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Stainless steel (SS) canisters used for storing spent nuclear waste are prone to chloride-induced stress corrosion cracking. Friction stir welding (FSW), being a low heat input process, can be used for repair welding of these canisters. In this study, an artificial crack was introduced on the 304L SS coupons by an electric discharge machining process. The artificial crack was repair-welded by FSW by maintaining the tool temperature a constant at two levels (725 and 825 °C). Friction stirring significantly reduced the grain size of the stirred zone (SZ) from about 47 to 2–4 μm. The fraction of low-angle grain boundaries increased in the SZ from 2 to 37–43%. On the other hand, the fraction of special grain boundaries (Σ3 and Σ9) that was ~ 50% in the base metal reduced to

Novel electrode coating materials are needed for coal-fired magnetohydrodynamic (MHD) direct-current extraction power plants. Addition of potassium salt to increase the conductivity of coal plasma combined with high temperature render the service conditions extremely arduous. Therefore, the electrode coating materials must possess certain properties including high-temperature corrosion resistance, high-temperature oxidation resistance, and adequate electrical conductivity. In this work, those very properties of three different compositions of transition metal diboride solid solutions of HfB2-ZrB2 are studied. Two main approaches of oxidation mitigation methods were employed: adding chemical dopants, such as Hf and La, and performing pre-anodization on the samples to create a protective oxide coating. Thermogravimetric experiments were run at different partial pressures of oxygen to determine the high-temperature oxidation resistance of these boride ceramics. In addition, two different salt coatings (KCl and K2SO4) were applied to the surface of the samples to test their hot corrosion resistance. High-temperature polarization resistance tests were also conducted to measure the electrical resistivity. The addition of lanthanum aided in long term oxidation and corrosion protection, with a parabolic rate constant, Kp = 1.33 × 10−5 kg2/m4 s, a 40% improvement over the sample without the addition of lanthanum under the same conditions. Anodization, however, proved disadvantageous when combined with either lanthanum or a corrosive environment.

Passivation kinetics of two Mg-RE alloys, such as Mg-Nd-Gd-Zn-Zr (EV31A), and Mg-Y-Nd-Gd-Zr (WE43C) were investigated in two different heat treated conditions (solution treated and overaged) in 0.01 - 1.0 M NaOH solutions under potentiostatic conditions. Negative reaction order was observed in dilute NaOH which transitioned to positive values as the passivation time increased and in the 1 M NaOH as well. The passive layers showed platelet morphology and the size of the platelets decreased with increase in the NaOH concentration. The hydrogen evolution reaction (HER) kinetics was not improved on the passive layer covered surface of the Mg-RE alloys in contrast to the improvements reported on the hydroxide covered pure magnesium. The electrochemical impedance increased with increase in the NaOH concentration in the solution treated condition of both Mg-RE alloys, whereas the overaged EV31A alloy showed a reverse trend. The passive layer of EV31A showed almost 100% higher charge carrier density than the film formed on the WE43C in the overaged condition. A better passivation behavior was observed in the solution treated condition than that in the overaged condition which could be attributed to the uniform distribution of the RE elements in the solution treated specimens. The WE43C alloy revealed better corrosion resistance in the alkaline solution than the EV31A alloy. Keywords: Magnesium rare earth alloy, Passivation, Alkaline solution, Hydrogen evolution

Electrochemical energy storage of TiO 2 anodic oxide with a novel honeycomb morphology was characterized and compared with that of standard TiO 2 nanotubes. The new morphology consists of several smaller-diameter (∼20 nm) nanotubes stacked inside of each honeycomb like hemisphere. The honeycomb like hemispheres are 160–200 nm in diameter with inter-wall thicknesses of 20–50 nm grown onto a planar barrier layer. The dual-layered honeycomb oxide has a total thickness of about 350–500 nm and high surface area. Cyclic voltammetry and galvanostatic charge-discharge tests were carried out on these oxide samples in as-anodized and thermally annealed conditions using 0.1 M NaOH and 0.1 M LiCl + 0.1 M HCl electrolytes. The honeycomb arrays showed an areal capacitance of 56 mF cm −2 at 100 mV s −1 scan rate, and 0.75 mF cm −2 at a current density of 0.1 mA cm −2 . The areal capacitance of the honeycomb structured samples were about 60% higher than that of the regular TiO 2 nanotubes. The honeycomb structured TiO 2 also showed 33% higher capacitance retention after 10,000 cycles than that of regular TiO 2 nanotubes. The higher capacitance retention could be attributed to the enhanced structural stability of the honeycomb structures.

