g-C₃N₄ was synthesized using analytical-grade urea (CO(NH₂)₂, $\ge$99% purity) as the precursor. In a typical synthesis, 10 g of urea was placed in a covered alumina crucible and thermally treated in a programmable muffle furnace (Nabertherm LHT 04/17, Nabertherm GmbH, Lilienthal, Lower Saxony, Germany) at temperatures ranging from 400 °C to 650 °C for 3 h in ambient air, employing a heating rate of 5 °C min^-1. During the thermal polymerization process, urea decomposed into gaseous by-products such as NH₃ and CO₂, resulting in the formation of a yellow solid corresponding to bulk g-C₃N₄, as illustrated in Fig. 1. The obtained material was collected, finely ground, and stored in airtight containers for subsequent use. Exfoliated g-C₃N₄ nanosheets were prepared via a simple and efficient electrochemical exfoliation approach. The electrochemical cell comprised two electrodes: a graphite electrode fabricated from recycled carbon sources (e.g., spent batteries) and a g-C₃N₄ electrode synthesized from urea. An acidic electrolyte, typically H₂SO₄ at various concentrations, was employed, and a DC power supply delivering 10–12 V was applied to promote interlayer expansion and exfoliation of g-C₃N₄, as shown in Fig. 1. Upon completion of the exfoliation process, the resulting suspension was allowed to stand until complete sedimentation of the solid product. The precipitate was then collected, repeatedly washed with deionized water, and dried in a laboratory oven (Memmert UN55, Memmert GmbH, Schwabach, Bavaria, Germany) at 50–60 °C to obtain fine g-C₃N₄ nanosheet powder. To evaluate the influence of post-thermal treatment, the exfoliated nanosheets were subsequently annealed at temperatures between 400 °C and 650 °C under controlled conditions [16, 17, 18]. This annealing process enabled a systematic investigation of the effects of heat treatment on the crystallinity, stacking order, and physicochemical properties of the g-C₃N₄ nanosheets, as discussed in the Results and Discussion section.

Fig. 1.

Schematic illustration of the synthesis of g-C₃N₄ nanosheets from Urea using a novel method. g-C₃N₄, Graphitic carbon nitride.

The g-C₃N₄ nanosheets were subjected to $\gamma$-irradiation using a ⁶⁰Co source at the National Center for Radiation Research and Technology (NCRRT), Cairo, Egypt [19, 20]. Irradiation was performed at room temperature under ambient atmospheric conditions, with absorbed doses ranging from 5 to 50 kGy. The dose rate was accurately calibrated prior to each irradiation run to ensure uniform energy delivery to the samples. During exposure, the nanosheets were sealed in quartz vials to prevent contamination and to preserve their structural integrity. This controlled $\gamma$-irradiation process was designed to induce defect states, modify surface characteristics, and promote structural rearrangements within the g-C₃N₄ framework. Systematic variation of the irradiation dose enabled a reliable evaluation of dose-dependent changes in crystallinity, electronic structure, optical properties, and photocatalytic performance, as discussed in the Results and Discussion section.

X-ray diffraction (XRD) measurements were employed to investigate the crystallographic structure and irradiation-induced microstructural changes in bulk g-C₃N₄ and exfoliated g-C₃N₄ nanosheets. The diffraction patterns were recorded using (D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany, Cu K$\alpha$ radiation, $\lambda$ = 1.5406 Å) over a 2$\theta$ range of 5°–90°. Structural parameters were quantitatively evaluated using the most intense (002) reflection located at approximately 2$\theta$ $\approx$ 27.4°, which is associated with the interlayer stacking of the conjugated g-C₃N₄ framework. The crystallite size (D) was estimated using the Scherrer Eqn. 1 [21, 22]:

(1)$$\mathrm{D}=\frac{0.9\lambda }{\beta \cos \theta }$$

where $\lambda$ is the wavelength, $\beta$ is the full width at half maximum (FWHM) of the (002) peak expressed in radians, and $\theta$ is the Bragg angle. The lattice microstrain ($\epsilon$) was calculated following the Williamson–Hall approximation [6, 22]:

(2)$$\epsilon =\frac{\beta }{4\tan \theta }$$

The dislocation density ($\delta$), which reflects the defect concentration within the crystalline domains, was determined using the relation [22, 23]:

(3)$$\delta =\frac{1}{{\mathrm{D}}^2}$$

The derived parameters should be regarded as approximate values, as instrumental broadening effects were not explicitly subtracted. This approach is widely adopted for comparative studies aimed at elucidating relative structural changes induced by external treatments such as $\gamma$-irradiation. In addition, the XRD data were subjected to Rietveld refinement to extract detailed microstructural parameters using the MAUD software package (version 2.94, developed by University of Trento, Trento, Italy). Complementary peak analysis and phase identification were performed with X’Pert HighScore Plus software (version 4.9, Malvern Panalytical B.V., Almelo, The Netherlands) where required [9, 10].

The morphology and size of the synthesized nanostructures were examined using a high-resolution scanning electron microscope (SEM, JEOL JSM-IT200, Tokyo, Japan) operated at an accelerating voltage of 25 kV. The structural features and dimensional characteristics of bulk g-C₃N₄ and its exfoliated nanosheets were further investigated by high-resolution transmission electron microscopy (HRTEM, JEOL 3010, JEOL Ltd., Tokyo, Japan) operated at 200 kV [11]. Additional morphological analysis and uniformity assessment were performed using a ZEISS EVO-MA10 scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDX) detector (Carl Zeiss Microscopy GmbH, Oberkochen, Baden-Württemberg, Germany), enabling simultaneous evaluation of surface morphology and elemental composition. SEM images were processed using ImageJ software (version 1.53, National Institutes of Health, Bethesda, MD, USA) to determine particle size distributions based on statistical analysis of more than 100 particles per sample. Elemental composition obtained from EDX measurements was analyzed using the instrument-integrated AZtecEnergy EDX analysis software (version 3.3, Oxford Instruments NanoAnalysis Ltd., High Wycombe, Buckinghamshire, UK) [11, 12].

