Titanium oxide nanotubes prepared in phosphate electrolytes

In the present communication, we report the electrochemical formation of self-organized titanium oxide nanotubes (π-TiO₂) prepared in fluoride ion containing phosphate electrolytes. The morphology of the π-TiO₂ layers (particularly the pore diameter and length) is affected by the electrochemical conditions used (applied potential, electrolyte composition, pH, and anodizing time). Under specific sets of conditions highly self-organized titanium oxide nanotubes are formed with diameters varying from approx. 40 nm to 100 nm and length from approx. 100 nm to 4 μm. XPS investigations show that the nanotubes formed in phosphate solutions contain a significant amount of phosphorous species.     

Electrochemical formation of self-organized zirconium titanate nanotube multilayers

The present work reports the formation of multilayers of self-organized zirconium titanate nanotubes by anodizing a Ti–35Zr alloy in 1 M (NH₄)₂SO₄ + 0.5 wt% NH₄F electrolytes. It was found that multilayers consisting of different diameter nanotubes can be produced by repeated anodization steps under different conditions. Formation of new nanotubes starts in the gaps between the existing tubes. The process allows the formation of multilayer stacks consisting of layers of several 100 nm in length and adjustable nanotube diameters in a range from 50 to 180 nm.     

Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles

Ag and Au nanoparticles were found to significantly enhance the photocatalytic activity of self-organized TiO₂ nanotubular structures. The catalyst systems are demonstrated to be highly efficient for the UV-light induced photocatalytic decomposition of a model organic pollutant – Acid Orange 7. The metallic nanoparticles with a diameter of ∼10 ± 2 nm (Ag) and ∼28 ± 3 nm (Au) were attached to a nanotubular TiO₂ layer that consists of individual tubes of ∼100 nm of diameter, ∼2 μm in length and approx. 15 nm of wall thickness. Both metal particle catalyst systems enhance the photocatalytic decomposition significantly more on the nanotubes support than placed on a compact TiO₂ surface.     

Hierarchically organized titanium dioxide nanostructured electrodes: Quantum-sized nanowires grown on nanotubes

Titanium dioxide nanotube electrodes were fabricated by anodization of titanium and decorated with quantum-sized rutile nanowires (2 nm in diameter) by chemical bath deposition. The length of the nanotubes (120 nm in external diameter) was varied between 4 and 10 μm by changing the anodization time. The hierarchically organized electrodes present good mechanical properties and an enhanced capacity for reversible charge accumulation. The photoelectrocatalytic properties of such electrodes have been tested by photo-oxidizing both water and oxalic acid, turning out to be superior to those of bare nanotubes, which are ascribed to an enhanced interfacial area while keeping the favorable transport properties (for both electrons and chemicals) typical of nanotube electrodes.     

Layer-by-layer nanoparticle coatings on lignocellulose wood microfibers

The layer-by-layer (LbL) self-assembly technique was applied to deposit organized multilayers of TiO₂ or SiO₂ nanoparticles of 30–80 nm diameter, and 50-nm diameter halloysite clay nanotubes on softwood fibers. Fluorescent and scanning electron microscopy images showed complete nanoparticle coating on these fibers. The thickness of the two-layer coating was estimated as 46, 58, and 115 nm for TiO₂, SiO₂, and halloysite tubules, respectively, which corresponds to ca. 1 wt% nanoparticle loading of the fibers. The brightness test of paper handsheets prepared from nanocoated fibers showed that TiO₂ nanoparticle coating gave handsheet reflectance of 84% at 450 nm, which is 4% higher than the brightness of the control sample from virgin fibers. The paper handsheets prepared with nanoparticle-coated fibers had 30–50% higher porosity with tensile strength index retained close to the control sample.     

Fast growth of TiO2 nanotube arrays with controlled tube spacing based on a self-ordering process at two different scales

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.     

Photoelectrochemical water splitting using a Cu(In,Ga)Se2 thin film

The effects of surface modification and reaction conditions on the photoelectrochemical properties of polycrystalline Cu(In,Ga)Se₂ (CIGS) thin films for water splitting were studied. CIGS modified with platinum particles (Pt/CIGS) generated a cathodic photocurrent at potentials up to + 0.4 V vs. RHE at pH = 9.5. The photocurrent was stable for 16 h, which resulted in a turnover number of over 500. A CdS-inserted film (Pt/CdS/CIGS) had significantly improved properties compared to Pt/CIGS: a 0.3 V higher onset potential of cathodic photocurrent and a three-fold increase in the quantum efficiency. Our results suggest the feasibility of CIGS as a photocathode for biphotoelectrochemical water splitting.     

