Templated Synthesis Of Heteroatoms Doped Graphene-like Carbon Nano-materials Using Spent Montmorillonite

The special 2D nano-sized structure and exchangeable inorganic cations in the interlayer of montmorillonite(Mt) make it an environmentally-friendly, low-cost, and highly-efficient adsorbent for wide application in environmental remediation, especially in the dye wastewater treatment. However, proper disposal and resource recycling of the spent Mt after the adsorption of dyes is still a challenge holding back the contamination remediation application of Mt. Therefore, it is crucial to find proper methods to dispose the used Mt. On the other side, templated synthesis of carbon materials using inorganic templates are drawing increasing interests recently. In this process, the adsorbed organic compounds serve as carbon sources and can be directly transformed into carbon materials within the pores of the inorganic template after carbonization. Then, pure carbon materials can be obtained after removing the inorganic templates by acid washing. For example, when clay minerals with layered structure are selected as templates, carbon materials with 2D structure(i.e., graphenelike materials) can be synthesized. These graphene-like carbon materials have found potential applications in various fields.In this work, heteroatom-doped graphene-like carbon materials were successfully synthesized through pyrolysis in the protection of N2 followed by acid washing using the spent Mt after the adsorption of CV. The structure, composition, and morphology of the obtained materials were investigated by thermogravimetry analysis(TG), X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR), elemental analysis, Raman spectra, scanning electron microscopy(SEM), transmission electron microscopy(TEM), atomic force microscope(AFM), X-ray photoelectron spectroscopy(XPS) and so on. Furthermore, the electrocatalytic activities of the obtained carbon materials were also determined. Meanwhile, the influences of different carbonization temperatures on the structure and performance of the obtained graphenelike carbon materials were carefully studied.The main conclusion for this work are as follows:(1) Carbon monolayer–Mt nanocomposites were synthesized using the used Mt after the adsorption of crystal violet(CV) as precursor. The obtained results showed that interlayer spacing of the resulting C/Mt nanocomposites is only 0.36 nm, close to the thickness of a single graphene layer(0.34 nm). Moreover, the atomic ratio of carbon/nitrogen may be controllable by simply changing carbonization temperature. During carbonization intercalated CV and Mt layers have mutual influence: Mt layers as template can transform CV into carbon monolayer, while the formed carbon monolayer in turn can help preserve the layered structure of Mt layers.(2) N-doped graphene-like carbon materials were successfully synthesized by acid treatment with the resulting C/Mt nanocomposites to remove the Mt layers. After acid washing pure carbon materials with graphene-like structure were obtained. Raman spectra showed the presence of both D-band and G-band on the as-prepared carbon materials. Furthermore, the intensity ratio of D-band/G-band band increased evidently with increasing pyrolysis temperature, suggesting the Mt template somehow showed a “hindering” effect for the graphitization of carbon sources. The carbon materials has simultaneously pyridinic, pyrrolic, and quaternary N. With increasing pyrolysis temperature, pyridinic N transformed into more stable quaternary N. In terms of the peak potential, onset potential, and electron transfer number, the as-prepared N-doped graphene-like materials exhibited an enhanced electrocatalytic activity toward oxygen reduction reaction(ORR).(3) N and S-codoped graphene-like carbon materials were successfully synthesized using the used Mt after the adsorption of methylene blue(MB) as precursor. With the pyrolysis temperature increasing the interlayer spacing of the carbon/Mt composites gradually decreased to 0.44 nm, a little bit larger than the thickness of a single graphene layer. This phenomenon was attributed to the reason that S atom was too large to fit in the sp2-hybridized carbon framework and protruded out of the sheet. The morphology analysis of the obtained materials confirmed the presence of graphenelike structure. The XPS results exhibited the presence of C, S, and N atoms on the carbon materials. N atoms were doped into graphene network with three kinds of binding configurations(pyridinic, pyrrolic, and graphitic N), while S atoms were in a major form of thiophene-S. Finally, the obtained N and S-codoped graphene-like carbon materials showed efficient electrocatalytic activity and stability, suggesting their potential application in fuel cells.