A novel S-scheme ZnO/Ce-g-C3N5 heterojunctions with enhanced photocatalytic activity

doi:10.21203/rs.3.rs-4543014/v1

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A novel S-scheme ZnO/Ce-g-C3N5 heterojunctions with enhanced photocatalytic activity

https://doi.org/10.21203/rs.3.rs-4543014/v1

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published 20 Jul, 2024

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Creating a high-performance photocatalyst is critical for enhancing the degradation of organic contaminants in wastewater. A novel ZN/Ce-CN heterojunctions, in which ZnO nanoparticles were uniformly deposited on the rare earth metal cerium doped g-C3N5 (Ce-CN) nanosheets, was fabricated in a two-step process (hydrothermal and calcination). The Mott-Schott test revealed S-scheme heterojunctions formed between Ce-CN and ZnO rather than type II. Regarding methylene blue (MB) degradation, the as-prepared heterojunctions demonstrated significantly more photocatalytic activity than Ce-CN and ZnO. The presence of heterojunctions in ZN/Ce-CN improves the separation efficiency of photo-induced e-h+ pairs, resulting in ZN/Ce-CN's superior photocatalytic activity. The primary active species engaged in photocatalytic degradation have been discovered to be h+, •OH, and •O2- based on the research results using electron spin resonance spectra and radical trapping. Furthermore, ZN/Ce-CN had excellent reusability and stability. These results demonstrate the remarkable photocatalytic activity of S-scheme photocatalyst in the degradation of organic contaminants.

ZnO

g-C3N5

Rare earth metal

Heterojunctions

Photocatalysis

Construction of rare earth metal cerium-doped g-C₃N₅ and ZnO S-scheme heterojunctions photocatalyst.
ZN/Ce-CN was 2.9 and 5.0 times for MB degradation efficiency than ZnO and g-C₃N₅.
The interplay between doping and heterojunctions increased photocatalytic activity.
Provide a green, economical and efficient way to treat water resources.

The rapid development of modern industry and population increase have resulted in water contamination, which is gaining more and more attention worldwide. Organic dyes have emerged as one of the most significant water contaminants [1]. Many methods have been used to remove organic dyes from wastewater [2]. Still, photocatalytic degradation of semiconductors is regarded as a promising and sustainable technology due to its ease of operation, high efficiency, economy, environmental friendliness, and good recyclability [3].

So far, many inorganic photocatalysts, such as TiO₂ [4], ZnO [5], and SnO₂ [6], have been extensively researched for organic dye removal. Among them, ZnO is one of the most widely used semiconductors for photocatalysis because of its remarkable properties such as low toxicity, low cost, and high oxidizing capacity [7, 8]. Unfortunately, owing to its wide band gap (3.37 eV) and low electron-hole separation efficiency, direct application of ZnO is limited [9]. Building a heterojunctions with other semiconductors with a narrower band gap has been reported as a desirable technique to overcoming the aforementioned ZnO-related disadvantages [10]. As it is well known, a band gap below 2.65-2.8 eV could absorb the maximum portion of the visible light spectrum.

Organic semiconductors have significant advantages over inorganic semiconductors due to their low level of toxicity, inexpensive cost, and plentiful reserves. g-C₃N₅, a nitrogen-rich graphitic carbon nitride, was successfully created with a greater nitrogen content (62.5 at%) than g-C₃N₄ (57.1 at%) [11]. In addition, g-C₃N₅ has a larger conjugated system than g-C₃N₄, attributed to the conjugation of the heptazine moiety to the lone electron pairs on the N atom [12, 13]. Accordingly, g-C₃N₅ possesses a narrower band gap (≈ 2.0 eV) than g-C₃N₄ (≈ 2.7 eV) [14, 15], which facilitates the improvement of solar energy utilization and photoactivity. Its wide application, however, is restricted by the smaller specific surface area and quick charge complexation. Doping with rare earth elements is thought to be an effective way to improve the light usage and energy conversion of g-C₃N₅. Rare earth metals have a unique valence electron structure that precludes photoelectron/hole complexation [10, 16-18]. Among the rare earth metals, cerium (Ce) has better electrical conductivity, and the mixed valence states of 3-valence and 4-valence makes Ce-based materials attractive in heterogeneous catalysis [19, 20], so Ce dopped g-C₃N₅ leads to different optical properties, which in turn leads to a high charge mobility to improve photocatalytic efficiency.

