Porous TiO2 Nanotubes Research Articles

Forming efficiency of porous anodic oxide and formation mechanism of nanopores

Porous anodic alumina (PAA) and porous anodic TiO2 nanotubes have received considerable attention because of their applications in a number of fields. The formation mechanisms of nanopores and nanotubes in these porous anodic oxides, however, have remained unclear until now. The interactions between porous structural features and current-time transients in anodizing process cannot be successfully explained. Based on the mechanism of dielectric breakdown of the compact anodic alumina (CAA), the differences and internal relations in their forming processes between CAA and PAA are contrasted in detail. From this innovative standpoint, according to the divergence of PAA and CAA in their current-time curves (or voltage-time curves), two essential causes which induce the decrease of the forming efficiency of oxide in the anodizing process, that is, the generation of the electronic current and the oxygen evolution, are presented in the paper. The evidences of the round hollows within the CAA films, show that the regularly embryo pores result from the oxygen bubbles. According to the aluminum anodizing in the mixed-electrolyte, the results show that once oxygen evolution stopping, the pore growth must be stopped, and the pores must be sealed by the above compact oxide. A novel composite film of the anodic oxide is presented. All of the above conclusively show that in the forming process of PAA, an appropriate magnitude of electronic current ensures the oxygen evolution and the pores formation, an appropriate magnitude of ionic current ensures the oxide formation and growth of pore walls.

Influence of Reducer on the Nanopore Structure of Porous Anodic Alumina

In recent years, attention has been focused on adjustment and control of the nanostructures of porous anodic alumina (PAA) and porous anodic TiO2 nanotubes (PATNT). Because the formation mechanism of PAA and PATNT is still unclear, it is difficult to adjust the nanostructures of PAA and PATNT. To validate the novel viewpoint of the nanopore resulting from an oxygen bubble mold, an innovative chemical approach was used to adjust the PAA nanostructures. One successful approach is to use a reducer to absorb the oxygen bubbles in the nanopores. A novel anodic alumina film was obtained in a mixed solution of the reducer and oxalic acid. The influence of the reducer on the PAA nanostructures which formed in H3PO4 solution was investigated in detail. The experimental results showed that the regularity and the diameters of the nanopores in the PAA decreased as the reducer content increased. The differences in the voltage-time curves between electrolytes with and without the reducer were analyzed quantitatively. The results showed that the conductivity of the anodic oxide film that formed in the electrolyte with the reducer was better than that in the electrolyte without the reducer. When aluminum anodizes in a sealed case, oxygen bubbles are easily absorbed by the reducer, the oxygen bubble mold effect disappears, and a compact alumina film is obtained. Overall, these results clearly demonstrate that nanopores result from the oxygen bubble mold effect.

Preparation of highly porous TiO2nanotubes and their catalytic applications

A simple procedure for preparing highly porous TiO2 nanotubes is reported. The nanotubes were prepared in the form of a nonwoven mat by emulsion electrospinning a solution containing poly(vinyl pyrrolidone), titanium tetraisopropoxide and oil, followed by calcination in air at 500 °C. The mixed crystalline material comprised anatase and rutile TiO2 particles, whose diameters were about 11 nm and 21 nm, respectively. The highly porous TiO2 nanotube membranes, which had large specific surface areas and excellent ratios of anatase phase to rutile phase, were shown to have excellent catalytic activities. Also the mixture of crystal forms improved the efficiency of photocatalysis because at the mixed interface electrons and holes are separated effectively. The new method for producing highly porous TiO2 nanotubes is versatile and could be extended to the fabrication of various types of highly porous nanotubes.

Photoinduced charge separation enhanced by the confinement of electron donor and acceptor at different surfaces of porous TiO2 nanotubes and its application in olefin oxidation

A novel photocatalyst, TiO2 nanotubes with the confinement of the electron donor and acceptor at the inner and outer surface, respectively, was fabricated by a universal method. The as-designed nanostructure afforded a void space separation state of electron donors and acceptors, which remarkably enhanced the photoinduced charge separation, resulting in the enhancement of the catalytic activity for the photooxidation of olefins with O2.

