Inhalt

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   Abstract
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   Table of contents
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   1 Introduction 1
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   2.1 Cryogenic distillation 5
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   2.3.1 Membrane technology 6
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   2.4 Two-dimensional membranes 13
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   2.4.1 Origin of 2D membranes 13
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   3 Materials 25
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   3.1 Carbon nanomembranes 25
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   3.1.1 Synthesis of carbon nanomembranes 25
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   3.2 2D covalent organic frameworks 48
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   3.2.1 Synthesis of 2D covalent organic frameworks 48
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   3.3 Bilayer oxides 62
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   3.3.1 Synthesis of bilayer oxides 62
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   4 Gas and vapor permeation in carbon nanomembranes 79
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   4.1 Introduction 79
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   4.2.1 Preparation of the nanomembranes 80
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   4.3 Results 83
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   5 Gas and vapor permeation in 2D covalent organic framework 119
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   5.1 Introduction 119
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   6 Gas and vapor permeation in bilayer silica 137
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   6.1 Introduction 137
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   7 Summary and conclusions 172
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   1 Introduction
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   The COVID-19 pandemic has embraced the world since the beginning of 2020. The new coronavirus infection can be severe leading to pneumonia and death. Presently, the number of infections and deaths worldwide exceed 242.3 million and 4.9 million respectively.1 Artificial ventilation is essential for the survival of people with severe pneumonia caused by the COVID-19, so that the availability of equipment and the supply of medical oxygen are a matter of life and death. The pandemic has increased the demand for
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  2 Theory
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   2.1 Cryogenic distillation
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   2.2 Adsorptive separation
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   The first industrial-scale machine designed by Carl von Linde was run in 1902 for air separation. Contemporary cryogenic separation units are based on the same principles allowing to separate gases from a mixture according to their boiling points. Firstly, the filtered air is compressed and passed through adsorbents to exclude water, carbon dioxide and hydrocarbons. Secondly, the process air is cooled by a heat exchanger and undergoes Joule-Thomson expansion resulting finally in a liquefaction after several
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   The second most popular way to separate air is a non-cryogenic adsorption process. This method is based on the selective adsorption of the gas on the adsorbent removing that component from a mixture. The choice of adsorbents depends on the nature of gases because the adsorption strength is conditioned by the boiling point of the substance as well as its polarizability, dipole and quadrupole moments. The adsorption processes are identified by their regeneration method. Zeolites selectively adsorb nitrogen fr
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  2.3 Membrane separation
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   2.3.1 Membrane technology
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   Membrane technologies emerged on the market in the 1980s as the third way to separate gas mixtures. The membrane system consists of a compressor for pressurizing, a coalescing filter and membrane modules. The advantages of the membrane separation are its simple operation, the absence of moving parts, low capital costs and energy consumption compared to other methods. The membranes play the role of a barrier that is much more permeable to one substance in contrast to others. Therefore, the membrane separatio
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   F = – D × dCdx, (2.1)
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  Depending on the porosity of membranes, different transport mechanisms determining the diffusion coefficient can realize. Sometimes the membrane has such a morphology that the passage of a gas can be described by two or more transport models. The simplest transport model is a viscous flow shown in Figure 2.1a. It occurs in relatively large pores when their sizes exceed the mean free path of the gas molecules (around 100 nm at atmospheric pressure). In this case, the diffusivity is inversely proportional to
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   2.3.2 Transport models in membranes
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   molecular velocity which is the higher the lighter the molecule is. The diffusion coefficient for the component is inversely proportional to square root of its molecular mass as a result of which the selectivity is quite low. If the pores are close to molecules in sizes, size exclusion known as molecular sieving might occur. This most desirable transport mechanism is presented in Figure 2.1c where the membrane is absolutely selective towards the smallest molecules being a hindrance for the larger molecules.
