03 Fakultät Chemie
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Item Open Access Designing covalent organic framework‐based light‐driven microswimmers toward therapeutic applications(2023) Sridhar, Varun; Yildiz, Erdost; Rodríguez‐Camargo, Andrés; Lyu, Xianglong; Yao, Liang; Wrede, Paul; Aghakhani, Amirreza; Akolpoglu, Birgul M.; Podjaski, Filip; Lotsch, Bettina V.; Sitti, MetinWhile micromachines with tailored functionalities enable therapeutic applications in biological environments, their controlled motion and targeted drug delivery in biological media require sophisticated designs for practical applications. Covalent organic frameworks (COFs), a new generation of crystalline and nanoporous polymers, offer new perspectives for light‐driven microswimmers in heterogeneous biological environments including intraocular fluids, thus setting the stage for biomedical applications such as retinal drug delivery. Two different types of COFs, uniformly spherical TABP‐PDA‐COF sub‐micrometer particles and texturally nanoporous, micrometer‐sized TpAzo‐COF particles are described and compared as light‐driven microrobots. They can be used as highly efficient visible‐light‐driven drug carriers in aqueous ionic and cellular media. Their absorption ranging down to red light enables phototaxis even in deeper and viscous biological media, while the organic nature of COFs ensures their biocompatibility. Their inherently porous structures with ≈2.6 and ≈3.4 nm pores, and large surface areas allow for targeted and efficient drug loading even for insoluble drugs, which can be released on demand. Additionally, indocyanine green (ICG) dye loading in the pores enables photoacoustic imaging, optical coherence tomography, and hyperthermia in operando conditions. This real‐time visualization of the drug‐loaded COF microswimmers enables unique insights into the action of photoactive porous drug carriers for therapeutic applications.Item Open Access On the thermal dimorphy of the strontium perrhenate Sr[ReO4]2(2024) Conrad, Maurice; Bette, Sebastian; Dinnebier, Robert E.; Schleid, ThomasHygroscopic single crystals of a new hexagonal high‐temperature modification of Sr[ReO4]2 were prepared from a melt of Sr[ReO4]2 ⋅ H2O and SrCl2 ⋅ 6 H2O. The structure analysis of the obtained crystals by X‐ray diffraction revealed that the title compound crystallizes in the ThCd[MoO4]3‐type structure with the hexagonal space group P63/m and the lattice parameters a=1023.81(7) pm and c=646.92(4) pm (c/a=0.632) for Z=2 in its quenchable high‐temperature form. Two crystallographically independent Sr2+ cations are coordinated by oxygen atoms forming either octahedra or tricapped trigonal prisms, whereas the Re7+ cations are found in the centers of discrete tetrahedral meta‐perrhenate units [ReO4]-. Temperature‐dependent in‐situ PXRD studies of dry powder samples of Sr[ReO4]2 exhibited its thermal dimorphy with a phase‐transition temperature at 500-550 °C from literature‐known m‐Sr[ReO4]2 into the newly discovered h‐Sr[ReO4]2 (hexagonal).Item Open Access Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids(2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, SabineCovalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.Item Open Access Defect chemistry and photo-ionic effects in bromide and iodide perovskites(2023) Wang, Ya-Ru; Maier, Joachim (Prof. Dr.)Bromide and iodide perovskites, especially their mixtures, hold great potential for opto-electronic application due to their optical absorption properties in the visible range. While it is established that these materials are mixed ionic-electronic conductors, their ionic transport properties both in the dark and under light are poorly understood. The present work deals with the defect chemistry and photo-ionic effects in halide perovskites, including iodide and bromide perovskites with special focus on the photo induced phase separation (photo de-mixing) in mixed bromide - iodide perovskites. The first part covers the defect chemical study of bromide perovskites, including 3D MAPbBr3 and 2D Dion-Jacobson (PDMA)PbBr4. The results reveal that both 3D MAPbBr3 and 2D (PDMA)PbBr4 are mixed ionic-electronic conductors. 