Photodegradation of Wastewater Containing Organic Dyes Using Modified G-C 3 N 4 -Doped ZrO 2 Nanostructures: Towards Safe Water for Human Beings

: Historically, the photocatalytic efficacy of graphitic carbon nitride (g-C 3 N 4 ) has been constrained by a rapid charge recombination rate and restricted sensitivity to visible light. To overcome these limitations and enhance the performance of g-C 3 N 4 , the strategic formation of heterojunctions with semiconductor materials is deemed the optimal approach. The present study employed a facile sonication-assisted pyrolysis method to synthesize a g-C 3 N 4 @ZrO 2 nanocomposite photocatalyst. This hybrid material was characterized extensively using a comprehensive suite of analytical techniques, including XRD, SEM, EDX, FTIR, and UV-Vis DRS. A comparative analysis of photocatalytic applications under identical conditions was conducted for all synthesized materials, wherein they were subjected to UVc light irradiation. The photocatalytic degradation of various dye models, such as MB, EY, and a combination of dyes, was assessed using the prepared nanocomposites. The g-C 3 N4@ZrO 2 photocatalysts showcased superior photocatalytic performance, with a particular variant, g-CNZ 6 , exhibiting remarkable activity. With a bandgap energy of 2.57 eV, g-CNZ 6 achieved impressive degradation efficiencies of 96.5% for MB and 95.6% for EY within 40 min. Following previous studies, the superoxide radical anions (O 2 − . and h + ) were largely accountable for the degradation of MB. Therefore, the observed efficacy of the g-C 3 N4@ZrO 2 nanocomposite photocatalyst can be attributed to the increased generation of these reactive species.


Introduction
Human history has been marked by an escalating stress on natural resources, with the past century seeing the most acute increase, principally due to the dramatic surge in the population and the significant demand to feed and sustain such a populace [1].Numerous global challenges have come to the fore, including mounting demands for, and the scarcity of, freshwater sources, a crisis precipitated by rampant industrialization, a burgeoning population, and relentless aridity [2].Imperative to environmental preservation is removing hazardous dyes from the wastewater discharge of the industries of textiles and pharmaceuticals [3].morphology and crystal structure of the g-C 3 N 4 @ZrO 2 nanocomposite and how this subsequently affected the activity and selectivity of the photodegradation of MB, EY, and mixed dye under UV light illuminations.

XRD Analysis
X-ray diffraction (XRD) analysis is a crucial technique in inspecting the heterojunctions and phase structures of pristine materials.Illustrated in Figure 1 are the XRD spectra obtained from the g-CN nanosheet samples.This layer contains an aromatic component, sp2, displaying a hybridized three-s-triazine structure [41].Concurrently, the (002) planes of the graphitic carbon nitride phase of the hexagonal crystal for [JCPDS 87-1526] resulted in two diffraction peaks, specifically at 12.8 • and 27.3 • for g-CN [42].

Morphological Analysis
The scanning electron microscopy (SEM) analysis was meticulously conducted on the g-CN and g-CNZ nanocomposite samples to discern the surface topography and underlying microstructure.Figure 2a distinctly illustrates the lamellar framework and multiplex aggregation phenomena characteristic of pure g-CN [47].The strikingly divergent morphologies of the two materials are readily distinguishable under the lens of the scanning electron microscope, facilitating differentiation.Accumulated research has postulated that this unique structure could potentially provide a wealth of active sites, augment the migration of the resultant carriers, facilitate the optimal catalyst selection process, and stimulate the evolution of a photocatalyst heterojunction [48].As revealed in Figure 2b-h, the irregular mass is an assembly of ZrO2 enveloped by a layer of flaky g-C3N4, signifying that the two constituents are meticulously compounded.The particulate dimensions fall within a range of 100 to 260 nm.The distinctive morphologies of the ZrO2 and g-CN enable the straightforward identification of the two semiconductors in the SEM images.It is observable within the composites that the ZrO2 nanoparticles are enrobed on the g-CN sur- Notably, the (002) diffraction peak appears relatively weaker for the synthesized g-CN nanosheet and particle.This observation considers the surface area ratio between two distinct peaks and suggests a decrease in the layered structural elements.The diffraction peak of the (002) plane in the g-CN nanosheet undergoes a slight shift from 27.3 • to 28.1 • , signifying the thermal exfoliation of the g-CN [43].
To determine the average crystallite size, lattice strain, and dislocation density of the samples, Debye-Scherrer's formula was applied [46]: where D is the size of the crystallite, β is the full width at half maximum (FWHM), λ is the wavelength, and lattice strain (ε) was determined [46]: while dislocation density (δ) was obtained through the following: Table 1 lists the average crystallite size values derived, the lattice strain determined from the XRD spectra, and the dislocation for all the manufactured samples.Additionally, the FWHM is β, and the diffraction peak angle is θ, respectively.The lattice strain, dislocation, and average crystallite size values computed using the XRD spectra for the first phase was determined to be between 10.011 and 17.029 nm, whereas the average crystallite size for the ZrO 2 phase was between 7.422 and 33.167 nm.It should be observed that the size of the crystallites tends to grow as the strain decreases.Equation ( 4) demonstrates that the lattice strain value relies on both β and cosθ, and are given in Table 1, together with the mean values of the lattice strain for each phase obtained.The range of the g-C 3 N 4 phase's lattice strain means was 2.683 × 10 −3 to 3.456 × 10 −3 .At the same time, for the ZrO 2 phase, this varied from 1.84 × 10 −3 to 4.670 × 10 −4 .As stated in Equation ( 5), the dislocation density (δ) can also be used to determine crystallinity.The g-C 3 N 4 phase varied between 6.511 and 10.615 × 10 −3 and between 1.045 and 8.816 × 10 −3 for the ZrO 2 phase.It was indicating that linked ZrO 2 levels were rising.The XRD data generally supported the creation of the gC 3 N 4 @ZrO 2 heterojunction photocatalyst using the one-pot synthesis method.

