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- ItemSynthesis of High Crystalline TiO2 Nanoparticles on a Polymer Membrane to Degrade Pollutants from Water(Basel : MDPI, 2018-9-5) Fischer, Kristina; Schulz, Paulina; Atanasov, Igor; Abdul Latif, Amira; Thomas, Isabell; Kühnert, Mathias; Prager, Andrea; Griebel, Jan; Schulze, AgnesTitanium dioxide (TiO2) is described as an established material to remove pollutants from water. However, TiO2 is still not applied on a large scale due to issues concerning, for example, the form of use or low photocatalytic activity. We present an easily upscalable method to synthesize high active TiO2 nanoparticles on a polyethersulfone microfiltration membrane to remove pollutants in a continuous way. For this purpose, titanium(IV) isopropoxide was mixed with water and hydrochloric acid and treated up to 210 °C. After cooling, the membrane was simply dip-coated into the TiO2 nanoparticle dispersion. Standard characterization was undertaken (i.e., X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, water permeance, contact angle). Degradation of carbamazepine and methylene blue was executed. By increasing synthesis temperature crystallinity and photocatalytic activity elevates. Both ultrasound modification of nanoparticles and membrane pre-modification with carboxyl groups led to fine distribution of nanoparticles. The ultrasound-treated nanoparticles gave the highest photocatalytic activity in degrading carbamazepine and showed no decrease in degradation after nine times of repetition. The TiO2 nanoparticles were strongly bound to the membrane. Photocatalytic TiO2 nanoparticles with high activity were synthesized. The innovative method enables a fast and easy nanoparticle production, which could enable the use in large-scale water cleaning.
- ItemRole of Reaction Intermediate Diffusion on the Performance of Platinum Electrodes in Solid Acid Fuel Cells(Basel : MDPI, 2021) Lorenz, Oliver; Kühne, Alexander; Rudolph, Martin; Diyatmika, Wahyu; Prager, Andrea; Gerlach, Jürgen W.; Griebel, Jan; Winkler, Sara; Lotnyk, Andriy; Anders, André; Abel, BerndUnderstanding the reaction pathways for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) is the key to design electrodes for solid acid fuel cells (SAFCs). In general, electrochemical reactions of a fuel cell are considered to occur at the triple-phase boundary where an electrocatalyst, electrolyte and gas phase are in contact. In this concept, diffusion processes of reaction intermediates from the catalyst to the electrolyte remain unconsidered. Here, we unravel the reaction pathways for open-structured Pt electrodes with various electrode thicknesses from 15 to 240 nm. These electrodes are characterized by a triple-phase boundary length and a thickness-depending double-phase boundary area. We reveal that the double-phase boundary is the active catalytic interface for the HOR. For Pt layers ≤ 60 nm, the HOR rate is rate-limited by the processes at the gas/catalyst and/or the catalyst/electrolyte interface while the hydrogen surface diffusion step is fast. For thicker layers (>60 nm), the diffusion of reaction intermediates on the surface of Pt be-comes the limiting process. For the ORR, the predominant reaction pathway is via the triple-phase boundary. The double-phase boundary contributes additionally with a diffusion length of a few nanometers. Based on our results, we propose that the molecular reaction mechanism at the electrode interfaces based upon the triple-phase boundary concept may need to be extended to an effective area near the triple-phase boundary length to include all catalytically relevant diffusion processes of the reaction intermediates. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemEffect of morphology on the photoelectrochemical activity of TiO2 self-organized nanotube arrays(Basel : MDPI, 2020) Ennaceri, Houda; Fischer, Kristina; Hanus, Kevin; Chemseddine, Abdelkrim; Prager, Andrea; Griebel, Jan; Kühnert, Mathias; Schulze, Agnes; Abel, BerndIn the present work, highly ordered titanium dioxide (TiO2) nanotube anodes were grown using a rapid anodization process. The photoelectrochemical performances of these electrodes strongly depend on the anodization conditions. Parameters such as electrolyte composition, anodization potential and anodization time are shown to affect the geometrical parameters of TiO2 nanotubes. The optimal anodization parameters are determined by photocurrent measurements, linear sweep voltammetry and electrochemical impedance spectroscopy. The thickness of the tube wall and its homogeneity is shown to strongly depend on the anodization potential, and the formation mechanism is discussed. This study permits the optimization of the photocurrent density and contributes to further improvement of the photoelectrochemical water-splitting performance of TiO2 nanotube photoelectrodes. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemLow-temperature atmospheric pressure plasma conversion of polydimethylsiloxane and polysilazane precursor layers to oxide thin films(Weinheim : Wiley VCH, 2023) Rudolph, Martin; Birtel, Peter; Arnold, Thomas; Prager, Andrea; Naumov, Sergej; Helmstedt, Ulrike; Anders, André; With, Patrick C.We study the conversion of two polymeric silicon precursor compound layers (perhydropolysilazane and polydimethylsiloxane) on a silicon wafer and polyethylene terephthalate substrates to silicon oxide thin films using a pulsed atmospheric pressure plasma jet. Varying the scan velocity and the number of treatments results in various film compositions, as determined by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The mechanism suggested for the conversion process includes the decomposition of the precursor triggered by plasma-produced species, the oxidation of the surface, and finally, the diffusion of oxygen into the film, while gases produced during the precursor decomposition diffuse out of the film. The latter process is possibly facilitated by local plasma heating of the surface. The precursor conversion appears to depend sensitively on the balance between the different contributions to the conversion mechanism.
