S.K Manik

Bio: S.K Manik is an academic researcher from University of Burdwan. The author has contributed to research in topics: Ball mill & Rietveld refinement. The author has an hindex of 6, co-authored 7 publications receiving 290 citations.

Annealing effect on nano-ZnO powder studied from positron lifetime and optical absorption spectroscopy

Abstract: Mechanical milling and subsequent annealing in air at temperatures between 210 and 1200°C have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from 76±1to22±0.5nm) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above 425°C annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature (>700°C) annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies.

X‐ray microstructure characterization of ball‐milled nanocrystalline microwave dielectric CaZrO3 by Rietveld method

Abstract: The microwave dielectric CaZrO3 has been prepared in nanocrystalline form by high-energy ball milling an equimolar powder mixture of CaO and monoclinic (m) ZrO2. The formation of CaZrO3 is noticed in the X-ray diffraction pattern of 2 h ball-milled powder. At the early stage of milling, up to 4 h, the weight fraction of CaZrO3 increases rapidly and then slowly reaches almost the saturation value of ∼73 wt% within 18 h of milling. It is observed that in the course of milling, the weight fraction of c-ZrO2 increases continuously from its starting value, ∼2.5 wt%, to ∼43 wt% within 2 h of milling, and then reduces slowly to its starting value. It seems that the c-ZrO2 phase has been stabilized by CaO and takes part in the formation of CaZrO3 by mechanochemical reaction of nanocrystalline CaO and c-ZrO2 crystallites. However, post-annealing of 2, 6 and 18 h ball-milled samples results in the formation of ∼95 wt% CaZrO3 phase. The formation mechanism, phase transition kinetics, structure and microstructure changes in terms of lattice imperfections and relative phase abundances of individual phases have been estimated from the analysis of X-ray powder diffraction data of unmilled, ball-milled and post-annealed samples by Rietveld powder structure refinement.

Oxygen Vacancy Clusters Promoting Reducibility and Activity of Ceria Nanorods

Abstract: CeO2 is a catalytic material of exceptional technological importance, and the precise role of oxygen vacancies is crucial to the greater understanding of these oxide materials. In this work, two ceria nanorod samples with different types and distributions of oxygen vacancies were synthesized. A direct relationship between the concentration of the larger size oxygen vacancy clusters and the reducibility/reactivity of nanosized ceria was revealed. These results may be an important step in understanding and designing active sites at the surface of metal oxide catalytic materials.

Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency.

Abstract: TiO2 nanocrystals with tunable bulk/surface defects were synthesized and characterized with TEM, XRD, BET, positron annihilation, and photocurrent measurements. The effect of defects on photocatalytic activity was studied. It was found for the first time that decreasing the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes, thus significantly enhancing the photocatalytic efficiency.

Characterization of Oxygen Vacancy Associates within Hydrogenated TiO2: A Positron Annihilation Study

Abstract: This paper introduces a novel method for characterizing the oxygen vacancy associates in hydrogenation-modified TiO2 by using a positron annihilation lifetime spectroscopy (PALS). It was found that a huge number of small neutral Ti3+–oxygen vacancy associates, some larger size vacancy clusters, and a few voids of vacancy associates were introduced into hydrogenated TiO2. The defects blurred the atomic lattice high-resolution transmission electron microscopy (HRTEM) images and brought about the emergence of new Raman vibration. X-ray photoelectron spectroscopy (XPS) measurement indicated that the concentration of oxygen vacancies was 3% in the TiO2 lattice. The photoluminescence (PL) spectroscopy, photocurrent, and degradation of methylene blue indicated that the oxygen vacancy associates introduced by hydrogenation retarded the charge recombination and therefore improved the photocatalytic activity remarkably.