Morphological Analysis
The scanning electron microscopy (SEM) analysis was meticulously conducted on the g-CN and g-CNZ nanocomposite samples to discern the surface topography and underlying microstructure.Figure 2a distinctly illustrates the lamellar framework and multiplex aggregation phenomena characteristic of pure g-CN [47].The strikingly divergent morphologies of the two materials are readily distinguishable under the lens of the scanning electron microscope, facilitating differentiation.Accumulated research has postulated that this unique structure could potentially provide a wealth of active sites, augment the migration of the resultant carriers, facilitate the optimal catalyst selection process, and stimulate the evolution of a photocatalyst heterojunction [48].As revealed in Figure 2b-h, the irregular mass is an assembly of ZrO 2 enveloped by a layer of flaky g-C 3 N 4 , signifying that the two constituents are meticulously compounded.The particulate dimensions fall within a range of 100 to 260 nm.The distinctive morphologies of the ZrO 2 and g-CN enable the straightforward identification of the two semiconductors in the SEM images.It is observable within the composites that the ZrO 2 nanoparticles are enrobed on the g-CN surface, corroborating that the two components are intimately combined [48].The composite's energy-dispersive X-ray spectroscopy (EDX) findings are shown in Figure 2. The evenly distributed constituents-namely, C, N, Zr, and O-illustrated in Figure 2e ′ -h ′ , bolster the presence of the dopant and thus corroborate the successful synthesis of the composite.Following aggregation, the photocatalysts' morphologies exhibit a uniform dispersion, indicative of the crystalline ZrO 2 's effect inherent in the composite photocatalyst, a conclusion that aligns with the XRD outcomes [49].

HRTEM Analysis
Figure 3 shows the TEM images of the g-CNZ 6 nanocomposite.g-C 3 N 4 is shown as a nanosheet with its heterojunctions.As observed in Figure 3a, g-C 3 N 4 exhibits a large and ultrathin wrinkled nanosheet morphology, which is because of the thermal exfoliation of the CN bulk, and the ZrO 2 irregular spherical nanostructure is deposited on the g-C 3 N 4 nanosheet to form g-C 3 N 4 @ ZrO 2 heterojunctions.The particle diameters of 6-8 nm of the ZrO 2 nanostructures in the heterojunction samples are also shown.The surface energy affected by the size and shape of the ZrO 2 nanostructures can be influenced by their amount and distribution over the g-C 3 N 4 nanosheet.Close interfacial contact between g-C 3 N 4 nanosheet and ZrO 2 nanoparticle is expected to improve the transfer and separation of the excited e − CB -h + VB pairs and increase the heterojunction photocatalytic performance [48].Compared with the non-crystalline structure of g-C 3 N 4 , the g-CNZ nanocomposite displayed crystallinity.The ZrO 2 structure, shown in Figure 3b, with an interplanar spacing of 0.31 nm, was related to the (−111) planes of the ZrO 2 phases, as shown in Figure 3b [49].In the SAED patterns in Figure 3c, the bright spots confirm the high crystallinity of the sample.The well-resolved crystalline rings in the SAED correspond to the (111) and (−111) planes of the monoclinic zirconia nanoparticles, which match well with the observed XRD patterns.Figure 3d-h shows the mapping of all the elements in the g-CNZ 6 nanocomposite.The photocatalyst confirms that the composite g-CNZ 6 samples contain C, N, Zr, and O elements.

Functional Group Analysis
Figure 4 delineates the pure g-C 3 N 4 and g-C 3 N 4 @ZrO 2 nanocomposites with their associated FT-IR spectra.Due to the -NH stretching frequencies and incomplete condensation of the NH 2 group, pure g-C 3 N 4 distinctly displays a peak within the range of 3500 to 3000 cm −1 [32].The vibrational peaks at 1230, 1323, and 1416 cm −1 are attributable to the stretching vibrations of the aromatic C-N, whereas the peak at 1633 cm −1 indicates the tensile stretching of the C=N [50,51].The characteristic triazine structural unit manifests a notable vibrational peak at 807 cm −1 [52].The characteristic ZrO 2 peaks at 715 cm −1 are predominantly due to the Zr-O molecular chain vibration [53].Both the g-C 3 N 4 and ZrO 2 distinctive peaks are discernible in the g-C 3 N 4 @ZrO 2 nanocomposite spectrum, thus corroborating the successful ZrO 2 incorporation into g-C 3 N 4 .Additionally, as the quantity of ZrO 2 doping increases, the prominence of the ZrO 2 absorption peak is enhanced, as the XRD data substantiates.
Catalysts 2024, 13, x FOR PEER REVIEW 6 of 21 dispersion, indicative of the crystalline ZrO2's effect inherent in the composite photocatalyst, a conclusion that aligns with the XRD outcomes [49].[49].In the SAED patterns in Figure 3c, the bright spots confirm the high crystallinity of the sample.The well-resolved crystalline rings in the SAED correspond to the (111) and (−111) planes of the monoclinic zirconia nanoparticles, which match well with the observed XRD patterns.Figure 3d-h shows the mapping of all the elements in the g-CNZ6 nanocomposite.The photocatalyst confirms that the composite g-CNZ6 samples contain C, N, Zr, and O elements.

