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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.

Hematite, α-Fe2O3, is considered as one of the most promising materials for sustainable hydrogen production via photoelectrochemical water splitting with a theoretical solar-to-hydrogen efficiency of 17%. However, the poor electrical conductivity of hematite is a substantial limitation reducing its efficiency in real experimental conditions. Despite of computing models suggesting that the electrical conductivity is extremely anisotropic, revealing up to 4 orders of magnitude higher electron transport with conduction along the (110) hematite crystal plane, synthetic approaches allowing the sole growth in that direction have not been reported yet. Here, we present a strategy for controlling the crystal orientation of very thin hematite films by adjusting energy of ion flux during advanced pulsed reactive magnetron sputtering technique. The texture and effect of the deposition mode on the film properties were monitored by XRD, conversion electron Mössbauer spectroscopy, XPS, SEM, AFM, PEC water splitting, IPCE, transient photocurrent measurements, and Mott-Schottky analysis. The precise control of the synthetic conditions allowed to fabricate hematite photoanodes exhibiting fully textured structures along (110) and (104) crystal planes with huge differences in photocurrents of 0.65 and 0.02 mA cm(-2) (both at 1.55 V versus RHE), respectively. The photocurrent registered for fully textured (110) film is among record values reported for thin planar films. Moreover, the developed fine-tuning of crystal orientation having a huge impact on the photoefficiency would induce further improvement of thin hematite films mainly if cation doping will be combined with the controllable texture.

In the present work, we investigate the self-organized anodic formation of nanotubular/porous hematite structures from Fe thin films on fluorine doped tin oxide (FTO) substrates. We show, for different metal film thicknesses, that transparent layers of an aligned 1D oxide morphology can be grown by complete anodization of sputtered iron films. The nanoporous or nanotubular structures show very different potentials for use as a photoanode for solar water splitting. Best performance under AM 1.5 (100 mW cm−2) conditions were found for a nanoporous hematite structure obtained after anodizing a 570-nm-thick iron film and using combined air/Ar annealing to maintain the nanoscale morphology.

We optimize the deposition of CH3NH3PbI3 perovskite into mesoporous electrodes consisting of anodic TiO2 nanotube layers. By a simple spin coating approach, complete filling of the tube scaffolds is obtained, that leads to interdigitated perovskite structures in conformal contact with the TiO2 tube counterparts. Such assemblies can be used as solid state solar cells in hole-transporting material-free configuration, i.e., the tube scaffold serves as electron collector and blocking layer, while the perovskite acts as visible light absorber and hole transporting material. We show that the complete filling of the tube scaffold with the perovskite is essential to improve the solar cell efficiency.

In the present work vertically aligned TiO2 nanorods were fabricated by anodization of Al/Ti dual layer structures. Anodization of Al leads to the formation of a porous alumina layer which is used as a template for the growth of TiO2 nanorods. We have studied the characteristics of these nanorods after annealing by high resolution TEM, showing a core–shell structure along the length of the nanorods formed from the expansion of the TiO2 nanorod into the alumina template during growth. Modification of the nanorods by spin coating of a solution of titanium isopropoxide (TIPO) in isopropanol yields enhanced photocatalytic activity up to 12 times due to the increase in surface area, light absorption volume, and the creation of a crystalline coating with an anatase-phase.

The fabrication of crystalline α-Fe2O3 hematite thin films by means of novel high power impulse magnetron sputtering (HiPIMS) and high power hollow cathode plasma jets system is reported. The coatings are based on low temperature pulsed-plasma reactive sputtering. These methods were compared with a more common method of medium frequency (MF) pulsed reactive DC magnetron sputtering. Although both high power methods yielded crystalline structure of the films already during the depositions, the films had to be thermally treated at elevated temperature in order to improve their physical (crystallinity) and electronic properties. The deposition methods used and the effect of the post deposition thermal annealing were judged on the basis of physical properties such as crystalline structure, optical absorption, surface topography, electronic properties, and electrical behavior. The functional properties were investigated under simulated photoelectrochemical water splitting conditions. Despite the revealed hematite phase of the as-deposited films, these were almost photoelectrochemically inactive. The annealing improved crystal structure of the deposited films and increased their dark conductivity. Furthermore the annealing initiated the diffusion of tin atoms from FTO (fluorine doped tin oxide) substrate into the film increasing its extrinsic conductivity. These improvements lead to higher photoefficiency.

