TiO2 Nanotube Layers Research Articles

Noble-Metal-Free Photocatalytic H2 Generation: Active and Inactive 'Black' TiO2 Nanotubes and Synergistic Effects.

Over the past five years a number of different synthesis approaches has been reported to obtain so-called 'black' titania. One of the outstanding features of the material is that certain synthesis processes lead to the formation of an intrinsic co-catalytic center and thus enable noble-metal free photocatalytic H2 -generation. In the present work, using TiO2 nanotube layers, we compare three common 'blackening' approaches, namely i) the original high-pressure hydrogenation (HPT-H2 ), ii) a classic high temperature reduction in Ar, and iii) an electrochemical (cathodic) reduction. We demonstrate that except for high pressure hydrogenation also cathodic reduction leads to an activation of TiO2 - that is, it exhibits noble-metal-free photocatalytic H2 generation. Moreover, we show that a combination of cathodic reduction/high pressure hydrogenation leads to a synergistic effect, that is, a significant enhancement of the combined co-catalytic activity.

Achieving homogeneous anodic TiO2 nanotube layers through grain refinement of the titanium substrate

In this work we used two-step electrochemical anodization to produce self-assembled layers of titanium dioxide nanotubes (TNT) on commercially pure titanium substrates with various grain sizes between 140nm to 35μm. The refined grain sizes were obtained using high pressure torsion (HPT). Optical microscopy, transmission electron microscopy, scanning electron microscopy and wettability testing showed that the homogeneity, morphology, thickness and wettability of TNT layers are critically dependent on the substrate grain size. With HPT processing up to ten turns, significant grain refinement was achieved with increasing dislocation density, leading to gradually thicker TNT layers up to 2μm and improved homogeneity with decreasing standard deviation from 10 to 5nm. HPT processing also changed the dissolution rate of oxide during anodization, which resulted in a different top morphology as well as an increased aqueous contact angle of TNT layers.

Influence of annealing temperatures on the properties of low aspect-ratio TiO2 nanotube layers

The present work reports on the effect of thermal annealing on electrical, optical and structural properties of low aspect ratio self-organized TiO2 nanotube layers formed by anodization of Ti. The average inner diameter and length of the nanotubes was 100nm and 1μm, respectively. From the whole range of temperatures (300–600°C), annealing at 400°C leads to the formation of crystalline TiO2 nanotube layers with anatase structure offering best electrical and photo-electrochemical properties. Conducting atomic force microscopy studies have been explored for the first time to identify the annealing temperature of nanotube TiO2 layers with best electrical properties.

Anodic TiO2 nanotube arrays directly grown on quartz glass used in front‐ and back‐side irradiation configuration for photocatalytic H2 generation

In the present work, we explore the front‐ and back‐side performance of a photocatalytic platform consisting of self‐organized TiO2 nanotube layers formed by complete anodization of Ti metal films evaporated on quartz slides. The adhesion and light transmission of the tube layers on the quartz surfaces are optimized by a suitable anodization procedure. After their growth, the nanotube arrays were converted into crystalline structures and sputter‐coated with co‐catalytic Pt nanoparticles. The optically transparent quartz substrates enable the use of the Pt‐decorated tube layers for photocatalytic H2 generation under either front‐ or back‐side illumination configurations. The nanotube films on quartz are characterized in view of their physico‐chemical properties, including their light transmission features measured using different light sources. The results show that the front‐side illumination under optimized factors, i.e., the amount of loaded co‐catalyst, yields a maximized photocatalytic performance in terms of H2 generation.

Addendum: Free-Standing Membranes to Study the Optical Properties of Anodic TiO2 Nanotube Layers.

In the present work we investigate various optical properties (such as light absorption and reflectance) of anodic TiO2 nanotubes layers directly transferred as self-standing membranes onto quartz substrates. This allows investigation in a transmission geometry which provides significantly more reliable data than measurements on the metallic Ti substrate. Light transmission and reflectance measurements were carried out for layers of thicknesses varying from 1.8 to 50 micrometer, and the layers were investigated in their amorphous and crystalline form. A series of wavelength-dependent light attenuation coefficients are extrapolated and found to match the photocurrent vs. irradiation wavelength behavior. However, a feature specific to anodic nanotubes is that their intrinsic carbon content causes a sub-bandgap response that is proportional to the carbon contamination content in the TiO2 nanotubes. Overall the extracted data provide valuable basis and understanding for the design of photo-electrochemical devices based on TiO2 nanotubes.