Raman spectra were recorded using an inVia Raman spectrometer (Renishaw plc, Gloucestershire, UK) equipped with a thermoelectrically cooled charge-coupled device (CCD) detector maintained at –80 °C. A He–Ne laser with an excitation wavelength of 532 nm was employed, with the laser power restricted to 1 mW to prevent sample heating. The integration time for each measurement was set to 100 s.

The optical absorption properties of the prepared g-C₃N₄ nanosheets were examined by ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) using a Shimadzu UV-2600 UV–Vis spectrophotometer (Shimadzu Corporation, Kyoto, Japan) equipped with an integrating sphere accessory (ISR-2600Plus). The measurements were carried out at room temperature over the wavelength range of 200–800 nm. Barium sulfate (BaSO₄) was employed as a non-absorbing reflectance standard, and all spectra were collected under identical conditions to ensure reliable comparison among samples subjected to different $\gamma$-irradiation doses. The obtained diffuse reflectance data were converted to the corresponding absorption spectra using the Kubelka–Munk function (Eqn. 4) [12, 13]:

(4)$$F(R)=\frac{{(1-R)}^2}{2R}$$

where R is the measured reflectance. The optical band gap energies were estimated by constructing Tauc plots based on the Kubelka–Munk-transformed data, assuming both direct and indirect electronic transitions for g-C₃N₄. Linear extrapolation of the absorption edge to the energy axis was used to determine the band gap values. All DRS measurements were repeated to confirm reproducibility. This characterization provides essential information on irradiation-induced changes in light absorption behavior and electronic structure of the g-C₃N₄ nanosheets.

The specific surface area and pore structure of the prepared g-C₃N₄ nanosheets were evaluated by nitrogen adsorption–desorption measurements using a Micromeritics ASAP 2020 surface area and porosity analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). Prior to analysis, all samples were degassed under vacuum at 150 °C for 12 h to remove physically adsorbed moisture and residual gases [12, 13, 14]. Nitrogen adsorption–desorption isotherms were recorded at 77 K over a relative pressure (P/P₀) range of 0.01–0.99. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method within the relative pressure range of 0.05–0.30, where a linear BET plot was obtained. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure close to unity (P/P₀ $\approx$ 0.99). Pore size distribution curves were derived from the adsorption branch of the isotherms using the Barrett–Joyner–Halenda (BJH) method, assuming cylindrical pore geometry [11, 13, 15]. All measurements were conducted under identical experimental conditions to ensure meaningful comparison among samples subjected to different $\gamma$-irradiation doses. The BET analysis provides insight into irradiation-induced changes in surface area, pore volume, and pore size distribution, which are key parameters influencing adsorption capacity and photocatalytic performance.

Photoluminescence (PL) spectroscopy was employed to investigate the electronic structure, defect-related states, and charge-carrier recombination behavior of the prepared g-C₃N₄ nanosheets before and after $\gamma$-irradiation. Steady-state PL measurements were carried out at room temperature using an Edinburgh Instruments FLS1000 fluorescence spectrometer (Edinburgh Instruments Ltd., Livingston, Scotland, UK) equipped with a continuous-wave xenon lamp as the excitation source. The excitation wavelength was fixed at 350 nm, corresponding to the intrinsic absorption of g-C₃N₄, while the emission spectra were recorded over the wavelength range of 380–650 nm [12, 16]. All spectra were collected under identical experimental conditions to ensure reliable comparison among samples subjected to different $\gamma$-irradiation doses. Time-resolved photoluminescence (TRPL) measurements were performed using the same system operated in time-correlated single-photon counting (TCSPC) mode. A pulsed diode laser with an excitation wavelength of 375 nm was employed, and the emission decay profiles were monitored at the dominant PL emission wavelength of each sample. The obtained decay curves were fitted using a multi-exponential decay model to extract the average lifetime and individual decay components, which provide insight into the relative contributions of radiative recombination and defect-assisted non-radiative processes [17, 18]. All PL and TRPL measurements were repeated to confirm reproducibility. The extracted lifetimes and emission intensities were analyzed comparatively to evaluate the influence of $\gamma$-irradiation dose on charge-carrier recombination dynamics. This PL characterization approach enables a reliable assessment of irradiation-induced defect states and their role in modulating the photophysical properties of g-C₃N₄ nanosheets.

The photocatalytic activity of the synthesized g-C₃N₄ nanosheets was evaluated through the degradation of methylene blue (MB) under visible-light irradiation. A 300 W xenon lamp equipped with a cutoff filter ($\lambda$ $>$420 nm) was employed as the light source to simulate visible solar irradiation. In a typical photocatalytic experiment, 50 mg of the photocatalyst was dispersed in 100 mL of an aqueous MB solution with an initial concentration of 10 mg L^-1 under continuous magnetic stirring to ensure homogeneous suspension.

Prior to light exposure, the reaction mixture was stirred in the dark for 30 min to establish adsorption–desorption equilibrium between MB molecules and the catalyst surface. Upon visible-light irradiation, aliquots of 3 mL were withdrawn at regular intervals of 20 min, centrifuged to remove suspended catalyst particles, and subsequently analyzed using a UV–Vis spectrophotometer. The residual concentration of MB was determined by monitoring the characteristic absorption maximum at 664 nm [19, 20, 21]. The photocatalytic degradation efficiency (D%) was calculated using the following Eqn. 5:

(5)$$D\%=\frac{C_0-C_{\text{t}}}{C_0}\times 100$$

where C₀ and C_t represent the MB concentrations at the initial time and at irradiation time t, respectively. The degradation kinetics were analyzed using a pseudo-first-order kinetic model, expressed as Eqn. 6 [16]:

(6)$$\ln \left(\frac{C_0}{C_{\text{t}}}\right)=kt$$

where k (min^-1) denotes the apparent first-order rate constant. This kinetic approach enables a quantitative comparison of the photocatalytic performances of g-C₃N₄ samples prepared under different synthesis temperatures and $\gamma$-irradiation doses, while providing a consistent framework for evaluating their relative activity, stability, and reusability under visible-light irradiation.