2D (PDMA)PbBr4 show three orders of magnitude lower ionic conductivity compared with 3D MAPbBr3. This implies that dimensionality reduction is an effective strategy for reducing ion migration in these systems. From a bromine partial pressure dependent study, it is concluded that MAPbBr3 and 2D(PDMA)PbBr4 are both P-type conductor and that the surface reaction is the limiting process for the incorporation and exporation of the Br2 gas. A non-monotonic dependence of the electronic conductivity on bromine partial pressure is detected for both 2D and 3D bromide perovskites. It can be attributed to the reversible formation and dissociation of AuBrx on the gold electrode and perovskites interface. The second part covers the investigation of the thermodynamic properties of the 2D mixed halide perovskites under light. It has been shown that light can be used as a knob for inducing photo de-mixing from single phase 2D mixed halide perovskites to to I-rich and Br-rich phases. In the dark, the photo de-mixed phases re-mix with complete reversibility of both their optical and structural properties, demonstrating the full miscibility of mixed bromide-iodide perosvkites in the dark. The temperature-dependence of absorption spectra for the photo de-mixed phases gave clear evidence for a miscibility gap under light, from which photo de-mixed phases’ compositions are extracted. The photo-miscibility-gap is mapped and confirmed by various methods. The shape of the photo-miscibility-gap shows limited variation in the 0.01 - 0.1 sun illumination intensity range. The non-encapsulation of surface, however, demonstrated a widening of the photo-miscibility gap. The third part covers the kinetic analysis and mechanistic investigation of photo de-mixing in 2D mixed halide perovskites. Simultaneous monitoring of the electrical conductivity and optical absorption allows for a local probe of electronic and ionic charge carriers, and the composition evolution. Furthermore, time dependent phase distribution is investigated with the aid of top view SEM, showing that I-rich nanodomains forming along the grain boundaries at early times after light exposure with further formation of such domains also within the grain at longer times. Local elementary distribution is probed with TEM. From the temperature dependent de-mixing half-time, an activation energy for photo de-mixing of 0.39 eV is obtained. Finally, together with DFT calculation on defect formation energy of the mixture of different defect type, a mechanistic description for photo de-mixing, both from molecular and microscopic level is proposed. The last part deal with the phase stability study of mixed halide perovskites in other dimensionalities, including 3D and nanocrystal based thin films. 3D mixed halide perovskites show that similar to 2D mixed halide perovskites, photo de-mixing occur in two stage. Different from the full reversibility of 2D, photo degradation of 3D perovskites into PbI2 in the dark over long time scales is observed. The nanocrystalline mixed halide perovskite (BA-MAPb(I0.5Br0.5)3) thin films show no de-mixing in contrast to 2D and 3D under same measurement conditions. Nanocrystals mixtures show superior phase stability under the same illumination condition, with neither degradation nor de-mixing. This thesis contributes to the understanding of the defect chemistry and ion transport properties of bromide and iodide perovskites, with specific focus on photo de-mixing in mixed halide perovskites. These findings will aid compositional engineering related to halide mixtures to enable optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo-ionic effects.Item Open Access The FCIQMC sign problem in the real-space Hubbard model(2023) Liebermann, Niklas; Alavi, Ali (Prof. Dr.)Full configuration interaction quantum Monte Carlo (FCIQMC) is an electronic structure method that has been applied to a variety of ab initio molecular and solid-state systems as well as the Hubbard model in delocalised bases. In this thesis, the behaviour of FCIQMC in the Hubbard model in the real-space formulation is investigated. A special emphasis is put on the consequences of the fermionic sign problem. Firstly, a classification of Hubbard lattice geometries based on their strength of the sign problem is performed. It is discovered that the commonly used ground state of the so-called stoquastic version of the Hamiltonian is not a good predictor for the difficulty to resolve the sign problem in FCIQMC in general. The notion of size-extensive and non-size-extensive behaviour of the sign problem is established. It is shown that although the vast majority of non-trivial fermionic systems suffer from the fermion sign problem when attempting to solve them using quantum Monte Carlo (QMC) methods, there are certain system configurations in the Hubbard model systems that are sign-problem-free. In principle, this allows for the unbiased treatment of systems with very large Hilbert space sizes in FCIQMC. However, attempting to solve these systems uncovers a new systematic bias in the FCIQMC algorithm, the population control bias. This is a bias that has been observed previously in other QMC methods, like diffusion Monte Carlo. A method that allows for the removal of