Anodically Grown TiO2 Nanotube Arrays Used As Substrates for Thermal Dewetting of Pt Show Significantly Enhanced Photocatalytic H2 Generation with Minimal Co-Catalyst Amounts

TiO2 nanostructures have recently attracted much attention due to their wide range of applications, such as photocatalysis, photovoltaics (dye-sensitized solar cell, photo-electrochemistry) and gas sensing. Particularly, electrochemically formed TiO2 nanotube arrays were shown to exhibit improved photocatalytic activity compared to other TiO2 nanostructures, this being ascribed to their directional electron transport and orthogonal charge carrier separation.1 The advantage of an electrochemical growth of the TiO2 nanotube layers is not only the simplicity of the process, but also the possibility of controlling the tube geometry (length, diameter, wall thickness) by selecting suitable anodization parameters (applied voltage, anodization time, temperature, fluoride concentration). Nevertheless, TiO2 alone is not effective in photocatalytic H2 generation: it requires the presence of noble metal co-catalysts (Pt, Au, Pd).2,3 Among the various noble metals, Pt is the most effective co-catalyst: it enables an efficient electron transfer at the semiconductor interface by providing a favorable solid state junction to TiO2 and it enhances the hydrogen recombination reaction. There are several well explored methods to decorate Pt onto TiO2 nanotubes, mostly leading to a decoration of the full tube length with nanoparticles (the most frequently used up to date is photodeposition). With our contribution we show a different approach: we decorate only the outermost surface of anodic TiO2 nanotubes with very thin Pt films (nominally few nm-thick) by using a simple plasma sputtering technique.4 Then, by a controlled thermal treatment, the Pt films are dewetted at the mouth of the tubes into arrays of Pt nanoparticles that are as small as of few nm. This site-selective deposition (only at the tubes top) allows for minimizing the amounts of costly noble metal co-catalyst, while at the same time leads to a significantly enhanced photocatalytic H2 generation compared to tubes that carry along their full length a classic Pt nanoparticle decoration. [1] K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 2014, 114, 9385–9454.[2] N. T. Nguyen, J. Yoo, M. Altomare, P. Schmuki, Chem. Commun. 2014, 50, 9653–9656.[3] N. T. Nguyen, M. Altomare, J. Yoo, P. Schmuki, Adv. Mater. 2015, 27, 3208–3215.[4] N. T. Nguyen, M. Altomare, J. E. Yoo, N. Taccardi, P. Schmuki, Adv. Energy Mater. 2015, doi: 10.1002/aenm.201501926. Figure 1

A new method for non-destructive stripping of coatings on titanium substrates

Abstract This work proposed a new method to prepare undamaged cross-sectional samples of coatings on titanium substrates. First, various kinds of coatings were fabricated on titanium substrates by electrochemical, chemical and physical methods. Then the coated samples were anodized in glycerol electrolyte containing NH4F (1 wt.%) + H2O (10 vol.%) at 50 V for 2 h, which resulted in coating detachment. Afterwards, coating cross-sectional samples were prepared with detached coating fragments and observed by scanning electron microscopy. The results showed that original internal microstructures of the coatings can be observed from the freshly-broken cross-sections, which might not be observed from samples prepared by existing methods. Finally, we concluded that it was the TiO2 nanotube layer formed between the coating and titanium substrate that caused the coating's detachment. The new method may be promising in detection, removing or transfer of coatings on titanium substrates.

