High Aspect Ratio TiO2 Nanotube Research Articles

Investigating the thickness-effect of free-standing high aspect-ratio TiO2 nanotube layers on microwave-photoresponse using planar microwave resonators

One-dimensional TiO2 nanotube (TNT) layers are a promising candidate for UV detection due to their distinctive anisotropic geometry which is effective for light harvesting and rapid carrier transport. Here, the photosensitivity efficiency of TNT layers with various thicknesses of 15, 50, 80, and 110 µm was utilized at a microwave frequency regime by modeling and experimentally. A planar microwave split ring resonator (PMSRR) was designed and fabricated to operate at ∼8 GHz to study TNT layers by monitoring the scattering parameter (S21) of the PMSRR under a constant UV irradiation power of ∼96.4 µW/cm2. According to the results, the 80 µm thick TNT layers demonstrated the highest resonant amplitude variation for the customized PMSRR. The change of the resonant amplitude was mainly attributed to the conductivity variation contributed by perturbation of trapped electron concentration, as the dominant factor under UV illumination, and their electromagnetic wave interaction. The main advantage of the proposed method of PMSRR for microwave photosensitivity monitoring over the conventional direct current (DC) conductivity measurements is to eliminate the effect of contact resistance between the TNT layers and metal electrodes utilizing the contactless aspect of wave interactions with the TNT layers at microwave regime to perform electrode-less measurements.

Gigahertz-Based Visible Light Detection Enabled via CdS-Coated TiO2 Nanotube Layers

Detection of visible light is a key component in material characterization techniques and often a key component of quality or purity control analyses for health and safety applications. Here in this work, to enable visible light detection at gigahertz frequencies, a planar microwave resonator is integrated with high aspect ratio TiO2 nanotube (TNT) layer-sensitized CdS coating using the atomic layer deposition (ALD) technique. This unique method of visible light detection with microwave-based sensing improves integration of the light detection devices with digital technology. The designed planar microwave resonator sensor was implemented and tested with resonant frequency between 8.2 and 8.4 GHz and a resonant amplitude between -15 and -25 dB, depending on the wavelength of the illuminated light illumination on the nanotubes. The ALD CdS coating sensitized the nanotubes in visible light up to ∼650 nm wavelengths, as characterized by visible spectroscopy. Furthermore, CdS-coated TNT layer integration with the planar resonator sensor allowed for development of a robust microwave sensing platform with improved sensitivity to green and red light (60 and 1300%, respectively) compared to the blank TNT layers. Moreover, the CdS coating of the TNT layer enhanced the sensor's response to light exposure and resulted in shorter recovery times once the light source was removed. Despite having a CdS coating, the sensor was capable of detecting blue and UV light; however, refining the sensitizing layer could potentially enhance its sensitivity to specific wavelengths of light in certain applications.

High Aspect Ratio TiO2 Nanotube Layers Obtained in a Short Time

In the last two decades, anodic TiO2 nanotube (TNT) layers have gained considerable attention for both their unique physical and chemical properties practical applications [1-3].One of the key parameters that characterize TNT layers is the aspect ratio, which is the ratio between the TNT layer thickness and the nanotube diameter. In particular, TNT layers with a high aspect ratio (HAR) due to their 1D ordered structure, an exceptional morphology and a high surface area, have a great potential in different applications, such as filtration and micro-photo reactors [4,5]. In the last decade, ethylene glycol (EG)-based electrolytes containing small amounts of NH4F and H2O [6] were the most widely used electrolytes to produce high-aspect ratio TNT layers. However, using these electrolytes very long anodization times i.e., several hours or up to several days [7], are needed. Apparently, the synthesis of long and extremely robust HAR TNT layers in a relatively short time remains very challenging until now. Recently, it has been shown that the addition of lactic acid (LA) to EG-based electrolytes can shift the dielectric breakdown potential to significantly higher potentials due to the strong adsorption of lactic acid on TiO2 during the anodization [8]. Thus, considerably higher anodization potentials and increased electrolyte temperatures can be employed and HAR TNT layers can be obtained within less than one hour of anodization.In this presentation we will demonstrate that HAR TNT layers can be obtained within short anodization times using an optimized anodization protocol. The key feature is the use of NH4F/H2O/EG electrolytes containing LA with optimized electrolyte [9].Experimental details and some recent anodization results will be presented and discussed.

