From Characterisation to Validation: A Journey through Master’s Level Analytical Chemistry

The master’s degree in Applied Analytical Chemistry at University College London (UCL) includes valuable teaching input from the UK National Measurement Laboratory for Chemical and Bio-Measurement hosted at LGC. The course starts by introducing accuracy, sensitivity, specificity, trueness, and precision for validating analytical chemistry measurement methods. The principles of proficiency tests, quality control, ruggedness, and associated statistics are practiced using a wide variety of case studies

Phase control during the synthesis of nickel sulfide nanoparticles from dithiocarbamate precursors

Square-planar nickel bis(dithiocarbamate) complexes, [Ni(S2CNR2)2], have been prepared and utilised as single source precursors to nanoparticulate nickel sulfides. While they are stable in the solid-state to around 300 °C, heating in oleylamine at 230 °C, 5 mM solutions afford pure α-NiS, where the outcome is independent of the substituents. DFT calculations show an electronic effect rather than steric hindrance influences the resulting particle size. Decomposition of the iso-butyl derivative, [Ni(S2CNiBu2)2], has been studied in detail. There is a temperature-dependence of the phase of the nickel sulfide formed. At low temperatures (150 °C), pure α-NiS is formed. Upon raising the temperature, increasing amounts of β-NiS are produced and at 280 °C this is formed in pure form. A range of concentrations (from 5–50 mM) was also investigated at 180 °C and while in all cases pure α-NiS was formed, particle sizes varied significantly. Thus at low concentrations average particle sizes were ca. 100 nm, but at higher concentrations they increased to ca. 150 nm. The addition of two equivalents of tetra-iso-butyl thiuram disulfide, (iBu2NCS2)2, to the decomposition mixture was found to influence the material formed. At 230 °C and above, α-NiS was generated, in contrast to the results found without added thiuram disulfide, suggesting that addition of (iBu2NCS2)2 stabilises the metastable α-NiS phase. At low temperatures (150–180 °C) and concentrations (5 mM), mixtures of α-NiS and Ni3S4, result. A growing proportion of Ni3S4 is noted upon increasing precursor concentration to 10 mM. At 20 mM a metastable phase of nickel sulfide, NiS2 is formed and as the concentration is increased, α-NiS appears alongside NiS2. Reasons for these variations are discussed

Insight into nature of iron sulfide surfaces during the electrochemical hydrogen evolution and CO2 reduction reactions

Greigite and other iron sulfides are potential cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (Fe3S4) modified electrode were determined at −0.6 V vs. SHE at pH 7, under conditions of the HER. In situ X-ray Absorption Spectroscopy (XAS) was employed at the Fe K-edge to show that iron-sulfur linkages were replaced by iron-oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core-shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH− as a product of the HER. Under these conditions iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ IR spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when −0.6 V vs. SHE is applied. Saturation of the solution with CO2 resulted in inhibition of the hydroxide formation, potentially due to surface adsorption of HCO3−. This study shows that the true nature of the greigite electrode under conditions of the HER is a core-shell greigite-hydroxide material and emphasises the importance of in situ investigation of the catalyst under operation in order to develop true and accurate mechanistic models

Macroscopic heat release in a molecular solar thermal energy storage system

The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. One approach is the development of energy storage systems based on molecular photoswitches, so-called molecular solar thermal energy storage (MOST). Here we present a novel norbornadiene derivative for this purpose, with a good solar spectral match, high robustness and an energy density of 0.4 MJ kg-1. By the use of heterogeneous catalyst cobalt phthalocyanine on a carbon support, we demonstrate a record high macroscopic heat release in a flow system using a fixed bed catalytic reactor, leading to a temperature increase of up to 63.4 \ub0C (83.2 \ub0C measured temperature). Successful outdoor testing shows proof of concept and illustrates that future implementation is feasible. The mechanism of the catalytic back reaction is modelled using density functional theory (DFT) calculations rationalizing the experimental observations

Synthesis of ternary sulfide nanomaterials using dithiocarbamate complexes as single source precursors