Functional Group Analysis
Figure 4 delineates the pure g-C3N4 and g-C3N4@ZrO2 nanocomposites with their associated FT-IR spectra.Due to the -NH stretching frequencies and incomplete condensation of the NH2 group, pure g-C3N4 distinctly displays a peak within the range of 3500 to 3000 cm −1 [32].The vibrational peaks at 1230, 1323, and 1416 cm −1 are attributable to the stretching vibrations of the aromatic C-N, whereas the peak at 1633 cm −1 indicates the tensile stretching of the C=N [50,51].The characteristic triazine structural unit manifests a notable vibrational peak at 807 cm −1 [52].The characteristic ZrO2 peaks at 715 cm −1 are predominantly due to the Zr-O molecular chain vibration [53].Both the g-C3N4 and ZrO2 distinctive peaks are discernible in the g-C3N4@ZrO2 nanocomposite spectrum, thus corroborating the successful ZrO2 incorporation into g-C3N4.Additionally, as the quantity of ZrO2 doping increases, the prominence of the ZrO2 absorption peak is enhanced, as the XRD data substantiates.

Optical Analysis
Using the UV-Vis diffuse reflectance spectroscopy (DRS) method, the capacity of the photocatalysts to absorb light and generate charge carriers in the g-CN and g-CNZ nanocomposites is assessed.Using the cut line approach, the absorption wavelength threshold

Optical Analysis
Using the UV-Vis diffuse reflectance spectroscopy (DRS) method, the capacity of the photocatalysts to absorb light and generate charge carriers in the g-CN and g-CNZ nanocomposites is assessed.Using the cut line approach, the absorption wavelength threshold is established by creating a tangent and intersection with the abscissa.The raw g-CN absorption edge, visible in Figure 5a, is like that of paper and has a wavelength of about 407 nm.It is located within the visual zone [54].The percentage reflectance increases with rising ZrO 2 concentrations at extremely low concentrations, peaking for the g-CNZ 3 sample.After that, a definite diminishing trend is seen with increasing ZrO 2 concentrations.The absorption wavelength threshold values for the g-CNZ 3 sample are red-shifted and have a maximum band edge of 388 nm, whereas the pure g-C 3 N 4 can absorb light up to a maximum wavelength of 407 nm.The reflectance rises with rising ZrO 2 concentrations at extremely low concentrations, peaking for the g-CNZ 3 sample.After that, an obvious decrease is seen with increasing ZrO 2 concentrations.The highest band edge for the g-CNZ 3 sample is 388 nm, whereas the pure g-C 3 N 4 can absorb light up to a maximum wavelength of 407 nm.The values of the absorption wavelength threshold are red-shifted within the range of (388-407 nm).Therefore, adding ZrO 2 makes g-C 3 N 4 's ability to absorb visible light more effective.The interaction of ZrO 2 with g-C 3 N 4 may lead to chemical bonds being formed between the two semiconductors, improving the optical characteristics and producing charge carriers [55].From the Tauc's relation provided in the following Equation ( 6), the calculated band gap E g of the samples was determined [56]: where hυ, α, A, and E g are defined as the incident light frequency, the absorption coefficient, a constant parameter, and the band gap energy, respectively.Also, the value of n depends on the optical transition type in a semiconductor, which is determined to be ½ and 2 for the CNZ 0 and g-CNZ nanocomposites, respectively [56][57][58].The n value is ½ for a direct bandgap semiconductor, while for the indirect bandgap materials, the n value will equal 2. The (αhν) 2 and (αhν) 1/2 vs. (hν) graphs were plotted for the direct and indirect bandgap semiconductor, as shown in Figure 5b,c.In the graphs, a straight line is fitted for the straight segment.The straight line to the E axis gives the band gap values.e.g., the values were found to be decreased.The rise in the carrier concentration leads to the development of certain chemical bonds between the g-CNZ 0 and g-CNZ , which is expected to cause a decrease in the band gap and improve the optical characteristics.maximum wavelength of 407 nm.The reflectance rises with rising ZrO2 concentrations at extremely low concentrations, peaking for the g-CNZ3 sample.After that, an obvious decrease is seen with increasing ZrO2 concentrations.The highest band edge for the g-CNZ3 sample is 388 nm, whereas the pure g-C3N4 can absorb light up to a maximum wavelength of 407 nm.The values of the absorption wavelength threshold are red-shifted within the range of (388-407 nm).Therefore, adding ZrO2 makes g-C3N4's ability to absorb visible light more effective.The interaction of ZrO2 with g-C3N4 may lead to chemical bonds being formed between the two semiconductors, improving the optical characteristics and producing charge carriers [55].From the Tauc's relation provided in the following Equation ( 6), the calculated band gap  of the samples was determined [56]: where hυ, α, A, and Eg are defined as the incident light frequency, the absorption coefficient, a constant parameter, and the band gap energy, respectively.Also, the value of n depends on the optical transition type in a semiconductor, which is determined to be ½ and 2 for the CNZ0 and g-CNZ nanocomposites, respectively [56][57][58].The  value is ½ for a direct bandgap semiconductor, while for the indirect bandgap materials, the n value will equal 2. The (αhν) 2 and (αhν) 1/2 vs. (hν) graphs were plotted for the direct and indirect bandgap semiconductor, as shown in Figure 5b,c.In the graphs, a straight line is fitted for the straight segment.The straight line to the E axis gives the band gap values.e.g., the values were found to be decreased.The rise in the carrier concentration leads to the development of certain chemical bonds between the g-CNZ0 and g-CNZ, which is expected to cause a decrease in the band gap and improve the optical characteristics.Table 2 lists all the estimated values for the direct and indirect bandgaps, respectively.Methylene Blue (MB) and eosine yellow (EY), acting as model pollutants, were engaged to evaluate the photocatalytic performance under simulated UVc light irradiation.A thoroughly dark environment was ensured to conduct the adsorption assays of the MB and EY dyes.Approximately half an hour later, a dynamic equilibrium state was achieved, satisfying the stability prerequisites for the adsorption/desorption processes of the tested catalyst specimens.
Figure 6 delineates the absorption intensity recorded at 663 nm, further augmented by the absorption spectra of the MB dye interacting with diverse catalysts throughout the photocatalytic response.Once a photocatalyst is incorporated into the catalytic system, a marked reduction in the MB concentration is observed, which intensifies in direct proportion to the prolonged duration of the catalytic process, as depicted in Figure 6.The efficacy of the MB dye degradation, observed over 40 min of photo-irradiation, is presented in Figure 7a.Table 3 exhibits the perceptible degradation of MB as a consequence of the reaction with the g-CNZ nanocomposites.Following the application of Equation ( 1), the MB degradation efficiency was determined to be 77.5% and 96.5% for the g-CNZ 0 and g-CNZ 6 nanocomposites, respectively.A substantial enhancement in the photodegradation of the photocatalyst may have been facilitated by the introduction of 0.5 g of ZrO 2 nanoparticles onto the g-CN sheets.This enhancement can be attributed to a robust interfacial interaction, expedited charge mobility, a heightened propensity for charge carrier segregation, and a reduced band gap compared to the g-CNZ 0 and g-CNZ 6 photocatalysts.Photodegradation rate constants, represented by Equation (2) as per [59], were employed to deduce the photodegradation rate constants.The pure g-CNZ 0 displayed a degradation rate of 0.035 min −1 .Integrating g-C 3 N 4 with ZrO 2 resulted in a remarkable augmentation of the electron-hole separation efficiency and UV light absorption capacity.A concurrent increase in the ZrO 2 compounding quantity led to an escalated degradation rate of 0.088 min −1 for the g-CNZ 6 nanocomposites, thereby outperforming the rate of the pure g-C 3 N 4 by a factor of 2.5.
As shown in Figure 8, the absorption spectra of the EY dye with different catalysts during the photocatalytic reaction for the experiment showed an absorption intensity at 520 nm.The EY was used as color pollution, and the photocatalytic performance of the as-prepared catalysts was evaluated.Where deterioration was placed 40 min after photoirradiation, the g-CNZ nanocomposite-enhanced photocatalytic performance is demonstrated by the high degradation efficiency and degradation rate constant (k) values in Table 3.As shown in Figure 9a-c, the degradation efficiency of the g-CNZ 6 nanocomposite within 40 min of light irradiation is up to 95.6%, with a high degradation rate constant (0.0767 min −1 ).As shown in Figure 8, the absorption spectra of the EY dye with different catalysts during the photocatalytic reaction for the experiment showed an absorption intensity at 520 nm.The EY was used as color pollution, and the photocatalytic performance of the asprepared catalysts was evaluated.Where deterioration was placed 40 min after photoirradiation, the g-CNZ nanocomposite-enhanced photocatalytic performance is demonstrated by the high degradation efficiency and degradation rate constant (k) values in Table 3.As shown in Figure 9a-c, the degradation efficiency of the g-CNZ6 nanocomposite  As shown in Figure 8, the absorption spectra of the EY dye with different catalysts during the photocatalytic reaction for the experiment showed an absorption intensity at 520 nm.The EY was used as color pollution, and the photocatalytic performance of the asprepared catalysts was evaluated.Where deterioration was placed 40 min after photoirradiation, the g-CNZ nanocomposite-enhanced photocatalytic performance is demonstrated by the high degradation efficiency and degradation rate constant (k) values in Table 3.As shown in Figure 9a-c, the degradation efficiency of the g-CNZ6 nanocomposite

