Contribution of Functional Groups to the Raman Spectrum of Nanodiamond Powders

In depth understanding of Raman spectra of carbon nanomaterials led to the extension of this technique from simple carbon allotrope detection (fingerprinting) to analysis of the dimensions and ordering of graphene, graphite and nanotubes. In characterization of nanodiamond powders, which have been attracting attention as one of the most promising carbon nanomaterials, Raman spectroscopy is still mainly used only for detecting the diamond phase because of poor understanding of other spectral features. In this paper, we critically examine different explanations of the broad asymmetric Raman band between 1500 and 1800 cm−1 present in all nanodiamond powders and provide an assignment of the contributing peaks, solving one of the major remaining mysteries in Raman spectroscopy of nanodiamond. By using nanodiamond powders with different and well-controlled surface chemistries, as well as in situ Raman measurements at elevated temperatures, we show that these peaks originate from O−H bending vibrations either from the surface functional groups or adsorbed water with contributions (shoulders) coming from sp2 carbon and C═O stretching vibrations. The observation of a strong O−H contribution to the Raman spectrum of nanodiamond raises concerns regarding the use of water as a coolant during the Raman spectra acquisition, because water may affect Raman spectra in the wavenumber ranges corresponding to bending and stretching O−H vibrations.