To gain insight into the reactive species involved in the photocatalytic degradation process, scavenger experiments were conducted under otherwise identical experimental conditions. Isopropyl alcohol (IPA, 10 mM, $\ge$99.5%, Cat. No. 278475, Sigma-Aldrich, St. Louis, MO, USA), p-benzoquinone (BQ, 1 mM, $\ge$98%, Cat. No. B10358, Sigma-Aldrich, St. Louis, MO, USA), and ethylenediaminetetraacetic acid (EDTA, 2 mM, $\ge$99%, Cat. No. E5134, Sigma-Aldrich, St. Louis, MO, USA) were employed as selective quenchers for hydroxyl radicals (•OH), superoxide radicals (•O₂^–), and photogenerated holes (h⁺), respectively. The scavengers were added to the reaction system after the adsorption–desorption equilibrium was reached and before visible-light irradiation. The resulting changes in photocatalytic degradation efficiency were analyzed to elucidate the relative contributions of the different reactive species to the MB degradation process [13, 16, 17].

Fig. 8 presents the UV–Vis DRS of g-C₃N₄ nanosheets synthesized at 550 °C and subjected to varying $\gamma$-irradiation doses (0–50 kGy). All samples exhibit a strong absorption response in the visible region, characteristic of the intrinsic electronic structure of g-C₃N₄. The absorption edge is located around the blue-green region of the spectrum, confirming the suitability of the material for visible-light-driven applications.

Fig. 8.

DRS of g-C₃N₄ nanosheets prepared at 550 °C under different $\gamma$-irradiation doses. DRS, diffuse reflectance spectroscopy.

With increasing $\gamma$-irradiation dose, a subtle but systematic shift in the absorption edge is observed, accompanied by minor variations in reflectance intensity. These changes suggest irradiation-induced modifications in the electronic structure, which may originate from defect formation, lattice distortion, or altered charge distribution within the g-C₃N₄ framework. Importantly, the overall spectral profile remains well preserved across all irradiation doses, indicating that the fundamental optical characteristics of g-C₃N₄ nanosheets are retained despite irradiation treatment. The controlled tuning of optical absorption through $\gamma$-irradiation highlights an effective strategy for tailoring the electronic and light-harvesting properties of g-C₃N₄ nanosheets without compromising their structural integrity, which is advantageous for photocatalytic and optoelectronic applications.

The optical band structure of $\gamma$-irradiated g-C₃N₄ nanosheets was evaluated using the Kubelka–Munk function derived from the UV–Vis DRS. The reflectance data were transformed according to the Kubelka–Munk formalism, enabling a reliable assessment of the absorption behavior and band gap evolution of the material. As shown in Fig. 9, all samples exhibit a well-defined absorption edge in the visible region, confirming the semiconducting nature of g-C₃N₄ nanosheets.

Fig. 9.

Effect of $\gamma$-irradiation on the Kubelka–Munk function of g-C₃N₄ nanosheets.

With increasing $\gamma$-irradiation dose, a gradual shift of the absorption edge toward longer wavelengths is observed, accompanied by subtle variations in reflectance intensity. This behavior indicates irradiation-induced modification of the electronic structure, which can be reasonably attributed to the formation of localized defect states and lattice perturbations within the g-C₃N₄ framework. Importantly, the overall spectral profile remains largely unchanged across the investigated dose range, suggesting that $\gamma$-irradiation induces controlled electronic tuning without causing significant structural degradation. Such moderate band structure modulation is advantageous for enhancing visible-light absorption and improving charge carrier utilization, which are critical factors governing the photocatalytic performance of g-C₃N₄-based materials. The optical band structure of $\gamma$-irradiated g-C₃N₄ nanosheets was further evaluated using the Kubelka–Munk function derived from UV–Vis DRS. The reflectance data were converted to the corresponding absorption coefficient using the Kubelka–Munk equation (Eqn. 4), which allows reliable estimation of the optical band gap for semiconducting materials [20, 30].

The UV–Vis absorption spectra of $\gamma$-irradiated g-C₃N₄ nanosheets, derived from diffuse reflectance measurements, are presented in Fig. 10. All samples exhibit strong absorption in the near-UV and visible regions, with a well-defined absorption edge characteristic of the semiconducting nature of g-C₃N₄. As the $\gamma$-irradiation dose increases, a gradual red shift of the absorption edge is observed, together with slight changes in absorbance intensity.

Fig. 10.

UV–Vis absorption spectra of g-C₃N₄ nanosheets prepared at 550 °C under different $\gamma$-irradiation doses. UV–Vis, ultraviolet–visible.

These spectral variations indicate irradiation-induced modification of the electronic structure, which can be attributed to the formation of localized defect states and subtle lattice distortions within the g-C₃N₄ framework. Importantly, the overall absorption profile remains largely preserved across the investigated dose range, suggesting that $\gamma$-irradiation enables controlled tuning of the optical properties without causing significant structural damage. Such moderate enhancement of visible-light absorption is beneficial for improving photoexcited charge generation and is expected to contribute positively to the photocatalytic performance of g-C₃N₄ nanosheets [15, 31].

The optical band gap of g-C₃N₄ nanosheets synthesized at 550 °C was evaluated using Tauc plots derived from DRS, as shown in Fig. 11, for samples subjected to $\gamma$-irradiation doses ranging from 0 to 50 kGy. The well-defined linear regions of the Tauc plots indicate that the optical absorption is governed predominantly by direct electronic transitions, from which band gap energies in the range of approximately 3.0–3.2 eV were determined. The pristine g-C₃N₄ nanosheets exhibit a band gap of about 3.0 eV, whereas a gradual increase to nearly 3.2 eV is observed with increasing $\gamma$-irradiation dose, evidencing a subtle blue shift in the absorption edge.