In summary, the key features of an excellent photocatalyst are a robust redox ability, high charge-separation efficiency, broad absorption range, and long-term stability. However, all of these features are unattainable in a single photo catalyst. As a result, organic and inorganic semiconductor composited photocatalyst is commonly employed to improve photocatalytic degradation of contaminates [21, 22]. However, photogenerated carrier's redox ability is limited by the use of conventional heterojunctions [23]. In contrast, the newly developed S-scheme heterojunctions, which improves photocatalytic effectiveness by facilitating the charge transfer in nanocomposites, can alleviate the above issues while retaining the photogenerated charges' high redox capacity [24, 25]. As a result, it is critical to develop new S-scheme heterojunctions photocatalytic materials capable of efficiently using photon energy in the visible range.

In this thesis, ZN/Ce-CN heterojunctions photocatalysts with different cerium doping content were synthesized by hydrothermal and calcination methods for the degradation of MB pollutants under light irradiation. Compared to pure g-C₃N₅ and ZnO, ZN/Ce-CN, which featured a unique nanoparticle/flake coupling configuration, demonstrated a greater photocurrent response and superior photocatalytic activity for MB removal. The boosted photocatalytic activity of ZN/Ce-CN was attributed to wide light response range and the improved charge separation resulting from the synergistic effect of the construction of the S-scheme heterojunctions between Ce-CN and ZnO. The ZN/Ce-CN developed in this study opens a new avenue for the development of a unique S-scheme heterojunctions photocatalyst for photo- and electrocatalytic material with a wide range of applications.

2.1 Materials

Analytical grade zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), 3-amino-1,2,4-triazole (3-AT), ethanol, isopropanol (IPA), methylene blue (MB), benzoquinone (BQ), sodium hydroxide (NaOH), concentrated sulfuric acid (H₂SO₄), potassium iodide (KI).

2.2 Synthesis of g-C₃N₅

To prepare the g-C₃N₅, 1.5 g of 3-AT was dissolved in 30 mL of distilled water and stirred at 80 °C on a magnetically heated stirrer until dry and crystallized, then ground into a powder and placed in a crucible for calcination in a muffle furnace at 5 °C·min^-1 up to 520 °C for 180 minutes.

2.3 Synthesis of Ce doped g-C₃N₅

The preparation of Ce doped g-C₃N₅ is identical to that of g-C₃N₅, in that Ce(NO₃)₃·6H₂O with mass fractions of 0.25%, 0.5%, 1%, 2%, and 4% is completely dissolved before adding 1.5g of 3-AT. The remaining steps are the same. The resulting sample is referred to as Ce-CN.

2.4 Preparation of ZnO

The first is the preparation of solution A, 2.97 g of Zn(NO₃)₂·6H₂O was completely dissolved in 30 mL of distilled water, and anhydrous ethanol (20 mL) was added. Subsequently, 0.8 g of NaOH was dissolved in 15 mL of distilled water and progressively added dropwise to the aforementioned liquid A. The resultant solution was agitated for 30 minutes at room temperature, sonicated for 30 minutes, and then hydrothermally reacted at 100 °C for 10 hours. The substance was then washed, dried, and processed to produce a fine white powder ZnO.

