This paper reviews the recent development of surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique for the growth of TiO2 one-dimensional nanostructures. SPCVD uses separated TiCl4 and H2O precursor pulses, and the anisotropic growth of TiO2 crystals is attributed to the combined effects of surface recombination and HCl restructuring at high temperature during elongated purging time. Therefore, the crystal growth is effectively decoupled from precursor vapor concentration, which allows uniform growth of TiO2 nanorods (NRs) inside highly confined spaces. The phase of TiO2 NRs can be tuned from anatase to rutile by raising the deposition temperature. Au catalysts are able to enhance the growth rate and led to bifurcated nanowire (NW) morphology. A high density three-dimensional (3D) NW architecture was created by SPCVD growing TiO2NRs inside dense Si NW forests. Such 3D structures offer both large surface area and excellent charge transport property, which substantially improved the efficiency of photoelectrochemical devices.

This article describes the multifunctional applications of TiO2. It substantiates the universality of the anodization process to grow well-ordered TiOxnanotube (T–NT) of hollow cylindrical shape on a variety of planar and nonplanar substrates. It highlights an approach to effectively bring down the cost of anodization via utilization of a small volume of electrolyte. The multifunctionality of these nanostructures is highlighted through representative examples that illustrate wide ranging optical, electronic, and catalytic properties. Combining the T–NT with other materials such as metals and photoactive additives to form composite nanostructures has been shown to benefit photocatalysis, photovoltaics, biological processes, and environment-related applications. This article also demonstrates the applicability of T–NT as an agent to produce dissolved oxygen in simulated blood—an application that can assist in the development of artificial lungs. Key results from the research group, collaborations, and recent articles are highlighted.

The influence of different TiO2precursors (laboratory-prepared sol–gel powders, dried and thermally treated at 400 °C, and commercial powders such as anatase by Aldrich and P25 Aeroxide by Evonik) on the structure and morphology of the titanate nanotubes obtained by hydrothermal method was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. In all cases, titanate-based nanotubes were obtained, having similar structure and morphology. The effect of the thermal treatment on the structural stability was also studied by differential thermal analysis and thermogravimetry analysis (TG/DTA) and differetial scanning calorimetry (DSC) measurements up to 600 °C, to determine the transformation of titanate nanotubes into other phases. Complementarily, by XRD investigation, the phases that develop after the thermal treatment of the titanate nanotubes in 110–400 °C temperature range were identified. The TEM micrographs come to confirm the deterioration of morphology and transformation from nanotubes to particles. The thermal stability of the investigated nanotubes led to the conclusion that this property depends not only on the sodium content but also on the type of precursor. The highest thermal stability was noticed for the nanotubes synthesized starting with anatase (Aldrich) commercial powder.

Use of nanostructured TiO2 for photocatalysis is a cost-effective and sustainable technology. However, to make this an attractive viable technology will require the design of TiO2 photocatalyst capable of harnessing the energy of visible light. One possible solution is the doping of TiO2 to reduce its band gap. In this paper, the effect of Cr-doping by ion implantation on the in situ crystallization and phase stability of anodic TiO2 nanotubes at elevated temperature is described. Cr-doping has dramatically reduced the fraction of anatase-to-rutile transformation and lowered the crystallization temperature of anatase from 600 to 400 °C and rutile from 600 to 500 °C. Ion beam analysis by Rutherford backscattering spectrometry has confirmed the existence of Cr ions composition gradation in doped TiO2 nanotubes. The real doping of Ti lattices with Cr ions was evidenced by the analyses of surface compositions and chemical states of the nanotubes using x-ray photoelectron spectroscopy.

