Excitation and Charge Transfers in Functionalized Nanodiamonds in Water

Generating solvated electrons from sunlight using diamond material has been proposed as a promising strategy to achieve CO2 or N2 reduction in liquid phase [Zhu, D. et al, Nat. Mater (2013), Zhang, L. et al. Angew. Chem. (2014)]. The high conduction band energy and negative electron affinity of diamond surfaces facilitate easy electron transfer to water. Despite the diamond's large band gap (5.47 eV), reports have demonstrated CO2 photo(electro)chemical reduction using visible light [Knittel, P. et al. <i>ChemCatChem</i>. (2020)] as well as the emission of solvated electrons from nanodiamonds [Buchner, F. et al. <i>Nanoscale</i> (2020)]. Our recent investigation of the sub-bandgap excitation on diamond materials has demonstrated the implication of the surface states in the photon absorption and the charge transfers in air [Chemin, A et al. <i>Small Methods</i> (2023)] by combining surface sensitive X-ray absorption spectroscopy (XAS) to surface photovoltage. However, ex-situ measurements have shown limitation in explaining charge transfers in liquid.<br/><br/>In this work, we investigate <i>in-situ</i> the nanodiamond-water interface to unravel the charge transfers and excitation process of detonation nanodiamonds functionalized with Ru complex, which showed promising photoelectrocatalytic properties [Kiendl, et al. Preprint (2022)]. While XAS characterisation is easily performed into vacuum, measurement in water is much more challenging due to the short penetration depth of soft X-ray in liquid. A recently introduced detection mode for XAS in liquid [Schön, D. et al.<i> J. Phys. Chem. Lett.</i> (2017)], performed at BESSY II synchrotron in Berlin, is used to characterize the unoccupied states at the nanodiamond surface as well as the structure of the water molecules at the interface with nanodiamonds. By comparing this measurement to XAS in vacuum, we determined that the interaction with the liquid lead to strong modification of the surface states. By coupling this information with UV-Vis absorption spectroscopy and photocurrent spectroscopy, we discuss further the impact of the water interface in the possible excitation path and charge transfer of the functionalized nanodiamonds.<br/><br/>This project has received funding from the European Commission under the Horizon 2020 grant agreement 665085 (DIACAT) and Volkswagen Foundation under the Freigeist Fellowship No. 89592.