Fig. 11.

Tauc plot analysis for direct band gap determination of $\gamma$-irradiated g-C₃N₄ nanosheets.

This slight widening of the band gap can be ascribed to irradiation-induced structural distortions and the formation of defect states, which alter the local electronic environment and influence optical transition energies. Importantly, despite these irradiation-induced modifications, the intrinsic semiconducting framework of g-C₃N₄ remains largely preserved, highlighting its structural and electronic robustness under high-dose $\gamma$-irradiation. The controlled modulation of the band gap via $\gamma$-irradiation thus represents a viable strategy for fine-tuning the optical properties of g-C₃N₄ nanosheets for advanced photocatalytic and optoelectronic applications [28].

Fig. 12 presents the Tauc plots used to evaluate the indirect optical band gap of g-C₃N₄ nanosheets synthesized at 550 °C and exposed to $\gamma$-irradiation doses ranging from 0 to 50 kGy. The indirect band gap energies were determined by extrapolating the linear regions of the ($\alpha$h$\nu$)^1/2 versus photon energy plots. The pristine g-C₃N₄ nanosheets exhibit an indirect band gap of approximately 2.45 eV, while the irradiated samples show a gradual increase to values in the range of 2.4–2.7 eV with increasing irradiation dose. These modest upward shifts in band gap energy suggest that $\gamma$-irradiation induces localized structural distortions and defect states that subtly influence the electronic transition pathways without significantly altering the intrinsic band structure of the material. Importantly, the consistent position of the absorption edge across all irradiation doses confirms the robust semiconducting nature and structural stability of g-C₃N₄ under high-dose $\gamma$-irradiation. The observed fine-tuning of the indirect band gap highlights $\gamma$-irradiation as an effective and controllable strategy for tailoring the optical and electronic properties of g-C₃N₄ nanosheets, which is particularly beneficial for photocatalytic and energy conversion applications where optimized band alignment is critical for performance [29].

Fig. 12.

Indirect band gap estimation of g-C₃N₄ nanosheets at 550 °C derived from Tauc plot analysis under varying $\gamma$-irradiation doses.

The variation in the BET specific surface area of g-C₃N₄ nanosheets synthesized at 550 °C as a function of $\gamma$-irradiation dose is presented in Fig. 13. The pristine sample exhibits a moderate surface area characteristic of layered g-C₃N₄ materials. Upon $\gamma$-irradiation, a gradual increase in surface area is observed with increasing dose, reaching a maximum at intermediate irradiation levels before showing a tendency toward saturation at higher doses. This behavior can be attributed to irradiation-induced exfoliation and the generation of microstructural defects, which promote partial delamination of stacked nanosheets and increase the availability of accessible surface sites [30, 31]. At higher irradiation doses, the stabilization of surface area suggests a balance between exfoliation and defect recombination or partial structural relaxation. Importantly, no abrupt loss in surface area is detected across the investigated dose range, indicating that the overall framework of g-C₃N₄ remains structurally robust under $\gamma$-irradiation. The irradiation-driven enhancement in surface area is expected to be beneficial for photocatalytic applications, as increased surface exposure facilitates improved adsorption of reactant species and more efficient utilization of active sites during photocatalytic reactions.

Fig. 13.

BET surface area of g-C₃N₄ nanosheets as a function of $\gamma$-irradiation dose. BET, Brunauer–Emmett–Teller.

Fig. 14 illustrates the variation in total pore volume of g-C₃N₄ nanosheets synthesized at 550 °C as a function of $\gamma$-irradiation dose. The pristine sample exhibits a relatively low pore volume, consistent with the dense stacking typically observed in bulk g-C₃N₄. Upon $\gamma$-irradiation, a gradual increase in total pore volume is observed with increasing dose, indicating irradiation-induced microstructural modification. This enhancement is primarily attributed to partial exfoliation, defect formation, and the creation of interlayer voids arising from the disruption of van der Waals interactions between adjacent nanosheets. At higher irradiation doses, the pore volume tends to stabilize, suggesting that a dynamic equilibrium is reached between defect generation and structural relaxation. Importantly, no abrupt collapse or degradation of the porous framework is detected, confirming the structural resilience of g-C₃N₄ under $\gamma$-irradiation. The observed increase in pore volume, together with the enhanced surface area, is expected to facilitate improved mass transport and greater accessibility of active sites, which are critical factors for enhancing photocatalytic efficiency and reaction kinetics [29, 30].

Fig. 14.

Total pore volume of g-C₃N₄ nanosheets as a function of $\gamma$-irradiation dose.

Fig. 15 presents the variation in average pore diameter of g-C₃N₄ nanosheets synthesized at 550 °C as a function of $\gamma$-irradiation dose. The pristine sample exhibits a relatively small average pore diameter, reflecting the compact stacking and limited interlayer spacing characteristic of non-irradiated g-C₃N₄. With increasing $\gamma$-irradiation dose, a gradual enlargement of the average pore diameter is observed, indicating progressive modification of the porous architecture. This behavior can be attributed to irradiation-induced exfoliation and defect formation, which promote partial delamination of the nanosheets and the expansion of interlayer voids. At higher doses, the increase in pore diameter becomes less pronounced, suggesting the establishment of a structural balance between defect generation and framework stabilization [31]. Importantly, the absence of abrupt pore collapse or excessive pore coalescence confirms the robustness of the g-C₃N₄ framework under $\gamma$-irradiation. The moderate enlargement of pore diameter, combined with enhanced surface area and pore volume, is expected to improve reactant diffusion and active-site accessibility, thereby favorably influencing photocatalytic reaction kinetics and overall performance.

Fig. 15.

Average pore diameter of g-C₃N₄ nanosheets as a function of $\gamma$-irradiation dose.