Key factors for an improved lithium ion storage capacity of anodic TiO2 nanotubes

In the present work we investigate key factors that influence the Li-storage capacity of anodically grown self-organized TiO2 nanotube layers. First, the influence of both tube length and top morphology are investigated regarding the rate performance of the nanotube layers. Then, factors such as the presence of initiation (grass) layers and the influence of the double walled morphology of conventional TiO2 nanotube layers are elaborated. At high C-rates a grass layer even shows a beneficial contribution, mainly by adding to the double layer capacitance. The presence of a double wall morphology and the associated carbon contamination lead to a lower intercalation ability. Two different approaches are taken to eliminate the inner carbon-rich layer: Cathodic polarization and tube formation in DMSO based electrolytes. This results in an enhancement of the areal capacity by 17% and 33% compared to unmodified tube layers for the two approaches.

Influence of the Ti microstructure on anodic self-organized TiO2 nanotube layers produced in ethylene glycol electrolytes

The relationship between the microstructure of Ti substrates and the anodic growth of self-organized TiO2 nanotube layers obtained upon their anodization in the ethylene glycol based electrolytes on these substrates is reported for the first time. Polished Ti sheets with mirror-like surface as well as unpolished Ti foils were considered in this work. Grains with a wide range of crystallographic orientations and sizes were revealed by Electron Backscatter Diffraction (EBSD) and correlated with nanotube growth on both types of substrates. A preferred grain orientation with [0001] axis perpendicular to the surface was observed on all substrates. Surfaces of all substrates were anodized for 18h in ethylene glycol electrolytes containing 88mM NH4F and 1.5% water and thoroughly inspected by SEM. By a precise comparison of Ti substrates before and after anodization, the uniformity of produced self-organized TiO2 nanotube layers was evaluated in regard to the specific orientation of individual grains. Grains with [0001] axis perpendicular to the surface turned out to be the most growth-promoting orientation on polished substrates. No orientation was found to be strictly growth-retarding, but sufficient anodization time (24h) was needed to obtain uniform nanotube layers on all grains without remnant porous initial oxide. In contrast with polished Ti sheets, no specific orientation was found to significantly promote or retard the nanotube growth in the case of unpolished Ti foils. Finally, the difference between the average nanotube diameters of nanotubes grown on various grains was investigated showing non-negligible differences in the diameter for different grain orientations and substrates.

Bioactivity of self-organized TiO2 nanotubes used as surface treatment on Ti biomaterials

Titanium and its alloys are widely used as implants due to their excellent mechanical properties, corrosion resistance and biocompatibility. TiO2 nanotubes have been studied as surface treatment to increase the specific area and to improve osseointegration. However, the thermodynamic stability and bioactivity of these nanostructures must be evaluated. The objective of this research was to obtain nanotubes oxides on Ti6Al4V alloy and to analyze the electrochemical stability in physiological solution at 37 °C and the bioactive response of the biomaterial. The nanotubes were obtained by potentiostatic anodization. The morphology of the oxides was evaluated by scanning electron microscopy. The chemical characterization was analyzed by energy dispersive spectroscopy and x-ray photoelectron spectroscopy techniques. The electrochemical stability was analyzed by open circuit potential (OCP) and the bioactivity by biomimetic test in a simulated body fluid (SBF) solution. The OCP of the nanotubes oxides was shown to be more noble and stable than the compacted oxides. The biomaterial covered with theses oxides showed sealing by Ca and P after 30 d immersion in artificial blood. And after 15 d of immersion in SBF, the hydroxyapatite could be seen on the non-sealed nanotubes. TiO2 nanotube layers could improve the superficial chemical stability and also the osseointegration process.

Inverted polymer solar cells with employing of electrochemical-anodizing synthesized TiO2 nanotubes

An inverted structure of polymer solar cells based on Poly(3-hexylthiophene)(P3HT):[6-6] Phenyl-(6) butyric acid methyl ester (PCBM) with using thin films of TiO2 nanotubes and nanoparticles as an efficient cathode buffer layer is developed. A total of three cells employing TiO2 thin films with different thickness values are fabricated. Two cells use layers of TiO2 nanotubes prepared via self-organized electrochemical-anodizing leading to thickness values of 203 and 423.7 nm, while the other cell uses only a simple sol–gel synthesized TiO2 thin film of nanoparticles with a thickness of 100 nm as electron transport layer. Experimental results demonstrate that TiO2 nanotubes with these thickness values are inefficient as the power conversion efficiency of the cell using 100-nm TiO2 thin film is 1.55%, which is more than the best power conversion efficiency of other cells. This can be a result of the weakness of the electrochemical anodizing method to grow nanotubes with lower thickness values. In fact as the TiO2 nanotubes grow in length the series resistance (Rs) between the active polymer layer and electron transport layer increases, meanwhile the fill factor of cells falls dramatically which finally downgrades the power conversion efficiency of the cells as the fill factor falls.

