Lakshman Neelakantan

Bio: Lakshman Neelakantan is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topic(s): Corrosion & Alloy. The author has an hindex of 14, co-authored 40 publication(s) receiving 496 citation(s). Previous affiliations of Lakshman Neelakantan include Ruhr University Bochum & Max Planck Society.

Electrochemical and semiconducting properties of thin passive film formed on titanium in chloride medium at various pH conditions

Abstract: The electrochemical behavior of titanium has been investigated in chloride electrolyte of different pH values (2.1, 5.2 and 7.4). Potentiodynamic polarization studies showed wide passive behavior irrespective of change in pH. Passive films were grown by applying different passive potentials (0.2 to 0.6 V (vs SCE)) by recording chronoamperometric curves for duration of 8 h. Electrochemical impedance spectroscopy (EIS) measurements showed higher impedance values for the oxide layer grown in neutral pH condition and for oxides formed at higher formation potentials. Emphasis is made mainly on the properties of titanium oxide estimated by Mott-Schottky analysis, which shows that passive film formed, is of n -type semiconducting film and the donor concentration is in the order of 10 20 cm − 3 . The calculated diffusivity of point defects is in the range of (0.5 to 2.5) × 10 ˗16 cm 2 s − 1 and tends to decrease with increase in electrolyte pH. Surface morphology of the passive films was examined using scanning electron microscope and surface roughness was characterized using confocal microscope. Ex- situ ellipsometry measurements were performed to analyze the optical constants of the oxide layer and to determine the oxide thickness. The thickness is in the range of 3.5 to 5.8 nm and comparable to the calculated values.

Influence of crystallite size and surface morphology on electrochemical properties of annealed TiO 2 nanotubes

Abstract: The current study investigates the effect of crystallite size and surface morphology of TiO2 nanotubes on their wettability and electrochemical properties. Self-organized amorphous TiO2 nanotubes were synthesized by anodization process in an acidic (0.5 wt% HF) and a neutral electrolyte (1 M Na2SO4 + 0.5 wt% NaF). Subsequently, the nanotubes were annealed at 450 °C to achieve crystalline phase. Scanning electron microscope micrographs revealed that nanotubes formed from the neutral bath are four times longer (1.2 μm) than the ones synthesized from the acidic bath (325 nm). The charge consumed during anodization is greater under the acidic conditions implying the severity of the attack on the nanotubes by the electrolyte. X-Ray diffraction analysis showed that after annealing TiO2 crystallizes in the tetragonal lattice as anatase structure. Peak fitting method for line profile analysis was employed to estimate the crystallite size and the micro strain. The oxide nanotubes formed in neutral medium showed smaller crystallite size (28.91 nm) than the one formed in acidic medium (43.37 nm). Wettability measurements showed wetting angles

Selective surface oxidation and nitridation of NiTi shape memory alloys by reduction annealing

Abstract: The current investigation aims in understanding the effect of short-term (300 s) annealing of NiTi shape memory alloys (SMAs) in a reducing atmosphere of N 2 –10% H 2 . The influence of temperature on the resulting surface morphology and chemistry is elucidated. On annealing at 600 °C, the surface is covered with a thin layer of titanium oxide, which is 7.5 nm thick, while at 800 °C, the surface is covered with a golden-yellow layer of TiN of thickness more than 100 nm. The surface analysis carried out by XPS on the specimen annealed at 800 °C confirms the formation of TiN and more notably, the surface is devoid of Ni.

Titania nanotubes from weak organic acid electrolyte: fabrication, characterization and oxide film properties.