2.5 Preparation of ZN/Ce-CN

0.1 g of ZnO was added to 30 mL of anhydrous ethanol and agitated for 10 minutes at room temperature to obtain a milky white suspension. A brownish-yellow suspension was obtained by adding 0.1 g of Ce-CN to the solution, followed by stirring for 60 minutes. Next, the suspension underwent a 10-hour hydrothermal reaction at 140°C. After washing, drying, and grinding, a solid powder that is light brownish yellow is produced. The resulting sample is referred to as ZN/Ce-CN. It is sufficient to substitute g-C₃N₅ for the Ce-CN powder in the synthetic process for ZN/CN, and all other steps are the same. The preparation scheme of ZN/Ce-CN is shown in Scheme 1.

2.6 photocatalytic degradation of MB

The degradation trials were conducted under a xenon lamp (500 W), and the reaction temperature was maintained at 20 °C by a water recirculation system. To achieve the adsorption-desorption equilibrium in the dark, 30 mg of catalyst was added to a 20 mg·L^-1 MB solution and agitated for 30 min. The photocatalytic reaction started when the xenon lamp was turned on and 3.0 mL of the solution was sampled periodically throughout the degradation process. After centrifugation to remove the catalyst, the concentration of methylene blue (MB) supernatant was determined in a UV-Vis spectrophotometer (Labtech 9100B) based on the absorption peak at 665 nm.

2.7 Characterization

The phase structure of all samples were determined by X-ray diffraction (XRD). Fourier transform infrared (FT-IR) spectra were measured on a PerkinElmer infrared spectrometer Frontie. Transmission microscopy (TEM; JEOL, JEM-2100) was used to examine the structure and morphology of all samples. The UV-Vis diffuse reflectance spectra of the materials were acquired utilizing a Hitachi U-4100 UV-Vis spectrometer (Shimadzu, UV-2600). X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., UK) was used to analyze the prepared samples' elemental composition and bonding information. The photoluminescence (PL) intensity of the prepared samples was characterized by fluorescence spectroscopy (PL). Electron paramagnetic resonance (EPR) measurements were performed on a Bruker ER200-SRC.

3.1 XRD patterns of different samples

X-ray diffraction (XRD) was used to analyze the crystalline characteristics and the phase structure of g-C₃N₅, ZnO, Ce-CN, ZN/CN, and ZN/Ce-CN (Fig. 1(a,b)). The diffraction peak of CN at 2θ = 27.4° was generated by the stacking of conjugated aromatic systems, corresponding to the (002) plane. No peak corresponding to Ce species was observed in Ce-CN due to the trace quantity and homogeneous dispersion of Ce single atoms [26]. However, for Ce-CN catalyst, obvious shifts toward a higher 2θ value (about 0.2) were observed. This could be due to Ce doping, which induces distortion in the g-C₃N₅ lattice, similar to Ce-C₃N₄ [27]. All peaks in the range of 2θ = 30 ~ 80° were indexed according to the hexagonal wurtzite ZnO structure (JCPDS No. 99-0111) [28]. ZnO remains in its crystalline phase even after g-C₃N₅ hybridization. As a result, the diffraction peaks of ZN/Ce-CN contain each separate peak for Ce-CN and ZnO with different intensities. Furthermore, the absence of any other new peaks shows that the effective synthesis of ZN/Ce-CN was accomplished.