this bias entirely with negligible computational overhead, mainly through introducing importance sampling to FCIQMC, is presented. This allows for the calculation of ground-state energies of the one-dimensional Hubbard model with up to 150 sites at and close to half-filling in the difficult intermediate interaction regime. Also, the fundamental many-particle gaps between the ground states of the half-filled and the system with one hole are calculated for up to 102 sites. Moving to sign-problematic systems, it is shown that the usual method of controlling the sign problem in FCIQMC, the initiator method, performs poorly in weakly sign-problematic Hubbard systems. Instead, it is demonstrated how applying the newly developed importance-sampled FCIQMC together with the exact non-initiator algorithm greatly reduces the minimum number of walkers necessary to obtain an unbiased ground-state energy in real-space Hubbard models. This allows for the calculation of numerically exact ground-state energies for width-two Hubbard ladders - which exhibit a size-extensive yet very weak sign problem - in the intermediate interaction regime at half-filling and with one hole. Again, this makes the calculation of the fundamental many-particle gaps possible. Finally, to deal with full two-dimensional Hubbard systems, a way to define fixed initiator subspaces in FCIQMC based on analytic wavefunction ansatzes is presented. This leads to far superior results compared to the usual population-based initiator criterion. Additionally, the newly developed two-shift method allows for the perturbative inclusion of the entire non-initiator space. This new scheme is shown to be compatible with importance sampling. Furthermore, an extrapolation scheme to the exact ground-state energy is presented. This allows for the estimation of the ground-state energy for systems up to 32 sites in the honeycomb lattice geometry.Item Open Access Insights into the first multi-transition-metal containing Ruddlesden-Popper-type cathode for all-solid-state fluoride ion batteries(2024) Vanita, Vanita; Waidha, Aamir Iqbal; Vasala, Sami; Puphal, Pascal; Schoch, Roland; Glatzel, Pieter; Bauer, Matthias; Clemens, OliverPromising cathode materials for fluoride-ion batteries (FIBs) are 3d transition metal containing oxides with Ruddlesden-Popper-type structure. So far, the multi-elemental compositions have not been investigated, but it could alternate the electrochemical performance similar to what has been found for cathode materials for lithium-ion batteries. In this study, we investigate RP type La2Ni0.75Co0.25O4.08 as an intercalation-based active cathode material for all-solid-state FIBs. We determine the structural changes of La2Ni0.75Co0.25O4.08 during fluoride intercalation/de-intercalation by ex situ X-ray diffraction, which showed that F- insertion leads to transformation of the parent phase to three different phases. Changes in the Ni and Co oxidation states and coordination environment were examined by X-ray absorption spectroscopy and magnetic measurements in order to understand the complex reaction behaviour of the phases in detail, showing that the two transition metals behave differently in the charging and discharging process. Under optimized operating conditions, a cycle life of 120 cycles at a critical cut-off capacity of 40 mA h g-1 against Pb/PbF2 was obtained, which is one of the highest observed for intercalation electrode materials in FIBs so far. The average coulombic efficiencies ranged from 85% to 90%. Thus, La2Ni0.75Co0.25O4.08 could be a promising candidate for cycling-stable high-energy cathode materials for all-solid-state FIBs.Item Open Access Lithium storage in titania films : unification of intercalation electrode and supercapacitor concepts(2023) Xiao, Chuanlian; Maier, Joachim (Prof. Dr.)Lithium intercalation batteries and supercapacitors are two indispensable components in nowadays’ mobile, information-abundant society, which rely on electrochemical processes. The core concept of such energy storage technologies is mass storage. In typical insertion electrodes for batteries, the capacity is determined by bulk storage within the electroactive particles, which is comparatively well investigated and understood. In contrast, supercapacitor electrodes are dominated by interfacial storage at interfaces, which is well addressed experimentally. However, the charge carrier chemistry (defect chemistry) especially in the latter case is not fully acknowledged. Consequently, the intercalation storage and supercapacitive storage are usually considered as independent phenomena and the two important fields appear to be unnecessarily separated. Therefore, the core of the present thesis is