Swomitra K. Mohanty

Redox-Induced Enhancement in Interfacial Capacitance of the Titania Nanotube/Bismuth Oxide Composite Electrode

Bismuth oxide (Bi2O3) decorated titania nanotube array (T-NT) composite materials were synthesized by a simple, yet versatile electrodeposition method. The effects of deposition current density and time on morphology evolution of the bismuth oxide phase were analyzed. It was found that an optimum deposition condition in terms of current density and time could be reached to achieve uniform and equiaxed crystal morphology of the deposited oxide phase. The morphology, shape, size distribution, and crystal structure of the bismuth oxide phase were evaluated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopic techniques. The electrochemical capacitance of the T-NT/Bi2O3 composites was studied by conducting cyclic voltammetry and galvanostatic charge-discharge experiments. These studies indicated that the capacitance behavior of the composite material was dependent on the morphology and distribution of the bismuth oxide phase. The capacitance was greatly enhanced for the composite having equiaxed and uniformly distributed bismuth oxide particles. The maximum interfacial capacitance achieved in this study was approximately 430 mF cm(-2). Galvanostatic charge-discharge experiments conducted on the composite materials suggested stable capacitance behavior together with excellent capacitance retention even after 500 cycles of continuous charge-discharge operation.

Effects of rectangular microchannel aspect ratio on laminar friction constant

In this paper, the effects of rectangular microchannel aspect ratio on laminar friction constant are described. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 150 micrometers to 600 micrometers and heights ranging from 22.71 micrometers to 26.35 micrometers . A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record the pressure drop. The experimental data obtained for water for flows at Reynolds numbers below 10 showed an approximate 20% increase in the friction constant for a specified driving potential when compared to macroscale predictions from the classical Navier-Stokes theory. When the experimental data are studied as a function of aspect ratio, a 20% increase in the friction constant is evident at low aspect ratios. A similar increase is shown by the currently available experimental data for low Reynolds number (< 100) flows of water.

Light-assisted anodized TiO2 nanotube arrays

Self-assembled arrays of titania nanotubes are synthesized via electrochemical anodization of Ti foils under the presence of UV-vis irradiation. Compared to control samples (anodized without light), the light-assisted anodized samples exhibit larger diameters as well as thicker nanotube walls, whereas the length of the nanotubes remains the same under otherwise similar synthesis conditions. Enhanced photoelectrochemical performance with light-assisted anodized samples under simulated AM 1.5 irradiation is observed by an increase in photocurrent density of 45-73% at 1.23 V (RHE). The enhanced photoelectrochemical performance is correlated to improved charge separation analyzed by Mott-Schottky. A mechanism on the photoeffect during anodization is presented. The morphology and improved properties obtained from the synthesis methodology may also find application in other fields such as sensing and catalysis.

Site-specific and patterned growth of TiO2 nanotube arrays from e-beam evaporated thin titanium film on Si wafer

Growth of TiO(2) nanotubes on thin Ti film deposited on Si wafers with site-specific and patterned growth using a photolithography technique is demonstrated for the first time. Ti films were deposited via e-beam evaporation to a thickness of 350-1000 nm. The use of a fluorinated organic electrolyte at room temperature produced the growth of nanotubes with varying applied voltages of 10-60 V (DC) which remained stable after annealing at 500 °C. It was found that variation of the thickness of the deposited Ti film could be used to control the length of the nanotubes regardless of longer anodization time/voltage. Growth of the nanotubes on a SiO(2) barrier layer over a Si wafer, along with site-specific and patterned growth, enables potential application of TiO(2) nanotubes in NEMS/MEMS-type devices.

Growth and characterization of TiO2 nanotubes from sputtered Ti film on Si substrate.

In this paper, we present the synthesis of self-organized TiO2 nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) sputtering. Organic electrolyte was used to demonstrate the growth of stable nanotubes at room temperature with voltages varying from 10 to 60 V (D.C.). The tubes were about 1.4 times longer than the thickness of the sputtered Ti film, showing little undesired dissolution of the metal in the electrolyte during anodization. By varying the thickness of the deposited Ti film, the length of the nanotubes could be controlled precisely irrespective of longer anodization time and/or anodization voltage. Scanning electron microscopy, atomic force microscopy, diffuse-reflectance UV–vis spectroscopy, and X-ray diffraction were used to characterize the thin film nanotubes. The tubes exhibited good adhesion to the wafer and did not peel off after annealing in air at 350 °C to form anatase TiO2. With TiO2 nanotubes on planar/stable Si substrates, one can envision their integration with the current micro-fabrication technique large-scale fabrication of TiO2 nanotube-based devices.

Hollow metallic micromachined needles with multiple output ports

In this paper, hollow metallic micromachined needles with multiple output ports are designed, fabricated, characterized, and packaged. The hollow metallic needles include design features such as tapered needle tips and multiple output ports on the bottom and top of each needle. The needle tip and shaft are formed by microelectroformed metal. The flow characteristics of the needles are currently being experimentally investigated and modeled using a finite element numerical model. The experimental results and theoretical models will be presented as part of this paper. The micromachined needles can be fabricated on a variety of substrates and can use micro-electroformed palladium as the structural material. The use of palladium as a structural material provides high mechanical strength and durability, as well as, biocompatibility for use in biomedical applications. The cross-sectional dimensions of individual needle tips begin at less than 10 micrometers in width and 15 micrometers in height and then taper to 200 micrometers in width and 60 micrometers in height. The significance of this work includes the development of hollow metallic micromachined needles for biomedical applications, as well as, a discussion of structural, fluidic, and packaging design considerations.