Addition of glycerol as a viscosity modifier in concrete is proposed to decrease the permeability of corrosion-inducing ions such as chloride and sulfate. In addition to controlling the permeability of concrete, glycerol could perform as an inhibitor of corrosion of rebar steel. Cyclic polarization studies were carried out on metallographically polished rebar steel specimens in actual concrete solutions at two different pH conditions (pH 12.5 and 9.0) and different chloride concentrations. The threshold concentration of chloride for passivity breakdown at pH 12.5 was greater than 50 × 10−3 mol/L in the absence of glycerol addition. The threshold increased to 81 × 10−3 mol/L upon addition of 2 wt% glycerol. The threshold chloride concentration for passivity breakdown in pH 9.0 cement solution was 0.2 × 10−3 mol/L without glycerol addition. No beneficial effect of glycerol was observed in the low pH condition. However, glycerol enhanced the passivation kinetics of the rebar steel in saturated cement solution, but did not affect the electronic properties of the passive layer. The passive layers exhibited n-type semiconductivity with a charge carrier density in the range of 2–7.5 × 1020 cm−3. Polarization of the specimens to potentials is higher than oxygen evolution potential, resulted in transition top-type semiconducting character due to an accumulation of holes. This phenomenon could be related to the passivity breakdown.

Spark plasma sintered transition metal diborides such as HfB 2 , ZrB 2 and their solid solutions were investigated as electrode materials for electrochemical hydrogen evolutions reactions (HER) in 1 M H 2 SO 4 and 1 M NaOH electrolytes. HfB 2 and ZrB 2 formed complete solid solutions when mixed in 1:1, 1:4, and 4:1 ratios and they were stable in both electrolytes. The HER kinetics of the diborides were slower in the basic solution than in the acidic solutions. The Tafel slopes in 1 M H 2 SO 4 were in the range of 0.15–0.18 V/decade except for pure HfB 2 which showed a Tafel slope of 0.38 V/decade. In 1 M NaOH the Tafel slopes were in the range of 0.12–0.27 V/decade. The composition of Hf x Zr 1−x B 2 solid solutions with x=0.2–0.8, influenced the exchange current densities, overpotentials and Tafel slopes of the HER. The EIS data were fitted with a porous film equivalent circuit model in order to better understand the HER behavior. In addition, modeling calculations, using density functional theory approach, were carried out to estimate the density of states and band structure of the boride solid solutions.

Cyclic voltammetry CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy measurements were carried out in 1 and 5 wt% TeCl4 containing LiCl-KCl eutectic molten salt at 450 °C using tungsten and glassy carbon as working electrodes. Reduction of Te(IV) to Te(0) occurred in three steps at potentials more negative than −0.4 VAg/AgCl. The Te(0) was further reduced to Te2− at more negative potentials. Formation of Te2− was observed at potentials more negative than −2.2 VAg/AgCl. Diffusivities of Te4+, Te2+, and Te2− and their formal potentials were estimated from the CV data. The diffusion coefficients of Te4+, Te2+ and Te2− were 0.9 × 10−5, 3.8 × 10−5, and 1.5 × 10−5 cm2 s−1, respectively in the 1 wt% TeCl4 containing LiCl-KCl molten salt. The diffusivity of Te species generally decreased with increase in the concentration of TeCl4.

