Surface studies of hydroxylated multi-wall carbon nanotubes

CVD grown MWCNTs, of typical diameter 5 to 50 nm and with approximately 15-20 concentric graphene layers in the multi-walls, have been surface functionalised using the Fenton hydroxylation reaction. HRTEM reveals little physical difference between the treated and untreated materials; images from both exhibit similar multi-wall structure and contain evidence for some low-level disruption of the very outermost layers. Raman spectra from the two types of nanotubes are almost identical displaying the disorder (D) peaks at approximately 1350 cm-1 and graphite (G) peaks at approximately 1580 cm-1, characteristic of graphene-based carbon materials, in approximately equal intensity ratios. Equilibrium adsorption data for nitrogen at 77 K leads to BET surface areas of 60.4 m2 g-1 for the untreated and 71.8 m2 g-1 for the hydroxylated samples; the increase in area being due to separation of the tube-bundles during functionalization. This is accompanied by a decrease in measured porosity, mostly at high relative pressures of nitrogen, i.e. where larger (meso 2-5 nm and macro >5 nm) pores are being filled, which is consistent with an attendant loss of inter-tube capillarity. X-ray photoelectron spectroscopy (XPS) shows that hydroxylation increases the nanotube surface oxygen level from 4.3 at.% to 22.3 at.%; chemical shift data indicate that approximately 75% of that oxygen is present as hydroxyl (-OH) groups. Water vapour adsorption by the hydroxylated surfaces leads to Type II isotherms which are characteristic of relatively high numbers of hydrogen bonding interactions compared to the untreated materials which exhibit Type III curves. This difference in polar surface energy is confirmed by calorimetric enthalpies of immersion in water which are -54 mJ m-2 for the untreated and -192 mJm-2 for the hydroxylated materials. The treated materials therefore have significantly increased water wettability/dispersivity and a greater potential for cross-linking with matrix compounds. The mechanism by which hydroxylation occurs i.e. free radical (OH·) attack and subsequent electrophilic addition at C=C bonds in the graphene basal planes, is discussed.