TiO2 Nanotube Layers 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.

2D FeSx Nanosheets by Atomic Layer Deposition: Electrocatalytic Properties for the Hydrogen Evolution Reaction.

2-dimensional FeSx nanosheets of different sizes are synthesized by applying different numbers of atomic layer deposition (ALD) cycles on TiO2 nanotube layers and graphite sheets as supporting materials and used as an electrocatalyst for the hydrogen evolution reaction (HER). The electrochemical results confirm electrocatalytic activity in alkaline media with outstanding long-term stability (>65 h) and enhanced catalytic activity, reflected by a notable drop in the initial HER overpotential value (up to 26 %). By using a range of characterization techniques, the origin of the enhanced catalytic activity was found to be caused by the synergistic interplay between in situ morphological and compositional changes in the 2D FeSx nanosheets during HER. Under the application of a cathodic potential in alkaline media, the as-synthesized 2D FeSx nanosheets transformed into iron oxyhydroxide-iron oxysulfide core-shell nanoparticles, which exhibited a higher active catalytic surface and newly created Fe-based HER catalytic sites.

White and black anodic TiO2 nanotubes: Comparison of biological effects in A549 and SH-SY5Y cells

In this work, the cytotoxicity of black TiO2 nanotubes in comparison to their white counterparts is exploited for the first time. To provide a comprehensive overview, the toxicity is also assessed in comparison to multi-walled carbon nanotubes (MWCNTs) using two different cell lines. The TiO2 nanotube layers were prepared by anodic oxidation of Ti foils and investigated in the as-prepared white form, and in the black form, achieved by reduction via comproportionation reaction of Ti and TiO2. Additionally, a part of the black TiO2 nanotube layers was coated with a thin TiO2 coating with a nominal thickness of ~0.275 nm by Atomic Layer Deposition (ALD). Pulmonary A549 cells and neural SH-SY5Y cells were exposed to a wide range of TiO2 nanotube concentrations (1–500 μg/mL) for 24 and 48 h. Then, the cytotoxicity was evaluated using cell viability and glutathione assessments. Essentially, no reduction in cell viability was observed after incubating the cells with TiO2 nanotubes up to 100 μg/mL, while MWCNTs decreased the cell viability at a similar concentration significantly. In addition, ALD TiO2 coating on the black TiO2 nanotubes caused disappearance of mild toxic effects of white and black TiO2 nanotubes due to the shading of carbon and fluorine species incorporated within the TiO2 nanotube walls.

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.

Pt-single atom decorated TiO2: Tuning anodic TiO2 nanotube structure and geometry toward a high-performance photocatalytic H2 production

Atomically dispersed Pt single atoms (SAs) on titania have been reported to be a highly effective co-catalyst in photocatalytic H2 generation. In this report, we grew layers of anodic TiO2 nanotubes (NTs) varying the lengths of the NTs from 1 to 18 μm as well as their crystalline state by thermal annealing over a wide range of temperatures (350 °C – 800 °C). We then decorated the nanotubes with single atom (SA) Pt co-catalysts deposited from H2PtCl6 solutions with various concentrations of 0.001–2 mM Pt and evaluate the photocatalytic H2 generation performance for the different NT layers under UV and solar illumination. The highest photocatalytic performance is achieved for 15–18 μm long anatase NTs decorated with Pt SAs from a 0.01 mM H2PtCl6 solution (leading to a Pt SA surface concentration of 5.6 × 105 /µm2). Except for a sufficient SA Pt loading and an anatase structure, the key parameter is the photon absorption length for near-band-gap UV and AM 1.5 light. Such optimized Pt SA@TiO2 NTs show a most efficient H2, outperforming many other photocatalytic 3D titania structures, such as titania-based powders, flakes, and Metal–Organic Frameworks (MOFs) tested under the same illumination conditions.

New insights into the mechanism of coupled photocatalysis and Fenton-based processes using Fe surface-modified TiO2 nanotube layers: The case study of caffeine degradation