Electrochemical corrosion tests were carried out on AZ31 magnesium alloy specimens in pH: 4.5, 9.5, and 13.0 solutions with 0-2000 ppm of chloride additions at room temperature. No passivity breakdown was observed during cyclic polarization in pH:13 solutions containing up to 1500 ppm of chloride. Addition of sodium sulfate and sodium dihydrogen phosphate as supporting electrolytes offset the chloride effect on the corrosion of AZ31 in pH 4.5 and 9.5 solutions. The Mott-Schottky analysis showed the presence of a duplex surface layer consisting of an n-type MgO1−x inner layer (x = 0.024-0.05), and a p-type outer layer which thickened with time at the expense of the inner layer.

Transition metal borides are being considered for use as potential electrode coating materials in magnetohydrodynamic direct power extraction plants from coal-fired plasma. These electrode materials will be exposed to aggressive service conditions at high temperatures. Therefore, high-temperature oxidation resistance is an important property. Consolidated samples containing an equimolar solid solution of ZrB2-HfB2 with and without the addition of 1.8 mol pct LaB6 were prepared by ball milling of commercial boride material followed by spark plasma sintering. These samples were oxidized at 1773 K (1500 °C) in two different conditions: (1) as-sintered and (2) anodized (10 V in 0.1 M KOH electrolyte). Oxidation studies were carried out in 0.3 × 105 and 0.1 Pa oxygen partial pressures. The anodic oxide layers showed hafnium enrichment on the surface of the samples, whereas the high-temperature oxides showed zirconium enrichment. The anodized samples without LaB6 addition showed about 2.5 times higher oxidation resistance in high-oxygen partial pressures than the as-sintered samples. Addition of LaB6 improved the oxidation resistance in the as-sintered condition by about 30 pct in the high-oxygen partial pressure tests.

This work presents novel photochemical reactors with integrated titania photocatalyst for the UV photodecomposition of gaseous methyl iodide present in simulated vessel off-gas (VOG) in a nuclear fuel reprocessing facility. Continuous flow reactors composed of fluorinated ethylene propylene (FEP) were designed and constructed to allow transmission of UV light into the reactor while also providing resistance to the corrosive nature of methyl iodide. Titania nanotube photocatalyst increases methyl iodide photolysis rate by two orders of magnitude compared to that with UV light alone. The rate constant for methyl iodide photolysis with and without catalyst directly depends on the intensity of the light at the location of the reaction, which is inversely proportional to the distance from the linear light source. The titania photocatalyst is significantly active at low light intensity where no detectable reaction occurs in absence of catalyst. For example, at an initial concentration of 400 ppb methyl iodide, a residence time of 9 s, and a light intensity of 0.5 mW/cm2, conversions with and without titania photocatalyst are >99% and 0%, respectively. The UV light source is a low-pressure mercury vapor lamp with primary emission at 254 nm, the wavelength that is in the range of highest cross-sectional absorbance for methyl iodide. Parameters investigated in this work include UV lamp to reactor distance (light intensity), impact of humidity, gas phase composition, and catalyst form (nanotube vs powder). This work also demonstrates that titania possesses a characteristic activation period on the order of approximately one hour to reach its full steady-state activity.

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.

