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[Display omitted] • Lignite is used to prepare value-added graphene quantum dots (L-GQDs) via the chemical oxidation process. • The L-GQDs are hybridized with bismuth oxybromide (BiOBr) to form composites by a semi-solvothermal method. • L-GQDs/BiOBr composites achieve enhanced degradation of NO at ppm level under visible light irradiation. • A type II heterojunction is formed between L-GQDs and BiOBr. • The mechanism of NO photodegradation using L-GQDs/BiOBr composites is explored extensively. In this work, lignite was used to prepare value-added graphene quantum dots (L-GQDs) via the chemical oxidation process. The prepared L-GQDs were then hybridized with bismuth oxybromide (BiOBr) to form composite photocatalysts by a semi-solvothermal method. The composites catalytically degraded NO under visible-light irradiation. Multiple characterizations demonstrated that L-GQDs were successfully deposited onto BiOBr without changing the valence states of the elements in BiOBr. The addition of L-GQDs not only boosted the NO removal efficiency from 48.44% to 80.17%, but also significantly improved the environmental friendliness of the catalyst. Experimental investigations and theoretical calculations confirmed the formation of a type II heterojunction between L-GQDs and BiOBr. Such a heterojunction facilitated the separation of photogenerated electrons and holes, thus enhancing the photocatalytic activity. Furthermore, the prepared composite catalyst demonstrated good stability, maintaining 94.40% of its original photocatalytic activity after five cycles. The synthesis and environmentally conscious application of the value-added L-GQDs in this work provides a cleaner alternative for utilizing low-rank coals. [ABSTRACT FROM AUTHOR]

• GQDs/BiOCl composites are prepared by a semi-solvent thermal method. • GQDs/BiOCl composites achieve enhanced NO degradation at ppm level under visible light irradiation. • The mechanism of NO degradation using GQDs/BiOCl composites is explored extensively. • GQDs in GQDs/BiOCl composite act as photoelectron acceptors. This work focuses on modifying BiOCl using graphene quantum dots (GQDs) via a semi-solvothermal method. The formed composites (GQDs/BOC) nanomaterials have enhanced catalytic activity in NO photocatalysis. Multiple characterizations are performed on the prepared GQDs/BOC to describe their crystallization, morphology, and photoelectric characteristics. In addition, photocatalyst activity of GQDs/BOC nanocomposites are assessed by treating 30–32 ppm of NO under visible-light illumination. Within 1 h of photocatalytic NO degradation, GQDs30/BOC behaves the best. Its conversion rate of NO reaches 81.42 % and the DeNOx index is 0.55; they are 1.81 and 2.20 times higher than the counterparts of BiOCl, respectively. The addition of GQDs increases the specific surface area of the BiOCl catalyst, improves the absorption of visible light, accelerates photogenerated carrier transport and inhibits their recombination. These roles of GQDs improve photocatalytic performance. The analysis of the degradation of NO by GQDs/BOC is explored experimentally and theoretically. GQDs in the GQDs/BOC composites are confirmed to act as electron acceptors. In addition, GQDs30/BOC retains excellent catalytic activity after four cycles of testing, evidencing good stability. [ABSTRACT FROM AUTHOR]

Visible light-driven photocatalysis for the degradation of nitric oxide (NO) has attracted considerable attention. Graphene quantum dots/graphite phase carbon nitride (GQDs/CN) heterojunctions were proposed as efficient visible-light responsive photocatalysts for NO treatment under the guidance of density function theory (DFT) simulation. Then, the GQDs/CN was synthesized via hydrothermal combination of GQDs and g-C 3 N 4 (CN) produced from pyrolysis of citric acid (CA) and melamine, respectively. Afterwards, the GQDs/CN composite samples were characterized using XRD, XPS, FE-SEM, TEM, HR-TEM, BET, UV–Vis DRS, PL, photocurrent tests, and EIS. Later, the photocatalytic activity of the prepared GQDs/CN composites was evaluated by degrading NO at ppm level under visible light irradiation at conditions of 20 ppm of NO, 5% of O 2 , and 50% of relative humidity. Results show that 9GQDs/CN outperforms its counterparts. Compared to CN, the 9GQDs/CN increases the NO conversion rate from 61% to 90%, the selectivity of nitrate formation from 53% to 74%, and the value of DeNO x index from −0.25 to 0.19. The enhanced performance results from the inclusion of GQDs, which increases the specific surface area, promotes the absorption of light, boosts the separation of photogenerated electrons and holes, and inhibits their recombination. Furthermore, we used active species detection experiment, IC, in-situ DRIFTS, ESR measurement, XPS VB, Mott-Schottky, and DFT calculations to systematically explore the mechanism behind the degradation of NO using GQDs/CN, and confirmed the formation of Type-Ⅱ heterojunction between GQDs and g-C 3 N 4. In addition, 9GQDs/CN maintains a high NO conversion rate after 5 cycles of experiments, indicating its potential in industrial applications. [Display omitted] • GQDs/CN heterojunctions were proposed to remove nitric oxide based on DFT simulation. • Enhanced photocatalytic activity of GQDs/CN nanocomposites for the degradation of gaseous nitric oxide at ppm level. • A Type-Ⅱ heterojunction is formed between GQDs and CN nanomaterials. • The mechanism of photocatalysis of GQDs/CN nanocomposites was extensively explored. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel method for the preparation of graphene quantum dots/bismuth tungstate (GQDs/BWO) composite photocatalysts. • Performance evaluation of enhanced sunlight responsive photocatalytic performance of GQDs/BWO composite for nitric oxide degradation. • The formation of Z-scheme heterojunction between GQDs and Bi 2 WO 6. • The mechanism of photocatalysis of the GQDs/BWO photocatalysts. Composite (GQDs/BWO) materials for enhanced photocatalysis are hydrothermally produced by combining graphene quantum dots (GQDs) with bismuth tungstate (Bi 2 WO 6). These GQDs/BWO nanomaterials are evaluated for photocatalysis, at different GQDs loadings, for the degradation of gaseous nitric oxide with concentrations of 10–11 ppm. Results show that 10GQDs/BWO has the best photocatalytic performance among competitors for NO degradation. The NO conversion rate reaches 73% during 30 min, which is 3.84 times higher than that of pure Bi 2 WO 6. This is because GQDs not only have a good electron transfer ability, but also form a direct Z-scheme heterojunction with Bi 2 WO 6. These features promote electron-hole separation efficiency of the composite catalyst, and inhibit their recombination. This performance results from the increase in the surface area of the composite and to the increase in light absorption. The selectivity for the formation of NO x - (S N O x - ) increased from 66 to 88%, and the DeNO x index increased from −0.003 to 0.43 after the introduction of GQDs. Active species scavenger experiment, Mott-Schottky (MS) tests and density functional theory (DFT) calculations are used to evaluate the mechanism of photocatalytic NO. Furthermore, cyclic tests show that 10GQDs/BWO catalysts maintain their chemical stability for NO degradation over 8000s. [ABSTRACT FROM AUTHOR]