Ellipsometric and mechanical characterization of nanostructured anodic oxide film formed on Ti-6Al-7Nb alloy

In this work, we grow TiO2 nanotube layers by using the single-step direct anodization of Ti-6Al-7Nb alloy in aqueous electrolytes containing F− ions. Nanotube layers are characterized by spectroscopic ellipsometry (SE) and field-emission gun scanning electron microscopy (FEG-SEM). We also use SE to monitor the anodization process for TiO2 nanotube layers on biocompatible Ti-6Al-7Nb alloy. In addition, we study mechanical properties by nanoindentation.

Wettability Switch of Anodic Titanium Dioxide Nanotubes with Various Diameters

Surface topography and physicochemical properties such as adhesion, cohesion and wettability is presumed to alter/facilitate the protein binding, cell adhesion and proliferation, thereby reducing post-operative complications with increased lifespan of biomedical implants.1 Current study examines the ageing behaviour of titanium dioxide (TiO2) nanotubes with various diameters and their switch in wettability by gaseous plasma treatment.Self-organized TiO2 nanotube layers were fabricated according to the electrochemical anodization method published earlier.2 The ethylene glycol based electrolytes were used for growing the nanostructures which contains specific amount of water and specific concentration of hydrofluoric acid. Anodization parameters such as applied voltage as well as experimental time were optimized to get desired diameters (15nm, 50nm and 100nm) of TiO2 nanotubes. As-anodized surfaces are hydrophilic and tend to age with time to more hydrophobic. Water contact angle measurements were performed during seven weeks period to study ageing behaviour as well as switch in wettability after gaseous plasma treatment. All the contact angle measurements were performed with demineralized water. The contact angle was measured by fitting the water drop on the surface. Surface wettability indeed switched after plasma treatment as the surface became hydrophilic. Changes in wettability after ageing were explained by organic contamination from ambient atmosphere, while hydrophilic character was obtained after plasma treatment due to removal of these contaminants. Moreover storing the samples in sealed container also influenced on their ageing profile.This current work provides solution and guidelines by which switch in wettability of TiO2 nanotubes can be achieved and can be employed in medical applications where hydrophilic character is desired for specific application, such as orthopedic implants.

Pt-Decorated g-C3N4/TiO2 Nanotube Arrays with Enhanced Visible-Light Photocatalytic Activity for H2 Evolution.

Aligned TiO2 nanotube layers (TiNTs) grown by self‐organizing anodization of a Ti‐substrate in a fluoride‐based electrolyte were decorated with graphitic‐phase C3N4 (g‐C3N4) via a facile chemical vapor deposition approach. In comparison with classical TiO2 nanotubes (anatase), the g‐C3N4/TiNTs show an onset of the photocurrent at 2.4 eV (vs. 3.2 eV for anatase) with a considerably high photocurrent magnitude in the visible range. After further decoration with Pt nanoparticles, we obtained a visible‐light responsive platform that showed, compared with g‐C3N4‐free TiNTs, a strong enhancement for photoelectrochemical and bias‐free H2 evolution (15.62 μLh−1 cm−2), which was almost a 98‐fold increase in the H2 production rate of TiNTs (0.16 μLh−1 cm−2). In a wider context, the g‐C3N4‐combined 3 D nanoporous/nanotubular structure thus provides a platform with significant visible‐light response in photocatalytic applications.

