Krishnan S. Raja

Self-organized TiO2 nanotubular arrays for photoelectrochemical hydrogen generation: effect of crystallization and defect structures

The effect of crystallization and surface chemistry of nanotubular titanium dioxide (TiO2) in connection with the photoelectrochemical process is reported in this investigation. TiO2 nanotubular arrays were synthesized by a simple anodization process in an acidified fluoride electrolyte at room temperature. The TiO2 nanotubes were amorphous in as-anodized condition; their transformation to crystalline phases was a function of annealing temperature and gaseous environment. The anatase phase was observed predominantly after annealing in non-oxidizing atmospheres, whereas annealing in an oxygen environment showed a mixture of anatase and rutile phases. X-ray photoelectron spectroscopy was used to determine the chemical environment of the surface, which revealed the presence of phosphate, oxygen vacancies and pentacoordinated Ti in hydrogen annealed samples. Diffuse reflectance photospectrometry of non-oxygen annealed samples showed long absorption tails extending in the visible region. The photoelectrochemical response of the TiO2 nanotubes annealed in different conditions was investigated. Photoelectrochemical performance under simulated solar light was improved by annealing the nanotubular TiO2 samples in non-oxidizing environment.

Nanostructured anodic iron oxide film as photoanode for water oxidation

Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm−2 at 0.2 VAg/AgCl and 1.8 mA cm−2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 VAg/AgCl), which are the properties required for promising photoanode materials.

Low-cost photoelectrocatalyst based on a nanoporous oxide layer of low-carbon steel

Low-carbon steel is a commonly used structural material in a wide variety of applications. An anodic oxide layer of this inexpensive alloy has been noted to have interesting photoelectrochemical behaviour similar to that of α-Fe2O3 prepared using other expensive starting materials. An ordered nanoporous oxide layer has been grown on to the low-carbon steel surface by a simple electrochemical anodization process in different electrolytes such as ethylene glycol containing 0.05M NH4F and 3–10 vol% water and 0.5M phosphoric acid solution containing 0.05M NH4F. After anodization, the nanoporous anodic oxide layer has been transformed to α-Fe2O3 by a low-temperature annealing process. Photoelectrochemical characterization of the anodic iron oxide materials has been carried out in 1M KOH electrolyte under a solar simulated illumination using Air Mass (AM) 1.5. The ordered nanoporous oxide layer prepared in ethylene glycol-based electrolyte showed a photocurrent density of about 85 µA cm−2 at 0.4 VAg/AgCl. Whereas the anodic iron oxide prepared by anodization of the low-carbon steel in 0.5M H3PO4 + 0.05M NaF solution showed a photocurrent density of 800 µA cm−2 at 0.4 VAg/AgCl. The improved photoactivity of the phosphate-modified oxide layer could be attributed to the high charge carrier concentration, low charge transfer resistance and better ability to expend holes in the oxygen evolution reaction.

Self-ordering dual-layered honeycomb nanotubular titania: a study in formation mechanisms

A new method for creating a honeycomb morphology of TiO2 nanotubular arrays has been proposed using a two-step anodization process. The first anodization step utilizes fluoride containing ethylene glycol as an electrolyte (EG-solution). The anodic oxide formed during the first anodization step was removed to expose hemispherical nanoindentations created on the substrate. These nanoimprints acted as templates for honeycomb morphology during the second step of anodization that was carried out in fluoride containing phosphoric acid solution (PA-solution). Several small diameter nanotubes were grown in each honeycomb cell during the second anodization step. The formation of such honeycomb morphology required a threshold concentration of Ti4+ in the anodization electrolyte. Below the threshold Ti4+ concentration (4 × 10−3 M in this investigation), a good honeycomb morphology was not observed. The formation mechanism proposed is based on weakly linear surface perturbation theory aided by compressive growth stresses and a decrease in surface energy due to the presence of [TiF6]2− species.