The synthesis, structure, and electrical performances of titanium dioxide (TiO2 and also doped TiO2) thin films, a capacitor dielectric for dynamic random access memory (DRAM) and a resistance switching material in resistance switching RAM (ReRAM), are reviewed. The three-dimensionality of these structures and the extremely small feature sizes (<20 nm) of these memory devices require the synthesis method of TiO2-based layers to exhibit high degree of conformality. Atomic layer deposition is, therefore, the method of choice in respect of film growth for these applications. The unique arrangement of the TiO6-octahedra in the rutile structure, which results in the value for dielectric constant of the dielectric layer, εr (>100), makes the material especially attractive as the capacitor dielectric layer in DRAM. Removing some of the oxygen ions from the rutile structure and arranging the resulting oxygen vacancies on a specific crystallographic plane results in the so called Magnéli phase materials, which show distinctive conducting semiconductor or metallic characteristics. External electrical stimuli can cause the repeated formation and rupture of conducting channels that consist of these Magnéli phase materials in the insulating TiO2 matrix, and this aspect makes the material a very feasible choice for applications in ReRAM. This article reviews the material properties, fabrication process, integration issues, and prospect of TiO2 films for these applications.

Titanium dioxide (TiO2) nanomaterials, as important photocatalysis materials, have been synthesized with many approaches. In this study, we reported the synthesis of TiO2 nanomaterials by reacting titanium isopropoxide with ethylene glycol under basic condition followed by calcination at high temperatures. The structural, optical, and photocatalytic properties of the TiO2 nanomaterials were studied with x-ray diffraction, Raman spectroscopy, transmission electron microscopy, differential scanning calorimetry, Fourier-transformed infrared spectroscopy, x-ray and ultraviolet (UV) photoemission spectroscopy, UV–vis diffusive reflectance, and photocatalytic decomposition of methylene blue. We found that the titanium ethylene glycolide decomposes at 330 °C and transforms into pure anatase TiO2 around 400 °C. The anatase phase further transforms into core/shell rutile/anatase TiO2 composite at 550 °C and displays the highest photocatalytic activity among the samples prepared. The high photocatalytic activity can be attributed to the improved charge separation at the rutile/anatase n/n junction interface and the high crystallinity of the sample after calcination.

High purity anatase titanium dioxide (TiO2) and iron (Fe)-doped TiO2 nanocrystals were prepared by a continuous flow synthesis method using isopropanol-water mixtures as solvent in supercritical or near-critical conditions. The method allows complete control of size (5–20 nm) and crystallinity (10–100%) of the nanoparticles and provides quick synthesis with a residence time of ∼10 s that can be scaled up to commercial production. It is found that the average crystallite size can be easily controlled by adjusting the ratio between isopropanol and water in the solvent, whereas the crystallinity is mainly controlled by the reaction temperature. As-prepared Fe-doped TiO2 nanoparticles appear to be single phase, but Fe3+ ions most likely do not occupy the Ti4+ sites in the anatase TiO2 crystal structure.

Nanocrystalline anatase was synthesized, using both domestic and laboratory microwave ovens, from different precursors. Nanoparticulate anatase was obtained after microwave irradiation of tetra-butyl orthotitanate solution in benzyl alcohol. As-synthesized samples have orange color due to the presence of organics that were eliminated after annealing at 500 °C, whereas the size of small anatase nanocrystals (around 8 nm) was preserved. Other nanocrystalline anatase samples were obtained from hexafluorotitanate-organic salt ionic liquid-like precursors. In this case, use of a domestic microwave oven and very short processing times (1–3 min irradiation time) were involved. Good specific capacity values and capacity retention at high C rates for insertion/deinsertion of Li+were recorded when testing such nanoparticles as electrode material in lithium cells. The electrochemical performances were found be strongly dependent on the phase composition, which in turn could be tuned through the synthetic procedure.

A new low-cost synthesis of brookite TiO2nanoparticles using isopropanol as both the solvent and ligand is described here. Other ligands can be bound to the titania surface during or postsynthesis to tailor the particles’ functionality. The often extremely rapid hydrolysis of titanium isopropoxide has been successfully controlled so that nanoparticle growth is achieved. The resulting 4-nm particles are nonagglomerated, stable in solution, and have a low polydispersity. The synthesis is scalable and enables the simple fabrication of large amounts of titania nanoparticles that do not scatter visible light and are highly suited for incorporation into optical composites.