Photocurrent generated on a carotenoid-sensitized TiO2 nanocrystalline mesoporous electrode

Photocurrent was observed upon monochromatic illumination of an ITO electrode coated with a TiO₂ nanocrystalline mesoporous membrane with carotenoid 8′-apo-β-caroten-8′-oic acid (ACOA) deposited as a sensitizer (illuminated area 0.25 cm²) and immersed in an aqueous 10 mM hydroquinone (H₂Q), 0.1 M NaH₂PO₄ solution (pH = 7.4) purged with argon, using a platinum flag counter electrode (area 3.3 cm²) and a SCE reference electrode. The carotenoid-sensitized short-circuit photocurrent reached 4.6 μA/cm² upon a 40 μW/cm² incident light beam at 426 nm, with an IPCE (%, incident monochromatic photon-to-photocurrent conversion efficiency) as high as 34%. The short-circuit photocurrent was stable during 1 h of continuous illumination with only a 10% decrease. An open-circuit voltage of 0.15 V was obtained (upon 426 nm, 40 μW/cm² illumination) which remained at a constant value for hours. The observed open-circuit voltage is close to the theoretical value (0.22 V) expected in such a system. The action spectrum resembled the absorption spectrum of ACOA bound on the TiO₂ membrane with a maximum near 426 nm. No decay of the ACOA on the TiO₂ surface was observed after 12 h, presumably because of rapid regeneration of ACOA from ACOA⁺ at the surface by electron transfer from H₂Q.     

High resolution LAPS and SPIM

The resolution of photocurrent measurements at field-effect capacitors as used in light-addressable potentiometric sensors (LAPS) and scanning photo-induced impedance microscopy (SPIM) has been investigated using silicon on sapphire (SOS) substrates illuminated at different wavelengths. Using a two-photon effect in silicon (λ = 1250 nm) to generate the photocurrent, genuine submicrometer resolution has been demonstrated for LAPS and SPIM. Improved sensitivity for both LAPS and SPIM was obtained using a 6.7 nm thick gate oxide on SOS anodically grown in 0.1 M HCl.     

Effect of Hg2+doping and of the annealing temperature on the nanostructural properties of TiO2 thin films obtained by sol–gel method

This work is a study of Hg²⁺-doped TiO₂ thin films deposited on silicon substrates prepared by sol–gel method and treated at temperatures ranging between 600 to 1000 °C for 2 h. The structural and optical properties of thin films have been studied using different techniques. We analyzed the vibrations of the chemical bands by Fourier transform infrared (FTIR) spectroscopy and the optical properties by UV–Visible spectrophotometry (reflection mode) and photoluminescence (PL). The X-ray diffraction and Raman spectra of TiO₂ thin films confirmed the crystallization of the structure under the form of anatase, rutile, mercury titanate (HgTiO₃) as a function of the annealing temperature. The observation by scanning electron microscopy (SEM) showed the changing morphology, with respect to nanostructures, nanosheets, nanotubes, with the annealing temperature. The diameters of nanotubes ranged from 50 nm to 400 nm. The photoluminescence and reflectance spectra indicated that these structures should enhance photocatalytic activity.     

Molybdic acid doped polyaniline micro/nanostructures via a self-assembly process

Molybdic acid (H₂MoO₄, MA) doped polyaniline (PANI) micro/nanostructures were prepared by a self-assembly process in the presence of ammonium persulfate ((NH₄)₂S₂O₈, APS) as the oxidant. The morphology of PANI-MA changed from nanofibers or nanotubes (～160 nm in diameter) to co-existence of nanofibers and microspheres (～3 μm in diameter) and that accompanied an enhancement of the conductivity from 5.42 × 10^?3 S cm^?1 to 2.8 × 10^?1 S cm^?1as the molar ratio of MA to aniline varied from 0.01 to 1.5. With increasing the polymerization time, moreover, the pH value of the reaction solution not only decreased due to sulfuric acid produced during the course of the polymerization, but also accompanied a change in morphology from microspheres to nanofibers. All above-mentioned observations could be interpreted by spherical and cylindrical micelle composed of MA as the “soft-template” in forming the micro/nanostructures.     