High aspect ratio TiO2 nanotube layers obtained in a very short anodization time

Over the past years, the growth of high aspect ratio (HAR) anodic TiO2 nanotubes in a very short time (minute scale) has remained challenging. In the present report, TiO2 nanotube (TNT) layers with HAR ≈ 450 were obtained during only 15 min of optimized anodization. The key feature is the use of NH4F/H2O/ethylene glycol (EG) electrolytes with the addition of lactic acid (LA) that prevents the dielectric breakdown and enables anodization at higher potentials compared to the classical NH4F/H2O/EG electrolyte without LA. The thickness and average diameter of the obtained TNT layers for 15 minutes’ anodization at 160 V were approximately 80 µm and 170 nm, respectively. Furthermore, we show for the first time that high temperature anodizing, previously introduced as a driving force for the growth of HAR nanotubes in a short time, can be avoided by an accurate optimization of the anodization conditions at room temperature. The results clearly show that electrolytes containing LA can serve as a promising candidate for the ergonomic and economic production of HAR TNT layers on Ti substrates in very short anodization times, i.e. with the addition of LA, the anodization time can be significantly reduced from several hours or several days to 15–30 min.

ALD growth of MoS2 nanosheets on TiO2 nanotube supports

Two-dimensional MoS2 nanostructures are highly interesting and effective in a number of energy-related applications. In this work, the synthesis of ultra-thin MoS2 nanosheets produced by the thermal Atomic Layer Deposition (ALD) process is reported for the first time using a previously unpublished set of precursors, namely bis(t-butylimido)bis(dimethylamino)molybdenum and hydrogen sulfide. These nanosheets are homogenously deposited within one-dimensional anodic TiO2 nanotube layers that act as a high surface area conductive support for the MoS2 nanosheets. The decoration of high aspect ratio TiO2 nanotube layers with MoS2 nanosheets over the entire nanotube layer thickness is shown for the first time. The homogeneous distribution of the MoS2 nanosheets is proved by STEM/EDX. This resulting new composite is employed as anode for Li-ion microbatteries. The MoS2-decorated TiO2 nanotube layers show a superior performance compared to their counterparts without MoS2. Compared to electrochemical performance of pristine TiO2 nanotube, a more than 50% higher areal capacity and a coulombic efficiency of 98% are obtained on the MoS2 decorated TiO2 nanotube layers, demonstrating clear synergic benefits of the new composite structure.

Self-organized, free-standing TiO2 nanotube membranes: Effect of surface electrokinetic properties on flow-through membranes

In the present work we investigate the effect of the surface electrokinetic properties and presence of background ions on the flow of a marker dye through TiO2 nanotube membranes. We believe the results to be of high significance not only for filtration but also for the design of microphotoreactor application based on photoactive TiO2 nanotubes membrane. First, both-side open, high aspect ratio TiO2 nanotube membranes were obtained by fast anodization of Ti to self-aligned TiO2 nanotube layers, followed by a lift-off process. Then we investigated the permeation through the TiO2 nanotube membranes by diffusion of acid orange 7 (AO7) and extracted the dye diffusion rates. The effects of pH, ionic concentration, and size of ions were investigated, and the results were compared with theoretical modeling of the surface charge of TiO2 and the neighbouring electric double layer as a function of different species of ions; the modeling confirms the experimental data. We observed a remarkable influence of the background ion species, as well as of ion concentrations and pH in the feed solution on the diffusion rate of AO7. The results of modeling are well in line with the observed influence of TiO2 nanotube inner surface charge and effective size (hydrodynamic radius) of the ions in the background solution. It is also observed that the absolute permeate flux and the membrane's permeability strongly depend on the electric and wetting conditions of the membrane surface.

Magnetic field enhanced photoelectrochemical response of a nanostructured titanium dioxide anode

The concept of permanent-magnet photoelectrochemical fuel cell is proposed. The fuel cell contains a TiO2 nanotube anode and a platinum cathode. Water was used as the fuel. The anode has a self-organized high aspect ratio TiO2 nanotube layer made through electrochemical oxidation of Ti. Photocatalysis and permanent magnetic field excitation experiments were performed. The photoelectrochemical responses of the anode to visible (Vis) and ultraviolet (UV) light were studied to examine the magnetic field effect. The open circuit voltages were 0.0564, 0.1625, 0.1898 and 0.376V, under the excitation of Vis, UV, magnet and magnet+UV, respectively. Current density was measured under different linear potential scanning conditions. It reached a value of 5.23mA/m2 just under the excitation of the permanent magnet at a bias voltage of 2V. Combining the magnetic action and the photovoltaic effect, the titanium dioxide photosensitive anode showed a maximum photocurrent density of 296.05mA/m2 at 2V bias. It is concluded that the absolute value of open circuit voltage (OCV) of the fuel cell due to the exposure to the magnet field is much higher than that due to either UV or Vis light illumination. However, UV light excitation results in higher current density in the measurement.

