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Register a new userCluster Assisted Water Dissociation Mechanism in MOF-74 and Controlling it Using Helium
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We show that the water dissociation reaction H$_2$O$rightarrow$OH+H in the confined environment of MOF-74 channels can be precisely controlled by the addition of the noble gas He. Elucidating the entire reaction process with ab initio methods and infrared (IR) spectroscopy, we prove that the interaction between water molecules is critical to the formation of water clusters, which reduce the dissociation barrier by up to 37% and thus influence the reaction significantly. Our time-resolved IR measurements confirm that the formation of these clusters can be suppressed by introducing He gas, providing unprecedented control over water dissociation rates. Since the water dissociation reaction is the cause of the structural instability of MOF-74 in the presence of water, our finding of the reaction mechanism lays the groundwork for designing water stab
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Metal organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties for many technologically important processes. However, their instability under humid conditions severely restricts practical use. We show that this instability and the accompanying reduction of the CO$_2$ uptake capacity of MOF-74 under humid conditions originate in the water dissociation reaction H$_2$O$rightarrow$OH+H at the metal centers. After this dissociation, the OH groups coordinate to the metal centers, explaining the reduction in the MOFs CO$_2$ uptake capacity. This reduction thus strongly depends on the catalytic activity of MOF-74 towards the water dissociation reaction. We further show that-while the water molecules themselves only have a negligible effect on the crystal structure of MOF-74-the OH and H products of the dissociation reaction significantly weaken the MOF framework and lead to the observed crystal structure breakdown. With this knowledge, we propose a way to suppress this particular reaction by modifying the MOF-74 structure to increase the water dissociation energy barrier and thus control the stability of the system under humid conditions.
Using infrared spectroscopy combined with ab initio methods we study reactions of H$_2$O and CO inside the confined spaces of Zn-MOF-74 channels. Our results show that, once the water dissociation reaction H$_2$O$;rightarrow;$OH+H takes place at the metal centers, the addition of 40 Torr of CO at 200 $^{circ}$C starts the production of formic acid via OH+H+CO$;rightarrow;$HCO$_2$H. Our detailed analysis shows that the overall reaction H$_2$O+CO$;rightarrow;$HCO$_2$H takes place in the confinement of MOF-74 without an external catalyst, unlike the same reaction on flat surfaces. This discovery has several important consequences: It opens the door to a new set of catalytic reactions inside the channels of the MOF-74 system, it suggests that a recovery of the MOFs adsorption capacity is possible after it has been exposed to water (which in turn stabilizes its crystal structure), and it produces the important industrial feedstock formic acid.
Magnetic particle hyperthermia, in which colloidal nanostructures are exposed to an alternating magnetic field, is a promising approach to cancer therapy. Unfortunately, the clinical efficacy of hyperthermia has not yet been optimized. Consequently, routes to improve magnetic particle hyperthermia such as designing hybrid structures comprised from different phase materials are actively pursued. Here we demonstrate enhanced hyperthermia efficiency in relative large spherical Fe/Fe-oxide core/shell nanoparticles through the manipulation of interactions between the core and shell phases. Experimental results on exemplary samples with diameters in the range 30-80 nm indicated a direct correlation of hysteresis losses to the observed temperature elevation rate with a maximum efficiency of around 0.9 kW/g. The absolute particle size, the core/shell ratio, and the interposition of a thin wustite interlayer, are shown to have powerful effects on the specific absorption rate. By comparing our measurements to micromagnetic calculations we have unveiled topologically non-trivial magnetisation reversal modes under which interparticle interactions become negligible, aggregates formation is minimized, and the energy that is converted into heat is increased. This information has been overlooked till date and is in stark contrast to the existing knowledge on homogeneous particles.
Monolayers of transition metal dichalcogenides (TMDs) have been established in the last years as promising materials for novel optoelectronic devices. However, the performance of such devices is often limited by the dissociation of tightly bound excitons into free electrons and holes. While previous studies have investigated tunneling at large electric fields, we focus in this work on phonon-assisted exciton dissociation that is expected to be the dominant mechanism at small fields. We present a microscopic model based on the density matrix formalism providing access to time- and momentum-resolved exciton dynamics including phonon-assisted dissociation. We track the pathway of excitons from optical excitation via thermalization to dissociation, identifying the main transitions and dissociation channels. Furthermore, we find intrinsic limits for the quantum efficiency and response time of a TMD-based photodetector and investigate their tunability with externally accessible knobs, such as excitation energy, substrate screening, temperature and strain. Our work provides microscopic insights in fundamental mechanisms behind exciton dissociation and can serve as a guide for the optimization of TMD-based optoelectronic devices.
Instability of perovskite photovoltaics is still a topic which is currently under intense debate, especially the role of water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) in explicit water. It is found that the initial dissolution of MAPbI$_3$ is a complex multi-step process triggered by the departure of I$^-$ ion from the CH$_3$NH$_3$I-terminated surface. Reconstruction of the free energy landscape indicates a low energy barrier for water dissolution of MAPbI$_3$. In addition, we propose a two-step thermodynamic cycle for MAPbI$_3$ dissolution in water at a finite concentration that renders a spontaneity of the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI$_3$ dissolution in water explain why the water immediately destroys pristine MAPbI$_3$. The dissolution thermodynamics of all-inorganic CsPbI$_3$ perovskite is also analyzed for comparison. Hydration enthalpies and entropies of aqueous ions play an important role for the dissolution process. Our findings provide a comprehensive understanding to the current debate on water instability of MAPbI$_3$.
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