Sergiu P. Albu

Bamboo-Type TiO2 Nanotubes : Improved Conversion Efficiency in Dye-Sensitized Solar Cells

Bamboo-type TiO2 nanotube layers were produced by alternating voltage anodization of Ti. In comparison to smooth TiO2 morphologies the stratified nanotube structure shows significantly increased light harvesting and conversion efficiencies when used in dye sensitized solar cells.

Morphological instability leading to formation of porous anodic oxide films

Electrochemical oxidation of metals, in solutions where the oxide is somewhat soluble, produces anodic oxides with highly regular arrangements of pores. Although porous aluminium and titanium oxides have found extensive use in functional nanostructures, pore initiation and self-ordering are not yet understood. Here we present an analysis that examines the roles of oxide dissolution and ionic conduction in the morphological stability of anodic films. We show that patterns of pores with a minimum spacing are possible only within a narrow range of the oxide formation efficiency (the fraction of oxidized metal atoms retained in the film), which should exist when the metal ion charge exceeds two. Experimentally measured efficiencies, over diverse anodizing conditions on both aluminium and titanium, lie within the different ranges predicted for each metal. On the basis of these results, the relationship between dissolution chemistry and the conditions for pore initiation can now be understood in quantitative terms.

Growth of Aligned TiO2 Bamboo-Type Nanotubes and Highly Ordered Nanolace†

Synthesis of carbon nanotubes by Iijima in 1991 stimulated intense research activity worldwide due to the anticipated technological impact of this unique combination of material, directionality, and dimension that could be exploited to enhance a plethora of materials properties. In the following years the chemical synthesis of various non-carbon nanotubes, in particular transition metal oxide nanotubes such as TiO2 (titanate) and V2O5 was achieved, mainly by using hydrothermal techniques. More recently a simple, cheap, and straightforward approach was reported that leads to ordered TiO2 nanotube arrays that grow self-aligned during anodization of Ti. The layers can be grown with tube dimensions varying from a few micrometers in length up to several hundreds of micrometers, and with diameters varying from about 10 to 200 nm. These layers of self-organized oxide nanotube structures, which are attached to the Ti substrate or suspended as membranes have created significant interest due to their anticipated impact in various applications. In particular, the properties of the material have been tailored for solar-energy conversion with dye sensitization or doping and display applications with enhanced electrochromic switching. Other reports focused on use as a photocatalyst 21] or in sensing, and due to the high biocompatibility of TiO2, interactions of biological cells with the tubular surface or enhanced bone formation on the material were studied. These oxide nanotubes are grown by anodization of a Ti metal sheet in dilute fluoride electrolytes at a certain voltage for a given time. Typically the tube diameter is controlled by the applied anodization voltage, and the length can be varied by means of the anodization time and by using different electrolytes. Anodization under constant-voltage conditions leads to an ordered layer consisting of a distinct morphology of smooth tubes with defined cylindrical or hexagonal cross section. 26] Herein we show how novel morphologies such as bamboo-type reinforced nanotubes and two-dimensional (2D) nanolace sheets can be obtained if the anodization process is carried out under specific alternating-voltage (AV) conditions. Figure 1 shows an examples of smooth and bamboo-type structures. The key to this approach is exploiting the fact that, during the initial stages of anodization, two fundamentally different oxide morphologies grow successively on the Ti surface. In the first stage, a compact anodic oxide is formed, and only after a certain period of time does the growth of ordered nanotubular structures take place. During the initial growth phase, pH and diffusion gradients are established that finally allow a tubular morphology to grow. 9] If this system is altered by a step to lower voltage, the concentration profiles will be adjusted to less steep gradients. Therefore, a subsequent step to the higher values will again lead to formation of an Figure 1. SEM images of anodically grown nanotube layers of 10-mm thickness. a) With smooth walls, grown under constant-voltage conditions (120 V). b) Bamboo-type tubes, grown under AV conditions, with a sequence of 1 min at 120 V and 5 min at 40 V. c) As in (b) but with an AV sequence of 2 min at 120 V and 5 min at 40 V. The left image shows the whole layer, and the right image a magnification of the tubes. The inset in Figure 1a (right) shows the initial compact layer formed at short times (30 s) at 120 V. d) Higher magnification of the tube reinforcement; the right image shows a cross section through an opened bamboo structure.

TiO2 Nanotubes in Dye‐Sensitized Solar Cells: Critical Factors for the Conversion Efficiency

Particle vs tube: The present paper systematically investigates a range of fundamental geometrical and structural features of TiO(2) nanotube layers and their effect on the dye-sensitized solar cell conversion efficiency, to deduce the most promising strategies for improvement. It is found that the performance of the cells strongly depends on the morphology and crystalline structure of the nanotubes.