TiO2 nanotube (TNT) layers are prepared via electrochemical anodization of Ti foil in ethylene glycol-based electrolyte containing fluoride ions. Using spin-coating of Fe(III) precursor solutions of different concentrations (10 µM to 10 mM), iron surface-modified TNT (Fe-TNT) layers are formed after annealing at 450 °C for 2 h in air. The Fe-TNT layers are comprehensively characterized by XRD, SEM, DRS and XPS. To identify and assess the contribution of photocatalysis and Fenton-based processes that are simultaneously triggered by Fe-TNT layers, degradation of caffeine is investigated in the dark and under UVA with and without the presence of 500 µM H2O2. It has been found that in the presence of H2O2 under UVA, 100 µM Fe-TNT can degrade 67 % of caffeine after 3 h, of which 4 % is from photolysis, 23.5 % from photocatalysis and 39.5 % from Fenton-based processes. It is important to note that Fenton-based reactions are due to the iron leaching, while the main reactive oxygen species are identified as hydroxyl radicals. In addition, the main degradation by-products are identified by LC-MS and IC-MS techniques, thus leading to the proposal of different degradation pathways of caffeine. To sum up, the research's findings provide novel insights into the concept of photochemical versatility, which is only obliquely explored in the corpus of previous research.

Pt single atoms dispersed in a hybrid MOFox-in-nanotube structure for efficient and long-term stable photocatalytic H2 generation

Hierarchical structures produced by thermal conversion of MOFs within the anodic TiO2 nanotube layers combine the excellent light harvesting, carrier transport, and Pt SA anchoring to achieve a remarkable photocatalytic H2 evolution performance.

Anodic TiO2 Nanotube Layers for Wastewater and Air Treatments: Assessment of Performance Using Sulfamethoxazole Degradation and N2O Reduction.

The preparation of anodic TiO2 nanotube layers has been performed using electrochemical anodization of Ti foil for 4 h at different voltages (from 0 V to 80 V). In addition, a TiO2 thin layer has been also prepared using the sol-gel method. All the photocatalysts have been characterized by XRD, SEM, and DRS to investigate the crystalline phase composition, the surface morphology, and the optical properties, respectively. The performance of the photocatalyst has been assessed in versatile photocatalytic reactions including the reduction of N2O gas and the oxidation of aqueous sulfamethoxazole. Due to their high specific surface area and excellent charge carriers transport, anodic TiO2 nanotube layers have exhibited the highest N2O conversion rate (up to 10% after 22 h) and the highest degradation extent of sulfamethoxazole (about 65% after 4 h) under UVA light. The degradation mechanism of sulfamethoxazole has been investigated by analyzing its transformation products by LC-MS and the predominant role of hydroxyl radicals has been confirmed. Finally, the efficiency of the anodic TiO2 nanotube layer has been tested in real wastewater reaching up to 45% of sulfamethoxazole degradation after 4 h.

Amorphous NiCu Thin Films Sputtered on TiO2 Nanotube Arrays: A Noble‐Metal Free Photocatalyst for Hydrogen Evolution

AbstractIn this work, NiCu co‐catalysts on TiO2 are studied for photocatalytic hydrogen evolution. NiCu co‐catalyst films are deposited at room temperature by argon plasma sputtering on high aspect‐ratio anodic TiO2 nanotubes. To tune the Ni : Cu atomic ratio, alloys of various compositions were used as sputtering targets. Such co‐catalyst films are found to be amorphous with small nanocrystalline domains. A series of parameters is investigated, i. e., i) Ni : Cu relative ratio in the sputtered films, ii) NiCu film thickness, and iii) thickness of the TiO2 nanotube layers. The highest photocatalytic activity is obtained with 8 μm long TiO2 nanotubes, sputter‐coated with a 10 nm‐thick NiCu films with a 1 : 1 Ni : Cu atomic ratio. This photocatalyst reaches a stable hydrogen evolution rate of 186 μL h−1 cm−2, 4.6 and 3 times higher than that of Ni‐ and Cu‐TiO2, respectively, demonstrating a synergistic co‐catalytic effect of Ni and Cu in the alloy co‐catalyst film.

Intrinsic properties of anodic TiO2 nanotube layers: In-situ XRD annealing of TiO2 nanotube layers

In this work, in-situ annealing of TiO2 nanotube (TNT) layers in an XRD chamber was carried out to investigate the crystallization process of the TNT layers in details. TNT layers of different thicknesses, i.e. 1, 5 and 20 μm, and of different morphology, i.e. double wall vs. single wall, were annealed in the temperature range from room temperature to 500 °C with two distinctly different annealing rates. Additionally, three different annealing atmospheres were explored, namely air as an oxidative, N2 as an inert, and 5%H2/95%N2 as a reductive atmosphere. XRD patterns were measured every 15 °C of the heating ramp and the crystallite sizes of anatase phase were calculated from the anatase peak using Scherer equation at each data point. The differences in crystallization behavior for the different TNT layers as well as the effect of the annealing atmosphere for 5 μm thick TNT layers are described and discussed.

