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TiO2-based catalyst supports in polymer electrolyte membrane fuel cells (PEMFCs) are widely investigated and provide an alternative to corrosion-susceptible carbon-based catalyst supports. For PEMF...

In the present work, we investigate the efficiency of TiO2 photoelectrode layers on boron doped diamond foil (BDDF) in comparison with a classic conducting glass (FTO) back contact. Crystalline thin TiO2 layers were prepared on the substrates by two different methods: (i) deposition of metallic Ti thin films followed by thermal oxidation to form TiO2 (TO-TiO2), (ii) reactive sputter deposition of TiO2 thin films and crystallization of these layers (SP-TiO2). Optimized layers show that TO-TiO2 films on BDDF deliver a significantly higher incident photon to current efficiency (IPCE) compared to directly sputtered SP-TiO2 layers and these layers on BDDF also outperform conducting glass (FTO) as a back contact. We ascribe this beneficial effect of the BDDF back contact to the formation of an intermediate conductive phase of Ti-carbides at the TO-TiO2/BDDF interface.

In the present work we introduce a simple approach to produce porous PtPdAg alloy nanoparticles on TiO2 nanotubes by a simple combined sputtering-dewetting-dealloying process. In a first step, three individual metals (Pt, Pd and Ag) were sequentially sputter-deposited on spaced TiO2 nanotubes, followed by a thermal dewetting treatment in Ar to convert these metal layers into ternary PtPdAg alloyed particles. Dealloying was carried out by a selective chemical dissolution of Ag in piranha solution that finally generated porous alloyed nanoparticles located on the TiO2 nanotube walls. The porous PtPdAg-decorated TiO2 nanotubes show enhanced photocatalytic H2 generation in comparison with porous Pt (67% increase) or any monometallic decoration (>120% increase). The photocatalytic enhancement is not only due to the increased number of active sites (larger surface area) of the porous particles but also due to the synergistic effects of the different alloying elements.

In this work we describe the synthesis and electrochemical properties of nitridated hierarchical TiO$_2$ nanotubes as an electrode for supercapacitors. The hierarchical TiO$_2$ nanostructures are formed by a controlled layer-by-layer TiO2 nanoparticle decoration on self-organized spaced TiO$_2$ nanotubes. These structures are then annealed in NH$_3$ atmosphere at elevated temperature to convert the material to a nitride structure -- this drastically enhances their electron-transport properties. The areal capacitance of hierarchical structures can be tuned by changing the number of decorated TiO$_2$ nanoparticle layers. The capacitance enhancement of the hierarchical structures reaches a maximum when the surface area through nanoparticle deposition is highest and the conductivity via nitridation is optimized.

In this work, we report on the electrochemical behavior of nitrided spaced TiO$_2$ nanotubes conformally coated with a nanocrystalline Nb$_2$O$_5$ layer and find for these hierarchical structures an excellent supercapacitor performance. Highly aligned conductive 1D electrodes were obtained by a three step process: i) growth of self-organized nanotubes with defined and adjustable intertube spacing, ii) conformal Nb$_2$O$_5$ decoration in the tube interspace (while providing full electrolyte access to the entire active area), and iii) high temperature nitridation. Key is the growth of a nanotube array with regular tube-to-tube interspacing that enables an optimized decoration with secondary materials such as Nb$_2$O$_5$. We observe an increase in electrode capacitance from 158 $\mu$F cm-2 for bare TiO$_2$ NTs, to 1536 $\mu$F cm-2 for TiO$_2$/Nb$_2$O$_5$ NTs, to finally 37 mF cm-2 for Nb$_2$O$_5$ decorated and then nitrided nanotubes. This drastic increase can be ascribed firstly to the defined spacing established between the tube arrays that then allows for a conformal coating with a secondary active coating. Secondly, nitridation causes a drastic increase of the electron conductivity of the entire scaffold and thus reduces resistive losses.

In the present work, we investigate two distinctly different anodic TiO2 nanotubular morphologies, spaced and close-packed arrays. While the close-packed tubular arrays are formed in ethylene glycol, spaced NTs, which have a regular gap between individual nanotubes, grow in a diethylene glycol or dimethyl sulfoxide based electrolyte. Depending on the electrolyte used for anodization, the morphology, crystal structure and chemical composition of the resulting nanotubular layer vary from one another. This influences the electrochemical and photoelectrochemical activity of nanotubes. Overall, we find that spaced NTs not only can provide enhanced charge transfer characteristics but also can show beneficial light absorption characteristics when used as a photo or light harvesting electrode.

