Controlling and monitoring interfacial chemical reactions is crucial for electrochemistry and photochemistry. Here, we developed a new type of interface-sensitive magnetic resonance technique by combining qPlus-based scanning probe microscopy (SPM) and quantum sensing based on a single nitrogen-vacancy center near the surface of a diamond. Using this technique, we investigated water dissociation at the hydrophilic diamond surface and monitored its elementary steps, including electron transfer, bond breaking, as well as water and hydrogen diffusion. The SPM tip was used to locally inject electrons into the interfacial water, and the resulting hydrated electrons (${\mathrm{e}}_{(\mathrm{aq})}^{-}$) were found to have specific configurations at the interface with a hyperfine interaction of about 28 MHz, which agrees with density-functional theory calculations. We observed that ${\mathrm{e}}_{(\mathrm{aq})}^{-}$ can induce water dissociation and the reaction product hydroxides diffuse $\sim 2.3$ times faster than water molecules. The diffusion coefficients of water and the reaction product hydroxides at the interface are about 3 orders of magnitude smaller than those in the bulk phase, but their ratio almost remain the same. These results show that the combination of SPM and quantum sensing provides a new platform to reveal the detailed electronic and nuclear processes of interfacial chemical reactions with nanometer resolution.

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• Chemical Physics & Physical Chemistry
• NV centers
• Quantum sensing
• Liquid-solid interfaces
• Water
• Scanning probe microscopy