The PL spectra of g-C₃N₄ nanosheets exhibit a strong dependence on $\gamma$-irradiation dose, as illustrated in Fig. 16. The pristine sample (0 kGy) shows a characteristic emission band centered at approximately 455 nm, which gradually intensifies with increasing irradiation dose and reaches a maximum at around 20–25 kGy. This enhancement is mainly attributed to the formation of irradiation-induced shallow defect states, such as nitrogen vacancies and localized lattice distortions, which act as temporary charge-carrier trapping centers.

Fig. 16.

Normalized PL spectra of g-C₃N₄ nanosheets at varying gamma irradiation doses. PL, Photoluminescence.

These shallow traps promote exciton localization and suppress rapid non-radiative recombination, thereby increasing the probability of radiative transitions. When the $\gamma$-irradiation dose exceeds 25 kGy, a pronounced decrease in PL intensity is observed. This behavior indicates the onset of defect saturation and the generation of deep trap states that function as efficient non-radiative recombination centers [32, 33, 34]. Excessive irradiation may also induce structural disorder within the conjugated g-C₃N₄ framework, further accelerating carrier quenching and reducing emission efficiency. Overall, these results demonstrate that controlled $\gamma$-irradiation provides an effective means to tailor the electronic structure and excitonic recombination behavior of g-C₃N₄ nanosheets, where an optimal balance between beneficial shallow defect formation and structural preservation is essential for maximizing optical performance.

Fig. 17 presents the variation in the average PL lifetime of g-C₃N₄ nanosheets synthesized at 550 °C as a function of $\gamma$-irradiation dose. The lifetime displays a clear non-monotonic behavior, initially increasing at low irradiation doses and reaching a pronounced maximum of approximately 4.6 ns at 25 kGy. This enhancement indicates that moderate $\gamma$-irradiation introduces a suitable density of shallow defect states, which act as temporary charge-carrier traps and effectively suppress fast non-radiative recombination, thereby prolonging carrier lifetime.

Fig. 17.

Variation of average lifetime of g-C₃N₄ nanosheets as a function of gamma irradiation dose.

In contrast, further increasing the irradiation dose beyond 25 kGy results in a noticeable reduction in the average PL lifetime. This decrease is associated with defect saturation and irradiation-induced structural disorder, leading to the formation of deep trap states that facilitate rapid non-radiative recombination. These observations highlight that controlled $\gamma$-irradiation plays a crucial role in optimizing charge-carrier dynamics in g-C₃N₄ nanosheets, whereas excessive irradiation adversely affects their optical performance and recombination behavior [32, 33].

Fig. 18 illustrates the variation in PL peak intensity of g-C₃N₄ nanosheets synthesized at 550 °C as a function of $\gamma$-irradiation dose. The PL intensity initially increases with increasing dose and reaches a maximum at approximately 15–20 kGy. This improvement arises from irradiation-induced shallow defect states, such as nitrogen vacancies and localized lattice distortions, which enhance exciton localization and favor radiative recombination by temporarily trapping charge carriers and suppressing rapid non-radiative processes. In this dose range, defect generation remains well controlled and does not significantly disrupt the conjugated heptazine framework, resulting in improved electronic interactions and enhanced emission intensity. With further increase in $\gamma$-irradiation dose beyond 20 kGy, the PL intensity progressively decreases. This decline indicates the onset of defect saturation and the formation of deep trap states that act as efficient non-radiative recombination centers. Excessive irradiation may also induce structural distortions within the g-C₃N₄ network, accelerating carrier quenching and reducing luminescence efficiency. Overall, the observed dose-dependent PL behavior reflects a critical balance between beneficial defect generation and irradiation-induced structural damage, demonstrating that moderate $\gamma$-irradiation optimizes the optical performance of g-C₃N₄ nanosheets, whereas higher doses lead to deterioration of their luminescence properties.

Fig. 18.

Variation of PL peak intensity of g-C₃N₄ nanosheets with gamma irradiation dose.

Fig. 19 presents the TRPL decay profiles of g-C₃N₄ nanosheets synthesized at 550 °C and subjected to different $\gamma$-irradiation doses [33]. All decay curves display a rapid initial drop in PL intensity followed by a slower decay tail, which is characteristic of multi-exponential recombination behavior involving both radiative and non-radiative processes. This behavior reflects the coexistence of free exciton recombination and defect-assisted carrier trapping within the g-C₃N₄ framework. At low to moderate irradiation doses (5–25 kGy), the decay profiles become slightly prolonged, indicating suppression of fast non-radiative recombination pathways. This effect is attributed to the formation of irradiation-induced shallow defect states that temporarily trap charge carriers, thereby extending exciton lifetime and enhancing radiative recombination probability. In contrast, at higher irradiation doses ($\ge$30 kGy), the decay curves converge toward shorter lifetimes, suggesting the emergence of deep-level trap states and increased structural disorder. These deep traps act as efficient non-radiative recombination centers, accelerating carrier quenching and diminishing luminescence efficiency. Overall, the TRPL results confirm that moderate $\gamma$-irradiation effectively optimizes charge carrier dynamics in g-C₃N₄ nanosheets, whereas excessive irradiation induces unfavorable defect states and structural distortions that compromise recombination behavior and optical performance.

Fig. 19.

Normalized PL decay curves of g-C₃N₄ nanosheets at different gamma irradiation doses.

Fig. 20 shows the dependence of the longer decay component ($\tau$₂) of g-C₃N₄ nanosheets synthesized at 550 °C on $\gamma$-irradiation dose. The $\tau$₂ lifetime increases gradually from approximately 3.5 ns for the pristine sample to a maximum value of about 4.4 ns at 25–30 kGy. This prolongation indicates effective suppression of non-radiative recombination pathways and enhanced stabilization of photogenerated charge carriers, which can be attributed to the controlled formation of irradiation-induced shallow defect states that temporarily trap carriers and delay recombination [34]. When the irradiation dose exceeds 30 kGy, the $\tau$₂ lifetime decreases progressively. This reduction reflects the accumulation of irradiation-induced structural disorder and the emergence of deep-level trap states that act as efficient non-radiative recombination centers, thereby accelerating carrier recombination. These results clearly demonstrate the dual role of $\gamma$-irradiation in tailoring the photophysical properties of g-C₃N₄ nanosheets and hence moderate irradiation improves exciton lifetime and charge carrier dynamics, while excessive exposure leads to deterioration of optical performance due to defect saturation and structural damage.

