Doohun Kim

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.

TiO2-WO3 composite nanotubes by alloy anodization: growth and enhanced electrochromic properties.

The present work demonstrates that uniform and highly ordered arrays of TiO(2)-WO(3) nanotubes can be grown by anodization of Ti alloys in an ethylene glycol/fluoride based electrolyte under selected electrochemical conditions. These aligned mixed oxide nanotube structures are highly suitable for enhanced electrochromic reactions; in particular we show that already small amounts of WO(3) (such as 0.2 at%) present in the tube oxide drastically improve the electrochromic properties (contrast, onset potential, cycling stability) of nanotube layer based devices.

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.

Anodic Formation of Thick Anatase TiO2 Mesosponge Layers for High-Efficiency Photocatalysis

We report a process for the fabrication of an anatase TiO(2) mesosponge (TMS) layer by an optimized Ti anodization process in a hot glycerol electrolyte followed by a suitable etching process. Such layers can easily be grown to >10 microm thickness and have regular channels and structural features in the 5-20 nm range. The layers show high photocatalytic activity and are mechanically very robust. The layers therefore open new pathways to the wide field of TiO(2)(anatase) applications.

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.

Formation of a Non-Thickness-Limited Titanium Dioxide Mesosponge and its Use in Dye-Sensitized Solar Cells

In 1998, Melody et al. introduced an electrochemical anodization process that was reported to lead to so-called nonthickness-limited (NTL) oxide growth for some refractory metals (in particular Ta). By anodization of Ta at 150–180 8C in glycerol/K2HPO4 solutions, oxide layers several tens of micrometers thick could be grown without observing a drop in the growth rate with time. In further work, this process was used with Ti, but was not successful. Herein, we show that by anodizing Ti under adequate conditions, a non-thicknesslimited oxide can indeed be grown. Moreover, by a subsequent selective etching treatment of these layers, a connected, ordered, and mesoporous TiO2 network can be obtained and is suitable for application in high-efficiency dye-sensitized solar cells. Over the past 30 years, TiO2 has attracted wide interest from both the scientific and the technological communities because of its high number of potential applications. In particular, its unique electronic properties make the material attractive, for example for photocatalysis, as well as for solar energy conversion. For this type of application, a large specific surface area is required, and therefore efficient photovoltaic or catalytic electrodes are commonly based on sintered or compacted anatase nanoparticles. In Gr tzel-type solar cells, particle sizes are typically around 10 nm, which are assembled to approximately 10 mm thick porous layers by doctor-blading or spin-coating approaches. The layers then are sensitized with a suitable dye and mounted into various solar cell configurations. 10, 11,16] Another versatile technique for the production of defined oxide layers is anodization of a suitable metal substrate. However, in the case of Ti, anodic layers of TiO2 are formed under most conditions with a compact morphology that is typically limited to a thickness of some 100 nm. To date, only the anodic growth of TiO2 nanotube layers seemed promising for the production of nanostructured geometries that have thicknesses in the 10 mm range and are interesting for solar cell applications. Herein, we introduce an anodization and selective etching approach to form a robust and ordered mesoporous TiO2 network (TiO2 mesosponge) that is tens of micrometers thick and is formed directly on a Ti substrate. Figure 1 shows a SEM cross section of a 15–18 mm thick layer of TiO2 grown by anodization of a Ti sheet in 10 wt% K2HPO4 in glycerol at (180 1) 8C. It was crucial to carefully optimize the experimental conditions in order to achieve the growth of such layers. In particular, the temperature, preanodization time, and anodization current have to be accurately controlled (details are given in the Supporting Information). Under the optimized conditions and after extended anodization, such layers can be grown to thicknesses greater than 50 mm. If the conditions are not sufficiently maintained, only compact or nanoporous oxide layers of some 100 nm thickness could be observed. Thick layers, as shown in Figure 1, have a comparably tight oxide morphology with some nanoscopic channels that are apparent in SEM (scanning electron microscopy; Figure 1b) and TEM (Figure 1c,d) images. The SEM image in Figure 1b shows that the channels typically have a preferred orientation perpendicular to the surface. This orientation is further confirmed by the TEM image (Figure 1c) and the HRTEM image (Figure 1d), which were taken in plan-view geometry and show most of the channels in an edge-on orientation. The width of these pores is in the range 5–10 nm. A main drawback in terms of applications is that the spacing between the nanoscopic channels is wide (ca. 20–50 nm), as apparent from the SEM inset in Figure 1b. However, if this structure is adequately chemically etched, a highly regular and defined sponge structure as shown in the SEM image of Figure 1 e, f is obtained. This structure was etched for 1 h in 30 wt % H2O2 under ultrasonication. A very regular mesoporous morphology is obtained over the entire sample thickness (Figure 1e), with typical TiO2 sizes in the range 5–10 nm and pores of approximately 10 nm (Figure 1 f). In comparison with the “asformed” layer, a much more open structure is present. Figure 2a shows the XRD pattern of the layers of Figure 1. The results reveal that the “as-formed” porous layers before and after etching are amorphous but contain some anatase and rutile crystallites. The diffractogram shown as inset in Figure 1d confirms the mostly amorphous nature but furthermore indicates that some metallic Ti may still be present in the structure. To make use of TiO2 in applications based on photoexcitation, a crystalline structure is desired to eliminate defects associated with the amorphous material. 21] To crystallize the mesoporous TiO2 layers, we [*] D. Kim, K. Lee, Dr. P. Roy, Dr. P. Schmuki Department of Materials Science and Engineering, WW4-LKO University of Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen (Germany) Fax: (+ 49)9131-852-7582 E-mail: schmuki@ww.uni-erlangen.de Homepage: http://www.lko.uni-erlangen.de/

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.

Size-Selective Separation of Macromolecules by Nanochannel Titania Membrane with Self-Cleaning (Declogging) Ability

We report on a simple and self-organizing process for the fabrication of TiO(2) nanochannel membranes with a channel width of 8-10 nm that can be used for size selective separation of macromolecules (proteins). The membrane, consisting of self-aligned oxide channels, is formed by complete anodization of a thin Ti foil under specific electrochemical conditions in a glycerol-phosphate electrolyte. Due to self-cleaning properties of TiO(2), clogged membranes (for example due to extended use) can easily be fully reopened and thus are reusable. As the TiO(2) after anodic formation directly contains anatase crystallites (the most photoactive TiO(2) crystal form) no thermal treatment of the membrane is required (avoiding the danger of thermally induced cracking).

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

Anodically formed transparent mesoporous TiO2 electrodes for high electrochromic contrast

In the present work, we show that metallic Ti thin films on FTO glass can be completely anodized to a mesoporous oxide layer using a glycerol–K2HPO4 electrolyte at elevated temperatures (∼180 °C). This results in highly transparent mesoporous oxide coated electrodes that provide a significantly enhanced transparency compared with the classic nanoparticle layers or TiO2 nanotube layers. We show that in electrochromic switching experiments these electrodes therefore exhibit a significantly higher contrast compared to other TiO2 nanostructures. Moreover, this type of anodic titania layer is produced in a fluoride-free electrolyte and adheres very well to the FTO glass, both features that are highly beneficial in view of an application in electrochromic devices.