Self-organized TiO2 nanotube arrays can be grown under a wide range of electrochemical conditions. In the present work, we evaluate the occurrence of spacing between tubes and the connection of this effect to organization of tubes on two-size scales. The results show that tube-spacing is initiated in the very early stages of anodization between individual pore morphologies. Furthermore, the spacing, as well as the organization on two-size scales can be controlled by changing the anodization conditions, e.g., electrolyte composition, applied voltage and temperature. Namely, adjustment of H2O content, electrode temperature and voltage can lead to spaced nanotubes, and allow to control spacing. Finally, we draw conclusions on the possible mechanism relevant to the growth of spaced tubes.

In the present work, we introduce a technique to achieve rapid growth of self-ordered anodic nanotubes with a well-defined tube-to-tube spacing (spaced tubes) and single-wall morphology. By optimizing the anodization conditions (electrolyte, temperature, etc.), the growth rate of spaced tubes can be ≈25 times faster than in conventional approaches while maintaining a tube-to-tube spacing of ≈100 nm. We show that the origin of the tube-to-tube spacing is self-ordering of nanotubes on two different scales – the primary large tubes are embedded in a matrix of secondary, very short nanotubes with a small diameter. Preferential etching of the small tubes during anodic growth leaves behind an ordered array of spaced individual tubes with a well-defined tube-to-tube spacing. Keywords: Anodization, TiO2 nanotubes, Two-size scale, Fast-growth, DMSO

Titanium dioxide (TiO2) nanotube coated substrates have revolutionized the concept of implant in a number of ways, being endowed with superior osseointegration properties and local drug delivery capacity. While accumulating reports describe the influence of nanotube diameter on cell behavior, little is known about the effects of nanotube lateral spacing on cells involved in bone regeneration. In this context, in the present study the MC3T3-E1 murine pre-osteoblast cells behavior has been investigated by using TiO2 nanotubes of ~78 nm diameter and lateral spacing of 18 nm and 80 nm, respectively. Both nanostructured surfaces supported cell viability and proliferation in approximately equal extent. However, obvious differences in the cell spreading areas, morphologies, the organization of the actin cytoskeleton and the pattern of the focal adhesions were noticed. Furthermore, investigation of the pre-osteoblast differentiation potential indicated a higher capacity of larger spacing nanostructure to enhance the expression of the alkaline phosphatase, osteopontin and osteocalcin osteoblast specific markers inducing osteogenic differentiation. These findings provide the proof that lateral spacing of the TiO2 nanotube coated titanium (Ti) surfaces has to be considered in designing bone implants with improved biological performance.

We report on how to grow and control self-organized TiO2 nanotube arrays that show defined and regular gaps between individual nanotubes. For this we use electrochemical anodization of titanium in fluoride containing di-ethylene glycol (DEG) electrolytes, with variations in voltage and water content in the electrolyte. In these specific electrolytes, such nanotubes show a true spacing, i.e. nanotubes are spaced both at top and at bottom in regular intervals, this in contrast to classic nanotubes obtained in other organic electrolytes showing a close-packed organization. We identify critical parameters that define the “region of existence” i.e. under which condition tube spacing occurs as well as the intertube distance, to be the voltage and the water content. Using these findings allows to grow tubes where diameter and spacing can even be independently controlled. Keyword: Spaced TiO2 nanotubes, Anodization, TiO2 arrays

During self-organized nanotubular oxide growth by electrochemical anodization of Ti in lactic acid (LA) containing dimethyl sulfoxide (DMSO) electrolytes, we observe the occurrence of regular current oscillations (COs). Key factors that influence COs are the LA concentration, the surface roughness and diffusion limitation. The oscillation characteristics (amplitude and period) depend on both LA concentration and applied voltage. Under optimized oscillation conditions a sequential growth/detachment of the TiO2 layers is observed that leads to a self-stacking of TiO2 nanostructured multilayers. Keywords: Anodization, TiO2 nanotubes, Electrochemical oscillations, Multilayers