High-contrast electrochromic switching using transparent lift-off layers of self-organized TiO2 nanotubes.

Electrochromic devices rely on the electric switchability of the spectral absorption behavior of a thin active layer on a transparent substrate. To produce a system that is fully transparent in the unbiased state, usually the active layer is sandwiched between a conductive glass substrate and a transparent conductive top electrode. Active materials that are used for electrochromic switching are, for example, WO3, [1,2] Nb2O5, [3] and TiO2. [4,5] These are wide-bandgap semiconductors with a lattice structure that allows easy field-aided proton intercalation. The intercalation of protons (or other small ions) leads to additional electronic states in the bandgap of the material, which in turn affect the optical properties of the material to cause light absorption in the visible spectral range. The intercalation of ions is typically limited to a very thin surface layer of the host material. To achieve a substantial contrast it is therefore required to create an electrochromic structure with a multifold and high surface area. This is usually achieved by using porous structures prepared from compacted TiO2 nanoparticles, [6] such as commercially available Degussa P25, which is sintered to a porous layer on conductive glass. Herein, we report the production of lift-off layers of anatase vertically oriented TiO2 nanotubes (NTs) that are transparent and can be transferred onto conductive glass, and demonstrate their application in high-contrast electrochromic display devices, as depicted schematically in Figure 1a. Earlier work showed that self-organized TiO2-NT layers (see Figure 1b and c) can be grown by using simple electrochemical anodization of Ti in F-containing electrolytes under optimized conditions. Thus, the NT geometrical parameters, such as tube diameter, length, and wall thickness, can be controlled by fine-tuning the electrochemical growth parameters. The resulting TiO2-NT layers have been used in various applications: the tube layers show a high photoconversion efficiency, enhanced electrochromicity, remarkable biocompatibility, and catalytic activity. Moreover, freestanding TiO2-NT layers with both sides open

Multilayer TiO2–Nanotube Formation by Two-Step Anodization

In this work we report on the growth of a closely stacked double layer of a self-organized TiO 2 nanotubes. For that we first anodize Ti in acidic electrolyte containing hydrofluoric acid to form thin nanotube layers. Afterwards we start a second anodization in a different electrolyte, a glycerol/NH 4 F mixture. This procedure allows us to grow the second layer directly underneath the first one. From scanning electron microscopy and transmission electron microscopy investigations we revealed that the second growth occurs via the tube bottoms of the first layer. These stacked multilayers generate new possibilities to vertically tailor the properties of the self-organized TiO 2 nanotube layers.

Enabling the Anodic Growth of Highly Ordered V2O5 Nanoporous/Nanotubular Structures

