Fig 1

(a) Schematic representation of a CDs' core and its small domains made of fully sp3- and sp2-hybridized carbon atoms. Blue and red spheres are C atoms, and white spheres are H atoms. The angle between the C–C bonds is 109.5° and 120°. (b) The HOMO and LUMO states of a fully sp2-hybridized carbon domain (η = 1) and the tripolar plate capacitance model used to estimate domain layer displacement during electron density redistribution [29]. Copyright 2019 American Chemical Society. (c) Atomic structures calculated using the HSE functional, depicting sp2 carbon rings confined by surrounding hydroxyl groups, along with their corresponding local density of states (LDOS). The scheme shows the change in the local band gap of the subdomain and the formation energy of hydroxyl groups as the number of confined sp2 carbon rings increases [31]. Copyright 2016 WILEY-VCH. (d) Graphene quantum dot (GQD) models: hexagonal, rectangular, triangular with zigzag edges, and triangular with armchair edges. (e) Schematic arrangement of sp3 carbon atoms within sp2-hybridized GQDs [34]. Copyright 2025 American Chemical Society."

Fig 2

(a) Dependence of HOMO and LUMO energy levels on the particle size of CQDs [37]. Copyright 2022 American Association for the Advancement of Science. (b) Schematic diagram of absorption and emission of CDs [38]. Copyright 2022 Springer Nature."

Fig 3

(a) B-CQDs and (b) R-CQDs: UV-Vis absorption spectra (black curves) and PL emission spectra (colored curves) [41]. Copyright 2024 Elsevier B.V. (c) UV-Vis absorption spectra of N-CDs and N, P-CD film [42]. Copyright 2022 Royal Society of Chemistry. (d) Absorbance changes of R-CDs and MB after UV light irradiation for different times [44]. Copyright 2021 American Chemical Society. (e) PL emission intensity with different time durations (A) under a tungsten lamp and (B) aqueous medium [43]. Copyright 2023 Elsevier B.V."

Fig 4

(a) Schematic illustration of the photo-basic effect in nitrogen-doped CDs. (b) Under protonic conditions, photoexcited basic CDs exhibit two distinct phenomena: In the first case, the photoexcited electron, driven by the existence of a local energy minimum of the protonated species (B*−H), can emit back to the empty ground state of the protonated CDs (B−H). In the second scenario, the photogenerated electrons, instead of returning to the ground state, can participate in reduction reactions to produce H2 (from B*−H) [48]. Copyright 2024 John Wiley and Sons. (c) Absorption spectra of PNZ (black) and PNZ-H2 (red) in acetonitrile. (d) Absorption spectra of aqueous PNZNA dispersion (blue), aqueous CND dispersion (magenta), and aqueous P-CND dispersion (green). (e) Schematic diagram of photocatalytic hydrogen production based on the PNZ intermediate.[49]. Copyright 2025 Wiley-VCH."

Fig 5

(a) Schematic illustration of the preparation of O-CDs, O, N-CDs, and O, Cl-CDs. (b) Schematic diagram of band bending from the interior to the surface of O-CDs, N-CDs, and Cl-CDs. (c) Schematic of O, Cl-CDs with high photocatalytic activity [53]. Copyright 2013 Royal Society of Chemistry. (d) Fluorescence decay curves of CDs derived from CA (citric acid precursor), CA-f (different fractions of CA), N-U (citric acid and urea precursors), N, S-Cys (citric acid and cysteine precursors), N, S-CysAm (citric acid and cysteamine precursors), N, S-Glu (citric acid and glutathione precursors), and N, S-Meth (citric acid and methionine precursors) in water. (e) Conversion of 9, 10-anthracenediyl-bis(methylene)dicarboxylic acid by singlet oxygen generated from different CDs. (f) Hydroxyl radical-mediated hydroxylation reaction of 2-hydroxyterephthalic acid by different CDs [54]. Copyright 2024 Elsevier B.V."

Fig 6

(a–f) Optimized structures and corresponding models of pure CQD and doped CQDs. (g–l) Projected density of states (PDOS) of pure CQD and doped CQDs. (m–r) Corresponding electrostatic potentials. Ev, EF, and Φ represent the vacuum level, Fermi level, and work function, respectively. Gray = C; Blue = N; Orange = P; White = H [55]. Copyright 2020 Elsevier B.V."