Visible-Light Photodegradation of Dye on Co-Doped Titania Nanotubes Prepared by Hydrothermal Synthesis

Highly porous Co-doped TiO2nanotubes synthesized from a hydrothermal treatment were used to photodecompose methylene blue (MB) in liquid phase under visible light irradiation. The anatase-type titania nanotubes were found to have high specific surface areas of about 289–379 m2/g. These tubes were shown to be hollow scrolls with outer diameter of about 10–15 nm and length of several micrometers. UV absorption confirmed that Co doping makes the light absorption of nanotubes shift to visible light region. With increasing the dopant concentration, the optical band gap of nanotubes became narrower, ranging from 2.4 eV to 1.8 eV, determined by Kubelka-Munk plot. The Co-doped nanotubes exhibit not only liquid-phase adsorption ability, but also visible-light-derived photodegradation of MB in aqueous solution. The synergetic effect involves two key factors in affecting the photocatalytic activity of Co-doped titania nanotubes under fluorescent lamp, that is, high porosity and optical band gap. The merit of the present work is to provide an efficient route for preparing Co-doped TiO2nanotubes and to clarifying their adsorption and photocatalytic activity under fluorescent lamp.

Fabrication of 3D interconnected porousTiO2 nanotubes templatedby poly(vinyl chloride-g-4-vinyl pyridine) for dye-sensitized solar cells

Porous TiO2 nanotube arrays with three-dimensional (3D) interconnectivity wereprepared using a sol–gel process assisted by poly(vinyl chloride-graft-4-vinylpyridine), PVC-g-P4VP graft copolymer and a ZnO nanorod template. A7 µm long ZnO nanorod array was grown from the fluorine-doped tinoxide (FTO) glass via a liquid phase deposition method. TheTiO2 sol–gel solutiontemplated by the PVC-g-P4VP graft copolymer produced a random 3D interconnection betweenthe adjacent ZnO nanorods during spin coating. Upon etching of ZnO,TiO2 nanotubes consisting of 10–15 nm nanoparticles were generated, as confirmed by wide-anglex-ray scattering (WAXS), energy-filtering transmission electron microscopy (EF-TEM) andfield-emission scanning electron microscopy (FE-SEM). The ordered and interconnectednanotube architecture showed an enhanced light scattering effect and increased penetrationof polymer electrolytes in dye-sensitized solar cells (DSSC). The energy conversionefficiency reached 1.82% for liquid electrolyte, and 1.46% for low molecular weight(Mw) and 0.74%for high Mw polymer electrolytes.

Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodizationof Ti sputtered on conductive glass

Self-organized porous TiO2 nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titaniumin an NH4F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 °C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, withan average external diameter of 92 nm, were grown; this gave a high conversion rate ofoxide from titanium (1.9), with a 220 nm thick layer of titanium, which was notoxidized, located at the base of the tubes. The NTs revealed a mainly amorphousstructure, which transformed mostly to anatase upon thermal treatment in air at450 °C.The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporatedinto liquid dye solar cells (DSCs) and further tested under back-side illumination. High values ofVoc (714 mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude andlarge recombination resistance and electron lifetime. In addition, typical values of fillfactors (of the order of 0.62) were attained, in agreement with the estimated ohmicresistance of the cells in combination with low electron transfer resistance at theplatinum/electrolyte interface. The overall moderate power conversion efficiency(of the order of 0.3%) was mainly due to the low short-circuit photocurrents (Jsc = 0.68 mA cm−2),which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitudeof Jsc was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading(due to the limited thickness of anodic titania) and the high charge transfer resistance at theTiO2 /conductive substrate due to the presence of barrier layer(s) underneath the tubes.These preliminary results encourage the DSC community to explore further thegalvanostatic anodizing of titanium in order to produce highly efficient porousTiO2 NTs directly on conductive glass. Current work is focusing on achieving complete anodizingof the metal substrate and full transparency for the photoelectrode in order to increase andoptimize the resultant cell efficiencies.

Fabrication of Mesoporous Titanium Oxide Nanotubes Based on Layer-by-Layer Assembly

Titanium oxide (TiO2)/polyacrylic acid (PAA) composite nanotubes were firstly fabricated through the sol-gel process of titanium alkoxide in the inner pores of alumina template followed layer-by-layer assembly with polyacrylic acid (PAA). Mesoporous TiO2 nanotubes could be obtained after the removal of PAA component by calcination and etching of the template with concentrated sodium hydroxide aqueous solution. The surface area of as-prepared porous TiO2 nanotubes was measured as twice larger than that of the conventional TiO2 nanotubes and the pore diameter in the wall of the tubes is several nanometers. Such assembled mesoporous nanotubes can serve as carriers for catalysis release and biomolecules.