Magnetically guidable single TiO2 nanotube photocatalyst: Structure and photocatalytic properties

The influence of annealing temperature, the crystallinity, textural properties of magnetically guidable anodic TiO2 nanotubes in form of single tube powders on the photocatalytic decomposition of model organic dye is investigated for the first time. The powders were prepared from self-organized anodic TiO2 nanotube layers (grown on Ti) by etching in piranha solution, detaching them from the Ti substrate, and annealing in a wide range of temperatures (400–900 °C). The nanotubular shape was preserved up to 800 °C. Powders annealed up to 800 °C did not contain any detectable rutile phase, which is unique among other TiO2 nanomaterials at this temperature. Part of the obtained TiO2 ST-NT powders was decorated with Fe3O4 nanoparticles using an oleic acid-based synthesis. The TiO2 ST-NT and TiO2 ST-NT@Fe3O4NPs powders were characterized in terms of morphology, crystallinity, and specific surface area. Finally, all materials were exploited for the photocatalytic decomposition of a model dye. The best photocatalytic performance of TiO2 ST-NT and TiO2 ST-NT@Fe3O4NPs powders were obtained at annealing temperatures of 600 and 700 °C, respectively. The synergy resulting from annealing, crystallite size, and specific surface area enhances the photocatalytic activity of TiO2 ST-NT and TiO2 ST-NT@Fe3O4NPs powders under UV light. This approach goes beyond the classical Kasuga alkaline hydrothermal approach, enabling the preparation of large quantity of TiO2 nanotubes without any impurities (such as e.g. Na or Ca content resulting from the Kasuga approach).

Large-Scale Synthesis of Photocatalytic TiO2 Nanotube Layers

Self-organized TiO2 nanotube layers have attracted considerable scientific and technological interest over the past 15 years motivated for their wide range of applications including (photo-) catalysis, hydrogen generation and biomedical uses [1,2]. The synthesis of the 1D TiO2 nanotube layers is carried out by a conventional electrochemical anodization of valve Ti metal sheets in various electrolytes.By now, the synthesis of TiO2 nanotube layers with different nanotube dimensions (diameter, length) and the application of these nanotube layers is well established on the laboratory scale. However, for real applications outside the laboratory scale, such as photocatalysis [3], TiO2 nanotube layers of much larger size are necessary. To date, just a few efforts were carried out to scale up the size of the nanotube layers to more than a few cm2 [3-7] due to the difficulty of controlling the parameters of the anodization process on a larger scale. The main challenge is the huge current received during the anodization of large substrates. This, in turn, leads to an increase in the electrolyte temperature that may result in dielectric breakdown of the growing nanotube layers.In this presentation, the preparation of TiO2 nanotube layers on larger area titanium substrates (dozens of cm2) will be discussed, taking into account the control of the anodization parameters. The TiO2 nanotube layers with different thicknesses and nanotube diameters were employed for gas phase photocatalysis, following the ISO standards [3,8]. Experimental details and photocatalytic results will be presented and discussed.

Bipolar Electrochemistry for the Synthesis of Anodic TiO2 Nanotube Layers

Self-organized TiO2 nanotube (TNT) layers have attracted considerable scientific and technological interest over the past 17 years motivated for their wide range of applications including (photo-) catalysis, hydrogen generation and biomedical uses [1,2]. The synthesis of these TNT layers is carried out by electrochemical anodization of valve Ti metal substrates in various F--containing electrolytes using a conventional 2-electrode set-up with the Ti substrate as anode and a Pt-foil as cathode.Instead of using a conventional set-up, also bipolar electrochemistry can be employed for the anodization of valve metals [3-7]. In this set-up, the Ti substrate is placed in an electrolyte between two so-called feeder electrodes, which are connected to the potentiostat. Due to the electric field between the feeder electrodes, the Ti substrate is polarized. If the applied potential between the feeder electrodes is high enough, redox reactions can be driven on the Ti substrate. This method opens new doors for the preparation of TNT layers with gradients in dimensions (i.e. diameter and length of the nanotubes) [3-5] as well as for the preparation of TNT layers on Ti substrates, which cannot be directly connected to the potentiostat, e.g. Ti spheres [8].In this presentation, the preparation of TNT layers on small Ti spheres [8] as well as on 3D printed Ti substrates [9] using bipolar electrochemistry will be shown and discussed. Furthermore, we will also show the possibility of using closed bipolar electrochemical cells, consisting of two closed half-cells with the Ti substrate as obstacle between the cells [10].