Fig 7

(a) The calculated absorption spectra of pristine CDs, amino CDs, CDs hydroxyl, CDs carboxyl, and CDs carbonyl. The inset pictures are their optimized CDs structures [58]. Copyright 2021 Royal Society of Chemistry. (b) A schematic diagram showing the presence of different functional groups on the surface of the synthesized carbon dots. (c) UV-Vis absorption (dashed lines) and PL excitation (solid lines) spectra of prepared CQDs [59]. Copyright 2021 American Chemical Society. (d) UV (365 nm) irradiating time and PL spectra of the nano-CDs [60]. Copyright 2023 American Chemical Society. (e) Recyclability of the as-prepared CDs for the photocatalytic oxidation of benzylamine [61]. Copyright 2024 American Chemical Society."

Fig 8

(a) Cross-section of the conceptual multilayer model for quasi-spherical Asp-CDs nanoparticles featuring a dual-domain structure comprising a hydrophobic core (deep sky blue) and a solvent-sensitive hydrophilic shell (light blue), with a schematic diagram illustrating the considered photophysical processes demonstrated through PL and LEPR experiments. Horizontally oriented black and gray bars represent π-conjugated carbon regions and heteroatom-rich flexible functional groups within the small (S), medium (M), and large (L) layers, respectively. (b) Light-induced electron paramagnetic resonance (LEPR) spectra of Asp-CDs in aqueous, aqueous/2-propanol (50 : 50 v/v), and benzene solutions. Simulated spectra for distinct spin species are denoted as electron (e−), hole (h+), and triplet (3T) species, plotted with red, dark green and light green, light blue and dark blue, and deep purple lines, respectively [62]. Copyright 2023 Wiley-VCH. (c) Schematic diagram of the photocatalytic H2O2 production process [64]. Copyright 2020 Elsevier B.V. (d) Schematic illustrating the relationship between nitrogen content in carbon dots and efficiency for photocatalytic organic reactions [66]. Copyright 2025 Springer Nature."

Fig 9

(a) Time course of produced O2 evolution for CoSAS@CD, CoSAS@CD-acid, VB12, Co@CD, and TTP@CD. X-ray absorption near edge spectroscopy (XANES) profiles (b) and Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) spectra (c) at the Co K-edge of CoSAS@CD, CoSAS@CD-acid, Co foil, CoO, CoOOH, and Co porphyrin (noted as Po) [69]. Copyright 2019 American Chemical Society. (d) Mechanism of visible light-promoted Fenton reaction by CD-COOFe [70]. Copyright 2025 Elsevier B.V. (e) Mechanism of DHP photooxidation by ZnCu-CDs [71]. Copyright 2018 American Chemical Society. (f) UV-Vis spectra of CDs, CD-AgNP, and CD-AuNP in water. (g) Fluorescence spectra of CDs, CD-AgNP, and CD-AuNP in water [74]. Copyright 2023 American Chemical Society. (h) Relative dielectric constants of CDs, Pd-CDs, and Pt-CDs [75]. Copyright 2025 Elsevier B.V."

Fig 10

(a) Schematic diagram of the photocatalytic hydrogen production reaction using nitrogen-doped CDs [76]. Copyright 2020 Springer Nature. (b) Illustration of charge transfer and reaction mechanisms under photocatalytic conditions [77]. Copyright 2025 John Wiley and Sons. (c) Mechanism diagram of the photocatalytic hydrogen evolution reaction (HER) for CD-SO3H and CD-OH [78]. Copyright 2024 Wiley-VCH. (d) Schematic diagram of hydrogen production by aggregated CDs under near-infrared light irradiation [79]. Copyright 2025 John Wiley and Sons."

Fig 11

(a) Reaction mechanism in the photocatalytic process of Fe-CDs [80]. Copyright 2024 Elsevier B.V. (b) Schematic diagram of charge separation and transfer in N-CQDs photocatalysis [81]. Copyright 2024, Elsevier B.V. (c) Illustration of interactions between CDs and dyes during photocatalytic dye degradation [82]. Copyright 2019 Elsevier B.V."

Fig 12

(a) Schematic diagram of the photocatalytic H2O2 production process using CQDs-NH [84]. Copyright 2024 Elsevier B.V. (b) Schematic diagram of the photocatalytic H2O2 production process using EDTA-CQDs [85]. Copyright 2024 Elsevier B.V."

Fig 13

(a) Pathway for synthesizing Cu(Ⅰ)-CQDs and the UV light-induced release of Cu(Ⅰ) from CQDs catalyzing a representative HUISGEN 1, 3-dipolar cycloaddition reaction [89]. Copyright 2017, Royal Society of Chemistry. (b) The selective regeneration mechanism of N, S-CDs for 1, 4-NADH [90]. Copyright 2023, American Chemical Society. (c) Reaction sequence for the hydrogenation of furfural acetone using Ru@CDs under hydrogen gas and light irradiation [92]. Copyright 2022, American Chemical Society."