Journal of neurology, neurosurgery and psychiatry

The supramolecular assembly of a series of copolymers combining poly­(ethylene oxide)-rich hydrophilic and fluorinated CO₂-philic sequences is analyzed by synchrotron small-angle X-ray scattering (SAXS) in supercritical CO₂, as well as in water/CO₂ emulsions. These copolymers were designed to have the same molecular weight and composition and to differ only by their macromolecular architecture. The investigated copolymers have random, block, and palm-tree architectures. Besides, thermoresponsive copolymer is also analyzed, having a hydrophilic sequence becoming water-insoluble around 41 °C, i.e., just above the critical point of CO₂. At the length scale investigated by SAXS, only the random copolymer appears to self-assemble in pure CO₂, in the form of a disordered microgel-like network. The random, block, and thermoresponsive copolymers are all able to stabilize water/CO₂ emulsions but not the copolymer with the palm-tree architecture, pointing at the importance of macromolecular architecture for the emulsifying properties. A modeling of the SAXS data shows that the block and the thermoresponsive copolymers form spherical micelle-like structures containing about 70% water and 30% polymer

In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity

Ni-based oxygen evolution catalysts (OECs) are cost-effective and very active materials that can be potentially used for efficient solar-to-fuel conversion process toward sustainable energy generation. We present a systematic spectroelectrochemical characterization of two Fe-containing Ni-based OECs, namely nickel borate (Ni­(Fe)−B_i) and nickel oxyhydroxide (Ni­(Fe)­OOH). Our Raman and X-ray absorption spectroscopy results show that both OECs are chemically similar, and that the borate anions do not play an apparent role in the catalytic process at pH 13. Furthermore, we show spectroscopic evidence for the generation of negatively charged sites in both OECs (NiOO^–), which can be described as adsorbed “active oxygen”. Our data conclusively links the OER activity of the Ni-based OECs with the generation of those sites on the surface of the OECs. The OER activity of both OECs is strongly pH dependent, which can be attributed to a deprotonation process of the Ni-based OECs, leading to the formation of the negatively charged surface sites that act as OER precursors. This work emphasizes the relevance of the electrolyte effect to obtain catalytically active phases in Ni-based OECs, in addition to the key role of the Fe impurities. This effect should be carefully considered in the development of Ni-based compounds meant to catalyze the OER at moderate pHs. Complementarily, UV–vis spectroscopy measurements show strong darkening of those catalysts in the catalytically active state. This coloration effect is directly related to the oxidation of nickel and can be an important factor limiting the efficiency of solar-driven devices utilizing Ni-based OECs

Active Nature of Primary Amines during Thermal Decomposition of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles

Although [Ni­(S₂CNBuⁱ₂)₂] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 °C in oleylamine to give pure NiS nanoparticles, while in <i>n</i>-hexylamine at 120 °C a mixture of Ni₃S₄ (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral <i>trans-</i>[Ni­(S₂CNBuⁱ₂)₂(RNH₂)₂] result as shown by <i>in situ</i> XANES and EXAFS and confirmed by DFT calculations. Heating to 90–100 °C leads to changes consistent with the formation of amide-exchange products, [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­R}] and/or [Ni­{S₂CN­(H)­R}₂]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni­{S₂CN­(H)­Hex}₂] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 °C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni­{S₂CN­(H)­Hex}₂]. DFT modeling of [Ni­{S₂CN­(H)­R}₂] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S₂CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni­{S₂CN­(H)­R}­(μ-SH)]₂. These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amide-exchange that account primarily for the formation of the observed nanoparticulate nickel sulfides