Fabrication of TiO2@MnO2 nanotube arrays by pulsed electrodeposition and their application for high-performance supercapacitors

The fabrication of TiO2@MnO2 nanotube arrays (NTAs) structure is accomplished via a pulsed electrodeposition technique. The successful preparation of this structure is owing to the high current pulse used, which creates a sufficient nucleation trend on the wall of the entire TiO2 nanotube layer, that results in the continuous deposition of MnO2 along the nanotube walls. The synthesized TiO2@MnO2 NTAs sample is found to exhibit excellent electrochemical behavior with high specific capacitance of 425.0Fg−1 and areal capacitance of 150.9mFcm−2, ranking in the top level ever achieved by a MnO2-TiO2 composite. Furthermore, a symmetric supercapacitor device is assembled using two strips of the TiO2@MnO2 NTAs electrode that yielded energy density as high as 7.4Whkg−1, a much higher value than most of the values reported for symmetric MnO2//MnO2 supercapacitors. The remarkable electrochemical properties of the TiO2@MnO2 NTAs sample make it an attractive candidate for the application of constructing high-performance supercapacitors.

Free‐Standing Membranes to Study the Optical Properties of Anodic TiO2 Nanotube Layers

In the present work we investigate various optical properties (such as light absorption and reflectance) of anodic TiO2 nanotube layers directly transferred as self-standing membranes onto quartz substrates. This allows investigation in a transmission geometry which provides significantly more reliable data than measurements on the metallic Ti substrate. Light transmission and reflectance measurements were carried out for layers of thickness varying from 1.8 to 50 μm, and the layers were investigated in their amorphous and crystalline forms. A series of wavelength-dependent light attenuation coefficients are extrapolated and found to match the photocurrent versus irradiation wavelength behavior. A feature specific to anodic nanotubes is that their intrinsic carbon contamination content causes a proportional sub-bandgap response. Overall, the extracted data provide a valuable basis and understanding for the design of photo-electrochemical devices based on TiO2 nanotubes.

Enhanced osteogenic activity and anti-inflammatory properties of Lenti-BMP-2-loaded TiO₂ nanotube layers fabricated by lyophilization following trehalose addition.

To enhance biocompatibility and osseointegration between titanium implants and surrounding bone tissue, numerous efforts have been made to modify the surface topography and composition of Ti implants. In this paper, Lenti-BMP-2-loaded TiO2 nanotube coatings were fabricated by lyophilization in the presence of trehalose to functionalize the surface. We characterized TiO2 nanotube layers in terms of the following: surface morphology; Lenti-BMP-2 and trehalose release; their ability to induce osteogenesis, proliferation, and anti-inflammation in vitro; and osseointegration in vivo. The anodized TiO2 nanotube surfaces exhibited an amorphous glassy matrix perpendicular to the Ti surface. Both Lenti-BMP-2 and trehalose showed sustained release over the course of 8 days. Results from real-time quantitative polymerase chain reaction studies demonstrated that lyophilized Lenti-BMP-2/TiO2 nanotubes constructed with trehalose (Lyo-Tre-Lenti-BMP-2) significantly promoted osteogenic differentiation of bone marrow stromal cells but not their proliferation. In addition, Lyo-Tre-Lenti-BMP-2 nanotubes effectively inhibited lipopolysaccharide-induced interleukin-1β and tumor necrosis factor-α production. In vivo, the formulation also promoted osseointegration. This study presents a promising new method for surface-modifying biomedical Ti-based implants to simultaneously enhance their osteogenic potential and anti-inflammatory properties, which can better satisfy clinical needs.

Feasibility of a Chronic Foreign Body Infection Model Studying the Influence of TiO<sub>2</sub> Nanotube Layers on Bacterial Contamination

Bacterial infections on the surface of medical devices are a significant problem in therapeutic approach, especially when implants are used in the living. In cardiology, pacemaker generator pocket surfaces, made in titanium alloy can be colonized by pathogen microorganism. This contamination represents a major risk of sepsis, endocarditis and localized infections for patients. A way to limit this bacterial contamination is to modify the surface topography using nano-structuration process of the titanium alloy surface of the implanted devices. The aim of this study is to evaluate the influence of TiO2 nanotube layers on bacterial infection in the living, considering the feasibility of an animal model of chronic foreign body infection. TiO2 nanotube layers prepared by electrochemical anodization of Ti foil in 0.4 wt% hydrofluoric acid solution were implanted subcutaneously in Wistar rats. Three weeks after implantation, TiO2 implants were contaminated by a Staphylococcus epidermilis strain using two different concentrations at 106 and 108 colony forming unit (CFU) in order to induce a sufficient infection level and to avoid unwanted over infection consequences on rats health during the experiments. After 28 days in the living, 75% of nanotube layers initially submitted to the 108 CFU inoculum were contaminated while only 25% nanotube layers initially submitted to the 106 CFU inoculum remained infected. This significant result underlines the influence of TiO2 nanotube layers in decreasing the infection level. Our in vitro experiments showed that the synthesized TiO2 nanotubes indeed decreased the Staphylococcus epidermilis adhesion compared to unanodized Ti foil.