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   C = S × 𝑓 = S × 𝛾 × p, (2.2)
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   F = D × S × p1 - p2Δℓ = 𝒫 × ΔpΔℓ, (2.3)
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   α(A,B) = 𝒫A𝒫B = DADB × SASB, (2.4)
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  It was noticed that membranes cannot achieve high selectivity and high permeability simultaneously. The trade-off between selectivity and permeability is known as the Robeson limit – a gain in selectivity leads to a loss in permeability and vice versa.32,33 The characteristics of membranes expressed in a logarithmic graph of the permeability-selectivity are located below a certain line, Robeson upper bound, that was determined empirically. The slope of this line depends on the difference in the kinetic diam
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   2.3.3 Disadvantages of membranes
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   bound was revised and the progress found was not so significant. Rigid ladder-type polymers and fluorinated polymers provided the shift. It was suggested that changing the permeation mechanism from solution-diffusion to molecular sieving could give an outstanding performance, but in reality an increase of the pore size leads to Knudsen diffusion with low selectivity.33 The Robeson upper bounds in 1991 and 2008 for O2/N2 separation are shown in Figure 2.2. The region of most polymeric membranes is schematica
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   2.3.4 State-of-the-Art for industrial membranes
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   Typically, industrial membranes are polymeric while the choice of the material depends on the preferred selectivity and the target gas mixtures: polyimide for O2/N2, polysulfone for H2/N2, cellulose acetate for CO2/CH4, etc.7,28 Conventional membranes demonstrate a satisfactory performance, but each membrane is permeable to other components of the mixture more or less due to the trade-off. In this way, membranes are competitive to purify gas mixtures with a high concentration of the target component produci
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   2.4 Two-dimensional membranes
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  The interest towards 2D materials has been growing after the isolation of a graphene monolayer from graphite crystals in 2004 that proved the existence of stable materials of one atom in thickness. The possibility to produce such materials of a large area prompted the idea of using them as membranes for gas separation.11,37 A nanoporous 2D membrane is a material with a thickness of several atoms which has many distributed pores.11,38 Generally, nanopores are understood as pores with a size of 1 – 100 nm. Ac
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   2.4.1 Origin of 2D membranes
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   Π = QΔp × A = FΔp= QLA, (2.5)
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   αA,B = ΠAΠB , (2.6)
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   where ΠA is the permeance for gas A, ΠB is the permeance for gas B.11,12 Besides the permeance Π, a 2D membrane can be characterized with an effective porosity Φ – fraction (in percent) of the open area Aopen available for the gas passage:
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   Φ = 100% × AopenAtotal = 100% × Π × 2𝜋RTM = 100% × QLAtotal× 2𝜋RTM , (2.7)
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  Graphene is a 2D material composed of sp2-hybridized carbon atoms in a hexagonal crystal lattice which has a Young's modulus of 1 TPa. High mechanical strength, chemical stability and thickness of one atom make graphene an excellent candidate as a 2D membrane. The central hexagonal “pore” being around 6.4 pm is too small for passage through it.11,37,40 Lennard-Jones diameters of gaseous molecules calculated by gas viscosity are presented for clarity in Table 2.1.41 Although some ripples, wrinkles and local
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   2.4.2 State-of-the-Art for 2D membranes
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   deflating over time was measured by atomic force microscopy (AFM) after removal the sample from the pressure chamber as shown in Figure 2.3a. The opposite happened
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   in the second method – the evacuated sample was displaced into a pressure chamber, and the microcavity was filled with gas that led to a decrease in tension of the graphene as shown in Figure 2.3b. The resonant frequency of the membrane depends on the mechanical tension and can be detected with a laser beam. In the issue, hydrogen and carbon dioxide permeated noticeably through sub-nanometer pores whose mean size was declared to be close to the kinetic diameter of argon (~ 340 pm). The estimated H2/CH4 sele
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   2.4.3 Intrinsically porous 2D materials
