Photocatalytic activity of self-heterojunctioned intermediate phases in HCl protonated and HNO3 deconjugated g-C3N4 nanostructures

This research involved the different acid-treatment conditions of graphitic carbon nitride and its modified nanostructures through thermal polycondensation of urea at various temperatures. X-ray diffraction patterns revealed that processing at a lower temperature than 500 °C resulted in melem and its derivatives, indicating incomplete transformation of urea to g-C3N4. However, treatment at higher temperatures and the HCl acid treatment led to the formation and expansion of g-C3N4 networks, as evidenced by notable differences in peak intensities observed in their Fourier-transform infrared and Raman spectra. Scanning electron microscopy analysis illustrated a transition from the granular morphology of melamine to the layered structure characteristic of g-C3N4. The nanoparticle morphology observed in the HNO3 acid treatment sample was attributed to the deconjugation of nanosheets through the highly oxidative acid medium. The most suitable photocatalytic activity for Methylene Blue (MB) degradation under UV and visible light illumination was observed for the samples prepared at 550 °C and HCl post-processed nanostructures. It is proposed that the enhanced photocatalytic activity observed in these samples is most likely attributed to the reduced recombination of photogenerated charge carriers facilitated by heterojunctions formed between different intermediate phases. These findings highlight the potential of modified g-C3N4 and its derivatives as promising photocatalytic materials for water purification applications.

Urea-driven g-C3N4 nanostructures for highly efficient photoreduction of Cr(vi) under visible LED light: effects of calcination temperature

Graphitic carbon nitride (g-C3N4) nanostructures were synthesized via the calcination of urea at various temperatures ranging between 400 and 600 °C and were utilized for photoreduction of Cr(vi) in aqueous medium. Due to the low adsorption of Cr(vi) on the g-C3N4 surface, a more accurate assessment of the photocatalytic performance of the samples was carried out. Although the characterization showed that the specific surface of samples increased as the calcination temperature increased, the most efficient product in terms of the photoreduction duration of Cr(vi) was produced through the calcination process carried out at 450 °C, which reduced the concentration by more than 99% in 40 minutes. These results demonstrate that the structural and surface properties of g-C3N4 are critical factors that impact the photocatalytic performance. Alongside the calcination temperatures, the concentration of citric acid as a hole scavenger, the source of illumination, pH levels, and the recycling ability of the produced specimen at 450 °C were also investigated. Conspicuously, the photocatalyst works better when more citric acid is present and the pH level decreases. Out of all the cases studied regarding the light source, the 400 nm LED light source was found to be the most efficient. Additionally, even after going through the photoreduction process four times, the photocatalyst still remained highly efficient.

Novel g-C3N4/PrFeO3 nanocomposites with Z-scheme structure and superior photocatalytic activity toward visible-light-driven removal of tetracycline antibiotics

In the presented work, heterostructured nanocomposites based on g-C3N4 and PrFeO3 with different mass content of PrFeO3 (0-10 wt%) were prepared by ultrasonic processing to study their photocatalytic activity in the process of antibiotic degradation under visible light. The study of phase composition, structural, morphological and textural characteristics carried out by powder X-ray diffraction, scanning electron microscopy and adsorption-structural analysis confirmed the presence of two phases - graphite-like C3N4 and orthorhombic PrFeO3 with average crystallite sizes of 5 and 21 nm and mesoporous structure with specific surface area of 57.2-68.6 m2/g and average pore size of 20 nm. The measured values of the forbidden bandwidth for the obtained nanocomposites were ∼3 eV, indicating potential activity under visible light irradiation. The efficiency of antibiotic removal under visible light was evaluated in the degradation of TCHCl. It was found that 5 % PrFeO3 content was optimal and increased the TOF by 5 times compared to pure g-C3N4. The results of photocatalytic test with absorbers showed that photocatalysis occurs by Z-scheme mechanism. The results obtained allow us to consider this nanocomposite as an effective and stable photocatalyst for pharmaceutical wastewater treatment.

Photocatalytic water purification under visible light using carbon nitride materials and β-Bi2O3 immobilized on electrospun polyvinyl acetate fibers

We report on the immobilization of carbon nitride (CN) materials and β-Bi2O3 on electrospun polyvinyl acetate (PVAc) fiber substrates using a dispersion based dip coating process. The spinning process was optimized by variation of several parameters to finally obtain continuous droplet-free fibers at 15 kV and a flow rate of 50 µL min−1 using a needle with 1.2 mm diameter. The polymer substrates were coated with the β-Bi2O3 and CN materials, which were characterized using SEM and applied in the photocatalytic degradation of organic pollutants such as Rhodamine B (RhB), ethinyl estradiol (EE2) and triclosan using visible light irradiation. The pollutants were degraded with up to 50% of the initial concentration within 8 h. Different amounts of CN material were deposited to evaluate the photocatalytic activity per mass. Immobilized CN materials were shown to be of higher activity (2.0 × 10−10 mol mg−1 min−1) than β-Bi2O3 (1.3 × 10−10 mol mg−1 min−1) and the mixture CN/β-Bi2O3 (1.6 × 10−10 mol mg−1 min−1). Reference samples with CN particles partially embedded in the polymer fleece showed minor degradaton rates (18% RhB degradation within 8 h) as compared to coated fiber substrates (47% RhB degradation within 8 h). Minor leaching of the carbon nitride material and no leaching of β-Bi2O3 occurs as shown by NPOC (non purgeable organic carbon) and ICP-MS measurements.

NiO-decorated graphitic carbon nitride toward electrocatalytic hydrogen production from ethanol

In this study, exfoliated g-C3N4/NiO nanocomposites were synthesized by the heat treatment of urea and subsequent ultrasonic exfoliation of the colloidal solution with the introduction of nickel acetate. Ultrafine nanocomposites were obtained after repeated heat treatment and were marked as initial g-C3N4, g-C3N4/NiO 2.5%, g-C3N4/NiO 5.0%, g-C3N4/NiO 7.5%, and g-C3N4/NiO 10%. The successful attachment of NiO to the surface of g-C3N4 was further confirmed by the results of TEM and SAED. The average sizes of the coherent scattering region, determined by the broadening of the reflex (002), were 11.6, 10.4, 10.4, 9.9 and 9.9 nm for the initial, 2.5%, 5.0%, 7.5%, 10% samples, respectively. The obtained powder of graphite-like carbon nitride and the NiO-composites, according to the results of low-temperature nitrogen adsorption, had a mesoporous structure and was characterized by an average pore size of 16.6-20.8 nm and a porosity of 0.40-0.57 cm3 g-1. It was found that increasing the amount of nickel oxide in the composite had a positive effect on the electrochemical characteristics of the electrode during electro-catalytic reforming - hydrogen evolution from a water-alcohol solution. The g-C3N4/NiO 7.5% nanocomposite showed the best results. Based on voltammetry, it was found that the overpotential of the hydrogen evolution reaction on graphitic carbon nitride equalled 215 mV (at 10 mA cm-2) and the Tafel slope was 95 mV dec-1. The results of the cyclic voltammetry of the electrode based on exfoliated g-C3N4 indicated its high stability.