Photodegradation of Mixed Dye
Combining two organic dyes (MB and EY) made it possible to imitate the appearance of real water contamination.The only variation between the experimental setup and the setup for the individual contaminants was the extended adsorption-desorption duration of 30 min.The photocatalyst was dissolved in 200 mL of a solution containing 10 ppm of the MB and EY dye and 0.01 g of the photocatalyst.The results of the evaluation of the g-CNZ 6 photocatalyst's photocatalytic performance for the mixed pollutants are displayed in Figure 10.By observing the UV-vis maximum absorbance peaks of MB and EY at 664 nm and 518 nm, respectively, the photocatalytic performance of the g-CNZ 6 nanocomposite towards the matrix of the pollutant was assessed.As shown in Figure 10.their maximum absorbance peaks diminish as the UVc light exposure time increases.At 120 min, photocatalytic rates of above 45% for MB and above 92% for EY were attained.Compared to other cationic dye pollutants (see Table 4), the g-CNZ 6 nanocomposite was more effective at removing anionic dyes.According to the above results, our g-CNZ 6 photocatalyst can be used in actual wastewater samples even if the better removal efficiency of those pollutants was only attained after a longer irradiation time.setup for the individual contaminants was the extended adsorption-desorption duration of 30 min.The photocatalyst was dissolved in 200 mL of a solution containing 10 ppm of the MB and EY dye and 0.01 g of the photocatalyst.The results of the evaluation of the g-CNZ6 photocatalyst's photocatalytic performance for the mixed pollutants are displayed in Figure 10.By observing the UV-vis maximum absorbance peaks of MB and EY at 664 nm and 518 nm, respectively, the photocatalytic performance of the g-CNZ6 nanocomposite towards the matrix of the pollutant was assessed.As shown in Figure 10.their maximum absorbance peaks diminish as the UVc light exposure time increases.At 120 min, photocatalytic rates of above 45% for MB and above 92% for EY were attained.Compared to other cationic dye pollutants (see Table 4), the g-CNZ6 nanocomposite was more effective at removing anionic dyes.According to the above results, our g-CNZ6 photocatalyst can be used in actual wastewater samples even if the better removal efficiency of those pollutants was only attained after a longer irradiation time.To further understand the g-CNZ6 nanocomposite's photocatalytic process, free radical capture studies were also conducted.As indicated in Figure 11, four scavengers were  To further understand the g-CNZ 6 nanocomposite's photocatalytic process, free radical capture studies were also conducted.As indicated in Figure 11, four scavengers were added to the pollutant solution before the photocatalytic experiment: isopropyl alcohol (IPA: .OH), sodium chloride (NaCl: h + ), ascorbic acid (ASC: O 2 −. ), and sodium nitrate (NaNO 3 : e − ).The amount of MB degradation was not significantly impacted by ascorbic acid (ASC: O 2 −. scavenger).Since sodium chloride (NaCl) is a h + scavenger, the degradation percentage dramatically decreases when it is introduced, going from 96.5 % to 20 %.The degradation percentage fell to 10% when ascorbic acid (ASC: O 2 −. scavenger) was present.This implies that h + and O 2 −. are two important reactive species in the photodegradation of MB by the g-CNZ 6 nanocomposites.
The degradation percentage fell to 10% when ascorbic acid (ASC: O2 − .scavenger) was present.This implies that h + and O2 − .are two important reactive species in the photodegradation of MB by the g-CNZ6 nanocomposites.The following formula describes how the active species in the chemical process in which the semiconductors VB and CB are involved are different [65,66]: The Eg is the band gap, X is the absolute electronegativity, and the values of g-C3N4 and ZrO2 for X are 4.73 eV and 5.91 eV, respectively.E0 is a constant, regarding the standard H electrode with a value of 4.5 eV [67][68][69].As a result, the predicted E values for the CN nanosheet and ZrO2 nanoparticle are −1.02 and −0.99 eV, respectively.Additionally, the E of the samples of the CN nanosheets and ZrO2 nanoparticles is +1.49 and +3.81 eV, respectively.Figure 12 delineates the anticipated method for Methylene Blue (MB) dye degradation by applying a g-CNZ6 nanocomposite under ultraviolet C (UVc) light irradiation.The underpinning of this mechanism is derived from valence band (VB) and conduction band (CB) potentials, extrapolated from diffuse reflectance spectroscopy (DRS) analysis in conjunction with the results from active species experimentation.
