Nitrogen-functionalized graphene oxide (N-FGO) was developed by reacting graphene oxide (GO) with supercritical ammonia (scNH3
). Functionalization was done by combining as-prepared GO with NH3
aq (28%) in an Inconel batch reactor for 1 hour by subjecting it to supercritical conditions with reaction temperature of 2500
C and operating pressure of 30mPa. At this condition, H2
O content of the NH3
aq, will still be in subcritical condition. To investigate the effect of H2
O content two methods were used. Assessment were based on the nitrogen content for each method after functionalization. In the first method, the system was in water-rich phase (denoted by N-FGO-1), GO was directly combined with NH3
aq, enabling H2
O to react with GO during functionalization. On the other hand, the second method was in ammonia-rich phase (denoted by N-FGO-2), where GO was placed in a holder to limit the reaction with H2
O. The successful functionalization was proven by characterizing GO properties before and after functionalization using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS), elemental analysis (EA), X-ray diffraction spectroscopy (XRD). Higher N content was achieved in N-FGO-2, however, both method were effective in the functionalization. Reaction mechanisms that describes the functionalization process were proposed based from characterization results. In this study, it was assumed that only replacement of terminal carbon occurred during the functionalization process which was due to the nucleophilic substitution reaction aided by the supercritical conditions. At supercritical condition, NH3
forms radicals, such as -NH2
, -NH and –N. These radicals may react with C atoms forming new functionalities such as pyrrolic N, amino N (amine, imides and amides), and pyridinic N. Functionalization with carboxylic group involves subsequent reactions with scNH3
, dehydration and decarboxylation process. Reactions with epoxy group involves ring opening and hydroxyl group undergo dehydration forming aromatic amine.
Furthermore, carbon dioxide (CO2) adsorptive properties were evaluated using thermal gravimetric (TG) apparatus. From results, N-FGO-2 resulted to the highest CO2 adsorption capacity around 0.5 mmol CO2/g adsorbent. While, N-FGO-1 can adsorbed around 0.45 mmol CO2/g adsorbent and GO can adsorbed around 0.3 mmol CO2/g adsorbent. Results indicate that the higher the N content of the adsorbent the higher the adsorptive capacity.
The study has presented promising results which can make notable contributions in the fields of carbon and graphene chemistry. The proposed novel process of graphene oxide functionalization with N groups can be applied to various types of materials and other carbon-based materials which in turn can be potentially useful for relevant applications such as electronics, energy conversion, storage devices, catalytic support and carbon capture.