Cs⁺ incorporation into CH₃NH₃PbI₃ perovskite:substitution limit and stability enhancement

In this study we systematically explored the mixed cation perovskite Csx(CH3NH3)1-xPbI3. We exchanged the A-site cation by dipping MAPbI3 films into a CsI solution, thereby incrementally replacing the MA+ in a time-resolved dipping process and analysed the resulting thin-films with UV-Vis, XRD, EDAX, SEM and optical depth-analysis in a high-throughput fashion. Additional in situ UV-Vis and time-resolved XRD measurements allowed us to look at the kinetics of the formation process. The results showed a discontinuity during the conversion. Firstly, small amounts of Cs+ are incorporated into the structure. After a few minutes, the Cs content approaches a limit and grains of δ-CsPbI3 occur, indicating a substitution limit. We compared this cation exchange to a one-step crystallisation approach and found the same effect of phase segregation, which shows that the substitution limit is an intrinsic feature rather than a kinetic effect. Optical and structural properties changed continuously for small Cs incorporations. Larger amounts of Cs result in phase segregation. We estimate the substitution limit of CsxMA1-xPbI3 to start at a Cs ratio x = 0.13, based on combined measurements of EDAX, UV-Vis and XRD. The photovoltaic performance of the mixed cation perovskite shows a large increase in device stability from days to weeks. The initial efficiency of mixed CsxMA1-xPbI3 devices decreases slightly, which is compensated by stability after a few days.</p

Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence

© 2019, The Author(s), under exclusive licence to Springer Nature Limited. Metal halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes.S.F. acknowledges funding from the Studienstiftung des deutschen Volkes and EPSRC, as well as support from the Winton Programme for the Physics of Sustainability. S.M. acknowledges funding from an EPSRC studentship. M.A.-J. thanks Nava Technology Limited, Cambridge Materials Limited and EPSRC (grant number: EP/M005143/1) for their funding and technical support. S.P.S. acknowledges funding from the Royal Society Newton Fellowship and EPSRC through a program grant (EP/M005143/1). T.A.S.D. acknowledges the National University of Ireland (NUI) for a Travelling Studentship and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement number 756962). K.F. acknowledges funding from a George and Lilian Schiff Foundation Studentship, an EPSRC studentship and a scholarship from the Winton Programme for the Physics of Sustainability. E.R. acknowledges funding from an ERC starting grant (no. 804523). R.H.F. acknowledges support from the Simons Foundation (grant 601946). Research work in Mons was supported by the Fonds de la Recherche Scientifique de Belgique - Fund for Scientific Research (F.R.S.-FNRS) and the EU Marie-Curie IEF project ‘DAEMON’. Computational resources have been provided by the Consortium des Équipements de Calcul Intensif (CÉCI). D.B. is an FNRS Research Director. S.D.S. acknowledges the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (HYPERION, grant agreement number 756962), the Royal Society and Tata Group (UF150033). F.D. acknowledges funding from the Winton Programme for the Physics of Sustainability

Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence

Metal halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes

The Cesium Oxide Mercuride Cs18Hg8O6

By reacting Cs with Cs2O and elemental mercury the new double salt Cs18Hg8O6 could be obtained. Its crystal structure (cubic, space group I23 with a=13.3920(10) angstrom, Z=2, R1=0.026/0.032 for I >= 2 sigma(I)/all I, respectively) comprises isolated oxide anions in octahedral coordination by cesium cations next to the mercuride anion [Hg-8](6-). This first isolated anion of mercury has cubic shape and is coordinated by cesium atoms capping the faces and the edges of the [Hg-8](6-) cube. The ionic character of the double salt is shown in DFT calculations of the electronic structure. Raman spectra show distinct features in the low energy region arising from the vibrations of the mercuride anion. Cs18Hg8O6 has very close structural analogies to the thallide oxide Cs18Tl8O6

The use of columns of the zeolite clinoptilolite in the remediation of aqueous nuclear waste streams

