Modification of the electronic structure of g-C3N4 using urea to enhance the visible light-assisted degradation of organic pollutants.

Graphitic carbon nitride has been proven to be a good candidate for using solar energy for photo-induced pollutant degradation. However, the high photo-induced holes-electron recombination rate, unfavorable morphology, and textural properties limited their application. In this study, we present a novel g-C3N4 with a novel electronic structure and physiochemical properties by introducing a single nitrogen in the graphitic network of the g-C3N4 through a novel method involving step-by-step co-polycondensation of melamine and urea. Through extensive characterization using techniques such as XPS, UPS-XPS, Raman, XRD, FE-SEM, TEM, and N2 adsorption-desorption, we analyze the electronic and crystallographic properties, as well as the morphology and textural features of the newly prepared g-C3N4 (N-g-C3N4). This material exhibits a lower C/N ratio of 0.62 compared to conventional g-C3N4 and a reduced band gap of 2.63 eV. The newly prepared g-C3N4 demonstrates a distinct valance band maxima that enhances its photo-induced oxidation potential, improving photocatalytic activity in degrading various organic pollutants. We thoroughly investigate the photocatalytic degradation performance of N-g-C3N4 for Congo red (CR) and sulfamethoxazole (SMX), and removal of up to 90 and 86% was attained after 2 h at solution pH of 5.5 for CR and SMX. The influence of different parameters was examined to understand the degradation mechanism and the influence of reactive oxygenated species. The catalytic performance is also evaluated in the degradation of various organic pollutants, and it showed a good performance.

Electronic structure modulation of multi-walled carbon nanotubes using azo dye for inducing non-radical reaction: Effect of graphitic nitrogen and structural defect.

Multiwalled carbon nanotube (MWCNT) have a great potential for advanced oxidation process as a metal free catalyst. However, there catalytic activity is very low and needs to be appropriately tuned. Herein, we demonstrate a novel synthesis method for tuning the defect and surface functionality of MWCNT using azo dyes and the catalytic performance was tested for the degradation of different organic contaminates using PMS as an oxidant. The content, type of heteroatom functional groups, and the defect parameters were optimized by varying the pH and concentration of the organic dye. The quenching effect showed that singlet oxygen (1O2) is the primary reactive species generated by graphitic nitrogen, which can be boosted by the degree of graphitic structure disruption in MWCNT. The Linear sweep voltammetry (LSV) also confirmed that extrinsic doping enhanced the non-radical degradation by increasing the direct charge transfer rate from MB to PMS. Moreover, the designed catalyst showed a fast degradation performance with 35.1 kJ/mol activation energy and achieved the highest dye degradation rate and even surpassed some state-of-the-art metal-based and metal-free catalysts. The effect of inorganic anions study has also confirmed its industrial applicability.

Ground coffee waste-derived carbon for adsorptive removal of caffeine: Effect of surface chemistry and porous structure.

Carbon-based adsorbents show high adsorption capacity towards caffeine due to their porosity and surface functionality. However, the main limiting factor for high performance has not been addressed; furthermore, the adsorption interaction with different active sites needs to be explored. In this study, we synthesized a hierarchical porous nitrogen-doped carbon with unique surface functionality by single-step calcination of coffee waste with KOH under N2. The porous structure, nitrogen content, and types are optimized by varying calcination temperature and KOH concentration. The result of the adsorption experiments shows that both the nitrogen type and the pore size distribution are the limiting factors to adsorption. In addition, the effect of acidic and basic functional groups is studied in detail. The adsorption of caffeine on CW-C is dominantly governed by EDA interaction between the resonance structure of pyridonic-N and the electron-withdrawing group of the caffeine, and the dispersive force caused by the oxidized-N and delocalized π electron of caffeine. Furthermore, we demonstrate that the surface of CW-C is not suitable for the formation of electrostatic and non-electrostatic interaction with caffeine. The maximum adsorption capacity of caffeine at 25 °C is 274.2 mg/g. Moreover, we demonstrate that the unique physio-chemical properties of CW-C are capable of adsorbing other emerging contaminants such as diclofenac, where maximum adsorption capacity of 242.3 mg/g diclofenac is recorded.

Azo-dye derived oxidized-nitrogen rich carbon sheets with high adsorption capability for dye effluent under both batch and continuous conditions.

The removal of methyl blue (MB) from wastewater using graphene and its derivative is very successful due to their high aromaticity which drives adsorption via π-π and electron-donor-acceptor (EDA) interactions; however, graphene is expensive and difficult to synthesize, which limit its practical application. Meanwhile, low aromatic carbon materials (LACM) derived from farm-water and other materials are cheaper and easier to synthesize but have limited π-π and EDA interactions and low adsorption capacity. Herein, we demonstrate that LACM with oxidized-nitrogen (N-O-) functionality overcomes this limitation via chemisorption of MB through a combination of hydrophobic-hydrophobic interactions and EDA interactions. This is confirmed using XPS analysis of LACM/N-O- post MB adsorption. Consequently, a remarkable adsorption capacity of 3904 mg g-1 is achieved under batch condition which is the highest ever reported for any MB adsorbent. Furthermore, LACM/N-O- works equally well under continuous-flow adsorption conditions which shows its practicability. Amongst several LACM precursors tested, only Azo-dyes are able to generate LACM/N-O- implying that the NN moiety is key to N-O- formation. A carbonization temperature of 700 °C generates the highest N-O- sites hence the highest adsorption capacity. Characterization of LACM/N-O- is done mainly using BET, XPS, Raman, TGA, and FTIR analysis.