Anodic TiO2 nanotube layers decorated by Pd nanoparticles using ALD: An efficient electrocatalyst for methanol oxidation

Herein, we report the performance of Pd nanoparticles (NPs) prepared by Atomic Layer Deposition (ALD) as a catalyst for methanol electro-oxidation. Pd NPs were decorated onto anodic TiO2 nanotube (TNT) layers as supporting material that possess a large available surface area and direct electrical contact via the underlying titanium foil. Different Pd loadings (150 – 300 – 450 – 600 ALD cycles) show different particles sizes ranging between 7 and 12 nm, as revealed by transmission electron microscopy. Coalescence dominated visibly from 450 ALD cycles, which led to a porous Pd layer all along the TNT walls rather than the growth of individual particles. Electrocatalytic performance was investigated by cyclic voltammetry (CV), where the catalytic activity increased proportional with Pd loading up to the highest values for 400 and 450 cycles, whereas a further increase in the number of ALD cycles (NALD) did not show any additional improvement in methanol oxidation current densities. TNT layers decorated with 400, 450 and 600 Pd ALD cycles show featureless curves suggesting complete anti-poisoning ability or possibly a proof of a direct conversion from CH3OH to CO2 (without any intermediate byproducts). The lack of an oxidation peak during the anodic scan and therefore a reduction peak during the cathodic scan, confirms Pd NPs (stabilized by TiO2) efficiently utilize OHads and chemisorbed CH3OH in a way that its CO poisoning was inhibited. As a result, the tuned high surface area TNT layers exhibited excellent performance as a supporting material for Pd NPs against formation of electrochemical poisoning species. Finally, the mechanism of the TNT layers interaction with Pd NPs, which led to the propelling methanol oxidation reaction without loss in performance over cycling is postulated.

Large-Scale Synthesis of Photocatalytic TiO2 Nanotube Layers

Self-organized TiO2 nanotube layers have attracted considerable scientific and technological interest over the past 15 years motivated for their wide range of applications including (photo-) catalysis, hydrogen generation and biomedical uses [1,2]. The synthesis of the 1D TiO2 nanotube layers is carried out by a conventional electrochemical anodization of valve Ti metal sheets in various electrolytes.By now, the synthesis of TiO2 nanotube layers with different nanotube dimensions (diameter, length) and the application of these nanotube layers is well established on the laboratory scale. However, for real applications outside the laboratory scale, such as photocatalysis [3], TiO2 nanotube layers of much larger size are necessary. To date, just a few efforts were carried out to scale up the size of the nanotube layers to more than a few cm2 [3-7] due to the difficulty of controlling the parameters of the anodization process on a larger scale. The main challenge is the huge current received during the anodization of large substrates. This, in turn, leads to an increase in the electrolyte temperature that may result in dielectric breakdown of the growing nanotube layers.In this presentation, the preparation of TiO2 nanotube layers on larger area titanium substrates (dozens of cm2) will be discussed, taking into account the control of the anodization parameters. The TiO2 nanotube layers with different thicknesses and nanotube diameters were employed for gas phase photocatalysis, following the ISO standards [3,8]. Experimental details and photocatalytic results will be presented and discussed.

Bipolar Electrochemistry for the Synthesis of Anodic TiO2 Nanotube Layers

Self-organized TiO2 nanotube (TNT) layers have attracted considerable scientific and technological interest over the past 17 years motivated for their wide range of applications including (photo-) catalysis, hydrogen generation and biomedical uses [1,2]. The synthesis of these TNT layers is carried out by electrochemical anodization of valve Ti metal substrates in various F--containing electrolytes using a conventional 2-electrode set-up with the Ti substrate as anode and a Pt-foil as cathode.Instead of using a conventional set-up, also bipolar electrochemistry can be employed for the anodization of valve metals [3-7]. In this set-up, the Ti substrate is placed in an electrolyte between two so-called feeder electrodes, which are connected to the potentiostat. Due to the electric field between the feeder electrodes, the Ti substrate is polarized. If the applied potential between the feeder electrodes is high enough, redox reactions can be driven on the Ti substrate. This method opens new doors for the preparation of TNT layers with gradients in dimensions (i.e. diameter and length of the nanotubes) [3-5] as well as for the preparation of TNT layers on Ti substrates, which cannot be directly connected to the potentiostat, e.g. Ti spheres [8].In this presentation, the preparation of TNT layers on small Ti spheres [8] as well as on 3D printed Ti substrates [9] using bipolar electrochemistry will be shown and discussed. Furthermore, we will also show the possibility of using closed bipolar electrochemical cells, consisting of two closed half-cells with the Ti substrate as obstacle between the cells [10].