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   Aforementioned 2D membranes demonstrate a wide pore size distribution that prevents outstanding selectivity and does not appear to be monodisperse. The fabrication of nanopores in 2D materials is carried out using expensive equipment, and this process is complicated by the need to have pores in a very narrow range of sizes. In addition, a great permeation rate can be achieved only with high pore density, but perforated two-dimensional materials lose in mechanical stability compared to pristine ones. There a
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  3 Materials
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   3.1 Carbon nanomembranes
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  The carbon nanomembrane (CNM) is an amorphous 2D material which is usually formed after the electron irradiation of a self-assembled monolayer (SAM). This carbon material with a thickness of the order of a nanometer can be obtained free-standing on a centimeter scale. CNM’s properties can be customized by the precursor selection and synthesis conditions which allows one to obtain a functional 2D material. The reparation of CNMs can be presented in three steps: (i) formation of the SAMs, (ii) cross-linking v
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   3.1.1 Synthesis of carbon nanomembranes
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   conditions. The S-H bond in thiols is able to dissociate and bind to the Au surface atom by a quite strong covalent Au-S bond with energy ~ 200 kJ·mol-1 while hydrogen leaves the interface irreversibly.52,55 The formation of a thiolate SAM from thiols is shown schematically in Figure 3.1. The use of molecules with different terminal functional groups allows the preparation of a monolayer with adjustable properties of its surface. The thickness of the SAM can be defined by the length of the molecule and the
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   was declared to have no phase contrast in transmission electron microscopy.70 The manufacturing of carbon nanomembranes based on BPT molecules is carried out according to the proven method shown schematically in Figure 3.2e.59,68–70 The 300 nm-thick gold layer grown on mica is preliminarily cleaned in the UV/ozone cleaner. Polycrystalline gold films on quartz substrates can also be used instead of Au/mica substrates.71 The gold surface is rinsed with ethanol, dried under a nitrogen flow and immersed into a
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   AB + e- → AB- → A + B-. (3.1)
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   coated with a PMMA layer, then the gold was etched in an I2/KI aqueous solution and the PMMA/CNM film was transferred onto TEM grids. The protective PMMA film was dissolved in acetone which was removed in a critical point dryer with carbon dioxide leaving the free-standing CNM intact in stage (iii). Besides the mentioned compounds, self-assembly on gold was performed with pyridine-4-thiol molecules in an alkaline aqueous solution.59
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    crosslinked network, and the molecules in SAMs should have at least two phenyl rings in a direction perpendicular to the substrate for the conversion to a carbon nanomembrane. All prepared CNMs were annealed in an UHV chamber at 1170 K for 30 minutes to convert them into graphene. The second group having a higher density of carbon atoms gave thicker nanocrystalline graphene sheets than the first group with oligophenyls. In the case of pyrolyzed CNMs based on a bulky molecule (HPB), half of the area was sin
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   The morphology of a carbon nanomembrane was explored with STM and AFM methods by Yang et al. Two domains of a highly-oriented TPT SAM on gold can be distinguished in the STM image in Figure 3.4k where the yellow line marks domain boundary. The line profiles presented in the inset in Figure 3.4k demonstrate the distinct periodicity along the two crystallographic directions of the adsorbate structure: 0.58 ± 0.01 nm for line 1 and 1.03 ± 0.02 nm for line 2. The molecules in the monolayer underwent a serious r
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  Since CNMs demonstrated a good mechanical stability, their mechanical properties were studied using the bulge test on a pressure cell.68,87,88 The nanomembranes were suspended over holes in a silicon wafer. The silicon wafer was mounted in a sealed pressure cell as shown in Figure 3.5a. Polydimethylsiloxane (PDMS) was used for the gas-tight sealing. The position of a nanomembrane was identified by the AFM tip. Figure 3.5b presents an AFM image with the profile of a free-standing CNM without an applied press
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   3.1.2 Properties of carbon nanomembranes
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   p = c1 × σ0 × ta2 × h + c2 × E × t(1−ν) × a4 × h3, (3.2)
