Showing papers by "Somnath C. Roy published in 2022"

Enhanced methane yield by photoreduction of CO2 at moderate temperature and pressure using Pt coated, graphene oxide wrapped TiO2 nanotubes

Abstract: Photocatalytic conversion of CO2 into hydrocarbons is one of the most sought-after approaches as it not only helps for its mitigation and utilization but does so through the renewable solar energy. Development of suitable materials and protocols to achieve better efficiency in photoconversion has been projected as one of the grand challenges in the 21st century. However, the stability of CO2 leads to a high thermodynamic barrier for its conversion into value added chemicals. In this work, we report the design and fabrication of novel three-component nanostructured photocatalyst material comprising TiO2 nanotube arrays, reduced graphene oxide and Pt nanoparticles. TiO2 nanotubes fabricated by electrochemical anodization are functionalized by chemically deposited graphene oxide layers and Pt particles deposited by sputtering technique. The photocatalysts are characterized for the crystallinity, morphology, and composition through XRD, FE-SEM and EDAX studies. Such a combination ensures easier separation of photogenerated charges as well as better catalytic activity. Further, the photoreduction experiments are carried out at moderately higher temperature of about 80 °C and at about 1.8 atm pressure in an indigenously designed batch reactor to partly overcome the thermodynamic barrier. We have achieved a high methane generation rate of about 3.42 mmol/g/hr. The possible reaction mechanisms are discussed.

Improved omnidirectional polarisation‐insensitive optical absorption and photoelectrochemical water splitting using aperiodic and tapered slanted, kinked and straight silicon nanowires

Abstract: We have experimentally demonstrated that slanted, kinked and straight silicon nanowire arrays form a broadband omnidirectional light‐harvesting structure. The unique design of kinked and slanted nanowires allows them to trap more light effectively across a wide wavelength range than straight silicon nanowires (SiNWs). We report that the light absorption in slanted, kinked and straight wires is enhanced by controlling their geometrical parameters. The p‐type SiNWs have less reflection than n‐type SiNWs due to their larger porosity gradient structure. Aperiodic and tapered slanted, kinked and straight SiNWs are remarkably photoactive and promising low‐cost materials for photoelectrochemical water splitting applications.

Localized thermal spike driven morphology and electronic structure transformation in swift heavy ion irradiated TiO2 nanorods

Abstract: Irradiation of materials by high energy (∼MeV) ions causes intense electronic excitations through inelastic transfer of energy that significantly modifies physicochemical properties. We report the effect of 100 MeV Ag ion irradiation and resultant localized (∼few nm) thermal spike on vertically oriented TiO2 nanorods (∼100 nm width) towards tailoring their structural and electronic properties. Rapid quenching of the thermal spike induced molten state within ∼0.5 picosecond results in a distortion in the crystalline structure that increases with increasing fluences (ions per cm2). Microstructural investigations reveal ion track formation along with a corrugated surface of the nanorods. The thermal spike simulation validates the experimental observation of the ion track dimension (∼10 nm diameter) and melting of the nanorods. The optical absorption study shows direct bandgap values of 3.11 eV (pristine) and 3.23 eV (5 × 1012 ions per cm2) and an indirect bandgap value of 3.10 eV for the highest fluence (5 × 1013 ions per cm2). First principles electronic structure calculations corroborate the direct-to-indirect transition that is attributed to the structural distortion at the highest fluence. This work presents a unique technique to selectively tune the properties of nanorods for versatile applications.

Optimization of etching and sonication time to prepare monolayer Ti3C2T MXene flakes: A structural, vibrational, and optical spectroscopy study

Abstract: Two-dimensional (2D) Ti3C2Tx MXene and its heterostructure exhibit promising performances in several applications such as energy storage, optoelectronics, electrocatalysis, water purification, biomedical, etc. Our investigation shows that an etching duration of 24 h is sufficient to remove Al layers from Ti3AlC2 and to obtain a well-separated accordion-like structure. Different lateral size monolayer MXene flakes ranging from 2 μm to 35 nm are obtained by tuning the sonication time from 2 to 8 h. The monolayer nature of the flakes is confirmed by both Atomic Force Microscope (AFM) and Transmission Electron Microscope (TEM). Raman vibrational modes and X-ray photoelectron spectroscopy (XPS) show that the flakes are containing –O, –F, and –OH functional groups. A unique type of absorption spectra is observed that comes from both interband and intraband transition. An in-depth structural, vibrational, and optical property study gives a clear idea to use these flakes for heterostructure formation with other materials for various applications.