In 1995 Masuda and Fukuda demonstrated that highly ordered, self-organized porous alumina structures can electrochemically be grown by anodizing aluminium in an oxalic acid electrolyte under a set of optimized electrochemical conditions. This initiated a large amount of follow-up work that used these structures either directly (e.g. as filters or photonic materials) or indirectly as a template for the deposition of a wide range of materials as nanowires, nanorods, or nanotubes. Self-organized porous oxide growth was believed to be constrained to alumina, until in 1999 Zwilling et al. introduced the growth of self-organized TiO2 nanotubes from Ti electrodes when anodized in a fluoride-containing electrolyte. In the following years, the “dilute” fluoridebased electrolytes were refined, not only to allow for an ever increasing control of the TiO2 nanotube geometry, [4] but dilute fluoride solutions were also found to be extremely versatile to grow highly ordered anodic nanotubes or nanoporous layers on other metals such as Zr, Hf, Nb, Ta, and a wide range of alloys. Common to all these anodic oxide growth procedures is that water is used as a source for oxide formation and fluorides are used to solubilized excess cations—this establishes a formation–dissolution steady-state situation. To achieve self-organizing oxide tubes or pore-growth conditions, the H2O and F contents in the electrolyte need to be optimized. A difficulty is that the dilute fluoride solutions also lead to chemical etching of the generated oxide structure, that is, for optimized conditions the chemical resistance of the formed oxide against fluoride and H2O etching may become crucial. This is no problem for oxides such as Ta2O5 or Nb2O5 (and only mildly for TiO2), but an extremely high etching susceptibility prevented (in spite of many attempts) the growth of defined ordered anodic layers from one of the most important transition-metal oxides, V2O5. Here, we show how to overcome this problem by using complex fluoride electrolytes such as [BF4] or [TiF6] 2 , which allow for the first time, to successfully grow self-organized nanoporous and nanotubular V2O5 structures. This is of special significance as V2O5 is one of the most investigated transition-metal oxides because of its application in catalysis, lithium batteries, electrochromics, and sensors. For many of these applications nanoscale geometries bear significant advantages in view of electronic, magnetic, catalytic, and ion intercalation properties. Up to now the synthesis of V2O5 nanotubes was mainly achieved by hydrothermal treatments, which yield randomly oriented assemblies (a nanotube powder). The key challenge for the preparation of self-organized V2O5 nanotubes or any vanadium oxide by electrochemical techniques is the instability of vanadium oxide in any watercontaining electrolyte and the ease of formation of highly soluble complexes with a wide range of anions. Some work has shown the feasibility to grow compact layers or films of anodic vanadium oxide in specific nonaqueous electrolytes. But over the past few years numerous attempts failed to anodically grow V2O5 nanotubes or ordered porous layers. [10] Virtually any electrolyte that is typically used for fabricating other transition-metal oxide nanotubes or ordered pore arrays was explored but failed (an overview of such attempts is compiled in the Supporting Information, Table S1). Here, we demonstrate that using complex fluoride salt electrolytes such as [TiF6] 2 and [BF4] enable self-organized anodization. In a first approach, we formed [TiF6] 2 species by dissolving pure titanium in HF and then dissolving this solution in ethylene glycol which was then used for anodization. After parameter screening for the dissolved Ti content, HF concentration, water content, and applied voltage we established conditions to grow highly ordered nanoporous and nanotubular vanadium oxide structures as shown in Figure 1. The vertically aligned vanadium oxide nanoporous layer shown in Figure 1a has a thickness of 13 mm and a pore diameter around 15 nm and was fabricated by anodization of a vanadium foil in an ethylene glycol (EG) containing 0.2m HF and 300 ppm Ti electrolyte at 120 V for 2 h. By extending the anodization duration to 24 h, a tubular structure as shown in Figure 1b can be formed (the top-view SEM images of typical tubular structures versus porous structures are given in Figure S1 in the Supporting Information). From the thickness–time curve shown in Figure 2a, it is found that a steady increase of oxide growth takes place and, for example, after 12 h of anodization a highly ordered nanoporous/tubular layer of approximately 45 mm thickness can be achieved (see Figure S2 in the Supporting Information). The growth rate of the porous layer becomes slower with extended anodization duration, which is a typical for self-organized anodic layers. We investigated the influence of the [TiF6] 2 concentration in the range from 200 (equivalent to 4.7 mm [TiF6] 2 ) to 1500 ppm Ti (equivalent to 35 mm [TiF6] 2 ). An etched layer [*] Dr. Y. Yang, S. P. Albu, Dr. D. Kim, Prof. Dr. P. Schmuki Department of Materials Science and Engineering, WW4-LKO University of Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen (Germany) E-mail: schmuki@ww.uni-erlangen.de Homepage: http://www.lko.uni-erlangen.de

Current dependent formation of PEDOT inverse nanotube arrays

PEDOT nanostructures with distinct patterns can be formed by means of electrodeposition in titania nanotubes. The deposition process critically depends on the current protocol, leading to selective filling of the inner and/or outer space of the tubes. The approach allows for synthesis of vertically aligned PEDOT–titania composites or polymer nanostructures in the form of PEDOT inverse-nanotube-arrays and PEDOT nanopore-arrays.

Excited state properties of anodic TiO2 nanotubes

Charge carriers, that is, holes as well as trapped and “free” electrons were investigated by means of time resolved spectroscopy in anodic TiO2 nanotubes that were heat treated at different temperatures. The lifetimes of the charge carrier were compared with those generated in reference layers of Solaronix Ti-Nanoxide D and Degussa P25 nanoparticles. Remarkably long lived “free” electrons were only noted in the TiO2 nanotubes. These findings have significance in view of any photoelectrochemical applications of TiO2 nanotubes.

Transport properties of single TiO2 nanotubes

We investigated the electric transport properties of single TiO2 nanotubes separated from an anodic titania nanotube array. The temperature dependence of the resistance measured with the conventional four point method of all investigated samples show a Mott variable range hopping behavior. The results obtained with two contacts indicate the existence of a potential barrier between the Cr/Au contacts and samples surfaces, which influence is clearly observable at temperatures <150 K. Impedance spectroscopy in the frequency range of 40 Hz to 1 MHz carried out at room temperature indicates that the electronic transport of these polycrystalline tubes is dominated by the grain cores.