Abstract: In this study, TiO 2 nanotubes were fabricated using anodic oxidation in fluoride containing weak organic acid for different durations (0.5 h, 1 h, 2 h and 3 h). Scanning electron microscope (SEM) micrographs reveal that the morphology of titanium oxide varies with anodization time. Raman spectroscopy and X-ray diffraction (XRD) results indicate that the as-formed oxide nanotubes were amorphous in nature, yet transform into crystalline phases (anatase and rutile) upon annealing at 600 °C. Wettability measurements show that both as-formed and annealed nanotubes exhibited hydrophilic behavior. The electrochemical behavior was ascertained by DC polarization and AC electrochemical impedance spectroscopy (EIS) measurements in 0.9% NaCl solution. The results suggest that the annealed nanotubes showed higher impedance (10 5 –10 6 Ω cm 2 ) and lower passive current density (10 − 7 A cm − 2 ) than the as-formed nanotubes. In addition, we investigated the influence of post heat treatment on the semiconducting properties of the oxides by capacitance measurements. In vitro bioactivity test in simulated body fluid (SBF) showed that precipitation of Ca/P is easier in crystallized nanotubes than the amorphous structure. Our study uses a simple strategy to prepare nano-structured titania films and hints the feasibility of tailoring the oxide properties by thermal treatment, producing surfaces with better bioactivity.

Role of crystallinity on the nanomechanical and electrochemical properties of TiO2 nanotubes

Abstract: In order to study the effect of annealing on the nanomechanical and electrochemical properties, TiO 2 nanotubes synthesized via anodic oxidation were subjected to thermal treatment at various temperatures. Structural studies revealed that as-anodized nanotubes are amorphous, but they become crystalline when subjected to thermal treatment in air. TiO 2 nanotubes annealed up to 450 °C crystallizes in pure anatase phase, whereas a mixture of anatase and rutile phase was observed for sample annealed at 600 °C. Interestingly, the evolution of rutile phase at 600 °C leads to decrease in crystallite size of the anatase phase. Nanoindentation studies showed that crystallite size plays a strong role in hardness, whereas porosity has significant influence on the elastic modulus of the nanotubes. Presence of two time constants in electrochemical impedance spectroscopy measurement presumably implies the formation of bilayer passive film. Mott Schottky analysis showed n -type semiconducting behavior with decrease in donor concentration (10 19 cm − 3 ) upon increasing the annealing temperature. However, the evolution of rutile phase decreases the donor concentration drastically by an order of a magnitude (10 18 cm − 3 ). Potentiodynamic polarization studies revealed that rutile phase favors enhanced passivation behavior exhibiting lower passive current density (10 − 7 A cm − 2 ) compared to the pure anatase nanotubes (10 − 6 A cm − 2 ). The enhanced passivation behavior of rutile phase was confirmed by Raman spectroscopy.

Heat treatment effect on the microstructure and corrosion behavior of 316L stainless steel fabricated by selective laser melting for proton exchange membrane fuel cells

Abstract: Microstructure and corrosion behavior of 316L stainless steel fabricated by selective laser melting (SLM) for bipolar plate were investigated and the subsequent heat treatment effect was also clarified. Results showed that sub-grains widely existed in the SLM 316L and the dislocations in the grain boundary migrated and disappeared after heat treatment. The as produced SLM 316L exhibited inferior corrosion resistance than the wrought in 0.5 M H2SO4 solution with 50 ppm Cl− and 2 ppm F− ions at 70 °C due to the SLM defects (molten pool boundaries, non-equilibrium phases etc.), and short, stress relief annealing did not homogenize the non-uniform structures. However, recrystallization heat treatment can improve the durability due to the more uniform structure and thicker passive film. Meanwhile, the proportion of the oxides in passive film formed on SLM 316L increased after heat treatment, especially for the chromic oxide, leading to a better anti-corrosive property.

A Review on TiO2 Nanotubes: Influence of Anodization Parameters, Formation Mechanism, Properties, Corrosion Behavior, and Biomedical Applications

Abstract: In this article, influence of anodization parameters on the formation of tubes, tube dimensions, formation mechanism, properties of TiO2 nanotubes (TNT), and their applications in biomedical field are reviewed. The fabrication of TNT of a different shape such as pore size, length, and wall thickness by varying anodization parameters including electrolytes, pH, voltage, electrolyte bath temperature, and current density is examined and discussed. The crystallographic nature of the nanotube obtained by various methods has also been discussed. Finally, the article concludes by examining the key properties including the corrosion aspect and various applications in biomedical field in depth.

