Molten Salts-Driven Discovery of a Polar Mixed-Anion 3D Framework at the Nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting

Mixed-anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn₄Si₂O₇Cl₂. We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon-based connectors from a non-oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H₂ evolution and water oxidation. This work then paves the way to a new family of mixed-anion solids     

Weighing Photons,I

For over fifty years astronomers sought inconclusively to confirm Albert Einstein's prediction of a "gravitational red shift" by studies of solar and stellar spectra. Although it was expected by the 1950s that the combination of atomic clocks with vehicles sent into space would finally enable a conclusive test, the development of precise examples of the resonant absorption of gamma rays between nuclei bound in solids, discovered by Rudolf Mössbauer in 1958, provided a basis for an experiment within an earthbound laboratory. I describe the trials and tribulations of making the necessary extension of the phenomenon, including the discovery of the unanticipated effect of temperature as causing relativistic time dilation, that led to a successful confirmation in March of 1960 of a shift amounting to only 2.5 2 10^⪣ to less than a 10% uncertainty.     

NMR Spectroscopy in the Optimization and Evaluation of RAFT Agents

The selection of a suitable mediating agent in Reversible Addition-Fragmentation Chain Transfer (RAFT) mediated polymerization is crucial to the degree of control that can be achieved. An overview of work from the Stellenbosch group is presented in which the use of NMR spectroscopy as a tool for evaluating RAFT-agents is highlighted. The occurrence of selective initialization, i.e. the selective conversion of a RAFT-agent into its single monomer adduct is discussed for various classes of monomers, as well as for copolymerization. One of the general rules for living polymerization is that chains should start growing early in the polymerization reaction. Selective initialization is claimed to be the extreme case where all chains have begun growing after the conversion of only one monomer equivalent per RAFT-agent.     

Molecular-weight dependence of the glass transition temperature of freely-standing poly(methyl methacrylate) films

We have used transmission ellipsometry to measure the glass transition temperature, T_g, of freely-standing films of atactic and syndiotactic poly(methyl methacrylate) (PMMA). We have prepared films with different molecular weights, MW, (159×10³ < M _w < 1.3×10⁶) and film thicknesses, h, ( 30nm < h < 200 nm). For the high-MW ( M _w > 509×10³) atactic PMMA films, we find that T_g decreases linearly with decreasing h, which is qualitatively similar to previous results obtained for high-MW freely-standing polystyrene (PS) films. However, the overall magnitude of the T_g reduction is much less (by roughly a factor of three) for the high-MW freely-standing PMMA films than for freely-standing PS films of comparable MW and h. The observed differences between the freely-standing PMMA and PS film data suggest that differences in chemical structure determine the magnitude of the T_g reduction and we discuss the possible origins of these differences. Our analysis of the MW-dependence of the T_g reductions suggests that the mechanism responsible for the MW-dependent T_g reductions observed in the high-MW freely-standing films is different than that responsible for the MW-independent T_g reductions observed in the low-MW freely-standing and supported films.     

Unsymmetrical bifunctional trithiocarbonate as unexpected by-product in the synthesis of a dithioester RAFT agent

The trithiocarbonate 2-(benzylsulfanylthiocarbonylsulfanyl) propanoic acid is formed as minor by-product in the synthesis of the dithioester 2-((2-phenylthioacetyl)sulfanyl) propanoic acid via the Grignard route. The mechanism for this side reaction is not clear. The isolated trithiocarbonate may act as unsymmetrical but bifunctional RAFT agent in the aqueous polymerization of N,N-dimethyl acrylamide. Therefore, it is important to separate it completely from the dithioester before engaging the latter in controlled free radical polymerization to guarantee a maximum control.     

Gel-permeation chromatographic studies of cellulose degradation. I. Treatment with hydrochloric acid

The molecular weight distribution in various celluloses degraded with hydrochloric acid has been studied by gel-permeation chromatography. Measurements were made on the residue after hydrolysis, on the degradation products solubilized by the acid, and on whole degraded samples comprising the product soluble in the acid plus the residue. It is shown that the total number of crystallites decreases during hydrolysis while the chain length distribution remains constant. The crystallites are gradually broken down into molecular fragments which show a size distribution centered on species with a degree of polymerization of about 8. Possible interpretations of these results are discussed.     

Weighing Photons, II

After completion of the initial experiment confirming the gravitational red shift by an application of the gamma-ray resonance of ⁵⁷Fe as described in "Weighing Photons, I," Physics in Perspective 2 (2000), 224-268. I proceeded to design and carry out a new version with improved components that would allow better control over sources of systematic error. This second article describes that effort, which was carried out between 1960 and 1964 with the collaboration, briefly, of R. W. P. Drever and then of Joseph L. Snider. It led to a result further confirming the anticipated fractional frequency shift of 2.45᎒^⪣, with an uncertainty of less than 1%, for the height of our "tower." Studies of external reflection of X rays and of possibly better isotopes, aiming toward a possible third-round experiment, were carried out with the collaboration of William T. Vetterling.     

Suppression of slow gain recovery in ultralong quantum-dash semiconductor optical amplifier emitting at 1.55 μm

Gain dynamics in two quantum-dash semiconductor optical amplifiers of different lengths emitting in the 1.55 μm region are investigated experimentally and compared. It is shown that slow gain recovery due to total carrier relaxation is totally suppressed in the ultralong amplifier. Consequently, the 10%-90% gain recovery time is drastically reduced from about 40 ps (short sample) to 10 ps (long one).     

Hyphenation of atmospheric pressure chemical ionisation mass spectrometry to supercritical fluid chromatography for polar car lubricant additives analysis

Car lubricant additives are added to mineral or synthetic base stocks to improve viscosity and resistance to oxidation of the lubricant and to limit wear of engines. As they belong to various chemical classes and are added to a very complex medium, the base stock, their detailed chromatographic analysis is very difficult and time consuming. In a previous paper, it was demonstrated that supercritical fluid chromatography (SFC) allows the elution of common low-molecular-weight additives. Since their total resolution could not be achieved owing to the limited peak capacity of packed columns, the hyphenation of selective and informative detection methods such as atomic emission detection (AED) was required. Further to results obtained in SFC-AED, this work describes the hyphenation of SFC to atmospheric pressure chemical ionisation ion trap mass spectrometry (MS). SFC–MS hyphenation is detailed: temperature, flow rates of gas and mobile phase introduced in the source, position of the restrictor, ionisation additives and conditions of autotune are studied. Car lubricant monitoring requires negative and positive ionisation modes with or without the addition of ionisation auxiliary solvent according to the nature of additives. Moreover, when sensitivity is of major concern for a selected additive, the autotuning routine of the MS has to be performed in conditions as close as possible to analytical conditions, i.e. under subcritical conditions. Unambiguous identification and structure elucidation of several additives in formulated car lubricants are also presented.