Flexible self-organization of two size-scales oxide nanotubes on Ti45Nb alloy

We report on the growth of anodic oxide nanotubes grown on a simple Ti45Nb alloy that show self-organization on two size-scales. The well-ordered nanotube layers consist of large and small diameter tubes that are arranged in an alternating two-dimensional (2D) structure. This bimodal morphology and the thickness of the nanotube layer are affected by the electrochemical conditions used. The bimodal feature can be obtained over a wide potential range (between 15 and 55 V), resulting in a linear diameter variation from 80 to 330 nm for the larger and 50–180 nm for the smaller tubes. The organization into two size-scales takes place during the first hour of the anodization process. This result of flexible two size-scale self-organization using a simple two element alloy is of significant importance not only for fundamental research but also for specific applications that can benefit from bimodal properties.     

Titania nanotube supported tin anodes for lithium intercalation

A new type of nanostructured titania nanotube supported tin anode was prepared for lithium ion batteries. The as-prepared titania nanotubes are in the anatase phase with diameters of about 12 nm. Tin nanoparticles are dramatically decorated on the titania nanotubes and have a particle size of about 10 nm. This new structure promises good retention of reversible capacity on cycling for lithium intercalation. By charge/discharge measurements, the reversible capacity of the titania nanotubes supported tin anode for lithiation and de-lithiation was found to be 312 mA h/g (cycled between 0.05 and 2.0 V) and 203 mA h/g (cycled between 0.05 and 1.3 V) after 50 cycles with around 100% columbic efficiency.     

Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes: Preparation,characterization, and application to photoelectrochemical cells

A novel electrodeposited CdS nanoparticle-modified highly-ordered TiO₂ nanotube-array photoelectrode and its application to photoelectrochemical cells is reported. Results show formation of a thin, nanoparticulate CdS layer, comprised of sphere-like 10–20 nm diameter nanoparticles, on the anodic synthesized TiO₂ nanotube-array (inner diameter of 70 nm, wall thickness 25 nm and ca. 400 nm length) electrode. The resulting CdS–TiO₂ photoelectrode has an as-fabricated bandgap of 2.53, and 2.41 eV bandgap after sintering at 350 °C in N₂ ambient. Photoelectrochemical properties are described in detail.     

Computational simulations of pore nucleation in silicon(1 1 1)

Float zone n-Si(1 1 1) was electrochemically etched in diluted NH₄F to form porous nuclei. The experimental results were compared with computational simulations of pore nucleation and growth. Electrochemical etching of silicon(1 1 1) results in pore nucleation preferentially localized on the edges of atomic terraces. The initial pore nuclei have diameter and depth of 17 nm and 0.3 nm, respectively. We find a correlation between H-terminated Si(1 1 1) atomic surface morphology and electric field distribution on pore nucleation and growth mechanism. The H-terminated surface is composed from wide (100–200 nm) atomic terraces with steps of 0.3 nm height. Electric field enhancement occurs at the terrace edges leading to focusing the holes trajectories. This leads to weakening of the Si–Si backbonds resulting in easy atom removing. The maximum electric field was observed at terrace edges and at the semispherical pore bottom.     

Superior storage performance of carbon nanosprings as anode materials for lithium-ion batteries

Carbon nanosprings (CNSs) with spring diameter of ～140 nm, carbon ring diameter of ～100 nm and pitch distance of ～150 nm, synthesized by using a catalytic chemical vapor deposition technology, have been investigated for potential applicability in lithium batteries as anode materials. The electrochemical results demonstrate that the present CNSs are superior anode materials for rechargeable lithium-ion batteries with high-rate capabilities, as well as long-term cycling life. At a current density as high as 3 A g^?1, CNSs can still deliver a reversible capacity of 160 mA h g^?1, which is about six times larger than that of graphite and three times larger than that of multi-wall carbon nanotubes under the same current density. After hundreds of cycles, there is no significant capacity loss for CNSs at both low and high current densities. The much improved electrochemical performances could be attributed to the nanometer-sized building blocks as well as the unusual spring-like morphology.     