Electrochemical detection of methyl nicotinate biomarker using functionalized anodized titania nanotube arrays

Sensing and detection of volatile organic compounds (VOCs) from exhaled breath is a possible method for early diagnosis of several pulmonary diseases. The use of solid-state TiO2 nanotube array sensors for VOC sensing applications has been of great interest. In this study, titania nanotubular arrays (TNAs) were synthesized through electrochemical anodization and used for the electrochemical detection of methyl nicotinate biomarker vapor. Functionalization of the TNA with cobalt was found to be necessary for methyl nicotinate detection. Titanium dioxide films synthesized through high temperature oxidation and functionalized with cobalt were also compared with cobalt functionalized TNA. The ordered TNA demonstrated itself to be an effective substrate for cobalt deposition and subsequent biomarker detection over thin titanium dioxide films. Surface analysis of the cobalt functionalized TNA by x-ray photoelectron spectroscopy (XPS) studies observed cobalt deposits exist as cobalt hydroxide on the surface. Exposure of the sensor surface to methyl nicotinate vapor results in the reduction of cobalt hydroxide to cobalt metal on the surface. Two mechanisms have been proposed to describe the binding of the nicotinate biomarker to cobalt functionalized TNA consistent with the XPS studies and band theory.

Theoretical and experimental study of sensing triacetone triperoxide (TATP) explosive through nanostructured TiO2 substrate

The present study focuses on understanding of the principle of interaction of explosive molecule triacetonetriperoxide (TATP) with metal sensitized TiO₂ nanotube composite material through theoretical modeling. This effort has also been extended in developing a laboratory scale sensor set up to detect TATP based on comprehensive computational modeling outcome and subsequent experimentation. Sensing mechanism depends on the nature of metal, where the TATP interaction with metal functionalized TiO₂ prompts a change in conductivity of the sensor platform. Therefore, a metal with higher affinity towards TATP would enhance the conductance, thereby promoting the efficiency of the sensor platform. DFT methodology has been used to identify metal with high affinity to TATP. It was found that Co(2+) metal ion shows significantly higher affinity towards TATP, selected from an array of metal ions with different valency, from monovalent to tetravalent. The preliminary experimental data also suggests that Co(2+) ion detects TATP by inducing a change in conductivity of the sensor substrate.

Electrochemical capacitance of iron oxide nanotube (Fe-NT): effect of annealing atmospheres

The effect of annealing atmosphere on the supercapacitance behavior of iron oxide nanotube (Fe-NT) electrodes has been explored and reported here. Iron oxide nanotubes were synthesized on a pure iron substrate through an electrochemical anodization process in an ethylene glycol solution containing 3% H2O and 0.5 wt.% NH4F. Subsequently, the annealing of the nanotubes was carried out at 500 °C for 2 h in various gas atmospheres such as air, oxygen (O2), nitrogen (N2), and argon (Ar). The morphology and crystal phases evolved after the annealing processes were examined via field emission scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. The electrochemical capacitance properties of the annealed Fe-NT electrodes were evaluated by conducting cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy tests in the Li2SO4 electrolyte. Based on these experiments, it was found that the capacitance of the Fe-NT electrodes annealed in air and O2 atmospheres shows mixed behavior comprising both the electric double layer and pseudocapacitance. However, annealing in N2 and Ar environments resulted in well-defined redox peaks in the CV profiles of the Fe-NT electrodes, which are therefore attributed to the relatively higher pseudonature of the capacitance in these electrodes. Based on the galvanostatic charge-discharge studies, the specific capacitance achieved in the Fe-NT electrode after annealing in Ar was about 300 mF cm(-2), which was about twice the value obtained for N2-annealed Fe-NTs and three times higher than those annealed in air and O2. The experiments also demonstrated excellent cycle stability for the Fe-NT electrodes with 83%-85% capacitance retention, even after many charge-discharge cycles, irrespective of the gas atmospheres used during annealing. The increase in the specific capacitance was discussed in terms of increased oxygen vacancies as a result of the enhanced transformation of the hematite (α-Fe2O3) phase to the magnetite (Fe3O4) phase for the electrodes annealed in the N2 and Ar atmospheres.

Effect of Sputtering Parameters on the Morphology of TiO 2 Nanotubes Synthesized From Thin Ti Film on Si Substrate

In this paper, we present the analysis of the properties of direct current (dc) magnetron sputtered Ti thin film that affect the morphology of TiO2 nanotubes synthesized by electrochemical anodization. Si wafer with thermally grown silicon dioxide was used as the substrate for deposition of Ti films. By varying the properties of the sputtered film, morphology of the anodized film can be varied from tubular to nanoporous TiO2. Three sputtering parameters that affect the properties of the film were studied, which include sputtering power, process gas (argon) pressure, and substrate temperature. Anodization of these films was carried out at 30 V (dc) using an ethylene glycol-based electrolyte. We show that the properties of thin film such as grain size and residual stress (bi-axial) do not affect the morphology of the anodized film and density alone influences the morphology of the anodized film. Most of the applications demonstrated by TiO2 nanotubes require annealing at high temperatures (350-800 °C) for calcination. Low residual stress in the thin film is required to prevent delaminating of the nanotubes from the substrate when exposed to high temperatures. We demonstrate that by varying the sputtering parameters, Ti films with low stress can be deposited which is required to have stable TiO2 nanotubes or nanoporous structure, based on the requirement of the application.