The narrow bandgap of pure graphitic carbon nitride (g-C3N4) triggers the generation of electron-conduction band (e − CB) and hole-valence band (h + VB) pairs upon exposure to UVc light, a characteristic of individual semiconductors.However, the subsequent rapid recombination of these pairs results in limited photocatalytic activity in pure g-C3N4, attributable to its relatively small bandgap (2.51 eV).Conversely, the large bandgap of pure ZrO2 nanoparticles, estimated at 4.8 eV, inhibits its excitation under UVc light exposure The following formula describes how the active species in the chemical process in which the semiconductors VB and CB are involved are different [65,66]: The E g is the band gap, X is the absolute electronegativity, and the values of g-C 3 N 4 and ZrO 2 for X are 4.73 eV and 5.91 eV, respectively.E 0 is a constant, regarding the standard H electrode with a value of 4.5 eV [67][68][69].As a result, the predicted E CB values for the CN nanosheet and ZrO 2 nanoparticle are −1.02 and −0.99 eV, respectively.Additionally, the E VB of the samples of the CN nanosheets and ZrO 2 nanoparticles is +1.49 and +3.81 eV, respectively.Figure 12 delineates the anticipated method for Methylene Blue (MB) dye degradation by applying a g-CNZ 6 nanocomposite under ultraviolet C (UVc) light irradiation.The underpinning of this mechanism is derived from valence band (VB) and conduction band (CB) potentials, extrapolated from diffuse reflectance spectroscopy (DRS) analysis in conjunction with the results from active species experimentation.
The narrow bandgap of pure graphitic carbon nitride (g-C 3 N 4 ) triggers the generation of electron-conduction band (e − CB ) and hole-valence band (h + VB ) pairs upon exposure to UVc light, a characteristic of individual semiconductors.However, the subsequent rapid recombination of these pairs results in limited photocatalytic activity in pure g-C 3 N 4 , attributable to its relatively small bandgap (2.51 eV).Conversely, the large bandgap of pure ZrO 2 nanoparticles, estimated at 4.8 eV, inhibits its excitation under UVc light expo-sure [64].Consequently, when UVc light is incident on the g-CNZ nanocomposite surface, the valence band (VB) electrons in both semiconductor constituents (g-C 3 N 4 and ZrO 2 ) are instantaneously excited to their respective conduction bands (CBs), leaving behind holes in the VB.The disparity in CB edge potentials between the two materials, with g-C 3 N 4 exhibiting a more negative potential than ZrO 2 , facilitates the migration of photoexcited electrons from the CB of g-C 3 N 4 to the CB of ZrO 2 .The resultant electrons reside at the CB of ZrO 2 , instigating interactions with surface oxygen to form superoxide radicals (O 2 − .), which subsequently interact with MB.Simultaneously, photogenerated holes at the VB of ZrO 2 migrate to the VB of g-C 3 N 4 due to the higher VB potential of ZrO 2 than g-C 3 N 4 .Consequently, these newly formed holes remain in the VB of g-C 3 N 4 , where they instigate their reactions with MB.The formation of a type II heterojunction significantly decreases the recombination rate while promoting the separation of photoinduced electrons and holes at the interface between g-C 3 N 4 and ZrO 2 .The photocarriers at the g-C 3 N 4 @ ZrO 2 interfaces still maintain the Z-scheme transfer mechanism, like in the case of bare photocatalysis.More interestingly, under the action of the polarization field, the bulk photoelectrons of g-C 3 N 4 and ZrO 2 are driven to Z-scheme transfer [65].The efficiency of the charge transport and separation of electron-hole pairs is contingent on the interaction between ZrO 2 and g-C 3 N 4 .An excess of ZrO 2 can accumulate on the g-C 3 N 4 layer, diminishing the interaction between the two materials and thus compromising the charge separation efficiency.As such, an optimal ZrO 2 concentration is deemed crucial.The anticipated mechanism for the degradation of MB by the g-CNZ 6 nanocomposite, as represented in Equations ( 9)-( 13), can be described as follows: 2e Catalysts 2024, 13, x FOR PEER REVIEW 15 of 21 [64].Consequently, when UVc light is incident on the g-CNZ nanocomposite surface, the valence band (VB) electrons in both semiconductor constituents (g-C3N4 and ZrO2) are instantaneously excited to their respective conduction bands (CBs), leaving behind holes in the VB.The disparity in CB edge potentials between the two materials, with g-C3N4 exhibiting a more negative potential than ZrO2, facilitates the migration of photoexcited electrons from the CB of g-C3N4 to the CB of ZrO2.The resultant electrons reside at the CB of ZrO2, instigating interactions with surface oxygen to form superoxide radicals (O2 − .), which subsequently interact with MB.Simultaneously, photogenerated holes at the VB of ZrO2 migrate to the VB of g-C3N4 due to the higher VB potential of ZrO2 than g-C3N4.
Consequently, these newly formed holes remain in the VB of g-C3N4, where they instigate their reactions with MB.The formation of a type II heterojunction significantly decreases the recombination rate while promoting the separation of photoinduced electrons and holes at the interface between g-C3N4 and ZrO2.The photocarriers at the g-C3N4@ ZrO2 interfaces still maintain the Z-scheme transfer mechanism, like in the case of bare photocatalysis.More interestingly, under the action of the polarization field, the bulk photoelectrons of g-C3N4 and ZrO2 are driven to Z-scheme transfer [65].The efficiency of the charge transport and separation of electron-hole pairs is contingent on the interaction between ZrO2 and g-C3N4.An excess of ZrO2 can accumulate on the g-C3N4 layer, diminishing the interaction between the two materials and thus compromising the charge separation efficiency.As such, an optimal ZrO2 concentration is deemed crucial.The anticipated mechanism for the degradation of MB by the g-CNZ6 nanocomposite, as represented in Equations ( 9)-( 13), can be described as follows:

