In Situ High-Energy Synchrotron Radiation Study of Boehmite Formation, Growth, and Phase Transformation to Alumina in Sub- and Supercritical Water

In situ X-ray diffraction study of the solvothermal formation mechanism of gallium oxide nanoparticles

Gallium oxides are of broad interest due to their wide band gaps and attractive photoelectric properties. Typically, the synthesis of gallium oxide nanoparticles is based on a combination of solvent-based methods and subsequent calcination, but detailed information about solvent based formation processes is lacking, and this limits the tailoring of materials. Here we have examined the formation mechanisms and crystal structure transformations of gallium oxides during solvothermal synthesis using in situ X-ray diffraction. γ-Ga₂O₃ readily forms over a wide range of conditions. In contrast, β-Ga₂O₃ only forms at high temperatures (T > 300 °C), and it is always preceded by γ-Ga₂O₃, indicating that γ-Ga₂O₃ is a crucial part of the formation mechanism of β-Ga₂O₃. The activation energy for formation of β-Ga₂O₃ from γ-Ga₂O₃ is determined to be 90-100 kJ mol^-1 in ethanol, water and aqueous NaOH based on kinetic modelling of phase fractions obtained from multi-temperature in situ X-ray diffraction data. At low temperatures GaOOH and Ga₅O₇OH form in aqueous solvent, but these phases are also obtained from γ-Ga₂O₃. Systematic exploration of synthesis parameters such as temperature, heating rate, solvent and reaction time reveal that they all affect the resulting product. In general, the solvent based reaction paths are different from reports on solid state calcination studies. This underlines that the solvent is an active part of the solvothermal reactions and to a high degree determines different formation mechanisms.

Moderate oxidation levels of Ru nanoparticles enhance molecular oxygen activation for cross-dehydrogenative-coupling reactions via single electron transfer

Ruthenium nanoparticles with altered surface oxidation states showed a volcano shaped relationship in molecular oxygen activation via single electron transfer for cross-dehydrogenative-coupling reactions.

In situ studies of solvothermal synthesis of energy materials

Solvothermal and hydrothermal synthesis, that is, synthesis taking place in a solvent at elevated temperature and pressure, is a powerful technique for the production of advanced energy materials as it is versatile, cheap, and environmentally friendly. However, the fundamental reaction mechanisms dictating particle formation and growth under solvothermal conditions are not well understood. In order to produce tailor-made materials with specific properties for advanced energy technologies, it is essential to obtain an improved understanding of these processes and, in this context, in situ studies are an important tool as they provide real time information on the reactions taking place. Here, we present a review of the use of powder diffraction and total scattering methods for in situ studies of synthesis taking place under solvothermal and hydrothermal conditions. The experimental setups used for in situ X-ray and neutron studies are presented, and methods of data analysis are described. Special attention is given to the methods used to extract structural information from the data, for example, Rietveld refinement, whole powder pattern modelling and pair distribution function analysis. Examples of in situ studies are presented to illustrate the types of chemical insight that can be obtained.

In situ X-ray diffraction study of the formation, growth, and phase transition of colloidal Cu(2-x)S nanocrystals

The formation, growth, and phase transition of colloidal monodisperse spherical copper sulfide nanocrystals synthesized in dodecanethiol have been followed by in situ synchrotron powder X-ray diffraction (PXRD). The formation of nanocrystals involves a thermal decomposition of the crystalline precursor [CuSC12H25], which upon heating forms an isotropic liquid that subsequently turns into colloidal β-chalcocite phase Cu2S nanocrystals. The redox reaction step in the precursor solution has been studied by proton NMR. Upon heating, high digenite phase nanocrystals are formed through a solid-state rearrangement phase transition of the β-chalcocite phase nanocrystals at temperatures above 260 °C. TEM and PXRD reveal that the nanocrystal size is independent of synthesis temperature and stabilizes after the phase transition has completed. Spherical monodisperse nanocrystals are obtained in all experiments, with the nanocrystals in the β-chalcocite phase (7 nm) being smaller than those in high digenite phase (11 nm).

Evolution of atomic structure during nanoparticle formation

Understanding the mechanism of nanoparticle formation during synthesis is a key prerequisite for the rational design and engineering of desirable materials properties, yet remains elusive due to the difficulty of studying structures at the nanoscale under real conditions. Here, the first comprehensive structural description of the formation of a nanoparticle, yttria-stabilized zirconia (YSZ), all the way from its ionic constituents in solution to the final crystal, is presented. The transformation is a complicated multi-step sequence of atomic reorganizations as the material follows the reaction pathway towards the equilibrium product. Prior to nanoparticle nucleation, reagents reorganize into polymeric species whose structure is incompatible with the final product. Instead of direct nucleation of clusters into the final product lattice, a highly disordered intermediate precipitate forms with a local bonding environment similar to the product yet lacking the correct topology. During maturation, bond reforming occurs by nucleation and growth of distinct domains within the amorphous intermediary. The present study moves beyond kinetic modeling by providing detailed real-time structural insight, and it is demonstrated that YSZ nanoparticle formation and growth is a more complex chemical process than accounted for in conventional models. This level of mechanistic understanding of the nanoparticle formation is the first step towards more rational control over nanoparticle synthesis through control of both solution precursors and reaction intermediaries.

Real-time synchrotron powder X-ray diffraction study of the antisite defect formation during sub- and supercritical synthesis of LiFePO4and LiFe1−xMnxPO4nanoparticles

In situsynchrotron powder X-ray diffraction (PXRD) is used to study the formation of LiFePO4nanoparticles during hydrothermal synthesis from LiOH, H3PO4, and two different iron precursors, FeSO4and (NH4)2Fe(SO4)2. Furthermore, the synthesis of Li(Fe1−xMnx)PO4(x= 0.25, 0.50 and 0.75) from LiOH, H3PO4and FeSO4/MnSO4is studied. The reactions involve an unknown intermediate phase, which is not the previously observed intermediate NH4FePO4·H2O. The intermediate phase quickly transforms into LiFePO4and Li(Fe1−xMnx)PO4even at rather low temperatures. The presence of ammonium enhances the formation of LiFePO4, and it also leads to a significant reduction in the concentration of Li–Fe antisite defects. Thein situPXRD technique allows one to follow the influence of time, temperature and manganese doping on the antisite defect concentration, and it is shown that even under supercritical conditions a reaction time of several minutes is required to suppress the defects. This makes flow synthesis of defect-free LiFePO4and Li(Fe1−xMnx)PO4nanoparticles challenging.

Experimental setup forin situX-ray SAXS/WAXS/PDF studies of the formation and growth of nanoparticles in near- and supercritical fluids

The growing interest in inorganic nanoparticles for a wide range of applications is spurring a need for synthesis methods that allow a highly specific tailoring of material properties. Synthesis in supercritical fluids holds great promise for solving this problem, but so far the fundamental chemical processes taking place under these conditions are to a large extent unknown. Here the design, construction and application of a versatile experimental setup are reported; this setup enablesin situsynchrotron small-angle X-ray scattering/wide-angle X-ray scattering/pair distribution function (SAXS/WAXS/PDF) studies of the formation and growth of nanoparticles under supercritical fluid conditions.