Showing papers by "Somnath C. Roy published in 2021"

Designing TiO2 nanostructures through hydrothermal growth: influence of process parameters and substrate position

Abstract: Synthesis conditions and processing parameters profoundly affect the growth and morphology of nanostructures. In particular, when nanostructures are fabricated through a chemical technique such as hydrothermal, the process parameters such as reaction time, temperature, precursor concentration, and substrate orientation play a crucial role in determining the structure-property relationships. In this work, we report the hydrothermal growth of Titanium dioxide (TiO2) nanostructures as a function of these parameters and show that specific morphologies can be obtained by a variation of these parameters. A systematic study is carried out to understand the influence of reaction time (from 0.5 h to 3.0 h), reaction temperature (180 °C–200 °C), titanium precursor concentration (0.25 ml and 0.50 ml in 20 ml solution of HCl and deionized water) and substrate orientation (horizontal and tilted at an angle), and we show that significant variation in morphology- from nanowires to nanorods and then dandelions can be achieved. In particular, we demonstrate that high surface area multidirectional growth of nanorods leading to flower-like nanostructures or dandelions resulting from precipitation during the hydrothermal process. This is in contrast with previous reports on similar structures, where the role of precipitations was not analyzed. The work shows a possibility to control such growth by manipulating substrate position inside the autoclave during the hydrothermal process and will be useful for surface-dependent applications.

Enhanced H2 evolution through water splitting using TiO2/ultrathin g-C3N4: A type II heterojunction photocatalyst fabricated by in situ thermal exfoliation

Abstract: Designing a photocatalyst material with reduced recombination of photogenerated charges is one of the most important aspects of hydrogen generation through solar water splitting. Here, we report hydrogen generation using the TiO2/ultrathin g-C3N4 (U-g-CN) heterostructure fabricated using a unique in situ thermal exfoliation process. Multilayer g-CN is converted into U-g-CN having a high surface (∼190 m2/g) area by calcination at ∼550 °C through oxygen-induced exfoliation, which also forms a robust heterostructure with TiO2. In addition, the presence of g-CN also inhibits further growth of TiO2 nanoparticles, thereby retaining a high specific surface area. The presence of U-g-CN causes a redshift (∼0.13 eV) in the absorption edge of heterostructure compared to that of bare TiO2, which extends the light absorption capability. Addition of 40 wt. % of multilayer g-CN to TiO2 shows an enhanced H2 evolution rate, which is ∼15 times and ∼4 times higher compared to that of bare TiO2 and U-g-CN, respectively. Photoluminescence (PL) and time-resolved PL (TRPL) studies indicate a reduced recombination rate of photogenerated charge carriers with an increase in the average lifetime from 10.53 (TiO2) to 13.32 ns (TiO2/U-g-CN40). The interfacial charge transport characteristics studied through impedance spectroscopy reveal a reduced charge transfer resistance at the semiconductor–electrolyte interface, which facilitates faster charge separation due to the heterostructure formation. The band edge positions are estimated through flatband potential from the Mott–Schottky measurements and optical absorption data, indicating a type-II heterojunction. More light absorption and enhanced separation of photogenerated charges at the heterojunction interface lead to better photocatalytic H2 generation.

Anomalous diameter dependent electrical transport in individual CuO nanowire

Abstract: Cupric oxide (CuO) nanostructure arrays have been extensively investigated for solar energy harvesting, electrochemical energy storage, chemical sensing, field-effect transistors, etc. Although most of these applications depend on the collective behavior of an array of such structures, analysis of electrical transport in a single nanostructure, which are the building blocks, is essential for understanding both the fundamental aspects and device performance. Here we report the electrical conduction mechanism in thermally grown single CuO nanowire (NW), which reveals that the current density has an anomalous dependence on the diameter of the NWs—decreasing with an increase in diameter. An analysis of the electrical behavior at room temperature shows that the current density in CuO NWs has different slopes in different regions of the applied bias indicating distinct types of charge transport, which are characterized as near Ohmic (lower voltage), trap controlled, and space charge limited conduction (higher applied voltage). Further, the trap density and activation energy are calculated from the temperature-dependent current density data, which shows higher values (9.38 × 1015cm−3, 79.4 meV) in thicker NWs compared to that in the thinner ones (3.96 × 1015 cm−3, 63.9 meV). Investigation of the NWs with Raman and photoluminescence spectra establishes the presence of Cu2O phase in thicker NWs, which act as hole traps to hinder the charge transport in p-type CuO and resulting in lower conductivity at higher diameters. This study helps to design and fabricate prototype nanodevices with desired conductivity based on CuO NWs.

