This paper reviews the state of the art of anodized titanium dioxide nanotubes (TiO2 NTs), with an emphasis on the growth mechanism leading to their formation and the effect of heat treatment on their structure and properties. The discussion is primarily focused on TiO2 NTs grown in fluoride containing electrolytes, although the mechanism of formation of NTs in fluoride free solutions via Rapid Breakdown Anodization (RBA) is briefly covered. After an initial overview of progress made on the synthesis of anodized TiO2 NTs the review provides an analysis of the factors affecting the anodizing process (fluoride concentration, electrolyte type, applied potential and anodizing time). Details of the current-time transient, the chemistry of the process and the chemical composition of the anodic films are described which provide key information to unveil the nanotube growth mechanism. The main debate is whether NTs growth in fluoride containing solutions occurs via field-assisted plastic flow (i.e. a constant upward displacement of the oxide to form the NTs) combined with field-assisted ejection of the Ti4+ ions (i.e. ions are ejected into the electrolyte without oxide formation) or via field-assisted dissolution (i.e. preferential dissolution at the pore base where the field is stronger) or whether both processes play a role. Whenever anodization takes place in organic solutions the experimental evidence supports the plastic flow model, whereas in aqueous media field-assisted (and chemical) dissolution occur. The mechanism of rib formation on the walls of the NTs is also reviewed, and it clearly emerges that the applied potential and water content in the electrolyte are key factors in determining whether the NTs are ribbed or smooth. There also appears to be a relationship between the presence of ribs and the evolution of oxygen bubbles at the anode. The impact of thermal treatment on the properties of the NTs is also described. A variety of crystalline structures are present in the NTs (i.e. anatase or rutile), depending on the heat treatment temperature and atmosphere and the resulting electrical properties can be varied from dielectric to semi-metallic. A heat treatment temperature limit ranging from 500 to 800 °C exists, depending on preparation history, above which sintering of nanoscale titania particles occurs leading to collapse of the NTs structure. Future work should aim at using annealing not just to influence the resulting crystalline phase, but also for generating defects to be exploited in specific applications (i.e. photocatalysis, water splitting and photovoltaics).

/pdf/a-review-of-growth-mechanism-structure-and-crystallinity-of-2ov7whusr1.pdf

A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotubes

We report a facile method to synthesize Fe3O4@polydopamine (PDA)-Ag core–shell microspheres. Ag nanoparticles (NPs) are deposited on PDA surfaces via in situ reduction by mussel-inspired PDA layers. High catalytic activity and fast adsorption of a model dye methylene blue (MB) at different pH values are achieved mainly due to the presence of monodisperse Ag NPs and electrostatic interactions between PDA and MB. The as-prepared Fe3O4@PDA-Ag microspheres also show high cyclic stability (>27 cycles), good acid stability, and fast regeneration ability, which can be achieved efficiently within several minutes by using NaBH4 as the desorption agent, showing great potentials in a wide range of applications.

Highly Regenerable Mussel-Inspired Fe3O4@Polydopamine-Ag Core–Shell Microspheres as Catalyst and Adsorbent for Methylene Blue Removal

Surface zwitterionization of graphene oxide (GO) was firstly conducted by grafting poly(sulfobetaine methacrylate) (PSBMA) onto the GO surface via reverse atom transfer radical polymerization (RATRP). Then, a novel type of GO-PSBMA/polyethersulfone (PES) loose nanofiltration membrane (NFM) was constructed by mixing with modified GO composites via phase inversion. FTIR, XRD, TEM, XPS and TGA were applied to analyze the chemical composition and morphology, confirming a favorable synthesis of GO-PSBMA composites. Besides, the effect of the embedded GO-PSBMA nanoplates on the morphology and overall performance of the hybrid membranes was systematically investigated based on the SEM images, water contact angle, zeta potential, and fouling parameters. It was found that the water flux of the hybrid membrane was greatly enhanced from 6.44 L m−2 h−1 bar−1 to 11.98 L m−2 h−1 bar−1 when the GO-PSBMA content increased from 0 to 0.22 wt%. The antifouling tests revealed that the GO-PSBMA embedded membranes had an excellent antifouling performance: a high flux recovery ratio (ca. 94.4%) and a low total flux decline ratio (ca. 0.18). Additionally, the hybrid membranes exhibited a distinct advance in the mechanical strength due to the addition of highly rigid GO. Notably, compared with unmodified membranes, the hybrid membranes had a higher retention of Reactive Black 5 (99.2%) and Reactive Red 49 (97.2%), and a lower rejection of bivalent salts (10% for Na2SO4) at an operational pressure of 0.4 MPa, rendering the membranes promising for dye/salt fractionation.