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   Young’s modulus for the 2MP CNM compared to the NPTH CNMs. The smallest Young’s modulus for a TPT CNM was explained by the lower degree of cross-linking due to dense packing of the TPT SAM. The bulky molecules (HPB, HBC-Br) formed low-ordered SAMs and become a nanoporous CNM after cross-linking. The HPB CNM being less porous exhibited a higher stiffness than the HBC-Br CNM with a larger porosity.88 The Young’s modulus of an ODT CNM was found to be around 0.6 GPa because the alkanethiolate SAMs has a lower d
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   Πcomposite = 1Π2D + 1Πsupport , (3.3)
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   where Πcomposite is the total permeance for the composite membrane, Π2D is the permeance for the 2D membrane, Πsupport. is the permeance for the bulk porous support. In fact, HIM analysis revealed micron-sized defects in the CNMs laying on PDMS. The areal fraction of defects (bare support) was estimated by microscopy to extract the permeation rates Π2D in CNMs from the total permeance in the composite membrane. The gas permeation rates in isolated CNMs are presented in Figure 3.6a. The processes of permeati
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   Π = ∆mM × A × t × ∆p , (3.4)
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   Π = pref × Isample × Arefpsample × Iref × Asample × 12𝜋m0kBT × NA , (3.5)
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   that was explained by probable higher adsorbate numbers on the CNMs surface with an increasing vapor pressure as displayed in Figure 3.6b. The permeance values found for the saturated water vapor from two the methods were quite close. The inert helium atoms crossed CNMs in 2500 times slower than the water molecules. Moreover, the permeation of other gases was not detected despite quite large nanochannels of ~ 0.7 nm estimated by AFM. Firstly, it was supposed that liquid water has a higher chance to reach a
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   3.2 2D covalent organic frameworks
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  Covalent organic frameworks (COFs) are a class of crystalline extended materials that are assembled from organic precursors (linkers) in a chemical condensation reaction. Although COFs with 3D lattices are known, most COFs have two-dimensional lattices where the building blocks are connected by a strong covalent bond, and these layers are stacked together by weaker interactions like van der Vaals forces, aromatic stacking or hydrogen bonding.94,95 A 2D lattice (layer) of COFs is composed of linkers located
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   3.2.1 Synthesis of 2D covalent organic frameworks
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   Figure 3.7c. The diameter of the hexagonal micropore was ~ 1.5 nm, but a staggered stacking led to ~ 0.7 nm-width channels considering van der Waals radii in COF-1. COF-5 with a structure similar to boron nitride consists of eclipsed AA layers with mesopores of ~ 2.7 nm in diameter as can be seen in Figure 3.7d. Distinct stacking was attributed to preferable π-π interactions in HHTP units in COF-5, while van der Waals interactions between B3O3 cycles in adjacent layers dominated in COF-1. The first represen
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   2D COF nanosheets has average lateral size of ~ 200 nm and thickness of 0.3 – 1.3 nm according to TEM and AFM methods.114 Zhu et al. used a nitrating mixture to exfoliate triazine-based COF. The method is presented in Figure 3.8d. The layered COF was soaked in either oxidizing H2SO4 or non-oxidizing H3PO4 for 5 minutes to obtain the intercalation compounds. Then the COF with intercalated acids was exposed to a mixture of H2SO4 and HNO3 for 10 minutes under heating and microwave irradiation. Nitronium ions N
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   Tta. The crystallinity was confirmed by XRD. The surfaces of the films were heterogeneous, and the films with sheet-like morphology consisted of layers with lateral size above 100 µm, according to SEM images.129 The thickness of the COF films can be controlled by varying the concentration of monomers, while the area is determined by cross-section of the beaker. Later, it was reported about a thickness of 2.5 – 5 nm in the 2D COF films obtained via the liquid-liquid interface-assisted synthesis.130,131 Li et
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  Researches about gas transport in 2D COF-based membranes have been carried out rarely, and a thickness of frameworks exceeds 100 nm in most studies.136 Li et al. studied gas permeation in exfoliated COF-1 nanosheets supported on macroporous alumina in 2017. AB-stacked COF-1 was synthesized under solvothermal conditions followed by dispersion in CH2Cl2 and sonification to produce 2D COF nanosheets. The AFM method revealed the thickness of a single nanosheet to be ~ 0.5 nm that corresponded to one layer. The
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   3.2.2 Properties of 2D covalent organic frameworks
