Strain Sensing Coatings for Large Composite Structures Based on 2D MXene Nanoparticles.

Real-time strain monitoring of large composite structures such as wind turbine blades requires scalable, easily processable and lightweight sensors. In this study, a new type of strain-sensing coating based on 2D MXene nanoparticles was developed. A Ti3C2Tz MXene was prepared from Ti3AlC2 MAX phase using hydrochloric acid and lithium fluoride etching. Epoxy and glass fibre-reinforced composites were spray-coated using an MXene water solution. The morphology of the MXenes and the roughness of the substrate were characterised using optical microscopy and scanning electron microscopy. MXene coatings were first investigated under various ambient conditions. The coating experienced no significant change in electrical resistance due to temperature variation but was responsive to the 301-365 nm UV spectrum. In addition, the coating adhesion properties, electrical resistance stability over time and sensitivity to roughness were also analysed in this study. The electromechanical response of the MXene coating was investigated under tensile loading and cyclic loading conditions. The gauge factor at a strain of 4% was 10.88. After 21,650 loading cycles, the MXene coating experienced a 16.25% increase in permanent resistance, but the response to loading was more stable. This work provides novel findings on electrical resistance sensitivity to roughness and electromechanical behaviour under cyclic loading, necessary for further development of MXene-based nanocoatings. The advantages of MXene coatings for large composite structures are processability, scalability, lightweight and adhesion properties.

SERS Performance of Ti3C2Tx MXene-Based Substrates Correlates with Surface Morphology.

The surface-enhanced Raman scattering (SERS) properties of low-dimensional semiconducting MXene nanoflakes have been investigated over the last decade. Despite this fact, the relationship between the surface characteristics and SERSing performance of a MXene layer has yet to be comprehensively investigated and elucidated. This work shows the importance of surface morphology on the overall SERS effect by studying few-layer Ti3C2Tx MXene-based SERS substrates fabricated by vacuum-assisted filtration (VAF) and spray coating on filter paper. The VAF deposition results in a dense MXene layer suitable for SERS with high spot-to-spot and substrate-to-substrate reproducibility, with a significant limit of detection (LoD) of 20 nM for Rhodamine B analyte. The spray-coated MXenes film revealed lower uniformity, with a LoD of 50 nM for drop-casted analytes. Moreover, we concluded that the distribution of the analyte deposited onto the MXene layer is affected by the presence of MXene aggregates created during the deposition of the MXene layer. Accumulation of the analyte molecules in the vicinity of MXene aggregates was observed for drop-casted deposition of the analyte, which affects the resulting SERS enhancement. Ti3C2Tx MXene layers deposited on filter paper by VAF offer great potential as a cost-effective, easy-to-manufacture, yet robust, platform for sensing applications.

Wettability of MXene films.

HYPOTHESIS: One of the highlighted properties of Ti3C2Tx MXene compared to other 2D nanomaterials is its hydrophilicity. However, the broad range of static contact angles of Ti3C2Tx reported in the literature is misleading. To elucidate the experimental values of the static contact angles and get reproducible contact angle data, it is wiser to perform the advancing and receding contact angle measurements on smooth and compact Ti3C2Tx layers and focus on deep understanding of the physical basis behind the wettability, which is provided by contact angle hysteresis. EXPERIMENTS: Measurements of the advancing and receding contact angle on mono-, bi, and trilayer Ti3C2Tx on two different substrates were performed. As substrates, UV-ozone treated silicon wafer and silicon wafer functionalized by (3-aminopropyl)triethoxysilane, were used. FINDINGS: The values of the advancing contact angle on Ti3C2Tx on both substrates were proved to be independent of the number of Ti3C2Tx layers, demonstrating a negligible effect of the background substrate wettability. In addition, a giant contact angle hysteresis (44-52 °) was observed on very smooth surface, most likely as a result of chemical heterogeneity arising from the diversity of surface terminal groups (F, O, and OH). The findings reported in this study provide a comprehensive understanding of the wettability of MXene.