3.2 IR

FTIR spectra were also utilized to explore the characteristics of the as-prepared ZN/Ce-CN at each step (Fig. 2). Similar characteristic peaks are detected for typical CN networks in graphitic of virgin g-C₃N₅ and Ce-CN materials. The IR bands at 1240 cm^-1, 1318 cm^-1, 1411 cm^-1, 1560 cm^-1, and 1640 cm^-1 are ascribed to the C-N stretch (νC-N) of heptazine (C₆N₇) aromatic nucleus in g-C₃N₅ as similar to g-C₃N₄ material. Whilst the characteristic broad peaks at 3154 cm^-1 is attributed to symmetric and asymmetric stretching vibrations of the -NH₂ (νN-H) and -OH (νO-H) groups [29]. The sharp peak observed at 812 cm^-1 was related to the breathing mode of triazine units in g-C₃N₅ [30], and it was obviously found in the ZN/Ce-CN composite. There are no further vibration modes found for Ce-CN, indicating that the structure of g-C₃N₅ is unaffected by cerium doping. The entire graphite-like structure of cerium-doped is essential for the π-delocalized electron system, which aids in the generation and transportation of photogenerated carriers [31]. O-H stretching vibration accounts for the intense broadband at about 3450 cm^-1 in the FTIR spectra of undoped ZnO [32]. The loading of ZnO broadens the peak bands of the O-H stretching vibration in ZN/CN and ZN/Ce-CN significantly. A well-defined band appears at 599-400 cm^-1, which could be due to the stretching vibration of the Zn-O bond [10], and it was clearly identified with a little shift in the ZN/CN and ZN/Ce-CN composite. Notably, the successful synthesis of heterojunctions in ZN/Ce-CN can be confirmed by the characteristic peaks of ZN/Ce-CN containing both Ce-CN and ZnO.

3.3 Morphological properties

TEM and HRTEM measurements were done to better understand the structure and content of the produced samples. It is evident from the ZN/Ce-CN TEM images (Fig. 3a) that ZnO nanoparticles were successfully deposited on Ce-CN nanosheets, demonstrating the composite of ZnO and Ce-CN, which is a fundamental requirement for heterojunctions production. Additionally, using HRTEM analysis, the interaction between the nanocomposites was investigated; the outcomes are displayed in Fig. 3b. HRTEM images further confirmed the presence of heterojunctions, with inter-planar distances of 0.281 and 0.247 nm corresponding to the (100) plane and (101) plane of ZnO, respectively [33]. Because of the low crystallinity, the lattice streaks of Ce-CN were not detectable. In addition, the utilization of energy dispersive spectroscopy (EDS) verified the existence of Zn, Ce, C, N, and O (Fig. 3(d-i)). The homogeneous distribution of all the elements suggests that the ZN/Ce-CN structure was successfully constructed.

3.4 XPS analysis

The ZN/Ce-CN nanocomposite's surface chemical composition and valence states were ascertained by XPS analysis; the outcomes are shown in Fig. 4. X-ray photoelectron spectroscopy (XPS) survey spectrum was utilized to confirm the presence of elements in ZN/Ce-CN nanocomposite which include C, O, N, Ce, and Zn, as shown in Fig. 4(a), which coincides with the EDX analysis (Fig.4i). In the HR-XPS spectrum of C 1s (Fig. 4(b)), the peaks at 284.9 eV and 287.9 eV which represent the C-C (sp³) and N-C=N (sp²) bonding respectively [34]. The sp³ carbon peak was caused by the presence of residual unreacted precursor, turbostratic carbon, and adventitious carbon, whereas the sp² carbon peak was caused by aromatic carbons building heptazine units (C₆N₇) of g-C₃N₅ matrix. The peak positioned at 398.5 eV in the HR-XPS spectra of N 1s (Fig. 4(c)) was assigned to the additive contribution of the secondary C=N-C/anionic nitrogen in the O-Zn-N linkage [19]. The typical peak at the binding energies of 399.4 eV corresponds to the tertiary (N-(C)₃) nitrogen of the heptazine (C₃N₄) moiety. As a result of the cumulative contribution of bridging azo (C-N=N-C) nitrogen and residual -NH₂ on the edge, a peak with a band energy value of 400.4 eV was detected. HR-XPS spectra of ZN/Ce-CN in the O 1s region (Fig. 4(d)) revealed two unique deconvoluted peaks at BE ≈ 530.6 and 531.7 eV. The former corresponds to the lattice oxygen of ZnO, whereas the latter corresponds to surface-adsorbed OH species. The position of the Zn 2p_3/2 and Zn 2p_1/2 peaks for the ZN/Ce-CN (Fig. 4(e)) are around 1022.2 eV and 1045.3 eV, respectively, demonstrating that the Zn element resides largely in the form of Zn²⁺ [8, 35]. The Ce 3d spectrum is seen in Fig. 4(f), which is associated with the Ce⁴⁺ 3d_3/2 cerium state and has peaks with binding energies of 898.4 and 882.02 eV. The thick layer of g-C₃N₅ covering the ZN/Ce-CN surface is the reason for the faint Ce signal [36]. Hence, the ZnO/Ce-CN nanocomposite's XPS spectrum revealed a robust chemical connection between ZnO and Ce-CN, which leads to the heterojunctions formation.