The fate and role of in situ formed carbon in polymer-derived ceramics

Abstract: Polymer-derived ceramics (PDCs) have been intensively studied for nearly 50 years due to their unique advantages to producing ceramic fibers, coatings, foams, nanocomposites and for additive manufacturing. A phenomenon associated with the polymer-to-ceramic transformation process using organo-substituted silicon polymers as the starting material has been widely reported, namely, in situ formation of carbon within the generated silicon-based ceramic matrix. Interestingly, the precipitation of carbon depends to a great extent on the molecular structure of the preceramic polymer and significantly affects the composition, crystallization and decomposition behavior, microstructural evolution as well as the related structural and functional properties of PDCs. Thus, this review article highlights the recent progress in the PDC field with the focus on the fate and role of the in situ formed carbon. Firstly, a brief summary of the synthesis and processing of PDCs is provided, followed by the microstructural characterization of the formed ceramics. The in situ formation of carbon, precursor-carbon-morphology relation and high-temperature evolution of the carbon will be summarized. Secondly, the influence of the segregated carbon on the microstructure and its associated properties of the PDCs will be comprehensively highlighted. Finally, potential advanced structural and functional applications of the PDCs related to the carbon are evaluated.

Enhanced photocatalytic, electrochemical and photoelectrochemical properties of TiO2 nanotubes arrays modified with Cu, AgCu and Bi nanoparticles obtained via radiolytic reduction

Abstract: TiO2 nanotubes arrays (NTs), obtained via electrochemical anodization of Ti foil, were modified with monometallic (Cu, Bi) and bimetallic (AgCu) nanoparticles. Different amounts of metals’ precursors were deposited on the surface of NTs by the spin-coating technique, and the reduction of metals was performed via gamma radiolysis. Surface modification of titania was studied by EDS and XPS analysis. The results show that AgCu nanoparticles exist in a Agcore-Cushell form. Photocatalytic activity was examined under UV irradiation and phenol was used as a model pollutant of water. Over 95% of phenol degradation was achieved after 60 min of irradiation for almost all examined samples, but only slight difference in degradation efficiency (about 3%) between modified and bare NTs was observed. However, the initial phenol degradation rate and TOC removal efficiency was significantly enhanced for the samples modified with 0.31 and 0.63 mol% of Bi as well as for all the samples modified with Cu and AgCu nanoparticles in comparison with bare titania nanotubes. The saturated photocurrent, under the influence of simulated solar light irradiation, for the most active Bi- and AgCu-modified samples, was over two times higher than for pristine NTs. All the examined materials were resistant towards photocorrosion processes that enables their application for long term processes induced by light.

The Study of Thin Films by Electrochemical Impedance Spectroscopy

Abstract: The capabilities and advantages of electrochemical impedance spectroscopy (EIS) as a useful and non-destructive technique are discussed. EIS provides the time dependent quantitative information about the electrode processes. The description of EIS is given in comprehensive way beginning from the theoretical basics of EIS and data interpretation in the frames of various equivalent electric circuits. The practical applications of EIS are described for the following thin film types: (i) cathodic metals/alloys films deposition; (ii) anodization of metals and characterization of oxide films and its growth by EIS including information provided by Mott-Schottky plots; (iii) underpotential deposition of metals; (iv) characterization of organic films onto metals; (v) application in development of biosensors and biofuel cells. The original data of EIS on cathodic electrodeposition of Co and Co-W are provided and reduction mechanisms involving adsorbed intermediates are discussed. The advantages of EIS in the oxide films characterization and their electrochemical properties are shown. EIS can be successfully applied for the characterization of biosensing surfaces and/or in evaluation of bioanalytical signals generated by biosensors. The glucose oxidase (GOx) based biosensor could be successfully analyzed by merged scanning electrochemical microscopy (SECM) and EIS techniques. Such combining study by SECM and EIS could be very attractive in order to evaluate the biofuel cell efficiency and in the modeling of biosensor action, because it is unavailable to obtain by other convenient electrochemical methods.