Stability and Recycling
Repeated assessments were executed on M) degradation, extending up to the fifth cycle to validate the scalability of the synthesized g-CNZ 6 nanocomposite.Following each cycle, the photocatalyst was retrieved through centrifugation, rinsed with distilled water, and dried in an oven set at a temperature of 80 • C. Figure 13 illustrates a marginal reduction in the photocatalytic efficacy of the g-CNZ 6 nanocomposite towards MB photodegradation.However, despite this slight decrement in performance, it was demonstrated that the g-CNZ 6 nanocomposite maintains considerable photocatalytic activity over multiple iterations.This observed resilience solidifies g-CNZ 6 's position as a competent photocatalyst.The composite can be recycled and reused multiple times while demonstrating substantial activity in a photocatalytic reaction, reinforcing its potential for sustained utilization.Repeated assessments were executed on M) degradation, extending up to the fifth cycle to validate the scalability of the synthesized g-CNZ6 nanocomposite.Following each cycle, the photocatalyst was retrieved through centrifugation, rinsed with distilled water, and dried in an oven set at a temperature of 80 °C. Figure 13 illustrates a marginal reduction in the photocatalytic efficacy of the g-CNZ6 nanocomposite towards MB photodegradation.However, despite this slight decrement in performance, it was demonstrated that the g-CNZ6 nanocomposite maintains considerable photocatalytic activity over multiple iterations.This observed resilience solidifies g-CNZ6's position as a competent photocatalyst.The composite can be recycled and reused multiple times while demonstrating substantial activity in a photocatalytic reaction, reinforcing its potential for sustained utilization.