3.5 UV-Vis

Using UV–vis DRS (Fig. 5(a)), the optical characteristics of the as-obtained pure g-C₃N₅, ZnO, Ce-CN, ZN/CN, and ZN/Ce-CN nanocomposite were determined. ZnO's absorption edge is approximately 397 nm, while g-C₃N₅'s is approximately 725 nm. g-C₃N₅ has a higher visible-light absorption capacity than pure ZnO, indicating that it can be activated by visible light and that it is a photo-responsive material capable of improving ZnO's light absorption capability. Because of an extended π conjugated network resulting from the overlap between N 2p in a heptazine π conjugated system and N 2p orbitals of bridging azo moieties, g-C₃N₅ absorbs substantially in the visible region, with band tails reaching 660 nm [29]. While the absorption edge of Ce-CN is roughly 713 nm, compared to pure g-C₃N₅, the Ce-CN absorption band has a blue shift, which could be caused by the quantum confinement effect and Ce-CN's defect state following cerium doping [37]. However, when compared to pure ZnO, the visible light absorption of ZN/Ce-CN nanocomposite increased considerably, and a large redshift was observed. It suggests that the nanocomposite are good photocatalytic materials. The Tauc model was utilized to further estimate the band gap values of the photocatalysts [38], as demonstrated by the Tauc plot presented in Fig. 5(b). For g-C₃N₅, ZnO, Ce-CN, ZN/CN, and ZN/Ce-CN, the observed band gaps (Eg) were 1.96, 2.71, 1.77, 2.66, and 2.31 eV, in that order. Ce-CN doping results in a significant improvement in the light absorption intensity (band gap decreased to 2.31 eV) of ZN/Ce-CN, which supports higher photocatalytic activity.

3.6 PL

Fig. 6 depicts the optical photoluminescence (PL) emission spectra stimulated at 389 nm, which were utilized to determine the carrier transfer efficiency in the photocatalytic process. The noticeable fluorescence emission peaks of g-C₃N₅ indicate significant photogenerated electrons and holes recombination rates. Ce is added to g-C₃N₅ to lessen the intensity of its PL emission. The PL emission peak of ZN/Ce-CN was further reduced after composite with ZnO, indicating that the recombination of carriers in the heterogeneity of the composite photocatalyst was significantly suppressed, effectively promoting carrier migration and separation [39].

3.7 Photo-electrochemical properties

In general, the greater the intensity of the photocurrent, the greater the electron-hole separation and migration efficiency. The effectiveness of charge separation was further investigated using transient photocurrent response and electrochemical impedance spectroscopy (EIS). The photocurrent transient response curve (I-t) (Fig. 7(a)) shows that when exposed to radiation, the photocurrents generated by ZN/Ce-CN are substantially stronger than those generated by pure g-C₃N₅ and ZnO. The photocurrent responses of pure g-C₃N₅, ZnO, and ZN/Ce-CN reverted to their initial values once the irradiation was discontinued. This reveals that the productivity of an electrons and holes dissociation has enhanced after combining g-C₃N₅ and ZnO due to higher photo-excited electrons production and transmission capacities [40, 41].

