X-Ray Absorption Spectroscopy on Nanodiamonds in Water

Easy production of solvated electrons from solar illumination would be a game changer for green chemistry and environmental research. The use of widely available synthetic diamonds to generate solvated electrons upon light irradiation in water has been proposed as a promising strategy to achieve CO2 or N2 reduction in liquid phase [1-2]. Free electrons can easily transfer to water because diamonds have a high energetic level of the conduction band and a negative electron affinity when its surface is hydrogenated. At first glance, the large band gap of diamonds (5.5 eV) should only enable excitation from deep UV light, limiting the electrons production using sun. However, CO2 photo(electro)chemical reduction with visible light has been previously reported [3]. Electron emission mechanisms from visible light radiation remains poorly understood but seems strongly related to the surface state of the diamonds and the interaction with water.<br/>The surface chemistry of the diamonds in water as well as its functionalization, for example with Ru dye, plays a dramatic role on electron excitation and emission. It affects the interfacial properties such as electron affinity or the electron emission and solvation. X-ray absorption spectroscopy (XAS) can be used to characterize the surface states. XAS corresponds to the resonant excitation by X-rays from the core electrons into unoccupied electronic levels allowing a high chemical sensitivity. This provides knowledge on the chemical state of the surface atoms, the density of states and the photoelectron yield. While this characterisation is easily performed into vacuum, measurement in water is much more challenging due to the vacuum requirement for soft X-ray propagation.<br/>In this work, we apply a new technique of XAS in liquid [4] to the characterization of the nanodiamonds film in interaction with water. The nanodiamonds are detonation nanodiamonds functionalized with Ru complex, which showed good photocatalytic properties. A similar method was previously applied to dispersed carbon dots [5]. Measurements were performed at BESSY II synchrotron in Berlin. We used a two electrodes liquid flow-cell with an ultra-thin membrane separating the liquid from the vacuum. The nanodiamonds are deposited on a gold-coated membrane used as working electrode. Here, we show that we can measure the absorption spectrum of nanodiamonds deposited on the membrane and of the water molecules in interaction with the nanodiamonds. The signature from both diamond core and of the functionalized molecules on the nanodiamond surface can be detected. By comparing this measurement to XAS in vacuum, we determine that the interaction with the liquid lead to strong modification of the surface states giving a new insight on possible pathways for the production of the solvated electrons in water.<br/><b>References</b><br/>Zhu, D. et al, Photo-illuminated diamond as a solid-state source of solvated electrons in water for nitrogen reduction. <i>Nat. Mater.</i> 12, 836–841. (2013).<br/>Zhang, L. et al. Selective Photoelectrochemical Reduction of Aqueous CO₂ to CO by Solvated Electrons. <i>Angew. Chem.</i> 53, 9746–9750. (2014)<br/>Knittel, P. et al. Nanostructured boron doped diamond electrodes with increased reactivity for solar-driven CO₂ reduction in room temperature ionic liquids. <i>ChemCatChem</i> cctc.202000938. (2020)<br/>Schön, D. et al. Introducing Ionic-Current Detection for X-ray Absorption Spectroscopy in Liquid Cells. <i>J. Phys. Chem. Lett.</i> <i>8</i>, 9, 2087-2092. (2017)<br/>Ren, J. et al. Uncovering the Charge Transfer between Carbon Dots and Water by in Situ Soft X-ray Absorption Spectroscopy.<i> J. Phys. </i><i>Chem. Lett.</i> 10, 14, 3843–3848 (2019)<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.