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   according to DFT calculations. Alumina substrates with a diameter of 1.8 cm and a thickness of 1 mm were used as macroporous supports (~ 70 nm pores) for 2D COFs. A hot-drop coating method was applied for membrane making: the suspension was slowly dropped onto heated at 120 °C Al2O3 support. The thicknesses of COF films were estimated to be ~ 10 nm. Although eclipsed AA-stacking was confirmed by XRD analysis for bulk TpPa-1, TpPa-2 and TpHz COFs, staggered AB-stacking of 2D COF nanosheets on alumina support
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   3.3 Bilayer oxides
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  Bilayer oxides represent a class of two-dimensional materials synthesized on metal surfaces. These structures are bound with the metal interface only via weak van der Waals interactions, while planar metal supports play guiding roles for the growth of two-dimensional structures. Similar to bulk oxides, in particular silicates, bilayer oxides exist in various phases of crystalline and vitreous structures. Being self-containing materials with thickness below 1 nm and known short-range order, bilayer structure
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   3.3.1 Synthesis of bilayer oxides
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   a SiO2.5/Mo(112) system as a reference under XPS control. Finally, the films were annealed at 1200 K in 3 × 10-6 mbar of O2. The coverage with 1 ML resulted in a hexagonal monolayer structure (2 × 2) with lattice parameter of 5.42 Å on Ru(0001) similar to the structure on Mo(112). The IRRAS spectra had intense peaks at 1134 cm-1 corresponding to Si-O-Ru vibrations instead of ~ 1300 cm-1 for bilayer structures. Both vibration modes at 1134 cm-1 and 1300 cm-1 coexisted after deposition of 1.5 ML that indicate
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   In oppose to crystalline phase, vitreous BS exhibited complex network with four-, five-, six-, seven-, eight-, nine-membered rings. Six-membered rings are more common than others leading to characteristic Si-Si-Si angle of 120.2 ± 14.7° and the average distance between silicon atoms of 3.01 ± 0.12 Å.152,154 The STM image with atomic model of crystalline-vitreous interface is shown in Figure 3.12g. The 1.6 nm-length buffer region was consisting of five- and seven-member rings.153 According to DFT calculation
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   3.3.2 Properties of bilayer oxides
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   At present, mechanical properties of bilayer oxides were measured only in theory. A bilayer silica should be quite a stable and stiff structure according to DFT calculations.174 The Young’s moduli were estimated by molecular dynamics simulations to be 416.2 GPa for crystalline BS and 270.2 GPa for vitreous BS that makes them comparable in strength to graphene. Brittle ruptures were predicted for crystalline phase, while vitreous bilayer silica was supposed to be a ductile 2D material.175 The high mechanical
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   barriers for argon and krypton seemed to be higher than for other species (Figures 3.13b – 3.13d). The barrierless transmission was predicted for all gases in nine-membered rings as shown in Figure 3.13e. Considering activated transport through pores in bilayer silica, the H2/CO2 selectivity was predicted to be ~ 1011 for crystalline phase and ~ 20 for vitreous phase. In general, the low selectivity was attributed to vitreous phase compared to crystalline BS. The permeation rates were estimated on the assum
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  4 Gas and vapor permeation in carbon nanomembranes
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   4.1 Introduction
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   Since membrane technology exhibits the lowest energy costs for an industrial separation, 2D membranes were predicted to be an ideal solution ensuring the shortest path for transmembrane flows. The transport of gases and vapors across ultrathin nanoporous materials is of interest for fundamental investigations and practical problems of membrane separation. Unlike bulk, planar materials allow one to highlight surface phenomena of a transmembrane passage. In this way, intrinsically porous carbon nanomembranes
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  4.2 Experimental
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   4.2.1 Preparation of the nanomembranes
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   Epitaxial 300 nm-thick gold substrates on mica were cleaned with ozone and rinsed with ethanol. To prepare thiolate SAM, an outgassed Au/mica substrate was poured with anhydrous DMF, and 1,1',4'1''-terphenyl-4-thiol (TPT) was added to make a nanomolar solution. The self-assembly took 24 hours at 75 °C in an inert atmosphere. The TPT SAMs were rinsed with DMF, ethanol and blown with dry nitrogen.