Mud Hills clinoptilolite has been used in an effluent treatment plant (SIXEP) at the Sellafield nuclear reprocessing site. This material has been used to remove Cs-134/137 and Sr-90 successfully from effluents for 3 decades. Samples of the zeolite have been tested in column experiments to determine their ability to remove radioactive Cs+ and Sr2+ ions under increasing concentrations of competing ions, Ca2+, Mg2+, Na+ and K+. These ions caused increased elution of Cs+ and Sr2+. Ca2+, Mg2+ and K+ were more effective competitors than Na+. For Na+, it was found that if concentration was reduced, then column performance recovered rapidly.Peer reviewe

The effect of cationic surfactants on improving natural clinoptilolite for the flotation of cesium

Flotation using cationic surfactants has been investigated as a rapid separation technique to dewater clinoptilolite ion exchange resins, for the decontamination of radioactive cesium ions (Cs+) from nuclear waste effluent. Initial kinetic and equilibrium adsorption studies of cesium, suggested the large surface area to volume ratio of the fine zeolite contributed to fast adsorption kinetics and high capacities (qc = 158.3 mg/g). Adsorption of ethylhexadecyldimethylammonium bromide (EHDa-Br) and cetylpyridinium chloride (CPC) surfactant collectors onto both clean and 5 ppm Cs+ contaminated clinoptilolite was then measured, where distribution coefficients (Kd) as high as 10,000 mL/g were evident with moderate concentrations CPC. Measurements of particle sizes confirmed that adsorption of surfactant monolayers did not lead to significant aggregation of the clinoptilolite, while 4, highlighting the great viability of flotation to separate and concentrate the contaminated powder in the froth phase

Bacterial ion effects and their relation to salt tolerance

Previously held under moratorium from 25 June 2018 until 24 February 2022Extremophiles are organisms that are able to tolerate conditions that would otherwise inhibit or even kill non-extremophilic organisms – such extremes include acidity, high salt concentrations, high temperatures and high pressure. Specifically, halophiles are organisms that have a requirement for high concentrations of salt for growth. These organisms have been found to use either of two adaptation strategies, known as ‘salt-in’ (accumulation of inorganic ions) and ‘salt-out’ (removal of inorganic ions and accumulation of neutral molecules). In the current study, the relationship between the level of salt tolerance of an organism and its ion metabolism was investigated in order to gain insight into halo-adaptation and mechanisms of bacterial salt tolerance. This was accomplished by analysing the effects of a variety of salts (21 different combinations) on a halophile (Salinibacter ruber), non-halophile (Escherchia coli) and halotolerant (Echinicola vietnamensis) organism, which was achieved via an analysis of the effects of salts on bacterial growth, intracellular cation accumulation, enzymatic activity and and bioinformatics analysis. It was found that cation preferences were directly related to the level of salt tolerance of the organism, which is hypothesised to be a product of proteome acidity as well as the presence of specific membrane cation transporters. Specifically, the preference of S. ruber for the higher charge density Na+ over K+ may be rationalised based on the Hofmeister effect –i.e. this cation may provide better stabilisation of intracellular enzymes at the optimal salt concentrations for growth of S. ruber, but may be destabilising if accumulated at higher concentrations, and for non-salt adapted organisms. The ability of E. vietnamensis to tolerate and utilise many non-physiological ions supports this theory. Additionally, E. vietnamensis was postulated to use a ‘hybrid’ osmotic adaptation strategy – this organism may have industrial applications due to its large salt concentration tolerance range and high tolerance for non-physiological cations. Crucially, it was also found that E. vietnamensis and S. ruber contained membrane cation transporters that may be essential for their salt tolerance, giving insight into the essential nature of these proteins for the possession of salt resistance, which may have potential to be utilised for the transfer of salt- tolerance to commercially important organisms. Finally, one specific salt combination tested, equimolar LiCl + KBr proved to totally inhibit bacterial growth and may show promise as an antimicrobial agent, for which a patent application has been initiated. The results of the current study can have various applications, including those within industry, medicine and astrobiology.Extremophiles are organisms that are able to tolerate conditions that would otherwise inhibit or even kill non-extremophilic organisms – such extremes include acidity, high salt concentrations, high temperatures and high pressure. Specifically, halophiles are organisms that have a requirement for high concentrations of salt for growth. These organisms have been found to use either of two adaptation strategies, known as ‘salt-in’ (accumulation of inorganic ions) and ‘salt-out’ (removal of inorganic ions and accumulation of neutral molecules). In the current study, the relationship between the level of salt tolerance of an organism and its ion metabolism was investigated in order to gain insight into halo-adaptation and mechanisms of bacterial salt tolerance. This was accomplished by analysing the effects of a variety of salts (21 different combinations) on a halophile (Salinibacter ruber), non-halophile (Escherchia coli) and halotolerant (Echinicola vietnamensis) organism, which was achieved via an analysis of the effects of salts on bacterial growth, intracellular cation accumulation, enzymatic activity and and bioinformatics analysis. It was found that cation preferences were directly related to the level of salt tolerance of the organism, which is hypothesised to be a product of proteome acidity as well as the presence of specific membrane cation transporters. Specifically, the preference of S. ruber for the higher charge density Na+ over K+ may be rationalised based on the Hofmeister effect –i.e. this cation may provide better stabilisation of intracellular enzymes at the optimal salt concentrations for growth of S. ruber, but may be destabilising if accumulated at higher concentrations, and for non-salt adapted organisms. The ability of E. vietnamensis to tolerate and utilise many non-physiological ions supports this theory. Additionally, E. vietnamensis was postulated to use a ‘hybrid’ osmotic adaptation strategy – this organism may have industrial applications due to its large salt concentration tolerance range and high tolerance for non-physiological cations. Crucially, it was also found that E. vietnamensis and S. ruber contained membrane cation transporters that may be essential for their salt tolerance, giving insight into the essential nature of these proteins for the possession of salt resistance, which may have potential to be utilised for the transfer of salt- tolerance to commercially important organisms. Finally, one specific salt combination tested, equimolar LiCl + KBr proved to totally inhibit bacterial growth and may show promise as an antimicrobial agent, for which a patent application has been initiated. The results of the current study can have various applications, including those within industry, medicine and astrobiology