Cadmium zinc telluride (CZT) nanowire sensors for detection of low energy gamma-ray detection

Bulk single crystals of CdZnTe compound semiconductor is used for room temperature radiation detection in commercial radiation sensors. A large volume of detector material with low defect density is required for increasing the detection efficiency. Manufacture of such a bulky detector-quality material with low defect density is expensive. In this communication, synthesis of nanowires arrays of CdZnTe that can be used for detecting low energy radiation is reported for the first time. CdZnTe ternary compound semiconductor, referred as CZT, was electrodeposited in the form of nanowires onto a TiO2 nanotubular template in non-aqueous electrolytes using a pulse-reverse process at 130 °C. Very high electrical resistivity of the CZT nanowires (in the order of 1010 Ω-cm) was obtained. Such a high resistivity was attributed to the presence of deep defect states such as cadmium vacancies created by the anodic cycle of the pulse-reverse electrodeposition process. Stacks of series connected CZT nanowire arrays were impressed with different bias potentials. The leakage current was in the order of tens of PicoAmperes. When exposed to a radiation source (Am -241, 60 keV), the current flow in the circuit increased. The preliminary results indicate that the CZT nanowire arrays can be used as radiation detector materials at room temperature with a much low bias potential (0.7 - 2.3 V) as against 300 - 500 V applied to the bulk detector materials.

Stability of the Nanoporous Bismuth Oxide Photoanodes for Solar Water Splitting

Bismuth oxide has well-dispersed valence bands that show enhanced mobility of charge carriers, high refractive index, and large dielectric constant. These properties are attractive for photocatalysis. Bismuth oxide has widely been investigated for photo degradation of dyes for safeguarding the environment. However, not much work has been reported on bismuth oxide as a photo electrode material for solar water splitting. Photo decomposition of Bi2O3 is a concern when used as a photo cathode. However, Bi2O3 can be obtained as an n-type semiconductor by stabilizing other polymorph: β-Bi2O3, which is a metastable phase. Thin films of nanoporous bismuth oxide were synthesized by a simple electrochemical anodization of bismuth substrate. Annealing the anodic nanoporous Bi2O3 at 240 °C for 2 h resulted in stabilization of the β-Bi2O3 phase. Longer annealing times resulted in formation of the more stable α-Bi2O3 phase that showed monoclinic lattice structure and p-type semiconductivity. The photo stability of these materials is discussed based on the photoelectrochemical measurements.

Synthesis and characterisation of CZT nanowire arrays for gamma ray detection

A single step pulsed potential electrodeposition of cadmium zinc telluride (CZT) nanowires was carried out in propylene carbonate solution at 130°C onto a nanotubular TiO2 template. Various combinations of potential pulses were investigated and the electrochemical parameters were optimised to obtain nanowires with a stoichiometry of Cd0.96Zn0.04Te. The CZT nanowires showed an electrical resistivity of 4.48 × 109 Ω-cm and an electronic band gap of 1.53 eV after annealing at 350°C for 1 h in argon atmosphere. The CZT nanowires grown on semiconductor nanotubular TiO2 templates can be integrated into a detection device for sensing low-medium energy gamma rays.

Photoelectrochemical characterization of dual-layered anodic TiO2 nanotubes with honeycomb morphology

Titanium dioxide (TiO2) nanotubes having a novel honeycomb like morphology were synthesized by a two-step anodization process and characterized for photoelectrochemical behavior. The titania nanotubes with honeycomb morphology showed at least 32% higher photocurrent density than the regular vertically oriented titania nanotubes at any given bias potential. The enhanced photoactivity of the honeycomb morphology was attributed to the better charge transport properties and the presence of a hemispherical surface morphology that enhanced the light harvesting behavior.

Particle Distribution and Tool Wear in Friction Stir Processed Al-SiCp Coatings

Abstract Al-Mg alloy was friction stir processed (FSP) incorporating two different sized SiC particles. Reduction of particle size after FSP was very significant with coarser SiC particles than finer. The density of SiC particles in FSP nugget was found to be uniform from top to bottom with coarser SiC particles, whereas it was more at the bottom than the top with finer. Tool wear after FSP was found to be relatively more with coarser particles than finer.

Sonoelectrochemical synthesis of low band gap titania nanotubes for photoelectrochemical generation of hydrogen

The sonoelectrochemical method is a highly efficient technique for the synthesis of well ordered and robust titanium dioxide nanotube arrays. Self ordered arrays of TiO2 nanotubes of various diameters and length can be rapidly synthesized under an applied potential of 5-20 V in the presence of organic electrolyte solvents like ethylene glycol. The TiO2 nanotubes prepared in the organic electrolytes and annealed under N2 atmospheres give a TiO2-xCx type of semiconductor materials having a band gap of 2.0 eV. The hybride nanotubes demonstrated promising efficiency in splitting water in the presence of solar light. In addition, the modeling of titania nanotubes using the first principles of the Density Functional Theory (DFT) approach is underway for calculating electronic properties of the TiO2 nanotubular structure. It is envisioned that the DFT modeling will yield valuable information in developing improved titania photoanodes for high efficiency photoelectrochemical splitting of water.