Fig. 20.

Variation of $\tau$₂ model lifetime of g-C₃N₄ nanosheets with gamma irradiation dose.

Fig. 21 illustrates the evolution of the PL emission peak of g-C₃N₄ nanosheets synthesized at 550 °C as a function of $\gamma$-irradiation dose. The pristine sample exhibits an emission maximum at approximately 469 nm, whereas the irradiated nanosheets show a pronounced blue-shift to about 456 nm at an irradiation dose of 30 kGy. This blue-shift originates from quantum confinement effects induced by $\gamma$-irradiation, arising from nanosheet thinning and localized lattice distortions, which modify the spatial confinement of charge carriers. In addition, irradiation reduces the effective conjugation length and alters defect-related energy levels through the formation of shallow defect states, collectively leading to a widening of the effective bandgap and emission at shorter wavelengths. At higher irradiation doses, a slight red-shift is observed at 40 kGy, followed by a minor blue-shift at 50 kGy, indicating the coexistence of competing effects related to partial defect passivation and increasing structural disorder. Overall, these systematic variations in emission wavelength demonstrate the strong sensitivity of the electronic structure of g-C₃N₄ nanosheets to $\gamma$-irradiation and confirm that controlled radiation exposure provides an effective approach for tuning their optical properties [35].

Fig. 21.

Variation of PL peak wavelength of g-C₃N₄ nanosheets with gamma irradiation dose.

The influence of $\gamma$-irradiation on the radiative recombination behavior of g-C₃N₄ nanosheets was further evaluated through PL quantum yield analysis (Fig. 22). The pristine sample exhibits the highest PL quantum yield, approximately 0.32, indicating a relatively high probability of radiative electron–hole recombination. Upon $\gamma$-irradiation, the PL quantum yield decreases progressively with increasing dose, declining to about 0.29–0.26 at intermediate doses and reaching a minimum value of approximately 0.18 at 50 kGy [35, 36]. This monotonic reduction signifies an effective suppression of radiative recombination pathways, which can be attributed to irradiation-induced structural or electronic modifications, such as the formation of defect states or trap sites that favor non-radiative charge transfer. At higher doses, the pronounced decrease in PL quantum yield implies substantial inhibition of electron–hole recombination, a feature generally considered beneficial for photocatalytic performance. It should be emphasized that the reported PL quantum yield values are discussed in a comparative sense to illustrate irradiation-induced trends rather than absolute quantum efficiencies. Nonetheless, the clear dose-dependent behavior demonstrates that controlled $\gamma$-irradiation provides an effective strategy for tuning charge carrier dynamics in g-C₃N₄ nanosheets.

Fig. 22.

Variation of PL quantum yield of g-C₃N₄ nanosheets prepared at 550 °C as a function of $\gamma$-irradiation dose.

Fig. 23 illustrates the effect of $\gamma$-irradiation on the relative PL efficiency and PL suppression behavior of g-C₃N₄ nanosheets prepared at 550 °C. The relative PL efficiency, normalized to the pristine sample, decreases gradually with increasing irradiation dose, indicating a progressive reduction in radiative electron–hole recombination. This trend is accompanied by a corresponding increase in PL suppression, which becomes particularly pronounced at higher doses. Such behavior suggests that $\gamma$-irradiation induces modifications in the electronic structure of g-C₃N₄ that favor charge carrier separation, likely through the introduction of defect states or trapping centers that facilitate non-radiative pathways. Importantly, these results are discussed comparatively, reflecting relative changes in emission behavior rather than absolute quantum efficiencies [37]. Nevertheless, the inverse relationship between PL efficiency and PL suppression provides clear evidence that controlled irradiation effectively modulates charge carrier recombination dynamics, a feature that is generally advantageous for enhancing photocatalytic performance.

Fig. 23.

Relative PL efficiency and PL suppression of g-C₃N₄ nanosheets prepared at 550 °C as a function of $\gamma$-irradiation dose.

Fig. 24 presents the time-dependent evolution of the UV–Vis absorption spectra of MB during its photocatalytic degradation in the presence of g-C₃N₄ nanosheets synthesized at 550 °C under non-irradiated conditions. The initial MB solution displays a strong absorption band centered at approximately 664 nm, along with a weaker shoulder around 610 nm, both of which are characteristic of the electronic transitions of MB molecules. Upon visible-light irradiation, a progressive decrease in the intensity of these absorption features is observed as the reaction time increases from 0 to 120 minutes, indicating the gradual degradation of the dye. The continuous attenuation of absorbance reflects the effective photocatalytic activity of the g-C₃N₄ nanosheets. During the process, photogenerated charge carriers participate in successive oxidation and reduction reactions, leading to the breakdown of MB into smaller intermediate species and, ultimately, to mineralized products. These results demonstrate the intrinsic photocatalytic capability of non-irradiated g-C₃N₄ nanosheets and provide a reliable reference point for evaluating the performance enhancement achieved through gamma irradiation [36, 37].

Fig. 24.

UV–Vis absorption spectra of MB solution recorded at different irradiation times in the presence of g-C₃N₄ nanosheets prepared at 550 °C under different $\gamma$-irradiation doses. MB, methylene blue.