- ItemBiocatalytic Self-Cleaning Polymer Membranes(Basel : MDPI, 2015) Schulze, Agnes; Stoelzer, Astrid; Striegler, Karl; Starke, Sandra; Prager, AndreaPolymer membrane surfaces have been equipped with the digestive enzyme trypsin. Enzyme immobilization was performed by electron beam irradiation in aqueous media within a one-step method. Using this method, trypsin was covalently and side-unspecific attached to the membrane surface. Thus, the use of preceding polymer functionalization and the use of toxic solvents or reagents can be avoided. The resulting membranes showed significantly improved antifouling properties as demonstrated by repeated filtration of protein solutions. Furthermore, the biocatalytic membrane can be simply “switched on” to actively degrade a fouling layer on the membrane surface and regain the initial permeability. The membrane pore structure (pore size and porosity) was neither damaged by the electron beam treatment nor blocked by the enzyme loading, ensuring a stable membrane performance.
- ItemRadiation-Induced Graft Immobilization (RIGI): Covalent Binding of Non-Vinyl Compounds on Polymer Membranes(Basel : MDPI, 2021) Schmidt, Martin; Zahn, Stefan; Gehlhaar, Florian; Prager, Andrea; Griebel, Jan; Kahnt, Axel; Knolle, Wolfgang; Konieczny, Robert; Gläser, Roger; Schulze, AgnesRadiation-induced graft immobilization (RIGI) is a novel method for the covalent binding of substances on polymeric materials without the use of additional chemicals. In contrast to the well-known radiation-induced graft polymerization (RIGP), RIGI can use non-vinyl compounds such as small and large functional molecules, hydrophilic polymers, or even enzymes. In a one-step electron-beam-based process, immobilization can be performed in a clean, fast, and continuous operation mode, as required for industrial applications. This study proposes a reaction mechanism using polyvinylidene fluoride (PVDF) and two small model molecules, glycine and taurine, in aqueous solution. Covalent coupling of single molecules is achieved by radical recombination and alkene addition reactions, with water radiolysis playing a crucial role in the formation of reactive solute species. Hydroxyl radicals contribute mainly to the immobilization, while solvated electrons and hydrogen radicals play a minor role. Release of fluoride is mainly induced by direct ionization of the polymer and supported by water. Hydrophobic chains attached to cations appear to enhance the covalent attachment of solutes to the polymer surface. Computational work is complemented by experimental studies, including X-ray photoelectron spectroscopy (XPS) and fluoride high-performance ion chromatography (HPIC).
- ItemCharge Separating Microfiltration Membrane with pH-Dependent Selectivity(Basel : MDPI, 2018-12-20) Breite, Daniel; Went, Marco; Prager, Andrea; Kuehnert, Mathias; Schulze, AgnesMembrane filters are designed for selective separation of components from a mixture. While separation by size might be the most common approach, other characteristics like charge can also be used for separation as presented in this study. Here, a polyether sulfone membrane was modified to create a zwitterionic surface. Depending on the pH value of the surrounding solution the membrane surface will be either negatively or positively charged. Thus, the charged state can be easily adjusted even by small changes of the pH value of the solution. Charged polystyrene beads were used as model reagent to investigate the pH dependent selectivity of the membrane. It was found that electrostatic forces are dominating the interactions between polystyrene beads and membrane surface during the filtration. This enables a complete control of the membrane’s selectivity according to the electrostatic interactions. Furthermore, differently charged beads marked with fluorescent dyes were used to investigate the selectivity of mixtures of charged components. These different components were successfully separated according to their charged state proving the selectivity of the invented membrane.