Furthermore, as shown in Fig. 7(b), the photo-generated carriers' transfer efficiency was thoroughly investigated using electrochemical impedance spectroscopy (EIS). The radius of the arc is closely related to the transport efficiency of the photogenerated carriers, the smaller the radius, the higher the transport efficiency due to less charge transfer resistance [42]. In the EIS Nyquist plots, the arc radius of the ZN/Ce-CN composite was smaller than that of Ce-CN and ZnO, implying an increase in electronic conductivity. Furthermore, ZN/Ce-CN possessed the highest electronic conductivity, which might be attributable to the formation of sufficiently tight interfacial connection between ZnO and CN. Photoelectrochemical study clearly showed that combining ZnO and Ce-CN can effectively improve interfacial electron transport, resulting in the generation of sizable free electrons that can further activate the MB efficiently, thus greatly enhancing the catalytic efficacy of the system [42].

3.8 Photocatalytic degradation activity of MB

As shown in Fig. 8(a), the produced nanoparticles' photocatalytic activity was assessed using methylene blue dye as a model pollutant. Initially, the dye solution was agitated with the photocatalyst in the dark for 30 minutes to achieve adsorption-desorption equilibrium. The pristine ZnO NPs and pure g-C₃N₅ nanosheets exhibited a poor photodegradation efficiencies percentage under 90 minutes irradiation at 69% and 55% respectively. It indicates that the recombination rate of photo-induced electron-hole pairs is faster in the one-component system of ZnO and g-C₃N₅. In comparison to single-component materials, all doped and composite photocatalysts demonstrated noticeably improved photocatalytic performance. ZN/(1%)Ce-CN nanocomposite shown the highest MB degradation efficiency of 97%, which was relatively higher than that of Ce-CN(76%), ZN/CN(77%), ZN/(0.25%)Ce-CN(92%), and ZN/(0.2%)Ce-CN(86%), under 90 minutes of UV-visible light irradiation. As a result, the effective production of heterojunctions under the synergistic effect of ZnO and Ce-CN plays a critical role in methylene blue degradation.

Next, Fig. 8b shows that the catalyst's degradation time of MB and -ln(C/C₀) is proportionate, in line with the first-order kinetic equation [43, 44]. The rate constant of (1%)ZN/Ce-CN nanocomposite was 0.312 min^-1, which is 5 times higher than that of g-C₃N₅, 2.9 times higher than that of ZnO, 3 times higher than that of Ce-CN, and 2.5 times higher than that of ZN/CN. The results revealed that (1%)ZN/Ce-CN nanocomposite had a high photocatalytic degradation efficiency for removing MB.

The role of pH on UV-Vis assisted degradation of MB in the presence of ZN/Ce-CN nanocomposite is shown in Fig. 8c. At pH 13, ZN/Ce-CN nanocomposite exhibited the most photocatalytic activity, reducing MB to around 99%. The ZN/Ce-CN nanocomposite degraded from 99% to 42% when the pH was lowered from 13 to 2, suggesting that the alkaline environment aids in the adsorption and photodegradation of MB. This is because the active sites of ZN/Ce-CN nanocomposite amass a significant amount of H⁺ ions in the pH range of 2-4.8, rendering them positively charged and repelling MB (a cationic organic molecule). With a pH of 10-13, there will be a lot of negative charge on the surface of the photocatalyst, which increases the strong electrostatic attraction between the electrocatalyst and the positively charged MB and promotes photocatalytic efficiency. However, with a pH of 10-13, there will be a lot of negative charge on the surface of the photocatalyst, which increases the strong electrostatic attraction between the ZN/Ce-CN nanocomposite and the positively charged MB and promotes photocatalytic efficiency [45].