Preparation of g-C3N4@ZrO2
Porous graphitic carbon nitride (g-CN) was prepared via thermal polycondensation of urea.A predefined amount of 6 g from urea and thiourea was finely ground and placed in a closed crucible and subjected to a controlled heating regimen at a rate of 5 °C/min up to a final temperature of 550 °C, which was maintained for post-cooling to ambient cycles, and the product was finely ground in an agate mortar.
The synthesis of g-C3N4@ZrO2 nanocomposites was conducted following the protocol illustrated in Figure 14.A total of 0.5 g from g-C3N4 was added to 50 mL DDW and placed in an ultrasonic probe for 15 min.A homogenous mixture was created by adding 10 g of Zirconium oxychloride to 10 g of citric acid, into which specific proportions of ZrO2 was added, and was stirred for 30 min to establish homogeneity in the resulting solution.This mixture was subsequently dried at 100 °C for 24 h.The dried composite was then calcinated at 550 °C for 2 h in a muffle furnace and later crushed to a uniform powder.The final composites, bearing different proportions of ZrO2 (0.001, 0.005, 0.01, 0.05, 0.1, 0.5, and 1 g), were designated as g-CNZ0 through g-CNZ7, respectively.

Preparation of g-C 3 N 4 @ZrO 2
Porous graphitic carbon nitride (g-CN) was prepared via thermal polycondensation of urea.A predefined amount of 6 g from urea and thiourea was finely ground and placed in a closed crucible and subjected to a controlled heating regimen at a rate of 5 • C/min up to a final temperature of 550 • C, which was maintained for post-cooling to ambient cycles, and the product was finely ground in an agate mortar.
The synthesis of g-C 3 N 4 @ZrO 2 nanocomposites was conducted following the protocol illustrated in Figure 14.A total of 0.5 g from g-C 3 N 4 was added to 50 mL DDW and placed in an ultrasonic probe for 15 min.A homogenous mixture was created by adding 10 g of Zirconium oxychloride to 10 g of citric acid, into which specific proportions of ZrO 2 was added, and was stirred for 30 min to establish homogeneity in the resulting solution.This mixture was subsequently dried at 100 • C for 24 h.The dried composite was then calcinated at 550 • C for 2 h in a muffle furnace and later crushed to a uniform powder.The final composites, bearing different proportions of ZrO 2 (0.001, 0.005, 0.01, 0.05, 0.1, 0.5, and 1 g), were designated as g-CNZ 0 through g-CNZ 7, respectively.

Characterization Techniques and Devices
X-ray diffraction (XRD) was employed to determine the crystalline structure of the synthesized photocatalysts.Scanning electron microscopy (SEM, model JSM-6360, Tokyo, Japan) operating at 20 kV, supplemented with energy-dispersive X-ray spectroscopy (EDX), was used to probe the surface morphology and elemental composition.The functional chemical bonds were identified from FT-IR Perkin Elme(Waltham, MA, USA) r in the range of 4000 to 400 cm −1 wavenumber.The optical properties were observed using UV-visible DRS (JASCO V-570, Tokyo, Japan).

Characterization Techniques and Devices
X-ray diffraction (XRD) was employed to determine the crystalline structure of the synthesized photocatalysts.Scanning electron microscopy (SEM, model JSM-6360, Tokyo, Japan) operating at 20 kV, supplemented with energy-dispersive X-ray spectroscopy (EDX), was used to probe the surface morphology and elemental composition.The functional chemical bonds were identified from FT-IR Perkin Elme(Waltham, MA, USA) r in the range of 4000 to 400 cm −1 wavenumber.The optical properties were observed using UV-visible DRS (JASCO V-570, Tokyo, Japan).

Photoreactor Design and Photocatalytic Activity of g-C3N4@ZrO2 Nanocomposites for Photodegradation
The photocatalytic performance of the synthesized g-C3N4@ZrO2 nanocomposites was measured under UVC radiation for the model pollutant using a wooden photoreactor at room temperature, designed by I.S. Yahia and his group at NLEBA/ASU/Egypt; more details about the photoreactor are mentioned in Hussien et al. [70].The dyes methylene blue (MB) and eosin yellow (EY) were employed as contaminants for this assessment.The structure, chemical formula and type are shown in Table 5.A total of 0.01 g of each composite was dispersed in a 200 mL solution containing either dye at a concentration of 10 ppm, and the mixture was subjected to dark conditions for approximately 30 min to attain adsorption-desorption equilibrium.After this period, samples were extracted and centrifuged at 3000 rpm for 10 min, and the residual mixture was re-illuminated using UVc lamps.Photocatalytic activity was quantified at intervals of 10 min during irradiation, and photodegradation was monitored within the range of 400 to 800 nm using a UV-Vis spectrophotometer (Agilent, Santa Clara, CA, USA).Degradation efficiency and apparent rate constants were calculated using Equations ( 1) and (2) [71]; the degree of dye degradation, apparent degradation rate constant ( ) in min , and photodegradation efficiency (%) were systematically calculated. and  denote the dye concentration at the initial time and at a desired time  post-irradiation, respectively.The photocatalytic performance of the synthesized g-C 3 N 4 @ZrO 2 nanocomposites was measured under UVC radiation for the model pollutant using a wooden photoreactor at room temperature, designed by I.S. Yahia and his group at NLEBA/ASU/Egypt; more details about the photoreactor are mentioned in Hussien et al. [70].The dyes methylene blue (MB) and eosin yellow (EY) were employed as contaminants for this assessment.The structure, chemical formula and type are shown in Table 5.A total of 0.01 g of each composite was dispersed in a 200 mL solution containing either dye at a concentration of 10 ppm, and the mixture was subjected to dark conditions for approximately 30 min to attain adsorption-desorption equilibrium.After this period, samples were extracted and centrifuged at 3000 rpm for 10 min, and the residual mixture was re-illuminated using UVc lamps.Photocatalytic activity was quantified at intervals of 10 min during irradiation, and photodegradation was monitored within the range of 400 to 800 nm using a UV-Vis spectrophotometer (Agilent, Santa Clara, CA, USA).Degradation efficiency and apparent rate constants were calculated using Equations ( 1) and (2) [71]; the degree of dye degradation, apparent degradation rate constant (k app ) in min −1 , and photodegradation efficiency (PDE%) were systematically calculated.A o and A denote the dye concentration at the initial time and at a desired time t post-irradiation, respectively.

