Toward An Advanced Methodology for Reliable Nanodiamond Characterization: The Strength of XPS/UPS/REELS Techniques in Assessing The Impact of Surface Terminations

Photoelectron spectroscopies are particularly suitable techniques to provide physicochemical information on nanomaterials since the escape depth of photogenerated electron in the matter is in the nanometer range. The versatile surface chemistry of nanodiamonds (NDs) plays a major role in their electronic and physicochemical properties^[1] and strongly govern their interactions with the substrate and other nanoparticles.<br/>The present study aims to combine several surface analysis tools, such as REELS, XPS and UPS in order to get insights about these differences for different surface terminations. In this work, milled NDs (MNDs) with mean diameters around 30 nm were investigated. MNDs were oxidized by annealing in air at atmospheric pressure ^[2,3]. Hydrogenation was then carried out by annealing in a hydrogen flow at 750°C. The crucial role of the substrate selection based on these surface terminations will be demonstrated. Indeed, the hydrogenated nanoparticle (H-MNDs) layer clearly appears more uniform on hydrophobic substrates while the wettability of oxidized ND (Ox-MND) dispersion is higher on hydrophilic substrates. The assistance of charge neutralizer (flood gun) during XPS acquisition will also be extensively discussed. REELS showed that hydrogenation does not lead to a significant formation of sp² carbon at the ND surface (the π-π* transition is not detected) but leads to the formation of C-H bonds. The XPS/UPS analyses reveal that the influence of the surface termination of the NDs with a clear shift of the Fermi level (order of 2.5 eV) ^[3], thereby modifying their conductivity. UPS measurements evidence a p-type conductivity associated with a negative electron affinity for the H-MND layers.Combined <i>in situ</i> UPS/XPS investigations of H-MNDs after thermal treatments confirm their sp³ hybridization and enable the reconstruction of the MND energy diagram. These results indicate that the exceptional conductivity of H-MNDs is attributed to the simultaneous presence of adsorbates and hydrogen terminated MND surface.<br/>Overall, this work is in line with the requirement for targeted reliability of photoemission and related techniques for the characterization of carbon-based materials. These approaches will be confronted to electrochemical impedance spectroscopy (EIS) and extended to less crystalline detonated ND (bottom-up approach) which are promising candidates for photocatalysis<br/><br/><br/><u>References</u><br/>[1] D. Miliaieva et al, Nanoscale Adv., 2023, 5, 4402–4414<br/>[2] J.C. Arnault, H.A. Girard, Current Opinion in Solid State and Materials Science 21 (2017), 10-16<br/>[3] L. Saoudi et al, Carbon 202 (2023), 438–449<br/><br/><u>Acknowledgements</u><br/>The authors thank the CHARM3AT and NanoSaclay LabEx for C. Njel funding. This work has also benefited from the support of the French National Research Agency (ANR) with the grant ANR-19-CE09-0025 (COCONUT project).