unifying battery and supercapacitor concepts using TiO2 thin films as master example. Following and extending our quantitative concept of job-sharing storage, a generalized picture that includes bulk and space charge storage (intercalation electrode and supercapacitor storage) is developed. The first part of the thesis reviews the classic space charge theory and extends it in terms of discrete modeling of space charge zones in solids, which is a sensible approach for handling pronounced space charge potentials as well as non-idealities in realistic solid state systems such as the battery system under concern in this thesis. In addition to issues of internal consistency, the continuum approach is questionable if extremely steep profiles close to the interface occur, and analytical corrections are not very helpful. In this context, the space charge behavior is studied in a discretized manner rather than by using the analytical Poisson-Boltzmann function. Combining discrete modeling with the continuum description provides a particularly powerful method, with the help of which non-idealities in the first layers (variation in structure, elastic effects, saturation effects, changes in dielectric constant) are directly addressed. Various examples of practical value for functional ceramics are discussed. In this way a more precise definition and demarcation of electrode and double layer capacity is achieved. The second part of the thesis, which is in fact the core part, investigates the unification of bulk storage and interfacial storage by carefully investigating the storage of lithium in titania films on various substrates as a function of thickness. The full picture in terms of charge carrier concentrations as function of spatial coordinate with cell voltage and substrate conditions as parameters is obtained. First, materials (TiO2 thin films) preparation and characterization are addressed. TiO2 thin films on various electron-accepting substrates (Nb doped SrTiO3 (N-ST), undoped SrTiO3 (ST), Fe doped SrTiO3 (F-ST), Ruthenium) were deposited utilizing three methods: atomic layer deposition (ALD), pulsed laser deposition (PLD) and molecular beam epitaxy (MBE). The substrates and the deposited films were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), which show that the grown TiO2 films are of good quality with sharp interface. Film thickness (one of the key parameters) was determined precisely using various techniques (x-ray reflectivity, TEM, stylus profilometry). Second, the battery storage capacity measurements of TiO2 thin films are supplemented by bias dependent impedance measurements, yielding interfacial resistance as well as interfacial capacitance. Combining with aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) measurements, independent information on electron and Li distribution is obtained. As a result, not only bulk and boundary storage contributions are precisely deconvoluted, but they can be traced back to a common thermodynamic conception. In fact, the entire profile including interfacial and bulk effects is derived by taking account of only a few bulk materials parameters: the (free) energies of the electronic carriers in TiO2 and the substrate as well as the formation (free) energy of the Li-ions in TiO2. Unifying intercalation electrode and supercapacitor concepts provides a way of better understanding and mitigating the energy and power density conflict of storage devices, which becomes particularly important for nanoionic systems.Item Open Access Novel stochastic methods in electronic structure theory and their application(2024) Weser, Oskar; Alavi, Ali (Prof. Dr.)This cumulative thesis is based on four publications that describe newly developed methods within full CI quantum Monte-Carlo (FCIQMC) and their application to challenging systems of biochemical interest. FCIQMC is a well parallelised, stochastic electronic structure method that enables the correlation of many electrons by taking advantage of the sparsity of the wave function expressed in a finite basis. The new methods are about: a.) an improved excitation generator, that benefits the performance of FCIQMC, in particular for localised orbital bases, b.) fine-grained control over the structure of the wave function by developing a stochastic version of the generalized active space (GAS) method, including its mean-field optimised variant GASSCF, and c.) targeting states of desired spin in a basis of Slater determinants (SDs). The improved excitation generator is an extension of the precomputed heat bath (PCHB) strategy with more effective sampling of double excitations and a novel approach for non-uniform sampling of single excitations. The non-uniform sampling of single