Dipankar Saha

Bio: Dipankar Saha is an academic researcher from Aarhus University. The author has contributed to research in topics: Powder diffraction & Rietveld refinement. The author has an hindex of 14, co-authored 27 publications receiving 440 citations. Previous affiliations of Dipankar Saha include Indian Institute of Technology Guwahati & Indian Institute of Science.

In Situ Total X-Ray Scattering Study of WO3Nanoparticle Formation under Hydrothermal Conditions

Abstract: Pair distribution function analysis of in situ total scattering data recorded during formation of WO3 nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO6 polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO3.

Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticles

Abstract: Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO2 nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studied in situ by X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr—O and Zr—Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly into m-ZrO2, or more gradually into mixed m- and t-ZrO2 with a concurrent increase of the shortest Zr—Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO2 favors the formation of almost phase-pure m-ZrO2 nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO2 phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO2 beyond empirical approaches.

Formation Mechanisms of Nanocrystalline MnO2 Polymorphs under Hydrothermal Conditions

Abstract: Understanding and controlling the polymorphism of manganese dioxide (MnO2) is of vital importance in many nanoscale applications. Here in situ powder X-ray diffraction (PXRD) in combination with in situ total X-ray scattering are used to reveal the formation mechanism as well as polymorph evolution of MnO2 under hydrothermal synthesis conditions. A “PXRD invisible” amorphous phase with a local structure resembling α-MnO2 (denoted α-MnO2(A)) is observed at all reaction stages, and it never fully disappears from the reaction solution. The MnO2 phase evolution involves initial formation of δ-MnO2, which transforms to α-MnO2, and then subsequently to β-MnO2. The phase transformations between different polymorphs do not involve dissolution–recrystallization, but they occur via solid-state mechanisms. However, the amorphous α-MnO2(A) phase plays a key role since it is consumed in growing both the α- and β-MnO2 polymorphs. Overall, the polymorphism of the crystalline product can be controlled through reaction ti...

Formation Mechanisms of Pt and Pt3Gd Nanoparticles under Solvothermal Conditions: An in Situ Total X-ray Scattering Study

Abstract: Platinum-based nanoparticles play a crucial role as catalysts, and solvothermal synthesis methods are attractive due to the excellent control of nanoparticle characteristics such as size, crystallinity, and morphology, which strongly affect the chemical and physical properties. Insight into the reaction mechanism leading to nanoparticle formation under solvothermal conditions generally remains elusive. This is mainly due to the experimental difficulties that lie in obtaining atomistic information on the nanoscale during the progression of the synthesis. Using in situ total X-ray scattering and pair distribution function (PDF) analysis with a time resolution of 1 s, we unravel the formation mechanisms of Pt and Pt3Gd nanoparticles under solvothermal conditions. We demonstrate that an octahedral Pt4+ platinic acid precursor complex is reduced in two steps. Both Pt and Pt3Gd nanocrystal formation proceeds via conversion of a square-planar Pt2+ complex to an unsaturated nanocluster (Pt0), which subsequently g...

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

Abstract: It is conventionally assumed that the growth of monodisperse colloidal nanocrystals requires a temporally discrete nucleation followed by monomer attachment onto the existing nuclei. However, recent studies have reported violations of this classical growth model, and have suggested that inter-particle interactions are also involved during the growth. Mechanisms of nanocrystal growth still remain controversial. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution onto the existing particles or by coalescence between the particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow. We suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems.

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Abstract: Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1–100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and ...

Ceria‐Based Solid Catalysts for Organic Chemistry

Abstract: Ceria has been the subject of thorough investigations, mainly because of its use as an active component of catalytic converters for the treatment of exhaust gases. However, ceria-based catalysts have also been developed for different applications in organic chemistry. The redox and acid-base properties of ceria, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation. Pure ceria is used in several organic reactions, such as the dehydration of alcohols, the alkylation of aromatic compounds, ketone formation, and aldolization, and in redox reactions. Ceria-supported metal catalysts allow the hydrogenation of many unsaturated compounds. They can also be used for coupling or ring-opening reactions. Cerium atoms can be added as dopants to catalytic system or impregnated onto zeolites and mesoporous catalyst materials to improve their performances. This Review demonstrates that the exceptional surface (and sometimes bulk) properties of ceria make cerium-based catalysts very effective for a broad range of organic reactions.

Solution combustion synthesis of metal oxide nanomaterials for energy storage and conversion

Abstract: The design and synthesis of metal oxide nanomaterials is one of the key steps for achieving highly efficient energy conversion and storage on an industrial scale. Solution combustion synthesis (SCS) is a time- and energy-saving method as compared with other routes, especially for the preparation of complex oxides which can be easily adapted for scale-up applications. This review summarizes the synthesis of various metal oxide nanomaterials and their applications for energy conversion and storage, including lithium-ion batteries, supercapacitors, hydrogen and methane production, fuel cells and solar cells. In particular, some novel concepts such as reverse support combustion, self-combustion of ionic liquids, and creation of oxygen vacancies are presented. SCS has some unique advantages such as its capability for in situ doping of oxides and construction of heterojunctions. The well-developed porosity and large specific surface area caused by gas evolution during the combustion process endow the resulting materials with exceptional properties. The relationship between the structural properties of the metal oxides studied and their performance is discussed. Finally, the conclusions and perspectives are briefly presented.