Effect of TiO2 Nanotube Layers Thickness on Periodontal Ligament Cells

Periodontal ligament cells (PDLCs) remaining in the tooth-extraction socket are involved in osseointegration after immediate implantation; however, their interaction with different implant surfaces has not been investigated. The aim of this study was to compare PDLC growth on substrates composed of differently sized titanium dioxide (TiO2) nanotubes with that on flat Ti. PDLC growth on Ti nanotubes was evaluated in terms of cell adhesion, proliferation, and osteogenic differentiation based on the expression of alkaline phosphatase (ALP), type 1 collage (COL-1), osteopontin (OPN), and Runt-related transcription factor 2 (RUNX2). We found that TiO2 nanotubes of different diameters (30, 70, and 120 nm) manufactured on Ti surface by anodisation had a well-defined structure. PDLCs grown on nanotube layers demonstrated polygonal morphology with more filopodia than those on flat Ti, which was especially evident on the nanotubes of larger diameters. However, cell adhesion and proliferation was the highest on the smallest 30 nm nanotubes. Similarly, mRNA levels of the ALP, COL-1, OPN, and RUNX2 genes increased in PDLCs cultured on 30 nm TiO2 nanotube layers compared to those in the cells grown on 70 nm and 120 nm nanotubes. In conclusion, small diameter (30 nm) nanotube layers can support PDLC adhesion, proliferation, and differentiation better than larger sized nanotubes, and consequently, have higher potential to promote bone formation and integration of an immediate implant.

Impact of Annealing Treatment on the Behaviour of Titanium Dioxide Nanotube Layers

In this work, we study the influence of the annealing treatment on the behaviour of titanium dioxide nanotube layers. The heat treatment protocol is actually the key parameter to induce stable oxide layers and needs to be better understood. Nanotube layers were prepared by electrochemical anodization of Ti foil in 0.4 wt% hydrofluoric acid solution during 20 minutes and then annealed in air atmosphere. In-situ X-ray diffraction analysis, coupled with thermogravimetry, gives us an inside on the oxidation behaviour of titanium dioxide nanotube layers compared to bulk reference samples. Structural studies were performed at 700°C for 12 h in order to follow the time consequences on the oxidation of the material, in sufficient stability conditions. In-situ XRD brought to light that the amorphous oxide layer induced by anodization is responsible for the simultaneous growths of anatase and rutile phase during the first 30 minutes of annealing while the bulk sample oxidation leads to the nucleation of a small amount of anatase TiO2. The initial amorphous oxide layer created by anodization is also responsible for the delay in crystallization compared to the bulk sample. Thermogravimetric analysis exhibits parabolic shape of the mass gain for both anodized and bulk sample; this kinetics is caused by the formation of a rutile external protective layer, as depicted by the associated in-situ XRD diffractograms. We recorded that titanium dioxide nanotube layers exhibit a lower mean mass gain than the bulk, because of the presence of an initial amorphous oxide layer on anodized samples. In-situ XRD results also provide accurate information concerning the sub-layers behavior during the annealing treatment for the bulk and nanostructured layer. Anatase crystallites are mainly localized at the interface oxide layer-metal and the rutile is at the external interface. Sample surface topography was characterized using scanning electron microscopy (SEM). As a probe of the photoactivity of the annealed TiO2 nanotube layers, degradation of an acid orange 7 (AO7) dye solution and 4-chlorophenol under UV irradiation (at 365 nm) were performed. Such titanium dioxide nanotube layers show an efficient photocatalytic activity and the analytical results confirm the degradation mechanism of the 4-chlorophenol reported elsewhere.