Synthesis of long TiO2 nanotube arrays with a small diameter for efficient dye-sensitized solar cells

The surface/volume ratio of TiO2 nanotube arrays determines their electrical, optical and chemical properties for various applications. For dye-sensitized solar cells (DSSCs), it is very important to fabricate long TiO2 nanotube arrays with a larger surface area in order to have more dye loading and achieve higher conversion efficiencies accordingly. In this paper, a convenient anodization method is introduced to synthesize high aspect ratio TiO2 nanotubes at a low voltage and a high temperature. With a high reaction temperature, the diffusion rate of ions in the anodization process was enhanced. At the optimum temperature, nanotubes longer than 16 μm with outside diameters as small as 75 nm could be obtained at a voltage of 20 V. Significantly, using these resulting TiO2 nanotube arrays to fabricate front-side illuminated DSSCs, a conversion efficiency as high as 5.98% could be achieved due to the large surface area and the enhanced attachment of the dye.

Preparation of Pt catalysts decorated TiO2 nanotube arrays by redox replacement of Ni precursors for proton exchange membrane fuel cells

With Ni as the precursor, by using pulse electrodeposition technique, Pt nanoparticles were synthesized and deposited onto the high aspect ratio TiO2 nanotube arrays support for fuel cells. The influence of pulse electrodeposition parameters on the morphology of Ni nanoparticles was investigated, and the prepared Ni nanoparticles with diameter of about 30nm were advantageous to the deposition and dispersion of Pt catalysts within TiO2 nanotubes support. The modified electrode exhibited high activity at half cell test. Furthermore, the electrochemical surface area of this electrode had reduced by 28% after an accelerated durability test compared to 57% for commercial Pt/C (JM). In this work, a TiO2 nanotube arrays as catalyst support in was tested under single cell test condition. The maximum power density reached 557mWcm−2 when this novel electrode was used as the anode of fuel cells.

Facile Fabrication of Open-Ended High Aspect-Ratio Anodic TiO<sub>2</sub> Nanotube Films and their Applications

In the present work, we demonstrated a facile process to prepare an open-ended high aspect-ratio TiO2 nanotube films through separating the anodic TNT array from the Ti substrate by a small reverse bias and opening the tube bottom by a chemical etching. The possible mechanisms of film detachment and pore opening processes have been briefly discussed. Such a process allows controlling the open-ended morphology by the straightforward chemical etching, which shows great potential in many applications, such as flow-through photocatalytic reactions, biofiltration, and diffusion controlling, and so on. An example using the open-ended TNT films is finally given as a flow-through photocatalytic reactor. The photocatalytic film has been shown to have multiple functions such as physical separation of contaminants, filtration, and decomposition of organic pollutants during diffusion.

Electrochemical synthesis of doped TNT as a nano photocatalyst for color degradation applications

The preparation of high aspect-ratio TiO2 nanotubes and their photocatalytic activity were demonstrated in this study. The high aspect-ratio TiO2 nanotube thin films were produced by electrochemical anodic oxidation of Ti in chloride-containing electrolytes. Nanotubes were doped with different concentrations of ZnO particles through anodization. The catalytic behavior was evaluated under batch reactor with photo-degradation test of Red Dye. The experimental results collectively demonstrate the successful ZnO doping of the resultant nanotube layers with significant abundant OH groups on their increased surfaces. The nanotubes doped with high content combined with an anatase as a two phase semiconductor led to the formation of very active photocatalyst with highly surface reaction sites. In contrast, to the undoped TNT, its anatase/rutile phase ratios are increased. These effects could be attributed to the enhanced critical active-site density on the surface, which provides better photocatalytical properties.

Synthesis and deposition of ultrafine Pt nanoparticles within high aspect ratio TiO2 nanotube arrays: application to the photocatalytic reduction of carbon dioxide

Using a rapid microwave-assisted solvothermal approach ultrafine Pt nanoparticles are synthesized and deposited in situ within high aspect ratio nanotube arrays. Adjusting the initial concentration of metal ion precursor inside the nanotube support controls the resulting Pt nanoparticle sizes. The Pt-nanoparticle/TiO2 nanotube composite is shown to greatly promote the photocatalytic conversion of carbon dioxide and water vapor into methane, a behavior attributed to the homogeneous distribution of metal co-catalyst nanoparticles over the TiO2 nanotube array surface providing a large number of active reduction sites. The novelty and flexibility of the technique, described herein, could prove useful for the deposition of metal, metal alloy, or metal oxide nanoparticles within a variety of nanotubular or nanoporous material systems with the resulting nanocomposites useful in catalysis, photocatalysis, photovoltaic, and photoelectrochemical applications.