Electrochemical energy storage using nanostructured metal oxides has been a topic of intensive investigation among materials scientists and electrochemists. Several transition metal oxides such as TiO2, WO3, Fe2O3, and V2O5 in the form of ordered nanoporous or nanotubular structures have been considered as active electrode materials for battery and electrochemical capacitor applications. Bismuth oxide (Bi2O3), because of its reasonable conductivity, high dielectric constant, catalytic behavior, non-toxicity, and abundance finds applications in a wide range of areas like gas sensors, optical coatings, microelectronics, photocatalysts, solid state electrolytes, and superconductors. Electro-deposited bismuth oxide (on various substrates and in various morphologies) has been investigated as a super capacitor material. The specific capacitance of the electro-deposited Bi2O3 varies from 60 -160 F/g. In the present investigation, Bi2O3 nanoporous films were synthesized by electrochemical anodization in the electrolyte solutions containing citric acid, ethylene glycol and glycerol. Nanoporous bismuth oxide films were formed by anodizing bismuth circular discs of 3 mm thick and 12.7 mm diameter. Anodization was carried out at various potentials ranging from 3 V to 60 V for different time durations ranging from 0.5 to 2 h. After anodization, the samples were thermally annealed at 200 °C for 2h. The influences of anodization time, electrolyte concentration and applied voltages on morphology have been investigated in this study. The electrochemical energy storage of the anodic bismuth oxide was investigated by carrying out cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy (EIS), and Mott-Schottky measurements in different electrolytes in the pH range of 7-14. All the depositions were examined under a FEI Quanta 200F scanning electron microscope. Figure 1 shows the nanoporous morphology of the oxide layer formed at 3V for 30minutes in citric acid electrolyte. The diameters of the pores were in the range of 20 -50 nm and total thickness of the film was about 500nm. It was observed that pore diameter and film thickness changed with the change in applied potential, time and electrolyte concentration. The XRD analysis showed that the nanoporous anodic oxide film after annealing was tetragonal β-Bi2O3. The nanoporous Bi2O3 contained a defect concentration in the range of 7x1016 – 4x1018 cm-3 for various anodized conditions. Figure 2 shows the typical results of cyclic voltammetry of as-anodized bismuth oxide nanoporous in 1 M KOH at different scan rates in a wide potential range. Oxidation of Bi (III) was not observed until 1.0 VAg/AgCl. However, a reduction wave was observed at potentials more negative than -0.6 VAg/AgCl, which could be attributed to the hydrogen ion intercalation in the oxide. Figure 3 shows the potential transient profile during the galvanostatic charge-discharge that was carried out at 10 mA/cm2.The high current density resulted in a sharp potential drop. The capacitance was calculated to be 78 mF/cm2 and the charge retention efficiency was 92%. The capacitance reported for the electrodeposited α-Bi2O3 was in the range of 12 - 22 mF/cm2 [1]. The anodic nanoporous oxide layer showed at least four times higher capacitance than the electro-deposited bismuth oxide. A detailed discussion will be provided in the final presentation on the electrochemical behavior of the Bi2O3 nanoporous structure as a function of morphology, and defect and electronic structures. References 1. T.P. Gujar, V.R. Shinde, D. Lokhande, Sung-Hwan Han, Journal of Power Sources 161 (2006) 1479–1485

ZrO2–WO3 mixed oxide nanotubes were synthesized by a simple electrochemical anodization route. The oxide nanotubes contained a mixture of metastable hexagonal WO3 and monoclinic (and orthorhombic) ZrO2 phases, as well as a mixed-oxide ZrW2O8 phase that showed a metastable tetragonal symmetry. Evaluation of photo-activity of the materials showed generation of photo-potentials of −85 mV and −230 mV in the as-anodized and annealed conditions. Because of the mismatch in the band edge positions of the WO3 and ZrO2 phases and the resultant relaxation of photo-generated charge carriers, no significant photo-current density could be observed. The arrays of oxide nanotubes are considered for electrochemical capacitor application because of their morphology-assisted fast charge/discharge kinetics and large surface area. Presence of a large concentration of charge defects (on the order of 1021 cm−3) and the reported high proton conductivity of the ZrO2–WO3 mixed oxide rendered high capacitance, which decreased with an increase in the scan rate of cyclic voltammetry. The highest measured capacitance was 40.03 mF/cm2 at a scan rate of 10 mV/s and the lowest was 1.93 mF/cm2 at 1 V/s in 1 M sulfuric acid solution.

Nanoporous oxide layer was formed on the NdFeB permanent magnets by a simple anodization process in fluoride containing ethylene glycol at 50 V for 15 minutes. The electrochemical polarization behavior of the commercial permanent magnetic material was evaluated in 0.1 M NaH2PO4 solution (pH: 4.75) containing 0–0.017 M of chloride at room temperature. The material was evaluated in magnetized and demagnetized condition with different surface preparation conditions such as: Ni-Cu-Ni coated, uncoated (bare), and anodized. The nickel coated magnetic sample showed reasonable pitting corrosion resistance up to 0.0017 M chloride addition. The uncoated samples showed passivity in chloride free phosphate solution. Chloride additions caused pitting and the pitting protection potential was equal to the corrosion potential with 0.0017 M chloride addition. Demagnetization did not alter the electrochemical polarization. However, the passive film formed on the magnetic sample showed higher charge carrier density than the film formed on the demagnetized sample.