In the present work we grow self-organized TiO2 nanotube arrays with a defined and controlled regular spacing between individual nanotubes. These defined intertube gaps allow one to build up hierarchical 1D-branched structures, conformally coated on the nanotube walls using a layer by layer nanoparticle TiO2 decoration of the individual tubes, i.e. having not only a high control over the TiO2 nanotube host structure but also on the harvesting layers. After optimizing the intertube spacing, we build host–guest arrays that show a drastically enhanced performance in photocatalytic H2 generation, compared to any arrangement of conventional TiO2 nanotubes or conventional TiO2 nanoparticle layers. We show this beneficial effect to be due to a combination of an increased large surface area (mainly provided by the nanoparticle layers) with a fast transport of the harvested charge within the passivated 1D nanotubes. We anticipate that this type of hierarchical structures based on TiO2 nanotubes with adjustable spacing will find even wider application, as they provide an unprecedented controllable combination of surface area and carrier transport.

In the present work, we report the use of TiO2 nanotube (NT) layers with a regular intertube spacing that are decorated by Pt nanoparticles through the atomic layer deposition (ALD) of Pt. These Pt‐decorated spaced (SP) TiO2 NTs are subsequently explored for photocatalytic H2 evolution and are compared to classical close‐packed (CP) TiO2 NTs that are also decorated with various amounts of Pt by using ALD. On both tube types, by varying the number of ALD cycles, Pt nanoparticles of different sizes and areal densities are formed, uniformly decorating the inner and outer walls from tube top to tube bottom. The photocatalytic activity for H2 evolution strongly depends on the size and density of Pt nanoparticles, driven by the number of ALD cycles. We show that, for SP NTs, a much higher photocatalytic performance can be achieved with significantly smaller Pt nanoparticles (i.e. for fewer ALD cycles) compared to CP NTs.

In the present work, we report on the use of organized TiO

Regularly spaced TiO2 nanotubes were prepared by anodizing a titanium substrate in triethylene glycol electrolyte at elevated temperature. In comparison to conventional TiO2 nanotubes, spaced nanotubes possess an adjustable spacing between the individual nanotubes; this allows for controlled buildup of a hierarchical nanoparticle‐on‐nanotube structure. Here, we use this principle for layer‐by‐layer decoration of the tubes with TiO2 nanoparticles. The hierarchical structure after N doping and NH3 treatment at 450 °C shows a significant enhancement of visible‐light absorption, although it only carries a low doping concentration of nitrogen. For optimized N‐doped and particle‐decorated spaced TiO2 nanotubes, a considerable improvement in photocatalytic activity is obtained in comparison with conventional N‐doped TiO2 nanotubes or comparable N‐doped nanoparticle films. This is attributed to an enhanced visible‐light absorption through the N‐doped nanoparticle shell and a fast charge separation between the shell and the one‐dimensional nanotubular core.

The present review gives an overview of the highlights of more than 10 years of research on synthesis and applications of ordered oxide structures (nanotube layers, hexagonal pore arrangements) that are formed by self-organizing anodization of metals. In particular we address the questions after the critical factors that lead to the spectacular self-ordering during the growth of anodic oxides that finally yield morphologies such as highly ordered TiO2 nanotube arrays and similar structures. Why are tubes and pores formed—what are the key parameters controlling these processes? Keywords: TiO2 nanotube, Self-organization, Anodization, Key parameters

In the present work we grow self-organized TiO

In the present work, we report on the use of organized TiO2 nanotube (NT) layers with a regular intertube spacing for the growth of highly defined α-Fe2O3 nano-needles in the interspace. These α-Fe2O3 decorated TiO2 NTs are then explored for Li-ion battery applications and compared to classic close-packed (CP) NTs that are decorated with various amounts of nanoscale α-Fe2O3. We show that NTs with tube-to-tube spacing allow uniform decoration of individual NTs with regular arrangements of hematite nano-needles. The tube spacing also facilitates the electrolyte penetration as well as yielding better ion diffusion. While bare CP NTs show a higher capacitance of 71 μAh cm−2 compared to bare spaced NTs with a capacitance of 54 μAh cm−2, the hierarchical decoration with secondary metal oxide, α-Fe2O3, remarkably enhances the Li-ion battery performance. Namely, spaced NTs with α-Fe2O3 decoration have an areal capacitance of 477 μAh cm−2, i.e. they have nearly ~8 times higher capacitance. However, the areal capacitance of CP NTs with α-Fe2O3 decoration saturates at 208 μAh cm−2, i.e. is limited to ~3 times increase.