Morphology dependent electrical conduction and breakdown in single TiO2 nanotubes

Abstract: Understanding the electrical conduction properties of a single nanostructure is essential for gaining insight into the fundamental charge transport through 1D materials and also for exploring the collective behavior of an array of such nanostructures. TiO2 nanostructures, such as electrochemically grown nanotubes, have been widely studied in recent times for several applications. The electrolyte plays a vital role in deciding the morphology, which, in turn, governs the charge transport behavior. Here we present a comparative study of the charge transport through a single TiO2 nanotube grown by electrochemical anodization using ethylene glycol and dimethyl sulphoxide electrolytes. The individual nanotubes are assembled into nanodevices using photolithography without relying on complex and sophisticated process like electron beam lithography or focused ion beam deposition. The electric field dependent charge transport properties show Schottky emission at a lower field regime and Poole–Frenkel emission in the higher region. The temperature-dependent electrical conduction (110 K–410 K) is mediated by two thermal activation processes, attributed to shallow impurities in the low-temperature range (T 230 K). The activation energies for EG based nanotubes are found to be higher than those for DMSO nanotubes owing to the double wall morphology of the formed tubes. Also, the study of the electrical breakdown phenomena of these nanotubes reveals three distinct categories of collapse. ‘Model A’ type breakdown is characterized by a stepwise rise of the current up to the breakdown point and a fall to zero following a non-uniform step by step decrease, which is driven by crack formation near the electrode interface and its propagation. ‘Model B’ shows a transient rise and fall in current, leading to breakdown due to electromigration, whereas ‘Model C’ type breakdown observed in a bundle of nanotubes shows a mixed trend of ‘Model A’ and ‘Model B’. The data and analysis provide insight into the current limit through an individual nanotube or bundle of nanotubes and will be useful for designing prototype nanodevices from titania nanostructures.

Application of a High Order Accurate Meshless Method to Solution of Heat Conduction in Complex Geometries.

Abstract: In recent years, a variety of meshless methods have been developed to solve partial differential equations in complex domains. Meshless methods discretize the partial differential equations over scattered points instead of grids. Radial basis functions (RBFs) have been popularly used as high accuracy interpolants of function values at scattered locations. In this paper, we apply the polyharmonic splines (PHS) as the RBF together with appended polynomial and solve the heat conduction equation in several geometries using a collocation procedure. We demonstrate the expected exponential convergence of the numerical solution as the degree of the appended polynomial is increased. The method holds promise to solve several different governing equations in thermal sciences.

Enhanced photoelectrochemical performance of CeO2 functionalized TiO2 nanotube arrays with Ag coating

Abstract: The photoelectrochemical performance of Ag coated CeO2 functionalized TiO2 nanotube arrays (AgCeTNT) is investigated here. The sample is prepared via three steps, such as synthesis of TiO2 nanotube by electrochemical anodization, CeO2 functionalization by hydrothermal and Ag coating by Successive Ionic Layer Adsorption and Reaction (SILAR) method. X-ray diffraction (XRD) analysis showed the anatase phase formation after annealing of TiO2 nanotubes. Field Emission Scanning Electron Microscopic (FESEM) images showed the conformal coverage of CeO2 and Ag nanoparticles on the surface of TiO2 nanotubes. Energy Dispersive X-Ray (EDX) analysis confirmed the presence of Ce, Ag, Ti and O in the AgCeTNT sample. Diffuse Reflectance Spectroscopy (DRS) study revealed the enhancement in visible light sensitivity with CeO2 functionalization and further with Ag coating. The photoelectrochemical measurement showed that the AgCeTNT sample exhibits improved photoelectrochemical performance compared to bare TiO2 nanotubes (TNT), CeO2 functionalized TiO2 nanotubes (CeTNT) and Ag coated TiO2 nanotubes (AgTNT) under the illumination of sunlight. This improvement in photoelectrochemical performance happeneddue to the combined effect of favorable band alignment of CeO2 and TiO2 and Surface Plasmon Resonance (SPR) effect of Ag nanoparticles, which lead to superior charge separation, efficient charge transport and enhanced visible light response.

