One-Step Synthesis of N/N-S Graphene from Fruitwastes: Chemistry and Mechanism

: The present work explored the synthesis of N-S containing graphene from nitrogen-rich fruit wastes in the presence of urea and thiourea. Since pure graphene exhibited limited electrocatalytic activity due to a low number of active sites, it is highly recommended to introduce multi-heteroatoms into the graphitic lattice, increasing electrical conductivity and interlayer spacing graphene. The results suggested that urea and thiourea played a dual role as they were involved in the cyclization and aromatization to form a graphitic lattice and introduced nitrogen and sulfur atoms within the lattice pyrolysis. The utilization of urea and thiourea facilitated the development of N-S containing graphene from fruit wastes and prevented oxygen's attack between 250-350 o C under normal atmospheric conditions.

In our previous works, emphasis was mainly given to ensure the selectivity of different carbon and nitrogen-rich fruit wastes towards the synthesis of nitrogen-containing graphene, bypassing the use of any additional N-dopants [43]. After the critical investigation of the chemistry and mechanism of nitrogen-containing fruit waste in developing N-graphene derivatives, it was planned to enhance the nitrogen contents within the graphitic lattice and develop N-S graphene for enhanced electrocatalytic efficiency. The role of easily available urea and thiourea in the formation of N-S graphene under normal atmospheric conditions was studied in detail, and interestingly, the results indicated the involvement of sulfur and diamine structures of urea and thiourea in the cyclization and aromatization of the monomer molecules present in fruit wastes to form nanosheets of N-S graphene. This approach will offer more potential resources for producing N/N-S graphene from various waste materials, without graphite, harsh chemicals, and inert atmosphere, and stimulate competent applications in the biosensor, solar cell, water purification, energy storage, etc.

Materials and Methods
Papaya seeds, watermelon rinds, and banana peels were obtained from the local market. The resultant products obtained from all the precursors were characterized by X-ray diffraction (Bruker D8 advance diffractometer) in the 2θ range of 5 -60 o , FE-SEM (Sigma-300, Carl Zeiss) along with EDX and HRTEM (Jeol JEM-2100 microscope) at an acceleration voltage of 200 kV.

Preparation of N/N-S graphene from fruit wastes.
Three fruit wastes viz., papaya seeds, watermelon rinds, and banana peels were chosen to prepare N-Gr, washed thoroughly with deionized water, cut into small pieces, and dried for 72 hrs. These were further ground to give fine powders, separately mixed with urea at the ratio of 1:5 and heated to 250 o C temperature. The black powders obtained after 2 hrs from all the precursors were washed with distilled water followed by 2 M nitric acid to remove the impurities. The resultant products were filtered and dried in the oven for 24 hrs. Similar to urea, the role of thiourea was also studied in the preparation of N-S graphene. Hence, the fine powders obtained after grinding the three fruit wastes were mixed with thiourea in a 1:5 ratio and subjected to pyrolysis at 250 o C and 350 o C. After 2 hrs, black powders formed in each case were washed, filtered, and dried in the oven for 24 hrs.

Results and Discussion
To study the involvement of diamine structures of urea in the one step formation of N-Gr, three fruit wastes viz. banana peels, papaya seeds, and watermelon rinds were subjected to pyrolysis at the temperature of 250 o C, which resulted in the development of B-N-Gr, P-N-Gr, and W-N-Gr from all the three fruit wastes. The XRD patterns showed the peaks in the range 2θ of 26.5-26.9 o at 250 o C (Figure 1), indicating the direct conversion of fruit wastes to N-Gr in each case. Urea successfully prevented the functionalization of nitrogen-containing graphene by hindering the attack of oxygen.  The FESEM images showed the formation of nanosheets of N-Gr (B-N-Gr, P-N-Gr, and W-N-Gr) in all the cases (Figure 2). The EDX analysis exhibited the carbon content from 43.5-46.2 %, whereas the nitrogen content was exceptionally high (40.0-42.9 %) in all the cases. The high amount of nitrogen in B-N-Gr, P-N-Gr, and W-N-Gr confirmed the involvement of the bent structure of urea in the cyclization and aromatization along with the monomer molecules present in fruit wastes. As a result, the heat treatment at 250 o C resulted in the direct conversion of fruit wastes to nitrogen-containing graphene in a single step. The presence of oxygen (~15%) indicated that the glucose molecules were intact within the graphitic lattice throughout the heat treatment. The TEM images showed the nanosheets in cases of B-N-Gr, P-N-Gr, and W-N-Gr (Figure 3 (a-c)). The mechanism of formation of N-Gr has been presented in Figure 4, which indicated the formation of N-Gr from all the three fruit wastes as confirmed by the XRD pattern. In our earlier work, it was observed that these three nitrogen-rich fruit wastes played an important role in the progress of graphitic structure and formed nitrogen-containing graphene without using any additional nitrogen dopants [43]. Herein, the utilization of bent-structured urea was found to enhance the nitrogen content during cyclization with hydroxyl groups of monomer molecules, resulting in the dehydrogenation of existing polyaromatic rings of N-graphene.  Figure 5(a-c) showed the formation of graphene at 350 o C from the same precursors in the presence of thiourea. Due to the high melting temperature of thiourea (182 o C), it was not able to convert the fruit wastes into N-S-Gr (B-N-S-Gr, P-N-S-Gr, and W-N-S-Gr) at 250 o C. However, when the temperature was increased to 350 o C, all the three fruit wastes were directly converted into nitrogen-sulfur-containing graphene in the presence of thiourea with an intense peak at 2θ of 26.8-27.1 o (Figure 5(a-c)). The FESEM images showed the formation of nanosheets in all the fruit wastes ( Figure  6(a-c)). The EDX showed the presence of 41.2-44 % of carbon, 38.2-40 % of nitrogen, and 11.3-15.2 % of sulfur in the resulted products ( Figure 6(d-f)). This also confirmed that the reducing nature of thiourea prevented the formation of functionalized graphene. The TEM images confirmed the formation of nanosheets for all three cases at 350 o C (Figure 7(a-c)). Like urea, bent-structured thiourea also participated in the cyclization and aromatization of the monomeric glucose molecules present in fruit wastes, introducing nitrogen and sulfur within the graphitic lattice at 350 o C ( Figure 8).

Conclusions
One-step formation of N and N-S containing graphene from three fruit wastes was reported in the presence of urea and thiourea, which facilitated the direct conversion of precursors to N/N-S Gr at 250 and 350 o C, respectively. The results confirmed the participation of bent structures of both the urea and thiourea in the development of hexagonal ring systems of graphene. This approach can effectively synthesize multi heteroatoms containing graphene without the usage of toxic chemicals and an inert atmosphere. The proper utilization of waste materials for developing N/N-S graphene derivatives may prove to be cost-effective and ecofriendly. Hence, it can be the solution to fulfill the demand for graphene and its derivatives in various research fields.

Funding
This research received no external funding.