Atomic force microscopy based characterization of functional interfaces

Abstract: Functional materials are greatly demanded for advanced applications in industry and our daily life such as adaptiveness to environment and energy harvesting. The study of functional interfaces is one important aspect for developing functional materials. Atomic force microscope (AFM) is a powerful tool for revealing the structure of designed interfaces and other properties such as the surface potential. In this work, the structures of different functional interfaces for applications in catalysis and solution filtration are resolved with AFM imaging and the triboelectric effect for energy harvesting is studied with Kelvin Probe Force Microscopy (KPFM) and AFM-based force spectroscopy.
The first project focused on the functional membrane-bound [NiFe] hydrogenases (MBH), which were immobilized on the surface of antimony doped tin oxide (ATO) coated Si substrates for catalysis of hydrogen oxidation. With AFM imaging, the thicknesses of two tagged MBHs immobilized on the ATO surfaces, streptavidin-tagged (strep-MBH) and polyhistidine-tagged (his-MBH), were determined with 0.1 nm precision and the results confirm a monolayer formation of both MBHs on the ATO surfaces.
In the second project, the effect of UV-mediated cross-linking on the instability of porous block copolymer (BCP) poly((butylmethacrylate-co-benzophenone methacrylate-co-methyl methacrylate)-block-(2-hydroxy-ethyl methacrylate)) (poly(BMA-co-BPMA-co-MMA)-b-poly(HEMA)) membranes was investigated with AFM imaging in air, pure water, water-ethanol mixture and pure ethanol. The pore size of the membranes was selected as a characteristic parameter for this study. Due to the hydrophilic blocks poly(HEMA), the cross-linked and noncross-linked membranes swelled and showed smaller pore sizes in all the tested solutions compared to that in air. The pore sizes of the membranes decreased with more ethanol content in the solutions because the hydrophobic blocks poly(BMA-co-BPMA-co-MMA) also swelled in ethanol. With cross-linking, membranes gained larger resistance to water and ethanol. In addition, the formation of pores in BCP polyisoprene-block-polyvinylpyridine (PI-b-P2VP) bulk films by ozonolysis was confirmed with topographic AFM imaging, but no significant dependence on the time span for ozonolysis was found. Moreover, the stabilization by cross-linking with 1,4-diiodobutane (DIB) did not affect the formation of pores.
For the triboelectric effect, an AFM based triboelectrification assay at micro-scale (TEAMS) was established to understand the charge transfer process between interfaces and further to optimize the performances of triboelectric nanogenerators (TENGs). In TEAMS, KPFM was performed for detecting surface potentials and force spectroscopy was used for contacting cantilever tips or colloidal probes with surfaces. The effect of the unloading velocity was investigated with polystyrene (PS), glass or polytetrafluoroethylene (PTFE) colloidal beads against gold (Au) surfaces. The unloading velocity did not show a substantial effect on the contact induced surface potential change ∆(∆Φ) in this work. The investigation of different materials indicated that material transfer for PTFE was involved during triboelectrification. For glass, likely surface adsorbed water had a strong effect on triboelectrification due to the hydrophilicity of the glass surface. In order to know how the moisture in the environment affects the tribo-charges, the contact experiments of PTFE beads against gold (Au) or aluminum (Al) surfaces were performed in air and in N2. For Au surfaces, it was observed that with a lower humidity, the value of ∆(∆Φ) was getting smaller, which indicates a larger triboelectric effect. For Al surfaces, an opposite trend appeared. In an attempt to enhance the triboelectric effect thiol terminated redox active compounds were functionalized onto Au coated cantilever tips and patterned Au surfaces for contact experiments. Four compounds were tested, 11,11,12,12-tetracyano-9,10-anthraquino-dimethane (TCAQ) as acceptor as well as tetrahiafulvalene (TTF), N-methylphenothiazine (PTZ) and triphenylamine (TPA) as donors. The redox pairs TCAQ-TTF, TCAQ-PTZ and TCAQ-TPA gave a significant triboelectric effect, among which TCAQ-TTF showed the greatest. This shows us that the redox active compounds are promising candidates for TENGs.
Lastly, all the findings for triboelectric effect convince us that TEAMS is a useful methodology for the investigation of the contact induced charge transfer and will further contribute to a molecular understanding of the triboelectric effect

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• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2023

Classification and Topics

• Keyword: Rasterkraftmikroskopie ; Rastersondenmikroskopie ; Oberfläche ; Nanostrukturiertes Material ; Oberflächenphysik ; Oberflächenanalyse ; Rasterkraftmikroskopie

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2023
• Creator: Hu, Qiwei
• Contributor: Hugel, Thorsten ; Pastewka, Lars ; Koslowski, Thorsten ; Albert-Ludwigs-Universität Freiburg. Institut für Physikalische Chemie ; Albert-Ludwigs-Universität Freiburg. Fakultät für Chemie und Pharmazie ; Living, Adaptive and Energy-autonomous Materials Systems (livMatS)

Further information

• DOI: 10.6094/UNIFR/235974
• URN: urn:nbn:de:bsz:25-freidok-2359742
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.