Improved photoelectrochemical performance of ultra thin g-C3N4 nanosheet: A comparative study from bulk to nanoscale

Abstract: Ultrathin g-C3N4 (g-CN) nanosheets are prepared by a two-step thermal exfoliation process in an air atmosphere. X-ray diffraction pattern (XRD) shows that ultrathin g-CN nanosheets have the same crystal structure as that of the bulk g-CN with a slight shift in the peak positions due to the reduction of sheet thickness. Field emission scanning electron microscope (FESEM) images show that the bulk g-CN is converted into ultrathin g-CN nanosheets due to thermal oxidation in an atmosphere, which increases the number of reactive active sites for water splitting. Due to a reduction in sheet thickness of g-CN, quantum confinement happened and thereby increase the bandgap from 2.88 (bulk g-CN) eV to 3.10 eV (ultrathin g-CN). Steady-state photoluminescence (PL) shows a blue shift, and time-resolve photoluminescence (TRPL) shows that the average lifetime of photogenerated charge carrier increases when bulk g-CN is converted into ultrathin g-CN nanosheets. Enhancement in photoelectrochemical performance is observed in ultrathin g-CN nanosheets compared to bulk g-CN due to the increased average lifetime of photogenerated charge carriers and a large number of reactive active sites.

Numerical simulation of flow of a shear-thinning Carreau fluid over a transversely oscillating cylinder

Abstract: Flow of a shear-thinning fluid over curved and moving geometries demands special attention owing to its industrial and biological relevance. The present study reports a numerical simulation of the motion of a Carreau fluid over a transversely oscillating cylinder for a range of oscillation amplitudes and frequencies at Reynolds number ). This difference in the flow structures is explained through the behaviour of the diffusion terms in the vorticity transport equation. Further, the effect of the Carreau fluid properties on the flow structures and vorticity dynamics are also discussed.

Facile synthesis of high aspect ratio gold coated magnetic nanowires

Abstract: Magnetic nanoparticles are of immense importance in various biological applications such as bio-imaging techniques, drug delivery, hematologic disease detection, biomolecules separation (Figure 1). However, one of the limitations faced while using magnetic nanoparticles is to synthesize small size, high surface to volume ratio particles via simple approach. Magnetic nanoparticles in general are synthesized using hydrothermal method, template based method or electrochemical methods. These conventional synthesis approaches are not only complex to carry out but are also time consuming. Herein, we report a wet chemical method to synthesize high aspect ratio magnetite (Fe3O4) nanowires. The benefit of this method is facile, rapid and cost-effective process. The method involves a chemical reduction of ferrous sulphate and ferric chloride. The resultant product was obtained in two portions – the precipitate and the supernatant. Physical appearance of both the precipitate and supernatant was observed as brown in colour. These synthesized magnetic nanoparticles in supernatant were light in weight and completely dispersible in water for more than 6h. Surface morphology analysis of these nanoparticles demonstrated that the precipitate was aggregate of nanoparticles whereas the particles present in the supernatant were observed to be thin nanowires of dimension approximately ranged from 200 to 275nm with calculated aspect ratio of approximately eleven (Figure 2). Magnetic property was observed in these nanowires using a simple bar magnet. Further, to enhance its biocompatibility, a thin gold coating was carried out on its surface. A reaction was performed by citrate capping followed by reduction using sodium borohydride in presence of chloroauric acid[1]. Gold coating was confirmed by a UV-vis absorbance spectrum that showed a distinct plasmonic peak at 540 nm (Figure 3). The prospective application planned in this study is for efficient separation of exosomes from bodily fluids. Conventionally exosome isolation is challenging in terms of process complexity, cost and accuracy[2]. Gold coated magnetic nanowires will allow direct attachment of a large number of exosomes by affinity interactions which can significantly enhance the capture efficiency. The magnetic activity allows magnetic nanowire based mixing, separation and purification via magnetic isolation and purification steps[3]. Alongside the light weight and dispersible nature of these nanowires prevents their agglomeration in the cell suspension allowing easy separation after exosome attachment using the external magnetic field. Overall these as synthesized high aspect ratio magnetic nanowires can be used as highly efficient nanocarriers for direct isolation of bulk exosomes from bodily fluids via magnetic separation method and thus can avoid the use of expensive commercial kit based multi-step pre-extraction procedures. Figure 1. Applications of magnetic nanoparticles Figure 2. SEM image of nanowires Figure 3. Absorption spectra of gold coated nanowires