Insight into Nucleation and Growth of Bi_2–<i>x</i>Sb_<i>x</i>Te₃ (<i>x</i> = 0–2) Nanoplatelets in Hydrothermal Synthesis

Controlled wet chemical synthesis of nanomaterials with tailored structures and properties, especially by hydrothermal/solvothermal methods, is difficult due to the complicated nucleation and growth processes. Therefore, it is important to gain a deeper understanding of the formation mechanisms involved in hydrothermal/solvothermal processes. In the present study, the formation mechanisms of Bi_2–<i>x</i>Sb_<i>x</i>Te₃ (<i>x</i> = 0–2) nanoplatelets under hydrothermal conditions are studied by <i>in situ</i> synchrotron radiation powder X-ray diffraction (SR-PXRD). Synthesis using glucose as the reducing agent results in direct nucleation of Bi₂Te₃ from Bi and Te precursors without the presence of any intermediate products, and the nucleation stage is completed in only about 1 min at 250 °C. When Sb is added, the nucleation process for Bi_2–<i>x</i>Sb_<i>x</i>Te₃ becomes slower with increasing Sb content. Depending on the Sb content and reaction temperature, an intermediate product of elemental Te nanostructures forms, and it is found to direct the morphology of the final products. Within the duration of the measurements, Sb₂Te₃ only forms at relatively high reaction temperatures due to the slower kinetics of the reaction. A two-stage nucleation mechanism is found for the formation of the ternary BiSbTe₃ with an initial nucleation stage of predominately Bi₂Te₃ followed by a second stage of slower nucleation and alloying process to BiSbTe₃. Both the composition and reaction temperatures affect the crystallization kinetics and crystallite growth mechanisms during synthesis of Bi_2–<i>x</i>Sb_<i>x</i>Te₃ nanoplatelets

High-Pressure, High-Temperature Studies of Phase Transitions in SrOsO₃Discovery of a Post-Perovskite

Using a recently developed method for in situ high-pressure, laser heating experiments in diamond anvil cells, we obtained a novel post-perovskite phase of SrOsO3. The phase transition from perovskite SrOsO3 was induced at 44 GPa and 1350 K in a diamond anvil cell and characterized with synchrotron powder X-ray diffraction. The newly obtained post-perovskite is quenchable and Le Bail refinements under ambient conditions yielded the unit cell parameters: a = 3.152(9) Å, b = 10.82(2) Å, c = 7.27(1) Å, V = 248.1(1) Å3. In addition, the compression of perovskite SrOsO3 at ambient temperature was investigated up to 66 GPa in a diamond anvil cell using synchrotron powder X-ray diffraction. The compression at ambient temperature showed that pressure alone does not induce the first-order phase transition to the post-perovskite structure. However, at 36 GPa, a continuous phase transition to monoclinic (P21/n) symmetry was detected, persistent up to 58 GPa, where the perovskite transitioned back to orthorhombic (Pbnm) symmetry. Fitting a third-order Birch–Murnaghan equation of state to the obtained P–V data for perovskite SrOsO3 yielded a bulk modulus of K0 = 187.4(15) GPa. Density functional theory calculations were performed to support the experimental findings in the compression study at ambient temperature. This work shows that transformations to the post-perovskite structure can be obtained for a wider range of perovskites than simple empirical rules otherwise suggest

Development of a Dual-Stage Continuous Flow Reactor for Hydrothermal Synthesis of Hybrid Nanoparticles

This paper provides a comprehensive description of the design and commissioning of a dual-stage flow reactor for hydrothermal synthesis, notably heterogeneous nanomaterials such as core–shell particles or nanocomposites. The design is based on the hypothesis that the next frontier of studies within continuous, hydrothermal synthesis lies as much with scalability as it does with the materials properties and performance in applications. Therefore, this reactor belongs to the up-scaled end of a laboratory system with a synthesis capacity of up to 50 g/h. Commissioning was accomplished with TiO₂ nanoparticles as a model material. Results comply with earlier ones obtained from single-stage reactors. Dual-stage synthesis of a TiO₂@SnO₂ nanocomposite was performed by adding a SnCl₄ solution to newly formed 9 nm TiO₂ nanoparticles, yielding deposition of 2 nm rutile SnO₂. Synthesis of pure SnO₂ produced much larger nanocrystals, indicating that TiO₂ nanoparticles provide the nucleation sites for SnO₂ and impede the growth beyond 2 nm

Pulsed-Flow Near-Critical and Supercritical Synthesis of Carbon-Supported Platinum Nanoparticles and In Situ X‑ray Diffraction Study of Their Formation and Growth

The formation and growth of carbon (C)-supported platinum (Pt) nanoparticles in a high-temperature, high-pressure ethanol solution have been studied by in situ synchrotron radiation powder X-ray diffraction (PXRD). Supercritical synthesis is shown to be an efficient way to prepare Pt nanoparticles, and the crystallite size of Pt nanoparticles is much smaller when formed with supporting C material compared with synthesis without C. On the basis of the time-resolved in situ PXRD data, a surface stress of 2.65 N/m is derived from the size dependence of the cell parameters. As proof of concept, C-supported Pt nanoparticles were subsequently synthesized in a pulsed-flow supercritical reactor, which offers complete control of the reaction temperature, pressure, and residence time. Well-dispersed Pt nanoparticles decorated on the supporting C material can be obtained by adjusting the reaction conditions, and the electrocatalytic activity of the samples is explored. A mass activity of 0.1209 A/mg_Pt at a potential of 0.9 V is obtained for the products prepared at 400 °C for a residence time of 20 s. The pulsed-flow supercritical method is a facile method to synthesize ligand-free C-supported Pt nanoparticles with high electrocatalytic activity

Surface-Dominated Transport on a Bulk Topological Insulator

Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, it is challenging to achieve surface-dominated transport. Here we use nanoscale four-point setups with a variable contact distance on an atomically clean surface of bulk-insulating Bi₂Te₂Se. We show that the transport at 30 K is two-dimensional rather than three-dimensional, that is, surface-dominated, and we find a surface state mobility of 390(30) cm² V^–1 s^–1 at 30 K at a carrier concentration of 8.71(7) × 10¹² cm^–2

Selective CO₂ Reduction to CO in Water using Earth-Abundant Metal and Nitrogen-Doped Carbon Electrocatalysts

Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO₂ reduction reaction (CO₂RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO₂RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO₂-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO₂RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO₂RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO₂ valorization