Recent Advancements in Morphologies of TiO2 Nanotube Layers and Their Photocatalytic Performance

Self-organized TiO2 nanotubes arrays, formed by electrochemical anodization of Ti, have attracted tremendous scientific and technological attention, due to their remarkable properties such as the tunability of dimensions, their directionality, high surface area, low density, and ability to absorb significant amount of incident light [1,2].The most widely used electrolyte for the synthesis of TiO2 nanotube layers is based on ethylene glycol containing small amounts of water and fluoride ions. However, nanotubes prepared in these electrolytes present a double-walled structure, with the outer wall consisting of almost pure TiO2 and the inner wall consisting of TiO2 contaminated with carbon and fluoride species [3,4]. In the recent years, the selective etching of the inner nanotube wall on Ti layer enabled synthesis of single-wall nanotubes that showed a superior photo-electrochemical performance compared to their double-walled counterparts [5]. An extensive etching enabled also the synthesis of single tube powders, which could be effectively decorated with Fe3O4 nanoparticles to act as magnetically guidable photocatalyst [6,7].The presentation will focus on the influence of all these nanotube morphologies on the photocatalytic degradation in the liquid phase. In particular, we will discuss, how the annealing temperature of single nanotubes influences their photocatalytic performance [7] and how the wall thickness of the single wall nanotubes can be optimized for the best performance [8]. Experimental details and photocatalytic results will be presented and discussed.

Scaling up anodic TiO2 nanotube layers – Influence of the nanotube layer thickness on the photocatalytic degradation of hexane and benzene

In this work, the preparation of homogenous TiO2 nanotube (TNT) layers with different thicknesses via anodization on Ti substrates with a large geometrical area of two times 5 cm x 10 cm (i.e. both sides of the Ti substrate) is shown for the first time. TNT layers with four different thicknesses of ∼0.65 µm, ∼1 µm, ∼7 µm, and ∼14 µm were prepared with excellent conformality and homogeneity over the anodized area. These TNT layers were successfully employed as photocatalysts for the degradation of hexane and benzene as model compounds in the gas phase under ISO standards, showing an increase of the conversion for both model compounds with the TNT layer thickness. While a stable hexane conversion was observed for all TNT layers during the measuring time of three hours, in case of benzene degradation an initial conversion decrease was monitored before the conversion stabilized. Despite this trend, SEM and XPS analyses did not reveal any significant amount of reaction products on the TNT layer surface.

Efficient fabrication of robust and highly ordered free-standing TiO2 nanotube layers

A free-standing TiO2 nanotube layer (FSL-TiO2NTs) was prepared via a three-step process, including the electrochemical anodization of Ti foil, calcination at 500 °C for 2 h in air, and detachment with 5.0 wt.% H2O2. The physicochemical properties of TiO2NTs grown on a Ti substrate (TiO2NTs/Tisub) and FSL-TiO2NTs obtained from electrolytes aged for different times (i.e., 0, 4, 8, or 12 h) were characterized by current density measurements, FE-SEM, TEM, XRD, UV–Vis absorbance spectroscopy and transient photocurrent measurement. In general, TiO2NTs synthesized in electrolytes aged for 8 and 12 h possessed not only a highly-ordered structure but also sufficient mechanical hardness to resist the detachment drag, resulting in FSL-TiO2NTs with a complete nanotubes layer structure. The materials prepared in this work demonstrate excellent potential for future applications.

Enhancement of Biofunctionalization by Loading Manuka Oil on TiO2 Nanotubes.

Metallic implants (mesh) for guided bone regeneration can result in foreign body reactions with surrounding tissues, infection, and inflammatory reactions caused by micro-organisms in the oral cavity after implantation. This study aimed to reduce the possibility of surgical failure caused by microbial infection by loading antibacterial manuka oil in a biocompatible nanostructure surface on Ti and to induce stable bone regeneration in the bone defect. The manuka oil from New Zealand consisted of a rich β-triketone chemotype, leptospermone, which showed strong inhibitory effects against several bacteria, even at very low oil concentrations. The TiO2 nanotubular layer formed by anodization effectively enhanced the surface hydrophilicity, bioactivity, and fast initial bone regeneration. A concentration of manuka oil in the range of 0.02% to less than 1% can have a synergistic effect on antibacterial activity and excellent biocompatibility. A manuka oil coating (especially with a concentration of 0.5%) on the TiO2 nanotube layer can be expected not only to prevent stenosis of the connective tissue around the mesh and inflammation by microbial infection but also to be effective in stable and rapid bone regeneration.