NaAc Effect on the Anodization Formation of TiO<sub>2</sub> Nanotube Arrays in Glycerol Based Electrolytes

Effect of NaAc on the anodic growth of TiO2 nanotube arrays is described. NaAc-added approach yields longer nanotubes relative to samples grown from NaAc-free electrolyte. And the growth rate of TiO2 nanotubes has pH independency in NaAc-added electrolytes. The key to achieve a high aspect ratio TiO2 nanotube arrays is to decrease the chemical dissolution rate at the mouth of the tube by adding NaAc as protective coating. Adsorption of Ac- species on the TiO2 surface is shown to markedly decrease the chemical dissolution rate of the tube mouth, resulting in longer nanotube length.

Synthesis and variable temperature electrical conductivity studies of highly ordered TiO2 nanotubes

Rafts of aligned, high aspect ratio TiO2 nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225-400 °C. Length, outer diameter and wall thickness of the nanotubes were approximately 60-80 ∝m, 160 nm and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO2 nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment at temperatures as low as 250 ϒC in air. The activation energy for conductivity over the temperature range 250 - 350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.

Comparison of photoelectrochemical properties of TiO2-nanotube-array photoanode prepared by anodization in different electrolyte

Self-organized, well-crystallized and high aspect-ratio TiO2 nanotube arrays (TNAs) have been prepared by anodic oxidation in dimethyl sulfoxide (DMSO) containing 5 wt% HF at 40 V (vs. Pt). A 50 h anodization results in a nanotube arrays approximately 19.4 μm in length, referred as long tube. As a comparison, the short titania nanotube arrays, about 500 nm in length, was obtained by anodization in HF aqueous solution, referred as short tube. Different characterization techniques (viz. FESEM, TEM, XRD and DRS) are used to study the nanotubular microstructure. The morphology of the nanotube electrodes shows an evident influence on their photocatalytic (PC) and photoelectrochemical reactivity. The long tube reveals enhanced photocurrent response and PC degradation efficiency of organic compounds. The kinetic constant of PC degradation of methylic orange (MO) for long tube electrode is found 1.55 times as high as the short tube. A significant photoelectrochemcial synergetic effect in MO degradation was observed on the long tube electrode and the photoelectrocatalytic (PEC) degradation of MO on long tube is 27% higher than its PC process.

Electrophoretically Deposited TiO2 Nanotube Light-Scattering Layers of Dye-Sensitized Solar Cells

We report herein the enhanced light-to-electricity conversion efficiency of a dye-sensitized solar cell using a new bilayer structure of the TiO2 electrode. The bilayer structure consists of a light-absorbing TiO2 nanoparticle layer together with a light-scattering TiO2 nanotube layer formed upon. High aspect-ratio TiO2 nanotubes (α-TNTs) with a diameter of 20 nm were prepared via anodization of a Ti sheet in a perchloric acid solution, and the α-TNT layer was electrophoretically deposited onto the sintered TiO2 nanoparticle layer. The light-scattering property of the α-TNT layer was comparable to that of the commonly used TiO2 sub-micron nanoparticle layer. The α-TNT layer provided a large surface area for dye-adsorption as well as an efficient transport pathway for photo-generated carriers. These effects allowed a higher incident photon-to-current efficiency of the bilayer TiO2 structure with the nanotube light-scattering layer over the whole spectral range relative to that with a sub-micron nanoparticle layer.

Fabrication of open-ended high aspect-ratio anodic TiO2 nanotube films for photocatalytic and photoelectrocatalytic applications

A facile process is introduced to flake high aspect-ratio anodic TiO(2) nanotube (TiNT) arrays off Ti substrates and then chemically remove the bottom caps to obtain open-ended TiNT films that exhibit high activity to photocatalytic degradation of methylene blue and efficient hydrogen production from photoelectrocatalytic water splitting.

Smooth anodic TiO2 nanotubes: annealing and structure

AbstractThe influence of an annealing process on the structure and the composition of smooth high aspect ratio TiO2 nanotube layers, grown by anodization of Ti in viscous glycerol electrolytes containing fluorides, is reported. XRD and HRTEM measurements demonstrate that the as‐formed high aspect ratio TiO2 nanotubes have an amorphous structure that can be entirely transformed to single phase – anatase – upon thermal annealing at 450 °C for 3 hours. XPS and EDX analyses furthermore show that annealing process is accompanied with an elimination of fluoride ions (present in the structure after formation). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)