Formation of iron oxide nanotubes on to pure iron substrate by an electrochemical anodization method was investigated in fluoride containing electrolytes. Anodization of iron foil in fluoride containing borate solution resulted in stacked nano-ring type oxide morphology. Nanoporous oxide layer was observed at low pH and a granular oxide layer was formed at higher pH of phosphate + fluoride solutions. Formation of either nanoporous or nanotubular oxide layer was observed in ethylene glycol (EG) solution containing 0.05–0.1 M fluoride + 1.5–3.0 vol.% water. Transition from nanoporous structure to nanotubular structure was critically controlled by anodization potential, water addition and fluoride concentration of the EG solution. The potential required for this transition decreased with increase in the water content up to 7 vol.% beyond which enhanced dissolution occurred. Annealing of the nanotubes at 500 °C resulted in predominantly α-Fe2O3 crystal structure. The annealed Fe2O3 samples consisting of a single layer of nanotubular structure showed a photo current density of 0.4 mA/cm2 at 0.5 V Ag/AgCl in 1 M KOH solution under simulated solar light illumination.

Phase transformation analysis of the phosphate containing and carbon doped titania nanotubes, prepared by a simple anodization method, reveals complete transformation from amorphous to anatase phase in air between 360 and 400 °C. Activation energies for formation of anatase phase are evaluated and compared for the two types of titania nanotubes. A detailed analysis of the phase transformation characteristics and stability of the anatase phase is reported.

Electron−hole (e-h) recombination loss is a major practical problem in using TiO2 as a photocatalyst. This paper describes the use of organic additives to reduce e-h recombination losses which significantly improved the photocurrent density of the integrated TiO2 nanotube/Ti photoanode. Studies on photoelectrochemical hydrogen generation using nanotubular arrays of TiO2 photoanodes were carried out in 1 M KOH with the addition of three different organic additives, namely, methanol (one hydroxyl group), ethylene glycol (two hydroxyl groups), and glycerol (three hydroxyl groups) using a simulated solar light. Ethylene glycol was found to be the best among the investigated organic additives to reduce electron hole recombination. The addition of ethylene glycol produced a photocurrent density of 3.3 mA/cm2 at 0.2 VAg/AgCl compared to 0.87 mA/cm2, using a 87 mW/cm2 light intensity in 1 M KOH solution. On the other hand, methanol and glycerol showed a photocurrent density of 2.43 and 2.55 mA/cm2 under the same ...

Self ordered arrays of titanium manganese mixed oxide nanotubes were prepared by anodization of Ti8Mn alloy (UNS R56080) under ultrasonication in diluted ethylene glycol containing fluoride. The dimensions of the nanotubes (diameter: 20–100 nm and length: 0.5–2.0 μm) could be tuned by changing the synthesis parameters. The as-anodized nanotubes showed a stoichiometry of (Ti,Mn)O 2 . Upon annealing at 500 °C in oxygen atmosphere, the nanotubes contained a mixture of anatase + rutile phases of TiO 2 and Mn 2 O 3 . The composition of the oxide nanotubes was influenced by the chemistry of the phases present in the alloy. More manganese content was observed in the oxide formed on the β-phase than in the oxide layer of α-phase. Anodization in the ultrasonic field increased the kinetics of nanotubular oxide formation and resulted in homogeneous ordering of the nanotubular arrays as compared to the anodization by conventional stirring in the fluoride containing ethylene glycol solution. Whereas, anodization in aqueous acidified fluoride solutions resulted in severe attack of the β-phase and did not show presence of nanotubular oxide structure.