Surface zwitterionic functionalized graphene oxide for a novel loose nanofiltration membrane

Hollow nano-objects have raised interest in applications such as sensing, encapsulation, and drug-release. Here we report on a new class of porous materials, namely inorganic nanotube aerogels that, unlike other aerogels, have a framework consisting of inorganic hollow nanotubes. First we show a preparation method for titanium dioxide, zinc oxide, and aluminum oxide nanotube aerogels based on atomic layer deposition (ALD) on biological nanofibrillar aerogel templates, that is, nanofibrillated cellulose (NFC), also called microfibrillated cellulose (MFC) or nanocellulose. The aerogel templates are prepared from nanocellulose hydrogels either by freeze-drying in liquid nitrogen or liquid propane or by supercritical drying, and they consist of a highly porous percolating network of cellulose nanofibrils. They can be prepared as films on substrates or as freestanding objects. We show that, in contrast to freeze-drying, supercritical drying produces nanocellulose aerogels without major interfibrillar aggregation even in thick films. Uniform oxide layers are readily deposited by ALD onto the fibrils leading to organic-inorganic core-shell nanofibers. We further demonstrate that calcination at 450 °C removes the organic core leading to purely inorganic self-supporting aerogels consisting of hollow nanotubular networks. They can also be dispersed by grinding, for example, in ethanol to create a slurry of inorganic hollow nanotubes, which in turn can be deposited to form a porous film. Finally we demonstrate the use of a titanium dioxide nanotube network as a resistive humidity sensor with a fast response.

Inorganic Hollow Nanotube Aerogels by Atomic Layer Deposition onto Native Nanocellulose Templates

Effects of TiO2 nanotubes with different diameters on gene expression and osseointegration of implants in minipigs.

We report the design and synthesis of two hematite (α-Fe2O3)-based nanomaterials based on effective hydrothermal conversion of chemically metastable K1.33Mn8O16 nanowires (KWs) in Fe(NO3)3 aqueous solution. Insights are gained into the functions of sodium dodecyl benzene sulfonate (SDBS) and the mechanisms for generating large quantities of α-Fe2O3 hollow structures (FHSs) and K1.33Mn8O16@α-Fe2O3 heterostructured nanowires (KFHWs) in solution phase. The controllable growth dynamics allows convenient control over the morphology and production of the hematite-based nanostructures. Adsorption experiments indicate that the resulting hematite-based materials are powerful nanosorbents for swift removal of Congo red from wastewater at room temperature. The adsorption kinetics and adsorption isotherm are also investigated and the findings indicate that the as-prepared KFHW and FHS hold great potential as environmentally friendly filter materials for water purification and organic waste removal.

Designed synthesis of hematite-based nanosorbents for dye removal

In-situ preparation of multi-layer TiO2 nanotube array thin films by anodic oxidation method

Highly ordered TiO2 nanotube arrays were fabricated on a conducting glass substrate in NH4HF2/glycol electrolyte via anodization of titanium film which was deposited by direct current magnetron sputtering (DCMS) at different temperatures. The results showed that Ti films with good homogeneity and high denseness could be formed under the conditions Ar pressure 0.35 Pa, direct current 3.5 A, and 2 h at 300 °C. Characterization of the TiO2 nanotube arrays was investigated comparatively, by altering anodization time. The surface morphology of the samples changed as the anodization time was prolonged from 10 to 150 min at 30 V. The TiO2 thin film was amorphous and could be transformed into anatase by annealing at 450 °C. On the basis of UV–visible transmission spectra the bandgap of the thin film was calculated to be 3.12 eV, and its tail extended to 2.6 eV.

Characterization of TiO2 nanotube arrays prepared via anodization of titanium films deposited by DC magnetron sputtering

Based on anodic aluminum oxide (AAO) templates prepared in different acidic solutions, highly ordered aligned titania nanotubes array films have been successfully prepared by the liquid phase deposition method. The effect of AAO template type on the microstructure of titania film have been studied. Using the template with a certain volume fraction of Al2O3 (less than 0.71), ordered aligned titania nanotubes were obtained, characterized with an outer diameter of 200 nm and an inner diameter of 100 nm, respectively. However, titania existed as ordered aligned nanorods with the diameter of 100 nm when the template with large volume fraction of Al2O3 (larger than 0.71) was used. TiO2 thin films calcined at 400°C for 4 h have an anatase phase and exhibit good photocatalytic activity, i.e., 75% methylene blue could be degraded under ultraviolet irradiation for 2 h.

In situ synthesis and characterization of TiO2 nanoarray films

TiO2/SiO2 composite nanotube photocatalysts supported on Al substrates were prepared by the anodic aluminum oxide (AAO) liquid-phase deposition method for degradation of 100 mg/L methyl blue (MB). AAO templates were prepared in H3PO4 acidic electrolytes through the two-step anodic oxidation method. The photocatalysts were characterized by field-emission scanning electron microscopy, thermal analysis (TG-DTA), and Fourier-transform infrared, x-ray photoelectron, and UV–Vis diffuse spectroscopies. The results showed that the presence of SiO2 in TiO2 inhibited the crystal phase transformation temperature effectively, with a greater wall thickness of 80–100 nm. It was found that Ti–O–Si bonds were formed on the surface and the surface hydroxyl concentration was increased, resulting in enhanced photocatalytic activity, being about 20% higher than for pure TiO2 films. The calculated band structure for TiO2/SiO2 was investigated using first-principle calculations. The TiO2/SiO2 composite exhibited a wider conduction band which would effectively prohibit recombination of photogenerated electrons and holes.

Kelin Huang

Papers

Designed synthesis of hematite-based nanosorbents for dye removal

In-situ preparation of multi-layer TiO2 nanotube array thin films by anodic oxidation method

Characterization of TiO2 nanotube arrays prepared via anodization of titanium films deposited by DC magnetron sputtering

In situ synthesis and characterization of TiO2 nanoarray films

Fabrication and photocatalytic activity of TiO2/SiO2 composite nanotubes