Changes in the colloid-chemical and photocatalytic properties of titania nanoparticles by attrition milling in the presence of glycine (Gly) and subsequent heat treatment were examined. By milling at 1500 rpm for 6 h, the average particle size was decreased from 123 to 85 nm, with simultaneous decrease in the specific surface area from 35.1 to 23.5 m2/g. Interfacial reactions between titania and Gly were confirmed by Fourier transform infrared spectroscopy, from the blue shift of the COO− related vibrational bands by 25 cm−1, relative to the same band from the pristine Gly. The bimodal N1s x-ray photoelectron spectroscopy peak similar to that from the reported titania—amino acid complex is another indication of the complex formation with the participation of nitrogen. When the dispersion was dried and calcined at 500 °C in air, the powder exhibited pale yellow color. Diffuse reflectance spectroscopy showed significant visible light absorption, suggesting nitrogen incorporation into titania. The fired product showed high photocatalytic antibacterial activity by irradiation of blue light centered at around 440 nm, using Escherichia coli as a specimen of bacterial species. Thus, the present Gly-modified titania nanoparticles could be used for eliminating indoor bacteria under soft blue illumination. The series of interfacial chemical processes involved are also discussed.

In this article, we review our recent research efforts on the synthesis, crystallinity control, and photocatalysis of nanostructured titanium dioxide (TiO2) shells. First, we introduce several synthetic methods for preparing TiO2 shell structures using either template-free or template-assisted approaches. Several methods to change the structures from amorphous to crystalline and subsequently ways to enhance the crystallinity are then discussed, including those involving the “silica-protected calcination” and “partial etching and recalcination” strategies. We also discuss the photocatalytic applications of the TiO2 nanoshells and the methods for improving their catalytic activities. Finally, we conclude with a summary and our perspective on the further development of the nanostructured TiO2 shells. It is believed that more rational design and modification strategies such as well-controlled nonmetal doping, plasmonic metal decoration and the hybridization with other semiconducting materials will significantly enhance the photocatalytic efficiency of TiO2-based catalyst materials.

The increased use of viscous electrolytes in dye-sensitized solar cells (DSSCs) has revitalized the interest in three-dimensionally ordered macroporous (3DOM) structures as potential electrodes. The simplest approach to a 3DOM structure, such as inverse opals, is colloidal assembly followed by precursor infiltration and calcination. However, current assembly methods are often optimized for very narrow particle size ranges due to the colloidal forces specific to the method used. Using five particle sizes, ranging from 0.5 to 10 μm, we have studied the particle-size dependence of six commonly used colloidal assembly methods and its effect on the resulting inverse opal structure using scanning electron microscopy and Fast Fourier Transforms. Our results indicate a clear correlation between particle size and colloidal assembly method. The information provided by our study will enable systematic studies on inverse opal TiO2 electrodes with various pore sizes, in which the influence of the inverse opal preparation methods used is minimized.

The hydrophobic rutile titania aerogels were successfully prepared by nonalkoxide ambient pressure drying through a modification process. The resulted materials were characterized by x-ray diffraction, scanning electronic microscope, transmission electron microscope, contact angle analyzer, Brunauer–Emmett–Teller specific surface area, and ultraviolet (UV)–visible diffuse reflection spectrum. The experimental results demonstrated that the as-prepared samples nanoparticles with rutile crystalline structure were uniformly distributed. The UV-resistant hydrophobic samples having high surface area were used as photocatalysts for dye degradation reaction.

Exceptionally dense titanium dioxide (TiO2) films were prepared via dip coating from a sol containing poly(hexafluorobutyl methacrylate) as the structure-directing agent. The films were grown on glass, F-doped SnO2, and crystalline silicon (111) faces, either pure or with a thin layer of SiO2. The TiO2 films cover perfectly even rough surfaces, which was ascribed to thixotropic properties of the precursor gel. The films provide antireflection function to crystalline Si wafers for photovoltaic applications. The optical reflectance in visible to near-infrared (NIR) wave lengths region is considerably smaller for Si wafers covered by TiO2/SiO2 film compared with that of SiO2/Si. The dense TiO2 films are amorphous with small amount of anatase and monoclinic TiO2(B). These two phases withstand calcination at 900 °C in films deposited on Si. For comparison, porous TiO2 films were grown by the same dip-coating protocol, but with alternative organic additives, either polymers or ionic liquids.