This talk will present self-organized vanadium oxy-fluoride nanotubular layers grown by electrochemical anodization of vanadium in a [BF4]- electrolyte. In a thermal conversion of this amorphous VOxFy precursor structure in ammonia gas, aligned vanadium oxy-nitride (VOxNy) nanoporous/nanotubular layers could be formed. We established conditions to reach conversion of precursor structures to a well crystallized VOxNy nanotube structure, without the formation of any secondary phases or the collapse of the ordered morphology. The formed VON structures were investigated by SEM, EDX, XRD, XPS and electrochemical measurements for capacitor applications. The overall capacitance of the vanadium oxy-nitride structures showed a value of 250 F/g. Reference: Y. Yang, R. Kirchgeorg, R. Hahn, P. Schmuki Electrochemistry Communications, accepted

In the present work, we investigate the performance of anodic TiO2 nanotube layers in dye-sensitized solar cells under front- and back-side illumination configurations. To fabricate transparent TiO2 nanotube electrodes, we evaporated 5 μm thick metallic Ti layers on FTO glass—then the metal layers are completely anodized to form aligned nanotube layers. We compare different types of FTO glass (conductivity and transparency) and use them for working electrodes as well as transparent platinized counter electrodes. The results show that for TiO2 nanotube electrodes the resulting light conversion efficiency in DSSCs is highly affected by the type of glass used (efficiency range—depending on configuration from 4.62% to 7.58%).

Host-guest systems such as hematite/SnO2 have attracted a great deal of interest as photoanodes for photoelectrochemical water splitting. In the present work we form an ordered porous tin oxide layer formed by self-organizing anodization of Sn films on a FTO substrate. Subsequently the anodic tin oxide nanostructure is doped with antimony (ATO) by a simple impregnation and annealing treatment, and then decorated with hematite using anodic deposition. Photoelectrochemical water splitting experiments show that compared to conventional SnO2 nanostructures, using a Sb doped nanochannel SnO2 as a host leads to a drastic increase of the water splitting photocurrent response up to 1.5 mA cm(-2) at 1.6 V (vs. RHE) in 1 M KOH under AM 1.5 (100 mW cm(-2) ) conditions compared to 0.04 mA cm(-2) for the non-Sb doped SnO2 scaffold.

Thermal oxidation of Fe to nanostructured hematite (wires, flakes) is currently widely investigated to produce efficient photoanodes for photoelectrochemical water splitting. The process carried out on pure iron, however, has the key drawback that not only hematite but a layered structure of Fe2O3–Fe3O4–FeO is formed where the thick suboxide layer underneath the Fe2O3 is highly detrimental to the photoresponse. In the present work, we show that suboxide formation can be largely suppressed if hematite nanowires/nanoflakes are thermally grown on Fe–Si alloys. For hematite structures grown on a Fe–Si alloy with 5 at% Si, a photocurrent onset potential as low as 0.6 VRHE can be reached (under AM 1.5 illumination and 1 M KOH). We believe that the results represent a key finding towards the formation of optimized hematite nanostructures using a thermal oxidation method.

In the current work, we present a new self-organizing anodization approach of metallic Sn layers to obtain vertically aligned tin oxide nanochannel structures. For this, we use a sulphide-containing electrolyte and a set of optimized anodizing parameters. The resulting high aspect ratio nanochannel morphologies can be converted into crystalline SnO2 by high temperature annealing and show highly promising H2 sensing properties. We show that these anodic layers can operate at relatively low temperatures (∼80 °C), detecting concentrations as low as 9 ppm, and with extremely fast response and recovery times. This excellent gas-sensing performance is ascribed to the advanced structure, characterized by a crack-free, straight and top-open nanochannel geometry.

Article history:Received 29 July 2013Received in revised form 4 August 2013Accepted 5 August 2013Available online 19 August 2013Keywords:Ga 2 O 3 Zn dopingSelf-organizationNanoporous structuresWater splitting The present work introduces the formation of a highly organized one dimensional Ga 2 O 3 nanoporous architec-ture by anodization of Ga metal at −10 °C in a mixture of phosphoric acid and ethylene glycol. In addition topureGa 2 O 3 ,weperforminsituZndopingbyalloyingGawith1 at.%Zn.Afterzincdoping,theGa 2 O 3 nanoporouslayers demonstrate promising properties for photo-induced hydrogen generation from water.© 2013 Elsevier B.V. All rights reserved. 1. IntroductionSince the pioneering works of Masuda and Fukuda on highly orga-nized porous Al 2 O 3 and Assefpour-Dezfuly et al. and Zwilling et al.on anodic TiO 2 nanotubes [1], great attention has been paid to theelectrochemical self-organization of one dimensional nanotubular ornanoporous structures of metal oxides, especially TiO