Fig. 25 illustrates the UV–Vis absorption spectra of the MB solution after 120 min of photocatalytic treatment over g-C₃N₄ nanosheets synthesized at 550 °C and exposed to different $\gamma$-irradiation doses. The pristine MB solution exhibits its characteristic main absorption band centered at approximately 664 nm, accompanied by a weaker shoulder around 610 nm, which are typical signatures of the chromophoric structure of MB [36]. Upon photocatalytic treatment in the presence of g-C₃N₄, a progressive attenuation of these absorption features is observed, with the extent of intensity reduction strongly dependent on the applied irradiation dose. In particular, nanosheets subjected to moderate $\gamma$-irradiation doses in the range of 10–25 kGy show the most pronounced decrease in MB absorbance, indicating enhanced photocatalytic degradation efficiency. This improvement can be reasonably attributed to irradiation-induced defect states that facilitate charge separation and increase the availability of active sites without severely disrupting the material framework. In contrast, samples exposed to higher irradiation doses ($\ge$40 kGy) retain relatively stronger absorption bands after irradiation, suggesting a decline in degradation efficiency, which is likely associated with defect saturation and partial structural deterioration of the nanosheets. Overall, these results demonstrate that controlled $\gamma$-irradiation plays a beneficial role in promoting the photocatalytic degradation of MB, whereas excessive irradiation adversely affects performance.

Fig. 25.

UV–Vis absorption spectra of MB degradation over g-C₃N₄ nanosheets (550 °C) under different $\gamma$-Irradiation at 120 min.

Fig. 26 depicts the UV–Vis absorption spectra of the MB solution after 120 min of photocatalytic treatment using g-C₃N₄ nanosheets synthesized at different preparation temperatures in the absence of $\gamma$-irradiation. The residual MB solution exhibits a characteristic absorption band centered at approximately 664 nm, accompanied by a weaker shoulder at lower wavelengths, reflecting incomplete dye degradation under suboptimal catalytic conditions. A clear dependence on synthesis temperature is evident from the variation in absorbance intensity [22, 37]. Nanosheets prepared at 550 °C display the lowest residual absorbance, indicating the highest photocatalytic degradation efficiency among the examined samples. Samples synthesized at 600 °C also show relatively strong degradation performance, albeit slightly inferior to that of the 550 °C sample. In contrast, materials prepared at lower temperatures (400–450 °C) retain higher absorbance intensities, suggesting reduced photocatalytic activity, which can be reasonably attributed to incomplete condensation and limited structural development of the g-C₃N₄ framework. Meanwhile, the sample synthesized at 650 °C exhibits diminished degradation efficiency, likely resulting from partial structural deterioration and loss of active sites at excessive calcination temperatures. Overall, these results demonstrate that synthesis temperature plays a critical role in determining the photocatalytic performance of g-C₃N₄ nanosheets, with an optimal preparation window centered around 550 °C under non-irradiated conditions.

Fig. 26.

UV–Vis absorption spectra of MB degradation over g-C₃N₄ nanosheets synthesized at different temperatures without $\gamma$-irradiation after 120 min.

Fig. 27 displays the UV–Vis absorption spectra of the MB solution after 120 min of photocatalytic treatment over g-C₃N₄ nanosheets synthesized at different temperatures and subjected to $\gamma$-irradiation at a dose of 20 kGy. For all samples, the characteristic MB absorption band centered near 664 nm is markedly attenuated compared with the non-irradiated condition, confirming that $\gamma$-irradiation enhances the overall photocatalytic degradation process. A pronounced dependence on synthesis temperature is clearly observed. In particular, nanosheets prepared at 550 °C exhibit the lowest residual absorbance intensity, indicating the most efficient MB degradation under irradiated conditions [37, 38]. Samples synthesized at 600 °C also demonstrate relatively high photocatalytic activity, although slightly inferior to that of the 550 °C sample. By contrast, materials prepared at lower temperatures (400–450 °C) retain higher absorbance levels, reflecting weaker degradation efficiency, which can be attributed to incomplete polymerization and suboptimal structural development of the g-C₃N₄ framework. Similarly, the sample synthesized at 650 °C shows reduced performance, likely due to partial structural degradation and excessive defect formation induced at elevated calcination temperatures. Overall, these results demonstrate that, under moderate $\gamma$-irradiation, synthesis temperature plays a decisive role in governing photocatalytic efficiency, with an optimal preparation window centered around 550–600 °C [38].

Fig. 27.

UV–Vis absorption spectra of MB degradation over g-C₃N₄ nanosheets synthesized at different temperatures after 120 min and under 20 kGy $\gamma$- irradiation.

Fig. 28 illustrates the photocatalytic degradation kinetics of MB over g-C₃N₄ nanosheets synthesized at 550 °C and subjected to different $\gamma$-irradiation doses under visible-light illumination. For all investigated samples, the normalized concentration ratio (C_t/C₀) decreases progressively with irradiation time, confirming continuous and effective photocatalytic degradation of MB throughout the reaction period. A pronounced dependence on $\gamma$-irradiation dose is evident. Nanosheets treated with moderate irradiation doses (10–25 kGy) exhibit a more rapid decline in C_t/C₀, indicating enhanced degradation kinetics and improved photocatalytic efficiency. This behavior can be reasonably attributed to the introduction of irradiation-induced defect states that facilitate charge carrier separation and migration while preserving the structural integrity of the g-C₃N₄ framework. In contrast, samples exposed to higher irradiation doses ($\ge$40 kGy) show a comparatively slight decrease in C_t/C₀, suggesting suppressed photocatalytic activity, which is likely associated with excessive defect formation and the emergence of charge-recombination centers. The non-irradiated sample displays intermediate kinetic behavior, highlighting the beneficial yet dose-dependent role of $\gamma$-irradiation. Overall, these results demonstrate that controlled $\gamma$-irradiation at moderate doses effectively optimizes the photocatalytic degradation kinetics of MB, whereas excessive irradiation adversely affects the activity of g-C₃N₄ nanosheets [39].

Fig. 28.

Degradation profiles of MB (C_𝐭/C₀) over g-C₃N₄ nanosheets synthesized at 550 °C under varying $\gamma$-irradiation.