3.9 Determination of reusability of composite ZN/Ce-CN

One of the most important features for degrading organic dyes is the material's stability [46]. To evaluate the stability of ZN/Ce-CN, the recycled photodegradation removal experiment of MB was studied. The photocatalytic degradation efficiency of ZN/Ce-CN composite stayed constant at 87.67% after five cycles (as seen in Fig. 9(a)), varying by about 2% to 4% per cycle, suggesting that it still possesses high photocatalytic activity. In addition, The XRD measurement results (Fig. 9(b)) demonstrated that the fresh ZN/Ce-CN and the utilized ZN/Ce-CN have similar XRD signals, indicating that ZN/Ce-CN had good structural stability. The results of this experiment revealed that the photodegradation efficiency of the ZN/CN nanocomposite slightly decreased after cycling experiments, which could be due to the fact that the photocatalyst was washed with distilled water several times and used for a long time, reducing their surface area and thus limiting the arrival of methylene blue to the active site. However, it does not affect the good stability and reusability of ZN/CN in the photocatalytic degradation process.

3.10 Trapping of reactive species

In order to identify the primary active species, free radical scavenger experiments were also used. Potassium iodide (KI), isopropanol (IPA), and p-benzoquinone (p-BQ) were respectively added to capture h⁺ (holes) [47], •OH (hydroxyl radical) [48] and •O₂^- (superoxide radical) [49].

According to Fig. 10(a), without the use of a trapping agent, the degradation rate of MB was 97%, but it reduced dramatically with the addition of potassium iodide (KI), from 97% to 52%, showing that the h⁺ play a prominent role in the photocatalytic degradation process. The degradation rate of MB reduced from 97% to 67% when isopropanol (IPA) was introduced, demonstrating that •OH constitute a major component impacting degradation. Additionally, the degradation rate of MB dropped marginally from 97% to 81% with the addition of p-benzoquinone (p-BQ) solution to the reaction solution, indicating that only a small portion of the degradation reaction is caused by •O₂^-. According to the findings, the principal active ingredient is h⁺ in the photocatalytic degradation reaction, while •OH also play a significant role, with some contribution from •O₂^-.

To confirm that the degradation process generates the aforementioned reactive oxidants, a possible free radical detection was performed utilizing an ESR test using DMPO as a probe (Fig. 10(b-d)). Under dark conditions, h⁺ showed a strong activity signal, which gradually decreased with increasing light time, due to the oxidation of TEMPO to TEMPO⁺ by h⁺, which has a strong oxidizing ability [50]. It can be seen that there are h⁺in the degradation reaction, and the ZN/Ce-CN nanocomposite can maintain a significant quantity of h⁺ in the light. It implies that the h⁺ species plays the most important function in the photocatalytic process.

Fig. 10(c,d) show that under dark conditions, ZN/Ce-CN had almost no active species characteristic signals, but after 10 minutes of light irradiation, DMPO-•O₂^- and DMPO-•OH appeared as significant characteristic peaks, producing more photogenerated carriers, which provides significance for the photocatalytic reaction [51].

3.11 Band structure of ZN/Ce-CN composite and charge transfer mechanism

The Mott-Schottky technique was used to determine the type of semiconductor and the flat band potential (Ef). The Mott-Schottky curves (Fig. 11(a,b)) for ZnO and Ce-CN have positive slopes, suggesting that they are both n-type semiconductors [52]. In the meantime, ZnO and Ce-CN had Ef values of -0.49 eV and -0.59 eV, respectively, concerning the Ag/AgCl electrode and pH of 7. That is, ZnO and Ce-CN had conduction band potentials of -0.29 eV and -0.39 eV, respectively, concerning NHE, pH = 7. Additionally, ZnO and Ce-CN had band gaps (Eg) of 2.71 and 1.77 eV, respectively, according to Fig. 5(b). As a result, their corresponding valence band potentials were 2.42 and 1.38 eV, respectively.