The present work reports the enhancement of the photoelectrochemical water splitting performance of in-situ silicon (Si)-doped nanotubular/nanoporous (NT/NP) layers. These layers were grown by self-organizing anodization on Fe-Si alloys of various Si content. The incorporation of Si is found to retard the layer growth rates, leads to a more pronounced nanotubular morphology, and most importantly, an improved photoelectrochemical behavior. By increasing Si content from 1, 2 to 5 at.% in the iron oxide NT/NP photoanodes, the photocurrent onset potential shifts favorably to lower values. At 1.3 V vs. RHE, hematite layer with 5 at.% Si shows a 5-fold increase of the photocurrent, i.e. 0.5 mA cm−2 in comparison to 0.1 mA cm−2 for the undoped samples. The study also reveals that a suitable layer thickness is essential to achieve a beneficial effect of the Si doping. Keywords: Fe2O3, Si doping, Water splitting, Photoelectrochemistry

A two-step anodization of a Ti50Zr alloy results in a various sized nanotube oxide structures, which show an improved antibacterial activity. The nanotubes were formed in glycol with 15 vol.% H 2 O and 0.2 M NH 4 F by two-step anodization. The oxide layer grown during 2 h was removed by sonication in deionized water and anodized again for 1 h at the same conditions as in the first step. The removed layer acts as a nano-prepatterned surface, where higher ordered and open nanotubes can be achieved. The surface morphologies were analyzed by SEM and AFM, the surface wettability by contact angle measurements. The diameter and the length of the grown nanotubes are potential dependent between 20 and 100 nm in diameter and 2.3 and 5.7 μm in length, respectively. The antibacterial properties were evaluated in vitro on the formed nanotubes on the TiZr alloy against gram negative Escherichia coli bacteria. The E. coli (ATCC 8738) were cultured in a tube containing Luria Bertani medium at 37 °C. The optical density was determined after 18 h of incubation. In comparison, the smallest nanotubes exhibited the most efficient antibacterial behavior against E. coli bacterium. This suggests the use of small diameter nanotubes on TiZr for antimicrobial surface applications, which are susceptible for biofilms and microbial cultures.

In the present work, we investigate the performance of anodic, self-organized TiO2 mesoporous structures (TMSs) in dye-sensitized solar cells (DSSCs) under front-side illumination conditions. In order to fabricate structures usable in a front-side (FS) configuration, TMSs were formed on FTO glass by a full anodic conversion of evaporated Ti films on the glass substrate in a glycerol/K2HPO4 electrolyte at 180 °C. The resulting mesoporous layers can be assembled to FS dye sensitized solar cells which show a thickness of only 3 μm and a remarkable efficiency of 4.34%. This is significantly higher than, for example, anodic TiO2 nanotubes of the same length even after efficiency enhancing TiCl4 treatments (3.01%). The best efficiency value for TMS-based DSSCs that we currently reached is for a 7 μm thick TMS layer, this is ≈6.82% which is highly promising for further improvements. Keywords: TiO2 nanotube, Mesoporous, Anodic oxidation, Dye-sensitized solar cells

In the past decades, a variety of self-aligned functional oxides have successfully been grown on a wide range of metals using electrochemical, self-organizing anodization. The earliest reports on highly ordered oxide structures included porous aluminum oxide layers grown by optimized anodization of Al in oxalic acid.1 These nanoporous oxides found a considerable number of direct applications, such as size-exclusion filters2 and waveguide structures,3 or sacrificial uses, such as templates for secondary material deposition for the production of nanowires and tubes.4, 5

A NiFe layered double hydroxide (LDH) co-catalyst coated Ta3N5 nanorod electrode is fabricated by vapor deposition of Al on a Ta foil followed by anodizing the Al layer in oxalic acid, (40 V, 0.5 h), pore widening of the as-prepared porous Al2O3 by immersion in 5% aq.

We introduce the use of pure molten ortho-phosphoric acid (o-H3PO4) as an electrolyte for selforganizing electrochemistry. This electrolyte allows for the formation of self-organized oxide architectures (one-dimensional nanotubes, nanochannels, nanopores) on metals such as tungsten that up to now were regarded as very difficult to grow self-ordered anodic oxide structures. In this work, we show particularly the fabrication of thick, vertically aligned tungsten oxide nanochannel layers, with pore diameter of ca. 10 nm, and illustrate their potential use in some typical applications.

Over the past decades, self-organized structures of va-rious oxide nanomaterials have received a wide interest in research and technology, as they provide a unique combi-nation of geometry and physical and chemical properties [1, 2]. Such structures may find, for example, applications in solar cells, catalysis, sensing, or bio-relevant coatings [3–5]. A most convenient growth technique for aligned nano-tubular or nanoporous oxide structures is self-organizing electrochemical oxidation reactions on metal substrates. In 1995 Masuda et al. reported on the formation of highly or-ganized porous alumina layers using the anodization of Al in oxalic acid under a specific set of experimental param-eters [6]. In 1999, Zwilling et al. introduced fluoride based electrolytes to grow ordered nanotubular TiO

In the present work we show that a suitable high temperature ammonia treatment allows for the conversion of single-walled TiO2 nanotube arrays not only to a N-doped photoactive anatase material (which is already well established), but even further into fully functional titanium nitride (TiN) tubular structures that exhibit semimetallic conductivity.

