Porous TiO2 Nanotubes Research Articles

Crater nanochannels of anodic aluminum oxide fabricated in NH4F electrolyte

In this work, porous anodic aluminum oxide with crater nanochannels is discovered for the first time while Al is anodized in NH4F electrolyte at high voltages. The unique structure of crater nanochannels is distinctly different from that of general porous anodic aluminum oxide (AAO) and anodic TiO2 nanotubes. A central channel appears and a circle of nanopores surround the channel while there is only one kind of pore in typical AAO and anodic TiO2 nanotubes. This finding may significantly facilitate the fabrication of special structures in AAO and anodic TiO2 nanotubes.

Derivation of a Mathematical Model for the Growth of Anodic TiO2 Nanotubes under Constant Current Conditions

Due to certain limitations of traditional models, the growth mechanism of porous anodic TiO2 nanotubes has not been well determined currently. Herein, for the first time, a mathematical model of voltage-time transient curves under constant current conditions is derived theoretically, based on the conception of ionic and electronic currents and Ohm's law. The simulation results show high fidelity to the experimental curves, and illustrate the linear correlation between nanotube length and ionic current. Further, based on this model, the avalanche breakdown can be explained, which shows advantage over the former derivations on compact films. And this model indicates that the discrepancy between compact and porous oxide films lies on the magnitude of electronic current during anodization. Moreover, the proportion of the ionic and electronic current is then calculated during constant current anodization. It can be concluded that the ionic current contributes to the oxide growth while the electronic current gives rise to the oxygen bubble evolution which acts as the growth mould of the oxide. The present results promote the understanding of the growth kinetics of porous anodic oxides from qualitative interpretation to quantitative analyses.

Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx Cocatalysts Produced by Atomic Layer Deposition for Photocatalytic Hydrogen Production.

Efficient separation of photogenerated electrons and holes, and associated surface reactions, is a crucial aspect of efficient semiconductor photocatalytic systems employed for photocatalytic hydrogen production. A new CoOx /TiO2 /Pt photocatalyst produced by template-assisted atomic layer deposition is reported for photocatalytic hydrogen production on Pt and CoOx dual cocatalysts. Pt nanoclusters acting as electron collectors and active sites for the reduction reaction are deposited on the inner surface of porous TiO2 nanotubes, while CoOx nanoclusters acting as hole collectors and active sites for oxidation reaction are deposited on the outer surface of porous TiO2 nanotubes. A CoOx /TiO2 /Pt photocatalyst, comprising ultra-low concentrations of noble Pt (0.046 wt %) and CoOx (0.019 wt %) deposited simultaneously with one atomic layer deposition cycle, achieves remarkably high photocatalytic efficiency (275.9 μmol h-1 ), which is nearly five times as high as that of pristine TiO2 nanotubes (56.5 μmol h-1 ). The highly dispersed Pt and CoOx nanoclusters, porous structure of TiO2 nanotubes with large specific surface area, and the synergetic effect of the spatially separated Pt and CoOx dual cocatalysts contribute to the excellent photocatalytic activity.

Integrating photosystem II into a porous TiO2 nanotube network toward highly efficient photo-bioelectrochemical cells

A photosystem II-based photoanode coupled with a porous TiO2 nanotube network is constructed for solar energy conversion into electrical power.

Porous Layers Composed of Oxide Crystallites Formed by the Combination of Laser Ablation and Anodization of Metal

High-voltage anodization is well known for producing nanostructures such as porous alumina and TiO2 nanotubes. The formation mechanisms involve an intricate balance between oxide growth and dissolution in which pore initiation and propagation is convoluted with the need for strain relief. Laser ablation with a nanosecond pulsed Nd:YAG laser (532 nm) produces texture e.g. arrays of pillars, macropores and ripples. Nanoparticles produced in the ablation plume can also be deposited on the target or a remote substrate. These substrates are anodized in solutions typically used for porous film and nanotube production (including viscous fluoride-containing electrolytes). A number of metals have been investigated, with emphasis here on Al and Ti as well as Zn. Laser pre-texturing results in the formation of structures unattainable by anodization alone. Deposited nanoparticles of Al and Zn are found to act as seeds for the growth of oxide crystallites that fuse into porous crystallite films.