Space charge limited conduction in anatase and mixed-phase (anatase/rutile) single TiO2 nanotubes

Abstract: TiO2 nanotube arrays are used for several applications; however, in-depth analysis of charge conduction through an individual nanotube is necessary to predict the behavior of devices and to achieve higher efficiency. Here we report successful isolation and charge transport study through individual nanotubes annealed at different conditions. Anatase or anatase/rutile mixed-phase nanotubes are obtained by annealing in air or nitrogen at temperatures 450 and 650 °C, respectively. Current-voltage measurements in the range of 213 K–413 K indicate that the charge transport through single nanotubes follows Ohmic conduction in lower voltage and trap-assisted space charge limited conduction (SCLC) at higher voltage. Charge transport is controlled by two thermal activation processes resulting in two different values of activation energy, which changes with applied voltage. The activation energy in mixed-phase sample is higher than in the anatase nanotube. Calculated values of trap density vary from 9.28 × 1013/cm3 for anatase nanotubes to 6.60 × 1015/cm3 for mixed-phase ( ∼ 52% rutile) and 3.34 × 1016/cm3 in mixed-phase nanotubes ( ∼ 76% rutile). The variation of trap density with an increase in rutile phase is attributed to electron trapping by rutile regions arising from a corresponding band alignment. This is also reflected in room temperature conductivity which decreases with increase in rutile content.

Photocatalytic Carbon Dioxide Conversion by Structurally and Materially Modified Titanium Dioxide Nanostructures

Abstract: TiO2 has aroused considerable attentions as a promising photocatalytic material for decades due to its superior material properties in several fields such as energy and environment. However, the main dilemmas are its wide bandgap (3–3.2 eV), that restricts the light absorption in limited light wavelength region, and the comparatively high charge carrier recombination rate of TiO2, is a hurdle for efficient photocatalytic CO2 conversion. To tackle these problems, lots of researches have been implemented relating to structural and material modification to improve their material, optical, and electrical properties for more efficient photocatalytic CO2 conversion. Recent studies illustrate that crystal facet engineering could broaden the performance of the photocatalysts. As same as for nanostructures which have advantages such as improved light absorption, high surface area, directional charge transport, and efficient charge separation. Moreover, strategies such as doping, junction formation, and hydrogenation have resulted in a promoted photocatalytic performance. Such strategies can markedly change the electronic structure that lies behind the enhancement of the solar spectrum harnessing. In this review, we summarize the works that have been carried out for the enhancement of photocatalytic CO2 conversion by material and structural modification of TiO2 and TiO2-based photocatalytic system. Moreover, we discuss several strategies for synthesis and design of TiO2 photocatalysts for efficient CO2 conversion by nanostructure, structure design of photocatalysts, and material modification.

WS2 nanosheets functionalized Fe2O3 nanorod arrays as a type II heterojunction for photoelectrochemical water splitting

Abstract: Fe2O3 is a stable and low bandgap photocatalyst capable of absorbing a wide range of solar spectra. However, a shorter hole diffusion length impedes its performance as an efficient photocatalyst. The formation of a type II heterojunction is an effective approach to facilitate the quick separation of photogenerated carriers. In this work, we report photo-electrochemical characteristics of WS2 functionalized Fe2O3 nanorod arrays fabricated on FTO coated glass substrate. The Fe2O3 nanorods fabricated by chemical bath deposition and WS2 nanosheets by the hydrothermal technique are characterized by X-ray diffraction, scanning and transmission electron microscopy, energy dispersive X-ray analysis (EDS), X-ray photoelectron spectroscopy, optical absorption, Raman spectra, and FT-IR. A heterojunction architecture formed between these resulted in a higher photocurrent density compared to that of bare Fe2O3 nanorods. Electrochemical impedance spectroscopy and Mott-Schottky measurements reveal lower charge transfer resistance and higher interfacial charge density for WS2 functionalized Fe2O3 nanorods. An energy band diagram for the heterojunction has been proposed to show the charge separation at the interface.