Anodization of Ti using fluoride containing polyhydric alcohols such as ethylene glycol or glycerol as electrolyte results in ordered arrays of TiO2 nanotubes with a smooth surface and a very high aspect ratio. However, the reproducibility of the result is affected by many experimental parameters, notably the water content. In this investigation, anodizations of Ti foil in anhydrous ethylene glycol +0.2 wt% NH4F solution (EG solution) with 0–1.0 wt% water additions were carried out at 20 V for 45 min in a dry-argon filled controlled-atmosphere glove box. It was observed that a minimum amount of 0.18 wt% of water addition was required to form a well ordered TiO2 nanotubular arrays. When the anhydrous EG solution was reused for third time, ordered arrays of nanotubes started to form. When the water addition to the EG solution was more than 0.5 wt%, formation of ridges was observed on the nanotubes. XPS results showed presence of un-anodized Ti element in the anhydrous condition and presence of organic and (NH4)2TiF6 type compounds in all the anodized samples in addition to the regular TiO2 phase. The results underline the influence of water content and local pH condition to form the ordered nanotubular arrays. Keywords: Nanotubular TiO2, Anodization, Ethylene glycol, Oxide film

This new method describes the application of sonoelectrochemistry to quickly synthesize well-ordered and robust titanium dioxide (TiO2) nanotubular arrays. Self-ordered arrays of TiO2 nanotubes of 30–100 m in diameter and 300–1000 nm in length can be rapidly synthesized under an applied potential of 5–20 V. The rate of formation of the TiO2 nanotubes by the sonoelectrochemical method is found to be almost twice as fast as the magnetic stirring method. It also demonstrates that high-quality nanotubes can be prepared using high viscous solvents like ethylene glycol under ultrasonic treatment. The TiO2 nanotubes prepared in the organic electrolytes (ethylene glycol) are then annealed under H2 atmosphere to give TiO2−xCx types material having a band gap of around 2.0 eV. This process is found to be highly efficient for incorporating carbon into TiO2 nanotubes. Various characterization techniques (viz., FESEM, GXRD, XPS, and DRUV–vis) are used to study the morphology, phase, band gap, and doping of the nanotubes. The photoelectrocatalytic activity of these materials to generate H2 by water splitting is found to be promising at 0.2 V vs Ag/AgCl.

A single step non-aqueous electrodeposition of cadmium zinc telluride (CZT) nanowires on nanoporous TiO 2 substrate was investigated under pulsed-potential conditions. Propylene carbonate was used as the non-aqueous medium. Cyclic voltammogram studies were carried out to understand the growth mechanism of CZT. EDAX and XRD measurements indicated formation of a compound semiconductor with a stoichiometry of Cd 1− x Zn x Te, where x varied between 0.04 and 0.2. Variation of the pulsed-cathodic potentials could modulate the composition of the CZT. More negative cathodic potentials resulted in increased Zn content. The nanowires showed an electronic band gap of about 1.6 eV. Mott-Schottky analyses indicated p-type semiconductor properties of both as-deposited and annealed CZT materials. Increase in Zn content increased the charge carrier density. Annealing of the deposits resulted in lower charge carrier densities, in the order of 10 15 cm −3 .

A potential-control circuit has been devised to measure passive-current (corrosion rate) on stainless alloys in conditions simulating the usual noble drift of the open-circuit corrosion potential in the presence of passivating dissolved oxidizers. Open-circuit corrosion potential and passive-current decay were measured with and without dissolved oxygen in dilute sulfuric acid solutions as a function of time on austenitic stainless steel and a Ni-base Cr–Mo–W alloy. The results confirm the long-standing assumption that constant-potential potentiostatic tests give a good approximation of passive-film growth kinetics in the presence of passivating dissolved oxidizers. Mott–Schottky measurements showed that passive films on each alloy have similar semiconducting properties whether formed potentiostatically or by dissolved oxygen. In both cases, the passive film on stainless steel seems to be composed of an inner n-type barrier layer covered with an outer p-type layer. The passive film on the nickel-base alloy is primarily n-type with only weak evidence of p-type behavior in an outer layer. Results are in general agreement with predictions of the point-defect passive-film model proposed by Macdonald and coworkers.