Efficient water management of composite membranes operated in polymer electrolyte membrane fuel cells under low relative humidity

High performance and durable electrolyte membrane operated in polymer electrolyte membrane fuel cells (PEMFCs) under low relative humidity (RH) has been achieved by incorporating various diameter sizes of mesoporous hygroscopic TiO2 nanotubes (TNT) in a perfluorosulfonic acid (Nafion®) membrane. Porous TNTs with different tube diameters are synthesized by thermal annealing the electrospun polymer containing titanium precursor mat at 600°C under an air atmosphere. The diameter of the TNT is significantly controlled by changing the concentration of the precursor solution. Compared to a commercial membrane (Nafion, NRE-212), the Nafion-TNT-10 composite membrane operated under 100% RH at 80°C generates about 1.3 times higher current density at 0.6V, and 3.4 times higher maximum power density operated under dry conditions (18% RH at 80°C). In addition, the Nafion-TNT-10 composite membrane also exhibits stable and durable operation under dry conditions. The remarkably high performance of the Nafion-TNT-10 composite membrane is mainly attributed to the significant reduction of the ohmic resistance as well as the improvement of cathode catalyst utilization by incorporating TNTs, which greatly enhances the water retention and the water management capability through the membrane. Furthermore, Nafion–TNT membranes exhibit superior mechanical property.

Porous Layers Composed of Oxide Crystallites Formed by the Combination of Laser Ablation and Anodization of Metal

Self-limiting and self-organizing high voltage anodization is well known for making oxide nanostructures such as porous alumina and TiO2 nanotubes. The formation mechanisms of both of these materials involve an intricate balance between oxide growth and dissolution in which pore initiation and propagation is convoluted with the need for strain relief because of the expansion inherent to oxide formation. Laser ablation with a nanosecond pulsed Nd:YAG laser operating at 532 nm produces texture in the form of arrays of pillars, macropores and ripples. Nanoparticles produced in the ablation plume can also be deposited on the target or a remote substrate. These substrates are anodized in solutions typically used for porous film and nanotube production (including viscous fluoride containing electrolytes). Results for a number of metals including Al, Sn, Ti and Zn will be discussed. Laser pre-texturing results in the formation of structures unattainable by conventional anodization. Deposited nanoparticles, of e.g. Al, are found to act as seeds for the growth of oxide crystallites that fuse into porous films composed of these crystallites, as shown in the image below. Figure 1

Effective approach to strengthening TiO2 nanotube arrays by using double or triple reinforcements

Porous anodic TiO2 nanotube arrays (TNTAs) are fragile and also susceptible to be damaged during physical manipulation. Few studies have involved the improvement of the poor interfacial adhesion of TNTAs to the Ti substrate. Here, the poor adhesion of TNTAs was dramatically improved by appending an additional compact layer (ACL) formed at the interface between TNTAs and the Ti substrate. The adhesion of TNTAs with single-ACL increased with the increase of the ACL thickness. Furthermore, the reinforced TNTAs with double-ACL and triple-ACL have been successfully developed for the first time. The experimental results indicated that the critical load of the TNTAs with triple-ACL is roughly 5.8 times higher than that of the untreated TNTAs. The present results may be helpful to assemble less brittle and large area TNTAs for extensive applications.

Facile synthesis of porous metal oxide nanotubes and modified nafion composite membranes for polymer electrolyte fuel cells operated under low relative humidity.

We describe a facile route to fabricate mesoporous metal oxide (TiO2, CeO2 and ZrO1.95) nanotubes for efficient water retention and migration in a Nafion membrane operated in polymer electrolyte fuel cell under low relative humidity (RH). Porous TiO2 nanotubes (TNT), CeO2 nanotubes (CeNT), and ZrO1.95 (ZrNT) were synthesized by calcining electrospun polyacrylonitrile nanofibers embedded with metal precursors. The nanofibers were prepared using a conventional single spinneret electrospinning technique under an ambient atmosphere. Their porous tubular morphology was observed by SEM and TEM analyses. HR-TEM results revealed a porous metal oxide wall composed of small particles joined together. The mesoporous structure of the samples was analyzed using BET. The tubular morphology and outstanding water absorption ability of the TNT, CeNT, and ZrNT fillers resulted in the effective enhancement of proton conductivity of Nafion composite membranes under both fully humid and dry conditions. Compared to a commercial membrane (Nafion, NRE-212) operated under 100% RH at 80 °C, the Nafion-TNT composite membrane delivered approximately 1.29 times higher current density at 0.6 V. Compared to the Nafion-TiO2 nanoparticles membrane, the Nafion-TNT membrane also generated higher current density at 0.6 V. Additionally, compared to a NRE-212 membrane operated under 50% RH at 80 °C, the Nafion-TNT composite membrane exhibited 3.48 times higher current density at 0.6 V. Under dry conditions (18% RH at 80 °C), the Nafion-TNT, Nafion-CeNT, and Nafion-ZrNT composite membranes exhibited 3.4, 2.4, and 2.9 times higher maximum power density, respectively, than the NRE-212 membrane. The remarkably high performance of the Nafion composite membrane was mainly attributed to the reduction of ohmic resistance by the mesoporous hygroscopic metal oxide nanotubes, which can retain water and effectively enhance water diffusion through the membrane.