Together with the Eg values of Ce-CN and ZnO in the DRS diagram and the band potentials form Mott-Schottky test (Fig. 11), a potential mechanism is conjectured (Fig. 12). For Ce-CN and ZnO, the CB potentials are -0.39 and -0.29 eV, respectively, and the VB potentials are 1.38 and 2.42 eV, respectively, for pH = 7 relative to NHE. Ce-CN could form S-scheme or a type II heterojunctions with ZnO because its CB and the VB potentials were more negative than ZnO's. Which was it, out of the two? A lot relied on the specific electron transfer mechanism in this case. The electrons generated on the CB of Ce-CN would flow to the CB of ZnO when exposed to light if a typical type II heterojunctions formed (Fig. 12, left). However, as the CB potential of ZnO (-0.29 eV) is more positive than the redox potential of O²/•O₂^- (-0.39 V versus NHE, pH = 7) [53], •O₂^- cannot be generated. In a similar manner, the h⁺ produced on ZnO's VB would flow to Ce-CN's VB. However, it is evident that •OH cannot be formed as the VB potential of Ce-CN (1.38 eV) is more negative than the redox potential of •OH/OH- (2.42 V vs NHE, pH = 7) [54], which is inconsistent with the experimental data. Consequently, it is possible to rule out the creation of type II heterojunctions in ZN/Ce-CN composites (Fig. 12, left).

On the contrary, in the S-scheme heterojunctions (Fig. 12, right), the photogenerated electrons are transported from ZnO to Ce-CN. The e- produced on ZnO’s CB would flow to Ce-CN’s VB and combine with the h⁺ on it. The e- on the CB of Ce-CN would convert O² to •O₂^-, while the h⁺ on the VB of Ce-CN would react with OH- to form •OH. Ultimately, the MB and its degradation intermediates were oxidized by the generated reactive species h⁺, •OH, and •O₂^-. The results of reactant scavenging tests and ESR spin trapping confirm the aforesaid observations. The above analyses all point to the creation of S-scheme heterojunctions in the ZN/Ce-CN composite.

The effective, ecologically acceptable, and stable photocatalytic material ZN/Ce-CN was successfully created using a simple calcination-hydrothermal technique for the photodegradation of methylene blue (MB). The ZN/Ce-CN heterojunctions photocatalyst shown better photocatalytic activity, with a degradation rate of up to 97% at 90 minutes when compared to pure ZnO, g-C₃N₅, Ce-CN, and ZN/CN. The kinetic constant for methylene blue (MB) degradation on the optimized ZN/Ce-CN heterojunctions photocatalyst was 2.9 times higher than that of ZnO and 5.0 times higher than that that of g-C₃N₅. Cycling experiments demonstrated that the ZN/Ce-CN heterojunctions photocatalyst retained good stability and repeatability after five cycles. ZN/Ce-CN has lower charge transfer resistance and higher charge separation efficiency in electrochemical testing. Active trapping experiments verified that h⁺ and •OH play a substantial role for degradating MB. Finally, the Mott-Schott test further revealed that the presence of S-scheme heterojunctions in the ZN/Ce-CN composite. In summary, ZN/Ce-CN, S-scheme heterojunctions photocatalyst lay the foundation for the continuous development of efficient green catalysts in the fields of environment and energy, and are also promising candidates for future water treatment.

CRediT authorship contribution statement

Jia Jia: Visualization; formal analysis; writing - review and editing. Lili Huang: Conceptualization; Visualization; formal analysis; writing - review and editing. Yumin Yan: Conceptualization; data curation; methodology; project administration. Haiqiao Wang: Investigation; data curation. Mingxia Tian: Formal analysis; writing - review and editing. Jianhui Jiang: Conceptualization; funding acquisition; investigation; methodology; supervision; validation.

Data availability statement

The data used to support the findings of this study are available from the corresponding author upon request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The work was financially supported by the Bingtuan Science and technology Program (2022ZD099) and the presidential research fund of Tarim University (TDZKCX202208).

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Scheme 1 is available in the Supplementary Files section.

No competing interests reported.

scheme1.png
Scheme 1 Preparation scheme of ZnO/Ce-C₃N₅ (ZN/Ce-CN) composites.
GA.png
Graphical abstract Proposed photocatalytic dye degradation mechanism of ZN/Ce-CN

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A novel S-scheme ZnO/Ce-g-C3N5 heterojunctions with enhanced photocatalytic activity

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