Fig. 29 illustrates the photocatalytic degradation efficiency of MB as a function of irradiation time over g-C₃N₄ nanosheets synthesized at 550 °C and exposed to different $\gamma$-irradiation doses under visible-light illumination. For all samples, the degradation efficiency increases steadily with irradiation time, confirming the continuous and effective photocatalytic removal of MB during the reaction process. A pronounced dependence on $\gamma$-irradiation dose is observed. Nanosheets treated with moderate irradiation doses (10–25 kGy) exhibit the highest degradation efficiencies throughout the reaction period, reaching markedly higher removal percentages after 120 min compared with the pristine sample. This enhancement can be reasonably attributed to irradiation-induced defect states that improve charge carrier separation and utilization while maintaining the structural integrity of the g-C₃N₄ framework. In contrast, samples exposed to higher irradiation doses ($\ge$40 kGy) display noticeably lower degradation efficiencies, indicating that excessive defect formation and partial structural disruption adversely affect photocatalytic performance [38]. The non-irradiated sample shows intermediate behavior, highlighting the beneficial yet dose-dependent influence of $\gamma$-irradiation. Overall, these results demonstrate that controlled $\gamma$-irradiation at moderate doses significantly enhances the photocatalytic degradation efficiency of MB, whereas excessive irradiation diminishes the activity of g-C₃N₄ nanosheets.

Fig. 29.

Degradation efficiency of MB over g-C₃N₄ nanosheets under different gamma irradiation doses.

To clarify the contribution of the main reactive species involved in the photocatalytic degradation of MB, scavenger experiments were carried out under visible-light irradiation, and the results are summarized in Fig. 30. In the absence of scavengers, the $\gamma$-irradiated g-C₃N₄ nanosheets exhibit a clear dependence of photocatalytic performance on irradiation dose, with the degradation efficiency increasing up to an intermediate dose and subsequently declining at higher doses. This behavior suggests that $\gamma$-irradiation modifies the photocatalytic activity without fundamentally altering the reaction pathway. Upon the addition of p-benzoquinone (BQ), a pronounced suppression of degradation efficiency is observed across the entire irradiation dose range, indicating that superoxide radicals (•O₂^–) play a dominant role in the photocatalytic degradation process. The introduction of PA also leads to a noticeable decrease in photocatalytic activity, though less severe than that induced by BQ, implying that hydroxyl radicals (•OH) contribute as secondary oxidative species. In contrast, the presence of EDTA results in a comparatively moderate reduction in degradation efficiency, suggesting a less dominant but still measurable involvement of photogenerated holes (h⁺).

Fig. 30.

Effect of reactive species scavengers on the photocatalytic degradation of MB over $\gamma$-irradiated g-C₃N₄ nanosheets.

Importantly, the relative suppression trends induced by the different scavengers remain consistent across all $\gamma$-irradiation doses, indicating that $\gamma$-irradiation primarily influences charge separation efficiency and surface reactivity rather than introducing a new photocatalytic mechanism. The predominance of •O₂^– radicals is consistent with the electronic structure of g-C₃N₄, whose conduction band position favors the reduction of dissolved oxygen under visible-light excitation. Overall, the scavenger results provide experimental support for the proposed photocatalytic pathway and demonstrate that $\gamma$-irradiation acts as an effective strategy to tune the photocatalytic efficiency of g-C₃N₄ nanosheets without altering the fundamental degradation mechanism.

Fig. 31 presents the variation of the apparent pseudo-first-order rate constant (k_app) for MB degradation over $\gamma$-irradiated g-C₃N₄ nanosheets as a function of successive photocatalytic cycles. For all samples, a gradual decline in the rate constant is observed with increasing cycle number, indicating partial deactivation of the photocatalyst during repeated use. Nevertheless, nanosheets exposed to moderate $\gamma$-irradiation doses (10–25 kGy) consistently exhibit higher rate constants throughout the cycling tests compared with the pristine material, confirming their superior and more stable photocatalytic activity [39, 40]. The enhanced performance at these doses can be attributed to an optimized defect structure that promotes efficient charge carrier separation while preserving the integrity of the g-C₃N₄ framework. In contrast, samples irradiated at higher doses ($\ge$40 kGy) display significantly lower rate constants and a more pronounced decline upon reuse, suggesting that excessive irradiation induces structural disorder and non-productive recombination centers. Despite the observable decrease in activity over repeated cycles, the overall trends demonstrate that controlled $\gamma$-irradiation at appropriate doses improves both the photocatalytic efficiency and the reusability of g-C₃N₄ nanosheets, whereas over-irradiation is detrimental to long-term performance [41].

Fig. 31.

Apparent rate constants of MB degradation over g-C₃N₄ nanosheets at different $\gamma$-irradiation doses across successive cycles.

The cycling stability of the prepared g-C₃N₄ nanosheets was systematically assessed by tracking the retention of the apparent first-order rate constant (k_n/k₁) over successive photocatalytic runs, as illustrated in Fig. 32. Across all samples, a slight but progressive reduction in k_n/k₁ is observed with increasing cycle number, which can reasonably be attributed to minor catalyst loss during recovery, partial surface coverage by residual intermediates, and gradual attenuation of accessible active sites. Importantly, the overall decrease remains limited, indicating that the structural integrity and intrinsic photocatalytic functionality of g-C₃N₄ are largely maintained throughout repeated use. In comparison with the pristine material, the $\gamma$-irradiated samples—particularly those treated at moderate doses—exhibit noticeably higher retention of the apparent rate constant, reflecting improved resistance to deactivation under cyclic operation. This enhanced stability is plausibly linked to irradiation-induced modifications of the electronic structure and surface states, which promote more robust charge separation while mitigating irreversible surface deterioration. Collectively, these findings are regarding catalyst durability and confirm that controlled $\gamma$-irradiation offers a viable strategy for improving both the activity and long-term reusability of g-C₃N₄-based photocatalysts [41].

Fig. 32.

Retention of apparent first-order rate constant (k_𝐧/k₁) of $\gamma$-irradiated g-C₃N₄ nanosheets over successive photocatalytic cycles for MB degradation.