Fabrication of TiO2rod in tube nanostructure with enhanced photocatalytic activity: investigation of the effect of the states of the precursor on morphology

Here, we report a special rod in tube nanostructured TiO2 prepared by a moderate esterification reaction. The morphology evolution with reaction time was observed by FSEM and FETEM. Comparing to the as-reported sphere in sphere nanostructured TiO2, the formation of a rod in tube nanostructure was attributed to the use of a solid precursor. The nucleation-dissolution-anisotropic grain growth-recrystallization mechanism is responsible for the formation of the rod in tube nanostructure. Moreover, the photocatalytic properties of different morphologies of samples were also compared, indicating that the rod in tube nanostructured TiO2 possessed the best photocatalytic activity. The improvement of photocatalytic activity is mainly attributed to the enhancement of light-harvesting and the separation of electron–hole pairs. Finally, the present work also provided a template-free method to synthesize hierarchical porous TiO2 nanotube through prolonging reaction duration.

Efficient suppression of nanograss during porous anodic TiO 2 nanotubes growth

When Ti foil was anodized in fluoride-containing electrolyte for a long time, undesired etching-induced “nanograss” would inevitably generate on the top of porous anodic TiO2 nanotubes (PATNTs). The nanograss will hinder the ions transport and in turn yield depressed (photo) electrochemical performance. In order to obtain nanograss-free nanotubes, a modified three-step anodization and two-layer nanostructure of PATNTs were designed to avoid the nanograss. The first layer (L1) nanotubes were obtained by the conventional two-step anodization. After washing and drying processes, the third-step anodization was carried out with the presence of L1 nanotubes. The L1 nanotubes, serving as a sacrificed layer, was etched and transformed into nanograss, while the ultralong nanotubes (L2) were maintained underneath the L1. The bi-layer nanostructure of the nanograss/nanotubes (L1/L2) was then ultrasonically rinsed in deionized water to remove the nanograss (L1 layer). Then much longer nanotubes (L2 layer) with intact nanotube mouths could be obtained. Using this novel approach, the ultralong nanotubes without nanograss can be rationally controlled by adjusting the anodizing times of two layers.

Facile fabrication of anatase TiO2 nanotube arrays having high photocatalytic and photovoltaic performances by anodization of titanium in mixed viscous solvents

Highly ordered, vertically aligned, one side-opened, and regularly porous anatase TiO2 nanotube arrays (TNAs) have been facilely grown by anodizing Ti foil in mixed viscous solvents of ethylene glycol (EG) and glycerol. By changing the volume ratio of two solvents, we have controlled the structural properties of TNAs such as tube diameters, wall thicknesses, and tube lengths. Our prepared TNAs have been found to have enhanced (004) planes, which are reactive in catalysis reactions. We have demonstrated that TNAs grown in 2:1 (v/v) EG and glycerol have the lowest band-gap energy and the largest mean crystallite diameter. TNAs grown on Ti foil have been directly employed for photocatalytic materials and the working electrode of photovoltaic dye-sensitized solar cells. Among our prepared samples, TNAs grown in 2:1 (v/v) EG and glycerol have shown the best photocatalytic activity for the degradation of methylene blue and the highest photovoltaic conversion efficiency of 4.08 %.