Room temperature anodization of titanium foil specimens was carried out in 0.5 M phosphoric acid solution with addition of various halide ions. Addition of 0.138 M HF or NaF resulted in self-ordered nano-tubular oxide structure. Addition of bromide and chloride ions initiated only pitting and nano-pores were not observed during anodization. Acidified fluoride solution is found to be necessary to obtain ordered nano-structure as neutral fluoride solution did not form nano-pores. Instability of the oxide layer during anodization and formation of the self-ordered structure can be explained by the perturbation theory. Separation of individual nano-tubes of titanium oxide layer from the inter-connected nano-pores could be attributed to the possible repulsion forces of the cation vacancies.

Nanotubular anodic oxide layer was grown on titanium surface by room temperature anodization in 0.5 M H3PO4 + 0.138 M fluoride solution. The inner surface of the nanotubular oxide walls contained adsorbed anions from the solution that were predominantly phosphate. Hydroxy apatite was electrodeposited on the anodized surface of Ti. Adsorbed phosphate ions facilitated nucleation of nanophase calcium phosphate material inside the nanotubular structure that resulted in vertical growth of apatite crystals. Bond strength of the calcium phosphate coating was found to be good, and cohesive mode of failure of the coating was observed after tensile testing.

Surface modification treatments were carried out on Alloy 22, a nickel base Ni–22Cr–13Mo–4W alloy, using shot peening process followed by low-temperature annealing. Electrochemical corrosion experiments were carried out on the surface modified specimens in hot acid chloride solution. Passivation and Mott–Schottky (M–S) analysis were carried out in sulfuric acid solution at room temperature. Improved passivity and better electronic properties of passive film were observed on the surface modified specimens that could be attributed to the surface nanocrystallization.

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.

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.

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 oC. Very high electrical resistivity of the CZT nanowires (in the order of 10 10 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.

The sonoelectrochemical method is a highly efficient technique for the synthesis of well ordered and robust titanium dioxide nanotube arrays. Self ordered arrays of TiO 2 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 TiO 2 nanotubes prepared in the organic electrolytes and annealed under N 2 atmospheres give a TiO 2 – x C x 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 TiO 2 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. Keywords: titanium dioxide nanotube, photoelectrochemical water splitting, density functional theory

Anodization of Ti in acidified fluoride solution resulted in a vertically oriented and an ordered nanotubular titanium oxide surface. Annealing of the TiO2 nanotubular arrays in a carbonaceous or nitrogen containing atmosphere presumably resulted in band-gap states, which enhanced the photo-activity. Composite electrode of nanotubular TiO2 + carbon doping resulted in a photocurrent density of more than 2.75 mA/cm2 at 0.2 V(Ag/AgCl) under simulated solar light illumination. The enhanced photo-activity of the carbon-modified nanotubular TiO2 is highly reproducible and sustainable for longer duration. The charge carrier densities, calculated based on the Mott-Schottky analyses, were in the range of 1-3 x 1019 cm-3 for both the carbon modified and the nitrogen-annealed nanotubular TiO2 samples. The asanodized and oxygen-annealed samples showed a charge carrier density of 5 x 1017 and 1.2 x1015 cm-3 respectively. In this study, the measured photo current density was not directly related to the charge carrier densities of the nanotubes. Presence of different phases, such as amorphous, anatase and rutile, influenced the photo activity more than the charge carrier density.