[EN] In the present work, we show the feasibility to grow highly ordered anodic Nb2O5 nanochannel structures from a thin Nb-layer on conducting glass (fluorine doped tin oxide, FTO). This is achieved using a glycerol-phosphate electrolyte at 180 degrees C and a suitable anodization voltage that allows a rapid and complete conversion of the entire metallic layer into a transparent oxide layer of an extremely well defined directional nature with 20-30 nm wide open vertical channels. These layers can be used, either in the amorphous state or after thermal crystallization, for efficient reversible electrochromic switching devices. (c) 2012 The Electrochemical Society. All rights reserved., The authors acknowledge DFG and the Erlangen DFG cluster of excellence (EAM) for financial support.

Self-organized nanoporous oxide layers were grown on a W-Ta alloy by electrochemical anodization. These nanostructured mixed oxide layers show an ultra-fast electrochromic switching kinetics. Compared with porous WO(3) nanostructures more than 10 times higher switching frequencies are reached along with a significantly enhanced lifetime and cyclability.

Self organized nanotubular structures of transition metal oxides grown on their metallic substrates, especially titanium [1], have attracted great scientific and technological interest due to the possibility to exploit their functional properties (such in photo-catalysis, electro-catalysis, solar energy conversion and as host for Li storage) in a nanotubular morphology. In contrast to the conventional way to grow anodic TiO2 nanotubes, which is an optimized electrochemical treatment of Ti in fluoride containing electrolyte (for an overview see [2]) this work reports on the growth of nanotubes by so-called RBA (rapid breakdown anodization).[3] This nanotube growth-process is in comparison extremely fast (seconds instead of hours) and leads to surface coatings consisting of dense packed bundles of nanotubes with very high aspect ratios (tube length up to 50 micrometer, tube diameter of ~40 nanometer). In addition, this method provides a new route for an insitu electrochemical doping during the growth of TiO2 nanotubular layers with various transition metals to adjust i.e. the photocatalytic activity [4]. The second part of the presentation will demonstrate that alternatively the electronic and optical properties [5] of these TiO2 nanotubes can be significantly modified, showing semi-metallic behavior by a high temperature reduction treatment in acethylene or ammonia creating a suitable platform for highly conductive surface area electrodes which is important i.e. sensing applications. [6] Literature: [1] V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Electrochim. Acta, 45, 921 (1999). [2] S. Berger, R. Hahn, P. Roy, P. Schmuki Phys. Status Solidi B247, 10, 2424 (2010). [3] R. Hahn, J. M. Macak, P. Schmuki, Electrochem. Commun.,9, 947 (2007). [4] M. Stark, R. Hahn, M.S. Killian, P. Schmuki Catal. Sci. Technol., 3, 1765 (2013). [5] R. Hahn, F. Schmidt-Stein, J. Salonen, S. Thiemann, Y.Y. Song, J. Kunze, V.-P. Lehto, P Schmuki, Angewandte Chemie - International Edition 48 (39), 7236 (2009). [6] R. Hahn et al submitted

This talk will present the formation of highly aligned nanotubular TiSx electrodes consisting of self-organized TiO2 nanotubes formed by thermal heat treatment in H2S gas. Depending on the annealing time and temperature several mixtures of titanium oxy-sulfides can be formed. Even a complete transformation into TiS2 is possible under the right conditions. The formed TiSx layers show a slightly modified tubular structure compared to the original TiO2 nanotubes, but still offer a high surface area in form of tubular structure. The formed TiSx were characterized by SEM, XRD, XPS and tested regarding their battery performance as a cathode material for lithium ion batteries. References concering high temperature treatments in reactive gas envoirment: Y.-C. Nah, I. Paramasivam, R. Hahn, N. K Shrestha and P. Schmuki, Nanotechnology 21 (2010) 105704 R. Hahn, F. Schmidt-Stein, J. Salonen, S. Thiemann, Y. Song, J. Kunze, V.-P. Lehto and Patrik Schmuki, Angew. Chem. Int. Ed. 48 (2009) 7236 –7239

In the present work we show that a suitable high temperature ammonia treatment allows for the conversion of single-walled TiO2 nanotube arrays not only to a N-doped photoactive anatase material (which is already well established), but even further into fully functional titanium nitride (TiN) tubular structures that exhibit semimetallic conductivity.

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

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.

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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.