78m2g−1).

Porous TiO2(B)/anatase microspheres with hierarchical nano and microstructures for high-performance lithium-ion batteries

Novel hierarchical porous TiO2(B)/anatase microspheres have been prepared by a facile solvothermal approach and evaluated as anode materials for advanced lithium-ion battery applications. The obtained porous microspheres, with diameters ranging from 2.5 to 5.5μm, are assembled by porous TiO2 nanosheets with the lateral size of a few micrometers and thickness of ~13nm; whilst, the nanosheets are formed by aggregations of nanosized primary TiO2 crystallites (~5–7nm). The hierarchical porous structures are verified by BET test results with a typical type-IV isotherm curve, a high surface area (~186.2m2g−1) and two kinds of pores (~4 and 8.3nm). The hierarchical porous TiO2 microspheres present excellent electrochemical performance with high Li storage capacity and excellent high-rate cycling capability (a specific capacity of 117mAhg−1 at the rate of 4000mAg−1 after 4500 cycles), which might be attributed to the enhanced Li+ diffusion and electronic conductivity induced by the hierarchical microstructures, the TiO2(B)/anatase heterojunction, and the pseudocapacitance of TiO2(B). Such hierarchical porous materials might be promising for applications in advanced power type lithium-ion batteries.

Three-dimensional sea-urchin-like hierarchical TiO2 microspheres synthesized by a one-pot hydrothermal method and their enhanced photocatalytic activity

Novel three-dimensional sea-urchin-like hierarchical TiO2 superstructures were synthesized on a Ti plate in a mixture of H2O2 and NaOH aqueous solution by a facile one-pot hydrothermal method at a low temperature, followed by protonation and calcination. The results of series of electron microscopy characterizations suggested that the hierarchical TiO2 superstructures consisted of numerous one-dimensional nanostructures. The microspheres were approximately 2–4μm in diameter, and the one-dimensional TiO2 nanostructures were up to 600–700nm long. A two-stage reaction mechanism, i.e., initial growth and then assembly, was proposed for the formation of these architectures. The three-dimensional sea-urchin-like hierarchical TiO2 microstructures showed excellent photocatalytic activity for the degradation of Rhodamine B aqueous solution under sunlight irradiation, which was attributed to the special three-dimensional hierarchical superstructure, and increased number of surface active sites. This novel superstructure has promising use in practical aqueous purification.

ChemInform Abstract: Hierarchical TiO2 Microspheres: Synthesis, Structural Control and Their Applications in Dye‐Sensitized Solar Cells

AbstractReview: 77 refs.

Enhanced photocatalytic H2 production on hierarchical rutile TiO2 microspheres

Hierarchical rutile TiO2 microspheres composed of nanorods have been synthesized by a facile hydrothermal method. The as-synthesized rutile sample shows highly enhanced photocatalytic activity towards H2 production from water. The superior photocatalytic performance can be attributed to the improved crystalline quality, the higher percentage of reductive {110} facets exposed and increased pore sizes for the samples obtained under higher temperatures.

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.

Mesoporous TiO2 microspheres synthesized with assistance of different structural controlling reagents and their photovoltaic properties

In this paper, hierarchical mesoporous TiO2 microspheres composing of connected nanoparticles were synthesized by the aids of aniline, lauryl alcohol and nonanoic acid. The dye sensitized solar cells (DSSCs) based on the mesoporous TiO2 microspheres demonstrated photoelectric conversion efficiencies of 3.9–6.5%, which are dramatically higher than that based on commercial P25 (3.26%). Mediated by different structural controlling reagents, the as-formed mesoporous TiO2 microspheres exhibited obvious difference in photovoltaic properties. Mainly due to the higher dye adsorption capacity, higher light scattering capability, as well as better inter- and intra-sphere connection, the microspheres synthesized with the assistance of aniline offered the highest photovoltaic efficiency.

Preparation of hierarchical TiO2 microspheres for enhancing photocurrent of dye sensitized solar cells

Hierarchically structured TiO2 microspheres were prepared at a low temperature by combining a sol-gel process with a solvothermal route and characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Results indicate that the phase structure of the as-prepared TiO2 products undergoes a transformation, which changes from amorphous microspheres with a smooth surface in the sol-gel process to hierarchical anatase ones consisting of nanocrystallines after the solvothermal treatment. The hierarchical anatase TiO2 microsphere shows large surface areas and good light scattering effects as the photoelectrodes for dye sensitized solar cells (DSSCs). DSSCs based on TiO2 microspheres exhibit an improvement power conversion efficiency of 6.58% and a high short current density of 13.83 mA/cm2 as compared to the commercial P25 based DSSCs with a power conversion efficiency of 4.94% and a high short current density of 10.28 mA/cm2.

Hierarchical TiO2 microspheres: synthesis, structural control and their applications in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs), as a type of promising energy conversion device, have received much attention in the past two decades due to their high efficiency and low cost. In order to achieve higher energy conversion efficiency, different TiO2 structures have been employed to fulfill the requirements of large surface area, light scattering capability and effective electron collection. In recent years, TiO2 hierarchical microspheres (HMSs) emerged as a class of robust photoanode materials to integrate the above-mentioned qualities, taking advantage of their unique structures. This review summarises the vast array of methods for the fabrication and structural control of the TiO2 HMSs. Subsequently, it focuses on illustrating the structural advantages of the HMSs, such as high surface area, submicron-sized diameter and interconnected primary building blocks over their performances when used in DSSCs. Finally, in the perspective, we intensively discuss the reasons preventing the HMS based DSSC from making striking improvements in power conversion efficiency. Moreover, the further optimization of the HMSs and their potential applications in other solar energy conversion devices are also discussed.

Hierarchical TiO2 spherical nanostructures with tunable pore size, pore volume, and specific surface area: facile preparation and high-photocatalytic performance

Anatase TiO2 hierarchical nanostructural microspheres with tunable pore size, pore volume, and specific surface area were prepared by a facile two-step method of electrospray and hydrothermal treatment. Compared to the calcination, the hydrothermal treatment can transfer the electrosprayed TiO2 microspheres to porous hierarchical nanostructures. Adding ammonia in the hydrothermal process has a great effect on the pore structure of TiO2 microspheres. The hydrothermal-treated samples with >2.0 ml ammonia being added are composed of both big and small nanocrystals. Some of the large nanocrystals grow in the [001] direction and contain step-like {101} surfaces. The large nanoparticles are formed through the combination of small particles by dehydration, which finally leads to the change of TiO2 microspheres from mesoporous to large-porous structure. The effects of the specific surface area, the pore volume, and the pore size on the photocatalytic activity are studied. It is considered that the pores on the surface layer of TiO2 microspheres are like a door that can control the diffusion of reactants between the outside and the inside. If the size of pores on the surface is big enough to allow the fast diffusion of reactants, the photocatalytic activity will increase with the increase of specific surface area and pore volume. In addition, further calcination on the TiO2 spheres after hydrothermal treatment can increase the photocatalytic activity, which is better than the commercial P25.

Efficient dye-sensitized solar cells based on hierarchical rutile TiO2 microspheres

Dye-sensitized solar cells (DSSCs) are fabricated based on rutile TiO2 microspheres that are synthesized by a hydrothermal route. We found that, with increasing deposition time, semi microspheres get converted into microspheres. Our results show that the TiO2-based cells exhibit a noticeable improvement in the overall efficiency: maximum 3.81% versus 0.67% for the reference cell made of a rutile TiO2 semispherical nanocrystalline film. This extraordinary result is attributed to the effective light trapping and dye loading resulting in the highest efficiency 3.81%.