We report the use of cheap, readily accessible and easy to handle di-isobutyl-dithiocarbamate complexes, [M(S(2)CN(i)Bu(2))(n)], as single source precursors (SSPs) to ternary sulfides of iron–nickel, iron–copper and nickel–cobalt. Varying decomposition temperature and precursor concentrations has a significant effect on both the phase and size of the nanomaterials, and in some instances meta-stable phases are accessible. Decomposition of [Fe(S(2)CN(i)Bu(2))(3)]/[Ni(S(2)CN(i)Bu(2))(2)] at ca. 210–230 °C affords metastable FeNi(2)S(4) (violarite) nanoparticles, while at higher temperatures the thermodynamic product (Fe,Ni)(9)S(8) (pentlandite) results. Addition of tetra-isobutyl-thiuram disulfide to the decomposition mixture can significantly affect the nature of the product at any particular temperature-concentration, being attributed to suppression of the intramolecular Fe(iii) to Fe(ii) reduction. Attempts to replicate this simple approach to ternary metal sulfides of iron–indium and iron–zinc were unsuccessful, mixtures of binary metal sulfides resulting. Oleylamine is non-innocent in these transformations, and we propose that SSP decomposition occurs via primary–secondary backbone amide-exchange with primary dithiocarbamate complexes, [M(S(2)CNHoleyl)(n)], being the active decomposition precursors

Designing photoswitches for molecular solar thermal energy storage

Solar energy conversion and solar energy storage are key challenges for a future society with limited access to fossil fuels. Certain compounds that undergo light-induced isomerisation to a metastable isomer can be used for storage of solar energy, so-called molecular solar thermal systems. Exposing the compound to sun light will generate a high energy photoisomer that can be stored. When energy is needed, the photoisomer can be catalytically converted back to the parent compound, releasing the excess energy as heat. This Letter gives examples of selected molecular solar thermal systems found in the literature. The focus of the Letter is on examples where molecular design has been used to improve the performance of the molecules, and as such it may serve as an inspiration for future design. The selected examples cover five widely studied systems, notably: anthracenes, stilbenes, azobenzenes, tetracarbonyl-fulvalene-diruthenium compounds and norbornadienes

Evaluating Dihydroazulene/Vinylheptafulvene Photoswitches for Solar Energy Storage Applications

Efficient solar energy storage is a key challenge in striving toward a sustainable future. For this reason, molecules capable of solar energy storage and release through valence isomerization, for so-called molecular solar thermal energy storage (MOST), have been investigated. Energy storage by photo-conversion of the dihydroazulene/vinylheptafulvene (DHA/VHF) photothermal couple has been evaluated. The robust nature of this system is determined through multiple energy storage and release cycles at elevated temperatures in three different solvents. In a nonpolar solvent such as toluene, the DHA/VHF system can be cycled more than 70 times with less than 0.01% degradation per cycle. Moreover, the [Cu(CH(3)CN4] P-6-catalyzed conversion of VHF into DHA was demonstrated in a flow reactor. The performance of the DHA/VHF couple was also evaluated in prototype photoconversion devices, both in the laboratory by using a flow chip under simulated sunlight and under outdoor conditions by using a parabolic mirror. Device experiments demonstrated a solar energy storage efficiency of up to 0.13% in the chip device and up to 0.02% in the parabolic collector. Avenues for future improvements and optimization of the system are also discussed

Active nature of primary amines during thermal decomposition of nickel dithiocarbamates to nickel sulfide nanoparticles

Although [Ni(S2CNBu2i)(2)] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 degrees C in oleylamine to give pure NiS nanoparticles, while in n-hexylamine at 120 degrees 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 trans-[Ni(S2CNBu2i)(2)(RNH2)(2)] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90-100 degrees C leads to changes consistent with the formation of amide-exchange products, [Ni(S2CNBu(2)(i)){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(S2CNBu2i){S2CN(H)Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 degrees 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}(mu-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

Doping group IIB metal ions into quantum dot shells via the one-pot decomposition of metal-dithiocarbamates

Almost half of solar energy reaching the Earth is in the infrared, and for solar cells, IR absorbing/emitting quantum dots are highly effective photovoltaic materials. As a possible approach to generating such materials, an investigation into the incorporation of group IIB metal ions during the shelling of II-VI and III-V semiconductor core/shell quantum dots is presented. Quantum dot shells consist of ZnS and an additional metal sulphide, obtained from the decomposition of metal dithiocarbamate single-source precursors. Resultant quantum dots are characterized and interrogated using transmission electron microscopy, high-resolution transmission electron microscopy, electron diffraction, time-of-flight-secondary ion mass spectroscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, photoluminescence emission and lifetime spectroscopy, and UV-vis spectroscopy. It is demonstrated that on incorporation of an additional metal sulphide during shelling, photoluminescence properties change dramatically according to the element and indeed, its concentration. Tunable infrared emission is achieved for Hg addition, thus a one-pot method for the synthesis of infrared emitting quantum dots from visible luminescent cores is hereby developed