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   4.2.2 Characterization of the nanomembranes
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   The infrared spectrometer VERTEX 70 (Bruker) was connected with a polarization-modulation module PMA 50 (Bruker) to measure the PM-IRRAS spectra for the SAMs, CNMs and IL/CNMs. The spectra were recorded in dry nitrogen flow at a resolution of 4 cm-1. A solution of the ionic liquid in acetonitrile was applied dropwise onto the CNM/Au/mica for a quantitative analysis via PM-IRRAS. The spectra were recorded after drop casting and spin coating of the acetonitrile solution, and the concentrations of [bmim][Tf2N]
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   The vacuum setup for permeation measurements is depicted in Figure 4.1. The Si3N4 chip with a CNM was glued onto a copper ring. Then the sample was inserted in the membrane cell of the vacuum setup, i.e. clamped between two conflat flanges. The free-standing nanomembrane divides the inner space of the permeation apparatus into the downstream compartment (the UHV detection chamber with a quadrupole mass spectrometer) and the upstream compartment (a sample channel and mixing chamber). Besides the sample chann
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   ZW = pu2𝜋mkBT , (4.1)
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  where ZW is the number of gas molecules impinging on an unit area per second (s-1·m-2), pu is the pressure in the upstream compartment (Pa), m is the mass of the gas molecule (kg), kB is the Boltzmann constant (m2·kg·s-2·K-1), T is the absolute
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   4.2.3 Permeation measurements
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   temperature (K). The QMS reference signal Iref (counts·s-1) is recorded in each permeation experiment, while the sample channel is closed by a chemical resistant valve. In this way, the permeance Π (mol·s-1·m-2·Pa-1) in the nanomembrane can be determined as follows:
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   Π = JsampleA × (pu−pd) ≅ Jref × IsampleA × pu × Iref , (4.2)
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  As previously reported, the CNMs were shown to be highly permeable for water.85 To compare water permeance with other components, the permeation experiments with binary mixtures at ~ 20 % of relative humidity were carried out. The partial pressure of D2O was set to be 5 mbar, while the partial pressures of N2 and CHCl3 were varied from 0 to 45 mbar. In this way, the molar fraction of heavy water was decreasing with an increase in the total pressure. These experiments revealed the constant water permeance, b
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   4.3 Results
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   surface and desorbed back into the gas phase inside the upstream compartment as shown in Figure 4.2, case I. Steric hindrances prevent most gases from permeating, while the smallest helium atoms seem to pass through the CNM with a higher probability.
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   D2OgasuKD2OadsukD2Ogasd , (4.3)
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   where D2Ogasu is the molecule in a gas phase on the upstream side, D2Oadsu is the adsorbed molecule on the membrane (on the upstream side), K is the adsorption-desorption equilibrium constant, D2Ogasd is the desorbed molecule on the downstream side, k is the effective first-order rate constant (s-1) accounting for the surface and the transmembrane diffusion together. The adsorption-mediated transport for condensable molecules is schematically presented in Figure 4.2, case II). Although it is assumed that a
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   J = kmono×θmono×n0 + kmulti×θmulti×n0 , (4.4)
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  4.4 Conclusion
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   To summarize the above, carbon nanomembranes being prepared from the thiolate and carboxylate aromatic SAMs appear to have pores with tortuous and narrow interiors at the sub-nanometer scale. The molecular transport across the CNMs can be realized in three ways depending on the size of molecules and condensability of the substances.
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  4.5 Publication: Vapour permeation measurements with free-standing nanomembranes
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   This article is reprinted with permission from Royal Society of Chemistry. The original paper appeared as:
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  4.6 Publication: Water-Assisted Permeation of Gases in Carbon Nanomembranes
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   This article is reprinted with permission from American Chemical Society. The original paper appeared as:
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  4.7 Publication: Carbon Nanomembranes from Aromatic Carboxylate Precursors
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   This article is reprinted with permission from John Wiley and Sons. The original paper appeared as:
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   4.8 Publication: Molecular transport in ionic liquid/nanomembrane hybrids
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   This article is reprinted with permission from Royal Society of Chemistry. The original paper appeared as:
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  5 Gas and vapor permeation in 2D covalent organic framework
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   5.1 Introduction
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   While the production of monodisperse pores in graphene, boron nitride, molybdenum disulfide, etc. remains challenging, chemical synthesis allows to create inherently porous 2D covalent organic frameworks (COFs). The sizes of regular monodisperse pores with a record density are designed through a choice of building blocks that constitute COFs potential in membrane separation. However, COF crystals are usually hundreds of nanometers in size which complicates a fundamental transport study in them. Recently, th
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  Although the areal fraction of crystal in the boronate ester COF films was about 50 %, the elongated shapes of the crystalline domains resulted in a lower chance to capture a free-standing crystal than an amorphous region. In this way, six out of ten samples were found to be impermeable for helium and p-xylene because dense amorphous regions without the COF crystals covered the orifices in the Si3N4/Si chips. The amorphous and crystal phases were distinguished using helium ion microscopy (HIM) in the transm
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   5.2 Results
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   αi,j = Mj/Mi , (5.1)
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   where Mi and Mj are the molar mass of gases (kg·mol-1). The permeation rates inversely proportional to the square root of the mass corresponded to the effusion mechanism: the lighter molecules moved faster than heavier species.