A series of TiO2 samples were prepared from the precursor P25 using impregnation method followed by a calcination. The phase compositions of the samples were controlled by addition of various additives, including NaNO3, NaHCO3, Na2SO4, Na3PO4, Na2SiO3, and Na2MoO4. X-ray diffraction patterns demonstrate that the phase transformation from anatase to rutile can be suppressed to a different extent by these additives. Photocatalytic H2 production reactions were performed in methanol aqueous solution. The catalysts, with the addition of NaNO3, NaHCO3, and Na2SO4, show a great improvement in the H2 production activity, while others show a negative effect. This indicates that both the phase structure and the properties of the additives are important for the photocatalytic activity. Infrared spectra of pyridine adsorption on the catalysts reveal that the decreased acidity of the samples may be responsible for the suppressed CO production in the photocatalytic methanol reforming.

Using titanium sulfate, Ti(SO4)2, as precursor and sodium hydroxide, NaOH, as adjusting reagent, pure brookite, pure anatase, and mixed-phase titanium dioxide (TiO2) with tunable brookite/anatase ratios were synthesized via a hydrothermal process. The samples were characterized by x-ray diffractionspectrometry, ultraviolet-visible diffuse reflectance spectrometry, transmission electron microscopy, and Brunauer-Emmett-Teller measurement. Photocatalytic degradation of Rhodamine B in aqueous solution served as a probe reaction to evaluate the photocatalytic activity of the as-prepared nanocomposites under visible irradiation (λ > 400 nm). The mixed-phase TiO2 exhibits higher photodegradation activity than single phase TiO2. The sample with 63.1% brookite and 36.9% anatase shows the highest degradation activity. Possible mechanism attributing to the enhanced activity was proposed based on the strucutre and surface property of the samples.

We report on Ni-modified TiO2 nanotubes with improved photocatalytic properties. Using the as-anodized TiO2 nanotubes as templates, Ni was electrodeposited using pulsed current waveforms. It was found that the Ni deposition was first initiated at the bottoms of the intertubular voids and then grew upward, resulting in a Ni/TiO2 coaxial nanostructure with Ni wrapping around the TiO2 nanotubes. Moreover, the tube inside was kept empty and tube openings unclogged for the fabricated Ni/TiO2 nanocomposites. Further photodegradation tests using methyl red revealed that the fabricated Ni/TiO2 nanocomposites possess higher photocatalytic efficiency than the counterparts of pristine TiO2 nanotubes. The observed improved photocatalytic efficiency is ascribed to the Schottky barriers formed between Ni and TiO2.

Photoanodes based on TiO2-polyheptazine (TiO2-PH) hybrids are, due to the energetics of photogenerated charges, very promising for solar water splitting in terms of possibly reduced need for external electric bias. Visible (λ > 420 nm) light-driven photooxidation of water at TiO2-PH electrodes loaded with two different metal oxide cocatalysts was investigated. As compared with TiO2-PH photoanodes loaded with colloidal [iridium (IV) oxide] IrO2 deposited by colloidal deposition, photoelectrodes modified with CoOx oxygen-evolving cocatalyst (Co-Pi) deposited by photoassisted deposition precipitation method showed both higher photocurrents and more efficient oxygen evolution under prolonged irradiation. The minimum external electric bias needed to observe complete photooxidation of water to dioxygen at TiO2-PH photoanodes modified with Co-Pi was estimated to be ∼0.6 V at pH 7. The key factor limiting the photoconversion efficiency at low bias potentials is the fast primary recombination of photogenerated charges.

We report results of comparative study of photocatalytic properties of polycrystalline TiO2 nanotubes and single-crystalline nanorods. It is demonstrated that single-crystalline nanorods show superior photocatalytic properties compared to polycrystalline nanotubes due to low recombination of photoexcited carriers. Grain boundaries in polycrystalline nanotubes act as a barrier of the effective carrier pathway. Visible light activity of the TiO2 nanostructures is enhanced by the sensitization of CdS nanoparticles on TiO2. Subsequent heat treatment of the CdS/TiO2 heterostructures led to the dramatically enhanced photoresponse under both white and visible light irradiation, which was attributed to the improved crystallinity of CdS nanoparticles and TiO2nanostructures.