Visible light active hydrogen modified (HM)-n-TiO2 thin films for photoelectrochemical splitting of water

Visible light active hydrogen modified n-type titanium oxide (HM-n-TiO₂) thin films were synthesized by thermal oxidation of Ti metal sheet (Alfa Co. 0.25 mm thick) in an electric oven followed by incorporation of hydrogen electrochemically under cathodic polarization at ?1.6 V vs Pt. The photoresponse of the HM-n-TiO₂ was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen in terms of photocurrent density, J_p. The optimized electric oven-made n-TiO₂ and HM-n-TiO₂ photoelectrodes showed photocurrent densities of 0.2 mA cm^?2 and 1.60 mA cm^?2, respectively, at a measured potential of ?0.4 V vs Pt at illumination intensity of 100 mW cm^?2 from a 150 W xenon lamp. This indicated an eightfold increase in photocurrent density for HM-n-TiO₂ compared to oven-made n-TiO₂ at the same measured electrode potential. The band-gap energy of HM-n-TiO₂ was found to be 2.7 eV compared to 2.82 eV for electric oven-made n-TiO₂ and a mid-gap band at 1.67 eV above the valence band was also observed. The HM-n-TiO₂ thin film photoelectrodes were characterized using photocurrent density under monochromatic light illumination and UV–Vis spectral measurements.     

Nanorods of transition metal oxalates: A versatile route to the oxide nanoparticles

A versatile route has been explored for the synthesis of nanorods of transition metal (Cu, Ni, Mn, Zn, Co and Fe) oxalates using reverse micelles. Transmission electron microscopy shows that the as-prepared nanorods of nickel and copper oxalates have diameter of 250 nm and 130 nm while the length is of the order of 2.5 μm and 480 nm, respectively. The aspect ratio of the nanorods of copper oxalate could be modified by changing the solvent. The average dimensions of manganese, zinc and cobalt oxalate nanorods were 100 μm, 120 μm and 300 nm, respectively, in diameter and 2.5 μm, 600 nm and 6.5 μm, respectively, in length. The aspect ratio of the cobalt oxalate nanorods could be modified by controlling the temperature.The nanorods of metal (Cu, Ni, Mn, Zn, Co and Fe) oxalates were found to be suitable precursors to obtain a variety of transition metal oxide nanoparticles. Our studies show that the grain size of CuO nanoparticles is highly dependent on the nature of non-polar solvent used to initially synthesize the oxalate rods. All the commonly known manganese oxides could be obtained as pure phases from the single manganese oxalate precursor by decomposing in different atmospheres (air, vacuum or nitrogen). The ZnO nanoparticles obtained from zinc oxalate rods are ～55 nm in diameter. Oxides with different morphology, Fe₃O₄ nanoparticles faceted (cuboidal) and Fe₂O₃ nanoparticles (spherical) could be obtained.     

Dye-sensitized solar cells using anodic TiO2 mesosponge: Improved efficiency by TiCl4 treatment

In the present work, we produce 15 μm thick titania mesosponge layers (TMSL) by a Ti anodization/etching process and use the layers in dye-sensitized solar cells (DSCs). We show that the solar cell efficiency can considerably be improved by a TiCl₄ hydrolysis treatment (increase of approx. 40% to an overall value of 4.9% under AM 1.5 illumination). This beneficial effect is due to the decoration of the ～10 nm wide channels present in TMSL with TiO₂ nanoparticles of approx. 3 nm diameter, which allow for a significantly higher specific dye loading of the TMS structure.     

Determination of hydrogen sulfide in gasoline by Au nanoclusters modified glassy carbon electrode

A promising hydrogen sulfide (H₂S) sensor was prepared by electrodeposition of Au nanoclusters on glassy carbon electrode (GCE) and the surface structure was characterized by SEM and EDAX. These flower-like form Au nanoclusters, which were made up of highly dense clustering Au nanorods with an average diameter of 20 nm and length up to 80 nm, had an average size about 600 nm and uniformly distributed on the GCE surface. The electrocatalytic oxidation of H₂S in gasoline was performed on this modified electrode, which had a satisfactory liner response to H₂S in the range of 1–80 ppm and a detection limit of 0.45 ppm (s/n = 3). This sensor was sensitive, selective and stable.