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   5.3 Conclusion
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   The high pore density in the COF resulted in the superior effective porosity of 40 % that was evaluated from gas permeation experiments. Since square-like pores are larger than gaseous species, the transport in an effusion regime was observed for the atmospheric gases. The free molecular flows were characterized by high permeation rates and low selectivity in accordance with Graham’s law of diffusion.
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   5.4 Publication: Mass Transfer in Boronate Ester 2D COF Single Crystals
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   This article is reprinted with permission from John Wiley and Sons. The original paper appeared as:
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  6 Gas and vapor permeation in bilayer silica
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   6.1 Introduction
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  Bilayer silica (BS) is an intrinsically porous material that has a potential for membrane separation. DFT calculations predicted a great selectivity for the crystalline phase as well as facile gas permeation for vitreous BS. However, it was not previously reported about permeation measurements in the free-standing material. The SiO2 film on Au/mica substrates were grown by plasma enhanced atomic layer deposition (PEALD) in the working group “Inorganic Materials Chemistry” of Prof. Anjana Devi at the Ruhr-Un
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  6.2 Results
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   After the deposition of SiO2 and annealing, the characteristic peak at 1290 cm-1 in the PM-IRRAS spectra was considered as evidence of the presence of bilayer silica on gold. The samples from the first series were prepared at 60 °C in the PEALD reactor, and the maximally intense vibration was found after 6 ALD cycles. The signal from the bilayer structure was reduced with an increase in the number of ALD cycles and it disappeared at 14 ALD cycles, while the new feature from 3D SiO2 at 1250 cm-1 began rising
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  6.3 Conclusion
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   The bilayer silica films were obtained on Au/mica substrates after PEALD at 240 °C followed by annealing at ~ 1000 K in air. The samples were transferred onto Si3N4/Si chips with ~ 2 µm orifices, and the molecular permeation was studied in vitreous BS in situ. Contrary to predictions, the poorly condensable gases did not pass through vitreous bilayer silica with detectable rates due to steric hindrances, and only the helium passage was observed despite the presence of relatively large nine-membered rings in
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   6.4 Publication: Molecular Permeation in Freestanding Bilayer Silica
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   This submitted article is reprinted from the manuscript. The original paper will appear as:
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   7 Summary and conclusions
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   As two-dimensional membranes are believed to ensure high permeation rates and molecular sieving selectivity, this thesis explored the transport of gaseous species in three types of intrinsically microporous planar nanomaterials with various pore morphologies. Carbon nanomembranes (CNMs) with an areal pore density of 6 × 1013 cm-2 were prepared from self-assembled aromatic precursors via electron-induced cross-linking. While the pore geometry in CNMs is tortuous exhibiting the size distribution from 0.4 to 1
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   Bibliography
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   1 WHO Coronavirus (COVID-19) Dashboard | WHO Coronavirus (COVID-19) Dashboard With Vaccination Data, https://covid19.who.int/, (accessed May 4, 2021).
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   2 A. D. Usher, Medical oxygen crisis: a belated COVID-19 response, NLM (Medline), 2021, vol. 397.
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   Declaration of autorship
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   Hereby, I declare that this thesis is my original work. No resources were used other than mentioned literature.
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   Bielefeld, November 2021
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   Scientific publications and contributions of the author
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   As a result of the doctoral study, the following articles have been published.
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   Acknowledgements
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   I express my deep gratitude to Dr. Petr Dementyev, my mentor in a new scientific field. I thank him for valuable advices and motivation to finish my thesis in three years.