excitations relies on spatially decaying integrals and matrix elements. An overall efficiency gain by a factor of two to four, as measured by variance reduction per wall-clock time, is shown. In the GAS method the active space is partitioned into multiple disjoint subspaces. A full configuration interaction (CI) expansion is generated for each subspace, while the interspace excitations are restricted using chemically motivated constraints on the occupation numbers per subspace. Within FCIQMC these constraints are efficiently encoded in precomputed probability distributions which removes nearly all runtime overheads of GAS. Stochastic GAS reduced density matrices (RDMs) are stochastically sampled, allowing orbital relaxations via stochastic GASSCF, and direct evaluation of properties that can be extracted from density matrices, such as the spin expectation value. Restricted active space (RAS) or other truncated wave function schemes are special cases of the GAS strategy, thus they are promptly available by an appropriate choice of the GAS subspaces and corresponding constraints. The efficient implementation of the stochastic GAS method, using hybrid parallelisation, allowed e.g. uncontracted stochastic MRCISD calculations on the quintet - triplet spin gap in a Fe-porphyrin model complex, with up to 96 electrons and 159 orbitals and a large CAS(32, 34) active space reference wave function, greatly improving previous estimates. The spin-purification method allows targeting states of desired spin in SDs. This is achieved by using a modified Hamiltonian H' = H + J S², with a suitable J > 0 that artificially enforces anti-ferromagnetic order. While a basis of configuration state functions (CSFs) can target spin states by construction, there are conceptual and practical advantages of using a SD basis while ensuring the correct spin. It can be directly coupled with other rich theory and codes that are (not yet) available in a CSF basis; an incomplete list includes: transcorrelated Hamiltonians, tailored coupled cluster, or stochastic perturbation theory. In addition, while convergence with respect to walker number is usually faster for CSFs, the SD basis is numerically cheaper and allows more walkers for the same computational effort. A particular notable application of the new method is a trinuclear [Mn3O4] metal complex, serving as a biomimetic for the active centre of the oxygen evolving complex (OEC), with a non-monotonic spin ladder whose particular electronic features could be reproduced within a CAS(55, 38) model active space.Item Open Access Li4Ln[PS4]2Cl : chloride-containing lithium thiophosphates with lanthanoid participation (Ln = Pr, Nd and Sm)(2023) Lange, Pia L.; Bette, Sebastian; Strobel, Sabine; Dinnebier, Robert E.; Schleid, ThomasThe synthesis and structural analysis of three new chloride-containing lithium thiophosphates(V) Li4Ln[PS4]2Cl with trivalent lanthanoids (Ln = Pr, Nd and Sm) are presented and discussed. Single crystals of Li4Sm[PS4]2Cl were obtained and used for crystal structure determination by applying X-ray diffraction. The other compounds were found to crystallize isotypically in the monoclinic space group C2/c. Thus, Li4Sm[PS4]2Cl (a = 2089.31(12) pm, b = 1579.69(9) pm, c = 1309.04(8) pm, β = 109.978(3)°, Z = 12) was used as a representative model to further describe the crystal structure in detail since Li4Pr[PS4]2Cl and Li4Nd[PS4]2Cl were confirmed to be isotypic using powder X-ray diffraction measurements (PXRD). In all cases, a trigonal structure in the space group R3̲ (e.g., a = 1579.67(9) pm, c = 2818.36(16) pm, c/a = 1.784, Z = 18, for Li4Sm[PS4]2Cl) displaying almost identical building units worked initially misleadingly. The structure refinement of Li4Sm[PS4]2Cl revealed bicapped trigonal prisms of sulfur atoms coordinating the two crystallographically distinct (Sm1)3+ and (Sm2)3+ cations, which are further coordinated by four anionic [PS4]3- tetrahedra. The compounds also contain chloride anions residing within channel-like pores made of [PS4]3- units. Eight different sites for Li+ cations were identified with various coordination environments (C.N. = 4-6) with respect to chlorine and sulfur. EDXS measurements supported the stoichiometric formula of Li4Ln[PS4]2Cl, and diffuse reflectance spectroscopy revealed optical band gaps of 2.69 eV, 3.52 eV, and 3.49 eV for Li4Sm[PS4]2Cl, Li4Nd[PS4]2Cl, and Li4Pr[PS4]2Cl, respectively. The activation energy for Li+-cation mobility in Li4Sm[PS4]2Cl was calculated as Ea(Li+) = 0.88 eV using BVEL, which indicates potential as a Li+-cation conductor.Item Open Access Role of ion redistribution at selected halide perovskite interfaces(2023) Jung, Mina; Maier, Joachim (Prof. Dr.)