Multifunktionale Membranen aus Graphenoxid und TiO2-Nanokristallen: Steuerung elektrischer, mechanischer und elektromechanischer Eigenschaften durch photokatalytische Reduktion
Zusammenfassung der Projektergebnisse
In this project we fabricated nanocomposite films of graphene oxide (GO) and titania nanocrystals (TNCs) with thicknesses ranging from 15 to 150 nm via layer-by-layer spin-coating (LbL-SC). TNCs of different sizes and shapes (dots, TNDs; rods, TNRs; plates, TNPs) were used for film assembly. Based on the TNCs’ photocatalytic activity, the GO could be converted to reduced graphene oxide (rGO) by exposing the films to UV radiation (254 nm). The formation of rGO was confirmed by UV/vis-, Raman-, ATR-IR-spectroscopy, and XPS. Furthermore, a significant increase in the films’ conductivity was observed upon UV exposure. Compared to the GO/TNP films, this conductivity increase was more pronounced for the GO/TND and GO/TNR films. Since the degree of GO reduction was controlled by the UV exposure time, the physical and chemical properties of produced (rGO/GO)/TNC films could easily be adjusted. Furthermore, such tuning of the films’ properties can be combined with conventional photolithographic patterning techniques. These features make (rGO/GO)/TNC composite films interesting materials for the development of novel electronic devices, microelectromechanical systems (MEMS), and sensors. According to one objective of this project, we studied the possible application of (rGO/GO)/TNC films as resistive humidity sensors. Depending on the degree of GO reduction and the level of applied relative humidity levels, these sensors showed different directions of their resistive responses, different sensitivities, and different response kinetics. Hence, by controlling the degree of GO reduction it is possible to adjust the sensor’s performance to application-specific humidity ranges. In addition, we studied the responses of these chemiresistors to different volatile organic compounds, revealing that their chemical selectivity can be tuned by varying the degree of GO reduction. In summary, these findings suggest that (rGO/GO)/TNC films are well suited for the facile fabrication of chemical sensor arrays using conventional UV photolithography. Another objective of this project was to characterize the mechanical properties of freestanding (rGO/GO)/TNC membranes and to evaluate their possible application in novel types of MEMS sensors. In order to produce freestanding membranes with reproducible properties, a semiautomated print process was developed to transfer the GO/TNC films from their initial substrates onto substrates featuring apertures or cavities. However, our initial attempts to fabricate freestanding (rGO/GO)/TNC membranes revealed that these composites were not robust enough to form freestanding membranes with diameters of ~100 µm. To solve this problem, we fabricated films with a double layer structure comprising a silk fibroin (SF) enforced (GO/SF) bottom layer and a (GO/TNR/GO) top layer. When transfer-printing these films onto substrates with circular apertures, they formed stable freestanding membranes. AFM bulge experiments revealed elastic moduli in the range of ~30 GPa (biaxial moduli: ~45 GPa), typical prestress values ranging from ~10 to ~20 MPa, and ultimate biaxial strength values of ~380 MPa. In order to demonstrate an electrostatically driven resonator, a 34 nm thick GO/SF/TNR membrane was transfer-printed onto a silicon substrate featuring square cavities with ~240 µm edge length. Reducing the GO of the (GO/TNR/GO) top layer via UV exposure provided sufficiently high conductivity to enable the membrane’s electrostatic actuation. By sweeping the drive voltage frequency and measuring the membrane’s deflection using a laser vibrometer, the amplitude spectrum was recorded and revealed a resonance peak at 430 kHz. Hence, we demonstrated a membrane resonator with tunable physical and chemical properties based on a (rGO/GO)/SF/TNC membrane. Such tunable actuators are of great interest for the development of highly sensitive MEMS sensors with electromechanical signal transduction (cf. US patent US11293900B2 and pending patent applications).
Projektbezogene Publikationen (Auswahl)
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Transfer Printing of Freestanding Nanoassemblies: A Route to Membrane Resonators with Adjustable Prestress. ACS Applied Materials & Interfaces, 13(34), 40932-40941.
Hartmann, Hauke; Beyer, Jan-Niklas; Hansen, Jan; Bittinger, Sophia C.; Yesilmen, Mazlum; Schlicke, Hendrik & Vossmeyer, Tobias