Controlled fabrication of porous double-walled TiO2nanotubes via ultraviolet-assisted anodization

Double-walled TiO2 nanotubes with porous wall morphologies are fabricated by anodization under ultraviolet (UV) irradiation. TiO2 formed by anodization of Ti is activated to generate electrons and holes by UV and the anodization process is influenced by the photo-generated charges. As a consequence, morphologies of the fabricated TiO2 nanotubes can be adjusted by controlling the UV illumination. Double-walled TiO2 nanotubes or single-walled nanotubes can be selectively formed by switching on/off the UV illumination. The thickness of the inner and outer walls of the double-walled nanotubes can be tailored by changing the UV power. Due to their larger surface areas compared to single-walled nanotubes, the porous double-walled nanotubes exhibit an enhanced photo-degradation rate for methylene blue (MB). The mechanism of the porous double-walled TiO2 nanotubes is proposed based on the photoactive semiconducting property of the as-growing TiO2 nanotubes under UV.

Influence of anodizing voltage mode on the nanostructure of TiO2 nanotubes

The nanostructure of self-ordered porous anodic TiO2 nanotubes (PATNTs) has extraordinary influence on their physical and chemical properties. For this reason, extensive attention has been paid on pulse anodization to regulate the nanostructure of PATNT. However, the relationships between the nanostructures and current curves still remain unclear. Based on the traditional potentiostatic and pulse anodizations, five different modes (i.e., potentiostatic, pulse, triangle wave, decrease, and increase step by step) of applied voltage and their influences on the nanostructures of PATNT have been investigated in detail. The growing rates of the nanotubes anodized under five different modes were compared for the first time. The results show that the growing rate of pulse voltage anodization is the fastest, reaching 116.4 nm min−1. The slowest is triangle wave voltage anodization, only 59.3 nm min−1. When the applied voltage decreases step-by-step, branched nanotubes can be formed in the bottom of PATNT. Yet, when the applied voltage increases step-by-step, triple-layer nanotubes with different diameters are formed, and the forming mechanism of this special nanostructure is discussed. The present results may be helpful to understand the mechanism of PATNT and facilitate the assembling diverse nanostructures for extensive applications in photocatalysis, dye-sensitized solar cells, and biomedical devices.

Self-Ordering Process of Hexagonal Cells in Porous Anodic TiO<sub>2</sub> Nanotubes

Cylindrical TiO2nanotubes and hexagonal TiO2nanotubes were obtained in the anodizing process of titanium in fluoride-containing electrolyte. Based on the experimental findings and viscous flow model, a mechanism of self-ordering process of hexagonal cells arrangement for porous anodic TiO2nanotubes (PATNT) is proposed in this paper. The analysis results show that oxygen evolution in the pore bottoms plays an important role in the self-ordering process. Oxide grows around the oxygen bubbles in the pore bottoms, which results in the formation of cylindrical TiO2nanotubes. Volume expansion of TiO2takes place during the anodizing process. Plastic deformation and repulsive force caused by volume expansion are responsible for self-ordering process of PATNT hexagonal cells.

Facile formation of mixed phase porous TiO2 nanotubes and enhanced visible-light photocatalytic activity

A simple and stable procedure for preparing mixed phase porous TiO2 nanotubes via an emulsion electrospinning is reported. The obtained porous TiO2 nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction pattern (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectrum (PL) and lifetime. These TiO2 nanotubes have a highly porous structure with an average diameter of 200nm. The results show that the small grain size of the rutile crystallites in the porous TiO2 nanotubes would make more catalytic “hot spots” at the rutile/anatase interface than that at the mixed phase TiO2 nanofibers and Degauss P25 under visible light irradiation. According to the decay curve, it implies these “hot spots” would restrict electron/hole recombination and leading a longer life time of 3.11ns in this porous nanotube compared to nanofibers and Degauss P25. Thus, these “hot spots” would stabilize the charge separation and enhance the photocatalytic activity of methyl orange (MO) under visible light irradiation. This method could be extended to the fabrication of various types of highly porous nanotubes.

