Sumanta Sain

Structural interpretation of SnO2 nanocrystals of different morphologies synthesized by microwave irradiation and hydrothermal methods

The detailed understanding of the crystal structure and microstructure of nanomaterials is useful to predict the properties of nanomaterials. SnO2 nanocrystals of different shapes like particles, spheres, rods and pyramids are chemically synthesized. Here, we report the crystal structure and microstructure of SnO2 nanocrystals of different morphologies using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). It is interesting to note that the tetragonal phase is found in particle, sphere and rod shaped SnO2 nanocrystals and pyramid shaped nanocrystals are composed of two types of orthorhombic phases. The Rietveld method has been used for refining simultaneously the atomic structure and microstructure of different SnO2 nanocrystals, and the growth mechanisms in different shapes are found to be different due to different preferential growth. Structural defects such as oxygen vacancies, deformation of unit cell and more importantly texturing effects are associated with change in bond length, bond angle and crystallite morphology. Size and lattice strain of different kinds of SnO2 nanocrystals are also studied in detail by using the Rietveld method of analysis and transmission electron microscopy (TEM).

Influence of Size and Shape on the Photocatalytic Properties of SnO2 Nanocrystals

Tuning the functional properties of nanocrystals is an important issue in nanoscience. Here, we are able to tune the photocatalytic properties of SnO2 nanocrystals by controlling their size and shape. A structural analysis was carried out by using X-ray diffraction (XRD)/Rietveld and transmission electron microscopy (TEM). The results reveal that the number of oxygen-related defects varies upon changing the size and shape of the nanocrystals, which eventually influences their photocatalytic properties. Time-resolved spectroscopic studies of the carrier relaxation dynamics of the SnO2 nanocrystals further confirm that the electron-hole recombination process is controlled by oxygen/defect states, which can be tuned by changing the shape and size of the materials. The degradation of dyes (90%) in the presence of SnO2 nanoparticles under UV light is comparable to that (88%) in the presence of standard TiO2 Degussa P-25 (P25) powders. The photocatalytic activity of the nanoparticles is significantly higher than those of nanorods and nanospheres because the effective charge separation in the SnO2 nanoparticles is controlled by defect states leading to enhanced photocatalytic properties. The size- and shape-dependent photocatalytic properties of SnO2 nanocrystals make these materials interesting candidates for photocatalytic applications.

Structural interpretation, growth mechanism and optical properties of ZnO nanorods synthesized by a simple wet chemical route†

ZnO nanorods are synthesized at room temperature through a simple chemical process without using any template or capping agent. ZnO nanopowders used in this synthesis are synthesized by mechanically alloying the ZnO powder. Here, we report primarily the crystal structure and microstructure interpretations of ZnO nanorods by analyzing X-ray diffraction patterns employing Rietveld refinement, field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray (EDX) spectroscopy techniques. Uniformly shaped pure ZnO nanorods with different lengths and diameters are synthesized within 5 h of reaction time. The Rietveld refinement and HRTEM images ascertain the growth of ZnO nanorods along the plane. STEM-HAADF images and EDX spectra and imaging of nanorods confirm the chemical composition and reveal the uniform elemental distributions of Zn and O over the entire nanorod. UV-visible spectra analyses of ZnO nanopowder and nanorods reveal a small decrease in optical band gap of nanorods due to morphological change. Photoluminescence (PL) spectra of both powder and rod-shaped ZnO reveal the presence of excess of oxygen in nanorods. Rietveld analysis corroborates the findings of PL and quantifies the content of oxygen in ZnO nanorods.

Photoswitching and Thermoresponsive Properties of Conjugated Multi-chromophore Nanostructured Materials

Conjugated multi-chromophore organic nanostructured materials have recently emerged as a new class of functional materials for developing efficient light-harvesting, photosensitization, photocatalysis, and sensor devices because of their unique photophysical and photochemical properties. Here, we demonstrate the formation of various nanostructures (fibers and flakes) related to the molecular arrangement (H-aggregation) of quaterthiophene (QTH) molecules and their influence on the photophysical properties. XRD studies confirm that the fiber structure consists of >95% crystalline material, whereas the flake structure is almost completely amorphous and the microstrain in flake-shaped QTH is significantly higher than that of QTH in solution. The influence of the aggregation of the QTH molecules on their photoswitching and thermoresponsive photoluminescence properties is revealed. Time-resolved anisotropic studies further unveil the relaxation dynamics and restricted chromophore properties of the self-assembled nano/microstructured morphologies. Further investigations should pave the way for the future development of organic electronics, photovoltaics, and light-harvesting systems based on π-conjugated multi-chromophore organic nanostructured materials.

Microstructure Characterization of Nanocrystalline Magnesium Ferrite Annealed at Elevated Temperatures by Rietveld Method

Nanocrystalline magnesium ferrite is synthesized at room temperature by high energy ball milling the MgO and α-Fe2O3 (1 : 1 mol fraction) powders. The Rietveld structure and microstructure refinements of X-ray powder diffraction data of 9 h milled sample reveal the presence of mixed and nearly inverse spinel nanocrystalline Mg-ferrite phases. Postannealing of nanocrystalline powder within 873–1473 K reveals continuous change in cation distribution among the tetrahedral and octahedral sites of mixed spinel lattice leading to nearly inverse spinel structure with increasing temperature. Mixed spinel structure finally transformed into inverse spinel structure after 1 h of annealing at 1073 K. The ferrite phase becomes completely stoichiometric by solid-state diffusion of unreacted (∼0.3 mol fraction) α-Fe2O3 into spinel lattice after 1 h of annealing at 1273 K. Interestingly, particle sizes of ferrite phases do not increase considerably up to 1073 K and increase suddenly after transformation of mixed spinel into nearly inverse spinel structure.

Microstructure and photoluminescence properties of ternary Cd0.2Zn0.8S quantum dots synthesized by mechanical alloying

Ternary Cd0.2Zn0.8S quantum dots with a mixture of both cubic (zinc blende) and hexagonal (Wurtzite) phases have been prepared by mechanical alloying, the stoichiometric mixture of Cd, Zn and S powders at room temperature in a planetary ball mill under Ar. The Rietveld analysis of the X-ray diffraction patterns gives an insight of the relative phase abundances of both cubic and hexagonal phases present in sample. Microstructure parameters like change in lattice parameters, the variation of lattice strain, particle size, stacking faults of different kinds are quantitatively determined. High resolution transmission electron microscopy (HRTEM) image analysis corroborates well with the results obtained from the Rietveld analysis. A core–shell structure has been found in HRTEM images where major cubic phase remains at the core and minor hexagonal phase constitutes the shell. Optical band gap measurement using UV–Vis spectroscopy confirms the quantum confinement effects. Steady-state and time-resolved photoluminescence study have also been carried out for better understanding the optical property of this material. Atomic structure modelling helps to reveal the structural changes happening during different milling times.