Exploring metal nanoparticles dispersed in water with intense terahertz spectroscopy

• Applicant: Fabio Novelli, Ph.D.
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Experimental Condensed Matter Physics ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 509442914

This project aims at exploring the basic physio-chemical properties of colloids of metal nanoparticles (NP) in water with intense and transient terahertz spectroscopy:1) How does charge transport develop inside metal nanoparticles in a liquid?It is important to measure the NP conductivity at the nanoscale because, as stated in (Hartland et al., ACS Energy Lett. 2, 1641, 2017), “the direct experimental measurements of the spectral distributions […] are still needed.” While THz spectroscopy is sensitive to the conductivity of the charge carriers, such experiments have not been performed on metal NP in solution. I propose to measure intense terahertz time-domain (INTHz-TDS) spectra as a function of NP size, concentration, and shape. I will benchmark this approach by measuring widely used nanoparticles such as gold. In particular, by changing the NP size, it will be possible to probe carriers’ that are either weakly (10-100 nm) or strongly localized (≤5 nm). 2) What are the properties of the extended hydration layer around nanoparticles?While it has been suggested that the hydration layers can affect the NP structure, magnetism, adsorption, and radical production, the experimental results are scarce. We recently found evidence of a reduced cooperativity in the hydrogen bond network around gold nanoparticles, but we were not able to probe all the solvation layers (J. Phys. Chem. B 123, 6521, 2019). Here I will gain more insight with high-resolution THz spectroscopy. I believe this experimental method to be appropriate because, as stated in (Zobel, Acta Cryst. A72, 621, 2016), it “can access hydrogen-bond rearrangements and relaxations in the picosecond timescale” but “thus far, the method has been mainly employed to study biological systems”. 3) Is it possible to trigger and detect water dynamics at the nanoparticles interface?As stated in (Carrasco et al., Nat. Mater. 11, 667, 2012), “Water/solid interfaces are important to an incredibly broad range of everyday phenomena and scientific and technological processes […]”. Recently, the Benderskii group (Nature 594, 62, 2021) showed that the hydrogen bond network is disrupted nearby a charged graphene electrode, while Fumagalli et al. (Science 360, 1339, 2018) showed that the dielectric constant of the interfacial water layer is small. However, little is known about the dynamics of the hydration water layers, especially around metal nanoparticles that are characterized by small surfaces with high curvature. In order to shed more light on this open issue, here I propose to perform optical-pump terahertz-probe experiments on colloids of gold NP in water.

• DFG Programme: Research Grants