Fabrication and Formation Mechanism of Triple-Layered TiO2 Nanotubes

The formation mechanism of porous anodic TiO2 nanotubes (PATNT) still remains unclear. A special approach is proposed in this paper to investigate the forming process of nanopores in the preformed nanotubes. A novel and not easily brittle nanostructure, called triple-layered TiO2 nanotube array, has been fabricated by changing the electrolytes during the electrochemical anodizing processes. The first porous layer was fabricated in fluoride-containing electrolyte, the middle compact layer was formed in fluoride-free electrolyte and the second porous layer was formed in the same fluoride-containing electrolyte. The results show that middle compact layer becomes thicker with the increase of the third time anodizing voltage. At the same time, it needs more time for the fourth time anodization to reach the equilibrium current, where the nanotubes begin to develop steadily. Furthermore, a possible mechanism for the growth of the triple-layered nanotubes is discussed by comparison with the normal PATNT. The present results may be helpful to understand the mechanism of PATNT and facilitate assembling diverse nanostructures for extensive applications in photocatalysis, dye-sensitized solar cells, and biomedical devices.

Electrospinning directly synthesized metal nanoparticles decorated on both sidewalls of TiO2 nanotubes and their applications

The hybrid structure of nanoparticle-decorated nanotubes has the advantage of both large specific surface areas of nanoparticles and anisotropic properties of nanotubes, which is desirable for many applications. In this study, Ag nanoparticles on highly porous TiO2 nanotubes (NTs) on both internal and external sidewalls (Ag@TiO2@Ag NTs) are directly synthesized by emulsion electrospinning and thermal evaporation for the first time. The Ag@TiO2@Ag NT heterostructures, which have large surface-to-volume ratios, improved electrical conductivity, and an excellent material synergetic effect, demonstrate excellent electrochemical properties and superior photocatalytic performance. The new method for the synthesis of Ag@TiO2@Ag NT heterostructures can be applied to fabricate various types of other novel metal nanoparticles (for example Au and Pt) on highly porous nanotubes on both internal and external sidewalls. The possible growth mechanism and reasons for excellent electrochemical properties and superior photocatalytic performance were discussed in detail.

WO3 nanoparticles decorated on both sidewalls of highly porous TiO2 nanotubes to improve UV and visible-light photocatalysis

The hybrid structure of nanoparticle-decorated nanotubes has the advantage of both large specific surface areas of nanoparticles and anisotropic properties of nanotubes, which is desirable for many applications. In this study, WO3 nanoparticles decorated on highly porous TiO2 nanotubes along both internal and external sidewalls (WO3@TiO2@WO3 heterostructures) were synthesized through emulsion electrospinning, thermal evaporation, and thermal annealing. The WO3@TiO2@WO3 heterostructures had large specific surface areas, high porous structure and excellent interface (between WO3 nanoparticles and TiO2 nanotubes). Three other samples, TiO2 nanofibers, TiO2 nanotubes, and TiO2 nanofibers decorated by WO3 nanoparticles, were prepared in order to compare with the WO3@TiO2@WO3 heterostructures for photocatalysis with both UV and visible light irradiation. The new material (WO3@TiO2@WO3 heterostructures) had a wide range of light absorption and demonstrated the best photocatalytic performance. The possible growth mechanism and reasons for high photocatalysis are discussed in detail.

Research Progress in Formation Mechanism of TiO<sub>2</sub> Nanotubes and Nanopores in Porous Anodic Oxide

Self-ordered porous anodic TiO2 nanotubes and other porous anodic oxides (PAO) have received considerable attention because of their potential for high technological application in a number of fields. The anodization of valve metals has been widely investigated over the last eight decades. The formation mechanisms of hexagonal cells and nanotubes, however, have remained unclear until now. Simply reviewing the mechanisms of PAO formation was not the aim of this research and we were more interested in reviewing the forming processes of compact anodic oxides (CAO) and investigating the relationship between the PAO and CAO, to better understand the pore generating mechanisms. The present work introduces the differences between PAO and CAO films, as well as reviewing the traditional theories of PAO films and their deficiencies. Recent progress in the formation mechanism of PAO, including the viscous flow, breakdown, equifield strength, and oxygen bubbles models has been reviewed in detail. The perspective on future developments for the PAO forming mechanism has been tentatively discussed. Based on sufficient analysis of the latest findings, it has been proposed that several approaches may be employed for investigating the pore forming and self-ordering mechanisms. These new approaches include ultrasound-assisted anodizing, anodizing under vacuum or high pressure and adding sodium carbonate or a reducing agent to the PAO-forming electrolytes. An investigation of changes in the current and anodizing efficiencies would also be an effective approach for better understanding the physical nature of field-assisted dissolution (FAD).