Characteristics Methylene Blue Eosin Yellow
Structure: Chemical Formula: C16H18N3S Cl C20H8Br4NaO5 Type of dye: Cationic dye Anionic dye

Conclusions
In this investigation, we have developed an augmented photocatalyst, a g-C3N4@ZrO2 composite, exhibiting an amplified response to direct UVc light exposure, synthesized via a simple pyrolysis process of fabricated g-C3N4@ZrO2 samples, complemented by sonication.Upon exposure to direct UVc radiation, the g-CNZ6 nanocomposite demonstrated superior photocatalytic efficacy in degraded MB, EY, and a mixed dye.Compared to the pure g-C3N4, the degradation efficiency of the g-CNZ6 nanocomposite was remarkably higher, with respective yields of 96.5% and 95.6% for the MB and EY dyes.This significant increase in the photocatalytic performance can be attributed to the synergistic interaction between ZrO2 and g-C3N4, culminating in efficient electron-hole pair separation, a critical driver of photocatalysis.An extensive series of reusability experiments unveiled that the g-CNZ6 nanocomposite maintained its photocatalytic stability and robustness, indicating promising potential for sustainable applications in dye degradation and water treatment.

Characteristics Methylene Blue Eosin Yellow
Structure: Chemical Formula: C16H18N3S Cl C20H8Br4NaO5 Type of dye: Cationic dye Anionic dye

Conclusions
In this investigation, we have developed an augmented photocatalyst, a g-C3N4@ZrO2 composite, exhibiting an amplified response to direct UVc light exposure, synthesized via a simple pyrolysis process of fabricated g-C3N4@ZrO2 samples, complemented by sonication.Upon exposure to direct UVc radiation, the g-CNZ6 nanocomposite demonstrated superior photocatalytic efficacy in degraded MB, EY, and a mixed dye.Compared to the pure g-C3N4, the degradation efficiency of the g-CNZ6 nanocomposite was remarkably higher, with respective yields of 96.5% and 95.6% for the MB and EY dyes.This significant increase in the photocatalytic performance can be attributed to the synergistic interaction between ZrO2 and g-C3N4, culminating in efficient electron-hole pair separation, a critical driver of photocatalysis.An extensive series of reusability experiments unveiled that the g-CNZ6 nanocomposite maintained its photocatalytic stability and robustness, indicating promising potential for sustainable applications in dye degradation and water treatment.

Figure 1 .
Figure 1.X-ray diffraction patterns of g-CN and g-CNZ nanostructured samples.

Figure 1 .
Figure 1.X-ray diffraction patterns of g-CN and g-CNZ nanostructured samples.

Figure 7 .
Figure 7. (a) Photodegradation efficiency toward MB, (b) the photocatalytic rate curves, and (c) the kinetics rate of the g-CNZ nanocomposite.

Figure 7 .
Figure 7. (a) Photodegradation efficiency toward MB, (b) the photocatalytic rate curves, and (c) the kinetics rate of the g-CNZ nanocomposite.

Figure 7 .
Figure 7. (a) Photodegradation efficiency toward MB, (b) the photocatalytic rate curves, and (c) the kinetics rate of the g-CNZ nanocomposite.

Figure 9 . 2 . 6 . 2 .Figure 9 .
Figure 9. (a) Photodegradation efficiency toward EY, (b) the photocatalytic rate curves, and (c) kinetics rate of the g-CNZ nanocomposites.2.6.2.Photodegradation of Mixed DyeCombining two organic dyes (MB and EY) made it possible to imitate the appearance of real water contamination.The only variation between the experimental setup and the

Figure 10 .
Figure 10.Photodegradation of organic dyes in the presence of g-CNZ6 nanocomposite.

Figure 10 .
Figure 10.Photodegradation of organic dyes in the presence of g-CNZ 6 nanocomposite.

Figure 11 .
Figure 11.Scavenger of MB dye in the presence of g-CNZ6 nanocomposite.2.6.4.Possible Mechanistic Pathway for Degradation of MB Dye

Figure 11 .
Figure 11.Scavenger of MB dye in the presence of g-CNZ 6 nanocomposite.2.6.4.Possible Mechanistic Pathway for Degradation of MB Dye

eFigure 12 .
Figure 12.A possible mechanism for degradation of MB in UVc by MB by g-CNZ6 nanocomposite.

Figure 12 .
Figure 12.A possible mechanism for degradation of MB in UVc by MB by g-CNZ 6 nanocomposite.

Figure 13 .
Figure 13.Reusability curve for degradation of MB by g-CNZ6 nanocomposite.

Figure 14 .
Figure 14.Schematic route of the synthesis pathway of the g-C 3 N 4 @ZrO 2 NCs.

Table 1 .
The computed mean values of the crystallite size, dislocation density, and strain from the XRD spectra for CN and CNZ NCs samples.

Table 2
lists all the estimated values for the direct and indirect bandgaps, respectively.

Table 2 .
The direct and indirect bandgap of g-CNZ nanocomposites.

Table 2 .
The direct and indirect bandgap of g-CNZ nanocomposites.

Table 3 .
Degradation efficiency and kinetic rate of MB and EY dyes.

Table 4 .
Some g-C 3 N 4 /ZrO 2 composite photocatalysts and photodegradation efficiency of organic pollutants.Reactive Species Involved in Photodegradation of MB Dye by g-CNZ 6 Nanocomposites