Cs⁺ incorporation into CH₃NH₃PbI₃ perovskite:substitution limit and stability enhancement

In this study we systematically explored the mixed cation perovskite Csx(CH3NH3)1−xPbI3.</p

Catalyst Deactivation in Copper-Catalysed C-N Cross-Coupling Reactions

This thesis details research into the mechanism of the copper-catalysed cross-coupling reaction, with a focus on the N-arylation of secondary amines and amides (Ullmann-Goldberg reaction). Issues limiting the application of the Ullmann-Goldberg reaction on industrial scale are uncovered and understood from a mechanistic point of view, to provide a platform for more efficient copper catalysts. In situ kinetic monitoring using 1H NMR spectroscopy is used to understand the role of each key component in the N-arylation of piperidine using a copper catalyst in a fully homogeneous system. Key roles of the amine and catalyst are observed, whilst product inhibition was found to significantly inhibit the reaction. Solvent effects are evaluated by repeating the in situ kinetic investigation in d7-DMF and d3-MeCN. The kinetic findings indicate deviation of the mechanism from the accepted literature mechanism, with a rate-limiting amine coordination proposed. Kinetic studies are also used to show the key role that the solubility of inorganic bases such as Cs2CO3 and K3PO4 play in rate-determining equilibria between copper species. Slow catalyst turnover was observed for up to 14 hours at lowered catalyst loadings, before a dramatic increase in the rate of reaction. A link between particle size, solubility and the deprotonation of the cyclic amide substrate are made, with the results having important implications on the use of inorganic bases in cross-coupling reactions. Inhibition of the reaction from inorganic side products and precipitation of copper from solution is combined with findings of a novel interaction between copper and cesium cation to give evidence for a non-innocent role of the cesium cation in copper-catalysed N-arylation. Heterogeneous catalysis is explored, where it is shown that copper precipitated from the reaction is capable of catalysis, introducing a question of a pseudo-heterogeneous mechanism and phase equilibria. A multi-edge XAFS (X-ray absorption fine structure) study is combined with laboratory experiments to uncover the activation, deactivation and reactivation pathways of an immobilised iridium transfer hydrogenation catalyst. The key role of an Ir-Cl bond is shown, where initial ligand exchange activates the catalyst, followed by further, slow ligand exchange, leading to deactivation of the catalyst