Metal-oxide semiconductors have received a wide attention as photo anodes since the demonstration of water splitting by TiO2 electrode under UV illumination [1]. Significant research efforts were focused on developing various semiconductors with band-gap engineering for efficient photo water splitting. Bismuth oxide (Bi2O3) with a band gap of 2.8eV is a promising photo anode which exhibits good electrical conductivity, oxygen ion conductivity, and high dielectric permittivity. Due to its distinctive properties, Bi2O3 finds applications in a wide range of areas like gas sensors, optical coatings, microelectronics, photocatalysts, solid state electrolytes, superconductors etc. In spite being a non-toxic material with appropriate band gap and valence band edge position (+3.13 V vs. NHE), Bi2O3 demonstrates a poor hydrogen evolution due to its lower conduction band edge position (+0.33 V vs. NHE). The general strategies employed to overcome these limitations were simultaneous doping and nano-sizing of the material. In the present investigation, Bi2O3 nanoporous films were synthesized by electrochemical anodization in the electrolyte solutions containing citric acid, ethylene glycol and glycerol. Nanoporous bismuth oxide films were formed by anodizing bismuth circular discs of 3 mm thick and 12.7 mm diameter. Anodization was carried out at various potentials ranging from 3 V to 60 V for different time durations ranging from 0.5 to 2 h. After anodization, the samples were thermally annealed at 200 °C for 2h. The influences of anodization time, electrolyte concentration and applied voltages on morphology have been investigated in this study. Potentiodynamic, potentiostatic, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis studies were carried out with and without illuminated conditions. All the depositions were examined under a FEI Quanta 200F scanning electron microscope. Figure 1 shows the nanoporous morphology of the oxide layer formed at 3V for 30minutes in citric acid electrolyte. The diameters of the pores were in the range of 20 -50 nm and total thickness of the film was about 500nm. It was observed that pore diameter and film thickness changed with the change in applied potential, time and electrolyte concentration. Photo electrochemical studies were carried out using a potentiostat (Gamry, Reference 600) with platinum as the counter electrode. Potential Vs current plots were constructed by scanning the potential of the sample from the open circuit potential to 0.5 V at a scan rate of 5mV/s in 1 M KOH solution. Potentiostatic measurements were carried out in 1 M KOH solution at 0.2 V and 0.5 VAg/AgCl. A solar simulator with an AM 1.5 filter was used for illuminating the samples. The thickness of the oxide layer increased with the increase in the anodization potential. The photo current density of the nanoporous bismuth oxide increased with increase in the thickness of the oxide. The dark current density decreased with the increase in thickness of the oxide layer. The maximum photo current density (Iilluminated-Idark) recorded at an applied potential of 0.5 VAg/AgCl was 1 mA/cm2 for the sample anodized at 20 V. The sample anodized at 60 V showed about 20 µA/cm2 dark current density and 0.8 mA/cm2 photo current density at 0.5 VAg/AgCl. The photo activity of the nanoporous bismuth oxide is comparable to that of nanotubular TiO2 oxide photo anodes. The nanoporous Bi2O3 contained a defect concentration in the range of 7x1016 – 4x1018 cm-3 under dark condition for various anodized conditions. Upon illumination, the defect density increased to values in the range of 4x1017 to 2x1019 cm-3. A detailed discussion will be provided in the final presentation on the photo electrochemical behavior of the Bi2O3 nanoporous structure as a function of morphology, and defect and electronic structures. References A. Fujishima and K. Honda, Nature 238 (1972) 37

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.

Ordered arrays of fluoride incorporated iron oxide nanotubes were synthesized by a simple electrochemical anodization technique. An iron foil was anodized at 50 - 60 V in ethylene glycol solution containing 0.3 wt% NH4F and 1.5 vol% distilled water for 15 - 30 minutes. During the anodization process, oxygen anions were partially substituted with fluoride anions. The self-ordered, vertically oriented, and free-standing iron oxide based nanotubes showed enhanced Li-ion storage capacity, and faster kinetics for Li-ion diffusion. The fluoride incorporation in the oxide was believed to expand the lattice, thereby increasing the ability of the lattice to accommodate enhanced lithium ion intercalation. Furthermore, increased conductivity was noted because of adsorption of carbonaceous species on the nanotubes while anodization and subsequent incorporation of carbon in the Fe-O-F lattice structure after thermal annealing. The proposed configuration of nanotubes showed a reversible nominal capacity > 700 mAh/g.

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.

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.

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.