- ItemTailoring Membrane Surface Charges: A Novel Study on Electrostatic Interactions during Membrane Fouling(Basel : MDPI, 2015) Breite, Daniel; Went, Marco; Prager, Andrea; Schulze, AgnesIn this work we aim to show that the overall surface potential is a key factor to understand and predict anti-fouling characteristics of a polymer membrane. Therefore, polyvinylidene fluoride membranes were modified by electron beam-induced grafting reactions forming neutral, acidic, alkaline or zwitterionic structures on the membrane surface. The differently charged membranes were investigated regarding their surface properties using diverse analytical methods: zeta potential, static and dynamic water contact angle, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Porosimetry measurements proved that there is no pore blocking due to the modifications. Monodisperse suspensions of differently charged polystyrene beads were synthesized by a radical emulsion polymerization reaction and were used as a model fouling reagent, preventing comparability problems known from current literature. To simulate membrane fouling, different bead suspensions were filtered through the membranes. The fouling characteristics were investigated regarding permeation flux decline and concentration of model fouling reagent in filtrate as well as by SEM. By considering electrostatic interactions equal to hydrophobic interactions we developed a novel fouling test system, which enables the prediction of a membrane’s fouling tendency. Electrostatic forces are dominating, especially when charged fouling reagents are present, and can help to explain fouling characteristics that cannot be explained considering the surface wettability.
- ItemBio-Inspired Polymer Membrane Surface Cleaning(Basel : MDPI, 2017-3-9) Schulze, Agnes; Breite, Daniel; Kim, Yongkyum; Schmidt, Martin; Thomas, Isabell; Went, Marco; Fischer, Kristina; Prager, AndreaTo generate polyethersulfone membranes with a biocatalytically active surface, pancreatin was covalently immobilized. Pancreatin is a mixture of digestive enzymes such as protease, lipase, and amylase. The resulting membranes exhibit self-cleaning properties after “switching on” the respective enzyme by adjusting pH and temperature. Thus, the membrane surface can actively degrade a fouling layer on its surface and regain initial permeability. Fouling tests with solutions of protein, oil, and mixtures of both, were performed, and the membrane’s ability to self-clean the fouled surface was characterized. Membrane characterization was conducted by investigation of the immobilized enzyme concentration, enzyme activity, water permeation flux, fouling tests, porosimetry, X-ray photoelectron spectroscopy, and scanning electron microscopy.
- ItemReduction of biofouling of a microfiltration membrane using amide functionalities-Hydrophilization without changes in morphology(Basel : MDPI, 2020) Breite, Daniel; Went, Marco; Prager, Andrea; Kühnert, Mathias; Schulze, AgnesA major goal of membrane science is the improvement of the membrane performance and the reduction of fouling effects, which occur during most aqueous filtration applications. Increasing the surface hydrophilicity can improve the membrane performance (in case of aqueous media) and decelerates membrane fouling. In this study, a PES microfiltration membrane (14,600 L m−2 h−1 bar−1) was hydrophilized using a hydrophilic surface coating based on amide functionalities, converting the hydrophobic membrane surface (water contact angle, WCA: ~90°) into an extremely hydrophilic one (WCA: ~30°). The amide layer was created by first immobilizing piperazine to the membrane surface via electron beam irradiation. Subsequently, a reaction with 1,3,5-benzenetricarbonyl trichloride (TMC) was applied to generate an amide structure. The presented approach resulted in a hydrophilic membrane surface, while maintaining permeance of the membrane without pore blocking. All membranes were investigated regarding their permeance, porosity, average pore size, morphology (SEM), chemical composition (XPS), and wettability. Soxhlet extraction was carried out to demonstrate the stability of the applied coating. The improvement of the modified membranes was demonstrated using dead-end filtration of algae solutions. After three fouling cycles, about 60% of the initial permeance remain for the modified membranes, while only ~25% remain for the reference.